Historical Perspective: [301440]
Introduction:
[anonimizat]- dictated by social pressure and patients’ needs- had changed all perspectives of dental treatment.
Approaching the end of the last century, a [anonimizat]. [anonimizat], both dental materials and dental technologies have progressed remarkably.
[anonimizat], [anonimizat], adequate clinical function and longevity.
[anonimizat] a [anonimizat]. It represents a versatile material that holds a [anonimizat].
[anonimizat], the mechanism is also responsible for its low temperature degradation (LTD). [anonimizat], micro-cracking and strength degradation.
[anonimizat]’s relatively opaque and monochromatic in color. This limits its use solely without an esthetic veneering ceramic since the eye is able to easily distinguish between a [anonimizat] a ceramic crown.
[anonimizat], [anonimizat] Y-[anonimizat].
[anonimizat]-up of the final anatomic shape and color characterization by manual veneering is necessary for restorations that require a high degree of individual coloring and esthetics.
Owing to the inertness and surface stability of Y-TZP establishing a durable chemical or mechanical bond between zirconia core and veneering porcelain was proven to be a difficult task. [anonimizat] . Accordingly, [anonimizat]-veneer bond strength has to be of a certain minimal strength.
Different surface treatment methods of the zirconia core have been investigated trying to improve the bond strength between the zirconia core and veneering ceramic, but up to date no conclusion has been reached regarding the exact bonding mechanism.
Though it’s quite uncommon for zirconia-veneered restorations to undergo bulk fracture of the zirconia framework itself where it had been reported in a 4 –year clinical study that zirconia framework failure rate was only 1%, yet it’s still uncertain whether the enhanced mechanical properties of the core material would improve the clinical performance of the ceramic-veneered restoration.
Bearing in mind the brittle nature of dental ceramics and their sensitivity to tensile stresses, different tests have been designated to test their mechanical properties with each of those tests having their own advantages and limitations.
Historical Perspective:
Zirconia-Based Ceramics:
Zirconia is a crystalline dioxide of zirconium whose first biomedical application was when it was introduced for the manufacture of ball heads for total hip replacements. It was later introduced in the dental field due its excellent mechanical properties and improved esthetic properties compared to metal-ceramic restorations. Its first use for root canal dowels was in 1989, for orthodontic brackets in 1994, for implant abutments in 1995 and for all-ceramic fixed partial dentures was in 1998 .
Pure, unalloyed zirconia is polymorphic and allotropic – at ambient pressure – occurring in three crystalline forms. At room temperature, pure zirconia is monoclinic and remains stable at this phase up to 1170C. Above this temperature, it transforms into tetragonal phase and then into cubic phase at 2370C. Upon cooling, phase transformation from tetragonal to monoclinic occurs and is accompanied by 4-5% increase in volume that is sufficient enough to cause catastrophic failure. Alloying pure zirconia with oxides such as CaO, MgO, Y2O3, CeO2 allows the retention of the tetragonal phase at room temperature, thus controlling the stress-induced tm transformation, efficiently arresting the crack propagation and improving the fracture toughness .
There have been many types of zirconia-based ceramics used in dentistry which are;
Glass-infiltrated zirconia toughened alumina: Uses high temperature sintered alumina glass-infiltrated copings. The flexural strength of the framework material ranges from 236 to 600 MPa, and the fracture toughness ranges between 3.1 and 4.61 MPa(m) ½.
Magnisuim partially stabilized zirconia (Mg-PSZ): It consists of clusters of tetragonal crystals within stabilized cubic zirconia matrix with the added stabilizer being MgO (8-10 mol %). This material isn’t widely used due to its high porosity, large grain size, low stability and low overall mechanical properties.
Yttrium partially stabilized tetragonal zirconia polycrystals (3Y-TZP): It has 3 mol% Yttria added as a stabilizer and consists of small equi-axed grains (0.2-0.5m in diameter) depending on the sintering temperature. It has superior mechanical properties with flexural strength 900-1200 MPa and fracture strength 9-10 MPa( m)1/2 .
Aging of zirconia:
For phase transformation toughening (ptt) to happen, a basic requirement is the presence of t-ZrO2 grains in a thermodynamic metastable state at the conditions at which the crack is trying to propagate. This thermodynamic metasatability is the same principle responsible for low thermal degradation (LTT) or aging of zirconia. LTD is defined as the spontaneous t-m transformation occurring over time at low temperature and humid atmosphere when the t-m transformation isn’t triggered by any local stresses produced at the tip of the advancing crack. The process usually begins at the surface and then proceeds to the bulk of the material.
Susceptibility to LTD is controlled by multiple material’s properties such as density, purity, grain size and type and content of stabilizer. Chevalier et al in 2007 stated that as the density of material-being affected by the manufacturing process- decreases, liability for LTD increases due to the ease of water penetration inside the material. Susceptibility to LTD is also influenced by stabilizer concentration where it was shown Chevalier et al in 2009 that LTD decreases as stabilizer concentration increases, while on the other hand, increased phase stability restricts the stress-induced transformation toughening and decreases the fracture toughness of the material as stated by Kondoh et al in 2004. As for the effect of grain size on liability for LTD, many authors showed that increased grain size causes the tetragonal phase to be less stable and thus increases the susceptibility for LTD. According to multiple studies, critical grain size- i.e. the grain size below which t-m transformation is greatly impeded – of 55-360 nm was defined.
Multiple mechanisms had been proposed trying to explain the phenomenon of LTD;
As proposed by Lange et al; water reacts with Y2O2 to form clusters rich in Y(OH)3 which consequently leads to depletion of the stabilizer in the surrounding zirconia grains that are then free to transform into monoclinic phase.
Yoshimura et al suggested that water vapour attacks the Zr-O bond breaking it and leading to stress accumulation due to movement of (_OH) which in turn elicits lattice defects acting as nucleating agents for further t-m transformation. This was confirmed by Lughi V and Sergo V who stated that water vapor possibly causes and certainly accelerates LTD.
Finally, according to Chevalier et al, it was suggested that (O2-) originating from the dissociation of water is responsible for filling of the oxygen vacancies that are believed to be one of the causes of destabilization and thus for LTD.
Despite the suggested mechanism for LTD, it starts at the surface and proceeds inwards causing surface uplifting with subsequent surface roughness and esthetic degradation. It then opens the possibility for water penetration below the surface allowing t-m transformation to propagate to the bulk of the material causing major cracks that end up in catastrophic failure..
An in vitro research was aimed at comparing failure loads of standardized zirconia 3-unit FPDs before and after exposure to an artificial aging process by means of a mastication simulator, corresponding to a 5-years of clinical function (about 1,200,000 cycles of thermo-mechanical fatigue in liquid environment). Following such treatment, failure loads of all test samples were reduced with significant differences due to different fabrication techniques for each system, but such a reduction ranged within clinically acceptable values. In fact, all test specimens showed minimum failure loads higher than 1000N, both before and after fatigue loading, thus widely exceeding average masticatory loads .
In the same paper, the authors addressed a warning to clinicians about the risks of intentionally leaving a Y-TZP framework without ceramic veneering at the level of the gingival side of the FPDs, as suggested by other researchers, in order to enhance the core strength. They stated that this would expose the zirconia framework to the intraoral salivary environment, increasing, at the same time, the potential for plaque retention and reducing resistance to low temperature degradation and service life.
On the other hand, a study was carried out by Nakamura K. in 2015 to assess the effect of low temperature degradation induced by autoclaving on the mechanical and microstructural properties of tooth colored Y-TZP. It was found out that the biaxial flexural strength values increased after ten hours of autoclaving, but then after 100 hours, values decreased becoming almost the same with or without autoclaving.
The effect of adding certain dopants to zirconia and its effect on resisting low temperature degradation was investigated. Nakamura T. et al in 2011 and 2012 investigated the effect of doping Y-TZP with 2 mol % of Si before sintering and its effect on low temperature degradation. They concluded that adding small amount of silica (0.5 wt %) improves the material’s resistance to low temperature degradation through the presence of round grains that decrease the internal stresses.
Esthetics of Zirconia-Based Restorations:
Color Reproduction and Coloring Techniques:
The integration of restoration with biologic tissues and the attainment of normal function are the goals that clinicians and technicians aspire to in every dental practice.
Since the apparent color of natural teeth is the result of the reflectance from dentin modified by absorption, scattering and thickness of enamel, thus, successful aesthetic restorations require the integration of several factors as the individual’s perception of color, the light source used for color evaluation, the surface and structural characteristics of both the tooth and the restorative materials used and knowledge of some basic principles of color perception..
Color is complex and encompasses both subjective and objective phenomena and is the result of response of the eye and the brain to a very narrow segment of electromagnetic spectrum. Since Newton’s experiments in the 18th century, it has been difficult to explain the scientific basis of color and attempts were always searched to translate it into numerical data for industrial perfection, repeatability and predictability. At the beginning of the 20th century, professor Albert H. Munsell introduced the color wheel showing three dimensions of color; hue, chroma and value. Hue is used to describe the pigment of the tooth or dental restoration. Chroma is the intensity of the color tone where the more the wavelengths of the color that are reflected, the higher the chroma of the hue. Value is the relative darkness and lightness of the hue where the higher the amount of light reflected from the surface, the higher is the value.
The paper from Clark 1933 , based mainly on the Munsell color scale of 1905, was the first attempt to organize dental colors. In 1950s, the first dental shade guides based on a rational arrangement of shade tabs were introduced to the dental profession.
Sproull in the early 1970s published a series of articles in which the three dimensional nature of color and its relationship with dental shades was studied, and a series of theoretical and practical indications were given in order to improve color matching in dentistry. These articles, that represented for a long time, the state of the art for dental color matching, pointed out how the procedure for shade taking was negatively influenced by several factors, among which was the shade guides that were regarded as poor and inadequate in relation to the complexity of the appearance of the teeth.
In 1976 and 1978, the Comission International de l’Eclairage developed a new system called CIE Lab* in which, for the first time, it was possible to express color by numbers and calculate the difference between two colors in a way that corresponded to the visual perception. In this system, color is expressed by three coordinates: L* value is the degree of lightness of an object, a* value is the degree of redness/greenness and b* value is the degree of yellowness/blueness .
In the CIELab* system a formula is used to calculate color differences;
∆E=[(L1-L2)2+(a1-a2)2+(b1-b2)2]1/2
The ∆E values are used to describe whether the changes in the overall shade are perceivable to the human observer. This magnitude of the color difference is based on the human perception of color, where color differences greater than 1 ∆E unit are visually detectable by 50% of human observers. Yet, under uncontrolled clinical conditions, such small differences in color would be unnoticeable because average color differences below 3.7 have been rated as a match in the oral environment . Despite the much effort, the identification of ∆E value for the clinically acceptable differences is a very difficult task and the establishment of a widely accepted limit is still controversial.
In order to minimize the color mismatch due to visual estimation and improve the color selection procedure, shade taking devices were developed where instrumental determination of color offer readings that are objective, quantified and more rapidly obtained .
A number of color measuring instruments are commercially available and can be divided into colorimeters and spectrophotometers. Colorimeter is generally a relatively simple instrument designed to measure color on the basis of three axes or stimuli by using a filter that simulates the human eye .
The spectrophotometer is a more sophisticated instrument, built to measure by reflection or transmission an observed object, giving the entire spectral curve and is limited for color measurement to the visible frequency range. Although the first spectrophotometers were already very accurate, they were quite complex, difficult to manage or accurately calibrate and their use in dentistry was limited to research purposes. In the last decade, advances in electronics, more sophisticated charge-coupled devices (CCDs) and better fiber optic technology have resulted in development of clinical shade taking devices.
One of the best rated benchmark instrument is the VITA Easyshade Compact, released in 2009 as an evolution of the former Easyshade is engineered relying on large diameter fiber optics arranged in a specific pattern in a stainless steel probe of 5-mm diameter that can both illuminate a tooth and receive light that is internally scattered by the enamel layer and reflected from the dentin layer of the tooth . Due to the array of light fibers, different measurement modes are possible with Easyshade Compact: tooth single mode, tooth area mode (cervical, middle and incisal shades), restoration color verification (provides ΔE values and includes lightness, chroma and hue comparison) and shade tab mode (practice/training mode) .
Browning et al 2009 reported that the Easyshade device, when compared for accuracy and consistency to three experienced clinicians, was at least comparable if not better than the dentist. In particular, the degree of matching was 91% for Easyshade and 69%, 85% and 79% for the three clinicians, respectively. Kim-Pusateri et al in 2009 reported accuracy of 93% for Easyshade. It should be noted that the results seemed to improve with time .
Though color of raw Y-TZP is white (73), however it can be easily colored through customized coloring liquids or being supplied in a pre-shaded form.
In pre-shaded Y-TZP, metal oxides are mixed with the starting Y-TZP powder before sintering at high temperature, this technique has successfully led to the reproduction of human teeth shades as mentioned by Cales B in 1998. Manufacturers reported advantages for their pre-shaded zirconia products such as; producing a homogenous color in the restoration with more predictable results and increasing the chameleon effect together with reducing the post sintering modification needed.
Though, Aboushelib et al 2010 stated that pre-colored zirconia frameworks did not offer any advantage of increased predictability over the uncolored ones. However, Hjerppe et al 2008 reported that biaxial strength is dropped with coloring and the amount of decrease is both shade and time dependent
KATANA Zirconia ML full contour zirconia from Kurary offered the world‘s first polychrome zirconia in 2014 with integrated color shift; starting with a deeply chromatic cervical shade, 2 transitional shades, and ending with an enamel shade.
Y-TZP blocks could be custom-colored where this technique involves the infiltration of machined restorations at the pre-sintered stage that have been brought into a highly porous state with special coloring solutions to produce work pieces of various shades.
The solutions are preferably water- or alcohol-based. Suitable salts or complexes are preferably those from the group of the rare earths or the 2nd or 8th subgroups, in particular Pr, Er, Fe, Co, Ni, Cu. Iron (Fe) is used for brown (main ingredient of A shaded coloring liquid), erbium(Er) is used for light violet, neodynium (Nd) is used for light pink, cerium (Ce) is used for cream and/or orange, terbium (Tb) is used for light orange, manganese (Mn) is used for black, and praseodymium (Pr) is used for dark yellow. Ions of these salts appear in liquids as acetate, chloride, nitrates, and nitrides. Stabilizing agents or complexing agents are used to stabilize the metal salts in their oxidation stage and in solution, and finally grinding auxiliaries as well as organic dye stuff pigments are there to facilitate matching of the color.
These solutions could be applied mainly through immersion of the work-piece in solution for a defined time and concentration, deposition of the solution by means of spraying process or through deposition of solutions by means of suitable application instruments as; brush, swab.etc.
After drying and at the initial stage of heating of the immersed porous pre-sintered zirconia block, the anions (acetic, chloric, and nitric ions) probably burn out or vaporize, and disappear on the surface of the pores of the zirconia block. The metal ions form an oxide layer on the surface of the pores of the zirconia block. It can be assumed that these metal ions rarely react with zirconia below the pre-sintering temperature. This means that these metal oxides form a new product with zirconia via a solid-solid reaction or remain as oxides at the boundaries around zirconia grains in the final sintering stage (1350-1600șC).
The effect of different solutions, concentrations, time and method of applications on the final esthetic outcome was not sufficiently studied in literature. Suttor et al 2004 reported that the depth of color is dependent on the effect of concentration of coloring liquid rather than time of application. PH value changes and the release of ions were mentioned also as secondary factors controlling depth of the color.
These techniques of infiltration with a coloring liquid although were thought to have theoretically more customization and characterization few drawbacks as lack of homogeneity and infilteration of the surface layers more than the bulk material had been reported. Shah et al 2008 referred to this as a result of increased coloring liquid concentrations, that a large amount of solvent was evaporated with sintering, leading to an increased dopant concentration on the top surface of the discs. That would explain the deeper chroma observed on the same surface compared to the bottom surface. Other authors related this phenomenon to Oxygen vacancies and diffusivity of Oxygen in interfaces or grain boundaries.
Translucency of Core-based Restorations:
The esthetic value of a ceramic crown is based on its ability to harmonize with the natural tooth. Key optical factors that permit a pleasing harmony are color, surface texture, and translucency. Many authors note translucency as a key component for ceramic restorations. Translucency occurs when a light beam passing through a material, is partly scattered, reflected, and transmitted through the object; the greater the quantity of light that passes through the object, the higher the translucency. Scattering of light is generated by many factors, such as different refractive indices among ceramic phases, voids and porosities, high crystalline content, and crystal number and size, especially when the crystal particles are slightly larger than the wavelength of the incident light.
A material’s translucency could be measured in terms of contrast ratio (CR), absolute translucency, or translucency parameter (TP). Contrast ratio (CR=Yb/Yw) is defined as the ratio of the illuminance (Y) of the test material when it’s placed over a black background (Yb) to the illiminance of the same material when it’s placed over a white background (Yw). This ratio tends towards unity for opaque materials and towards zero for transparent materials. It’s calculated using any instrument capable of quantitatively measuring visible light intensity. Absolute translucency, on the other hand, measures light transmission percent. Hefferenan et al compared the translucency of various full-coverage, all-ceramic restorations. They measured the relative translucency of the core samples and found no difference between the light transmitted through the metal and the zirconia. Based on visual examination, full-coverage, all-ceramic restorations with zirconia cores appear to transmit light yet in their study the contrast ratio was 1.00 indicating an opaque specimen.
This outcome raises questions about the value of contrast ratios when comparing ceramics. Spink L. et al stated that the relationship between absolute and relative translucency is sensitive only down to 50% transmission. Once the translucency of a material drops below 50%, contrast ratios converge to 1.0.
On the other hand, translucency parameter (TP) is defined as the color difference of a material of a given thickness over white and black backgrounds, and corresponds directly to common visual assessments;
TP= [(Lb* – Lw*) 2 + (ab* – aw*) 2 + (bb* – bw*) 2 ]1/2
Where TP value of zero corresponds to a completely opaque material and the greater the TP value the higher the actual translucency of the material. Despite of these studies, there is no standard or consensus on the method of choice to quantify translucency of aesthetic restorative materials.
Kelly et al demonstrated that core translucency was one of the primary factors in achieving good esthetics and that it affected the shade of artificial restorations.
However, zirconia- containing core materials have poor translucency and are difficult to satisfy the esthetic requirements because of the chemical nature, the amount of crystals, the particles’ size, the pores and the sintered density that determine the amount of light that is reflected, transmitted and absorbed and thus influencing the optical properties of the core materials. Less crystalline content and a refractive index close to that of the matrix cause less scattering of light and since Y-TZP is polycrystalline and has a different refractive index than the matrix, most of the light passing through it is intensely scattered and diffusely reflected leading to an opaque appearance.
Jiang Li et al stated that addition of alumina Y-TZP for improving the mechanical property could result in greatly decreasing the transmittance. This was attributed to the finding that alumina addition leads to decreased relative density where the relative density represents the porosity of the material. It was stated that even a minor amount of residual porosity could significantly prevent translucency in oxide ceramics.
The limited translucency of the conventional zirconia materials lead to the introduction of the modified monolithic zirconia. Beuer et al in 2012 evaluated the light transmission of full contour zirconia crowns compared to powder build-up veneered zirconia substructures and CAD/CAM-veneered zirconia sub-structures and found out that polished full-contour zirconia restorations showed significantly higher light transmittance than the rest of the groups.
Nanocrystallyine Zirconia:
Regarding the esthetic drawbacks of conventional yttria-stabilized zirconia, many attempts have been made trying to optimize the production conditions to improve its optical properties where some studies suggested that microcrystals and full densification could enhance the optical properties and light transmittance
Translucency of zirconia is related to the amount and type of additives, the sintering temperature, the atmospheric conditions during the sintering process, and the heating methods.
In particular, the final temperature of the sintering process and the heating method used are direct determinants of the density, porosity, and grain size of zirconia where as temperature rose, particles were sintered together, pores on the grains’ boundaries were reduced and sintered density increased .
Li Jiang et at stated that Y-TZP could gain nearly full density and 17–18% transmittance at the final sintering temperature of 1,450-1500 C and that Y-TZP of the smaller nano-particles had higher density and transmittance than that of the larger particles.
Attempts trying to introduce translucent zirconia had been through consolidating nano-powders to full density with nano-crystals through the industrial sintering technique such as hot-isostatic pressing (HIP), microwave and millimeter wave sintering, and spark plasma sintering (SPS) .
Electric current assisted powder consolidation techniques have become of the most successful routes for producing fully dense bulk oxide ceramics with crystal sizes in the nanocrystalline range. Recently, a high pressure version of the technique has been shown to be capable of producing dense materials with a remarkable <20 nm grain size.
The novelty of the technique lied in the simultaneous application of high current densities and pressures in addition to the traditional processing parameters of temperature and time. In the literature, this technique is often called spark plasma sintering (SPS). Maximum pressure plays a crucial role in overall density..
This unique technique gives a chance to decrease the heating and the cooling time of the sintering process which in turn minimizes the amount of grain growth in the material and maintains the nanometric grains of the powder .
However, it is important to emphasize that the consolidation of ultrafine powder is very difficult because generally, as the size decreases below ≈0.5μm, the particles exhibit a greater tendency to interact, giving rise to the formation of agglomerates. One consequence of the presence of agglomerates is that the packing of the consolidated powders can be quite non-uniform and, during the firing step, little benefit can be obtained over that of a coarse powder. In order to make use of the unique properties of bulk nano-crystalline materials, the nanometer range powders have to be densified with minimal microstructural coarsening and/or undesirable microstructural transformations .
While many of the nano-zirconia powders available in the market today are comprised of hard-agglomerated nano-crystals with a primary crystal size of approximately 30 nm, Glidewell Laboratories' program manager and lead researcher, developed a method for producing non-agglomerated 3 nm nano-crystalline zirconia powder using a revolutionary bottom-up nanotechnology technique known as "gas-phase condensation." .
The focused effort of the nano-zirconia research team has resulted in new discoveries about the nature of sub-5 nm nano-zirconia crystals. Glidewell Laboratories has filed a U.S. patent application on the new ceramic nanotechnology (patent pending). This method consists of colliding high-energy yttrium, zirconium and oxygen ions together in an energetic gaseous phase and condensing yttria zirconia nano-crystal particles resulting from atomic collisions during flight in the gas phase. The condensed yttria zirconia nano-crystal particles are separated from the gas phase and collected in the form of nanocrystalline powder.
According to Knapp, "The key to making transparent polycrystalline zirconia material is starting with a non-agglomerated yttria zirconia primary crystal size less than 5 nm”. The bottom-up nanotechnology method builds up nanoscale materials atom by atom or molecule by molecule. Additionally, the bottom-up-produced nanocrystalline structures are not altered during the process of forming the nanoscale crystals, whereas top-down methods alter the crystal structure and surface chemistry. Nanozirconia material produced by the gas-phase condensation method overcomes the inherent sub-5 nm crystal size production barrier and hard-agglomeration formations found in conventional nanocrystalline ceramic processing
Common zirconia dental ceramics are translucent and not transparent as a result of light-scattering during transmission by birefringence and porosity. Light-scattering by birefringence is an intrinsic property of polycrystalline optical materials with an anisotropic crystalline index of refraction. Birefringence is reduced dramatically when the sintered grain size is reduced below 100 nm. Porosity causes light-scattering in the visible spectrum between 400-700 nm, which reduces the zirconia optical transparency .
Previous work on ceramics densified from powders has confirmed that scattering is the primary reason for the opacity of most ceramics and it has been found that light scattering is largely a function of the porosity of a material. Apetz et al showed that the absorption of transparent alumina depends on the porosity and pore size of a sample. Anselmi-Tamburini et al also found that density (porosity) plays a large role in the transparency of YSZ .
However, Alaniz J. et al. believe that absorption in the visible range was attributed primarily to oxygen vacancies, and that grain boundary or grain boundary associated defects do not significantly alter the optical properties in the visible range. Their conclusion was based on the fact that their test samples were all 99.9% dense and had very small grain sizes of 55 nm and the fact that the pore diameter is usually significantly smaller than the average grain size. Anselmi-Tamburini et al. , calculated that only pores larger than 50 nm cause significant scattering and thus reduction of transmission.
Further evidence supporting their opinion that the porosity does not play a major role in the optical behavior was seen when annealing the samples in air at 750 ˚C dramatically increased the transmission.
They assumed that the dominant absorption centers in the nanocrystalline samples were oxygen vacancies created during the densification process . Annealing the YSZ samples in air had the effect of diffusing oxygen back into the sample and removing the oxygen vacancies that were created at high temperatures where there was remarkable increase in transmission when the samples were annealed for various times.
They supported their claim that grain boundaries were not a significant source of scattering by the fact that both the grain boundary width (1 nm) and grain diameter (55 nm) were significantly smaller than the wavelength of light in the transparent range. In addition, YSZ has an isotropic refractive index so the grain boundaries are not expected to behave as refractive interfaces separating grains with dissimilar orientations. They claimed that the lack of absorption by grain boundaries (or their associated defects) is due to the inherently low oxygen vacancy concentrations in the grain boundary region.
The lack of absorption at grain boundaries is important for the development of nanocrystalline ceramics that have very high concentration of grain boundaries. These findings could be technologically significant because nanocrystallinity has been previously shown to improve the mechanical properties .
Surface Treatment Methods of Zirconia Ceramics:
Though the adhesion mechanism between metal and porcelain is well-established and is believed to be through micro-mechanical bond, compatible coefficient of thermal expansion (CTE) match, Van der Waals force, and mainly the suitable oxidation of metal and inter-diffusion of ions between the metal and porcelain yet, the bonding mechanisms for veneering ceramic to the zirconia are up to now unclear .
Different surface treatment methods of zirconia core surface were suggested with varying outcomes on the core-veneer bond quality. Some of these surface treatments rely on creating micro-mechanical interlocking with veneering porcelain, chemical adhesion or both.
Surface Treatment Methods Causing Micro-mechanical Interlocking:
Grinding:
It’s a commonly used surface treatment method to improve mechanical bonding with zirconia. Several methods are used for surface grinding as; abrasive paper or wheels, particle air abrasion using Al2O3 or other abrasive particles ranging in size from 50 to 250 μm or grinding using a diamond bur .
One advantage of these surface grinding methods is that they are generally easy to apply in a dental environment. In addition to their ease of application, it was shown that surface grinding results in a tetragonal to monoclinic phase change on the surface of zirconia. This can theoretically produce a compressive stress layer that counteracts the flaw-induced reduction in strength. Work by Guazzato et al. and Kosmač et al. showed that sandblasting produced the most effective tetragonal to monoclinic phase change when compared to fine polishing, grinding with an abrasive wheel, or grinding using a diamond bur. It was determined that sandblasting was able to induce transformation at low temperature, with minimal surface damage.
Care has to be taken with the amount of surface grinding, as an excess amount can diminish the strength-enhancing effect. Care also has to be taken when heat treating surface-ground ZrO2. It has been shown that heat treatment temperatures for bonding veneering porcelain to ZrO2 substructures, around 900–930°C, can cause a decrease in flexural strength. The temperature applied during heat treatments, coupled with existing residual stresses, can sometimes be sufficient to cause a transformation of the monoclinic phase back to the tetragonal phase, which relieves the compressive stress in the surface layer and reduces the flexure strength of ZrO2 .
On the other hand, these some of these grinding techniques can create surface microcracks where it was shown that particle air-abrasion increased the surface roughness of the framework by creating sharp scratches and grooves on the surface and extending up to 30-50 Um deep resulting in grain pull-out and microcracking. This surface damage resulted in reduction of the flexural strength as well as the fatigue resistance of zirconia. It’s worth mentioning that this deteriorating effect becomes more evident when coarse aluminum particles were used (100-120 Um)
Hot Chemical Etching:
Based upon the metallic nature of pure zirconium, an experimental hot chemical etching solution has been advocated for treating zirconia. The action of the hot etching solution is basically a corrosion-controlled process where it selectively etches the zirconia, enabling micro-retention. It modifies the grain boundaries through preferential removal of the less arranged, high energy peripheral atoms.Casucci et al stated that the application of the hot experimental etching solution increased the zirconia surface roughness and created micro spaces that would optimize the overall bonding mechanism.
Laser
The most important interaction between laser and substrate is the absorption of laser energy by the substrate which is determined by the surface pigmentation, water content and other irradiated characteristics of the irradiated surface. Since zirconia ceramic is water-free and relatively opaque in color, absorption of the laser energy might be influenced, and irradiation might not obviously improve the micromechanical interlocking.
Different studies showed that Nd:YAG laser causes a smooth surface with irregular microcracks on it when used on surface of zirconia ceramics and higher output power was shown to cause larger and deeper surface cracks due to the wide melting areas formed during local temperature changes.
Li L et al in their study found that high output of Nd:YAG laser induced higher surface roughness than lower output powers, but was still lower than that of the air abrasion group . Their study indicated that surface roughness and bond strength might be enhanced by laser irradiation with stronger output power (> 3W) which on the other hand may cause ceramic defects and reduce the mechanical properties of zirconia ceramics .
Nano-Structured Alumina Coating:
It is based on applying a nano-structured alumina coating with a high surface area and good wetting ability thus creating a micro-mechanical interlocking.
The nano-structured alumina coating on zirconia ceramics is achieved by the hydrolysis of aluminum nitride to form lamellar boehmite (AlOOH) onto the zirconia surface. A series of heat treatments subsequently follow where the boehmite undergoes a series of phase transitions into alumina. It results in the formation of a discontinuous nano-structured alumina coating (240 nm thick) on to the zirconia surface which was found to significantly improve the bond strength compared to air-abrasion surface treatment .
A different technique of alumina and aluminium nitride coating by reactive magnetron sputtering was evaluated by Kulunk T et al.. A thin, uniform coating structure of 0.4 µm- 1.5 µm was formed on the zirconia core surface and was found ineffective in increasing the bond strength.
Zirconia Coating:
Teng J et al in 2012 evaluated the effect of a novel surface conditioning method on core-veneer bond strength of zirconia ceramic system. In their experiment, pre-sintered zirconia blanks were coated with slurry of less than 3 µm zirconia particles mixed with adhesive. The scanning electron miscroscope (SEM) images for the zirconia modified specimens revealed a rugged surface and abundant microporosities that would allow the penetration of the veneering porcelain.
Aboushelib M. in 2012 performed a study to evaluate the effect of fusion sputtering of zirconia on the microtensile bond strength of zirconia to resin. As-sintered control group was compared with fusion sputtering group and particle abrasion with 50 µm alumina group. For the fusion sputtering group, 10 gm of unsintered zirconia particles 7-12 µm were mixed with 10 ml of 50% ethyl alcohol and placed in a compression glass container. The mixture was applied on the zirconia specimens for 5 seconds at a distance of 20 mm and with air pressure adjusted to 0.3 MPa. SEM images of the fusion sputtering group showed the formation of 8-12 µm highly retentive beads made of zirconia dioxide. These beads provided sufficient micromechanical retention with the resin cement and increased the microtensile bond strength values even after 6 months of water storage.
2-Surface Treatments Methods Causing Chemical Adhesion:
Adhesion Promoters:
Special functional monomers have been used to improve the adhesion to ZrO2. These materials present a chemical affinity for metal oxides and can be included both in resin cement and adhesives or applied directly over the ceramic surface. Phosphate ester monomers, such as 10-methacryloyloxyidecyl-dihyidrogenphosphate (MDP), chemically react with ZrO2, promoting a water-resistant bond to densely sintered zirconia ceramics.
Gas Fluorination:
Attempts to modify the zirconia surface creating a more reactive surface and thus facilitating chemical bonding were always investigated. A simple florination technique of zirconia surface was introduced in which an oxyflouride conversion layer was created on zirconia surface and thus enhancing its reactivity. Piascik J et al investigated the effect of this surface treatment on the bond strength between zirconia and resin cements and concluded that the oxiflouride conversion layer formed was receptive to the organosilane chemical attachment.
Surface Treatment Methods Causing Chemical Bonding and Micro-mechanical Interlocking:
Silica coating:
Si deposition methods started in 1984 with the silicoater technology to improve the bond with resin cement where it involved pyrolytically applying a silica coating on a substrate surface, followed by application of silane before bonding using a resin cement. It has been successful in improving the bond strength of resin cements to metals and decreasing the degradation of bond strength after thermocycling. Yet, it was expensive and too complex to be commercially available for standard dental applications .
Silica coating using tribochemistry is a common practice for coating metal alloys, and alumina-and zirconia-based dental ceramics. Tribochemistry involves creating chemical bonds by applying kinetic energy, without any application of additional heat or light. This is a cold silicatization method, with the energy needed in the silicatization process transferred to the object material in the form of kinetic energy, which generates frictional heat locally at the impact focus.
Rocatec ( laboratory device introduced to the marked in 1989) and CoJet (chair side device) systems are the most heavily favored commercial products utilized for applying the coating..
The Rocatec system (Espe, Seefeld, Germany), applies two sandblasting steps to the surface before the application of a resin. The steps in this tribochemical silica coating processes include the following: (1) sandblasting with 110 pm aluminum oxide (Rocatec-Pre powder) to clean the surface and (2) formation of a silica layer by sandblasting with a special silica particle containing 110-µm aluminum oxide. Another modified silica coating procedure is the Silicoater MD system (Heraeus Kulzer, Wehrheim, Germany), in which after sandblasting with aluminum oxide, a “chromium oxide-dotted silica layer,” is baked onto the surface in a special furnace.
Tribochemical silica-coating or silicatization using CoJetTM at the dentist’s surgery is a widely used and accepted conditioning method in ceramic and metal alloy structure reparation and cementing. The CoJet system is based on airborne micro-blasting sand, which is especially silica-modified aluminum trioxide and provides the ceramics with a reactive silica-rich outer surface
A convenient silica-coating method developed in Europe was used to improve the bond strength between metal and resin composite. This method was adopted later in 2012 by Oguri T et al for joining dental porcelain to zirconia based ceramics where it depends on the formation of silicate layer by spraying with fused silicon-dioxide for 10 seconds using Silano-Pen.
Improvement of bond strength between zirconia based ceramics and resin cement was also investigated through the application of different silica-containing materials. These methods are believed to be successful as well for improving the bond strength between veneering porcelain and zirconia based ceramics because of the silica content present in the veneering porcelain.
The application of silica containing materials on zirconia surface has been investigated and reported by many authors where the application of a 5 µm layer of fused glass micro-pearls to the zirconia surface has been shown to increase the bond strength with resin cement. On the other hand, Kitayama et al introduced the internal coating technique (INT) based on coating the internal surface of zirconia with a thin layer of fusing silica based ceramic. A different technique for application of silica containing materials was the application of glaze layer 120 µm thick on the fitting surface of zirconia ceramics as shown be Everson et al who compared its effect to tribochemical silica coating on bond strength with resin cements.
In another study, zirconia surface was painted with a slurry of micro-pearls and fired in a furnace at 720șC without vaccum resulting in the formation of a fused glass film 5μm thick that increased the surface roughness of ZrO2 and allowed increased micro-retention..
Zhang Y et al in 2009, introduced the functionally graded glass/zirconia/glass structure (GGZ) based on the idea of grading a brittle material with a material of lower modulus of elasticity at the external surface and thus minimize the tensile stresses at the outer plate surface making the structure less prone to fracture. These glass graded zirconia structures showed improved damage resistance, in addition to good esthetics relative to monolithic Y-TZP, better cementation results with the use of HF acid etching and silanization as well as decreased wear of the opposing dentition.
These Graded G/Z/G structures were fabricated using a glass-ceramic infiltration technique with the infilterarting glass consisting of of SiO2 (65.5 wt.%), Al2O3 (11.7 wt.%), K2O (10.0 wt.%), Na2O (7.3 wt.%), CaO (3.0 wt.%), and Tb4O7 (1.wt.%). This composition was selected so that the final product had a high melting point coupled with an excellent resistance to crystallization during the cooling from the elevated temperatures. The CTE of the selected glass composition was around 10.4 × 10−6 °C−1 (from 25 to 450 °C), similar to that of Y-TZP (10.4 × 10−6 °C−1, from 25 to 450 °C)..
The top and bottom surfaces of presintered Y-TZP were coated with a slurry of the powdered glass composition and glass infiltration and densification were carried out simultaneously at 1450°C for 2 hrs during zirconia sintering. SEM images of G/Z/G showed a thin, outer surface of residual glass layer followed by a graded glass-zirconia layer at both the top and bottom surfaces, sandwiching a dense Y-TZP. The outer surface residual glass layer was approximately 20 μm thick at both the top and bottom surfaces. The matched CTE between the infilterating glass and Y-TZP produces continuous transition from the external glass layer deep to the interior dense Y-TZP without causing excessive thermal stresses.
Selective Infiltration Etching
Thermodynamic behavior of Y-TZP allows the manipulation of the structure of the surface grains by controlling both temperature and heating time. Based upon this fact, Aboushelib M in 2007 introduced a method for improving the bond strength between Y-TZP and resin cements. In his experiment, he introduced the idea of heat-induced maturation (HIM) of zirconia causing stressing of the grain boundary regions by two short thermal cycles, but without providing sufficient energy for grain growth or cubic phase formation. This process allows the grain boundaries to become pre-stressed and easily infiltrated by other materials. In addition to HIM, the grain boundary regions are further widened by applying a thin layer of semi-liquid glass that infiltrates selectively between the boundaries of the surface grains exerting surface tension and capillary forces allowing re-arrangement movements of the surface grains and creates 3-D network of inter-grain nano-spaces that were evident by the scanning electron microscope. It was shown by results obtained from this study that HIM/SIE method of zirconia surface treatments allowed superior zirconia-resin bond strength that was not affected by artificial etching.
Veneering Problems:
Chipping or fracture of the veneering porcelain has been the most commonly observed problem in zirconia-basesd restorations. It could be classified into two groups, being either within the veneer itself or originating at the interface between the core and the veneer.
Guazzato et al stated that the strength of a non-homogenous all ceramic structure was determined by its weakest component which was usually the core-veneer interface or the veneering material itself. Thus, the veneering material had to be strong enough to withstand stresses of mastication to prevent delamination and fracture of the veneering material.
Agustin et al showed that the predominant failure type for zirconia core restorations was cohesive failure (71.6%) compared to metal core restorations that showed only adhesive fractures.
Saito et al stated that the most frequent fracture type for porcelain-veneered zirconia cores was cohesive fracture being 88.8%.
Also Sundh et al, in a laboratory study, stated that zirconia layered crowns failed basically by delamination of the veneer from intact core structure, while crowns made of layered lithium disilicate core material failed by fracture of both the core and veneering ceramic, which meant that the core-veneer bond strength was also material dependent.
Guess et al conducted a study to evaluate the shear bond strength between various commercial zirconia cored and veneering ceramics. They stated that delaminations with exposure of the zirconia core ceramic and minor chip-off fractures of the veneering ceramic were described as the most frequent reason for failures of zirconia fixed partial dentures. Chip-off fracture rates at 15% after 24 months, 25% after 31 months and 8% and 13% after 36 and 38 months respectively, were observed.
In a study carried out by Smith et al to gain insight into the fracture behavior of prosthesis under incisally directed load, it was shown that 50% of all ceramic InCeram crowns failed by delamination of the veneering glass alone leaving a thin layer residual glass on the core surface. They stated that the term “delamination” indicated complete debonding or adhesive failure, which didn’t occur in the presence of good bond between a compatible core and the veneering material.
Many factors may influence that chipping of the veneering porcelain such as the mismatch in the coefficient of thermal expansion between the core and the veneer, firing shrinkage of the ceramic, flaws on the veneering porcelain and poor wetting by the veneer on the core, types of core or veneering ceramic, surface finish of the core, application of a liner and the method of veneering.
Being mainly composed of glass, most veneering ceramics have low fracture toughness values (0.7 – 0.9 MPa∙m1/2) which are at least eight times lower than that of the zirconia core ceramics. In addition to their inherent low fracture toughness, conventional condensation and sintering technique used in fabricating the veneering layer may also contribute to the low fracture resistance of veneering ceramics. High porosity during condensation may cause critical flaws and veneer fracture. Not only the processing procedures that can induce defects into ceramic materials where, during mastication, dental restorations are also subjected to cyclic and variable rates of loading that may lead to crack initiation on the contact surfaces leading to fatigue failure.
While heat treatment of bilayer ceramics to the temperature near the glass transition temperature of the veneer and then rapid cooling to room temperature could produce residual compressive stresses within the veneer layer providing a strengthening effect, the residual tensile stress caused from slow cooling could decrease the fracture resistance of a veneer layer especially when combined with the local residual tensile stresses caused from contact damage. The residual tensile stresses may also develop due to the thermal expansion mismatch between the core and veneer. These residual tensile stresses can lower the fracture resistance of a veneering material. The thermally compatible core-veneer system has been suggested to have a thermal contraction mismatch approximately ≤ 1.0 ppm/K..
For a zirconia core and a compatible veneering ceramic obtained from the same manufacturer, the bond strength of a bilayered restoration ranged between 26 to 37 MPa which were comparable to the veneer strengths .
Mechanical Testing of Zirconia-Veneered Restorations:
Since zirconia has been widely used- recently- for the fabrication of ceramic fixed partial dentures, it’s becoming essential to examine its use clinically as a successful restorative material. Multiple clinical studies have been carried out to assess the success and survival of zirconia restorations with the most prevalent technical complication being chipping of the veneering porcelain followed by framework fracture. In a systematic review carried out by Raigrodski A J et al to assess the survival and complications of zirconia, the percentage of complete failures due to framework fracture or loss of retention was less than 10%. On the other hand, chipping of the veneering porcelain was the most common concern noted in all of the studies included in the review.
Different test methodologies have been used to evaluate the mechanical properties of ceramics. They ranged from microtensile bond strength test, three and four point loading tests, biaxial flexural test and shear bond strength test with each of these having its specific advantages and disadvantages.
To measure the bond strength of all-ceramic systems, shear tests or microtensile tests are generally used to evaluate the influence of the substrate surface on the bond quality
Shear bond strength test is defined as a test in which two materials are connected via an adhesive agent and loaded in shear till separation occurs.
Despite the fracture resistance of all-ceramic restorations being one of the highly important factors in determining the clinical success of theses restorations, yet no standard method exists for such measurements due to the complex geometry of the restorations. The most common methods applied for measurement of ceramics’ strength are the uni-axial bending tests including three-point and four-point bending tests’ and biaxial bending tests.
Fischer J et al in 2008 compared the flexural strengths for ten different veneering ceramics for zirconia using three-point flexural strength and bi-axial flexural strength as well as four-point flexural strength. According to their study, it was concluded that the four-point flexural strength values for all materials were significantly lower than those obtained with the three-point flexural test while the bi-axial strength values ranged in-between those of the above two test.
For biaxial tests, there are different designs including ball-on-ring, ring-on-ring and piston-on-three-ball test with the latter considered the ASTM standard for bi-axial flexural testing where one of its main advantages is that the supporting three balls ensure contact, even with warped specimens and that it allows the flat surface of the loading piston to be parallel to the specimen surface
Cheng M.et al in 2001 tested seven different compositions of 8-YSZ of different thicknesses using the piston-on-3-ball method and proved that the thickness of the specimen does not have an effect on the failure probability distribution.
Shear Bond Strength between Zirconia Core and Veneering Porcelain:
Saka M and Yuzugullu B. in 2013 evaluated the effect of different surface treatments of microwave and conventionally sintered zrconia cores on the shear bond strength between the veneering porcelain and the zirconia cores. After having 2 groups of zirconia; microwave or conventially sintered discs, each group was divided into four subgroups including; a control untreated group, sandblasting with 50 Um alumina at 0.2 MPa for 10 seconds, liner application and a fourth sub-group that was sandblasted followed by liner application. They concluded that sandblasting followed by liner application on conventionally sintered zirconia cores could be preferred for improving the bond strength.
Umer S et al in 2013 carried out a research comparing the shear bond strength of zirconia core and veneer with that of lithium disilicate core and veneering porcelain. They found out that the inter-ceramic bond between the zirconia core and veneer was higher-but with no significant difference- than that between lithium disilicate and veneering porcelain.
Gasparic L B in 2013 examined the correlation between surface roughness and shear bond strength in zirconia veneering ceramics. In their study, two groups of presintered Y-TZP IPS e-max ZirCad blocks were fully sintered All samples were then ground under water spray jet incorporated in hand-piece with a 90 µm grit diamond bur at maximum revolutions of 200,000 rpm with minimal pressure to ensure consistent grinding speed. One group of samples was left untreated for surface roughness measurement and veneering while the other group was further sandblasted with 110 µm alumina particles at 2.5 bar for 5 seconds and then left for surface roughness measurement and veneering. For all specimens, surface roughness and shear bond strength were. Their results showed that there was positive correlation between shear bond strength values and surface roughness of the core where higher shear bond strength values were obtained with the ground and sandblasted samples and veneering porcelain remained on the zirconia surface under significantly higher shear forces.
Oguri T et al in 2012 examined the effect of convenient silica coating on shear bond strengths of porcelain veneers on zirconia-based ceramics. Four groups of fully sintered zirconia/alumina nanocomposites stabilized with ceria were examined for shear bond strength with veneering porcelain after receiving different types of surface treatments. One group was ground with carborundum point for ten seconds, 2nd group was sandblasted with 50 µm alumina particles at pressure 0.2 MPa for ten seconds and the third group received a silicate layer through spraying with fused silicon dioxide for ten seconds using Silano-Pen. A fourth group of untreated specimens was left as control. Surface roughness for all groups was measured using surface-texture and contour measuring instrument. Porcelain veneering was built up using a metal cylinder mould with a hollow 6 mm diameter centered over each zirconia sample and fired according to the manufacturer’s recommendations. All samples were tested for shear bond strength using a universal testing machine at a crosshead speed 1 mm/min. they concluded that silica coating with Silano-Pen was efficient in increasing the bond strength between zirconia based ceramics and veneering porcelain without excessively damaging the surface.
Teng J et al in 2012 compared the shear bond strength between veneering porcelain and zirconia core after receiving different surface treatments. Pre-sintered Y-TZP ceramic core specimens of dimensions were sintered according to the manufacturer’s recommendations and then divided into three groups; one group was ground with silicon carbide paper up to 1200 grit under water coolant, the second group was ground in the same manner as the first group then air-abraded with 110 µm alumina particles at pressure 0.3 MPa at a distance of 10 mm for 10 seconds. A third group of pre-sintered Y-TZP samples was modified by powder coating with a slurry of Y-TZP of particle size less than 3 µm. The slurry was applied twice with a thin brush on the surface of pre-sintered zirconia surface and then coated specimens were sintered according to the manufacturer’s recommendations. Veneering porcelain in a stainless steel mould. A fourth group of metal-ceramic was prepared as a control for comparison. After veneering firing, all specimens were subjected to shear bond strength test where load was applied with a chisel-shaped piston parallel to the long axis of the specimen and close to the interface as possible till failure occurred. They concluded from their research outcome that modifying the surface with zirconia coating could significantly improve the shear bond strength with the veneering porcelain.
Mosharraf R et al in 2011 investigated the effect of zirconia grinding under water spray jet with 90 µm grit bur, air-borne particle abrasion with 110 µm, and the effect of liner material application after being air-abraded on the shear bond strength between zirconia and veneering porcelain. They showed that different surface treatments had different effects with grinding dramatically decreasing the shear bond strength.
Kim H et al in 2011 studied the effect of different surface treatments on the shear bond strength between zirconia core and veneering ceramic. Specimens were either air-borne particle abraded with 110 µm alumina particles, received a liner material after being air-abraded following the previous protocol or a liner was applied directly on the ground specimens without air-borne particle abrasion. Results showed that air-borne particle abrasion samples displayed significantly higher shear bond strength values than ones on which liner was applied.
Fischer J et al in 2010 evaluated the shear bond strength of different veneering ceramics to ceria-stabilized tetragonal alumina nanocomposites after different surface treatments. Ceria/alumina nanocomposiye cubes were prepared and one face was polished to 3 µm using diamond paste. Half of the specimens were then air-abraded with 110 µm alumina particles while the other half was only polished. A group of hot-isostatic post compacted Y-TZP was prepared as a control group. For each of these groups, a second series was produced with the application of liner before veneering. All specimens were then subjected to shear bond strength test. According to their research, they concluded that liner application as well as air-borne particle abrasion significantly decreased the shear bond strength between veneering ceramics and ceria stabilized zirconia core. They showed that shear bond strength was significantly different between Y-TZP and ceria stabilized zirconia and veneering ceramic were Y-TZP showed higher values.
Fischer J et al in 2008 tested the effect of zirconia surface treatments on the shear strength of zirconia/veneering ceramic composites. Densely sintered zirconia specimens were prepared and polished with diamond paste up down to 3 µm. One group was sandblasted with 110 Um alumina particles, a second group was coated with silica using Rocatec Pre and Rocatec Plus for 11 and 12 seconds subsequently both at 0.28 MPa. Each of these groups was subsequently divided into two subgroups according to whether liner was applied before veneering of not. Surface roughness of polished, sandblasted and silica coated samples was measured before veneering. All specimens were subjected to shear bond strength tests. They found out that neither increased surface roughness nor liner application improved the shear bond strength.
Guess P et al in 2008 investigated the effect of different types of zirconia cores and veneering ceramics, and the effect of thermocycling on the shear bond strength between core and veneering porcelain. Three different types of zirconia; Cercon Base, Vita-Inceram YZ and DC zircon were used to create three groups of zirconia cores. Specimens in each group were then fully sintered and veneered with the corresponding veneering porcelain after being pre-treated according to the manufacturer’s recommendations. The Cercon Base cores were sandblasted with 110 µm alumina particles at 2.5 bar before veneering and all specimens in all groups received a layer of liner material before application of veneering porcelain. A fourth group of metal-ceramic was prepared and served as a control group. Half of the specimens in each group were subjected to thermocycling for 200,000 cycles at temperature alternating between 5 and 55˚ C with immersion time 45 seconds. All specimens were then subjected to shear bond strength test where load was applied parallel to the long axis of the specimen through a wedge at the core-veneer interface. They found out that neither the zirconia core type nor the thermocycling had a significant effect on the shear bond strength between the core and the veneering porcelain in all the ceramic groups. Metal-ceramic group showed significantly higher value for SBS than the three all-ceramic groups.
Flexural Strength of All-ceramic Restorations:
Fonseca R C et al in 2014 examined the effect of grinding, air-borne particle abrasion using either; 1- 30 µm silica-modified-alumina particles (Rocatec Soft), 110 µm silica-modified-alumina particles (Rocatec Plus) or 3- 120 µm alumina particles followed Rocatec Plus, and heat-treatment on biaxial flexural strength and phase transformation of Y-TZP ceramics. As concluded from their research; grinding resulted in decrease of the bi-axial flexural strength but without promoting any phase transformation. On the other hand, the three protocols of air-borne particle abrasion didn’t influence the bi-axial flexural strength of the non-heat treated groups, but resulted in increase in the monoclinic phase percentage.
El-Korashy D and El-Refai D in 2014 carried out a research evaluating the effect of different chemico-mechanical surface treatments of Y-TZP surface on, flexural strength of Y-TZP. They examined the effect of; air-abrasion with 110 µm alumina, silica coating with 30 µm silica modified alumina particles, hot-etching solution and finally the same hot-etching followed by silica-coating of the surface using the same protocol as in the second group. It was shown that silica coating induced a significant increase in bi-axial flexural strength compared to other groups.
Marelli M et al in 2013 investigated the effect of surface finishing and coloring process of Y-TZP on flexural strength of Y-TZP core as well as the effect of different types of veneering porcelains on the flexural strength of bi-layered zirconia-veneer specimens. First and in order to evaluate the effect of surface finishing and coloring process, four groups of pre-sintered Y-TZP were prepared. For first group, specimens were sintered only while in second group specimens were colored then sintered. In the third group, specimens were sintered then polished with carborundum disks under water coolant while in the last group, specimens were colored, sintered then polished. For all groups, surface roughness was measured using contact measuring system then flexural strength was measured using three-point bending test. To test the effect of different types of veneering ceramics, different Y-TZP were prepared and coated with veneering ceramic of thickness 1.1 mm to reach a total thickness of 2.2 mm. All samples were subjected to three-point bending test. Results showed that surface polishing had significant effect on producing a smoother surface. They concluded that surface roughness had an important effect on mechanical strength of the material where flexural strength increased significalntly with highly polished surfaces while there was no significant effect for coloring process on the mechanical strength. They also showed that different types of veneering porcelains exhibited different mechanical behavior and failure modes.
Lin W et al in 2012 evaluated the effect of the zirconia veneering technique being heat-pressed or powder/liquid layering on the biaxial flexural strength of the bi-layered restorations and showed that layering porcelain diminishes the strength and reliability of bi-layered ceramics.
Yilmaz H et al in 2011 performed a research testing the effect of fatique on bi-axial flexural strength of bi-layered discs of Lava and Cercon base veneered with their corresponding feldspathic porcelain. Two groups of core-veneered discs were prepared according to the core material used and then half of the specimens of each group were subjected to fatique testing under cyclic loading. All specimens were later tested for bi-axial flexural strength using piston on three-balls technique and the stresses generated at the core and the veneer, at the top and the bottom surfaces and the interface of the bi-layered disc were calculated. It was shown that cyclic loading of 20,000 cycles didn’t cause any statistically significant difference in the strength of Cercon-based group while it caused significant increase in the strength of Lava-based group and the same was true for the strength values at the interface.
Wang H et al in 2008 investigated the effect of different surface treatments on the flexural strength of Y-TZP bars and the relation between the surface roughness and strength of the material. Two groups of zirconia bars were prepared through either CAD/CAM milling using Cercon milling machine or cut using diamond coated cutting disc saw. After sintering, some of the ground bars were polishesd using ascending grit silicon carbide paper. They showed that polished zirconia displayed the highest strength values and concluded that there was a direct relationship between flexural strength and surface roughness of the test specimens.
Effect of Surface Roughness on Mechanical Properties of Zirconia-Based Restorations:
Different surface treatment methods of zirconia core substrate cause different degrees of surface roughness. The exact influence of surface roughness on bond strength and mode of failure is still controversial and has to be further examined
Surface roughness could be measured in terms of Ra value which describes the average surface roughness as a mean of the elevations and depressions measured from an estimated surface, the RP value which represents the average vertical elevations measured from the estimated average surface, and the RV value which is the surface depressions measured from the estimated surface.
Jiang T. et al in 2014 compared the surface roughness of four groups of zirconia. The first group acted as a control group and received no treatment at all, the second group was air-abraded with using 50-μm aluminum oxide particles, the third group was air abraded with 50 μm aluminum oxide particles followed by selective infilteration etching where the surface was fused with two layers of glass ceramic, and then etched with 10% HF for 10 minutes and a forth group was treated using a modified glass agent without airborne particle abrasion. Specimens of each group were imaged using the atomic force microscope for surface roughness analysis. According to their results, it was shown that the control untreated group displayed the lowest Ra values with a relatively smooth surface while both groups receiving SIE-with or without air abrasion- displayed the highest values. It was thus concluded that the SIE treatment with a modified glass agent using standard dental equipment increased the surface roughness of zirconia without the need for complicated techniques or specific devices and was easily controlled.
El-Korashy D and El-Refai D in 2014 in their research evaluating the effect of different chemico-mechanical surface treatments of Y-TZP surface on surface roughness, found out that that surface roughness was significantly different among different groups with the highest value shown in the group subjected to hot chemical etching followed by air-abrasion.
Saka M and Yuzugullu B in 2013 evaluated the effect of conventional sintering of Y-ZTP and microwave sintering on the surface roughness of zirconia surface. They showed that conventionally sintered Y-TZP showed higher Ra values than microwave sintered group but was of no significant effect on the shear bond strength between zirconia core and the veneering porcelain..
Oguri T. et al in 2012 examined the effect of different surface treatments on surface roughness of fully sintered zirconia/alumina nanocomposites stabilized with using surface-texture and contour measuring instrument. They concluded that surface roughness values of Silano-Pen treated group was very comparable and not significantly different from that of control untreated group.
Fisher J et al in 2008 evaluated the effect of surface roughness after different surface treatments of zirconia in shear bond strength with veneering porcelain. According to their study, it was shown that both sandblasting and silica coating significantly increased the surface roughness values but didn’t improve the shear bond strength.
Despite the ongoing efforts, still the primary factors behind the bonding mechanism between zirconia and veneering ceramic are unclear and the clinical studies on success and failure of zirconia-veneered restorations are still limited .
Since different manufacturers recommend different surface treatments of zirconia, further studies are still needed to understand the bonding mechanism and improve the bond strength between the zirconia core and veneering porcelain for core-veneered restorations.
Statement of the Problem:
Inertness and chemical stability of partially stabilized zirconia makes it difficult to attain a durable bond between the zirconia and the veneering ceramic leading to failure of bi-layered zirconia-veneered restorations under function. In an attempt to overcome such problem, nano-crystalline translucent partially stabilized zirconia, was introduced, that could be used in a fully anatomical form with no need for veneering. Despite its higher translucency and better esthetic properties than the conventional Y-TZP, it couldn’t be used in the anterior region of the mouth in its fully anatomical form without veneering for better esthetic results. This raises the same problem of bond durability between it and the veneering porcelain.
Aim of the Study:
This study aims at evaluating the effect of different surface treatment methods of nano-crystalline zirconia on bonding with veneering porcelain through the following tests ;
Shear bond strength between nano-crystalline zirconia and veneering porcelain.
Biaxial flexural strength of the bi-layered restoration..
Correlelation between shear bond strength and biaxial flexural strength of the restoration will be examined.
Surface treatment methods that are to be employed are;
Silica Coating ( Tribochemical coating).
Glass Grading.
Zirconia powder deposition.
Materials and Methods:
Materials:
inCoris TZI Blocks:
These are nano-crystalline tetragonal Yttria- stabilized zirconia blocks of dimensions 40mm x 19mm. Blocks are initially manufactured in a partially sintered state; then enlarged by the inLab CAD/CAM system, and finally, densely sintered.
Table 1: Chemical composition of inCoris TZI
Table 2: Technical data of inCoris TZI.
Figure 1: inCoris TZI block 40/19
inCoris TZI Coloring Liquid:
Aqueous coloring liquid of shade A2 for coloring zirconia restorations. It contains different metal ions that diffuse into the porous zirconia structure in the pre-sintered stage. Stabilizing agents agents are used to stabilize the metal salts and ensure that the ions remain in the zirconia crystal lattice during drying. In the sintering process step, this stabilizing agent is completely burned out.
Figure 2: inCoris coloring liquid
Veneering Porcelain VITAVM 9 ENL:
VITA VM 9 has been designed as a special ceramic featuring a fine structure for yttrium partially-stabilized ZrO2 sub-structures with a coefficient of thermal expansion ( CTE) of approximatly. 10.5×10-6K-1. VITA VM 9 is composed of fine leucite crystals that are homogenously distributed in the glass phase surface is created, which provides VITAVM 9 with excellent milling and polishing properties.
Table 3: Physical properties of VITA VM9
Figure 3: VITAVM9 ENL powder
VITA AKZENT Glaze:
Low-fusing glass ceramic in powder and liquid form.
Figure 4:VITA AKZENT glaze powder and liquid
Cojet Powder:
Silicatized sand particle size 30 μm. This fine particle size allows a much lower abrasion rate than with conventional abrasives where even fine edges can be treated without damage.
Table 4:Standard composition of Cojet powder
Figure 5:Cojet powder
Nano-crystalline Zirconia Powder:
3 mol % Y2O3 stabilized nano-powder of average primary particle size 30-60 nm
Table 5: Physical properties of nano-crystalline Yettria stabilized nano-zirconia powder
Figure 6: Nano-crystalline YTZP powder
Hydrofluoric Acid Etch:
Viscous hydroflouoric acid (8%)
Table 6: Dentobond porcelain etch composition
Figure 7: Dentobond porcelain etch
Methodology:
One hundred and twenty nano-crystalline yttria-stabilized zirconia samples were constructed with different configurations according to the factor under research. Samples were classified according to whether they would be tested for shear bond strength or biaxial flexural strength, color reproduction and translucency parameter measurements.
For shear bond strength test, samples were cut in the form of zirconia plates of final dimensions 12mm x 15 mm x 2 mm and veneered with porcelain discs of dimensions 3 mm x 3mm.
For biaxial flexural strength test, color reproduction and translucency parameter measurement, samples were cut in the form of zirconia discs 12 mm in diameter and 0.8mm thick-after sintering- veneered with a layer of porcelain of 0.7 mm thickness and of the same diameter as the zirconia discs.
Before veneering, all zirconia discs were colored by dipping in aqueous zirconia coloring solution and then received different surface treatments-being tribochemical silica coating, zirconia powder deposition or glass grading- before being sintered. Half of the samples of each group of each type were subjected to hydrothermal aging before testing.
Samples Grouping:
Samples were classified into into four equal groups (A,B,C,D) according to the type of surface treatment received. For shear bond strength test, each group had 20 samples (n=20) that were further sub-divided into two sub-groups to be tested before and after autoclave aging.
For biaxial flexural strength test, color reproduction and translucency parameter measures, each group had ten samples (n=10) that were further subdivided into two equal sub-groups to be tested before and after aging.
Table 8: Samples grouping for biaxial flexural strength test, color reproduction and translucency parameter measurment
Samples Preparation:
Slicing of the Zircona Samples:
Samples for Shear Bond Strength Test:
A hard stainless steel disc of 8cm diameter and 0.3 mm width, and fine tooth was mounted on a manually adjusted custom made milling machine (fig.8,9). The machine was adjusted to mill plates- 2.5 mm thick each- out of the pre-sintered inCoris TZI blocks of size 40/19. Thickness was adjusted to allow for 20% shrinkage during sintering so that the final plate thickness would be 2 mm.
Samples for Bi-axial Flexural Strength Test:
Zirconia samples were cut in the same way as for shear bond strength test samples, but of thickness 1 mm to allow for 20% shrinkage during firing and thus resulting in final thickness 0.8 mm.
Zirconia plates were then fixed to a circular mould and adjusted into a circular form of diameter 15 mm using a carbide diamond stone under water coolant.
Figure 8: Stainless steel disc mounted on a milling machine for cutting of zirconia samples
Figure 9: Hardened stainless steel disc
Figure 10: Milled zirconia plate for shear bond strength test and disc bi-axial flexural strength test
II. Finishing and Thickness Adjustment of the Milled Specimens:
Each zirconia sample was finished wet using water-proof silicon carbide sandpaper of different grit size ranging from 320 to 1200 with Grinder Polisher machine in order to adjust the final thickness and provide a smooth surface .
Each specimen thicknesses was checked using a digital caliper. The samples were washed under running water and ultrasonically cleaned in distilled water for five minutes to wash out any debris that might affect the surface.
III. Coloring of the Zirconia samples:
Specimens coloring was done by immersion technique in TZI sirona coloring liquid of shade A2. Each sample was immersed solely at a time in a dipping container for 2 minutes (fig.11).
The samples were removed from the coloring liquid with a pair of tweezers and were hung for 30 seconds in a vertical position to allow dropping of excess liquid traces before being placed on a non-absorbable surface. Samples were then left to bench dry for 24 hours before applying different surface treatment protocols.
Figure 11: Dipping of zirconia samples in coloring solution
Surface Treatments Application:
Tribochemical Silica Coating:
Pre-sintered zirconia samples were silica coated with 30 μm silicatized sand particles at pressure of 3 bars using air prophy unit mounted on the dental unit. Specially designed molds (15 mm width × 19 mm length × 2 mm height for the pre-sintered plates and 15 mm diameter x 1 mm height for pre-sintered discs) were constructed to fix the samples and prevent their movement during the silica coating.
A specially designed holder for the air prophy unit was constructed. It contains a central bar that is composed of two sliding parts, one moving inside the other, with a key to facilitate adjusting the distance between the zirconia plate and the nozzle opening of the air prophy unit. The outer sliding bar contains a metallic shelf in order to put the mold containing the zirconia sample on it.
A graduated meter holding the specially designed mold was fixed to the shelf for standardization purpose. The inner sliding part ends with 45ș sloped opening to accommodate the angled end of the air prophy unit nozzle and arrange it at a right angle to the plate The air prophy unit nozzle was screwed inside the sloped opening using a key lock. The zirconia sample was fixed inside the mold and placed on the metallic shelf of the device.
The distance between the nozzle of the air prophy unit and the zirconia sample was fixed at 1cm by a key lock after raising the metallic shelf guided by a ruler (fig.12).
The mold holding the zirconia sample was moved along the graduated meter to ensure even coating of the surface and each sample was silica coated for 10 seconds.
Figure 12: Tribochemichal coating of zirconia disc
Zirconia Powder Deposition:
Pre-sintered zirconia samples were coated with 30 nm Y-TZP powder particles at pressure of 3 bars using air prophy unit mounted on the dental unit
Same sample-holding mold and air prophy unit holder used for silica coating were used, but with distance between the nozzle of the air prophy unit and the pre-sintered zirconia sample set to 2 cm (fig.13). Each sample was coated for 5 seconds.
Figure 13: zirconia powder coating of zirconia disc
Glass Grading:
Each pre-sintered zirconia sample was caoted with a single layer of powdered glass slurry. Akzent glaze powder and liquid were mixed together according to the manufacturer’s recommendations and a single layer was applied to one surface of each sample using a brush (fig.14). Firing of the porcelain glass layer took place during the same cycle of zirconia sintering.
After sintering, each sample was manually ground with fine-grit round end diamond bur under water coolant. For final thickness adjustment, each sample was then placed in a special sample holder 12 mm in diameter and 0.9 mm in height and finished with water proof silicon carbide sandpaper of grit size 1200 (fig.15).
All samples were then ultra-sonically cleaned in distilled water for 5 minutes.
Glass-graded and ground surface of each sample was then etched with hydroflouric acid 8 % for 1 minute then washed under running water and air-dried.
Figure 14: Glass grading of zirconia disc
Figure 15: Sample holder for thickness adjustment of the glass grading layer.
Samples Sintering:
Samples were placed on the untreated surface onto the sintering tray of the temperature furnace Infire HTC speed (SDS) . The samples were supported by glass beads to avoid deformation. The "Super Speed" program permanently installed in the furnace was chosen for speed sintering. The duration of the program run was 90 minutes and sintering temperature was 1540șC.
Samples of each group were arranged spaced from each other to allow even shrinkage (fig.16). Groups were sintered simultaneously, each in a single sintering cycle.
Figure 16: zirconia samples supported by beads on the firing tray
Checking and Verification:
After sintering, samples were left to cool down to room temperature before final dimensions were checked-where 20% shrinkage from the original dimensions was expected- using a digital caliper.
The final thickness of the samples was 12 x 0.8 mm for the zirconia discs used for biaxial flexural strength testing and 15mm length x 12mm width x 2mm height for the zirconia plates used for shear bond strength testing.
Environmental Scanning Electron Microscope:
One random representative sintered sample from each group was scanned under Environmental Scanning Electron Microscope (ESEM) (fig.17) for surface assessment, with magnification of 100, 1000, 5000 X.
Figure 17: Scanning electron microscope
Surface Roughness Measurement:
For surface roughness measurment, specimens were photographed using USB Digital microscope with a built-in camera connected with an IBM compatible personal computer using a fixed magnification of 120X. The images were recorded with a resolution of 1280 × 1024 pixels per image. Digital microscope images were cropped to 350 x 400 pixels using Microsoft office picture manager to specify/standardize area of roughness measurement. The cropped images were analyzed using WSxM software (Ver 5 develop 4.1, Nanotec, Electronica, SL)
Within the WSxM software, all limits, sizes, frames and measured parameters are expressed in pixels. System calibration was done to convert the pixels into absolute real world units and subsequently, a 3D image of the surface profile of the specimens was created. WSxM software was used to calculate average of heights (Ra) expressed in μm , (fig.18).
Figure 18: Digital microscope connected on a special software for measurement of Ra value
Veneering:
Shear Bond Strength Samples:
A special stainless steel mold with a central hole 3 mm in diameter and 3 mm in height was constructed for build-up of the veneering porcelain discs to ensure standerdized thickness of the veneering layer.
Veneering porcelain VM9 Enamel Light (ENL) powder and liquid were mixed according to the manufacturer’s instructions, condensed layer by layer in the stainless steel mold-secured to the zirconia plates- using a flat ended spatula under hand pressure and excess moisture was blotted by adapting facial tissue on the porcelain surface. Each porcelain disc-secured to the zirconia plate- was then pushed out of the mold using a special flat-ended piston of the same hole diameter 3 mm.
Four veneering porcelain discs were built up on each zirconia sample. Samples were then fired in porcelain furnace (Programat P300/G2) according to the manufacturer’ recommendations.
After the first bake, each specimen was returned to the mold, checked and then the mold was refilled and the specimen fired for correction to compensate for firing shrinkage.
After the second bake, all veneering porcelain discs were checked for final thickness (3 mm diameter x 3 mm height) using a digital caliper.
Table 9: Firing chart of VITAVM9
B:pre-drying temp, S: pre-drying time, t: heating rate, t’’: heating time T: firing temperature, H: holding time, , L: long-term cooling.
Figure 19: Sintered zirconia plate afte rfiring of the porcelain discs
Bi-axial Flexural Strength Test Samples:
A two-part teflon mold having the same diameter as the sintered zirconia discs (12 mm diameter) and 1.5 mm height was constructed and used for veneering of the zirconia discs (fig.20).
Veneering porcelain VM9 Enamel Light (ENL) powder and liquid were mixed according to the manufacturer’s instructions. Each zirconia disc was placed inside the specially designed mold and the veneering porcelain was condensed on it layer by layer under hand pressure (fig.21). Excess moisture was blotted by adapting facial tissue on the porcelain surface. Veneering was carried out in two- firing cycles in porcelain furnace Programat P300/G2) to compensate for firing shrinkage.
After the first firing cycle, each sample was checked for thickness using a digital caliper then re-seated in the mold where veneering porcelain was applied in the same previous steps as in the first bake. Discs were then removed from the mold and fired.
Figure 20: Two-part teflon mold for build-up of veneering porcelain
Figure 21: Veneering of zirconia discs in a specially designed mold
Polishing and Glazing:
After veneering, all core-veneered zirconia discs were finished and polished using a fine polishing disc of porcelain polishing kit in a straight hand-piece. All specimens were polished by the same operator while attempted to apply constant pressure.
After polishing, samples were glazed with VITA AKZENT glaze (fig.22) and fired according to the following cycle;
Table 10: Firing chart of VITA AKZENT glaze
B:pre-drying temp, S: pre-drying time, t: heating rate, T: firing temperature, H: holding time, L: long-term cooling.
Figure 22: Glazing of zirconia discs
Measurement of Color and Translucency:
Color and translucency measurement were carried out on the same core-veneered zirconia discs constructed for bi-axial flexural strength testing.
Color Reproduction:
Color reproduction was measured using VITA Easyshade Compact spectrophotometer (fig.23). The VITA Easyshade Compact* was set to the “Restoration mode” and shade A2 was selected. The spectrophotometer aperture was centralized on the center of each sample over a white background (fig.24) with maximum intimate contact between the aperture and the flat specimen surface. The button was pressed to measure the difference in color (ΔE) between the specimen and the selected shade.
The device was calibrated in the calibration slot before each measurement for maximum standardization. Three measurements were taken for each specimen and their average was recorded.
Figure 23: VITA Easyshade Compact
Figure 24:Color measurment over white background using VITA Easyshade Compact
Translucency:
Each sample was measured against both black and white backgrounds using tooth single mode of EasyShade where in each time, CIE Lab values were calculated (fig 25).
The EasyShade device was calibrated before each measurement in order to standardize the reproducibility. Each sample was measured three times on each background and an average was recorded.
The translucency parameter (TP) was obtained by calculating the color difference of the specimen over the black and white backgrounds with the following equation;
TP = [(L∗B− L∗W)2+ (a∗B− a∗W)2+ (b∗B− b∗W)2]1/2
Where subscript “B” refers to the color coordinates over a black background and the subscript “W” refers to those over a white background.
Figure 25:Measurement of color parameters over black background
Mechanical Tests:
Shear Bond Strength Test:
Each zirconia plate with the porcelain veneer discs was tightened to a metal holder in universal testing machine (Lloyd Instruments) (fig.26,27).
Load was applied parallel to the long axis of the specimen through a chisel shaped- piston at the core/veneer interface at crosshead speed of 1 mm/min till delamination of the veneering ceramic occurred.
Shear strength was calculated from the load at fracture (N) and the surface area of the zirconia-veneer interface.
Figure 26: Sample fixed on LLoyd universal testing machine for shear bond strength test
Figure 27: Chisel-shaped piston directed at the zirconia-veneer interface
Bi-axial Flexural strength Test:
All samples were tested for bi-axial flexural strength using piston-on-three-balls test as described in the ISO standard 6872 for dental ceramics.
Three hardened stainless steel balls with a diameter 3.2 mm were positioned 120˚ apart on a support circle with a diameter 10 mm (fig.28). Specimens were placed concentrically on the supporting balls so as to ensure that the load was centrally applied.
Load was applied with a universal testing machine (Lloyd Instruments) through a flat punch with tip diameter 2 mm at the centre of the specimen at cross-head speed 1 mm/min with the core surface being placed in tension for all specimens (fig.29).
The load at the point of fracture was recorded and biaxial flexural strength was calculated through the following equation;
Where;( σ) is the bi-axial flexural stress,( ta) and (tb) are thicknesses of the two material layers where (a) is the material on top and (b) is the material at the bottom, Ea and Eb are the Young’s of the two materials which is 66.5 GPa for VITAVM9 and 210 GPa for inCoris TZI.( υ ) is Poisson’s ratio which is 0.25 for both materials. (M) is the maximum bending moment calculated from the equation;
Where (W) is the load, (R) is the equivelant radius of loading and (A) is the radius of of the circle of the support points which is 5 mm and
R= √1.6B2+d2 -0.675d
Where (B) is the radius of the tip of the piston which is 1mm and (d) is the thickness of the specimens which is 1.5mm.
Figure 28: Three hardened stainless steel balls positioned 120˚ apart on a support circle with a diameter 10 mm.
Figure 29: zirconia disc concentrically placed over the the three balls and fixed to the LLoyd universal testing machine for bi-axial flexural strength test
Aging of Zirconia Samples:
All samples were aged in a steam autoclave (Sturdy SA-260MA- Class B) (fig. 30,31) at 134 ˚C with 2 bars pressure for 3 consecutive cycles (each cycle being 45 minutes) which was clinically representative of 9 years of intra-oral service.
Figure 30: Autoclave Sturdy SA-260-MA
Figure 31: Veneered samples placed for autoclave aging
Post-aging Measurements:
After aging procedure, the second phase of measurements was held with the same steps discussed earlier as follows;
Shear bond strength
Biaxial flexural strength
Color reproduction
Translucency
Statistical Methods:
All Data were collected, tabulated and subjected to statistical analysis. Statistical analysis was performed by SPSS in general (version 17), also Microsoft office Excel was used for data handling and graphical presentation.
Quantitative variables were described by the Mean, Standard Deviation (SD), Standard Error (SE), the Range (Maximum – Minimum) and 95% confidence interval of the mean.
Qualitative categorical variables were described by proportions and Percentages.
Independent samples t-test was used for comparing the means of the same variable between aged and un-aged groups.
One way analysis of variance (ANOVA) was used for comparing the means of different groups both for aged and un-aged conditions. Dunnett’s test was used for comparing each group with the control. Multiple comparison Bonferroni method was used for comparing each two groups together.
Significance level was considered at P < 0.05 (S); while for P < 0.01 was considered highly significant (HS).
Two-tailed tests were assumed through- out the analysis for all statistical tests.
Surface characterization (morphological and elemental composition analysis) :
Uncoated Zirconia Plate:
Scanning Electron Microscope:
Figure 32:SEM image of untreated zirconia plate. (1000X)
Zirconia surface showed scratches introduced by milling and polishing procedures during preparation of the plates.
EDAX analysis:
Table 11: Elemental composition of untreated zirconia plate.
It was noted that the highest and most dominant peak was for zirconia that was 79 wt%.
Tribochemical Silica Coated Plate (Cojet):
Scanning Electron Microscope:
Surface of Zirconia plate treated with tribochemical silica coating was covered with fine particles. Prominent micro pores and irregularities were also observed.
EDX Analysis:
Figure 35: Elemental composition of tribochemical coated zirconia plate.
Table 12: Elemental composition of tribochemical coated zirconia plate.
Peaks of silica and alumina were detected after application of tribochemical silica coating where the silica content was 0.32wt%
Zirconia PlateTtreated with Nano -zirconia Powder:
Scanning Electron Microscope:
Figure 36: SEM image of zirconia plate treated with zirconia powder deposition.(1000X)
SEM revealed the presence of retentive beads made of zirconia dioxide material the surface of the zirconia plate also appeared to be rugged with abundant microporosities.
EDX Analysis:
Table 13: Elemental composition of zirconia plate treated with zirconia powder deposition.
The highest peak was recorded for Zirconia that was 79.49 wt. %
Zirconia plate treated with glass grading:
Scanning Electron Micrsoscope:
Figure 38:SEM image of glass graded graded zirconia plate(1000X).(a) After fusing with glass ceramic (b) After etching ewith 10% HF
The zirconia surface took on a smooth glass-like structure after fusing with glass ceramic. When the ceramic layer was removed by etching with 10% HF for 1 min, micropore spaces appeared between the zirconia grains, causing a rough surface appearance.
EDX Analysis:
Table 14: Elemental composition of glass graded zirconia plate.
Peaks of silica, sodium, potassium and calcium were recorded with EDX analysis. The highest peak recorded was the silica peak 46.29 wt%
Surface Roughness Measurements:
Effect of Different Surface Treatment Methods on Surface Roughness:
Table 15: Mean (SD) of surface roughness values of different surface treatment groups.
Figure 40: Bar chart showing means of (Ra) value in relation to different surface treatments before aging.
Table 16: One way analysis of variance (ANOVA) showing the effect of different surface treatments on surface roughness values..
(*) indicates presence of significance.
One way analysis of variance showed that different surface treatment groups had significant effect over the surface roughness values.
Table 17: Multiple comparison tests showing the effect of different surface treatments on surface roughness; Dunnett-t for comparison with the control group and Boneferroni for comparison between the groups.
(*) indicates presence of significance.
The multiple comparison tests showed that there was significant difference in the surface roughness values between the control group and each of the other three groups.
Bi-axial Flexural Strength Measurements:
Effect of Different Surface Treatment Methods on Bi-axial Flexural Strength:
Table 18: Mean (SD) of bi-axial flexural strength values in different surface treatment groups before and after aging.
Figure 41: Bar chart showing means of bi-axial flexural strength values in relation to different surface treatments before and after aging.
Table 19: One way analysis of variance (ANOVA) for testing the means of bi-axial strength values of un-aged sub-groups.
One way analysis of variance showed that there was no significant difference in bi-axial strength values for all un-aged sub-groups within all different surface treatment groups.
Table 20: One way analysis of variance (ANOVA) for testing the means of bi-axial strength values of aged sub-groups.
One way analysis of variance showed that there was no significant difference in bi-axial strength values for all aged sub-groups within all different surface treatment groups.
Effect of Aging on Bi-axial Flexural Strength Values in Different Surface Treatment Groups:
Table 21: Independent samples t-test comparing the mean (TP) between un-aged and aged sub-groups in different surface treatment groups.
(*) indicates presence of significance
Paired sample t-test showed that there was significant difference in bi-axial strength values before and after aging only in the control group and the zirconia powder deposition group. Results showed that aging caused significant reduction in bi-axial strength values in those groups.
In both cojet and glass grading groups, aging caused reduction in bi-axial strength values but with no significant difference.
Shear Bond Strength Measurements:
. Effect of Different Surface Treatment Methods on Shear Bond Strength Values:
Table 22: Mean ( SD) shear bond strength values of different surface treatment groups.
Figure 42: Bar chart showing means of shear bond strength in relation to different surface treatments before and after aging.
Table 23: One way analysis of of variance (ANOVA) for testing the mean values of shear bond strength on un-aged sub-groups.
(*) indicates presence of significance.
One way analysis of variance showed that there was significant difference in shear bond strength values for all un-aged sub-groups within all different surface treatment groups
Table 24: Multiple comparison tests showing the effect of different surface treatments on shear bond strength values of un-aged sub-groups; Dunnett-t for comparison with the control group and Boneferroni for comparison between the groups.
(*) indicates presence of significance.
Multiple comparison tests showed that there was significant difference in shear bond strength values of all un-aged sub-groups except between glass graded and zirconia deposition groups as well as between control and cojet groups.
Table 25: One way analysis of variance (ANOVA) for testing the mean values of shear bond strength on aged sub-groups.
(*) indicates presence of significance.
One way analysis of variance showed that there was significant difference in shear bond strength values for all aged sub-groups within all different surface treatment groups.
Table 26: Multiple comparison tests showing the effect of different surface treatments on shear bond strength values of aged sub-groups; Dunnett-t for comparison with the control group and Boneferroni for comparison between the groups.
(*) indicates presence of significance.
Multiple comparison tests showed that there was significant difference in shear bond strength values of all aged sub-groups except between glass graded and zirconia deposition groups as well as between control and glass graded groups.
Effect of Aging on Shear Bond Strength Values in Different Surface Treatment Groups:
Table 27: Independent samples t-test comparing the shear bond strength means between un-aged and aged sub-groups of each surface treatment group.
(*) indicates presence of significance.
Paired samples t-test showed that there was significant difference in shear bond strength values before and after aging in both the control group and the cojet treated groups. It was shown also that the control group showed the highest mean value before aging while the lowest mean value was recorded for the zirconia powder deposition group.
Color Measurements:
CIELab and ∆E values Measurements:
CIELab Values:
L* Value Measurements:
. Effect of Different Surface Treatment Methods on L* Value:
Table 28: Mean (SD) L* values of different surface treatment groups before and after aging.
Figure 43: Bar chart showing mean of L* value in relation to different surface treatments before and after aging
Table 29: One way Analysis of variance (ANOVA) comparing the means of L* values of unged sub-groups of different surface treatment groups
(*) indicates presence of significance.
One way analysis of variance (ANOVA) showed that there was significant difference in L* values among the un-aged sub-groups of different surface treatment groups.
Table 30: Multiple comparison tests showing the effect of diiferent surface treatments on L* value of un-aged subgroups; Dunnett-t for comparisons with the control group and Bonferroni for comparisons between groups
(*) indicates presence of significance.
Multiple comparison tests showed that that there was significant difference of the L* value between the un-aged sub-groups of all groups except between the control and zirconia powder deposition groups and between the cojet and zirconia powder deposition groups.
Table 31: One way analysis of variance (ANOVA) comparing the means of L*values of aged sub-groups for different surface treatment groups
(*) indicates presence of significance.
One way analysis of variance (ANOVA) showed that there was significant difference in L* values among the un-aged sub-groups of different surface treatment groups.
Table 32: Multiple comparison tests showing the effect of different surface treatments on L* values of aged sub-groups; Dunnett-t for comparisons with the control group and Bonferroni for comparisons between groups.
(*) indicates presence of significance.
Multiple comparison tests showed that there was significant difference in L* values of aged sub-groups of all groups except between control and zirconia powder deposition groups and between cojet and zirconia powder deposition groups.
. Effect of Aging on L* Value in Different Surface Treatment Groups:
Table 33: Independent samples t-test comparing the L* value means between un-aged and aged sub-groups of each surface treatment groups
Paired samples t-test showed that there was no significant difference in L* value before and after aging in all surface treatments groups. It was shown also that the control group displayed the lowest L* value while the glass graded group displayed the highest L* value.
. a* Value Measurements:
Effect of Different Surface Treatment Methods on a* value:
Table 34: Mean (SD) of a* value of different surface treatment groups before and after aging.
Figure 44: Bar chart showing means of a* value in relation to different surface treatments before and after aging.
Table 35: One way analysis of variance (ANOVA) comparing the a* value means of un-aged sub-groups of different surface treatment groups.
(*) indicates presence of significance.
One way analysis of variance (ANOVA) showed that there was significant difference in the a* value of un-aged sub-groups of different surface treatment groups.
Table 36: Multiple comparison tests showing the effect of different surface treatment methods on a* value of un-aged sub-groups; Dunnett-t for comparison with the control group and Bonferroni for comparison between groups.
(*) indicates presence of significance.
Multiple comparison tests showed that there was a significant difference between all groups except between the control and zirconia powder deposition groups.
Table 37: One way analysis of variance (ANOVA) comparing the a* value means of aged sub-groups of different surface treatment groups.
(*) indicates presence of significance
One way analysis of variance (ANOVA) showed that there was significant difference in a* value of aged sub-groups of different surface treatment groups.
Table 38:Multiple comparison tests showing the effect of different surface treatments on a* value of aged sub-groups; Dunnett-t for comparison with the control group and Bonnferroni for comparison between groups.
(*) indicates presence of significance
Multiple comparison tests showed that there was significant difference in the a* value of aged sub-groups between control and glass graded groups, cojet and glass graded groups as well as between zirconia powder deposition and glass graded groups.
Effect of Aging on a* Value in Different Surface Treatment Groups:
Table 39: Independent samples t-test comparing the a* value means between the un-aged and aged sub-groups of different surface treatment groups.
(*) indicates presence of significance.
Paired sample t-test showed that there was significant difference in the a* value before and after aging in both the control group and the zirconia powder deposition groups where in both groups aging caused a significant shift towards the red zone of color.
On the other hand, aging had no significant effect on the a* value in both the cojet and glass graded groups.
b* Value Measurements:
. Effect of Different Surface Treatment Methods on b* Value:
Table 40: Mean (SD) of b* value of different surface treatment groups before and after aging.
Figure 45: Bar chart showing means of b* value in relation to different surface treatments before and after aging.
Table 41: One way analysis of variance (ANOVA) comparing the b* value means of an-aged sub-groups of different surface treatment groups
(*) indicates presence of significance.
One way analysis of variance showed that there was significant difference in b* value between un-aged sub-groups of different surface treatment groups.
Table 42: Multiple comparison tests showing the effect of different surface treatments on b* value of unaged sub-groups; Dunnett-t test for comparisons with the control group and Bonferronni for comparisons between groups.
(*) indicates presence of significance
Multiple comparison tests showed that there was significant difference in b* value between all groups except between the cojet and zirconia powder deposition groups.
Table 43: One way analysis of variance (ANOVA) comparing the b* value means of aged sub-groups of different surface treatment groups
(*) indicates presence of significance.
One way analysis of variance showed that there was significant difference in the b* value between all groups.
Table 44: Multiple comparison tests showing the effect of different surface treatments on the b* value of aged sub-groups; Dunnett-t for comparisons with the control group and Bonferroni for comparisons between groups.
(*) indicates presence of significance
Multiple comparison tests showed that there was significant difference in the b* value between all groups except between cojet and zirconia powder deposition groups.
Effect of Aging on b* Value in Different Surface Treatment Groups:
Table 45: Independent samples t-test comparing the b* value means between unaged and aged sub-groups of different surface treatment groups.
(*) indicates presence of significance
Paired sample t-test showed that there was no significant difference in the b* value before and after aging for all groups except in the glass graded group where aging caused a significant shift in the yellow zone of color.
Aging had no significant difference on the b* value on the other three groups where b* value was always in the yellow zone of color.
. ∆E Measurements:
Effect of Different Surface treatment Methods on ∆E Values:
Table 46: Mean and (SD) of delta E values for different surface treatment groups before and after aging.
Figure 46: Bar chart showing means of ∆E in relation to different surface treatments before and after aging.
Table 47: One way analysis of variance (ANOVA) comparing the delta E means of un-aged sub-groups of different surface treatment groups.
(*) indicates presence of significance.
One way analysis of variance showed that there was significant difference in the ∆E value of un-aged sub-groups of different surface treatment groups
Table 48: Multiple comparison tests showing the effect of different surface treatments on the delta E value of un-aged sub-groups; Dunnett-t test for comparison with the control group and Bonnferroni for comparison between the groups.
(*) indicates presence of significance
Multiple comparison tests showed that there was significant difference in the delta E value between un-aged sub-groups of all groups except between cojet and zirconia powder deposition groups.
Table 49: One way analysis of variance (ANOVA) comparing the delta E means of aged sub-groups of different surface treatment groups.
(*) indicates presence of significance.
One way analysis of variance showed that there was significant difference in the ∆E value of aged sub-groups of different surface treatment groups.
Table 50: Multiple comparison tets showing the effect of different surface treatments on the delta E value of aged sub-groups; Dunnett-t test for comparison with the control group and Bonnferroni for comparisons between the groups.
(*) indicates presence of significance.
Multiple comparison tests showed that there was significant difference in the delta E value between aged sub-groups of all groups except between cojet and zirconia powder deposition groups.
. Effect of Aging on ∆E Values in Different Surface Treatment Groups:
Table 51: Independent samples t- test comparing delta E value means between un-aged and aged sub-groups of each surface treatment group.
(*) indicates presence of significance.
Paired sample t-test showed that aging had no significant effect over the delta E values except in the glass graded group. For all groups-either before or after aging- the delta E values were far above both the perceptible and acceptable range.
Translucency Parameter Measurements:
. Effect of different surface treatments on translucency parameter (TP):
Table 52: Mean (SD) of translucency parameter (TP) of different surface treatment groups before and after aging.
Figure 47: Bar chart showing means of (TP) in relation to different surface treatments before and after aging.
Table 53: One way analysis of variance (ANOVA) comparing the (TP) means of un-aged sub-groups of different surface treatment groups.
(*) indicates presence of significance
One way analysis of variance showed that there was significant difference in the TP value of the un-aged sub-groups of different surface treatment groups.
Table 54: Multiple comparison tets showing the effect of different surface treatments on the TP value of un-aged sub-groups; Dunnett-t test for comparison with the control group and Bonnferroni for comparisons between the groups.
(*) indicates presence of significance
Multiple comparison tests showed that there was significant difference in the delta E value of un-aged sub-groups for all surface treatment groups except between the control and cojet groups.
Table 55: One way analysis of variance (ANOVA) comparing the (TP) means of aged sub-groups of different surface treatment groups.
(*) indicates presence of significance
One way analysis of variance showed that there was significant difference in the TP value of the aged sub-groups of different surface treatment groups.
Table 56: Multiple comparison tests showing the effect of different surface treatments on the TP value of aged sub-groups; Dunnett-t test for comparison with the control group and Bonnferroni for comparisons between the groups.
(*) indicates presence of significance
Multiple comparison tests showed that there was significant difference in the (TP) value of aged sub-groups for all surface treatment groups except between the control and cojet groups.
Effect of Aging on (TP) Value in Different Surface Treatment Groups:
Table 57: Independent samples t-test comparing the mean (TP) between un-aged and aged sub-groups in different surface treatment groups
Paired sample t-test showed that aging caused no significant difference in the (TP) value for all surface treatment groups. I(TP) value was highest for the control group and lowest for the glass graded group.
Discussion:
Core-veneered restorations have been introduced in the dental field in an attempt to achieve an all-ceramic restoration that provides both strength and esthetics.. With the advances in computer aided design and computer aided technologies; the high strength ceramic systems have become increasingly popular. Zirconia and especially Yttria-stabilized tetragonal zirconia polycrystals, with its unsurpassed mechanical properties has had its clinical application expanded from single crowns and short span fixed partial dentures (FPDs) to multi-unit and full-arch zirconia frameworks.
Despite its well-documented mechanical properties, systematic reviews have reported an increased incidence of technical complications with zirconia-veneered restorations such as fracture of the veneering ceramic. These fractures don’t necessarily imply restoration replacement, but may need some intervention in order to keep the restoration functioning. This could be seen in the typical failure pattern where a thin layer of veneering ceramic remains adhering to the zirconia framework revealing a weakness in the veneering porcelain itself. In addition to this, complete chipping of the veneering porcelain from the zirconia framework could be seen indicating the presence of adhesive failure between the zirconia and the veneering porcelain.
In an attempt trying to reduce chipping and fracture of the veneering porcelain, and with the help of the recent innovations in materials’ technologies, nano-crystalline Yttria-stabilized zirconia has been introduced for construction of monolithic restorations. Clinical studies have shown increased values of strength and toughness for monolithic zirconia compared to zirconia frameworks with laminate veneering. No bulk fractures or failures in the framework had been reported in the literature with a follow-up of 8 years.
Though Beuer et al evaluated the translucency and fracture strength of fully anatomical zirconia crowns and proved them to have better translucency and fracture strength than veneered crowns, yet an up-to-date complaint of full-contour zirconia, is its relative opacity and lack of translucent ivory color of natural teeth. For this reason, and for the sake of producing an esthetically optimized restoration with pronounced anatomical structure design, it’s advised to use individually veneered structures made of translucent zirconia restorations in cases of high esthetic standards such as the anterior teeth, canines and premolars.
Since the veneering ceramic is generally fused to the core at high temperatures, significant residual stresses could be generated in both layers if the coefficient of thermal expansion (CTE) matching is not satisfied. Consequently, special veneer ceramics that have lower or same CTE as zirconia have to be used to minimize this unfavorable complication of veneer chipping/cracking.
Previous research had shown that in zirconia veneered restorations, unlike metal-ceramic systems, excessive stresses that arise from CTE mismatch cause more destructive stress to be formed in the veneer layer of zirconia-based restorations due to the high rigidity of the zirconia framework. Thus, the strength of the veneering ceramic is a crucial parameter for long-term clinical success. Based on that fact, VITAVM9 veneering porcelain was chosen for the current study where it has a coefficient of thermal expansion of approximately 10.5×10-6K-1.
With zirconia being a polycrystalline material with no glass phase, core bonding with the veneering porcelain becomes difficult. Different surface treatment methods of the zirconia core have been tried in order to enhance the core-veneer bond strength, but still the bonding mechanisms between the zirconia core and the veneer are up to now unclear.
This study aimed at investigating the effect of different surface treatment methods of the nano-crystalline zirconia core on the shear bond strength (SBS) between the core and the veneering porcelain as well as the bi-axial flexural strength, color reproduction and translucency parameter of zirconia veneered restorations. Three surface treatment methods were chosen for this study in addition to a forth group in which samples were left un-treated to serve as a base-line for comparison with the rest of the groups.
Different techniques for silica coating have been reported in literature such as tribochemical silica coating and convenient silica coating using Silano Pen. In this study tribochemical silica coating using Cojet powder was chosen as one of the surface treatment methods. The tribochemical silica coating technique was done according to Gomes et al where the zirconia surface was treated using 30 μm alumina coated silica particles for 20 seconds, at working distance 10 mm. Distance was kept fixed using a specially designed holder and the pressure was adjusted to 2.8 bars.
The application of silica containing materials on zirconia surface has been investigated and reported by many authors.
In our study the concept of functionally graded glass/zirconia/glass structure (GGZ) introduced by Zhang et al in 2009 was used. Since the infiltrating glass must have (CTE) matching with that of Y-TZP and should have high melting point coupled with an excellent resistance to crystallization during cooling from the elevated temperatures, VITA AKZENT Glaze was used in our study. Trying to optimize a minimum thickness layer, the glaze ceramic was applied as a single layer using a brush. After sintering, each sample treated surface was manually ground with fine-grit round end diamond bur under water coolant and then acid-etched with hydraufloric acid 8% for 1 minute.
Based on the idea of fusion sputtering of nano-zirconia-introduced by Abousheleib M in 2012 for bonding zirconia ceramics with resin cements, and earlier by Teng J et al -the use of nano-sized Y-TZP powder was evaluated as a surface treatment method for the veneering surface of zirconia in the current study. 30 nm Y-TZP powder was deposited on the nano-crystalline zirconia samples for 5 seconds, at working distance 20 mm. Distance was kept fixed using a specially designed holder and the pressure was adjusted to 3 bars in order to create a controlled layer thickness. According to Aboushelib M, after sintering, zirconia particles become structurally fused with underlying framework and create undercuts suitable for establishing mechanical retention
A representative sample from each group was examined under the environmental scanning electron microscope (SEM) -after application of the surface treatment layer- to gain insightful information about the surface topography after different surface treatments. Environmental scanning electron microscope doesn’t need gold sputtering and thus was chosen so that the surface layer wouldn’t be affected and consequently the bonding with the veneering porcelain. The EDX attachment was used for the chemical analysis of the surface layer.
For evaluating the core-veneer bond strength, shear bond strength (SBS) test was chosen in the current study. Though multiple test methods are available, SBS test showed more standardized data since the applied forces are perpendicular to the bonding area, and the small cross-sectional area of the bonded surface eliminates the incorporation of structural flaws which significantly affect the test readings.
It was reported that the ultimate strength of all-ceramic systems could only be assessed by evaluating the mechanical behavior of the core-veneer composite. Based upon that and bearing in mind that the stress distribution in core-veneer all-ceramic restorations is more complex than in single component structures, different tests were developed for evaluating the strength of these systems.
Anatomical structures that mimic crowns and bridges offer the advantage of being similar in geometry to the main prosthesis and thus offer better imitation of vivo cases. However, they have their major drawbacks where there’s a difficulty in preparing specimens with reproducible dimensions and in data comparison where finite element analysis is needed. Moreover, comparison of observations between studies is usually complicated by the differences in tooth preparation, framework design, material thickness at areas of high tensile stress and the core-veneer ratio. Accordingly, standardized non-anatomical specimens offer more controlled environment for laboratory tests and consequently for controlling the basic mechanical properties.
Multiple tests are available for testing the flexural strength of all-ceramic restorations among which are; three-point flexural test, four-point flexural test and bi-axial flexural test. A previous study showed that all three test designs provided nearly the same relative order of results with different types of ceramics, yet four-point test showed relatively lower values which was explained by the higher probability of the presence of surface cracks between the two loading pistons.
On the other hand, in bi-axial flexural test, force is applied to the center of the specimen sparing the effect of edge defects that usually lead to early failure. Bi-axial flexural test also allows meaningful comparisons of strength values among different studies when comparable specimen preparation techniques and test parameters are used. Based upon the previous facts, bi-axial flexural strength test using a piston-on-three-balls technique was employed in our study. Specimens were designed in the form of bi-layered discs of 0.8 mm core thickness and 0.7 mm veneer thickness representing a total thickness of 1.5mm. Cheng et al in a previous study showed that the thickness of the specimen had no effect on failure distribution of the piston-on-three balls test, yet these thicknesses were chosen in our study in an attempt to imitate the real life cases where the recommended framework thickness for anterior FPD is 0.8 mm at the tapering restoration edge.
Results of the mechanical strength tests of bi-layered specimens differ according to the specimen configuration tested with whether the core material or the veneering porcelain is on the bottom surface. Investigations of clinically failed all-ceramic restorations have shown that the fracture origin is located at the tensile surface of the restoration. It was also stated that strength, reliability and mode of fracture of bi-layered composites is determined by the material on the bottom surface under bi-axial tensile stress all of which justified our choice to place the zirconia core surface as the tensile side for our test.
Regarding color and translucency measurements, previous studies used flat disc-shaped specimens in order to facilitate the process of obtaining controlled thicknesses of the different ceramic layers. Accordingly, this was applied in the following study where flat disc-shaped specimens were used for measurement of color reproduction and translucency parameter.
A white background was used in the current study, with reference to some previous researches, for the purpose of color measurement in order to eliminate the influence of the background on the measured color. In other previous studies, neutral grey background had been used.
Color measurement was done using Vita EasyShade Compact spectrophotometer whose CIELab output is based on D65 illuminant and 2-degree standard observer to resemble clinical situation.
The‖ restoration mode was used to obtain ΔE between the sample and data set in its software about shade A2. CIELab measurements make it possible to evaluate the amount of perceptible color change in each specimen. It’s a uniform 3-dimensional color order system. The L*coordinate represents the lightness-darkness of the specimen where the greater the L*, the lighter the specimen is. The a* coordinate expresses the chroma along the red-green axis where a positive a* relates to the amount of redness and a negative a* relates to the amount of greenness of the specimen. The b*coordinate measures chroma along the yellow-blue axis where a positive b* relates to the amount of yellowness while a negative b* relates to the amount of blueness of the specimen.
Because the human eye has a limited capacity to recognize small differences in color and the interpretation of visual color comparisons is subjective, the threshold level for visually perceivable or clinically acceptable color differences varies based on individual report.
Based on in vitro conditions, Douglas R.D. et al stated that the threshold for an acceptable color difference between metal ceramic crowns was reported to be 1.7 ∆Eab units while in another study based on composite resin specimens, 3.3 ∆Eab units was considered an acceptable threshold. As a clinical perceptible threshold, ∆Eab value of 3.7 units was judged as a perfect match based on composite resin veneer restorations and their comparison teeth. Based on a recent clinical study, it was reported that the perceivable color difference for 50% of the dentists was ∆Eab 2.6 while that at which 50% of the dentists would go for remake of the restoration due to color mismatch was 5.5 units, a guidance which we followed in the current study.
For translucency measurements, translucency parameter (TP) was chosen instead of contrast ratio (CR) measurement as it corresponds directly to the common visual assessment of translucency. CR is the ratio of the reflectance of a specimen over a black backing to that over a white backing of a known reflectance, and is an estimate of the opacity of a 1 mm thick specimen. Since the translucency of a substance is a function of wavelength, the reduction of a translucency spectrum (wavelength-dependent CR values) to a single parameter (TP) provides a simpler method to compare translucency. Besides Spink et al concluded that CR, which measures diffuse reflectance, does not detect small changes in light transmission, when materials present high scattering and absorption coefficients. CR could be used only for ceramic materials with a percent of total transmission of at least 50% .
Looking on the opposite side of the coin and just like displaying the unique property of phase transformation toughening (PTT) mechanism, low-temperature degradation (LTD) has been associated with several Y-TZP-based biomaterials. This phenomenon can have detrimental effects on the mechanical properties of zirconia, where once the zirconia crystals transform into m-polymorph phase, they can’t exhibit PTT anymore.
However, it is difficult to simulate LTD in the laboratory. Since autoclave treatment was proven to induce some degree of aging, it was a good method to propose an accelerated test for LTD. Chevalier et al reported that 1hour of autoclaving at 134°C had- theoretically- the same effect as 3 to 5 years in vivo, yet literature is debating if this is enough to trigger t-m transformation where further factors to which zirconia restorations are exposed in the oral environment (e.g. cyclic mechanical and thermal loading) are not considered in this calculation. Accordingly, ageing may probably proceed more rapidly in vivo.
Based upon the current international ISO standards, the maximum acceptable amount of monoclinic zirconia crystals is 25%. where this standard implies that a maximum of 25 wt % of monoclinic zirconia is present after an accelerated aging test conducted for 5h at 134 ◦C and 2 bar. Lughi V and Sergo V stated that it takes ten years for 25% of monoclinic zirconia to develop. Since it has been well-established in literature that the evaluation of all-ceramic restorations over five years of service is considered the gold standard and since a period of ten years is considered a reasonable lifetime for dental restoration to function in the oral cavity, samples-in the current study- were subjected to autoclave aging for 3 hours at 134 ◦C and 2 bar. However, these standards do not provide any information about the actual lifetime at/ near room temperature, Lughi V and Sergo V stated that lifetime prediction at room or body temperature according to the ISO standards relies on the assumption that the activation energy is approximately the same for all stabilized zirconia ceramics—a risky assumption, considering; residual stresses, grain size and dopant used.
In the current study; surface roughness of all specimens was assessed and expressed in terms of (Ra) value which describes the average surface roughness as a mean of the elevations and depressions measured from an estimated surface.
Ra values were shown to be the highest for the group that received cojet surface treatment and lowest for the control group with significant difference between the control group and each of the other three groups.
Since Cojet sand is basically ordinary alumina particles coated with silica using sol-gel technology, the significantly higher Ra value for the Cojet group may be attributed to the prominent micro-pores, irregularities and grooves caused by alumina particles as evident in the SEM micrographs. Similar results were reported in previous work when Aboushelib M compared the effect of sandblasting using 50 µm alumina particles and fusion sputtering with Y-TZP zirconia powder on zirconia surface and showed that sandblasted group had the highest surface roughness values compared to other groups.
Having a look on the idea of glass grading of zirconia, it could be compared to the selective infilteration etching technique based on creating a three-dimensional network of inter-grain porosity. This happens when fully sintered zirconia is heated causing stressing of the grain boundary regions and allowing molten glass to infilterate selectively between the boundaries of the surface grains. Based on the same concept and as revealed by the SEM micrographs, it was assumed that the low fusing porcelain, in the glass graded group, infilterarted between the grain boundaries of the surface layer during zirconia sintering creating a three-diminsional network. Further application of hydrofluoric acid etching on the silica-rich porcelain layer formed on the surface caused selective removal of silica particles creating micro-pores between the grains and increased the surface roughness. This could explain the significantly higher Ra value for the glass graded group as compared to the control group in our study.
Jianga T et al showed that zirconia surface treated with selective infilteration etching showed the highest Ra value when compared to both control untreated group and air-borne particle abraded group with 50 µm alumina particles. Their results come different from the results of our study which might be attributed to the fact that they applied hydrofluoric acid etching for 10 minutes while in our study acid etching was carried out for one minute only which might have caused total removal of silica particles and thus deeper micro-pore spaces and higher surface roughness.
In case of the group treated with zirconia powder deposition, zirconia powder deposition was applied using the same technique of Cojet treatment using air prophy unit at pressure 0.2 MPa. The technique of application together with the use of nano-zirconia particles might have caused the formation of zirconia clusters on the surface. This was confirmed by SEM micrographs and by previous work of Aboushelib M where fusion sputtering of zirconia powder resulted in formation of 8-12 µm high retentive beads made of zirconia dioxide material.
Since long-term clinical success of the multi-layered restoration depends not only on the mechanichal strength of the core, but is also determined by the weakest component of the structure, light has to be shed on the on the relation between the core and the veneer. It has been previously reported that the core-veneer bond strength is a weak point in a multi-layered restoration and that it highly affects its clinical success.
According to our results, different surface treatment methods of the zirconia surface caused significant differences in the values of shear bond strength with the veneering porcelain. The control untreated group showed the highest shear bond strength value followed by the cojet treated group which were both significantly higher than the glass graded group and the zirconia powder deposition treated group that showed the lowest values before aging of the samples.
As stated in literarture, many variables may affect the core-veneer bond strength such as the surface finish of the core, which can affect mechanical retention, residual stresses generated by mismatch in thermal expansion coefficient (TEC), development of flaws and structure defects at core-veneer interface and wetting properties and volumetric shrinkage of the veneer. The individual and the combined effects of such variables can influence the core veneer bond strength.
A previous study by Fischer et al has shown that surface finish and surface roughness of the core material had no effect on the bond strength to the veneering porcelain. We don’t agree with that speculation where according to our results, the control untreated group having the lowest surface roughness values showed the highest mean shear bond strength while both the glass graded group and the zirconia powder deposition groups showed the lowest shear bond strength values. It could be speculated that high surface roughness of the surface may cause poor wettability by the veneering porcelain on the core surface and thus development of microporosities at the interface that could affect the bond strength.
These findings are almost opposite to what was stated by Oguri T et al who showed that increased surface roughness enhanced the shear bond strength.
It was stated previously in literature that some kind of chemical bond occurs between the core and veneering porcelain where some components from the veneering porcelain where found to diffuse into the superficial zirconia layer. This could explain why the control untreated group showed high shear bond values where no other factors such inter-mechanical interlocking were needed to improve the bond.
Since it was proved previously that the CTE of the veneering porcelain should be slightly lower than that of the core material in-order to develop a layer of compressive stresses in the porcelain layer and thus enhance the bond strength between them, it is most probably the mismatch in the CTE between the veneering porcelain and the low fusing porcelain layer on the zirconia surface that is causing the low strength values for that group.
According to the SEM images, it was shown that there are areas of micro beads and zirconia clusters on surface of zirconia in the zirconia powder treated group. These clusters might have played a role in decreasing the wettablity by the veneering porcelain and increasing the micro-porosities at the interface decreasing the bond strength at the interface for that group.
As for the effect of aging on different surface treatment groups, it was noted that autoclave aging caused a significant increase in the shear bond strength values for the cojet treated group while causing significant reduction in the bond values for the control group.
The significant decrease in the bond values for the control group could be explained by the effect of low temperature degradation in causing phase transformation of the surface zirconia layer into monoclinic phase. It has been suggested that a superficial layer of monoclinic phase results in tensile stresses in the veneering porcelain layer and thus adversely affecting the bond strength since the CTE of monoclic zirconia (7.5×10-6/k) is significantly lower than that of tetragonal zirconia (10.8×10-6/k).
Previous studies have reported that adding a small amount of silica to zirconia resists water penetration and thus hydrolysis therefore reducing the effect of low temperature degradation. This finding comes in accordance with our results where it was noted that aging caused significant improvement in the shear bond strength values.
Regarding the flexural strength results, the control group receiving no treatment showed the highest value, but with no significant difference among groups either before or after aging. This higher value for the control group could be explained by its significantly lower surface roughness than the remaining three groups which coincides with results obtained from the work of Mrrelli M et al who stated that there was a strong effect of surface roughness on mechanical strength of zirconia ceramics where they recorded a massive increase in the average flexural strength of Y-TZP plates after mechanical polishing treatment Wang H et al also proved that lower surface roughness values could increase the mechanical strength of the material.
Considering the effect of aging, bi-axial strength values were significantly lower for the aged sub-groups than the unaged in both the control and the zirconia deposition groups. These results are opposite to what was suggested by Nakamura K where the mean strength of colored Y-TZP increased after 10 hours of autoclaving. His results were also in accordance with multiple previous studies that suggested that when zirconia is subjected to LTD by autoclaving, a layer of compressive stresses in generated on the surface due to the superficial phase transformation. This layer resists crack propagation and results in increased strength before generation of further monoclinic phase causing micro-cracks and subsequent strength reduction.
On the other hand, the other two groups showed no significant reduction in the flexural strength values after aging. A suggested reason for that could be the effect of silica that could have penetrated at the grain boundaries and reduced hydrolysis thus suppressing the occurrence of degradation as stated by Nakamura T et al. In another research, Nakamura et al concluded that addition of small amount of silica to Y-TZP decreases the internal stresses through the presence of rounded grains which subsequently improved the resistance to LTD without actually affecting the resistance to crack propagation.
With esthetics of a restoration being a paramount concern, the effect of different surface treatments on the CIELab values and color reproduction of the core-veneered discs was investigated before and after aging.
For L* value representing the degree of lightness and darkness, it was noted that there was no significant difference before and after aging in each surface treatment group.
In the unaged sub-groups, glass graded samples showed the highest values while the control group specimens had the lowest values. This could be attributed to the whitish color of the low fusing porcelain used for glass grading. Color changes of ceramic materials may also occur from metal oxides that are added to ceramic materials in order to obtain acquired color shades.
There was significant difference between all groups-either before or after aging- except between control group and zirconia powder deposition and between cojet and zirconia powder deposition with the high values for the cojet group being probably due to the color of the cojet powder.
No significant difference was noted between the control and the zirconia powder deposition groups since both surfaces are composed of nano-crystalline yttria-stabilized zirconia.
Regarding the a* value representing the red-green axis scale of color, all groups showed significant difference when compared before aging except control and the zirconia deposition groups that showed no significant difference in a* value when compared together.
It was noted that for all unaged sub-groups, except glass grading group, the a* value was in the red zone of color with the cojet group showing a shift in the red zone away from the base-line probably due to the effect of cojet powder color. In case of glass graded group, there was notable shift in color towards the green zone which might be attributed to the coloring pigments present in the low fusing porcelain.
Control and zirconia deposition groups showed no significant difference regarding the a* value because both surfaces are composed of nano-crystalline zirconia.
After aging, no significant difference in a* value was reported between the cojet and control groups and between the cojet and zirconia groups because aging caused surface uplifting, micro-cracks and enhanced water penetration causing shift in the a* value in both the control and zirconia deposition groups while the cojet group wasn’t affected by aging due to the effect of silica in suppressing water penetration.
No significant difference was seen before and after aging in each group except in the zirconia and the control groups where aging had a significant effect causing notable shift in the reddish zone. This could be due to the phase transformation at the surface of the specimens which would change its optical reflection.
For the b*value representing the yellow-blue axis of color, all groups appeared to fall in the yellow zone of color with the glass graded group showing the lowest b* value which is probably due to the coloring pigments present in the low fusing porcelain.
There was significant difference between all groups both before and after aging- except between cojet and zirconia deposition groups.
There was no significant difference in each group before and after aging except in the glass graded group where aging had caused a higher shift towards the yellowish zone.
Based on in vitro conditions, Douglas R et al stated that the threshold for an acceptable color difference between metal ceramic crowns was reported to be 1.7 ∆Eab units while in another study based on composite resin specimens, 3.3 ∆Eab units was considered an acceptable threshold. As a clinical perceptible threshold, ∆Eab value of 3.7 units was judged as a perfect match based on composite resin veneer restorations and their comparison teeth. Based on a recent clinical study, it was reported that the perceivable color difference for 50% of the dentists was ∆Eab 2.6 while that at which 50% of the dentists would go for remake of the restoration due to color mismatch was 5.5 units, a guidance which we followed in the current study.
In the current study, the ∆ E values showing the color change with different surface treatment methods before and after aging all ranged between 11.6 and 21.6 which were far beyond both the perceptible and acceptable values. It was seen that there was significant difference in all groups-in both un-aged and aged sub-groups- except between cojet and zirconia deposition groups.
No significant difference was reported in each group of surface treatment before and after aging except in the glass graded group in which aging caused a higher change in color due to the effect of aging on causing a shift towards the yellowish zone.
Translucency parameter representing the translucency of samples was shown to be highest for the control group receiving no surface treatment at all and lowest for the glass graded group with all groups showing significantly different values in both sub-groups except between the control and cojet.
In previous studies, Aboushelib M reported the presence of 8-12 µm retentive beads made of zirconia dioxide material on the surface when viewed under the SEM. Also, Teng J et al stated the presence of rugged surface with abundant microporosities when coated zirconia specimens were examined under the SEM.
On the other hand, glass graded group showed significantly lower translucency values due to the high opacity of the low fusing porcelain used for glass grading
For the present study, significantly lower TP value for the zirconia deposition group may be attributed to the formation of clusters of zirconia powder during blasting. Such clusters-having indefinite arrangement- might cause scattering of light and decrease of translucency. These clusters might also decrease the wettability of the veneering porcelain on the zirconia surface causing voids formation that act as site for light entrapment, preventing passage of light and decreasing the material’s translucency.
Inside each surface treatment group, there was no significant difference in the TP value before and after aging showing that aging had no effect on the samples’ translucency.
Summary and Conclusions:
Summary:
During the last few decades, the demand for metal-free restorations, being conditioned by both social pressures and interests of dental profession has increased dramatically and a climate of non-acceptance of metal alloys in the oral cavity prevailed.
Trying to meet those highly increasing esthetic demands while at the same time providing a long-term successful and biocompatible restoration, several types of all-ceramic systems have been developed.
With the advances in computer aided design (CAD) and computer aided manufacturing (CAM) technologies, the high-strength ceramic systems have become increasingly popular and Yttria-stabilized tetragonal zirconia (Y-TZP), with its unsurpassed mechanical properties and biocompatibility has had its clinical applications expanded in the field of fixed prosthodontics. However, its whitish opaque color has imposed an esthetic problem and the need for veneering with a more esthetic glass ceramic became imperable.
Advances in industrial facilities and the improvements in nano-technology, led to the evolution of nano-structured polycrystalline zirconia in an attempt to add esthetic value to the mechanical supremacy. Though nano-crystalline full-contour zirconia is reported to have high transcluceny, yet for better esthetic results, it’s advised to use individually veneered structures.
Minor Chipping or delamination of the veneering porcelain from the zirconia framework has been reported to be one of the major causes of failure of zirconia-based restorations. Different surface treatment methods of zirconia surface have been tried in an attempt to overcome this probplem, but still the exact mechanism of bonding between the zirconia core and the veneering porcelain is unknown.
This study was conducted to evaluate the effect of different surface treatment methods on shear bond strength between nano-crystalline Yttria-stabilized zirconia and veneering porcelain, as well as bi-axial flexural strength, color reproduction and translucency parameter of the core-veneered restoration.
Nano-crystalline yttria-stabilized zirconia samples were constructed with different configurations according to the factor under research and classified according to whether they would be tested for shear bond strength or biaxial flexural strength, color reproduction and translucency parameter measurements.
For shear bond strength test, samples were cut using a hard stainless steel disc mounted on a milling machine in the form of zirconia plates of final dimensions 12mm x 15 mm x 2 mm and veneered with porcelain discs of dimensions 3 mm x 3mm.
For biaxial flexural strength test, color reproduction and translucency parameter measurement, samples were cut in the form of zirconia discs 12 mm in diameter and 0.8mm thick-after sintering- veneered with a layer of porcelain of 0.7 mm thickness and of the same diameter as the zirconia discs.
Before veneering, all zirconia discs were colored by dipping in aqueous zirconia coloring solution of shade A2 and then subjected to tribochemical silica coating, zirconia powder deposition or glass grading- before being sintered. Half of the samples of each group of each type were subjected to hydrothermal aging using autoclave before testing.
Results showed that different surface treatment methods applied had significant over shear bond strength, surface roughness, color reproduction and translucency parameter, while they had no signignificant effect on bi-axial flexural strength. This was true when for both the un-aged and aged sub-groups of different surface treatment goups.
Conclusions:
Different surface treatments had a significant effect on surface roughness of zirconia surface where untreated group displayed the smoothest surface.
Control untreated having the lowest surface roughness displayed the highest flexural strength values denoting that biaxial flexural strength could be affected dierectly by surface roughness of the framework surface.
Despite the high initial flexural strength of the control group, autoclave aging caused significant reduction in strength values which shows the determental effect that aging might have on zirconia-based restorations.
Both control untreated group and cojet treated group dispalyed the highest shear bond stregth values initially,yet after autoclave aging, the control group showed significant reduction in shear bond strength values while cojet treated group showed significant increase in values. This implies that tribochemical silica coating could be benificial in improving the shear bond strength over time.
Different types of surface treatment implied in the previous study affected the color reproduction and translucency negatively where the control untreated group showed the highest translucency parameter and color reproduction, yet aging had no effect.
Clinical Recommendations:
In cases of high esthetic standards where individually veneered structures might be needed, it’s best recommended to apply the veneering porcelain layer over the zirconia surface directly with no special surface treatments.
Tribochemical silica coating using cojet powder might only be used due to its effect in improving the shear bond strength with the veneering porcelain over time.
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List of Tables
Table 1: Chemical composition of inCoris TZI 51
Table 2: Technical data of inCoris TZI. 51
Table 3: Physical properties of VITA VM9 53
Table 4:Standard composition of Cojet powder 55
Table 5: Physical properties of nano-crystalline Yettria stabilized nano-zirconia powder 56
Table 6: Dentobond porcelain etch composition 57
Table 8: Samples grouping for biaxial flexural strength test, color reproduction and translucency parameter measurment 59
Table 9: Firing chart of VITAVM9 71
Table 10: Firing chart of VITA AKZENT glaze 73
Table 11: Elemental composition of untreated zirconia plate. 85
Table 12: Elemental composition of tribochemical coated zirconia plate. 87
Table 13: Elemental composition of zirconia plate treated with zirconia powder deposition. 89
Table 14: Elemental composition of glass graded zirconia plate. 91
Table 15: Mean (SD) of surface roughness values of different surface treatment groups. 92
Table 16: One way analysis of variance (ANOVA) showing the effect of different surface treatments on surface roughness values.. 93
Table 17: Multiple comparison tests showing the effect of different surface treatments on surface roughness; Dunnett-t for comparison with the control group and Boneferroni for comparison between the groups. 93
Table 18: Mean (SD) of bi-axial flexural strength values in different surface treatment groups before and after aging. 94
Table 19: One way analysis of variance (ANOVA) for testing the means of bi-axial strength values of un-aged sub-groups. 95
Table 20: One way analysis of variance (ANOVA) for testing the means of bi-axial strength values of aged sub-groups. 95
Table 21: Independent samples t-test comparing the mean (TP) between un-aged and aged sub-groups in different surface treatment groups. 96
Table 22: Mean ( SD) shear bond strength values of different surface treatment groups. 97
Table 23: One way analysis of of variance (ANOVA) for testing the mean values of shear bond strength on un-aged sub-groups. 98
Table 24: Multiple comparison tests showing the effect of different surface treatments on shear bond strength values of un-aged sub-groups; Dunnett-t for comparison with the control group and Boneferroni for comparison between the groups. 98
Table 25: One way analysis of variance (ANOVA) for testing the mean values of shear bond strength on aged sub-groups. 99
Table 26: Multiple comparison tests showing the effect of different surface treatments on shear bond strength values of aged sub-groups; Dunnett-t for comparison with the control group and Boneferroni for comparison between the groups. 99
Table 27: Independent samples t-test comparing the shear bond strength means between un-aged and aged sub-groups of each surface treatment group. 100
Table 28: Mean (SD) L* values of different surface treatment groups before and after aging. 101
Table 29: One way Analysis of variance (ANOVA) comparing the means of L* values of unged sub-groups of different surface treatment groups 102
Table 30: Multiple comparison tests showing the effect of diiferent surface treatments on L* value of un-aged subgroups; Dunnett-t for comparisons with the control group and Bonferroni for comparisons between groups 102
Table 31: One way analysis of variance (ANOVA) comparing the means of L*values of aged sub-groups for different surface treatment groups 103
Table 32: Multiple comparison tests showing the effect of different surface treatments on L* values of aged sub-groups; Dunnett-t for comparisons with the control group and Bonferroni for comparisons between groups. 103
Table 33: Independent samples t-test comparing the L* value means between un-aged and aged sub-groups of each surface treatment groups 104
Table 34: Mean (SD) of a* value of different surface treatment groups before and after aging. 105
Table 35: One way analysis of variance (ANOVA) comparing the a* value means of un-aged sub-groups of different surface treatment groups. 106
Table 36: Multiple comparison tests showing the effect of different surface treatment methods on a* value of un-aged sub-groups; Dunnett-t for comparison with the control group and Bonferroni for comparison between groups. 106
Table 37: One way analysis of variance (ANOVA) comparing the a* value means of aged sub-groups of different surface treatment groups. 107
Table 38:Multiple comparison tests showing the effect of different surface treatments on a* value of aged sub-groups; Dunnett-t for comparison with the control group and Bonnferroni for comparison between groups. 108
Table 39: Independent samples t-test comparing the a* value means between the un-aged and aged sub-groups of different surface treatment groups. 109
Table 40: Mean (SD) of b* value of different surface treatment groups before and after aging. 110
Table 41: One way analysis of variance (ANOVA) comparing the b* value means of an-aged sub-groups of different surface treatment groups 111
Table 42: Multiple comparison tests showing the effect of different surface treatments on b* value of unaged sub-groups; Dunnett-t test for comparisons with the control group and Bonferronni for comparisons between groups. 111
Table 43: One way analysis of variance (ANOVA) comparing the b* value means of aged sub-groups of different surface treatment groups 112
Table 44: Multiple comparison tests showing the effect of different surface treatments on the b* value of aged sub-groups; Dunnett-t for comparisons with the control group and Bonferroni for comparisons between groups. 112
Table 45: Independent samples t-test comparing the b* value means between unaged and aged sub-groups of different surface treatment groups. 113
Table 46: Mean and (SD) of delta E values for different surface treatment groups before and after aging. 114
Table 47: One way analysis of variance (ANOVA) comparing the delta E means of un-aged sub-groups of different surface treatment groups. 115
Table 48: Multiple comparison tests showing the effect of different surface treatments on the delta E value of un-aged sub-groups; Dunnett-t test for comparison with the control group and Bonnferroni for comparison between the groups. 115
Table 49: One way analysis of variance (ANOVA) comparing the delta E means of aged sub-groups of different surface treatment groups. 116
Table 50: Multiple comparison tets showing the effect of different surface treatments on the delta E value of aged sub-groups; Dunnett-t test for comparison with the control group and Bonnferroni for comparisons between the groups. 116
Table 51: Independent samples t- test comparing delta E value means between un-aged and aged sub-groups of each surface treatment group. 117
Table 52: Mean (SD) of translucency parameter (TP) of different surface treatment groups before and after aging. 118
Table 53: One way analysis of variance (ANOVA) comparing the (TP) means of un-aged sub-groups of different surface treatment groups. 119
Table 54: Multiple comparison tets showing the effect of different surface treatments on the TP value of un-aged sub-groups; Dunnett-t test for comparison with the control group and Bonnferroni for comparisons between the groups. 119
Table 55: One way analysis of variance (ANOVA) comparing the (TP) means of aged sub-groups of different surface treatment groups. 120
Table 56: Multiple comparison tests showing the effect of different surface treatments on the TP value of aged sub-groups; Dunnett-t test for comparison with the control group and Bonnferroni for comparisons between the groups. 120
Table 57: Independent samples t-test comparing the mean (TP) between un-aged and aged sub-groups in different surface treatment groups 121
List of Figures
Figure 1: inCoris TZI block 40/19 52
Figure 2: inCoris coloring liquid 52
Figure 3: VITAVM9 ENL powder 54
Figure 4:VITA AKZENT glaze powder and liquid 54
Figure 5:Cojet powder 55
Figure 6: Nano-crystalline YTZP powder 56
Figure 7: Dentobond porcelain etch 57
Figure 8: Stainless steel disc mounted on a milling machine for cutting of zirconia samples 60
Figure 9: Hardened stainless steel disc 60
Figure 10: Milled zirconia plate for shear bond strength test and disc bi-axial flexural strength test 61
Figure 11: Dipping of zirconia samples in coloring solution 62
Figure 12: Tribochemichal coating of zirconia disc 64
Figure 13: zirconia powder coating of zirconia disc 65
Figure 14: Glass grading of zirconia disc 66
Figure 15: Sample holder for thickness adjustment of the glass grading layer. 66
Figure 16: zirconia samples supported by beads on the firing tray 67
Figure 17: Scanning electron microscope 68
Figure 18: Digital microscope connected on a special software for measurement of Ra value 69
Figure 19: Sintered zirconia plate afte rfiring of the porcelain discs 71
Figure 20: Two-part teflon mold for build-up of veneering porcelain 72
Figure 21: Veneering of zirconia discs in a specially designed mold 72
Figure 22: Glazing of zirconia discs 73
Figure 23: VITA Easyshade Compact 75
Figure 24:Color measurment over white background using VITA Easyshade Compact 75
Figure 25:Measurement of color parameters over black background 76
Figure 26: Sample fixed on LLoyd universal testing machine for shear bond strength test 77
Figure 27: Chisel-shaped piston directed at the zirconia-veneer interface 78
Figure 28: Three hardened stainless steel balls positioned 120˚ apart on a support circle with a diameter 10 mm. 80
Figure 29: zirconia disc concentrically placed over the the three balls and fixed to the LLoyd universal testing machine for bi-axial flexural strength test 80
Figure 30: Autoclave Sturdy SA-260-MA 81
Figure 31: Veneered samples placed for autoclave aging 81
Figure 32:SEM image of untreated zirconia plate. (1000X) 84
Figure 33: Elemental composition of untreated zirconia plate. 85
Figure 34: SEM image of zirconia tribochemichal silica coated zirconia plate. (1000X) 86
Figure 35: Elemental composition of tribochemical coated zirconia plate. 87
Figure 36: SEM image of zirconia plate treated with zirconia powder deposition.(1000X) 88
Figure 37: Elemental composition of zirconoa plate treated with zirconia powder deposition. 89
Figure 38:SEM image of glass graded graded zirconia plate(1000X).(a) After fusing with glass ceramic (b) After etching ewith 10% HF 90
Figure 39: Elemental composition of glass graded zirconia plate. 91
Figure 40: Bar chart showing means of (Ra) value in relation to different surface treatments before aging. 92
Figure 41: Bar chart showing means of bi-axial flexural strength values in relation to different surface treatments before and after aging. 94
Figure 42: Bar chart showing means of shear bond strength in relation to different surface treatments before and after aging. 97
Figure 43: Bar chart showing mean of L* value in relation to different surface treatments before and after aging 101
Figure 44: Bar chart showing means of a* value in relation to different surface treatments before and after aging. 105
Figure 45: Bar chart showing means of b* value in relation to different surface treatments before and after aging. 110
Figure 46: Bar chart showing means of ∆E in relation to different surface treatments before and after aging. 114
Figure 47: Bar chart showing means of (TP) in relation to different surface treatments before and after aging. 118
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