Review of Literature [301602]

[anonimizat]-ceramic restorations have the potential to be a more effective selection when compared to metal ceramic restorations. [anonimizat] a more translucent look that replicates the appearance of the natural tooth (1). All-ceramic fixed partial dentures (FPDs) [anonimizat] a clinical use. [anonimizat] (2). However, [anonimizat] (3). [anonimizat], such as a glass-[anonimizat], a lithium-disilicate-[anonimizat]-based materials. [anonimizat]-stabilized zirconia polycrystals (3Y-TZP) (4). This zirconia contains 3 mol% of yttria (Y2O3) as a stabilizer. The major advantage of this material is its high fracture resistance which is represented by their superior flexural strength (900-1000 MPa) and fracture toughness (5.5 – 7.4 MPa∙m1/2) [anonimizat] (5). The processing procedures of 3Y-TZP usually use a CADCAM technology for machining a presintered zirconia blank to a desired size and shape of a prosthesis and subsequent firing at 1350 – 1550℃ which is carried out to produce a densely sintered product. Compensation for 20 – 30% firing shrinkage is made during a CAD procedure. There is a growing interest in the use of Zirconia oxide ceramics as substitutes for metal core structures (6).

As Zirconia is relatively opaque and monochromatic in color, a layer of veneering ceramic is built on to provide the restoration with the required esthetics. [anonimizat] a lengthy and a time-consuming process. [anonimizat], [anonimizat], [anonimizat]–veneer interface remain unavoidable. [anonimizat] (7).

Establishing a [anonimizat] (8). Several surface roughening and coating methods have been used to optimize the surface of Zirconia and to improve its bond strength.

[anonimizat]-[anonimizat]. Also selective infiltration etching (SIE), was developed for dental and biomedical applications. (9).

In spite of being the golden standard in terms of strength and toughness, 3Y-[anonimizat]ity, which has been a major issue for medical use and has led to the replacement of several zirconia femoral heads in orthopedic patients. Low-temperature degradation (LTD) has been associated with several 3Y-TZP-based biomaterials, but is difficult to simulate in the laboratory. Aging occurs experimentally in zirconia, mostly in humid atmosphere or in water. An accelerated ageing test using steam and pressure has been developed to simulate LTD, s-LTD (simulated LTD) (10).

The bond strength between zirconia and its veneering ceramic depends on so many interrelated factors and the effects of these factors together on the bond between veneering ceramic and Zirconia framework is currently the subject of comprehensive investigations.

Thu s the purpose of this study is to investigate the effect of a new method of surface treatment of Zirconia on its bond strength with the ceramic veneering ceramics with different veneering techniques and before and after artificial aging.

Review of Literature

Dental crown and bridge restorative materials are changing from porcelain fused to metal (PFM) to all-ceramic systems. Techniques, which have been developed over decades in the case of PFM systems, must now be redeveloped for all-ceramic systems (11).

All-ceramic systems can provide a better esthetic result for a wider range of patients than can metal-ceramic systems, because a wider range of translucency and opacity (value) can be achieved. (12)

Ceramics as dental materials have desirable characteristics such as chemical stability, biocompatibility, high compressive strength, and a coefficient of thermal expansion similar to that of tooth structure. In addition, dental ceramics have esthetic properties that simulate the appearance of natural dentition; however, they are susceptible to fracture, which is a result of material characteristics and surface and bulk defects (13).

Ceramics fall into three main composition categories: predominantly glass; particle-filled glass; and polycrystalline. Zirconia is a polycrystalline ceramic without a glassy phase and exists in several forms.

History of zirconia:

Zircon has been known as a gem from ancient times. The name of the metal, zirconium comes from the Arabic Zargon (golden in colour) which in turn comes from the two Persian words Zar (Gold) and Gun (Colour). Zirconia, the metal dioxide (ZrO2), was identified as such in 1789 by the German chemist Martin Heinrich Klaproth in the reaction product obtained after heating some gems, and was used for a long time blended with rare earth oxides as pigments for ceramics. (14)Good chemical and dimensional stability, mechanical strength and toughness, coupled with a Young’s modulus in the same order of magnitude of stainless steel alloys was the origin of the interest in using zirconia as a ceramic biomaterial. In the early stages of the development, several solid solutions (ZrO2-MgO, ZrO2-CaO, ZrO2-Y2O3) were tested for biomedical applications. But in the following years the research efforts appeared to be more focused on zirconia-yttria ceramics, characterised by fine grained microstructures known as Tetragonal Zirconia Polycrystals (TZP).

Zirconia is a well-known polymorph that occurs in three forms: monoclinic (M), cubic (C) and tetragonal (T). Pure zirconia is monoclinic at room temperature. This phase is stable up to 1170șC. Above this temperature it transforms into tetragonal and then into cubic phase at 2370șC. During cooling, a T-M transformation takes place in a temperature range of about 100șC below 1070șC. The phase transformation taking place while cooling is associated with a volume expansion of approximately 3-4%. Stresses generated by the expansion originate cracks in pure zirconia ceramics that, after sintering in the range 1500-1700șC, break into pieces at room temperature. It was Ruff and co-workers 1929 (15) that showed the feasibility of the stabilisation of C-phase to room temperature by adding to zirconia small amounts of CaO. The addition of stabilizing oxides, like CaO, MgO, CeO2, Y2O3, to pure zirconia allows to generate multiphase materials known as Partially Stabilized Zirconia (PSZ) whose microstructure at room temperature generally consists (16) of cubic zirconia as the major phase, with monoclinic and tetragonal zirconia precipitates as the minor phase. These precipitates may exist at grain boundaries or within the cubic matrix grains.

In Garvie and Nicholson 1972 (17) showed that the mechanical strength of PSZ was improved by a homogeneous and fine distribution of the monoclinic phase within the cubic matrix. The development of zirconia as an engineering material was marked by Garvie et al. 1975 (18) , who in their article ‘Ceramic Steel?’ showed how to make the best of T-M phase transformation in PSZ improving mechanical strength and toughness of zirconia ceramics. The tetragonal metastable precipitates was observed to be finely dispersed within the cubic matrix were able to be transformed into the monoclinic phase when the constraint exerted on them by the matrix was relieved, i.e. by a crack advancing in the material. In that case, the stress field associated with expansion due to the phase transformation acts in opposition to the stress field that promotes the propagation of the crack. An enhancement in toughness is obtained, because the energy associated with crack propagation is dissipated both in the T-M transformation. This remarkably increases the fracture toughness of the material by hindering, but not preventing, the propagation of a crack; tensile stress concentration converts the transformation from (t) phase to the (m) phase(19). Increasing the crystal volume, constrained by the surrounding ones, leads to a favourable compressive stress. This limits growth of cracks. Transformation toughening or “phase transformation toughening” (PTT) is the reported mechanism for exceptional flexural strength and fracture toughness of zirconia among all the other ceramics(20). At room temperature, the transformation from tetragonal to monoclinic is a one-way process. This means that, once it takes place, the crack-hindering effect cannot be exploited for limiting further fractures, “like a used match cannot be lit again” (21). Heating the material at a temperature between 900 ◦C and 1000 ◦C for a short time, can reverse the process (22) ; in this case, the phase transition from monoclinic back to tetragonal form, rather than making crystals available again for further transformation and crack repair, generates a relaxation of the advantageous compressive stress at the surface, reducing the material toughness From this point of view, the high temperature thermal process of veneering zirconia with feldspathic ceramic should be taken into account as a possible risk of such a detrimental reverse transformation (4) .

The grain size dramatically influence the mechanical behaviour of zirconia, in that higher temperatures and longer sintering times produce larger grain sizes (23). The critical crystal size is approximately 1µm: above such dimension, zirconia is more prone to spontaneous PTT due to lower stability, whereas a smaller grain size makes zirconia less susceptible to this phenomenon, although below 0.2µm PTT does not happen and zirconia fracture toughness decreases. Consequently, the sintering conditions are paramount since they influence the crystal size, strongly affecting the mechanical properties and the stability of zirconia and have to be strictly controlled in the whole production process (24).

Characteristics of Zirconia:

Biological characteristics of zirconia:

Biocompatibily and bioinercia:

In vitro and in vivo studies have confirmed a high biocompatibility of zirconia, especially when it is completely purified of its radioactive contents(25). Zirconia based ceramics are chemically inert materials, allowing good cell adhesion, and while no adverse systemic reactions have been associated with it. (26) However, particles from the degradation of zirconia at low temperature (LTD) or from the manufacturing process can be released, promoting an immune localized inflammatory reaction.(27)

Degree of toxicity

In vitro tests have shown that zirconia has a lower toxicity than titanium oxide and similar to alumina. Cytotoxicity, carcinogenicity, mutagenic or chromosomal alterations in fibroblasts or blood cells has not been observed(28).

Optical characteristics

Degree of opacity and translucency

The ceramic systems used in dentistry must have adequate translucency to achieve good dental esthetics while at the same time provide adequate strength during chewing. Considering the currently available ceramic materials, these two properties cannot be obtained by a single material, especially in the manufacturing of fixed prostheses. Thus, an oxide ceramic material should be used as an infrastructure, while a glass or feldspathic ceramic must be used as an esthetic coating material. The infrastructures of zirconia provide good masking of darkened substrates due to an adequate level of opacity, and they also allow a controlled translucency after lamination, due to their homogeneity and high density (residual porosity <0.05%), even limited in thickness (0.5mm). Its opaque optical behavior can be attributed to the fact that the grain size is greater than the length of light, and also that zirconia has a high refractive index, low absorption coefficient and high opacity in the visible and infrared spectrum. (29) Therefore, zirconia cannot be used as a restorative material alone. Because of its opacity and the current processing technologies, zirconia must be covered with translucent ceramics which exhibit characteristics that may look like natural teeth(30).

Mechanical characteristics

The mechanical performance of zirconia were extensively investigated on both single crowns and 3- and 4-unit FPDs, with variable reported data, due to a noticeable difference of experimental conditions and measurements. Mechanical properties of zirconia were proved to be higher than those of all other ceramics for dental use, with a fracture toughness of 6–10MPa/m1/2, a flexural strength of 900–1200MPa and a compression resistance of 2000MPa (4,14). An average load-bearing capacity of 755N was reported for zirconia restorations (31). Fracture loads ranging between 706N (31), 2000N (32) and 4100N (33) were reported; all of the studies demonstrated that in dental restorations zirconia yields higher fracture loads than alumina or lithium disilicate. A recent in vitro investigation on zirconia FPDs evidenced failure loads ranging between 379 and 501MPa, thus higher than average human biting force, confirming a satisfactory serviceability of such frameworks (34).

Flexural strength

Flexural strength is an important mechanical property that aids in predicting the performance of fragile materials. It can be defined as the final force required to cause fracture and is strongly affected by the size of flaws and defects on the surface of the material tested(35). Micro cracks and defects that inherently grow during the thermal and mechanical processes can significantly influence the measurement of resistance. However, data alone cannot be extrapolated to predict the clinical performance of a material. Resistance values are significant when incorporated into context, through knowledge of the material microstructure, processing history, methodology, test environment and failure mechanisms. Structural failure probabilities are determined by additional failure, variables that describe the stress distribution and sizes of defects, which may be considered as single or multiple failure modes. An understanding of the current clinical failure modes is absolutely necessary before the results of in vitro resistance testing can be considered with clinical validity(36). Values of mechanical strength of fragile materials usually exhibit a large dispersion of values (above 50%), even for high performance ceramics(37). This known phenomenon is based on the distribution of defects or failures. The hypothesis that surface defects and micro-cracks in ceramic Y-TZP zirconia are made internally on the surface machined by CAD-CAM technique was confirmed by Luthardt et al. 2004(38). Milling may introduce residual surface compressive stresses that can significantly increase the resistance of zirconia ceramics. On the other hand, severe wear can make profound defects, which act as stress concentrating areas. Alternative methods, such as the partially sintered method of ceramics manufacturing, as well as wear-free procedures, should be developed to obtain crowns and bridges of the Y-TPZ system that increases strength and reliability. Another important fact is that the accumulation of microcracks resulting from loading in an aqueous environment (such as that found in the oral cavity), can cause surface defects that act as enhancers of tension in areas of local concentration, facilitating the initiation of fracture under low level applied stresses (39).

Fracture toughness

Fracture toughness is defined as the level of critical stress at which a particular defect starts to grow. This property indicates the material's ability to resist rapid crack propagation and catastrophic fracture(40). It also measures the ease of crack growth from an initial failure. Steel and ductile metals show values above 50MPa m1/2.Ceramics cover a range of fracture toughnesses which rarely exceeds 5MPa m1/2. Not surprisingly, these low values affect their clinical performance(41). In zirconia, the process of phase transformation induces compressive stress at the crack tip and shear stresses that act against the stress field generated in this region. The addition of an oxide stabilizes the system transformation of zirconia in the tetragonal phase and retains a layer of compressive stresses, resulting in the formation of a tougher stabilized tetragonal zirconia polycrystal(42).

Subcritical crack growth

The subcritical crack growth (SCG), which consists of a slow propagation of failures, is one of the major causes of damage to ceramics and usually occurs as a function of time. SCG under constant load is due to the corrosive action in the region under stress at the crack tip. (43) The amount of SCG is affected by different factors that add to the strain rates. The format, depth and width of the defects within the material affect the tension intensity factor. In ceramics, the cyclic loading also accelerates the crack propagation and decreases its threshold due to degradation of toughening mechanisms(44). A faster spread of the crack is observed in the presence of water, which can be attributed to a high concentration of water molecules around the crack. This environment increases the rate of crack growth because it facilitates the Zr-O-Zr union cleavage at the end of the crack (27).

Aging

Aging or zirconia low temperature degradation (LTD) is a progressive and spontaneous phenomenon that is exacerbated in the presence of water, steam or fluids. The consequences of the material aging process are many, including surface deterioration, micro cracks and decreased resistance in medium and long term periods. Aging occurs through a slow surface transformation to the monoclinic stable phase. This transformation begins in individual particles on the surface through a mechanism of stress corrosion. The initial transformation of specific particles can be related to a state of imbalance: greater particle size, lower yttria content, specific guidance from the surface, the presence of residual stress, or even the presence of a cubic phase. The transformation occurs through nucleation and growth processes. This phenomenon leads to a cascade of events occurring in neighbouring particles, leading to an increase in volume that stresses the particles and results in subcritical crack growth (SCG), offering a way for water to penetrate inside the material. The stage of growth again depends on various microstructure patterns, such as: porosity, residual stresses, and particle size, among others(45). The attempt to minimize the degradation at low temperature (LTD) of 3Y-TZP includes reducing the particle size, increasing the content of a stabilizing oxide, or even the formation of composites with aluminum oxide (Al2O3). The addition of alumina particles prevents the relaxation of the network of tetragonal zirconia under stress during the aging process, since relaxation is responsible for degradation (39). In the literature, there is a major concern in evaluating the influence of superficial(46) and termic treatments(43) on the mechanical properties of partially stabilized zirconia. Fine polishing of the surface can reduce surface defects created in the finishing, improving the mechanical properties of the surface. However, prior to polishing, surface modifications such as adjustments and finishing, can introduce a compressive surface tension, initially increasing the flexural strength, but then changing the phase integrity of the material and increasing the susceptibility to aging (47).

Zirconia in dentistry:

In the biomedical field zirconia is used as hip prosthesis replacing femur heads.The introduction of zirconia (zirconium dioxide, ZrO2) as a dental material has generated considerable interest in the dental community(4). Zirconia is widely used to build single crowns, anterior and posterior fixed partial dentures, splinted crowns, zirconia implants and crowns on Implants, cantilever bridges, inlay and onlay bridges, and resin bonded bridges.

The most common type of zirconia used in dentistry is yttrium partially-stabilized tetragonal zirconia polycrystal (3Y-TZP). This type of zirconia is made of transformable, t-shaped grains stabilized by the addition of 3 mol% yttrium-oxides (Y2O3). It is placed in category 4-polycrystalline solids (alumina and zirconia) and has no glassy components. All the atoms are packed into a regular pattern making it dense and stronger(48). At the moment it is the most popular and frequently used form of zirconia commercially available for dental applications.

Manufacturing zirconia for dental application:

Computer aided design /Computer aided manafacture CAD/CAM zirconia dental frameworks can be produced according to two different techniques: “soft machining” of presintered blanks or “hard machining” of fully sintered blanks (4).

The soft machining process is the most diffused manufacturing system for 3Y-TZP, based on milling of pre-sintered blanks that are fully sintered at a final stage. Such zirconia blanks, at the so-called “green state”, are produced by compacting zirconia powders (in presence of a binder that will be eliminated in the following pre-sinterization step) through a cold, isostatic pressing process; this results in a very narrow pore size (20–30 nm) and quite homogeneous distribution of the components inside the blank(4). Processing at a proper pre-sintering temperature of zirconia is a crucial factor since this parameter affects hardness, machinability and roughness of the blanks. From the manufacturers’ point of view about the choice of the most convenient production technique, hardness and machinability act as opposite factors: an adequate hardness is necessary to manipulate the 3Y-TZP blanks safely, but, if excessive, it is detrimental to a proper machinability. Moreover, higher pre-sintering temperatures create rougher blank surfaces (4).

After scanning a stone die of the supporting abutment(s) (or directly the wax pattern of the crown/FPD), a virtual, enlarged framework is designed by sophisticated CAD softwares(23). Then, through a CAM milling procedure, a framework with enlarged accurately controlled dimension is machined out of the blank. At the end, the sinterization is completed at high temperature the zirconia framework acquires its final mechanical properties in that it undergoes a linear volume shrinkage of about 25%, so regaining its proper dimensions. Such processing is known to produce very stable cores containing a significant amount of tetragonal zirconia with surfaces virtually free from monoclinic phase(4). Nevertheless, a certain amount of cubic zirconia may be present due to an uneven distribution of yttrium oxide. The cubic phase is richer in stabilizing oxides than the surrounding tetragonal crystals and this may negatively influence the stability of the material (23).

Frameworks can be colored either adding minimum amounts of metal oxides to the zirconia powder or, after machining, by soaking the core in solutions of metal salts (like cerium, bismuth or iron); the framework coloration seems neither to induce PTT nor to decrease the mechanical performance of the restorations(4). Soft-machining is the preferred process by the majority of the manufacturers, like Procera Zirconia (Nobel Biocare AB, Goteborg, Sweden), Lava (3MESPE, Seefeld, Germany),Cercon (Dentsply Degudent, Hanau, Germany), and Crerec (Sirona dental system, Germany).

In the hard machining technique, on the other hand, the 3Y-TZP blocks are previously densely sintered through a process called “hot isostatic pressing”: at high temperatures (1400–1500 ◦C) and high pressure in inert gas environment, very hard, dense and homogeneous blocks of fully sintered zirconia are produced(49), out of which the frameworks are shaped to the proper, desired form and to the right, final dimension by using powerful and resistant milling machines with diamond abrasives. Hard-machining of Hipressed (“HiPed”) Zirconia is utilized by Denzir (Decim AB, Skelleftea, Sweden) and DC-Zirkon (DCS Dental AG, Allschwill, Germany).

The issue of which technique is suitable to get the better outcomes still remains a controversial topic. The major drawback of soft-machining is the problem of matching the sintering shrinkage of the framework to the enlargement amount programmed by the software as precisely as possible(50). In any case, some in vitro investigations have confirmed high fracture toughness and flexural strength with different production techniques, using both hot and cold isostatic pressed zirconia blanks(5).

It is clear that, compared to the soft-machining, the hardmilling procedure is more time consuming and requires cutting devices that have to be very tough and resistant to wear; the fully sintered 3Y-TZP blocks are much harder and less machinable of both fully sintered zirconia and densely sintered alumina blocks, making milling time much longer and the production procedure more expensive (21).

From an operative point of view, moreover, milling zirconia blanks at thin sections is very difficult and can lead to unpredictable results (51). Finally, it has been demonstrated that grinding such blocks introduces various kinds of surface microcrack and defects (52), both of the “brittle” and the “ductile” type, depending on various factors, such as the grain size of the diamond burs or the rotation speed: fine burs determine a more “ductile” damage compared to the “brittle” fractures due to the coarse ones, while high speed grinding procedure allows to reduce the applied force to the block and minimize the dimension and depth of the surface defects (52). In any case, there is high level of evidence that all surface treatments creating stress, like grinding, sandblasting or indentations on the zirconia surface determine some degree of (t) > (m) transformation before clinical use(21) being detrimental to the long-term serviceability of zirconia restorations(53). Surface grinding can determine deep defects that reduce toughness(22), decrease the strength(5), and the consequent exposure of the processing flaws to wetness may have further detrimental effects (54); the resultant alteration of phase integrity is reported to increase the susceptibility of the material to aging (47).

Frameworks produced by hard machining exhibit a considerable amount of monoclinic zirconia, associated with higher susceptibility to LTD and surface microcracking, resulting in a less stable material (23). In any case, since there is no standardization of the treatments utilized, it is very difficult to compare the results of the studies focused on the surface treatments of zirconia(4).

Soft machining procedures provide predictable stability of the framework, as long as its surface is not damaged after sintering (e.g. by an occlusal adjustment). To date, the surface state after processing is still a controversial issue, particularly after hardmachining, although there is wide agreement on the fact that microcracking due to processing flaws or occlusal adjustments is among the main causes of fatigue damage and failure(4). Residual stress, like that arising when zirconia is fired at high temperature and then rapidly cooled down or when ceramic material with different coefficient of thermal expansion (CTE) is used for veneering, was found to be a more critical factor than final surface roughness in inducing LTD(47).

As regards the framework thickness, almost all manufacturers agree in considering 0.5mm the minimum thickness for copings, in order to prevent core deformation (4). It is a well accepted concept that framework thickness and shape should be optimized and individualized to achieve an even thickness of veneering ceramic (33) as well as a suitable support for it. Another important aspect determining the mechanical properties of zirconia-based FPDs is connector shape and size. In some clinical trials, fractures of zirconia FPDs have been shown to be associated with insufficient connector height(20). Flexural strength has to be high enough to withstand occlusal loads, since connectors are under applied tensile stress, so the dimensions of connectors are paramount factors for the long-term success of zirconia FPDs; however, they are limited in height by the presence of the periodontal soft tissues(55). Notwithstanding the lack of strong scientific evidence about the ideal connector size, some in vitro analyzes recommended minimum diameters of about 3.0–6.0mmfor 3-unit, 4.0–6.0mmfor 4-unit and 5.0–6.0mmfor 5-unit zirconia FPDs(56) and these are the recommended figures by most manufacturers.

Veneering of zirconia:

All zirconia substructures finally undergo particular CAM processing. After milling, these frameworks have to be veneered with feldspathic or glass ceramics by means of the layering, the press, or the digital veneering technique. Although the application of full-contour zirconia restorations is currently discussed as an alternative to commonly veneered restorations(5), esthetically superior results can only be achieved by applying veneering materials with mechanical properties inferior to those of the frameworks. Current processing technologies cannot make zirconia frameworks as translucent as natural teeth, nor can they provide internal shade characterization or facilitate customized shading. Following the development of dental material science, the translucency of zirconia has been improved. By internal and external stain techniques, full-contour zirconia restorations can now be used therefore, zirconia cores are either veneered or not veneered.

No veneer zirconia

They are zirconia cores that can provide internal shade characterization or facilitate customized shading to full contour restoration without veneering. However, the clinical indication of full zirconia restorations is limited to posterior regions with little esthetic demand, and excess wear of the opposing teeth had become a concern because of the high strength and hardness of zirconia(57).

Veneered zirconia

On the other hand veneered zirconia can be done by one of the following techniques:

Layering technique:

Since Y-TZP substructure lacks color properties and offers less light transmission, it is necessary to veneer its surface to ensure the esthetic value of restorations. (58)

Several veneering techniques, such as traditional layering technique veneered by condensing and sintering veneering porcelain were suggested. Layering technique is the most commonly used technique for veneering zirconia frameworks. To achieve natural appearance of all-ceramic restorations, it is necessary to incorporate layers of porcelain of different opacity and shade. Various veneering ceramics are specially developed for zirconia core material fluoroapatite, aluminum oxide or leucite. The veneering concept of zirconia frameworks with feldspathic glass ceramics is empirical and recommended procedures may vary from one manufacturer to another. Depending on the system, a thin “liner” material can be fired on the framework as an intermediate layer with the veneering ceramic, similarly to opaque used for PFM restorations. The liner is a partially opaque feldspathic ceramic, it provides some chroma and fluorescence and may also assure wetting of the framework. The properties of veneering ceramics developed for zirconia-based restorations were copied and pasted from veneering ceramics designed for the PFM concept, with the CTE of veneering ceramics adapted to be slightly lower than the zirconia(59).

It consists of many layers. First layer used to color or shade the core for the white zirconia “core-shaded porcelains.” testing also found that the bond of the normal body porcelains fired and normal temperatures created a weak bond of the veneering porcelain to the zirconia framework. The materials developed to solve both of these problems were essentially high-chroma opaque materials used to shade the core and create a “bonding” layer to which the porcelain fused on subsequent porcelain firings. The materials are fired at a high enough temperature to melt the material to effectively wet the surface of the zirconia, creating both a micromechanical and chemical bond. After firing, the core basically looks like opaqued metal. Subsequent layers consist of Dentine Base and enamel similar to that of metalo-ceramic restoration(60).

Press-on technique

This technique is basically similar with the conventional heat-pressed technique, except for the zirconia framework. The protocol includes waxing up, forming the mold by lost wax process, melting ceramic ingots at high temperature, and injecting the melted ceramics into the mold by pressure, followed by divesting, staining, and glazing. The press-on technique is primarily used to improve veneer ceramic fracture resistance by reducing bubble formation and multiple firing. However, according to in vitro (61) and in vivo experiments, the press-on technique does not result in greater fracture reduction compared with the hand-layering technique.

Christensen 2010 (62) found that the effect of the press on technique varies with different systems. Improvement in fracture resistance was not ideal in Christensen's IPS e.max ZirPress group but was significant in the Noritake CZR Press group (leucite-containing system), suggesting that the ingredients of pressed ingots possibly play a role in fracture behaviour.

Choi 2011 (63) also proved that leucite-containing systems, such as CZR Press and Vita PM9, exhibit better physical properties than other systems for the press-on technique.

CAD/CAM veneering technique

The manufactured CAD/CAM veneer will be joined to the zirconia framework by fusion glass ceramic or by using resin cement (64). Lithium disilicate has been proposed to be connected to zirconia framework by glass fusion ceramic (65).

Veneer ceramics can be fabricated by CAD/CAM technique, through which porosity and time of firing are reduced and materials with improved strength can be used.

Beuer 2009 (66) also favoured this technique over the hand-layering method. In Watzke's 2011 (67) clinical report on the CAD-on technique,20 single crowns and 5 three-unit bridges were included. There was no veneer fracture observed after one year of use. According to Schmitter 2012, (65) higher fracture resistance is achieved with the CAD-on technique than with the hand-layering technique. In another study comparing restorations made with the RLT and CAD-on techniques, CADon products revealed higher strength than RLT products with the same thickness

Because of the high strength of ceramics reinforced by lithium disilicate, manufacturers suggest a minimum veneer ceramic thickness of 0.7 mm for CADoand 1 mm for RLT.

Vita Rapid Layer Technology (RLT) and IPS e.max CAD-on technique are usually employed for bridge fabrication. In RLT, veneer ceramics is fabricated with feldspathic

TriLuxe forte (flexural strength of about 150 MPa). The veneer ceramics is sintered then attached to the sintered zirconia framework by resin cement.

Meanwhile, e.max CAD (flexural strength of about 360 MPa) made primarily with lithium disilicate is used in the CAD-on technique. After milling, the veneer porcelain is not sintered immediately. A special fusing glass ceramics is added between the two layers; the two layers are joined together after fusion firing.

The two techniques described above have been developed in recent years, so relevant scientific information is scarce.

Bonding to zirconia:

The traditional methods of mechanical and adhesive bonding used on silica-based ceramics are not applicable for use with high-strength ceramics, i.e., zirconia. The difficulty of bonding to zirconia has resulted in alternative methods of adhesion being developed. Strong bonding relies on micromechanical interlocking and chemical bonding to the ceramic surface, which requires surface roughening for mechanical bonding and surface activation for chemical bonding. Non-destructive methods for treating inert ceramics to produce an activated/functionalized surface are desirable. Concerning the conditioning systems, several ceramic surface treatments have been suggested to overcome this issue. Treatments can be divided into: chemical surface treatments, mechanical surface treatments, and alternative treatments (68).

Chemical surface treatment

Hydrofluoric Acid Etching:

The most common surface treatment used for bonding to ceramic restorations is based on micromechanical bond obtained with HF etching enabling a micromechanical interlocking. However, HF etching does not produce any change in arithmetic roughness (Ra) of ZrO2 (69). The negligible effect of the HF on the ZrO2 surface occurs due to the absence of glassy matrix, resulting in low bond strength values (70).

Mechanical Surface Treatments

Air-Abrasion with Aluminum Oxide Particles:

Surface grinding is a commonly used alternative for roughening the surface of ZrO2 to improve mechanical bonding. There are several methods used for surface grinding: grinding using abrasive paper or wheels (SiC or Al2O3), particle air-abrasion using Al2O3 or other abrasive particles ranging in size from 50 to 250 μm (71), and grinding using a diamond bur. The advantage of these surface grinding methods is that they are generally easy to apply in a dental environment (72).

Air-abrasion with aluminum oxide particles (Al2O3) has been studied since the nineties and its effectiveness is closely related to the sandblasted ceramic surface and the air abrasion method.

Air abrasion with Al2O3, is affected by: particle size, pressure, distance from ceramic surface, working time, impact angle and the type of the ZrO2 ceramics may be mandatory. On a yttrium stabilized tetragonal zirconia (Y-TZP) material, the use of greater particle size (from 50μm to 150μm) results in a rougher surface but no significant alteration in bond strength. (73)

Through a scanning electron microscopy (SEM) evaluation in Borges et al 2003 (74) showed that the air-abrasion with 50 μm Al2O3 during 5 s at 4-bar pressure is able to create irregularities on the surface of glass ceramics; however, the same procedure did not change the surface of In-Ceram Alumina, In-Ceram Zirconia, and Procera.

During an evaluation with an optical profilometer, Della Bona et al 2007 (69) showed an increase in the arithmetic roughness (Ra) of In-Ceram Zirconia (from 207nm to 1000 nm) after the use of 25 μm Al2O3 air-abrasion at a distance of 10mm for 15 s, at a pressure of 2.8 bars. De Oyagüe et al. 2009 (75) employed 125μm Al2O3 air-abrasion for 10 s at approximately 5-bar pressure, which resulted in 45.77 nm for Ra, against 9.39nm of the control group (no treatment), on a yttrium-stabilized tetragonal zirconia (Y-TZP) material .

Si Deposition Methods:

Si deposition methods started in 1984 with the silicoater technology, and in 1989 the Rocatec system, a laboratory device, was developed and later the CoJet system, a chairside device was introduced into the market. These systems are based on the use of 110 μm (Rocatec) or 30 μm (CoJet) Si-coated alumina particles that are blasted onto the ceramic surface. Sandblasted ceramics acquires a reactive Si-rich outer surface prone to silanization and the following adhesive cement. Its use requires silane application before cementation. The tribochemical Si-coating on ceramic surfaces increases the bond strength of resin cement to glass-infiltrated ZrO2 or Y-TZP. Usually, 2.5–2.8-bar air-abrasion pressures are used; however, higher pressure results in higher bond strength with CoJet(76).

Alternative zirconia surface Treatments:

Different alternative methods to treat ZrO2 surfaces have been proposed and evaluated in order to produce a reliable adhesion, especially in long term. A large range of mechanical, chemical, or both approaches have been tried to modify the ZrO2 surface to increase the surface bond area, surface energy, or wettability.

Plasma spraying:

A method that was proposed by Derand et al. 2005 (77) where plasma spraying (hexamethyldisiloxane) using a reactor (Plasma Electronic, Germany), was proposed, increasing the bond strength of resin cement to ZrO2. The authors related that plasma is a partially ionized gas containing ions, electrons, atoms, and neutral species. However, the mechanism of surface modification and rise of the bond strength remain unclear, and the authors suggested that the improvement in bond strength might be explained by covalent bonds.

Laser application:

Some studies have suggested the use of erbium-dopedyttrium aluminum garnet (Er:YAG) or CO2 laser to enhance the bond strength to resin cement; therefore, the effect of laser on the ZrO2 could be tested with the same aim. Laser application removed particles by microexplosions and by vaporization, a process called ablation. However, bond strength results indicated that the effect of laser irradiation is contradictory(78).

The applications of micropearls of low fusing porcelain:

The application of fused glass micro-pearls to the surface of ZrO2 has been shown to increase the bond strength of resin cements to ZrO2. In these studies, a slurry of micro-pearls was painted on a ZrO2 surface and fired in a furnace. The fused glass film increased surface roughness of ZrO2, allowing increased micro-retention. The silica-rich film also allows for silanization of ZrO2 before bonding, making it possible to form siloxane bonds to resin cement. Derand et al. 2005 (77) showed that use of this fused micro-pearl film significantly increased the bond strength of ZrO2 (11.3–18.4 MPa) compared to untreated or silanized ZrO2 (0.5–1.5 MPa).

Vapour deposition of silicon tetrachloride:

Vapour deposition of silicon tetrachloride (SiCl4) is another

types of silicatization methods that have been used, showing improved bond strength (77). Recently, Piascik et al. 2009(79) reported improvement in adhesion to zirconia ceramics via vapor-phase deposition technique, whereby silicon tetrachloride is combined with water vapor to form an ultra-thin silicate layer on the zirconia surface. With the confirmation of the effectiveness of the chloro-silane pretreatment, the long-term usage of the vapor-phase deposition technique needs to be investigated.

Selective infiltration etching (SIE):

The most innovative surface treatment for ZrO2 was introduced by Aboushelib et al. 2006 and tested with respect to microtensile bond strength in 2007 (80). This method was named selective infiltration etching (SIE) and uses principles of heat-induced maturation and grain boundary diffusion to transform the relatively smooth non retentive surface of Y-TZP into a highly retentive surface. A low temperature molten glass is applied on selected ZrO2 surfaces and submitted to a heat-induced infiltration process, determining zirconia crystal rearrangements. After that, the glass is removed with a 5% hydrofluoric acid solution bath, leaving intergrain nanoporosities where low-viscosity resin materials may flow and interlock after polymerization. Aboushelib et al. 2007 (81) showed that using SIE on ZrO2 resulted in increased microtensile bond strength (49.8 ± 2.7 MPa) when compared to particle air-abraded ZrO2 (33.4 ± 2.1 MPa). The use of SIE improved nano-mechanical retention of zirconia by increasing the surface area available for bonding. This was confirmed by AFM work done by Casucci et al. 2009 (82) showing that the surface roughness of ZrO2 is significantly greater after SIE, when compared to particle air-abrasion or HF etching.

Hot etching solution:

Considering the metallic nature of pure zirconium, it can be assumed that treatments originally performed for conditioning metals or alloys may be somewhat beneficial for etching zirconium dioxide crowns or bridge frameworks. Recently an experimental hot chemical etching solution has been advocated for treating zirconia, a protocol previously proposed by Ferrari for conditioning the wings of Maryland bridges The optimal temperature for conditioning the substrate is 100ș C, for a period of 10 minutes. The action of the hot etching solution is basically a corrosion-controlled process. It selectively etches the zirconia, enabling for micro-retentions by modifying the grain boundaries through preferential removal of the less arranged, high energy peripheral atoms. (82)

A recent study by Casucci et al. 2009 (82) concluded that the effects of the chemo-mechanical zirconia surface treatments are material dependent. The application of the hot experimental etching solution increased the zirconia surface roughness of all the tested materials and created micro spaces that would optimize the overall bonding mechanism. However, further studies are necessary to confirm the effects of this novel solution on the bond strengths of resin luting agents to pretreated zirconium ceramics.

Bonding of veneering ceramic to zirconia:

A high percentage of clinical failures of ZrO2-based dental prosthetics reported in the literature are attributed to debonding and/or fracture of veneering ceramic. Failure rates due to veneer debonding and/or fracture as high as 15% for ZrO2 restorations 2–5 years old have been reported (83).

Besides framework design, the durability of the interface between the ZrO2 framework and veneer depends on many factors related to the two different material phases, including chemical bonding, mechanical interlocking, and extent of interfacial stress generated via thermal expansion mismatch and glass transition temperature differences (84).

Since ceramics are extremely susceptible to tensile stresses, achieving a slight compressive stress in the veneering ceramic is preferred, as in metal ceramic (PFM) restorations. For this to occur, the veneering material must have a thermal expansion coefficient lower than the core material. Zirconia ceramics have coefficients of thermal expansion (CTE) ranging from approximately 9–11 μm/m°K, depending on stabilizing oxide and other variables, while specialty porcelains can have CTE values ranging from 7–13 μm/m°K, depending on compositional variations. Achieving an appropriate CTE match/mismatch is possible, but other factors related to intimate and uniform contact, and adhesion between the two ceramic phases is also important for success (85).

There is evidence that chemical bonding between ZrO2 substructures and porcelain veneering materials is important in achieving a durable interface, even to the extent that surface roughening of the ZrO2 prior to veneer application might not be necessary (43).

The bond strength of porcelain veneers to ZrO2 has been examined using shear and microtensile bond strength test. It was determine that bond strength of veneer to ZrO2 is comparable to that of veneer to metal. Differences in materials and testing condition could lead to differences in veneer bond strengths when comparing metal to ZrO2, however bond strength of veneers to ZrO2 is thought to be sufficient for dental applications(85).

Clinical performance of zirconia as fixed partial denture core material:

The choice of zirconia as a core material for single crown, both in the anterior and in the posterior sites, has been increasing over time with clinical results that seem quite comparable to metal–ceramic single restorations. Zirconia single crowns showed a success rate of 93% after a 2 years observation period, with a favorable soft tissue response, in a limited sample size of 15 Cercon crowns (Dentsply Degudent, Hanau, Germany)(86). Ortorp 2009 (79) had a longer observational period (3 years), performed on 204 Procera zirconia single crowns delivered in a private practice, showed a survival rate of 93%; in this study, 16% of complication were recorded (6% loss of retention, 2.5% extraction of abutment teeth, 5% persistent pain, 2% porcelain chipping) (87).

Comprehensive systematic reviews of the literature on the survival rates of all-ceramic single crowns and fixed partial dentures in comparison with metal–ceramic restorations have been published, reporting, after 5 years of observation, favorable survival rates (95.6%) for metal–ceramic prostheses, to be compared to 93.3% for all-ceramic restorations, among which zirconia based prostheses showed the best clinical performances and resulted as the most reliable all-ceramic systems. Zirconia was affected only by cracking or chipping of veneering ceramic (88).

Conversely, crazing or chipping (superficial cohesive fractures) of the veneering porcelain were reported by the majority of the studies as the most frequent complications affecting zirconia-based prostheses, mainly at level of posterior teeth and independently from the type of restoration. According to Sailer et al. 2007 (89), after 5 years of clinical service, 3–5-unit posterior zirconia FPDs supported by natural teeth exhibited secondary caries as the most common cause of (biologic) failure, affecting 21.7% of the restorations, whereas porcelain chipping occurred in 15.2% of the prostheses.

Only in very few cases of major chipping or when serious esthetic problems arose, the restorations needed a total replacement (87). Although minor cohesive fracture of veneering ceramic being the most frequent typology of failure reported in the majority of the cited clinical studies, exposure of the underlying zirconia core was rarely observed and, in any case, it is very hard to detect by the naked eye.. The causes of porcelain chipping may be material-related to some extent; on the other hand, factors may be also dependent on the prosthetic design, such as core-porcelain thickness ratio and framework architecture. An incorrect shaping of the framework does not provide adequate, uniform support to the veneering ceramic and this could play a critical role in porcelain chipping. Moreover, surface damages can represent a starting point for the onset of fractures(24).

Aging of zirconia

The low temperature degradation (LTD), or “aging”, of zirconia is a well known process, strictly related to the phase transformation toughening (PTT), of which represents the other side of a same coin: it consists in a spontaneous, slow transformation of the crystals from the tetragonal phase to the stabler monoclinic phase in absence of any mechanical stress. This phenomenon decreases the physical properties of the material and exposes zirconia frameworks at the risk of spontaneous catastrophic failure (42). Mechanical stresses and wetness accelerate zirconia LTD; other factors affecting such a process are: grain size, temperature, vapor, surface defects of the material, type, percentage and distribution of stabilizing oxides and processing techniques (54).

The main features of ageing are summarised as follows:

Transformation proceeds most rapidly at 200– 300 șC and is time dependent.

The degradation is caused by the t–m transformation, accompanied by micro-cracking.

Water or water vapour enhances the transformation.

Transformation proceeds from the surface to the bulk of zirconia materials.

Higher stabilising content or finer grain size increase the resistance to transformation (90).

Although LTD is to be considered as a risk factor for mechanical prosthetic failures, to date such a direct relationship has not been demonstrated by scientific evidence in the clinical service (45). Even though the long-term effects of LTD on zirconia in dental restorations have not been completely investigated yet, aging is regarded as likely to induce detrimental changes in the mechanical behaviour of the material, like micro cracking, strength decrease, enhanced wear rates with release of zirconia grains in the surrounding environment(91), as well as surface roughening, with further degradation of mechanical and esthetic properties (92).

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 thermomechanical fatigue in liquid environment). Such a treatment reduced the failure loads of all test samples, 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 (83). In the same paper, a cautionary warning was addressed 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; this would expose the framework zirconia 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 (83). Such an issue is still debated and controversial; in any case, further investigations will be necessary to elucidate the relationship between aging of zirconia and long-term survival of the products (45).

Dissolution of ceramic can happen through 2 ways: by ionic exchange during exposure to an acidic solution, or by breakdown of Si-O network in a basic solution(93). The intensity of chemical deterioration is related to the glass matrix composition and the ratio of crystal incorporation. This will result in slow crack growth and may lead to failure of ceramic restorations in the oral cavity complex situation. So some concerns are assessed regarding zirconium dioxide structural stability when it's exposed to oral environment(85).

Ceramic corrosion in aqueous environments occurs mainly through ion-exchange processes(94). The phase transformation is controlled by the chemical reaction between water and Zr-O-Zr bonds on the surface. On the basis of the corrosion mechanism, the reaction between water and Zr-O-Zr suggests that Zr-OH bonds are formed in the lattice of zirconia and it starts the phase transformation(95).

To date, there is no accepted mechanism to explain the phenomenon, but only informed speculations:

Lange et al. 1986 (96) propose, based on some transmission electron microscope observations, that water reacts with Y2O3 to form clusters rich in Y(OH)3; this leads to a depletion of the stabilizer in the surrounding zirconia grains which are then free to transform to monoclinic.

According to Yoshimura et al. 1987 (97), water vapor attacks the Zr–O bond, breaking it and leading to a stress accumulation due to movement of –OH; this in turn generates lattice defects acting as nucleating agents for the subsequent t–m transformation.

Chevalier et al. 2009 (98) propose that O2− originating from the dissociation of water, and not OH−, is responsible for the filling of oxygen vacancies which is believed to be one of the causes of destabilization and, hence, of LTD and for the long diffusional path.

Low-temperature degradation (LTD) has been associated with several 3Y-TZP-based biomaterials, but is difficult to simulate in the laboratory. Aging occurs experimentally in zirconia, mostly in humid atmosphere or in water. An accelerated ageing test using steam and pressure has been developed to simulate LTD. No ceramics studied, including Y-TZP, have been found to be chemically inert in water. More recently, when 3Y-TZP disks were implanted in a denture and worn for 24 h/day for 1 year, the increase in the percentage of monoclinic phase was similar to that of zirconia aged in an autoclave at 134°C for 6 h (10).

Testing veneer zirconia bond strength:

The bond strengths of restorative materials to dental hard tissues is usually reported as the load at failure divided by the cross-sectional area of the bonded interface (F/A). Strength values calculated in this way are referred to as the "nominal strength" values, but this is valid only if the applied load is equally distributed throughout the entire bonded interface. Therefore, a crucial factor in evaluation of the usefulness of a specific bond strength test is a thorough awareness of the stress patterns involved in bond failure (99).

Finite element analysis (FEA) studies have demonstrated that the manner in which loads are generally applied in shear test or tensile bond strength tests results in non-uniform stress patterns. With shear loading, severe stress concentrations arise near the loading site, as well as tensile stresses caused by a bending moment). With the tensile bond strength test, where specimens are pulled away from a larger flat surface, there are pronounced stress concentrations at the periphery of the interface, due to the change in the geometry and material properties of the materials bonded together. These stress concentrations could explain the frequent cohesive failures within the substrate and the discrepancy between the actual nominal strength of the substrate and the apparent low stress measured. Stress inhomogeneities due to geometry differences can be reduced significantly by bonding two-rod specimens of uniform cross section together and by pulling them at the top and bottom surfaces. The specimens used in the microtensile bond strength test µTBS test have a uniform geometry at the bonding interface as well, but the tensile load in most investigations is not applied at top and bottom surfaces. Rectangular bar-shaped specimens are commonly attached by being stuck to by one of their flat lateral sides to the test set-up. Hourglass-shaped specimens, with either a cylindrical or a rectangular bonding area, are mounted by means of specially designed holders enclosing the specimens. Some of these studies showed an inverse relationship between the µTBS and specimen size, and the higher values found for specimens with a smaller cross-sectional area were explained by a lower occurrence of internal defects and surface flaws (99).

Different test methods have been suggested for testing the core-veneer bond strength, e.g. shear bond strength, three and four point loading test, biaxial flexure strength, and other commonly used methods as direct compression. Each test method has its advantages and disadvantages, but a common limitation of most of the test methods is the difficulty to determine the core-veneer bond strength from the applied force at failure on the sample in the specific test setup (100). Testing the core-veneer bond strength in real tension is not often done as it is challenged by the problem of the fixation of the test specimens of these brittle materials in the test setup. However, once these problems are solved it will give the opportunity to describe the bond strength and the mode of failure of core-veneered all-ceramic systems. A possible test method of core-veneered all-ceramic systems is the microtensile bond strength test. This test has proven to be a reliable test for evaluating the bond strength of composite materials to a variety of substrates (7).

Thus the microtensile bond strength of zirconia and its veneer combines all the aforementioned factors to determine its quality.

Aim of the Work

This in vitro study was conducted to demonstrate the effect of surface treatments of Zirconia on the bond strength with veneering ceramics within the following variables:

Different surface treatment:

No treatment.

Air abrasion

Selective infiltration technique

Acid etching

Different veneering techniques:

Layering technique

CAD\CAM Cap veneer sintered on Zirconia.

CAD\CAM Cap veneer adhered to Zirconia.

The study was held in two phases: a mechanical phase followed by aging phase.

Materials and Methods

MATERIALS

Materials used in this study:

InCoris ZI blocks

VITAVM9 veneering porcelain material

CEREC Blocs

NX3 light cure adhesive resin cement

Zirconia blocks:

InCoris ZI blocks were used (Figure 1).

Figure (1): InCorisZi blocks

The blocks are initially manufactured in a partially sintered state; they are individually processed to specification, and finally, densely sintered. Densely sintered single-unit products are then veneered. It has the following Chemical composition:

ZrO2+HfO2+Y2O3 ≥ 99.0%, Y2O3 5.2%, HfO2 2%,

Al2O3 ≤ 0.35% and Fe2O3 ≤ 0.3%.

The size of the blocks used was 40 x 19 x 15mm.

Veneering materials:

Two types of veneering materials were used:

VITAVM9 veneering porcelain material

CEREC Blocs

VITAVM9 veneering porcelain material:

Used for conventional layering technique. VITAVM9(figure2) has been designed as a high-fusing, fine-structure feldspar ceramic for ZrO2 substructures partially stabilized with yttrium in the CTE range of approx. 10.5. The CTE is precisely adjusted to zirconium dioxide materials. The material is composed chemically of (wt%): SiO2(60-64) ; Al2O3(13-15);K2O (7-11) ; Na2O (4-6); B2O3(3-5); CaO(1-2) ; BaO(1-3) ; SnO2<0.5 , TiO2<0.5; ZrO2(0-1); and Mg-,Fe-,P- Ox<1.

Figure (2): Vm9 base dentine and effect bonder.

CEREC Blocs:

Used for CAD on veneering technique. Fine-structured feldspathic ceramic blocks used with CEREC or inLab. They have selective composition, fine microstructure and are industrially sintered. They have good polishability and outstanding enamel-like abrasion properties. Chemically they are composed of : SiO2 (56 – 64); Al2O3 (20 – 23) ;Na2O 6 – 9; K2O ( 6 – 8); CaO( 0,3 – 0,6) and TiO2 (0,0- 0,1). Oxide % of total weight the chemical composition values specified above are batch-dependent. Oxides, contained in very low concentrations and used e.g. for colouring, are not specified here. The blocks used were 8 x 10 x 15 mm (figure 3).

Figure (3): Cerec blocks for CAD –on veneer.

NX3 light cured adhesive resin cement:

NX3 Nexus Third Generation (figure 4) is a color-stable adhesive resin cement with enhanced bond compatibility for both total-etch and self-etch adhesive techniques, Bonds to all substrates with Excellent adhesion to dentin, enamel, CAD/CAM blocks, ceramic, porcelain, resin and metal. With the following composition: BisGMA, UDMA, EBPADMA, TEGDMA, proprietary redox initiator, barium aluminosilicate filler, nano-ytterbium fluoride, nano-silica.

Figure (4): NX3 Nexus third generation

METHODS

Preparing Zirconia blocks:

InCoris ZI blocks 40 x 19 x 15mm were cut vertically into 4mm slices using a precision cutting machine Micracut150 (figure 5). The digital micrometer attached to the machine was adjusted to 4mm (figure 6). 36 slices were prepared. Slices were then checked with hand held digital micrometer (figure 7). All slices were ultrasonically cleaned for 15 minutes in distilled water, dried using stream air then sent to the laboratory for sintering process.

Figure (5): Micracut precision cutting device

Figure (6): Micrometer attached to the device adjusted to 4mm.

Figure (7): Checking slices after cutting.

Sintering of zirconia:

Sintering was carried out according to manufacturer recommendations. It was carried out in Sirona infireHTC furnace in dry conditions at 1540șC for 2hours.

After sintering the slices shrunk approximately 25% and 3mm slices were obtained (Figure 8).

Figure (8): Checking slices after sintering

Grouping of specimens:

Slices were then divided into 4 groups (table 1) according to the surface treatment to be received:

The first group: slices were not treated at all and to be used as control.(C) (n=9)

The second group: slices were air abraded .(A) (n=9)

The third group: slices were silica infiltrated on its surface (S) (n=9)

The fourth group: slices were acid etched.(E) (n=9)

After surface treatment each group was further subdivided into 3 subgroups according to veneering techniques:

First subgroup received: layering veneering technique (l) (n=3).

Second subgroup received: CAD –on veneering technique sintered to the surface (s) (n=3).

Third subgroup received: CAD –on veneering technique adhered to the surface (a) (n=3).

Table (1): Grouping of slices

Surface treatment of the specimens:

Air abrasion:

For standardization purposes, a specially designed crystal glass box was constructed to work as a blasting room with magnetic door, with dimensions (30cm x 30cm x40 cm) with 1cm glass wall thickness. The glass box has two laser-cut ovoid holes in its side walls and specially designed laser welded hexagonal attachment fixed to the middle of the top surface to closely fit the sand blasting nozzle (figure 9).

Figure (9): Specially designed glass box.

A specially designed holder was constructed to hold the zirconia discs at a fixed distance (1cm) from the sandblasting nozzle. This holder is composed of a wooden base with a central copper bar fixed to it. The central bar is composed of two sliding parts, one move inside the other, with a key lock to facilitate adjusting the distance between the specimen and the sandblasting nozzle. The holder ending with a flat horizontally positioned copper plate (figure 10).

Figure (10): Specially designed holder

A specially designed copper split mold was constructed to fix the disc and prevent its movement while receiving surface treatment. This split mold consisting of three parts namely; one piece copper base and two splitted copper parts that could be joined together and fixed to the copper base using four metal screws (figure 11).

Figure (11): Copper split mold

The holder was placed inside the glass blasting room and the disc was positioned over the copper plate after screwing it inside the copper split mold. The nozzle of the sandblaster was fitted in the hexagonal attachment in the top of the glass blasting room and the distance between the disc and the sandblasting nozzle opening was adjusted at 1cm by the aid of the sliding part of the specimen holder and guided by a specially designed metal rod of 1cm length (figure 12).

Figure (12): Sand blasting nozzle fixed to the glass box

The zirconia discs were airborne particle abraded with 110μm aluminum oxide particles (Al2O3) at pressure of 2.8MPa using sandblaster device and the specimens were faced to the sand beam with an angle of 90ș at a distance of 1cm for 15sec followed by ultrasonic cleaning in distilled water for 10min (figure 13).

Figure (): Sand blaster nozzle and its gauge

Silica infiltration:

In a chemical laboratory a specially formulated sodium silicate powder (60%) was prepared. The powder of the glass was mixed with water to give a thin, creamy mixture, a layer was applied on the surface of the specimens using a thin brush to totally coat it and left to air dry for 12 hours.

The specimen with the applied coat was then heated to 60oC in hot air oven for 2 hours. The coated surface is then treated with hydrochloric acid (36.5 molar mass) for 10 minutes.

Na SiO3 +2 HCl 2NaCl+ H2SiO3 +SiO2x H2O

Then it is heated to 600oC for one more hour.

SiO2x H2O heat 600șC SiO2+H2O

Where at the end the silica precipitate on the surface and the water evaporates by heat. This technique is adopted from selective infiltration etching technique(9).

Acid etching:

In a chemical laboratory a specially formulated mixture of HNO3 26wt% and HF 9wt% acids were prepared with ratio 4HF:1HNO3 this concentration was chosen based on a pilot study were different concentration were tested and the one that showed the better surface alteration on the scanning electron microscope were chosen. This technique was adopted from chemical surface treatment of stainless steel(101).

. Solution was placed in plastic containers and specimens were immersed in the solution for 10 minutes. Specimens were washed with distilled water then left drying in heat oven at 100șC for 1 hour to evaporate the water. All slices after surface treatments were ultrasonically cleaned for 10 minutes and were examined under scanning electron microscope SEM and atomic force microscope AFM.

Zirconia discs were evaluated under an atomic force microscopy (AFM, Multimode Nanoscope IIIa, Digital Instruments, Veeco Metrology group, Santa Barbara, CA, USA). Images were taken in air. The tapping mode was performed using a 1–10 Ωcm phosphorus (n) dopes Si tip (at 50µm). Changes in vertical position provide the height of the images that were registered as bright and dark regions. The tip-sample was maintained stable through constant oscillation amplitude (set-point amplitude). Fields of view at 5 µm _ 5 µm scan size were considered and recorded with a slow scan rate (0.1 Hz). A single operator analyzed the average surface roughness (Ra) of the ceramic substrate after different surface treatments, expressing it as a numeric value (in nanometers) using specific software (NanoscopeV530R35R). The images were taken to compare the surface roughness of different groups

Veneering of the specimens:

Layering VM9 technique:

A copper split mold was designed to exactly fit the zirconia specimen with inner walls of 6mm17mm x15mm in dimensions (figure14). Mold was coated with separating medium. Zirconia specimen were placed inside the mold and secured inside the mold. Only base dentine layering was used in this study (figure15).

Figure (14): Copper split mold

Figure (15): Zirconia slice in the split mold

The layering technique was done according to the following steps:

Binder cycle: VM9 effect bonder powder was mixed with its liquid according to manufacturer instructions. A thin layer was applied using porcelain brush and baked in VITA VACUMAT 30 furnace following the specially programmed baking cycle (table2).

Table (2): Recommended baking cycle for effect bonderVM9.

1: Predrying temperature

2: Predrying holding time

3: Heating up rise in temperature time

4: Rise in temperature per minute

5: End temperature

6: Holding time for end temperature

7: Vacuum time

First bake: VM9 dentine powder was mixed with its liquid according to manufacturer instructions, layer by layer was applied in the mold (figure16), mold was slightly over filled and baked in VITA VACUMAT 30 furnace following the specially programmed baking cycle (table 3).

Table (3): Recommended baking cycle for base dentin VM9 first bake

Figure (16): Application of Vm9 in the mold.

Second bake: after the first bake the specimen is returned to the mould (figure 17), checked and then refilled and fired for correction to compensate for shrinkage according to the following cycle (table 4):

Table (4): Recommended second bake cycle for base dentin VM9

Figure (17): Slices repositioned in mold, porcelain refilled and baked.

Specimens were then checked for 6mm thickness (figure 18).

Figure (18): Checking of specimen after last cycle

Glaze cycle:

The specimens were then glazed (figure 19) with VITA AKZENT glaze and fired according to the following cycle (table 5):

Table (5): Recommended glazing cycle

Figure (19): Final layered specimen

Preparing CAD-on blocks:

Cerec blocks 8 x 10 x 15 mm were used. They were cut vertically in a precision cutting machine Micracut150 (figure 20). The digital micrometer attached to the machine was adjusted to 3mm and slices were cut accordingly and checked by handheld digital micrometer (figure 21).

Figure (20): Micrometer of micracut adjusted to 3mm.

Figure (21): CAD on slices thickness rechecked

The slices were then glazed with VITA AKZENT glaze and fired according to the recommended glazing cycle.

Slices were then divided into two groups first group were used for sintered veneering and the second group were used for adhered veneering.

CAD-on sintered veneering:

A surface treated sintered zirconia slice was brushed with a thin layer of VM9 effect bonder powder mixed with its liquid according to manufacturer instructions, then a slice of glazed cerec was placed horizontally on it, slightly tapped to remove any air bubbles, placed under vertical loading device with 1kg load applied. Both slices were then baked in VITA VACUMAT 30 furnace following the specially programmed effect bonder baking cycle. After that thickness was checked for 6mm (Figure 22).

Figure (22): Checking of thickness

Figure (23): Final CAD-On sintered specimen

CAD-on adhered veneering:

The glazed porcelain slices were treated with ultradent porcelain etch for 60 seconds, and then thoroughly washed and air dried. Kerr silane primer was then applied for 60 seconds and left to air dry. A layer of Universal single bond was applied air dried for 2-3 seconds then light cured for 10 seconds.

Then a layer of nx3 light cured was applied on the zirconia slice surface, the treated glazed porcelain slice was placed on it and both were placed in the vertical loading device with 3kg load placed on it.

The assembly was light cured for 20 seconds from each aspect and then another 20 seconds after load removal.

The thickness was then checked for 6mm thickness (figure 24).

Figure (24): Thickness checked

Figure (25): Final CAD-on adhered specimen

Preparation for testing:

Veneered specimens were then prepared for micro tensile testing and cut 1mm X 1mm X 6mm. In micra cut 150 where the micrometer (figure 26) was adjusted to 1mm. Diagram in (figure 27) illustrated the cutting procedure. Rods were checked for quality and thickness (figure 28).

Figure (26): Micrometer of micracut adjusted to 1mm.

Figure (27): Diagram showing cutting procedure

Figure (28): Checking thickness of microbars and final microbar

Twelve bars from each group were prepared and then divided into two groups, the first one was tested directly and the other was aged first (n=6).

Aging of specimen:

Each group of specimens were aged in the autoclave for eight consecutive cycles each for 45 min at134°C (270°F) /186 kPa (27 psi),so that the total aging time was 6 hours (10).

Testing of specimens:

In order to test specimens a specially designed attachment was fabricated. The attachment consist of two parts, first upside-down u shaped part A enclosing an exactly fitting (frictionless) part B with a reduced free space between them 0.8 mm, attached together with a plastic sheet at the backside of both parts. The plastic sheet allows hinge movement of B when forces (F) are applied to B via a steel rod loosely fitted in an outlet of (A) (figure 29,30).

Figure (29): Attachment used for micro tensile testing front view,
rear view and top view.

Figure (30): Diagram showing attachment parts

For each group (n=6) the microbars were tested after 24 hours distilled water storage each microbar was attached to the test attachment using cyanoacrylate adhesive (ZAPIT) (figure31) taking care to place the interface at the free space between the attachment parts (figure 32).

Figure (31): Cyanoacrylate adhesive used

Figure (): Microbar adhered to the attachment

The attachment was then placed in the universal testing machine (LLOYD) with a load cell 5KN. Data was recorded using computer software. A tensile load with compression mode of force was applied via universal testing machine at a crosshead speed of 0.5 mm/min. The load required for fracture of each specimen was recorded in MPa (Newton divided by the area) (figure 33,34).

Figure (33): Microbar and attachment in the universal testing machine.

Figure (34): Diagram illustrating fracture of microbars during testing

Failure analysis:

Fractured microbars were ultra sonically cleaned and dried and then viewed under digital microscope with magnification 40X and type of failure was specified recorded as number and percentage in each subgroup.

Statistical Analysis

Data were presented as mean and standard deviation (SD) values. Micro-tensile bond strength data showed parametric distribution; so regression model using three-way Analysis of Variance (ANOVA) was used in testing significance for the effect of surface treatment, veneering technique, aging and their interactions on mean micro-tensile bond strength. Tukey’s post-hoc test was used for pair-wise comparison between the groups when ANOVA test is significant.

The significance level was set at P ≤ 0.05. Statistical analysis was performed with IBMSPSS Statistic Version 20 for Windows.

Results

Statistical Results

Three-way ANOVA results

The results showed that surface treatment, veneering technique, aging and the interaction between the three variables had a statistically significant effect on mean micro-tensile bond strength.

Table (6): Regression model results for the effect of different variables on mean micro-tensile bond strength in MPa.

df: degrees of freedom = (n-1), *: Significant at P ≤ 0.05

Effect of surface treatment:

Effect of surface treatments regardless of other variables

Silica infiltration showed the statistically significantly highest mean micro-tensile bond strength 75.5MPa. Followed by the control group 61.2MPa, and then lower mean micro-tensile bond strength was observed with air abrasion 58.2MPa. Acid etched showed the statistically significantly lowest mean micro-tensile bond strength 43.2MPa.

Table (7): Effect of surface treatment on micro-tensile bond strength in MPa regardless of other variables:

*: Significant at P ≤ 0.05, Different superscripts indicate statistically

significant difference

Figure (35): Bar chart showing mean values for the effect of surface treatment on micro-tensile bond strength in MPa regardless of other variables.

Effect of surface treatments interactions with other variables

Without aging

With layering technique, Silica infiltration showed the statistically significantly highest mean micro-tensile bond strength70.2MPa. There was no statistically significant difference between Air Abrasion and Acid etched; both showed the statistically significantly lowest mean micro-tensile bond strength values35.1MPa and 35.3MPa.\

With sintering technique Air Abrasion showed highest mean micro-tensile bond strength values65.1MPa. Silica infiltration showed statistically significantly lower mean micro-tensile bond strength55.5MPa. Acid etched showed the statistically significantly lowest mean micro-tensile bond strength 40.5MPa.

With Adhering technique, Silica infiltration showed the statistically significantly highest mean micro-tensile bond strength 88.8MPa, followed by Air Abrasion 49.6MPa. Acid etched showed the statistically significantly lowest mean micro-tensile bond strength 23.6MPa.

Table (8): Effect of surface treatments on micro-tensile bond strengths in MPa with different variables’ interactions without aging

Different superscripts in the same row indicate statistically significant differences

Figure (36): Bar chart representing mean values for effect of surface treatments on micro-tensile bond strengths in MPa without aging

With aging

With Layering technique, there was no statistically significant difference between, Silica infiltration and Acid etched; all showed the statistically significantly highest mean micro-tensile bond strength values 69.3MPa, 71.4Mpa respectively. Air Abrasion showed the statistically significantly lowest mean micro-tensile bond strength 44.7Mpa.

With Sintering technique, there was no statistically significant difference between Air abrasion and Silica infiltration ; both showed the statistically significantly highest mean micro-tensile bond strength values 84.5MPa, 90.4MPa respectively. Acid etched; showed t statistically significantly lowest mean micro-tensile bond strength values72.2MPa.

With Adhering technique, Silica infiltration showed the statistically significantly highest mean micro-tensile bond strength 78.7MPa. Air Abrasion showed statistically significantly lower mean micro-tensile bond strength 70MPa. Acid etched showed the statistically significantly lowest mean micro-tensile bond strength 16.4MPa.

Table (9): Effect of surface treatments on micro-tensile bond strengths in MPa with different variables’ interactions with aging

Different superscripts in the same row indicate statistically significant differences

Figure (37): Bar chart representing mean values for effect of different surface treatment on micro-tensile bond strengths in MPa with aging.

In general, Air abrasion showed statistically significant same mean micro-tensile bond strength as that of Control, Silica infiltration higher than control whereas acid etched showed lower than control.

With Layering technique, Air abrasion showed statistically significant lower mean micro-tensile bond strength than that of Control and same as Acid etch without aging and lower than control with aging, Silica infiltration shows statistically significant higher mean micro-tensile bond strength than that of Control without aging and same as Control with aging, whereas Acid etched showed statistically significant lower mean micro-tensile bond strength than that of Control and same as Air abrasion without aging and same as Control with aging.

With Sintering technique, Air abrasion showed statistically significant same mean micro-tensile bond strength as that of control without aging and higher than Control with aging, Silica infiltration shows statistically significant lower mean micro-tensile bond strength than that of Control but higher than Acid etched without aging and higher than Control with aging, whereas Acid etched showed statistically significant lower mean micro-tensile bond strength than that of Control and Silica Infiltration without aging and same as control with aging.

With Adhering technique, Air Abrasion showed statistically significant lower mean micro-tensile bond strength than that of Control and higher than Acid etched without aging and higher than Control lower than Silica infiltration with aging, Silica infiltration shows statistically significant higher mean micro-tensile bond strength than that of Control without aging and higher than Control higher than Air Abrasion with aging, whereas Acid etched showed statistically significant lower mean micro-tensile bond strength than that of Control lower than Air Abrasion without aging and lower than Control with aging.

Effect of Veneering Technique

Effect of veneering techniques regardless of other variables

Sintering showed the statistically significantly highest mean micro-tensile bond strength 67.7MPa. There was no statistically significant difference between Layering and Adhering both showed the statistically significantly lowest mean micro-tensile bond strength values 56.7MPa, 54.2Mpa respectively.

Table (10): Effect of veneering techniques on micro-tensile bond strengths in MPa regardless of other variables

*: Significant at P ≤ 0.05, Different superscripts indicate statistically significant differences

Figure (38): Bar chart representing mean values for effect of veneering techniques micro-tensile bond strengths in MPa.

Effect of veneering techniques interaction with other variables

Without aging

Without surface treatment (Control), there was no statistically significant difference between Layering and Sintering techniques; both showed the statistically significantly highest mean micro-tensile bond strength values 61.2MPa, 64.3MPa respectively. Adhering technique showed the statistically significantly lowest mean micro-tensile bond strength56.9MPa but with non-statistically significant difference from layering technique.

With air abrasion, Sintering showed the statistically significantly highest mean micro-tensile bond strength65.1MPa. Adhering showed statistically significantly lower mean micro-tensile bond strength49.6MPa. Layering showed the statistically significantly lowest mean micro-tensile bond strength35.1MPa.

With silica infiltration, Adhering showed the statistically significantly highest mean micro-tensile bond strength 88.8Mpa. Layering showed statistically significantly lower mean micro-tensile bond strength 70.2Mpa. Sintering showed the statistically significantly lowest mean micro-tensile bond strength 55.5Mpa.

With acid etched, there was no statistically significant difference between Layering and Sintering techniques; both showed the statistically significantly highest mean micro-tensile bond strength values 35.3Mpa, 40.5MPa respectively. Adhering technique showed the statistically significantly lowest mean micro-tensile bond strength 23.6MPa.

Table (11): Effect of veneering techniques on micro-tensile bond strengths in MPa with different variables’ interactions without aging

Different superscripts in the same row indicate statistically significant differences

Figure (39): Bar chart representing mean values for effect of veneering techniques on micro-tensile bond strengths in MPa without aging.

With aging

Without surface treatment (Control), there was no statistically significant difference between Layering and Sintering techniques; both showed the statistically significantly highest mean micro-tensile bond strength values 66.5MPa, 68.6MPa respectively. Adhering technique showed the statistically significantly lowest mean micro-tensile bond strength but with non-statistically significant difference from layering technique49.8MPa.

With Air Abrasion Sintering showed the statistically significantly highest mean micro-tensile bond strength 84.5 Mpa. Adhering showed statistically significantly lower mean micro-tensile bond strength 70Mpa. Layering showed the statistically significantly lowest mean micro-tensile bond strength 44.7Mpa.

With silica infiltration, Sintering showed the statistically significantly highest mean micro-tensile bond strength 90.4MPa. Adhering showed statistically significantly lower mean micro-tensile bond strength78.7. Layering showed the statistically significantly lowest mean micro-tensile bond strength 69.3MPa.

With acid etched, there was no statistically significant difference between Layering and Sintering techniques; both showed the statistically significantly highest mean micro-tensile bond strength values 71.4MPa, 72.2MPa respectively. Adhering technique showed the statistically significantly lowest mean micro-tensile bond strength 16.4MPa.

Table (12): Effect of veneering techniques on micro-tensile bond strengths in MPa with different variables’ interactions with aging.

Different superscripts in the same row indicate statistically significant differences

Figure (40): Bar chart representing mean values for effect of veneering techniques on micro-tensile bond strengths in MPa without aging.

In general, Sintering showed statistically significant higher mean micro-tensile bond strength than both Layering and Adhering veneering technique.

With Control (no surface treatment), Layering technique showed statistically significant same mean micro-tensile as Sintering higher than adhered without aging and same as Sintering with aging, Sintering technique showed statistically significant same mean micro-tensile as Layering higher than Adhering without aging and same as Layering with aging whereas Adhering technique showed statistically significant lower mean micro-tensile than Sintering and Layering without aging and lower than sintering not statistically significant lower than Layering with aging.

With Air Abrasion, Layering technique showed statistically significant lower mean micro-tensile than Sintering lower than adhering without aging and with aging, Sintering technique showed statistically significant higher mean micro-tensile than Layering and Adhering without aging and with aging whereas Adhering technique showed statistically significant lower mean micro-tensile than Sintering and higher than Layering without aging and with aging.

With Silica infiltration, Layering technique showed statistically significant higher mean micro-tensile than Sintering lower than Adhering without aging and lower than Sintering and Adhering with aging, Sintering technique showed statistically significant higher mean micro-tensile than Layering lower than adhered without aging and higher than Layering and Adhering with aging whereas Adhering technique showed statistically significant higher mean micro-tensile than Sintering and Layering without aging and lower than sintered higher than Layering with aging.

With Acid etched, Layering technique showed statistically significant same mean micro-tensile as Sintering higher than Adhering without aging with aging, Sintering technique showed statistically significant same mean micro-tensile as Layering higher than adhered without aging and with aging whereas Adhering technique showed statistically significant lower mean micro-tensile than Sintering and Layering without aging and with aging.

Effect of Aging:

Effect of aging on micro-tensile bond strength regardless of other variables

Aging showed statistically significantly higher mean micro-tensile bond 65.2MPa strength than without aging 53.8MPa.

Table (13): Effect of aging on micro-tensile bond strengths in MPa regardless of other variables.

*: Significant at P ≤ 0.05

Figure (41): Bar chart representing mean values for effect of aging on micro-tensile bond strengths in MPa.

Effect of aging on micro-tensile bond strength with other interactions:

With layering technique

Without surface treatment (Control), there was no statistically significant difference between mean micro-tensile bond strength without and with Aging 61.2MPa, 66.5MPa respectively.

With Air Abrasion, Aging showed statistically significantly higher mean micro-tensile bond strength 44.7MPa than without Aging 35.1MPa.

With silica infiltration, there was no statistically significant difference between mean micro-tensile bond strength without and with Aging 70.2Mpa, 69.3Mpa respectively.

With acid etched, Aging showed statistically significantly higher mean micro-tensile bond strength 71.4MPa than without Aging 35.3MPa.

Table (14): Effect of aging on micro-tensile bond strengths in MPa with different variables’ interactions after using layering technique

Different superscripts in the same row indicate statistically significant differences

Figure (42): Bar chart representing mean values for effect of aging on micro-tensile bond strengths in MPa after using layering technique.

With Sintering technique

Without surface treatment (Control), there was no statistically significant difference between mean micro-tensile bond strength without and with Aging 64.3MPa, 68.6MPa respectively.

With Air Abrasion, Aging showed statistically significantly higher mean micro-tensile bond strength 84.5MPa than without Aging 65.1MPa.

With silica infiltration, Aging showed statistically significantly higher mean micro-tensile bond strength 90.4MPa than without Aging 55.5MPa.

With acid etched, Aging showed statistically significantly higher mean micro-tensile bond strength 72.2MPa than without Aging 40.5MPa.

Table (15): Effect of aging on micro-tensile bond strengths in MPa with different variables’ interactions after using sintering technique

Different superscripts in the same row indicate statistically significant differences

Figure (43): Bar chart representing mean values for effect of aging on micro-tensile bond strengths in MPa after using Sintering technique

With Adhering technique

Without surface treatment (Control), there was no statistically significant difference between mean micro-tensile bond strength without and with Aging 56.9MPa, 49.8MPa respectively.

With Air Abrasion, Aging showed statistically significantly higher mean micro-tensile bond strength 70MPa than without Aging 49.6MPa.

With Silica infiltration, without Aging showed statistically significantly higher mean micro-tensile bond strength 88.8MPa than with Aging 78.7MPa.

With Acid etched, there was no statistically significant difference between mean micro-tensile bond strength without and with Aging 23.6MPa, 16.4MPa respectively.

Table (16): Effect of Aging on micro-tensile bond strengths in MPa with different variables’ interactions after using adhering technique

Different superscripts in the same row indicate statistically significant differences

Figure (44): Bar chart representing mean values for effect of Aging on micro-tensile bond strengths in MPa after using Adhering technique.

In general, Aging showed statistically significant higher mean micro-tensile bond strength than Without Aging.

With Control (no surface treatment), no statistically significant difference in mean micro-tensile bond strength between aging and without aging with all veneering techniques.

With Air abrasion, Aging showed statistically significant higher mean micro-tensile bond strength than without aging with all veneering technique.

With Silica infiltration, in both layering and Sintering technique Aging showed statistically significant higher mean micro-tensile bond strength than without Aging while with Adhering technique Aging showed statistically significant lower mean micro-tensile bond strength than without aging.

With acid etching, in both Layering and Adhering technique there was no statistically significant difference in mean micro-tensile bond strength between Aging and without Aging while with Sintering technique Aging showed statistically significant higher mean micro-tensile bond strength than without Aging.

Fracture Analysis

In general three types of failure appeared during testing: cohesive in the veneering and was referred to as (CV) layer, cohesive at the junction (CJ) and adhesive between veneer and core material (A). No cohesive failure in core ever appeared during testing.

Figure (45): Figure showing cohesive failure (CV) in veneer where A is the core and B is the veneer under digital microscope 40X.

Figure (46): Figure showing cohesive failure in veneer (CV) where A is the core and B is the veneer under SEM 100X.

Figure (47): Figure showing cohesive failure at the junction (CJ) where A is the core , B is the veneer under and C is the junction under digital microscope 40X.

Figure (48): Figure showing cohesive failure at the junction(CJ) where A is the core , B is the veneer under and C is the junction under SEM 100X.

Figure (49): Figure showing adhesive failure (A) where A is the core and C is the junction under digital microscope 40X.

Figure (50): Figure showing adhesive failure (A) where A is the core and C is the junction under SEM 100X.

The frequency of occurrence of type of failure number were calculated total number of specimens were 6 (n=6) and percentage were calculated and formulated in the following table.

Table (17): Frequency of type of failure and its percentage in all groups

In layering technique:

The CV is the only type of failure that appeared with all surface treatment and in not aged and aged groups (100%).

In Sintering technique:

Two types of failure appeared during testing CV and CJ.

Within the Control and Air Abrasion group: 33.3% were CV in both aged and not aged group where as 66.7% were CJ in both Aged and not aged group.

Within Silica infilteration and Acid etched group: in not Aged group 50% were CV and 50% were CJ, whereas in Aged group 33.3% were CV and 66.7% were CJ.

In adhering technique:

Two types of failure appeared during testing CV and A.

In Control, Air Abrasion and Silica infiltration: in both not aged and aged group 33.3%were CV and 66.7% were A.

In Acid etched: The A is the only type of failure that appeared with not aged and aged groups (100%).

Figure (51): Bar graph representing percent of type of fracture in each tested group.

Microscopic Results

Scanning electron microscope SEM of surface treated core material:

Control:

Figure (52): SEM image showing zirconia surface with no surface treatment
at A: 500X and B:4000X

Air abrasion:

Figure (53): SEM image showing zirconia surface with Air Abrasion
at A: 500X and B: 4000X.

Silica infiltrated:

Figure (54): SEM image showing zirconia surface with silica infiltration
at A: 500X and B:4000X

Acid etched:

Figure (55): SEM image showing zirconia surface with acid etched
at A: 500X and B:4000X

Atomic force microscope AFM of surface treated core material:

Control:

Figure (56): AFM image showing surface topography of the control group. Mean surface roughness Ra measured was 163.717 nm.

Air Abrasion:

Figure (57): AFM image showing surface topography of Air Abrasion group. Mean surface roughness Ra measured was 335.856 nm.

Silica infiltration:

Figure (58): AFM image showing surface topography of Silica infiltration group Mean surface roughness Ra measured was 334.281nm.

Acid etched:

Figure (59): AFM image showing surface topography of Acid etched group. Mean surface roughness Ra measured was 360.263nm.

From the previous images it can be deduced that all surface treatment used in this study has altered the surface roughness of the zirconia compared to the control group with no surface treatment mean surface roughness 163.7nm. Acid etched showed the highest mean surface roughness of 360.2 nm, followed by Air Abrasion 335.8 nm and Silica infiltration 334.3 nm.

Scanning electron microscope SEM of core veneer rods:

Layering technique:

Figure (60): SEM image showing layering technique where
A is the Zirconia core and B is the veneer 500X and 4000X.

Sintering CAD on technique:

Figure (61): SEM image showing Sintering CAD on technique where A is the Zirconia core and B is the veneer 500X and 4000X

Adhering CAD on technique:

Figure (62): SEM image showing Adhering CAD on technique where A is the Zirconia core and B is the veneer 500X and 4000X.

Discussion

Even though reports indicate that zirconia frameworks have excellent fracture resistance, chipping fracture, delamination of the ceramic veneer have been reported as the most common technical problem facing the clinical performance of zirconia frameworks. Studies have identified factors that may initiate fracture of the veneering ceramic, including inappropriate framework design, mismatched thermal properties of the veneering ceramic and zirconia core, and the use of incompatible veneering ceramics. However, inadequate bonding between the ceramic veneer and zirconia framework might be the most relevant factor that results in chipping of the veneer. The bond strength between zirconia and its veneering ceramic is highly affected by the type of zirconia, the type of veneering ceramic used, zirconia surface treatment, liner material application and incorporation of flaws during veneering(102).

Micromechanical interactions between the core and the veneering layer are essential to obtain a stable bond. However, there are no clear data with respect to the bond stability between zirconia and ceramic veneers under loads, or data relating to the behaviour of veneering ceramics when the zirconia bond is reduced or increased(103).

So the aim of this study was to investigate the quality of bond between zirconia and its veneering ceramic under different surface treatments and veneering techniques with and without aging.

When designing this study, it was put into the consideration that most framework fractures in all-ceramic FPDs were reported in the connector region. Therefore, connector dimensions are crucial for fracture resistance as it is advised in the literature that connector dimension to be 3×3 mm in order to increase the fracture strength of zirconia-based FPDs by 20%. Therefore the zirconia core thickness was selected to 3mm(85).

Surface treatments were carried out under controlled conditions: first a Control group was used with no treatment at all to act as a baseline where the effects of other treatments were compared to. Secondly: Air Abrasion was carried out in a specially designed chamber to ensure that the nozzle is on equal distance from the specimen at all times and at 90ș from the specimen, in literature on a yttrium stabilized tetragonal zirconia (Y-TZP) material, the use of greater particle size (from 50μm to 150μm) results in a rougher surface but no significant alteration in bond strength(104) so for this study we used the 110 μm to ensure enough surface roughness occurrence. Thirdly Silica infiltration method: this technique is modification from selective infiltration etch where the later uses glass-conditioning agent composed of silica (65% wt), alumina (15% wt), sodium oxide (10% wt), potassium oxide (5% wt), and titanium oxide (5% wt)(9), based on the promising results of this technique and the previously used silica coating techniques we used a glassy powder containing only silica compound to test the efficiency of silica separately without the other components. Fourthly Acid etched: this technique uses an acid solution that combines HF and HNO3. Basically this mixture is used for stainless steel pickling; the rational was that one acid potentiates the action of the other and zirconia is considered ceramic steel as both share the non reactive surface.

Specimens were then veneered using 3 different techniques in an attempt to standardize the results we used feldspathic porcelain namely VM9 and Cerec blocks so that the difference in results would be due to difference in technique only. It has been recommended that the veneer thickness does not exceed two-fold of the core thickness (85), so we used 3mm of veneer thickness to be equal to that of the core thickness.

Several test approaches, such as shear bond strength and 3- or 4-point flexure, were previously selected for measuring core veneer bond strength. A common disadvantage of these approaches is that they require a relatively large specimen size, and in the case of ceramic materials, this would result in higher incorporation of structural flaws, which lead to premature failure of the specimens before the bond strength level is reached. Additionally, the inhomogeneous stress distribution in these tests results in cohesive failure of the veneer ceramic, giving a misleading feeling of superior core veneer bond strength(105). The adoption of micro-tensile bond strength test for measuring bond strength has many advantages over other testing methods like shear bond strength(106), as the applied tensile stresses are vertical to the bonded area thus failure becomes directly a function of the tested bond strength(107). Moreover, the small cross-section of the microbars ensures less incorporation of structural defects, resulting in a low percentage of variance (η = 0.99) of the data. On the other hand, it is a very tedious test, which requires investing much time and effort, especially during cutting the microbars, to avoid accidental damage to the specimens(80). Moreover, this method subjects the zirconia– veneer interface to direct tension, a state that rarely occurs during functional loading, where the interface is subjected to different forces that change in magnitude and in direction. Nevertheless, weak zirconia veneer micro-tensile bond strength would suggest a higher probability of delaminatation failure, especially under the influence of fatigue and in the presence of water(108).

In spite of being the golden standard in terms of strength and toughness, Y-TZP may lack long-term stability, which has been a major issue for medical use and has led to the replacement of several zirconia femoral heads in orthopaedic patients. Low-temperature degradation (LTD) has been associated with several Y-TZP-based biomaterials, but is difficult to simulate in the laboratory. Ageing occurs experimentally in zirconia, mostly in humid atmosphere or in water. An accelerated ageing test using steam and pressure has been developed to simulate LTD. No ceramics studied, including Y-TZP, have been found to be chemically inert in water. More recently, when Y-TZP disks were implanted in a denture and worn for 24 h/day for 1 year, the increase in the percentage of monoclinic phase was similar to that of zirconia aged in an autoclave at 134°C for 6 h(10).

The Nature of the core veneer interface in Y-TZP compared to that of metal ceramic bond is not totally clear, The adhesion mechanism between metal and porcelain is believed to be the 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. Data presented in literature has shown the bond strength of ceramic to metal substrates to be in the range of 54 – 71 MPa(109), and a sufficient bond for metal-ceramic has been accepted when the fracture stress is greater than 25 MPa(110). However, the bonding mechanisms for veneering ceramic to the zirconia are up to now unclear. According to investigations depends on the wettability of the zirconia core with the veneering ceramic, micromechanical interactions were merely regarded. Moreover, there are less information available on the bond strength values between the all-ceramic core and veneering materials, and there exists no accurate test method for obtaining information on core/veneer adhesion in bi-layered all-ceramic materials in dentistry(111). The properties of veneering ceramics developed for zirconia-based restorations were copied and pasted from veneering ceramics designed for the PFM concept. They were adapted to zirconia frameworks performing Coefficient of linear Thermal Expansion (CTE) measurements and thermal shock testing (4). Indeed, the CTE mismatch between the bulk materials was designed to mimic the ceramo-metal CTE mismatch, with the CTE of veneering ceramics adapted to be slightly lower than the zirconia. Based on the principle that compressive stress improves the mechanical behaviour of the veneering ceramic, this approach was intended to develop residual compressive stress within the veneer during the cooling process (112). Lately some manufacturers introduced slow cooling procedures of the veneering ceramic to induce some additional residual compressive stress development and to reduce veneer fracture. Recently residual stress profiles were measured in veneering ceramic layered either on metal or zirconia disk frameworks(113). Manijota et al. 2013(60), suggested in his study that zirconia based samples behave differently than metal-based samples. As expected metal-based samples exhibited exclusively compressive stress, either in surface or in depth of the veneer layer, while, surprisingly, zirconia-based samples exhibited frequently tensile stress in the veneering ceramic lying near the framework. The presence of interior tensile stress was related to slow cooling rate and to high veneer/framework thickness ratio(60).

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(19).

For the layered restoration to gain the full benefit of the underlying core material, the bond between the weaker ceramic and the stronger framework must be of a certain minimum value and toughness to allow proper transfer of loading stresses between the two materials. During mastication, the restoration receives functional stresses, which induce a sort of temporary deformation of the restoration and result in the generation of strain energy, which becomes stored in the system. During unloading, the restoration elastically recovers to its original shape, and the stored energy is released. With cyclic loading, the interface between zirconia and the veneer ceramic must resist these changes, and this is where the bond between the two materials comes into function(19).

In previous work, the micro-tensile strength of a variety of widely used core and veneering ceramics was evaluated. In addition, the bond strength between the core and the veneer was evaluated. One interesting finding was, despite the high micro-tensile strength (340MPa) of zirconia, that its bond strength (29MPa) with veneering ceramic was inferior when compared to, e.g. IPS Empress Eris system (45MPa). As the strength of composite structures is equal to the weakest part in the structure, low core–veneer bond strength can result in chipping and/or delamination of the veneer ceramic (19).

The results of this study showed that surface treatment, veneering technique and aging affects the mean micro-tensile bond strength of the tested specimen.

Effect of surface treatment on mean micro tensile bond strength:

Both SEM and AFM had shown alteration in the surface of zirconia core compared to the control group with all surface treatments used.

The highest mean surface roughness was achieved by the Acid etched group. HF removes the glassy matrix of glass ceramics creating a high surface energy substrate with micro porosities enabling a micromechanical interlocking (74). However, HF etching does not produce any change in arithmetic roughness (Ra) of ZrO2 (69). The negligible effect of the HF on the ZrO2 surface occurs due to the absence of glassy matrix, resulting in low bond strength values.(104) When combining two acids, HNO3 and HF, the etching solution resulted in modifications to the zirconia surface, and significant increase in surface roughness. The action of this acid solution is basically a corrosion-controlled process. It can be speculated that the acid solution may determine a chemical dissolution of the grain structure on the zirconia surface, (114) enlarging the grain boundaries throughout the preferential removal of the less-arranged, high-energy peripheral atoms. (115)Etching rate depends on solution movement over the ceramic surface and on both mixing ratios and immersion time. Although Acid etched showed the highest mean surface roughness, it didn’t reflect positively on the mean micro-tensile bond strength, it showed the lowest. This can be attributed to the fact that the veneer –core bonding mechanism depends on the wettability of the core zirconia with the veneer ceramic and the increased roughness didn’t enhance the wetting and left some voids.

Air Abrasion is supposed to roughen the zirconia, increasing the bonding area and modifying the ceramic surface energy and wettability, thus facilitating the formation of resin-zirconia micromechanical interlocks. This method is effective for both improving the bonding strength of luting cements and in bonding to veneering porcelain(116). However, the mechanical stress involved in Air Abrasion initiates the phase transition from tetragonal to monoclinic zirconia, and the phase transition gives rise to volume expansion and reduction in coefficient of thermal expansion, subsequently resulting in compressive stress which decreased the bond strength(117). In our current study the Air abrasion showed mean surface roughness slightly higher than that of Silica infiltration but lower than that of Acid etched, but it showed lower mean micro-tensile bond than silica infiltration similar to that of control.

Silica infiltration: the technique used here in this study is a modification of the selective infiltration etching SIE technique proposed by Aboushelib et al. 2007(81). It is based on the infiltration of the ceramic substrate by organic oxides creating a more irregular surface with intergrains micro-retentive spaces. Despite its chemical inertness, diffusion of small dopant phases, such as silica, sodium, potassium, and magnesium has been reported at grain boundary regions in several studies(116). This grain boundary diffusion is directly influenced by controlled heating of zirconia to moderate temperature ranges (700 to 900◦C). The presence of the dopant phases at grain boundary regions exerts high capillary and surface tension forces, which further enhance grain sliding, splitting, and rearrangement movements previously described, (118) indicating that the surface grains of zirconia could be manipulated to create a retentive surface by controlling its dynamic properties. The new surface treatment method, selective infiltration etching (SIE), was developed for dental and biomedical applications. It transforms the surface of zirconia from a dense, non retentive, relatively smooth, and low energy surface to a highly active and retentive surface. In this method, the surface of zirconia is coated with a glass-containing (119) conditioning agent and heated above its glass transition temperature where grain boundary diffusion of the glass is optimized. The presence of the molten glass at grain boundaries results in sliding and splitting of the surface grains and exerts surface tension and capillary forces, which overcome the grain boundary energy levels. After cooling to room temperature, the glass is dissolved in an acidic bath, exposing the newly created retentive surface. (120) AFM analysis of the current study revealed a significant improvement in average surface roughness with no signs of ceramic degradation and showed a homogenous surface with no sharp peaks. The presence of such peaks could be a site for crack propagation. The surface area available for bonding and the presence of retentive spaces make this treatment promising for conditioning zirconia ceramic surface. It can be further speculated that this method may leave chemically reactive islets on the surface of zirconia, enhancing its bonding potentials. In our current study this technique has shown the highest mean micro-tensile bond strength of all surface treatment groups. The presence of the proposed chemically reactive islets of silica oxides facilitates both mechanical interlocking as well as chemical fusion with the veneering porcelain.

Effect of veneering technique on mean micro-tensile bond strength:

Manufacturing crowns and fixed dental prostheses (FDPs) generates residual stresses within the veneering ceramic and framework during the cooling process. These “lockedin” stresses add to functional loads and are an important predictive factor for the mechanical behaviour of restorations, as compressive stresses reinforce ceramic and tensile stresses facilitate the initiation and the propagation of cracks. Knowledge of the residual stress distribution within the veneering ceramic as a function of depth is a key factor for understanding and predicting chipping and delaminations. Such fractures are reported as an important cause of short-term clinical failure with Yttria-tetragonal-zirconia polycrystal (Y-TZP) based FPDs (121). These complications are more frequent in zirconia-based restorations than in ceramic fused- to-metal structures (PFMs).

Stress profile in the veneering ceramic is generated by the successive effects of the thermal gradients occurring during the cooling/solidification period of the veneer liquid phase sintering process, and the mismatch in thermal expansion properties between core and veneering ceramic(122).

In a study conducted by Mainjot et al. 2012(113), he described the stress profile along the veneer thickness compared to (PFMs), opposite ways explains the residual stress profile in metal- and in zirconia-based structures. A chronological two-step approach is discussed to explain residual stress development in metal-based restorations, and a three-step approach, comprising and additional step, is proposed for zirconia-based restorations. These three steps are, respectively:

1. The tempering effect, which explains the development of compressive stresses in the surface for both types of restoration, these stresses decreasing with depth. This effect is influenced by thermal gradient, explained by the lower thermal conductivity of zirconia, which induces a higher thermal gradient in the sample

2. The CTE mismatch effect, which explains the development of interior compressive stresses near the framework in both restorations. The resulting stress profile measured in metal-based restoration describe a typical curve, starting with compressive at the ceramic surface, decreasing with depth to 0.5–1.0 mm from the surface, and then becoming compressive again. Veneer thickness decreases the CTE mismatch effect, increases visco-relaxation, but does not influence the tempering effect in the surface of metal based restorations. Whereas, in Zirconia-based restoration there appear to be a gradual shift to tensile residual stresses at 0.9 mm depth from the surface, when the veneer was 1.5 mm thick. This could be a potential explanation to delamination encountered in clinical practice.

3. The stress-induced crystalline transformation effect, which is a hypothesis to explain the inverted variation of measured interior stresses with veneer thickness in zirconia-based restorations, in comparison with metal-based restorations.

Our current study had shown that Sintering CAD –on technique had showed highest mean micro-tensile bond strength followed by Layering and then Adhering CAD-on technique with no significant difference between the last two techniques.

In previous studies Sundh et al. 2004(123), showed that fractographically assessed failure mechanisms of all-ceramic restorations, the core–veneer interface was sometimes reported to be the crack initiation site. Another study showed that during testing, the veneer ceramic absorbs the delivered energy and transmits it to the supporting framework. If the total amount of the delivered energy, exceeds the strength of the restoration, fracture results. For CAD-veneered zirconia specimens, the initiated occlusal crack resulted in minor chipping of the veneer ceramic leaving behind an intact restoration. When delivering an equal amount of energy to a manually layered zirconia restoration, the initiated crack was able to cross the full thickness of the veneer ceramic and to deflect and propagate at the zirconia–veneer interface, resulting in delamination failure. All other variables being equal, the presence of structural defects located at the zirconia–veneer interface could greatly increase the chances of delamination failure during function. Naturally enough, the extent of the fracture would have clinical implications regarding the repair method of choice, as small chipping failures could be more easily handled (124). Ceramic structures tend to fail because of surface tension, where cracks and flaws propagate by slow crack growth leading to the catastrophic failure (125). In all-ceramic systems, the flaw population (size, number and distribution) can be related to the material, or be affected by the fabrication process. Thus, it might be expected that CAD CAM veneering introduces fewer flaws than layering, resulting in better strength properties, as it is a more controlled procedure. By comparison, the layering technique is more sensitive and subject to variability due to the individual building and firing procedures (126).

According to Schmitter et al. 2012(65), higher fracture resistance is achieved with the CAD-on technique than with the hand-layering technique. In Beuer et al. 2009 (127) also favored this technique over the hand-layering method. The fracture strength of specimens with a sintered veneer cap was significantly higher than that of the other groups tested. The CAD/CAM-process uses high quality material with a minimum of flaws compared to the manual procedures of veneering or heat pressing.

Layering technique is done by preparing workable ceramic slurry that is operator-dependent, and variations in the powder: liquid ratio and mixing technique are known to affect the density, the strength, the percentage of structural defects, and the number and size of air bubbles in the fired veneer(108). Moreover the difference in thermal expansion coefficient could be responsible for the generation of tensile pre-stresses resulting in weakening the zirconia. Fabrication of conventional dental porcelains consists of a frit condensation followed by a sintering process. Sintering may introduce thermally induced residual stresses. The moisture content of the veneering material during sintering might induce changes in the zirconia/veneering interface and provoke transformation from the tetragonal phase to the monoclinic phase (128), which in term leads to decrease in the mechanical properties.

Adhering Cad-on technique, this technique is novel and depends on the retention of the veneering ceramic by means of resin cement adhered to the zirconia surface it encounter the problems of resin cement adhesion with zirconia surface. Traditional silane chemistry is not truly effective with zirconia, as it possesses a relatively non-polar surface, is more chemically stable than silica containing ceramics, and not easily hydrolyzed(129). So the bond strength with the resin cement depends mainly on the mechanical interlocking and no chemical reaction can be detected. Also the bond resin bond with zirconia surface has no long term stability and decrease significantly with thermocycling(130).

Fracture Analysis

As the bilayer ceramic composite comprise two layers of ceramics with very different mechanical performances, it is of outmost importance to identify the crack origin and the crack propagating trajectory for understanding the fracture mechanism(s) and for improving the performances of dental restorations. During mechanical testing the fracture of bilayer all-ceramic specimens occurs instantly when a critical stress is reached, which makes it impossible to follow the evolution of propagating cracks.

In our current study it can be assumed the predominant type of failure that is almost apparent in all groups is the cohesive failure in veneer .two other types of failure appeared are the cohesive failure at junction (interfacial) and adhesive failure.

Cohesive failure: It is reasonable that the failure mode of zirconia-based all-ceramic restorations veneered with a relatively weak porcelain – assuming a good bond – tends more to cohesive chipping of the porcelain at lower fracture loads whereas higher-strength veneer material provokes to a certain extent total fracture at higher loads. The relatively weak veneering porcelain (90MPa) led to cohesive fractures, where a thin porcelain layer still remained on the zirconia coping. This type of failure indicates the good interfacial bond between the core and the veneer material that is critical for the success of these composite structures (126)

Adhesive failure: When the fracture pattern was almost completely adhesive, it was shown that stress distribution in the microbars was not homogenous due to the different E-moduli of the core and the veneer, and also the way the specimens were attached to the device (19). Delamination can be the result of the use of the weak veneer ceramic or due to a weak bond between the core and veneer(116). In our current study the adhesive failure always appeared with the Adhering CAD on technique and this may be due to the weak bond achieved with the resin cement and zirconia.

Cohesive failure at the junction (interfacial): When the pattern of fracture was mainly interfacial, again due to the inhomogeneous stress distribution in the microbars (19). One of the expected causes of increased interfacial failure in some test groups is the generation of tensile stresses at the interface accompanied compressive stresses generated in the veneer ceramic as a result of difference in CTE (131).

If the failure mode is cohesive, one can only estimate that the bond strength between the core and veneer is stronger than a certain value. The estimated bond strength between the core and veneer is at least twice the reported values if the failure mode is cohesive (116).

As the thermal expansion coefficient of various types of dental ceramics is nonlinear, the difference in TEC may result in unexpected high pre-stress at the interface which could be the reason for interfacial failure observed in some of the tested veneers.

Additionally, interface toughness plays a significant role and directly affects the failure pattern. When interface toughness exceeds applied tensile stresses, the bilayered structure acts as a homogenous material and the initiated crack crosses the interface. On the other hand when the interface toughness is low a propagating crack may deflect at this weak region and spread along the interface causing delamination of veneer ceramic (132). All previously described factors may be exaggerated in the oral condition due to fatigue and structural corrosion especially under the presence of water (116).

Effect of Aging

In our current study, the autoclave induced aging increased the mean micro-tensile bond strength.

The results of our study coincide with the conclusions of Chun Li et al. (2013)(133), Fracture primarily occurred within the porcelain adjacent to the zirconia–porcelain interface and this was not influenced by autoclave induced monoclinic transformation of the zirconia surface prior to veneering In this study he did the autoclave aging prior to veneering in our study we did the autoclave aging after veneering. In his study fracture was not affected but our study showed increase in the micro-tensile which can be attributed to the fact that the bond strength of the core veneer depends on the wettability of the veneer to the core surface so both the increased temperature and phase transformation allowed the veneer to flow better on the surface of the core.

Surprisingly, despite its reputation as a steel ceramic, zirconia is a dynamic material at an ultra structural level. Besides its unique tetragonal monoclinic transformation, other dynamic changes occur in response to mechanical and thermal stresses(18). Additionally, it would result in structural changes of the surface of zirconia as surface lifts, increased surface roughness, and grain pull-out (19). A common issue associated with zirconia is the unknown effects of low-temperature degradation (LTD). Tholey et al. 2009(128) did observe that the moisture induced LTD accompanied with glass induced dissolution from porcelain sintering at the Y-TZP interface resulted in severe surface faceting to occur. These surface facets have been shown by atomic force microscopy techniques (134) to be monoclinic (martensitic) self-accommodating variants which are induced through LTD. Furthermore, Choi et al. 2011 (63) suggested that zirconia and glass ceramic established a thermodynamically stable equilibrium by dissolution of the zirconia ions into the veneering material. Therefore, at this stage, the bonding characteristics between Y-TZP and the veneering material appear to involve both chemical and mechanical interaction. (133)

A known method to accelerate in a controlled manner the tetragonal to monoclinic transformation is by autoclave treatment (135). This technique allows us to critically observe the effect on the veneering adhesion as the amount of monoclinic zirconia at the surface is increased. The 900◦C porcelain firing cycle performed after the auto-clave cycles indicated that a typical firing schedule was sufficient for most of the monoclinic phase to revert to the tetragonal phase for all specimens, evident by the absence of the monoclinic peak. This outcome was not surprising as the firing temperature of 900◦C was well within the tetragonal phase field. Therefore, even without the regeneration firing recommended by some manufacturers, a porcelain firing schedule was sufficient to completely convert the monoclinic phase induced by the autoclave cycles, back to tetragonal.

From what has been previously discussed it can be deduced that the veneer core bond strength is multi-factorial and interrelated bond whereas the nature of the bond itself is still controversial, so further investigations needs to be done in revealing the nature of this bond

Summary

The introduction of yttrium partially stabilized tetragonal zirconia polycrystal (Y-TZP) to the dental field opened the design and application limits of all-ceramic restorations with greater confidence and success rates. With its superior mechanical properties, three or four-unit fixed partial dentures (FPDs) are no longer the safe limit for the construction of core veneered all ceramic restorations. Establishing a strong and stable bond with Zirconia has proven to be difficult, as the material is acid resistant and does not respond to common etching and silanation procedures used with other glass containing ceramic materials which react to silane coupling agents. As Zirconia is relatively opaque and monochromatic in colour, a layer of veneering ceramic is built on to provide the restoration with the required esthetics. Low-temperature degradation (LTD) has been associated with several 3Y-TZP-based biomaterials. The bond between veneering ceramic and Zirconia framework is currently the subject of comprehensive investigations.

Thus the purpose of this study is to investigate the effect of a new method of surface treatment of Zirconia on its bond strength with the ceramic veneering ceramics with different veneering techniques and with and without s-LTD aging.

A total of 144 microbars were prepared. Each 36 have received a different surface treatment namely: no treatment (control), Air Abrasion with 110μm aluminum oxide particles (Al2O3), Silica infiltration with silicate compound and heat, and acid etch with a mixed solution of HF and HNO3.Then each 36 were further divided into three groups each 12 received different veneering technique Layering, Sintering CAD on technique and Adhering CAD on technique. Then each 12 microbar were divided into two groups were one group didn’t receive any treatment and the other groups were aged using s-LTD aging in an autoclave at 134șC for 6 hours. The bars were attached to a specially designed attachment and tested in the universal testing machine (LLOYD). The load required for fracture of each specimen was recorded in MPa. Data were presented as mean and standard deviation (SD) values, a regression model using three-way Analysis of Variance (ANOVA) was used in testing significance for the effect of surface treatment, veneering technique, aging and their interactions on mean micro-tensile bond strength. Tukey’s post-hoc test was used for pair-wise comparison between the groups when ANOVA test is significant. Fractured microbars viewed under digital microscope with magnification 40X and type of failure was specified recorded as number and percentage in each subgroup.

The results of this study revealed that that surface treatment, veneering technique, aging and the interaction between the three variables had a statistically significant effect on mean micro-tensile bond strength. Air abrasion showed statistically significant same mean micro-tensile bond strength as that of Control, Silica infiltration higher than control whereas acid etched lower than control. Sintering CAD-on showed statistically significant higher mean micro-tensile bond strength than both Layering and Adhering CAD-on veneering technique. Aging showed statistically significant higher mean micro-tensile bond strength than Without Aging.

Conclusion

Within the limitation of the current study the following conclusions could be derived:

Silica infiltration surface treatment has proven to be an efficient way for increasing the bond strength between YTZP-zirconia and its veneering ceramic; it showed the highest mean micro-tensile bond strength

Sintering CAD-on technique for YTZP- zirconia veneering showed the highest mean micro-tensile strength among the veneering techniques, and the most common type of fracture appearing is the cohesive at junction which implies that the veneer bulk is stronger than layering technique.

The Acid etch solution used in this study has produced the highest mean micro roughness in the YTZP- zirconia core surface but the lowest mean micro-tenile bond strength further studies are required to investigate its effect on the mechanical properties of the core material and bonding to zirconia.

Adhering CAD-on technique can be used as cold technique for YTZP-zirconia veneering showed nearly same micro-tensile bond strength as that of layering technique; moreover the type of failure is the adhesive type of fracture compared to cohesive veneer failure in layering technique.

Clinical Implications

Silica infiltration can be used as an effective surface treatment for YTZP-zirconia, further research are needed in order to facilitate its commercial use.

Sintering CAD- on is a good alternative for veneering YTZP- zirconia core material, but further investigations are required to find a specially formulated fusing porcelain to bond it with YTZP-zirconia core and to compare it with the pressing technique.

Adhering CAD- on can be a promising veneering technique but need to be studied thoroughly using different type of cements or formulating a special kind of cement to insure a better bond with the core material.

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الملخص العربى

المقدمة

بعض المواقف الإكلينيكية تتطلب تركيبات تجميلية عالية الجودة, فى هذه الحالة تكون التركيبات الخزفية الكاملة أفضل من التركيبات المعدنية المكسوة بالخزف ,حيث تسهم تركيبة السبيكة المعدنية فى إعطاء مظهر معتم للتركيبة بينما التركيبات الخزفية تكون أكثر شفافية مما يجعلها تعطى مظهر اقرب إلى الأسنان الطبيعية.

هناك إهتمام متزايد باستخدام الخزف الذى يحتوى على أكسيدات الزرقونيوم كبديل للب المعدنى و ذلك لأن الخزف المحتوى على أكسيدات الزرقونيوم له خواص ميكانيكية أعلى بالإضافة إلى تطور التككنولوجي فى صناعة الخزف الزرقونى الى جعله أسهل و أكثر إستخداما. تكمن المشكلة فى إستخدام الخزف الزرقونى فى صعوبة الحصول على رابطة قوية وثابتة, حيث أنها مقاومة للحمضيات ولا تستجيب لطريقة التخريش بالحمض أو بمادة السيالين.

هناك العديد من التقنيات استخدمت لتخشين سطح الخزف الزرقونى مثل السحل بالهواء المَضغوط و التخشين بواسطة الرؤوس الماسية، ولكنها لم تنجح في تحقيق رابطة ميكانيكية ثابتة على سطح الخزف الزرقونى.

ظهرت طريقة جديدة لمعالجة سطح الخزف الزرقونى "التخريش الإرتشاحى الإنتقائى "وهى تقوم بتحويل سطح الخزف الزرقونى من سطح املس، غير قادر على تكوين رابطة ثابتة إلى سطح متفاعل و قادر على تكوين روابط ثُابتة.

كذلك يعتبر الخزف الزرقونى من المواد المعتمة أحادية اللون لذلك يجب وضع كسوة خزفية على سطحها لإعطائها مظهرا أقرب إلى الطبيعة.

بالرغم من دقة التقنيات المستخدمة في تصنيع الكسوة الخزفية فإنها تستهلك الكثير من الوقت وكذلك تسمح بوجود عيوب بنيوية مثل فقاعات الهواء والثغرات الدقيقة مما يجعل القدرة على كسر أو إزالة هذة الكسوة أسهل.

لذلك فإن الرابط بين الخزف الزرقونى و الكسوة الخزفيةهي تحت الدراسة و لإختبار الموضوعى.

الهدف:

الهدف من هذه الدراسة هو دراسة تاثير طرق معالجة السطح على الرابطة بين الخزف الزرقونى والكسوة الخزفية مع إستخدام طرق مختلفة لعمل الكسوة الخزفية واختبار تاُثير التعتيق.

المواد والطرق المستخدمه:

تم إعداد ما مجموعه 144 عينه وتقسيمها الي 4 مجموعات. كل مجموعة من36 تلقت معالجة السطحية مختلفة :

اول مجموعه لم تتلق اي علاج

ثاني مجموعه تلقت خربِشه باستخدام جزيئات أكسيد الألومنيوم μm) 110)

ثالث مجموعه تلقت ارتشاح جزيئات من السيليكا علي السطح

المجموعه الرابعه تلقت تخريش باستخدام خليط حمَضي من حمض الهيدروفلوريك والنيتريك.

وبعد ذلك تم تقسيم كل 36 الى ثلاث مجموعات كل 12 تلقت كسوة خزفيه بطريقه مختلفة:

طريقة الطبقات

غطاء مصنع بطريقة ال- CAD ملبد علي السطح

غطاء مصنع بطريقة ال- CAD ملصق علي السطح

ثم تم تقسيم كل 12عينه إلى مجموعتين مجموعه لم تتلق أي علاج والمجموعه الأخرى تلقت تعتيق باستخدام الأوتكلاف علي درجة حرارةC 134ș لمدة 6 ساعات.

تم تعليق العينات علي مرفق مصنع خصيصا واختبارها في آلة اختبار العالمي (لويد). وسجلت الحمل اللازمة لكسر كل عينة في MPa. تم تحليل البيانات احصائيا عن طريق المتوسط والانحراف المعياري في نموذج الانحدار باستخدام تحليل ثلاثي التباين (ANOVA). تم اختبار العينات المكسورة تحت المجهر الرقمي مع التكبير X 40 وتم تحديد نوع الكسر وتسجيله كعدد ونسبة في كل مجموعة فرعية.

نتائج البحث:

كشفت هذه الدراسة عن الاتي:

كل من المعالجة السطحيه وتقنيه الكسوة الخزفيه والتعتيق لها تاثير علي الرابط بين الزكونيوم والكسوة الخزفية.

من طرق معالجة السطح المختلفة فان ارتشاح جزيئات من السيليكا علي السطح اعطت افضل تاثير علي الرابطه.

مع الكسوات الخزفية المختلفة فإن المجموعه التي تلقت غطاء مصنع بطريقة ال- CAD ملبد علي السطح اعطت افضل تاثير علي الرابطه.

التعتيق أدى الى زيادة الرابطة بين الزركونيوم والكسوة الخزفية.

جودة الرابطة بين الزركونيوم و الكسوة الخزفية تحت تأثيرطرق مختلفة للمعالجة السطحية باستخدام تقنيات متنوعة لتصنيع الكسوة الخزفية

رسالة مقدمه إلى
قسم التيجان والجسور كلية طب اسنان ، جامعة عين شمس للحصول على درجة دكتوراة فى التركيبات السنية المثبتة.

مقدمة من

غادة عبد الفتاح عبد الستار

بكالوريوس طب اسنان ، ماجستير تيجان وجسور

مدرس مساعد بقسم التيجان والجسور

كلية طب أسنان – جامعة عين شمس.

كلية طب أسنان

جامعة عين شمس

2015

المشرفون

أ. د امينة محمد حمدي

أستاذ التيجان والجسور

كلية طب أسنان – جامعة عين شمس.

أ.د محمد عادل الدملاوى

أستاذ التيجان والجسور

كلية طب أسنان – جامعة عين شمس

أ.م.د طارق صلاح مرسى

أستاذ مساعد التيجان والجسور

كلية طب أسنان – جامعة عين شمس

Quality Of Zirconia\Veneer Bond Under Different Surface Treatments Using Variable Veneering Techniques

Thesis

Submitted for fulfillment of Doctor Degree requirement in fixed Prosthodontics, Faculty of Dentistry,

Ain Shams University

By

Ghada Abdel Fattah Abdel Sattar

B.D.S, M.D.Sc

Assistant lecturer in Crown and Bridge department

Faculty of Dentistry, Ain Shams University

Faculty of Dentistry

Ain Shams University

2015

Supervisors

Dr. Amina Mohamed Hamdy

Professor of Crown and Bridge

Faculty of Dentistry, Ain Shams University.

Dr. Mohamed Adel El-Demellawy

Professor of Crown and Bridge

Faculty of Dentistry, Ain Shams University

Dr. Tarek Salah Morsi

Assistant Professor of Crown and Bridge

Faculty of Dentistry, Ain Shams University

Acknowledgement

First and foremost I would like to thank Allah the most merciful and greatest beneficent for all his blessings.

I would like to express my deepest gratitude to
Dr. Amina Mohamed Hamdy, Prof. of fixed prosthodontics, Faculty of Dentistry, Ain shams university, for her sincere effort, great confidence, meticulous advice, and valuable comments throughout this work.

My deepest appreciation to Dr. Mohamed Adel El-Demellawy Prof. of fixed prosthodontics, Ain Shams University, for his guideness and dedicated work specially on the AFM imaging of this work.

I would like to express my heartfull thanks and deepest gratitude to Dr. Tarek Salah Morsi, Ass. Prof. and head of Fixed Prosthodontics Department, Fixed Prosthodontics, for his help throughout the details of every part of this work, his time and effort, his support and guidance ,cooperation and continuous advice to complete this work.

Personal appreciation and thanks to Dr. Mohamed Selim, national center of research, for his innovations in the surface treatments used in this research.

I would like to express my appreciation to Dr. Marwa El Wahsh, Lecturer of Crown and Bridges, Ain Shams universities, for her help and advices throughout the work.

Special thanks to Dr. Haitham Mohamed Amro, for his guidance throughout the practical part of this research

Finally I would like to thank the staff member of Crown and Bridges department, Ain Shams University each and every one of them for their thorough understanding and support during the course of this work.

Dedication

I would like to dedicate this work to my Parents, my family, my husband and my two lovely daughters Alia & Hana.

List of Contents

Title Page No.

List of Tables i

List of Figures iii

Introduction 1

Review of Literature 4

Aim of the Work 38

Materials and Methods 39

Results 64

Discussion 96

Summary 114

Conclusion 116

References 118

Arabic summary

List of Tables

Table No. Title Page No.

Table (1): Grouping of slices 45

Table (2): Recommended baking cycle for effect bonderVM9. 51

Table (3): Recommended baking cycle for base dentin VM9 first bake 52

Table (4): Recommended second bake cycle for base dentin VM9 53

Table (5): Recommended glazing cycle 54

Table (6): Regression model results for the effect of different variables on mean micro-tensile bond strength. 64

Table (7): Effect of surface treatment on micro-tensile bond strength regardless of other variables: 66

Table (8): Effect of surface treatments on micro-tensile bond strengths with different variables’ interactions without aging 68

Table (9): Effect of surface treatments on micro-tensile bond strengths with different variables’ interactions with aging 70

Table (10): Effect of veneering techniques on micro-tensile bond strengths regardless of other variables 72

Table (11): Effect of veneering techniques on micro-tensile bond strengths with different variables’ interactions without aging 74

Table (12): Effect of veneering techniques on micro-tensile bond strengths with different variables’ interactions with aging. 76

Table (13): Effect of aging on micro-tensile bond strengths regardless of other variables. 78

Table (14): Effect of aging on micro-tensile bond strengths with different variables’ interactions after using layering technique 80

Table (15): Effect of aging on micro-tensile bond strengths with different variables’ interactions after using sintering technique 82

List of Tables (Cont…)

Table No. Title Page No.

Table (16): Effect of Aging on micro-tensile bond strengths with different variables’ interactions after using adhering technique 84

Table (17): Frequency of type of failure and its percentage in all groups 88

List of Figures

Fig. No. Title Page No.

Figure (1): InCorisZi blocks 39

Figure (2): Vm9 base dentine and effect bonder. 40

Figure (3): Cerec blocks for CAD –on veneer. 41

Figure (4): NX3 Nexus third generation 41

Figure (5): Micracut precision cutting device 42

Figure (6): Micrometer attached to the device adjusted to 4mm. 42

Figure (7): Checking slices after cutting. 43

Figure (8): Checking slices after sintering 43

Figure (9): Specially designed glass box. 46

Figure (10): Specially designed holder 47

Figure (11): Copper split mold 47

Figure (12): Sand blasting nozzle fixed to the glass box 48

Figure (13): Sand blaster nozzle and its gauge 48

Figure (14): Copper split mold 50

Figure (15): Zirconia slice in the split mold 51

Figure (16): Application of Vm9 in the mold. 52

Figure (17): Slices repositioned in mold, porcelain refilled and baked. 53

Figure (18): Checking of specimen after last cycle 53

Figure (19): Final layered specimen 54

Figure (20): Micrometer of micracut adjusted to 3mm. 55

Figure (21): CAD on slices thickness rechecked 55

Figure (22): Checking of thickness 56

Figure (23): Final CAD-On sintered specimen 56

Figure (24): Thickness checked 57

Figure (25): Final CAD-on adhered specimen 58

Figure (26): Micrometer of micracut adjusted to 1mm. 58

Figure (27): Diagram showing cutting procedure 59

Figure (28): Checking thickness of microbars and final microbar 59

Figure (29): Attachment used for micro tensile testing front view, rear view and top view. 60

Figure (30): Diagram showing attachment parts 61

Figure (31): Cyanoacrylate adhesive used 61

Figure (32): Microbar adhered to the attachment 62

Figure (33): Microbar and attachment in the universal testing machine. 62

Figure (34): Diagram illustrating fracture of microbars during testing 63

List of Figures (Cont…)

Fig. No. Title Page No.

Figure (35): Bar chart showing mean values for the effect of surface treatment on micro-tensile bond strength regardless of other variables. 66

Figure (36): Bar chart representing mean values for effect of surface treatments on micro-tensile bond strengths without aging 68

Figure (37): Bar chart representing mean values for effect of different surface treatment on micro-tensile bond strengths with aging. 70

Figure (38): Bar chart representing mean values for effect of veneering techniques micro-tensile bond strengths 72

Figure (39): Bar chart representing mean values for effect of veneering techniques on micro-tensile bond strengths without aging. 74

Figure (40): Bar chart representing mean values for effect of veneering techniques on micro-tensile bond strengths without aging. 76

Figure (41): Bar chart representing mean values for effect of aging on micro-tensile bond strengths. 78

Figure (42): Bar chart representing mean values for effect of aging on micro-tensile bond strengths after using layering technique. 80

Figure (43): Bar chart representing mean values for effect of aging on micro-tensile bond strengths after using Sintering technique 82

Figure (44): Bar chart representing mean values for effect of Aging on micro-tensile bond strengths after using Adhering technique. 84

Figure (45): Figure showing cohesive failure (CV) in veneer where A is the core and B is the veneer under digital microscope 40X. 86

Figure (46): Figure showing cohesive failure in veneer (CV) where A is the core and B is the veneer under SEM 100X. 86

Figure (47): Figure showing cohesive failure at the junction(CJ) where A is the core , B is the veneer under and C is the junction under digital microscope 40X. 86

Figure (48): Figure showing cohesive failure at the junction(CJ) where A is the core , B is the veneer under and C is the junction under SEM 100X. 87

List of Figures (Cont…)

Fig. No. Title Page No.

Figure (49): Figure showing adhesive failure (A) where A is the core and C is the junction under digital microscope 40X. 87

Figure (50): Figure showing adhesive failure (A) where A is the core and C is the junction under SEM 100X. 87

Figure (51): Bar graph representing percent of type of fracture in each tested group. 89

Figure (52): SEM image showing zirconia surface with no surface treatment at A: 500X and B:4000X 90

Figure (53): SEM image showing zirconia surface with Air Abrasion at A: 500X and B: 4000X. 90

Figure (54): SEM image showing zirconia surface with silica infiltration at A:500X and B:4000X 90

Figure (55): SEM image showing zirconia surface with acid etched at A:500X and B:4000X 90

Figure (56): AFM image showing surface topography of the control group. Mean surface roughness Ra measured was 163.717 nm. 92

Figure (57): AFM image showing surface topography of Air Abrasion group. Mean surface roughness Ra measured was 335.856 nm. 92

Figure (58): AFM image showing surface topography of Silica infiltration group Mean surface roughness Ra measured was 334.281nm. 93

Figure (59): AFM image showing surface topography of Acid etched group. Mean surface roughness Ra measured was 360.263nm. 93

Figure (60): SEM image showing layering technique where A is the Zirconia core and B is the veneer 500X and 4000X. 95

Figure (61): SEM image showing Sintering CAD on technique where A is the Zirconia core and B is the veneer 500X and 4000X 95

Figure (62): SEM image showing Adhering CAD on technique where A is the Zirconia core and B is the veneer 500X and 4000X. 95

Introduction

Review of Literature

Aim of the Study

Materials and Methods

Results

Discussion

Conclusions

Summary

References

Arabic Summary