Engineered Nanomaterials Used In Dental Implantology

Engineered Nanomaterials used in Dental Implantology

Authors: Georgiana Dolete, Cristina Florina Ilie, Irina Florentina Nicoară, George Mihail Vlăsceanu

Abstract

During the years, a consistent amount of materials have been used for the obtaining of implants meant to replace missing or damaged teeth. Along with the fast development of technology, the quality of the implants have been improving, mainly due to the intense research in the field of materials science. Generally speaking, the classes of materials employed in the fabrication of implants were metals, at first, followed by polymers and recently ceramics and composites. A major obstacle in obtaining efficient, stable, long-time functioning implants lies in the infections associated with the implantation which have been increasing in the healthcare system. Thus, there are have been developed more nanotechnology and nanomaterials based strategies to prevent and annihilate antibiotics resistant pathogens and biofilms. In addition, new manufacturing techniques, such as CAD/CAM have gained a lot of interest and are continously developing since they are able to ensure precise route of shaping personalized implants for precise destinations. Most complications are incident short after the surgical interventions and are related to deficient recovery practices; however, some may occur a long time after the surgery and be difficult to trace back to their actual origin. The success of a dental implant depends on the surgeon’s skills, the correlation between the design and chemical composition of the implant and its purpose and the patients’ transition to their regular lifestyle after surgery. To improve the properties of the dental composites for a safer and easier transition for the patient, several approaches have been studied: direct incorporation of antibacterial agents, immobilization of antibacterial monomers/polymers, silver, fluoride or calcium phosphate doping, some with remarkable results.

Introduction

Nanomaterials, zero, one, two or three dimensional, have been intensively studied since the subjection of the concept of nanotechnology as they were found to fit perfectly in applicative fields like medicine, chemistry, informatics and communication, industry, energy generating. Nanosized particles in medicine can be used for diagnosis, drug targeting and controlled delivery or tissue engineering. Moreover, depending on their biological behavior, some bioactive or bioresorbable nanomaterials could be successfully used for both tissular reconstructions and state-of-the-art implant biomaterials doping or coatings for improved chemical stability, host cell adhesion, antimicrobial conduct, mechanical properties, esthetical aspects, pursuant to the envisioned features of the designed devices (Bhardwaj, 2013)

From ancient times, the need of replacing missing or damaged teeth has existed; the accessibility to the insertion area was one of the most important aspects which eased the procedure and it is of no surprise that the first implants human kind developed were dental. Despite the fact that modern concepts of biocompatibility were not at hand, it is still remarkable the engeniosity people had when it came to model and insert primitive materials implants and the anticipation of the properties the replacing materials should exhibit. From the ancient civilizations until today, the development of dental implantology has been mainly based on the development of the quality and diversity of the materials used in the manufacturing process. Generally speaking, the classes of materials employed in the fabrication of implants were metals, at first, followed by polymers and recently ceramics and composites, which have been found to exhibit a versatile behavior, easing the shaping and modeling of various devices (Saini, 2015).

A major obstacle in obtaining efficient, stable, long-time functioning implants lies in the infections associated with the implantation which have been increasing in the healthcare system. Thus, there are have been developed more nanotechnology and nanomaterials based strategies to prevent and annihilate antibiotics resistant pathogens, like species able to protect themselves by outlining a biofilm made of extracellular polymeric substances crosslinked into a stable matrix (Laverty, 2015). Dental bacterial infections are loosely caused by 10 of the more than 300 different bacterial encompassed in the human oral flora (Asikainen, 1993). The risks of infection are even higher after orthodontic therapy or implantation due to the dental plaque formation and mineralization, a complex process yet to be completely understood (Rakhshan, 2015; Wong 2007).

For that matter, studies have been conducting to determine a smart approach regarding the use of nanoparticles enriched dental composites for better antibacterial behavior and also for improving the mechanical properties of the implant. In addition, new manufacturing techniques, such as CAD/CAM have gained a lot of interest and are under a fast and continuous evolvement since they are able to ensure a quick and precise route of shaping personalized implants with a very strong control of its features, for precise destinations.

To sum up, all the research revolving around the use of nanoparticles in stomatology has another important goal, besides the immediate need of replacing missing structures in the oral cavity; integrating features such as better antimicrobial behavior, increased mechanical properties, personalized design helps developing an implant with less chances of being the center of a post-surgical complication. Since bacterial proliferation on the surface of teeth can lead to a damaged structure and poor mechanical properties, as well as fissures caused by physio-chemical agent lead to infection spreading, engineering a dental nanocomposite with augmented properties is an important goal many diverse studies have (James, 2004; Mehdawi, 2013).

INTRODUCTION – BIOMATERIALS USED IN TIME FOR DENTAL IMPLANTS

During the years, a consistent amount of materials have been used for the obtaining of implants meant to replace missing or damaged teeth. Along with the fast development of technology, the quality of the implants have been improving, mainly due to the intense research in the field of materials science. As far as we can tell , the first implants can be traced to ancient civilizations, like the Egyptian or the Central American. From the first implants made of stone or ivory, the dental implantology has come a long way, in terms of both quality and quantity of the materials employed in the process; from the Gold and ivory implants dated to the 16th century, until the Iridium, Tantalum and various alloys reported in the early last century, a broad spectrum of polymers had also been tried as potential raw materials for the fabrication of more and more efficient implants. However, today, new biomaterials such as zirconia, roxolid and metal implants with modified surface are extensively studied as they ensure properties that are required for an optimal functioning of the implants and also which fulfills the esthetical needs (Saini, 2015).

ANCIENT ERA (through AD 1000) to PRESENT

Replacement of missing tooth with various materials dates back to ancient period of Greek, Egyptian or south American civilization where bone, carved ivory, shells, metal and even animal teeth were used (Shrestha, 2014). One of the most remarkable examples from that period is a skull from pre Columbian era in which artificial tooth is carved with dark stone. (Saini, 2015).

The beginning of endosseous oral implantology is marked in the Foundational period (1800-1910). To form a shape of tooth root, Maggiolo used gold in 1809. The use of teeth made of porcelain was reported by Harris in 1887. Zamenski combined in 1890 porcelain with gutta-percha, and rubber. Eight years later, a silver capsule was placed in the tooth socket by R.E Payne.

In the early 1900’s lambotte fabricated implants of aluminum, silver, brass, red copper, magnesium, gold and soft steel plated with gold and nickel (Saini, 2015).

Because of the better performance and more predictable results than the naturally derived materials, synthetic materials such as polymers, ceramics or metal alloys started replacing them in dawn of the modern era (1935-1978). The first one who achieved a 15 years implant survival was Strock, who used an vitallium screw anchored in bone and a porcelain crown mounted immediately after anchoring.

Polymers and Composites

Used for the first time in 1930, polymeric implants were made from polymethylmethacrylate and polytetrafluoroethylene. However, early use as implant materials has resulted in a failure in the most cases. After Milton Hodosh reported the biologically toleration of polymers, a polymethacrylate tooth-replica implant was developed and showed very good results in the restoration of function and appearance.

For bone augmentation and for the repair of periimplant bone defects, biodegradable polymers (Polyvinyl alcohol, polylactides or glycosides, cyanoacrylates or other hydrated forms) have been combined with biodegradable CaPO4 to obtain scaffolds, plates, screws. To transfer force to soft and hard tissue region, different forms of polymers were used in combination with other categories of synthetic biomaterials (HA, Al2 O3, Glass ceramics) as coatings. In osseointegrated implants, polymers are used as components between prosthesis and implant in order to absorb shocks and to simulate the natural tooth’s biomechanical function. As an exemple, The IMZ implants incorporate a polymethylene intramobile element (IME) placed between the implant and prosthesis which acts as an internal shock absorber. The results showed that this element also helps in reducing occlusal loads (Shrestha, 2014).

Metals and metal alloys

Besides biomechanical properties and the easiness of processability, metals offer the posibility of sterilization by the common sterilization procedure which is an important matter in the fabrication of implants. Due to advances in time, metals extensively used so far are replaced with more efficient materials. However, Titanium (Ti) and its alloys (mainly Ti-6Al-4V) are still remaining the most commonly used metals for dental implants, while prosthetic components are still made from gold alloys, stainless steel, and cobalt-chromium or nickel-chromium alloys (Saini, 2015).

Cobalt chromium alloys used in manufacture of customized implants, such as subperiosteal frames includes cobalt, chromium and molybdenum as the major elements. While Cobalt ensure a continuous phase for the basic properties, Chrome give corrosion resistance through the oxidized surface and Molybdenum provides bulk strength and corrosion resistance.

Iron-Chromium-Nickel Based Alloys contains nickel as a major element. In this regard, care must be taken to avoid pitting corrosion. Also, if biomaterials such as titanium, cobalt, zirconium or carbon are used, galvanic coupling and bio-corrosion can result.

Titanium and Titanium alloys Ti6Al4V

Four grades of commercially pure titanium and two titanium alloy are recognized by ASTM Committee F-4 on Materials for Surgical Implants and are commercially available as dental implants. The two alloys are Ti-6Hl-4V and Ti-6Hl-4V extra low interstitial (ELI). The commercially pure titanium materials, also referred to as unalloyed titanium, are commercially pure grade I titanium, commercially pure grade II titanium, commercially pure grade III titanium and commercially pure grade IV titanium (McCracken, 1999). The grades differ in their oxygen content. Grade 4 is having the most (0.4%) and grade 1 the least (0.18%) oxygen content.

Titanium is successfully used as an implant material due to its excellent biocompatibility and to the formation of stable oxide layer on its surface. In past few years the evolution of titanium as biomaterial for implant has dramatically increased because of its favorable combination of mechanical strength, chemical stability, and biocompatibility, making it the material of choice for intraosseous applications. The main disadvantage is due to gray color of titanium.

The first generation of titanium implants, with a history of 50 years of success, had a smooth surface texture. The second generation implants were created with surfaces designed to improve the molecular interactions, cellular response and osseointegration. Over time, this second generation of implants used clinically suffered mechanical blasting, bioactive coatings, anodized and, more recently, laser modified surfaces.

Pure titanium heated to 883°C undergoes a crystallographic change from alpha to beta phase. The alloys most commonly used for dental implants are of the alpha-beta variety. Aluminum and vanadium are added to stabilize the phases and to improve the mechanical properties of this metal. Aluminum, the alpha-phase stabilizer, increase the strength and decrease the weight of the alloy, while Vanadium, the beta-phase stabilizer, increase the ductility. The Ti-6Al-4V alloy is the most common one and it contains 6% aluminum and 4% vanadium. Recently, niobium has replaced vanadium due to the local adverse tissue reaction and immunological responses, and Ti-6Al 7Nb has been proposed as an alternative. Other elements such as zirconium, tantalum, palladium and indium are also researched to improve biocompatibility of Ti-6Al 4V (Shrestha, 2014).

Ceramics

Generally used in bulk forms and, recently, as coatings materials for metals, ceramics have been used in dentistry in different combinations. A number of properties like high strength, endurance to biodegradation, almost insignificant electrical and thermal conductivity or color made oxide ceramics a proper material for surgical implant devices (Shrestha, 2014). The main disadvantages of ceramics in dentistry were low ductility and brittleness. Aluminum, titanium and zirconium oxides, considered as high ceramics, were used to produce some dental implants (for example root form , endosteal plate form or pin-type implants) (Saini, 2015). Because of its poor survival rate, aluminium oxide was withdrawn from the market (Shrestha, 2014).

Calcium phosphate materials (CaPs) were considered a good option to promote accelerated bone healing and osteointegration around implants. From the category of bioactive ceramics we mention hydroxyapatite, TCP (tricalcium phosphate) and glass ceramics. After a various number of in vitro and vivo experiments, scientists came to the conclusion that dense or porous HA ceramics should be used as materials for long term or permanent bone implants, while porous TCP ceramics to be used in cases that require bioresorbable material. To stimulate bone-forming cells and to ensure bone-implant fixation, metallic implants surface can be coated with CaPs through plasma-spraying technique, which is the only coating method that has been used in clinical practice for dental implants made of titanium. Nowadays, biomimetic technologies can be used to add to the coating antibiotics, growing factors, drugs or other bioactive agents.

Modern implant dentistry is delineated from the period of mid 1930’s to the present. In recent years the treatment options and modalities for achieving optimal functional and aesthetic outcomes with implant restorations have clearly changed. Therefore, implant research has focused on discovering tooth-colored implant material that improves the aesthetic appearance of dental implants and, at the same time, is highly biocompatible and able to withstand the forces present in the oral cavity and therefore zirconia came into being (Saini, 2015).

Zirconia

The first report of zirconia was made in 1975 by Cranin and co. Yet, zirconia was used for dental prosthetic surgery with endosseous implants in early nineties. As an alternative to titanium implants from the aesthetic point of view, ceramic implants were introduced also for osseointegration and less plaque accumulation resulting in improvement of the soft tissue management.Zirconia’s polumorphism is characterized by three crystal forms( monoclinic on room temperature, tetragonal at 1170℃ and cubic at 2370 ℃). To avoid breaking into pieces on cooling, pure Zirconia can be stabilized by adding CaO, MgO, and Y2O3 (Yttrium). The result will be a partially stabilized zirconia (PSZ) – a multiphase material combining cubic, monoclinic, and tetragonal phases. By adding Yttrium at room temperature, tetragonal zirconia polycrystals (TZP) can be obtained. Containing tetragonal phase only, Yttria stabilized TZP is suitable for biomedical application due to the low porosity, high density, high bending and compression strength (Saini, 2015). Considering its mechanical properties suitable for high-load situations, the radiopacity, the ivory color similar to natural tooth’s color, zirconia processed with dental CAD/CAM systems is now becoming the first choice in treating esthetic implant cases (Shrestha, 2014).

Most studies on titanium and zirconia implants showed similarity of osseointegration and attachment to bone. Yet,there are a number of studies that sustain the idea that the contact between bone and implant is higher for zirconia than titanium. There are certain aspects – such as tensile strength and modulus of elasticity -of zirconia implants that can be brought into question due to lack of long term clinical trials. However, ongoing studies give encouraging results of zirconia because of aesthetically pleasing appearance, good biocompatibility and osseointegration (Shrestha, 2014).

Carbon and Carbon Silicone Compounds

Carbon based biomaterials with minimal host response have also been used as coatings on metallic implants.Better cell attachment was seen during in vitro testing on carbon coated zirconia compared to uncoated disc. Unlike other types of materials used before, these carbonaceous materials do not suffer from fatigue. The limitations in load bearing applications are referring to intrinsic brittleness and low tensile strength. However, higher fracture loads than forces in mastication were shown in a single type of carbon blade dental implant (Shrestha, 2014).

Titanium-zirconium alloy (Straumann Roxolid)

In comparation with pure titanium, titanium-zirconium alloys with 13%-17% zirconium (TiZr1317) presents improved mechanical properties, such as increased elongation and the fatigue strength. It has been proven that osseointegration is not negatively influenced by Titanium or Zirconium. Roxolid, a material 50% stronger than pure titanium developed by Straumann, satisfies most requirements in implantology. Due to its better mechanical properties and its similar good biocompatibility as pure titanium, TiZr1317 can be used to produce thin implants and implant components (Saini, 2015).

Future Trends

The advance of nanotechnology has offered the possibility of developing new approaches for controlling the implant surfaces. Lately, one supperior approach involving computer assisted moddeling eased the manipulation of the key features of polymer nanocomposites (PNC), such as shape and pore distribution and size. Nonetheless, it is yet to be established in time whether the nanopattering or micron scale pattering is the most convenient choice. Studies on flat surfaces have not really proven the advantages of nanopillar dense surface since there is the suspicion that nanopillars increase the hydrophobicity to an extent that might disfavor their use in implantology. Hence, further studies should be performed for establishing adequate coating composition and a correlation between the composite features and the specifics of implantation area, in terms of thickness, surface topography, cell response, as a first step for developing a comprehensive dental implants library for the treatment of various conditions (Shrestha, 2014).

INTRODUCTION – CURRENTLY USED DENTAL IMPLANTS

To ensure a better osseointegration between titanium and the bone tissue, a large spectrum of implant configurations have been employed. Commonly used are the endosseous (blade-like, pins, cylindrical, disk-like, screw shaped, and tapered with screw shaped), subperiosteal and transmandibular implants.

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Subperiosteal implants

Before the era of osseointegration, implants meant to support total or pation dentures came in different designs but even though some would be functional for long time periods, the rated of success was highly unpredictable. Goldbers and Gershkoff described in 1949 the first subperiosteal implant which of all the commonly used frameworks and devices, had the longest clinical trial. These devices are not osseointegrated as they are not fixed inside the bone tissue, but designed to cover what is left of the original bone structure (Albrektsson, 1986). Imagined to be used as removable devices, subperiosteal implants, the success rate was rather poor (50%) as the bone was continually remodeling (Noorthoek, 2013).

Constructed from metallic materials such as alumina, Vitallium, titanium and carbon coatings, subperiosteal implants had been used for total edentulous mandibles and maxillae. Complications had been reported, nonetheless: pain and swelling, lacerations, bone resorption. A long study concluded that this type of implant had unacceptable high failure rates and since it has not been used until today (Albrektsson, 1986).

The vitreous carbon implant

Made mostly of pure carbon (99.99%), vitreous carbon implants have a core of stainless steel. They may be used as single units or with the use of an adjacent teeth. Although promising results are coming from animal studies, none of clinical trials provided acceptable outcomes. There were reported cases of bone loss, osteomyelitis or paresthesia.

Blade-vent implants

Known as an endosseous implant, the “blade implant” was presented around the same time as the transosseous implant. Despite the fact that various types of blade-vent implants, made of various materials such as CrNiVa-alloy, aluminum oxides, titanium alloy or vitreous carbon – have been used clinically over the last decades, it is no longer used due the low success rate (under 50%). The problem with this type of implant was that it became loose, infected, and had to be removed (Moldovan, 2013). Complications of blade-vents (soft tissue problems, infections or loss of bone mass) determined most of the time the removal of the implant (Albrektsson, 1986).

The single-crystal sapphire implant

Made of aluminium oxide, single-crystal implant consists in one piece with three components: the screw, a collar and the gingival component. This implant is not dedicated for usual clinical applications since the research in this direction is still at preliminary stages. Although there are a few reports about the a good evolution in time of implants, they have less than five years (Albrektsson, 1986).

The Tübingen aluminum ceramic implant

First described by Schulte and Heimke, this aluminum oxide implant is used to replace teeth in both maxilla and the mandible. It is shaped like an irregular conical cylinder. With the surface lacunae to allow for osteocyte ingrowth , there are evidences in studies involving dogs showing a bond between Tübingen implants and bone, without involving soft tissue.

Although the aluminum ceramics is well tolerated into the body, there were some reports of tissue reactions caused by the material. The occasional loss is one of the few complications related with Tübingen implants, which can be easily removed in case of a fracture.

The TCP-implant

The TCP-implant is a cylindrical shaped biodegradable ceramic coated titanium device. The coating, made of tri- and tetra calciumphosphate, was proven during in vivo studies to ensure better biocompatibility as the bone cells are able to achieve direct contact to the implant, and a gradual replacement of the coating with the rate of bone tissue growth. With little but complications reported, but inconsistent proof of its outcome, this implant was not clinically used.

The TPS-screw

The TPS-screw, first described by Ledermann is made of titanium with a plasma-sprayed surface. Histological studies revealed that TPS-screws ensure a proper contact between the bone and the titanium, as not soft tissue is interposed between them. Recommended for edentulous mandible reconstruction, the TPS-screw was subjected to extensive clinical investigations, with acceptable results after both five and ten years and even longer.

The ITI hollow-cylinder implant

The ITI hollow-cylinder implant is a hollow device designed to ensure the increasing of implant-bone surfaced contact, as it presents holes along its body that connects the exterior to the interior which favor the immobilization of the implant in the regeneration bone tissue.

As the TPS-screw, the ITI hollow-cylinder is a titanium made device with a plasma-sprayed surface (Noorthoek, 2013).

Experimental studies reveal biological and physical data regarding these implants. Employed only for mandible insertions, the ITI implants are single (type K) or double-hollow cylinder (type F). The specialized literature reports positive outcomes of their use in dental implantology, and despite the fact that the five year studies have not yet published, there are indications that these implants will have a success rate of more than 85%, which is the minimal criteria for their commonly employment in clinical use (Noorthoek, 2013)..

The IMZ dental implant

The IMZ is a cylindrical shaped titanium which has been usually used for toot replacements in edentulous mandibles. So far, no specific complications caused by the IMZ implant have been reported. A disadvantage may be considered in the case of removal of a wing-type implant where the procedure would involve harming the bone.

The Core-Vent titanium alloy implant

The idea behind the development of an implant with the Core-Vent system was similar to the hollow ITI implant, as it would increase the contact area with the local tissue for an improved bone regeneration. Recommended for single or multiple tooth replacements, the Core-Vent implant is a screw shaped porous device made of Titanium-6 Aluminum-4 Vanadium alloy. In vivo preclinical histological tests reveal contact between the tissue and the implant, with no complications associated to the insertion.

The transosteal, mandibular staple bone plate

The transosseous implant was a dental concept introduces in 1968 by Dr. Small. The principle behind the idea is based on the mandible transversation of the implant for bottom to the top; primarily designed for mandible reconstructions, the trasnosseous implants were obtained from gold or titanium alloys, as well as ceramic coated stainless steel. The unfavourable outcome of the implant after 10 to 16 years lead to its clinical replacement with endosseous implants (Moldovan, 2013).

The Brånemark osseointegrated titanium implant

This implant is screw-shaped titanium with an unique machine-produced surface, featuring micro-irregularities. Created for both edentulous mandible and maxilla, the studies regarding its employment do not report severe complications. Implants that were not properly anchored and failed shortly after insertion were successfully replaced with new ones in the same socket. With one of the most impressive success rate, the development of Brånemark osseointegrated titanium implant is mentioned, after 50 years from its first report, as a limestone in dental implantology (Albrektsson, 1986).

GEORGIANA

CRISTINA

Avoiding post-surgical complications

The end of surgery does not necessarily imply the end of all the health problems related to the condition. During the following period of recovery, the patients are faced with reentering their normal routine. In some cases, the recovery only takes a few days or less, enough for the organism to regain its normal functional parameters. However, after major interventions, the recovery could be difficult and last several weeks, during which the patients should be careful while going back to their lifestyle in order to avoid complications related to the surgeries they have suffered (James, 2004).

Most complications are incident short after the interventions and are related to deficient recovery practices; however, some may occur a long time after the surgery and be difficult to trace back to their actual origin. The success of a dental implant depends on the surgeon’s skills, the correlation between the design and chemical composition of the implant and its purpose and the patients’ transition to their regular lifestyle after being subjected to surgery. Poor choice of materials is an important cause of dental implant failure. Most often, the insufficient mechanical properties can lead to the development of fissures or instability which can further cause damage to the surrounding area in the oral cavity: bleeding, tooth decay, biofilm and calculus formation, ion release. Even though in dental implantology is less common than in mandibular, maxillary or alveolar reconstruction, autologus bone grafts remain the golden standard, regardless the multifunctional modern composite materials exhibit. Nonetheless, researchers have been developing new composite biomaterials with improved properties for applications in dental medicine in order to surpass these inconveniencies (Matsunoa, 2010).

The antibacterial and biofilm inhibiting properties of the biocomposites used in dental implantology are linked to their mechanical properties more than for any other specific class of biomaterials as damages or degradation of their structural and mechanical integrity can easily lead to a bacterial infection that could determine further complications or the total failure of the implant. Dietary aspects and saliva are factor which influence the plaque (biofilm) development and also the mechanical stability in time of the biocomposites (Mehdawi, 2013).

In order to surpass the embedding of adhering microorganisms (species such as Streptococcus mutans, Lactobacili and Actinomyces) on the surface of teeth, widely used composite materials such as glass ionomer cements (GICs), compomers and resing modifies glass ionomer cements (RMGICs) were subjected to several in vitro studies to assess their cariogenic activity. The goal of these studies was to help determine the most adequate approaches for modifying their composition in order to ensure a better antibacterial behavior in terms of decreasing or inhibiting the biofilm development (Mehdawi, 2013).

Several approaches were attempted: direct incorporation of antibacterial agents, immobilization of antibacterial monomers/polymers, silver, fluoride or calcium phosphate doping. Both commercial and non-commercial dental composited were subjected to the study, for a comparative assessment. Pharmaceutical agents, like Triclosan, BAC (benzalkonium chloride) and CHX (chlorhexidine) inhibited the microorganism growth by interfering with the enzymatic cycles or bacterial cell membrane deterioration. However, it was observed a slight decrease mechanical properties. The bactericidal monomer MDPB (12-methacryloyloxydodecylpyridinium bromide) was shown to actively inhibit the negatively charged microorganism proliferation by disrupting the cell wall, with no biological, chemical or mechanical impact. On the other hand, it was proven to be effective at the surface of the implant and only upon contact, leading to the conclusion that it is not likely to avoid recurring development of biofilms (Mehdawi, 2013).

Composites minimally doped with silver fillers (silver silica gel, silver-zirconium phosphate or silver zeolite) exhibit good antibacterial properties but only upon contact with the microorganism. Moreover, it was determined that the colour stability and mechanical properties are affected. The fluoride containing composited are known to improve the resistance to biofilm development, but with lower rate of success. However the incorporation if amorphous calcium phosphate lead to developing composite with better remineralizing behavior and mechanical properties and a promising ability to inhibit carious enamel and dentine which should be further investigated (Mehdawi, 2013).

Silver nanoparticles have been widely used as doping or coating agents for their excellent antimicrobial properties determined in numerous studies. A group of researchers under the direction of Espinosa-Cristóbal aim to evaluate ex vivo the adherence rate of nanosilver exposed Streptococcus mutans on dental enamel. Three sizes of spherical and pseudospherical nanosilver was employed in this study. The inhibition of bacterial proliferation was massive when S. mutans was exposed to the small nanoparticles (9.3 nm, 21.3 nm) and rather poor for the 98 nm silver particles. It was suggested that the small nanoparticles penetrated the membrane the metabolic cycles of the cell and leading to cellular death (Espinosa-Cristóbal, 2013).

To reduce the bacterial proliferation and support the mineralization process, Chatzistavrou et al. a nanosilver doped bioactive glass for incorporation into dental composites. To assess the influence of the silver nanoparticles on the dental composite, bioactivity tests and mechanical measurements were performed. The bioactivity of the novel composite was evaluated by repeatedly immersing samples put in contact with natural tooth tisue in simulated body fluid. During the antibacterial tests performed on E. coli and S. mutans, significant results were registered. The mechanical tests revealed unimportant changes from the property evaluation of silver free dental composites. However promising, the study mentions a drawback from a wide use of this novel composite material; silver oxidation might prevent its use on the anterior side of the teeth, but will be useful for posterior reconstructions (Chatzistavrou, 2015).

Silver nanoparticles could also indirectly influence the mechanical properties of biomaterials used in dentistry. Asmussen et al. proved that the polymerization of methacrylate and epoxy resins used in dental implantology is enhanced by the presence of nanosilver. It was shown that due to the heat release when irradiated, silver nanoparticles enable and speed up the polymerization and also ensures a better degreeof polymerization. This novel technique stands out as a more environmental friendly approach since it requires a lower energy consumption and less special equipment. Moreover, the high degree of polymerization is a crucial factor that ensures superior mechanical properties (Asmussen, 2015).

The promising activity of calcium phosphate was evaluated in a study by Zhang et al. who subjected the idea of a calcium phosphate nanoparticles doped composite for long term ion release. The basic principle this proposal was constructed on is the need of Ca and P ions for continuous remineralization of the tooth as a way to resist the degradation in the oral cavity. Moreover, reminaralization would be able to neutralize the acids resulted from the biofilm development and better preserve the exterior structure of tooth and the avoid complications such as pulp perforation (Ling).

A similar concept was subjected by Wu et al., who proposed the incorporation of calcium phosphate nanoparticles in dental composites as a self-healing and antibacterial agent. The successful bactericidal properties were evaluated by measuring the biofilm lactic acid production and by counting the biofilm colony-forming units. The possession of self-healing properties was proved through reminaralization studies in case of superficial fractures (Wu, 2015).

Nanosilica is another material which has gained a lot of interest in dental restoration and implantology destined composites designing due to its suspected ability to improve their mechanical properties. Atai et al. synthesized porous 12 nm silica particles and incorporated them in a matrix of Bis-GMA and TEGDMA. Similarly, microsilica was employed as filler in similar matrices for a comparative evaluation of the impact size has on the composite’s flexural strength, elastic modulus and fracture toughness. Overall, the registered results were compared to the ones obtained for various commercially used nanocomposite, Filtek Supreme® Translucent (Atai, 2012). Improvement related to the mechanical properties of a commonly used dental composite was highlighted in a report by Samuel et al. The team of scientists correlatively investigated the effect of mesoporous and nonporous spherical silica on resin matrices, aiming to achieve a more resistant dental composite toward wear and hydrolysis. The mechanical measurements were equivalent to the ones subjected for a commercially available composite containing porous and nonporous fillers – SolitaireTM (Samuel, 2009).

Dental resins mechanical properties have also been reported improved by incorporating titanium dioxide nanoparticles. Sun et al. subjected a paper regarding the dramatic changes in elastic modulus and hardness (up to 48% for 0.06% mass fraction of nanoparticles) small amounts of titanium dioxide nanoparticles brought to common dental resins. Another advantage of their incorporation derives from their photoactivity and crosslinking potential which enhances the polymerization and ensures superior stability at hydrolysis (Sun, 2011). Moreover, Yan et al. studied the possibility of obtaining a superior composite coating for titanium dioxide nanotubes with a broad applicability in the field of biomaterials. They prepared a mixture of silver dopes chitosan and Hap for further coating TiO2 nanotubes. The porous, biocompatible and antibacterial structure they obtained presented essential properties (cell adhesion, mechanical strength) that could favor its safe use as implant coatings or hard tissue superficial scaffolds (Yan, 2015).

Promising results regarding post-implantation remineralization (Hap layers growth) were also obtained by a group of researchers lead by Kumar and Singh. They synthesized a leucite based glass ceramic composite with required mechanical properties for dental applications. The in vitro study the novel material was subjected to revealed adequate biocompatibility in terms of SCC-25 cells (human buccal epithelial cell line) proliferation; it was also demonstrated that it possessed an interesting cytotoxic effect towards tumor cell lines which could further be exploited for the development of a versatile restorative and anti-tumor dental material (Kumar, 2015).

In a long term study conducted by Weir et al., the improved resistance to wear and aging of calcium fluoride nanoparticles doped composite was investigated. Based on the promising remineralization properties both fluoride and calcium exhibit, the group of researchers doped commercial and experimental fillers with 53 nm CaF2 nanoparticles and glass particles. The wear of the novel composite was proven to be within the range accepted for commercially used standard materials. A significant aspect is, however, related to an improved stability of these materials in the oral environment which could recommend them for restorations associated with lower rates of aging and thus better caries inhibiting properties (Weir, 2012).

Polymer based composites have been widely used in modern dental implantology. Lately, mixtures of polymers and hydroxyapatite attracted more interest due to their improved properties gained thanks to the excellent biocompatibility of the calcium phosphate compound. Taheri et al. have researched more eligible similar composites which would exhibit a required behavior for dental applications and, further more, increased resistance to secondary carries as a result of a better stability when exposed to body fluids. In this regard, their novel design was based on manufacturing fluoride containing apatite in the shape of nanorods as a doping agent for dental fillers. The evaluation of mechanical and biological characteristics of the composite showed promising results as it complies with the properties of approved materials and exhibit good antibacterial behavior as a result of released fluoride ions mediated pH reducing. Another significant achievement of their research consists of manufacturing nanorods geometrically similar to the enamel for increased biocompatibility (Taheri, 2015).

A stronger resistance of the dental implants to physical and mechanical factors was linked to composite resins containing polymer grafted hydroxyapatite whiskers as filler. Nanowhiskers are a type of crystal nanofibers with a diameter of less than 100 nm and the ratio between their length and their diameter > 100. Liu et al. revealed that poly bisphenol A glycidyl methacrylate (Poly(Bis-GMA)) grafted to previously silanized Hap whiskers exhibit a biocompatible behavior associated to a remarkably enhanced flexural strength and good physical stability in terms of volume preservation which has positive impact on the durability of the implant. Although promising, further explorations are needed to determine the ideal ratio of polymer and inorganic phase for best property conservation (Liu, 2013).

CONCLUSIONS:

Nanotechnology and the restless struggle for developping new better nanomaterials with applications in the biomedical field has had an impact upon dentistry in terms of improving the quality of materials and also in the conceptualizing of upgraded shaping technologies used for implant manufacturing. In due time, oral health will probably be maintained through a most advanced level of biotechnology and nanorobots assisted life-long process. Nonetheless, despite the advantages of nanotechnology, it does not come without risks when employed. Straightforward directions should be established when the scientific community reaches a high level of understanding of the processes that happen at nanoscale, when nanotechnology can be strictly controlled when there is undoubtful proof of its safe use for the benefits of human health.

REFERENCES:

SAINI, J., SINGH, Y., ARORA, P., ARORA, V, JAIN, K., (2015) Implant biomaterials: A comprehensive review, World J Clin 16, 3(1), p. 52-57.

SHRESTHA, S., JOSHI, S., (2014) Current concepts in biomaterials in dental implant, Science Research 2(1), p.7-12.

MCCRACKEN, M., (1999) Dental Implant Materials: Commerciallv Pure Titanium and Titanium Alloys I, Journal Orthodontics, 8, 1 p. 40-43.

ALBREKTSSON, T., ZARB, G.,WORTHINGTON, P., ERIKSSON, A.R., (1986) The Long-Term Efficacy of Currently Used Dental Implants: A Review and Proposed Criteria of Success, JOMI, 1(1), p. 11-25.

NOORTHOEK, D.R., (2013) Macroscopic and Microscopic Dental Implant Design: A Review of the Literature, Thesis presented at the University of Florida for the Degree of Master of Science.

MOLDOVAN, S., (2013) Dental Implants: A Comprehensive Review, Continuing Education Course.

RAKHSHAN, H., RAKHSHAN, V., (2015) Effects of the initial stage of active fixed orthodontic treatment and sex on dental plaque accumulation: A preliminary prospective cohort study, The Saudi Journal for Dental Research, 6, 2, p.86–90.

WONG, L., Sissons, C.H., (2007) Human dental plaque microcosm biofilms: Effect of nutrient variation on calcium phosphate deposition and growth, Archives of oral biology, 52, p.280–289.

ASIKAINEN, S., ALALUUSUA, S., (1993), Bacteriology of dental infections, European Heart Journal, 14, p.43-50.

JAMES, M., ANDERSON, COOK, G., COSTERTON, B., HANSON, S.R., HENSTEN-PETTERSEN, A., JACOBSEN, N., JOHNSON, R.J., MITCHELL, R.N., PASMORE, M., SCHOEN, F.J., SHIRTLIFF, M., STOODLEY, P., (2004) Host Reactions to Biomaterials and Their Evaluation, BIOMATERIALS SCIENCE, An Introduction to Materials in Medicine 2nd Edition, Elsevier Inc.

MATSUNOA, T., OMATAA, K., HASHIMOTOB, Y., TABATAA, Y., SATOH, T., (2010) Alveolar bone tissue engineering using composite scaffolds for drug delivery, Japanese Dental Science Review, 46, p.188–192.

MEHDAWI, I.M., YOUNG, A., (2013) Antibacterial composite restorative materials for dental applications, Non-metallic Biomaterials for Tooth Repair and Replacement, 2013, Woodhead Publishing Limited.

ESPINOSA-CRISTÓBAL, L.F., MARTÍNEZ-CASTAÑÓN, G.A., TÉLLEZ-DÉCTOR, E.J., NIÑO-MARTÍNEZ, N., ZAVALA-ALONSO, N.V., LOYOLA-RODRÍGUEZ, J.P., (2013) Adherence inhibition of Streptococcus mutans on dental enamel surface usingsilver nanoparticles, Materials Science and Engineering C, 33, p.2197–2202.

CHATZISTAVROU, X., VELAMAKANNI, S., DIRENZO, K., LEFKELIDOU, A., FENNO, J.C., KASUGA, T., BOCCACCINI, A.R., PAPAGERAKIS, P., (2015) Designing dental composites with bioactive and bactericidal properties, Materials Science and Engineering C, 52, p.267–272.

ASMUSSEN, S.V., ARENAS, G.F., VALLO, C.I., (2015) Enhanced degree of polymerization of methacrylate and epoxy resinsby plasmonic heating of embedded silver nanoparticles, Progress in Organic Coatings, 88, p. 220–227.

LING, Z., WEIR, M.D, HACK, G., FOUAD, A.F, HOCKIN H.K., XU, H., .Rechargeable dental adhesive with calcium phosphate nanoparticles for long-term ion release, Journal of Dentistry http://dx.doi.org/10.1016/j.jdent.2015.06.009.

WU, J., WEIR, M.D., MELO, M.A.S., HOCKIN H.K., XU, H., (2015) Development of novel self-healing and antibacterial dental composite containing calcium phosphate nanoparticles, Journal of Dentistry.

ATAI, M., PAHLAVAN, A., MOIN, N., (2012) Nano-porous thermally sintered nano silica as novel fillers for dental composites, Dental Materials, 28, p.133-145.

SAMUEL, S.P., LI, S., MUKHERJEE, I., GUO, Y., PATEL, A.C., BARAN, G., WEI, Y., (2009) Mechanical properties of experimental dental composites containing a combination of mesoporous and nonporous spherical silica as fillers, Dental Materials, 25, p. 296-301.

SUN, J., FORSTER, A.M., JOHNSON, P.M., EIDELMAN, N., QUINN, G., SCHUMACHER, G., ZHANG, X.,WU, W., (2011) Improving performance of dental resins by adding titanium dioxide nanoparticles, Dental Materials, 27, p. 972-982.

YAN, Y., ZHANG, X., LI, C., HUANG, Y., DING, Q., PANG, X., (2015) Preparation and characterization of chitosan-silver/hydroxyapatitecomposite coatings onTiO2nanotube for biomedical applications, Applied Surface Science, 332, p.62–69.

KUMAR, P.H., SINGH, V.K., SRIVASTAVA, A., HIRA, S.K., KUMAR, P., MANNAB, P.P., (2015) Mechanochemically synthesized leucite based bioactive glass ceramic composite for dental veneering, Ceramics International.

WEIR, M.D, MOREAU, J.L., LEVINE, E.D., STRASSLER, H.E., CHOW, L.C., HOCKIN H.K., (2012) Nanocomposite containing CaF2 nanoparticles: Thermal cycling, wear and long-term water-aging, Dental Materials, 28, p.642-652.

TAHERI, M.M., KADIR, M.R.A., SHOKUHFAR, T., HAMLEKHAN, A., SHIRDAR, M.R., NAGHIZADEH, F., (2015) Fluoridated hydroxyapatite nanorods as novel fillers for improving mechanical properties of dental composite: Synthesis and application, Materials & Design, 82, p.119–125

LIU, F., WANG, R., CHENG, Y., JIANG, X., ZHANG, Q., ZHU, M., (2013) Polymer grafted hydroxyapatite whisker as a filler for dental composite resin with enhanced physical and mechanical properties, Materials Science and Engineering C, 33, p.4994–5000.