The hard dental tissues, mainly enamel and dentine, are natural composites made of collagen and hydroxyapatite (HA) micro crystals. The use of composite materials in dentistry has considerably grown in the last years.
Dental restorative material
Composite dental restorative materials have advanced considerably over the past 10 years. Although composites have not totally replaced amalgam, they have become a viable substitute in many situations. Problems still exist with polymerization contraction stress, large differences in the coefficient of thermal expansion of composites compared with tooth structure, and with some technique sensitivity; however, new expanding resins, nanofiller technology, and improved bonding systems have the potential to reduce these problems.
With increased patient demands for esthetic restorations, the use of direct filling composite materials will continue to grow. The one major caveat to this prediction is that clinicians must continue to use sound judgment on when, where, and how to use composite restoratives in their practices. The most critical aspects for long-term performance of dental composites are the dimensional stability, following the polymerization shrinkage, wear resistance, and mechanical properties. Wear and mechanical properties strongly depend on the filler-matrix adhesion; microcavities, resulting from material detachment, could play a decisive role in the adhesion of bacterial plaque. Polymer matrix composites are used in restorative dentistry to fill cavities, to restore fractured teeth and to replace missing teeth. In most applications, dental composite consists of a polymeric acrylic or methacrylic matrix reinforced with ceramic particles. The commercial formulations of matrices are mainly based on bisphenol-A-glycidyldimethacrylate (Bis-GMA); triethylenglycoldimeta-chrylate (TEGDMA) is added to reduce the viscosity.
Polysiloxane can also be used as matrix material. The major concern associated with the use of Polysiloxane as polymer matrices in dental restorative materials, is the generally modest mechanical properties of the polymers. However, it has long been demonstrated that thermal stability and mechanical properties of Polysiloxane can be substantially modified by incorporation of bulkier substituent such as phenyl groups or more polar groups in the chains. In study of Chia-Yin Chen, et al , epoxy acrylates modified with various stoichiometric amounts of urethane acrylates were used as the resin matrix and 3-(trimethoxysilyl) propyl methacrylate (MSMA)-modified monodisperse nano-sized SiO2 particles were synthesized and used as the inorganic reinforcing filler. Polymer composites suitable for using as dental restorative materials were formed by curing the resin matrices containing MSMA-SiO2 with visible-light. The viscosities of the obtained resin matrices, their polymerization shrinkages, hardness, flexural strengths, and flexural moduli of the obtained polymer composites are better in comparison with bisphenol-A/glycidyl dimethacrylate (Bis-GMA)-based dental restorative materials.
HA is the mineral component of natural hard tissue. It has been studied by Viviane V.Silva, et al extensively as a candidate biomaterial for its use in prosthetic applications. However, the main weakness of this material lies in its poor mechanical strength which makes it unsuitable for load-bearing applications. On the other hand, partially stabilized zirconia has been widely studied on account of its high strength and fracture toughness, and good biocompatibility. Therefore, it is believed that the addition of a particulate zirconia phase to a hydroxyapatite one may lead to an improvement of the mechanical properties of this kind of composite and will not affect its biocompatibility. In this study, two series of zirconia- hydroxyapatite composites, Z4H6 and Z6H4 with 40 and 60 vol.% of zirconia content respectively, were prepared by powder uniaxial pressing at 700 MPa and sintering in air at 1200-1500°C for 3 hours. The sintering behavior, microstructural characteristics and mechanical properties were evaluated. Variation of average grain size for the zirconia and hydroxyapatite phases with sintering temperature was observed. Relative densities ranging from 89 to 91% of the theoretical values were reached for the Z6H4 ceramic series and for the Z4H6 sample sintered at 1400°C, although micro cracks were present in all specimens sintered at 1400 and 1500°C. Values of ultimate compressive strength, Young’s modulus, micro-Vickers hardness and Poisson’s ratio near to those for human cortical bone and human tooth (dentine and enamel) were found for almost all samples investigated suggesting that these materials present potential applications as structural implants.
Currently used dental implants are made of dental alloys but due to cytotoxic problems, high strength ceramic materials appear as an interesting alternative to these alloys. In order to combine good mechanical properties and promotion of the osseointegration process, a ceramic composite material composed of 10 wt.% tricalcium phosphate (TCP) mixed with partially stabilized zirconia has been elaborated. Such a ceramic has been implanted into albino rats, the sites of implantation being the adrenal gland for the toxicity evaluation and the sphenoid bone to estimate the osteogenesis potential. These results have been compared to those obtained with implants made of alumina (Al2O3), zirconia (ZrO2) or polyethylene terephthalate (Dacron®). In all cases no rejection effect was observed. The histological study indicated that ZrO2-TCP ceramics induced marginal and subjacent cell disorganization. However, the existence of an irregular cellular band indicated a cellular colonization process on the TCP particles.
The microscopy study of the implants after removal confirmed the bioresorption of TCP. The biocompatibility of zirconia-alumina (ZA) nano-composites in load-bearing applications such as dental/ orthopedic implants was significantly enhanced by the addition of bioactive HA.
According to study of Fujihara K., et al, functionally graded composite dental post has continuously varied stiffness and this function successfully solved stress concentration at the root. Aesthetic composite archwires made of glass /epoxy unidirectional composite has targeted to obtain the highest bending performance by interface control. An epoxysilane coupling agent with 1.0 wt. % solvent showed the highest bending performance. Aesthetic composite bracket has fabricated by braided preform in order to reinforce tie-wing slot where orthodontic force is applied through an archwire. Composite brackets have indicated around 43% mechanical resistance of stainless-steel bracket at tie-wing slot. Bending performance of the braided composite compression bone plates is investigated. The result indicates that the braided composite plate with 2.6 mm thickness can be suitable for forearm treatment whereas the braided composite plate of 3.2 mm thickness is applicable to femur or tibia fixation.