Composites are a combination of two or more classes of materials. In dentistry, the most common compos- ite is a combination of a polymer and ceramic, where the polymer is used to bind ceramic particles. The polymer functions as the matrix in dental composites and the particles are reinforcing materials. Polymer matrix composites, also known as resin composites, are used as sealants, intracoronal and extracoronal restorations, provisional restorations, veneers, den- ture teeth, cements, and core buildups. As a class, dental composites are formable, can be made to be machineable, opaque or translucent, moderate in stiffness and hardness, thermal and electrical insu- lators, and sparingly soluble. Many advances have been made in particle technology and are discussed in Chapter 9.
In a composite the properties are intermediate between the two compositional materials. A benefit of combining two material classes is the ability to fabricate a new material that has desirable handling properties that are not achievable with one material alone. For example, individual ceramic particles do not adhere to each other, but the addition of a poly- mer to bind them enables the composite to be used as a paste. Use of a polymer alone will not achieve suffi- cient stiffness and stability, which are properties con- tributed by the ceramic particles. The polymer used in many dental composites, bisphenol A-glycidyl methacrylate (Bis-GMA), has shown no significant health risks when used in dental resin composites.
In industries and professions other than dentistry, other classes are combined to form composites, such as metal-ceramic composites used in aerospace. A commonly used material in the construction indus- try, concrete, is a composite of sand, gravel, and portland cement. As with dental resin composites, the cement in concrete is the binder for the sand and gravel particles. Composites differ from alloys in that, at the microscopic level, the individual compo- nents of the composite are visible.
In the case of concrete, a limiting factor is the adhesion between the portland cement and sand- gravel particles. At the surface, the cement washes away with use, leaving the particles incompletely surrounded by cement. These particles are then eas- ily dislodged, leaving a void in the surface of the con- crete. In dental resin composites, coupling agents are used to enhance the adhesion between the ceramic particles and polymer matrix, thereby increasing its wear resistance and long-term surface integrity.
During polymerization of resin composites, a volumetric contraction of the polymer matrix occurs that results in strain within the matrix. This contrac- tion strain is coupled with the increase in elastic modulus as the composite cures. The combination of contraction strain and development of elastic modu- lus produces stress within the composite because the periphery of the restoration is constrained by adhe- sion to the enamel and dentin cavity walls. Methods to reduce residual stress include the development of polymers with reduced shrinkage during cure and modifications to the clinical placement technique.
More detail on the measurement of polymerization shrinkage can be found in Chapter 5 and the mecha- nism for polymerization can be found in Chapter 9.
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123 Prevention is a foundation of dentistry. Low-level flu- orides in water supplies have provided tremendous benefit in reducing the incidence of dental caries in children. Fluorides can be introduced into community water supplies to ensure systemic ingestion during early life, when the teeth are forming. Fluoride can also be provided as a dietary supplement to inhibit caries where drinking water is not fluoridated. Patients who are at high risk for developing caries in spite of receiv- ing systemic fluoride can be given additional fluoride via topical application in toothpastes, mouth rinses, gels, and varnishes. A combination of systemic and topical fluoride applications has contributed to a dra- matic reduction in the prevalence of smooth surface caries since the 1960s. Pits and fissures on the occlusal surfaces of posterior teeth, however, are more resistant to fluoride uptake because of the irregular morphol- ogy of the occlusal surface. This, combined with the retention of food and the difficulty of proper brushing in the posterior segment, can lead to the start of a cari- ous lesion. A preventive therapy consisting of a sealant to fill in the occlusal irregularities can reduce the risk of caries by creating a smoother surface that is easier to clean and less likely to retain food and harbor bacteria.