Direct resin-based composite restorations—clinical challenges

With the increase in esthetic awareness and the widespread use of adhesive techniques, a resin composite has become the material of choice for anterior and posterior restorations. In spite of its advantages, polymerization shrinkage and the resulting stress are still a concern for clinicians. The shrinkage stress may cause cusp deflection, interfacial debonding or enamel cracks, leading to microleakage, post-operative sensitivity and secondary caries. The micro-mechanical nature of the interaction of dental adhesives with enamel and dentin is a result of the infiltration of resin monomers into the microporosities left by the acid etching and subsequent enveloping of the exposed enamel hydroxyapatite crystals and the dentin collagen fibers with the resulting polymer. The latest development in dental adhesives is based on simplification and reduced application time of the bonding procedure by using non-rinsing adhesives. These non-rinsing (or self-etch) adhesives do not require a separate acid-etch step as they condition and prime enamel and dentin simultaneously by infiltrating and partially dissolving hydroxyapatite to generate a resin-infiltrated zone with minerals and smear layer incorporated. In spite of their user-friendliness and low technique sensitivity, self-etch adhesives have resulted in low bonding effectiveness, behaving as semi-permeable membranes to allow movement of water across the dentin-resin interface and potentially leading to hydrolytic degradation. Recently, the role of metalloproteinases (MMPs), in inducing degradation of the dentin extracellular matrix components, has been highlighted in the literature along with the potential role of chlorhexidine in inhibiting the deleterious role of MMPs. The ultimate goal of a bonded restoration is to attain an intimate adaptation of the restorative material with the dental structure. This task is difficult to achieve as the bonding procedure is different for enamel and for dentin. The bonded interface undergoes degradation with time when a hermetic seal is not achieved. On the other hand, an improved sealing results in decreased post-operative sensitivity and higher resistance to the caries process. The stress resulting from the polymerization shrinkage of resin composites is also a factor that interferes with the bonding procedure as it may cause interfacial leakage and cusp deflection.