Reduction of polymerization shrinkage in adhesive restaurations

Abstract

Composite materials used for the restoration of lost dental tissues fulfill almost all functional and aesthetic requirements. However, polymerization shrinkage still remains the main problem of light-curing composite resins. The consequences of this phenomenon are microfractures and microleakage in the marginal areas of the composite restoration. The aim of this paper is to present the most important methods for reducing polymerization shrinkage of adhesive restorations, based on the contemporary literature available. These methods include changes in the chemical composition of the composite, correct application technique of the adhesive-composite system and selection of the appropriate light curing protocol. In the last few years, composite materials have experienced significant changes in the chemical composition in order to compensate for their major shortcomings. By introducing pre-polymerized filler particles, alternative matrices based on silorane and ormocer, and new germanium-based photoinitiators, the degree of polymerization shrinkage has been significantly reduced. Optimal adhesive bond between the filling and the tooth can compensate volumetric shrinkage of material which occurs during polymerization. Numerous studies have shown that total-etch adhesive systems, in comparison with self-etch, form a better adhesive bond to hard dental tissues. Polymerization shrinkage can also be reduced and marginal adaptation improved by applying a stress-absorbing layer and preheated composite. With the use of modified light-curing protocols, such as soft start with reduced initial light intensity, and pulse-delayed with relaxation period, the negative effects of polymerization shrinkage and stress development can be further reduced, and the risk of microfracture formation minimized.