[Year:2021] [Month:January-June] [Volume:6] [Number:1] [Pages:14] [Pages No:31 - 44]
Polymer-based dental composites are the widely used direct restorative materials. However, one of the disadvantages that include polymerization shrinkage, which leads to a cascade of effects like shrinkage stress, cusp deflection, enamel/dentinal cracks, marginal leakage, marginal discoloration, secondary caries, and postoperative sensitivity, needs eradication to improvise their performance. Polymerization shrinkage is the result of covalent bonds replacing van der Waals forces in a polymer. Many factors play a significant role in the polymerization shrinkage development, its magnitude, and the shrinkage stress. Viscosity, glass transition temperature, density, reaction kinetics, molecular mobility, modulus development, gel point, vitrification, degree of conversion, configuration (C) factor, and compliance of bonded surfaces dictate both the quantum of shrinkage and when it occurs and if and how its stresses are relieved. There are three efficient methods of reducing polymerization shrinkage and shrinkage stress. The first one focuses on reducing the reactive groups, the second one focuses on introducing new chemistries, and the third method focuses on controlling curing protocols. Increasing the molar volume and molecular weight of resins helps reduce reactive groups as is increasing the filler content. Alternative polymerization methods like ring-opening polymerization and step-growth polymerization have been very effective ways of reducing shrinkage. Debonding the fillers or using functionalized fillers or using different coupling agents based on chain transfer and dynamic covalent chemistry (DCC) are promising methods to relieve shrinkage stress. Modifying the available methacrylates; introducing certain chemicals; altering solvents, inhibitors, initiators, and coinitiators; using bulk-fill concept; and utilizing ormocer technology are also catching up with moderate to immense potential to reduce either shrinkage or shrinkage stress. Among filler modifications, nanogel incorporation, among matrix modifications, thiocarbonate and allyl sulfide introduction and among coupling agent modifications, using functionalized agents seem to be the best methods in either reducing shrinkage or shrinkage stress. Though laboratory investigations have been very much encouraging the above-mentioned methods, the real test of clinical studies is not as optimistic. Long-term clinical performance of materials based on these various methods has not been clinically significantly different from conventional materials. Nonetheless, some of the new materials have not yet gone through the rigor of long-term clinical studies and hence controlled clinical trials of such materials are highly recommended.