Comparative Evaluation of Balance between Polymerization Shrinkage and Hygroscopic Expansion in Bulk-fill Materials: An In Vitro Study

All resin-based materials, including hydrophobic composites, can absorb water and exhibit hygroscopic expansion, which may counteract polymerization shrinkage. 9However, excessive hygroscopic expansion can reverse shrinkage stress and lead to expansion-related failures. 10hile chemically cured resin composites were initially introduced to replace silicate cements, posterior composite restorations remain technique-sensitive and time-consuming due to their need for incremental layering and concerns about depth of cure. 11To address these challenges, bulk-fill composites have been developed. IntroductIonRestorative dentistry demands precise bonding strategies paired with the esthetic properties of resin-based materials. 1Over 500 million composite restorations are performed annually worldwide, making them one of the most common dental procedures. 2 When indirect restorations are needed, esthetic onlays and crowns bonded with resin cement are often the treatment of choice. 3 The success of both direct and indirect resin restorations heavily depends on adequate polymerization of the resin component. 4Polymerization converts monomers into a polymer network, resulting in a volumetric reduction known as polymerization shrinkage.This shrinkage occurs as the distance between monomer chains decreases, transforming weak van der Waals forces into stronger covalent bonds.As the resin hardens, viscosity increases, leading to the "gel point" and eventually vitrification, where the material loses its ability to flow.Before vitrification, some internal stresses may be partially relieved, but postgel shrinkage can still generate significant contraction stress. 5,6or over five decades, polymerization-induced shrinkage stress in resin-based restorative materials has been a clinical concern, contributing to microleakage, secondary caries, marginal breakdown, enamel cracking, and postoperative sensitivity. 7ortunately, some of these stresses are mitigated over time by stress relaxation and water absorption.