Determinants of in vitro gap formation of resin composites

Objectives. To investigate whether polymerization shrinkage, flow, modulus, and bond strength influence marginal gap formation of resin composite restorations in vitro. Methods. Eleven proprietary resin composites were studied. Shrinkage was measured by the ‘bonded-disk method’ (n=3). Flow was measured as the diameter of a constant volume of resin composite to which a load of 20 N had been applied for 60 s (n=3). Modulus was measured in 3-point bending (n=6). Bond strength mediated to human dentin by an adhesive system was measured in shear (n=6). Gap formation was measured in a light microscope along the margins of all-dentin, butt-joint cavities restored with the adhesive system and each resin composite and expressed as the widest gap in percent of the cavity diameter (n=6). Results. Significant differences were found between the resin composites regarding all determined properties. A significant linear correlation was found between flow and gap formation (r=−0.68, P<0.025). Three-dimensional regression analysis showed a significant correlation between polymerization shrinkage (X1), flow (X2), and gap formation (r=0.79, P1<0.05, P2<0.005). The highest coefficient of correlation was found when the first part of the polymerization shrinkage (from 0 to 10 s) was disregarded (r=0.90, P1<0.005, P2<0.0005). Conclusions. With the adhesive system used, polymerization shrinkage and flow were found to be significant determinants of gap formation around resin composite restorations in vitro.