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 (X(1)), flow (X(2)), and gap formation (r=0.79, P(1)<0.05, P(2)<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, P(1)<0.005, P(2)<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.