Polymerization contraction stresses of resin-based composite restorations within beveled cavity preparations of Class...

To evaluate internal stresses resulting from polymerization of resin-based composite materials in cavity preparations beveled at the internal line angles or the cavosurface margin, compared with those in conventional butt-joint box-shaped cavity preparations.

Contraction stress generated in resin-based composite restorations placed in an experimental cavity preparation model was determined using micro-photoelastic analysis. Three types of cavity preparations simulating Class I restorations with bevels at internal line angles (1 mm and 2 mm deep), and at the cavosurface margin (2 mm deep) were prepared in a posterior composite model in order to obtain no marginal defect with the restorative composite. The transparent restorative composite was bulk-filled into the preparations and light-cured for 80 seconds. Specimens 2 mm thick for micro-photoelastic analysis were cut perpendicular to the long axis of the preparation. Fringe patterns for directions and magnitudes of stresses were obtained using transmitted and reflected polarized light. Then, the photoelastic stress analysis was performed to examine stresses in the preparations. The data were statistically compared with the previous data in conventional butt-joint box-shaped preparations by ANOVA and Scheffé's F test (P < 0.05).

In 1 and 2 mm deep internal beveled cavity preparations, the maximum principal stress lines were parallel to the wall at the straight internal bevels. In the preparation with a round bevel at the cavosurface margin, the stress distribution in the bulk of the preparation was similar to that in the butt-joint preparation. Maximum stress values in 1 and 2 mm deep internal beveled preparations were 12.2 +/- 0.2 MPa and 18.8 +/- 2.8 MPa, respectively. For 2 mm-deep preparation, the maximum stress values in the internal beveled preparations were significantly lower than those values in butt-joint box-shaped cavity preparations. Maximum stress values in the preparations with the bevel at the cavosurface margin were 22.7 +/- 1.0 MPa, and were not significantly different from those values observed in the butt-joint preparations.