Comparison of post-gel shrinkage strains in light-polymerized composite resins

Statement of problem. Polymerization shrinkage is associated with light-polymerized and polyacid-modified composites. Purpose. This in vitro study compared the polymerization shrinkage of a light-polymerized hybrid composite, a posterior composite, and a polyacid-modified composite. Material and methods. Five disk-shaped specimens (1 cm in diameter and 2 mm in thickness) were prepared in a mold from each of the following test materials: a light-polymerized hybrid composite (3M Valux Plus), a posterior composite (Filtek P60), and a polyacid-modified composite (Dyract AP). The hybrid composite served as the control material. A linear strain gauge was placed at the center of each specimen. After the specimens were light-polymerized for 60 seconds under 400-mW/cm² light intensity, microstrains were recorded with the strain indicator continuously every 4 seconds for 120 seconds. The data were evaluated with 1-way analysis of variance (P<.05) followed by a post hoc least significant difference test. Results. During the first 20 seconds of light polymerization, all materials exhibited tensile strains between 6 and 55 με that quickly converted into compressive strains and continued rising after the light source was removed. The strains for all materials increased at a declining rate until they reached a constant level between 148 and 180 seconds. The polymerization shrinkage recorded for Filtek P60 was the lowest (-190.56 με), followed by 3M Valux Plus (-303.94 με) and Dyract AP (-345.64 με) (P<.05). The dimensional change between Filtek P60 and Dyract AP and between Filtek P60 and 3M Valux Plus was significant (P=.001). However, the difference between Dyract AP and 3M Valux Plus specimens was not significant (P=.208). Conclusion. Within the limitations of this study, the light-polymerized posterior composite tested demonstrated less polymerization shrinkage than the polyacid-modified and hybrid composites tested. Polymerization shrinkage velocity significantly affected the magnitude of strains.