Novel Method to Sequentially Measure Polymerization Shrinkage

Polymerization shrinkage of resin based restorative materials remains significantly problematic. Objective: This study measured and compared the polymerization shrinkage of a resin based composite resin to a resin modified glass ionomer after storage in 100% humidity. Methods: A new modification of the Accuvol (Bisco) was used to measure polymerization shrinkage with a removable keyed pedestal that allowed for precise replacement into the same original position, facilitating sequential measurements after storage in 100% humidity. Two groups of 10 each samples were fabricated: Group A- Aelite LS, a resin based low shrink composite and Group B- Fuji II LC, a resin modified glass ionomer. Each specimen was shaped and placed on the newly developed pedestal for the Accuvol and then the specimen was polymerized with a Bisco VIP light (output >750mw/cm2) for 40 seconds. Following polymerization and post curing (2 hours), the pedestal with specimen was placed in the Accuvol and the shape of the polymerized specimen was imaged. The polymerization shrinkage (volume %) of each specimen (n=10) was calculated and expressed as per cent volume shrinkage. After storage in 100% humidity at 37 Celsius the polymerization shrinkage was again determined for each group at one, two and three weeks. Results: Mean polymerization shrinkage for each material was tabulated and subjected to statistical analysis. ANOVA and Tukey's Multiple Test post-hoc tests (p=.05) determined that the expansion after hydration was significantly greater for the RMGI Group. The Aelite LS did not demonstrate any significant change for any of the time periods. Conclusions: RMGI reportedly has significantly greater polymerization shrinkage than low shrink resin based composite. After storage in 100% humidity at 37degrees Celsius, the RMGI group demonstrated significant expansion due to water absorption averaging 6%at one week, 6.2%at two weeks and 7.4%at three weeks.