Effect of post-polymerization heat-treatments on degree of conversion, leaching residual MMA and in vitro cytotoxicity of autopolymerizing acrylic repair resin

Objectives: This study evaluated the effect of post-polymerization heat-treatments on degree of conversion (DC), residual methyl methacrylate concentration (MMA_r) and in vitro cytotoxicity of autopolymerizing acrylic repair resin. Methods: A total of 336 specimens were prepared by bench- and hydroflask-curing and subjected to post-polymerization heat-treatments: a) water immersion at 60 °C for 30 min, b) microwaving at 500 W for 3 min, c) combined use of water immersion and microwaving d) no treatment (as control). Specimens were eluted in cell culture medium for 1, 2, 5 and 7 days. DC and MMA_r in eluates were measured by FTIR spectrometry and HPLC, respectively. In vitro cytotoxicity of eluates on L-929 fibroblasts was determined by XTT assay. Data were statistically analyzed with Dunn's multiple comparison and Pearson correlation tests (p ≤ 0.05). Results: DC was highest (99.9%) in bench- and hydroflask-cured groups which were subjected to water immersion. At all elution periods, MMA_r was detected in eluates of all treatment groups and were higher in bench-cured groups than hydro-flask cured groups. Cell proliferation values indicated slightly cytotoxic effect throughout 7 days; regardless of the curing method or post-polymerization treatment. The correlation between MMA_r and cell proliferation was negative after elution of 1, 2, 5 days and was only statistically significant (p < 0.05) at 5 days. At elution of 7 days, the correlation was positive with no significance. Significance: Post-polymerization heat-treatment of autopolymerizing acrylic repair resin by immersion in water at 60 °C for 30 min is clinically recommended to improve the DC while reducing the leaching residual MMA.