Why Resin Performance Should be Evaluated at Mouth Temperature

Objectives: The degree of polymerization of dental resins can be determined from microhardness recordings, or by calculating the degree of conversion from Fourier Transform Infrared Spectroscopy (FTIR) recordings. In the mouth resin polymerization occurs not at room temperature, but at approximately 35^oC. Temperature has an effect on resin polymerization, but the effect of temperature is often not considered in most studies.

Methods: Two millimeter thick specimens of composite, Tetric Evoceram A2, were cured using a SmartLite IQ2 light on a Tensor 27 FTIR machine. The temperature of the composite on the Attenuated Total Reflectance detector was set at either at 22^o or at 35^oC. The polymerization reaction and degree of conversion was monitored at the bottom of the composite in real time for the first 60 seconds and a final recording was made two hours later. The polymerization rates and final degree of conversion for the specimens cured at 22^o and 35^oC were compared. The specimens were then removed and the Knoop microhardness at the top and bottom was measured at 9 locations in the center of the specimen. Three specimens were made at each temperature.

Results: The final degree of conversion at 22^oC and 35^oC was 55.4% ± 0.6% and 64.7% ± 1.0% respectively. The Knoop microhardness at 22^oC and 35^oC was 39.3 ± 1.1 and 51.4 ± 1.0 respectively at the bottom. The temperature of the composite had a significant effect on the rate of polymerization, the final degree of conversion, and the Knoop hardness (p<0.05). Regression analysis showed an excellent correlation between degree conversion and hardness r² >0.9.

Conclusion: These results have significant implications when designing bonding, polymerization shrinkage, and physical property studies. To produce clinically relevant results, resins should be cured at mouth temperature and not at room temperature. NSERC and Dalhousie University supported this study.