Effect of temperature on composite polymerization stress and degree of conversion

摘要

Objective. To test the following hypotheses: (1) degree of conversion (DC) and polymerization stress (PS) increase with composite temperature (2) reduced light-exposure applied to preheated composites produces similar conversion as room temperature with decreased PS. Methods. Composite specimens (diameter: 5 mm, height: 2 mm) were tested isothermally at 22 ℃ (control), 40 ℃, and 60 ℃ using light-exposures of 5 or 20 s (control). DC was accessed 5 min after light initiation by FTIR at the specimen bottom surface. Maximum and final PS were determined, also isothermally, for 5 min on a universal testing machine. Non-isothermal stress was also measured with composite maintained at 22 ℃ or 60 ℃, and irradiated for 20 s at 30 ℃. Data were analyzed using two-way ANOVA/Tukey and Student's t-test(α = 5%). Results. Both DC and isothermal maximum stress increased with temperature (p＜ 0.001) and exposure duration (p＜ 0.001). Isothermal maximum/final stress (MPa) were 3.4±2.0b/3.4±2.0A (22 ℃), 3.7±1.5b/3.6±1.4A (40 ℃) and 5.1±2.0a/4.0±1.6A (60 ℃). Conversion values (%) were 39.2 ± 7.1c (22 ℃), 50.0 ± 5.4b (40 ℃) and 58.5 ± 5.7a (60 ℃). The reduction of light exposure duration (from 20 s to 5 s) with pre-heated composite yielded the same or significantly higher conversion (%) than control (22 ℃, 20s/control: 45.4 ± 1.8b, 40 ℃, 5 s s: 45.1 ± 0.5b, 60 ℃, 5 s s: 53.7 ± 2.7a, p＜0.01). Non-Isothermal conditions showed significantly higher stress for 60 ℃ than 22 ℃ (in MPa, maximum: 4.7 ±0.5 and 3.7 ±0.4, final: 4.6±0.6 and 3.6±0.4, respectively). Clinical significance: Increasing composite temperature allows for reduced exposure duration and lower polymerization stress (both maximum and final) while maintaining or increasing degree of conversion.