Fernanda Calabró Calheiros

Influence of irradiant energy on degree of conversion, polymerization rate and shrinkage stress in an experimental resin composite system

OBJECTIVE This study evaluated the degree of conversion (DC), maximum rate of cure (Rpmax), and polymerization stress (PS) developed by an experimental dental composite subjected to different irradiant energies (3, 6, 12, 24, or 48 J/cm2) under constant irradiance (500 mW/cm2). METHODS DC and Rpmax were monitored for 10 min on the bottom surface of 2-mm thick disks and on 150-microm thick films (representing the top of the specimen) using ATR-FTIR. PS was monitored for 10 min in 2-mm thick disks bonded to two glass rods (Ø=5mm) attached to a universal testing machine. One-way ANOVA/Tukey tests were used and differences in DC and R(p)(max) between top and bottom surfaces were examined using Students t-test. Statistical testing was performed at a pre-set alpha of 0.05. RESULTS For a given surface, DC showed differences among all groups, except at the top between 24 and 48 J/cm2. Rpmax was similar among all groups at the same surface and statistically higher at the top surface. PS also showed significant differences among all groups. Data for 48 J/cm2 were not obtained due to specimen failure at the glass/composite interface. SIGNIFICANCE Increases in irradiant exposure led to significant increases in DC and PS, but had no effect on Rpmax.

Effect of temperature on composite polymerization stress and degree of conversion.

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 pre-heated composites produces similar conversion as room temperature with decreased PS. METHODS Composite specimens (diameter: 5mm, height: 2mm) were tested isothermally at 22°C (control), 40°C, and 60°C using light-exposures of 5 or 20s (control). DC was accessed 5min after light initiation by FTIR at the specimen bottom surface. Maximum and final PS were determined, also isothermally, for 5min on a universal testing machine. Non-isothermal stress was also measured with composite maintained at 22°C or 60°C, and irradiated for 20s at 30°C. Data were analyzed using two-way ANOVA/Tukey and Students 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°C), 3.7±1.5b/3.6±1.4A (40°C) and 5.1±2.0a/4.0±1.6A (60°C). Conversion values (%) were 39.2±7.1c (22°C), 50.0±5.4b (40°C) and 58.5±5.7a (60°C). The reduction of light exposure duration (from 20s to 5s) with pre-heated composite yielded the same or significantly higher conversion (%) than control (22°C, 20s/control: 45.4±1.8b, 40°C, 5s s: 45.1±0.5b, 60°C, 5s s: 53.7±2.7a, p<0.01). Non-Isothermal conditions showed significantly higher stress for 60°C than 22°C (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.