Effect of ramped light intensity on polymerization force and conversion in a photoactivated composite

Purpose: This study evaluated the effect of ramped light intensity on the polymerization shrinkage forces and degrees of conversion (DC) of a hybrid composite. Materials and Methods: Composite samples were bonded between two steel rods (2.50 mm diameter, 1.25 mm apart, configuration factor = 1.0) mounted in a universal testing machine using a constant displacement mode. Polymerization contraction force was recorded for 250 seconds under four light exposure conditions: group 1, STD: (40 s ± 800 mW/cm²); group 2, EXP: (150 mW/cm² logarithmic increase to 800 mW/cm² over 15 s) + (25 s ± 800 mW/cm²); group 3, 2-STEP: (10 s × 150 mW/cm²) + (30 s × 800 mW/cm²); group 4, MED: (80 s × 400 mW/cm²). Maximum curing force (N_250s) and maximum force rate of the four groups were compared using one-way analysis of variance (ANOVA) (α= 0.05) and the Tukey test. Degrees of conversion obtained with STD, EXP, and MED cure modes were evaluated at three depths (top surface, 1 mm, and 2 mm) using Fourier transform infrared spectroscopy (FTIR). Results: Maximum rates of polymerization shrinkage force development and standard deviations (SD), in ascending order, were group 4, MED: 0.33 ± 0.03 N/s; group 2, EXP: 0.35 ± 0.06 N/s; group 1, STD: 0.44 ± 0.03 N/s; and group 3, 2-STEP: 0.46 ± 0.07 N/s. Maximum rates of polymerization shrinkage force development of group 2, EXP and group 4, MED were statistically equivalent and lower than those of group 1, STD and group 3, 2-STEP. Maximum shrinkage forces (± SD), in ascending order, were group 2, EXP: 20.4 ± 2.5 N; group 4, MED: 25.8 ± 1.0 N; group 3, 2-STEP: 27.4 ± 5.8 N, and group 1, STD: 30.5 ± 2.7 N. Maximum force of the EXP mode was statistically lower than MED, 2-STEP, and STD curing modes. The EXP ramp was successful in reducing the conversion rate at the top surface and at 1.0-mm depth, but it did not affect the total conversion compared to the STD 40-second cure mode. There was no difference in DC at the top surface and 1-mm depth with mode of cure. The MED cure mode resulted in a higher DC than the EXP mode at a depth of 2 mm. CLINICAL SIGNIFICANCE. Maximum shrinkage force and force rate exhibited during the first 250 seconds of polymerization were significantly lower using a ramped light intensity exposure. Ramped light intensity decreased conversion rate at the top surface and at 1.0-mm depth and did not affect the total extent of conversion compared to a standard 40-secondY single-intensity cure mode. The slower conversion rate resulting from ramped light intensity helped to reduce the rate and maximum polymerization stress, but would not be expected to compromise the physical properties for the restorative material, since similar degrees of conversion were obtained.