Contraction force rate of polymer composites is linearly correlated with irradiance

The force developed during cure of a composite represents the potential loads that can be induced into the dental adhesive and tooth structure that in turn affects the integrity of the dental adhesive and tooth. The purpose of this study was to evaluate the dependence of polymerization contraction force development on light energy density (product of irradiance and time).

Contraction force during polymerization was measured with a low compliance test fixture in which the composite specimen was placed between a glass plate and steel rod. The steel rod passed through a washer-type load cell that measured force development during cure. Six irradiance levels were evaluated as well as a 'pulse-delay' method. A generic composite consisting of a 1:1 blend of BisGMA and TEGDMA resin and 67 wt% unsilanated hybrid filler with 5 wt% fumed silica was used for all experiments. Contraction force was collected for 550 s. The first derivative of contraction force with respect to time (dF/dt) was calculated. Net contraction force at 550 s and max[dF/dt] was statistically analyzed as a function of irradiance and energy density (product of irradiance and time) with one-way ANOVA and Tukey's post-hoc test at the 0.05 level of significance.

Contraction force increased most rapidly immediately following light activation. Force resulting from the pulse-delay method was significantly different from all other methods (p < 0.001). Force resulting from irradiation at 600 mW/cm2 was significantly different (p < 0.01) from all other methods and 500 mW/cm2 was significantly different from 100 and 200 mW/cm2. Maximum df/dt (max[dF/dt] over full range of time) was linearly related to irradiance, linear regression r2 = 0.98. All pairs of irradiance were significantly different except pulse-delay and 200-300 mW/cm2 and 300 and 400 mW/cm2.

The pulse-delay method demonstrated contraction force rates lower than what would be expected using energy considerations and lower force rates at each of the two light exposures than their single exposure counterparts. Since the adhesive resin and dentin are viscoelastic and thus strain rate dependent, time dependent contraction force should be an important consideration.