Influence of specimen diameter on the relationship between subsurface depth and hardness of a light-cured resin composite.

In pilot studies of the relationship between subsurface depth and hardness of a light-cured resin composite, it was found that the resin composite was softer at a depth of 0.5 mm than at, for example, a depth of 1.0 mm. It is possible that the increase in hardness at intermediate subsurface depths compared with the hardness at small depths is due to the heat of polymerization, causing a greater increase in temperature at intermediate depths than at small depths. As the temperature rise increases with volume of the test specimen, it was hypothesized that the increase in hardness would increase with the diameter of the irradiated specimen. The hardness of a resin composite was measured as a function of subsurface depth for cylindrical specimens of 3, 4, and 6 mm diameter. It was found that the resin composite was softer at 0.5 mm than at 1.0-1.5 mm depth independent of specimen diameter. Possible explanations are oxygen inhibition of polymerization and high rate of cure of material at small subsurface depth. It was also found that, corresponding with increasing specimen diameters, the specimens became significantly softer at depths of 3.0 mm, 3.5 mm, and 4.0 mm, respectively. The heat production and reflection of light from the walls of the molds may explain the latter, but not the former finding.