Effects of different irradiance levels of the light curing unit on the dental resin composite's microhardness and cytotoxicity

Polymerization of composite resins could be affected by iradiance levels of light curing units. Furthermore, imperfect polymerization of resin composite polymerization could cause low surface hardness and high cytotoxicity. This study examined the effect of the irradiance levels of the light curing units on the microhardness of cured resin composites and their cytotoxicity A pulse-lighting curing unit with irradiance levels of 800, 900 and 1,000 mW/cm2 in either 10 or 20 sec was employed for this study and a commercially available continuous-lighting curing unit of 900 mW/cm2 in 20 sec was used as comparison. Resin composite specimens were irradiated using the curing units, and the microhardness with Knoop system and cytotoxicity by the MTT were subsequently determined. The irradiance of 900±10 mW/m2 cm2 emitted from the pulse lighting is the closest value to that emitted by the continuous-lighting curing unit. The resin composites irradiated at higher irradiance levels exhibited significantly higher (p < 0.05) microhardness than those irradiated at lower irradiance levels. Regarding their cytotoxicity values, higher significant differences (p < 0.05) were observed with increasing irradiance levels. Increased irradiance of the pulse-lighting curing unit raised the level of microhardness while minimizing the cytotoxicity of the tested resin composites. For clinical work, enough microhardness and cytotoxicity values obtained from the resin composites irradiated using the pulse lighting curing unit might provide a safe tooth condition.