Effects of cyclic temperature changes on hardness of composite restoratives

The clinical durability of some composite restorative materials may be significantly affected by cyclic temperature changes. This study investigated the effects of cyclic temperature changes on surface hardness of four commercial composite resins (Silux, Z100, Ariston and Surefil). Eighteen specimens of each material were divided into three treatment groups comprising a control and two different thermal cycling regimes. Control specimens were stored in distilled water at 35 degrees C for 178 hours. Thermal cycled specimens were stored in distilled water at 35 degrees C for 173 hours and subjected to five hours (300 cycles) of a thermal cycling regime consisting of the cycle ABAC, where A and B represent the fixed temperatures of 35 degrees C (28 seconds) and 15 degrees C (two seconds) and C, depending on the treatment group, either 45 degrees C or 60 degrees C (two seconds). All specimens were subsequently subjected to hardness testing (KHN) using a digital microhardness tester (load = 500 gf; dwell time = 15 seconds). Results were analyzed using ANOVA/Scheffe's test (p<0.05). The effect of thermal cycling on hardness was material-dependent. While thermal cycling significantly increased the surface hardness of Z100 and Surefil, it significantly decreased the hardness of Ariston. The hardness of Silux was not significantly affected by cyclic temperature changes. For all treatment groups, Z100 was significantly harder than the other composite resins evaluated and Surefil was significantly harder than Silux and Ariston. For both thermal cycled groups, Silux was significantly harder than Ariston.