Thermocycling is commonly employed in laboratory studies to simulate the in vivo aging of restorative materials. However, there is little consistency in the regimens used, and some researchers have questioned the clinical relevance and, hence, the necessity of including thermal stressing in in vitro protocols. This study examined the effects of five thermal stressing regimens on the flexural and dentin bond strengths of a hybrid resin composite.
Methods: For flexural strength tests, 95 rectangular specimens (15 mm x 2 mm x 2 mm) were fabricated using a stainless steel split mold, then light cured for 60 seconds. For bond strength tests, 75 caries-free molars were flattened occlusally to expose dentin, then polished through 600 grit SiC paper; dentin surfaces were etched, rinsed and blotted dry. A dentin adhesive was applied and light cured for 30 seconds; resin composite was condensed through a stainless steel split mold (4.3 mm diameter x 3.5 mm high), then light cured for 60 seconds. All specimens were stored in deionized water for 24 hours, then stressed for 100 hours according to one of five regimens: 1) cycled between 5 degrees C and 55 degrees C (9000 cycles; 20-second dwell time); 2) held at 5 degrees C constant; 3) held at 22 degrees C constant; 4) held at 55 degrees C constant; 5) held at 5 degrees C for 50 hours, then at 55 degrees C for 50 hours. Flexural strengths were measured using an Instron 5500R and three-point bending apparatus at a crosshead speed of 0.5 mm/minute. Shear bond strengths were measured using an MTS Bionix 200 at a crosshead speed of 0.5 mm/minute.
Results: ANOVA revealed no significant differences in either flexural strength or shear bond strength among the five thermal regimens.