Effect of pre-heating on depth of cure and surface hardness of light-polymerized resin composites

Purpose: To evaluate the depth of cure and surface hardness of two resin composites when subjected to three preheating temperatures, three polymerization times and two types of curing lights. Methods: Two resin composites were used in this study (Esthet-X and TPH), three polymerization times (10, 20, 40 seconds), three preheating temperatures (70, 100, 140°F/21.1, 37.7 and 60°C), and two curing lights (halogen and LED). For depth of cure measurements, 180 specimens (4 mm in diameter and 2 mm in depth) were made for 36 combinations of variables. Four Knoop hardness measurements were obtained from both the top and bottom surfaces. For the surface hardness, another 180 (4 × 6 mm) cylindrical specimens were fabricated. Each specimen was sectioned in half and hardness measurements were made at 0.5 mm intervals. Statistical analyses were performed using the multifactor ANOVA at a level of significance of α = 0.05. Results: For depth of cure, there was a statistical difference among all the main effects (time, temperature and curing light) for both composites (P> 0.001) when the % difference from the top was analyzed. Results indicate that there was an increase in hardness as the temperature of the composite was increased from 70 to 140°F for both composites for either the top or the bottom. The percent difference in hardness was greater when the LED curing light was used compared to the halogen curing light. Overall there was a greater change in hardness when the resin composite was polymerized at 140°F. Although the ISO standard was not met in many cases, there was a significant increase in hardness on both the top and bottom as temperature and curing time increased (P< 0.001). Results for the surface hardness showed that there was a significant statistical difference (P< 0.001) in hardness when the surface hardness at 0.5 and 3.5 mm were analyzed separately. There was a general increase in surface hardness for both the hybrid and microhybrid as time and temperature increased. For both hybrid and microhybrid groups, as the temperature increased, there was an increase in hardness and it was statistically different (P< 0.001). When the percent difference between 70 and 100°F or 70 and 140°F was evaluated, the greatest increase occurred between the 70 and 140°F and minimal increase between 100 and 140°F. Overall, the LED curing light provided a greater surface hardness for the hybrid at both depths than the halogen curing light. For the microhybrid, the halogen curing light provided the greatest surface hardness when the resin was polymerized for 40 seconds.