Effects of Curing Distance and Time on the Microhardness of Posterior Composites: An Experimental Study

Introduction Composite resins are commonly employed in restorative dentistry because of their aesthetic and functional properties, with microhardness serving as a key measure of their durability and clinical success. The degree of polymerization of the composites may be influenced by variables such as the distance between the curing tip and resin surface, as well as the duration of light exposure. Therefore, this in vitro study aimed to evaluate the combined effects of varying curing tip distances and exposure times on the microhardness of a posterior composite resin, providing insights into optimizing clinical protocols for enhanced restoration longevity. Materials and methods This in vitro study was conducted in the Department of Conservative Dentistry and Endodontics and used SureFil™ (Dentsply Sirona, Charlotte, NC, USA), a packable posterior composite with an 82% filler loading. Sixty cylindrical samples (6 mm diameter and 5 mm thickness) were prepared in a two-part split mold. Four groups (A, B, C, and D) were defined by curing tip distances of 1, 3, 5, and 8 mm, respectively, using metal spacer plates. Each group was subdivided into three subgroups based on exposure times of 40 s, 60 s, and 80 s, with five samples per subgroup. The samples were cured using a halogen curing unit (DENTSPLY®Caulk QHL75™, 450 mW/cm²). After 24 hours at 100% humidity, the samples were embedded in acrylic, polished, and tested for microhardness using a Mitutoyo HM-200 tester (Mitutoyo Corporation, Kawasaki, Japan) at depths ranging from 0.5 to 5.0 mm. The Vickers hardness number (VHN) was calculated, and the data were analyzed using two-way analysis of variance and Bonferroni post-hoc tests. Results Microhardness decreased significantly with increasing curing distance (p < 0.001) and was highest at 1 mm with 80 s of curing, whereas the lowest values were at 8 mm for 40 s. Longer exposure times (80 s) improved the microhardness across all distances, particularly at 5 mm. The curing distance explained 96% of the variance (p < 0.001), with a non-significant interaction effect (p = 0.065). Conclusion Minimizing the curing tip distance (1-3 mm) and extending the exposure time (≥60 s) significantly enhanced the microhardness of posterior composite resins, optimized polymerization, and improved clinical durability.