Effectiveness of different polymerization protocols for class II composite restorations

Objective: To investigate the effect of shortened curing times on Vickers Hardness (VH) of a Class II composite resin restoration. Methods: A human molar with a Class II inlay preparation was mounted in a standardized typodont set-up. In absence of an adhesive, composite resin was inserted and cured in three 2 mm thick increments. Two composites (Clearfil AP-X; Esthet-X) were polymerized with four light-curing units (halogen: Astralis 10; LED: LEDemetron I, The Cure and Smartlite) following four curing protocols. Curing time per layer: (1) 10 s; (2) 20 s; (3) 40 s (control); (4) 10 s combined with additional indirect lateral curing from buccal and palatinal for 10 seconds. In total 32 groups of 10 restorations each were obtained. After removal of the restoration, VH of the axial surface was determined at the top and bottom layers directly after light-curing and after 7 days dry and dark storage. To analyze VH and effects of experimental variables a backward linear stepwise regression analysis was performed. Results: Differences in VH were found for the composites, light-curing units, curing protocols and locations of measurement (top or bottom). Directly after light-curing VH was significantly influenced by curing protocols. After one week both composite materials showed increased VH due to post-curing, especially at the bottom, resulting in smaller differences between groups. After one week, curing protocols had no significant effect on VH of Clearfil AP-X. VH of Esthet-X was still influenced by curing time, but differences between curing protocols were smaller than directly after light-curing. Conclusion: With high intensity light-curing units, exposure times of 10 s might be sufficient, depending on materials and curing protocols. With additional lateral curing of a Class II composite resin restoration a higher degree of cure can be obtained in shorter time.