The effectiveness of different polymerization protocols for class II composite resin restorations

Objectives To investigate the effect of reduced light exposure times on Vickers hardness (VH) of class II composite resin restorations. Methods Class II restorations were made in vitro in three 2 mm thick increments in a human molar. Two composite resins (Clearfil AP-X; Esthet-X) were polymerized with four light-curing units (Halogen; Astralis 10, LED; The Cure, L.E.Demetron I, Smartlite) following four curing protocols. Three protocols with exposure times of 10 s, 20 s or 40 s (control) per layer. In the fourth protocol, 10 s irradiation per layer was combined with additional lateral curing for 10 s from buccal and palatal after removal of the metal matrix. VH of the axial surface was determined at top and bottom layers directly after light-curing and after 7 days storage. Linear regression analysis was performed to analyze the effect of protocol variables. Results Directly after light-curing VH of both composite resins was significantly influenced by curing protocols. After 7 days, curing protocols had no significant effect on VH of Clearfil AP-X, except for the Smartlite. VH of Esthet-X was still influenced by curing protocol, but differences were smaller than directly after light-curing. Conclusions With high intensity light-curing units, exposure times of 10 s/2 mm increment can be sufficient to obtain under in vitro conditions a high degree of conversion, 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 less time.