The reliability of standardized flexure strength testing procedures for a light-activated resin-based composite

Objectives To investigate the differences in the reliability of three-point flexure strength (TFS) and bi-axial flexure strength (BFS) data of a dental resin-based composite (RBC) irradiated by a handheld or an oven light-curing unit (LCU). Methods Three-point bar-shaped (25×2×2 mm3) and bi-axial disc-shaped (12 mm diameter, 2 mm thick) specimens of Filtek™ Z250 were polymerized utilizing either a handheld (n=20) or an oven-LCU (n=20). The mean TFS and BFS, associated Weibull moduli and degree of conversion (DC) for each curing regime were obtained following 24 h immersion in a lightproof water bath maintained at 37±1 °C. Results A significant decrease in TFS (129±15 and 127±13 MPa) compared with BFS (140±12 and 148±13 MPa) was identified for specimens irradiated with both LCU types (P<0.001). The Weibull moduli of TFS data associated with the handheld-was significantly decreased compared with the oven-LCU since confidence intervals did not overlap (7.5–9.4 and 9.5–10.6, respectively). In contrast, the Weibull moduli of the BFS data associated with either LCU were not significant (11.3–12.4 and 11.3–13.5). A significant decrease in the DC of three-point and bi-axial flexure specimens irradiated with the handheld compared with the oven-LCU was reported (P=0.031). Conclusions The improved experimental reliability combined with the increased clinical relevance in specimen geometry of disc-compared with bar-shaped specimens may advocate bi-axial flexure testing methodology as the standard to assess the strength of light-activated dental RBCs. The differences in extent of polymerization of RBC specimens cured with either LCU were not consistent with an equivalent dose of light energy density. This phenomenon may be attributed to differences in polymerization efficiency associated with the quantity of useful light energy emitted from the handheld- compared with the oven-LCU.