Effectiveness of surface treatments on fatigue and static flexural bond strength of a CAD-CAM resin composite

The aim of this in vitro study was to evaluate the effect of different surface treatments before repairing a CAD-CAM resin composite on fatigue and static flexural bond strength with a direct resin composite. For this, CAD-CAM resin composite blocks (Tetric CAD) were ground and divided into 3 groups: aluminum oxide air-abrasion (50-μm grain-size) + adhesive (AA + AD group), adhesive only (AD group), and silane (SIL group). Direct (DIR-RC) and indirect resin composite (IND-RC) were also tested for cohesive strength. Blocks were sectioned into beams (1 × 2 × 12 mm) and submitted to flexural strength (cohesive strength) and flexural bond strength (repaired sample) tests under static load (n= 10; 1 mm/min) and cyclic fatigue (n= 15; initial load= 5 N, 1.4 Hz, step-size= 5 N, 10,000 cycles per step) using a ball-in-hole device. Results were analyzed in MPa via finite element analysis. Failure analysis and topographic analysis were also conducted. The AA + AD and AD groups showed higher static flexural bond strength than SIL, but no differences were observed after fatigue. The IND-RC group exhibited the highest flexural strength under both static and fatigue conditions. All groups showed reduced bond strength after fatigue, with SIL presenting the most pronounced degradation. None of the surface treatments restored the original strength of CAD-CAM material. Surface grinding, whether followed by air abrasion or not, combined with adhesive application, yielded comparable outcomes. Therefore, the combination of adhesive application with surface grinding is an effective approach for CAD-CAM resin composite repair.