Evaluation of Surface Treatment Methods on Fatigue Resistance and Static Flexural Bond Strength in CAD-CAM Resin Composites

This in vitro study aimed to assess the influence of different surface treatments prior to repairing a CAD-CAM resin composite on both fatigue and static flexural bond strength when bonded to a direct resin composite. CAD-CAM resin composite blocks (Tetric CAD) were ground and divided into three treatment groups: (1) aluminum oxide air-abrasion (50-μm particle size) followed by adhesive application (AA + AD group), (2) adhesive application only (AD group), and (3) silane treatment (SIL group). For comparison, both direct (DIR-RC) and indirect resin composites (IND-RC) were tested to determine their cohesive strength. The composite blocks were sectioned into beams (1 × 2 × 12 mm) and subjected to flexural bond strength tests under static loading (n = 10; 1 mm/min) and cyclic fatigue loading (n = 15; initial load = 5 N, frequency = 1.4 Hz, step increment = 5 N, 10,000 cycles per step) using a ball-in-hole testing setup. Finite element analysis was used to analyze the results in megapascals (MPa). Additionally, failure modes and surface topography were evaluated. Under static loading, both the AA + AD and AD groups demonstrated significantly higher bond strength compared to the SIL group. However, after fatigue loading, no statistically significant differences were observed among the surface treatment groups. The IND-RC group exhibited the highest flexural strength under both static and fatigue conditions. Fatigue loading reduced bond strength in all experimental groups, with the SIL group showing the greatest reduction. Importantly, none of the surface treatments fully restored the original strength of the CAD-CAM resin composite. However, surface grinding, with or without air abrasion, followed by adhesive application, provided comparable and favorable results. Thus, adhesive application in combination with surface grinding appears to be an effective strategy for the clinical repair of CAD-CAM resin composites.