Different surface treatments prior to CAD-CAM resin composite repair: An analysis of stress distribution and flexural fatigue behavior

This in vitro study evaluated the stress distribution and fatigue behavior of repaired CAD-CAM resin composite after different surface treatments following diamond bur grinding. Discs (Ø= 12 mm, 1 mm thick) were prepared from Tetric CAD blocks, subjected to diamond bur grinding to simulate clinical damage, and divided into three groups: AA + AD (air-abrasion + adhesive), AD (adhesive only), and SIL (silane only). Repairs were performed with a 0.4 mm layer of direct resin composite (EvoCeram), followed by light-curing. After storage in water at 37°C from 24 h to 14 days, specimens underwent fatigue biaxial flexural testing (n = 15; 20 Hz; initial load= 50 N, step-size = 10 N, 10,000 cycles/step) until fracture. Stress distribution was analyzed using finite element analysis (FEA), and surfaces were characterized by topographic and fractographic analyses. As a result, no significant differences in stress distribution were observed among the groups in either the restorative substrate or the repair layer. However, the SIL group exhibited significantly lower fatigue longevity than AA + AD and AD. Air-abrasion promoted roughness for micromechanical retention, while adhesive created a smooth and uniform surface; silane alone had no topographic effect. Fractographic analysis revealed failures initiating in the restorative substrate and propagating to the repair layer. A void between the repair and restoration was noted in the SIL group, indicating compromised adhesion. The FEA confirmed higher stress concentrations in the substrate rather than the repair layer. Combining diamond bur grinding with adhesive, either with or without air-abrasion, improves the mechanical longevity of direct repairs in CAD-CAM resin composites.