Monotonic and fatigue flexural bond strength of CAD-CAM lithium disilicate and resin composites repaired with direct resin composites

Objective: To evaluate the flexural bond strength of CAD-CAM lithium disilicate (LD) and resin composite (RC) repaired with different direct resin composites.

Materials and methods: LD and RC blocks were sectioned (6 × 6 × 6 mm). LD was crystallized, and after, both materials were ground with a fine-grit diamond bur. LD blocks were etched with 5 % HF acid followed by silane application. RC blocks were air-abraded with Al₂O₃. A universal adhesive system was applied to both substrates. The blocks were repaired with nanohybrid (NANOH), bulk-fill (BULK), or flowable (FLOW) resin composite up to 6 mm additional length, and then cut into beams (1 × 2 × 12 mm). After storage in distilled water at 37 °C for 24 h, a flexural bond strength test was run with the ball-on-hole approach (10 mm span, 2 mm diameter) under monotonic (0.5 mm/min, n = 10) or cyclic fatigue (for LD/RC respectively: initial load 5/10 N; step size: 2.5/5 N, 10,000 cycles for each step, 1.4 Hz, n = 15). Finite element analysis was employed to calculate bond strength (MPa), and the data were statistically analyzed (α = 0.05).

Results: For repaired LD, LD-FLOW showed the highest bond strength values. Higher occurrence of premature failures was seen at LD. Among the repaired RC, RC-BULK and RC-FLOW exhibited the highest values in monotonic testing, meanwhile RC-FLOW alone achieved the highest results in fatigue testing.

Conclusion: When repairing CAD-CAM LD or RC, better bond strength is achieved using a flowable resin composite. The high incidence of premature failures in LD raises questions to the repair longevity.