How does hydrofluoric acid etching affect the cyclic load-to-failure of lithium disilicate restorations?

This study investigated the effect of etching with distinct hydrofluoric (HF) acid concentrations on the cyclic load-to-failure (C(Lf)) of simplified lithium disilicate glass-ceramic restorations adhesively cemented to a dentin analogue (n = 20): non-etched/control (CTRL), or etched for 20 s with HF acid at 3% (HF3), 5% (HF5), or 10% (HF10). A silane coating was then applied onto the ceramic surfaces. Fatigue tests followed the staircase approach (initial load= 720 N; step-size= 70 N; 500,000 cycles per sample; 20 Hz) using a hemispheric stainless-steel piston (O= 40 mm) under water. The C(Lf) data were analyzed using Dixon and Mood method. Topographic and fractographic analyses were conducted. C(Lf) (in N) of HF3 (1355 +/- 32.0) and HF5 (1335 +/- 58.8) groups were the highest and statistically similar; HF10 presented intermediate C(Lf) (1175 +/- 132.9), while the non-etched group had the lowest one (965 +/- 145.0). Topographical analysis showed that the higher the HF acid concentration, the more pronounced the topographical changes. All failures (radial cracks) started from the inner surface of the ceramic discs. Topographical changes promoted by intermediate HF acid concentrations (3% and 5%) may improve fatigue performance for adhesively-cemented lithium disilicate restorations.