Hydrofluoric acid concentrations: Effect on the cyclic load-to-failure of machined lithium disilicate restorations.

OBJECTIVES: To evaluate the effects of the etching with different hydrofluoric acid (HF) concentrations on the cyclic load-to-failure (C(Lf)) of machined lithium disilicate crowns cemented to dentin analogue material. METHODS: Pairs of dentin analogue prosthetic preparations and lithium disilicate ceramic crowns with simplified and standardized designs were machined (n=18). The preparations were etched with 10% HF (60s), followed by primer application. The intaglio surface of the ceramic crowns was treated as follows: non-etched (control, CTRL); or etched for 20s with different HF concentrations - 3% (HF3), or 5% (HF5), or 10% (HF10). A silane coating was then applied onto the treated ceramic surfaces, and they were adhesively cemented to the preparations. To perform the fatigue tests (staircase approach), a hemispheric stainless-steel piston (O=40mm) applied cyclic loads in the center of the crowns under water (initial load: 720N; step-size: 70N; cycles: 500,000; frequency: 20Hz). Additionally, topographic, fractographic, and fractal analyses were carried out. The fatigue data were analyzed using the Dixon and Mood method. RESULTS: Although the topographic and fractal analyses depicted the action of HF etching altering the superficial complexity and topography, the preponderant topography pattern was established by machining on CAD/CAM. All groups showed similar C(Lf) (in N) (CTRL=805.00+/-91.23; HF3=781.25+/-29.87; HF5=755.00+/-154.49; HF10=833.75+/-100.74). SIGNIFICANCE: Etching with different HF acid concentrations did not promote a deleterious effect on the cyclic load-to-failure of machined lithium disilicate crowns.