Effect of zirconia surface treatment, resin cement and aging on the load-bearing capacity under fatigue of thin simplified full-contour Y-TZP restorations

This study investigated the effect of zirconia surface treatment (air-abrasion with aluminum oxide or tribochemical silica coating) and aging on the fatigue behavior of thin monolithic Y-TZP (yttria-stabilized tetragonal zirconia polycrystal) restorations cemented with 2 types of resin cements, containing or not containing MDP, to a dentin-like substrate. Y-TZP ceramic (Zenostar T, diameter (Ø) 10 mm, 0.7 mm thick) and dentin-like discs (Ø 10 mm, 2.8 mm thick) were assigned into eight groups according to three factors: ‘zirconia surface treatment’ (aluminum oxide particles air-abrasion ‘AO’; or tribochemical silica coating via silica-coated aluminum trioxide particles air-abrasion + silanization ‘SC’); ‘MDP-containing resin cement’ (with: Panavia F2.0, ‘MDP’; or without: Multilink Automix, ‘nMDP’); and ‘aging’ (baseline; or aged - 'AG':12,000 thermal cycles + 60 days water storage). Y-TZP intaglio surface was conditioned and dentin-like substrate was etched with hydrofluoric acid prior to bonding. Aging was performed in half of the specimens before the fatigue testing (Staircase, 20 Hz; 250,000 cycles). Fractographic and topographic characteristics were analyzed by stereomicroscope and SEM. Prior to aging, no significant difference was found between the two surface treatments, irrespective to the cement. Samples bonded with resin cement containing MDP had a significant reduction in their fatigue failure load when Y-TZP was air-abraded with aluminum oxide particles and subjected to aging (MDP-AO = 2050.71A; MDP-AO/AG = 1756.67B). Other studied conditions were not affected by aging. Topographic images revealed a rougher surface for aluminum oxide air-abrasion. Fractography supports all failures as a radial crack starting at the Y-TZP intaglio surface. Bonded thin simplified Y-TZP restorations had a high load-bearing capacity, regardless of the studied factors. The MDP-containing resin cement applied on aluminum oxide air-abraded zirconia surface was not enough to maintain the fatigue performance after aging, while higher stability to aging was achieved by treating with the tribochemical silica coating method. When using MDP-free resin cement, the surface treatment and the aging did not impact the fatigue performance.