The effects of water on degradation of the zirconia-resin bond.

OBJECTIVES: 10-methacryloyloxydecyldihydrogenphosphate (MDP) containing primers improve bonding of yttria-stabilised tetragonal zirconia (Y-TZP) to methacrylate resins. The present study investigated the role played by water in the deterioration of MDP-mediated zirconia-resin bonds. METHODS: Grit-blasted Y-TZP plates were conditioned with two MDP primers and bonded with resin for shear bond strength (SBS) testing. Additional bonded plates were aged hydrothermally and compared with unaged Y-TZP after 24h of water-storage or 6 months of water/acid/alkali-storage. The monoclinic phase (m-ZrO(2)) in different groups was determined by X-ray diffraction. Hydrolytic stability of the coordinate bond between MDP and zirconia in neutral/acid/alkaline environment was analysed using thermodynamic calculations. Microleakage and release of the element phosphorus from MDP-mediated Y-TZP/resin-bonded interfaces were evaluated via methylene blue dye infiltration and inductively coupled plasma-mass spectrometry (ICP-MS). RESULTS: Hydrothermal ageing did not significantly alter SBS. Ageing in acidic or neutral medium led to significant decline in SBS. The m-ZrO(2) phase increased after hydrothermal ageing but no m-ZrO(2) was detected in the water/acid/alkali-aged specimens. A higher equilibrium constant was identified in the MDP-t-ZrO(2) complex when compared with the MDP-m-ZrO(2) complex. MDP-conditioning failed to prevent infiltration of the methylene blue dye. Phosphorus was detected by ICP-MS from the solutions used for soaking the resin-bonded specimens. CONCLUSIONS: Hydrolysis of the coordinate bond between MDP and ZrO(2), rather than t-->m phase transformation, weakens the bond integrity between MDP-conditioned Y-TZP and methacrylate resin. CLINICAL SIGNIFICANCE: Hydrolysis of the coordinate bond between MDP and zirconia is responsible for deterioration of the integrity of the bond between MDP-conditioned Y-TZP and methacrylate resin.