Artificial Enamel Wear After Prolonged Chewing Simulation Against Y-TZP Crowns

Objectives: Although monolithic yttrium-tetragonal zirconia polycrystal (Y-TZP) prostheses are steadily becoming a routine in the dental practice, the effect of the occlusion between zirconia and tooth structure on the wear of dental enamel is not known. The aim of this study was to evaluate the effect of zirconia-based crowns on wear of artificial enamel after 5-years simulated ageing.
Methods: Fifteen prepared bovine incisors were scanned for the manufacturing of Y-TZP crowns using CAD/CAM technique, according to the following groups (n=5): Polished (PM) and glazed (GM) monolithic zirconia crowns (1.5mm uniform thickness), and Bi-layer (BL) crowns (0.8mm zirconia coping, 0.7mm porcelain veneer). Crowns were cemented with resin cement, and chewing simulation (2.5 million cycles/80N/artificial saliva/37°C) was performed with steatite indenters (6mm diameter) as antagonists. Antagonists were scanned using a surface profilometer (Alpha-Step D-600, KLA Tencor Corp) and, assuming the uniformity of control samples (unaged), the material loss was calculated based on the height and diameter of the worn surface. Roughness of the occlusal surface of the crowns was also investigated using the profilometer. Scanning electron microscopy was used to characterize the abraded surface. One-way ANOVA and Tukey test (p=.05) were employed for analysis of wear results.
Results: A significant difference was observed amongst the groups (p<0.001). Artificial enamel abraded against porcelain (BL) had significantly higher material loss (0.217mm³± 0.015) than those abraded against polished zirconia (PM - 0.167mm³± 0.02). Enamel against glazed zirconia (GM) abraded the least (0.101mm³± 0.03). BL crowns presented higher roughness, followed by PM crowns. SEM indicated that GM surface became smoother after ageing, revealing removal of surface irregularities; PM surface presented deep scratches and had the highest variation in roughness between baseline and aged sample.
Conclusions: Surface finishing of monolithic Y-TZP crowns plays a role in antagonists’ wear. There is a direct relationship between roughness and artificial enamel wear.