Fracture of Porcelain Fused to Yttria-Tetragonal Zirconia Polycrystals Dental Crowns from Dual-motion Cyclic Loading in Aqueous Condition

Porcelain fused to Y-TZP dental crowns are widely used in dental restorations. The fracture damage from concentrated loading especially under dual-motion cyclic loading-simulating chewing remains doubtful. Flat occlusal surface porcelain/Y-TZP ceramic crowns were fabricated following general dental practice and were supported with dentine-like composite to simulate actual dental restorations. Specimens were subjected to eccentric forces in a Willytic chewing simulator with loads of 5, 7 and 8 Kg delivered by means of steel stylus of 4 mm diameter in wet condition. The damage evolution was observed at stepwise increase in cycles of 100,000 cycles each. Force measurements were taken using a 3D force sensor during dynamic loading of simultaneous axial-lateral movements. Localized surface damage which was observed at early stages was halted later after 500,000 cycles with distinctive crack patterns. Radial cracks were observed in three locations; the first location was at the porcelain inner layer under the indentation area. The second location was at the marginal extremists similar to lunar cracks observed when using soft intenders. The third location was the mesiobuccal area between the occlusal and axial sides of the crown. This latter finding may explain the chipping damage of the dental crowns. More interestingly at mesiobuccally area between the occlusal and the axial side of the crown which explain the chipping damage of dental crowns, it was believed that extension of radial cracks from indentation area are the major cause of such failure.