Failure modes of Y-TZP crowns at different cusp inclines

OBJECTIVES: To compare the reliability of the disto-facial (DF) and mesio-lingual (ML) cusps of an anatomically correct zirconia (Y-TZP) crown system. The research hypotheses tested were: (1) fatigue reliability and failure mode are similar for the ML and DF cusps; (2) failure mode of one cusp does not affect the failure of the other. METHODS: The average dimensions of a mandibular first molar crown were imported into CAD software; a tooth preparation was modelled by 1.5 mm marginal high reduction of proximal walls and occlusal surface by 2.0 mm. The CAD-based tooth preparation was milled and used as a die to fabricate crowns (n=14) with porcelain veneer on a 0.5 mm Y-TZP core. Crowns were cemented on composite reproductions of the tooth preparation. The crowns were step-stress mouth motion fatigued with sliding (0.7 mm) a tungsten-carbide indenter of 6.25 mm diameter down on the inclines of either the DF or ML cusps. Use level probability Weibull curve with use stress of 200 N and the reliability for completion of a mission of 50,000 cycles at 200 N load were calculated. RESULTS: Reliability for a 200 N at 50,000 cycles mission was not different between tested cusps. SEM imaging showed large cohesive failures within the veneer for the ML and smaller for the DF. Fractures originated from the contact area regardless of the cusp loaded. CONCLUSION: No significant difference on fatigue reliability was observed between the DF compared to the ML cusp. Fracture of one cusp did not affect the other

Dental ceramics and the molar crown testing ground

All ceramic crowns are highly esthetic restorations and their popularity has risen with the demand for life-like and cosmetic dentistry. Recent ceramic research has concentrated on developing a fundamental understanding of ceramic damage modes as influenced by microstructure. Dental investigations have elucidated three damage modes for ceramic layers in the 0.5-2 mm thickness using point contacts that duplicate tooth cuspal radii; classic Hertzian cone cracking, yield (pseudo-plastic behavior), and flexural cracking. Constitutive equations based upon materials properties have been developed that predict the damage modes operational for a given ceramic and thickness. Ceramic thickness or thickness of the stiff supporting core in layer crowns is critical in flexural cracking as well as the flaw state of the inner aspect of the crown. The elastic module of the supporting structure and of the luting cement and its thickness play a role in flexural fracture. Clinical studies of ceramics extending over 16 years are compared to the above relationships and predictions. Recommendations for clinical practice are made based upon the above