Mechanical Modeling and Laboratory Testing of Anatomically Correct All-Ceramic Crowns

Objective: Predict failure loads and modes of all-ceramic crowns using an anatomically correct FEA model and to construct and fatigue test standardized all-ceramic crowns to proof this model.

Methods: The average dimensions of a mandibular first molar crown were imported into CAD software; a tooth preparation was modeled by reducing proximal walls by 1.5 mm and occlusal surface by 2.0 mm. A cement, Y-TZP core (0.5mm thickness), and a porcelain veneer layer of a 5-cusp first lower molar configuration were designed and analyzed by FEA to determine maximum stress areas.

The CAD-based tooth preparation was milled and used as a die to fabricate 25 crowns with 1.0 mm porcelain veneered (Lava veneer, 3M/ESPE) on 0.5 mm Y-TZP cores (LAVA frame, 3M/ESPE). Crowns were cemented on composite (Z100, 3M/ESPE) reproductions of the tooth preparation. Three crowns were subjected to single cycle load to failure; 22 were subjected to step-stress mouth motion fatigue. All mechanical testing was performed by sliding a WC indenter of 6.25mm diameter 0.7 mm lingually down the mesio-distal cusp. A master Weibull curve and the reliability for completion of a mission of 50,000 cycles at 200 N load were calculated (Alta Pro, Reliasoft).

Results: High maximum principal stress occurred in the occlusal surface of both core and veneer in proximity to the loading position. High stress levels were also observed at the margins. Single load to failure showed fractures through the zirconia core at 1227N (average). Reliability for a 200NX50K cycle mission was 0.64 (0.79-0.43 90% CB). In fatigue loading, veneer chips but the core does not fracture, similar to clinical performance reports.

Conclusions: High stress concentrations in FEA predict fractures in single cycle loading. Different failure modes are expressed in response to single-cycle and fatigue loading; fatigue loading reproduces failure modes observed clinically. Supported by NIDCR-DE109676.