Fracture Origins of Clinically Failed Bilayer All-Ceramic Dental Crowns

Objectives: To perform a failure analysis of five clinically failed all-ceramic crowns, comprising to define the fracture circumstances, to disclose the fracture origins and crack paths, to classify the microscopic defects detected and, finally, to correlate the defects to certain manufacturing steps. Methods: Collecting bilayered all-ceramic crowns failed in vivo. The fractured surfaces of failed crowns were preliminarily examined by an optical microscope followed by detailed fractography investigations using a field emission scanning electron microscope. A standard practice for fractography failure analysis of advanced ceramics is applied to disclose the fracture origin. Results: Three types of fracture features are defined being either cracks initiated on the tooth margin, cracks initiated by the tooth occlusial contacts or veneer porcelain chipping-off/delamination. The presence of microscopic defects was identified as the major fracture origin in all cases. The cracks originating on the inner surface of the ceramic cores and progressing by tensile stresses yielded breakdown of the entire crown, while Hertzian cracks initiated on the surface of veneering porcelain by repetitive compressive bite loads usually resulted in porcelain chipping-off and delamination. The observed microscopic defects in this study are classified into six categories that all can be ascribed to a certain manufacturing step. Conclusions: It is found that all six categories of observed defects are not materials-originated but processes-related, implying the great potential available to improve the reliability of all ceramic restorations by further optimizing the manufacturing process. Consequently, it is recommended that the packing homogeneity of granules in the preparation of the ceramic cores is improved as well as the distribution of opalizers in the veneering porcelain powders. Cautions have to be taken to avoid contaminations both during the ceramic core manufacture and veneering processes.