FRACTURE SURFACE CHARACTERIZATION OF CLINICALLY FAILED ALL CERAMIC CROWNS

The goal of this study was to establish a protocol for the retrieval a nd fractographic analysis of failed restorations, and to compare the f racture surface features of clinically failed ceramic restorations and with those of controlled laboratory test specimens fabricated from th e same materials. Ten fractured Dicor(R) crowns and 12 fractured Ceres tore(R) crowns were retrieved and analyzed. Optical microscopy of the failed crowns revealed that the critical segments of nine of the 10 (9 0%) Dicor(R) crowns and nine of the 12 (75%) Cerestore(R) crowns were acceptable for fractographic analysis. Twelve disks of each material w ere fabricated as controls and fractured by bi-axial flexure for analy sis of the similarities and/or differences between the fractographic f eatures of fractured clinical crowns and the disks. Each of the 10 Dic or(R) crowns was observed to fail along the internal surface. For 78% of the Cerestore(R) crowns, failure initiation occurred at the porcela in/core interface or inside the core material. Critical flaw sizes of the failed Dicor(R) crowns ranged from 127 to 272 mu m. Failure stress es of the Dicor(R) crowns, estimated by fractographic techniques and f racture mechanics relationships, ranged from 65 to 94 MPa. Estimated f ailure stresses for two of the Cerestore(R) crowns which had failure i nitiation sites in the porcelain layer were 15 and 68 MPa. It is concl uded that the fracture initiation sites of dental ceramics are control led primarily by the location and size of the critical flaw, and not b y specimen thickness.