RESUMEN
BACKGROUND: An in vitro study was performed to assess the effect of three implant abutment angulations and three core thicknesses on the fracture resistance of overlaying computer-aided manufacturing (CAM) milled zirconia (Cercon(®) system) single crowns. METHODS: Three groups, coded A to C, with different implant abutment angulations (group A/0°, group B/15° and group C/30° angulation) were used to construct 15 crowns for each angulation. Forty-five overlay restorations were milled using the Cercon(®) system with zirconium core thicknesses of 0.4, 0.6 and 0.8 mm using five crowns for each angulation. The final restorations were prepared and stored in distilled water at mouth temperature (37°C) for 24 hours prior to testing. The restorations were cemented using Temp Bond(®) . The load required to break each crown and the mode of failure were recorded. All the results obtained were statistically analysed by the ANOVA test (level of significance p < 0.05). Tested crowns were examined using a stereomicroscope at 40X and selected crowns (five randomly selected from each group were further examined by scanning electron microscopy) to reveal the zirconia-ceramic interface and to determine the fracture origin. RESULTS: Implant abutment angulations significantly (p < 0.05) reduced the fracture resistance of overlaying CAM-milled zirconia single crowns. The fracture loads of Cercon(®) crowns cemented onto abutment preparations with a 30° angulation were the lowest of the groups tested. The core thickness (0.4 to 0.8 mm) did not significantly (p > 0.05) affect the fracture resistance of the CAM-milled zirconia single crowns. SEM showed that the origin of the fracture appeared to be located at the occlusal surfaces of the crowns and the crack propagation tended to radiate from the occlusal surface towards the gingival margin. CONCLUSIONS: The implant angulation of 30° significantly (p < 0.05) reduced the fracture resistance of overlaying CAM-milled zirconia single crowns. Reducing the core thickness from 0.8 mm to 0.4 mm did not affect (p > 0.05) the fracture resistance of overlaying CAM-milled zirconia single crowns.