Purpose: Many studies in the dental literature look at the effect of different surface treatment methods on the flexure strength of zirconia where polished zirconia has been used as control. However, zirconia is subjected to different types of surface damage as a result of the CAD/CAM milling procedure and also to damage produced by other laboratory procedures in use daily.
Aim of the work: The aim of this work was to evaluate the effect of different surface treatment methods and in particular the effect of the CAD/CAM milling procedure on the flexure strength of zirconia frameworks.
Materials and methods: At least 20 zirconia bars (17 mmx2 mmx1 mm) for each group were prepared by either cutting and polishing zirconia milling blocks or by using a CAD/CAM device (Cercon) which left behind characteristic surface features related to the milling process. The fully sintered bars received either of the following surface treatments: air-borne particle abrasion (with 50 and 120 microm aluminum oxide particles, or both). Some bars received a heat treatment commonly used in baking veneer ceramics before or after particle abrasion. The surface roughness was measured for all bars, which were finally loaded in a three-point device. The fractured bars were examined using scanning electron microscopy. Data were analyzed using one-way analysis of variance and survivability was estimated using Weibull analysis (alpha<0.05).
Results: There were significant differences in the flexure strength (in MPa) between the tested groups subjected to different surface treatments which can be categorized into four strength levels: (1074-1166 MPa) for polished zirconia and the CAD/CAM bars that were particle abraded (50 microm Al2O3) whether with or without heat treatment (936 MPa) for the ground bars that were particle abraded (50 microm Al2O3), (708-794 MPa) for CAD/CAM bars and for the polished bars that were particle abraded (120 microm Al2O3), and (546 MPa) for the ground bars that were particle abraded (120 microm Al2O3) being the weakest. There was a strong correlation between flexure strength and the severity of surface damage as indicated by surface roughness (R2=0.912). Scanning electron microscopy revealed different types of surface and subsurface damage produced by the different surface treatments.
Conclusions: The surface damage produced by the CAD/CAM milling procedure significantly reduced the strength of zirconia which could be further weakened by different surface treatment methods resulting in unexpected failures at stresses much lower than the ideal strength of the material. It is advised to consider the effect of the CAD/CAM procedure on the characteristic strength when designing zirconia-based fixed partial dentures.