Assessment of Fit for Fully-Contoured Zirconia Crowns on Minimal Preparations

Objectives: With increased aesthetic awareness, full gold crowns are becoming less acceptable to patients. Although all-ceramic or metal-ceramic crowns provide a good aesthetic alternative, they necessitate extensive tooth preparation. This in-vitro investigation examined whether, as a conservative tooth-coloured alternative, fully contoured zirconia crowns made on preparations designed for cast gold crowns (or even narrower) would provide clinically acceptable marginal fit (marginal opening <120µm) and axial contour.
Methods: A full-coverage crown preparation was made on an ivorine maxillary first molar, initially with a fine chamfer (FC) finish-line depth of 0.07-0.28mm and then modified to provide a preparation with a heavy chamfer (HC) of 0.32–0.44mm. Cast nickel-chrome replicas were made of both preparations. Impressions were obtained of each metal replica and ten fully-contoured zirconia crowns (LAVA 3DCORE) were fabricated on stone dies. Measurements of fit (absolute marginal opening) were made on the metal replicas under uniform load without cementation using a stereomicroscope and a 3-CCD camera at 200x magnification and assessed using image analysis software (ImagePro Express). Thickness of the crowns marginally and height of contour were also measured. Comparisons between FC and LC groups were made using Mann-Whitney U Test.
Results: The mean marginal fit of the samples in FC group (55.7±18.3µm) was not significantly different from that in the HC group (59.7±14.9µm)(p>0.05). Over-contour was evident mesially and distally in the FC group but otherwise was regarded as within normal parameters. The mean direct measurement of marginal crown thickness for the FC group was 0.31±0.01mm and for the HC group was 0.47±0.04mm (p>0.05). No significant height of contour differences were noted between the ivorine tooth and zirconia crowns (p>0.05)
Conclusions: The marginal fit and contour of fully-contoured zirconia crowns made on narrow preparations (FC and HC) were within the clinically acceptable limits described in the literature (McLean J Brit Dent J 1971;131:107-111). Based on these results, fully-contoured zirconia crowns might offer an aesthetic yet conservative alternative to cast metal restorations and this warrants further investigation