Geographic distribution of porcelain veneer preparation depth with various clinical techniques

Various clinical techniques have been advocated for uniform reduction of the tooth surface before a porcelain veneer restoration. Often these techniques do not produce a consistently uniform labial reduction.

The purpose of this study was to identify the degree of inconsistency on a geographic scale in the depth of labial reduction for porcelain veneers, resulting from the use of 3 clinical techniques. The technique of co-ordinate metrology was used to map the variations in the depth of the preparation.

A single operator using 3 techniques (dimples as depth guides, freehand, or depth grooves as depth guides) prepared 90 noncarious, unrestored extracted teeth to receive porcelain veneers (n=30). Impressions of the prepared and unprepared teeth were scanned with a co-ordinate measuring machine. In-house software was used to color-code the plotted images on the basis of the depth of preparation. Profile measurements were also made along the mid-labial sagital plane at the mid-labial, incisal, and cervical regions, as well as along the mesial proximal and distal proximal areas along the mid-labial horizontal plane. The ideal depth range for the labial reduction was chosen to be 0.4 to 0.6 mm. One-way analysis of variance and the Bonferroni test were performed to determine the significance (P<.05) in the difference between the means of reductions achieved with the 3 techniques.

There was no statistically significant difference in the mean percentage area prepared to the ideal depth range (0.4 mm-0.6 mm), between dimple (44.59%), freehand (36.35%), and depth groove (38.43%) techniques. The difference in the mean percentage area of reduction greater than 0.6 mm between dimple (12.98%), freehand (29.66%), and dimple and depth groove (37.32%) techniques were statistically significant (P=.0000), but not between freehand and depth groove techniques. With the profile measurements it was seen that there were statistically significant differences in the mean depth between dimple (0.45 mm) and depth groove (0.63 mm), and freehand (0.51 mm) and depth groove in the mid-buccal (P<.0004) and cervical (dimple = 0.48 mm, freehand = 0.52 mm, depth groove = 0.63 mm) (P<.0005) regions. There was statistically significant difference (P<.0000) in the mean depth between the dimple (0.39 mm) and freehand (0.30 mm), dimple and depth groove (0.50 mm), and freehand and depth groove techniques in the incisal area. In the mesial proximal region statistically significant difference (P<.0034) in the mean depth was found between the dimple (0.52 mm) and freehand (0.68 mm), and dimple and depth groove (0.64 mm) techniques only. In the distal proximal region, there was no statistically significant difference in the mean depth between dimple (0.55 mm), freehand (0.66 mm), and depth groove (0.64 mm) techniques.

The use of dimple technique showed a trend to greater consistency and fidelity in labial reduction to a depth of 0.4 to 0.6 mm. The 3 techniques for veneer preparations studied were associated with varying degrees of inconsistency in the distribution of depth of preparation within a tooth and between teeth in the same technique group.