Identification of optimal gingival displacement widths, finish line depths, and preparation designs for digitally...

Digital scans for single crowns have been reported to achieve accuracy similar to that of traditional impressions in certain patients, but criteria for acquiring high-quality scans are lacking.

The purpose of this in vitro study was to determine the optimal gingival displacement widths, finish line depths, and preparation designs for single crowns when using intraoral scanners (IOSs) to achieve clinically acceptable and high-quality recordings.

Three different preparation designs of computer-aided design and computer-aided manufacturing (CAD-CAM) polymethyl methacrylate (PMMA) abutments (round shoulder, shoulder, and sloped shoulder) were fitted with titanium sleeves of 4 different thicknesses (0.2 mm, 0.3 mm, 0.4 mm, and 0.5 mm) and magnetically attracted onto the spindle of an electronic spiral micrometer. Occlusal registration material was injected around the abutment to simulate gingival tissue. After setting, the titanium sleeves were removed to create gaps as different gingival displacement widths. The spiral micrometer was rotated to create 5 different finish line depths (supragingival 0.5 mm, equal gingiva, subgingival 0.5 mm, subgingival 1.0 mm, and subgingival 1.5 mm). Two IOSs (CEREC Primescan [PS] and TRIOS 3 [TS]) were used to scan 3 preparation designs with a combination of 4 gingival displacement widths and 5 finish line depths 5 times each (N=600). The files were imported into an engineering software program and superimposed with the corresponding reference scanned files of the original abutments. Tangent distances (TD) and marginal angle differences (MAD) between the testing groups and references were measured and analyzed using the Kolmogorov-Smirnov, Kruskal-Wallis, and Jonckheere-Terpstra tests (α=.05).

A significant linear trend of decreasing TD and MAD with increasing gingival displacement widths and shallower finish line depths was found. Statistically significant differences (P<.05) were found in TD and MAD between different preparation designs at finish line depths that were equigingival and 0.5-mm subgingival.

Clinically acceptable scans generally required a gingival displacement width of at least 0.3 mm and a finish line depth within subgingival 1.0 mm. Both IOSs produced high-quality scans for supragingival finish lines. Both IOSs required at least 0.4-mm gingival displacement width to achieve high-quality scans under equal gingival conditions. Only PS with at least 0.4-mm gingival displacement width was able to achieve high-quality scans for subgingival margins. The accuracy ranking of preparation designs was round shoulder (highest), shoulder, and sloped shoulder (lowest).