Accuracy and reproducibility of 3D digital tooth preparations made by gypsum materials of various colors

Abstract

Purpose: The study aimed to identify the accuracy and reproducibility of preparations made by gypsum materials of various colors using quantitative and semi-quantitative three-dimensional (3D) approach.

Materials and methods: A titanium maxillary first molar preparation was created as reference dataset (REF). Silicone impressions were duplicated from REF and randomized into 6 groups (n=8). Gypsum preparations were formed and grouped according to the color of gypsum materials, and light-scanned to obtain prepared datasets (PRE). Then, in terms of accuracy, PRE were superimposed on REF using the best-fit-algorithm and PRE underwent intragroup pairwise best-fit alignment for assessing reproducibility. Root mean square deviation (RMSD) and degrees of similarity (DS) were computed and analyzed with SPSS 20.0 statistical software (α=.05).

Results: In terms of accuracy, PREs in 3D directions were increased in the 6 color groups (from 19.38 to 20.88 µm), of which the marginal and internal variations ranged 51.36 - 58.26 µm and 18.33 - 20.04 µm, respectively. On the other hand, RMSD value and DS-scores did not show significant differences among groups. Regarding reproducibility, both RMSD and DS-scores showed statistically significant differences among groups, while RMSD values of the 6 color groups were less than 5 µm, of which blue color group was the smallest (3.27 ± 0.24 µm) and white color group was the largest (4.24 ± 0.36 µm). These results were consistent with the DS data.

Conclusion: The 3D volume of the PREs was predisposed towards an increase during digitalization, which was unaffected by gypsum color. Furthermore, the reproducibility of digitalizing scanning differed negligibly among different gypsum colors, especially in comparison to clinically observed discrepancies.

Keywords: Accuracy; Color; Dental gypsum materials; Reproducibility; Three-dimensional analysis.

Fig. 1. Experimental procedure for marginal, internal and overall discrepancy analysis. The reference dataset (REF) for the prepared tooth (A) were divided into marginal (A1) and internal (A2) area, which were separately performed with a best-fit virtual alignment and with the tooth prepared dataset (PRE) obtained by gypsum materials of various colors (B). Ultimately, marginal (C1), internal (C2), and overall (C) color-coded difference images of the prepared tooth were obtained. The color-coded difference images had 19 colored segments, where green or blue shades indicated a negative deviation, while yellow and red indicated a positive deviation.

Fig. 2. Degree of similarity (DS) calculated for color-coded difference images. Corresponding pixels were considered similar when their red, green, and blue distances were < 32 (2⁵) elements of the RGB-color-space. The number of similar pixels in relation to the total image size in pixels estimated the degree of similarity.

Fig. 3. Deviation analysis of color-coded difference images in internal (upper column) and marginal (lower column) surfaces of prepared tooth.

Fig. 4. Semi-quantitative deviation analyses for color variations between the blue color group and other color groups.