Evaluation of digital model accuracy and time-dependent deformation of alginate impressions

The aim of this study was to evaluate the accuracy of digital models produced with the three-dimensional dental scanner, and to test the dimensional stability of alginate impressions for durations of immediately (T0), 1 day (T1), and 2 days (T2).

A total of sixty impressions were taken from a master model with an alginate, and were poured into plaster models in three different storage periods. Twenty impressions were directly scanned (negative digital models), after which plaster models were poured and scanned (positive digital models) immediately. The remaining 40 impressions were poured after 1 and 2 days. In total, 9 points and 11 linear measurements were used to analyze the plaster models, and negative and positive digital models. Time-dependent deformation of the alginate impressions and the accuracy of the conventional plaster models and digital models were evaluated separately.

Plaster models, negative and positive digital models showed significant differences in nearly all measurements at T (0), T (1), and T (2) times (P < 0.01, P < 0.05, and P < 0.001). Arch perimeter measurements did not differ at T (0) and T (1) times (P > 0.05), but they demonstrated statistically significant differences at T (2) time (P < 0.05) between the models.

This study showed that measurements on negative digital models offer a high degree of validity when compared to measurements on positive digital models and plaster models; differences between the techniques are clinically acceptable. Direct scanning of the impressions is practicable method for orthodontists.