The accuracy of dual-arch impressions: a pilot study

Dual-arch impression trays often are used for addition silicone final impressions of fixed prosthodontic preparations, but concerns about distortion of the impression are common because such trays lack rigidity.

This in vitro pilot study was designed to determine the accuracy of addition silicone impressions made with custom trays or made with either passive or stressed dual-arch trays.

Complete crown preparations of a mandibular molar, premolar, and incisor were made on a dentoform. These tooth preparations received flat, parallel indexes on the facial and lingual axial walls for accurate and reproducible positioning of a digital caliper. Gypsum dies were produced with an addition silicone impression material in either custom trays or dual-arch trays seated passively or with induced flexure (3 dies per tray group). The facio-lingual dimensions of the dies were measured with a digital caliper accurate to +/-5 microm and compared to the dimensions of the original preparations. Flexure in the latter group was induced by contact of the tray with a simulated torus, made of resin, in the lingual vestibule of the dentoform. Data were analyzed with 2-way analysis of variance and Tukey's multiple comparison test (alpha=.05).

Dies fabricated with either the custom or passive dual-arch tray reproduced the facio-lingual dimensions of the preparations within a -27 to +13 microm range. Dies fabricated with the flexed dual-arch tray exhibited greater discrepancy, in the range of -47 to -67 microm relative to the preparations. Tray type was a significant factor (P=.002): the flexed tray group was significantly different than the other 2 groups, which did not differ from each other.

Within the limitations of this pilot study, dual-arch impressions were comparable in accuracy to impressions made with custom trays. Accuracy was reduced, however, when the trays were flexed during closure of the arches.