Wax Lining in an Impression Tray and Accuracy in Gypsum Cast Fabrication

Controversy exists as to whether lining a metal-perforated impression tray with wax will yield a distorted irreversible hydrocolloid impression. Two current textbooks have completely different recommendations, but there is no evidence to support either theory. This project evaluates distortion in gypsum casts that have been prepared from wax-lined and unlined metal impression trays. The purpose of this study was to assess the influence of wax on the border and in the palate of metal-perforated impression trays on the dimensional stability of the resulting irreversible hydrocolloid impression.

A dentaform was prepared with marks on the cusp tips of teeth 6 and 11 and distobuccal cusps of teeth 2 and 15. Using a standard maxillary rim lock impression tray, alginate impressions were made using a dentaform with no wax lining, wax lining the border of the tray, and wax lining the border and the palate (n = 10). Casts were randomized. Six measurements were taken using a stereographic measuring microscope, from tooth #6 to #11, #11 to #15, #15 to #2, #2 to #6, #6 to #15, and #2 to #11. An acrylic template was used to position the cast in a reproducible position on the microscope.

Trays lined with wax on the border and the palate yielded casts with significantly different dimensions when compared to those poured from unlined or border-lined trays (p < 0.05). Casts produced from unlined and border-lined trays were not significantly different in dimension from the dentaform (p > 0.05).

There is no difference between the metal tray with no wax, the metal tray with a wax around the border, and the dentaform. The clinician can make irreversible hydrocolloid impressions using wax on the periphery or without wax on the periphery and feel confident that the impression has not been compromised in its ability to accurately reproduce the maxillary arch when used. Caution should be exercised when adding wax to the palate of the impression tray.