A historical review of hydrocolloids and an investigation of the dimensional accuracy of the new alginates for crown and bridge impressions when using stock trays.

Reversible hydrocolloids have been used since 1937 and irreversible hydrocolloids since 1947 for the making of impressions for fixed prostheses; the impression techniques were similar to today's. The dimensional accuracy of reproduction of the irreversible hydrocolloids has been proved since 1947. In this paper the dimensional accuracy of three new alginates, Algi -X ( Algiflex Super, Howmedica Alginate), Ardent Alginate, Ultrafine and a combination material, Colloid 80/ Algiace ( Dentloid / Algiace ) were compared to two agar hydrocolloids and an addition silicone, when different stock trays were used as in the dental clinic. In most clinical situations the new alginates used in metal stock trays are as accurate as the "old" impression materials. The combination material is less accurate when many abutment preparations are to be reproduced. Whether perforated or nonperforated metal stock trays are used with the alginates is of no consequence to the accuracy. Alginates used in disposable plastic trays may cause severe inaccuracy. In narrow spaces with severe unblocked undercuts some of the alginates may be inferior to the other impression materials in this study.