A review of contemporary impression materials and techniques.
The contemporary restorative dentist has a host of impression materials available for making impressions in fixed prosthodontics,implant dentistry, and operative dentistry. With proper material selection and manipulation, accurate impressions can be obtained for fabrication of tooth- and implant-supported restorations. This article outlines the ideal properties of impression materials and explains the importance of critical manipulative variables. Available impression materials are analyzed relative to these variables, and several "specialized" impression techniques are described. Special attention is paid to polyvinyl siloxane impression materials because they have become the most widely used impression material in restorative dentistry.
Accuracy of casts generated from dual-arch impressions.
STATEMENT OF PROBLEM
Dual-arch trays are often used to make simultaneous impressions of a prepared tooth and the opposing teeth. Many dentists are concerned with the accuracy of the casts generated from this type of impression.


PURPOSE
The purpose of this investigation was to compare the accuracy of stone casts of a prepared tooth generated using 2 types of dual-arch impression tray/impression material combinations.


MATERIAL AND METHODS
The maxillary left first molar on a dentoform mounted on an articulator was prepared for a full-coverage gold crown. Ten impressions were made with either a plastic (P) or metal (M) tray and a polyether (PE) or vinyl polysiloxane (VPS) impression material. Each impression was cast in improved dental stone, and the buccolingual dimension of the die was measured at the midpoint of the buccal and lingual gingival margins. The prepared tooth (T) served as the control. The data were analyzed using a 2-factor analysis of variance (ANOVA) alpha=.05.


RESULTS
The P/VPS combination (10.673 mm) produced the largest die, followed by P/PE (10.602), T (10.508), M/PE (10.484), and M/VPS (10.472). The 2-factor ANOVA showed a significant difference between the tray types but not between the impression materials.


CONCLUSION
The metal trays produced dies smaller than the tooth, and the plastic trays produced dies that were larger.
Accuracy of second pour casts using dual-arch impressions.
This study compared the accuracy of the second pour of addition silicone impressions made using custom acrylic resin complete-arch trays and two types of dual-arch trays. Thirty impressions were made of a typodont using metal dual-arch, plastic dual-arch and acrylic resin complete-arch custom trays (10 impressions/group). Tooth #30 (first molar) and tooth #32 (third molar) in the typodont were custom metal dies made in the shape of complete crown preparations. On the metal dies were scribed reference marks permitting measurements to be made in the mesial-distal, buccal-lingual and occlusal-gingival dimensions. In addition, an inter-tooth measurement between tooth #30 and #32 was made. Following impression procedures, gypsum casts were poured. The casts were removed from the impression and, 24-hours later, second pours were completed. The distances between reference marks on each cast were measured using a measuring microscope. The measurements from each second pour cast were used to compute a group mean, with values expressed as a percent deviation from the master model. A two way analysis of variance was used to compare the groups. No statistically significant differences were found in accuracy among the three type trays except in the inter-tooth measurement where the metal dual-arch tray was more accurate than the other trays.