A Comparison of Dimensional Accuracy of Addition Silicone of Different Consistencies with Two Different Spacer Designs - In-vitro Study.
INTRODUCTION
Dimensional accuracy of impression materials is crucial for the production of working casts in Fixed Prosthodontics. The accurate replication of tooth preparations and their arch position requires impression materials that exhibit limited distortion.


METHODS
This study was conducted to comparatively evaluate the dimensional accuracy of additional silicones by comparing two different techniques and spacer designs, by measuring the linear changes in interpreparation distance. The impressions were made from a stainless steel master die simulating a three unit bridge. A total 80 die stone (type IV, Ultrarock) models were obtained from the impressions made using two different parameters. The two different parameters are Multimix and Monophasic technique and different spacer designs.


RESULT
The interpreparation distance of the abutments in the casts was measured using a travelling microscope. Each sample was measured thrice and the mean value was calculated. The results obtained were statistically analysed and the values fall within the clinically acceptable range.


CONCLUSION
The most accurate combination is multimix technique with spacer design which uses less bulk of impression material.
Accuracy of three polyvinyl siloxane putty-wash impression techniques.
STATEMENT OF PROBLEM
There is much discussion in the dental literature concerning the effect of the impression technique on the accuracy of cast restorations.


PURPOSE
This study assessed the accuracy of 3 putty-wash impression techniques using the same impression material (polyvinyl siloxane) in a laboratory model.


MATERIAL AND METHODS
The 3 putty-wash impression techniques used were (1) 1-step (putty and wash impression materials used simultaneously); (2) 2-step with 2-mm relief (putty first as a preliminary impression to create 2-mm wash space with prefabricated copings. In the second step, the wash stage was carried out); and (3) 2-step technique with a polyethylene spacer (plastic spacer used with the putty impression first and then the wash stage). For each technique, 15 impressions were made of a stainless steel master model that contained 3 complete crown abutment preparations, which were used as the positive control. Accuracy was assessed by measuring 6 dimensions (intraabutment and interabutment) on stone dies poured from impressions of the master model.


RESULTS
One-way analysis of variance showed statistically significant differences among the 3 putty-wash impression techniques, for all intraabutment and interabutment measurements (P <.001). Overall discrepancies of the 2-step technique with 2-mm relief putty-wash impression technique were significantly smaller than that in the 1-step and polyethylene putty-wash impression techniques.


CONCLUSION
The polyvinyl siloxane 2-step, 2-mm, relief putty-wash impression technique was the most accurate for fabricating stone dies.
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.