Effect of the impression margin thickness on the linear accuracy of impression and stone dies: an in vitro study.

The space available for impression material in gingival sulcus immediately after the removal of retraction cord has been found to be 0.3-0.4 mm. However after 40 s only 0.2 mm of the retracted space is available. This is of concern when impression of multiple abutments is to be made. Hence a study was planned to determine the minimum width of the retracted sulcus necessary to obtain a good impression. Five metal dies were machined to accurately fit a stainless steel block with a square cavity in the center with spaces, 1 mm deep and of varying widths (0.11-0.3 mm) away from the block. Polyvinyl siloxane impressions were made and poured using a high strength stone. Using traveling microscope, length and widths of abutment, impression and die were measured and compared for linear accuracy and completeness of impression. Results showed 1.5-3 times greater mean distortion and larger coefficient of variance in the 0.11 mm group than in the wider sulcular groups. ANOVA test for distortion also showed statistically significant differences (P < 0.05). 75 % of impressions in 0.11 mm group were defective compared to less than 25 % of impressions in other width groups. It is not always possible to predictably obtain accurate impressions in sulcus width of 0.11 mm or lesser. Dimensionally accurate and defect free impressions were obtained in sulcus width of 0.15 mm and wider. Hence clinicians must choose retraction methods to obtain a width greater than 0.35 mm. Further immediate loading of the impression material after cord removal may improve accuracy.