At King Saud University's College of Dentistry, final impressions for removable partial dentures are made using polysulfide rubber, fast-set and regular set irreversible hydrocolloid impression materials. Custom trays used are either single-spaced or double spaced. Further, impression material attachment to the tray could be through tray perforations or the use of adhesives. Literature to support each one of these methods separately to obtain an "accurate impression" is available. There is, however, a dearth on literature reporting on the influence of these combined variables. The purpose of this investigation was to simultaneously study the influence of impression material type, tray space and method of tray attachment on the accuracy of the resulting cast dimensions. A factorial experimental design was used. One master model representing a Kennedy Class III Modification 2 partial edentulous arch was used. Three reference points A, B, and C were placed on the master cast and reproduced on all experimental casts. Ten combined variables of tray space (2), impression material (3), and method of tray attachment (2) were used. Eight casts were made utilizing each of the combined variables resulting in 80 experimental casts. The dimensions AB, AC and BC were accurately measured (+10 /jm) on the master model and all experimental casts. Each measurement was repeated twice and averaged resulting in 480 experimental measurements (80 casts x 3 dimensions x 2 readings). One way analysis of variance (ANOVA) was utilized to discern differences between the dimensions of the master model and those of casts produced by each of the combined variables including the effect of the impression material. Tukey's standardized range test was used to discern accuracy differences among the combined variables. Statistical analyses revealed no significant differences for the dimensions A-B, A-C, or B-C between the master model and those produced by the ten combined variables. Nonetheless, the mean B-C dimensions on casts produced by the ten combined variables tended to be smaller (though not significantly) than for the master model. Further, while there were no significant differences for the dimensions A-B and A-C among casts produced by the ten combined variables, significant differences were detected for the B-C dimension. Casts poured in regular set alginate impressions in single spaced perforated trays showed significantly larger B-C dimension than those derived from fast-set alginate impressions in double spaced perforated trays (P<0.05). Provided that impressions are well attached to their trays and poured immediately (within 5 minutes), tray conditions tested in this study had no effect on accuracy of casts poured from either polysulfide or either of the irreversible hydrocolloid impression materials.

Saudi Dental Journal 1999;11(1):2-7.