Accuracy of alginate and elastomeric impression materials.

The accuracy of three alginates and seven elastomeric impression materials was determined. Impressions were taken of a truncated cone and poured with stone die. A steel ring fitting the steel cone was placed on the dies, and the discrepancy between the top surface of the ring and the die was measured. From these measurements the deviation between the base diameter of the die and that of the cone was calculated and used to express the accuracy of an impression material. All impressions showed a net shrinkage, resulting in too large a die stone and incomplete seating of the steel ring. The inaccuracy of the alginates varied between 44 and 188 microns. Blueprint regular was found to be significantly more accurate than the two other alginates investigated, and had a degree of accuracy comparable with that of the more accurate elastomeric impression materials. Within this group of materials the inaccuracy varied between 39 and 130 microns. Generally, the addition-curing silicones were more accurate than the polyethers. Baysilex and President regular body had the highest degree of accuracy of the elastomeric impression materials.