Accuracy of irreversible hydrocolloids (alginates) for fixed prosthodontics. A comparison between irreversible hydrocolloid, reversible hydrocolloid, and addition silicone for use in the syringe-tray technique.

One reversible, seven irreversible hydrocolloids and two addition silicones were examined. The aim was to study their ability to reproduce six differently shaped abutments of a full arch stainless steel master model correctly, by measuring: 1) the accuracy of irreversible hydrocolloid impressions with different storage periods of 15 min, 2 h, 24 h and 95 h, reversible hydrocolloid stored 15 min and 2 h, and Type III addition silicones stored 24 h when the syringe-tray technique was used; and 2) whether mixing technique or tray design had any influence of the accuracy of irreversible hydrocolloid impressions. All the tested impression materials had difficulties in producing full arch casts, where the six abutments were correctly reproduced, because 58% of the heights and 45% of the diameters of the occlusal surfaces were shorter than the corresponding distances of the master model. Storage time, mixing technique and tray design were of significant importance for the irreversible hydrocolloids. Concerning the accuracy at the gingival margin, a shorter storage time resulted in improved accuracy, and mechanical mixing without a vacuum and a tray designed similar to a perforated stock tray gave most accurate impressions.