Irreversible hydrocolloids for crown and bridge impressions: Effect of different treatments on compatibility of irreversible hydrocolloid impression material with type IV gypsums

Objectives. The aims of this research were: 1) to determine if the compatibility between irreversible hydrocolloids (alginates) and type IV gypsums (die stones) is affected by different treatments of the impressions before pouring, and 2) to present a rating system based on Rygeʹs system for clinical evaluation in order to classify the quality of gypsum casts using detail reproduction and surface structure as criteria. Methods. Impressions were taken on a Plexiglas staircase (heights of the steps 15–82 μm) with six irreversible hydrocolloids designed for crown and bridge and ten type I and II irreversible hydrocolloids. Impressions of each brand were divided into four groups. One group was treated with 2% potassium sulfate, the second with water, the third with an alginate stabilizer, Sorbanat (Guilini Chemie), and as a control, a fourth group which was not allowed to come into contact with any liquid, i.e., the impressions were poured dry. The differently treated impressions were poured with ten type IV gypsums. The quality of all gypsum casts was then judged according to the rating system. Results. All the irreversible hydrocolloids, if correctly treated before pouring with a compatible gypsum, could reproduce details of 20 μm. Detail reproduction did not improve with the use of 2% potassium sulfate solution compared to water or if the impressions were poured “dry”. In general, the alginate stabilizer produced casts with significantly better detail reproduction and surface structure. Significance. The results show that dentists and dental technicians need to know how each specific irreversible hydrocolloid should be treated and also with which type IV gypsum it is compatible. This research also indicates that an irreversible hydrocolloid impression should not come into contact with any liquid within the first 15 min.