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Clinical Cases

Influence of delayed pouring on irreversible hydrocolloid properties
Stéfani Becker Rodrigues
Carolina Rocha Augusto
Vicente Castelo Branco Leitune
Susana Maria Werner Samuel
Fabrício Mezzomo Collares
Dental Materials Laboratory, School of Dentistry, Univ Federal do Rio Grande do Sul - UFRGS, Porto Alegre, RS

Due to their low cost and easy handling, irreversible hydrocolloids are one of the most commonly used impression materials in dental practice. Currently, they are more commonly used for initial impressions to obtain a preliminary model used for diagnostic purposes, treatment planning, and fabrication of provisional prostheses or custom trays. The alginate impression materials contain sodium alginate, calcium sulfate, trisodium phosphate, diatomaceous earth, zinc oxide, and potassium titanium fluoride, all in the form of a powder. When mixed with water, a sol is formed through a reaction with the sodium or potassium salts of alginic acid and calcium sulfate. After this chemical reaction takes place, a fast-setting gel is formed as a result of the replacement of the monovalent calcium by sodium and potassium cations, allowing crosslinking of the alginic salts. There are many commercial variations of alginate that vary in consistency, setting time, elasticity, strength, and dimensional stability; manufacturers also add fillers, which can present an impact on its properties, application, setting time, and pouring time.
Declaration of Interests: The authors certify that they have no commercial or associative interest that represents a conflict of interest in connection with the manuscript.
The physical, mechanical, and chemical properties of alginate can be affected by how long they are stored and by the storage conditions prior to the production of the gypsum cast. Compared to other impression materials, irreversible hydrocolloids have the disadvantages of low dimensional stability and reduced capacity for detail reproduction. Dental casts obtained from impressions made with irreversible hydrocolloids tend to present decreased detail reproduction, particularly in sharp line-angle areas, compared to those from other impression materials, such as elastomers. The dimensional stability of alginate is required to obtain reliable gypsum models of the molded arches. It is recommended that irreversible hydrocolloid impressions be poured immediately or within a few minutes of removal from the mouth to prevent distortion. Syneresis or imbibition when exposed to air or water could influence alginate’s dimensional stability, leading to less accurate casts.

To overcome this problem of a short recommended pouring time after impression, new irreversible hydrocolloids have been developed with increased pouring times, even allowing for up to five days of storage. Because pouring time is an important factor in the dimensional stability of irreversible hydrocol
loids and the resulting stone casts, an investigation of these new materials is needed. The aim of this study was to evaluate the properties of irreversible hydrocolloid impression materials poured immediately and after specific storage periods.