![]() ![]() After depletion of the phosphate ions, the calcium reacts with soluble alginate to form the irreversible calcium alginate gel. Adding water causes the calcium from calcium phosphate dehydrate to react with the phosphate ions from sodium phosphate and pyrophosphate to form insoluble calcium phosphate. In an alginate impression material the powder contains calcium sulfate dehydrate, soluble alginate, and sodium phosphate. Unlike the agar hydrocolloids, the formation of a gelation stage prevents the material from being reliquified. 4Īlginate impression materials (irreversible hydrocolloid) change from a sol phase to a gel phase by means of a chemical reaction. The potassium sulfate should not be washed out of the impression, but should be carefully blown away with a stream of air. The potassium sulfate counteracts the effect of the borax, resulting in a more accurate die. The borax retards the set of the die stone, creating a weak surface. ![]() When the impression is left on the bracket table for a few minutes, water leaches out with some borax. (The potassium sulfate solution will result in a harder surface of the stone die material.) Many agar hydrocolloid materials contain borax as the filler. The impression is placed in a glass or stainless steel covered dish containing a 2% potassium sulfate solution, and the impression is left in the solution for 5 to 20 minutes. When using agar (reversible hydrocolloid) impression material, the tray is removed with a snap-out method. The disadvantages are the initial start-up cost of the conditioning unit, casts must be poured immediately, and the material has a low tear strength when impressing narrow, deep sulci. Reversible hydrocolloids offer the following advantages: a custom tray is not required no separate components are mixed, thus the chance for error is reduced they are relatively hydrophilic and inexpensive they are pleasant tasting and clean to use casts are easily removed from the impression and compatibility with stone allows the creation of bubble-free casts. Therefore, these impressions should be removed quickly with a snap action. Higher compressive and tear strengths occur at higher loading, and tearing decreases with the increasing rate of impression removal from the mouth. 2 The material is viscoelastic, and its tear and compressive strengths are dependent on how quickly the material is removed from the mouth. Agar is an organic, hydrophilic colloid that is derived from seaweed and is a sulfuric ester of linear polymer galactose. The material liquefies between 70☌ and 100☌ and becomes a gel again at 30☌ to 50☌. 1 Agar hydrocolloid materials are derived from reversible agar gels. This material has been considered the standard for dental impressions because a casting made from one impression will fit the die from a second impression. Sears introduced reversible hydrocolloid in 1937. Agar hydrocolloid (reversible), alginate hydrocolloid (irreversible), and a variety of elastomeric impression materials (polyethers and polyvinyl siloxanes) are the most commonly used impression materials. Desirable properties of an impression material include the following: ease of use, readily wets the oral tissues, adequate tear strength, no permanent deformation after strain, dimensional stability, accuracy, compatibility with die materials, and easy disinfection. Impression materials are used to make accurate reproductions of the hard and soft tissues. This article pre-sents a technique for using reversible-irreversible hydrocolloid impression material to fabricate an indirect onlay at a single visit. In many cases, the indirect fabrication of inlays and onlays offers certain clinical advantages when compared to direct fabrication, and if an indirect restoration can be fabricated and seated in a single visit, the patient benefits in terms of efficiency and convenience. Obtaining an accurate impression is a critical aspect of treatment to provide an indirect restoration. ![]()
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