It is more difficult to mix, has a bad odor, stains clothing, and undergoes some dimensional changes so the timing of pouring the cast is of concern. While its introduction was a step forward in impression-making, polysulfide rubber base also possesses undesirable characteristics. The introduction of polysulfide rubber base provided accuracy, reasonably good tear-resistance, and better dimensional stability. Their disadvantages were a relatively low tear-resistance and a lack of dimensional stability over time, even when carefully stored. The introduction of reversible and irreversible hydrocolloid materials offered significantly im-proved accuracy. They had obvious disadvantages, including distortion (waxes) and an inability to flex over and around undercuts with breakage (compound, plaster, and zinc oxide-eugenol). Dental impression compound and other waxes as well as plaster and zinc oxide-eugenol have served as impression materials. Historically, many different materials have been used. Because models are required for extraoral diagnostic procedures, analysis of occlusal functions, and fabrication of indirect restorations and appliances, some reliable method of making those models is required. Impressions are a part of every day for general practitioners and for many specialists. New Technologies for Easier and More Accurate Impressions Thomas Berry Gary Radz
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