Impressions for prosthodontic restorations reproducing narrow spaces and severe undercuts

Distortion of an impression material may occur in special clinical situations in which the material's elastic limit is exceeded. Impressions were made of a master model designed to test such situations. Three different hydrocolloids and an addition silicone were used in the experiment. An extended setting time had to be used for one material, to prevent it from tearing. When this extended time was used, this material was the dimensionally most accurate material in the test. All materials were accurate in reproducing normally positioned abutments, while some of the impressions failed to reproduce adjacent abutments with particularly narrow interspaces; narrow passages may therefore require further tooth preparation. In the most severely undercut area differences existed among the materials, and sometimes tears occurred. When the papilla is missing, the undercut area should be blocked out to prevent plastic deformation of the impression material, causing inaccurate restorations.