Removal time: a factor in the accuracy of irreversible hydrocolloid impressions

Controversy exists regarding the correct time of removal of an alginate impression from the mouth. To investigate the effect of time of removal on accuracy, a series of alginate impressions were made of the lower left quadrant of a subject having a cast gold crown with four reference marks on tooth no. 19. The impressions were removed from the mouth at 1-5, 2, 3, 4-5 and 7 min after the insertion of the tray. Accuracy of the impressions was assessed by comparing the distances between the reference marks in stone casts with standard measurements taken from the gold crown prior to cementation. Impressions removed at 1-5 and 7 min produced the greatest distortions buccolingually. For the mesio-distal lengths all time intervals produced casts which were larger than the standard. The reasons for these findings are discussed and possible explanations are presented.