Photoelastic Stress Analysis of Forces Opposing Withdrawal of Impressions

¹ Department of Operative Dental Surgery, University of Sheffield, Sheffield, England

The causes of distortion of elastic impression materials were discussed. A method of using photoelastic stress analysis to demonstrate traction stress was described. The results of examining the effects of different shapes resembling teeth, with and without cavities, on the gelatin were given, and a graphical interpretation that facilitates comparison was included. From the results obtained, it was suggested that, wherever possible, undercut areas around the teeth and in cavities should be eliminated before the impression is taken. It was suggested that line and point angles should be made slightly rounded rather than sharp. The danger of incorporating air in the impression material was illustrated.