ABSTRACT
The application of die- spacer to crown preparation dies prior to the fabrication of cast crowns is an acceptable procedure that improves the fit of the restorations. Previous studies have shown that the retention of the restoration will be improved, unchanged or reduced when an appropriate thickness of die- spacer is used. But in our country shellac is used instead of die- spacer. No study has compared the effect of using standard die-spacer and shellac on the retention of fixed prosthesis. This study compares the effect of using these two materials on the retention of fixed prosthesis. Ninety eight cast copings were fabricated on dies coated with die-spacer and shellac, cemented on the dies and removed using an Instron Universal Testing Machine. T-test showed that there is statistically significant difference between the mean force required to remove crowns relieved with die- spacer and relieved with shellac. With regard to some practical difficulties in application of shellac and the results of this study; it appears that application of die-spacer is better than shellac