Die relief, seating methods and fit of full crowns.

This study investigated the influence of different frequencies of vibration supplied by an electropneumatic condenser on the seating rate and vertical discrepancy of full crowns with four different internal configurations: A) unrelieved; B) four layers of die spacer; C) eight layers of die spacer and D) unrelieved but vented occlusally. Crowns were luted with zinc phosphate cement under a static 5 Kg load alone and with three frequencies of vibration. The seating process was monitored electronically to determine seating rate and final fit. Venting and die relief enhanced seating under static load. Vibration increased the seating rate and reduced vertical discrepancy of groups B to D crowns compared to group A crowns. Medium frequency vibration with group C and D crowns gave the smallest vertical discrepancies. Vibration did not improve the final seating of group A crowns. This suggests that vibration is only effective when an escapeway of some kind is provided.