Techniques to improve the seating of castings.

A convergence of 20 degrees for full crowns is the most likely to be seen clinically, as determined by random measurements taken at a dental laboratory. Castings did not seat without cement, by an average of 215 micron at 10 degrees of convergence, or by 99 micron at 20 degrees of convergence. Biting forces will seat a casting approximately 150 micron, but will cause random concentrations of force against tooth structure, thus compressing it. All castings tend to rebound from this position. Zinc phosphate and polycarboxylate cemented crowns seated to 33 micron and 20 micron respectively when relieved, but each were elevated to 112 micron when not relieved. A silicophosphate cement displaced crowns with 20 degrees of convergence 122 micron under "ideal" clinical conditions, even when relieved; and with CBA 9080 cement, more than 500 micron. A die relief method was found to be the most suitable of the three casting compensation techniques. Casting retention, after cementation, was increased by 25%.