• retention;
  • convergence;
  • film thickness


Although traditional ideal convergence (the sum of taper of the opposite sides) for crown proparation has been arbitrarily set at 4° to 10°, some believe absolute parallelism yields the highest retention. This study examined the relationship between the degree of convergence of a machined metal die and the retention of its casting.

Materials and Methods

The method used was that of cementing cast metal crowns onto full crown preparations on brass dies with varying convergence angles, and then recording the force required to remove the crowns from the dies in a vertical direction using a Tate-Emery Testing Machine and Load Indicator.


It was found that retention (ie, the force needed to remove the cemented castings from the die in their common long axis) increases from O° convergence to peak between 6° to 12° convergence. It also seems that a critical film thickness does exist for optimum retention, and that film thicknesses smaller than the critical thickness may be responsible for the phenomenon that we have observed and directly related to the convergence angle itself.


There seems to be experimental data supporting the use of traditionally taught convergence. Our study found that convergence angles between 6° and 12° seem to be optimum for tooth crown preparation when one plans to use zinc phosphate cement. Convergence angles of less than 6° may not be desirable even if they can be clinically achieved. The results of our study indicate that a relationship exists between the convergence angle and the critical cement thickness that is necessary to realize the maximum strength properties of zinc phosphate cement.