Evaluation of resistance form for prepared teeth.

Resistance form of preparations for which castings were made in a large dental laboratory was evaluated. Each preparation was also evaluated for the placement of grooves. Evaluation of resistance form was done by grasping the casting between thumb and index finger and attempting to roll the uncemented casting off the die with finger pressure. Each preparation was tested for resistance form in all directions (buccal, lingual, mesial, and distal), from a fixed point of rotation. Evaluation was by tooth groups: incisors, canines, premolar, and molars. The preparations were found to have resistance form in 96% of the incisors, 92% of the canines, 81% of the premolars and only 46% of the molars. Only nine of 294 preparations evaluated had grooves. Mesial or distal grooves provided resistance to buccal and lingual dislodgment, and buccal or lingual grooves provided resistance to mesial and distal dislodgment.