Bipolar head design: inner bearing range of motion and disassociation.

To address the clinical problems of joint stiffness, acetabular pain, and component wear, recent bipolar heads have been designed to achieve increased inner bearing range of motion. We tested four designs to determine if this compromises component integrity. Inner bearing ranges of motion were determined and the components then mechanically tested to determine inner bearing pull-out disassociation strengths as well as static and dynamic impingement forces for disassociation. Inner bearing ranges of motion with a 22 mm head were between 65 degrees to 84 degrees for the four prostheses. Pull-out forces for disassociation ranged between 700 N to 1475 N; static impingement forces were 20 Nm to 49 Nm and dynamic impingement forces were 5 Nm to 24 Nm. There was no relation between bipolar head inner bearing range of motion and the potential for component disassociation; however, one design modification produced a lowered disassociation strength. Design modifications must be evaluated by a variety of test methods to adequately determine their effects on bipolar head integrity.