Mechanics of edge loading in total hip implants: Understanding severe wear and squeaking in ceramicon-ceramic bearings

Wear rates for ceramic-on-ceramic (CoC) hip implants are often much higher in vivo than predicted from conventional hip simulator tests. Also, many CoC implants begin squeaking 14–26 months postoperatively. These problems may arise because in vivo usage can impart edge loading that causes severe contact stresses, a condition that is not reproduced in standard in vitro tests. Several in vitro wear studies have subjected CoC implants to the severe conditions of edge loading, but since these test methods are not standardized, it is difficult to compare independent results. In particular, the detailed mechanics of edge loading have not been clearly published.;This research elucidates some key aspects of edge-loading mechanics and investigates the effects that the combination of severe and mild (dual-severity) loading conditions—namely, edge loading and loaded concentric articulation—have when applied to CoC implants. Specifically, correlations between dual-severity conditions and problems such as severe wear and squeaking were studied.;Free Body Diagram analysis was used to develop an analytical model that approximates the normal contact force for a simple case of edge loading. The model represented a custom apparatus designed to conduct fully-lubricated wear tests. Commercially-available CoC implants were subjected to alternating cycles of dual-severity conditions; both severe wear and audible squeaking (rare for lubricated in vitro tests) resulted. Wear progression was measured using a stylus profilometer, and vibrations corresponding with squeaking were measured with a laser vibrometer. The analytical model was validated with force measurements taken during edge loading.;Results showed that the analytical model closely approximates the edge-loading contact force for pristine and worn implants. Furthermore, the analytical model and the test results demonstrated that small increases in the friction coefficient caused the peak contact force to rise significantly. For large friction coefficients, the high contact force often prevented reduction from occurring. In one test, severe wear developed on the liner's inner diameter surface, and squeaking occurred, but only when the resultant force during concentric articulation was oriented with the wear scar on the femoral head. Many of the problems with CoC implants are better understood by evaluating the conditions described by edge-loading mechanics.