Frictional torque in surface and conventional hip replacement.

UNLABELLED: The frictional torque of the Trapezoidal-28 total hip replacement and the Tharies SR-3 and SR-5 surface replacements were measured in the University of California at Los Angeles hip-joint simulator in calf serum. Loads of as much as 890 newtons were applied while the femoral component oscillated through a 60-degree arc at forty cycles per minute. Frictional torque, as measured by a ring transducer at a fixed distance from the center of rotation, was seen to diminish as the thickness of the polyethylene increased and was found to be relatively proportional to the diameter of the femoral component. The torques measured in this study were found to be twenty to 100 times smaller than the static torques to failure for acetabular fixation that have been reported in the literature. Finally, no permanent deformation of the thin acetabular shells attributable to the cementing process was apparent when the recommended technique of surgical insertion was utilized. CLINICAL RELEVANCE: Surface replacement components have been shown to generate greater frictional torques at the acetabular bone-cement interface than do smaller-diameter conventional hip replacements. This indicates that the local mechanical-loading conditions of the hip joint are not the same for both types of replacements, and this should be considered when selecting the type of arthroplasty for an individual patient.