The Contribution of Frictional Torque to Loosening at the Cement-Bone Interface in Tharies Hip Replacements*

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Abstract
We evaluated the results in a series of 170 osteoarthrotic hips (156 patients) at a median of 10.0 years (range, 3.0 to 16.4 years) after a Tharies resurfacing arthroplasty performed with cement. Our purpose was to assess the role of frictional torque in loosening at the cement-bone interface. The hips were divided into three groups on the basis of the diameter of the bearing surface: small (thirty-six, thirty-nine, or forty-three millimeters), medium (forty-seven millimeters), and large (fifty-one or fifty-four millimeters). Comparisons were made with Kaplan-Meier survivorship analysis, with stepwise discriminant analysis of risk factors for aseptic loosening. Revision for aseptic loosening was the end point for survivorship analysis. Despite higher frictional torques due to the increased diameter of the bearing surface and the increased average load, the larger prostheses survived significantly longer than the smaller ones (p=0.002). Stepwise covariate discriminant analysis indicated that the size of the bearing surface was the only factor identified that significantly affected survival. In six patients who had had a bilateral surface replacement with a component of a different size in each hip, the smaller prosthesis always failed first (p=0.001), at an average of seventy-seven months; the larger prosthesis failed at an average of 113 months. Nine patients had had a bilateral replacement with components of the same size, and there was no significant difference in the durations of survival of the two prostheses. Analysis of radiographs and retrieved specimens indicated that, regardless of the size of the component, the mechanism of loosening on both the acetabular and the femoral side of this so-called double-cup replacement was progressive resorption of bone induced by polyethylene wear particles that compromised the fixation of the components. More time was required for the process to disrupt++ the larger fixation area of the larger components. These data indicate that frictional torque was not the primary factor in the loosening of these prostheses with a large bearing surface and that higher friction and frictional torques can be tolerated if the generation of wear debris is sufficiently limited. These findings may be important as alternatives to polyethylene bearing surfaces are investigated.

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