Lessons learned from loosening of the McKee-Farrar metal-on-metal total hip replacement

Clinical and radiographic data for 15 McKee-Farrar hip replacements that ha d failed because of aseptic loosening (4 stem loosening, 9 cup loosening, a nd 2 loosening of both components) between 0.6 and 21 years (average, 8.3 y ears) were compared with 15 hips in which the McKee-Farrar total hip replac ement has survived between 21 and 26 years. Hips that loosened were biomech anically disadvantaged compared with those that demonstrated long-term surv ival. Radiographic evaluation demonstrated that in hips that were revised f or aseptic femoral loosening, the offset was decreased by a mean of 1.4, wh ereas it was increased by a mean of 4.0 mm in the surviving hips (P = .04). Further, in hips revised for aseptic loosening, the center of rotation was medialized by a mean of only 1.4 mm, whereas the center of rotation was me dialized by a mean of 6.4 mm in the surviving hips (P = .1). Unfavorable bi omechanics results in increased joint reaction forces that could contribute to loosening of these prostheses. Five of 6 McKee-Farrar stems revised for aseptic loosening compared with 7 of 15 surviving stems were in varus (P = .1) and, as a result, had cement mantle defects in zones III and VII. Thus , in the McKee-Farrar; similar to what has bren seen in hips with metal-on- plastic bearings, curved stems are associated with varus positioning, cemen t mantle defects, and loosening. Wear of the metal-on-metal articulation do es not appear to be the cause of failure in these cases. Wear could not be detected radiographically. At revision surgery, there was no indication of excessive bearing wear or gross metal staining of periprosthetic tissues. M icroscopic analysis of tissue sections demonstrated both metal and polymeth ylmethacrylate particles of variable size and shape. The variability of the particles suggests that they are likely the result of loosening and that t hey were not generated by bearing surface wear that could cause loosening. Although it is hoped that improvements in the wear resistance of the bearin g will increase survivorship, this experience and analysis of the McKee-Far rar total hip replacement illustrates the importance of the implant design, biomechanics of the reconstruction, and role of surgical implantation tech nique.