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Current status of acetabular fixation in primary total hip arthroplasty.

Factors that influence the outcome of acetabular replacement are design materials, means of fixation, operative technique, and patient-related parameters (e.g., etiology of osteoarthrosis). Whereas improved cementing techniques have produced a marked reduction in the rate of femoral component loosening, the incidence of acetabular loosening has been only slightly influenced by such improvements. Presently, uncemented porous-coated acetabular components represent the state of the art in total hip arthroplasty. Experimental and clinical data have shown in histologic, radiologic, clinical, and survivorship studies that hemispheric cups are superior to other designs and that primary stability can be better maintained by creating "intrinsic" stability (e.g., "oversized cup") rather than by screw fixation. Threaded cups have failed to demonstrate any improvement in results and have been virtually abandoned in the United States. The idea of metal backing has some obvious theoretical advantages. However, metal backing has failed to provide any improvement with respect to cemented cups. There are great reservations concerning metal backing in cementless fixation. Although there is some enthusiasm about hydroxyapatite, a "wait-and-see" attitude is justified because of the brittleness of the material, its questionable strength of bonding to substrate, and its unproven long-term behavior in vivo. Polyethylene as a bearing surface remains problematic, and the future will show whether new technologies are able to solve the problems encountered with metal-to-metal combinations. For the size of the femoral head, a compromise between smaller (22 mm) and larger (32 mm) components seems to be most effective.