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A study was begun in 1983 to determine the efficacy of a threaded acetabular cup. Fifty-five patients who had a total of sixty-eight threaded titanium cups had a complete clinical and radiographic evaluation yearly. Fifty-two of the arthroplasties had been primary and sixteen, revisions. The average duration of follow-up was six years (range, five to nine years). Seventeen cups had to be revised at an average of sixty-two months (range, twenty-seven to 108 months) after the index operation. Nine additional cups were loose and revision was pending at the most recent follow-up examination. Failure was defined as revision or pending revision. Thus, twenty-six (38 per cent) of the sixty-eight cups failed. Sixteen (31 per cent) of the fifty-two primary arthroplasties failed and ten of the sixteen revision arthroplasties failed. Radiographic changes that were evident in patients who had a failed cup consisted of superomedial migration of the cup with osteolysis in Zone 3, as classified by DeLee and Charnley. These radiographic changes preceded symptoms in most patients. Because of the high rate of failure of this acetabular component at six years, we believe that its use is not warranted.