Loosening of threaded acetabular cups in arthroplasty of the hip. The association with different types of coxarthrosis.

Seventy-nine patients underwent hip arthroplasty using a threaded acetabular cup during 1988 and 1989. Out of 71 cups implanted for coxarthrosis, radiographs of 62 have been reviewed at a mean follow up of 69 months. The survival rate at 5.5 years was 77.5%. The mean age at operation was 61 years. Loosening of the cup occurred in 7 cases with pain on walking; 5 had a revision operation. In another 28 patients, radiolucent lines greater than 1 mm were found around the cup and 7 of these were unstable. The previous coxarthrosis was classified as atrophic with 5 loose cups, normotrophic with 2 loose cups and hypertrophic with no loosening. This difference between loosening in the atrophic and hypertrophic types of preexisting coxarthrosis was statistically significant. Radiolucent lines were present in 61% of the atrophic cases, 53.4% of the normotrophic and 25% of the hypertrophic, which was also significant. If the cases of loosening are added to those with radiolucent lines, the types of coxarthrosis show highly significant differences. Threaded cups had a high percentage of failure which was greater in cases with atrophic coxarthrosis.