Ectopic ossification in hip arthroplasty. A retrospective study of predisposing factors in 637 cases.

We investigated predisposing factors for the development of heterotopic ossification in a retrospective study of 637 hip arthroplasties, of which 484 were unilateral, 62 bilateral and 29 revision operations. The frequency of heterotopic ossification after a primary hip arthroplasty was 57 percent. In a univariate analysis, men, patients with hypertrophic arthrosis, and cemented arthroplasty were all at risk of developing heterotopic ossification. After a multivariate analysis, the male sex and the cemented arthroplasty remained as significant factors. In bilateral operations, the contralateral side developed heterotopic ossification in 82 percent when the primary hip operation had already caused ossification. There was no increase in ossifications after the contralateral operation. Half of the revision operations had an increase of heterotopic ossification from 1 to 4 Brooker classes.