The risk of acetabular protrusion following prosthetic replacement of the femoral head.

A total of 24 patients treated with hemiarthroplasty showed a crude frequency of acetabular protrusion of 26 per cent. Two factors were associated with the development of protrusion: the length of the follow-up period and previous hip operation. The frequency of protrusion was estimated using logistic regression techniques based on information concerning the age of the patients, the length of the follow-up period and any previous hip operation. The frequency ranged from 4.8 per cent for a patient of 75 years or under, with a follow-up period of less than 12 months and no previous hip operation, to 61 per cent for patients of over 75 years, with a follow-up period exceeding 12 months, and a previous hip operation. Patients of over 75 years were 1.7 times more likely to develop protrusion than younger patients, patients previously operated on were 2.3 times more likely to develop protrusion than those not operated on previously, and patients with a follow-up time period exceeding 12 months were 7.9 times more likely to develop protrusion than patients with a shorter follow-up time.