A widely variable prevalence of dislocation after total hip arthroplasty has been reported, partly because of varying durations of follow-up for this specific end-point. The effect of demographic factors on the long-term risk of dislocation as a function of time after total hip arthroplasty is not well understood. The purpose of the present study was to determine the risk of dislocation as a function of time after Charnley total hip arthroplasty and to investigate the demographic factors that influence the cumulative risk of dislocation.

Six thousand six hundred and twenty-three consecutive primary Charnley total hip arthroplasties were performed in 5459 patients at one institution between 1969 and 1984. The patients included 2869 female patients and 2590 male patients with a mean age of sixty-three years. All procedures were performed with a 22-mm femoral head, and all femoral and acetabular components were fixed with cement. The patients were followed at routine intervals and were specifically queried about dislocation. The cumulative risk of dislocation was calculated with use of the Kaplan-Meier method.

Three hundred and twenty hips (4.8%) dislocated. The cumulative risk of a first-time dislocation was 1% at one month and 1.9% at one year and then rose at a constant rate of approximately 1% every five years to 7% at twenty-five years for patients who were alive and had not had a revision by that time. Multivariate analysis revealed that the relative risk of dislocation for female patients (as compared with male patients) was 2.1 and that the relative risk for patients who were seventy years old or more (as compared with those who were less than seventy years old) was 1.3. Three underlying diagnoses-osteonecrosis of the femoral head, acute fracture or nonunion of the proximal part of the femur, and inflammatory arthritis-were associated with a significantly greater risk of dislocation than osteoarthritis was.

The cumulative long-term risk of dislocation after total hip arthroplasty is considerably greater than has been reported in short-term studies. The incidence of dislocation is highest in the first year after arthroplasty and then continues at a relatively constant rate for the life of the arthroplasty. Patients at highest risk are female patients and those with a diagnosis of osteonecrosis of the femoral head or an acute fracture or nonunion of the proximal part of the femur. Level of Evidence: Prognostic study, Level II-1 (retrospective study). See Instructions to Authors for a complete description of levels of evidence.

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