Heterotopic bone formation in total hip arthroplasty: predisposing factors, classification and the significance for clinical outcome.

PURPOSE OF THE STUDY: Heterotopic bone formation in the area around the femoral head is a recognised complication following total hip arthroplasty. The aim of the study was to determine the prevalence of heterotopic ossification and to select a method of classification. Also, we studied the potential association of heterotopic bone with the following variables: sex, age at operation, operative technique, type of prosthesis and previous surgery on the ipsilateral hip. MATERIAL: The study involved 103 consecutive patients who underwent total hip replacement at our department in 1995. 175 patients were excluded from the study. Among 103 patients there were 35 male and 68 female patients. METHOD: Antero-posterior radiographs of the hips were reviewed preoperatively, six weeks after the operation and at one year after surgery. The degree of heterotopic bone formation was determined using the classifications of Brooker et al. (Grades 0-IV) and Kjaersgaard-Andersen et al. (Grades I and II). RESULTS: Heterotopic bone has developed in 27 (26.2%) patients, in 37% of male patients, mean age 59.8 years, and in only 20.5% female patients mean age 68.3 years. Brooker et al. Grade I ossification developed in 37% of the patients, Grade II in 44.4% and Grade III in 18.6%. According to the Kjaersgaard-Andersen et al. classification 22 patients developed Grade I and 5 patients Grade II ossification. The leading indication for surgery was osteoarthrosis, followed by aseptic necrosis (7.4%). Heterotopic bone formation was found in 20% of all patients with aseptic necrosis of the femoral head. Only 2.7% of patients operated on the ipsilateral hip developed ossifications. DISCUSSION: The study has shown that the incidence of ossifications is relatively low, yet still within the range of percentages reported in the literature. The finding that heterotopic bone most frequently develops in men and women older than 65 years both with osteoarthrosis is consistent with some authors. We found no association between the incidence of heterotopic ossification and the operative approach used or the type of prosthesis implanted, which has been confirmed by other investigators. Authors confirm that clinical parameters were significantly affected only in the most severe cases of ossification. CONCLUSION: The study has identified male patients and female patients older than 65 years both with osteoarthrosis as risk factors for the formation of heterotopic bone. The prevalence did not depend on the operative approach or type of prosthesis used. Heterotopic ossification was not associated with previous operative treatment of the hip. Both classification systems used in this study have proved reliable.