The height-to-width index for the assessment of femoral head deformity following osteonecrosis in the treatment of developmental dysplasia.

BACKGROUND: The principal complications that follow the treatment of developmental dysplasia of the hip are redislocation and growth disturbance of the femoral head and neck as a result of osteonecrosis of the femoral epiphysis. Growth disturbance secondary to osteonecrosis is difficult to determine until long after the treatment episode has passed. Consequently, the treating surgeon has little early feedback regarding the long-term consequences of management interventions. We therefore sought to devise a quantitative method to identify early evidence of growth disturbance related to osteonecrosis. METHODS: The width and height of the epiphyses were measured on anteroposterior radiographs of the pelvis made twelve to eighteen months after successful closed reduction and on the latest available radiograph for each patient (mean age, 8.6 years). The epiphyseal index was calculated by dividing the height by the width. The radiographs were also scored for osteonecrosis with use of the Kalamchi and MacEwen classification system and were also assessed for sphericity with use of Mose rings. RESULTS: Forty-seven patients with late-presenting developmental dysplasia of the hip who subsequently underwent successful closed reduction were included. An index of <0.357 on the twelve to eighteen-month post-treatment radiograph strongly predicted the development of a nonspherical femoral head on the latest radiograph (sensitivity, 0.83; specificity, 0.95; positive predictive value, 0.55; and negative predictive value, 0.99). CONCLUSIONS: The height-to-width index appears to be a simple and quantifiable measurement of the severity of growth disturbance as a consequence of osteonecrosis following treatment for developmental dysplasia of the hip. It is predictive of asphericity at the time of intermediate-term follow-up and appears likely to predict asphericity at maturity, but this must be confirmed with follow-up to maturity. Unlike the currently used methods of assessing osteonecrosis, the index allows for the quantifiable evaluation of growth disturbance within a few years after the corrective procedure.