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Contralateral hip in patients with unilateral nontraumatic osteonecrosis of the femoral head.

To clarify the risk of the development of osteonecrosis of the femoral head in the contralateral hip of patients with unilateral nontraumatic osteonecrosis of the femoral head, a followup study was performed using magnetic resonance imaging. Forty-six asymptomatic contralateral hips with no evidence of osteonecrosis on magnetic resonance imaging, in patients with unilateral osteonecrosis, were subjects of this study. The period from the onset of unilateral osteonecrosis to entrance in this study ranged from 1 month to 20 years (mean, 3.1 years). There were 29 males and 17 females ranging in age from 16 to 71 years (mean, 41 years). Twenty-three patients had previous steroid therapy for various conditions: 16 had alcoholism and 7 had no etiologic factors. The followup term ranged from 3 to 8 years (average, 5.5 years). During the followup period, 1 patient (2%) had a band of low signal intensity demarcating the lesion of normal fat intensity on T1 weighted images, which progressed to collapse of the femoral head. These results suggest that in unilateral osteonecrosis cases, osteonecrosis rarely develops in the contralateral hip that has appeared normal on magnetic resonance imaging. The contralateral hip with a negative magnetic resonance imaging scan is not in Stage 0 of osteonecrosis, but is normal.