Magnetic resonance imaging for early diagnosis of idiopathic osteonecrosis of the femoral head

Twenty-one patients with silent hip (26 bone joints) in whom X-rays failed to detect abormality were proven to have abnormality on magnetic resonance imaging (MRI). Using frontal and sagittal MRI, the 21 patients were thus examined to determine which types of patients develop osteonecrosis. Collapse of the femoral head developed in 10 patients (12 joints), all of whom showed ring (R) or band (B) pattern on either frontal or sagittal MRI view. The occurrence time of collapse ranged from 6 months to 3 years and 3 months with a mean of 2 years and 2 months after the initial scanning of MRI. When restricting to either view, neither R nor B pattern was detected in 4 joints. This suggested that MRI in two views is useful. When the outer edge of R or B pattern exceeded the outer margin of the acetabulum in the frontal view, and when it exceeded the anterior 1/2 of the femoral head in the sagittal view, collapse occurred in a high incidence. In predicting collapse of the femoral head, the presence and location of R or B pattern should be determined from both frontal and sagittal views. (N.K.)