Fatty marrow conversion of the proximal femoral metaphysis in osteonecrotic hips.

To determine whether fatty marrow conversion of the proximal femoral metaphysis is related to osteonecrosis of the femoral head using a marrow conversion index ([signal intensity of the proximal femoral metaphysis/signal intensity of the greater trochanter] x 100 in T1 weighted magnetic resonance images), a case control study was conducted on 42 osteonecrotic hips in 28 patients. The 28 patients (42 osteonecrotic hips) were matched with 84 control patients (84 normal hips) for gender, age (5-year range), and time of presentation (1-year range). The marrow conversion index was measured in each hip studied. The index was 90.2% (standard deviation, 8.2%) in osteonecrotic hips and 75.1% (standard deviation, 9.1%) in matched controls. By conditional logistic regression, a 5% increase in the index was associated with 3.6 times increase of the odds ratio of osteonecrosis and a 10% increase with a 12.9 times increase of the odds ratio. The marrow conversion index, which reflects the ratio of fatty marrow conversion of the proximal femoral metaphysis to that of the greater trochanter measured on T1 weighted magnetic resonance images, is increased in osteonecrotic hips.