Quantifying the Extent of Femoral Head Osteonecrosis: A Modified Kerboul Method Using MRI
IKH Koo¹, YC Ha², HJ Kim¹, JJ Yoo¹, YM Kim^1
1Department of Orthopaedic Surgery, Seoul National University College of Medicine, Seoul, Korea, and ²Department of Orthopaedic Surgery, Gyeong-Sang National University School of Medicine, Chinju, Korea

We tested the hypothesis that combined necrotic angle measurement using MRI scans predicts the subsequent risk of collapse of femoral head osteonecrosis. Thirty-seven hips with early-stage osteonecrosis in 33 consecutive patients were investigated. The arc of the necrosis was measured by the method of Kerboul et al using mid-coronal and mid-sagittal MRI scans of the femoral head instead of anteroposterior and lateral radiographies, and the two angles were added. Hips were classified into four categories based on the magnitude of the added angle; grade 1 (<200°), grade 2 (200°-249°), grade 3 (250°-299°), and grade 4 (>300°). After the initial evaluations, the hips were randomly assigned to core-decompression group or conservatively-treated group. Patients underwent regular follow-up until femoral head collapse or for a minimum of five years. Seven hips in grade 4 and 16 hips in grade 3 developed femoral head collapse in 36 months; six out of nine hips in grade 2, and none of five hips in grade 1 developed collapse (log rank test, p<0.01). In a retrospective analysis, none of four hips with combined necrotic angle <190° (low risk group) collapsed, all 25 hips with combined necrotic angle >240° (high risk group) collapsed, and four (50%) of eight hips with combined necrotic angle between 190° and 240° (moderate risk group) collapsed during the study. The Kerboul combined necrotic angle ascertained by MRI scans instead of radiographies is a major predictor of future collapse.