• BACKGROUND
    • The hypothesis that the combined necrotic angle measurement from magnetic resonance imaging scans predicts the subsequent risk of collapse in hips with femoral head necrosis was tested.
  • METHODS
    • Thirty-seven hips with early stage osteonecrosis in thirty-three consecutive patients were investigated. With use of the modified method of Kerboul et al., we measured the arc of the femoral surface involved by necrosis on a midcoronal as well as a midsagittal magnetic resonance image (rather than an anteroposterior and a lateral radiograph) and then calculated the sum of the angles. On the basis of the magnitude of the resulting combined angle, hips were classified into four categories: grade 1 (<200 degrees), grade 2 (200 degrees to 249 degrees), grade 3 (250 degrees to 299 degrees), and grade 4 (>/=300 degrees). After the initial evaluations, the hips were randomly assigned to a core decompression group or a nonoperative group. Patients underwent regular follow-up until femoral head collapse or for a minimum of five years.
  • RESULTS
    • Seven grade-4 hips and sixteen grade-3 hips had development of femoral head collapse by thirty-six months. Six of the nine grade-2 hips and none of the five grade-1 hips collapsed (log-rank test, p < 0.01). None of the four hips with a combined necrotic angle of /=240 degrees (the high-risk group) collapsed, and four (50%) of the eight hips with a combined necrotic angle between 190 degrees and 240 degrees (the moderate-risk group) collapsed during the study period.
  • CONCLUSIONS
    • The Kerboul combined necrotic angle, as ascertained with use of magnetic resonance imaging scans instead of radiographs, is a good method to assess future collapse in hips with femoral head osteonecrosis.
  • LEVEL OF EVIDENCE
    • Prognostic Level I. See Instructions to Authors on jbjs.org for a complete description of levels of evidence.