Prediction of collapse in femoral head osteonecrosis: a modified Kerboul method with use of magnetic resonance images.

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 </=190 degrees (the low-risk group) collapsed, all twenty-five 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.