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°),
grade 2 (200° to 249°), grade 3 (250° to 299°), and grade 4
(=300°). 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
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° (the low-risk
group) collapsed, all twenty-five hips with a combined necrotic angle of
=240° (the high-risk group) collapsed, and four (50%) of the eight hips
with a combined necrotic angle between 190° and 240° (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.