The Discrepancy in the Posterior Boundary of Necrotic Lesion Between Axial and Oblique Axial Slices of MRI in Patients with Osteonecrosis of the Femoral Head.

BACKGROUND

Little is known concerning differences in assessment of anteroposterior osteonecrotic lesion of the femoral head (ONFH) between the axial plane parallel to the body axis (axial) and the axial plane parallel to the femoral neck axis (oblique axial) using magnetic resonance imaging (MRI). This study aimed to compare the posterior boundary of necrotic lesions between the axial and oblique axial planes on MRI.

METHODS

This study retrospectively reviewed 120 consecutive hips in 71 patients (47 males and 24 females; mean age, 42 years) with ONFH, for which both axial and oblique axial MRI slices were available. The posterior boundaries of the necrotic lesions were calculated as the ratio of posterior intact circumference to anteroposterior circumference of the femoral head using mid-axial and mid-oblique axial MRI slices. The necrotic angles, calculated using the modified Kerboul method, were compared between cases with progression and nonprogression of collapse.

RESULTS

The mean posterior intact ratio in the axial slice was 49.2% (range, 8.6 to 85.1%), while that in the oblique axial slice was 33.5% (7.5 to 79.2%). The posterior intact ratio in the oblique axial slice significantly decreased compared to that in the axial slice (p < 0.0001). In cases with progression of collapse, the mean necrotic angle was significantly larger in the oblique axial slice than in the axial slice (p < 0.0001). The sensitivity and specificity of the necrotic angle with regard to the prognosis of the femoral head were both higher when using the oblique axial slice (sensitivity: 88.4%, specificity: 82.3%) than when using the axial slice (sensitivity: 85.1%, specificity: 62.0%). The posterior intact ratio at a mean of 6.8 mm superior to the mid-axial slice corresponded to that of the mid-oblique axial slice.

CONCLUSION

Our findings suggest that the posterior boundaries of necrotic lesions differ between axial and oblique axial slices, and examination of the oblique axial MRI slice might be more suitable for predicting the prognosis of the femoral head.

LEVEL OF EVIDENCE

Diagnostic-Investigating a diagnostic test. Level IV (Case series).