Correlation of alpha angle between various radiographic projections and radial magnetic resonance imaging for cam deformity in femoral head-neck junction.

PURPOSE: Radial magnetic resonance imaging (MRI) along the axis of the femoral head neck is the gold standard for detection of cam deformity of the proximal femur. This study was performed to identify which plain radiographic projection was best correlated with radial MRI.

METHODS

Five different plain radiographic projections and 18 slices of radial MRI were applied to 35 consecutive hips with groin pain and positive impingement sign. Alpha angles were measured to detect the asphericity of the femoral head-neck junction in all images. Radiographs were taken in anteroposterior pelvis, cross-table lateral, 90° Dunn, 45° Dunn and modified 45° Dunn views. Pearson's correlation coefficients were determined to assess the association between the alpha angle obtained from radial MRI and each radiographic technique. The sensitivity, specificity, positive and positive predictive values and accuracy of plain radiographic alpha angle measurements were assessed using a threshold alpha angle value of 50.5° for cam deformity.

RESULTS

Pearson's correlation coefficients in the alpha angle values between MRI and plain radiographic projections were 0.45, 0.70, 0.62, 0.81 and 0.69 for the anteroposterior pelvis, cross-table lateral, 90° Dunn, 45° Dunn and modified 45° Dunn views, respectively. In terms of sensitivity and accuracy, the 45° Dunn view had the greatest values.

CONCLUSIONS

Alpha angle of the 45° Dunn view was best correlated with that of radial MRI. The 45° Dunn view had better sensitivity and accuracy than other radiographic projections. The 45° Dunn view may be preferable for screening of cam deformity.

LEVEL OF EVIDENCE: II.