How useful is the alpha angle for discriminating between symptomatic patients with cam-type femoroacetabular impingement and asymptomatic volunteers?

PURPOSE

To compare the alpha-angle measurements in volunteers and patients with femoroacetabular impingement (FAI) and to develop potential threshold values.

MATERIALS AND METHODS

This study was approved by the institutional review board; all individuals signed informed consent. Magnetic resonance (MR) images at 1.5 T in 106 individuals (ages 20-50 years) were analyzed in 53 patients (33 cam- and 20 mixed-type FAI) and 53 age- and sex-matched asymptomatic volunteers. Alpha angles were measured on radially reformatted MR images of the proximal femur by two independent readers. Intraclass correlation coefficient (ICC) and receiver operating characteristic (ROC) were calculated.

RESULTS

Mean alpha angles were highest in the anterosuperior segment: 65.4° ± 11.5 [standard deviation] and 65.2° ± 7.3 for readers 1 and 2 in patients and 53.3° ± 9.6 and 55.0° ± 8.8 in volunteers, respectively (P < .001, patients vs volunteers). Alpha angles greater than 55° were measured in 20 (38%) and 33 (62%) of 53 volunteers for readers 1 and 2, respectively. Maximal alpha angle in any segment was substantially different (P < .001) in patients and volunteers (70.3° ± 11.2 vs 57.9° ± 10.5 for reader 1; 69.4° ± 8.8 vs 58.7° ± 8.9 for reader 2), with a large overlap. Overall interobserver agreement was good (ICC, 0.712). ROC showed the largest area under the curve at the anterosuperior segment: 0.791 and 0.824 for readers 1 and 2, respectively (P < .001). A 55° alpha-angle threshold value gave a sensitivity and specificity of 81% and 65% for reader 1 and of 90% and 47% for reader 2, respectively. A 60° alpha-angle threshold value gave a sensitivity and specificity of 72% and 76% for reader 1 and 80% and 73% for reader 2, respectively.

CONCLUSION

There is substantial overlap in the alpha-angle measurements between volunteers and patients with cam-type deformities. Discrimination is best at the anterosuperior segment. Increasing the alpha-angle threshold value from 55° to 60° reduces false-positive results while maintaining a reasonable sensitivity.