How Useful Is the Alpha Angle for Discriminating between Symptomatic Patients with Cam-type Femoroacetabular Impingement and Asymptomatic Volunteers?

Abstract

MBA 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° 6 11.5 [standard deviation] and 65.2° 6 7.3 for readers 1 and 2 in patients and 53.3° 6 9.6 and 55.0° 6 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° 6 11.2 vs 57.9° 6 10.5 for reader 1; 69.4° 6 8.8 vs 58.7° 6 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.q RSNA, 2012 1

Patient-Specific Anatomical and Functional Parameters Provide New Insights into the Pathomechanism of Cam FAI

“…1A). The radial alpha angle was obtained through a 1:30 clockface rotation about the longitudinal femoral neck axis [39,43], observing the anterosuperior quadrant (Fig. 1B).…”

“…1B). An axial alpha angle greater than 50.5°or radial alpha angle greater than 60°w as considered as cam deformity [6,43]. The femoral neck-shaft angle, from the frontal plane, was formed between the femoral neck and shaft axes (Fig.…”

“…Defining who is at risk of impingement and cartilage damage has been mainly based on the size and location of the cam deformity, which has been traditionally quantified by the alpha angle on multiplane imaging [3,7,36,37,39,43,45]. Several authors have shown that the severity of the cam deformity (that is, a higher alpha angle) is associated with an increased risk of hip pain and joint degeneration [7,28,33].…”

“…Typically, a cam deformity is indicated by an alpha angle greater than 50.5°and 60°in the oblique-axial and radial plane, respectively [7,36,37,39]. However, the accuracy and sensitivity of the alpha angle in determining the risk of developing symptomatic FAI is inconsistent [6,30,34,37,43]. Moreover, other common radiographic measures of the femoral head and neck and acetabulum have been associated with symptomatic FAI [5,10,26,40], but it remains unclear as to which combination of parameters plays a role in identifying patients at risk of developing symptoms.…”

“…Many individuals with cam deformities may not experience mechanical impingement or pain symptoms, thus qualifying them as asymptomatic individuals [11,18,26,43]. Therefore, we postulated that FAI symptoms may be related to other anatomical parameters that can exacerbate mechanical impingement in patients at risk of developing symptoms.…”