Relationship between the alpha and beta angles in diagnosing CAM-type femoroacetabular impingement on frog-leg lateral radiographs

Alpha and beta angles are commonly used radiographic measures to assess the sphericity of the proximal femur and distance between the pathologic head–neck junction and the acetabular rim, respectively. The aim of this study was to explore the relationship between these two measurements on frog-leg lateral hip radiographs. Fifty frog-leg lateral hip radiographs were evaluated by two orthopaedic surgeons and two radiologists. Each reviewer measured the alpha and beta angles on two separate occasions to determine the relationship between positive alpha and beta angles and the inter- and intra-observer reliability of these measurements. There was no significant association between positive alpha and beta angles, [kappa range −0.043 (95 % CI −0.17 to 0.086) to 0.54 (95 % CI 0.33–0.75)]. Intra-observer reliability was high [alpha angle intra-class correlation coefficient (ICC) range 0.74 (95 % CI 0.58–0.84) to 0.99 (95 % CI 0.98–0.99) and beta angle ICC range 0.86 (95 % CI 0.76–0.92) to 0.97 (95 % CI 0.95–0.98)]. There is no statistical or functional relationship between readings of positive alpha and beta angles. The radiographic measurements resulted in high intra-observer and fair-to-moderate inter-observer reliability. Results of this study suggest that the presence of a CAM lesion on lateral radiographs as suggested by a positive alpha angle does not necessitate a decrease in clearance between the femoral head and acetabular rim as measured by the beta angle and thus may not be the best measure of functional impingement. Understanding the relationship between these two aspects of femoroacetabular impingement improves a surgeon’s ability to anticipate potential operative management. III.