Introduction: Cam impingement is characterized by abnormal contact between the proximal femur and acetabulum caused by a non-spherical femoral head, known as a cam deformity. A cam deformity is usually quantified by the alpha angle; greater alpha angles substantially increase the risk for osteoarthritis (OA). However, there is no consensus on which alpha angle threshold to use to define the presence of a cam deformity. Aim: To determine alpha angle thresholds that define the presence of a cam deformity and a pathological cam deformity based on development of OA. Methods: Data from both the prospective CHECK cohort of 1002 individuals (45-65 years) and the prospective population-based Chingford cohort of 1003 women (45-64 years) with respective follow-up times of 5 and 19 years were combined. The alpha angle was measured at baseline on anteroposterior radiographs, from which a threshold for the presence of a cam deformity was determined based on its distribution. Further, a pathological alpha angle threshold was determined based on the highest discriminative ability for development of end-stage OA at follow-up. Results: A definite bimodal distribution of the alpha angle was found in both cohorts with a normal distribution up to 60°, indicating a clear distinction between normal and abnormal alpha angles. A pathological threshold of 78° resulted in the maximum area under the ROC curve. Conclusion: Epidemiological data of two large cohorts shows a bimodal distribution of the alpha angle. Alpha angle thresholds of 60° to define the presence of a cam deformity and 78° for a pathological cam deformity are proposed.

Alpha angle, Cam deformity, Cam impingement, Definition, Femoroacetabular impingement, Hip OA,
Osteoarthritis and Cartilage
Department of Orthopaedics

Agricola, R, Waarsing, J.H, Thomas, G.E, Carr, A.J, Reijman, M, Bierma-Zeinstra, S.M, … Arden, N.K. (2014). Cam impingement: Defining the presence of a cam deformity by the alpha angle. Data from the CHECK cohort and Chingford cohort. Osteoarthritis and Cartilage, 22(2), 218–225. doi:10.1016/j.joca.2013.11.007