Cam impingement: defining the presence of a cam deformity by the alpha angle: data from the CHECK cohort and Chingford cohort

Osteoarthritis Cartilage. 2014 Feb;22(2):218-25. doi: 10.1016/j.joca.2013.11.007. Epub 2013 Nov 21.


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.

Keywords: Alpha angle; Cam deformity; Cam impingement; Definition; Femoroacetabular impingement; Hip OA.

Publication types

  • Multicenter Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Aged
  • Cohort Studies
  • England / epidemiology
  • Female
  • Femoracetabular Impingement / complications
  • Femoracetabular Impingement / diagnostic imaging
  • Femoracetabular Impingement / epidemiology
  • Femoracetabular Impingement / pathology*
  • Femur Head / diagnostic imaging
  • Femur Head / pathology
  • Hip Joint / diagnostic imaging
  • Hip Joint / pathology
  • Humans
  • Male
  • Middle Aged
  • Netherlands / epidemiology
  • Osteoarthritis, Hip / epidemiology
  • Osteoarthritis, Hip / etiology
  • Osteoarthritis, Hip / pathology
  • Radiography
  • Reproducibility of Results