title: Cam impingement: defining the presence of a cam deformity by the alpha angle: data from the CHECK cohort and Chingford cohort.
abstract: BACKGROUND
  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.
  
  
  OBJECTIVE
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Orthopaedics, Erasmus MC University Medical Centre, Rotterdam, The Netherlands. Electronic address: r.agricola@erasmusmc.nl.
authors.forename: R
authors.initials: R
authors.lastname: Agricola
authors.affiliation: Department of Orthopaedics, Erasmus MC University Medical Centre, Rotterdam, The Netherlands. Electronic address: e.waarsing@erasmusmc.nl.
authors.forename: J H
authors.initials: JH
authors.lastname: Waarsing
authors.affiliation: Nuffield Department of Orthopaedics, Rheumatology and Musculoskeletal Sciences, NIHR Musculoskeletal Biomedical Research Unit, University of Oxford, Nuffield Orthopaedic Centre, Oxford, England, UK. Electronic address: geraint.thomas@ndorms.ox.ac.uk.
authors.forename: G E
authors.initials: GE
authors.lastname: Thomas
authors.affiliation: Nuffield Department of Orthopaedics, Rheumatology and Musculoskeletal Sciences, NIHR Musculoskeletal Biomedical Research Unit, University of Oxford, Nuffield Orthopaedic Centre, Oxford, England, UK. Electronic address: andrew.carr@ndorms.ox.ac.uk.
authors.forename: A J
authors.initials: AJ
authors.lastname: Carr
authors.affiliation: Department of Orthopaedics, Erasmus MC University Medical Centre, Rotterdam, The Netherlands. Electronic address: m.reijman@erasmusmc.nl.
authors.forename: M
authors.initials: M
authors.lastname: Reijman
authors.affiliation: Department of Orthopaedics, Erasmus MC University Medical Centre, Rotterdam, The Netherlands; Department of General Practice, Erasmus MC University Medical Centre, Rotterdam, The Netherlands. Electronic address: s.bierma-zeinstra@erasmusmc.nl.
authors.forename: S M A
authors.initials: SM
authors.lastname: Bierma-Zeinstra
authors.affiliation: Nuffield Department of Orthopaedics, Rheumatology and Musculoskeletal Sciences, NIHR Musculoskeletal Biomedical Research Unit, University of Oxford, Nuffield Orthopaedic Centre, Oxford, England, UK. Electronic address: sion.glyn-jones@ndorms.ox.ac.uk.
authors.forename: S
authors.initials: S
authors.lastname: Glyn-Jones
authors.affiliation: Department of Orthopaedics, Erasmus MC University Medical Centre, Rotterdam, The Netherlands; Department of Biomechanical Engineering, Delft University of Technology, Delft, The Netherlands; Department of Orthopaedics and Rheumatology, University Medical Centre Utrecht, Utrecht, The Netherlands. Electronic address: h.h.weinans@umcutrecht.nl.
authors.forename: H
authors.initials: H
authors.lastname: Weinans
authors.affiliation: Nuffield Department of Orthopaedics, Rheumatology and Musculoskeletal Sciences, NIHR Musculoskeletal Biomedical Research Unit, University of Oxford, Nuffield Orthopaedic Centre, Oxford, England, UK. Electronic address: nigel.arden@ndorms.ox.ac.uk.
authors.forename: N K
authors.initials: NK
authors.lastname: Arden
mesh_terms: D000368:Aged; D015331:Cohort Studies; D004739:England / Q000453:epidemiology; D005260:Female; D057925:Femoracetabular Impingement / Q000150:complications / Q000000981:diagnostic imaging / Q000453:epidemiology / Q000473:pathology*; D005270:Femur Head / Q000000981:diagnostic imaging / Q000473:pathology; D006621:Hip Joint / Q000000981:diagnostic imaging / Q000473:pathology; D006801:Humans; D008297:Male; D008875:Middle Aged; D009426:Netherlands / Q000453:epidemiology; D015207:Osteoarthritis, Hip / Q000453:epidemiology / Q000209:etiology / Q000473:pathology; D011859:Radiography; D015203:Reproducibility of Results
keywords: Alpha angle; Cam deformity; Cam impingement; Definition; Femoroacetabular impingement; Hip OA