title: Where is the neck? Alpha angle measurement revisited.
abstract: OBJECTIVE
  The alpha angle is the most used measurement to classify concavity of the femoral head-neck junction. It is not only used for treatment decisions for hip impingement, but also in cohort studies relating hip morphology and osteoarthritis. Alpha angle measurement requires identification of the femoral neck axis, the definition of which may vary between studies. The original "3-point method" uses 1 single point to construct the femoral neck axis, whereas the "anatomic method" uses multiple points and attempts to define the true anatomic neck axis. Depending on the method used, the alpha angle may or may not account for other morphological characteristics such as head-neck offset.
  
  
  METHODS
  We compared 2 methods of alpha angle measurement (termed "anatomic" and "3-point") in 59 cadaver femora and 83 cross-table lateral radiographs of asymptomatic subjects. Results were compared using Bland-Altman plots.
  
  
  RESULTS
  Discrepancies of up to 13 degrees were seen between the methods. The 3-point method had an "equalizing effect" by disregarding femoral head position relative to the neck: in femora with high alpha angle, it resulted in lower values than anatomic measurement, and vice versa in femora with low alpha angles. Using the anatomic method, we derived a reference interval for the alpha angle in normal hips in the general population of 30-66 degrees.
  
  
  CONCLUSIONS
  We recommend the anatomic method because it also reflects the position of the femoral head on the neck. Consensus and standardization of technique of alpha angle measurement is warranted, not only for planar measurements but also for CT or MRI-based measurements.
authors.affiliation: Departments of Orthopaedic Surgery , Haga Hospital, the Hague , the Netherlands .
authors.forename: Heinse
authors.initials: H
authors.lastname: Bouma
authors.forename: Niels-Jan
authors.initials: NJ
authors.lastname: Slot
authors.forename: Paul
authors.initials: P
authors.lastname: Toogood
authors.forename: Tom
authors.initials: T
authors.lastname: Pollard
authors.forename: Paulien
authors.initials: P
authors.lastname: van  Kampen
authors.forename: Tom
authors.initials: T
authors.lastname: Hogervorst
mesh_terms: D015331:Cohort Studies; D005260:Female; D005269:Femur / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D005270:Femur Head / Q000033:anatomy & histology* / Q000000981:diagnostic imaging; D005272:Femur Neck / Q000033:anatomy & histology* / Q000000981:diagnostic imaging; D006801:Humans; D008297:Male; D011859:Radiography