title: Quantifying the contribution of pincer deformity to femoro-acetabular impingement using 3D computerised tomography.
abstract: OBJECTIVE
  To provide a simple, reliable method for the three-dimensional quantification of pincer-type hip deformity.
  
  
  METHODS
  Computerised tomography scans of 16 normal female hips and 15 female hips with clinical femoro-acetabular impingement (FAI) and radiographic signs of pincer secondary to acetabular protrusio were analysed. After orientating the pelvis in the anterior pelvic plane, the acetabular centre was determined, and the ratios of its coordinates to the corresponding pelvic dimensions were calculated. Acetabular coverage of the femoral head and centre-edge angles were also measured for the two groups.
  
  
  RESULTS
  In hips with a pincer, the hip was medialised by 37 % (p = 0.03), more proximal by 5 % (p = 0.05) and more posterior by 9 % (p = 0.03) compared with the normal hips. Coverage of the femoral head in protrusio hips was significantly greater than normal (average 71 % vs 82 %, p = 0.0001). Both the lateral centre-edge angle and the combined anterior-posterior centre-edge angle were greater in protrusio hips than in the normal ones (48° vs 37 °, p < 0.001; and 216° vs 176°, p < 0.0001 respectively).
  
  
  CONCLUSIONS
  Displacement in acetabular protrusio occurs in all planes. This CT-based method allows for the accurate and standardised quantification of the extent of displacement, as well as 3D measurement of femoral head coverage. In the adult female population, a combined centre-edge angle of over 190° suggests an acetabulum that is too deep and a potential cause of symptoms of femoro-acetabular impingement. Conversely, an acetabulum that has a combined centre-edge angle of less than 190° may be considered to be of normal depth, and therefore not contributing a pincer to FAI should it occur.
authors.affiliation: Department of Orthopaedic Surgery, Imperial College London, Charing Cross Hospital, 7th Floor, East Wing, Fulham Palace Road, London W6 8RF, UK. w.dandachli@imperial.ac.uk
authors.forename: Wael
authors.initials: W
authors.lastname: Dandachli
authors.forename: Ali
authors.initials: A
authors.lastname: Najefi
authors.forename: Farhad
authors.initials: F
authors.lastname: Iranpour
authors.forename: Jonathan
authors.initials: J
authors.lastname: Lenihan
authors.forename: Alister
authors.initials: A
authors.lastname: Hart
authors.forename: Justin
authors.initials: J
authors.lastname: Cobb
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000465:Algorithms; D005260:Female; D057925:Femoracetabular Impingement / Q000000981:diagnostic imaging* / Q000209:etiology; D006621:Hip Joint / Q000002:abnormalities* / Q000000981:diagnostic imaging*; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000379:methods*; D008297:Male; D008875:Middle Aged; D011856:Radiographic Image Enhancement / Q000379:methods; D011857:Radiographic Image Interpretation, Computer-Assisted / Q000379:methods*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014057:Tomography, X-Ray Computed / Q000379:methods*; D055815:Young Adult