title: Posterior acetabular arc angle of the femoral head assesses instability of posterior fracture-dislocation of the hip.
abstract: OBJECTIVE
  Unstable posterior fracture-dislocation of the hip is determined by the wall defect or acetabular fracture index. The unstable hip is a result of inadequate posterior acetabular coverage of the femoral head from the posterior acetabular wall fracture. In order to measure total posterior acetabular coverage of the femoral head and avoid using the contralateral acetabulum as a calculation reference, the posterior acetabular arc angle of the femoral head was measured to assess stability of posterior fracture-dislocation of the hip.
  
  
  METHODS
  Using coronal computed tomography (CT) scan of the normal contralateral acetabulum at the level of the widest acetabular diameter and thinnest medial wall of 60 acetabular fractures, posterior acetabular arc angles of the femoral head in intact, 20 % and 50 % defects of posterior acetabular walls were measured. The angles were measured from the acetabular centre to the thinnest medial wall and to the top, inner cortex of 80 % and 50 % posterior acetabular walls.
  
  
  RESULTS
  Average intact, 80 % and 50 % posterior acetabular walls were 33.82 ± 4.30, 26.88 ± 3.33 and 16.91 ± 2.15 mm which corresponded to 92.25 ± 11.34, 77.42 ± 10.04 and 50.63 ± 6.58° of posterior acetabular arc angles of the femoral head. The intraclass correlation coefficient (ICC) of the measurements including correlation of conversion of posterior acetabular wall depths to posterior acetabular arc angles of the femoral head were more than 0.82 and 0.89.
  
  
  CONCLUSIONS
  The measurement technique of posterior acetabular arc angle of the femoral head has strong reliability. Therefore, stable or unstable posterior fracture-dislocation of the hip can be determined in terms of more than 77 degrees or less than 50 degrees of posterior acetabular arc angles of the femoral head instead of less than 20 % or more than 50 % posterior acetabular wall defect.
authors.affiliation: Department of Orthopaedic Surgery, Faculty of Medicine Siriraj Hospital, Mahidol University, Nakhon Pathom, Thailand. tmthr@mahidol.ac.th
authors.forename: Thossart
authors.initials: T
authors.lastname: Harnroongroj
authors.forename: Purinon
authors.initials: P
authors.lastname: Suangyanon
authors.forename: Theerawoot
authors.initials: T
authors.lastname: Tharmviboonsri
authors.forename: Thos
authors.initials: T
authors.lastname: Harnroongroj
mesh_terms: D000077:Acetabulum / Q000000981:diagnostic imaging*; D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D005260:Female; D005270:Femur Head / Q000000981:diagnostic imaging*; D006617:Hip Dislocation / Q000150:complications* / Q000000981:diagnostic imaging; D006620:Hip Fractures / Q000150:complications* / Q000000981:diagnostic imaging; D006621:Hip Joint / Q000000981:diagnostic imaging*; D006801:Humans; D007593:Joint Instability / Q000175:diagnosis* / Q000000981:diagnostic imaging / Q000209:etiology*; D008297:Male; D008875:Middle Aged; D008953:Models, Anatomic; D012015:Reference Standards; D015203:Reproducibility of Results; D012189:Retrospective Studies; D014057:Tomography, X-Ray Computed; D055815:Young Adult
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