title: Acetabular retroversion in Down syndrome.
abstract: BACKGROUND
  The management of the unstable Down syndrome hip is challenging, and there is controversy about the anatomic factors that may contribute to the instability. It has been our observation that children with Down syndrome often have a deficient posterior acetabular wall. This is different from other congenital acetabular dysplasia where the anatomic deficiency is typically anterolateral. These observations suggest that the acetabulum in Down syndrome hip dysplasia may be relatively retroverted. The purpose of this study was to determine the acetabular version in children with Down syndrome and compare this with matched controls from both normal and developmental dysplasia of the hip (DDH) populations.
  
  
  METHODS
  A cohort of Down patients treated surgically for acetabular dysplasia and/or hip instability was matched by age, sex, and side to a group of normal controls and compared with a cohort of patients who had undergone periacetabular osteotomy for DDH. For all patients, preoperative computed tomography scans were used to measure acetabular version through the joint center. Statistical differences were determined using analysis of variance with α=0.05.
  
  
  RESULTS
  We identified 16 subjects in each cohort. The average acetabular version in the normal control group was 13±5 degrees and in the DDH cohort was 21±7 degrees. In contrast, the mean version in the group of patients with Down syndrome was 2±11 degrees, indicating increased acetabular retroversion; this result was significantly different from both the normal group (P=0.02) and those with DDH (P<0.001). According to the criteria described by Tönnis for computed tomography measured retroversion, 10/16 patients with Down syndrome were severely retroverted compared with only 3/16 normal controls and 1/16 patients with DDH (P=0.002).
  
  
  CONCLUSIONS
  Patients with Down syndrome and hip instability seem to have more retroverted acetabula than normal controls and patients with DDH. In patients with Trisomy 21, axial imaging should be performed to evaluate acetabular version when planning the optimal corrective osteotomy for instability and/or acetabular deficiency.
  
  
  METHODS
  Level III (prognostic, retrospective case-control).
authors.affiliation: The Children's Hospital of Philadelphia, Philadelphia, PA, USA.
authors.forename: Wudbhav N
authors.initials: WN
authors.lastname: Sankar
authors.forename: Jonathan G
authors.initials: JG
authors.lastname: Schoenecker
authors.forename: Matthew E
authors.initials: ME
authors.lastname: Mayfield
authors.forename: Young-Jo
authors.initials: YJ
authors.lastname: Kim
authors.forename: Michael B
authors.initials: MB
authors.lastname: Millis
mesh_terms: D000077:Acetabulum / Q000002:abnormalities* / Q000601:surgery; D000293:Adolescent; D000704:Analysis of Variance; D016022:Case-Control Studies; D002648:Child; D015331:Cohort Studies; D004314:Down Syndrome / Q000150:complications*; D005260:Female; D006618:Hip Dislocation, Congenital / Q000209:etiology / Q000503:physiopathology* / Q000601:surgery; D006801:Humans; D007593:Joint Instability / Q000209:etiology / Q000503:physiopathology* / Q000601:surgery; D008297:Male; D010027:Osteotomy / Q000379:methods; D012189:Retrospective Studies; D012720:Severity of Illness Index; D014057:Tomography, X-Ray Computed; D055815:Young Adult