title: Pelvic positioning creates error in CT acetabular measurements.
abstract: BACKGROUND
  CT allows for accurate measurement of acetabular orientation and shape, but malpositioning of the pelvis may lead to measurement variance.
  
  
  OBJECTIVE
  We therefore sought to determine: (1) whether acetabular anteversion measurements using the femoral head centers differed from those using the posterior ischia, and (2) the extent to which changing obliquity, rotation, and tilt of a pelvis in a CT scanner affected the measurement of acetabular variables.
  
  
  METHODS
  A radiopaque human pelvis model with articulated hips was suspended from a plastic sheet as part of an adjustable frame. Changes in the transverse and sagittal planes created rotation and tilt, while rotating the frame in the coronal plane created obliquity. CT scans were obtained, varying the combinations of obliquity, rotation, and tilt by intervals of 5°, up to 20°. Acetabular anteversion (AA), anterior acetabular sector angle (AASA), posterior acetabular sector angle (PASA), and horizontal acetabular sector angle (HASA) were measured.
  
  
  RESULTS
  The two methods for measuring AA yielded values differing by 1° to 4° but correlated (r = 0.981) across the spectrum of pelvis positioning. Pelvic obliquity and tilt were linearly associated with changes in the measurements. For each 1°-increase in pelvic obliquity, AA changed -0.4°, and AASA, PASA, and HASA changed 1.93°, 0.99°, and 2.80°, respectively. For each 1°-increase in pelvic tilt, AA changed 0.8°, and AASA, PASA, and HASA changed -1.07°, 0.52°, and -0.51°, respectively. Rotation had no affect on the variables.
  
  
  CONCLUSIONS
  Small changes in pelvic obliquity and tilt were associated with variances in acetabular measurements. The measured changes were directly proportional to the changes in obliquity and tilt, and were additive. Pelvic rotation created no changes in measurement.
authors.affiliation: Department of Orthopaedic Surgery, Shriners Hospital for Children, 3551 North Broad Street, Philadelphia, PA 19140, USA. hvanbosse@shrinenet.org
authors.forename: Harold J P
authors.initials: HJ
authors.lastname: van Bosse
authors.forename: Duron
authors.initials: D
authors.lastname: Lee
authors.forename: Eric R
authors.initials: ER
authors.lastname: Henderson
authors.forename: Debra A
authors.initials: DA
authors.lastname: Sala
authors.forename: David S
authors.initials: DS
authors.lastname: Feldman
mesh_terms: D000077:Acetabulum / Q000000981:diagnostic imaging* / Q000601:surgery; D019644:Arthroplasty, Replacement, Hip / Q000379:methods*; D006622:Hip Prosthesis; D006801:Humans; D008953:Models, Anatomic; D056888:Patient Positioning; D010384:Pelvic Bones / Q000000981:diagnostic imaging* / Q000601:surgery; D012399:Rotation; D014057:Tomography, X-Ray Computed