title: Misinterpretation of cup anteversion in total hip arthroplasty using planar radiography.
abstract: BACKGROUND
  Anteroposterior pelvic radiographs are routinely used to monitor cup orientation in total hip arthroplasty (THA). Analysis of planar radiographs leads to a certain degree of measurement error for the cup anteversion (AV). With the current study, we wanted to clarify whether planar radiography can be used for accurate evaluation of the THA position.
  
  
  METHODS
  The postoperative orientation of pelvic implants in 42 patients was analyzed according to five documented mathematical algorithms using planar radiographs. Postoperative computed tomography (CT) pelvis scans were available for all patients. A CT-based navigation system was used to determine AV.
  
  
  RESULTS
  The comparison showed that all five formulas presented substantial variations for the AV angle. Of these, Widmer's algorithm presented the smallest difference compared to the CT. Misinterpretation of postoperative planar radiographs is a common problem in THA.
  
  
  CONCLUSIONS
  Planar radiographs are too imprecise for exact evaluation of the correct cup AV after THA. CT-based analysis may be necessary if exact values are required.
authors.affiliation: Department of Orthopaedic Surgery, University of Duisburg-Essen, Hufelandstr. 55, 45122 Essen, Germany. axel@marx.ac
authors.forename: Axel
authors.initials: A
authors.lastname: Marx
authors.forename: Marius
authors.initials: M
authors.lastname: von Knoch
authors.forename: Jörg
authors.initials: J
authors.lastname: Pförtner
authors.forename: Matthias
authors.initials: M
authors.lastname: Wiese
authors.forename: Guido
authors.initials: G
authors.lastname: Saxler
mesh_terms: D019644:Arthroplasty, Replacement, Hip; D003951:Diagnostic Errors; D005260:Female; D006622:Hip Prosthesis; D006801:Humans; D007719:Knee Joint / Q000000981:diagnostic imaging* / Q000601:surgery*; D008297:Male; D008433:Mathematics; D012189:Retrospective Studies; D014057:Tomography, X-Ray Computed