title: Segment choice and cup stability influence wear measurements using radiostereometric analysis: a radiostereometric study comparing wear measured by markers in the polyethylene with markers in the periacetabular bone.
abstract: BACKGROUND
  Radiostereometry is a well documented method to measure the polyethylene wear after total hip replacements. Wear is measured according to the point motion of the head center in relation to the polyethylene as the reference segment. Increasing head sizes and new cup materials may diminish visibility of markers deteriorating the segment and leading to study drop outs. Alternatively markers in the periacetabular bone may be easier to detect rendering this segment more stable. Our aim was to compare wear measurements against the cup, the acetabuluar bone and a calculated wear estimation including cup migration.
  
  
  METHODS
  A prospective randomized controlled trial comparing reverse hybrid with cemented total hip replacement was conducted. 31 patients had tantalum markers in the polyethylene and in the periacetabular bone making it possible to measure wear using both as reference segments. We used a uniplanar radiostereometric technique.
  
  
  RESULTS
  Wear in the y-axis was overestimated by 53% when markers in the periacetabular bone were used (P<0.001). Proximal wear was 0.34 mm (95% CI of mean: 0.29-0.38) when using the polyethylene as the reference and 0.52 mm (95% CI of mean: 0.38-0.65) using the acetabular bone. Migration of the cup seemed to influence the wear calculations, overestimating wear when markers in the periacetabular bone were used as the reference segment.
  
  
  CONCLUSIONS
  Wear measured with periacatebular bone markers is influenced by cup migration, overestimating wear measurements. We therefore recommend not using the acetabular bone as the reference segment.
authors.affiliation: Lovisenberg Diaconal Hospital, Department of Orthopedic Surgery, Norway. eli@lds.no
authors.forename: Einar
authors.initials: E
authors.lastname: Lindalen
authors.forename: Lars
authors.initials: L
authors.lastname: Nordsletten
authors.forename: Stephan M
authors.initials: SM
authors.lastname: Röhrl
mesh_terms: D000077:Acetabulum / Q000000981:diagnostic imaging* / Q000601:surgery*; D000368:Aged; D019544:Equipment Failure Analysis / Q000379:methods; D005260:Female; D057918:Fiducial Markers; D006622:Hip Prosthesis; D006801:Humans; D007593:Joint Instability / Q000000981:diagnostic imaging* / Q000517:prevention & control*; D008297:Male; D008875:Middle Aged; D011474:Prosthesis Design; D011856:Radiographic Image Enhancement / Q000379:methods*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D016896:Treatment Outcome