title: Simultaneous measurements of knee motion using an optical tracking system and radiostereometric analysis (RSA).
abstract: OBJECTIVE
  Invasive methods are more reproducible and accurate than non-invasive ones when it comes to recording knee kinematics, but they are usually less accessible and less safe, mainly due to risk of infection. For this reason, non-invasive methods with passive markers are widely used. With these methods, varying marker sets based on a number of single markers, or sets of markers, known as clusters, are used to track body segments. We compared one invasive method--radiostereometric analysis--with a non-invasive method, an optical tracking system with 15 skin-mounted markers.
  
  
  METHODS
  9 subjects (10 knees) were investigated simultaneously with a dynamic RSA system and a motion-capture system while performing an active knee extension.
  
  
  RESULTS
  For flexion/extension, there was good agreement on an individual basis and at the group level. For internal/external rotation, the group mean was fairly similar, up to 25 degrees of flexion. Recordings of abductions and/or adductions revealed a systematic mean difference of 2-4 degrees during the range of flexion measured. The correlation between the 2 methods in the horizontal and frontal planes was poor.
  
  
  CONCLUSIONS
  Our skin-marker model provided reliable data for flexion/extension. Recordings of internal/external rotation and abduction/adduction were less accurate on an individual basis than at the group level, most probably due to soft-tissue motion and the presence of small true motion in these planes.
authors.affiliation: Department of Orthopaedics, Sahlgrenska University Hospital, Mölndal, Sweden. roy.tranberg@bredband.net
authors.forename: Roy
authors.initials: R
authors.lastname: Tranberg
authors.forename: Tuuli
authors.initials: T
authors.lastname: Saari
authors.forename: Roland
authors.initials: R
authors.lastname: Zügner
authors.forename: Johan
authors.initials: J
authors.lastname: Kärrholm
mesh_terms: D015502:Absorptiometry, Photon; D000368:Aged; D019645:Arthroplasty, Replacement, Knee / Q000009:adverse effects / Q000379:methods; D001696:Biomechanical Phenomena; D005260:Female; D006801:Humans; D021621:Imaging, Three-Dimensional; D007719:Knee Joint / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D009068:Movement / Q000502:physiology; D016059:Range of Motion, Articular / Q000502:physiology; D012307:Risk Factors; D012399:Rotation