title: The Validity of a New Low-Dose Stereoradiography System to Perform 2D and 3D Knee Prosthetic Alignment Measurements.
abstract: BACKGROUND
  The EOS stereoradiography system has shown to provide reliable varus/valgus (VV) measurements of the lower limb in 2D (VV2D) and 3D (VV3D) after total knee arthroplasty (TKA). Validity of these measurements has not been investigated yet, therefore the purpose of this study was to determine validity of EOS VV2D and VV3D.
  
  
  METHODS
  EOS images were made of a lower limb phantom containing a knee prosthesis, while varying VV angle from 15° varus to 15° valgus and flexion angle from 0° to 20°, and changing rotation from 20° internal to 20° external rotation. Differences between the actual VV position of the lower limb phantom and its position as measured on EOS 2D and 3D images were investigated.
  
  
  RESULTS
  Rotation, flexion or VV angle alone had no major impact on VV2D or VV3D. Combination of VV angle and rotation with full extension did not show major differences in VV2D measurements either. Combination of flexion and rotation with a neutral VV angle showed variation of up to 7.4° for VV2D; maximum variation for VV3D was only 1.5°. A combination of the three variables showed an even greater distortion of VV2D, while VV3D stayed relatively constant. Maximum measurement difference between preset VV angle and VV2D was 9.8°, while the difference with VV3D was only 1.9°. The largest differences between the preset VV angle and VV2D were found when installing the leg in extreme angles, for example 15° valgus, 20° flexion and 20° internal rotation.
  
  
  CONCLUSIONS
  After TKA, EOS VV3D were more valid than VV2D, indicating that 3D measurements compensate for malpositioning during acquisition. Caution is warranted when measuring VV angle on a conventional radiograph of a knee with a flexion contracture, varus or valgus angle and/or rotation of the knee joint during acquisition.
authors.affiliation: Department of Orthopedics, University of Groningen, University Medical Center Groningen, Groningen, The Netherlands.
authors.forename: Marrigje F
authors.initials: MF
authors.lastname: Meijer
authors.affiliation: Department of Radiology, University of Groningen, University Medical Center Groningen, Groningen, The Netherlands.
authors.forename: Ton
authors.initials: T
authors.lastname: Velleman
authors.affiliation: Department of Orthopedics, University of Groningen, University Medical Center Groningen, Groningen, The Netherlands.
authors.forename: Alexander L
authors.initials: AL
authors.lastname: Boerboom
authors.affiliation: Department of Orthopedics, University of Groningen, University Medical Center Groningen, Groningen, The Netherlands.
authors.forename: Sjoerd K
authors.initials: SK
authors.lastname: Bulstra
authors.affiliation: Center for Human Movement Sciences, University of Groningen, University Medical Center Groningen, Groningen, The Netherlands.
authors.forename: Egbert
authors.initials: E
authors.lastname: Otten
authors.affiliation: Department of Orthopedics, University of Groningen, University Medical Center Groningen, Groningen, The Netherlands.
authors.forename: Martin
authors.initials: M
authors.lastname: Stevens
authors.affiliation: Department of Trauma Surgery, University of Groningen, University Medical Center Groningen, Groningen, The Netherlands.
authors.forename: Inge H F
authors.initials: IH
authors.lastname: Reininga
mesh_terms: D019645:Arthroplasty, Replacement, Knee / Q000379:methods*; D001696:Biomechanical Phenomena; D006801:Humans; D007719:Knee Joint / Q000601:surgery*; D007720:Knee Prosthesis; D016059:Range of Motion, Articular / Q000502:physiology