title: Knee rotation influences the femoral tunnel angle measurement after anterior cruciate ligament reconstruction: a 3-dimensional computed tomography model study.
abstract: OBJECTIVE
  Femoral tunnel angle (FTA) has been proposed as a metric for evaluating whether ACL reconstruction was performed anatomically. In clinic, radiographic images are typically acquired with an uncertain amount of internal/external knee rotation. The extent to which knee rotation will influence FTA measurement is unclear. Furthermore, differences in FTA measurement between the two common positions (0° and 45° knee flexion) have not been established. The purpose of this study was to investigate the influence of knee rotation on FTA measurement after ACL reconstruction.
  
  
  METHODS
  Knee CT data from 16 subjects were segmented to produce 3D bone models. Central axes of tunnels were identified. The 0° and 45° flexion angles were simulated. Knee internal/external rotations were simulated in a range of ± 20°. FTA was defined as the angle between the tunnel axis and femoral shaft axis, orthogonally projected into the coronal plane.
  
  
  RESULTS
  Femoral tunnel angle was positively/negatively correlated with knee rotation angle at 0°/45° knee flexion. At 0° knee flexion, FTA for anterio-medial (AM) tunnels was significantly decreased at 20° of external knee rotation. At 45° knee flexion, more than 16° external or 19° internal rotation significantly altered FTA measurements for single-bundle tunnels; smaller rotations (± 9° for AM, ± 5° for PL) created significant errors in FTA measurements after double-bundle reconstruction.
  
  
  CONCLUSIONS
  Femoral tunnel angle measurements were correlated with knee rotation. Relatively small imaging malalignment introduced significant errors with knee flexed 45°. This study supports using the 0° flexion position for knee radiographs to reduce errors in FTA measurement due to knee internal/external rotation.
authors.affiliation: Department of Orthopedic Surgery, University of Pittsburgh, Kaufman Building Suite 1011, 3471 Fifth Avenue, Pittsburgh, PA, 15213, USA.
authors.forename: Jing
authors.initials: J
authors.lastname: Tang
authors.forename: Eric
authors.initials: E
authors.lastname: Thorhauer
authors.forename: Chelsea
authors.initials: C
authors.lastname: Marsh
authors.forename: Freddie H
authors.initials: FH
authors.lastname: Fu
authors.forename: Scott
authors.initials: S
authors.lastname: Tashman
mesh_terms: D000328:Adult; D016118:Anterior Cruciate Ligament / Q000000981:diagnostic imaging* / Q000601:surgery*; D059549:Anterior Cruciate Ligament Reconstruction / Q000379:methods*; D001696:Biomechanical Phenomena; D016022:Case-Control Studies; D005269:Femur / Q000000981:diagnostic imaging / Q000601:surgery; D006801:Humans; D021621:Imaging, Three-Dimensional; D007719:Knee Joint / Q000000981:diagnostic imaging* / Q000503:physiopathology*; D008297:Male; D008875:Middle Aged; D010027:Osteotomy / Q000379:methods; D016059:Range of Motion, Articular; D012399:Rotation; D014057:Tomography, X-Ray Computed / Q000379:methods*