title: Identifying the Functional Flexion-extension Axis of the Knee: An In-Vivo Kinematics Study.
abstract: OBJECTIVE
  This study aimed to calculate the flexion-extension axis (FEA) of the knee through in-vivo knee kinematics data, and then compare it with two major anatomical axes of the femoral condyles: the transepicondylar axis (TEA) defined by connecting the medial sulcus and lateral prominence, and the cylinder axis (CA) defined by connecting the centers of posterior condyles.
  
  
  METHODS
  The knee kinematics data of 20 healthy subjects were acquired under weight-bearing condition using bi-planar x-ray imaging and 3D-2D registration techniques. By tracking the vertical coordinate change of all points on the surface of femur during knee flexion, the FEA was determined as the line connecting the points with the least vertical shift in the medial and lateral condyles respectively. Angular deviation and distance among the TEA, CA and FEA were measured.
  
  
  RESULTS
  The TEA-FEA angular deviation was significantly larger than that of the CA-FEA in 3D and transverse plane (3.45° vs. 1.98°, p < 0.001; 2.72° vs. 1.19°, p = 0.002), but not in the coronal plane (1.61° vs. 0.83°, p = 0.076). The TEA-FEA distance was significantly greater than that of the CA-FEA in the medial side (6.7 mm vs. 1.9 mm, p < 0.001), but not in the lateral side (3.2 mm vs. 2.0 mm, p = 0.16).
  
  
  CONCLUSIONS
  The CA is closer to the FEA compared with the TEA; it can better serve as an anatomical surrogate for the functional knee axis.
authors.affiliation: Center for Joint Surgery, Southwest Hospital, the Third Military Medical University, Chongqing, People's Republic of China.
authors.forename: Li
authors.initials: L
authors.lastname: Yin
authors.affiliation: Center for Joint Surgery, Southwest Hospital, the Third Military Medical University, Chongqing, People's Republic of China.
authors.forename: Kaining
authors.initials: K
authors.lastname: Chen
authors.affiliation: Center for Joint Surgery, Southwest Hospital, the Third Military Medical University, Chongqing, People's Republic of China.
authors.forename: Lin
authors.initials: L
authors.lastname: Guo
authors.affiliation: Department of Radiology, Southwest Hospital, the Third Military Medical University, Chongqing, People's Republic of China.
authors.forename: Liangjun
authors.initials: L
authors.lastname: Cheng
authors.affiliation: Center for Joint Surgery, Southwest Hospital, the Third Military Medical University, Chongqing, People's Republic of China.
authors.forename: Fuyou
authors.initials: F
authors.lastname: Wang
authors.affiliation: Center for Joint Surgery, Southwest Hospital, the Third Military Medical University, Chongqing, People's Republic of China.
authors.forename: Liu
authors.initials: L
authors.lastname: Yang
mesh_terms: D000328:Adult; D001696:Biomechanical Phenomena / Q000502:physiology; D005260:Female; D005269:Femur / Q000033:anatomy & histology / Q000502:physiology*; D006801:Humans; D007719:Knee Joint / Q000033:anatomy & histology / Q000502:physiology*; D008297:Male; D008953:Models, Anatomic; D016059:Range of Motion, Articular / Q000502:physiology*; D012399:Rotation; D014057:Tomography, X-Ray Computed; D016474:Weight-Bearing