title: [Mathematical analysis of the influences of femoral and tibial bone cuts on the dynamic alignment in total knee arthroplasty].
abstract: OBJECTIVE
  To investigate the influences of the distal femoral cut, the anterior and posterior femoral cuts, the proximal tibial cut on the dynamic alignment of the lower extremity in total knee arthroplasty.
  
  
  METHODS
  Based on the three-dimensional geometric model, imitating the flexion movement of the knee without axial rotation after total knee arthroplasty, the influence of each of the three bone cuts on the dynamic alignment was analyzed with the assumption of standard bone cuts of the other two and equality of the medial and lateral soft tissue balancing. The dynamic alignment was defined as the angle between the mechanical axis of the tibia and the sagittal plane of the body when the knee was in any angle of flexion. With two of the three major bone cuts standard, the track of the tibial movement was established when the other one bone cut deviated from the ideal section of angle A. Based on the principle of geometry, the mathematical formula were established to present the influences of three bone cuts on the dynamic alignment of the lower extremity.
  
  
  RESULTS
  All of the three kinds of bone cuts in total knee arthroplasty influenced the dynamic alignment of the lower extremity not just in one static position, but during the whole range of motion. At the θ angle of knee flexion, the alignment of the lower extremity was arcsin(cosθsinA) when the varus/valgus femoral component alignment was A; the alignment of the lower extremity was arcsin(sinθsinA) when the rotational femoral component alignment was A; the alignment of the lower extremity was A when the varus/valgus tibial component alignment was A.
  
  
  CONCLUSIONS
  The influences of the distal femoral cut, the anterior and posterior femoral cuts, the proximal tibial cut on the dynamic alignment of the lower extremity in total knee arthroplasty are dynamically changed during the flexion movement of the knee.
authors.affiliation: Department of Osteoarthropathy, Yantaishan Hospital, Yantai 264001, China.
authors.forename: Yuchi
authors.initials: Y
authors.lastname: Zhao
authors.forename: Shijun
authors.initials: S
authors.lastname: Wang
authors.forename: Zhongyuan
authors.initials: Z
authors.lastname: Zhao
authors.forename: Kun
authors.initials: K
authors.lastname: Song
authors.forename: Shudong
authors.initials: S
authors.lastname: Zhang
mesh_terms: D019645:Arthroplasty, Replacement, Knee; D005269:Femur / Q000601:surgery*; D006801:Humans; D007719:Knee Joint; D016059:Range of Motion, Articular; D012399:Rotation; D013977:Tibia / Q000601:surgery*