title: Abnormal axial rotations in TKA contribute to reduced weightbearing flexion.
abstract: BACKGROUND
  Previous in vivo fluoroscopy studies have documented that axial rotation for patients having a TKA was significantly less than those having a normal knee. In fact, many subjects having a TKA experience a reverse axial rotation pattern where the femur internally rotates with increasing flexion. However, no previous studies have been conducted to determine if this reverse axial rotation pattern affects TKA performance.
  
  
  OBJECTIVE
  The purposes of this study were: (1) Do normal and reverse axial rotation patterns of a TKA affect the maximum flexion angle postoperatively? (2) Does the axial rotation angle of the knee at maximum flexion during weightbearing impact the magnitude of the maximum flexion achieved in weightbearing?
  
  
  METHODS
  One hundred twenty patients having TKA, previously analyzed under in vivo conditions using fluoroscopy and a three-dimensional model-fitting software package, were further evaluated to determine if reverse axial rotation patterns limit weightbearing TKA flexion. In this retrospective cohort, we identified 58 patients who had a normal axial rotation pattern (greater than 15° normal rotation). Sixty-two patients experienced greater than 3° of reverse axial rotation, defined as internal rotation of the femur relative to the tibia.
  
  
  RESULTS
  Patients having a normal axial rotation achieved greater weightbearing knee flexion than those with reverse axial rotation (115° versus 109°, p = 0.02). Additionally, patients with greater than 3° of normal axial rotation at maximum flexion had more flexion than those with less than 3° of normal axial rotation at ending flexion (115° versus 107°, p < 0.001).
  
  
  CONCLUSIONS
  These findings show reverse axial rotation and a smaller magnitude of normal axial rotation reduce weightbearing knee flexion. This is likely the result of increased posterior movement of the lateral condyle and is an important consideration in future implant designs.
authors.affiliation: Department of Mechanical, Aerospace & Biomedical Engineering, University of Tennessee, 301 Perkins Hall, 1506 Middle Drive, Knoxville, TN, 37996, USA, bmeccia@cmr.utk.edu.
authors.forename: Bradley
authors.initials: B
authors.lastname: Meccia
authors.forename: Richard D
authors.initials: RD
authors.lastname: Komistek
authors.forename: Mohamed
authors.initials: M
authors.lastname: Mahfouz
authors.forename: Douglas
authors.initials: D
authors.lastname: Dennis
mesh_terms: D019645:Arthroplasty, Replacement, Knee; D001696:Biomechanical Phenomena / Q000502:physiology; D006801:Humans; D007719:Knee Joint / Q000000981:diagnostic imaging / Q000502:physiology / Q000601:surgery*; D007720:Knee Prosthesis; D011474:Prosthesis Design; D011859:Radiography; D016059:Range of Motion, Articular / Q000502:physiology*; D012189:Retrospective Studies; D012399:Rotation; D016474:Weight-Bearing / Q000502:physiology*
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