title: The effect of geometric variations in posterior-stabilized knee designs on motion characteristics measured in a knee loading machine.
abstract: BACKGROUND
  In different posterior-stabilized (PS) total knees, there are considerable variations in condylar surface radii and cam-post geometry. To what extent these variations affect kinematics is not known. Furthermore, there are no clearly defined ideal kinematics for a total knee.
  
  
  OBJECTIVE
  The purposes of this study were to determine (1) what the kinematic differences are caused by geometrical variations between PS total knee designs in use today; and (2) what design characteristics will produce kinematics that closely resemble that of the normal anatomic knee.
  
  
  METHODS
  Four current PS designs with different geometries and one experimental asymmetric PS design, with a relatively conforming medial side, were tested in a purpose-built machine. The machine applied combinations of compressive, shear, and torque forces at a sequence of flexion angles to represent a range of everyday activities, consistent with the ASTM standard test for measuring constraint. The femorotibial contact points, the neutral path of motion, and the AP and internal-external laxities were used as the kinematic indicators.
  
  
  RESULTS
  The PS designs showed major differences in motion characteristics among themselves and with motion data from anatomic knees determined in a previous study. Abnormalities in the current designs included symmetric mediolateral motion, susceptibility to excessive AP medial laxity, and reduced laxity in high flexion. The asymmetric-guided motion design alleviated some but not all of the abnormalities.
  
  
  CONCLUSIONS
  Current PS designs showed kinematic abnormalities to a greater or lesser extent. An asymmetric design may provide a path to achieving a closer match to anatomic kinematics.
  
  
  CONCLUSIONS
  One criterion for the evaluation of PS total knees is how closely the kinematics of the prosthesis resemble that of the anatomic knee, because this is likely to affect the quality of function.
authors.affiliation: Department of Orthopaedic Surgery, New York University-Hospital for Joint Diseases, New York, NY, USA, Peter.Walker@nyumc.org.
authors.forename: Peter S
authors.initials: PS
authors.lastname: Walker
authors.forename: Michael T
authors.initials: MT
authors.lastname: Lowry
authors.forename: Anoop
authors.initials: A
authors.lastname: Kumar
mesh_terms: D019645:Arthroplasty, Replacement, Knee; D001696:Biomechanical Phenomena / Q000502:physiology; D006801:Humans; D007717:Knee / Q000601:surgery; D007719:Knee Joint / Q000601:surgery*; D007720:Knee Prosthesis; D011474:Prosthesis Design; D016059:Range of Motion, Articular / Q000502:physiology
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