title: In vivo determination of cam-post engagement in fixed and mobile-bearing TKA.
abstract: BACKGROUND
  Kinematics vary, sometimes in important ways, among the different types of total knee arthroplasty (TKA) designs, yet differences between the in vivo mechanisms of cam-post engagement in rotating-platform posterior-stabilized (PS) TKA, bicruciate-stabilized TKA, and fixed-bearing PS TKA designs remain largely uncharacterized.
  
  
  OBJECTIVE
  The objective of this study was to determine the cam-post mechanism interaction for subjects implanted with three different TKA designs.
  
  
  METHODS
  In vivo, analysis was conducted for patients implanted with nine rotating-platform PS TKAs, five knees with a fixed-bearing PS TKA, and 10 knees with a bicruciate-stabilized TKA while performing a deep knee bend. Three-dimensional kinematics of the implant components were determined by analysis of fluoroscopic images during flexion. The distances between the interacting surfaces were measured throughout flexion and instances and locations of contact were identified.
  
  
  RESULTS
  Seven of the 10 bicruciate-stabilized knees analyzed had the femoral component engaged with the anterior aspect of the tibial post at full extension. Posterior cam-post engagement occurred at 34° for the bicruciate-stabilized (range, 17°-68°), 93° for the fixed-bearing PS (range, 88°-100°), and at 97° (range, 90°-104°) for rotating-platform PS TKA. In bicruciate-stabilized and fixed-bearing PS knees, the contact initially occurred on the medial aspect of the posterior surface of the tibial post and then moved centrally and superiorly with increasing flexion. For rotating-platform PS TKA, it was located centrally on the posterior surface of the post at all times.
  
  
  CONCLUSIONS
  This study suggests that mobility of the polyethylene might play an important role in ensuring central cam-post interaction in PS TKA. The polyethylene insert rotates axially in accordance with the rotating femur, maintaining central cam-post contact. This phenomenon was not observed in the fixed-bearing PS TKAs analyzed in this study.
  
  
  CONCLUSIONS
  We speculate that this centralized symmetrical contact between the cam and posterior surface of the post could be beneficial clinically in terms of reducing wear of the posterior surface and particularly at the medial extremes of it.
authors.affiliation: Center for Musculoskeletal Research, University of Tennessee, 301 Perkins Hall, Knoxville, TN, 37996, USA, szingde@cmr.utk.edu.
authors.forename: Sumesh M
authors.initials: SM
authors.lastname: Zingde
authors.forename: Filip
authors.initials: F
authors.lastname: Leszko
authors.forename: Adrija
authors.initials: A
authors.lastname: Sharma
authors.forename: Mohamed R
authors.initials: MR
authors.lastname: Mahfouz
authors.forename: Richard D
authors.initials: RD
authors.lastname: Komistek
authors.forename: Douglas A
authors.initials: DA
authors.lastname: Dennis
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D019645:Arthroplasty, Replacement, Knee / Q000295:instrumentation / Q000379:methods*; D001696:Biomechanical Phenomena / Q000502:physiology; D005260:Female; D006801:Humans; D007719:Knee Joint / Q000502:physiology / Q000601:surgery*; D007720:Knee Prosthesis; D008297:Male; D008875:Middle Aged; D011474:Prosthesis Design; D016059:Range of Motion, Articular / Q000502:physiology*; D016896:Treatment Outcome; D016474:Weight-Bearing / Q000502:physiology