title: Comparison of stability and kinematics of the natural knee versus a PS TKA with a 'third condyle'.
abstract: OBJECTIVE
  The goal of this study was to compare the kinematics of knees before and after total knee arthroplasty (TKA) that relies on an inter-condylar 'third condyle'. The hypothesis was that the 'third condyle' provides sufficient flexion stability and induces a close to normal femoral rollback, thus granting natural joint kinematics.
  
  
  METHODS
  Intra-operative navigation data were collected from 29 consecutive cases that received a cemented TKA (HLS Noetos, Tornier SA, France) designed with an inter-condylar 'third condyle' that engages within the tibial insert beyond 35° flexion. Operations were guided by a non-image-based system (BLU-IGS, Orthokey Italia srl, Italy) that recorded relative femoral and tibial positions in native and implanted knees during: passive range of motion, anterior drawer test at 90° flexion, and varus-valgus stress tests at full extension and at 30° flexion.
  
  
  RESULTS
  The total internal tibial rotation during flexion was similar for native (8.2 ± 4.2°) and implanted knees (8.0 ± 5.4°). The lateral femoral condyle was more posterior in implanted knees (1.2 ± 9.4 mm) than in native knees (9.5 ± 3.6 mm) throughout early flexion (p < 0.01), but this difference diminished beyond 100° flexion (n.s.). The implanted knees did not exhibit paradoxical external tibial rotation. Varus-valgus laxity in full extension was lower for implanted knees than for native knees (p = 0.0221), but at 30° flexion was almost identical for both native and implanted knees. Anteroposterior laxity was similar in implanted and native knees.
  
  
  CONCLUSIONS
  The 'third condyle' TKA provides similar anteroposterior and mediolateral stability to the natural knee. This feature granted an adequate balance between laxity and constraint to reproduce natural joint kinematics, including smooth femoral rollback, without causing paradoxical external tibial rotation.
authors.affiliation: Clinica Ortopedica e Traumatologica II, Laboratorio di Biomeccanica ed Innovazione Tecnologica, Istituto Ortopedico Rizzoli, Via di Barbiano 1\10, 40136, Bologna, Italy.
authors.forename: Stefano
authors.initials: S
authors.lastname: Zaffagnini
authors.forename: Simone
authors.initials: S
authors.lastname: Bignozzi
authors.forename: Mo
authors.initials: M
authors.lastname: Saffarini
authors.forename: Francesca
authors.initials: F
authors.lastname: Colle
authors.forename: Bharat
authors.initials: B
authors.lastname: Sharma
authors.forename: Plamen Slavov
authors.initials: PS
authors.lastname: Kinov
authors.forename: Maurilio
authors.initials: M
authors.lastname: Marcacci
authors.forename: David
authors.initials: D
authors.lastname: Dejour
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D019645:Arthroplasty, Replacement, Knee; D001696:Biomechanical Phenomena; D005260:Female; D006801:Humans; D007593:Joint Instability / Q000503:physiopathology*; D007719:Knee Joint / Q000502:physiology* / Q000503:physiopathology* / Q000601:surgery; D007720:Knee Prosthesis; D008297:Male; D008875:Middle Aged; D016059:Range of Motion, Articular