What clinical characteristics and radiographic parameters are associated with patellofemoral instability after kinematically aligned total knee arthroplasty?
Thirteen patients presented with patellofemoral instability out of 3212 knees treated with kinematically aligned total knee arthroplasty (KA TKA) during a nine year period. We determined the clinical characteristics and post-operative radiographic parameters associated with patellofemoral instability, and whether re-operation and patient reported outcome measures are different between patients with and without patellofemoral instability.



Patients with patellofemoral instability were matched 1:3 to a control cohort based on date of surgery (±3 months), age (±10 years), sex, pre-operative knee deformity (varus or valgus), and implant brand. We analyzed clinical characteristics and seven post-operative radiographic parameters.



Patellofemoral instability presented atraumatically (12 of 13) at 5 ± 4.7 months for a 0.4 % incidence at a mean follow-up of 43 ± 36 months. No pre-operative clinical characteristics were associated with instability. Patients with patellofemoral instability had greater flexion of the femoral component (11° versus 5°; p = 0.0012), a trend toward greater external rotation of the tibial component (2° versus 0°; p = 0.2704), more reoperations (9 versus 0; p = 0.0026) and a lower Oxford Knee Score (36 versus 42; p = 0.0045) than controls.



Patellofemoral instability after kinematically aligned TKA is infrequent, presents atraumatically, and is associated with greater flexion of the femoral component than the control group.



Minimizing flexion of the femoral component might reduce the risk of patellofemoral instability by promoting early engagement of the patella in the trochlear during knee flexion.
Department of Orthopaedics, University of California, Davis, 4860 Y Street, Suite 3800, Sacramento, CA, 95817, USA. nedopil@ucdavis.edu.
Alexander J
AJ
Nedopil
Biomedical Engineering Graduate Group, University of California, Davis, Davis, CA, 95616, USA.
Stephen M
SM
Howell
Department of Mechanical Engineering, University of California, Davis, CA, 95616, USA.
Maury L
ML
Hull
D000368:Aged; D000369:Aged, 80 and over; D019645:Arthroplasty, Replacement, Knee / Q000009:adverse effects*; D001696:Biomechanical Phenomena; D016022:Case-Control Studies; D005260:Female; D006801:Humans; D007593:Joint Instability / Q000175:diagnosis* / Q000209:etiology; D007719:Knee Joint / Q000601:surgery*; D007720:Knee Prosthesis / Q000009:adverse effects; D008297:Male; D008875:Middle Aged; D016059:Range of Motion, Articular; D012086:Reoperation
Flexion of the femoral component; Kinematic alignment; Knee arthroplasty; Oxford knee score; Patellofemoral instability