• Marco A Marra, Marta Strzelczak, Heesterbeek Petra J C PJC Sint Maartenskliniek Research, Postbus 9011, 6500 GM, Nijmegen, The Netherlands., Sebastiaan A W van de Groes, Dennis Janssen, Koopman Bart F J M BFJM Department of Biomechanical Engineering, University of Twente, Postbus 217, 7500 AE, Enschede, The Netherlands., Nico Verdonschot, and Ate B Wymenga.
• Orthopaedic Research Laboratory, Radboud Institute for Health Sciences, Radboud University Medical Center, Postbus 9101, 6500 HB, Nijmegen, The Netherlands. mamarra@outlook.com.
• Knee Surg Sports Traumatol Arthrosc. 2018 Nov 1; 26 (11): 3377-3385.

PurposeWhen downsizing the femoral component to prevent mediolateral overhang, notching of the anterior femoral cortex may occur, which could be solved by flexing the femoral component. In this study, we investigated the effect of flexion of the femoral component on patellar tendon moment arm, patellofemoral forces and kinematics in posterior-referencing CR-TKA. Our hypothesis was that flexion of the femoral component increases the patellar tendon moment arm, reduces the patellofemoral forces and provides stable kinematics.MethodsA validated musculoskeletal model of CR-TKA was used. The flexion of the femoral component was increased in four steps (0°, 3°, 6°, 9°) using posterior referencing, and different alignments were analysed in combination with three implant sizes (3, 4, 5). A chair-rising trial was analysed using the model, while simultaneously estimating quadriceps muscle force, patellofemoral contact force, tibiofemoral and patellofemoral kinematics.ResultsCompared to the reference case (size 4 and 0° flexion), for every 3° of increase in flexion of the femoral component the patellar tendon moment arm increased by 1% at knee extension. The peak quadriceps muscle force and patellofemoral contact force decreased by 2%, the patella shifted 0.8 mm more anteriorly and the remaining kinematics remained stable, with knee flexion. With the smaller size, the patellar tendon moment arm decreased by 6%, the quadriceps muscle force and patellofemoral contact force increased by 8 and 12%, and the patellar shifted 5 mm more posteriorly. Opposite trends were found with the bigger size.ConclusionFlexing the femoral component with posterior referencing reduced the patellofemoral contact forces during a simulated chair-rising trial with a patient-specific musculoskeletal model of CR-TKA. There seems to be little risk when flexing and downsizing the femoral component, compared to when using a bigger size and neutral alignment. These findings provide relevant information to surgeons who wish to prevent anterior notching when downsizing the femoral component.

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