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Understanding the differences in knee kinematic descriptions is important for comparing data from different laboratories and observing small but important changes within a set of knees. The purpose of this study was to identify how differences in fixed body femoral coordinate systems affect the described tibiofemoral and patellofemoral kinematics for cadaveric knee studies with no hip present. Different methods for describing kinematics were evaluated on a set of seven cadaveric knees during walking in a dynamic knee simulator. Three anatomical landmark coordinate systems, a partial helical axis, and an experimental setup-based system were examined. The results showed that flexion-extension was insensitive to differences in the kinematic systems tested, internal-external rotation was similar for most femoral coordinate systems although there were changes in absolute position, varus-valgus was the most sensitive to variations in flexion axis direction, and anterior-posterior motion was most sensitive to femoral origin location. Femoral coordinate systems that define the sagittal plane using anatomical landmarks and locate the flexion axis perpendicular to the femur's mechanical axis in the frontal plane were typically similar and described kinematics most consistently.

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Despite the overall successful outcomes following primary total knee arthroplasty (TKA) and the concept that a well-balanced TKA yields a more successful result, concerns still remain in the reported literature regarding the patellofemoral joint. Diminished outcomes have been associated with poorly balanced or placed patella implants. The effect of different techniques to achieve flexion-extension balance and the use of posterior stabilized (PS) versus cruciate retaining (CR) implant designs on patellofemoral balancing has not been previously studied. The purpose of this study is to utilize a validated computational analysis software to simulate the effects of varying implant positions and sizes of femoral components. The patellofemoral retinaculum (PFR) load was significantly affected by some conditions, while others did not reach significance. The proximal-distal implant position with knee flexion angle ( < 0.001), the implant size ( < 0.001), and the implant bearing type (CR/PS) ( < 0.05) were significant. For the proximal-distal implant position and knee flexion angle, a more proximal implant position (elevating the joint line) increased the PFR load from 15 to 30°, and a more proximal implant position reduced retinaculum load from 60 to 135°. However, at 45°, implant position does not affect retinaculum load. Achieving the appropriate balance between the dynamic nature of both the tibiofemoral and the patellofemoral interaction in TKA has proven to be complex and challenging to manage. Balancing of a TKA is essential to the proper functioning and overall longevity of the implant. These results demonstrate that patellofemoral balance is affected by implant size and position during flexion-extension gap balancing.