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The lack of awareness of the exact number of instantaneous centers of knee flexion/extension rotation leads to the presence in the market of total knee arthroplasty (TKA) femoral components designed under different hypotheses. Although single radius (SR) designs are thought to replicate the physiological behavior in a more realistic way, surgeons do not always agree about the veracity of their theoretical advantages with respect to the multiple radii components (J-curve (JC) design). Apart from clinical studies, up to now, any literature study biomechanically and exhaustively compares these two TKA solutions, thus a finite element analysis (FEA) has been carried out. In particular, two models were defined to analyze the performance of a SR design and a JC design with the same tibial component during gait cycle and squat motor task. Tibio-femoral kinematics and kinetics have been investigated comparing the resulting contact area between components, internal–external (IE) rotation, position and magnitude of the center of total forces due to contact pressure and polyethylene von Misses stresses. Results demonstrate that, for low demanding tasks, there are no significant differences between the two designs, however, during the squat motor task, some changes in contact force and increases in polyethylene stress were identified for the SR solution.

Preface: Understanding knee kinematics is a fundamental prerequisite to address restoration in pathological joints; these performances represent the goal to achieve after the treatment. Experimental activities addressing knee kinematics often involve Motion Capture systems to acquire information, both for in vivo and ex vivo studies. This technique allows a complete analysis of the joint kinematics and is able to provide useful results for the comparison mentioned above. Objectives: The definition of a reproducible and straightforward protocol represents a beneficial factor to improve both reliability and the time required: in this study a new protocol for kinematics experimental activities on ex vivo knee specimens was developed, validated and tested. Methods: Synthetic bones were chosen for the analysis and different Total Knee Arthroplasties models were selected (a Cruciate Retaining, a Posterior Stabilized, a Constrained Condylar Knee and a Rotating Hinge). A dedicated frame was used to support and secure the knee specimens and pair the extensor mechanism to a motor. The Motion Capture System was assembled and paired with dedicated marker-sets to be fixed to the specimens. A post-processing tool was developed to analyze the outputs and was validated with a goniometer. A series of force-driven tasks were defined, implemented and run in the motor system for all the different prosthesis configurations and kinematics output were analyzed, comparing the outputs with the expected results. Results: The validation of the system returned satisfying results in terms of correspondence between the angles imposed and the ones measured, with an average error below $3^{\circ }$ and a standard deviation below $1^{\circ }$ for each kind of rotation. The results from the different testing were coherent with the type of specimens and prostheses tested. Conclusions: This study proved that the protocol and testing set-up developed for knee kinematics in vitro analysis are able to provide reliable and coherent data, as proven by the post-processing validation and by the testing campaign on synthetic specimens.