In vivo kinematics of a Fixed and Mobile Bearing TKA in the same patient; a case report

Background: Total knee arthroplasy (TKA) polyethylene can be subdivided in two design groups: fixedbearing (FB), where the polyethylene tibial insert is locked with the tibial tray, and mobile‐bearing (MB), which facilitate movement of the insert relative to the tray. Purpose/Aim of Study: To describe in vivo kinematics of a PS MB TKA and a PS FB TKA implanted in the same patient using Model Based dynamic RSA. Materials and Methods: One patient (72 yo) with the implantation of both insert designs was evaluated at 2 years of follow up after surgery. The FB was implanted in the left knee, the MB in the right one. Dynamic RSA was used to evaluate the kinematics while the patient performed a sit‐to‐stand. Kinematical patterns were evaluated using Grood and Suntay decomposition and Low point Method. Data were related to the flexion angle versus Internal‐ External (IE), Varus‐ Valgus (VV) rotations and Antero‐Posterior (AP) translations of the femur with respect to the tibia. Findings/Results: During the sit‐to‐stand of the FB, from 80° to 30° a slight lateral pivot was observed, then both condyles moved simultaneously forward until full extension. The MB design shows a wider lateral pivot during the whole movement, with a greater anterior translation of both condyles. Concerning IE rotations, FB design made an internal rotation trend during the whole movement. On the contrary the MB shows an internal rotation from 80° to 40°, then an anterior translation was observed until full extension. VV rotations of both designs didn’t show any significative pattern, in according with a correct soft tissues balancing. Conclusions: A different kinematic result between FB and MB designs was found. Despite of that, the clinical outcome of our patient was excellent for both knees. The theoretical advantages for MB design have not been demonstrated by any outcome studies