Can a medially stabilized TKA design approach a natural knee kinematics?

Background: It is known that to obtain a stable kinematics is the most challenging goal to achieve after TKA. The femoral surface of a medially stabilized (MS) TKA is spherical on the medial side in the first degrees of flexion, thus the radius closes slightly. The medial part of the polyethylene surface is more congruent respect to the medial condyle, while the lateral one is flat anteroposteriorly. These features would theoretically provide an anterior‐posterior stability and a greater rotation of the medial condyle. Purpose/Aim of Study: To describe the in vivo kinematics of the knee after MS Fixed Bearing TKA (GMK Sphere(TM) Medacta International AG, Castel San Pietro, Switzerland) using Model Based dynamic RSA. Materials and Methods: A cohort of 14 patients (mean age 70,7 yr) was evaluated at 9 months of follow up after surgery. The patients performed two motor tasks: a sit‐to‐stand and a lunge. Kinematical data were evaluated using Grood and Suntay decomposition and Low point method. Data were related to the flexion angle versus internal‐external, varus‐valgus rotations and antero‐posterior translations of the femur with respect to the tibia. Findings/Results: During sit‐to‐stand a clear medial pivot from 80° until full extension was observed. A slight internal rotation of the femur was evaluated during the whole movement. Concerning the lunge, from the beginning to the end of the bending, the condyles make a posterior translation simultaneously, with an external rotation. During the extension phase, an anterior translation, a slight medial pivot and an internal rotation were observed. Conclusions: The kinematic data of the investigated prosthesis are similar in both motor tasks. This behaviour reproduces a medial pivot, as in the native knee joint.