Internal femoral component malrotation in TKA significantly alters tibiofemoral kinematics

Femoral component malrotation in total knee arthroplasty (TKA) is clinically proven to cause dissatisfaction and impaired function. This study is an attempt to characterize the tibiofemoral kinematics following femoral malrotation in posterior stabilized (PS) TKA. It was hypothesized that internal malrotation would introduce the most pronounced changes.

Six fresh-frozen cadaver specimens were mounted in a kinematic rig. Three motion patterns were applied with the native knee and following PS TKA (passive motion, open chain extension, and squatting) while infrared cameras recorded the trajectories of markers attached to femur and tibia. Three different femoral implants were tested: a conventional posterior stabilized component, and adapted components of the same implant with 5° of intrinsic external and internal rotation, respectively.

The implantation of the PS TKA resulted in less tibial internal rotation (squat 33–70°, p < 0.05) and the medial femoral condyle shifted posteriorly especially in deep flexion (squat 84–111°, p < 0.05). Internal component malrotation caused internal rotation and abduction of the tibia in flexion (squat 33–111°, p < 0.05), an elevated (squat 43–111°, p < 0.05) and more anterior (passive 61–126°, p < 0.05) located medial femoral condyle and a lateral femoral condyle located more posterior and inferior (squat 73–111°, p < 0.05) than in the neutrally aligned TKA. External component malrotation caused only little changes under passive motion. Under a squat there was less internal rotation and more adduction to the tibia (33–111°, p < 0.05). The medial femoral condyle was moved more posterior (squat 59–97°, p < 0.05), the lateral femoral condyle more superior (squat 54–105°, p < 0.05) than in the neutrally aligned TKA.

The greatest differences to the native tibiofemoral kinematics were introduced by internal rotation of the femoral component. Also neutrally and externally rotated femoral components introduce kinematic changes, but to a lesser extent. With respect to the alterations introduced to kinematics internal malrotation should be avoided when performing PS TKA.