Restoration of Joint Inclination in Total Knee Arthroplasty Offers Little Improvement in Joint Kinematics in Neutrally Aligned Extremities

Abstract

Kinematically aligned total knee replacements have been shown to better restore physiological kinematics than mechanical alignment and also offer good postoperative satisfaction. The purpose of this study is to evaluate the extent to which an inclined joint line in a kinematically aligned knee can alter the postoperative kinematics. A multi-body dynamic simulation was used to identify kinematic changes in the joint. To accurately compare mechanical alignment, kinematic alignment and a natural knee, a "standard" patient with neutral alignment of the lower extremities was selected for modeling from a joint database. The arthroplasty models in this study were implanted with a single conventional cruciate-retaining prosthesis. Each model was subjected to a flexion movement and the anteroposterior translation of the femoral condyles was collected for kinematic analysis. The results showed that the mechanical alignment model underwent typical paradoxical anterior translation of the femoral condyles. Incorporating an inclined joint line in the model did not prevent the paradoxical anterior translation, but a 3° varus joint line in the kinematic alignment model could reduce the peak value of this motion by about 1 mm. Moreover, the inclined joint line did not restore the motion curve back to within the range of the kinematic curve of the natural knee. The results of this study suggest that an inclined joint line, as in the kinematic alignment model, can slightly suppress paradoxical anterior translation of the femoral condyles, but cannot restore kinematic motions similar to the physiological knee. This finding implies that prostheses intended to be used for kinematic alignment should be designed to optimize knee kinematics with the intention of restoring a physiological motion curve.

Keywords: computational simulation; joint line inclination; kinematic alignment; knee kinematics; mechanical alignment; total knee arthroplasty.

FIGURE 1

Full-length weight-bearing radiograph of the “standard” patient showing the values of mLDFA and MPTA.

FIGURE 2

Schematic diagram of the benchmark TKA model (Model 1). MFFC and LFFC are the medial flexion facet center and the lateral flexion facet center of the corresponding femoral component’s posterior condyle, respectively. The knee flexion axis was defined as the connecting line between MFFC and LFFC.

FIGURE 3

Schematic diagram of the Model 1 for validation. Kinematic data recorded from the “Key point” marker was used for model validation.

FIGURE 4

Models constructed in this study with different joint alignment. The joint line inclination and internal rotation of the implants were increased in 1° increments between models, up to Model 6 (5° varus of joint line, 2° internal rotation of implant). Model 4 reproduced the original tibiofemoral articular geometry of the “standard” patient, and therefore met the technical requirements of kinematic alignment (KA). ER and IR represent internal rotation and external rotation relative to the posterior condylar axis, respectively.

FIGURE 5

Kinematic curve of Model 1 plotted against in vivo data for model validation. (A) Femoral translation coupled with knee flexion, with positive values indicating posterior translation relative to the reference position. (B) Tibial axial rotation coupled with knee flexion, with positive values indicating internal tibial rotation relative to the femur.

FIGURE 6

Kinematic comparison of different joint line inclinations showing anterioposterior translation of the medial (A) and lateral (B) FFCs coupled with knee flexion. The “0” value on the vertical axis represents the initial position of the model before mechanical equilibrium, positive values indicate forward positioning of the FFCs, and negative values indicate backward positioning of the FFCs.

FIGURE 7

Kinematic comparison between Model 1 (MA-TKA), Model 4 (KA-TKA) and the “living” knee (Johal et al., 2005). The motion curves of the medial FFCs (A) and lateral FFCs (B) were compared. The values on the vertical axis indicate the distance between the FFCs and the ipsilateral posterior edge of tibia condyles (for the natural knee), or the distance between the FFCs and the ipsilateral posterior edge of the polyethylene insert (for the models).