Abstract
INTRODUCTION
Contemporary PCL sacrificing Total Knee Arthroplasty (TKA) implants (CS) consist of symmetric medial and lateral tibial articular surfaces with high anterior lips designed to substitute for the stability of the native PCL. However, designs vary significantly across implant systems in the level of anteroposterior constraint provided. Therefore, the goal of this study was to investigate kinematics of two CS designs with substantially different constraint levels. The hypothesis was that dynamic knee simulations could show the effect of implant constraint on kinematics of CS implants.
METHODS
LifeModeler KneeSIM software was used to analyze contemporary CS TKA (X) with a symmetric and highly dished tibia and contemporary CS TKA (Y) with a symmetric tibia having flat sections bounded by high anterior and posterior lips, during simulated deep knee bend and chair sit. The flat sections of CS-Y implant are designed to allow freedom prior to motion restriction by the implant lips. Components were mounted on an average knee model created from Magnetic Resonance Imaging (MRI) data of 40 normal knees. Relevant ligament/tendon insertions were obtained from the MRI based 3D models and tissue properties were based on literature values. The condyle center motions relative to the tibia were used to compare the different implant designs. In vivo knee kinematics of healthy subjects from published literature was used for reference.
RESULTS
Prior publications on in vivo kinematics of healthy knees showed that normal knee motion is characterized by an overall medial pivot. This includes greater, consistent posterior rollback of the lateral condyle than medial (Fig 3). In contrast, CS implant X showed symmetric motion including paradoxical anterior sliding until 120° flexion. This caused a more anterior location for both femoral condyles in flexion as opposed to the posterior location seen in healthy knees. CS implant Y with flat sections showed even greater anterior sliding than CS-X. These trends were seen for chair sit activity as well. Thus, while CS-X showed less paradoxical sliding, both implants suffered from kinematic deficits due to absence of the PCL (Fig. 1 and Fig. 2).
CONCLUSION
The two CS implants showed different kinematic performance confirming the hypothesis that implant design affects kinematics of CS TKA. Absence of the PCL in contemporary CS implants resulted in kinematic deficits. In particular, a symmetric implant with flat sections connecting the anteroposterior implant lips showed excessive paradoxical anterior sliding. These data showed both the need and opportunity for novel designs to address the limitations of contemporary CS implants.
