Abstract
INTRODUCTION
ACL retaining (BCR) Total Knee Arthroplasty (TKA) provides more normal kinematics than ACL sacrificing (CR) TKA. However, in the native knee the ACL and the asymmetric shape of the tibial articular surface with a convex lateral plateau are responsible for the differential medial/lateral femoral rollback (medial pivot). Therefore, the hypothesis of this study was that an asymmetric biomimetic articular surface together with ACL preservation would better restore native knee kinematics than retention of the ACL alone. Normal knee kinematics from bi-planar fluoroscopy was used to reverse engineer the tibial articular surface of the biomimetic implant. This was achieved by moving the femoral component through the healthy knee kinematics and removing material from a tibial template.
METHODS
LifeModeler KneeSIM software was used to analyze a biomimetic BCR implant (asymmetric tibia with convex lateral surface), a contemporary BCR (symmetric shallow dished tibia) and a contemporary CR (symmetric dished tibia) implant during simulated deep knee bend and chair sit. Components were mounted on an average bone model created from Magnetic Resonance Imaging (MRI) data of 40 normal knees. The soft-tissue insertions were obtained from the average knee model and the mechanical properties were obtained from literature. Femoral condyle center motions relative to the tibia were used to compare different implant designs. In vivo knee kinematics of healthy subjects from published literature was used for reference.
RESULTS
During simulated deep knee bend, the ACL sacrificing contemporary CR implant showed initial posterior femoral subluxation due to the absent ACL, followed by paradoxical anterior sliding until 90° flexion, and no medial pivot rotation. Retention of the ACL in the contemporary BCR implant reduced the initial posterior shift of the femur in extension. However, medial pivot rotation and steady posterior rollback could not be achieved. In contrast, the biomimetic BCR implant showed knee motion very similar to that reported for healthy knees in vivo, with medial pivot rotation and greater, consistent rollback of the lateral femoral condyle than the medial condyle (11 mm medial vs. 16 mm lateral, Fig. 1 and Fig. 3). Similar trends were seen for all implants during simulated chair sit (Fig. 2 and Fig. 3).
CONCLUSION
An ACL preserving biomimetic TKA implant was able to restore normal knee kinematics unlike contemporary ACL retaining and ACL sacrificing implants, during the simulated activities. This confirmed the hypothesis that a biomimetic articular surface together with ACL preservation is required to restore normal knee kinematics.
