Abstract
INTRODUCTION
In native knees anterior cruciate ligament (ACL) and asymmetric shape of the tibial articular surface with a convex lateral plateau are responsible for differential medial and lateral femoral rollback. Contemporary ACL retaining total knee arthroplasty (TKA) improves knee function over ACL sacrificing (CR) TKA; however, these implants do not restore the asymmetric tibial articular geometry. This may explain why ACL retention addresses paradoxical anterior sliding seen in CR TKA, but does not fully restore medial pivot motion. To address this, an ACL retaining biomimetic implant, was designed by moving the femoral component through healthy in vivo kinematics obtained from bi-planar fluoroscopy and sequentially removing material from a tibial template. We hypothesized that the biomimetic articular surface together with ACL preservation would better restore activity dependent kinematics of normal knees, than ACL retention alone.
METHODS
Kinematic performance of the biomimetic BCR design (asymmetric tibia with convex lateral surface), a contemporary BCR implant (symmetric shallow dished tibia) and a contemporary CR implant (symmetric dished tibia) was analyzed using KneeSIM software. Chair-sit, deep knee bend, and walking were analyzed. Components were mounted on an average bone model created from magnetic resonance imaging (MRI) data of 40 normal knees. Soft-tissue insertions were defined on the average knee model based on MRI data, and mechanical properties were obtained from literature. Femoral condyle center motions relative to the tibia were tracked to compare different implant designs.
RESULTS
During simulated chair-sit, the biomimetic BCR implant showed knee motion similar to that reported for healthy knees in vivo including medial pivot rotation with greater rollback of the lateral femoral condyle (5 mm medial vs. 11 mm lateral). The CR implant showed posterior femoral subluxation in extension, paradoxical anterior sliding until 60° flexion followed by limited rollback until 105° with no medial pivot rotation. The conventional BCR implant reduced initial posterior shift of the femur in extension, however, medial pivot rotation and steady posterior rollback was not achieved. Similar trends were also found for deep knee bend activity.
During walking the CR implant showed posterior subluxation in extension followed by anterior motion similar to the chair-sit activity. Both BCR implants showed less femoral excursion without posterior subluxation similar to published in vivo kinematics data for bi-uni patients.
CONCLUSION
By simulating a variety of daily activities with different ranges of knee motion we were able to show that the ACL preserving biomimetic TKA implant could restore activity dependent normal knee kinematics unlike contemporary ACL retaining and ACL sacrificing implants. For chair-sit activity there was a clear medial pivot pattern for the biomimetic BCR design (unlike any other implant), while for lower flexion activities there was no medial pivot apparent in our simulations. These activity dependent knee motions are consistent with published in vivo kinematics and confirmed our hypothesis that biomimetic articular surface together with ACL preservation may be required to restore normal knee function.
The biomimetic BCR design with its anatomical articular surface together with ACL preservation may provide patients with a more normal feeling knee following TKA surgery.
