Current research with total knee replacements is directed at refining the design to improve patient function. In this project, implant design innovations in the Encore 3DKnee were experimentally tested to determine if their intended functional improvements were achieved.

The first design feature focused on in this project was the lateral pivot of the tibial insert. It has a congruent lateral compartment and wide medial compartment to encourage internal-external rotations about the lateral condyle during knee motion to improve kinematics by allowing axial rotations. The second design feature addressed was the implementation of a single radius of curvature design for the posterior femoral condyles. This increases the extensor moment arm and has the potential to restore the extensor mechanism by enhancing quadriceps function and improving patellofemoral mechanics. The third focus of this project was the effect of PCL sacrifice with this implant which was designed to spare the PCL.

Three experimental tests were conducted to see the effects of the implants design. Four knee conditions were tested in each experiment so results could be compared against those of specimens when intact, ACL deficient, implanted with the PCL spared, and implanted with the PCL sacrificed. The first experiment conducted on specimens was three cycles of passive flexion and extension of the knee from 0 to 90 degrees while collecting the specimen’s kinematic motions. In the second test a 500 N compressive load was applied to the joint. Kinematics and data from a pressure sensitive film inserted in the tibio-femoral articulation were recorded. In the third experiment an increasing quadriceps load was applied to the knees until a 50 Nm extension moment was generated about the knee’s flexion-extension axis. During this experiment the quadriceps load required, the kinematic motions of the knees, and joint contact pressures from the patellofemoral articulation were acquired.

Results from cadaver testing determined that under passive flexion and extension the lateral pivot design is able to reproduce internal-external rotations during flexion and extension. Intact knees had an average internal-external rotation of 6.23 degrees, and after implantation the specimens still rotated 4.15 degrees. Internal-external rotations were also present under compressive loading at discrete angles. From an initial flexion angle of 0 degrees to 90 degrees, intact knees rotated an average of 19.8 degrees, implanted specimens rotated an average of 15.8 degrees. These rotations were much higher than those seen in passive flexion and extension which never exceeded 9 mm.

The extensor mechanism was restored with respect to enhancing quadriceps function, but patellofemoral mechanics were not restored. The contact area between the patella and femur was reduced greatly after resurfacing of the patella during TKA. For healthy knees the average contact area at 90 degrees of flexion was 587.75 mm², ACL deficient was 448 mm². At this same angle the implanted knee had a contact area of 256.83 mm², and the PCL deficient implanted knee had 189.67 mm². Reduced contact areas produced increased pressures transmitted though the articulation in the implanted knees. Increased pressure and load concentrations have the potential to cause patellar complications such as implant loosening, component wear, and anterior knee pain. Once implanted, the quadriceps loads required to generate a moment at discrete angles remained on par with intact specimens and often required a lesser load. This result has the potential to improve both short term and long term TKA results. Reduced quadriceps forces may aid in postoperative rehabilitation times, as well as in activities which rely on the extensor mechanism like rising from a chair and stair climbing. A reduced quadriceps load also correlates to reduced forces transmitted through the patellofemoral joint. Reduced forces may lead to less wear, which would result in fewer revisions and a longer implant life.