Summary
A novel robotic testing method shows effects of TKR design on knee stability and kinematics
Abstract
Objectives: Instability after TKA, caused by patient factors, implant design or surgical technique is still a cause of patient dissatisfaction. The goal of this work was to assess the stability of three different implant design types in vitro, whilst minimising surgical variability. This was accomplished by first testing the implants ‘in isolation’, without soft tissues present, followed by a cadaveric study.
Methods
Three different implant designs were tested: Depuy Synthes Attune MS (gradual radius change on femur), Zimmer-Biomet Persona MC, (multi-radius) and Stryker Triathlon (Single-radius).
The first experiment attached the tibial tray and insert to a KUKA 6-DOF robotic arm controlled by simVITRO software, whilst the femoral component was attached to a rigid base. This set-up allowed the implants to be tested from 0-140 degrees of flexion under a constant compressive force of 710N (1 BW). The implants were tested without any extra external tibial load (neutral path), then 90N anterior tibial force and then 90N posterior tibial force. Each implant was tested 6 times.
The second, cadaveric study started by testing 8 fresh-frozen cadaveric knees. The tibia and femur were potted using PMMA and the tibia was attached to the robotic end-effector, with the femur mounted on the rigid base. The native knee was flexed-extended 0-90o under 50N compressive load to find the ‘passive flexion’ arc and then 710N compressive load to find the loaded flexion arc. The knee underwent stability testing with the following loads: 90N anterior/posterior tibial force, 5Nm internal/external rotation, 8Nm varus/valgus torque. After this initial test, the knee underwent a PCL-retaining TKA. Three implant designs, with identical bone cuts, but differing articulating geometries to represent gradual radius, multi radius and single radius designs, underwent the same stability testing in each knee.
Results
Each of the implants showed different kinematics/stability in the isolated implant test, and all of them were different from the native knee in the cadaveric test. Isolated implant testing showed greater stability, but with significant differences between them, for the multi-radius and gradual radius change designs than with the single-radius design. The single-radius implant showed much higher AP laxity (11mm) and less consistent kinematics than the other implants (5mm at 20o flexion) in the cadaveric tests. Tractive rolling of the single-radius design caused the femur to roll anteriorly-posteriorly during extension-flexion in both the isolated implant and cadaveric tests, but when tested in the knee its laxity limits were shifted anteriorly versus the isolated implant.
Conclusion
This novel robotic testing method shows that implant design affects the isolated implant stability and also the implanted cadaveric knee. Robotic testing allows flexion-extension under compressive force, providing more clinically relevant data. Testing several TKR designs with differing bearing geometry showed how inherent implant stability and geometry affect kinematics and stability of the loaded knee. Some implant designs rely more on soft tissues for stability. More rigorous implant testing and a better understanding of differences in kinematics and stability between designs should allow surgeons to make more informed decisions surrounding implant design choice.