Novel Method for Simulating Patellofemoral Joint Mechanics Using a Six-Axis Joint Simulator with Virtual Ligament Restraints

ABSTRACT Although there is an international standard for simulating patellofemoral joint mechanics to evaluate patellar implant durability (ISO 14243-5, Implants for Surgery: Wear of Total Knee Prostheses Durability Performance of the Patellofemoral Joint), the parameters in the standard may produce higher contact forces than in a well-aligned total knee arthroplasty (TKA) due to the baja (low) initial placement of the patellar implant on the femur and overconstraint of the patellar implant. The method presented here focuses on producing similar joint reaction forces to those in literature for well-aligned TKA by placing the patella in a natural extension position and allowing passive patella mediolateral translation through virtual soft tissue restraints. A commercial six-axis joint simulator applied superoinferior patellar translation, anteroposterior compression, and femoral flexion during simulated walking and squat-type activities. Legally marketed patellar and femoral implants were subjected to simulated walking and squat kinematics to validate the resulting reaction forces and passive mediolateral translations against those in the literature. The peak superoinferior and mediolateral contact forces measured during the walking simulation (74.6 N ± 4.8 N and 129.9 N ± 4.3 N, respectively) and during the squat simulation (609.1 N ± 3.6 N and 70.4 N ± 0.9 N, respectively) were similar to those documented in literature. The patellar implant translated medially 2.2 mm ± 0.2 mm and then back during walking, which is a similar range and direction to that reported in literature. During the squat activity, patellar implant translation increased to 5.5 mm ± 0.1 mm medially before translating back toward the intercondylar notch at higher flexion angles. Although the results are limited to one femoral implant and one patellar implant design, the method presented here has accurately replicated patellofemoral kinematics, reaction forces, and soft tissue mechanics for the evaluated patellar and femoral implant geometry, aligning with published literature.