Combined measurement and modeling of specimen-specific knee mechanics for healthy and ACL-deficient conditions

Abstract

Quantifying the mechanical environment at the knee is crucial for developing successful rehabilitation and surgical protocols. Computational models have been developed to complement in vitro studies, but are typically created to represent healthy conditions, and may not be useful in modeling pathology and repair. Thus, the objective of this study was to create finite element (FE) models of the natural knee, including specimen-specific tibiofemoral (TF) and patellofemoral (PF) soft tissue structures, and to evaluate joint mechanics in intact and ACL-deficient conditions. Simulated gait in a whole joint knee simulator was performed on two cadaveric specimens in an intact state and subsequently repeated following ACL resection. Simulated gait was performed using motor-actuated quadriceps, and loads at the hip and ankle. Specimen-specific FE models of these experiments were developed in both intact and ACL-deficient states. Model simulations compared kinematics and loading of the experimental TF and PF joints, with average RMS differences [max] of 3.0° [8.2°] and 2.1° [8.4°] in rotations, and 1.7 [3.0] and 2.5 [5.1] mm in translations, for intact and ACL-deficient states, respectively. The timing of peak quadriceps force during stance and swing phase of gait was accurately replicated within 2° of knee flexion and with an average error of 16.7% across specimens and pathology. Ligament recruitment patterns were unique in each specimen; recruitment variability was likely influenced by variations in ligament attachment locations. ACL resections demonstrated contrasting joint mechanics in the two specimens with altered knee motion shown in one specimen (up to 5mm anterior tibial translation) while increased TF joint loading was shown in the other (up to 400N).

Keywords: Cruciate deficient; Finite element; Ligament; Patella; Walking.

Figure 1

Knee cadaver mounted in the Kansas Knee Simulator (KKS) (left), and its computational representation (middle) with specimen-specific TF and PF soft tissue structures (right): anterior cruciate ligament (ACLam, ACLpl), posterior cruciate ligament (PCLal, PCLpm), lateral collateral ligament (LCL), popliteofibular ligament (PFL), medial collateral ligament (MCL), superficial medial collateral ligament (DMCL), posterior oblique ligament (POL), anterolateral structure (ALS), posterior capsule (PCAPM, PCAPL)

Figure 2

Comparison of model (dashed) and experimental (solid) TF kinematics in the KKS simulator for intact and ACL-resected conditions in two specimens

Figure 3

Comparison of model (dashed) and experimental (solid) PF kinematics in the KKS simulator for intact and ACL-resected conditions in two specimens.

Figure 4

Comparison of model (dashed) and experimental (solid) quadriceps force in the KKS simulator for intact and ACL-resected conditions in two specimens.

Figure 5

Total TF and PF contact force (left) and contact center of pressure with force vectors (right) shown for two specimens in intact and ACL-deficient conditions.

Figure 6

Total (vector sum of all ligament force) and individual ligament recruitment as a function of knee flexion (left), and total ligament shear and tensile forces (right) for intact (solid) and ACL-deficient (dashed) conditions in two specimens.