Patient dependent knee modeling at several flexion angles: A study on soft tissues loadings.

OBJECTIVE: With cartilage physiopathology, knee stability and therefore the subsequent loadings on ligaments have a strong impact in the development of knee osteoarthritis. To understand this phenomenon, authors are proposing different knee models. Most of them are without any flexion, some have been introduced by controlling actively muscular groups. In this case, the forces used to achieve the flexion may not be physiological, and the pressure on the articulation can be questioned. We propose a mixed approach, both using medical imaging (MRI, EOS X-ray system) and force platform in conjunction with a finite element model.

MATERIAL AND METHODS: One healthy volunteer underwent a MRI and an EOS imaging of the knee. EOS images gave the exact position of the bones for five flexion angles, the force platform ensuring consistent boundary conditions. A 3D geometrical model of the bones and cartilages was segmented from the MRI stacks; this model was meshed and smoothed for finite element analysis (FEA). The bones of the model were fitted to those of the EOS images to have the physiological positions of the knee. The FEA was carried out with the EOS imaging experimental boundary conditions, ensuring the global knee mechanical equilibrium.

RESULTS: To validate this patient-specific model, its bony structure was confronted with the EOS images once the mechanical equilibrium reached. The difference of position was within the error range of the image registration, showing a satisfying equilibrium state. Last, this model gave us an estimation of the tension in the ligaments for every flexion as well as a pressure map on the cartilages.

DISCUSSION/CONCLUSION: The FEA model main assumption lies on the absence of menisci and muscular active control. Nevertheless, the model is suitable to achieve a reasonable equilibrium. It has been used successfully in the study of ligament tension during the flexion cycle. It will then be used as a reference in studying arthritic knee joints for clinical cases.