Specimen-Specific Finite Element Analysis to Study Cruciate Ligament Mechanics

Katherine Stemmer, Department of Biomedical Engineering, Lerner Research Institute, Cleveland Clinic and Department of Biomedical Engineering, Ohio State University

Ahmet Erdemir, Computational Biomodeling (CoBi) Core and Department of Biomedical Engineering,Lerner Research Institute, Cleveland Clinic

Connecting the tibial and femoral bones in the lower limb are the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL). These ligaments prevent the bones from sliding anteriorly and posteriorly relative to each other, therefore providing mechanical stabilization of the joint. Knowing the mechanical function of the ACL and PCL helps researchers and clinicians develop surgical devices and procedures to repair torn cruciate ligaments. Additionally, understanding these ligaments helps therapists create more effective exercises for patients with knee injuries to restore healthy mechanical function of the knee.

The purpose of this project is to create a specimen-specific finite element representation of the cruciate complex of the knee joint in order to study the mechanics of the ACL and PCL under prescribed tibiofemoral joint movements. By creating a computer model, users may discover the stresses, strains, and displacements of the cruciate ligaments under varying conditions which may not be acquired in vivo or in vitro. In a clinical setting, this will allow surgeons to learn the specific mechanical behavior of these ligaments in the patient rather than gross measurements of the joint function. This knowledge will be particularly beneficial during ACL reconstruction when the surgeon must decide which muscle tendon to use as a graft, the appropriate thickness of the graft, and placement of the graft. The computer model will allow researchers to evaluate the current replacements used for ACL and PCL reconstruction surgery and determine which substitute ligament is most effective mechanically.

The specific components of the knee joint analyzed for the model were the ACL, PCL, femur, and tibia. Starting with magnetic resonance images1 in the axial, sagittal, and coronal planes of the knee joint, the components were manually segmented using the open source software program, Slicer 3D2, to apply color labels to the different tissues in the magnetic resonance images. Moreover, volumes were generated using the model maker option in Slicer 3D2. Next, geometries were reconstructed using the open source program Meshlab3 by smoothing and remeshing triangulated surface models of the tissues of interest. Finally, using the open source program Salome4, three-dimensional volume meshes of different densities, using tetrahedral finite elements, were created in order to allow finding the optimal mesh density which balances between accuracy and speed of computation.

In upcoming modeling work, the mechanical properties of both ligaments, along with prescribed tibiofemoral joint movements (as obtained from experiments) will then be applied in the open source program PreView5 and simulated using FEBio6. Simulation results will be explored further using post-processing tool, PostView7 to evaluate deformations, stresses, and forces within cruciate ligaments under varying motions of the human knee. A preliminary finite element analysis was conducted on a low density ACL mesh. A neo-Hookean material property was used, a fixed boundary condition was applied to the tibia attachment site, and a 5mm displacement was prescribed for the femur attachment site. The analysis results displayed effective stresses, total displacments, and max shear stresses that were consistent with the prescribed conditions.

1Chokhandre, S., Bennetts, C., Halloran, J., Colbrunn, R., Bonner, T., Jones, M. H., & Erdemir, A.

(n.d.). Comprehensive identification of tibiofemoral joint anatomy and mechanical response:

Pathway to multiscale characterization. In ASME 2012 Summer Bioengineering Conference (pp.

1089-1090). http://dx.doi.org/10.1115/SBC2012-80200

2Slicer 3D open source software http://www.slicer.org/

3Meshlab open source software http://meshlab.sourceforge.net/

4Salome open source software www.salome-platform.org

5PreView - FEBio Software Suite open source software http://febio.org/preview/

6FEBio open source software http://febio.org/febio/

7PostView - FEBio Software Suit open source software http://febio.org/postview/