A Finite Element Model of a Realistic Foot and Ankle for Flatfoot Analysis

Abstract

Adult-Acquired Flatfoot is a degenerative condition in which the ligaments and tendons supporting the arch deteriorate eventually leading to arch collapse. This deterioration can occur at various locations along the arch creating a number of different patterns of collapse. Surgical treatment for adult acquired flatfoot consists of a combination of various osteotomies. Although general guidelines exist, there is no systematic way to determine which combination of osteotomies should be used to correct a given foot deformity. Computer simulation with finite element analysis might provide an analytical tool to optimize the choice of osteotomy location and size. By dividing a complex problem into simpler components, finite element analysis allows for the solution of complex problems by solving a large set of simple equations. Finite element analysis has previously been used to study effects of diabetes, shoe design, and gait analysis in the foot. These studies have oversimplified geometry and material properties of foot tissues which limits the true mechanical behavior. The goal of this study was to create an anatomically and physiologically correct finite element model of the foot and ankle. To create a healthy foot model, CT scans were collected from one cadaver foot to create a three-dimensional cortical bone model in 3D Slicer software. The cortical bone model was imported into SolidWorks to create the geometry for trabecular bone, cartilage, ligaments, and tendons. Journal articles, textbooks, and other resources were used in order to create realistic cartilage, ligament, and tendon models (Netter & Colacino, 1997; see also Boss & Hintermann, 2002; Campbell et al., Apr. 2014; Golanó et al., 2010; Mahadevan). Final approval of the model’s geometry was obtained from the orthopaedic surgeon supervising this study. After completing the anatomically correct geometry of the foot, it was imported into finite element software (ANSYS, http://www.ansys.com/). The model was meshed with solid elements only: tetrahedral elements for the foot and hexahedral elements for the ground support. Linear elastic material properties were assigned to all bodies. Boundary conditions and contacts were created including a fixed ground support and bonded and frictionless contacts. A body weight force was applied to the tibia and tendon forces were applied to simulate loading during midstance. The frictionless contacts created a nonlinear problem that caused the simulation to fail to converge to a solution. Abnormally high stresses and deformation were found in the results. The foot model failed to converge to realistic results because of the current model’s complexity. An anatomically correct foot model was successfully created, but simplifications need to be made to the model in the future for convergence. Recommendations for simplification include modeling ligaments as truss elements, adding spring elements to tendons, and adding soft tissue or fat pads to the model. After simplifications are completed and realistic results are obtained, Flatfoot conditions and surgeries can be simulated and analyzed.