Finite Element Modeling of the Foot

Abstract

Finite element (FE) foot modeling is a useful tool for investigating foot and ankle biomechanics. Such models can provide insight into internal soft tissue behavior of the foot and are ideal for conducting parametric analyses. Many of the previous FE foot models simulated quasi-static loads and imposed major simplifications on anatomy and material properties, which could compromise their accuracy and usefulness. The purpose of this study was to develop and validate two subject-specific FE foot models (normal and diabetic) capable of simulating both quiet stance (quasi-static loading) and the stance phase of gait (dynamic loading), and to explore the plantar pressure and internal soft tissue stress distribution under these two loading conditions. The models included subject-specific bone, skin, muscle and fat anatomy obtained from CT and MR imaging data. Subject-specific hyperelastic material properties for each soft tissue were determined from inverse FE analysis of gated MRI compression experiments. Ligaments, tendons, and joint cavities were modeled more accurately based on the medical images. Four regions of the plantar aponeurosis were modeled with non-linear material properties obtained from cadaveric mechanical testing. The models were validated with in vivo experimental data collected in-house and with literature data under three loading conditions: passive compression, quiet stance, and the stance phase of gait, by analyzing bone kinematics, ground reaction forces (GRFs), plantar pressure, plantar aponeurosis force and ankle joint force. The subject-specific models were capable of simulating physiologic quiet stance and dynamic gait conditions. The vertical GRF, plantar pressure distribution, bone kinematics, plantar aponeurosis force and ankle joint force were reproduced in the model. The von Mises and hydrostatic stress predictions provided insight into the specific location and time of the peak internal stress, which suggested regions of elevated injury risk as well as possible mechanisms of injury. The model's ability to effectively perform parametric analyses was demonstrated by investigating the effect of soft tissue simplifications on plantar pressure and internal stress. The protocols and mechanical test data outlined in this study will serve as a guide line for future FE foot model development. The models of the normal and diabetic subject developed in this dissertation are useful tools for exploring and generating clinically effective treatments for foot and ankle pathology.