Investigating the internal stress/strain state of the foot using magnetic resonance imaging and finite element analysis

It is known that mechanical forces acting within the soft tissues of the foot can contribute to the formation of neuropathic ulcers. Unfortunately, only surface measurements (plantar pressure) are used clinically to estimate foot risk due to mechanical loading. In the absence of equipment to monitor the internal stress and strain states, it is currently unknown how these surface measurements relate to what is happening inside of the foot.

Magnetic resonance imaging (MRI) has the potential to provide high resolution in vivo images of the internal structure of the foot. When combined with a device capable of applying loads to the limb during imaging, MRI can be used to visualize 3-dimensional internal strains. If the load applied during imaging is known, finite element models can be used to estimate the internal stress state which corresponds to the visualized deformation.

This dissertation describes the development and use of an MRI-compatible loading device to perform an in vivo load-deformation experiment on the human forefoot, a common site of neuropathic ulceration. The collected load-displacement field data was used in conjunction with a novel, 3-dimensional, layered-tissue finite element model of the forefoot to simultaneously optimize the material properties of three tissue layers: skin, plantar fat pad, and muscle.

2-dimensional and 3-dimensional finite element models of the forefoot incorporating separate skin, fat, and muscle tissue sections were used to investigate the relationship between peak plantar pressure and peak internal stress. To determine the effect of the clinical goal of lowering peak plantar pressures on the internal stress and strain measures, a cushioning foam mat was modeled underneath the forefoot.

The location of peak internal stresses (sub-metatarsal 3) did not agree with the location of peak plantar pressure (sub-metatarsal 2). Inserting a cushioning foam mat decreased the peak plantar contact pressure by 66%, but did not change the location of peak internal stresses and only decreased their magnitudes by about 2%. Internal stresses were, however, reduced by 78.5% in the skin near the site of peak plantar pressure. The 3-dimensional multi-tissue model developed in this study is the first of its kind and provides novel insight into the mechanical responses of foot tissues.