Process of estimating the material properties of human heel pad sub-layers using inverse finite element analysis and some model applications

The human heel pad is subject to repetitive loading and plays an important role in absorbing shocks which may cause injuries. The heel pad has a composite biological structure consisting of the fat pad and the skin. The fat pad tissue is organised into a superficial micro-chamber layer and a deep macro-chamber layer. The heel pad sub-layers have different structures and properties. Hence, to understand the contribution of each layer to the heel problem, it is essential to develop a model with discrete structure. Currently, only plantar pressure measurements are used for diagnosis and treatment of the heel problems, whereas it has been shown that high internal tissue stress is an important factor. Because of complex geometry, discrete structure and nonlinear material behaviour of the heel pad, the external force applied to the heel may result in inhomogeneous internal stress condition. Therefore, the relationship between the plantar pressure and internal stress does not seem to be simple. Since there is no equipment to allow measurement of internal stress, a detailed multi-layered FE model of the heel pad can be used as a solution to predict the internal stress. The main objective of this work was to obtain the hyperelastic and viscoelastic material properties of the subject-specific heel pad sub-layers in-vivo. For this purpose, a combined methodology of finite element modeling and experimentation was developed. An anatomically detailed 3D FE model of the human heel area was developed using MR images of the right foot of a female subject. A combined ultrasound and indentation system was used to apply series of slow and rapid compression tests on the same foot. The forcestrain responses of the heel pad and its sub-layers were used as input to the FE model to estimate properties of the heel pad sub-layers using inverse FEA. The hyperelastic and viscoelastic FE models were then implemented to investigate the effects of experimental and geometrical factors on the heel pad responses. The model was also used to assess the robustness of the hyperelastic FE model when predicting the behaviour of other heels with different geometries. Finally, this model was used with Taguchi method to evaluate the effect of footwear design factors on the compressive stress in the heel pad tissue. There were some key limitations in this study. For example, the properties of the heel pad sub-layers were estimated only for a specific heel pad. Also, whilst it is preferred to use xviii automatic segmentation and solid modeling to improve repeatability of some FE processes, some parts of the modeling process were performed manually.