Cases

P.J. Antunes, G.R. Dias, A.T. Coelho, F. Rebelo, T. Pereira

Finite element analysis can be a very powerful tool for understanding stress distribution on the feet and its implications for human comfort. Shoe insoles that are mechanically optimized to simultaneously support the body weight without foot deviations, and act as contact pressure reducers in the precarious plantar zones, can enhance foot comfort. The geometrical complexity of the foot structure implies an adequate 3D modelling of the foot structure in order to precisely simulate the biomechanical behaviour of the human foot. The study below illustrates how Mimics software assisted researchers to quickly and easily generate an accurate, anatomically detailed 3D foot model for non-linear finite element analysis.

Fast translation of CT scan data into complete 3D models for FEA

Using Materialise’ interactive medical imaging control system (MIMICS), the research team processed CT scan data of the foot region of a 26 year old male. After importing the medical data (DICOM images), the team employed a thresholding technique based on Hounsfield units to separate the bone and the soft tissues. They separated the segmentation mask for each bone with manual editing and the region growing tool. As a result, every bone now corresponds to an individual mask, which can be distinguished by the different applied mask colours. This process allows the generation of independent geometrical files and 3D models. For the complete definition of the bone foot structure and soft tissues, 30 different regions were defined. (see figure 2)

Each region was then reconstructed in 3D with Mimics, capturing each bone and the encapsulated soft tissues volume that geometrically define the foot structure. (see figure 3)

In order to create 3D CAD models, each model was exported to an *.STL file. Using CATIA software, the cartilages that were not reconstructed in the segmentation process were then modelled, and volume Boolean operations were performed to achieve a volume of soft tissues. (see figure 4). This procedure guaranteed perfect alignment of the cartilages on the bones, an important consideration for future finite element model generation.

Next, the researchers imported the assembled model of the bone structure of the foot, the cartilage models and the Boolean-generated soft tissues volume in the non-linear FEA package ABAQUS. The model was then prepared for non-linear analysis, achieved specifically through the definition of loads, boundary conditions, material properties, kinematic constraints and mesh discretization processes. The resulting FEA model allows the output of several variations that can be used to test the comfort of shoe insoles or to study other biomechanical aspects of the foot. (see figure 5)

The FEA contact pressure values were experimentally verified by the use of equipment that measures podologic pressure. The non-linear FEA model generated in this study can serve as a tool for design optimization of shoe insoles or other foot support devices. With this tool, a wide variety of insole geometries and materials can be tested in order to study and improve foot comfort. This can be achieved by modifying the insole’s geometrical design and/or the formulation of the insole’s materials.

Mimics has again proven itself an invaluable tool in creating an accurate, detailed and anatomically correct 3D model based on scanner data. Its user-friendliness and high performance design lifts every project to a higher level.