FEM in Foot Surgery

On the right track - AnyBody and ANSYS in Foot Surgery

Orthopedic foot surgery primarily corrects mechanical foot problems, the diagnosis of which takes top priority for foot specialists. Since clinical examination methods often fail here, a new path has been opened up that can produce quantifiable and reproducible results. In addition, previously concealed mechanical pathology results can be shown in details: reactive forces and moments are to be calculated in all foot joints at various times in the stance phase with the help of FEM.

FEM in Foot Surgery
This innovative method for investigating a mechanical problem is split into five stages: Stages 1 – 3 are measurements of the kinetics, i.e. the kinematics and the muscle electrical activities when walking. In the 4th stage the muscle forces are calculated using the AnyBody program, and these are then introduced into the ANSYS calculation in the 5th stage with the floor's reactive forces and movements as boundary conditions.

Kinetics

The kinetic investigation consists of a dynamic measurement of pressure on the pressure plate and the 3D measurement of force with the force measuring plate. The patient walks at her/his own speed and steps on the measuring plate with the 2nd step.

Kinematics

The motion analysis was carried out with a 3D motion analysis system using 6 cameras. Only the stance phase is measured in each case. The examination is always carried out separately on one side. The marker setup covers the 5 segments for the standard examination. The setups and methods were validated in the laboratory of the Aarau Canton Hospital in during a master thesis project (ETH Zurich).

Electromyography

The electromyography essentially serves as a control in the sense of post-processing in the subsequent muscle modeling.

Muscle modeling

The AnyBody Modeling System program is used for muscle modeling. Since data is only collected from one side, on account of the kinetic setup, the 2-sided model is modified e.g. the removal of the second leg from the model calculations (Figure 1). The parameters measured in the kinematics stages are transferred to AnyBody using MATLAB routines.  Subsequently, the muscular forces of all the muscles in the lower extremity are calculated.

FEM models

It requires substantial time to create an FEM model of an individual foot. Therefore, a standard foot model was generated in ANSYS using computer tomography data (Figure 2).

Nevertheless, in order to achieve a certain "individuality" in the calculations, scaling is carried out.  In other words, the length of the standard foot model was adapted to the actual length of the participant’s foot from x-ray measurements. The positions of the individual bones were also transferred to the foot model by measuring the x-rays and then rotating accordingly in the ANSYS Design Modeler.

The joints between the bones are normally defined as hinge joints. The axes of the joints are defined on the basis of data from the pertinent literature – wherever available. In the case of smaller joints, where no such data is usually available , the origin and the direction of the joint axis has to be determined according to the existing geometry.

The coordinates of the muscular insertions and origins are defined in the model on the basis of the anatomical position according to literature. Since the total force of the muscle and the local position of the tendons are known from AnyBody, the individual force components could be calculated at both the insertion and the origin points.  Simultaneously, the deflection is also calculated in the FEM model.

Quasi-static investigations are still being carried out at present. The model was therefore calculated in 10 different stance phases, whereby the position of the individual foot segments was adapted to each kinematic measurement.

The material properties were defined as isotropically elastic, whereby the individual bones were treated as solids. This simplification appeared legitimate since only reactive forces and reactive moments were primarily calculated in the joints.

What is new about this procedure is the fact that, unlike other foot models, the muscular forces are used as boundary conditions. The effects of the forces on the deflection points of the muscle are also taken into account. Since the effect of force on the metatarso-phalangeal joint of the big toe is considerable, the omission of this force would appear to be too much of a simplification. 

The linear calculation methods, linear material properties, quasi-static approach, and the restriction of the rigid body movements are still not ideal in the procedure described here. This only allows an interpretation of the reactive forces and moments in the primary joints, similar to analytical models. Contractions and extensions have to be interpreted with reservations. In contrast, the reactive forces and moments can be determined in any joint in the foot, something that is not possible with AnyBody alone.

A key difference to earlier models is the fact that the current model is three-dimensional.

3-D FEM simulations require a relatively large amount of time to execute, even when a standard foot model is utilized. Substantial effort is needed to execute the multitude of geometrical changes to represent the respective skeletal positions of a quasi-static state.

This is why the focus of these simulations in foot surgery currently lies in the field of research and not in everyday clinical work.

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