Effect of Varying the Angle of Rotation of the Heel Model

To assess the effect of variation in the angle of rotation of the foot on the heel pad responses, the angle of rotation of the heel model (reference value = 17.6 ) was varied for ±1 , ±5 and ±10 .All structures in the FE model were assigned with the material properties as per Table 4.2. Similar to previous parametric studies, the heel pad model was compressed up to strain of 40% with displacement rate of 5mm/s to reproduce the physical tests conditions (see section 5.2.3). The model predicted force-strain results for reference angle of rotation and different angles are shown in Figures 4.14-4.16.

As can be seen in Figure 4.14, the predicted force-strain relationship for the models with angle of rotations of ±1 are generally coincident with the force-strain result for the model with the reference angle of rotation (RMS error<1.5% max force). According to Figures 4.15 and 4.16 extend changes to the angle of rotation of the heel model (±5 and ±10 ) bring some small variations (RMS error<4% max force). Figures 4.15 and 4.16 show that for more than 25% strain, by increasing the angle of rotation the force decreases for the same depth of indentation. It can be explained by the fact that by increasing the amount of rotation of the model less material is compressed in high strains. In low strains, the amount of materials, which are required to be compressed for the same depth of indentation, are approximately equivalent for different rotation of the model.

The differences between the force-strain results obtained for the model with reference and different angles of the rotation are calculated and represented by RMS error in Table 4.4. The results indicates that for maximum 10 degree difference in angle of rotation which is ~48% average measurement error of the amount of the heel pad coming out from the aperture

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induced less than 4% maximum force error to predicted force-strain results. Understanding the effect of angle of rotation of the FE model, the measurements error was kept small by repeating the examination and measurement of the amount of the heel protruded from the foot brace at different times.

Figure 4.14: Effect of varying the angle of rotation of the FE model for ±1 .

Figure 4.15: Effect of varying the angle of rotation of the FE modelfor ±5

0 10 20 30 40 50 0% 5% 10% 15% 20% 25% 30% 35% 40% Forc e (N ) Strain

Angle of rotation of the heel 17.6

_±1

16.6 degree 17.6 degree 18.6 degree

0 10 20 30 40 50 0% 5% 10% 15% 20% 25% 30% 35% 40% Forc e (N ) Strain

Angle of rotation of the heel 17.6

_±5

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Figure 4.16: Effect of varying the angle of rotation of the FE modelfor ±10

Table ‎4.4: Difference between the force-strain curves obtained for the model with the reference angle of rotation and the models with different angle of rotations.

Angle of rotation 18.6º (+1º) 16.6º (-1º) 22.6º (+5º) 12.6º (-5º) 27.6º (+10º) 7.6º (-10º) RMS error (N) 0.47 0.31 1.40 0.99 1.64 1.43

RMS error (% Max force) 1.04 0.68 3.12 2.15 3.62 3.15

4.4 CONCLUSIONS

In recent years, several 3D FE models of the human foot have been generated to investigate the biomechanical behaviour of the heel pad. Although the macro-chamber, micro-chamber and skin have been distinguished different structures and material properties, in most of 3D FE models, they have been fused together to represent the soft tissue in the heel region (6, 73, 87, 102, 109, 110, 115). In some limited 3D FE models, the skin layer was separated from the macro-chamber and micro-chamber to investigate the material properties of the skin layer and gain better understanding of the heel pad biomechanical behaviour (7, 101, 103).

The aim of this chapter was to develop the 3D FE model of the heel region with size of 9.25cm from the back and 4.5cm from the bottom of the heel. The anatomically detailed

0 10 20 30 40 50 0% 5% 10% 15% 20% 25% 30% 35% 40% Forc e (N ) Strain

Angle of rotation of the heel 17.6

_±10

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model of the heel region composed of the muscle tissue, plantar fascia, macro-chamber, micro-chamber and skin was generated based on created solid geometries in Chapter 3. The different structures in the model were meshed using hexahedral elements. The heel pad model was rotated for 17.6 to replicate the position of the heel inside the test rig during physical tests. The indenter and the load cell were modelled and positioned under the calcaneus tuberosity to reproduce the compression tests conditions (see chapter 5). Loading and boundary conditions were applied to the model so that simulated the actual experimental conditions. To complete the FE model, the material properties obtained from the literature were applied to the FE model structures.

Some issues arose such as finding reliable material properties for the foot muscle tissue and the plantar fascia. In most of developed 3D FE model in the literature, the foot muscle tissue was merged with other soft tissues in the foot so identical material properties were assigned to them (73, 102, 109-111). The literature does not contain sufficient data regarding to the material properties of the foot muscle tissue and the results found for the plantar fascia were only tension material properties in its longitudinal axis (73, 97, 109, 110). Therefore, for muscle and plantar fascia parameters available in the literature were used initially and parametric studies were conducted to estimate the possible errors which might be induced to the FEA as results of not using accurate material properties. The parametric studies showed that the predicted force-strain responses of the heel pad were generally insensitive to the variation of the stiffness of the muscle tissue and plantar fascia in the commonly accepted ranges.

Another issue that was faced during the development of the FE model was measuring the angle of rotation of the heel in the test rig. Because of the narrow space between the foot brace and the heel pad indenter, it was not possible to use tools with high accuracy to measure the amount of rotation of the heel region during experiments. In addition, the compliance

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within the brace system allowed small changes in the position of the foot inside the test rig. Therefore, another parametric study was performed to assess the effect of the rotation of the angle of the FE model on the predicted force-strain results. These results showed that the predicted results were only slightly influenced by variation of the angle of rotation by 1 degree (RMS error<1.5% max force). Larger changes were noticed for 5 and 10 degrees (RMS error<4% max force). By repeating the measurement at different times, the minimisation of the measurement error was ensured.

After completion of the study it was noticed that the hyperelastic Ogden material properties for macro-chamber, micro-chamber and skin layers obtained from literature as initial parameters to start FEA, were determined using a formula different from what was used in Ls-Dyna. Specifically, the shear modulus from literature (µl) was equal to (½ µlsα), where µls represents the shear modulus in Ls-Dyna and α is identical in both systems. This means that instead of 16.45kPa as shear modulus in Ls-Dyna, 4.82kPa should have been used. Further studies in Chapter 6 showed that when using 16.45kPa as shear modulus of the macro- chamber, the force-strain response of the tissue was still underestimated. Therefore, since this parameter was used as initial parameter to start FEA and did not affect the final results of this project, it was not necessary to repeat the studies using the corrected value.

The FE model with specifications described in this chapter was used to estimate the material properties for the heel pad sub-layers (see Chapter 6) and showing its application in studying the effects of experimental and geometrical conditions on the heel pad responses and also investigating the effect of foot wear design factors on the heel stress (see Chapter 7).

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