The heel pad is under repetitive high stresses relating to cycles of loading during locomotion. Plantar pressures should be relieved in patients with plantar heel pain to both address clinical symptoms and address stress experienced by inflamed tissue. There are a range of strategies related to use of different materials and geometries around the heel to reduce load applied to the heel and related tissues. Accordingly there is a great interest in design of insoles and footwear products that might reduce the load, pressure or stress related to the heel pad. This might be through use of energy absorbing material under the heel, or changes in the geometry of the footwear/insoles around the heel to change the heel pad geometric responses to loading or a combination of both approaches.
A variety of research has been done to design footwear products using measures of plantar pressure. This information is used in designing footwear in order to redistribute and reduce
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high plantar pressure under the heel and metatarsal regions. These measurements were obtained using pressure platforms and flexible pressure-sensing insoles (85, 86). However, whilst these studies can identify the changes in external loads due to variations in footwear or insole designs, they are expensive and time-consuming. Large number of physical testing on different subjects with different footwear is required to achieve reliable results. A combined FE model of the foot and footwear can provide biomechanical information about influence of the footwear on behaviour of the heel pad and thus reduce the risk of injury and tissue damage. It also allows quick and easy modification of footwear, which results in a more rapid search for optimum designs. A number of studies have used FEA to understand the effects of footwear design.
Chen et al. developed a 3D model of the foot of 24 years old subject to investigate effects of the total contact insole on redistribution of the plantar stresses (87). A homogeneous hyperelastic material model was used to represent the heel pad soft tissue (Figure 2.25). The combined heel pad and insole model was compressed by a rigid plate up to total displacement of 2.4mm at velocity of 20mm/s. They tested changes in plantar stress when using flat and two sets of insoles that their geometries were designed to match the shape of the foot and thus maximise the contact area under the foot with different material combinations. The flat insole was made of Microcell Puff. The first set of total contact insole was made of three layers: top layer of PPT; mid-layer of Microcell Puff; bottom layer of Thermocork. The second set of total contact insole was made of two layers: top layer of medium Plastazote; bottom layer of PPT. The analysis predicted reductions in peak plantar pressure and average normal stresses of 19.8% to 56.8% on various foot regions (except the mid-foot and hallux regions) by wearing total contact insoles compared to flat insole. This method had the benefit of comparing the effects of different insole conditions on a plantar pressure using the same foot
53 Loading Heel pad Contact with friction Sidewalls Contact with friction Insole Midsole Rigid floor
geometry, loading conditions etc. They reported that the two sets of total contact insoles used in this study reduce the plantar pressure in the heel region.
Figure 2.25: A 3D FE model of the foot and the insole: (A) The complete foot model of the bone and the soft tissue; (B) The insole model. Derived from (87).
Goske et al. used a 2D plane strain FE model of the heel pad to investigate effects of insole design parameters such as shape, material
and thickness of insoles on the heel plantar pressure (88). Microcell Puff, Microcell Puff Lite and Poron cushioning were used for insole design while firm crepe and leather were used to model midsole and sidewalls respectively.
Figure 2.26: A footwear model interacting with the heel pad. Derived from (88).
FEA allowed studying 27 insole designs systematically. The single layer heel pad model was developed based on MRI data (Figure 2.26) and modelled the heel pad as a hyperelastic material. The compressive load of 678N was applied to the heel pad to simulate loading at first step of walking. They reported that the geometric conformity of the insole to the shape of the heel pad is the most important design factor affecting the heel plantar pressure, whereas
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the insole material selection has the least influence on the plantar pressure. The results showed that the peak pressure was relieved by 24% using flat insole compared to barefoot condition. This value increased to 44% using full conforming insoles.
Spears et al. used a 2D plane strain two layers FE model of the heel pad to investigate the heel counter effect on the peak stress (Figure 2.27) (11). Two rigid and deformable behaviours were assigned to the heel counter with using leather material for deformable condition. They reported that in the presence of the heel counter peak compressive stress increased (16%) whereas tensile and shear stresses decreased (33% and 26%) in the skin layer. In the fat pad, compressive and shear stresses decreased (22% and 58%) and tensile stress remained negligible. These trends were mirrored by high friction, increase of thickness of the heel pad and using deformable heel counter.
Figure 2.27: A 2D model of the combination of the heel pad and the footwear. Derived from (11).
Cheung et al. developed a 3D FE model of the human foot and ankle based on MR images (Figure 2.28). The aim of their project was to combine FEA and Taguchi method to study the sensitivity of five design parameters including arch type, insole thickness, midsole thickness, insole material and midsole material on plantar pressure reduction (74). The heel pad was
Insole Midsole
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modelled as a single layer hyperelastic material and its responses were studied under 550N ground reaction force.
The foot orthosis was made of insole, midsole and outsole layers. Polyurethane (Poron-L24 and Poron-L32) and Eva foam (Nora-SL and Nora-SLW) were used for insole and midsole material. High density Eva foam (Nora-AL) was assigned to outsole layer. According to their findings, among the five design factors, arch type and insole stiffness were the first and second important factors for peak pressure reduction. The insole with custom-moulded shape and softer material had the greatest reduction on plantar pressure.
Figure 2.28: A 3D finite element model of the foot and the footwear. Derived from (74).
Luo et al. created a 2D axisymmetric model to investigate the influence of insole geometry and insole material on the heel plantar stress (Figure 2.29). The heel pad was modelled as a composite hyperelastic material with two layers namely the fat pad and the skin. Two types of insole geometries were investigated in this study: flat insoles with or without reliefs (cavities under the heel); custom contoured insoles. Three insole materials were used to study their effects on the plantar pressure: material with stiffness equal to the subject’s heel pad; material with 2.5 times the stiffness of the subject’s heel pad; and material with 6 times the stiffness of the subject’s heel pad.
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The responses of the heel pad under 200N load, about of the subject’s body weight were
studied. The results showed that flat insoles with reliefs reduce the pressure beneath the heel pad more than the flat insoles without reliefs. However, custom insoles provide the most reduction with value of 79%-92% in the heel plantar stress. Comparing the insole material the maximum reductions in tissue stress were 25% when the stiffness of insole material was equal to the stiffness of the subject’s heel pad in flat insoles (77). This result is agreement with findings of the study conducted by Cheung et al. and Goske et al, that the softer insole material has more effect on reduction of the heel plantar pressure (74, 88).
Figure 2.29: : A FE model of the heel pad interacting with different insoles: (A) Meshed model of the heel pad; (B) Flat insole with conical relief at bottom; (C) Custom contoured insole with conical relief at top. Derived from (77).
Reviewing the studies which used FEA for footwear design showed that FEA could assist design processes towards an optimum without burden of carrying out a large number of experiments. Furthermore, compared to experimental studies, not only does FEA reveal the responses of the heel pad at its outer surface (of the model) but also shows the internal reactions of the heel pad sub-layers tissues.
Apart from the use of FEA of the heel region in footwear design there are many more applications which can provide easy and quick way to reach valuable conclusions for better understanding foot functionalities and also improving physical experimentation procedures. Some of applications of the FE model of the heel region are as follows: (1) investigation of
Skin Insole
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effects of various experimental conditions (e.g. indenter size and shape) and the heel pad geometry (e.g. heel pad thickness and maximum width) on heel pad biomechanical responses. These results can be beneficial in explaining why experimental data available in the literature differs from each other and how we should be careful using and comparing them; (2) studying effects of other factors than design features such as friction, which might be influential on the heel pad behaviour; and finally (3) studying effects of stiffness of each of the heel pad sub- layers on the heel pad biomechanical behaviour. The characteristics and applications of the FE models reviewed earlier are summarized in Table 2.7.
Table 2.7: Summery of FE models used for footwear design
Research Geometry Material Application
Chen et al. (2003)
3D (foot bones, cartilage, ligaments, soft
tissue, foot orthosis)
Bones, cartilages, ligaments, soft tissue (linear elastic), insole, midsole (hyperelastic)
Flat and total contact insole on stress and plantar pressure
distribution Goske et al.
(2006)
2D (heel pad, heel counter, insole, midsole)
Heel counter (elastic), heel pad, insole, midsole
(hyperelastic)
3 insole conformity levels, 3 insole materials, 3 insole thicknesses on plantar pressure Spears et al.
(2007)
2D (skin, fat pad, insole, midsole, heel counter)
Skin, fat pad, insole, midsole (hyperelastic), heel counter
(elastic)
2 heel counter materials on peak stress
Cheung et al. (2008)
3D (foot bones, soft tissue, cartilage, ligaments, fascia, foot
orthosis, ground)
Bones, cartilage, ligaments, fascia, ground (elastic), soft tissue, orthosis (hyperelastic)
4 arch type levels, 4 insole thicknesses, 4 insole materials,
4 midsole thicknesses, 4 midsole materials on plantar
pressure Luo et al.
(2011)
2D (skin, fat pad, insole, ground)
Skin, fat pad, insole (hyperelastic), ground
(elastic)
5 insole shapes, 3 insole materials on stress, strain and
strain energy density
Reviewed FE models have shown their applications in understanding of influence of heel supports on the biomechanical behaviour of the heel pad. More detailed geometry of the foot and more accurate material properties of the FE model components can advance the representation of the combination of the foot and footwear. It is no surprising to find a wide range of variations in effects of the foot support on the internal stress and plantar pressure amongst the reviewed studies. The sources of the divergence might be difference between: material properties and geometry of the footwear model, loading and boundary conditions and the degree of simplification of material properties and geometry of the foot model. Although
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the reviewed models obtained different amounts of the stress or pressure relief using foot supports, similar agreements can be found in their studies: (1) decrease in stiffness of the insole material increases the reduction in stress and plantar pressure in the heel pad; (2) increase in thickness of insole material elevates the heel stress and pressure relief; (3) full conforming insole is an effective factor in pressure reduction by increasing contact area; (4) heel counter decreases the stress in the heel by preventing heel pad from excessive deformation and maintaining more soft tissue material under the calcaneus.