Prosthetic feet are an essential part of lower limb prostheses and are aimed to restore mobility to people with lower limb amputation [38-39]. Forefoot and heel stiffness properties of the prosthetic foot have great effects on the functional mobility of lower limb users [40]. The stiffness of a prosthetic foot can have crucial effects on gait of amputees. The decreased heel stifness (i.e., a softer heel) has been related to prosthetic side reduced knee flexion and reduced internal extensor moment during early-stance [3941-4243]. Also it is associated with increased energy return [44-4546], and faster time to foot-flat, hence improve the quality of stability [47]. Similarly, decreased prosthetic forefoot stiffness (i.e., softer forefoot) has been associated with increased ankle peak push-off power and work [45, 48] and increased knee extensor and hip musculature compensation [49]. Also it is related to and increased intact limb loading [45-46] and increased side energy storage and return [48-49].
There are large number of commercially available prothetic feet with a wide range of design, mechanical properties and materials available [50 -515253]. Using the spring like mechanism in prosthetic feet led to greater elastic energy storage that contribute to push-off during unloading of the foot [54-55]. The stiffness of the foot prosthesis heel decreased by using a spring component, hence knee flexion was reduced [40], [44] and elastic energy return was increased [46-4648], [56 -57585960].
The foot prosthesis was modelled in SolidWorks such as it has a spring like mechanism in order reduce metabolic enegy expenditure [45], as shown in Fig. 1. In order to have a spring like mechanism that has intermediate stiffness, a steel material was chosen for the foot part of the prosthesis. The energy expenditure was lowest with a spring of intermediate stiffness. Disadvantages of the softest spring was include greater push-off and excessive hell displacement [45], [54-55]. The foot model was designed as it has dimensions on x, y and z axes 104.16 mm, 19.02 mm, 22.5 mm respectively. Hence the mass of the foot prosthesis was given as 0.37 kg.
After modelling in SolidWorks, the foot prosthesis model was imported in to ANSYS and it was meshed as shown in Fig. 2. A steel material was chosen, in order to make the foot prosthesis strong enough against loads, reaction forces, damping forces and friction forces. The mechanical properties of the foot prosthesis were given in Table 1.
Table 1. Mechanical properties of the foot prosthesis.
Young’s
Modulus
|
Poisson’s
Ratio
|
Elastic
Modulus
|
Shear
Modulus
|
200 GPa
|
0.33
|
166.67 GPa
|
76.923 GPa
|
The triangle mesher was chosen together with the minimum edge length of 0.0001 m. Hence, the foot model was meshed as it will have 43865 mesh points and 23320 mesh elements as shown in Fig. 2. It was rather important to determine the right position of the applied force in order to make a realiable stress and elongation analyses. As it was mentioned ealier in this paper that the prosthesis was designed for a 90 kg person. The prosthesis model carry loads which were more than the twice of the 90 kg (approximately 1750 N) person’s weight. For this reason, the foot model will be safe for the user and much more stronger against loads and forces which will have acted on it.
2.1.The Foot Prosthesis Analyses in ANSYS
As shown in Fig. 3, the force, 1750 N was appplied to the heel of the prosthesis model at the point B. There was not a applied force at the point A. However at this point, the foot prosthesis components were montajed to each other, such as they can slide each other surface without any friction forces (Fig. 3). The maximum von-Mises stress was measured at the spring part of the foot prosthesis which has the value of 174.49 MPa (Fig. 4).
The chosen steel material has the Young’s modulus of 200 GPa and the yielding stress of 650 MPa. For this reason, the foot prosthesis may stand approximately 3 times more of the maximum von-Mises stress that was occured on the spring part of it. This was happened due to the fact that the spring element may get shorter or longer elastically under the loads very easily owing to its elastic properties. Hence, due to the spring like mechanism which has intermediate stiffness was reduced the energy expenditure [40], [44], [46-48] and [56-60]. The total deformation distribution of the foot model in ANSYS was shown in Fig. 5.
According to this figure maximum deformations were occured on the heel part of the foot prosthesis which has the value of 12.68 mm. It could be easily seen that those deformations were elastic and will not cause any damage to the foot prosthesis due to the fact that the von-Mises stress (Fig. 4) on those areas below the yielding stress of the steel material. From the equivalent von-Mises stess and deformation analyses it was envisaged that the chosen steel material for the foot model was the right desicion and it could be use in real life prosthesis application safely as well.