Abstract
BACKGROUND:
The prosthetic foot is an essential component of the prosthetic limb used by people who suffer from amputation. The prosthetic foot or limb is expensive in developing countries and cannot be used by most people with special needs.
OBJECTIVE:
In this study, an uncomplicated prosthetic foot is designed that can be manufactured at low costs using 3D printer technology and can be provided to a wide range of amputees. The foot was designed using CAD software and analyzed using ANSES.
METHODS:
Carbon fiber material was chosen to be suitable for the manufacturing process using 3D printer technology. The selected material was tested in tensile and fatigue tests to determine its mechanical properties. The numerical analysis was carried out assuming the use of an artificial foot by a patient weighing 85 kg.
RESULTS:
The results showed that the material proposed for manufacturing has good mechanical properties for this application. The results of the engineering analysis also showed that the model has successfully passed the design process and is reliable for use by amputees.
CONCLUSION:
The success model designed in this study in the numerical analysis process gives reliability to the use of this design to manufacture the prosthetic foot.
Introduction
A prosthetic foot is a remarkable technological advancement in the field of orthopedics, designed to restore mobility and functionality to individuals who have experienced limb loss, typically due to injury, illness, or congenital conditions [1]. This innovative device serves as a substitute for the natural foot, providing users with the ability to walk, run, and engage in various activities that contribute to a more fulfilling and independent life. The primary indication for the use of a prosthetic foot is to enhance the overall quality of life for individuals with limb loss [2]. Amputations may occur as a result of trauma, such as accidents or injuries, or as a necessary medical intervention in cases of severe diseases or congenital anomalies [3]. Prosthetic feet are tailored to meet the specific needs of each user, taking into consideration factors such as the level of amputation, the individual’s lifestyle, and their functional requirements.
In this study, we delve into the comprehensive definition of prosthetic feet, shedding light on the intricacies of their design and functionality. Moreover, we examine the diverse indications for their use, emphasizing the transformative impact they have on the lives of those who benefit from these remarkable prosthetic advancements. The importance of designing and manufacturing prosthetic feet for amputees lies in achieving several important goals, including economics and engineering. The economical aspect is to reduce the cost while maintaining durability, as low-priced prosthetic feet can greatly affect the accessibility of a large number of people with special needs.
The basis of this study was to evaluate the amputees clinically and learn about their requirements. The preparation and implementation were based on the opinions of amputees and their requirements to improve their lives and practice their daily activities. When looking at the cases of clinical amputees in prosthetic support centers, the amputees requested to be provided with lightweight prosthetic feet so as not to make them tired while walking, especially for older people, as well as for them to be highly sturdy to bear their weight and to be reliable while using them. There were also requests from amputees to have feet available to them at economical prices, especially in developing countries, so that they could be easily acquired by amputees with limited income. As for the engineering aspect, the design, material selection, and manufacturing processes play an important role in producing and providing the prosthetic foot. Researchers try to find lightweight materials that are characterized by durability and strength, as well as precision in design, to manufacture the prosthetic foot to be comfortable for the amputee during use and compatible with walking dynamics.
Many previous scientific and engineering studies focused on the prosthetic foot (e.g. [4]). Yousif et al. focused on the influence of temperature on the mechanical properties of a newly designed prosthetic foot [5]. Sehar et al. studied the lamination effects on the mechanical strength of carbon-fiber composites used in the fabrication of prosthetic feet [6]. Kadhim et al. suggested three common materials that could be used to make prosthetic feet: carbon fiber, glass fiber, and a hybrid composite material that is made up of resin, carbon fiber, and glass fiber. These materials were tested to find the best one to fabricate prosthetic feet [7]. Al-Aboodi and Aboud investigated the impact of deliberately modifying the stiffness of the keel of a prosthetic foot and the ankle joint on the ability to maintain a standing posture in patients who have undergone amputations below the knee on one leg [8]. Shomran et al. identified certain patterns in the fatigue behavior of a composite structure consisting of reinforced carbon fiber with AL2O3 nanoparticles, specifically in the context of a prosthetic foot [9]. Tryggvason et al. studied the implementation of dynamic finite element analysis in the design modification and power consumption evaluation of a variable stiffness prosthetic foot [10]. From reviewing previous studies, it was noted that the research path in the field of manufacturing prosthetic feet is in continuous development because of its importance in improving the lives and daily activities of a large segment of people with special needs. It was noted in previous studies that the development of the prosthetic foot is limited to two paths, one of which is the development of the materials involved in the manufacture of the foot, and the other is developing the design for the foot. In this study, an attempt was made to develop the prosthetic foot through the manufacturing process to be easy, fast, and inexpensive. This study also focused on choosing the material to manufacture the foot after it had been tested and ensured its reliability. It also focused on making a new design that is less complicated than the existing designs.
The specimen dimension according to standard ASTM-D638-V.
The printed specimens of the tensile test.
The fatigue specimen dimension.
The fatigue machine device.
The prosthetic foot model design.
The boundary condition is applied on the foot at the heel strike stage.
The boundary condition is applied on the foot at the mid-stance stage.
Material use
In this study, carbon fiber polylactic acid (PLA) filament was chosen as the proposed material for the manufacture of prosthetic feet. The chemical composition consists of a matrix of PLA polymer 85% and carbon fibers 15% where the carbon fibers are embedded in the PLA matrix with the diameter of filament 1.8 mm. To mechanically test the material, tensile and fatigue test samples will be printed to be tested and the designed model analyzed numerically to ensure the suitability of the material for this application. Carbon fiber filament composite material was chosen because it is stronger, sturdier, dimensionally more stable, lightweight and has appropriate mechanical properties. It is also suitable for manufacturing using 3D printer technology. The carbon fiber filaments had a diameter of 1.8 mm and were printed at temperatures 200 °C–220 °C.
Methods
Tensile test
To find the mechanical properties of carbon fiber filament, a tensile test must be performed. Three samples of carbon fiber were printed using a 3D printer to conduct a tensile test. The dimensions of the samples were printed according to the standard dimensions ASTM-D638-V [11], as shown in Fig. 1. Samples were printed at 100% density to obtain the highest mechanical properties as shown in Fig. 2. The tensile test was performed by using the Testometric device with a loading rate of 5 mm/min and strain rate of 0.01 S−1.
The boundary condition is applied on the foot at the toe-off stage.
Summary of the tensile test results
The stress-strain curves of a printed specimen.
The S-N curve of printed carbon fiber filament.
The stress analysis of the model at heel strike stage.
The deformation analysis of model at heel strike stage.
The safety factor value of the model at heel strike stage.
The stress analysis of model at mid-stance stage.
The deformation analysis of model at mid-stance stage.
The safety factor value of the model at mid-stance stage.
The stress analysis of model at toe-off stage.
The deformation analysis of model at mid-stance stage.
The safety factor value of the model at mid-stance stage.
When using a prosthetic foot while walking, the foot will be exposed to cyclic stress, so a fatigue test must be performed. Eight carbon fiber samples were printed for testing. The density of the printed samples is 100% to obtain the highest endurance stress. The dimensions of the printed samples were taken from measuring the dimensions of the device samples as shown in Fig. 3. The device used in the test is (HI-TEICH) [12], as shown in Fig. 4. The type of load is alternative bending loading and the stress ratio R = −1.
Prosthetic foot model
The prosthetic foot model was created using SolidWorks software. The prosthetic foot model was created taking into account that it should be simple in design, free from complexity, and consist of one part that is lightweight and low in cost. The heel area was also strengthened because it was exposed to a stronger shock during the walking cycle to avoid failure occurring in this area. The dimensions of the foot in terms of length, width, and height, were taken from the anatomical dimensions of the human foot. In this study, more than four models were created, but each model suffered from a specific defect and was threatened with failure. Developments were made until it the final model was obtained (Fig. 5), which is characterized by its durability and ability to withstand stress and the patient’s weight while walking and performing other activities.
Numerical analysis
The purpose of carrying out the engineering analysis process is to test the designed model and predict what will happen to the model when loads are applied and to treat areas of failure and strengthen weak areas. The engineering analysis process helps to understand what will happen to the model in terms of mechanical changes and risks without the need to manufacture the model and expose it to failure. Rather, the failure of the model is treated before it is manufactured, thus saving the time needed to reach solutions and also saving the cost of the manufactured model for testing. To begin the numerical analysis process, the results of mechanical tests and the geometric shape of the model in the CAD software need to be obtained, and applied to the boundary conditions of the model, such as load applying and fixation areas, to obtain the regions of distribution stress and deformations in the foot and also obtain the value of the safety factor.
In this study, the boundary conditions were set in three cases according to the stages that the foot goes through during the gait cycle: the first is in the heel-strike stage, in which the load is applied at the ankle joint region and the foot is fixed in the heel region, as shown in Fig. 6. The second case is in the mid-stance stage, in which the load is placed in the ankle joint region, and fixed at the rearfoot and forefoot, as shown in Fig. 7. As for the third case, which is the toe-off stage, the foot is fixed from the forefoot, and the load is applied to the ankle joint region, as shown in Fig. 8.
Results and discussion
After three samples of carbon fiber were tested in tensile testing, the results are summarized in Table 1. The average of the three tests was taken and represented in the stress-strain curve as shown in Fig. 9. The average results of the tensile test showed that the ultimate stress value = 55 MPa, the yield stress value = 38 MPa, and the Young’s modulus value = 1.27 GPa. The results of the fatigue after test eight specimens are shown in Fig. 10 which represents the relationship between the applied stress and the number of cycles, with the endurance stress value = 21 MPa.
The results of the numerical analysis were divided into three groups. The first group is the results of the stress resulting from the application of boundary conditions on the foot in the three stages of the walking cycle. The second group is the results of the deformations that occur in the foot as a result of the application of the load. The third group knows the value of the safety factor for foot design at each stage of the gait cycle.
The results showed that the value of von-misses stress generated in the prosthetic foot is 20.46 MPa in the heel strike stage, 6.13 MPa in the mid-stance stage, and 30.06 MPa in the toe-off stage, as shown in Figs 11, 14 and 17, respectively.
The results showed that the values of deformations generated in the prosthetic foot model are 0.825 mm in the heel strike stage, 0.853 mm in the mid-stance stage, and 2.32 mm in the toe-off stage, as shown in Figs 12, 15 and 18, respectively.
The results for the value of safety factor for the prosthetic foot model is 3 in the heel strike stage, 4.2 in the mid-stance stage, and 2.26 in the toe-off stage, as shown in Figs 13, 16 and 19, respectively.
From the results of the stress analysis, it was found that there is a large gap between the value of the maximum generated von Mises stress and the yield stress. The yield stress represents the maximum load that the material can bear before failure, which in this study is equal to 55 MPa.
The results of deformities in the prosthetic foot, they are acceptable because the deformities must occur when the patient’s weight is applied to the prosthetic foot. As for the safety factor values, they are acceptable at all stages because they exceed 1.25 and indicate the reliability of the design and the ability of the foot to bear the weight of the patient without any failure. The design is safe if the safety factor exceeds 1.25 [13].
In any study that is intended to be applied clinically in the field of prosthetics, it must be engineering success in terms of design and durability in the first place to give reliability in its use when manufactured so that it does not cause injury to the amputee in the event of its use and engineering failure, which could cause a psychological reaction to the amputee by not using the prosthetic again. Therefore, this study must verify the use of the appropriate material for manufacturing and adopt the successful design of the foot. According to the results above, the success of the prosthetic foot model was verified, and this paves the way for its manufacture and clinical use by the amputee later.
Conclusion
The following conclusions can be drawn from the study: The selected material (carbon fiber filament) is suitable for manufacturing the prosthetic foot due to its good mechanical properties that can bear the weight of the patient, as the yield stress is 55 MPa, the ultimate stress is 38 MPa, and the Young’s modulus is 1.27 GPa. Since the carbon fiber material was successfully chosen to manufacture the prosthetic foot, the foot can be manufactured using additive manufacturing techniques. Using the technique of manufacturing the prosthetic foot using a 3D printer will be better than the currently used manufacturing methods because of the speed of completing the model manufacturing, the ability to manufacture complex shapes, the cleanliness of the model surface, and the low cost of manufacturing. The success of the prosthetic foot model designed in this study in the numerical analysis process gives reliability to the use of this design to manufacture the prosthetic foot. The model designed for the prosthetic foot is safe to use and does not suffer from mechanical failure for amputees who weigh 85 kg or less, although the clinical validity such as biomechanical alignment, gait dynamic, energy expenditure, and residual limb health remains to be a future problem.