Localization of composite prosthetic feet: manufacturing ...

Globally, thousands suffer every year from complications of various diseases, such as diabetes, circulatory and vascular disease, trauma, and cancer which could lead to limb amputations1. Limb amputation significantly reduces the quality of life (QOL)2 particularly in case of lower limb amputation as it impedes amputees’ mobility3. Amputation levels, as shown in Figure 1, are classified into upper and lower limbs. According to4, lower limb amputations represent almost 97% of amputations in the United States out of 1.7 million amputees’ population. This indicates the significance of lower limb problem to be handled. On the Egyptian local level, databases about amputation levels are not comprehensively updated. However, the trend of higher lower limb extremity, represented in the United States, can be expected to take place also in Egypt. A supportive argument for this expectation is the high percentage of Diabetes in Egypt where almost 0.5% of them suffer lower limb extremity5. Diabetes, with its consequences of vascular diseases, results in more than 90% of the lower limb extremity6. Out of the Trade map website about export and import data; the documented import bill of Prosthetics and Orthotics (P&O) costs Egypt at least 600 Million L.E. in 2020 based on trade map statistics for HS-code: 9021–10,31–39 up to 1 billion L.E. according to governmental figures7. A greater portion of P&O import bill is not documented because of the non-counted direct purchase of the amputees abroad. This results in an untrusted database about the amputation levels and the P&O parts. The Egyptian state declared its intention in tackling this problem as represented in the conference of “Different, We Are Able’’ which is held in Cairo 2018. The state started to merge the efforts of all related local authorities and individual experts into single consortium. The activities of this consortium are putting rules for providing high-level of medical services, integrating amputees in society, establishing a comprehensive database for people with physical disabilities, and following up the process of an integrated industrial complex to localize and transfer technology for manufacturing prosthetic limbs to overcome the market gap8. This process requires; from another point, building well-trained manpower resources, to have an accredited professional education program and to start local Research and Development R&D capacity to sustain the localization process of P&O industry. This orientation is ensured by funding scientific and applied projects from the Egyptian state as mentioned in the acknowledgment.

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Levels of amputation in upper and lower limbs.

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In order to improve the amputee’s QOL with lower limb extremity; lower prosthetic limbs are designed to retrieve some movement functions9 and substitute the missing limb10. During rehabilitation, temporary lower limb prosthetics are used to make amputees accustomed to walking and performing daily life activities safely. The prosthetics are designed to satisfy the functions of the lost limb namely, shock absorption, weight-bearing stability, and progression.

More concern is given to the ankle foot and the below knee amputation due to their higher statistics11. It goes without saying that ankle foot manufacturing is a subset of the whole below knee set.

The required functions of the lost limb depend on the expected activities of the amputee. The prosthetic foot affects the posture, walking correctness, and the loading degree on the joints. Prosthetic foot takes different shapes depending on the severity of disability and functionality and therefore suitable designs and materials are chosen for each case12. These variables affect the prosthetic foot design and consequently the manufacturing technology adopted in Egypt.

In brief, the localization chances of lower limb prosthetic foot are studied in terms of the available technologies in Egypt. The factors affecting the status of the Egyptian manufacturing capabilities are discussed, namely the number of target amputees, definition of amputation levels, prosthetics’ design concepts, material selection, technology readiness level in Egypt for the different manufacturing alternatives. Consequently, a value chain, to control such an industry in the Egyptian state, is proposed.

Amputation level of lower limb prosthesis: material and design classification

As a normal procedure in selecting a suitable prosthetic foot design, the potential level of amputees; according to their mobility and capability to use lower limb prosthesis, is assessed by a physician then amputees are assigned a K-level which has values of K0, K1, K2, K3, or K4. This classification determines the ability of amputees to safely utilize the prosthetic foot, where K0 represents amputees who do not have the ability to walk safely without assistance while K4 represents amputees who can use prosthesis effortlessly and perform dynamic activities13. For low K-level amputees, solid-ankle-cushion-heel (SACH) foot is considered the most suitable option. SACH foot is the most basic type of prosthetic foot, consisting of a solid foot shaped block, usually made of wood, and combined with an aluminum pylon to join the foot to the socket. This type of prosthetic foot provides supporting function and basic mobility through a simple hinge to mimic the ankle joint motion in sagittal plane, see Fig. 1. In the late 1950s, SACH was evolved towards better simulating functions of the human foot and ankle complex. This prosthetic foot is manufactured from poplar wood keel with plywood reinforcement. Multi-axial ankle–foot mechanism was designed to accommodate uneven terrain not just plantar and dorsiflexion, as in \* MERGEFORMAT Fig. 2, in the sagittal plane. This design used a stiff anterior keel or leaf spring, made initially of Delrin and subsequently of phenolic and Fiberglas materials and high strength carbon plates, to store spring potential energy through deformation of the keel in mid to late stance and return a portion of this energy for propulsion in the absence of active ankle plantar flexors.

Anatomical terms of the lower limb movement.

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However, SACH foot is not suitable for higher K-level amputees as it loses a large amount of energy during gait cycle, therefore a new design was created for high K-level amputees. Energy-storage-and-return (ESR) foot is the new design which started after the launching of the Seattle Foot14. ESR provides mobility and convenience for users with high K-levels as it is designed with elastic materials. These materials deform under loading, then stores potential energy that is later released in the gait cycle which allows the foot to return to its original shape15.

Composites are used normally as materials in fabrication of ESR. These composites are reinforced with either carbon or glass fibers. Compared to other materials, composites are characterized by superior strength to weight and exceptional biocompatibility16. Composites can improve the gait efficiency by cumulating, storing, and then releasing energy during the gait cycle which is essential in ESR design. The efficiency primarily depends on the prosthetic foot design as well as the composite parameters such as fiber selection, fiber form, type of combination, mass content, as well as the design of the prosthesis17.

In brief, the design of the lower limb prosthetic has traditional and modern approaches. The traditional one goes back to 1980, where the foot types, as mentioned earlier, are classified upon the number of axes namely, single axis SACH, multi-axis, and dynamic response. This traditional classification comprises what is called conventional foot (CF) types. The modern classification is based on the energy timeline which divide the prosthetic foot into CF, ESR, and bionic foot18. Despite the ESR prosthetic foot being able to store and release mechanical energy, there is no net positive output work to help the amputee in forward progression. This ESR prosthetic foot does not have the ability to adapt to different terrain. The amputees with passive foot prostheses suffer and face difficulties during walking on slopes19,20. Also, the smooth roll-over shape of the human ankle–foot which was presented by Hansen21 affects the walking efficiency and performance. Hence, the prosthetic foot should have roll-over characteristics similar to human. Hansen also22 developed the third type of feet in the modern classification which is the bionic foot. It is the ankle–foot prosthesis, which is capable of automatically adapting to different walking surfaces and changing the ankle joint impedance from low to high throughout stance phase. The main problem of adding actuators to the ankle–foot prosthesis is the increasing of the total prosthetic weight which affects the amputee comfort.

The following paragraph will discuss which type of feet is more appealing to the planned industrial complex regarding the expected demand statistics, its economic burden and needed technology.

Technology readiness level for prosthetic manufacturing

Technology readiness level (TRL) is an agreed-upon method to assess the maturity of certain technology. It is a nine-level system as shown in Table 1. Level 1 is just observation of basic principles. Then TRL develops across different levels of the concept formulation, proof, validation till level 9 of practical proof in an operational environment23.

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As mentioned previously, the aim of this work is to evaluate the current technologies available in Egypt to start localization of manufacturing the lower limb. This evaluation depends accordingly on the TRL of each type of the prosthetic feet prosthesis illustrated in Table 2. Based on previous literature12,14,16,24,25,26, Table 2 presents the four types of feet according to the timeline accompanied with their descriptions in terms of their advantages and disadvantages. The TRL of each type is estimated regarding the manufacturing capabilities in the Egyptian market as shown in \* MERGEFORMAT Table 3. TRL of the first type is 7 and the technology required for the CF foot is considered "not advanced". However, the second type of the prosthetic feet is selected for localization in Egypt due to the three following reasons:

1. a.

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   The simple non-advanced technology in the low-income countries is the governing reason for selecting CF foot due to its relatively low price and simple maintenance. However, CF foot meets the needs of utmost K2 amputees. Getting the amputee feels more natural walking pattern (gait) requires mimicking the dynamics of an anatomical foot. The second type of feet in Table 2, ESR foot, fulfills partially these dynamics. But cost wise, it is expensive and costs more than 5 thousand USD depending on the material and the design such as multi-axial and microprocessor. Even the bionic foot in Table 2 may cost more than 100 thousand USD in western countries27,28. In other words, keeping a low level of mimicking the natural foot meets the cost limitation of the low-income country amputee. However, it reduces the QOL and negatively affects the surrounding relatives and the whole community’s efficiency. Therefore, selecting a relatively higher degree technology would influence positively the QOL of the amputees and their relatives.

2. b.

   The import bill of the prosthetic foot to Egypt is mainly attributed to the higher added value of advanced technology foot. This is attributed to the presence of manufacturing centers for CF foot in Egypt. Also, this is evidenced by the market report about growing market in the Middle East especially for advanced prosthetic feet29.

3. c.

   TRL of ESR foot is promising. Except for the carbon fiber part, the manufacturing technologies of the foot components are available and mature in Egypt. The carbon foot part itself is processed manually. The chain of the processes comprises of fabric cutting, orientation, stacking, resin infusion, curing, trimming, and machining. The manufacturing process of ESR using composite material has gone through different phases. Starting from manual hand layup which is considered the simplest technique to produce layers of laminates in the composite as it is a low-cost tool and uses room temperature-cured resins. However, this technique is time-consuming and the composite is prone to air bubble formation30. The opportunity to automate part of this chain is highly potential by some technologies not available in Egypt like resin transfer molding RTM or Resin pre-impregnated Fabric PREPREG. RTM is more potential to be realized as it is not complicated technology involves the pumping of resin into a closed die filled with stacked carbon fiber fabric in the required foot preform as a one part or more according to the design, see \* MERGEFORMAT Fig. 3. Vacuum Assisted Resin Transfer Molding (VARTM) is another form of RTM technology. VARTM technology is used for production on a small scale, therefore it is utilized for producing prototypes31. The most advanced techniques are pressurized Resin Transfer Molding (RTM)32 and the use of pre-impregnated carbon fabrics with resin (PREPREG)33 to ensure good quality.

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Schematic of RTM Principle.

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Methodology of prosthetic ESR foot manufacturing

Following a modular design would help in the industrialization of the P&O parts and in the definition of the required TRL that should be met. The prothesis consists of several components1. Figure 4 shows the selected common size of 27 under investigation. The components of the foot can be classified as modular parts. The modular components are like the pyramid metallic adaptor and the bolts. The upper and lower parts of the foot itself are also considered modular products as they can be classified to specific modular sizes. Non modular part is like the socket which connects the pyramid with the amputee remaining part of the leg.

Selected common size of prosthesis foot.

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The proposed methodology of localizing the prosthetic foot manufacturing in Egypt is as follows:

• Designing of the prosthetic foot where the foot breakdown consists of modular parts to help mass production, maintenance and interchangeability. The proposed design is checked by modeling regarding the foot endurance to the expected stresses;

• Selection of the manufacturing method with respect to the TRL in Egypt and the product complexity;

• Testing of the ESR foot;

• Developing a value chain regarding the results and discussion.

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