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 based on trade map statistics for HS-code: &#;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 . 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 s, 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 , 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.

   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;

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• 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.

Of the 1.6 million persons living in the United States with limb loss, approximately 1.3 million (86%) have an amputation of the lower limb.1,2 These individuals vary tremendously with respect to age, sex, amputation level, and etiology, comorbid health conditions, physical presentation, ambulatory potential, and daily activity levels. Accordingly, a range of prosthetic foot types have been developed to reflect this variation, allowing for the appropriate pairing of prosthetic foot type to end user.

The prosthetic foot is an integral component of any lower-limb prosthesis after major lower-limb amputation (i.e., ankle disarticulation level or proximal). In attempting to best restore the functionality previously provided by the anatomical foot and ankle, prosthetic feet have many mechanical design variations. For example, design features attempt to replicate the shock absorption associated with loading response, the coronal adaptations experienced in midstance, the rigid forefoot leverage required in terminal stance, and the propulsion associated with terminal stance through preswing. These functions are pursued through a number of mechanisms, including mechanical joint axes, compressive foams, and bumpers. In addition, many feet now have elastic materials designed to deform under load and then return to their original shape, releasing the energy stored during deformation to provide power to the gait cycle. The costs associated with prosthetic foot types vary with the technologies used to meeting these functional goals.

The number of prosthetic feet available within the rehabilitation community can be overwhelming. However, prosthetic feet are generally classified into several key categories reflecting basic differences in technologies, functional performance limitations, and costs (Table 1).3 The solid-ankle-cushion-heel (SACH) foot is the simplest category of prosthetic foot consisting of a solid ankle block with a rigid forefoot and a compressive material within the heel. The single-axis foot integrates a single mechanical hinge to replicate the function of the ankle joint in the sagittal plane. The multiple-axis foot includes flexible elements to allow dampened movement in all planes of motion. The flexible-keel foot introduces flexible elements to the forefoot of the prosthesis to enhance the sagittal progression of the center of pressure through stance phase. A broad category of energy-storage-and-return (ESAR) feet is constructed of elastic materials that deform under load, storing potential energy that is released later in the gait cycle when these elements return to their original shape.3

Basic prosthetic foot categories for lower-limb prostheses

Clinical practice guidelines (CPGs) are increasingly common in health care, with the US Agency for Healthcare Research and Quality (AHRQ) now housing over practice guidelines in its National Guideline Clearinghouse.4 Yet, the field of orthotics and prosthetics is underrepresented in this area, with only a single CPG listed in the AHRQ database. Encouragingly, the field has begun to develop and publish practice guidelines across a range of care episodes including the management of plagiocephaly,5 postoperative care after transtibial amputation,6 prosthetic foot selection for individuals with lower-limb amputation,7 prescription guidelines for microprocessor-controlled prosthetic knees in the South East England,8 and a two-part &#;Dutch evidence-based guidelines of amputation and prosthetics of the lower extremity.&#;9,10

The scope and depth of CPGs are variable, with direct implications on their resultant clinical relevance and ultimate incorporation into practice. The current effort is modeled after the CPGs of the American College of Physicians,11 with necessary adaptations to accommodate the emerging evidence base of orthotic and prosthetic care. The stated goals of this approach are to &#;provide clinicians with clinical based guidelines based upon the best available evidence; to make recommendations on the basis of that evidence; to inform clinicians of when there is no evidence; and finally, to help clinicians deliver the best health care possible.&#;11(p194)

Clinical utility is of paramount importance in this effort, culminating in a small number of succinct, actionable, evidence-based recommendations.12 Notably, within this framework, although the resultant CPGs represent a comprehensive overview of available literature, deficits in the available literature preclude CPGs within this framework from providing comprehensive clinical guidance.

The purpose of this guideline is to present the available evidence with respect to determining the most appropriate prosthetic foot type for individuals with lower-limb amputation in consideration of their clinical presentation. The target audience for this guideline includes prosthetists, surgeons, physicians, physical therapists, and policy makers. The target patient population comprises individuals who have experienced major lower-limb amputation (i.e., ankle-disarticulation level or proximal), who have experienced adequate healing at the primary wound site to permit prosthetic fitting, and who have the desire and potential or demonstrated ability to ambulate with a prosthesis.

A Medline search was conducted on through April to locate sources of evidence statements within the published literature. The following search terms were used: &#;lower limb amputation&#; AND &#;prosthesis,&#; &#;prosthetic feet&#; OR &#;components,&#; AND &#;systematic review&#; OR &#;meta-analysis.&#; This search yielded 96 abstracts. Of these, four papers were identified as secondary knowledge sources (i.e., meta-analysis, systematic review, or evidence-based guidelines) that synthesized published findings of primary knowledge related to the performance characteristics of prosthetic foot types. These publications include a Cochrane Review,13 published national evidence-based guidelines,10 a systematic review,6 and an evidence-based narrative review and meta-analysis.14 An additional recent systematic review with meta-analysis that had not yet been indexed but had been published was also identified and included.15

In more recent publications, where authors provided explicit evidence statements, these were extracted for subsequent synthesis. If explicit evidence statements were not provided, well-supported narrative statements were extracted. Extracted statements are summarized in Table 2. Statements addressed the following key considerations:

Evidence statements taken from secondary knowledge sources

1. Comparative effectiveness: Where available, statements related to the comparative efficacy of various foot types were extracted from secondary knowledge sources.
2. Benefits of treatments: Benefits described in the evidence base include such considerations as self-selected walking velocity, increased stride length, favorable kinematics and kinetics, metabolic improvements, and subjective benefits and preferences.
3. Harms of treatments: Harms described in the evidence base include the peak vertical impact forces experienced by the sound-side limb, residual limb pain, skin problems and shock and stress at the hip and knee.

COMPARATIVE EFFECTIVENESS

Statements of comparative efficacy have largely been drawn between ESAR feet and the other foot types described above and contained in Table 1.

BENEFITS

The benefits associated with prosthetic foot types are shown in Table 2 with their sources of evidence. In general, the SACH foot has served as the base of comparison for other foot types. The benefits associated with the single-axis foot type include its ability to rapidly accommodate the ground in the sagittal plane with attendant benefits to stability during loading response.15,16 To the extent that multiple-axis and flexible-keel feet have been represented in secondary knowledge sources, they are seen as a base of comparison for ESAR feet. A single secondary knowledge publication has addressed the benefits of multiaxial ankle function when such elements are attached proximally to a range of prosthetic feet.15

The benefits of ESAR feet include increases in self-selected walking speed10,14,16 and both perceived and measured improvements in walking efficiency.14&#;16 Favorable gait measures include an extended stride length.10,14&#;16 Favorable kinetics include increased propulsive properties and walking efficiency during level-ground ambulation,15 the negotiation of environmental obstacles such as stairs and ramps,13,15 and at elevated activity levels.10,13,14

HARMS

The potential harms associated with prosthetic foot type in secondary knowledge sources include both objective measurements and patient-reported outcomes. The magnitude of the initial peak vertical ground reaction force on the sound-side limb has been observed to decrease with the use of ESAR feet.14 Such forces have been often associated with overuse strain and injury to the contralateral limb.17 These objective findings are reinforced by patient-reported outcomes of decreased limb pain, skin problems, and shock or stress at the hip and knee with the use of ESAR feet.14

CONSIDERATIONS BY PATIENT TYPE

The benefits ascribed to single-axis feet are limited to those patients with limited ambulatory ability and/or potential whose sagittal plane stability is threatened during loading response. There is no evidence to suggest that this is a beneficial characteristic for more active walkers with adequate strength and balance. Similarly, the benefits of ESAR feet are largely confined to observations during level-ground walking at active walking velocities, elevated walking speed and activity levels, or during the negotiation of ramps and stairs.

RECOMMENDATIONS

Recommendation 1: For patients ambulating at a single speed who require greater stability during weight acceptance because of weak knee extensors or poor balance, a single-axis foot should be considered.

Studies suggest a more rapid sagittal plane rotational acceleration of the foot about the ankle during weight acceptance with the use of a single-axis foot, bringing the prosthetic foot into full contact with the floor more quickly than other prosthetic foot options.15,16 This increased surface area may provide greater stability to those patients with poor balance. In addition, this ankle movement draws the ground reaction force anteriorly, reducing the magnitude of the external knee flexion moment during weight acceptance. This reduces the likelihood of a knee-buckling event, creating a more stable environment for users with weak knee extensors or transfemoral prostheses. However, the abrupt plantarflexion observed with simple single-axis feet may compromise the progression of the center of pressure through stance phase, disrupting the smoothness of gait among patients capable of elevated and/or variable speeds of ambulation.

Recommendation 2: Patients at elevated risks for overuse injury (i.e., osteoarthritis) to the contralateral lower limb and lower back are indicated for an ESAR foot to reduce the magnitude of the cyclical vertical impact forces experienced during weight acceptance.

Studies suggest consistent reductions in the peak ground reaction force experienced by the sound-side limb during weight acceptance with the use of ESAR feet.14 Patients with higher risks for overuse injury because of elevated activity levels, greater self-selected walking velocities, or younger age (thus anticipating comparatively more years of prosthetic ambulation) stand to benefit the most from the reduced loading forces associated with ESAR feet. Self-report outcomes of decreased pain, skin problems, shock, and stress at the hip and knee with the use of ESAR feet are consistent with these laboratory findings.14 The shock-absorbing characteristics of ESAR feet seem to be more apparent at speeds exceeding self-selected walking velocities.14 Likely contributions to the mechanism of impact reduction include consistent observations of increased propulsion from ESAR feet14,15 and greater tibial progression into terminal stance without sacrificing ankle moment,16 both of which collectively reduce the dropoff from the prosthetic foot onto the contralateral limb at the conclusion of the prosthetic step.

Recommendation 3: Neither patient age nor amputation etiology should be viewed as primary considerations in prosthetic foot type.

Several of the comparative benefits associated with ESAR feet relative to alternate feet designs, including elevated self-selected walking speeds and greater limb symmetry in terminal stance, have been observed among patients with both traumatic and dysvascular amputation etiologies.14,16 Therefore, amputation etiology does not in itself seem to limit the potential beneficial effects associated with ESAR feet. Similarly, studies suggest that the benefits of ESAR feet in increasing self-selected walking speed seem to encompass a broad age range and may be independent of either amputation etiology or age.14

Recommendation 4: Patients capable of variable speed and/or community ambulation are indicated for ESAR feet.

Compared with the other foot designs described in Table 1, ESAR feet have been associated with both perceived and measured increases in self-selected walking speeds.10,14,16 Studies suggest that this may be the result of increased step length, predominantly in the step length of the contralateral limb.10,13,14 Users have subjectively reported increased stability with this foot type relative to other prosthetic foot categories.14,15 This willingness to increase contralateral step length relative to other foot designs may reflect the user's increased perceived stability when using an ESAR feet.

Lengthening the contralateral step may partially explain the modest reductions experienced in reported and measured oxygen cost, which is the energy expelled to traverse a given distance.14&#;16 The comparative benefits of ESAR feet with respect to reduced energy costs of ambulation are more pronounced at elevated walking speeds,13 during the negotiation of inclines and declines,13 and during stair ascent.15

INCONCLUSIVE AREAS OF EVIDENCE

Most of the published literature on prosthetic foot design consists largely of comparative efficacy trials between SACH feet and ESAR feet. Single-axis, multiple-axis, and flexible-keel feet, although utilized in prosthetic rehabilitation, are underrepresented in the academic literature. Despite high volumes of use of these foot types, the scientific community has lagged behind in reporting empirical evidence that may coincide or refine anecdotal evidence. Lacking any empirical evidence, the flexible-keel foot type in particular lacks the objective support to justify its specific mention in the guideline beyond its current mention as an existing category of prosthetic foot.

More recent prosthetic foot technologies, including hydraulic ankle-foot units, microprocessor-regulated ankle/feet, and externally powered propulsive ankle/feet continue to emerge in both prosthetic rehabilitation and its associated evidence base. However, these were not included within the scope of the source publications or the resultant CPG. Supplements to prosthetic feet, including vertical shock pylons and multiple-axis ankle units, have received limited treatment within secondary knowledge sources but were not included within the scope of this review.

Importantly, it is recognized that patients are individuals with unique presentations. As such, the noted clinical practice guidelines are meant to serve only as &#;guides.&#; They may not apply to all patients and clinical situations. Thus, they are not intended to replace clinical judgment. In addition, it is recognized and planned that the clinical practice guidelines will need to be updated as new evidence emerges surrounding prosthetic feet.