This systematic literature review concentrates on the design elements of prosthetic limbs intended for individuals with lower-limb amputations. The review's coverage encompasses peer-reviewed journal articles, conference proceedings, and pertinent materials from the gray literature. The review encompasses research conducted in various disciplines, such as biomechanics, engineering, rehabilitation, and clinical practice. The inclusion criteria for this review involve studies that address the design of prosthetic limbs, including but not limited to socket design, alignment techniques, material selection, control systems, and user-centered design approaches. Studies evaluating functional outcomes, user satisfaction, and quality of life measures related to prosthetic limb design are also included. This review excludes studies that focus solely on surgical techniques, rehabilitation protocols, or clinical outcomes unrelated to prosthetic limb design. Additionally, studies that do not provide sufficient information on the design aspects or lack empirical data are excluded from the review.
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The objective of conducting a systematic literature review on the design of amputated lower limbs is to provide a comprehensive and evidence-based analysis of the existing knowledge in this field. The review aims to identify and evaluate relevant research studies, articles, and publications that address various aspects of prosthetic limb design for individuals with lower-limb amputations. The review aims to identify and analyze the key design factors that influence the development of prosthetic limbs for amputated lower limbs. This includes examining aspects such as biomechanical considerations, material selection, alignment techniques, socket design, interface technology, and customization options. The present work seeks to assess the impact of different design approaches on functional outcomes for individuals with lower-limb amputations. This includes analyzing gait analysis, energy expenditure, mobility, stability, balance, and performance in various activities of daily living. This involves examining factors such as comfort, fit, aesthetics, cosmesis, psychosocial integration, and overall user experience. The review aims to identify design features that contribute to higher user satisfaction and improved psychosocial well-being. The review seeks to explore and discuss emerging technologies and innovations in prosthetic limb design for amputated lower limbs. This includes examining the potential applications of robotics, sensor technology, artificial intelligence (AI), and wearable devices in enhancing the functionality and usability of prosthetic limbs.
The significance of ideal design is magnified by ongoing technological progress. Prosthetic limb design stands to gain from breakthroughs like microprocessors, sensors, and robotics, allowing for intelligent prosthetic systems that dynamically adjust to users' motions, enhancing control, stability, and responsiveness. Technological advancements also facilitate the integration of wearable devices and smart interfaces, allowing users to monitor their activity levels, adjust settings, and receive real-time feedback. These features enhance the functionality and usability of prosthetic limbs, promoting a seamless interaction between the user and the device ( Siddiqui et al., b ; Deathe and Miller, ).
Comfort is a vital aspect of prosthetic limb design. A well-designed prosthesis considers factors such as socket fit, cushioning, and interface pressure management to minimize discomfort and skin-related issues. Proper weight distribution and alignment of the prosthetic limb alleviate excessive pressure on the residual limb, reducing the risk of pain, skin breakdown, and long-term complications. Customization is another critical component of optimal design. Each individual's residual limb is unique in terms of size, shape, and sensitivity. A customized prosthetic limb ensures a precise fit and accommodates the specific needs and functional requirements of the user. Customization also extends to aesthetic considerations, allowing individuals to personalize their prosthetic limbs, contributing to their self-esteem and body image ( Ma et al., ; Pei et al., ). Prosthetic limb design significantly impacts an individual's psychological and social well-being. Aesthetics and cosmesis play a vital role in promoting body image, self-confidence, and social acceptance. Advancements in design techniques, materials, and coverings allow for the creation of realistic-looking prosthetic limbs that closely resemble natural limbs, reducing the stigma associated with limb loss. User-centered design approaches empower individuals by involving them in the design process, considering their preferences and addressing their psychosocial needs. This collaboration fosters a sense of ownership and promotes a positive user experience, enhancing user satisfaction and overall well-being. By providing functional and aesthetically pleasing prosthetic limbs, individuals can feel more confident, actively engage in social interactions, and regain a sense of normalcy in their lives ( Cohen and Hoberman, ; Dunn, ; Gallagher and MacLachlan, ; Behel et al., ).
The pivotal role of optimal design in crafting prosthetic limbs cannot be overstated, directly influencing functionality, comfort, and the well-being of those with limb loss. Prosthetic limb design surpasses aesthetics, encompassing a holistic grasp of biomechanics, user requirements, and technological progress. This work highlights the importance of optimal design for prosthetic limbs and its significant impact on enhancing mobility, promoting independence, and improving the overall well-being of individuals. One of the primary goals of prosthetic limb design is to restore and enhance mobility for individuals with limb loss. Optimal design takes into account the biomechanical principles of human locomotion, ensuring that the prosthesis closely mimics the natural movement of the missing limb ( Jönsson et al., ). By providing appropriate joint dynamics, alignment, and weight distribution, prosthetic limbs enable users to engage in various activities, such as walking, running, and climbing stairs. An optimal design ensures a seamless integration between the residual limb and the prosthetic component, allowing for efficient energy transfer and reducing the effort required during locomotion. This results in improved walking efficiency, reduced fatigue, and enhanced overall functionality, enabling individuals to regain their independence and actively participate in daily activities ( Taylor et al., ). Chen et al. () studied a robust gait phase estimation method using thigh angle models to avoid measurement errors. A Kalman filter-based smoother is designed to further enhance the estimation. The proposed method is evaluated through offline analysis and validated in real-time experiments.
Toe or partial foot amputation type of amputation involves the removal of one or more toes or a portion of the forefoot. It is commonly performed for conditions such as gangrene, infections, or deformities that affect a localized area of the foot. Transmetatarsal amputation involves the removal of the forefoot up to the metatarsal bones. It is performed when there is a need to remove a larger portion of the foot, while preserving the ankle joint and the ability to bear weight on the residual limb. Lisfranc or Chopart amputation involves the removal of the midfoot, including the metatarsal bones, tarsal bones, and the corresponding articulations. This type of amputation is typically performed in cases of severe trauma or deformities affecting the midfoot. Syme amputation is a surgical procedure that involves the removal of the foot and ankle joint while preserving the heel pad. This procedure aims to provide a weight-bearing surface for better prosthetic fitting and improved functional outcomes. Transtibial amputation refers to the removal of the lower leg, including the tibia and fibula bones, while preserving the knee joint. It is one of the most common types of lower-limb amputations and is performed for various reasons, including trauma, vascular diseases, or complications of diabetes. Knee disarticulation involves the removal of the lower-limb at the knee joint level, preserving the femur bone. This type of amputation is typically performed when preserving the knee joint is beneficial for maintaining stability, allowing for better prosthetic fitting and functional outcomes. Transfemoral amputation, also known as above-knee amputation, involves the removal of the entire lower limb, including the femur bone. This is a more complex procedure that requires the use of a prosthetic knee joint for functional mobility. Hip disarticulation is the most extensive form of lower-limb amputation, involving the removal of the entire lower limb along with the hip joint. This procedure is performed in rare cases where there is extensive disease or trauma involving the hip joint.
Lower-limb amputation involves the removal of a part or the entire lower extremity, including bones, muscles, and soft tissues. The extent of the amputation can vary depending on the underlying conditions, the extent of tissue damage, and the goals of the procedure. Amputations can be categorized into different levels based on the location of the amputation relative to anatomical landmarks ( Wong et al., ). Figure 1 shows the major classification of lower-limb amputation. Following are the classification of lower-limb amputation:
In addition to the biomechanical considerations, the design of amputated lower limbs has a significant psychosocial impact on individuals. Prosthetic limb design can greatly influence an individual's psychological well-being, self-esteem, and social integration ( Jackson et al., ). The appearance and aesthetics of prosthetic limbs play a crucial role in user acceptance and confidence. Designing prostheses that closely resemble the natural limb can help amputees regain a sense of normalcy and promote positive body image. Additionally, advances in prosthetic limb aesthetics, such as the use of realistic skin-like coverings and customizable designs, contribute to the overall acceptance and integration of prostheses into the individual's self-identity. User-centered design approaches are vital in ensuring user satisfaction and quality of life outcomes. Engaging amputees in the design process, considering their unique needs, preferences, and functional requirements, allows for personalized and tailored prosthetic limb solutions. User involvement empowers individuals, fosters a sense of ownership, and promotes a more positive experience with the prosthetic limb. Psychosocial integration is another significant aspect influenced by prosthetic limb design. A well-designed prosthetic limb can enable individuals to participate in social activities, enhance their self-confidence, and reduce stigmatization. Providing individuals with functional and aesthetically pleasing prostheses contributes to their overall well-being, allowing them to engage in various activities and roles within their communities ( Datta et al., ; Pezzin et al., ; Johannes et al., ; Safari, ; Yu et al., ).
The study selection process for the systematic literature review on the design of amputated lower limbs was as follows. The titles and abstracts of the identified studies were screened to determine their relevance to the research question and inclusion criteria. Studies that clearly did not meet the inclusion criteria or were irrelevant to the topic were excluded at this stage. The remaining studies from the initial screening underwent a full-text assessment. The full-text articles were carefully reviewed to determine if they met all the inclusion criteria and provided relevant information on the design aspects of prosthetic limbs for amputated lower limbs. Studies that did not meet the inclusion criteria or lacked the required information were excluded. The included studies underwent data extraction, where relevant information such as study characteristics (author, year of publication), study design, sample size, methodology, key findings, and outcomes were extracted and organized in a standardized format. This process ensured that important information from each study was captured for analysis. The quality and risk of bias of the included studies were assessed using appropriate tools or checklists. This assessment helped evaluate the strength and reliability of the evidence provided by each study and considered potential sources of bias that may have affected the validity of the findings. The extracted data were synthesized and analyzed to identify common themes, patterns, and trends in the design of prosthetic limbs for amputated lower limbs. This synthesis may have included a narrative synthesis or, if appropriate, a meta-analysis of the quantitative data.
Identify the key design factors and considerations involved in the development of prosthetic limbs for individuals with lower-limb amputations, including biomechanical considerations, material selection, alignment techniques, socket design, interface technology, and customization options. Evaluate the impact of different design approaches on functional outcomes, including gait analysis, energy expenditure, mobility, stability, balance, and performance in various activities of daily living for individuals with lower-limb amputations. Assess user satisfaction and quality of life outcomes associated with different prosthetic limb designs, including factors such as comfort, fit, aesthetics, cosmesis, psychosocial integration, and overall user experience. Explore emerging technologies and innovations in prosthetic limb design for amputated lower limbs, including robotics, sensor technology, AI, and wearable devices, and their potential applications in enhancing functionality and usability. Identify gaps in the existing literature and provide recommendations for future research and development in the design of prosthetic limbs for individuals with lower-limb amputations. By addressing these research objectives, this study aims to provide valuable insights into the design factors and considerations that contribute to optimal prosthetic limb designs. The findings can inform clinical practice and prosthetic limb development, and ultimately improve the functional outcomes, user satisfaction, and quality of life for individuals with lower-limb amputations.
The design of prosthetic limbs for individuals with lower-limb amputations is a complex process that requires a comprehensive understanding of biomechanical principles. Biomechanics plays a crucial role in determining the functionality, comfort, and overall performance of these devices. Present review provides an overview of the biomechanical factors that influence the design of amputated lower-limb prosthetics, including socket design, alignment, joint mechanics, and gait analysis ( Horgan and MacLachlan, ; Deathe and Miller, ; Kahle et al., ; Highsmith et al., ; Siddikali and Sreekanth, ; Pinhey et al., ). The socket is a critical component of the prosthetic limb that interfaces with the residual limb. Its design significantly affects the fit, stability, and weight-bearing distribution ( Köhler et al., ). The socket must be customized to the shape of an individual's residual limb to ensure a precise fit, maximize contact area, and distribute forces evenly. It should also provide adequate support and promote efficient energy transfer during walking and other activities. Proper socket design reduces pressure points, enhances comfort, and minimizes the risk of skin breakdown and discomfort. Alignment refers to the correct positioning of the prosthetic limb in relation to the user's anatomy. Proper alignment is crucial to achieve optimal biomechanical function and gait symmetry. Alignment factors include the angular positioning of the knee, ankle, and foot, as well as the sagittal, coronal, and transverse planes. Precise alignment helps maintain proper joint mechanics, reduces stress on the residual limb, and improves stability and balance during walking and other movements ( Schmalz et al., ; Zhang et al., , ).
Prosthetic limbs must replicate the natural joint mechanics of the lower limb to ensure smooth and efficient movement. The mechanical behavior of prosthetic joints, such as the knee and ankle, should closely mimic the natural range of motion, joint axes, and kinematics. This allows users to perform activities such as walking, running, and climbing stairs with minimal deviations from normal biomechanics. Proper joint mechanics facilitates a more natural gait pattern, reduces energy expenditure, and enhances overall functionality and user satisfaction ( Schmalz et al., ; Orendurff et al., ; Bae et al., ). Gait analysis is a valuable tool for evaluating the biomechanical performance of prosthetic limbs ( Ferrari et al., ; Peters et al., ; El Habachi et al., ; Leardini et al., ). It involves the measurement and assessment of various parameters during walking, such as step length, stride length, cadence, ground reaction forces, and joint angles. By analyzing gait patterns, clinicians and researchers can identify biomechanical deviations and assess the effectiveness of prosthetic limb designs. Gait analysis helps optimize alignment, socket fit, and component selection, leading to improved walking efficiency, reduced fatigue, and enhanced functional outcomes ( Stagni et al., , ; Koh et al., ; Li et al., ; Gasparutto et al., ).
Optimal weight distribution is crucial for comfortable and efficient use of prosthetic limbs. Uneven weight distribution can lead to discomfort, pressure points, and skin irritation. Prosthetic limb designs should distribute weight evenly across the residual limb and the prosthetic components to minimize excessive loading and prevent overuse injuries ( Simon et al., ). A balanced weight distribution also helps users maintain stability, balance, and control during activities, contributing to enhanced mobility and overall functionality. To ensure the prolonged durability of prosthetic limbs, specific recommendations and best practices can be implemented. Regular maintenance routines, including thorough cleaning and lubrication of components, can prevent premature wear and damage. Proper usage techniques and weight management are essential to avoid excessive strain on the prosthetic. Consideration of these biomechanical factors is essential in the design of amputated lower-limb prosthetics. An integrated approach that combines socket design, alignment, joint mechanics, and gait analysis results in optimal prosthetic limb functionality and improved user outcomes. By understanding the biomechanical principles and their impact on design, prosthetists and engineers can create personalized and efficient prosthetic limb solutions that maximize mobility, comfort, and overall quality of life for individuals with lower-limb amputations ( Duprey et al., ). Table 1 summarizes some recent studies carried out in prosthetic design.
Table 1:
No.AuthorsObjectiveMethodologyOutcome1 Wang et al. () Design an adjustable frame-type prosthetic socket with constant force to adapt to stump volume fluctuations.Design a constant force device based on shape memory alloy for maintaining constant stump'socket interface stress.An adjustable prosthetic socket that allows users to adjust the socket volume, maintains constant interface stress, and adapts to stump volume fluctuations.2 Fidelis and Arowolo () Design and implement a mechanical, body-powered, transfemoral prosthetic device for affordable functional ambulation.Utilize anthropometric measurements to design an ergonomic prosthetic device using AutoCAD rendering and engineering methods like casting and welding.Successful fabrication of a transfemoral prosthetic limb consisting of a polypropylene socket, galvanized iron knee joint, and perlite foot, restoring ambulatory function for the amputee.3 Van Der Stelt et al. () Develop a workflow for producing low-cost 3D-printed transtibial prosthetic sockets in LMICs.Use CAD and CAM to scan and 3D-print prosthetic sockets, with locally sourced foot and imported prosthetic parts.Cost-effective production of transtibial prosthetic sockets, with a 3D-printed socket costing $20 and total material cost of the prosthesis amounting to approximately $100. Potential to sell a 3D-printed prosthesis for $170, benefiting individuals in LMICs.4 Tang et al. () Optimize socket design to reduce local load on residual limb.Divide residual limb into load-bearing regions and apply modifications to socket design based on carrying capacity.Reduced contact interface pressures in specific regions, increased walking distance, and improved pressure distribution.5 Dickinson et al. () Assess the repeatability of plaster casting and 3D scanning for prosthetic socket design.Conduct a comparative reliability assessment in participants with transtibial amputation.Deviation analysis shows high repeatability for plaster casting and varying reliability for different 3D scanners.6 Vásquez and Pérez () Design a low-cost alignment device for lower transfemoral prostheses.Implement conceptual design methodology, model the device in SolidWorks, and analyze its mechanical resistance using finite element analysis in ANSYS.Carbon film material shows promising results for a 3D printed alignment device prototype, with potential for mass production and implementation in prosthetic centers worldwide.7 Sturma et al. () Develop a structured rehabilitation protocol after TMR surgery.Conduct a Delphi study involving European clinicians and researchers in upper limb prosthetic rehabilitation, utilizing a web-based survey to gather expert consensus on rehabilitation steps and their importance.A 16-step rehabilitation protocol for TMR patients was established, emphasizing the need for multiprofessional teamwork and patient selection and education.8 Ratnakar and Ramu () Utilize CT scan-based technology and finite element analysis to develop and evaluate 3D models of prosthetic sockets for lower-limb amputations.Develop 3D models from CT images using image-processing software, modify them in CAD, and convert them to the STL format. Analyze the models in ANSYS under static and dynamic conditions to evaluate stress distribution. Prepare a 3D model with a 3D printer based on simulation results.Enhanced evolution and prefabrication of prosthetic sockets, providing a development method for healthcare providers. Positive patient satisfaction with transtibial prosthetics made using 3D printers.9 Gubbala and Inala () Design and develop a prosthetic socket for lower-limb amputation using 3D printing technology.Utilize CT-based 3D models and perform finite element-based simulation and analysis. Extract models from CT images using image-processing software, modify them in CAD software, and convert them to STL format. Conduct static and dynamic analyses of the prosthetic stump and socket using ANSYS. Prepare a 3D model with a 3D printer based on simulation results.Estimation of pressure distribution, evaluation of below-knee prosthesis function under static and dynamic conditions. Advancement in prosthetic socket evolution and prefabrication using finite element approach. Contribution to an overall prefabrication evaluation system for healthcare providers. Positive patient satisfaction with transtibial prosthetics made using 3D printers and simulation process.10 Cabibihan et al. () Evaluate the functionality, durability, and cost of popular designs of body-powered, 3D printed prosthetic hands for long-term usage.Selected representative sample of 3D printed prosthetic hands and assessed their suitability for grasping postures, durability, and cost. Tested the prosthetic hand's ability to perform 3 out of 33 grasping postures. Evaluated the material and cables used in the hand for their ability to withstand a 22 N normal grasping force. Analyzed the cost model to determine the production cost of a 3D printed hand.The selected 3D printed prosthetic hand design demonstrated the ability to perform functional grasping postures for everyday objects. The material and cables used were able to withstand the required grasping force. The cost model indicated that a 3D printed hand could be produced at a low cost, as low as $19. These findings provide a baseline for the development of functional and affordable prosthetic hands, benefiting children with congenital missing limbs and war-wounded individuals.11 Siddiqui et al. (a) Investigate the effect of different materials on the functionality of prosthetic running blades.Conduct finite element modeling and numerical simulations to study the static behavior of a prosthetic running blade under various load conditions. Evaluate the performance based on total deformation, equivalent stress, and strain energy. Compare the use of titanium alloy (grade 5), carbon fiber, stainless steel (AISI 316), and aluminum alloy ( T4) as materials for running blades.Titanium alloy exhibits high durability and tensile strength but is costly for manufacturing running blades. Aluminum alloy ( T4) blades experience more deformation compared to titanium and carbon fiber blades. Carbon fiber offers excellent mechanical properties and has outstanding tensile properties, making it suitable for creating running blades.Socket design is a critical aspect of prosthetic limb development, as it serves as the interface between the residual limb and the prosthesis ( Gerschutz et al., ; Gariboldi et al., ). The socket plays a crucial role in distributing forces, providing stability, and ensuring user comfort. This text provides an overview of the key principles and considerations involved in socket design for individuals with lower-limb amputations. One of the primary objectives of socket design is to achieve an individualized fit for each user. Residual limbs come in various shapes, sizes, and contours, requiring custom-made sockets to ensure proper contact and load distribution. Individualized fit minimizes pressure points, reduces shear forces, and enhances overall comfort. To achieve this, prosthetists employ techniques such as plaster casting, 3D scanning, and digital modeling to capture the unique anatomy of the residual limb ( Rai et al., ). Efficient load distribution is crucial for optimal functionality and comfort. The socket must evenly distribute forces across the residual limb to prevent localized pressure and potential skin breakdown. Various strategies are employed to achieve proper load distribution, such as the use of pressure-relief areas, padding, and flexible materials. Pressure mapping technologies and computer simulations help evaluate the load distribution characteristics of different socket designs ( Lenhart et al., ; Siddikali and Sreekanth, ). Socket design must provide adequate stability and suspension to ensure secure attachment of the prosthetic limb to the residual limb. Stability refers to the control of rotational and translational movements, while suspension involves maintaining the prosthesis in position during various activities. Various suspension methods, including suction, vacuum-assisted, and strap-based systems, are utilized to achieve secure and comfortable suspension. The socket design should accommodate the necessary suspension mechanism while ensuring stability and minimizing unwanted movements ( Quinlan et al., a, b).
The choice of materials and construction techniques significantly impact the performance of the socket. Materials should be lightweight, durable, and compatible with the user's skin. Commonly used materials include thermoplastics, carbon composites, and silicone liners. Advanced manufacturing methods, such as 3D printing, allow for complex socket designs and customization. Proper fabrication techniques, including lamination, molding, and thermoforming, ensure the desired shape, strength, and durability of the socket ( Gerschutz et al., , ). User comfort is a critical consideration in socket design. Discomfort or pain can significantly affect the user's adherence and satisfaction with the prosthetic limb. Socket design should consider factors such as cushioning, pressure distribution, breathability, and temperature control ( Gariboldi et al., ). Innovative technologies, including gel liners, adjustable interfaces, and modular components, help enhance comfort by reducing pressure points and improving socket fit and adjustability. Proper alignment of the socket is essential for optimal biomechanical function and gait symmetry ( Marinopoulos et al., ). The alignment ensures that the prosthesis replicates the natural anatomical position and joint axes of the lower limb. Precise alignment contributes to improved stability, reduced energy expenditure, and more efficient gait patterns. Alignment adjustments may be necessary during the fitting process to fine-tune the socket's orientation and optimize user performance. Socket design should allow for adjustability and adaptability to accommodate changes in the residual limb, such as volume fluctuations, muscle atrophy, or bony prominences. Adjustable components, modular systems, and interchangeable interfaces enable prosthetists to make necessary modifications without requiring a complete socket replacement ( Dickinson et al., ). This adaptability extends the lifespan of the socket and ensures an optimal fit and function over time. The interface between the residual limb and the socket, along with the suspension system, significantly influences socket performance ( Quinlan et al., a). Soft interfaces, such as silicone liners, help improve comfort, cushioning, and moisture management. Suspension systems, including suction, vacuum, or strap-based systems, provide secured attachment and control of the prosthetic limb.
In recent years, there has been a paradigm shift toward patient-centric design in the field of prosthetics. Recognizing the importance of meeting the unique needs and preferences of individual users, prosthetic design has evolved to prioritize the patient's comfort, functionality, and overall satisfaction. Here we explore the key considerations in patient-centric design, including customization, user involvement, comfort, functionality, aesthetics, and psychosocial factors. One of the fundamental aspects of patient-centric design is customization. Every individual has unique anatomical characteristics, functional requirements, and personal preferences. Customization allows prosthetists to tailor the design, fit, and functionality of prosthetic limbs to meet the specific needs of each patient. This includes considerations such as residual limb shape, size, and volume, as well as the alignment, components, and interface materials used in the prosthesis. Customization ensures a better fit, improved comfort, and enhanced overall functionality for the individual user ( Beekman and Axtell, ; Reinbolt et al., ; Akarsu et al., ). Involving the patient in the design process is crucial for achieving patient-centric outcomes. By actively engaging individuals with amputations in decision-making, prosthetists can gain valuable insights into their unique needs, goals, and expectations. Patient involvement allows for open communication, shared decision-making, and a collaborative approach to design. Prosthetists can gain a better understanding of the user's lifestyle, preferences, and activities, enabling them to create prosthetic solutions that align with the user's specific requirements and optimize their functional outcomes.
Comfort is a paramount consideration in patient-centric design. Prosthetic limbs should be comfortable to wear for extended periods, minimizing discomfort, pressure points, and skin irritation. Factors such as socket design, padding, suspension systems, and interface materials play a crucial role in enhancing comfort. Customized socket design ensures a proper fit and weight distribution, reducing pressure on the residual limb. The use of cushioning materials, such as silicone liners or gel interfaces, improves comfort and reduces friction. Attention to detail in design and fabrication helps minimize discomfort and maximize overall satisfaction for the user. Patient-centric design places a strong emphasis on improving the functionality of prosthetic limbs. The aim is to enable users to perform a wide range of activities, including walking, running, climbing stairs, and engaging in sports or recreational activities. Prosthetic components, such as knees, feet, and power systems, are chosen based on the user's functional requirements and activity level. The alignment, joint mechanics, and range of motion should closely mimic the natural limb to facilitate more natural movement and gait patterns ( Adouni et al., ). By focusing on functionality, patient-centric design empowers individuals to regain their independence and participate fully in daily activities. The visual appearance of prosthetic limbs is an essential consideration in patient-centric design ( Hall and Dornan, ). Aesthetics can have a significant impact on an individual's self-esteem, body image, and social acceptance. Prosthetists work closely with patients to create prosthetic limbs that match their skin tone, incorporate realistic features, and align with their personal preferences. Advances in cosmetic covers, realistic silicone skin, and patterned socket designs allow for greater customization and aesthetic appeal. By considering the aesthetics, patient-centric design seeks to address not only the physical but also the psychological well-being of individuals with amputations. Patient-centric design acknowledges the psychosocial impact of prosthetic limbs on individuals' lives. It recognizes the importance of addressing psychological, emotional, and social aspects alongside physical considerations. Prosthetic limbs should enhance self-confidence, body image, and quality of life for the user. Factors such as ease of use, reliability, and social acceptance are crucial. Peer support, counseling, and psychological interventions are also integrated into patient-centricity.
Amputation of the limb or extremity of a limb, either from the upper extremity amputation (UEA) or lower extremity amputation (LEA) of the human body, affects people's quality of life. Globally in , the highest number of trauma amputations was in East and South Asia, followed by Western Europe, North Africa and the Middle East, increasing in North America and Eastern Europe [ 1 ].
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In a 28-year study (') on the incidence of lower extremity amputations in 19 countries (EU15+) by Hughes et al. [ 2 ], significant geographic and temporal variability was observed. According to this study, Australia had the highest incidence of LEA in women and men during the study period. However, this incidence decreased steadily in the Netherlands and the USA, and a higher percentage reduction is found in the USA. Another recent study examined Germany's LEA incidence from to [ 3 ]. According to this study, in , compared to , the incidence of major amputations decreased by 7.3%, while the incidence of minor amputations increased by 11.8%, and there was also a decrease in the incidence of women to men.
In terms of the causes that led to the amputation of the lower limbs, demographically, there are many differences between developed and developing countries. For example, compared to developed countries where most amputations are due to disease processes such as diabetes, the causes of amputations are more related to environmental factors, living conditions, or wars in developing countries [ 4 ].
Individuals who have had lower limb amputations face many other physical challenges that can compromise their health and mobility. Hence the need for a technical replacement (prosthesis) that restores the biomechanical function of the amputated element and the body's integrity. Management of lower limb prostheses for these individuals is a complicated issue. Unfortunately, the candidates who want to benefit from the usefulness of a prosthesis are a heterogeneous group with distinct ages and needs. In addition, the choice of the perfect candidate depends on the following factors: the aetiology of limb loss, the level of amputation, comorbidities and health, the postoperative stage, and the state of rehabilitation. Lower limb prostheses can be classified into three types of devices: passive, semi-powered or powered. For example, individuals with transtibial amputation (the term transtibial indicates that the amputation occurred between the knee and the ankle) are usually prescribed a prosthesis for storage and return of passive elastic energy, made of carbon fibre and works as an arc without the ability to generate energy again or to articulate. Most ankle and foot prostheses available on the market until the early s were completely passive. The simplest solution for transtibial amputations is a solid prosthesis of the foot without an ankle joint [ 5 ]. As a result, the mechanical properties did not adapt to the user's walking speed and terrain type. Individuals with transtibial amputation often adopt compensatory gait strategies. These can lead to significant changes in gait dynamics, the joints loading and working, and the muscular activity of the affected and unaffected leg [ 6 ]. In the last three decades, approaches in prosthetic technology have led to significant advances, especially regarding biomechanical and user comfort [ 7 ] and in walking symmetry and energy cost [ 8 ]. An excellent example of a marketable passive prosthetic foot is the C-Walk [ 9 ], equipped only with passive components but combined suitably. As a result, it is more effective from an energy point of view compared to other prostheses in this category.
12,13,14,15,16,17,The first electrically powered ankle-foot prosthesis was built in and was pneumatically operated [ 10 ]. Subsequently, from until now, studies have focused on designing and developing autonomous energy supply systems [ 11 18 ]. The main factors considered for the design of prostheses are both the mechanical properties [ 19 20 ] and the length of the prosthesis [ 21 ]. Another factor that should not be overlooked is the weight of the prosthetic components [ 22 ]. A higher weight also increases the stress on the socket'the residual connection of the limbs, which is one of the most critical elements in the prosthesis [ 23 ]. In addition, most studies in the literature have been conducted on the evaluation of kinematic and kinetic gait [ 24 ] and foot plantar pressure [ 25 26 ].
Magnetorheological fluids (MR) were successfully introduced into prosthetic devices after . In , a patent was published [ 27 ] for a variable torque magnetorheological knee prosthesis produced by Ossur Inc, Los Angeles, CA, USA [ 28 ]. Herr and Wilkenfeld [ 29 ], in , presented a magnetorheological knee prosthesis that automatically adjusts the cushioning of the knee to walking amputated using only local detection of the knee, torque and strength position. In , a study related to [ 27 ] was carried out, which was intended to be part of a project to create models with finite elements of the knee [ 30 ]. Another invention patent [ 31 ] was published in generally relating to powered human augmentation devices, such as lower-extremity prosthetic, orthotic, or exoskeleton apparatus, and/or humanoid robotic devices designed to emulate human biomechanics. Among the most current studies is [ 32 ], which analyzes the energy consumption of a magnetorheological active knee actuator that has been designed for transfemoral prostheses. The system was developed as an operational motor unit consisting of an EC motor, a harmonic drive, and a magnetorheological clutch (MR) parallel with an MR brake.
34,By analyzing the above literature, it can be said that significant advances have been made in the research and development of prostheses for the lower limbs, which has led to an increase in the function and quality of life for many people with amputations of the lower limbs living in developed countries. However, one downside of this new research and development is that many potential users live in developing countries and cannot benefit from this new technology. This fact is due to multiple causes: cost, durability, maintenance or access to these prostheses. Under these conditions, research needs to focus on designing and developing cost-effective foot prostheses that meet economic, environmental and physical standards to cope with unfavourable climates and working conditions. So far, many cheap prosthesis projects have been done to support the lower limbs, such as [ 33 35 ]. Our team aims to design and implement a low-cost prosthesis to support people with lower limb amputation (especially transtibial amputation). In the first phase, two goals were set: to design the smart ankle prosthesis and implement a solution to determine the weight distribution on the sole. The smart ankle prosthesis is intended to be developed as a passive one (from the articulation actuation point of view) and controllable (by using a magnetorheological fluid and controlling its properties to obtain a controllable damping effect inside the articulation). For the weight distribution on the sole, in this stage of our research, we will propose an alone sandal with eight pressure sensors. The first objective we set for the current study is to propose a smart ankle prosthesis design; the second objective is to design, develop, and validate a solution to determine the weight distribution on the sole; the third objective is to conduct a series of measurements which allow us to test the data acquisition system firstly and to compare the acquired data against other systems presented in the literature, and secondly to create a dataset which will be used to design a future algorithm which to detect the gait phases. With these parameters determined, it is possible to control the optimal position of the prosthesis joints, depending on the phase and the type of movement. The determinations were made for people without amputated limbs. The appropriate values for an amputated limb can be determined by mediating their values. This simplified and optimized system can be integrated into a prosthesis for its configuration in real-time, regardless of the phase or type of movement.
The research effort in this field focuses on improving the characteristics of the artificial ankle to closely simulate the human ankle's functionality. The design of an artificial ankle involves many scientific and technical areas such as medicine, robotics and mechatronics, biomechanics, material science, mechanical engineering, electronics, and others. The challenge of the ankle prosthesis design is to find the means to achieve the functions of an intact ankle, especially the role of power generation.
This study presents an innovative solution for a smart ankle prosthesis based on smart fluids that will simulate the functionality of the human ankle for both walking and running activities.
The mechanical structure consists of the mechanical elements of a standard prosthesis ( Figure 1 ). The innovative element will be a spherical joint based on smart fluids (class 4), which replaces the human ankle joint. The spherical joint allows two rotations, corresponding to the up and down movement and the lateral rotation of the foot. For each rotation, the spherical joint consists of two concentric hemispherical shells between which there is a magnetorheological fluid. The volume between the two spherical shells is divided in two by a fixed belt. Also, a spherical cap rotates between the two spheres. It is rigidly attached to the prosthesis elements, rotating with them and generating the rotational movement of the ankle. Angle 0 divides the fluid into two equal volumes for the relaxation position.
At the rotation between the two hemispheres, the fluid is circulated from one space to another through a magnetorheological stop valve (outside the joint). Due to the incompressibility of the fluid through the control of the stop valve, the control of the rotation of the spherical cap between the two hemispheres is obtained, so implicitly, the control of the rotation of the joint. For the second axis of rotation, proceed similarly. The joint is shown in Figure 2 . Its detailed description is presented in the works [ 36 ]. Also, the mechanical system contains the reconstruction of the leg components that have been amputated (ankle, leg, etc.).
The actuator system: The prosthesis uses elastic elements that maintain the foot's position in the relaxation position. Also, the ankle joint allows rotation (lateral and vertical) under the action of body weight (support on foot, walking, running). A stop-valve controls both movements of the joint with rheological fluid. After the cessation of the effort (due to the move), the elastic elements bring the paw to the position of relaxation.
The sensorial system: The sensory system provides information about the position of the articulation of the prosthesis and the force exerted on it due to the body's movement (size, direction). The information is provided to the control system. It consists of incremental rotation sensors (associated with the spherical joint) and force sensors (associated with the foot paw). In addition, the value of the working pressures for the stop valve is given by pressure sensors.
The control system: Prosthesis control systems must accomplish multiple tasks, such as recognizing the amputee's intended movements (high-level control), applying an appropriate control law based on the amputee's intent (mid-level control), and using local feedback to command the actuation systems within the prosthesis (low-level control). The control system will be capable of controlling the ankle joint across various ambulation modes (level-ground walking, ramp ascent/descent, stair ascent/descent, running); however, these control strategies are highly sophisticated.
There is a known ankle prosthesis that solves this problem through two constructive solutions, namely:
The most used solution is the prosthetics leg for normal daily activities (upright position, walking, etc.). Generally, they are designed strictly for one person (weight, dimensional) and a spherical joint of class 5 that allows vertical movements of the foot. Most of these prostheses are passive, and the movement control is performed (strictly mechanical) by elastic elements or hydraulic/pneumatic cylinders. These have the disadvantage of the need for design strictly reported to a beneficiary. They also allow only one type of activity (e.g., walking) [ 37 ].
Another solution is the prosthetic for particular activities (sports activities: running, jumping). They are generally built from a single elastic body without containing the rotating joint. Therefore, they are dedicated only to sports activities, designed only for certain types of requests, strictly for one person. To switch to daily activities, it is necessary for this person to change the prosthesis [ 38 ].
The classical spherical joints do not control the movement of interconnected elements, having only the role of a passive kinematic couple.
Our spherical joint based on smart fluids was proposed in a national patent application: Spherical joint based on intelligent fluids'A// [ 39 ] and was a gold medalist at Euro Invent .
The rest of the paper is structured as follows: Section 2 presents the design of the prosthesis and the sensory system, as well as the data validation of the sensory system; Section 3 shows the results obtained from the simulations; Section 4 presents the challenges associated with the development of such systems and their potential solutions, as well as a discussion about the future research perspectives are given; finally, Section 5 is devoted to the conclusions.
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