Structural Design and Analysis of Below-knee Prosthetic Leg in Mechanical System

doi:10.21203/rs.3.rs-3249738/v1

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Research Article

Structural Design and Analysis of Below-knee Prosthetic Leg in Mechanical System

https://doi.org/10.21203/rs.3.rs-3249738/v1

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Background

A prosthetic leg is not a counterfeit appendage but rather than an artificial limb, that is connected where the leg has been excised due to an injury accident, sickness, or other medical reasons. Presently, prosthetics offer an option for people with an amputee to enable them living in a conventional life. Prosthetic legs can vary in complexity and design depending on the level of amputation, needs and goals of the individual as well as their lifestyle and activities.

Research Design and Methods:

In this study, we aimed to develop a structural design and analysis of below knee prosthetic leg that can provide adequate support and mobility to amputees. The 3D experience software Solidworks is used for structural design of the prosthetic and Finite Element Analysis (FEA) is employed to simulate the performance under various loading conditions.

Results

The analytical result demonstrates the functionality of the prosthetic leg and highlights the safety measure of the body structure that meets high standards of performance, durability, and regulatory compliance.

Limitations:

The design and drafting of the model is done only in silico (computer) methods. Further, static analysis of the prototype model is evaluated using Solidworks simulation (version 2016). Actual analysis (real-life analysis) of the prototype is not structured in this paper.

Conclusion

This study represents a modern generation and approvingly operational pneumatic prosthetic leg. Technological evolution has led to the development of more advanced prosthetics to improve the human mankind who have amputees.

Below-knee Prosthetic

Brainstorming

FEA

Static Analysis

Solidworks Simulation

A prosthesis is an artificial part of the body, and the purpose of the prosthesis is to replace a lost body part or to improve the functionality of a body component. The word “Prosthesis” is a Greek term that means “addition, application, and attachment” (Chaudri et al. (2014)). Around 3000 years ago an Egyptian mummy was discovered with a wooden and leather prosthetic toe, which is the oldest example of a prosthesis in existence (Shahar et al. (2019)). In the field of medicine, it refers to a tool that restores a lost bodily component, which might have been lost as a result of trauma, illness, or a congenital defect. As stated by Craelius. (2022), usually four key types of prosthesis are considered for the human body; they are Transradial, Transfemoral, Transtibial, and Transhumeral. Another different type is also applicable like replacing a missing foot and hand in a partial process which occurred due to an accident or replacing a finger, eye, nose, and other body parts. In this large prosthesis segment, we choose to focus on the leg prosthesis system. In the market, three basic types of prosthetic legs are available: (i) Systems for prosthetic ankles and feet, (ii) Systems for knee replacement, and (iii) Systems for Sports-specific knee and foot prosthesis. Further, the process of the knee prosthesis is classified into two categories, one is above-knee amputation and the other one is below-knee amputation. Both processes have come under the category of lower limb amputation (Nehler et al (2003)). The bellow knee amputations are much more common than those of the above knee, and more easily restored to near-normal ambulation. According to Winter and Sienko (1988), more people in our population are living with below-knee amputations due to surgery for peripheral vascular disease and ageing. The foot, ankle joint, distal tibia, fibula, and associated soft tissue components are all amputated as part of a transtibial amputation which is known as a bellow knee amputation process (www.ncbi.nlm.nih.gov). The intersection of the upper and middle third parts is often where the bellow knee amputation is carried out. As the muscles that trigger the shank are still there, the procedure almost restores complete knee function. The below-knee residue shouldn't be taken into consideration after the stump since the socket supports weight over its whole length. Mostly, the human knee can rotate from 0° to outside the 45°, and up to 160 degrees of flexion in the sagittal plane (Johal et al. (2005)). Li et al. (2020) proposed a design for human bellow-knee amputation with different essential measurements. Proper measurement, angle of rotation, and weight, these things are highly significant for the design of a prosthetic leg.

A successful work system must consider human nature, ability, and limits to be done effectively, safely, and comfortably; Ergonomics is a systematic discipline of study that does this. From the theory of ergonomics, a prosthetic leg is a classic example of a human-machine system (Hong-Liu et al. (2011)). In this system, the dynamic interaction between the human body and the prosthetic leg creates a high need for an accurate structure. Ergonomics aids in reducing discomfort, building muscles, and boosting blood flow (Stecker et al. (2019)). Combining these things can increase the mental clarity of any user. Ergonomically designed prosthetic legs are focused on the exact location of the body; the user’s strength, and weight; the type of material used which is easily available; and the cost of the products. For example, Liu et al. (2011) design a prosthetic leg that is based on ergonomics and produces a testing device that is capable to identify the gait symmetry for any below-knee prosthetic leg. A traditionally designed prosthetic leg cannot stimulate the human body's gait because this type of prosthetic leg has a knee joint locking mechanism. On the other side, the intelligent prosthetic leg which is designed with ergonomics features can easily balance the body's gait. Day by day the acceptance and importance of ergonomics design increases because of its perfection in the system. The main goal of ergonomics is to make an indifferent posture for every user, reduce the overly strong force, and make everything accessible (Kroemer and Kroemer, (2016)).

The crucial part of designing a below-knee prosthetic is choosing the appropriate materials. As per Sundararaj and Subramaniyan (2021) for every customer, a constant need exists for an affordable prosthetic. The below-knee prosthetic is structured through analysis of the tension, strain, and deformation. The major focus to select the material is it should be lightweight and of optimum quality. Because if the material is heavy weight, then it does not work properly (Chakravarthy et al. (2017)). Aluminum, Titanium, Magnesium, Copper, Steel, and many more components are used in a prosthetic leg. But the goal of the selection process is to maintain the safety issues. Selection of proper elements and designing a prosthetic leg is very challenging where ergonomics and engineering play a crucial role. The designers focus on the construction of apparatus and equipment for fundamental research in human motion. Safety, function, control, efficiency, beauty, comfort, simplicity, and durability are just a few of the numerous aspects that designers must consider while designing a prosthetic device. As they are all interrelated, it is difficult to rank the significance of these elements (Savsani et al. (2023)). Due to excessive demand for below-knee amputations, we aimed to focus on this area based on various literatures. Our study emphasis the structural design and simulation analysis of below knee prosthetic using FEA technique. The material selection for the prosthetic is done on key factors basis as discussed below. The static analysis proves the importance of selected materials based on their strength, durability, weight, and cost.

This paper presents a novel approach to designing and analyzing prosthetic legs for individuals with below-knee amputations. The novelty of this paper lies in several key aspects such as,

While there are many papers that discuss prosthetic limbs, this paper specifically focuses on designing and analysis of a below-knee prosthetic leg. This is significant because the requirements and challenges for below-knee amputees are different from those for above-knee amputees.
The paper places significant emphasis on the structural design and analysis of prosthetic leg. This includes selecting appropriate materials through benchmarking technique and analyzing structural integrity of the leg.
A detailed analysis of the prosthetic leg which involves simulating its behavior under different loads and conditions has been presented that helps to ensure the safety, reliability, and effectiveness of the prosthetic leg.

This paper offers a unique perspective on designing and analyzing prosthetic leg with a specific focus on below-knee amputees and simulating the behavior of prototype model under different static loads based on Finite Element Analysis (FEA) approach.

The rest of the paper is organized as follows: the related studies of below knee prosthetic leg is presented in section 2; the methodological study is presented in section 3; in section 4, we have discussed the FEA technique in prosthetic amputation. Further in section 5, we present the material selection and their properties. Section 6 defines the experimental simulation and result analysis of different parts of prosthetic leg. Finally, section 7 concludes the study.

The designing process of a prototype prosthetic leg depends on the concept of ergonomics. Because only ergonomics can give an appropriate measurement to develop a product. After a lot of research work and brainstorming a prosthetic leg can be structured that should be accurate by every side, and also accepted by the users. In this segment, the writing firmly identified with our work can be delegated to three primary classes: (1) Design of the below-knee prosthetic leg, (2) The impact of Ergonomics in design, and (3) The use of simulation process.

2.1. Design of the Below-knee Prosthetic Leg

The connection between a human body and a mechanical support system is made via a prosthetic socket. The socket's design and fit ultimately affect its energy consumption, comfort, support, and patient acceptability. Different design requirements depend on the user's demands and the production process. Higgs et al. (2022) define some design specifications which have a major impact on the design of below-knee prosthetics. They are the range of socket wall thickness, proper movement of sockets, capacity to bear the body weight, user’s actions, and hold body pressure. Material selection to make accuracy in design is a significant part of the process of manufacturing. The production system indicates building a below-knee prosthetic that tolerates maximum loading and minimum dislocation. Edwards (2000) talks about the earliest design of prosthetics, which is named Patella Tendon Bearing (PTB). In the early stage, PTB works with a unique methodology. The use of a corset placed on the thigh to help with loading was no longer necessary for the amputee; whereas to allow for comfortable loading, it made use of a unique weight-bearing form. According to Nze et al. (2017) in a below-knee amputation, the designed ankle joint improved the prosthesis' functioning, leading to better amputee rehabilitation than with a standard prosthesis that features a direct pylon-foot assembly without any consideration for an ankle joint. Due to high demand, Abd Al-razaq et al. (2016) developed a new concept for below-knee prosthetics which is known as a modular socket system. It applied direct lamination to patients' remaining limbs. To investigate the mechanical characteristics of socket materials, tensile and creep tests were performed at 500 degrees Celsius. The below-knee prosthesis was developed primarily to provide amputees with more comfortable sockets. The design objective was not achieved if the user perceives the socket to be unattractive. The prosthetic socket's ultimate design was practical and comfortable, as well as visually beautiful.

2.2. The Impact of Ergonomics in Prosthetic Design

The fundamental goal of ergonomics analysis in the design process is to include human interests as early as possible so that they may be incorporated into every creative and innovative aspect of a man-made item. These days ergonomics plays an important role to meet the demand for creating human below-knee prosthetics with improved functionality and controllability. Corti (2022) introduced these generations to a new concept regarding ergonomics in the design field. As per a survey statement which is proposed by Chipol et al. (2018); a low percentage of users are suffering from using prosthetics due to a lack of ergonomics design. The application of ergonomics is fulfilled when a user has the capacity to move at least 45 metres or more than 45 meters without the use of any other support. In a below-knee prosthetic device, five features are added; (i) Suspension: which helps to secure the prosthetic limb to the body’s remaining limb, (ii) Socket: which helps to fit the body’s limb into an artificial limb, (iii) Liner: this is a replaceable inner socket composed of flexible material that is often used for prosthetic lower limbs below the knee, (iv) Shin Tube: this is located between both the socket and the artificial foot, and (v) Prosthetic Foot & Ankle. These all features have come under ergonomics. Herdiman et al. (2020) face new challenges in designing prosthetics for ankle joints in utilizing all concepts of ergonomics. Users preferred the new design because it meets prosthetic device technology's requirements. Cost, longevity, tradition, and atmosphere are all factors that applied technology must consider. The use of the right technology offers ways to get around prosthesis constraints. The user's condition, as well as the various sizes and components utilized in the creation of below-knee prosthetics, are only a few of the variables and circumstances that require consideration in design development. According to Annisa et al. (2022) ‘Anthropometry’ is one ergonomic method that may be used in equipment design to create the best items based on the user's body proportions. A below-knee prosthesis that was ergonomically constructed provided more comfort and eliminated the need for walking aids like sticks.

2.3. The use of Simulation Process

The simulation technique compares various designs and solutions to evaluate the performance of a current system or forecast the performance of a planned system. That is the reason for using simulation in the prosthetic design section. The objective of any prosthetic design is to nearly resemble the real physiological appearance, functionality, and feel of the replacement body part as feasible. Haider et al. (2022) introduced a new vision for designing prosthetics in the CAD model and using SolidWorks software to analyze the kinematic and dynamic study of the virtual model's transfemoral prosthesis. The use of this simulation was done over the joint of a knee. Sometimes, a static structure of a prosthetic is optimized through SolidWorks to calculate the force of the device (Torres-SanMiguel (2022)). From today's aspect, the design work for a below-knee prosthetic is very familiar. Luengas et al. (2022) approach research of prosthesis gait using simulations produced by a modelling technique. Through 3D design work, done with the help of SolidWorks, an analytical model of an active transtibial prosthesis was created. LaPrè et al. (2018) proposed conventional gait analysis techniques that estimated the motion and loads at the residuum-socket interface using a global optimization methodology and anatomical limitations. An innovative, semi-active prosthetic knee has been developed which shows the result as across the whole gait, the knee torque symmetry index is less than 10% (Zhang et al. (2021)). Most of the users are adopting motion-based prosthetics. The properties of the prosthetic as discovered via testing are utilized to parametrize and validate the model. Research done by Tryggvason et al. (2017) that, the conventional testing technique may be simulated, and changes to the model indicate expected functional adjustments. Strbac and Popovic (2012) outline the process for enhancing the prosthetic foot's rigidity. The process enables the choice of the foot's components and the components used in its design. The method is based on optimization, and the objective is to minimize the gap between knee joint torques when a person is walking normally and when they are walking with a transfemoral prosthesis. Simulating the torque and angle of the human knee is the prosthetic knee's significant technological advancement. In a below-knee prosthetic design, simulation is added just to provide a proper design that can help to eliminate errors by developing the design in final processing.

The design process is a multistage process involving identification of appropriate problem followed by brainstorming, model solution, testing and analysis (Chapman, (2001)). In this paper, design consideration of below knee prosthetic leg is proposed with knee and ankle joint mechanism. The basic design of prosthetic leg and related parts are designed using 2D and 3D CAD models. Further, structural analysis is achieved to determine the stress, strain and deformation based on a Finite Element Analysis (FEA) method.

3.1. Problem Identification:

When people lose limbs through accidents, genetic abnormalities, or battle, prosthetic limbs are required. In both humans and animals, a prosthetic leg or limb serves as a synthetic substitute for a lost biological limb. Regrettably, because of poverty, exorbitant costs, and a lack of technicians, many patients have no way to obtain prosthetic devices (Mabry et al., (2020)). Though mechanical and automated prosthetics can be bridged with bionic limbs, their excessive weight and high cost is a disadvantage for both above and below knee amputation (Sundararaj and Subramaniyan, (2021)). The knee and ankle joints are the two main anatomical components of the human leg. The knee, which allows mobility between the femur, tibia, and patella, is the intermediate joint of the lower limb (Vaienti et al. (2017)). Amputees face several challenges when it comes to finding the perfect prosthetic leg. Here are some of the most common problems:

Proper fit

Prosthetic legs must fit properly to be comfortable and functional. However, due to differences in anatomy and the shape of residual limbs, finding a perfect fit can be difficult. Some amputees may need custom-made prosthetic legs to ensure a proper fit (Manz et al. (2022)).

Comfort

Prosthetic legs can be heavy, and they can cause discomfort or even pain if not fitted properly. Additionally, they can cause irritation or rubbing, leading to skin breakdown or ulcers. Finding the right balance between comfort and functionality is critical (Mohd Hawari et al. (2017)).

Mobility

Prosthetic legs must be designed to support the weight of the body and allow for mobility. However, the type of prosthetic leg needed may vary depending on the level of amputation, the activity level of the amputee, and other factors (Mileusnic et al. (2021)).

Cost

Prosthetic legs can be expensive, and not all insurance plans cover the full cost. This can make it difficult for some amputees to afford the prosthetic leg they need (Amsan et al. (2019)).

Durability

Prosthetic legs must be durable and able to withstand the wear and tear of everyday use. However, they can wear out over time, requiring repairs or replacement (Andrysek, (2010)).

Adaptability

As the needs of the amputee change, their prosthetic leg may need to be adjusted or replaced to accommodate those changes. For example, as an amputee loses or gains weight, their prosthetic leg may need to be adjusted to maintain a proper fit (Wurdeman et al. (2014)).

Psychological impact

Losing a limb can have a significant psychological impact, and adjusting to a prosthetic leg can be challenging. Some amputees may need support from mental health professionals to cope with the emotional and psychological effects of limb loss and prosthetic use (Saradjian et al. (2008)).

Based on the above literatures, it can be concluded that the below knee prosthesis is more commonly used than the above knee prosthesis. The knee joint is complex and weight-bearing, making the biomechanics of above knee prosthesis more complex. Below knee amputations are more common, and advancements in prosthetic technology have led to the development of more lightweight, comfortable, and functional below knee prostheses. Therefore, in this paper the authors mainly focused on the below knee amputation addressing the above problems and challenges faced by the majority of amputees. Below knee prostheses offer greater stability and control as the knee joint is preserved, allowing for better balance and control of the prosthesis. Additionally, below knee prostheses tend to be less complex and less expensive than above knee prostheses, providing more surface area for the prosthetic to attach and more leverage for the user. Improving the design and functionality of below knee prostheses can enhance the quality of life for amputees. In the below section, brainstorming is evaluated to address the design requirements and material selection for the prosthetic.

3.2. Brainstorming:

Brainstorming is a process of generating new and creative ideas through group discussion and collaboration. It is a technique used to encourage creative thinking and problem-solving by allowing individuals to freely express their thoughts and ideas without fear of criticism or judgment (Al-Samarraie and Hurmuzan, (2018)). In the context of prosthetic leg design, brainstorming is required to explore and generate new ideas that can be incorporated into the design of a prosthetic leg (Manero et al. (2019)). This process helps in identifying the various design requirements and challenges that need to be addressed. Brainstorming also helps in exploring different design options and evaluating their feasibility, effectiveness, and potential impact on the users. The design of a prosthetic leg involves multiple considerations such as the functionality, comfort, safety, durability, cost, and user needs. Brainstorming can help in exploring these considerations and identifying potential solutions to improve the design of the prosthetic leg (Daly et al. (2016)).

The Delphi technique is a commonly used method for gathering information and opinions from experts in a particular field. This technique has been particularly useful in the field of market research, where it has been used to identify customer needs and requirements (Habibi et al. (2014)). The Delphi technique was employed in the development of a prosthetic leg design, dimension setup, and material selection. A panel of experts consisting of prosthetists, orthopaedic surgeons, and biomedical engineers was selected to provide their opinions and expertise on the topic. The experts were first provided with a questionnaire to gather their initial opinions and suggestions on prosthetic leg design, dimension setup, and material selection. The responses were analysed, and the areas of agreement and disagreement were identified. The results were then presented to the experts for further input, and the process was reiterated until a consensus was reached. The final outcomes of this study included a list of recommended design features, optimal dimension setups, and material selections for prosthetic leg development, which can help to improve the overall function and quality of life for amputees. The Delphi technique proved to be an effective tool for integrating expert opinions and identifying key factors for successful prosthetic leg design, dimension setup, and material selection. Figure 1 illustrates the Delphi technique in action, with a group of experts providing their opinions on what customers want and need in a prosthetic leg.

3.3. Ergonomics and Anthropometry in prototype design:

Prosthetic leg design plays a crucial role in the rehabilitation and quality of life of individuals with lower limb amputations. To ensure optimal functionality, comfort, and biomechanical alignment, prosthetic legs must be designed with careful consideration of ergonomics and anthropometry (Annisa et al. (2022)). Ergonomics involves designing products or systems to fit the capabilities, limitations, and needs of the amputees. In the context of prosthetic leg design, ergonomics aims to create a limb that closely replicates the biomechanics and functionality of a natural leg, ensuring maximum comfort and ease of use (Hong-Liu et al. (2011)). Whereas, Anthropometry refers to the measurement of human body dimensions, proportions, and physical characteristics (Jervas, (2015)). It plays a vital role in prosthetic leg design as it helps create customized limbs that fit the unique dimensions of each individual. Figure 2 refers to the foot anthropometry of the human leg.

3.4. Design consideration:

The below knee prosthetic legs are designed to help individuals who have lost their lower leg due to trauma, disease, or congenital deformity to regain their mobility and independence (Robinson et al. (2010)). The primary goal of a below knee prosthetic leg is to mimic the function of natural leg and allow user to perform activities of daily living (Grimmer and Seyfarth, (2014)). In this paper, we have proposed the design over clip spring type lower leg support, tie-based thigh guard, hydraulic based knee support, and individuals having spinal height. Ergonomics of human knee has been used in current review. During ground level walking the human step can be arranged into various stages. For the design purpose basic three requirements are present; (i) ability to provide torque in an efficient way, (ii) ability to fit with lower limb anatomical envelope, and (iii) the ability to mimic the ergonomics of human knee. The artificial prosthetic leg design is majorly incorporated with its structure, limbs, and knee joint (Yusof et al. (2021)). The input desired position trajectory provides a force reaction that initializes the parameters from the user. Secondly, the actuator of the user will move the prosthetic leg itself. Solidworks, a 3D experience software is used to design the prototype of prosthetic leg as shown in Fig. 3. Followed by Fig. 4 shows the design and dimension of main parts for the prototype model. Further, static analysis of the designed prototype is done by Solidworks simulation which follows the finite element analysis to simulate the 3D model and forecast a product’s physical behavior in real life.

The basic design and drafting of the proposed prototype model is initially designed in AutoCAD software to probe the dimensions used for 3D protype model and further analysis. Figure 5 illustrates the 2D drafting and kinetic diagram of the proposed model. The abbreviation used to validate the drafting design are stated as: Length of the higher leg part up to knee (l₂), Length of the lower leg part up to ankle (l₁), Mass of link 2 (m₂), Mass of link 1 (m₁), Torque to hip joint (T₂), Torque to knee joint (T₁); and the dimension abbreviations for kinetic diagram are as: Angle between calf and foot (θ₁), Angle between thigh and calf (θ₂), Distance between the screw1 and pivot (E₁), Part of prosthetic thigh to fasten the pivot (E₂), Distance between the pivot in E1 and ankle (F₁), Part which sticks to the remnant human thigh’s part (F₂), Distance between two pivots of foot (G₁), Distance between two pivots in the higher leg part (G₂), Distance between the pivot in foot and E1 (H₁), Distance between the screw 2 and the pivot (H₂), Distance between the pivot and the screw nut 1 (L₁), Distance between the pivot and the screw nut 2 (L₂), Angle between E2 and F2 (α₂).

3.4.1. Torque analysis:

The knee angle θ₂ and angular velocity $\overrightarrow{{\omega }}$ are shown for level ground walking, and the relationship between the knee angle and knee torque is depicted in further details. The largest torque (absolute value) happens at the end of stance flexion. The largest knee angular velocity occurs during the two swing phases than in the stance phase.

In Fig. 6,

Length and mass of AB link: l₂ and m₂

Length and mass of BC link: l₁ and m₁

$${W}_{2}\widehat{{F}_{1}} : {m}_{2}g\text{sin}{\alpha }_{2}$$

$${W}_{2}\overrightarrow{o} : {m}_{2}g\text{cos}{\alpha }_{2}$$

$${W}_{1}\widehat{n} : {m}_{1}g\text{sin}[\pi -({\alpha }_{2}+{\theta }_{2}\left)\right]$$

$$R\widehat{m} : {m}_{2}g\text{cos}[\pi -\left({\alpha }_{2}+{\theta }_{2}\right)]= {W}_{1}\widehat{m}=Foot Reaction$$

$$\overrightarrow{{F}_{1}} : {\overrightarrow{F}}_{\left(load\right)}+ {W}_{2}{\widehat{F}}_{1} or {\overrightarrow{F}}_{\left(load\right)}+ {m}_{2}g\text{sin}{\alpha }_{2}$$

$${F}_{1x} : {(\overrightarrow{F}}_{\left(load\right)}+ {m}_{2}g\text{sin}{\alpha }_{2})\text{sin}{\alpha }_{2}$$

$${F}_{1y} : {(\overrightarrow{F}}_{\left(load\right)}+ {m}_{2}g\text{sin}{\alpha }_{2})\text{cos}{\alpha }_{2}$$

$${T}_{2} : \left[{(\overrightarrow{F}}_{\left(load\right)}+ {m}_{2}g\text{sin}{\alpha }_{2}){l}_{2}\text{cos}{\alpha }_{2}-{m}_{2}g\text{cos}{\alpha }_{2}\frac{{l}_{2}\text{cos}{\alpha }_{2}}{2}\right]-{I}_{2}\overrightarrow{\alpha }$$

$$or,\left[\begin{array}{c}{(\overrightarrow{F}}_{\left(load\right)}+ {m}_{2}g\text{sin}{\alpha }_{2}){l}_{2}\text{cos}{\alpha }_{2}-\\ {m}_{2}g\text{cos}{\alpha }_{2}\frac{{l}_{2}\text{cos}{\alpha }_{2}}{2}\end{array}\right]-\left[\frac{{l}_{2}^{2}}{12}{m}_{2}\frac{{d}^{2}{\overrightarrow{\theta }}_{2}}{d{t}^{2}}\right]$$

$${T}_{2} : \left[\begin{array}{c}{(\overrightarrow{F}}_{\left(load\right)}+ {m}_{2}g\text{sin}{\alpha }_{2}){l}_{1}\text{cos}\left[\pi -\left({\alpha }_{2}+{\theta }_{2}\right)\right]-\\ {m}_{1}g\text{sin}[\pi -\left({\alpha }_{2}+{\theta }_{2}\right)]\frac{{l}_{1}\text{cos}\left[\pi -\left({\alpha }_{2}+{\theta }_{2}\right)\right]}{2}\end{array}\right]-{I}_{1}\overrightarrow{\alpha }$$

$$or,\left[{(\overrightarrow{F}}_{\left(load\right)}+ {m}_{2}g\text{sin}{\alpha }_{2}){l}_{2}-{m}_{2}g\text{cos}{\alpha }_{2}\frac{{l}_{2}\text{cos}{\alpha }_{2}}{2}\right]-\left[\frac{{l}_{2}^{2}}{12}{m}_{2}\frac{{d}^{2}\overrightarrow{\pi -\left({\alpha }_{2}+{\theta }_{2}\right)}}{d{t}^{2}}\right]$$

Finite Element Analysis (FEA) is a computational method that involves diving a complex physical system into small elements to analyze and solve various mechanical problems (Kim et al. (2018)). FEA is extensively used in design and analysis of prosthetic leg where it can simulate the stress and strain that occur during normal use (Roylance, (2001)). This can help designers to optimize the design of prosthetic legs ensuring that it is safe, durable, and comfortable for the wearer. For example, FEA can be used to evaluate the effect of different materials and geometries on the performance of prosthetic legs (Omasta et al. (2012)). Once the 3D CAD model is prepared, it is divided into small elements called finite elements. The number of finite elements used depends on the complexity of the model and accuracy required. Each finite element has its own set of material properties and is connected to neighboring elements to simulate the behavior of the prosthetic leg.

Software packages like SOLIDWORKS and ANSYS have become indispensable tools in efficiently solving engineering problems (Jweeg, (1983); Price et al. (2019)). These packages employ the finite element analysis (FEA) technique, which is a computerized method for tackling complex issues in various domains such as vibration, heat, fluid flow, and other physical phenomena (Hughes, (2012)). By utilizing FEA, one can determine potential solutions to difficult problems and evaluate the behavior of products under different conditions. SOLIDWORKS is a widely used software program for finite element analysis and, in this particular study, it was employed to generate an analysis of a below knee prosthetic leg. This allowed for the flexibility to change the material used and facilitated the creation of an accurate representation of the socket's real shape by inputting its actual dimensions into SOLIDWORKS software (version 2016).

The material selection of a prosthetic leg is a critical process that can significantly impact the function, durability, and comfort of the prosthesis. The material for each part of the prototype design is illustrated in Fig. 7 as bill of material.

A variety of materials can be used in making prosthetic legs such as metals, polymers, polyethylene, and composites (Rizqillah, (2022)). But it depends on some key factors discussed below.

i. Strength and durability: The materials used in prosthetic leg must be strong enough to resist the forces and stress that occurred on prosthesis on daily use. Metals such as titanium or aluminum alloys and composites such as carbon fiber are used to control high strength and durability.

ii. Weight: Prosthetic legs must be lightweight to avoid excessive strain on the user’s residual limb. Materials such as carbon fiber and other composites are commonly used due to their lightweight properties.

iii. Comfort: The materials used in prosthetic leg must be comfortable to wear for extended periods of time. Materials such as silicone or thermoplastic elastomers (TPE) can be used in liners or sockets to provide a comfortable fit and reduce friction or pressure points.

iv. Cost: The cost of the materials used in prosthetic leg can vary widely with some materials being more expensive than others. It is important to balance cost considerations with need for quality and durability in selection of materials for prosthesis.

Depending on the above factors, we have selected the materials that can resist high stress and displacement and are also cost effective with comfortable weight. The materials properties used for different parts of prosthetic leg is represented in Table 1.

In this paper, we have presented the structural design and simulation analysis of prosthetic leg that is subjected to the action such as standing, walking, running, and jumping. For example, The bio-medical stress of femur bone is experimented with 1410 N in running condition proposed by the study of Nithin et al. (2015). In our study, 3D experience software Solidworks is adopted to enable simulation on different parts of virtual prototype prosthetic leg as a replacement of physical model. Solidworks simulation is a structural analysis tool that utilizes FEA to simulate the protype model and predict the physical behavior of the model in real-life. The simulation based on prototype-based model also helps to generate a quarter of data in materials and structures subjected to varying force. The results show that the static characteristics of pneumatic artificial muscle can be epitomized to determine the simulation control.

The FEA analysis for the different parts of prototype-based prosthetic leg is represented below. Essentially the static test is performed to ensure the random force and fatigue strength generated on prosthetic leg can be encountered over its lifespan.

6.1. Analysis of Ankle assembly part:

The ankle assembly is a critical component in the design of a prosthetic leg and plays a crucial role in providing stability, shock absorption, and a natural gait pattern for user. By incorporating a variety of components (such as ankle joint, foot plate, shock absorber etc.) designers can create a highly effective and functional prosthetic leg that allows users to enjoy a comfortable and effective lifestyle. The volumetric properties of different ankle assembly parts are given in Table 2.

6.1.1. Load and Fixtures:

Applied forces and fixed portion for ankle assembly is presented in Table 3. The resultant reaction force for ankle part is 200.049 N. It can be seen that 500N normal force is applied on four faces and respective stress, strain and deformation analysis are represented in subsequent sections.

6.1.2. Von-mises stress:

The von-mises stress encountered in ankle assemble part is shown in Table 4. The stress type is selected as von-mises type for the simulation based on the model in Solidworks. As per Table 4, the maximum stress is 1.5336$\times$10² MPa and minimum is 3.776$\times$10⁻⁴ MPa. Based on the stress result the ankle part is predicted to be strong enough to withstand the static force.

6.1.3. Equivalent strain:

Equivalent strain subject to different loading condition of prosthetic leg is given in Table 5 which shows minimum strain of 1.611$\times$10⁻⁸ and maximum of 8.726$\times$10⁻⁴. The strain displacement being negligible, it will be suitable for below knee prosthetic leg.

6.1.4. Resultant displacement:

Resultant displacement refers to the overall or net displacement of the object. The resultant displacement for prototype-based prosthetic leg is represented in Table 6. The maximum deformation is 6.904 mm which occurs during running.

6.1.5. Factor of safety:

The factor of safety defines the ratio of the capacity of a system to the maximum load or stress that it can resume. The factor of safety for prosthetic leg is represented in Table 7. which shows the maximum value of 1.972$\times$10⁶ and minimum of 4.848.

6.2. Analysis of Pneumatic Assembly:

The volumetric properties of pneumatic shaft assembly is given in Table 8 which specifies the effective means of using pneumatic shaft or pneumatic cylinder type actuator to power and support the prosthetic ankle. The volumetric data helps to facilitate the analysis and refer for further improvement.

6.2.1. Load and Fixtures:

The load and fixture associated with pneumatic shaft is given in Table 9. The resultant reaction force for pneumatic assembly is 4806.24 N. It can be seen that 10 MPa downward pressure is applied on shaft with a phase angle of 0 degree that will generate suitable motion of shaft while running. The respective static analysis is described in the sections below.

6.2.2. Von-mises stress:

In the case of pneumatic shaft, von-mises stress depends on internal pressure of the shaft, material properties and design condition. The von-mises stress for pneumatic shaft assembly is presented in Table 10. From the test, we obtained the maximum and minimum stress as 7.039$\times$10¹ MPa and 4.123$\times$10⁻⁷ MPa respectively. The stress analysis using Solidworks is also attached in Table 10. This probs to be sufficient to endure the strength analysis of pneumatic shaft.

6.2.3. Equivalent strain:

The equivalent strain in pneumatic shaft is a function of combined axial effect, hoop and shear strain that occurs due to internal pressure or external loads applied to the shaft. In our analysis, the obtained equivalent strain is represented in Table 11 with analysis diagram.

6.2.4. Resultant displacement:

The resultant displacement of a pneumatic shaft signifies the overall deformation of the shaft under the influence of internal pressure or external force. The displacement simulates the behavior of the shaft under distinctive condition and predicts resultant deformations. It depends on geometry and material properties of the shaft, internal pressure or external pressure applied to the shaft. The resultant displacement during running and rest position is given in Table 12.

6.2.5. Factor of Safety:

The factor of safety (FoS) is the measure of how much stronger a structure or component is than it needs to be supported. In context of a pneumatic shaft in a prosthetic, FoS would depend on the type of materials used, expected load and stress and the safety margin. The FoS for pneumatic shaft in given in Table 13.

6.3. Analysis of Limb shell:

As per the previous load analysis and fixture, static analysis for the limb shell is performed. The main load that affects the limb cell is the upward and downward force generated by ankle and pneumatic assembly parts. The volumetric properties of limb shell treated as solid body is given in Table 14.

Based on that static analysis of limb shell, the maximum and minimum von-mises stress generated is 5.664$\times$10² MPa and 1.378$\times$10⁻¹ MPa respectively. Similarly, maximum, and minimum equivalent strain generated on the limb cell is 3.37$\times$10⁻⁷ and 3.496$\times$10⁻⁹ respectively. The maximum resultant displace is occurred while running which is of 2.325$\times$10⁻⁵ mm. The fixture detail of the limb shell is given in Table 15.

The result shows that the limb cell model is strong enough to endure the static strngth test. The respective analysis is depicted in Fig. 8, Fig. 9, and Fig. 10.

The simulations of each part and static analysis are developed based on the model in Solidworks software treated as a solid body. From the complete simulation, the volumetric provides the key features to facilitate these new controls for analyzing and referring for improvement and weight of the proposed model comes up with a lower range of 1.5 to 2.3 kg than commercially available in market (ranges from 1 kg to 5 kg (Sundararaj and Subramaniyan (2021))). The static analysis of different parts of the prototype model using Solidworks shows effectiveness and robustness of the model and the overall acceptance of the prototype. The market price of all the considered materials is very low. Since the design of prosthetic does not consume complex geometry, the cost associated with the manufacturing process may get reduced. The exact manufacturing cost for this prosthetic is not evaluated in this study.

This study represents a modern generation and approvingly operational pneumatic prosthetic leg. The structural design of prosthetic leg including the limbs and knee joint was optimized through a synthetic design approach. This paper presents the structural design of prosthetic leg prototype using AutoCAD, Solidworks and analyze as well as simulate the behavior of prototype model under different static loads based on Finite Element Analysis (FEA) approach. The static test such as stress, strain, deformation, and factor of safety for each main part is performed to measure the relationship between output pressure and input pressure. The simulation results demonstrate that the prosthetic leg followed the trajectory satisfactorily and may be a good choice for users. This new technology could potentially improve the quality of lime for amputees by providing them with more functional and comfortable prosthetic limbs. The authors only proposed silico (computer) based model for this study. The design and drafting of the model is done using AutoCAD (version 2019) and Solidworks (version 2016). Further, static analysis of the prototype model is evaluated using Solidworks simulation (version 2016). Real-life analysis of prototype model is not discussed in this paper.

For future study, the authors may focus on the cost analysis and further simulation of the proposed prosthetic leg. The above knee prosthetic can also be a suitable choice for simulation analysis in future aspects.

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Tables 1 to 15 are available in the Supplementary Files section.

No competing interests reported.

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Structural Design and Analysis of Below-knee Prosthetic Leg in Mechanical System

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Abstract

Background

Research Design and Methods:

Results

Limitations:

Conclusion

Figures

1. Introduction

2. Literature Review

2.1. Design of the Below-knee Prosthetic Leg

2.2. The Impact of Ergonomics in Prosthetic Design

2.3. The use of Simulation Process

3. Methodology

3.1. Problem Identification:

3.2. Brainstorming:

3.3. Ergonomics and Anthropometry in prototype design:

3.4. Design consideration:

3.4.1. Torque analysis:

4. Finite Element Analysis (FEA) Method

5. Material selection and properties

6. Experimental Simulation and Result Analysis

6.1. Analysis of Ankle assembly part:

6.1.1. Load and Fixtures:

6.1.2. Von-mises stress:

6.1.3. Equivalent strain:

6.1.4. Resultant displacement:

6.1.5. Factor of safety:

6.2. Analysis of Pneumatic Assembly:

6.2.1. Load and Fixtures:

6.2.2. Von-mises stress:

6.2.3. Equivalent strain:

6.2.4. Resultant displacement:

6.2.5. Factor of Safety:

6.3. Analysis of Limb shell:

7. Conclusion

References

Tables

Additional Declarations

Supplementary Files

Status:

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