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Evolutions in Mechanical Engineering

An Investigation into a Spring Based Ankle Joint Designed for Lower Limb Prosthesis

Sayem Hossain Bhuiyan*

School of Mechanical Engineering, Engineering Campus, University of Science Malaysia, Malaysia

*Corresponding author:Sayem Hossain Bhuiyan, Senior Lecturer, School of Mechanical Engineering, Engineering Campus, University of Science Malaysia, Malaysia

Submission: December 14, 2020;Published: March 26, 2021

DOI: 10.31031/EME.2021.03.000561

ISSN 2640-9690
Volume3 Issue3

Abstract

A spring based ankle joint is designed to improve the performance of mechanical type prostheses. The torsional and compression springs with the help of some foot plate arrangement is used to enable the prosthesis in producing desired flexion and extension and also storing and returning energy by the ankle joint during the gait cycle. The motion analysis and Finite-Element Analysis (FEA) of ankle joint components are carried out to assess the feasibility of the design. The FEA results are then compared with the real data obtained from the literature for a similar subject. The pattern of motion analysis results has shown a great resemblance with the gait cycle of a healthy biological limb, and the design is identified to be safe with factory of safety of 1.9 from the FEA results.

Keywords: Prosthesis; Ankle joint; Biomechanics; Kinematics; Kinetics; Finite element analysis

Introduction

The typical prosthetic ankle joints are either hinge type or single piece solid plate type [1]. These are cheap and robust; however, they have limitation in providing adequate stability (for hinge type), flexion and extension (for single plate type) to the prosthesis due to their construction. The advanced type of electronically controlled ankle joints are, therefore, very good at producing desired movement with enough stability, which are expensive. The proposed design of an ankle joint will enable the mechanical type of ankle joint overcoming the limitation of stability, flexion and extension within an affordable price. This will enhance the range of motion of the mechanical type of prosthesis without incorporating any expensive electronic devices into the ankle joint. Unlike to the typic al mechanical prosthesis, the new design will allow the prosthesis to bend forward and backward to any desired angle with enough stability. In addition to this, the proposed design will enable the prosthesis absorbing more shocks than any other type of ankle joints available in the market. A spring based ankle joint will make the prosthesis imitating the movement of the biological limb closely further.
The hip, knee and ankle joints are essentially involved in balancing and controlling the lower-limb locomotion during different movements in walking. In recent transtibial amputees, a reduced reliance on the hip strategy and an increased utilization of the ankle strategy is expected over time during dynamic balancing [2]. The Natural Ankle-Foot System (NAFS) plays a major role in the performance of human locomotion, contributing to the maintenance of upright support and generation of forward momentum [3]. The ankle motion during ground-level walking is quasi-periodic and is usually divided into two main phases: the stance phase and the swing phase. An ideal gait cycle is typically defined as starting with the heel strike of one foot and ending at the next heel strike of the same foot. The shape and duration of the gait cycle varies from step to step and it strongly depends on the gait speed, subject morphology, subject weight, and terrain conditions [4]. Prostheses are required to imitate the gait cycle of the healthy biological limb to move in a more natural way. To mimic unimpaired ankle joint function during gait, prosthetic devices for individuals with Transtibial Amputation (TTA) must approximate unimpaired ankle Range of Motion (ROM), torque, and power with similar synchrony and magnitude [5,6]. To recreate the gait cycle movements in the prosthesis, the ankle joint construction, it kinematics and kinetics are crucially important [7]. Ankle kinematics indicates the movement of the ankle component in space without consideration of the forces that cause that movement, whereas the ankle kinetics indicate the movement and the forces that cause that movement [8]. The stiffness of prosthetic ankle is also identified contributing to balance control [9]. During levelground steady state walking, the natural ankle joint undergoes a phase of energy storage or absorption [i.e., negative work done via eccentric muscle activity] during early to midstance, followed by a phase of energy generation [i.e., positive work done via concentric muscle activity] during late stance [10]. The efficiency of the ESR prostheses depends on the ratio of energy storing and returning capacity [3,11]. A closer replication of kinematics and kinetics of natural ankle by the prosthetic ankle joint would make it more effective. A compression and a torsional spring-based ankle joint would help to obtain an ESR prosthesis by storing energy during compression and twisting and releasing during extension and decoiling respectively. Besides, these springs will help the amputee to overcome the difficulties in producing required angle of rotation in their prosthetic feet without affecting the stiffness. The compression spring at the foot will allow the amputee to absorb more shock, whereas the torsion spring will enable the ankle joint to rotate in a controlled way to any desired angle without demanding any additional setup.

Design of Spring Base D Ankle Joint

The design of the spring-based ankle joint is unlike to any other ankle joint currently available in the market. The typical mechanical ankle joints for normal walking are hinge type or fixed type joint, whereas the ankle joints for running and sprinting are single piece solid plate type. The newly designed ankle joint is comprised of number of components-one shank rod, one hinge knuckle, one bushing pin, two ball bearing, one sleeve bearing, one upper foot plate, one lower foot plate , one torsional spring, two torsional spring holder, one compression spring, one compression spring holder, one compression spring guide and some bolts and nuts. The lower foot plate and upper foot plate are bolted together at frontal end and there is a compression spring positioned between two plates at the posterior end. The compression spring is mounted on a spring holder attached to the lower foot plate and being held by a spring guide attached to the upper foot plate from the top.
Due to the shape of the upper foot plate, it acts as a leaf spring. The foot plates and the compression spring assembly jointly behave as a shock absorber. At the same time, this assembly acts as an Energy Storing and Returning (ESR) system for the ankle joint. The hinge knuckle is mounted on the top of the upper foot plate where the shank rod is connected to a busing pin and two ball bearings from two opposite sides. A sleeve bearing is put in between the busing pin and shank rod connecter hole. This has been done to protect the busing pin from early damage by heavy load due to the amputee weight. The torsional spring is placed at the hinge joint of shank rod and knuckle with the help of two spring holders from two ends. The torsional spring provides enough stiffness to the ankle joint when rotate and also can store and return energy during compression and expansion respectively. In an attempt to replicate the functions of muscles and tendons in storing and returning energy, ankle joints are developed by utilizing the elasticity and stiffness characteristics of spring material, where energy is stored when the material deforms, and energy is returned when the material returns to its initial shape. Different views of the newly designed spring based ankle joint are shown in the Figure 1. The ankle-foot system of the prosthesis is enabled producing flexion at the ankle by introducing the torsional spring at the joint. The torsional spring of the ankle joint permit to generates any angle of rotation between 340 and -150 degrees of flexion, which is usual range of flexion of a healthy ankle joint [12,13]. The torsional spring movement is maximum at 340 of rotation, which is zero when the shank is upright. The compression spring installed in between the foot plates helps to absorb shocks and store energy during stance phase and return during swing phase. The mass of the knee-ankle-foot system is 3.5kg. The heel-toe length is 0.3m. The different components of the spring based ankle joint are shown in the following Figure 2.

Figure 1: Design of ankle joint a) frontal view, and b) isometric view.


Figure 2:The exploded view of the different ankle joint components.


The prosthetic ankle joint has been designed with an aim to upgrade the existing mechanical type of ankle joints, eliminate their limitations and thus to make them serving like a healthy natural ankle joint. The biomechanics of a healthy ankle joint has been imitated with an arrangement of some compression and torsional springs, and some foot plates of leaf spring. A greater range of flexion and extension had been achieved with help of some hinge joint incorporated in the ankle joint. The required stiffness and stability of the ankle joint have been obtained with the torsional spring introduced at the hinge joint of the prosthetic ankle. The shock absorption by the prosthesis has been achieved with the foot plates, and the compression spring placed in-between. The most important phenomenon, which makes a prosthesis moving like a healthy biological limb during walking is the features of Energy Storing and Returning (ESR). This property of ESR has been obtained with the resultant effect of compression and torsional springs. The ESR prosthetic foot absorbs shock and stores energy when loaded and then releases stored energy at push-off. Both the static and dynamic analyses of the ankle joint have been conducted to see the movement of the prosthetic joint. The finite-element analysis of the joint components has been carried out to predict the changes in the mechanical properties of the joint components and also their ability to withstand against the applied load. The viability of the design has been evaluated with a comparative study conducted between the simulation results, and the real data obtained from the literature for a subject with similar anthropometric variables.

Ankle Joint Analysis

The ankle-foot complex plays an important role in human locomotion. The ankle flexor and extensor muscles are crucial to provide vertical support and forward progression of the body. A closer imitation of the flexion and extension movements of the ankle joint would make the prosthesis moving in more natural way. For low speed walking, providing additional energy in ambulation of a prosthesis is not significant; however, for normal and fast walking speeds, providing additional energy by the ankle-foot arrangement for propulsion at the plantar flexion phase is crucially important. A motion analysis of the ankle joint would permit one to see the energy storing and returning capacity of the ankle-foot system and also the ability of the foot in following gait movement during walking.

Motion analysis of ankle joint

Motion analysis of the ankle joint is carried out to see the movements of the ankle-foot assembly and its different components under applied external load. The movements and forces calculated for the ankle-foot system will help to carry out a structural analysis of the ankle components and thus to ensure ankle performance. Two types of analysis are carried out in motion analysis-kinematic analysis and dynamic analysis.

Kinematic analysis: For kinematic analysis of the ankle joint, the angular displacement and angular velocity under applied force and torque are observed, which are then compared with the practical test results to see the deflections. At the stance phase of the gait cycle, no rotation is observed at the ankle joint. Then the entire load acting on the ankle joint is Y-directional, which is the body weight of the amputee, i.e. equal to the body weight of the amputee and the value of applied torque will be zero. However, at the swing phase, the applied load will be the Y-component of the amputee body weight for the accrued angle of rotation during flexion and extension movements, and the value of the applied toque will be the torque due to the X-component of amputee weight for that particular rotation angle. The variables used in the simulation are based on the features of subject, which are tabulated in Table 1. The weight of the subject is

Table 1:Anthropometrical variable of subjects.

𝑊𝑊= 𝑚𝑎=82.2𝐾𝑔∗9.81𝑚⁄𝑠2=806.38𝑁=𝐹=𝐴𝑝𝑝𝑙aa𝑒𝑑 𝑙𝑜𝑎𝑑.


Both the torsional and compression springs play significant role in creating movement at the ankle joint. In ankle joint, the torsional spring carries partial load of the user during swing phase of walking and also when to produce bending movement in the ankle joint, whereas the compression spring carries proportion of load at stance phase of gait cycle. The load applies to the torsional spring=the proportion of amputee weight shared by the torsional spring during swing phase and flexion and extension movements. During swing phase of a biological lower limb, the finger joints help to produce desired rotation in the foot. However, there is no finger joint incorporated in the proposed design, the ankle joint has to yield that movement in the foot arrangement.

The rotation angle varies depending on the mode and speed of walking and also on the type of activities performed. The maximum possible angles of rotation are considered as design paramet