Static structural testing of trans-tibial composite sockets

Numerical simulation and experimental testing for static failure prediction in additively manufactured below-knee prosthetic sockets

The socket of a transtibial prosthesis is a structural part customized to a patient's amputated residual lower limb. The free-form geometry of the socket can be suitable for additive manufacturing (AM) to save time and cost. However, the mechanical fracture of additively manufactured lower limb prostheses is not yet fully understood. A novel experimental method and numerical approach by finite element method (FEM) to test the strength and fracture behavior of a lower limb prosthetic socket of acrylonitrile butadiene styrene (ABS), reverse-engineered using computer-aided design (CAD) from the actual amputee's residual limb and manufactured using fused filament fabrication (FFF) are proposed in the present work. The mechanical behavior, von Mises stress distribution, and the damage status of layered AM sockets of different thicknesses were simulated by FEM using Hashin's transversely isotropic mechanical damage model, initially developed for composite materials. The experimental work showed that the fracture failure initiated at the corner of the lobe in the 4 mm thickness socket at a failure load of 918.5 N. The FEM results predicted this failure load to be 896.6 N, with only a 2.45% error as compared to the experiment. The failure loads predicted by FEM in the sockets with thicknesses of 3, 5, and 6 mm were 618.1, 1008.6, and 1105.2 N, respectively. The present work provides a dependable method for testing a below-knee prosthetic socket against static failure and arriving at a factor-of-safety (FoS) based socket thickness selection for any amputee.

Strength Assessment of PET Composite Prosthetic Sockets

A prosthesis is loaded by forces and torques exerted by its wearer, the amputee, and should withstand instances of peak loads without failure. Traditionally, strong prosthetic sockets were made using a composite with a variety of reinforcing fibres, such as glass, carbon, and Kevlar. Amputees in less-resourced nations can lack access to composite prosthetic sockets due to their unavailability or prohibitive cost. Therefore, this study investigates the feasibility of polyethylene terephthalate (PET) fibre-reinforced composites as a low-cost sustainable composite for producing functional lower-limb prosthetic sockets. Two types of these composites were manufactured using woven and knitted fabric with a vacuum-assisted resin transfer moulding (VARTM) process. For direct comparison purposes, traditional prosthetic-socket materials were also manufactured from laminated composite (glass-fibre-reinforced (GFRP)), monolithic thermoplastic (polypropylene (PP) and high-density polyethylene (HDPE)) were also manufactured. Dog-bone-shaped specimens were cut from flat laminates and monolithic thermoplastic to evaluate their mechanical properties following ASTM standards. The mechanical properties of PET-woven and PET-knitted composites were found to have demonstrated to be considerably superior to those of traditional socket materials, such as PP and HDPE. All the materials were also tested in the socket form using a bespoke test rig reproducing forefoot loading according to the ISO standard 10328. The static structural test of sockets revealed that all met the target load-bearing capacity of 125 kg. Like GFRP, the PETW and PETK sockets demonstrated higher deformation and stiffness resistance than their monolithic counterparts made from PP and HDPE. As a result, it was concluded that the PET-based composite could replace monolithic socket materials in producing durable and affordable prostheses.

Static strength of lower-limb prosthetic sockets: an exploratory study on the influence of stratigraphy, distal adapter and lamination resin

Knowledge about the mechanical properties of lower-limb prosthetic sockets fabricated with resin infusion lamination and composite materials is limited. Therefore, sockets can be subject to mechanical failure and over-dimensioning, both of which can have severe consequences for patients. For this reason, an exploratory study was conducted to analyze the effect of stratigraphy (layup and fibers), matrix (resin) and mechanical connection (socket distal adapter) on socket static strength, with the objectives of: 1) implementing a mechanical testing system for lower-limb prosthetic sockets based on ISO 10328:2016 and provide the mechanical design of the loading plates, 2) apply the testing system to a series of laminated sockets, and 3) for each type of distal adapter, identify the combinations of stratigraphy and matrix with acceptable strength and minimum weight. Twenty-three laminated sockets were produced and tested. Sixteen met the required strength, with ten exhibiting an excessive weight. Among the remaining six, four combinations of stratigraphy and resin were identified as best option, as they all overcame ISO 10328 P6 loading level and weighted less than 600 g. The selected stratigraphies had limited or absent amount of Perlon stockinettes, which seems to increase weight without enhancing the mechanical strength. Sockets based on Ossur MSS braids and connector show the best compromise between strength and weight when the amount of carbon braids is halved.

Numerical Analysis of a Transtibial Prosthesis Socket Using 3D-Printed Bio-Based PLA

Lower-limb prosthesis design and manufacturing still rely mostly on the workshop process of trial-and-error using expensive unrecyclable composite materials, resulting in time-consuming, material-wasting, and, ultimately, expensive prostheses. Therefore, we investigated the possibility of utilizing Fused Deposition Modeling 3D-printing technology with inexpensive bio-based and bio-degradable Polylactic Acid (PLA) material for prosthesis socket development and manufacturing. The safety and stability of the proposed 3D-printed PLA socket were analyzed using a recently developed generic transtibial numeric model, with boundary conditions of donning and newly developed realistic gait cycle phases of a heel strike and forefoot loading according to ISO 10328. The material properties of the 3D-printed PLA were determined using uniaxial tensile and compression tests on transverse and longitudinal samples. Numerical simulations with all boundary conditions were performed for the 3D-printed PLA and traditional polystyrene check and definitive composite socket. The results showed that the 3D-printed PLA socket withstands the occurring von-Mises stresses of 5.4 MPa and 10.8 MPa under heel strike and push-off gait conditions, respectively. Furthermore, the maximum deformations observed in the 3D-printed PLA socket of 0.74 mm and 2.66 mm were similar to the check socket deformations of 0.67 mm and 2.52 mm during heel strike and push-off, respectively, hence providing the same stability for the amputees. We have shown that an inexpensive, bio-based, and bio-degradable PLA material can be considered for manufacturing the lower-limb prosthesis, resulting in an environmentally friendly and inexpensive solution.

Mechanical testing of transtibial prosthetic sockets: A discussion paper from the American Orthotic and Prosthetic Association Socket Guidance Workgroup

BACKGROUND

The advent of novel manufacturing technologies, materials, and socket design concepts could introduce risks to prosthetic limb users, as the existing knowledge base for safe fabrication may not apply. Moreover, although structural test standards exist for mass-produced prosthetic components, they are not applicable to prosthetic sockets.

METHODS

The "AOPA Socket Guidance Workgroup" was formed in 2020 to provide the prosthetic community with evidence-based clinical best practices and methods in the field of prosthetic socket structural analysis. This multidisciplinary expert workgroup undertook a critical analysis of the knowledge gaps regarding the requirements for mechanical testing of lower limb prosthetic sockets.

RESULTS

The Workgroup identified knowledge gaps in 4 domains. Domain 1 describes the shape and composition of a mock residual limb, required to support and generate in vivo representative loading within the socket. Domain 2 concerns prosthetic socket coordinate systems and alignment. Domain 3 regards the components and requirements of test specimens. Finally, Domain 4 considers test conditions, loading parameters, and acceptance criteria.

CONCLUSIONS

This paper describes these knowledge gaps in detail and recommends potential solution approaches based on literature review, group consensus around existing knowledge, or the formation of new study groups to fill each knowledge gap. Our intent is for the recommendations arising from this paper to support the community (e.g., researchers in the clinic, academia, industry, and funders) in addressing these knowledge gaps.

3D printed transtibial prosthetic sockets: A systematic review

The prosthetic socket, which transfers load from the residual limb to the prosthesis, is an integral part of the prosthesis. 3D printing has emerged as a potentially viable alternative to traditional fabrication for producing sockets that effectively transfer loads. We conducted a systematic review to better understand the current state of this newer fabrication method, with a focus on the structural integrity of 3D printed sockets and factors that can affect the strength of 3D printed sockets when tested using ISO 10328 standards. Literature searches were carried out in five databases (PubMed, Scopus, CINAHL, Web of Science and Google Scholar). Two reviewers independently performed the literature selection, quality assessment, and data extraction. A total of 1023 unique studies were screened in accordance with inclusion and exclusion criteria. Of 1023 studies, 12 studies met all inclusion criteria, with failure data for 15 3D-printed sockets and 26 standard laminated sockets. Within 3D printed sockets, the addition of composite materials such as carbon fiber particles and distal reinforcement using a compositing infill technique appears to improve socket strength. In light of the considerable amount of heterogeneity between studies in terms of materials and alignment used, the absolute values for failure could not be established for 3DS nor directly compared between 3DS and LCS. However, there is some evidence that the probability of a failure at a given load may be comparable between 3DS and LCS up to the P8 level. For all sockets, whether a laminated composite socket or a 3D printed socket, failure mainly occurred at the distal end of the socket or the pyramid attachment, which is consistent with the ISO testing protocol. Improving the strength of the 3D printed sockets through design modifications at the distal end and implementing emerging printing technologies could help to promote 3D printed sockets as a viable option, particularly when cost or access to care is limited.

Digital healthcare technologies: Modern tools to transform prosthetic care

INTRODUCTION

Digital healthcare technologies are transforming the face of prosthetic care. Millions of people with limb loss around the world do not have access to any form of rehabilitative healthcare. However, digital technologies provide a promising solution to augment the range and efficiency of prosthetists.

AREAS COVERED

The goal of this review is to introduce the digital technologies that have the potential to change clinical methods in prosthetic healthcare. Our target audience are researchers who are unfamiliar with the field of prostheses in general, especially with the newest technological developments. This review addresses technologies for: scanning of amputated limbs, limb-to-socket rectification, additive manufacturing of prosthetic sockets, and quantifying patient response to wearing sockets. This review does not address biomechatronic prostheses or biomechanical design practices.

EXPERT OPINION

Digital technologies will enable affordable prostheses to be built on a scale larger than with today's clinical practices. Large technological gaps need to be overcome to enable the mass production and distribution of prostheses digitally. However, recent advances in computational methods and CAD/CAM technologies are bridging this gap faster than ever before. We foresee that these technologies will return mobility and economic opportunity to amputees on a global scale in the near future.

Multi-scale semi-analytical model for fatigue life prediction of trans-tibial prosthetic sockets

The trans-tibial socket is an essential component of the prosthesis that connects it to the residual limb. Socket misalignments and permanent deformations reduce the comfort of the amputee. In order to forestall such issues, sufficient information about the socket lifespan needs to be acquired, which is fairly difficult given the lack of calculation methods that estimate the fatigue life in the literature. In this paper a semi-analytical model is proposed based on experimental results. It highlights the effects of the amputee average daily walking time on the socket fatigue life. A proportionality relationship is demonstrated linking the Burgers model parameters at the structural scale to those at the macroscopic scale. Hence, it becomes possible to evaluate the permanent deformation that induces a misalignment in the socket. These results are useful for the designers to predict the fatigue life of the socket, and also for clinicians to monitor the mechanical degradation of the trans-tibial socket and schedule maintenance or replacement.

Preliminary Material Evaluation of Flax Fibers for Prosthetic Socket Fabrication

Composite prosthetic sockets are typically made of fiberglass or carbon fiber. These fibers have good mechanical properties, but relatively poor vibration damping. Flax fibers are claimed to have exceptional vibration damping properties, with the added benefit of being a natural renewable resource and a cost-effective alternative to synthetic fibers. Flax fibers could prove beneficial for prosthetic sockets, providing lightweight sockets that reduce vibrations transmitted to the body during movement. This research used impact testing (impulse hammer and custom drop tower) on flat and socket shaped composite samples to evaluate composite layer options. Sample vibration dissipation was measured by a combination of accelerometers, load cells, and a dynamometer. Composite sockets made purely of flax fibers were lighter and more efficient at damping vibrations, reducing the amplification of vibrations by a factor of nearly four times better than sockets made purely of carbon fiber. However, the bending stiffness, elastic moduli, and flexural strength of flax sockets fabricated using the traditional socket manufacturing method were found to be ten times lower than theoretical values of flax composites found in the literature. By increasing fiber volume fraction when using the traditional socket manufacturing method, the composite's mechanical properties, namely, vibration damping, could improve and flax fiber benefits could be explored further.

Epoxy based sandwich composite using three-dimensional integrally woven fabric as core strengthened with additional carbon face-sheets

Sandwich composites are three-dimensional (3D) composite structures that offer higher stiffness with overall low density. However, they suffer from low strength; thus, not suited for load bearing applications. In this work, an attempt is made to develop a high strength lightweight sandwich composite suited for load-bearing applications. A sandwich composite based on 3D integrally woven fabric with thickness 3 mm as the core and strengthened with additional 2x2 twill woven carbon fabric face-sheets is reported. The samples were manufactured by wet hand lay co-lamination process using Araldite® LY 1564 epoxy as the matrix polymer and with fiber fraction of 50% by weight. The number of additional carbon face-sheets over the core was varied from two to eight in steps of two. The composite samples were experimented under three-point bending and edgewise compression tests to determine the flexural and compressive strengths in both warp and weft directions. The weft direction samples yielded higher flexural and compressive strengths due to the continuous arrangement of the core pile yarn. The samples with six carbon face-sheets tested along the weft direction offered the highest specific strengths of ~409 kN m/kg and 259 kN m/kg in bending and compression tests. Similarly, the flexural strength was ~340 MPa, and compressive strength was ~217 MPa. A detailed fractography study revealed no core crushing or compression failure of the core during bending tests.

Physical sciences

In the original edition of Prosthetics and Orthotics International, Dr Sidney Fishman identified what he anticipated as foundational educational needs for the emerging field of clinical prosthetics and orthotics. Within the broader construct of the physical sciences, this included mathematics, physics, chemistry, biomechanics, and material sciences. The clinical application of these disciplines to expanding the collective understanding within the field is described, including the biomechanics of able-bodied and prosthetic gait, the material science of socket construction, the physics of suspension and load distribution, and the engineering of prosthetic components to mimic human biomechanics. Additional applications of the physical sciences to upper limb prosthetics and lower limb orthotics are also described. In contemplating the continued growth and maturation of the field in the years to come, mechatronics and statistics are suggested as future areas where clinical proficiency will be required.

Using mechanical testing to assess the effect of lower-limb prosthetic socket texturing on longitudinal suspension

To function effectively, a lower limb prosthetic socket must remain securely coupled to the residual limb during walking, running and other activities of daily living; this coupling is referred to as suspension. When this coupling is insufficient longitudinal pistoning of the socket relative to the residual limb occurs. Increasing friction of the socket/liner interface may improve socket suspension and textured sockets may be fabricated relatively easily with 3D printing. The aim of this study was to investigate longitudinal displacement of sockets with different types of textures under two suspension conditions: passive suction and active vacuum. In order to do this, we developed a mock residual limb and mechanical testing protocol. Prosthetic sockets, 14 textured sockets and an Original Squirt-Shape (OSS) Socket, were fabricated from polypropylene copolymer using the Squirt-Shape^™ 3D Printer and compared to a smooth socket thermoformed from polypropylene copolymer. Sockets were mounted onto a dual durometer mock residual limb and subjected to four levels of distraction forces (100 N, 250 N, 500 N and 650 N) using a hydraulic material testing system. There was a statistically significant three-way interaction between suspension, force level and texture (p < 0.0005). Longitudinal displacements between textured and reference sockets, for all force levels and both suspension conditions, were significantly different (p < 0.0005). Using these newly developed mechanical testing protocols, it was demonstrated that texturing of polypropylene copolymer sockets fabricated using Squirt-Shape significantly decreased longitudinal displacements compared to a smooth socket. However, none of the novel textured sockets significantly reduced longitudinal displacement compared to the OSS socket under passive suction suspension.

Using mechanical testing to assess texturing of prosthetic sockets to improve suspension in the transverse plane and reduce rotation

Creating a secure and comfortable linkage between the residual limb and prosthetic socket in persons with lower limb amputation is a critical factor for successful rehabilitation, including ambulation and other activities of daily living. Unwanted rotation within the socket can be a clinical problem for prosthesis users. One way of addressing issues experienced with transverse plane control of the socket may be through increased friction interface forces. It has been proposed that friction at the residual limb/socket interface may be increased by adding texture to interface components. Three-dimensional (3D) printing may be used to fabricate sockets with texture patterns added to the inner socket surface. Hence, the aim of this study was to investigate the effects of socket texturing on transverse plane rotation of the socket on a mock residual limb under two suspension conditions: passive suction and active vacuum. To conduct this study, we developed a mechanical testing protocol as no standardized tests currently exist to assess prosthetic sockets. Sockets with 14 different texture patterns were fabricated using the Squirt-Shape^™ 3D printer. Textured sockets were compared to an Original Squirt-Shape (OSS) socket and a smooth thermoformed socket. Sockets were fitted with a mock residual limb and bi-axially loaded to 350 N compression with simultaneous rotation (2.5°, 5° and 7.5°) using a custom rotation assembly attached to a uniaxial hydraulic material testing system. There was a statistically significant three-way interaction between suspension, angle and texture (p < 0.0005). Torques between textured and reference sockets, for all rotation angles and both suspension conditions, were significantly different (p < 0.0005). Using newly developed testing protocols, it was demonstrated that some texture patterns significantly increased torque (i.e., resistance against unwanted rotation) in the transverse plane compared to both OSS and smooth sockets, especially for passive suction. Rotation testing of sockets may provide insight into socket design to improve suspension in the transverse plane.

Prosthetic limb sockets from plant-based composite materials

BACKGROUND

There is a considerable demand for lower limb prostheses globally due to vascular disease, war, conflict, land mines and natural disasters. Conventional composite materials used for prosthetic limb sockets include acrylic resins, glass and carbon fibres, which produce harmful gasses and dust in their manufacture.

OBJECTIVES

To investigate the feasibility of using a renewable plant oil-based polycarbonate-polyurethane copolymer resin and plant fibre composite, instead of conventional materials, to improve safety and accessibility of prosthetic limb manufacture.

STUDY DESIGN

Experimental, bench research.

METHODS

Test pieces of the resin with a range of plant fibres (10.0% by volume) were prepared and tensile strengths were tested. Test sockets of both conventional composite materials and plant resin with plant fibres were constructed and tested to destruction.

RESULTS

Combinations of plant resin and either banana or ramie fibres gave high tensile strengths. The conventional composite material socket and plant resin with ramie composite socket failed at a similar loading, exceeding the ISO 10328 standard. Both wall thickness and fibre-matrix adhesion played a significant role in socket strength.

CONCLUSIONS

From this limited study we conclude that the plant resin and ramie fibre composite socket has the potential to replace the standard layup. Further mechanical and biocompatibility testing as well as a full economic analysis is required.

CLINICAL RELEVANCE

Using readily sourced and renewable natural fibres and a low-volatile bio-resin has potential to reduce harm to those involved in the manufacture of artificial limb sockets, without compromising socket strength and benefitting clinicians working in poorer countries where safety equipment is scarce. Such composite materials will reduce environmental impact.

Fatigue test of low-cost flexible-shank monolimb trans-tibial prosthesis

Monolimb refers to a kind of trans-tibial prostheses with the socket and shank moulded into one piece of thermoplastic material. If properly designed, the shank of a monolimb can deflect which may compensate for the lost ankle plantarflexion and dorsiflexion to some extent. However, provision of shank flexibility is usually accompanied by reduced structural strength of the entire prosthesis. In the recent work using finite element analysis and the Taguchi method, the dimensions of the shank for the monolimb were derived which aimed at giving high shank flexibility and reasonable strength to resist static load. Yet, fatigue testing has not been performed. Fatigue failure may happen when a relatively low level of load is applied repeatedly. This study aimed to document the fatigue life of two flexible-shank monolimbs, by applying cyclic force of 800 N at the forefoot region for 500,000 cycles. Results showed that the design of the foot bolt adaptor played an important role in the structural integrity of the monolimb. One monolimb completed the fatigue test of 500,000 cycles without visual material yield, but with 3.8 degrees change in dorsiflexion angle when the load was removed.

A numerical approach to evaluate the fatigue life of monolimb

Monolimb refers to a transtibial prosthesis with the prosthetic socket and the shank being molded into one piece of thermoplastic material. Shank flexibility of a monolimb can improve gait and comfort. However, fatigue failure of monolimbs under cyclic walking load is an important concern. This study is to evaluate the fatigue life of a monolimb designed for a transtibial amputee, based on finite element analysis, the statistical Miner's rule and reliability analysis. Stress uncertainty due to modeling error and the scatter in fatigue test data were considered. Results indicated that the accuracy of fatigue life evaluation of monolimb depends significantly on the precision of stress estimation. In addition, relationship between fatigue failure probability and the number of walking steps was suggested providing a reference for clinicians to determine the interval of the inspection for the monolimb.

Structural integrity of polypropylene prosthetic sockets manufactured using the polymer deposition technique

Rapid prototyping (RP) technology has been used recently as a means for automated socket fabrication. Although the technology has proven to be promising and has truly automated the socket fabrication process, the structural integrity of RP sockets remains questionable. For the long term, unsupervised use of these 'unconventional' sockets, their material properties and structural integrity must be determined. This study investigated the structural integrity of polypropylene sockets manufactured using a polymer deposition technique, in which a socket is formed by a continuous strand of partially melted polypropylene that is spirally deposited according to the socket's cross-sectional contour. To investigate the problem of delamination of the socket, the tensile properties of the socket material were determined according to ASTM D638-99. The ultimate tensile strength was found to be approximately 13-23 per cent lower than that of polypropylene sheets that are at present normally used for socket fabrication. In order to improve the load-bearing capacity of the socket, it was reinforced using a double-wall arrangement at the distal region, where failure normally occurs. The structural integrity of the complete prosthesis was then investigated according to ISO 10328 (loading condition II). The prosthesis passed the static loading test registering only 12 mm permanent deformation, and it successfully completed a preliminary cyclic test of 250,000 cycles with no observable failure.