A parametric design and optimization approach to enhance ...

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A Parametric Design and Optimization Approach to Enhance the Fatigue Life of a Male Pyramid Socket Adapter

P.A. le Roux; R.F. Laubscher

IPhD student, Department of Mechanical Engineering, University of Johannesburg, South Africa.
II Department of Mechanical Engineering, University of Johannesburg, South Africa.

ABSTRACT

This research paper presents a parametric design approach to optimize a male pyramid socket adapter for a transtibial prosthetic limb, aimed at enhancing its fatigue life. These adapters often experience premature failure in athletic scenarios. The study employs a parametric design and optimization method, analyzing the current design for structural integrity using finite element analysis (FEA) aligned with ISO code of practice load criteria. By identifying and enhancing high-stress areas, the method mitigates the maximum first principal stress while adhering to industry standards pertinent to pyramid socket adapter design. Experimental comparisons of the current and optimized designs, through low cycle fatigue testing, validate the optimization, revealing significant improvements in fatigue life for the optimized socket shape, in keeping with established SN curves for Ti6Al4V material. This parametric optimization technique demonstrates its effectiveness and applicability across various mechanical applications.

Additional keywords: Parametric design, Low-cycle fatigue testing, Transtibial prosthesis, Male pyramid socket adapter.

Nomenclature

PAD Peripheral arterial disease

SACH Solid-ankle-cushioned-heel

WHO World Health Organization

ISO International Organization for Standardization

FEA Finite Element Analysis

1 Introduction

As global average lifespans increase due to healthcare advancements, the demand for artificial limbs rises (Ziegler-Graham et al.). While modern medicine often prevents amputations via wound infection management, limb loss remains prevalent, largely due to severe injuries (National Health Services in England). The primary cause of amputation is Peripheral Artery Disease (PAD), which results in restricted blood flow to the limbs, potentially leading to gangrene or other complications (Johns Hopkins Medicine). PAD primarily affects adults aged 50-75, frequently linked to diabetes or atherosclerosis (Johns Hopkins Medicine).

Post-amputation, individuals can opt for prostheses that mimic natural limb functions, though considerable physiotherapy and rehabilitation are essential for adjustment. Prostheses typically require more energy to operate, as the residual limb compensates for the absence of bone and muscle (Health Net).

Transtibial (below-knee) amputation is the most common form of amputation today (National Health Services in England; Prosthetic & Orthotic care). By preserving the knee joint, patients can regain or even enhance activities pre-amputation.

A typical transtibial prosthesis consists of:

Socket: Serves as the interface between the patient and prosthesis, tailored to ensure the transfer of body weight without discomfort.

Pylon and modular components: These replicate bone functions, crafted from materials like titanium or aluminum depending on patient needs.

Foot: Comprises of numerous bones and ligaments, adapting based on patient weight and activity level.

The modular components connecting the pylon to the foot usually remain stable in adults, but children experience rapid changes that affect prosthesis length and alignment, potentially leading to premature failures without adjustments.

Heavier patients generally face reduced service life for prosthetic components due to increased stress. Recent studies indicate significant portions of the population are overweight or obese (World Health Organization). The minimum required lifespan for lower limb structures is three million loading cycles, in accordance with ISO standards. Premature failures often result from alignment issues or increased physical activities.

The male pyramid socket adapter is a universal component for prosthetic limbs, typically made from stainless steel or titanium, determined by specific patient requirements. It connects the residual limb to the pylon and is critical for structural stability.

Common failure scenarios associated with socket adapters were investigated, focusing on components sourced from a prosthetics lab. These investigations often reveal that misalignment and excessive activity contribute to premature failures.

Overalignment can lead to separation between the adapter components, resulting in elevated stress. Evidence of this includes wear marks from localized movement and failure to transfer loads correctly through the designated load lines.

Typical socket adapter failures were largely attributed to fatigue, with hairline cracks forming due to repeated loads, weakening structural integrity leading to eventual fractures.

3 Parametric Design Optimization

The process begins with establishing design aims such as improving structural integrity and fatigue life. Constraints include maintaining weight and ensuring geometric compatibility. The evaluation commences via FEA based on various load scenarios as dictated by international codes.

Notable findings from the FEA indicated high-stress concentrations at the adapter's apex, prompting design iterations aimed at reducing these stress levels through geometric adjustments.

Through optimized design iterations, significant reductions in maximum stresses were achieved (from 774 MPa to 578 MPa). The finalized design demonstrated an improvement in fatigue life, ensuring that the adapter can withstand expected loads during typical use.

The optimization success indicates applicability across a wider range of mechanical components, potentially benefitting various engineering and prosthetic designs.

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