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A fatigue assessment technique for modular and pre-stressed orthopaedic implants

A fatigue assessment technique for modular and pre-stressed orthopaedic implants
A fatigue assessment technique for modular and pre-stressed orthopaedic implants
Orthopaedic implants experience large cyclic loads, and pre-clinical analysis is conducted to ensure they can withstand millions of loading cycles. Acetabular cup developments aim to reduce wall thickness to conserve bone, and this produces high pre-stress in modular implants. As part of an implant development process, we propose a technique for preclinical fatigue strength assessment of modular implants which accounts for this mean stress, stress concentrating features and material processing.

A modular cup’s stress distributions were predicted computationally, under assembly and in-vivo loads, and its cyclic residual stress and stress amplitude were calculated. For verification against damage initiation in low-cycle-fatigue (LCF), the peak stress was compared to the material’s yield strength. For verification against failure in high-cycle-fatigue (HCF) each element’s reserve factor was calculated using the conservative Soderberg infinite life criterion.

Results demonstrated the importance of accounting for mean stress. The cup was predicted to experience high cyclic mean stress with low magnitude stress amplitude: a low cyclic load ratio (Rl = 0.1) produced a high cyclic stress ratio (Rs = 0.80). Furthermore the locations of highest cyclic mean stress and stress amplitude did not coincide. The minimum predicted reserve factor Nf was 1.96 (HCF) and 2.08 (LCF). If mean stress were neglected or if the stress ratio were assumed to equal the load ratio, the reserve factor would be considerably lower, potentially leading to over-engineering, reducing bone conservation.

Fatigue strength evaluation is only one step in a broader development process, which should involve a series of verifications with the full range of normal and traumatic physiological loading scenarios, with representative boundary conditions and a representative environment. This study presents and justifies a fatigue analysis methodology which could be applied in early stage development to a variety of modular and pre-stressed prosthesis concepts, and is particularly relevant as implant development aims to maximise modularity and bone conservation.
titanium, fatigue, soderberg, arthroplasty, preclinical analysis
1350-4533
72-80
Dickinson, A.S.
10151972-c1b5-4f7d-bc12-6482b5870cad
Browne, M.
6578cc37-7bd6-43b9-ae5c-77ccb7726397
Roques, A.C.
d1fa28ab-03a9-476c-abaa-0fcb22ece985
Taylor, A.C.
39974814-4868-4c73-a3fa-2adfa4be3e46
Dickinson, A.S.
10151972-c1b5-4f7d-bc12-6482b5870cad
Browne, M.
6578cc37-7bd6-43b9-ae5c-77ccb7726397
Roques, A.C.
d1fa28ab-03a9-476c-abaa-0fcb22ece985
Taylor, A.C.
39974814-4868-4c73-a3fa-2adfa4be3e46

Dickinson, A.S., Browne, M., Roques, A.C. and Taylor, A.C. (2014) A fatigue assessment technique for modular and pre-stressed orthopaedic implants. Medical Engineering & Physics, 36 (1), 72-80. (doi:10.1016/j.medengphy.2013.09.009).

Record type: Article

Abstract

Orthopaedic implants experience large cyclic loads, and pre-clinical analysis is conducted to ensure they can withstand millions of loading cycles. Acetabular cup developments aim to reduce wall thickness to conserve bone, and this produces high pre-stress in modular implants. As part of an implant development process, we propose a technique for preclinical fatigue strength assessment of modular implants which accounts for this mean stress, stress concentrating features and material processing.

A modular cup’s stress distributions were predicted computationally, under assembly and in-vivo loads, and its cyclic residual stress and stress amplitude were calculated. For verification against damage initiation in low-cycle-fatigue (LCF), the peak stress was compared to the material’s yield strength. For verification against failure in high-cycle-fatigue (HCF) each element’s reserve factor was calculated using the conservative Soderberg infinite life criterion.

Results demonstrated the importance of accounting for mean stress. The cup was predicted to experience high cyclic mean stress with low magnitude stress amplitude: a low cyclic load ratio (Rl = 0.1) produced a high cyclic stress ratio (Rs = 0.80). Furthermore the locations of highest cyclic mean stress and stress amplitude did not coincide. The minimum predicted reserve factor Nf was 1.96 (HCF) and 2.08 (LCF). If mean stress were neglected or if the stress ratio were assumed to equal the load ratio, the reserve factor would be considerably lower, potentially leading to over-engineering, reducing bone conservation.

Fatigue strength evaluation is only one step in a broader development process, which should involve a series of verifications with the full range of normal and traumatic physiological loading scenarios, with representative boundary conditions and a representative environment. This study presents and justifies a fatigue analysis methodology which could be applied in early stage development to a variety of modular and pre-stressed prosthesis concepts, and is particularly relevant as implant development aims to maximise modularity and bone conservation.

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More information

Accepted/In Press date: 18 September 2013
e-pub ahead of print date: 19 October 2013
Published date: 9 January 2014
Keywords: titanium, fatigue, soderberg, arthroplasty, preclinical analysis
Organisations: Bioengineering Group

Identifiers

Local EPrints ID: 357068
URI: http://eprints.soton.ac.uk/id/eprint/357068
ISSN: 1350-4533
PURE UUID: bdfa330a-aacf-406c-b9f8-e7dfef57e1de
ORCID for A.S. Dickinson: ORCID iD orcid.org/0000-0002-9647-1944
ORCID for M. Browne: ORCID iD orcid.org/0000-0001-5184-050X

Catalogue record

Date deposited: 03 Oct 2013 13:36
Last modified: 18 Feb 2021 17:08

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