Annulus fibrosus functional extrafibrillar and fibrous mechanical behaviour: experimental and computational characterisation.

Marlène Mengoni,Sebastien N F Sikora,Diane E Gregory,Fernando Y Zapata-Cornelio,Ruth K Wilcox,Oluwasegun Kayode

doi:10.1098/rsos.170807

Marlène Mengoni, Sebastien N F Sikora + Show 4 more

Open Access

https://doi.org/10.1098/rsos.170807

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Abstract

The development of current surgical treatments for intervertebral disc damage could benefit from virtual environment accounting for population variations. For such models to be reliable, a relevant description of the mechanical properties of the different tissues and their role in the functional mechanics of the disc is of major importance. The aims of this work were first to assess the physiological hoop strain in the annulus fibrosus in fresh conditions (n = 5) in order to extract a functional behaviour of the extrafibrillar matrix; then to reverse-engineer the annulus fibrosus fibrillar behaviour (n = 6). This was achieved by performing both direct and global controlled calibration of material parameters, accounting for the whole process of experimental design and in silico model methodology. Direct-controlled models are specimen-specific models representing controlled experimental conditions that can be replicated and directly comparing measurements. Validation was performed on another six specimens and a sensitivity study was performed. Hoop strains were measured as 17 ± 3% after 10 min relaxation and 21 ± 4% after 20–25 min relaxation, with no significant difference between the two measurements. The extrafibrillar matrix functional moduli were measured as 1.5 ± 0.7 MPa. Fibre-related material parameters showed large variability, with a variance above 0.28. Direct-controlled calibration and validation provides confidence that the model development methodology can capture the measurable variation within the population of tested specimens.

Highlights

Degeneration or trauma of intervertebral discs accounts for almost half of causal diagnosis of low back pain [1]
Material parameters are often derived from experimental data of excised tissue, e.g. the behaviour of the fibres [4,9,10,11,12] or the extrafibrillar matrix (EFM) [11,12,13] in the annulus fibrosus have been derived from mechanical tests performed on specimens of single lamellae that have been prepared so that load is either oriented in the direction of the fibres or perpendicular to them
Physiological hoop strain in the annulus fibrosus was measured in fresh tissue for the first time, and a functional or physiological mechanical behaviour was defined for its extrafibrillar matrix

Summary

Introduction

Degeneration or trauma of intervertebral discs accounts for almost half of causal diagnosis of low back pain [1]. It can provide systematic methods to analyse the outcome of a therapy and its potential for success, to inform experimental testing and target key factors needing control, to test scenario accounting for patient-specific or disease-specific changes in anatomy and tissue behaviour For such models to be reliable in a clinical or product development environment, a relevant description of the mechanical properties of the different tissues and their role in the functional mechanics of the disc is of major importance. Material parameters are often derived from experimental data of excised tissue, e.g. the behaviour of the fibres [4,9,10,11,12] or the extrafibrillar matrix (EFM) [11,12,13] in the annulus fibrosus have been derived from mechanical tests performed on specimens of single lamellae that have been prepared so that load is either oriented in the direction of the fibres or perpendicular to them. The role of residual stress or pre-strain in soft tissues has been highlighted early in the literature [14] but only limited work has been done on evaluating pre-strain in the annulus [15,16], and, to the authors’ knowledge, none on fresh tissue

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