Elsevier

Journal of Biomechanics

Volume 67, 23 January 2018, Pages 24-31
Journal of Biomechanics

Tensile behaviour of individual fibre bundles in the human lumbar anulus fibrosus

https://doi.org/10.1016/j.jbiomech.2017.11.016Get rights and content

Abstract

Disc degeneration is a common medical affliction whose origins are not fully understood. An improved understanding of its underlying mechanisms could lead to the development of more effective treatments.

The aim of this paper was to investigate the effect of (1) degeneration, (2) circumferential region and (3) strain rate on the microscale mechanical properties (toe region modulus, linear modulus, extensibility, phase angle) of individual fibre bundles in the anulus fibrosus lamellae of the human intervertebral disc. Healthy and degenerate fibre bundles excised from different circumferential regions in the outer anulus (posterolateral, lateral, anterolateral, anterior) were tensile tested at slow (0.1%/s), medium (1%/s) and fast (10%/s) strain rates using a micromechanical testing system.

Our preliminary results showed that neither degeneration nor circumferential region significantly affected the fibre bundles’ mechanical behaviour. However, when the fibre bundles were tested at higher strain rates, this resulted in significantly higher linear moduli and lower phase angles.

These findings, compared with data from other studies investigating single and multiple lamellae sections, suggest that degeneration has minimal effect on outer anulus mechanics irrespective of structural level, and the inter- and intra-lamellar arrangement and continuity of the fibre bundles may influence the lamellae’s regional behaviour and viscoelasticity.

Introduction

Low-back pain is a common musculoskeletal disorder that can be caused by a number of different factors, such as intervertebral disc degeneration (Luoma et al., 2000). Degenerate discs are characterised by various structural anomalies, including increased dehydration and decreased disc height (Hadjipavlou et al., 2008). The disease's origins are not fully understood and current treatments aim to alleviate the symptomatic pain instead of targeting the root cause (An et al., 2003, Andersson et al., 2006, Brox et al., 2006, Mirza and Deyo, 2007, Raj, 2008, Zdeblick, 1995). An improved understanding of the disease’s underlying mechanisms could potentially lead to the development of more effective treatments.

One region that contributes to loading response is the anulus fibrosus (AF), which lies on the disc’s periphery, and consists of 10–20 fibrous layers, or lamellae (Bogduk, 2002). The mechanical properties of multiple and single lamella sections with respect to degenerative grade, circumferential region and strain rate have been explored in the literature.

Neither degeneration nor strain rate had any significant effect upon mechanical behaviour (Acaroglu et al., 1995, Fujita et al., 1997, Gregory and Callaghan, 2010, Guerin and Elliott, 2006, Holzapfel et al., 2005, O'Connell et al., 2009).

Regarding region, studies testing in the circumferential direction found that anterior sections had significantly higher linear (‘Young’s’, E) modulus values than those taken from the posterolateral region (Acaroglu et al., 1995, Ebara et al., 1996). Higher E indicates the tissue has greater elasticity and strength, and could be related to the fibre bundles in the anterior lamellae having a tighter, more complete structure and higher percentage of endplate-to-endplate continuity (Adams and Green, 1993, Marchand and Ahmed, 1990, Pezowicz, 2010, Pezowicz et al., 2005, Tsuji et al., 1993). The anterior region is subjected to much lower levels of axial strain compared with the posterior region (Stokes, 1987); hence, these structural differences could be associated with mechanical demand. Initial toe region modulus (ET) was consistent between regions (Fujita et al., 1997, Smith et al., 2008), indicating that the initial straightening of crimped collagen fibres and elongation of elastin fibres is uniform throughout the AF.

Single anterior lamellae also exhibited the highest E values (Holzapfel et al., 2005, Skaggs and Weidenbaum, 1994), and strain rate did not significantly affect loading response (Holzapfel et al., 2005). Degeneration in single lamellae has not yet been explored.

These studies provide an overview of the AF’s macroscale mechanics; however, the microscopic, collagen-rich fibre bundles, which are the major structural components of the lamellae, are yet to be investigated. These discrete fibre bundles, bound together by ground substance, are a distinct feature of the lamellae, as their orientation alternates with each successive lamella, giving the AF torsional resilience (Bogduk, 2002, Marchand and Ahmed, 1990).

This study aimed to examine the mechanical properties of healthy and degenerate fibre bundles extracted from the outermost lamellae with respect to circumferential region, degenerative grade and strain rate. Specifically, E, ET, extensibility (eM, the point of transition between E and ET) and phase angle (ɸ, energy absorption, where 0° denotes purely elastic behaviour and 90° denotes purely viscous behaviour) were measured. These parameters quantify the elastic and viscoelastic response of fibre bundles. Comparisons with lamella properties may help validate multi-scale computational models of the disc, and contribute to the development of tissue-engineered scaffolds.

Section snippets

Specimen preparation

Ten human cadaver lumbar spines (mean age: 51 years, range: 16–91 years), were sourced from the National Disease Research Interchange (Pennsylvania, USA) and the Ray Last Laboratories, University of Adelaide (South Australia, Australia), cut into motion segments (two adjacent vertebrae and intervening discs), sealed in plastic bags and stored at −30 °C prior to use. One motion segment was taken from each of eight spines, with three motion segments taken from each of the remaining two spines.

Results

The mean ± 95% CI sample width and thickness of the extracted fibre bundles were measured to be 1.75 ± 0.12 mm and 0.50 ± 0.04 mm, respectively. The stress-strain cycles for the fibre bundles were viscoelastic in nature and showed signs of hysteresis, as expected for soft tissue (Fig. 4).

The main effect for strain rate (Fig. 5) was not significant for ET (p = 0.284) or eM (p = 0.071); however, E (p = 0.003) and ɸ (p < 0.001) were significantly affected. For E, pairwise comparisons showed that

Discussion

This study investigated the mechanical behaviour of individual fibre bundles in healthy and degenerate human AF, and compared it to those in multiple and single lamellae.

There were several limitations. Firstly, an ideal tensile test should stretch the tissue to failure. However, despite using fabric tabs and cyanoacrylate adhesive on the tissue, and lining the clamps with emery paper, the failure point could not be reached without the high-strength tissue slipping from the jaws of the clamps, a

Conclusion

For the first time, the mechanical properties of healthy and degenerate fibre bundles from the outer AF, as a function of circumferential region and strain rate, were compared, and the possibility of tensile testing individual fibre bundles using a micromechanical testing system was demonstrated. Our preliminary findings suggested that degeneration and circumferential region did not alter fibre bundle mechanical behaviour, whereas strain-rate dependence was observed for E and ɸ, indicating

Acknowledgements

The authors acknowledge Mr Richard Stanley for his valuable assistance with sample preparation and testing apparatus design.

Conflict of interest

The authors of the submitted paper ‘Tensile behaviour of individual fibre bundles in the human lumbar anulus fibrosus’ have no financial and/or personal relationships with other people or organisations that could lead to a conflict of interest.

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