Tensile behaviour of individual fibre bundles in the human lumbar anulus fibrosus
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.
References (41)
- et al.
Lumbar instrumented fusion compared with cognitive intervention and exercises in patients with chronic back pain after previous surgery for disc herniation: a prospective randomized controlled study
Pain
(2006) - et al.
Degeneration affects the fiber reorientation of human annulus fibrosus under tensile load
J. Biomech.
(2006) - et al.
Biomechanical properties of native and tissue engineered heart valve constructs
J. Biomech.
(2014) - et al.
Interfibrillar shear stress is the loading mechanism of collagen fibrils in tendon
Acta Biomater.
(2014) - et al.
Theoretical model and experimental results for the nonlinear elastic behavior of human annulus fibrosus
J. Orthop. Res.
(2004) - et al.
Degeneration and aging affect the tensile behavior of human lumbar anulus fibrosus
Spine
(1995) - et al.
Tensile properties of the annulus fibrosus: I. The contribution of fibre-matrix interactions to tensile stiffness and strength
Eur. Spine J.
(1993) - et al.
Biochemical aspects of development and ageing of human lumbar intervertebral discs
Rheumatology
(1977) - et al.
Summary statement: emerging techniques for treatment of degenerative lumbar disc disease
Spine
(2003) - et al.
Treatment of intractable discogenic low back pain. A systematic review of spinal fusion and intradiscal electrothermal therapy (IDET)
Pain Physician
(2006)
Clinical Anatomy of the Lumbar Spine and Sacrum
Tensile properties of nondegenerate human lumbar anulus fibrosus
Spine
Anisotropic and inhomogeneous tensile behavior of the human anulus fibrosus: experimental measurement and material model predictions
J. Biomech. Eng.
Collagen: Structure and Mechanics
Radial tensile properties of the lumbar annulus fibrosus are site and degeneration dependent
J. Orthop. Res.
An examination of the influence of strain rate on subfailure mechanical properties of the annulus fibrosus
J. Biomech. Eng.
A comparison of uniaxial and biaxial mechanical properties of the annulus fibrosus: a porcine model
J. Biomech. Eng.
The pathophysiology of disc degeneration: a critical review
J. Bone Joint Surg. (British)
Biophysical and physiological investigations on cartilage and other mesenchymal tissues VI. Characteristics of human nuclei pulposi during aging
Acta Orthopaedica
Nutrition of the intervertebral disc: solute transport and metabolism
Connect. Tissue Res.
Cited by (9)
Detailed mechanical characterization of the transition zone: New insight into the integration between the annulus and nucleus of the intervertebral disc
2022, Acta BiomaterialiaCitation Excerpt :The moving cell linear regression method was used to determine the most linear region and calculate young's and toe moduli. Both moduli were defined by the regression line with the largest coefficient of determination (r2), while the regression commenced at zero strain and 70% of maximum stress for toe and young's modulus, respectively [42–44]. Within the toe region, fibrous elements of the tissue were most likely to be uncoiled while young's modulus represented the region where the majority of fibers in the TZ region were fully recruited.
Saline-polyethylene glycol blends preserve in vitro annulus fibrosus hydration and mechanics: An experimental and finite-element analysis
2022, Journal of the Mechanical Behavior of Biomedical MaterialsCitation Excerpt :Finite element models were developed to represent rectangular, uniaxial AF test specimens oriented along the circumferential-axial direction and were modeled at a one-to-one scale to the average initial experimental specimen dimensions (Supplemental Fig. 1). The lamellar thickness (0.4 mm) was determined based on values from single-lamellar studies [Holzapfel et al., 2005; Pham et al., 2018]. A notch was modeled at the midlength to replicate the experimental specimen geometry.
Fiber engagement accounts for geometry-dependent annulus fibrosus mechanics: A multiscale, Structure-Based Finite Element Study
2021, Journal of the Mechanical Behavior of Biomedical MaterialsCitation Excerpt :To illustrate representative differences between specimen types, one specimen from each type was selected for comparison: specimen representative Type A (w = 2.75 mm, l = 4 mm; Type A), specimen representative Type B (w = 2.75 mm, l = 5 mm; Type B), and specimen representative Type C (w = 2.75 mm, l = 6 mm; Type C). Fiber engagement was evaluated at 1.09 specimen stretch for separate models based on the reported transition stretch of type I collagen (Haut 1986; Gentleman et al., 2003; Kato et al., 1989), as well as the reported mean AF fiber bundle transition stretch along the fiber direction (Pham et al., 2018). Engagement was assessed for each fiber element using a stress-based criterion.
New findings confirm the viscoelastic behaviour of the inter-lamellar matrix of the disc annulus fibrosus in radial and circumferential directions of loading
2018, Acta BiomaterialiaCitation Excerpt :Toe modulus represented the initial region of the loading curve where the elastic fiber network in the ILM was uncoiling, and linear modulus represented the region where the majority of fibers in the ILM were fully recruited. Extensibility was calculated as the strain at the intersection of the toe and linear stiffness lines [40]. The average loading modulus represented the entire loading curve, and average modulus represented the entire load-unload curve.