Multiscale mechanical investigations of the human anulus fibrosus

Author: Diana Pham

Pham, Diana, 2015 Multiscale mechanical investigations of the human anulus fibrosus, Flinders University, School of Computer Science, Engineering and Mathematics

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Low-back pain is a commonly reported musculoskeletal disorder, with one possible cause being intervertebral disc degeneration. Presently, this disease is incurable, whilst the available treatments are not always effective and aim to alleviate the symptomatic pain instead of targeting the root cause. A more solid understanding of the mechanisms underlying disc degeneration could potentially lead to the development of more effective treatments.

One region that notably contributes to the disc’s loading response is the anulus fibrosus (AF), which lies on the periphery of the disc, and consists of fibrous, collagen-type-I-rich layers, or lamellae. Previous studies have investigated the mechanical properties of multiple and single lamella sections of the AF. It has been observed that disc degeneration does not significantly affect the mechanical behaviour of multiple or single lamellae, though lamellae in the anterior region tend to have higher Young’s Moduli. Moreover, an increased strain rate correlates with an increased Young’s Modulus and lower hysteresis.

To provide a more comprehensive, multiscale analysis of the disc, and clarify the effect that disc degeneration might have upon the different components within the disc, it would be pertinent to investigate its mechanical properties at lower hierarchical levels, such as the microscale and nanoscale. These are the two main foci of this research.

The microscale study investigated the collagen type I bundles which constitute the AF lamellae. Individual fibre bundles were extracted from healthy and degenerate human outer AF in four different anatomical regions: posterolateral, lateral, anterolateral and anterior. Each sample was tensile tested under hydrated conditions and subjected to slow (0.1%/s), medium (1%/s) and fast (10%/s) strain rates. Four mechanical parameters were measured: toe modulus, Young’s Modulus of Elasticity, extensibility and phase shift. It was found that disc degeneration and anatomical region had no significant effect on these parameters, though increased strain rate produced higher Young’s Moduli and lower phase shifts. The phase shifts of the fibre bundles were also smaller in magnitude than those at the lamella level.

The nanoscale study investigated collagen type I fibrils extracted from the fibre bundles used in the microscale study. Young’s Modulus was indirectly derived by nanoindenting the fibrils in ambient conditions using an atomic force microscope. Due to limitations with nanoindentation, no other parameters were considered at the nanoscale. As in the microscale study, it was found that disc degeneration and anatomical region had no significant effect upon the Young’s Moduli of the fibrils.

The results from these two studies indicate that disc degeneration did not affect the microscale nor nanoscale of the AF. Moreover, the regional inconsistencies observed at the lamella level were not evident at the fibre bundle or fibril level, suggesting that the properties of the disc become more uniform when more homogeneous structures are considered, and that their intra-lamellar arrangement could influence the overall mechanical properties of the lamellae. Finally, the smaller phase shifts at the microscale demonstrate that the fibre bundles exhibited more elastic behaviours than the lamellae.

This research was constrained by relatively small sample sizes and limitations in the testing apparatus; thus, the results need to be interpreted with caution. Nonetheless, this body of work may provide a precedent for future investigations into AF mechanics at the microscale and nanoscale, two hierarchical levels that were previously unexplored.

Keywords: spine, intervertebral disc, collagen, degeneration, anulus fibrosus, atomic force microscopy

Subject: Medical Biotechnology thesis, Biotechnology thesis

Thesis type: Doctor of Philosophy
Completed: 2015
School: School of Computer Science, Engineering and Mathematics
Supervisor: A/Prof John Costi