Reduced Shear Modulus and Altered Lamellar Morphology of the Outer Annulus Fibrosus in Painful Intervertebral Disc Degeneration Compared With Tissue From Non-Surgical Controls.

BACKGROUND

Stability of the spine and intervertebral disc (IVD) integrity is enabled by the highly organized fibrocartilaginous annulus fibrosus (AF). The shear properties of the AF are important in maintaining IVD integrity. AF shear mechanics in degenerative disc (DD) remain underexplored, especially in comparing minimally degenerative (non-DD) and symptomatic DD individuals. This study measured tissue mechanical properties (AF simple shear modulus and dynamic shear properties) and examined structure (with optical coherence tomography (OCT)) in surgical DD and non-DD control individuals.

METHODS

Whole AF tissue samples were collected from non-DD donors (N = 13) and DD surgical individuals (N = 30). Two anterior outer AF (OAF) 5 mm cubes were sectioned from each sample and subjected to shear in two orientations, radial (coronal plane, G1) and circumferential (sagittal plane, G2). Tissues underwent static shear and dynamic shear protocols to a maximum of 10% shear strain. Following mechanical tests, average lamellar thickness was assessed using OCT.

RESULTS

Static shear moduli were significantly reduced for DD tissue compared to non-DD in both the radial (G1) (non-DD: 83.0 ± 41.3 kPa, DD: 24.1 ± 23.7 kPa) and the circumferential (G2) (non-DD: 226.2 ± 81.9 kPa, DD: 54.0 ± 40.2 kPa) orientations (p < 0.05). Further dynamic mechanical alterations were detected in hysteresis, phase shift, and dynamic modulus. Shear moduli correlated negatively with lamellar thickness (G1: rs = -0.63, G2: rs = -0.71).

CONCLUSIONS

There were significant alterations in AF shear moduli and dynamic properties in DD individuals when compared to non-DD controls. Structural correlations highlight the role of the highly organized AF lamellar structure on shear modulus values. These findings suggest that altered AF mechanics may contribute to DD pathology and associated low back pain, warranting further investigation into structural and functional AF changes in symptomatic individuals.