Anular delamination strength of human lumbar intervertebral disc

Progression of intervertebral disc (IVD) herniation does not occur exclusively in a linear manner through the anulus fibrosus (AF), but can migrate circumferentially due to localized AF delamination. Consequently, resistance to delamination is an important factor in determining risk of herniation progression. The inter-lamellar matrix located between the AF layers is responsible for resisting this delamination; however, its mechanical properties are largely unknown. This study aimed to determine the mechanical properties of the inter-lamellar matrix in human AF samples via a peel test. Seventeen human IVDs (degeneration grades of 2-3) were obtained from six lumbar spines. From these 17 discs, 53 tissue samples were obtained from the superficial and deep regions of the anterior and posterior AF. Samples were dissected into a 'T' configuration to facilitate a T-peel test (or 180-degree peel test) by initiating delamination between the two middle AF layers. Peel strength was found to be 33 % higher in tissues obtained from the superficial AF region as compared with the deep region (p = 0.047). This finding may indicate a higher resistance to delamination in the superficial AF, and as a result, delamination and herniation progression may occur more readily in the deeper layers of the AF.