This study demonstrates a far greater complexity to the interlamellar architecture of the disc annulus than has previously been recognized. Our findings are clearly relevant to disc biomechanics. Significant degrading of the translamellar bridging network may result in annular weakening leading potentially to disc failure. Most importantly this work opens the way to a much clearer understanding of the microanatomy of the disc wall.

Little is known about the complex forces acting on the deformable multi-layered annulus at a microstructural level as the spine is compressed, flexed and twisted. The recently described translamellar bridging network radially linking many lamellae at discrete locations around the disc wall could be expected to play a significant biomechanical role. In this study, segments of annular wall that were sectioned at a range of angles (oblique, in-plane, sagittal and transverse) were examined using differential interference contrast microscopy to fully elucidate the fibrous detail of the translamellar bridging structures. Typically encompassing a width of 300-600 μ m, translamellar bridging fibres proceed radially in the interbundle space within an individual lamella. Upon traversing the lamella, the bulk of these radial fibres bend through 90 ° to merge with the fibres of the adjacent lamellae. The central fibres of this bridging system continue into the equivalent bridging structures in the adjacent lamellae. As well as exposing structural details that underpin the biomechanical properties of the disc wall, this study has also exposed the limitations of using standard section planes commonly employed by disc researchers.

The present investigation has offered a glimpse of an anular wall apparently capable of remodeling in response to perturbations in its normal mechanical environment. The translamellar cross-bridges undergo adaptations in structure, in response to altered stresses locally at the anular defect site but also distantly in the contralateral AF in the destabilized disc. It is currently not known whether such changes in anular microarchitecture, however, predispose the anulus to further mechanical damage or have a stabilizing role to play in this structure.

Although age-and degeneration-related changes in the morphology and biochemistry of the annulus fibrosus have been extensively reported, studies of tensile strength changes show only a weak correlation with maturity. Given that the disc is a tissue system in which significant levels of deformation occur with normal physiological loading, there may be structure-related properties that provide a better indicator of the influence of ageing on its function. This study is a morphological investigation of lamellar interfibre cohesivity with respect to maturity. Anterior segments of ovine lumbar discs in two age groups were cut at one of two section angles to generate intralamellar and interlamellar slices. These slices of hydrated annular tissue were subjected separately to microtensile and swelling forces, and examined using differential interference contrast microscopy. There were distinct differences in microstructural responses to transverse extension between the immature and mature intralamellar slices. The immature tissue exhibited a diffuse expansion of the array to form a fine fibrous net. In contrast, the mature tissue displayed a discontinuous expansion with the development of clefts and localized fibre buckling. A difference was also observed in the free-swelling response; the immature slices remained planar, whereas the cropped lamellar fibres in the mature slices exhibited a folded, buckled morphology. Morphological evidence from these experiments infers differences in fibre cohesivity between the immature and mature tissues, consistent with biochemical and histological studies. More extreme levels of deformation in the mature tissue could result in discontinuous opening of the fibrous arrays, which might have the potential to lead to cleft formation. These clefts may, in turn, provide micropaths through which nuclear material could extrude. Importantly, with many animal studies carried out on immature discs, the results here suggest that some caution is required with respect to extrapolating annular behaviour beyond this age group.

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