How age influences unravelling morphology of annular lamellae - a study of interfibre cohesivity in the lumbar disc.

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.