Tensile properties of the annulus fibrosus. I. The contribution of fibre-matrix interactions to tensile stiffness and strength.

We investigated the tensile properties of samples of human lumbar annulus fibrosus. Here we consider the effect of sample size, and hence collagen disruption, on the results obtained. Vertical slices, 5 mm thick and 30 mm wide, were cut from the lateral margins of the annulus and adjacent vertebral bodies. The bony ends of each slice were secured in a materials testing machine so that the annulus could be stretched vertically, as occurs during bending movements of the spine in life. Tensile stiffness was measured repeatedly after successive vertical cuts in the annulus had reduced the effective size of the sample. Stiffness (per unit cross-sectional area) decreased as the specimen size decreased. The mean length of collagen fibre bundles in the specimens was calculated from a geometrical model and shown to be proportional to the tensile stiffness. Extrapolation of the results suggested that the vertical stiffness and strength of 15-mm-wide specimens of annulus would be about 44% of their values in situ. We conclude that collagen fibres need not be continous to reinforce the annulus and that fibre-matrix interactions make a large contribution to the tensile stiffness and strength.