Collagen organisation in the interspinous ligament and its relationship to tissue function.

X-ray diffraction and determination of optical anisotropy show that collagen fibres in pig and human lumbar interspinous ligaments tend to be orientated parallel to the spinous processes. There is a distribution of fibre orientations about this preferred direction. Flexion of the spine does not change the direction of preferred orientation but the angular spread of fibres increases. This pattern of collagen fibre orientations is consistent with the interspinous ligaments being able to transmit tension from the thoracolumbar fascia to the vertebrae. Since the collagen fibres tend to be aligned antero-posteriorly, they provide a high efficiency of reinforcement in this direction. The lack of fibres orientated perpendicular to the spinous processes will lead to low strength in the cranio-caudal direction so that the ligament can provide little resistance to flexion of the spine.