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Relationship Between Structure and Mechanical Function of the Tissues of the Intervertebral Joint1

Synopsis This paper reviews our current understanding of the relationship between the structures and properties of the tissues of the spine and their mechanical functions. Emphasis is on the human lumbar spine. Vertebrae consist of a core of cancellous bone (low density) surrounded by a shell of cortical bone (high stiffness); as a result they have high stiffness but low mass. The intervertebral disc is able to withstand compression because of the swelling pressure exerted by the nucleus pulposus which is constrained, radially, by the annulus fibrosus. Thus the disc acts as a thick-walled pressure vessel. Collagen fibers within the annulus provide reinforcement during compression, bending and torsion of the disc. Collagen fibers also provide tensile reinforcement and prevent tears spreading across ligaments. The ligamenta flava contain elastic fibers (low stiffness and low strength) with collagen fibers (high stiffness and high strength). In the unstretched ligamenta flava, the collagen fibers have almost random orientations but they become aligned as the ligament is stretched. This structure enables the high extensibility of elastic fibers to be exploited but protects them from damage at high strains. The structure of the interspinous ligament suggests that its main function is to attach the thoracolumbar fascia to the posterior spine. Thus the fascia is maintained in tension when stretched by the abdominal muscles. This and other observations indicate the importance of muscles for maintaining the stability of the spinal column.