The ANNULUS FIBROSUS makes up the outer part of the disc. It consists of collagen fibers containing significant amounts of water and proteoglycans. These fibers are arranged in concentric layers called LAMELLAE. The lamellae are thicker and more numerous in the anterior part of the disc.
Within each lamella, the collagen fibres are oriented obliquely about 30º to the horizontal. The orientation is reversed in each contiguous layer, resulting in a 120º change from layer to layer. This interstriation of fibers imparts great tensile strength and allows for multiplanar motion. The structure of the disc is similar to a radial automobile tire, which also has great strength and simultaneous compression and tension capabilities.
The NUCLEUS PULPOSUS is the internal substance of the disc. The inner fibers of the annulus fibrosus gradually blend with the nucleus pulposus.
The nucleus pulposus is more gelatinous than the annulus and has a higher water and proteoglycan content. The water content can be 85% in young people; in older people, the water content of the nucleus pulposus may be less than 25%. The nucleus pulposus acts to resist compression in axial loading forces.
Besides allowing for motion, the discs act as shock absorbers to prevent injury to the vertebral bodies and neurological struc-tures. However, the discs are so strong that in compression, the vertebral body will fail before the discs fail.
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