The spine has slightly movable cartilaginous joints between the vertebral bodies and between the articular facets. Between the vertebral bodies are the intervertebral discs, each consisting of a soft mucoid central core, the nucleus pulposus, rimmed by the tough fibrous tissue of the annulus fibrosis. The intervertebral discs cushion movement between vertebrae and allow the vertebral column to curve, flex, and bend. The flexibility of the spine is largely determined by the angle of the articular facet joints relative to the plane of the vertebral body, and varies at different levels of the spine. Note that the vertebral column angles sharply posterior at the lumbosacral junction and becomes immovable. The mechanical stress at this angulation contributes to the risk for disc herniation and subluxation, or slippage (spondylolisthesis), of L5 on S1.

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