Growth of human intervertebral discs and vertebral bodies.

In normal children, who usually learn to walk unaided before the age of two, the resultant change from a primary to secondary lumbar curvature is accompanied by a change in the relative position of the nucleus pulposus within the intervertebral disc, from a predominantly posterior situation to a central situation. During the third and fourth years of life, an increase in the rate of vertical growth of the L 4-5 "total disc", as measured at its centre, is associated with the gradual change from convexity to concavity in the shape of the bony vertebral end surfaces bounding the disc. In non-ambulant spastic children, the increased vertical growth of L 4-5 discs and the change in shape of the lumbar vertebral bodies, described in normal children, are not seen. In these non-ambulant children, though normal vertebral body heights are attained, the heights of the L 4-5 discs are severely reduced compared with normal heights for corresponding ages, and the anteroposterior diameters of lower lumbar vertebral bodies and discs are also more than two standard deviations below the corresponding norms. It is suggested that while vertical growth of the central region of lumbar vertebral bodies may be genetically determined and independent of mechanical factors, vertical growth of lumbar intervertebral discs and anteroposterior growth of lumbar vertebral bodies and discs are dependent on the activity associated with weight-bearing in the erect posture.