[The spine under typical work-load. Aspects of biomechanics]

The varied pressure effects in the transverse plane of a lordotic and kyphotic vertebral segment are described, showing the effects on the height of the intervertebral space. Loss of height indicates the origin of pathologic mechanical development: Fixed lordosis during the first phase, with its destructive influence on the posterior segment of the fibrous ring and the sudden, irritating protrusion or penetration of nuclear tissue in the second phase at the moment of anterior flexion, pushing the nucleus into the annulus which is damaged by pressure. The possibility of further loss of height of the vertebral bodies themselves is pointed out. This results from a decline in supportive power of cortex and cancellous bone and reduced hydration of the colloidal spaces of the vertebral body.