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segments, if damaged, and treatment often involves surgical fusion or newer forms of stabilisation. Fusion of one motion segment has a profound effect on the adjacent motion segment in the lumbar spine (demanding increased motion to compensate), but in the thoracic spine it has little effect if any. This is because the lumbar spine is more mobile and the thoracic spine is a relatively stiff region in the normal spine.

57.6 The function spinal unit (FSU) The functional spinal unit is defined as a pair of adjacent vertebrae and the connecting joints and ligaments but devoid of musculature.² The FSU is frequently referred to in the description of the biomechanical properties of a motion segment.

57.7 Junctional areas of the spine The lower back includes three regions (thoracic, lumbar and sacral) with two intervening junctional areas: the thoraco-lumbar and the lumbo-sacral junctions.

57.8 In general the thoracic spine is a relatively rigid segment and the lumbar spine is a relatively mobile segment of the spine. The five sacral vertebrae are fused together and no movement is possible between them.

57.9 The junctions between a mobile and a stiff segment of the spine are biomechanically important and are more prone to injury.

57.10 The thoraco-lumbar junction [T12/L1] Three anatomical changes take place at this junction.

It is the junction between the thoracic kyphosis (forward curve) and the lumbar lordosis (backward curve).

The thoracic spine has the rib cage to support it from the sides. The lumbar spine does not.

There is also a change in the orientation of the facet joints at this junction.

57.11 The rib cage The stiffness of the thoracic spine may largely be attributed to the rib cage. Besides the three main joints and seven other ligaments connecting each FSU in the thoracic spine, the rib cage acts as additional link, joining the