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57.4 The motion segment The adjacent vertebrae with the disc, facet joints, the connecting ligaments and muscles together form a motion segment. Between the atlas (C1) and the sacrum (S1) the spine consists of 24 such motion segments. The range and direction of movement in individual motion segment varies and depends on the anatomical peculiarities of that segment.
57.5 Activity related back pain can arise from one or more of these motion 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 |
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