Page 345 - Clinical Anatomy

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330 The head and neck

may displace forward on its neighbour below with either dislocation or
fracture of the articular facets between the two (fracture dislocation) and
with rupture of the interspineous ligaments.
The cervical vertebrae (particularly C7), may be fractured or, more com-
monly, dislocated by a fall on the head with acute flexion of the neck, as
might happen on diving into shallow water. Dislocation may even result
from the sudden forward jerk which may occur when a motorcar or aero-
plane crashes. Note that the relatively horizontal intervertebral facets of the
cervical vertebrae allow dislocation to take place without their being frac-
tured, whereas the relatively vertical thoracic and lumbar interverbral
facets nearly always fracture in forward dislocation of the dorsolumbar
region.
2◊◊The comparatively thin posterior part of the annulus fibrosus may
rupture, either due to trauma or to degenerative changes, allowing the
nucleus pulposus to protrude posteriorly into the vertebral canal— the so-
called ‘prolapsed intervertebral disc’ (Fig. 233). This may sometimes occur
at the lower cervical intervertebral discs (C5/6 and C6/7), very occasion-
ally in the thoracic and upper lumbar region or, by far the most commonly,
at the L4/5 or L5/S1 disc. The diagnosis of this and other spinal conditions
has been greatly facilitated by the introduction of MRI scans which give
excellent anatomical details of this region (Fig. 233b).
Aprolapsed L4/5 disc produces pressure effects on the root of the 5th
lumbar nerve, that of the L5/S1 disc on the 1st sacral nerve. Pain is referred
to the back of the leg and foot along the distribution of the sciatic nerve. Hip
flexion with the leg extended (‘straight leg raising’) is painful and limited
due to the traction which this movement puts upon the already irritated
and stretched nerve root. There may be a weakness of ankle dorsiflexion
and numbness over the lower and lateral part of the leg and medial side of
the foot (L5) or the lateral side of the foot (S1). L5 involvement may cause
weakness of extension of the great toe (extensor hallucis longus). If S1 is
affected, the ankle jerk may be diminished or absent and there may be
weakness of plantar flexion.
Occasionally the disc prolapses directly backwards, and, if this is exten-
sive, may compress the whole cauda equina, producing paraplegia.
3◊◊Lumbar puncture—see page 338.