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Anatomy relevant to epidural and subarachnoid blockade

Created: 12/12/2004

The typical thoracic vertebra has a heart-shaped body (Figure 1) bearing one or two facets for articulation with the head of a rib. Its vertebral foramen is smaller and more circular than those of the cervical and lumbar regions. The two pedicles bear long and strong transverse processes. It articulates with its neighbouring vertebra with articular processes that bear nearly vertical facets facing (superior) posteriorly and (inferior) anteriorly. Its spinous process is long and slopes posteroinferiorly so that its tip overlies the level of the vertebral body below.

Figure 1

Thoracic vertebra

The typical lumbar vertebra
has a larger kidney-shaped body and its vertebral foramen is larger than that of the thoracic vertebra (Figure 2). Its transverse processes are long and slender and its articular processes are directed (superior) posteromedially and (inferior) anterolaterally. Its spinous process is shorter, broader and more horizontal than those of the thoracic vertebrae.

Figure 2

Lumbar vertebra

Joints and ligaments of the vertebrae

the articular surfaces of the bodies of adjacent vertebrae are covered by hyaline cartilage and united by a thick fibrocartilaginous intervertebral disc. These are strong cartilaginous joints designed for weight-bearing. The disc is a shock absorber, its centre, the nucleus pulposus, is gelatinous and surrounded by a fibrous part, the annulus fibrosus. Adjacent vertebrae articulate by two synovial facet joints between the paired articular processes.

Ligaments: the vertebral bodies are also united by anterior and posterior longitudinal ligaments (Figure 3). The anterior ligament extends from the occiput to the sacrum and is attached to each vertebra. Its deep fibres blend with and are attached to the intervertebral discs. The posterior ligament extends from the occipital bone to the sacrum and is attached to each vertebral body and disc. Additional support is provided by the ligamenta flava (Figure 4), which unite adjacent laminae. These ligaments contain a large amount of yellow elastic tissue and are strong. Because of their elasticity they remain taut during flexion and extension of the spine, maintaining the curvatures of the spinal column and supporting it when it is flexed. The elasticity of these ligaments permits the ‘interlaminar gap’ to widen (and facilitate lumbar puncture during flexion of the spine). The ligaments have no sensory supply and can be pierced painlessly when a lumbar puncture is performed.

Figure 3

Anterior view of thoracic vertebrae

Figure 4

Lateral view of lumbar vertebra

The supraspinous, interspinous (Figure 5) and intertransverse ligaments help unite adjacent vertebrae. The supraspinous is a strong ligament connecting the tips of the spinous processes from C7 to the sacrum. Above C7 it forms the ligamentum nuchae, which attaches to the occipital protuberance and provides attachment to neck muscles. The intertransverse ligaments are weak but best developed in the lumbar region. The articular ligaments lie around the joints of the articular facets. All these ligaments contribute to supporting the spinal column when it is in the flexed position.

Figure 5

Dorsolateral view of thoracic vertebra

The vertebral canal
is bounded anteriorly by the vertebral bodies and the intervertebral discs, each covered by the posterior longitudinal ligament, which is continuous from the back of the body of the axis to the sacrum. Posteriorly it is bounded by the laminae, ligamenta flava and the arch of the vertebra. The vertebral canal is usually larger in the cervical and lumbar regions. Dura covers the spinal cord, which lies free in the canal. Between the spinal dura and the walls of the canal is the epidural space which is generally about 5 mm wide, though wider in the lumbar region.

Applied anatomy

The anatomy of the spinal cord is described in the following article.

Surface anatomy:
the effective use of epidural anaesthesia depends on precise knowledge of the segmental arrangement of the motor and sensory nerves, an appreciation of the increasing obliquity of the nerve roots and knowledge of the surface anatomy that relates to the vertebral canal. Only when all are understood precisely can the level of the anaesthetic block be that which was planned. An approximate depiction of segmental levels is shown in Figure 6. In the thoracic region, the spinous processes of the vertebrae are palpable; the spine of T7, which overlies the level of the body of T8, and is at the level of the spinal cord segment of T9/10, lies at the level of the inferior angle of the scapula when the arms are held by the sides. By counting the vertebral spines from this point one can identify the spines of the other thoracic vertebrae. The spine of L4 usually lies on a line drawn between the highest points of the iliac crests (the intercristal plane). From this point other vertebral levels can be identified. The spinal cord ends at the level of the spine of the first lumbar vertebra (i.e. between the bodies of L1 and L2). The dural sac ends at the second sacral body and this level corresponds to a line joining the buttock dimples, which overlie the posterior superior iliac spines.

Figure 6

Surface markings

The epidural space
contains the dural sac, the spinal nerve roots, the extradural venous plexus, spinal arteries, lymphatics and fat (Figure 7 and see Figure 6 on page 146). The veins are valveless and distend when the patient strains or coughs. They drain via the azygos vein to the inferior vena cava and may distend when there is obstruction to the vena cava (e.g. in advanced pregnancy or with a large abdominal tumour). They are continuous below with the pelvic veins and above with the intracranial veins. A misplaced epidural injection that inadvertently enters the extradural veins can cause local anaesthetic agent or air to track intracranially. The extradural space is widest posteriorly and at this point is occasionally divided by a fold of dura mater into two or three compartments that do not always communicate with each other. The consequence of this infrequent abnormality may be patchy analgesia after an epidural anaesthetic. Although the dura is attached superiorly to the margins of the foramen magnum this cannot be relied on to prevent inadvertent passage of analgesic into the cranial cavity. Prolongations of the dura surround the nerve roots (dural cuffs) and fuse with them as they traverse the intervertebral foraminae. The anterior and posterior nerve roots cross the epidural space before they join in the intervertebral foramina and thus can be anaesthetized by the epidural route.

The subdural space:
this thin potential space between the dura and arachnoid may be entered inadvertently when performing an epidural anaesthetic. The result may be patchy inadequate anaesthesia or an abnormally extensive nerve block because, though the space does not communicate with the subarachnoid space, the nerve roots that cross it are affected by the injected anaesthetic agent.

Figure 7

The epidural space

The subarachnoid space
(Figure 8) is bounded by pia mater, which invests the spinal cord, roots of the spinal nerves and the spinal blood vessels, and lines the arachnoid. The pia mater gives some support to the spinal cord by denticulate ligaments, which extend in pairs, from the foramen magnum to the T12/L1 level, from the lateral surfaces of the cord to be attached to the dura.

Figure 8

Transverse section of an intervertebral disc

Copyright © 2004 The Medicine Publishing Company Ltd

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