Part twenty-two. Vertebral canal
The vertebral canal (spinal canal) is a smooth-walled tubular space formed by the whole series of vertebral foramina lying one above the other (see p. 421). Its anterior boundaries are the bodies of the vertebrae, intervertebral discs and the posterior longitudinal ligament. Posteriorly are the vertebral laminae and ligamenta flava, while at the sides are the pedicles of the vertebrae and the large intervening spaces, the intervertebral foramina. The canal contains the spinal meninges and the spinal cord with its nerve roots and blood vessels. Its lower end becomes continuous with the sacral canal (see p. 437).
The bony walls of the canal are separated from the contained meninges by the epidural space (extradural space) which contains loose connective tissue, fat and veins. The adipose tissue is mainly in the posterior part of the epidural space and extends laterally into the intervertebral foramina with the nerve roots and their dural sheaths. The veins form the internal vertebral venous plexus (Fig. 6.110). The plexus receives its tributaries mostly from the large basivertebral veins draining the active red marrow in the bodies of the vertebrae (see p. 428). The plexus lies mainly in the anterior part of the epidural space and comprises a collection of longitudinally aligned veins with numerous transverse connections. The internal vertebral plexus sends its efferent veins (the intervertebral veins) through the intervertebral foramina and between adjacent ligamenta flava to drain into the external vertebral venous plexus and thence into the segmental veins. The internal vertebral plexus provides a venous bypass of the diaphragm. It functions when the inferior vena cava cannot cope with a sudden flush of blood resulting from a sudden increase of intra-abdominal pressure (e.g. in coughing or abdominal straining). Thus pelvic and abdominal venous blood is momentarily squirted up the plexus above the diaphragm, into posterior intercostal veins, and thereby into the superior vena cava.
Figure 6.110 Anterior aspect of vertebral canal exposed by removal of neural arches to show the internal vertebral venous plexus. The posterior longitudinal ligament is usually narrower behind the vertebral bodies than seen here.
Spinal meninges
The spinal dura mater, or theca, is a prolongation of the inner layer of the dura mater of the posterior cranial fossa. It extends downwards through the foramen magnum to the level of S2 vertebra. It is attached rather firmly to the margin of the foramen magnum, to the tectorial membrane and to the
posterior longitudinal ligament on the body of the axis vertebra. Elsewhere it lies free in the spinal canal, apart from fine fibrous bands to the posterior longitudinal ligament in places, especially towards the caudal end of the canal. The spinal dura is pierced segmentally by the anterior and posterior roots of the spinal nerves and is prolonged over these roots as sleeve-like projections which enter the intervertebral foramina and fuse with the epineurium of the mixed spinal nerves.
The spinal arachnoid mater lines the inner surface of the spinal dura, with only a potential space between these two membranes. Below the level of the spinal cord (i.e. over the cauda equina) the arachnoid is nothing but a delicate membrane that is supported by the dura mater, but over the spinal cord itself the arachnoid sends many delicate processes across the subarachnoid space to the pia mater on the cord, forming a lace-like arrangement.
The spinal pia mater, as in the cranium, invests the surface of the central nervous system. It clothes the spinal cord and lines the anterior median sulcus. It is prolonged over the spinal nerve roots until where the dura blends with the epineurium of the mixed spinal nerves. It is projected below the apex of the conus medullaris, whence it extends as the filum terminale to perforate the spinal theca at the level of S2 vertebra. It then descends to the back of the coccyx (Fig. 6.98). The filum terminale lies centrally in the cauda equina, but is not part of the cauda which consists of nerve roots only. A lateral projection of pia mater on each side forms the denticulate ligament. This flange crosses the subarachnoid space between the anterior and posterior nerve roots and, piercing the arachnoid, connects the side of the spinal cord to the dura mater. It is attached in an unbroken line along the spinal cord from the foramen magnum to the conus medullaris, but its lateral edge has a series of teeth-like projections, which are attached to the dura between successive nerve roots (Fig. 6.111 ). There are usually 21 such dentate ligaments on each side. The highest is attached to the dura just above the foramen magnum, behind the vertebral artery and in front of the spinal root of the accessory nerve. The lowest dentate ligament lies between the twelfth thoracic and first lumbar nerve roots.
Figure 6.111 Lower end of the spinal cord exposed by opening the dura and arachnoid mater from behind. On the left the nerve roots and denticulate ligament have been removed. The nerve roots on
the right are shown devoid of their pial covering.
The spinal subarachnoid space is relatively large, accommodating about half of the total volume of cerebrospinal fluid (75 mL out of 150 mL). It communicates through the foramen magnum with the subarachnoid space of the posterior cranial fossa. Some cerebrospinal fluid percolates away along the meningeal sheaths of the spinal nerves.
Below the level of the conus medullaris the space contains only the cauda equina and filum terminale, in addition to cerebrospinal fluid, and it ends at the level of S2 vertebra.
Lumbar puncture and spinal and epidural anaesthesia
In lumbar puncture the needle is generally inserted in the midline between the spines of L3 and L4 or L4 and L5 vertebrae when the patient's back is flexed, usually when curled up lying on one side. The needle passes through the supraspinous and interspinous ligaments and through or between ligamenta flava before penetrating the dura. An alternate paramedian route through the muscles of the back passes through the ligamentum flavum but not through the supraspinous and interspinous ligaments. Since the spinal cord ends at the level of L1 vertebra, it is in no danger.
I n spinal anaesthesia, the anaesthetic solution is injected into the subarachnoid space (with the needle in a similar position to that used for lumbar puncture), so mixing with the cerebrospinal fluid surrounding the nerve roots and percolating into them.
I n epidural anaesthesia the solution is injected at the clinically indicated level into the epidural (extradural) space without penetrating the dura, and the solution infiltrates through the meningeal sheaths containing the nerve roots. The approach is similar to that for lumbar puncture, but in the thoracic region the paramedian route may have to be used on account of the overlap of long thoracic spinous processes. An alternative less-used approach is through the sacral hiatus into the sacral canal. Through any of these routes a catheter may be inserted for the continuous infusion of an analgesic solution.