anatomy of the thorax and abdomen



Applied anatomy of the thorax and abdomen



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The thoracic spine



The vertebra


The thoracic spine has a primary dorsal convexity (Fig. 1) associated with intrauterine life – a phylogenetic kyphosis – whereas the cervical and lumbar spine have a compensatory lordosis.




The 12 thoracic vertebrae are intermediate in size between those in the cervical and lumbar regions. They are composed of a vertebral body – a cylindrical ventral mass of bone – continuing posteriorly into a vertebral arch (Fig. 2). The typical thoracic vertebral body is heart-shaped in cross-section and has on each of its lateral aspects a superior and inferior costal facet for articulations with the ribs (costovertebral joints). The arch is constructed out of two pedicles and two short laminae, the latter uniting posteriorly to form the spinous process. Laminae and spinous processes lie obliquely covering each other like the tiles of a roof, so protecting the posterior cord posteriorly. The pedicles carry the articular and transverse processes.




The posterior aspect of the vertebral body and the arch enclose the vertebral foramen. The spinal cord at the thoracic level is rounder and smaller than at the cervical level, and in consequence the vertebral foramina are also smaller.


Where the pedicles and laminae unite the transverse process is found, slightly posterior to the articular process, pedicle and intervertebral foramen. There is also an oval facet for the ribs on all the transverse processes, except for T11 and T12, to which ribs are not attached.


The spinous processes at mid-thorax are long and very steeply inclined: each transverse process is at a level one and a half vertebrae higher than the tip of the corresponding spinous process. In the upper and lower thorax, the spinous processes are less inclined; here, the corresponding transverse process is located approximately one level higher.


The oval intervertebral foramina are located behind the vertebral bodies and between the pedicles of the adjacent vertebrae and contain the segmental nerve roots. In the thoracic spine, these are situated mainly behind the inferoposterior aspect of the upper vertebral body and not just behind the disc. This makes a nerve root compression by a posterolateral displacement less likely at the thoracic level, whereas at the lumbar level nerve root compressions by posterolateral disc protrusions are quite common (Fig. 3, see Standring, Fig. 42.27).




The location of the intervertebral foramen depends on the level. In the upper and lower thoracic area, it is level with the tip of the spinous process of the vertebra above, i.e. level above. At mid-thorax there is a difference of about image levels.



The intervertebral disc


A fibrocartilaginous disc forms the articulation between two vertebral bodies. The anatomy and behaviour of discs are discussed in Chapter 31, Applied anatomy of the lumbar spine. However, it is worth noting here that thoracic discs are narrower and flatter than those in the cervical and lumbar spine. Disc size gradually increases from superior to inferior. The nucleus is rather small in the thorax. Therefore protrusions are usually of the annular type, and a nuclear protrusion is very rare in the thoracic spine.



The ligaments


The longitudinal ligaments run anteriorly and posteriorly on the vertebral bodies (Fig. 4, see Standring, Fig. 54.10). The anterior ligament covers the whole of the vertebral bodies’ anterior aspect and some of their lateral aspect. It is firmly connected to the periosteum but only loosely to the discs. The posterior longitudinal ligament is strongly developed at the thoracic level and is wider than in the lumbar region, although it covers only a part of the posterior aspect of each vertebral body. It has some lateral expansions, which are firmly attached to the discs.




The ligamentum flavum, interposed between the laminae, extends laterally as far as the medial part of the inferior articular process. At each level the ligamentum flavum has lateral extensions to both sides to form the capsules of the facet joints (see Standring, Fig. 42.42). The transverse processes are connected to each other by the intertransverse ligaments. The supra- and interspinal ligaments bridge the gap between the spinous processes.




Content of the spinal canal


The spinal canal is formed by the vertebral foraminae of the successive vertebrae, the posterior aspects of the discs, the posterior longitudinal ligament, the ligamenta flava and the anterior capsules of the facet joints. In contrast to the cervical and lumbar regions, where the canal is triangular in cross-section and offers a large lateral extension to the nerve roots, the thoracic spinal canal is small and circular. It can be divided into three zones: the upper (T1–T3), and lower (T10–T12) zones are transitional, respectively, the cervical and the thoracic spine, and the thoracic and the lumbar spine. Between these is the mid-thoracic zone (T4–T9), where the spinal canal is at its narrowest (Fig. 5).




The spinal canal contains the dural tube, within which are the spinal cord, the spinal nerves and the epidural tissue (see Standring, Fig. 43.3).




Spinal cord


The dura mater contains the spinal cord, which ends approximately at the L1 level. The spinal cord occupies the space of the spinal canal maximally at the thoracic level. This, together with its poor vascularization renders the thoracic spine very vulnerable to damage by extradural processes and vertebral trauma.


The cord depends for its blood supply on these arterial circles. The inner circle is of three longitudinal arterial channels, which run from the medulla oblongata to the conus medullaris. Their perforating arteries to the spinal cord are larger and more numerous at the cervical and lumbar level than at the thoracic level. Moreover, the inner arterial circle is characterized in the thoracic spine by a lack of anastomoses.


One of the two outer arterial circles is located in the extradural space, the other in the extravertebral tissues. These give rise to the ‘medullary feeders’, which arise at the cervical spine mainly from the vertebral arteries and in the thoracic and lumbar spine from the intercostal and lumbar arteries, which are segmental branches of the aorta. It is at mid and lower thorax that the spinal cord has the least profuse blood supply: the so-called critical vascular zone. When surgery is contemplated here, strict care must be taken not to impede blood flow.



Nerve roots and innervation


As at the cervical and lumbar levels, the thoracic spinal nerves emerge from the cord as a ventral and a posterior ramus, which join together to form the short spinal nerve root. The lateral part of the spinal canal that envelops the nerve root is the radicular canal. It is formed anteriorly by the posteroinferior aspect of the upper vertebra and a small part of the intervertebral disc, both covered by the posterior longitudinal ligament. The posterior boundary is formed by the lamina and the superior articular facet. The intraspinal course of the upper thoracic nerve roots is almost horizontal, as in the cervical spine. Therefore, a nerve root can only become compressed by its corresponding disc. However, the more caudal it is in the spine, the more oblique is the nerve root’s course. The T12 nerve root within the spinal canal is at the height of the eleventh vertebral body, and therefore courses downwards, outwards and slightly anteriorly behind the T11 disc and the T12 vertebral body, to leave the foramen at the inferior margin of body T12. As a consequence, the lowest thoracic nerve roots can be compressed by disc lesions of two consecutive levels (T12 root by T11 or T12 disc).

Jun 5, 2016 | Posted by in ORTHOPEDIC | Comments Off on anatomy of the thorax and abdomen
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