10
The thoracic wall and diaphragm
Chapter contents
10.1 The thoracic cage
The thoracic wall is made up of skeletal elements that form the thoracic cage (or more commonly, but less accurately, the rib cage) and muscles that move the components of the thoracic cage relative to each other for ventilation and postural movement. The thoracic cage is made up posteriorly by the thoracic part of the vertebral column, laterally and anteriorly by the ribs and costal cartilages, and by the sternum in the anterior mid-sternal area.
10.1.1 The thoracic vertebral column
The thoracic vertebral column is made up of 12 thoracic vertebrae and their intervertebral discs. The thoracic vertebrae are not arranged in a straight line, but are concave anteriorly as shown in Figure 9.2.
All vertebrae have the following general configuration as shown in Figure 10.1A:
• A heart-shaped body with two backward projections, the pedicles, either side of the vertebral foramen. The foramen forms the spinal canal with the foramina of other vertebrae. Note in Figure 10.1C that the pedicles are slightly shallow above and strongly grooved below to form intervertebral foramina with adjacent vertebrae for the passage of spinal nerves;
• Two stout transverse processes running laterally and slightly posteriorly;
• Two flat plates called laminae which join to form a long spinous process—you can feel the tips of the spinous processes very easily under the skin in the midline of your back;
• Superior and inferior articular processes at the junction of the pedicles and laminae. In thoracic vertebrae, the superior facets are set vertically with the facets on the superior processes facing posterolaterally and those on the inferior processes anteromedially; the relative movement of the vertebrae is thus mainly rotary, but there is very little actual movement in the thoracic part of the vertebral column.
The thoracic vertebrae are modified from this basic pattern to articulate with the ribs through several more articular facets as shown in Figure 10.1 A, B, and C. They carry on each side:
• Shown most clearly in Figure 10.1 C, a superior and inferior demi-facet (a half facet) on each side of the body for the heads of two ribs in the case of T2–T9 or a single complete facet for the head of one rib in the case of T1 and T10–T12;
• Shown in Figure 10.1 A and B, a facet near the tip of each transverse process for the tubercle of a rib (except T11 and T12).
Vertebrae are separated from each other by an intervertebral disc as shown in Figure 10.2C. Each intervertebral disc has an outer ring of fibrocartilage, the annulus fibrosus, which encloses the nucleus pulposus, an ovoid gelatinous mass. The intervertebral discs are a shock-absorbing mechanism; sudden downward forces will tend to compress the nucleus vertically, but the simultaneous lateral expansion will be resisted by the fibres of the annulus. The slight degree of movement possible between adjacent vertebral bodies occurs by a rolling movement on the relatively incompressible nucleus with the annulus being stretched on one side and compressed on the other. The effects of overload on the intervertebral discs are described in Box 10.1.
Fig. 10.1 A typical thoracic vertebra seen in: A) Superior; B) Anterior; C) Lateral views.
Sometimes forces generated are so great that the nucleus of the intervertebral disc herniates through the annulus; this is a slipped disc and is most usually in the lumbar region of the vertebral column. The herniated nucleus may compress the spinal cord and inhibit nerve transmission; the body would be paralysed and anaesthetic below the level of compression. More commonly, the herniated nucleus extrudes laterally and impinges on the spinal nerve that exits at that level on that side, causing pain to radiate down the distribution of the nerve. With increasing age, the nucleus becomes thinner and less aqueous and is eventually converted to fibrocartilage; some degree of mobility in the vertebral column is lost as these age changes occur.
10.1.2 Ribs and costal cartilages
A typical rib has the following features shown from above and anteriorly in Figure 10.2A and B respectively. Most of the length of the rib is formed by the shaft which is convex externally. The superior border is rounded whereas the inferior border is sharp; these features are best seen in Figure 10.2B. The sharp lower border is the lateral edge of the costal groove on the medial surface, a shallow groove running along the lower border that houses the intercostal artery, vein, and nerve. The radius of curvature of the shaft decreases at the angle, seen best in Figure 10.2A. In Figure 10.2A, observe the tubercle, a posterior projecting elevation just medial to the angle. It has a small facet which articulates with the transverse process of a vertebra; this feature is absent from ribs 11 and 12. The stout neck connects the tubercle and the head which carries superior and inferior facets that articulate with the bodies of the vertebrae as shown in Figure 2.6B; note that ribs 1, 10, 11, and 12 have only one facet. The first rib is very atypical; it is described in Box 10.2 for those who require such detail.
Each rib is continued anteriorly by a costal cartilage that articulates either directly or indirectly with the sternum anteriorly.
Fig. 10.2 The articulations between a rib and vertebrae. A) Superior view. B) Anterior views.
The first rib is atypical as you can see in Figure 10.3; it is flattened from above downwards and is shorter and more curved than any other rib. It shows prominent rough areas on its upper surface where the scalene muscles, arising from the cervical vertebrae, attach to it. The first rib is often grooved between the muscle attachments by the roots of the brachial plexus and the subclavian artery. A similar marking may be made by the subclavian vein in front of the attachment of scalenus anterior.
Costal joints
As illustrated in Figure 10.2, each rib has several articulations.
• The major articulation of the head of each rib with the vertebrae is seen best in Figure 10.2B. The inferior facet on the head of each of ribs 2–9 articulates with the superior demi-facet on the body of the corresponding vertebra. The superior facet articulates with the inferior demi-facet of the vertebra above and with the intervertebral disc between them. The heads of ribs 1, 10 (usually), 11, and 12 have a rounded head with a single articular facet that articulates only with the corresponding vertebra. These joints are all synovial.
• As seen in Figure 10.2A, the tubercles of the first ten ribs form synovial joints with the facets of the transverse processes of the corresponding vertebrae. Strong costotransverse ligaments unite each rib and vertebra.
The double articulation of each rib with the bodies of two vertebrae and the transverse process of its corresponding vertebra and the relative positions of these attachments means that the ribs twist on their articulations rather than slide up and down (see Section 11.4)
10.1.3
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