The thoracic cage

Chapter 4

The t h o r ax

CHAPTER CONTENTS

The thoracic cage 107 The diaphragm 111 The pleura 114 The trachea and bronchi 116 The lungs 118 The mediastinal divisions 124 The heart 125 The great vessels 133 The oesophagus 136 The thoracic duct and mediastinal lymphatics 139 The thymus 139 The azygos system 140 Important nerves of the mediastinum 142 The mediastinum on the chest radiograph 143 Cross-sectional anatomy 145

THE THORACIC CAGE

The roof of the thoracic cage is formed by the suprapleural membrane and the diaphragm is its floor. The walls are made of the skeleton and attached muscles. The bones involved are: (i) 12 thoracic vertebrae (see section on vertebral column); (ii) 12 ribs and their costal cartilages and (iii) the sternum.

The ribs

There are 12 pairs of ribs - 7 true, 3 false and 2 floating. Occasionally a normal subject has only 11 pairs.

The typical rib (Fig. 4. 1) A typical rib has a head, neck, tubercle and shaft.

The head has two facets for articulation w i th vertebral bodies, for example the sixth rib articulates w i th the bodies of T5 and T6 vertebrae. These costovertebral joints are synovial joints.

The neck of the rib is attached by a ligament to the transverse process of the vertebra above.

The tubercle has a facet medially for articulation w i th its own transverse process. This costotransverse joint is also a synovial joint. The tubercle also has a nonarticular part laterally for ligament attachment.

The shaft has a posterior angle and a much less prominent anterior angle. It has a subcostal groove that is much more prominent posteriorly. This lodges the intercostal vessels and nerves. Twelfth rib

This has only one articular facet on its head; it has no tubercle and no subcostal groove.

Costal cartilages

These are the unossified anterior ends of the ribs. They slope upwards to the sternum, where they form synovial sterno¬ chondral joints (except the first, which forms a primary cartilaginous joint w i th the sternum). The joint between the rib and costal cartilage is a primary cartilaginous joint. The costal cartilages of the first seven ribs articulate w i th the sternum. The eighth to tenth ribs articulate w i th the costal cartilages of the ribs above. The eleventh and twelfth costal cartilages have pointed ends and end in the muscles of the abdominal wall.

Atypical ribs

First rib (Fig. 4. 2) This is the shortest, flattest and most curved rib. It articulates w i th T1 only. A tubercle on its inner border marks the attachment of the scalenus anterior muscle. It is grooved by the subclavian vein anteriorly. Posteriorly, another groove marks where the lowest trunk of the brachial plexus and the subclavian artery - that is, the nerve trunk and not the artery - lies in contact w i th the bone.

Second rib

This is less curved and twice as long as the first rib. It has a tubercle on its external (lower) border, which is often well marked on a chest radiograph, at the site of attachment of the second head of the scalenus anterior muscle.

Tenth rib

This differs from the typical ribs by having only one articular facet on its head.

Eleventh rib

This also has only one articular facet on its head. It has no tubercle for articulation w i th the transverse process. The intercostal space and vessels This is bridged by the muscles - the external, internal and innermost intercostal muscles. The neurovascular bundle lies between the internal and innermost muscle layers.

Intercostal arteries

Posterior • Upper two spaces supplied by superior intercostal arteries from the costocervical branch of the subclavian artery; • Lower nine from the thoracic aorta.

Anterior

• Two branches to most intercostal spaces: — Upper six spaces supplied by the internal thoracic branch of the subclavian artery;

— Next three by the musculophrenic artery, the continuation of the internal thoracic artery. The lower two spaces have no anterior intercostal artery.

Intercostal veins

Posterior (see Fig. 4. 36) • First intercostal vein arches over the pleura to drain into the brachiocephalic vein; • Second to fourth drain to a superior intercostal vein which drains to the azygos vein on the right and to the brachiocephalic vein on the left; • Fifth to eleventh on the right drain to the azygos vein; • Left fifth to eighth drain to the accessory hemiazygos vein and ninth to eleventh to the hemiazygos vein. Anterior • Veins accompany arteries to internal thoracic and musculophrenic veins.

Muscles of the thoracic cage (see Figs 4. 42 and 4. 43) The external, internal and innermost intercostal muscles occupy the spaces between the ribs. Subcostal muscles on the deep surface of the lower ribs span two or three ribs, w i th fibres that run in the same direction as the internal and the innermost intercostal muscles. The innermost inter¬ costals and the subcostals separate the intercostal neurovascular bundles from the pleura. The transverse thoracic muscle arises on the deep surface of the sternum and adjacent lower costal cartilages and passes superolaterally to the deep surface of the anterior ribs.

Superficial to the ribs three muscle groups posteriorly, the costal levators, attach the ribs to the vertebrae: from the transverse processes of the vertebrae to the posterior part of the rib below, serratus posterior superior; from the spinous processes of the vertebrae superolaterally to the posterior ribs and serratus posterior inferior; and from the spinous processes inferolaterally to the posterior aspect of the lower ribs.

Other muscles have attachments to the thoracic cage and can be seen on axial cross-sectional imaging, including the pectoral muscles anteriorly, the serratus anterior and the teres major and subscapularis laterally and posteriorly, and the rhomboids, the erector spinae and trapezius posteriorly.

Radiological features of the thoracic cage (Fig. 4. 3) • Normal variants: ribs may fuse or bifurcate; they may be splayed or hypoplastic. Variation is commoner in the upper ribs, especially the first. • On a PA chest radiograph the subcostal groove may appear as a fine line below the rib, especially posteriorly (not to be mistaken for a periosteal reaction or pneumothorax). It may also appear as shallow indentation posteriorly near the neck (not to be mistaken for rib notching). • A prominence is often seen on the second rib on a chest radiograph, which is usually symmetrical right and left.

This is due to insertion of part of the scalenus anterior muscle. • The costal cartilages, especially the first, may calcify or ossify from early adulthood. In males the calcification is usually in marginal bands, and in females in a central tongue. • Cervical ribs: these are bony or fibrous bands between

C7 and the first rib and are found in 1-2% of subjects.

Of these, 50% are bilateral and often they are asymmetrical. They may be distinguished from the first rib by the orientation of the transverse process that of C7 points downward, whereas that of T1 is horizontal or points upward. A fibrous band is likely to be present if the anterior tubercle of the transverse process of C7 is prominent on a radiograph. The brachial plexus is likely to be prefixed (that is, arise from C4-C8 rather than C5-T1 if the cervical rib is well developed).

This situation is less likely to cause neurological symptoms.

Lumbar ribs: the transverse process (which is the costal element of the lumbar vertebra), may fail to fuse w i th the vertebral body and retain a synovial joint w i th the neural arch. They are symptomless. • Ossification of the ribs: a bony centre arises at the angle of the rib in the eighth fetal week. Secondary centres occur at the head and at the tubercle at 15 years of age and fuse at 25 years of age.

The sternum (Fig. 4. 4) The sternum has: • A manubrium opposite T3 and T4 which articulates with the clavicle and w i th one-and-a-half costal cartilages;

A sternal angle, which is a secondary cartilaginous joint and lies opposite T4 /5 disc space;

A body opposite T5-T9, made up of four stenebrae which articulate w i th five-and-a-half costal cartilages; and • A xiphoid process, which remains cartilaginous well into adult life.

Radiological features of the sternum

Plain films On a PA chest radiograph the manubrial borders may simulate mediastinal widening. The remainder of the sternum is not seen. Oblique views are necessary to project the sternum away from the heart. These project it over the lungs, whose markings may cause confusing shadows. Lateral views are also helpful.

Variation in sternal configuration include: (i) depression of the lower end, known as pectus excavatum; and (ii) prominence of the midportion, known as pectus carinatum.

Fig. 4. 3 Ribs:

(a) subcostal groove clearly visible below the second left rib (arrowheads). Note also the scalene tubercle on the first rib (arrow);

(b) prominence on upper surface of the second rib due to the insertion of part of scalenus anterior muscle;

(c) cervical rib - a well developed bony cervical rib on the left side (arrows).

Ossification of the sternum

Bony centres for the manubrium and stenebrae appear from above downwards from the fifth to the ninth fetal months. Between 15 and 25 years of age stenebrae fuse from below upwards. The xiphoid process fuses w i th the body at 40 years of age and the body and manubrium fuse in old age, if at all.

Computed tomography On CT images of the thorax the manubrium is usually angled w i th respect to the gantry, and this may cause cortical lack of sharpness. The body of the sternum is usually perpendicular to the beam and well demarcated.