Thoracic Cage-The Ribs

The thoracic cage is formed by the vertebral column behind, the ribs and intercostal spaces on either side and the sternum and costal cartilages in front. Above, it communicates through the ‘thoracic inlet’ with the root (BASE) of the neck; below, it is separated from the abdominal cavity by the diaphragm (Fig. 1).

The Ribs

The greater part of the thoracic cage is formed by the twelve pairs of ribs.

Of these, the first seven are connected anteriorly (FROM THE FRONT) by way of their costal cartilages to the sternum, the cartilages of the 8th, 9th and 10th articulate (COMMUNICATE)each with the cartilage of the rib above (‘false ribs’) and the last two ribs are free anteriorly (‘floating ribs’).

Each typical rib (Fig. 5) has a head bearing two articular facets (FACES, SURFACES), for articulation with the numerically corresponding vertebra and the vertebra above, a stout neck, which gives attachment to the costotransverse ligaments, a tubercle with a rough non-articular portion and a smooth facet which is for articulation with the transverse process of the corresponding vertebra, and a long shaft flattened from side to side and divided into two parts by the ‘angle’ of the rib. The angle demarcates (DEFINES) the lateral limit of attachment of the erector spinae muscle.

The following are the significant features of the ‘atypical’ ribs;

1st Rib (Fig. 6). This is flattened from above downwards. It is not only the flattest but also the shortest and most curvaceous of all the ribs. It has a prominent tubercle on the inner border of its upper surface for the insertion of scalenus anterior. In front of this tubercle, the subclavian vein crosses the rib; behind the tubercle is the subclavian groove where the subclavian artery and lowest trunk of the brachial plexus lie in relation to the bone. It is here that the anaesthetist can infiltrate the plexus with local anaesthetic.

Crossing the neck of the first rib from the medial to the lateral side are the sympathetic trunk, the superior intercostal artery (from the costocervical trunk) and the large branch of the first thoracic nerve to the brachial plexus.

The 2nd rib is much less curved than the 1st and about twice as long.

The 10th rib has only one articular facet on the head.

The 11th and 12th ribs are short, have no tubercles and only a single facet on the head.

The 11th rib has a slight angle and a shallow subcostal groove;

the 12th has neither of these features.

Clinical Features

Rib fractures

The chest wall of the child is highly elastic and therefore fractures of the rib in children are rare. In adults, the ribs may be fractured by direct violence or indirectly by crushing injuries; in the latter the rib tends to give way at its weakest part in the region of its angle. Not unnaturally, the upper two ribs, which are protected by the clavicle, and the lower two ribs, which are unattached and therefore swing free, are the least commonly injured.

In a severe crush injury to the chest several ribs may fracture in front and behind so that a whole segment of the thoracic cage becomes torn free (‘stove-in chest’). With each inspiration (INHALE) this loose flap sucks in, with each expiration (EXHALE) it blows out, thus undergoing paradoxical (INCONSISTENT) respiratory movement.

The associated swinging movements of the mediastinum (THE MASS OF TISSUES AND ORGANS SEPARATING THE TWO PLEURAL SACS, BETWEEN THE STERNUM IN FRONT AND THE VERTEBRAL COLUMN BEHIND, CONTAINING THE HEART AND ITS LARGE VESSELS, TRACHEA, ESOPHAGUS, THYMUS, LYMPH NODES, AND OTHER STRUCTURES AND TISSUES)produce severe shock and this injury calls for urgent treatment by insertion of a chest drain with underwater seal, followed by endotracheal intubation, or tracheostomy, combined with positive pressure respiration.

Aortic Coarctation  Fig. (34b) 

In coarctation of the aorta, the intercostal arteries derived (COMING, MADE) from the aorta receive blood from the superior intercostals (from the costocervical trunk of the subclavian artery), from the anterior intercostal branches of the internal thoracic artery (arising from the subclavian artery) and from the arteries anastomosing around the scapula.

Together with the communication between the internal thoracic and inferior epigastric arteries, they provide the principal collaterals (MAIN PROTECTION) between the aorta above and below the block. In consequence, the intercostal arteries undergo dilatation and tortuosity (BECOME BENT AND TWISTED) and erode the lower borders of the corresponding ribs to give the characteristic irregular notching of the ribs, which is very useful in the radiographic confirmation of this lesion (INJURY, WOUND, LACERATION).

Cervical rib

A cervical rib (Fig. 7) occurs in 0.5% of subjects and is bilateral (TWO SIDED) in half of these cases. It is attached to the transverse process of the 7th cervical vertebra and articulates with the 1st (thoracic) rib or, if short, has a free distal (FURTHEST AWAY) extremity (PORTION OF ELONGATED STRUCTURE) which usually attaches by a fibrous (LEATHERY) strand to the (normal) first rib.

Pressure of such a rib on the lowest trunk of the brachial plexus may produce paraesthesiae (ABNORMAL SKIN SENSATION) along the ulnar border of the forearm and wasting of the small muscles of the hand (T1). Less common vascular changes, even gangrene, may be caused by pressure of the rib on the overlying subclavian artery. This results in post-stenotic dilatation of the vessel distal to the rib in which a thrombus (BLOOD CLOT) forms from which emboli (BLOCKAGE IN THE BLOOD FLOW) are thrown off.