Thoracic Cage-Costal Cartilage and Sternum

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 costal cartilages
These bars of hyaline cartilage serve to connect the upper seven ribs directly to the side of the sternum and the 8th, 9th and 10th ribs to the cartilage immediately above. The cartilages of the 11th and 12th rib merely join the tapered extremities of these ribs and end in the abdominal musculature (ARRANGEMENT AND CONDITION OF THE MUSCLES).

Clinical features
1◊◊The cartilage adds considerable resilience to the thoracic cage and protects the sternum and ribs from more frequent fracture.
2◊◊In old age (and sometimes also in young adults) the costal cartilages undergo progressive ossification (THE HARDENING OR CALCIFICATION OF SOFT TISSUE INTO A BONELIKE MATERIAL); they then become radio-opaque and may give rise to some confusion when examining a chest radiograph of an elderly patient.



The sternum
This dagger-shaped bone, which forms the anterior part of the thoracic cage, consists of three parts. The manubrium is roughly triangular in outline and provides articulation for the clavicles and for the first and upper part of the 2nd costal cartilages on either side. It is situated opposite the 3rd and 4th thoracic vertebrae. Opposite the disc between T4 and T5 it articulates at an oblique angle at the manubriosternal joint (the angle of Louis), with the body of the sternum (placed opposite T5 to T8). This is composed of four parts or ‘sternebrae’ which fuse between puberty and 25 years of age. Its lateral border is notched to receive part of the 2nd and the 3rd to the 7th costal cartilage. The xiphoid process is the smallest part of the sternum and usually remains cartilaginous well into adult life. The cartilaginous manubriosternal joint and that between the xiphoid and the body of the sternum may also become ossified after the age of 30.

Clinical features
1◊◊The attachment of the elastic costal cartilages largely protects the sternum from injury, but indirect violence accompanying fracture dislocation of the thoracic spine may be associated with a sternal fracture. Direct violence to the sternum may lead to displacement of the relatively mobile body of the sternum backwards from the relatively fixed manubrium.
 2◊◊In a sternal puncture a wide-bore needle is pushed through the thin layer of cortical bone covering the sternum into the highly vascular spongy bone beneath, and a specimen of bone marrow aspirated with a syringe.
3◊◊In operations on the thymus gland, and occasionally for a retrosternal goitre, it is necessary to split the manubrium in the midline in order to gain access to the superior mediastinum. A complete vertical split of the whole sternum is one of the standard approaches to the heart and great vessels used in modern cardiac surgery.

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.

Surface markings of the more important thoracic contents (Figs 2–4)

The Trachea

The trachea commences in the neck at the level of the lower border of the cricoid cartilage (COMPLETE RING OF CARTILAGE AROUND THE TRACHEA (C6)) and runs vertically downwards to end at the level of the sternal angle of Louis (T4/5), just to the right of the mid-line, by dividing to form the right and left main bronchi. In the erect position and in full inspiration the level of bifurcation (THE SPLITTING OF A MAIN BODY INTO TWO PARTS) is at T6.

The Pleura

The cervical pleura can be marked out on the surface by a curved line drawn from the sternoclavicular joint (JOINT BETWEEN THE CLAVICLES AND THE STERNUM) to the junction of the medial and middle thirds of the clavicle; the apex of the pleura is about 2.5cm above the clavicle. This fact is easily explained by the oblique slope of the first rib. It is important because the pleura can be wounded (with consequent pneumothorax) by a stab wound — and this includes the surgeon’s knife and the anaesthetist’s needle—above the clavicle.

The lines of pleural reflexion pass from behind the sternoclavicular joint on each side to meet in the midline at the 2nd costal cartilage (the angle of Louis). The right pleural edge then passes vertically downwards to the 6th costal cartilage and then crosses:

•◊◊the 8th rib in the midclavicular line;

•◊◊the 10th rib in the midaxillary line;

•◊◊the 12th rib at the lateral border of the erector spinae (VERTEBRAL MUSCLES).

On the left side the pleural edge arches laterally at the 4th costal cartilage and descends lateral to the border of the sternum, due, of course, to its lateral displacement by the heart; apart from this, its relationships are those of the right side.

The pleura actually descends just below the 12th rib margin at its medial extremity (LIMIT) — or even below the edge of the 11th rib if the 12th is unusually short; obviously in this situation the pleura may be opened accidentally in making a loin incision to expose the kidney, perform an adrenalectomy or to drain a subphrenic abscess.

The Lungs

The surface projection of the lung is somewhat less extensive than that of the parietal pleura (OUTER LINING OF THE THORAX) as outlined above, and in addition it varies quite considerably with the phase of respiration. The apex of the lung closely follows the line of the cervical pleura (THE DOME-SHAPED LAYER OF PARIETAL PLEURA LINING THE CERVICAL EXTENSION OF THE PLEURAL CAVITY) and the surface marking of the anterior border of the right lung corresponds to that of the right mediastinal pleura (SECTION OF THE PARIETAL PLEURA).

On the left side, however, the anterior border has a distinct notch (the cardiac notch) which passes behind the 5th and 6th costal cartilages. The lower border of the lung has an excursion of as much as 5–8cm in the extremes of respiration, but in the neutral position (MIDWAY BETWEEN INSPIRATION AND EXPIRATION) it lies along a line which crosses the 6th rib in the midclavicular line (VERTICAL LINE CROSSING THROUGH THE CLAVICLE), the 8th rib in the midaxillary line (PERPENDICULAR LINE DRAWN DOWNWARD FROM THE APEX OF THE AXILLA), and reaches the 10th rib adjacent (NEIGHBOURING) to the vertebral column posteriorly (FROM THE BACK).

The oblique fissure, which divides the lung into upper and lower lobes, is indicated on the surface by a line drawn obliquely downwards and outwards from 2.5cm lateral (ADJACENT) to the spine of the 5th thoracic vertebra to the 6th costal cartilage about 4cm from the midline. This can be represented approximately by abducting the shoulder to its full extent; the line of the oblique fissure then corresponds to the position of the medial (IN THE CENTER) border of the scapula.

The surface marking of the transverse fissure (separating the middle and upper lobes of the right lung) is a line drawn horizontally along the 4th costal cartilage and meeting the oblique fissure where the latter crosses the 5 th rib.

The heart

The outline of the heart can be represented on the surface by the irregular quadrangle bounded by the following four points (Fig. 4):

1◊◊the 2nd left costal cartilage 12mm from the edge of the sternum;

2◊◊the 3rd right costal cartilage 12mm from the sternal edge;

3◊◊the 6th right costal cartilage 12mm from the sternum;

4◊◊the 5th left intercostal space 9cm from the midline (corresponding to the apex beat).

The left border of the heart (indicated by the curved line joining points 1 and 4) is formed almost entirely by the left ventricle (the auricular appendage (ADDITION) of the left atrium peeping around this border superiorly), the lower border (the horizontal line joining points 3 and 4) corresponds to the right ventricle and the apical part of the left ventricle; the right border (marked by the line joining points 2 and 3) is formed by the right atrium (see Fig. 24a).

A good guide to the size and position of your own heart is given by placing your clenched right fist palmar surface down immediately inferior (LOWER, UNDER) to the manubriosternal junction.

 Note that the heart is about the size of the subject’s fist, lies behind the body of the sternum (therefore anterior to thoracic vertebrae 5–8), and bulges over to the left side. The surface markings of the vessels of the thoracic wall are of importance if these structures are to be avoided in performing aspiration of the chest.

The internal thoracic (internal mammary) vessels run vertically downwards behind the costal cartilages half an inch from the lateral border of the sternum. The intercostal vessels lie immediately below their corresponding ribs (the vein above the artery) so that it is safe to pass a needle immediately above a rib, dangerous to pass it immediately below (see Fig. 8).

Surface anatomy and surface markings-The Thorax

The experienced clinician spends much of his working life relating the

surface anatomy of his patients to their deep structures

The following bony prominences (BULGES) can usually be palpated (EXAMINED BY TOUCH) in the living subject (matching vertebral levels are given in brackets):

•◊◊superior angle of the scapula (SHOULDER BLADE) (T2);

•◊◊upper border of the manubrium sterni(BROAD UPPER STRENUM), the suprasternal notch (fig 1.2)(DIP IN THE STERNUM)(T2/3);

•◊◊spine of the scapula (T3);

•◊◊sternal angle (of Louis)— the transverse ridge at the manubrio-sternal

Junction(BUMP AT STERNAL JUNCTION) (T4/5);

•◊◊inferior angle of scapula (T8);

•◊◊xiphisternal joint (LOWEST JOINT IN STERNUM)(T9);

•◊◊lowest part of costal margin—10th rib (the subcostal line passes through

L3).

Note from Fig. 1 that the manubrium(BROAD UPPER STERNUM) corresponds to the 3rd and 4th

Thoracic (PART OF BODY BETWEEN HEAD AND ABDOMEN) vertebrae and overlies the aortic arch, and that the sternum corresponds to the 5th to 8th vertebrae and neatly overlies the heart.

Since the 1st and 12th ribs are difficult to feel, the ribs should be enumerated (COUNTED)

from the 2nd costal cartilage (CONNECTS STERNUM AND RIBS), which articulates with the sternum at the angle of Louis.

The spinous processes (LUMPY BITS) of all the thoracic vertebrae can be palpated in

the midline posteriorly (FROM THE BACK TO THE FRONT), but it should be remembered that the first spinous process that can be felt is that of C7 (the vertebra prominens).

The position of the nipple varies considerably in the female, but in the male it usually lies in the 4th intercostal space (BETWEEN THE RIBS) about  10cm from the midline. The apex beat (BEAT OF HEART FELT OVER APEX), which marks the lowest and outermost point at which the cardiac impulse can be palpated, is normally in the 5th intercostal space 9cm from the midline (just below and medial (TO THE CENTRE) to the nipple).

The trachea (WINDPIPE) is palpable in the suprasternal notch (fig. 1.2) midway between the heads of the two clavicles.

Fig. 1.2 the suprasternal notch-the dip imbetween the two clavicles and top of the sternum