Overview of the thorax and diaphragm and surgical anatomy of the chest wall

Development of the chest wall

Several congenital anomalies of the chest wall that lend themselves to surgical correction are seen in clinical practice.

Ribs develop from the costal processes of the thoracic vertebrae. It is important to note that costal processes of other vertebrae do not form ribs, possibly as a result of differential gene expressions and cellular interactions in the precursor tissues; when this process is disturbed, extra ribs may form, as in the case of cervical or lumbar ribs. The sternum develops from paired longitudinal condensations of somatic mesoderm that fuse in the midline to form a cartilaginous plate. Ossification centres form along this cartilaginous model in a segmental fashion. While those for the body fuse, that of manubrium remains separate; the xiphoid centre does not ossify until after birth.

The diaphragm develops from four sources: the lateral body wall; septum transversum; pleuroperitoneal canal; and mesentery of the oesophagus. Deficits in any of these sources produce a hernia: for example, a posterolateral defect as a result of a deficit in the contribution made by the pleuroperitoneal canals.

The ribs at birth are horizontal and the muscle efficiency is low: the newborn and infant ventilation is therefore predominantly abdominal. With the growth of the abdomen and as the ribs acquire their oblique orientation, the diaphragm increases the dome configuration while the baby's crawling and walking movements allow the abdominal breathing at birth to improve the component of chest ventilation gradually.

Surface anatomy of the chest wall

The manubriosternal angle is usually more pronounced in men than women. It is palpable at the junction of the manubrium with the sternal body and serves as a useful landmark for the level of the sternal plane and the medial ends of the second costal cartilages, and so offers an accurate point at which to start counting ribs. In most adults, the sternal angle is located between the fourth thoracic vertebra and the upper half of the fifth thoracic vertebra (it varies from the lower half of the second thoracic vertebra to the lower half of the sixth thoracic vertebra, a distance of about 8 cm); the plane lies at a slightly higher vertebral level in women. The sternal plane represents the lower border of the superior mediastinum, and at this point, the right and left pleurae are in contact with each other: it is therefore a useful starting point when delineating the surface markings of the parietal pleura. The sternal plane is conventionally described as lying over the tracheal bifurcation, the concavity of the aortic arch and the point where the azygos vein enters the superior vena cava. CT analysis places these three major surface landmarks typically a little lower, at the level of the fifth or sixth thoracic vertebra ( Fig. 42.1 ).

The xiphisternal joint and xiphoid process are palpable at the inferior end of the sternum; the joint usually lies at the level of the ninth thoracic vertebra. The costal margin is palpable passing inferolaterally from the xiphisternum. Posteriorly, the free ends of the eleventh and twelfth ribs may be palpable and the spinous processes of the thoracic vertebrae are palpable. The spinous process of the first thoracic vertebra sits below that of the seventh cervical vertebra (vertebra prominens) and is often more prominent. Tracing the twelfth rib superomedially aids identification of the spinous process of the twelfth thoracic vertebra. The angles of the ribs are palpable several centimetres lateral to the spinous processes of the vertebrae. The surface anatomy of the posterior chest wall is covered in Chapters 30 and 33 . Radiological studies of cross-sectional imaging have revealed variations in surface anatomy affected by age, gender, posture, phase of respiration, build and ethnicity ( Table 42.1 ).

Clinical anatomy of the Thorax

The thorax is the upper part of the trunk. It consists of an external musculoskeletal cage, the thoracic or chest wall, and an internal cavity that contains the heart, lungs, oesophagus, trachea and principal bronchi, thymus, vagus and phrenic nerves, right and left sympathetic trunks, thoracic duct, lymph nodes, and major systemic and pulmonary blood vessels. Inferiorly, the thorax is separated from the abdominal cavity by the diaphragm; superiorly, it communicates with the neck and the upper limbs ( Video 42.1 ).

The skin and soft tissues of the chest wall cover a musculoskeletal frame consisting of twelve pairs of ribs, which articulate with twelve thoracic vertebrae posteriorly and (except for the last two pairs of ribs) with the sternum anteriorly, via their costal cartilages; intrinsic muscles and muscles that connect the chest wall with the upper limb and the vertebral column; and numerous blood and lymphatic vessels and nerves that supply the components of the musculoskeletal frame and the overlying skin and breast tissue.

The skeletal framework of the thoracic wall provides extensive attachment sites for muscles associated functionally with the neck, abdomen, back and upper limbs. Some of them (scalenes, infrahyoid strap muscles, sternocleidomastoid, serratus anterior, pectoralis major and minor, external and internal obliques, and rectus abdominis) function as accessory muscles of respiration, although they are usually active only during forced respiration. The thoracic wall offers protection to some of the abdominal viscera: the greater part of the liver lies under the right dome of the diaphragm; the stomach and spleen lie under the left dome of the diaphragm; and the posterior aspects of the superior poles of the kidneys lie on the diaphragm and are anterior to the twelfth rib on the right, and to the eleventh and twelfth ribs on the left.

Bones and joints

The thoracic skeleton consists of twelve thoracic vertebrae and their intervening intervertebral discs (midline, posterior); twelve pairs of ribs and their costal cartilages (predominantly lateral); and the sternum (midline, anterior). When articulated, they form an irregularly shaped osteocartilaginous cylinder, reniform in horizontal section, which is narrow above, broad below, flattened anteroposteriorly and longer behind. Laterally, the thoracic cage is convex and is formed by the ribs; anteriorly, it is slightly convex and is formed by the sternum and the distal parts of the ribs and their costal cartilages.

Thoracic vertebrae

The twelve thoracic vertebrae have the following typical features: small bodies that bear costal facets for articulation with the head of ribs; transverse processes with articular facets for the tubercles of ribs; round vertebral foramina; and long thoracic spines. The pedicles form the intervertebral foramina for nerve passages while the laminae serve as attachment points for the interlaminar ligaments.

Ribs

The ribs form the greater part of the thoracic skeleton and articulate posteriorly with the thoracic vertebrae. Their number may be increased by cervical or lumbar ribs or reduced if the twelfth pair is absent. The first seven pairs of ‘true’ ribs articulate with the sternum by costal cartilages; the lower five ‘false’ ribs either join the superjacent costal cartilage or ‘float’ free at their anterior ends as relatively small and delicate structures tipped with cartilage. The tenth rib may also float: the incidence varies from 35% to 70%, depending on ancestry.

The ribs and costal cartilages provide the necessary stiff support for the extrinsic muscles (pectoralis major and minor, serratus anterior, diaphragm) to exert outward force of the chest, thus expanding the lungs and resulting in inhalation. Without the ribs, the chest will collapse down on to the lung. This can be exploited by thoracoplasty, a surgical procedure to remove the ribcage and collapse the chest wall on to a destroyed lung, such as may occur with severe infections of the lung ( Fig. 42.2 ).

A typical rib has a shaft and anterior (costal) and posterior (vertebral) ends. The smooth internal surface of the shaft is marked by a costal groove, bounded below by the inferior border. The posterior end has a head, neck and tubercle. The head presents two facets, separated by a transverse crest; the lower and larger facet articulates with the body of the corresponding vertebra, and the crest is attached to the intervertebral disc above. In situ , typical ribs present medial and lateral surfaces and superior and inferior borders. The first rib has superior and inferior surfaces and medial and lateral borders. The posterolateral curvature of the ribs, from their vertebral ends to the angles, produces a deep internal groove, the paravertebral gutter, on either side of the vertebral column. The ribs and costal cartilages are separated by intercostal spaces, which are deeper anteriorly and between the upper ribs. Each space is occupied by three layers of flat muscles and their apo­neuroses, neurovascular bundles and lymphatic channels.

The medial border of the first ribs on each side, together with the body of the first thoracic vertebra posteriorly and the superior border of the manubrium sterni anteriorly, forms the bony margins of the thoracic inlet; structures that pass between the thorax and the upper limb therefore pass over the first rib, the apices of the lungs and the apical pleurae, and may be damaged in pathologies affecting the first rib, including fractures and tumours.

Thoracic outlet syndrome

In this condition, the subclavian vessels and/or the brachial plexus are compressed as they exit the thorax via the space between the first rib, the clavicle and the scalene triangle. Patients typically complain of pain and paraesthesiae in the arm, hand or finger. Thoracic outlet syndrome is most commonly neurogenic; venous and arterial thoracic outlet syndromes are less common. Treatment involves resection of the first rib and scalenectomy.

In the core common axillary approach, the patient is placed in the lateral position with the axilla, arm and chest wall prepared. The arm is held in place using retractors. A transverse incision is made, extending between latissimus dorsi and pectoralis major, taking care not to injure the long thoracic or intercostobrachial nerves. The brachial plexus and subclavian artery are visualized within the axilla ( Ch. 38 ). All muscular attachments are dissected from the first rib using blunt dissection and periosteal elevation; once the lower border is free, the scalene muscles are identified, isolated and divided, and the first rib is resected using bone cutters. If a cervical rib is present, it is excised at this time. The subclavian vein anteriorly, the first thoracic nerve root posteriorly and the pleura are all at risk during this procedure.

Sternum

The sternum is an elongated, flattened bone that forms the middle portion of the anterior wall of the thorax and the anterior boundary of the mediastinum. It articulates with the clavicles at the sterno­clavicular joints and with the cartilages of the first seven pairs of ribs. It is divided into a manubrium, body and xiphoid process. The manubrium is level with the third and fourth thoracic vertebrae; its superior border contains a central jugular (suprasternal) notch between two oval fossae, the clavicular notches, for articulation with the sternal ends of the clavicles (see Fig. 42.1 ). The body of the sternum is level with the fifth to ninth thoracic vertebrae. It articulates with the manubrium at the level of the sternal angle (manubriosternal joint, angle of Louis), which typically lies opposite the inferior border of the body of the fourth thoracic vertebra. Asymmetry is common: the manubrio­sternal joint may not form an angle or the joint/angle may be displaced more inferiorly. The xiphoid process is in the epigastrium.

The anterior surface of the sternum is subcutaneous and facilitates procedures through the sternum, including punctures and access to the heart. The arch of the aorta and its main branches, the brachiocephalic vein, pleura and lungs are posterior relations of the manubrium. The heart and pericardium, sternopericardial ligaments and thymus are posterior relations of the body of the sternum. The internal thoracic (mammary) artery, a key source of grafts for artery revascularization, runs close to the lateral parasternal line. A number of muscles are attached to the sternum: sternothyroid, sternohyoid, sternocleido­mastoid, pectoralis major, transversus thoracis (sternocostalis) and the diaphragm.

Sternal clefts and bifid xiphoid processes represent products of sternal maldevelopment that occasionally presents challenges to clinicians.

Intervertebral joints are described in Chapter 31 .

Muscles of the chest wall

Muscles of the anterior chest wall

The intrinsic muscles of the chest wall are the intercostals, subcostales, transversus thoracis, levatores costarum, serratus posterior and, occasionally, sternalis. The extrinsic muscles are the scapular muscles and the muscles that connect the upper limb, chest wall and vertebrae: that is, trapezius, latissimus dorsi, rhomboids major and minor, levator scapulae, pectorales major and minor, subclavius, supra- and infraspinatus, teres major and teres minor. Various combinations of these muscles will be encountered during different surgical approaches to the thorax.

Intercostal muscles

Eleven pairs of external intercostal muscles extend from the tubercles of the ribs, where they blend with the posterior fibres of the superior costotransverse ligaments, almost to the costal cartilages, where each continues forwards to the sternum as an aponeurotic layer, the external intercostal membrane. Eleven pairs of internal intercostal muscles begin anteriorly at the sternum, in the interspaces between the cartilages of the true ribs, and at the anterior extremities of the cartilages of the ‘false’ ribs; they are thickest in this intercartilaginous (parasternal) part. The internal intercostal muscles continue back as far as the posterior costal angles, where each is replaced by an aponeurotic layer, the internal intercostal membrane, continuous posteriorly with the anterior fibres of a superior costotransverse ligament, and anteriorly with the fascia between the internal and external intercostal muscles. Each muscle descends from the floor of a costal groove and adjacent costal cartilage, and inserts into the upper border of the rib below; their fibres are directed obliquely, nearly at right angles to those of the external intercostal muscles.

Transversus thoracis spreads over the internal surface of the anterior thoracic wall. It is attached to the lower third of the posterior surface of the sternum, xiphoid process and costal cartilages of the lower three or four true ribs near their sternal ends. Its fibres diverge and ascend laterally as slips that pass into the lower borders and inner surfaces of the costal cartilages of the second, third, fourth, fifth and sixth ribs. The lowest fibres are horizontal and are contiguous with the highest fibres of transversus abdominis; the intermediate fibres are oblique; and the highest are almost vertical. Transversus thoracis varies in its attachments, not only between individuals but even on opposite sides of the same individual.

The intercostal muscle is a convenient muscle to be used as a muscle graft based on the intercostal artery for its blood supply. It is employed mostly to reinforce a bronchial anastomosis after, for example, resection of the right upper lobe and reimplantation of the bronchus intermedius to the carina, as in Fig. 42.3 .