CHEST WALL

The muscular, tendinous, and bony structures of the chest serve several functions. The chest wall must be rigid enough to protect the thoracic viscera and serve as a fixation point against which the muscles of the upper extremity and abdomen can work yet flexible enough to expand and contract with vigorous respirations.

With gentle respirations, the chest wall is a cylinder with the diaphragm as its piston. With inspiration, the diaphragm contracts, its dome is flattened, and, like a piston, it descends in the chest. This motion increases the volume of the thorax, and actively expands the lungs by drawing in air through the trachea. The lungs are very elastic and tend to collapse without outward forces keeping them expanded. With exhalation, the diaphragm relaxes, the elasticity of the lungs causes lung volume to decrease, and air is expelled. Ultimately, the tendency of the lung to collapse is countered by the outward force/rigidity of the chest wall. With vigorous respirations, the intercostal muscles, scalenes, and other accessory muscles of respiration elevate the ribs and increase the thoracic volume much more than usual. With vigorous respirations, the chest wall and diaphragm act in concert like a bellows increasing thoracic volume and then relaxing and allowing the elasticity of the lung to decrease thoracic volume.

The bony structures of the chest wall include 12 ribs, 12 thoracic vertebrae, and the sternum. All ribs articulate posteriorly with the transverse processes and vertebral bodies of their respective thoracic vertebrae and the vertebral body directly superior (Figure 1). Ribs 1 through 7 are called true ribs because they articulate anteriorly directly with the sternum through their own costal cartilage. Ribs 8, 9, and 10 are called false ribs because they articulate anteriorly to the costal cartilage of the rib above. This creates a construct of stairstepping costal cartilages, which ultimately articulates with the sternum and creates the costal arch or costal margin. Ribs 11 and 12 are called floating ribs because they do not articulate with any structure anteriorly (Figure 2). Rather, they attach to the abdominal wall musculature, primarily the internal oblique muscle.

Figure 1 Costovertebral junction. Lateral view showing two left ribs and three vertebrae. Note that ribs articulate with transverse process and body of one vertebrae and body of vertebrae above.

(Redrawn from Grant’s Atlas of Anatomy, 11th ed., Philadelphia, Lippincott Williams & Wilkins, 2005, figures 1.13–1.14, pp. 14–15.)

Figure 2 Bony chest wall. Anterior view.

(Redrawn from Grant’s Atlas of Anatomy, 11th ed., Philadelphia, Lippincott Williams & Wilkins, 2004, figure 1.8, p. 9.)

Because ribs 1 through 10 are fixed anteriorly and posteriorly, they function much like a bucket handle (Figure 3A). When performing a tube thoracostomy, as you approach the sternum anteriorly and the transverse processes posteriorly, the size of the interspace becomes fixed and narrow. Laterally, away from these points of attachment, the ribs separate and the interspace opens. The widest portion of the interspaces can be found at the lateral apogee or “keystone” of the rib. Tube thoracostomies placed laterally will be easier to place through the interspace and more comfortable for the patient (Figure 3B). Also, when creating a thoracotomy, division of the intercostal muscles far anterior and posterior will create a larger working space without tearing the intercostal muscle or fracturing a rib with placement of the rib spreader. The skin need only be divided over the working space, not over the entire intercostal incision.

Figure 3 Bucket handle motion of ribs. Ribs are fixed anteriorly at the sternum and posteriorly at the vertebrae. The ribs will move like a “bucket handle.” The widest space between the ribs will be at the lateral apogee or “keystone.”

(Redrawn from Pearson FG, editor: Thoracic Surgery, 2nd ed. Philadelphia, Churchill Livingstone, 2002, figure 48-3, p. 1327.)

The sternum has three parts, the manubrium, the body, and the xiphoid process. The manubrium is thick and broad, articulating with the clavicle, first rib, and sharing the second rib articulation with the body of the sternum. The sternoclavicular articulation is the only bony articulation of the thorax to the shoulder girdle (see Figure 2). Understanding the angle of the clavicle, manubrium, and first rib is important in safe placement of central venous catheters into the subclavian vein. The subclavian vein and artery leave the arm and enter the thoracic inlet over the top of the first rib and under the clavicle. Once under the clavicle, a needle directed parallel to the clavicle and first rib will not enter the chest and cause a pneumothorax before finding the subclavian vein. A needle directed too steeply in its approach will quickly enter and exit the triangle where the subclavian vein is found, penetrate the intercostal space, and puncture the lung (Figure 4).

Figure 4 Central venous cannulation of subclavian vein. (A) The clavicle and rib cage form a triangle through which the subclavian vein courses. The vein runs roughly parallel to both the clavicle and the rib cage. (B) A needle, once passed under the clavicle, directed parallel to the rib cage and clavicle will have a far greater chance of finding the vein. A needle directed too steeply will enter and exit this triangle, penetrate the intercostal space, and puncture the lung.

(Redrawn from Grant’s Atlas of Anatomy, 11th ed., Philadelphia, Lippincott Williams & Wilkins, 2004, figures 1.8, 6.29, pp. 9, 379.)

The second rib inserts into the sternomanubrial junction (angle of Louis). This can be easily palpated in most people as a horizontal ridge in the sternum or where the two planes that make up the sternum intersect (Figure 5). The interspace immediately below the angle of Louis is the second interspace. The angle of Louis serves as a landmark to rapidly locate the second rib and second interspace for placement of a catheter to decompress a tension pneumothorax or to place an anterior tube thoracostomy for an apical pneumothorax.

Figure 5 Sternum, lateral view. Angle of Louis can be palpated in the midline as a raised horizontal ridge or as the point where the plane of the manubrium and body intersect. The second rib articulates directly lateral to the angle of Louis.

(Redrawn from Gray’s Anatomy, 39th ed., figure 57.5, p. 954.)

The first rib is short, broad, flat, and arches sharply from posterior to anterior (Figure 6). The second rib is longer than but very similar to the first rib (Figure 7). The first slip of the serratus anterior muscle attaches to the second rib approximately one-third of the arc from posterior to anterior—this slip also attaches to the inferior aspect of the first rib. Posterior to this attachment, the scalenus posterior attaches to the second rib.

Figure 6 Right first rib.

(Adapted from Gray’s Anatomy, 20th ed., plate 124.)

Figure 7 Right second rib.

(Adapted from Gray’s Anatomy, 20th ed., plate 125.)

When performing a thoracotomy, counting ribs can identify the correct interspace. Once the latissimus dorsi muscle has been divided and the serratus anterior muscle divided or swept anterior, the scapula is elevated. Thin fibrous attachments hold the undersurface of the scapula to the chest wall. A hand placed deep to the scapula, posterior near the spine, and apically can palpate ribs. The first rib is identified by its conspicuously broad and flat contour. Inferior to this, the second rib can be identified by the attachment of the scalenus posterior muscle. This muscle body is palpable by sweeping the finger from posterior to anterior along the second rib (Figure 8). Less distinct will be the third rib, which seems to “turn the corner” from the apex of the chest to the lateral chest wall (Figure 9). In a lateral decubitus position, the tip of the scapula overlies the sixth interspace. In a male, the nipple overlies the fourth interspace.

Figure 8 Counting ribs. Once the rib cage is visualized, the scapula is elevated, and a hand placed posterior and superior to palpate and count ribs. Note that the first rib is broad, short, and flat; the second rib has the insertion of the scalenus posterior; and the third rib “turns the corner” from apex to lateral chest wall.

Figure 9 Third rib “turns the corner.” Anteroposterior chest radiograph illustrating how the rib cage forms a loose box with the third rib at a corner. Arrows denote third rib.

MUSCLES OF THE CHEST WALL

Integral to safe thoracentesis, placement of a tube thoracostomy, or a thoracotomy, is understanding the layers of the chest wall and the anatomy of the interspace.

The paired pectoralis major muscles cover the majority of the anterior chest wall. The pectoralis major muscle originates from the clavicle and anterior aspects of ribs 1 through 6 inserting on the proximal humerus. Its origin from the chest wall is broad and an anterior thoracotomy will divide or separate its fibers. Inferiorly, the rectus abdominus muscle inserts onto the costal cartilages of ribs 5 through 7 and the xiphoid process. Lateral to this, the muscle fibers of the external oblique insert onto ribs 5 through 12. The external oblique muscle interdigitates with the serratus anterior muscle as it inserts on ribs 1 through 8 (Figure 10). Most thoracotomies do not traverse the interspaces guarded by the rectus abdominus and external oblique. These muscles will be encountered with thoracoabdominal incisions crossing the costal margin.

Figure 10 Muscles of thorax: left lateral view.

(Adapted from Gray’s Anatomy, 20th ed., plate 392.)

Laterally and posteriorly, two musculo-fascial layers guard the ribs. The more superficial layer contains the latissimus dorsi muscle laterally. Posteriorly, at the ausculatory triangle, or posterior border of the latissimus dorsi, this layer becomes a thin but tough layer of fascia, which more posteriorly envelopes the trapezius muscle. The second musculo-fascial layer contains the serratus anterior muscle laterally, becoming a broader sheet of thin but tough fibrous tissue posteriorly and then becoming the rhomboid major muscle then the rhomboid minor muscle posteriorly and superiorly (Figure 11). A tube thoracostomy will traverse these muscle layers to reach the ribs and interspaces. Knowing where you are in these layers allows precious time to be saved in traversing them and getting to where you need to be to complete the procedure.

Figure 11 Muscles of the thorax. (A) Superficial layer containing latissimus dorsi m. and trapezius m. (B) Deep layer containing serratus anterior m., rhomboid major m., and rhomboid minor m.

(Redrawn from Grant’s Atlas of Anatomy, 11th ed., Philadelphia, Lippincott Williams & Wilkins, 2004, figures 4.47, 4.48, 6.13, pp. 233, 234, 367.)

A typical tube thoracostomy is placed in the fifth interspace at the anterior axillary line. The muscle bodies traversed are thinner here. From superficial to deep, the surgeon will separate skin, subcutaneous fat, the latissimus dorsi/trapezius musculo-fascial layer, and then the serratus anterior musculo-fascial layer. At this depth, the shiny surface of the periosteum of the ribs and the oblique fibers of the external intercostal muscle can be seen. As discussed later, tube thoracostomies are performed over the superior aspect of the rib. It is much easier to locate the superior aspect of the rib when you do not have intervening layers of muscle and fascia.

A thoracotomy can be fashioned to divide or spare these muscles as needed in order to gain access to the rib cage. A full thoracotomy will divide the latissimus dorsi laterally and the trapezius posteriorly. The incision sweeps from horizontal across the lateral chest to vertical and parallel to the spine posteriorly (Figure 12). Deep to this layer, the serratus anterior can be swept anterior or divided. Posteriorly, the fascial layer coming off the serratus anterior is divided and then the rhomboid major and rhomboid minor muscles are divided. The innervation of the trapezius muscle and rhomboid muscles runs from medial to lateral. The more muscle body that is left medially, the more muscle function will be retained. Enough muscle needs to be left attached to the scapula to allow suture repair of the muscle, and the muscle should not be stripped from the scapula. The posterior and vertical aspect of this incision where the trapezius and rhomboids are divided is done to elevate the scapula off the chest wall, to access the interspaces underneath.

Figure 12 Full posterolateral thoracotomy.

(Redrawn from Grant’s Atlas of Anatomy, 11th ed., Philadelphia, Lippincott Williams & Wilkins, 2004, Figs. 4.47, 4.48, 6.13, pp. 233, 234, 367.)

A thoracotomy can be extended anterior, dividing the pectoralis major muscle overlying the interspace of interest. The sternum can be split transversely, and a thoracotomy continued on the contralateral side. This is termed a “clam shell” thoracotomy. The left and right mammary artery will be found 1 cm lateral to and on either side of the sternum, deep to the ribs and intercostal muscles, but superficial to the pleura. These vessels can be cauterized if speed is needed, but are prone to spasm and late bleeding, and should be sought and ligated when possible.

INTERCOSTAL SPACE

Each intercostal space from superficial to deep, has two layers of muscle; an artery, a vein, and a nerve; and a diminutive inner layer of muscle. The external intercostal muscles run obliquely with fibers in the same orientation as the external oblique muscle of the abdomen (fingers in pockets). Deep are the internal intercostal muscles running in the opposite direction. The intercostal artery, vein, and nerve run along the inferior aspect of each rib, occasionally running underneath a ledge in the costal groove. To avoid injury to these three structures, tube thoracostomies and thoracotomies are directed over the superior aspect of each rib or through the middle of the interspace, but not the inferior aspect of the rib (Figure 13). The innermost intercostal muscles are located deep to the neurovascular bundle and run in the same direction as the internal intercostal muscles. While mentioned in anatomy texts, surgically, the innermost intercostal muscles do not need to be considered separately from the internal intercostal muscle (Figure 14). The intercostal arteries originate as segmental branches off the descending aorta. The intercostal space, including the underlying pleura, can be harvested as a posteriorly based pedicled muscle flap (Figure 15). This flap is useful for reinforcing bronchial or esophageal repairs.

Figure 13 Technique for tube thoracostomy. Tubes placed emergently for trauma are placed along the anterior axillary line, fourth or fifth interspace. The intercostal space is entered on the superior aspect of the rib. Finger palpation confirms entrance into the pleural cavity and avoids inadvertent subscapular or intraabdominal placement of tubes as well as injury to adhesed lung.

(From Moore FA, Moore EE: Trauma resuscitation. In Wilmore DN, Brennan MF, Harken AH, et al., eds. Care of the Surgical Patient. New York, Scientific American, 1989.)

Figure 14 Intercostal space in cross-section. Note main intercostal bundle along the inferior aspect of the rib. The collateral nerve and artery, while present, are diminutive.

(Adapted from Crafts RC: A Textbook of Human Anatomy, 3rd ed., New York, John Wiley & Sons, 1985.)

Figure 15 (A) Intercostal muscle flap. Based on intercostal artery with pedicle posterior. (B) Transverse view of this flap.

The internal mammary artery originates from the subclavian arteries bilaterally, and descends on the inside of the chest wall, approximately 1 cm lateral to the sternum bilaterally (Figure 16).

Figure 16 Internal mammary arteries as viewed from inside the chest.

(From Pearson FG, editor: Thoracic Surgery, 2nd ed. Philadelphia, Churchill Livingstone, 2002, figure 48-8, p. 1330.)

PLEURAL SPACE

Normally the lung is coupled to the chest wall by the vacuum, which exists between the visceral and parietal pleura. With penetration of the chest wall air is allowed into the pleural space from the outside or more commonly, penetration of the lung allows air to escape from air spaces within the lung (alveoli, bronchioles, bronchi) into the pleural space. The coupling of the visceral and parietal pleura is broken and the potential space, which is the pleural space, becomes a real space. The elasticity of the lung causes it to collapse and a pneumothorax is formed. The pleural space extends superiorly to where it rises above the circumference of the first rib to inferiorly where the diaphragm inserts on the costal margin and the 12th rib. Lung may or may not be present between the diaphragm and ribs in the lower most recesses of the pleural space. Anterior to the pericardium and posterior to the sternum, the two pleural cavities can abut but rarely communicate.

DIAPHRAGM

The diaphragm is the movable dome-shaped partition between the thoracic and abdominal cavities. With full exhalation, the dome of the diaphragm can rise to the level of the fourth interspace anteriorly (nipple level). With full inhalation, the diaphragm flattens, bringing the thoracic cavity down to the level of the costal margin anteriorly and the 12th rib posteriorly. The muscle fibers of the diaphragm originate from the sternum, the ribs, and the vertebral column. All three groups insert on a tough, fibrinous central tendon. Fibers of the sternal portion are short, arising as small slips from the back of the xiphoid process. Laterally on either side of the xiphoid, fibers originate from the inner surface of the lower six costal cartilages (costal margin). Posteriorly, fibers originate from a thick band arching over the quadratus lumborum (lateral arcuate ligament) and the psoas major (medial arcuate ligament). The paired lateral arcuate ligaments extend from the tip and lower margin of the 12th ribs and arch over the quadratus lumborum muscle to the transverse processes of L1. The paired medial arcuate ligaments complete the journey, arching over the psoas major from the tip of the transverse process of the first lumbar vertebrae to the tendinous portion of each diaphragmatic crus (Figure 17).

Figure 17 Diaphragm as viewed from the abdomen. The diaphragm originates bilaterally from the xiphoid process, costal margin, lateral arcuate ligament, and medial arcuate ligament, and inserts into the central tendon. The left and right crus originate from the lumbar vertebral bodies and insert into the central tendon.

(Adapted from Langley LL, Telford IR, Christensen JB: Dynamic Anatomy and Physiology, 5th ed., New York, McGraw-Hill, 1980.)