The thoracic skeleton consists of the thoracic vertebrae posteriorly, the ribs laterally, and the sternum and costal cartilages anteriorly. The costal cartilages secure the ribs to the sternum. The thoracic cage forms a protective cage around vital organs such as the heart, lungs, and great blood vessels. The thoracic skeleton provides attachment points for the muscles of the back and chest that allow support of the shoulder girdle (scapula and clavicle) and movement of the upper limbs.
The 12 thoracic vertebrae articulate with the 12 pairs of ribs. Thoracic vertebrae typically bear two costal facets on each side, one at the superior edge and the other at the inferior edge of the vertebral body, where they receive the heads of the ribs (Figure 2-4A). The bodies of T10 to T12 vary from this pattern, however, by having only a single facet for their respective ribs. In addition, the T1 to T10 transverse processes have costal facets that articulate with the tubercles of the ribs.
A. Superior view of a thoracic vertebra articulating with ribs. B. Lateral and anterior views of the thoracic rib cage. C. Anterior and posterior views of a typical rib. D. Anterior view of the sternum.
Twelve pairs of ribs form the flared sides of the thoracic cage and generally extend anteriorly from the thoracic vertebrae to the sternum (Figure 2-4B). Rib pairs 1 to 7 are known as the true ribs and attach directly to the sternum by individual costal cartilages. The remaining five pairs of ribs are called false ribs because they either attach indirectly to the sternum or lack a sternal attachment entirely. Rib pairs 8 to 10 attach to the sternum indirectly by joining each other via costal cartilages immediately above. Rib pairs 11 and 12 are called floating ribs because they have no anterior attachments; instead, they are embedded in the muscles of the lateral body wall.
Oblique course of ribs.
The ribs course in an oblique, inferior direction from their thoracic vertebral articulation to their anterior sternal articulation. For example, rib 2 articulates with the T2 vertebra posteriorly but with the sternal angle at the T4 vertebral level anteriorly. Therefore, an axial section of the thorax, such as the one seen in a CT scan, intersects several ribs.
Ribs 2 to 10 are considered typical ribs and have the following bony landmarks (Figure 2-4C):
- Head and neck. Form costovertebral joints by articulating with the costal demifacets of adjacent thoracic vertebral bodies and intervertebral discs.
- Tubercle. Articulates with the costal facets of adjacent thoracic vertebral transverse processes (with the exception of ribs 11 and 12).
- Shaft. The long portion of the rib consisting of a smooth superior border and a sharp, thin inferior border possessing a costal groove housing the intercostal veins, arteries, and nerves. The distal end of the rib shaft articulates with the costal cartilage, forming costochondral joints.
Fracture of a rib
commonly occurs just anterior to the angle, the weakest point of the rib, and may puncture the parietal pleura, resulting in a pneumothorax
Ribs 1, 11, and 12 are atypical ribs. Rib 1 is not palpable because it lies deep to the clavicle. It has the scalene tubercle on the upper surface for the inferior attachment of the anterior scalene muscle. The groove for the subclavian vein is anterior to the scalene tubercle; the groove for the subclavian artery is posterior to the tubercle. Ribs 11 and 12 do not articulate with the sternum and receive the name “floating ribs.”
The term “costal” means rib or rib-like part.
The sternum, or breastbone, is flat and consists of the following structures (Figure 2-4D):
- Manubrium. Along its superior border, the manubrium contains the jugular notch at the T2 vertebral level. The notch is flanked on each side by a synovial sternoclavicular joint, which provides the only bony attachment between the upper limb and axial skeleton. Although the other joints between the costal cartilages and the sternum are synovial, the first sternochondral joint with rib 1 is cartilaginous. The upper half of rib 2 attaches to the body of the sternum at the manubriosternal joint, better known as the sternal angle.
- Sternal angle. The sternal angle (of Louis) is the junction between the manubrium and the sternal body. The junction can be palpated as a prominent transverse ridge largely because the posterior facing angle between the manubrium and the body is less than 180 degrees. The sternal angle is an important clinical landmark for two reasons. First, rib 2 articulates with the sternum at the level of the sternal angle. Accordingly, palpation of the sternal angle permits bilateral identification of rib 2 and all of the lower ribs. Second, the sternal angle marks the following important clinical levels:
- The level of the T4 and T5 vertebrae in an axial plane.
- The start and end of the aortic arch.
- The tracheal bifurcation into the right and left primary bronchi.
- The azygos vein courses over the right primary bronchus to join the superior vena cava.
- The border between the superior and middle mediastinum is demarcated.
- The thoracic lymphatic duct transitions from the right to the left side of the thoracic cavity.
- Sternal body. The sternal body articulates with the second through seventh costal cartilages, the manubrium, and the xiphoid process.
- Xiphoid process. The shape of the xiphoid process varies. It is located at the T9 vertebral level and articulates with the sternal body. The linea alba, a thickened vertical cord of connective tissue in the midline of the abdominal wall, attaches to the xiphoid process.