The thorax, consisting of the thoracic vertebrae, the ribs, and the sternum (Figs. 5–1A and B), has several important functions. It provides a base for the muscle attachment of the upper extremities, the head and neck, the vertebral column, and the pelvis. The thorax also provides protection for the heart, lungs, and viscera. Therefore, there needs to be a certain amount of inherent stability to the thorax. The structure of the rib cage significantly increases the stability of the thoracic spine in flexion/extension, lateral bending, and rotation.1,2,3,4 Probably the most important function of the chest wall is its role in ventilation. The process of ventilation depends on the mobility of the bony rib thorax and the ability of the muscles of ventilation to move the thorax.5,6
Anterior (A) and posterior (B) views of the thorax are shown, including its component parts: the sternum, 12 pairs of ribs and their costocartilages, and the thoracic vertebrae.
Function, especially ventilatory function, can be affected when pathology interferes with the structure of the bony thorax. For example, scoliosis is a pathological lateral curvature of the spine frequently associated with rotation of the vertebrae.7 A right thoracic scoliosis (named for the convex side of the thoracic curve) results in left lateral flexion of the thoracic spine (Fig. 5–2A). The coupled rotation in a typical right thoracic scoliosis causes the bodies of the vertebrae to rotate to the right and the spinous processes to rotate to the left. The right transverse processes of the vertebrae rotate posteriorly, carrying the ribs with them (Fig. 5–2B). This is the mechanism that causes the classic posterior rib hump of scoliosis. On the concave side of the scoliotic curve, the effects are just the opposite. The transverse processes of the vertebrae move anteriorly, bringing the articulated ribs forward. The rib distortion that results from the vertebral rotation is evident bilaterally in Figure 5–2A and 5–2B. These musculoskeletal abnormalities limit the range of motion of the rib cage and the spine and, therefore, decrease ventilatory abilities.8 The coupling and interaction of the bony thorax and the ventilatory muscles and their relationship to ventilation will be the focus of this chapter.
A. A right thoracic scoliosis (named for the side of the convexity) of 52° shows the evident rib distortion that results from accompanying rotation of the involved vertebrae. There is also a lumbar curve of 32°. B. The bodies of the thoracic vertebrae in a right scoliosis typically rotate to the right, resulting in posterior displacement of the right transverse process and the attached right rib, as well as anterior displacement of the opposite transverse process and left rib.