Anatomy, Thorax, Wall

Excerpt

The thoracic wall consists of a bony framework held together by twelve thoracic vertebrae posteriorly, giving rise to ribs that encircle the lateral and anterior thoracic cavity. The first nine ribs curve around the lateral thoracic wall and connect to the manubrium and sternum. Ribs 10 to 12 are relatively short and attach to the costal margins of the ribs just above them. Ribs 10 to 12, due to their short course, do not reach the sternum.

The first seven ribs are called true ribs and attach to the manubrium and directly attach to the body of the sternum. Ribs eight to ten only attach to the inferior part of the sternum via the costal cartilage. Ribs 11 to 12 are termed floating ribs because they do not attach directly to the sternum. Ribs eight to ten are known as false ribs because they lack direct attachment to the sternum. At the level of the spine, the ribs articulate with the costal facet of two opposing vertebrae. An articular capsule surrounds the head of each rib, and the attachment to the transverse process is made with the help of the radiate ligament. Once the ribs leave the vertebrae, they gently curve around the lateral thoracic wall and approach the anterior wall of the thoracic cavity.

The vertical bone of the chest, the sternum, defines the anterior chest wall. The three separate bone segments of different sizes and shape that make up the sternum include 1) the thick manubrium, 2) the long body of the sternum, and 3) the xiphoid process. The sternum develops independently of the ribs. In sporadic cases, the sternum may not fully form, and the underlying heart may be exposed.

The most superior portion of the sternum is the manubrium, and it is also the first to form during embryogenesis. The sternal body and xiphoid process soon follow the manubrium in development. Anatomically, the manubrium is located at the level of thoracic vertebral bodies T3 and T4. The manubrium is also the widest and thickest segment of the sternum. During a physical exam of the chest, one noticeable feature of the manubrium is the presence of the suprasternal notch. On either side of this notch, one will feel the thick attachment from the clavicles. Some thoracic surgeons will only make a midline incision in the manubrito access to the superior mediastinum, suprasternal goiter, or thymus.

The sternal body is located at the level of vertebral bodies T5 to T9, covers a significant portion of the mid-chest, and is very strong. To access the chest cavity, surgeons usually cut through the sternum with a mechanical saw.

The xiphoid process is a thin and very small bone. The bone's size may vary from 2 to 5 cm, and its shape is also variable. The xiphoid may appear bifid, oval, or curve inward or outward. The xiphoid is mostly cartilaginous in younger individuals but is nearly wholly ossified by age 40. By the age of 60 and over, the xiphoid is almost certainly completely calcified. To perform pericardiocentesis safely, the needle has to be placed directly underneath the xiphoid because the heart is just a few fingerbreadths below.