The thorax is enclosed by the thoracic wall (also called the thoracic cage). The bony parts of the thoracic wall are: vertebrae (T1-T12), ribs, intervertebral discs, and the sternum. In this article we shall look at the anatomy of these component parts.
The 12 pairs of ribs form the majority of the protective cage of the thorax. They are curved, flat bones, which continue anteriorly as cartilage, called costal cartilage.
All the ribs are attached posteriorly to the vertebral column, but their anterior attachment varies:
- Ribs 1-7 attach independantly to the sternum
- Ribs 8 – 10 attach the costal cartilages superior to them
- Ribs 11 and 12 do not attach anywhere, and end in the abdominal muscles. They can be known as floating ribs.
Ribs can be classified as typical or atypical. Ribs 1,2,11 and 12 are atypical – but before we look at those, we’ll go over the standard structure of a rib:
The head is wedge shaped, and has two articular facets separated by a wedge of bone. One facet articulates with the numerically corresponding vertebrae, and the other articulates with the vertebrae above. The neck contains no bony prominence, just simply connects the head with the body. Where the neck meets the body there is a roughed tubercle with a facet for articulation with the transverse process of the corresponding vertebrae. The body, or shaft of the rib is flat, and curved. The internal surface of the shaft has a groove for the neurovascular supply of the thorax, protecting it from damage.
Rib 1 is shorter and fatter than the other ribs. It only has one facet on its head for articulation with its corresponding vertebrae (there isn’t a thoracic vertebrae above it). The superior surface is marked by two grooves, which make way for the subclavian vessels
Rib 2 is thinner and longer than rib 1, and has two articular facets on the head as normal. It has a roughened area on its upper surface, where the serratus anterior attaches.
Ribs 11 and 12 have no neck, and only contain one facet, which is for articulation with the corresponding vertebrae
Clinical Relevance: Rib Fractures
Rib fractures most commonly occur in the middle ribs. This is a consequence of crushing injuries or direct trauma to the ribs. A common complication of rib injury is further soft tissue injury from the broken end. Possible complications are injuries to the lungs, spleen or diaphragm.
If more than one rib is fractured, the affected area is no longer under control of the wall, and moves paradoxically. This condition is known as flail chest. It can be very serious, as it impairs full expansion of the rib cage, affecting the oxygen content of the blood
The vertebrae that articulate with the ribs are termed thoracic vertebrae. They are distinctive from other classifications of vertebrae:
On the body of the vertebrae there are two costal facets, which articulate with the head of its respective rib, and the rib inferior to it. On the transverse processes of the thoracic vertebrae there is a costal facet for articulation with its respective rib. The spinous processes are slanted inferiorly and anteriorly. This offers increased protection to the spinal cord, preventing an object like a knife entering the spinal canal through the intervetebral discs.
The sternum (or breastbone) is split into three parts; the manubrium, the sternum and the xiphoid process.
The manubrium is the most superior portion of the sternum. It articulates with the clavicle on its superior surface, forming a distinct depression called the jugular notch. On the lateral edges there is a facet for articulation with the costal cartilage of the 1st rib, and a demifacet (meaning half facet) for articulation with part of the costal cartilage of the 2nd rib.
Immediately inferior to the manubrium is the body. It is flat and elongated. Like the manubrium, the lateral edges of the body have facets for articulation with the costal cartilage of ribs 3-6, and demifacets for articulation with ribs 2 and 7.
The xiphoid process is the most inferior and smallest part of the sternum. It is variable in shape and size, and ossifies completely by the age of 40. Its inferior end is found at the vertebral level of T10
Clinical Relevance: Fractures of the Sternum
Sternal fractures are relatively uncommon, and are usually associated with severe trauma. The fracture is classified as comminuted, as the bone breaks into several pieces. Despite the degree of damage to the sternum, the fragments are not usually displaced, due to the attachment of the pectoral muscles.
Sternal fractures have a high mortality rate (25-45%). This is not due to the fracture itself, but usually as a result of heart and lung injuries, which are likely to occur simultaneously with the primary fracture. Because of this, it is crucial to check patients with sternal fractures for visceral injury.