The ribs are a set of twelve paired bones which form the protective ‘cage’ of the thorax. They articulate with the vertebral column posteriorly, and terminate anteriorly as cartilage (known as costal cartilage).
As part of the bony thorax, the ribs protect the internal thoracic organs. They also have a role in ventilation; moving during chest expansion to enable lung inflation.
In this article, we shall look at the anatomy of the ribs – their bony landmarks, articulations and clinical correlations.
There are two classifications of ribs – atypical and typical. The typical ribs have a generalised structure, while the atypical ribs have variations on this structure.
The typical rib consists of a head, neck and body:
The head is wedge shaped, and has two articular facets separated by a wedge of bone. One facet articulates with the numerically corresponding vertebra, and the other articulates with the vertebra above.
The neck contains no bony prominences, but 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 vertebra.
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 the vessels and nerves from damage.
Ribs 1, 2, 10 11 and 12 can be described as ‘atypical’ – they have features that are not common to all the ribs.
Rib 1 is shorter and wider than the other ribs. It only has one facet on its head for articulation with its corresponding vertebra (there isn’t a thoracic vertebra 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, from which the serratus anterior muscle originates.
Rib 10 only has one facet – for articulation with its numerically corresponding vertebra.
Ribs 11 and 12 have no neck, and only contain one facet, which is for articulation with their corresponding vertebra.
The majority of the ribs have an anterior and posterior articulation.
All the twelve ribs articulate posteriorly with the vertebra of the spine. Each rib forms two joints:
- Costotransverse joint – Between the tubercle of the rib, and the transverse costal facet of the corresponding vertebra.
- Costovertebral joint – Between the head of the rib, superior costal facet of the corresponding vertebra, and the inferior costal facet of the vertebra above.
The anterior attachment of the ribs vary:
- Ribs 1-7 attach independently to the sternum.
- Ribs 8 – 10 attach to the costal cartilages superior to them.
- Ribs 11 and 12 do not have an anterior attachment and end in the abdominal musculature. Because of this, they are sometimes called ‘floating ribs’.
Clinical Relevance: Rib Fractures
Rib fractures most commonly occur in the middle ribs, as a consequence of crushing injuries or direct trauma. A common complication of a rib fracture is further soft tissue injury from the broken fragments. Structures most at risk of damage are the lungs, spleen or diaphragm.
If two or more fractures occur in two or more adjacent ribs, the affected area is no longer under control of the thoracic muscles. It displays a paradoxical movement during lung inflation and deflation. This condition is known as flail chest. It impairs full expansion of the ribcage, thus affecting the oxygen content of the blood. Flail chest is treated by fixing the affected ribs, preventing their paradoxical movement.