The thoracic cage is mostly formed by the twelve pairs of ribs.
In general, the ribs can be categorized into two groups:
1) Based on articulations
- true (1 to 7),
- false (8 to 10), and
- floating (11 and 12) ribs
2) Based on structure
- typical (3 to 9) and
- atypical (1, 2, 10, 11, and 12) ribs
Classification based on articulations
Ribs 1 – 7 are true ribs (vertebrosternal ribs) because the costal cartilage from each of these ribs attaches directly to the sternum.
Ribs 8–12 are called false ribs (vertebrochondral ribs). The costal cartilages from these ribs do not attach directly to the sternum. For ribs 8–10, the costal cartilages are attached to the cartilage of the next higher rib. Thus, the cartilage of rib 10 attaches to the cartilage of rib 9, rib 9 then attaches to rib 8, and rib 8 is attached to rib 7.
The last two false ribs (11–12) are also called floating ribs (vertebral ribs). These are short ribs that do not attach to the sternum at all. Instead, their small costal cartilages terminate within the musculature of the lateral abdominal wall.
Classification based on structure
Ribs 3-9 are classified as typical ribs.
The ribs have different components which start from posterior to anterior. The structures of typical ribs are:
1. Head (caput costae)
Each typical rib has a head which is wedge-shaped and has two articular facets. These facets are separated by a slither of bone called the crest of the head. The bigger facet articulates with the numerically corresponding vertebral body. The smaller facet articulates with the vertebral body above.
Commonly unremarkable! It is stout – fat and thick. It gives attachment to the costotransverse ligaments.
This is a bony prominence (in other words – bump) between neck and body and projects posteriorly. It has a smooth articular part which attaches to the transverse process of the corresponding vertebrae, and a roughened non-articular portion which forms the attachments for the costotransverse ligament.
Also known as the shaft. It is thin, flat and curved. The shaft has the greatest degree of curvature at its angle of rib or costal angle which is just lateral to the tubercle. The angle of the rib will align with the medial border of the scapula in the anatomical position, and marks the attachments for some muscles of the back. If you look at the internal concave surface of the shaft there is a shallow costal groove inferiorly for passage of nerve and blood vessels.
Ribs 1 , 2, 10, 11, and 12 are atypical ribs. Let us look at the structures that make them atypical.
1. First rib
The first rib is the shortest, widest and thickest rib. It only has one articulating facet and articulates with the T1 vertebra body – there is no thoracic vertebrae above it. The superior surface is marked by two grooves for subclavian vessels – laterally for subclavian vein and medially for the subclavian artery. They are separated by The scalene tubercle which gives attachment to the anterior scalene muscle.
2. Second rib
The second rib is thinner and slightly longer than the first rib. The main atypical feature of this rib is the roughened tuberosity on the superior surface which also function as attachment for anterior scalene muscle.
3. 10th rib
Just like the first rib, the 10th rib also only has one articulating facet with T10.
4. 11th and 12th ribs
Both these ribs only have one articulating facets for their corresponding vertebra – T11 and T12 respectively. Additionally they have no neck and no tubercles.