Content Reviewers:Scott Caterine, BSc (Hons.), MSc, MB, BCh, BAO (Hons.), Viviana Popa, MD
The thorax is the area between the neck and abdomen and consists of the thoracic cavity, its contents, and the walls that surround it.
The thoracic skeleton forms the osteocartilaginous thoracic cage that protects the thoracic viscera and some of the abdominal organs.
This skeleton is made up of the sternum, 12 pairs of ribs and associated costal cartilages, as well as 12 thoracic vertebrae and intervertebral discs interposed between them.
The ribs and costal cartilages form the largest part of the thoracic cage and both can be identified numerically, going from 1 to 12 for the ribs, and going from 1 to 10 for the costal cartilage, as ribs 11 and 12 do not have an associated costal cartilage.
Now, the true thoracic wall includes the thoracic cage and the muscles between the ribs and the skin, as well as subcutaneous tissue, muscles and fascia that cover its anterolateral aspect.
The structures that cover its posterior aspect are considered to belong to the back.
Additionally, the mammary glands of the breasts lie within the thoracic wall subcutaneous tissue, and the anterolateral axioappendicular muscles are also considered part of the thoracic wall.
Okay, let’s start talking about the ribs, which form most of the thoracic cage.
They’re curved, flat, lightweight, and highly resilient bones.
Each rib has a spongy interior that contains bone marrow or hematopoietic tissue which forms blood cells.
There are three types of ribs based on their connection to the sternum: true, false, and floating ribs.
The true or vertebrosternal ribs are the first 7 pairs of ribs, and they attach directly to the sternum through their own costal cartilages.
Consequently, their connection to the sternum is indirect.
The floating ribs or vertebral or free ribs are the 11th, the 12th pair of ribs, and sometimes the 10th pair of ribs.
The cartilages of these ribs don’t connect anteriorly to the sternum at all; instead they end up in the posterior abdominal musculature.
Ribs can further be classified as either typical or atypical ribs.
Typical ribs are the 3rd through the 9th pair of ribs and have the following components: head, neck, tubercle, and body.
On a posterior view, the head of a typical rib is wedge shaped and has two facets - superior and inferior - that are separated by the crest of the head.
The inferior facet articulates with the numerically corresponding vertebra, and the superior facet articulates with the vertebra above it.
The neck connects the head with the body at the level of the tubercle.
The tubercle, located at the junction of the neck and body, has a smooth articular part that connects with the corresponding transverse process of the vertebra and a rough nonarticular part where the costotransverse ligament attaches.
The body is thin, flat, and curved, especially at the costal angle, where the rib makes an anterolateral turn.
This angle also demarcates the lateral limit of attachment of the deep back muscles to the ribs.
The internal surface of the body is concave and has a costal groove that’s parallel to the inferior border of the rib, which provides protection for the intercostal nerves and vessels at that level.
Now, the 1st, 2nd, 10th through 12th rib are atypical ribs because they are unlike the typical ribs in various ways.
So, the 1st rib is the broadest, shortest, and most sharply curved of the seven true ribs.
Its body is the widest and is nearly horizontal.
The grooves are separated by a scalene tubercle and ridge to which the anterior scalene muscle is attached.
It also has a tubercle, which articulates with the transverse process of T1.
Now, the 2nd rib has a thinner and less curved body, and it’s longer than the 1st rib.
Its head has two facets for articulation with the bodies of T1 and T2 vertebrae.
The 10 through 12th ribs, like the 1st rib, only have one facet on their heads and articulate with a single vertebra.
The 11th and 12th ribs are short and don’t have a neck or tubercle.
Now, the head of each rib articulates with thoracic vertebrae via costovertebral joints, which are synovial plane joints.
Regarding ribs 2 through 9, the head of each rib articulates with the superior costal facet of the vertebral body of the same number and with the inferior costal facet of the vertebral body above it and to the intervertebral disc between them.
So, for example, the head of rib 4 articulates with the superior costal facet of the vertebral body of T4 and with the inferior costal facet of the vertebral body of T3 and with the intervertebral disc between those two vertebrae.
Unlike ribs 2 through 9, the 1st, 11th, 12th, and sometimes the 10th rib only articulate with a single vertebral body at the same vertebral level as the rib.
For example, rib 11 articulates with T11.
Then there are the costotransverse joints, which are also synovial plane joints.
Regarding ribs 1 through 10, the tubercle of the rib articulates with the transverse process of the vertebra of the same number.
Ribs 11th and 12th don’t articulate with the transverse process of vertebrae of the same number.
Now, let’s take a deep breath and play a fun game.
Let’s see if you can identify which atypical rib we’re showing in these images and what the structures are that form a typical rib.
Moving on to the sternum, which is a flat, elongated bone that forms the middle of the anterior part of the thoracic cage.
The sternum is the protector of the mediastinal viscera, especially the heart.
It consists of three parts: manubrium, body, and xiphoid process.
Let’s now identify each structure and their components one by one in both anterior and lateral views.
The manubrium is a trapezoidal bone and is the widest and thickest part of the sternum.
The jugular notch - or the suprasternal notch - is the superior and concave border of the manubrium and can be easily palpated.
The notch is deepened by the medial ends of the clavicles, which are much larger than the small clavicular notches in the manubrium found laterally on either side.
Now, the sternoclavicular joint is a saddle type of synovial joint and the joints formed between the ends of the clavicles and the clavicular notches of the manubrium as well as 1st costal cartilages.
Inferior and lateral to the clavicular notch, there’s the first sternocostal joint which is the costal cartilage of the 1st rib attached to the lateral border of the manubrium of the sternum, which is classified as a primary cartilaginous joint.
Between the manubrium and body of the sternum, there’s the manubriosternal joint, which is a secondary cartilaginous joint.
Now, the manubrium and body of the sternum lie in different planes superior and inferior to their junction.