Sternum | Chest bone
Anatomy of sternum:
It consists of three parts;
- Manubrium sterni: The uppermost part
- Body (shaft): The middle part
- Xiphoid process: The lowest part
Ossification of sternum:
Ossification occurs by a total of 6 centers. One is for manubrium, four for body and one for xiphoid process.
It is the angle formed between the manubrium and body of sternum. The vertex of this angle lies at the manubriosternal joint.
Anatomy of Sternum:
The sternum (chest bone) lies in the midline of the anterior chest wall. It is a flat bone that can be divided into three different parts: Manubrium sterni, Body of sternum and Xiphoid process.
It is the upper-most part of the sternum that articulates with:
- The right and left clavicles
- First costal cartilages of both sides
- Upper part of the second costal cartilage of both sides
It is quadrangular in form and lies at the level of third and fourth thoracic vertebrae of the spinal cord. The superior border of manubrium is thick, actually the thickest part of the bone and forms the jugular notch. On the surface of human body, the jugular notch can be easily seen as depression just above the manubrium. The anterior surface of this part of the bone is convex from side to side while the posterior surface is concave.
Body of sternum:
It is longer, narrower and thinner as compared to the manubrium sterni, with which it articulates superiorly at the manubriosternal joint, which is of fibrocartilaginous type. At the lower border, it articulates with the xiphoid process at the xiphisternal joint. The body of sternum is of flat contours and contains notches on each side for articulation with the costal cartilages. There are a total of 12 costal cartilages but only first 7 articulate with the sternum directly.
It is the smallest and lowest part of the bone and is actually a thin plate of hyaline cartilage. It may become ossified at its proximal end in the adult life but it is purely cartilaginous in children and teenagers. It is not attached to ribs or costal cartilages.
Ossification of Sternum:
The sternum originally consists of two cartilaginous bars, situated one on either side of the median plane and connected with the cartilages of the upper ribs. These two bars fuse with each other along the middle line to form the cartilaginous sternum which is ossified from six centers:
- One for manubrium sterni: Appears at the sixth month and rarely unites with other centers except at the old age.
- Four for the body: The first of these four appears at sixth month, second and third at 7th month and fourth appears one year after birth. They unite with each other after puberty.
- One for the xiphoid process: It appears at 5th to 18th year of life but remains partly cartilaginous.
It is also known as Angle of Louis and is formed by the articulation of the body of sternum with the manubrium sterni. It is present in the form of ridge at the site of the manubriosternal joint and lies opposite the intervertebral disc between third and fourth thoracic vertebrae. The sterna angle lies adjacent to the second intercostals space fairly consistently and for this reason, it is used as a guide to count the ribs and intercostals spaces.