The sternum, also called the breastbone, is a relatively small, somewhat T-shaped bone that lies in the anterior midline of the thorax. It participates in forming the anterior wall of the thoracic cage and is connected directly or indirectly to the ribs via their costal cartilages. Additionally, the sternum links the shoulder girdle to the thorax via the clavicle.
Overall, the sternum protects the internal organs of the thorax, such as the heart and lungs, from mechanical injury. Also, it provides attachment sites for various muscles and ligaments, such as the sternocleidomastoid and sternohyoid muscles and the anterior sternoclavicular ligament.
Parts and landmarks
The sternum consists of three parts:
- Manubrium - broad most superior portion; this part is massive and appears quadrangular-shaped with four borders; it is also the thickest and widest part of the bone; it articulates with both clavicles and the first rib pair;
- Jugular notch - a midline depression that is found at the center of the superior border of the manubrium; it is also known as the suprasternal or presternal notch;
- Clavicular notches - two indentations that are found at the superior corners of the manubrium; they are located on either side of the jugular notch; these notches serve as the sites at which the left and right clavicles connect with the sternum, forming the sternoclavicular joints;
- First costal notches - two indentations that are found on the sides of the manubrium below the clavicular notches; they serve as the sites at which the cartilages of the first ribs attach to the manubrium, forming the sternochondral joints;
- Body (corpus) - middle and central portion of the sternum that articulates with the second to seventh ribs; it is the longest part, and it may completely or partially fuse with the manubrium; also, unfused variation between both parts is possible;
- Costal notches - several depressions that are found on the sides of the body; they represent the articulation sites for the costal cartilages of the second to seventh ribs; the manubrium and the body together form the articulation sites for the second costal cartilages (both parts have demi-facets); the body and the xiphoid process together form the articulation sites for the seventh costal cartilages (both portions have demi-facets);
- Transverse ridges - they represent the lines of fusion between the sternal segments and can be seen as horizontal lines on the anterior surface of the body; they pass through the right and left costal notches for the costal cartilages of the ribs 3 - 5;
- Xiphoid process (xiphisternum) - narrow triangular-shaped tip of the sternum; it is the most inferior part that appears pointed; it is also the most variable portion and is often fused together with the body.
The lower border of the manubrium articulates with the superior border of the body, forming an angle called the sternal angle. The articulation between the manubrium and the body is called the manubriosternal joint, while the joint between the body and the xiphoid process is known as the xiphisternal joint.
Note: Up to 10% of the population may have a small opening in their sternal bodies. In that case, it is called the sternal foramen.