The sternum is a flat bone in the midline of the anterior chest wall, meaning the front wall of the chest. It lies between the right and left ribs and connects with the clavicles, or collarbones, and the costal cartilages, the cartilage bars connecting ribs towards the front of the chest.
Where is the sternum located?
The sternum occupies the sternal part of chest, the central region of the front chest wall. Its broad upper end lies between the medial ends of the clavicles, meaning the ends nearest the midline. Its lower end extends towards the junction between the chest and abdomen.
Within the skeletal system, the sternum forms the anterior anchor of the rib cage. The ribs curve forwards from the vertebral column, or spine. The first seven pairs connect to the sternum through their own costal cartilages; the eighth to tenth pairs usually connect indirectly through the cartilage above, while the eleventh and twelfth pairs have no anterior connection to the sternum. The sternum is wider above and narrower at its lower tip.
Behind it is the mediastinum, the central compartment of the chest between the lungs. The pericardium, the sac surrounding the heart, and the heart lie behind its lower portion, with intervening connective tissue. The pleurae, the membranes associated with the lungs, approach its posterior surface from either side.
What are the parts of the sternum?
The sternum has three parts, arranged from above downwards. Their shapes and connections distinguish them even though they form a continuous central support for the chest wall.
| Part | Position and shape | Main connections |
|---|---|---|
| Manubrium | Broad upper part | Clavicles, first costal cartilages and part of each second costal cartilage |
| Body of sternum | Elongated middle part | Second to seventh costal cartilages |
| Xiphoid process | Small lower projection | Body of sternum, part of each seventh costal cartilage and attachments of muscles and connective tissue |
The manubrium and body of sternum meet at a slight angle rather than forming a completely straight plate. The xiphoid process varies in shape and may be pointed, rounded or divided at its tip. It is initially cartilaginous and usually ossifies with age, although the timing and extent of ossification vary.
What are the main sternal landmarks and surfaces?
The jugular notch is the central indentation along the superior border of the manubrium; superior means towards the head. It is also called the suprasternal notch. The clavicular notches lie on either side and receive the medial ends of the clavicles.
The sternal angle is the palpable ridge at the junction of the manubrium and body of sternum; palpable means detectable by touch. The second costal cartilage meets the sternum at this level, making the angle a standard landmark for identifying ribs. In the anatomical position, a transverse plane, a horizontal plane through the body, passing through the angle is conventionally described as reaching the disc between the fourth and fifth thoracic vertebrae, the vertebrae of the chest region.
The anterior surface faces forwards and provides muscle attachments. The posterior surface faces into the chest. Along the lateral borders, meaning the sides, costal notches receive the costal cartilages; the body may also retain transverse ridges marking fusion of its developing segments.
Which joints connect to the sternum?
A joint is a connection between skeletal elements. The sternum has connections with the clavicles, costal cartilages and its own neighbouring parts.
- Sternoclavicular joints: these join the clavicles to the manubrium. They are synovial joints, meaning they have a joint cavity enclosed by a capsule, and each contains a fibrocartilaginous disc, a pad of tough cartilage.
- Sternocostal joints: these join the first seven costal cartilages to the sternum. The first is a primary cartilaginous joint, in which the elements are united by cartilage without a joint cavity. The second to seventh are generally synovial joints.
- Manubriosternal joint: this joins the manubrium to the body of sternum. It is usually a symphysis, a joint united by fibrocartilage, and may become fused.
- Xiphisternal joint: this joins the body of sternum to the xiphoid process. It is a primary cartilaginous joint that may ossify, meaning become replaced by bone.
The second costal cartilage attaches at the manubriosternal junction. This explains why the sternal angle and the second rib are closely related anatomical landmarks.
Which muscles attach to the sternum?
The pectoralis major muscle, a large muscle on the front of the chest, takes part of its origin from the anterior sternum. An origin is an attachment that is usually relatively fixed during a muscle's action. The sternocleidomastoid muscle, which helps move the head and neck, has a sternal attachment on the anterior manubrium.
The sternohyoid and sternothyroid muscles, which help position structures in the front of the neck, attach to the posterior manubrium. The transversus thoracis muscle attaches to the posterior lower sternum and xiphoid process and extends towards costal cartilages. The diaphragm, the main muscle of breathing, and the rectus abdominis muscle of the anterior abdominal wall also attach to the xiphoid process.
What does the sternum do?
The sternum supports the front of the rib cage and helps protect the heart and other structures behind it. Its clavicular connections link the upper limbs to the central skeleton, while its muscle attachments support movements of the upper limb, neck and trunk.
During breathing, movement of the ribs and their cartilages changes the dimensions of the chest. The sternum moves with these connections, contributing to expansion of the chest from front to back during inspiration, the drawing of air into the lungs.
How can you locate the sternum in the 3D model?
In the 3D model next to this article, rotate to the front of the chest, zoom towards the midline and select the sternum. You can isolate it or fade neighbouring structures to examine its position.
Rotate towards a side view and adjust the zoom. You can also hide neighbouring structures, switch layers or cut a section along the sagittal, coronal or transverse plane. Use these controls together with selection and isolation to change the view.