Breastbone
Sternum
location_on Anterior midline of the thorax, connecting the ribs
The sternum is a flat, dagger-shaped bone in the center of the anterior chest wall composed of three parts: the manubrium, body, and xiphoid process. It articulates with the clavicles and the first seven pairs of ribs (directly or indirectly), forming the anterior boundary of the thoracic cage. The sternum protects vital mediastinal structures and is the site of bone marrow biopsy in adults.
Key Anatomical Features
- Manubrium is the superior widest part, bearing the jugular (suprasternal) notch
- Sternal angle (angle of Louis) at the manubriosternal joint marks the level of the second rib, T4-T5 disc, and the tracheal bifurcation
- Body (gladiolus) articulates with costal cartilages 2-7
- Xiphoid process is the inferior cartilaginous tip that ossifies in adulthood
- Clavicular notches on the manubrium articulate with the medial clavicular ends
- Contains red marrow throughout life, making it accessible for bone marrow aspiration
Muscle Attachments
| Muscle | Attachment | Action |
|---|---|---|
| Pectoralis major | Anterior surface of manubrium and body | Adducts, flexes, and medially rotates the arm |
| Sternocleidomastoid | Superior manubrium | Flexes the neck and rotates the head |
| Sternohyoid | Posterior manubrium | Depresses the hyoid bone |
| Sternothyroid | Posterior manubrium | Depresses the larynx |
| Transversus thoracis | Posterior surface of body and xiphoid | Depresses the costal cartilages |
| Diaphragm (sternal part) | Posterior xiphoid process | Primary muscle of respiration |
Joints and Articulations
| Joint | Type | Connects to |
|---|---|---|
| Sternoclavicular joint | Synovial saddle with articular disc | Clavicle |
| Manubriosternal joint (sternal angle) | Cartilaginous symphysis (secondary) | Manubrium to body |
| Sternocostal joints | Synovial (2nd-7th) and synchondrosis (1st) | Costal cartilages of ribs 1-7 |
Common Pathologies
Sternal fracture
Usually from direct anterior chest trauma such as steering wheel impact. Associated with myocardial contusion and thoracic spine fractures. Pain worsens with breathing.
Pectus excavatum
Congenital depression of the sternum and lower costal cartilages creating a funnel chest. The most common chest wall deformity, occurring in about 1 in 400 births.
Pectus carinatum
Congenital protrusion of the sternum creating a pigeon chest. Less common than pectus excavatum but may cause psychosocial distress.
Median sternotomy complications
Sternal dehiscence and mediastinitis following cardiac surgery through a midline sternotomy incision. A serious complication occurring in 1-5% of sternotomies.
Clinical Relevance
The sternal angle (angle of Louis) is one of the most important surface landmarks in clinical medicine: it marks the level of the second costal cartilage (starting point for rib counting), the T4-T5 intervertebral disc, the tracheal bifurcation, the superior limit of the heart, and the beginning and end of the aortic arch. Bone marrow aspiration is commonly performed at the manubrium or body in adults because the sternum retains red marrow throughout life.
Development and Ossification
The sternum develops from fusion of two cartilaginous sternal bars in the midline. Ossification proceeds from six centers: one for the manubrium, four for the body (sternebrae), and one for the xiphoid. The sternebrae begin to fuse from inferior to superior starting at puberty, completing in adulthood. The xiphoid may not ossify until the 4th decade.
Did You Know?
- The sternal angle is named after French physician Pierre Charles Alexandre Louis, not King Louis
- The xiphoid process can sometimes be mistaken for an abdominal mass by patients who discover it by palpation
- In CPR, chest compressions are performed on the lower half of the sternum, compressing the heart between the sternum and spine
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