Sternum

The sternum (plural: sterna or sternums) completes the anterior chest wall as the ventral breastplate.

Gross anatomy

The sternum is composed of a manubrium, a body and the inferior xiphisternum (a.k.a. xiphoid process). They articulate via secondary cartilaginous joints via hyaline cartilage with a fibrocartilaginous intervening disc: 

• the manubrium is flat and four-sided, wider superiorly
• the body is a flat rectangular bone that has grooves (facets) on its lateral border for articulation with the ribs:
  • approximately 20 cm long
  • 3-4 cm wide
  • 1 cm thick
• the xiphoid process is a thin bony projection inferiorly
  

Articulations

• superiorly the manubrium attaches to the neck where the two deep layers of cervical investing fascia insert
• the manubrium articulates with the first rib (primary cartilaginous), clavicle (atypical synovial), and body of the sternum (secondary or primary cartilaginous joint) ⁶
• the body articulates with the second rib at the sternomanubrial angle (of Louis , which is in the transthoracic plane of Ludwig) as well as the 3^rd to 7^th rib and costal cartilages
  • the articulations of 2^nd and 7^th ribs are shared as demifacets between each of the manubrium and the body, and the body and the xiphoid respectively 
• inferiorly articulates with xiphisternal joint, a secondary cartilaginous joint ⁶ (symphysis)

Attachments

Musculotendinous

• muscular attachments: sternocleidomastoid, intercostal, pectoralis major, sternohyoid, and sternothyroid muscles
• transversus thoracis muscle arises from the posterior surface of the body
• the xiphisternum attaches to linea alba

Ligamentous

• sternopericardial ligaments secure the fibrous pericardium to it
• interclavicular ligament
• anterior and posterior sternoclavicular ligaments (thickenings of the sternoclavicular joint capsule)

Arterial supply

• nutrient branches from internal thoracic (mammary) artery
• deep to the sternum is the internal thoracic artery laterally

Venous drainage

• tributaries of the internal thoracic veins, posterolateral to the sternum

Innervation

• nerve supply is via intercostal nerves which arise from the anterior rami of thoracic spinal nerves

Lymphatic drainage

• chain of internal mammary lymph nodes

Variant anatomy 

• episternal ossicle
• manubriosternal and sternoxiphoidal fusion
• sternal and xiphoid foramina
• sternal cleft and band
• sternal defect (notch)
• tilted sternum: oblique instead of horizontal orientation in the mediolateral plane ⁵
• suprasternal tubercle
• unfused sternal body segments
• xiphoid shape variations

See also

• pectus carinatum
• pectus excavatum