The sternum (plural: sterna or sternums) articulates with clavicles and ribs, completing the anterior chest wall as the ventral breastplate.
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Gross anatomy
The sternum comprises three bones which are firmly connected by symphyses (secondary cartilaginous joints).
The broader superior surface of the quadrilateral manubrium has a midline notch, the sternal notch. The manubrium is connected to the deep cervical fascia, three neck muscles and is notched at the sternoclavicular and first costochondral joints.
The body is a flat rectangular bone approximately 20 cm long, 3-4 cm wide and 1 cm thick that has concavities (facets) on its lateral border at the rib articulations.
The xiphoid process is a thin bony projection inferiorly.
Articulations
the manubrium articulates with the costal cartilage of the first rib (sternocostal joint; primary cartilaginous/synchondrosis joint), clavicle (sternoclavicular joint; atypical synovial joint), and body of the sternum (manubriosternal joint; secondary cartilaginous/symphysis joint typically) 6
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the body articulates with the costal cartilage of the second rib at the sternal angle (of Louis, which is in the transthoracic plane of Ludwig), as well as the costal cartilages of the 3rd to 7th ribs
the articulations of 2nd and 7th costal cartilages are shared as demifacets between the manubrium and the body, and the body and the xiphoid, respectively
inferiorly articulates with xiphisternum (xiphisternal joint, a secondary cartilaginous/symphysis joint) 6
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
tilted sternum: oblique instead of horizontal orientation in the mediolateral plane 5
unfused sternal body segments (sternebrae)
Development
Ossification
The manubrium forms from a single primary ossification center present at birth. The body of the sternum typically develops from four sternebrae (horizontal bony bars) with the first sternebra most often arising from a single primary ossification center prenatally and the second to fourth sternebrae most often arising from two or more primary ossification centers within each sternebra postnatally 8. Ossification is observed to occur in a superior to inferior direction, whilst fusion of the ossification centers between adjacent sternebrae occurs in an inferosuperior direction. Ossification continues up to 18-19 years of age with fusion commencing as early as 7 years in some individuals. The xiphisternum ossifies quite independently later in life.
A number of sternal variants arise due to morphological differences in ossification and fusion such as a sternal foramen, unfused sternebrae, or honeycomb sternum.
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