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Sternum

Other Terms: Breast bone

Type

flat

Description

The sternum is the ventral bone of the thoracic wall. It forms from six segmental elements, or sternebrae, that fuse during development. The bone has the appearance of a sword with a wide handle called the manubrium, a tapering blade or body, and a sharp point-like apex named the xiphoid process. A distinct angle forms at the junction of the manubrium and the body. This angle is called the sternal angle. A horizontal plane extended posteriorly intersects the disc between the fourth and fifth vertebrae. This is an important plane that marks the lower end of the superior mediastinum. The lateral margins of the bone are notched for reception of the costal cartilages and clavicles. Its anterior surface is slightly convex, while the posterior surface is weakly concave. The bone is typical of flat bones in having thin outer lamellae of compact bone with a spongy core filled with red bone marrow.

Etymology

This word comes from the Greek word sternon meaning breast. It was first used by Claudius Galen in the 2nd Century. The Latinized form sternum, which was not used by the Romans, came into the English in the 17th Century.

Articulations

The sternum articulates with sixteen bones: the paired clavicles and the first seven pairs of ribs via the costal cartilages. The paired clavicular notches, facing posterolaterally on the superior margin of the manubrium, articulate with the rounded sternal ends of the clavicles. Along each lateral margin of the sternum are seven costal notches that receive the distal ends of the costal cartilages in a synovial joint capsule.

Ossification

Ossification of the sternum corresponds to the six developmental elements of the bone. The cartilaginous anlagen for the bone form as two vertical bars next to the median plane. Early in embryonic life these anlagen fuse to form the unpaired cartilaginous sternum. Six ossification centers appear in the intervals between the rib junction points. The first centers appear in the two superior segments during the sixth fetal month. During the seventh fetal month the next two segments appear. The fifth segment begins to ossify soon after birth. Appearing last, the center for the xiphoid process arises anywhere between the fifth and twentieth year. In early life the sternebrae of the vertebral body are separated by cartilage. Near puberty these segments begin to ossify and are usually completed by the twenty-fifth year. The xiphoid process is slow to ossify and usually does not join the body until after the fortieth year. The manubrium usually remains separate from the body. They are separated by a thin plate of cartilage. Sometimes they may fuse superficially, but below the superficial plate of bone their remains a central cartilaginous plate. This typical pattern of ossification can show considerable variation, with many additional ossification centers arising.

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