Normal Sternum
Structure	The sternum is composed of highly vascular cancellous tissue, covered by a thin layer of compact bone, which is thickest in the manubrium between the articular facets for the clavicles.
Ossification	The sternum originally consists of two cartilaginous bars (paired mesenchymal bands), situated one on either side of the median plane and connected with the cartilages of the upper nine ribs of its own side. These two bars fuse with each other along the middle line (at approximately 8 wks) to form the cartilaginous sternum. The cartilaginous sternum is ossified from six centers: · one for the manubrium · four for the body · one for the xiphoid process. The ossification centers appear in the intervals between the articular depressions for the costal cartilages, in the following order: · the manubrium and first piece of the body (month 6) · the second and third pieces of the body (month 7) · the fourth piece, during the first year after birth · the xiphoid process, between the fifth and eighteenth years of life. The centers make their appearance at the upper parts of the segments, and proceed gradually downward.  There may be two small episternal centers, which make their appearance one on either side of the jugular notch. Occasionally some of the segments are formed from more than one center, the number and position of which vary. Thus, the first piece may have two, three, or even six centers. ssure. Union of the various centers of the body begins about puberty, and proceeds from below upward. By the age of twenty-five they are all united. The xiphoid process may become joined to the body before the age of thirty, but this occurs more frequently after forty; on the other hand, it sometimes remains ununited in old age
Articulations	The sternum articulates on either side with the clavicle and upper seven costal cartilages.