March 12, 2010
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Sternum

General features - The sternum (figs. 298 to 300) is along flat bone, forming the median portion of the anterior wall of the thorax. Its average length is about 17cm, and is rather greater in the male than in the female. Its upper end supports the clavicles, and its margins articulate with the cartilages of the first seven pairs of ribs. It consists of three parts, named from above downwards, the manubrium, the body and the xiphoid process; in early life the body consists of four segments or sternebrae. In its natural position the inclination of the bone is oblique from above, downwards and forwards. It is slightly convex in front, and concave behind; it is broad above, narrow at the junction of the manubrium with the body, below which it gradually widens as far as the level of the articulations of the cartilages of the fifth ribs, and then narrows quickly to its lower end.

The manubrium sterni is of a somewhat triangular form, broad and thick above, narrow below at its junction with the body. Its anterior surface is smooth, convex from side to side and concave from above downwards. Its posterior surface is concave and featureless. The superior border is thick, and presents at its centre the suprasternal (jugular) notch; on each side of this notch there is an oval articular surface, directed upwards, backwards, and laterally, for articulation with the sternal end of the clavicle, and termed the clavicular notch. The inferior border, oval and rough, is covered in the recent state with a thin layer of cartilage, for articulation with the upper end of the body. The lateral borders are each marked above by a depression for the reception of the first costal cartilage, and below by a small articular facet, which, with a similar one on the upper angle of the body, forms a notch for the sternal end of the costal cartilage of the second rib. Between the depression for the first costal cartilage and the facet for the second, the narrow curved edge slopes from above downwards and medially. The widest part of the sternum is at the level of the first costal cartilages.

The body of the sternum is longer, narrower, and thinner than the manubrium, and attains its greatest breadth close to the lower end. Its anterior surface, nearly flat, is directed forwards and upwards, and is marked by three transverse ridges,* which indicate the lines of fusion of four originally separate segments. A sternal foramen, of varying size and form, is occasionally seen at the junction of the third and fourth pieces of the body. The posterior surface, slightly concave, is also marked by three transverse lines, less distinct, however, than those on the anterior surface. The upper end is oval and articulates with the manubrium, the junction of the two forming the sternal angle, which can be felt through the skin without difficulty. The lower end is narrow, and articulates with the xiphoid process. Each lateral border (fig. 300), at its superior angle, has a small notch, which, with a similar one on the manubrium, forms a cavity for the reception of the sternal end of the cartilage of the second rib; below this four costal notches articulate with the sternal ends of the cartilages of the third, fourth, fifth and sixth ribs; the inferior angle has a small facet, which, with a similar one on the xiphoid process, forms a notch for the reception of the cartilage of the seventh rib. These articular depressions are separated by a series of curved edges, which diminish in length from above downwards, and correspond to the anterior ends of the intercostal spaces.

^* Paterson (The Human Sternum, 1904), found that these ridges were absent in 26-7 per cent.; that a ridge existed opposite the third costal cartilage in 69 per cent.; opposite the fourth in 39 per cent ; and opposite the fifth in 4 percent.

Figure 298

The xiphoid process is the smallest piece of the sternum and is thin and elongated. It is cartilaginous in youth, but, at its upper part, more or less ossified in the adult. Above, it articulates with the lower end of the body of the bone, and on the front of each superior angle there is a facet for a part of the cartilage of the seventh rib. The xiphoid process varies greatly; it may be broad and thin, pointed, bifid perforated, curved or deflected to one or other side.

Particular features- The manubrium lies opposite the third and fourth thoracic vertebrae. Its anterior surface on each side, gives attachment to the sternal origins of the pectoralis major and sternomastoid muscles. It's posterior surface gives origin to the sternothyroid muscle, opposite the first costal cartilage; above this level the most medial fibers of the sternohyoid usually arise from the bone (fig. 299). This surface forms the anterior boundary of the superior mediastinum and its lower part is related to the arch of the aorta, and its upper part to the innominate, left common carotid and left subclavian arteries. Its lateral portions are related to the lungs and pleurae. The suprasternal notch gives attachment to some of the fibers of the interclavicular ligament. On the lateral border no joint cavity is interposed between the manubrium and the first costal cartilage, and the union is of the nature of a primary cartilaginous joint.

Figures 299 - 300
Figures 299-300

The body lies opposite the fifth to the ninth thoracic vertebrae. Its anterior surface gives attachment on each side, to the articular capsules of the sternocostal joints and to the sternal origin of the pectoralis major muscle. Its posterior surface gives origin inferiorly to the sternocostalis (transversus thoracic) muscle, and presents numerous important relationships. On the right side of the median plane it is related to the right pleura and the thin, anterior border of the right lung, which intervene between it and the pericardium. To the left of the median plane the upper two pieces are related to the left pleura and lung. but the lower two are directly related to the pericardium. The borders give attachment to the anterior intercostal membranes in the intervals between the costal notches. With the exceptions of the first and the sixth the cartilages of the true ribs articulate with the sternum at the lines o€ junction of its primitive component segment; this is well seen in many of the lower animals, where the parts of the bone remain ununited longer than in man.

The Xiphoid process lies in the floor of the epigastric fossa. Its anterior surface gives insertion to the most medial fibers of the rectus abdominis and to the aponeuroses of the external and internal oblique muscles, its lower end gives attachment to the linea alba, and its borders to the aponeuroses of the internal oblique and transverses abdominis muscles. Its posterior aspect gives origin, on each side to some of the fibers of the diaphragm, and is related to the anterior surface of the liver.

Structure.-The sternum is composed of highly vascular spongy substance covered by a layer of compact bone, which is thickest on the manubrium between the clavicular notches.

Ossification.--In early fetal life the sternum consists of two cartilaginous sternal plates, one on each side of the median plane. Opposite the first eight pairs of ribs these plates fuse in the median plane about the eighth week to form the cartilaginous sternum; which is ossified from six center : one for the manubrium, four for the body, and one for the xiphoid process (fig. 301).

Figures 301 - 302
Figures 301-302

These centers appear in the intervals between the costal notches in the following order: in the manubrium and first piece of the body during the sixth month of fetal life; in the second and third pieces of the body, during the seventh month of fetal life; in the fourth piece, shortly before birth; and in the xiphoid process, in the third year or much later. Two small episternal centers sometimes appear, one on each side of the suprasternal notch ; they are probably vestiges of the episternal bone of the monotremes and lizards. The manubrium may have two, three, or more centers; when two are present, one is generally situated above the other, the upper being the larger. The first segment of the body seldom has more than one centre; but the second, third, and fourth segments are often ossified from two laterally placed centers.

Union between the centers for the body begins about puberty and proceeds from below upwards (fig. 302) ; by the age of twenty-five they are all united. The xiphoid process usually fuses with the body about the age of forty years, but may remain ununited in old age. In advanced life the manubrium is occasionally joined to the body by bone, but when this occurs only the superficial part of the intervening cartilage is converted into bone ; the central part remains unossified.