Sternum

STERNUM (sternum), represents a flat bone that closes the chest cavity in front, together with seven or eight pairs of ribs attached to it. Its upper edge, incisura jugularis, in an adult at moderate exhalation is located at the level of the intervertebral cartilage between the I-II thoracic vertebrae, the lower (processus xiphoideus) at the level of IX-X. The position of the sternum changes depending on the phase of respiration. In children the sternum stands higher, in old people it descends due to a change in the position of their ribs. Along its long axis it is inclined (obliquely downward and forward) so that the lower end is farther from the spine than the upper end and is slightly convex forward. Three parts are distinguished in it: the manubrium (manubrium sterni), the body (corpus), and the xiphoid process (processus ensiformis s. xiphoideus) (see figure). The first two are joined by a fibrous cartilage (synchondrosis sternalis), which in the adult age (with the exception of the costal cartilage of the I rib) fuses the manubrium and the body; the xiphoid process remains cartilaginous or partially ossifies with age. The connection of the manubrium and the body usually occurs at an angle, differently expressed and open posteriorly (angulus sternalis Ludovici). The manubrium (the widest part of the bone) on its upper edge has three semilunar notches: the middle - incisura semilunaris s. jugularis, and two lateral - incisurae claviculares, covered with cartilage and serving for connection with the sternal ends of the clavicles (articulatio sterno-clavicularis), between the articular surfaces of which a joint cartilage (discus articularis) is laid, dividing the joint cavity into two departments. In this cartilage sometimes there are two small bones (ossicula suprasternalia), representing a rudiment of the suprasternal apparatus (episternum) of ganoid fish. Immediately below are the areas for attachment of the first ribs (incisura costalis I), the cartilages of which usually, not forming a joint, directly pass into the sternum (synchondrosis). At the lower end of the manubrium there is a half-notch for the second ribs - incisura costalis secunda (dimidia), the other half of which is located on the adjoining end of the body of the sternum. The body of the sternum slightly expands downward and on its anterior surface has transversely located, differently expressed ridges, three in number, remnants of former cartilaginous layers separating each pair of ribs. On the edges, besides the aforementioned semilunar notch, there are five or six depressions - incisurae costales, where the cartilages of the ribs articulate. In the joint cavity of the II rib there is the interarticular sternocostal ligament (lig. sterno-costale interarticulare); sometimes this is observed in the other ribs articulating with the sternum, but usually after the age of 30 their connection turns into a synchondrosis. The xiphoid process varies greatly in shape and size and is sometimes bifurcated or has an opening. According to shape two variants of the sternum can be distinguished: 1) with a relatively large length there is a short manubrium and a uniformly wide body, and 2) with a generally small length the manubrium is long, and the body of uneven width; between them transitional forms (Reches) are possible. The bone consists of an outer and an inner plate and spongy substance laid between them. The structure and location of the latter are different depending on age and shape of the sternum, since large-celled fields, increasing in area with age, are located concentratedly in the first variant of shape and scattered in the second. In most cases the anterior compact plate is expressed more significantly and is stronger than the posterior. The nutrition of the bone occurs at the expense of branches of the internal mammary artery (a. mammariae int.), which penetrate the bone as periosteal branches, usually either at a right angle or retrograde. The following muscles are attached to the sternum. The pectoralis major (m. pectoralis major) on its anterior surface (sometimes its fibers approach those of the pectoralis major of the opposite side). The sternocleidomastoid (m. sterno-cleido-mastoideus) of the sternal head attaches to the upper edge and anterior surface of the manubrium of the sternum (manubrium sterni); the sternohyoid (m. sterno-hyoideus) on the posterior surface of the manubrium of the sternum and the sternoclavicular joint (artic. sterno-clavicularis); the sternothyroid (m. sterno-thyreoideus) in the same place, but somewhat more medially; the transverse thoracic muscle (m. transversus thoracis s. triangularis sterni) to the posterior surface of the xiphoid process of the sternum near its caudal edge; the rectus abdominis (m. rectus abdominis) with an individually differently developed portion to the ventral surface of the xiphoid process. The sternum develops from a paired rudiment of so-called sternal ridges, which are formed as a result of the expansion and subsequent fusion of the anterior ends of the V-VIII ribs, which are still cartilaginous at that time (Ratlike, Haswell, Parker). At first these ridges are separated by a layer of connective (membranous) tissue, and later, approaching each other in the median plane, fuse into an unpaired rudiment of the sternum. In the 6th month of intrauterine life centers of ossification appear in the form of one, less often two bone nuclei in the manubrium and several paired and one unpaired (total from 4 to 13) in the body of the sternum and one in the xiphoid process (Schwegel). The sternum is absent in fish and dipnoans, but in amphibians, reptiles and birds, as well as in mammals, it is a product of the thoracic ribs. The sternum of birds has certain peculiarities: in Ratitae it has the appearance of a plate to which ribs are attached, but in most Carinatae a ridge (carina) stands out along its median line, arising also from paired rudiments and serving for attachment of the wing muscles (m. pectoralis). Developing, these rudiments envelop the trachea, which is thus enclosed between the walls of the ridge. The ridge is present in flying fossil birds, in bats and is weakly expressed in non-flying Carinatae. Congenital changes in the shape of the sternum are conditioned by irregularities in its development, consisting in the cessation of growth of the sternal ridges, as a result of which their fusion does not occur or is incomplete. The presence of a wide longitudinal slit throughout the length of the sternum (fissura sterni congenita, thoracoschisis medialis) is associated with a significant displacement of organs and is observed only in non-viable acephala. In viable children, partial slits are more often observed, mainly in the upper part, in the form of a triangle with the base up and filled with connective tissue mass. Slits of the sternum are often accompanied by ectopia cordis (protrusion of the heart), in which cases usually a part of the cardiac sac is also absent. Changes in shape of secondary nature are conditioned by various deformations of the chest cavity (kyphoses, scolioses, rachitic curvatures - pectus carinatum, pectus excavatum).

Fractures of the sternum occur in the articulation of the manubrium with the body, but more correctly they should be referred to the group of fractures. The latter are very rare (0.1%), usually transverse (at the level of the synchondrosis, less often at the level of attachment of the III and IV ribs), and are observed simultaneously with severe injuries of the chest cavity and spine. Only in isolated cases are isolated fractures known, which are a consequence of forced contraction of the abdominal press with excessive extension of the trunk or as a result of direct and immediate violence. In these cases the lower fragment comes forward and presses on the upper one; very rarely displacement of fragments is not observed. In fractures pneumothorax, anterior mediastinal emphysema and injury of the internal mammary artery are possible. Due to the unfavorable architecture of the branches of the internal mammary artery, nourishing the sternum, osteomyelitis of the latter occurs relatively rarely and in young age is localized mainly in the upper part of its body, since it is here and at this age that there is the largest area of small-celled fields filled with red marrow, representing the locus electionis of osteomyelitic processes. As a frequent complication, penetration of pus into the anterior mediastinum is noted, which is connected with the anatomical structure of the compact plates. Tuberculous osteitis and osteomyelitis are more often of secondary nature, since the process, starting at the articulations of the ribs, then involves the sternum. Erosions of the sternum can be a consequence of syphilis (gummas) and occur relatively rarely. Thinning and even defects of the sternum have been observed many times due to constant pressure of an aneurysm of the ascending aorta, which develops and reaches significant sizes. Among tumors, giant-cell sarcomas and single cases of chondroma are described. Metastases of cancers (of the mammary gland, bronchi), hypernephromas and other malignant neoplasms can also develop secondarily in the sternum. Lit.: Vekhes G., Anatomicheskie dannye dlya predraspolozheniya i form techeniya osteomielitov grudini, Yekaterinoslavskiy med. zhurn., 1925, No. 7-9; Anthony R., Notes sur la morphogenie du sternum chez les mammiferes, Bull, et mem. de la Soc. d'anthropologie de Paris, 1901, No. 1.

The sternum is a flat bone forming the anterior wall of the thoracic cavity, articulating with the clavicles and ribs. It consists of three parts: the manubrium, body, and xiphoid process. Development begins from paired sternal ridges, ossifying centers appear in the manubrium and body. Fractures are rare, usually transverse, often associated with chest trauma. Osteomyelitis is uncommon due to the vascular supply from the internal mammary artery. Congenital defects like sternal clefts are rare and usually non-viable. Pathological changes include erosion from syphilis, thinning from aortic aneurysm, and metastases from cancers.