Generality
The sternum is a long and flat bone, located at the center of the thorax and representing one of the fundamental parts of the thoracic cage; the others are: the 12 thoracic vertebrae, the 12 pairs of ribs and the costal cartilages tied to the ribs.
The handlebar is the highest region; it has a trapezoidal shape and hosts the insertion for the clavicles and the first two pairs of costal cartilages (one pair is shared with the body).
The body is the intermediate region; it has an elongated shape and offers anchoring for six pairs of costal cartilages (of these, however, only four reside entirely in the body).
Finally, the xiphoid process is the lowest region; it has a small depression, which, together with a similar area on the body, guarantees the insertion of the seventh pair of costal cartilages.
The function of the sternum is to protect, along with the other elements of the thoracic cage, the heart, lungs, esophagus and thoracic blood vessels.
What is the sternum?
The sternum is the long and flat bone, located in the center-median position of the thorax and constituting one of the main parts of the thoracic cage .
ANATOMICAL RECALL ON THE MAIN COMPONENTS OF THE THORACIC CAGE
The thoracic cage is the skeletal structure placed in the upper part of the human body, exactly between the neck and the diaphragm, which serves to protect vital organs (such as the heart and lungs) and important blood vessels (aorta, veins, cavities, etc.).
According to the anatomy manuals, in addition to the sternum in the anterior position, it includes:
- Later, the 12 thoracic vertebrae .
- Latero-anteriorly, 12 pairs of ribs (or ribs ). Each pair of ribs is connected to one of the 12 thoracic vertebrae; obviously, the left ribs emerge from the left side of the aforementioned vertebrae, while those on the right from the corresponding right side.
- In front, between the sternum and the ribs, the costal cartilages .
Looking at the rib cage from top to bottom, the first 7 pairs of ribs project towards the sternum and make contact with it through the costal cartilages.
The eighth, the ninth and the tenth pairs are joined only indirectly to the sternum, as their corresponding costal cartilages flow towards the costal cartilages of the immediately higher coasts. In other words, the costal cartilages of the eighth pair join those of the seventh; the costal cartilages of the ninth pair join those of the eighth; finally, the costal cartilages of the tenth pair join those of the ninth.
The ribs that form the eleventh and twelfth pair are free and are also significantly shorter than the previous ones.
Anatomy
Similar to a tie, the sternum has three regions of a certain relevance, regions that the doctors have called:
handlebar, body and xiphoid process .
Before moving on to the specific description of these three components, it is useful to remember some general characteristics of the sternum:
- It is an uneven, long and flat bone.
- Its upper part supports both clavicles . Furthermore, it is the point of origin of one of the two ends of the sternocleidomastoid muscles . The sternocleidomastoid muscle is the muscular element that allows the head to flex and tilt laterally, turning it from the opposite side.
- The two side areas serve as anchor points for the first 7 pairs of costal cartilages.
- At its inner surface, the so-called sternopercardial ligaments are attacked . These fix the pericardium (which otherwise would be free to move) to the sternum.
- Seen from the side, the sternum has a semiarch shape. Starting from the handlebar, the structure projects forward and downward.
- In an adult individual, the sternum is about 17 centimeters long on average. In men it is longer than in women.
From the site: www.yorku.ca
HANDLE
Trapezoidal in shape, the handlebar is the highest part of the sternum .
On the upper side, in the center, it has a concavity, which can be found by touch, which is called jugular incisura . On the sides of the jugular notch lie two large ditches lined with cartilaginous tissue. These two pits house the medial ends of the clavicles, forming the so-called sternoclavicular joints .
On each side edge of the handlebar, there are two depressions (or facets ): one upper and one lower. The depression in the upper position acts as an anchor point for the costal cartilage of the first coast; the depression in the lower position, on the other hand, houses the costal cartilage of the second coast. Therefore, the costal cartilages of the first two pairs of ribs are connected to the sternal handlebar.
There is a substantial difference between the two depressions of each side: the upper one belongs entirely to the handlebar, while the lower one is in communion with the body (NB: in fact, the most correct term to define the lower facet is semi-faceted ).
Both the lateral regions, including, between the two facets, converge towards the center; in other words they go together.
On the inner surface of the handlebar, the upper sternopericardial ligament takes place, the first of this group of ligaments that hold the pericardium in place.
Finally, on the lower side of the handlebar, in the center, there is an oval area, covered with cartilage, which articulates with the second portion of the sternum starting from the top, ie the body.
The articulation present therein is called the handlebar-sternal articulation .
BODY
With a flat shape, the body is the central and longest portion of the sternum .
Its upper side is called the sternal angle and represents the articulation area with the overhanging handlebar. In some people, the sternal angle may be concave or rounded to the touch.
On the outer surface of the body, perpendicular to the direction of the sternum, there are three raised areas, called transverse ridges . In the spaces between each transverse ridge, the major pectoral muscles are attached.
Three zones similar to the transversal crests are repeated also on the inner surface of the sternum, but are less evident than the previous ones.
Moving then on the lateral edges of the body, each of these presents, from top to bottom:
- The semi-facet that, together with that of the handlebar, allows the housing of the costal cartilage of the second coast;
- Four depressions (or facets ), similar to those present on the sides of the handlebar and used to house the costal cartilages of the third, fourth, fifth and sixth coasts;
- A second semi-facet that constitutes, along with a similar area on the xiphoid process, the anchor point for the costal cartilage of the seventh coast.
In other words, starting from the top, on the edges of the body, there are the spaces that serve to house the costal cartilages of six pairs of ribs : from the second pair to the seventh pair.
The body undergoes a strong narrowing at the level of the lower side. Here, there is an area that allows articulation with the xiphoid process.
XIFOIDEO PROCESS
The xiphoid process is the terminal and smaller portion of the sternum .
Typically, it resides at the level of the tenth thoracic vertebra.
Its composition is mainly cartilaginous at least up to 40 years. An ossification process then takes place.
From the structural point of view it has two particularities:
- Laterally, it has the semi-facet that, with that of the body, composes the anchorage space for the costal cartilage of the seventh coast.
- Later, it guarantees the insertion to the inferior sternopericardial ligament, which, together with the superior, keeps the pericardium in position.
JOINTS
In addition to the articulations described in the previous paragraphs, it is important to remind readers that every costal cartilage is joined to the sternum by means of so - called costal -sternal joints .
DEVELOPMENT OF THE STERNO
Until a certain period of fetal life, the sternum is a cartilaginous structure, divided into two bar-like elements: the right bar and the left bar.
About the sixth month of intrauterine life, its six ossification centers (one on the handlebar, four in series on the body and one on the xiphoid process) begin their activity:
- At the sixth month of fetal life, the ossification center of the handlebar and the first ossification center located on the body are activated.
- At the seventh month of fetal life, the second and third ossification center of the body are put into motion.
- During the first year of life, the fourth center of ossification of the body begins its action.
- Between 3 and 8 years of life, the ossification center of the xiphoid process is activated.
Functions
Being a fundamental part of the thoracic cage, the sternum contributes to the protection of: heart, lungs, esophagus and blood vessels located in the chest.
Furthermore, it performs a fundamental support action for the clavicles and costal cartilages.
Diseases of the Sternum
Sternal fractures are the main and most common problems that can affect the sternum.
Relatively rare, these conditions are generally the result of impact trauma to the chest (these are typical consequences of motor vehicle accidents).
After an impact of a certain magnitude, the sternum can break at various points; however, the most fragile area and the one that undergoes most fractures is that between the handlebar and the sternum, where there is the handlebar-sternal joint.
Sternal fractures have a high mortality (between 25 and 45%). This is due to the fact that the rupture of the sternal bones can create pointed ends, capable of perforating the heart or the underlying lungs. This situation is more likely to occur when the impact on the sternum is very violent.
