Generality
The sternum is the long and flat bone, located in the center of the thorax and representing one of the fundamental parts of the rib cage; the others are: the 12 thoracic vertebrae, the 12 pairs of ribs and the costal cartilages linked to the ribs.
The dumbbell is the highest region; it has a trapezoidal shape and houses 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 provides anchorage 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, ensures the insertion of the seventh pair of costal cartilages.
The function of the sternum is to protect, along with the other elements of the rib 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 rib cage.
ANATOMICAL RECALL ON THE MAIN COMPENENTS OF THE THORACIC CAGE
The rib cage is the skeletal structure located 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, hollow veins, 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 the right ribs emerge from the corresponding right side.
- Anteriorly, between the sternum and 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, ninth and tenth pairs join only indirectly to the sternum, as their corresponding costal cartilages converge towards the costal cartilages of the immediately superior ribs. 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, and finally, the costal cartilages of the tenth pair join those of the ninth.
The ribs forming the eleventh and twelfth pair are free and are also decidedly shorter than the previous ones.
Anatomy
Similar to a tie, the breastbone has three regions of some significance, regions that doctors have called:
dumbbell, 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 unequal bone, long and flat.
- Its upper part supports both collarbones. Furthermore, it is the point of origin of one of the two heads of the sternocleidomastoid muscles. The sternocleidomastoid muscle is the muscle element that allows the head to be flexed and tilted laterally, making it rotate on the opposite side.
- The two lateral zones serve as an anchor point for the first 7 pairs of costal cartilages.
- To its inner surface, the so-called sternoperocardial ligaments attach themselves. These fix the pericardium (which would otherwise be free to move) to the breastbone.
- Seen from the side, the sternum has a semi-arch shape. Starting from the handlebar, the structure projects forward and downward.
- In an adult individual, the breastbone is about 17 centimeters long on average. In men it is longer than in women.
From the site: www.yorku.ca
HANDLEBAR
Trapezoidal in shape, the manubrium is the highest part of the sternum.
On the upper side, in the center, it has a concavity, detectable to the touch, which takes the name of jugular notch. On the sides of the jugular notch reside two large pits lined with cartilage tissue. These two pits house the medial ends of the clavicles, forming the so-called sternoclavicular joints.
On each lateral edge of the dumbbell, 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 rib; the depression in the lower position, on the other hand, houses the costal cartilage of the second rib. Therefore, the costal cartilages of the first two pairs of ribs are connected to the sternal manubrium.
C "is a substantial difference between the two depressions on each side: the upper one belongs entirely to the handlebar, while the lower one is in communion with the body (N.B: in fact, the more correct term to define the lower facet is semi-facet).
Both lateral regions, included, between the two facets, converge towards the center; in other words, they huddle together.
On the inner surface of the manubrium, the superior sternopericardial ligament takes place, the first of this group of ligaments that hold the pericardium in place.
Finally, on the lower side of the manubrium, 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 joint present therein is called the manubrium-sternal joint.
BODY
Flat in shape, the body is the central and longest portion of the breastbone.
Its upper side is called the sternal angle and represents the joint area with the handlebar above. In some people, the sternum corner may feel 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 engage.
Three areas similar to the transverse ridges are also repeated on the inner surface of the sternum, but are less evident than the previous ones.
Moving therefore on the lateral edges of the body, each of these presents, from top to bottom:
- The semi-facet which, together with that of the handlebar, allows the housing of the costal cartilage of the second rib;
- Four depressions (or facets), similar to those present on the sides of the handlebar and designed to house the costal cartilages of the third, fourth, fifth and sixth ribs;
- A second semi-facet which, together with a similar area on the xiphoid process, constitutes the anchor point for the costal cartilage of the seventh rib.
In other words, starting from the top, on the edges of the body, there are the spaces that are used to house the costal cartilages of six pairs of ribs: from the second pair to the seventh pair.
The body undergoes a strong shrinkage at the level of the lower side. Here, there is an area that allows for articulation with the xiphoid process.
XIFOID PROCESS
The xiphoid process is the terminal and smallest portion of the sternum.
Typically, it resides at the level of the 10th thoracic vertebra.
Its composition is mainly cartilaginous at least up to the age of 40. After that, an ossification process takes place.
From a structural point of view it has two peculiarities:
- Laterally, it has the semi-facet which, together with that of the body, makes up the anchoring space for the costal cartilage of the seventh rib.
- Posteriorly, it ensures insertion into the inferior sternopericardial ligament, which, together with the superior, holds the pericardium in position.
ARTICULATIONS
In addition to the joints described in the previous paragraphs, it is important to remind readers that each costal cartilage joins the sternum by means of the so-called costo-sternal joints.
DEVELOPMENT OF THE STERNUM
Up to a certain period of fetal life, the sternum is a cartilage structure, divided into two bar-like elements: the right bar and the left bar.
Around the sixth month of intrauterine life, its six ossification centers (one on the manubrium, four in series on the body and one on the xiphoid process) begin their activity:
- In the sixth month of fetal life, the ossification center of the manubrium and the first ossification center located on the body are activated.
- In the seventh month of fetal life, the second and third ossification centers of the body are put into motion.
- During the first year of life, the fourth ossification center 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 rib cage, the sternum contributes to the protection of: the heart, lungs, esophagus and blood vessels located in the chest.
In addition, it performs a fundamental support action for the collarbones and costal cartilages.
Diseases of the sternum
Sternal fractures are the main and most common problems that can affect the sternum.
Relatively quite rare, these conditions are generally the result of impact trauma to the chest (they are typical consequences of motor vehicle accidents).
After a serious impact, the breastbone can rupture in various places; however, the most fragile area and the one that suffers the most fractures is that between the manubrium and the sternum, where there is the manubrium-sternal joint.
Sternal fractures have a "high mortality rate (between 25 and 45%). This is due to the fact that breaking the sternal bones can create pointed ends, capable of puncturing the underlying heart or lungs. This situation is more likely to occur. occur when the impact on the sternum is very violent.
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