What is the Sternum?
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The sternum is a part of the skeletal system consisting of a vertical flat bone. It is a single bone that is long, narrow and flat. Present in the middle of the anterior thoracic region, it measures about 6 inches long with a thickness of a fraction of an inch. Many muscles that facilitate movement in the head, neck and arms originate in the sternum.
It protects vital organs such as heart, thymus gland, aorta, vena cava, etc. and maintains the stability of the thoracic skeleton. It also serves as a keystone to the rib cage and resembles a necktie. Another name that is used to refer to the sternum is the breastbone. It is larger in males than in females.
Parts of the Sternum
The sternum develops in the following distinct parts:
- Manubrium: It is the most superior and widest part of the sternum at the level of the 3rd and 4th thoracic vertebrae. Through the costal cartilages of the first and second pair of ribs, it can form joints between them and the clavicles. A sternoclavicular joint is a place where the pectoral girdle of the shoulder and the axial skeleton of the thorax connect, making it the only point of skeletal attachment between the two parts.
Formation of the sternoclavicular joint results when the manubrium meets the clavicles at the concave clavicular notches. The lateral sides of this part of the sternum have indentations serving as attachment points for the costal cartilages of the ribs. The joint of the costal cartilage of the second ribs is the place where the manubrium meets the second part of the sternum known as the body.
Further, it has a slight posterior bend which forms a sterna angle. The pectoralis major and sternocleidomastoid muscles find attachment on the anterior surface of the manubrium, while the sternohyoid and sternothyroid muscles find attachment on the posterior surface.
- Body: It is the longest part of the sternum also known as corpus sterni or gladiolus. However, it is narrower than the manubrium and is located at the level of T5 – T9 vertebrae. It is almost rectangular and has three horizontal ridges that pass through the 3rd, 4th and 5th articular facets. The sternal foramen can be seen between the 3rd and 4th parts of the sternal body. Formation of a bulk of the rib cage occurs as a result of the connection of the costal cartilages of the second through tenth ribs to the body. To prevent rib separation, the body provides slight concave indentations on the lateral sides to serve as a strong attachment point for costal cartilages. The pectoralis major is attached to the flat anterior surface of the sternal body.
- Xiphoid process: Lying at the levels of the T10 vertebra, the xiphoid process is the most inferior and smallest portion of the sternum. During the initial years of life, it remains as a region of flexible hyaline cartilage. It does not serve as an area of attachment for either the costal cartilages or the ribs. Throughout childhood and adulthood, until the individual is about 40 years of age, the xiphoid process continues to undergo ossification.
In the end, the cartilage hardens to form a bone. The attachment between the sternal body and the superior border is possible through the xiphisternal joint. The articular facets on either side provide attachment to the 7th rib, while the linea alba emerges from the inferior border. The attachments of the anterior costoxiphoid ligament and part of the rectus abdominis are present on the anterior surface. The posterior surface includes the joints of costoxiphoid ligament, diaphragm and transverses thoracic.
Clinical Importance of Sternum
- Bone marrow biopsy: A hematopoietic tissue refers to a site where the production of blood cells takes place. The sternum is a hematopoietic tissue. Thus obtaining a sample for a biopsy of the bone marrow is possible through the sternum. It includes the introduction of a large bore needle into the sternum to aspirate a sample of the bone marrow.
- Sternal angle: The fusion of the manubrium and the body of the sternum forms a sternal angle also known as the angle of Louis. It is at the level of the 2nd costal cartilage and between the 4th and 5th thoracic vertebrae. It is an important location as this is where the ribs and costal cartilages are first counted. An estimate of the position of other parts of the area can be made concerning this angle which adds to its importance.
- Surgery: The sternum may be split into half along its axis to gain better access to the heart during surgery in the thoracic cavity. The operation may be to repair the heart, thymus gland, retrosternal goiter and great vessels. After the completion of the surgery, the doctors are most likely to use a stainless steel wire to join the two halves of the sternum together.
- Xiphoid process: Like the sternal angle, the xiphoid process is also an important landmark among other structures present in the thoracic cavity. The xiphoid process marks vital locations such as the anatomical end of the anterior thoracic cage, formation of the infrasternal angle with the subcostal margins, the central tendon of the diaphragm, inferior border of the heart and superior border of the liver.
Problems Related to the Sternum
- Sternum pain: The sternum links many joints together and any damage in the form of dislocation, injury, trauma, etc. to the sternum can cause severe pain. Its location is at the center of the thoracic cavity. Therefore, no matter how small the wound or damage may be, it has severe repercussions on the sternum and inflicts immense pain. Moreover, movement in the thoracic cage occurs continuously for involuntary actions such as respiration which may intensify the pain that an individual experiences.
- Sternal fracture: The costal cartilages act as a cushion and protects the sternum from injury and trauma. However, exerting too much pressure can result in fracture or dislocation of the sternum. If the professional performing the CPR does not use proper technique and places his/her hands in the wrong location on the chest, it may lead to a fracture of the xiphoid. In some cases, a part of the xiphoid may break off and pierce a tender organ, causing severe medical complications.
- Post-surgery complications: Surgery on any of the organs within the thoracic cavity may require splitting the sternum to gain access to the particular organ. After the surgery, a stainless steel wire is used to join the two halves of the sternum, to prevent separation. However, sometimes exerting extreme stress on the sternum by engaging in activities such as lifting heavy objects can result in significant damage to the sternum due to the wires cutting through the bony tissue.