The angle of Louis is the eponymous name given to the sternal angle which is the palpable anatomical feature formed from the manubriosternal junction. The joint of the sternal body and the manubrium.
This joint is a synarthrotic joint, a fibrous connection between the two bones of the manubrium and the sternal body which does not allow any significant movement.
In a cadaveric study of preserved skeletal specimens, the sternal angle was found to range from 149.0 degrees to 177.0 degrees with an average of 163.4 in men and 165.0 in women.
A small amount of movement occurs, particularly in younger people where the fibrous joint has increased flexibility. Complete fusion has been shown to occur at around 30 years of age. The movement in forced breathing has been measured at 4.4 degrees by in vivo measurements.
The angle of Louis forms part of the ribcage. It performs generic functions of the skeletal tissues; protection, mechanical leverage for movement, and support for other organs. Importantly, the ribcage provides support for and allows ventilation through movement of the thoracic cage. The movement at the sternal angle allows the body of the sternum to move anteriorly and superiorly. This increases the volume of the intrathoracic cage and in particular, allows transverse expansion in the lower thoracic cage leading to maximal air flow.
The sternum develops at the same time as the rest of the ribcage from mesenchymal bands or bars which develop chondritic tissues as they move ventrally and medially forming cartilaginous shapes of the adult bones. They later ossify in a craniocaudal direction.
The blood supply to the sternum has been investigated through contrast cross-sectional imaging both in vivo and in cadavers. Cadaveric dissection has added to this knowledge. These studies have revealed the presence of a perforator artery into the sternum at each intercostal level as a branch from the internal mammary arteries. Associated veins have been identified following a similar course. Importantly in patients having internal mammary harvesting, these branches anastomose with the intercostal and therefore indirectly with the posterior intercostal arteries providing a possible collateral blood flow. Despite this studies have still shown that immediately after harvesting of the internal mammary artery there can be a period of ischemia affecting the sternum.
The sternum and manubrium are innervated by the intercostal nerves which are part of the somatic nervous system. These nerves arise from the anterior rami of spinal nerves from segments T1-T11. The first two nerves supply the proximal sternum and manubrium. These nerves play a role in the contraction of the intercostal muscles as well as providing sensation to the skin. Unlike the lateral thorax, the manubrium and sternum have fewer nerves- and this explains why a sternotomy incision is less painful than a thoracotomy.
The intercostal space superior and inferior to the angle of Louis is spanned by a triple layer of muscle. The outmost intercostal muscles have fibers running in an oblique direction. These fibers run in from an anterior to the inferior direction and play a role in the elevation of the rib during inspiration. The next set of muscles, the internal intercostals, are also oriented in an oblique fashion, but the fibers are from posterior to inferior direction. This muscle layer plays a role in depressing the ribs during expiration. The most inner set of intercostals are thin and are similar to the internal intercostal muscles in their orientation. In between these runs the neurovascular bundle.
Pectoralis major has its origin across the anterior surface of the sternum and the sternocostal articulations of the superior ribs its fibers origin, and therefore, includes the sternal angle.
Sternalis, a rare anatomical variation occurs in roughly 8% of the total population. It originates from the lower costal cartilages as tendinous fibers and runs superiorly parallel to the sternal body it inserts into the sternal angle. It's proposed action is to help in the expansion of the thoracic cage.
Many different sternal anomalies can occur following abnormal development. A complete sternal cleft can occur when the two sagittal bars of the sternum do not fuse. This can sometimes allow the heart to protrude through the sternum. An incomplete fusion can cause a “sternal foramen” to be left within the sternum.
The sternum can protrude in pectus carinatum (known as pigeon breast due to its similarity to an avian shape of the ribcage). The sternum can recede in pectus excavatum (known as funnel chest) these are due to both abnormal angles of the sternal angle but also due to curvatures of the sternal body. These abnormalities often become more pronounced during childhood.
Significant pectus excavatum or carinatum is sometimes repaired surgically; these repairs are often performed where the sternal malformation occurs in conjunction with significant scoliosis. However, studies have shown that these repairs do not always lead to improvements in scoliosis and ribcage remodeling. The manubrium and proximal are routinely opened up during open heart surgery. Because of the strength of the bone, a mechanical saw is required to access the chest cavity. In children, strong sutures can be used to put the sternum back, but in all individuals above the age of 2, stainless steel wires are required to realign and close the sternum. If the blood supply is poor or if the adjacent ribs have been traumatized, the wires can loosen, and this can result in sternal dehiscence. If there is an infection, the wires need to be pulled out, and a plastic surgery consult has to be made so that the sternum can be closed with a muscle flap.
The sternal angle is an important clinical landmark for identifying many other anatomical points:
The source of the eponymous name “the angle of Louis” is believed to have originated from either Antoine Louis a French clinician or Wilhelm Friedrich von Ludwig, a German physician. However, there is no definitive evidence of either, and a recent 2013 review suggested it originates from another doctor, Pierre Charles Alexandre Louis.
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