Anatomy, Angle of Louis

The intercostal space superior and inferior to the angle of Louis is spanned by a triple layer of muscle. The outermost intercostal muscles (external intercostals) have fibers running in an oblique direction. These fibers course in an anterior and inferolateral 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, orthogonally to the external intercostals. The innermost 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, and therefore, includes the sternal angle.

Sternalis, a rare anatomical variation, occurs in a single-digit percentage of the population (some estimates pointing to roughly 4%).[11] It commonly originates from the lower costal cartilages as tendinous fibers and runs superiorly parallel to the sternal body, inserting into the sternal angle. A proposed action of sternalis 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 also recede in pectus excavatum (known as funnel chest). These variations are due to both abnormal angles of the sternal angle and 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 sternum 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 together, but in all individuals above the age of 2, stainless steel wires are required to realign and close the sternum. If the blood supply to the sternum 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 may need to be pulled out, and a plastic surgery consult generally must 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:

• It marks the point at which the costal cartilages of the second rib articulate with the sternum. This is particularly useful when counting ribs to identify landmarks as rib one is often impalpable. The counting of ribs is essential when one is attempting to make a thoracic incision. If the wrong rib is counted, access to the internal chest organs can be difficult. Additionally, making an incision at the first or second rib interspace can result in damage to large, important blood vessels and the brachial plexus. Identification of the second rib and thus the second intercostal space inferiorly is also useful when auscultating heart sounds. The optimal location for auscultation of the aortic valve is generally the right second intercostal space, whereas the optimal location for auscultation of the pulmonic valve is generally the left second intercostal space.
• It is at the level of the T4-T5 intervertebral disc.
• It marks the level of the transverse thoracic plane which divides the mediastinum into the superior and inferior mediastinum.
• It overlies the aortic arch on the left and the superior vena cava on the right.
• The pericardium extends from just superior to the angle of Louis to the level of the xiphisternal joint.
• It is roughly at the level of the bifurcation of the trachea.
• The tracheal carina is deep to the sternal angle.
• It is roughly at the level of the bifurcation of the pulmonary trunk.
• Ligamentum arteriosum attaches to the aortic arch deep to the angle of Louis.

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 origin, and some speculation even suggests it originates from another doctor, Pierre Charles Alexandre Louis.