Sternal angle

The sternal angle, which varies around 162 degrees in males,[3] marks the approximate level of the 2nd pair of costal cartilages, which attach to the second ribs, and the level of the intervertebral disc between T4 and T5.[2] In clinical applications, the sternal angle can be palpated at the T4 vertebral level.

The sternal angle is used in the definition of the thoracic plane. This marks the level of a number of other anatomical structures: 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.

The superior articular surface is located on the inferior border of the manubrium. The oval inferior margin is roughened for the attachment of the articular disc.

The inferior articular surface is located on the superior border of the body of sternum. Both articular surfaces are irregularly shaped and covered by hyaline cartilage.

Between these two facets, there is an articular disc composed of fibrocartilage. In most cases, it ossifies with age.

This joint is also known as the manubriosternal angle (angle of Louis) that serves as an important anatomical landmark. It is located approximately 7 cm below the upper margin of the manubrium. Being the location of the medial end of the second costal cartilage, this manubriosternal angle is a point at which clinicians can start palpating and counting ribs. For example, cardiologists use this anatomical landmark as the starting point for the physical exam since the angle of Louis is 5 cm above the right atrium. Also, the horizontal plane that passes through the joint and the articular disc between the fourth and fifth thoracic vertebrae divides the mediastinum into superior and inferior parts.

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 angle also marks a number of other features:

• Carina of the trachea is deep to the sternal angle
• Passage of the thoracic duct from right to left behind esophagus
• Ligamentum arteriosum
• Loop of left recurrent laryngeal nerve around aortic arch

The angle is in the form of a secondary cartilaginous joint (symphysis).

This is where the 2nd rib joins with the sternum. A clinically useful feature of the (manubriosternal) joint is that it can be palpated easily. This is because the manubrium normally angles posteriorly on the body of the sternum, forming a raised feature referred to as the sternal angle.

The sternal angle is also called the angle of Louis, but the reason for that name was lost. Once thought to be after Antoine Louis or Wilhelm Friedrich von Ludwig, it is now believed to be after Pierre Charles Alexandre Louis.[4]

• Thoracic plane
• List of medical mnemonics

• Anatomy photo:18:st-0212 at the SUNY Downstate Medical Center - "Thoracic Wall: Bones"