The Angle of Louis, also known as the sternal angle, is a distinct anatomical landmark on the anterior chest wall. This palpable structure is the joint where two major segments of the breastbone, or sternum, meet. Its location is a reliable guide for identifying other structures in the thoracic cavity, making it a frequent starting point for physical examinations. Accurately locating this angle is a foundational skill, particularly for counting ribs.
Anatomical Identity of the Sternal Angle
The Sternal Angle is the junction where the manubrium, the uppermost section of the sternum, articulates with the body of the sternum. This joint is a synarthrosis, which is slightly flexible in younger people but often fuses completely around age 30. The angle presents as a slight, transverse ridge that can be felt beneath the skin on the front of the chest.
The bony prominence of the Angle of Louis aligns with the second costal cartilage on either side of the sternum. This attachment point is important because the first rib is difficult to feel beneath the clavicle. The Sternal Angle thus serves as the definitive starting point for correctly numbering all the other ribs.
Posteriorly, the Sternal Angle horizontally corresponds to the level of the intervertebral disc between the fourth and fifth thoracic vertebrae (T4/T5). This horizontal plane, known as the transverse thoracic plane, divides the contents of the chest cavity into distinct regions. The difference in plane between the manubrium and the sternal body creates the angle, which typically measures about 162 to 170 degrees in adults.
Step-by-Step Guide to Palpation
To locate the Angle of Louis, begin by identifying the suprasternal notch, the concave dip at the top of the sternum between the collarbones. This notch is easily found by applying gentle pressure with a finger. Palpation should be performed using the pads of the fingers to maximize sensitivity.
From the suprasternal notch, slowly slide the fingers inferiorly along the midline of the manubrium. Continue this downward movement until the fingers encounter a distinct, horizontal ridge or bump that feels like a slight bend in the bone. This palpable ridge is the Angle of Louis.
Once the ridge is clearly felt, confirm the location by finding the second rib. While keeping a finger on the Sternal Angle, slide the finger laterally. This movement will lead directly onto the costal cartilage of the second rib, where it attaches to the sternum.
Locating the second rib confirms the bony ridge is the Angle of Louis. This landmark is the only place where a rib can be definitively identified without counting down from the impalpable first rib. After locating the second rib, the next palpable space inferiorly is the second intercostal space, and immediately below that is the third rib, allowing for the systematic counting of the remaining ribs.
Clinical and Anatomical Significance
The Angle of Louis is a reference point for clinical examination and internal anatomy. Its most common use is accurately counting the ribs during a physical assessment, such as when placing a stethoscope for heart or lung sounds. Since the second rib articulates at this readily palpable joint, it provides a reliable starting point for identifying the intercostal spaces below it.
Anatomically, the horizontal plane that passes through the Sternal Angle and the T4/T5 disc serves as the boundary separating the superior and inferior parts of the mediastinum, which is the central compartment of the chest cavity. This division is important for organizing the organs and vessels contained within the chest. Several major internal structures are aligned with this plane, highlighting its organizational importance.
For instance, the trachea, the windpipe, bifurcates into the left and right main bronchi at this level, a structure known as the carina. The aortic arch, the curved portion of the body’s largest artery, also begins and ends near this plane. Furthermore, the point where the azygos vein drains into the superior vena cava, a major vein returning blood to the heart, is often found at the same horizontal level. These alignments demonstrate how the Angle of Louis acts as an external marker for complex internal arrangements within the chest.