The sternal angle, also known as the angle of Louis, is a prominent anatomical landmark located on the human chest. It serves as a point on the sternum, or breastbone, which helps in identifying other structures within the thoracic cavity. This angle plays a role in both physical examinations and medical procedures, a valuable reference point for healthcare professionals. Its fixed position provides a consistent starting point for assessments.
Anatomy of the Sternal Angle
The sternal angle is the junction where two main parts of the sternum meet: the manubrium, which is the upper portion, and the body of the sternum, the central and largest part. This articulation forms a slight projection or ridge that is often palpable and sometimes visible on the chest surface. It is a cartilaginous joint, known as the manubriosternal joint. The sternal angle is particularly significant because it lies at the level where the costal cartilages of the second pair of ribs attach to the sternum. This connection to the second rib makes it a reliable reference point.
Locating and Using the Sternal Angle
Locating the sternal angle on the chest involves palpation. One can begin by finding the jugular notch, the indentation at the top of the sternum between the collarbones. Moving fingers downwards from this notch along the sternum, a slight horizontal ridge or change in angle will be felt. This prominence is the sternal angle.
The sternal angle’s most direct practical use is as a starting point for counting ribs. Since the second costal cartilage attaches at this level, identifying the sternal angle immediately leads to the second rib, allowing for sequential counting of the ribs and intercostal spaces below.
This landmark also serves as a reference for a horizontal plane, known as the transverse thoracic plane, which divides the chest cavity into superior and inferior regions. This plane corresponds to the level of the intervertebral disc between the fourth and fifth thoracic vertebrae (T4-T5) posteriorly.
Clinical Significance and Applications
The sternal angle is important in clinical practice due to its consistent anatomical position. Healthcare professionals use it during physical examinations, particularly for auscultating heart sounds. The second intercostal spaces, found immediately adjacent to the sternal angle, are optimal locations for listening to the aortic and pulmonic valves.
The sternal angle also helps in identifying deeper anatomical structures within the chest. For example, it marks the level of the tracheal bifurcation, where the trachea divides into the main bronchi. The beginning and end of the aortic arch are also found at this level.
The sternal angle guides medical procedures and diagnostic imaging. Its position assists in localizing injuries or masses within the thoracic cavity. In procedures like thoracocentesis, where fluid is removed from around the lungs, the sternal angle helps count ribs to identify a safe needle insertion point, minimizing risk to surrounding structures.
Radiologists also rely on this landmark for interpreting chest X-rays and CT scans, as it helps count ribs and assess thoracic structure alignment. Understanding the sternal angle’s location and its relation to other structures is important for diagnosis and patient care.