The sternal angle is a slight but highly significant ridge on the human chest that serves as a fundamental landmark for anatomical orientation. This small protuberance on the breastbone is one of the most reliable external guides to the complex structures within the chest cavity. Because it is easily found on the body’s surface, the sternal angle acts as a crucial reference point for medical professionals during physical examination and diagnostic procedures. Understanding this structure is paramount to accurately locating important features of the heart, lungs, and surrounding vessels.
Precise Anatomical Location and Nomenclature
The sternal angle, formally known as the Angulus sterni, is the fibrocartilaginous joint where the manubrium and the body of the sternum meet. This junction, technically a symphysis, creates a minor bend in the bone that projects slightly forward. The angle created at this point is typically obtuse, measuring approximately 160 to 170 degrees in adults. This landmark is also recognized as the Angle of Louis. The sternal angle is readily identifiable because the two segments of the sternum lie in slightly different planes, causing the manubrium to angle posteriorly on the sternal body. This difference in alignment forms the distinct, palpable ridge on the anterior chest wall.
The Practical Guide to Palpation
Locating the sternal angle starts at the top of the chest. Begin by placing your finger pads at the base of your neck in the small, visible dip located between your collarbones, known as the jugular notch. This notch sits at the superior edge of the manubrium. From the jugular notch, slowly slide your fingers straight down the midline of the breastbone. After moving inferiorly a short distance, you will feel a distinct horizontal ridge or a slight bony bump. This projection is the sternal angle, and it is typically the most prominent bony landmark on the upper chest. Once identified, move your fingers laterally to confirm the location of the second rib. Following the angle outward allows you to feel where the costal cartilage of the second rib attaches directly to the sternum at this precise point. This is a key step, as the first rib is often obscured by the clavicle and cannot be used as a starting reference point for counting.
Crucial Anatomical Alignments
The significance of the sternal angle extends far beyond the surface of the chest because it aligns with a number of major internal structures. This external landmark corresponds internally to the intervertebral disc located between the fourth and fifth thoracic vertebrae, designated as T4-T5. This line running from the sternal angle anteriorly to the T4-T5 disc posteriorly is called the transverse thoracic plane. This imaginary plane serves as the precise level for several vital transitions within the cardiopulmonary system.
Major Internal Transitions
The sternal angle marks the level where:
- The trachea divides into the left and right main bronchi (tracheal bifurcation).
- The aortic arch begins and ends, transitioning from the ascending aorta to the thoracic aorta.
- The superior vena cava enters the pericardial sac.
- The pulmonary trunk bifurcates into the right and left pulmonary arteries.
Consequently, the sternal angle provides a reliable surface guide to concentrated transitions of the cardiopulmonary system.
Clinical Importance as a Landmark
Healthcare providers rely heavily on the sternal angle as a fixed, unmoving reference point for clinical examinations and procedures. Its primary utility is serving as the starting location for accurately counting the ribs and the intercostal spaces between them. By locating the second rib’s attachment at the angle, clinicians can systematically count downwards to identify the location of every other rib and the spaces used for listening to the heart and lungs. The transverse thoracic plane defined by the sternal angle is also used to conceptually divide the central chest cavity, known as the mediastinum, into superior and inferior sections. This division simplifies the complex organization of the chest’s internal anatomy for diagnostic purposes, such as interpreting chest X-rays or CT scans. Furthermore, the angle’s reliable position makes it an excellent external marker for guiding the placement of medical devices and for localizing heart valve sounds.