The sternum, or breastbone, forms the central, flat bone at the front of the chest. It serves as an anchor for the rib cage, which collectively creates the protective framework of the thorax. The Sternal Angle is an important anatomical feature within this framework. This junction acts as an easily accessible external guidepost that healthcare professionals use to locate and understand the intricate internal arrangement of the chest cavity. Finding this angle allows for the mapping of several deep, non-visible structures, transforming a simple bony ridge into a foundational reference point for both physical examinations and medical procedures.
The Protective Framework of the Thorax
The rib cage, also called the thoracic cage, is a bony enclosure made up of 12 pairs of ribs, the sternum, and 12 thoracic vertebrae. This structure forms the core of the chest cavity and is designed to be both rigid and flexible. The ribs connect to the sternum through pliable costal cartilage, allowing the cage to expand and contract.
The primary function of this framework is the physical protection of the vital organs housed within the chest. It shields the heart, lungs, and major blood vessels from external trauma. The rib cage also provides attachment points for the muscles involved in breathing, including the intercostal muscles and the diaphragm.
The semi-rigid nature of the cage is necessary for the mechanics of respiration. As the diaphragm contracts and the rib muscles elevate the ribs, the volume of the thoracic cavity increases, which draws air into the lungs.
Identifying the Angle of Louis
The Sternal Angle, formally known as the manubriosternal joint, is often referred to by its historical name, the Angle of Louis. This landmark is defined as the distinct horizontal ridge formed where the upper segment of the sternum, called the manubrium, joins the main body of the sternum. Because the manubrium and the sternal body lie in slightly different planes, their junction creates a noticeable angle that projects slightly forward.
A person can easily palpate this angle on their own chest, making it a reliable surface anatomy landmark. To locate it, start at the suprasternal notch, the soft depression at the very top of the breastbone between the collarbones. Sliding a finger gently downward along the midline of the sternum, the first prominent, horizontal bony bump encountered is the Sternal Angle.
This palpable ridge provides an immediate, external reference point for the complex anatomy hidden beneath the skin. Its prominence ensures that even in individuals with significant soft tissue, this bony structure remains an accessible guide for examination.
Aligning Internal Anatomy
The Sternal Angle is considered a primary anatomical landmark because it aligns precisely with a number of deep, non-palpable internal structures. This single horizontal plane passes from the angle anteriorly to the space between the fourth and fifth thoracic vertebrae (T4/T5) posteriorly, acting as a geographical divider for the chest.
The most practical alignment is the attachment of the costal cartilage of the second rib. Unlike the first rib, which is often hidden behind the clavicle and difficult to feel, the second rib attaches directly at the Sternal Angle. This connection provides a definitive starting point for counting all the other ribs.
This imaginary plane divides the central chest compartment, known as the mediastinum, into two major regions. Everything above this line is classified as the superior mediastinum, while structures below it are in the inferior mediastinum. This division is significant because the plane marks the level where the trachea bifurcates, or splits, into the right and left main bronchi that lead to the lungs. It also corresponds to the beginning and end of the arch of the aorta, the body’s largest artery, as it loops over the heart.
Using the Angle in Diagnosis and Procedures
The Sternal Angle’s fixed anatomical relationship to internal structures gives it practical value in medicine. Since the second rib is consistently found at the angle, clinicians use this point to systematically count ribs and intercostal spaces (the gaps between the ribs). Counting ribs is fundamental for accurately identifying the location of pathology, such as a rib fracture, or for targeted procedures.
The angle is also utilized as a reference point for auscultation, the process of listening to internal sounds with a stethoscope. The second intercostal spaces, found directly beneath the second ribs, are the standard locations for listening to the sounds produced by the aortic and pulmonary heart valves. Using the Sternal Angle to find the second intercostal space ensures medical professionals are listening at the correct location to assess heart function.
Furthermore, the angle guides more involved procedures, such as placing drainage tubes or identifying the correct vertebral level for imaging studies. Because it reliably marks the T4/T5 intervertebral disc, it allows radiologists and surgeons to correlate external surface features with precise internal depth and location.