The Angle of Louis, also known as the sternal angle, is a slightly raised, transverse ridge located on the front of the chest. This anatomical landmark is a reliable reference point in clinical medicine and anatomical studies. It allows healthcare professionals to count ribs and identify the positioning of several major internal structures within the chest cavity.
Defining the Angle: Structure and Location
The sternum, or breastbone, forms the central part of the chest and is composed of three segments: the manubrium, the body, and the xiphoid process. The Angle of Louis is the joint where the manubrium meets the body of the sternum, known anatomically as the manubriosternal joint. This junction forms a slight outward angle, making it palpable as a distinct ridge beneath the skin.
This joint is slightly movable, especially in younger individuals, but often fuses in older adults. It is located approximately 5 centimeters below the suprasternal notch, the dip at the top of the manubrium. By running a finger down the center of the chest, the Angle of Louis can be easily identified as the first noticeable bony protrusion encountered. The angle varies slightly among individuals but measures around 162 to 170 degrees.
The Starting Point: Using the Angle for Rib Counting
The Angle of Louis serves as the starting point for counting ribs on the anterior chest wall. The lateral edges of the sternal angle are precisely where the costal cartilage of the second pair of ribs attaches to the sternum. This attachment makes the Angle of Louis a reliable guide during a physical examination.
Once the angle is located, the examiner can feel for the second rib and then slide a finger down into the second intercostal space immediately below it. This landmark is necessary because the first rib is often obscured and challenging to palpate, as it is tucked behind the clavicle. Using the Angle of Louis to confirm the second rib allows all subsequent ribs to be accurately counted.
Internal Alignments
Beyond rib counting, the Angle of Louis represents a horizontal plane aligning with several major internal structures. This imaginary line, often called the thoracic plane, extends from the sternal angle anteriorly to the intervertebral disc between the fourth and fifth thoracic vertebrae (T4/T5) posteriorly. This alignment helps orient clinicians and surgeons to anatomical events occurring deep within the body.
The plane of the Angle of Louis divides the mediastinum, the central compartment of the chest, into two sections: the superior mediastinum (above the plane) and the inferior mediastinum (below the plane). It also marks the precise level where the trachea bifurcates into the right and left main bronchi, a point known as the carina.
Several major blood vessels and ducts align with this level. The arch of the aorta, the large vessel carrying oxygenated blood from the heart, begins and ends at the Angle of Louis. The azygos vein, which drains blood from the posterior chest wall, arches over the right main bronchus to empty into the superior vena cava at this same horizontal plane. This convergence of internal structures makes the Angle of Louis a centralized navigational point for the thoracic cavity.