The manubrium is the superior segment of the sternum, or breastbone, a flat bone situated in the center of the chest. The sternum is divided into three parts: the manubrium, the central body, and the inferior xiphoid process. The sternum’s primary function is to anchor the clavicles and upper ribs, protecting underlying thoracic organs like the heart and lungs.
Anatomical Position and Description
The manubrium is positioned at the highest point of the sternum within the thoracic cage, superior to the sternal body. It is a broad, nearly quadrilateral bone that is wider at the top and tapers toward its inferior connection with the sternal body. This segment generally aligns posteriorly with the third and fourth thoracic vertebrae (T3 and T4).
A notable feature of its superior border is a shallow, concave depression known as the jugular notch or suprasternal notch, which is easily felt at the base of the neck between the collarbones. On either side of the jugular notch are the clavicular notches, which are the attachment sites for the clavicles. The manubrium forms the superior boundary of the anterior mediastinum, the compartment in the chest containing the heart and major blood vessels.
Key Connections and Articulations
The manubrium forms several important joints that anchor the upper limbs and the rib cage to the central skeleton. Laterally, bilateral clavicular notches articulate with the medial ends of the clavicles, forming the sternoclavicular joints. These joints allow for a wide range of motion in the shoulder girdle, connecting the arm to the trunk.
Inferiorly, the manubrium articulates with the costal cartilage of the first pair of ribs, securing the uppermost ribs to the sternum. This joint is a primary cartilaginous joint that is relatively immobile and helps stabilize the upper chest. The manubrium also connects with the sternal body below it at the manubriosternal joint, a secondary cartilaginous joint that may fuse completely into a single bony structure later in life.
The Manubrium as a Topographical Landmark
The junction between the manubrium and the sternal body is known as the Sternal Angle, or Angle of Louis, which is visible and palpable as a slight, transverse ridge on the chest. This angle is a significant reference point for clinicians and anatomists. The Sternal Angle is the precise location where the costal cartilage of the second rib attaches to the sternum.
Since the first rib is often hidden beneath the clavicle, the palpable second rib attachment at the Sternal Angle provides the starting point for accurately counting all the ribs in a physical examination. This anatomical level corresponds posteriorly to the intervertebral disc between the fourth and fifth thoracic vertebrae (T4-T5). The Sternal Angle serves as the imaginary horizontal plane that divides the mediastinum into its superior and inferior compartments. This level also aligns with major internal events, including the beginning and end of the aortic arch and the bifurcation of the trachea.