Role in the Thoracic Cage
The sternum is a critical component of the bony trunk skeleton. The entire thoracic cage (Skeleton thoracis) is formed by three main components:
- Sternum (Sternum);
- Ribs (Costae);
- Thoracic vertebrae (Vertebrae thoracicae).
This complex fulfills several critical functions. Primarily, it creates a reliable shield to protect the thoracic cavity organs (heart, lungs, and great vessels). Additionally, the thoracic cage participates directly in respiratory mechanics and serves as a solid anchor for the muscles of the pectoral girdle.
Anatomical Parts of the Sternum
Topographically, this bone is divided into three distinct segments:
- Manubrium of sternum (Manubrium sterni) — the superior, widest part.
- Body of sternum (Corpus sterni) — the middle, longest segment.
- Xiphoid process (Processus xiphoideus) — the inferior segment of the bone.
Interestingly, the shape of the xiphoid process is highly variable. It depends directly on the pattern of ossification center development during embryogenesis. Consequently, the process may be pointed, spade-shaped, bifid at the tip, or even fenestrated with apertures.
A characteristic junction forms between the manubrium and body—the sternal angle (Angulus sterni). Notably, in some individuals, the synchondrosis between the manubrium and body never fully ossifies, allowing these elements to remain separate bones throughout life.
Notches and Rib Articulations
The margins of the sternum feature a series of notches designed for skeletal articulations:
- Along the superior border of the manubrium lies the unpaired jugular notch (Incisura jugularis). This is an important clinical landmark easily palpable in the neck.
- Flanking the jugular notch are the paired clavicular notches (Incisurae claviculares) for articulation with the clavicles.
- Inferiorly, along the lateral borders of the bone, run the costal notches (Incisurae costales).
The distribution of costal notches corresponds strictly to specific ribs. The manubrium contains the notch for the 1st rib cartilage. At the level of the sternal angle (the manubriosternal junction), the notch for the 2nd rib is located. The remaining costal notches, accommodating the 3rd through 7th ribs, are located along the margins of the sternal body and xiphoid process.
Anatomical Orientation
To correctly orient an isolated sternum relative to the anatomical position, follow three simple rules:
- The wide manubrium must point strictly superiorly.
- The variable xiphoid process must point inferiorly.
- The sternal angle must project anteriorly — serving as the definitive landmark to identify the anterior surface of the bone.