Sternum
Sternum
Also known as: breastbone, manubrium, xiphoid process, sternal angle, angle of Louis
The flat midline bone of the anterior chest wall (≈17 cm long), composed of the manubrium, body and xiphoid process. It anchors the clavicles and the costal cartilages of the true ribs.
Overview
The manubrium is the broad upper part, at the level of T3–T4. Its superior border shows the jugular (suprasternal) notch (level of T2/T3) flanked by the clavicular notches for the sternoclavicular joints; its lateral border receives the 1st costal cartilage and, at its lower corner, part of the 2nd. The manubrium meets the body at the manubriosternal joint (a secondary cartilaginous joint), which projects as the palpable sternal angle (of Louis).
The body (gladiolus), formed by fusion of four sternebrae, is longer and narrower; its lateral borders have notches for costal cartilages 2–7. The small xiphoid process at the lower end is cartilaginous in youth and ossifies around the age of 40; it gives attachment to the linea alba and the diaphragm and lies at T10 (xiphisternal joint ≈T9).
The sternal angle is a key surface landmark at the level of the T4–T5 intervertebral disc: here the 2nd costal cartilage articulates (for counting ribs and intercostal spaces), and this horizontal plane marks the boundary between the superior and inferior mediastinum, the beginning and end of the aortic arch, the bifurcation of the trachea (carina), the bifurcation of the pulmonary trunk, the azygos vein entering the superior vena cava and the ligamentum arteriosum.
- Location
Anterior midline of the thorax.
Parts & subdivisions
- ManubriumJugular notch, clavicular notches, 1st costal cartilage.
- Sternal angle (of Louis)Manubriosternal joint at T4–T5; 2nd costal cartilage.
- Body (gladiolus)Costal notches for cartilages 2–7.
- Xiphoid processCartilaginous in youth, ossifies ≈40 years; diaphragm and linea alba attach.
- Jugular (suprasternal) notchPalpable at T2–T3; trachea lies directly behind it.
Anatomy
- Articulations
Clavicles (sternoclavicular joints), costal cartilages 1–7 (1st synchondrosis; 2–7 synovial sternocostal joints), manubriosternal and xiphisternal joints.
- Ossification
Endochondral from 6 centres (1 manubrium, 4 body, 1 xiphoid); the sternebrae of the body fuse from below upward between puberty and ≈25 years; the manubriosternal joint usually persists.
- Development
From paired sternal bars of lateral plate mesoderm that fuse in the midline; incomplete fusion causes sternal clefts or a sternal foramen.
Function
- Protects the heart, great vessels and thymus
- Anchors the clavicles and the true ribs
- Attachment for pectoralis major, sternocleidomastoid, infrahyoid muscles, diaphragm and rectus abdominis
- Contains red marrow throughout life
Clinical significance
Median sternotomy
The sternum is split in the midline for cardiac surgery and wired together afterwards.
Sternal fracture
Typically from steering-wheel or seat-belt impact; warrants assessment for blunt cardiac injury.
Sternal foramen and marrow biopsy
A congenital sternal foramen (≈5%) in the lower body makes sternal marrow aspiration dangerous (cardiac tamponade) — the iliac crest is preferred.
Mnemonics
At the sternal angle (T4/5) — "RAT PLAn"
Rib 2 (costal cartilage), Aortic arch begins and ends, Trachea bifurcates (carina), Pulmonary trunk bifurcates, Ligamentum arteriosum, Azygos vein joins the SVC — and the plane divides the superior from the inferior mediastinum.
Test yourself
The sternal angle (of Louis) is a key landmark because it lies at the level of:
- The 1st costal cartilage and T2
- The 4th costal cartilage and T6
- The xiphisternal joint and T9
- The 2nd costal cartilage and the T4–T5 intervertebral disc
Answer: The 2nd costal cartilage and the T4–T5 intervertebral disc
The manubriosternal joint lies opposite the 2nd costal cartilage (used to count ribs) and the T4–T5 disc. This plane marks the start and end of the aortic arch, the tracheal bifurcation, the pulmonary trunk bifurcation and the superior/inferior mediastinum boundary.
Compiled from standard anatomy and physiology references — see the Skeletal System sources. For education only; not medical advice.