Structure
The sternum has three parts, the manubrium above, the body in the middle and the xiphoid process below, joined by cartilage.
Manubrium
The manubrium is the broad, thick upper part. Its upper border forms the jugular (suprasternal) notch, level with the lower border of T2. On each side a clavicular notch receives the medial end of the clavicle at the sternoclavicular joint. Below this the 1st costal cartilage joins the lateral border, and at its lower corner the manubrium shares a notch with the body for the 2nd costal cartilage.
Body
The body is longer and narrower. It forms from four segments, the sternebrae, whose fusion lines can be seen as faint transverse ridges on the front. Its lateral borders carry notches for the 2nd to 7th costal cartilages; the 7th cartilage joins at the xiphisternal junction.
Xiphoid process
The xiphoid process is the small, variable tip. It is cartilaginous in young people and ossifies in adult life. It may be bifid or perforated. The xiphisternal joint lies at the level of T9 and marks the lower limit of the thoracic cavity in front, the upper surface of the liver and the central tendon of the diaphragm.
Joints and the sternal angle
The sternal angle is the palpable ridge at the manubriosternal joint, and it marks the level of several important thoracic events.
The manubriosternal joint is a secondary cartilaginous joint (symphysis) that allows slight hinging during breathing and may ossify in older age. The 2nd costal cartilage attaches at this level, so counting ribs and intercostal spaces always starts here. A horizontal plane through the sternal angle to the T4/T5 intervertebral disc marks:
- the boundary between the superior and inferior mediastinum
- the beginning and end of the arch of the aorta
- the bifurcation of the trachea (carina)
- the bifurcation of the pulmonary trunk
- the azygos vein arching over the right main bronchus to join the superior vena cava
- the thoracic duct crossing from right to left behind the oesophagus
The xiphisternal joint is a primary cartilaginous joint that usually ossifies in later life. The sternoclavicular joint is a saddle-type synovial joint with an articular disc.
Relations and attachments
Behind the manubrium lie the great vessels of the superior mediastinum; behind the body lie the pericardium and heart.
- Behind the manubrium: the left brachiocephalic vein crossing to the right, the arch of the aorta and its three branches, and the thymus or its fatty remnant. Sternohyoid and sternothyroid arise here.
- Behind the body: the anterior mediastinum, the pericardium with the right ventricle, and the anterior margins of the pleura and lungs. Transversus thoracis arises from the lower body and xiphoid.
- Laterally: the internal thoracic vessels run vertically behind the costal cartilages, a short distance from the sternal border, giving perforating branches to the sternum and breast.
- In front: pectoralis major arises from the anterior surface, and the sternal head of sternocleidomastoid from the front of the manubrium.
- Below: rectus abdominis inserts on the xiphoid and the 5th to 7th costal cartilages, the linea alba attaches to the xiphoid, and the sternal fibres of the diaphragm arise from its back.
Clinical relevance
The sternum is the surgical door to the heart and a landmark for resuscitation and examination.
- Median sternotomy: the standard approach for most cardiac surgery; the bone is split in the midline and closed with wires. The internal thoracic arteries lie close to the sternal border and are harvested for coronary grafts.
- Sternal fracture: usually from a steering wheel or seatbelt; the concern is underlying myocardial contusion.
- Chest compressions: are delivered over the lower half of the sternum, above the xiphoid.
- Bone marrow aspiration: the sternum contains red marrow throughout life and can be sampled, although the posterior iliac crest is preferred. A congenital sternal foramen from incomplete fusion is a hazard for needles, which could enter the pericardium.
- Pectus excavatum and carinatum: depression or protrusion of the sternum and costal cartilages.
- Auscultation: the aortic and pulmonary areas lie in the 2nd intercostal spaces right and left of the sternum, found from the sternal angle.