Page 5 of 7

AN21.1-11 | Thoracic cage — Part 4

Costochondral and Interchondral Joints (AN21.10)

The anterior ends of the ribs don't reach the sternum directly. Instead, they connect via costal cartilages — bars of hyaline cartilage that provide flexibility to the thoracic cage.

Costochondral joints — where the bony rib meets its costal cartilage:
• Type: primary cartilaginous joint (synchondrosis)
• Movement: none (the bone and cartilage are directly continuous)
• No joint cavity, no synovial membrane — the periosteum of the rib blends with the perichondrium of the cartilage
• These joints can separate in trauma (costochondral separation) — painful but often missed on X-ray because cartilage is radiolucent

Sternocostal joints — where the costal cartilage meets the sternum:
• 1st sternocostal joint: primary cartilaginous (synchondrosis) — no movement, very firm
• 2nd–7th sternocostal joints: synovial — allow gliding movement during breathing
• The 2nd sternocostal joint is at the sternal angle — this is why the sternal angle is your rib-counting landmark

Interchondral joints — between adjacent costal cartilages of ribs 6-7, 7-8, 8-9 (and sometimes 9-10):
• Type: synovial — small, plane joints
• They link the costal cartilages together, contributing to the rigid costal margin
• They allow slight gliding during breathing

Clinical note — Tietze syndrome: Painful swelling of the costochondral or sternocostal joints (especially 2nd and 3rd), often mistaken for cardiac chest pain. It's benign but alarming. Palpation reproduces the pain — a key distinguishing feature from cardiac causes.

The Mediastinum — The Compartment Between the Lungs (AN21.11)

The mediastinum is the central compartment of the thoracic cavity — everything between the two lungs. It extends from the sternum (anteriorly) to the vertebral column (posteriorly), and from the thoracic inlet (superiorly) to the diaphragm (inferiorly).

It is divided into four subdivisions by an imaginary horizontal plane passing through the sternal angle anteriorly and the T4/T5 intervertebral disc posteriorly:

1. Superior mediastinum — above the plane
Contents: arch of aorta and its three branches (brachiocephalic trunk, left common carotid, left subclavian), SVC (and brachiocephalic veins), trachea and its bifurcation, oesophagus, thoracic duct, vagus nerves, phrenic nerves, left recurrent laryngeal nerve (loops under the aortic arch), and the thymus (prominent in children, involutes in adults).

2. Anterior mediastinum — below the plane, in front of the pericardium
Contents: fat, lymph nodes, remnants of the thymus, sternopericardial ligaments. This is a small space.

3. Middle mediastinum — contains the pericardium and its contents
Contents: the heart, pericardium, ascending aorta, pulmonary trunk, SVC (lower part), main bronchi, phrenic nerves (on the pericardium).

4. Posterior mediastinum — below the plane, behind the pericardium
Contents: descending thoracic aorta, oesophagus (lower part), azygos and hemiazygos veins, thoracic duct, vagus nerve (oesophageal plexus), sympathetic trunks and splanchnic nerves.

Mnemonic for the sternal angle as the dividing line: "All Thoracic Structures Meet at the Sternal Angle" — it divides superior from inferior mediastinum.

You'll revisit the mediastinum in detail when you study the heart (next module), the great vessels, and the respiratory system. For now, focus on the four subdivisions and the sternal angle as the landmark that divides them.

Diagram illustrating the superior, anterior, middle, and posterior mediastina of the thorax.. Brave (teachmeanatomy.info). Used for educational purposes.