Describe the intercostal space with its muscles, vessels and nerves. Add a note on the thoracic cage and applied aspects.

The Thoracic Cage

• Formed by 12 thoracic vertebrae, 12 pairs of ribs, costal cartilages and the sternum
• True ribs (1–7) — articulate directly with the sternum; false ribs (8–10) — through the cartilage above; floating ribs (11, 12) — no anterior attachment
• Typical rib (3–9) — head with two facets, neck, tubercle, angle, shaft with a costal groove on the inferior border
• Atypical ribs — 1, 2, 10, 11, 12. The first rib is the shortest, broadest and most curved, and bears the scalene tubercle with grooves for the subclavian vessels

Muscles of the Intercostal Space

• The neurovascular bundle runs between the internal and innermost intercostal muscles — the plane that matters clinically
• Also present — subcostalis and transversus thoracis

The Neurovascular Bundle

• The bundle is protected by the costal groove only as far forward as the angle of the rib; anteriorly it lies more centrally in the space
• A collateral branch runs along the upper border of the lower rib, which is why bleeding can still occur

Arteries of the Intercostal Space

• The internal thoracic (mammary) artery runs about 1 cm lateral to the sternal border, and divides at the 6th space into the superior epigastric and musculophrenic arteries
• It is the graft of choice in coronary artery bypass, having far better long-term patency than a saphenous vein graft

Intercostal Nerves

• Ventral rami of T1–T11; the T12 is the subcostal nerve
• T1 — mostly joins the brachial plexus
• T2 — gives the intercostobrachial nerve to the medial side of the arm
• T7–T11 — thoraco-abdominal: they leave the intercostal space and continue into the anterior abdominal wall
• Branches — rami communicantes, collateral, lateral cutaneous (mid-axillary line) and anterior cutaneous terminal branches, and muscular and pleural branches

Dermatomes of the Trunk

• T2 — sternal angle; T4 — nipple; T6 — xiphisternum; T10 — umbilicus; L1 — inguinal region
• These are the standard reference levels for testing a sensory level in spinal injury

Movements of Respiration

• Quiet inspiration — diaphragm and external intercostals
• Quiet expiration is passive, by elastic recoil of the lungs and chest wall
• Forced inspiration — adds the sternocleidomastoid, scalenes, pectorals and serratus anterior (the accessory muscles); their use is a clinical sign of respiratory distress
• Forced expiration — abdominal muscles and internal intercostals

Applied Aspects

• Intercostal drainage and pleural aspiration — enter along the upper border of the lower rib. The "safe triangle" is bounded by the lateral border of pectoralis major, the anterior border of latissimus dorsi and a horizontal line at the level of the nipple
• Intercostal nerve block is given at the angle of the rib, remembering the collateral branch — two adjacent spaces must also be blocked because of overlap
• Herpes zoster follows a single dermatome and does not cross the midline — a striking demonstration of segmental innervation
• Flail chest — multiple ribs fractured in two places move paradoxically, impairing ventilation; the underlying pulmonary contusion is the greater danger
• Rib fractures most often involve the middle ribs at the angle; the first and second are protected, so their fracture implies severe trauma and possible injury to the subclavian vessels or brachial plexus
• Cervical rib — an extra rib from C7 compressing the lower trunk of the brachial plexus and the subclavian artery, giving thoracic outlet syndrome
• Internal thoracic artery injury is a risk in sternal puncture and anterior chest procedures, since it lies only 1 cm from the sternal edge
• Sternal puncture for marrow aspiration is done at the level of the 2nd or 3rd space in the midline, where the manubrium is thickest
• Costochondritis (Tietze syndrome) causes localised tenderness at the costochondral junction and is a common benign cause of chest pain
• Thoracotomy is usually performed through the 5th intercostal space, and the periosteum stripped from the upper border of the rib below to avoid the bundle
• Thoracic wall pain differs from visceral pain — it is sharp, well localised, and worsened by movement and by pressure over the affected segment