Thoracic wall and intercostal spaces

Ribs

• Floating ribs are false ribs too. A loin incision near the 12th rib can open the pleura, because the pleura extends below the medial part of that rib.
• Typical rib: head with two facets (its own vertebra and the one above), neck, tubercle, angle (the weakest point) and a costal groove on the inner lower border sheltering the neurovascular bundle.
• First rib: shortest, broadest, flattest; scalene tubercle for scalenus anterior, with the subclavian vein in front and the subclavian artery and lower trunk of the brachial plexus behind. Its fracture implies very high energy.
• Rib fractures are commonest in the middle ribs (about 5–9) at or near their angles. Upper rib (1–2) fractures suggest great vessel or brachial plexus injury. Lower rib (9–12) fractures suggest liver, spleen or kidney injury, so hypotension after left lower rib fractures means a ruptured spleen until proved otherwise.
• Flail chest: two or more adjacent ribs broken in two or more places; the segment moves paradoxically (in on inspiration). The underlying pulmonary contusion causes most of the hypoxia.
• A child's chest wall is very compliant, so energy passes to the lung without breaking ribs: a hypoxic child with patchy lung opacity and no rib fractures has a pulmonary contusion. Rib fractures in a child imply very high energy or non-accidental injury.

Sternum and anterior relations

• Manubrium, body and xiphoid. The sternal angle (manubriosternal joint) lies at the second costal cartilage and the T4/5 disc: tracheal bifurcation, beginning and end of the aortic arch, azygos joining the SVC, and the plane between superior and inferior mediastinum.
• A displaced fracture of the body of the sternum (steering wheel) demands exclusion of blunt cardiac injury: ECG and troponin. A normal ECG on arrival makes significant injury unlikely.
• Penetrating wounds beside the sternum: just right of the sternum in the 4th–5th spaces enters the right atrium; just left of the sternum in the 4th–5th spaces enters the right ventricle (which forms most of the anterior surface). The internal thoracic artery runs about 1 cm from the sternal edge and may also be cut.
• The fundus of the gallbladder lies behind the tip of the right 9th costal cartilage.

Layers of the intercostal space

From outside in: skin, superficial fascia, (chest wall muscles), external intercostal, internal intercostal, innermost intercostal, endothoracic fascia, parietal pleura.

• The neurovascular bundle runs in the costal groove between the internal and innermost intercostals, from above down vein, artery, nerve.
• The layer immediately deep to the innermost intercostal, just outside the parietal pleura, is the endothoracic fascia. A bullet or blade passing the intercostals meets it before the pleura.

Intercostal vessels and nerves

• Posterior intercostal arteries: spaces 1–2 from the superior intercostal artery (costocervical trunk), the rest from the thoracic aorta.
• Anterior intercostal arteries: from the internal thoracic artery (spaces 1–6) and its musculophrenic branch (7–9). When ligating the internal thoracic artery near its origin, protect the phrenic nerve, which crosses it.
• Posterior intercostal veins: the first drains into the brachiocephalic (or vertebral) vein; the 2nd–3rd form the superior intercostal vein (right into the azygos, left into the left brachiocephalic); the rest drain to the azygos on the right and the hemiazygos veins on the left.
• Intercostal nerves are the ventral rami of T1–T11; T12 is the subcostal nerve. Each gives a lateral cutaneous branch near the mid-axillary line and ends as an anterior cutaneous branch. T10 supplies the umbilicus, and lower intercostals supply the abdominal wall, so rib or pleural pain can be felt in the abdomen.
• The intercostobrachial nerve is the lateral cutaneous branch of T2; it crosses the axilla to the inner arm and is often cut in axillary clearance, leaving numbness of the armpit and inner upper arm with normal shoulder movement.

Pleura and lung surface markings

Between the two borders lies the costodiaphragmatic recess. A needle in the 9th space in the mid-axillary line may pass the pleural recess into the diaphragm and liver (right) or spleen (left), so aspirate higher.

Needles, drains and blocks

• Insert needles just above the upper border of the rib below to avoid the bundle.
• Chest drain in the safe triangle: anterior border of latissimus dorsi, lateral border of pectoralis major, a horizontal line at the level of the nipple (5th intercostal space), apex below the axilla; usually the 4th–5th space in the mid-axillary line.
• Needle decompression of tension pneumothorax: 2nd space mid-clavicular line, or the 4th–5th space just anterior to the mid-axillary line in adults.
• Intercostal nerve block: given at the angle of the rib, posteriorly, below the lower border. This site is proximal to the lateral cutaneous branch, so the lateral chest wall is numbed too; a block anterior to the mid-axillary line leaves lateral fractures painful.
• Escharotomy: a circumferential full-thickness burn of the chest forms a rigid eschar that restricts expansion; in a ventilated patient airway pressures rise and tidal volumes fall. Urgent escharotomy releases it; the eschar is dead skin, so little anaesthesia is needed.

Breathing movements

• Pump handle: upper ribs lift the sternum, increasing the anteroposterior diameter. Bucket handle: lower ribs swing outwards, increasing the transverse diameter.
• The diaphragm is the main muscle of quiet inspiration and keeps working when ribs are broken; accessory muscles (sternocleidomastoid, scalenes, pectorals) join in distress.
• Lower external intercostals blend with the external oblique; serratus anterior interdigitates with the external oblique on the lateral chest wall.

Remember Vein, artery, nerve under each rib, between the internal and innermost layers: go in above the rib below.

Exam trap False ribs are 8–12; floating ribs (11–12) are a subset of them.

How FCPS asks about this

• Which ribs are false or floating: false 8–12; floating 11–12.
• Layer deep to the innermost intercostal: endothoracic fascia, then parietal pleura; the bundle lies between internal and innermost.
• Pleura crosses the mid-axillary line at: 10th rib (8th midclavicular, 12th paravertebral).
• Numb armpit and inner arm after axillary clearance: intercostobrachial nerve (T2).
• Why block at the rib angle: proximal to the lateral cutaneous branch.
• Sternal body fracture after steering wheel impact: exclude blunt cardiac injury; child hypoxic without rib fractures has pulmonary contusion.
• Ventilated patient with circumferential chest burn and rising airway pressure: escharotomy.