Thoracic wall and diaphragm

Thoracic Cage - Framework for Breathing

• Components: Bony framework formed by 12 thoracic vertebrae, 12 pairs of ribs, and the sternum.
• Sternum: Manubrium, Body, Xiphoid Process.
  • Sternal Angle (of Louis): Key clinical landmark at T4/T5 level. Articulates with the 2nd rib. Marks the tracheal bifurcation and aortic arch.

• Ribs:
  • True Ribs (1-7): Attach directly to the sternum via their own costal cartilage.
  • False Ribs (8-10): Attach to the cartilage of the rib above.
  • Floating Ribs (11-12): Do not connect to the sternum.

⭐ Flail Chest: Occurs when ≥3 consecutive ribs are fractured in ≥2 places, leading to paradoxical chest wall movement during respiration. This segment moves inward during inspiration and outward during expiration, impairing ventilation.

Diaphragm - The Breathing Muscle

• Attachments: Originates from the xiphoid process, lower six costal cartilages, and upper lumbar vertebrae. Inserts into the central tendon.
• Innervation: Phrenic nerve ($C3, C4, C5$). 📌 "C3, 4, 5 keeps the diaphragm alive."
• Action: Primary muscle of inspiration. Contraction flattens the dome, increasing the vertical diameter of the thorax.
• Apertures: Key structures pass at specific vertebral levels. 📌 Mnemonic: "I Ate Ten Eggs At Twelve."
  • Caval (T8): Inferior Vena Cava.
  • Esophageal (T10): Esophagus, Vagus Nerves (CN X).
  • Aortic (T12): Aorta, Thoracic Duct.

⭐ Clinical Pearl: Unilateral diaphragmatic paralysis, often from phrenic nerve injury, causes paradoxical upward movement of the affected hemidiaphragm during inspiration (positive "sniff test").

Respiratory Mechanics - Pump & Bucket

• Pump Handle Motion (Upper Ribs 2-6):
  • Increases anterior-posterior (AP) diameter.
  • Sternum & upper ribs move up and forward.
  • Muscles: Scalenes, Sternocleidomastoid (accessory).
• Bucket Handle Motion (Lower Ribs 7-10):
  • Increases transverse (lateral) diameter.
  • Lateral shaft of ribs moves up and out.
  • Muscles: External intercostals.
• Diaphragm:
  • Primary muscle of inspiration; flattens on contraction, ↑ vertical diameter.
  • 📌 "C3, 4, 5 keeps the diaphragm alive" (Phrenic nerve).

⭐ Flail Chest: Paradoxical motion where a segment of the chest wall (due to ≥3 consecutive rib fractures in ≥2 places) moves inward during inspiration and outward during expiration, impairing ventilation.

Clinical Correlates - Wall & Floor Faults

• Thoracic Outlet Syndrome (TOS):
  • Compression of brachial plexus & subclavian vessels at the superior thoracic aperture.
  • Symptoms: Neurogenic (pain, paresthesia) or vascular (swelling, discoloration).
  • Provocative tests: Adson's, Wright's, Roos.
• Flail Chest:
  • Fracture of ≥3 adjacent ribs in ≥2 places, causing paradoxical chest wall motion.
  • Leads to inefficient breathing and respiratory failure.
• Diaphragmatic Hernias:
  • Congenital: Bochdalek (posterolateral), Morgagni (anteromedial). 📌 Bochdalek = Back & to the side.
  • Acquired: Hiatal (sliding/paraesophageal), traumatic.

⭐ A left-sided Bochdalek hernia is the most common congenital diaphragmatic hernia (~85%), often presenting with severe respiratory distress in newborns due to pulmonary hypoplasia.

• The intercostal neurovascular bundle (VAN) runs between the internal and innermost intercostal muscles in the costal groove.
• Thoracentesis is performed superior to the rib in the mid-axillary line (e.g., 8th-9th ICS) to avoid this bundle.
• The phrenic nerve (C3, C4, C5) provides all motor innervation to the diaphragm; irritation causes referred shoulder pain.
• Key diaphragmatic apertures: IVC (T8), esophagus & vagus (T10), aorta & thoracic duct (T12).
• Flail chest involves multiple rib fractures, causing paradoxical chest wall motion.