intercostal muscles

There are three layers of intercostal muscles: external, internal, and innermost intercostals, running between adjacent ribs. The external intercostal muscles fibers run obliquely downward and forward (hands-in-pockets direction) and are primarily active during inspiration, elevating the ribs and increasing thoracic volume (bucket-handle and pump-handle movements). The internal intercostal muscles fibers run perpendicular to the external layer (downward and backward) and are important in forced expiration, depressing the ribs. The innermost intercostals are similar in orientation to the internal intercostals but are separated by the neurovascular bundle (intercostal nerve, artery, and vein, running in the costal groove in the order VAN from superior to inferior).

Clinically, the intercostal muscles are important in the mechanics of breathing, particularly during increased respiratory effort (e.g., exercise, respiratory distress) when accessory muscles like the internal intercostals become more active for forced expiration. Intercostal muscle retractions are a clinical sign of respiratory distress, especially in pediatric patients, indicating increased work of breathing. The intercostal space is also a critical anatomical landmark for procedures like thoracentesis, chest tube placement, and needle decompression—these procedures are performed along the superior border of the rib to avoid damaging the neurovascular bundle located along the inferior border of the rib above. Intercostal nerve blocks are used for pain management following thoracic surgery or rib fractures. Rib fractures can also cause intercostal muscle spasm and pain, exacerbating hypoventilation and atelectasis risk.