Posterior intercostal artery

A posterior intercostal artery is one of the paired segmental arteries that arise from the descending thoracic aorta and supply the intercostal spaces, typically from the 3rd through 11th intercostal spaces. There are usually nine pairs of posterior intercostal arteries arising from the thoracic aorta (the first two posterior intercostal arteries arise from the supreme intercostal artery, a branch of the costocervical trunk of the subclavian artery).

Origin: These arteries originate from the posterior or posteromedial aspect of the descending thoracic aorta.The origin of the posterior intercostal arteries in the upper thoracic level is situated at most about two levels caudal to the feeding level, whereas the origins in the lower levels are just caudal to the corresponding levels.Anatomical variations are common, with approximately 70% of individuals having at least one common trunk where paired posterior intercostal arteries arise together.

Course and segments: The posterior intercostal artery is divided into four segments based on neurovascular branching patterns: vertebral, costal, intermuscular, and rectus. The artery courses laterally within the intercostal space, conventionally running in the intercostal groove along the inferior border of the superior rib. However, the artery is actually exposed within the intercostal space for the first 6 cm lateral to the spine before becoming shielded by the superior rib. 

Branches: Dorsal branches arise from the vertebral segment, while musculocutaneous perforators and lateral branches originate from the costal segment. These arteries provide blood supply to the intercostal muscles, ribs, parietal pleura, skin, and contribute to spinal cord perfusion.