Adventitious bursa of the first metatarsal

The adventitious bursa of the first metatarsal is an acquired, non-native bursal structure that develops in response to chronic friction or pressure, most commonly over the medial aspect of the first metatarsal head, particularly in the setting of hallux valgus deformity. Unlike native bursae, which are lined by synovial membrane and located at predictable anatomical sites, adventitious bursae lack a true synovial lining and form at sites of abnormal mechanical stress.

Anatomically, the adventitious bursa is typically situated between the skin and the medial eminence of the first metatarsal head. It is superficial to the joint capsule and medial collateral ligament complex, and may overlie the abductor hallucis tendon. The bursa is unilocular, compressible, and may contain simple or complex fluid, as seen on ultrasound imaging. Its walls are composed of fibrous tissue with a single layer of attenuated cells, rather than true synovial cells.

The formation of this bursa is most frequently associated with hallux valgus, where the medial prominence of the first metatarsal head is subjected to repetitive friction from footwear. This can result in inflammation, distention, and sometimes secondary infection or hemorrhage. The bursa does not communicate with the first metatarsophalangeal joint and is distinct from the native intermetatarsal bursae, which are located deeper and have a synovial lining.