Subcutaneous bursa of lateral malleolus

The subcutaneous bursa of the lateral malleolus is an adventitious bursa located superficial to the lateral malleolus of the fibula, interposed between the skin and the underlying bony prominence. This bursa is not a native synovial-lined structure but rather develops in response to chronic friction or pressure, such as from footwear or trauma, and serves to reduce friction between the skin and the lateral malleolus.

Anatomically, the bursa is situated directly over the lateral malleolus, with its boundaries defined by the skin superficially and the periosteum of the fibula deep. It is typically unilocular, compressible, and may be visualized as a fluid-filled sac on ultrasound, appearing anechoic or hypoechoic depending on its contents. The bursa is extra-articular and does not communicate with the ankle joint.

Important adjacent structures include the superficial fibular (peroneal) nerve, which courses anterior to the lateral malleolus, and the sural nerve, which runs just posterior to the lateral malleolus.