Subcutaneous bursa of medial malleolus

The subcutaneous bursa of the medial malleolus is an adventitious bursa located superficial to the medial malleolus, within the subcutaneous tissue. Anatomically, it lies between the skin and the bony prominence of the medial malleolus of the tibia, typically at the level where the medial malleolus is most prominent. This bursa is not a native synovial-lined structure but rather develops in response to chronic friction or pressure, such as from footwear or repetitive trauma, and serves to reduce friction between the skin and the underlying bone.

On imaging, particularly MRI, the subcutaneous bursa of the medial malleolus appears as a unilocular, fluid-filled sac within the subcutaneous fat overlying the medial malleolus. It may be associated with varying degrees of subcutaneous edema, and in asymptomatic individuals, the area may show only normal fat or minimal edema. When distended, the bursa is typically compressible and contains simple or complex fluid, distinguishing it from other cystic structures such as ganglia, which are usually multilocular and non-compressible.

The bursa does not communicate with the ankle joint and is extra-articular. Its walls are thin and may not always be lined by true synovium, reflecting its adventitious nature. The anatomical boundaries are defined by the skin superficially, the medial malleolus deep, and the surrounding subcutaneous fat laterally and medially.