Subtendinous calcaneal bursa

The subtendinous calcaneal bursa (retrocalcaneal bursa) is a native synovial bursa consistently present and is located between the anterior surface of the Achilles tendon and the posterior surface of the calcaneal tuberosity. It functions to reduce friction during tendon movement. Anatomically, the bursa is typically 1–1.5 mL in volume and contains a 1–2 cm synovial fold originating from its upper wall, which extends distally between the tendon and bone. The inner surface of the superior and posterior bursal walls is lined by a unilayered synovial membrane, which projects into synovial villi of variable shapes and sizes. The ceiling of the bursa may contain delicate skeletal muscle fibers radiating into the synovial fold. The floor of the bursa is covered by a 200–500 μm layer of fibrous cartilage, into which the Achilles tendon inserts; this cartilaginous layer continues anteroproximally to cover the bursal surface of the calcaneal tuberosity, where the cortical bone is thinned to approximately 50 μm.

Imaging studies show the retrocalcaneal bursa as a unilocular, compressible structure, with normal dimensions in asymptomatic individuals averaging 1 mm anteroposteriorly, 6 mm transversely, and 3 mm craniocaudally. Enlargement beyond these dimensions is associated with pathology, such as retrocalcaneal bursitis. Histologically, the bursa is lined by synovium and is distinct from the subcutaneous calcaneal bursa, which lies more superficially.