Physiology of the retrocalcaneal bursa.

To clarify the function of the retrocalcaneal bursa the hindfoot was studied by magnetic resonance imaging at various positions of the ankle joint. In normal individuals a tongue-like extension of the retromalleolar fat pad entered the bursa during plantar flexion as the angle between Achilles tendon and calcaneus widened. The reverse occurred in dorsiflexion. In contrast, in a patient with spondyloarthritis and retrocalcaneal bursitis excessive cavitary fluid prevented the intrusion of the fat pad. The sliding motion of the fat pad in and out of the bursa during ankle motion allows a more caudal, advantageous insertion of the Achilles tendon into the calcaneus.