Retrocalcaneal Bursitis is swelling of the fluid-filled sac (bursa) at the back of the heel bone under the Achilles tendon.
A bursa acts as a cushion and lubricant between tendons or muscles sliding over bone. There are bursas around most large joints in the body, including the ankle. The retrocalcaneal bursa is located in the back of the ankle by the heel. It is where the large Achilles tendon connects the calf muscles to the heel bone.