One of the most common causes is due to injury of the Achilles tendon or injury to structures around the Achilles tendon. A tendon is a structure that attaches a muscle to bone. The Achilles tendon attaches the calf muscles to the back of the heel bone, known as the calcaneus. It is also possible to have a bony deformity of the calcaneus called a Haglund’s deformity, which can rub on the Achilles tendon and irritate it. Near the attachment of the Achilles tendon to the calcaneus, there is a fluid-filled sac called a bursa, which can be particularly painful when inflamed. Sometimes, it is possible to have all three of these problems occurring at the same time.