(on the upper surface of the heel is a bony protruberence called the Haglund Bump aka "pump bump". At the back of the heel, the achilles tendon attaches and we can see calcific deposits or spurs into the attachment area. In between these two structures, lies the retrocalcaneal bursa.)
The heel contains the largest bone in the foot called the Calcaneus. At the back part of the calcaneus is the attachment of the Achilles tendon. This large tendon is very strong and aids in walking and jumping. The retrocalcaneal bursa lies between the Achilles tendon and the calcaneus. It is a fluid filled sac which allows the tendon to glide smoothly over the calcaneal bone. Bursae can be found all over the body where tendons slide over bone.
The Haglunds syndrome occurs when the retrocalcaneal bursa is inflamed due to repeated pressure on the back of the heels by shoes. The bursa and the Achilles tendon are impinged between the Haglunds bump and the shoes. This leads to inflammation of these structures, swelling and pain. With repeated impingments, the Achilles tendon may calcify at its attachment with the calcaneus. This inherently weakens the tendon and predisposes it to ruptures.