This lateral x-ray of the calcaneus shows an avulsion fracture at the insertion of the Achilles tendon, with marked separation of fragments.
Avulsion fractures of the calcaneal tuberosity are rare, accounting for only 3% of all calcaneal fractures. There is a strong association with diabetes, where they may occur spontaneously, and are thought to be due to peripheral neuropathy. They also occur in osteoporosis and hyperparathyroidism. Treatment is typically with open reduction and internal fixation, although minimally displaced fractures may be treated in a closed fashion.

Reference: McNaughton GW, Thomas L. Spontaneous Avulsion Fracture Of The Calcaneus In A Non-Diabetic Patient. SMJ 2006 51(3): 50

Credit: Dr Laughlin Dawes, Sue Bachelor