The heel bone is the largest bone in the foot and absorbs the most amount of shock and pressure. A heel spur develops as an abnormal growth of the heel bone. Calcium deposits form when the plantar fascia pulls away from the heel area, causing a bony protrusion, or heel spur to develop. The plantar fascia is a broad band of fibrous tissue located along the bottom surface of the foot that runs from the heel to the forefoot. Heel spurs can cause extreme pain in the rear foot, especially while standing or walking. Heel pain may also be due to other causes, such as a stress fracture, tendonitis, arthritis, nerve irritation, or, rarely, a cyst. Because there are several potential causes, it is important to have heel pain properly diagnosed.