A heel spur forms as a calcium deposit on the heel bone, or calcaneus, of the foot, as explained by WebMD. Heel spurs are often caused by wear and tear, and it may be associated with osteoarthritis.
Chronic inflammation of the foot where soft tissue tendons attach to the heel are a cause of heel spurs, according to MedicineNet. The plantar fascia that stretches from the heel to toe has a poor blood supply when inflamed, leading to slow healing and formation of soft calcium deposits, as Runner's World describes. Heel spurs cause foot pain in about 5 percent of people, and OrthoInfo indicates that one in 10 people have heel spurs.