A callus is an area of hard, thickened skin that can occur across the ball of the foot, on the heel, or on the outer side of the big toe. While many consider them a skin problem, they actually are systemic of a problem with the bone. Calluses have painful nerves and bursal sacs (fluid-filled balloons that act as shock absorbers) beneath them, causing symptoms ranging from sharp, shooting pain to dull, aching soreness.
Calluses form from repeated friction and pressure as the shoe (or ground) rubs against a bony prominence (bone spur) on the toe or foot. The skin thickens in response to this pressure, small amounts of friction or pressure over long periods of time cause a corn or callus. A great deal of friction or pressure over shorter periods of time can cause blisters or open sores. Calluses typically develop under a metatarsal head (the long bone that forms the ball of the foot) that is carrying more than its fair share of the body weight, usually due to it being dropped down or due to its longer length. thickened and hardened part of the skin or soft tissue, especially in an area that has been subjected to friction.