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.
A plantar callus forms when one metatarsal bone is longer or lower than the others, and it hits the ground first-and with more force than it is equipped to handle-at every step. As a result, the skin under this bone thickens like a rock in your shoe. Plantar calluses that are recurring are sometimes removed surgically in a procedure called an osteotomy, which relieves pressure on the bone.
A condition called Intractable Plantar Keratosis (IPK) is a deep callus directly under the ball of the foot. IPK is caused by a “dropped metatarsal,” which happens when the metatarsal head drops to a lower level than the surrounding metatarsals and protrudes from the bottom of the foot. This results in more pressure being applied in this area and causes a thick callus to form.
A “dropped metatarsal” can either be a congenital abnormality, a result of a metatarsal fracture, or a structural change that may have occurred over time.