Plantar Keratosis is a discrete callus, usually about 1 centimeter in diameter, directly under a metatarsal head, under the ball of your foot. This is caused by a "dropped metatarsal" which causes the metatarsal head to lie at a lower level than the surrounding metatarsals and protrude from the bottom of the foot. This results in more pressure being applied in this area and causes a thick callus to form. Patients usually complain of pain directly under the affected metatarsal at the location of the callus, made worse by prolonged periods of standing or activity. This pain is described initially as an ache, but can get more painful with continued thickening of the callus.
This problem can usually be diagnosed by your doctor with a physical examination of your foot. A plantar wart, a skin growth caused by a virus infection and not by pressure, can sometimes mimic this problem, but these two conditions have a different appearance and can usually be differentiated by exam. Often the affected metatarsal head can be felt to be lower in position than the surrounding metatarsals. Sometimes an x-ray may be taken of the front part of your foot, which will show the relative positions of the metatarsal heads.
This picture illustrates an IPK after reduction of the surface layer of callus. Note the small "core" (nucleation) at the center. Direct pressure on this area is very painful. Further reduction involved enucleation with a small sharp instrument. This is not usually painful.
A "dropped metatarsal" can either be a congenital structural abnormality, a result of a metatarsal fracture that healed in a displaced position, or a structural change that may have occurred over time. A contributing factor may be a hammertoe that contracts and puts retrograde downward pressure on the metatarsal head. This causes the metatarsal head to be driven into the ground and increases the pressure and friction on the underlying callus.
Treatment and Prevention
The callus contributes to the pain by causing excessive body weight to be borne by the injured underlying area. You may try smoothing it down with a pumice stone or emery board when you get out of the bathtub or shower and your skin is soft and wet. Switch to athletic shoes or shoes with a soft sole and good forefoot cushioning. Also of benefit may be insoles to add more cushioning to the bottom of your foot, if your shoes have enough extra room to accommodate them.
Depending on your overall health and circulation status, your doctor may recommend periodic trimming of the callus and fit you with a insole or custom arch support that provides padding around the callus and prevents or decreases direct pressure being put on the callus when you walk. Your doctor will discuss other options that may include surgically realigning the affected lower metatarsal head, so it lies at a higher level, more in line with the other metatarsals. Correction of an associated hammertoe will also prevent the backwards-downward pressure that may be adding to this problem. Surgery will be designed to actually correct the structural problem itself that is causing the callus and pain.
By following the treatment plan your doctor recommends, you can minimize your future problems with this deformity.