This article is about calluses and corns of human skin, in plant cell biology callus refers to a mass of undifferentiated cells.

What are corns and calluses?

Most commonly, callus is an area of skin, usually on the hand or foot which has become relatively thick and hard, from rubbing and/or pressure. Calluses may lead to other problems such as serious infection. Shoes that fit well can keep calluses from forming. A special case of callus is a corn. Corns are much like calluses but usually occur on the top of the toes or fingers on thin, or glabrous skin surfaces. Sometimes they do occur within the thicker palmar or plantar skin surfaces.

History of the corn

The name corn comes from the appearance under the microscope. In the days when lenses and microscopes were first invented, scientists put everything under the microscope. When corns were removed, the hard part, the center of the corn, appears to look like the inside of a funnel with a broad raised top and pointed bottom. It looks, when examined under magnification, much like barley seed. In parts of Europe, barley is called 'corn', hence the name stuck. The scientific name is heloma. Hard corns are called heloma durum. Soft corns are called heloma molle.

Development of skin corns and calluses

How calluses develop

A callus is thickening of the skin. Although usually found on the foot, they can occur anywhere on the body where there is pressure. It is a natural reaction of the thick skin on the palmar or plantar surfaces to react to pressure by producing a callus.

Players of stringed instruments such as the guitar will develop calluses on their fingers if they play frequently enough. These help the player, because the thicker skin of the callus protects the skin of the fingertips; extended play is often painful before calluses develop. If a beginning player plays too hard, a blister may be produced before a callus.

People who have bunion deformities of the great toe find that they have painful calluses behind the second or third toe on the plantar surface of the foot. These are caused by unequal pressure placed on the smaller metatarsals rather than under the larger first metatarsal. These types of pressure induced calluses can be very painful and often do not respond to trimming of the callus, soft materials nor orthotic devices but require surgical change in the function of the foot.

How corns develop

Corns are a special case of hyperkeratosis of the skin. They are formed when there is pressure against the skin and the pressure point or the skin traces an elliptical or semi-elliptical path. This forms a swirl of tissue, the center of which is at the point of pressure gradually widening. There is stimulation of the tissues which produce a corn. In that way, even when the corn is removed or the pressure surgically removed, sometimes the skin continues to grow corns for some time.

What is the difference between soft and hard corns?

The difference between soft and hard corns is where they occur. Hard corns occur on dry flat surfaces of skin. Soft corns, frequently between two toes, stay moist, keeping the skin surrounding the hard center of the corn soft. The corn itself is not soft.

What can be done about calluses and corns?

Most corns and calluses under the foot are caused from pressure of the bones in the foot pressing against or holding the skin from moving against the shoe or the ground. While changing or using well fitting shoes will help some of these problems, most of the time some intervention is required. The most basic treatment is to put a friction reducing insole or material into the shoe or against the foot. Sometimes this will reduce pain caused by heat buildup without making the callus or corn go away. Sometimes a change in the function of the foot by use of an orthotic device is required. This reduces friction and pressure allowing the skin to rest and stop forming protective skin coverings. Sometimes surgical correction of the pressure is needed. Sometimes the only thing that relieves the pain is to shave the calluses down and perhaps pad them. This is usually done by a podiatrist.

One of the well knows and very common cures for corns that appear on the feet of the patient, involves the wrapping of potatoe peels around the affected area and leaving it overnight. For best results this should be done on a night of full moon.

Shoes and their role in corns and calluses

Shoes and callus formation

Although well fitting shoes may keep the feet from being painful, they rarely change the way the foot functions. Well fitting shoes can keep some problems from advancing and can keep problems from being painful.

Shoes and corn formation

Shoes can produce corns by rubbing against the top of the toes or foot. Stretching the shoe over that area may reduce the pressure and make the corn stop hurting. If the foot continues to move in the area the corn may stay but not hurt.

Other causes of callus and corns

Some callus occurs where there is no rubbing and no pressure. These hyperkeratoses can be caused by a variety of things. Some toxins like arsenic can cause thick palms and soles. Some diseases such as syphillis can cause thickening of the palms and soles as well as pin point hyperkeratoses.

Corns at the nail edge

The nails, particularly toenails, are very stiff. If the toenail or fingernail rubs against the skin and pinches it between another surface for a period of time, a corn can form at the edge of the nail. These are difficult to treat because frequently it is the nail itself which is the cause of the corn.

Corns on the hands, legs, or other spots

Much like calluses, corns can form in other areas for other reasons. There is a benign condition called Keratosis palmaris et plantaris which produces corns in the creases of the fingers and non-weight bearing spaces of the feet. Most of this type of corn are called senile keratoses and occur with age and with hormonal shifts.

Corns, calluses and the diabetic

Diabetics face special skin challenges. Because diabetes affects the capillaries, the small vessels which feed the skin its blood supply, thickening the skin increases the difficulty to supply nutrients to the skin. Additionally, the shear and pressure forces that cause corns and calluses, tear the capillaries, causing bleeding within the callus or corn.

Often bleeding within a callus is an early sign of diabetes, even before elevated blood sugars. Although the bleeding can be small, sometimes small pools of blood or hematoma are formed. The blood itself is an irritant, a foreign body within the corn or callus making the area burn or itch. If the pool of blood is exposed to the outside, the area can become infected. If the area becomes infected, the tissue can breakdown and fail to heal or become ulcerated. This process can be prevented at several places. Infections can become limb and life threatening. Diabetic foot infections are the leading cause of diabetic limb amputation.