A callus is a thick, hardened area of skin. They form when the skin is subject to constant friction, pressure or other irritation. They are quite commonly found on the feet, but can be found frequently on the hands of people who frequently use tools, such as carpenters, but can even form in small areas of people who frequently use writing instruments like pens and pencils, or who play string instruments like the guitar or violin. .

In most cases, a callus is harmless and needs no treatment. However, if subject to further friction, they can often form skin ulcers and can open a path for infection.

Calluses are formed by an accumulation of dead keratin forming cells which form cross-links with each other resulting in a very friction and pressure resistant surface.

A callus is only medically significant in a diagnosis where there is no evidence of repetitive wear. For example, both arsenic and syphillis can result in calluses.

A "corn" is a callus that does not smoothly follow the contour of the structure it is attached to. They commonly form on the outside edge of the small toe, particularly in reaction to wearing tight shoes.

Calluses are difficult to treat after they form, so prevention is an important consideration. Shoes should fit properly. People who work with their hands should wear protective gloves wherever possible. However, they can be dissolved, self-treated with a pumice stone or callus shaver, or removed by a podiatrist.

Callus at Wikipedia

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