Both corns and calluses involve excessive production of dead skin cells. This is the uppermost layer of the skin that protects it from external injury. For corns and calluses, this production is the result of intermittent pressure, from shoes or weight bearing. The skin responds to this pressure by thickening. Initially this "toughening" of the skin is helpful, but over time it builds up and becomes an irritant. Increased mass of the lesion results in increased pressure and discomfort. 

A vicious cycle develops which is only broken by relieving the pressure or reducing the lesion. Corns usually are on the toes, and result from shoe pressure. Calluses are usually on weight bearing areas on the bottom (plantar) surface of the foot, and result from weight bearing and/or abnormal alignment of the metatarsal bones in the ball of the foot. Both terms are expressions of the same type of lesion, which is medically termed a hyperkeratosis.

Both corns and calluses may have a deeper central core, known to the podiatrist as a nucleation. This can be the site of exquisite tenderness. This core is not the "root" of the lesion, in the sense that removing this will keep the lesion from returning. It is simply the area of greatest pressure, often corresponding to a prominence of the underlining bone. Reducing this with sharp instruments and applying accommodative pads and tape, the podiatrist relieves discomfort and dissipates pressure. Unless something is done to permanently relieve pressure, the lesion will redevelop. Permanent relief is sometimes achieved by changing shoes, orthotics, or surgery to remove bony prominences and realign bones.

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