Corns and calluses are hyperkeratotic lesions that form as a result of persistent pressure or frictional forces at a particular skin site. While the foot is the most predisposed site of involvement, lesions may occur at any location subjected to excessive mechanical force. Characteristic sites may be seen in laborers, musicians, and athletes and may serve as an “occupational mark.” Examples include the “drummer’s digit” or the “pulling boat hands” of the crew team. Hyperkeratosis is a physiologic process intended to form a protective barrier against further soft-tissue damage. There is reactive proliferation of keratinocytes, leading to stratum corneum thickening. Lesions become pathologic only when they become painful or limit a patient’s function. In cases of severe peripheral neuropathy or vascular disease, such as that seen in diabetes, hyperkeratotic lesions may be a harbinger of ulceration.

There are both intrinsic and extrinsic factors that may contribute to the development of corns and calluses. Intrinsic factors include the presence of bony prominences or abnormal foot mechanics, either hereditary or acquired. This may lead to excessive mechanical force on a neighboring digit or unbalanced pressure on the sole of the foot. Extrinsic factors that predispose one to hyperkeratotic lesions include the use of poorly fitted or damaged footwear. Women who wear high-heeled shoes with a small toe box are particularly at risk. In the absence of anatomical deformity or inappropriate equipment use, high levels of activity, as in athletics and laboring professions, is also a frequent cause.

Corns and calluses display slightly different clinical characteristics and often occur at different sites. The corn (heloma) is distinguished from the callus by its small size, well-circumscribed nature, and presence of a central core (Fig. 8-1). The central core is a keratin plug that presses into the dermis, frequently leading to pain. Corns have been historically classified as a hard corn (heloma durum) or a soft corn (heloma molle); many authorities prefer description based solely on the anatomical location. The hard corn is more common, and typically occurs on the dorsolateral aspect of the fifth toe or the dorsum of the interphalangeal joints of the lesser toes (Fig. 8-2). The soft corn occurs as a macerated, extremely painful lesion in the interdigital spaces, most often between the fourth and fifth toes. A breach in the epidermal barrier at this site predisposes to secondary fungal and bacterial infections, as well as underlying sinus formation. A “kissing lesion” is characterized by two opposing lesions within the same interdigital site.

The callus (tyloma, clavus) is identified by its large size and poorly circumscribed nature (Fig. 8-3). It characteristically lacks a central core and is of uniform thickness throughout. Depending on the location and severity, a callus may or may not elicit pain. It is most often found at sites of weight-bearing, which typically includes the heel and ball of the foot. Calluses are also often

found at the site of placement of musical instruments or at the location of repetitive friction during vigorous athletic activity.