Chapter 98. Corns and Calluses

Every human being, with the exception of the nonweight-bearing infant, is vulnerable to the development of corns and calluses, because the skin is subjected to regular mechanical stress. The prevalence of corns and calluses can be readily appreciated by the number of nonprescription products aimed at reducing or preventing them—a billion-dollar market annually.

The earliest known discussion of these lesions can be found in the writings of Cleopatra, who authored a textbook on cosmetics.1 Corns and calluses have plagued humankind since antiquity, affecting those at all socioeconomic levels.

Certain foot types and regions are prone to mechanically induced skin thickening, regardless of race, gender, or age.

Corns and calluses result from the prolonged application of excessive mechanical shear or friction forces to the skin. In theory, these forces induce hyperkeratinization, which leads to a thickening of the stratum corneum, although the precise mechanism by which this occurs remains unknown. If the abnormal forces are distributed over a broad area (i.e., more than 1 cm2), a callus develops. In contrast, a corn will form if the same forces are applied to a focused location, with the lamellae of the stratum corneum becoming impacted to form a hard central core known as the radix or nucleus (Fig. 98-1).

Mechanical keratoses are not determined genetically. Heredity does play a role, however, in configuring the individual's skeletal architecture. A family history of bony abnormality or ligamentous laxity predisposes the person to the presence of sites of increased cutaneous friction or shear. The prevalence of these lesions has also proven to be significantly higher in females, certain ethnic groups, and mentally ill patients.2,3

History

Corns and calluses produce painful symptoms often described as burning, especially when the affected area is weight bearing and/or shoes are worn. This discomfort is thought to result from microtearing of the thickened, inflexible skin.

Cutaneous Lesions

Corns [clavi or helomata (singular: clavus or heloma)] and calluses [tylomata (singular: tyloma)] are, respectively, keratotic papules and plaques that occur in areas that are subject to sustained excessive mechanical shear or friction forces.

Related Physical Findings

The lesions occur in predictable pedal locations, corresponding to a structural deformity or biomechanical fault. Crookedness of the lesser toes leads to prominence of the proximal and/or distal interphalangeal joints. Keratoses can therefore form either dorsal to those joints, between the toes, at the distal end of the toe, or on the lateral aspect of the fifth toe and/or toenail (lateral toenail corn, also known as Durlacher's corn; see eFig. 98-1.1). Interdigital corns can be hard when they are adjacent to the interphalangeal joint(s) (see eFig. 98-1.2) or soft when deep within the fourth interdigital space. The softness of latter last corn results from trapped perspiration, which leads to maceration of the keratotic tissue (Fig. 98-2).

In patients with bunions (hallux valgus), a callus usually forms at the medioplantar aspect of the hallux. During gait, the individual rolls off that portion of the great toe because of its incorrect position. The skin is subsequently pinched to form a “pinch callus” (see eFig. 98-2.1). In addition, the first metatarsal often does not bear its fair share of the weight-bearing load. Weight is therefore transferred laterally to the second metatarsal head, which usually leads to the development of an additional corn or callus beneath that bone.

Other favored locations for lesser metatarsal head keratosis include the following:

• Beneath the first and fifth metatarsal heads in cavus foot types
• Beneath the fifth metatarsal head alone in persons with tailor's bunions (bunionette)
• Erratic locations (e.g., beneath the third and fifth metatarsal heads, isolated third or fourth metatarsal head, second and fourth metatarsal heads) in individuals with structural abnormalities such as brachymetatarsia or dislocated metatarsophalangeal joints, as in rheumatoid arthritis or neuroarthropathy (Fig. 98-3)
• Beneath the second through fourth metatarsal heads when multiple hammertoes, hallux valgus, or an equinus deformity coexists (Fig. 98-4)

Another variant of corn is that referred to as heloma miliare or seed corn. This title is derived from the clinical appearance of these corns: multiple guttate keratoses that are easily pared. When seamed nylon hosiery was fashionable, this garment was considered the causative factor. However, patients still present with seed corns even though they have never worn seamed stockings.

In contrast to nonmechanically induced keratoses, corns and calluses exhibit changes within the epidermis, dermis, and adipose layer. Corns demonstrate a parakeratotic plug within the stratum corneum, with a pressure-related loss of the stratum granulosum as well as atrophy of the stratum malpighii. The dermis displays significant fibrosis, dilated eccrine ducts and blood vessels, hypertrophied nerves, and scar tissue replacement of subcutaneous fat. Overall, the histologic changes in calluses are less pronounced, and include a thickened stratum corneum but intact stratum granulosum.4

Because corns and calluses are the result of mechanical friction and shear alone, there are no associated hematologic, chemical, serologic, or immunohistochemical abnormalities.

Diagnosis and Differential Diagnosis

(See Box 98-1)

If left untreated, corns and calluses result in painful ambulation and also in subhelomal bursitis and blistering that can rupture to the surface. Because of the close proximity of some corns to joints and bone, septic arthritis and/or osteomyelitis can ensue. The mechanical forces that cause corns and calluses can also rupture portions of the subcutaneous vascular plexus, which leads to hemorrhage into keratotic tissue. In healthy patients, such findings are of minor significance, but in other cases (e.g., in diabetic patients and patients with connective tissue disease), they may herald extensive skin ulceration or vasculitis.

The need for lower extremity amputation is a dominant fear in most diabetic patients. Such amputations are most often preceded by a history of foot ulceration (see Chapter 151). Although a number of comorbidities contribute to the development of ulceration (e.g., peripheral vascular disease, neuropathy, and limited joint mobility), minor trauma via repetitive pressure is the pivotal precipitating event. As markers of repetitive friction and shear, corns and calluses in the diabetic foot are of special significance. Simple débridement of these hyperkeratotic lesions decreases peak plantar pressures by as much as 26%.5 In a retrospective review of more than 200 diabetic foot ulcerations, patients who had their corns and calluses pared frequently experienced a statistically significant decrease in the incidence of foot ulceration, hospitalization, and surgical intervention.6 Hemorrhage within a corn or callus is an especially ominous sign, indicating subcutaneous breakdown with a strong potential for ulceration. Therefore, ulcer care should include paring of calluses. The use of proper footwear by the diabetic as well as the nondiabetic patient may also play a role in not only preventing but also reducing the development of callosities. Shoes should be correctly sized to accommodate the width and length of the patient's foot, and the heel should be elevated minimally, if at all, to prevent pathology and pain.7,8

(See Box 98-2)

Corns and calluses can be prevented only by reducing or eliminating the mechanical forces that created them. Usually, this is a daunting, if not impossible, task. Repetitive occupational motions are often unavoidable, patients are commonly reluctant to alter shoe styles, and osseous architecture is predetermined through heredity.