Calluses, Corns & Plantar Warts

(Consolidates the former “Plantar Warts” note.) Three hyperkeratotic foot lesions distinguished by site, skin lines and what paring back reveals.

	Callus	Corn	Plantar wart
Site	Pressure areas (under metatarsal heads)	Pressure areas (chronic irritation)	Anywhere, not necessarily pressure areas
Skin lines	Preserved	Preserved	Lost (interrupted)
Squeeze	Painless	Painful on direct pressure	Painful on squeezing
Paring back	Normal skin	White avascular keratotic core	Pinpoint bleeding (the “iceberg” — most of the wart is beneath)

Two clinical clues separate a plantar wart from a corn/callus at the same site: location of a plantar wart is not restricted to pressure sites whereas a plantar callus or corn is always at a pressure site. Tenderness is maximal with lateral pressure for a plantar wart whereas a corn or callus is more tender with direct pressure.

Calluses and corns

Corns and calluses are caused by response to friction and pressure. Repetitive injury results in the skin trying to protect itself from blistering. The basal epidermal cells (keratinocytes) increase in number resulting in thicker prickle cell layer and thicker stratum corneum. A corn (clavus, heloma) is inflamed and painful; a callus (tyloma) is painless; a “soft corn” (heloma molle) occurs where surface skin stays damp and peeling, e.g. between squashed-together toes.

Management — relieve the pressure first, then reduce the thickened skin:

• Relieve pressure: well-fitting, comfortable, flat footwear; a protective corn plaster or cushion to spread pressure more evenly; separate toes with soft cotton, lamb’s wool, moleskin or web spacers; made-to-measure orthotics if recurrent.
• Reduce skin thickness: soak in warm water ≥10 minutes, then sandpaper, file or pumice the surface; a specially designed corn trimmer can pare down the central core; keratolytic creams or heel balms containing urea, salicylic acid or lactic acid.
• Painful fissures: petroleum jelly or a sealant (liquid bandage, nail glue), antibiotic ointment if infected, cover with a thick adhesive plaster.
• Avoid salicylic acid in diabetes and in patients with peripheral vascular disease or the frail elderly (ulceration/infection risk from its keratolytic/chemical-burn action on compromised skin) — refer these patients to podiatry instead of self-treating.
• Podiatry review for recurrent lesions, high-risk feet, or if protruding bone (e.g. a bunion) is the underlying cause — occasionally needs orthopaedic surgical correction.

Plantar warts (and other cutaneous viral warts)

A viral wart is a very common benign lesion caused by infection with human papillomavirus (HPV), spread by direct skin-to-skin contact or autoinoculation (scratching can seed a line of new warts, or a subungual wart under a nail). The incubation period can be as long as twelve months. Warts are particularly common in school-aged children, in dermatitis (defective skin barrier), and in drug- or disease-induced immunosuppression. Presentations besides plantar warts include common warts (verruca vulgaris — hands, knees, periungual), plane warts (face/hands/ shins, often spread by shaving), and filiform warts.

Plantar warts (verruca plantaris) include tender inwardly growing myrmecia on the sole caused by HPV 1, and clusters of superficial less painful mosaic warts due to HPV 2.

Natural history — doing nothing is a legitimate option. In children, even without treatment, 50% of warts disappear within six months, and 90% are gone in 2 years. Warts are more persistent in adults but usually clear eventually, and recur more often in smokers and in immunosuppressed patients (e.g. transplant recipients).

Treatment — a 2012 Cochrane review of 85 trials (8,815 participants) found that salicylic acid (SA) had a definite but modest beneficial effect compared to placebo, effective for warts at all sites with few adverse effects, though it may take several weeks of daily use to work. Cryotherapy performed no better than placebo overall in the review’s meta-analysis. Aggressive cryotherapy appeared more effective than gentle cryotherapy, but with increased adverse effects (more blistering and pain). Combining the two modalities helped — SA and cryotherapy combined appeared more effective than SA alone. Duct tape, once thought a safe simple option, is not as effective as first thought, once more trials were done.

• Salicylic acid (topical wart paint/paste): soften the wart by soaking, rub the surface with pumice/emery board, apply the paint daily to the wart only (a rim of normal skin included), then remove and reapply the next day — persistence matters, since HPV infects the basal epidermis and warts regrow fast if not eradicated. About 70% of warts resolve within twelve weeks of daily applications.
• Cryotherapy with liquid nitrogen, repeated every 1–2 weeks; expect blistering/discomfort for days afterwards. Success is in the order of 70% after 3–4 months of regular freezing.
• Combine SA + cryotherapy for a better response than either alone if first-line monotherapy is slow to work.
• Second-line/resistant warts: electrosurgery (curettage and cautery — leaves a scar, ~20% recur), or for recurrent/extensive disease bleomycin injections, laser, or a contact sensitiser (diphencyprone, dinitrochlorobenzene, squaric acid). Imiquimod is approved for anogenital warts but is usually ineffective for cutaneous warts.
• No treatment is universally curative — set expectations accordingly, especially in adults.