Conway McLean, DPM, Journal columnist

You may have had this experience, it’s a common occurrence. A subtle, pressure-like discomfort is noted from the bottom of your foot, apparently out of nowhere. The same area produces that pressure sensation again a few days later, although it seems more noticeable, more insistent. A week passes and then actual pain is experienced. Your foot now has your attention.

Life is hectic, and health issues that aren’t life threatening can easily be forgotten or cast aside. A sharp sensation originating at the bottom of the foot will often be symptomatic only intermittently. Time will often pass before a self-examination is performed. The area of pain (sometimes difficult to visualize depending on the flexibility of the sufferer) will often demonstrate the presence of a small spot of discoloration, which appears mildly callused. The obvious question: what is it?

There exist numerous diagnoses explaining a small spot on the bottom of the foot. This is a unique part of the body, enduring the stresses of weight bearing, a task borne by no other structure. To withstand the rigors of bipedal ambulation, our integument (our outer covering, i.e. skin) has developed extra layers to deal with the pressure and shear forces. But these forces are substantial over the course of your typical day, 10,000 steps and all. This is especially true beneath the heel bone and metatarsals which bear most of our weight.

These pressures have been measured and they are tremendous when evaluated over time. Any irregularity at the bottom of the foot, especially one that is small, hard, and well-defined, will reliably produce pain. Although less than the hand, the distribution of nerve endings in the feet is quite high, which means one of these little callused spots can elicit significant discomfort. Is it the callus that causes pain, or the lesion producing the callus? Again, another appropriate question.

Callus production is a natural response to irritation and pressure. If you have ever shaken hands with a carpenter, you’ll have experienced this phenomenon. This is a protective mechanism, and the aforementioned wood worker is grateful. But in nature, too much of anything is a bad thing, and continued callus formation in a localized area will invariably result in pain.

When a callus causes pain, most people will attempt to treat it by removing some of the callus material, which may be temporarily helpful but can be dangerous depending on the method. Unfortunately, callus build-up is just a symptom of excess force. Many factors go into the production of the dry, thickened dead material which is callus. Predictably, hereditary input is important in how quickly callus tissue is produced, although genetics also determine one’s foot architecture. A metatarsal bone that is positioned too low will exert greater force on the skin under the bone. Trimming the callus won’t alter the position of the bone. And to someone with diabetes, a thick callus often precedes the development of an open sore, frequently allowing an infection and potential limb loss.

Treatment for a pressure-induced callus may involve callus removal, but a more effective, preventative approach is to lessen the pressure experienced by changing the supporting surface. This is best done with some type of innersole, arch support, or, even better, a custom foot orthotic. Padding is not the solution since more material, no matter how “cushiony,” will add force to the spot.

Quite often this dark spot is not pressure-induced but something else. The possibilities are numerous but certain pathologies are more common. Although often mistaken for a wart, chronic force to some area of skin frequently causes a callus resulting from a plugged oil gland. There are thousands of these structures in our skin, helping to keep it healthy and flexible. But when one of these tiny glands becomes obstructed, it presses on the skin, leading to a tiny knot of callus. Although benign, these calluses become vaguely pointed, in effect poking the individual with every step.

Also very common (and often inaccurately identified) is the true plantar wart. These are a viral infection of the skin and are extremely resistant to the traditional therapies. These methods have consisted of tissue destruction, akin to treating a splinter with a blowtorch. A newer and very elegant solution involves the application of tiny bursts of energy, which educates the individual’s immune system to kill off the wart. Recurrence, a huge problem with all the traditional methods, is seen less than 1 percent of the time with this technology. Fortunately, it effectively treats the plugged gland problem as well.

Much less common than warts is the presence of skin cancer on the foot. Because many do not suspect a malignancy, healthcare providers included, these are diagnosed too often too late. Malignant melanoma is reliably deadly without timely identification, and these are not always darkly pigmented. Various skin cancers can occur on the feet, but who examines their feet? After all, they are hidden away, unseen inside our shoes. As per a relevant cliche, out of sight, out of mind.

This brief list only touches the surface of the long list of lesions appearing on the foot. If you are having pain from a new spot on the foot, don’t assume it is benign and will disappear on its own. If it is callusing, this will most likely be a progressive problem as the skin learns to make callus better over time. As with many medical conditions, lesions on the bottom of the foot get harder to treat as time passes by. Treatment starts with an accurate diagnosis, typically aided by consulting a specialist who will be acquainted with the optimal therapies. But don’t live with foot pain; you’ll walk less, and that’s not good for anyone.

EDITORS NOTE: Dr. Conway McLean is a podiatric physician now practicing foot and ankle medicine in the Upper Peninsula, having assumed the practice of Dr. Ken Tabor. McLean has lectured internationally on surgery and wound care, and is board certified in both, with a sub-specialty in foot orthotic therapy. Dr. McLean welcomes questions, comments and suggestions at drcmclean@penmed.com.