Conway McLean, DPM, Journal columnist

Skin is interesting stuff…..perhaps you know some of the relevant statistics? It’s the largest organ in the body, for one. Another is that your skin accounts for about 15% of your body weight and covers 21 square feet, on average. Contained within the skin are 7 miles of blood vessels. Our skin serves many functions, some of them essential for life. (This is why burns are so often deadly.) Perhaps one of the most important is our skin’s ability to protect us from the physical environment. Think of the trauma suffered by the skin of the feet, step after step, day after day, whether shoeless or shod.

The body’s primary response to this kind of physical insult is to thicken via the formation of a material colloquially known as callus. Interestingly, calluses have a complex social history. Not long ago, they were a sign of the lower social classes, working folk, the uneducated. Someone who toils long hours performing hard physical tasks likely had rough, thickened skin on their hands. A carpenter’s hands, for example, are often tremendously roughened, and likely they are thankful since this skin reaction protects them from their work hazards.

Predictably, this natural process, the production of extra, thicker skin, can lead to problems. As in many components of nature, too much of anything is a bad thing. Many painful and harmful conditions can result from this common development, callusing of the skin. But callus formation is a natural result of skin irritation, whether it comes in the form of pressure, shear forces, or any such stress, over time (a key component of this process). Since callus is primarily composed of keratin, callus formation is technically referred to as keratinization, keratin being one of the main proteins of our skin.

Most calluses form somewhere on the feet, predictable since the stress of carrying the human body around entails tremendous physical forces. With ten thousand steps a day (the amount many of us take on average), there’s plenty of opportunity for excessive pressures to some prominent bone or bent toe. Cramming the foot into a rigid, narrow shoe is also a frequent component of callus development. The reasons a painful callus arises are numerous indeed.

One of the most consistent sites for a callus is under the ball of the foot. This is the area on the bottom of the front of the foot behind where the toes attach. But the specific cause for a callus in this area varies tremendously. Some of the explanations for calluses growing under the ball of the foot may not be readily apparent. For example, when the arch rolls down too much, more pressure is endured by the inner part of this surface. Thus, a thick callus is found here on many of us. Commonly, the specialized padding under these bones thins over time and more concentrated pressure is experienced by the skin under the associated metatarsal bone(s).

A plethora of other factors exist leading to symptomatic callus formation. One less predictable is a tight Achilles tendon. The lever principle comes into play here, whereby a shortened tendon pulls up more on the back of the heel, and the foot becomes a lever with the ankle as the axis. Like a seesaw, pulling up on the heel pushes the ball of the foot down harder. Or perhaps one of the metatarsal bones is slightly lower, relative to the ground and the adjacent bones. It’s going to get more pressure, and now you know the result.

The pressure may come from a toe that has contracted up sufficiently, usually the result of a progressing hammertoe deformity. When a toe begins to pull up at the ball of the foot, it can come to sit on top of the metatarsal bone more than on the end, pushing it down. It becomes basic physics, with the skin under the head of the metatarsal receiving years of excessive force.

As is always the case in all such discussions, genetic variability is critically important. Some people are just callus formers, and have skin that produces extra keratin, aka callus tissue, with even slight irritation. Then there is the matter of an individual’s skin care regimen. When skin is better hydrated, it builds callus more slowly. Someone who uses a moisturizer regularly is going to have less build-up than another, all other factors being equal. Then there is the matter of which moisturizer is being used. They are not all created equally.

Perhaps the most devastating example of the consequences of a callus is in a diabetic who has developed some of the nerve changes that occur to so many with this widespread disease. They often won’t be aware of the pressure pain of a callus in this area, and so won’t compensate for it. How does someone with normally functioning nerves adapt to these symptoms? Pain is a powerful stimulus, leading to any number of changes. Hopping, limping, shoe change, use of padding, even seeing a doctor, all are some of the many ways to deal with this kind of foot pain.

In the neuropathic diabetic, these won’t occur and continued pressure too often leads to skin death. Again, this may not hurt so the afflicted diabetic won’t be aware of it. The inside of a shoe, socks, also floors, all are environments harboring a multitude of microorganisms. With the lack of attention, in combination with the impaired defenses of the diabetic, these all too commonly result in infection, hospitalization, and too often amputation.

This is a topic that could go many places. We haven’t touched on treatments. Therapies for these range from simple topical ointments to reconstructive surgery. And calluses cause symptoms in many other parts of the foot besides the ball. We haven’t touched on calluses on the toes or the heel, but most calluses do occur on the foot.

Calluses are one of the most common foot skin complaints experienced by people of all ages. They often become a greater problem with time, as the skin learns to make callus better, over the years. Heart function decreases so that the skin gets less blood, tissues not as healthy. The years of pressure at the site of a callus often have consequences to the tissues composing the skin. Learning how to care for a mild but consistent callus can prevent it from ever becoming a significant one.

They can disappear when the source of the irritation and pressure are addressed, identified, and accommodated. Whether its shoe modifications or prescription foot orthoses (custom arch supports), innumerable, time-tested methods for alleviating, or at least minimizing, the long term effects of chronic calluses are available. The site of the lesion, that would be the callus, determines the most appropriate specialist to see about the condition. Need I say more?