The Mechanics Involved

Calluses that form on the bottom of feet often occur on the ball of the foot, an area formed by the ends of the five long bones connecting to our toes. These bones, the metatarsals, are similar in structure to the analogous bones in our hands, the metacarpals. Make a fist: the bumps that form the big knuckles at the end of our hands are the metacarpal heads. Metatarsals, in our feet, also have these nearly round heads, and play a big role in bearing the weight of our bodies when we stand or walk. Sometimes one of these metatarsal bones is unusually long, or sits lower than the ones next to it. The result is that a given metatarsal head bears a disproportionate amount of the load. It is hitting the ground first and hardest, is bearing more weight, and doing it for a longer period of time than the other metatarsals. The result? There is a dramatic increase in the pressure applied in that particular area. Again, the skin has two choices, and when able to do so, it will always choose to grow. The callus forms to protect the skin from the breakdown that will occur if this reinforcement is not attempted. Once the callus forms however, we begin to feel the discomfort. Indeed, the callus is there for all the right reasons, but its very presence increases the amount of pressure in that area, which causes pain.

The pain caused by corns and calluses is a blessing: it is a signal from the body to take action. This action is either to remove the callus or remove the pressure from the area causing it. Both options work well, and the choice of which approach to employ is as individualized as each person and the specific reasons their corns and calluses formed. Among many other reasons, plantar (bottom of the foot) calluses can develop because of an area of scar tissue, previous fracture, the presence of a wart, a plugged sweat gland, different types of arthritis, or contractures of the toes (termed hammertoes, or mallet toes). Hammertoes are of particular interest, as they are implicated in both calluses under metatarsal heads and corns on and between the toes themselves.

Formation of hammertoes is usually due to a muscular imbalance within the foot. A contracted toe, flexed at the first or second knuckle, causes a rising of the toe, and a simultaneous lowering of the metatarsal head behind the toe. The result can be a callus under the metatarsal head, and a corn over the flexed knuckle from increased shoe pressure. A callus can also form at the tip of the toe, if the toe is bent in such a way that the tip, rather than the pulpy bottom of the toe, is absorbing the weight of standing and walking. Corns can also form between toes due to the bony prominences of two adjacent toes rubbing together. The reasons for corn formation are the same as any other hyperkeratosis; here the reasons are usually shoe pressure or shear, or adjacent toe pressure, rather than the force of the ground. The pain that results is a call to action. Unfortunately, not everyone is equipped with this signaling system.

Some people do not have the benefit of good sensation in their feet, like many people living with diabetes. As a result, they may not even be aware of potentially harmful pressures to their feet. For these people, the very presence of calluses requires attention. Without the pain signals, a callus left untreated could lead to skin breakdown, or ulceration, which can ultimately be limb-threatening. All people with diabetes or diminished sensation in their feet should be regularly examined by their podiatrist. This physician specializes in the diagnosis and treatment of conditions of the foot. The podiatrist can inform these patients more about their condition, and provide effective treatment that can assist the patient in avoiding harmful complications.