Calluses result from hyperkeratosis, the formation of an accumulation of several layers of the horny layer of epithelium. Cornification is the process by which stratified squamous epithelia are formed, or the formation of the so-called ‘horny layer’ of the skin. Thus, incomplete cornification or differentiation may occur in the stratum corneum in the callused skin.
Hyperkeratosis is caused by an increase in keratinocyte activity associated with stimulation of the epidermis which is a normal physiologic response of the skin to chronic excessive pressure or friction on the skin caused for example by a deformity of the foot or tight shoes.
The cells of skin react to persistent abnormal pressure on the foot by increasing keratinization that turns into callus. Friction due to pressure on prominent bony areas of the foot or hammertoes, inappropriate, badly fitting tight shoes, abnormal foot mechanics (deformity of the foot exerting abnormal pressure) result in pressure and shear stress while walking. Dry skin and high levels of activity produce pressure and friction that lead to calluses.
Hyperkeratotic lesions are secondary to increased mechanical stress. As mechanical stresses on the skin increase, the body attempts to protect irritated skin by forming a hyperkeratotic lesion, such as a callus. Hyperkeratosis is a normal protective response of the skin, which becomes pathologic when the callus grows so large that it becomes the source of symptoms.
However, a hyperkeratotic lesion will increase the pressure in a tight shoe, thus creating a vicious cycle. Increased pressure increases the formation of calluses, which further increases the pressure. Agressive sharp scalpel debridement of hyperkeratotic lesion in turn stimulates keratinocyte activity leading to swift regrowth of more callus.