What is a foot callus?

Our well-being is associated with healthy skin on the soles of our feet. The ability to walk and stand depends on the proper function of the skin on the sole of the feet. Plantar foot skin exhibits unique biophysical properties that are distinct from skin on other areas of the body. Hyperkeratotic lesions represent one of the most prevalent foot problems in older people. Approximately 90% of older people exhibit callus, a form of hyperkeratotic lesion which is a thickening of the outer layer of the skin. Plantar pressure is significantly increased in callused regions of skin in older people. Raised pressure may play a role in the development of plantar calluses by accelerating the turnover rate of epidermal keratinocytes, the major cell type of the epidermis which is the outermost of the layers of the skin.

Callus formation

Calluses are painful and unsightly lesions which can have a detrimental impact on the mobility and independence of older people. Symptoms may be severe enough to seriously affect a person’s gait and choice of footgear or activities. People with diabetes often have dry feet and trouble with calluses. Callus predispose to foot ulceration in diabetics. There are two basic types of calluses, the diffuse-shearing and the discrete-nucleated. The discrete-nucleated callus is referred to as plantar corn because this localized and painful lesion contains a central keratin plug. 

Podiatrists are the provider of choice

Some of the most common mechanical injuries of the lower extremity present as calluses. A callus is a broad-based or diffuse hyperkeratotic lesion of relatively even thickness, usually found under the metatarsal heads at a site of repetitive friction, continual rubbing, and pressure. Podiatrists are the provider of choice for calluses. 84% of people who seek treatment for corns and calluses on their foot will visit a podiatrist.

What is the second metatarsal head?

Excessive pressure and shear stress while walking cause a risk of callus formation. Repeated pressure and shear stress during walking contribute to callus formation on the plantar region. Callus under the second metatarsal head has been associated with increased shear stress/pressure ratios. 

The metatarsals are the long bones in the midfoot that connect the toes to the hind foot. The metatarsals are numbered one through five, starting with the big toe. So the second metatarsal is the long bone of the second toe. Callus under the fifth metatarsal head has been associated with increased shear stress.

What is the desquamation?

During normal keratinization, viable epidermal cells differentiate into horn cells which are lost from the surface of the stratum corneum, the outermost layer of the skin, during desquamation, the shedding of the outermost layer of the skin. Callus forms as a result of hyperproliferation, cells dividing more rapidly, and incomplete differentiation, the process by which cells acquire specialized features, of epidermal keratinocytes, cells that form the superficial layer of the skin. 

In some conditions there is a failure of the normal loss of binding forces between corneocytes near the surface, and a build-up of these cornilied cells, leading to hyperkeratosis which is considered to be a protective mechanism that prevents damage to the deeper tissues by dispersing the applied force over a large area and volume of skin.

In response to repetitive friction or pressure, normal healthy skin undergoes accelerated keratinization, production of large amounts of a protein called keratin. A callus is a local thickening of skin characterized by accelerated keratinization and a reduced rate of desquamation, the shedding of the epidermis which is the outermost layer of the skin. However, normal desquamation is important for a normal skin appearance. The generation of thickened skin that covers the soles of feet in response to elevated pressure, places individuals with diabetes at risk of ulceration.

Calluses typically form when the skin is exposed to high friction from excessive pressure and shear stress while walking. Such stimuli initiate a process of hyperproliferation of keratinocytes in the deepest layer of the epidermis, the stratum basale. The final step is the conversion of living cells into corneocytes, which are scaffolded by cell envelopes. The keratinocytes migrate towards the outermost layer, the stratum corneum, but instead of differentiating into flat and anucleate corneocytes, they maintain a more voluminous, non-squamous shape. The mean volume of corneocytes from callused skin is increased, indicating that corneocytes from callused skin are thicker (or rounder) than those of normal plantar skin.

The number and size of these keratinocytes, in combination with increased expression of adhesion molecules and decreased hydration make calluses thick and durable. Thus, callus formation is dependent on intrinsic adhesiveness within the stratum corneum of the epidermis of the foot.

Are plantar calluses natural?

Broadly distributed plantar calluses are natural in habitually barefoot individuals. Extensive calluses being an evolutionarily normal condition for humans. Footwear only emerged about 30.000 years ago. The majority of time since then, humans have worn minimalist footwear designed to protect the sole of the foot. Thus until the relatively recent invention and wide availability of footwear, plantar calluses functioned effectively providing a protective layer of variable thickness while maintaining the ability to perceive a broad range of tactile stimuli during locomotion. The ability of the foot to increase callus thickness in response to environmental stimuli such as friction, combined with the absence of a trade-off between protection and tactile perception, make calluses a remarkable example of engineering by natural selection.

Hunter-gatherers took approximately three to five times more steps per day on average than humans in post-industrial societies indicating that many people today do not experience evolutionarily normal loading from walking. Besides the reduced frequency of steps there is the cushioning of modern footwear that reduces the walking forces by two to three times.

Footwear—as with many recent cultural innovations— provides many benefits, including protection, comfort and style, but also has costs. Humans have regularly worn footwear since at least the Early Upper Palaeolithic, also called the Late Stone Age that dates 50,000 to 12,000 years before the present. The oldest direct evidence of a leather moccasin-type shoe was found in Armenia that dates to the Chalcolithic, 3627–3377 calendar years before the present.

Footwear has been used originally as a protective covering in the form of woven sandals for the foot for thermal insulation in cold climates and for protection of the plantar foot from ground surface.

Adequate Shoe

Today a large proportion of the population wear incorrectly sized footwear.

Buldt, AK, Menz, HB. Incorrectly fitted footwear, foot pain and foot disorders: a systematic search and narrative review of the literature. J Foot Ankle Res. 2018;11:43.

A 2017 study found that, among older adults with a history of foot lesions, such as corns and calluses, only 14% were wearing the right size shoes.

Palomo-López P, Becerro-De-Bengoa-Vallejo R, Losa-Iglesias M, Rodríguez-Sanz D, Calvo-Lobo C, López-López D. Footwear used by older people and a history of hyperkeratotic lesions on the foot. Medicine. 2017. 96(15):e6623.

57% of people with callus wear incorrectly fitting shoes.

Wearing shoes of fit size and width has the potential to prevent callus formation. Custom footwear with confirmed efficacy in preventing callus formation is available. 

However, a large proportion of the population is wearing inappropriately sized footwear based on length and width measurements. The majority is wearing footwear that is too narrow. 

Shoes getting tight?

Feet may get wider with age. Feet get wider with age while the length remains the same. Tissue becomes less tight, causing sagging of the arches resulting in the increased width. The arch of the foot sags with age. 

The most problems are with the front (forefoot) of the foot. This makes the width and height of the front of the shoe important. The use of differently sized shoes for each foot may be necessary to prevent the onset of foot deformities. 

Callus and shoes of narrow width

The pressure and shear stress are significantly lower when putting on shoes of fit size compared with larger sizes. The increased peak shear under the fifth metatarsal head is significantly smaller when putting on shoes of fit width compared with those of narrow width. Increased peak shear stress under the fifth metatarsal head has been associated with callus formation.

Callus is associated with an increased shear stress/normal stress (pressure) ratio under the second metatarsal head and with higher peak shear stress under the fifth metatarsal head. Callus formation under the second metatarsal head is caused by wearing large shoes, and that callus formation under the fifth metatarsal head is associated with wearing narrow shoes.

What is an appropriate shoe?

Wearing shoes of suitable size and width is effective to reduce pressure and shear stress under the second and fifth metatarsal head. Patients should be advised to wear low-heeled shoes with a soft upper portion and a roomy toebox. Irregularities within the shoe should not be overlooked, because a poorly-positioned seam or stitching may be the mechanical irritation responsible for creating a lesion. 

What causes callus?

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.

How do you remove a callus?

Most callus lesions will usually disappear following the removal of the causative mechanical forces and can be managed conservatively by proper footwear, orthoses, padding to redistribute mechanical forces and, if necessary, regular paring. The ‚gold standard‘ of callus treatment is sharp scalpel debridement by a trained foot care specialist, podiatry treatment, as has been demonstrated by a three-armed randomised, comparative trial using biophysical outcome measures. 

Callus removal is a key step in preventing diabetic foot ulcer. Early and regular removal of hyperkeratotic tissue is important in patients with neuropathy and peripheral vascular disease. The most important therapeutic goals are relieving pressure points and avoiding or reducing callus formation. Podiatrists play a role in preventing and correcting the abnormal mechanical stresses and managing calluses by regular scalpel debridement.

The principles of treatment should be to provide symptomatic relief. A mainstay of palliative callus treatment is sharp debridement using a blade to pare down these callus lesions and reduce the amount of hyperkeratotic tissue. 

Therapeutic padding can alleviate the patient’s symptoms by reducing the amount of mechanical irritation to the site of the callus. Plantar calluses caused by weight-bearing stresses on the metatarsal heads may be relieved or eliminated by accommodative metatarsal pads to transfer weight away from the painful area to the uninvolved areas of the foot.