Hyperkeratosis of the foot: part 2.
One of the most common conditions seen in podiatry practice is corns and callus, accounting for up to 75% of podiatrists' workload (1,2). They often form painful skin lesions and can be the main reason for seeking treatment. Although, seemingly a trivial condition, it has been shown that older people, in particular, who have plantar calluses have difficulty with walking and using stairs (3). Calluses and painful feet have also been associated with an increased risk of falls (3,4). Within the diabetic population, plantar callus has been identified as a risk factor for the development of ulceration (5) and therefore a considerable amount of research has been carried out in this area. This article will cover the formation, aetiology and potential treatment of mechanically induced callus, corns will be covered in a later article. Non-mechanical causes of callus were presented in 'Hyperkeratosis of the foot: part 1' in the June 2008 edition of Podiatry Now.
Key Point: callus may account for up to 75% of a practitioner's workload
What is a callus?
Callus has been defined by Mackie (6) as 'an excessive formation of the normal keratin for the body site in question' and may be associated with disorders such as psoriasis, eczema and mycosis, as well as mechanical causes.
The formation of symptom-free callus is thought to be a normal process which helps protect the foot from trauma (7-9), and is known as physiological callus. If the traumatic stimulus causing the callus formation is removed, the skin, in particular the stratum corneum, reverts in time to its original structure (9). However, when the callus accumulates sufficiently to cause discomfort, and persists despite the removal of the stimulus, it is referred to as pathological callus (8,10). This may progress to ulceration, particularly when the nutrition of the tissues is impaired, for example in diabetes mellitus (5,11-13).
Key Point: callus is a risk factor for developing ulceration in the diabetic foot
Incidence and sites of callus formation
Within the UK it is estimated that 75-80% of people over 65 present with callus or corns (14,15), and, not surprisingly, calluses are more common in women (15,16). Within the working population, the estimate of people suffering from calluses is 18% (16).
The distribution of plantar calluses varies across the metatarsal heads, with the second metatarsal head being the most common site, see Table 1.
The incidence and location of calluses differ between men and women. It has been found that men are more likely to have calluses over the first and fifth metatarsal heads and women over the second (17). There are a number of potential reasons for this, such as footwear and foot shape and structure, but this is not based on research evidence.
Key Point: up to 80% of individuals over 65 suffer with callus, most commonly located under the second metatarsal head
Footedness has been suggested to be an influencing factor in foot pathology (18,19) and callus formation (20), where footedness has been described as the tendency to prefer the use of a consistent foot in performing voluntary motor acts. The preferred or dominant limb is characterised as the mobilizing or manipulating limb, whereas the non-preferred limb is the stabilizing one. For example, in kicking a football, the preferred limb would be the one that strikes the ball, while the non-preferred limb stabilizes the body (20). There are varying degrees of footedness, which may depend on the individual and the task in question, resulting in a number of methods which have been devised to determine footedness (20). So, although Springett et al (16) found that calluses formed uni-or bilaterally, regardless of the side of footedness, they did not mention how they determined the preferred side.
Key Point: the presence of callus and foot pain is associated with falls in the elderly
How does callus form?
While the formation of callus has been investigated over a number of decades, there is still much that is unknown. To be able to appreciate some of the cellular aspects of callus formation, it is useful to start with the normal process of keratinisation. This process ensures a balance of newly generated cells from the basal layer, which pass upward through the spiny and granular layer, arriving in the stratum corneum where the cells spend the remaining half of their life before being desquamated. This process takes 4-6 weeks and is regulated by cytokines and epidermal growth factors (9).
However, with calluses, this process is disrupted. The cell proliferation in the basal layer has been found to increase by 2-3 times, but this is not compensated by an increased desquamation rate (9). The rate of cell transition through the different layers of the epidermis is more rapid, resulting in immature cells in the stratum corneum, which still have connections (desmosomes) with surrounding cells. These connections increase the cell-to-cell cohesion, which makes it more difficult for the cells to be shed from the stratum corneum (9). This culminates in a build up of cells in the stratum corneum, increasing the thickness in this area; hence a callus is formed. The presence of the callus itself causes the release of cytokines and epidermal growth factors, thereby perpetuating the cycle of callus formation.
Due to the thickness of the stratum corneum at the site and the increased levels of linoleic acid (which maintains the barrier function of the stratum corneum) (21), the movement of water through the skin is reduced, leading to a rigid structure or plaque (22). During normal walking, this plaque moves against the surrounding tissues which become traumatised and release inflammatory mediators and growth factors, which causes more cells to be generated from the basal layer, and the process continues.
Why does callus form?
Springett (22) proposed a model that stated that an excess duration and magnitude of mechanical pressures (that is greater pressures over longer times during the gait cycle) were predominant factors in stimulating the cyclical process of callus formation. The increase in pressure stimulates the release of cytokines and epidermal growth factors in the epidermis, so more cells are produced and the cycle of callus begins. This suggests that if this excess pressure can be altered or reduced, it may be possible to prevent or limit the formation of callus.
In order for the excess pressure or foot mechanics to be modified or altered, firstly they need to be identified. Several factors have been suggested which may give rise to callus formation and they have been broadly categorised into intrinsic (unique factors of an individual) and extrinsic (arising from the outside world) factors (8,23). Intrinsic factors include anatomical variants which can give rise to altered gait and faulty foot mechanics (24), for example rear and forefoot abnormalities, digital deformities, bony prominences. It also includes disorders of the spine and skin, and systemic disease (23). Extrinsic factors include footwear (poorly fitted, over-worn, inappropriate design), hosiery, and sporadic exercise regimes, which may lead to excessive loading on certain areas of the body (23). It is also possible for a combination of intrinsic and extrinsic factors to be responsible for callus formation.
Key Point: there are many proposed aetiologies for callus but so far there is no clear evidence to support one theory
Two studies have investigated plantar foot pressures in people with and without callus, who were otherwise healthy (25,26). Both studies found that those people with callus had higher pressures than those without (by 25% and 12% respectively), Potter and Potter (25) went on to remove the callus in the callus group and found that there was no difference in the pressures before or after callus removal, concluding that although people with callus have higher foot pressures than those without, these pressures do not reduce when the callus is removed. This suggests that, in otherwise healthy individuals, the callus has formed due to an alteration in foot pressures. This would indicate treatment targeted at altering foot mechanics, which will be addressed later.
Callus and foot pressures
In individuals without any underlying systemic condition, it has been found that removing callus did not change the plantar pressures. However, it people with diabetes, this does not seem to be the case. A number of studies have found that following callus debridement plantar pressures have reduced by 26% (13), 32% (27) and 58% (28), which given the associated risk of developing an ulcer (5,29), indicates that callus in people with diabetes should be debrided.
The picture is different again for those people with rheumatoid arthritis. Due to the nature of the disease, the metatarsal heads become prominent, with reduced fatty padding and callus formation. Two studies have investigated the effect of callus debridement on plantar pressures (30,31); both found no significant alteration in pressure after debridement.
Despite the fact that high plantar pressures have been shown to be associated with calluses, it is not possible for clinicians to estimate reliably where these high pressure are occurring (32). Guldemond and colleagues (32) investigated the ability of podiatrists, pedorthotists and orthotists to reliably distinguish areas of the foot with elevated plantar pressures in participants with metatarsalgia. In general, plantar pressures under the hallux were underestimated and those under the metatarsals were over estimated, across and within all professions. Therefore the only way of establishing areas of high pressure reliably is with appropriate foot pressure measuring systems, which are expensive and beyond the reach of many practitioners.
The studies on foot pressures have only considered vertical pressure, although undoubtedly, shear has a part to play in callus formation (33), but as yet no robust foot pressure system is able to measure shear and reliably quantify and identify where it is occurring on the foot.
Classification of callus
Although callus is usually considered as an homogenous entity, it has been recognised that different forms exist. From a cellular point of view, Thomas et al (9) described all calluses as having superficial strippable layers (loosely held together), with deeper cohesive underlying layers (firmly held together). But from a clinical point of view calluses can be seen to be different shapes (from well circumscribed to diffuse), sizes and textures (dry, glassy, moist) (34) and this can be more noticeable in the diabetic foot (33). Sgarlato (35) attempted to classify callus into 7 categories, concentrating on 'shearing' callus. While this was comprehensive, it would be difficult to categorize commonly occurring plantar calluses into more than three of the descriptions. More recently Potter and Aitken (36) conducted a study to gain consensus on 10 typical types of callus. Although consensus was reached for heel and digital callus, plantar callus did not reach consensus.
It has been demonstrated that calluses re-grow at different rates; fast types make 80% re-growth three weeks following debridement, and slow types make 18% re-growth over the same period (34). Although it was not easy to identify clinically which callus would exhibit a fast or slow type, it was found that 89% of fast type calluses were either over the first or fifth metatarsal head and 55% of slow type calluses were over the second metatarsal head. While this is a relatively new finding, it does not help in determining the optimum time for callus debridement. Pitei et al (27) investigated two time frames for callus debridement in a diabetic population, 3-4 weeks and 6-8 weeks. They measured the amount of callus debrided and found no significant difference. They concluded that the longer interval would be safe regarding ulceration, but only in those with a slower rate of callus formation.
A common reason for debriding callus is pain. Redmond et al (37) investigated the pain scores pre and post callus debridement in 79 participants (using a Visual Analogue Scale). Although they found a significant reduction in the foot pain after debridement, they acknowledged that this only assessed the immediate effect of callus debridement and not the pain management over a typical interval of 6-12 weeks. Timson and Spooner (38) took this a stage further. They investigated two groups of participants, one group receiving scalpel debridement alone and the other, insole therapy alone. The immediate post-intervention scores in both groups were significantly reduced. After 6 weeks pain scores were taken again. The debridement group's pain scores were similar to their original pre-treatment score, with 32% reporting increased pain scores. However, for the insole group, their scores were significantly lower than either of their original scores, which demonstrated their continued pain relief with insoles 6 weeks after issue. This is still a short time frame and a combination of scalpel debridement and insole therapy needs to be investigated.
Within the rheumatoid population, pain relief from callus debridement has also been investigated. Although debridement provides immediate relief, it has been found to last only for 2-7 days (30,31).
Management of calluses
Given the background to the current research in callus, it is now possible to explore how this impacts on clinical practice.
In order to be able to treat a person who presents with painful callus (it is suggested that painless calluses should not be removed, unless the individual has diabetes), it is necessary to determine the cause. This returns the discussion to the intrinsic and extrinsic factors which may cause the callus and taking a detailed medical history followed by a musculoskeletal assessment of the lower limb. Once the potential cause has been identified, management will revolve around removing or altering the excess pressure.
This is usually achieved by the use of either chairside appliances, deflective or cushioning padding, simple or casted insoles. Caselli et al (39) found that using a Poron insole in conjunction with callus debridement reduced pain more than debridement alone up to 4 weeks after issue, while Timson and Spooner (38) found that pain was reduced up to 6 weeks after issue of an insole only, with callus debridement. Colagiuri et al (40) investigated the use of scalpel debridement and functional orthoses in a group of people with diabetes and found that more calluses improved in the orthoses group. It would seem, therefore that insole therapy is the way forward in managing calluses.
If the causes are found to be extrinsic, for example footwear, then appropriate, tailored advice should be given.
Once the cause of the callus has been addressed, it is now possible to consider relieving the symptoms. Several studies have looked at scalpel reduction and found that it provides immediate pain relief (37-39), although not necessarily long lasting.
Other options for reducing the pain caused by calluses include topical preparations which encompass emollients, urea creams and hydrocolloid dressings. All these will hydrate the callus, making it more pliable and therefore more comfortable (41). There is a variety of over-the-counter products available, some containing urea or salicylic acid, which have been discussed in an earlier CPD article (42). Springett et al (43) investigated using hydrocolloid dressings, which were found to improve corns, callus and heel callus.
Filing or pumicing has been advocated over a number of years, although there is no research evidence to support or refute its practice. If this is going to be advised, then specific instructions should be issued, for example only file or pumice once a week or once a fortnight, using a wet file or pumice and for up to 3 minutes. However, a note of caution; callus is thought to be stimulated by excess mechanical stress and filing or pumicing could replicate this stress and therefore contribute to the callus formation, rather than reduce it.
Foot surgery is an option when conservative measures fail (44), although the details are beyond the scope of this article.
Whatever treatment is carried out, it is essential to evaluate it. A number of studies have used simple pain scales, using the Visual Analogue Scale (VAS), which measures pain intensity only (37,38). However, there are more sophisticated podiatry specific outcome measures now available, which consider quality of life measures, foot function and type of pain (45-47), and these should start being the mainstay of our treatment evaluations. The details of these will be covered in a future article.
Key Point: evaluation of outcomes are an important step in establishing effectiveness of treatments provided by practitioners
The treatment of calluses should:
1 identify the cause, remove or alter if possible
2 provide symptomatic relief, either by scalpel debridement or encouraging self-help in the form of using creams
3 evaluate the outcome of the treatment, in conjunction with the patient and amend if necessary
After reading this CPD article spend some time reflecting on its content, using the sub-headings as prompts. Keep the reflections, notes and key points in your CPD portfolio
1. How would you define hyperkeratosis?
2. What are the main causes of hyperkeratosis on the feet?
3. What percentage of my own caseload may have hyperkeratosis attributable to causes other than mechanical?
4. Will this article change my practice at all? If so how?
5. How will this impact on the care of my patients / service users?
6. After reading this, have I identified any new CPD needs (for example revision of specific topics, acquisition of new skills etc).
Original article published in Podiatry Now September 2008 The Institute would like to thank Julia Potter, Ivan Bristow and the Society of Chiropodists and Podiatrists for permission to reproduce the article in irs entirety.
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Julia Potter, Senior Lecturer, School of Health Sciences, University of Southampton.
Table 1 Percentage distribution of plantar callus from 3 surveys (rounded values) Site Springett Springett et Grouios et al  al   Survey 1 Survey (n=115) (n=1000) 2 (n=319) 1st metatarsal 21 27 23 head (MTH) 2nd MTH 30 36 32 3rd MTH 12 5.5 10 4th MTH 8 6 7.5 5th MTH 11 13.5 12.5 2nd and 3rd MTH 6.5 6 7.5 2nd, 3rd and 4th MTH 3.5 5.5 5
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|Date:||Mar 1, 2015|
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