Chapter 29 Foot Problems and Care

“There’s language in her eye, her cheek, her lip; Nay, her foot speaks.”

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The American Alpine Club has estimated there are more than 100,000 climbers in the United States with 1 to 2 million days in the mountains a year.²⁴ Combined with the 73.3 million people (roughly one-third of the population) in the United States who went hiking in 2002,³ it is no surprise that friction blisters are arguably the most common medical problem encountered in the wilderness. For some, a foot blister may be considered merely a nuisance; for others, it may ruin an outing, necessitate immobility, or even lead to cellulitis or sepsis (Figure 29-1).^2,²⁹

FIGURE 29-1 Aggressively spreading leg cellulitis.

(Courtesy Grant S. Lipman, MD.)

Blistering rates in the hiking community range from 7% to 54%,^14,⁸⁰ with incidence often inversely proportional to the experience levels of the hikers studied. Blister occurrence in marathons range from 0.2% to 39%,⁴⁷ whereas in multiday adventure events, foot care represents the most common (32% to 45%) reported injury.^48,⁷⁹ In the authors’ experience providing medical support for more than 1800 miles of ultraendurance races in four countries with more than 1200 documented medical visits, 70% of chief complaints were related to foot care.³⁹

In the military, foot problems have historically been recognized as one of the most common training complications and preventable debilitating injuries. In 1895, the Duke of Connaught reported 824 cases of blistered feet during maneuvers in the New Forest.⁷⁸ Cadets at the U.S. Military Academy undertaking a 21-km (13-mile) hike reported a 48% occurrence of blisters,³⁷ and over 65% of military personnel after a 10-km (6.2-mile) road march had observed blisters.⁴² Although most blisters are of minor medical significance, they can impair concentration, decrease athletic performance, and potentially lead to debilitating infections. A studied Navy recruit population was estimated to have an 84% incidence of cellulitis secondary to foot blisters, with an average loss of 8 full days of training per case.²⁹ A recent study of over 2,000 U.S. Marine recruits showed that those with blisters were 50% more likely to experience additional training-related injuries.⁹

Blister Pathophysiology

The earliest etymology of the word friction, described from 1704, is “to rub; to crumble; they injure.”⁵⁰ The mechanism of a friction blister injury is the repeated action of skin rubbing against another surface. As the external contact of either sock or footwear moves across the skin, the frictional force (F_f) opposes this movement. Amonton described the magnitude of frictional resistance in 1699 as “directly proportional to the load and independent of the area of contact between the surfaces.” Frictional force increases with increasing external force. When external force exceeds the frictional force, movement occurs at the interface. In addition to magnitude of the frictional force, frequency of the cycling of the object across the skin leads to higher probability of blister development.⁵⁶ There is an inverse relationship between these two variables; at higher frictional forces, it takes fewer cycles to form blisters.^1,^11,^12,⁵⁶

The skin surface is subjected to normal forces (F_n), the perpendicular or “force of contact,”³⁸as well as nonperpendicular or tangential shear forces (Figure 29-2). The magnitude and relationship of a frictional force is defined by the following formula:⁸ F_f = µ • F_n, where µ is the coefficient of friction or the ratio of the shear force to normal force that needs to be overcome to initiate movement of the materials relative to the skin.⁶⁷ There is wide variation in the coefficients of friction between the skin and various substances (Figure 29-3, online), and frictional properties of the skin depend on inherent suppleness and hydration, as well as a host of external factors, including temperature and humidity.⁸⁴

FIGURE 29-2 Cross-section illustration of skin and resultant vector direction of a friction force and vector direction of a shear force.

(Reproduced with permission from Polliack AA, Scheinberg S: A new technology for reducing shear and friction forces on the skin: Implications for blister care in the wilderness setting, Wilderness Environ Med 17:109, 2006.)

FIGURE 29-3 Laboratory product comparisons using a custom-made friction-measurement apparatus.

Shear forces extend horizontally between skin layers (Figure 29-4), skin and sock interface, between socks, and between socks and footwear. When the forces overcome resistance, sliding occurs. Repeated sliding at a friction point causes exfoliation of the stratum corneum and erythema in and around this zone.^38,⁵⁶ This is experienced as a initial sensation of heat—the “hot spot.” Continued friction on a hot spot causes epidermal cells in the stratum spinosum to delaminate and split.^2,⁷⁶ With continued rubbing, a sensation of stinging or burning occurs as a narrow pale area forms around the central reddened region. This enlarges inward.³⁸ This skin becomes elevated as the underlying epidermis fills with fluid, and a blister is created (Figure 29-5).⁷⁶ The intact superficial cells of the stratum corneum and stratum granulosum form the blister’s “roof.” The separated cleft in the area under the blister roof quickly fills with a low-protein electrolyte-rich transudate as a result of hydrostatic pressure.¹³ The underlying basal skin layer and associated epidermal–dermal interface is usually unaffected and undamaged.

FIGURE 29-4 Epidermal layers.

(From http://www.ratbehavior.org.)

FIGURE 29-5 Longitudinal section of the epidermis showing a friction blister.

(Reproduced with permission from Knapik JJ, Reynolds KL, Duplantis KL, et al: Friction blisters: Pathophysiology, prevention and treatment, Sports Med 20:136, 1995.)

Healing of the blister is rapid if one can reduce further friction and exacerbation of the injury. At 6 hours, cells at the base of the blister increase uptake of amino acids and nucleosides, precursors of RNA and DNA. After 24 hours, there is high mitotic activity in the blister wound, and at 48 hours, new stratum granulosum can be seen. By 120 hours, a new stratum corneum is made.²⁰ In the presence of continued friction and pressure, as is often the case in the backcountry, where continued ambulation and activity are required, the body needs outside assistance to heal.

Blister Prevention

The perpendicular directed normal force between the foot and the insole is determined by the weight of the hiker and the weight being carried. Reducing the magnitude of the forces on the feet that cause a friction blister can be as simple as reducing the carried load ³⁶ either by shedding a few pounds of adipose tissue or taking a few items out of the backpack. Another way to minimize force on the feet is to use a padded insole or arch support. There are myriad generic and custom moldable insoles on the market. Although an insole or orthotic does not technically reduce the perpendicular forces on the feet, it helps to evenly distribute pressure over the plantar surface of the foot. Greater pressure occurs when there is a high load on a smaller surface area of the foot, which may cause that area of skin to be susceptible to blister formation.

Either increasing or decreasing the ease with which two surfaces rub against each other can reduce the chances of blister development. If the coefficient of friction is small, the resulting F_f will be minimized, with greater ease of sliding and reduction of the shear forces and chances for blister formation. Another option is to maximize frictional forces through well-fitted footwear, resulting in little or no movement at the skin–surface interface.

Reduction of Movement Within the Footwear System

Shoes or boots should fit properly and comfortably. Shoes that are too tight can increase contact points of pressure on the foot. Those that are too loose can allow excess movement that allows generation of frictional forces. Narrow shoes can cause blisters on the large and small toes. Loose shoes can create blisters on the tips of toes from sliding and jamming the tips into the toe box. A toe box that is too shallow can cause blisters on the tops of the toes from repeated contact. It is important to fit a shoe in the evening, because feet tend to swell throughout the day and the fit will be similar to the natural swelling that feet undergo while active. When trying on shoes or boots, wear the same socks and/or insoles or orthotics that you will be using on the trails. Size boots to compensate for thicker socks. Allowing for ample time to break in a new set of footwear before use increases flexibility in the material, thereby reducing potential high-friction areas against the foot.

Increasing Movement Within the Footwear System—Socks

Various sock layer combinations can create a nonspecific weak shear layer, exploiting the coefficients of friction and minimizing the friction forces against the skin of the foot itself. The goal is to have friction occur between the two layers of socks, not between the skin and socks. A smooth, thin, snug-fitting synthetic sock worn as an inner layer against the foot will move with the foot, whereas a thick, woven sock tends to move with the footwear and cushions against shocks. Socks that have a low frictional coefficient worn close to the foot have been found by computer modeling to reduce plantar shear stress. This reduction in shear force is compounded by also wearing a sock that has high friction against the insole.¹⁶ This combination is commonly used by many outdoor enthusiasts and has been found to produce the least amount of blisters in military populations.³⁵ The thinner synthetic liner sock will also assist in humidity control by retaining less moisture and wicking moisture and perspiration away from the skin surface.^26,²⁷

A longitudinal double-blind study was conducted to determine the effect of sock fiber on frequency and size of blisters. Two different visually identical socks were tested—one with 100% acrylic fibers, the other with 100% natural cotton fibers. Acrylic fiber socks were associated with fewer blister events and size when compared with cotton fiber socks.²⁶

Minimizing Moisture

Repetitive rubbing on moist skin produces higher F_f than on very wet or dry skin.^2,^56,⁷⁶ Cutaneous hydration leads to increased contact area, adhesion, and maceration, resulting in more frequent blisters (Figure 29-6). Very wet skin has a low incidence of blister formation, likely due to the lubricating effects of water on the skin surface. Frictional forces on dry skin may exfoliate superficial cells of the stratum corneum, lubricating the feet in a manner analogous to the lubricating effects of graphite powder.³⁸

FIGURE 29-6 Moist macerated feet.

(Courtesy Mark Ellis, MD.)

High-technology oversocks combine waterproof materials with traditional socks to help keep feet dry when repeatedly exposed to water. Combining Gore-Tex oversocks with wicking liner socks and foot antiperspirant are tricks to reduce foot moisture. Consider the addition of gaiters to help eliminate dirt, gravel, sand, and rocks from entering the sock-shoe system. Consistently moist skin or sweaty feet may require frequent changing of socks.

Feet Preparation

Repeated low-intensity F_f exposure results in cellular proliferation and epidermal thickening. These adaptations to underlying skin may reduce the likelihood of developing blisters. A soft and supple foot will be more adept at withstanding frictional stress than will cracked, horny feet. Many podiatrists and ultraendurance athletes recommend preparing feet with Bag Balm, a moisturizer, Vaseline, or other softening agent for months before an event (Figure 29-7, online). Other clinicians and athletes have the opinion that “dry is better” and take serial footbaths in povidone-iodine or tannic acid.⁴⁷ Calluses should be filed down with a pumice stone or emery board to prevent them from tearing off, which would leave an open wound. Blisters deep to a callus are extremely painful and difficult to drain (Figure 29-8). Toenails should be kept short and beveled downward to reduce the incidence of subungual hematomas. Before a big event, consider having a professional manicure. Having this done at least a week before the planned outing allows time for the manipulations to the epidermis and cuticles to heal, to prevent potential bacterial entry and infection on the trail.

FIGURE 29-8 Subcallus blister.

(Courtesy Grant S. Lipman, MD.)

FIGURE 29-7 Blister-free feet after running 150 miles in 6 days. Bag Balm had been prophylactically applied for several months before the event.

(Courtesy Grant S. Lipman, MD.)

Blister Prevention Compounds

The key to preventing blisters involves decreasing shear forces on the skin. Despite extensive studies regarding the impact of shear forces on development of blisters, there are few studies examining the efficacy of various modalities (e.g., powders, antiperspirants, lubricants, tapes, pads). Theoretically, each of these offers a distinct advantage. Lubricants, tapes, and pads are proposed to decrease the friction that leads to blisters, whereas powders and antiperspirants are thought to limit perspiration and resulting cutaneous hydration associated with activity that lead to increased blister incidence. Antiperspirant mechanism of action is by the metallic salt that dissolves at the skin surface, forming an acidic solution that hydrolyzes upon contact with more alkaline sweat, precipitating and “plugging” the acrosyringium of the eccrine sweat gland.^60,⁶¹

Preventive Taping and Pads

A preventive barrier between the footwear and potential point of blister formation is a proactive treatment that can prevent a blister. Barriers are best used as preventive measures before blisters form, either at the beginning of the day’s activities or as soon as a hot spot develops. The barrier needs to be adhesive so it can remain fixed to skin, despite the action of frictional forces, heat, or humidity that will likely occur inside the footwear acting on the barrier. The concept of prevention is to have a layer over the skin such that the following resulting shear will occur between the barrier and the footwear, not the footwear and the skin. The shelves of drugstores and running stores are filled with products that can be applied in areas that historically have developed blisters. These include Micropore paper tape, cloth tape, Elastikon, Kinesio tape, Moleskin, Spenco Blister Pads, Blist-O-Ban, and duct tape (Table 29-1). Use of an adhesive-like tincture of benzoin or Pedi-Pre Tape spray adherent will help keep the barrier fixed to the skin. ENGO Performance Patches are smooth fabric-film composite patches that go on the inside of the shoe or insole. Silicone gel toecaps and sheaths reduce friction at the tips and between toes.

TABLE 29-1 Foot Care Tapes and Pads

Product	Description	Advantage
Blist-O-Ban	Ultrathin patented BursaMed dome reported to decrease the forces along the skin by deflecting forces away from the skin. Useful over hot spots or mild blisters.	Ultrathin, easy to apply, hypoallergenic, multiday use, water resistant
Compeed	Sterile hypoallergenic plaster reported to protect vulnerable skin from friction damage.	Easy to apply, water resistant, flexible, multiday use
Duct tape	Polyethylene tape with flexible shell and pressure-sensitive adhesive quality; however, not very breathable and difficult to remove; may be used over any surface.	Strong, easy to apply, multiday use
Elastikon	Flexible tape made of a porous high-twist, cotton elastic cloth tape with a rubber-based adhesive; useful over heels and plantar surface of foot.	Breathable, easy to apply to uneven surfaces
Leukotape	Slightly thicker tape with zinc oxide and rayon backing; may be used over any part of the foot.	Breathable, easy to apply
Micropore paper tape	Breathable pressure-sensitive adhesive tape. Useful for preventive taping and underneath thicker tape; however, not well suited for wet environments.	Easy to apply and remove, hypoallergenic, multiday use
Moleskin	A heavy cotton fabric sheared on one side with adhesive backing on the other. Useful over relatively even surfaces to offset the direct pressure on large blisters.	Easy to apply, hypoallergenic
Spenco 2nd Skin	Series of products ranging from nonwoven cotton tape to a hydrogel pad designed to assist with fluid absorption and wound healing. The pads are very useful for complicated blisters without any skin cover.	Easy to apply, hypoallergenic, multiday use

Taping is a relatively easy and cost-effective method to prevent hot spots and subsequent blister injuries at various sites on the foot. Ideally, taping products should be thin, easy to apply, adhere well, and provide limited seams that may themselves be friction points. Paper-like tape (Figure 29-9, online) can be easily placed over and around the toes, heel, plantar, and dorsal aspects of the foot (Figure 29-10). Due to its low cost, ease of use, and silky feel, it is the authors’ first-line product to apply on a hot spot to prevent blister formation. Elastikon is a sturdier and wider tape that may be used over the planter surfaces of the feet, heels, and dorsa of the feet. Because of the rough feel of this tape, it should not be used in the spaces between digits. Duct tape is used with variable success in areas such as the heel of a foot. However, there are innate qualities that make this product less than ideal for blister prevention. It is nonporous, leading to increased moisture content and maceration of skin beneath the tape; it wrinkles easily, leading to pressure points (Figure 29-11); and the adhesive backing sticks too well, often removing epidermis along with the tape. Detailed descriptions on taping techniques are discussed in the blister treatment section of this chapter.

FIGURE 29-10 Paper-tape prophylactically applied to heels to prevent hot spots.

(Courtesy Ashlie Emmett.)

FIGURE 29-11 Wrinkled duct tape that can promote blister formation.

FIGURE 29-9 Paper tape.

(Courtesy 3M.)

Like taping, pads (which are a combination of adhesive dressing with a central thicker composite pad) are relatively easy to apply to prevent or treat developing blisters. Pads can be extremely easy to apply over relatively smooth surfaces such as the Achilles region or metatarsal area. However, these products are usually bulkier than tape, making them more difficult to apply on uneven surfaces or in small areas, such as between the toes. Improper application can lead to excessive friction around the product and further epidermal injury. Pads are exponentially more expensive than tape and potentially take up more space in the pack, limiting their use on multiday trips, where weight and space may be at a premium. A product made popular by British athletes, Compeed, is notorious for its strong adhesion qualities (Figure 29-12), which becomes apparent when removing layers of epidermis along with the bandage after activity (Figure 29-13).

FIGURE 29-12 Removing a Compeed bandage that had been placed over a blister, showing strong adhesive qualities.

(Courtesy Grant S. Lipman, MD.)

FIGURE 29-13 Torn epidermis resulting from Compeed bandage removal.

(Courtesy Grant S. Lipman, MD.)

There are very few studies assessing the efficacy of any these products in preventing blister formation.^63,⁷⁰ The relatively new adhesive bandage Blist-O-Ban has been prospectively studied affixed at specific sites (e.g., heel or metatarsal head). The product appears to decrease the incidence of new blisters in treated feet compared with untreated feet and adheres well in humid conditions. However, these studies did not address blister formation in common areas, such as the toes and intertriginous spaces, that can be difficult to treat with abrasive or bulky dressings.

Any chapter on foot care would be remiss without mentioning Moleskin, the ubiquitous product that has been a fixture in hikers’ first-aid kits for generations (Table 29-2). The technique of using Moleskin is to affix a doughnut-shaped patch around a hot spot or blister, securing the material with the irritated epidermis in the opening, in the hope that friction will act on the perimeter of feltlike material rather than the original spot. Although large or open heel blisters may benefit from being surrounded by Moleskin or Molefoam (Figure 29-14), the authors do not recommend this family of products for treatment or prevention of hot spots or blister treatment. Anecdotally, it has a high failure rate, is difficult to anchor, and the volume of the product in a well-fitted shoe box may cause a “contrecoup” blister on the toes or even under the Moleskin product itself because of its high coefficient of friction.⁵⁹

TABLE 29-2 Personal Foot Care Kit

FIGURE 29-14 Moleskin “doughnut” placed around a large open heel blister.

(Courtesy Paul Langer, DPM.)

Antiperspirants/Powders

Antiperspirants and powders (Table 29-3) have been proposed as preventive measures to decrease the amount of moisture at the foot-sock interface. Limited studies provide conflicting findings in regard to blister prevention and the impact on foot perspiration. A small prospective, nonblinded study assessed the effectiveness of two different antiperspirants (aluminum based) in reducing sweat accumulation and preventing blisters. Each of the subjects applied one of the antiperspirants and then completed a 1-hour treadmill march in a warm environment. Overall, both groups experienced a significant decrease (greater than 50%) in foot-sweat accumulation and a nonsignificant trend toward fewer blisters. However, there was an increase in the incidence of irritant dermatitis.¹⁷ A randomized study of 23 healthy males examined the impact of antiperspirants with emollients by applying one of three preparations (a 20% aluminum-based antiperspirant with emollient additive; emollient alone; or nothing) for 4 days before ambulating on a treadmill in a warm environment while carrying a load over a 4-hour period. All groups experienced a similar incidence of blisters and sweat accumulation with no reported skin irritation.⁶⁴

TABLE 29-3 Foot Care Antiperspirants and Powders

The largest of the prevention studies was a double-blind trial of 667 U.S. military cadets randomized to either a 20% aluminum-based antiperspirant or placebo for 5 days before a 21-km (13-mile) hike. For compliant subjects who used the preparation for at least 3 nights, the incidence of blister formation was 21% in the antiperspirant group versus 48% for the placebo group. However, these results were tempered by a 57% incidence of skin irritation in the antiperspirant group compared with only 6% in the placebo group.³⁷ The gained antiperspirant benefit from antiperspirant may not be worth the risk for resulting irritation.

Overall, there are conflicting data to support or refute the use of antiperspirants in preventing foot blisters. In theory, antiperspirants reduce the amount of perspiration and resulting friction, leading to reduced blister formation. However, this must be balanced with the high incidence of skin irritation. Further research is needed to better define the proper dilution of compounds in the antiperspirants and frequency of application to decrease the incidence of foot blisters. Antiperspirants are most likely helpful for those who suffer from excessively sweaty feet (hyperhidrosis). If the decision is made to use an antiperspirant, a trial should be attempted at least a week before the planned outing to avoid having skin irritation in an outdoor setting.

A variety of powders are designed to help with foot odor and perspiration. These powders are typically composed of an astringent to assist with itching and an inorganic compound (talc, sodium bicarbonate) to assist with wetness. Despite widespread marketing of these compounds, there is no published scientific evidence to suggest that these products prevent foot blisters.⁶ In addition, it has been suggested that powders may increase the risk for blisters because of their tendency to clump with perspiration. Despite anecdotal support for use of powders, they are likely best used to dry out feet in the evening, rather than being used on the trail.

Lubricants

Lubricants ostensibly prevent blister formation by decreasing friction at the foot-contact material interface. Lubricants have been developed that are more advanced than traditional Vaseline, which is greasy and tends to attract grit particles that irritate and may increase friction and blister production (Table 29-4). Lubricating agents that have a subjective “greasiness” leave skin with a higher coefficient of friction than those with a sensation of “slipperiness.”⁵⁴ Advanced lubricants use silicone and petrolatum mixes that have a silky feel and are thought to work by altering skin smoothness and moisture content of the skin. Several studies have shown that after applying lubricating substances to skin, there is an initial decrease of the coefficient of friction, but that within an hour, it returns to baseline with a subsequent increase in friction 35% over baseline over the next 4 to 6 hours (Figure 29-15).^12,⁵⁴ The studies suggest that with prolonged exercise, the use of lubricants might contribute to blister formation, so if used, they need to be reapplied frequently.

TABLE 29-4 Foot Care Lubricants

FIGURE 29-15 Effect of lubricant cosmetic ingredient on skin friction coefficient. Amount applied to each material: approximately 2 mg cm⁻².

(Reproduced with permission from Sivamani RK, Goodman J, Gitis NV, et al: Coefficient of friction: Tribological studies in man—an overview, Skin Res Technol 9:227, 2003.)

More recently, analysis of the effect of skin creams on the skin of a rat model found that the cream appeared to reduce roughness of the surface while increasing hydrophilic properties of the skin. In addition, the authors noted an increase in the coefficient of friction for the cream-treated skin when the thickness of the cream was lower than a specific thickness.⁷⁷ Future clinical studies are needed to assess if newer-generation lubricants or creams prevent blisters, need re-application at certain intervals, or actually contribute to blister formation.