Medicine for the Outdoors
Medicine for the Outdoors

Dr. Paul Auerbach is the world's leading outdoor health expert. His blog offers tips on outdoor safety and advice on how to handle wilderness emergencies.

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Foot Blisters 1

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The Wilderness Medical Society Annual Meeting & 25th Anniversary held in Snowmass, Colorado from July 25-30, 2008 was fantastic in every aspect. The meeting was attending by more than 300 individuals, and included lectures, workshops, evening presentations and a celebration banquet complete with costume ball. I can’t imagine how the meeting might have been better, unless each and every one of you could have been in attendance.

In an extended series of posts, I am going to feature information adapted from the meeting presentations and syllabus, graciously contributed by speakers, all of whom are WMS members. They are all to be commended by the innumerable volunteer hours they contribute to wilderness medicine education, making the outdoors safer and a more enjoyable experience for all of us.

The first two posts from the meeting are about one aspect of backcountry foot care – blisters - based upon information provided by Dr. Grant S. Lipman, who is a Clinical Assistant Professor of Surgery in the Division of Emergency Medicine at Stanford University.

The cause of a friction blister is the repeated action of skin rubbing against another surface. As the external contact, such as a coarse, sweat- and dust-impregnated sock, moves across the skin, the opposing force is called the frictional force (Ff). The combination of the magnitude of the Ff and the frequency of the rubbing of the object across the skin determines the probability of a blister development. Therefore, the greater the Ff, the lower the number of rubbing cycles needed for blister development.

The outermost layer of the skin is known as the epidermis. The epidermal skin layers are, from superficial to deep, called the stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale. In terms of foot blister formation, shear forces extend horizontally between skin layers, between the skin and sock interface, between socks, and between socks and footwear. When the forces within a shoe or boot overcome resistance, sliding occurs. Repeated sliding at a friction point causes an initial sensation of heat – the so-called “hot spot.” Further friction on a hot spot causes blister formation. Skin cells in the stratum spinosum tear and separate. The superficial cells of the overlying layers remain relatively intact, forming a blister’s “roof.” The underlying skin layer, and associated interface between the epidermis and dermis, are usually unaffected. The separated space in the area under the blister roof quickly fills with fluid.

Thick skin like that found on the palms and soles is more likely to undergo blister formation. The epidermis in these areas is usually thick, tightly adhered to underlying structures, and relatively immobile, which causes greater friction than that induced in thinner, more supple skin, such as that found on the back of the forearm.

Blister healing is rapid if one can reduce further friction and worsening of the injury. In a mere 24 hours after blister formation, there is regenerative growth in the blister wound, and at 48 hours, evidence of healing in the basal layer. However, in the presence of continued friction and pressure, as is often the case in the backcountry, the body benefits from medical attention that provides healing assistance.

As in all of medicine, prevention is paramount. To prevent blisters, one must minimize friction generated by the normal biomechanical forces of walking and the contributors to friction. The force between the foot and the insole is determined by the weight of hiker as well as any weight being carried. Reducing the magnitude of the forces on the feet can be as simple as reducing the carried load, whether that means losing personal weight or shedding pounds from the backpack. Another way to minimize the force on the feet is to use a padded insole or arch support. This does not technically reduce the force, but it helps to evenly distribute pressure over the bottom surface of the foot, which thereby decreases the tendency for blister formation.

Either increasing or decreasing the ease with which two surfaces rub against each other can reduce frictional forces. If there is easy sliding, then coefficient of friction is small, reducing the chances for blister formation. At the opposite extreme, if there is no rubbing, then there is no movement at the skin-surface interface.

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 friction. Overly narrow shoes typically 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.

In general it is best to fit (size) shoes in the evening, because feet tend to swell throughout the day. When trying on shoes or boots, make sure to wear the same socks and/or insoles or orthotics that you will be using on the trails. Size boots to compensate for thicker socks. Allow for ample time to break in new footwear. This will stretch the material, sometimes loosen it and increase flexibility, and thereby reduce friction points against the foot. The breaking-in period also conditions the skin itself by causing epidermal thickening.

Soft and supple feet are better able to withstand frictional stress than are cracked and horny feet. Many podiatrists recommend preparing feet with Bag balm, a moisturizer, petrolatum, or other softening agent. Calluses should be filed down to prevent them from tearing off or contributing to the development of deep blisters underneath that are extremely painful and difficult to drain. Toe nails should be kept trimmed short and beveled downwards to reduce the development of bleeding underneath the nails (subungal hematomas).

It is possible to reduce shear forces in the footwear system by deliberately creating a weak shear layer using two pairs of socks. The goal is to have friction occur between the two layers of socks, not between the skin and the socks. A smooth, thin, snug fitting synthetic sock worn as an inner layer against the foot will move with the foot, while a thick, woven sock worn as an outer layer tends to move with the footwear and cushion against shocks. The thinner synthetic liner sock will also assist in moisture control by wicking moisture and perspiration away from the skin surface.

A preventive barrier between the footwear and a potential point of blister formation can save many an outing. Barriers are best utilized as preventive measures before blisters form, either at the beginning of the day 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 friction, warmth and/or moisture. Micropore paper tape, cloth tape, Elastikon elastic tape, moleskin, Spenco Blister Pads, Blist-O-Ban, and duct tape are methods to prevent blister development. Using an adhesive such as tincture of benzoin or Pedi-Pre Tape Spray will help keep the barrier adherent to the skin.

A cardinal rule of taping is to smooth out any wrinkles, and cut off “dog ears” that may lead to further pressure points. ENGO Blister Prevention Patches are slick fabric-film composite patches that are placed on the inside of the shoe or insole. Silicon gel toecaps and sheaths reduce friction between the toes and therefore blister formation in this common and frustrating location to.

Keep the skin clean and dry to minimize friction. Skin hydration leads to increasing contact area and friction, so moist skin results in more frequent blisters. However very wet skin has a low incidence of blister formation, likely due to the lubricating effects of water on the skin surface.

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 is a method to reduce foot moisture. Consider the addition of gaiters to help eliminate dirt, gravel, sand, and rocks from entering the sock-shoe system. If your feet are often moist or sweaty, then try to change socks frequently.

Drying powders decrease moisture for short periods of time and are useful in the evening to dry out feet, but after about one hour may actually increase the friction between surfaces. Lubricants have been developed that are more advanced than traditional Vaseline, which is greasy and tends to trap grit particles, which are irritating and may increase friction and blister production. Advanced lubricants that combine silicone and petrolatum have a silky feel, prevent friction, and repel moisture from the skin. Lubricants can be applied preemptively, or over tape when hot spots develop. However, after about 3 hours, friction is increased as the lubricants are absorbed into the skin and socks. Lubricants should be tested before use on the trail to assess for allergic reaction, and if used, reapplied frequently.

Antiperspirants irritate and block sweat ducts, reducing the amount of perspiration. People who suffer from a condition called hyperhidrosis experience excessive foot perspiration and subsequently have extremely moist feet. These people may benefit the most from antiperspirants.

The next post in this series will be about blister treatment.

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About the Author

Dr. Paul S. Auerbach is the world’s leading authority on wilderness medicine.

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