Looking Beneath the Skin to Manage Friction Blisters

Friction blisters of the feet commonly occur in individuals engaged in running, hiking, military training, and other intense sports. Depending on their severity, they result in discomfort, pain, disruption in training, or even a complete cessation of activity. Although factors contributing to their formation may include pressure, friction, and moisture, ultimately blisters of the skin are caused by shear. This article summarizes the key concepts presented in a pair of recent papers that address blister causation and prevention. 1, 2 Shear Force Shear occurs when layers of a material move sideways, and shear force creates stress in the material itself. This happens when parallel lateral forces oppose each other. In the case of the foot, the motion of the bones relative to the ground creates shear within the layers of the skin. Consider the effect of a force (F) acting at an angle to the supporting surface, such as at the instant of heel strike (Figure 1). The inclination of the force can be divided into a vertical component (Fy) and a horizontal component (Fx). The vertical force (Fy), often measured as pressure, causes compression of the soft tissue and indicates how hard the limb hits the ground. The horizontal force (Fx) produces a sliding motion and is resisted by friction. When the surface of the skin resists sliding but the bones move, an internal shear stress is created. With sufficient force or repetition, this leads to tearing in the stratum spinosum, one of the five layers of the epidermis. With time this intraepidermal tear fills with a plasma-like thin, colorless fluid, and a blister is formed. Starting as a small, puffy raised pocket, it may lead to a larger and painful delayering of the skin. It is the continuous motion of the bone beneath that causes the deformation. Blister Prevention It is important to distinguish that blisters are not the result of an abrasion to the outer surface of the skin—they are not caused by rubbing. Understanding that blisters occur from the inside, beneath the surface of the skin, there are ways to address the causative factors and limit the likelihood of their formation. Combinations of strategies that reduce the number of shear cycles, increase the resiliency of the skin, and lower the compressive forces are valid ways to protect against blisters. Table 1 is a summary of the main prevention mechanisms. 2 Shear Reduction As blisters occur due to excessive shear, the prevention techniques aim to lessen the magnitude and effects of the deforming force. The following are some of the more common approaches, listed alphabetically. Antiperspirants: Blister formation is associated with skin friction, which is known to increase in the presence of moisture. Several studies have examined the rate of blister incidence while using antiperspirants. The results were mixed, indicating antiperspirants may help but do not universally reduce the risk. In certain cases, antiperspirants can lead to irritating dermatitis. Lubricants: The scientific evidence to support the use of lubricants in preventing friction blisters is also limited, and their effect on the foot has not been studied directly. Several papers indicate that while the application of a lubricant to skin may initially reduce friction, the benefit appears to diminish and vanish over the span of a couple of hours. To be effective for endurance events, lubricants need to be reapplied frequently. Padded apertures: Adhesive-backed felt padding and moleskin can be cut into circular donuts and U shapes. Applying these pads directly to the skin around protrusions reduces peak pressure spots, presumably lessening the likelihood for blisters at those specific sites. Powders: Using foot powder potentially absorbs moisture and may also act as a dry lubricant to reduce friction. However, research on the benefit of powders is inconclusive. Additionally, powders may clump when they become damp, which is a distinct disadvantage in wet environments and for excessively sweaty feet. Surface materials: Teflon (PTFE) tape, commercially available as ShearBan, is known to have a very low coefficient of friction (CoF). It is adhesive backed and can be applied directly to areas of the insole or footwear. Other materials such as Plastazote, Poron, and Spenco also have a role to play in protecting the skin as they can deform and absorb force. Although not as smooth as ShearBan, the CoF of Plastazote remains fairly consistent regardless of surface moisture content, whereas the CoF of Poron and Spenco increases. Tapes and dressing: Tape, moleskin, and other dressings are often used to prevent blister formation on the skin. When successful, it is believed they offer a combination of reducing friction and diffusing the shear force. Applying tape to bony prominences can protect the skin by spreading the deforming load across a broader area. However, if the padding is too thick, it may have the opposite effect and actually increase the pressure locally. One set of studies indicated the use of thin, nonelastic paper tape potentially provides the best option. Footwear and Insoles Appropriate footwear is an essential component of any blister prevention strategy. Shoes that are too tight put pressure on the skin, whereas loose fitting footwear may lead to excessive motion. Shoes must be properly fitted by measuring both feet and fit to the longest foot. However, the basic measure of foot length alone is a crude guide. Pedorthists will also consider the width at the ball of the foot, the length from the heel to the first metatarsophalangeal joint, the volume of the foot, the width of the heel, and several other key measures. Given the variety of unique shoe lasts, upper materials, and construction techniques, the best footwear choice often depends on the specific activity for which it is intended. Dress, athletic, and therapeutic shoes each have different functions that inform the design and sizing. For example, many distance runners wear sneakers at least a half size up to protect their toes from blisters and bruising, as their feet can swell during long runs. On the other hand, soccer players prefer tight fitting boots to give them better contact with the ball and minimize internal foot movement. Extra-depth therapeutic shoes come in multiple widths to accommodate wide feet, edema, and orthotic inserts. Shoes with expandable uppers more easily accommodate sensitive bunions and hammertoes. It is very important to have a break-in period for new footwear before engaging them in full activity such as a wilderness hike or a road race. Finally, shoes that are overworn can also cause problems when they become stiff, creased, or lose their shape. Cushioning materials are another way to reduce pressure and absorb some of the shear force. When an insole deforms, it lowers both the compression force and the friction at the surface. Among various materials used for orthotics, closed-cell neoprene rubber (Spenco) has been studied the most. The application of a very thin nylon mesh lowers the surface friction, making it easier to slide the foot inside the shoe. Neoprene has been shown to be successful in blister prevention. Likewise, polyurethanes (Poron) appear to offer some benefit too, but are not as effective. Socks and Hosiery Another important consideration in blister management is hosiery. Socks act as a barrier between the epidermis of the skin and the lining of the footwear, serving several functions. Socks prevent blisters by lowering the CoF at the skin’s surface. They achieve this by wicking and reducing moisture content. They can also be woven with shear-absorbing fabrics. Depending on their construction and layering, they provide shock absorption, lowering contact pressure. Lastly, through thermal conduction they can help regulate foot temperature and perspiration. The moisture-wicking ability of a material depends on both the fibers and the weave. Socks are made from a wide variety of yarns, natural and synthetic, including blends of these. Cotton socks are comfortable, but they easily retain moisture, which makes them a poor choice for physically demanding activities. Wool is capable of absorbing more moisture, keeping the surface of the skin dry, and it acts as a better insulator. Wool also maintains cushion when wet, resisting compression. In one set of studies, socks made with acrylic fibers were shown to be superior to cotton in limiting blister formation. The authors believed the advantages of acrylic came from a combination of lower friction force on the skin, due to superior moisture management, plus the dense padding. Other reports suggest cushion-sole socks offer a benefit possibly by dissipating pressure. Attempting to optimize the properties of different materials, manufacturers produce many blended fabrics incorporating combinations of cotton, wool, acrylic, polyester, polypropylene, and olefin. The real effectiveness may depend on a number of independent factors including: the environment created inside the shoe due to the specific activity; the material, construction, and ventilation of the shoe’s upper; and the sweatiness of the particular user. Finally, the actual fit of the sock has a role to play. Under challenging conditions, better designed well-fitting socks are less likely to bunch. Cheaper one-size-fits-all styles may easily crease and have rougher seams, creating pressure points. Conclusion Blister formation is attributable to tearing within a layer of the skin as a result of internal shear stress. The combination of repetitive bone movement beneath the skin and high friction at the surface creates the necessary conditions. Moisture, heat, and friction play a role, but they are secondary to the destructive force of bone motion. Blister prevention strategies seek to decrease the magnitude of the shear force, lower the number of harmful cycles, and increase the resiliency of the skin. The formation of blisters may be patient-specific, activity-specific, or footwear-specific, which needs to be addressed accordingly with the proper selection of footgear, socks, and other modalities. For further information and education on blister prevention you can visit the Blister Prevention Pro site for footcare specialists (pro.blister-prevention.com). The author gratefully acknowledges the help and contribution of Douglas Richie Jr., DPM, FACFAS, in preparing this article. Séamus Kennedy, BEng (Mech), CPed, FAAOP(A), is president and co-owner of Hersco Ortho Labs, New York. He can be contacted at seamus@hersco.com or by visiting hersco.com. References Rushton, R. and D. Richie. 2024. Friction blisters of the feet: A new paradigm to explain causation. Journal of Athletic Training 59(1):1-7. Rushton, R. and D. Richie. 2024. Friction blisters of the feet: A critical assessment of current prevention strategies. Journal of Athletic Training 59(1):8-21. Goffkein/stock.adobe.com

Looking Beneath the Skin to Manage Friction Blisters

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Looking Beneath the Skin to Manage Friction Blisters

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Biotensegrity and Prosthetic Interface Design

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