Functional Limitations From Pain Caused by Repetitive Loading on the Skin: A Review and Discussion for Practitioners, With New Data for Limiting Friction Loads

In Brief Reddened painful skin, areas where the outermost layer of skin has been rubbed off (abrasions), and blisters are very common problems observed daily by orthotists, prosthetists, pedorthists, athletic trainers, and a host of other caregivers. These examples of acute skin trauma and the pain that precedes and/or accompanies them are often what determine the limits of an individual’s functional performance. When these problems occur within footwear, orthoses, or prosthetic sockets, the cause is repetitive loading—a combination of peak load magnitude and number of loading cycles sufficient to produce significant skin trauma. Walking, running, and many other activities involve movements of skeletal elements relative to surfaces such as shoe insoles, orthoses, and prosthetic sockets. Those relative movements are an inevitable consequence of transferring load through soft tissue and across the skin interface. The loads contain both normal (perpendicular to the skin surface) components and friction (parallel to the skin surface) components. Historically, most instances of skin trauma have been attributed directly to excess pressure and have been dealt with using pressure-management techniques exclusively. Pressure-reduction techniques do often lead to some improvement. However, the assumption that those problems are directly governed by excess pressure is substantially in error, leading to partial solutions and missed opportunities to achieve much higher levels of safe, pain-free function. Research has established the friction component of repetitive loadings to be the major factor governing how many load cycles a given area of skin can tolerate before the onset of pain and acute trauma. To increase comfortable, pain-free function, the magnitude of friction load peaks applied each loading cycle must be reduced. This can be accomplished by reducing peak normal (pressure) load magnitudes and/or reducing the coefficient of friction (COF) operating between the skin and support surface in the problem area. Friction forces over the vast majority of the loaded skin surface area are useful for stability, suspension, and the transmission of functional forces. Friction creates a problem only in very limited areas. This is the rationale for reducing the operative COF in problem areas only. Management of the “coefficient of friction” has not generally been considered on a par with pressure management as a way to reduce skin trauma and achieve greater comfort, function and performance. This discussion illustrates how both peak pressure and COF factor into skin trauma.