Friction and shear are mechanical forces contributing to pressure ulcer formation. The tissue injury resulting from these forces may look like a superficial skin insult. Shear and friction are 2 separate phenomenons, yet they often work together to create tissue ischemia and ulcer development.
Shear is a “mechanical force that acts on an area of skin in a direction parallel to the body’s surface. Shear is affected by the amount of pressure exerted, the coefficient of friction between the materials contacting each other, and the extent to which the body makes contact with the support surface.” 1 Think of this as pulling the bones of the pelvis in one direction and the skin in the opposite direction. The deeper fascia slides downward with the bone; the superficial fascia remains attached to the dermis. This insult and compromise to the blood supply creates ischemia and leads to cellular death and tissue necrosis. Shear and friction go hand in hand—one rarely occurs without the other.
Shear injury will not be seen at the skin level because it happens beneath the skin. Elevation of the head of the bed increases shear injury in the deep tissue, and may account for the number of sacral ulcers seen in practice.
Unlike shear injury, friction injury will be visible. Friction is the “mechanical force exerted when skin is dragged across a coarse surface such as bed linens.” 1 Simply, it is 2 surfaces moving across one another. A skin insult caused by friction looks like an abrasion or superficial laceration. Friction, however, is not a primary factor in the development of pressure ulcers. It can contribute to an insult or stripping of the epidermal layer of the skin, creating an environment conducive to further insult. An alteration in the coefficient of friction increases the skin’s adherence to the outside surface (the bed). Friction then combines with shearing forces and the ultimate outcome may be a pressure ulcer. Tissues subjected to friction are more susceptible to pressure ulcer damage. The 3 mechanical forces (pressure, friction, and shear) may act in concert to create tissue damage. Other patients at risk for pressure ulcers from friction are elderly people, those with uncontrollable movements, such as spastic movements, and those who use braces or appliances that rub against the skin.
Adapted from Ayello E, Baranoski S, Lyder C, Cuddigan J. Pressure ulcers. In: Baranoski S, Ayello EA, editors. Wound Care Essentials: Practice Principles. Springhouse, PA: Lippincott Williams & Wilkins; 2004. p 243.
1. Bergstrom N, Bennett MA, Carlson CE, et al. Treatment of Pressure Ulcers. Clinical Practice Guideline, No. 15. AHCPR Publication No. 95-0652. Rockville, MD: Agency for Health Care Policy and Research; December 1994.