Pressure ulcers are defined as localised injury to the skin and/or underlying tissue, usually over a bony prominence, as a result of pressure, or pressure in conjunction with shear. An important change to this definition is the elimination of friction as a cause of pressure ulcers. Patients can develop friction injury to the heel from the constant movement of the heel on their bed linen. The change to this definition stems from an understanding that frictional forces are superficial and lead to heat in the tissues, producing serum-filled blisters; friction does not involve pressure. Shear forces — the combination of pressure and movement — can and do lead to pressure ulcers.