Shearing
Shearing forces are exerted parallel to the skin and are generated by forces acting in opposite directions. Force is generated by patient movement through gravity and sliding or by incorrect manual handling.

Friction
Friction is resistance to movement between the patient’s skin and the external support surface. This force acts in a direction that is opposite to patient movement. Abrasions of the epidermis and dermis occur as a result of friction between the skin and the bed surface. Tissues, which are attached to the bone, are compressed, obstructed and torn in both shear and friction situations. Shearing and friction occur when the:

• patient slides in bed/chair
• patient’s bed-head is elevated beyond 30 degrees (the most common cause of shear injury)
• patient is pulled across bed/chair as a result of incorrect manual handling, causing friction between the skin and bed/chair’s surface
• patient is restless or has limb spasms, leading to friction between the skin and the bed surface.

Moisture
Moisture alters the resistance of the epidermis to external forces by softening the skin’s surface and reducing the tensile strength. Urine, faeces, perspiration and/or wound drainage may soften the skin, making it more susceptible to pressure, shear and friction.