Bed sores , also called pressure sores or decubitus ulcers, are caused by pressure against the skin that inhibits an adequate supply of blood to skin and underlying tissues. Other factors related to limited mobility can make the skin vulnerable to damage and contribute to the development of pressure sores. There are three primary contributing factors Sustained pressure. When your skin and the underlying tissues are trapped between bone and a surface such as a wheelchair or bed, the pressure may be greater than the pressure of the blood flowing in the tiny vessels (capillaries) that deliver oxygen and other nutrients to tissues. Deprived of these essential nutrients, cells of the skin and other tissues are damaged and may eventually die. This kind of pressure tends to happen in areas that aren't well padded with muscle or fat and that lie just over a bone, such as your spine, tailbone (coccyx), shoulder blades, hips, heels and elbows.
Friction. Friction is the resistance to motion. When a person changes position or is handled by care providers, friction may occur when the skin is dragged across a surface. The resistance to motion may be even greater if the skin is moist. Friction between skin and another surface may make fragile skin more vulnerable to injury.
Shear. Shear occurs when two surfaces move in the opposite direction. For example, when a hospital bed is elevated at the head, a person can slide down in bed. As the tailbone moves down, the skin over the bone may stay in place — essentially pulling in the opposite direction. This motion may damage tissue and blood vessels, making the site more vulnerable to damage from sustained pressure.