Pressure ulcer

A pressure ulcer, more commonly known as a bedsore, is an inflammation of the skin that appears when pressure is consistently applied to a spot directly covering a bone. The inflamed skin eventually develops an open sore. They are caused by a combination of a lack of blood flow to the affected area (causing necrosis), often combined with a shearing force applied due to slight shifts of the body which, when repeated, tear the skin open. Bedsores are most often seen in bedridden individuals, particularly the elderly, although they can happen to coma patients, long term hospital patients, and patients in traction to treat bone injuries. However, there are about 100 known risk factors, including some diseases. A pressure ulcer can develop in as little as a few hours.
Although the ulcers are not dangerous in and of themselves, they do provide a route for opportunistic pathogens to bypass the skin.
Pressure ulcers can be treated in a similar manner to other wounds, but any necrotic tissue should be removed before dressing. It is more important to stress prevention when dealing with high risk patients. The most common prevention method is to move the patient's position every two hours, which prevents formation of the ulcers. In addition, modern studies have shown that high-quality mattresses that distribute weight more evenly also prevent the pressure that can cause ulcer formation. 
Pressure ulcer at Wikipedia