A pressure sore (bed sore) is an injury to the skin and/or the tissues under the skin. Constant pressure on an area of skin reduces blood supply to the area. Over time, it can cause the skin to break down and form an open sore (ulcer). Pressure sores are more likely to form if you or a person you are caring for is in the hospital or is confined to a chair or bed.

Pressure sores most often form on the skin over bony areas where there is little cushion between the bone and the skin. Most pressure sores form on the lower part of the body, including over the tailbone and on the back along the spine, on the buttocks, on the hips, and on the heels. Other common spots are the back of the head; the backs of the ears; the shoulders, elbows, and ankles; and between the knees where the legs rub together.

Pressure sores can range from red areas on the surface of the skin to severe tissue damage that goes deep into muscle and bone. These sores are hard to treat and slow to heal. Other problems, such as bone, blood, and skin infections, can develop when pressure sores do not heal properly.  WebMD