A bed sore occurs due to pressure. Hence, it is also called "pressure injuries". Bedsore risk increases when the blood supply to the skin is blocked for 2-3 hours. It happens when delicate blood vessels get compressed between outside pressure over the skin over bony prominences. Bedsores occur in areas of skin over bones such as the back of the head, shoulder blades, elbows, buttocks, hip, tailbone, sides of the knees, over the heels, and toes.