Pressure sores are caused by continuous pressure on the local tissues of the body, resulting in impaired blood circulation, lack of nutrition and loss of normal function, which eventually leads to degeneration, ulceration and necrosis. They are more likely to occur in people who are bedridden for long periods of time, who are frail, who have difficulty turning over, and who have sensory impairments in their limbs. Pressure sores are caused by a variety of factors, including external and internal causes. The details are as follows: 1. external factors: including pressure, shear, friction, and moisture; 2. internal factors: including age, skin condition, mobility, nutrition, and tissue perfusion. The pressure on the skin at the bony bulge lacks muscle and fat tissue protection, thus causing impaired blood circulation and pressure sores.