Pressure ulcers may be formed by a number of different processes. The common cause of a pressure ulcer is unrelieved pressure that squeezes the blood vessels and cuts off the supply of oxygen and nutrients to the skin.

When body parts do not receive enough oxygen and nutrients, they begin to die and form ulcers. Pressure is worst in bony areas, where bones cause a great deal of force on the skin and tissue and press them against an outside surface.

Pressure ulcers can also form when tissues shift against each other or in the opposite direction as the bone (e.g., when sliding down in a bed or a chair). The blood vessels may stretch or bend, cutting off circulation. External pressure is generally more likely to cause an ulcer than these shearing forces alone, but the two factors often combine to exacerbate the risk.

Ulcers may also form due to friction that wears away the top layers of the skin. Poorly fitting clothing can cause this friction. Other causes of ulcers include:

• Impaired circulation. If the blood does not circulate properly, certain body parts may be deprived of needed oxygen and nutrients. Ulcers caused by impaired circulation alone are not pressure ulcers. However, impaired circulation can increase the risk of pressure ulcers.
  
  
• Anemia. A reduction in red blood cells that reduces the amount of oxygen delivered to body parts. Ulcers caused by anemia alone are not pressure ulcers. However, anemia can increase the risk of pressure ulcers.

Some people have an increased risk of developing pressure ulcers. Risk factors include:

• Old age or fragile skin. Elderly people tend to have thinner skin. When the skin is thin or fragile, it tears more easily, which may result in pressure ulcers.
  
  
• Immobility. People confined to a bed or wheelchair may be unable to shift their weight, resulting in pressure ulcers.
  
  
• Smoking. Nicotine impairs circulation and slows healing.
  
  
• Malnourishment. Lack of protein, vitamins and other nutrients impairs skin integrity and slows healing.
  
  
• Excessive thinness or weight loss. If there is not enough fat to cushion pressure, pressure ulcers may be more likely to form.
  
  
• Urinary or fecal incontinence (loss of bladder or bowel control). These conditions result in excessive moisture remaining on the skin, making it more fragile and more easily torn. The areas are also not clean, increasing the risk of infection.
  
  
• Contractures (shortening of the muscles, tendons and ligaments around a joint). This condition causes increased friction and shearing forces.

People who cannot feel or recognize discomfort or pain may not be able to shift their weight often enough, resulting in pressure ulcers. People with nerve damage cannot sense discomfort or pain causing the tissue breakdown.

This is why people with diabetes, which often causes nerve damage, are at higher risk for developing pressure ulcers. Other illnesses that may increase the risk of ulcer development include:

• Sickle cell disease (an inherited blood disease)
  
  
• Syphilis (an infectious disease that results in ulcers and rashes)
  
  
• Lupus (a chronic inflammatory disease that can affect the skin, as well as the joints, blood, kidneys and various other body parts)
  
  
• Leprosy (an infectious disease characterized by skin lesions and nerve damage)