Pressure ulcers form where bone causes the greatest force on the skin and tissue and squeezes them against an outside surface. This may be where bony parts of the body press against other body parts, a mattress, or a chair. In persons who must stay in bed, most pressure ulcers form on the lower back below the waist (sacrum), the hip bone (trochanter), and on the heels. In people in chairs or wheelchairs, the exact spot where pressure ulcers form depends on the sitting position. Pressure ulcers can also form on the knees, ankles, shoulder blades, back of the head, and spine.

Nerves normally tell the body when to move to relieve pressure on the skin. Persons in bed who are unable to move may get pressure ulcers after as little as 1-2 hours. Persons who sit in chairs and who cannot move can get pressure ulcers in even less time because the force on the skin is greater.

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