In hospitalized patients who are bedridden or chairbound, risk factors include hypoalbuminemia, fecal incontinence, and fractures. In general, impaired mobility is the most important risk factor for the development of pressure ulcers. Higher pressures require a shorter time to cause ulceration of skin than lower pressures. Shearing forces cause an opposite parallel sliding motion in the planes of the skin, resulting in an ulcer from traction of the skin and interruption of blood supply. Shearing forces occur, for example, when the head of the patient's bed is raised and his or her body slides down, creating traction over the skin.
Friction applies mechanical forces to the skin, resulting in increased susceptibility to ulceration.
Friction is the force that results in skin abrasion when a patient is dragged across bedsheets, for instance. Moisture secondary to incontinence or perspiration can result in skin maceration and predispose the patient to pressure ulcers. Furthermore, patients with cognitive impairment, depression, or spinal cord injury are likely to be immobile or unable to report symptoms of pain and discomfort. In stage IV ulcers, undermining and sinus tracts may also be present. The most common sites for pressure sores in elderly patients include the scapula, iliac crest, sacrum, ischium, trochanter, lateral malleolus, heel, and lateral edge of the foot.