The reported prevalence of pressure ulcers among bedridden hospitalized patients is 18 percent. Bedridden and chairbound patients have an eight percent incidence of new pressure ulcer formation within three weeks of hospitalization. Most prospective studies identifying risk factors for stage II or greater pressure ulcers in hospitalized patients have implicated impaired mobility, incontinence, undernutrition, and impaired consciousness as significant risk factors. 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.