Pressure injuries result from sustained mechanical loading that exceeds capillary closing pressure (approximately 32 mmHg), causing tissue ischemia. Compressed tissue undergoes anaerobic metabolism, producing lactic acid and reactive oxygen species. When pressure is relieved, reperfusion injury compounds the damage through oxidative stress and inflammatory cascade activation. Shear forces (when the skeleton slides against resistance of skin adhered to a surface) cause mechanical deformation of blood vessels, accelerating ischemia. Friction removes the protective stratum corneum, making skin vulnerable. Moisture from incontinence alters skin pH, macerates the epidermis, and increases friction coefficient. Deep tissue injury often begins at the bone-muscle interface and extends outward, meaning surface appearance may underestimate true damage depth.