Pressure injuries, once commonly known as pressure ulcers or bedsores, represent a significant health concern for patients with limited mobility. The breakdown of skin and underlying tissue involves multiple mechanical forces acting upon the body. While clinicians treat the final injury, understanding the distinct forces that cause the damage is necessary for effective prevention. The relationship between these forces clarifies that mechanical forces like shearing are the cause, not the injury itself.
Defining Pressure Injuries
A pressure injury is defined as localized damage to the skin and soft tissue, typically occurring over a bony prominence or related to a medical device. This type of injury can present as intact skin or as an open ulcer. The fundamental mechanism involves a lack of blood flow, or ischemia, to the affected area. Sustained mechanical force compresses blood vessels, depriving the tissue of necessary oxygen and nutrients. This deficit leads to cell death (necrosis), which manifests as the visible pressure injury. The injury results from intense or prolonged pressure, or pressure combined with a shearing force.
The Mechanism of Shearing
Shearing is a mechanical force that acts parallel, or tangential, to the body’s surface. This force occurs when two adjacent layers of tissue move in opposite directions relative to one another.
A common example is when a patient is sitting up in bed with the head elevated and begins to slide down toward the foot of the bed. In this scenario, the skin layer may stick to the bed sheet due to friction, while the underlying bone and deep tissues continue to move downward with gravity. This differential movement causes the internal tissue structures to stretch and distort.
The resulting lack of blood flow in the deep tissue, often at the bone-muscle interface, leads to a “bottom-up” injury. This internal damage often first appears as a Deep Tissue Pressure Injury (DTPI), presenting as a localized area of deep red, maroon, or purple discoloration on intact skin. Because the damage begins internally, a shear-related injury can be quite advanced by the time it becomes visible on the surface.
How Shearing Differs from Friction and Direct Pressure
Pressure injuries are caused by a triad of mechanical forces: direct pressure, friction, and shearing, each damaging tissue through a distinct mechanism. Direct pressure involves a force applied perpendicular to the body’s surface, such as when a person lies on their sacrum or heel. This perpendicular force compresses capillaries, impeding blood flow and causing ischemia that typically starts near the surface and progresses inward.
Friction is the resistance encountered when the skin is dragged across a rough external surface, like a bed sheet. This force causes superficial damage, manifesting as an abrasion or loss of the top layer of skin, often described as a “top-down” injury. Friction usually results in a partial-thickness wound. Shearing is the unique internal distortion caused by opposing parallel forces, leading to the stretching and tearing of blood vessels deep within the tissue. While friction often accompanies shear, friction damages the epidermis superficially, whereas shear damages the deep muscle and soft tissue by compromising its blood supply.
Preventing Shearing Damage
Preventing damage from shearing forces requires specific interventions focused on minimizing the relative movement between the skin and underlying skeleton. A primary strategy involves managing the angle of the head of the bed. Elevating it over 30 degrees significantly increases the risk of the patient sliding and creating shear; limiting the incline to 30 degrees or less when the patient is not eating helps maintain a safer position.
Caregivers must use proper lifting and transfer techniques, avoiding the practice of dragging or pulling a patient across a surface. Using specialized friction-reducing sheets or assistive devices helps the patient’s body move as a single unit, preventing the skin from being left behind. Specialized support surfaces, like pressure-redistributing mattresses, are also used to minimize sliding and evenly distribute the mechanical forces.