Shearing of the skin happens when deeper layers of tissue slide in one direction while the surface skin stays in place, or moves the opposite way. Unlike a scrape or abrasion you can see on the surface, shear damage occurs underneath the skin, where internal layers stretch, distort, and lose blood supply. It is one of the primary forces behind pressure injuries (sometimes called bedsores or pressure ulcers), and it most commonly affects people who spend extended time in a bed or wheelchair.
How Shearing Works Inside the Skin
Skin is made of multiple layers. The outermost layer (epidermis) sits on a thicker layer (dermis), which sits on fatty tissue and connective tissue called fascia. Beneath all of that are muscle and bone. Shearing occurs when the skeleton moves in one direction and the skin attached to an external surface resists that movement. The classic example: a person in a hospital bed with the head elevated slides downward. Their tailbone shifts toward the foot of the bed, pulling deeper tissue with it, while the skin over the sacrum stays stuck to the sheets. The layers between bone and skin stretch and twist apart.
This internal pulling is what makes shear so damaging. It kinks and compresses blood vessels in the deep portion of the tissue, cutting off oxygen delivery to the cells. Research has shown that when shear forces are applied on top of normal pressure, blood flow and oxygen levels in the skin drop more sharply than with pressure alone. That oxygen starvation, called ischemia, leads to cell death and tissue breakdown from the inside out.
Shear vs. Friction
People often confuse shear and friction, but they injure the skin in opposite directions. Friction is a surface-level force: skin dragged across a rough material like bed linen, producing a visible abrasion or superficial scrape. It works from the top down. Shear, by contrast, works from the bottom up. It acts parallel to the skin’s surface but deep within the tissue layers, and its damage is invisible at first because it happens beneath intact skin.
In practice, the two forces almost always occur together. Dragging a person across sheets creates friction on the surface and shear underneath. But friction alone typically causes only minor surface wounds. Shear is the force that leads to the deep tissue destruction characteristic of serious pressure injuries.
What Shear Damage Looks Like
Because shear injury starts below the skin surface, there may be no visible wound in the early stages. The first signs are often deep bruising, a purple or dark discoloration of intact skin, or a boggy, spongy feeling in the tissue over a bony area like the sacrum, heels, or shoulder blades. As damage progresses, the tissue can break down from the inside, sometimes creating a wound with “undermining,” where the opening on the surface is small but a larger pocket of destroyed tissue exists beneath it.
International pressure injury staging systems recognize shear as a direct cause of these wounds alongside pressure. Clinicians diagnosing a pressure injury are expected to confirm the presence of pressure and/or shear as a causative factor before classifying the wound’s severity.
Who Is Most at Risk
Anyone who cannot reposition themselves independently faces shear risk, but certain factors make the skin far more vulnerable.
- Aging skin. Older adults produce less collagen, elastin, and subcutaneous fat. Their skin loses elasticity, dries out from reduced oil and sweat gland activity, and the capillaries become more fragile. These changes mean the skin and underlying tissue are less able to absorb and recover from mechanical forces. Even minor shearing can trigger vascular damage and bruising.
- Dehydration. The skin’s barrier function depends on proper hydration to maintain its structure and resilience. Dehydrated skin loses the ability to respond effectively to mechanical stress, making it easier for internal layers to separate.
- Malnutrition. Without adequate protein and nutrients, the body cannot repair tissue or maintain the structural integrity of skin layers.
- Reduced mobility. People who are bedbound, wheelchair-dependent, or recovering from surgery are exposed to sustained pressure and shear, especially if they cannot shift their weight on their own.
- Moisture. Excessive skin moisture from sweat, incontinence, or wound drainage softens the outer layer of skin and increases the friction coefficient between skin and surfaces, which in turn amplifies shear forces in deeper tissue.
- Sensory loss. People with nerve damage from conditions like diabetes or spinal cord injury may not feel the discomfort that would normally prompt them to shift position.
- Very young skin. Premature and newborn infants have underdeveloped skin with poor bonding between the outer and deeper layers, placing them at increased risk as well.
Where Shearing Happens Most Often
The sacrum (the flat bone at the base of the spine) is the most common site for shear injury because of how body mechanics work in a bed. When the head of the bed is elevated, gravity pulls the body downward. The skeleton slides, but skin grips the mattress. Raising the head of the bed to 30 degrees or more generates significant shear force concentrated at the sacrum.
Other common sites include the heels (which press into the mattress and slide during leg movement), the shoulder blades, and the elbows. In wheelchair users, the ischial tuberosities, the bones you sit on, are a frequent target because the body tends to slide forward on the seat cushion.
How to Reduce Shearing Forces
Prevention centers on minimizing the conditions that allow skin and deeper tissue to move in opposite directions.
Bed Positioning
Clinical guidelines recommend keeping the head of the bed at the lowest elevation that the person’s medical condition allows. Every degree of elevation increases the gravitational pull that drives shearing at the sacrum. When the head must be raised, preventing the person from sliding downward is critical. This can mean bending the knees slightly, using a footboard, or adjusting the bed’s break point so the bend occurs at the hips rather than the waist.
Repositioning Technique
One of the most common sources of shear is the act of moving a patient in bed. Pulling or dragging a person across sheets creates intense friction and shear simultaneously. Low-friction slide sheets, made from slippery synthetic fabric, allow caregivers to reposition someone by sliding them rather than dragging them. These sheets reduce the grip between the person’s skin and the bed surface so that the body moves as a unit instead of the skin catching while deeper tissue shifts. After repositioning, the slide sheet is removed so it does not remain under the person during rest.
Support Surfaces
Specialized mattresses and wheelchair cushions are designed to redistribute pressure and reduce shear. Guidelines from wound care organizations specify that the support surface should be matched to the individual’s needs for pressure redistribution, shear reduction, and microclimate control (managing heat and moisture at the skin surface). Foam, air, and gel surfaces all work slightly differently, but the goal is the same: spread force over a larger area and reduce the internal stretching of tissue layers.
Skin Care
Keeping skin clean, properly moisturized, and dry from excess moisture helps maintain the barrier’s resilience. Barrier creams can protect against incontinence-related moisture. Adequate hydration and nutrition support the skin’s structural integrity from the inside, giving it more capacity to tolerate mechanical stress without breaking down.
Why Shearing Matters Beyond the Surface
Shear is deceptive because the skin can look completely intact while serious damage develops underneath. A deep tissue pressure injury can progress rapidly once it starts, and by the time a wound becomes visible on the surface, substantial tissue has already been destroyed. This is why prevention matters so much more than treatment for shear-related injuries. Healing a deep pressure wound can take weeks to months, may require surgical intervention, and carries significant risk of infection. Understanding the mechanics of shear, and taking straightforward steps to reduce it, prevents a cascade of damage that is far harder to reverse than it is to avoid.