Shearing most often occurs over bony prominences where skin is compressed against a surface, with the sacrum (lower back), coccyx (tailbone), heels, and ischial tuberosities (the bones you sit on) being the most common sites. These injuries happen when bone and deep tissue slide in one direction while the skin stays fixed in place, cutting off blood flow beneath the surface. Understanding where and why shearing occurs is key to preventing serious pressure injuries.
Body Sites Most Vulnerable to Shearing
Shearing gravitates toward areas where bone sits close to the skin’s surface, creating a pinch point between the skeleton and whatever surface the body rests on. The sacrum and coccyx are the most frequent locations because these bones bear significant weight during sitting and semi-reclined positions. When a person slides down in a hospital bed or wheelchair, the tailbone moves in one direction while the skin over it is held in place by friction against the mattress or seat. That opposing force is shearing.
The heels are another high-risk area, particularly in people who are bedridden. The heel bone is prominent and has very little cushioning tissue around it, making it especially susceptible when legs rest against a mattress for extended periods. The ischial tuberosities, the two bony points at the base of the pelvis that bear your weight when sitting, are a primary concern for wheelchair users. Any time the body shifts or slides in a chair, these bones can drag internal tissue in a different direction than the skin, setting up the conditions for a shearing injury.
How Shearing Damages Tissue
What makes shearing particularly dangerous is that the damage happens beneath the skin, where you can’t see it. The mechanical force acts parallel to the body’s surface. Picture the bones of the pelvis pulling downward while the skin stays anchored to the bed: the deeper layer of connective tissue slides with the bone, but the outer layer stays attached to the skin. This stretches and distorts the tiny blood vessels running between those layers, choking off oxygen supply to the tissue. The resulting lack of blood flow causes cells to die, and tissue begins to break down from the inside out.
This is what distinguishes shearing from friction. Friction acts on the skin’s surface and can cause abrasions or skin tears you can see immediately. Shearing works deeper, damaging tissue and blood vessels well below the surface before any visible wound appears. The two forces almost always occur together, but shearing is the one responsible for the deeper, more serious injuries that can develop into full pressure ulcers.
Hospital Beds and Patient Positioning
Hospital beds are one of the most common environments where shearing injuries develop. The biggest culprit is raising the head of the bed too high. When the backrest is elevated beyond 30 degrees, gravity pulls the body downward toward the foot of the bed while the skin on the back and sacrum stays stuck to the sheet. This creates a constant shearing force on the sacrum and tailbone that worsens the longer the position is held. Clinical guidelines recommend keeping the head of the bed at or below 30 degrees to minimize this effect.
Repositioning and transfers also introduce shearing risk. Sliding a patient up in bed, rolling them to one side, or moving them from bed to chair can drag skin over bony prominences if not done carefully. Using the bed’s knee break (the section that bends at the knees) helps prevent patients from gradually sliding downward. Slide sheets, which are low-friction fabrics placed under the patient, reduce the drag on skin during repositioning and are a standard prevention tool in wound care.
Wheelchair Seating and Shearing Risk
Wheelchair users face a distinct set of shearing challenges, especially over long periods of sitting. The ischial tuberosities bear the bulk of seated body weight, and any shift in posture can create opposing forces between bone and skin. Reclining the wheelchair backrest is a common strategy to redistribute pressure, but it comes with a tradeoff. Research has found that reclining the backrest by 20 degrees increases surface shear forces by about 7%, and a 30-degree recline increases them by roughly 25%. The body tends to slide forward in the seat as the back tilts, pulling internal tissue away from the skin.
Tilt-in-space systems, which angle the entire seat backward without changing the hip angle, can reduce pressure more effectively. However, tilting beyond about 25 degrees may also increase shearing, this time from the body sliding backward into the chair. The balance between pressure relief and shear reduction is a central challenge in wheelchair seating design, and getting the angles right matters for long-term skin health.
Who Is Most at Risk
Older adults are disproportionately affected by shearing injuries. Aging skin loses elasticity, the tissue layers become thinner, and blood vessels are more fragile, all of which lower the threshold at which shearing causes damage. People who cannot reposition themselves independently are at the highest risk because they rely on caregivers to move them, and every transfer or adjustment is an opportunity for shearing forces to develop if technique is poor.
Limited mobility from spinal cord injuries, stroke, or post-surgical recovery places people in sustained contact with bed or chair surfaces, extending the duration of any shearing forces. Moisture from sweating or incontinence further raises the stakes by increasing the coefficient of friction between skin and surface, making the skin more likely to “grip” while deeper tissue continues to move. The combination of prolonged immobility, thin skin, and moisture creates the highest-risk scenario for shearing injuries.
Practical Prevention Strategies
Preventing shearing centers on two principles: reducing the forces that cause it and minimizing the time those forces act on vulnerable tissue. Keeping the head of the bed at or below 30 degrees is one of the simplest and most effective measures. Using the bed’s knee break prevents the slow downward slide that builds shearing force on the sacrum over hours. During any repositioning or transfer, slide sheets or friction-reducing fabrics should be placed under the patient so the skin isn’t dragged across the surface.
For wheelchair users, regular weight shifts and properly fitted seating systems help distribute forces more evenly. Cushions designed to reduce both pressure and shear at the seat surface are widely available and should be matched to the individual’s body shape and mobility level. Keeping skin clean and dry reduces the friction component that amplifies shearing, and regular skin checks over the sacrum, coccyx, heels, and sitting bones help catch early signs of damage before a deep injury develops.