Shearing most often occurs at the sacrum, the bony area at the base of your spine just above the tailbone. It also commonly affects the heels, shoulder blades, and the back of the head. These are all places where bone sits close to the skin surface, creating the conditions for deeper tissue layers to slide against each other when the body is repositioned or gravity pulls it downward against a support surface.
What Shearing Actually Does to Tissue
Shear is a mechanical force that acts parallel to the skin’s surface, pulling deeper layers of tissue in one direction while the outer skin stays put. Picture someone semi-reclined in a hospital bed: gravity pulls the pelvis downward and forward, but the skin on the sacrum stays stuck to the sheet. The deeper tissue, fascia, and blood vessels attached to the bone slide downward while the surface skin remains anchored. This stretches and distorts the small blood vessels running between those layers, cutting off blood flow. Without blood supply, the affected tissue becomes starved of oxygen, and cells begin to die.
This is fundamentally different from friction, which is two surfaces rubbing against each other. Friction damages the outer layer of skin and looks like an abrasion or scrape. Shear injury, by contrast, is invisible at the skin surface because the damage happens underneath. The tissue between the bone and the skin is where the real distortion occurs. When shear-related injuries do become visible, they often appear as deep, discolored patches or elongated horizontal stripes across the tissue rather than a surface wound.
The Sacrum Is the Highest-Risk Site
The sacrum bears the brunt of shearing force more than any other body part. When the head of a hospital bed is elevated, gravity constantly pulls the skeleton downward while the skin resists that slide. The sacrum sits at the pivot point of this force. Raising the head of the bed beyond 30 degrees significantly increases shear load on the sacral area, which is one reason sacral pressure injuries are so common in clinical settings.
The pelvis is also structurally vulnerable. The sacroiliac joint, where the base of the spine meets the hip bones, is oriented in a way that naturally channels the weight of the trunk into shear forces rather than simple compression. In an upright posture, the load from your spine transfers through this joint largely as vertical shear. Your body manages this through deep core muscles, particularly the transverse abdominis, which wraps around and essentially clamps the sacrum between the hip bones to keep it stable. When those muscles are weak, inactive, or a person is immobilized, the joint loses that stabilization and shear forces go unchecked.
Other Common Shearing Sites
The heels are another frequent location. When someone lies in bed, the weight of the leg presses the heel into the mattress while any sliding motion drags the deeper tissue forward. Because the heel has very little padding between bone and skin, even small shearing forces can compromise blood flow quickly.
The shoulder blades and the back of the head are vulnerable in similar ways, especially in people who are positioned on their backs for extended periods. Any bony prominence where tissue is thin and the body’s weight creates downward or lateral pull is a potential shearing site. The elbows and the spine over the thoracic vertebrae can also be affected, particularly in very thin individuals or those who are malnourished and have lost protective subcutaneous fat.
Positions and Situations That Increase Shearing
The single biggest contributor to shearing in a clinical setting is head-of-bed elevation. When the bed is angled above 30 degrees, the body’s tendency to slide toward the foot of the bed increases dramatically. Current prevention guidelines recommend keeping the head of the bed at or below 30 degrees whenever possible. If elevation above that angle is medically necessary (for breathing support, eating, or reducing aspiration risk), placing a protective foam dressing over the sacrum and using the bed’s knee break to prevent the body from sliding are standard countermeasures.
Transferring a person by dragging rather than lifting is another common cause. Pulling someone across bed linens creates simultaneous friction on the skin surface and shear in the deeper tissues. The coefficient of friction between skin and hospital fabrics ranges from about 0.27 for standard hospital linen to 0.38 for bed protectors, and natural fabrics like wool can reach 0.75. Higher friction means the skin grips the surface more firmly, which actually worsens the internal shearing because the outer skin is held in place while everything beneath it shifts.
Sliding down in a wheelchair or recliner produces the same effect. A person who gradually slumps forward in a wheelchair experiences continuous low-grade shear across the sacrum and ischial tuberosities (the “sit bones”). Poorly fitting wheelchairs, slippery seat cushions, and long periods without repositioning all amplify the problem.
Who Is Most Vulnerable
People who cannot reposition themselves are at the highest risk. This includes those recovering from surgery, individuals with spinal cord injuries, people who are sedated or on ventilators, and elderly patients with limited mobility. Reduced sensation compounds the danger because the person cannot feel the discomfort that would normally prompt them to shift their weight.
Moisture also plays a role. Skin that is damp from sweat, incontinence, or wound drainage has a higher coefficient of friction against linens, which increases the grip on the skin surface and magnifies the shearing effect underneath. Malnutrition and dehydration reduce skin elasticity and thin the subcutaneous tissue layer, leaving less of a buffer between bone and skin.
How Shear Injuries Differ From Surface Wounds
Because shearing damages tissue beneath the skin, the earliest signs are often subtle. A deep tissue injury caused by shear may first appear as a persistent area of dark purple or maroon discoloration that doesn’t blanch when you press on it. The shape of the discoloration can be a clue: shear-related injuries tend to be elongated or stripe-shaped rather than round, reflecting the direction the tissue was pulled.
In some cases, shearing separates the outer skin layers, creating a skin tear with a flap of lifted tissue. This can look similar to a blister, making it easy to confuse with a less serious wound. The key difference is what’s happening deeper. A friction burn affects only the epidermis and heals relatively quickly. A shear injury may involve tissue death in the fat, fascia, or muscle layers below, even when the surface skin looks mostly intact. By the time a shear injury fully declares itself on the surface, significant damage may already be present underneath.
Reducing Shearing Forces
Limiting bed-head elevation to under 30 degrees is the most straightforward way to reduce sacral shear. When higher elevation is unavoidable, using the bed’s knee break prevents the body from sliding toward the foot of the bed. Lifting rather than dragging during repositioning eliminates the combination of friction and shear that occurs when skin is pulled across linens. Slide sheets and transfer boards reduce the force needed to move someone and keep the skin from gripping the surface.
Support surfaces matter too. Pressure-redistribution mattresses and overlays reduce the overall contact pressure, which in turn reduces the shearing component. Keeping skin clean and dry lowers the friction coefficient, making it less likely that the skin will “stick” while deeper tissue slides. Barrier creams and moisture-wicking fabrics help maintain that lower-friction environment. For the heels specifically, elevating them off the mattress entirely with pillows or heel-suspension devices removes both pressure and shear from the equation.