Pressure ulcers form most often over bony prominences, the places where bone sits close to the skin surface and compresses soft tissue against a mattress, chair, or other surface. Three locations account for roughly 70% of all pressure ulcers: the sacrum (the flat bone at the base of the spine), the ischial tuberosities (the “sit bones” deep in each buttock), and the greater trochanters (the bony points on the outer hips). Beyond these top three, heels, the back of the head, shoulder blades, elbows, ankles, and ears are all well-documented sites.
The specific spot that’s most vulnerable depends almost entirely on body position, how long that position is held, and whether any medical devices are pressing against the skin.
Why Bony Prominences Are the Problem
A pressure ulcer develops when sustained force squeezes skin and the tissue beneath it against a hard structure, cutting off blood flow. Bone creates that hard structure from the inside. Where the layer of fat and muscle between bone and skin is thinnest, even moderate pressure can choke off circulation in minutes. The tissue starves, cells begin to die, and damage works its way from the surface inward or, in some cases, from deep tissue outward.
The earliest sign is a patch of skin that stays red (or, on darker skin, stays darker or feels warmer) even after pressure is removed. This is a Stage 1 injury. If pressure continues, the skin can break open into a shallow wound with a pink or red base, sometimes appearing as a blister. Deeper stages involve damage to fat, muscle, and even bone.
Lying on Your Back: Sacrum and Heels
The supine position puts the greatest load on the sacrum, which is why it consistently ranks as the single most common pressure ulcer site. In a three-year ICU study, 39% of all pressure ulcers occurred at the sacrum. A separate clinical survey found sacral ulcers made up about 36% of cases. The sacrum is broad and flat with relatively little padding, so body weight concentrates there when a person lies face up.
Heels are the second major concern in this position. They’re small, bony, and bear a surprising amount of contact force against a mattress. Clinical guidelines from the National Pressure Injury Advisory Panel specifically recommend offloading heels, meaning floating them above the bed surface entirely using pillows or foam devices, rather than simply cushioning them. One clinical study found heel ulcers accounted for about 15% of all pressure injuries in hospitalized patients.
The back of the head (the occiput) is also at risk when lying supine, particularly in people who are sedated or unable to shift their head. In ICU data, roughly 2% of pressure ulcers form behind the head, but in infants and toddlers, the occiput is one of the most common sites because a baby’s head is proportionally larger and heavier relative to the body.
Lying on Your Side: Hips and Ankles
When someone is positioned on their side, pressure shifts to the greater trochanter, the knob of bone at the outer hip. This is one of the top three ulcer sites overall. The lateral position also loads the bony bump on the outside of the ankle (the lateral malleolus) and the outer edge of the knee. Placing a pillow between the knees and ankles separates these bony surfaces and reduces the risk significantly.
Shoulder and ear ulcers can also develop in the lateral position, especially if the head sinks into a firm pillow and the ear folds under its own weight for hours.
Lying Face Down: Knees, Feet, and Face
Prone positioning is common in intensive care for patients with severe respiratory failure. It creates a unique pattern of pressure injuries that hits areas most other positions spare. A French clinical trial tracking ICU patients in the prone position found that knees were the most frequently affected site, with ulcers developing in about 45% of patients studied. The tops of the feet followed at roughly 24%, and the lips at about 20%. The forehead, nose, chin, cheeks, and chest also showed injuries, though less frequently.
These numbers are notably high and reflect the reality that prone positioning applies sustained force to soft, sensitive areas of the face and the front of the body that aren’t built to bear weight for extended periods.
Sitting: The Ischial Tuberosities
For anyone who spends long hours in a wheelchair or seated position, the ischial tuberosities carry the highest risk. These are the two small, rounded bones at the bottom of each side of the pelvis. When you sit upright, your full upper-body weight funnels down through these two points into the seat cushion. In the United States, nearly 50% of the roughly 1.4 million people who rely on wheelchairs for mobility develop serious tissue breakdown at the ischial tuberosities or greater trochanters.
Research on seating mechanics shows that in a normal upright posture, pressure concentrates sharply at the ischial tuberosities while the thighs bear much less load. Tilting the seat or using specialized cushions can redistribute that pressure forward onto the thighs, which have more muscle and fat to absorb it. Regular pressure relief, lifting yourself off the seat for a few seconds or shifting your weight side to side, remains one of the simplest and most effective protections.
Medical Devices Create Their Own Risk
Pressure ulcers don’t only come from body weight against a surface. Medical devices pressing against skin can cause identical injuries, and they often show up in locations that wouldn’t otherwise be vulnerable. In one hospital study, the most common device-related ulcer site was the ears, accounting for 35% of these injuries, typically caused by oxygen tubing looping over the top of the ear or clip-on oxygen monitors pinching the earlobe.
Other device-related sites include the lips (from breathing tubes), the bridge of the nose and cheeks (from face masks used for ventilation), the arms (from arterial line tubing), and the fingertips. A pressure ulcer on the bridge of the nose, the lip, or the webbing of the thumb has essentially no explanation other than a device pressing there. For this reason, clinical protocols call for regularly repositioning breathing tubes and removing face masks to inspect the skin underneath.
How Location Differs in Children
In adults, the sacrum, buttocks, and heels dominate. In children, the pattern shifts. The back of the head is one of the most common sites in babies, infants, and toddlers because of the proportionally large, heavy skull resting against a crib mattress or car seat. Ears are also more vulnerable in young children.
Beyond those differences, children share many of the same risk areas as adults: the sacrum, hips, buttocks, heels, elbows, spine, and shoulder blades. Children in casts, splints, or braces face additional risk directly under those devices, and the same applies to any medical equipment that contacts the skin for prolonged periods.
Matching Prevention to Position
Because location tracks so closely with body position, the most practical prevention strategy is position-specific. For someone lying on their back, sacral protection through frequent small shifts in position (called microshifts) and heel offloading with pillows are the priorities. For side-lying, cushioning between the knees and ankles protects the trochanter and lateral malleolus. For prolonged sitting, regular pressure relief from the ischial tuberosities matters most.
Repositioning every two hours is the widely cited standard for bedridden individuals, though some people with thinner tissue or poorer circulation may need more frequent changes. Skin checks at all high-risk bony prominences, especially the sacrum, heels, and hips, are the fastest way to catch a Stage 1 injury before it progresses. On darker skin tones, redness can be difficult to see, so checking for warmth, firmness, or tenderness in those areas is more reliable than relying on color changes alone.