Clinical guidelines name pressure and shear as the cause of pressure ulcers, but rarely explain what the words mean. Drawing on thirty years in rehabilitation engineering, I unpack pressure, shear, stress and strain in plain language, show why the skin check can look normal while damage develops over the bone, and explain why the heels are so vulnerable. This one of our longer articles.

Physiotherapists inherit the consequences of the first weeks after a stroke, a brain injury or a spinal cord injury twice over.

The first consequence is at the ankle itself. A foot that has spent weeks driven into plantarflexion by tone and gravity arrives in the rehabilitation gym short of the range it needs. A foot that cannot reach plantigrade cannot take weight properly, and everything that needs attention (standing practice, transfers, gait work) gets harder.

The second inherited problem relates to the skin. A heel pressure ulcer acquired during the acute phase can halt weight-bearing for weeks or months, right at the point in recovery when time matters most. Every day that the patient remains in bed results in loss of muscle mass. We also know that such an ulcer, even when healed, is much more likely to recur due to changes in the tissue's mechanical properties.

Most writing about heel pressure relief devices treats these as separate problems, one belonging to tissue viability and one to physiotherapy. In my experience, if we want continuity of care, they are best considered as part of the same problem seen from two directions, and the device on the end of the bed is where they meet. This article is about that meeting point, including some evidence that does not say what many people assume it says.

In 1994, I was visiting the Cleveland Clinic and saw a product design I came to know as the PRAFO (pressure relief ankle-foot orthosis). I recognised this as a practical orthosis being used to reduce the risk of heel pressure ulcers developing in at-risk patients following orthopaedic procedures. It's true that I liked the concept so much that we created a company, Anatomical Concepts (UK), to bring the products to the UK. Thirty years have now gone by, and we still deal with the PRAFO range. We still see the challenge that heel pressure ulcers present to affected individuals and our healthcare systems. For decades, we've emphasised prevention, yet acquired ulcers remain a widespread problem. We have some understanding of the mechanical and medical factors that heighten risk and lead to pressure ulcers, but something is missing.

Heel pressure injuries are one of those problems where the evidence has been ahead of everyday practice for years. We have known for many years that pillows and improvised supports rarely keep a heel clear of the bed for long, and that a heel touching anything is a heel under pressure and shear. The 2025 International Pressure Injury Guideline (the fourth edition produced by NPIAP, EPUAP, and PPPIA) has now caught up to that reality, and in doing so it has changed the language clinicians and commissioners should use when they think about heel protection.

The guideline introduces a phrase worth noticing: "floating heels."

It is not a marketing line. It is a clinical description of what an effective heel offloading intervention has to achieve, taken from the guideline itself. And it has practical implications for any service that has to choose, fund, or audit heel protection equipment.

The pillow remains perhaps the most commonly used heel elevation method in hospitals worldwide. I guess this is because they are readily available, cost nothing beyond what's already supplied for patient comfort, and require no special equipment or training. They are also inadequate for heel protection and can compromise continuity of care.

The evidence is now clear: an Australian multi-centre ICU trial found that purpose-designed heel offloading devices achieved a 0.4% pressure injury incidence, compared to 8.4% with standard care, which typically means pillows and repositioning [1]. That's a twenty-fold difference. Translated into practical terms: for every 1,000 patients, 79 fewer will develop heel pressure injuries when proper offloading devices are used instead of pillows.

This isn't a criticism of clinical staff who use pillows—they're working with what's available and following long-established practice. It's an observation about biomechanics: what a pillow can and cannot achieve when the goal is heel offloading.

Understanding why pillows often fail points toward what effective heel protection actually requires.

When clinicians think about pressure ulcer prevention, they typically focus on pressure—the perpendicular force that compresses tissue against a surface. This is understandable. The condition is called a pressure ulcer. Pressure is in the name.

But pressure tells only part of the story. Shear—the force acting parallel to the support surface that distorts tissue layers relative to each other—may be even more damaging than pressure alone. Research dating back to Bennett's seminal 1979 study demonstrated that when shear is present, the pressure required to produce vascular occlusion is reduced by approximately 50%. At shear levels of roughly 100 g/cm², the pressure needed to stop blood flow was half that required when little shear was present.

This finding has profound implications for heel protection. A device that reduces pressure but doesn't address shear may leave the heel vulnerable to the very damage it was meant to prevent.