Why does a heel that looked fine go purple two days later?
Pressure is something applied to the skin. Damage is something that happens to the cells underneath. Who the person is sets the level of risk; what presses on the heel, and for how long, is what turns that risk into an ulcer. The Heel Risk Explorer lets you change what is applied, and who it is applied to, and watch how the risk of heel pressure ulceration responds.
The mechanics: we can show you numbers
How hard the surface presses, whether the skin is sliding or damp, and above all how long the load stays on: these have been measured. Cell death thresholds have been mapped against pressure and time in rat muscle, shear has been shown in human hand tissue to roughly halve the pressure that shuts off blood flow, and brief lifts have been shown in animal work not to reset the damage already accumulating. The explorer uses those published numbers, and labels the species behind every one.
The person: we can only show you directions
Diabetes, poor circulation, spinal cord injury, age, nutrition, a previous ulcer: each is known to raise risk, and for each the mechanism is understood. What nobody has, anywhere in the published literature, is a validated way to weigh them against the mechanics or against each other. So the explorer moves them the only way the evidence allows: each factor shifts the picture in its documented direction, with the mechanism explained, and the risk band never moves more than one step on their account.
Why we can't confidently give you a single risk score
A tool that blended all of this into one confident percentage would be inventing knowledge that does not exist. The two panels above are built from different kinds of evidence, and the explorer keeps them visibly different: numbers where numbers are published, directions where they are not. That distinction is not a limitation of this page. It is the state of the science, and seeing it clearly is the point.
Two damage processes run at the loaded heel
Sustained deformation kills cells in minutes to hours. Blocked blood flow starves tissue over hours. The explorer shows both processes, and where your scenario sits against the damage threshold curve measured in rat muscle: the reason a heel can be injured long before anything shows at the surface, and why the purple appears two days late.
Curve shown as published, not scaled to humans. Every figure in the explorer carries its source and species.
An educational model of published mechanisms, built for clinicians, carers and curious patients. It does not assess or predict risk for any individual, it produces no score with clinical meaning, and it does not replace structured risk assessment or clinical judgement. If you are worried about a real heel, speak to the clinical team looking after it.