Heel ulcers: Investigating injurious tissue load thresholds in humans, based on a patient-specific computational heel model

Heel ulcer (HU) is the second most common type of ulcer, formed when the skin and fat that envelope the calcaneus are deformed for extended periods between the bone and a support, eventually causing structural damage to the tissues. No specific data exist regarding the in vivo mechanical stress and strain thresholds for the onset and progression of HUs within clinically relevant time periods (e.g., while anesthetized in the operation room, bed-bound in nursing homes, etc.). We used an MRI heel scan of a 72-year-old male with a clearly demonstrated deep tissue injury at the right heel, to develop a three-dimensional finite element model of the heel, which include the calcaneal bone, subcutaneous fat, insertion and distal region of the Achilles tendon, and the skin. Biomechanical properties of all tissues were adopted from the literature. A preinjury anatomy was simulated using healthy tissue mechanical attributes. Thresholds of the mechanical loads that led to the onset and progression of HU were accessed by back calculating the mechanical conditions in the wound, caused by the foot weight, as it is not expected to significantly change during the injury-relevant time frame. Shear due to support inclination and foot weight was simulated by displacing the superior surface of the calcaneus downward and horizontally. Von Mises stress and Lagrangian strain injury thresholds were offered, along with compressive to tensile proportions of the total strain. The influence of shear stress, foot weight, and bed angle on the thresholds was analyzed. Strain energy density was examined as a predictor for ulcer formation.