Shore hardness is a more representative measurement of bulk tissue biomechanics than of skin biomechanics.

Abstract Shore hardness (SH) is a non-invasive, cost-effective measurement of a tissue’s resistance to indentation that can enhance clinical research and practice in areas like skin pathologies or diabetic foot ulceration. Even though the measurement itself is relatively simple, correctly interpreting its outcome is challenging. To support the effective use of SH this study investigates whether SH should be interpreted as a measurement of skin or of bulk tissue biomechanics. A 3D Finite Element model of the heel and a validated model of a Shore-00 durometer were used to simulate testing for different combinations of stiffness and thickness in the skin and subcutaneous tissue. Twenty scenarios of altered tissue stiffness or thickness relative to the reference condition were investigated. The results of this numerical analysis showed that SH is significantly more sensitive to changes in skin thickness, relatively to subcutaneous tissue, but equally sensitive to changes in the stiffness of either tissue. Indicatively, 25% reduction in skin thickness (0.3mm thickness change) or in subcutaneous tissue thickness (5.9mm thickness change) reduced SH by 7% or increased SH by 2% respectively. At the same time 25% reduction in the initial stiffness of skin (10.1MPa stiffness change) or of subcutaneous tissue (4.1MPa stiffness change) led to 11% or 8% reduction in SH respectively. In the literature, SH is commonly used to study skin biomechanics. However, this analysis indicates that SH quantifies the deformability of bulk tissue, not of skin. Measurements of skin thickness are also necessary for the correct interpretation of SH.