Corroboration in vivo mechanical strain distribution in soft tissues for pressure ulcer prevention: A comparative analysis between a simplified finite element analysis and experimental strain fields

Basic research into the aetiology of pressure ulcers suggests that the concentration of mechanical strain in biological soft tissues is critical to their development. Direct measurement of in vivo strain is not compatible with clinical routine. To overcome this problem, several finite element models (FEM) have been proposed by the biomechanical community to estimate strain from imaging data. However, no direct experimental validation of the underlying relationships between mechanical loading and soft tissue strain distribution predicted by the model has been performed, and such validation evidence must be obtained prior to any clinical evaluation. Building on the experimental results obtained in N = 10 healthy volunteers (Zappalá et al., 2024), the relevance of the modelling hypotheses of the finite element model proposed in this study, which is based on a simplified geometric representation of the ischial region (Macron et al., 2020), was investigated. A methodology was proposed to estimate the different parameters needed to construct the model from the available MRI masks. The FEM was then used to estimate in vivo compressive and shear strains. The resulting strains were then compared with experimental data. The results show that the model assumptions lead to an overall overestimation of the compressive and shear strains in the muscle tissue, especially directly under the ischial tuberosity. Similarly, the model underestimates the strain in adipose tissue (mean error in shear strain of -0.2). This study highlights the fact that the assumptions usually made for the geometric modelling of muscle tissue (homogeneous soft tissue layer) lead to an incorrect estimation of peak strain localisation. Further work should be done to improve the representation of muscle tissue.