Estimating the material properties of heel pad sub-layers using inverse Finite Element Analysis

Abstract

Detailed information about the biomechanical behaviour of plantar heel pad tissue contributes to our understanding of load transfer when the foot impacts the ground. The objective of this work was to obtain the hyperelastic and viscoelastic material properties of heel pad sub-layers (skin, micro-chamber and macro-chamber layers) in-vivo. An anatomically detailed 3D Finite Element model of the human heel was used to derive the sub-layer material properties. A combined ultrasound imaging and motorised platform system was used to compress heel pad and to create input data for the Finite Element model. The force-strain responses of the heel pad and its sub-layers under slow compression (5mm/s) and rapid loading-hold-unloading cycles (225mm/s), were measured and hyperelastic and viscoelastic properties of the three heel pad sub-layers were estimated by the model. The loaded (under ∼315N) thickness of the heel pad was measured from MR images and used for hyperelastic model validation. The capability of the model to predict peak plantar pressure was used for further validation. Experimental responses of the heel pad under different dynamic loading scenarios (loading-hold-unloading cycles at 141mm/s and sinusoidal loading with maximum velocity of 300mm/s) were used to validate the viscoelastic model. Good agreement was achieved between the predicted and experimental results for both hyperelastic (<6.4% unloaded thickness, 4.4% maximum peak plantar pressure) and viscoelastic (Root Mean Square errors for loading and unloading periods <14.7%, 5.8% maximum force) simulations. This paper provides the first definition of material properties for heel pad sub-layers by using in-vivo experimental force-strain data and an anatomically detailed 3D Finite Element model of the heel.

An in vivo model for overloading-induced soft tissue injury

“…The effect of overloading in deeper tissues was not assessed because SW elastography cannot produce reliable measurements close to bony surfaces 4 , 5 . The use of alternative elastography techniques could enable the investigation of the effect of overloading on different plantar soft tissue layers 19 . More research will also be needed to test the applicability of the proposed methodology for shear overloading, which is another important contributor to overloading injuries 20 .…”

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

“…In reality, the subcutaneous tissue of the heel consists of two distinct layers of visco-hyperelastic tissues: the first being the microchamber layer, which is a thin layer of small septa comprised of elastin fibres, and the second, the macrochamber layer, which is a thick layer of larger septa comprised of roughly equal amounts of elastin fibres and collagen [21][22][23][24]. These two layers have been shown to exhibit different mechanical behaviour [25] and have different functional roles [26]. Simulating the anisotropic viscohyperelastic mechanical behaviour of skin, microchamber and macrochamber layers could expand on the association between the measurement of SH and the mechanical properties of the skin and different subcutaneous layers.…”

A novel dynamic mechanical analysis device to measure the in-vivo material properties of plantar soft tissue and primary finite elementary analysis results

“…On the other hand, finite element analysis (FEA) has also been widely used in various biomechanical problems in recent years, especially in the study of soft tissue materials. Based on the parameter data mainly obtained from in-vitro specimen, scholars have constructed a large number of foot FE models to analyze the mechanical behavior of plantar soft tissue [12]. However, the FEA method depends on the initial parameter input of the material, and how to accurately obtain these parameters is a fundamental question.…”