A clinically applicable non-invasive method to quantitatively assess the visco-hyperelastic properties of human heel pad, implications for assessing the risk of mechanical trauma

Abstract

Cite this article as: Sara Behforootan, Panagiotis E. Chatzistergos, Nachiappan Chockalingam and Roozbeh Naemi, A clinically applicable non-invasive method to quantitatively assess the visco-hyperelastic properties of human heel pad with implications for assessing the risk of mechanical trauma, Journal of the Mechanical Behavior of Biomedical Materials, http://dx.doi.org/10.1016Materials, http://dx.doi.org/10. /j.jmbbm.2017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Purpose: To develop a clinically viable non-invasive method of assessing the mechanical properties of the heel pad. Furthermore the effect of non-linear mechanical behaviour of the heel pad on its ability to uniformly distribute foot-ground contact loads in light of the effect of overloading is also investigated. Methods:An automated custom device for ultrasound indentation was developed along with custom algorithms for the automated subject specific modeling of heel pad. Non-time-dependent and time-dependent material properties were inverse engineered from results from quasistatic indentation and stress relaxation test respectively. The validity of the calculated coefficients was assessed for five healthy participants. The implications of altered mechanical properties on the heel pad's ability to uniformly distribute plantar loading were also investigated in a parametric analysis. Results:The subject specific heel pad models with coefficients calculated based on quasi-static indentation and stress relaxation were able to accurately simulate dynamic indentation.Average error in the predicted forces for maximum deformation was only 6.6 ± 4.0%. When the inverse engineered coefficients were used to simulate the first instance of heel strike the error in terms of peak plantar pressure was 27%. The parametric analysis indicated that the heel pad's ability to uniformly distribute plantar loads is influenced both by its overall deformability and by its stress/ strain behaviour. When overall deformability stays constant, changes in stress/strain behaviour leading to a more "linear" mechanical behaviour appear to improve the heel pad's ability to uniformly distribute plantar loading. Conclusions:The developed technique can accurately assess the visco-hyperelastic behaviour of heel pad. It was observed that specific change in stress-strain behaviour can enhance/weaken the heel pad's ability to uniformly distribute plantar loading that will increase/decrease the risk for overloading and trauma.

An In-Vivo Model For Overloading-Induced Soft Tissue Injury

“…Tissue stiffening was assessed separately for the edges of the loaded area since the pressure in those areas was expected to be the highest 15 . This was also confirmed for the specific loading scenario presented here using a previously developed finite element model 11 of the heel (Figure 2b,c).…”

“…A finite element analysis of the overloading scenario presented here was performed by adapting a previously published model of the healthy heel which was validated against in-vivo measurements of plantar pressure 11 . The purpose of this analysis was not to provide a quantitative assessment of the absolute value of interface pressure, but to provide a relative assessment of the difference between the pressure at the edges and at the centre of the loaded area.…”

Shear wave elastography can assess the in-vivo nonlinear mechanical behavior of heel-pad

“…A linear array ultrasound probe (4-15 MHz, SL 15-4 Linear transducer, SuperSonic Imagine Ltd), which is acting also as the indenter, was moved slowly towards the foot and the initial thickness of the heel-pad was measured from the first ultrasound image where the calcaneus was visible. The indenter was moved using a motor which could be programmed to realize a predefined loading protocol (Sara Behforootan et al, 2017b Every test was preceded by seven preconditioning loading/ unloading cycles to maximum compression at 0.5 mm/ s to minimize the effect of loading history (Behforootan et al, 2017). After preconditioning, heel-pad was compressed in five steps to a maximum of 50% of its original thickness.…”

Localized pressure stimulation using turf‐like structures can improve skin perfusion in the foot

“…A bespoke indentation device was utilized to stimulate each ROI with the different samples and skin perfusion was measured before and immediately after stimulation (Figure ). More specifically, the indentation device stimulated, in separate tests, the rearfoot, midfoot, and forefoot regions of the right foot of the participants with 10 load/unload cycles . The loading/unloading rate was set at 20 mm/s to simulate walking .…”