A method for subject-specific modelling and optimisation of the cushioning properties of insole materials used in diabetic footwear

Abstract

This study aims to develop a numerical method that can be used to investigate the cushioning properties of different insole materials on a subject-specific basis. Diabetic footwear and orthotic insoles play an important role for the reduction of plantar pressure in people with diabetes (type-2). Despite that, little information exists about their optimum cushioning properties. A new in-vivo measurement based computational procedure was developed which entails the generation of 2D subject-specific finite element models of the heel pad based on ultrasound indentation. These models are used to inverse engineer the material properties of the heel pad and simulate the contact between plantar soft tissue and a flat insole. After its validation this modelling procedure was utilised to investigate the importance of plantar soft tissue stiffness, thickness and loading for the correct selection of insole material. The results indicated that heel pad stiffness and thickness influence plantar pressure but not the optimum insole properties. On the other hand loading appears to significantly influence the optimum insole material properties. These results indicate that parameters that affect the loading of the plantar soft tissues such as body mass or a person's level of physical activity should be carefully considered during insole material selection.

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

“…The inability to rigidly fix the calcaneus in vivo is considered as an inherent challenge of in vivo testing. This limitation can render unreliable calculations of deformation that are based on the displacement of the indenter (Chatzistergos et al, 2015;Erdemir et al, 2006;Naemi et al, 2016). The use of ultrasound imaging to directly measure heel pad deformations was deemed necessary and was implemented in this study.…”

“…A custom image processing algorithm (MATLAB and Statistics Toolbox Release 2014b, The MathWorks, Inc., Natick, Massachusetts, US) was utilised to outline the boundaries of the calcaneus on both planes (Chatzistergos et al, 2015). The two, 2D curves representing the boundaries of the calcaneus on the frontal and sagittal plane were imported into Ansys (ANSYS® Academic Research, Release 16) and the 3D surface of the calcaneus was reconstructed by dragging the sagittal boundary along the frontal one.…”

“…Therefore, quantifying the mechanical properties of plantar soft tissue has been an exciting and evolving topic for the past few years. Several approaches involving in vivo and in vitro testing have been utilised to measure and quantify the subject specific mechanical behaviour of the plantar soft tissue (Chatzistergos et al, 2015;Erdemir et al, 2006;Miller-young et al, 2002;Naemi et al, 2016;Pai and Ledoux, 2010).…”

“…In this type of test, a load cell is used in series with an ultrasound probe enabling the simultaneous measurement of externally applied force and the internal deformations of plantar soft tissue (Chatzistergos et al, 2015;Erdemir et al, 2006).…”

“…For this purpose, an axisymmetric model of the indentation test was used to inverse engineer the hyperelastic material coefficients of bulk heel pad tissue. In order to increase the subject specificity of the inverse engineering process, Chatzistegos et al utilised the ultrasound images that were collected during the indentation test to reconstruct the geometry of the heel pad and to design subject specific 2D models of the indentation tests (Chatzistergos et al, 2015).…”