Patient-specific optimization of insole material properties: A pilot study

Diabetes mellitus (type 2) is the most frequent cause of non-traumatic lower-limb amputations. It is
indicated that footwear significantly affects the course of the diabetic foot syndrome. This usually works
by using an orthotic insole designed to distribute pressure evenly over the plantar surface. As it stands,
the prescription of diabetic footwear is intuitive in nature and is mainly provided through subjective
measurements [1]. In this context the present study aims at developing a computational tool to facilitate
the patient-specific, evidence-based choice of insole material for people with diabetes.
For this purpose a novel in-vivo measurement based procedure was designed for insole material
optimization. During the first step of this procedure the
morphology and the macroscopic mechanical behaviour
of the plantar soft tissues is analysed using a specialized
device for ultrasound indentation[2]. This device
combines ultrasound and dynamometry to measure the
tissue’s force/deformation curve. The indentation tests
are simulated using a 2D finite element (FE) model
(Fig.1b). The foot is considered to consist of rigid bone
and a bulk soft tissue. The geometry of this tissue is
reconstructed from the ultrasound images (Fig.1a) and
its mechanical behaviour is defined using the Ogden
hyperelastic model. The material coefficients are
inverse engineered using the data from in-vivo tests to
create a model that imitates the plantar tissue’s
macroscopic response to loading. This model is
finally used to simulate the contact between foot and
insole and to calculate their contact pressure (Fig.1c).
The mechanical behaviour of the insole is defined
using the Ogden HyperFoam material model. An optimization
algorithm is employed to calculate the material
coefficients that minimize the plantar pressure.
As a first step this procedure was employed to analyse the mechanical behaviour of the heel pad of
a single subject and to find the optimum material properties for insoles that have different
thicknesses. The results showed that the optimum insole material can reduce plantar pressure up to
21% compared to a rigid insole. Moreover the values of the optimum material coefficients are
strongly influenced by the insole’s thickness indicating that a combination of different insole
thickness and material properties could be more effective in minimising plantar pressure.