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

https://doi.org/10.1016/j.medengphy.2015.03.009Get rights and content

Highlights

  • We developed a new method for the subject specific modelling of the heel pad.

  • Validation showed it can estimate the pressure reduction achieved by insole materials.

  • Its purpose is to inform insole material selection on a subject-specific basis.

  • Altered plantar tissue stiffness or thickness is not important for insole selection.

  • Subject specific loading is very important for insole material selection.

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.

Introduction

The diabetic foot disease is one of the most common complications of type-2 diabetes. Previous reports highlight that approximately 15% of people with diabetes world-wide will at some stage develop diabetic foot ulceration that could lead to amputation [1]. The complications of diabetes (type-2) are the most frequent cause of non-traumatic lower-limb amputations [1]. While in the UK up to 100 people/week have a limb amputated as a result of diabetes, it is indicated that up to 80% of these amputations could have been prevented with correct management [2].

Even though it is clear that certain areas of the foot have a significantly higher risk for ulceration (i.e. metatarsal head area, the heel and the hallux) [3] the mechanisms behind ulceration are not yet fully understood. Foot ulcers in people with diabetes are multi-factorial and linked to a variety of clinical risk factors, like peripheral neuropathy and vascular insufficiency [4], as well as biomechanical risk factors, such as increased plantar pressure [3].

Previous in-vivo studies performed with age-matched groups of non-diabetic and diabetic volunteers have found that diabetic plantar soft tissue tends to be thicker [5], stiffer [5], [6], harder [7] and to return less energy after a load/unload cycle (i.e. higher energy dissipation ratios) [8]. Moreover recent in-vivo results revealed statistically significant correlations between the stiffness of the heel pad of people with diabetes (type-2) and their blood sugar and triglycerides levels [9].

One of the most common experimental techniques used to study the in-vivo mechanical behaviour of plantar soft tissues is ultrasound indentation. During the indentation test tissue deformation is measured from the ultrasound images [5,[8], [9], [10]] and the applied force is measured from a load sensor enabling the calculation of a force/deformation curve. This curve describes the macroscopic response of the plantar soft tissue to loading and is influenced by the morphology of the tissue as well as by the size and shape of the indenter. The effect of indenter size was numerically investigated by Spears et al. [11] to conclude that larger indenters can produce more reliable and robust measurements compared to smaller ones.

In order to produce a more accurate and objective technique for the material characterisation of plantar soft tissue, Erdemir et al. [10] combined the in-vivo indentation test with finite element (FE) modelling. Axisymmetric FE models of the indentation test were used to inverse engineer the values of the material coefficients of a simplified hyperelastic bulk soft tissue.

One of the main therapeutic objectives for the management of the diabetic foot syndrome is the reduction of plantar pressure. Although, therapeutic footwear and orthotic insoles play an important role in redistributing the plantar load [12], [13], [14], [15], very little information exists on the optimum cushioning properties of the materials used as foot beds, insoles or a sole. Whilst the criteria for the selection of orthotic insole materials, which were devised some time ago, identify stiffness [16] and the material's “pressure distributing properties” [17] as critical factors for selection, no quantitative method exists to identify the most appropriate material on a subject-specific basis [18], [19]. As it stands there is no guideline on how “soft” or “stiff” an insole should be. Despite that, currently there are a huge number of commercially available insole materials and new ones are produced every year.

In this context the purpose of this study is to set the basis for an integrated procedure for the subject-specific FE modelling of the heel pad upon which the investigation of the mechanical compatibility between heel and insole would be possible. Such procedure would allow the optimal cushioning of the insole to be determined based on subject-specific characteristics.

Section snippets

Ultrasound indentation

A healthy volunteer (age = 38 years, body mass = 82 Kg) was recruited for the purpose of this study. Ethical approval was sought and granted by the University Ethics Committee and the subject provided full informed consent.

An ultrasound indentation device (Fig. 1) comprising an ultrasound probe connected in series with a load cell (3 kN, INSTRON) was utilised to perform indentation tests at the area of the apex of the calcaneus [9]. The instrumented probe was mounted on a rigid metallic frame

Ultrasound indentation

The preliminary plantar pressure measurements showed that the average(±stdev) peak pressure for all 10 trials of barefoot standing on a rigid surface was equal to 176 kPa (±7.6 kPa) while the average(±stdev) net compressive force applied to a section of the heel that is similar to the one imaged during the indentation test was 80 N (±4 N).

The main output of the indentation test was the average force/deformation curve of the heel pad (Fig. 3). The reconstructed outline of the calcaneus is shown

Discussion

Even though current literature is rich with elaborate geometrically detailed FE models of the entire foot [21], [22], [24], [25] and of the heel [26], the design and use of these models is labour intensive, computationally expensive and requires a significant amount of information in terms of tissue geometry and mechanical properties. This makes the extensive use of geometrically detailed FE models impractical for clinical applications or the optimisation of footwear design. The use of

Conflict of Interest

None declared.

Funding

This work is supported through the project titled DiabSmart funded by the European Commission (grant agreement no. 285985, Industry Academia partnerships and Pathways (FP7-PEOPLE-2011-IAPP)).

Ethical Approval

Ethical approval was sought and granted by the Staffordshire University Ethics Committee.

References (40)

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