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