The relationship between the mechanical properties of heel-pad and common clinical measures associated with foot ulcers in patients with diabetes

https://doi.org/10.1016/j.jdiacomp.2014.03.011Get rights and content

Abstract

Aim

The present study aims at investigating the correlation between the mechanical properties of the heel-pad of people with type-2 diabetes and the clinical parameters used to monitor their health and ulceration risk.

Methods

A new device for the in-vivo testing of plantar soft tissues was built and pilot-tested. This device consists of an ultrasound probe connected in series with a dynamometer. Loading is applied manually using a ball-screw actuator. A total of 35 volunteers with type-2 diabetes were recruited and the thickness, stiffness of their heel-pads as well as the energy absorbed during loading were assessed. The participants with diabetes also underwent blood tests and measurements of Ankle Brachial Index and Vibration Perception Threshold.

Results

Pearson correlation analysis revealed strong correlations between triglycerides and heel-pad stiffness (r = 0.675, N = 27, p < 0.001) and between triglycerides and energy (r =  0.598, N = 27, p = 0.002). A correlation of medium strength was found between Fasting Blood Sugar (FBS) and stiffness (r = 0.408, N = 29, p = 0.043).

Conclusions

People with type-2 diabetes and high levels of triglycerides and FBS are more likely to have stiffer heel-pads. Increased stiffness could limit the tissues’ ability to evenly distribute loads making them more vulnerable to trauma and ulceration.

Introduction

Recent reports indicate that approximately 15% of people with diabetes world-wide will at some stage develop diabetic foot ulceration (Boulton, 2000). Indeed diabetes mellitus (type-2) is the most frequent cause of non-traumatic lower-limb amputations. In the UK up to 100 people per week have a limb amputated as a result of diabetes even though up to 80% of these amputations could have been prevented with correct management (Diabetes UK, 2011). The severity of this fact becomes even more pronounced considering that eight out of ten people die within five years of having an amputation (Khanolkar, Bain, & Stephens, 2008).

Foot ulcers in people with Diabetes are multi-factorial and linked to a variety of risk factors like peripheral neuropathy, vascular insufficiency and physiological measures (Crawford, Inkster, Kleijnen, & Fahey, 2007). Whilst, some of the epidemiological studies demonstrate that the indicators of neuropathy like impaired sensation, Vibration Perception Threshold (VPT) are predictors of ulceration (Crawford et al., 2007, Frykberg et al., 1998), other studies show that peripheral vascular disease indicated by Ankle Brachial Index (ABI), glycohaemoglobin (HbA1c) level and duration of diabetes are the main contributing factors to ulcers (Boyko, Ahroni, Cohen, Nelson, & Heagerty, 2006). Although these major risk factors are known to contribute to foot ulceration, it is not completely understood how they affect the mechanical properties of plantar soft tissue.

From previous research, it is clear that diabetes can affect the internal structure and the mechanical properties of the plantar soft tissues. Some in-vivo studies performed with age-matched groups of non-diabetic and diabetic volunteers have found that diabetic plantar tissue tends to be thicker (Chao, Zheng, & Cheing, 2011), stiffer (Chao et al., 2011, Klaesner et al., 2002), harder (Piaggesi et al., 1999) and shows higher energy dissipation ratios (i.e. the ratio of the energy-input over the energy-return after the end of a load/unload cycle) (Hsu et al., 2002, Hsu et al., 2007, Hsu et al., 2000). On the other hand Erdemir, Viveiros, Ulbrecht, and Cavanagh (2006) studied the mechanical behaviour of heel-pad using a novel methodology which combined in-vivo testing and finite element modelling to inverse engineer the tissue’s material coefficients. The authors of that study found no statistically significant stiffness or thickness difference between age-matched groups of people with diabetes and non-diabetic volunteers (Erdemir et al., 2006).

Despite this there is a clear paucity of studies which explore the relationship between the mechanical properties of plantar soft tissue and commonly employed clinical and biochemical measures. Hsu et al. (2000) found a weak correlation between the tissue’s energy dissipation ratio and the patient’s neuropathy score measured based on 10 g monofilament test. These authors also indicated a strong correlation between energy dissipation ratio and the duration of diabetes.

The most popular technique for the in-vivo study of plantar soft tissues’ mechanical behaviour is the combined use of ultrasonography and dynamometry. Previous research have developed different devices to perform in-vivo indentation (Chao et al., 2011, Erdemir et al., 2006, Hsu et al., 2000, Hsu et al., 2007, Tong et al., 2003, Zheng et al., 1999) or bulk compression tests (Hsu et al., 2009, Rome et al., 1998, Zheng et al., 2012). Typically, during an indentation test the plantar soft tissue is compressed between the indenting device and a bony prominence. During the test the applied force is recorded using a load sensor (i.e. dynamometer or load cell) and tissue deformation is measured from the ultrasound images.

Given this background, the overall aim of the present study is to investigate if significant correlation exists between the mechanical properties of the heel-pad of people with diabetes and the clinical parameters used to monitor their health and ulceration risk.

Section snippets

Patients and method

A total of seventeen (17) volunteers with no known musculoskeletal disease or diabetes (Group 1) with average age 35.0(± 5.8) years, average height 158.3(± 9.8) cm and average body mass 65(± 14) kg were recruited to pilot-test the in-vivo loading procedure and produce reference data. Moreover, thirty five (35) volunteers with type-2 diabetes (Group 2) with average age 54.8(± 9.1) years, average height 167.4(± 9.3) cm, average body mass 73(± 16) kg and average duration of diabetes 13.9(± 7.8) years

Results

The average value of the deformation rate calculated during the pilot-testing of the device was equal to 0.96 mm/s ± 0.14 mm/s which results in an inter-subject variability of 14%. The average intra-subject variability of the deformation rate was equal to 7%. The reproducibility of the mechanical measurements assessed by a test–retest procedure was 5% for tissue thickness, 4% for tissue stiffness and 8% for energy. The comparison between the results from different load cycles showed that the

Discussion

The present study aimed at investigating the correlation between the mechanical behaviour of heel-pad and common parameters measured routinely in a clinical practice (Table 1). For this purpose a custom ultrasound-based device was designed and built to load the foot and study the in-vivo mechanical behaviour of plantar soft tissues. A linear array ultrasound probe with a relatively big foot-print (≈ 6 cm2) was connected in series with a dynamometer and mounted on a rigid frame. The instrumented

Acknowledgments

Funding from DiabSmart project is acknowledged. DiabSmart project was funded by the European Commission, Grant Agreement Number 285985, under Industry Academia Partnerships and Pathways (FP7-PEOPLE-2011-IAPP). This project has a focus on development of a new generation of Diabetic footwear using an integrated approach and Smart materials.

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    Competing interests: None declared.

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