Biomechanical Properties of the Foot Sole in Diabetic Mellitus Patients: A Preliminary Study to Understand Ulcer Formation

Biomechanical Properties of the Foot Sole in Diabetic Mellitus Patients: A Preliminary Study to Understand Ulcer Formation

V. B. Narayanamurthy (Division of Biomedical Engineering, Department of Applied Mechanics, Indian Institute of Technology, Madras, India), Richa Poddar (Sundaram Medical Foundation, India) and R. Periyasamy (National Institute of Technology Raipur, India)
DOI: 10.4018/978-1-5225-3158-6.ch019
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Abstract

This aricle evaluates and quantifies the biomechanical properties of the foot sole like – loss of protective sensation, hardness of the foot sole and pressure distribution parameter called Power ratio (PR) and its alterations, which have a direct effect on ulcer formation. A new parameter PRS Index is developed to understand the interplay between these parameters and its role in ulcer formation. All diabetic subjects attending the Diabetic foot clinic from Dec2003 to June 2007 undergo a standard foot examination. A total of 652 diabetic patients including 57 ulcer patients are taken for our study. The biomechanical properties include loss of protective sensation (LOPS) which is measured by 10 gm Semmes Weinstein Monofilament (SWMF). Hardness of the foot sole or absence of suppleness is tested using the Durometer (ASTM-D 2240 standards). Plantar pressure measurement is done using the PedoPowerGraph(p) which measures pressure distribution parameter PR. Foot wear properties like hardness of the insole affecting the formation of plantar ulcers was also measured. The above mentioned important parameters can be measured objectively and calculate PRS index value for diabetic with history of previous ulcer patients. We found a single entity of either the PR or shore independently cannot predict the risk for ulcer formation. In this study we found new PRS index value for diabetic with history of previous ulcer patients show significant correlation (i.e. p<0.05 level) between footwear shore and PRS index for history of previous ulcer patients. No significant correlation was shown for diabetic without history of previous ulcer patients and this may be due to diabetic patients are wearing footwear randomly with different degree Shore. From the case studies we found that the PRS index values and other biomechanical parameter of the foot sole can be reversed if the patients wear proper MCR footwear with 20 degree Shore. Use of appropriate footwear has shown that these easily measurable parameters and thus prevent ulcer formation as mentioned in the earlier studies. Several methods are used previously for predicting ulceration in DM patients. But in this study the new index PRS was studied and its role in predicting ulceration. Use of appropriate footwear will reverse the hypertrophic response; this can be quantified by the PRS index. We have found that there is decrease in PRS index by proper off loading the pressure using 20-degree shore MCR footwear.
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Introduction

Diabetes mellitus (DM) is recognised as a major health problem in India. Diabetic foot ulceration is one of the well-recognised long-term sequels among 15% of diabetic patients (Pecoraro, Reiber, & Burgess, 1990). Neuropathy is the precursor of ulceration in all cases and this induces changes in the biomechanical properties of the foot sole (Thomas, Patil, Radhakrishnan, Murthy, & Parivalavan, 2003). Ulceration in the foot in diabetic patients is a constant risk factor in infected foot. A lot of protocols are available for detecting the foot at risk. Plantar pressure, hardness of the foot sole and footwear (Thomas, Patil, Radhakrishnan, Murthy, & Parivalavan, 2003; Charanya, Patil, Narayanamurthy, Parivalavan, & Visvanathan, 2004), which may have a predictive value in detecting foot at risk of ulceration. Loss of protective sensation (LOPS) to 10gm Semmes Weinstein monofilament (SWMF) has been recognised as a very well known parameter to detect the foot at risk (Bell-Krotoski, Weinstein, & Weinstein, 1993; Weinstein, 1993). However this only divides the diabetic population into two groups – those having sensation and those who do not have sensation to 10gm SWMF who have higher risk of development of ulcer formation. It does not detect the subgroup that will develop ulceration. Elevated plantar pressures have been known to be another cause of plantar ulceration. There is no absolute threshold pressure, as ulcers develop (Boulton et al., 1985; Cavanagh, Simoneau, & Ulbrecht, 1993) even below the accepted high level of peak plantar pressures. Only peak pressures have been considered in all the pressure measuring equipments (Wertsch et al., 1992), which measure foot pressure using discrete transducers. These have a 33% margin of error due to the use of discrete transducers having low spatial resolution of the pressure measurement (Lord, Hosein, & Williams, 1992). Therefore in this investigation, foot pressure distribution is recorded using PedoPowerGraph(P) system based on optical Pedobarograph principle having high spatial resolution of pressure measured using a new parameter PR. The new parameter PR measures the standing plantar pressure distribution in both the feet simultaneously. This is a more accurate method of measuring the foot pressure, because this method involves the measurement of foot pressure using PedoPowerGraph and calculating foot pressure parameter PR using actual foot pressure distribution rather than using only peak pressure. Hardness of the sole of the foot measured by Durometer (or) Shoremeter is a useful parameter (Pigassesi et al., 1999), which shows the degree of hypertrophy of the cells in response to increased plantar pressures i.e. the formation of callus. Callus formation is a hypertrophic response to increasing pressure and foot at this stage is said to be at compensated stage (Charanya, Patil, Narayanamurthy, Parivalavan, & Visvanathan, 2004; Brand, 1989). Callus therefore distributes the increased pressure in space and time so that the pressure in the sole of the foot is neutralised based on Kelvin principle. This happens up to a certain level beyond which the hypertrophic response breaks down resulting in the formation of ulcer. The specialised skin is the end organ of the foot and when ulcer forms the foot is said to have decompensated and failed. The Foot sole thickness is also a known factor in ulcer formation, which responds to sensory and motor function (Charanya et al., 2004). Measurement of foot sole thickness involves ultrasound examination, which is time consuming and expensive (Thomas, Patil, Radhakrishnan, Murthy, & Parivalavan, 2003; Charanya, Patil, Narayanamurthy, Parivalavan, & Visvanathan, 2004). All these responses are dynamic processes and correspond to the duration of diabetes, vascularity, control of diabetic status, deformities and many other factors. The final pathway is increase in pressure in an area of the foot, which is compensated by hypertrophic response and formation of callus (Thomas, Patil, Radhakrishnan, Murthy, & Parivalavan, 2003). Increasing plantar pressure and callosity have a very close relationship (Narayanamurthy et al., 2009). This is given by a new index called the PRS index (the product of the highest pressure parameter PR in the foot and the highest Shore level (S) in the foot). The hardness of the insole of the footwear used and worn by the patient at the time of clinical examination has a direct bearing on the hardness of the foot sole (Charanya et al., 2004). In this study, we have looked at the plantar pressure; hardness, sensation of the foot sole and hardness of the insole of the foot wear and also tried to find their interrelationships. PRS index seems to clearly segregate the high-risk group who can develop ulcers. Reversal of the hypertrophic response and disappearance of callus should happen with appropriate off loading footwear. Effectiveness of offloading footwear or other offloading devices should show decrease in PRS index. A representative case, which shows decrease in the PRS index with clinical documentation of disappearance of the callus, is presented. PRS indices for various groups of diabetic are presented in this study.

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