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Research Papers

# The Diabetic Foot Load Monitor: A Portable Device for Real-Time Subject-Specific Measurements of Deep Plantar Tissue Stresses During Gait

[+] Author and Article Information
Eran Atlas

Department of Biomedical Engineering, Tel Aviv University, Tel Aviv 69978, Israel

Ziva Yizhar

Department of Physical Therapy, Tel Aviv University, Tel Aviv 69978, Israel

Amit Gefen1

Department of Biomedical Engineering, Tel Aviv University, Tel Aviv 69978, Israelgefen@eng.tau.ac.ul

1

Corresponding author.

J. Med. Devices 2(1), 011005 (Mar 10, 2008) (10 pages) doi:10.1115/1.2891241 History: Received May 31, 2007; Revised February 11, 2008; Published March 10, 2008

## Abstract

Elevated stresses in deep plantar tissue of diabetic neuropathic patients were associated with an increased risk for foot ulceration, but only interfacial foot pressures are currently measured to evaluate susceptibility to ulcers. The goals of this study were to develop a real-time patient-specific plantar tissue stress monitor based on the Hertz contact theory. The biomechanical model for stress calculations considers the heel and metatarsal head pads, where most ulcers occur. For calculating stress concentrations around the bone-pad interface, plantar tissue is idealized as elastic and incompressible semi-infinite bulk (with properties measured by indentation), which is penetrated by a rigid sphere with the bone’s radius of curvature (from X-ray). Hertz’s theory is used to solve the bone-pad mechanical interactions, after introducing correction coefficients to consider large deformations. Foot-shoe forces are measured to solve and display the principal compressive, tensile, and von Mises plantar tissue stresses in real time. Our system can be miniaturized in a handheld computer, allowing plantar stress monitoring in the patient’s natural environment. Small groups of healthy subjects $(N=6)$ and diabetic patients $(N=3)$ participated in an evaluation study in which the differences between free walking and treadmill walking were examined. We also compared gait on a flat surface to gait on an ascending/descending slope of $3.5deg$ and when ascending/descending stairs. Peak internal compression stress was about threefold greater than the interface pressure at the calcaneus region. Subjects who were inexperience in treadmill walking displayed high gait-cycle variability in the internal stresses as well as poor foot loading. There was no statistical difference between gait on a flat surface and gait when ascending/descending a slope. Internal stresses under the calcaneus during gait on a flat surface, however, were significantly higher than when ascending/descending stairs. We conclude that the present stress monitor is a promising tool for real-time patient-specific evaluation of deep tissue stresses, providing valuable information in the effort to protect diabetic patients from foot ulceration. Clinical studies are now underway to identify which stress parameters can distinguish between diabetic and normal subjects; these parameters may be used for establishing injury threshold criteria.

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Copyright © 2008 by American Society of Mechanical Engineers
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## Figures

Figure 1

Definition of the contact problem between a rigid sphere and an elastic surface

Figure 2

An example of the assumptions and definitions on a lateral X-ray of human calcaneus. The yellow line is the plantar aspect of the calcaneus. The ROI is at the around the contact point.

Figure 3

Technological concept of real-time subject-specific stress monitor

Figure 4

The physical apparatus for model validity

Figure 5

(a) Phantom; (b) phantom during loading by Instron machine; (c) comparison graph between phantom measurements and model prediction of internal compressive stress

Figure 6

Portable real-time subject-specific stress monitor

Figure 7

Example of the stress monitor distribution maps for Subject 1 (right) and Subject 2 (left)

Figure 8

Model results of peak principal compression stress at the heel pad during gait in Healthy Subjects 1, 4, and 5 and Diabetic Subject 2

Figure 9

Comparison between gait pattern while free walking on (a) level surface to walking on (b) treadmill, for Subjects 1, 3, and 4. It can be seen that Subject 3 did not load his feet properly while treadmill walking and Subject 4 dragged her feet during treadmill walking.

Figure 10

Example of comparison between gait types in Subject 3 for superficial stress (top), peak internal shear stress (middle), and peak internal von Mises stress (bottom)

Figure 11

Explaning how the mean value of stress is computed. The ROI is devided to matrix of nodes and element. Stresses are computed analytically at each node (N) and we want to calculate the value of the stresses at the center point of the each element.

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