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.

1.
Jeffcoate
,
W. J.
, and
Harding
,
K. G.
, 2003, “
Diabetic Foot Ulcers
,”
Lancet
0140-6736,
361
(
9368
), pp.
1545
1551
.
2.
Gefen
,
A.
, 2003, “
Plantar Soft Tissue Loading Under the Medial Metatarsals in the Standing Diabetic Foot
,”
Med. Eng. Phys.
1350-4533,
25
, pp.
491
499
.
3.
Gefen
,
A.
, and
Linder-Ganz
,
E.
, 2004, “
Diffusion of Ulcers in the Diabetic Foot is Promoted by Stiffening of the Plantar Muscular Tissue Under Excessive Bone Compression
,”
Orthopade
0085-4350,
33
, pp.
999
1012
.
4.
Yarnitzky
,
G.
,
Yizhar
,
Z.
, and
Gefen
,
A.
, 2006, “
Real-Time Subject-Specific Monitoring of Internal Deformations and Stresses in the Soft Tissue of the Foot: A New Approach in Gait Analysis
,”
J. Biomech.
0021-9290,
39
, pp.
2673
2689
.
5.
Gefen
,
A.
,
Megido-Ravid
,
M.
,
Itzchak
,
Y.
, and
Arcan
,
M.
, 2000, “
Biomechanical Analysis of the Three Dimensional Foot Structure During Gait: A Basic Tool for Clinical Applications
,”
ASME J. Biomech. Eng.
0148-0731,
122
, pp.
630
639
.
6.
Hertz
,
H.
, 1882, “
Uber die Beruhrunng Fester Elasticher Korper
,”
J. Reine Angew. Math.
0075-4102,
92
, pp.
156
171
.
7.
Dingwell
,
J.
,
Ovaert
,
T.
,
Lemmon
,
D.
, and
Cavanagh
,
P. R.
, 1997, “
Analytical Approaches to the Determination of Pressure Distribution Under a Plantar Prominence
,”
Clin. Biomech. (Bristol, Avon)
0268-0033,
12
, pp.
S14
S15
.
8.
Wang
,
Y. C.
, and
Lakes
,
R.
, 2002, “
Analytical Parametric Analysis of the Contact Problem of Human Buttocks and Negative Poisson’s Ratio Foam Cushions
,”
Int. J. Solids Struct.
0020-7683,
39
, pp.
4825
4838
.
9.
Hills
,
D. A.
,
Nowell
,
D.
, and
Sackfield
,
A.
, 1993,
Mechanics of Elastic Contacts
,
Butterworths
,
London
, pp.
187
197
and
203
204
.
10.
Huber
,
M. T.
, 1904, “
Zur Theorie der Beruhrung Fester Elastischer Korper
,”
Ann. Phys.
0003-3804,
319
(
6
), pp.
153
163
.
11.
Spears
,
I. R.
, and
Miller-Young
,
J. E.
, 2006, “
The Effect of Heel-Pad Thickness and Loading Protocol on Measured Heel-Pad Stiffness and Standardized Protocol for Inter-Subject Comparability
,”
Clin. Biomech. (Bristol, Avon)
0268-0033,
21
, pp.
204
212
.
12.
Zhang
,
M.
,
Zheng
,
Y. P.
, and
Mak
,
A. F. T.
, 1997, “
Estimating the Effective Young’s Modulus of Soft Tissues From Indentation Test-Nonlinear Finite Element Analysis of the Effects of Friction and Large Deformation
,”
Med. Eng. Phys.
1350-4533,
19
(
6
), pp.
512
517
.
13.
Spears
,
I. R.
,
Miller-Young
,
J. E.
,
Waters
,
M.
, and
Rome
,
K.
, 2005, “
The Effect of Loading Conditions on Stress in the Barefooted Heel Pad
,”
Med. Sci. Sports Exercise
0195-9131,
37
(
6
), pp.
1030
1036
.
14.
White
,
S. C.
,
Yack
,
H. J.
,
Tucker
,
C. A.
, and
Lin
,
H.-Y.
, 1998, “
Comparison of Vertical Ground Reaction Forces During Overground and Treadmill Walking
,”
Med. Sci. Sports Exercise
0195-9131,
30
(
10
), pp.
1537
1542
.
15.
Dingwell
,
J. B.
,
Curumano
,
J. P.
,
Cavanagh
,
P. R.
, and
Sternad
,
D.
, 2001, “
Local Dynamic Stability Versus Kinematic Variability of Continuous Overground and Treadmill Walking
,”
ASME J. Biomech. Eng.
0148-0731,
123
, pp.
27
32
.
16.
Jacob
,
S.
, and
Patil
,
M. K.
, 1999, “
Stress Analysis in Three-Dimensional Foot Models of Normal and Diabetic Neuropathy
,”
Front Med. Biol. Eng.
0921-3775,
9
, pp.
211
227
.
17.
Segal
,
A.
,
Rohr
,
E.
,
Orendurff
,
M.
,
Shofer
,
J.
,
O’Brien
,
M.
, and
Sangeorzan
,
B.
, 2004, “
The Effect of Walking Speed on Peak Plantar Pressure
,”
Foot Ankle Int.
1071-1007,
25
(
12
), pp.
926
933
.
18.
Burnfield
,
J. M.
,
Few
,
C. D.
,
Mohamed
,
O. S.
, and
Perry
,
J.
, 2004, “
The Influence of Walking Speed and Footwear on Plantar Pressure in Older Adults
,”
Clin. Biomech. (Bristol, Avon)
0268-0033,
19
, pp.
78
84
.
19.
Gefen
,
A.
, 2003, “
The In Vivo Elastic Properties of the Plantar Fascia During the Contact Phase of Walking
,”
Foot Ankle Int.
1071-1007,
24
(
3
), pp.
238
244
.
20.
Fischer
,
A. C.
, 1999, “
The Hertzian Contact Surface
,”
J. Mater. Sci.
0022-2461,
34
(
1
), pp.
129
137
.
21.
Gefen
,
A.
, 2007, “
Risk Factors for a Pressure-Related Deep Tissue Injury: A Theoretical Model
,”
Med. Biol. Eng. Comput.
0140-0118,
45
, pp.
563
573
.
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