Biomedical Engineering Reference
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should also redistribute and reduce the excessive plantar pressure or shear from the vulnerable areas
to more tolerable areas by enabling total contact with the plantar foot of the patient. In addition, foot
orthoses should support and align the joints of the foot in a proper position for weight bearing and
propulsion and accommodate or correct foot deformities.
Using our developed FE model, flat and custom-molded insole supports were simulated. The
custom-molded insole was made from the unloaded shape of the subject's bare foot. The bare-
foot shape was obtained by an impression cast with the subject sitting in the neutral position. The
positive cast was digitized and imported to SolidWorks to form the solid models of the insole.
A 5-mm-thick insole was meshed into three-dimensional brick elements with a Poisson ratio of
0.4 and a varied Young's modulus of 0.3 (soft), 1 (firmer), and 1000 MPa (rigid) for simulation of
(1) open-cell polyurethane foams, such as Professional Protective Technology's PPT material; (2)
high-density ethylene vinyl acetate; and (3) polypropylene materials, respectively. A very rigid,
1-mm-thick bottom layer was used to simulate the ground support and to facilitate the application of
concentrated ground reaction forces. Figure 1.9 shows the peak plantar pressures over three regions
of the foot during balanced standing using six different insoles.
From the FE parametric analysis of the foot orthosis, the use of a custom arch support was found
to be an important design factor for reducing peak plantar pressure. Another important factor for
reducing pressure is the use of a soft cushioning insole material. The FE model predicted a pro-
nounced inverse relationship between insole thickness and peak plantar pressure, consistent with
the experimental findings in the literature (Linge 1996). To find the optimal insole thickness one
needs to consider the issue of reduced proprioception and propulsion efficiency, which deserves
further investigation.
By and large, a custom pressure-relieving foot orthosis should provide the best total contact fit of
the plantar foot during weight bearing. The cushioning insole layer should contribute the majority
of the thickness of the foot orthosis to reduce the peak pressure. The use of extra-depth footwear
for patients with diabetes is highly recommended to comfortably accommodate a thick, cushion-
ing insole layer. The FE analysis suggested that the custom-molded total-contact foot orthosis was
effective in reducing peak plantar pressures and would play an important role in the prevention of
foot ulceration in diabetic patients.
It should be noted that the design guidelines for pressure-relieving foot orthoses were established
from FE simulations and validation of a single subject, representing individual geometrical proper-
ties. In addition, only a single instance of the stance phase of gait was simulated in a static condition
and the influence of the shoe-orthosis-foot interface was not considered. Therefore, the orthotic
performance during different load-bearing and dynamic conditions and the generalization of the
current FE simulations for the response of the general population deserves further computational
and experimental investigation.
0.3
0.25
0.2
0.15
0.1
0.05
0
F0.3
F1.0
F1000
T0.3
T1.0
T1000
Forefoot
Midfoot
Rear foot
Foot region
FIgure 1.9
Effects of stiffness of flat and custom-molded insoles on peak plantar pressure. The key indi-
cates insole type (F, flat; M, custom molded) and rigidity (0.3, soft; 1.0, firmer; 1000, rigid).
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