Offloading Device for the Reduction of Foot Complications in Patients with Diabetes
Rod Aliabadi1, Patrick Gonzales1, Michael Osborne1, Colin Smith2
Advisors: Dr. William Dougherty3, Dr. Paul King1, Dr. Kevin Robinson4, Richard Brown, C.O., C. Ped.5
Department of Biomedical Engineering1, Department of Mechanical Engineering2, Department of Plastic Surgery3, Vanderbilt University, Nashville, TN, Department of Physical Therapy4, Belmont University, Nashville, TN, Nashville Orthotic and Prosthetic Services5, Nashville, TN
Design Description
Problem Statement
Self-customizable Perforated Orthotic for Patients with Diabetes
The physiological effects of diabetes can lead to excessive
vertical plantar pressure during normal gait. This pressure
can result in ulcerations in the feet of patients and may lead
to amputation. Devices to reduce this pressure and prevent
ulceration are often very expensive and have limited
versatility. Among non-customized products on the market
designed to reduce the occurrence of foot ulcerations in
diabetic patients, none are a viable alternative to laborintensive customized inserts. This project seeks to develop
such an alternative.
The device is composed of four heat-moldable material layers. Using innovative perforation-based
customizable features each patient will be able to adjust the insert to meet the specific needs of his or
her unique feet without intensive labor costs. After preliminary testing it was decided to use a quadlam (four layer) design consisting of Thermo-cork (uniformly perforated with diamonds 9/16” per
side), Multi-cell Puff (specifically perforated with circles in areas of high pressure), PORON, and pink
Plastazote in order from the bottom up. The top three layers are the three materials that compose the
diabetic tri-lam that is used in existing products for offloading and heat forming ability. The cork
provides unique offloading that is not available on the market without customization.
Material Selection
Material Offloading of First Metatarsal Head
Perforation Shape Determination: Static Test
Diabetes and Foot Complications
Diabetes affects nearly 20.8 million Americans and is
characterized by insufficient insulin production (Type
I) or insufficient insulin action at the cellular level
(Type II). There is no known cure for the progressive
disease, but patients can be treated by insulin therapy
to suppress many of the symptoms. As the disease
progresses, patients may experience neuropathy, or
degeneration of the peripheral nervous system. The
resultant loss of sensation is most severe in the
peripheral limbs, signifying the loss of muscular
innervation and a loss of flexion in muscles of the
feet. This results in deformation of the skeletal
system, or a loss of skeletal structure, that can lead to
areas of high plantar pressure and ulceration. Ulcers
can also form as a result of ischemia, a reduced level
of oxygen in the blood, because of the reduced
capacity for healing that results from decreased
circulation. Thus, a small injury or infection due to
trauma can rapidly progress into a dangerous ulcer
without any patient awareness. Preventative devices
typically consist of customized insoles that conform
perfectly to maintain total contact with a patients foot
and incorporate recesses in areas of deformation to
redistribute the pressure and prevent the development
of ulcerations.
Cost to Manufacture
In order to accurately calculate the manufacturing cost of our proposed product, the group assessed the costs
of producing 2400 pairs of inserts. Expenses from materials, labor, and perforating the Thermo-cork and the
Multi-cell Puff are exhibited in the table below and analyzed to determine an estimate for the market price.
Figure 1 shows the typical areas of high pressure
that result in ulceration in the diabetic foot.
a)
b)
Part
Qty
Price Each ($)
Total ($)
Thermo-cork sheets (Acor)
100
156
15600
Dual laminate sheets (Acor)
100
120
12000
Multi-cell puff sheets (Acor)
100
48
4800
Glue (Rubber Cement)
25
2.50
67.50
Perforations (H and K perforations)
200
25
5000
Labor costs (at 6 pairs/hr)
400
15/hr
6000
Figure 3 shows the basic design of the prototype and the reasoning behind it. On the left is our final design shown from the bottom and
the top views. The perforations on the bottom allow the customization demonstrated on the heel in the photos at the top right while
maintaining total contact with the foot. The bottom left shows a flow chart that describes the current process of insole customization by a
certified professional and highlights the steps that can be made more efficient in order to reduce costs.
Evaluation of Inserts-Vertical Plantar Pressure vs. Time
c)
d)
Figure 2 shows typical diabetic foot deformations and
ulcerations. a) a severe ulcer on the first metatarsal
head b) foot drop, which prevents patients from
being able to hold their feet up c) hammer toe, which
increases pressure on a toe after it curls towards the
foot d) Charcot foot, which results in the loss of arch
and increased pressure in the center of the foot.
Ideal Design
The ideal design should have minimal thickness (<1/2”) to fit in any standard extra-depth shoe. It
should be as durable (4 month useful life) and as effective at offloading as current custom inserts,
but it should be customizable at a cost of less than $100 per pair. The insert must be able to be
changed over time to compensate for morphological change and be available in all sizes.
Our Design Process
The group consulted experts and literature to learn about the effects of diabetes and the market for a
device to reduce plantar pressure. The second phase involved brainstorming and using Innovation
Workbench. After preliminary material testing, a prototype was designed and tested before being
modified for final testing and comparison to existing devices. Safety and costs were considered.
Advantages, Disadvantages, and Costs of Example Competitor Products
Acor QuikFit Pro
• Total contact, ideal materials for
diabetics per literature
• Require sizing and modification;
do not provide offloading; not
durable; extra cost to modify
• $25/pair for 3-4 month use
Custom Insert
• Maintains total contact and
offloads areas of high pressure,
advocated by research.
• Intensive professional labor
• $260+/pair for 4 month use
Four custom inserts were made with the help of a
certified pedorthist, a foot device specialist. The left
insert was made for a normal foot, and the right was
made to compensate for a loaded first metatarsal
head. Using the FSM, we tested the custom and New
Balance inserts to determine which materials
provided optimal offloading. Two nickels were
placed under the left first metatarsal head to simulate
loading. It was verified in the top figure that the Trilam inserts that are advocated in literature provide the
lowest maximum pressure. The base materials did
not vary much. Thermo-cork was chosen because it
would be easiest to perforate and would provide the
most structure. A second test explored different
perforation schemes in tri-lam with static testing.
Based on the results seen in the bottom graph, a
general scheme was chosen for the base with a
specific scheme for the second layer.
Acor QuikFormables
• Total contact; customizable,
choice of materials
• Labor intensive construction;
requires sizing
• Varied prices for 4 month use
Gel or Air Filled Insoles
• Conforms to foot morphology
• Short useful life; no offloading;
not total contact; foot can move
in shoe; puncture risk, little aid
•$15/pair for 1 month use
The insert was designed to be 3/8” at the toes
and was shaped to fit in any standard extradepth shoe of a specified size. The Thermocork layer was ground to about 1/16”. The
Multi-cell Puff was 1/16”. The other layers
were 1/8”. The materials that were used are
known to have a useful life of at least four
months in custom models. Using a general
perforation scheme in the cork allows
offloading at any site, and specific perforation
of the Puff compensates in areas of typical
high pressure. In areas of typical high
pressure, like across the metatarsal heads, the
diamonds can be split into equilateral
triangles to provide more specific offloading.
The figure assesses the performance of the prototype, a custom insert of the same materials, and a
generic insert during free selected gait. This data is from a region of interest beneath the first
metatarsal head that had been loaded to simulate a diabetic foot. The prototype with removed cork
(green) reduced vertical plantar pressure as effectively as the custom insert (blue) but seemed to
worsen without Puff (red). Perforations present an alternative to customization for pressure reduction.
F-Scan Vertical Plantar Pressure Testing
Test 1L Max
50
10
40
20
30
30
20
40
10
50
0
60
5
10
15
20
The F-Scan Mobile (FSM) system by Tekscan, Inc., shown to
the left, allowed the wireless measurement of vertical plantar
pressure during free selected gait. The FSM used a 0.15 mm
thick resistive transducer to obtain pressures between the foot
and insert with a spatial resolution of 0.25 cm2. The data
acquired was loaded into Matlab and analyzed using a
program created to determine areas of peak pressure in the
pressure map, shown bottom left, and allow the analysis of
regions of interest over the time of the gait. The FSM data
was used to determine the ideal material, ideal perforation
design, and the efficacy of the final prototype.
Sum
Cost of Production per Pair
Estimated Market Price per Pair
43467.50
$18.11
$35
This cost is about 13% of the cost for custom inserts and only $10 more per pair than the less robust QuikFit inserts.
Market
In the United States alone, approximately 1.25 million diabetic patients can be expected to
develop peripheral ulcerations related to their disease and spend approximately $30,000 a year on
treatment, leading to a multi-billion dollar domestic market to prevent such maladies. The device
would also comply with current Medicare legislation to supply customized footwear to all needy
patients that suffer from diabetes and its complications.
Safety Issues
This device is a Class I Device according to the FDA, making approval likely. Concerns include
the length of useful life, which could be tested, and improper modification, which could be
minimized by warnings and professional training.
Social Impact
These inserts utilize materials typically used in customized inserts to address neuropathy,
calluses, pre-ulcerative conditions, and ulcers. According to Richard Brown, C.O., C. Ped, “The
inserts balance and correct joint alignment to optimize gait and act as a critical first line of
defense to prevent conditions that occur with patients that suffer the devastating complications of
diabetes. The layers with perforations allow clinicians to easily modify the inserts to offload
specific areas of the foot without the need for an extensive lab.” This design has immense
application in rural and international markets with limited access to the professionals and
facilities needed to make custom insoles, and the reduced cost makes effective inserts available
to more low-income patients than existing technology.
Conclusion
Our group has met its goal of designing an inexpensive, versatile device for reducing vertical
plantar pressure and the incidence of peripheral ulcerations. This product has great potential to
provide an alternative to customization for needy patients without proper health care access. We
have tested the design using an F-Scan system and have planned a manufacturing process. From
this work, the device could potentially be manufactured and marketed; however, the project could
be continued to design a “smart” insert that can monitor vertical pressure during its use.
Acknowledgements
Nashville Orthotic & Prosthetic Services
Department of Physical Therapy Dr. Naji Abumrad, Michele Frisbie,
Horace Watson
Dr.