Design of an Ankle Foot Orthotic
Christopher Sullivan
1
Abstract
The Ankle Foot Orthotic (AFO) has been around for centuries, created to substitute functionality
of an ankle that had been lost to injury or disease. A common reason a patient might be prescribed an
AFO is a condition called foot-drop, a disorder involving lack of ability to dorsiflex the foot. Foot-drop
can be caused by many factors, but most have to do with diseases and injury to the nervous system.
People who have had a stroke, for example, may retain the ability to “push off,” but have no ability to
raise their toe during swing. Hard plastic AFOs that hold a patient’s foot in a neutral position are the
current standard for combating foot-drop. Unfortunately the restrictive nature of the AFO can cause
unnatural movements in the patient’s stride. The purpose of this project is to create a prototype active
AFO that will still support the foot, but aid in the ability to regulate rotation of the ankle during sections
of the gait cycle. Adding the ability to rotate to the brace should increase the comfort level of the brace,
while still replacing the lost functionality of the ankle. To accomplish this, a prototype will be created,
and its ability to support the foot will be assessed in both lab and in clinical testing.
2
Background information:
Foot Drop
Foot drop is only a symptom; it is the inability to dorsiflex the foot. Without dorsiflexion the foot
can drag on the ground which inevitably leads to tripping and over exertion of the hip and knee in
attempts to compensate for the new height that the foot must reach to clear the ground. Foot drop can
be caused by many different diseases, such as polio, multiple sclerosis, cerebral palsy, and stroke (1), all
of which effect neurological functions or are muscular disorders. There are 6.5 million stroke survivors
each year and out of them 20% suffer from a lasting symptom of foot drop (2).
Basics of the Gait Cycle
Walking is the cyclical motion of our legs as we shift our weight from one foot to the other in an
attempt to make a forward progression. This cyclical process as viewed from a single foot is called the
gait cycle. The gait cycle can be split into two major functions; the stance phase, where the foot is
initially planted and then pushes off; and the swing phase, where the toe is pulled up towards the shin
and hovers over the ground while swinging to its next destination (3). The foot has two kinds of motion
during this cycle, dorsiflexion and plantar flexion. Dorsiflexion is anytime the foot is being brought closer
to the shin; plantarflexion is any movement when the foot is being pointed towards the ground (3).
The gait cycle, although complicated, has been studied extensively. The average angles of the
limbs have been compiled in many different papers (4, 5, 6, 7) using many different techniques. Data
from (4) was collected by recording the movements of a subject who had been outfitted with an
assortment of markers indicating joint angles. The angle of the pelvis was taken with respect to the
ground and all other angles were measured relative to the pelvis angle, allowing for any angle to be
determined with respect to the ground. This data has been used for initial calculations of the torque that
is generated by the center of mass of the foot, without inertial forces.
Stance
Initial DoubleLimb Support
Periods
% of
Cycle
Foot
Strike
Swing
Second DoubleLimb Support
Single-Limb
Stance
Opposite
Toe-Off
Opposite
Foot Strike
Initial
Swing
Toe-Off
62%
0%
Figure 1. Gait Cycle Graphic Explanation
3
Foot
Clearance
Terminal
Swing
MidSwing
Tibia
Vertical
Foot
Strike
100%
Hip Flexion Extension θH
Pelvic Tilt θP
Angle Data
Opposite Toe-Off
Opposite Foot Strike
Toe-Off
20
10
Angle Data
Opposite Toe-Off
Opposite Foot Strike
Toe-Off
60
Angle(Degrees)
Angle (Degree)
30
40
20
0
0
0
20
40
60
80
0
100
20
60
80
Angle Data
Opposite Toe-Off
Opposite Foot Strike
Toe-Off
30
60
Angle (Degree)
Angle Data
Opposite Toe-Off
Opposite Foot Strike
Toe-Off
40
20
10
-10
0
20
-30
0
0
20
40
60
100
Plantar Dorsi Flexion θF
Knee Flexion-Extension θK
Angle (Degree)
40
% of Cycle
% of Cycle
80
40
60
80
100
% of Cycle
100
% of Cycle
Angle of Foot to Ground θBC
Torque at the Ankle TR
1.00
-15
-35
0.80
0
20
40
60
Angle Data
Opposite Toe-Off
Opposite Foot Strike
Toe-Off
80
100
Torque (Nm)
Angle (Degree)
5
-55
-75
0.60
Torque Data
Opposite Toe-Off
Opposite Foot Strike
Toe-Off
0.40
0.20
0.00
% of Cycle
0
20
40
60
% of Cycle
Figure 2. Gait data plots (13)
4
80
100
Gait Mechanics
An initial look at a simple free body diagram of the foot has been conducted, with the purpose of
benchmarking the amount of torque needed by the ankle to keep the foot aloft. Joint angle data from
(4), specifically the pelvic (the only angle that is
taken with respect to ground), the hip, the
knee, and the foot angles (θP, θH, θN, and θF,
respectively) were used to calculate the angle
of the foot relative to ground. The Center of
Gravity of the foot was calculated from
anthropometric data (8). In combination with
the angle of the foot with respect to ground;
the greatest moment that is required to
support a normal average foot (8) during the
swing cycle was calculated to be .90 N·m. This
initial analysis does not include inertial effects,
and is just a static analysis. The next phase of
Figure 3. Angle explanation
the project will involve a dynamic analysis of the leg and
foot. More data is needed about the patent to determine
anything else about the reactions that an AFO would feel
when the foot is on the ground, but this is negligible for
now because in our design the AFO will be free to move
Figure 4. FBD of the Foot
during the stance phase.
Commercially available AFOs
Only in the last two decades have there been real strides made in full
replacement of the function of the limb, with a solid brace. These braces are
called Ankle Foot Orthotics (AFOs). Most commercially available AFOs are passive,
in that they only support the foot and add no energy to the system. Most are
5
Figure 5. Hard Plastic AFO (9)
made from thermosetting plastics and are molded to fit the patent’s own leg. These AFOs are light and
rigid. This rigidity has been identified as a possible hindrance for patients to adopt their new AFOs, as it
can cause un-natural gaits by not emulating the
movements of a healthy ankle.
In addition to rigid orthotics there are also other
solutions that attempt to give small shocks (5V max) to
the peroneal nerve, causing dorsiflexion (10). This form of
therapy is commonly known as Electrical Stimulation (10).
Electrical Stimulation is considered to be a viable
replacement for the AFO (10), but the devices are not
without their problems; they are expensive, considered
Figure 6. Bioness Knee and Foot Electrical Simulation Device (1)
an invasive technology, and the long term effects of
electrical stimulation are unknown. These devices are also contraindicated in many patients (11). For
example, the peroneal nerve is not always in an appropriate spot where electrical stimulation could
work without discomfort, electrical stimulation should not be used on anyone with a heart condition or
any kind of pacemaker, and the leg itself that is being stimulated to should not have any recent fracture
or dislocation. While these types of devices are considered active AFOs, they are outside the planned
scope of this project (11).
Recently Investigated AFOs
Active AFOs that have been created for
research are abundant, but most have been
hindered by their need to be tethered to a
computer or external power supply, and they are
usually too complicated to bridge the gap as a
commercially viable alternative to the inherently
simple passive AFOs.
Figure 7. i-AFO Unit (12)
An “i-AFO” was constructed in 2010 by researchers at Yamagata University, in Japan (12). Its
purpose was to better control the gait of a patient who had flaccid paralysis of the ankles. “i-AFO” used
a rotational breaking system to variably dampen the system. This particular AFO was in its 3rd
generation and was still too bulky to fit into an unmodified shoe. But it was still useful in that it helped
to show the advantages that can be achieved by dampening alone.
6
In 2008 A more complex AFO was created by
Svensson, and Holmberg from Halmstad University. The AFO
was created to help patients walk up an inclined surface,
stairs, or flat ground. They used a magneto rheological type
dampener. This is type of dampener is variable, that provides
various levels of dampening based of a voltage. During the
swing phase the AFO would lock up holding up the foot, and
Figure 8. Variable Dampening AFO for Inclined Surfaces (13)
during the stance phase the AFO would release its hold. This AFO was important because its goal was to
recreate a normal gait cycle, even when being used on inclined surfaces. The power supply for this AFO
is not mentioned in the paper, so the practical effectiveness of this kind of damper is hard to measure.
Air muscles are not new, but their excellent strength to weight ratio has
renewed interest in the technology during recent years. In 2005 air muscles
were used by university of Michigan researchers to create an AFO (14). It
tackled the problem of plantar flexion, which is the motion opposite typical foot
drop. The AFO was successful in that it did generate plantar flexion but it was
never tested on patients and the AFO had to be tethered because of
computational and air supply limitations. The AFO was later adapted to include
dorsiflexion, but the added air muscles made it impossible to wear a shoe at all.
Researchers at UIUC recently developed an AFO that used a bladder to
Figure 9. Air Muscle AFO (14)
generate all the air pressure needed for the AFO calling it the Pneumatic
Power Harvesting Ankle-foot Orthosis (15). This AFO used a very small
piston to mechanically lock the foot into a preferable position. This AFO
is self-contained, and runs purely on the mechanical action of walking to
trigger the different states that it had. Unfortunately the design was
bulky and there was no variability to it: the device was either on or off.
Models of the foot are invaluable in the design of an AFO. Using
a simplified 2-D approach, approximate models of the ankle-foot joint,
Figure 10. Pneumatic Power Harvesting AFO (15)
as well as simple actuators, have been made in the past. The effects of drop foot on the gait cycle have
also been simulated. By generating an accurate model of the injured system it should be possible to add
on simplified models of an AFO. The effects on the gait could be predicted. So far this has only been
done to illustrate how a pre decided actuator would have to work, and has not been used for
comparison. (3)
7
Gap in Research
The research that has been done in the past relied heavily on an assumed actuator, and because
of this there has been no investigation to seek the best actuator for the AFO, or the best means to apply
this actuation. This project will go through a process similar to senior design; initially, interviews with
medical professionals and patients will be held, and after this modeling of a healthy leg and an injured
leg can be done. The information gathered from the interviews and from the simulations will help to
determine the appropriate actuator for the job.
There has yet to be a serious look into the different pros and cons of the actuators as applied to
an AFO, such as other forms of dampeners, Air muscles, variable material stiffness, combination springs
and dampeners, or even electric motors. Couple these with self-generating power from, either a bladder
or some kind of piezoelectric plate, harvesting of the energy of the stride can be carried out. Opening
the design process to more than one possibility promise to provide an AFP that is self-contained,
untethered, and can fit inside a shoe.
Characterization of Actuators and Energy Storage/Conversion Devices
In the past it has been necessary to pick actuators based on their inherent abilities in
comparison to each other. Tables and plots of different physical and mechanical properties, as well as
operating characteristics, of these devices are readily available, and excerpts have been provided (16,
17, 18, 19). This data is very important, but are only useful when coupled with the needs specified by
the customer. The devices identified as being key to the system architecture are energy storage, energy
conversion, and an actuator. The important indices to characterize these devices are power density,
strain, density, and efficiency. Some of the power generation devices, such as piezoelectric elements are
fabricated as thin plates, and their energy density is calculated in terms of energy generated per unit
area.
Efficiency
Density
Energy or Power Density
Energy Storage
Energy Conversion
Actuator
𝜂𝑠 = 𝐸𝑜𝑢𝑡 ⁄𝐸𝑖𝑛
𝜂𝑐 = 𝑊𝑜𝑢𝑡 ⁄𝑊𝑖𝑛
𝜂𝑎 = 𝑊𝑜𝑢𝑡 ⁄𝑊𝑖𝑛
𝜌𝑠 = 𝑚⁄𝑉
𝜌𝑠 = 𝑚⁄𝑉
𝜌𝑎 = 𝑚⁄𝑉
𝐸𝜌 = 𝐸𝑜𝑢𝑡 ⁄𝑚
𝑃𝜌 = 𝑊𝑜𝑢𝑡 ⁄𝑚
𝑃𝜌 = 𝑊𝑜𝑢𝑡 𝜌𝑎 ⁄𝑚
𝜖 = ∆𝐿⁄𝐿𝑚𝑖𝑛
Strain
8
Energy Storage and Conversion
Potential Energy
Storage
Energy
Density
50-100 Wh/kg
40-175 Wh/kg
150-250 Wh/kg
140-300 Wh/kg
Efficiency
85-90%
Lead Acid battery
90-95%
NiCad battery
90-95%
NiMH battery
90-95%
Li-ion battery
40-70%
Compressed Air
2.78 Wh/kg
90%
Super-capacitor
Table 1. Data from tables was collected from (16,17)
Power
Generation
Electromagnetic
generator
Piezo element
Power
Density
Efficiency
200-300 W/kg
20-70%
2
.01-.02 W/m
2
0.17 W/km
.5-5%
2-5%
Seebeck elements
Table 2. Data from tables was collected from (16,17)
Actuators
Actuator Type
Muscle Power
Power
Density
0.5 MW/m3
Actuator
Strain
Density
Efficiency
.3-.7
1000-1100 kg/m3
20-25%
Piezo Electric Low
Strain
100-1000 MW/m3
5E-6 - 3E-5
2600-4700 kg/m3
98%
Piezo Electric High
Strain
90-500 MW/m3
5E-5 - 2E-4
7500-7800 kg/m3
90-98%
100-700 MW/m3
.0006-.002
6500-9100 kg/m3
80-98%
.7-100 MW/m3
.007-.07
6400-6600 kg/m3
1-2%
.01-.04 MW/m3
.1-.4
3800-4400 kg/m3
50-80%
3
.1-1
3
30-40%
5 MW/m3
.1-1
180-250 kg/m3
90-98%
3
32-50%
Magnetostrictor
Shape memory
alloy
solenoid
hydraulic piston
pneumatic piston
500 MW/m
3
1 MW/m
0.35
Air muscle
Table 3. Data for table was collected from (16, 17, 18, 19)
9
1600-2000 kg/m
500-1000 kg/m
Budget
The funding for the project has already been acquired. A full time Co-op Block during the summer of
2011 and part time work have been approved for this project. $1000 has been allocated for the
construction and testing of a prototype. This money will be put towards any materials that are needed
to construct a prototype, as well as any specialized manufacturing that might need to take place (due to
the natural geometry involved in the human body). External sensors not meant to be permanently
attached to the AFO can be purchased; these sensors will be necessary for almost any kind of clinical
testing that could take place.
Statement of Work
The Following is a statement of the work that will be completed throughout the course of this project.
1. Customer needs must be set, and associated with design criteria. This information will be
gathered by interviewing local medical professionals, orthopedic specialists, and patients.
2. An analytical model of the foot is to be created. The model is going to be used for
benchmarking, and approximation. The model should help push the design process in an
appropriate direction.
3. Create a System level design, of what the AFO needs to accomplish, and how each part of the
design could be tested.
4. Follow up with stakeholders to receive feedback on initial design ideas, as well as informing the
stakeholders of progress made so far. Feedback will be incorporated as the project moves into
the detailed design phase.
5. Perform detailed design and analysis of the AFO to be constructed.
6. Present final design to stakeholders. This will constitute, at a minimum, a CAD package, bill of
materials, and a proposed test plan. The test plan should encompass how the design will be
qualified, and how it will be tested in a clinical setting.
7. The AFO will be built and tested in the lab according to the test plan.
8. Once lab test results are within specifications, the AFO will be tested with a group of patients
with foot drop, under the supervision of their clinical providers.
10
Timeline:
Winter 2010

Project identified

Initial literature search Begun

Thesis Proposal Started

Rapid Prototyping Taken
Spring 2011 9hr/week

Finish up Undergraduate Required Courses

Thesis Proposal Finished

Finish Literature search

Quasi-Static modeling of foot
Summer 2011 full-time

Co-Op working on project

Dynamic modeling of foot

Interviews Patients and Clinicians
o

Proof of Concept prototypes created
o

Information gathered and added to model
Preliminary Design
o

Review of Data Collected
Present design concept to stakeholders (patients, clinicians, and thesis committee)
Detailed Analysis and Design
o
Presentation to stakeholders (patients, clinicians, and thesis committee)

Building AFO Started

Writing Thesis
Fall 2011 9hr/week

Take Senior Design 1, Math 1, and Controls Systems

Finish Building AFO

Bench Testing of AFO

Writing Thesis
Winter 2011 9hr/week

Take Senior Design 2, Math 2
11

Finish Bench Testing

Clinical Testing

Compile Work

Writing Thesis
Spring 2012 9hr/week

Finish Clinical Testing
o
Compare Results with Model and Bench Testing

Finish Writing Thesis

Thesis Defense
12
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