Active Ankle-Foot Orthotic: Tethered Air MuscleNathan Couper, Bob Day, Patrick Renahan, Patrick Streeter

Faculty Guide: Elizabeth DeBartolo, PhD

This material is based upon work supported by the National Science Foundation under Award No. BES-0527358. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation.

OutlineCurrent ankle-foot orthotics (AFOs) are successfully used to improve mobility in individuals with foot drop, however these devices are restrictive to the user and do not support natural gait cycles in all scenarios. AFO users note difficulty specifically when descending stairs and ramps. A natural gait cycle while descending includes a toe then heel foot strike. AFOs prevent the user from plantarflexing, resulting in a heel then toe foot strike. This leads to an awkward and unbalanced gait while descending stairs and ramps. [1]

The device developed for this project utilizes air muscle driven actuation to provide users with the necessary plantarflexion to descend stairs and ramps. It requires minimal modification to a standard AFOs, and can be easily installed or removed. The device is capable of supporting a normal pace while descending stairs and ramps and is adjustable to support individuals of various sizes and weights. The device is tethered and is designed for use in a clinical setting, and the prototype requires manual activation.

Foot DropFoot drop is the inability to dorsiflex – or raise – the foot. During the gait cycle this disorder causes the effected foot to drag on the ground, leading to trips and falls. In order to compensate for foot drop affected individuals are forced to use an unnatural gait, placing extra stress on the knee and hip joints.

Foot drop is often caused by neurological dysfunction stemming from injury, or disease such as stroke, or MS. Foot drop is manifested in many ways; some are able to partially dorsiflex the foot, some may experience spasms of the foot , and some may lose the ability to both dorsiflex and plantarflex the foot.

An assistive AFO is the preferred method for supporting those with foot drop. These AFOs may be rigid or include an elastomer hinge at the ankle joint. In the later case the hinge will be loaded such that when the foot is off the ground the toes are held slightly elevated. While not every individual can use a hinged AFO, it does provide for a more natural gait by allowing the ankle to flex during the plant phase of the gait cycle.

Design CriteriaDefining the User

The user maintains minimal muscle control over dorsiflexion AND plantarflexion. The user has a hinged AFO to support their foot. The user’s size is between that of a 5th percentile female and a 95th percentile male per

the ANSUR database.

Defining the Device The device must be attachable to the AFO without compromising fit or comfort The device will be used while the foot and AFO are in a shoe. The device must be easy to install and remove from the AFO. The device must not compromise the integrity of the AFO The device will be tethered, and for use in a clinical setting. The device will be capable of operating in an aquatic setting. The device will responsive enough to support the gait of a healthy individual When not actuated, the device does not interfere with the operation of the AFO.

AcknowledgementsWe would like to thank the following people for their generous donations and assistance: Dr. Kathleen Lamkin-Kennard, Dr. J.J. Mowder-Tinney and Nazareth College Physical Therapy Clinic, Gordon Potter Jr. C.O. and Cayuga Orthotics and Prosthetics Inc., Rochester Orthopedic Labs, Inc., Colton Scott, and Christopher Sullivan and the National Science Foundation

Air MusclesAlso known as pneumatic artificial muscles, air muscles consist of a long, inflatable, expanding bladder inside a woven sheath. The bladder is plugged and clamped at one end, while the other end is connected to a power source – such as a compressed gas canister.

When the bladder is inflated it expands against the sheath, resulting in the air muscle contracting. The air muscle becomes wider and shorter, just like a human muscle. This action can be used to actuate smooth motion in whatever the air muscle is attached to.

Air muscles were chosen for their lightweight nature, inexpensive materials, and likeness to a human muscle. The speed and force with which an air muscle acts is adjustable to a users needs by varying the size of the air muscle as well as the air pressure and flow used in the contraction.

On average, a healthy individual descending stairs will use a 47.4 degree range of motion about the ankle. [2] It was demonstrated that this device can facilitate at least a 64.0 degree range of motion. The AFO used in prototyping incorporates no stops to prevent hyperextension, however the device will not interfere with such a structure. By changing the air muscle size and adjusting the cable length, the range and orientation of the motion can be regulated. In general, testing showed that the device is capable of assisting most users in achieving the necessary range of motion for a normal gait cycle while descending stairs.

Testing was performed to verify the ability of air muscles to support normal gait speeds. It was found that air muscles are capable of actuating and releasing at acceptable rates. By regulating air pressure and flow the speed and force with which the air muscle acts can be adjusted appropriately to an individual. Lifetime tests were conducted on both anchors and air muscles. The anchor points performed well, with both the device anchors and the AFO itself showing no significant wear. Testing was performed to simulate three years of clinical use, this time frame was chosen because typical insurance policies will cover a new AFO every three years. [3] Air muscles were found durable enough to support an estimated 95 percent uptime in clinical use.

The device is tethered, and in its current form would only be suitable for use in a clinical setting during rehabilitation and physical therapy. In order to be useful in daily life the device needs to be paired with a power supply as well as a terrain and gait sensing unit that can accurately coordinate the air muscle contractions. A 48 cubic inch, 3000 psi compressed air canister would support approximately 2050 contractions, or about 240 flights of stairs. Such canisters are available in weights under 4 pounds. Wearable terrain sensors have been shown feasible and effective.

Left: Distal Anchor and Lower Plug

Right: Proximal Anchor

Proximal and Distal Anchors

1) Outer Sleeve Contains bladder

during expansion2) Bladder

Expandable material, inflates when activated

3) Clamp4) Plug

Air Muscle Dissection

Active Ankle-Foot Orthotic: Tethered Air MuscleRochester Institute of Technology

Kate Gleason College of EngineeringRochester, NY 14623

This material is based upon work supported by the National Science Foundation under Award No. BES-0527358. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation.

Citations[1] Sullivan, Christopher R., 2012. “Design of a Terrain Detection System for Foot Drop.” meresearch.rit.edu[2] Bovi, G., Rabuffetti, M., et al., 2011. “A multiple-task gait analysis approach: Kinematic, kinetic and EMG reference data for healthy young adult subjects.” Gait and Posture. 33.[3] STS Innovative Casting Technology, 2012, "The Podiatrist's Guide to Medicare Billing for Custom Fabricated Ankle-Foot Orthoses." http://www.stssox.com/PodAFOmanual.pdf.

1) Proximal Anchor: Connects the device to the calf cradle of the AFO. Provides pneumatic connection point to the device.

2) Distal Anchor: Connects the device to the heel cradle of the AFO. Slotted to provide connection point for cable.

3) Air Muscle: Provides actuation to the device. When inflated the Distal andProximal Attachments are pulled towards each other.

4) Lower Plug Seals the end of the air muscle Anchors cable to air muscle Allows cable length to be adjusted per the user

5) Cable Connects the air muscle to the Distal Attachment

ConclusionsOn average, a healthy individual descending stairs will use a 47.4 degree range of motion about the ankle. [2] It was demonstrated that this device can facilitate at least a 64.0 degree range of motion. The AFO used in prototyping incorporates no stops to prevent hyperextension, however the device will not interfere with such a structure. By changing the air muscle size and adjusting the cable length, the range and orientation of the motion can be regulated. In general, testing showed that the device is capable of assisting most users in achieving the necessary range of motion for a normal gait cycle while descending stairs.

Testing was performed to verify the ability of air muscles to support normal gait speeds. It was found that air muscles are capable of actuating and releasing at acceptable rates. By regulating air pressure and flow the speed and force with which the air muscle acts can be adjusted appropriately to an individual. Lifetime tests were conducted on both anchors and air muscles. The anchor points performed well, with both the device anchors and the AFO itself showing no significant wear. Testing was performed to simulate three years of clinical use, this time frame was chosen because typical insurance policies will cover a new AFO every three years. [3] Air muscles were found durable enough to support an estimated 95 percent uptime in clinical use.

The device is tethered, and in its current form would only be suitable for use in a clinical setting during rehabilitation and physical therapy. In order to be useful in daily life the device needs to be paired with a power supply as well as a terrain and gait sensing unit that can accurately coordinate the air muscle contractions. A 48 cubic inch, 3000 psi compressed air canister would support approximately 2050 contractions, or about 240 flights of stairs. Such canisters are available in weights under 4 pounds. Wearable terrain sensors have been shown feasible and effective.

Left: As the user steps down with the unaffected leg, the air muscle is relaxed. This allows the foot to naturally dorsiflex as the knee moves forward towards the next step.Right: The user steps down with the affected leg. Here the air muscle is contracted. The air muscle overcomes the dorsiflexing force of the hinge to plantarflex the foot. This leads to a toe first foot plant onto the next step.

Left: Distal Anchor and Lower Plug

Right: Proximal Anchor

Proximal and Distal Anchors

Designed to be installed once, though they can be removed with a hex key

Inside connections to be countersunk into AFO, creating a safe, comfortable inner surface for the user

Air muscle is threaded into the Proximal Anchor. Cable is slotted into the Distal Anchor.

Lifetime testing simulated three years of clinical use. Minimal wear was observed on the AFO, no wear was observed on either anchor.

The AFO is designed to allow the foot a wide range of motion. When the foot is on the ground the hinge allows the foot to rotate. It is important that the air muscle does not restrict this motion. The picture – below, left – illustrates the flexibility of the air muscle to allow dorsiflexion.

1) Outer Sleeve Contains bladder

during expansion2) Bladder

Expandable material, inflates when activated

3) Clamp4) Plug

Air Muscle Dissection

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3

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