"Push-Off" Energy Storing Prefabricated AFO

This Ankle Foot Orthosis was designed to replace the
function of the calf muscles. There are numerous pathologies and traumatic incidents that impair or complicate the
normal calf muscle function. The posterior calf muscles
provided normal knee stability in stance, standing balance.
It also provides propulsion in late stance phase of gait, for
the transfer of weight to the other extremity.
Calf muscle deficit in function is not limited to the lack of
firing of the muscle.. The deficit also included the inability
to control the firing or contraction of the muscle. The calf
muscle fires in a combination of eccentric and concentric
contractions through out stance phase. Varying firing patterns to accommodate variable cadence.
This may present as a weakness in strength in conventional
strength testing. In many cases the patient can fire the
contraction of the muscle during testing but does not have
the fine control to vary the firing.
The “Push Off” Energy Storing AFO utilizes a flat carbon
fiber leaf spring that extends from the foot plate to the posterior calf. The upright or leaf spring bends in the forward
direction of walking. Bending the carbon upright creates a
rebound resistance similar to that of the calf muscle in normal ambulation. The orientation of the upright on the posterior of the calf allows the patient to utilize this rebound
force through mid to late stance phase.

Push-Off” Energy Storing AFO

The “Push-Off” Energy Storing Ankle Foot Orthosis is constructed
of a Carbon Fiber composite lamination (variable layers). This system of carbon fabrication produces a rebound action. When a flat
carbon structure is bent, it springs back to its formed position with a
consistent amount of force. The farther it is bent, the greater the rebound force. By adjusting the number of layers in construction, you
can change the amount of force the structure will rebound

 

This design consist of a Foot Plate, an Upright and a Calf Band…

Foot-plate:
The flat foot plate extends from the heel to the distal
aspect of the toes,. The heel section and the toe section
have variable layers to make those areas more flexible
while the center of the foot plate is stiff. The flexibility of
the heel portion of the foot plate, the posterior 1.5 inches,
bends at “Heel Strike” reducing the knee flexion moment
in gait. The area of the foot plate in the center is rigid to
the metatarsal heads, this accommodates the attachment of
the upright. The area from the metatarsal heads to the end
of the toes is flexible to create a rebound force. As the patient progresses over the foot in late stance

Upright:
The flat upright extends from the foot plate up to the posterior calf. The flat Upright attaches to the foot
plate on the lateral side of the foot-plate, about 1.5 inches from the posterior aspect. The flat upright spirals
from the lateral side of the heel, around to the posterior of the calf. The attachment of the upright to the lateral
side of the foot plate allows for the posterior aspect of the foot plate to bend at heel strike. This reduces the
knee flexion moment at heel strike, that is evident in most ankle foot orthosis designs.
The flat upright spirals from the lateral side of the foot plate to the back of the calf. This controls the direction
of flexion to match the forward progression of the knee over the foot during gait. The spiral orientation of the
upright works in conjunction with the rigid center section of the foot plate. As the patient transfers weight to
the foot, it secures the foot plate flat. As the patient then progresses over the planted foot, it bows the upright,
loading the upright. As the patient reaches the end of stance phase the loaded upright produces a controlled
amount of plantar force, Push off.

The formed position of the upright matches the slight plantar positioning of the ankle at “Foot Flat”
phase of gait. As the patients’ leg progresses forward through “Mid Stance” the upright bends creating a
rebound force. As the heel leaves the ground at “Push Off “phase of gait the rebound force is between
300–350 Newton Meters (120-200 lbs). This amount of rebound force is not a set amount but an arc of
increasing rebound. The patient is able to allow the heel to rise earlier in late stance to reduce the rebound
force as needed. The patient is also able to adjust their step length to again alter the amount of rebound
force. The combination of these minimal alterations to gait, allow the patient to use the rebound force to
find the most efficient gait and to vari their cadence.

 

Calf Band:
The flat calf band curves around the back of the leg to make a larger area of pressure. The strap around the
anterior of the calf holds the patient to the calf band. The security of this connection between the patient and
the calf band is key to the function of the rebound of the orthosis. As the patient takes a step, every degree of
motion loads the structure of the orthosis. Any gapping between the patient and the orthosis reduces the rebound force of the orthosis.
The “Push Off” design produces up to 350 newton meters (200lbs) of force. The rebound force is gradually
increased across the anterior strap as the leg progress over the planted foot. This amount of force on the crest
of the tibia would not be tolerable. The design entails a custom molded anterior shell. The shell consist of a
1/8 inch Kydex plastic. Moldable at low temp (180 degrees). The shell requires the practitioner to heat up the
shell and mold it to the patients leg. Accommodating a relief of the anterior crest of the tibia by moving the
pressure to the sides.

The anterior shell has a padded sleeve to further accommodate the pressure and to set the position of the shell
on the strap. Protection has to be taken not to put the hot moldable shell directly to the patients leg.
The strapping on the orthosis runs through a chafe to insure it secures the fitted tightness to the leg. The anterior shell is molded and then Velcro attaches it to the strap . This insure placement and repeatable tightness
during donning by the patient.

Functional Outcomes:
Conventional AFO designs specifically limit motion during stance phase to stabilize the ankle or knee under
weight bearing or to limit a specific motion.
The “Push Off” design utilizes the lightweight and flexible rebound characteristics of carbon fiber. This
makes for less weight and bulkiness for the patient. The “Rebound” characteristics allow the patient to go
through a near normal range of motion during stance phase. It also reduces the knee flexion moment at heel
strike to almost nothing. The design loads the energy of the forward momentum of the patient and rebounds
this energy in late stance. The amount of rebound assistance, 300-350 Newton Meters (150-200lbs), assisting
the patient in propulsion in late stance. As such, replacing missing calf muscle function throughout stance
phase
The rebound is reduced if the degree of bend of the upright is reduced. This accommodates a lesser amount at
static midstance. The patient can utilize the lesser rebound for standing stability.
The 90 degree formed position of the orthosis maintains the ankle positioning through “Swing” phase, insuring the clearance of the foot.
In Summary, conventional ankle foot orthosis designs primarily function by holding the foot at a relative, 90
degrees during stance and swing phase of gait. A variety of materials were used to reduce the weight and
bulk of the orthosis, including carbon fiber, with notable success in those areas. Even with reduction of
weight and bulk, the static 90 degree positioning of conventional designs continue to create a functional deficit by limiting the patients normal range of motion in Stance phase of gait. The patient then needs to compensate for the lack of motion.
The “Push Off” design, utilizes the spiral design and the rebound characteristics of carbon fiber to allow the
patient a near full range of motion throughout Stance phase. At the same time the “Push Off” AFO captures
this range of motion to load the orthosis and then rebounds this energy to assist the patient propulsion in late
stance and tappers off as they move into swing phase. It then passively maintaining a 90 degree position
through Swing phase.

Measuring Rebound resistance:
In the Stance phase of gait the ankle moves through a 40 degree range of motion. From “Heel Strike” to
“Foot Flat” the ankle plantarflexes 30 degrees. From “Mid Stance” to “Heel Off” the ankle dorsiflexes 10
degrees. Through the stance phase the ankle progresses from the 30 degrees plantar position to the 10 degree dorsiflexed position.
At Heel Strike the forefoot drops to the floor. At that moment there is the greatest distance from the knee
center to the metatarsal heads on the foot. At “Heel Off”, the moment the heel leaves the ground, the knee
center is the closest to metatarsal heads. The common difference in these two measurements is 1 1/4 to 1
3/4 inches. The difference is dependent on the height of the person. Or more specifically to the length from
the floor to the knee center. This reduction of distance is the available range of motion an orthosis design
can utilize to load the structure of the orthosis.

In accordance with this, we measure the rebound resistance of the orthosis utilizing these distances. The orthosis is placed on a force sensor. The upper back of the calf band is then compress towards the metatarsal heads,
the predetermined distance of 1 1/4 inches to 1 3/4 inches. This give an arc of resistance as well as a total pressure.

If the rebound force is less than 50 newton meters (35 lbs), the orthosis is not strong enough to maintain a 90
degree positioning in swing phase or to effect push off during stance phase. 80—100 Newton meters (50 lbs)
seems to be most effective with foot drop. In excess of 150 newton meters of force, The orthosis begins to reduce the range of motion in Stance phase.
The “Push Off” design produces a 300-350 Newton Meters (up to 200lbs) of rebound resistance at 1.25 to
1.75 inches of compression. This force is not a set amount but an arc of increasing rebound. The patient is able
to allow the heel to rise earlier in late stance to reduce the rebound force as needed. The patient is also able to
adjust their step length to alter the amount of rebound force. Utilizing the force plate sensor allows us to
graph the arc of the resistance , similar to the way the patient would feel the rebound force.