Energy Storing Foot Drop AFO
This Ankle Foot Orthosis was designed to Que the function
of the calf muscles in late stance. 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. Conventional orthoses designs limit the motion of the foot to Swing phase to
clear the toe. Unfortunately this also limits motion during
Stance phase witch reduces the patients ability to push off
and initiate early Swing phase.
The “Lift” Energy Storing– Foot Drop 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. The carbon leaf spring bends
to allow the normal range of motion during stance phase.
Bending the carbon upright creates a small rebound resistance that assist or Ques the calf muscle in late stance
phase. This facilitates the a normal acceleration into earl
swing phase.
The “LIFT” 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”, 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
phase of gait, they bend (load) the toe plate. The rebound of the toe plate, in conjunction with the rebound of
the upright, assist the patient into early swing phase. The formed position of the toe section of the foot plate
has a rise that assits in clearing the toe in mid swing phase.
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 footplate, 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 80–100 Newton
Meters (30-40 lbs). This amount of rebound force is assistive at queing the patient in accelerating the limb into
swing phase. In conjunction with the rebound of the toe plate, this makes for a smooth step and better toe
clearance through swing phase.
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.
A soft padding on the crest of the tibia is recommended for the patient to tolerate the flexible force with comfort.
Functional Outcomes:
Conventional orthoses design in plastic and other materials, including the Spiral design, commonly function
to hold the ankle at a 90 degree position. This is assistive to hold the foot at 90 degrees during swing phase.
preventing the “Foot Drop” patient from catching the toe or having to elevate the knee to clear the toe. These
conventional designs restrict the normal range of motion of the ankle during both swing and stance phase.
The 90 degree positioning that is assistive to the patient in swing phase, limits the patients normal motion in
stance phase. The 90 degree position at Heel Strike does not allow the patient to drop the forefoot. This creates a knee flexion moment at heel strike, forcing the patient to alter there normal smoothness of their gait
pattern. From Mid Stance to Push Off, the 90 degree position decreases the patients ability to push off. This
decreases the patients ability to begin accelerating the leg into swing phase, ultimately making it harder for
the patient to clear the toe during forward swing. Although plastics are light weight they are still bulky and
the limitation to push off makes them feel heavy to the patient.
The Lift 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 the
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 small amount of assistance, 80-100 Newton Meters, helps the patient initiate acceleration into swing phase. 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 90 degrees
during swing phase. 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 range of
motion in Stance phase of gait. The Lift design, utilizes the spiral design and the rebound characteristics of
carbon fiber to allow the patient a full range of motion in Stance phase. At the same time the Lift AFO captures this range of motion to load the orthosis and then rebounds this energy to assist the patient as they move
into swing phase. All while also maintaining a 90 degree position in 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 (15lbs), the orthosis is not strong enough to maintain a 90
degree positioning in swing phase or to effect Push Off from stance phase. 80—100 Newton meters (30-40
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.
Fitting Criteria:
It is imperative for the proper function of the orthosis, that the upright extends as high as possible.
The longer the lever arm of the orthosis extends up
the calf the lower the pressure against the leg to
load the orthosis. There are common sizes of legs
but they all vary between the length of the leg and
the size/length of the foot.
There are three determinate measures that are key
for the Lift orthosis; the height of the orthosis (A),
the length of the foot to the metatarsal heads (B)
and the total length of the foot (C)
The “LIFT”.AFO is built with a foot plate to accommodate a 1/2 inch heel height shoe. A shoe
with a larger or smaller heel height can complicate
the function of the orthosis.
Some patients have a shorter leg and longer foot
while others have the opposite. Custom fitting of
the orthosis can allow for trimming the length and
width of the foot plate. The height of the orthosis,
the attachment point of the strut to the foot plate
and the length of the foot plate to the metatarsal
heads; can not be adjusted.
Consistency and Durability:
There are standard fabrication methods in the orthotic/prosthetic industry. Thermal-Forming is the method of
drape molding plastic into a formed shape. Thermal-Set is the method of layering composite materials and
then saturating them with a resin. The resin is mixed chemical catalyst that sets the resin into the formed
shape. Thermal-Set fabrication is commonly done with carbon fiber composite materials for strength and
lightweight characteristics.
“Pre-Preg” composite fabrication is newer to the industry. This entails layering composite materials that are
pre-impregnated with resin. Most commonly epoxy resin. This allows for the ability to control the exact
amount of resin being used. The mold and layup of composite material is then placed under pressure.
Squeezing the composite layup to the form shape. The resin is then cured or hardened with heat. This process of fabrication was pioneered by the aviation industry. This process makes a structure that will bend and
rebound with a controlled resultant force. This process also makes a consistent repeatable structure. This enables the testing of one structures durability and then to replicate the process to achieve reliable consistency.
The Lift orthosis is fabricated by a Pre-Preg Composite fabrication to create a repeatable rebound strength.
Orthosis durability, test at more than ten million cycles, with 40 degrees of flexion without failure. This correlates with five years of daily walking averages. Other methods of fabrication can be utilized to form Carbon fiber and epoxy resin to make the same flat orientation structures. Although the orthosis would look the
same in fit and appearance the consistent rebound strength and durability would be inconsistent.