1 LOWER EXTREMITY ORTHOTICS TO ENHANCE AMBULATION

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LOWER EXTREMITY ORTHOTICS TO ENHANCE AMBULATION
INTRODUCTION
The first orthotic objective is to design an orthosis that addresses the biomechanical
needs of the client by providing support and substitution for lost muscle function or provide
control of excessive spasticity. The principle of the intervention with the least amount of control
necessary is achieved by limiting motion at any joint only if it will assist in providing improved
joint stability and creating a stable base of support. The second objective is to provide a stable
skeletal alignment. Long-term effects of skeletal misalignment include the acquisition of
pathomechanical deformities. (1) While the biomechanical needs of the patient are routinely
considered, alignment problems leading to chronic pathomechanical deformities are frequently
overlooked. For example, a patient may be fitted with an off the shelf posterior leaf spring ankle
foot orthosis (AFO) to obtain clearance of the foot during swing phase but the orthosis may not
provide adequate support of the subtalar joint during stance phase once weight is applied to the
leg. This alignment will over time lead to excessive pronation composed of subtalar eversion,
midtarsal depression or pronation and forefoot abduction. In the patient population where
muscle loss and imbalance are common, skeletal alignment as well as muscle stability is of
utmost importance.
GOALS FOR ORTHOTIC INTERVENTION
The most obvious use for an ankle foot orthosis is control of the ankle joint in the sagittal
plane. The AFO can sustain clearance of the foot during swing phase if there is inadequate
strength of the ankle dorsiflexors including the tibialis anterior, extensor hallicus longus, and
extensor digitorum longus. The AFO can also substitute for push off during stance phase if the
ankle plantarflexors are weak. Less obvious goals of an AFO include controlling the position of
the ankle in the sagittal plane to control mild knee hyperextension, as well as knee flexion
instability, due to weakness of the quadriceps.
Coronal plane stability of the subtalar joint can be achieved with a well-designed plastic
AFO as well as coronal plane supination and pronation of the forefoot. Transverse plane control
must also be considered when designing an orthotic system. With proper stabilization of the
subtalar joint, transverse plane control of forefoot abduction and adduction can be obtainable.
The more difficult component to control of transverse rotation, is internal rotation of the femur
and tibia, which can be addressed by careful material selection and design principles. The
materials and components must not allow transverse rotation structurally to occur and the force
systems must be appropriately placed for effective force couple systems to prevent unwanted
movement.
Through optimal skeletal alignment of the person along with appropriate biomechanical
controls in our AFO design, we hope to create a stable base of support to allow safe and efficient
ambulation and prevent the development of future pathomechanical deformities.
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BIOMECHANICAL CONTROLS FOR AFO’S
Three-Point Force Systems
The controls incorporated in orthotic systems are based on three-point force systems to
affect alignment by controlling two adjacent skeletal segments. (Fig. 1) The corrective force is
located on the convex side of the curve at the joint addressed (b). Two counteractive forces are
positioned on the opposite side above (c) and below (a) the corrective force. As the distance of
the counteractive force from the corrective force increases so do the lever arms and therefore the
effectiveness. Based on the principle, Pressure=Total Force/ Area of Force Application, the
objective is to distribute the forces over a larger area to decrease the resultant pressures. (2) A
well fitting total contact orthosis avoiding bony prominences and utilizing effective three-point
force systems will assist in achieving this objective.
Fig. 1 Three-Point Force System
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To provide mediolateral stability at the subtalar joint and control excessive subtalar
eversion, the three-point force system (Fig. 2) has the corrective force applied proximal to the
medial malleolus (b) and at the sustentaculum tali (c). Due to the fact that pressure cannot be
applied to the bony medial malleolus, the corrective force must be applied over two adjacent
areas. The sustentaculum tali (ST) is located on the calcaneus and if stabilized correctly by a ST
modification or pad provides a horizontal ledge to support the talus. (3) The two counteractive
forces at the distal lateral calcaneus (a) and proximal lateral calf (d) are above and below the
joint and as far away from the joint as possible to produce longer lever arms.
Subtalar inversion (Fig. 3) from an unopposed tibialis anterior is controlled by the
corrective force placed proximal to the lateral malleolus (c) and over the cuboid (b) if possible.
Again, we are unable to apply a direct force over the lateral malleolus and must place the
corrective forces adjacent. The two counteractive forces are located at the distal medial
calcaneus (a) and the medial proximal flare (d).
Fig. 2 Subtalar eversion
force system
Fig. 3 Subtalar inversion
force system
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Plantarflexion Stop
A plantarflexion stop or posterior stop in an AFO (Fig. 4) is designed to substitute for
inadequate strength of the ankle dorsiflexors including the tibialis anterior, extensor hallicus
longus, and the extensor digitorum longus during swing phase of gait. This stop is effective by
limiting the plantarflexion range of motion of the talocrural joint. The three-point force system
has the corrective force at the shoe instep or ankle strap and two counteractive forces, one at the
plantar surface at the ball of the foot and the second on the posterior calf region. An important
concept when evaluating the ankle position is the tibial angle to the floor (Fig. 5)
Fig. 4 Articulated AFO with
a plantarflexion stop
Fig. 5 Tibial angle to floor
We define the tibial angle to the floor by bisecting the distal one-third of the tibia in the
sagittal plane and measuring this angle in relationship to the floor. Each shoe has a “heel height”
or the difference of the height of the heel minus the thickness of the material at the ball of the
foot. This resultant slope affects the tibial angle to the floor once the AFO is inserted into the
shoe. The tibial angle to the floor must be measured with the shoe on when evaluating the
stability and function during ambulation. This angle will be altered with the use of shoes with
varying heel heights. For example, a tibia placed in relative dorsiflexion to the floor while
wearing a shoe produces a knee flexion moment at loading response and can decrease a mild to
moderate knee hyperextension moment during stance phase of gait or create knee flexion
instability at loading response when walking.
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Dorsiflexion Stop
A dorsiflexion stop or anterior stop in an AFO (Fig. 6) is used to simulate push off and
substitutes for weak ankle plantarflexors. The stop will limit tibial advancement during
midstance providing stability in the sagittal plane by limiting the doriflexion range of motion of
the talocrural joint. Limitation of dorsiflexion to neutral or in slight plantarflexion also
influences the stability of the knee and is of assistance when the quadriceps strength is grade fair
minus. With restraint of the tibia, the body’s center of mass moves anterior to the knee joint axis
and due to the resultant ground reaction force vector a knee extension moment is created.
Fig. 6 Laminated AFO with a
dorsiflexion stop
Fig. 7 Articulated AFO with a
dorsiflexion assist
Dorsiflexion Assist
A dorsiflexion assist joint can be composed of a spring arrangement (Fig. 7) or a flexure
joint. Both components function to bring the talocrural joint through dorsiflexion range of
motion, thus providing clearance of the foot during swing phase. They also allow plantarflexion
range of motion at loading response therefore decreasing the knee flexion moment which may
destabilize the knee and increase the potential for falls. (7)
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AFO DESIGNS
Conventional AFO Designs
A conventional design AFO (Fig. 8) is composed of a shoe, stirrup, ankle joint,
sidebar/upright, calfband, and calf closure. The control of the subtalar joint and foot depends on
the stability and integrity of the shoe. Once the shoe is worn, the effectiveness decreases. A
soleplate extending to the metatarsal heads is added between the midsole and the outer shoe of
the shoe to produce an effective lever arm. Due to the lack of total contact, the conventional
AFO is not an effective design for controlling coronal or transverse plane motion. A foot insert
or UCBL foot orthosis may be added inside the shoe to improve the control and alignment of the
subtalar and midtarsal joints.
Fig. 8 Conventional AFO
Fig. 9 Double adjustable
ankle joint
A double adjustable ankle joint (Fig. 9) allows a greater degree of adjustability. The dual
channel system enables the practitioner to utilize the following controls at the ankle: 1) fixed
position of the ankle in the sagittal plane, 2) limited range of motion, 3) controlled plantarflexion
at loading response due to a spring in the posterior channel and a dorsiflexion stop via a pin in
the anterior channel as shown in Figure 9.
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Plastic AFO Designs
The biomechanical functions of plastic AFO’s are described by their trimlines. The
trimlines reflect the rigidity in relationship to the range of motion they allow at the talocrural
joint. They range from a solid ankle design (Fig. 10) positioning the ankle in a fixed position to
a posterior leaf spring design (Fig. 11). A solid ankle design is used with combined dorsiflexion
and plantarflexion muscle loss or weakness with a trimline at the ankle region anterior to the
malleoli. It affords maximal stability in the sagittal, coronal, and transverse planes at the ankle
joint, subtalar, and midtarsal joints by placing the joint in a fixed position by utilizing multiple
three-point force systems or force couples. To safely control the knee with this AFO the
individual will need grade fair strength of the quadriceps and a tibial angle to the floor of 0-5
degrees of relative dorsiflexion when positioned in the shoe.
A posterior leaf spring AFO is trimmed posterior to the malleoli allowing 1) controlled
plantarflexion at loading response, 2) dorsiflexion range of motion during late midstance through
terminal stance, and 3) providing clearance of the foot during swing phase of gait. Many current
pre-fabricated designs are extremely flexible and offer no stability of the subtalar and midtarsal
joints during weight bearing. A custom fabricated posterior leaf spring design AFO can be
designed to offer a more refined amount of resistance and improved control of the subtalar and
midtarsal joints by the trimlines and casting features. The most common function or goal when
using this AFO design is the limitation of plantarflexion range of motion during swing phase
when an individual has weakness of the ankle dorsiflexors.
Fig. 10 Solid Ankle AFO
Fig. 11 Posterior Leaf Spring AFO
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The term, ground reaction AFO (Fig. 12) has historically been used to describe the plastic
AFO composed of a solid ankle design with a pretibial shell. Ground reaction force vectors
induce a knee extension moment at the end of stance phase when a dorsiflexion stop or anterior
stop limiting the dorsiflexion range of motion is incorporated into the design. As the center of
mass of the individual is moving forward and tibial advancement is limited by the AFO, a knee
extension moment is created. The tibial angle to the floor contributes to determining knee
stability as well as the length of the foot plate. Stability at midstance is achieved with the ankle
at 90 degrees to the floor or slightly posteriorly tilted or plantarflexed. A knee flexion moment
will be accentuated at loading response as the dorsiflexion angle of the AFO is increased. The
length of the footplate may be extended distally past the usual length at the metatarsal heads to
increase the knee extension moment arm during midstance through terminal stance. The ground
reaction AFO design is indicated with quadriceps strength of fair minus (4). Another AFO
design offering this control is composed of double adjustable ankle joints, a footplate, and a
pretibial shell. (Fig. 13) The ankle joint may be designed with stops in both channels or a stop in
the anterior channel and a spring in the posterior channel. As discussed previously, the spring in
the posterior channel will allow controlled plantarflexion range of motion at loading response
and will reduce the knee flexion moment that may cause knee instability.
Figure 12 Solid Ankle Ground
Reaction AFO
Figure 13 AFO with pretibial shell,
Double adjustable ankle
joints
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References
1.
2.
3.
4.
Fish DJ, Nielsen JP. Clinical Assessment of Human Gait. JPO 1993; Vol. 5, No. 2: 2736.
Fess EE, Philips CA. Hand Splinting: Principles and Methods, 2nd Edition St. Louis,
MO: C.V. Mosby, 1987; 126.
Carlsen MJ, Berglund G. An Effective Orthotic Design for Controlling the Unstable
Subtalar Joint: Orthotics and Prosthetics, 1979; 33 (1): 31-41.
Yang GW, Chu DS. Floor Reaction Orthosis: Clinical Experience. Orthotics and
Prosthetics 1986; Vol. 40, No. 1, 33-37.