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          One of the first objectives when developing an orthotic treatment plan for an individual is to
identify the biomechanical needs of the client. Determining the biomechanical needs will assist in
providing support and substitution for lost muscle function or provide control of undesired motion
due to spasticity. Ideally, the optimal situation is to limit motion of a joint only if it is necessary and
if this limitation will provide improved joint stability and create 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 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. This alignment may lead to excessive pronation and forefoot
abduction. In the patient population where muscle loss and imbalance are common, skeletal
alignment as well as stability is of utmost importance.

        The most obvious use for an ankle foot orthosis is control of the ankle joint in the sagittal
plane. The AFO can offer 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 and knee flexion instability due to weakness of the quadriceps.
        Coronal plane stability of the subtalar joint and supination and pronation of the forefoot may
be achieved with a well-designed and well-fitting plastic AFO. With proper stabilization of the
subtalar joint, transverse plane control of the forefoot is obtainable and is demonstrated by the
position of forefoot abduction and adduction. The more difficult component of transverse rotation to
control is internal rotation of the tibia. Only with a system that itself is rigid and does not allow any
transverse rotation will the force systems in the AFO be effectively transferred to the skeletal system.
        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 development of pathomechanical deformities.
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

 Fig. 1 Three-Point Force System
         To provide mediolateral stability at the subtalar joint and control excessive subtalar eversion,
the three-point force system (Fig. 2) indicates a 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                                      Fig. 3 Subtalar inversion
       force system                                                  force system
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 includes 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                           Fig. 5 Tibial angle to floor
       a plantarflexion stop

        The tibial angle to the floor is determined by bisecting the distal one-third of the tibia in the
sagittal plane and measuring this angle in relationship to the floor. The tibial angle to the floor must
be measured with the shoe on when evaluating the stability and function during ambulation. This
angle will change with the use of shoes with varying heel heights and must be considered when
selecting shoewear in order to maintain a stable knee alignment during walking. A tibia placed in
relative dorsiflexion while wearing a shoe and an AFO with a plantarflexion stop will exhibit a knee
flexion moment at loading response and may control a mild to moderate knee hyperextension
moment during midstance phase of gait.
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 inhibit 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                                   Fig. 7 Articulated AFO with a
       dorsiflexion stop                                             dorsiflexion assist

Dorsiflexion Assist
       A dorsiflexion assist joint can be composed of a spring arrangement (Fig. 7) or a flexure joint.
Both designs function to 1) bring the talocrural joint through dorsiflexion range of motion, thus
providing clearance of the foot during swing phase, and 2) allowing plantarflexion range of motion at
loading response therefore decreasing the knee flexion moment which may destabilize the knee. (7)
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.

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.
Plastic AFO Designs
         The biomechanical functions of plastic AFOs 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
form a solid ankle design positioning the ankle in a fixed position to a posterior leaf spring design
(Fig. 10). A solid ankle design is used with combined dorsiflexion and plantarflexion muscle loss
with a trimline at the ankle region anterior to the malleoli. It affords maximal stability in the sagittal,
coronal, and transverse planes at the talocrural, subtalar, and midtarsal joints by placing the joint in a
fixed position utilizing multiple three-point force systems. 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.
A posterior leaf spring AFO is trimmed posterior to the malleoli allowing controlled plantarflexion at
loading response. The flexibility allows dorsiflexion range of motion during late midstance and
terminal stance. The main function of this AFO design is to limit plantarflexion range of motion
during swing phase when an individual has weakness of the ankle dorsiflexors. Unfortunately the
inherent flexibility of this design will not control excessive subtalar eversion, midtarsal pronation, or
forefoot abduction.

 Fig. 10 Posterior Leaf Spring

        The ground reaction AFO (Fig. 11) has historically been used to describe the plastic AFO
composed of a solid ankle design with a pretibial shell. Ground reaction forces induce a knee
extension moment at the end of stance phase by a dorsiflexion stop or anterior stop limiting the
dorsiflexion range of motion. 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
while wearing the AFO and shoe is critical in determining knee stability. Stability is achieved with
the ankle at 90 degrees to the floor or slightly posteriorly tilted. The length of the footplate may be
extended to the end of the toes to increase the knee extension moment arm. The ground reaction AFO
design is indicated with quadriceps strength of fair minus (4).

1.   Fish DJ, Nielsen JP. Clinical Assessment of Human Gait. JPO 1993; Vol. 5, No. 2: 27-
2.   Fess EE, Philips CA. Hand Splinting: Principles and Methods, 2nd Edition St. Louis,
     MO: C.V. Mosby, 1987; 126.
3.   Carlsen MJ, Berglund G. An Effective Orthotic Design for Controlling the Unstable
     Subtalar Joint: Orthotics and Prosthetics, 1979; 33 (1): 31-41.
4.   Yang GW, Chu DS. Floor Reaction Orthosis: Clinical Experience. Orthotics and
     Prosthetics 1986; Vol. 40, No. 1, 33-37.

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