Orthotics Lower Limb

7/31/2019 Orthotics Lower Limb

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LOWER EXTREMITY ORTHOTICS TO ENHANCE AMBULATION

INTRODUCTION

The first orthotic objective is to design an orthosis that addresses the biomechanicalneeds 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 improvedjoint stability and creating a stable base of support. The second objective is to provide a stableskeletal alignment. Long-term effects of skeletal misalignment include the acquisition of

pathomechanical deformities. (1) While the biomechanical needs of the patient are routinelyconsidered, 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 anklefoot 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 theleg. This alignment will over time lead to excessive pronation composed of subtalar eversion,

midtarsal depression or pronation and forefoot abduction. In the patient population wheremuscle 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 inadequatestrength 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 theankle 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 flexioninstability, 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 controlmust also be considered when designing an orthotic system. With proper stabilization of thesubtalar 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 femurand 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 forcesystems 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 efficientambulation and prevent the development of future pathomechanical deformities.


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BIOMECHANICAL CONTROLS FOR AFOS

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 islocated 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 ofthe 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, theobjective 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-pointforce 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 themedial 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 adjacentareas. 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 counteractiveforces 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 medialcalcaneus (a) and the medial proximal flare (d).

Fig. 2 Subtalar eversion Fig. 3 Subtalar inversion

force system force system


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Plantarflexion Stop

A plantarflexion stop or posterior stop in an AFO (Fig. 4) is designed to substitute forinadequate 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 bylimiting 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 theplantar 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

We define the tibial angle to the floor by bisecting the distal one-third of the tibia in thesagittal 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 thefoot. 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 thestability 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 whilewearing 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 flexioninstability 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 andsubstitutes 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 ofthe 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 fairminus. With restraint of the tibia, the bodys 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 AssistA 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 ofmotion, 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 maydestabilize 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 onthe 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 ofthe 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 insertor 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 adjustableankle 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) fixedposition 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 inthe anterior channel as shown in Figure 9.


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Plastic AFO Designs

The biomechanical functions of plastic AFOs are described by their trimlines. Thetrimlines 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 dorsiflexionand plantarflexion muscle loss or weakness with a trimline at the ankle region anterior to themalleoli. 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 multiplethree-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 0-5degrees of relative dorsiflexion when positioned in the shoe.

A posterior leaf spring AFO is trimmed posterior to the malleoli allowing 1) controlledplantarflexion 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 currentpre-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 bedesigned 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 whenusing 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 plasticAFO 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 anteriorstop 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 kneeextension 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 ankleat 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. Thelength 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 groundreaction 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 apretibial 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 inthe 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 Figure 13 AFO with pretibial shell,Reaction AFO Double adjustable ankle

joints


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References

1. Fish DJ, Nielsen JP. Clinical Assessment of Human Gait. JPO 1993; Vol. 5, No. 2: 27-36.

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 UnstableSubtalar Joint: Orthotics and Prosthetics, 1979; 33 (1): 31-41.

4. Yang GW, Chu DS. Floor Reaction Orthosis: Clinical Experience. Orthotics andProsthetics 1986; Vol. 40, No. 1, 33-37.

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Orthotics Lower Limb