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LOWER EXTREMITY ORTHOTICS TO ENHANCE AMBULATION INTRODUCTION 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. 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 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. 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 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) 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. 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 AFO 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). 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 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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