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Alternative Strategies in Tone-Reducing AFO Design

Michael Lohman, MEd, CO, OTR/L
Helene Goldstein, MA, OTR/L

ABSTRACT

A review of recent literature indicates several divergent perspectives are used currently in managing patients with spastic lower extremities. The intent of this article is to analyze methods presented in published clinical orthotic studies regarding biomechanical and neurophysiological approaches to tonereducing orthoses. Each management rationale will be briefly summarized, including biomechanical, primitive reflex activity, pressure over muscle tendons, prolonged stretch, and orthokinetics. Results of lower-extremity orthotic studies will be reviewed. Discussion will focus on current research efforts and indications for future clinical practice.

Introduction

Spasticity resulting from upper motor neuron lesion dysfunction poses a significant challenge in thermoplastic ankle-foot orthosis (AFO) design. Traditionally, conventional biomechanical design principles that combat deformity-as displayed in the designs of the Seattle, Engen and Teufel AFOs- have been recommended for patients with increased muscle tone (1). Recent literature has reflected an increased interest in neurophysiological design principles (2). These principles focus not only on joint mobility and stability criteria but also attempt to use the sensory feedback provided by the orthosis to alter or reduce muscle tone (3,4). The theoretical basis for tone-reducing orthoses includes several divergent rationales that attempt to justify specific design strategies. These rationales include inhibition of reflexes, pressure over muscle insertions, stretch and orthokinetics.

Some of these tone-reducing designs have been evident in recent orthotic literature. But additional techniques documented in allied health literature have not been applied widely within the orthotic profession. As members of the multidisciplinary team, orthotists should be in the forefront of new developments regarding orthotic alternatives for the spastic patient. Controversy still surrounds various neurophysiological approaches due to the lack of solid scientific research about the underlying causes of spasticity and its management.

McPherson states spasticity is indicative of central nervous system dysfunction and may result in exaggerated or hyperactive stretch reflexes (5). These reflexes can be triggered actively by a dynamic response-the force and velocity of movement involved while the muscle is being stretched-or can be influenced by a static response to the muscle's maintained state of stretch. Muscle tone is the resistance a muscle offers to passive stretch or elongation. Hypertonicity implies that the resistance or force of the muscle is sufficient to move the limb toward a contracted or abnormal resting position (5). The biomechanical force exerted by an AFO on spastic plantarflexors is an attempt to reduce the passive component or inherent visco-elastic properties of the muscles to thereby prevent or correct contracture. The dynamic component or tension within the muscle during stretch is critical to consider in orthotic evaluation and selection.

The purpose of this discussion is twofold:

• to concisely and objectively describe various AFO design rationales that have been reported as effective in reducing the passive component of hypertonus and
• to examine rationales that focus on the prevention of the triggering abnormal reflex activity associated with spasticity (6-8). The literature reflects a lack of consensus regarding a uniform or quantitative method of measuring tone (9-17). Many authors did not monitor tone directly, relying instead on a qualitative description of the effects of the AFO on functional skills, such as gait or transfer (8,10,12). Some design rationales are equally applicable to both upper- and lower-extremity orthotic design (13,14,18).

Although some authors limited their population samples to either developmentally acquired (e.g., cerebral palsy) or adult injury (e.g., cerebral vascular accident), Bronkhurst and Lamb included both groups in their research (2,4,12,13,15,19,20). Further, some authors have incorporated more than one rationale in their experimental design, which may confound the clarity of results (10,11). Each of the four broad rationales (reflexes, insertions, stretch and orthokinetics) that will be discussed are interrelated and contribute to a fuller understanding of tone-reducing design and to the critical importance of a comprehensive patient evaluation.

Primitive Reflex Activity

A reflex consists of a motor act that is elicited by some specific sensory input. Primitive reflexes appear at birth and become integrated once higher-order reflexes leading to more complicated movements emerge. When the central nervous system-which is responsible for normal voluntary control-is damaged, these primitive reflexes will again dominate motor activity and contribute to patterns of spasticity (6).

Orthotic literature cites Duncan as a primary reference for reflexes of the foot that influence AFO design (3,4,7). Four of the reflexes he named are the toe (plantar) grasp reflex, positive supporting action, and the inversion and eversion reflexes.

The toe grasp (plantar grasp) reflex is triggered by pressure over the ball of the foot and results in marked increase of tone in toe flexion or ankle plantarflexion. AFO design features that reportedly reduce stimulus pressure include spastic inhibitor bars, foam toe separators and metatarsal arch supports to unweigh metatarsal heads (see Figure la ,Figure 2 ,and Figure 3a ) (2,4,8,9).

The positive supporting reaction, also triggered by pressure over the ball of the foot, results in a total extensor pattern with a noted increase of tone in plantarflexion and inversion. AFO design features that reportedly reduce stimulus pressure include toe extensions, toe hyperextensions and pressure under metatarsal heads (see Figure 3b and Figure 4a ) (8,10). Bronkhurst and Lamb report the use of an internal heel may serve to unweigh the ball of the foot although the authors say this modification primarily facilitates ankle dorsiflexion (see Figure 1b ) (4).

The inversion reflex is triggered by pressure over the first metatarsal head along the medial border of the foot while, conversely, the eversion reflex is triggered by pressure over the fifth metatarsal head along the foot's lateral border. An AFO design feature that reportedly reduces the abnormal tone )f these patterns is stimulating the antagonistic reflex to balance the deformity by extending the metatarsal pad o the foot's extreme lateral margin, hereby triggering the eversion reflex 2,3). Conversely, by extending the metatarsal pad medially, the inversion reflex would be triggered (see Figure 3c ).

Pressure over Muscle Insertion

In 1974, Farmer reported that continuous firm pressure at the point of insertion has a tone-reducing effect (11). Recent orthotic literature has incorporated this AFO design by applying pressure on either side of the tendo-calcaneus and at the insertion of the gastrocnemius-soleus muscle group (see Figure 4b ) (2,10,12). Increased muscle tone in this group frequently is accompanied by excessive plantar flexion. This rationale might also be applied to control excessive knee extension tone. By incorporating a patella tendon-bearing design into a floor reaction AFO, pressure would be maintained at the insertion of the quadriceps (see Figure 5 ).

Active and Static Prolonged Stretch

Tone-reducing AFOs and inhibitory casting have been observed to decrease reflex tone by providing mechanical stabilization of the joint and altering properties of the muscle spindle through static immobilization (10,12, 17). These goals have been achieved through a variety of designs that provide total ankle-foot contact. Such designs include plaster serial casts, supramalleolar orthoses and bivalved thermoplastic AFOs (see Figure 6 and Figure 7 ) (3,9,12,14,16,19,21). Adjustable designs allow for a graduated change in the position of joint range at which static force is applied (3,18).

Fracture brace joint components with adjustable stops have been used successfully to maintain consistent force (18). Some authors propose using a dynamic force of spring steel or elastic as a more effective method of providing the slow, continuous stretch necessary to effect a reduction of the passive component of hypertonia (20,22). Both studies found dynamic stretch more effective than static stretch. These authors focused on reducing wrist and finger flexor spasticity, but these upper-extremity designs do not have to contend with floor reaction forces created during gait (18, 20, 22).

Orthokinetics

Originally developed in 1927 by Julius Fuchs, an orthopedic surgeon, the orthokinetic rationale focuses on the physical effects of materials placed over muscle bellies (23). Passive field materials (those that are cool, rigid and smooth), including thermoplastics, tend to produce an inhibitory effect. These materials do not mechanically deform or elongate underlying muscles, which would stimulate or increase muscle tone.

Active field materials (those that are warm, expansive and textured) include materials such as foam, elastic and hook/loop straps and tend to produce a facilitatory effect. During muscle contraction these materials tend to expand, providing minute pinching motions to the dermatone over the active muscle. This facilitation is referred to as exteroproprioceptive stimulation, and its principles have been incorporated in AFO design (see Figure 7 ) (11). The foam interface on the anterior shell of the Chattanooga articulating AFO provides active field stimulation of anterior tibialis that would encourage dorsiflexion. The unpadded posterior shell of a conventional AFO provides passive field inhibition to gastronemius, reducing spasticity to plantarflexors. It is important to understand these dual orthokinetic concepts are interrelated and should be applied simultaneously. When attempting to provide passive reduced stimulus over the hypertonic muscle, always apply active increased stimulus over the antagonist muscle group.

Conclusion

Orthotic literature reflects a wide spectrum of treatment with AFO designs reported to be effective in tone reduction. As professional practitioners, orthotists must provide direction to the clinical decision-making process in applying these devices. Although many of these rationales appear to be mutually supportive as a collective strategy, some refinement of application standards is indicated. Some tone-reducing variables requiring further investigation include:

• location of metatarsal bar
• extent of toe extension
• amount of tendon insertion pressure
• effectiveness of orthokinetic principles

The continued development of neurophysiological AFO orthotic design reflects broad multidisciplinary collaboration. While current research has not yet resolved specific application controversies, advancements in this field have expanded orthotic options and improved the quality of life for the patients and families served.

Michael Lohman, MEd, CO, OTR/L, is a staff practitioner for Orthopedic Services Inc., 7820 Wakeley Plaza, Omaha, NE 68114. Lohman is also assistant clinical professor at Creighton University's Department of Occupational Therapy.

Helene Goldstein, MA, OTR/L, is assistant professor of occupational therapy at Creighton University, 24th at California St., Omaha, NE 68178.

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