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JPO Journal of Prosthetics & Orthotics:
doi: 10.1097/JPO.0b013e3182173a61
Article

Design and Effect of Ankle-Foot Orthoses Proposed to Influence Muscle Tone: A Review

Kobayashi, Toshiki PhD; Leung, Aaron K.L. PhD; Hutchins, Stephen W. PhD

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Author Information

TOSHIKI KOBAYASHI, PhD, is affiliated with the Orthocare Innovations, Mountlake Terrace, Washington.

AARON K.L. LEUNG, PhD, is affiliated with the Department of Health Technology and Informatics, The Hong Kong Polytechnic University, Hong Kong, People's Republic of China.

STEPHEN W. HUTCHINS, PhD, is affiliated with the Centre for Health, Sport and Rehabilitation Sciences Research, University of Salford, Salford, United Kingdom.

Disclosure: The authors declare no conflict of interest.

This work was supported by The Hong Kong Polytechnic University International Postgraduate Scholarships for PhD studies.

Correspondence to: Toshiki Kobayashi, PhD, 6405 218th SW, Suite 301, Mountlake Terrace, WA 98043; e-mail: tkobayashi@orthocareinnovations.com.

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Abstract

Ankle-foot orthoses (AFOs) designed and proposed to influence muscle tone are generally called as “tone-reducing” AFOs, “tone-inhibiting” AFOs, or “dynamic” AFOs. These orthoses were originally evolved from the use of plaster casts to influence the positive support reflex or tonic reflex, which were either triggered by pressing reflexogenous areas on the plantar surface of the foot or suppressed by offloading them. The effects of wearing AFOs to influence muscle tone have mainly been studied in patients with cerebral palsy, stroke, or head injury. Although different AFO designs exist, it seems that there is a lack of evidence to demonstrate that these AFOs can actually reduce or inhibit spastic muscle tone. This article specifically reviews the classification of patient groups recruited in previous studies, the design characteristics of AFOs, and the clinical and biomechanical effects reported. The results of this review suggested that the level of evidence for AFOs being able to influence muscle tone was very low. Therefore, further research with randomized controlled trials is required to investigate their clinical effects.

Spasticity is one of the most common neurological impairments, which may occur after an upper motor neuron lesion. It is defined as “disordered sensorimotor control, resulting from an upper motor neuron lesion, presenting as intermittent or sustained involuntary activation of muscle.”1 This usually involves a lesion of both the pyramidal and parapyramidal systems, which may be caused in association with stroke, spinal cord injury, multiple sclerosis, brain trauma, cerebral palsy, or head injury.2 Joints that are affected by spastic muscles may develop deformity, pain, weakness, and abnormal movement.2

Ankle-foot orthoses (AFOs) designed and proposed to influence muscle tone (AFOs with tone-influencing designs) are widely known as “tone-reducing” AFOs, “tone-inhibiting” AFOs, or “dynamic” AFOs (DAFOs). These orthoses were evolved from techniques used in the application of plaster casts for patients with spastic muscle tone. These plaster casts were designed to influence the positive support response or tonic reflex, which had been observed in response to cutaneous stimulation of reflexogenous areas on the plantar surface of the feet in patients with cerebral palsy.3 One of the first attempts in designing such a cast for patients with cerebral palsy was reported by Sussman and Cusik.4 It was followed by a series of subsequent studies on individuals with cerebral palsy.5–10

The theory was then applied to AFO designs. Following the report of their effect on gait in patients with head injury by Zachazewski et al.,11 various AFO designs were proposed and investigated. In their review article, Lohman and Goldstein12 identified four potential tone reduction features, which would be worthy of further investigation. These were a) the addition and location of a metatarsal bar or dome; b) the extent of toe extension induced by the design; c) the amount of loading added to areas adjacent to the Achilles tendon insertion point; and d) the effectiveness of the orthokinetic principles applied in the design.

Although a number of articles on AFOs with tone- influencing designs have been published since then, an up-to-date review of available studies was thought warranted in the recent need of evidence-based practice in orthotic interventions. This article aims to present a concise review of the current state of knowledge to clinicians and researchers by specifically investigating: a) the classification of patient groups recruited into the studies selected; b) the design characteristics of the AFOs used; and c) their clinical effect.

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METHODS

A literature search was conducted using Google Scholar, ISI web of knowledge, Medline, Scopus and RECAL legacy, and a thorough review of cited references from appropriate articles. Key words used were AFO, head injury, cerebral palsy, tone, inhibiting, orthosis, orthotics, reducing, spasticity, spinal cord injury, and stroke. Inclusion criteria for the article of this review article were as follows:

1. Studies were performed on AFOs with tone-influencing designs.

2. AFO designs used in the study were described.

3. Publication of the study was in a peer-reviewed journal in English.

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RESULTS

The results of the literature search identified 19 suitable articles (Table 1).11,13–30 These articles were analyzed and compared to produce the following summary.

Table 1
Table 1
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PATIENT GROUPS RECRUITED IN PREVIOUS STUDIES

Most of the volunteer subjects included in the articles reviewed were children with cerebral palsy,14–16,18,21–23,25–28 although patients with head injury,11,13,15,16 stroke,13,17,19–20,24,30 and spinal cord injury29 were also recruited (Table 2).

Table 2
Table 2
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DESIGN CHARACTERISTICS

The design characteristics of the AFOs with tone- influencing designs used in the reviewed studies are summarized in Table 3. These included nonarticulated AFOs,11,14,15,17,20,21,24,29 articulated AFOs,22,30 supramalleolar orthoses,14,16,18,19,22–23,25–28 and neurophysiological AFOs.13 However, variations were found in certain pertinent AFO design features within these categories.

Table 3
Table 3
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THE AMOUNT OF EXTENSION OF THE TOES

The majority of studies used orthoses with a toes piece either extended11,15,18,20,22,24,29 or kept horizontal.16,19,21,23,25–28,30 However, the amount of toe elevation or extension varied in the literature. In one study, they were elevated by approximately 10°,18 whereas another recommended their elevation by 0.5 cm.22 A 5-mm-thick boomerang-shaped bar was applied on patients with stroke.24 Zachazewski et al.11 claimed that the toes piece should be hyperextended, whereas Bronkhorst and Lamb15 stated an opposite opinion.

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LOADING ON THE METATARSAL HEADS

Although most articles described plaster model rectifications to relieve loading from the metatarsal heads,13,15,16,19–23,25–30 two articles suggested to increase loading on them.11,22 The addition of felt material that extended from under the toes to a more proximal position underneath the metatarsophalangeal joints was believed to help prevent plantar grasping.11 A 0.5 cm of elevation beneath the metatarsal heads was used in an attempt to reduce excessive tone in plantarflexion and inversion.22

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ALIGNMENT OF THE ANKLE WITHIN THE AFO

The ankle joint was kept at 90° with the subtalar joint in a neutral position in most AFO designs.11,15,17,20,21 The subtalar joint was again positioned at its neutral position but with free plantar/dorsiflexion ankle movements made available in DAFOs.16,19,23,25–28 Mediolateral ankle stability was useful in providing control of plantar/dorsiflexion in them. The neurophysiological AFOs positioned the subtalar, midfoot, and forefoot to their neutral position and tuned plantar/dorsiflexion ankle movements by the stiffness of the orthoses.13 The ankle joint was kept at 90° with plantarflexion stop and free dorsiflexion in those adopted articulated AFO designs.22,30

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LOADING ON THE HEEL

Loading on the heel was relieved in the majority of studies,16,19,21,23,25–28,30 although loading on the heel was augmented in one study assuming that it would assist dorsiflexion of the ankle by increasing cutaneous stimulation of the reflexogenous area.15

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LOADING ON THE TENDON INSERTION

Loading was applied medially and laterally to the Achilles tendon insertion area in an attempt to improve contact and afford more rigid immobilization.11,29

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CLINICAL EFFECTS

The clinical effects of the AFOs reported on temporal and spatial gait parameters, kinetics and kinematics, foot loading, balance, posture, function, and alteration to tone are summarized in Table 4.

Table 4
Table 4
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GAIT

Increase in gait velocity17,20,24,29 and in step or stride length,17,20,21,26,29 decrease or increase in cadence,17,20,21,24 reduction in excessive knee flexion,18 improvement in foot loading pattern and support,19,20 improvement in ankle kinematics,11,21 decrease in positive support reflex,11,15 and reduction in time spent in double stance phase29 have all been reported.

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BALANCE, POSTURE, AND FUNCTION

Improvement in posture, balance, and standing were reported14,16,25 along with improvement in gross motor functional measures.27

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NEUROPHYSIOLOGICAL EFFECTS

No conclusive neurophysiological effects were reported when walking with the AFOs.26,29,30 Spasticity assessed by studying the median frequency26 and the ratio of maximum Hoffman reflex amplitude to maximum muscle response amplitude30 of electromyography (EMG) signals did not show reductions with the use of such AFOs. However, the mean EMG activity of the gastrocnemius muscle was reported to be better modulated when walking with the AFOs in one study.29

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CARRYOVER EFFECTS

A potential carryover effect was reported in two studies.15,19

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COMPARISON WITH STANDARD AFOs

Studies in which comparisons with other types of AFOs were performed did not show conclusive evidence that AFOs with tone-influencing designs were superior to others. Only two studies17,20 showed more positive effects on gait parameters in comparison with standard AFOs, whereas others21,22,26 did not show any significant difference.

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DISCUSSION

This article concisely reviewed the current state of knowledge on the effect of the AFOs with tone-influencing designs. The literature search demonstrated that the relevant studies mainly involved patients with cerebral palsy, followed by those with head injury or stroke. Although spasticity is common among these patient groups, it is not clear which of them would benefit the most from these AFOs.

Marked variations in AFO design parameters were found, such as 1) the type of AFO (i.e., articulated, nonarticulated, supramalleolar, or neurophysiological) used; 2) the amount of toe extension; 3) the amount of loading on metatarsal heads; 4) the amount of loading at the heel; 5) the amount of loading on the Achilles tendon insertion area; and 6) the alignment of an ankle joint. Although more recent studies19,23,25–28 adopted DAFOs whose design characteristics were proposed by Hylton,16 the efficacy of these design parameters requires further investigation.

The AFOs may have positive effect on gait, posture, and EMG data in patients with spasticity. However, it was not conclusive whether they could actually reduce or inhibit muscle tone. Currently, there is no definitive method to quantify spasticity or muscle tone.31,32 This is one of the obstacles in evaluating so-called tone-reducing or tone-inhibiting effects. No significant effect in the median frequency signal26 or Hoffman reflex amplitude30 of the EMG have been reported with the use of such AFOs. The findings of these studies would question their neurophysiological effect. In addition, some studies showed that AFOs with tone-influencing designs did not have any significant effects in comparison with standard AFO designs.21,22,26 Some studies have also suggested that the AFOs may be recommended for use as an adjunct to appropriate physiotherapy, but they might not be effective if they are used alone.16,18,25

On the basis of the results of this literature review, we conclude that the efficacy of the AFOs with tone-influencing designs (i.e., in reducing or inhibiting muscle tone) has not been sufficiently proven because of the very low level of evidence despite the positive clinical effect reported in previous studies. This is because the study design adopted in precedence studies was not robust enough. The articles reviewed showed an equivalent range of level of evidence of Grade C proposed by Sacket.33 Indeed, it is difficult to make a strong argument for a particular position on whether AFOs are able to influence muscle tone when the evidence levels are so low. This is in agreement with previous publications.34,35 Thus, it would not be appropriate to use the term tone-reducing or tone-inhibiting for such AFOs until their efficacy is confirmed with grade A evidence level.33

In summary, the following issues need to be further investigated regarding the use of AFOs with tone-influencing designs:

1. The classification of patient groups that may benefit from these types of AFOs needs to be more accurately defined.

2. The design parameters used in these types of AFOs need to be more quantitatively defined.

3. The statistical and clinically significant effects of these types of AFOs in comparison with standard AFOs need to be demonstrated with randomized controlled trials.

The authors would therefore recommend that future studies should carefully investigate these aspects systematically.

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CONCLUSION

More systematic research with randomized controlled trials is necessary to determine whether AFOs with tone-influencing designs would truly have positive clinical effects. Moreover, it is required to reexamine whether such AFOs do in fact have tone-reducing or tone-inhibiting effects while ambulating. If their positive effects are confirmed in future studies, it will be essential to investigate their optimal designs and to demonstrate which group of patients would benefit the most from the AFOs.

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REFERENCES

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31. Kobayashi T, Leung AK, Akazawa Y, et al. Quantitative measurement of spastic ankle joint stiffness using a manual device: a preliminary study. J Biomech 2010;43:1831–1834.

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34. Bowers R, Ross K. A review of the effectiveness of lower limb orthoses used in cerebral palsy. In: Morris C, Condie D, eds. Recent Developments in Healthcare for Cerebral Palsy: Implications and Opportunities for Orthotics. Copenhagen, Denmark: International Society for Prosthetics and Orthotics; 2009:235–297.

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KEY INDEXING TERMS: ankle-foot orthosis; brain injury; tone; muscle; cerebral palsy; spasticity; stroke

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