When prescribing an ankle foot orthosis or neuroprosthesis for a patient with acute drop foot following stroke, lower extremity practitioners should consider the device’s potential effects on neural plasticity and motor relearning in addition to its potential effects on gait.

By Chad Lairamore, PhD, PT, CBISt

Cerebrovascular accident (CVA) is a leading cause of disability in the US¹ and frequently results in hemiparesis, impairing the patient’s ability to ambulate.^2-5 The hemiparesis often affects dorsiflexion of the ankle, resulting in foot drop and decreased gait velocity.^2-5

One method often employed to treat foot drop is the use of an ankle foot orthosis (AFO).  AFOs have been shown to provide many benefits for improving hemiparetic gait, including increased gait velocity,^5-14 a more symmetrical gait pattern, improved foot clearance during swing,^6,9,11,13,15 and decreased energy expendi­ture.^8,12 Traditionally, a definitive custom plastic AFO is not prescribed until improvements in gait have reached a plateau, four to six months after CVA, as recommended by McCollough et al.¹⁶

This raises the question of what should be done to treat foot drop when an individual is in the initial recovery phase following CVA. Some authors have suggested using a hinged metal upright AFO during the initial recovery period to allow therapeutic adjustments to the AFO as the patient improves.¹⁶ A rigid Valens AFO also has been suggested as an integral part of CVA rehabilitation.¹⁵ However, many practitioners are reluctant to prescribe an AFO for patients with an acute stroke out of fear that using an orthosis encourages disuse of the tibialis anterior muscle, which may result in long-term dependence on bracing.^15,17

The tibialis anterior muscle contributes significantly to control of the foot during the loading and swing phases of the gait cycle.^18,19 Persistent impairments in the ability to contract the tibialis anterior muscle result in foot drop or foot slap and cause subsequent gait abnormalities.¹⁸ During the swing phase of gait the tibialis anterior muscle is maximally contracted to dorsiflex the foot and maintain dorsiflexion throughout the swing phase, providing foot clearance as the leg is advanced.^18,19 If an individual is unable to contract the tibialis anterior muscle sufficiently they will exhibit foot drop and have difficulty advancing the leg and may exhibit excessive hip flexion, circumduction, lateral trunk lean, or vaulting to advance the limb.¹⁸ During the loading phase of gait the ankle plantar flexes and the tibialis anterior muscle is eccentrically active to control the rate of plantar flexion.^18,19 If the tibialis anterior muscle is weak, an individual will exhibit foot slap, which disrupts the heel rocker, forward progression, and shock absorption.¹⁸

Several studies have demonstrated a decrease in tibialis anterior muscle activity during gait with use of an AFO. Hesse et al¹⁵ recorded a decrease in tibialis anterior muscle activation when individuals post-CVA walked with a rigid AFO compared with walking without an AFO. Geboers et al¹⁷ found an immediate decrease in tibialis anterior muscle firing when a rigid AFO was used in healthy individuals and those with peripheral paresis. A study by Crabtree and Higginson²⁰ confirmed these clinical findings through an AFO computer model that predicted torque based on ankle angle and velocity during walking. The authors estimated tibialis anterior muscle activity would decrease across the entire gait cycle with use of a posterior leaf AFO.²⁰ These studies support practitioners’ notions that use of an AFO may lead to decreased use of the tibialis anterior muscle.

Using functional magnetic resonance imaging, current research regarding CVA recovery has demonstrated plasticity of cortical areas of the brain related to walking.^21-24 A major construct of neural plasticity for promotion of motor relearning is to use the affected limb after an upper motor neuron injury.^25-28 In fact, the first two principles of experience-dependent plasticity espoused by Kleim and Jones are simply stated as: “use it or lose it” and “use it and improve it.”²⁹ These principles of neuroplasticity suggest that use of an orthosis and the resulting decrease in muscle activity will likely result in a decreased opportunity for motor relearning during the recovery phase after CVA. This potential decrease in opportunity for motor relearning may make practitioners reluctant to use an orthosis for patients with an acute CVA.

Dynamic ankle orthosis

Clinicians have attempted to design orthoses to provide ankle stability and dorsiflexion while minimizing the effects on decreasing muscle recruitment during gait to prevent learned nonuse.³⁰ Included in this group of orthoses is the dynamic ankle orthosis (DAO) (Figure 1).³⁰ A DAO is constructed from Polyflex II and consists of a calcaneal foot plate, a short medial upright, and a cuff.  It theoretically provides medial ankle stability and a proper base of support for weight acceptance, allows for normal tibial advance­ment during stance phase of gait, and assists in foot clearance during swing phase.³⁰

Researchers from the University of Central Arkansas in Conway recruited 15 poststroke individuals who were recently discharged from inpatient rehabilitation to investigate the effects of DAO usage on hemiplegic gait. All participants were six months or less poststroke (mean days poststroke, 86.3 ± 50.41; range, 16-208 days). Investigators used a repeated measure design and participants walked at a self-selected speed under three conditions: wearing a DAO, wearing a posterior leaf spring AFO, or wearing no orthosis. Three-dimensional motion analysis and electromyography (EMG) of the tibialis anterior muscle were performed.

The findings, published in December in Prosthetics & Orthotics International, demonstrated a significant decrease in tibialis anterior muscle EMG activity during the swing phase of gait with the DAO compared with walking without an AFO.³¹ This suggests that even though the DAO is designed to be less restrictive than traditional AFOs, that design does not translate into increased tibialis anterior muscle activity during gait. In fact, there was no difference in muscle activity between the DAO and the posterior leaf spring AFO. While the DAO has a short footplate, functionally it is coupled with the shoe, and together they provide sagittal plane stability and subsequently reduce demand on ankle dorsiflexors.

Clinicians should take the decrease in tibialis anterior muscle activity into consideration. The decrease in muscle activity suggests that individuals recovering from a CVA may have less opportunity for motor relearning compared with walking without an orthosis. Yet clinicians should also consider the potential benefits of orthosis usage as well as the potential for an increase in the amount of walking which could offset any decrease in tibialis anterior activity.

Carbon fiber AFOs

Another type of AFO commonly used clinically for patients with foot drop following stroke is a carbon fiber AFO. A carbon fiber AFO stores energy throughout stance and has a spring-like effect, increasing the plantar flexion moment and power generation during push off.^32-34 Additionally, use of this type of AFO in a population with chronic stroke has been shown to increase gait speed and decrease energy required to ambulate.^35,36 However, research has shown the use of a carbon fiber AFO directly takes over the work of the ankle.³⁵

As with use of the DAO, the reduced demand on the ankle musculature may result in decreased opportunity for motor relearning, and clinicians should consider this when treating a patient during the initial recovery phase after a CVA. Conversely, it is plausible that use of a carbon fiber orthosis, which increases the capacity for gait by decreasing energy demand, may be more beneficial than walking without an AFO. Currently, there is limited research on carbon fiber AFOs and their use as a modality during rehabilitation and there is a need for longitudinal studies investigating the effect of carbon fiber AFO usage on relearning gait to inform clinical decision-making.

Functional electrical stimulation

Another method gaining popularity for eliciting dorsiflexion during gait in individuals with CVA is functional electrical stimulation (FES). FES is a technique in which a small electrical current is applied to the peroneal nerve and the tibialis anterior muscle during the swing phase of gait to elicit a contraction of the tibialis anterior muscle when the foot is advanced. Studies have shown the use of FES for its orthotic effect as a neuroprosthesis increases gait velocity, decreases the physiological demand of walking, improves gait symmetry, increases balance during gait, and improves social integration for individuals with CVA.^37-46

In addition, use of FES has demonstrated a therapeutic effect on walking performance up to 11 months after its use in patients with chronic CVA.^44,45 FES has been shown to facilitate normal tibialis anterior muscle electromyography and improve gait velocity in individuals with CVA.^40,42,47 Additionally, long-term use of FES increases activation of motor cortical areas in the brain.⁴⁸ It has also been suggested that regular use of FES can improve control over voluntary neural motor pathways and inhibit abnormal reflexes.⁴⁹ Additionally, Dimitrijevic et al⁵⁰ and Weingard et al⁵¹ both suggest use of FES during early gait training may promote motor relearning.

In a recent unpublished study investigating the effects of FES on tibialis anterior muscle activity, researchers from the University of Central Arkansas performed a single-blinded randomized controlled trial with 26 participants. All participants had foot drop secondary to an acute nonprogressive neurological injury and were enrolled in an acute inpatient rehabilitation facility. Participants were divided into two groups: one received FES during gait training and the other received sham stimulation during gait training. For the duration of the participants’ stay in the facility, FES or sham FES was administered during gait training three times a week in 45-minute sessions. Investigators used the Bioness L300 (Figure 2) to administer stimulation to the FES group as well as minimal sensory stimulation over the tibia to the sham stimulation group. Before and after intervention researchers evaluated EMGs of the tibialis anterior muscle and spatiotemporal gait parameters as participants walked 10 feet. The therapeutic effect of the FES was in question, therefore participants were tested without use of the FES in pre- and post-testing.

Although there were no differences between groups at the onset of the study, the researchers found the group receiving FES exhibited significantly more tibialis anterior muscle activity during the swing phase of gait at the end of the study compared with the sham stimulation group. Given the evidence suggesting use of an orthosis decreases tibialis anterior muscle activity, the FES-facilitated increase in tibialis anterior muscle activity may play a role in improving motor relearning as opposed to performing gait training without FES or with an orthosis.

Another option for individuals with decreased ankle stability is to use FES in combination with an AFO.  Frequently individuals with hemiparesis demonstrate mediolateral ankle instability during stance in conjunction with foot drop during swing. For these individuals it may be beneficial to use an AFO in conjunction with FES, as the technique does not provide stimulation or stability during the stance phase of gait.

FES improves gait and foot clearance without diminishing tibialis anterior muscle activity, but it is not for everyone. Currently, there is minimal insurance coverage for an FES neuroprosthesis, and the cost is significantly greater than that of an AFO and may be prohibitive for many individuals. In addition, the tingling sensation associated with FES is not tolerated well by all individuals and contraindications, such as a demand-type pacemaker or excessive edema, may also limit use.

Conclusion

When making decisions about prescribing a device for treating foot drop during the initial recovery phase after a CVA one should consider how the orthosis or neuroprosthesis will influence recovery and brain plasticity, in addition to its effect on gait. Although further longitudinal studies are needed, the most recent literature suggests FES can provide foot clearance and improve gait characteristics while promoting motor relearning.

In situations in which a patient requires use of an orthosis, clinicians should consider the potential for decreased tibialis anterior muscle activity that can be associated with device use. Conversely, clinicians should also consider the benefits of different orthoses, including the possibility that orthosis use can be associated with increased activity levels and improved symmetry of gait, which could offset any decrease in muscle activity.

Chad Lairamore, PhD, PT, CBISt, is assistant professor at the University of Central Arkansas in Conway, and a physical therapist at Baptist Health Rehabilitation Hospital in Little Rock, AR.

Disclosure: The author has no affiliation with products, devices, or companies mentioned in the article.