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Understanding the mechanics of foot drop conditions guides the way to more effective treatment

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Vol. 23 • Issue 16 • Page 16

Foot Drop

Foot drop refers to the downward point of toes from a plantar-flexed ankle, resulting from weakness or contracture around the ankle joint. In the language of the ICD it is an impairment of body structure that influences activities and participation. Causes of foot drop include a spinal cord injury, cerebral palsy, an acquired brain injury such as a traumatic brain injury or stroke, injury to tendons or muscles, or a peripheral nerve injury.

The dropped foot results from weakness of the tibialis anterior. Often the extensor digitorum longus and extensor hallucis longus are involved. Involvement of the invertors and evertors results in an equus appearance of the foot. The consequence is difficulty clearing the involved foot during swing phase.

In normal gait, the swing foot moves away from the floor following push off and clears the floor as the hip and knee move into extension. The foot moves into full dorsiflexion and the hip flexes forward. It remains in dorsiflexion as the heel moves toward the floor for heel strike. In the contralateral limb, the forefoot lowers to the ground following heel contact. The tibia moves forward over the stationary foot via the heel rocker. As the foot continues to move forward, the heel rises off the floor while the ankle plantar flexes and the toes extend for push off.

When the foot remains in a plantar-flexed position, gait mechanics are affected. The most noticeable changes are a shortened stride length, decreased gait velocity, decreased stability and decreased shock absorption at the knee.

Compensatory Movement

With drop foot, forefoot or toe contact occurs instead of heel strike. Following contact the foot immediately goes into flat foot, causing the knee to hyperextend. This prevents the rocker system of the foot to function as the tibia moves forward. Instead of a smooth motion, the tibia is only able to move through available passive range of motion (ROM). Limb advancement is abridged, resulting in shortened stance duration. Heel off occurs once tibial movement halts. The foot is able to move into position for push off, but weak or absent toe extensors limit the amount of force generated. Sometimes forward trunk lean occurs to assist with moving the leg forward over the foot.

When the limb is in swing phase, toe clearance is impaired. In mild cases the toes may clear the floor but with little or no room to spare. In severe cases the toes and forefoot can drag along the floor. In order to compensate, gait deviations develop to assist with clearance of the foot. Compensations at the hip include exaggerated flexion or circumduction. The trunk will lean laterally away from the affected leg and may be accompanied by hip vaulting. The purpose of these deviations is to increase the amount of space between the foot and the toes.

A secondary effect of foot drop is increased energy expenditure for gait. Normal gait follows a pendulum pattern in which energy is transferred from limb to limb at heel strike. Combined with effective use of gravity, minimal energy is exerted to walk. When gait mechanics are altered, there is increased muscular activity. Instead of relying on the transference of energy, the body must generate the energy necessary to move forward and control balance. The increased energy demand can limit endurance and increase the

Archive ImageA

The author demonstrates a treatment technique for a patient with foot drop.

potential for falls.

Management of foot drop focuses on minimizing further loss of ROM, limiting the impact on gait, and preventing damage to the bones and soft tissue of the foot. A secondary goal is decreasing the risk of falls. Depending on the cause, this is accomplished by development of an exercise program and orthotics. Functional electrical stimulation (FES) and neuroprotheses have also been effective.

In cases where foot drop is the result of a peripheral nerve injury, a solid plastic ankle-foot orthosis (AFO) is used. The purpose is to prevent damage to the foot and maintain ROM while the nerve regenerates. No movement is permitted at the ankle. Strengthening exercises are begun once muscle function returns. Sometimes surgery is required to repair or relocate damaged nerves.

Treatment following a neuromuscular or acquired brain injury is more complex. In this case, both the underlying body impairment and the functional loss must be addressed. An AFO is used to position the foot in either neutral or slight dorsiflexion to assist with toe clearance. A built-up medial wall can be used to control an equus deformity. In order to facilitate normal movement and positioning during transfers, an AFO with a hinge is used. The hinge allows the ankle to dorsiflex so the foot can be positioned beneath the patient. It also creates a more normal gait pattern. Dorsiflexion assist can be added if active dorsiflexion is present.

Use of AFOs

The use of an AFO can be temporary or permanent. If the patient develops activity ankle control, the AFO may eventually be discarded once the muscles are strong enough to function independently. One of the secondary consequences of foot drop is shortening of the Achilles tendon. Use of an AFO along with resting splints can reduce the risk of contracture.

FES can be used separately or as part of the AFO. It is applied to the dorsiflexors to facilitate activation during swing. Depending on the patient's cognition and functional level, it can be applied as part of an exercise program for strengthening or used during gait training. Activation of the stimulation can be performed manually by the therapist or via a heel switch in the patient's shoe. Weight bearing through the heel activates the switch.

A neuroprothesis is an FES-based orthosis. The stimulation facilitates dorsiflexion while the prosthesis is worn. The stimulation is provided via surface electrodes over the muscle belly of the tibialis anterior. The result is a lifting of the toes as weight comes off the heel. The lift is maintained during swing phase. As weight moves onto the heel, the stimulation stops firing and the dorsiflexors relax. The amount and severity of gait deviations are reduced, as is energy expenditure.

Several studies are showing a relationship between the use of FES and neuroprotheses to motor learning during gait. Continued practice of dorsiflexion with each step facilitates motor learning of the action. As the patient continues to walk, the practice continues. Over time, improvement is seen even when not using the stimulation.

While AFOs can be used for almost any patient, FES and neuroprotheses have some limitations. The patient must have adequate cognition to understand the purpose of the stimulation and provide feedback about the intensity. Fitting a neuroprothesis can be time consuming. Appropriate electrode placement must be determined on an individual basis. Not everyone tolerates the sensation of electricity. Some people have allergic reactions to the electrodes. Compared to AFOs and FES alone, the cost is significantly greater and may not be covered by insurance.

Treatment for foot drop resulting from cerebral palsy is different. Some children rely on their extensor tone for ambulation. Inhibiting it via an AFO would decrease their functional abilities. Continued growth of the child means AFOs must be refabricated to keep up with changes in structure size and shape. Tendon-release surgery is sometimes performed to increase ROM of the ankle. Although the surgery is delayed as long as possible, it is often repeated as the child grows. 

Resources

Graham, J. (2010). Foot drop: Explaining the causes, characteristics and treatment. British Journal of Neuroscience Nursing, 6(4), 168-172.

The Pathokinesology Service and the Physical Therapy Department of Rancho Los Amigos National Rehab Center. Observational gait analysis. (2001). Los Amigos Research and Education Institute Inc. Downey, CA.

Perry, J., & Burnfield, J. (2010). Gait analysis: Normal and pathological function, 2nd edition. Thorofare, NJ: Slack Incorporated.

Sackley, C., Disler, P., Turner-Stokes, L., Wade, D., Brittle, N., & Hoppitt, T. (2009). Rehabilitation interventions for foot drop in neuromuscular disease (review). The Cochrane Library.

Toni Patt is an ADVANCE blogger based in the Houston area.




     

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