Nearly everyone reading this column has “tripped over his own foot” at one time or another. For most of us, it happened because of our less-than-gold-medal-level athletic prowess. But foot drop is a permanent disability for a growing number of Americans due to a variety of age-related conditions.
Foot drop is a significant weakness in the muscles of the ankle and toe which control dorsiflexion; the tibialis anterior, extensor hallucis longus, and extensor digitorum longus. These muscles allow the body to lift the foot during the swing phase of walking, thus preventing a person from truly tripping over his own foot. Weakness in this group of muscles causes the foot to drop in an unsupported manner when the leg is lifted by the hip and knee muscles. Compensatory exaggerated flexion of the hip and knee must occur to prevent the toes from catching onto or dragging on the ground in order to safely walk.
Therapy for foot drop depends on the etiology. Some patients are treated with surgical release of the pressure on the nerve, such as removing a herniated disc or releasing the pressure of a compartment syndrome. Others are treated to control the primary disease, such as diabetes or multiple sclerosis. If the foot drop does not respond to a medical or surgical therapy, an ankle foot orthosis (AFO) is often used. There are a variety of AFOs available, ranging from simple rigid plastic “braces” to articulated varieties. They are constructed of various materials ranging from leather and canvas to various plastics — and now even Kevlar and high technology metals. Their primary function is to splint the ankle to keep the foot from dropping and thereby prevent tripping.
An electrical stimulation device may offer people who still have a functioning common peroneal nerve a more useful solution.
There are a number of third-generation devices marketed in the United States, the two latest being the WalkAide External Functional Neuromuscular Stimulator by Innovative Neurotronics and the NESS L300 Foot Drop System by Bioness. Both require a doctor’s prescription.
The WalkAide has a tilt sensor embedded in the leg cuff that senses the movement of the tibia. The leg cuff is attached just below the knee with a hook-and-loop strap. When a specific angle of tilt has occurred, it fires the stimulator, which leads to contraction of the muscle. When another angle is articulated at the end of the stride, the firing ceases and the foot is allowed to drop to the ground to start the entire process again. The electrodes are positioned to fire the muscles that are not functioning normally. The entire device is programmed for each patient by a trained clinician who can make fine adjustments.
The WalkAide is programmed for walking, but can be programmed to allow the patient to exercise while at rest to strengthen the muscles. The patient has intensity controls but cannot reprogram the system without the clinician.
The NESS L300 uses a slightly different technology. It consists of three pieces: a leg cuff with adjustable electrodes placed over the peroneal nerve below the patella, a sensor, and a handheld control unit. The gait sensor is attached to the shoe near the ankle and uses radio waves to communicate with the stimulator on the leg cuff.
When the heel lifts, wireless signals from the gait sensor, which automatically adjust to changes in speed and terrain, are sent to the leg cuff stimulator, causing the foot to dorsiflex (the movement that decreases the angle between the foot and the leg) with just a bit of eversion (the movement of the sole of the foot away from the median plane). Patient parameters including stimulation intensity, waveform, and stimulation ramping are set wirelessly by a trained clinician. The small hand-held control unit captures patient-specific parameters such as intensity and waveform. It is given to the patient for home use where it can be used to turn the device on and off as well as to make minor stimulation adjustments. The control unit tracks the number of steps a patient takes in a given day as a well as the time it took to travel that distance.
A recent study published in the American Journal of Physical Medicine & Rehabilitation demonstrated statistically significant improvements in gait speed, stability, and symmetry in patients who suffered a stroke or traumatic brain injury.
All of the devices currently available are cleared by the Food and Drug Administration under a process that determines that the device was “substantially equivalent to legally marketed predicate devices marketed in interstate commerce prior to May 28, 1976, the enactment date of the Medical Device Amendments or to devices that have been reclassified in accordance with the provisions of the Federal Food, Drug and Cosmetic Act that do not require approval of a pre-market approval application.”
There is no efficacy process for this clearance review that corresponds to the rigid criteria used for new drug approvals, forcing most health plans to look at the available data to make coverage determinations.
Managed care implications
The baby boomers are unlikely to accept the inevitable consequences of stroke and other causes of foot drop. This group is determined to defy the results of aging at all costs, placing the health care decision makers directly in their sights. Devices such as the NESS L300 and the WalkAide improve function and allow more freedom. Devices described in this article are bound to be popular with physicians and patients alike as they offer the hope of normalized function. They also cost more, a lot more than alternatives. A traditional AFO costs a few hundred dollars, but an electrical or neuromuscular stimulator runs $4,500–$6,000.
A number of studies have been published by rehabilitative specialists, yet medical policy for several of the largest national health plans states that this technology is considered investigational or unproven. Other plans are paying for it, creating a problem for physicians caring for these patients.
Why are there great differences between coverage criteria and approval? What evidence will health plans require before these products are considered medically necessary? Are studies demonstrating enhanced gait or dynamic stability sufficient? Will there be special requirements for the inevitable software updates? Are there specific functional measures managed care organizations will desire? What about patient satisfaction and quality of life — are they compelling for managed care organizations to approve payment? What will be the response to denials by patients who demand these devices?
We are now entering into the age where microprocessors can replace damaged nervous system processes. As more function-improvement devices are developed to replace simple adaptive devices, it is obvious that these and other questions will continue to fill the pages of Tomorrow’s Medicine.