The basic goal of orthotic management is to improve toe clearance during swing and provide stability during stance, but new technologies—from energy-storing composites to functional electrical stimulation—do much more.
by Jeremy Farley, CPO/L
“Drop foot” is a condition affecting the lower limb where there is insufficient ability on the part of the individual to adequately dorsiflex or pick the foot up, characterized by equinus during the swing phase of gait. Drop foot gait or high-steppage gait is often characterized by excessive hip and knee flexion along with uncontrolled plantar flexion of the foot after heel contact. Poor toe clearance during swing can increase a patient’s risk of tripping or falling. In addition, excessive equinus may predispose the affected foot to initiate contact with the toe rather than the heel, and the resulting altered gait pattern can also contribute to the risk of injury or falls.
The symptoms of drop foot can be caused by weakness of the muscles controlling dorsiflexion of the foot (tibialis anterior, extensor hallucis longus, and extensor digitorum longus) or injury to the nerves controlling the muscles. “Drop foot” itself is not a disease, but a symptom of another underlying cause. This condition is associated with a wide variety of diseases and disorders, including cerebrovascular accident or stroke, traumatic brain injury, spinal cord injury, spinal stenosis, disc herniation, multiple sclerosis, poliomyelitis, diabetes mellitus, or direct injury to the peroneal nerve.
Regardless of the mechanism of injury, treatment of drop foot typically involves bracing with an ankle foot orthosis, or AFO. The goal of orthotic management is to provide toe clearance while the affected limb is swinging and stability while the affected foot is on the ground. The AFO functions by limiting the speed at which the foot plantarflexes during loading response (foot slap) and prevents the foot from dropping during the swing phase of gait (drop foot).1,2 This prevents the toe of the foot from coming in contact with the floor and decreases the risk of stumbling.
AFOs accomplish this by creating a frame around the foot and ankle. The AFO typically extends from distal to the metatarsal heads to just distal to the head of the fibula. The AFO can be fabricated of a variety of materials, including plastics, metal, leather, and carbon composite. Plastic AFOs can be either off the shelf (for short term use) or custom molded from a cast (for complicated cases or long term use). Metal and leather AFOs are typically used when skin contact must be kept at a minimum or heavy use and wear are expected. Hybrid designs incorporating plastics and metal do exist and can be used to gain the advantages of both systems. Typically plastic style or hybridized designs are used in North America, due to a greater degree of patient acceptance and circumferential control.3
The frame may be solid, as in a posterior leaf spring AFO, in which a plastic shell supports the posterior leg and plantar surface of the foot, and range of motion is dependent on the flexibility of the distal shank. Although the posterior leaf spring AFO is not hinged, resistance to plantar flexion can be controlled by adjusting the trim lines at the ankle. Articulated AFOs typically combine a lightweight thermoplastic shell material with an anatomically aligned mechanical ankle joint that either blocks or resists plantar flexion. More recently, energy-storing AFOs have been developed to both assist with dorsiflexion and facilitate propulsion at push-off in patients with weak plantar flexor muscles. These devices are made of a material with some flex to it—initially thermoplastics, now often carbon composites—that stores potential energy during early stance phase and releases it at toe-off.4 Research suggests that this spring-like action facilitates ankle and knee kinematics that are more physiologically normal.5
Problems using AFOs may include sizing, difficulty in obtaining proper fitting shoe gear, and general discomfort due to warmth because wearing the brace often makes the wearer feel hot. If a patient’s lower limb volume fluctuates, such as in the case of edema, an off-the-shelf thermoplastic AFO or even a custom molded version may no longer fit properly. The bulk of the orthosis in the shoe may require a larger shoe, anywhere from one half to a full size.1
Functional electrical stimulation
Recent developments in functional electrical stimulation (FES) over the past several years have led to the emergence of neuroprosthetic devices, which provide electrical stimulation to the nerves that control the dorsiflexor muscles. The first use of a transcutaneous peroneal nerve stimulator to improve a gait pattern in a stroke patient was reported in 1961;6 since then, several other techniques for stimulating the peroneal nerve have been developed.7-9 The fundamental techniques used in these initial devices are strikingly similar to devices available today. Improvement in electronics and manufacturing processes allowed for the production of smaller, faster, and more efficient devices. For example, the original device used a foot switch, similar in function to that of today’s devices but connected to the controller using a wire; modern versions utilize remote sensors.
Several manufacturers have developed these devices to aid in toe pickup with muscle stimulation. Innovative Neurotronics was the first to market with the WalkAide, Bioness has the Ness 300 foot drop system, and Odstock Medical Limited has the Odstock Dropped Foot Stimulator, just to name a few (for more information, see sidebar, page XX). All of these devices use a small electronic package, typically worn on the leg, to deliver an electrical current to the common peroneal nerve and initiate dorsiflexion by activating the muscles in the anterior compartment (tibialis anterior, extensor hallucis longus, and extensor digitorum longus) of the tibia. The devices employ a heel switch to determine when the affected limb comes into contact with the ground. When there is weight on the heel, the device is off. When weight is off the heel, the device turns on, causing the ankle to dorsiflex. Alternate methods to activate the device have also been explored, including EMG sensors, natural sensors, and tilt sensors.7-9 The commercially available WalkAide uses a tilt sensor to determine the orientation of the leg relative to vertical, initiating stimulation when the leg is tilted back (signifying late stance phase) and terminating stimulation when the leg is tilted forward (signifying the end of swing phase).9
Pros and cons
The most obvious benefit of using a neuroprosthetic device is the ability to provide benefits similar to an AFO without the need for actual bracing. The decreased weight and improved cosmesis of the device compared to a conventional AFO can be significant. Benefits of using a nerve stimulator include a decrease in spasticity,10 increased speed in walking,11,12 decreased effort in walking and “training effect.”13,14 The training effect is also referred to as carry over, or the occurrence that benefits gained from using the device often remain in place after the device is removed. These improvements have been attributed to several factors15: decreased activity of tendon reflexes (both Achilles tendon and patellar tendon), decreased spastic co-contraction, and increased muscle strength. Other benefits reported include improved gait symmetry and improved long term effects compared to a conventional AFO.16
Using a neuroprosthetic device is also associated with some disadvantages. The most common problems reported were accurately locating the electrodes and adequate patient training.10 Other common problems reported include cost, reliability, and ease of use.13 Costs of the devices vary depending on manufacturer, as well as insurance provider, but typical costs for neuroprosthetic devices can be eight to 10 times that of a traditional posterior leafspring AFO. Neuroprosthetic devices have a narrow field of application, in that they cannot be used in patients with more proximal joint involvement such as knee instability, which limits clinical application.11 The devices cannot be used with disorders that affect the peripheral nervous system, such as the common peroneal nerve, which must be intact in order for the device to function.10,13 Some studies have reported issues with patients’ ability to tolerate the electrical stimulation.10
The development and application of neuroprosthetic devices continues to advance as the body of research grows. As the devices are used in clinical settings and familiarity improves, improved patient training protocols will develop. More evidence based research with large subject populations investigating the performance of these devices is still needed, especially in terms of long-term effects. Future developments include incorporating functional electrical stimulation into conventional orthotic devices to provide improved function in daily activities. As technology continues to develop and manufacturing processes continue to improve, the devices themselves will get smaller, more efficient, and more durable.
Implantable electrodes are being investigated as a means of improving accuracy of electrode placement and eliminating patient difficulties in correctly donning the existing devices. Surface electrodes reside on the surface of the skin and require nothing more than a method to maintain contact with the skin. Contact can be maintained by body adhesives or through the use of a strapping material. All implantable electrodes require some sort of separate surgical procedure to attach the electrodes to the body, which helps to promote accurate placement of the electrode and ensure the maximum effect.
Several different styles of implantable electrodes exist15: percutaneous intramuscular, implantable intramuscular, epimysial, and nerve cuff electrodes. Percutaneous intramuscular electrodes are typically inserted through the skin by hypodermic needle and rest within the muscle belly. These electrodes are typically used for research and experimental situations, as they are not as durable as other implantable types. Implantable intramuscular electrodes are a more durable version of the percutaneous intramuscular electrode, mostly due to a more robust design. Epimysial electrodes are sewn directly to the surface of the muscle. Nerve cuff electrodes stimulate nerve cells by surrounding the cells circumferentially.
Neuroprosthetic devices in their current form have demonstrated to be at least as effective as AFOs for the treatment of drop foot. Issues with cost still provide a significant hurdle to overcome, especially in today’s healthcare scene. As the body of research grows supporting the devices grows, so will acceptance.
Jeremy Farley, CPO/L, is a clinical prosthetist for Fillauer in Chattanooga, TN.
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