Sensory neuropathy is the most common form of diabetic neuropathy. Nerve damage results from poorly managed and chronically high levels of blood sugar. In patients who have type 1 diabetes, which usually affects those 25 years and younger, there is insulin deficiency. In regard to people with type 2 diabetes, their insulin production inadequately meets the body’s daily need to metabolize sugar and starches found in such foods as bread, potatoes, rice and corn.
In sensory polyneuropathy, nerve damage occurs many years after the onset of type 1 diabetes and poor glycemic management whereas nerve dysfunction is more accelerated among those with type 2 diabetes.
Nerve damage may occur due to small nerve fiber compression by the Schwann cells. This compression occurs as enzymes in the Schwann cells catalyze sugar into a crystalline alcohol that absorbs water. Water saturation causes the cells to swell, compress and thereby strangle the nerve. Unless the process is reversed, both the Schwann cell and the small nerve fiber slowly die.
The end result of small nerve fiber compression commonly includes pain in the form of burning, prickling and/or shock-like sensation to the feet and/or to the extremities.
Nerve damage can also be caused by neuroischemia, a lack of adequate blood supply to maintain large nerve fiber vitality. By destroying large nerve fibers, neuroischemia can lead to a loss of balance and a deadening sensory loss in the extremities.
The inability of the distal or lower extremity small nerve fibers to distinguish coarse from smooth, temperature variations and other sensory stimuli aggravates anatomical deterioration.1 Accordingly, one may see the development of calluses, swelling (edema), foot ulcers and/or gangrene. The patient’s inability to distinguish changes in the body’s balance due to large nerve fiber demise leads to reduced proprioception and coordination. This may impair daily living activities and increase the risk of falls and fractures.2
Nerve damage from diabetic neuropathy also greatly increases the risks of joint dislocations and stress fractures to bone. In patients with diabetes, bone stress injuries of the foot due to polyneuropathy are atypical since these injuries present as load-related swelling rather than load-related pain. Delayed treatment or overuse may cause irreversible joint and bone damage.3
What You Should Know About Distal Neuropathy And Diabetic Drop Foot
Distal neuropathy, the most frequent type of sensory, peripheral nerve damage, may also involve the motor functions of the foot and ankle. Distal motor neuropathy progressively inhibits the muscle stretch reflexes of the ankle and knee, and results in muscle weakness. The deterioration of the peroneal nerve compromises the function of the foot and ankle muscles. It also creates a void that the Achilles tendon will fill by abnormally pronating the midtarsal joint of the midfoot.
These patients will subsequently have an equinovarus deformity of the hindfoot with ankle pronation of the forefoot. They will also have an inefficient gait characterized by the foot slapping the floor at the beginning and toe dragging at the end of a step. This is otherwise known as diabetic drop foot.
When Gait Is Affected By Sensorimotor Neuropathy
During gait, the force of the heel at heel strike exceeds that of the body’s weight. The direction or vector of that force passes behind the ankle and knee. If the patient controls that force, the foot gently but firmly begins the process of planting itself to support the body throughout the weightbearing part of the gait cycle. However, if the force exerted at heel strike is uncontrolled due to sensorimotor neuropathy, the foot gives the floor a big, heavy slap with each step. It will also cause an excessive pronation moment during the mid-stance segment of the gait cycle. The result is a compromised gait pattern and load-related pressures that are greatly abnormal.
Conversely, at the end of the weightbearing segment of stance phase (toe off), sensorimotor neuropathy short circuits the ability of the foot to clear the floor fully in preparation for the next step. Accordingly, these patients may subsequently be prone to toe scuffing and stumbling.
Can AFOs Be Beneficial?
One may utilize an ankle-foot orthosis (AFO) to alleviate the problem. In regard to using AFOs for diabetic sensorimotor neuropathy and footdrop, they can control plantarflexion of the foot during the swing and stance phases of the gait cycle.
Some AFOs will also assist the dorsiflexors to lift the foot. These devices are effective when the foot is able to achieve a plantigrade attitude when no ankle contracture is present and when the range of motion (ROM) of the ankle and foot are within normal limits. Some AFOs are made to fit within a shoe. These AFOs are generally prefabricated or custom fabricated from lightweight polypropylene or carbon fiber composite material. The footpiece of this plastic appliance provides minimal medial lateral support of the foot and extends to the posterior aspect of the calf, encompassing half of its circumference. Its length extends 7 cm distal to the fibular head.
When one utilizes these AFOs for diabetic sensorimotor footdrop, they provide a posterior leaf spring (PLS) effect by narrowing to approximately 4 cm on each side of the lower leg midline, posterior to the malleoli. The amount of leafspring or dorsiflexion assistance depends on the rigidity, thickness and diameter of that critical area as well as the height, weight, strength and activity level of the patient. By default, it prohibits plantarflexion footdrop. A proximal anterior Velcro strap keeps it in place.
Weighing The Pros And Cons Of Custom And Prefabricated AFOs
Custom polypropylene plastic AFOs may also use dorsiflexion assist ankle joints. The ankle joints bisect the sagittal midline of the anatomical ankle joint and its center of rotation keyed to the distal medial malleoli. The shape of the brace provides more medial and lateral stability to the lower leg by default.
Prefabricated carbon fiber dorsiflexion assist AFOs are lighter in weight and provide a longer service life but their shape is somewhat different. These devices provide no medial lateral support to the foot and cover the half of the circumference of the proximal lower leg from the anterior aspect. They provide dorsiflexion assist and foot support by way of a serpentine shape that runs from the medial proximal part of the brace to the lateral part of the footplate between the mid- and hindfoot. A posterior Velcro strap holds it in place.
A metal AFO is attached to a stirrup or a caliper that is riveted to the heel area of the shoe. The stirrup or caliper set on the shoe reflects the amount of toe out on the contralateral side. Some metal AFOs will have a single, medial aluminum upright whereas others will have bilateral uprights that bisect the sagittal midline of the malleoli and the lower leg. The upright(s) attach to a calf band positioned approximately 7 cm distal to the fibula head. An anterior Velcro strap and a buckle attached to the uprights secure the AFO to the patient.
A metal dorsiflexion assist ankle joint AFO, which provides free range of motion to 90 degrees in the sagittal plane, is indicated for the diabetic patient with sensorimotor neuropathy and drop foot. The center of rotation and position of the ankle joint AFO is the same as the plastic type. Both ankle joint AFOs prevent footdrop while the metal AFO assists the lifting of the foot from toe off through the swing phase of the gait cycle.
The aforementioned AFOs have their own risks and potential compromises. One may mitigate some of these issues by utilizing accessory components to the AFO or to the shoe. The lateral T-strap on the shoe may be necessary to maintain the foot’s alignment with the line of progression and to counteract equinovarus of the hindfoot and the ankle. Another accessory involving a shoe modification is the solid ankle cushioned heel (SACH). The SACH modification dampens the force exerted by the foot during heel strike and approximates controlled plantarflexion to foot flat when one employs this modification with an AFO for footdrop.
Most AFOs resist, if not completely eliminate, controlled plantarflexion. While that does away with the problem of foot slapping and toe scuffing drop foot, it greatly accelerates the moment in the gait cycle between heel strike and mid-stance, virtually skipping foot flat, the intervening moment of plantarflexion between the two. Without the modification of a SACH to the shoe to compensate for the restricted or eliminated plantarflexion caused by the brace, a compromised gait pattern is still evident. Accordingly, there is further risk of pressure injuries to the sensorimotor neuropathic diabetic foot.
Other risks and compromises are limited to the specific type of brace. The PLS/AFO and its carbon fiber cousin have cosmetic appeal because both are far less noticeable when worn in public. Both function as effectively as the metal variety of AFOs and are somewhat lighter in weight than the metal brace.
However, the footplate of the polypropylene plastic AFO is hard whereas the carbon fiber composite footplate is less so with it being thinner and having more of an edge. When a hard footplate is situated inside of a shoe containing a sensorimotor neurologically impaired diabetic foot with a compromised gait pattern at best, there is a significantly increased risk of further injury to the foot.
A Closer Look At The MLS/AFO Device
The orthotic-prosthetic lab at the Veterans Affairs Medical Center in Miami, directed by Tomas Dowell, CPO, LPO, developed another AFO option for sensorimotor diabetic neuropathy and drop foot.
The Miami Leaf Spring ankle-foot orthosis (MLS/ AFO) synthesizes elements of the PLS/AFO, the carbon fiber composite and metal types by combining a carbon fiber composite with a posterior leaf spring AFO that attaches to the heel area of the shoe by way of a metal caliper. The brace accesses the shoe heel posteriorly and is transferable to other shoes that have the caliper modification.
It is a safer AFO than the prefabricated carbon fiber composite or either type of hard plastic AFO, the posterior leaf spring or the one with dorsiflexion assist ankle joints. The MLS/AFO attaches to the posterior heel of the shoe to provide the leverage needed to correct the drop foot condition. It also keeps the sensorimotor neuropathic foot in an environment that is accommodative and dedicated entirely to its need. One may also include a custom molded arch support, a depth inlay shoe, or custom molded shoes with inserts for the insensate foot.
Another advantage of the MLS/AFO over the PLS/AFO is its performance. The laminated fiber carbon material of the MLS has a greater memory. In most cases, the plastic PLS/AFO stretches posteriorly because the PLS provides a very limited amount of plantarflexion at heel strike in the gait cycle. While that is initially good in regards to the gait cycle, it loses the memory of its original shape over time.
Accordingly, this AFO loses its original purpose to correct equinovarus through swing phase and keep the toe of the shoe from scuffing during the toe off phase of the gait cycle. In this regard, the MLS/AFO is safer and more effective. It not only provides more plantarflexion bias at heel strike than the PLS/AFO but it maintains its original shape for a significantly longer amount of time. It does not stretch.
In contrast, the metal AFO, either the single or double upright AFO, and the plastic AFO with dorsiflexion assist ankle joints provide no plantarflexion bias without a SACH modification to the shoe. One may add the same modification to the shoe heel of the MLS/AFO to enhance that bias to approach something closer to normal without compromising its purpose. Aside from this, most people would find the MLS/AFO more cosmetically appealing and slightly lighter in weight.
Another AFO that is comparable in function and design to the MLS/AFO is the VAPC clip-on type. It is prefabricated with a distal clip that clips on to the heel counter of a shoe. The device also has an anterior strap attached to the cuff that grasps the proximal, posterior calf. It also has a semi-rigid stay between the cuff and the clip that provides leverage to alleviate the drop foot condition. Its main detriment is that the clip will not reliably remain in place on the shoe. It “clips off” too frequently.
People with diabetic sensorimotor neuropathy and drop foot are in a precarious state of health. They require a meticulous approach to treatment and lifestyle changes to maintain hope for their well being. With these patients, there is a direct correlation with PVD and they have a heightened risk for foot complications such as loss of protective sensation, infection, ulcer, stress fracture to bone and joint subluxation.
Selecting the right type of AFO is important for restoring a more efficient, healthier gait pattern and reducing the risk of further complications. Some AFOs do this better than others. Ideally, it becomes a choice of which type restores the patient’s gait pattern in the most effective manner while minimizing the risk of further injury.
The MLS/AFO approaches this with its posterior placement to the shoe heel with a caliper modification isolated from the compromised condition of the foot. By virtue of its plantarflexion bias without the need for a SACH modification to the shoe, this AFO exceeds other AFOs in function and durability while maintaining its capacity to lift the foot and provide dorsiflexion assist throughout the gait cycle.
Mr. Zernich is a certified Orthotist-Prosthetist at the Veterans Affairs Medical Center in Miami. He frequently writes on orthotic, prosthetic and rehabilitative medicine subjects.
Mr. Dowell is Chief of the Orthotic and Prosthetic Lab at the Veterans Affairs Medical Center in Miami.
Dr. Tolchin is the Chief of Physical Medicine and Rehabilitation for the Miami VA Healthcare System. He is also a Voluntary Clinical Assistant Professor of Rehabilitation Medicine within the Miller School of Medicine at the University of Miami.
Dr. Steinberg is an Assistant Professor in the Department of Plastic Surgery at the Georgetown University School of Medicine in Washington, D.C. Dr. Steinberg is a Fellow of the American College of Foot and Ankle Surgeons.
1. Armstrong DG, Holtz-Neiderer K, Wendel C, Mohler MJ, Kimbriel HR, Lavery LA. Skin temperature monitoring reduces the risk for diabetic foot ulceration in high-risk patients. Am J Med 120(12):1042-6, Dec 2007.
2. Casellini CM, Vinik AI. Clinical manifestations and current treatment options for diabetic neuropathies. Endocrine Practice, 13(5) 550-66, Sept 2007.
3. Chantelau E, Richter A, Ghassem-Zadeh N, Poll LW. “Silent” bone stress injuries in the feet of diabetic patients with polyneuropathy: a report of 12 cases. Arch Orthopaedic Trauma Surg 127(3):171-7, April 2007.