Lower-limb bracing is often targeted at addressing some form of neuromuscular compromise. Individuals burdened by stroke, multiple sclerosis, traumatic brain injury, polio, or paralysis are referred to orthotists who fit the patients with rigid or semirigid structures to stabilize paralyzed joints, assist weakened muscle groups, or resist abnormal tonic motions. As a profession, we’ve collected a lot of experience stabilizing lower-limb joints that lack normal musculoskeletal control.
Less frequently, our interventions are also called upon to consider sensory deficits. People with spina bifida or spinal cord injuries are often treated with devices that address not only lower-limb paralysis or weakness, but also provide proximal sensory input to the compromised extremities.
But what if there is no musculoskeletal compromise? Recently, a small body of evidence has begun to develop in the area of using AFOs to address sensory, rather than motor, deficits and strike a balance between providing assistive sensory input without imposing overly restrictive mechanical forces. This article reviews that body of evidence and summarizes the current understanding of how AFOs might be beneficial in managing sensory deficits associated with peripheral neuropathy.
The Laboratory Origins
The concept of using AFOs to provide sensory input in cases of peripheral neuropathy can be traced at least as far as a clinical trial conducted by Noel Rao and Alex Aruin, published in 2006.1 Their study began with the premise that auxiliary sensory inputs are known to assist with balance and posture in the presence of distal neuropathies. For example, light touch on a stable tabletop or the sensory input from a single-point cane are known to reduce postural sway. Similarly, stable contact against the forehead, nose, or leg have also been shown to reduce sway values in these populations. Could contact against the proximal shank of the leg provide a similar secondary source of sensory input?
To find out, the authors assembled a cohort of 11 patients with peripheral sensory neuropathy. For seven of the subjects, the neuropathy was due to diabetes. Sensory compromise was measured using Semmes-Weinstein monofilaments, demonstrating diminished protective sensation in all but three subjects. Limb strength was also recorded; six of 11 subjects presented with 4/5 or 5/5 ankle strength, and two of 11 subjects presented with 1/5 or 2/5 ankle strength.
The subjects underwent a Sensory Organization Test (SOT) in which they stood on a force platform with visual surround, with and without wearing custom leaf spring AFOs, while experiencing four different testing conditions. In the first two tests of static balance, the platform was stable and the subjects stood with their eyes open and then again with their eyes closed. In the third and fourth tests of dynamic balance, the platform moved with the subjects’ body sway in the sagittal plane, again in eyes-open and eyes-closed conditions. Sway values were measured through the force platform and equilibrium scores were assigned for each condition. Scores closer to 100 indicate very little sway, while scores closer to 0 suggest that the subject is approaching the limits of his or her stability.
Several predictable trends were observed. Sway values increased in both of the eyes-closed conditions and during both of the dynamic balance assessments where the force platform was no longer stationary. In addition, the use of the leaf spring AFOs consistently reduced sway values across all four test conditions. In general, the more challenging the test, the greater the benefit derived from the AFO. Thus, in the eyes-open, static assessment, AFO use only increased the mean equilibrium scores by about 14 percent. For the eyes-closed, static and eyes-open, dynamic tests, the benefits were more pronounced—36 percent and 39 percent improvements respectively. During the most challenging, eyes-closed, dynamic balance test, use of the AFO increased equilibrium scores by an average of 60 percent.1
The authors were cautious in their interpretation of their data. Improvements in balance scores were clearly associated with AFO usage. However, were these improvements due to mechanical stabilization of the ankle joints or the provision of auxiliary sensory information? The answer to this question would have to wait for another clinical trial by the same investigators.
Sensory Input or Mechanical Stabilization?
In their second trial, published in 2011, Rao and Aruin assembled a cohort of 12 individuals with clinically confirmed diabetes-related peripheral neuropathy.2 All subjects presented with lower-limb strength of at least 3/5, yet experienced difficulty maintaining their balance. Sensation was tested in the feet and calves using Semmes-Weinstein monofilaments and vibration from a tuning fork. The results suggested diminished or absent sensation in the feet with normal, protective sensation in the calves.
Thus the study cohort could be characterized as a rather homogeneous group of individuals with compromised balance despite good lower-limb strength, and compromised distal sensation due to diabetes-related neuropathy. The subjects experienced an SOT regimen similar to that performed in the original trial. However, instead of standard-design leaf spring AFOs, the subjects wore modified custom AFOs in which the calf section was connected to the footbed by an extremely flexible element that provided less than 1 newton of pressure. Restated, the device was capable of providing sensory information to the calf and shin but was too flexible to provide any mechanical stabilization.2
The resulting observations with regard to postural sway values found that equilibrium scores were, on average, higher across all four test conditions with the use of the adapted AFOs. However, for the stationary platform tests, the benefits were almost nominal. It was only on the dynamic platform tests that the benefits became more marked. In the eyes-open condition, use of the adapted AFOs increased the average equilibrium scores by roughly 8 percent. In the more challenging, eyes-closed, moving platform test, equilibrium scores were quite low, but their average increased by over 80 percent with the adapted AFOs.
The purpose of the SOT is to systematically eliminate certain balance inputs (e.g., visual, somatosensory, and vestibular) to better isolate the impacts of each input to corrective postural reactions. In the case of the dynamic balance tests, somatosensory inputs were compromised when the force platform was sway-referenced (i.e., moved with the sagittal sway of the test subjects). In the fourth test condition, somatosensory and visual inputs were distorted or absent. This explains the severely lowered equilibrium scores in this condition and the marked impact of auxiliary sensory input that were translated past the distal areas of sensory compromise (foot and ankle) to the intact sensory preceptors of the calf.2 These observations suggest that, particularly in cases where visual and somatosensory inputs are compromised, such as low lighting or uneven, unstable surfaces, the auxiliary sensory inputs provided by an AFO may provide benefits to postural stability and retention.
Moving From the Lab to the Clinic
The clinical utility of the previous study was inherently limited by several factors. The devices in question were highly modified to systematically provide sensory input without mechanical stabilization. In addition, their effect was measured in a controlled setting and only examined the effects of the AFOs on postural stability and reaction. The next trial, conducted by a different research team, sought to apply the laboratory observations of the prior research to a more clinically relevant scenario.3
In this trial, a similar cohort of 12 subjects with peripheral neuropathy was assembled. As with the prior trials, a lack of protective sensation was verified using Semmes-Weinstein monofilaments, with all subjects demonstrating a profoundly lowered sensory threshold. All subjects were then fit with a correctly sized, off-the-shelf, carbon fiber ground reaction AFO (Allard ToeOFF) before undergoing further testing.
Testing began with a similar evaluation of posturography in quiet standing on a stable, firm surface, with and without wearing the AFO. As before, sagittal sway values in both eyes-open and eyes-closed conditions improved with AFO usage. As expected, the effect was more pronounced in the eyes-closed condition (roughly 17 percent less sway) than in the eyes-open condition (roughly 13 percent less sway). Perhaps surprisingly, sway values in the coronal plane were even more affected by the addition of the AFOs, improving almost 30 percent in both the eyes-open and eyes-closed conditions.3
These benefits established, the authors proceeded to evaluate the effects of the AFOs on more dynamic elements of balance. Subjects were asked to perform the 14-item Mini Balance Evaluation Systems Test (Mini-BESTest), assessing such things as sit to stand, rising on the toes, rapid compensatory stepping, pivot turns, and dynamic gait. Their gait speed was assessed using a self-selected and fast as possible ten-meter walk test (10MWT). Finally, dynamic gait and balance were assessed using the Timed Up and Go (TUG) test.
The fundamental concern of the trial was to determine if the additional sensory input and mechanical stability provided by an AFO during static posture assessment might come at a cost, compromising dynamic stability during balance and gait assessments with greater clinical utility and real-world application.
The answer was a complex one. On average, there were no significant differences between the AFO and no AFO conditions in the different clinical assessments. However, the effects were quite variable between subjects. Some subjects appeared to benefit from the AFO intervention while others demonstrated decreases in dynamic balance. The clinical message of this research underscores the importance of evaluating the individual effects of AFOs on a patient-specific basis when seeking to address deficits in balance and sensation. Ultimately, clinicians must ensure that any benefits to postural stability are not unduly countered by restrictions to dynamic stability and gait.3
Coupling the Clinic With Fall Concerns
A final publication reports on a similar effort to quantify the effect of clinically available AFOs on posture, dynamic gait, and fall concerns. The cohort used in this study differed somewhat from the studies described thus far. In this instance the cohort was a convenience sample of 30 older adults (> 65 years old) who could walk 20 meters without an assistive device. Ultimately, the average age of the study subjects was 73. Just under half presented with diabetes, with 13 of the 30 subjects presenting with peripheral neuropathy (verified as in the earlier trials with vibration perception thresholds).4
The subjects were fit with custom AFOs (Moore Balance Braces), described as a “flexible, open ankle posterior leaf style gauntlet design which is intended to allow ankle stabilization without inhibiting sagittal plane motion.” As with previously described studies, postural sway was recorded during quiet standing, with and without wearing the AFO and in eyes-open and eyes-closed scenarios. As with the other trials, sway values improved with AFO usage in both visual conditions, with average values reduced by just under 50 percent compared to shoes alone.4
The subjects were also asked to complete the TUG test. As with the previously described study, average TUG values were essentially unchanged, reported at 13.75 seconds for the shoes-alone condition and 14.08 seconds for the shoes-plus-AFO condition, suggesting that the static balance benefits of the AFOs were not coupled with substantially negative impacts on dynamic balance. In addition to these variables, the authors collected data on fall history and fall concerns within the study cohort. Despite the lack of commonly encountered musculoskeletal treatment diagnoses, both were found to be rather high. Fourteen of the subjects reported no falls in the previous 12 months, with ten subjects reporting a single fall, and six subjects reporting multiple falls. Fall concerns were even more prevalent, with 19 of the 30 subjects reporting high concern for a fall, seven reporting moderate concern, and only four subjects reporting no concern.
After the testing with the AFOs was completed, subjects were asked to answer the following questions using a ten-point Likert scale:
- The AFO makes me feel less likely to fall when standing.
- I am likely to continue to wear the AFO daily.
With a score of 10 representing "strongly agree," the average responses to the questions were 7.59 and 6.28 respectively.4
Compliance with lower-limb orthoses is always a concern. Compliance among a cohort of older adults with no defined treatment diagnosis other than their age and, in roughly half of the cases, diabetes-related peripheral neuropathy, could be reasonably questioned. The relatively high scores to the two questions reinforce the potential benefits that might be gained with an AFO that provides auxiliary sensory input in both balance and balance confidence.
Summary
While AFOs have largely been used to provide mechanical stability and assistance to lower limbs characterized by musculoskeletal deficits, there is a growing body of literature that supports their use to address sensory deficits even when weakness is not an immediate concern. Translating sensory input from the floor, past the compromised distal extremity (foot and ankle) to areas of the body where sensory organs are intact (calf and shin), AFOs appear to provide additional sensory input that aids postural balance. These benefits must be weighed on a case-by-case basis against any deficits that may occur during dynamic function when range of motion at the ankle is restricted. Particularly among patients with distal sensory compromise who describe elevated fall history or fall concerns, bracing for balance is a treatment modality that should receive due clinical consideration.
Phil Stevens, MEd, CPO, FAAOP, is in clinical practice with Hanger Clinic, Salt Lake City. He can be reached at .
References
- Rao, N. and A. S. Aruin. 2006. Automatic postural responses in individuals with peripheral neuropathy and ankle-foot orthoses. Diabetes Research and Clinical Practice 74 (1):48-56.
- Rao, N. and A. S. Aruin. 2011. Auxiliary sensory cues improve automatic postural responses in individuals with diabetic neuropathy. Neurorehabilitation and Neural Repair 25 (2):110-7.
- Bigelow, K.E. and K. Jackson. 2014. Immediate influence of carbon composite ankle-foot orthoses on balance and gait in individuals with peripheral neuropathy. Journal of Prosthetics and Orthotics 26 (4):220-7.
- Yalla, S.V., R. T. Crews, A. E. Fleischer, G. Grewal, J. Ortiz, and B. Jahafi. 2014. An immediate effect of custom-made ankle foot orthoses on postural stability in older adults. Clinical Biomechanics 29 (10):1081-8.
