Lower extremity weakness resulting in foot drop is one of the most common characteristics of Charcot-Marie-Tooth disease. An ankle-foot orthosis (AFO) can help improve gait for these patients, but traditional AFOs are often bulky or stiff. Although custom-made carbon fiber AFOs have anecdotally been shown to improve gait for patients with Charcot-Marie-Tooth (CMT) disease, these are often difficult to fabricate and fit, and minimal research has been conducted to support their use.

To investigate this further, researchers from the University of Nevada, Las Vegas (UNLV) studied the functional effect of carbon fiber AFOs on this patient population.

“A local practitioner had developed an energy storing and releasing brace that used carbon fiber composite materials, and this was an attempt to bring some science into the design of that brace,” Edward S. Neumann, PhD, PE, CP, FAAOP, emeritus professor in the Department of Civil and Environmental Engineering and Construction at UNLV, told O&P Business News. “In particular, we were looking at the extent to which the brace stored and released energy.”

Braced vs. unbraced

The study included eight participants with CMT with bilateral foot drop who were able to ambulate without an orthosis or external support. The participants were each given a minimum of 10 weeks to adjust to the new orthosis.

“One of the things that is important in use of this particular design, as in the case of any design, is the need to allow the individuals to accommodate to the device,” Janet Dufek, PhD, FACSM, associate professor in the department of kinesiology and nutrition sciences at UNLV, said. “In our current work, we had individuals accommodate for at least 10 weeks before we did the retest so that they can actually learn how to best use the storage and release energy relative to their unique gait characteristics.”

Once the participants adapted to the new AFO, they walked both with and without it at a self-selected speed over a force platform and instrumented walkway while being tracked by a 12-camera motion capture system.

Edward S.
Neumann

Janet Dufek

“We instrumented three pairs of braces with strain gauges to measure the amount of strain taking place and then estimated the amount of force necessary to produce that given amount of strain,” Neumann said. “Then we developed a finite element model that could be used to model the stress strain relationships that were occurring.”

The researchers also analyzed strength, spatiotemporal data and lower extremity joint kinetic parameters in patients when they wore the AFO and when they did not.

They found that all of the participants with the weakest muscles walked faster and with increased stride length and frequency with the AFO than without it, and as velocity increased, maximum joint moments shifted from the hip to the ankle and knee. The researchers also found an inverse relationship between ankle joint strength and improvements in walking velocity.

“There was nothing in the literature to discuss the phenomenon of energy storage and release with the carbon composite fiber AFOs,” Neumann said. “We found a strong correlation between the amount of energy storage and release and the velocity increase of the subjects. I wouldn’t say this was exactly a surprise, but we had to examine the data to realize that this relationship existed.”

Future AFO designs

The study results, which found that carbon fiber composite construction provides a resistance force-vs-deflection relationship that makes energy storage possible, resulting in improved gait, could affect the future design of AFOs for patients with CMT.

“I would say that there is a need to first establish how much energy storage and release a particular patient will need based on the extent to which the disease has progressed and affected their muscular strength,” Neumann said. “And second, there is a need to be able to design an AFO that will deliver that amount of energy storage and release.”

Neumann also emphasized the importance of fabricating custom-made devices for this patient population.

“I don’t think anything involving just taking something routinely off the shelf is going to hit the mark,” Neumann said. “I see the CMT population as being specialized and each patient having a unique set of characteristics, as does the amputee population when it comes to socket design and other aspects of prescribing prostheses.” — by Megan Gilbride

Disclosure: This study was funded by an Applied Research Initiatives grant from UNLV, the Heredity Neuropathy Foundation and Ortho Rehab Designs. The authors have no relevant financial disclosures.