Clinicians are already embracing carbon-fiber ankle foot orthoses, which provide a dynamic assist and may even help strengthen calf muscles in patients with foot drop. Now, slowly, research is exploring how these devices can best be used.

by Cary Groner

Clinicians are gathering data, both anecdotally and through studies, that elucidate the advantages of using carbon-fiber ankle-foot orthoses (AFOs) to manage foot drop instead of the more traditional plastic posterior supports that cross the heel. It’s been known for a while that carbon-fiber AFOs provide energy return at toe-off; what’s increasingly intriguing is evidence that the devices allow the calf muscles to fire, preventing atrophy and often allowing muscle tissue to regenerate.

“This use of carbon is a paradigm shift in the orthotic industry,” said Carey Jinright, CO, of Precision Medical Solutions in Montgomery, AL. “We’re going from creating static alignment to creating a functional environment.”

Jinright explained that plastic posterior longitudinal supports (PLSs) were never intended to provide energy return or help normalize gait.

“As clinicians, we became very narrowly focused,” Jinright said. “We saw the patient dragging their toes during swing phase and thought, for safety reasons, that we wanted those toes to clear the floor. The problem was that if you do that with static alignment, what happens at heel strike? The patient is locked into a 90-degree position, and a lot of times they overcome that fixated angle through excessive flexion of the knee.”

A carbon-fiber AFO helps facilitate a more normal gait, Jinright said.

“If the patient can achieve controlled motion, why would we want to take that away? Our goal should be to control excessive motion—pronation or supination—not the useful motion they already have,” he said.

According to Randy Stevens, BOCPD, CFO, who practices in Harrisburg, PA, patients wearing plastic posterior AFOs sometimes demonstrate recognizable gait patterns that practitioners can take as a cue for a different type of intervention.            “You’ll often see a little more hip hike, then not as much toe off at the end of the gait cycle,” he said. “The energy restoring aspect of carbon AFOs increases range of motion in the knee and hip, and leads to less abducted gait patterns. More muscles are firing, so we’re not contributing to weakening of the gastrocs. Oftentimes a more normal gait brings with it a more erect posture, as well.”

Identifying causes

In adult patients, a number of factors may lead to the foot drop these devices are designed to address. Post-stroke hemiplegia is a frequent cause, as is nerve damage during spinal surgery, traumatic injury, Charcot-Marie-Tooth syndrome, and multiple sclerosis (MS).

Jinright, however, emphasized that diagnosis doesn’t necessarily determine which device is best for an individual.

“Every patient is different,” he said. “I just like to bring them in, try them in the newest technology, and see what happens. I want the patient’s abilities to determine whether this is the right product.”

One advantage is that patients can usually wear their normal shoe size instead of the larger size sometimes necessitated by other designs, he added. This makes them less likely to trip, further increasing safety.

Multiple planes

Carbon-fiber AFOs typically feature a flat footplate so that a foot orthosis can be placed over it.

“This AFO is basically a sagittal-plane device,” said Jinright. “Some people tend to look at it as the entire instrument, and it’s not; it’s just one tool. If the patient has a frontal-plane defect—if they’re an excessive pronator or supinator—you have to address that with a foot orthotic.”

According to Stevens, neuropathic patients may do well in carbon-fiber AFOs because the concurrent use of foot orthoses allows for a superior degree of customization.

“Patients who’ve had, say, a history of plantar ulcerations have a hard time with plastic AFOs, because when they push off the foot plate, the big toe can break down,” he said. “With a carbon-fiber foot plate, you can use a custom orthosis and off-load those areas more easily.”

Stevens noted that footwear is important in this context, as well.

“You need to consider heel elevation, pitch, and so forth,” he said. “If I notice that the knee is hyperextending, I can control that with a heel lift underneath the AFO.”

Research

In recent years, research—typically from Europe—has begun to define and quantify the ways in which carbon-fiber AFOs work best.

For example, a 2004 Swedish study reported that, in hemiparetic stroke patients, use of a carbon-fiber AFO increased walking speed about 20% and decreased energy cost (oxygen consumed per meter) by roughly 12%, versus unbraced walking.¹ At the 2010 International Society of Prosthetics and Orthotics World Congress in Leipzig, Germany, the Swedish researchers presented new data showing similar results, though they noted that only the increase in walking speed was likely to be clinically significant.²

A 2007 study from Germany reported that the use of a carbon-fiber AFO significantly increased energy return during the third rocker phase of gait and provided support during the complete stance phase, in a group of primarily spina bifida patients. Analysis of ankle and knee kinematics revealed that the spring contributed to a more physiologic gait.³

In 2008, a study by the same researchers, published in Gait & Posture, reported that in patients with spina bifida, the more physiological ankle and knee kinematics associated with a carbon-fiber AFO suggested a functional improvement versus a more traditional orthotic device, but that kinetics and kinematics during stance phase were significantly influenced by alignment with footwear, which should be carefully checked.⁴

In another presentation at the 2010 ISPO conference, scientists from the Netherlands reported that in computer simulation, an AFO’s stiffness affected the timing of energy return, which is important in determining the energy cost of walking. Optimal stiffness for stored energy wasn’t directly correlated with that for energy cost: stiffness associated with the highest ankle push-off velocity just before contralateral foot strike led to the lowest energy cost.⁵

In the United States, David Ruthsatz, CO, ATC, has been conducting a study in patients who switch from a plastic PLS to a carbon-fiber AFO. Ruthsatz, who practices at Capital Prosthetics & Orthotics in Columbia, SC, noticed that his patients—most of whom are non-athletes with a wide range of etiologies—were reporting that the change seemed to lead to stronger calf muscles, so he decided to find out. When he switched them to the carbon-fiber AFO, he measured the girth of their gastrocs, then had them come back eight weeks later for another assessment.

“In almost every case we found increased girth,” he said, although he and his colleagues aren’t yet sure why. “Is it because the AFO has a front stoppage instead of a calf?” he asked. “Or is it because we have an open calcaneus? We see on EMG studies that the gastroc and soleus actually fire more when they should be firing, when initial contact occurs. We think it’s because it slows the femur and the tibia anteriorly, acting as a braking system so there isn’t too much knee bend.”

Ruthsatz is planning additional research to more precisely define the muscle-building mechanism.

Customization options

Because carbon-fiber AFOs come with an assortment of strut configurations, clinicians have to decide which—if any—is best for a given patient.

Patients with medial ankle instability would be better candidates for an AFO with a lateral strut, whereas someone who tends to roll outward would be better suited to a medial strut, Ruthsatz said.

“If you can determine where the patient is least stable, that helps determine the kind of orthosis you use,” Ruthsatz said. “For example, if you have an MS patient with no posterior muscle mass, you may use a solid ankle AFO on them. If the patient has a physical therapist, I want to see those notes, to assess how they match what I’m seeing.”

Ruthsatz now prescribes carbon-fiber AFOs about 80% of the time.

“We used to think in terms of immobilizing a joint,” he said. “Now we want to assist the motion that the patient has, help them find a safe gait.”

According to Josh Ahlstrom, CPO, who practices in Indianapolis, clinicians have different levels of customization available, depending on their preferences and the patient’s needs.

“In an off-the-shelf device, you can adjust toe plate length, or where the toe break is,” he said. “For example, if you have a patient where the strut isn’t hitting them in the right spot, or the rockers aren’t matching the shoe properly, you can trim the toe to move the AFO forward, or you can put wedges under the heel to change ground reaction forces at initial contact so you can create more of a flexion moment.”

Ahlstrom also likes being able to adjust foot plate fit so that athletic patients can get a stronger, more rigid brace to suit their activity levels. In one young basketball player, for example, a carbon AFO as originally prescribed wasn’t up to the rigors of the court.

“[It] worked well in the office, but when he played, the forces were too much and he cracked it,” he said. “So we put him in a bigger size and he does tremendously well. He just needed something a little more rigid to take the beating when he’s active.”

Ahlstrom also appreciates the flexibility off-the-shelf devices offer in terms of controlling motion in multiple planes.

“It gives you variability in controlling sagittal plane function and frontal plane function independently of one another,” he said. “It lets me incorporate an off-the-shelf foot orthosis, or sometimes even a supramalleolar orthosis (SMO), if it’s indicated.”

A few companies make fully custom carbon-fiber AFOs, as well. Because they tend to be costly, Ahlstrom has developed a way to decrease mistakes and remakes.

“For one of our patients, we needed to be sure that the dorsiflexion angle was perfect before putting them in the brace, so we had the [custom] company send a test brace made of thermoplastic so we could fit it and make adjustments,” he explained. “Then we sent that back and had them use that to fabricate the final carbon brace.”

Contraindications

Despite their obvious advantages, carbon-fiber AFOs aren’t for everyone. Very large calves can cause problems, according to Ruthsatz, as can the long stride typical of very tall people.

“Carbon AFOs seem to handle only so much ankle-to-tibia flexion,” he said. “With a long stride length, there is more knee flexion, then more ankle or dorsiflexion. With that type of stride, when you repeatedly overextend the AFO, the carbon will tend to delaminate and weaken.”

The devices may not be optimal for patients with spasticity or tight Achilles tendons, either.

“That’s when you have to get physical therapy involved,” Ruthsatz said. “If you can stretch them out of that spasticity, there’s potential; but if you can’t break that tone, the patient is plantar flexed and pushing down on the foot plate, and it’s going to jam the knee backward.”

Jinright, too, chooses more traditional plastic AFOs in patients with spasticity issues.

“If you have an overactive gastroc, and it’s pushing the foot into plantar flexion, sometimes the rigidity of a dorsal plastic brace can limit that,” he said. “The gastroc won’t fight against it as much as it would a carbon brace, because the latter has more freedom.”

Chris Toelle, LO, CO, practice manager with Hanger in Winter Haven, FL, agrees.

“In a patient with a lot of contracture, or with a severe deformity, you may want to use carbon as a reinforcing tool, but it may not be the best choice for the primary AFO,” he said. “I’ve primarily gone to carbon-fiber AFOs for patients with simple drop foot and minor medial or lateral instability, though. I just feel that they have a much higher functional return for the patient in terms of getting back to normal gait.”

Kids

Clinicians particularly appreciate the freedom carbon-fiber AFOs offer children, and research supports the potential for improving gait in pediatric patients.

For example, Swedish scientists reported in 2007 that carbon-fiber AFOs enhanced gait function in most subjects by improving ankle plantar flexion moment and stride length, among other variables.(6) A Belgian research group reported that in children with hemiplegia, a carbon-fiber AFO produced significantly larger ankle range of motion and velocity during push-off, and an increased plantar flexion moment and power generation at pre-swing, compared to a PLS.(7) In 2008, the same group reported that carbon-fiber AFOs allowed a more physiological third ankle rocker versus a PLS, as well as a significantly higher maximal hip flexion moment in stance.(8)

“We deal with a large number of pediatric patients, so for us the versatility to deal with them by separating the two planes [i.e., sagittal and frontal] has been really helpful,” said Ahlstrom.

In children with mitochondrial disease, for example, who may need more support later in the day, when fatigued, clinicians can put together a combination of approaches.

“We can put them in an SMO early in the day, then add something simple that the teacher can slide into their shoe as they start to wear out,” he said. “That’s been really beneficial in allowing them to continue utilizing their muscles through the course of the day while maintaining the same gait pattern.”

Cary Groner is a freelance writer based in Northern California

References

1. Danielsson A, Sunnerhagen K. Energy expenditure in stroke subjects looking with a carbon composite ankle foot orthosis. J Rehabil Med 2004;36(4):165-168.

2. Danielsson A, Sunnerhagen K, Willen C. Comparison of energy cost of walking with and without a carbon composite ankle foot orthosis in stroke subjects. Poster presented at 13^th ISPO world conference, Leipzig, Germany, May 2010.

3. Alimusaj M, Knie I, Wolf S, et al. [Functional impact of carbon fiber springs in ankle foot orthoses.] Orthopade 2007;36(8):752-756.

4. Wolf SI, Alimusaj M, Rettig O, Doderlein L. Dynamic assist by carbon fiber spring AFOs for patients with myelomeningocele. Gait Posture 2008;28(1):175-177.

5. Bregman DJ, van der Krogt MM, de Groot V, et al. The effect of ankle foot orthosis stiffness in the energy cost of walking: a simulation study. Poster presented at 13th ISPO world conference, Leipzig, Germany, May 2010.

6. Bartonek A, Eriksson M, Gutierrez-Farewik EM. Effects of carbon fiber spring orthoses on gait in ambulatory children with motor disorders and plantarflexor weakness. Dev Med Child Neurol 2007;49(8):615-620.

7. Desloovere K, Molenaers G, Van Gestel L, et al. How can push-off be preserved during use of an ankle foot orthosis in children with hemiplegia? A prospective controlled study. Gait Posture 2006;24(2):142-151.

8. Van Gestel L, Molenaers G, Huenaerts C, et al. Effect of dynamic orthoses on gait: a retrospective control study in children with hemiplegia. Dev Med Child Neurol 2008;50(1):63-67.