A Step Short: Why We Need More Research on Prefabricated Carbon Composite Ankle-Foot-Orthoses

By Lindi Mitsou, MSPO, CPO;  Nicholas Carr, BS; Jordan Dunn, BS; Jayne Kernodle, BS;  Yelizaveta Kozlov, BS;  Jillian Picard-Busky, ATC, LAT;  Abigail Riley, BS;  and Alex Wright, BS, NREMT

As the population ages and the twin epidemics of diabetes and obesity compromise mobility in ever-more patients, clinicians will need to understand the design specifics of these devices. But what do we know and is it enough?

Many patients with gait abnormalities utilize lower limb orthoses to provide support, correction, and enhance function of the lower extremity. A variety of design options from joint level(s) to material selection are available and must be appropriately selected for individual patient treatment plans. A popular design choice for patients experiencing difficulties with clearance in swing and forward propulsion in terminal stance are prefabricated carbon fiber composite ankle foot orthoses (C-AFOs). Carbon fiber composites provide energy storage and return as well as high stiffness to weight ratios making them an ideal material for many patients. As a result, there are numerous prefabricated C-AFOs on the market, varying in design and stiffness. As this market continues to grow, it is important for clinicians to understand how these devices function as a group and differ from each other to appropriately select interventions for their patients.

Utilizing Gait Parameters and Joint Angles in Gait Research

The main goal of human gait is efficient locomotion, requiring four distinct functions: propulsion, shock absorption, stance stability, and conservation of energy.1 Locomotion is achieved through the reciprocal pattern of gait as defined by Jacquelin Perry and subsequent researchers.1,2 By determining and describing each period and phase of the gait cycle, Perry and others have codified normal human gait in such a way that allows clinicians and researchers to effectively analyze gait observationally and in lab settings.

Numerous biomechanical parameters can be utilized to examine gait and it is important that researchers select those that provide relevant information. In a 2017 systematic review, Roberts et al determined that temporospatial parameters are the most frequently collected parameter in research of human gait.3 Temporal and spatial parameters refer to the kinematic information regarding time and distance, respectively. The most common temporal parameters include cadence and walking velocity.3,4 Frequently investigated spatial gait parameters include step length, stride length, step width, and foot progression angle. In research of non-pathological human gait, the most cited spatial parameter is step or stride length.3,4

Temporospatial gait parameters provide clinicians and researchers with clear information regarding reciprocation, or lack thereof, between the lower limbs. Asymmetries in temporospatial parameters are often cited in pathologic gait as deviations and examining changes in these parameters is integral to patient treatment plans. When analyzing gait, clinicians assess gait parameters through observation, comparing them to healthy gait criteria as defined by Jacquelin Perry and others. A 1993 study by the US Department of Veterans Affairs collected temporospatial gait parameter data in healthy adults to provide as reference for future clinical and research application.4 In their study, researchers defined several characteristics that influence cadence, velocity, and step length: environment, sex, and age. The environment was determined to be influential as walking velocity decreased with indoor, short walkways versus walking outdoors and on longer walkways. With regards to sex, female participants walked with shorter step lengths, at slower overall velocities, and had higher cadence values than their male counterparts. Finally, increasing age resulted in reduced gait velocity and step length with minimal changes in cadence. The same study also aimed to define normal values for step length, walking velocity, and cadence for numerous defined groups based on age, sex, and task (i.e., self-selected speed instructions: slow, normal, and fast). It is important to consider each of these variables when examining gait, both clinically and in lab settings.

Assessing joint angles has also been established in gait analysis.2,5 Often presented as angle-angle diagrams, total joint excursion (range of motion), or joint angle versus time graphs,5 joint angle kinematics provide valuable information regarding the overall movement of the body and its limbs during the gait cycle. The joints of the lower limb aid in the transmission of forces and loads during weight acceptance and in the maintenance of stability during mid-stance of gait.1,6 In swing phase, proper joint angles are important for toe clearance and limb advancement.1 It is necessary to consider all joints of the lower limb as positioning alters muscle force generation and segment loading. Hip and knee joint position can impact ankle range of motion and muscle force production at the ankle and vice versa.6,7 Therefore, altering range of motion with orthotic intervention will impact the joint kinematics of not only the braced joint(s), but those of the entire limb.

Though clinicians and researchers can approximate joint angles with observational gait analysis, motion capture is the most reliable tool in analyzing body segment positioning and joint angles.2,5 Researchers have established normal values for joint angles in each phase of gait via motion capture analysis systems (or electro-goniometry).1,5,6 A year after the 1993 study, the US Department of Veterans Affairs released reference data for knee and hip joint angles examining the same variables as in the temporospatial parameters study. They determined that sex significantly affected knee excursion in all parameters measured with greater values for male participants. Though less consistent, sex may also affect hip flexion-extension and rotation. Minimal but significant changes were noted with increase in age resulting in a reduction of knee flexion in mid-stance by 0.5° per decade and in total knee excursion by 0.5-0.8° per decade.5 Finally, increasing gait speed resulted in significant increases in angles and excursion of all joints assessed.

Existing Research on Prefabricated C-AFOs

Generally, prefabricated orthoses of various types are frequently used for common pathology presentations due to their ease of use and relatively low cost. Clinically, these devices may be billed as off-the-shelf or custom-fit, the latter requiring “more than minimal self-adjustment” and the expertise of a certified clinician per the Medical Directors of the Durable Medical Equipment Medical Administrative Contractors.8 Incorporating the dynamic, stiffness, and light weight properties of carbon composites into the design of ankle foot orthoses (AFOs) has resulted in a ubiquitous nature of C-AFOs in various rehabilitation settings. With numerous manufacturers producing prefabricated C-AFOs and the frequency of their use, clinicians must have appropriate knowledge of how each orthosis will affect patient gait.

Currently, many clinicians rely on the knowledge that most C-AFOs are relatively similar functionally and mostly vary only in design. Design variations among manufacturers include cuff placement (anterior/posterior), strut placement (medial, lateral, posterior), strut design, starting neutral angle, and carbon fiber composite blend (altering stiffness and energy return capabilities). Numerous studies have investigated the stiffness of  C-AFOs.9–12 However, unlike the current testing and manufacturing standards for carbon fiber composite prosthetic components, no testing standards exist for C-AFOs resulting in slight variations in reported stiffness.9,13 When comparing five different stiffness tests on a variety of prefabricated C-AFOs from different manufacturers, Shuman et al found significant differences in the calculated rotational stiffness among all testing methods.9 (See page 41.) While general trends in measured stiffness have been established, without standardization for C-AFO stiffness testing, it is impossible to generalize study results to all C-AFOs.

Medical practitioners and clinicians must be accurately informed on how custom orthoses compare to prefabricated orthoses both quantitatively and qualitatively in function. This can be achieved by comparing temporospatial gait parameters and joint angle data between prefabricated and custom orthoses. A study conducted in 2014 compared the use of custom thermoplastic AFOs to prefabricated C-AFOs used acutely post stroke. The data showed that both interventions significantly improved velocity, cadence, stride length, and step length when compared to conditions with no intervention. There was no significant variation between orthotic interventions functionally. However, the study found that participants significantly preferred custom orthoses over prefabricated devices in a 2:1 ratio. A qualitative survey showed that participants felt significantly safer, more comfortable, and better balanced in custom interventions. The survey also demonstrated a significant improvement in perceived ease of swing and aesthetic preference specifically with custom AFOs.14 Further research is required to determine if the variation in gait parameter data between prefabricated and custom orthoses is statistically and clinically significant.

Previous research examining the efficacy of prefabricated C-AFOs has continuously supported their use in a variety of patient populations. In 2004, Danielsson et al examined the effects of prefabricated C-AFOs on energy expenditure in patients post stroke (average age 52 years). The researchers found that self-selected gait speed increased by 20% when patients wore C-AFOs as compared to no orthosis at all. They also determined that energy expenditure decreased by 12% with C-AFO use.15 A subsequent study was conducted in 2019 with pediatric patients experiencing unilateral drop foot secondary to cerebral palsy. This study investigated the efficacy of 2 different prefabricated C-AFOs and determined that both orthoses improved toe clearance in swing phase and initial heel contact.16

Available data regarding joint kinematics suggests that AFOs created with carbon composites may improve joint range of motion and joint power generation. A 2007 study conducted in Switzerland examined the impact of AFOs on the gait of children with motor disorders such as myelomeningocele, arthrogryposis, and neuropathy. Each child was provided with both a custom solid ankle foot orthosis and a custom carbon strut orthosis, then allowed several weeks to acclimate to the device. Once familiarized with the devices, each child underwent 3D gait analysis with both interventions. In children with myelomeningocele, custom carbon strut orthoses significantly increased dorsiflexion and plantarflexion, power generation at the ankle, and hip flexion. Conversely, children with arthrogryposis multiplex congenita did not experience the same improvements.18 This study shows that carbon fiber composites can be useful in improving the mobility of certain patient populations, but further investigation is required to better understand the applicability of these devices. In 2018, a systematic review of 27 studies evaluated available research examining the effects of various AFOs on the gait of post- hemiplegic stroke patients. Seven studies within the systematic review showed a significant improvement in peak ankle dorsiflexion throughout the gait cycle in patients with drop foot. All studies showed that any orthotic intervention at the ankle was better than having no intervention in relation to drop foot.19

Based on present findings, no studies have examined the effect of prefabricated C-AFOs on non-pathological gait. Most studies with C-AFOs have examined pathologic gait in a variety of populations14–16 or have been conducted with custom fabricated thermoplastic AFOs utilizing carbon struts.17,18 Consensus among researchers and clinicians is that C-AFOs are effective in improving gait parameters, reducing energy expenditure, and providing toe clearance in swing phase and effective propulsion at terminal stance.14–19 Examining non-pathological gait provides foundational knowledge of the function of these devices by removing the extraneous variables presented by pathological gait abnormalities.

A Call for More Research

Although C-AFOs are readily available and frequently used in practice, there is still a need for foundational research pertaining to their effect on gait. Existing research has solidified the contributions of carbon composite materials in orthotic devices and demonstrated the efficacy of utilizing carbon composites in AFO design for select patient populations. However, the foundational understanding of how prefabricated C-AFOs as a group affect gait is still unknown. Additionally, while no testing standards exist for C-AFO manufacturing and testing, existing research and understanding of their functioning remains relative and lacks impact.

Future research should collect data regarding gait parameters and joint kinematics to demonstrate the functional effect of these devices on gait. Studies involving a greater variety of patient populations are needed to determine C-AFO efficacy as current research is not generalizable. Research should be conducted with numerous manufactured options, rather than just 1 or 2, to provide generalizable conclusions across C-AFO options. Testing protocols must be explicitly defined, especially regarding material properties, due to the variability amongst testing options. Finally, gathering information in non-pathological patients can provide foundational knowledge regarding the effects of these devices.

Only with stronger research backing can clinicians and healthcare professionals make appropriate decisions regarding which C-AFO will have the most appropriate impact on the gait and functioning of their patient.

Lindi Mitsou, MSPO, CPO, is an Assistant Professor  in the Department of Rehabilitation Sciences at the University of Hartford in West Hartford, Connecticut.

Nicholas Carr, BS, will begin clinical residency for the Kansas City Area with Hanger Clinic after earning his MSPO from the University of Hartford in May 2023.

Jordan Dunn, BS, will earn her MSPO from the University of Hartford in May 2023 and begin residency with Hanger Clinic in Allentown, PA.

Jayne Kernodle, BS, will begin a research residency as a Chicago Area Resident for Hanger Clinic after matriculating with her MSPO from the University of Hartford in May 2023. 

Yelizaveta Kozlov, BS,   will complete her MSPO with the University of Hartford in May 2023 and begin her clinical residency with New England Orthotic and Prosthetic Systems in New Haven, CT.

Jillian Picard-Busky, ATC, LAT, will begin a clinical residency in Rochester, MN, with Hanger Clinic after earning her MSPO from the University of Hartford in May 2023.

Abigail Riley, BS, will begin clinical residency at Coastal Prosthetics and Orthotics in Chesapeake, VA, after earning her MSPO from the University of Hartford in May 2023.

Alex Wright, BS, NREMT, a current Patient Care Technician at Backus Hospital, will continue working toward his O&P certification after completing his MSPO from the University of Hartford in May 2023