Ankle Foot Orthosis Stiffness Research Articles

Trunk Movements Should be Considered When Optimizing Ankle-Foot Orthosis Stiffness in Children with Cerebral Palsy

To investigate whether varying Ankle-Foot Orthosis (AFO) stiffness leads to changes in trunk movements, and whether these changes are related to the walking energy cost.

Effect of Shoes on Stiffness and Energy Efficiency of Ankle-Foot Orthosis: Bench Testing Analysis.

Understanding the mechanical properties of ankle-foot orthoses (AFOs) is important to maximize their benefit for those with movement disorders during gait. Though mechanical properties such as stiffness and/or energy efficiency of AFOs have been extensively studied, it remains unknown how and to what extent shoes influence their properties. The aim of this study was to investigate the effect of shoes on stiffness and energy efficiency of an AFO using a custom mechanical testing device. Stiffness and energy efficiency of the AFO were measured in the plantar flexion and dorsiflexion range, respectively, under AFO-alone and AFO-Shoe combination conditions. The results of this study demonstrated that the stiffness of the AFO-Shoe combination was significantly decreased compared to the AFO-alone condition, but no significant differences were found in energy efficiency. From the results, we recommend that shoes used with AFOs should be carefully selected not only based on their effect on alignment of the lower limb, but also their effects on overall mechanical properties of the AFO-Shoe combination. Further study is needed to clarify the effects of differences in shoe designs on AFO-Shoe combination mechanical properties.

Impact of ankle foot orthosis stiffness on Achilles tendon and gastrocnemius function during unimpaired gait

Ankle foot orthoses (AFOs) are designed to improve gait for individuals with neuromuscular conditions and have also been used to reduce energy costs of walking for unimpaired individuals. AFOs influence joint motion and metabolic cost, but how they impact muscle function remains unclear. This study investigated the impact of different stiffness AFOs on medial gastrocnemius muscle (MG) and Achilles tendon (AT) function during two walking speeds. We performed gait analyses for eight unimpaired individuals. Each individual walked at slow and very slow speeds with a 3D printed AFO with no resistance (free hinge condition) and four levels of ankle dorsiflexion stiffness: 0.25Nm/°, 1Nm/°, 2Nm/°, and 3.7Nm/°. Motion capture, ultrasound, and musculoskeletal modeling were used to quantify MG and AT lengths with each AFO condition. Increasing AFO stiffness increased peak AFO dorsiflexion moment with decreased peak knee extension and peak ankle dorsiflexion angles. Overall musculotendon length and peak AT length decreased, while peak MG length increased with increasing AFO stiffness. Peak MG activity, length, and velocity significantly decreased with slower walking speed. This study provides experimental evidence of the impact of AFO stiffness and walking speed on joint kinematics and musculotendon function. These methods can provide insight to improve AFO designs and optimize musculotendon function for rehabilitation, performance, or other goals.

Precision orthotics: optimising ankle foot orthoses to improve gait in patients with neuromuscular diseases; protocol of the PROOF-AFO study, a prospective intervention study

IntroductionIn patients with neuromuscular disorders and subsequent calf muscle weakness, metabolic walking energy cost (EC) is nearly always increased, which may restrict walking activity in daily life. To reduce walking...

An individual approach for optimizing ankle-foot orthoses to improve mobility in children with spastic cerebral palsy walking with excessive knee flexion

Ankle-Foot Orthoses (AFOs) are commonly prescribed to promote gait in children with cerebral palsy (CP). The AFO prescription process is however largely dependent on clinical experience, resulting in confusing results regarding treatment efficacy. To maximize efficacy, the AFO's mechanical properties should be tuned to the patient's underlying impairments. This study aimed to investigate whether the efficacy of a ventral shell AFO (vAFO) to reduce knee flexion and walking energy cost could be improved by individually optimizing AFO stiffness in children with CP walking with excessive knee flexion. Secondarily, the effect of the optimized vAFO on daily walking activity was investigated. Fifteen children with spastic CP were prescribed with a hinged vAFO with adjustable stiffness. Effects of a rigid, stiff, and flexible setting on knee angle and the net energy cost (EC) [Jkg−1m−1] were assessed to individually select the optimal stiffness. After three months, net EC, daily walking activity [stridesmin−1] and knee angle [deg] while walking with the optimized vAFO were compared to walking with shoes-only. A near significant 9% (p=0.077) decrease in net EC (−0.5Jkg−1m−1) was found for walking with the optimized vAFO compared to shoes-only. Daily activity remained unchanged. Knee flexion in stance was reduced by 2.4° (p=0.006). These results show that children with CP who walk with excessive knee flexion show a small, but significant reduction of knee flexion in stance as a result of wearing individually optimized vAFOs. Data suggest that this also improves gait efficiency for which an individual approach to AFO prescription is emphasized.

The influence of ankle-foot orthosis stiffness on walking performance in individuals with lower-limb impairments

BackgroundPassive-dynamic ankle-foot orthoses utilize stiffness to improve gait performance through elastic energy storage and return. However, the influence of ankle-foot orthosis stiffness on gait performance has not been systematically investigated, largely due to the difficulty of manufacturing devices with precisely controlled stiffness levels. Additive manufacturing techniques such as selective laser sintering have been used to successfully manufacture ankle-foot orthoses with controlled stiffness levels. The purpose of this study was to use passive-dynamic ankle-foot orthoses manufactured with selective laser sintering to identify the influence of orthosis stiffness on walking performance in patients with lower-limb neuromuscular and musculoskeletal impairments. MethodsThirteen subjects with unilateral impairments were enrolled in this study. For each subject, one passive-dynamic ankle-foot orthosis with stiffness equivalent to the subject's clinically prescribed carbon fiber orthosis, one 20% more compliant and one 20% more stiff, were manufactured using selective laser sintering. Three-dimensional kinematic and kinetic data and electromyographic data were collected from each subject while they walked overground with each orthosis at their self-selected velocity and a controlled velocity. FindingsAs the orthosis stiffness decreased, ankle range of motion and medial gastrocnemius activity increased while the knee became more extended throughout stance. Minimal changes in other kinematic, kinetic and electromyographic quantities were observed. InterpretationSubjects effectively compensated for changes in ankle-foot orthosis stiffness with altered gastrocnemius activity, and the stiffness levels analyzed in this study had a minimal effect on overall walking performance.

Defining the Mechanical Properties of a Spring-hinged Ankle Foot Orthosis to Assess its Potential Use in Children With Spastic Cerebral Palsy

A rigid ventral shelf ankle foot orthosis (AFO) may improve gait in children with spastic cerebral palsy (SCP) whose gait is characterized by excessive knee flexion in stance. However, these AFOs can also impede ankle range of motion (ROM) and thereby inhibit push-off power. A more spring-like AFO can enhance push-off and may potentially reduce walking energy cost. The recent development of an adjustable spring-hinged AFO now allows adjustment of AFO stiffness, enabling tuning toward optimal gait performance. This study aims to quantify the mechanical properties of this spring-hinged AFO for each of its springs and settings. Using an AFO stiffness tester, two AFO hinges and their accompanying springs were measured. The springs showed a stiffness range of 0.01-1.82 N · m · deg(-1). The moment-threshold increased with increasing stiffness (1.13-12.1 N · m), while ROM decreased (4.91-16.5°). Energy was returned by all springs (11.5-116.3 J). These results suggest that the two stiffest available springs should improve joint kinematics and enhance push-off in children with SCP walking with excessive knee flexion.

How does ankle-foot orthosis stiffness affect gait in patients with lower limb salvage?

Ankle-foot orthoses (AFOs) are commonly prescribed during rehabilitation after limb salvage. AFO stiffness is selected to help mitigate gait deficiencies. A new custom dynamic AFO, the Intrepid Dynamic Exoskeletal Orthosis (IDEO), is available to injured service members but prescription guidelines are limited. In this study we ask (1) does dynamic AFO stiffness affect gait parameters such as joint angles, moments, and powers; and (2) can a given dynamic AFO stiffness normalize gait mechanics to noninjured control subjects? Thirteen patients with lower limb salvage (ankle arthrodesis, neuropathy, foot/ankle reconstruction, etc) after major lower extremity trauma and 13 control subjects who had no lower extremity trauma and wore no orthosis underwent gait analysis at a standardized speed. Patients wore their custom IDEO with posterior struts of three different stiffnesses: nominal (clinically prescribed stiffness), compliant (20% less stiff), and stiff (20% stiffer). Joint angles, moments, powers, and ground reaction forces were compared across the varying stiffnesses of the orthoses tested and between the patient and control groups. An increase in AFO compliance resulted in 20% to 26% less knee flexion relative to the nominal (p = 0.003) and stiff (p = 0.001) conditions, respectively. Ankle range of motion and power generation were, on average, 56% (p < 0.001) and 63% (p < 0.001), respectively, less than controls as a result of the relatively fixed ankle position. Patients with limb salvage readily adapted to different dynamic AFO stiffnesses and demonstrated few biomechanical differences among conditions during walking. None of the stiffness conditions normalized gait to controls. The general lack of differences across a 40% range of strut stiffness suggests that orthotists do not need to invest large amounts of time identifying optimal device stiffness for patients who use dynamic AFOs for low-impact activities such as walking. However, choosing a stiffer strut may more readily translate to higher-impact activities and offer less chance of mechanical failure.

The effect of AFO stiffness on the knee joint during the heel strike phase of gait in stroke patients (919.12)

Stroke patients suffering from foot drop are often prescribed ankle‐foot orthoses (AFOs) to improve foot clearance during gait. However, excessive restriction of plantarflexion (PF) during the heel strike phase of gait has been shown to cause biomechanical changes at the knee joint. This pilot study aimed to determine how increasing the PF stiffness of an AFO affects knee joint kinematics and kinetics during heel strike in hemiplegic stroke patients with varying levels of active ankle range of motion. We hypothesized that increased PF resistance would cause an increase in anterior tibial rotation at heel strike with resulting increases in knee flexion angle and knee extension moment. To test this hypothesis, 5 stroke patients with foot drop were clinically evaluated, instrumented with reflective markers, and fitted with a spring modified AFO (Orthocare Innovations). A compression spring in the posterior aspect of the AFO was changed to increase the external dorsiflexion torque from 0.3 Nm/deg (Spring 1) to 2.0 Nm/deg (Spring 2). Kinematic and kinetic data were collected during walking on an instrumented treadmill (Bertec) in a motion analysis lab. Five steps with each spring were examined. Two patients exhibited an increase in knee flexion angle and knee extension moment with the stiffer spring. Clinical findings revealed that these 2 patients had minimal active PF while the other 3 patients had full active PF. From these initial results, we cautiously conclude that stroke patients with decreased active PF cannot overcome excessive PF resistance during the heel strike phase of gait, and as a result exhibit compensation strategies at the knee joint. These results will be used to create a more objective method for the prescription of AFOs in hopes to improve the long‐term quality of life for stroke patients.Grant Funding Source: Supported by National Institutes of Health (2R44HD069095‐02)

Design of a stiffness-adjustable ankle-foot orthosis and its effect on ankle joint kinematics in patients with stroke

Ankle-foot orthoses (AFOs) are commonly prescribed to improve gait. The stiffness of an AFO is central for successful prescription; however, the recommended level of stiffness is currently based on the experience of clinicians. Therefore, the aim of this study was to design an experimental AFO (EAFO) whose stiffness was adjustable using commercially available oil-damper joints, and to demonstrate its potential capability in investigating the effects of altering AFO stiffness on gait. The influence of the EAFO stiffness on ankle joint kinematics in sagittal plane was evaluated in 10 patients with stroke by altering the stiffness of its oil-damper- type orthotic ankle joints using the four levels pre-set and defined by the manufacturer in dorsi- and plantarflexion directions independently. The mean peak plantarflexion angle was reduced by 105%, showing a change from 8.18 (3.14) degrees of plantarflexion to 0.38 (4.17) degrees of dorsiflexion, whilst the mean peak dorsiflexion angle was reduced by 44%, showing a change from 11.46 (5.57) degrees of dorsiflexion to 6.47 (5.23) degrees of dorsiflexion by altering the EAFO stiffness. The EAFO would therefore serve as a convenient tool when investigating the influence of AFO stiffness on gait in both clinical and research settings.

Development of a method for fabricating polypropylene non-articulated dorsiflexion assist ankle foot orthoses with predetermined stiffness

A non-articulated plantarflexion resist ankle foot orthosis (AFO), commonly known as a posterior leaf spring AFO, is indicated for patients with motor impairment to the dorsiflexors. The AFO is often custom molded to a patient's lower limb anatomy and fabricated from polypropylene. There are no established guidelines for fabricating this type of AFO with predetermined stiffness of the ankle region for normal walking speeds. Therefore an AFO may not meet the biomechanical needs of the patient. Quantify the biomechanical ankle stiffness requirement for an individual with complete dorsiflexor impairment and develop a method for fabricating an AFO with ankle stiffness to meet that requirement. Experimental, bench research. The literature on sagittal biomechanics of non-pathological adults was reviewed to derive the stiffness of the ankle during loading response. Computer models of 144 AFOs were created with geometric variations to account for differences in human anthropometrics. Computer-based finite element analysis was employed to determine the stiffness and safety factor of the models. Stiffness of the AFOs ranged from 0.04 to 1.8 Nm/deg. This ample range is expected to account for the stiffness required for most adults with complete dorsiflexor impairment. At 5° deflection the factor of safety (ratio of strength to stress) ranged from 2.8 to 9.1. A computer program was generated that computes AFO stiffness from user-input variables of AFO geometry. The stiffness is compared to a theoretically appropriate stiffness based on the patient mass. The geometric variables can be modified until there is a close match, resulting in AFO design specification that is appropriate for the patient. Through validation on human subjects, this method may benefit patient outcomes in clinical practice by avoiding the current uncertainty surrounding AFO performance and reducing the labor and time involved in rectifying a custom AFO post-fabrication. This method provides an avenue for improving patient outcomes by avoiding the current uncertainty surrounding non-articulated plantarflexion resist ankle foot orthosis performance. The ability to quantify the biomechanical ankle stiffness requirement for an individual with complete dorsiflexor impairment provides insight into how other AFO types should be designed as well.

Trimline Severity Significantly Affects Rotational Stiffness of Ankle-Foot Orthosis

In Brief Ankle-foot orthoses (AFOs) are prescribed to patients suffering from lower limb disorders. Usually made of polypropylene, these braces have the ability to be custom cut and formed to meet the patient's needs, based on how much support they need in certain planes of ankle motion. A study was conducted using a previously designed AFO testing apparatus to test the stiffness of a set of AFOs along the three planes of ankle motion. Three different trimlines (conservative, moderate, and aggressive) were applied to these samples, and data were collected. Data were analyzed using a one-way analysis of variance with Tukey comparison. It was found that all nine groups of data (grouped with respect to plane of motion and trimline depth) yielded significantly different stiffness measures, except for the moderate and aggressive cuts along the abduction/adduction plane. A reproducibility study was also performed, and the range of standard deviations for stiffness measures collected along each axis was found to be 0.068 to 0.460 Nm/°, showing that the testing apparatus can be used by a variety of users and with different specimens with limited variability. In addition, a linear model was fit to trimline length versus average stiffness measure along each plane. These results will serve to further develop a mathematical model to aid in assisting physicians in recommending a proper trimline for a patient, requiring specific stiffness measures along different axes of motion. Ankle foot orthoses (AFOs) are tailored to fit individual patient needs, and can range from stiff and rigid to flexible and articulated, but are rarely quantified for functional measures of mechanical stiffness or range of motion. This study reveals that there is a statistically significant change between the conservative, moderate and aggressive trimlines in stiffness performance. The realization that each trimline produces a significantly different stiffness is vital to the applicability of AFOs to different situations. Information gathered in this study may contribute to the future development of a system for clinicians to determine the appropriate trimline for the patient before fabricating the AFO, thus helping to save time and money.

EFFECTS OF PROSTHETIC MASS DISTRIBUTION ON MUSCULOSKELETAL SYSTEM DURING AMPUTEE GAIT

Ankle foot orthoses (AFOs) are designed to improve gait for individuals with neuromuscular conditions and have also been used to reduce energy costs of walking for unimpaired individuals. AFOs influence joint motion and metabolic cost, but how they impact muscle function remains unclear. This study investigated the impact of different stiffness AFOs on medial gastrocnemius muscle (MG) and Achilles tendon (AT) function during two walking speeds. We performed gait analyses for eight unimpaired individuals. Each individual walked at slow and very slow speeds with a 3D printed AFO with no resistance (free hinge condition) and four levels of ankle dorsiflexion stiffness: 0.25 Nm/°, 1 Nm/°, 2 Nm/°, and 3.7 Nm/°. Motion capture, ultrasound, and musculoskeletal modeling were used to quantify MG and AT lengths with each AFO condition. Increasing AFO stiffness increased peak AFO dorsiflexion moment with decreased peak knee extension and peak ankle dorsiflexion angles. Overall musculotendon length and peak AT length decreased, while peak MG length increased with increasing AFO stiffness. Peak MG activity, length, and velocity significantly decreased with slower walking speed. This study provides experimental evidence of the impact of AFO stiffness and walking speed on joint kinematics and musculotendon function. These methods can provide insight to improve AFO designs and optimize musculotendon function for rehabilitation, performance, or other goals.

Analysis of correlation between reduction of ankle planterfexion moment and stiffness of ankle foot orthosis in hemiplegics

Analysis of correlation between reduction of ankle planterfexion moment and stiffness of ankle foot orthosis in hemiplegics

805 片麻痺者足関節の力学特性および歩行特性に基づく短下肢装具力学特性の検討(GS-2,16 医療機具,研究発表講演)

An ankle-foot orthosis (AFO) is a device widely used by hemiplegic patients to assist their gait. The aim of this study is to estimate an adequate mechanical property of an AFO based on a passive property of ankle. First, we examine mechanical properties of hemiplegic ankle and their AFO prescribed suitably, and then evaluate the correlation between these two properties. The results show that AFO plantar stiffness has significant correlations (p<0.05) with ankle resistive moment at a knee-extended posture and with ankle dorsal stiffness at a knee-flexed posture. The former correlation gives reasonable estimation at initial stance phase, and the latter at swing phase. Next, we examine a relationship between the AFO stiffness and gait characteristics of hemiplegic subjects. The results show that the regression derived from the correlations are useful for estimation of adequate AFO plantar stiffness.

1204 短下肢装具関節剛性の設計指針(OS12 バイオメカニクカルデザインI)

1204 短下肢装具関節剛性の設計指針(OS12 バイオメカニクカルデザインI)

A three centre study of the variability of ankle foot orthoses due to fabrication and grade of polypropylene.

This study investigates if fabrication techniques employed at different orthotic centres affect the characteristics of the manufactured plastic orthoses. Plaster models were formed from the same master mould. The thickness and bending stiffness of the supplied polypropylene sheets were measured prior to fabrication. An orthotic technician at each of the 3 orthotic centres manufactured 4 homopolymer and 4 copolymer polypropylene ankle-foot orthoses (AFOs), following each centre's fabrication practice. Another technician at one of the orthotic centres manufactured an additional 4 homopolymer and 4 copolymer AFOs. The thickness, the dorsiflexion stiffness and plantarflexion stiffness of the 32 fabricated AFOs were monitored and compared. Analysis of the results suggests: Copolymer polypropylene sheets are supplied marginally thicker than homopolymer polypropylene sheets. The difference between the thickness of the 16 copolymer and 16 homopolymer AFOs was not significant. The thickness of the AFOs manufactured in copolymer was less consistent than homopolymer. Dorsiflexion stiffness of the copolymer AFOs was less consistent than homopolymer AFOs. Although the bending stiffness of the copolymer and homopolymer sheets differed significantly, there was no significant difference between the dorsiflexion stiffness of the copolymer and homopolymer AFOs. Plantarflexion stiffness was consistent for both the copolymer and the homopolymer AFOs and there was no significant difference between the plantarflexion stiffness of the copolymer and homopolymer AFOs. The thickness and flexural stiffness of the AFOs manufactured by 2 technicians at the same centre did not differ. These results are useful benchmarks for the flexural stiffness of AFOs.

Assessment of the non-linear behaviour of plastic ankle foot orthoses by the finite element method

The stiffness characteristics of plastic ankle foot orthoses (AFOs) are studied through finite element modelling and stress analysis. Particular attention is given to the modelling and prediction of non-linear AFO behaviour, which has been frequently observed in previous experimental studies but not fully addressed analytically. Both large deformation effects and material non-linearity are included in the formulation and their individual influence on results assessed. The finite element program is subsequently applied to the simulation of a series of tests designed to investigate the relation between AFO trimline location and stiffness for moderate and large rotations. Through careful consideration and identification of key modelling parameters, the developed finite element solution proves to be a reliable and effective alternative means of assessing variations of a typical plastic AFO design so that particular patient requirements could be met, in the long term.

Mechanical Characteristics of Ankle Foot Orthosis and its Effect on Human Gait.

Ankle foot orthosis (AFO) assists the gait motion of persons with disability such as of hemiplegia, and the biomechanical function is the primary concern of AFO. In conventional prescription of AFO, the attention has been mainly focused on the ankle joint support at a secure position against the stumbling due to toe contact with floor during the swing phase of gait cycle, but little attention has been paid for its function during the stance phase. In this study, the mechanical characteristics of shoehorn-type AFO is examined in the context of the gait motion analysis during the stance phase of normal subjects, and by experimental analysis of the ankle joint stiffness in sagittal plane on a universal testing machine. Deformation of AFO is characterized by the relative angle between the shank and foot planes, and it is found that the stiffness of AFO depends strongly on the foot angle. The joint moment during the stance phase is evaluated for slow-speed gait with and without AFO. It is found that the knee extension moment of the gait with AFO is significantly larger than that without AFO. It is expected that the ankle joint constraint imposed by AFO results in a gait pattern different from the normal, although the resisting moment of AFO is not so significant in the joint moment.

Viscoelastic Properties of Plastic Pediatric AFOs

ABSTRACT The viscoelastic behavior of pediatricsized polypropylene ankle-foot orthoses (AFOs) has been investigated by measuring the stiffness and buckling of AFOs before, during and after cyclic loading. Cyclic loading simulates normal gait by forcing the AFOs into dorsiflexion (10 degrees) and plantarflexion (15 degrees). Mechanical properties were measured after three 24-hour periods of 187,200 cycles. After cyclic loading, mechanical properties were again measured at six discrete time intervals to determine if the plastic recovered or remained permanently degraded. The cycling rate simulated a relatively rapid walking pace of 130 steps per minute. The AFOs exhibited a significant reduction in mechanical properties—30 percent reduction in stiffness and 4.4 percent increase in malleolar diameter— after just one 24-hour period of cycling. Within 15 minutes after cycling had ceased, however, the AFOs began to recover their mechanical integrity and within an hour had recovered fully. Microfracturing (crazing) was observed in the sagittal walls of the AFO specimens. This was attributed to excessive stress as the plastic curved outward to accommodate the malleoli prominences.