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International Forum: Development of a New Ankle-Foot Orthosis with Dorsiflexion Assist, Part 1: Desirable Characteristics of Ankle-Foot Orthoses for Hemiplegic Patients

Sumiko Yamamoto, PhD
Masahiko Ebina
Shinji Miyazaki, PhD
Hideo Kawai
Toshio Kubota, MD

ABSTRACT

The mechanical characteristics of ankle-foot orthoses that are important for hemiplegic gait are the magnitude of the assist moment and the initial ankle angle. An experimental ankle-foot orthosis that allows for easy adjustment of the magnitude of the assist moments and the initial ankle angle was developed.

The gaits of hemiplegic patients using the experimental orthosis were measured, and the plantarflexion assist moment was found to be unnecessary for the hemiplegic gait in most cases. The hemiplegic gaits for various magnitudes of the dorsiflexion assist moment and initial ankle angles were examined, and the orthosis characteristics appropriate for the individual patients were selected. Based on these results, desirable characteristics of ankle-foot orthoses for hemiplegic patients were determined.

Key Words: Ankle-Foot Orthosis; Hemiplegic Patient; Assist Moment; Gait Analysis.

Introduction

Ankle-foot orthoses (AFOs) are commonly used by hemiplegic patients, and various types of AFOs, mainly made of plastics, have been developed. Currently, posterior-type plastic AFOs are prescribed most frequently in Japan (1).

The stiffness of posterior AFOs most influences hemiplegic gait, and various procedures for measuring the stiffness of posterior AFOs have been developed (2-4). The stiffness of posterior AFOs changes markedly depending on the trimlines around the ankle joint. Since the shape of the ankle varies greatly among patients, it is difficult to precisely adjust the stiffness of posterior AFOs to suit individual patients.

The stiffness of AFOs is represented by the magnitude of the assist moment generated by the device. The authors developed an experimental AFO that allows the magnitude of the assist moments and the initial ankle angle to be changed easily; the authors subsequently measured the gaits of hemiplegic patients wearing these AFOs (5,6). These studies revealed the most important mechanical characteristics of AFOs are the magnitude of the dorsiflexion assist moment and the initial ankle angle.

The purpose of the present study was to determine targets for the development of a new AFO based on this finding. First, the assist moment of the experimental AFO was determined with reference to the mechanical characteristics of conventional AFOs. Next, the authors determined whether the plantarflexion assist moment of AFOs is necessary for hemiplegic gait. The authors then measured the gait of hemiplegic patients wearing the experimental AFO for various magnitudes of the dorsiflexion assist moment and initial ankle angles; they subsequently determined the characteristics of the AFO appropriate for the individual patients. Finally, the authors identified targets of development for a new AFO.

The Role of an AFO's Assist Moment

AFOs most commonly are prescribed to hemiplegic patients to ensure toe clearance and prevent excessive inversion of the ankle joint during the swing phase of gait; absorb the body-weight impact at the initial stance; and support forward propulsion of the body during the mid- to late stance phase. These functions mainly are achieved by controlling the magnitude of the assist moment of AFOs in the sagittal plane. Inversion of the ankle joint can be prevented through appropriate control of plantarflexion because inversion always is accompanied by plantarflexion.

Figure 1 shows the relationship between the dorsi/plantar assist moment of an AFO and the muscle moment in each phase of gait. At the initial stance, the AFO undergoes plantarflexion and generates a dorsiflexion assist moment that assists the dorsiflexors (see Figure 1a) . From the midstance to the late stance phase, the AFO undergoes dorsiflexion and generates a plantarflexion assist moment (see Figure 1b) . During the swing phase, the AFO generates a dorsiflexion assist moment that opposes the gravitational force acting on the foot (see Figure 1c) . In previous studies, the authors found the dorsiflexion assist moment at the initial stance to be quite important for hemiplegic gait (5,6).

Structure and Characteristics of the Experimental AFO

To determine the effect of the assist moment on hemiplegic gait, the authors developed an experimental AFO that allows the magnitude of the assist moments in dorsiflexion and plantarflexion to be changed easily and independently. The experimental AFO is shown schematically in Figure 2 as type I. This AFO consists of a plastic foot, two aluminum uprights, two double Klenzak joints without springs, and two spring systems on both the medial and the lateral sides of the AFO. The springs anterior to the ankle joint generate a dorsiflexion assist moment in accordance with the degree of plantarflexion induced by the rotation of the uprights, and the posterior springs generate a plantarflexion assist moment in accordance with the degree of dorsiflexion. The magnitudes of the assist moments in the dorsiflexion and plantarflexion directions can be changed independently. The initial ankle angle at which the springs begin to elongate and thus generate the assist moment is controlled by adjusting the lengths of the spring systems.

The mechanical characteristics of the experimental AFO were determined with reference to those of conventional AFOs. The mechanical characteristics of two types of conventional AFOs are shown in Figure 3 . The procedure for measuring the AFOs is described in Reference 3.

In Figure 3a , the characteristics of a posterior AFO, which is the most rigid plastic AFO available, and those of a plastic spiral AFO, which is the most flexible plastic AFO available, are shown by the solid line and the dotted line, respectively. In Figure 3b , the characteristics of an AFO with Klenzak joints, those of an AFO with double Klenzak joints, and those of an AFO with joints and rubber dorsiflexion straps are shown by the solid line, dotted line and dashed line, respectively. The horizontal axis in the graphs depicts the rotation angle of the ankle joint, and the vertical axis depicts the assist moment generated by the AFO.

Based on these results, four types of springs were selected to approximate the function of the conventional AFOs. The springs are composed of wires of the following diameters with the following associated spring constants: 1) 1.0 mm and 0.06 kgf/mm; 2) 1.2 mm and 0.16 kgf/mm; 3) 1.4 mm and 0.37 kgf/mm; and 4) 1.6 mm and 0.77 kgf/mm.

The overall characteristics of the experimental AFO are shown in Figure 4 . The solid lines indicate the characteristics for plantarflexion, and the dotted lines represent those for dorsiflexion. In the graph, each line represents the characteristics of four conditions achieved using various combinations of springs, i.e., No. 1 is the combination of 1.4 mm 3 2; No. 2 is the combination of 1.6 mm 3 2; No. 3 is the combination of 1.6 mm 3 2 + 1.4 mm 3 2; and No. 4 is the combination of 1.6 mm 3 4. Arbitrary combinations of these characteristics are possible. The functional characteristics of conventional AFOs can be modeled by appropriate adjustment of the combination of the springs of the experimental AFO.

Necessity of the Plantarflexion Assist Moment

Most thermoplastic AFOs generate a plantarflexion assist moment during dorsiflexion of the ankle joint from mid- to late stance phase. From results obtained in previous studies, the authors concluded the plantarflexion assist moment is not necessary for hemiplegic gait. In normal gait the triceps surae muscle resists dorsiflexion to stabilize the ankle and knee joints during midstance (7). The literature supports the theory that the plantarflexion assist moment substitutes for this muscle function in flaccid paralysis (8-10). However, an AFO that can undergo dorsiflexion can reduce gait spasticity in children with cerebral palsy (11). It also has been reported the anterior stop assists the activation of the triceps surae muscle to achieve heel-off in hemiplegic gait (12), but the authors challenge the validity of this statement. Hemiplegic patients show a comparatively large degree of activation of the triceps surae muscle due to an abnormal stretch reflex that occurs too early during the stance phase (13). Therefore, the authors performed the following experiment to test for the necessity of the plantarflexion assist moment for pathologic hemiplegic gait.

First, the pathologic gait of hemiplegic patients wearing the experimental AFO (type 1) with various dorsiflexion assist spring combinations and for various initial ankle angles was measured so the magnitude of the dorsiflexion assist moment and the initial ankle angle appropriate for each patient could be determined. Next, the plantarflexion assist springs were installed on the experimental AFO. The magnitude of the plantarflexion assist moment was determined with reference to the function of conventional posterior AFOs. The gaits then were measured again, and the two sets of data were compared. The angular displacements of the ankle and knee joints of the paretic side were measured using goniometers, and temporal factors and the joint moment around the ankle joint were measured using capacitive transducers as described in Reference 5. The initial pilot subjects were four hemiplegic patients who used plastic AFOs daily.

None of the patients experienced forward thrust of the knee joint when they wore the experimental AFO without plantarflexion assist springs. Moreover, all of the patients reported feeling uncomfortable when they walked while wearing the experimental ankle-foot orthosis with plantarflexion springs.

As an example of gait data, the angular displacements for one subject are shown in Figure 5 . The horizontal axis represents the time in percentage of the duration of the gait cycle. The data are average data for 10 gait cycles. The angular displacements of the knee joint when no plantarflexion assist spring was used were not significantly different from the displacements when such a spring was employed; however, the dorsiflexion of the ankle joint from mid- to late stance was restricted with use of a plantarflexion assist spring. In addition, the ankle joint remained plantarflexed during the swing phase when a plantarflexion assist spring was used.

Similar results were obtained for the other three patients. These results coincide with those obtained by Thilmann et al., who reported it was necessary to allow dorsiflexion for transfer of the center of gravity forward during midstance (14).

Modifications to the AFO

The results of this study demonstrate the plantarflexion assist moment is not necessary for and may in fact negatively affect hemiplegic gait. Thus, the plantarflexion assist spring system was removed from the experimental AFO. A schematic of the modified experimental AFO is shown in Figure 2 as type II. The ankle joint of this experimental AFO rotates freely in dorsiflexion because there is no plantarflexion assist device. The gaits of hemiplegic patients wearing this experimental AFO were measured, and the magnitude of the dorsiflexion assist moment and the initial ankle angle appropriate for each patient were determined. The method of gait measurement was the same as that in the first experiment. Together with the gait data, patients' impressions of gait and video images of the gait of the patients wearing the type II AFO with various mechanical characteristics were recorded.

The dorsiflexion assist springs work to reduce the knee flexion moment by ensuring smooth plantarflexion (15) and prevent quick plantarflexion due to insufficient force generated by the dorsiflexors at the initial stance phase. The appropriate function of the type II AFO for each individual patient was determined by trial and error. The authors used the following criteria to adjust the mechanical characteristics of the type II AFO for each patient:

  1. If forward thrust or an unstable knee joint at initial stance was observed, adjustments were made to reduce the dorsiflexion assist moment to set the initial ankle angle at the neutral position in which the shank is approximately perpendicular to the floor while the patient is standing.
  2. If flexion of the knee joint and excessive dorsiflexion of the ankle joint at the initial stance were observed, adjustments were made to set the initial ankle angle at the neutral position.
  3. If hyperextension of the knee joint from mid- to late stance was observed, adjustments were made to increase the dorsiflexion assist moment.
  4. If excessive elongation of pushoff phase was observed, adjustments were made to set the initial ankle angle at the dorsiflexion position in which the shank is inclined slightly forward while the patient is standing.
  5. If outward rotation or excessive flexion of the hip joint at late stance was observed, adjustments were made to reduce the dorsiflexion assist moment.
  6. If insufficient toe clearance during the swing phase was observed, adjustments were made to set the initial ankle angle at the dorsiflexion position.

Methods

The subjects were 33 hemiplegic patients who used plastic ankle-foot orthoses on a daily basis. It took approximately 30 minutes to determine the best conditions, i.e., the best dorsiflexion assist moment magnitude and initial ankle angle of the type II AFO, for each patient. The gait data varied among the patients, but the walking velocity coincided with the patients' impression of the gait. In most cases the velocity was minimum under the best conditions as determined by the patients. Under these conditions the patients who showed hyperextension of the knee had minimum knee extension angles, and the patients whose knee joints were unstable demonstrated minimum knee flexion angles.

The results are shown in Figure 6 . These histograms show the number of subjects with each condition. The magnitude of the dorsiflexion assist moment was selected from among those generated by the combinations of springs shown in Figure 4 . The maximum dorsiflexion assist moment that can be generated using this experimental ankle-foot orthosis might be insufficient for correction of a severely spastic gait; so correction of such a gait is optimal with the use of a rigid posterior ankle-foot orthosis as reported in the literature (16). There was no relation between the selected magnitude of the dorsiflexion assist moment and the selected initial ankle angle.

Results

The magnitude of the dorsiflexion assist moment and the initial ankle angle appropriate for each patient are influenced by many factors, including the patient's gender, degree of disability, muscle strength and weight. Since some of these parameters may fluctuate, modifiability of the dorsiflexion assist moment magnitude and initial ankle angle of the AFO after fabrication of the AFO is essential. The desirable characteristics of AFOs for hemiplegic patients are summarized below based on the results of the authors' previous studies as well as the current one:

  1. The AFOs should have an articulated ankle joint and a moderate corrective ability in the inversion/eversion direction.
  2. The initial ankle angle of the AFOs should be adjustable in the range of 0 to 10 degrees of dorsiflexion.
  3. The range of dorsiflexion of the ankle joints should be 30 degrees from the initial ankle angle, taking into consideration rotation during gait, ascent and descent of stairs and slopes, and squatting.
  4. The AFOs should generate no plantarflexion assist moment during dorsiflexion.
  5. The range of plantarflexion of the ankle joints should be 10 degrees from the initial ankle angle, taking into consideration rotation during gait and descent of stairs and slopes.
  6. The AFOs should generate a dorsiflexion assist moment during plantarflexion. The magnitude of the dorsiflexion assist moment should be variable in the range of 5 to 20 Nm per 10 degrees of plantarflexion.

The authors developed an AFO that incorporates all of the characteristics mentioned above. Further information on this new AFO is given in a separate article, to be published in an upcoming issue of the JPO.

Acknowledgements

The authors are very grateful to Mr. S. Kubo, Mr. T. Hayashi and Mr. M. Iwasaki of the Tokyo Metropolitan Prosthetic and Orthotic Research Institute and Mr. T. Yamaguchi of the Nakaizu Rehabilitation Center for their assistance in the gait measurement and the fabrication of the experimental AFOs. The authors also would like to thank Meidenkousan Co. Ltd. and Kyowa Electronic Instruments Co. Ltd. for financial support.


References:

  1. Ofir R, Sell H. Orthoses and ambulation in hemiplegia: a 10-year retrospective study. Arch Phys Med Rehab 1980;61:216-20.
  2. Condie DN, Meadows CB. Some biomechanical considerations in the design of ankle-foot orthoses. Orth Pros 1970;3:45-52.
  3. Yamamoto S, Ebina M, Iwasaki M, Kubo S, Kawai H, Hayashi T. Comparative study of mechanical characteristics of plastic AFOs. JPO 1993;5:2:59-64.
  4. Sumiya T, Suzuki Y, Kasahara T. Stiffness control in posterior-type plastic ankle-foot orthoses: effect of ankle trimline. Part 1: a device for measuring ankle moment. Pros Orth Intl 1996;20:129-31.
  5. Yamamoto S, Miyazaki M, Kubota T. Quantification of the effect of the mechanical properties of ankle-foot orthoses on hemiplegic gait. Gait & Posture 1993;1:27-34.
  6. Yamamoto S, Ebina M, Kubo S, Kawai H, Hayashi T, Iwasaki M, Kubota T, Miyazaki S. Quantification of the effect of dorsi-/plantar flexibility of ankle-foot orthoses on hemiplegic gait: a preliminary report. JPO 1993;5:3:88-94.
  7. Perry J. Gait analysis: normal and pathological function. New York: SLACK, 1992.
  8. Lehmann JF, Ko MJ, Delateur BJ. Double-stopped ankle-foot orthosis in flaccid peroneal and tibial paralysis: evaluation of function. Arch Phys Med Rehab 1980;61:536-41.
  9. Lehmann JF, Condon SM, Delateur BJ, Smith JC. Ankle-foot orthoses: effect on gait abnormalities in tibial nerve paralysis. Arch Phys Med Rehab 1985;66:212-18.
  10. Perry J, Fontaine JD, Mulroy S. Findings in postpoliomylitis syndrome. JBJS 1995;77A:1148-53.
  11. Middleton EA, Hurley GRB, McIlwain JS. The role of rigid and hinged polypropylene ankle-foot orthoses in the management of cerebral palsy: a case study. Pros Orth Intl 1988;12:129-35.
  12. Lehmann JF, Condon SM, Price R, Delateur BJ. Gait abnormalities hemiplegia: their correction by ankle-foot orthoses. Arch Phys Med Rehab 1987;68:763-71.
  13. Knutsson E. Analysis of gait and isokinetic movements for evaluation of antispastic drugs or physical therapies. Motor Control Mechanisms in Health and Disease. Raven Press, 1983.
  14. Thilmann AF, Fellows SL, Ross HF. Biomechanical changes at the ankle joint after stroke. J Neurol Neurosurg Psych 1991; 54:134-9.
  15. Lee K, Johnston R. Biomechanical comparison of 90-degree plantarflexion stop and dorsiflexion assist ankle braces. Arch Phys Med Rehab 1973;54:302-6.
  16. Sumiya T, Suzuki Y, Kasahara T. Stiffness control in posterior-type plastic ankle-foot orthoses: effect of ankle trimline. Part 2: orthosis characteristics and orthosis/patient matching. Pros Orth Intl 1996;20:132-7.