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Development of an Ankle-Foot Orthosis with Dorsiflexion Assist, Part 2: Structure and Evaluation

Sumiko Yamamoto, PhD
Masahiko Ebina, CPO
Shigeru Kubo, Eng.
Takeo Hayashi
Yoshiyuki Akita, CPO
Yasuyuki Hayakawa, CPO

ABSTRACT

An ankle-foot orthosis (AFO) for hemiplegic patients called the DACS (dorsiflexion assist controlled by spring) AFO was developed. Previous studies have shown the DACS AFO to have the following desirable characteristics: 1) the magnitude of the dorsiflexion assist moment and the initial ankle angle of the AFO can be changed easily, and 2) no plantarflexion assist moment is generated. DACS AFOs were used daily by 5 hemiplegic patients, and their evaluations were favorable. In this paper, the structure and the mechanical characteristics of the DACS AFO and the gait improvement of hemiplegic patients with the DACS AFO are shown.

Key Words: ankle-foot orthosis, gait analysis, hemiplegic patient

Introduction

The most important parameters concerning ankle-foot orthoses (AFOs) for hemiplegic patients are the dorsiflexion assist moment and the initial ankle angle. Ideally, the dorsiflexion assist moment should be adjustable for the condition of each patient in the range from 5 to 20 N·m per 10° of plantarflexion. The largest magnitude of the dorsiflexion assist moment is approximately 5 times the magnitude of the dorsiflexion assist moment generated by the conventional Klenzak joints. We determined the desirable characteristics of AFOs quantitatively by analyzing hemiplegic gait of patients with AFOs (Table 1 ). Hemiplegic gait can be improved remarkably if these characteristics are adjusted to each patient's condition. Moreover, it is quite important that these characteristics can be changed arbitrarily in the course of the gait training, even after the fabrication of the AFOs.¹

We have developed an orthosis that incorporates the desired characteristics, called the DACS (dorsiflexion assist controlled by spring) AFO. The DACS AFO was evaluated through daily use by 5 hemiplegic patients, and their rating was favorable. In this paper, we discuss the structure and the mechanical characteristics of the DACS AFO, and the results of gait analysis of patients using the DACS AFO.

Structure of the DACS AFO

Figure 1 shows the appearance of the DACS AFO, which was developed to incorporate the characteristics listed in Table 1 . The DACS AFO consists of two plastic parts, the foot and the shank connected at the ankle joint. An assist device that generates the dorsiflexion assist moment is set at the rear of the shank. Dorsiflexion correction is achieved via the compression force of a spring within the assist device.

The mechanical principles of the operation of the DACS AFO are shown in Figure 2 . When the ankle joint rotates into plantar flexion, a piston compresses the spring and the assist device generates the assist moment proportional to the plantar flexion angle. The dorsiflexion assist moment can be changed easily by using 4 springs which have different spring coefficients. When the ankle joint rotates into dorsiflexion, it rotates freely because a slider in the piston can move with little friction. The initial ankle angle is an angle at which the dorsiflexion assist moment starts to develop. The initial ankle angle can be changed by altering the length of the assist device, as shown in Figure 3 . When the length of the assist device is short, the dorsiflexion assist moment starts to develop at the neutral angle (0° of dorsiflexion), and when it is long, the dorsiflexion assist moment develops at the dorsiflexion position. The range of the initial ankle angle was set from 0° to 10° of dorsiflexion.

Methods

Structure and Mechanical Characteristics

For determination of the mechanical characteristics of the DACS AFO, it was fixed at the sole and a force was applied to the shank with a wire. The force magnitude and the ankle joint angle were measured simultaneously using a strain gauge and a potentiometer, respectively. The equipment was simple and was developed at our institute.

Gait Analysis

The gaits of hemiplegic patients fitted with the DACS AFO, a conventional AFO with metal uprights and mechanical plantarflexion stoppers, and a shoehorn AFO, were measured by using the 3-dimensional position sensor system (VICON 370, Oxford Metrics). The order of each test was determined randomly. Informed consent was obtained from all patients before the start of the study. The metal AFO and the posterior AFO had previously been used by each patient. The dorsiflexion assist moment and the initial ankle angle of the DACS AFO were adjusted for each patient by observing the gait before the gait measurement. Joint kinematic patterns have been used to describe the gait of hemiplegic patients.² The rotation angle of the foot in the horizontal plane was calculated as the angle between the longitudinal axis of the foot and the progressional direction. Each set of results is the averaged data for 5 gait cycles.

It is said that the walking velocity is a predictor of improved locomotion of hemiplegic patients.³ The maximum walking velocity of hemiplegic patients with the posterior AFOs and the DACS AFOs were measured to evaluate the effect of the AFO design in a simple manner. The walking velocity without any AFOs was also measured, when possible.

Patient Evaluation

Patients' evaluations were obtained by written questionnaire. Patients were questioned about the comfortability of walking and other comments about the daily use of the DACS AFO.

Results

Mechanical Characteristics

The mechanical characteristics of the DACS AFO are shown in Figure 4 . The horizontal axis shows the ankle joint angle, and the vertical axis the assist moment. In the figure, results of 4 different springs are shown. The DACS AFO generates the dorsiflexion assist moment during plantarflexion, but does not generate plantarflexion assist moment during the dorsiflexion. The range of the dorsiflexion assist moment is from 2 to 17 N·m per 10° of plantarflexion. The weight of the assist device was approximately 80 g, and the total weight of the DACS AFO was approximately 300 g. The weight of the commonly used posterior-type plastic AFO was approximately 200 g.

Gait Analysis

Table 2 shows the patients' profiles, and the dorsiflexion assist moment and the initial ankle angle selected by each patient. As an example of gait data, the ankle joint angle (Figure 5a ), the knee joint angle (Figure 5b ) and the rotation angle of the foot in the horizontal plane (Figure 5c ) of the paretic side of a hemiplegic patient (No. 3 in Table 2 ) are shown.

The dorsiflexion of the ankle joint during the stance phase was restricted when the patient wore the posterior AFO, but it was improved when she wore other AFOs, as shown in Figure 5a . The knee-joint angle showed hyperextension through almost the entire stance phase, except for the slight flexion at the initial stance. The magnitude of hyperextension was reduced in the case of the DACS AFO as compared to cases where other AFOs were used. It was found that the patient showed marked external rotation at the time of foot off. This rotation must have been the result of excessive external rotation of the hip joint. Because of the synergic pattern, patients must rotate the hip joint externally to achieve flexion of the hip and the knee joints at the end of the stance phase. The rotational angle was very large when the posterior AFO was used, but it was reduced when the DACS AFO was used. This result coincided with the patient's impression that "when I walk using the posterior AFO I have to use my whole body, but it feels just like my own foot when I use the DACS AFO."

Figure 6 shows the forward progression of the hip joint of the paretic side of another hemiplegic patient (No.5 in Table 1 ). She started the gait training 3 weeks before the measurement, but she chose the DACS AFO because of the flexibility of the ankle joint. The progression of the hip joint was delayed at the end of the stance phase when she wore the posterior AFO, but it was improved when she wore the DACS AFO. This means that the DACS AFO facilitates the smooth toe-off due to the absence of the plantarflexion assist moment.

As shown in Figure 7 , the walking velocity with the DACS AFOs was higher than that with the posterior AFOs for all the patients.

Patient Evaluation

None of the hemiplegic patients using the DACS AFO complained about its weight. There was no problem in wearing shoes when the bottom of the assist device was higher than 7 cm from the sole. Hemiplegic patients could put on the DACS AFO themselves as easily as the shoehorn AFO.

Discussion and Conclusion

The magnitude of the dorsiflexion assist moment and the initial ankle angle can be changed in the DACS AFO, and no plantarflexion assist moment was generated. From the results of the gait analysis of patients using various AFOs, it was found that a smoother gait could be achieved with the use of the DACS AFO compared to using conventional AFOs. The performance of the DACS AFO is now being assessed through daily use by approximately 70 hemiplegic patients.

The DACS AFO could not generate 20 N·m of dorsiflexion assist moment per 10° of rotation, which was the target of the development. This was because deformation of the plastic parts was unavoidable due to the use of a nonelastic spring. The spring coefficient of the most rigid spring is 4.3 kgf/mm, and the amount of the force generated by this spring is approximately 450 N when the DACS AFO rotates into 10° in plantarflexion.

The DACS AFO is currently being subjected to mechanical endurance tests. Continuous plantarflexion is applied to the DACS AFO to check the durability of each part. At present, more than two million repetitions of plantar flexion have been applied and no serious problems have arisen. The results of the endurance test will be used to improve the design of the DACS AFO. The DACS AFO can be used by hemiplegic patients daily, and it is also useful for gait training due to the ease of alteration of various characteristics. A moderately large dorsiflexion assist moment and dorsiflexion initial ankle angle facilitate the increase of knee extension muscle forces that prevent forward thrust during the initial stance. For this purpose, it is necessary to improve the method of exchanging springs; we are currently investigating this.

We have produced manuals and videos to show the prescription criteria and the fabrication procedures of the DACS AFO. We hope theses will be effective for promoting the use of the DACS AFO.

Acknowledgements

The authors are very grateful to Mr. Y. Kitamura of NHK Spring Co., Ltd. for designing and fabricating the assist device for the DACS AFO.

References:

1. Yamamoto S, Ebina M, Miyazaki S, Kawai H, Kabota T. Development of a new ankle foot orthosis with dorsiflexion assist. Part I: Desireable characteristics of ankle foot orthosis for hemiplegic patients. In Journal of Prosthetics and Orthotics 1997;9:4:174-179.
2. Olney SJ, Richards C. Hemiparetic gait following stroke. Part I: Characteristics. In Gait and Posture. 1996;4(2):136(148.
3. Richards C, Malouin F, Dumas F, Tardif D. Gait velocity as an outcome measure of locomotor recovery after stroke. In: Craik RL, Oatis C, eds. Gait Analysis: Theory and Application. St Louis: Mosby; 1995:355-364.