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Comparative Study of Mechanical Characteristics of Plastic AFOs

Sumiko Yamamoto, PhD
Masahiko Ebina, CPO
Mitsuo Iwasaki, CPO
Shigeru Kubo
Hideo Kawai, RPT
Takeo Kayashi, CPO

ABSTRACT

Ankle-foot orthoses (AFOs) are categorized four ways with respect to their shape. posterior spring, anterior spring, side-stay and spiral. In this article the flexibility of these types of AFOs fitted to the limb is measured using a muscle training machine. A comparison of the flexibility of different types of AFOs is made.

Introduction

Various types of plastic ankle-foot orthoses (AFOs) have been developed using advanced fabrication techniques and plastic materials. AFOs control ankle joints and supply adequate corrective forces to limbs in appropriate phases during gait. The corrective forces are generated by the deformation of the ankle joint area of AFOs. Mechanical properties of AFOs are characterized by the relationship between the forces acting on an AFO and its deformation. This relationship will be referred to as flexibility.

Several researchers have studied the flexibility of AFOs (1,2,3). In these studies the relationship between the force applied to an AFO using a wire and the deformation angle was measured for empty AFOs and for AFOs fitted to prosthetic limbs. Flexibility of an empty AFO may differ from that of an AFO fitted to a patient's limb, which has viscoelastic properties. This is particularly so in spiral and shoehorn-type AFOs that do not have a single rotational center. It is evident AFO flexibility affects AFO function.

Method

Measuring equipment

To measure the relationship between the ankle-joint angle of a limb fitted with an AFO and the resistive movement, a special piece of equipment was developed (see Figure 1 ). The patient's foot is fixed to a foot plate attached to a pulley (1,2). The position of the rotational center of the pulley is adjustable in vertical and lateral directions so the subject's joint axis can be aligned with the pulley's axis. The patient is asked to sit, and his shank is fixed with a supporting pad (3). Special care is given to ensure the natural deformation of an AFO with a supporting pad. The pulley's axis is connected to a driving shaft of a muscle-training machine (4)1. The axis of the pulley rotates at a constant velocity in accordance with the eccentric contraction training program of the muscle-training machine. The rotational angle and the resistive movement applied to the axis are registered by computer.

Procedure

The ankle joint, which has a complex anatomical structure, moves as the result of the combined movements of several joints. In these movements, dorsi and plantarflexion is achieved by the talocrural joint, and inversion/eversion is accomplished by the subtalar joint (4). The axes of these joints are shown in Figure 2 . The flexibility of the AFOs in both dorsi- and plantarflexion and inversion/eversion were measured under conditions shown in Figure 3 . In each condition, the corresponding joint axis was set in line with the rotational axis of the equipment.

When the ankle joint was moved without muscle contraction, the movement measured was the sum of the resistive movement of the AFO and the resistive movement of passive components of the ankle joint (generated by ligaments and other tissues). The resistive movement of the passive components of the ankle joint were measured in the same condition but without the AFO. The difference between these two results represented the resistive movement of the AFO. The patient was instructed to completely relax his limb muscles, and the state of relaxation was confirmed by monitoring surface EMG signals of dorsi- and plantarflexors.

Because of the viscous property of plastic materials, it was supposed that the flexibility of AFOs depended on the deformation velocity. Therefore, measurements were repeated in three velocities: 5, 10 and 50 degrees per second. For reference, the maximum velocity of the ankle joint during gait is about 10 degrees per second, which occurs at the heel contact of a normal subject. Range of the ankle joint angle was measured from 20 degrees plantarflexion to 15 degrees dorsiflexion and from 15 degrees inversion to 10 degrees eversion.

AFOs Measured

All AFOs except ready-made Ortop were made using a positive plaster model of a normal subject from poiypropylene of 4mm thickness. The following AFOs were used: posterior spring type-three shoehorn-type AFOs with different trimlines in the ankle joint area; anterior-spring type- two KU half AFOs, with and without ankle joint, and Yunoko AFOs; sidestay type-two Saga plastic AFOs with different-shaped ankle joints and an Ortop; and spiral type - hemi-spiral AFO and full-spiral AFO. All AFOs are shown in Figure 4 . Eleven AFOs were measured.

Results

Although measurements were made at three different velocities, it was evident the velocity (from 5 to 50 degrees per second) did not affect the flexibility of the AFOs. Thus, all data provided are the result of measurement at 10 degrees per second. An example is shown in Figure 5 . In this graph, the abscissa indicates the ankle joint angle, positive values represent dorsiflexion and negative values represent plantarflexion. The ordinate indicates the resistive movement of an AFO, positive values represent dorsiflex movement and negative values represent plantarflex movement. Here, steep curve means low flexibility, i.e., high rigidity of an AFO. In Figure 5a , the resistive movement of the ankle joint with an AFO is shown by a solid line, and the resistive movement of the ankle joint without an AFO is shown by a dotted line. The data obtained by subtracting the dotted line from the solid one reveals the resistive movement of the AFO itself, which is shown in Figure 5b . All data showed hysteresis loop and the resistive movement of a bare foot concurs with data obtained by another researcher (5). Similar hysteresis loops were obtained not only in dorsi- and plantarflexion but also in inversion/ eversion. In the following graphs, the ordinate shows the resistive movement generated by an AFO. Flexibilities of different kinds of AFOs are shown in Figure 6 . In these graphs, flexibility in dorsi- and plantarflexion is shown by solid lines and flexibility in inversion/eversion by dotted lines. The following characteristics are observed from Figures 6a , 6b , 6c , 6d , 6e , 6f , 6g , 6h , 6i , 6j , 6k .

  • Shoehorn-type AFOs (the posterior-spring type) have very rigid properties in dorsi- and plantarflexion, especially in plantarflexion, when compared with other AFOs. They also have a relatively rigid property in inversion/eversion.

  • In shoehorn-type AFOs, the width of the ankle joint area alters flexibility in plantarflexion. The height of the mediolateral walls in the area of ankle joint affects mainly flexibility in dorsiflexion.

  • The anterior spring-type AFOs (except an AFO with a joint) have lower flexibility in dorsiflexion than in plantarflexion.

  • KU half AFOs, the anterior-spring types, generally have high flexibility. The KU half AFO with a joint has extremely high flexibility in dorsiflexion because its resistive movement is caused by only the friction of the joint part. The resistive movement in plantarflexion slightly increases when an assistive strap is used.

  • Saga plastic AFOs, the side-stay type, have similar flexibility both in dorsi- and plantarflexion. The solid line shifts to the dorsiflexed direction because the neutral position of the AFO was slightly dorsiflexed. The hysteresis loop shows symmetrical shape with respect to the ordinate. This suggests Saga AFOs can flex smoothly without excess stress on the ankle joint area because their rotational centers closely coincide with the ankle joint center. They are as rigid as shoehorn-type AFOs in inversion/eversion.

  • The hemi-spiral AFO is rather rigid in plantarflexion and flexible in dorsiflexion. It shows a rigid property in inversion/eversion in spite of its asymmetric shape.

  • The full-spiral AFO is very flexible in dorsi- and plantarflexion.

  • Ortop AFO is slightly rigid in plantarflexion and extremely flexible in dorsiflexion. It is flexible in inversion/ eversion in comparison with the custom AFOs.

Discussion

To compare the flexibility of various AFOs, the slope of curves in Figures 6a , 6b , 6c , 6d , 6e , 6f , 6g , 6h , 6i , 6j , 6k is shown in Figure 7 . The slope was calculated by approximating each curve in dorsi- and plantarflexion and inversion/eversion by straight lines. In Figure 7 , large value implies a rigid property in that direction. In this graph, the abscissa indicates flexibility in inversion/eversion and the ordinate flexibility in the dorsi- and plantarflexion.

The flexibility of an AFO in dorsi and plantarflexion corresponds to the control of the ankle joint in sagittal plane during gait. The corrective force of an AFO in the dorsiflexed direction supplements weak dorsiflexors at the heel contact, when the maximum plantarflexion occurs. The correcting force of an AFO in the plantarflexed direction assists weak plantarfiexors in the 'middle and last parts of the stance phase when maximum dorsiflexion occurs.

Therefore, the flexibility of an AFO in dorsi- and plantarflexion must match the degree of paralysis of dorsiflexors and of plantarfiexors of each patient. Shoehorn-type AFOs, the posterior-spring type, have the most rigid properties among all AFOs. Anterior-spring type AFOs, Yunoko AFO and KU half AFOs have a more rigid property in dorsiflexion than in plantarflexion. The spiral AFOs are very flexible, especially in dorsiflexion.

Flexibility in inversion/eversion is related to the supportability of the subtalar joint. Rigidity in this direction ensures high stability in the frontal plane during the midstance of gait. The posterior spring-type AFOs (shoehorn-type AFOs) and the side stay-type AFOs (Saga plastic AFOs) show high rigidity, and restrain the movement of the ankle joint. The anterior spring-type AFOs (Yunoko AFO and KU half AFOs) have lower supportability in this direction than the posterior spring-type AFOs and the spiral-type AFOs because they have open trimline in the ankle joint area.

Conclusion

Flexibilities of AFOs were measured in dorsi- and plantarflexion and inversion/eversion using a muscle-training machine. Eleven AFOs of posterior spring, anterior spring, side stay and spiral were measured, clarifying the flexibility characteristics of different types of AFOs.

Acknowledgements

The authors are grateful to Tokuda Prosthetic Manufactory and Saga Arizono Manufactory for manufacturing the AFOs and to Prof. H. Watanabe of Saga Medical University for his valuable advice.


Sumiko Yamamotois a researcher at Tokyo Metropolitan Prosthetic and Orthotic Research Institute.

Masahiko Ebina, CPO a researcher at Tokyo Metropolitan Prosthetic and Orthotic Research Institute.

Mitsuo Iwasaki, CPO a researcher at Tokyo Metropolitan Prosthetic and Orthotic Research Institute.

Shigeru Kubo a researcher at Tokyo Metropolitan Prosthetic and Orthotic Research Institute.

Hideo Kawai, RPT a researcher at Tokyo Metropolitan Prosthetic and Orthotic Research Institute.

Takeo Kayashi, CPO a researcher at Tokyo Metropolitan Prosthetic and Orthotic Research Institute.

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