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

Abstract

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.

Introduction

Stroke is a major cause of disability, and ankle-foot orthoses (AFOs) are routinely prescribed to assist and improve gait [1], [2]. Provision of an AFO with optimal stiffness is one of the key elements for a successful prescription; however, its recommendation is currently made based on subjective assessment and personal experience. Therefore, the availability of a device that could support the prescription of an AFO by quantifying its appropriate stiffness could be potentially useful.

The stiffness of an AFO must match the condition of spasticity and weakness in each patient [3]. Therefore, a number of attempts to measure its stiffness have been made [4], [5]. It has been suggested that the plantarflexion resistive stiffness of an AFO to enable smooth plantarflexion at initial contact is the most important function [6]. On the other hand, the dorsiflexion resistive stiffness of an AFO may support insufficient push-off [7], whilst also having the potentially unwanted effect of restricting ankle dorsiflexion [8]. Therefore, the effect of AFO stiffness on gait requires further investigation, and this could potentially be achieved using an AFO whose stiffness could be systematically adjustable.

An AFO designed with an oil-damper joint was developed [8] and has proven to be effective in assisting the heel rocker function during loading response [9]. Compared to the other AFOs designed to control sagittal plane stiffness [10], [11], the use of an oil-damper joint appeared more realistic and feasible.

The aim of this study was therefore to develop a more advanced experimental AFO (EAFO), whose stiffness could be adjustable both in dorsi- and plantarflexion directions, and to demonstrate its effect on ankle joint kinematics in patients with stroke.