Academic journal article Journal of Rehabilitation Research & Development

Stance Control Knee Mechanism for Lower-Limb Support in Hybrid Neuroprosthesis

Academic journal article Journal of Rehabilitation Research & Development

Stance Control Knee Mechanism for Lower-Limb Support in Hybrid Neuroprosthesis

Article excerpt

INTRODUCTION

A considerable effort has been made to dev elop and integrate a controllable knee mechanism in lower-limb bracing that supports the knee during the stance phase of gait and allows for free movement during swing. These stance control knee mechanis ms (SCKMs) have utilized a myriad of design approaches, including bail lock ing [1], pin locking [2-4], a ratchet/pawl [5-6], cam locking [7], belt clamping [8], hydraulics [9], a magnetic particle brake [10], a wrap-spring clutch [11], a dog clutch via circular ratchet plates [12], a roller clutch [3], lever locking, and spring stiffness switching [13]. Many of these mechanisms are difficult to unlock under load [3 ,8,14], which may be necessary during the transition from the stance phase to the swing phase of gait. Users with weak or fatigued knee extensors may have difficulty extending the knee to unload the join t to unlock the mechanism.

This is also a problem when bracing is combined with functional neuromuscular stimulation (FNS) in a hybrid neuroprosthesis (HNP) [15] for gait assistance in individuals with par aplegia [2]. The commercially available SCKMs do not provide sufficient control for this user population because they either require consistent full extension or a pre set orientation of the thigh to operate. Others only lock at discrete angles [14], which may result in a minimal degree of unsupported knee flexion after locking and thus complicate the contralateral swing leg clearance.

The objective of this study was to develop a new SCKM to provide improved reliability and functionality over existing mechanisms specifically for individuals with spinal cord injury (S CI) whose gait is powered by electrical stimulation of their paralyzed muscles. This article describes the development of the knee mechanism--from the design concept, through bench testing, to safety testing with nondisabled individuals--and the effectiveness of stance knee control during walking in paraplegia [16].

DESIGN

Objective

The objective of the SCKM is to fully support the knee during standing and the stance phases of gait while allowing unhindered knee movement during stepping. The primary goal is to eliminate the need for muscle activity during static load supporting tasks or to reduce the duty cycle of electrical stimulation to the knee extensor muscles in an HNP sys tem. This should delay the onset of fatigue by prolonging the rest period s between successive contractions [10] and, therefore, extend operating times and walking distances. Consequently, the SCKM was specified to have high mechanical impedance during stance to prevent falling from knee collapse or buckling resulting in insufficient foot-to-floor clearance of the contralateral leg during swing. On the other hand, low mechanical impedance is critical during swing to minimize the torque generated by electrical stimulation of the paralyzed muscle s necessary to drive the mechanism and thereby reduce muscle fatigue. Finally, the SCKM must be capable of transitioning between states of high and low impedance responsively and consistently according to the dynamic requirements of gait.

Conceptual Mechanism Design

The design of the SCKM consists of a miniature hydraulic system attached across the knee joint to the thigh and leg up rights of the knee-ankle-foot orthosis (KAFO) via revolute joints in a four-bar linkage arrangement for linear-to-rotary transmission (Figure 1). A twoway, two-position, normally closed solenoid valve inline between the ports of a single rod, double-acting hydraulic cylinder locks the knee mechanism without consuming power. The mechanism was designed to be locked only against knee flexion. Knee extension, c orresponding to cylinder extension, can cause the pressure at port A relative to port B to exceed the valve cracking pre ssure, which forces the valve to open and thereby ineffective to lock against extension.

[FIGURE 1 OMITTED]

A single-acting, spring-loaded cylinder was employed as an accumulator to take up the fluid volume of the piston rod when flow is directed from the blind to the rod side of the cylinder during knee flexion. …

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