Objective: To evaluate a selective implantable drop foot stimulator (ActiGait) in terms of effect on walking and safety. Design: A phase II trial in which a consecutive sample of participants acted as their own controls. Subjects: People who had suffered a stroke at least 6 months prior to recruitment and had a drop-foot that affected walking were recruited from 3 rehabilitation centres in Denmark. Methods: Stimulators were implanted into all participants. Outcome measures were range of ankle dorsiflexion with stimulation and maximum walking speed and distance walked in 4 minutes. Measurements were applied before implantation, at 90 days and at a long-term follow-up assessment. Changes over time and with and without stimulation are reported. Safety was evaluated by nerve conduction velocity and adverse events. Results: Fifteen participants were implanted and 13 completed the trial. Long-term improvements were detected in walking speed and distance walked in 4 minutes when stimulated, and the orthotic effect of stimulation showed statistically significant improvement. The device did not compromise nerve conduction velocity and no serious device-related adverse events were reported. Technical problems were resolved by the long-term follow-up assessment at which further improvement in walking was observed. Conclusion: This trial has evaluated the safety and performance of the device, which was well accepted by patients and did not compromise safety.
When functional neuromuscular stimulation (FNS) is used to restore the use of paralyzed limbs after a spinal cord injury or stroke, it may be possible to control the stimulation using feedback information relayed by natural sensors in the skin. In this study we tested the hypothesis that the force applied on glabrous skin can be extracted from the electroneurographic (ENG) signal recorded from the sensory nerve. We used the central footpad of the cat hindlimb as a model of the human fingertip and recorded sensory activity with a cuff electrode chronically implanted around the tibial nerve. Our results showed that the tibial ENG signal, suitably liltered, recmed, and smoothed carries detailed static and dynamic information related to the force applied on the footpad. We derived a mathematical model of the force-ENG relation that provided accurate estimates of the ENG signal for a wide range of force proliles, amplitudes, and frequencies. Once fitted to data obtained in one recording session, the model could be made to fit data obtained in other sessions from the same cat, as well as from other cats, by simply adjusting its overall gain and offset. However, the model was noninvertible; i.e., the force could not be similarly predicted from the ENG signal, unless additional assumptions or restrictions were introduced. We discuss the reasons for these findings and their implications on the potential use of nerve signals as a source of continuous force feedback information suitable for closed-loop control of FNS.
A tetraplegic volunteer was implanted with percutaneous intramuscular electrodes in hand and forearm muscles. Furthermore, a sensory nerve cuff electrode was implanted on the volar digital nerve to the radial side of the index finger branching off the median nerve. In laboratory experiments a stimulation system was used to produce a lateral grasp (key grip) while the neural activity was recorded with the cuff electrode. The nerve signal contained information that could be used to detect the occurrence of slips and further to increase stimulation intensity to the thumb flexor/adductor muscles to stop the slip. Thereby the system provided a grasp that could catch an object if it started to slip due to, e.g., decreasing muscle force or changes in load forces tangential to the surface of the object. This method enabled an automatic adjustment of the stimulation intensity to the lowest possible level without loosing the grip and without any prior knowledge about the strength of the muscles and the weight and surface texture of the object. Index Terms-Functional electrical stimulation (FES), hand grasp, natural sensory feedback, nerve cuff electrode, neural prostheses I. INTRODUCTION M OTOR function of a paralyzed hand can be partially restored by means of functional electrical stimulation (FES) [1]-[3]. The present systems are feedforward-controlled, and the user relies on his/her experience and on visual feedback. When picking up an object, the user has to estimate the appropriate stimulation intensity, which produces enough force to hold the object. This is difficult and causes many patients to use excessive force [4]. Further, the system is not able to adapt to slow changes in force output of the stimulated muscles caused by, e.g., fatigue, electrode drift, and length-tension properties of the muscles at different hand orientations. To deal with these problems, it has been proposed to use force sensors, position sensors, or combinations thereof to provide closed-loop control of the grasp. This provides more linear control of grasp force and grasp opening [5]. Several investigators have looked into the possibilities of using artificial sensors for measurement of finger position and grasp force
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