Morten Kristian Haugland

Cutaneous whole nerve recordings used for correction of footdrop in hemiplegic man

One hemiplegic patient with a dropfoot was chronically implanted with a cuff on the sural nerve, and recordings were made regularly during a period of two years. The results showed that the human sural nerve responded in a similar way to mechanical inputs applied on the skin, as the tibial nerve did in cats (M.K. Haugland, et al., ibid., vol. 2, p. 18-28, 1994), indicating that previous experimental systems using natural sensory feedback for closed-loop FNS are possible to adapt to humans. During walking, the recorded nerve signal modulated strongly and gave a clearly detectable response at foot contact and a silent period when the foot was in the air through the swing phase of the walking cycle. A portable system was built that used the recorded signal to control a peroneal stimulator to correct for footdrop. There were two reasons for this: first, to show in a relatively simple system the possibility of solving the practical problems involved in recording nerve activity during stimulation and second, to remove the external heel switch used in existing systems for footdrop correction, thereby making it possible to use such systems without footwear and preparing it to be a totally implantable system. The method for removing artifacts, as developed in animal experiments, was adapted to provide usable nerve signals while stimulating the ankle dorsiflexor muscles.

Skin contact force information in sensory nerve signals recorded by implanted cuff electrodes

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 the authors tested the hypothesis that the force applied on glabrous skin can be extracted from the electroneurographic (ENG) signal recorded from the sensory nerve. They 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. Their results showed that the tibial ENG signal, suitably filtered, rectified, and smoothed carries detailed static and dynamic information related to the force applied on the footpad. The authors derived a mathematical model of the force-ENG relation that provided accurate estimates of the ENG signal for a wide range of force profiles, 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. The authors 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. >

Control of FES thumb force using slip information obtained from the cutaneous electroneurogram in quadriplegic man

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.

Biopotentials as command and feedback signals in functional electrical stimulation systems.

Today Functional Electrical Stimulation (FES) is available as a clinical tool in muscle activation used for picking up objects, for standing and walking, for controlling bladder emptying, and for breathing. Despite substantial progress in development and new knowledge, many challenges remain to be resolved to provide a more efficient functionality of FES systems. The most important task of these challenges is to improve control of the activated muscles through open loop or feedback systems. Command and feedback signals can be extracted from biopotentials recorded from muscles (Electromyogram, EMG), nerves (Electroneurogram, ENG), and the brain (Electroencephalogram (EEG) or individual cells). This paper reviews work in which EMG, ENG, and EEG signals in humans have been used as command and feedback signals in systems using electrical stimulation of motor nerves to restore movements after an injury to the Central Nervous System (CNS). It is concluded that the technology is ready to push for more substantial clinical FES investigations in applying muscle and nerve signals. Brain-computer interface systems hold great prospects, but require further development of faster and clinically more acceptable technologies.

PHASE II TRIAL TO EVALUATE THE ACTIGAIT IMPLANTED DROP-FOOT STIMULATOR IN ESTABLISHED HEMIPLEGIA

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.

Degeneration and regeneration in rabbit peripheral nerve with long-term nerve cuff electrode implant: a stereological study of myelinated and unmyelinated axons

Abstract Selective and dynamically co-ordinated functional electrical stimulation (FES) of paralysed/paretic limbs in upper motor neuron lesioned people depends on optimal contact at the neural interface. Implanted nerve cuff electrodes may form a stable electrical neural interface, but may also inflict nerve damage. In this study the immediate and long-term effects of cuff implantation on the number and sizes of myelinated and unmyelinated axons have been evaluated with unbiased stereological techniques. Cuff electrodes were implanted in rabbit tibial nerves just below the knee joint, and the stereological analyses were carried out 2 weeks and 16 months after implantation. Myelinated axons were analysed at standardised levels proximal to, underneath, and distal to the cuff; unmyelinated axons underneath the cuff. A 27% loss of myelinated axons was found underneath and distal to the nerve cuff 2 weeks post surgery. All axonal sizes were equally lost except for the very smallest. At 16 months post surgery the number of myelinated axons was restored to control values at both levels. Except for the presence of regenerative sprouts at 2 weeks post surgery, no changes in the number or sizes of unmyelinated axons were revealed at either 2 weeks or 16 months post surgery. Our study demonstrates that implanted cuff electrodes may cause an initial loss of myelinated axons but with subsequent regeneration.

A flexible method for fabrication of nerve cuff electrodes

A method for construction of cuff electrodes is presented. The method is based on using platinum foil electrodes fixed by rubber bands on a Teflon coated mandrel, that is then dip-coated with silicone. The method allows for design of cuff electrodes of practically any size, shape and electrode configuration, and simple cuffs can be built in less than one hour of work Cuff electrodes have been built that range in size from 0.5 to 10 mm ID and 5 to 70 mm length and number of electrodes from one to twelve. Tri-polar recording electrodes have been tested in more than 30 chronic implantations in rabbits (tibial nerve) and for various acute experiments, where different electrode configurations were investigated. None of the chronic implants have failed and impedances have been stable for one year.

Natural neural sensing and artificial muscle control in man

In the intact organism, specialised sensors in the skin, muscles and joints provide sensory feedback information that is normally used by the central nervous system to regulate and update the motor output. Many sensory functions remain intact after spinal cord injury or stroke. Here we demonstrate that natural sensory nerve activity in man can be chronically recorded, and that the recorded neural activity can provide sensory feedback signals for an event-driven artificial reflex control of paralysed muscles.

Implantable telemeter for long-term electroneurographic recordings in animals and humans

A system is described that amplifies an electroneurographic signal (ENG) from a tripolar electrode nerve cuff and transmits it from the implanted amplifier to an external drive box. The output was raw ENG, bandpass filtered from 800 to 8000 Hz. The implant was powered by radio-frequency induction and operated for coil-to-coil separations up to 30 mm. The testing and performance of the system is described. The input-referred noise was never more than 1μV RMS, and, at some positions of the radio-frequency field, was 0.7μV, close to the expected value for the amplifier used. The common-mode rejection ratio (CMRR) depended on the impedance imbalance from the cuff and the length of input cable. Devices with a short cable and low source impedance had CMRR of 84 dB, but, with 31 cm of cable and a real cuff, the CMRR fell to 66 dB. Recovery from a stimulus artifact took 5ms. The responses of the cuff to external potential gradients and to common-mode signals are described theoretically or by simulation. The devices are available for use in neuroprosthetic or neurophysiological research.

A Chip for an Implantable Neural Stimulator

This paper describes a chip for a multichannel neural stimulator for functional electrical stimulation (FES). The purpose of FES is to restore muscular control in disabled patients. The chip performs all the signal processing required in an implanted neural stimulator. The power and digital data transmission to the stimulator passes through a 5 MHz inductive link. From the signals transmitted to the stimulator, the chip is able to generate charge-balanced current pulses with a controllable length up to 256 μs and an amplitude up to 2 mA, for stimulation of nerve fibers. The quiescent current consumption of the chip is approx. 650 μA at supply voltages of 6–12 V, and its size is 3.9×3.5 mm2. It has 4 output channels for use in a multipolar cuff electrode.