Timothy A. Perkins

FES cycling may promote recovery of leg function after incomplete spinal cord injury.

Study design: Single subject pilot.Objectives: (i) To see whether strength and endurance for recreational cycling by functional electrical stimulation (FES) are possible following spinal cord injury (SCI). (ii) To develop the equipment for FES-cycling.Setting: England.Methods: Near-isometric or cycling exercise was performed by the incomplete SCI subject at home.Results: After training for an average of 21 min per day for 16 months, the stimulated muscles increased in size and the subject was able to cycle for 12 km on the level. Surprisingly, there was a substantial increase in the measured voluntary strength of the knee extensors and the subject reports improved leg function.Conclusion: FES-cycling may promote recovery after incomplete spinal cord injury. If so, it offers the possibility of being a convenient method for widespread use.Sponsorship: Spinal Injuries Unit, Royal National Orthopaedic Hospital, Stanmore.

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 Stimulator ASIC Featuring Versatile Management for Vestibular Prostheses

This paper presents a multichannel stimulator ASIC for an implantable vestibular prosthesis. The system features versatile stimulation management which allows fine setting of the parameters for biphasic stimulation pulses. To address the problem of charge imbalance due to rounding errors, the digital processor can calculate and provide accurate charge correction. A technique to reduce the data rate to the stimulator is described. The stimulator ASIC was implemented in 0.6-μ m high-voltage CMOS technology occupying an area of 2.27 mm2. The measured performance of the ASIC has been verified using vestibular electrodes in saline.

Recruitment by motor nerve root stimulators: significance for implant design

Three paraplegics have been implanted with stimulators of the lumbar anterior roots. Twelve roots were trapped in slots, each with three electrodes, a central cathode and two anodes, but the anodes in all the slots were connected together to reduce the number of wires. Cross-talk between roots was observed at lower levels than expected. Cross-talk was assessed from the ratio of the roots threshold to the threshold of the contralateral response (expected ratio: 72). Two hypothetical reasons for this low ratio were: that the cathode current was not equally shared by the anodes; or that the contralateral responses were reflex. Experiments showed that neither explanation was valid. The ratio of the contralateral to ipsilateral threshold for individual slots (K(1)) was sometimes low because the ipsilateral threshold was high. By taking the ratio of the lowest contralateral response to lowest ipsilateral response, for all roots in each subject (K(2)), the ratio should approach the theoretical value. However, for the two subjects with small slots, it was 7.9 and 15.3, much less than 72, suggesting that the original theory was incorrect. Approximate calculations of the activation function suggest that the reason may be that roots which run close to a slot, but not through it, may pass through a virtual anode region outside the ends of the slots, and that anodal break stimulation in those regions causes the cross-talk. Our estimate is that this cross-talk would be expected to occur at intensities above 5.3 times the cathodal threshold. If the roots are stimulated in pairs, below the levels of cross-talk, experimental results show that the moments obtained in response are additive to within 5%.

Implant provision of key, pinch and power grips in a C6 tetraplegic

An 11-channel multiplexed stimulator of nerves and muscles in the left forearm was implanted for hand control in January 1986 in a 21 year old woman who, after sustaining a C6 spinal lesion 7 years earlier, had voluntary shoulder and elbow movement but paralysed hands, trunk and legs. The patient controls the stimulation via a microcomputer control box and an RF transdermal link. We have investigated the control of her stimulated hand with a joystick under her contralateral hand which she moves from the shoulder and elbow. Since 1986, we have tried a variety of joystick control schemes involving power and key grips. Currently, for grip adjustment, forward and backward joystick movements correspond to thumb extension and abduction, respectively giving in addition both finger and wrist extension, whereas right and left joystick movements yield first closure and thumb opposition/adduction and flexion, respectively. Useful grasps are available by moving the joystick forward and then left (key grip), by moving the joystick backward and left (pinch grip), or by moving the joystick back and right (power grip). Thus, three distinct grips may be selected using these three quadrants of joystick movement. An additional control mode was found to be desirable to augment the patients limited voluntary wrist positioning and provide wrist stability while adjusting finger grip.

A fast passive phase shift keying modulator for inductively coupled implanted medical devices

This paper presents an integrated modulator for inductively coupled biomedical telemetry. The circuit implements passive phase shift keying (PPSK) modulation and is designed to work at 13.56 MHz with a single pair of coils for both data transmission and power delivery. The data link can reach data rates up to 1/16 of the carrier frequency, i.e., 847.5 kbps in this case. To our knowledge, it is the fastest data rate achieved by a single wireless link used simultaneously for power delivery and communication in implanted medical devices. The circuit was fabricated in a 0.6-μm CMOS technology, occupies a silicon area of 0.7 mm2 and dissipates about 2 mW from a 5 V power supply at full speed of operation.

Apparatus to measure simultaneously 14 isometric leg joint moments. Part 2 : Multi-moment chair system

An apparatus has been developed that measures isometrically the 14 lower limb joint moments corresponding to the degrees of freedom of the hips, knees and ankles. This is the second of two papers describing the development of the multi-moment chair system (MMCS). It presents the overall design and changes that were implemented to compensate for problems. These were primarily to improve the accuracy of hip joint moments; a compromise between accuracy and practicalities, because of force-moment responses being measured at the ankles. All joint moment errors have been calculated to be of the order of a few newton metres. Since these represent errors of less than 10% when cosindering peak joint moment responses, this is considered sufficiently accurate for the proposed application. The MMCS is being used in a programme to investigate the restoration of lower limb functions, principally standing, in paraplegics by electrical stimulation of the lumbosacral anterior roots.

Stimulation management for a multichannel vestibular neural prosthesis

This paper describes the stimulation management unit for a multichannel vestibular neural prosthesis. The unit is designed as part of a stimulator ASIC in the implantable subsystem of the prosthesis. This digital unit provides the stimulator ASIC the ability to generate biphasic current pulses at specified amplitude, duration and pulse rate to drive electrodes in the semicircular canals. The circuit was implemented in 0.6-μm CMOS technology and post-layout simulations are presented to show its operation.

Selecting candidates for a lower limb stimulator implant programme: A patient-centred method

Objective: To develop an effective selection procedure for lower limb functional neurostimulation (LLFNS) for standing in paraplegia.Design: The selection procedure and exclusion criteria were based on the previous experience of two clinical centres with experience of LLFNS.Setting: Two Regional Spinal Injuries units in southern England.Subjects: 254 fully rehabilitated paraplegics living in the community.Intervention: Patients were invited to participate in the programme, and if suitable to subject themselves to a rigorous staged selection procedure from which they could withdraw at any time.Outcome measure: Functionally successful home standing using closed-loop surface electrical stimulation.Results: 57/254 patients were suitable on paper and were accessible. 19 of these (CI=10–28) were interested in the project and attended one of the spinal centres for details. Twelve (CI=5–19) of these fulfilled the selection criteria and started on the training programme; and 10 of them completed the muscle training programme successfully. Seven patients (CI=2–12) achieved closed-loop standing in the laboratory and four patients (CI=1–8) did so at home.

An implantable 3-D vestibular stimulator with neural recording

This paper presents an implantable vestibular stimulator for restoring three-dimensional (3-D) balance sensation. Advanced stimulation management is used to control independent parallel stimulation on the three semicircular canals. A switched-capacitor technique is implemented to allow stimulation on the three canals with minimized crosstalk. Realtime neural recording function is also included in the design. The stimulator was fabricated in a 0.6-μm HV CMOS technology. Measured results are presented to demonstrate the operation of the circuit.