10 th Annual Conference of the International FES Society July 2005 – Montreal, Canada Initial results with fully implanted Neurostep TM FES system for foot drop Hoffer JA 1,2 , Baru M 2,3 , Bedard S 2,3 , Calderon E 2 , Desmoulin G 1,2 , Dhawan P 4 , Jenne G 2 , Kerr J 1,2 , Whittaker M 4 , Zwimpfer TJ 5 1 School of Kinesiology, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6 2 Neurostream Technologies, Port Coquitlam, British Columbia, Canada V3B 1A8 3 Victhom Human Bionics, Saint-Augustin-de-Desmaures, Quebec, Canada G3A 2J9 4 G.F. Strong Rehabilitation Centre, Vancouver Hospital, Vancouver, British Columbia, Canada V5Z 2G9 5 Dept. of Surgery, Faculty of Medicine, Univ. of British Columbia, Vancouver, BC, Canada V5Z 4E3 hoffer@sfu.ca Abstract original, telemetry-based, Neuromuscular Assist prototype [1,2]: they include an external power In a pilot feasibility study, a prototype closed- source and RF transmitter, implanted receiver loop FES walking assist system (Neurostep TM ) connected to electrodes placed on or near the was fully implanted in the thigh of a 70 yr old peroneal nerve, and an external sensor (usually male hemiplegic subject with hypotonia and a heel switch) used to initiate stimulation during foot drop who, 3 yr after the stroke, required swing. Telemetry-based systems have several ankle and knee braces and contact guard known limitations. Notably, Dr. Waters himself assistance and could only walk 5-10 m without predicted that broad acceptance of a foot drop fatiguing. Two multi-chambered Neurocuffs TM device would require a fully implanted system , were implanted on the common peroneal (CP) as was shown earlier with heart pacemakers [2]. and tibial (TIB) nerves and connected to a The concept of implanting nerve cuff electrodes control unit that included custom low-power, to obtain electroneurographic (ENG) signals low- noise ASIC amplifiers and a pacemaker battery. The Neurostep TM was programmed via generated by natural sensory receptors in the body and use ENG as feedback to control FES, telemetry to automatically turn On when the was originally proposed by Hoffer [3], tested in subject stood up, detect heel contact (HC) and pilot human subjects in Aalborg and shown to toe lift (TL) events from the sensed nerve provide gait-related signals robust enough to signals and stimulate selected nerve channels control FES for treating foot drop [4]. The long- to control the foot trajectory during walking. standing challenge was to design a closed-loop An “Exercise Mode” provided trains of stimuli FES system in a fully implantable package that that strengthened the disused dorsiflexor has no external parts and is always ready to go. muscles and supported walking. The subject was tested and trained with Neurostep TM in the With this in mind we developed Neurostep TM , laboratory and received gait re-education for the first totally implanted, closed-loop FES 10 wk before he started to use the device at device for foot drop. Key innovations that home. After 6 months, he was able to routinely enabled this prototype Neurostep TM system walk 250 m without fatigue. Although the implementation include multi-chamber record/ long-term benefit of using this prototype was stimulate Neurocuffs TM [5], custom low-power, limited by a connector malfunction, this pilot low-noise, low-leakage, fully integrated ASIC study shows the feasibility of treating foot drop bandpass amplifiers and rectifiers [6], and a with a totally implanted, neuroelectrically computationally simple, low-power gait event controlled FES system. The Neurostep TM detection algorithm [patents pending]. technology is now being refined and expanded We summarize here our findings from an eight- into an implantable assistive device platform month pilot feasibility study. The Neurostep TM for disabilities such as foot drop, incontinence, subject was a 70 y.o. male 3 yr post-stroke with limb amputation, paraplegia and chronic pain. severe foot drop and flexor hypotonia that were 1. INTRODUCTION managed with a knee brace in addition to an ankle-foot orthosis (AFO). He could only walk Waters et al. showed 30 yr ago the feasibility of 5-10 m without fatiguing and required contact correcting foot drop with a partially implanted guard assistance. This study, approved by the peroneal nerve stimulator [1]. However, there is Health Canada Medical Device Board and five still no commercial, implantable FES system institutional human research ethics committees, for foot drop. A reason may be that all device was carried out in Vancouver in 2003-2004. designs to date basically replicate Medtronic’s
10 th Annual Conference of the International FES Society July 2005 – Montreal, Canada the TIB Neurocuff TM (“HC”) were measured by 2. METHODS an internal Neurostep TM circuit and telemetered Figure 1 shows the Neurostep TM prototype and out of the body. Impedances initially fluctuated its dimensions. Circuitry, battery and telemetry and stabilized about 3 weeks after Neurocuff TM antenna are sealed in a pacemaker-like welded implant (Fig. 3). The lower impedance in Ch. 2 titanium case. Two 30 mm Neurocuffs TM attach is attributed to shorts in the header connectors. to the device header via 25 cm long flexible Stimulation of each CP channel with brief trains leads and 3-pole modified IS-1 connectors. of 100 µ s x 150-250 µ A pulses elicited tingling sensations that the subject consistently referred to specific locations of the left leg. Perceptual thresholds (Fig. 4) slowly doubled in Ch. 3 and 4 over the first nine weeks. The rise in Ch. 2 threshold was attributed to a shorted connector. Sensory Thresholds (uA*100usec) Stimulation Intensity (uA) 1000 Figure 1 Figure 2 800 Ch.1 600 Ch.2 In a first surgery, through a 6 cm posterior thigh 400 Ch.3 Ch.4 200 incision (Fig. 2; arrow ) the surgeon isolated the 0 common peroneal (CP) and tibial (TIB) nerves, 0 10 20 30 40 50 60 70 Days after 1st Implant (Day #) measured nerve perimeters with a flexible ruler, Figure 4. Sensory stimulation thresholds vs. time. implanted suitably sized, multi-chambered TIB and CP Neurocuffs TM , and sealed each cuff with As expected, motor thresholds were higher than a suture through interdigitated closing elements sensory thresholds. Motor thresholds also rose [7]. The cuff lead connectors were inserted in a gradually for Ch. 3 & 4 and suddenly for Ch. 2 dummy device placed in a subcutaneous pocket (Fig. 5). Stimulation of Ch. 3 and 4 produced in the medial thigh and the incision was closed. strong dorsiflexion, indicating greater proximity The dummy device had a percutaneous cable to to deep peroneal nerve axons. Stimulation of allow external recordings of signals during the Ch. 1 sometimes elicited a limb flexion reflex. first 10 d. After the signals were evaluated, the Motor Thresholds (ua*100usec) incision was re-opened and the dummy device Stimulation Intensity (uA) and its percutaneous wires were removed and 1000 800 replaced with an implanted Neurostep TM unit. Ch.1 600 Ch.2 The Neurostep TM was turned On or Off with a 400 Ch.3 Ch.4 magnet or telemetry programming interface and 200 0 programmed to automatically wake up when the 0 10 20 30 40 50 60 70 subject stood up, detect heel contact (HC) and Days after 1st Implant (Day#) toe lift (TL) events from the nerve signals Figure 5. Motor stimulation thresholds vs. time. sensed while walking and stimulate selected CP channels to control foot trajectory during swing. Stimulation Strength vs. Days after Implant 80 Im p e d a n c e s S u b je c t 1 2 -7 4 -2 ) 70 (N e 60 c 8 r o 50 F 7 Average of initial four n 40 io Stim Peaks (N) 6 C H1 x 30 ifle R (kOhm) 5 C H2 20 s r 4 o C H3 D 10 3 C H4 0 HC 2 0 20 40 60 1 Time (Days after implant) 0 0 5 10 17 21 26 31 35 40 47 54 59 Figure 6. Improvement in dorsiflexion force elicited D ay by similar Ch. 3 and Ch. 4 stimulation, vs. time. Figure 3. Resistive part of electrode impedances During the second month of Neurostep TM plotted vs. days after Neurocuff TM implant. mediated stimulation the ankle dorsiflexor force 3. RESULTS elicited by CP stimulation of standard intensity greatly increased (Fig. 6), indicating a reversal Impedances between the recording/stimulating of the disuse atrophy in the paralyzed muscles. electrode in each CP Neurocuff TM channel with The fatigue resistance in the ankle dorsiflexors respect to its indifferent electrode, and between also improved (Fig. 7). the recording and the indifferent electrodes in
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