Abstract:
A ministimulator is positioned adjacent to the deep peroneal nerve and electrically actuated to elicit balanced dorsiflexion, without eversion, of the ankle of a patient having foot drop.

Description:
REFERENCE TO RELATED APPLICATION  
       [0001]     The present application is related to U.S. Provisional Patent Application Ser. No. 60/540,234, entitled “A Method to Produce a Balanced Dorsiflexion During the Gait of Patients with Foot Drop”, filed Jan. 28, 2004, which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates to implanting a miniature electrical stimulator (“microstimulator”) in the leg of a patient and applying stimulation for the purpose of alleviating foot drop.  
       BACKGROUND OF THE INVENTION  
       [0003]     Foot drop is a common consequence of diseases affecting peripheral nerves or areas of the central nervous system that normally produce dorsiflexion of the ankle during the swing phase of walking. If the appropriate nerves are not activated, the foot drops and may drag on the ground instead of swinging smoothly through the air. If the cause of the problem is central in origin the peripheral nerves are still available for stimulation.  
         [0004]     Liberson et al 1  in 1961 first proposed that the stimulation of the common peroneal (“CP”) nerve could be timed appropriately using a heel switch to turn on when the heel leaves the ground and turn off when the heel again hits the ground. More recently, Stein, in U.S. Pat. No. 5,814,093, incorporated herein by reference, taught that a tilt sensor built into a foot drop stimulator could improve CP nerve stimulation using external electrodes by relating leg position during gait with the initiation and termination of stimulation. The patent taught an electronic circuit for so controlling stimulation in response to tilt sensor readings.  
         [0005]     However, externally applied stimulation of the CP nerve innervates muscles that flex the ankle (e.g. the tibialis anterior, “TA,” muscle and the extensor digitorum longus, “EDL”, muscle) and others (e.g. peroneus longus, “PL”, muscle) that evert the ankle (i.e. rotate it outward). Furthermore, the PL muscle is innervated by the superficial branch of the CP nerve and so is more easily stimulated from skin surface. Obtaining a balanced dorsiflexion (that is, without significant eversion) of the ankle with surface electrodes is therefore difficult.  
         [0006]     There have been attempts described in the literature for resolving this problem by surgically implanting electrodes near the CP nerve or near the motor points of more than one muscle (O&#39;Halloron et al 2 , 2003; Rozman et al 3 , 1990; Waters et al 4 , 1973). The former approach does not solve the problem since stimulating the whole nerve via surface electrodes or subcutaneously will still produce eversion as well as flexion. The latter approach is problematic since the motor points of these muscles are often quite distributed in space and several muscles and motor points may need to be stimulated.  
         [0007]     In recent years, small injectable microstimulators were developed by Schulman et al and disclosed in U.S. Pat. Nos. 5,324,316 and 5,405,367 (both incorporated herein by reference). These microstimulators can be inserted into tissue using a hypodermic needle, without surgery.  
       SUMMARY OF THE INVENTION  
       [0008]     This invention is based on the discovery that if a microstimulator is implanted in a patient&#39;s leg at a location or implantation site adjacent to the deep peroneal (“DP”) nerve and remote from the common peroneal (“CP”) nerve and its superficial branch (known as the superficial peroneal nerve or “SP” nerve), and if the microstimulator is energized to stimulate the DP nerve during the swing phase of gait, then such stimulation will elicit balanced dorsiflexion of the patient&#39;s ankle, substantially without eversion.  
         [0009]     In order to take advantage of this discovery, it was necessary to develop: 
        a workable system for positioning and delivering the microstimulator to the aforementioned site, where it would preferably lie generally parallel and adjacent to and at substantially the same depth as the DP nerve, at a locus beneath the PL muscle and spaced forwardly of the anterior tibial artery; and     a hardware system for controlling and implementing stimulation of the DP nerve in relation to gait.        
 
         [0012]     This involved: mapping, in connection with the patient&#39;s leg, the path or course of the CP nerve, the branch point for the SP and DP nerves, and the course of the latter nerve; selecting an implantation site along the course of the DP nerve; determining the depth of the DP nerve at the site; and implanting the microstimulator at the site so as to lie adjacent to and preferably alongside the DP nerve.  
         [0013]     It further involved: monitoring the position of the patient leg during gait; and initiating and terminating electrical stimulation of the DP nerve alone during the swing phase of gait so as to elicit balanced dorsiflexion of the ankle.  
         [0014]     In one embodiment, the invention is concerned with a method for treating a patient with foot drop, comprising: implanting a microstimulator, adjacent to the DP nerve of the patient&#39;s leg, for delivering stimulus thereto; and electrically stimulating the DP nerve to elicit balanced dorsiflexion of the patient&#39;s ankle.  
         [0015]     In another embodiment, the invention is concerned with a method for treating a patient with foot drop, comprising: mapping, in connection with the patient&#39;s leg, the course of the CP nerve, the branch point for the SP nerve and the DP nerve from the CP nerve, and the course of the DP nerve; selecting an implantation site adjacent to the course of the DP nerve; determining the depth of the DP nerve at the site; implanting a microstimulator at the site adjacent the DP nerve; and electrically stimulating the DP nerve to elicit balanced dorsiflexion of the patient&#39;s ankle. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a side view of part of the anatomy of a human leg, showing the proposed insertion path and implantation site for a ministimulator, in accordance with the invention;  
         [0017]      FIG. 2  is a photograph showing a patient leg in the process of mapping the courses of the CP nerve, the SP nerve and the DP nerve;  
         [0018]      FIG. 3  is a photograph showing the leg in the process of determining the depth of the implantation site;  
         [0019]      FIG. 4  is a schematic block diagram of the system used to monitor leg gait and control nerve stimulation in response thereto; and  
         [0020]      FIG. 5  is a more detailed schematic block diagram of the system shown in  FIG. 4 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]     This description teaches: 
        a technique for accurately positioning and implanting a microstimulator  1  at a desired site  2  in a patient&#39;s leg  3 ; and     a system  4  for energizing and controlling the timing and duration of stimulation.        
 
         [0024]     More particularly, a suitable implantable microstimulator  1  is the Schulman et al device previously mentioned and available from the Alfred E. Mann Foundation for Scientific Research, Sylmar, Calif. and the Alfred E. Mann Institute at the University of Southern California, Los Angeles, Calif. This microstimulator  1  and its associated equipment is identified by the trade-mark BION. The microstimulator  1  is energized and controlled using radio frequency signals from a custom circuit, forming part of the supplied BION equipment. This microstimulator  1  can be implanted through a hypodermic needle, such as an Angiocath™ needle. This device comprises of a plastic sheath surrounding a hypodermic needle. Once the needle is withdrawn, when its tip has reached the desired site a Bion microstimulator can be pushed down the plastic sheath by using a plunger, to sit at the location formerly occupied by the needle tip.  
         [0025]     The microstimulator  1  is to be positioned in the patient&#39;s leg, substantially parallel and adjacent to the DP nerve  5  at an implantation site  2  immediately beneath the PL muscle  6  and spaced forwardly of the anterior tibial artery  7 . The selected site  2  should be sufficiently remote from the CP and SP nerves  8 , 9  and sufficiently close to the DP nerve  5  so that low intensity stimulation (e.g. 1-3 microamps) by the microstimulator  1  will activate the TA and EDL muscles  11 ,  12  while the PL muscle  6  remains quiescent.  
         [0026]     This can be achieved in the following manner.  
         [0027]     By way of overview, electromyographic (“EMG”) recordings from several muscles are used to map the courses of the CP, SP and DP nerves  8 ,  9 ,  5 . These recordings are developed in the following manner.  
         [0028]     Two sets  13 ,  14  of surface self-adhesive EMG recording electrodes  15  are placed on the skin of the patient&#39;s leg  3 . One set  13  is placed over the belly of the TA muscle  11 . This set  13  also records to some extent from the nearby EDL muscle  12 . The other set  14  is placed over the PL muscle  6 . The recording electrode sets  13 ,  14  are placed directly over the relevant motor point, which is usually located  4  finger breadths distal to the tibial tuberosity  16  in the case of the TA muscle  11  and  7  finger breadth below the fibular head  17  in the case of the PL muscle  6 .  
         [0029]     After the EMG electrode sets  13 ,  14  and associated conventional EMG equipment are so positioned and operatively connected, a bipolar hand-held stimulator is used to surface stimulate the CP nerve  8  and its branches—the DP and SP nerves  5 ,  9 . Further refinement of the precise location of the site  2  can be achieved by moving the recording electrode until a location where the amplitude of the maximum motor (“M”) wave produced by stimulating the nerves is greatest and the rising slope of the wave is sharpest. A reference electrode  18  is placed 5 cm distally to each of the relevant recording electrodes. This bipolar configuration helps to minimize noise in the recording and improves selectivity of the recorded target muscles. The goal is to record from muscles innervated by the DP and SP nerves as selectively as possible.  
         [0030]     The courses of the CP, SP and DP nerves  8 ,  9 ,  5  and the location of the branch point  19 , between the popliteal fossa  20  and the proximal calf  21 , are mapped out by moving the stimulating electrode and finding the locations at which the largest M-wave can be elicited using the lowest stimulus intensity. Activation of the nerve can only be achieved by low stimulus currents when the stimulating electrode is in close proximity to the nerve. Further confirmation of activating of the target nerves can also be obtained by observing the mechanical twitch of the innervated muscles. As long as the stimulus is over the CP nerve  8  large M-waves will be recorded from both sets  13 ,  14  of EMG electrodes. Once the branch point is passed, and the stimulating electrode is over the DP nerve, only a large TA muscle M-wave will be recorded. Conversely, if the stimulating electrode is over the SP nerve only a large PL muscle M-wave will be recorded.  
         [0031]     Once the courses of each of the nerves  8 ,  9 ,  5  and the branch point  19  have been mapped, the depth of the DP nerve  5  under the skin can be established using a fine monopolar needle electrode  23 .  
         [0032]     The needle electrode  23  is inserted close to the DP nerve  5 , about 2 cm beyond the branch point  19 . Once the needle electrode  23  has been inserted perpendicularly to the skin, the stimulus intensity is gradually increased until a clear, reproducible M-wave can be elicited. This intensity is a measure of the distance of the electrode  23  from the nerve and will decrease as the distance decreases.  
         [0033]     The needle electrode  23  is then carefully advanced perpendicularly into the leg tissue in small increments. At each new depth, stimulation is repeated and the stimulus intensity needed to produce an M-wave of the same amplitude is noted. Further advancement of the needle electrode  23  is halted when a point at which very low stimulus intensity requirement (in the region of 1 to 3 mA with a rectangular pulse width of 200 microseconds) is reached. A nerve can only be activated at such a low intensity if the electrode is very close to the nerve.  
         [0034]     This first needle electrode  23  is then left in place. The depth of the needle tip can be estimated by measuring the length of the remaining part of the needle electrode protruding above the skin.  
         [0035]     Thus, the target implantation site  2  to which to direct the microstimulator is known in three dimensions, two along the skin surface and the third in terms of the depth of the nerve below the skin.  
         [0036]     Insertion of the implantation tool, a hypodermic needle  25 , is now initiated. The hypodermic needle  25  is a modified 12 gauge Angiocath™ needle that allows electrical stimulation through the trocar tip. The hypodermic needle  25  is inserted along the path  24  shown in  FIG. 1 . This path  24  follows the CP nerve  8  past the branch point  19  and then along the DP nerve  5 . At each step, single stimulation pulses are applied  
         [0037]     As the hypodermic needle  25  is advanced along the insertion path  24 , it initially excites both TA and PL muscles  11 ,  6 , since it is following the path of the CP nerve  8 . However, one can feel a difference in resistance to insertion when the needle  25  reaches the tendinous origin of the PL muscle  6 . Once the needle  25  goes through the PL muscle  6 , it again moves more easily and the TA muscle  11  is stimulated selectively at levels similar to that obtained by the original needle electrode  23 . Then, the two needles  23 ,  25  are close to each other and to the DP nerve  5 . The tip of the hypodermic needle  25  is now at the desired site  2  and the needle electrode  23  can be removed.  
         [0038]     When the tip of the hypodermic needle  25  has been placed at the desired microstimulator implantation site  2 , the trocar is removed. A microstimulator is inserted into the lumen of the needle  25 . A plunger is then used to apply a light pushing force to the back end of the microstimulator to eject it into the leg tissue.  
         [0039]     The hypodermic needle  25  is then removed and the microstimulator is tested for functionality and the motor threshold is measured. Testing is done by placing the microstimulator coil  26  over the implant site  2 . Stimulation pulses are applied in increasing steps until a noticeable muscle twitch in TA muscle  11  is produced. Increasing the stimulation intensity should produce a brisk muscle twitch and a large TA muscle M-wave with little or no PL muscle M-wave. This indicates that the microstimulator is in the desired position. Then, the stimulation is discontinued for 4-6 days to allow the surrounding tissue to heal. If the microstimulator is not properly positioned to give selective stimulation of the TA muscle, the process can be repeated with a second microstimulator A similar threshold test is performed to create a history of thresholds for each microstimulator, if more than one have been implanted.  
         [0040]     Having reference now to  FIGS. 4 and 5 , there are shown general and more specific schematic block diagrams of the system for driving implanted microstimulators and controlling the timing and duration of stimulations. This system combines the BION™ hardware and the Walk Aide 2™ hardware available from Biomotion Ltd., Edmonton, Alberta and described in U.S. Pat. No. 5,814,093.  
         [0041]     In connection with the Walk Aide 2 hardware, a tilt of the leg shank backwards relative to the body at the end of the stance phase of the walking cycle activates tilt sensor circuitry  30  that sends a signal representing tilt angle to microcontroller  31 . If the tilt signal exceeds a predetermined threshold and some other logic conditions are met, for example that stimuli have not been generated for a period known as the “Wait” period, a stimulus gate signal is generated. This signal is formatted as a code sequence that can be decoded by the Bion microstimulator  1  to produce a pattern of stimuli with the desired amplitude and duration. In the preferred implementation the sequence of commands is formatted efficiently using a non-return to zero invert (NRZI) formatter  32 . The coded sequence is then sent to the BION coil driver circuit  33  and then to the coil  26 . The microstimulator internal circuitry decodes this sequence and produces a prescribed sequence of stimulus pulses. The BION microstimulator  1  contains no batteries, so the external coil  26  must supply power as well as the sequence of control pulses. The block diagram of  FIG. 5  also shows other sensors and controls that enhance the flexibility of the overall design.  
         [0042]     In greater detail, a lithium ion battery (7.2V)  28  is used to power both the coil driver  33  and the other electronics (after regulation to 5V). The coil driver  33  is tuned to the preferred radio frequency of the microstimulator  1  and is shaped to fit in a cuff around the leg, so that it covers the implanted microstimulator(s)  1 .  
         [0043]     A combination of sensors is used to control the timing of the stimulation. More particularly, a tilt sensor  30  (Analog Devices ADXL202) measures the orientation of the leg with respect to gravity, a foot sensor  34  (Interlink Technology force sensing resistor FSR-20) measures the pressure of the heel on the ground and a hand switch  35  can be used by a clinician to set up the initial timing of the stimuli.  
         [0044]     A linearizing amplifier  29  corrects the otherwise non-linear response of the foot sensor  34 .  
         [0045]     The output of the tilt sensor  30  is filtered (not shown) to remove sharp transients such as the deceleration of the foot hitting the ground.  
         [0046]     The microcontroller  31  (Microchip PIC16LF 876) processes inputs and generates outputs based on a state engine that includes timing constraints, as described in U.S. Pat. No. 5,814,093. The microcontroller  31  has on-board non-volatile memory for storage of parameters used by the state machine. The parameters can be adjusted using a Windows™ program, Walk Analyst™, which allows the stimulation current, the duration and the frequency of the stimuli produced by the microstimulator(s)  1  to be varied as desired for optimum function. The parameters are read and written via serial communications with the microcontroller  31  using the optically isolated RS232 interface isolator  36 .  
         [0047]     The stimulus button  37  allows the operation of the electronics and the positioning of the coil  26  to be tested, as well as allowing the users to adjust the intensity control to the desired level.  
         [0048]     Indicators  38  are provided for off/on status, stimulation and low battery conditions.  
         [0049]     In previous implementations for surface stimulation the microcontroller produced pulses that were amplified to produce stimuli directly to the muscles through the skin. In the current implementation, a string of non-return to zero invert (NRZI) encoded data is generated that modulates the frequency of the coil  26  and conveys the stimulus parameter to the microstimulator  1 . The NRZI formatter  32  offloads the overhead of encoding and maintaining an ‘idle’ (recharge) power condition in the microstimulator  1  by using a recirculating shift register. The formatter circuit therefore reduces the speed requirements for the microcontroller in communicating with the microstimulator, by taking care of synchronization and data encoding issues that would usually be done with firmware. This results in power savings by allowing a lower system clock speed that would otherwise be needed to supply the coil with data for the microstimulator.  
         [0050]     Although particular devices used have been identified in the foregoing description, the invention is not limited to these devices. Other implanted stimulation devices that are sufficiently small to fit in the space available should also work. Also, other foot drop stimulators could be modified to drive the microstimulator appropriately.  
       REFERENCES  
       [0000]    
       
          (1) Liberson, W. T., Holmquest, H. J., Scott, D., and Dow, M. 1961. Functional electrotherapy, stimulation of the peroneal nerve synchronized with the swing phase of the gait of hemiplegic patients. Arch. Phys. Med., 42: 101-105;  
          (2) O&#39;Halloran, T., Haugland, M., Lyons, G. M., and Sinkjaer, T. 3004. Modified implanted drop foot stimulator system with graphical user interface for customized stimulation pulse-width profiles. Med. Biol. Eng. Comput, 41(6): 701-709;  
          (3) Rozman, J., Stanic, U., Malezic, M., Acimovic-Janezic, R., Kljajic, M., and Kralj, A. 1990. Implantable electrical stimulation and technology of Jozef Stefan Institute in Ljubljana. In Advances in External Control of Human Extremities. Nauka, Belgrade, Yugoslavia. pp. 617-626.  
          (4) Waters, R. L., McNeal, D., and Perry, J. 1975. Experimental correction of foot drop by electrical stimulation of the peroneal nerve. Journal of Bone and Joint Surgery, 57A: 1047-1054.