Abstract:
An electrically sensing and stimulating outer sheath for ensuring accurate surgical placement of a microsensor or a microstimulator near a nerve in living tissue is disclosed. The electrically sensing outer sheath may also be used to verify the function of the microstimulator or microsensor during surgical placement but before the outer sheath is removed. In the event that the microstimulator is not optimally placed near the nerve, or if the microstimulator is malfunctioning, this can be determined prior to removal of the outer sheath, thus reducing the possibility of nerve or tissue damage that might be incurred during a separate operation to remove the microstimulator.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of commonly assigned U.S. Provisional application No. 60/330,165, filed Oct. 19, 2001. This application is related to but in no way dependent on commonly assigned U.S. Patent application, System and Method for Removing Implanted Devices, filed on even date herewith and incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to placement of a nerve stimulator or sensor in living tissue. 
     BACKGROUND OF THE INVENTION 
     Microstimulators are small, implantable electrical devices that pass a small signal to living tissue in order to elicit a response from a nerve or muscle. Microsensors are similar electrical devices except that they detect electrical and other signals that are generated by living tissue. The term microstimulator is intended to apply equally to both microstimulators and microsensors. The use of microstimulators or microsensors which are implanted in living tissue to stimulate a muscle function by either stimulating a nerve or the muscle itself are well known. The microstimulators receive power and control signals by inductive coupling of magnetic fields generated by an extracorporeal antenna rather than requiring any electrical leads. See for example, U.S. Pat. Nos. 5,193,539; 5,193,540; 5,324,316; 5,405,367; 6,175,764; 6,181,965; 6,185,452; 6,185,455; 6,208,894; 6,214,032; and 6,315,721, each of which is incorporated in its entirety by reference herein. These microstimulators are particularly advantageous because they can be manufactured inexpensively and can be implanted non-surgically by injection. Additionally, each implanted microstimulator can be commanded, at will, to produce a well-localized electrical current pulse of a prescribed magnitude, duration and/or repetition rate sufficient to cause a smoothly graded contraction of the muscle in which the microstimulator is implanted. 
     While primarily designed to reanimate muscles so that they can carry out purposeful movements such as locomotion, the low cost, simplicity, safety and ease of implantation of these microstimulators suggests that they may additionally be used to conduct a broader range of therapies in which increased muscle strength, increased muscle fatigue resistance and/or increased muscle physical bulk are desirable; such as therapies directed to muscle disorders. For example, electrical stimulation of an immobilized muscle in a casted limb may be used to elicit isometric muscle contractions that prevent atrophy of the muscle for the duration of the casting period and facilitate rehabilitation after the cast is removed. Similarly, repeated activation of microstimulators injected into the shoulder muscles of patients suffering from stroke enable the paretic muscles to retain or develop bulk and tone, thus helping to offset the tendency for such patients to develop subluxation at the shoulder joint. Use of microstimulators to condition perineal muscles increases the bulk and strength of the musculature in order to maximize its ability to prevent urinary or fecal incontinence. See for example, U.S. Pat. No. 6,061,596, which is incorporated in its entirety by reference herein. 
     Microstimulators, as exemplified by the BION® of Advanced Bionics Corporation, are typically elongated devices with metallic electrodes at each end that deliver electrical current to the immediately surrounding living tissues. The microelectronic circuitry and inductive coils that control the electrical current applied to the electrodes are protected from the body fluids by a hermetically sealed capsule. This capsule is typically made of a rigid dielectric material, such as glass or ceramic, that transmits magnetic fields but is impermeable to water. 
     Often, while placing the miniature microstimulator in living tissue, the orientation of the microstimulator changes slightly such that the microstimulator is not in fact in electrical contact with the nerve, requiring reorientation of the microstimulator. The microstimulator may move at any point in the surgical implantation procedure. If the microstimulator has moved, it may be at a significant distance from the nerve that is to be stimulated. Consequently, more energy is needed from the microstimulator to stimulate the nerve, unless the microstimulator is repositioned closer to the nerve. While such microstimulators may be injected, the actual placement requires first locating the desired end point near the nerve or muscle. The known method of placement involves locating the nerve with an electric probe, placing a hollow implantation tool over the electric probe and removing the electric probe to allow the miniature microstimulator to be passed down the length of the hollow implantation tool. The implantation tool is then removed, leaving the microstimulator implanted at or near the desired location. If there is a problem with the function or location of the microstimulator, then additional surgery must be performed to remove or relocate the microstimulator, imposing risk, discomfort and potential tissue damage to the patient. 
     Using a known implantation tool, as disclosed in U.S. Pat. No. 6,214,032, to implant a microstimulator, may lead to the device being located remotely from the desired nerve. In this approach, an electrically stimulating trocar is first used to locate the desired nerve. The trocar is removed, after a cannula is slid along the trocar to be next to the nerve. Then the microstimulator is placed next to the nerve by inserting the microstimulator into the cannula and pushing the microstimulator to the end of the cannula, where it is ejected and is left behind, after the cannula is removed. The problem is that once the electrically stimulating trocar is removed, there is no way to detect movement of the cannula. Thus, the microstimulator may be left some distance from desired location, as was located by the stimulating trocar. This displacement from the optimum stimulating site unacceptably increases the power requirements and diminishes the battery life of the microstimulator. 
     Therefore, it is desired to have a method of implantation that ensures that the microstimulator is functioning properly and is implanted in an optimum position prior to removing the implantation tools that are utilized during surgery to place the microstimulator. 
     OBJECTS OF THE INVENTION 
     It is an object of the invention to locate an outer sheath near a nerve by monitoring muscle response from an electrical sensing or stimulating outer sheath. 
     It is an object of the invention to enable placement of a microstimulator or microsensor near a nerve by using an electrical path through the outer sheath. 
     It is an object of the invention to reliably place a microstimulator or microsensor near a nerve during surgery. 
     It is an object of the invention to verify that a microstimulator or microsensor is properly functioning during surgical placement of the microstimulator or microsensor. 
     It is an object of the invention to provide a tool for insertion of a microstimulator in living tissue. 
     It is an object of the invention to facilitate placement of a microstimulator in living tissue. 
     Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a stimulating electrode near a nerve. 
     FIG. 2 illustrates an outer sheath with sheath electrode surrounding an electrode probe near a nerve. 
     FIG. 3 illustrates an outer sheath with sheath electrode near a nerve. 
     FIG. 4 illustrates a microstimulator in an outer sheath. 
     FIG. 5 illustrates a microstimulator as the outer sheath is withdrawn. 
     FIG. 6 illustrates a stimulating electrode probe near a nerve. 
     FIG. 7 illustrates a stimulating electrode probe surrounded by an inner sheath and an outer sheath near a nerve. 
     FIG. 8 illustrates an outer sheath with a sheath electrode positioning a microstimulator near a nerve. 
     FIG. 9 illustrates an implanted microstimulator after removal of the outer sheath. 
     FIG. 10 illustrates an electrode probe surrounded by an inner sheath that is located near a nerve. 
     FIG. 11 depicts an electrode probe surrounded by an inner sheath that is surrounded by an outer sheath that is near a nerve. 
     FIG. 12 depicts an outer sheath and sheath electrode near a nerve. 
     FIG. 13 depicts an outer sheath and sheath electrode near a nerve with a microstimulator being inserted by a blunt-end push rod. 
     FIG. 14 depicts an implanted microstimulator near a nerve. 
     FIG. 15 illustrates an outer sheath and sheath electrode near a nerve with a microstimulator that is contained in a silk tube being inserted by a blunt-end push rod. 
     FIG. 16 illustrates an electrode probe with a dilator outer sheath and sheath electrode positioned near a nerve. 
     FIG. 17 illustrates a dilator outer sheath with a sheath electrode containing a microstimulator for placement near a nerve. 
     FIG. 18 illustrates a microstimulator being ejected from a dilator outer sheath near a nerve. 
     FIG. 19 illustrates a microstimulator ejection tool. 
     FIG. 20 illustrates a cross-sectional view of the implantation tool. 
     FIG. 21 illustrates a cross-sectional view of the implantation tool ejecting a microstimulator. 
     FIG. 22 depicts a cross-sectional view of the outer sheath and ring electrode near a nerve. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A. Two Part System for Insertion of a Microstimulator 
     A solution to the problems that have been encountered in precisely placing a microdevice in living tissue is to monitor the position of the implant device continuously by observing the muscle response to electrical stimulation during implantation of the microdevice, between the time when the probe is removed and when the microdevice is released. Loeb, at al. describe an alternative approach to placing a microstimulator near a nerve. See U.S. Pat. No. 6,214,032, which is incorporated herein in its entirety by reference. See also U.S. Pat. No. 6,345,202, which is incorporated herein in its entirety by reference, which discusses verifying the location of the insertion needle by electrical stimulation of a removable trocar within the hollow sheath of the needle. 
     A preferred embodiment of the invention is illustrated in FIGS. 1-5, wherein FIG. 1 illustrates the electrode probe  2  locating the nerve  6  by electrically stimulating the nerve  6  and observing the muscle response. The electrical signal is generated by the electrical stimulator  12 , e.g., a pulse generator. It is obvious that the electrode probe  2  could be a detector and electrical stimulator  12  could be a signal amplifier. The signal passes along electrode probe wire  10 , along electrically insulated electrode probe  2  to conducting tip  14 . Return electrode probe wire  11  preferably completes the electrical path by connecting between the skin  4  and electrical stimulator  12 . Electrode probe  2  is electrically insulated along its entire length, except that the conducting tip  14  is not insulated, allowing the electrical signal to pass into the living tissue. Visual observation of the contracting muscle indicates when the conducting tip  14  is located next to nerve  6 . Location marks  28 , that circumscribes electrode probe  2 , provides a visual indication of the precise location of the nerve. 
     After the nerve  6  is located, electrode probe wire  10  is detached from the electrode probe  2  and an outer sheath  16 , as illustrated in FIG. 2, is slid over and along the electrode probe  2 , to penetrate the living tissue. The outer sheath  16  is inserted until it aligns with depth indicator  29 , a selected one of the location marks  28 . The outer sheath  16  contains a sheath lead wire  20 , which is electrically insulated along its length. The sheath lead wire  20  passes along the length of outer sheath  16 , preferably on its inner diameter along the wall. The lead wire  20  terminates at the sheath electrode  18 , which is preferably located on the end of the outer sheath  16  that contacts the nerve  6 . The sheath electrode  18  preferably receives an electrical signal from the electrical stimulator  12  by a current that passes along sheath lead wire  20  to the sheath electrode  18 . A return electrode is preferably attached to the skin  4  and the electrical circuit is completed by return electrode probe wire  11 . 
     The outer sheath  16  is inserted to align with an electrode location mark  28  such that the sheath electrode  18  is located near the nerve  6 . The position of the sheath  16  is optimized by electrically pulsing the nerve  6  and observing the response of the associated muscle. When electrode probe  2  is removed, the position of the outer sheath  16  is confirmed by electrically pulsing the nerve  6 , as previously discussed. 
     Once the electrode probe  2  is removed from the outer sheath  16 , FIG. 3, the outer sheath  16  is ready to receive the microstimulator  22  (see FIG.  4 ). Alternatively as previously discussed, the microstimulator  22  may be a sensor of signals from the living tissue. FIG. 4 illustrates the outer sheath  16  with the microstimulator  22  being pushed into the outer sheath  16  with blunt-end push rod  24 . The push rod  24  is inserted to a location mark  25  such that the microstimulator  22  is located at the end of outer sheath  16 , near the nerve  6 . 
     The position of the microstimulator  22  can be verified by testing it before the outer sheath  16  is removed. If a problem is discovered, then the microstimulator  22  may be easily removed with the outer sheath  16 . If no problem is discovered and if it is desired to implant the microstimulator  22 , then the outer sheath  16  is removed, as illustrated in FIG. 5, by holding the microstimulator  22  in position near the nerve  6  with the push rod  24  while the outer sheath  16  is removed. 
     B. Three-Part System for Placement of a Microstimulator 
     An alternative embodiment of the invention is illustrated in FIGS. 6-9. FIG. 6 illustrates the electrode probe  102  locating the nerve  106  by electrically stimulating the nerve  106 . The response of the associated muscle is observed. Electrode probe  102  is electrically insulated along its length, but conducting tip  114  is not insulated, allowing the electrical signal to pass into the living tissue. The location marks  128  that circumscribe electrode probe  102  provide a precise location of the nerve depth. 
     The electrical signal is generated by the electrical stimulator  112 . The electrical stimulator  112  may be hand-operated or it may be operated by a foot-control lever  113  that is moved by the foot of the surgeon or an assistant. The signal passes along electrode probe wire  110 , along electrically insulated electrode probe  102  to conducting tip  114 . Return electrode probe wire  111  preferably completes the electrical path by connecting between the skin  4  and electrical stimulator  112 . 
     After the nerve  106  is located, electrode probe wire  110  is detached from the electrode probe  102  (see FIG. 6) and sheath lead wire  120  is attached to sheath electrode  118  (see FIG.  7 ). Then, an inner sheath  108  and outer sheath  116  are slid along the electrode probe  102 , as shown in FIG.  7 . The inner sheath  108  is sharp and enters the skin  104  and other living tissue at insertion point  26 , enlarging the hole for the implantation, until the top of inner sheath  108  aligns with depth indicator  129  on electrode probe  102  (a selected one of the location marks  128 ), thereby indicating that the tip of the inner sheath  108  is aligned with and is next to the nerve  106 . 
     The electrode probe  102  is then removed from the inner sheath  108 . Next, the inner sheath  108  is removed from the outer sheath  116 . The location of the outer sheath  116 , with respect to the nerve  106 , is determined by passing an electrical signal from the electrical stimulator  112  along electrode probe wire  120 , which is preferably embedded in the interior wall of the outer sheath  116 , as illustrated in FIG.  7 . Alternately, the electrode probe wire  120  may pass along the outside of outer sheath  116  or it may be embedded in the wall of outer sheath  116 . Outer sheath  116  is preferably electrically insulated or is comprised of a nonconductive material, such as plastic, to ensure that the electrical pulsing signals that are used to locate the nerve pass into the living tissue and not into the outer sheath  116 . 
     After the electrode probe  102  and the inner sheath  108  have been removed from the outer sheath  116 , the outer sheath  116  can no longer be readily relocated because the outer sheath  116  is not designed to penetrate living tissue. Saline solution is injected into outer sheath  116  to ensure that electrical conductivity is established when the microstimulator  122  is placed in outer sheath  116  (see FIG.  8 ). Outer sheath  116  contains a plurality of holes  117  that are located to facilitate electrical contact between the microstimulator  122  and the living tissue. As described in the incorporated patents, the microstimulator  122  preferably has an axial dimension of less than 60 mm and a lateral dimension of less than 6 mm. In a preferred embodiment, the microstimulator  122  has microstimulator electrodes  123  located on each end. The sheath electrode  118  may be electrically pulsed to ensure that the location of outer sheath  116  has not changed significantly, relative to the nerve  106 , while the microstimulator  122  is placed in the outer sheath  116 . 
     FIG. 8 illustrates the microstimulator  122  as it has been placed inside outer sheath  116  and urged toward nerve  106  by blunt-end push rod  124 . Blunt-end push rod  124  contains push rod location marks  125 , which indicate the position of the microstimulator  122  during insertion. Push rod depth indicator  130  (a selected one of the location marks  125 ), which indicates when the microstimulator has arrived at the end of outer sheath  116 , and is therefore near nerve  106 . Alternatively, the microstimulator may be urged along outer sheath  116  by the electrode probe  102  or by inner sheath  108 . It is beneficial that any alternative push rod have location marks to indicate when the microstimulator  122  has arrived at the end of the outer sheath  116 . 
     Before the microstimulator  122  is ejected from the outer sheath  116 , its position may be confirmed by stimulation of the sheath electrode  118 . Furthermore, the function of the microstimulator  122  may be checked by causing stimulation pulses to be emitted from the electrodes of the microstimulator. 
     Once its position and function are confirmed, the microstimulator  122  is ejected from the outer sheath  116 , FIG. 9, by holding the push rod  124  in place as the outer sheath  116  is withdrawn away from the nerve  106  and out of the living tissue at insertion point  26 . Typically, this apparatus implants the microstimulator  122  a distance from the nerve  106  that is approximately equal to the distance from the sharp tip of the inner sheath  108  to the tip of outer sheath  116 . 
     C. Improved Three-Part System for Placement of a Microstimulator 
     An alternative embodiment of the invention is presented in FIGS. 10-14. FIG. 10 provides a side view of the electrode probe  2 , which is used to initially locate the nerve  6  (and/or muscle tissue) by means of inserting the probe  2  into the living tissue, preferably at an angle to the skin  4  through an insertion point  26  in the skin  4  and into the living tissue. The electrode probe  2  is a sharp device that is electrically insulated along its length but that is not electrically insulated at its conducting tip  14 . The electrode probe  2  is used to electrically stimulate the living tissue near the tip  14 , thereby locating the desired nerve  6  by eliciting a specific response, such as contraction of a nearby muscle. It is understood that this approach can equally well be used to stimulate muscle tissue. 
     The electrode probe  2  is attached by electrode probe wire  10  to an electrical stimulator  12 , which can be pulsed manually to locate the nerve  6 . The electrical path is completed by return electrode probe wire  11 , that is preferably attached to skin  4 . It is preferred that the electrical stimulator  12  be controlled by foot control  13 , although it may be controlled by a hand control in the alternative. The electrode probe  2  location with respect to the nerve  6  and/or the muscle tissue is determined by observing the muscle response when the electrode probe  2  is electrically stimulated. After the electrode probe conducting tip  14  is optimally located, the inner sheath  8  is slid along the electrode probe  2  to enlarge the opening in the tissue (see FIG.  10 ). In an alternative embodiment, the inner sheath  8  and outer sheath  16  may be simultaneously slid along the pre-positioned electrode probe  2  into the living tissue. 
     In a preferred embodiment (see FIG.  11 ), the electrode probe  2  is held in close proximity to the nerve  6  while a cylindrically hollow outer sheath  16  is slid over the inner sheath  8 . The inside diameter of inner sheath  8  has a diametral dimension that is preferably slightly larger than the outer diameter of electrode probe  2 , e.g., by 5% to 20%, while the outside diameter of inner sheath  8  preferably is approximately equal to the outside diameter of microstimulator  22 , e.g., within about 5% (see FIG.  13 ). A thin electrically conductive sheath lead wire  20 , having a diameter of about one-thousandth of an inch, is located in the wall of outer sheath  16  connecting the sheath electrode  18  and the electrical stimulator  12 . The sheath electrode  18  is located on the end of the outer sheath  16  that is nearest the nerve  6 . 
     This device offers the additional improved feature that both the outer sheath  16  and the inner sheath  8  are near the nerve  6 , thus allowing the ultimate position of the implanted microdevice to be near the nerve  6 . The closer the implanted microdevice is to the nerve, generally, the less power is consumed in its operation and the longer the device will survive without battery replacement. 
     As shown in FIG. 12, the electrode probe  2  and inner sheath  8  are removed from the living tissue while the position of the outer sheath  16  is maintained next to the nerve  6  by electrically pulsing the nerve  6  with a current from sheath electrode  18  and observing the response of the muscle associated with the nerve  6 . In order to ensure that there is no interference with electrical stimulation of the nerve  6 , both the inner sheath  8  and the outer sheath  16  must be non-conductors or must be electrically insulated from the living tissue. Accordingly, in a preferred embodiment, the inner sheath  8  and the outer sheath  16  are made of plastic. 
     The sheath lead wire  20  may be located in alternative locations in or along the wall of the outer sheath  16 . The sheath lead wire  20  may be located in the wall, which is preferred, or along the outside of the hollow outer sheath  16 , or inside the outer sheath  16 , e.g., in a groove. The sheath lead wire  20  can then be used to conduct an electrical signal to stimulate the nerve  6  and to confirm the position of the outer sheath  16  relative to the nerve  6 . 
     Prior to insertion of the microstimulator  22 , the outer sheath  16  may be flushed with saline solution. Holes  17  are located in the outer sheath at locations to ensure good electrical contact between the microstimulator  22 , after it is inserted into the outer sheath  16 , and the living tissue. 
     A microstimulator  22  (see FIG. 13) is typically a small tubular device that contains an electronic package and communication means, for modifying or affecting a body parameter, when it is located near a nerve  6  or muscle to be stimulated. In a preferred embodiment, the microstimulator  22  has microstimulator electrodes  23  located on each end. 
     FIG. 13 illustrates the microstimulator  22  being inserted into the outer sheath  16  using the blunt-end push rod  24 . Alternately, the microstimulator can be inserted into the outer sheath  16  by using the electrode probe  2  or inner sheath  8 . The blunt-end push rod  24  has location mark  28  that circumscribes the push rod  24  such that the location of the microstimulator  22  in the outer sheath  16  can be ascertained by reference to the location mark  28 . 
     Once the microstimulator  22  is placed in contact with the nerve  6 , by passing the microstimulator  22  down the length of the inner sheath  8 , the microstimulator  22  is activated and powered via an externally provided RF signal and the muscle that responded before is observed to see if it is still responding when stimulated by the microstimulator  22 . In an alternative embodiment, the microstimulator  22  may be activated by an RF signal or powered by means other than via an RF signal, such as by an internal battery. If the muscle is responding properly, the outer sheath  16  is pulled back while restraining the microstimulator  22  with the blunt-end push rod  24  (see FIG.  13 ). The microstimulator  22  is free of the outer sheath  16  and both the outer sheath  16  and blunt-end push rod  24  are removed from the living tissue. The microstimulator  22  remains in position next to the nerve  6  and at the base of insertion point  26 , as illustrated in FIG. 14, after the outer sheath  16  and the blunt-end push rod  24  have been removed. 
     D. Removal of a Microstimulator with a String Loop 
     In a preferred embodiment, the microstimulator  22  (see FIG. 13) contains removal loop  30 , e.g., an eyelet, on the end nearest the skin  4  to facilitate attachment of removal string  32  to the microstimulator  22 . The removal string  32  may be left in the living tissue near the insertion point  26  (see FIG. 14) or it may be left outside the living tissue. The removal string may be used to locate and/or to remove the microstimulator by pulling on it. This technique is effective for a few days post-surgery to remove the microstimulator  22  without risking further damage or trauma to the implant area, until the tissue begins to heal and adheres to the microstimulator. 
     E. Removal of a Microstimulator with a Fabric Sock 
     An alternative embodiment to the removal system using the removal string  32  connected to the removal loop  30  on the microstimulator  22  (see FIGS. 13 and 14) is to place the microstimulator  22  in a porous, non-soluble, biocompatible fabric tube  100  (see FIG.  15 ). A preferred material for biocompatible fabric tube  100  is a silk tube, which is essentially a “sock” or closed end tube. Silk is a preferred material because it is biocompatible and does not bond readily to the living tissue. As an alternative to silk, any closely woven material made of non-soluble material may be used. Alternatives include dialysis membrane materials. The ideal material is porous to allow solute materials to penetrate and flood the microstimulator surfaces for optimum electrical contact, however the structure of the materials must be so fine that the body&#39;s connective tissue cannot penetrate and lock the fabric tube  100  into place. Should the microstimulator  22  need to be removed, then the end of the fabric tube  100  is located either protruding from the skin  4  or implanted beneath the skin  4  near insertion point  26 , and slowly withdrawn from the living tissue with the microstimulator  22  inside. 
     F. Two-Part System with Expanding Aperture for Placement of a Microstimulator 
     A further embodiment of an insertion system for placing a microstimulator or microsensor into living tissue is presented in FIGS. 16-18. In an analogous process to that previously discussed the electrically insulated electrode probe  202  is first inserted in the living tissue through the skin  204  at insertion point  26  in order to locate a nerve  206  by electrically stimulating the nerve  206  and visually observing the muscle response. The electrical signal is generated by an electrical stimulator  212  and the signal passes along a wire (not illustrated) to the electrode probe  202  and to the exposed electrically conductive tip  214  of the electrode probe  202 . The circuit is completed by return electrode probe wire  211  that is preferably attached to the skin  204 . The insulated wire  210  is removed from the electrode probe  202  after the probe  202  has located the nerve  206 . 
     As illustrated in FIG. 16, the dilator outer sheath  216  is inserted over electrode probe  202  and into the living tissue until the aperture tip  230  of the dilator outer sheath  216  is approximately aligned with the conducting tip  214  of the electrode probe  202 . The dilator outer sheath  216  has a sharp end to facilitate insertion into the living tissue. The sharp end forms aperture  230 . 
     The proper alignment is achieved by visually aligning the dilator outer sheath  216  with the location mark  228 . The electrode probe  202  is removed and the location, relative to the nerve  206 , of the dilator outer sheath  216  is confirmed by passing an electrical signal from the electrical stimulator  212  along the electrically insulated wire  210 , which in a preferred embodiment extends along the inside wall of the dilator outer sheath  216 . The insulated wire  210  terminates in sheath electrode  218 , which is located near aperture  230 . The circuit is completed by return electrode probe wire  211  that is preferably attached to the skin  204 . 
     In alternative embodiments, the wire  210  may be located along the outside wall or may be replaced with a conductive path along the outside wall of the dilator outer sheath  216  or along the inside wall of the dilator outer sheath  216 . The nerve  206  is pulsed with an electrical signal from the sheath electrode  218  and the response of the muscle is observed. 
     Preferably, the dilator outer sheath  216  is electrically insulated to avoid conduction of electricity into the dilator outer sheath  216  and away from nerve  206 . The dilator outer sheath  216  is preferably comprised of plastic. Dilator outer sheath  216  preferably contains a plurality of holes  217  that pass through the wall near the aperture  230  (see FIG.  17 ). The holes  217  are preferably located to provide an electrically conductive path between the living tissue and the microstimulator  222 . 
     FIG. 17 illustrates the dilator outer sheath  216  with the microstimulator  222  inserted therein and next to the aperture  230  that is next to the nerve  206 . The microstimulator  222  is shown inserted part way along the inside of the dilator outer sheath  216  in FIG.  17 . 
     In a preferred embodiment (see FIG.  17 ), the microstimulator  222  has microstimulator electrodes  223  located on each end. The microstimulator  222  will be inserted until the nerve-end of the microstimulator  222  is approximately even with the aperture  230  formed by dilator outer sheath  216 . When the microstimulator  222  is fully inserted in dilator outer sheath  216 , the microstimulator  222  is near nerve  206 . The inside diameter of the dilator outer sheath  216  is preferably larger than the outside diameter of the microstimulator  222 , e.g., by 5% to 20%, allowing the microstimulator  222  to pass along the length of the dilator outer sheath  216  with moderate pressure from the blunt-end push rod  224 . In a preferred embodiment, the microstimulator  222  is positioned by using the blunt-end push rod  224 , although the electrode probe  202  or another comparable probe with location marks can be used. 
     Since the dilator outer sheath  216  may move after electrode probe  202  is removed and during the insertion of microstimulator  222 , the location of the dilator outer sheath  216 , and more particularly the aperture  230 , next to the nerve  206  is verified by preferably pulsing nerve  206  with a current from conducting tip  218  and observing the response of the muscle. 
     Prior to removing dilator outer sheath  216  and leaving the microstimulator  222  implanted next to nerve  206 , the function of the microstimulator  222  is confirmed by checking its electrical functions. If there is a problem with the microstimulator  222  or if the dilator outer sheath  216  moved and is no longer located next to the nerve  206 , then the microstimulator  222  may be removed by withdrawing the dilator outer sheath  216  from the living tissue. 
     If it is desired to implant the microstimulator  222 , then the dilator outer sheath  216  is removed from the living tissue by holding the microstimulator  222  in place with the blunt-end push rod  224  and moving the dilator outer sheath  216  along the push rod  224  and out of the living tissue (see FIG.  18 ). Aperture  230  enlarges as microstimulator  222  is forced through the aperture. 
     The microstimulator  222 , shown in FIG. 18, has been partially ejected from dilator outer sheath  216 . The aperture  230  expandably conforms to the outside diameter of microstimulator  222  during the ejection process. In a preferred embodiment, the dilator outer sheath  216  is comprised of an electrical insulator, such as plastic, that conforms to allow ejection of the microstimulator  222 . The microstimulator  222  is completely ejected by removing the dilator outer sheath  216  from the living tissue and leaving the microstimulator  222  in place next to the nerve  206 . 
     G. Device for One-Handed Placement of a Microstimulator 
     Placement of a microstimulator  322  in living tissue may be facilitated by using the implantation tool  300  of FIG.  19 . This implantation tool  300  enables one-handed placement of a microstimulator  322  near a nerve (not illustrated). The procedure begins with electrode probe  302  being used to locate the desired nerve by using electrical stimulation, as previously described. Electrode probe  302  is electrically insulated along its length to eliminate electrical shorts and is electrically conductive at its tip to pass an electrical signal to the stimulating site near the nerve. The implantation tool  300  is then slid over electrode probe  302 . The electrode probe  302  is held steady until the aperture  330  is next to the nerve, as determined by observing the mark  304  on the electrode probe  302 . 
     The electrode probe  302  is removed from the implantation tool  300  and the position of implantation tool  300  relative to the nerve (not illustrated) is determined by observing the muscle response when the nerve is stimulated by pulsing the electrical stimulator  312  (see FIG.  20 ). The electrical signal passes along sheath electrode wire  310 , which passes down the length of implantation tool  300  along outer sheath  316  and to sheath electrode  318 , which is located at the end of the implantation tool  300 , next to the nerve being stimulated. The electrical stimulator  312  is preferably controlled by a foot control. A return electrode probe wire  311 , attached from the skin to the electrical stimulator  312  near the implantation site, completes the electrical circuit. 
     Saline is preferably injected into the implantation tool  300 . The saline facilitates obtaining a good electrical connection between the nerve, living tissue, and the microstimulator  322  which is about to be implanted. In a preferred embodiment (see FIG.  20 ), the microstimulator  322  has microstimulator electrodes  323  located on each end. 
     The plunger  360  is withdrawn from the implantation tool  300  (see FIG. 20) by moving ratcheting lever  350  with respect to handle  348 , until the microstimulator  322  is moved into ejection position by ejection spring  306 . The plunger  360  is then moved into the implantation tool  300  by reversing the direction set switch (not illustrated) and then moving ratcheting lever  350  with respect to handle  348 . When plunger  360  is moved to a predetermined position, as indicated by a mark  308  on the plunger  360 , then the microstimulator  322  is next to the aperture  330 , as illustrated in FIG.  21 . 
     In a preferred embodiment, the outer sheath  316  and the plunger  360  are made of an electrically non-conductive material, such as plastic. The outer sheath  316  and plunger  360  must be insulated or must be nonconductors to ensure that the electrical pulsing signals that are used to locate the nerve are not electrically shorted. 
     The holes  317 , that are preferably located near the tip of the implantation tool  300  nearest the nerve, pass through the wall of the outer sheath  316 . The holes  317  are located to correspond with the microstimulator  322  when it is ready to be ejected from the implantation tool  300 , as illustrated in FIG. 21, to enable electrical contact between the microstimulator  322  and the living tissue. 
     The electrical functions of the microstimulator  322  are preferably verified while it is retained in the outer sheath  316 , near the nerve (see FIG.  21 ). The microstimulator  322  is ejected by continuing to move ratcheting lever  350  to force the microstimulator  322  through the aperture  330  by means of the plunger  360 . During the ejection process, the implantation tool is slowly withdrawn from the living tissue and the microstimulator  322  is ejected to remain at the same relative position to the nerve. 
     The outer sheath  316  is removable from the implantation tool  300  by disassembling disconnect  370 . This allows the outer sheath  316  portion of the implantation tool  300  to be removed and discarded or cleaned separately from the rest of the tool  300 . 
     H. Ring Electrode for Placement of a Microstimulator 
     FIG. 22 depicts an alternative embodiment of the invention wherein there is a ring electrode  418  that is attached circumferentially at the sharpened tip of outer sheath  416  that is nearest the nerve  406 . The outer sheath  416  passes through the skin  404  at the insertion point  426 . The outer sheath  416  contains holes  417  which are located in the wall of the outer sheath  416  to facilitate electrical contact between the microstimulator (not shown) and the living tissue during insertion of the microstimulator in the tissue, but before the microstimulator has been ejected from the outer sheath  416 . An electrical signal is generated by the electrical stimulator  412  that passes along sheath lead wire  420  to ring electrode  418 . Ring electrode  418  is a conductive material that may be plated, deposited, mechanically bonded, or attached by any of the known processes for making a conductor that is integrally bonded to or a part of the sharpened tip of outer sheath  416 . The sharpened tip end (i.e., inserted end) is referred to as the distal end of the outer sheath  416 , while the opposite end of the outer sheath  416  is referred to as the proximal end. An advantage of having a ring electrode  418  over a single point electrode is that the possibility of damaging the nerve  406  with an electric pulse is reduced when the size of the electrode is increased. 
     I. Ring Return Electrode for Placement of a Microstimulator 
     FIG. 22 additionally depicts an alternative embodiment for a ring return electrode, wherein the ring return electrode  422  is located circumferentially around the outside of sheath  416 . The ring return electrode  422  preferably acts as the cathode return element and completes the electrical circuit via the return electrode probe wire  411 , which in turn connects to the electrical stimulator  412 . The ring return electrode  422  is preferably located between the sharpened tip or distal end (i.e., the inserted end) and the proximal end of the sheath  416 . 
     A benefit of utilizing the ring electrode  418  in conjunction with the ring return electrode  422  is that by locating ring return electrode  422  a distance from ring electrode  418  that approximates the distance between the electrodes on the microstimulator (not illustrated), the electrical resistivity that the microstimulator will encounter after being implanted in the living tissue can be measured before the microstimulator is ejected from the outer sheath  416 . This allows a prediction of the battery life of the implanted microstimulator and gives the surgeon an opportunity to modify the implantation location, if the predicted life or performance of the microstimulator is not adequate. 
     The following nonlimiting example sets forth an exemplary procedure for implantation of a miniature implantable stimulator or sensor, e.g., the BION® that is available from Advanced Bionics Corporation, by using an embodiment of the present invention. 
     EXAMPLE 
     Microstimulator Implantation Procedure, Anterior Approach, for Sleep Apnea 
     1. Instruct the patient to lie down in the supine position. 
     2. Prepare the patient for surgery using standard surgical preparation. 
     3. Anesthetize the skin and subcutaneous tissue with 1% xylocaine/1:100,000 epinephrine at and around the insertion site in the neck. 
     4. Anesthetize one nostril and the nasopharynx with topical lidocaine/oxymetazoline solution and insert a laryngoscope to observe tongue movement during hypoglossal nerve stimulation. 
     5. Mark the midpoint of the hyoid bone and mark a point about 1 cm anterior/superior to the hyoid with a sterile pen. Make an incision about 1 cm wide parallel to the hyoid extending down into the subcutaneous tissue about 5 mm mid center over the 1 cm anterior point. Use an intravenous sedative as required. 
     6. Attach the electrical stimulator cathodal connecting lead to the proximal end of the blunt tipped electrode probe. The electrical stimulator anode lead is attached to a surface electrode placed on the exposed shoulder. 
     7. Insert the probe into the incision about 5-6 mm off the midline at a right angle to the skin. Advance the probe slowly inward at about 15 degrees laterally off the perpendicular toward the hypoglossal nerve. 
     8. Turn the electrical stimulator on (at approximately 30 pulses/sec, 3 mA, 200 μsec) and advance the probe slowly inward toward the hypoglossal nerve (HGN) until a contraction of the tongue is observed. Increase the stimulation current to 5-10 mA for brief periods, if required, to optimally position the probe. Check with the patient to ensure comfort at this level. 
     9. Remove the cathodal connecting lead from the probe. Connect the sheath lead wire to the electrical stimulator. Slide the inner sheath and outer sheath near the tip of the probe by observing location marks on the probe. 
     10. Turn the electrical stimulator on (at approximately 30 pulses/sec, 3 mA, 200 μsec) and advance the inner sheath and the outer sheath slowly toward the optimum position near the hypoglossal nerve (HGN) until a contraction of the tongue is observed. It may be necessary to increase the stimulation current to 5-10 mA for brief periods while searching for the optimum location for the best response of the muscle. Check with the patient to ensure comfort at this level. 
     11. While holding the inner sheath and outer sheath, pull the probe gently out of the inner sheath. Detach the outer sheath from the inner sheath. Holding the outer sheath, withdraw the inner sheath 3-4 cm. 
     12. Attach a 5 ml syringe, filled with normal sterile saline (0.9% NaCl), to the inner sheath and inject a few drops into the inner sheath, then remove the inner sheath. Then, insert the microstimulator into the outer sheath. The microstimulator is positioned by pushing it with the inner sheath, which is marked on its shaft to indicate when the tip microstimulator is at the tip of the outer sheath. Add more saline into the outer sheath through the inner sheath, ensuring that the anode will make electrical connection to the tissue through the small holes in the outer sheath&#39;s wall. 
     13. To ensure proper microstimulator position, turn the electrical stimulator on and confirm that a contraction of the tongue is observed when it is stimulated with the sheath electrode. Then activate the microstimulator external coil and controller. If the microstimulator does not contract the genioglossus muscle (GGM) adequately, then withdraw the microstimulator while it is still in the outer sheath. Then reposition the microstimulator using the outer sheath and sheath electrode to determine the optimum position. If the response is similar to that evoked using the electrical stimulator and probe, then pull the outer sheath gently up to the second mark on the inner sheath, while holding the inner sheath and microstimulator stationary in the fixed position, so the microstimulator is extruded and placed in position. After the microstimulator is extruded, remove the outer sheath and inner sheath from the patient, and then test the microstimulator again for position near the nerve using the external coil and controller. If the microstimulator has moved after being extruded from the outer sheath (verified by stimulation and poor GGM response while the microstimulator pickup electrodes indicate good coupling), then withdraw the microstimulator by the attached removal loop, and reintroduce using steps 10-13. 
     14. If the microstimulator is in the correct location and is able to stimulate the GGM satisfactorily, then the emerging removal loop is threaded onto a small curved needle and sewn to the subcutaneous tissues. Close the subcutaneous layer with dissolvable sutures and the skin with monofilament nylon sutures. Keep the skin sutures in place for approximately 10 days. 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. For example, while the examples have generally referenced implantation of devices for nerve stimulation to invoke muscle stimulation, it is recognized that the muscle may be stimulated directly. Thus, any stimulation or sensing of any neuro-muscular pathway, i.e., nerve or muscle, with a microdevice, i.e., a microstimulator or microsensor, is applicable to the present invention. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.