# Patent

## Title
System and methods for performing dynamic pedicle integrity assessments

## Abstract

The present invention involves systems and related methods for performing dynamic pedicle integrity assessments involving the use of neurophysiology.

## Background
CROSS-REFERENCES TO RELATED APPLICATIONS
This is a continuation of U.S. patent application Ser. No. 11/061,184 filed on Feb. 18, 2005 by Miles et al., now issued as U.S. Pat. No. 7,657,308, which is a continuation of PCT Patent Application Serial No. PCT/US2004/025550 filed on Aug. 5, 2004 and published as WO/2005013805, which claims priority to: (1) U.S. Provisional Patent Application No. 60/493,024 entitled “Systems and Methods for Performing Pedicle Integrity Assessments During Spinal Surgery” and filed on Aug. 5, 2003, and (2) U.S. Provisional Patent Application No. 60/540,083 entitled “Insulation Sheath for Medical Instruments and Related Methods” and filed on Jan. 28, 2004. The entire contents of these earlier applications are hereby expressly incorporated by reference into this disclosure as if set forth fully herein.


BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to a system and methods generally aimed at surgery. More particularly, the present invention is directed at a system and related methods for performing dynamic pedicle integrity assessments involving the use of neurophysiology.
II. Description of Related Art
A trend in spinal surgery is toward performing surgery in a minimally invasive or minimal access fashion to avoid the trauma of so-called open or “direct access” procedures. A specific area of interest is in the placement of pedicle screws (percutaneous and open), which are typically employed to effect posterior fixation in spinal fusion procedures. While great strides are being made in this area, a risk exists that the pedicle may become breached, cracked, or otherwise compromised due to the formation and/or preparation of the pilot hole (designed to receive a pedicle screw) and/or due to the introduction of the pedicle screw into the pilot hole. If the pedicle (or more specifically, the cortex of the medial wall, lateral wall, superior wall and/or inferior wall) is breached, cracked, or otherwise compromised, the patient may experience pain or neurologic deficit due to unwanted contact between the pedicle screw and exiting nerve roots. This oftentimes necessitates revision surgery, which is disadvantageously painful for the patient and costly, both in terms of recovery time and hospitalization.
Various attempts have been undertaken at performing pedicle integrity assessments. As used herein, the term “pedicle integrity assessment” is defined as detecting or otherwise determining whether a part of a pedicle has been breached, cracked, or otherwise compromised due to the formation and/or preparation of the pilot hole (designed to receive a pedicle screw) and/or due to the introduction of the pedicle screw into the pilot hole. “Formation” is defined as the act of creating an initial pilot hole in a pedicle, such as through the use of a drill or other hole-forming element. “Preparation” is defined as the act of refining or otherwise acting upon the interior of the pilot hole to further prepare it to receive a pedicle screw, such as by introducing a tap or reamer element into the initial pilot hole. “Introduction” is defined as the act of inserting or otherwise placing a pedicle screw into the initially formed and/or prepared pilot hole, such as by screwing the pedicle screw into the pilot hole via a screw driver or similar element.
Among the attempts, X-ray and other imaging systems have been employed, but these are typically quite expensive and are oftentimes limited in terms of resolution such that pedicle breaches may fail to be detected.
Still other attempts involve capitalizing on the insulating characteristics of bone (specifically, that of the walls of the pedicle) and the conductivity of the exiting nerve roots themselves. That is, if a wall of the pedicle is breached, a stimulation signal applied to the pedicle screw and/or the pilot hole (prior to screw introduction) will cause the various muscle groups coupled to the exiting nerve roots to contract. If the pedicle wall has not been breached, the insulating nature of the pedicle will prevent the stimulation signal from innervating the given nerve roots such that the associated muscle groups will not twitch. Traditional EMG monitoring systems may be employed to augment the ability to detect such innervation. A drawback with such prior art systems is that they do not lend themselves to assessing pedicle integrity in a dynamic fashion (that is, during the formation, preparation and/or introduction stages of pedicle screw fixation, whether in open or percutaneous procedures). A similar drawback exists, particularly in open cases, where fluids (e.g. blood and/or interstitial fluid) at or near the pedicle target site may cause shunting as the stimulation signal is applied during the formation, preparation and/or introduction stages of pedicle screw fixation.
The present invention is directed at addressing this need and eliminating, or at least reducing, the effects of the shortcomings of the prior art as described above.
SUMMARY OF THE INVENTION
The present invention overcomes the drawbacks of the prior art by providing, according to one broad aspect of the present invention, a method for performing pedicle integrity assessments, comprising the steps of: (a) establishing electrical communication between a stimulation element and an interior of a pedicle hole during at least one of pilot hole formation, pilot hole preparation, and pedicle screw introduction; (b) applying a stimulation signal to said stimulation element; and (c) monitoring to assess whether nerves adjacent said pedicle are innervating as a result of the step of applying said application of stimulation signal to said stimulation element.
The present invention overcomes the drawbacks of the prior art by providing, according to another broad aspect of the present invention, a system for performing pedicle integrity assessments comprising a medical instrument for use in at least one of pedicle hole formation, hole preparation, and pedicle screw insertion at a pedicle target site, said pedicle target site in the general location of neural structures. A sensor is configured to detect a voltage response from muscles associated with said neural structures. A control unit is coupled to said medical instrument and the sensor, the control unit being configured to (a) transmit a stimulation signal to said medical instrument, (b) receive a voltage response from the sensor, and (c) determine whether said neural structures are innervating as a result of the step of applying said application of stimulation signal to said medical instrument element.
But for the systems and methods of the present invention, patients may be released and subsequently experience pain and/or neurologic deficit due to unwanted contact between the exiting nerve root and misplaced pedicle screws, which oftentimes requires another costly and painful surgery.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of an exemplary surgical system 20 capable of performing dynamic pedicle integrity assessments according to the present invention;
FIG. 2 is a block diagram of the surgical system 20 shown in FIG. 1;
FIG. 3 is a side view illustrating the use of first and second exemplary systems for assessing pedicle integrity according to the present invention;
FIG. 4 is a side view illustrating the use of third and fourth exemplary systems for assessing pedicle integrity according to the present invention;
FIG. 5 is a side view illustrating the use of a fifth and sixth exemplary system for assessing pedicle integrity according to the present invention;
FIG. 6 is a perspective view of the first exemplary system for assessing pedicle integrity according to the present invention as shown in FIG. 3, comprising a K-wire insulator electrically coupled to a handle assembly;
FIG. 7 is a perspective view of the third exemplary system for assessing pedicle integrity according to the present invention as shown in FIG. 4, comprising a universal insulating assembly including a handle assembly coupled to an insulating cannula according to the present invention;
FIG. 8 is a perspective view illustrating an exemplary electrical coupling mechanism capable of being disposed within the handle assembly shown in FIG. 7;
FIGS. 9-11 are perspective views illustrating insulating cannulas of varying sizes and dimensions for use with the handle assembly of FIG. 7 according to the present invention;
FIG. 12 is a perspective view of the fifth exemplary system for assessing pedicle integrity according to the present invention as shown in FIG. 5, comprising a universal coupling assembly having a spring-loaded contact plunger and an electrical cable for electrically connecting the contract plunger to the handle assembly;
FIGS. 13A-13C are top and side views of the universal coupling assembly of FIG. 5, illustrating the universal coupling assembly by itself (FIG. 13A) and coupled to an exemplary tool (FIGS. 13B and 13C);
FIGS. 14-16 are side views of the sixth exemplary sixth system for assessing pedicle integrity according to the present invention as shown in FIG. 5, comprising an insulating sheath dimensioned to be used in an open procedure to prevent current shunting between an instrument delivering electrical stimulation and the surrounding tissues and/or fluids;
FIG. 17 is a graph illustrating a plot of a stimulation current pulse capable of producing a neuromuscular response (EMG) of the type shown in FIG. 18;
FIG. 18 is a graph illustrating a plot of the neuromuscular response (EMG) of a given myotome over time based on a current stimulation pulse (such as shown in FIG. 17) applied to a nerve bundle coupled to the given myotome;
FIG. 19 is an illustrating (graphical and schematic) of a method of automatically determining the maximum frequency (FMax) of the stimulation current pulses according to one embodiment of the present invention;
FIG. 20 is a graph illustrating a plot of EMG response peak-to-peak voltage (Vpp) for each given stimulation current level (IStim) forming a stimulation current pulse according to the present invention (otherwise known as a “recruitment curve”);
FIG. 21 is a graph illustrating a traditional stimulation artifact rejection technique as may be employed in obtaining each peak-to-peak voltage (Vpp) EMG response according to the present invention;
FIG. 22 is a graph illustrating the traditional stimulation artifact rejection technique of FIG. 21, wherein a large artifact rejection causes the EMG response to become compromised;
FIG. 23 is a graph illustrating an improved stimulation artifact rejection technique according to the present invention;
FIG. 24 is a graph illustrating an improved noise artifact rejection technique according to the present invention;
FIG. 25 is a graph illustrating a plot of a neuromuscular response (EMG) over time (in response to a stimulus current pulse) showing the manner in which voltage extrema (VMax or Min), (VMin or Max) occur at times T1 and T2, respectively;
FIG. 26 is a graph illustrating a histogram as may be employed as part of a T1, T2 artifact rejection technique according to an alternate embodiment of the present invention;
FIGS. 27A-27E are graphs illustrating a current threshold-hunting algorithm according to one embodiment of the present invention;
FIG. 28 is a series of graphs illustrating a multi-channel current threshold-hunting algorithm according to one embodiment of the present invention;
FIGS. 29-30 are exemplary screen displays illustrating one embodiment of the pedicle integrity assessment feature of the present invention; and
FIGS. 31-33 are exemplary screen displays illustrating another embodiment of the pedicle integrity assessment feature of the present invention.



DESCRIPTION OF THE SPECIFIC EMBODIMENTS
Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. The systems disclosed herein boast a variety of inventive features and components that warrant patent protection, both individually and in combination.
The present invention is directed at performing dynamic pedicle integrity assessments. In its most fundamental form, the present invention involves establishing electrical communication between a stimulation source and the interior of a pedicle hole during the hole formation, hole preparation, and/or screw introduction steps of pedicle screw fixation. By applying a stimulation signal (such as the current-controlled signal described below) during these steps, and monitoring the neuro-muscular responses resulting from this stimulation, the system of the present invention can automatically detect and communicate to the user whether the integrity of the pedicle has been compromised during to the steps of hole formation, hole preparation and/or screw introduction. In so doing, the present invention advantageously allows the surgeon to immediate appreciate the breach or potential breach of the pedicle and correct the screw placement. This avoids the problem of patients being released only to subsequently experience pain and/or neurologic deficit due to unwanted contact between the exiting nerve root and misplaced pedicle screws.
Although shown and described within the context of a particular exemplary system having a stimulation source and monitoring capacity, it will be appreciated by those skilled in the art that any number of systems for providing a stimulation signal and for monitoring to assess pedicle breach may be employed without departing from the scope of the present invention.
FIGS. 1-2 illustrate, by way of example only, a surgical system 20 provided in accordance with a broad aspect of the present invention. The surgical system 20 includes a control unit 22, a patient module 24, an EMG harness 26 and return electrode 28 coupled to the patient module 24, and a host of pedicle screw test accessories 30 capable of being coupled to the patient module 24 via an accessory cable 32 in combination with a handle assembly 36. In the embodiment shown, the pedicle screw test accessories 30 include (by way of example only) a K-wire insulator 34, a universal insulating assembly 38, and a universal electrical coupler 35. As will be described in greater detail below, a K-wire 37 and a tap member 39 are shown, by way of example, as exemplary stimulation elements according to the present invention. The K-wire 37 may be electrically coupled to the control unit 22 and/or patient module 24 (so as to receive a stimulation signal) through the use of the K-wire insulator 34, the universal insulating assembly 38 and/or the electrical coupler 35 (provided the K-wire 37 is insulated in some manner if used in percutaneous procedure). The tap member 39 may be electrically coupled to the control unit 22 and/or patient module 24 (so as to receive a stimulation signal) through the use of the universal insulating assembly 38, the electrical coupler 35 (provided the tap member 39 is insulated in some manner if used in a percutaneous procedure) and/or by bringing a stimulation element into contact with the tap member 39, such as by (for example) providing a longitudinal cannulation within the tap member 39 and disposing an electrically coupled K-wire 37 therein.
The control unit 22 includes a touch screen display 40 and a base 42, which collectively contain the essential processing capabilities for controlling the surgical system 20. The patient module 24 is connected to the control unit 22 via a data cable 44, which establishes the electrical connections and communications (digital and/or analog) between the control unit 22 and patient module 24. The main functions of the control unit 22 include receiving user commands via the touch screen display 40, activating stimulation, processing signal data according to defined algorithms (described below), displaying received parameters and processed data, and monitoring system status and reporting fault conditions. The touch screen display 40 is preferably equipped with a graphical user interface (GUI) capable of communicating information to the user and receiving instructions from the user. The display 40 and/or base 42 may contain patient module interface circuitry that commands the stimulation sources, receives digitized signals and other information from the patient module 24, processes the EMG responses to extract characteristic information for each muscle group, and displays the processed data to the operator via the display 40.
As will be described in greater detail below, the surgical system 20 is capable of performing pedicle integrity assessments in a dynamic fashion, that is, during the formation and/or preparation of the pilot hole and/or during pedicle screw placement. Surgical system 20 accomplishes this by having the control unit 22 and patient module 24 cooperate to send stimulation signals to one or more stimulation electrodes or electrode regions on the various pedicle screw test accessories 30. Depending upon effect of pilot hole formation, pilot hole preparation and/or pedicle screw introduction (namely, on the bone forming the pedicle), the stimulation signals may cause nerves adjacent to or in the general proximity of the K-wire 37 and/or tap member 39 to innervate, which, in turn, can be monitored via the EMG harness 26. The pedicle integrity assessment feature of the present invention are based on assessing the evoked response of the various muscle myotomes monitored by the surgical system 20 via EMG harness 26.
The accessory handle assembly 36 includes a cable 55 for establishing electrical communication with the patient module 24 (via the accessory cable 32). In a preferred embodiment, each pedicle screw test accessory 30 (namely, K-wire insulator 34, universal insulating assembly 38, and electrical coupler 35) includes a proximal electrical connector 56, a distal electrical connector (described below), and an electrical cable 57 extending therebetween. The proximal electrical connector 56 is preferably threaded and designed to engage with the distal end 59 of the handle assembly 36. In this fashion, the screw test accessories 30 may be quickly and easily coupled (electrically and mechanically) to the accessory handle assembly 36. The distal electrical connector of the K-wire insulator 34 and universal insulating assembly 38 may comprise any number of suitable mechanisms for establishing electrical communication with an instrument passing therethrough (such as a K-wire 37 passing through the K-wire insulator 34 and/or the universal insulating assembly 38, and such as a tap member 39 extending through the universal insulating assembly 38). In a preferred embodiment, the distal electrical connectors within the universal insulating assembly 38 will be capable of expanding, moving or otherwise accommodating instruments of varying diameters according to the present invention. The distal electrical connector of the coupler 35 may include any number of suitable electrode or electrode regions (including protrusions) on or about the distal (or pinching) ends of the clamp arms 61 forming the coupler 35. Corresponding regions (such as electrodes or electrode regions—including indentations) may be provided on the K-wire 37, the tap member 39, such as where such devices are to be directly coupled to the handle assembly 36 (i.e. where K-wire 37 and/or tap member 39 are disposed through insulating elements that do not include distal electrical connectors, for percutaneous procedures) according to the present invention.
In all situations, the user may operate one or more buttons of the handle assembly 36 to selectively initiate a stimulation signal (preferably, a current signal) from the patient module 24 to the pedicle probe 56. According to the present invention, this stimulation signal is applied with the K-wire 37 and/or tap member 39 touching the interior wall during the formation and/or preparation of the pilot hole and/or with the K-wire 37 touching a pedicle screw during introduction. This serves to test the integrity of the wall(s) of the pedicle. That is, a breach or compromise in the integrity of the pedicle will allow the stimulation signal to pass through the pedicle and innervate an adjacent nerve root. By monitoring the myotomes associated with the nerve roots (via the EMG harness 26 and recording electrode 27) and assessing the resulting EMG responses (via the control unit 22), the surgical system 20 can assess whether a pedicle breach occurred during hole formation, preparation and/or screw introduction. If a breach or potential breach is detected, the user may simply withdraw the misplaced pedicle screw and redirect to ensure proper placement.
FIG. 3 illustrates two exemplary manners of performing pedicle integrity assessments according to the present invention (both percutaneous by way of example), one employing the K-wire insulator 34 and one employing the electrical coupler 35. With combined reference with FIG. 6, the K-wire insulator 34 according to the present invention includes an elongate insulating body 60 having a tapered distal end 63, open distal and proximal ends, and a lumen or cannulation extending therebetween dimensioned to receive and pass the K-wire 37. A cap element 64 is provided for placement in the proximal end of the insulating body 60. The cap element 64 has a lumen therewithin dimensioned to pass the K-wire 37 and includes the distal electrical connector (not shown) coupled to the electrical cable 57. As shown in FIG. 3, the K-wire insulator 34 may be advanced to the pedicle target site in a percutaneous fashion, by either establishing a virgin approach to the pedicle target site or by passing through a previously established percutaneous corridor (such as may be left or formed by commercially available percutaneous pedicle screw placement systems). This process may be facilitated by first establishing a pilot hole through the use of a so-called Jam-Sheede needle (comprising an inner rigid needle element disposed within a rigid outer needle element), after which point the inner rigid needle element is removed such that the K-wire 37 may be introduced into the pilot hole. The outer rigid needle element of the Jam-Sheede device may then be removed, leaving the K-wire 37 in place. The K-wire insulator 34 may then be advanced over the K-wire 37. Once the distal end 63 of the K-wire insulator 34 abuts the opening of the pedicle pilot hole, buttons 64 on the handle member 36 may be employed to apply the stimulation signal to the K-wire 37. In this fashion, the majority of the K-wire 37 is insulated from the surrounding tissue, while the distal end of the K-wire 37 may be brought into direct contact with the pilot hole to perform pedicle integrity assessments according to one embodiment of the present invention. As will be appreciated, this same technique could be employed to bring the stimulation electrode or electrode region of the K-wire 37 into contact with a portion of a fully inserted pedicle screw (not shown). As will also be appreciated, the outer rigid needle element of a Jam-Sheede device may itself be insulated, such that the K-wire 37 may be disposed through the Jam-Sheede needle and still perform according to the present invention.
FIG. 3 also illustrates that the electrical coupler 35 may be employed to perform pedicle integrity assessments, by way of example only, by establishing electrical communication between the tap member 39 during preparation of the pedicle hole. The electrical coupler 35 accomplishes this by engaging the electrode or electrode regions on the opposing clamping arms 61 against a portion of the proximal end of the tap member 39. To facilitate this, the tap member 39 may be equipped with indentations or similar features for matingly engaging with corresponding features on the distal regions of the clamping arms 61. In the embodiment shown, an insulated cannula 66 is provided for insulating all but the exposed distal and proximal ends of the tap member 39 (due to the percutaneous approach shown by way of example). As with the body 60 of the K-wire insulator 34, the insulated cannula 66 is preferably equipped with a tapered distal end 67. In use, the tap member 39 will be advanced through the insulated cannula 66 (such as by being passed over a K-wire 37 via an internal cannulation) and rotated to prepare threads along the interior of the pilot hole. During preparation of the pilot hole in this manner, the handle member 36 may be used to apply the stimulation signal to the electrical coupler 35 which, in turn, transmits this stimulation signal to the interior of the pilot hole to perform pedicle integrity assessments according to another embodiment of the present invention. If the pedicle has not been breached during the preparation process, the tap member 39 may then be removed and a pedicle screw introduced into the prepared pilot hole. By selecting a pedicle screw having the same approximate characteristics (i.e. pitch, thread height, diameter, length, etc. . . . ) as the tapping (distal) portion of the tap member 39, the need to perform further pedicle integrity assessments during the introduction of the pedicle screw may be obviated or minimized.
FIG. 4 illustrates two more exemplary manners of performing pedicle integrity assessments according to the present invention (both percutaneous by way of example), one employing the universal insulating assembly 38 and one employing the electrical coupler 35. With combined reference to FIGS. 7-11, the universal insulating assembly 38 includes a handle assembly 68 and an insulated cannula 70 extending from the distal portion of the handle assembly 68. As best seen in FIG. 7, the handle assembly 68 includes a housing member 71 and an electrical connector port 72 for connection with the electrical cable 57. With reference to FIG. 8, the housing member 71 contains a universal electrical coupling mechanism 73 comprising, by way of example, a plurality of contact elements 74 (in this case springs extending between posts 75). A lumen 76 is provided (by way of example only) in the approximate center of (and extending between) upper and lower base members 77. The contact elements 74 are positioned in a transverse fashion such that they intersect generally in the same plane as the center of the lumen 76. In this fashion, any metallic or conductive instrument passed through the lumen 76 will be brought into contact with the contact elements 74, thereby providing the ability to apply an electrical signal to the instrument. Moreover, the contact elements 74 are capable of moving, expanding, or otherwise accommodating instruments having a variety of diameters. As best shown in FIGS. 9-11, the insulated cannula 70 may be provided having any number of different lengths and widths, depending upon the device to be passed through it. A threaded base member 78 is preferably coupled to each insulated cannula 70 to facilitate coupling the particular insulated cannula 70 to a corresponding threaded portion on the distal region of the housing member 71. In this fashion, a surgeon may quickly and easily change between any of a variety of insulating cannulas 70 depending upon the application (i.e. depth to the pedicle target site) and the device to be passed therethrough (i.e. the tap member 39 as shown in FIG. 4).
The insulating cannula 70 serves to isolate a portion of the instrument as it is passed through the handle assembly 68. In this fashion, the insulating cannula 70 may be advanced to a pedicle target site, such as to the opening of a pedicle pilot hole as shown in FIG. 4. Although not shown, it is to be readily appreciated that the present invention also contemplates advancing the distal end of the insulating cannula 70 over or in general abutment with a proximal portion of a percutaneously placed pedicle screw pedicle screw. In either instance, an instrument or device (such as, by way of example, K-wire 37 or the tap member 39, depending upon the situation) may be passed through the handle member 68 during the steps of hole formation, hole preparation and/or screw introduction. The insulating cannulas 70 are of varying size depending upon the particular target site and surgical application, but may preferably be provided ranging from 0 inches to 24 inches in length and of any diameter suitable to pass the instrument of interest.
FIG. 4 also illustrates a variant of the embodiment shown in FIG. 3, except that the insulated cannula 66 is specifically dimensioned to pass the K-wire 37, as opposed to larger diameter instruments such as the tap member 39 as shown in FIG. 3. In this instance, the electrical coupler 35 may be used to establish electrical communication between the K-wire 37 and the interior of a pedicle hole. With the distal end of the K-wire 37 in such electrical communication with the interior of the pilot hole, the handle assembly 36 may be employed to apply the stimulation signal to perform a pedicle integrity assessment according to the present invention. Placement of the K-wire 37 within the pilot hole, and the advancement of the insulated cannula 66, may be the same as described above with reference to the Jam-Sheede device described above.
FIG. 5 illustrates an alternate embodiment of the universal electrical coupler 35 shown above (designated 35′ for clarity), employed in both a percutaneous procedure (on the left) and an open procedure (on the right). With brief reference to FIGS. 12-13, the electrical coupler 35′ comprises a contact plunger 41 disposed within a housing member 43. The plunger 41 is generally cylindrical in shape and includes an electrical contact region 45 at its distal end, as best viewed in FIGS. 13A-13C. The electrical contact region 45 is electrically coupled to the electrical cable 57, which extends from the proximal end of the plunger 41 (through a hole in the proximal end of the housing member 43). The plunger 41 is preferably spring-loaded relative to the housing member 43 such that it is normally biased into a closed position (best seen in FIG. 13A). The housing member 43 includes a generally cylindrical body 47 (dimensioned to slideably receive part or all of the plunger 41 therein) and a distal arm 49. The distal arm 49 includes a generally arcuate interior surface 51, and the electrical contact region 45 includes a generally arcuate surface 53 (both viewed best in FIG. 13A).
To use the coupler 35′, a user need only pull on the electrical cable 57 (which may be reinforced and/or provided with a strain-relief as necessary) to retract the plunger 41, thereby opening a region between the surface 51 of arm 49 and surface 53 of electrical contact region 45. With the plunger 41 in this retracted state, any number of generally cylindrical devices (as shown generically at 65 in FIGS. 13B-13C) may be positioned within this open region. The plunger 41 may thereafter be released, which under spring-loading pushes the electrical contact region 45 of the plunger 41 into abutment (and electrical communication) with the device 65. It will be appreciated that device 65 may comprise any number of suitable instruments for use in the hole formation, hole preparation and/or screw introduction processes of pedicle screw fixation (such as, by way of example only, the K-wire 37 and/or the tap 39 shown in FIG. 5).
Returning to FIG. 5, once the electrical coupler 35′ of the present invention is coupled to the particular instrument, such as the K-wire 37 and/or the tap 39, it may be used as follows in accordance with the present invention. In the percutaneous procedure, the electrical coupler 35′ is employed to establish electrical communication between the tap member 39 during preparation of the pedicle hole. In the embodiment shown, an insulated cannula 66 is provided for insulating all but the exposed distal and proximal ends of the tap member 39 (due to the percutaneous approach shown by way of example). As with the body 60 of the K-wire insulator 34, the insulated cannula 66 is preferably equipped with a tapered distal end 67. In use, the tap member 39 will be advanced through the insulated cannula 66 (such as by being passed over a K-wire 37 via an internal cannulation) and rotated to prepare threads along the interior of the pilot hole. During preparation of the pilot hole in this manner, the handle member 36 may be used to apply the stimulation signal to the electrical coupler 35′ which, in turn, transmits this stimulation signal to the interior of the pilot hole during preparation to perform pedicle integrity assessments according to another embodiment of the present invention. If the pedicle is not breached as preparation progresses, then tap member 39 may be removed and a pedicle screw introduced into the prepared pilot hole. By selecting a pedicle screw having the same approximate characteristics (i.e. pitch, thread height, diameter, length, etc. . . . ) as the tapping (distal) portion of the tap member 39, the need to perform further pedicle integrity assessments during the introduction of the pedicle screw may be obviated or minimized.
In an open procedure (or minimally invasive relative to the percutaneous approach described above), the electrical coupler 35′ may—according to a further aspect of the present invention—be employed an insulating sheath 110. With brief reference to FIGS. 14-16, the insulation sheath 110 includes an elongated sheath 120 and a molded expandable seal-tip 130, which collectively provide the ability to insulate an electrified medical instrument (e.g. the K-wire 37 in FIG. 5 or the awl 180 in FIGS. 14-16) so as to avoid unwanted shunting according to this aspect of the present invention. To accomplish this, a continuous lumen is provided extending between a proximal opening 124 formed in the elongated sheath 120 and a distal opening 136 formed in the seal tip 130. In use, described by way of example with reference to awl 180, the tip 186 of the awl 180 is advanced into the proximal opening 124, through the inner lumen, and out the distal opening 136 in the seal tip 130. When positioned as such, all regions of the awl 180 (except the distal tip 186) will be insulated from fluids (e.g. blood and/or interstitial fluid) during the hole formation step, with the shaft 184 disposed within the lumen of the sheath 110 and the handle 182 disposed outside the sheath 110. It will be appreciated that, while described with reference to an awl, the insulation sheath 110 of the present invention may be employed with any number of different instruments and may find use during the steps of hole preparation and/or screw introduction as well.
The elongated sheath 120 may be comprised of any material capable of providing a flexible layer of protection from bodily fluids and insulation from electric current. By way of example only, elongated sheath 120 may comprise a thin polyurethane film. Alternative embodiments of elongated sheath 120 may be composed of a variety of insulative materials, including but not limited to rubber, plastic, resilient polymers, elastomers, polyesters, polyimides, silicicones, fluoropolymers, and teflon. Preferably, elongated sheath 120 is flexible and/or compressible to accommodate different sizes of medical instruments and the areas of the body in which said instruments are used. Elongated sheath 120 may be made by seam welding at least one flat polyurethane (or other suitable material) sheet to make an enclosed corridor of various shapes. In such a case, elongated sheath 120 will include at least one seam along the length of the corridor. Preferably, the elongated sheath 120 is of sufficient length such that proximal end 122 will begin to bunch up (due to its flexible character) once the medical instrument is fully inserted into sheath 110 (e.g. once the distal tip of the instrument is protruding from distal opening 136). Distal opening 136 is preferably resilient yet pliable enough so that it will remain generally flush against shaft of the medical instrument such that a seal preventing leakage of fluid inside sheath 110 is formed.
As noted above, the system 20 described generally above is exemplary of a system including a stimulation source and monitoring capacity for use in performing pedicle integrity assessment according to the present invention. It will be appreciated by those skilled in the art, however, that any number of systems for providing a stimulation signal and for monitoring to assess pedicle breach may be employed without departing from the scope of the present invention. That said, the following discussion elaborates on the particular algorithms and principles behind the neurophysiology for performing pedicle integrity assessments according to the exemplary embodiment shown (system 20 of FIGS. 1-2) according to the present invention.
FIGS. 17 and 18 illustrate a fundamental aspect of the present invention: a stimulation signal (FIG. 17) and a resulting evoked response (FIG. 18). By way of example only, the stimulation signal is preferably a stimulation current signal (IStim) having rectangular monophasic pulses with a frequency and amplitude adjusted by system software. In a still further preferred embodiment, the stimulation current (IStim) may be coupled in any suitable fashion (i.e. AC or DC) and comprises rectangular monophasic pulses of 200 microsecond duration. The amplitude of the current pulses may be fixed, but will preferably sweep from current amplitudes of any suitable range, such as from 2 to 100 mA. For each nerve and myotome there is a characteristic delay from the stimulation current pulse to the EMG response (typically between 5 to 20 ms). To account for this, the frequency of the current pulses is set at a suitable level such as, in a preferred embodiment, 4 Hz to 10 Hz (and most preferably 4.5 Hz), so as to prevent stimulating the nerve before it has a chance to recover from depolarization. The EMG response shown in FIG. 18 can be characterized by a peak-to-peak voltage of Vpp=Vmax−Vmin.
FIG. 19 illustrates an alternate manner of setting the maximum stimulation frequency, to the extent it is desired to do so rather than simply selecting a fixed maximum stimulation frequency (such as 4.5 Hz) as described above. According to this embodiment, the maximum frequency of the stimulation pulses is automatically adjusted. After each stimulation, Fmax will be computed as: Fmax=1/(T2+TSafety Margin) for the largest value of T2 from each of the active EMG channels. In one embodiment, the Safety Margin is 5 ms, although it is contemplated that this could be varied according to any number of suitable durations. Before the specified number of stimulations, the stimulations will be performed at intervals of 100-120 ms during the bracketing state, intervals of 200-240 ms during the bisection state, and intervals of 400-480 ms during the monitoring state. After the specified number of stimulations, the stimulations will be performed at the fastest interval practical (but no faster than Fmax) during the bracketing state, the fastest interval practical (but no faster than Fmax/2) during the bisection state, and the fastest interval practical (but no faster than Fmax/4) during the monitoring state. The maximum frequency used until Fmax is calculated is preferably 10 Hz, although slower stimulation frequencies may be used during some acquisition algorithms. The value of Fmax used is periodically updated to ensure that it is still appropriate. For physiological reasons, the maximum frequency for stimulation will be set on a per-patient basis. Readings will be taken from all myotomes and the one with the slowest frequency (highest T2) will be recorded.
A basic premise behind the neurophysiology employed in the present invention is that each nerve has a characteristic threshold current level (IThresh) at which it will depolarize. Below this threshold, current stimulation will not evoke a significant EMG response (Vpp). Once the stimulation threshold (IThresh) is reached, the evoked response is reproducible and increases with increasing stimulation until saturation is reached. This relationship between stimulation current and EMG response may be represented graphically via a so-called “recruitment curve,” such as shown in FIG. 20, which includes an onset region, a linear region, and a saturation region. By way of example only, the present invention defines a significant EMG response to have a Vpp of approximately 100 uV. In a preferred embodiment, the lowest stimulation current that evokes this threshold voltage (VThresh) is called IThresh. As will be described in greater detail below, changes in the current threshold (IThresh) may be indicative of a change in the degree of electrical communication between a stimulation electrode and a nerve. This is helpful in assessing if a screw or similar instrument has inadvertently breached the cortex of a pedicle. More specifically, where an initial determination of (IThresh), such as by applying a stimulation current to the interior of a hole during formation and/or preparation, is greater than a later determination of (IThresh), such as by applying a stimulation current to the pedicle screw during insertion, the decrease in IThresh, if large enough, may indicate electrical communication between the pedicle screw and the nerve. Based on the insulation properties of bone, such electrical communication would indicate a breach of the pedicle.
In order to obtain this useful information, the present invention must first identify the peak-to-peak voltage (Vpp) of each EMG response corresponding a given stimulation current (IStim). The existence stimulation and/or noise artifacts, however, can conspire to create an erroneous Vpp measurement of the electrically evoked EMG response. To overcome this challenge, the surgical system 20 of the present invention may employ any number of suitable artifact rejection techniques, including the traditional stimulation artifact rejection technique shown in FIG. 21. Under this technique, stimulation artifact rejection is undertaken by providing a simple artifact rejection window T1WIN at the beginning of the EMG waveform. During this T1 window, the EMG waveform is ignored and Vpp is calculated based on the max and min values outside this window. (T1 is the time of the first extremum (min or max) and T2 is the time of the second extremum.) In one embodiment, the artifact rejection window T1WN may be set to about 7.3 msec. While generally suitable, there are situations where this stimulation artifact rejection technique of FIG. 21 is not optimum, such as in the presence of a large stimulation artifact (see FIG. 22). The presence of a large stimulation artifact causes the stimulation artifact to cross over the window T1WIN and blend in with the EMG. Making the stimulation artifact window larger is not effective, since there is no clear separation between EMG and stimulation artifact.
FIG. 23 illustrates a stimulation artifact rejection technique according to the present invention, which solves the above-identified problem with traditional stimulation artifact rejection. Under this technique, a T1 validation window (T1−VWIN) is defined immediately following the T1 window (T1WIN). If the determined Vpp exceeds the threshold for recruiting, but T1 falls within this T1 validation window, then the stimulation artifact is considered to be substantial and the EMG is considered to have not recruited. An operator may be alerted, based on the substantial nature of the stimulation artifact. This method of stimulation artifact rejection is thus able to identify situations where the stimulation artifact is large enough to cause the Vpp to exceed the recruit threshold. To account for noise, the T1 validation window (T1−VWIN) should be within the range of 0.1 ms to 1 ms wide (preferably about 0.5 ms). The T1 validation window (T1−VWIN) should not be so large that the T1 from an actual EMG waveform could fall within.
FIG. 24 illustrates a noise artifact rejection technique according to the present invention. When noise artifacts fall in the time window where an EMG response is expected, their presence can be difficult to identify. Artifacts outside the expected response window, however, are relatively easy to identify. The present invention capitalizes on this and defines a T2 validation window (T2−VWIN) analogous to the T1 validation window (T1−VWIN) described above with reference to FIG. 23. As shown, T2 must occur prior to a defined limit, which, according to one embodiment of the present invention, may be set having a range of between 40 ms to 50 ms (preferably about 47 ms). If the Vpp of the EMG response exceeds the threshold for recruiting, but T2 falls beyond the T2 validation window (T2−VWIN), then the noise artifact is considered to be substantial and the EMG is considered to have not recruited. An operator may be alerted, based on the substantial nature of the noise artifact.
FIG. 25 illustrates a still further manner of performing stimulation artifact rejection according to an alternate embodiment of the present invention. This artifact rejection is premised on the characteristic delay from the stimulation current pulse to the EMG response. For each stimulation current pulse, the time from the current pulse to the first extremum (max or min) is T1 and to the second extremum (max or min) is T2. As will be described below, the values of T1, T2 are each compiled into a histogram period (see FIG. 26). New values of T1, T2 are acquired for each stimulation and the histograms are continuously updated. The value of T1 and T2 used is the center value of the largest bin in the histogram. The values of T1, T2 are continuously updated as the histograms change. Initially Vpp is acquired using a window that contains the entire EMG response. After 20 samples, the use of T1, T2 windows is phased in over a period of 200 samples. Vmax and Vmin are then acquired only during windows centered around T1, T2 with widths of, by way of example only, 5 msec. This method of acquiring Vpp automatically rejects the artifact if T1 or T2 fall outside of their respective windows.
Having measured each Vpp EMG response (as facilitated by the stimulation and/or noise artifact rejection techniques described above), this Vpp information is then analyzed relative to the stimulation current in order to determine a relationship between the nerve and the given stimulation element transmitting the stimulation current. More specifically, the present invention determines these relationships (between nerve and the stimulation element) by identifying the minimum stimulation current (IThresh) capable of resulting in a predetermined Vpp EMG response. According to the present invention, the determination of IThresh may be accomplished via any of a variety of suitable algorithms or techniques.
FIGS. 27A-27E illustrate, by way of example only, a threshold-hunting algorithm for quickly finding the threshold current (IThresh) for each nerve being stimulated by a given stimulation current (IStim). Threshold current (IThresh), once again, is the minimum stimulation current (IStim) that results in a Vpp that is greater than a known threshold voltage (VThresh). The value of is adjusted by a bracketing method as follows. The first bracket is 0.2 mA and 0.3 mA. If the Vpp corresponding to both of these stimulation currents is lower than VThresh, then the bracket size is doubled to 0.2 mA and 0.4 mA. This doubling of the bracket size continues until the upper end of the bracket results in a Vpp that is above VThresh. The size of the brackets is then reduced by a bisection method. A current stimulation value at the midpoint of the bracket is used and if this results in a Vpp that is above VThresh, then the lower half becomes the new bracket.
Likewise, if the midpoint Vpp is below VThresh then the upper half becomes the new bracket. This bisection method is used until the bracket size has been reduced to IThresh mA. IThresh may be selected as a value falling within the bracket, but is preferably defined as the midpoint of the bracket.
The threshold-hunting algorithm of this embodiment will support three states: bracketing, bisection, and monitoring. A stimulation current bracket is a range of stimulation currents that bracket the stimulation current threshold IThresh. The width of a bracket is the upper boundary value minus the lower boundary value. If the stimulation current threshold IThresh of a channel exceeds the maximum stimulation current, that threshold is considered out-of-range. During the bracketing state, threshold hunting will employ the method below to select stimulation currents and identify stimulation current brackets for each EMG channel in range.
The method for finding the minimum stimulation current uses the methods of bracketing and bisection. The “root” is identified for a function that has the value −1 for stimulation currents that do not evoke adequate response; the function has the value +1 for stimulation currents that evoke a response. The root occurs when the function jumps from −1 to +1 as stimulation current is increased: the function never has the value of precisely zero. The root will not be known exactly, but only with a level of precision related to the minimum bracket width. The root is found by identifying a range that must contain the root. The upper bound of this range is the lowest stimulation current IThresh where the function returns the value +1, i.e. the minimum stimulation current that evokes response. The lower bound of this range is the highest stimulation current IThresh where the function returns the value −1, i.e. the maximum stimulation current that does not evoke a response.
The pedicle integrity assessment function may begin by adjusting the stimulation current until the root is bracketed (FIG. 27B). The initial bracketing range may be provided in any number of suitable ranges. In one embodiment, the initial bracketing range is 0.2 to 0.3 mA. If the upper stimulation current does not evoke a response, the upper end of the range should be increased. The range scale factor is 2. The stimulation current should preferably not be increased by more than 10 mA in one iteration. The stimulation current should preferably never exceed the programmed maximum stimulation current. For each stimulation, the algorithm will examine the response of each active channel to determine whether it falls within that bracket. Once the stimulation current threshold of each channel has been bracketed, the algorithm transitions to the bisection state.
During the bisection state (FIGS. 27C and 27D), threshold hunting will employ the method described below to select stimulation currents and narrow the bracket to a selected width (for example, 0.1 mA) for each EMG channel with an in-range threshold. After the minimum stimulation current has been bracketed (FIG. 27B), the range containing the root is refined until the root is known with a specified accuracy. The bisection method is used to refine the range containing the root. In one embodiment, the root should be found to a precision of 0.1 mA. During the bisection method, the stimulation current at the midpoint of the bracket is used. If the stimulation evokes a response, the bracket shrinks to the lower half of the previous range. If the stimulation fails to evoke a response, the bracket shrinks to the upper half of the previous range. The proximity algorithm is locked on the electrode position when the response threshold is bracketed by stimulation currents separated by the selected width (i.e. 0.1 mA). The process is repeated for each of the active channels until all thresholds are precisely known. At that time, the algorithm enters the monitoring state.
After identifying the threshold current IThresh, this information may be employed to determine any of a variety of relationships between the screw test accessory and the nerve. For example, as will be described in greater detail below, when determining the current threshold Ithresh of a nerve during pedicle integrity assessment, the relationship between the pedicle testing assembly 36 and the nerve is whether electrical communication is established therebetween. If electrical communication is established, this indicates that the medial wall of the pedicle has been cracked, stressed, or otherwise breached as a result of pilot hole formation, pilot hole preparation, and/or screw introduction. If not, this indicates that the integrity of the medial wall of the pedicle has remained intact. This characteristic is based on the insulating properties of bone.
In a significant aspect of the present invention, the relationships determined above based on the current threshold determination may be communicated to the user in an easy to use format, including but not limited to, alpha-numeric and/or graphical information regarding pedicle integrity assessments, stimulation level, EMG responses, instrument in use, set-up, and related instructions for the user. This advantageously provides the ability to present simplified yet meaningful data to the user, as opposed to the actual EMG waveforms that are displayed to the users in traditional EMG systems. Due to the complexity in interpreting EMG waveforms, such prior art systems typically require an additional person specifically trained in such matters which, in turn, can be disadvantageous in that it translates into extra expense (having yet another highly trained person in attendance) and oftentimes presents scheduling challenges because most hospitals do not retain such personnel.
When employed in lumbar spinal procedures, for example, such EMG monitoring would preferably be accomplished by connecting the EMG harness 26 to the myotomes in the patient's legs corresponding to the exiting nerve roots associated with the particular spinal operation level. (This may similarly be performed during cervical spinal procedures by employing the system of the present invention to monitor the myotomes on the arms of the patient). In a preferred embodiment, this is accomplished via 8 pairs of EMG electrodes 27 placed on the skin over the major muscle groups on the legs (four per side), an anode electrode 29 providing a return path for the stimulation current, and a common electrode 31 providing a ground reference to pre-amplifiers in the patient module 24. Although not shown, it will be appreciated that any of a variety of electrodes can be employed, including but not limited to needle electrodes. The EMG responses measured via the EMG harness 26 provide a quantitative measure of the nerve depolarization caused by the electrical stimulus. By way of example, the placement of EMG electrodes 27 may be undertaken according to the manner shown in Table 1 below for spinal surgery:












TABLE 1







Color
Channel ID
Myotome
Spinal Level









Blue
Right 1
Right Vastus Medialis
L2, L3, L4



Violet
Right 2
Right Tibialis Anterior
L4, L5



Grey
Right 3
Right Biceps Femoris
L5, S1, S2



White
Right 4
Right Gastroc. Medial
S1, S2



Red
Left 1
Left Vastus Medialis
L2, L3, L4



Orange
Left 2
Left Tibialis Anterior
L4, L5



Yellow
Left 3
Left Biceps Femoris
L5, S1, S2



Green
Left 4
Left Gastroc. Medial
S1, S2










With reference again to FIGS. 1-2, the surgical system 20 performs pedicle integrity assessments via, by way of example only, the use of pedicle testing accessories 30 in combination with the handle assembly 36. More specifically, upon pressing the button on the screw test handle 36, the software will execute a testing algorithm to apply a stimulation current to the particular target (i.e. pilot hole formation instruments and/or preparation instruments and/or the pedicle screw introduction instruments), setting in motion the pedicle integrity assessment function of the present invention. The pedicle integrity assessment features of the present invention may include, by way of example only, an “Actual” mode (FIGS. 29-30) for displaying the actual stimulation threshold 91 measured for a given myotome, as well as a “Relative” mode (FIGS. 31-32) for displaying the difference 92 between a baseline stimulation threshold assessment 93 of a bare nerve root and an actual stimulation threshold assessment 91 for a given myotome. In either case, the surgical accessory label 84 displays the word “SCREW TEST” to denote use of the pedicle testing assembly 36 for performing pedicle integrity assessments. The screw test algorithm according to the present invention preferably determines the depolarization (threshold) current for all responding EMG channels. In one embodiment, the EMG channel tabs 82 may be configured such that the EMG channel having the lowest stimulation threshold will be automatically enlarged and/or highlighted and/or colored (EMG channel tab R3 as shown in FIG. 29) to clearly indicate this fact to the user. As shown in FIG. 25, this feature may be overridden by manually selecting another EMG channel tab (such as EMG channel tab R1 in FIG. 30) by touching the particular EMG channel tab 82 on the touch screen display 40. In this instance, a warning symbol 94 may be provided next to the EMG channel tab having the lowest stimulation threshold (once again, EMG channel tab R3 in FIG. 29) to inform the user that the stimulation threshold 91 is not the lowest stimulation threshold.
Any number of the above-identified indicia (such as the baseline stimulation 93, actual stimulation 91, difference 92, and EMG channel tabs 82) may be color-coded to indicate general safety ranges (i.e. “green” for a range of stimulation thresholds above a predetermined safe value, “red” for range of stimulation thresholds below a predetermined unsafe value, and “yellow” for the range of stimulation thresholds in between the predetermined safe and unsafe values—designating caution). In one embodiment, “green” denotes a stimulation threshold range of 9 milliamps (mA) or greater, “yellow” denotes a stimulation threshold range of 6-8 mA, and “red” denotes a stimulation threshold range of 6 mA or below. By providing this information graphically, a surgeon may quickly and easily test to determine if the integrity of a pedicle has been breached or otherwise compromised, such as may result due to the formation and/or preparation of a pedicle screw hole and/or introduction of a pedicle screw. More specifically, if after stimulating the screw hole (during formation and/or preparation) and/or pedicle screw itself (during introduction) the stimulation threshold is: (a) at or below 6 mA, the threshold display 40 will illuminate “red” and thus indicate to the surgeon that a breach is likely; (b) between 6 and 8 mA, the threshold display 40 will illuminate “yellow” and thus indicate to the surgeon that a breach is possible; and/or (c) at or above 8 mA, the threshold display 40 will illuminate “green” and thus indicate to the surgeon that a breach is unlikely. If a breach is possible or likely (that is, “yellow” or “red”), the surgeon may choose to withdraw the pedicle screw and redirect it along a different trajectory to ensure the pedicle screw no longer breaches (or comes close to breaching) the medial wall of the pedicle.
While this invention has been described in terms of a best mode for achieving this invention's objectives, it will be appreciated by those skilled in the art that variations may be accomplished in view of these teachings without deviating from the spirit or scope of the present invention. For example, the present invention may be implemented using any combination of computer programming software, firmware or hardware. As a preparatory step to practicing the invention or constructing an apparatus according to the invention, the computer programming code (whether software or firmware) according to the invention will typically be stored in one or more machine readable storage mediums such as fixed (hard) drives, diskettes, optical disks, magnetic tape, semiconductor memories such as ROMs, PROMs, etc., thereby making an article of manufacture in accordance with the invention. The article of manufacture containing the computer programming code is used by either executing the code directly from the storage device, by copying the code from the storage device into another storage device such as a hard disk, RAM, etc. or by transmitting the code on a network for remote execution. As can be envisioned by one of skill in the art, many different combinations of the above may be used and accordingly the present invention is not limited by the scope of the appended claims.

## Claims

1. A method of delivering stimulation signals to patient tissue during surgery for monitoring neuromuscular evoked potentials, comprising the steps of: (a) attaching an electrical coupling mechanism having a plunger with an electrical contact surface electrically coupled to a stimulation source to a surgical instrument, (b) advancing the surgical instrument into the patient tissue; (b) applying a stimulation signal through the electrical coupling mechanism to the surgical instrument; and (c) monitoring to assess whether nerves are innervating as a result of the step of applying said stimulation signal.

2. The method of claim 1, wherein the plunger is enclosed within a housing.

3. The method of claim 2, wherein the plunger is generally cylindrical in shape.

4. The method of claim 2, wherein the plunger is spring-loaded relative to the housing.

5. The method of claim 4, wherein the plunger is biased in a closed position.

6. The method of claim 5, wherein the housing includes a distal arm situated opposite the electrical contact surface.

7. The method of claim 4, wherein the electrical contact surface is generally arcuate.

8. The method of claim 5, wherein the electrical coupling surface is coupled to an electrical cable that is coupled to the stimulation source.

9. The method of claim 8, wherein pulling on the cable moves the plunger to an open position.

10. The method of claim 1, wherein monitoring to assess whether nerves are innervating as a result of the step of applying the stimulation signal includes detecting voltage responses of a predetermined magnitude with sensors positioned on or within muscles associated with said nerves.

11. The method of claim 10, wherein the stimulation source and the sensors are communicatively linked to a control unit.

12. The method of claim 11, wherein the control unit identifies a relationship between the stimulation signal applied and the voltage response of predetermined magnitude.

13. The method of claim 12, wherein the relationship is the lowest stimulation amplitude required to evoke the voltage response of predetermined magnitude.

14. The method of claim 13, wherein said relationship provides an indication of pedicle integrity.

15. The method of claim 1, wherein said surgical instrument is configured for at least one of pedicle hole formation, pedicle hole preparation, and pedicle screw insertion.

16. A system for monitoring electromyographic (EMG) evoked potentials during surgery, comprising:
a plurality of sensors that detect EMG responses from muscles associated with nerves located proximate to a surgical target site;
an electrical coupling mechanism having a plunger with an electrical contact surface electrically coupled to a stimulation source and coupleable to a surgical instrument to deliver stimulation signals from the stimulation source to the surgical instrument; and
a control unit in communication with the plurality of sensors and the stimulation source, the control unit being operable to (a) direct transmission of the stimulation signals; (b) receive voltage responses from the plurality of sensors, and (c) determine whether any of the nerves are innervating as a result of the step of applying the stimulation signals to the surgical instrument.

17. The system of claim 16, wherein the surgical instrument is configured for at least one of pedicle hole formation, pedicle hole preparation, and pedicle screw insertion.

18. The system of claim 16, wherein the plunger is enclosed within a housing and spring-loaded relative to the housing.

19. The system of claim 16, wherein the electrical contact surface is generally arcuate.

20. A system for performing pedicle integrity assessments, comprising:
a surgical instrument for use in at least one of pedicle hole formation, hole preparation, and pedicle screw insertion at a pedicle target site, said pedicle target site in the general location of neural structures;
an insulation member through which said surgical instrument is passed comprising a flexible hollow elongated sheath having a proximal end, a distal end, an interior lumen, and a substantially enclosed seal tip attached to said distal end of said hollow elongated member, wherein said seal tip includes an expandable aperture for receiving said surgical instrument;
a sensor configured to detect a voltage response from muscles associated with said neural structures; and
a control unit coupled to said medical instrument and the sensor, the control unit being configured to (a) transmit a stimulation signal to said surgical instrument, (b) receive a voltage response from the sensor, and (c) determine whether said neural structures are innervating as a result of the step of applying said application of stimulation signal to said surgical instrument.