US20100317989A1 - Systems and Methods for Performing Neurophysiologic Assesments With Pressure Monitoring - Google Patents

Abstract

Systems and methods for performing neurophysiologic assessments of neural tissue including nerve pathology monitoring which may or may not be augmented by adding the ability to assess or monitor the pressure being exerted upon a nerve or nerve root before, during and/or after retraction.

Description

CROSS REFERENCE TO RELATED APPLICATIONS
• This application is a divisional of U.S. patent application Ser. No. 11/490,717, filed Jul. 20, 2006 which claims priority to U.S. Provisional Patent Application Ser. No. 60/701,305, filed on Jul. 20, 2005, the entire contents of which are expressly incorporated by reference into this disclosure as if set forth herein in their entireties.
BACKGROUND OF THE INVENTION
• 1. 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 neurophysiologic assessments with additional pressure monitoring.
• 2. Description of Related Art
• It has been estimated that somewhere between 50 and 70 million people suffer from chronic back pain in the United States. In most cases, conservative therapies, including, for example, bed rest and physical therapy will succeed in alleviating or at least significantly reducing the back pain. Still, a significant number of patients are unaided by conservative therapies alone and undergo spinal surgery before finding relief. The rate at which caregivers and patients opt for surgery also continues to grow as medical technology advances and surgical options increase. In all, approximately 750,000 spine surgeries are performed per year in the United States alone.
• When necessary, spine surgery may provide great benefit to the patient, often allowing patients to resume activities long since abandoned because of the debilitating pain. Spine surgery, however, is not without risk. Operating on or near the spine generally means operating in close proximity to delicate neural tissue, such as the spinal cord and nerve roots. Often, in order to reach the surgical target site the delicate nerve tissue must be retracted out of the surgical corridor. A typical nerve retractor serves to pull or otherwise maintain the nerve outside the area of surgery, thereby protecting the nerve from inadvertent damage or contact by the “active” instrumentation used to perform the actual surgery. While generally advantageous in protecting the nerve, it has been observed that such retraction can cause nerve function to become impaired or otherwise pathologic over time due to the retraction. In certain surgical applications, such as spinal surgery, it is not possible to determine if such retraction is hurting or damaging the retracted nerve until after the surgery (generally referred to as a change in “nerve health” or “nerve status”).
• The present invention is directed at eliminating, or at least reducing the effects of, the above-described problems.
SUMMARY OF THE INVENTION
• The present invention includes a system and methods capable of performing a variety of neurophysiologic assessments, and particularly nerve pathology monitoring (via Nerve Retractor mode), by combining neurophysiology monitoring with any of a variety of instruments used in or in preparation for surgery (referred to herein as “surgical accessories”). Other assessments performed by thesystem10 may include one or more of, but not necessarily limited to, neuromuscular pathway status (Twitch Test), pedicle integrity testing (Screw Test), nerve proximity testing during surgical access (Detection), nerve pathology monitoring (Nerve Retractor), and detection of spontaneous muscle activity (Free Run EMG-which may be conducted alone or in conjunction with any other assessment). The nerve pathology monitoring of the present invention may be augmented by equipping a nerve root retractor with one or more pressure sensing technologies and/or providing an additional system or device for assessing or monitoring the pressure being exerted upon a nerve or nerve root before, during and/or after nerve retraction.
• The fundamental method steps for performing the nerve pathology assessments (as well as many of the other assessments which may be performed) according to the present invention include: (a) stimulating one or more nerves with the surgical accessory (e.g. nerve retractor among others); (b) measuring the response of nerves innervated by the stimulation of step (a); (c) determining a relationship between the surgical accessory and the nerve based upon the response measured in step (b); and (d) communicating this relationship to the surgeon in an easy-to-interpret fashion.
• The step of stimulating one or more nerves may be accomplished by applying any of a variety of suitable stimulation signals to an electrode(s) on the surgical accessory, including voltage and/or current pulses of varying magnitude and/or frequency. The stimulating step may be performed at different times depending upon the particular surgical accessory in question. For example, with regard to neural pathology monitoring, stimulation may be performed before, during and/or after retraction of the nerve root, regardless of whether additional pressure sensing features for monitoring the pressure being exerted upon a retracted nerve or nerve root are employed.
• The step of measuring the response of nerves innervated by the stimulation step may be performed in any number of suitable fashions, including but not limited to the use of evoked muscle action potential (EMAP) monitoring techniques (that is, measuring the EMG responses of muscle groups associated with a particular nerve). According to one aspect of the present invention, the measuring step is preferably accomplished via monitoring or measuring the EMG responses of the muscles innervated by the stimulated nerve(s).
• The step of determining a relationship between the surgical accessory and the nerve based upon the measurement step may be performed in any number of suitable fashions depending upon the manner of measuring the response, and may define the relationship in any of a variety of fashions (based on any number of suitable parameters and/or characteristics). By way of example, for neural pathology assessments according to the present invention, the relationship may be, by way of example only, whether the neurophysiologic response of the nerve has changed over time. Such changes may be quickly determined using a hunting algorithm to determine a stimulation threshold current level at various times during the procedure. According to one embodiment, these parameters may be augmented with information regarding the pressure being exerted upon a retracted nerve or nerve root.
• The step of communicating this relationship to the surgeon in an easy-to-interpret fashion may be accomplished in any number of suitable fashions, including but not limited to the use of visual indicia (such as alpha-numeric characters, light-emitting elements, and/or graphics) and audio communications (such as a speaker element). By way of example, for nerve pathology monitoring, the determined threshold value may be visually displayed as a simple numerical value on the display. In addition, color coded graphics may be displayed to indicate the relative safety level indicated by the threshold value. When pressure sensing capabilities are added to the nerve root retractor according to one aspect of the present invention, the step of communicating the relationship to the user may also include information about the pressure being exerted upon a retracted nerve or nerve root, such as the retraction duration, the extent of retraction, and/or the resulting pressure.
• The nerve pathology monitoring function of the present invention may be augmented with additional pressure sensing capabilities. This may be accomplished by positioning one or more pressure sensors on the nerve retractor and communicatively linking the pressure sensor to pressure mapping software on the control unit or other processor.
BRIEF DESCRIPTION OF THE DRAWINGS
• Many advantages of the present invention will be apparent to those skilled in the art with a reading of this specification in conjunction with the attached drawings, wherein like reference numerals are applied to like elements and wherein:
• FIG. 1 is a flow chart illustrating the fundamental steps of the neurophysiology-based surgical system according to the present invention;
• FIG. 2 is a perspective view of anexemplary system10 capable of performing neurophysiologic assessments such as for example, detecting pedicle breaches, nerve proximity (detection), nerve pathology, neuromuscular pathway status, and spinal cord health.
• FIG. 3 is a block diagram of thesystem10 shown inFIG. 2;
• FIG. 4 is a graph illustrating an exemplary single pulse stimulation signal according to one embodiment of the present invention;
• FIG. 5 is a is a graph illustrating an exemplary multipulse stimulation signal according to one embodiment of the present invention;
• FIG. 6 is a graph illustrating an exemplary EMG response to the stimulus ofFIG. 3 or4;
• FIG. 7 is a graph illustrating a plot of peak-to-peak voltage (Vpp) for each given stimulation current level (I_Stim) forming a stimulation current pulse train according to the present invention (otherwise known as a “recruitment curve”);
• FIGS. 8-9 are perspective and side views, respectively, of an exemplary nerve root retractor assembly according to one embodiment of the present invention;
• FIG. 10 is a perspective view of an exemplary nerve root retractor forming part of the assembly ofFIGS. 8 and 9, according to one embodiment of the present invention;
• FIG. 11 is an exemplary screen display illustrating one embodiment of the Nerve Retractor mode for performing neural pathology monitoring according to one embodiment of the present invention;
• FIG. 12 is a perspective view of an exemplary nerve root retractor including a pressure sensor according to one embodiment of the present invention;
• FIG. 13 is a perspective view of the system ofFIG. 2 adapted to perform additional pressure sensing functions according to one embodiment of the present invention;
• FIG. 14 is an exemplary multi-function screen display of the pressure sensing features of thesystem10 according to one embodiment of the present invention;
• FIG. 15 is an exemplary full screen display of the 2-dimensional pressure mapping feature according to one embodiment of the present invention;
• FIG. 16 is an exemplary full screen display of the 3-dimensional pressure mapping feature according to one embodiment of the present invention;
• FIG. 17 is an exemplary full screen display of the pressure distribution chart feature according to one embodiment of the present invention;
• FIG. 18 is an exemplary full screen display of the pressure vs. time chart feature according to one embodiment of the present invention;
• FIG. 19 is a perspective view of the system ofFIG. 2 adapted to perform additional pressure sensing functions according to a different embodiment of the present invention; and
• FIG. 20 is an exemplary screen display illustrating one embodiment of the Nerve Retractor mode for performing neural pathology monitoring augmented by pressure monitoring according to one embodiment 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 capable of performing a variety of neurophysiologic assessments, and particularly nerve pathology monitoring (via Nerve Retractor mode), by combining neurophysiology monitoring with any of a variety of instruments used in or in preparation for surgery (referred to herein as “surgical accessories”). Other assessments performed by thesystem10 may include one or more of, but not necessarily limited to, neuromuscular pathway status (Twitch Test), pedicle integrity testing (Screw Test), nerve proximity testing during surgical access (Detection), and detection of spontaneous muscle activity (Free Run EMG-which may be conducted alone or in conjunction with any other mode). As will be described in greater detail below, the nerve pathology monitoring of the present invention may be augmented by equipping anerve root retractor29 with one or more pressure sensing technologies and/or providing an additional system or device for assessing or monitoring the pressure being exerted upon a nerve or nerve root before, during and/or after nerve retraction. It is expressly noted that, although described herein largely in terms of use in spinal surgery, theneuromonitoring system10 and related methods of the present invention are suitable for use in any number of additional surgical procedures where neurological impairment due to nerve retraction is a concern.
• FIG. 1 illustrates the fundamental method steps for performing the nerve pathology assessments (as well as many of the other assessments which may be performed) according to the present invention, namely: (a) stimulating one or more nerves with the surgical accessory (e.g. nerve retractor); (b) measuring the response of nerves innervated by the stimulation of step (a); (c) determining a relationship between the surgical accessory and the nerve based upon the response measured in step (b); and (d) communicating this relationship to the surgeon in an easy-to-interpret fashion.
• FIGS. 2-3 illustrate, by way of example only, asurgical system10 provided in accordance with a broad aspect of the present invention. Thesurgical system10 includes acontrol unit12, apatient module14, anEMG harness16 including 8 pairs ofEMG electrodes18 and areturn electrode22 coupled to thepatient module14, and a host ofsurgical accessories24 capable of being coupled to thepatient module14 via one or moreaccessory cables26. In the embodiment shown, thesurgical accessories24 include (by way of example only) ascrew test assembly27 and a nerveroot retractor assembly28. Other surgical accessories not shown here may include, but are not necessarily limited to, a K-wire, a sequential dilation access system (e.g. dilating and working cannulae), taps, pedicle access probes, tissue retractor assemblies, and surface electrodes etc. . . . . Additional components, such as for example an MEP stimulator (not shown) may also be connected to thesystem10.
• Thecontrol unit12 includes atouch screen display30 and abase32, which collectively contain the essential processing capabilities for controlling thesurgical system10. Thepatient module14 is connected to thecontrol unit12 via a data cable34 (or optionally via wireless communication), which establishes the electrical connections and communications (digital and/or analog) between thecontrol unit12 andpatient module14. The main functions of thecontrol unit12 include receiving user commands via thetouch screen display30, activating stimulation in the selected mode (e.g. Nerve Retractor), processing signal data according to defined algorithms (described below), displaying received parameters and processed data, and monitoring system status and reporting fault conditions. Thetouch screen display30 is preferably equipped with a graphical user interface (GUI) capable of communicating information to the user and receiving instructions from the user. Thedisplay30 and/orbase32 may contain patient module interface circuitry that commands the stimulation sources, receives digitized signals and other information from thepatient module14, processes the EMG responses to extract characteristic information for each muscle group, and displays the processed data to the operator via thedisplay30.
• The step (a) of stimulating one or more target tissues with thesurgical accessory24 is accomplished be coupling one or moresurgical accessories24 equipped with an electrode region(s) to a stimulation source (e.g. the patient module) communicatively linked to thecontrol unit12. Preferably, multiple coupling options are included with thesystem10 to accommodate the variety of surgical accessories and functions performed by thesystem10. By way of example only, a stimulation handpiece36 (FIG. 1) may connect to thepatient module14 via anaccessory cable26 at one end, and couple a surgical accessory24 (e.g. nerve retractor29—thereby formingnerve retractor assembly28; or a pedicle probe member—thereby forming screw test assembly27) at the other end. Thestimulation handpiece36 may include one ormore buttons38 for selectively initiating stimulation according to the selected function. In a preferred embodiment thestimulation handpiece36 is reusable and sterilizable. By way of further example only, an electric coupling device such as, by way of example only,stimulation clip40 and/or stimulation clip42 (FIG. 1) may be provided to couple surgical accessories24 (such as for example, a tap member, access probe, tissue retractor assembly, and/or various cannulae) to thesystem10 such that stimulation signals may be transmitted through the tool during use. The electric coupling device may be used alone or using in conjunction withstimulation handpiece36, as is shown. Various other connectors may also be employed to couple the surgical accessory to the stimulation source, such as for example, a male/female type socket connection and other commonly know electrical connectors.
• The step (b) of measuring the response of nerves innervated by the stimulation of step (a) may be performed in any number of suitable fashions, including but not limited to the use of evoked muscle action potential (EMAP) monitoring techniques (that is, measuring the EMG responses of muscle groups associated with a particular nerve). In a preferred embodiment, EMG response monitoring is accomplished via 8 pairs EMG electrodes18 (placed on the skin over the muscle groups to be monitored), acommon electrode20 providing a ground reference to pre-amplifiers in thepatient module14, and ananode electrode22 providing a return path for the stimulation current. A preferred EMG electrode for use with thesystem10 is a dual surface electrode which is shown and described in detail in the commonly owned and co-pending U.S. patent application Ser. No. 11,048,404, entitled “Improved Electrode System and Related Methods,” filed on Jan. 31, 2005, which is expressly incorporated by reference into this disclosure as if set forth in its entirety herein. It should be appreciated however, that any of a variety of known electrodes can be employed, including but not limited to surface pad electrodes and needle electrodes. It should also be appreciated that EMG electrode placement depends on a multitude of factors, including for example, the spinal cord level and particular nerves at risk and user preference, among others. In one embodiment (set forth by way of example only), the preferred EMG configuration is described for Lumbar surgery in Table 1, Thoracolumbar surgery in Table 2, and Cervical surgery in Table 3 below:

• TABLE 1
  Lumbar

  Color	Channel	Myotome	Nerve	SpinalLevel
  Red	Right
  1	Right Vastus Medialis	Femoral	L2, L3,L4
  Orange	Right
  2	Right Tibialis Anterior	Common	L4, L5
  			Peroneal
  Yellow	Right
  3	Right Biceps Femoris	Sciatic	L5, S1,S2
  Green	Right
  4	Right Medial Gastroc.	Post Tibial	S1,S2
  Blue	Left
  1	Left Vastus Medialis	Femoral	L2, L3,L4
  Violet	Left
  2	Left Tibialis Anterior	Common	L4, L5
  			Peroneal
  Gray	Left
  3	Left Biceps Femoris	Sciatic	L5, S1,S2
  White	Left
  4	Left Medial Gastroc.	Post Tibial	S1, S2

• TABLE 2
  Thoracolumbar

  Color	Channel	Myotome	Nerve	SpinalLevel
  Red	Right
  1	Right Abductor Pollicis Brevis	Median	C6, C7, C8,T1
  Orange	Right
  2	Right Vastus Medialis	Femoral	L2, L3,L4
  Yellow	Right
  3	Right Tibialis Anterior	Common Peroneal	L4,L5
  Green	Right
  4	Right Abductor Hallucis	Tibial	L4, L5,S1
  Blue	Left
  1	Left Abductor Pollicis Brevis	Median	C6, C7, C8,T1
  Violet	Left
  2	Left Vastus Medialis	Femoral	L2, L3,L4
  Gray	Left
  3	Left Tibialis Anterior	Common Peroneal	L4,L5
  White	Left
  4	Left Abductor Hallucis	Tibial	L4, L5, S1

• TABLE 3
  Cervical

  Color	Channel	Myotome	Nerve	SpinalLevel
  Red	Right
  1	Right Deltoid	Axilliary	C5,C6
  Orange	Right
  2	Right Flexor Carpi	Median	C6, C7, C8
  		Radialis
  Yellow	Right
  3	Right Abductor Pollicis	Median	C6, C7, C8, T1
  		Brevis
  Green	Right
  4	Right Abductor Hallucis	Tibial	L4, L5,S1
  Blue	Left
  1	Left Deltoid	Axillary	C5,C6
  Violet	Left
  2	Left Flexor Carpi	Median	C6, C7, C8
  		Radialis
  Gray	Left
  3	Left Abductor Pollicis	Median	C6, C7, C8, T1
  		Brevis
  White	Left
  4	Left Abductor Hallucis	Tibial	L4, L5, S1

• The step (c) of determining a relationship between the surgical accessory and the nerve based upon the response measured in step (b) may be performed in any number of suitable fashions depending upon the manner of measuring the response, and may define the relationship in any of a variety of fashions based on any number of suitable parameters and/or characteristics). By way of example only, the step of determining a relationship, within the context of a nerve pathology assessment, may involve identifying what stimulation current level is required to evoke a significant muscle response (i.e. the relationship between the surgical accessory (and more specifically the stimulation signal emitted from the surgical accessory) and the nerve may be defined as the stimulation threshold current level, described below).
• A basic premise underlying the methods employed by thesystem10 for much of the neurophysiologic monitoring conducted is that neurons and nerves have characteristic threshold current levels (I_Thresh) at which they will depolarize, resulting in detectable muscle activity. Below this threshold current, stimulation signals, such as the single pulse signal shown by way of example only inFIG. 4 and the multi-pulse signal shown by way of example only inFIG. 5, will not evoke a significant EMG response. Each EMG response can be characterized by a peak-to-peak voltage of V_pp=V_max−V_min, shown inFIG. 6. Once the stimulation threshold (I_Thresh) is reached, the evoked response is reproducible and increases with increasing stimulation until saturation is reached as shown inFIG. 7. This is known as a “recruitment curve.” In one embodiment, a significant EMG response is defined as having a V_ppof approximately 100 uV. The lowest stimulation signal current, I_stimthat evokes this threshold voltage (V_Thresh) is called I_Thresh. Finding I_threshis useful in making neurophysiologic assessments because it provides a relative indication as to the degree of communication between a stimulation signal and nerve tissue. For example, as the degree of electrical communication between a stimulation signal and a nerve decreases, I_threshwill increase. Conversely, as the degree of communication between the stimulation signal and a nerve increases, I_threshwill decrease.
• Thesystem10 capitalizes on and enhances the information derived from I_threshby quickly, accurately, and efficiently finding I_threshand comparing the determined value against predetermined safety indicator levels. Armed with the useful information conveyed by thesystem10, the surgeon may detect early on any problem or potential problem and then act to avoid and/or mitigate the situation. By way of general example only, an excessively high I_threshor an increase over a previous I_threshmeasurement during Nerve Retractor mode may indicate a deterioration of nerve root function caused by excessive and/or prolonged retraction. On the opposite hand, a decrease in Ithresh over previous measurements may indicate During Screw Test and Detection modes, a low I_threshvalue may indicate a breach in the pedicle, or the close proximity of a nerve, respectively.
• To quickly determine I_thresh, thesystem10 may employ a variety of suitable algorithms and techniques which are described in detail in the “NeuroVision Applications,” all of which are incorporated by reference below, as if they were set forth herein in their entireties. One exemplary threshold hunting algorithm is described hereafter in only brief detail. The threshold hunting algorithm utilizes a bracketing method and a bisection method to find I_thresh. The bracketing method finds a range (bracket) of stimulation currents that must contain I_thresh. To accomplish this, the algorithm directs stimulation to begin at a predetermined current level (based on the selected function). For each subsequent stimulation, the current level is doubled from the previous current level. This doubling continues until a until a stimulation current recruits, that is, results in an EMG response with a V_ppgreater or equal to V_thresh(e.g. 100 uV). This first stimulation current to recruit, together with the last stimulation current to have not recruited, forms the initial bracket. If the stimulation current threshold, I_thresh, of a channel exceeds a maximum stimulation current, that threshold is considered out of range.
• After the bracket containing the threshold current I_threshhas been determined, the initial bracket is successively reduced via the bisection method to a predetermined width. This is accomplished by applying a first bisection stimulation current that bisects (i.e. forms the midpoint of) the initial bracket. If this first bisection stimulation current recruits, the bracket is reduced to the lower half of the initial bracket. If this first bisection stimulation current does not recruit, the bracket is reduced to the upper half of the initial bracket. This process is continued for each successive bracket until I_threshis bracketed by stimulation currents separated by the predetermined width. In one embodiment, the midpoint of this final bracket may be defined as I_thresh; however, any value falling within the final bracket may be selected as I_threshwithout departing from the scope of the present invention.
• During some functions (e.g. Screw Tests and Detection) stimulations may stop after I_threshis determined for the channel possessing the lowest I_thresh. For other functions (e.g. Nerve Retractor), however, it may useful to determine I_threshfor every channel. To accomplish this quickly, the hunting algorithm may employ additional methods allowing it to omit certain stimulations, thereby reducing the number of stimulations and time required to obtain an I_threshvalue on each channel. I_threshis still found using the bracketing and bisection methods described above, however the algorithm will omit stimulations for which the result is predictable from data previously acquired. When a stimulation signal is omitted, the algorithm proceeds as if the stimulation had taken place. This permits the algorithm to proceed to the next required stimulation immediately, without a time delay inherently associated with each stimulation signal. To further reduce the number of stimulations required over the time frame of an entire surgical procedure, the algorithm may confirm previously obtained I_threshvalues (e.g. by stimulation at current levels just below and at/or just above I_threshand determining whether the resulting responses are consistent with the previously acquired I_threshvalue), rather than initiating stimulations from the beginning each time a function is performed.
• The final step (d) of communicating this relationship to the surgeon in an easy-to-interpret fashion may be accomplished in any number of suitable fashions, including but not limited to the use of visual indicia (such as alpha-numeric characters, light-emitting elements, and/or graphics) and audio communications (such as a speaker element). By way of example only, the determined I_threshvalue may be visually displayed as a simple numerical value ondisplay30. In addition, color coded graphics may be displayed to indicate the relative safety level indicated by the I_thresh(e.g. “green” for a range of stimulation thresholds below (or above, depending on the selected mode) a predetermined safe value, “red” for a range of stimulation thresholds above (or below, depending on the mode) a predetermined unsafe value, and “yellow” for the range of stimulation thresholds in between the predetermined safe and unsafe values-designating caution). When pressure sensing capabilities are added to the nerve root retractor according to one aspect of the present invention, the step of communicating the relationship to the user may also include information about the pressure being exerted upon a retracted nerve or nerve root, such as the retraction duration, the extent of retraction, and/or the resulting pressure. In one embodiment, set forth by way of example only, the information shown on thedisplay30 may include at least some of the following components (depending on the active mode) as set forth in Table 1:

• TABLE 1
  Screen
  Component	Description
  Spine Image	An image of the human body/skeleton showing the electrode placement
  	on the body, with labeled channel number tabs on each side (1-4 on the
  	left and right). Left and right labels will show the patient orientation.
  	The channel number tabs may be highlighted or colored depending on
  	the specific function being performed.
  Myotome & Level	A label to indicate the Myotome name and corresponding Spinal
  Names	Level(s) associated with the channel of interest.
  Menu	A drop down navigation component for toggling between functions.
  Display Area	Shows procedure-specific information including stimulation results.
  Color Indication	Enhances stimulation results with a color display of green, yellow, or
  	red corresponding to the relative safety level determined by the system.
  Mode Indicator	Graphics and/or name to indicate the currently active mode (Twitch
  	Test, Free-Run EMG, Basic Screw Test, Dynamic Screw Test,
  	Difference Screw Test, Detection, Nerve Retractor). In an alternate
  	embodiment, Graphics and/or name may also be displayed to indicate
  	the instrument in use, such as the dilator, K-wire, retractor blades,
  	screw test instruments, and associated size information, if applicable,
  	of the cannula, with the numeric size. If no instrument is in use, then
  	no indicator is displayed.
  Stimulation Bar	A graphical stimulation indicator depicting the present stimulation
  	status (i.e. on or off and stimulation current level)
  Sequence Bar	Shows the last seven stimulation results and provides for annotation of
  	results.
  EMG waveforms	EMG waveforms may be optionally displayed on screen along with the
  	stimulation results.
  Pressure Indicia	Data regarding pressure being exerted upon a retracted nerve or nerve
  	root, such as the duration of retraction, the extent of retraction, and/or
  	the resulting pressure.

• By way of example only, the various functional modes capable of being performed bysystem10 may include, but is not necessarily limited to, the Twitch Test, Free-run EMG, Basic Screw Test, Difference Screw Test, Dynamic Screw Test, MaXcess® Detection, Nerve Retractor, MEP Auto, MEP manual, and SSEP modes, all of which are described only briefly hereafter. The Twitch Test mode is designed to assess the neuromuscular pathway via the so-called “train-of-four test” test to ensure the neuromuscular pathway is free from muscle relaxants prior to performing neurophysiology-based testing, such as bone integrity (e.g. pedicle) testing, nerve detection, and nerve retraction. This is described in greater detail within Int'l Patent App. No. PCT/US2005/036089, entitled “System and Methods for Assessing the Neuromuscular Pathway Prior to Nerve Testing,” filed Oct. 7, 2005, the entire contents of which is hereby incorporated by reference as if set forth fully herein. The Basic Screw Test, Difference Screw Test, and Dynamic Screw Test modes are designed to assess the integrity of bone (e.g. pedicle) during all aspects of pilot hole formation (e.g., via an awl), pilot hole preparation (e.g. via a tap), and screw introduction (during and after). These modes are described in greater detail in Int'l Patent App. No. PCT/US02/35047 entitled “System and Methods for Performing Percutaneous Pedicle Integrity Assessments,” filed on Oct. 30, 2002, and PCT/US2004/025550, entitled “System and Methods for Performing Dynamic Pedicle Integrity Assessments,” filed on Aug. 5, 2004 the entire contents of which are both hereby incorporated by reference as if set forth fully herein. The MaXcess® Detection mode is designed to detect the presence of nerves during the use of the various surgical access instruments of theneuromonitoring system10, including the k-wire62,dilator64,cannula66,retractor assembly70. This mode is described in greater detail within Int'l Patent App. No PCT/US02/22247, entitled “System and Methods for Determining Nerve Proximity, Direction, and Pathology During Surgery,” filed on Jul. 11, 2002, the entire contents of which is hereby incorporated by reference as if set forth fully herein. The MEP Auto and MEP Manual modes are designed to test the motor pathway to detect potential damage to the spinal cord by stimulating the motor cortex in the brain and recording the resulting EMG response of various muscles in the upper and lower extremities. The SSEP function is designed to test the sensory pathway to detect potential damage to the spinal cord by stimulating peripheral nerves inferior to the target spinal level and recording the action potential from sensors superior to the spinal level. The MEP Auto, MEP manual, and SSEP modes are described in greater detail within Int'l Patent App. No. PCT/US2006/003966, entitled “System and Methods for Performing Neurophysiologic Assessments During Spine Surgery,” filed on Feb. 2, 2006, the entire contents of which is hereby incorporated by reference as if set forth fully herein. The Nerve Retractor mode is designed to assess the health or pathology of a nerve before, during, and after retraction of the nerve during a surgical procedure. This mode is described in greater detail within Int'l Patent App. No. PCT/US02/30617, entitled “System and Methods for Performing Surgical Procedures and Assessments,” filed on Sep. 25, 2002, the entire contents of which is hereby incorporated by reference as if set forth fully herein.
• Nerve pathology monitoring as it may be performed by thesystem10 during Nerve Retractor mode is described hereafter in more detail. Thesystem10 preferably accomplishes neural pathology monitoring via the Nerve Retractor mode, specifically by determining a baseline stimulation threshold (again, stimulation threshold is the value I_thresh) with direct contact between thenerve retractor29 and the nerve, prior to retraction. Subsequent stimulation thresholds are determined during retraction and they are compared to the baseline threshold. Significant changes in the stimulation threshold may indicate potential trauma to the nerve caused by the retraction and are displayed to the user on thedisplay30. An increase in I_threshover time is an indication that the nerve function is deteriorating. By monitoring this the surgeon may intra-operatively assess if the retracted nerve is being damaged or otherwise compromised (such as due to a prolonged surgery), such that it can be temporarily released to allow it to recover before returning to retraction to continue with the surgery. It's believed that releasing the nerve root in this fashion will prevent or reduce the adverse effects (nerve function compromise) that may otherwise result from prolonged retraction. Changes in I_threshin the opposite direction (i.e. decreases in I_threshover time) may indicate that the surgical procedure is improving or aiding a previously unhealthy nerve, such as during spinal decompression surgery. For accurate and timely information, I_threshmay preferably be determined for each channel according to the multi-channel hunting algorithm described above.
• The nerveroot retractor assembly28, shown in a preferred embodiment inFIGS. 8-10, comprises thestimulation handpiece36 employed with a selectively removablenerve root retractor29. Thenerve root retractor29 has a generally angled orientation relative to the longitudinal axis of thestimulation handpiece36. Thedistal end44 is generally curved and includes an arcuatenerve engagement surface46 equipped with, by way of example only, at least onestimulation electrode48. As best shown inFIG. 10, thenerve root retractor29 is preferably removable from thestimulation handpiece36. To accomplish this, thestimulation handpiece36 includes adetachable cap member50.Threads52 are provided on the proximal end of thenerve root retractor29 to allow a threaded coupling engagement between thestimulation handpiece36 and thenerve root retractor29. During such engagement,electrical contacts54 on thenerve root retractor29 become electrically coupled to thestimulation handpiece36 such that, upon activation of one or more of the buttons38 a stimulation current signal will be transmitted from thecontrol unit12 and/orpatient module14 and delivered to the stimulation electrode(s)48 on thenerve root retractor29 for the purpose of performing neural pathology monitoring according to the present invention. Thenerve root retractor29 is preferably disposable and, as described above, thestimulation handpiece36 is preferably reusable and sterilizable. It should be understood that while thenerve retractor29 is shown primarily in use with thestimulation handpiece36 to formnerve retractor assembly28,nerve retractor29 may be used with any suitable handle and/or may include a fixed handle. Whennerve retractor29 is used withoutstimulation handpiece36 it may be coupled to thesystem10 via one of theelectric coupling devices40,42, a male/female type electrical coupler, or any other form of electric coupling.
• In use, thenerve root retractor29 is introduced into or near a surgical target site in order to hook and retract a given nerve out of the way. According to the present invention, the nerve root may be stimulated (monopolar or bipolar) before, during, and/or after retraction in order to assess the degree to which such retraction impairs or otherwise degrades nerve function over time. To do so, the user may operate one ormore buttons38 of thestimulation handpiece36 to selectively transmit a stimulation current signal, according to the algorithm described above, from thepatient module14 to the electrode(s)48 on theengagement surface46 of thenerve root retractor29. By monitoring the myotome(s) associated with the nerve root being retracted (via the EMG harness16) and assessing the resulting EMG responses to preferably determine I_thresh(via the control unit12), thesystem10 can assess whether (and the degree to which) such retraction impairs or adversely affects nerve function over time. With this information, a user may wish to periodically release the nerve root from retraction to allow nerve function to recover, thereby preventing or minimizing the risk of long-term or irreversible nerve impairment. Similarly, an unhealthy nerve may be monitored in the same manner to determine if nerve function improves due to a particular procedure, such as spinal nerve decompression surgery.
• The nerve retraction monitoring feature of the present invention is best viewed with regard toFIG. 11. The screen display for the Nerve Retractor mode may include any of a variety of indicia capable of communicating parameters associated with the nerve retraction monitoring feature of the present invention to a surgeon, including but not limited to, channel windows containing one or more of the channel number, myotome name, spinal level, baseline I_thresh, previous I_thresh, current I_thresh, and the associated waveform, and various selection tabs for one or more of starting baseline stimulation, starting retraction stimulation, stopping stimulation, activating free-run EMG monitoring and adjusting the sensitivity of the free-run EMG, noting the retraction site, mode selection, annotating results, and remote messaging.
• As described above, the nerve pathology assessments (Nerve Retractor mode) conducted by thesystem10 may be further augmented via the use of any number of pressure sensing technologies working in addition to the stimulation based nerve monitoring described above and throughout this disclosure. The pressure sensing features contemplated herein offer added safety and qualitative assessment features by providing the ability to monitor how much pressure is being applied to a particular nerve during nerve root retraction. This pressure may be a function of, among other factors, the degree of retraction (that is, distance the nerve is moved during retraction) and the duration of retraction. In any case, the pressure resulting on a nerve during retraction—if too high in magnitude and/or too long in duration—may result in neurologic deficit of varying degrees. By monitoring the pressure according to the present invention, a user may selectively reduce the degree of retraction (such as by moving theretractor assembly28 and nerve closer to the “natural” position of the nerve) and/or periodically releasing the nerve to avoid and/or minimize any resulting neurologic deficit due to retraction.
• The pressure sensing technologies may include any number of commercially available and/or publicly known pressure sensing technologies, and/or those later developed. The commercially available and/or publicly known pressure sensing technologies include, but are not necessarily limited to, the piezoelectric-based pressure sensing technique shown and described in U.S. Pat. No. 5,769,781 to James Chappuis, the capacitive-based pressure mapping system shown and described in U.S. Pat. No. 5,010,772 to Bourland et. al., the strain gauge-based pressure sensing technology shown and described in U.S. Pat. No. 4,784,150 to Voorhies et. al., the pressure sensitive ink-based technique shown and described in U.S. Pat. No. 5,989,700 to Krivopal, and that shown and described in U.S. Pat. No. 6,272,936 to Oreper et. al.
• Augmenting the nerve pathology monitoring with pressure sensing capabilities may be accomplished in any number of suitable manners, including but not limited to equipping the nerve retractor assembly28 (and more specifically, one or more of thenerve root retractor29, thestimulation handpiece36, thedistal end44, and the nerve engagement surface46) with one or more pressure sensing technologies. By way of example only, with reference toFIG. 9, this may be done by equipping thenerve engagement surface46 with apressure sensor56. By way of example only, thepressure sensor56 illustrated inFIG. 12 is a capacitive foam grid (comprising any number of suitable grid units) adhered tonerve engagement surface46 ofnerve retractor29. Whilepressure sensor56 is shown herein having a small number of relatively large grid units, this is done for illustrative purposes. It will be readily understood by those skilled in the art that that any number of grid units and unit sizes may be utilized, andsensor56 preferably comprises a large number of relatively small grid units to increase precision. To communicatively link thepressure sensor56 to a processor unit, a data cable (not shown) may be run along thenerve retractor29, and is preferably integrated within the interior ofnerve retractor29, and from there may be connected to the processing unit viacable58. Alternatively, wireless communication may be used to link the pressure sensor and processing unit. As will be discussed below, the processor may becontrol unit12 or an additional dedicated processor may be employed for pressure sensing features. In use, pressure applied to the capacitive foam grid induces voltage responses corresponding to the affected grid units. The voltage responses are relayed to the processor which translates the response into one or more of numerical and/or graphical (e.g. color) indicia relating to pressure, which may be displayed to the surgeon.
• According to a further embodiment of the present invention, a separate system or device may be employed for assessing or monitoring the pressure being exerted upon a nerve or nerve root before, during and/or after nerve retraction. Such a separate system may supplant (that is, take the place of) and/or augment (that is, serve in along with) thenerve root retractor29 equipped with pressure sensing capabilities described above.
• With reference toFIG. 13, according to one embodiment, set forth by way of example only, the pressure sensing features of thesystem10 are controlled via aseparate processing unit60 while the EMG based nerve monitoring features are controlled via control unit12 (as described throughout the description). Processingunit60 is preferably a personal workstation or laptop computer (by way of example only) running pressure sensing software designed to interface with thepressure sensor56 and display pressure data to the user on theprocessor screen62. One such software program, by way of example only, is the X3 Series software distributed by XSENSOR™ Technology Corporation, Calgary Canada. In use, processingunit60 is preferably arranged near thecontrol unit12 such that the surgeon may view nerve pathology data from thedisplay30 and pressure data fromscreen62 at the same time.
• As previously mentioned, pressure applied to the retracted nerve may be a function of, among other factors, the degree of retraction (that is, distance the nerve is moved during retraction) and the duration of retraction, either of which may result in neurologic deficit of varying degrees. With reference toFIG. 14, various pressure mapping features of the present embodiment that allow the surgeon to asses both the extent and duration of pressure applied to the retracted nerve are illustrated.FIG. 14 illustrates by way of example, a multi-function screen view of the pressure sensing software. The multi-function screen may include (by way of example only) a 2-dimensional (2-D)grid map64, a 3-dimensional (3-D)grid map66, and apressure distribution chart68, and a pressure vs. time (PvT)chart70, alegend72, and atool bar74. Thetool bar74 andlegend72 may be found on all screen views of the pressure mapping software. Thetool bar74 allows the user to select between different view screens, among other functions (such as for example, selecting the desired sensor range and precision). Thelegend74 indicates the numerical pressure value associated with a specific color (e.g. in one example red indicates the highest pressure of the selected range, 15.0 PSI inFIG. 14).
• FIGS. 15-18 illustrate exemplary embodiments of the full screen displays.FIG. 15 is a full screen display of the 2-D grid map64, wherein various colors are used (according to the legend72) to indicate the amount of pressure measured for each grid unit on thesensor56. Areference marker76 is included which corresponds to a specific corner of thepressure sensor56, to help orient the viewer.FIG. 16 illustrates the full screen display of the 3-D grid map, wherein various colors (according to the legend72) are again used to indicate the amount of pressure measured on the grid units ofsensor56.FIG. 17 illustrates the full screen display of thepressure distribution graph68, wherein the vertical axis indicates the percentage of grid units at the given moment which are under a measured pressure falling within the pressure intervals on the horizontal axis.FIG. 18 shows the full screen display of a PvT graph, wherein the maximum pressure measured onsensor56 is charted against the retraction time.
• Utilizing the various functions of the pressure mapping software, the surgeon may keep be kept aware to the extent which pressure is applied to a nerve throughout retraction, and also the length of time which a nerve is retracted. In addition, the 2-D and 3-D maps64,66 also allow the surgeon to quickly assess not only the extent and duration of the pressure, but also the position, orientation, and center mass of thenerve retractor29 on the nerve. By way of example only, should either of the grid maps indicate that pressure is highest around one edge it may indicate that the nerve is not resting in the ideal position (e.g. centered and flat against the nerve engagement surface46) and the surgeon may act to reposition the nerve relative to the nerve retractor. Pressure data collected by theprocessor60 may preferably be viewed in real time, however, theprocessor60 may also save the data for the entire procedure and the surgeon may go back and view any data from the same procedure at any time.
• In another embodiment, set forth herein by way of example only and depicted inFIG. 19, the pressure sensing features of thesystem10 are controlled via thecontrol unit12 and the pressure features may be fully integrated with the Nerve Retractor mode described above. In this embodiment, both thepressure sensor56 and thestimulation electrode48 are communicatively linked to thepatient module14 and pressure data is preferably viewed from within the Nerve Retractor screen display (discussed above with reference toFIG. 11). An embodiment of the Nerve Retractor screen display whensystem10 is augmented with pressure sensing capabilities according to this embodiment is illustrated by way of example only inFIG. 20. In the example shown, the screen display includes, among its other features, apressure selection tab78 for initiating pressure monitoring (preferably at the same time nerve retraction) begins, amaximum pressure readout80, and atimer readout82. Selecting thepressure selection tab78 using the GUI activates thepressure sensor56 and starts the timer. The running time is displayed to the user in thetimer readout82. The maximum pressure measured on thesensor56 is displayed with constant real time updating. Although not shown herein, it will be appreciated that any of a number of various features such as the 2-D and 3-D grid maps, pressure distribution, and pressure vs. time graph may also be generated and displayed in this embodiment. This may be accomplished, for example only, by providing one or more pop-up windows displaying the data and/or rearranging the screen view. Pressure data measured by thesystem10 is recorded and saved such that it may be accessed again if necessary. Pressure data may also be included in procedure reports generated by thesystem10 such that an accurate record is easily obtained.
• While the pressure sensing features herein have been described above in terms of monitoring retraction pressure on nerves it will be understood that monitoring retraction pressure may be useful for any number of different body tissues which must be retracted out of the way during surgery. By way of example only, it may be extremely beneficial to monitor retraction pressure on the larynx and/or esophagus which must be retracted during anterior cervical procedure.
• It may also be advantageous to communicate pressure related data captured by thesystem10 to persons not present in the operating room. It is contemplated that the data may be transmitted (along with the nerve pathology and other neurophysiologic assessment data) to one or more remote locations and viewable by authorized persons. This may be accomplished by any number of data transmission methods. In one example, the data may be transmitted to a remote user via remote monitoring software such as that described in detail in the commonly owned and co-pending U.S. patent application Ser. No. 11/418,589, entitled “System and Methods for Performing and Monitoring Neurophysiologic Assessments,” filed on May 5, 2006, the entire contents of which are incorporated by reference herein as if set forth in its entirety.
• 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 (11)

1. A method for assessing the pathology of a nerve, comprising the steps of:

retracting a nerve within a patient using a nerve retractor equipped with a pressure sensor;

stimulating said nerve with an electrical stimulation signal delivered by at least one stimulation electrode; and

using at least one sensor to detect neuromuscular responses evoked by said stimulation signal.

2. The method ofclaim 1 wherein said pressure sensor is linked to a processing unit and said processing unit calculates the pressure exerted on said nerve by said nerve retractor.

3. The method ofclaim 2, wherein said processing unit further includes a display, wherein said display provides at least one of alpha-numeric and graphic indicia relating to said calculated pressure.

4. The method ofclaim 3, wherein said alpha-numeric and graphic indicia includes at least one of a maximum pressure value, time value, color, 3-dimensional pressure map, 2-dimensional pressure map, chart, and a graph.

5. The method ofclaim 2, comprising the additional steps of:

using a control unit communicatively linked to said stimulation electrode and said sensor to identify a relationship between said stimulation signal and said neuromuscular response.

6. The method ofclaim 5, wherein said control unit includes a display and said display provides at least one of alpha-numeric and graphic indicia relating to said identified relationship.

7. The method ofclaim 5, wherein said identified relationship is the lowest stimulation current necessary to evoke a neuromuscular response.

8. The method ofclaim 7, wherein said control unit is configured to perform a threshold hunting algorithm to identify the lowest stimulation current necessary to evoke a neuromuscular response.

9. The method ofclaim 1, wherein said stimulation electrode is positioned on said nerve retractor.

10. The method ofclaim 5, wherein said control unit and said processing unit are a single unit.

11. The method ofclaim 5, wherein said control unit and said processing unit are distinct units.

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