Patent Publication Number: US-2022233121-A1

Title: Directional dilator for intraoperative monitoring

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 15/881,624 filed Jan. 26, 2018, which is a continuation of U.S. patent application Ser. No. 13/830,508 filed Mar. 14, 2013, now U.S. Pat. No. 9,888,859 issued Feb. 13, 2018, which claims priority to U.S. Provisional Application Ser. No. 61/612,195, filed Mar. 16, 2012 and titled “Stationary Directional Dilators”, the complete disclosure of which is hereby incorporated by reference into this application as if set forth fully herein. 
    
    
     BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     The present invention relates generally to surgery and, more particularly, to directional dilation during intraoperative monitoring through the use of a stationary directional dilator. 
     II. Discussion of the Prior Art 
     Intraoperative monitoring is commonly employed during surgeries which involve passing surgical instruments near or through tissues or areas having neural structures which, if contacted, may result in neurological deficit for the patient. Spine surgery is but one example and may be employed to address any number of different spinal disorders. To do so, it is necessary to create an operative corridor extending between an incision site and the spinal column. Depending on the approach or trajectory to the spine (e.g. anterior, posterior, lateral, etc.), different tissues will need to be traversed in order to establish the operative corridor. 
     The XLIF® procedure by NuVasive, Inc. is an exemplary surgical procedure, which involves establishing an operative corridor from a lateral approach to the lumbar spine while traversing through the psoas muscle. The psoas muscle is known to contain nerve roots which exit from the spinal cord. To safely establish an operative corridor through the psoas muscle, Nu Vasive, Inc. has developed certain systems and methods, such as that shown and described in U.S. Pat. No. 7,905,840 (hereinafter “the&#39;840 patent”), the entire content of which is hereby expressly incorporated by reference into this disclosure as if set forth fully herein. 
     The &#39;840 patent includes an electromyographic (EMG) intraoperative monitoring system and an access system comprising sequential dilators and a split-blade retractor, which collectively provide the ability to establish a so-called “less invasive” or “minimally disruptive” operative corridor through the psoas muscle to a surgical target site in the lumbar spine. Each sequential dilator has an electrode at the distal end which, when coupled to the intraoperative monitoring system, provides the ability to send a stimulation signal into the surrounding tissue to help determine the presence of nerves. Each sequential dilator may be physically rotated about its longitudinal axis during such stimulation to help determine the direction of the nerve(s) relative to the electrode and thus the dilator. This nerve proximity and nerve direction information may be used by the surgeon to help inform his or her surgical decision-making. 
     The present invention presents an alternate manner of directional dilation during surgeries involving intraoperative monitoring, including but not limited to spine surgery. 
     SUMMARY OF THE INVENTION 
     The present invention accomplishes this goal by providing a stationary directional dilator. In one aspect, the stationary directional dilator includes a stationary dilator and a rotatable electrode at the distal end of the stationary dilator. By “stationary” it is meant that during the process of nerve detection the dilator is maintained in a generally static or still position relative to any rotation about its longitudinal axis. In use with an intraoperative monitoring system, a surgeon may hold the stationary dilator in this stationary, “non-rotating” manner while rotating the electrode to detect the presence and/or direction of surrounding nerves during the process of advancing the dilator through tissue towards the surgical target site (e.g., the intervertebral disc in the case of lateral, trans-psoas spinal fusion surgery). This nerve proximity and nerve direction information may, in turn, be used by the surgeon to help inform his or her surgical decision-making. 
    
    
     
       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: 
         FIGS. 1-2  are perspective and front views, respectively, of a stationary directional dilator according to one aspect of the present invention; 
         FIGS. 3-4  are perspective and front views, respectively, of an outer dilator forming part of the stationary directional dilator of  FIG. 1 ; 
         FIGS. 5-6  are perspective and front views, respectively, of a rotatable electrode structure forming part of the stationary directional dilator of  FIG. 1 ; 
         FIGS. 7-8  are top and bottom views, respectively, of the rotatable electrode structure of  FIG. 5 ; 
         FIGS. 9-10  are perspective and front views, respectively, of a rotatable electrode structure according to an alternate aspect forming part of the stationary directional dilator of  FIG. 1 ; 
         FIGS. 11-13  are perspective, top and bottom views, respectively, of a cap member forming part of the stationary directional dilator of  FIG. 1 ; 
         FIGS. 14-15  are perspective and front views, respectively, of an electrical coupling element forming part of the stationary directional dilator of  FIG. 1 ; 
         FIGS. 16-17  are perspective and front views, respectively, of a stationary directional dilator according to an alternate aspect; 
         FIGS. 18-19  are perspective and front views, respectively, of a rotatable electrode structure forming part of the stationary directional dilator of  FIGS. 16-17 ; 
         FIGS. 20-21  are perspective and front views, respectively, of a stationary directional dilator according to an alternate aspect; 
         FIGS. 22-23  are perspective and front views, respectively, of a rotatable electrode structure forming part of the stationary directional dilator of  FIGS. 20-21 ; 
         FIG. 24  is a diagram of an exemplary intraoperative monitoring system for use with a stationary directional dilator of the present invention; and 
         FIG. 25  is a schematic diagram illustrating the functional components of the intraoperative monitoring system set forth in  FIG. 24 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     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&#39; 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 stationary directional dilator and associated methods disclosed herein boast a variety of inventive features and components that warrant patent protection, both individually and in combination. 
       FIGS. 1 and 2  illustrate, by way of example only, a stationary directional dilator  10  according to one aspect of the present invention. The stationary directional dilator  10  includes a stationary dilator  12  and a rotatable electrode structure  14  with an electrode  16  at the distal end. By “stationary” it is meant that the dilator  12  is maintained in a generally static or still position relative to any rotation about its longitudinal axis. The rotatable electrode structure  14  includes an electrode marker  48  located towards the proximal end of the electrode structure  14 , which is in alignment with the electrode  16  located at the distal end of the electrode structure  14 . An electrical coupler  18  extends from a cap member  19  and is electrically connected to the electrode  16 . The electrical coupler  18  is intended to be connected to an intraoperative monitoring system of the type shown and described in the &#39;840 patent. As will be described in greater detail below, the cap member  19  is used to help couple the rotatable electrode structure  14  and the stationary dilator  12  to enable the selective rotation of the electrode structure  14  (and with it electrode  16 ) relative to the stationary dilator  12 . 
     When coupled to an intraoperative monitoring system, the stationary directional dilator  10  of the present invention may be used to detect the presence and/or direction of surrounding nerves during the process of advancing the dilator  10  through tissue towards the surgical target site. More specifically, the intraoperative monitoring system accomplishes this by sending electrical stimulation signals to the electrode  16  on the stationary directional dilator  10 . Depending upon the location of the stationary directional dilator  10  within a patient (and more particularly, to any neural structures), the stimulation signals may cause nerves adjacent to or in the general proximity of the dilator  10  to depolarize. This causes muscle groups to innervate and generate EMG responses, which can be sensed via the EMG electrodes on the legs of the patient. By monitoring these myotomes associated with the nerves and assessing the resulting EMG responses, the intraoperative monitoring system is capable of detecting the presence of such nerves. 
     To determine the direction to the nerves, the user need simply hold the dilator  12  in a generally stationary position while rotating the electrode  16  as the stimulation signal is being emitted to the surrounding tissue. The closer the nerve is to the electrode  16 , the lower the electrical stimulation (e.g. current) required to innervate the nerve. As such, the user may simply rotate the electrode  16  to identify the point in its rotation relative to the dilator  12  at which the stimulation signal required to innervate the nerve is the lowest. This may be accomplished by visualizing graphical indicia (e.g. numbers, colors, etc. . . . ) indicative of the stimulation signal level and/or listening to sounds generated by the intraoperative monitoring system indicative of the stimulation signal level. This point is the direction of the nerve relative to the stationary dilator  12 . The electrode marker  48  may be used to help determine the location of the nerve to the stationary dilator  12 , given that the electrode  16  will be located within the patient and thus not readily visible. 
       FIGS. 3 and 4  detail the stationary outer dilator  12  forming part of the stationary directional dilator  10  of  FIG. 1 . The outer dilator  12  includes a generally elongated lower or distal section  20  extending away from an upper or proximal section  22 . The proximal section  22  includes an upper ring member  24 , a lower ring member  26 , and a plurality of struts  28  extending between the upper and lower ring members  24 ,  26 . A plurality of openings  30  are defined between the struts  28  and the upper and lower ring members  24 ,  26 . The distal section  20  and proximal section  22  are both generally tubular and hollow in construction with a contiguous lumen  32  extending through both sections  20 ,  22 . The diameter of the lumen  32  is slightly wider in the proximal section  22  than in the distal section  20 , which (as will be described in greater detail below) is designed to accommodate and enable rotation of the proximal end of the rotatable electrode structure  14  (described below). 
       FIGS. 5 and 6  detail the rotatable electrode structure  14  forming part of the stationary directional dilator  10  of  FIG. 1 . The rotatable electrode structure  14  includes an upper collar member  40 , a lower ring member  42 , and a plurality of struts  44  extending there between. With combined reference to  FIGS. 5-7 , the upper collar member  40  includes an upper electrode surface  46 , the electrode marker  48 , and a lumen  50  extending there through. With combined reference to  FIGS. 5-6 and 8 , the lower ring member  42  includes electrode  16  disposed on a beveled surface  54 , and a lumen  56  extending there through. The diameter of the lumen  50  and the diameter of the lumen  56  are preferably the same or approximately so. This will permit the stationary directional dilator  10  of the present invention to be slidably advanced over a previously placed dilator having an outer diameter slightly less than the inner diameter of the stationary directional dilator  10 . It will be appreciated that any such previously placed dilator may be a standard dilator (i.e. without electrodes), an electrified dilator of the type shown (by way of example only) in the &#39;840 patent, or a stationary directional dilator  10  of the present invention only having a smaller outer diameter than shown in the Figures. It will also be appreciated that the dilator  10  may also be dimensioned larger than shown so as to serve as a subsequent dilator within a series of sequential dilators. 
     The upper electrode surface  46  is electrically coupled to the electrode  16 , which may be accomplished in any number of suitable manners. By way of example only, this may be accomplished through the use of an electrical wire or lead extending through and/or along the strut  44   e  (for “electrical”) which is in alignment between the electrode marker  48  and the electrode  16 . During use the electrode surface  46  will be brought into electrical communication with the intraoperative monitoring system through the use of the electrical coupler  18  of  FIG. 1 . Based on the electrical communication between the electrode surface  46  and the electrode  16 , then, the electrode  16  will be in communication with the intraoperative monitoring system which, as described above, may be used to detect the presence and/or direction of surrounding nerves during the process of advancing the dilator  10  through tissue towards the surgical target site. 
       FIGS. 9-10  detail a rotatable electrode structure  14 ′ according to alternate aspect of the present invention. Based on the commonality of construction and components, and for the sake of brevity, only those features and functions varying from the rotatable electrode structure  14  of  FIGS. 5-6  will be described. Electrode structure  14 ′ is virtually identical to that shown in  FIGS. 5-6 , except that a generally tubular and hollow section  45  replaces the struts  44 . In this embodiment, an electrical wire or lead for connecting the surface electrode  46  and the electrode  16  may extend through and/or along the hollow section  45 . During use the electrode surface  46  will be brought into electrical communication with the intraoperative monitoring system through the use of the electrical coupler  18  of  FIG. 1 . Based on the electrical communication between the electrode surface  46  and the electrode  16 , the electrode  16  will be in communication with the intraoperative monitoring system which, as described above, may be used to detect the presence and/or direction of surrounding nerves during the process of advancing the dilator  10  through tissue towards the surgical target site. 
       FIGS. 11 through 13  detail the cap member  19  forming part of the stationary directional dilator  10  of  FIG. 1 . The cap member  19  includes an upper surface  60 , a lower surface  62 , an outer side wall  64 , an inner side wall  66 , and a central lumen  68  extending between the supper and lower surfaces  60 ,  62 . The diameter of the central  68  is preferably the same (or approximately so) as the diameter of the lumens  50 ,  56  of the rotatable electrode structure  14 . As described above, this permits the stationary directional dilator  10  of the present invention to be slidably advanced over a previously placed dilator having an outer diameter slightly less than the inner diameter of the stationary directional dilator  10 . It will be appreciated that any such previously placed dilator may be a standard dilator (i.e. without electrodes), an electrified dilator of the type shown (by way of example only) in the &#39;840 patent, or a stationary directional dilator  10  of the present invention only having a smaller outer diameter than shown in the Figures. The cap member  19  also includes an offset lumen  70  extending between the upper and lower surfaces  60 ,  62 . The offset lumen  70  is dimensioned to receive the electrical coupler  18 , which will be described in greater detail below. The upper and lower surfaces  60 ,  62  are preferably insulated and/or constructed from non-conductive material (e.g. plastic) such that any electricity flowing through the electrical connector  18  during use will not shunt or otherwise become misdirected to other components, the patient, or the surgeon. 
       FIGS. 14 and 15  detail the electrical coupling element  18  forming part of the stationary directional dilator  10  of  FIG. 1 . The electrical coupling element  18  includes a lower region  72 , an upper region  74 , and a middle region  76 . The lower region  72  is dimensioned to be received at least partially within the offset lumen  70  of the cap member  19  and includes a lower surface  78 . The lower surface  78  is dimensioned such that, in use, it will be brought into physical (and thus electrical) contact with the upper electrode surface  46  of the collar member  40  of the rotatable electrode structure  14 . The middle region  76  is dimensioned to be coupled to a clip or other electrical connector of the type shown and described in the &#39;840 patent for the purpose of establishing electrical communication between the electrical coupler  18  and the intraoperative monitoring system such as that shown in the &#39;840 patent. The middle region  76  has a slightly reduced diameter relative to that of the upper region  75 , which serves to prevent such an electrical clip or connector (not shown) from inadvertently disengaging from the electrical coupler  18  as may otherwise occur if it did not include the “stop” formed by the upper region  75 . 
       FIGS. 16-17  illustrate, by way of example only, a stationary directional dilator  10 ′ according to another aspect of the present invention. The stationary directional dilator  10 ′ includes a stationary dilator  12  and a rotatable electrode structure  14  with an electrode  16  at the distal end. The outer dilator  12  is generally tubular and hollow along its entire length (it does not include any proximal section  22  as found in  FIGS. 1-4 ) with an inner lumen (not shown, but the same as lumen  32  in  FIGS. 3-4 ) designed to accommodate and enable rotation of the rotatable electrode structure  14 . As shown in  FIGS. 18-19 , the rotatable electrode structure  14  is essentially identical to the embodiment shown and described above with reference to  FIGS. 9-10 , with the exception that the collar member  40  receives the electrical coupler  18  in an offset lumen  73 . In the interest of brevity, the common features and components need not be repeated. 
       FIGS. 20-21  illustrate, by way of example only, a stationary directional dilator  10 ″ according to another aspect of the present invention. The stationary directional dilator  10 ″ includes a stationary dilator  12  and a rotatable electrode structure  14  with an electrode  16  at the distal end. The outer dilator  12  is generally tubular and hollow along its entire length like that shown in  FIGS. 16-17  with an inner lumen (not shown, but the same as lumen  32  in  FIGS. 3-4 ) designed to accommodate and enable rotation of the rotatable electrode structure  14 . As shown in  FIGS. 22-23 , the rotatable electrode structure  14  is essentially identical to the embodiment shown and described above with reference to  FIGS. 18-19 , with the following exceptions. First, the collar member  40  includes a recess  41  having an electrode surface  47  dimensioned to be coupled to a “clamp” or “clip” style electrical connector to establish electrical communication between the intraoperative monitoring system and the stationary directional dilator  10 ″. Second, the electrode marker  48  is broken into two sections  48   a  (above recess  41 ) and  48   b  (below the recess  41 ), both of which are in alignment with the electrode  16 . In the interest of brevity, the other common features and components need not be repeated. 
     The components forming the stationary directional dilators  10 ,  10 ′,  10 ″ may be constructed from any number of suitable materials for carrying out the intended purpose of performing directional dilation during intraoperative monitoring. In this regard, all components other than the electrodes (e.g.  16 ,  18 ,  46 ,  47 ) may be made of plastic, carbon fiber or metal (e.g. aluminum) so long as any such metallic non-electrode components are adequately insulated to prevent unwanted shunting that may otherwise occur. The non-electrode structures (e.g. dilator  12 , electrode structure  14 , cap member,  19 , etc. . . . ) may be manufactured such that the electrodes are separate components fixed onto and/or molded into the respective part or portion of the non-electrode structure, such as where those structures are manufactured from non-conductive material (e.g. plastic, carbon fiber, etc.). Alternatively, if manufactured from a conductive material (e.g. aluminum, etc.), the non-electrode structures may be coated or otherwise treated with insulation to effectively creative the electrodes by covering all surface areas other than those areas intended to serve as electrodes. 
     The stationary directional dilators  10 ,  10 ′,  10 ″ of the present invention may be dimensioned according to the intended application. For example, these dilators  10 ,  10 ′,  10 ″ may be provided having a relatively short length if intended for use in posterior spine surgery procedures where the distances between the skin incision and surgical target site (e.g. disc space), ranging for example from 40 mm to 80 mm. If intended for use in lateral spine surgery, the length will be longer to accommodate the longer distance between the skin incision and the surgical target site, ranging for example from 90 mm to 150 mm. If intended to serve as a sequential dilator within series of sequential dilators, the stationary directional dilators  10 ,  10 ′,  10 ″ may be provided for each size of the series of sequential dilators or, alternatively, to serve as some but not all of the sequential dilators. Lastly, it will be appreciated that the stationary directional dilators  10 ,  10 ′,  10 ″ may be dimensioned to serve as the actual working corridor to the surgical target site. If intended for use as a working corridor, the stationary directional dilator may be dimensioned such that the inner lumen to pass the appropriate instruments and/or implants to the surgical target site. 
     In any of the foregoing embodiments ( 10 ,  10 ′,  10 ″), rotation of the electrode structure  14  may be manual, such as by manually twisting the collar member  40  in between the thumb and forefinger of a user (e.g. surgeon). By looking at the electrode marker  48 , the user will be able to tell the direction of the electrode  16  within the patient. Although not shown, the rotation of the electrode structure  14  may also be automated, such as by providing a motor or other transmission mechanism to drive the electrode structure  14  into rotation. Such an automated rotation mechanism may be part of the stationary directional dilators of the type described herein, or may be part of a robotic surgery system which drive the rotation of the electrode  16  relative to the stationary dilator  12 . In either case, such an automated rotation mechanism may obviate the need to manually twist the collar member  40  into motion or at least augment the speed of manual rotation. In either event, to accomplish this the intraoperative neuromonitoring system may be used to provide feedback to the motor to selectively rotate the ring member  42  to position the directional electrode  16  in order to determine the direction of the associated nerve or neural structure relative to the distal region of the dilator. 
       FIGS. 24-25  illustrate, by way of example only, a monitoring system  120  of the type disclosed in the &#39;840 patent suitable for use with the stationary directional dilator  10  of the present invention. (Although described in use with dilator  10 , it will be appreciated that use with the dilators  10 ′ and  10 ″ will be operate according to the same principles.) The monitoring system  120  includes a control unit  122 , a patient module  124 , and an EMG harness  126  and return electrode  128  coupled to the patient module  124 , and a cable  132  for establishing electrical communication between the patient module  124  and the stationary directional dilator  10  of the present invention. Although not shown, the stationary directional dilator  10  may be used along with various other surgical access instruments to create an operative corridor from a lateral approach through the psoas muscle to a surgical target site in the lumbar spine (e.g. K-wires, sequential dilators (smaller or larger diameter than that of dilator  10 ), split blade retractors, etc.). 
     Electrical communication can be achieved by providing, by way of example only, a hand-held stimulation controller  152  capable of selectively providing a stimulation signal (due to the operation of manually operated buttons on the hand-held stimulation controller  152 ) to one or more connectors  156   a ,  156   b ,  156   c . The connectors  156   a ,  156   b ,  156   c  are suitable to establish electrical communication between the hand-held stimulation controller  152  and (by way of example only) the stimulation electrode  16  on the stationary directional dilator  10 , as well as any electrodes on any other surgical access instruments having electrodes for use in nerve detection and/or proximity. 
     It will be appreciated that any number different types of connectors  156  may be used depending upon the manner of engaging or coupling to the stationary directional dilator  10 . By way of example only, connector  156   a  is a “plunger” type, connector  156   b  is a “clip” type, and connector  156   c  is a “pin” type. Connector  156   a  has a generally cylindrical contact  157  housed partially within a generally cylindrical body  158  and spring loaded towards an arm member  159 . The contact  157  may be moved away from the arm member  159  in order to be placed over an instrument to be electrified (for example electrode  18  of  FIGS. 1-2 ), at which point the contact  157  may be released in order to effectively pinch the instrument in between the contact  157  and arm member  159  to establish the desired electrical contact. Connector  156   b  includes a pair of arms which are spring loaded towards one another and include at least one electrode such that, when opened and closed over an instrument to be electrified (for example electrode  47  of  FIGS. 20-21 ), the arms effectively pinch the instrument to establish the desired electrical contact. Connector  156   c  includes a pin which may be introduced into a female electrical connector on the instrument to be electrified. 
     In order to use the monitoring system  120 , at least one of the connectors  156   a ,  156   b  and  156   c  will need to be coupled to the stationary directional dilator  10 , such as (by way of example) connecting connector  156   a  to the electrical coupler  18  in  FIG. 1-2  or by connecting connector  156   b  to the electrode  47  in  FIGS. 20-21 . The user may thereafter selectively initiate a stimulation signal (preferably, a current signal) from the control unit  122  to the electrode  16  of the stationary directional dilator  10 . Stimulating the electrode  16  on the stationary directional dilator  10  before, during and/or after establishing operative corridor will cause nerves that come into close or relative proximity to the surgical access instruments to depolarize, producing a response in a myotome associated with the innervated nerve. 
     The control unit  122  includes a touch screen display  140  and a base  142 , which collectively contain the essential processing capabilities (software and/or hardware) for controlling the monitoring system  120 . The control unit  122  may include an audio unit  118  that emits sounds according to a location of a surgical instrument with respect to a nerve. The patient module  124  is connected to the control unit  122  via a data cable  144 , which establishes the electrical connections and communications (digital and/or analog) between the control unit  122  and patient module  124 . The main functions of the control unit  122  include receiving user commands via the touch screen display  140 , activating stimulation electrodes on the surgical access instruments, processing signal data according to defined algorithms, displaying received parameters and processed data, and monitoring system status and report fault conditions. 
     The touch screen display  140  is preferably equipped with a graphical user interface (GUI) capable of communicating information to the user and receiving instructions from the user. The display  140  and/or base  142  may contain patient module interface circuitry (hardware and/or software) that commands the stimulation sources, receives digitized signals and other information from the patient module  124 , processes the EMG responses to extract characteristic information for each muscle group, and displays the processed data to the operator via the display  140 . 
     In one aspect, the monitoring system  120  is capable of determining nerve direction relative to the stationary directional dilator  10  before, during and/or following the creation of an operative corridor to a surgical target site. Monitoring system  120  accomplishes this by having the control unit  122  and patient module  124  cooperate to send electrical stimulation signals to the electrode  16  on the stationary directional dilator  10 . 
     Depending upon the location of the stationary directional dilator  10  within a patient (and more particularly, to any neural structures), the stimulation signals may cause nerves adjacent to or in the general proximity of the dilator  10  to depolarize. This causes muscle groups to innervate and generate EMG responses, which can be sensed via the EMG harness  126 . By monitoring the myotomes associated with the nerves (via the EMG harness  126  and recording electrodes  127 ) and assessing the resulting EMG responses (via the control unit  122 ), the system  120  is capable of detecting the presence of such nerves. 
     The stationary directional dilator  10  of the present invention allows a user to determine the direction to nerves. To do so, the user need simply hold the dilator  12  in a generally stationary position while rotating the electrode  16  as the stimulation signal is being emitted to the surrounding tissue. The closer the nerve is to the electrode  16 , the lower the electrical stimulation (e.g. current) required to innervate the nerve. As such, the user may simply rotate the electrode  16  to identify the point in its rotation relative to the dilator  12  at which the stimulation signal required to innervate the nerve is the lowest. This is facilitated by visualizing the electrode marker  48 , given the location of the electrode  16  within the patient. This point is the direction of the nerve relative to the stationary dilator  12 . 
     By determining the direction to adjacent nerves, the stationary directional dilator  10  provides the ability to actively negotiate around or past such nerves to safely and reproducibly form the operative corridor to a particular surgical target site. In spinal surgery, for example, this is particularly advantageous in that the stationary directional dilator  10  may be particularly suited for detecting the presence and direction of nerves during the process of establishing an operative corridor through the psoas muscle to an intervertebral target site in the lumbar spine. 
     Any of the features or attributes of the above the above described embodiments and variations can be used in combination with any of the other features and attributes of the above described embodiments and variations as desired. Various modifications, additions and other alternative embodiments are possible without departing from the true scope and spirit. The embodiments presented herein were chosen and described to provide an illustration of various principles of the present invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the present invention as determined by the appended claims when interpreted in accordance with the benefit to which they are fairly, legally, and equitably entitled.