Abstract:
Systems and method for accessing a spinal target site involving a plurality of tissue displacement members configured to be advanced to a spinal target site and moved relative to one another to provide surgical access to the spinal target site, and an expandable member disposed at least partially about at least a portion of the plurality of tissue displacement members and configured to prevent tissue from migrating into the spinal target site during surgical access.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]     The present application claims priority from commonly owned and co-pending U.S. patent application Ser. No. 11/489,020 filed Jul. 18, 2006, which claims priority from commonly owned U.S. Pat. No. 7,079,883 filed May 7, 2003 and U.S. Pat. No. 6,564,078 filed Jun. 4, 1999. Additionally, the present application claims benefit under 35 U.S.C. § 119(e) from U.S. Provisional Patent Application Ser. No. 60/113,651 filed Dec. 23, 1998; U.S. Provisional Patent Application Ser. No. 60/120,663 filed Feb. 12, 1999; and U.S. Provisional Patent Application Ser. No. 60/123,268 filed Mar. 8, 1999; the complete disclosures of which are hereby incorporated herein by reference in their entirety for all purposes. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     I. Field of the Invention  
         [0003]     The present invention relates to nerve surveillance systems and to cannulae systems for use in minimally invasive spinal surgery.  
         [0004]     II. Discussion of the Prior Art  
         [0005]     A significant danger of performing intervertebral operations or accessing an intervertebral space during spine surgery is that of inadvertently contacting or damaging the para-spinal nerves, including the exiting nerve roots, traversing nerves and the nerves of the cauda equina. The exact location of these para-spinal nerves cannot be determined prior to the commencement of surgery. Moreover, intervertebral spaces in the spine have other sensitive nerves disposed at locations which are not entirely predictable prior to insertion of the surgical tool into the intervertebral area. Accordingly, the danger of pinching or damaging spinal nerves when accessing an intervertebral space has proven to be quite limiting to the methods and devices used during minimally invasive spinal surgery. In addition, as cannulae are received through the patient&#39;s back, such as when performing minimally invasive spinal surgery, minor blood vessels are ruptured, thereby blocking the surgeon&#39;s vision inside the intervertebral region after the cannula has been inserted.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention provides nerve surveillance probes which are adapted to assist the surgeon in identifying the presence and location of para-spinal nerves as the probe is advanced during minimally-invasive surgery, thus providing a device for guiding the path of other surgical instruments to be inserted into this intervertebral space. In a preferred aspect of the present invention, an expandable tip cannula system is provided which functions both as an access portal for spinal surgery and as a system for nerve surveillance such that the presence and relative position of para-spinal nerves can be detected as the expandable tip cannula is inserted through the patient&#39;s facia and para-spinal musculature. An advantage of determining the position of the para-spinal nerve with respect to the distal tip of the cannula in particular is that the para-spinal nerve can be avoided or gently moved out of the surgeon&#39;s way while inserting the cannula. Accordingly, in a preferred aspect, the present invention provides a cannulated system which is adapted to assist the surgeon in guiding the path of surgical instruments received into the intervertebral space, while identifying the presence and location of para-spinal nerves as the cannula is advanced to a patient&#39;s intervertebral space during minimally invasive surgery.  
         [0007]     Optionally, the present nerve surveillance expandable tip cannula may also be adapted to selectively electrically induce cauterization of severed blood vessels when the cannula or other surgical instruments sever small blood vessels when they are inserted percutaneously into the patient and are advanced along a path into the patient&#39;s intervertebral space. An additional advantage of the present cannula system therefore is that, prior to piercing the annulus of an intervertebral disc, vessels on the surface of the disc may be cauterized to assure clear vision inside the disc after surgical entry is made.  
         [0008]     In one embodiment, the present expandable tip nerve surveillance cannula preferably comprises a hollow tubular body with an expandable tip portion mounted at its distal end. In a preferred aspect of the invention, the expandable tip portion comprises a plurality of generally triangular shaped petals which are held together in a radially-inwardly tapering arrangement by breakable seals disposed between adjacent petals. Since the expandable tip portion of the cannula tapers to a narrow blunt end, the cannula can be easily pushed through the patient&#39;s facia and spinal musculature using blunt dissection, while minimizing the amount of cutting and tearing of such structures.  
         [0009]     Alternatively, a central electrode can be disposed on a central obturator passing though the cannula and a second electrode can be disposed on a distal end of a second cannula, wherein the second cannula is used to open the petals.  
         [0010]     An obturator shaft which is slidably received within the hollow tubular cannula body provides support for the cannula, giving the cannula sufficient strength such that the cannula can be inserted percutaneously through the patient&#39;s facia and para-spinal musculature. Preferably, the obturator has a large solid handle which allows the surgeon to grasp and push the cannula through the resistance of the facia and para-spinal musculature.  
         [0011]     After the cannula has been inserted and is resting on the patient&#39;s annulus, an inner cannula or rod which is slidably received within the cannula is then used to separate the breakable seals, opening the petals radially outwards to a distance sufficient to provide access for surgical instruments passing therethrough.  
         [0012]     In some preferred aspects, an electrode is disposed in each of the petals, and most preferably at or near the distal end of each of the petals. In other aspects of the invention, a plurality of electrodes are radially disposed about the distal end of the obturator and the electrodes protrude out of a small hole defined by truncated petals, as will be explained.  
         [0013]     In various aspects of the present invention, the electrodes can be powered at a low level to thereby sense the position of a para-spinal nerve through continuous real time electromyographic monitoring, or alternatively, the electrodes can be powered at a higher level such that they operate to cauterize blood vessels. Safety systems ensure that power levels sufficient to cause cauterization are not activated if a nerve is sensed to be near the electrodes at the distal end of the cannula.  
         [0014]     In alternate embodiments, the present invention comprises an elongated nerve surveillance probe having one or more electrodes at its distal tip. In such aspects, the nerve surveillance probe is preferably advanced to the patient&#39;s intervertebral space through a cannula. In other alternate embodiments, the present nerve surveillance probe is received into the patient through various cannulae and expandable mesh trocars. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  is a side perspective view of a first nerve surveillance probe of the present invention.  
         [0016]      FIG. 2  is a sectional side elevation view of the first nerve surveillance probe positioned adjacent the spinal nerve with the first probe received within a first cannula which is itself received with an expandable mesh.  
         [0017]      FIG. 3  shows the probe of  FIG. 2 , but with the mesh expanded and a second cannula received thereover, (after the distal ends of the first cannula and expandable mesh have been advanced past the nerve).  
         [0018]      FIG. 4  is a sectional side elevation corresponding to  FIG. 3 , but with the first probe and first cannula removed.  
         [0019]      FIG. 5  is an end view corresponding to  FIG. 4 .  
         [0020]      FIG. 6  is a side perspective view of a second nerve surveillance probe of the present invention.  
         [0021]      FIG. 7  is a sectional side elevation view of a second nerve surveillance probe received within the second cannula.  
         [0022]      FIG. 8  is an end view corresponding to  FIG. 7 .  
         [0023]      FIGS. 9A, 9B  and  9 C sequentially show a schematic view of an expandable mesh system as moved from a contracted position ( FIG. 9A ) to an expanded position ( FIG. 9B ), and with an outer cannula received thereover ( FIG. 9C ).  
         [0024]      FIG. 10  is an end view of the nerve surveillance probe of  FIG. 6  pushing a nerve out of the way of an advancing cannula.  
         [0025]      FIG. 11  is an illustration of an expandable tip nerve surveillance probe of the present invention.  
         [0026]      FIG. 12  is a perspective distal view of the system of  FIG. 11 .  
         [0027]      FIG. 13  is a view of the distal tip of the system of  FIG. 12 , with the petals in a closed position.  
         [0028]      FIG. 14  is a view corresponding to  FIG. 13 , but with petals in an open position.  
         [0029]      FIG. 15  is a sectional view of the system of  FIG. 11 , with an obturator received therein and the petals in a closed position.  
         [0030]      FIG. 16  is a schematic illustration of the electrodes at the distal tip of the present invention, the electrodes being used to sense the position of a para-spinal nerve.  
         [0031]      FIG. 17  is a sectional view of the system of  FIG. 11  with an inner cannula received therein and the petals in an open position.  
         [0032]      FIG. 18  is a side view of an alternate embodiment of the distal tip region of the present invention having truncated petals.  
         [0033]      FIG. 19  is an end view corresponding to  FIG. 18 .  
         [0034]      FIG. 20  is a top plan view of a peel back expandable tip cannula.  
         [0035]      FIG. 21  is a side elevation view of the peel back cannula  FIG. 20 .  
         [0036]      FIG. 22  is a side sectional view of the peel back cannula of  FIG. 20  in a sealed position.  
         [0037]      FIG. 23  is a sectional side elevation view of the peel back cannula of  FIG. 20  in an open position.  
         [0038]      FIG. 24  is a top plan view corresponding to  FIG. 23 .  
         [0039]      FIG. 25  is a side elevation view of a curved petal nerve surveillance probe.  
         [0040]      FIG. 26  is a side elevation view corresponding to  FIG. 25 , but with the petals in an open position.  
         [0041]      FIG. 27  is a view corresponding to  FIG. 26 , but with an expandable elastomer shown wrapped around the distal end of the curved petals.  
         [0042]      FIG. 28  is a sectional elevation view of the distal end of an alternate nerve surveillance cannula.  
         [0043]      FIG. 29  is a perspective view of an alternate nerve surveillance probe.  
         [0044]      FIG. 30  shows the surveillance probe of  FIG. 29  with the petals opened by an inner cannula.  
         [0045]      FIG. 31  corresponds to  FIG. 30 , but with the internal obturator removed.  
         [0046]      FIG. 32  corresponds to  FIG. 30 , but with the internal obturator advanced distally.  
         [0047]      FIG. 33  corresponds to  FIG. 31 , but with the internal cannula advanced distally. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0048]     As will be set forth herein, the present invention encompasses both nerve surveillance probes which are received through cannulae, and various expandable tip cannulae comprising nerve surveillance probes at their distal ends.  
         [0049]     In a first preferred embodiment, as is seen in  FIG. 1 , an electromyography nerve surveillance probe  10  having a blunt end  11  is provided. Electrode  13  is disposed at the distal end of probe  10  and is charged by electrical contacts  15 . As electrode  13  approaches nerve  20  (as seen in  FIG. 2 ), the minimal threshold depolarization value elicited by the electrode will result in corresponding electromyography activity, such that the presence of nerve  20  can be sensed by standard electromyographic techniques, thus indicating the presence of the nerve. Specifically, using standard electromyographic techniques, the presence of nerve  20  will be sensed by appropriate needles or patches attached to the appropriate muscle as electrode  13  stimulates, and thereby depolarizes nerve  20 .  
         [0050]     In an exemplary method of application, (as is shown in  FIG. 2 ), the present nerve surveillance probe  10  can be advanced percutaneously through the patient&#39;s back in a posterolateral approach towards the patient&#39;s intervertebral space using the arrangement in which a first cannula  30  surrounds probe  10  as the probe is advanced. As probe  10  is advanced, it will then become positioned proximal nerve  20 . When this occurs, the presence of nerve  20  relative to probe  10  will be determined by the signal generated by electrode  13  as set forth above.  
         [0051]     In one preferred aspect of the present invention, an expandable mesh  32  is received over first cannula  30  such that expansion of this mesh from the contracted position shown in  FIG. 2  to the expanded position shown in  FIG. 3  will gently move nerve  20  out of the way.  
         [0052]     Also in a preferred aspect as shown in  FIG. 3 , a second cannula  34  can thereafter be received over expanded mesh  32 , thereby providing a large passageway  40  for intervertebral access when probe  10 , first cannula  30 , and expanded mesh  32  are removed as shown in  FIGS. 4 and 5 . Accordingly, the large passageway  40  into the intervertebral area provided by cannula  34  protects sensitive nerve  20  while providing access for surgical instruments therethrough, including such surgical instruments as intervertebral inserts, bone decorticators, cameras, articulating forceps, intervertebral inserts and intervertebral positioning systems.  
         [0053]     As is seen in  FIG. 6 , a second nerve surveillance probe  9  is also provided. Nerve surveillance probe  9  has a plurality of electrodes  12 ,  14 ,  16  and  18  disposed at radial locations adjacent to blunt distal end  8 , as is seen in  FIGS. 6, 7  and  8 . Radially disposed electrodes  12 ,  14 ,  16 , and  18  perform a variety of useful functions, as follows.  
         [0054]     Referring to  FIG. 8 , as electrodes  12 ,  14 ,  16 , and  18  are disposed at radial locations around the tip of probe  10 , the electrodes which are closest to nerve  20 , (in this case electrode  14 , and to a lesser degree electrodes  12  and  16 ), will operate to depolarize the nerve such that the presence of nerve  20  can be detected by standard electromyographic techniques. As such, a signal will be generated telling the operating surgeon that nerve  20  is proximal to electrode  14 . As can be appreciated, should nerve  20  instead be positioned in another orientation, the signal from electrodes  12 ,  14 ,  16  and  18  would instead indicate the presence of the nerve at a different location. Accordingly, probe  9  can be operated as a tool for inspecting the interior passageway of cannula  34  to determine if nerve  20  had become inadvertently trapped therein as cannulae  34  is advanced over expanded mesh  32 . Moreover, as the electrodes  12 ,  14 ,  16 , and  18  are disposed at radial locations around the distal end of the probe, it is possible to determine the exact location of nerve  20 . Preferably as well, each of electrodes  12 ,  14 ,  16 , and  18  will be activated in a repeating sequence with a sufficient delay time therebetween to detect an electromyographic response.  
         [0055]     In another aspect of the invention, radially disposed electrodes  12 ,  14 ,  16 , and  18  can be used for electrocoagulation of blood vessels, for example, blood vessels on the patient&#39;s annulus when accessing the patient&#39;s intervertebral region. Specifically, as a plurality of electrodes are disposed at the distal end of probe  9 , it is possible to pass current between various electrodes, thus cauterizing adjacent blood vessels.  
         [0056]     In another aspect of the invention, radially disposed electrodes  12 ,  14 ,  16 , and  18  can be used to assist in avoiding, or alternatively in moving, nerve  20  as follows. Referring to  FIG. 10 , nerve  20  will be determined to be adjacent to electrode  14  using the above set forth method. Probe  10  can then be gently moved in a radial direction away from electrode  14 , as is shown by arrow D, such that nerve  20  can then be gently pushed out of the way, providing safe access to the patient&#39;s intervertebral space. Alternatively, the movement of probe  10  in a direction opposite direction D will push the nerve out of the way such that a cannula can then be advanced past nerve  20  without damaging the nerve.  
         [0057]     In another aspect of the present invention as shown in  FIGS. 9A, 9B  and  9 C, the expansion of mesh  32  is controlled as follows. As is shown in  FIG. 9A , expandable mesh  32  is in a contracted position and is mounted on the end of a cannula  35 . A distal end of mesh  32  is positioned against the patient&#39;s annulus  40  or any other suitably hard bone structure. Pushing rod or cannula  35  in direction D 2  will compress mesh  34 , causing it to expand radially and shorten. This movement will displace nerve  20  (shown here in cross section). Following this, cannula  37  can be slid over expanded mesh  32  is seen in  FIG. 9B . Following this, cannula  37  can be advanced past nerve  20 , gently pushing nerve  20  still further out of the way, as shown in  FIG. 9C . Lastly, rod or cannula  35  and attached mesh  32  can be removed, leaving a large cannulated passageway to the annulus or intervertebral space.  
         [0058]     It is to be understood that the present nerve surveillance probes can be used without the expandable mesh system of  FIGS. 9A, 9B  and  9 C. Moreover, it is to be understood that the present method and apparatus of minimally invasive nerve surveillance can be used in any arthroscopic procedure.  
         [0059]     As can also be appreciated the present nerve surveillance probes are able to detect the presence of any other efferent skeletal motor nerve in addition to the spinal nerve and can thus be used in various surgical procedures. Alternatively, using evoked potential electromyography, the present nerve surveillance probes are also adapted to sense the presence of afferent sensory nerves in response to signals received in the spinal cord or cerebral cortex.  
         [0060]     In a second preferred embodiment, the present invention provides an expandable tip nerve surveillance cannula system  110  comprising an endoscopic hollow cannula shaft  112  having an expandable tip  113  comprised of a plurality of petals  114  (the details of petals  114  are better shown in  FIGS. 12, 13 , and  14 ). System  110  further comprises an obturator  120  which is slidably received within cannula shaft  112 . As is shown in  FIG. 15 , obturator  120  is a rigid structure which provides internal support to cannula shaft  112  such that cannula shaft  112  can be received percutaneously. Shaft  112  can have a cross section which is circular, oval, racetrack-shaped or any other design. By holding obturator handle  122 , the surgeon is able to advance cannula shaft  112  through the patient&#39;s para-spinal musculature and dock expandable tip  113  at the patient&#39;s annulus.  
         [0061]     As seen in  FIGS. 12 and 13 , expandable tip  113  is comprised of a plurality of petals  114 , held together by breakable seals  115 . Breakable seals  115  can be formed by an elastomeric material with predictable failure segments between the petals. In one preferred aspect each of petals  114  has an electrode  116  disposed therein as shown. Electrodes  116  serve the following important functions.  
         [0062]     First, electrodes  116  can be used for electromyography, and in particular to sense the presence and relative position of para-spinal nerves as cannula shaft  112  is advanced. Referring to  FIG. 16 , as can be seen electrodes  116   a ,  116   b ,  116   c ,  116   d ,  116   e  and  116   f  are disposed radially about cannula shaft  112 , with one electrode disposed in each of petals  114 , as has been described. Electrodes  116   a ,  116   b ,  116   c ,  116   d ,  116   e  and  116   f  assist in sensing the presence and location of para-spinal nerve  160  as follows. The electrodes closest to nerve  160  (in this case electrodes  116   b  and  116   c , and to a lesser degree, electrodes  116   a  and  116   d ) will operate to depolarize nerve  160  such that the presence of nerve  160  can be detected by electromyography. As such, shaft  112  can be moved in direction D, thereby avoiding nerve  160  as shaft  112  is inserted. Alternatively, of course, shaft  112  can be moved in the opposite direction to D, such that cannula shaft  112  gently moves nerve  160  out of the way. Moreover, when none of electrodes  116   a ,  116   b ,  116   c ,  116   d ,  116   e  and  116   f  sufficiently stimulate to depolarize the nerve, and thereby assist in its detection, shaft  112  can be safely advanced toward the patient&#39;s intervertebral space. Should each one of electrodes  116   a ,  116   b ,  116   c ,  116   d ,  116   e  and  116   f  depolarize the nerve, this would indicate that the nerve is directly in front of the advancing cannula shaft  112 . Accordingly, the cannula shaft could be moved such that contact with the nerve is avoided.  
         [0063]     Alternatively, when none of electrodes  116   a ,  116   b ,  116   c ,  116   d ,  116   e  and  116   f  indicate the presence of a nerve, electrodes  116   a ,  116   b ,  116   c ,  116   d ,  116   e  and  116   f  can be powered to a higher level such that a cauterization of minor blood vessels can be achieved by passing increased electric current between each of the various adjacent electrodes, thus cauterizing adjacent blood vessels. Preferably, the present invention comprises a safety system such that cauterization power levels for electrodes  116  are not activated when any of electrodes  116  sense the presence of a para-spinal nerve thereby.  
         [0064]     Preferably, each of electrodes  116   a ,  116   b ,  116   c ,  116   d ,  116   e  and  116   f  are operated in sequence, affording a sufficient latency period therebetween for the detection of an electromyographic signal.  
         [0065]     As seen in  FIG. 11 , button  121  can be used to activate the blood vessel cauterization functions. Buttons  121  and  123  are conveniently located on the near handle  122  such that they may be activated while the surgeon grips obturator handle  122 .  
         [0066]     Subsequent to being positioned at the patient&#39;s annulus, obturator  120  is removed from cannula shaft  112 . As seen in  FIG. 17 , inner cannula  130  is then inserted into cannula shaft  112 . Inner cannula  130  is then inserted into cannula shaft  112 . Inner cannula  130  is dimensioned to be of a size that, when fully inserted into shaft  112 , inner cannula  130  breaks apart seals  115 , forcing petals  114  to be displaced radially outwards to a distance of at least the internal diameter of shaft  112  as shown. Inner cannula  130  can alternately comprise a solid rod or obturator which is dimensioned to be received within shaft  112  to open petals  114 .  
         [0067]     As can be seen in  FIG. 13 , a notch  118  is found between adjacent petals  114  where petals  114  are mounted to the distal end  113  of cannula shaft  112 . Notches  118  operate to facilitate breakage of seals  115  by providing a stress relief region at the base of breakable seals  115 .  
         [0068]     In an alternate design, as shown in  FIGS. 18 and 19 , distal tip  113  comprises truncated petals  114   a  which, when sealed together by way of breakable seals  115 , meet at their distal end to define a small opening  117  at distal tip  113  of cannula shaft  112 . In this design, an obturator  120   a  is slidably received within cannula shaft  112 . Obturator  120   a  has a narrow distal end  113   a  which protrudes through opening  117 . Electrodes  119   a ,  119   b ,  119   c ,  119   d ,  119   e  and  119   f  are disposed radially about the narrow distal end  113   a  of obturator  120   a , functioning similar to the probe design shown in  FIG. 6 .  
         [0069]     In this alternate design of  FIGS. 18 and 19 , nerve surveillance and blood vessel cauterization functions as described above and as performed by electrodes  116  on petals  114  are instead performed by electrodes  119  on obturator  120   a . In this aspect of the invention, petals  114   a  are truncated and obturator  120   a  protrudes therethrough.  
         [0070]     In another alternate embodiment, a peel back cannula having an expandable tip is provided. Referring to  FIG. 20 , cannula  150  is provided. Cannula  150  has a tapered narrow distal end  152  and a tear away line  153  which is formed in the preferred polymeric material of cannula  150 . Tear away line  153  will split under tension as will be explained. Cannula  150  may also comprise electrodes  153  which perform a similar function to the electrodes  116  described herein. Electrodes  153  can be disposed axially along the length of cannula  150 , or radially around the distal end of cannula  150 , or some combination thereof.  
         [0071]     An advantage of being disposed axially along the cannula is that electrodes  153  will be able to sense the position of a nerve relative to the cannula in an axial dimension. Similarly, an advantage of being disposed radially around the cannula is that the electrodes will be able to sense the position of a nerve relative to the cannula in a radial dimension. It is to be understood that all embodiments of the present invention comprise the concept of nerve surveillance electrodes disposed both radially around and axially along the nerve surveillance cannula or obturator, and that the radial electrode placement shown in the design of  FIGS. 7, 8  and  11  to  19 , and the axial electrode placement shown in the design of FIGS.  20  to  23  is not limiting.  
         [0072]     In a preferred method of operation, cannula  150  is advanced such that its tapered end  152  is adjacent nerve  160  as is seen in  FIG. 22 . An obturator  155  is positioned within cannula  150 . Obturator  155  provides structural support for the cannula as it is being inserted or as it is moving a nerve. Obturator  155  provides structural support for the cannula as it is being inserted or as it is moving a nerve. Obturator  155  is thereafter removable such that cannula  150  operates as an open passageway as will be explained.  
         [0073]     A narrow inner cannula  157  may also be provided. Cannula  157  is received around obturator  155  and within cannula  150 . When the operator has determined it is safe and desirable to open cannula  150 , inner cannula  157  is advanced to the position shown in  FIGS. 23 and 24 . Specifically, inner cannula  157  pushes against the tapered end  152  of cannula  150  causing cannula  150  to split open along tear away line  153 . Accordingly, inner cannula  157  can be used to provide a cannulated passageway when obturator  157  has been withdrawn therefrom. Alternatively, inner cannula  157  can be replaced by suitably dimensioned obturator for opening cannula  150  along tear away line  153 .  
         [0074]     Tear away line  153  may be formed by scribing the polymeric material forming cannula  150 . Tear away line  153  preferably runs some distance along opposite sides of the open end  152  of cannula  150 . Alternatively, tear away line  153  can be disposed along the top and bottom of cannula  150  as shown.  
         [0075]      FIG. 25  is a side view of a curved petal design of the present invention in a closed position with cannula  220  having outwardly curved petals  212  at distal end  215 . A nerve  230  is disposed adjacent the ends of closed petals  212  as shown. Petals  212  are then opened, using methods described herein, as shown in  FIG. 26 . The opening of petals  212  causes nerve  230  to be generally displaced upward away from an operative site which may preferably comprise a patient&#39;s intervertebral disk  240 .  
         [0076]     As shown in  FIG. 27 , an elastomer  250  can be wrapped around the petals  212  such that nerves are not pinched in gaps  213  between the adjacent petals either when the petals are first opened or when the petals are closed during the removal of the cannula from the patient. It is to be appreciated that elastomer  250  could also be wrapped around the ends of any of the straight petal designs shown in FIGS.  11  to  19 .  
         [0077]     The operative site or target site may comprise a patient&#39;s intervertebral disk  240  when the present invention is used in minimally invasive spinal surgery. It is to be understood, however, that the present expandable tip cannula can be used in all manner of minimally invasive surgery and is especially useful for approaching any target site having sensitive nerves adjacent thereto since the present invention is specifically adapted to gently push the nerve out of the way as the petals are opened, thereby providing a cannulated access portal for the insertion and removal of various surgical devices through cannula  220 .  
         [0078]      FIG. 28  shows an alternate design of the distal end  302  of a nerve surveillance cannula  300 . Cannula  300  has a plurality of expanding petals  314 , with each petal  314  comprising an electrode  316  adapted for nerve surveillance or blood vessel cauterization as described above. In this aspect of the invention, an obturator  310  protrudes through an opening between petals  314 , as shown. As can be seen, obturator  310  may preferably be tapered to a narrow distal end  302 , which assists in easing cannula  300  through the patient&#39;s facia and para-spinal musculature and into the patient&#39;s intervertebral space. In addition, distal end  302  of obturator  310  can be shaped to latch against the ends of petals  314 , as shown, thereby assisting in holding together petals  314  as cannula  300  is advanced.  
         [0079]     Preferably, obturator  310  further comprises a centrally disposed electrode  320 . Electrode  320 , being axially displaced from electrodes  316  is adapted to sense the position of a nerve in the axial direction as probe  300  approaches the nerve. Subsequent to placement at the patient&#39;s intervertebral space, an internal cannula  315  can be advanced distally to open petals  314  with obturator  310  being advanced slightly to first un-latch the distal ends of petals  314  and then withdrawn from cannula  300 , providing a cannulated access to the patient&#39;s intervertebral space.  
         [0080]      FIG. 29  through  33  show an alternative nerve surveillance cannula and probe system  400 , comprising a cannula  402  having a plurality of radially outwardly extending petals  404 . An internal obturator  500  is received within cannula  402 . Obturator  500  has an electrode  502  disposed at its distal end as shown in  FIG. 30 . Electrode  502  can also be seen at distal end of cannula  402  in  FIG. 29 . Electrode  502  operates to stimulate and thereby, depolarize a nerve as cannula  402  is advanced towards the patient&#39;s intervertebral space.  FIG. 29  shows cannula  402  with petals  404  closed around electrode  502  as the cannula is advanced.  
         [0081]      FIG. 30  shows an inner cannula  550  which is advanced through cannula  402  to open petals  404  as shown. Inner cannula  550  preferably comprises an electrode  510  which is disposed around the distal end of the cannula, as shown. After inner cannula  550  has opened petals  404 , as shown, electrode  502  is turned off and obturator  500  is removed from inner cannula  550  as is shown in  FIG. 31 . Electrode  510  remains turned on such that it is adapted to detect whether a nerve is positioned close to entering within cannula  550 , or whether a surgical instrument advanced through cannula  550  would contact a nerve proximal electrode  510 .  
         [0082]     As is shown in  FIG. 32 , obturator  500  can thereafter be advanced through cannula  550  to bluntly divide and dilate the annulus of a disc. In this aspect of the invention, electrode  502  is turned off as the annulus is divided and dilated. Annular electrode  510  may preferably be turned on during this procedure to sense the presence of nerves adjacent the distal end of cannula  550 .  
         [0083]     As is seen in  FIG. 33 , after the annulus has been divided and dilated, obturator  500  can be withdrawn from cannula  550  with cannula  550  advanced distally into the hole cut into the annulus. As such, a safe cannulated access way into the annulus or other region of the patient&#39;s body is provided.