Patent Publication Number: US-2020281578-A1

Title: Surgical access system and related methods

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/820,134 filed Nov. 21, 2017 (currently pending), which is a continuation of U.S. patent application Ser. No. 15/059,215 filed Mar. 2, 2016 (now U.S. Pat. No. 9,820,729), which is a continuation of U.S. patent application Ser. No. 14/959,454 filed Dec. 4, 2015 (now U.S. Pat. No. 9,572,562), which is a continuation of U.S. patent application Ser. No. 14/599,237 filed Jan. 16, 2015 (now U.S. Pat. No. 9,204,871), which is a continuation of U.S. patent application Ser. No. 14/195,227 filed Mar. 3, 2014 (now U.S. Pat. No. 8,956,283), which is a continuation of U.S. patent application Ser. No. 14/018,173 filed Sep. 4, 2013 (now U.S. Pat. No. 8,663,100), which is a continuation of U.S. patent application Ser. No. 13/756,908 filed Feb. 1, 2013 (now U.S. Pat. No. 8,679,006), which is a continuation of U.S. patent application Ser. No. 13/486,093, filed Jun. 1, 2012 (now U.S. Pat. No. 8,512,235), which is a continuation of U.S. patent application Ser. No. 12/650,271 filed Dec. 30, 2009 (now U.S. Pat. No. 8,192,357), which is a continuation of U.S. patent application Ser. No. 10/682,568 filed Oct. 8, 2003 (now U.S. Pat. No. 8,137,284), which claims the benefit of priority from U.S. Provisional Patent Application Ser. No. 60/417,235 filed Oct. 8, 2002, the entire contents of these applications are hereby expressly incorporated by reference into this disclosure as if set forth fully herein. The present application also incorporates by reference the following patent applications in their entireties (collectively, the “NeuroVision Applications”): PCT App. Ser. No. PCT/US02/22247, entitled “System and Methods for Determining Nerve Proximity, Direction, and Pathology During Surgery,” filed on Jul. 11, 2002; PCT App. Ser. No. PCT/US02/30617, entitled “System and Methods for Performing Surgical Procedures and Assessments,” filed on Sep. 25, 2002; PCT App. Ser. No. PCT/US02/35047, entitled “System and Methods for Performing Percutaneous Pedicle Integrity Assessments,” filed on Oct. 30, 2002; PCT App. Ser. No. PCT/US03/02056, entitled “System and Methods for Determining Nerve Direction to a Surgical Instrument,” filed Jan. 15, 2003. 
    
    
     BACKGROUND 
     I. Field 
     The present invention relates generally to systems and methods for performing surgical procedures and, more particularly, for accessing a surgical target site in order to perform surgical procedures. 
     II. Description of Related Art 
     A noteworthy trend in the medical community is the move away from performing surgery via traditional “open” techniques in favor of minimally invasive or minimal access techniques. Open surgical techniques are generally undesirable in that they typically require large incisions and high amounts of tissue displacement to gain access to the surgical target site, which produces concomitantly high amounts of pain, lengthened hospitalization (increasing health care costs), and high morbidity in the patient population. Less-invasive surgical techniques (including so-called “minimal access” and “minimally invasive” techniques) are gaining favor due to the fact that they involve accessing the surgical target site via incisions of substantially smaller size with greatly reduced tissue displacement requirements. This, in turn, reduces the pain, morbidity and cost associated with such procedures. The access systems developed to date, however, fail in various respects to meet all the needs of the surgeon population. 
     One drawback associated with prior art surgical access systems relates to the ease with which the operative corridor can be created, as well as maintained over time, depending upon the particular surgical target site. For example, when accessing surgical target sites located beneath or behind musculature or other relatively strong tissue (such as, by way of example only, the psoas muscle adjacent to the spine), it has been found that advancing an operative corridor-establishing instrument directly through such tissues can be challenging and/or lead to unwanted or undesirable effects (such as stressing or tearing the tissues). While certain efforts have been undertaken to reduce the trauma to tissue while creating an operative corridor, such as (by way of example only) the sequential dilation system of U.S. Pat. No. 5,792,044 to Foley et al., these attempts are nonetheless limited in their applicability based on the relatively narrow operative corridor. More specifically, based on the generally cylindrical nature of the so-called “working cannula,” the degree to which instruments can be manipulated and/or angled within the cannula can be generally limited or restrictive, particularly if the surgical target site is a relatively deep within the patient. 
     Efforts have been undertaken to overcome this drawback, such as shown in U.S. Pat. No. 6,524,320 to DiPoto, wherein an expandable portion is provided at the distal end of a cannula for creating a region of increased cross-sectional area adjacent to the surgical target site. While this system may provide for improved instrument manipulation relative to sequential dilation access systems (at least at deep sites within the patient), it is nonetheless flawed in that the deployment of the expandable portion may inadvertently compress or impinge upon sensitive tissues adjacent to the surgical target site. For example, in anatomical regions having neural and/or vasculature structures, such a blind expansion may cause the expandable portion to impinge upon these sensitive tissues and cause neural and/or vasculature compromise, damage and/or pain for the patient. 
     This highlights yet another drawback with the prior art surgical access systems, namely, the challenges in establishing an operative corridor through or near tissue having major neural structures which, if contacted or impinged, may result in neural impairment for the patient. Due to the threat of contacting such neural structures, efforts thus far have largely restricted to establishing operative corridors through tissue having little or substantially reduced neural structures, which effectively limits the number of ways a given surgical target site can be accessed. This can be seen, by way of example only, in the spinal arts, where the exiting nerve roots and neural plexus structures in the psoas muscle have rendered a lateral or far lateral access path (so-called trans-psoas approach) to the lumbar spine virtually impossible. Instead, spine surgeons are largely restricted to accessing the spine from the posterior (to perform, among other procedures, posterior lumbar interbody fusion (PLIF)) or from the anterior (to perform, among other procedures, anterior lumbar interbody fusion (ALIF)). 
     Posterior-access procedures involve traversing a shorter distance within the patient to establish the operative corridor, albeit at the price of oftentimes having to reduce or cut away part of the posterior bony structures (i.e. lamina, facets, spinous process) in order to reach the target site (which typically comprises the disc space). Anterior-access procedures are relatively simple for surgeons in that they do not involve reducing or cutting away bony structures to reach the surgical target site. However, they are nonetheless disadvantageous in that they require traversing through a much greater distance within the patient to establish the operative corridor, oftentimes requiring an additional surgeon to assist with moving the various internal organs out of the way to create the operative corridor. 
     The present invention is directed at eliminating, or at least minimizing the effects of, the above-identified drawbacks in the prior art. 
     SUMMARY 
     The present invention accomplishes this goal by providing a novel access system and related methods which involve: (1) distracting the tissue between the patient&#39;s skin and the surgical target site to create an area of distraction (otherwise referred to herein as a “distraction corridor”); (2) retracting the distraction corridor to establish and maintain an operative corridor; and/or (3) detecting the existence of (and optionally the distance and/or direction to) neural structures before, during and after the establishment of the operative corridor through (or near) any of a variety of tissues having such neural structures which, if contacted or impinged, may otherwise result in neural impairment for the patient. 
     As used herein, “distraction” or “distracting” is defined as the act of creating a corridor (extending to a location at or near the surgical target site) having a certain cross-sectional area and shape (“distraction corridor”), and “retraction” or “retracting” is defined as the act of creating an operative corridor by increasing or maintaining the cross-sectional area of the distraction corridor (and/or modifying its shape) with at least one retractor blade such that surgical instruments can be passed through operative corridor to the surgical target site. It is expressly noted that, although described herein largely in terms of use in spinal surgery, the access system of the present invention is suitable for use in any number of additional surgical procedures, including those wherein tissue having significant neural structures must be passed through (or near) in order to establish an operative corridor. 
     According to one broad aspect of the present invention, the access system comprises a tissue distraction assembly and a tissue retraction assembly. The tissue distraction assembly (in conjunction with one or more elements of the tissue retraction assembly) is capable of, as an initial step, distracting a region of tissue between the skin of the patient and the surgical target site. The tissue retraction assembly is capable of, as a secondary step, being introduced into this distracted region to thereby define and establish the operative corridor. Once established, any of a variety of surgical instruments, devices, or implants may be passed through and/or manipulated within the operative corridor depending upon the given surgical procedure. 
     The tissue distraction assembly may include any number of components capable of performing the necessary distraction. By way of example only, the tissue distraction assembly may include a K-wire, an initial dilator of split construction, and one or more dilators of traditional (that is, non-split) construction for performing the necessary tissue distraction to receive the remainder of the tissue retractor assembly thereafter. One or more electrodes may be provided on one or more of the K-wire and dilator(s) to detect the presence of (and optionally the distance and/or direction to) neural structures during tissue distraction. 
     The tissue retraction assembly may include any number of components capable of performing the necessary retraction. By way of example only, the tissue retraction assembly may include one or more retractor blades extending proximally from the surgical target site for connection with a portion (more specifically, a pivot linkage assembly) of the speculum assembly. The retractor blades may be equipped with a mechanism for transporting or emitting light at or near the surgical target site to aid the surgeon&#39;s ability to visualize the surgical target site, instruments and/or implants during the given surgical procedure. According to one embodiment, this mechanism may comprise, but need not be limited to, providing one or more strands of fiber optic cable within the walls of the retractor blades such that the terminal (distal) ends are capable of emitting light at or near the surgical target site. According to another embodiment, this mechanism may comprise, but need not be limited to, constructing the retractor blades of suitable material (such as clear polycarbonate) and configuration such that light may be transmitted generally distally through the walls of the retractor blade light to shine light at or near the surgical target site. This may be performed by providing the retractor blades having light-transmission characteristics (such as with clear polycarbonate construction) and transmitting the light almost entirely within the walls of the retractor blade (such as by frosting or otherwise rendering opaque portions of the exterior and/or interior) until it exits a portion along the interior (or medially-facing) surface of the retractor blade to shine at or near the surgical target site. The exit portion may be optimally configured such that the light is directed towards the approximate center of the surgical target site and may be provided along the entire inner periphery of the retractor blade or one or more portions therealong. 
     The retractor blades may be optionally dimensioned to receive and direct a rigid shim element to augment the structural stability of the retractor blades and thereby ensure the operative corridor, once established, will not decrease or become more restricted, such as may result if distal ends of the retractor blades were permitted to “slide” or otherwise move in response to the force exerted by the displaced tissue. In a preferred embodiment, only the posterior and anterior retractor blades are equipped with such rigid shim elements, which are advanced into the disc space after the posterior and anterior retractor blades are positioned. The rigid shim elements are preferably oriented within the disc space such that they distract the adjacent vertebral bodies, which serves to restore disc height. They are also preferably advanced a sufficient distance within the disc space (preferably past the midline), which serves the dual purpose of preventing post-operative scoliosis and forming a protective barrier (preventing the migration of tissue (such as nerve roots) into the operative field and the inadvertent advancement of instruments outside the operative field). 
     According to yet another aspect of the present invention, any number of distraction assemblies and/or retraction assemblies (including but not limited to those described herein) may be equipped to detect the presence of (and optionally the distance and/or direction to) neural structures during the steps tissue distraction and/or retraction. To accomplish this, one or more stimulation electrodes are provided on the various components of the distraction assemblies and/or retraction assemblies, a stimulation source (e.g. voltage or current) is coupled to the stimulation electrodes, a stimulation signal is emitted from the stimulation electrodes as the various components are advanced towards the surgical target site, and the patient is monitored to determine if the stimulation signal causes muscles associated with nerves or neural structures within the tissue to innervate. If the nerves innervate, this indicates that neural structures may be in close proximity to the distraction and/or retraction assemblies. 
     This monitoring may be accomplished via any number of suitable fashions, including but not limited to observing visual twitches in muscle groups associated with the neural structures likely to found in the tissue, as well as any number of monitoring systems. In either situation (traditional EMG or surgeon-driven EMG monitoring), the access system of the present invention may advantageously be used to traverse tissue that would ordinarily be deemed unsafe or undesirable, thereby broadening the number of manners in which a given surgical target site may be accessed. 
    
    
     
       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 perspective view of a tissue retraction assembly (in use) forming part of a surgical access system according to the present invention, including a posterior retractor blade  12 , an anterior retractor blade  14 , and two supplemental retractor blades  16 ,  18 ; 
         FIGS. 2 and 3  are front and back views, respectively, of the posterior and anterior retractor blades  12 ,  14  shown in  FIG. 1 ; 
         FIGS. 4 and 5  are front and back views, respectively, of shim element  22 ,  24  according to the present invention, dimensioned to be engaged with the inner surface of the posterior and anterior retractor blades of  FIG. 1  for the purpose of positioning a shim extension  80  within the disc space; 
         FIG. 6  is a perspective view illustrating the back of the supplemental retractor blades  16 ,  18  shown in  FIG. 1  according to the present invention; 
         FIG. 7  is a perspective view illustrating the components and use of an initial distraction assembly  40  (i.e. K-wire  42 , an initial dilating cannula  44  with handle  46 , and a split-dilator  48  housed within the initial dilating cannula  44 ) forming part of the surgical access system according to the present invention, for use in distracting to a surgical target site (e.g. disk space); 
         FIG. 8  is a perspective view illustrating the K-wire  42  and split-dilator  48  of the initial distraction assembly  40  with the initial dilating cannula  44  and handle  46  of  FIG. 7  removed; 
         FIG. 9  is a posterior view of the vertebral target site illustrating the split-dilator  48  of the present invention in use distracting in a generally cephalad-caudal fashion according to one aspect of the present invention; 
         FIG. 10  is a perspective view illustrating the split-dilator  48  of the present invention in use distracting in a generally posterior-anterior fashion according to another aspect of the present invention and in use with a posterior retractor blade  12  forming part of the retractor assembly  10  of the present invention; 
         FIG. 11  is a perspective view illustrating a shim introducer  51  introducing the posterior shim element  22  such that the shim extension  80  is positioned within the disc space; 
         FIG. 12  is a perspective view illustrating an anterior retractor blade  14  forming part of the retractor assembly  10  of the present invention being introduced with a gripping assembly  53  in association with the anterior half  48   b  of the split dilator  48  of the present invention; 
         FIG. 13  is a perspective view of the anterior retractor blade  14  being moved away from the posterior retractor blade  12  through the use of a plurality of sequentially dilating cannula  50 ; 
         FIG. 14  is a perspective view illustrating the introduction of supplemental retractor blades  16 ,  18  according to the present invention; 
         FIG. 15  is a perspective view illustrating a shim introducer  51  introducing the anterior shim element  24  such that the shim extension  80  is positioned within the disc space; 
         FIG. 16  is a perspective view of an exemplary nerve monitoring system capable of performing nerve monitoring before, during and after the creating of an operative corridor to a surgical target site using the surgical access system in accordance with the present invention; 
         FIG. 17  is a block diagram of the nerve monitoring system shown in  FIG. 16 ; and 
         FIGS. 18-19  are screen displays illustrating exemplary features and information communicated to a user during the use of the nerve monitoring system of  FIG. 16 . 
     
    
    
     DETAILED DESCRIPTION 
     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 surgical access system disclosed herein boasts a variety of inventive features and components that warrant patent protection, both individually and in combination. 
     It is furthermore to be readily understood that, although discussed below primarily within the context of spinal surgery, the surgical access system and related methods of the present invention may find applicability in any of a variety of surgical and/or medical applications such that the following description relative to the spine is not to be limiting of the overall scope of the present invention. Moreover, while described below employing the nerve monitoring features described above (otherwise referred to as “nerve surveillance”) during spinal surgery, it will be appreciated that such nerve surveillance will not be required in all situations, depending upon the particular surgical target site (e.g. disk space, vertebral body, and/or internal organ) and surgical approach (e.g. lateral, posterior, anterior, and/or postero-lateral approaches to the spine). 
     The present invention is directed at a novel surgical access system and related methods which involve creating and maintaining an operative corridor to the surgical target site, and optionally detecting the existence of (and optionally the distance and/or direction to) neural structures before, during and/or after this process (including the steps of distraction and/or retraction). 
     Distraction followed by retraction is advantageous because it provides the ability to more easily position an operative corridor-establishing device through tissue that is strong, thick or otherwise challenging to traverse in order to access a surgical target site. The various distraction systems of the present invention are advantageous in that they provide an improved manner of atraumatically establishing a distraction corridor prior to the use of the retraction systems of the present invention. The various retractor systems of the present invention are advantageous in that they provide an operative corridor having improved cross-sectional area and shape (including customization thereof) relative to the prior art surgical access systems. Moreover, by optionally equipping the various distraction systems and/or retraction systems with one or more electrodes, an operative corridor may be established through (or near) any of a variety of tissues having such neural structures which, if contacted or impinged, may otherwise result in neural impairment for the patient. 
     The present invention involves accessing a surgical target site in a fashion less invasive than traditional “open” surgeries and doing so in a manner that provides access in spite of the neural structures required to be passed through (or near) in order to establish an operative corridor to the surgical target site. Generally speaking, the surgical access system of the present invention accomplishes this by providing a tissue distraction assembly and a tissue retraction assembly, both of which may be equipped with one or more electrodes for use in detecting the existence of (and optionally the distance and/or direction to) neural structures. These electrodes are preferably provided for use with a nerve surveillance system such as, by way of example, the type shown and described in the NeuroVision Applications referenced above, the entire contents of which are expressly incorporated by reference as if set forth herein in their entirety. 
     Generally speaking, this nerve surveillance is capable of detecting the existence of (and optionally the distance and/or direction to) neural structures during the distraction and retraction of tissue by detecting the presence of nerves by applying a stimulation signal to such instruments and monitoring the evoked EMG signals from the myotomes associated with the nerves being passed by the distraction and retraction systems of the present invention. In so doing, the system as a whole (including the surgical access system of the present invention) may be used to form an operative corridor through (or near) any of a variety of tissues having such neural structures, particularly those which, if contacted or impinged, may otherwise result in neural impairment for the patient. In this fashion, the access system of the present invention may be used to traverse tissue that would ordinarily be deemed unsafe or undesirable, thereby broadening the number of manners in which a given surgical target site may be accessed. 
     The tissue distraction assembly of the present invention (comprising a K-wire, an initial dilator, and a split-dilator disposed within the initial dilator) is employed to distract the tissues extending between the skin of the patient and a given surgical target site (preferably along the posterior region of the target intervertebral disc) such that, once distracted, the resulting void or distracted region within the patient is of sufficient size to accommodate the introduction of a posterior retractor blade forming part of a tissue retraction assembly of the present invention. This forms the posterior border of the resulting operative corridor. Following (or contemporaneous with) this, a posterior shim element (which is preferably slideably engaged with the posterior retractor blade) may be advanced such that a shim extension in positioned within the posterior region of the disc space. An anterior retractor blade may then be introduced (in generally abutting relation with the posterior retractor blade) and thereafter moved anteriorly to increase the AP (or “width”) dimension of the operative corridor. Once in the appropriate anterior position, the anterior retractor blade may be locked in position and, thereafter, an anterior shim element advanced therealong for positioning a shim extension within the anterior of the disc space. The shim elements serve to distract the adjacent vertebral bodies (thereby restoring disc height), to form protective barriers (against the migration of tissue into (or instruments out of) the operative site), and to rigidly couple the posterior and anterior retractor blades in fixed relation relative to the vertebral bodies. First and second supplemental retractor blades (disposed caudal and cephalad) are also preferably employed to establish and maintain the “height” dimension of the operative corridor. Once established, any of a variety of surgical instruments, devices, or implants may be passed through and/or manipulated within the operative corridor depending upon the given surgical procedure. 
       FIG. 1  illustrates a tissue retraction assembly  10  forming part of a surgical access system according to the present invention. The retraction assembly  10  includes a posterior retractor blade  12 , an anterior retractor blade  14 , and supplemental retractor blades  16 ,  18 , all of which are coupled to a mounting structure  20 . Posterior and anterior retractor blades  12 ,  14  establish an AP (or “width”) dimension of an operative corridor  15 . Posterior retractor blade  12  and anterior retractor blade  14  are equipped with shim elements  22 ,  24 , respectively. Shim elements  22 ,  24  serve to distract the adjacent vertebral bodies (thereby restoring disc height), form protective barriers (against the migration of tissue into (or instruments out of) the operative site), and rigidly couple the posterior and anterior retractor blades  12 ,  14  in fixed relation relative to the vertebral bodies. First and second supplemental retractor blades  16 ,  18  (disposed caudal and cephalad) establish and maintain the “height” dimension of the operative corridor  15 . 
     Any number of suitable mounting units (not shown) may be employed to maintain the retraction assembly  10  in a fixed and rigid fashion relative to the patient. The mounting structure  20  may be coupled to any number of mechanisms for rigidly registering the mounting structure  20  in fixed relation to the operative site, such as through the use of an articulating arm mounted to the operating table. The mounting structure  20  includes posterior and anterior struts  26 ,  28  disposed in hinged relation to fixed strut members  30 ,  32 . The hinged nature of struts  26 ,  28  allows the posterior and anterior retractor blades  12 ,  14  to be adjusted independently of one another. The proximal portion of each of the retractor blades  12 - 18  is preferably provided in a split or forked fashion to accommodate locking assemblies  36  for locking the position of the retractor blades  12 - 18  with respect to the mounting assembly  20 . 
     The retractor blades  12 ,  14  ( FIGS. 2-3 ) are dimensioned to detachably couple with the shim elements  22 ,  24  ( FIGS. 4-5 ), respectively. In one embodiment, shown best in  FIG. 3 , this is accomplished by providing the inner surface  72  of each retractor blade  12 ,  14  with an engagement groove  70  (having, by way of example, a female dove-tail configuration as shown) along the midline thereof and one or more recess  74  disposed at or near the distal end. As best seen in  FIG. 5 , the exterior surface  73  of each shim element  22 ,  24  is provided with an elongated engagement member  76  (having, by way of example, a male dove-tail configuration as shown) dimensioned to be slideably received within the engagement groove  70  of the retractor blades  12 ,  14 , along with a pair of generally square-shaped engagement members  78  dimensioned to “snap” into the recesses  74  disposed at the distal ends of the retractor blades  12 ,  14 . 
     The inner surface  75  of each shim elements  22 ,  24  ( FIG. 4 ) is provided with a generally concave region  77  having an aperture  79  formed therein for engagement with a shim introducer (shown generally in  FIGS. 11 and 15 ) for the purpose of controlling the engagement between the shim elements  22 ,  24  and the retractor blades  12 ,  14 , respectively, as well as the advancement of the distal regions  80  of the shim elements  22 ,  24  into the surgical target site (e.g. disc space). As best seen in  FIGS. 2 and 5 , the retractor blades  12 ,  14  and shim elements  22 ,  24  may be respectively provided with one or more electrodes  82 ,  84  for use in undertaking the nerve surveillance techniques described herein. The same is true for supplemental retractor blades  16 ,  18  ( FIG. 6 ), which may also be provided with one or more electrodes  86  for use in undertaking the nerve surveillance techniques described herein. 
     The retractor blades  12 - 18 , as well as the shim elements  22 ,  24  may optionally be equipped with any number of different mechanisms for transporting or emitting light at or near the surgical target site to aid the surgeon&#39;s ability to visualize the surgical target site, instruments and/or implants during the given surgical procedure. For example, one or more strands of fiber optic cable may be coupled to these components such that light may be delivered from a light source and selectively emitted into the operative corridor and/or the surgical target site. 
     This may be accomplished, by way of example only, by constructing the retractor blades  12 - 18  and/or shim elements  22 ,  24  of suitable material (such as clear polycarbonate) and configuration such that light may be transmitted generally distally through a light exit region formed along the entire inner periphery thereof and located in the general vicinity as the operative corridor  15 . This may be performed by providing the retractor blades  12 - 18  and/or the shim elements  22 ,  24  with light-transmission characteristics (such as with clear polycarbonate construction) and transmitting the light almost entirely within the walls of the retractor blades  12 - 18  and/or shim elements  22 ,  24  (such as by frosting or otherwise rendering opaque portions of the exterior and/or interior and coupling the light source thereto such as via a port) until it exits a portion along the interior surface thereof to shine at or near the surgical target site. 
     In one embodiment, a variety of sets of retractor blades  12 - 18  and/or shim elements  22 ,  24  may be provided, each having a different length to account for any number of possible surgical target sites and/or anatomical configurations. In a further embodiment, each set of retractor blades  12 - 18  and/or shim elements  22 ,  24  may be marked or color-coded to aid in indicating to the surgeon the particular length of the blade  12 - 18  and/or shim element  22 ,  24  (and/or extension  80  of the shim elements  22 ,  24 ) and/or the depth of the surgical target site. 
     The retractor blades  12 - 18  and shim elements  22 ,  24  may be constructed from any number of materials suitable for medical applications, including but not limited to plastics, metals, ceramics or any combination thereof. Depending on the construction, some or all of these devices may be disposable (i.e. single use) and/or reusable (i.e. multi-use). 
       FIG. 7  illustrates an initial distraction assembly  40  forming part of the surgical access system according to the present invention. The initial tissue distraction assembly  40  is employed to perform an initial distraction of tissue from the skin of the patient down to or near the surgical target site, prior to the introduction of the tissue retraction assembly  10  shown and described above with reference to  FIGS. 1-6 . By way of example, this is accomplished by providing the initial distraction assembly  40  as including a K-wire  42 , an initial dilating cannula  44  with handle  46 , and a split-dilator  48  housed within the initial dilating cannula  44 . The initial tissue distraction assembly  40  may be constructed from any number of materials suitable for medical applications, including but not limited to plastics, metals, ceramics or any combination thereof. Depending on the construction, some or all of the tissue distraction assembly  40  may be disposable (i.e. single use) and/or reusable (i.e. multi-use). As will be discussed below in greater detail), the K-wire  42 , initial dilating cannula  44 , and split-dilator  48  may be provided with electrodes  60 ,  62 ,  64 , respectively, for the purpose of determining the location of nerves or neural structures relative to these components as they are advanced towards or positioned at or near the surgical target site. 
     The K-wire  42  is preferably constructed having generally narrow diameter (such as, by way of example only, 1.5 mm) and sufficient rigidity and strength such that it can pierce the skin of the patient and be advanced through the intervening tissue to reach the surgical target site. The K-wire  42  also preferably includes indicia for determining the distance between a distal end thereof and the skin of the patient. The split-dilator  48  and dilating cannula  44  are inner and outer dilating elements, respectively, capable of being simultaneously introduced over the K-wire  42  for the purpose of further distracting the tissue previously distracted by the K-wire  42 . 
     The split-dilator  48  is preferably constructed having an inner diameter approximating the diameter of the K-wire  42  (such as, by way of example only, 1.5 mm), an outer diameter of increased dimension (such as, by way of example only, 6.5 mm), and indicia for determining the distance between a distal end  52  and the skin of the patient. The initial dilating cannula  44  is similarly preferably constructed having an inner diameter approximating the outer diameter of the split-dilator  48  (such as, by way of example only, 6.5 mm), an outer diameter of increased dimension (such as, by way of example only, 7.5 mm), and indicia for determining the distance between a distal end  54  and the skin of the patient. The respective lengths of the K-wire  42 , dilating cannula  44 , and split-dilator  48  may vary depending upon the given surgical target site (that is, the “depth” of the surgical target site within the patient). It will be similarly appreciated that the diameters and dimensions for these elements may also vary depending upon the particular surgical procedure. All such surgically appropriate variations (length, diameter, etc. . . . ) are contemplated as falling within the scope of the present invention. 
     In use, the K-wire  42  and split-dilator  48  are disposed within the initial dilating cannula  44  and the entire assembly  40  advanced through the tissue towards the surgical target site (e.g. disk space) as shown in  FIG. 7 . After the initial dilating assembly  40  is advanced such that the distal ends of the split-dilator  48  and initial dilating cannula  44  are positioned within the disc space ( FIG. 7 ), the initial dilator  44  and handle  46  are removed ( FIG. 8 ) to thereby leave the split-dilator  48  and K-wire  42  in place. As shown in  FIG. 9 , the split-dilator  48  is thereafter split such that the respective halves  48   a ,  48   b  are separated from one another to distract tissue in a generally cephalad-caudal fashion relative to the target site. The split dilator  48  may thereafter be relaxed (allowing the dilator halves  48   a ,  48   b  to come together) and rotated such that the dilator halves  48   a ,  48   b  are disposed in the anterior-posterior plane. Once rotated in this manner, the dilator halves  48   a ,  48   b  are again separated to distract tissue in a generally anterior-posterior fashion. 
     As shown in  FIG. 10 , the posterior retractor blade  12  is thereafter advanced along the posterior half  48   a  of the split dilator  48 . At this point, the posterior shim element  22  ( FIGS. 4-5 ) may be advanced along the posterior retractor blade  12  ( FIGS. 2-3 ) such that the shim extension  80  (distal end) is positioned in the posterior region of the disc space as shown in  FIG. 11  (with posterior half  48   a  of the split dilator  48  removed). The anterior retractor blade  14  may thereafter be advanced along the anterior half  48   b  of the split dilator  48  as shown in  FIG. 12 . At this point, secondary distraction may be undertaken according to the present invention by removing the anterior half  48   b  of the split dilator  48  and introducing a plurality of sequentially dilating cannula  50  between the posterior retractor blade  12  and the anterior retractor blade  14  as shown in  FIG. 14 . This serves to move the anterior retractor blade  14  anteriorly from the posterior retractor blade  12 . 
     The retraction of the present invention is performed by expanding, modifying, and/or maintaining the distraction corridor to establish and/or maintain an operative corridor to the surgical target site. As shown in  FIG. 15 , according to one embodiment, this is accomplished by introducing the anterior shim element  24  along the anterior retractor blade  14  such that the shim extension  80  (distal region thereof) extends into the anterior region of the disc space. Supplemental retractor blades  16 - 18  ( FIG. 6 ) may also optionally be introduced to define the cephalad and caudal sides of the operative corridor  15  as shown in  FIGS. 14-15 . Once positioned as such, the retractor blades  12 ,  14  and supplemental retractor blades  16 ,  18  may be locked in a position relative to the mounting structure  20  by tightening the respective nuts of the locking assemblies  36 . 
     The retraction assembly  10  of the present invention, and in particular the shim extensions  80  of the posterior and anterior shim elements  22 ,  24  serve to prevent the ingress of unwanted or sensitive biological structures (e.g., nerve roots and/or vasculature) into the surgical target site, as well as prevent instruments from passing outside the surgical target site and contacting surrounding tissues or structures. Once established, any of a variety of surgical instruments, devices, or implants may be passed through and/or manipulated within the operative corridor  15  depending upon the given surgical procedure. 
     According to yet another aspect of the present invention, any number of distraction components and/or retraction components (including but not limited to those described herein) may be equipped to detect the presence of (and optionally the distance and/or direction to) neural structures during the steps tissue distraction and/or retraction. This is accomplished by employing the following steps: (1) one or more stimulation electrodes are provided on the various distraction and/or retraction components; (2) a stimulation source (e.g. voltage or current) is coupled to the stimulation electrodes; (3) a stimulation signal is emitted from the stimulation electrodes as the various components are advanced towards or maintained at or near the surgical target site; and (4) the patient is monitored to determine if the stimulation signal causes muscles associated with nerves or neural structures within the tissue to innervate. If the nerves innervate, this may indicate that neural structures may be in close proximity to the distraction and/or retraction components. 
     Neural monitoring may be accomplished via any number of suitable fashions, including but not limited to observing visual twitches in muscle groups associated with the neural structures likely to found in the tissue, as well as any number of monitoring systems, including but not limited to any commercially available “traditional” electromyography (EMG) system (that is, typically operated by a neurophysiologist. Such monitoring may also be carried out via the surgeon-driven EMG monitoring system shown and described in the following commonly owned and co-pending “NeuroVision Applications” incorporated by reference into this disclosure above. In any case (visual monitoring, traditional EMG and/or surgeon-driven EMG monitoring), the access system of the present invention may advantageously be used to traverse tissue that would ordinarily be deemed unsafe or undesirable, thereby broadening the number of manners in which a given surgical target site may be accessed. 
       FIGS. 16-17  illustrate, by way of example only, a monitoring system  120  of the type disclosed in the NeuroVision Applications suitable for use with the surgical access system of the present invention. 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 surgical access system of the present invention. More specifically, this 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 electrodes on the K-wire  42 , the dilating cannula  44 , the split-blade dilator  48 , the retractor blades  12 - 18 , and/or the shim elements  22 ,  24  (collectively “Surgical Access Instruments”). 
     In order to use the monitoring system  120 , then, these Surgical Access Instruments must be connected to the connectors  156   a ,  156   b  and/or  156   c , at which point the user may selectively initiate a stimulation signal (preferably, a current signal) from the control unit  122  to a particular Surgical Access Instruments. Stimulating the electrode(s) on these Surgical Access Instruments 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 Access 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 embodiment, the monitoring system  120  is capable of determining nerve direction relative to one or more of the K-wire  42 , dilating cannula  44 , split-retractor  48 , retractor blades  12 - 18 , and/or the shim elements  22 ,  24  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 one or more of the stimulation electrodes provided on these Surgical Access Instruments. Depending upon the location 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 Surgical Access Instruments to depolarize. This causes muscle groups to innervate and generate EMG responses, which can be sensed via the EMG harness  126 . The nerve direction feature of the system  120  is based on assessing the evoked response of the various muscle myotomes monitored by the system  120  via the EMG harness  126 . 
     By monitoring the myotomes associated with the nerves (via the EMG harness  126  and recording electrode  127 ) and assessing the resulting EMG responses (via the control unit  122 ), the surgical access system of the present invention is capable of detecting the presence of (and optionally the distant and/or direction to) such nerves. This 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, as well as monitor to ensure that no neural structures migrate into contact with the retraction assembly  10  after the operative corridor has been established. In spinal surgery, for example, this is particularly advantageous in that the surgical access system of the present invention may be particularly suited for establishing an operative corridor to an intervertebral target site in a postero-lateral, trans-psoas fashion so as to avoid the bony posterior elements of the spinal column. 
       FIGS. 18-19  are exemplary screen displays (to be shown on the display  140 ) illustrating one embodiment of the nerve direction feature of the monitoring system shown and described with reference to  FIGS. 16-17 . These screen displays are intended to communicate a variety of information to the surgeon in an easy-to-interpret fashion. This information may include, but is not necessarily limited to, a display of the function  180  (in this case “DIRECTION”), a graphical representation of a patient  181 , the myotome levels being monitored  182 , the nerve or group associated with a displayed myotome  183 , the name of the instrument being used  184  (e.g. dilating cannula  44 ), the size of the instrument being used  185 , the stimulation threshold current  186 , a graphical representation of the instrument being used  187  (in this case, a cross-sectional view of a dilating cannula  44 ) to provide a reference point from which to illustrate relative direction of the instrument to the nerve, the stimulation current being applied to the stimulation electrodes  188 , instructions for the user  189  (in this case, “ADVANCE” and/or “HOLD”), and (in  FIG. 19 ) an arrow  190  indicating the direction from the instrument to a nerve. This information may be communicated 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). Although shown with specific reference to a dilating cannula (such as at  184 ), it is to be readily appreciated that the present invention is deemed to include providing similar information on the display  140  during the use of any or all of the various Surgical Access Instruments of the present invention, including the initial distraction assembly  40  (i.e. the K-wire  42 , dilating cannula  44 , and split dilator  48 ), the secondary distraction assembly  50  ( FIGS. 13-14 ), and/or the retractor blades  12 - 18  and/or shim elements  22 ,  24  of the retraction assembly  10 . 
     The initial distraction assembly  40  ( FIG. 7 ) may be provided with one or more electrodes for use in providing the neural monitoring capabilities of the present invention. By way of example only, the K-wire  42  may be equipped with a distal electrode  60 . This may be accomplished by constructing the K-wire  42  for a conductive material, providing outer layer of insulation extending along the entire length with the exception of an exposure that defines the electrode  60 . The electrode  60  has an angled configuration relative to the rest of the K-wire  42  (such as, by way of example only, in the range of between 15 and 75 degrees from the longitudinal axis of the K-wire  42 ). The angled nature of the electrode  60  is advantageous in that it aids in piercing tissue as the K-wire  42  is advanced towards the surgical target site. 
     The angled nature of the distal electrode  60  is also important in that it provides the ability to determine the location of nerves or neural structures relative to the K-wire  42  as it is advanced towards or resting at or near the surgical target site. This “directional” capability is achieved by the fact that the angled nature of the electrode  60  causes the electrical stimulation to be projected away from the distal portion of the K-wire  42  in a focused, or directed fashion. The end result is that nerves or neural structures which are generally closer to the side of the K-wire  42  on which the electrode  60  is disposed will have a higher likelihood of firing or being innervated that nerves or neural structures on the opposite side as the electrode  60 . 
     The direction to such nerves or neural structures may thus be determined by physically rotating the K-wire  42  at a particular point within the patient&#39;s tissue and monitoring to see if any neural stimulation occurs at a given point within the rotation. Such monitoring can be performed via visual observation, a traditional EMG monitoring, as well as the nerve surveillance system disclosed in the above-referenced NeuroVision Applications. If the signals appear more profound or significant at a given point within the rotation, the surgeon will be able tell where the corresponding nerves or neural structures are, by way of example only, by looking at reference information (such as the indicia) on the exposed part of the K-wire  42  (which reference point is preferably set forth in the exact same orientation as the electrode  60 ). 
     The dilating cannula  44  and split dilator  48  may also be provided with electrodes (flat electrodes  64  and angled electrodes  62 , respectively) for the purpose of determining the location of nerves or neural structures relative to the dilating cannula  44  and split-dilator  48  are advanced over the K-wire  44  towards or positioned at or near the surgical target site. Electrodes  62 ,  64  may be provided via any number of suitable methods, including but not limited to providing electrically conductive elements within the walls of the dilating cannula  44  and split dilator  48 , such as by manufacturing them from plastic or similar material capable of injection molding or manufacturing them from aluminum (or similar metallic substance) and providing outer insulation layer with exposed regions (such as by anodizing the exterior of the aluminum dilator). 
     The secondary distraction assembly (including the sequential dilation assembly  50  of  FIGS. 13-14 ) may be provided with one or more electrodes for use in providing the neural monitoring capabilities of the present invention. By way of example only, it may be advantageous to provide one or more electrodes on the dilating cannulae comprising the sequential dilation assembly  50  for the purpose of conducting neural monitoring before, during and/or after the secondary distraction. 
     The retractor blades  12 - 18  and the shim elements  22 ,  24  of the present invention may also be provided with one or more electrodes for use in providing the neural monitoring capabilities of the present invention. By way of example only, it may be advantageous to provide one or more electrodes on these components (preferably on the side facing away from the surgical target site) for the purpose of conducting neural monitoring before, during and/or after the retractor blades  12 - 18  and/or shim elements  22 ,  24  have been positioned at or near the surgical target site. 
     The surgical access system of the present invention may be sold or distributed to end users in any number of suitable kits or packages (sterile and/or non-sterile) containing some or all of the various components described herein. For example, the retraction assembly  10  may be provided such that the mounting assembly  20  is reusable (e.g., autoclavable), while the retractor blades  12 - 18  and/or shim elements  22 ,  24  are disposable. In a further embodiment, an initial kit may include these materials, including a variety of sets of retractor blades  12 - 18  and/or shim elements  22 ,  24  (and extensions  80 ) having varying (or “incremental”) lengths to account for surgical target sites of varying locations within the patient, optionally color-coded to designate a predetermined length. 
     As evident from the above discussion and drawings, the present invention accomplishes the goal of providing a novel surgical access system and related methods which involve creating a distraction corridor to a surgical target site, thereafter retracting the distraction corridor to establish and maintain an operative corridor to the surgical target site, and optionally detecting the existence of (and optionally the distance and/or direction to) neural structures before, during and/or after the formation of the distraction and/or operative corridors. 
     The steps of distraction followed by retraction are advantageous because they provide the ability to more easily position an operative corridor-establishing device through tissue that is strong, thick or otherwise challenging to traverse in order to access a surgical target site. The various distraction systems of the present invention are advantageous in that they provide an improved manner of atraumatically establishing a distraction corridor prior to the use of the retraction systems of the present invention. The various retractor systems of the present invention are advantageous in that they provide an operative corridor having improved cross-sectional area and shape (including customization thereof) relative to the prior art surgical access systems. Moreover, by optionally equipping the various distraction systems and/or retraction systems with one or more electrodes, an operative corridor may be established through (or near) any of a variety of tissues having such neural structures which, if contacted or impinged, may otherwise result in neural impairment for the patient. 
     The surgical access system of the present invention can be used in any of a wide variety of surgical or medical applications, above and beyond the spinal applications discussed herein. By way of example only, in spinal applications, any number of implants and/or instruments may be introduced through the working cannula  50 , including but not limited to spinal fusion constructs (such as allograft implants, ceramic implants, cages, mesh, etc.), fixation devices (such as pedicle and/or facet screws and related tension bands or rod systems), and any number of motion-preserving devices (including but not limited to nucleus replacement and/or total disc replacement systems). 
     While certain embodiments have been described, 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 application. For example, with regard to the monitoring system  120 , it may be implemented using any combination of computer programming software, firmware or hardware. As a preparatory act to practicing the system  120  or constructing an apparatus according to the application, the computer programming code (whether software or firmware) according to the application 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 application. The article of manufacture containing the computer programming code may be 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 application is not limited by the scope of the appended claims.