Patent Publication Number: US-11660082-B2

Title: Dilation system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 16/050,820, filed Jul. 31, 2018 which is a continuation of U.S. application Ser. No. 14/046,685, filed Nov. 1, 2012, now U.S. Pat. No. 10,039,540; which is a continuation-in-part of PCT/US2012/063061, filed Nov. 1, 2012; which claims benefit to U.S. Provisional Application Ser. No. 61/554,397, filed Nov. 1, 2011; the entire contents of each being hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE PRESENTLY DISCLOSED INVENTIVE CONCEPTS 
     1. Field of the Presently Disclosed Inventive Concepts 
     The inventive concepts disclosed and claimed herein relate to systems and methods for performing surgical procedures and, more particularly, but not by way of limitation, to systems and methods for accessing a surgical target site to perform surgical procedures. 
     2. Brief Description of Related Art 
     The present state of the art, when referencing a lateral surgical access approach, typically consists of using the following surgical instruments: neuromonitoring probe, dilators, and a retractor. Once an operative level is identified and an incision is created, dilators are used to create a surgical access site which is often followed by the use of a retractor or other specialized tools to create a surgical access corridor. 
     During a lateral approach to a patient&#39;s spine, a psoas muscle, which is located on either side of the spine, is separated in order to access the spine and, in particular, an intervertebral disc space or one or more vertebral bodies within a patient&#39;s spinal column. It is desirable to avoid neural elements or nerves of the lumbar plexus that lie within the psoas muscle during such procedures. The anterior third of the psoas muscle is typically considered a safe zone for muscle separation. 
     The neural elements or nerves of the psoas muscle may be mapped using a stimulating probe. In this manner, the most posterior neural or nerve free area of the psoas muscle may be located and identified. The stimulating probe may then be inserted through the psoas muscle via the most posterior neural or nerve free tissue area or through nearly any other region that is free of neural elements or nerves and toward the spine or into the intervertebral disc space in order to initiate safe tissue separation of the psoas muscle. Dilators are next placed over the probe to create and enlarge a surgical access site. Following the use of dilators, a retractor or other specialized tools are used to further enlarge the surgical access corridor. 
     Concentric dilators separate the muscle radially, and as such, dilate tissue on all both sides of the stimulating probe in a uniform fashion. This in turn may impinge on neural elements or nerves located outside of the safe zone. Directional dilators have been suggested to overcome the problems associated with concentric dilators. While directional dilation systems are effective have avoiding known neural elements, they are limited in their ability to continuously monitor nerve proximity and to create a surgical access site of a desired shape while at the same time reducing the amount of tissue damage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a partially exploded, perspective view of a dilation system constructed in accordance with the inventive concepts disclosed herein having three dilators and three electrode assemblies. 
         FIG.  2    is an elevational view of the dilation system of  FIG.  1   . 
         FIG.  3    is an elevational view of the dilation system of  FIG.  2    shown rotated 90 degrees. 
         FIG.  4 A  is a top plan view of the dilation system. 
         FIG.  4 B  is a bottom plan view of the dilation system. 
         FIG.  4 C  is an elevational view of a first dilator. 
         FIG.  4 D  is a top plan view of the first dilator. 
         FIG.  4 E  is a top plan view of another embodiment of a first dilator. 
         FIG.  5 A  is an elevational view of the dilators shown without the electrode assemblies and shown assembled in a concentric arrangement. 
         FIG.  5 B  is a bottom plan view of the dilators shown without the electrode assemblies and shown assembled in a concentric arrangement. 
         FIG.  6    is a bottom plan view of the dilators without the electrode assemblies and shown arranged in an eccentric arrangement. 
         FIG.  7    is an elevational view of another embodiment of a dilation system constructed in accordance with the inventive concepts disclosed herein having three dilators. 
         FIG.  8    is a bottom plan view of the dilation system of  FIG.  7   . 
         FIG.  9    is a top plan view of the dilation system of  FIG.  7    shown in a concentric arrangement. 
         FIG.  10    is a top plan view of the dilation system of  FIG.  7    shown in an eccentric arrangement. 
         FIG.  11 A  is an elevational view of an exemplary first dilator of the dilation system of  FIG.  7   . 
         FIG.  11 B  is a top plan view of the first dilator. 
         FIG.  12 A  is an elevational view of an exemplary second dilator of the dilation system of  FIG.  7   . 
         FIG.  12 B  is a top plan view of the second dilator. 
         FIG.  13 A  is an elevational view of an exemplary third dilator of the dilation system of  FIG.  7   . 
         FIG.  13 B  is an elevational view of the third dilator of  FIG.  13 A  shown rotated 90 degrees. 
         FIG.  13 C  is a top plan view of the third dilator. 
         FIG.  14    is a perspective view of another embodiment of a dilation system constructed in accordance with the inventive concepts disclosed herein. 
         FIG.  15 A  is a perspective view of an exemplary first dilator shown being inserted over a stimulating probe. 
         FIG.  15 B  is a perspective view of an exemplary second dilator being inserted over the first dilator. 
         FIG.  15 C  is a perspective view of an exemplary third dilator being inserted over the first dilator. 
         FIG.  16 A  is a front elevational view of the first dilator of the dilation system of  FIG.  14   . 
         FIG.  16 B  is a side elevatonal view of the first dilator. 
         FIG.  16 C  is a rear elevational view of the first dilator. 
         FIG.  16 D  is a sectional view taken along line  16 D- 16 D of  FIG.  16 C . 
         FIG.  17    is a sectional view taken along line  17 - 17  of FIG.  FIG.  16 B . 
         FIG.  18    is top end view of the first dilator. 
         FIG.  19    is a front elevational view of an exemplary second dilator of the dilation system of  FIG.  14   . 
         FIG.  20 A  is a sectional view taken along line  20 A- 20 A of  FIG.  19   . 
         FIG.  20 B  is a sectional view taken along line  20 B- 20 B of  FIG.  19   . 
         FIG.  21    is a front elevational view of an exemplary third dilator of the dilation system of  FIG.  14   . 
         FIG.  22 A  is a sectional view taken along line  22 A- 22 A of  FIG.  21   . 
         FIG.  22 B  is a sectional view taken along line  22 B- 22 B of  FIG.  21   . 
         FIG.  23 A  is a perspective view of an exemplary first dilator of another embodiment of a dilation system. 
         FIG.  23 B  is a perspective view illustration an exemplary second dilator connected to the first dilator. 
         FIG.  23 C  is a sectional view of the first and second dilator shown in combination with a retractor assembly. 
         FIG.  24 A  is a perspective view of another embodiment of a dilation system constructed in accordance with the inventive concepts disclosed herein shown in a closed condition. 
         FIG.  24 B  is a perspective view of the dilation system of  FIG.  24 A  shown in an expanded condition. 
         FIG.  25    is a sectional view taken along line  25 - 25  of  FIG.  24 B . 
         FIG.  26    is an exploded, perspective view of a link assembly. 
         FIG.  27 A  is an exploded, perspective view of a drive rod shown detached from the link assembly. 
         FIG.  27 B  is a perspective view of the drive shaft shown attached to the link assembly. 
         FIG.  28 A  is a perspective view of the dilation system of  FIG.  14    illustrated in the closed condition provided with an expandable sheath. 
         FIG.  28 B  is a perspective view of the dilation system of  FIG.  28 A  shown in the expanded condition. 
         FIG.  29    is a perspective view of another embodiment of a dilation system constructed in accordance with the inventive concepts disclosed herein shown in a closed condition. 
         FIG.  30    is a perspective view of the dilation system of  FIG.  28    shown in an expanded condition. 
         FIG.  31    is a perspective view of the dilation system of  FIG.  28    shown in the expanded condition with a secondary expander shown inserted therein. 
         FIG.  32    is a perspective view of an expander. 
         FIG.  33    is a perspective view of the secondary expander. 
         FIG.  34    is a perspective view of another embodiment of a dilation system constructed in accordance with the inventive concepts disclosed herein. 
         FIG.  35    is a perspective view of the dilation system of  FIG.  34    shown provided with an expandable sheath. 
         FIG.  36 A  is an elevational view of another embodiment of a dilation system constructed in accordance with the inventive concepts disclosed herein. 
         FIG.  36 B  is a top end view of the dilation system of  FIG.  36 A . 
         FIG.  37 A  is an elevational view of the dilation system of  FIG.  36 A  shown with a first expansion member. 
         FIG.  37 B  is a top end view of the dilation system of  FIG.  37 A . 
         FIG.  38 A  is an elevational view of the dilation system of  FIG.  37 A  shown with a second expansion member. 
         FIG.  38 B  is a top end view of the dilation system of  FIG.  38 A . 
         FIG.  39    is an exploded, perspective view of another embodiment of a dilation system constructed in accordance with the inventive concepts disclosed herein. 
         FIG.  40 A  is perspective view of a drive member. 
         FIG.  40 B  is a distal end view of the drive member of  FIG.  40 A . 
         FIG.  41 A  is a perspective view of the dilator system of  FIG.  39    shown in a closed condition with a stimulating probed positioned therein. 
         FIG.  41 B  is a perspective view of the dilator system of  FIG.  39    shown in a closed condition with a second stimulating probed positioned therein. 
         FIG.  41 C  is a perspective view of the dilator system of  FIG.  39    shown in a partially expanded condition. 
         FIG.  41 D  is a perspective view of the dilator system of  FIG.  39    shown in an expanded condition. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Before explaining at least one embodiment of the presently disclosed and claimed inventive concepts in detail, it is to be understood that the presently disclosed and claimed inventive concepts is not limited in its application to the details of construction, experiments, exemplary data, and/or the arrangement of the components set forth in the following description or illustrated in the drawings. The presently disclosed and claimed inventive concepts are capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for purpose of description and should not be regarded as limiting. 
     Certain exemplary embodiments of the invention will now be described with reference to the drawings. In general, such embodiments relate to dilation systems for accessing a patient&#39;s spinal column. 
     As generally understood by one of ordinary skill in the art, the dilation systems will be described in connection with accessing the spine to perform a surgical procedure, but the dilation systems will find use not only in orthopedic surgery, but in other surgical procedures in which a surgeon wishes to gain access to an internal cavity by cutting the skin and going through the body wall in order to keep the incision spread apart so that surgical instruments can be inserted. For example, the dilation systems may be used for anteriorly or posteriorly accessing the spine, for accessing the thoracic or cervical region of the spine, or for accessing nearly any other part of the body. 
     Referring to  FIGS.  1 - 6   , a dilation system  10  is illustrated. The dilation system  10  includes a plurality of sequential dilators  12 ,  14 , and  16 , and a plurality of electrode assemblies  18  and  20 . The sequential dilation system  10  may include more or less dilators such as, for example, one, two four, etc. The dilation system  10  is adapted to be used in combination with a monitoring K-wire or stimulating probe (not shown) known for transmitting an electrical pulse. 
     Referring to  FIGS.  4 C- 4 E,  5 A, and  5 B , the first dilator  12  is a tubular member having an outer surface  32 , a proximal end  33 , a distal end  34  and a bore  35  extending from the proximal end  33  to the distal end  34 . The first dilator is illustrated as having a substantially egg shaped transverse cross section whereby the first dilator  12  is provided with a lobe  36 . The bore  35  is offset from the longitudinal axis of the first dilator  12  away from lobe  36 . The bore  35  is sized to receive a stimulating probe (not shown). Alternatively, the first dilator  12  may be provided with a channel  38  formed near the tip of the lobe and extending the length of the first dilator  12  ( FIG.  6   ), or the first dilator  12  may have an open side  37  opposite the lobe  36  ( FIG.  4 E ). 
     In use, the first bore  35  removably receives the stimulating probe in an assembled configuration (e.g., when the stimulating probe is slidably received within the first bore  35  of the first directional dilator  12 ) so that a surgeon can stimulate the first dilator  12 . The axis of the stimulating probe may be coaxial with the axis of the bore  35  of the first dilator  12  in the assembled configuration. 
     With reference to  FIGS.  5 A and  5 B , the second dilator  14  is characterized as being an open sided tubular structure that has an outer surface  42 , a proximal end  43 , a distal end  44 , and a bore  45  extending from the proximal end  43  to the distal end  44 . The second dilator  14  is illustrated as having a substantially oval shaped transverse cross section. The bore  45  is offset from the longitudinal axis of the second dilator  14  toward the open side of the second dilator  14 . The bore  45  is shaped to slidingly receive the first dilator  12  so that the tip of the lobe  36  of the first dilator  12  is received in the open side of second dilator  14  and is substantially flush with the outer surface of the second dilator  14 . The second dilator  14  further includes a channel  46  formed at or near the outer surface  42  of the closed side of second dilator  14  extending the length of the second dilator  14  ( FIG.  6   ) for receiving an electrode in a manner to be described below. Because the bore of the second dilator  14  is offset toward the lobe  36  of the first dilator, inserting the second dilator  14  over the first dilator  12  causes the second dilator  14  to dilate the opening formed in the patient in a direction opposite the lobe  36  of the first dilator  12 . 
     The third dilator  16  is characterized as being an open sided tubular structure that has an outer surface  52 , a proximal end  53 , a distal end  54  and a bore  55  extending from the proximal end  53  to the distal end  54 . The third dilator  16  is illustrated as having a substantially circularly shaped transverse cross section. The bore  54  is offset from the longitudinal axis of the third  16  dilator toward the open side of the third dilator  16 . The bore  55  is shaped to slidingly receive the second dilator  12 . As such, in the exemplary embodiment, the bore  55  is oval shaped. The third dilator  16  further includes a plurality of channels  56  formed at or near outer surface of the third dilator  14  extending the length of the second dilator  14  ( FIG.  6   ) for receiving electrodes in a manner to be described below. 
     Because the bore  54  is oval shaped, the third dilator  16  may be inserted over the second dilator  14  in one of two ways. First, as illustrated in  FIG.  5 B , the third dilator  16  may be inserted over the second dilator  14  with the open side of the third dilator  16  positioned opposite the open side of the second dilator  14 . This causes the opening formed in the patient by insertion of the third dilator  16  to be concentrically positioned about the bore  35  of the first dilator  12 . Second, as illustrated in  FIG.  6   , the third dilator  16  may be inserted over the second dilator  14  with the open side of the third dilator  16  positioned in alignment with the opposite the open side of the second dilator  14 . This causes the opening formed in the patient by insertion of the third dilator  16  to be eccentrically positioned about the bore  35  of the first dilator  12  or in a direction away from the bore  35  of the first dilator  12 . 
     Referring again to  FIGS.  1 - 3   , the electrode assembly  18  includes an electrode  60  and an electrode holder  62 . The electrode  60  can be any electrode now or hereafter known for transmitting an electrical pulse. The electrode  60  includes an electrode tip  64  ( FIG.  4 B ). The electrode holder  62  is formed of an electrically insulating material and is substantially similar in shape to the second dilator  14  so as to be positionable about a portion of the first dilator  12 . The electrode  60  extends through the electrode holder  62  so that a proximal end of the electrode  62  extends from an upper end of the electrode holder  62 . 
     In use, the electrode holder  62  may function as a handle to facilitate insertion of the electrode  60  into the channel  46  of the second dilator  14  until the electrode holder  62  contacts the proximal end of the second dilator  14  and the electrode tip  64  is position near the distal end  44  of the second dilator  14  ( FIG.  4 B ). 
     The electrode assembly  18   a  includes three electrodes  70  and an electrode holder  72 . The electrodes  70  can be any electrode now or hereafter known for transmitting an electrical pulse. The electrodes  70  include an electrode tip  74 . The electrode holder  72  is substantially similar in shape to the third dilator  14  so as to be positionable about a portion of the second dilator  14 . The electrodes extend through the electrode holder  72  so that a proximal end of the electrodes  70  extends from an upper end of the electrode holder  72 . 
     In use, the electrode holder  72  may function as a handle to facilitate insertion of the electrodes into the channels  56  of the third dilator  16  until the electrode holder  72  contacts the proximal end of the third dilator  16  and the electrode tips  74  are position near the distal end of the third dilator  16  ( FIGS.  2 ,  3 , and  4 B ). 
     The dilation system  10  is shown to use four electrodes to determine nerve location during the preparation of a surgical corridor. Four electrodes at final assembly allow nerves to be located 360° around the surgical site without the additional step of rotating the dilators. The assembly procedure would be carried out one dilator at a time with the first dilator  12  placed over a stimulating probe. Once the first dilator  12  is in place, the probe may be removed and replaced with a K-wire, if desired. The second dilator  14  with a single disposable electrode attached is placed over both the first dilator  12  and k-wire. The single electrode can be utilized to determine nerve proximity and the assembly can be rotated as one unit. The third dilator  16  is placed over all the elements with three additional disposable electrodes attached. Once fully assembled, the third and second dilators place the four electrodes 90° apart allowing for nerve detection in four equally separated directions spanning 360°. 
     Additionally, the eccentric dilators are shaped in an oblong fashion to prevent rotation between elements and allow for a unique assembly orientation. The dilator assembly can be rotated at each step without causing damage to surrounding tissue as a result of rotating interfaces and eliminates the possibility of single electrode misplacement relative to the triple electrodes. This feature ensures that the four electrodes are placed 90° apart at final assembly and unwanted rotation between dilators does not occur during use. 
     The shape of the dilators can be made so the dilators are assembled in an alternating pattern, which allows for the k-wire channel to be located concentric to the final assembly. As well, the dilators can be assembled in an eccentric pattern, allowing for dilation to occur in tissue in a single direction. This can be advantageous to perform dilation in a direction away from nerve tissue. 
     Rotation between dilators can be eliminated via a keyhole slot, locking method or other geometry. Greater or fewer dilatation steps can be used. Electrodes can be integrated into the dilators for greater than one time use. Electrodes can be placed in an alternative pattern. Electrodes can be assembled in different combinations (e.g., two sets of two electrodes, four single electrodes, only two electrodes etc). Greater or fewer electrodes can be used. Dilator assembly could have a non-circular basis. 
     Referring now to  FIGS.  7 - 13   , another embodiment of a dilation system  10   a  is illustrated. The dilation system  10   a  includes a plurality of sequential dilators  12   a ,  14   a , and  16   a . The sequential dilation system  10   a  may include more or less dilators such as, for example, one, two four, etc. The dilation system  10   a  is adapted to be used in combination with a monitoring K-wire or stimulating probe (not shown) known for transmitting an electrical pulse. 
     As best shown in  FIGS.  11 A and  11 B , the first dilator  12   a  is characterized as being an open sided tubular structure having an outer surface  32   a , a proximal end  33   a , a distal end  34   a , and a bore  35   a  extending from the proximal end  33   a  to the distal end  34   a . The first dilator  12   a  is illustrated as being substantially oval shaped. The bore  35   a  is offset from the longitudinal axis of the first dilator  12   a . The bore  35   a  is sized to receive a K-wire (not shown). The first dilator  12   a  is provided with an electrode  38  at the distal end  34   a  generally opposite the bore  35   a . The first dilator  12   a  further has a corresponding connector point  39  ( FIG.  11 A ) for connecting the electrode to neural monitoring equipment. 
     As best shown in  FIGS.  12 A and  12 B , the second dilator  14   a  is characterized as being an open sided tubular structure having an outer surface  42   a , a proximal end  43   a , a distal end  44   a , and a bore  45   a  extending from the proximal end  43   a  to the distal end  44   a . The second dilator  14   a  is illustrated as having a substantially oval shaped transverse cross section. The bore  45   a  is offset from the longitudinal axis of the second dilator  14   a  toward the open side of the second dilator  14   a . The bore  45   a  is shaped to slidingly receive the first dilator  12   a  so that one side of the first dilator  12   a  is received in the open side of second dilator  14   a  and is substantially flush with the outer surface of the second dilator  14   a . Alternatively, the second dilator  14   a  can be positioned over the first dilator  12   a  with the open sides aligned. The second dilator  14   a  is provided with an electrode  48  at the distal end generally opposite the bore  45   a . The second dilator  14   a  further has a corresponding connector point  49  for connecting the electrode to neural monitoring equipment. 
     Referring now to  FIGS.  13 A and  13 B , the third dilator  16   a  is characterized as being an open sided tubular structure that has an outer surface  52   a , a proximal end  53   a , a distal end  54   a , and a bore  55   a  extending from the proximal end  53   a  to the distal end  54   a . The third dilator  16   a  is illustrated as having a substantially circularly shaped transverse cross section. The bore  55   a  is offset from the longitudinal axis of the third dilator  16   a  toward the open side of the third dilator  16   a . The bore  55   a  is shaped to slidingly receive the second dilator  12   a . As such, the bore is oval shaped. Because the bore is oval shaped, the third dilator  16   a  may be inserted over the second dilator  14   a  in one of two ways. First, as illustrated in  FIG.  9   , the third dilator  16   a  may be inserted over the second dilator  14   a  with the open side of the third dilator  16   a  positioned opposite the open side of the second dilator  14   a . This causes the opening formed in the patient by insertion of the third dilator  16   a  to be concentrically positioned about the bore of the first dilator  12   a . Second, as illustrated in  FIG.  10   , the third dilator  16   a  may be inserted over the second dilator  14   a  with the open side of the third dilator  16   a  positioned in alignment with the opposite the open side of the second dilator  14   a . This causes the opening formed in the patient by insertion of the third dilator  16   a  to be eccentrically positioned about the bore of the first dilator  12   a  or in a direction away from the bore of the first dilator  12   a . The third dilator  16   a  is provided with an electrode  58  at the distal end generally opposite the bore  55   a . The third dilator  16   a  further has a corresponding connector point  59  for connecting the electrode to neural monitoring equipment. 
     The electrodes of the first, second, and third dilators  12   a ,  14   a , and  16   a  are provided for the purpose of determining the location of nerves or neural structures relative to the each of the dilators  12   a ,  14   a , and  16   a  as they are advanced over the K-wire towards or positioned at or near the surgical target site. The dilators  12   a ,  14   a , and  16   a  may be equipped with the electrodes via any number of suitable methods, including but not limited to providing electrically conductive elements within the walls of the dilators such as by manufacturing the dilators from plastic or similar material capable of injection molding or manufacturing the dilators from aluminum (or similar metallic substance) and providing outer insulation layer with exposed regions (such as by anodizing the exterior of the aluminium dilator). 
     As best shown in  FIGS.  3  and  7   , the first dilators  12  and  12   a  have a length greater than the length of the second dilator  14  and  14   a , respectively, and the second dilators  14  and  14   a  have a length greater than the length of the third dilators  16  and  16   a , respectively. This stepped arrangement permits the proximal ends of the first dilators  12  and  12   a  to extend further out of the patient in the assembled and operational configurations to facilitate connection of the electrodes to the neural monitoring equipment and such that a surgeon may grasp and remove or otherwise manipulate the dilators. 
     It should be understood that the dilators described above may include a plurality of depth indicators located on the outer surface thereof. 
     A method of using the dilation systems  10  and  10   a  will now be described for accessing a patient&#39;s spine. The technique may be particularly desirable for accessing the lumbar region of the spine via a lateral approach, although a similar or the same method may be used in other parts of the patient&#39;s body. 
     Using a stimulating probe and an electromyograph (EMG) (not shown) in a manner similar to that described in U.S. 2011/0208226 and U.S. Ser. No. 13/887,838, which is hereby expressly incorporated herein by reference, the surgeon may map a safe zone, i.e., a zone generally free of any neural elements or nerves, on the tissue of interest (e.g., psoas muscle). For example, on the psoas muscle, the anterior third of the psoas muscle is generally considered a safe zone. 
     Once a safe zone is established, anatomical placement may be confirmed via intra-operative fluoroscopy. The surgeon inserts the stimulating probe through the psoas muscle toward the patient&#39;s spine. If the surgery is being performed on the intervertebral disc space, the distal end of the stimulating probe may be inserted into the annulus of the desired intervertebral disc space. The stimulating probe may be inserted via the most posterior portion of the safe zone. 
     The surgeon can insert or slide the first dilator  12 ,  12   a  over the stimulating probe so that the first longitudinal axis is located to one side of the stimulating probe, away from a sensed neural element or nerve, through the psoas muscle and into a position proximate the patient&#39;s spine. The surgeon can then insert the second dilator  14 ,  14   a , if necessary, to further dilate the tissue proximate the outside surface of the first dilator  12 ,  12   a  and in a desired direction. The surgeon can repeat this process as often as necessary. Finally, if desired, a retractor (not shown) can be inserted over the third dilator  16 ,  16   a  to subsequently retract the tissue and to permit removal of the dilation system  10 ,  10   a  and the stimulating probe. 
     Referring now to  FIG.  14   , another embodiment of a dilation system  100  is illustrated. The dilation system  100  includes a plurality of sequential dilators  112 ,  114 , and  116 . The dilation system  100  may include more or less dilators such as, for example, one, two four, etc. The dilation system  100  is adapted to be used in combination with a monitoring K-wire or stimulating probe  118  known for transmitting an electrical pulse. 
     Referring now to  FIGS.  15 A,  16 A- 16 D,  17 , and  18   , the first dilator  112  is an elongated, tubular member having an outer surface  120 , a proximal end  122 , a distal end  124  and a bore  126  extending from the proximal end  122  to the distal end  124 . The bore  126  is sized to receive the stimulating probe  118 . The first dilator  112  may be provided with a second bore or channel  138  extending parallel to the bore  126  and the length of the first dilator  112  for receiving an electrode  140 . 
     The first dilator  112  is further characterized as having a lower section  128 , an upper section  130 , and an intermediate section  132 . The lower section  128  is sized and configured to be extended through the psoas muscle. By way of example, the lower section  128  may have a diameter in a range of from about 4 mm to about 10 mm. In one embodiment, the lower section  128  has a generally keyhole shaped cross section. 
     The upper section  130  of the first dilator  112  is provided with a first guide  134  and a second guide  136  which are configured to slidably and matingly engage with the second dilator  114  and the third dilator  116 , respectively, in a manner to be discussed below. The first guide  134  is illustrated has having a dovetail configuration with a taper at a proximal end thereof to facilitate engagement with the second dilator  114 . Similarly, the second guide  136  is illustrated as having a dovetail configuration with a taper at a proximal end thereof to facilitate engagement with the third dilator  116 . In one embodiment, the first guide  134  and the second guide  136  extend radially outwardly in diametrically opposing directions. 
     It should be appreciated that the first guide  134  and the second guide  136  may be formed in a variety of shapes. For example, the first guide  134  and the second guide  136  may be formed as grooves configured to receive a dovetail tongue. Also, the first guide and the second guide may be formed so that the size of the first guide  134  is different from the size of the second guide  136  for reasons that will be described below. 
     The intermediate section of the first dilator  112  may be an open sided structure, and shaped similar to the lower section  128  (e.g., generally keyhole shaped cross section). As illustrated in  FIG.  16 D , the first dilator  112  may be constructed in multiple pieces. For example, the lower section  128 , the upper section  130 , and the intermediate section  132  may be formed as separate pieces which are adapted to be assembled with one another. It will also be understood that the first dilator  112  may be formed as a single piece. 
     With reference to  FIGS.  15 B,  19 ,  20 A, and  20 B , the second dilator  114  is configured to slide along the first dilator  112  ( FIG.  15 B ) and be positioned to create a semi-circular cross section that assists with slitting muscle fibers when a combination of the first dilator  112  and the second dilator  114  are rotated about the stimulated probe  118 . The second dilator  114  is generally characterized as being an elongated, open sided or C-shaped structure that has an outer surface  141 , an inner surface  142 , a proximal end  143 , a distal end  144 , and a bore  145  extending from the proximal end  143  to the distal end  144  for receiving an electrode, such as the electrode  140 , in a manner to be described below. 
     The second dilator  114  is further characterized as having a lower section  146 , an upper section  148 , and an intermediate section  150 . The inner surface  142  of the lower section  146  is sized and configured to be matingly received over a corresponding portion of the lower section  128  of the first dilator  112 . The inner surface  142  of the upper section  148  of the second dilator  114  is configured to slidably and matingly engage with the first guide of the first dilator  112 . To this end, the upper section  148  of the second dilator  114  has a groove  152  corresponding to the shape of the first guide  134 . Again, it will be appreciated that the first guide  134  and the groove  152  may be formed in a variety of shapes and that the position of the first guide  134  and the groove may interchanged. To facilitate sliding of the second dilator  114  over the first dilator  112 , the inner surface  142  of the intermediate section  150  of the second dilator  114  may be configured so that the inner surface  142  is in a non-contact relationship with the outer surface  120  of the first dilator  112 . 
     The second dilator  114  has a first longitudinal edge  154  and a second longitudinal edge  156 . The bore  145  may be formed proximate to the first longitudinal edge  154 . The second longitudinal edge  156  may be angled to form a wedge with the inner surface  142  of the second dilator  114  to facilitate rotation of the first dilator  112  and the second dilator  114  relative to the muscle fiber. 
     With reference to  FIGS.  15 C,  21 ,  22 A,  22 B , the third dilator  116  is configured to slide along the first dilator  112  and be positioned to transform the semi-circular cross section of the combination of the first dilator  112  and the second dilator  114  to a circular cross section of a selected diameter, e.g. 16 mm-22 mm. The third dilator  116  is generally characterized as being an elongated, open sided or C-shaped structure that has an outer surface  160 , an inner surface  162 , a proximal end  164 , a distal end  166  and a bore  168  extending from the proximal end  164  to the distal end  166  for receiving an electrode, such as the electrode  140 , in a manner to be described below. 
     The third dilator  116  is further characterized as having a lower section  170 , an upper section  172 , and an intermediate section  174 . The inner surface  162  of the lower section  172  is sized and configured to be matingly received over a corresponding portion of the lower section  128  of the first dilator  112 . The inner surface  162  of the upper section  174  of the third dilator  116  is configured to slidably and matingly engage with the second guide  136  of the first dilator  112 . To this end, the upper section  174  of the third dilator  116  has a groove  176  corresponding to the shape of the second guide  136 . Again, it will be appreciated that the second guide  136  and the groove  176  may be formed in a variety of shapes and that the position of the second guide  136  and the groove  176  may be interchanged. To facilitate positioning of the third dilator  116  over the first dilator  112 , the inner surface  162  of the intermediate section  176  of the third dilator  116  may be configured so that the inner surface  162  is in a non-contact relationship with the outer surface  120  of the first dilator  112 . 
     The third dilator  116  has a first longitudinal edge  178  and a second longitudinal edge  180 . The bore  168  may be formed proximate to the first longitudinal edge  178 . The first longitudinal edge  178  and the second longitudinal edge  180  of the third dilator  116  are configured to mate with the second longitudinal edge  156  and the first longitudinal edge  154  of the second dilator  114 , respectively. Because the first longitudinal edge  154  and the second longitudinal edge  156  of the second dilator  114  are angled to facilitate rotation within the psoas muscle, the first longitudinal edge  178  and the second longitudinal edge  180  of the third dilator  116  have an inverse shape to that of the first longitudinal edge  154  and the second longitudinal edge  156  of the second dilator  114  so as to mate with the first and second longitudinal edges  154  and  156  of the second dilator  114 . To facilitate insertion of the second dilator  114  on the first dilator  112  prior to insertion of the third dilator  116 , the first guide  134  and the second guide  136  may be sized differently from one another and the grooves  152  and  176  of the second and third dilators  114  and  116  may be sized to mate with the corresponding one of the first and second guides  134  and  136 . 
     A method of using the dilation systems  100  will now be described for accessing a patient&#39;s spine. The technique may be particularly desirable for accessing the lumbar region of the spine via a lateral approach, although a similar or the same method may be used in other parts of the patient&#39;s body. 
     Using a stimulating probe  118  and an electromyograph (EMG) (not shown), a surgeon may map a safe zone, i.e., a zone generally free of any neural elements or nerves, on the tissue of interest (e.g., psoas muscle). For example, on the psoas muscle, the anterior third of the psoas muscle is generally considered a safe zone. 
     Once a safe zone is established, anatomical placement may be confirmed via intra-operative fluoroscopy. The surgeon inserts the stimulating probe  118  through the psoas muscle toward the patient&#39;s spine. If the surgery is being performed on the intervertebral disc space, the distal end of the stimulating probe  118  may be inserted into the annulus of the desired intervertebral disc space. The stimulating probe  118  may be inserted via the most posterior portion of the safe zone. 
     The surgeon can insert or slide the first dilator  112  over the stimulating probe  118 , through the psoas muscle, and into a position proximate the patient&#39;s spine. The surgeon can insert a stimulating probe  140  into the bore  138  so that a surgeon can stimulate the first dilator  112  while the first dilator  112  is being inserted into position. The surgeon can then insert the second dilator  114  on the first dilator  112  by inserting the second dilator  114  laterally into to engagement with the first dilator  112  with the upper section of the second dilator  114  positioned above the first guide  134  of the first dilator  112  ( FIG.  15 B ). 
     The second dilator  114  may then be slid downwardly along the first dilator  112  in such a way that the groove  152  of the second dilator  114  is received over the first guide  134  of the first dilator  112  so as to axially guide the second dilator  114  through the psoas muscle and further dilate the tissue. Prior to positioning the second dilator  114 , the surgeon can remove the electrode  140  from the first dilator  112  and insert the electrode  140  into the bore  143  of the second dilator  114  to stimulate the second dilator  114  while the second dilator  114  is being inserted into position. The surgeon can repeat this process using the third dilator  116 . Finally, if desired, a retractor can be inserted over the second dilator  114  and the third dilator  116  to subsequently retract the tissue and to permit removal of the dilation system  100  and the stimulating probe  118  and the electrode  140 . 
     Referring now to  FIGS.  23 A- 23 C , another embodiment of a dilation system  200  is illustrated. The dilation system  200  includes a pair of dilators  202  and  204 . The dilation system  200  may include more or less dilators such as, for example, one, three four, etc. The dilation system  200  is adapted to be used in combination with a monitoring K-wire or stimulating probe (not shown) known for transmitting an electrical pulse. 
     Referring now to  FIG.  23 A , the first dilator  112  is an elongated member having an outer surface  206 , a proximal end  208 , a distal end  210 , and a bore  212  extending from the proximal end  208  to the distal end  210 . The bore  212  is sized to receive a K-wire or stimulating probe, such as the stimulating probe  118  illustrated in  FIG.  14   . The first dilator  202  can be manufactured such that the K-wire can be accommodated via a continuous thru-bore or an open slot located closer to the outer surface of the first dilator  202 . 
     The first dilator  202  is used to create the initial access corridor. The distal end  210  is tapered to facilitate insertion into tissue, and the distal end  212  is stimulatable via an exposed electrode  214 . The general shape of the first dilator  202  is atraumatic. A connector  216  is provided along one side of the first dilator  202 . The connector  216  is illustrated as being a dovetail groove. 
     In use, the first dilator  202  is inserted over the K-wire or stimulating probe with the connector  216  positioned in an anterior direction to monitor the proximity of nerves. 
     The second dilator  204  is used to further dilate tissue and can be advanced into position directly anterior to the first dilator  202 . The second dilator  204  is an elongated member having an outer surface  220 , a proximal end  222 , and a distal end  224 . Like the first dilator  202 , the second dilator  204  may be manufactured with a stimulatable electrode. The shape of the second dilator  204  is atraumatic as it passes through tissue. The second dilator  204  is configured to connect to and slide along the length of the first dilator  202 . To this end, the second dilator  204  has a connector  226 . 
     The connector  226  of the second dilator  204  is configured for sliding and mating engagement with the connector  216  of the first dilator  202 . In one embodiment, the connector  226  is a dovetail tongue that allows for the second dilator  204  to slide along the length of the first dilator  202  while maintaining attachment. 
     The dilation system  200  further includes a retractor blade assembly  230  ( FIG.  23 C ) which is shown to include three blades  232   a ,  232   b , and  232   c . The contour of the blades  232   a ,  232   b , and  232   c , when assembled, closely matches the outer profile of a combination of the first dilator  202  and the second dilator  204 . The posterior retractor blade  232   a  may have an embedded electrode and accompanying wiring to enable monitoring of nerves. The electrode and wiring can be embedded into the dilator blades and retractor blades or added afterwards by a sticky probe or conductive epoxy ink. 
     The dilators and the retractor blades can be machined, molded, or extruded and machined from materials such as stainless steel, anodized aluminum, PEEK, carbon fiber composite, or any biocompatible material suitable to maintain the shape and function of the components. 
       FIGS.  24 - 28    illustrate another embodiment of a dilation system  300  constructed in accordance with the inventive concepts disclosed herein. The dilation system  300  is used to create an initial corridor which may be expanded to a desired diameter, e.g., 16-22 mm, without inserting any additional instruments. In addition, the dilation system  300  is adapted to be used with a k-wire or stimulating probe, such as the stimulating probe  118  illustrated in  FIG.  14   . The dilation system  300  includes a base  302 , a first blade  304 , a second blade  306 , and an actuating mechanism  308  operably associated with the first blade  304  and the second blade  306  so as to cause the first blade  304  and the second blade  306  to move from a closed condition ( FIG.  24 A ) wherein the first blade  304  and the second blade  306  are positioned adjacent one another to permit insertion through selected tissue to an expanded condition ( FIG.  24 B ) wherein the first blade  304  and the second blade  306  are spread apart relative to one another to expand the tissue. 
     The first blade  304  has a proximal end  310  and a distal end  312 . The first blade  304  extends from the base  302  so that the distal end  312  of the first blade  304  extends away from the base  302  in such a way that the distal end  312  is deflectable relative to the proximal end  310 . Similarly, the second blade  306  has a proximal end  314  and a distal end  316 . The second blade  306  extends from the base  302  so that the distal end  316  of the second blade  306  extends away from the base  302  in such a way that the distal end  316  is deflectable relative to the proximal end  314 . Each of the first blade  304  and the second blade  306  has a generally arcuate cross section and the distal ends  312  and  316  may be tapered to facilitate insertion into a patient. Each of the first blade  304  and the second blade  306  may be provided with a bore or channel (not shown) extending the length of the first dilator  112  for receiving an electrode (not shown) to aid in stimulating adjacent tissues. 
     The actuating mechanism  308  includes a linkage  320 , a drive rod  322 , and a drive member  324 . In one embodiment, the linkage  320  includes a pair of cross links  326 . Each cross link  326  is identical in construction and has a body  328  with a pin  330  pivotally connectable to the distal ends  312  and  316  of the first blade  304  and the second blade  306  via a groove  331  formed in the distal ends  312  and  316  of the first blade  304  and the second blade  306 . Each cross link  326  further includes a pin  332  and hole  334  with the pin  332  being received through the hole  334  of the other cross link  326  and the hole  334  receiving the pin  332  of the other cross link  326 . The body  328  is provided with a longitudinal groove  336  for receiving a stimulating probe when the first blade  304  and the second blade  306  are in the closed condition. 
     The drive rod  322  has a proximal end  338  and a distal end  340 , and the drive rod  322  is positioned between the first blade  304  and the second blade  306 . The distal end  340  of the drive rod  322  is provided with pair of holes  342  for pivotally receiving the pins  332  of the cross links  326  such that axial movement of the drive rod  322  moves the first blade  304  and the second blade  306  from the closed condition to the expanded condition. 
     The drive member  324  is rotatably connected to the base  302 , and the proximal end  338  of the drive rod  322  is threadingly connected to the drive member  324  in such a way that rotational movement of the drive member  324  causes axial movement to the drive rod  322 . 
     The drive member  324  and the drive rod  322  have a central bore  344  sized to receive a stimulating probe. Further, the distal end  312  of the first blade  304  may have a first notch  346 , and the distal end  316  of the second blade  306  may have a second notch  348 . The first notch  346  and the second notch  348  are aligned to cooperate to form an opening  350  ( FIG.  24 A ) sized to receive the stimulating probe when the first blade  304  and the second blade  306  are in the closed condition. 
     Referring to  FIGS.  28 A and  28 B , the dilation system  300  may further include an expandable sheath  352  positioned about at least the distal ends of the first blade  304  and the second blade  306  to cover the spaces between the first blade  304  and the second blade  306 . The sheath  352  can be made of any type of expandable material, such as a mesh, fabric, polymer, or elastomeric polymer. 
     Referring now to  FIGS.  29 - 33   , another embodiment of a dilation system  400  is illustrated. The dilation system  400  is used to create an initial corridor which may be expanded to a desired diameter, e.g., 16-22 mm, without inserting any additional instruments. In addition, the dilation system  400  is adapted to be used with a k-wire or stimulating probe, such as the stimulating probe  401 . The dilation system  400  includes a base  402 , a first blade  404 , a second blade  406 , and an actuating mechanism  408  operably associated with the first blade  404  and the second blade  406  so as to cause the first blade  404  and the second blade  406  to move from a closed condition ( FIG.  29   ) wherein the first blade  404  and the second blade  406  are positioned adjacent one another a distance to permit insertion through selected tissue to an expanded condition ( FIGS.  30  and  31   ) wherein the first blade  404  and the second blade  406  are spread apart relative to one another a selected distance to expand the tissue. 
     The first blade  404  has a proximal end  410  and a distal end  412 . The first blade  404  extends from the base  402  so that the distal end  412  of the first blade  404  extends away from the base  402  in such a way that the distal end  412  is deflectable relative to the proximal end  410 . Similarly, the second blade  406  has a proximal end  414  and a distal end  416 . The second blade  406  extends from the base  402  so that the distal end  416  of the second blade  406  extends away from the base  402  in such a way that the distal end  416  is deflectable relative to the proximal end  414 . Each of the first blade  404  and the second blade  406  has a generally arcuate cross section and the distal ends  412  and  416  may be tapered to facilitate insertion into a patient. Each of the first blade  404  and the second blade  406  may be provided with a bore or channel (not shown) extending the length thereof for receiving an electrode (not shown) to aid in stimulating adjacent tissues. 
     As best illustrated in  FIGS.  32  and  33   , the actuating mechanism  408  includes a drive rod  418 , an expander member  420 , a guide  422 , and a second expander member  423  ( FIG.  33   ). The drive rod  418  has a proximal end  424  and a distal end  426 . The drive rod  418  extends between the first blade  404  and the second blade  406  and is rotatably connected to the base  402  in such a way that the drive rod  418  is rotatable through an angle of at least about 90 degrees. In one embodiment, the base  402  is provided with a pair of diametrically opposed slots  428  (only one slot  428  being visible), and the guide rod  418  is provided with a pair of diametrically opposed pins  430  extending radially therefrom and slidably received in the slots  428 . The drive rod  418  has a central bore  431  sized to receive the stimulating probe  401 . 
     The expander member  420  extends from the drive rod  418  in such a way that rotational movement of the drive rod  418  moves the first blade  404  and the second blade  406  from the closed condition to the expanded condition. As best illustrated in  FIG.  32   , the expander member  420  may include a pair of flanges or protrusions  432  that are sized and shaped to contact the first blade  404  and the second blade  406  and move the first blade  404  and the second blade  406  to the expanded condition upon rotation of the drive rod  418 . The guide member  422  may extend from the drive rod  418  in a substantially perpendicular relationship of the expander member  420  for guiding the second expander member  423  in a manner to be discussed below. 
     The second expander member  423  is slidably positionable over the drive rod  418  and between the first blade  404  and the second blade  406  so as to extend between the first blade  404  and the second blade  406  to fill the gap between the longitudinal edges of the first blade  404  and the longitudinal edges of the second blade  406 . The second expander  423  may include a base  434 , a first blade  436  extending from the base  434 , and a second blade  438  extending from the base  434  in a spaced apart, parallel relationship to the first blade  436  so that the first blade  436  and the second blade  438  are slidably positionable over the expander member  420  when the expander member  420  is rotated to the expanded condition. The first blade  436  and the second blade  438  may be provided with a slot  440  extending along at least a portion an inner surface thereof for slidingly receiving a portion of the guide member  422 . 
     Each of the first blade  436  and the second blade  438  has a generally arcuate cross section and the distal ends may be provided with a raised area  442  which is positionable substantially flush with the outer surfaces of the first blade  404  and the second blade  406  in the expanded condition so that the first blade  436  and the second blade  438  cooperate with the first blade  404  and the second blade  406  to form a circular cross section of a selected diameter, e.g., 16 mm-22 mm. 
     As an alternative to use of the second expander  423  to fill the gaps between the first blade  404  and the second blade  406 , it will be appreciated that the dilation system  400  may further include an expandable sheath positioned about at least the distal ends of the first blade  404  and the second blade  406  to cover the spaces between the first blade  404  and the second blade  406 . As described above, the sheath can be made of any type of expandable material, such as a mesh, fabric, polymer, or elastomeric polymer. 
       FIGS.  34  and  35    illustrate another embodiment of a dilation system  500  constructed in accordance with the inventive concepts disclosed herein. The dilation system  500  is used to create an initial corridor which may be expanded to a desired diameter, e.g., 16-22 mm, without inserting any additional instruments. In addition, the dilation system  500  is adapted to be used with a k-wire or stimulating probe, such as a stimulating probe  501 . The dilation system  500  includes a base  502 , a first blade  504 , a second blade  506 , and an actuating mechanism  508  operably associated with the first blade  504  and the second blade  506  so as to cause the first blade  504  and the second blade  506  to move from a closed condition wherein the first blade  504  and the second blade  506  are positioned adjacent one another a distance to permit insertion through selected tissue to an expanded condition ( FIGS.  34  and  35   ) wherein the first blade  504  and the second blade  506  are spread apart relative to one another a selected distance to expand the tissue. 
     The first blade  504  has a proximal end  510  and a distal end  512 . The first blade  504  extends from the base  502  so that the distal end  512  of the first blade  504  extends away from the base  502  in such a way that the distal end  512  is deflectable relative to the proximal end  510 . Similarly, the second blade  506  has a proximal end  514  and a distal end  516 . The second blade  506  extends from the base  502  so that the distal end  516  of the second blade  506  extends away from the base  502  in such a way that the distal end  516  is deflectable relative to the proximal end  514 . Each of the first blade  504  and the second blade  506  has a generally arcuate cross section and the distal ends  512  and  516  may be tapered to facilitate insertion into a patient. Each of the first blade  504  and the second blade  506  may be provided with a bore or channel (not shown) extending the length thereof for receiving an electrode (not shown) to aid in stimulating adjacent tissues. 
     The actuating mechanism  508  includes a drive rod  518  having a proximal end and a distal end. The drive rod is positioned between the first blade  504  and the second blade  506  and configured in such a way that axial movement of the drive rod  518  moves the first blade  504  and the second blade  506  from the closed condition to the expanded condition. The drive rod  518  may have a wedge shape so that axial movement of the drive rod  518  wedge will drive the first blade  504  and the second blade  506  in a radial direction to larger or smaller diameters. 
     The drive rod  518  may be moved in a variety of different ways. For example, the drive rod  518  may be axially moved manually by sliding the drive rod  518  between the first blade  504  and the second blade  506 . Also, the drive rod  518  may be moved in a manner similar to that described above with respect to the dilation system  200  where the drive rod  518  may be moved with an internally threaded assembly, by way of example. 
     The expansion of the first blade  504  and the second blade  506  will leave gaps as they go to the large circumferences so these gaps should be covered to effectively dilate the tissue. To this end, the dilation system  500  may include an expandable sheath  520  ( FIG.  35   ) positioned about at least the distal ends of the first blade  504  and the second blade  506  to cover the spaces between the first blade  504  and the second blade  506 . The sheath  520  may be laterally supported by a plurality of support member  522  (only one support member  522  being visible in  FIG.  35   ). As described above, the sheath  520  can be made of any type of expandable material, such as a mesh, fabric, polymer, or elastomeric polymer. The sheath  520  may alternatively be in the form of a plurality of metal slotted tubes connected to the blades  504  and  506 . The slotted tubes must be made out of an elastic material such as nitinol to accommodate the bending to a larger diameter. The tubes are initially nested within each other. As the blades are driven radially outward they interact with each other such that the diameters of tubing will increase. The sliding and expanding action will maintain an enclosed cylinder with some discontinuities, but no gaps. Alternatively, the drive rod  518  may be configured to file in the gaps. 
     Referring now to  FIGS.  36 - 38   , another embodiment of a dilation system  600  is illustrated. The dilation system  600  includes a first dilator  601 , an expandable dilator  602 , and a plurality of expansion dilators  604  positionable between the first dilator  601  and the expandable dilator  602  to enlarge the surgical corridor and create an enlarged working space. The first dilator  601  is a generally tubular member with a bore  606  for receiving a K-wire or stimulating probe, such as the stimulating probe  118  of  FIG.  14   . 
     The expandable dilator  602  includes an expandable sheath  608  and a plurality of support members  610  extending longitudinally along an inner surface of the sheath  608  in a spaced apart relationship to provide longitudinal support to the sheath  608  while allowing for the sheath  608  to expand radially. The sheath  608  may be constructed of any suitable elastomeric material. The support members  610  are sized to extend along the length of the sheath  608  may be connected to sheath  602  in any suitable manner, such as use of an adhesive, weaving the support arms in the sheath  602 , or embedding the support members  610  in the sheath  608 , for example. While the expandable dilator  602  is shown as having four support members  608 , it will be appreciated that the expandable dilator  602  may expand with the use of at least three support members  610 . However, the sheath  608  will created a rounder corridor the more support members  610  that are utilized. 
     In use, the first dilator  601  is inserted through tissue up to a disc annulus while monitoring neural structures. A K-wire may be inserted through the bore  606  of the first dilator  601  to dock the first dilator  601  to a disc annulus. The expandable dilator  602  is then inserted over the first dilator  601  as shown in  FIGS.  36 A and  36 B . A second dilator  604   a  ( FIGS.  37 A and  37 B ) may then be inserted between first dilator  601  and the expandable dilator  602 , thereby expanding the sheath  608 . As shown in  FIGS.  38 A and  38 C , a third dilator  604   b  may be inserted between the second dilator  604   a  and the expandable dilator  602 , thereby further expanding the sheath  608 . A retractor (not shown) may be inserted about the sheath  608 , and the dilation system  600  and the K-wire removed. 
     As shown, the support members  610  and the dilators  601 ,  604   a  and  604   b  may be provided with corresponding tongues  612  and grooves  614  for aligning the dilators to one another during expansion. The tongue and groove construction also allows the sheath  608  to expand uniformly. The dilation system  600  has an advantage in that only two dilators are inserted that pass through tissue which minimizes tissue trauma as the remaining dilators are used to expand the dilation system radially without any further insertion forces. This is due to the sheath remaining in contact with tissue for the remainder of the expansion. 
     Electrodes, such as electrode  616 , may be embedded in or on the first dilator  601  and the sheath  608  to allow for surrounding neural tissue to be identified, monitored, and assessed based on distance and pathology. 
     Referring now to  FIGS.  39 - 41   , another embodiment of a dilation system  700  is illustrated. The dilation system  700  is used to create an initial corridor which may be expanded to a desired diameter, e.g., 16-22 mm, without inserting any additional instruments. In addition, the dilation system  700  is adapted to be used with a k-wire or stimulating probe, such as the stimulating probe  701  ( FIGS.  41 A- 41 D ). The dilation system  700  includes a wedge assembly  702  and an actuating assembly  708 . 
     The wedge assembly  702  includes a base  703 , a first blade  704 , a second blade  706 . The actuating mechanism  708  is operably associated with the first blade  704  and the second blade  706  so as to cause the first blade  704  and the second blade  706  to move from a closed condition ( FIGS.  41 A and  41 B ) wherein the distal ends of the first blade  704  and the second blade  706  are positioned adjacent one another to form a generally wedge shaped structure to facilitate insertion through selected tissue to an expanded condition ( FIG.  41 D ) wherein the distal ends of the first blade  704  and the second blade  706  are spread apart relative to one another a selected distance to expand the tissue. 
     The base  703  serves to connect the first blade  704  to the second blade  706 . The base  703  may include an elongated tubular portion  705  (best shown in  FIG.  41 D ) for supporting and guiding the probe  701 . As shown in  FIG.  39   , in one embodiment the wedge assembly  702  may be formed from two blade sections  707   a  and  707   b  to facilitate assembly with the actuating assembly  708 . The blade sections  707   a  and  707   b  may be secured to one another at the base  703  in a suitable manner, such as with an adhesive or by welding, with the blades  704  and  706  straddling the actuating assembly  708 . 
     The first blade  704  has a proximal end  710  and a distal end  712 . The first blade  704  extends from the base  702  so that the distal end  712  of the first blade  704  extends away from the base  702  in such a way that the distal end  712  is deflectable relative to the proximal end  710 . Similarly, the second blade  706  has a proximal end  714  and a distal end  716 . The second blade  706  extends from the base  702  so that the distal end  716  of the second blade  706  extends away from the base  702  in such a way that the distal end  716  is deflectable relative to the proximal end  714 . Each of the first blade  704  and the second blade  706  has a generally arcuate cross section and the distal ends  712  and  716  may be tapered to facilitate insertion into a patient. To assist in insertion of the wedge assembly  702  to be described below, each of the first blade  704  and the second blade  706  may be provided with a finger indention  717  near the proximal ends thereof. 
     Each of the first blade  704  and the second blade  706  may be provided with a channel  718  ( FIG.  39   ) on an inner surface thereof extending to the distal end sized to receive the stimulating probe  701  when the first blade  704  and the second blade  706  are in the closed condition. At least one of the first blade  704  and the second blade  706  may also be provided with a longitudinal slot or channel  720  ( FIG.  39   ) along a portion of an outer surface thereof for receiving a second stimulating probe  722  ( FIG.  41 B ) when the first blade  704  and the second blade  706  are in the closed condition. The slot  720  may include one or more tabs  721  ( FIG.  41 A ) extending inwardly into the slot  722  for gripping the probe  722  when the probe  720  is positioned in the slot  720 . 
     The actuating mechanism  708  includes a drive member  730  positioned between the first blade  704  and the second blade  706  and configured in such a way that axial movement of the drive member  730  moves the first blade  704  and the second blade  706  from the closed condition to the expanded condition and causes the drive member  730  to extend between the first blade  704  and the second blade  706  to fill the gap between the longitudinal edges of the first blade  704  and the longitudinal edges of the second blade  706 . 
     The drive member  730  has a proximal base  732 , a distal base  734 , a first blade  736  extending between the proximal base  732  and the distal base  734 , a second blade  738  extending between the proximal base  732  and the distal base  734  in a spaced apart, parallel relationship to the first blade  736  so that the first blade  736  and the second blade  738  are slidably positionable between the first blade  704  and the second blade  706 . To assist in insertion of the drive member  730  in a manner to be described below, each of the first blade  736  and the second blade  738  may be provided with a finger indention  739  near the proximal ends thereof. 
     The longitudinal edges of the first blade  736  and the second blade  738  are configured to mate with the longitudinal edges of the first blade  704  and the second blade  706  so that the outer surfaces of the first blade  736  and the second blade  738  are substantially flush with the outer surfaces of the first blade  704  and the second blade  706  in the expanded condition ( FIG.  41 D ). As a result, the wedge assembly  702  and the drive member  730  cooperate to form a circular cross section of a selected diameter, e.g., 16-22 mm, about which a retractor assembly (not shown) may be disposed. 
     The proximal base  732  of the drive member  730  serves to connect the proximal ends of the first blade  736  and the second blade  738  to one another. In one embodiment, the proximal base  732  is generally U-shape to provide an open side which facilitates insertion of the probe  722  into the slot  720 . The proximal base  732  is configured to receive the tubular portion  705  of the base  703  and to contact the proximal end of one of the first and second blades  704  and  706  in way that the contact between the proximal base  730  and wedge assembly  702  serve to limit the movement of the actuating assembly  708  relative to the wedge assembly  702 . 
     Referring to  FIGS.  40 A and  40 B , the distal base  734  is configured to slidingly mate with the inner surface of the first and second blades  704  and  706  to cause the first and second blades  704  and  706  to move from the closed condition to the expanded condition when the distal base  734  is caused to be axially moved from the proximal ends of the first and second blades  704  and  706  to the distal ends of the first and second blades  704  and  706 . The distal base  734  is provided with an open sided hole  740  for supporting and guiding the probe  701 . 
     In use, the probe  701  is initially positioned through the psoas to the designated disc space to define a central location for subsequent instrumentation. Following positioning of the probe  701 , the dilator system  700  is inserted over the probe  701  with the dilator system  700  in the closed position ( FIG.  41 A ) by pushing the wedge assembly  702  axially along the probe  701 . With the wedge assembly  702  in the closed condition, the psoas is enlarged to an approximate oval shape. As illustrated in  FIG.  41 B , the slot  720  in the outer surface of one of the first and second blades  704  and  706  allows the second probe  722  to be used during the insertion process. The probe  722  can be manipulated, and the dilator system  700  rotated to assist with neural monitoring. 
     After the wedge assembly  702  has been positioned up to the disc space, the drive member  730  is pushed axially toward the distal ends of the first and second blades  704  and  706  in manner to move the first blade  704  and the second blade  706  from the closed condition to the expanded condition ( FIG.  41 D ) and cause the drive member  730  to extend between the first blade  704  and the second blade  706  to fill the gap between the longitudinal edges of the first blade  704  and the longitudinal edges of the second blade  706  ( FIG.  41 D ). When the drive member  730  is moved to the disc space, the outer surfaces of the first blade  736  and the second blade  738  are substantially flush with the outer surfaces of the first blade  704  and the second blade  706  in the expanded condition ( FIG.  41 D ) and thereby cooperate to form a circular cross section of a selected diameter about which a retractor assembly (not shown) may be disposed. 
     From the above description, it is clear that the inventive concepts disclosed and claimed herein are well adapted to carry out the objects and to attain the advantages mentioned herein, as well as those inherent in the invention. While exemplary embodiments of the inventive concepts have been described for purposes of this disclosure, it will be understood that numerous changes may be made which will readily suggest themselves to those skilled in the art and which are accomplished within the spirit of the inventive concepts disclosed and claimed herein.