Abstract:
The present invention relates to devices and methods directed towards accessing and forming holes in bone tissue including penetrating vertebral bones during surgical treatments of the spine, for example, to cannulate pedicles and form pilot holes for placing pedicle screws.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Application claims the benefit of priority from U.S. Provisional Patent Application Ser. No. 61/260,326, filed on Nov. 11, 2009, and U.S. Provisional Patent Application Ser. No. 61/299,866, filed on Jan. 29, 2010, the entire contents of which are each incorporated by reference into this disclosure as if set forth in their entireties herein. 
    
    
     FIELD 
     The present invention relates to devices and methods directed towards accessing and forming holes in bone tissue including penetrating vertebral bones during surgical treatments of the spine, for example, to cannulate pedicles and form pilot holes for placing pedicle screws. 
     BACKGROUND 
     An emerging trend in spinal surgery is to perform surgery in a minimally invasive or minimal access fashion to avoid the trauma of so-called open or “direct access” procedures. A specific area of interest is in the placement of pedicle screws, which are typically employed to effect posterior fixation in spinal fusion procedures. While great strides are being made in this area, a risk exists (as it does in open procedures) that the pedicle may become breached, cracked, or otherwise compromised during the procedure. If the pedicle (or more specifically, the cortex of the medial wall, lateral wall, superior wall and/or inferior wall) is breached, cracked, or otherwise compromised, the patient may experience pain or neurologic deficit due to unwanted contact between the pedicle screw and exiting nerve roots. This often necessitates revision surgery, which can be painful and costly, both in terms of recovery time and hospitalization. 
     Some attempts to minimize the risk of a pedicle breach involve capitalizing on the insulating characteristics of bone and the conductivity of the exiting nerve roots themselves to perform pedicle integrity assessments. That is, if the wall of the pedicle is breached, a stimulation signal applied to the pedicle screw and/or the pilot hole (prior to screw introduction) will cause the various muscle groups coupled to the exiting nerve roots to contract. If the pedicle wall has not been breached, the insulating nature of the pedicle will prevent the stimulation signal from innervating the given nerve roots such that the associated muscle groups will not twitch. Traditional EMG monitoring systems may be employed to augment the ability to detect such innervation. 
     One period during a pedicle screw procedure in which the risk of a pedicle breach is prevalent is during the initial access of the pedicle. Typically, initial access to a pedicle may be achieved by inserting a needle to the target site and driving the needle point into the pedicle, creating a pilot hole. Due to the size and shape of the typical needle, however, manipulation and maneuvering of the needle may be awkward or difficult, increasing the risk of complication. Additionally, the pedicle may be breached and nerve damage done during the initial drive of the needle into the pedicle, before a pedicle integrity test assessment may be performed. 
     A problem that may arise when various medical instruments are electrified and used with traditional EMG monitoring systems is that different instruments may produce different EMG stimulation thresholds. For example, an electrified needle may exhibit a threshold stimulation of approximately 5-6 mA, while a bone screw placed in the same location may exhibit a threshold stimulation of approximately 16-20 mA. This can be problematic in that an electrified needle may tend to indicate a breach in the pedicle wall when in fact the pedicle wall is intact. 
     The present invention is directed at eliminating, or at least improving upon, the shortcomings of the prior art. 
     SUMMARY 
     This application describes devices and methods for accessing and forming holes in tissue. The tissue may be bone tissue, and particularly spinal bones including vertebral pedicles. The devices can penetrate the vertebral bones during surgical treatments of the spine. Electrical stimulation signals may be deliverable to the tissue through the devices. The electrical stimulation signals may be used to monitor the integrity of the tissue as the devices are advanced into the tissue. 
     According to one example, a needle assembly system for use in a medical procedure is described. The needle assembly includes a cannula assembly having concentric inner and outer cannulas. The inner cannula has a distal end and a first length. The outer cannula has a distal end and a second length shorter than the first length. The cannula assembly also includes an insulated sheath situated between the inner cannula and the outer cannula. The insulated sheath is movable between a position in which a distal end of the insulated sheath is proximate the distal end of the inner cannula and a position in which a distal end of the insulated sheath is proximate the distal end of the outer cannula. The needle assembly also includes a stylet disposed through the inner cannula and having a shaped tip that protrudes from the distal end of the inner cannula. 
     According to another aspect of the system the insulated sheath is biased towards the position proximate the distal end of the inner cannula. A spring situated above the insulated sheath and between the inner cannula and the outer cannula that biases the insulated sheath towards the distal end of the inner cannula. 
     According to another aspect of the system the outer cannula is also insulated. 
     According to another aspect of the system a connector situated outside the outer cannula is attached to the insulated sheath. The connector may attach to the insulted sheath through at least one elongated slot proximate a distal end of the outer cannula. The outer cannula may also include graduated depth markers and the position of the connector relative to the depth marker provides an indication of penetration depth into tissue. 
     According to another aspect of the system the needle assembly also includes a handle. The handle may include an upper portion and a lower portion separable from the upper portion. The upper portion may be connected to the stylet and the lower portion may be connected to the cannula assembly. The upper portion and lower portion may be lockable to one another to lock the cannula assembly and the stylet together. 
     According to another aspect of the system the needle assembly includes a contact that receives electrical stimulation signals from a stimulator. The contact may be a conductive surface situated within the upper handle portion attached to the sylet. The contact may be configured to engage a stimulation clip associated with the stimulator. The upper portion of the handle may include a cutout region that securely engages the stimulation clip. 
     According to another aspect of the system the sylet is removable from the cannula assembly. A k-wire may also be included that is advanceable through the inner cannula after the sylet has been removed. 
     According to another aspect of the system the shaped tip and distal end of the inner cannula are adapted to form a hole in bone and a distal end of the insulated sheath is adapted to engage a surface of the bone when the shaped tip and distal end of the inner cannula are advanced into bone such that the insulated sheath slides relative to the inner cannula and outer cannula as said the shaped tip and distal end of the inner cannula are advanced deeper into the bone exposing the exterior of inner cannula to the interior of the bony. 
     According to another example, there is described a surgical needle assembly. The surgical needle assembly includes a cannula assembly having concentric inner and outer cannulas. The inner cannula has a distal end and a first length. The outer cannula has a distal end and a second length shorter than the first length. The cannula assembly also has a handle portion. The needle assembly also an insulated sheath situated between the inner cannula and the outer cannula. The insulated sheath is spring biased to a first position in which a distal end of the insulated sheath is proximate the distal end of the inner cannula. The insulated sheath is also movable between the first position and a second position in which a distal end of the insulated sheath is proximate the distal end of the outer cannula, and a handle portion. The needle assembly also includes a stylet removably disposed through the inner cannula and having a shaped tip adapted to penetrate bone that protrudes from the distal end of the inner cannula. The stylet also includes a handle portion lockingly mateable with the handle portion of the cannula assembly. 
     According to another aspect, the needle assembly include a connector situated outside the outer cannula is attached to the insulated sheath. The outer cannula may include graduated depth markers and the position of the connector relative to the depth marker may provide an indication of penetration depth into the bone. 
     According to another aspect, the needle assembly includes a contact that receives electrical stimulation signals from a stimulator. The contact may be a conductive surface situated within the upper handle portion attached to the sylet. The contact is configured to engage a stimulation clip associated with the stimulator. 
    
    
     
       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 plan view of an example of a pedicle access system according to one embodiment of the present invention; 
         FIG. 2  is a perspective view of a cannula forming part of the pedicle access system of  FIG. 1 ; 
         FIG. 3  is a perspective view of a coupling element forming part of the cannula of  FIG. 2 ; 
         FIG. 4  is a perspective view of a stylet forming part of the pedicle access system of  FIG. 1 ; 
         FIG. 5  is a perspective view of a locking cap forming part of the stylet of  FIG. 4 ; 
         FIG. 6  is a perspective view of the distal portion of the stylet of  FIG. 4  protruding from the distal region of the cannula of  FIG. 2 ; 
         FIG. 7  is a perspective view of the distal portion of the stylet of  FIG. 4  protruding from the distal region of the cannula of  FIG. 2 , with the distal region of the cannula having an uninsulated portion; 
         FIG. 8  is a perspective view of the distal portion of the stylet of  FIG. 4  protruding from the distal region of the cannula of  FIG. 2 , with the distal region of the cannula having a directional electrode; 
         FIGS. 9-10  are plan and perspective views, respectively, of the stylet of  FIG. 4  partially inserted into the cannula of  FIG. 2 ; 
         FIG. 11  is a is a plan view of the stylet of  FIG. 4  fully inserted into the cannula of  FIG. 2  in an unlocked position; 
         FIG. 12  is perspective view of the locking cap of the fully inserted stylet of  FIG. 11 , shown in an unlocked position; 
         FIG. 13  is a perspective view of the cannula and stylet combination in the unlocked position of  FIG. 11 ; 
         FIGS. 14-15  are perspective and plan views, respectively, of the cannula and stylet combination of  FIG. 13  in the locked position; 
         FIGS. 16-17  are perspective views of a T-handle forming part of the pedicle access system of  FIG. 1 ; 
         FIG. 18  is a perspective view of the pedicle access system of  FIG. 1  with the cannula and stylet combination of  FIG. 13  fully inserted and locked in the T-handle; 
         FIG. 19  is an exploded perspective view of a pedicle access system according to an alternative embodiment of the present invention; 
         FIG. 20  is a perspective view of the assembled pedicle access system of  FIG. 19 ; 
         FIGS. 21-22  are plan and perspective views, respectively, of a cannula forming part of the pedicle access system of  FIG. 20 ; 
         FIGS. 23-24  are plan and perspective views, respectively, of a coupling element forming part of the cannula of  FIG. 21 ; 
         FIG. 25  is a perspective view of a stylet forming part of the pedicle access system of  FIG. 20 ; 
         FIG. 26  is a perspective view of a handle forming part of the stylet of  FIG. 25 ; 
         FIG. 27  is a perspective view of the pedicle access system of  FIG. 20  including an enlarged view of a distal region thereof; 
         FIGS. 28-30  are perspective, top plan and bottom plan views, respectively, of a lock collar forming part of the pedicle access system of  FIG. 20 ; 
         FIG. 31  is an exploded perspective view of a pedicle access system according to a further alternative embodiment of the present invention; 
         FIGS. 32-33  are perspective views of an assembled pedicle access system of  FIG. 31 ; 
         FIG. 34  is a front view of the pedicle access system of  FIG. 32 ; 
         FIGS. 35-36  are side and perspective views, respectively, of a cannula forming part of the pedicle access system of  FIG. 31 ; 
         FIGS. 37-38  are side and perspective views, respectively, of a coupling element forming part of the cannula of  FIG. 35 ; 
         FIG. 39  is a perspective view of a stylet forming part of the pedicle access system of  FIG. 31 ; 
         FIG. 40  is a perspective view of a needle forming part of the stylet of  FIG. 39 ; 
         FIGS. 41-42  are perspective and plan views, respectively, of a handle forming part of the stylet of  FIG. 39 ; 
         FIGS. 43-44  are perspective and plan views, respectively, of a lock collar forming part of the pedicle access system of  FIG. 31 ; 
         FIG. 45  is a bottom plan view of a handle of  FIG. 46  in engagement with a lock collar of  FIG. 43 ; 
         FIG. 46  is a perspective view of a retractable insulation sheath forming part of the pedicle access system of  FIG. 31 ; 
         FIG. 47  is a perspective view of an insulation tube forming part of the retractable insulation sheath of  FIG. 46 ; 
         FIG. 48  is a perspective view of a retraction tube forming part of the retractable insulation sheath of  FIG. 46 ; 
         FIGS. 49-50  are perspective views of a sheath attachment element forming part of the pedicle access system of  FIG. 31 ; 
         FIG. 51  is an exploded perspective view of a pedicle access system according to another alternative embodiment of the present invention; 
         FIG. 52  is a perspective view of the pedicle access system of  FIG. 51 , showing a retractable sheath in a raised position; 
         FIG. 53  is a perspective view of the pedicle access system of  FIG. 51 , showing a retractable sheath in a lowered position; 
         FIGS. 54-55  are front and side views, respectively of the cannula of the pedicle access system of  FIG. 51  with the stylet in a detached position; 
         FIG. 56  is a perspective view of a cannula forming part of the pedicle access system of  FIG. 51 ; 
         FIG. 57  is a perspective view of a stylet forming part of the pedicle access system of  FIG. 51 ; 
         FIGS. 58-59  are enlarged front and bottom pictures of the proximal end of the cannula of  FIG. 56 ; 
         FIGS. 60-61  are enlarged front and bottom pictures of the proximal end of the stylet of  FIG. 57 ; 
         FIG. 62  is a perspective view of the cannula of  FIG. 56 ; 
         FIG. 63  is a perspective view of a retractable sheath assembly forming part of the pedicle access system of  FIG. 51 ; 
         FIG. 64 . is a perspective view of a pedicle access system of another embodiment of the present invention; 
         FIG. 65  a partially exploded view of the pedicle access system of  FIG. 64 ; 
         FIG. 66  is a partially exploded view showing the retractable sheath and cannula forming a part of the pedicle access system of  FIG. 64 ; 
         FIG. 67  is a side view of the pedicle access system of  FIG. 64 , showing a retractable sheath in an extended position; 
         FIG. 68  is a perspective view of the pedicle access system of  FIG. 64 , showing a retractable sheath in a retracted position; and 
         FIGS. 69-70  are enlarged front and bottom pictures of the proximal end of the cannula of  FIG. 65 ; 
         FIGS. 71-72  are enlarged front and bottom pictures of the proximal end of the stylet of  FIG. 65 ; and 
         FIG. 73  is a perspective view of an example of a neurophysiology system capable of connecting to the pedicle access systems of  FIGS. 1, 19, 31, 51 and 64  to conduct neurophysiological monitoring. 
     
    
    
     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 insulated pedicle access system and related methods disclosed herein boast a variety of inventive features and components that warrant patent protection, both individually and in combination. 
       FIG. 1  illustrates an example of a pedicle access system  10  according to one embodiment of the present invention. The pedicle access system  10  includes a cannula  12 , a stylet  14 , and a T-handle  16 . As will be described with greater detail below, the pedicle access system  10  may be used to percutaneously approach the pedicle, initiate pilot hole formation, and conduct a stimulation signal to the target site for the purposes of performing a pedicle integrity assessment during formation of the pilot hole. To do this, the cannula  12  and stylet  14  may be lockingly mated to form a cannula/stylet combination  15  which may be inserted through an operating corridor to the pedicle target site, using the T-handle  16  to facilitate easy movement and positioning of the cannula/stylet combination  15 . The cannula/stylet combination  15  may be driven into the bone at the target site to form a pilot hole while a stimulation signal is applied to the pedicle access system  10  and conducted to the target site to assess the integrity of the pedicle during hole formation. The T-handle  16  may be detached from the cannula/stylet combination  15  to facilitate the use of various surgical tools (such as by way of example only a forceps, mallet, or needle driver) after proper positioning of the cannula  12  and stylet  14 . Additionally, removal of the T-handle after proper positioning of the cannula/stylet combination  15  provides a less obstructed view of the operating corridor and surgical target site. As shown and described herein, the cannula  12  and stylet  14  are generally cylindrical in shape. However, it should be understood that cannula  12  and stylet  14  may be provided in any suitable shape having any suitable cross-section (e.g. generally oval or polygonal) without deviating from the scope of the present invention. 
       FIG. 2  illustrates an example of a cannula  12  forming part of pedicle access system  10 . Cannula  12  includes a coupling element  18  and an elongated shaft  20 . An interior lumen extends through the cannula  12  from a first opening  22  located at a proximal region  30  of the coupling element  18  to a second opening  24  located at a distal end  21  of the elongated shaft  20 . Elongated shaft  20  may be composed of any conductive material such as metal, for example. A polymeric coating is provided on a substantial portion of the exterior surface of elongated shaft  20  such that elongated shaft  20  comprises an insulated portion  26  and an uninsulated portion  28 . Although elongated shaft  20  is shown having a single uniform diameter, it will be appreciated that one or more diameter changes may be incorporated along the elongated shaft  20  without deviating from the scope of the present invention. 
     With reference to  FIG. 3 , coupling element  18  comprises a proximal region  30 , a center section  32 , and a base portion  34 . Proximal region  30  is dimensioned to engage with the stylet  14  (described below). Proximal region  30  may include at least one tab member  36  that protrudes in a generally lateral direction from the proximal region  30 . By way of example only, as shown in  FIG. 3  proximal region  30  includes two tab members  36  positioned opposite one another and adjacent to first opening  22 . As will be described in greater detail below, tab members  36  function to lock the cannula  12  and stylet  14  together. Center section  32  is dimensioned to be received within T-handle aperture  66  ( FIG. 16 ) as described in further detail below. Center section  32  may be provided with at least one cutout  38  dimensioned to receive a locking mechanism  68  ( FIG. 16 ) incorporated into T-handle  16  to secure the T-handle  16  to the cannula/stylet combination  15 , or optionally to the cannula  12  only. The base  34  has a circumference that is greater than the circumference of center section  32 , such that a ledge  39  is formed at the interface of center section  32  and the base portion  34 . The ledge  39  engages the rim  72  of T-handle  16  so as to minimize potential stress on a T-handle locking mechanism  68  discussed below. 
       FIG. 4  illustrates an example of a stylet  14  forming part of the pedicle access system  10 . Stylet  14  includes a locking cap  40  and a needle element  42 . Locking cap  40  has a similar size and shape to center section  32 , and is similarly dimensioned to be received within T-handle aperture  66 , discussed below. Locking cap  40  includes a distal end  44  and a proximal end  46 . As illustrated in  FIG. 5 , locking cap  40  includes a generally cylindrical aperture  48  having an opening at distal end  44  and extending in a proximal direction at least partially the length of locking cap  40 . Generally cylindrical aperture  48  is dimensioned to receive the generally cylindrical proximal region  30  of cannula  12 . Furthermore, locking cap  40  includes at least one longitudinal channel  50  (defined by an axis extending through the proximal and distal ends  46 ,  44  respectively) and at least one lateral channel  52  extending generally perpendicularly from longitudinal channel  50 . Longitudinal channel  50  and lateral channel  52  each extend from an exterior surface  54  through an interior surface  56  into aperture  48 . Preferably, the number of longitudinal channels  50  and lateral channels  52  correspond to the number of tab members  36  on cannula  12 . By way of example only (and as shown in  FIGS. 3-5 ), cannula  12  includes two tab members  36  and stylet  14  includes two longitudinal channels  50  and two lateral channels  52 . 
     Longitudinal channel  50  initiates at the distal end  46  and has a length corresponding to the length of the generally cylindrical  48 . Lateral channel  52  initiates at the proximal end of longitudinal channel  50  and extends generally perpendicularly therefrom such that together the longitudinal and lateral channels  50 ,  52  form a generally half-T shape. Longitudinal channel  50  and lateral channel  52  function to interact with the tab  36  on cannula  12 , so as to lock the stylet  14  and cannula  12  together. Longitudinal channel  50  has a width dimension sufficient to accommodate the length of tab member  36  and lateral channel  52  has a height dimension sufficient to accommodate the height of tab member  36  (best viewed in  FIG. 12 ). A ridge  58  (shown in  FIG. 12 ) may be positioned along the distal-most edge  53  and/or proximal-most edge  55  of the lateral channel  52  to engage with tab member  36  and provide a locking means for the cannula/stylet combination  15 . Additionally, a portion of exterior surface  54  adjacent to proximal end  46  may comprise a ramped surface  49  such that the circumference of distal end  44  is slightly greater than the circumference of proximal end  46 , so as to facilitate engagement with the T-handle  16 . 
     The needle element  42  comprises an elongated shaft  41  having a proximal region  43  and a distal region  45 . The proximal region  43  may be attached to the interior of locking cap  40  between proximal end  46  and aperture  48 . Elongated shaft  41  extends distally from proximal region  43  with a significant portion protruding generally perpendicularly from the opening of aperture  48 . Needle element  42  is dimensioned to be inserted through the interior lumen of cannula  12 . The distal region  45  generally includes a distal portion of elongated shaft  41  and a shaped tip  47  having any form or shape capable of being driven into the pedicle to create a pilot hole. By way of example only, shaped tip  47  may have a beveled or double diamond form. As illustrated in  FIG. 6 , when needle element  42  is fully inserted into cannula  12 , at least a portion of distal region  45  (including shaped tip  47 ) may protrude slightly from the second opening  24  of cannula  12 . Due to the insulated nature of cannula  12 , the portion of needle element  42  that protrudes from cannula  12  effectively constitutes a stimulation region  60 . The stimulation region  60  may include the distal region  45  and/or the shaped tip  47 . 
     According to a further aspect of the present invention, any part of the needle element  42  (e.g. the elongated shaft  41 , distal region  45  and/or shaped tip  47 ) may be provided with a coating to insulate and therefore limit or reduce the stimulation region  60  to a desired configuration. For example, the distal tip  47  may have an insulation coating to effectuate a stimulation region  60  consisting of the portion of the distal region  45  of the needle element  42  between the insulated cannula  12  and the insulated distal tip  47 . Alternatively, the entirety of needle element  42  may be provided with an insulative coating and the distal region  21  of cannula  12  may be provided with (for example) one or more uninsulated portions  29  ( FIG. 7 ) and/or one or more directional electrodes  31  ( FIG. 8 ) forming a stimulation region  61 . These alternative arrangements serve to mitigate an apparent phenomenon in which certain geometries (e.g. points and edges) tend to generate significantly higher current densities and therefore are much more efficient at exciting a nearby nerve, even through bone tissue. As a result, instrumentation having these geometries may show a lower stimulation threshold (and thus causing an EMG monitoring system to indicate a breach in an intact pedicle) unless this phenomenon is otherwise compensated for. 
     Needle element  42  may be composed of a conductive material, such as metal. Alternatively, needle element  42  may be composed of a non-conductive material with one or more embedded conductive elements at or near the distal end (e.g. distal region  45  and/or shaped tip  47 ) capable of being communicatively linked with a pedicle integrity testing system. 
       FIGS. 9-15  illustrate the formation of the cannula/stylet combination  15 . In  FIGS. 9-10  stylet  14  is introduced into cannula  12 . Needle element  42  of stylet  14  is inserted into the interior lumen of cannula  12  through the first opening  22  of coupling element  18 . The locking cap  40  of stylet  14  is positioned such that its longitudinal channels  50  are aligned with the tab members  36  of cannula  12 . The proximal region  30  of cannula  12  is received into the aperture  48  of locking cap  40 , and the tab members  36  pass through the longitudinal channels  50  as insertion of needle element  42  progresses. Insertion is complete when the proximal portion  30  is fully received by aperture  48 , leaving the locking cap  40  in the “unlocked” position illustrated in  FIGS. 11-13 . As mentioned above, the distal region  45  of needle element  42  including shaped tip  47  (and the stimulation region  60 ) may protrude from the second opening  24  of the elongated shaft  20  of cannula  12  when stylet  14  is fully inserted, shown in  FIG. 11 . In the unlocked position, tab members  36  are positioned at the proximal end of longitudinal channels  50  where the channels intersect lateral channels  52 . At this point, the corresponding shapes of the locking cap  40  of stylet  14  and center section  32  of cannula  12  are out of alignment. To lock stylet  14  in place and complete the combination, the locking cap  40  is rotated until it is aligned with the center section  32  as illustrated in  FIGS. 14-15 . As the lateral channels  52  rotate around the tab members  36 , ridges  58  come into contact with the tab members  36 . The ridges may not pass the tab members  36  if the locking cap  40  is not rotated with enough force to deform the ridges  58 . Once the ridges  58  have deformed, the rotation may continue towards the final position. The locking cap  40  and center section  32  become aligned and the ridges  58  may clear the tab members  36  and regain their original forms, thereby preventing inadvertent rotation of the locking cap  40  back to the unlocked position. 
       FIG. 16  illustrates an example of a T-handle  16  forming part of the pedicle access system  10 . T-handle  16  includes a grip region  64 , an aperture  66  for engaging the cannula  12  or cannula/stylet combination  15 , and a locking mechanism  68  for securing the T-handle to the cannula  12 . Grip region  64  may be provided in any number of suitable shapes and sizes that may aid the user in holding and manipulating the pedicle access system  10  during use. The T-handle aperture  66  is dimensioned to snugly receive both the locking cap  40  and center section  32  when they are aligned in the locked position as described above. The locking mechanism  68  preferably comprises a lever having one end that is integrated into the aperture wall and a free end  70  extending therefrom. The majority of the locking mechanism  68  (excluding free end  70 ) may comprise the same thickness as the aperture wall and does not protrude, interiorly or exteriorly, from the aperture wall. In its “natural” state, the interior surface of free end  70  protrudes into the aperture  66  space. The interior surface of free end  70  is dimensioned to engage the cutout  38  in the center section  32  of cannula  12 . Furthermore, as illustrated in  FIG. 17 , the interior surface of free end  70  may be slightly ramped, such that the edge further from the aperture opening protrudes further into the aperture than the edge closer to the aperture opening. The ramped portion works in concert with the ramped surface  49  at the proximal end  46  of locking cap  40  to force the free end  70  out of its natural state as the locking cap  40  of stylet  14  and center section  32  of cannula  12  are received into the T-handle aperture  66 . When the locking cap  40  and center section  32  are fully inserted into the T-handle aperture  66 , as illustrated in  FIG. 18 , the interior surface of free end  70  aligns with the cutout  38  in the center section  32  and free end  70  returns to its natural state, thus locking the T-handle  16  to the cannula  12 . Furthermore, as the locking cap  40  and center section  32  are fully inserted into the T-handle aperture  66 , the ledge  39  engages the rim  72 . This interaction functions to minimize potential stress on the T-handle locking mechanism  68  by increasing the surface area that receives force applied by the user. To remove the T-handle  16 , the free end  70  may be lifted to disengage with the cutout  38 , and the T-handle may be pulled off. Optionally, T-handle  16  may be cannulated (not shown) such that an interior lumen extends from an opening on the top of the handle into the aperture  66 . 
       FIGS. 19-20  illustrate an example of a pedicle access system  110  according to an alternative embodiment of the present invention. The pedicle access system  110  includes a cannula  112 , a stylet  114 , and a lock collar  116 . As described above in relation to pedicle access system  10 , pedicle access system  110  may be used to percutaneously approach the pedicle, initiate pilot hole formation, and conduct a stimulation signal to the target site for the purposes of performing a pedicle integrity assessment during formation of the pilot hole. To do this, the cannula  112  and stylet  114  may be lockingly mated and inserted through an operating corridor to the pedicle target site, using the handle portion  140  of the stylet  114  to facilitate easy movement and positioning of pedicle access system  110 . The pedicle access system  110  may be driven into the bone at the target site to form a pilot hole while a stimulation signal is applied and conducted to the target site to assess the integrity of the pedicle during hole formation. As shown and described herein, the cannula  112  and stylet  114  are generally cylindrical in shape. However, it should be understood that cannula  112  and stylet  114  may be provided in any suitable shape having any suitable cross-section (e.g. generally oval or polygonal) without deviating from the scope of the present invention. 
       FIGS. 21-22  illustrate an example of a cannula  112  forming part of pedicle access system  110 . Cannula  112  includes a coupling element  118  and an elongated shaft  120 . An interior lumen extends through the cannula  112  from a first opening  122  located at a proximal region  130  of the coupling element  118  to a second opening  124  located at a distal end  121  of the elongated shaft  120 . Elongated shaft  120  may be composed of any conductive material such as metal, for example. A polymeric coating may be provided on a substantial portion of the exterior surface of elongated shaft  120  such that elongated shaft  120  comprises an insulated portion  126  and an uninsulated portion  128  (the edge of the coating and thus the boundary between portions  126 ,  128  represented by callout  127  in  FIGS. 21-22 ). Elongated shaft  120  may include any number of diameter changes incorporated along its length without deviating from the scope of the present invention. In the alternative, elongated shaft  120  may be provided with a uniform diameter along its length. 
     With reference to  FIGS. 23-24 , coupling element  118  comprises a proximal region  130 , a center section  132 , and a distal portion  134 . Proximal region  130  includes an engagement region  131  dimensioned to engage with the handle portion  140  of the stylet  114  (as described in further detail below). The engagement region  121  may be provided in any suitable geometric configuration to allow for secure mating with the engagement tabs  144  of the handle  140 . By way of example only, the coupling element  118  is shown in  FIGS. 23-24  having a hexagonal engagement region  131 , however other shapes are possible. Proximal region  130  may include at least one tab member  136  that protrudes in a generally lateral direction from the proximal region  130 . By way of example only, as shown in  FIG. 24  proximal region  130  includes two tab members  136  positioned opposite one another and adjacent to first opening  122 . Tab members  136  may be utilized to attach supplemental instruments and/or apparatuses to the cannula  112 . Center section  132  may be provided with a diameter that is larger than the diameters of the proximal region  130  and distal portion  134 , and may be provided with a plurality of ridges  133  and/or other features for the purpose of providing a suitable gripping area for a user. The distal portion  134  is dimensioned to engage with the elongated shaft  120  of the cannula  112 . 
       FIG. 25  illustrates an example of a stylet  114  forming part of the pedicle access system  110 . Stylet  114  includes a handle portion  140  and a needle element  142 . Handle portion  140  may (by way of example) resemble a T-handle for providing a user with a suitable gripping means. Handle portion  140  may be provided with a pair of engagement tabs  144  extending distally from handle portion  140 . Engagement tabs  144  extend generally perpendicularly from the handle  140  and generally parallel to one another such that the engagement tabs  144  collectively form an interior space  146 . Interior space  146  is dimensioned to receive the proximal region  130  of the coupling element  118  of the cannula  112 . Each engagement tab  144  is provided with a medial (inwardly-facing) indentation  148  and a lateral (outwardly-facing) indentation  150 . Medial indentations  148  are dimensioned to engage the engagement region  131  of the coupling element  118 , described above. For this reason, the medial indentations  148  may be provided with any geometry complementary to the shape of the engagement region  131  such that when mated, the engagement tabs  144  (via the medial indentations  148 ) will prevent movement of the engagement region  131 , in effect locking the cannula  112  in place relative to the stylet  114 . The lateral indentations  150  are dimensioned to interact with the first and second protrusions  170 ,  172  of the lock collar  116  described in further detail below. 
     The needle element  142  comprises an elongated shaft  152  having a proximal region  154  and a distal region  156 . The proximal region  154  may be attached to the interior of handle portion  140 . Elongated shaft  152  extends distally from proximal region  154  and generally perpendicularly from the handle  140 . Needle element  142  is dimensioned to be inserted through the interior lumen of cannula  112 . The distal region  156  generally includes a distal portion of elongated shaft  152  and a shaped tip  158  having any form or shape capable of being driven into the pedicle to create a pilot hole. By way of example only, shaped tip  158  may have a beveled or double diamond form. As illustrated in  FIG. 27 , when needle element  142  is fully inserted into cannula  112 , at least a portion of distal region  156  (including shaped tip  158 ) may protrude slightly from the second opening  124  of cannula  112 . Due to the insulated nature of cannula  112 , the portion of needle element  142  that protrudes from cannula  112  effectively constitutes a stimulation region  160 . The stimulation region  160  may include the distal region  152  and/or the shaped tip  158 . 
     According to a further aspect of the present invention, any part of the needle element  142  (e.g. the elongated shaft  152 , distal region  156  and/or shaped tip  158 ) may be provided with a coating to insulate and therefore limit or reduce the stimulation region  160  to a desired configuration. For example, the distal tip  158  may have an insulation coating to effectuate a stimulation region  160  consisting of the portion of the distal region  156  of the needle element  142  between the insulated cannula  112  and the insulated distal tip  158 . This coating serves to mitigate an apparent phenomenon in which certain geometries (e.g. points and edges) tend to generate significantly higher current densities and therefore are much more efficient at exciting a nearby nerve, even through bone tissue. As a result, instrumentation having these geometries may show a lower stimulation threshold (and thus causing an EMG monitoring system to indicate a breach in an intact pedicle) unless this phenomenon is otherwise compensated for. 
     Needle element  142  may be composed of any conductive material, such as metal. Alternatively, needle element  142  may be composed of a non-conductive material with one or more embedded conductive elements at or near the distal end (e.g. distal region  156  and/or shaped tip  158 ) capable of being communicatively linked with a pedicle integrity testing system. 
     With reference to  FIGS. 28-30 , a lock collar  116  is provided to lockingly mate the cannula  112  and the stylet  114 . Lock collar  116  has a generally cylindrical overall shape, and includes a proximal portion  162 , a distal portion  164  and an interior lumen  166  extending therethrough. The proximal portion  162  may have a diameter greater than that of the distal portion  164  and is provided with a plurality of friction elements  168  to allow a user to grasp and turn the lock collar  116 . The distal portion  164  includes a generally oval-shaped opening  170  providing access to the lumen  166 . The opening  170  further includes a pair of opposing first protrusions  172  and a pair of opposing second protrusions  174  located along the inside edge of opening  170 . First protrusions  172  are located 180° from one another and are positioned at the long ends of the oval-shaped opening  170 . Second protrusions  174  are positioned at the narrow sides of the oval-shaped opening  170  (and thus are located at 90° intervals from the first protrusions  172  and 180° from one another). First and second protrusions  172 ,  174  are each dimensioned to engage the lateral indentations  150  provided on the engagement tabs  144  of the handle  140 , described above. 
     The interior lumen  166  is dimensioned to receive both of the engagement tabs  144  of the handle  140 . Initially, the pedicle access system  110  of the present invention may be provided with the locking collar  116  attached to the stylet  114  in an initial position. This initial position is defined by the first protrusions  172  resting in the lateral indentations  150  of the engagement tabs  144  of the handle  140 . Upon insertion of the needle element  142  into the cannula  112 , the distal region  130  of the coupling element  118  of cannula  112  will enter the space  146  of the handle  140  such that the medial indentations  148  are aligned with (but not yet engaging) the engagement region  131  of the coupling element  118 . At this point, a user would then rotate the lock collar  116  90° to a second position such that the second protrusions  174  rest in the lateral indentations  150 . Due to the oval-shaped nature of the opening  170 , upon rotation of the lock collar  116 , the engagement tabs  144  will be forced toward one another, and the medial indentations  148  will come in contact with and positively engage the engagement region  131 . As noted previously, this positive engagement prevents the cannula  112  from moving. At the same time, the lock collar  116  serves to lock the engagement tabs  114  in place, effectively locking the cannula  112  and the stylet  114  together. The pedicle access system  110  is now ready for use. 
       FIGS. 31-34  illustrate an example of a pedicle access system  210  according to a further alternative embodiment of the present invention. The pedicle access system  210  includes a cannula  212 , a stylet  214 , a lock collar  216  and a retractable insulation sheath  217 . As described above in relation to pedicle access systems  10  and  110 , pedicle access system  210  may be used to percutaneously approach the pedicle, initiate pilot hole formation, and conduct a stimulation signal to the target site for the purposes of performing a pedicle integrity assessment during formation of the pilot hole. To do this, the cannula  212  and stylet  214  may be lockingly mated and inserted through an operating corridor to the pedicle target site, using the handle portion  240  of the stylet  214  to facilitate easy movement and positioning of pedicle access system  210 . The pedicle access system  210  may be driven into the bone at the target site to form a pilot hole while a stimulation signal is applied and conducted to the target site to assess the integrity of the pedicle during pilot hole formation. The retractable insulation sheath  217  functions to ensure maximum efficiency of the stimulation signal as by limiting or preventing shunting of the signal during pilot hole formation. As shown and described herein, the cannula  212 , stylet  214  and retractable insulation sheath  217  are generally cylindrical in shape. However, it should be understood that cannula  212 , stylet  214  and sheath  217  may be provided in any suitable shape having any suitable cross-section (e.g. generally oval or polygonal) without deviating from the scope of the present invention. 
     The retractable insulation sheath  217  functions to ensure maximum efficiency of the stimulation signal as by limiting or preventing shunting of the signal during pilot hole formation. With specific reference to  FIGS. 32-34 , this is accomplished by providing a tubular insulation member  274  slideably mated with a housing member  276  described in greater detail below. In an initial position (shown in  FIGS. 33-34 ), the tubular insulation member  274  is fully extended such that it extends at least to the tip  258  of the stylet  214 . Upon formation of a pilot hole in a pedicle (or other piece of bone), the stylet  214  will advance into the bone while the insulation sheath remains outside the bone (a position shown by way of example in  FIG. 32 ). Due to the insulative properties of the sheath  217 , the electrical current when supplied will be directed into the pilot hole by the uninsulated portion of the cannula  212  and stylet  214  while prevented from shunting outside of the hole by the sheath  217 . 
       FIGS. 35-36  illustrate an example of a cannula  212  forming part of pedicle access system  210  of the present invention. Cannula  212  includes a coupling element  218  and an elongated shaft  220 . An interior lumen extends through the cannula  212  from a first opening  222  located at a proximal region  230  of the coupling element  218  to a second opening  224  located at a distal end  221  of the elongated shaft  220 . Elongated shaft  220  may be composed of any conductive material such as metal, for example. Elongated shaft  220  may include any number of diameter changes incorporated along its length without deviating from the scope of the present invention. In the alternative, elongated shaft  220  may be provided with a uniform diameter along its length. 
     With reference to  FIGS. 37-38 , coupling element  218  comprises a proximal region  230 , a center section  232 , and a distal portion  234 . Proximal region  230  includes an engagement region  231  dimensioned to engage with the handle portion  240  of the stylet  214  (as described in further detail below). The engagement region  231  may be provided in any suitable geometric configuration to allow for secure mating with the engagement tabs  144  of the handle  140 . By way of example only, the coupling element  218  is shown in  FIGS. 37-38  having a plurality of triangular-shaped indentations  233 , however other shapes are possible. Proximal region  230  may include at least one tab member  236  that protrudes in a generally lateral direction from the proximal region  230 . By way of example only, as shown in  FIG. 38  proximal region  230  includes two tab members  236  positioned opposite one another and adjacent to first opening  222 . Tab members  236  may be utilized to attach supplemental instruments and/or apparatuses to the cannula  212 . Center section  232  may be provided with a diameter that is larger than the diameters of the proximal region  230  and distal portion  234 , and may be provided with a plurality of ridges  235  and/or other features for the purpose of providing a suitable gripping area for a user. The distal portion  234  is dimensioned to engage with the elongated shaft  220  of the cannula  212  and may further be provided with a recess  237  for engagement with the sheath attachment element  292 , described in further detail below. 
       FIG. 39  illustrates an example of a stylet  214  forming part of the pedicle access system  210 . Stylet  214  includes a handle portion  240  and a needle element  242 . Referring to  FIGS. 41-42 , the handle portion  240  may (by way of example) resemble a T-handle for providing a user with a suitable gripping means. By way of example only, the handle portion  240  may have a substantially hollow interior that is not fully enclosed. Handle portion  240  includes an aperture  243  and a pair of engagement tabs  244  extending distally from handle portion  240 . Aperture  243  is dimensioned to allow passage of the needle element  242  from the handle portion  240 . Engagement tabs  244  extend generally perpendicularly from the handle  240  and generally parallel to one another such that the engagement tabs  244  collectively form an interior space  246 . Interior space  246  is dimensioned to receive the proximal region  1230  of the coupling element  218  of the cannula  212 . Each engagement tab  244  is provided with a medial (inwardly-facing) protrusion  248 . Medial protrusions  248  are dimensioned to engage the engagement region  231  of the coupling element  218 , described above. For this reason, the medial protrusions  248  may be provided with any geometry complementary to the shape of the engagement region  231  such that when mated, the engagement tabs  244  (via the medial protrusions  248 ) will prevent movement of the engagement region  231 , in effect locking the cannula  212  in place relative to the stylet  214 . 
     With reference to  FIG. 40 , the needle element  242  comprises an elongated shaft  252  having a proximal region  254  and a distal region  256 . The proximal region  254  includes an attachment element  257  configured to attach to the interior of handle portion  240 . The attachment element  257  is also configured to provide a point of contact for an electrical stimulation source (e.g. a clip attached to an electrical source). Elongated shaft  252  extends distally from proximal region  254  and generally perpendicularly from the handle  240  (and through aperture  243 ). Needle element  242  is dimensioned to be inserted through the interior lumen of cannula  212 . The distal region  256  generally includes a distal portion of elongated shaft  252  and a shaped tip  258  having any form or shape capable of being driven into the pedicle to create a pilot hole. By way of example only, shaped tip  258  may have a beveled or double diamond form. When needle element  242  is fully inserted into cannula  212 , at least a portion of distal region  256  (including shaped tip  258 ) may protrude slightly from the second opening  224  of cannula  212 . 
     Needle element  242  may be composed of any conductive material, such as metal. Alternatively, needle element  242  may be composed of a non-conductive material with one or more embedded conductive elements at or near the distal end (e.g. distal region  256  and/or shaped tip  258 ) capable of being communicatively linked with a pedicle integrity testing system. Although shown as separate parts, the stylet  214  is preferably provided as a single unit, with the needle element  242  and attachment element  257  molded in place in the handle  240 . 
     With reference to  FIGS. 43-44 , a lock collar  216  is provided to lockingly mate the cannula  212  and the stylet  214 . Lock collar  216  has a generally cylindrical overall shape, and includes a proximal portion  262 , a distal portion  264  and an interior lumen  266  extending therethrough. The proximal portion  262  may have a diameter greater than that of the distal portion  264  and is provided with a plurality of friction elements  268  to allow a user to grasp and turn the lock collar  216 . The distal portion  264  includes a generally oval-shaped opening  270  providing access to the lumen  266 . The opening  270  further includes a pair of opposing protrusions  272  located along the inside edge of opening  270 . Protrusions  272  are located 180° from one another and are positioned approximately midway between the “long ends” and the “narrow sides” of the oval-shaped opening  270 . Protrusions  272  are dimensioned to engage the sides of engagement tabs  244  of the handle  240 , described above. 
     The interior lumen  266  is dimensioned to receive both of the engagement tabs  244  of the handle  240 . Initially, the pedicle access system  210  of the present invention may be provided with the locking collar  216  attached to the stylet  214  in an initial position. This initial position is defined by the protrusions  272  resting alongside the engagement tabs  244  of the handle  240 . The engagement tabs  244  at this point are disposed in the “long ends” of the oval-shaped opening  270 . Upon insertion of the needle element  242  into the cannula  212 , the distal region  230  of the coupling element  218  of cannula  212  will enter the space  246  of the handle  240  such that the medial protrusions  248  are aligned with (but not yet engaging) the engagement region  231  of the coupling element  218 . At this point, a user would then rotate the lock collar  216  90° to a second position such that the protrusions  272  rest in against the engagement tabs  244  and the engagement tabs  244  rest in the “narrow sides” of the oval-shaped opening  270 , as shown in  FIG. 45 . Due to the oval-shaped nature of the opening  270 , upon rotation of the lock collar  216 , the engagement tabs  244  will be forced toward one another, and the medial protrusions  248  will come in contact with and positively engage the engagement region  231 . As noted previously, this positive engagement prevents the cannula  212  from moving. At the same time, the lock collar  216  serves to lock the engagement tabs  214  in place, effectively locking the cannula  212  and the stylet  214  together. 
     With reference to  FIGS. 46-50 , the pedicle access system  210  may be provided with a retractable insulation sheath  217  to electrically insulate the cannula  212  and stylet  214 . The insulation sheath  217  may be composed of a non-conductive material or coated with a non-conductive polymer coating to insulate the sheath  217 . This prevents shunting of electrical current during pilot hole formation, increasing the efficiency with which the stimulation current is delivered to the target area. Referring to  FIG. 46 , the insulation sheath  217  includes an insulation tube  274  and a housing member  276 . As seen in  FIG. 47 , the insulation tube comprises a cannulated, elongated and generally cylindrical member having a proximal end  278  and a distal end  280 . The proximal end  278  includes at least one tab  282  configured to slideably engage the housing member  276  as set forth below. In the example shown in  FIG. 47 , the insulation tube  274  includes a pair of tabs  282  positioned opposite one another, however any number of tabs  282  may be provided without departing from the scope of the invention. The distal end  280  may be provided with a generally tapered surface  284  to allow for an improved interface with the bone. 
     Referring to  FIGS. 46 &amp; 48 , the housing member  276  comprises an elongated generally cylindrical member having a proximal end  286 , a distal end  287  and an interior lumen  288 . The proximal end  286  includes a shaped engagement feature  289  (e.g. a recess as shown) dimensioned to engage a sheath attachment element  292  described in further detail below. The housing member  276  further includes at least one elongated track  290  in the form of a cutout section extending substantially the length of the housing member  276 . The track  290  is dimensioned to slideably receive the tabs  282  of the insulation tube  274  such that the insulation tube  274  is allowed to migrate within the lumen  288 . 
     With reference to  FIGS. 49-50 , an example of a sheath attachment element  292  is shown. The sheath attachment element  292  may be provided as a generally cylindrical member having an interior lumen  293 . Sheath attachment element  292  is dimensioned to provide a snap-fit engagement with both the housing member  276  and the coupling element  218  of cannula  212 . The lumen  293  is provided with a first ridge  294  near a distal end for secure engagement with recess  289  of the housing member  276 . Similarly, the lumen  293  is provided with a second ridge (not shown) near a proximal end for engagement with recess  237  of the coupling element  218  ( FIG. 37 ). During assembly of the pedicle access system  210 , the retractable insulation sheath  217  may be provided with the sheath attachment element  292  mated to the housing member  276 . The cannula  212  is then inserted into the insulation sheath  217  and sheath attachment element  292  will then engage the coupling element  218 , thus securely attaching the insulation sheath  217  to the pedicle access system  210 . 
     In use, the pedicle access system  210  is provided with the insulation tube  274  in a first, fully extended position (e.g.  FIG. 33 ). The insulation tube  274  will remain in this position as the pedicle access system  210  is advanced through an operative corridor to a bony target site (e.g. a pedicle). Upon initial engagement with the bony structure, the tip  258  of the needle element  242  and the distal end  280  of the insulation tube  274  may contact the bone at approximately the same time. At this point the user may want to begin monitoring the integrity of the pilot hole formation by using a stimulation signal as described below. As the needle  242  is advanced into the bone, forming a pilot hole, the distal end  280  remains engaged to the outside surface of the bone. At the same time, the proximal end  278  (including tabs  282 ) of the insulation tube will advance proximally along the track  290  of the housing member  276 . Due to the insulated nature of insulation tube  274 , the portion of needle element  242  and cannula  212  that protrude from insulation tube  274  effectively constitute a stimulation region  260  ( FIG. 32 ). As the needle  242  and cannula  212  are advanced into the bony structure (and the insulation tube  274  remains on the outside of the bony structure), the stimulation region  260  becomes larger. Upon completion of the pilot hole formation, the needle  242  and cannula  212  are withdrawn from the bony structure, and the pedicle access system  210  may be removed from the operative corridor. A spring (not shown) or other control mechanism may be provided to limit the extent of migration of the insulation tube  274  and/or provide a means for the insulation tube  274  to bias toward returning to the fully extended position upon removal of the needle  242  from the pilot hole in the pedicle. 
       FIGS. 51-63  illustrate an example of a pedicle access system  310  according to a further alternative embodiment of the present invention. The pedicle access system  310  includes a cannula  312 , a stylet  314 , and a retractable insulation sheath  317 . As shown and described herein, the cannula  312 , stylet  314  and retractable insulation sheath  317  are generally cylindrical in shape. However, it should be understood that cannula  312 , stylet  314  and sheath  317  may be provided in any suitable shape having any suitable cross-section (e.g. generally oval or polygonal) without deviating from the scope of the present invention. Pedicle access system  310  may be used to percutaneously approach the pedicle, initiate pilot hole formation, and conduct a stimulation signal to the target site for the purposes of performing a pedicle integrity assessment during formation of the pilot hole. To do this, the cannula  312  and stylet  314  may be lockingly mated and inserted through an operating corridor to the pedicle target site, using the handle portion  340  of the stylet  314  to facilitate easy movement and positioning of pedicle access system  310 . The pedicle access system  310  may be driven into the bone at the target site to form a pilot hole while a stimulation signal is applied and conducted to the target site to assess the integrity of the pedicle during pilot hole formation. The retractable insulation sheath  317  functions to ensure maximum efficiency and consistency of the stimulation signal when performing pedicle integrity assessments by limiting or preventing shunting of the signal to tissue above the pedicle while also avoiding current density issues that may be encountered when focusing the stimulation signal into the pilot hole through a fixed point. 
     With specific reference to  FIGS. 52-53 , the retractable sheath includes a tubular insulation member  374 , having a proximal end  378  and a distal end  380 , extending from a connecting member  376  described in greater detail below. In an initial position (shown in  FIG. 53 ), the tubular insulation member  374  is fully extended such that it extends at least to the tip  358  of the stylet  314 . When forming the pilot hole in a pedicle (or other piece of bone), the stylet  314  will advance into the bone while the insulation sheath remains outside the bone (a position shown by way of example in  FIG. 52 ). Due to the insulative properties of the sheath  317 , the electrical current will be directed into the pilot hole by the exposed portion of the cannula  312  and stylet  314  (the size of which increases to match the depth of the pilot hole as the sheath  317  retracts) while prevented from current shunting outside of the hole by the sheath  317 . 
       FIG. 56  illustrates an example of a cannula  312  forming part of pedicle access system  310  of the present invention. Cannula  312  includes a coupling element  318  and an elongated shaft  320 . An interior lumen extends through the cannula  312  from a first opening  322 , having a threaded luer lock  323  located at the proximal end  330  of the cannula  312  to a second opening  324  located at the distal end  321  of the elongated shaft  320 . The interior lumen is dimensioned to receive the stylet  314  therethrough. The threaded luer lock  323  may be used to connect other apparatus to the cannula  312 , for example, a syringe to withdraw or introduce aspirate or a cement delivery vehicle to deliver cement to the bone through the cannula  312 . 
     Elongated shaft  320  has an upper region  351  and a lower region  353 . The upper region  351  has a diameter approximating that of the inner diameter of the retractable insulation sheath  317  such that the retractable sheath may slide along the elongated shaft between the initial position and the exposed position. The lower region  353  has a diameter approximating that of the needle element  342  of stylet  314  such that the elongated shaft penetrates into the pedicle with the stylet  314 . Elongated shaft  320  may include any number of additional diameter changes incorporated along its length without deviating from the scope of the present invention. The elongated shaft  320  may be composed of any conductive material such as, for example, metal. 
       FIGS. 57 and 60-61  illustrate an example of a stylet  314  forming part of the pedicle access system  310 . Stylet  314  includes a handle portion  340  and a needle element  342 . The handle portion  340  may (by way of example) resemble a T-handle for providing a user with a suitable gripping means. By way of example only, the handle portion  340  may have a substantially hollow interior that is not fully enclosed. Locking cylinder  344  extends generally perpendicularly from the handle  340  and includes a pair of radial engagement ridges  348  protruding outward such that the end of the locking cylinder has a larger diameter according to a first direction in line the handle portion  340  and a smaller diameter according to a second direction perpendicular to the handle portion  340 . A boss  360  is located on the under surface  359  of the handle on each side of the locking cylinder  344 . By way of example only, the boss  360  is circular shaped, though other shapes may also be suitable As described further below, this configuration allows the stylet  314  and the cannula  312  to be lockingly engaged. 
     The needle element  342  comprises an elongated shaft  352  having a shaped tip  358  and an attachment element  357 . The attachment element  357  is situated within the hollow of handle portion  340  and provides a point of contact for an electrical stimulation source (e.g. a clip attached to an electrical source). Elongated shaft  352  extends distally from attachment region  357  (though a lumen in locking cylinder  344 , not shown) and generally perpendicularly from the handle  340 . Needle element  342  is dimensioned to be inserted through the interior lumen of cannula  312 . The shaped tip  358  may have any form or shape capable of being driven into the pedicle to create a pilot hole. By way of example only, shaped tip  358  may have a beveled or double diamond form. When needle element  342  is fully inserted into cannula  312 , at least a portion of the shaped tip  358  may protrude slightly from the second opening  324  of cannula  312 . Needle element  342  may preferably be composed of any conductive material such as, for example, metal. 
     With reference to  FIG. 58-59 , the coupling element  318  of cannula  312  comprises a capture region  364  located in between side extensions  366 . The capture region  364  is dimensioned to receive the locking cylinder  344  to securely mate the cannula  312  with the handle  340 . The capture region has a lower bore  367  diameter that approximates the larger diameter of the locking cylinder (i.e. the diameter including the engagement ridges) and a bore opening  369  diameter that approximates the smaller diameter of the locking cylinder (i.e. the diameter orthogonal to the engagement ridges). The locking cylinder  344  has an open central cavity  345  to receive the lure lock  323  when the cannula  312  and stylet  14  are engaged. Thus, to engage the stylet  314  with the cannula  312 , the handle  340  is aligned perpendicular to the coupling element  318  and the needle element  342  is inserted into the opening  322  though the lure lock  322  until the locking cylinder  344  completely receives the lure lock  323  therein. The handle  340  is then rotated relative to the connecting element  318  until the engagement ridges  348  lockingly engage within lower bore  367  of the capture region  364 . The upper surface  365  of each side extension  366  contains a detent  368  that is complementary to the bosses  360  on handle  340  and as the handle  340  is rotated into alignment with the coupling element  318  the detents  368  engage the bossed  360 , preventing unwanted unlocking of the handle  340  and coupling element  318 . 
     A friction bearing  370  situated on the upper region  351  is dimensioned to positively engage the inner diameter of the retractable sheath. By way of example, the friction bearing  370  is composed of a polymer material, such as nylon. The diameter of the friction bearing  370  is preferably such that the retractable sheath  374  will not move along the elongated shaft without the application of force to the sheath. In this manner, the surgeon may utilize the sheath  374 , and particularly the connector element  376 , to handle the pedicle access system  310  without the cannula  312  and/or stylet  314  moving relative to the sheath. According to another contemplated embodiment, the friction bearing may be positioned inside the retractable sheath  317  rather than on the elongated shaft  320 . By way of example, the inside of the sheath may be fitted with an O-ring. A ridge may be positioned on the elongated shaft proximal to the O-ring such that the sheath  374  will not travel beyond the initial position (e.g. such that the sheath cannot be removed from the elongated shaft  320  completely). According to another contemplated embodiment, the O-ring may be replaced with a coil spring (positioned on either the elongated shaft  320  or within the retractable sheath  317 . Depth markings provided on the upper region  351  indicate the depth to which the elongate shaft  320  is penetrated into the pedicle. 
     In use, the pedicle access system  310  is provided with the insulation tube  374  in a first, fully extended position (e.g.  FIG. 52 ). The insulation tube  374  will remain in this position as the pedicle access system  310  is advanced through an operative corridor to a bony target site (e.g. a pedicle). Upon initial engagement with the bony structure, the tip  358  of the needle element  342  and the distal end  380  of the insulation tube  374  may contact the bone at approximately the same time. At this point the user may want to begin monitoring the integrity of the pilot hole formation by delivering a stimulation signal as described below. As the needle  342  penetrates into bone forming a pilot hole, the distal end  380  engages the outside surface of the bone which prevents further advancement of the insulation tube  374  relative to the bone. Due to the insulated nature of insulation tube  374 , the portion of needle element  342  and cannula  312  that protrude from insulation tube  374  effectively constitute a stimulation region  390  ( FIG. 52 ). As the needle  342  and cannula  312  are advanced into the bony structure (and the insulation tube  374  remains on the outside of the bony structure), the stimulation region  390  becomes larger. Upon completion of the pilot hole formation, the needle  342  and cannula  312  are withdrawn from the bony structure, and the pedicle access system  310  may be removed from the operative corridor. A spring (not shown) or other control mechanism may be provided to limit the extent of migration of the insulation tube  374  and/or provide a means for the insulation tube  374  to bias toward returning to the fully extended position upon removal of the needle  342  from the pilot hole in the pedicle. 
       FIGS. 64-72  illustrate an example of a pedicle access system  410  according to a further alternative embodiment of the present invention. The pedicle access system  410  includes a cannula assembly  412 , a retractable sheath  417 , and a stylet  414 . As shown and described herein, the cannula  412 , stylet  414  and retractable insulation sheath  417  are generally cylindrical in shape. However, it should be understood that cannula  412 , stylet  414  and sheath  417  may be provided in any suitable shape having any suitable cross-section (e.g. generally oval or polygonal) without deviating from the scope of the present invention. 
     The pedicle access system  410  may be used to percutaneously approach the pedicle, initiate pilot hole formation, and conduct a stimulation signal to the pilot hole for the purposes of performing a pedicle integrity assessment during formation of the pilot hole. To do this, the cannula assembly  412  and stylet  414  may be lockingly mated (as shown in  FIG. 64 ) and inserted through an operating corridor to the pedicle target site. The retractable sheath  417  is preferably permanently assembled to the cannula assembly  412  (best shown in  FIG. 65 ) and is slidably mated to the cannula assembly  412  in order to allow the retractable sheath  417  to translate relative to the cannula assembly  412 . The pedicle access system  410  may be driven into the bone at the target site to form a pilot hole while a stimulation signal is applied to assess the integrity of the pedicle during pilot hole formation. The retractable insulation sheath  417  functions to ensure maximum efficiency and consistency of the stimulation signal when performing pedicle integrity assessments by limiting or preventing shunting of the signal to tissue above the pedicle while also avoiding current density issues that may be encountered when focusing the stimulation signal into the pilot hole through a fixed point. 
       FIG. 66  illustrates an example of a cannula  412  forming part of pedicle access system  410  of the present invention. Cannula  412  includes a coupling element  418 , an outer sheath  419  and an inner cannula  420 . An interior lumen extends through the inner cannula  420  from a first opening  422  through a threaded luer lock  423  located at the proximal end of the cannula  412  to a second opening  424  located at the distal end of the inner cannula  420 . The interior lumen is dimensioned to receive the stylet  414  therethrough. The proximal ends of the inner cannula  420  and outer sheath  419  are securely mated to the coupling element  418  with the inner cannula  420  concentric to the outer sheath  419 . Additionally, the inside diameter of the outer sheath  419  is larger than the outer diameter of the inner cannula  420  and allows sufficient space therebetween for the retractable sheath  417  to longitudinally slide between the outer sheath  419  and inner cannula  420 . The fixed outer sheath  419  provides increased handling characteristics to the pedicle access system  410  by providing for a fixed grasping surface along the majority of the length. The outer sheath is preferably insulated while the inner cannula  420  may be composed of any conductive material such as metal. 
     With specific reference to  FIGS. 67-68 , the retractable sheath includes a tubular insulation member  474 , having a proximal end  478  and a distal end  480 , extending from a connecting member  476 . In an initial position (shown in  FIG. 67 ), the tubular insulation member  474  is fully extended such that it extends close to or beyond the tip  458  of the stylet  414  and opening  424  of inner cannula  420 . When forming the pilot hole in a pedicle (or other piece of bone), the stylet  414  and inner cannula  20  will advance into the bone while the insulation sheath remains outside the bone (a position shown by way of example in  FIG. 68 ). Due to the insulative properties of the retractable sheath  417 , the electrical current will be directed into the pilot hole by the exposed portion of the cannula  412  and stylet  414  (the size of which increases to match the depth of the pilot hole as the retractable sheath  417  retracts) and be prevented from from shunting to tissue outside of the hole by the retractable sheath  317 . 
     A spring  450  housed in the cannula interacts with the proximal end of the retractable sheath  417  to bias the retractable sheath  417  towards the fully extended position until the stylet  414  and cannula  412  are driven into a bone. As the stylet  414  and cannula  412  penetrate the bone, the retractable sheath  417  retracts as the distal end  480  engages the bone surface. The tubular insulation member  474  of the sheath  417  longitudinally slides between the outer sheath  419  and inner cannula  420  of the cannula  412 . The connecting member  476  of the sheath  417  longitudinally slides along the outer surface of the outer sheath  419  of the cannula  412 . Tracks  442  that extend along a portion of the outer sheath  419  provide for a coupling pathway between the connecting member  476  and the tubular insulation member  474 . The connecting member  476  and tubular insulation member  474  are securely coupled to each other at coupling junctions  462  which extend through the tracks  442 . The tracks  442  also restrict the longitudinal travel of the retractable sheath  417 , thus preventing the retractable sheath  417  from translating to far relative to the cannula  412 . Depth markings  441  may be provided along the outer sheath  419  such that the position of the connecting member  476  (of the retractable sheath  417 ) relative to the depth markings indicates the approximate depth the cannula  412  and stylet  414  have penetrated into the pedicle. 
       FIG. 65  illustrates an example of a stylet  414  forming part of the pedicle access system  410 . Stylet  414  includes a handle portion  440  and a needle element  442 . The handle portion  440  may (by way of example) resemble a T-handle for providing a user with a suitable gripping means. By way of example only, the handle portion  440  may have a substantially hollow interior that is not fully enclosed. Locking cylinder  444  extends generally perpendicularly from the handle  440  and includes a pair of radial engagement ridges  448  protruding outward such that the end of the locking cylinder has a larger diameter according to a first direction in line the handle portion  440  and a smaller diameter according to a second direction perpendicular to the handle portion  440 . A boss  460  is located on the under surface  459  of the handle on each side of the locking cylinder  444 . By way of example only, the boss  460  is circular shaped, though other shapes may also be suitable As described further below, this configuration allows the stylet  414  and the cannula  412  to be lockingly engaged. 
     The needle element  442  comprises an elongated shaft  452  having a shaped tip  458  and an attachment element  457 . The attachment element  457  is situated within the hollow of handle portion  440  and provides a point of contact for an electrical stimulation source (e.g. a clip attached to an electrical source). Elongated shaft  452  extends distally from attachment region  457  (though a lumen in locking cylinder  444 , not shown) and generally perpendicularly from the handle  440 . Needle element  442  is dimensioned to be inserted through the interior lumen of inner cannula  420 . The shaped tip  458  may have any form or shape capable of being driven into the pedicle to create a pilot hole. By way of example only, shaped tip  458  may have a beveled or double diamond form. When needle element  442  is fully inserted into the inner cannula  420 , at least a portion of the shaped tip  458  may protrude slightly from the second opening  424 . Needle element  442  may preferably be composed of any conductive material such as, for example, metal. 
     With reference to  FIGS. 69-72 , the coupling element  418  of cannula  412  comprises a capture region  464  located in between side extensions  466 . The capture region  464  is dimensioned to receive the locking cylinder  444  to securely mate the cannula  412  with the handle  440 . The capture region has a lower bore  467  diameter that approximates the larger diameter of the locking cylinder (i.e. the diameter including the engagement ridges) and a bore opening  469  diameter that approximates the smaller diameter of the locking cylinder (i.e. the diameter orthogonal to the engagement ridges). The locking cylinder  444  has an open central cavity  445  to receive the lure lock  423  when the cannula  412  and stylet  414  are engaged. Thus, to engage the stylet  414  with the cannula  412 , the handle  440  is aligned perpendicular to the coupling element  418  and the needle element  442  is inserted into the opening  422  though the lure lock  422  until the locking cylinder  444  completely receives the lure lock  423  therein. The handle  440  is then rotated relative to the connecting element  418  until the engagement ridges  448  lockingly engage within lower bore  467  of the capture region  464 . The upper surface  465  of each side extension  466  contains a detent (not shown) that is complementary to the bosses  460  on handle  440  and as the handle  440  is rotated into alignment with the coupling element  418  the detents  468  engage the bosses  460 , preventing unwanted unlocking of the handle  440  and coupling element  418 . 
     The pedicle access systems  10 ,  110 ,  210 ,  310  and  410  described above may be used in combination with neurophysiology monitoring systems and methods to conduct pedicle integrity assessments while cannulating the pedicles. By way of example only, the pedicle access systems  10 ,  110 ,  210 ,  310  and  410  may be used in combination with the system and methods shown and described in Int&#39;l Patent App. Ser. Nos. PCT/US02/22247, filed on Jul. 11, 2002, Int&#39;l Patent App. Ser. No. PCT/US02/22247, PCT/2008/124079, filed on Apr. 3, 2008, PCT/US2008/012121, filed on Oct. 28, 2008 the contents of which are each hereby incorporated by reference into this disclosure as set forth herein in their entireties. With reference to  FIG. 73 , one such neurophysiology system  500  for performing pedicle integrity assessments is depicted, including an electric coupling device  502  capable of coupling to the embodiments  210 ,  310  and  410  (e.g. the embodiments with handle cutouts) to the pedicle access systems described above the system  500 . Alternate style connectors, including a plunger style connector  502  and a clamp style connector  504  are also shown and are capable of coupling all of the embodiments  10 ,  110 ,  210 ,  310  and  410  to the pedicle access systems described above the system  500 . 
     The neurophysiology system  500  performs pedicle integrity assessments by determining the amount of electrical communication between a stimulation signal and the nerve root lying outside the pedicle. To do this, a stimulation signal is applied to the pilot hole through one of the access needles as described above. Electrodes positioned over the appropriate muscles measure the EMG responses corresponding to the delivered stimulation signals. The relationship between the EMG response and the stimulation signal is then analyzed by the system and the results are conveyed to the user on the system display. The basic theory underlying the pedicle integrity test is that given the insulating character of bone, a higher stimulation current (or current density) is required to evoke an EMG response when the stimulation signal is applied to an intact pedicle as opposed to a breached pedicle. Thus, if EMG responses are evoked by stimulation currents (or current densities) lower than a predetermined safe level, the surgeon may be alerted that there is a possible breach. The neurophysiology system may be provided with software capable of compensating for multiple safe stimulation thresholds based on different current densities being applied to the pedicle by certain geometries of different instruments. 
     In another significant aspect of the present invention, the pedicle access system  10  may be used in conjunction with spinal fixation systems that require access to pedicle target sites and need pilot holes, including but not limited to those systems shown and described in commonly owned and co-pending U.S. patent application Ser. No. 11/031,506 filed Jan. 6, 2005, and commonly owned and co-pending Int&#39;l Patent App. Ser. No. PCT/US05/032300 filed Sep. 8, 2005. After positioning the pedicle access system  10  on the desired pedicle target site and safely forming a pilot hole as described above, the T-handle  16  and stylet  14  may be unlocked and removed from the cannula  12 , leaving the cannula  12  positioned in the pilot hole. Guide wires subsequently used by the spinal fixation systems may then be safely deployed to the pilot hole through the cannula  12 . Once the guide wire is in position the cannula  12  may be removed from the target site and the surgeon may commence use of the surgical fixation system. 
     While the invention is susceptible to various modification and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the invention is to cover all modifications, equivalents, and alternatives falling within the scope and spirit of the invention as defined herein.