CROSS REFERENCES TO RELATED APPLICATIONS 
     The present application is a utility patent application that claims priority to U.S. Provisional Application Ser. No. 61/600,576, filed on Feb. 17, 2012, the entire contents of which are hereby expressly incorporated by reference into this disclosure as if set forth fully herein. 
     FIELD 
     This application describes surgical instruments and methods for performing bi-cortical pedicle fixation. 
     BACKGROUND 
     Bones consist of cancellous bone covered by a thin layer of cortical bone as illustrated in  FIG. 1 . Cancellous bone is a sponge-like bone structure which is less dense, softer, and weaker when compared to cortical bone. Bone screws are utilized in surgery typically to stabilize and fix bone segments or to use as an anchor site within the bone. Most commonly, the screws are advanced through the outer cortical wall and anchored into the cancellous bone within. However, bi-cortical fixation can be used to achieve greater purchase, as the screw is fixed within the stronger cortical bone at two separate points, the proximal and distal ends of the screw. Doing so increases the screw&#39;s pull out strength, which may be desirable at higher load levels, such as in the lower lumbar and sacrum of the spine. 
     Safely achieving bi-cortical screw purchase is often difficult however. In the human vertebrae for example, the goal of bi-cortical pedicle screw fixation is to reach and thread the lead end of the screw into the anterior cortical wall. If the tip of the screw or associated instrumentation is advanced too far beyond the anterior cortical wall, the vital tissues that reside adjacent the anterior wall of the vertebrae, the great vessels for example, may be put at risk. Even with utilization of intraoperative fluoroscopy, safely gauging a screw&#39;s position can be difficult. As illustrated in  FIG. 2 , the curvature of the anterior cortical wall of the vertebral body may cause difficulty correctly determining the position of a screw from a lateral view, such that in the lateral fluoroscope image the distal end of the screw may appear to be contained within the vertebra since the final depth of the distal end may be less than the vertebral depth at the anterior most portion ( FIG. 2A ). However, the actual screw position,  FIG. 2B , is such that the distal end of the screw protrudes beyond the anterior cortical wall but at a position where the depth of the wall is less than the greatest depth near the center. 
     Current methods of bi-cortical screw fixation rely heavily on surgeon feel when forming and/or tapping the pilot hole through the vertebral body and/or during screw insertion. Thus, a need exists for instruments and methods to facilitate bi-cortical implantation of bone anchors. 
     SUMMARY 
     In preferred embodiments, the method of bi-cortical screw fixation utilizes a system of instruments with implants to achieve safe and repeatable bi-cortical fixation of screws. The method may be used for bi-cortical fixation in most bone segments and is well suited for use when securing pedicle screws in a vertebral body. In a preferred embodiment, a method is described for use in the sacrum. 
     The method begins by placement of a K-wire through the posterior cortical wall of a vertebral pedicle, via a Jamsheedi needle. One or more dilators are then inserted over the K-wire to dilate the tissues adjacent the K-wire. In this preferred example, the dilators include a first, second, and third dilator of increasingly larger diameter. The dilators are advanced until their lead end contacts the bone surface of the pedicle. 
     Optionally, a contour probe with reference scale may be advanced through the outer (e.g. third) dilator (after removal of the first and second dilators) to the pedicle. This instrument will assist the surgeon in measuring the magnitude of surface irregularity at the pedicle. The surgeon can then determine if there is a need for use of a bone reamer to create a flat pedicle surface and to gauge the depth of reaming desired. If needed, a cannulated bone reamer is guided down the K-wire and rotated sufficiently against the bone to the predetermined depth therein creating a uniform bone surface at the pedicle site. The resulting flat pedicle surface situated perpendicular to the guide wire serves as a level seat for the distal end of the second dilator, increasing the accuracy (if necessary) with which an exposed proximal end of the dilator can be used as reliable reference point to measure the depth of the vertebra later in the technique. Upon removal of the reamer, bone shavings may be removed by suction or other instruments. The second dilator is reinserted into the third dilator and advanced until seated against the bone or newly created uniform bone surface. 
     A cannulated blunt-tip probe is advanced over the guide wire and down the second dilator into the cortical wall pilot hole created by the Jamsheedi needle. As the name implies, the probe includes a blunted tip suitable to burrow through the cancellous bone within the vertebral body, extending the pilot hole and establishing a desired trajectory through the vertebra. While the blunt-tip probe effectively traverses through the softer cancellous bone, the probe is ineffective at puncturing the denser cortical bone. Thus, when the probe tip arrives at the anterior cortical wall, the probe experiences a hard stop and further advancement of the probe is inhibited. With the blunt-tip probe traversing the depth of the pedicle, reference markers near the proximal end of the probe are consulted (relative to the end of the dilator) to determine the depth to the anterior cortical wall, which can be later used to determine the desired tap depth and screw length. 
     With the blunt-tip probe defining the pilot hole trajectory, the third dilator is preferably fixed in position in alignment with the pilot hole trajectory. Fixing of the dilator may be achieved by attachment of a fixing arm to a fixator portion on the third dilator. The fixing arm may take several forms such as an A-arm attached to the operating table or other fixed device. Fixedly aligning the dilator with the pilot hole trajectory advantageously allows the K-wire to be removed during the subsequent tapping and screw insertion steps. 
     With the K-wire and blunt-tip probe removed, a tap is advanced through the second dilator which ensures alignment with the previously prepared pilot hole (by virtue of being constrained within the third dilator, which has a fixed trajectory; the second dilator is also fixed). Armed with the previously determined depth measurement, the desired tap depth to penetrate and tap the cortical wall without extending too far beyond the cortical wall can be determined, allowing for controlled piercing of the cortical wall. An adjustable safety stop on the tap is used to control the depth to which the tap can be received through the second dilator and thus also, the depth the tap can advance through the vertebra. Though these steps have been described with reference to a tap, in instances where self-tapping screws are used, the tap may be replaced with an awl including the same depth controlling features as the described tap. 
     The desired size pedicle screw may be chosen based on the determined depth of the vertebra. The pedicle screw is attached to the screw inserter then advanced down the third dilator (the second dilator having been removed) and under rotation advanced through the bone until reaching the desired bi-cortical position. The screw inserter may also include reference markings and/or adjustable depth stop as still an additional feature for controlling screw depth. 
     Reference markings on the instruments may be in a variety of forms, including numbers reflective of relative distances or depths, hash marks, grooves, ridges, color codes, or other visual or tactile indicator capable of providing measurement or sizing feedback to the user. The reference markings may represent a specified depth, or direct the user to a particular screw size or instrument choice. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view along a median sagittal plane of a human vertebra. 
         FIG. 2A  is a lateral view representative of a false negative indication of cortical wall breach that is possible on a lateral fluoroscopic image. 
         FIG. 2B  is a perspective view of the vertebra and screw of  FIG. 2A , illustrating the actual position of the pedicle screw extending beyond anterior cortical wall. 
         FIG. 3  is a front perspective view of an example embodiment of a first dilator with a K-wire. 
         FIG. 4  is a front perspective view of an example embodiment of a second dilator concentrically positioned over the first dilator and K-wire of  FIG. 3 . 
         FIG. 5  is a front perspective view of the nose of the first dilator of  FIG. 3 . 
         FIG. 6  is a top perspective view of an example embodiment of a third dilator. 
         FIG. 7  is a bottom perspective view of a the third dilator of  FIG. 6 . 
         FIG. 8  is a front perspective close up view of an example embodiment of a fixator used in a third dilator. 
         FIG. 9  is a top perspective view of the third dilator of  FIG. 6 . 
         FIG. 10  is a front perspective view of an example embodiment of a blunt-tip probe. 
         FIG. 11  is a front perspective close up view of the blunt tip of the probe illustrated in  FIG. 10 . 
         FIG. 12  is a front perspective view of an example embodiment of a bone tap with safety stop. 
         FIG. 13  is a close up view of a distal portion of the tap of  FIG. 12  with the safety stop mechanism. 
         FIG. 14  is a front perspective close up view of the tap tip of  FIG. 12 . 
         FIG. 15  is a front perspective view of an example embodiment of a tap&#39;s safety stop assembly. 
         FIG. 16  is a front perspective view of a release used within a safety stop. 
         FIG. 17  is a front perspective view of an example embodiment of a bone reamer. 
         FIG. 18  is a front perspective view of an example embodiment of a pedicle contour probe. 
         FIG. 19  is a front perspective close up view of the tip of the probe in  FIG. 18 . 
         FIG. 20  is a front perspective view of a pedicle screw. 
         FIG. 21  is a front perspective view of a pedicle screw with associated insertion instruments used in minimally invasive procedures. 
         FIG. 22  is a cross-sectional sagittal plane view through the pedicles of the lumbar spine illustrating proper placement of a guidewire, according to one example method for achieving bi-cortical screw fixation using the instruments of  FIGS. 3-21 . 
         FIG. 23  is a cross-sectional sagittal plane view through the pedicles of the lumbar spine illustrating placement of an first dilator, a second dilator, and a third dilator against the bone segment, according to the example method. 
         FIG. 24  is a cross-sectional sagittal plane view through the pedicles illustrating insertion of the reamer, according to the example method referenced in  FIG. 23 . 
         FIG. 25  is a lateral view of a blunt tip probe creating a pilot hole in the vertebrae, according to the example method referenced in  FIG. 23 . 
         FIG. 26  is a cross-sectional sagittal plane view through the pedicles of  FIG. 25  illustrating the blunt tip probe creating a pilot hole in the vertebrae. 
         FIG. 27  is a lateral view of a tap creating thread in the pilot hole in the vertebrae, according to the example method referenced in  FIG. 23 . 
         FIG. 28  is a cross-sectional sagittal plane view through the pedicles of  FIG. 27  illustrating the tap creating thread in the pilot hole in the vertebrae. 
         FIG. 29  is a lateral view of the spine illustrating a pedicle screw with attached insertion instruments advanced into the pedicle, according to the example method referenced in  FIG. 23 . 
     
    
    
     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 system and method for performing bi-cortical pedicle fixation disclosed herein boasts a variety of inventive features and components that warrant patent protection, both individually and in combination. 
     The present application describes a instruments and methods for performing bi-cortical pedicle fixation. Several instruments are utilized in the method disclosed herein for bi-cortical screw fixation.  FIGS. 3-21  illustrate various example embodiments of instruments used during the later described method.  FIG. 3  illustrates a first dilator advanced to the target pedicle (e.g. the S1 pedicle) over a K-wire (the K-wire having been positioned in the pedicle using a jamsheedi needle, not shown). After placement of the K-wire  100  (or similar guide wire) at the predetermined position in the bone, a plurality of dilators are used to dilate the tissues surrounding the K-wire to provide access to the pedicle. The K-wire defines an elongated axis ‘A’ that serves as a surgical guide path through the body to the entry point on the pedicle. 
     The dilator having the smallest outer diameter is the first dilator  101  comprising an elongated tube body  106  of sufficient length to extend from the surface of the bone to a distance above the skin. An outer surface  107  of dilator  101  resides on the exterior of the dilator body  106 . This surface  107 , preferably smooth, slides along the soft tissues of the body while radially stretching them to provide passage of the first dilator  101  down to the bone segment. 
     At the distal or lead end  103  portion of the first dilator  101  is the nose  104  portion. The nose  104  is preferred to be of a rounded cone or bullet shape. As the first dilator  101  is advanced, the surface of the leading smaller diameter portion of the nose  104  begins to gradually dilate the surrounding tissues to the full diameter of the nose  104 . 
     Central to the nose is an aperture  105  that extends the length of the first dilator  101  and defines an inner elongated wall  110  of the dilator  101  as illustrated in  FIG. 5 . The aperture  105  is of a diameter slightly larger than the K-wire  100  such that the first dilator  101  can freely slide down the wire  100  without permitting ingress of tissue between the dilator and K-wire. The aperture  105  diameter may increase in diameter as it moves along the body  106  towards the proximal end  108  portion to prevent binding between the K-wire and the inner walls  110  of the aperture  105 . At the distal end of the nose  104 , is distal stop surface  111  that abuts against the bone when fully advanced down surgical path A. 
     A grip portion  109  may be included on or inscribed into surface  107  at the proximal end  108  of elongated body  106 . The grip portion  109  may take a variety of forms to improve the surgeon&#39;s grasp on the dilator  101  as the dilator is directed toward the bone segment. In the example embodiment shown, the grip  109  is in the form of a knurled surface but alternatively may be in the form of a polymer sleeve pulled over a recessed area of the elongated body  106 . At the proximal end is proximal stop surface  112 . 
       FIG. 4  illustrates a second dilator  150  of the plurality of dilators, an intermediate dilator placed concentrically over the first dilator  101  and K-wire/guidewire  100 . The second dilator  150  in this preferred embodiment is a replica of the first dilator  101  but varies dimensionally in diameter and length. For example, the inner elongated wall  110  of the second dilator  150  is sized slightly larger in diameter than outer diameter of body  106  of the first dilator  101  wherein the second dilator  150  will glide over the first dilator  101 . Similarly, the body  106  of second dilator  150  comprises an outer diameter slightly smaller than the inner wall  110  of the third dilator  200  illustrated in  FIGS. 6-9  wherein third dilator  200  can freely glide over second dilator  150 . Although gaps between inner and outer dilator surfaces are sufficient to pass one dilator over the other, these gaps are minimized to prevent soft tissue from embedding within the gaps as increasingly larger dilators are advanced down the surgical axis. 
     The length of second dilator  150  is preferably sized wherein when stop surface  111  abuts against bone, grip portion  109  is fully exposed above the patient&#39;s skin as well as above the entire proximal end of third dilator  200 . The length of first dilator  101  exceeds both the second dilator  150  and third dilator  200  wherein when first dilator  101  stop surface  111  abuts against bone, first dilator  101  grip portion  109  is fully exposed above proximal end  108  of second dilator  150 . 
       FIGS. 6 ,  7 ,  8  &amp;  9  illustrate views of a preferred embodiment of the third dilator  200  (e.g. the final dilator according to the example embodiments described herein). The third dilator  200  comprises an elongated body  106  with an inner elongated wall  110  defining a central aperture  220 . This central aperture  220  is of sufficient diameter to slide over surface  107  of second dilator  150  as described previously and in addition is sufficient to provide passage for pedicle screw  151  and screw insertion instruments  153  such as those seen in  FIGS. 20 &amp; 21 . An outer surface  107  resides on the elongated body  106  of the third dilator  200 . The body  106  terminates at proximal screw face  224  on the proximal end  108 . 
     The third dilator  200  comprises one or more fixator portions  201 . In this embodiment, the fixator  201  is an extension of the proximal dilator body  106  in the form of a fixation boss  202 . The boss  202  comprises a top surface  204 , a bottom surface  205 , and a side wall  208 . An inner wall  203  defines an aperture  207  extending through the top  204  and bottom surfaces  205 . 
     The aperture  207  may comprise threads  206  and is configured to house a fixator lock  209  portion ( FIG. 8 ). The fixator lock  209  comprises an elongate body  212  to be received in aperture  207 . The outer surface of elongate body  212  has threads complementing those threads  206  in aperture  207  for a threaded engagement. Alternatively, fixator lock  209  may utilize a press fit when non-threaded. 
     An inner wall  213  defines a central threaded aperture through body  212 . A fixator face  210  is illustrated here in the form of radially spaced inclined teeth  211 . A stop  214 , in the form of a ridge abuts the fixator lock top surface  204  when fully seated into aperture  207 . A notch  215  partially houses interference locking pin  216  along with bore  218  in top surface  204  by press fit. The pin  216 , when pressed into position prevents derotation and thus loosening of fixator lock  209  once seated in fixation boss  202 . 
     Through the fixator face  210  is stabilization bore  219 . This bore  219  extends down from fixator face  210 . When used during surgery, the fixator lock  209  is the site for attachment of a fixation apparatus such as an A-arm which on one end is clamped to the surgical table or other immovable apparatus. The free end of the A-arm comprises a locking fixator with locking features complementing the fixator lock  209  described herein. For example, the free end of an A-arm may comprise a threaded fastener for advancing in the threaded inner wall  213 , along with a post for housing within stabilization bore  219 , and fixator face complementary to fixator face  210 . Tightening of said fastener draws the A-arm tight to the fixator lock therein securely fixing the fixator lock  209  to the A-arm. According to the example shown, a plurality of fixators  201  with various size fixator locks  209  are included. 
     In this embodiment wherein the fixator lock  209  is formed as a separate part of third dilator  200 , the opportunity exists to choose a material of manufacture having a strength and hardness that is highly resistant to wear. For example, the body  106  of dilator  200  may be manufactured from an anodized aluminum or a polymer like Radel, whereas the fixator lock  209  of  FIG. 8  may be a stainless steel. The fixator portion  201  of third dilator  200  may take many other forms suitable for fixing the dilator  200  in a predetermined position during surgery. For example, in an alternate embodiment (not shown), the fixator face  210  may be machined into top surface  204  along with stabilization bore  219  and threaded inner wall  213  wherein the A-arm clamps directly to the fixator face  210  integral with fixation boss  202 . 
     As an alternate form of fixator  201  (not shown), one or more elongated channels integral to outer dilator surface  107  and parallel with axis E may be utilized to house fixation pins that thread or penetrate directly into the bone therein holding third dilator tight to the bone surface. In yet another alternative, the fixator  201  may be in the form of a post extending outward radially about axis E from surface  107  at the proximal end  108  of third dilator  200 . In yet another alternative, with an absence of fixation bosses  202 , the fixator  201  may be in the form of dilator surface  107  at the proximal end  108  of third dilator  200 . In this alternative configuration, the free-end of the A-arm may comprise a circumferential clamp configured to encircle the outer circumference of the tube. In another alternate embodiment, instead of (or in addition to) a fixator, the third dilator may be provided with a handle that me be used by the surgeon or assistant to hold the third dilator in the desired position. 
     As illustrated in  FIG. 7 , the third dilator  200  has an outside taper  221  to improve movement through tissue that thins into scalloped teeth  222  at distal end  103 . The teeth  222  may be sharpened  223 . These teeth  222  lodge into bone when third dilator  200  is fully advanced into the surgical site and serve as yet another means to fix the dilator  200 . It is not necessary that teeth  222  all reside in the same plane since the pedicle bone surface may not necessarily be flat. Therefore the teeth  222  may be profiled to best fit the contour of the pedicle bone surface. Although the example embodiment of the third dilator includes a flat (unsloped) distal tip with teeth serrations, an alternative option may include sloped end (with or without teeth serrations) that would approximate the slope of the sacrum adjacent the S1 pedicle. 
     The dilators may be manufactured of materials such as polymers (e.g. Radel), carbon fiber, aluminum, titanium, or stainless steel alloys. The instruments used herein are preferably manufactured from aluminum, titanium or stainless steel alloys. Other materials having suitable performance characteristics may also be used. 
     Illustrated in  FIGS. 10 and 11  is a preferred embodiment of a blunt tip probe  300  configured to slide within inner cannula wall  110  of second dilator  150 . The probe  300  comprises an elongated body  301  with central cannula  302  along axis B extending the entire length of body  301 . The cannula  302  defines an inner wall  303  of said elongated body  301 . At the distal end  103  of probe  300 , is a blunt tip  304  illustrated in  FIG. 11  having a bulbous end  305 . Blunt tip  304  may include one or more external serrations  306  to assist with pilot hole extension when the instrument is advanced through cancellous bone. At distal end of blunt tip  304  is distal surface  308  utilized to push through cancellous bone during pilot hole extension. 
     Probe  300  is configured to slide within inner elongated walls  110  of second dilator  150 . The probe arm  309  portion is a distal end portion  103  of body  301  that narrows for a length of D which is sufficient to span from the outer surface of the pedicle to the anterior side of the anterior cortical wall for pilot hole extension through the cancellous bone. Proximal to the probe arm  309  may be a diameter transition  310  wherein the diameter of outer surface  311  of body  301  increases to a diameter just less than inner cannula diameter of the inner elongated wall  110  of second dilator  150 . This diameter transition  310  may be in different forms such as a fillet as illustrated in  FIG. 10 , a chamfer, or a step. The combination of the narrow probe arm  309  with blunt tip  305  and the thicker body  301  permits the probe to advance through cancellous bone to extend the pilot started by the Jamsheedi with enough rigidity to withstand bending (as opposed to typical ball tip probes), such that length measurements taken from the probe are not skewed, while lacking the ability under normal insertion forces to penetrate through cortical bone. 
     On outer surface  311  is probe reference  312 , illustrated in  FIG. 10  as a series of black circumferential etched lines but may take other forms. For example, probe reference  312  may be in the form of grooves, hash marks, or depressions, and may be color coded or marked with alpha-numeric characters. The reference marks  312  are tied to the length of the second dilator and are indicative of the length to which the distal end of the probe advances beyond the distal end of the second dilator  150 . Since the distal end of the second dilator rests against the pedicle surrounding the pilot hole, the distance the distal end of the probe  300  extends beyond the distal end of the second dilator  150  (when the probe is fully advanced through the vertebra to the anterior cortical wall) corresponds to the depth of the vertebra from the outer wall of the pedicle to the inner surface of the anterior cortical wall. 
     Probe  300  may also comprise a neuromonitoring connection  313  configured for attachment of neuromonitoring accessory (e.g. stimulation clip, not shown) for monitoring pedicle integrity (e.g. detecting breaches of the pedicle wall) during advancement of the probe through the pedicle. In this embodiment illustrated in  FIG. 10 , connector  313  is in the form of a conductive circumferential body  301  for attachment of a stimulation clip. Connector  313  is typically located near proximal end  108  to avoid interfering with the surgical entry site. 
     At the proximal end  108  of probe  300  is a grip portion  317  configured for gripping by the surgeon. In this embodiment, grip portion  317  is in the form of features for attachment of a removable handle (not shown). Body  301  comprises one or more torque faces  315  for transmitting torque from the handle through body  301 , one or more lock faces  314  for temporary locking of the handle to body  301 , and an axial face  318  to transmit axial forces from the handle down body  301 . Alternatively, body  301  may extend proximally and be formed into the shape of a handle or be configured to accept a handle thereon such as in the form of a rubber grip. 
     Illustrated in  FIGS. 12-14  is a preferred embodiment of a bone tap  400  configured to slide within inner cannula wall  110  of second dilator  150 . Bone tap  400  comprises an elongated body  401  with central cannula  402  along axis F extending the entire length of body  401 . The cannula  402  defines an inner wall  403  of said elongated body  401 . At the distal end  103  of bone tap  400 , is tap shaft  404  shown in  FIG. 12  and that further comprises fluted  409  tap tip  439  with radial cutting teeth  407  formed by tap thread  408  and cutting face  405  that includes one or more forward cutting teeth  406  to assist with penetration of the anterior cortical wall. 
     Bone tap  400  is configured to slide within inner elongated walls  110  of second dilator  150 . The tap arm  410  portion is a distal end portion  103  of body  401  that narrows for a length D as introduced earlier. Length D is sufficient in length to span from the outer surface of the pedicle to the anterior side of the anterior cortical wall for taping threads along pilot hole. Proximal to the tap arm  410  may be a diameter transition  419  wherein the diameter of outer surface  411  of body  401  increases to a diameter just less than inner cannula diameter created by the inner elongated wall  110  of second dilator  150 . For example, this diameter transition  419  may be in the form of a fillet as illustrated in  FIG. 12 , a chamfer, or a step. 
     On outer surface  411  is tap reference  412  illustrated in  FIG. 13  as a series of hash lines but may take other forms. For example, tap reference  412  may be in the form of grooves, circumferential etched lines, or depressions, and may be color coded or marked with alpha-numeric characters suitable for determining the depth of the instrument with respect to anatomical structures of the patient or to other instruments. 
     Tap  400  also includes a safety stop  421  also illustrated in  FIGS. 13 ,  15  and  16 . Safety stop  421  adjusts along depth ladder  422  corresponding to the tap reference  412 . In this embodiment, depth ladder  422  comprises a generally rectangular cross-section with a plurality of depth notches  423  on lateral sides of the rectangle configured to serve as incremental stop positions engaging safety stop  421 . As seen in  FIG. 15 , safety stop  421  comprises a housing  426  with radial surface  427  and a pair of opposing side surfaces  424 . Centered along axis K, ladder bore  425  with profile complementing depth ladder  422  extends through opposing side surfaces  424 . Generally perpendicular to ladder bore  425 , release bore  428  extends through radial surface  427  to house release  429 . Release  429  in this embodiment has a generally a square shaped ring body  430  with an exposed activation surface  431 , a pair of opposing legs  432 , and a bottom strut  433 . On the backside of bottom strut  433  is spring surface  434 . Within ring body  430 , resides ring bore  435  of a generally rectangular shape. Extending from inside the legs  432  and bottom strut  433  are cogs  436 . Each cog  436  has opposing side surface  437  and top surface  438 . 
     In use, release  429  is housed in release bore  428 . A biasing element (not shown), preferably in the form of a spring and situated within release bore  428  and behind spring surface  434 , biases release  429  outward causing cogs  436  to move towards central axis K for engagement of depth notches  423  therein causing safety stop  421  to lock in desired position along depth ladder  422 . Side surface  424  serves as a stop against proximal stop surface  112  of second dilator  150  wherein tap is limited to a depth predetermined by the user. Accordingly, the depth stop can be set based on the measured depth of the vertebra such that the distal end may be advanced into but not through the anterior cortical wall. 
     While shown according to one example embodiment, the safety stop  421  may take on a variety of forms. For example, it may be in the form of a resilient ring that expands upon force of the user, adjusted to a new position, then contracts back around a complementary depth ladder recess. As another alternative, stop  421  may be in the form of a threaded nut translating up and down a threaded depth ladder. Yet another alternative for the button is in the shape of a ball detent mechanism, in which this mechanism contains ball bearings that lock into mating grooves on the instrument shaft. Ball detent mechanisms are a popular choice in similar designs. 
     Tap  400  may also comprise a neuromonitoring connection  413  configured for attachment of a neuromonitoring accessory (e.g. stimulation clip, not shown) for monitoring pedicle integrity (e.g. detecting breaches of the pedicle wall) during advancement of the tap through the pedicle. In this embodiment illustrated in  FIG. 12 , connector  413  is in the form of a conductive circumferential body for attachment of a stimulation clip. Connector  413  is typically located near proximal end  108  to avoid interfering with the surgical entry site. 
     At the proximal end  108  of tap  400  is a grip portion  417  configured for gripping by the surgeon. In this embodiment, grip portion is in the form of features for attachment of a removable handle (not shown). Body  401  comprises one or more torque faces  415  for transmitting torque from the handle through body  401 , one or more lock faces  414  for temporary locking of the handle to body  401 , and at least one axial face  418  to transmit axial forces from the handle down body  401 . Alternatively, body  401  may extend proximally and be formed into the shape of a handle or be configured to accept a handle thereon such as in the form of a rubber grip. 
     A preferred embodiment of a bone reamer  500  is illustrated in  FIG. 17 . Reamer  500  comprises an elongate body  501  with outer surface  511 . A central cannula  502 , sufficient to house a guide wire, defines an inner wall of the cannula. At distal end  103  is reamer head  504  configured at the preferred trajectory for removing uneven or angled bone at the surface of the pedicle when driven under rotation against a bone surface. Reamer head  504  comprises a distal face  505  to abut against the bone surface, one or more axial reamer blades  506  for shaving the surface of the bone, a radial bone channel  507  to house bone chips as they are cut, and axial channel  508  as a path for bone chips to move into chip pocket  509 . At proximal end  108  the instrument is a handle portion  510  configured for grasping by the user. The handle may include a grip  512  here shown in the form of axial grooves or knurling in body  501 . 
       FIGS. 18-19  illustrate a preferred embodiment of a contour probe  600 . Contour probe  600  may be utilized to map irregularities of the pedicle surface if desired. This information may be used to determine whether reaming is desirable, the depth of reaming required, and bone to yoke  152  spacing that may be necessary for proper polyaxial motion of the pedicle screw yoke. Contour probe  600  comprises an elongate body  601  with an outer surface  602  of body  601 . Central to body  601 , an elongated cannula  605 , sufficient to receive a K-wire, defines an inner wall  606  of the cannula. At the proximal end  604 , a handle portion  607  may include a grip  608  here shown in the form of radial grooves or knurling in body  601  to improve grip of the instrument. At the distal end  603 , is contour tip  609  laterally offset from axis P. The tip  609  comprises an elongated tip arm  610  and is preferably rounded at contact surface  611 . A medial surface  612  resides on the inside of tip arm  610 . The elongate body  602  is configured with a diameter to pass through the inner elongated walls  110  of second dilator  150  or may alternatively be configured with larger outer surface  602  diameter when used within third dilator  200 . On outer surface  602  is contour reference  613  illustrated in  FIG. 18  as a series of spaced grooves but may take other forms as described previously. With the contour probe advanced to the pedicle through the second dilator, the height of the proximal end of the probe relative to the second dilator adjusts as the tip  609  is rotated around the pedicle. If the height variation is substantial the surgeon may optionally choose to use reamer  500  prior to inserting the blunt probe  300 , or prior to assessing the depth of the vertebra from the blunt tip probe prior to tapping. 
     The following exemplary steps of a procedure using the instruments described above provides an example method for safely and reproducibly achieving bi-cortical screw fixation at the S1 vertebral body. While described with relation to the fixation at the S1 body, the same method may be used other vertebral levels as well. In the preferred embodiment, the method is two-fold beginning with determining the distance from the top most surface of the pedicle to the inner surface of the anterior cortical wall and in using this information to safely pierce the anterior cortex (anterior cortical wall) without extending the tap or screw anteriorly beyond the cortex further than necessary. Second, methods are described for maintaining guide at a stable and consistent trajectory such that tapping and screw insertion may be performed without advancement over a K-wire (which can be inadvertently advanced through the anterior cortical during such steps). 
     In the preferred embodiment, the method begins with placement of a guidewire (K-wire) in a predetermined location in the sacral (S1) pedicle  700  ( FIG. 22 ). The K-wire  100  acts to guide instruments and establish the screw trajectory to this location. The skin may be incised over the pedicle at the desired entry point (e.g. approximately 1 cm lateral to the pedicle). A Jamsheedi needle (not shown) is inserted into the vertebra at the predetermined location. The stylet of the Jamsheedi is removed followed by insertion of the K-wire  100  though the remaining Jamsheedi cannula. The K-wire  100  is inserted a distance one half the depth of the vertebrae or a distance to assure it is firmly seated within the bone without the K-wire  100  tip piercing beyond the distal cortical bone wall. To prevent injury to tissues adjacent the distal cortical wall of the bone when inserting the Jamsheedi and/or guidewire, their position may be monitored by intra-operative fluoroscopy and neurophysiology monitoring equipment. 
     At least one, and preferably a series of sequential dilators are used to dilate down to the pedicle over the K-wire  100 . In the preferred embodiment, the surgeon grasps the first dilator  101  and directs aperture  105  over the loose end of K-wire  100 . The surgeon then advances the first dilator  101  down the surgical path stretching through the soft tissues surrounding the K-wire  100  until first dilator stop surface  111  abuts the bone. Inner elongated wall  110  of second dilator  150  is then directed over outer surface  107  of first dilator  101 , again stretching through the surrounding soft tissue until stop surface  111  of second dilator  150  abuts the targeted S1 pedicle. The central aperture  220  of third dilator  200  is then advanced down over second dilator surface  107  therein fully stretching surrounding soft tissue out of its path until teeth  222  contact the S1 pedicle bone surface. 
     As an option (not shown), the surgeon may utilize contour probe  600  to map the pedicle surface for irregularities. This is performed by removing the second dilator  150  and first dilator  101  away from the surgical site. Elongated cannula  605  of contour probe  600  is then advanced over K-wire  100  until contact surface  611  abuts the bone. At the anticipated screw trajectory, the user then monitors depth changes in reference  613  compared to proximal screw face  224  of third dilator  200  as contour probe  600  is rotated over the surface of the pedicle. Small to no reference change indicates little surface height variation whereas large reference changes indicate large changes in surface height. In the case of large changes in pedicle surface height, the surgeon may choose to level the pedicle surface using a bone reamer  500  to create a flat bone surface before reinsertion of second dilator  150  in later steps ( FIG. 24 ). Utilizing the bone reamer creates a flat bone surface against which the second dilator sits to facilitate depth measurement with the blunt tip probe  300 . The user advances central cannula  502  of bone reamer  500  over K-wire  100  until distal face  505  abuts the bone surface and places rotational and axial force through handle  510  toward the vertebrae causing reamer blades  506  to cut the bone and resulting in a level surface. The bone reamer  500  is then removed. Bone chips may be removed from the site by hand instruments or suction. Second dilator  150  is then reinserted down central aperture  220  of third dilator  200  until contacting pedicle bone surface. 
     In subsequent steps, the pilot hole initially created by the Jamsheedi needle through the posterior cortical wall of the pedicle is extended through the cancellous bone to the inner surface of the anterior cortical wall ( FIG. 25 ). In this step, neuromonitoring may be performed to ensure the pilot hole extends distally through the pedicle and does not breach the pedicle wall. Central cannula  302  of blunt-tip probe  300  is advanced over K-wire  100  into the pilot hole created by the jamsheedi. The surgeon, using grip portion  317 , continues with controlled advancement of probe  300  through the softer cancellous bone until a harder stop is felt through the instrument indicating abutment of distal surface  308  with the inner surface of the anterior cortical wall. A depth reading is noted from probe reference  312  in view of proximal stop surface  112  of second dilator  150 . In this embodiment, the references on the probe are calibrated wherein the user can directly read a depth ‘Y’ from the reference where the reference aligns with the proximal stop surface  112  indicating the depth of the distal surface  308  of probe  300  beyond the distal stop surface  111  of the second dilator  150 , which corresponds to the depth of the vertebra from posterior pedicle wall to inner surface of the anterior cortical wall. 
     With the blunt-tip probe  300  now defining the correct pilot hole trajectory, third dilator  200  is concentrically aligned to this path, by virtue of the second dilator  150  being aligned with the probe, and fixed in place by attachment of fixator lock  209  of third dilator  200  to an articulating arm (A-arm) or compatible handle. In this embodiment, the articulating arm (not shown) locks against fixator face  210  with screw fixation through stabilization bore  219  and threading into inner wall  213  ( FIGS. 6-9 ). For additional stability, the surgeon may choose to drive or tap proximal screw face  224  of third dilator  200  to seat teeth  222  in pedicle bone as illustrated in  FIG. 23 . By locking the third dilator  200  with an A-arm, the hole trajectory is defined therein providing for concentric alignment of the pilot hole, tap, and screw placement. K-wire  100  and blunt tip probe  300  are no longer necessary and are removed. 
     The pilot hole is now tapped and anterior cortical wall pierced ( FIG. 28 ). Bone tap  400  is utilized to tap the pilot hole in the bone in preparation of screw  151  insertion. Tap  400  is also used to provide a controlled method of piercing the anterior cortical wall. As discussed earlier, tap reference  412  and probe reference  312  may be calibrated to provide the same depth reading on each instrument when at identical bone depths while also indicating the depth of penetration into the bone. Bone tap  400  features optional safety stop  421 . In this embodiment, the safety stop is adjustable in 2.5 mm increments. 
     In the next step of the method, depth ‘Y’ is recalled. Assuming for example, the anterior cortical wall to be 2.5 mm thick, 2.5 mm is added to depth reading ‘Y’ for sum ‘Q’. Sum Q represents the tap depth required to pierce the anterior cortical wall. Distal facing side surface  424  of safety stop  421  is aligned with the tap reference  412  value equal to sum Q by depressing activation surface  431  and sliding safety stop  421  along depth ladder  422 . For example: If the second blunt-tip probe reference  312  reading is 45 mm, then distal facing side surface  424  is aligned with reading 47.5 mm. This step provides controlled piercing of the anterior cortical wall without the tap over extending anteriorly. 
     Neuromonitoring may again be performed during tapping to ensure the tap does not breach the pedicle wall. Tap shaft  404  of bone tap  400  is led to pilot hole through the second dilator, along the trajectory fixed via the third dilator, and advanced with rotation causing tap thread  408  to tap pilot hole. When distal facing side surface  424  abuts proximal stop surface  112  of second dilator  150 , the pilot hole is threaded to the desired depth. Rotation of tap can now be reversed and tap  400  removed from surgical path, followed by the second dilator. 
     Based on depth measures obtained earlier such as value Q or Y, the surgeon will then choose an appropriate screw length for bi-cortical purchase. The surgeon may choose a screw  151  length to compensate for any amount of spacing she may desire between yoke  152  and the pedicle bone surface for full poly-axial motion of the yoke  152 . The surgeon may also choose a slightly longer screw to assure threads have full purchase in the anterior cortical wall yet have minimal protrusion. 
     Pedicle screw  151  with attached insertion instruments  153  is now centered then advanced down screw path trajectory defined by central aperture  220  of fixed third dilator  200  and pre-threaded pilot hole. Because the screw length is selected based on the predetermined vertebra depth, monitoring insertion depth of the inserter is not necessary. However, the screw insertion instruments may also have an inserter reference  154  similar to that seen on other instruments. Because the second dilator  150  is removed and cannot be utilized as a depth reference, however, the reference on the screw inserter may be made to account for the difference in length between the second dilator and the third dilator. The above described steps may be completed for positioning of each pedicle screw to be implanted and the fixation construct may be completed with rod placement and construct locking. 
     While the present invention has been shown and described in terms of preferred embodiments thereof, it should be understood that this invention is not limited to any particular embodiment and that changes and modifications may be made without departing from the true spirit and scope of the invention as defined in the appended claims.
Surgical systems and methods are disclosed for safe bi-cortical bone screw placement within a bone segment. Included is a method of measurement to control advancement of instruments and implants to repeatedly obtain bi-cortical screw fixation while minimizing protrusion of the lead end of the screw beyond the distal cortical wall therein reducing incidence of injury to adjacent soft tissues.