Patent Publication Number: US-2013253599-A1

Title: Surgical Trajectory Monitoring System and Related Methods

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/301,233, filed on Jun. 3, 2009 (now issued as U.S. Pat. No. 8,442,621), which was the national stage entry of PCT/US07/11962, filed on May 17, 2007, which claims priority to U.S. Provisional Patent Application Ser. No. 60/801,488, entitled “Pedicle Access Probe and Related Methods,” and filed on May 17, 2006; U.S. Provisional Patent Application Ser. No. 60/918,955, entitled “Trajectory Aligned Pedicle Access,” and filed on Mar. 19, 2007; U.S. Provisional Patent Application Ser. No. 60/919,049, entitled “System and Methods for Orienting a Fluoroscope,” and filed on Mar. 19, 2007; and U.S. Provisional Patent Application Ser. No. 60/925,630, entitled “Surgical Access System and Methods for Orienting the Same,” and filed on Apr. 20, 2007; the entire contents of each of the above noted applications are expressly incorporated by reference into this disclosure as if they were set forth in their entireties herein. 
    
    
     BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     The present invention relates generally to determining a desired trajectory and/or monitoring the trajectory of surgical instruments and implants and, more particularly, doing so during spinal surgery, including but not limited to ensuring proper placement of pedicle screws during pedicle fixation procedures and ensuring proper trajectory during the establishment of an operative corridor to a spinal target site. 
     II. Discussion of the Art 
     Determining the optimal or desired trajectory for surgical instruments and/or implants and monitoring the trajectory of surgical instruments and/or implants during surgery have presented challenges to surgeons since the inception of surgery itself. One example is pedicle fixation, which is frequently performed during spinal fusions and other procedures designed to stabilize or support one or more spine segments. Pedicle fixation entails securing bone anchors (e.g. pedicle screws) through the pedicles and into the vertebral bodies of the vertebrae to be fixed or stabilized. Rods or other connectors are used to link adjacent pedicle screws and thus fix or stabilize the vertebrae relative to each other. A major challenge facing the surgeon during pedicle fixation is implanting the pedicle screws without breaching, cracking, or otherwise compromising the pedicle wall, which may easily occur if the screw is not properly aligned with the pedicle axis. 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 and/or neurologic deficit due to unwanted contact between the pedicle screw and delicate neural structures, such as the spinal cord or exiting nerve roots, which lie in close proximity to the pedicle. A misplaced pedicle screw often necessitates revision surgery, which is disadvantageously painful for the patient and costly, both in terms of recovery time and hospitalization. 
     The present invention is aimed primarily at eliminating or at least reducing the challenge associated with determining the optimal or desired trajectory for surgical instruments and/or implants and monitoring the trajectory of surgical instruments and/or implants during surgery. 
     SUMMARY OF THE INVENTION 
     The present invention facilitates the safe and reproducible use of surgical instruments and/or implants by providing the ability to determine the optimal or desired trajectory for surgical instruments and/or implants and monitor the trajectory of surgical instruments and/or implants during surgery. By way of example only, the present invention may be used to ensure safe and reproducible pedicle screw placement by monitoring the axial trajectory of surgical instruments used during pilot hole formation and/or screw insertion. Neurophysiologic monitoring may also be carried out during pilot hole formation and/or screw insertion. It is expressly noted that in addition to its uses in pedicle screw placement, the present invention is suitable for use in any number of additional surgical procedures where the angular orientation or trajectory of instrumentation and/or implants and/or instrumentation is important, including but not limited to general (non-spine) orthopedics and non-pedicular based spine procedures It will be appreciated then that while the surgical instruments are generally described below as pedicle access tools, cannulas, retractor assemblies, and imaging systems (e.g. C-arms), various other surgical instruments (e.g. drills, screw drivers, taps, etc. . . . ) may be substituted depending on the surgical procedure being performed and/or the needs of the surgeon. 
     A surgical trajectory system may include an angle-measuring device (hereafter “tilt sensor”) and a feedback device. The tilt sensor measures its angular orientation with respect to a reference axis (such as, for example, “vertical” or “gravity”) and the feedback device may display or otherwise communicate the measurements. Because the tilt sensor is attached to a surgical instrument the angular orientation of the instrument, may be determined as well, enabling the surgeon to position and maintain the instrument along a desired trajectory during use. 
     The tilt sensor may include a sensor package enclosed within a housing. The housing is coupled to or formed as part of a universal clip to attach the tilt sensor to a surgical instrument. The sensor package may comprise a 2-axis accelerometer which measures its angular orientation in each of a sagittal and transverse plane with respect to the acting direction of gravity. The sagittal orientation corresponds to a cranial-caudal angle and the transverse orientation corresponds to a medial-lateral angle. The sensor package is preferably situated such that when the tilt sensor is perpendicular to the direction of gravity, the inclinometer registers a zero angle in both the sagittal and transverse planes. Thus, when the tilt sensor is fixed perpendicularly to the longitudinal axis of the surgical instrument, the angular orientation of the longitudinal axis of the instrument is determined relative to gravity. Alternatively, a 3-axis sensor may be used. The 3-axis sensor may comprise a 2-axis accelerometer to measure sagittal and transverse orientation and either a gyroscope and/or one or more magnetometers (e.g. a single 3-axis magnetometer or a combination of a 1-axis magnetometer and a 2-axis magnetometer) to measure the longitudinal axial rotation of the instrument. 
     A universal clip may be provided to attach the tilt sensor to the surgical instrument. The universal clip comprises a fastener, which securely holds a surgical instrument to the clip, a sensor bed, which snugly holds the tilt sensor to the clip, a coupler, which connects the sensor bed to the fastener, and a collar, which travels along the coupler to engage the surgical instrument with the fastener. 
     A surgical instrument for use with the surgical trajectory system may comprise, by way of example only, a pedicle access probe. The instrument may generally comprise a probe member having a longitudinal axis and a handle. The probe member may be embodied in any variety of configurations that can be inserted through an operating corridor to a pedicle target site and bore, pierce, or otherwise dislodge and/or impact bone to form a pilot hole for pedicle screw placement. The probe member may be composed of any material suitable for surgical use and strong enough to impact bone to form a pilot hole. In one embodiment, the material may be capable of conducting an electric current signal to allow for the use of neurophysiologic monitoring. 
     The handle may be permanently or removably attached to the probe member and may be shaped and dimensioned in any of a number of suitable variations to assist in manipulating the probe member. In some embodiments, the handle includes a cutout region for accommodating attachment of the universal clip. In other embodiments, the handle includes an integral cavity for receiving the tilt sensor directly. In still other embodiments the tilt sensor is permanently integrated into the instrument handle. 
     A feedback device may be communicatively linked to the tilt sensor via a hard wire or wireless technology. The feedback device may communicate any of numerical, graphical, and audiofeedback corresponding to the orientation of the tilt sensor in the sagittal plane (cranial-caudal angle) and in the transverse plane (medial-lateral angle). The medial-lateral and cranial-caudal angle readouts may be displayed simultaneously and continuously while the tilt sensor is in use, or any other variation thereof (e.g. individually and/or intermittently). The feedback device may be placed next to the patient on the surgical table, or it may be affixed to any number of suitable objects in the operating room. Alternatively, a display may be provided that may be positioned in the practitioner&#39;s field of view during surgery. By way of example only, the display may be attached to one of the surgical instrument, the universal clip, and the practitioner&#39;s hand. 
     A bubble level device may be provided with the surgical trajectory system and used to ensure the tilt sensor is functioning correctly. The bubble level device comprises a handle with a bulls-eye level mounted in it. When the handle is placed on a flat surface with the tilt sensor inserted into it, an indicator ring should encircle a bubble captured within the glass. When the bubble is within the indicator ring, the tilt sensor display should read approximately zero-degrees for both the cranial-caudal and medial-lateral angle readouts. 
     In general, to orient and maintain the surgical instrument along a desired trajectory during pilot hole formation, the surgical instrument is advanced to the pedicle (through any of open, mini-open, or percutaneous access) while oriented in the zero-angle position. The instrument is then angulated in the sagittal plane until the proper cranial-caudal angle is reached. Maintaining the proper cranial-caudal angle, the surgical instrument may then be angulated in the transverse plane until the proper medial-lateral angle is attained. Once the feedback device indicates that both the medial-lateral and cranial caudal angles are matched correctly, the instrument may be advanced into the pedicle to form the pilot hole, monitoring the angular trajectory of the instrument until the hole formation is complete. 
     Before the pilot hole is formed, the desired angular trajectory (e.g. the cranial-caudal angle and the medial-lateral angle) must first be determined. Preoperative superior view MRI or CAT scan images are used to determine the medial-lateral angle. A reference line is drawn through the center of the vertebral body and a trajectory line is then drawn from a central position in the pedicle to an anterior point of the vertebral body. The resulting angle between the trajectory line and the reference line is the desired medial-lateral angle to be used in forming the pilot hole. 
     The cranial-caudal angle may be determined using an intraoperative lateral fluoroscopy image incorporating a vertical reference line. Again, a trajectory line is drawn from the pedicle nucleus to an anterior point of the vertebral body. The resulting angle between the trajectory line and the vertical reference line is the desired cranial-caudal angle to be used in forming the pilot hole. A protractor outfitted with a tilt sensor may be provided to assist in determining the cranial-caudal angle in the operating room. Alternatively, the cranial-caudal angle may be calculated preoperatively using imaging techniques that provide a lateral view of the spine. The medial-lateral and cranial-caudal angles should be determined for each pedicle that is to receive a pedicle screw. Alternate and/or additional methods for predetermining the pedicle angles are also contemplated and may be used without deviating from the scope of the present invention. 
     An alternate feedback device may be provided which can communicatively link multiple tilt sensors at once. Preferably, the alternate feedback device may be linked to three tilt sensors simultaneously, and angle measurement feedback may be provided simultaneously for all attached tilt sensors. In one example, this allows a tilt sensor to be engaged with a surgical instrument, a protractor, and a C-arm fluoroscope without the need for multiple displays and/or connecting and disconnecting the tilt sensor to the various devices during the procedure. 
     According to one embodiment of the present invention, a coupler may be provided to attach the tilt sensor to a standard C-arm. The coupler comprises a sensor bed which receives and holds the tilt sensor, and a mount which may attach to the C-arm. The sensor bed may be pivotally attached to the mount and may preferably pivot between a horizontal position and a vertical position with respect to the mount. This allows the tilt sensor to be aligned in the same starting orientation with respect the direction of gravity whether the C-arm is in the A/P position or the lateral position. It will be appreciated however that this is not always necessary, as in some instances (such as when determining the cranial-caudal angle using the C-arm) the sensor output is adequate if only one axis of the sensor is perpendicular to gravity. 
     A coupler may be configured to attach to the C-arm via a belt, strap, Velcro, tape, etc. . . . The coupler may be configured to attach to the sidewall of the C-arm, the face of the C-arm, or any other part of the C-arm. The coupler may also include a reticle/plumb line. The plumb line may comprise a radio-dense marker which will appear on the flouro-images. The plumb line may be oriented parallel to the direction of gravity to serve as a vertical reference line in the flouro-images. The radio dense marker may be configured in any number of arrangements to provide additional markings on the flouro-images. A target may also be provided to augment the plumb line. 
     In an alternate embodiment of the present invention, it is contemplated that the orientation of C-arm with respect to the patient may be adjusted by moving the patient rather than the C-arm. To accomplish this, by way of example only, a tilt sensor may be attached to the surgical table and the table may be adjusted rather than the C-arm. In still another embodiment, a tilt sensor may be attached directly to the patient and again, the table may be adjusted rather than the C-arm. 
     In addition to facilitating adjustments to a C-arm orientation, a C-arm equipped with the orientation system of the present invention may have other uses as well. By way of example only, the C-arm may be used to determine a pedicle axis and/or a starting point for pedicle penetration. 
     The cranial-caudal angle may be determined intraoperatively with a C-arm fluoroscope equipped with the orientation system of the present invention. The C-arm is oriented in a lateral position and then radially rotated until a vertical reference line is parallel to the pedicle axis, this is the trajectory lateral position. The angle measured by the tilt sensor is the cranial-caudal angle of the pedicle axis. 
     To select a starting point for pedicle penetration, the C-arm may be placed in the trajectory lateral position. From the trajectory lateral position the C-arm may be rotated back to the A/P position while maintaining the radial rotation imparted to achieve the trajectory lateral position. A surgical instrument may be advanced to the target site and positioned on the lateral margin of the pedicle, the preferred starting point according to this example. The depth of penetration of the surgical instrument may be checked during advancement by rotating the C-arm back to a trajectory lateral view. 
     Alternatively, the starting point may be determined using an “owls eye” view. The C-arm may be oriented such that it is aligned with both the medial-lateral and cranial-caudal angles as discussed above. The tip of the pedicle access instrument is placed on the skin so that the tip is located in the center of the pedicle of interest on the fluoro-image; and thereafter the instrument is advanced to the pedicle. Another flouro-image is taken to verify that the tip of the instrument is still aligned in the center of the pedicle. 
     Using the “owls eye” view, a standard surgical instrument may be guided along a pedicle axis without the use of an additional tilt sensor on the surgical instrument. In the “owls eye” image, a surgical instrument properly aligned with the pedicle axis will appear as a black dot. Once aligned, the surgical instrument may be advanced through the pedicle while ensuring that it continues to appear as only a dot on the fluoroscopy image. The depth of penetration may again be checked with a trajectory lateral image. 
     The surgical trajectory system may also be utilized a surgical access system. Using the surgical trajectory system can aid in both the insertion and positioning of the access instruments themselves, as well as, aiding in the later insertion of instruments and/or implants through the surgical access instruments. One significant advantage is the ability to later visually align surgical instruments and/or implants along the same trajectory by visually comparing the alignment of the instrument to that of the access instrument The 
     Neurophysiologic monitoring may be carried out in conjunction with the trajectory monitoring performed by the surgical trajectory system. The surgical trajectory system may be used in combination with neurophysiologic monitoring systems to conduct pedicle integrity assessments before, during, and after pilot hole formation, as well as to detect the proximity of nerves while advancing and withdrawing the surgical instrument from the pedicle target site. By way of example only, a neurophysiology system is described which may be used in conjunction with the surgical trajectory system. 
     The neurophysiology system includes a display, a control unit, a patient module, an EMG harness, including eight pairs of EMG electrodes and a return electrode coupled to the patient module, and a host of surgical accessories (including an electric coupling device) capable of being coupled to the patient module via one or more accessory cables. 
     To perform the neurophysiologic monitoring, the surgical instrument is configured to transmit a stimulation signal from the neurophysiology system to the target body tissue (e.g. the pedicle). As previously mentioned, the surgical instrument probe members may be formed of material capable of conducting the electric signal. To prevent shunting of the stimulation signal, the probe members may be insulated, with an electrode region near the distal end of the probe member for delivering the electric signal and a coupling region near the proximal end of the probe member for coupling to the neurophysiology system. 
     The neurophysiology system performs nerve monitoring during surgery by measuring the degree of communication between a stimulation signal and nerves or nerve roots situated near the stimulation site. To do this, the surgical instrument is communicatively linked to the neurophysiology monitoring system and stimulation signals are emitted from an electrode region on the surgical instrument. EMG electrodes positioned over the appropriate muscles measure EMG responses corresponding to the stimulation signals. The relationship between the EMG responses and the stimulation signals are then analyzed and the results are conveyed to the practitioner (e.g. audibly and/or visually on the neurophysiology display). More specifically, the system determines a threshold current level at which an evoked muscle response is generated (i.e. the lowest stimulation current that elicits a predetermined muscle response). Generally the closer the electrode is to a nerve, the lower the stimulation threshold. Thus, as the probe member moves closer to a nerve, the stimulation threshold will decrease, which may be communicated to the practitioner to alert him or her to the presence of a nerve. The pedicle integrity test, meanwhile, works on the underlying theory that given the insulating character of bone, a higher stimulation current 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 lower than a predetermined safe level, the surgeon may be alerted to a possible breach. 
     In one embodiment, the tilt sensor may be communicatively linked directly to the control unit of the neurophysiology monitoring system and data from the tilt sensor may audibly communicated and/or visually communicated alone and/or jointly with the neurophysiologic data. 
    
    
     
       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 an exemplary view of a surgical trajectory system, including a tilt sensor and an LCD feedback device, connected to a surgical instrument, according to one embodiment of the present invention; 
         FIG. 2  is a perspective view of a tilt sensor of the surgical trajectory system of  FIG. 1 , according to one embodiment of the present invention; 
         FIG. 3  is a perspective view of a tilt sensor, the outer housing shown in dashed lines to make visible the inclinometer situated within the housing, according to one embodiment of the present invention; 
         FIG. 4  is a perspective view depicting the bottom of the tilt sensor, according to one embodiment of the present invention; 
         FIGS. 5-7  illustrate a universal clip connector used to attach the tilt sensor of  FIG. 2  to a surgical instrument, according to one embodiment of the present invention; 
         FIG. 8  is an exploded view of the universal clip connector of  FIGS. 5-7 , according to one embodiment of the present invention; 
         FIG. 9  is a perspective view of the tilt sensor mated with the universal clip, according to one embodiment of the present invention; 
         FIGS. 10A-10B  illustrate a way in which the universal clip may securely couple a surgical instrument, according to one embodiment of the present invention; 
         FIG. 11  is a perspective view of a surgical instrument (e.g. jamsheedi needle) for penetrating pedicle bone to form a pilot hole, which is linked to the surgical trajectory system of  FIG. 1 , according to one embodiment of the present invention; 
         FIG. 12  is a perspective view of a surgical instrument (e.g. gear shift probe) for penetrating pedicle bone to form a pilot hole, which is linked to the surgical trajectory system of  FIG. 1 , according to another embodiment of the present invention; 
         FIG. 13  is a perspective view of the surgical instrument of  FIG. 12  coupled to the surgical trajectory system by a universal clip as opposed to the direct coupling seen in  FIG. 12 , according to yet another embodiment of the present invention; 
         FIG. 14  is a top view of a bubble level device optionally provided with the tilt sensor, according to one embodiment of the present invention; 
         FIG. 15  is an exemplary display unit, releasably or permanently coupled to the universal clip, such that the display feedback may be viewed in the surgical field, according to one embodiment of the present invention; 
         FIG. 16  is a front view of the display unit of  FIG. 15 , engaged to a surgical instrument, according to one embodiment of the present invention; 
         FIG. 17  illustrates the position of the display unit of  FIG. 16  relative to the manipulating hand of the practitioner, according to one embodiment of the present invention; 
         FIGS. 18A-18B  are front and side views, respectively, of the display unit of  FIG. 15  attached directly to the practitioner&#39;s manipulating hand, according to another embodiment of the present invention; 
         FIG. 19  illustrates a superior view preoperative MRI image used to determine the proper medial-lateral angle for hole formation, according to one embodiment of the present invention; 
         FIG. 20  illustrates an intraoperative lateral fluoroscopy image used to determine the proper cranial-caudal angle for hole formation, according to one embodiment of the present invention; 
         FIG. 21  is a perspective view of a bubble needle for ensuring a true vertical reference line in fluoroscopy images, according to one embodiment of the present invention; 
         FIG. 22  is a top view of a handle portion of the bubble needle of  FIG. 21  illustrating the level device used to orient the needle vertically; 
         FIG. 23  is a perspective view of a digital protractor device configured for use with the surgical trajectory system of  FIG. 1  to determine angle measurements, according to one embodiment of the present invention; 
         FIG. 24A  is a back view illustrating the protractor device of  FIG. 23  in a first position aligned with a vertical reference line in the intraoperative lateral fluoroscopy image of  FIG. 20 , according to one embodiment of the present invention; 
         FIG. 24B  is a back view illustrating the protractor device of  FIG. 23  in a second position aligned with the desired pedicle trajectory on the intraoperative lateral fluoroscopy image of  FIG. 20 , according to one embodiment of the present invention; 
         FIG. 25  is a display unit for providing feedback from multiple tilt sensors, according to one embodiment of the present invention; 
         FIG. 26  is an illustration of an operating theater equipped with a surgical table, C-arm fluoroscope, fluoroscope monitor, practitioner, and patient; 
         FIG. 27  is a front view of the C-arm of  FIG. 26  oriented in an A/P position for generating an A/P fluoroscopic image; 
         FIG. 28  is front view of the C-arm of  FIG. 26  oriented in a lateral position for generating a lateral fluoroscopic image; 
         FIGS. 29A-29B  are front views of the C-arm of  FIG. 26  oriented according to desired medial-lateral angles between the A/P position of  FIG. 27  and the lateral position of  FIG. 28 ; 
         FIGS. 30A-30B  are side views of the C-arm of  FIG. 26  oriented according to various cranial-caudal angles; 
         FIG. 31  is a perspective view of a coupler for coupling the surgical trajectory system of  FIG. 1  to the C-arm of  FIG. 26 , according to one embodiment of the present invention; 
         FIG. 32  is a front view of the coupler of  FIG. 31 , according to one embodiment of the present invention; 
         FIG. 33  is a side view of the coupler of  FIG. 31  with a sensor bed aligned in a horizontal position, according to one embodiment of the present invention; 
         FIG. 34  is a side view of the coupler of  FIG. 31  with a sensor bed aligned in a vertical position, according to one embodiment of the present invention; 
         FIG. 35  is a perspective view of the coupler of  FIG. 31  with the tilt sensor of  FIG. 2  engaged in the sensor bed, according to one embodiment of the present invention; 
         FIG. 36  is a side view of the coupler/tilt sensor combination of  FIG. 35  attached to a signal receiver of the C-arm of  FIG. 26  in the A/P position, according to one embodiment of the present invention; 
         FIG. 37  is a side view of the coupler/tilt sensor combination of  FIG. 35  attached to a signal receiver of the C-arm of  FIG. 26  in the lateral position, according to one embodiment of the present invention; 
         FIG. 38  is a perspective view of a coupler for coupling the surgical trajectory system of  FIG. 1  to the C-arm of  FIG. 26 , according to another embodiment of the present invention; 
         FIG. 39  is a side view of the coupler of  FIG. 38 , according to one embodiment of the present invention; 
         FIG. 40  is bottom view of the coupler of  FIG. 38 , according to one embodiment of the present invention; 
         FIG. 41  is a front view of the coupler of  FIG. 38  and tilt sensor of  FIG. 1  attached to the signal receiver face of the C-arm of  FIG. 26 , according to one embodiment of the present invention; 
         FIG. 42  is a side view of the coupler/tilt sensor combination of  FIG. 41  attached to a signal receiver of the C-arm of  FIG. 26  in the A/P position, according to one embodiment of the present invention; 
         FIG. 43  is a side view of the coupler/tilt sensor combination of  FIG. 41  attached to a signal receiver of the C-arm of  FIG. 26  in the lateral position, according to one embodiment of the present invention; 
         FIG. 44  is a perspective view of a target containing a radio-dense marker for augmenting a radio-dense marker included with the coupler of  FIG. 38 , according to one embodiment of the present invention; 
         FIG. 45  is a front view of the target of  FIG. 44  in use with the coupler of  FIG. 38 , according to one embodiment of the present invention; 
         FIG. 46  is a front view of the coupler of a coupler for coupling the surgical trajectory system of  FIG. 1  to the C-arm of  FIG. 6 , the coupler including 4 radio-dense markers positionable over the signal receiver, according to another embodiment of the present invention; 
         FIG. 47  is a front view showing a surgical table rotated laterally to adjust the orientation of the C-arm of  FIG. 26  with respect to the patient, according to another embodiment of the present invention; 
         FIG. 48  is a side view showing a surgical table rotated in the sagittal plane to adjust the orientation of the C-arm of  FIG. 26  with respect to the patient, according to another embodiment of the present invention; 
         FIG. 49  is a top view of a vertebral body showing the medial-lateral angle A 1  of the pedicle axis; 
         FIG. 50  is a side view of a vertebral body showing the cranial-caudal angle A 2  of the pedicle axis; 
         FIG. 51  is a true A/P image of the spine with a vertical reference line formed by the coupler of  FIG. 38 , according to one embodiment of the present invention; 
         FIG. 52  is a true lateral image of the spine with a vertical reference line formed by the coupler of  FIG. 38 , according to one embodiment of the present invention; 
         FIG. 53  is a trajectory lateral image of the spine where the C-arm has been rotated to align the vertical reference line with the pedicle axis, according to one embodiment of the present invention; 
         FIG. 54  is an A/P image of the spine used to determine the starting point for pilot hole formation through the pedicle, the A/P image being taken while maintaining the lateral trajectory of the C-arm as set as in  FIG. 53 , according to one embodiment of the present invention; 
         FIG. 55  is a trajectory lateral image for verifying the depth of penetration through a pedicle, according to one embodiment of the present invention; 
         FIG. 56  is an “owls eye view” (i.e. looking straight along the pedicle axis) of the pedicle generated by orienting the C-arm with both the medial-lateral and cranial-caudal angles of the pedicle axis, according to one embodiment of the present invention; 
         FIG. 57  is a side view illustrating the use of a tissue distraction assembly (comprising a plurality of dilating cannulae over a K-wire) to distract tissue between the skin of the patient and the surgical target site, according to one embodiment of the present invention; 
         FIG. 58  is a perspective view of a tissue retraction assembly (in use) forming part of a surgical access system, according to one embodiment of the present invention; 
         FIG. 59  is a side view of a retractor assembly according to the present invention, comprising a handle assembly having three (3) retractor blades extending therefrom disposed over a dilating assembly, according to one embodiment of the present invention; 
         FIG. 60  is a side view illustrating the use of a tissue distraction assembly (comprising a plurality of dilating cannulae over a K-wire) to distract tissue between the skin of the patient and the surgical target site with a tilt sensor attached to the distraction assembly to monitor the angle of insertion, according to one embodiment of the present invention; 
         FIG. 61  is a side view of a surgical trajectory system, including a tilt sensor attached via a universal clip to a post connected to a first retractor blade, an LCD feedback device, and a tissue retractor, according to one embodiment of the present invention; 
         FIG. 62  is a side view of a surgical trajectory system, including a tilt sensor attached via a universal clip to a post comprising part of an articulating arm to which the retractor is attached, an LCD feedback device, and a tissue retractor, according to one embodiment of the present invention; 
         FIG. 63  is an exemplary view of a surgical trajectory system, including a tilt sensor attached positioned directly onto the surface of the tissue retraction of  FIG. 60 , according to one embodiment of the present invention; 
         FIG. 64  is an exemplary view of a neurophysiology system capable connecting the pedicle access probe in order to conduct various nerve monitoring functions; according to one embodiment of the present invention; 
         FIG. 65  is a perspective view showing the surgical instrument of  FIG. 11  linked to both the surgical trajectory system of  FIG. 1  and the neurophysiology monitoring system of  FIG. 66 , according to one embodiment of the present invention; 
         FIG. 66  is a perspective view showing the surgical instrument of  FIG. 12  linked to both the surgical trajectory system of  FIG. 1  and the neurophysiology monitoring system of  FIG. 66 , according to one embodiment of the present invention 
         FIG. 67  is an exemplary screen display of the surgical trajectory system  10  incorporating both alpha-numeric and graphical indicia, according to one embodiment of the present invention; and  FIG. 68  is an exemplary screen display of the surgical trajectory system  10  incorporating both alpha-numeric and graphical indicia, according to another embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     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 systems disclosed herein boast a variety of inventive features and components that warrant patent protection, both individually and in combination. 
     Various embodiments are described of a trajectory monitoring system and surgical uses thereof for enhancing the safety and efficiency of surgical procedures. In one example, set forth by way of example only, the present invention may facilitate safe and reproducible pedicle screw placement by monitoring the axial trajectory of various surgical instruments used during pilot hole formation and/or screw insertion. In another example, set forth by way of example only, intraoperative imaging performance may be improved and radiation exposure minimized by monitoring the precise orientation of the imaging device. In yet another example, monitoring the orientation of surgical access instruments can aid in both the insertion and positioning of the access instruments themselves, as well as, aiding in the later insertion of instruments and/or implants through the surgical access instruments. While the above examples are described in more detail below, it is expressly noted that they are set forth by way of example and that the present invention may be suitable for use in any number of additional surgical actions where the angular orientation or trajectory of instrumentation and/or implants is important. By way of example only, the present invention may be useful in directing, among other things, the formation of tunnels for ligament or tendon repair and the placement of facet screws. Accordingly, it will be appreciated then that while the surgical trajectory system is generally discussed herein as being attached to instruments such as pedicle access tools, C-arms, dilating cannulas, and tissue retractors, other instruments (e.g. drills, screw drivers, taps, inserters, etc. . . . ) may be substituted depending on the surgical procedure being performed and/or the needs of the surgeon. In a further aspect of the present invention, the trajectory monitoring system may be used in conjunction with, or integrated into, a neurophysiology system for assessing one or more of pedicle integrity and nerve proximity, among others functions, as will be described below. 
     Details of the surgical trajectory system are discussed in conjunction with a first exemplary use thereof for monitoring pilot hole formation (and/or screw insertion) during pedicle screw placement. As used herein, pilot hole formation is meant to encompass any of, or any combination of, creating a hole in bone (such as, for example only, by awling, boring, drilling, etc. . . . ) and preparing a previously formed hole (such as, for example only, by tapping the hole). 
     With reference now to  FIG. 1 , there is shown, by way of example only, one embodiment of a surgical trajectory system  10  engaged with a surgical instrument  12  for accessing a pedicle. The surgical trajectory system  10  comprises an angle-measuring device (hereafter “tilt sensor”)  14  and a feedback device  16 . The tilt sensor  14  measures its own angular orientation with respect to a reference axis, such as vertical or gravity. The feedback device  16  displays the angle measurements obtained by the tilt sensor  14  for reference by a practitioner. By attaching the tilt sensor  14  to a surgical instrument in a known positional relationship, the angular orientation of the instrument may be determined with respect to the same reference axis. This enables the surgeon to position and maintain the instrument  12  along a desired trajectory path during use. For example, during pilot hole formation, surgical instrument  12  may be aligned and advanced along a pre-determined pedicle axis, thereby decreasing the risk of breaching the pedicle wall. 
     Tilt sensor  14 , illustrated in  FIGS. 2-4 , includes a sensor package  17  ( FIG. 3 ) enclosed within a housing  18 . The housing  18  may be made from a surgical grade plastic, metal, or any material suitable for use in the surgical field. Housing  18  may be dimensioned to snugly mate with a universal clip  26 , described below, for attaching tilt sensor  14  to surgical instrument  12 . As shown herein, housing  18  has a generally rectangular box shape. However, it should be understood that housing  18  may be provided in any suitable shape having any suitable cross-section (e.g. generally ellipsoidal, triangular, or other polygonal shape), provided that the sensor package  17  inside the housing may be positioned such that it is oriented orthogonally relative to the direction of gravity, without deviating from the scope of the invention. An alignment groove  20 , shown by way of example only on the top of housing  18  in  FIG. 2 , may be provided to assist in properly positioning tilt senor  14  relative to the instrument  12 . Specifically, groove  20  cooperates with a complementary structure, such as alignment ridge  44  on the universal clip  26  ( FIG. 6 ) such that the tilt sensor  14  may only attach to the clip in a single, known orientation. An engagement tab  22 , shown by way of example only on the bottom of housing  18  in  FIG. 4 , may be included to assist in securing tilt sensor  14  to universal clip  26 , thereby preventing movement and/or dislodgment of the tilt sensor  14 . One or more additional grooves  25  may be included on housing  18  to provide increased grip during handling (e.g. during insertion and removal of the tilt sensor  14 ). In another embodiment, (not shown) the tilt sensor  14  may be permanently attached to the instrument  12 . In still another embodiment (not shown) the tilt sensor may be integrated within the instrument. 
     In one embodiment, sensor package  17  comprises a 2-axis accelerometer that measures angular orientation with respect to the acting direction of gravity. The angular orientation of tilt sensor  14  is measured in a sagittal plane and a transverse plane. By way of example, the orientation of the tilt sensor  14  in the sagittal plane represents a cranial-caudal angle A 2 (i) with respect to the direction of gravity and the patient. Orientation in the transverse plane represents a medial-lateral angle A 1 (i) with respect to a patient and the direction of gravity. Sensor package  17  is preferably situated within housing  18  such that when housing  18  is perpendicular to the direction of gravity, the accelerometer registers zero angle in both the sagittal and transverse planes (i.e. the zero-angle position or A 1 (i)=0 and A 2 (i)=0). In other words, both the cranial-caudal angle and medial-lateral angle are equal to zero. Thus, when tilt sensor  14  is fixed perpendicular to the longitudinal axis of the surgical instrument  12 , the angular orientation of the instruments longitudinal axis may be determined relative to gravity. 
     Utilizing only a 2-axis accelerometer, the accuracy of the tilt sensor  14  may be adversely affected by movement around the third, rotational axis. To counter this, measurements should preferably be taken only when at least one of the longitudinal axis  178  and transverse axis  180  tilt sensor  14  are aligned with a selected reference frame, such as for example, the longitudinal axis of the patient&#39;s spine (i.e. the tilt sensor  14  should be in approximately the same rotational alignment for every measurement). In one embodiment, this may be accomplished effectively using visual aids to help keep the tilt sensor  14  in line with the reference frame and/or ensure measurements may be taken only when the tilt sensor  14  appears to be in this correct rotational position. In the event the surgical instrument  12  is inadvertently or purposely rotated during use, the practitioner need only continue, or reverse rotation until the tilt sensor  14  again appears to be perpendicular to the long axis of the spine. Alternatively (or in addition to), various markings or other indicia (not shown) may be included on one or more of the tilt sensor  14 , the surgical instrument  12 , and the universal clip  26 , to ensure proper alignment prior to obtaining measurements. 
     In an alternative embodiment, the sensor package  17  may be configured such that it may account for, or at least measure, rotation (e.g. a “3-axis sensor”). In one embodiment, the sensor package  17  includes a 2-axis accelerometer augmented by a gyroscope (not shown), which may comprise any number of commercially available gyroscopes. While the accelerometer again measures the angular orientation of the tilt sensor  14  with respect to gravity, the gyroscope detects movement about the rotational or z-axis. By monitoring the rate of rotation and time, the system  10  may determine the degrees of rotation imparted on the surgical instrument (and tilt sensor  14 ). The feedback device  16  may indicate to the user that the sensor  14  is not aligned in the correct reference frame such that the user may take steps to correct the alignment prior to taking measurements. The feedback device  16  may display feedback according to any number of suitable methods. By way of example, the feedback may utilize numeric indicia to indicate the degree of misalignment, color indicia, such as red or green indicating the rotational status (e.g. aligned or misaligned), audible alert tones (e.g. low frequency tones for non-alignment and high frequency tones for proper alignment or visa versa or any combination thereof), etc. . . . Alternatively, the system  10  may be configured to correct the angle data output based on the degree of rotation detected. In this manner, angle data from the tilt sensor may be acquired from any rotational position. A button (not shown) may be provided on the tilt sensor  14  and/or feedback device  16  to initially zero the sensor package  17  when it is aligned with the reference frame. 
     In another embodiment, the sensor package  17  accounts for rotational movement by utilizing magnetometers (not shown) in conjunction with the 2-axis accelerometers, where the magnetometer may comprise any number of commercially available magnetometers. The sensor package  17  includes a triplet of magnetic sensors oriented perpendicular to each other, one pointing in the x-axis, one in the y-axis, and third pointing in the z-axis. The magnetic sensors in the x and y axis act as a compass and calculate a heading of tilt sensor  14  relative to magnetic north. The third magnetometer in the z-axis and the x and y axis accelerometers monitor the tilt permitting the “compass” to work when it is not level to the ground. Since the sensor package  17  monitors for angular orientation in the x-axis and y-axis and maintains a constant heading reference, the system  10  may calculate the amount of axial rotation relative to an established reference frame (i.e. the patient). The feedback device  16  may again be configured to indicate the rotational status of the tilt sensor  14  to the user, allowing them to realign the sensor  17  with the proper reference frame prior to establishing a reading. The feedback device  16  may again utilize numeric indicia to indicate the degree of misalignment, color indicia, such as red or green indicating the rotational status (e.g. aligned or misaligned), audible alert tones (e.g. low frequency and/or volume tones for non-alignment and high frequency and/or volume tones for proper alignment or visa versa or any combination thereof), etc. 
     With reference to  FIGS. 5-10 , there is shown one example of a universal clip  26  for mating tilt sensor  14  to the surgical instrument  12 . Universal clip  26  comprises a sensor bed  28 , a coupler  30 , a fastener  32 , and a collar  34 . Sensor bed  28  has an exterior surface  36 , an interior surface  38 , and an opening  40 . Interior surface  36  and opening  40  collectively form cavity  42 . Sensor bed  28  is generally rectangular in shape, however, it should be understood that sensor bed  28  may be provided in any suitable shape having any suitable cross-section (e.g. generally ellipsoidal, triangular, or other polygonal shape) to receive the tilt sensor  14  therein in a known orientation. Cavity  42  is dimensioned to snugly receive at least a portion of tilt sensor  14 . An alignment ridge  44  may be provided along an upper region of interior surface  38  of cavity  42 , shown by way of example only along the top, to ensure tilt sensor  14  is positioned properly within the cavity  42  (e.g. with the top surface of housing  18  adjacent to the upper region of the interior surface  38 ). Alignment ridge  44  is complementary to alignment groove  20  on the tilt sensor housing  18 . In order to insert the tilt sensor  14  in cavity  42 , the alignment ridge  44  and alignment groove  20  must be aligned. Thus, tilt sensor  14  may preferably only fit together with the sensor bed  28  in a single, known orientation. To secure the tilt sensor  14 , a hole  45  is provided within a lower region of the interior surface  38  ( FIG. 7 ), which receives a complementary protrusion  24  situated on engagement tab  22  of sensor housing  18 . When the tilt sensor  14  is fully inserted into cavity  42 , protrusion  24  becomes seated in hole  45 , thereby preventing the unintentional disengagement of tilt sensor  14  from the sensor bed  28 . It should be understood that various other retaining mechanisms (not shown) may be used in place of, or in addition to, the protrusion/hole engagement described above. Other retaining mechanisms may include, but are not necessarily limited to, a ball-spring mechanism and a clasp. 
     Sensor bed  28  is attached to coupler  30 . For stability, sensor bed  28  and coupler  30  may preferably be formed of a single member. Coupler  30  comprises a generally tubular member including a first hollow portion  46  and a second hollow portion  47 . First hollow portion  46  attaches to sensor bed  28  and retains a receptacle  50 . Second hollow portion  48  retains an engagement end  52  of fastener  32 . The receptacle  50  and engagement end  52  may be communicatively linked within coupler  30 . The second portion  48  preferably has external threading  49  that is complementary to internal threading  53  on collar  34 , allowing the collar  34  to move laterally along the second portion by twisting the collar  34 . The free end  54  of fastener  32  is configured to tightly grasp instrument  12 . By way of example only, the free end may comprise a hook. If neurophysiology monitoring is to be used, as described below, receptacle  50  can be employed to couple the universal clip  26  to a neurophysiology monitoring system. Because the receptacle  50  and fastener  32  are communicatively linked within coupler  30 , stimulation signals may be passed from the neurophysiology monitoring system to the instrument  12 . 
     With reference to  FIGS. 10A-10B , to secure a surgical instrument  12  to universal clip  26 , a portion of surgical instrument  12  (shown in cross-section) is positioned within the hook of free end  54  ( FIG. 10A ). Collar  34  may then be twisted in a first direction to laterally displaced collar  34  toward the free end  54  so as to capture surgical instrument  12  between the collar  34  and free end  54  ( FIG. 10B ). When fully tightened, universal clip  26  is securely fastened to instrument  12  and extends perpendicular to the longitudinal axis of surgical instrument  12 . When tilt sensor  14  is engaged with clip  26 , tilt sensor  14  also extends perpendicular to the longitudinal axis of instrument  12 . To release the surgical instrument  12 , collar  34  may be twisted in the opposite direction ( FIG. 10A ). 
     A surgical instrument  12 , according to one embodiment, is illustrated in  FIG. 11 . Surgical instrument  12  may comprise a pedicle access probe. By way of example only, instrument  12  may be any of the insulated pedicle access probes described in detail in the commonly owned and co-pending U.S. patent application Ser. No. 11/448,237, entitled “Insulated Pedicle Access System and Related Methods,” and filed on Jun. 6, 2006, the entire contents of which is incorporated by reference as if set forth herein in its entirety. Instrument  12  comprises generally a probe member  60 , having a longitudinal axis  61 , and a handle  62 . Probe member  60  may be embodied in any variety of configurations that can be inserted through an operating corridor to a pedicle target site and bore, pierce, or otherwise dislodge and/or impact bone to form a pilot hole for pedicle screw placement. Probe member  60  may be generally cylindrical in shape, however, probe member  60  may be provided in any suitable shape having any suitable cross-section (e.g. generally oval, polygonal, etc. . . . ). A distal region  64  of probe member  60  may have a shaped tip  66  formed of any number of shapes generally suited to effect pilot hole formation, such as, by way of example only, a beveled point, double diamond, drill bit, tap, and a generally tapered awl. A proximal region  68  of probe member  60  may be configured to couple with universal clip  26 . Probe member  60  may be composed of any material suitable for surgical use and strong enough to impact bone to form a pilot hole. In one embodiment, the material may also be capable of conducting an electric current signal to allow for the use of neurophysiologic monitoring. By way of example only, probe member  60  may be composed of titanium, stainless steel, or other surgical grade alloy. 
     Handle  62  may be permanently or removably attached to probe member  60  along the proximal region  68 . Handle  62  may be shaped and dimensioned in any of a number of suitable variations to assist in manipulating probe member  60 . By way of example only, the handle  62  may be generally T-shaped such as the handle pictured in  FIG. 11 . Other suitable shapes for handle  62  may include, but are not necessarily limited to, generally spherical, ellipsoidal, and egg-shaped. Universal clip  26  may preferably grasp the proximal end  68  of probe member  60  through a cutout in the handle  62 . If the handle does not have a cutout the universal clip  26  may attach to the probe member  60  below the handle  62 . Universal clip  26  forms a sturdy connection with probe member  60  such that the tilt sensor  14  is maintained in a position perpendicular to the longitudinal axis  61  of probe member  60 . When the longitudinal axis  61  of probe member  60  is parallel to the direction of gravity, the tilt sensor  14  is perpendicular to the direction of gravity (i.e. the zero-angle position). In other words, when the longitudinal axis  61  of probe member  60  is parallel to the acting direction of gravity, both the cranial-caudal angle and the medial-lateral angle will be zero-degrees. 
     With reference to  FIG. 12 , there is shown another example of a pedicle access instrument that may be used with the surgical trajectory system  10 . Surgical instrument  80  is similar to a standard gearshift probe, which are commonly used to create holes in pedicle bone, modified for direct engagement with the tilt sensor  14 . Surgical instrument  80  comprises a probe member  82 , a handle  84 , an orientation shaft  86 , and a sensor cavity  88  for receiving and holding a tilt sensor  14 . Probe member  82  may be generally cylindrical in shape and include a distal end  90 , a proximal end  92 , and a longitudinal axis  94  extending through distal and proximal ends  90 ,  92 , respectively. However, it should be understood that probe member  82  may be provided in any suitable shape having any suitable cross-section (e.g. generally oval, polygonal, etc. . . . ). The distal end  90  may have a shaped end comprising any of a number of forms generally suited to effect pilot hole formation such as (by way of example only) a beveled point, drill bit, or as shown in  FIG. 12 , an awl with a generally tapered shape. Probe member  82  may be composed of, by way of example only, titanium, stainless steel, or other material strong enough to impact bone to form a pilot. In one embodiment the material of probe member  82  is capable of conducting an electric signal for employing neurophysiologic monitoring, as described below. Probe member  82  may include one or more markings  96  about the exterior surface that can be viewed to indicate the depth of penetration in a pedicle target site. 
     Handle  84  may be permanently or removably attached to probe member  82  at the proximal end  92 . Handle  84  may be shaped and dimensioned in any of a number of suitable variations to assist in manipulating probe member  82 . By way of example only, the handle  84  may be generally spherical shaped, T-shaped, or egg shaped, to name a few. With further reference to  FIG. 12 , an orientation shaft  86  may be positioned on handle  84 . Orientation shaft  86  extends from handle  84  perpendicularly to the longitudinal axis  94  of the probe member  82 . The handle  84  also includes a sensor cavity  88 . The sensor cavity  88  functions to receive and hold the tilt sensor  14  in proper position relative to the probe member  82 . The sensor cavity  88  may preferably be dimensioned to snugly receive at least a portion of the tilt sensor  14 , thereby preventing movement of the tilt sensor  28  within the cavity  88 . One or more mechanisms may be provided on the tilt sensor  14  and/or on or within the sensor cavity  88  to secure the tilt sensor  14  in position. Preferably, sensor cavity  88  may include a hole or recess (not shown) for capturing the protrusion  24  on tilt sensor  14 , described above. Other such mechanisms may include, but are not necessarily limited to, a ball-spring mechanism and a clasp mechanism 
     Orientation shaft  86  may be provided as a visual marker to assist in aligning the tilt sensor  14  and surgical instrument  80  with the proper reference frame (as described above) when obtaining angle measurements. Similar to that described above with reference to the universal clip  26 , the orientation shaft  86  may be aligned, by way of example only, perpendicular to the longitudinal axis of the spine when a measurement is read. Again this eliminates the risk of inaccurate angle measurements due to rotational movement of the surgical instrument when a 2-axis accelerometer is utilized. 
     In an alternate embodiment, the surgical instrument  80  may be provided without sensor cavity  88 . Instead, the universal clip  26  may attach to probe member  82 , as illustrated in  FIG. 13 . In another alternate embodiment, (not shown) the tilt sensor  14  may be permanently attached to the handle  80 . In still another embodiment (not shown) the tilt sensor may be integrated within the instrument  80 . 
     With reference to  FIG. 14 , a bubble level device  110  may be used to ensure the tilt sensor  14  is functioning correctly. The bubble level device  110  includes a handle  112  with a level  114  mounted in it. The tilt sensor  14  is inserted into the handle  112 , which then is placed on a flat surface so that an indicator ring  116  on the domed transparent surface  117  of level  114  encircles the bubble  118  captured within the domed surface  117 . When the bubble  118  is within the indicator ring  116 , the tilt sensor display should read approximately zero-degrees for both the cranial-caudal readout  99  and medial-lateral readout  98 . 
     Regardless of the manner of coupling the tilt sensor  14  to the respective instrument (e.g. probe  12 , gear shift  80 , or any other instrument), the feedback device  16  is communicatively linked to tilt sensor  14  to provide feedback to the surgeon regarding angle of the tilt sensor  14  and instrument relative to the desired angles (medial-lateral and cranial-caudal). This communication link may be accomplished via hard-wire (e.g. data cable  70  of  FIG. 1 ) and/or via wireless technology, in which case the tilt sensor  14  and feedback device  16  may include additional hardware commonly used for enabling such wireless communication. The feedback device  16  may be a computer or similar type processing unit (not shown). User input may be directed to the trajectory system  10  through feedback device  16 . In one embodiment, shown in  FIG. 15 , feedback device  16  includes an LCD display with at least one numerical readout  98  for presenting the medial-lateral angle determined by the tilt sensor  14  and at least one numerical readout  99  for presenting the cranial-caudal angle determined by the tilt sensor. Readouts  98  and  99  may be displayed in a single window or on separate windows designated for each readout. In one embodiment, the medial-lateral and cranial-caudal readouts  98 ,  99  are displayed simultaneously and continuously while the tilt sensor  14  is in use. If only one window is provided, a switch or button for toggling between the two angle measurements may be utilized (not shown). The feedback device  16  may be placed next to the patient on the surgical table, or it may be affixed to any number of suitable objects in the operating room, including, but not necessarily limited to, an IV pole, surgical table, prep table, fluoroscope, neurophysiologic monitoring system, etc. . . . If communicatively linked to the feedback device  16  via hard-wire, the position of the feedback device  16  should be such that the tilt sensor  14  may move freely without tensioning the data cable  70 . 
     At times it may not be convenient for the surgeon to look away from the surgical field to view the angle measurements displayed by the surgical trajectory system  10 . According to one embodiment, shown in  FIGS. 15-16 , this may be accomplished by providing the feedback device  16  having a smaller, thinner and/or miniaturized size relative to that shown in  FIG. 1 . By providing the feedback device in the manner shown in  FIGS. 15-16 , the practitioner may be able position the feedback device  16  close to his or her hand so as have the displays  98 ,  99  both in their general field of view while using the surgical trajectory system  10 . For example, with reference to  FIG. 17 , the feedback device  16  may be coupled to the instrument  12  via a universal clip  26  such that it rests above and slightly offset from the practitioner&#39;s hand, thus maintaining the feedback device  16  in the surgical field without obstructing the surgeon&#39;s view of the operative site. By way of another example, with reference to  FIGS. 18A-18B , feedback device  16  may be positioned on the back of the hand of the practitioner, which may be augmented via a strap  102 . Again, this advantageously positions the feedback device  16  in the surgical field without obstructing the surgeon&#39;s view of the operative site. 
     It is further contemplated that feedback device  16  may be configured to provide feedback based on indicia other than numerical readouts  98 ,  99 . By way of example, feedback device  16  may utilize a code based on the expression of color to indicate the angular orientation of the tilt sensor  14  and also thus, the orientation of the instrument to which it is attached. One such color code scheme will display the color red when the angles of the tilt sensor  14  fall outside an acceptable range from the correct orientation, the color yellow when the angles of the sensor  14  are close to the correct range but not yet within an optimal orientation range, and the color green when the angles of the tilt sensor  14  are aligned within an optimal orientation range. The correct orientation of the tilt sensor  14  (i.e. whether it is in the “green zone”, “yellow zone”, or “red zone”) may be measured against predetermined angle measurements previously inputted into the system  10  or, against a separate reference marker which is known to be positioned in the correct orientation and is communicatively linked to the trajectory system  10  (e.g. a C-arm as will be described below). A single color display could be used to indicate the overall orientation of the tilt sensor  14 , or, individual and independently operated color display could be used to indicate the orientation in the cranial-caudal direction and medial-lateral directions, respectively. Various implementation methods may be utilized to accomplish the display of the color code. By way of example only, the LCD displays could be configured to output a color and change the color according to the code based on input data from the sensor  14 . Alternatively, dedicated color displays comprising one of each color in the code may be arranged such that a single color is displayed based upon the input from the sensor  14 . Two displays for each color may be utilized in order to show the orientation status of both the cranial-caudal and medial-lateral angles independently. LED lights may be used instead of LCD displays and arranged according to any of the above configurations. Feedback based on color indicia may be used alone or in combination with the numerical data previously described. 
     By way of another example, the feedback device  16  may utilize a code based on the emission of audio tones to indicate the angular orientation of the tilt sensor  14  relative to predetermined reference angles corresponding to the pedicle axis. One method for implementing an audio code involves varying one or more of the volume, pitch, frequency, pulse rate, and length of the audio tone based on the determined orientation of the sensor  14  relative to the predetermined orientation ranges. Audio feedback may be used alone, or in combination with one or both of the numerical data and color indicia previously described. In one embodiment, a first audible signal may be indicative of an optimal variance between the trajectory of the instrument and at least one of the first and second determined angular relationships between the sensor  14  and said reference direction. A second audible signal may be indicative of an unacceptable variance between the trajectory of the instrument and at least one of the first and second determined angular relationships between the sensor  14  and the reference direction. A third audible signal may be indicative of an acceptable yet not optimal variance between the trajectory of the instrument and at least one of the first and second determined angular relationships between the sensor  14  and the reference direction. 
       FIG. 67-68  illustrate, by way of an example only, embodiments of a screen display  500  capable of receiving input from a user in addition to communicating feedback to the user. The screen display  500  incorporates both alpha-numeric and color indicia as described above. In this example (though it is not a necessity) a graphical user interface is utilized to enter data directly from the screen display. By way of example, measurements obtained for the medial-lateral angle A 1  may be entered into input boxes  504  and  506  for (for left and right pedicles, respectively). The data may be entered for each spinal level of interest by selecting the appropriate level from a menu  506  and then entering the correct values into boxes  504  and  506 . The entered values may be saved by the system such that during the procedure selecting the spinal level from menu  510  automatically recalls the inputted values. A box  508  is provided for inputting the cranial-caudle angle A 2  determined for each level. A C-arm window  512  contains data pertaining to a second tilt sensor  14  positioned on a fluoroscopic imager, as is described in more detail below. By way of example numeral boxes  514  and  516  display the numeric values determined by the C-arm tilt sensor  14 . When the numeric values corresponding to the C-arm sensor matches within an accepted range the predetermined target angles the C-arm window  512 , or a portion there of (such as the circle  518 ) may be saturated with the color green. Alphanumeric characters may also visually indicate that the target angles have been matched by the C-arm. If the c-arm is aligned with the pedicle axis (placed in the owls&#39; eye view) as is later described, the C-arm values A 1 (c) and A 2 (c) should approximate the pedicle axis angles A 1  and A 2 . The user may have the option to base feedback from the instrument  12 ,  80  mounted tilt sensor  14  (e.g. red, yellow, green indicia, etc. . . . ) on matching the C-arm sensor values, rather than the predetermined target values. This option may be exercised, by way of example only, by selecting the appropriate button in the C-arm window  512 . A status bar  520  may be provided to indicate the relative status of both the instrument  12 ,  80  and C-arm tilt sensors. By way of example only, the status bar  520  depicted in  FIGS. 67 and 68  indicate that both the instrument  12 ,  80  and the C-arm sensors are attempting to match the targeted angles. Other messages (not shown) may indicate for example, that the instrument  12 ,  80  is trying to target the C-arm angles, that the target angles are matched, or that a sensor is not in use.  FIGS. 67-68  depict varying embodiments of an instrument window. 
     The instrument window embodied in  FIG. 67 , employs a color coded target. The outer rings  524  of the target may be red. The middle rings  526  of the target may be yellow, and the inner circle  528  may be green. When the instrument is aligned with the target angles center circle may be saturated green, indicating that both the medial-lateral angle A 1  and cranial-caudal angles have been matched, or A 1 =A 1 (i) and A 2 =A 2 (i). Similarly if the instrument  12 ,  80  is matched to the C-arm (A 1 (c)=A 1 (i) and A 2 (c)=A 2 (i)) the center circle may be saturated green. The middle  526  and outer  524  rings may be divided into quadrants  530 ,  532 ,  534 , and  536  corresponding to right, left, cranial, and caudal, respectively. By way of example, if the instrument is aligned to far left of the target, the outer  524  or middle  526  ring in the left quadrant  530  will be saturated depending upon how misaligned the instrument is (i.e. whether it falls into the yellow or red range). Similarly, if the instrument  12 ,  80  is aligned to far cranially, the outer  524  or middle  526  ring in the upper quadrant  534  will be saturated depending upon how misaligned the instrument is. If the instrument  12 ,  80  has matched one of the targeted angles but not the other, only the quadrant corresponding to the misaligned angle will be saturated. The instrument window embodied in  FIG. 68 , employs a color coded display approximating the look of a bubble level. A free floating ring  538  moves relative to the movement of the instrument. The closer the bubble is to the center, the closer the instrument is to matching the target angle (or C-arm angle). When the instrument is within the range indicating proper alignment the ring  538  may be saturated green. 
     In general, to orient and maintain the surgical instrument  12 ,  80  along a desired trajectory during pilot hole formation, the distal end of surgical instrument  12 ,  80  may first be placed on the pedicle target site in the zero-angle position. The universal clip  26  or orientation shaft  86  should be set in the desired reference position, preferably perpendicular to the longitudinal axis of the spine. The surgical instrument  12 ,  80  may then be angulated in the sagittal plane until the desired cranial-caudal angle is reached. Maintaining the proper cranial-caudal angle, the surgical instrument  12 ,  80  may then be angulated in the transverse plane until the proper medial-lateral angle is attained. Once the feedback device  16  shows that both angles are correct, the instrument  12 ,  80  may be advanced into the pedicle to form the pilot hole. The instrument  12 ,  80  may be rotated back and forth to assist in the formation of the pilot hole. To keep the proper trajectory throughout formation, the instrument  12 ,  80  may occasionally be realigned so that the universal clip  26  or orientation shaft  86  is again perpendicular to the long axis of the spine and the angle measurements rechecked. This may be repeated until the pilot hole is complete. 
     To form a pilot hole in a vertebral pedicle with the aid of the surgical trajectory system  10 , the surgical instrument  12 ,  80  is advanced to the pedicle target site where the pilot hole is to be formed. This may be done through any of open, mini-open, or percutaneous access. The precise starting point for pilot hole formation may be chosen by the practitioner based upon their individual skill, preferences, and experience. One method which may be employed to select the starting point is described further below, in conjunction with methods for utilizing the tilt sensor  14  to orient a fluoroscope. In brief, a C-arm (used for fluoroscopic imaging) may be equipped with the tilt sensor  14 . The C-arm may then be oriented so that the fluoroscope&#39;s x-ray beam is parallel to the axis of the pedicle in one or both axes. A pedicle cross-section may be seen in the resulting images and a starting point may be selected. 
     Upon safely reaching the pedicle target site, the surgical instrument  12 ,  80  is manipulated into the desired angular trajectory.  FIGS. 19-20  illustrate one exemplary method for determining the desired trajectory angles, wherein a series of measurements are used to determine the pedicle axis of the pedicle (or more likely, pedicles) which will receive a pedicle screw. As shown in  FIG. 19 , preoperative superior view MRI or CAT scan images are obtained and used to determine the medial-lateral angle A 1 . A vertical reference line is drawn through the center of the vertebral body (in the A-P plane). A medial-lateral trajectory line is then drawn from a central position in the pedicle (e.g. a position within the soft cancellous bone, as opposed to the harder cortical bone forming the outer perimeter of the pedicle) to an anterior point of the vertebral body for the target pedicle. The resulting angle between the medial-lateral trajectory line and the reference line is measured and the result correlates to the medial-lateral angle A 1  of the pedicle axis of the target pedicle, and thus also the medial-lateral angle to be used in forming the pilot hole. The measurement is repeated for each pedicle and the results may be noted and brought to the operating room for reference during the surgery. As previously mentioned, in some embodiments the feedback device  16  includes and/or is communicatively linked to a processor having memory such that the predetermined measurements may be input into the system prior to surgery for easy retrieval and application later. 
     As shown in  FIG. 20 , the cranial-caudal angle A 2  may be determined using an intraoperative lateral fluoroscopy image. A vertical reference line is preferably captured in the lateral fluoroscopy image to ensure measurements are performed with respect to the direction of gravity. Fluoroscopy image outputs can generally be rotated 360° such that the image can appear on the monitor in any orientation and a vertical reference line prevents measurements from inadvertently being calculated from an incorrect reference position. One method for generating an accurate vertical reference line includes inserting a straight K-wire or needle into the spinous process at the desired vertebral level. The K-wire or needle may be oriented parallel to the acting direction of gravity, using the tilt sensor  14  and/or a bubble needle  140 . of the type shown, by way of example, in  FIGS. 21-22   
     The bubble needle  140  demonstrated in  FIGS. 21-22  comprises a needle or probe portion  142  and a handle  144 . The needle portion  142  is composes of a biocompatible radio-dense material such that it will show up in fluoroscopic images. The handle  144  may be removably or permanently attached to the needle portion  142 . Like the handle of bubble level device  110 , the handle of bubble needle  140  is outfitted with a level  146  mounted in it. The level  146  comprises a volume of fluid (e.g. water, oil, saline, etc. . . . ) contained within at least partially translucent enclosure  148  with a gas bubble  150  (e.g. oxygen, air, CO 2 , etc. . . . ) disposed within the domed closure  148 . The bubble  150 , when the instrument is positioned vertically, will move to the approximate center of the domed enclosure  148 . In this fashion, the bubble needle  140  may be employed to generate a true vertical reference line during imaging by first positioning the needle portion  142  on a desired target site within the site to be imaged, such as, for example only, docked on the spinous process and then using the level  146  to align the needle  142  vertically by holding the handle  144  with the bubble  150  at the center of the domed enclosure  148 . In one optional embodiment, the domed enclosure  148  may include one or more concentric circles  152  forming a “bulls-eye” on the domed enclosure  148  to aid in the positioning of the gas bubble  150  within the domed enclosure  148 . Although described herein as a “gas bubble” it will be appreciated that this “targeting” substance may be another liquid having a lighter molecular weight than the base fluid. 
     As an alternative, the vertical reference line may be generated by positioning a radio dense marker on the C-arm rather than the patient. Exemplary embodiments of such radio dense markers and methods for positioning them on the C-arm are described below in more detail. The lateral fluoroscopy image of  FIG. 20  is shown with the vertical reference line visible in the center of the image. A cranial-caudal trajectory line is drawn from the pedicle nucleus to an anterior point of the vertebral body for the target pedicle. The resulting angle calculated between the cranial-caudal trajectory line and the vertical reference line is the cranial-caudal angle A 2  of the pedicle axis and is the angle measurement to be used in forming the pilot hole. These steps should be repeated for each pedicle that is to receive a pedicle screw. It will be appreciated that while the cranial-caudal angle A 2  is generally described herein as being determine intraoperatively, it is also contemplated that angle A 2  may be determined preoperatively as well by combining various medical imaging and computer processing techniques to recreate the vertebra of interest and allowing the pedicle axis, to be calculated prior to surgery. 
     According to one embodiment, shown in  FIGS. 23-24 , a protractor  120  may be provided with system  10  to assist in determining the cranial-caudal angle from the intraoperative image. The protractor  120  includes a handle  122  and a blade  124 . A proximal end  126  of handle  122  comprises a sensor bed  128 . Sensor bed  128  is similar to sensor bed  28  described above with reference to universal clip  26 , and preferably engages tilt sensor  14  in the same manner. In other embodiments (not shown) the tilt sensor  14  may be permanently attached to or integrated within the handle  122  Blade  124  is generally flat and rectangular in shape with generally flat and triangular buttress element  125  extending generally perpendicularly from the top and distal region of the blade  124 . The distal edge of the blade  124  is coplanar with the distal edge of the buttress element  125  such that each may be disposed against a flat surface (e.g. fluoroscope monitor) at the same time. The blade  124  attaches at its proximal end to a distal end  130  of handle  122 . After taking the intraoperative lateral fluoroscopy image, the protractor  120  is used at the C-arm screen to measure the desired cranial-caudal angle. The blade  124  is lined up with the vertical reference line ( FIG. 24A ). Because the fluoroscopy image may not be aligned properly on the fluoroscope imaging monitor, the tilt sensor  14  may be set to zero when it is lined up with the vertical reference line. A button (not shown) may be provided on one of the tilt sensor  14  and feedback device  16  to selectively zero out of the sensor  14 . The protractor  120  is then rotated about its longitudinal axis until the blade  124  lines up with the pedicle trajectory line ( FIG. 24B ). Feedback device  16  will display the numerical readout  99  for the angle between the vertical reference line and the pedicle trajectory line (i.e. the cranial-caudal angle) as measured by the tilt sensor  14 . 
     With reference to  FIG. 25 , there is shown an alternate embodiment of a feedback device  134  which is configured to communicatively link multiple tilt sensors  14  at once. Feedback device  134  may be linked to three tilt sensors  14  individually or simultaneously and angle measurements may be provided individually or simultaneously for all attached tilt sensors  14 . In one example, this allows a tilt sensor  14  to be engaged with an instrument such as pedicle access probe  12 , the protractor  120 , and (not shown) a C-arm without the need for multiple displays and/or connecting and disconnecting the tilt sensor  14  to the various devices during the procedure. 
     Once the desired trajectory angles are determined for the necessary pedicles, pilot holes may be formed and screws inserted using the tilt sensor  14  to ensure the instruments and implants are aligned with the determined angles. As mentioned above, the safety and reproducibility of pilot hole formation may be further enhanced by employing neurophysiologic monitoring, as will be described in detail below, in conjunction with the trajectory monitoring performed by the surgical trajectory system  10 . 
     The surgical trajectory system  10  of the present invention may provide additional advantages during surgery when used to enhance intraoperative imaging commonly performed during many surgical procedures. C-arm fluoroscopes are used extensively during many surgical procedures. During spinal surgery for example, the C-arm is used frequently to help locate specific structures of the spine, to direct the positioning of surgical instruments and/or instrumentation, and to verify the proper alignment and height of vertebra, among other uses. As will be clear from the description herein, augmenting C-arm usage with the surgical trajectory system  10  may increase efficiency and reduce radiation exposure associate with using the C-arm by eliminating much of the guess work and trial and error that is often required to achieve the desired fluoroscopic image. 
     With reference to  FIG. 26 , there is shown a typical operating theatre  200  in which a practitioner  202  may perform surgical procedures on a patient  204 . The patient  204  is positioned on a radio-opaque operating table  206 . Arrayed around the table  206  are a standard C-arm  208 , comprising a frame  210 , a signal transmitter  212 , and a signal receiver/image intensifier  214 , and a flour-monitor  216 . In use, an x-ray beam  218 , having a central axis  220 , may be directed from the signal transmitter  212  through a desired area of patient  204  and picked up by the signal receiver  214 . An image of the patient&#39;s  204  body tissue located in the path of beam  218  is generated and displayed on fluoroscope imaging monitor  216 . It should be appreciated that while the C-arm  208  is discussed herein generally for use during spine surgery to capture images of the spine, such discussion is for exemplary purposes only. It will be understood that the C-arm  208  may be utilized for imaging in a wide variety of surgical procedures. The devices and methods for orienting the C-arm described herein may apply equally well to those other such procedures, and as such, fall within the scope of the present invention. 
     As illustrated in  FIGS. 27-30 , the C-arm frame  210  may be adjusted to alter the path of the beam  218 , and thus the image that is generated. In  FIG. 27  the frame  210  is oriented such that beam  220  travels parallel to the direction of gravity. With the patient in the prone position, as shown herein, this position of frame  210  generates an anterior-posterior (A/P) image. This position of C-arm  208  is referred to hereafter as the A/P position. Rotating the frame 90° in a medial-lateral direction (through a transverse plane), as depicted in  FIG. 28 , directs the beam  220  perpendicular to the direction of gravity and generates a lateral image. This position of the C-arm  208  is referred to as the lateral position. A/P and lateral images may both be useful during a spinal procedure and the C-arm may be adjusted between the A/P and lateral positions numerous times during the procedure. As illustrated in  FIGS. 29A-29B , the frame  210  may also be oriented in any position within the transverse plane between the A/P and lateral positions, such that the beam  220  forms an angle A 1 (c) (the medial-lateral angle) between zero and 90° with respect the direction of gravity. Furthermore, as illustrated in  FIGS. 30A-30B , the frame  210  may also be rotated in a cranial-caudal direction (within a sagittal plane) such that the beam  220  forms another angle A 2 (c) (the cranial-caudal angle) with respect to the direction of gravity. By way of example only, the C-arm may be oriented such that one or both of angles A 1 (c) and A 2 (c) correspond to the desired axis of trajectory of a pedicle bone, i.e. angles A 1  and A 2 , as will be discussed in more detail below. 
     By attaching the tilt sensor  14  to the C-arm  208  in a known positional relationship, the angular orientation of the C-arm with respect to the reference axis (gravity) may be determined. This enables the practitioner to quickly position the C-arm  208  in a known orientation, such as, by way of example only, the precise orientation in which a previous image was acquired. Doing so may eliminate the time and extra radiation exposure which is often endured while acquiring numerous images while “hunting” for the right image. Attaching the tilt sensor  14  to the C-arm may further enable the practitioner to determine the angular orientation of anatomical structures within the patient (e.g. vertebral pedicles), as will be described below. This may be advantageous, for example only, when the practitioner is performing pedicle fixation and preoperative images (such as the MRI or CAT images which may be used to determine the pedicle axis angle A 1 ) are not available for preoperative planning, as described above. 
       FIGS. 31-34  depict a coupler  222  for attaching the tilt sensor  14  to a standard C-arm  208  according to one embodiment of the invention. Coupler  222  comprises a sensor bed  224  configured to receive tilt sensor  14 , and a mount  226  configured to attach to the C-arm  208 . Mount  226  includes a base region  228  equipped with a slot  232  spanning the distance between the sides  234 . In use, a strap or belt  236  ( FIG. 36 ) may be passed through slot  232  and fastened around C-arm  208  to secure the coupler  222 , and thus the tilt sensor  14 , to the C-arm  208 . An upper region  238  of coupler  222  contains a cutout  242  in which sensor bed  224  is pivotally attached to coupler  222  via a pin  244 . Sensor bed  224  may preferably pivot between a horizontal position (with respect to base region  228 ), illustrated in  FIG. 33 , and a vertical position (with respect to base region  228 ), illustrated in  FIG. 34 . As will be described below, pivoting the sensor bed  224  between the vertical position and the horizontal position allows the tilt sensor  14  to be aligned in the same starting orientation (with respect the direction of gravity) whether the C-arm is in the A/P position or the lateral position. Ball-springs  240 , or any of a number of other suitable mechanisms, may be positioned within cutout  242  to prevent unintentional movement of sensor bed  224  and maintain it in either of the desired horizontal or vertical positions. 
     Sensor bed  224  is similar to sensor bed  28  of the universal clip  26  as both are designed to receive and secure tilt sensor  14 . Bed  224  has an exterior surface  246 , an interior surface  248 , and an opening  250 . Together, interior surface  248  and opening  250  form cavity  252 . Sensor bed  224  is generally rectangular in shape, however, it should be understood that sensor bed  224  may be provided in any suitable shape having any suitable cross-section (e.g. generally ellipsoidal, triangular, or other polygonal shape) without deviating from the scope of the invention. Cavity  252  is dimensioned to snugly receive at least a portion of tilt sensor  14 . An alignment ridge  254 , configured to engage tilt sensor  14  as described below, may be provided along the interior surface  248  (shown by way of example only along the top of sensor  14 ) to ensure tilt sensor  14  is positioned properly (e.g. right side up) within the cavity  252 . Additional engagement features, such as for example, a hole or indentation (not shown) may be provided to cooperate with protrusion  24  on tab  22  help secure the tilt sensor  14  within cavity  252 . 
     To attach tilt sensor  14  to the coupler  222 , the practitioner  202  or other user may grasp tilt sensor  14  and align the groove  20  of housing  18  with the alignment ridge  254  of cavity  252 . The tilt sensor  14  may then be slid into the cavity until the protrusion  24  engages the hole or indentation in cavity  252 . In other embodiments (not shown) the tilt sensor  14  may be permanently attached to or integrated within coupler  222 .  FIG. 35  illustrates the tilt sensor  14  fully engaged within the sensor bed  224  of the coupler  222  and in the horizontal position referenced with regard to  FIG. 33 . Thereafter, sensor bed  224  and tilt sensor  14  may be pivoted into one or the other of the horizontal and vertical positions as needed during the procedure.  FIG. 36  shows the coupler  222  attached to the C-arm  208  oriented in the A/P position, while  FIG. 37  shows the coupler  222  attached to the C-arm  208  oriented in the lateral position. In either case, the belt  236  is fastened around signal receiver  214  and the base region  228  of coupler  222  rests square and secure against a sidewall  256  of receiver  214 . The bottom surface  230  of mount  226  may be contoured or shaped to match the contour of the side wall  256 . For example, the bottom surface  230  may have a generally upside down “V” shape (best viewed in  FIG. 32 ) to securely rest against the rounded surface of side wall  256 . To orient the tilt sensor  14  in the zero-angle position with the C-arm  208  oriented in the A/P position of  FIG. 36 , the sensor bed  224  is positioned in the “vertical” position relative to the coupler  222  as shown in  FIG. 34 . To orient the tilt sensor  14  in the zero-angle position with the C-arm  208  in the lateral position of  FIG. 37 , the sensor bed  224  is arranged in the “horizontal” position relative to the coupler  222  as shown in  FIGS. 33 and 35 . In either case, this positions the tilt sensor  14  perpendicular to gravity. 
       FIGS. 38-40  depict a coupler  260  for attaching tilt sensor  14  to a standard C-arm  208  according to another example embodiment of the preset invention. Coupler  260  includes sensor bed  224  and a mount  262  having a base region  264  for attaching to the C-arm  208  and an upper region  266  for pivotally attaching sensor bed  224  to the mount  262 . In other embodiments (not shown) the tilt sensor  14  may be permanently attached to or integrated within coupler  260 . Sensor bed  224  is pivotally attached, via a pin  268 , within a cutout  270  in upper region  266 . Sensor bed  224  may preferably pivot between a horizontal position (with respect to base region  264 ) and a vertical position (with respect to base region  264 ) as described above with reference to coupler  222  in  FIGS. 33-35 . Ball-springs  272 , or any other suitable mechanism, may be positioned within cutout  270  to prevent unintentional movement of sensor bed  224  and maintain it in either of the desired horizontal and vertical positions. 
     Whereas mount  226  of coupler  222  was configured to preferably attach to the sidewall  256  of signal receiver  214 , mount  262  is configured to preferably mount to the face  258  of signal receiver  214 , as illustrated in  FIGS. 41-43 . In one embodiment, Velcro® may be used to releasably fix mount  262  to face  258 . Other attachment means, including, but not necessarily limited to, adhesive gel and adhesive tape may be used instead of Velcro®. As illustrated in  FIGS. 42-43 , since the orientation of coupler  260  varies from that of coupler  222  by 90° when fixed to the C-arm (because coupler  222  attaches to the sidewall  256  and coupler  260  attaches to the face  258 ), the roles of the horizontal and vertical positions of the sensor bed  224  are reversed. When C-arm  208  is in the A/P position as in  FIG. 42 , the sensor bed  224  is pivoted to the horizontal position such that the length and width of the tilt sensor  14  are perpendicular to the direction of gravity. Conversely, when C-arm  208  is in the lateral position, as in  FIG. 43 , the sensor bed  224  is pivoted to the vertical position such that the length and width of tilt sensor  14  are again perpendicular to gravity. 
     Coupler  260  may also include an integrated plumb line  274 , which may generate a reference line viewable in fluoroscopic images generated by C-arm  208 . Preferably, coupler  260  and plumb line  274  may be fixed to the C-arm such that the plumb line  274  forms a vertical reference line in lateral fluoroscopic images. Plumb line  274  comprises an elongated radio-dense marker  276  situated in a radio-opaque case  278 . Case  278  is fixed to coupler  222  at a first end  280 . A free end  282  may be attached to C-arm face  258 , via Velcro® or other suitable attachment means, to help prevent any movement of the plumb line  274  once it is positioned. 
     Coupler  260  is preferably fixed to the C-arm face  258  with the C-arm in the lateral position, which allows gravity to help correctly position the plumb line  274 . To attach coupler  260  to the signal receiver face  258  according to a preferred embodiment, a Velcro® pad may be adhered to the face  258  in a position  284  adjacent to the outer edge and centered along the vertical diameter of face  258 . A complementary Velcro® pad may be adhered to button  288  pivotally coupled to the bottom surface  286  of coupler  260  and the Velcro® pads stuck together. If it is not already in place, tilt sensor  14  is positioned in sensor bed  224 , which should be oriented in the vertical position (if C-arm is in the lateral position). The free end  282  of plumb line  274  may then be moved (e.g. via gravity of the user) such that plumb line  274  and coupler  260  pivot around button  288  until the tilt sensor is in the zero angle position. Once the zero-angle position is achieved, Velcro® pads may be used to attach another button  290 , situated on the free end  282  of plumb line  274 , to the face  258 . 
     In addition to the vertical reference line for which plumb line  274  may be utilized, other configurations are also contemplated. By way of example only,  FIG. 44  depicts a target  292  which may be used with plumb line  274 . Target  292  includes a generally circular radio-dense marker  294  situated in a radio-opaque target case  296 . Target case  296  includes an aperture slot  298  configured to receive and slidably engage case  278  of plumb line  274 . Once plumb line  274  is fixed in position, target  292  may be slid along plumb line  274  until a desired position along plumb line  274  is achieved, after which it may be fixed in place with Velcro® or other suitable attachment means.  FIG. 45  depicts target  292  positioned along the plumb line  274  at the center of face  258 . Circular radio-dense marker  294  has a diameter of approximately 1-inch. However, it will be understood that marker  294  may have any suitable size, ranging anywhere from just large enough to be seen, to just small enough to fit on face  258  of receiver  214 . It should also be understood that while target marker  294  is shown and described as generally circular, target marker  294  may be provided having any general shape, including but not necessarily limited to, generally rectangular, triangular, ellipsoidal, and polygonal. In addition, plumb line  274  may also comprise various other configurations. With reference to  FIG. 46 , by way of one example, case  278  may be shaped and dimensioned to approximate that of receiver face  258 , and radio-dense marker  276  may be provided according to any desirable layout, such as the four equal length bars positioned equidistantly around case  278  shown in  FIG. 46 . 
     While couplers  222  and  260  have been described as attaching the tilt sensor  14  in a pivotal relation, it should be appreciated that this is not always necessary. In some instances (such as when determining the cranial-caudal angle using the C-arm) the sensor output is adequate if only one axis of the sensor is perpendicular to gravity. Thus the tilt sensor  14  may be engaged to the coupler in a fixed orientation. Alternatively, the need to pivot the tilt sensor  14  may be overcome by equipping sensor package  17  with a second 2-axis accelerometer arranged perpendicular to the first. As the tilt sensor  14  is angled past the effective range of the first accelerometer the second accelerometer take over make assume the measurement function from the first accelerometer. 
     In an alternate embodiment of the present invention, it is contemplated that the orientation of C-arm  208  with respect to the patient  204  may be adjusted by moving the patient rather than the C-arm  208 . To accomplish this, by way of example only, trajectory system  10 , and specifically, tilt sensor  14  may be attached to the surgical table  206 . With the C-arm  208  in the A/P position, the bed, and thus the patient on the bed, may be rotated laterally (through a transverse plane), shown in  FIG. 47 , to change the medial-lateral angle at which the x-ray beam  220  travels through the patient. As shown in  FIG. 48 , the bed may also be rotated in a cranial or caudal direction (through a sagittal plane) to change the cranial-caudal angle at which the x-ray beam  220  travels through the patient. The trajectory system  10 , via feedback device  16 , will display the cranial-caudal and medial-lateral angles of the bed with respect to gravity. Since the beam  220  is parallel to the direction of gravity in the A/P position, the cranial-caudal and medial-lateral angles of orientation of the x-ray beam with respect to the bed  206  (and patient  204 ) are also known. To later obtain images from precisely the same position, the bed may be readjusted until the feedback device  16  again shows the angular readouts corresponding to the desired position. Although not shown, it is within the scope of the invention to attach the tilt sensor  14  directly to the patient  204  rather than the bed  206 . Again, with the C-arm  208  aligned in the A/P position, the bed may be rotated to change one or both of the medial-lateral angle and cranial-caudal angle of the patient&#39;s spine with respect to the x-ray beam  218 . Again, later images may be obtained from precisely the same position by readjusting the bed until the feedback device  16  again shows the correct angular readouts for the position desired. 
     Simple and reproducible orienting of the C-arm  208  in a known position while decreasing the x-ray exposure for everyone represents one general advantage of using the trajectory system  10  to enhance imaging techniques during surgery. Other benefits may also be gained by using the system  10  with a C-arm, such as the previously mentioned benefit of determining the angular orientation or trajectory of an anatomical structure within the patient. Without limiting the scope of the present invention, specific examples will be described for determining the axis of a vertebral pedicle, or in other words, the angles A 1  and A 2  described above with regard to forming a pilot hole for pedicle screw implantation. 
     In a first example, the trajectory system  10  equipped C-arm  208  may be used to help determine the axis of a pedicle as well as a good starting position for entering the pedicle in line with the determined axis. As mentioned, the implantation of pedicle screws without breaching, cracking, or otherwise compromising the pedicle wall is critical to the success of a procedure still presents a significant challenge to spinal practitioners. To mitigate this challenge according to one embodiment of the present invention, the orientation of the pedicle axis, defined by a medial-lateral angle A 1  (illustrated in  FIG. 49 ) and a cranial-caudal angle A 2  (illustrated in  FIG. 50 ), may be determined and the pedicle screw and/or related instruments may be advanced through the pedicle along the desired trajectory. 
     In this example the angle A 1 , or medial-lateral angle, of the pedicle is determined preoperatively in the same manner previously described with reference to  FIG. 19 . Specifically, a vertical reference line is drawn through the center of the vertebral body and a medial-lateral trajectory line is drawn from a central position in the pedicle (e.g. a position within the soft cancellous bone, as opposed to the harder cortical bone forming the outer perimeter of the pedicle) to an anterior point of the vertebral body. The angle formed between the medial-lateral trajectory line and the vertical reference line is measured and represents the angle A 1  value. The angle A 2 , or the cranial-caudal angle, may preferably be determined intraoperatively with a C-arm fluoroscope  208  equipped with the trajectory system  10  of the present invention. Angle A 2  is determined in essentially the same fashion as it was described previously. However, with the addition of the tilt sensor  14 , need to measure the angle from the fluoroscopic imaging monitor is obviated. Instead, when the vertical reference line formed by plumb line  247  lines up with the pedicle, the pedicle angle A 2  corresponds to the angle of the cranial-caudal trajectory angle of the C-arm tilt sensor  14 , which is displayed on feedback device  16 . 
     To determine the cranial-caudal angle A 2 , the C-arm  208  is first positioned so that the base is perpendicular to the spine of the patient  204 . This may be verified with an A/P fluoroscopy image as shown in  FIG. 51 . The base is perpendicular to the spine when the spinous process is equally spaced between the pedicles. Also, when the base of the C-arm  208  is perpendicular to the spine, the vertical reference line created by plumb line  274  is generally parallel to the vertebral endplates in the A/P fluoroscopy image. 
     Once the alignment of the base is verified, the C-arm  208  is rotated to the lateral position and the tilt sensor  14  may be pivoted into its corresponding position (i.e. the horizontal position if the coupler is fixed to the receiver sidewall  256  or the vertical position if the coupler is fixed to the receiver face  258 ). Starting from the lateral position, illustrated in  FIG. 52 , the C-arm  208  may be rotated radially (through a sagittal plane) until the vertical reference line is parallel to the pedicle axis. In this position, which may be referred to as the trajectory lateral position illustrated in  FIG. 53 , A 2  is equal to A 2 (c). When the practitioner is satisfied with the alignment of the vertical reference line to the pedicle axis, the correct A 2  value measured from the tilt sensor  14  may be recorded from the feedback device  16 . Again, this angle measurement corresponds to the cranial-caudal angle A 2  of the pedicle axis and the method may be repeated for each pedicle that is to receive a pedicle screw. 
     To select a starting point for pedicle penetration, the C-arm  208  may be placed in the trajectory lateral position for the pedicle of interest. This may be accomplished by again lining up the vertical reference line in a lateral fluoroscopy image with the pedicle axis or by simply rotating the C-arm until the tilt sensor  14  feedback readout  16  indicates that the previously determined trajectory lateral position has been attained (i.e. the angle measurement displayed by the tilt sensor  14  matches the angle measurement displayed when the trajectory lateral position was initially determined). From the trajectory lateral position, the C-arm  208  may be rotated back to the A/P position while maintaining the rotation imparted to achieve the trajectory lateral position. As pictured in  FIG. 54 , a pedicle penetration instrument, such as instrument  12  described above, may be advanced to the target site and positioned on the lateral margin of the pedicle, which is the preferred starting point according to this example. The surgical instrument  12  may then be oriented according to the medial-lateral and cranial-caudal angles previously determined (i.e. A 1 (i)=A 1  and A 2 (i)=A 2 ) and thereafter safely advanced through the pedicle. The depth of penetration of the surgical instrument  12  may be checked during advancement by rotating the C-arm back to a trajectory lateral view of  FIG. 53 . As pictured in  FIG. 55 , the surgical instrument  12  should appear parallel to the vertical reference line in the trajectory lateral view. 
     An alternate method for determining a preferred starting point for pedicle penetration utilizes the “owl&#39;s eye” view described below. Specifically, the C-arm is oriented to the “owls eye” view, or in other words both the medial-lateral and cranial-caudal angles of the C-arm  208  match the medial-lateral and cranial-caudal angles of the pedicle axis (A 1 (c)=A 1  and A 2 (c)=A 2 ). The tip of the pedicle access instrument  12  is placed on the skin so that the tip is located in the center of the pedicle of interest on the fluoroscope image; this marks the preferred incision site. The access instrument  12  is advanced to the pedicle and another fluoroscope image is taken to verify that the tip of the instrument is still aligned in the center of the pedicle. 
     To orient the C-arm  208  in the owl&#39;s eye position, the medial-lateral and cranial-caudal angles of the pedicle axis may each be determined as described with reference to FIGS.  21  and  51 - 53 . Next, from the trajectory lateral position, the C-arm is rotated back to the A/P position while maintaining the cranial-caudal orientation achieved in the trajectory lateral position. Finally, while monitoring the feedback device  16 , the C-arm  208  is orbitally rotated until the angle matches the medial-lateral angle derived from the preoperative planning (A 2 (c)=A 2 ). At this point the fluoroscopic image, illustrated in  FIG. 56 , displays an “owl&#39;s eye” view of the pedicle, which appears generally as a circle. 
     The owl&#39;s eye view illustrated in  FIG. 56  may be utilized for various reasons. By way of example, the owl&#39;s eye view may be used to determine the starting point for penetration of the pedicle as described. Also, in the owls eye image, a surgical instrument  12  properly aligned with the pedicle axis will appear as a black dot. Thus, once aligned, the surgical instrument may be advanced through the pedicle while ensuring that it continues to appear as only a dot on the fluoroscopy image. The depth of penetration may again be checked with a trajectory lateral image ( FIG. 55 ). Additionally, if preoperative planning was not conducted to determine angle A 1  of the pedicle axis, the C-arm can be manually (e.g. through trial and error) positioned in the owl&#39;s eye view. Once the C-arm is positioned in the owls eye position the tilt sensor  14  may be read to determine the angle A 1 . 
     Still another example of the gainful application of the surgical trajectory monitoring system  10  to enhance surgical safety and efficiency includes monitoring the orientation of surgical access instruments. Monitoring trajectory can aid in both the insertion and positioning of the access instruments themselves, as well as, aiding in the later insertion of instruments and/or implants through the surgical access instruments. One significant advantage is the ability to later visually align surgical instruments and/or implants along the same trajectory by visually comparing the alignment of the instrument to that of the access instrument Often times during a surgical procedure, the orientation of a surgical instrument or implant is of critical import to the success of the surgery. By way of example only, while placing bone screws through a pedicle (which is a small generally tubular structure connecting posterior elements of a vertebra to the vertebral body) it is critical to ensure the screw is contained within the pedicle and does not breach the outer pedicle wall. Since the pedicle is surrounded by delicate nervous tissue, a breach can have serious consequences for the patient, ranging from mild pain to paralysis. One way to mitigate the risk of a pedicle breach during screw placement (including preparation for screw placement, such as pilot hole formation and tapping) is to determine the angular orientation of the pedicle and thereafter advance the necessary instruments and screws along the determined trajectory. By orienting the surgical access components along the pedicle trajectory, the surgical instruments and pedicle screws may be simply and efficiently advanced along the same trajectory, and thus avoid a breach, by “eyeballing” alignment with the access components. 
     An exemplary surgical access system  300  is described, by way of example only, with reference to  FIGS. 57-59 . Surgical access system  300  includes a tissue distraction assembly  302  ( FIG. 57 ) and a tissue retraction assembly  304  ( FIG. 58 ) generally of the type shown and described in commonly assigned U.S. Pat. No. 7,207,949, the entire contents of which is hereby incorporated in this disclosure as if set forth in its entirety herein. 
     As shown in  FIG. 57 , the tissue distraction assembly  302  includes a K-wire  306 , an initial dilating cannula  308 , and a sequential dilation system  310 . In use, the K-wire  306  is disposed within the initial dilating cannula  308  and the assembly is advanced through the tissue towards the surgical target site (e.g. annulus). This is preferably accomplished while employing the nerve detection and/or direction features described below. As will be described in detail below, dilation may also be carried out along a desired angular trajectory path by using the surgical trajectory system  10  of the present invention. After the initial dilating assembly is advanced such that the distal end of the initial dilator  308  is positioned within the disc space, the sequential dilation system  310  consisting of one or more supplemental dilators  312 ,  314  may be employed for the purpose of further dilating the tissue down to the surgical target site. Each component of the sequential dilation system  310  (namely, the K-wire  306  and the supplemental dilators  312 ,  314 ) may, according to the present invention, be provided with one or more electrodes (preferably at their distal regions) equipped for use with a nerve surveillance system, such as, by way of example, the type associated with the neuromonitoring system  400  discussed below. 
     As shown in  FIG. 58 , the retraction assembly  304  includes a plurality of retractor blades extending from a handle assembly  334 . By way of example only, the handle assembly  334  is provided with a first retractor blade  336 , a second retractor blade  338 , and a third retractor blade  340 . Although shown and described below with regard to the three-bladed configuration, it is to be readily appreciated that the number of retractor blades may be increased or decreased without departing from the scope of the present invention. The retractor assembly  304  is shown in a fully retracted or “open” configuration, with the retractor blades  336 ,  338 ,  340  positioned a distance from one another so as to form an operative corridor  348  there between and extending to a surgical target site (e.g. an intervertebral disc). As will be described in detail below, the placement of the retraction assembly  304  may be carried out along a desired angular trajectory path by using the surgical trajectory system  10  of the present invention. Each component of the retraction system  304  (namely, the blades  336 - 340 ) may, according to the present invention, be provided with one or more electrodes (preferably at their distal regions) equipped for use with a nerve surveillance system, such as, by way of example, the type associated with the neuromonitoring system  400  discussed below. 
     To use the angle monitoring system  10  with the surgical access system  300 , the desired trajectory of the surgical corridor may be determined. If for example, the surgeon intends to implant pedicle screws through the resulting operative corridor  348 , it may be desirous to orient the surgical corridor  348  along the pedicle axis. Methods for determining the pedicle axis have been described previously and may be followed here again to obtain the desired angle values. 
     Once the pedicle trajectories have been determined the practitioner may begin advancing the surgical access system  300  according to the desired angular trajectory. The K-wire  306  is advanced into the pedicle. If a rigid K-wire is utilized the tilt sensor  14  may be attached to a proximal end of the K-wire  306  via universal clip  26 , and the angle of insertion guided by the feedback device  16 . After the K-wire  306  is docked in the vertebra, the clip  26  is removed (if it was used) and may be attached to the first dilator  308  for insertion, again following the desired trajectory via the feedback device  16 . This is repeated until the last dilator  310  is in place over the surgical target site. Next, the tilt sensor  14  may be removed from the final dilator and attached to the retractor  304  and as previously described, the surgical retractor assembly  304  may be advanced to the target site over the outer dilator with the blades  336 ,  338 ,  340  in the “closed” position. The dilator is removed and the blades may be retracted to an “open” position providing a corridor  348  for the surgeon to work. If the angle of the retractor  304  was not determined during insertion it may be determined at this point by attaching the tilt sensor  14  in the manner described and then reading the measurements. 
     After the retractor assembly  304  is set in the proper angular alignment, a pilot hole may be formed in the pedicle. By way of example only a gear shift probe or awl may be advanced to the pedicle. Once the tip of the instrument rests on the pedicle the shaft of the instrument should be adjusted until it is aligned both medial-laterally and cranial-caudally with the axis of the first retractor blade  336 . When aligned with the axis of the first blade, or in other words, with the longitudinal axis of the surgical corridor, the instrument tip may be driven into the pedicle along the pedicle axis, ensuring that the pedicle wall is not breached. This may be repeated for a tapping instrument and during screw insertion. After the first screw is inserted along the pedicle axis, the retractor system  304  may be adjusted until it is aligned with the axis of the next pedicle to receive a screw, and the steps are repeated again until the next screw is placed. Once every screw has been placed and connecting elements inserted and locked into the screws, the retractor system  304  may be removed. It will again be appreciated that although this method is described as orienting the retractor assembly along a pedicle axis and thereafter inserting instruments into the pedicle, there are any number of procedures which may be benefited by inserting an instrument along a specific trajectory and the method described applies equally thereto. 
     Coupling the tilt sensor  14  to the various components of the distraction assembly  302  and retraction assembly  304  may be accomplished in any of a variety of manners. With reference to  FIG. 60 , the universal clip  26  (and thus tilt sensor  14 ) may be attached to one of the sequential dilators  310 . In the same fashion as described above in relation to the pedicle access probe, a portion of the surgical distraction component, preferably near the proximal end, is positioned within the hook of free end  54  of clip  26 . Collar  34  may then be twisted to securely fasten the distraction component to the universal clip  26 . When completely fastened, the tilt sensor  14  extends perpendicularly to the longitudinal axis of the distraction component. Since the component is angled in either of the sagittal or transverse planes, the user may be apprised of the angle relative to gravity via feedback device  16 . It will be appreciated that while the tilt sensor  14  is shown in  FIG. 60  attached only to the outer or largest dilator, the clip may be attached to the K-wire and then each dilator in turn if desired. 
     With reference to  FIGS. 61-63 , the universal clip  26  (and thus the tilt sensor  14 ) may be attached to the tissue retractor assembly  304 . In  FIG. 61 , the universal clip  26  is attached to a post  312  extending vertically from the top of first retractor blade  336 . This embodiment is advantageous in that the post  312  may be in electrical communication with an electrode  350  at or near the distal end of retractor blade  336 . The universal clip  26  may be in electrical communication with a neuromonitoring system  400 , described below, such that electrical stimulations from the neuromonitoring system  400  may thus be delivered through the universal clip  26 , eliminating the need for connecting multiple devices to the retractor  304 . In  FIG. 65 , the universal clip  26  is attached to a post  313  used to attach the surgical retractor assembly  304  to an articulating arm  315  which provides secure connection to the operating table and holds the retractor assembly in place. 
     Whether universal clip  26  is attached to post  312  (as in  FIG. 61 ) or post  313  (as in  FIG. 62 ) it will be appreciated that it extends perpendicular to the longitudinal axis of the surgical corridor  348  created between the retractor blades. It will be appreciated that, if and when the second and third retractor blades  338 ,  340  are rotated out during surgery, the longitudinal axis of the surgical corridor remains the same and follows the axis of the first blade  336  which does not rotate. It should also be appreciated that though the clip  26  is shown in  FIGS. 61 and 62  extending over the retractor blades and the corridor  348  they create, this is done for the ease of viewing and preferably the surgical trajectory assembly  10  is oriented such that the clip  26  extends away from the surgical corridor when so as not to inhibit the surgeons ability to manipulate instruments and/or see the surgical wound. 
       FIG. 63  shows a tissue retractor connected directly to the handle assembly  334  of a tissue retractor  304  via any number of suitable manners, including but not limited to the use of a Velcro® patch, tape, strap, etc. . . . The tilt sensor  14  is preferably positioned on the handle assembly  334  such that it is perpendicular to the longitudinal axis of the spine, thus as it is shown in  FIG. 63 , the handle extensions would face out to the side of the patient, perpendicular to the longitudinal axis of the spine. If necessary, the tilt sensor may be detached and repositioned. 
     In addition to being physically coupled to the retractor  304 , it will be appreciated that the tilt sensor  14  may be formed as an integral part of the retractor  304  and/or any individual component thereof without departing from the scope of the present invention. For example, the tilt sensor  14  may be formed as part of any of the retractor blades  336 ,  338 ,  340 , any component of the handle assembly  334 , and/or any component of the dilation assembly  302 . In such an instance, the “tilt-sensor enabled component” may be have an electrical coupling and/or wireless communication technology to provide the tilt sensing functionality described herein and may be disposable or reusable as described herein. 
     The surgical trajectory system  10  described above may be used in combination with any number of neurophysiologic monitoring systems. These may include, but are not necessarily limited to, neurophysiologic monitoring system capable of conducting pedicle integrity assessments before, during, and after pilot hole formation, as well as to detect the proximity of nerves while advancing and withdrawing the surgical instrument (e.g. probe  12 , awl  80 , dilation assembly  302 , retraction assembly  304 ) from the pedicle target site. An exemplary neuromonitoring system  400  is shown by way of example in  FIG. 64 . Neuromonitoring system  400  has been described in detail elsewhere and will be described only briefly herein. By way of example only, the various functional modes of the neuromonitoring system  10  may include the Twitch Test, Free-run EMG, Basic Screw Test, Difference Screw Test, Dynamic Screw Test, MaXcess® Detection, and Nerve Retractor, all of which will be described briefly below. The Twitch Test mode is designed to assess the neuromuscular pathway via the so-called “train-of-four test” test to ensure the neuromuscular pathway is free from muscle relaxants prior to performing neurophysiology-based testing, such as bone integrity (e.g. pedicle) testing, nerve detection, and nerve retraction. This is described in greater detail within Int&#39;l Patent App. No. PCT/US2005/036089, entitled “System and Methods for Assessing the Neuromuscular Pathway Prior to Nerve Testing,” filed Oct. 7, 2005, the entire contents of which is hereby incorporated by reference as if set forth fully herein. The Basic Screw Test, Difference Screw Test, and Dynamic Screw Test modes are designed to assess the integrity of bone (e.g. pedicle) during all aspects of pilot hole formation (e.g., via an awl), pilot hole preparation (e.g. via a tap), and screw introduction (during and after). These modes are described in greater detail in Int&#39;l Patent App. No. PCT/US02/35047 entitled “System and Methods for Performing Percutaneous Pedicle Integrity Assessments,” filed on Oct. 30, 2002, and PCT/US2004/025550, entitled “System and Methods for Performing Dynamic Pedicle Integrity Assessments,” filed on Aug. 5, 2004 the entire contents of which are both hereby incorporated by reference as if set forth fully herein. The MaXcess® Detection mode is designed to detect the presence of nerves during the use of the various surgical access instruments of the neuromonitoring system  10 , including the k-wire  62 , dilator  64 , cannula  66 , retractor assembly  70 . This mode is described in greater detail within Int&#39;l Patent App. No PCT/US02/22247, entitled “System and Methods for Determining Nerve Proximity, Direction, and Pathology During Surgery,” filed on Jul. 11, 2002, the entire contents of which is hereby incorporated by reference as if set forth fully herein. The Nerve Retractor mode is designed to assess the health or pathology of a nerve before, during, and after retraction of the nerve during a surgical procedure. This mode is described in greater detail within Int&#39;l Patent App. No. PCT/US02/30617, entitled “System and Methods for Performing Surgical Procedures and Assessments,” filed on Sep. 25, 2002, the entire contents of which are hereby incorporated by reference as if set forth fully herein. Although not described herein, various other functional modes may be performed by the system  10 , such as for example only, MEP and SSEP functions which are described in detail within Int&#39;l Patent App. No. PCT/US2006/003966, entitled “System and Methods for Performing Neurophysiologic Assessments During Spine Surgery,” filed on Feb. 2, 2006, the entire contents of which are hereby incorporated by reference as if set forth fully herein. 
     With reference to  FIG. 64 , the neurophysiology system  400  includes a display  401 , a control unit  402 , a patient module  404 , an EMG harness  406 , including eight pairs of EMG electrodes  408  and a return electrode  410  coupled to the patient module  404 , and a host of surgical accessories  412 , including an electric coupling device  414  capable of being coupled to the patient module  404  via one or more accessory cables  416 . As shown in  FIGS. 65-66 , to perform the neurophysiologic monitoring, the surgical instrument  12 ,  80  is configured to transmit a stimulation signal from the neurophysiology system  400  to the target body tissue (e.g. the pedicle). As previously mentioned, the probe members  60 ,  82  may be formed of material capable of conducting the electric signal. To prevent shunting of the stimulation signal, the probe members  60 ,  82  may be insulated. By way of example, a substantial portion of probe members  60 ,  82  may be provided with an insulative coating. Uninsulated portions near the distal end  90  form a stimulation region (or “electrode”)  135 . An additional uninsulated region is also provided near the proximal end of the probe member to serve as a coupling point  137  for linking the neurophysiology system to the instrument  80 . In another embodiment, not shown, a retractable insulative sheath, as described in U.S. patent application Ser. No. 11/448,237, may be employed in place of, or in addition to the insulative coating. In still another alternative, probe members  60 ,  82  may be formed of a non-conductive material with one or more embedded conductive elements at or near the distal end capable of being communicatively linked with neurophysiology system  400 . 
     The neurophysiology system  400  performs nerve monitoring during surgery by measuring the degree of communication between a stimulation signal and nerves or nerve roots situated near the stimulation site. To do this, the surgical instrument is connected to the neurophysiology monitoring system  400  and stimulation signals are activated and emitted from electrode  135 . EMG electrodes  408  positioned over the appropriate muscles measure EMG responses corresponding to the stimulation signals. The relationship between the EMG responses and the stimulation signals are then analyzed by the system  400  and the results are conveyed to the practitioner on the display  401 . More specifically, the system  400  determines a threshold current level at which an evoked muscle response is generated (i.e. the lowest stimulation current that elicits a predetermined muscle response). Generally the closer the electrode  135  is to a nerve the lower the stimulation threshold will be. Thus, as the probe member  60 ,  82 , or surgical access members  302 ,  304  move closer to a nerve, the stimulation threshold will decrease, which may be communicated to the practitioner to alert him or her to the presence of a nerve. The pedicle integrity test, meanwhile, works on the underlying theory that given the insulating character of bone, a higher stimulation current 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 lower than a predetermined safe level, the surgeon may be alerted to a possible breach. 
     The surgical instrument  12 ,  80 ,  302 ,  304  may be connected to the neurophysiology system  400  by attaching a DIN cable  413  to the receptacle  50  of universal clip  26 . When tilt sensor  14  is attached directly to the surgical instrument (rather than using universal clip  26 ), as with instrument  80  described above, an electric coupling device  414  may be provided with the neurophysiology system  400 . The electric coupling device may be attached to the uninsulated coupling point  137  at the proximal end  64  of probe member  82 . The electric coupling device  414  may comprise a number of possible embodiments which permit the system  400  to attach to the coupling point  137  and transmit a stimulation signal through the probe member  82 . 
     One such embodiment of electric coupling device  414  utilizes a spring-loaded plunger to hold the coupling point  38  and transmit the stimulation signal. The plunger  418  is composed of a conductive material such as metal. A nonconductive housing  414  partially encases the plunger rod  418  about its center. Extending from the housing  420  is an end plate  424 . An electrical cable  426  connects the electric coupling device  414  to neurophysiology system  400 . A spring (not shown) is disposed within the housing  420  such that in a natural or “closed” state the plunger  418  is situated in close proximity to the endplate  424 . Exerting a compressive force on the spring (such as by pulling the cable  426  while holding the housing  420 ) causes a gap between the end plate  424  and the plunger  418  to widen to an “open” position, thereby allowing insertion of the coupling point  137  between the end plate  424  and plunger  418 . Releasing the cable  426  allows the spring to return to a “closed” position, causing the plunger  418  to move laterally back towards the endplate such that a force is exerted upon the coupling point  137  and thereby holds it in place between the endplate  424  and the plunger  418 . Thereafter, the electrical stimulus may be passed from the neurophysiology system  400  through the cable  426  and plunger  418  to the probe member  82 . 
     Alternatively, the electrical coupling device may be embodied in the form of a clip  428 . The clip  428  is comprised of two prongs hingedly coupled at a coupling point  430  such that the clip  428  includes an attachment end  432  and a non-attachment end  434 . A stimulation electrode  436  is disposed on the attachment end  432  and communicates with an electric cable  426  extending from the non-attachment end  434  to the neurophysiology system  400 . In a “closed” position the prong ends at the attachment end  432  touch. Depressing the prongs at the non-attachment end  434  in a direction towards each other causes a gap to form between the prong ends at the attachment end  432 . Positioning the “opened” attachment end  432  over the coupling point  137  and releasing the force on the non-attachment end  434  causes the attachment end  432  to pinch tight on the coupling point  137  and thereby allow the electrical stimulus to pass from neurophysiology system  400 , through the stimulation electrode  236 , to the probe member  82 . 
     During pilot hole formation, while the trajectory of the surgical instrument is being monitored to prevent the instrument from breaching the pedicle walls, pedicle integrity assessments may be performed to alert the practitioner in the event a breach does occur. Stimulation signals are emitted from electrode  135 , which should be at least partially positioned within the pedicle bone during hole formation. The stimulation threshold is determined and displayed to the surgeon via the neurophysiology monitoring system  400 . Due to the insulating characteristics of bone, in the absence of a breach in the pedicle wall, the stimulation threshold current level should remain higher than a predetermined safe level. In the event the threshold level falls below the safe level, the surgeon is alerted to the potential breach. When the pilot hole is fully formed, a final integrity test should be completed. 
     In one embodiment, tilt sensor  14  is communicatively linked directly to control unit  402  of the neurophysiology monitoring system. The display  401  of neurophysiology system  400  replaces feedback device  16  of the trajectory monitoring system  10  and data from the tilt sensor  14  is shown jointly with the neurophysiologic data. As described above, audio indicia may also be communicated to the user regarding the trajectory angles. 
     The systems and methods described above may be utilized by personnel during surgical procedures to help maximize efficiency and increase safety. Each of the applications for, and methods of, using the surgical trajectory system  10  described above have advantages when used on their own and it is expressly noted that combining some or all of the different uses and methods of the trajectory system may be especially beneficial. By way of example only, a trajectory assisted method and procedure for pedicle fixation utilizing multiple aspects of the trajectory system  10  will now be described. 
     The trajectory assisted pedicle fixation procedure begins with the proper positioning of the patient  204 . The patient is positioned on the surgical table  206  in the prone position and the affected spinal levels are adjusted until they are level with the floor. Once the patient is presumed to be properly positioned, one or more of a visual inspection, A/P fluorography, and a mechanical inspection with a level may be performed to verify the position. 
     After positioning of the patient  204  is complete the angles to be used for pilot hole formation may be determined. The first angles to be determined are the medial-lateral angles A 1 . As previously noted this step can be completed prior to the surgery and the medial-lateral angles for each pedicle to receive a screw are brought into the operating room for reference. To calculate the medial-lateral angle A 1 , the pedicle angle correction value should be determined (and loaded into CPU or the neuromonitoring system  400  if applicable), if necessary, by first determining the amount of rotation in the spine relative to the vertical reference in the MRI or CAT image. The angular difference between a line drawn connecting the posterior superior iliac spine (PSIS) and the line parallel to the bottom of MRI image box. Next, a line is drawn down the pedicle axis of each pedicle and a vertical reference line is drawn between them through the center of the vertebral body. A measuring device, such as a protractor may then be used to calculate the angular difference between the vertical reference line and the lines drawn through the axis of the pedicles. The values determined are the medial-lateral angles to use for pedicle cannulation during pilot hole formation. The step is repeated until the medial-lateral angle for each pedicle of interest is determined. Again if necessary, correct the angle values determined based on the pedicle angle correction value. 
     After determining the medial-lateral angles for each pedicle, the next step is to obtain the cranial-caudal angles A 2  for each pedicle as well. The superior endplate is positioned parallel to the reticle plumb line  274 . The angle of the pedicle axis is (or vertebral endplate) is measured relative to the C-arm gravity line. This is accomplished by first placing the C-arm  208  into the lateral orientation. The reticle line represents the gravity line and all cranial-caudal angles are measured relative to this reference line. The head or foot of the surgical table  206  may be raised or lowered to attain parallel alignment between the superior endplate of applicable vertebral body and this reference line, if desired. 
     The cranial and caudal angles of applicable pedicle axes or superior endplates are measured via one of two methods. First, a protractor  120  outfitted with a tilt sensor  14  may be lined up with the vertical reference line formed by the plumb line  274  and zeroed. Subsequently, the protractor  120  is rotated into alignment with the pedicle axis or desired trajectory. Subsequent angles are measured per vertebral level using the same method. 
     In the second method, the plumb/reticle line  274  and the associated tilt sensor  14  is utilized to measure the angles. The C-arm  208  is rotated in radial rotation until the plumb line  274  parallel to the pedicle axis or superior endplate of interest and the angle is noted. Then the plumb line  274  is rotated parallel to the superior endplate of the upper level vertebra and the angle is noted. The resulting cranial caudal angles are recorded and are used to facilitate later positioning of the C-arm into the A/P plane of the vertebra of interest. 
     The next main step of the trajectory assisted pedicle fixation procedure of the preset invention is to properly position the C-arm  208 . Initially all the C-arm articulations are set to zero and the horizontal arm is set at midline. Next the C-arm radial rotation is set to the predetermined cranial-caudal angle A 2  for the level of interest. The C-arm base is then translated cranially or caudally to align the plumb/reticle reference line  274  parallel to the superior endplate of the vertebral body of interest. Then the orbital rotation of the C-arm is altered slowly until the C-arm angle A 1 (c) matches the predetermined medial-lateral angle A 1 . Finally, the C-arm is translated using horizontal arm translation to center beam  218  over the right lower level pedicle. 
     Next the starting point for pedicle cannulation is determined. A tilt sensor  14  is attached to the pedicle access instrument  12 . The tip of the instrument  12  is placed on the skin so that the tip is located in the center of the pedicle and the location is marked with a pen. Using a scalpel a longitudinal 1.5 cm incision centered over the pen mark is made. 
     The instrument  12  is advanced oriented in the same owl&#39;s eye trajectory, through the incision all the way to the bone. A C-arm  208  image is used to see if the tip of the needle is near the axis of the pedicle. If not the tip is adjusted until it is directly in the center of the lower level pedicle. 
     The surgical instrument  12  is attached to the neuromonitoring system  400  and via the one of a coupling device  414  and DIN cable  413 . Once neuromonitoring capability is established the instrument is advanced to a depth of 1 cm and medial wall integrity is confirmed with a C-arm  208  shot at the 1 cm point. The instrument may be advanced while making any adjustments necessary to keep the trajectory in line with the pedicle axis. A final C-arm  208  shot may be taken at the final position for confirmation. After pedicle cannulation is performed on the right side of the lower level, the steps may be repeated again on the left side. 
     To move on to adjacent levels, owl&#39;s eye views may be obtained from previous levels by C-arm articulations and x-ray image sequences according to the following: first orient the C-arm by radial rotation to the upper level specific cranial-caudal angle; align the target level superior endplate with the reticle/plumb line  274  by C-arm base translation; next orient the C-arm by orbital rotation to specific medial-lateral angle; then translate the C-arm horizontal arm translation to center the pedicle of interest in the image; and finally, cannulate the pedicle. These steps are repeated until all pedicles have been cannulated. Implantation of the pedicle screws and supporting rods may now be implanted, preferably using a lateral fluoroscopy view. 
     While the invention is susceptible to various modifications and alternative forms, (such as the drill bit, needle points, and T-handle mentioned above) 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. By way of example, the method for determining the cranial-caudal A 2  has been described herein as taking place intraoperatively using lateral fluoroscopy imaging. However, the cranial-caudal angle may also be determined preoperatively employing various imaging and/or computer processing applications. For example, a 3-D model of a patient&#39;s vertebra (or other applicable body part) may be obtained using a combination of medical imaging and computer processing. From the 3-D model the angle A 2  may be calculated after which the determined value may be utilized by the surgical trajectory system and methods described above. It is further contemplated that computer processing of medical images may be used to extrapolate the pedicle axis angles A 1  and A 2  without the need for human intervention. Finally, it will be appreciated that the intraoperative monitoring discussed herein has generally focused on the use of a C-arm fluoroscopic imager, however, orienting the C-arm with a tilt sensor and providing a trajectory oriented reticle/plumb line using the methods and systems described herein may apply to any form of intraoperative monitoring.