Patent Publication Number: US-2021186617-A1

Title: Surgical Monitoring System and Related Methods For Spinal Pedicle Screw Alignment

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 15/792,377, filed Oct. 24, 2017, which is a continuation of U.S. application Ser. No. 14/841,270, filed Aug. 31, 2015, now U.S. Pat. No. 9,795,451, which is a continuation of U.S. application Ser. No. 12/739,950, filed Aug. 23, 2010, now U.S. Pat. No. 9,119,752, which is the U.S. national stage of PCT/US2008/012121, filed Oct. 24, 2008, and which claims the benefit of priority to U.S. provisional apps. 61/000,349 filed Oct. 24, 2007, and 61/196,266, filed Oct. 15, 2008, the entire contents each of which 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 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 control unit 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 audio feedback corresponding to the orientation of the tilt sensor in the sagittal plane (cranial-caudal angle) and in the transverse plane (medial-lateral angle). 
     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 control unit or secondary 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. 
     According to one embodiment of the present invention, a reticle may be provided to attach the tilt sensor to a standard C-arm. The reticle comprises an integrated sensor and a mount which may attach to the C-arm. The reticle further comprises an adjustable laser. The laser may be aimed along the C-arm axis. In use the laser cross-hair will mark a target incision site on a patient when the C-arm is oriented in line with the pedicle axis. Radiopaque markers are also integrated into the reticle. The markers provide a reference for properly aligning the fluoroscopic images. 
     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 fluoroscopic image; and thereafter the instrument is advanced to the pedicle. Another fluoroscopic 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. 
     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. By way of further example, the neurophysiology system and the surgical trajectory system may be integrated into a single system. Neurophysiology monitoring and trajectory monitoring may be carried out concurrently and the control unit may display results for each of the trajectory monitoring function and any of a variety of neurophysiology monitoring functions, including, but not necessarily limited to, Twitch Test, Free-run EMG, Basic Screw Test, Difference Screw Test, Dynamic Screw Test, Nerve Detection, Nerve Health, MEP, and SSEP. 
     The neurophysiology system includes a display, a control unit, a patient module, one or more of an EMG harness and an SSEP harness, a host of surgical accessories (including an electric coupling device) capable of being coupled to the patient module via one or more accessory cables. According to one embodiment, the electric coupling device may be the tilt sensor clip. 
     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. 
     According to one exemplary method of using the systems and methods described herein, a pedicle screw may be implanted in a target pedicle according to the following steps. First, preoperative measurements corresponding to the medial/lateral angle of the pedicle are determined using suitable imaging technology, such as for example, MRI. Level the C-arm and then attach the laser reticle to the C-arm using the integrated tilt sensor and LED indicators to align the laser reticle into proper position and the adjustable clamps to lock the reticle in place. Adjust the laser cross-hairs if necessary such that the center of the cross-hairs align with the calibrated center of the C-arm emitter. Select from the navigated guidance system whether fluoroscopy will be used and indicate which side of the body the C-arm is positioned. Ensure the patient is still aligned properly on the table and then measure the cranial/caudal angles. Use the virtual protractor to determine the cranial/caudal angles and then enter the predetermined medial/lateral angles into the system. Return the C-arm to the A/P position to verify again that the patient hasn&#39;t moved. Orient the C-arm into owls eye position. Mark the skin where laser cross-hairs direct and then repeat the steps for each pedicle to be instrumented. Make a hole with a pedicle access probe and advance the probe to the spine. Ensure that the pedicle probe is docked on a good starting point with the C-arm and then advance the probe into the pedicle, repeating again for each pedicle to be instrumented. 
    
    
     
       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 sensor clip, C-arm, laser reticle, surgical instrument and control unit, according to one embodiment of the present invention; 
         FIG. 2  is a perspective view of a tilt sensor clip 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 sensor 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; 
         FIG. 5  is a close up view of a handle portion of a surgical instrument, according to one embodiment of the present invention; 
         FIG. 6  and  FIG. 7  illustrate a sensor clip connector used to attach the tilt sensor to a surgical instrument, according to one embodiment of the present invention; 
         FIG. 8  is an illustration of an operating theater equipped with a surgical table, C-arm fluoroscope, fluoroscope monitor, practitioner, and patient; 
         FIG. 9  is a front view of the C-arm of  FIG. 8  oriented in an A/P position for generating an A/P fluoroscopic image; 
         FIG. 10  is front view of the C-arm of  FIG. 8  oriented in a lateral position for generating a lateral fluoroscopic image; 
         FIG. 11A  and  FIG. 1B  are front views of the C-arm of  FIG. 8  oriented according to desired medial-lateral angles between the A/P position of  FIG. 9  and the lateral position of  FIG. 10 ; 
         FIG. 12A  and  FIG. 12B  are side views of the C-arm of  FIG. 8  oriented according to various cranial-caudal angles; 
         FIG. 13 - FIG. 19  are exemplary views of a laser reticle equipped with an integrated tilt sensor, radiopaque cross-hairs, and laser light, according to one embodiment of the present invention; 
         FIG. 20  is a top view of a vertebral body showing the medial-lateral angle A 1  of the pedicle axis; 
         FIG. 21  is a side view of a vertebral body showing the cranial-caudal angle A 2  of the pedicle axis; 
         FIG. 22  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. 23  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. 24  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; 
         FIG. 25  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; 
         FIG. 26 - FIG. 36  are exemplary screen displays of the surgical trajectory system  10  incorporating alpha-numeric, graphical indicia, and fluoroscopic image data, and various control features according to another embodiment of the present invention; 
         FIG. 37 - FIG. 46  are exemplary screen displays of the surgical trajectory system  10  incorporating alpha-numeric, graphical indicia, and fluoroscopic image data, and various control features according to yet another embodiment of the present invention; and 
         FIG. 47  is a perspective view of an exemplary neuromonitoring system for use in conjunction with the surgical trajectory system of  FIG. 1 , according to one 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  including a tilt sensor clip  12  (also referred to as “tilt sensor”) engaged with a surgical instrument  14 , a feedback and control device comprising a control unit  16 , and a laser reticle  18  coupled to a C-arm  20 . The tilt sensor clip  12  measures its own angular orientation with respect to a reference axis, such as vertical or gravity. The control unit  16  provides feedback related to the angle measurements obtained by the tilt sensor clip  12  for reference by a practitioner and receives user input. The tilt sensor clip  12  attaches to surgical instrument  14  in a known positional relationship such that the angular orientation of the instrument  14  may be determined with respect to the same reference axis. This enables the surgeon to position and maintain the instrument  14  along a desired trajectory path during use. For example, during pilot hole formation, surgical instrument  14  may be aligned and advanced along a pre-determined pedicle axis, thereby decreasing the risk of breaching the pedicle wall. 
     Tilt sensor  12 , illustrated in  FIGS. 2-4 , includes a sensor package  22  ( FIG. 3 ) enclosed within a housing  24 . The housing  24  may be made from a surgical grade plastic, metal, or any material suitable for use in the surgical field. Housing  24  is configured to snugly couple with the surgical instrument  14  in a known positional relationship, as will be described below. 
     In one embodiment, sensor package  22  comprises a 2-axis accelerometer that measures angular orientation with respect to the acting direction of gravity. The angular orientation of tilt sensor  12  is measured in a sagittal plane and a transverse plane. By way of example, the orientation of the tilt sensor  12  in the transverse plane represents a medial-lateral angle A 1 ( i ) with respect to a patient and the direction of gravity. Orientation in the sagittal plane represents a cranial-caudal angle A 2 ( i ) with respect to the direction of gravity and the patient. Sensor package  22  is preferably situated within housing  24  such that when housing  24  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  12  is fixed perpendicular to the longitudinal axis of the surgical instrument  14 , 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  12  may be 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  26  and transverse axis  28  tilt sensor  12  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  12  should be in approximately the same rotational alignment for each measurement). In one embodiment, this may be accomplished effectively using visual aids to help keep the tilt sensor  12  in line with the reference frame and/or ensure measurements may be taken when the tilt sensor  12  appears to be in this correct rotational position. By way of example only, the sensor clip  12  attaches to the instrument  14  such that a free end of the clip may “point” to the patients feet when the sensor  12  is in the correct-rotational position. In the event the surgical instrument  14  is inadvertently or purposely rotated during use, the practitioner need only continue, or reverse rotation until the tilt sensor  12  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  12  and the surgical instrument  14  to ensure proper alignment prior to obtaining measurements. 
     In an alternative embodiment, the sensor package  22  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  22  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  12  with respect to gravity, the gyroscope detects movement about the rotational or z-axis. By monitoring the rate of rotation and time, the system may determine the degrees of rotation imparted on the surgical instrument  14  (and tilt sensor  12 ). The control unit  16  may indicate to the user that the sensor  12  is not aligned in the correct reference frame such that the user may take steps to correct the alignment prior to taking measurements. The control unit  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 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  12  may be acquired from any rotational position. A button (not shown) may be provided on the tilt sensor  12  and/or control unit  16  to initially zero the sensor package  22  when it is aligned with the reference frame. 
     In another embodiment, the sensor package  22  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  22  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  12  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  22  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 control unit  16  may again be configured to indicate the rotational status of the tilt sensor  12  to the user, allowing them to realign the sensor  22  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. 
     A surgical instrument  14 , according to one embodiment, is illustrated in  FIG. 4 . Surgical instrument  14  may comprise a pedicle access probe. By way of example only, instrument  14  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  14  comprises generally a probe member  30 , having a longitudinal axis  32 , and a handle  34 . Probe member  30  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  30  may be generally cylindrical in shape, however, probe member  30  may be provided in any suitable shape having any suitable cross-section (e.g. generally oval, polygonal, etc.). A distal region  36  of probe member  30  may have a shaped tip  38  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  40  of probe member  30 , accessible via a cutout  42  in the handle  34 , may be configured to couple the housing  24  of sensor clip  12 . Probe member  30  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  30  may be composed of titanium, stainless steel, or other surgical grade alloy. The distal region  36  may also be equipped with a retractable insulation sheath  44 . The sheath  44  ensures maximum efficiency of an electrical stimulation signal that may be delivered to the shaped tip  38  during neurophysiologic monitoring that may be conducted in conjunction with the surgical trajectory monitoring of system  10 , as described below. 
     Handle  34  may be permanently or removably attached to probe member  30  along the proximal region  40 . Handle  34  may be shaped and dimensioned in any of a number of suitable variations to assist in manipulating probe member  30 . By way of example only, the handle  34  may be generally T-shaped such as the handle pictured in  FIG. 4 . Other suitable shapes for handle  34  may include, but are not necessarily limited to, generally spherical, ellipsoidal, and egg-shaped. Sensor clip  12  forms a sturdy connection with probe member  30  such that the tilt sensor  12  is maintained in a position perpendicular to the longitudinal axis  32  of probe member  30 . When the longitudinal axis  32  of probe member  30  is parallel to the direction of gravity, the tilt sensor  12  is perpendicular to the direction of gravity (i.e. the zero-angle position). In other words, when the longitudinal axis  32  of probe member  30  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  FIGS. 2-7 , sensor clip  12  will be further described. To secure the sensor clip  12  to surgical instrument  14 , housing  24  includes a fastener end  46  dimensioned to snugly receive at least a portion of instrument  14 . By way of example only, fastener end  46  comprises an end hook  48 , and a handle receiver  50 . To maintain a snug fit with the instrument  14 , the end hook  48  is configured to snap on and tightly grasp the proximal region  40 , as illustrated in  FIG. 4 . To attach the clip  12  to instrument  14 , the end hook  48  is fitted onto the proximal end  40  of the instrument  14 . Thereafter the housing  24  is rotated until the handle receiver  50  fully engages with the handle  34  of the instrument  14  ( FIGS. 6-7 ).  FIG. 6  illustrates, by way of example only, a the proximal end  40  (shown in cross-section) tightly positioned within the end hook  48  and before engaging the handle  34  (also shown in cross-section) in the handle receiver  50 .  FIG. 7  illustrates the handle  34  (again in cross-section) after the clip  12  has been rotated into position with the handle  34  fully engaged in the handle receiver  50 . Once fully engaged, fastener end  46  is dimensioned to prevent the unintentional disengagement of instrument  14  from the sensor clip  12 . When engaged, sensor clip  12  extends perpendicular to the longitudinal axis  32  of instrument  14 . To release the surgical instrument  14 , the clip  12  may be rotated to disengage the handle receiver  50  from the handle  34 , and the end hook  48  may be released. 
     Also illustrated in  FIGS. 2-4 and 6-7  is a secondary feedback system  52 , integrated into the sensor clip  12 . By way of example only, secondary feedback system  52  comprises a collection of LED light indicators to provide an indication of the angular orientation of surgical instrument  14  relative to a reference orientation. The collection of LED&#39;s includes four LED directional lights  54 - 60 , a central LED light  62 , and a function LED light  64 . The four LED directional lights  54 - 60  are independently operated to provide an indication to the user of the sensors  12  (and hence, the instrument  14 ) angular position relative to a desired position (as determined, for example, by predetermined angle measurements captured by or inputted into the system  10 ). By way of example only, two opposing LED lights  54  and  56  may correspond to the orientation of the sensor  12  in the cranial-caudal direction and the other two opposing LED lights  58  and  60  may correspond to the orientation in the medial-lateral directions. According to one example, the LED directional lights  54 - 60  will light to indicate the direction in which the instrument  14  needs to be adjusted to align with the desired trajectory. Thus, (by way of example) if the instrument is properly aligned in the medial-lateral direction but not in the cranial-caudal direction then one if lights  54  and  56  will light up to indicate that the instrument needs to be moved in the direction of the lighted LED (either  54  or  56  depending upon whether the instrument needs to be adjusted in the cranial direction or the caudal direction), if however, the instrument  14  is not aligned in either the cranial-caudal direction or the medial-lateral direction, one each of LEDS  54 - 56  and  58 - 60  will light to indicate which direction (i.e. either cranial or caudal and either left or right, respectively) that the instrument  14  needs to be adjusted to align with the desired trajectory. 
     The control unit  16  is communicatively linked to tilt sensor clip  12  and functions to provide feedback to the surgeon regarding the angle of the tilt sensor  12  and instrument  14  relative to the desired angles (e.g. predetermined medial-lateral and cranial-caudal angles) as well as receive user input related to various aspects of the trajectory system  10 . Byway of example only, the control unit  16  includes a display  66  which may show one or more or alpha-numeric, graphic, and color indicia indicative of the sensor  12  trajectory, imported fluoroscopic or other images, patient and or user information, and other system related information. The control unit  16  may also receive user input, such as by way of example, user selectable parameters and/or preferences, procedure related data (including but not necessarily limited to predetermined medial-lateral angles, predetermined cranial-caudal angles), etc. By way of further example, the control unit  16  display  66  may include tools which may be utilized by the user, such as by way of example only, a virtual protractor for predetermining angles. Various features and aspects of the control unit  16  and display  66  functionality are discussed in more detail below. In addition to display  66 , the control unit  16  may be configured to utilize audio indicators as well. By way of example, the control unit  16  may utilize a code based on the emission of audio tones to indicate the angular orientation of the tilt sensor  12  relative to predetermined reference angles corresponding to the desired trajectory. 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  12  relative to the predetermined orientation ranges. Audio feedback may be used alone, or in combination with one or both of the alpha-numeric, graphic, 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  12  and the 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  12  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  12  and the reference direction. 
     The communication link between the sensor clip  12  and may be accomplished via hard-wire (e.g. data cable  68  of  FIG. 1 ) and/or via wireless technology, in which case the tilt sensor  12  and control unit  16  may include additional hardware commonly used for enabling such wireless communication. 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  12  may move freely without tensioning the data cable  68 . 
     According to another aspect of the present invention, the laser reticle  18  is attached to C-arm  20  (fluoroscope) to aid in orienting the C-arm  20  into an advantageous position. By way of example only, it may be advantageous during pedicle screw placement to orient the C-arm  20  in an owl&#39;s eye or oblique position (i.e. the trajectory of the x-ray beam is directly in line with the angular trajectory of the pedicle). The reticle  18  is equipped with an integrated sensor package  70 . Sensor package  70  comprises an accelerometer similar to the sensor package  22  of clip  12  (such that a repeat discussion is not necessary). Including a tilt sensor package  70  in the reticle allows the system  10  to determine the angular position of the reticle  18 , and hence the C-arm  20  to which it is attached, with respect to gravity. The C-arm  20  and laser reticle  18  will now be discussed in more detail. 
     With reference to  FIG. 8  there is shown a typical operating theatre in which a practitioner  72  may perform surgical procedures on a patient  74 . The patient  74  is positioned on a radiolucent operating table  76 . Arrayed around the table  76  are a standard C-arm  20 , comprising a frame  78 , a signal transmitter  80 , and a signal receiver/image intensifier  82 , and the control unit  16  which receives and displays video feed from the C-arm  20 , allowing live fluoroscopic images to be integrated with the trajectory monitoring system  10 . In use, an x-ray beam  84 , having a central axis  86 , may be directed from the signal transmitter  80  through a desired area of patient  74  and picked up by the signal receiver  82 . An image of the patient&#39;s  74  body tissue located in the path of beam  84  may be generated and displayed on the display  66 . It should be appreciated that while the C-arm  20  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  20  and laser reticle  18  combination may be utilized for imaging in a wide variety of surgical procedures. 
     As illustrated in  FIGS. 9-12 , the C-arm frame  78  may be adjusted to alter the path of the beam  84 , and thus the image that is generated. In  FIG. 9  the frame  78  is oriented such that beam  84  travels parallel to the direction of gravity. With the patient in the prone position, as shown herein, this position of frame  78  generates an anterior-posterior (A/P) image. This position of C-arm  20  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. 10 , directs the beam  84  perpendicular to the direction of gravity and generates a lateral image. This position of the C-arm  20  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. 11A-11B , the frame  78  may also be oriented in any position within the transverse plane between the A/P and lateral positions, such that the beam  84  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. 12A-12B , the frame  78  may also be rotated in a cranial-caudal direction (within a sagittal plane) such that the beam  84  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  20  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  (owl&#39;s eye or oblique view), as will be discussed in more detail below. 
     By attaching the reticle  18  with integrated tilt sensor package  70  to the C-arm 20  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  20  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 sensor  70  (via reticle  18 ) 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. 
     According to one embodiment laser reticle  18  may be attached to the receiver.  82  of the C-arm  20 , as pictured in  FIG. 1 .  FIG. 13  illustrates one embodiment of laser reticle  18  comprising a reticle frame  1152 , radiopaque cross hair marker  1154 , radiolucent front cover  1158 , radiolucent back plate  1159 , adjustable clamps  1160 , sensor tilt LED indicators  1164 , and an adjustable laser emitter system  96 .  FIG. 14  illustrates an exploded view of laser reticle  18 . The reticle  18  is configured to generate a reference cross-hair viewable in fluoroscopic images generated by C-arm  20 . Other benefits may also be gained by using laser reticle  18 , such as the benefit of producing a laser cross-hair target on the skin of the patient. This benefit will be discussed in greater detail below.  FIG. 13  illustrates one embodiment of laser reticle  18  comprising a reticle frame  86 , radiopaque cross hair marker  88 , radiolucent front cover  90 , radiolucent back plate  91 , adjustable clamps  92 , sensor tilt LED indicators  94 , and an adjustable laser emitter system  96 .  FIG. 14  illustrates an exploded view of laser reticle  18 . 
     Reticle frame  86  may be made of aluminum material in a generally circular configuration with an inner window opening  98 . The purpose of window opening  98  is to allow a fluoroscopic image to pass through window  98  unobstructed by the metal material of frame  86 . It should be understood that various other suitable materials and configurations may be used in place of, or in addition to, the reticle frame described above. Other reticle frames may include, but are not necessarily limited to, a generally rectangular configuration with square window opening. With reference to  FIGS. 13-14 , reticle frame  86  has a front, leading edge  100  and a back edge  102 . 
       FIG. 15  illustrates, by way of example only, one embodiment of radiopaque cross hair marker  88 . When attached to the receiver  82  of the C-arm  20 , radiopaque cross hair  88  is captured in the fluoroscopic image, giving the operator a reference point to the center of the receiver  82 . Moreover, the cross-hairs  88  provide vertical reference line in the fluoroscopic image, as discussed below. By way of example only, the radiopaque cross hair marker  88  may be produced from metal BB&#39;s  106  fixed onto the radiolucent back plate  91 . With reference to  FIG. 15 , the cross-hair pattern may comprise a single radiopaque BB  106  as the exact center, with four lines of BB&#39;s extending out from the center, along the vertical and horizontal axis of reticle  18 . Radiopaque cross hair marker  88  may also comprise a longer, vertical arrow which points to gravity when the reticle is-properly mounted and the C-arm is in the lateral position. Although the radiopaque marker  88  is described as being formed by metal BB&#39;s fixed in a cross-hair pattern onto a radiolucent case, it can be appreciated that other suitable materials and configurations may be used to produce the same radiopaque target effect. 
     Laser reticle  18  is preferably mounted to the C-arm  82  with the C-arm in the lateral position, which allows gravity to help correctly position the laser reticle  18  and the corresponding radiopaque cross-hair marker  88 . Laser reticle  18  includes a set of adjustable clamps  92 , a sensor package ( 70 ), and sensor tilt LED indicators  94  to assist in the positioning of laser reticle  18  on to C-arm  82 . As mentioned, a sensor package  70 , similar to sensor package  22  is integrated within the housing of the laser reticle  18 . The sensor package  70  is preferably situated such that it is orthogonal to the reference markers  88 , and when the tilt sensor  70  is perpendicular to the direction of gravity, the sensor registers a zero angle in both the sagittal and transverse planes. The sensor package is communicatively linked to sensor tilt LED indicators  94 , located near the superior edge of the laser reticle  18 , and the center LED indicator will light up when the tilt sensor is perpendicular to the direction of gravity. If the tilt sensor is not perpendicular to the direction of gravity, the sensor tilt LED indicators  94  will prompt the operator to tilt the laser reticle  18  in the direction of the lit LED indicator until the center LED indicator lights up, indicating that the sensor package  70  is perpendicular to the direction of gravity. Once the laser reticle  18  is leveled out in this position, the operator may tighten the adjustable clamps  92  to securely attach the laser reticle  18  on to the C-arm receiver  82 . The use of adjustable clamps  92  allows laser reticle  18  to attach to nearly any C-arm. Laser reticle  18  may be shaped in a generally circular pattern to correspond to the circular shape of the receiver  82 . However, laser reticle  18  may be shaped and dimensioned in any of a number of suitable variations including, but not necessarily limited to, generally rectangular, triangular, ellipsoidal, and polygonal. 
     Laser reticle  18  also includes an adjustable laser cross hair emitter, which may generate a cross-haired target onto the patient&#39;s skin at the surgical access site. Adjustment knobs may be used to adjust the laser emitter along the sagittal and transverse planes. The laser is generated from the center of the laser reticle  18 , and when looking down the axis protruding from the center of the reticle  18  the point of laser generation directly overlaps the center point of the radiopaque cross hair  88 . An advantage of providing an adjustable laser emitter is to allow the operator to point the laser down any desirable path (and thus correct for deformities in the C-arm frame that occur over time). In particular, the operator may adjust the laser emitter so that it propagates directly towards the center of the C-arm signal transmitter  80  and down the central axis  85  of the x-ray beam  84 . Thus, a perfect vector will be created down the central axis  85  of the x-ray beam 85 , which may assist the surgeon in determining a preferred starting point for skin penetration when performing, by way of example only, a pedicle screw placement procedure, as it will precisely mark with the laser cross hair the skin incision site above and in line with the pedicle axis when the C-arm  20  is oriented in the owls eye position When properly adjusted, the laser cross-hairs will be aligned with the radiopaque cross-hairs  88 . It will be appreciated that the perfect vector benefit 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. It is also appreciated that although the laser emitter is described as emitting a cross-haired pattern, the emitted laser may be shaped in any of a number of suitable variations including, but not necessarily limited to, a bulls-eye, or a single point. 
       FIGS. 16-19  illustrate an adjustable laser emitter system  96  of the laser reticle  18  that allows the laser emitter to move both up and down and left and right. The laser emitter system comprises a plastic (and radiolucent) light tube  108  extending to the center of the reticle  18  from an adjuster assembly  110  coupled to backplate  91 . To adjust the laser emitter  96  up and down, the light tube  108  is coupled to a rocker bar  112  forming part of adjuster assembly  110 . A pair of circular apertures (not shown), one in each end of rocker bar  112 , pivotally couple the rocker bar to circular set screws (also not shown) extending inward from anchors  114 . A vertical adjustment knob  116  translates a roller  118  along an incline ramp  120  extending from the rocker bar  112 . As the roller  116  engages the incline ramp  120  the rocker bar  112  pivots around the set screws coupling the rocker bar to anchors  114  and the light tube  108  moves along a vertical plane. A tensioned spring  122  causes the rocker bar  112  to return to its natural position when the roller  118  is translated back down the ramp  120 . A laser emitter situated in rocker bar  112  and coaxial with the light tube  108  directs laser light through the light tube  108 . An angled plastic mirror  124  at the distal end of the light tube  108  redirects the laser light though a hole  126  in the tube. Overlying hole  126  is a defractive optical element  128  (DOE). In a preferred embodiment, the DOE  128  is square shaped such that the laser light exits the DOE  128  in two perpendicular lines, forming a cross-hairs on the laser target. To adjust the laser emitter  96  laterally, the light tube  108  is rotated about its longitudinal axis. To accomplish this, a lateral adjustment knob  130  turns a gear  132  coupled to a complementary gear  134  associated with the proximal end of the light tube  108 . In a preferred embodiment, the laser reticle  18  is equipped with an internal power source to power the laser and the LED indicators  94 . According to one embodiment, the internal power source is a disposable battery  136 . 
     Having described the various components of the surgical trajectory monitoring system  10 , exemplary methods for utilizing the system  10  during surgery will now be described. By way of example, the system  10  is described herein for use in guided formation of one or more pedicle screw pilot holes for safe and reproducible pedicle screw implantation. It will be appreciated however, that the surgical trajectory monitoring system  10  may be used during any of a number of surgical procedures without deviating from the scope of this invention. In accordance with a first general aspect of the present invention, the surgical trajectory monitoring system may be used to orient and maintain surgical instrument  14  along a desired trajectory, for example, during pilot hole formation. The distal end of surgical instrument  14  may first be placed on the pedicle target site in the zero-angle position. The instrument  14  is rotated to the desired reference position, preferably with the longitudinal axis  26  of sensor clip  12  in line with the longitudinal axis of the spine. The surgical instrument  14  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  14  may then be angulated in the transverse plane until the proper medial-lateral angle is attained. Control unit  16  and/or secondary feedback system  52  will indicate to the user when the instrument  14  is aligned with the desired angles. Once the angular orientation of the instrument is correct, the instrument  14  may be advanced into the pedicle to form the pilot hole. The instrument  14  may be rotated back and forth to assist in the formation of the pilot hole. To keep the proper trajectory throughout formation, the instrument  14  may occasionally be realigned with the longitudinal axis  26  of the sensor clip  26  in line with 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  14  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. Methods for determining a starting point with the aid of surgical trajectory system  10  are described below. 
     Upon safely reaching the pedicle target site, the surgical instrument  14  is manipulated into the desired angular trajectory. By way of example the pedicle axis, defined by a medial-lateral angle A 1  (illustrated in  FIG. 20 ) and a cranial-caudal angle A 2  (illustrated in  FIG. 21 ), may be determined and the pedicle screw and/or related instruments may be advanced through the pedicle along the desired trajectory.  FIGS. 22-23  illustrate one exemplary method for manually 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. 22 , 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. Preferably the angles may be input into control unit  16  of system  10  during, as will be described below, for easy retrieval and application later. 
     As shown in  FIG. 23 , the cranial-caudal angle A 2  may be determined using an intraoperative lateral fluoroscopy image from C-arm  20 . A vertical reference line is preferably captured in the lateral fluoroscopy image to ensure measurements are performed with respect to the direction of gravity. In a preferred embodiment, this is accomplished through the use of laser reticle  18 . The vertical reference line is important as the 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. 
     Once the desired trajectory angles are determined for the necessary pedicles, pilot holes may be formed and screws inserted using the tilt sensor  12  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 . 
     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 and for directing pedicle hole formation along the pedicle axis, utilizing various features of the trajectory monitoring system  10 . It will be appreciated that various other methods may be utilized to carryout guided pedicle screw pilot hole formation in accordance with the various components of the present invention. By way of example only, various features, components, methods and/or techniques that may be used with the surgical trajectory monitoring system  10  are shown and described within the PCT Patent App. No. PCT/2007/011962, entitled “Surgical Trajectory Monitoring System and Related Methods,” filed May 17, 2007, the entire contents of which are hereby incorporated by reference as if set forth fully herein. 
       FIGS. 24-25  illustrate, by way of an example only, one embodiment of screen display  500  of control unit  16  capable of receiving input from a user in addition to communicating feedback information 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 (GUI) is utilized to enter data directly from the screen display. In a surgical procedure of pedicle screw placement, for example, it is advantageous to determine and record the medial-lateral (A 1 ) and cranial-caudal (A 2 ) angle of each pedicle at the different levels of the spinal, i.e. A 1  and A 2  of L 1 , A 1  and A 2  of L 2 , etc. The GUI of display  500  allows the user to enter the predetermined A 1  and A 2  angles of each spinal level and save this information into surgical system  10 . By saving such information, the system  10  may advantageously recall the predetermined angles (A 1  and A 2 ) for each spinal level at any given time. It is appreciated that the current integrated control system may also be utilized to determine the pedicle access angles (A 1  and A 2 ). This process is described below. It is also appreciated that in addition to its uses in pedicle screw placement, the current embodiment may be 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 
     With reference to  FIGS. 24-25 , measurements obtained for a pre-defined medial-lateral (M-L) angle A 1  may be entered into input boxes  504  and  506  for (for left and right pedicles, respectively). Multiple adjustment buttons may be used to set the pre-defined angles.  FIG. 24  illustrates a method, by way of example only, of adjusting the left and right M-L angles A 1  by using the angle adjustment button sets  503 .  FIG. 25  illustrates another method, by way of example only, of increasing or decreasing the M-L angles in increments of 10° using the angle adjustment buttons  505  labeled (by way of example only) “+10” and “−10”. More precise angle adjustments may be made by increasing or decreasing the pre-defined angle in increments of 1° using the angle adjustment buttons  507  labeled (byway of example only) “+1” and “−1”. Measurements obtained for the cranial-caudal (C-C) angle A 2  may also be entered into input box  510  and adjusted using the angle adjustment button set  511 . Level selection menu  508  allows the user to input the predetermined angle A 1  and A 2  for each spinal level. The entered values may be saved by the system such that during the procedure selecting the spinal level from level selection menu  508  automatically recalls the inputted values. 
     Control unit  16  display screen  500  may provide feedback information from multiple tilt sensors  12  associated with different devices (e.g. instruments  14 , C-arm  20 , laser reticle  18 , etc.). By displaying feedback information from multiple devices, the information may be used in conjunction with each other to assist a surgeon in safely performing complicated surgical procedures (e.g. pedicle screw implantation, etc.). It is appreciated that further advantages may be gained by combining the tilt sensor data with other relevant data (e.g. neurophysiologic monitoring data, fluoroscopic images, etc.) to provide an integrated system and/or methods for assisting in the performance of the surgical procedure. With reference to  FIGS. 24-25 , display screen  500  provides a C-arm angle window  512  containing-data pertaining to a second tilt sensor  12  positioned on a fluoroscopic imager. By way of example, numeral boxes  514  and  516  display the numeric values of the medial-lateral and the cranial-caudal angles as determined by the C-arm tilt sensor  12 . Numeric values  514  and  516  may be referenced by the user to help match the M-L and C-C values corresponding to the C-arm sensor within an accepted range of the pre-defined target angles A 1  and A 2  for each spinal level. If the C-arm is aligned with the pedicle axis (placed in the owls eye view) the C-arm values A 1 ( c ) and A 2 ( c ), indicated in windows  514  and  516 , should approximate the pedicle axis angles A 1  and A 2 . In another example, the C-arm window  512 , or a portion there of (such as the circle  518 ) may be saturated with the color green when the numeric values corresponding to the C-arm sensor matches within an accepted range of the predetermined target angles. 
     Display screen  500  may also provide feedback information from another tilt sensor  12  coupled with surgical instrument  14 . With reference to  FIGS. 24-25 , by way of example only, the angular orientation of instrument  14  may be communicated to the user in the instrument window  522 . Instrument window  522  may employ different embodiments to assist the user in matching the angular orientation of the instrument  14  to the predefined target angles for each level. With reference to  FIG. 24 , the control system display  500  employs a color coded target to provide feedback information of the angular orientation of surgical instrument  14 . 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 predetermined target angles, the center circle may be saturated green, indicating that both the medial-lateral angle A 1  and cranial-caudal angle A 2  have been matched, or A 1 =A 1 ( i ) and A 2 =A 2 ( i ). If the user wishes to match the angular orientation of the instrument  14  to the angular orientation of the C-arm, the user may make that selection in the “match instrument to” window  509 . When instrument  14  is matched to the C-arm (A 1 ( c )=A 1 ( i ) and A 2 ( c )=A 2 ( i )) the center circle  528  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 too 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 is aligned too 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 has matched one of the targeted angles but not the other, only the quadrant corresponding to the misaligned angle will be saturated. 
     In another embodiment of instrument window  522 ,  FIG. 25  employs a color coded display, approximating the look of a bubble level, to provide feedback of the angular orientation of the surgical instrument  14 . 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. When the instrument is within the range indicating proper alignment, the ring  538  may be saturated green. Similar to the embodiment of  FIG. 30 , the user may also have the option to match the angular orientation of the instrument  14  to 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 “match instrument to” window  509 . A status bar  520  may be provided to indicate the relative status of both the instrument  14  and C-arm tilt sensors. By way of example only, the status bar  520  depicted in  FIGS. 24 and 25  indicate that both the instrument  14  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. 26-36  illustrate, by way of example only, another embodiment of screen display  600  of an integrated control unit  16 .  FIGS. 26-36  illustrate multiple screen displays of an example embodiment of a “Pedicle Cannulation Assist” (PCA) program designed to integrate data from multiple sources. The PCA program may be utilized with an embodiment of the feedback device  16  comprising a computer or similar type processing unit (not shown) capable of receiving input from a user as well as communicating feedback to the user. In similar fashion to the display screen  500 , this example utilizes (though it is not necessary) a graphical user interface (GUI) to enter data directly on the screen displays. The exemplary screen display  600  represents a setup screen from which the user may select the desired technique (e.g. “owls eye” or “A/P&amp; Lateral”—described below) to be performed, as well as various configurations utilized within the technique (e.g. integration of live fluoroscopic images, orientation of the C-arm, etc.). Screen display  600  includes a header  602  that identifies the program and indicates the current configuration as selected by the user (e.g. Owls eye technique with integrated live fluoroscopy as depicted in  FIG. 26 ). Buttons in the technique field  604  may be used to select the desired technique to be applied. By way of example, the “Owl&#39;s Eye” button  606  may be touched to select the Owls Eye technique (described below) and the “A/P &amp; Lateral button” may be touched to select the A/P &amp; Lateral technique. In the imaging field  610 , buttons  612  and  614  may be touched to select between the options of integrating live fluoroscopic images or proceeding without integrated images, respectively. In the orientation field  616 , the user may set the orientation of the C-arm (i.e. whether the C-arm is positioned on the right or left side of the patient) that is to be utilized during the procedure. By way of example, the user may simply touch the C-arm depiction  618  to toggle from one orientation option to the next. The start button  620  locks in the selected configuration and advances the program. 
     In this embodiment, the feedback device  16  utilizes an image capture system (not shown) preferably incorporated within the hardware and/or software in order to retrieve images from the C-arm. When the live fluoroscopic image option is selected display screen  600  may be advance to a format viewing window to format the image (if necessary), as shown in  FIG. 27 . The instruction field  620  provides instructions for formatting the image into the appropriate size and/or alignment. Upon selecting the image feed button  622 , the fluoroscopic image  630  is retrieved and displayed in the viewing window  624  located in the image field  626 . As indicated by the instructions in the instruction field  620 , the image may be resized by, for example only, touching and dragging the bottom right corner of the viewing window  624 . The image may be aligned by touching (by way of example only) the top left corner of the viewing window  624  and dragging it until the image is aligned. Button sets  628  and  629  may be provided and utilized as alternative ways to align and resize the image  630 , respectively. The proceed button  632  locks in the viewing window  624  formatting and advances the program. 
     A “virtual protractor” display screen is illustrated in  FIGS. 28-30 . The virtual protractor screen may be utilized to input and/or determine the angles to be used during pilot hole formation (i.e. the cranial-caudal and medial-lateral angles discussed elsewhere herein). Data management field  634  may be used to view and input angle data in the integrated screen. The data management field includes an M/L window  636 , a C/C window  638 , and spinal level buttons  640 . Spinal level buttons  640  may be used to select and indicate the spinal level which corresponds to the data being input or displayed in the M/L and C/C windows  636  and  638  (e.g. level L 5  in  FIGS. 28 and 29 , level L 4  in  FIG. 30 ). As previously described, the medial-lateral angles for each pedicle to be instrumented are preferably determined preoperatively. The data may be taken to the OR and entered using the M/L window  636 . To enter the data, the proper spinal level is selected and the edit M/L angles button  644  is selected. As shown in  FIG. 29 , a keypad  646  appears in the data management field  634  and the angles may be entered and saved (or cleared and reentered) for the left and right pedicles. Toggling between the left and right pedicles may be done by selecting the appropriate buttons labeled, by way of example only, “left”  645  and “right”  647 . This may be done in turn for each applicable pedicle. Alternatively, the data may be input into the system prior to surgery or entered onto an external memory device (e.g. memory cord, USB flash drive, etc.) and transferred to the system in the OR in order to reduce the overall surgical time. 
     The cranial-caudal angles for each pedicle to be instrumented may be determined using the virtual protractor  648  superimposed on the fluoroscopic image  630 . To accomplish this, the C-arm is oriented in the lateral position such that the image  630  shown on the screen is a lateral image. A zero line  650  may be rotated into alignment with the vertical reference line generated in the fluoroscopic image (as previously described) by selecting (e.g. touching) and dragging it into position. The center point  649  of the virtual protractor  648  may then be centered over the appropriate pedicle by touching the image at the desired position. The protractor  648  will then position itself, centered on the position touched. Once positioned over the center of the pedicle, the virtual protractor may be rotated using the control bar  652  until it is aligned with the axis of the pedicle. Selecting the capture C/C button  637  will automatically input and save the angle into the integrated system as determined by the rotation of the virtual protractor  648  relative to the zero line  650 . With reference to  FIG. 37 , the user may also enter the C/C angle manually by selecting the edit button  639  in the C/C window  638 . After selecting the edit button  639 , a C/C keypad  654  appears and the user may select the appropriate “to foot” or “to head” button to finalize the angle input for the selected level. The C/C angles may be determined and entered for each applicable spinal level. The proceed button  632  will advance the program into the appropriate technique screen. 
     By way of example only,  FIGS. 31-34  illustrate a main screen display for the owls eye technique according to one exemplary embodiment. An indicator  656  shows the relative orientation of the C-arm  20 , either AP view or lateral view. The indicator  656  does not necessarily correspond to the true AP or true lateral orientations but is rather just a general indication. For example, as shown here the C-arm is oriented in the owl&#39;s eye position which is not a true AP view but is generally closer to a true AP view than a true lateral view. If the C-arm is rotated past a certain point, by way of example, 60 degrees, the indicator will change to indicate the opposite view (e.g. lateral). Selecting the option button  658  expands an option menu  659 , illustrated in  FIGS. 32-33 , which may include but is not necessarily limited to, a show or hide angle button  660 , a zoom button  662 , and a hide button  664 . The show or hide angle button  660  either opens or closes an instrument angle window  668  and C-arm angle window  670  ( FIGS. 33 and 34 ). The zoom button  662  zooms in on the fluoroscopic image  630 . The hide button  664  contracts the options menu  659 . A data management field  661  illustrates the selected spinal level and the cranial-caudal and medial-lateral angels previously input for the selected level. The angles may be edited in the data management field  661  via controls similar to those previously described with reference to virtual protractor screen of  FIGS. 28-30 . Instrument and C-arm target indicators,  672  and  678  respectively, are positioned opposite each other around the fluoroscopic image  630 . By way of example only,  FIG. 34  illustrates the main screen display  600  for the owls eye technique when the live fluoroscopy option is not selected. The display in  FIG. 34  is generally the same except that the fluoroscopic image  630  is replaced by a graphic representing the patient. 
     The instrument target indicator  672  includes a medial-lateral bar  676  and a cranial-caudal bar  674 . Individual segments of the target indicator  672  may be colored to represent the position of the instrument and relative to the previously determined target angles (displayed in the data management window  661 ). The indicator bar  672  may, for example, be shown generally as neutral color (e.g. gray). A single segment on each of the medial-lateral bar  676  and cranial-caudal bar  674  may be highlighted by a color (e.g. green) to indicate the relative position instrument to the target angle. By way of example, the closer the lighted segment is to the target circle, the closer the instrument is to being aligned with the corresponding predetermined angle. The size of the individual segments may be different and correspond to the range of values encompassed by the segment. By way of example only, the larger segments situated farthest from the target circles correspond to larger ranges. In one example, set forth by way of example only, the target circle has a range of 3 such that the cranial-caudal target circle will be highlighted when the instrument is aligned within 3° of the corresponding cranial-caudal target angle and the medial-lateral target circle will be highlighted when the instrument is aligned within 3° of the predetermined medial-lateral angle. In one embodiment, the entire medial-lateral bar  676  is highlighted in the appropriate color (e.g. green) when the instrument is aligned within the range of the target circle (e.g.  3  in this example). Similarly, the entire cranial-caudal bar  674  is highlighted in the appropriate color (e.g. green) when the instrument is aligned within the range of the target circle (e.g. again 3° in this example). Thus, when the instrument is aligned within 3° of the target medial-lateral angle and 3° of the target cranial-caudal angle, the entire instrument target indicator  672  may be highlighted in the appropriate color (e.g. green in this example). 
     In another method, the user may also match the angular orientation of instrument  14  to a predefined angular orientation is illustrated using the “ball and stick” target indicator  684  of changing length, illustrated in  FIG. 33 . The length and position of the ball and stick will indicate to the user the desired orientation of surgical instrument  14  in reference to a predefined angular orientation. As illustrated in  FIG. 33 , one end of the stick is positioned in the center of fluoroscopic image  630  and the other end extends outwards from the center into the top-left quadrant. This illustration indicates to the user that the orientation of instrument  14  is not matched up with the predefined angular orientation. Specifically, indicator stick  684  in  FIG. 33  specifies to the user that the angular orientation of instrument  14  is too far right in the M-L direction and too far towards the head of the patient in the cranial-caudal direction. By way of example only, the user will adjust instrument  14  in accordance to the position of the indicator stick. As the user adjusts the angular orientation of instrument  14  towards the desired angles, the stick indicator will shorten in length. Once the desired angular orientation is found, fluoroscopic image  630  may produce an image of a single dot at the center of the image. In another example, the entire fluoroscopic image  630 , or a portion thereof, may be saturated with the color green angular values corresponding to the instrument sensor matches within an accepted range of the predetermined target angles. It is appreciated that any suitable combination of the methods described, whether alone or in combination with another, may be used to indicate to the user the angular orientation of instrument  14  in reference to predefined angles. 
     Like the instrument target indicator  672 , the C-arm target indicator  678  includes a medial-lateral bar  680  and a cranial-caudal bar  924 . Individual segments of the target indicator  678  may be colored to represent the orientation of the C-arm relative to the previously determined target angles (displayed in the data management window  661 ). The C-arm target indicator  678  may, for example, be shown generally as neutral color (e.g. gray). A single segment on each of the medial-lateral bar  680  and cranial-caudal bar  682  may be highlighted by a color (e.g. purple) to indicate the relative position C-arm to the target angles. By way of example, the closer the lighted segment is to the target circle, the closer the C-arm is to being aligned with the corresponding predetermined angle. The size of the individual segments may be different and correspond to the range of values encompassed by the segment. By way of example only, the large segments situated farthest from the target circles correspond to larger ranges. In one example, set forth by way of example only, the target circle has a range of 3° such that the cranial-caudal target circle will be highlighted when the C-arm is aligned within 3° of the corresponding cranial-caudal target angle and the medial-lateral target circle will be highlighted when the C-arm is aligned within 3° of the predetermined target medial-lateral angle. In one embodiment, the entire medial-lateral bar  680  is highlighted in the appropriate color (e.g. purple) when the C-arm is aligned within the range of the target circle (e.g.  3  in this example). Similarly, the entire cranial-caudal bar  682  is highlighted in the appropriate color (e.g. purple) when the instrument is aligned within the range of the target circle (e.g. again 3° in this example). Thus, when the instrument is aligned within 3° of the target medial-lateral angle and 3° of the target cranial-caudal angle, the C-arm target indicator  678  may be highlighted in the appropriate color (e.g. purple in this example). 
     In use, the C-arm is easily oriented into the owls eye position using the C-arm target indicator  678  as a guide. Again, when the owls eye position is reached, both the medial-later bar  680  and cranial-caudal bar  682  will fully highlighted in the appropriate color (e.g. purple). Once the C-arm is in the owls eye position, the starting point for instrument insertion may be determined according to the owls eye method for starting point determination previously described above. If the live fluoroscopy option is not chosen, the starting point may be determined using the fluoroscopic image monitor  216  as previously described. When the instrument is positioned on the desired starting point, the instrument may be aligned with the pedicle axis by adjusting the instrument until both the medial-lateral indicator bar  676  and the cranial-caudal indicator bar  674  of the instrument target indicator are fully highlighted, indicating that the instrument is aligned with the target angles which preferably correspond to the pedicle axis. When the target indicator  672  shows correct alignment, the instrument may be advanced into and through the pedicle into the vertebral body. The process may be repeated for each pedicle to be instrumented. 
     With reference now to  FIGS. 41-42 , there is shown, by way of example only, main screens display for the A/P &amp; lateral technique option, respectively. The A/P &amp; lateral technique main display is generally similar to the owls eye main display above. The A/P &amp; lateral technique does not utilize predetermined target angles and thus the data management field from the owl&#39;s eye main display is replaced with a data capture window  686 . Selecting the capture instrument trajectory button  688  locks in the medial-lateral and cranial-caudal angles associated with the position of the instrument when the button is selected. Thereafter, the instrument target indicator  690  functions as described above with the “captured” angles filling the role of the predetermined target angles. Thus, the surgeon is free to determine a desired trajectory through any desired means. The instrument target indicator  690  will assist the surgeon in maintaining the selected trajectory thereafter. If the C-arm is rotated into the lateral orientation, as depicted in  FIG. 42 , the cranial-caudal bar  692  of the instrument indicator  690  disappears and a protractor  648  is superimposed on the fluoroscopic image  630 . The cranial-caudal orientation of the instrument is thereafter depicted via rotation of the protractor  648 . 
     One example method for using the exemplary A/P &amp; lateral main display uses predetermined medial-lateral angles as described previously. The M/L angles are recorded prior to surgery and brought to the OR for reference. The C-arm may be oriented in the lateral view position and the protractor  648  aligned with the axis of the pedicle. In this position the instrument is aligned in the proper cranial-caudal position. Maintaining the cranial-caudal position, the instrument may be adjusted until the instrument angle window  668  indicates that the instrument is aligned with the predetermined medial-lateral angle. Once in this position the capture instrument trajectory button  688  may be selected. Thereafter, the instrument may be advanced into and through the pedicle using the instrument target indicator  690  to maintain the trajectory. This may be repeated for each pedicle to be instrumented. 
       FIGS. 37-46  illustrate, by way of example only, yet another embodiment of screen display  700  of an display screen system capable of receiving input from a user in addition to communication feedback from multiple sources (e.g. instrument  15 , laser reticle  18 , C-arm  20 , etc.). In similar fashion to the display screen  500  and  600 , this example utilizes (though it is not necessary) a graphical user interface (GUI) to enter data directly on the screen displays. Screen display  700  includes a header  702  that identifies the program and indicates the current configuration as selected by the user (e.g. Navigated Guidance as illustrated in  FIG. 37 ). Display screen  700  also consists, by way of example, test menu bar  704 . From menu bar  704 , the user may select and/or change multiple options of the selected configuration. Test menu bar  706 , by way of example only, may open up a menu bar (not shown), from which multiple neurophysiologic test may be incorporated. In this setup screen, the user may select a predetermined reference angle (e.g. using pre-defined angles as a reference when implementing the navigated guidance function of the current system) by pressing reference button  708  and selecting a reference option from reference menu  710 . The user may also adjust the image screen of the display by selecting imaging button  712 . 
       FIG. 38  illustrates the proceeding screen display from selecting imaging button  712 . In the imaging field  714 , buttons  716  and  718  may be touched to select between the options of integrating live fluoroscopic images from the C-arm or proceeding without integrated images, respectively. In the imaging controls field, the user may set the orientation of the image by pressing the flip and rotate button sets,  722  and  724 , respectively. The user may also adjust the brightness and contrasting settings of the image by selecting the appropriate buttons. Display screen also consists of an instrument menu bar  730 , capable of allowing the user to make multiple adjustments to multiple integrated feedback instruments. From instrument menu bar  730 , the user may set the orientation of the C-arm (i.e. whether the C-arm is positioned on the right or left side of the patient) that is to be utilized during the procedure. The user may select C-arm button  732  labeled, by way of example only, “Reticle”.  FIG. 39  illustrates the screen display that follows the selection of button  732 . In the C-arm orientation field  734  the user may select the desired C-arm orientation by selecting one of the directional buttons  736 . C-arm orientation field  734  may also include an anatomical diagram  738  of a patient to assist the user in selecting the C-arm orientation. Although it is not described, it is appreciated that adjustments may be made for other communicatively linked instruments that may be selected from instrument menu bar  730 . The accept button  740  locks in the selected configuration and advances the program. The user may then choose to input the predefined M/L and C/C angles into the system by selecting the level button  742  on menu bar  704 . 
       FIG. 40  illustrates, by way of example only, the advancing screen from the selection of level button  742 .  FIG. 40  also illustrates the user&#39;s option of hiding the instrument menu bar  730  by pressing menu hide button  741 . From this screen the user may input the M/L angles (A 1 ) and C/C angle (A 2 ) for each pedicle level in the data management field  744 . Data management field  744  may be used to view and input angle data in the integrated screen. The data management field includes an M/L window  746 , a C/C window  748 , and spinal level buttons  750 . Spinal level buttons  750  may be used to select and indicate the spinal level which corresponds to the data being input or displayed in the M/L and C/C windows  746  and  748 . As previously described, the medial-lateral angles for each pedicle to be instrumented are preferably determined preoperatively. The data may be taken to the OR and entered using the M/L window  746 . From the M/L window  746 , the user may input the predefined M/L angles by increasing or decreasing the right or left M-L angles in increments of 10° using the angle adjustment buttons  752  labeled (by way of example only) “+10” and “−10”. More precise angle adjustments may be made by increasing or decreasing the pre-defined angle in increments of 1° using the angle adjustment buttons  754  labeled (by way of example only) “+1” and “−1”. Measurements obtained for the pre-defined cranial-caudal (C-C) angle A 2  may also be entered into C/C window  748 . By way of example only, pre-defined C/C angle A 2  may be entered though either the virtual protractor function (described in more detail below) or angle A 2  may be entered manually. The user may select either of these functions by pressing the “Virtual Protractor” button  756  and the “Edit Manually” button  758 , respectively. 
       FIGS. 41-42  illustrate, by way of example only, the subsequent virtual protractor onscreen display of the system when the user selects the “Virtual Protractor” button  756 . In this screen, the user is given another opportunity to make additional adjustments to the image of the fluoroscopic image from the imaging controls field  760 . It is appreciate that throughout the program, the user may make many adjustments to the system (e.g. adjust the fluoroscopic image, change the reference angles, make adjustments to instrument controls, etc.). The chief purpose of this integrated screen display is to determine the angles to be used during a surgical procedure, such as pilot hole formation (i.e. the cranial-caudal and medial-lateral angles discussed elsewhere herein). Spinal level buttons  750  may be selected to input the C/C angle for each level. The C/C angles for each pedicle to be instrumented may be determined using the virtual protractor  762  superimposed on the fluoroscopic image  770 . To accomplish this, the C-arm is oriented in the lateral position such that the image  770  shown on the screen is a lateral image. A zero line  764  may be rotated into alignment with the vertical reference line generated in the fluoroscopic image (as previously described) by selecting (e.g. touching) and dragging it into position. The center point  766  of the virtual protractor  762  may then be centered over the appropriate pedicle by touching the image at the desired position. The protractor  762  will then position itself, centered on the position touched. Once positioned over the center of the pedicle, the virtual protractor may be rotated using the control button  768  until it is aligned with the axis of the pedicle. Selecting the save image button  772  will input the angle determined by the rotation of the virtual protractor  762  relative to the zero line  764 . With reference to  FIG. 42 , the user may choose to give the image a file name in save field  774 . Virtual protractor screen may also consist of head and foot diagrams  776  to assist the user in understanding the orientation of the patient. The determined C/C angle may be displayed in C/C angle display window  778 . Return button  780  brings the user back to the screen display illustrated in  FIG. 40 . 
       FIG. 43  illustrates, by way of example only, the subsequent onscreen display of the system when the user selects the “Edit Manually” button  758  of  FIG. 40 . In this example, similar to the process of adjusting the pre-defined M-L angles A 1  above, the pre-defined C-C angle A 2  may be increased or decreased in increments of 10° and 1° by pressing the angle adjustment buttons,  782  and  784 , accordingly. Furthermore, the direction of the pre-defined C-C angle A 2  may be entered by pressing the cephalad (towards the head) button  786  and caudal (towards the feet)  788 . The user may also manually input the C/C angle at each spinal level by selecting one of the appropriate spinal level buttons  750 . By pressing the save button  772 , the entered values may be saved by the system such that during the procedure selecting the spinal level from “Reference” menu  517  automatically recalls the inputted values. 
       FIGS. 44-46  illustrate, by way of example only, onscreen displays of the system with feedback information from multiple sources. Viewing window  790  may capture a fluoroscopic image from the C-arm, as illustrated in  FIG. 44 , to assist the operator in determining the fixed angles of the pedicle in pedicle placement procedures. However, the user may choose not to display a fluoroscopic image, and instead utilize a graphic in its replacement, as illustrated in  FIGS. 45-46 . Viewing window  790  utilizes a cross-hair reference  792  (not visible in  FIG. 44 ) to indicate the center of the image from the C-arm. Center reference  792  may assist the user in procedures which require the surgeon to operate along a desired angular trajectory to the spine. Display  700 , in this embodiment, also consists of other feedback information windows to assist the surgeon during operation. Instrument window  794  provides feedback information to the user of the angular orientation of the attach instrument  14 . When the angular orientation of the instrument is in accordance with the predefined angular orientation, the system may alert the user of the match. By way of example only, instrument window  794  may be saturated with a color (e.g. green) to indicate the proper alignment of the instrument. Instrument window  794  may also provide alphanumeric feedback. When the angular orientation of the instrument is properly aligned with the predefined angles, the M/L angles and the C/C angles with match accordingly (A 1 ( i )=A 1 ). Menu bar  704  may also provide the predefined reference angles for each level to compare with the feedback information of the various instruments. C-arm window  796  may also communicate feedback information to the user. In similar fashion to instrument window  794 , feedback information from the angular orientation of the C-arm may be utilized.  FIGS. 44 and 45  illustrate an additional C-arm window  798  depicted the orientation of the C-arm as in relation to the patient. 
       FIG. 46  illustrates, by way of example only, the onscreen display  700  of a display screen system when running neurophysiologic test. Neurophysiologic button  797  allows the user to run neurophysiologic test. By way of example only, the user may select to run a dynamic stimulated EMG test while continuing to run the navigated guidance features of the current system with feedback information from multiple sources. Stop button  799  allows the user to stop stimulation when running a test.  FIG. 46  also illustrates the integrated system&#39;s ability to recall saved recordings from the history menu  795   
     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 systems 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  14  from the pedicle target site. By way of example, the surgical trajectory monitoring system  10  may be used in conjunction with the neuromonitoring system  400 , illustrated by way of example only in  FIG. 47 . A neuromonitoring system is shown and described in the commonly owned and co-pending U.S. patent application Ser. No. 12/080,630, entitled “Neurophysiology Monitoring System,” and filed on Apr. 3, 2008 the entire contents of which is hereby incorporated by reference as if set forth fully herein. Neuromonitoring system  400  may perform, by way of example, 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. Functionality of neuromonitoring system  400  has been described in detail elsewhere and will be described only briefly herein. The Twitch Test mode is designed to assess the neuromuscular pathway via the so-called “train-of-four” 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 PCT Patent Application 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 PCT Patent Application No. PCT/US2002/035047 entitled “System and Methods for Performing Percutaneous Pedicle Integrity Assessments,” filed on Oct. 30, 2002, and PCT Patent Application No. 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  400 , including the k-wire  427 , dilator  430 , cannula  431 , retractor assembly  432 . This mode is described in greater detail within PCT Patent Application No. PCT/US2002/022247, 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 PCT Patent Application No. PCT/JS2002/030617, 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. The MEP Auto and MEP Manual modes are designed to test the motor pathway to detect potential damage to the spinal cord by stimulating the motor cortex in the brain and recording the resulting EMG response of various muscles in the upper and lower extremities. The SSEP function is designed to test the sensory pathway to detect potential damage to the spinal cord by stimulating peripheral nerves inferior to the target spinal level and recording the action potential from sensors superior to the spinal level. The MEP Auto, MEP manual, and SSEP modes are described in greater detail within PCT Patent Application No. PCT/JS2006/003966, entitled “System and Methods for Performing Neurophysiologic Assessments During Spine Surgery,” filed on Feb. 2, 2006, the entire contents of which is hereby incorporated by reference as if set forth fully herein. 
     With reference to  FIG. 47 , 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 . To perform the neurophysiologic monitoring, the surgical instrument  14  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  30  may be formed of material capable of conducting the electric signal. To prevent shunting of the stimulation signal, the probe member  30  may be insulated. 
     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 the distal end. 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 is to a nerve the lower the stimulation threshold will be. Thus, as the probe member or surgical access members 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  14  may be connected to the neurophysiology system  400  by through sensor clip  12 . By way of example and with reference to  FIG. 2-3 , an additional cable  47  may couple the clip  12  to the neurophysiology system  400 . Attached to the cable  47 , inside the endhook  48 , is an exposed wire  49  that contact the exposed proximal portion  40  of instrument  14 . 
     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 the electrode, 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, the neurophysiology system  400  control unit and the surgical trajectory system  10  control unit  16  may be integrated into a single unit. Neurophysiology monitoring and trajectory monitoring may be carried out concurrently and the control unit may display results for each of the trajectory monitoring function and any of the variety of neurophysiology monitoring functions. Alternatively, the control unit  16  and control unit  402  may comprise separate systems and the sensor clip  12  may be communicatively linked directly to control unit  402  of the neurophysiology monitoring system and the control unit  16 . 
     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.