Abstract:
An adjustable awl for forming a hole for the insertion of a screw or other device in a pedicle or other body part. The awl comprises an elongated housing having an open end, and an elongated awl member movably mounted in the housing and being extendable beyond the open end to vary the length of the awl. The awl comprises means for locking the awl member in a selected position relative to the housing. The awl member is provided with markings thereon to indicate its position relative to the housing.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a divisional application of U.S. patent application Ser. No. 11/060,582 filed Feb. 18, 2005 now U.S. Pat. No. 7,235,076, the entirety of which application is incorporated by reference. 
     This application claims the benefit of U.S. Provisional Patent Application No. 60/545,903, filed on Feb. 20, 2004. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the general field of spinal surgery and, more particularly, to a computerized or automated method for the accurate sizing and placement of pedicle screws in spinal surgery. 
     BACKGROUND OF THE INVENTION 
     Placement of screws into the human spine is a common surgical procedure to allow for a multitude of spinal surgeries to be performed. Screws are typically placed into the pedicles of individual vertebra in the lumbar and sacral spine. Given their biomechanical advantages over other modes of fixation, surgeons are expanding the areas of the spine in which pedicle screws are placed. However, adjacent to the spine are numerous vital structures and organs, in particular the cervical and thoracic spine regions, which have very low tolerance for surgically created injuries that may ultimately lead to significant morbidity and/or mortality. For this reason the majority of research focus on placement of pedicle screws is centered on improving accuracy to maintain a screw within a bony (intraosseous) environment. 
     Image guided systems are evolving which are increasingly user friendly to assist a surgeon in accurately placing a screw. The critical parameters for placing a pedicle screw into the human spine are diameter, length, trajectory and then actual placement of the screw. To date many of the image guidance systems allow for manual determination of these parameters to improve a surgeon&#39;s manual performance in screw placement. Up to the present time, no system is available which will automatically determine ideal pedicle screw diameter, length and trajectory for accurate placement of pedicle screws. The present invention provides this capability akin to a pilot who flies an airplane with computer controlled aviation capabilities, and allows for placement of pedicle screws using either an open or percutaneous technique. 
     Patent Application Publication No. US 2004/0240715 A1, published on Dec. 2, 2004, relates to methods and computer systems for determining the placement of pedicle screws in spinal surgery. It discloses a method wherein the minimum pedicle diameter is first established for determining the optimum screw trajectory and then the maximum screw diameter and length using the optimum trajectory for each pedicle. Two dimensional transverse slice data is stacked to form three dimensional data points to determine optimum trajectory by linear least squares solution to fit the data, requiring the solution to go through the overall minimum transverse pedicle widths. A disadvantage of this method is that it allows for eccentric trajectory determination, particularly for distorted pedicle anatomy, with consequent smaller maximum diameter and length screw determinations resulting in biomechanically inferior constructions. In contrast, the new and improved method of the present invention always places the trajectory concentrically through the pedicle by the determination of optimum trajectory by using the center point of the smallest cross sectional area (isthmus) and projecting with a computer a line normal to this circumscribed area in opposite directions, as described more particularly hereinafter. The new and improved methods of the present invention allow for maximum screw diameter and length determinations for intraosseous placement. 
     SUMMARY OF THE INVENTION 
     The present invention utilizes three dimensional images and a computer or similar device to generate a table providing the maximum allowable pedicle screw diameter and length, summary data on trajectory, and also generates a schematic diagram illustrating this data for individual vertebral pedicles. The numerical data can be utilized by the surgeon for actual intraosseous pedicle screw placement by one of the following methods: 1. Manual screw placement by the surgeon&#39;s preferred method; 2. A pedicle base circumference outline method combined with intraoperative fluoroscopy; 3. Automated screw placement; or 4. Any commercially available registration software (e.g., computed tomography/fluoroscopy, etc.) The present invention also allows for extraosseous or extrapedicular pedicle screw placement if a surgeon should so desire based on a trajectory beginning at the same starting point from the anterior cortex but angled tangentially any distance or angle to the surgeon&#39;s desired preference. 
     The invention also facilitates safe and reliable access to any vertebral body through a transpedicular or peripedicular approach, such as during a vertebroplasty, kyphoplasty or vertebral body biopsy. 
     Furthermore, the invention forms a novel research tool for developing smaller or larger diameter or custom sized pedicle screws throughout the spine. 
     One method of the present invention generally comprises the following steps:
         1. A computed tomography scan (CT), magnetic resonance image (MRI), CT capable fluoroscopy or similar two dimensional imaging study of the spine area of interest may first be obtained.   2. A dimensionally true three dimensional computer image of the bony spine is generated from the CT, MRI or other studies, or in any other suitable manner.   3. The computer generated three dimensional individual vertebra are then hollowed out by a computer or other device similar to an eggshell transpedicular vertebral corpectomy to the specifications desired by the surgeon, e.g., thickness of cortical wall remaining in the vertebral body cortices or pedicle walls. The individual vertebra can be visualized as a structure which has been cored or hollowed out and the resulting remaining vertebral body is “electrified” or highlighted throughout its walls.   4. A computer then automatically determines the maximum allowable diameter screw to be placed by determining the narrowest diameter or smallest cross sectional area (isthmus) within any given pedicle based on surgeon pedicle cortical wall diameter preferences.   5. A computer then generates an elongated cylinder by starting at the center of the isthmus as a straight line which determines the ideal trajectory and extends in opposite directions e.g., perpendicular to the plane of the isthmus so that it is positioned concentrically as much as possible within the pedicle without touching the remaining “electrified” or highlighted cortex. This line is allowed to penetrate the dorsal or posterior pedicle cortex so that it can extend beyond the skin of a patient to any desired length. The line terminates inside the vertebral body to within a surgeon&#39;s predetermined distance from the predefined anterior inner cortical wall so that it cannot penetrate it.   6. A computer then builds the line concentrically in radial directions to a final maximum diameter which will not exceed the narrowest defined pedicle diameter based on surgeon preference pedicle cortical wall thickness. This concentric building grows into a visible cylinder which stops building when any point on its outer surface comes into “contact” with the “electrified” or highlighted inner cortical wall. This rule, however, does not apply to the posterior cortex adjacent to the exiting straight trajectory line generated from the isthmus.   7. A computer then determines the length of the screw by measuring the length of the cylinder starting at the predefined anterior inner cortex up to its intersection with the dorsal/posterior cortex. To facilitate the placement of screws in accordance with one of the automated methods described hereinafter, the cylinder may be extended beyond its intersection with the dorsal/posterior cortex.   8. A computer then provides a data summary table which displays the ideal pedicle screw diameter, length and trajectory for each individual vertebra pedicle and an idealized schematic drawing of same.   9. The tabulated data can then be utilized to determine the viability of using pedicle screws based on maximal pedicle screw diameter and length, and also for placement of screws by a surgeon&#39;s preferred method, such as one of the methods described hereinafter.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1   a  and  1   b  are three dimensional computer images of the side and back, respectively, of the bony spine made from CT, MRI or other studies of the spine area of interest; 
         FIG. 2  illustrates three dimensional computer images of individual vertebra undergoing a manual eggshell corpectomy from the spine area shown in  FIGS. 1   a  and  1   b;    
         FIG. 3  is a computer image of a hollowed out individual vertebra showing the narrowest diameter or cross sectional area (isthmus) within the pedicle; 
         FIG. 4  is a computer image view of a hollowed out individual vertebra showing the generation of the straight line through the center of the isthmus and extending in opposite directions through the posterior pedicle cortex and toward the anterior inner cortex; 
         FIG. 5  is a schematic drawing showing the generation of the cylinder by building the line extending through the center of the isthmus concentrically in radial directions; 
         FIGS. 6   a  and  6   b  are schematic images of hollowed out individual vertebra that are of symmetrical and irregular shape, respectively; 
         FIGS. 7   a  and  7   b  are schematic views showing the isthmus of straight and curved pedicles, respectively; 
         FIG. 8  is a schematic view of a hollowed out vertebra showing the length of the cylinder for determining pedicle screw length; 
         FIG. 9  is a schematic side elevational view of the individual vertebra labeled by a surgeon for pedicle screw installation; 
         FIG. 10   a  is a data summary table generated by a computer of maximum pedicle screw diameter and length, and also of the trajectory angle of the pedicle screw with respect to the sagittal and transverse planes; 
         FIG. 10   b  is a schematic side view of a vertebra showing the sagittal plane and the nature of the trajectory angles in  FIG. 10   a ; and 
         FIG. 10   c  is a schematic plan view of a vertebra showing the transverse plane and the nature of the trajectory angles in  FIG. 10   a;    
         FIG. 10   d  is a schematic rear view of a vertebra showing the coronal plane and the nature of the trajectory angles in  FIG. 10   a;    
         FIG. 11  is a computer generated schematic view of the ideal pedicle screw placements as identified in the data summary table of  FIG. 10   a  in AP plane demonstrating coronal trajectory; 
         FIG. 12  is a table of maximum available screw size parameters corresponding to the data in the summary table of  FIG. 10   a  and pedicle base circumference outlines (coronal planes) and pedicle distance points A-B; 
         FIG. 13  is a computer generated schematic view of the screw placements as identified in the table of  FIG. 12 ; 
         FIG. 14   a  is a schematic side elevational view of a vertebra showing the isthmus and the pedicle base circumference; 
         FIG. 14   b  is a schematic plan view of a vertebra showing the computer generated pedicle cylinder extending through the pedicle base circumference in the transverse and coronal planes; 
         FIGS. 14   c ,  14   d  and  14   e  are plan views of vertebra in the lumbar, thoracic and cervical regions, respectively, showing the relationship between the isthmus and the pedicle base circumference in each vertebra; 
         FIGS. 14   f  and  14   g  are schematic rear elevational views of a vertebra showing the positioning of an awl for creating the pedicle screw pilot hole in the vertebra; 
         FIG. 14   h  shows schematic and aligned plan and rear elevational views of a vertebra with a manually determined pedicle screw directional line extending through the center of the pedicle base circumference; 
         FIGS. 15   a ,  15   c  and  15   e  show schematic rear elevational views of a vertebra in different orientations with a computer generated pedicle screw cylinder extending through the pedicle base circumference thereof; 
         FIGS. 15   b ,  15   d  and  15   f  show schematic side elevational views of the vertebra illustrated in  FIGS. 15   a ,  15   c  and  15   e , respectively; 
         FIG. 16  shows CT transaxial views through the center of pedicles T 1 , T 2 , T 4  and T 5  demonstrating pedicle morphology, isthmus and determination of pedicle pilot hole entry points correlating with intraoperative AP fluoroscopic images of each respective vertebra; 
         FIGS. 17   a  and  17   b  are side elevational views of different embodiments of an adjustable awl of the present invention; 
         FIG. 18   a  is a schematic view of an intraoperative AP fluoroscopic image of individual vertebral and pedicle base circumferences; 
         FIG. 18   b  is a schematic view of computer generated three dimensional images of vertebra with computer placed pedicle cylinders and pedicle base circumferences; 
         FIG. 18   c  is a schematic view of the registered images of  FIGS. 18   a  and  18   b;    
         FIG. 19   a  is a schematic side elevational view of a dual ring pedicle screw aligning apparatus constructed in accordance with the present invention; 
         FIG. 19   b  is a front elevational view of the apparatus shown in  FIG. 19   a;    
         FIGS. 19   c  and  19   d  are schematic plan views of a vertebra showing the use of the dual ring pedicle screw aligning apparatus in a percutaneous environment and an open surgical environment, respectively; 
         FIG. 20  is a front elevational view of a modified dual ring pedicle screw aligning apparatus; 
         FIGS. 21   a  and  21   b  are side and front elevational views of the end portion of a first embodiment of a drilling cannula member for the dual ring aligning apparatus shown in  FIGS. 19   a  and  19   b;    
         FIGS. 22   a  and  22   b  are side and front elevational views of the end portion of a second embodiment of a drilling cannula member for the dual ring aligning apparatus shown in  FIGS. 19   a  and  19   b;    
         FIG. 23   a  is a perspective view of a slotted outer cannula for use with the dual ring aligning apparatus of  FIGS. 19   a  and  19   b;    
         FIG. 23   b  is a front elevational view of the slotted cannula shown in  FIG. 23   a  with an aligning ring disposed therein; 
         FIG. 24  is a schematic view of a hollowed out vertebra showing different pedicle screw trajectories in a centered or ideal trajectory and an extraosseous or extrapedicular trajectory that is offset tangentially from the centered trajectory; and 
         FIG. 25  is a schematic plan view of a vertebra showing the installation of a pedicle screw in accordance with the method of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The methods of determining pedicle screw size and placement in accordance with the present invention are set forth in more detail hereinafter. 
     Step 1 
     A computed tomography scan (CT), magnetic resonance image (MRI), CT capable fluoroscopy or similar two-dimensional imaging study of the spine area of interest may first be obtained. Thin cut sections are preferable to increase accuracy and detail. 
     Step 2 
     A dimensionally true three dimensional computer image of the bony spine is made from the CT, MRI or other studies or in any other suitable manner, as shown in  FIGS. 1   a  and  1   b.    
     Step 3 
     The three dimensional individual vertebra as shown in  FIG. 2  are then hollowed out by a computer, similar to an eggshell transpedicular vertebral corpectomy, to the specifications desired by the surgeon (i.e., thickness of cortical wall remaining in the vertebral body cortices or pedicle walls). These specifications allow for asymmetric thicknesses, such that, for example, anterior vertebral body cortex could be five millimeters thick, lateral vertebral body wall seven millimeters thick and the pedicle walls only one millimeter thick; or body cortical wall uniformly five millimeters thick and pedicle walls only one millimeter thick or the like. The individual vertebra can be visualized as a structure which has been cored or hollowed out and the resulting remaining vertebral body is “electrified” or highlighted in a suitable manner throughout its walls. 
     Step 4 
     A computer then automatically determines the maximum allowable diameter screw to be placed by determining the narrowest diameter or cross sectional area (isthmus) X within any given pedicle based on surgeon pedicle cortical wall diameter preferences, as shown in  FIG. 3 . 
     Step 5 
     A computer then generates an elongated cylinder by starting at the center of the isthmus X as a straight line  10  in  FIG. 4  which determines the ideal axis/trajectory and extending in opposite directions, e.g., perpendicular to the plane of the isthmus of the pedicle so that it is positioned concentrically as much as possible within the pedicle without touching the remaining cortex with the center of the isthmus being the fulcrum. This line is allowed to penetrate the dorsal or posterior pedicle cortex so that it can extend beyond the skin of a patient to any desired length. The line terminates inside the vertebral body to within a predetermined distance (e.g. 5 mm) from the predefined anterior inner cortical wall, as selected by the surgeon, so that it does not penetrate the anterior outer cortex and also maximizes screw diameter as described hereinafter. 
     Step 6 
     A computer then builds the line  10  concentrically in radial directions as shown schematically in  FIG. 5  to its final maximum diameter which will not exceed the narrowest defined pedicle diameter based on surgeon preference pedicle cortical wall thickness. This concentric building ultimately grows into a visible cylinder  12  which stops building when any point on its outer surface comes into “contact” with the highlighted inner cortical wall with the exception of the posterior pedicle cortex. The cylinder formed has at its center the beginning line  10  which may be identified in a different color or pattern than the concentrically built cylinder  12 . As described hereinafter, the cylinder  12  may be extended beyond its intersection with the dorsal/posterior cortex to facilitate the placement of screws in accordance with one of the automated methods described hereinafter. 
     Step 7 
     The maximal diameter allowed may actually be less than that determined by the narrowest diameter method for those pedicles which have irregular anatomy, as shown in  FIG. 6   b , such as curved pedicles ( FIG. 7   b ) or a similar deformity. This prevents cortical pedicle wall breach. 
     Step 8 
     A computer then determines the length of the screw by measuring the length of the cylinder  12  starting at the point D in  FIG. 8  adjacent to the predefined anterior inner cortex up to its intersection A with the dorsal/posterior cortex. 
     Step 9 
     A computer then provides a data summary table as shown in  FIG. 10   a  which displays the ideal pedicle screw diameter, length and trajectory (measured as an angle shown in  FIGS. 10   b  and  10   c  with respect to the transverse and sagittal planes with corresponding superior end plate  20  as the reference plane) for each individual vertebra pedicle, and also provides idealized schematic drawings as shown in  FIG. 11 . Individual vertebra are labeled by having the surgeon identify any specific vertebra as shown in  FIG. 9  and then the computer automatically labels the remaining vertebral bodies with the surgeon confirming accurate vertebral body labeling. 
     Step 10 
     This tabulated data can then be utilized at this juncture for determination of the viability of using pedicle screws based on maximal pedicle screw diameter and length, as shown in  FIG. 12 , and also for placement of screws by a surgeon&#39;s preferred method.  FIG. 12  also provides the individual pedicle base circumference outlines (coronal trajectory) from points A to B and their respective lengths. Actual screw sizes utilized will be based on surgeon selection of commercially available screws. A computer can automatically determine and generate this table once the surgeon provides the available screw size ranges in the selected pedicle screw system and can concomitantly generate an idealized schematic AP (coronal), lateral and transverse drawing with the data as shown in  FIG. 13 . Furthermore, this system provides surgeon override capabilities to choose a diameter different than the maximum available one on an individual vertebra basis and incorporates these override modifications into the summary data and diagrams. 
     Step 11—Manual Pedicle Screw Placement 
     The surgeon may then use the idealized schematic diagram and summary data for pedicle screw placement based on his or her preferred method. 
     Step 12a—Pedicle Base Circumference Outline Method—Manual Determination 
     This method takes advantage of radiographic vertebral body anatomical landmarks to match the ideal pedicle screw trajectory in the coronal plane as shown in  FIGS. 10   d  and  11 . Specifically, the radiodensity circular lines seen on standard anteroposterior x-ray or fluoroscopic images correspond to the pedicle base circumferences. The pedicle base circumference B is defined as the cortical junction between the pedicle wall and its transition into the vertebral body. This pedicle base circumference is distinctly different from the pedicle isthmus, but can in some instances be one and the same or super imposable for individual vertebra as seen in  FIGS. 14   a - 14   e.    
     For manual utilization of the pedicle base circumference technique, first the ideal trajectory through the pedicle isthmus X is manually determined using the corresponding transverse radiographic image through the pedicle as seen in  FIG. 14   b . The pedicle isthmus X is then measured to determine the maximum diameter pedicle screw. The trajectory is utilized for determination for the maximum pedicle screw length. The pedicle base circumference B is then determined by identifying the transition of the pedicle wall into the vertebral body as seen in  FIG. 14   b . Finally, the length A-B which corresponds to the starting point on the posterior cortex A up to the intersection with the pedicle base circumference B is measured and utilized for the calibration of a suitable tool such as a variable length awl to be described hereinafter. Point A and point B should be centered with respect to the pedicle base circumference from the top (cephalad) and bottom (caudad) edges of the pedicle base circumference, as shown in  FIG. 14   h . The ideal trajectory and pedicle base circumference are then combined to determine where the point A lies with respect to the anteroposterior projection of the pedicle base circumference and where the point B lies within the pedicle base circumference. This pedicle base circumference outline will have a circular configuration to resemble the anteroposterior radiographic image for each individual vertebra. 
     For manual placement of pedicle screws, a standard fluoroscopy unit can be used to align the superior endplate of the respective vertebral body parallel to the fluoroscopic imaging. Furthermore, the vertebral body is centered when its superior end plate is fluoroscopically visualized by symmetric disc space with the cephalad vertebral body, and when the vertebral body is equidistant from each pedicle by having the pedicle base circumference outlines visually identical on the fluoroscopic AP image. This centering can still occur when there are other than two pedicles per vertebral body, such as congenital anomalies, tumors, fractures, etc. An appropriately calibrated variable length awl or other suitable tool T is then placed onto the posterior cortex of the corresponding vertebral body at pedicle pilot starting hole point A under fluoroscopic imaging and advanced into the pedicle up to point B as seen in  FIGS. 14   f  and  14   g . This placement is confirmed fluoroscopically and represents two points A and B on a straight line that co-aligns with the ideal trajectory. The tool T can be readjusted to lengthen and further advance into the vertebral body to point D or exchanged for another pedicle probing awl or similar tool. The pedicle is then sounded for intraosseous integrity, the hole tapped and the appropriate diameter and length pedicle screw is placed transpedicularly into the vertebral body. 
     In accordance with Step 12a, CT transaxial views through center of pedicles T 1 , T 2 , T 4 , T 5 , as shown in  FIG. 16 , demonstrate pedicle morphology, isthmus and manual determination of pedicle pilot hole entry points correlating with intraoperative AP fluoroscopic images of each respective vertebra. Pedicle screw length, diameter and trajectory have already been determined. The pedicle base circumference outline is represented as the circle on the bottom right hand corner and is utilized as a consistent intraoperative marker for identifying pedicle pilot hole starting points. For example, the starting points A for both T 1  and T 2  pedicles are approximately 2 pedicle base circumferences and 1.25 pedicle base circumferences, respectively, as seen on the AP fluoroscopic pedicle base circumference seen intraoperatively (indicated by the dot within the circle). The T 4  and T 5  pedicle pilot holes are 0.9 and 0.8 pedicle base circumferences, respectively. 
     Step 12b—Pedicle Base Circumference Outline Method—Semi-Automated 
     This method is similar to Step 12a except that points A and B and pedicle base circumference outline is determined by a computer after building the computer generated pedicle cylinders concentrically. This data is then summarized as in  FIG. 12 . This data also includes the sagittal and transverse trajectory angles measured in degrees with respect to the superior endplate and midline vertebral body. A variable length awl or other tool, for example, may then be appropriately adjusted to specific pedicle length A-B summarized in  FIG. 12  and screws placed with standard fluoroscopy as described in Step 12a. 
     Step 12c—Pedicle Base Circumference Outline Method—Fully Automated 
     This method further expands the present technique to allow for real-time imaging and multiple vertebral body visualization for pedicle screw placement. The data generated is the same as in  FIG. 12  except that the pedicle base circumference outlines and identified points A and B are dynamic and do not require the vertebral body to be centered or have the superior end plate parallel to the fluoroscopic imaging as in Steps 12a and 12b. The fluoroscopically imaged vertebral bodies are registered by any suitable method to the computer generated vertebral bodies with their corresponding computer generated pedicle cylinders. The points A and B are then visualized as seen in  FIGS. 15   a ,  15   c  and  15   e  and displayed as in  FIG. 12  as updated real-time imaging. A variable length awl or other tool, for example, may then be adjusted to appropriate length for starting at point A and advancing to point B for each respective vertebra. It is noted that any suitable tool, such as a nonadjustable awl, may be used other than an adjustable awl in accordance with the methods of the present invention. 
     Step 13—Adjustable Variable Length Awl 
     The distance from point A to point B ( FIG. 14   b ), posterior cortex to intersection with pedicle base circumference, is utilized to set the length A-B on an adjustable variable length awl constructed in accordance with the present invention. This awl is used to establish the pedicle pilot hole under fluoroscopic imaging. The pedicle pilot hole forms the first step in a series of steps for actual placement of a pedicle screw. The pilot pedicle hole is started at the identified starting point A indicated by the computer generated pedicle cylinders and advanced to point B once it is fully seated. 
     Referring to  FIG. 17   a , the awl  100  comprises a cannulated radiolucent housing  102  with an open end which movably supports a radio opaque awl member  104 . The awl  100  is fully adjustable for variable lengths to correspond to length A-B and also configured to prevent advancement of the awl further than any distance A-B as seen in  FIG. 14   b  and other drawing figures. 
     A surgeon can adjust the awl to any length from point A to point D, the final screw length, in  FIG. 14   b  once the distance A-B has been radiographically confirmed. The awl  100  preferably is of such construction to tolerate being struck with a mallet or the like and is of a diameter narrow enough to be used percutaneously. To facilitate visualization of depth, the awl member  104  may be marked in color or otherwise at fixed increments  106 , such as 5 mm or 10 mm. 
     The awl  100  may be provided with a solid head  108  at its outer end for striking, and with any suitable locking mechanism  110 , such as a locking screw mechanism, for locking the awl member  104  in a desired position relative to the housing  102 . The awl may also be provided with a window  112  or other indicia for indicating the position or length of the awl member  104 .  FIGS. 14   f  and  14   g  show an awl being advanced into the pedicle to create the screw pilot hole. 
       FIG. 17   b  illustrates a modified adjustable awl  300  which comprises a cannulated or hollow awl member  304  and a head  308  with a central aperture  309  such that a guide wire  311  may extend through the head and through the awl member  304  to its inner end. After the pilot hole is formed by the awl  300 , the guide wire  311  may be left in position in the pilot hole to facilitate its location during subsequent steps leading to the installation of the pedicle screw. 
     Step 14—Dual Ring Co-Aligned Technique 
     For automated intraoperative pedicle screw placement the dimensionally true three dimensional spine model with computer automated placed pedicle screw cylinders defining length, diameter and trajectory is utilized. In addition, the pedicle base circumference outline data is utilized to facilitate registration with intraoperative imaging. 
     Real-time intraoperative fluoroscopy is utilized for accurate registration with the three dimensional model on an individual vertebral basis. This fluoroscopic vertebral body image is centered on the monitor and identified by the surgeon for its specific vertebral body identifier (i.e., T 2 , T 3 , etc.). The corresponding dimensionally true three dimensional individual vertebral model is registered to this fluoroscopic image as shown schematically in  FIGS. 18   a ,  18   b  and  18   c . This can be performed on either surgically exposed spines or percutaneously. 
     The registration occurs by utilizing internal vertebral body bony landmarks. These landmarks are the pedicle base circumferences seen on the fluoroscopic image which arise from the confluence of the pedicle cortical walls joining the vertebral body. As hereinbefore explained, these pedicle base circumferences form either circular or elliptical shapes which can change configuration and square area based on vertebral body rotation with respect to fluoroscopic imaging. 
     The intraoperative fluoroscopic and computer spine generated pedicle base circumference outlines are then registered. Precision of registration is obtained by assuring outlines are superimposed and measured square areas are equal and by assuring distance between pedicles is equal. This method of registration eliminates the requirement of having a radiographic marker anchored to the patient&#39;s skeleton, which is particularly disadvantageous for percutaneous applications. This method also allows for free independent movement of one vertebral body to another demonstrating compliance of this computer generated model, which is particularly useful in spines with instability. The surgeon confirms adequacy of registration of pedicle base circumferences intraoperatively in order to proceed with screw placement. This method allows for magnification or reduction of the computer generated model to match the intraoperative fluoroscopic image. 
     The full three dimensional image which now includes the computer generated pedicle base circumference and pedicle cylinder is then projected superimposed on the intraoperative fluoroscopic image. As shown in  FIGS. 19   a  and  19   b , the computer pedicle screw cylinder  200  is then projected out of the patient&#39;s body through the posterior cortex and is intercepted by and extends through two separate and collinear rings  202 ,  204 . The rings are mounted on a suitable support frame  206  anchored to the patient&#39;s bed or other support (not shown) and are sized to allow interception of the computer cylinder image and to allow placement of drilling cannulas. The first ring  202  intercepts the computer pedicle screw cylinder near the posterior cortical region  208  or just outside the body and the second ring  204  intercepts the computer pedicle screw cylinder at any desired distance from the first ring  202 . The longer the distance between the two rings the greater the accuracy of screw placement. The interception of the computer pedicle cylinder by the rings  202 ,  204  is displayed on a computer monitor which demonstrates movement of the rings with respect to the computer pedicle cylinder  200 . 
       FIGS. 19   c  and  19   d  illustrate the computer generated cylinder  200  and line  210  projecting out from a vertebral body VB through the rings  202 ,  204  in a surgically open environment and a percutaneous environment, respectively. 
     Interception of the pedicle cylinders occurs on two levels. The computer pedicle cylinders  200  are comprised of a central line  210  with surrounding cylinder. First, the rings  202 ,  204  need to be centered to both the central line  210  and pedicle cylinder  200 . Second, the rings are registered to the vertebral body so their movements can be followed on the computer monitor such as through LED devices. Third, the rings are constructed to have inner diameters to allow matching of diameters corresponding to the diameter of the computer generated pedicle cylinders  200 . A variety of removable rings with different diameters may be provided to allow utilization of any pedicle screw system desired by the surgeon. Fourth, the rings can be constructed to be adjustable in any suitable manner to allow for variable diameters to allow matching of diameters corresponding to the diameter of the computer generated pedicle cylinder as shown in  FIG. 20  where the ring  202  is formed of movably connected sections  212  that can be rotated to vary the ring diameter. Registration of the rings with the computer pedicle cylinder is identified and confirmed on the computer monitor. 
     The two co-aligned rings  202 ,  204  now form the conduit in which to place a drilling cannula  214  ( FIGS. 21   a  and  21   b ) which is also secured to the frame  206  anchored to the patient&#39;s bed or other support. Inside this drilling cannula  214  is placed a solid cannula member  216  ( FIGS. 21   a  and  21   b ), or a specialized inner cannula member  218  ( FIGS. 22   a  and  22   b ) may be used which has multiple narrow movable and longitudinal metal parallel pins  220  therein and is open centrally to allow for drill placement. The multiple pins  220  allow for the inner cannula member  218  to rest evenly on an uneven surface. This feature provides additional stability at the posterior cortex drilling area to avoid toggling of the drill bit. Additionally, the specialized inner cannula member  218  allows for retraction of the multiple parallel pins to allow fluoroscopic visualization of drilling within the pedicle. Either method may be used by the surgeon. 
     The pedicle is then drilled to its desired precalibrated depth and not exceeding the predetermined pedicle screw length. The pedicle is then sounded with a pedicle probe to assure osseous integrity. 
     For actual screw placement, a specialized slotted outer cannula  230  ( FIGS. 23   a  and  23   b ) is placed collinear and onto the co-aligned two rings  202 ,  204  which are removably mounted on the support frame. This specialized cannula  230  is also secured to the support frame or other anchoring device. The rings are then removed by rotating them approximately ninety degrees (not shown) and withdrawing them from the cannula  230 . The slotted cannula&#39;s adjustable inner diameter is sufficient to accommodate any pedicle screw diameter threaded and variable head size. The appropriate pedicle screw (not shown) is placed into its holding screwdriver, placed into the slotted cannula and then placed into its respective pedicle. 
     For the modified adjustable coaligned rings shown in  FIG. 20 , the slotted cannula  230  in  FIG. 23   a  can be used or alternatively, the rings  202 ,  204  may be left in position and adjusted to a fully open position to accommodate a screwdriver placed into and through the rings. 
     Step 15 
     There are currently commercially available software packages capable of producing intraoperative registration of intraoperative fluoroscopy images with preoperative three dimensional images of a patient&#39;s spine. Such capabilities can be integrated with the methods of the present invention to provide summary numerical data and idealized illustrated diagrams. The latter information will provide the basis for actual screw placement as described herein or by a surgeon&#39;s preferred choice. 
     Step 16 
     For surgeons who prefer to place screws extraosseous or extrapedicular because the pedicle screw sizes are too small to accommodate available screw sizes, planned eccentric screw placement in large pedicles or planned straight ahead versus anatomic axis screw placement, the present invention allows this capability. It accomplishes this by obtaining all idealized data and then allows a surgeon to offset the pedicle pilot hole entry placement at any desired distance tangentially from the ideal trajectory, i.e., the anterior screw position is the pivot point D from which a computer pedicle cylinder  12  is generated, as shown in  FIG. 24 . Furthermore, these changes will be automatically recorded to generate new idealized AP, lateral and transaxial schematic diagrams incorporating these changes. This data can be used for placement of screws by either the pedicle base circumference method, an automated aligning method or a commercially available CT/fluoroscopy registration method. For the pedicle base circumference method, new pilot hole lengths are determined to allow for proper length of an awl or other tool. 
     As an illustrative embodiment,  FIG. 25  shows schematically the installation of a pedicle screw  20  by a screwdriver  22  or the like through the center of the isthmus X in accordance with the present invention. 
     While many of the steps of the methods of the present invention are described as being computer-generated, it is noted that any suitable apparatus or device may be utilized to accomplish these steps in accordance with the methods of the present invention. 
     The invention has been described in connection with what is presently considered to be the most practical and preferred embodiments. It is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.