Abstract:
A gravity dependent pedicle screw tap hole guide comprises a guide shaft maintainable parallel to a drill bit during the drilling of a pedicle screw tap hole; a level indicator associated with a reference direction and responsive to gravity to provide feedback regarding an angular difference between an acting direction of gravity and the reference direction; and a mounting attaching the indicator to the shaft and establishing a positional relationship between the reference direction and the longitudinal axis. Using the guide involves angulating the shaft about its distal end adjacent the base of the superior articular process and the base and middle of the transverse process until the guide indicates that the angular orientation of the longitudinal axis matches the previously determined pedicle axis orientation with respect to the acting direction of gravity, and then drilling the tap hole along a trajectory established by the longitudinal axis of the shaft.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    The present application is a divisional application of U.S. patent application Ser. No. 10/103,079 (filed Mar. 21, 2002) entitled “Gravity Dependent Pedicle Screw Tap Hole Guide”. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates generally to devices and methods for inserting pedicle screws into the spine, and more specifically to devices and methods for accurately establishing a pedicle screw tap hole drilling trajectory.  
         BACKGROUND OF THE INVENTION  
         [0003]    The bones and connective tissue of an adult human spinal column consist of more than 20 discrete bones coupled sequentially to one another by a tri-joint complex which consist of an anterior disc and the two posterior facet joints, the anterior discs of adjacent bones being cushioned by cartilage spacers referred to as intervertebral discs. These more than 20 bones are anatomically categorized as being members of one of four classifications: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine, which comprises the top of the spine, up to the base of the skull, includes the first 7 vertebrae. The intermediate 12 bones are the thoracic vertebrae, and connect to the lower spine comprising the 5 lumbar vertebrae. The base of the spine is the sacral bones (including the coccyx). The component bones of the cervical spine are generally smaller than those of the thoracic and lumbar spine.  
           [0004]    The spinal column of bones is highly complex in that it includes these more than 20 bones coupled to one another, housing and protecting critical elements of the nervous system having innumerable peripheral nerves and circulatory bodies in close proximity. In spite of these complexities, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction. Genetic or developmental irregularities, trauma, chronic stress, tumors, and disease, however, can result in spinal pathologies which either limit this range of motion, or which threaten the critical elements of the nervous system housed within the spinal column. A variety of systems have been disclosed in the art that achieve this immobilization by implanting artificial assemblies in or on the spinal column. These assemblies may be classified as anterior, posterior, or lateral implants. As the classifications suggest, lateral and anterior assemblies are coupled to the anterior portion of the spine, which is the sequence of vertebral bodies. Posterior implants generally comprise pairs of rods, which are aligned along the axis along which the bones are to be disposed, and which are then attached to the spinal column by either hooks which couple to the lamina or attach to the transverse processes, or by screws which are inserted through the pedicles.  
           [0005]    The pedicles are the strongest parts of the vertebrae and therefore provide a secure foundation for the screws to which the rods are to be attached. In order to obtain the most secure anchor for the pedicle screws, it is essential that the screws be threaded in alignment with the pedicle axis and not be allowed to deviate therefrom. Misalignment of the pedicle screws during insertion can cause the screw body or its threads to break through the vertebral cortex and be in danger of striking surrounding nerve roots. A variety of undesirable symptoms can easily arise when the screws make contact with nerves after breaking outside the pedicle cortex, including dropped foot, neurological lesions, sensory deficits, or pain.  
           [0006]    Known surgical procedures to avoid misalignment of the pedicle screws involve recognizing landmarks along the spinal column for purposes of locating optimal tap hole entry points, approximating tap hole trajectories, and estimating proper tap hole depth. Some surgeons use a Kocher clamp applied to the vertebral bone for a reference mark and/or view radiographs or other medical images to better understand relative positions of the patient&#39;s anatomy. X-ray exposures and/or fluoroscopy can sometimes be used to monitor the advancement of a pedicle screws through the vertebra. Unfortunately, these procedures are subject to surgeon visual approximation errors, and anatomical landmarks are different for each patient. Further, prolonged radiation exposure to a patient is undesirable. U.S. Pat. No. 4,907,577 (Mar. 13, 1990) discloses a jig that is described therein as providing a safe route for drilling pedicle screw tap holes, by identifying a precise location for drilling to prevent deviation from the drilling direction so as to prevent injury during surgery to the nerve root or spinal cord. However, the jig has a variety of moving parts that must be adjusted and monitored simultaneously during the adjustments, making operation of the jig difficult and time consuming. Further, operation of the jig must occur during surgery, as it must be held adjacent the vertebral body to determine the proper adjustment settings. Finally, adjustment of the jig to the proper settings requires precise visual approximation by the surgeon, an activity that should be minimized to ensure that a misaligned trajectory is not established in place of a safe one.  
           [0007]    More technologically advanced systems such as the StealthStation™ Treatment Guidance System, the FluoroNav™ Virtual Fluoroscopy System (both available from Medtronic Sofamor Danek), and related systems, seek to overcome the need for surgeons to approximate landmarks, angles, and trajectories, by assisting the surgeons in determining proper tap hole starting points, trajectories, and depths. However, these systems are extremely expensive, require significant training, are cumbersome in operation, are difficult to maintain, and are not cost effective for many hospitals.  
           [0008]    U.S. Pat. Nos. 5,474,558 (Dec. 12, 1995) and 5,196,015 (Mar. 23, 1993) propose a procedure in which a screw opening is started in part of a skeletal region, e.g., a pedicle of a lumbar vertebra, and an electric potential of a certain magnitude is applied to the inner surface of the opening while the patient is observed for nervous reactions such as leg twitching. The opening continues to be formed while the electric potential is applied until a desired hole depth is obtained in the absence of nervous reaction to the potential. The direction in which the screw opening is being formed is changed to a direction other than the last direction, after observing patient reactions to the electric potential when the screw opening was being formed in the last direction. Unfortunately, this procedure is inherently reactive rather than proactive, in that the surgeon becomes aware of the misalignment after the patient exhibits a nervous reaction, and by that time the misaligned hole has been drilled.  
           [0009]    Therefore, there is a need for a simple device that eases the difficulties associated with safely placing pedicle screws. Specifically, there is a need for such a device that assists a surgeon in making more accurate the surgeon&#39;s assessment of the proper insertion trajectory of the pedicle screw. Further, there is a need for such a device that does not require the surgeon to rely on visual approximations. In addition, there is a need for such a device that proactively determines the desirable drilling trajectory rather than reactively informing the surgeon when an improper trajectory has been used.  
         SUMMARY OF THE INVENTION  
         [0010]    The needs identified above and other needs in the art are achieved by the present invention that provides a gravity dependent pedicle screw tap hole guide and methods of use thereof.  
           [0011]    One embodiment of a gravity dependent pedicle screw tap hole guide of the present invention has a shaft with a proximal end, a distal end, a longitudinal axis, and a fluid chamber attached to the shaft. A bubble in the fluid chamber indicates whether or not the chamber is level and/or to what degree it is not level. The translucent wall of the chamber has a reference mark positioned so that that when the bubble is centered under the reference mark, the longitudinal axis of the shaft is parallel to the acting direction of gravity. The wall also has a grid that, when the bubble is not centered under the reference mark, indicates an angular difference (preferably in two perpendicular planes) between the longitudinal axis of the shaft and the acting direction of gravity. Preferably, the longitudinal axis of the shaft extends perpendicular to a plane in which a platform holding the chamber extends. The chamber is preferably a hemispherical enclosure with a central axis that is parallel to the longitudinal axis of the shaft.  
           [0012]    In operation of this embodiment, the surgeon first exposes a vertebral bone and applies a Kocher clamp to the spinous process in a vertical position (where the longitudinal axis of the clamp is parallel to the acting direction of gravity) to his best visual approximation. Preferably, the guide of this embodiment is used here to make the vertical placement more accurate, by holding the shaft parallel to the longitudinal axis of the Kocher clamp while manipulating the shaft with the Kocher clamp so that when the bubble is centered under the reference mark, the surgeon knows that the Kocher clamp is in a vertical position.  
           [0013]    Next, a lateral radiograph is taken and used to approximate the cephalad-caudad declination of the pedicle of interest and the medial angulation of the pedicle is determined from preoperative transaxial MRI and/or CAT scan images. The surgeon then positions the distal end of the shaft against the exposed vertebral bone in the vicinity of the base of the superior articular process and the base and middle of the transverse process (referred to herein as the “preferred tap hole entry point”), and angulates the shaft until the angular difference between the longitudinal axis of the shaft and the acting direction of gravity matches the determined cephalad-caudad declination (in the cephalad-caudad plane), and the medial angulation (in the medial plane). During this angulation, the surgeon can view the bubble&#39;s position relative to the grid lines, to know when and in what direction additional angulational adjustment of the shaft is necessary to bring the shaft to the desired position.  
           [0014]    Once the shaft has been placed in the desired position, the surgeon can be confident that drilling into the vertebral bone along the trajectory established by the longitudinal axis of the shaft in the desired position will result in a pedicle screw tap hole that is formed to maximize the stability of a pedicle screw subsequently screwed thereinto. The shaft can be hollow to accommodate a drill bit for this purpose, or, if the shaft is not hollow, the distal end of the drill bit can be placed against the preferred tap hole entry point of the exposed vertebral bone, and the shaft can be held parallel to the longitudinal axis of the drill bit so that the shaft and the drill bit can be angulated in parallel together until the guide indicates the drill bit is at the desired angle.  
           [0015]    Another embodiment of a gravity dependent pedicle screw tap hole guide of the present invention also has a shaft with an attached fluid chamber housing a level-indicating bubble that rests under a reference mark when the chamber is level. The chamber is movably attached to the shaft and thereby positionable relative to the shaft. Specifically, the degree of perpendicularity of the longitudinal axis of the shaft relative to a plane defined by the chamber can be varied in at least two planes. In this regard, the movable attachment of the chamber to the shaft is achieved by two rotatable mountings, the first being between the shaft and the second rotatable mounting, and the second being between the first rotatable mounting and the chamber. The first rotatable mounting rotates about an axis extending perpendicular to the longitudinal axis of the shaft, and the second rotatable mounting rotates about an axis extending perpendicular to the plane defined by the chamber. Each of the rotatable mountings can be secured at any position to which it can be rotated. When each rotatable mounting is in its zero position, the plane of the chamber is perpendicular to the longitudinal axis of the shaft and, accordingly, when the enclosure is oriented such that the bubble is under the reference mark, the longitudinal axis of the shaft is parallel to the acting direction of gravity. Marks on the mountings preferably indicate the relative angle of rotation of the rotatable mounting with respect to the zero position, such that if either or both of the rotatable mountings are placed in a rotated position, the user can read the marks to determine the angular difference between the longitudinal axis of the shaft and the plane defined by the chamber when the chamber is oriented so that the bubble is under the circle.  
           [0016]    In operation of this embodiment, the surgeon proceeds as indicated above with regard to the first embodiment, but use of this embodiment to make the Kocher clamp vertical placement more accurate is as follows: The rotatable mountings are placed in their respective zero positions, and the shaft is held parallel to the longitudinal axis of the Kocher clamp while being manipulated with the Kocher clamp until the bubble is centered under the reference mark, at which time the surgeon knows that the Kocher clamp is in a vertical position.  
           [0017]    After determining the cephalad-caudad declination and medial angulation of the pedicle of interest, the surgeon places the first rotatable mounting into a rotated position at an angular offset matching the cephalad-caudad declination, and places the second rotatable mounting into a rotated position at an angular offset matching the medial angulation. During these rotations, the surgeon can view the marks to ensure that the mountings are rotated to the desired angles. Then, the surgeon positions the distal end of the shaft against the preferred tap hole entry point, and angulates the shaft until the bubble is under the reference mark. The surgeon can then safely drill the tap hole as desired along the trajectory established by the longitudinal axis of the shaft. Again, the shaft can be hollow and/or held in parallel to the drill bit as the drill bit is angulated against the preferred tap hole entry point.  
           [0018]    Yet another embodiment of a gravity dependent pedicle screw tap hole guide of the present invention is similar to the first embodiment discussed above, but uses an accelerometer instead of a fluid chamber housing a level-indicating bubble. An accelerometer is known in the art as an electronic device that can determine its angular orientation relative to the acting direction of gravity, and therefore can be used to determine, for any device in fixed relation to the accelerometer, the angular orientation of that device relative to the acting direction of gravity. The accelerometer can be connected to an analog or digital readout presenting the angular orientation of the accelerometer relative to the acting direction of gravity. Preferably, the shaft is attached in fixed relation to the accelerometer such that when the accelerometer indicates that there is no angular difference between the reference direction recognized by the accelerometer and the acting direction of gravity, the longitudinal axis of the shaft is parallel to the acting direction of gravity. Accordingly, as the shaft is oriented freely in space, the accelerometer indicates the angular difference (preferably in two planes) between the longitudinal axis of the shaft and the acting direction of gravity.  
           [0019]    Operation of this embodiment proceeds as indicated with regard to the first embodiment, with the accelerometer (rather than the fluid-containing enclosure in the first embodiment) indicating when the shaft is in the desired position, that is, when the angular difference between the longitudinal axis of the shaft and the acting direction of gravity matches the cephalad-caudad declination (in the cephalad-caudad plane) and medial angulation (in the medial plane) of the pedicle.  
           [0020]    Still another embodiment of a gravity dependent pedicle screw tap hole guide of the present invention is similar to that of the second embodiment described above, except that the fluid-containing enclosure of that embodiment is replaced with an accelerometer similar to the accelerometer of the third embodiment described above. Accordingly, when each rotatable mounting is in its zero position, and the accelerometer reads level, the longitudinal axis of the shaft is parallel to the acting direction of gravity. And, accordingly, if either or both of the rotatable mountings are placed in a rotated position, the user can, when the accelerometer is oriented level, read the marks to determine the angular difference between the longitudinal axis of the shaft and the acting direction of gravity.  
           [0021]    Operation of this embodiment proceeds as indicated with regard to the second embodiment, with the accelerometer indicating when the accelerometer is oriented level (and thus, if the rotatable mountings have been rotated to match the cephalad-caudad declination and medial angulation of the pedicle, that the shaft is at the desired angulation). 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIGS. 1 a - c  are side, top, and perspective views of an embodiment of a gravity dependent pedicle screw tap hole guide of the present invention, the guide utilizing a fluid chamber housing a level-indicating bubble.  
         [0023]    [0023]FIGS. 2 a - c  are front, side, and top views of another embodiment of a gravity dependent pedicle screw tap hole guide of the present invention, the guide utilizing a fluid chamber housing a level-indicating bubble and rotatable mountings.  
         [0024]    [0024]FIGS. 3 a - c  are side, top, and perspective views of yet another embodiment of a gravity dependent pedicle screw tap hole guide of the present invention, the guide utilizing an accelerometer.  
         [0025]    [0025]FIGS. 4 a - c  are front, side, and top views of still another embodiment of a gravity dependent pedicle screw tap hole guide of the present invention, the guide utilizing an accelerometer and rotatable mountings.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0026]    While the present invention will be described more fully hereinafter with reference to the accompanying drawings, in which particular embodiments and methods of implantation are shown, it is to be understood at the outset that persons skilled in the art may modify the invention herein described while achieving the functions and results of this invention. Accordingly, the descriptions that follow are to be understood as illustrative and exemplary of specific structures, aspects and features within the broad scope of the present invention and not as limiting of such broad scope. Like numbers refer to similar features of like elements throughout.  
         [0027]    Referring now to FIGS. 1 a - c , an embodiment of a gravity dependent pedicle screw tap hole guide of the present invention is illustrated. The guide in this embodiment has a shaft  100  that has a proximal end  101  and a distal end  102  and a longitudinal axis  105 , and a fluid chamber  110  attached to the shaft  100 . The fluid chamber  110  is partially filled with fluid  120 , and the fluid  120  is contained within the chamber  110 , such that a bubble  130  is present in the chamber  110 . Because the gas in the bubble  130  is lighter than the fluid in the chamber  110 , the bubble  130  floats in the chamber  110 , seeking to travel in a direction opposite the acting direction of gravity, but being prevented from leaving the chamber  110  because the chamber  110  is closed.  
         [0028]    The chamber  110  has a wall  135  through which the bubble  130  is visible. The wall  135  has a reference mark  160  positioned so that that when the bubble  130  is centered under the reference mark  160 , it is indicated that the longitudinal axis  105  of the shaft  100  is parallel to the acting direction of gravity.  
         [0029]    Further, the translucent wall  135  has at least one relative mark (grid  150 ) that can be read to determine the location of the center of the bubble  130  relative to the reference mark  160  when the bubble  130  is not centered under the reference mark  160 , the relative mark (grid  150 ) indicating an angular difference between the longitudinal axis  105  of the shaft  100  and the acting direction of gravity.  
         [0030]    Preferably, as shown, the longitudinal axis  105  of the shaft  100  extends in a direction perpendicular to a plane in which a platform  180  laterally attached to the shaft  100  extends. The chamber  110  is preferably a transparent hemispherical enclosure  110  having a central axis  170  (the axis  170  passing through the center top of the hemisphere  110  and being perpendicular to the platform  180 ) is parallel to the longitudinal axis  105  of the shaft  100 .  
         [0031]    Also preferably, the outer surface of the enclosure  110  is marked with a guide grid  150  formed by grid lines as shown. Grid lines in a first grid line set  140  are evenly spaced along the curved surface of the enclosure  110  and extend in respective planes parallel to the longitudinal axis  105  of the shaft  100 . (Only one grid line of this set is marked  140  merely for clarity in presentation of the figures; the reference numeral  140  applies to the entire set of grid lines). Grid lines in a second grid line set  142  are evenly spaced along the curved surface of the enclosure  110  and extend in respective planes parallel to the longitudinal axis  105  of the shaft  100  but perpendicular to the grid lines in the first set  140 . (Only one grid line of this set is marked  142  merely for clarity in presentation of the figures; the reference numeral  142  applies to the entire set of grid lines). The central grid line of each set intersects with the other to define the reference mark  160 .  
         [0032]    Accordingly, each grid line in the first set  140  indicates (when the bubble  130  is under the line) a respective angular difference between the longitudinal axis  105  of the shaft  100  and the acting direction of gravity in a first plane, and each grid line in the second set  142  indicates (when the bubble is under the line) a respective angular difference between the longitudinal axis  105  of the shaft  100  and the acting direction of gravity in a second plane perpendicular to the first plane. The lines are preferably labeled to assist the surgeon in quantifying the angular difference. In this embodiment, grid lines in the first set  140  are labeled in degrees, in reference to the first plane, −40, −30, −20, −10, 0, 10, 20, 30, 40, respectively. Also in this embodiment, grid lines in the second set  142  are labeled in degrees, in reference to the second plane, −40, −30, −20, −10, 0, 10, 20, 30, 40, respectively. It should be understood that other labeling, with greater or lesser angles, and/or greater or lesser increments, can also be used.  
         [0033]    In operation of this embodiment, the surgeon first exposes the vertebral bone into which the pedicle screw is to be placed. Next, the surgeon applies a clamp (e.g., a Kocher clamp) to the spinous process of the exposed vertebral bone, placing the Kocher clamp in a vertical position (parallel to the acting direction of gravity) to his best visual approximation. Preferably, the gravity dependent pedicle screw tap hole guide of this embodiment is used at this point in the procedure to make more accurate the surgeon&#39;s vertical placement of the Kocher clamp. That is, the shaft of the guide can be held parallel to the longitudinal axis of the Kocher clamp, manipulated with the Kocher clamp while being maintained in said parallel position, so that when the bubble  130  is centered under the reference mark  160 , the surgeon knows that the Kocher clamp is in the vertical position.  
         [0034]    Once the Kocher clamp in attached to the spinous process in the vertical position, a lateral radiograph is taken, and the cephalad-caudad declination of the pedicle of interest is determined by the surgeon to his best visual approximation using the longitudinal axis of the Kocher clamp in the radiograph image as the “zero” axis. Also, the medial angulation of the pedicle is determined from preoperative transaxial MRI and/or CAT scan images. Angular measurement devices known in the art can be used to make these angular assessments more accurate. Once the cephalad-caudad declination and the medial angulation have been determined, the surgeon positions the distal end  102  of the shaft  100  against the exposed vertebral bone in the vicinity of the base of the superior articular process and the base and middle of the transverse process (referred to herein as the “preferred tap hole entry point”), and angulates the shaft  100  until the angular difference between the longitudinal axis  105  of the shaft  100  and the acting direction of gravity in the first plane matches the determined cephalad-caudad declination, and the angular difference between the longitudinal axis  105  of the shaft  100  and the acting direction of gravity in the second direction matches the determined medial angulation. (It should be understood that alternatively, the device can be used with the grid lines in the set  140  begin use to match the medial angulation, and the grid lines in the set  142  being used to match the cephalad-caudad declination.) During this angulation, the surgeon can view the position of the bubble  130  under the guide grid  150 , and particularly the bubble&#39;s position relative to the grid lines, to know when and in what direction additional angulational adjustment of the shaft  100  is necessary to bring the shaft  100  closer to the desired position, and when the shaft  100  has reached the desired position.  
         [0035]    Once the shaft  100  has been placed in the desired position, the surgeon can be confident that drilling into the vertebral bone along the trajectory established by the longitudinal axis of the shaft  100  in the desire position will result in a pedicle screw tap hole that is formed to maximize the stability of a pedicle screw subsequently screwed thereinto. That is, the surgeon can be confident that the drilling is unlikely to result in penetration of the distal end of the drill bit to any outer surface of the vertebral bone, and is likely to result in the walls of the tap hole being relatively uniformly thick at any given cross-section. Drilling into the vertebral bone along the trajectory established by the longitudinal axis  105  of the shaft  100  in the desired position can be accomplished in that the shaft  100  can be hollow, as shown, with its internal diameter being sufficient to accommodate a drill bit suitable for drilling the tap hole, and with its length being shorter than the exposed length of the drill bit (the amount of the drill bit protruding from the drill) by an amount sufficient to allow the drill bit to go into the bone to the clinically desired depth before the drill hits the proximal end of the shaft  100 . The drill bit can therefore be passed into the shaft  100 , and can be rotated therein during the drilling, so that the tap hole is drilled along an extension of the longitudinal axis of the shaft  100  at the desired angle.  
         [0036]    Alternatively, the distal end of the drill bit can be placed against the preferred tap hole entry point of the exposed vertebral bone, and the shaft  100  can be held parallel to the longitudinal axis of the drill bit. (This parallel holding can be accomplished, for example, by using suitable attachments or mountings for the shaft against the drill.) The drill bit and the shaft  100  can be angulated together (while being maintained in relative parallel positions) until the angular difference between the longitudinal axis  105  of the shaft  100  and the acting direction of gravity in the first plane matches the determined cephalad-caudad declination, and the angular difference between the longitudinal axis  105  of the shaft  100  and the acting direction of gravity in the second plane matches the determined medial angulation. (It should be understood that alternatively, the device can be used with the grid lines in the set  140  begin use to match the medial angulation, and the grid lines in the set  142  being used to match the cephalad-caudad declination.) During this angulation, the surgeon can view the position of the bubble  130  under the guide grid  150 , and particularly the bubble&#39;s position relative to the grid lines, to know when and in what direction additional angulational adjustment of the drill bit (and parallel shaft  100 ) is necessary to bring the drill bit closer to the desired position, and when the drill bit has reached the desired position. Once the drill bit has been placed in the desired position, the surgeon can be confident that drilling into the vertebral bone along the trajectory established the longitudinal axis of the drill bit in the desired position will result in a pedicle screw tap hole that is formed to maximize the stability of a pedicle screw subsequently screwed thereinto.  
         [0037]    Referring now to FIGS. 2 a - c , another embodiment of a gravity dependent pedicle screw tap hole guide of the present invention is illustrated. The guide in this embodiment has a shaft  200  that has proximal end  201  and a distal end  202  and a longitudinal axis  205 , and a fluid chamber  210  attached to the shaft  200 . The fluid chamber  210  is partially filled with fluid  220 , and the fluid  220  is contained within the chamber  210 , such that a bubble  230  is present in the chamber  210 . Because the gas in the bubble  230  is lighter than the fluid in the chamber  210 , the bubble  230  floats in the chamber  210 , seeking to travel in a direction opposite the acting direction of gravity, but being prevented from leaving the chamber  210  because the chamber  210  is closed. Preferably, as shown, the chamber  210  defines a plane  215  that is perpendicular to the acting direction of gravity when the chamber  210  is held level. The chamber  210  has a translucent wall  235  through which the bubble  230  is visible. The translucent wall  235  has a reference mark  260  positioned so that that when the chamber  210  is held level, the bubble  230  is centered under the reference mark  260 . The chamber  210  is movably attached to the shaft  200  and thereby positionable relative to the shaft  200 . Specifically, the degree of perpendicularity of the longitudinal axis  205  of the shaft  200  relative to the plane  215  defined by the chamber  210  can be varied in at least two planes.  
         [0038]    Preferably, as shown, a platform  282  is laterally attached to the shaft  200 . The chamber  210  is a transparent cylindrical enclosure  210  mounted on the platform  282 , the bottom surface  215  of the chamber  210  defining the plane  215 . Also preferably, an upper surface  235  of the enclosure is centrally marked with a circle  260 . When the chamber  210  is oriented so that the bottom surface  215  is held level, the bubble  230  is under the circle  260 .  
         [0039]    Also preferably, the movable attachment of the chamber  210  to the shaft  200  is achieved by two rotatable mountings  270 ,  280  between the chamber  210  and the shaft  200 . The first rotatable mounting  270  is between the shaft  200  and the second rotatable mounting  280 . The second rotatable  280  mounting is between the first rotatable mounting  270  and the chamber  210 . The first rotatable mounting  270  rotates about an axis  275  extending perpendicular to the longitudinal axis  205  of the shaft  200 , and the second rotatable mounting  280  rotates about an axis  285  extending perpendicular to the plane  215  defined by the chamber  210 . Each of the rotatable mountings  270 ,  280  can be secured at any position to which it can be rotated. In this embodiment, the securing is accomplished in each rotatable mounting by a set screw that when loose, permits rotation, and when tight, prevents rotation by pressing the relatively moving surfaces of the rotatable mounting against one another. Alternative or additional securing mechanisms can be provided within the scope of the present invention.  
         [0040]    Also preferably, the angles of rotation that can be achieved by the rotatable mountings are indicated by two sets  240 ,  242  of angle marks associated respectively with each rotatable mounting  270 ,  280 . Each set has a zero mark, each zero mark indicating a zero position into which the associated rotatable mounting can be placed. When each rotatable mounting  270 ,  280  is in its zero position, the plane  215  of the enclosure  210  is perpendicular to the longitudinal axis  205  of the shaft  200  and, accordingly, when the enclosure  210  is oriented such that the bubble  230  is under the circle  260 , the longitudinal axis  205  of the shaft  200  is parallel to the acting direction of gravity.  
         [0041]    Additional marks in the set preferably indicate the relative angle of rotation of the rotatable mounting with respect to this zero position, such that if either or both of the rotatable mountings are placed in a rotated position, the user can read the marks to determine the angular difference between the longitudinal axis  205  of the shaft  200  and the plane  215  when the enclosure  210  is oriented so that the bubble  230  is under the circle  260 . Preferably, each set marks 10 degree increments, e.g., −40, −30, −20, −10, 0, 10, 20, 30, 40, with the first rotatable mounting marks indicating the angular difference in a first plane, and the second rotatable mounting marks indicating the angular offset in a second plane parallel to the first plane. It should be understood that other labeling, with greater or lesser angles, and greater or lesser increments, can also be used.  
         [0042]    In operation of this embodiment, the surgeon proceeds as indicated above with regard to the first embodiment, applying a Kocher clamp in a vertical position to the spinous process of the vertebral bone into which the pedicle screw is to be placed. The surgeon can again use his best visual approximation to apply the Kocher clamp vertically, or can preferably use the gravity dependent pedicle screw tap hole guide of this embodiment to make the placement more accurate. That is, the rotatable mountings  270 ,  280  of the guide can be placed in their respective zero positions, so that the plane  215  of the enclosure  210  is perpendicular to the longitudinal axis  205  of the shaft  200 , and the shaft  200  can then be held parallel to the longitudinal axis of the Kocher clamp, and manipulated with the Kocher clamp while being maintained in said parallel position similar to the use of the first embodiment discussed above, so that when the bubble  230  is centered under the circle  260 , the surgeon knows that the Kocher clamp is in a vertical position.  
         [0043]    Next, a lateral radiograph is taken and used to approximate the cephalad-caudad declination of the pedicle of interest, and the medial angulation of the pedicle is determined from preoperative transaxial MRI and/or CAT scan images. The surgeon then places the first rotatable mounting  270  into a rotated position at an angular orientation matching the cephalad-caudad declination, and places the second rotatable mounting  280  into a rotated position at an angular orientation matching the medial angulation. During these rotations, the surgeon can view the rotatable mounting marks  240 ,  242  to ensure that the mountings  270 ,  280  are rotated to the desired angles.  
         [0044]    Then, the surgeon positions the distal end  202  of the shaft  200  against the preferred tap hole entry point of the exposed vertebral bone, and angulates the shaft  200  until the bubble  230  is under the circle  260 . When the bubble  230  is under the circle  260 , this indicates to the surgeon that the angulation of the shaft  200  matches the angulation of the pedicle with respect to the vertical.  
         [0045]    Once the shaft  200  has been placed in the desired position, the surgeon can be confident that drilling into the vertebral bone along the trajectory established by the longitudinal axis  205  of the shaft  200  in the desired position will result in a pedicle screw tap hole that is formed to maximize the stability of a pedicle screw subsequently screwed thereinto. Drilling into the vertebral bone along an extension of the longitudinal axis  205  of the shaft  200  can be accomplished in that the shaft  200  can be hollow, as discussed with regard to the first embodiment, and the drill bit passed into and rotated in the shaft  200  during the drilling. Alternatively, also as discussed with regard to the first embodiment, if a hollow shaft is not used, the shaft  200  can be held parallel to the longitudinal axis of the drill bit, and the drill bit and the shaft  200  can be angulated together (while being maintained in relative parallel positions) until the bubble  230  is under the circle  260 . When the bubble  230  is under the circle  260 , this indicates to the surgeon that the angular orientation of the shaft  200  (and therefore the angular orientation of the drill bit) matches the angular orientation of the pedicle with respect to the vertical.  
         [0046]    Referring now to FIGS. 3 a - c , yet another embodiment of a gravity dependent pedicle screw tap hole guide of the present invention is illustrated. The guide in this embodiment has a shaft  300  that has a proximal end  301  and a distal end  302  and a longitudinal axis  305 , and an accelerometer  310  attached to the shaft  300 . The accelerometer  310  is an electronic device that can determine its angular orientation relative to the acting direction of gravity, and therefore can be used to determine, for any device in fixed relation to the accelerometer  310 , the angular orientation of that device relative to the acting direction of gravity. Although a variety of accelerometers exist and can be used with the present invention, one example of an accelerometer that can be used with the present invention has as its central functional mechanism a computer chip that determines the angular orientation of a reference direction relative to the acting direction of gravity, and further can be connected to other electronic devices to provide relevant data in that regard to such devices. A suitable accelerometer is sold by Analog Devices, Inc. (Norwood, Mass.) as product number ADXL202. Accordingly, and preferably as shown, an analog or digital readout  320  in communication with the accelerometer  310  is viewable to provide the angular orientation of the accelerometer  310  relative to the acting direction of gravity.  
         [0047]    Preferably, as shown, the shaft  300  is attached in fixed relation to the accelerometer  310  such that when the accelerometer  310  indicates that there is no angular difference between the reference direction recognized by the accelerometer  310  and the acting direction of gravity, the longitudinal axis  305  of the shaft  300  is parallel to the acting direction of gravity. Accordingly, as the shaft  300  is oriented freely in space, the accelerometer  310  indicates the angular difference (preferably in two dimensions) between the longitudinal axis  305  of the shaft  300  and the acting direction of gravity.  
         [0048]    Operation of this embodiment proceeds as indicated with regard to the first embodiment, with the accelerometer  310  (rather than the fluid-containing enclosure of the first embodiment) indicating when the shaft  300  is in the desired position, that is, when the angular difference between the longitudinal axis of the shaft  300  and the acting direction of gravity matches the cephalad-caudad declination (in the first plane) and medial angulation (in the second plane) of the pedicle.  
         [0049]    Referring now to FIGS. 4 a - c , still another embodiment of a gravity dependent pedicle screw tap hole guide of the present invention is illustrated. The guide in this embodiment is similar to that of the second embodiment described above, except that the fluid-containing enclosure  210  of that embodiment is replaced with an accelerometer  410  similar to the accelerometer  310  described in the third embodiment described above. Elements in this fourth embodiment that are similar to those in the second embodiment are referenced with like numbers, but in the four hundreds rather than the two hundreds. Accordingly, when each rotatable mounting  470 ,  480  is in its zero position, and the accelerometer  410  reads level, the longitudinal axis  405  of the shaft  400  is parallel to the acting direction of gravity. And, accordingly, if either or both of the rotatable mountings are placed in a rotated position, the user can read the marks in the mark sets  440 ,  442  to determine the angular difference between the longitudinal axis  405  of the shaft  400  and the acting direction of gravity when the accelerometer  410  is oriented level.  
         [0050]    Operation of this embodiment proceeds as indicated with regard to the second embodiment, with the accelerometer  410  indicating when the accelerometer  410  is oriented level (and thus, if the rotatable mountings  470 ,  480  have been rotated to match the cephalad-caudad declination and medial angulation of the pedicle, that the shaft  400  is at the desired angulation).  
         [0051]    While there has been described and illustrated specific embodiments of an intervertebral spacer device, it will be apparent to those skilled in the art that variations and modifications are possible without deviating from the broad spirit and principle of the present invention. The invention, therefore, shall not be limited to the specific embodiments discussed herein.