Abstract:
A system of instruments and a method for using these instruments for effective spinal stabilization using cannulated bone screws is described. In particular, the system and method provides accurate and efficient measurement of the appropriate screw length to be inserted in a patient with a lower spine injury. The system and method further enables accurate orientation or placement of the cannulated bone screws from outside the patient&#39;s body. The present method describes a surgical technique for the placement of cannulated bone screw for translaminar facet, transfacet, and general orthopedic applications.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not Applicable. 
     FIELD OF THE INVENTION 
     The invention relates generally to surgical tools for spinal stabilization, and to methods pertaining thereto. More particularly, this invention relates to a cannulated bone screw system for transfacet and translaminar stabilization of the spine. 
     BACKGROUND OF THE INVENTION 
     Thoracolumbar injury is a common pathology of the spine that is responsible for the generation of back or neck pain suffered by many patients. Injury to this area of the spine usually results from a degenerative disc disease, infection or tumor. Treatment of the injury often requires surgical intervention to restore the structural continuity of the spine. The most common surgery for treating such an injury is spinal fusion. Spinal fusion is the surgical joining of one vertebral body to another. This type of treatment often involves internal bracing and instrumentation to stabilize the spine to facilitate the efficient healing of the spine without deformity or instability, while minimizing any immobilization and post-operative care of the patient. 
     A method suggested by Magerl for treating a thoracic or lumbar pathology involves fixation of successive vertebrae using translaminar screws. Conventionally, these translaminar screws extend through the spinous process and then through the lamina at the facet joint into and through the pedicle of the successively inferior vertebrae. Another current approach utilizes standard fracture fixation techniques typically employed in orthopedic fracture applications. Such fixation techniques can include the use of lag screws to compress bone fragments together. A combination of bone plates and bone screws is also commonly implemented under current fracture fixation techniques. These bone screws and plates provide internal support as the fusion occurs. 
     With these conventional surgical procedures, screw length is determined by trial and error, probing into the surgical site, or by measurement forceps. Not only are these imprecise and inefficient ways to determine screw length, but result in excess trauma to the injury site and add to the time required to complete the surgery. Further, current systems and methods do not sufficiently enable a surgeon to identify the final screw trajectories to avoid screw collision. There is thus a need for a system and method that provides measurement of appropriate screw length in an easy and reliable manner. There is also a need for improved control over the trajectory of the screw during implantation. Finally, it is desirable to identify appropriate screw insertion sites to ensure that the final screw trajectories will not collide. 
     SUMMARY OF THE INVENTION 
     The present invention provides a system of instruments and a method for using these instruments for effective spinal stabilization using cannulated bone screws. In particular, the system and method provide accurate and efficient measurement of the appropriate screw length to be inserted in a patient undergoing spinal stabilization surgery. The system and method offers improved control over the trajectory of the screw during implantation. In addition, the system of the present invention minimizes both the need to use fluoroscopy in conjunction with the implantation process and the visibility problems that exist when installing conventional bone screws in certain patients, e.g., severely obese patients, by enabling accurate orientation or placement of the cannulated bone screws. Moreover, the present invention facilitates proper screw placement by allowing the identification of appropriate screw insertion sites to ensure that the final screw trajectories will not collide. 
     One method of using the instruments of the present invention involves an improved surgical technique for the placement of cannulated bone screws for use in orthopedic and spinal applications. Specifically, the present method is directed to a surgical technique for the placement of cannulated bone screws for translaminar facet, transfacet, and general orthopedic applications. 
     The present invention provides a cannulated screw system for effecting the posterior stabilization of the spine. The screw system also acts as an internal fixation device during the time interval required for arthrodesis. The screws can be placed either translaminar or directly through the facets for posterior spine fixation. The bone screws provide internal support as fusion and healing occur. 
     The system of the present invention includes a guide instrument, an obturator, and a series of cannulae which slide over the obturator and index with the fiducial markings on the obturator to indicate appropriate screw length. In one embodiment of the invention, there is provided an anatomic guide instrument for use with a cannulated guide pin obturator for placement of a guide wire. The anatomic guide references the desired location of the screw tip, and holds and aligns the guide pin obturator. When assembled to the anatomic guide instrument, the obturator references the desired location of the screw head. By placing a surgical cannula over the obturator, measurement marks indicate the length that spans the intended trajectory of the bone screw, which is identified by the distal prong of the guide instrument and the distal end of the obturator. 
     In addition, the guide instrument incorporates a middle prong which can either reference bony anatomy or provide a visual cue to determine screw trajectory. This orientation can be important for certain applications, e.g., spinal applications, because it establishes a safe trajectory to avoid injury to the surrounding anatomy and nervous tissues. In addition, the use of a guide wire minimizes the risk of damage or fracture of the facet by providing a stable guide for the bone screw with solid bony support. 
     In another aspect of the present invention, the guide instrument incorporates a multi-position ratchet mechanism which allows the instrument to be manipulated into several geometric configurations, thereby providing the surgeon with an instrument geometry suited to avoid interference with a patient&#39;s surrounding soft tissue anatomy. 
     Further features of the invention, its nature and various advantages, will be more apparent from the accompanying drawings and the following detailed description of the drawings and the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded view of an anatomic guide instrument of the present invention; 
     FIG. 2 is a plan view of the anatomic guide instrument of FIG. 1; 
     FIG. 3 depicts the anatomic guide instrument of FIG. 2 oriented with respect to a vertebral body; 
     FIG. 4 depicts the anatomic guide instrument of FIG. 2 and a sliding arm instrument of the present invention; 
     FIG. 5 is a perspective view of the sliding arm instrument of FIG. 4; 
     FIG. 6 depicts the anatomic guide instrument, sliding arm instrument, and guidewire of the present invention; 
     FIG. 7A is a perspective view of an obturator of the present invention; 
     FIG. 7B is a plan view of the obturator of FIG. 7A, with detailed view of the proximal end; 
     FIG. 7C is a side view of the obturator of FIGS. 7A and 7B, with detailed view of the distal end; 
     FIG. 8 is a plan view of the guidewire of FIG. 6, with detailed view of the distal end; 
     FIG. 9A is a perspective view of a cannula of the present invention; 
     FIG. 9B is a perspective view of another cannula of the present invention; 
     FIG. 9C is a perspective view of yet another cannula of the present invention; 
     FIG. 10 depicts the removal of the anatomic guide instrument of FIG. 2 from the surgical site; 
     FIG. 11 is a plan view of a drill stop of the present invention; 
     FIG. 12 is a plan view of a drill bit of the present invention, with detailed views; 
     FIG. 13 is a perspective view of a washer of the present invention; 
     FIG. 14 is a perspective view of a partially threaded cannulated screw of the present invention; 
     FIG. 15 is a perspective view of a substantially fully threaded cannulated screw of the present invention; 
     FIG. 16A is a plan view of a screw insertion instrument of the present invention; 
     FIG. 16B is a cross-sectional view of the screw insertion instrument of FIG. 16A, along lines A—A; 
     FIG. 17A is an exploded view of a screwdriver of the present invention; 
     FIG. 17B is a cross-sectional view of the screwdriver of FIG. 17A; 
     FIG. 18A is a cross-sectional view of the screws of FIGS. 14 and 15 fully inserted; and 
     FIG. 18B is another cross-sectional view of the screws of FIGS. 14 and 15 fully inserted. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides a cannulated screw system for posterior stabilization of the spine. The screw system acts as an internal fixation device during the time interval required for arthrodesis. The screws can be placed either translaminar or directly through the facets for posterior spine fixation. The bone screws provide internal support for the vertebrae as fusion and healing occur. 
     The system of the present invention includes an anatomic guide instrument  100  as shown in FIGS. 1 and 2. The guide instrument is used to determine and ensure the proper screw trajectory and to help determine the length of a bone screw to be used. The guide instrument  100  comprises a three-prong assembly  102  attached to a shaft  104 . As shown in FIG. 1, shaft  104  is detachably connectable to a grip  124 . The assembly  102  comprises a distal prong  106 , a middle prong  108 , and a proximal prong  110 . Distal prong  106  is distal-most when properly oriented with respect to a vertebral body  10  during a surgical procedure and has a pointed tip  112  that is adapted to contact the dorsal surface of the base of the transverse process  12 , as shown in FIG.  3 . Middle prong  108  has a scalloped edge  114  at its distal end that is adapted to register with the dorsal inferior third of the lamina  14 . The proximal prong  110  has an obturator block  116  that is intended to be positioned adjacent to the lateral surface of the spinous process  16  contralateral to the facet to be instrumented. Together, the placement of these prongs determines the trajectory and location of the screw itself. 
     The proximal prong  110  and its obturator block  116  function as a holding block and de facto aiming mechanism for screw insertion. The block  116  includes a locking mechanism  120  for holding and securing elongate instruments therein, and a slot  118  for quick release of the elongate instruments. In one embodiment, the locking mechanism can include a set screw (not shown). 
     A ratchet connection  122  is formed where the three-prong assembly  102  attaches to shaft  104 . The ratchet connection allows the orientation of the three-prong assembly  102  to be changed up to about 60° relative to the shaft  104 . Guide instrument  100  is preferably made of surgical grade stainless steel or other corrosive-resistant metals. 
     To initiate the procedure, a first incision  20  about 5 to 6 inches long is made and then the guide instrument  100  is positioned on the relevant features of the patient&#39;s spine  10 , as illustrated in FIG.  4 . Once the guide instrument  100  is in place, a small, second incision  30  is made lateral to the first incision  20 . The location of the second incision  30  can be determined with the aid of a sliding arm instrument  130 , shown in FIG. 5, and a guidewire  160 , shown in FIG.  8 . The sliding arm instrument  130  has a substantially U-shape, with both the proximal arm  132  and distal arm  134  each having a lug  136  extending laterally at an approximately right angle to the arm  132 ,  134 . Lug  136  on the distal arm  134  of the sliding arm instrument  130  is insertable into the obturator block  116  of the anatomic guide instrument  100 . Each lug  136  has a channel, or aperture  138  that is aligned with one another. In use, the sliding arm instrument  130  provides a location and alignment mechanism between the second incision  30  and the obturator block  116  of the proximal prong  110  of the guide instrument  100 . Together, the guide instrument  100  and the sliding arm instrument  130  enable the surgeon to determine screw trajectory from outside the patient&#39;s body. 
     FIG. 6 illustrates how the location of the second incision can be determined. Once the sliding arm instrument  130  is attached to the anatomic guide instrument  100 , a guidewire like the one shown in FIG. 8 can be passed through the proximal arm  132  of sliding arm instrument  130 . Guidewire  160  has a proximal end  162  and a distal end  164  having a trocar tip  166  that allows the guidewire  160  to be self-drilling. To determine the location of the second incision  30 , the guidewire  160  is slid through the lug  136  on the proximal arm  134  until the trocar tip  166  touches the patient&#39;s skin, indicating the location of the second incision  30 . 
     After the second incision  30  is made, the sliding arm instrument  130  is removed and a cannulated obturator  140  is passed through the second incision  30  and through the obturator block  116  of the proximal prong  110  of the guide instrument  100  until the distal end of the obturator  140  contacts bone (e.g., the lateral side of the spinous process contralateral to the facet to be instrumented). Obturator  140 , as shown in FIGS. 7A to  7 C, has a proximal end  142 , a tapered distal end  144 , and a ridged distal face  146  shown in detail in FIG. 7C to provide traction on the bony surface of the spinous process. The obturator  140  also includes a measurement window  148 , representing an opening in the wall of the obturator  140  that aids in determining the desired screw length. As shown in detail in FIG. 7B, marks, or indicia  150  representing screw lengths surround the window  148 . Obturator  140  can be inserted so that the distal end  144  extends to the middle prong  108 . The obturator  140  can also pass through the scalloped end  114  and extend all the way to the distal prong  110 . 
     Once the obturator  140  is in place within the obturator block  116 , a first cannula  170  similar to the one shown in FIG. 9A is placed over the obturator  140 . The obturator  140  includes calibrated indicia  150  that, with the aid of the first cannula  170 , helps the surgeon determine the proper length of the bone screw to be used. The first cannula  170  includes a notched proximal end  172  and a tapered distal end  174  to facilitate tunneling through soft tissue. The desired screw length can be determined by aligning the proximal end  172  of the first cannula  170  with the indicia  150  on the obturator  140 . This arrangement enables the proper screw length to be determined by assessing where the proximal end  172  of the cannula  170  registers with marks  150  formed on the obturator  140 . This reading approximates the length of the screw as it represents the distance between the distal prong  106  of the guide instrument  100  (the location of the screw tip) and the distal end  144  of the obturator (the location of the screw head). The first cannula  170  can be removed or allowed to remain in place after the screw length is determined. 
     Next, guidewire  160  is introduced through the obturator  140 . The guidewire includes a trocar tip  166  at the distal end  164  that allows it to be self-drilling. Guidewire  160  can extend all the way through obturator  140 , through the scalloped edge  114  of middle prong  108  of the guide instrument  100 , until the trocar tip  166  rests at the pointed tip  112  of the distal prong  106 . The location of the distal prong  106  represents the location of the tip of the screw to be inserted. The axis of guidewire  160  is located approximately 2-3 mm away from the axis that connects the tips of the middle prong  114  and distal prong  112  of guide instrument  100 . This offset constrains the screw trajectory to be properly oriented within the center of the lamina of the posterior spine. 
     After guidewire  160  is inserted into obturator  140 , the locking mechanism  120  of the guide instrument  100  is unlocked and the obturator  140  is disengaged from the guide instrument  100 . Preferably, guide instrument  100  can be removed at this point. To effect removal of the guide instrument  100 , the obturator  140  can be slightly retracted towards the surgeon, while guidewire  160  stays in place and the guide instrument  100  is lifted away from the vertebral body, as shown in FIG.  10 . Following the introduction of the guidewire  160 , the first cannula  170 , if previously removed, is repositioned over the obturator  140  until its distal end  174  contacts the guide instrument  100  while the obturator  140  remains in contact with the lateral surface of the spinous process. When first cannula  170  is in position, the obturator  140  can be removed, leaving the guidewire  160  and first cannula  170 . 
     At this point, a drill stop  200  shown in FIG. 11 is assembled onto a cannulated drill bit  210  as shown in FIG.  12 . Cannulated drill bit  210  has proximal end  212  connectable to a surgical drill. Distal end  214  may include specialized cutting flutes  216  as shown in detail in FIG. 12 to facilitate drilling into hard bone. Near proximal end  212  on drill bit  210  are markings  218  shown in detail that correspond to screw lengths. The drill bit  210  is placed over the guidewire  160  and used to drill to the desired depth. The cannulae  170 ,  180 ,  190  are sized at an appropriate length to limit drilling depth as determined by adjustment of the drill stop  200  as referenced by indicia  218 . The drill bit  210  is then removed and a second cannula  180  shown in FIG. 9B is placed over the first cannula  170  and the first cannula  170  is removed. Second cannula  180  includes a tapered distal end  184  and a notched proximal end  182 . Thereafter, a third cannula  190  is placed over the second cannula and the second cannula is removed. Third cannula  190  also includes a tapered distal end  194  and a notched proximal end  192 . As clearly shown in FIGS. 9A to  9 C, cannulae  170 ,  180  and  190  are graduated in size. The use of cannulae with progressively larger diameters helps prevent coring of soft tissue by gently displacing tissue to larger open diameters. 
     Once the third cannula  190  is in place, an optional washer  220  such as the one shown in FIG. 13 can be inserted over the guidewire  160  and through the cannula  190  to the proper position adjacent the bone  14 . Use of a polyaxial washer  220  optimizes the contact area with bone. However, if the washer  220  is not desired, the third cannula  190  need not be used. Next, a screw  230 ,  240  is inserted over the guidewire  160  and through the cannula  190  and the washer  220  (if present) into the predrilled hole in the vertebral bone  10 . 
     Two types of screws  230 ,  240  as illustrated in FIGS. 14 and 15 are available in the present system. The first is a substantially fully threaded screw  240 , while the second is a partially threaded screw  230  with a non-threaded lag region. The screws  230 ,  240  are available in sizes between 15 and 60 mm (partially threaded screws  220  are only available in sizes between 25 and 60 mm), thus accommodating a wide anatomical range. Screws  230 ,  240  are preferably made of a titanium alloy, but could be made of any suitable biocompatible metal or plastic. 
     In a translaminar-transfacet procedure, a partially-threaded screw  230  as illustrated in FIG. 14 and a substantially fully threaded screw  240  as shown in FIG. 15 are used in combination, to avoid interference. Both are cannulated, to be inserted around guidewire  160 . Partially-threaded screw  230  includes head  232 , tapered end  234 , and through-hole  236 . Likewise, substantially fully threaded screw  240  has head  242 , tapered end  244 , and through-hole  246 . 
     The screws  230  and  240  can be inserted over guidewire  160  with screw placement instrument  250 . As seen in FIGS. 16A and 16B, screw placement instrument  250  includes shaft  252  having a lip  254  at one end. As illustrated in FIG. 16B, within shaft  252  are a spring  254  and a plunger  260  having a receiving end  256  and a head  262 . Receiving end  256  clasps the screw  230 ,  240  until released by depressing head  262  against lip  254  of the screw placement instrument  250 . 
     A screwdriver  270  as shown in FIGS. 17A and 17B can be implemented to implant screws  230 ,  240 . Screwdriver  270  includes shaft  272  and a distal end  276  having a hexagonal head  280  attached thereto. The proximal end  274  is coupled to grip  282 . Both shaft  272  and grip  282  have through-holes  278 ,  284  that are aligned, as shown in FIG.  17 B. As such, the screwdriver  270  is able slide over guidewire  160  to engage and effect tightening of screws  230 ,  240 . The screw  230 ,  240  is fully inserted when the distal edge of grip  282  contacts the proximal end  192  of the third cannula  190 . 
     The screws are inserted through the lateral surfaces of the spinous process, cross within the spinous process, pass through the contralateral lamina and facet joint, and terminate at the entrance to the pedicle. It is recommended that a substantially fully threaded screw  240  and a partially threaded screw  230  be used at each level to minimize screw interference at the region where the screws cross, as shown in FIGS. 18A and 18B. It is critical for appropriate screw insertion sites to be identified to ensure that the final screw trajectories will not collide. 
     The following outlines suggest exemplary surgical techniques for a translaminar-facet and transfacet procedure using the present invention. 
     Translaminar-Facet Procedure 
     1. In preparation for implant placement, prepare the surgical site to provide adequate visualization of the anatomy that includes both facet joints  12 , the lamina  14 , spinous process  16 , and the base of the transverse processes. If desired, carefully remove the joint capsule and joint cartilage from the facet joints and pack with bone graft. Avoid removing subchondral bone. 
     Adjust the angle of the shaft  104  of the guide assembly  100  as needed to avoid interference with surrounding soft tissue anatomy. 
     2. Place the distal prong  106  of the guide assembly  100  on the dorsal surface of the base of the transverse process. Register the scalloped end, or notch  114  of the middle prong  108  on the dorsal inferior third of the lamina  14 , as shown in FIG.  3 . The placement of these prongs defines the trajectory and location of the guidewire  160 . The placement of the guidewire  160  then controls screw placement. 
     3. A secondary incision  30  is used through which the guidewire  160 , obturator  140 , cannulae  170 ,  180 ,  190 , washer  220 , and screw  230 ,  240  are passed. The secondary incision  30  is lateral to the primary incision  20 . The sliding arm instrument  130  should be used to identify the site of the secondary incision  30  to provide an optimal trajectory for the obturator  140  and cannulae  170 ,  180 ,  190  to lead to the site of the primary incision. 
     4. Insert the lug  136  of the sliding arm instrument  130  into the obturator block  116  of the anatomic guide instrument  100 . It is not necessary to engage the set screw  120  on the anatomic guide instrument  100 . The opposite (distal) lug  136  is outside of the surgical site, and is automatically aligned with the final trajectory for screw placement. By placing the guidewire  160  through the distal lug  136 , the location of a secondary incision site  30  can be identified. Then, a small one centimeter stab wound incision is made at this site. Use a blunt dissection instrument to establish a path between this secondary incision  30  and the primary incision  20 . This minimizes disruption to subcutaneous connective tissues and clears space for the obturator  140  and other guide instruments. 
     5. Introduce the obturator  140  into the anatomic guide instrument  100  and advance the obturator  140  until its tip contacts the lateral surface of the spinous process  16 , contralateral to the facet to be instrumented. Insertion is facilitated by rotating the obturator  140  as it is located within obturator block  116 . The obturator tip  144  must be dorsal to the intersection of the lamina with the spinous process. This entrance point is a critical point and occasionally the middle prong  108  may move dorsally and will no longer contact the lamina. 
     However, the distal prong  106  must remain on the dorsal surface of the base of the transverse process  12 . Tighten the set screw  120  on the anatomic guide instrument  100  to lock the obturator  140  in place. 
     6. Place the first cannula  170  over the obturator  140 . Make sure that the cannula  170  contacts the anatomic guide instrument  100  at the obturator block  116  and that the obturator  140  contacts the lateral surface of the spinous process  12  when reading the screw length measurement. The desired screw length can be determined where the end  172  of the cannula registers with the index marks  150  on the obturator  140 . This distance is a measure of the length between the distal prong  106  of the anatomic guide instrument  100  (located at the screw tip  234 ,  244 ) and the obturator tip  144  (located at the screw head  232 ,  242 ). 
     If it is desired to engage additional screw length beyond the location of the distal prong  106  of the anatomic guide instrument  100 , add the incremental length to that measured using the cannula  170  and obturator  140 . 
     7. Remove the first cannula  170  by sliding it back over the obturator  140 . 
     8. Insert the guidewire  160  through the obturator  140 . Advance the guidewire  160  until the black marking  168  on the guidewire  160  registers with the index mark  150  on the obturator  140  that corresponds with the desired screw length. 
     9. Disengage the obturator  140  from the anatomic guide instrument  100  by unlocking the set screw  120  on the anatomic guide instrument  100 . Slide the obturator  140  back until it is completely disengaged from the guide instrument  100 . Remove the anatomic guide instrument  100  while leaving the obturator  140  and guidewire  160  in place (the slot  118  on the obturator block  116  of the anatomic guide instrument  100  allows for disassembly from guidewire  160 ). Slide the obturator  140  back down over the guidewire  160  to once again contact the spinous process  12 . 
     10. Reinsert first cannula  170  over the obturator  140 . 
     11. Remove the obturator  140 . 
     12. Assemble the cannulated drill stop  200  onto the cannulated drill. Set the drill stop  200  on the drill bit  210  to the desired screw length. The index markings  218  on the drill bit  210  indicate drilling depth. Note: The drill stop  200  is firmly locked when the floating ring is engaged over the collet to lock the drill stop  200  to the drill bit  210 . Failure to appreciate this step could result in overdrilling. 
     Because this is a cannulated system, use a cannulated drill system. 
     13. Place the drill bit  210  over the guidewire  160  and through the first cannula  170 . Drill until the drill stop  200  contacts the end of the cannula  172 . 
     14. Remove the drill. Occasionally the guidewire  160  will come out with the drill. If so reinsert the guidewire  160  into the hole in preparation for cannulated screw insertion. Slide the second cannula  180  over the first cannula  170 . Remove the first cannula  170 , leaving the second cannula  180  in place. In a similar manner, replace the second cannula  180  with the third cannula  190 . 
     Graduating the cannulae  170 ,  180 ,  190  in this manner will help prevent maceration or “coring” of soft tissues as progressively larger cannulae are inserted. At times, the guidewire  160  may loosen when the drill is removed. In that case, reinsert the guidewire  160  into the previously-drilled hole. 
     15. Insert the washer  220  over the guidewire  160 . Pick the desired screw  230 ,  240  from the screw tray using the screw placement instrument  250  and slide the screw  230 ,  240  over the guidewire  160 . Then, slide the screwdriver  270  over the guidewire  160 , engage the hex opening of the screw head, and insert the screw  230 ,  240 . The screw  230 ,  240  is fully inserted when the handle  282  of the screwdriver  270  contacts the  192  end of the cannula. Be careful not to advance the guidewire  160  with the drill  210 . 
     16. Remove the guidewire  160 . It is recommended that screw placement be verified in coronal and sagittal planes with x-ray or fluoroscopy. 
     17. Repeat steps 1-14 to place a second screw  230 ,  240  on the opposite side of the spinous process  12 . Choose the entry site of the second screw carefully so as to avoid any interference with the first screw. In general, the entrance site should be a minimum of 5 mm cephalad of the centerline of the first screw. It is recommended that the second screw be a fully threaded screw  240 . 
     18. Close in standard fashion. 
     Transfacet Procedure 
     1. In preparation for implant placement, smooth and clear surface of bony prominences to optimize visualization of the anatomy. Adjust the angle of the shaft  104  on the guide instrument  100  to promote clear access to the surgical site. 
     2. Place the distal prong  106  of the guide instrument  100  on the base of the transverse process  12  at the junction of the superior facet. 
     3. Determine the screw entry point so that the facet joint can be instrumented. Assemble the obturator  140  into the guide instrument  100  with the tip of the obturator  140  contacting the facet at the determined entry point. Tighten the set screw  120  to lock the obturator  140  in place. 
     4. Place the first cannula  170  over the obturator  140 . The position of the end  172  of the cannula  170 , relative to the depth index marks  150  on the obturator  140 , indicates screw length to the distal prong  106  from the entrance point on the facet as determined in step 3. 
     5. Remove the first cannula  170 . 
     6. Place the guidewire  160  through the obturator  140 . 
     7. Insert the guidewire  160  through the obturator  140 . Advance the guidewire  160  until the guidewire  160  indexes with the desired screw length as indicated on the obturator  140 . 
     8. Unlock the set screw  120 , leaving the obturator  140  on the guidewire  160  but disengage the guidewire  160  from the anatomic guide instrument  100 . 
     9. Remove the anatomic guide instrument  100 , leaving the guidewire  160  and obturator  140  in place. Slide the obturator  140  over the guidewire  160  to the surface of the facet. 
     10. Reinsert the first cannula  170  over the obturator  140 . 
     11. Remove the obturator  140 . 
     12. Assemble the drill. Set the drill stop  200  at the desired screw engagement depth. The drill stop  200  is firmly locked when the floating ring is moved over the expanding collet to the desired drill depth, as marked on the drill bit  210 . 
     13. Drill through the first cannula  170  until the drill stop  200  makes contact with the distal end of  174  the cannula  170 . Be careful not to advance the guidewire  160  with the drill. 
     14. The screw  230 ,  240  and washer  220  can be inserted through the third cannula  190 . Use the following sequence: 
     a. Remove the drill and slide the second cannula  180  over first cannula  170 . 
     b. Next, replace the first cannula  170  with the second cannula  180 . 
     c. Replace the second cannula  180  with the third cannula  190 . 
     d. Graduating the cannulae in this manner will help protect soft tissues by preventing “coring”. 
     e. Remove the first and second cannulae  170 ,  180 . 
     15. Insert the washer  220  over the guidewire  160  (optional). Select the desired screw  230 ,  240  and slide it over the engaged guidewire  160 . Next slide the screwdriver  270  over the guidewire  160  and engage the hex of the screw. Insert the screw. 
     The screw  230 ,  240  is fully inserted when the handle  282  of the screwdriver  270  contacts the distal end  194  of the cannula  190 . Do not allow the guidewire  160  to be advanced unintentionally with either the drill or screws. 
     16. Remove the guidewire  160 . Verify screw placement with x-ray or fluoroscopy is desired. 
     17. Repeat on opposite side. 
     18. Close in standard fashion. 
     It will be understood that the foregoing is only illustrative of the principles of the invention, and that various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention. All references cited herein are expressly incorporated by reference in their entirety.