Abstract:
An apparatus for implementing an improved spinal fusion cage have a centrally-located fixation screwhole of diameter minimized to accommodate a variable-angle, intervertebral, cannulated fixation screw is disclosed. The system includes a rigid connection between the spinal fusion cage to the shaft of the fixation screw, and a variable angle drill targeting device for directing the fixation screw through the on-visually acquirable, centrally-located fixation screwhole without X-ray or other imaging guidance.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This international application claims priority to U.S. application Ser. No. 13/388,993, filed on Feb. 5, 2012, which in turn claims priority to International Application No. PCT/US2010/055531, filed on Nov. 5, 2010, which in turn claims priority to U.S. Provisional Application No. 61/280,621, filed on Nov. 6, 2009, where the entire contents of all applications are incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to a surgical apparatus, and a procedure and method for using that apparatus. 
     BACKGROUND OF THE INVENTION 
     A spinal fusion is a surgical procedure that promotes two back bones, or vertebrae, growing together into one bone.  FIG. 1  models the front or anteroposterior view of two such vertebrae L5 ( 104 ) and S1 ( 108 ) separated by a disc  112 .  FIG. 2  models the side or lateral view of the same. As shown in  FIG. 3 , in the course of the spinal fusion, a fusion cage  304  is at times required. 
     The fusion cage  304  is a medical implant that is used to replace the removed disc between two vertebrae or to replace one or more vertebrae and their adjacent disc. Fenestrations  308  in these cages are usually filled with a bone grafting material that promotes a bony fusion between the bone above the cage and the bone below the cage. A screwhole  312  is usually provided in the front of the cage to accommodate an insertion handle used in positioning the cage into the disc space. Fusion cages are supplied in various sizes to fit the space between the bones being fused. As shown in  FIG. 4 , a trial cage  404  can be used to select the desired size fusion cage to fit into the disc space. 
     In some situations it is desirable to mechanically fix together the bones above and below the cage in order to limit movement between the bones and cages, thus promoting successful fusions and preventing cage displacement. This fixation can be accomplished as shown in  FIG. 5  by passing a guide pin  504  into the bone on the near side of the cage, through a fenestration  308  in the interior of the fusion cage  304 , and then into the bone on the far side of the cage. A fixation screw  508  can then be inserted over the guide pin  504  after which the guide pin is then removed. As  FIG. 6  illustrates, the cage fenestration  308  cannot be visually acquired with the cage  304  in place in the disc. The trajectory of the guide pin  504 , starting from the visually acquired guide pin entry point  604 , must instead be directed with the use of x-rays. 
     In order to visualize the location of the cage fenestration  308  ( FIG. 5 ) through which a fixation screw  508  is to pass, metal markers are placed in the wall of the fusion cage  304  that are immediately adjacent to this fenestration  308 . 
       FIG. 7  illustrates the front view of the spine shown in  FIG. 6  as it would appear on an x-ray. The L5 backbone  104  and S1 backbone  108  would be seen. Since the fusion cage  304  and the remaining disc  112  are invisible on x-ray, an empty space would appear in their place. Because, like bone, metal is visible on x-ray, metal markers in the walls of the cage fenestration  308  would be visible. The surgeon would be able to acquire the right side wall marker  704  and the left side wall marker  708 . 
       FIG. 8  illustrates the side view of the spine pictured in  FIG. 6  as it would appear on an x-ray. The L5 backbone  104  and the S1 backbone  108  would be visible. Again, an empty space would appear in the place of fusion cage  304  and any remaining disc  112 . On the x-ray, the surgeon would be able to acquire the additional front wall marker  804  and the rear wall marker  808 . As depicted in  FIG. 8 , these markers can be made in a different shape in order to distinguish them from the side wall markers  704  and  708  shown in  FIG. 7 . 
     Being metallic, the guide pin  504  can also be seen on an x-ray. Using the front x-ray view exemplified by that shown in  FIG. 7 , a surgeon would direct the guide pin  504  between the right side wall marker  704  and the left side wall marker  708 . Using the side x-ray view  FIG. 8 , the surgeon simultaneously directs the guide pin  504  between the front wall marker  804  and the rear wall marker  808 . 
     The surgeon would thus be assured that s/he has passed the guide pin  504  through the L5 backbone  104 , through the cage fenestration  308  in the fusion cage  304 , and into the S1 backbone  108 . The surgeon can then insert the fixation screw  508  down over the guide pin  504  as shown in  FIG. 7  and  FIG. 8 . The guide pin  504  would then be removed, leaving the fixation screw  508  in position passing through the cage fenestration  308  in the fusion cage  304 . 
     Unfortunately, due to the difficulty in directing a guide pin  504  through a screwhole fenestration  308  in a fusion cage  304  using x-rays, the screwhole fenestration  308  must be significantly larger than the fixation screw  508 . As a result, it is possible for the fusion cage  304  to partially displace out of the disc  112 . This displacement can then result in excessive movement between the L5 backbone  104  and the S1 backbone  108 , resulting in a failure of the spinal fusion. As shown in  FIG. 5 , a large screwhole fenestration  308  leaves any remaining fenestrations in the fusion cage  304  to be small. This results in most of the bone grafting material being placed in the screwhole fenestration  308 , which is unwanted. 
     Further, passage of the fixation screw  508  through the screwhole fenestration  308  can disturb this bone grafting material and adversely impact a successful spinal fusion. It is therefore desirable to make the screwhole fenestration  308  as small as possible in order to prevent cage migration, and to allow the remaining fenestrations to be as large as possible and to carry the majority of the bone grafting material. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an apparatus and method for performing specific types of spinal surgeries. The apparatus and method assist in permanently locating a fusion cage within a spine. It is an additional object of the present invention to make the surgical procedures easier and safer to perform. These and other objects and advantages of the invention will become readily apparent as the following description is read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 2  show front and side views, respectively, of two vertebrae; 
         FIG. 3  shows an example fusion cage; 
         FIG. 4  shows an example trial cage; 
         FIG. 5  shows an example guide pin and fixation screw within a fusion cage such as that shown in  FIG. 3 ; 
         FIG. 6  shows a view of a spine with a fusion cage inserted therein; 
         FIGS. 7 and 8  show the front and side views of the spine shown in  FIG. 6 ; 
         FIGS. 9A ,  9 B,  9 C, and  9 D show, respectively, plan, front, right-side, and cross-sectional right-side views of a modified fusion cage; 
         FIG. 9E  shows the modified fusion cage of  FIGS. 9A-9D  including front and rear trajectories; 
         FIG. 9F  shows the modified fusion cage of  FIGS. 9A-9E  including right and left trajectories; 
         FIG. 10A  shows a plan view and cross-sectional right side view of a second version of a modified fusion cage; 
         FIG. 10B  shows a snap ring incorporating an anti-spin tab which fits into the anti-spin recess within the modified fusion cage of  FIG. 10A ; 
         FIGS. 10C ,  10 D,  10 E, and  10 F illustrate a snap ring within the modified fusion cage of  FIGS. 10A-10B ; 
         FIG. 11A  shows a rigid drill targeting device; 
         FIGS. 11B and 11C  show a drill target inserted through a utility screwhole in a modified fusion cage; 
         FIG. 12A  shows a left side view of an articulating drill targeting device; 
         FIG. 12B  shows an example of an insertion handle that screws into a modified fusion cage; 
         FIG. 12C  shows a back view of the articulating drill targeting device of  FIG. 12A ; 
         FIG. 12D  shows a left side view and a cross-sectional view of a guide body; 
         FIGS. 12E-12F  show back and front views, respectively, of the guide body of  FIG. 12D ; 
         FIG. 12G  shows an exploded perspective view of the guide body of  FIGS. 12D-12F ; 
         FIG. 12H  shows a drill guide having depth gauge markings and numbers, and a flexible guide pin tissue protector that can be screwed onto or otherwise attached to the drill guide; 
         FIG. 12I  shows a flexible screw insertion tissue protector that can be slid over a guide pin; 
         FIG. 13  depicts a potential method for implementing the preferred embodiments; 
         FIG. 14  shows excising a diseased disc, thus creating a space for a cage; 
         FIG. 15  shows trialing the disc space of  FIG. 14  by attaching the trial cage to an insertion handle; 
         FIG. 16  shows inserting a modified fusion cage into a space using an insertion handle; 
         FIG. 17A  shows an example position of a rigid drill targeting device rotated to the left side; 
         FIG. 17B  shows the rigid drill targeting device of  FIG. 17A  rotated to the right side; 
         FIGS. 17C and 17D  shows the rigid drill targeting device rotated toward the front and back directions, respectively; 
         FIG. 17E-17H  show example positions of an articulating drill targeting device; 
         FIG. 18  illustrates obtaining an example measurement for a fixation screw length; 
         FIG. 19  shows an example use of a guide pin operated in conjunction with the articulating drill targeting device of  FIGS. 17E-17H ; 
         FIG. 20  shows an example removal of the articulating drill targeting device of  FIGS. 17E-17H ; 
         FIG. 21  shows an example insertion of a fixation screw; 
         FIG. 22  shows an example of the fixation screw of  FIG. 21  being in a final position; and 
         FIG. 23  shows an example of the fixation screw of  FIGS. 21-22  being locked. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before explaining the disclosed embodiment of the present invention in detail it is to be understood that the invention is not limited in its application to the details of the particular arrangement shown, since the invention is capable of other embodiments. Also, the terminology used herein is for the purpose of description and not of limitation. 
     The embodiments disclosed herein involve the use of a rigid drill targeting device  1104  such as that shown in  FIG. 11A  or an articulating drill targeting device  1204  such as shown in  FIG. 12A  combined with a modified fusion cage  904   FIG. 9A , or with a modified fusion cage  1004   FIG. 10A . As noted earlier, a fusion cage is a medical implant that is to be permanently installed within a human spine. The targeting devices disclosed herein and other elements are combined to direct a guide pin  504  (e.g.  FIG. 8 ) along the desired trajectory for placing a screw through the bone on the near side of the modified fusion cage  904  or modified fusion cage  1004 , through a fixation screwhole  914  in modified fusion cage  904  (e.g.  FIG. 9A ) or a fixation screwhole  1014  in a modified fusion cage  1004  (e.g.  FIG. 10A ), and into the bone on the far side of the modified fusion cage  904  or modified fusion cage  1004 . The drawings herein illustrating the various embodiments generally show to the L5 and S1 vertebrae. However, it is to be noted that the L5 and S1 vertebrae are but examples, for illustration only. The embodiments discussed herein can also be used on other combinations of vertebrae besides those explicitly shown in the drawings. 
     A first advantage is that X-ray guidance of the guide pin  504  is not required. The improved accuracy of insertion of guide pin  504  allows the fixation screwhole  914  or fixation screwhole  1014  to be made in a diameter that is near or the same as that of the fixation screw  508 . 
     A second advantage is that the risk of cage migration is eliminated. The remaining cage fenestrations can be made as large as possible and will contain all of the bone grafting material. There is no bone grafting material to be disturbed by the insertion of the fixation screw  508 . 
       FIG. 9A  shows the plan view of a modified fusion cage  904  having a front  906 , back  908 , right side  910 , and left side  912 . A centrally-located fixation screwhole  914  accommodates passage of a guide pin  504  followed by a fixation screw  508  as was illustrated at least within  FIGS. 7 and 8 . As shown in  FIG. 9A , additional fenestrations  308  accommodate bone grafting material. 
       FIG. 9B  shows the front view of the modified fusion cage  904  incorporating a utility screwhole  920  which connects with the fixation screwhole  914 .  FIG. 9C  shows the right side view of the modified fusion cage  904  with the taller front  906  and shorter back  908  which fit the tapered shape of disc  112  that the modified fusion cage  904  replaces.  FIG. 9D  shows the cross-sectional right side view of the modified fusion cage  904  illustrating an hourglass shape of the fixation screwhole  914 . Also shown is the connecting utility screwhole  920 . 
       FIG. 9E  shows how this hourglass shape is necessary if the fixation screwhole  914  is to be made the smallest diameter possible yet still accommodate a fixation screw  508  being inserted along a trajectory which can vary through arc  924  from a maximum front trajectory  926  to a maximum rear trajectory  928 . 
     Also illustrated within  FIG. 9E  is how the connecting utility screwhole  920  can accommodate the insertion of a locking screw  930  which can engage the fixation screw  508  and prevent it from subsequently migrating in or backing out of the fixation screwhole  914 . In  FIG. 9F , a cross-sectional front view of the modified fusion cage  904  shows how this same hourglass shape keeps the diameter of the fixation screwhole  914  to a minimum while accommodating a fixation screw  508  being inserted along a trajectory which can vary from a maximum left trajectory  936 , through an arc  934 , to a maximum right trajectory  938 . 
       FIG. 10A  shows a plan view and cross-sectional right side view of a second version of a modified fusion cage  1004  having the same features as the modified fusion cage  904 , except for a further modification of fixation screwhole  914  resulting in the fixation screwhole  1014 . This fixation screwhole  1014  includes a bearing or snap ring recess  1016  which accommodates a tilting bearing or snap ring  1018 . The bearing or snap ring recess  1016  may incorporate an anti-spin recess  1017 . 
       FIG. 10B  shows the snap ring  1018  incorporating an anti-spin tab  1019  which fits into the anti-spin recess  1017  located at the back of the bearing or snap ring recess  1016 . The anti-spin recess is sufficiently large to allow the bearing or snap ring  1018  to tilt in the bearing or snap ring recess  1016 , but small enough to prevent the bearing or snap ring  1018  spinning while fixation screw  508  is being inserted. The bearing or snap ring  1018  incorporates a central screwhole  1020  which is the same diameter as the fixation screw  508 . 
     This same anti-spin feature maybe accomplished by making the bearing or snap ring recess  1016  non-circular in shape and making the bearing or snap ring  1018  have a matching non-circular outer shape. 
     While the hourglass shape of the fixation screwhole  914  in the modified fusion cage  904  reduces the diameter of the fixation screwhole  914  to a minimum, screw passage from a variety of angles as described at least within  FIGS. 9E and 9F  results in the minimum diameter at the waist of the hourglass-shaped fixation screwhole  914  still being slightly larger than the diameter of the fixation screw  508 . 
     Undesired residual movement between the modified fusion cage  904  and the fixation screw  508  can be further reduced or eliminated by the locking screw  930 .  FIGS. 10C ,  10 D,  10 E, and  10 F illustrate how use of the tilting bearing or snap ring  1018  in the modified fusion cage  1004  achieves a fixation screwhole  1014  having a minimum diameter exactly matching the diameter of the fixation screw  508 . 
       FIG. 10C  shows a cross-sectional right side view of the modified fusion cage  1004 . The fixation screw  508  is being inserted along the maximum front trajectory  926 . As the fixation screw  508  starts to thread itself through the central screwhole  1020  in the bearing or snap ring  1018 , the bearing or snap ring  1018  will tilt to the front until it becomes perpendicular to the fixation screw  508 . 
     Since the inner diameter of the bearing or snap ring  1018  matches the outer diameter of the fixation screw  508 , there is no side-to-side movement possible between the two. Further, since the outer dimension of the tilting bearing or snap ring  1018  matches the inner dimension of the bearing or snap ring recess  1016 , there is no side-to-side movement possible between these two either. The combined result ensures that no side-to-side movement is possible between the fixation screw  508  and the modified fusion cage  1004 . As in the modified fusion cage  904 , the locking screw  930  can then be inserted into the utility screwhole  920  to engage the fixation screw  508  and prevent its migrating in or backing out of the fixation screwhole  1014 . 
       FIG. 10D  shows the tilting bearing or snap ring  1018  tilted to the back accommodating insertion of a fixation screw  508  along the maximum back trajectory  928 . Similarly, the cross-sectional front views at least within  FIGS. 10E and 10F  illustrate how the tilting bearing or snap ring  1018  can tilt respectively left accommodating the maximum left trajectory  936  or right accommodating the maximum right trajectory  938  for insertion of the fixation screw  508 . 
     The tilting snap ring  1018  can be made with the diameter of the central hole  1020  (e.g.  FIG. 10B ) slightly smaller than the outer diameter of the fixation screw  508  and with the outer dimension of the snap ring  1018  similarly smaller than the inner dimension of the bearing or snap ring recess  1016 . As the fixation screw  508  advances into the tilting snap ring  1018 , the central hole  1020  will expand to the outer dimension of the fixation screw  508 . The outer dimension of the snap ring  1018  will similarly expand to the inner dimension of the bearing or snap ring recess  1016 . When the fixation screw  508  is in its final position, it can still be gripped tightly enough by the expanded snap ring  1018  to make the locking screw  930  unnecessary. 
     As shown in  FIG. 5 , insertion of a guide pin  504  through a large fenestration  308  using x-ray guidance can be challenging for the surgeon. Bringing an x-ray machine into the operative field requires the surgeon and his assistants to move out of the way while still trying to hold retractors and sharp instruments, such as guide pin  504 , in position. 
     As shown in  FIG. 7 , excellent hand-eye coordination is required for the surgeon to look at the relative position of the fusion cage  304 , as denoted by the right side marker  704  and left side marker  708 , and the guide pin  504  on the front x-ray view of the spine. The trajectory of the guide pin  504  in this side-to-side direction must be adjusted accordingly. The surgeon must then maintain the guide pin  504  in this exact position while the x-ray machine is re-positioned to expose the side view of the spine as shown in  FIG. 8 . Additional movement of surgeon and assistants maybe required to accommodate this repositioning of the x-ray machine. The surgeon must again rely on good hand-eye coordination while he looks at the side x-ray view of the spine and adjusts the trajectory of the guide pin  504  in this front-to-back direction before advancing the guide pin  504  into the backbone  104 . 
     The surgeon then stops advancing the guide pin  504  and repeats the front and side x-rays and makes further adjustments to the trajectory. These further adjustments can be difficult because the guide pin  504  must be pulled back almost out of the backbone  104  before it can be re-directed. When then advanced in the corrected direction, the guide pin  504  may deflect back down the previous mis-aligned drill path. The guide pin  504  may then have to be completely removed from the back bone  104  and a new entry site for guide pin insertion selected. The entire previously described guide pin insertion process must then be started all over again. 
     While the placement depicted in  FIG. 5  of a guide pin  504  through a large fenestration  308  in fusion cage  304  using the described x-ray guidance method is challenging, it is still possible in the hands of a sufficiently skilled surgeon. Conversely, the placement of a guide pin  504  through a minimal diameter fenestration, such as the fixation screwhole  914  in the modified fusion cage  904  or the fixation screwhole  1014  in modified fusion cage  1004 , using the described x-ray guidance method, is sufficiently difficult as to be impractical. Consequently, in order to achieve the benefits of the tight fit between fixation screw  508  and modified fusion cage  904  or modified fusion cage  1004 , an alternative guidance method for placement of the guide pin  504  is required. 
     As shown in  FIG. 11A , a rigid drill targeting device  1104  consists of a small sliding pipe or drill guide  1108  mounted in a guide body  1120 . A drill arm  1112  connects a semi-circular drill target  1116  to the guide body  1120 . The sliding drill guide  1108  is secured by a thumbscrew  1118 . The drill guide  1108  directs the guide pin  504  precisely through the notch in the drill target  1116  on the opposite end of the drill arm  1112 . 
       FIGS. 11B and 11C  show how the drill target  1116  can be inserted through the utility screwhole  920  in the modified fusion cage  904  to engage the front side of fixation screwhole  914 . With the drill target  1116  thus positioned, the guide pin  504  is directed by the drill guide  1108  precisely through the center of the fixation screwhole  914 . 
     The rigid drill targeting device  1104  can be rotated front-to-back while maintaining the drill target  1116  aligned in the center of the fixation screwhole  914 . The trajectory of the guide pin  504  can thus be varied through a front-to-back arc  924  as depicted at least within  FIG. 9E . 
     As depicted in  FIG. 9F  the rigid drill targeting device  1104  can also be rotated side-to-side while maintaining the drill target  1116  aligned in the center of the fixation screwhole  914 . The trajectory of the guide pin  504  can thus be varied through the side-to-side arc  934 . 
     In either variation of trajectory, the rigid drill targeting device  1104  will maintain the direction of the guide pin  504  precisely through the center of the fixation screwhole  914 . 
     The drill target  1116  can similarly be placed through the utility screwhole  920  in modified fusion cage  1004  (e.g.  FIG. 10A ) to engage the front side of the central hole  1020  of rotating bearing or snap ring  1018  (e.g.  FIG. 10B ). The guidance benefits describe for placing a guide pin  504  through the fixation screwhole  914  in modified fusion cage  904  can also be achieved for the fusion cage  1004 . 
     By using the rigid drill targeting device  1104 , x-ray guidance is not required and therefore movement of surgeon and assistants away from the operative field is not required. Also, it is not required that the surgeon have extraordinary hand-eye coordination or exceptional skill. Also, operative time for placing the guide pin  504  is reduced. Eliminating the x-ray machine from the operative field and reducing the operative time combine to reduce the likelihood of wound contamination and infection. 
     The placement of a fixation screw  508  in a minimum diameter fenestration, such as fixation screwhole  914  in modified fusion cage  904  or fixation screwhole  1014  in modified fusion cage  1004 , can be accomplished with repeatable speed and precision and with relative ease. The benefits of a tight fit between fixation screw  508  and modified fusion cage  904  or modified fusion cage  1004  can thus be realized. 
     As depicted in  FIG. 11B , movement of the rigid drill targeting device  1104  in the front-to-back direction is limited by the diameter and length of the utility screwhole  920  relative to the diameter of the drill arm  1112 . Rotation of the drill targeting device  1104  toward the front of the modified fusion cage  904 , when trying to achieve the maximum front trajectory  926  (e.g.  FIG. 9E ), can be stopped short by the drill arm  1112  contacting the bottom side of the utility screwhole  920 . Similarly, a rotation of the rigid drill targeting device  1104  toward the back of the modified fusion cage  904 , when trying to achieve the maximum back trajectory  928  (e.g.  FIG. 9E ), can be stopped short by the drill arm  1112  contacting the top side of the utility screwhole  920 . When this happens, the available arc  924  ( FIG. 9E ) for varying the trajectory of a guide pin  504  in the front-to-back direction is reduced. In turn the ability of the surgeon to pick his desired entry point into a backbone  104  (e.g.  FIG. 8 ), and thus to avoid important adjacent anatomical structures, is reduced. 
     Additionally, as the drill arm  1112  passing through the utility screwhole  920  is made smaller in diameter, the available arc  924  (e.g.  FIG. 9E ) is increased. The likelihood, however, of inadvertently bending the drill arm  1112  increases. A bent drill arm  1112  would result in a mis-direction of the guide pin  504 . 
     Further, the drill target  1116  can inadvertently slip off of its engagement on the front side of the fixation screwhole  914  (e.g.  FIG. 11B ) or on the front side of the central hole  1020  in the rotating bearing or snap ring  1018  (e.g.  FIG. 10B ). Such a disengagement, if not recognized by the surgeon, could result in a mis-direction of the guide pin  504 . 
     As shown in  FIG. 11A , the rigid drill targeting device  1104  has the advantage of simplicity of construction. It also has the advantage of rigidity of its components which best maintains the alignment of the drill guide  1104  with the drill target  1116  following repeated use. However, while much better than no targeting device, the previously described design limitations of the rigid drill targeting device  1104  somewhat reduce its utility. Consequently, an alternative targeting device that overcomes these limitations is desirable. 
       FIG. 12A  shows the left side view of an articulating drill targeting device  1204 . At the front is a drill guide  1208  that slides in a guide body  1212  and is secured by a locking lever  1216 . The guide body  1212  is linked to an inserter body  1220  at the bottom by two horizontal arms  1224  at the top and by two vertical arms  1228  at the back of the instrument. All arm connections are made with hinge pins  1232 . 
     The articulating drill targeting device  1204  is used in conjunction with a modified fusion cage  904  or a modified fusion cage  1004 , an insertion handle  1236 , a spacer sleeve  1240 , a driver cap  1244 , and a guide pin  504 . 
     As shown in  FIGS. 12A and 12B , an insertion handle  1236  is screwed in to a modified fusion cage  904  or into a modified fusion cage  1004  using an open-end wrench applied to the hex section  1252 . The articulating drill targeting device  1204  is slid down the insertion handle  1236  by way of a hole  1248  through the inserter body  1220  until it abuts the hex section  1252  of the insertion handle  1236 . A spacer sleeve  1240  slides over the insertion handle  1236  and abuts the inserter body  1220 . A driver cap  1224  is screwed onto the insertion handle  1236  and abuts the spacer sleeve  1240 . 
     The inserter body  1220  is thus maintained firmly in the desired position relative to the modified fusion cage  904  or modified fusion cage  1004 . As  FIG. 12A  shows, the drill guide  1208  directs a guide pin  504  precisely along the trajectory  1256  through the center of the fixation screwhole  914  in modified fusion cage  904  or the fixation screwhole  1014  in modified fusion cage  1004 . 
     The articulating drill targeting device  1204  is in the geometric configuration of two interconnected parallelograms as defined by the horizontal arms  1224  and guide body  1212  interconnected by hinge pins  1232  to the vertical arms  1228  and the inserter body  1220 . Utilizing this parallelogram principal, the articulating drill targeting device  1204  can be rotated front-to-back while maintaining the guide pin  504  in precise alignment with the center of the fixation screwhole  914 . 
     The further described embodiments of the articulating drill targeting device  1204  when used with modified fusion cage  904  are identical when used with fusion cage  1004 . 
     As shown in  FIG. 12A , when rotated toward the front of modified fusion cage  904 , the articulating drill targeting device  1204  can easily achieve the alignment of the guide pin  504  along the maximum front trajectory  926  without impediment. Similarly, rotation of the articulating drill targeting device  1204  toward the back of the modified fusion cage  904  achieves alignment of guide pin  504  with the maximum back trajectory  928  without impediment. 
     As  FIG. 12A  illustrates, the articulating drill targeting device  1204  is connected to the modified fusion cage  904  by the robust insertion handle  1236 . There is no small diameter drill arm  1112  (e.g.  FIG. 11B ) as used in the rigid drill targeting device  1104  that is at risk of becoming bent and misguiding a guide pin  504 . 
     As shown in  FIGS. 12A and 12B , the screw connection of insertion handle  1236  to the fusion cage  904  maintains the articulating drill targeting device  1220  to be securely attached to modified fusion cage  904 . An inadvertent disengagement between the targeting device and cage, as can occur with the rigid drill targeting device  1104 , is thus avoided. 
       FIG. 12C  shows the back view of the articulating drill targeting device  1204  with the horizontal arm  1224  and the vertical arms  1228  connected with the hinge pins  1232  to the guide body  1212  and the inserter body  1220 . The articulating drill targeting device  1204  slides onto the insertion handle  1236  using hole  1248  in the inserter body  1220 . Drill guide  1208 , and therefore guide pin  504 , are aligned with hole  1248  as depicted by guide pin trajectory  1256  (also shown at least within  FIG. 12A ). 
     As shown in  FIGS. 9B and 10A , when insertion handle  1236  is screwed into utility screwhole  920  in modified fusion cage  904 , insertion handle  1236  becomes aligned with the center of fixation screwhole  914 . With the drill guide  1208  being in alignment with hole  1220  as shown in  FIG. 12C , drill guide  1208  is thus also in alignment with insertion handle  1236  and, therefore, also in alignment with the center of fixation screwhole  914  in modified fusion cage  904 . 
     As shown in  FIG. 12C , since the articulating drill targeting device  1204  rotates side-to-side around the center of insertion handle  1236  which is aligned with the center of fixation screwhole  914 , then the articulating drill targeting device  1204  also rotates about the center of fixation screwhole  914 . 
     The articulating drill targeting device  1204  is thus unimpeded in rotating side-to-side while maintaining alignment of drill guide  1208  with fixation screwhole  914 . Articulating drill targeting device  1204  easily maintains this alignment while rotating from the maximum left guide pin trajectory  936 , through arc  934 , to the maximum right guide pin trajectory  938 . 
       FIG. 12D  shows a left side view and a cross-sectional view of the guide body  1212 .  FIG. 12E  shows the back view and  FIG. 12F  show the front view of guide body  1212 . The drill guide  1208  is able to slide up and down in hole  1258 . A spring-loaded locking lever  1216  engages serrations  1209  in the side of drill guide  1208 . The geometry of this engagement is such that the drill guide  1208  can be pushed down, without impediment, firmly into position against the backbone  104  at the desired guide pin entry point  604  (e.g.  FIG. 6 ). 
     The locking lever  1216  prevents the drill guide  1208  from backing up. The articulated drill targeting device  1204 , therefore, remains firmly in position by acting as a C-clamp in squeezing the L5 backbone  104  between the modified fusion cage  904  and the drill guide  1208 . 
       FIG. 12G  shows an exploded perspective view of guide body  1212 . A spring  1215  fixed by pin  1218  maintains the top end  1217  of locking lever  1216  to default to a raised position. The bottom end  1219  of locking lever  1216  is thereby maintained depressed against the serrations  1209  on the drill guide  1208  (e.g.  FIG. 12D ) until such time as the surgeon depresses the top end  1217  of the locking lever  1216 . 
     Following insertion of the guide pin  504 , the clamping action of the articulated drill targeting device  1204  is released by the surgeon depressing the top end  1217  of the locking lever  1216  which rotates on pin  1218  to raise the bottom end  1219  away from the serrations  1209  on drill guide  1208 . The drill guide  1208  can now be pulled up out of the drill body  1212  and off of the guide pin  504 . 
       FIGS. 12D ,  12 F, and  12 G show a guide pin exit slot  1262  connecting the front side of the guide body  1212  with the drill guide hole  1258 . Following removal of the drill guide  1208 , the articulating drill targeting device  1204  is rotated to the back allowing the guide pin  504  to exit the drill guide hole  1258  by way of this guide pin exit slot  1262 . The articulating drill targeting device  1204  can now be removed along with the inserter handle  1236  in the reverse order of their attachment to modified fusion cage  904 . 
       FIG. 12H  shows the drill guide  1208  with an alignment fin or pin  1207  that engages guide pin exit slot  1262  as drill guide  1208  slides up and down through the hole  1258  in guide body  1212 . Drill guide  1208  is thereby prevented from rotating in the hole  1258 . The serrations  1209  on the side of drill guide  1208  are thereby maintained facing and engaging locking lever  1216 . 
       FIG. 12H  shows depth gauge markings  1210  and numbers  1211  on drill guide  1208 . When the drill guide  1208  is clamped into position against the backbone  104  at the desired guide pin entry point  604  (e.g.  FIG. 6 ), the distance from the guide pin entry point  604  to the center of fixation screwhole  914  in modified fusion cage  904  can be read off the drill guide  1208  where the depth gage markings  1210  align with the top of guide body  1212 . 
     Knowing the height of the selected modified fusion cage  904  and knowing the desired distance that fixation screw  508  needs to extend past this cage, the surgeon can easily calculate the required length of fixation screw  508 . 
     By knowing the required length of the fixation screw  508 , the surgeon knows to what depth guide pin  504  must be drilled. Repeated use of x-rays to follow the advance of guide pin  504  to the desired depth is not required. Measuring the length of guide pin  504  thus inserted, in order to determine the required length of fixation screw  508 , is not necessary. Thus, the previously described negative factors associated with the use of x-ray guidance are further avoided. 
       FIG. 12H  shows a flexible guide pin tissue protector  1213  that can be screwed onto or otherwise attached to drill guide  1208 . This guide pin tissue protector  1213  prevents important adjacent tissues from being damaged by winding around the spinning guide pin  504  as it is being drilled into the bone. 
       FIG. 12I  shows a flexible screw insertion tissue protector  1266  that can be slid over guide pin  504  following insertion of guide pin  504 , removal of drill guide  1208 , and removal of articulating drill targeting device  1204 . This flexible screw insertion tissue protector  1266  accommodates the passage of a flexible drill bit  1270 . It also accommodates the passage of fixation screw  508  and attached flexible screwdriver  1274  down over guide pin  504 . Adjacent soft tissues are thus protected from damage by the spinning drill bit  1270 . Adjacent soft tissues are also protected by the spinning fixation screw  508  and screwdriver  1274  during screw insertion. 
     A Potential Method for Implementing Various of the Preferred Embodiments 
       FIG. 13  depicts an example method  1300  for implementing various of the embodiments disclosed herein. This example method is applicable to both modified fusion cages  904  and  1004 , and can be briefly summarized as follows. Step  1304  comprises excising the diseased disc. Step  1308  comprises trialing the disc space. Step  1312  comprises inserting the modified fusion cage. Step  1316  comprises positioning the drill targeting device. Step  1320  comprises measuring for fixation screw length. Step  1324  comprises pinning with the guide pin. Step  1328  comprises removing the drill targeting device. Step  1332  comprises screwing in the fixation screw. Step  1336  comprises extracting the guide pin. Step  1340  comprises locking the fixation screw. 
     The method  1300  will now be described in more detail.  FIG. 14  shows excising the diseased disc  112  from between the backbones  104  and  108  to be fused, thus creating a space  1404  for the cage. 
       FIG. 15  shows trialing the disc space by attaching the trial cage  404  to an insertion handle  1504 . The varying sizes of modified trial cages are repeatedly inserted into the disc space until a proper fit is obtained. 
       FIG. 16  shows inserting the modified fusion cage  904  into space  1404  using an insertion handle  1236 . 
       FIG. 17A  shows positioning the rigid drill targeting device  1104  by inserting the drill target  1116  through the utility screwhole  920  to engage the front side of fixation screwhole  914 . In such an environment, the rigid drill targeting device  1104  can now be rotated in the side-to-side direction and front-to-back direction to place the entry point for the guide pin  504  insertion at a position of the surgeon&#39;s choosing, while maintaining the alignment of the drill guide  1108  with the fixation screwhole  914  in the modified fusion cage  904 . 
       FIGS. 17A and 17B  show movement in the side-to-side direction. In  FIG. 17A  the rigid drill targeting device  1104  has been rotated to the left side to select an entry point  1704 . In  FIG. 17B  the rigid drill targeting device  1104  has been rotated to the right side to select a different entry point  1708 . At either entry point, the drill guide  1108  remains precisely aligned with the drill target  1116  and fixation screwhole  914 . 
       FIGS. 17C and 17D  show movement in the front-to-back direction. In  FIG. 17C  the rigid drill targeting device  1104  has been rotated toward the front. In  FIG. 17D  the rigid drill targeting device  1104  has been rotated toward the back. Rotation in this front-to-back direction is carried out simultaneously with rotation in the side-to-side direction in order to place the drill guide  1108  at the entry point of the surgeon&#39;s choosing. 
     In  FIG. 17A  the drill guide  1108  has been slid down against the bone at the chosen entry point  1704  and locked in position with the thumbscrew  1118  (e.g.  FIG. 11A ). The position of the drill targeting device is stabilized by the clamping of the backbone  104  between the drill guide  1108  and drill target  1116 . 
       FIG. 17E  shows positioning the articulating drill targeting device  1204 . The inserter handle  1236  is already in position having been used in step  1312  inserting the modified fusion cage  904 . The articulating drill targeting device  1204  is now assembled onto insertion handle  1236  as described in connection at least with  FIGS. 12A and 12B . 
     As with the rigid drill targeting device  1104 , the articulating drill targeting device  1204  can now be rotated in the side-to-side direction and in the front-to-back direction to place the entry point for guide pin  504  insertion at a location of the surgeon&#39;s choosing, while maintaining the alignment of the drill guide  1208  with the fixation screwhole  914  in the modified fusion cage  904 . 
       FIGS. 17E and 17F  show this movement in the side-to-side direction. In  FIG. 17E  the articulating drill targeting device  1204  is rotated to the left side to select an entry point  1704 . In  FIG. 17F  the articulating drill targeting device  1204  is rotated to the right side to select a different entry point  1708 . At either entry point the drill guide  1208  remains precisely aligned with the fixation screwhole  914 . 
       FIGS. 17G and 17H  show movement in the front-to-back direction. In  FIG. 17E  the articulating drill targeting device  1204  has been rotated to the front. In  FIG. 17H  the articulating drill targeting device  1204  has been rotated to the back. Rotation in this front-to-back direction is carried out simultaneously with rotation in the side-to-side direction in order to place the drill guide  1208  at the entry point of the surgeon&#39;s choosing. 
     In  FIG. 17E  the drill guide  1208  has been slid down against the bone at the chosen entry point  1704  and is held locked in position by the locking lever  1216 , as described at least in connection with  FIGS. 12D ,  12 E, and  12 F. 
       FIG. 18  shows measuring for fixation screw length by reading the value of the depth gauge marking  1210  on drill guide  1208  that is aligned with the top of guide body  1212  as described in connection at least with  FIG. 12H . The required depth of insertion of guide pin  504  is next calculated as described in connection at least with  FIG. 5  and determining the required length of the fixation screw  508 . The same procedure is used with the rigid drill targeting device  1104 . 
       FIG. 19  shows pinning the modified fusion cage  904  with a guide pin  504  using the articulating drill targeting device  1204 . The guide pin  504  is inserted into the drill guide  1208  and drilled through the backbone  104  on the near side of the modified fusion cage  904 , through the fixation screwhole  914  in the modified fusion cage  904 , and then into the backbone  108  on the far side of the modified fusion cage  904 . The guide pin  504  is inserted to the predetermined depth calculated in step  1320 . The same procedure is used with the rigid drill targeting device  1104 .  FIG. 19  shows the final position of guide pin  504 . 
       FIG. 20  shows removing the articulating drill targeting device  1204  as described in connection at least with  FIGS. 12D ,  12 F, and  12 G. The rigid drill targeting device  1104  is removed from guide pin  504  in a similar fashion. Thumbscrew  1118  (e.g.  FIG. 11A ) is loosened. Drill guide  1108  is slid up and off from the guide pin  504  and the rigid drill targeting device  1104  is rotated back and off from the guide pin  504  in a manner identical to the articulating drill targeting device  1204 . The drill target  1116  is then disengaged from fixation screwhole  914  and extracted out of the utility screwhole  920 . 
       FIG. 21  shows screwing in the fixation screw  508 . Depending on surgeon preference, drilling over guide pin  504  using cannulated drill  1270  (e.g.  FIG. 12I ) may first be performed. Fixation screw  508  is then inserted over guide pin  504  using the cannulated flexible screwdriver  1274  (e.g.  FIG. 12I ). If the surgeon prefers, these steps can be performed through the flexible screw insertion tissue protector  1266  as described in connection at least with  FIG. 12I . 
       FIG. 22  shows extracting the guide pin  504  leaving the fixation screw  508  in its final position, thereby firmly fixing modified fusion cage  904  in position between the L5 backbone  104  and the S1 backbone  108 . 
       FIG. 23  shows locking the fixation screw  508  to the modified fusion cage  904  by inserting locking screw  930  into utility screwhole  920  using for example a hex screwdriver  2304 . 
     Additional Information 
     The embodiments disclosed herein allow the accurate visual identification of a variable entry point into bone, the accurate mechanical identification of a fixed screwhole in a fusion cage positioned in a disc space, and the accurate control of the drill path between these two points. 
     Repeated X-rays are not required to adjust trajectory as the guide pin is advanced. Radiation exposure to the patient and to the surgeon is thereby reduced. 
     The continuous presence of an X-ray machine in the operative field is not required and therefore the surgeon&#39;s view of and access to the operative site during a critical phase of the surgery are not blocked. 
     Repeated adjustments of the trajectory during drilling are not required thus avoiding deflection of the guide pin down a misdirected drill path or drilling at a new entry point. 
     The ability to safely and accurately achieve bone fixation by placing the screw through the cage in this fashion avoids alternative fixation methods. These methods may involve making a separate incision. They may involve closing the present incision and turning the patient over to make a separate incision on the opposite side of the body. Avoiding the requirement to make a separate incision or to turn the patient over shortens the operative time for the patient and reduces the surgical insult to the patient. 
     A fusion cage may displace when a patient is turned over in the operating room in order to put alternative fixation in through a separate incision on the opposite side of the body. Fixing the cage with the screw in this fashion at the same time and through the same incision as the cage is placed avoids this risk of cage displacement. 
     The prominence of large implants used as an alternate fixation method on the surface of the bone, which can irritate or injure adjacent structures, is avoided. 
     A smaller diameter screwhole in the fusion cage results in a reduced distance the cage can displace before being stopped by the side of the screwhole contacting the screw. 
     A smaller diameter screwhole facilitates the design of a mechanism to lock the cage to the screw. 
     Having a fusion cage with a fenestration dedicated to the screw only and made as small in diameter as the screw, avoids partial cage displacement, damage to the graft material by the screw, and maximizes the area of remaining fenestrations for graft. This lessens the risk of a loose cage which can lead to failure of fusion. 
     Passing the screw through such a small screwhole is only feasible utilizing the drill targeting device. 
     The hourglass shape of the screwhole in modified cage  904  gives the smallest diameter fixed screwhole that will allow screw entry from multiple directions. 
     Connecting the utility screwhole to the center of the fixation screwhole allows the center of the screwhole to be accessed by the target of a rigid drill targeting device. The targeting device can then rotate on this center point allowing the guide pin to approach from varied trajectories yet still direct the guide pin through the center of the screwhole. This feature allows the use of a locking screw which decreases the risk of the fixation screw migrating in or backing out. It further diminishes any movement between the cage and fixation screw. 
     The modified cage  1004  with the tilting, bearing, or snap ring fixation screwhole allows the screwhole to be the same diameter as the fixation screw while still permitting screw entry from multiple trajectories. Having the fixation screwhole the same diameter as the fixation screw prevents any undesired movement between the screw and cage. The snap ring can act as a locking mechanism on the fixation screw so the separate locking screw is not necessarily required. The metal tilting, bearing, or snap ring can protect the fusion cage, which is usually made of plastic, from damage by the guide pin as it passes through the screwhole. If the guide pin is slightly off center, it can deflect off the metal bearing or snap ring rather than dig into the side wall of a plastic cage with the fixed hole. 
     Unlike previous cages, use of the x-ray machine is not required. Radiation exposure to the patient and operating room personnel is reduced. Surgeon and assistants having to move out of the way of the x-ray machine at the critical point in the operation is avoided. The guide pin can be placed on the first pass. Multiple trajectory changes are avoided. The operation is performed faster. Not bringing the x-ray machine into the operative field and performing the operation faster both decrease the risk of wound contamination and infection, decrease blood loss, and decrease expense of operating room time. No exceptional hand-eye skills are necessary on the surgeon&#39;s part in order to drill the guide pin through the cage. 
     The drill targeting device makes it possible to pass the guide pin on the first attempt case after case. No trajectory adjustments are required. Deflection of the guide pin down a misdirected drill path is avoided. Restarting pin entry at a new entry site to avoid an old drill path is not required. The rigid drill targeting device allows the surgeon to select from multiple possible entry points. He can select the entry point that best avoids important adjacent anatomical structures and at the same time pass the pin through the center of a minimum diameter screwhole. 
     The articulated drill targeting device has the advantages of the rigid. In addition, the articulated has a wider unrestricted range of movement in the front-to-back direction. Since it is not necessary to pass a narrow targeting arm into the center of the cage, the risk of a bent target arm misdirecting the guide pin is avoided. Since it is mounted to the insertion handle which is screwed into the cage, the connection of the targeting device to the cage is secure. The risk of the targeting device disengaging from the cage and misdirecting a screw is avoided. The ratcheting locking lever on the drill guide allows the surgeon to push the drill guide down against the bone with one hand without having to depress the locking lever. The drill guide is then firmly clamped onto the bone with the cage in the disc space on one end and the drill guide engaging the bone on the other. The locking lever will prevents the targeting device from coming unclamped by the drill guide inadvertently backing up. 
     Incorporating a guide pin exit slot allows the targeting device to be removed from the pin without sliding it up and off of the pin. At times it is necessary for the guide pin to enter the abdominal wall through a small incision separate from the larger incision in which the cage is placed into the disc and through which the targeting device is attached to the cage. The targeting device cannot be slid up and off of the pin in this circumstance. The guide pin exit slot makes it possible for the guide pin to be placed through this small separate incision yet still makes it possible for the surgeon to remove the targeting device from the pin. 
     The alignment pin on the side of the drill guide controls rotation of the drill guide in the guide block and keeps the serrations on the drill guide oriented facing the locking lever. 
     Having depth markings and numbers on the drill guide enables the targeting device to double as a measuring caliper. The depth to which the guide pin should be inserted and the length of the fixation screw that is required can be determined using this feature avoiding the necessity of doing it with an x-ray as has been historically required. 
     The ability to pass a screw through a fusion cage through the same incision on the front of the abdomen avoids having to make a separate incision on the back of the patient to insert other types of fixation screws that join the bones together. Fewer incisions mean shorter operative times, less blood loss, less risk of infection. Injury to the back muscles that occurs with this separate incision on the back is avoided. Displacement of the cage occurring when the patient is being turned over to put screws in from the back is avoided. 
     It is anticipated that various changes may be made in the arrangement and operation of the system of the present invention without departing from the spirit and scope of the invention, as defined by the following claims.