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 which 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 . first , the patient spine is exposed through an anterior approach ( i . e . the surgeon creates an access hole which permits direct interaction with the anterior and / or anterio - lateral portion of the intervertebral bodies ). in the case of scoliosis , as well as in other disorders in which the intervertebral space requires distraction and / or repositioning , the surgeon removes the intervertebral disc material , usually leaving some portion of the annulus ( the cylindrical weave of fibrous tissue which normally surrounds and constrains the softer cartilage cushion of the disc material ). the surgeon then , in succession , inserts a series of intervertebral trial spacers of defined width . each of the series of spacers is of a progressively wider thickness , resulting in the continual widening of the space until restoration of the proper disc height has been achieved . proper disc height restoration is determined by surgical experience , and by observation of the annulus . ( often , the tightening of the annulus indicates that the proper disc height has been reached , inasmuch as the annulus is much less likely to be distorted by the same disruption that caused the intervertebral disc to collapse in the first place .) more particularly , with respect to the specific instruments disclosed herein , a series of solid trial spacer elements and an instrument for their insertion and removal is now provided . each trial spacer is a generally cylindrical disc having a deep annular groove at its midpoint , which forms a central trunk and radial flanges at each end of the trunk . stated alternatively , two cylindrical upper and lower halves of the disc are held in a closely coaxial spaced apart association by the central trunk , which forms a coaxial bridge between the upper and lower halves . the annular groove is particularly useful for holding the spacer using the spacer insertion instrument of the invention , described below , in that the holding end of the insertion instrument fits within the groove . a variety of features of embodiments of the trial spacer elements are disclosed . in some embodiments , such as the first and second embodiments described below , support portions ( the portions that are in contact with the adjacent vertebral bodies when the spacer is disposed between the bodies ) of the top and bottom surfaces are parallel . spacers having this feature are generally described herein as “ constant thickness ” trial spacers . in other embodiments , such as the third and fourth embodiments described below , the support portions are not parallel , providing an overall taper to the spacer at an angle . spacers having this feature are generally described herein as “ tapered thickness ” trial spacers . the tapered thickness trial spacers are particularly useful for treating scoliosis , as described below . other features of embodiments of the trial spacer elements include beveled flanges and non - parallel annular groove walls . more specifically , in some embodiments , such as the second and fourth embodiments described below , the flanges are radially beveled in that an outer edge of the top surface of the disc is tapered toward an outer edge of the bottom surface of the disc . in other embodiments , such as the first and third embodiments described below , the flanges are not radially beveled in this manner . the radial beveling feature can be particularly useful for easing the insertion of the spacer in between collapsed vertebral bodies , as described below . further , in some embodiments , such as the first and third embodiments described below , the walls of the annular groove are parallel , such that the floor of the groove is as wide as the opening of the groove . in other embodiments , such as the second and fourth embodiments described below , the walls of the annular groove are tapered toward one another with the increasing depth of the groove , such that the floor of the groove is narrower than the opening of the groove . each type of annular groove is useful , depending on the particular surgical application and on the particular embodiment of the spacer insertion instrument that is used to insert the spacer . collections of trial spacer elements are provided by the invention . preferably , each spacer in a particular set maintains the same diameter as the other spacers in the set . ( it shall be understood that different collections of spacers may be provided such that the diameter of the selected collection of trial spacers is appropriate for the specific patient being treated . for example , the diameters of the trial spacers in a collection that is suitable for use with pediatric patients would be smaller than the diameters of the trial spacers in a collection that is suitable for use with adult patients .) also preferably , each spacer in a particular set has a predetermined depth that differs from the depth of the other spacers in the set . the predetermined depth is provided in that while each spacer in the set shares the same annular groove dimensions ( so that each can be held by the same insertion instrument ), each spacer has a different flange thickness ( in sets where the spacers are constant thickness spacers ). for sets of tapered thickness spacers , the predetermined maximum depth and predetermined minimum depth ( the two depths providing the overall taper ) are provided in that while each spacer in the set shares the same annular groove dimensions ( so that each can be held by the same insertion instrument ), each spacer has a different maximum flange thickness and a different minimum flange thickness . preferably in sets of tapered thickness spacers , the overall taper angle is the same for each spacer in the set . the usefulness of providing sets of spacers similar in most respects except for the depth dimension will be described in greater detail below . referring now to fig1 a - c , a first embodiment of an intervertebral trial spacer 100 of the invention is illustrated in side , top and side cutaway views , respectively . the spacer 100 is a cylindrical disc with an annular groove 102 that forms a central trunk 103 and radial flanges 104 , 106 at each end of the trunk 102 . in this embodiment , support portions 108 , 110 of the top and bottom surfaces 112 , 114 of the disc are parallel . further in this embodiment , the walls 120 , 122 of the annular groove 102 are parallel , such that the floor 124 of the groove 102 is as wide as the opening 126 of the groove 102 . further in this embodiment , the spacer 100 has a central bore 128 . referring now to fig1 d , a set of intervertebral spacers 100 a - 1 of the invention are illustrated in a side view . each spacer 100 a - 1 is formed generally similarly to the intervertebral spacer 100 of fig1 a - c , except that each spacer 100 a - 1 has a predetermined depth ( denoted by the preferred dimension identified adjacent each spacer ) provided in that while each spacer 100 a - 1 shares the same annular groove dimensions as the other spacers , each spacer 100 a - 1 has a different flange thickness dimension . for example , the flanges 1041 , 1061 are thicker than the flanges 104 a , 106 a . referring now to fig2 a - c , a second embodiment of an intervertebral spacer 200 of the invention is illustrated in side , top and side cutaway views , respectively . similarly to the spacer 100 , the spacer 200 is a cylindrical disc with an annular groove 202 that forms a central trunk 203 and radial flanges 204 , 206 at each end of the trunk 202 . however , in this embodiment , the flanges 204 , 206 are radially tapered in that support portions 208 , 210 of the top and bottom surfaces 212 , 214 of the disc are parallel , while an outer edge 216 of the top surface 212 is tapered toward an outer edge 218 of the bottom surface 214 . further in this embodiment , in contrast to the spacer 100 , the walls 220 , 222 of the annular groove 202 are tapered toward one another with the increasing depth of the groove 202 , such that the floor 224 of the groove 202 is more narrow than the opening 226 of the groove . further in this embodiment , the spacer 200 has a central bore 228 . referring now to fig2 d , a set of intervertebral spacers 200 a - 1 of the invention are illustrated in a side view . each spacer 200 a - 1 is formed generally similarly to the intervertebral spacer 200 of fig2 a - c , except that each spacer 200 a - 1 has a predetermined depth ( denoted by the preferred dimension identified adjacent each spacer ) provided in that while each spacer 200 a - 1 shares the same annular groove dimensions as the other spacers , each spacer 200 a - 1 has a different flange thickness dimension . for example , the flanges 2041 , 2061 are thicker than the flanges 204 a , 206 a . with regard to the instrument for the insertion and removal of the trial spacer elements , a first embodiment ( particularly useful for inserting constant thickness trial spacers ) of a spacer insertion tool includes an elongated shaft and a handle at one end of the shaft . the distal end of the shaft includes semi - circular hook that is adapted to hold a trial spacer within an enclosure formed by the hook . the angle swept out by the hook is slightly greater than 180 degrees , but the inner diameter of the hook is only slightly larger than the central trunk of the trial spacer . therefore , the trial spacer may be snapped into the enclosure , but maintains complete rotational freedom within its grasp . a loading tool may be provided to assist in the loading and unloading of the trial spacer from the trial spacer insertion instrument of this embodiment . this loading tool comprises a forked hook having two tines separated by a notch that engages the shaft of the insertion tool as the tines engage the flanges of the trial spacer , to force the trial spacer into the enclosure . alternatively and / or additionally , the same device may be utilized to remove the spacer from the enclosure , by reversing the position of the forked hook relative to the insertion tool and the spacer . referring now to fig5 a , a first embodiment of a spacer insertion tool 500 of the invention is illustrated in a side view . the insertion tool 500 includes an elongated shaft 502 and a handle 503 at one end of the shaft 502 . at the other end of the shaft 502 , the insertion tool 500 includes a semi - circular hook 504 that is adapted to hold an intervertebral spacer of the invention within an enclosure 506 of the hook 504 . the central trunk of the spacer can be snapped into the enclosure 506 of the hook 504 so that the extent of the hook 504 fits loosely within the annular groove of the spacer and is flanked by the flanges of the spacer . the central trunk of the spacer can also be snapped out of the enclosure 506 . in this regard , the hook 504 has an opening 508 that temporarily expands when the central trunk of the spacer is forced through the opening 508 . that is , the outer diameter of the central trunk is greater than the width of the opening 508 , so that the central trunk cannot pass through the opening 508 without force . the application of a force sufficient to cause the opening 508 to expand when confronted with the central trunk causes the central trunk to pass through the opening 508 . after the central trunk has cleared the opening 508 , the opening 508 will contract . the temporary expansion in this embodiment is provided by the hook 504 being formed of a material having a low elasticity and the hook 504 being provided with a stress notch 510 on the extent ( preferably located opposite the opening 508 for maximum efficiency ) to ease the expansion . once the spacer is loaded into the enclosure , the opening 508 , having contracted back to its resting width , prevents the central trunk from exiting the enclosure radially through the opening , because , as stated above , the outer diameter of the central trunk is greater than the width of the opening 508 . further , by flanking the extent of the hook 504 , the flanges of the spacer prevent the spacer from exiting the enclosure laterally . the hook 504 therefore holds the spacer loosely in the enclosure so that the spacer can rotate about the cylindrical axis of the central trunk while being held by the hook 504 . referring now to fig5 b , a cutaway view of the insertion tool 500 of fig5 a holding the spacer 100 of fig1 a - c shows the extent of the hook 504 in cross - section and fitting within the annular groove of the spacer . it can be seen that to enable the spacer 100 to be loosely held in the enclosure , the width of the extent is smaller than the width of the annular groove , and the depth of the extent is less than the depth of the annular groove if it is desirable for the flanges to fully flank the extent . preferably , as shown , the outer diameter of the hook 504 is substantially equal to the outer diameter of the spacer 100 . referring now to fig6 a - b , an embodiment of a loading accessory 600 for a spacer insertion tool of the invention is illustrated in side and top views , respectively . the loading accessory 600 can be used to ease the passing of the central trunk of the spacer through the opening of the spacer insertion tool , both for loading the spacer into the enclosure and unloading the spacer from the enclosure . the loading accessory 600 includes an elongated shaft 602 and a forked hook 604 at an end of the shaft 602 . a notch 606 having a base 608 separates the tines 610 , 612 of the forked hook 604 . the width of the notch 608 separating the tines 610 , 612 is wide enough to accommodate the width of the hook 504 of the insertion tool 500 and the width of the shaft 502 of the insertion tool 500 , but narrow enough so that the tines 610 , 612 can engage the edges of the flanges of the spacer . preferably , as shown , the curvature of the tines 608 , 610 follows the curvature of the edges of the flanges . referring now to fig6 c , the loading accessory 600 of fig6 a - b is shown m operation to load the spacer 100 of fig1 a - c into the spacer insertion tool 500 of fig5 a . initially , the spacer 100 is positioned adjacent the opening 508 of the insertion tool 500 . then , the tines 610 , 612 of the loading accessory 600 are passed on either side of the shaft 502 of the insertion tool 500 such that the notch 606 accommodates the shaft 502 and until the base 608 of the notch 606 contacts the shaft 502 . then , the loading accessory 600 is rotated , using the contact between the shaft 502 and the base 608 as a fulcrum , to cause the tines 610 , 612 to engage the flanges 104 , 106 of the spacer 100 and push them into the enclosure 506 of the tool 500 . applying a force to the rotation , sufficient to cause the opening 508 of the tool 500 to expand when confronted with the central trunk of the spacer , causes the central trunk to pass through the opening 508 . referring now to fig6 d , the loading accessory 600 of fig6 a - b is shown in operation to unload the spacer 100 of fig1 a - c from the spacer insertion tool 500 of fig5 a . initially , with the spacer 100 held by the tool 500 , the tines 610 , 612 of the loading accessory 600 are passed on either side of the shaft 502 of the insertion tool 500 such that the notch 606 accommodates the shaft 502 and until the base 608 of the notch 606 contacts the shaft 502 . then , the loading accessory 600 is rotated , using the contact between the shaft 502 and the base 608 as a fulcrum , to cause the tines 610 , 612 to engage the flanges 104 , 106 of the spacer 100 and push them out of the enclosure 506 of the tool 500 . applying a force to the rotation , sufficient to cause the opening 508 of the tool 500 to expand when confronted with the central trunk of the spacer , causes the central trunk to pass through the opening 508 . the width of the notch 606 accommodates the width of the hook 504 as the spacer 100 is being pushed out of the enclosure 506 . the insertion tool of this first embodiment can be used to insert a series of constant thickness trial spacers ( some of which may have beveled flange edges for easing the insertion between the collapsed bones and into the space to be distracted ). more specifically , thinner trial spacers can initially be inserted into the spacer , followed successively by thicker trial spacers until the desired spacing is achieved . once the appropriate spacing has been achieved , immobilization of the spine by fixation , fusion , or non - fusion techniques and devices , such as those set forth in co - pending u . s . patent application ser . nos . 09 / 906 , 117 and 09 / 906 , 118 , entitled “ an intervertebral spacer device having a wave washer force restoring element ” and “ an intervertebral spacer device having a spiral wave washer force restoring element ”, respectively , as well as u . s . pat . no . 5 , 989 , 291 , entitled “ an intervertebral spacer device ”, each of which has been assigned to the same assignee as this present invention , the specifications of which are all fully incorporated herein by reference , may be desirable . while simple distraction to a constant height across the intervertebral space is appropriate for standard disc compression pathologies , in the case of scoliosis , simple constant thickness distraction is insufficient to remediate the pathological condition . what is necessary is the distraction of the sequence of spaces , each to an appropriate angle and height , such that the overall spinal configuration is anatomically correct . tapered trial spacers , such as those disclosed in the present application , are the first such distraction tools to provide such a tailored correction of the misangulation of the spinal bones . more particularly , the surgeon inserts the tapered trial spacers into the intervertebral space ( presumably from the anterior , or anterio - lateral , approach ) with the narrow edge of the trial spacer forming a wedge and sliding between the adjacent bones . by utilizing either a second or third embodiment of the spacer insertion tool , described more fully hereinafter with respect to fig7 a - c and 8 a - c respectively , the surgeon may turn the spacer around its axis within the intervertebral space to find the most appropriate rotational position ( corresponding to the most desirable straightening effect on the spinal column ). stated alternatively , each of the tapered trial spacers has an overall wedge shape that generally corresponds to the pathological tapering of the adjacent bones that characterizes scoliosis . by rotating the wedge - shaped spacer after it has been placed between the adjacent bones , the overall disc alignment may be compensated , restoring appropriate anatomical status . it should be understood that additional rotation of the spacer may restore lordosis to the spine , and that over - rotation ( if the particular spine is flexible enough ) of the spacer would result in a pathological curvature in the opposite direction . referring now to fig3 a - c , a third embodiment of an intervertebral spacer 300 of the invention is illustrated in side , top and side cutaway views , respectively . similarly to the spacer 100 , the spacer 300 is a cylindrical disc with an annular groove 302 that forms a central trunk 303 and radial flanges 304 , 306 at each end of the trunk 303 . however , in this embodiment , support portions 308 , 310 of the top and bottom surfaces 312 , 314 of the disc are not parallel , providing an overall taper to the spacer 300 at an angle . still , similarly to the spacer 100 , the walls 320 , 322 of the annular groove 302 are parallel , such that the floor 324 of the groove 302 is as wide as the opening 326 of the groove 302 . further in this embodiment , the spacer 300 has a central bore 328 . referring now to fig3 d , a set of tapered intervertebral spacers 300 a - j of the invention are illustrated in a side view . each spacer 300 a - j is formed generally similarly to the intervertebral spacer 300 of fig3 a - c , except that each spacer 300 a - j has a predetermined maximum depth ( denoted by the preferred maximum depth dimension identified adjacent each spacer ) and a predetermined minimum depth ( denoted by the preferred minimum depth dimension identified adjacent each spacer ), each provided in that while each spacer 300 a - j shares the same annular groove width dimension as the other spacers , each spacer 300 a - j has a different maximum flange thickness dimension and a different minimum flange thickness dimension . for example , the flanges 304 j , 306 j have a thicker maximum flange thickness dimension and a thicker minimum flange thickness dimension than the flanges 304 a , 306 a . referring now to fig4 a - c , a fourth embodiment of an intervertebral spacer 400 of the invention is illustrated in side , top and side cutaway views , respectively . similarly to the spacer 200 , the spacer 400 is a cylindrical disc with an annular groove 402 that forms a central trunk 403 and radial flanges 404 , 406 at each end of the trunk 403 . however , in this embodiment , support portions 408 , 410 of the top and bottom surfaces 412 , 414 of the disc are not parallel . still , similarly to the spacer 200 , the flanges 404 , 406 are radially tapered in that an outer edge 416 of the top surface 412 is tapered toward an outer edge 418 of the bottom surface 414 . further in this embodiment , similarly to the spacer 200 , the walls 420 , 422 of the annular groove 402 are tapered toward one another with the increasing depth of the groove 402 , such that the floor 424 of the groove 402 is more narrow than the opening 426 of the groove . further in this embodiment , the spacer 400 has a central bore 428 . referring now to fig4 d , a set of tapered intervertebral spacers 400 a - j of the invention are illustrated in a side view . each spacer 400 a - j is formed generally similarly to the intervertebral spacer 400 of fig4 a - c , except that each spacer 400 a - j has a predetermined maximum depth ( denoted by the preferred maximum depth dimension identified adjacent each spacer ) and a predetermined minimum depth ( denoted by the preferred minimum depth dimension identified adjacent each spacer ), each provided in that while each spacer 400 a - j shares the same annular groove width dimension as the other spacers , each spacer 400 a - j has a different maximum flange thickness dimension and a different minimum flange thickness dimension . for example , the flanges 404 j , 406 j have a thicker maximum flange thickness dimension and a thicker minimum flange thickness dimension than the flanges 404 a , 406 a . it should understood that the various features of the different embodiments of the intervertebral spacer of the invention discussed above can be used in various combinations and permutations to form the illustrated embodiments and other embodiments of the intervertebral spacer of the invention . in some embodiments , the walls of the annular groove are parallel . in other embodiments , they are not parallel . in some embodiments where they are not parallel , they are tapered toward one another with the increasing depth of the groove . in other embodiments where they are not parallel , they are tapered toward one another with the decreasing depth of the groove . in some embodiments , the support portions of the top and bottom surfaces are parallel . in other embodiments , they are not parallel . in some embodiments , the flanges are radially tapered in that the outer edge of the top surface is tapered toward an outer edge of the bottom surface . in other embodiments , the flanges are not radially tapered . in some embodiments , the spacer has a central bore . in other embodiments , the spacer does not have a central bore . it should be understood that while in the illustrated embodiments where spacers in a set have an overall taper , the angle of the overall taper of each spacer in the set is the same as the angle of the overall taper of the other spacers in the set , the invention encompasses a set of spacers in which the angle of the overall taper of each spacer in the set is different than the angle of the overall taper of at least one other spacer in the set . it should be understood that while in the illustrated embodiments where the spacer has an overall taper , the angle of the overall taper can be predetermined , such that the maximum flange thickness and the minimum flange thickness can be selected to achieve a desired overall taper angle . it should be understood that while in the illustrated embodiments the spacers are shown as having a cylindrical shape , it should be understood that in other embodiment , the spacers can have oval , square , or rectangular cross - sections , or cross - sections of other shapes , provided that any corners are rounded as necessary to prevent damage to surrounding tissue . as suggested previously , the insertion , rotation and removal of the tapered trial intervertebral spacers requires an alternate spacer insertion tool . this second embodiment of the spacer insertion tool includes a handle and an elongated dual shaft , the dual shaft culminating in a trial spacer grasping pincer , rather than the simple hook of the first embodiment . this pincer differs from the hook of the first embodiment of the trial spacer insertion tool described above , inasmuch as the dual shaft includes a fixed shaft and a selectively engagable shaft which , together , form pincer . more specifically , the fixed shaft includes a semicircular hook portion of the pincer at its distal end , having an enclosure within which a trial spacer can be placed . the selectively engagable shaft includes the complementary portion of the pincer , which moves toward the hook portion to grasp and hold the trial spacer when the engagable shaft is engaged , and moves away from the hook portion to release the trial spacer when the engagable shaft is disengaged . ( the spacer can be unloaded and loaded when the engagable shaft is disengaged .) the engagement action prevents the spacer from moving relative to the tool , and therefore permits the surgeon to rotate the tapered spacer in between the vertebral bodies ( by contrast , the first embodiment of the trial spacer insertion instrument permitted the spacer to rotate freely in the enclosure of the hook ). referring now to fig7 a , another embodiment of a spacer insertion tool 700 of the invention is illustrated in a side view . the insertion tool 700 includes an elongated shaft 702 and a handle 704 at one end of the shaft 702 . the insertion tool 700 further includes a compression assembly that is adapted to hold an intervertebral spacer of the invention at the other end of the shaft 702 so that the spacer cannot move when held . the insertion tool 700 further includes a release assembly that is adapted to release the spacer from being held . the compression assembly includes a semicircular hook 706 at the other end of the shaft 702 and a compression surface 708 adjacent the hook 706 . the hook 706 has an enclosure 709 defined by the extent of the hook 706 and an opening 710 through which the central trunk can pass freely to be placed into the enclosure 709 . that is , the width of the opening 710 is greater than the diameter of the central trunk . when the central trunk is placed within the enclosure 709 , the extent of the hook 706 fits loosely within the annular groove of the spacer . the compression assembly further includes a compression trigger 712 mechanically connected to the hook 706 such that as the compression trigger 712 is placed in an engaged position , the hook 706 is pulled toward the compression surface 708 . the mechanical connection includes a rod 714 connected at one end to the hook 706 and at the other end to a plate 716 . a rod 718 protruding from the plate 716 is engaged by a slot 720 in a lever 722 attached to the compression trigger 712 . when the compression trigger 712 is engaged , the rod 714 of the lever 722 pulls the plate 716 by the slot 720 . the plate 716 in turn pulls the rod 714 , which in turn pulls the hook 704 toward the compression surface 708 . when the hook 706 is pulled toward the compression surface 708 when the central trunk of the spacer is in the enclosure 709 , the central trunk is compressed within the enclosure 709 between the hook 706 and the compression surface 708 so that the spacer cannot move . the release assembly includes a spring 724 biasing the compression trigger 712 to a disengaged position . therefore , after the compression trigger 712 is released , it moves to the disengaged position . however , so that the central trunk remains compressed within the enclosure even after the compression trigger 712 is released ( e . g ., so that the surgeon does not need to continue holding the compression trigger 712 to effect the compression ), the compression assembly further includes teeth 726 on the rod 714 and corresponding teeth 730 that confront the rod teeth 726 to prevent the rod 714 from retreating , to maintain the compression . the release assembly further includes a release trigger 732 that can be engaged to release the rod teeth 726 from the corresponding teeth 730 to allow the rod 714 to return to its rest position , thereby alleviating the compression . more specifically , the release trigger 732 has the corresponding teeth 730 and the release assembly further includes a spring 734 that biases the release trigger 732 toward a position in which the corresponding teeth 730 engage the rod teeth 726 . this arrangement allows the release trigger 732 to be engaged by pressing the release trigger 732 with a force great enough to overcome the bias of the spring 734 , so that the corresponding teeth 730 are disengaged from the rod teeth 726 . therefore , when the release trigger 732 is pressed , the compression is alleviated , and the central trunk of the spacer can be freely passed through the opening 710 to be taken out of the enclosure 709 . referring now to fig7 b , a cutaway view of the insertion tool 700 of fig7 a holding the spacer 400 of fig4 a - c shows the extent of the hook 706 in cross - section and fitting within the annular groove of the spacer as the spacer is compressed between the compression surface 708 and the hook 706 . it can be seen that the width of the extent of the hook 706 is smaller than the width of the annular groove , and the depth of the extent is less than the depth of the annular groove if it is desirable for the flanges to fully flank the extent . preferably , as shown , the outer diameter of the hook 706 is substantially equal to the outer diameter of the spacer 400 . referring now to fig8 a - b , yet another embodiment of a spacer insertion tool 800 of the invention is illustrated in open and closed side views , respectively . the insertion tool 800 includes an elongated shaft 802 and a handle 804 at one end of the shaft 802 . the insertion tool 800 further includes a compression assembly that is adapted to hold an intervertebral spacer of the invention at the other end of the shaft 802 so that the spacer cannot move when held . the insertion tool 800 further includes a release assembly that is adapted to release the spacer from being held . the compression assembly includes a claw 806 at the other end of the shaft 802 having opposing pincers 807 a , 807 b , each providing one of opposing compression surfaces 808 a , 808 b . the claw 806 has an enclosure 809 defined by the extents of the pincers 807 a , 807 b and an opening 810 through which the central trunk can pass freely to be placed into the enclosure 809 when the claw 806 is open ( i . e ., when the opposing pincers 807 a , 807 b are separated ). that is , the width of the opening 810 is greater than the diameter of the central trunk when the claw 806 is open . when the central trunk is placed within the enclosure 809 , the extents of the pincers 807 a , 807 b fit loosely within the annular groove of the spacer . the compression assembly further includes a compression slide 812 that when moved to an engaged position ( here , a forward position shown in fig8 b ) closes the claw 806 . the closure of the claw 806 by the compression slide 812 is effected as follows . one of the pincers 807 a is in a fixed position relative to the elongated shaft 802 whereas the other pincer 807 b is adapted to rotate about an axis transverse to the shaft 802 . in this embodiment , the rotation is provided by a pin 813 passing through each pincer at a rotation point along the transverse axis . one position of the movable pincer 807 b along the rotation path ( shown in fig8 a ) defines the opened claw 806 in that the pincers 807 a , 807 b are separated . another position of the movable pincer 807 b along the rotation path ( shown in fig8 b ) defines the closed claw 806 in that the pincers 807 a , 807 b are brought together . when the pincers 807 a , 807 b are separated , an engagement surface 814 of the movable pincer 807 b is placed in an available compression path of an engagement surface 816 of the compression slide 812 . the engagement surface 814 is tapered so that when the compression slide 812 is moved to the engaged , the engagement surface 816 of the compression slide 812 moves along the available compression path and engages the tapered surface 814 to push the surface 814 aside and thereby cause a rotation of the movable pincer 807 b to the position defining the closed claw 806 . when the pincers 807 a , 807 b are thereby brought together to close the claw 806 when the central trunk of the spacer is in the enclosure 809 , the compression surfaces 808 a , 808 b come to bear on the central trunk to compress it within the enclosure 809 so that the spacer cannot move . the release assembly includes a spring 818 biasing the movable pincer 807 b to the rotation path position defining the open claw 806 . therefore , when the compression slide 812 is moved to a disengaged position ( here , a backward position ), the engagement surface 816 of the compression slide 812 moves along an available release path ( here , a backtracking along the compression path ) and frees the engagement surface 814 of the movable pincer 807 b to allow the engagement surface 814 to return to a place in the available compression path by the biasing action of the spring 818 . when the claw 806 is open , the compression is alleviated and the central trunk of the spacer can be freely passed through the opening 810 to be taken out of the enclosure 809 . the release assembly further includes at least one barrier 820 a , 820 b that limits the biasing action of the spring 818 by preventing the movable pincer 807 h from rotating beyond the position that places the engagement surface 814 in the available compression path . in this embodiment , confrontation surfaces 822 a , 822 b on the movable pincer 807 b confront the barriers 820 a , 820 b as the pincer 807 b rotates toward the rotation path position defining the open claw 806 under the biasing force of the spring 818 . when the engagement surface 814 is returned to the place in the available compression path , the barriers 820 a , 820 b prevent the confrontation surfaces 822 a , 822 b from advancing further . the spring 818 and the barriers 820 a , 820 b maintain the movable pincer 807 b in this position until the compression slide 812 is advanced toward the engaged position by a force great enough to overcome the biasing force of the spring 818 . referring now to fig8 c , a cutaway view of the insertion tool 800 of fig8 a - b holding the spacer 400 of fig4 a - c shows the extents of the pincers 807 a , 807 b in cross - section and fitting within the annular groove of the spacer as the spacer is compressed between the compression surfaces 808 a , 808 b . it can be seen that the width of each extent is smaller than the width of the annular groove , and the depth of each extent is less than the depth of the annular groove if it is desirable for the flanges to fully flank the extents . preferably , as shown , the outer diameter of the claw 806 is substantially equal to the outer diameter of the spacer 400 . there are alternative insertion and rotating instruments that may be designed , so long as they selectively and alternatingly release or hold the trial spacer securely against rotation ( the spacer can &# 39 ; t rotate freely if it is to be turned in the intervertebral space ). the tapered trial spacers themselves can include angle markers 430 that clearly indicate to the surgeon the amount of rotation that was necessary for the correction of the spinal deformity . such angle markers 430 can also serve as a guide for the implantation of a secondary bone graft ( e . g ., a femoral ring ) or another intervertebral spacer device . once the surgeon has determined the appropriate geometry for the surgical implants via the trial spacers , he or she is ready to immobilize the spine in that position . while multiple ways for immobilizing the spine are disclosed in the prior art , any one of which may be suitable for the specific surgical patient &# 39 ; s treatment , three alternative ways are herein described . first , the trial spacers may be left in the patient while rod fixation apparatuses ( anterior or posterior ) are mounted to the spine , thereby holding the spine in its desired orientation even after the trial spacers are subsequently removed . alternatively , surface plating and / or intervertebral cage devices may be mounted to the spine to promote fusion without the need for bulky rod assemblies . ( while this approach may seem more surgically desirable , questions regarding the long - term stability of these constructs have led to some surgeons to choose combinations of rodding and cages .) a third approach to immobilizing the corrected spine is to insert a shaped bone graft , or suitably contoured porous metal spacer , into the properly distracted intervertebral space , and either plating or using rod fixation to hold the construct stable as the spine fuses . the insertion of a femoral ring allograft , or porous metal implant , into an intervertebral space is described more fully in u . s . pat . nos . 6 , 558 , 387 and 6 , 471 , 725 entitled “ a porous interbody fusion device having integrated polyaxial locking interference screws ” and “ porous intervertebral distraction spacers ”, assigned to the same assignee as the present invention , the specifications of each being incorporated herein by reference . the tapered trial spacers may also serve as precursors ( measuring instruments ) for another spacer ( e . g ., a porous metal spacer ), similarly shaped , which is inserted into the intervertebral space by the same instrument .
an insertion tool for inserting an intervertebral spacer is provided . the tool includes first and second pincers . the first pincer is fixed and the second pincer is pivotably coupled to the first pincer at a pivot location in open - biased opposition to the first pincer . the tool includes a compression slide which may be selectively translated into and out of engagement with the second pincer . when the compression slide is in a predetermined engagement position with the second pincer , the first and second pincers are in a closed position for holding the intervertebral spacer . when the compression slide is out of engagement with the second pincer , the first and second pincers are in an open position for releasing the intervertebral spacer .