text: while the invention will be described more fully hereinafter with reference to the accompanying drawings , 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 the invention . accordingly , the descriptions that follow are to be understood as illustrative and exemplary of specific structures , aspects and features within the broad scope of the invention and not as limiting of such broad scope . like numbers refer to similar features of like elements throughout . a preferred embodiment of a cervical disc replacement device of the present invention , for use with the instrumentation of the present invention , will now be described . referring now to fig1 a - 3 f , a top element 500 of the cervical disc replacement device 400 is shown in anterior ( fig1 a ), lateral ( fig1 b ), and bottom ( fig1 c ) views ; a bottom element 600 of the cervical disc replacement device 400 is shown in anterior ( fig2 a ), lateral ( fig2 b ), and top ( fig2 c ) views ; and an assembly 400 of the top and bottom elements 500 , 600 is shown in top ( fig3 a ), lateral ( fig3 b ), anterior ( fig3 c ), posterior ( fig3 d ), antero - lateral perspective ( fig3 e ), and postero - lateral perspective ( fig3 f ) views . the cervical disc replacement device 400 is an alternate embodiment of the cervical disc replacement device of the &# 39 ; 702 application . the illustrated alternate embodiment of the cervical disc replacement device is identical in structure to the cervical disc replacement device 100 in the &# 39 ; 702 application ( and thus like components are like numbered , but in the 400 s rather than the 100 s , in the 500 s rather than the 200 s , and in the 600 s rather than the 300 s ), with the exception that the vertebral bone attachment flanges are configured differently , such that they are suitable for engagement by the instrumentation described herein . ( it should be noted that , while the &# 39 ; 702 application illustrated and described the cervical disc replacement device 100 as having an upper element flange 506 with two bone screw holes 508 a , 508 b , and a lower element flange 606 with one bone screw hole 608 , the &# 39 ; 702 application explained that the number of holes and the configuration of the flanges could be modified without departing from the scope of the invention as described in the &# 39 ; 702 application .) more particularly , in this alternate embodiment , the upper element 500 of the cervical disc replacement device 400 has a vertebral body attachment structure ( e . g ., a flange ) 506 that preferably extends upwardly from the anterior edge of the upper element 500 , and preferably has a lateral curvature that approximates the curvature of the anterior periphery of the upper vertebral body against which it is to be secured . the attachment flange 506 is preferably provided with a flat recess 507 , centered on the midline , that accommodates a clip 1150 a ( described below ) of the present invention . the attachment flange 506 is further provided with at least one ( e . g ., two ) bone screw holes 508 a , 508 b , preferably symmetrically disposed on either side of the midline . preferably , the holes 508 a , 508 b have longitudinal axes directed along preferred bone screw driving lines . for example , in this alternate embodiment , the preferred bone screw driving lines are angled upwardly at 5 degrees and inwardly ( toward one another ) at 7 degrees ( a total of 14 degrees of convergence ), to facilitate a toenailing of the bone screws ( described below and shown in fig1 a - h ). centrally between the bone screw holes 508 a , 508 b , at least one mounting feature ( e . g ., a mounting screw hole ) 509 is provided for attaching the upper element 500 to an insertion plate 700 ( described below ) for implantation . similarly , in this alternate embodiment , the lower element 600 of the cervical disc replacement device 400 also has a vertebral body attachment structure ( e . g ., an oppositely directed and similarly configured vertebral body attachment flange ) 606 that preferably extends downwardly from the anterior edge of the lower element 600 , and preferably has a lateral curvature that approximates the curvature of the anterior periphery of the lower vertebral body against which it is to be secured . the attachment flange 606 is preferably provided with a flat recess 607 , centered on the midline , that accommodates a clip 1150 b ( described below ) of the present invention . the attachment flange 606 is further provided with at least one ( e . g ., two ) bone screw holes 608 a , 608 b , preferably symmetrically disposed on either side of the midline . preferably , the holes 608 a , 608 b have longitudinal axes directed along preferred bone screw driving lines . for example , in this alternate embodiment , the preferred bone screw driving lines are angled downwardly at 5 degrees and inwardly ( toward one another ) at 7 degrees ( a total of 14 degrees of convergence ), to facilitate a toenailing of the bone screws ( described below and shown in fig1 a - h ). centrally between the bone screw holes 608 a , 608 b , at least one mounting feature ( e . g ., a mounting screw hole ) 609 is provided for attaching the lower element 600 to the insertion plate 700 ( described below ) for implantation . prior to implantation of the cervical disc replacement device , the surgeon will prepare the intervertebral space . typically , this will involve establishing access to the treatment site , removing the damaged natural intervertebral disc , preparing the surfaces of the endplates of the vertebral bones adjacent the intervertebral space , and distracting the intervertebral space . ( it should be noted that the cervical disc replacement device of the present invention , and the instrumentation and implantation methods described herein , require minimal if any endplate preparation .) more particularly , after establishing access to the treatment site , the surgeon will remove the natural disc material , preferably leaving as much as possible of the annulus intact . then , the surgeon will remove the anterior osteophyte that overhangs the mouth of the cervical intervertebral space , and any lateral osteophytes that may interfere with the placement of the cervical disc replacement device or the movement of the joint . using a burr tool , the surgeon will then ensure that the natural lateral curvature of the anterior faces of the vertebral bodies is uniform , by removing any surface anomalies that deviate from the curvature . also using the burr tool , the surgeon will ensure that the natural curvature of the endplate surface of the upper vertebral body , and the natural flatness of the endplate surface of the lower vertebral body , are uniform , by removing any surface anomalies that deviate from the curvature or the flatness . thereafter , the surgeon will distract the intervertebral space to the appropriate height for receiving the cervical disc replacement device . any distraction tool or method known in the art , e . g ., a caspar distractor , can be used to effect the distraction and / or hold open the intervertebral space . additionally or alternatively , the cervical disc replacement trials of the present invention can be used to distract the intervertebral space ( as described below ). referring now to fig4 a - f , a cervical disc replacement trial 1200 of the present invention is shown in top ( fig4 a ), lateral ( fig4 b ), lateral ( head only ) ( fig4 c ), posterior ( fig4 d ), anterior ( fig4 e ), antero - lateral perspective ( head only ) ( fig4 f ), and postero - lateral perspective ( head only ) ( fig4 g ) views . preferably , a plurality of cervical disc replacement trials are provided primarily for use in determining the appropriate size of a cervical disc replacement device to be implanted ( or whether a particular size of the cervical disc replacement device can be implanted ) into the distracted cervical intervertebral space ( e . g ., the cervical disc replacement device 400 of fig1 a - 3 f ). preferably , for each cervical disc replacement device to be implanted , a plurality of sizes of the cervical disc replacement device would be available . that is , preferably , a plurality of the same type of cervical disc replacement device would be available , each of the plurality having a respective footprint and depth dimension combination that allows it to fit within a correspondingly dimensioned intervertebral space . for example , the plurality of cervical disc replacement devices could include cervical disc replacement devices having oval footprints being 12 mm by 14 mm , 14 mm by 16 mm , or 16 mm by 18 mm , and depths ranging from 6 mm to 14 mm in 1 mm increments , for a total of 27 devices . accordingly , preferably , each of the plurality of trials for use with a particular plurality of differently sized cervical disc replacement devices would have a respective oval footprint and depth dimension set corresponding to the footprint and depth dimension set of a respective one of the plurality of differently sized cervical disc replacement devices . for example , the plurality of trials for use with the set of cervical disc replacement devices described , for example , could include trials having oval footprints being 12 mm by 14 mm , 14 mm by 16 mm , or 16 mm by 18 mm , and depths ranging from 6 mm to 14 mm in 1 mm increments , for a total of 27 static trials . it should be understood that the cervical disc replacement devices and / or the trials can be offered in a variety of dimensions without departing from the scope of the invention , and that the dimensions specifically identified and quantified herein are merely exemplary . moreover , it should be understood that the set of trials need not include the same number of trials for each cervical disc replacement device in the set of cervical disc replacement devices , but rather , none , one , or more than one trial can be included in the trial set for any particular cervical disc replacement device in the set . each of the cervical disc replacement trials ( the cervical disc replacement trial 1200 shown in fig4 a - g is exemplary for all of the trials in the plurality of trials ; preferably the trials in the plurality of trials differ from one another only with regard to certain dimensions as described above ) includes a shaft 1202 having a configured distal end 1204 and a proximal end having a handle 1206 . preferably , the proximal end is provided with a manipulation features ( e . g ., a hole 1216 ) to , e . g ., decrease the weight of the trial 1200 , facilitate manipulation of the trial 1200 , and provide a feature for engagement by an instrument tray protrusion . the distal end is configured to approximate relevant dimensions of the cervical disc replacement device . more particularly in the illustrated embodiment ( for example ), the distal end 1204 has a trial configuration ( e . g ., a head 1208 having an oval footprint dimensioned at 12 mm by 14 mm , and a thickness of 6 mm ). the upper surface 1210 of the head 1208 is convex , similar to the configuration of the vertebral body contact surface of the upper element 500 of the cervical disc replacement device 400 ( but without the teeth ). the lower surface 1212 of the head 1208 is flat , similar to the configuration of the vertebral body contact surface of the lower element 600 of the cervical disc replacement device 400 ( but without the teeth ). the illustrated embodiment , therefore , with these dimensions , approximates the size of a cervical disc replacement device having the same height and footprint dimensions . the cervical disc replacement trial , not having the teeth , can be inserted and removed from the intervertebral space without compromising the endplate surfaces . the cervical disc replacement trial 1200 further has an over - insertion prevention features ( e . g ., a vertebral body stop 1214 ) preferably disposed at the anterior edge of the head 1208 , to engage the anterior surface of the upper vertebral body before the trial 1200 is inserted too far into the intervertebral space . the body of the trial 1200 preferably has one or more structural support features ( e . g ., a rib 1216 extending anteriorly from the head 1208 below the shaft 1202 ) that provides stability , e . g ., to the shaft 1202 for upward and downward movement , e . g ., if the head 1208 must be urged into the intervertebral space by moving the shaft 1202 in this manner . further , preferably as shown , the head 1208 is provided with an insertion facilitation features ( e . g ., a taper , decreasing posteriorly ) to facilitate insertion of the head 1208 into the intervertebral space by , e . g ., acting as a wedge to urge the vertebral endplates apart . preferably , as shown , the upper surface 1210 is fully tapered at approximately 5 degrees , and the distal half of the lower surface 1212 is tapered at approximately 4 degrees . accordingly , the surgeon can insert and remove at least one of the trials ( or more , as necessary ) from the prepared intervertebral space . as noted above , the trials are useful for distracting the prepared intervertebral space . for example , starting with the largest distractor that can be wedged in between the vertebral bones , the surgeon will insert the trial head 1208 ( the tapering of the trial head 1208 facilitates this insertion by acting as a wedge to urge the vertebral endplates apart ), and then lever the trial handle 1206 up and down to loosen the annulus and surrounding ligaments to urge the bone farther apart . once the annulus and ligaments have been loosened , the surgeon removes the trial head 1208 from the intervertebral space , and replaces it with the next largest ( in terms of height ) trial head 1208 . the surgeon then levers the trial handle 1206 up and down to further loosen the annulus and ligaments . the surgeon then proceeds to remove and replace the trial head 1208 with the next largest ( in terms of height ) trial head 1208 , and continues in this manner with larger and larger trials until the intervertebral space is distracted to the appropriate height . this gradual distraction method causes the distracted intervertebral space to remain at the distracted height with minimal subsidence before the cervical disc replacement device is implanted . the appropriate height is one that maximizes the height of the intervertebral space while preserving the annulus and ligaments . regardless of the distraction method used , the cervical disc replacement trials are useful for finding the cervical disc replacement device size that is most appropriate for the prepared intervertebral space , because each of the trial heads approximates the relevant dimensions of an available cervical disc replacement device . once the intervertebral space is distracted , the surgeon can insert and remove one or more of the trial heads to determine the appropriate size of cervical disc replacement device to use . once the appropriate size is determined , the surgeon proceeds to implant the selected cervical disc replacement device . a preferred method of , and instruments for use in , implanting the cervical disc replacement device will now be described . referring now to fig5 a - f , an insertion plate 700 of the insertion instrumentation of the present invention is shown in top ( fig5 a ), lateral ( fig5 b ), anterior ( fig5 c ), and posterior ( fig5 d ) views . fig5 e and 5 f show anterior ( fig5 e ) and antero - lateral perspective ( fig5 f ) views of the insertion plate 700 mounted to the cervical disc replacement device 400 . the insertion plate 700 has a base 702 with a first mounting area 704 a ( preferably an upwardly extending flange ) and a second mounting area 704 b ( preferably a downwardly extending flange ), and a primary attachment feature ( e . g ., an anteriorly extending central stem ) 706 . the connection of the stem 706 to the base 702 preferably includes an axial rotation prevention feature , e . g ., two oppositely and laterally extending key flanges 708 a , 708 b . the stem 706 preferably has a proximal portion 710 that is tapered to have a decreasing diameter away from the base 702 . that is , the tapered proximal portion 710 has an initial smaller diameter that increases toward the base 702 gradually to a final larger diameter . the base 702 preferably has a posteriorly extending ledge 716 that has a flat upper surface and a curved lower surface . the insertion plate 700 is mounted to the cervical disc replacement device 400 to facilitate the preferred simultaneous implantation of the upper and lower elements 500 , 600 . the upper and lower elements 500 , 600 are held by the insertion plate 700 in a preferred relationship to one another that is suitable for implantation . more particularly , as shown in fig3 a - f , 5 e , and 5 f , the elements 500 , 600 are preferably axially rotationally aligned with one another , with the element perimeters and flanges 506 , 606 axially aligned with one another , and held with the bearing surfaces 512 , 612 in contact . the ledge 716 maintains a separation between the anterior portions of the inwardly facing surfaces of the elements 500 , 600 to help establish and maintain this preferred relationship , with the flat upper surface of the ledge 716 in contact with the flat anterior portion of the inwardly facing surface of the upper element 500 , and the curved lower surface of the ledge 716 in contact with the curved anterior portion of the inwardly facing surface of the lower element 600 . while any suitable method or mechanism can be used to mount the elements 500 , 600 to the insertion plate 700 , a preferred arrangement is described . that is , it is preferred , as shown and as noted above , that the flanges 506 , 606 of the elements 500 , 600 ( in addition to having the bone screw holes 508 a , 508 b , 608 a , 608 b described above ) each have at least one mounting feature ( e . g ., mounting screw hole 509 , 609 ), and the insertion plate 700 has two ( at least two , each one alignable with a respective mounting screw hole 509 , 609 ) corresponding mounting features ( e . g ., mounting screw holes 712 a , 712 b ), spaced to match the spacing of ( and each be collinear with a respective one of ) the mounting screw holes 509 , 609 on the flanges 506 , 606 of the elements 500 , 600 of the cervical disc replacement device 400 when those elements 500 , 600 are disposed in the preferred relationship for implantation . accordingly , mounting screws 714 a , 714 b or other suitable fixation devices are secured through the collinear mounting screw hole pairs 509 , 712 a and 609 , 712 b ( one screw through each pair ), such that the elements 500 , 600 are immovable with respect to the insertion plate 700 and with respect to one another . thus , in this configuration , the upper element 500 , lower element 600 , and insertion plate 700 construct is manipulatable as a single unit . preferably , for each size of cervical disc replacement device , the described configuration is established ( and rendered sterile in a blister pack through methods known in the art ) prior to delivery to the surgeon . that is , as described below , the surgeon will simply need to open the blister pack and apply the additional implantation tools to the construct in order to implant the cervical disc replacement device . preferably , the configuration or dimensions of the insertion plate can be modified ( either by providing multiple different insertion plates , or providing a single dynamically modifiable insertion plate ) to accommodate cervical disc replacement devices of varying heights . for example , the positions of the mounting screw holes 712 a , 712 b on the flanges 704 a , 704 b can be adjusted ( e . g ., farther apart for replacement devices of greater height , and close together for replacement devices of lesser height ), and the size of the flanges 704 a , 70 b can be adjusted to provide structural stability for the new hole positions . preferably , in other respects , the insertion plate configuration and dimensions need not be modified , to facilitate ease of manufacturing and lower manufacturing costs . it should be noted that the described configuration of the construct presents the cervical disc replacement device to the surgeon in a familiar manner . that is , by way of explanation , current cervical fusion surgery involves placing a fusion device ( e . g ., bone or a porous cage ) in between the cervical intervertebral bones , and attaching a cervical fusion plate to the anterior aspects of the bones . widely used cervical fusion devices ( an example single level fusion plate 1300 is shown in anterior view in fig1 a and in lateral view in fig1 b ) are configured with a pair of laterally spaced bone screw holes 1302 a , 1302 b on an upper end 1304 of the plate 1300 , and a pair of laterally spaced bone screw holes 1306 a , 1306 b on a lower end 1308 of the plate 1300 . to attach the plate 1300 to the bones , two bone screws are disposed through the upper end &# 39 ; s bone screw holes 1302 a , 1302 b and into the upper bone , and two bone screws are disposed through the lower end &# 39 ; s bone screw holes 1306 a , 1306 b and into the lower bone . this prevents the bones from moving relative to one another , and allows the bones to fuse to one another with the aid of the fusion device . accordingly , as can be seen in fig5 e , when the upper and lower elements 500 , 600 of the cervical disc replacement device 400 are held in the preferred spatial relationship , the flanges 506 , 606 of the elements 500 , 600 , and their bone screw holes 508 a , 508 b , present to the surgeon a cervical hardware and bone screw hole configuration similar to a familiar cervical fusion plate configuration . the mounting of the elements 500 , 600 to the insertion plate 700 allows the elements 500 , 600 to be manipulated as a single unit for implantation ( by manipulating the insertion plate 700 ), similar to the way a cervical fusion plate is manipulatable as a single unit for attachment to the bones . this aspect of the present invention simplifies and streamlines the cervical disc replacement device implantation procedure . as noted above , the cervical disc replacement device 400 and insertion plate 700 construct is preferably provided sterile ( e . g ., in a blister pack ) to the surgeon in an implant tray ( the tray preferably being filled with constructs for each size of cervical disc replacement device ). the construct is preferably situated in the implant tray with the stem 706 of the insertion plate 700 facing upwards for ready acceptance of the insertion handle 800 ( described below ). referring now to fig6 a - e , an insertion handle 800 of the insertion instrumentation of the present invention is shown in top ( fig6 a ), lateral ( fig6 b ), anterior ( fig6 c ), and postero - lateral ( distal end only ) ( fig6 d ) views . fig6 e shows an antero - lateral perspective view of the insertion handle 800 attached to the stem 706 of the insertion plate 700 . fig6 f shows a magnified view of the distal end of fig6 e . the insertion handle 800 is provided primarily for engaging the stem 706 of the insertion plate 700 so that the cervical disc replacement device 400 and insertion plate 700 construct can be manipulated into and within the treatment site . the insertion handle 800 has a shaft 802 with an attachment feature ( e . g ., a longitudinal bore ) 804 at a distal end 806 and a manipulation feature ( e . g ., a flange ) 810 at a proximal end 808 . preferably , the longitudinal bore 804 has an inner taper at the distal end 806 such that the inner diameter of the distal end 806 decreases toward the distal end 806 , from an initial larger inner diameter at a proximal portion of the distal end 806 to a final smaller inner diameter at the distal edge of the distal end 806 . the distal end 806 also preferably has an axial rotation prevention feature , e . g ., two ( at least one ) key slots 814 a , 814 b extending proximally from the distal end 806 . each slot 814 a , 814 b is shaped to accommodate the key flanges 708 a , 708 b at the connection of the base 702 to the stem 706 when the distal end 806 is engaged with the stem 706 . the material from which the insertion handle 800 is formed ( preferably , e . g ., ultem ™), and also the presence of the key slots 814 a , 814 b , permits the diameter of the hollow distal end 806 to expand as needed to engage the tapered stem 706 of the insertion plate 700 . more particularly , the resting diameter ( prior to any expansion ) of the hollow distal end 806 of the insertion handle 800 is incrementally larger than the initial diameter of the tapered proximal portion 710 of the stem 706 of the insertion plate 700 , and incrementally smaller than the final diameter of the tapered proximal portion 710 of the stem 706 of the insertion plate 700 . accordingly , longitudinally aligning the insertion handle shaft 802 with the stem 706 , and thereafter pushing the hollow distal end 806 of the insertion handle shaft 802 toward the insertion plate 700 , causes the hollow distal end 806 to initially readily encompass the tapered proximal portion 710 of the stem 706 ( because the initial diameter of the tapered proximal portion 710 is smaller than the resting diameter of the hollow tapered distal end 806 ). with continued movement of the insertion handle shaft 802 toward the insertion plate base 702 , the hollow distal end 806 is confronted by the increasing diameter of the tapered proximal portion 710 . accordingly , the diameter of the hollow distal end 806 expands ( by permission of the shaft 802 body material and the key slots 814 a , 814 b as the slots narrow ) under the confrontation to accept the increasing diameter . eventually , with continued movement under force , the inner surface of the hollow distal end 806 is friction - locked to the outer surface of the tapered proximal portion 710 . each of the key slots 814 a , 814 b straddles a respective one of the key flanges 708 a , 708 b at the connection of the base 702 to the stem 706 . this enhances the ability of the insertion handle 800 to prevent rotation of the insertion handle shaft 802 relative to the insertion plate 700 ( about the longitudinal axis of the insertion handle shaft 802 ). it should be understood that other methods or mechanisms of establishing engagement of the stem 706 by the insertion handle 800 can be used without departing from the scope of the invention . once the insertion handle 800 is engaged with the insertion plate 700 , manipulation of the insertion handle shaft 802 effects manipulation of the cervical disc replacement device 400 and insertion plate 700 construct . the surgeon can therefore remove the construct from the implant tray , and insert the construct into the treatment area . more particularly , according to the implantation procedure of the invention , after the surgeon properly prepares the intervertebral space ( removes the damaged natural disc , modifies the bone surfaces that define the intervertebral space , and distracts the intervertebral space to the appropriate height ), the surgeon inserts the cervical disc replacement device 400 into the intervertebral space from an anterior approach , such that the upper and lower elements 500 , 600 are inserted between the adjacent vertebral bones with the element footprints fitting within the perimeter of the intervertebral space and with the teeth of the elements &# 39 ; vertebral body contact surfaces 502 , 602 engaging the vertebral endplates , and with the flanges 506 , 606 of the upper and lower elements 500 , 600 flush against the anterior faces of the upper and lower vertebral bones , respectively . ( as discussed above , the flanges 506 , 606 preferably have a lateral curvature that approximates the lateral curvature of the anterior faces of the vertebral bones .) referring now to fig7 a - e , an insertion pusher 900 of the insertion instrumentation of the present invention is shown in top ( fig7 a ), lateral ( fig7 b ), and anterior ( fig7 c ) views . fig7 d shows an antero - lateral perspective view of the insertion pusher 900 inserted into the insertion handle 800 . fig7 e shows a magnified view of the proximal end of fig7 d . once the construct is properly positioned in the treatment area , the surgeon uses the insertion pusher 900 to disengage the insertion handle shaft 802 from the stem 706 of the insertion plate 700 . more particularly , the insertion pusher 900 has a longitudinal shaft 902 having a preferably blunt distal end 904 and a proximal end 906 preferably having a flange 908 . the shaft 902 of the insertion pusher 900 has a diameter smaller than the inner diameter of the insertion handle shaft 802 , such that the shaft 902 of the insertion pusher 900 can be inserted into and translated within the longitudinal bore 804 of the insertion handle shaft 802 . ( the longitudinal bore 804 preferably , for the purpose of accommodating the insertion pusher 900 and other purposes , extends the length of the insertion handle shaft 802 .) the shaft 902 of the insertion pusher 900 is preferably as long as ( or , e . g ., at least as long as ) the longitudinal bore 804 . accordingly , to remove the insertion handle shaft 802 from the insertion plate 700 , the shaft 902 of the insertion pusher 900 is inserted into the longitudinal bore 804 of the insertion handle shaft 802 and translated therein until the blunt distal end 904 of the pusher shaft 802 is against the proximal end of the tapered stem 706 of the insertion plate 700 . because the shaft 902 of the insertion pusher 900 is as long as the longitudinal bore 804 of the insertion handle shaft 802 , the flange 810 of the insertion handle 800 and the flange 908 of the insertion pusher 900 are separated by a distance ( see fig7 d and 7 e ) that is equivalent to the length of that portion of the stem 706 that is locked in the distal end 806 of the insertion handle shaft 802 . accordingly , a bringing together of the flanges 810 , 908 ( e . g ., by the surgeon squeezing the flanges 810 , 908 toward one another ) will overcome the friction lock between the distal end 806 of the insertion handle shaft 802 and the stem 706 of the insertion plate 700 , disengaging the insertion handle shaft 802 from the insertion plate 700 without disturbing the disposition of the cervical disc replacement device 400 and insertion plate 700 construct in the treatment area . referring now to fig8 a - e , a drill guide 1000 of the insertion instrumentation of the present invention is shown in top ( fig8 a ), lateral ( fig8 b ), and anterior ( fig8 c ) views . fig8 d shows an antero - lateral perspective view of the drill guide 1000 inserted onto the stem 706 of the insertion plate 700 . fig8 e shows a magnified view of the distal end of fig8 d . once the insertion handle 800 has been removed , the surgeon uses the drill guide 1000 to guide the surgeon &# 39 ; s drilling of the bone screws ( described below ) through the bone screw holes 508 a , 508 b and 608 a , 608 b of the upper 500 and lower 600 elements &# 39 ; flanges 506 , 606 and into the vertebral bones . more particularly , the drill guide 1000 has a longitudinal shaft 1002 having a configured distal end 1004 and a proximal end 1006 with a manipulation feature ( e . g ., lateral extensions 1008 a , 1008 b ). the lateral extensions 1008 a , 1008 b are useful for manipulating the shaft 1002 . the distal end 1004 is configured to have a shaft guiding feature ( e . g ., a central bore 1010 ) suitable for guiding the shaft 1002 in relation to the stem 706 of the insertion plate 700 therethrough . for example , the central bore 1010 accommodates the stem 706 so that the drill guide 1000 can be placed on and aligned with the stem 706 . the longitudinal axis of the bore 1010 is preferably offset from the longitudinal axis of the drill guide shaft 1002 . the distal end 1004 is further configured to have two guide bores 1012 a , 1012 b that have respective longitudinal axes at preferred bone screw drilling paths relative to one another . more particularly , the central bore 1010 , drill guide shaft 1002 , and guide bores 1012 a , 1012 b , are configured on the distal end 1004 of the drill guide 1000 such that when the central bore 1010 is disposed on the stem 706 of the insertion plate 700 ( see fig8 d and 8 e ), the drill guide shaft 1002 can be rotated on the stem 706 into either of two preferred positions in which the guide bores 1012 a , 1012 b are aligned with the bone screw holes 508 a , 508 b or 608 a , 608 b on either of the flanges 506 or 606 . stated alternatively , in a first preferred position ( see fig8 d and 8 e ), the drill guide 1000 can be used to guide bone screws through the bone screw holes 508 a , 508 b in the flange 506 of the upper element 500 , and in a second preferred position ( in which the drill guide is rotated 180 degrees , about the longitudinal axis of the stem 706 , from the first preferred position ), the same drill guide 1000 can be used to guide bone screws through the bone screw holes 608 a , 608 b in the flange 606 of the lower element 600 . when the drill guide 1000 is disposed in either of the preferred positions , the longitudinal axes of the guide bores 1012 a , 1012 b are aligned with the bone screw holes 508 a , 508 b or 608 a , 608 b on the flanges 506 or 606 , and are directed along preferred bone screw drilling paths through the bone screw holes . accordingly , to secure the upper element flange 506 to the upper vertebral body , the surgeon places the drill guide shaft 1002 onto the stem 706 of the insertion plate 700 , and rotates the drill guide 1000 into the first preferred position . preferably , the surgeon then applies an upward pressure to the drill guide 1000 , urging the upper element 500 tightly against the endplate of the upper vertebral body . using a suitable bone drill and cooperating drill bit , the surgeon drills upper tap holes for the upper bone screws . once the upper tap holes are drilled , the surgeon rotates the drill guide shaft 1002 on the stem 706 of the insertion plate 700 until the guide bores 1012 a , 1012 b no longer cover the upper bone screw holes 508 a , 508 b . the surgeon can then screw the upper bone screws into the upper tap holes using a suitable surgical bone screw driver . additionally , to secure the lower element flange 606 to the lower vertebral body , the surgeon further rotates the drill guide shaft 1002 on the stem 706 of the insertion plate 700 until the drill guide 1000 is in the second preferred position . preferably , the surgeon then applies a downward pressure to the drill guide 1000 , urging the lower element 600 tightly against the endplate of the lower vertebral body . using the suitable bone drill and cooperating drill bit , the surgeon drills lower tap holes for the lower bone screws . once the lower tap holes are drilled , the surgeon rotates the drill guide shaft 1002 on the stem 706 of the insertion plate 700 until the guide bores 1012 a , 1012 b no longer cover the lower bone screw holes 608 a , 608 b . the surgeon can then screw the lower bone screws into the lower tap holes using the suitable surgical bone screw driver . it should be noted that the bone screws ( or other elements of the invention ) may include features or mechanisms that assist in prevent screw backup . such features may include , but not be limited to , one or more of the following : titanium plasma spray coating , bead blasted coating , hydroxylapatite coating , and grooves on the threads . once the elements 500 , 600 are secured to the adjacent vertebral bones , the surgeon removes the drill guide 1000 from the stem 706 of the insertion plate 700 and from the treatment area ( see fig9 a ). using a suitable surgical screw driver , the surgeon then removes the mounting screws 714 a , 714 b that hold the insertion plate 700 against the elements &# 39 ; flanges 506 , 606 , and removes the insertion plate 700 and the mounting screws 714 a , 714 b from the treatment area ( see fig9 b ). referring now to fig1 a - f , a retaining clip 1150 a of the present invention is shown in top ( fig1 a ), lateral ( fig1 b ), posterior ( fig1 c ), anterior ( fig1 d ), postero - lateral perspective ( fig1 e ), and antero - lateral perspective ( fig1 f ) views . ( the features of retaining clip 1150 a are exemplary of the features of the like - numbered features of retaining clip 1150 b , which are referenced by b &# 39 ; s rather than a &# 39 ; s .) referring now to fig1 a - e , a clip applicator 1100 of the insertion instrumentation of the present invention is shown in top ( fig1 a ), lateral ( fig1 b ), and anterior ( fig1 c ) views . fig1 d shows a postero - lateral perspective view of the clip applicator 1100 holding two retaining clips 1150 a , 1150 b of the present invention . fig1 e shows an antero - lateral perspective view of fig1 d . referring now to fig1 a - h , the clip applicator 1100 is shown applying the retaining clips 1150 a , 1150 b to the cervical disc replacement device 400 . fig1 b - h show anterior ( fig1 b ), posterior ( fig1 c ), top ( fig1 d ), bottom ( fig1 e ), lateral ( fig1 f ), antero - lateral perspective ( fig1 g ), and postero - lateral perspective ( fig1 h ) views of the cervical disc replacement device 400 after the retaining clips 1150 a , 1150 b have been applied . once the mounting screws 714 a , 714 b and the insertion plate 700 are removed , the surgeon uses the clip applicator 1100 to mount the retaining clips 1150 a , 1150 b on the flanges 506 , 606 to assist in retaining the bone screws . as shown in fig1 a - f , each of the clips 1150 a , 1150 b preferably has an applicator attachment feature ( e . g ., a central attachment bore 1152 a , 1152 b ) and , extending therefrom , a pair of bone screw retaining features ( e . g ., oppositely directed laterally extending flanges 1156 a , 1156 b and 1158 a , 1158 b ) and a flange attachment feature ( e . g ., an upwardly ( or downwardly ) extending hooked flange 1160 a , 1160 b ). the extent of the hook flange 1160 a , 1160 b is preferably formed to bend in toward the base of the hook flange 1160 a , 1160 b , such that the enclosure width of the formation is wider than the mouth width of the formation , and such that the extent is spring biased by its material composition toward the base . the enclosure width of the formation accommodates the width of the body of a flange 506 , 606 of the cervical disc replacement device 400 , but the mouth width of the formation is smaller than the width of the flange 506 , 606 . accordingly , and referring now to fig1 b - h , each clip 1150 a , 1150 b can be applied to an element flange 506 , 606 such that the hook flange 1160 a , 1160 b grips the element flange 506 , 606 , by pressing the hook &# 39 ; s mouth against the edge of the element flange 506 , 606 with enough force to overcome the bias of the hook flange &# 39 ; s extent toward the base , until the flange 506 , 606 is seated in the hook &# 39 ; s enclosure . the attachment bore 1152 a , 1152 b of the clip 1150 a , 1150 b is positioned on the clip 1150 a , 1150 b such that when the clip 1150 a , 1150 b is properly applied to the flange 506 , 606 , the attachment bore 1152 a , 1152 b is aligned with the mounting screw hole 509 , 609 on the flange 506 , 606 ( see fig1 b - h ). further , the posterior opening of the attachment bore 1152 a , 1152 b is preferably surrounded by a clip retaining features ( e . g ., a raised wall 1162 a , 1162 b ), the outer diameter of which is dimensioned such that the raised wall 1162 a , 1162 b fits into the mounting screw hole 509 , 609 on the element flange 506 , 606 . thus , when the clip 1150 a , 1150 b is so applied to the element flange 506 , 606 , the element flange 506 , 606 will be received into the hook &# 39 ; s enclosure against the spring bias of the hook &# 39 ; s extent , until the attachment bore 1152 a , 1152 b is aligned with the mounting screw hole 509 , 609 , at which time the raised wall 1162 a , 1162 b will snap into the mounting screw hole 509 , 609 under the force of the hook &# 39 ; s extent &# 39 ; s spring bias . this fitting prevents the clip 1150 a , 1150 b from slipping off the flange 506 , 606 under stresses in situ . each of the laterally extending flanges 1156 a , 1156 b and 1158 a , 1158 b of the clip 1150 a , 1150 b is sized to cover at least a portion of a respective one of the bone screw heads when the clip 1150 a , 1150 b is attached in this manner to the flange 506 , 606 ( see fig1 b - h ), so that , e . g ., the clips 1150 a , 1150 b help prevent the bone screws from backing out . referring again to fig1 a - e , the clip applicator 1100 has a pair of tongs 1102 a , 1102 b hinged at a proximal end 1104 of the clip applicator 1100 . each tong 1102 a , 1102 b has an attachment feature ( e . g ., a nub 1108 a , 1108 b ) at a distal end 1106 a , 1106 b . each nub 1108 a , 1108 b is dimensioned such that it can be manually friction locked into either of the attachment bores 1152 a , 1152 b of the retaining clips 1150 a , 1150 b . thus , both clips 1150 a , 1150 b can be attached to the clip applicator 1100 , one to each tong 1102 a , 1102 b ( see fig1 d and 11 e ). preferably , as shown in fig1 d and 11 e , the clips 1150 a , 1150 b are attached so that their hook flanges 1154 a , 1154 b are directed toward one another , so that they are optimally situated for attachment to the element flanges 506 , 606 of the cervical disc replacement device 400 ( see fig1 a ). preferably , the clips 1150 a , 1150 b are attached to the clip applicator 1100 as described above prior to delivery to the surgeon . the assembly is preferably provided sterile to the surgeon in a blister pack . accordingly , when the surgeon is ready to mount the clips 1150 a , 1150 b to the element flanges 506 , 606 of the cervical disc replacement device 400 , the surgeon opens the blister pack and inserts the tongs 1102 a , 1102 b of the clip applicator 1100 ( with the clips 1150 a , 1150 b attached ) into the treatment area . accordingly , and referring again to fig1 a - h , the clips 1150 a , 1150 b can be simultaneously clipped to the upper 500 and lower 600 elements &# 39 ; flanges 506 , 606 ( one to each flange 506 , 606 ) using the clip applicator 1100 . more particularly , the mouths of the clips 1150 a , 1150 b can be brought to bear each on a respective one of the flanges 506 , 606 by manually squeezing the tongs 1102 a , 1102 b ( having the clips 1150 a , 1150 b attached each to a set of the distal ends of the tongs 1102 a , 1102 b ) toward one another when the mouths of the clips 1150 a , 1150 b are suitably aligned with the flanges 506 , 606 ( see fig1 a ). once the clips 1150 a , 1150 b have been attached to the flanges 506 , 660 with the raised walls 1162 a , 1162 b fitting into the mounting screw holes 509 , 609 of the flanges 506 , 606 , the clip applicator 1100 can be removed from the clips 1150 a , 1150 b by manually pulling the nubs 1108 a , 1108 b out of the attachment bores 1152 a , 1152 b , and the clip applicator 1100 can be removed from the treatment area . after implanting the cervical disc replacement device 400 as described , the surgeon follows accepted procedure for closing the treatment area . referring now to fig1 a - e , an alternate dual cervical disc replacement device configuration and alternate insertion plates for use therewith , suitable , for example , for implantation in two adjacent cervical intervertebral spaces , are illustrated in exploded perspective ( fig1 a ), anterior ( fig1 b ), posterior ( fig1 c ), lateral ( fig1 d ), and collapsed perspective ( fig1 e ) views . referring now also to fig1 a - c , an alternate upper element of the configuration is shown in posterior ( fig1 a ), anterior ( fig1 b ), and antero - lateral ( fig1 c ) views . referring now also to fig1 a - c , an alternate lower element of the configuration is shown in posterior ( fig1 a ), anterior ( fig1 b ), and antero - lateral ( fig1 c ) views . referring now also to fig1 a - c , an alternate , upper , insertion plate of the configuration is shown in anterior ( fig1 a ), posterior ( fig1 b ), and antero - lateral ( fig1 c ) views . referring now also to fig1 a - c , an alternate , lower , insertion plate of the configuration is shown in anterior ( fig1 a ), posterior ( fig1 b ), and antero - lateral ( fig1 c ) views . more particularly , the alternate dual cervical disc replacement device configuration 1350 is suitable , for example , for implantation into two adjacent cervical intervertebral spaces . the configuration preferably , as shown , includes an alternate , upper , cervical disc replacement device 1400 ( including an upper element 1500 and an alternate , lower , element 1600 ), for implantation into an upper cervical intervertebral space , and further includes an alternate , lower , cervical disc replacement device 2400 ( including an alternate , upper , element 2500 and a lower element 2600 ), for implantation into an adjacent , lower , cervical intervertebral space . the illustrated alternate , upper , embodiment of the cervical disc replacement device is identical in structure to the cervical disc replacement device 400 described above ( and thus like components are like numbered , but in the 1400 s rather than the 400 s , in the 1500 s rather than the 500 s , and in the 1600 s rather than the 600 s ), with the exception that the flange 1606 of the lower element 1600 is configured differently and without bone screw holes . the illustrated alternate , lower , embodiment of the cervical disc replacement device is identical in structure to the cervical disc replacement device 400 described above ( and thus like components are like numbered , but in the 2400 s rather than the 400 s , in the 2500 s rather than the 500 s , and in the 2600 s rather than the 600 s ), with the exception that the flange 2506 of the upper element 2500 is configured differently and without bone screw holes . more particularly , in the alternate , upper , cervical disc replacement device 1400 of this alternate configuration , the flange 1606 of the lower element 1600 does not have bone screw holes , but has at least one mounting feature ( e . g ., a mounting screw hole ) 1609 for attaching the lower element 1600 to the alternate , upper , insertion plate 1700 ( described below ). similarly , and more particularly , in the alternate , lower , cervical disc replacement device 2400 of this alternate configuration , the flange 2506 of the upper element 2500 does not have bone screw holes , but has at least one mounting feature ( e . g ., a mounting screw hole ) 2509 for attaching the upper element 2500 to the alternate , lower , insertion plate 2700 ( described below ). as can be seen particularly in fig1 a - c , 15 b , 16 b , 17 a , and 18 a , the extent of the flange 1606 is laterally offset to the right ( in an anterior view ) from the midline ( and preferably limited to support only the mounting screw hole 1609 ), and the extent of the flange 2506 is laterally offset to the left ( in an anterior view ) from the midline ( and preferably limited to support only the mounting screw hole 2509 ), so that the flanges 1606 , 2506 avoid one another when the alternate lower element 1600 of the alternate , upper , cervical disc replacement device 1400 , and the alternate upper element 2500 of the alternate , lower , cervical disc replacement device 2400 , are implanted in this alternate configuration ( fig1 a - e ). it should be noted that the alternate , upper , cervical disc replacement device 1400 does not require both elements 1500 , 1600 to be secured to a vertebral body . only one need be secured to a vertebral body , because due to natural compression in the spine pressing the elements &# 39 ; bearing surfaces together , and the curvatures of the saddle - shaped bearing surfaces preventing lateral , anterior , or posterior movement relative to one another when they are compressed against one another , if one element ( e . g ., the upper element 1500 ) is secured to a vertebral body ( e . g ., to the upper vertebral body by bone screws through the bone screw holes 1508 a , 1508 b of the element flange 1506 ), the other element ( e . g ., the alternate , lower , element 1600 ) cannot slip out of the intervertebral space , even if that other element is not secured to a vertebral body ( e . g ., to the middle vertebral body ). similarly , the alternate , lower , cervical disc replacement device 2400 does not require both elements 2500 , 2600 to be secured to a vertebral body . only one need be secured to a vertebral surfaces together , and the curvatures of the saddle - shaped bearing surfaces preventing lateral , anterior , or posterior movement relative to one another when they are compressed against one another , if one element ( e . g ., the lower element 2600 ) is secured to a vertebral body ( e . g ., to the lower vertebral body by bone screws through the bone screw holes 2608 a , 2608 b of the element flange 2606 ), the other element ( e . g ., the alternate , upper , element 2500 ) cannot slip out of the intervertebral space , even if that other element is not secured to a vertebral body ( e . g ., to the middle vertebral body ). accordingly , the alternate , upper , insertion plate 1700 is provided to facilitate a preferred simultaneous implantation of the upper and lower elements 1500 , 1600 of the alternate , upper , cervical disc replacement device 1400 into the upper intervertebral space . similarly , the alternate , lower , insertion plate 2700 is provided to facilitate a preferred simultaneous implantation of the upper and lower elements 2500 , 2600 of the alternate , lower , cervical disc replacement device 2400 into the lower intervertebral space . the upper and lower elements 1500 , 1600 are held by the insertion plate 1700 ( preferably using mounting screws 1714 a , 1714 b ) in a preferred relationship to one another that is suitable for implantation , identical to the preferred relationship in which the upper and lower elements 500 , 600 are held by the insertion plate 700 as described above . similarly , the upper and lower elements 2500 , 2600 are held by the insertion plate 2700 ( preferably using mounting screws 2714 a , 2714 b ) in a preferred relationship to one another that is suitable for implantation , identical to the preferred relationship in which the upper and lower elements 500 , 600 are held by the insertion plate 700 as described above . the illustrated alternate , upper , insertion plate 1700 is identical in structure to the insertion plate 700 described above ( and thus like components are like numbered , but in the 1700 s rather than the 700 s ), with the exception that the lower flange 1704 b is offset from the midline ( to the right in an anterior view ) to align its mounting screw hole 1712 b with the offset mounting screw hole 1609 of the alternate lower element 1600 of the alternate , upper , cervical disc replacement device 1400 . similarly , the illustrated alternate , lower , insertion plate 2700 is identical in structure to the insertion plate 700 described above ( and thus like components are like numbered , but in the 2700 s rather than the 700 s ), with the exception that the upper flange 2704 a is offset from the midline ( to the left in an anterior view ) to align its mounting screw hole 2712 a with the offset mounting screw hole 2509 of the alternate upper element 2500 of the alternate , lower , cervical disc replacement device 2400 . accordingly , the upper and lower elements 1500 , 1600 , being held by the insertion plate 1700 , as well as the upper and lower elements 2500 , 2600 , being held by the insertion plate 2700 , can be implanted using the insertion handle 800 , insertion pusher 900 , drill guide 1000 , clips 1150 a , 1150 b ( one on the upper element flange 1506 , and one on the lower element flange 2606 , because only the upper element 1500 and the lower element 2600 are secured by bone screws ), and clip applicator 1100 , in the manner described above with respect to the implantation of the cervical disc replacement device 400 . it should be noted that the described alternate configuration ( that includes two cervical disc replacement devices ) presents the cervical disc replacement devices to the surgeon in a familiar manner . that is , by way of explanation , current cervical fusion surgery involves placing a fusion device ( e . g ., bone or a porous cage ) in between the upper and middle cervical intervertebral bones , and in between the middle and lower vertebral bones , and attaching an elongated two - level cervical fusion plate to the anterior aspects of the bones . widely used two - level cervical fusion devices ( an example two level fusion plate 1350 is shown in anterior view in fig1 c and in lateral view in fig1 d ) are configured with a pair of laterally spaced bone screw holes 1352 a , 1352 b on an upper end 1354 of the plate 1350 , a pair of laterally spaced bone screw holes 1356 a , 1356 b on a lower end 1358 of the plate 1350 , and a pair of laterally spaced bone screw holes 1360 a , 1360 b midway between the upper and lower ends 1354 , 1358 . to attach the plate 1350 to the bones , bone screws are disposed through the bone screw holes and into the corresponding bones . this prevents the bones from moving relative to one another , and allows the bones to fuse to one another with the aid of the fusion device . accordingly , as can be seen in fig1 b , when the upper and lower elements 1500 , 1600 of the cervical disc replacement device 1400 , and the upper and lower elements 2500 , 2600 of the cervical disc replacement device 2400 , are held in the preferred spatial relationship and aligned for implantation , the upper element flange 1506 and lower element flange 2606 , and their bone screw holes 1508 a , 1508 b and 2608 a , 2608 b , present to the surgeon a cervical hardware and bone screw hole configuration similar to a familiar two level cervical fusion plate configuration ( as described above , a middle pair of bone screws holes is not needed ; however , middle bone screw holes are contemplated by the present invention for some embodiments , if necessary or desirable ). the mounting of the elements 1500 , 1600 to the insertion plate 1700 allows the elements 1500 , 1600 to be manipulated as a single unit for implantation ( by manipulating the insertion plate 1700 ), similar to the way a cervical fusion plate is manipulatable as a single unit for attachment to the bones . similarly , the mounting of the elements 2500 , 2600 to the insertion plate 2700 allows the elements 2500 , 2600 to be manipulated as a single unit for implantation ( by manipulating the insertion plate 2700 ), similar to the way a cervical fusion plate is manipulatable as a single unit for attachment to the bones . this aspect of the present invention simplifies and streamlines the cervical disc replacement device implantation procedure . while there has been described and illustrated specific embodiments of cervical disc replacement devices and insertion instrumentation , it will be apparent to those skilled in the art that variations and modifications are possible without deviating from the broad spirit and principle of the invention . the invention , therefore , shall not be limited to the specific embodiments discussed herein .