now referring to fig1 , in some embodiments , the bone anchor 1 is a temporary fixation pin that is attached to an anterior surface of the vertebral body . the pin typically has a distal end portion 2 , an intermediate portion 7 comprising a threaded shaft 3 adapted for bony fixation , and a proximal end 5 . so that endplate coverage is maximized , preferably , the bone anchor of each marker is inserted into a midline of its respective vertebra . extending from the intermediate portion of the temporary fixation pin is a pair of lateral arms 11 or wings . these arms are dimensioned to define the outer boundaries of the disc prosthesis and / or the lateral edges of the disc space . now referring to fig2 a and 2 b , in preferred embodiments , the arms 11 form an angle α of between about 90 and 180 degrees . providing the arms to form an angle α of between about 90 and less than 180 degrees preferably allows the surgeon to place the anchor safely into the anterior face of the vertebra , while allowing each arm to extend down towards the vertebral endplate , thereby allowing the lateral edge of each arm to be adjacent the vertebral endplate and shortening the distance between the opposing edges and easing the task of visualizing the window . now referring to fig2 c , in preferred embodiments , each arm has an outer lateral edge 13 , and the lateral edges are substantially parallel to each other . when a pair of such markers are inserted into upper and lower vertebral bodies , they may be oriented so that the lateral edges of the upper and lower markers are substantially parallel to each other , and the edges should run directly towards the opposing edges on the opposing marker , thereby making easier the task of visualizing the window of permissible entry . now referring to fig3 , in one embodiment , the marker is a two piece design , and includes a standard distraction pin 15 having a shaft 20 and a modular ring 17 having a pair of arms 19 extending therefrom . the distraction pin can have an engagement surface 21 adapted to engage and hold the inner surface 23 of the modular ring . in use , the distraction pin is inserted into a vertebral body and the modular ring is inserted down and friction fit upon the shaft of the distraction pin . the ring can also be be locked onto the shaft via an anti - rotation feature such as a nub . therefore , in accordance with the present invention , there is provided a modular lateral edge marker , comprising a ) a standard distraction pin having a shaft , and b ) a ring having a pair of arms extending therefrom , wherein the ring is adapted to be received upon the shaft of the distraction pin . the lateral edge marker is sized such that it can accommodate all sizes of disc prostheses and / or vertebral body widths . the lateral arms can be sized to span a variety of overall distances to accommodate all sizes of disc prosthesis and vertebral body widths . in some embodiments , the arms or wings of the lateral edge marker are made of a material that is flexible . this flexibility enables the surgeon intraoperatively modify the arm edge to arm edge dimension and accommodate all desired widths . now referring to fig4 , a pair of the lateral edge markers of the present invention are used in tandem by placing the bone anchor portion of each marker directly upon the midline of the respective upper and lower vertebral bodies . now referring to fig5 a - 5 c , the lateral edge marker of the present invention is preferably placed into a vertebral body with the help of insertion tool 51 . the insertion tool has a proximal handle 52 , an intermediate shaft 53 , and a distal end portion 55 . the distal end portion of the tool has a flange 43 extending laterally therefrom and a rectangular throughhole 45 passing axially through the flange . in use , this axial throughhole is passed over a midline marker that has been placed upon the midline in the intervertebral disc , thereby centering the insertion tool upon the midline . the distal end portion also has an axial recess that comprises a central deep recess 61 for accepting the proximal portion of the bone anchor and a pair of more shallow lateral recessed portions 63 for accepting an inner portion of each arm . now referring to fig . 6 , the mating of the lateral edge marker features with the recesses on the bottom of the insertion tool will cause alignment of the arms of the lateral edge marker with the centering hole of the insertion tool , thereby allowing the orientation of the arms to key off the placement of the centering hole upon the midline marker . in some embodiments , and now referring to fig7 a , the proximal end portion 71 of the bone anchor is axially elongated so that it can function as a caspar pin as well as a lateral edge marker . in some embodiments , and now referring to fig7 b , the arms of caspar - type lateral edge marker are removable . the removable arms can be placed upon the casper pin at the point of care , thereby allowing modularity in the size offerings . in fig7 b , there is provided a modular lateral edge marker , comprising a ) a caspar distraction pin 81 having an elongated proximal shaft 83 , a distal threaded shaft 84 adapted for insertion into bone , a intermediate base 85 , and tongue 87 disposed upon the proximal shaft , and b ) a ring 91 having an inner annular surface 93 having a recess 92 adapted for fixation to the tongue , an outer surface 95 , and pair of arms 97 extending from the outer surface of the ring . in other embodiments , the arms of the caspar - type lateral edge marker are bendable to accommodate variations in size . these bendable legs are preferably made of wire and are molded to fit the specific application . in the some embodiments of using the lateral edge marker of the present invention , the lateral edge marker pin is first inserted at the approximate midline of the vertebral body . using fluoroscopy , verify that the outer boundaries of the lateral edge marker are consistent with the lateral edges of the disc space . lastly , use the outer boundaries created from the lateral edge marker as guidance during the discectomy , endplate preparation and final implant placement . in some embodiments , the lateral edge marker of the present invention is used to help the surgeon with endplate preparation prior to the insertion of the motion disc . as the step of rasping the vertebral endplates is the most imprecise step of the implantation procedure , it would be useful to develop a method of increasing the precision of this step . keying the rasping off a properly placed midline marker is of limited utility for two reasons . first , the midline marker is often visually obscured during the rasping procedure . second , the midline marker offers little guidance to the surgeon as to the outer boundaries of the region to be rasped . by rasping with the guidance of the lateral markers of the present invention , these drawbacks are eliminated . because the lateral markers of the present invention are advantageously used in the implantation of intervertebral motion disc prostheses , in some embodiments of the present invention , there is provided a kit comprising : i ) a bone anchor having a distal end portion adapted for fixation to a vertebral body , a proximal end and a longitudinal shaft therebetween , and ii ) a pair of arms extending from the transversely from the bone anchor , and
a device that provides lateral markers for the surgeon as an indication of the permissible outer boundary limits of insertion of an intervertebral motion disc into the disc space .