Abstract:
A bone plating system is provided that permits maintenance of a compression force while also accommodating bony subsidence, among other features. Methods of implantation are also provided that improve alignment and placement during implantation and avoid maneuvers that weaken the vertebral bodies. A modular distraction screw is placed during the initial stages of surgery when all relevant landmarks are still intact. After completion of the surgical bone work, a proximal end of the distraction screw is detached, leaving a protruding distal segment implanted in the centerline of the vertebral bodies above and below the newly fused disc space. A bone plate is guided into proper position relative to the upper and lower vertebra by attaching the bone plate to the protruding distal segments. The distal segments of the distraction screws are tightened onto the plate and the plate is held stationary while bone screws are placed. The bone plating system is also extendable, allowing additional bone plates to be placed and coupled with existing plate components to create a multi-level plating system. Additional bone plates may be placed contemporaneously or during a subsequent surgical procedure.

Description:
RELATED APPLICATION 
   The present application claims priority to co-pending U.S. provisional patent application Ser. No. 60/439,030 filed on Jan. 10, 2003, and is a continuation-in-part of co-pending U.S. patent application Ser. No. 10/683,325 filed on Oct. 10, 2003, each of which is incorporated herein by reference in its entirety. 

   BACKGROUND 
   1. Field of the Invention 
   The present invention is directed at skeletal plating systems, components thereof, and method of implant placement. These systems are used to adjust, align and maintain the spatial relationship(s) of adjacent bones or bony fragments during healing and fusion after surgical reconstruction of skeletal segments. Such systems may be comprised of bone distraction devices, skeletal plates, bone screws and/or bone cables, bone screw-to-plate locking mechanisms, and any additional instruments needed for implant placement. 
   2. Related Art 
   Whether for degenerative disease, traumatic disruption, infection or neoplastic invasion, surgical reconstructions of the bony skeleton are common procedures in current medical practice. Regardless of anatomical region or the specifics of the reconstructive procedure, many surgeons employ an implantable skeletal plate to adjust, align and maintain the spatial relationship(s) of adjacent bones or bony fragments during postoperative healing. These plates are generally attached to the bony elements using bone screws or similar fasteners and act to share the load and support the bone as osteosynthesis progresses. 
   Available plating systems used to fixate the cervical spine possess several shortcomings in both design and implantation protocols. These plates are manufactured and provided to the surgeon in a range of sizes that vary by a fixed amount. This mandates that a large number of different size plates must be made and inventoried—adding to cost for manufacturer, vendor, and end user (hospitals). More importantly, the pre-manufactured sizes may not precisely fit all patients forcing surgeons to choose between a size too small or too large. 
   Plates used to attach three or more vertebrae after removal of two or more discs are manufactured with an equal distance between screw holes in the vertical plane. For example, the distance between the pair of C3 (cervical bone #3) and the pair of C4 (cervical bone #4) screws is equal to the distance between the C4 and C5 (cervical bone #5) screws as well as the distance between the C5 and C6 (cervical bone #6) screws and so forth. This not only ignores the known anatomical difference in size between bones at different levels but also fails to anticipate that a patient&#39;s unique pathology may require more extensive bony resection at one or more levels, further adding to these differences. Thus, selection of a plate with a suitable total length may still produce improper fit at one or more levels. 
   Current cervical plates are not modular, and will not permit addition of one plate to another for extension of the bony fusion at a future date. It is accepted that fusion of a specific spinal level will increase the load on, and the rate of degeneration of, the spinal segments immediately above and below the fused level. As the number of spinal fusion operations have increased, so have the number of patients who require extension of their fusion to adjacent levels. Currently, the original plate must be removed and replaced with a longer plate in order to fixate the additional fusion segment. This surgical procedure necessitates re-dissection through the prior, scarred operative field and substantially increases the operative risk to the patient. Further, since mis-alignment of the original plate along the vertical axis of the spine is common, proper implantation of the replacement plate often requires that the new bone screws be placed in different bone holes. The empty holes that result may act as stress concentration points within the vertebral bodies, as would any empty opening or crack within a rigid structural member, and lead to bone fracture and subsequent screw/plate migration. 
   Current plates may provide fixation that is too rigid. Since bone re-absorption at the bone/graft interface is the first phase of bone healing, fixation that is too rigid will not permit the bone fragments to settle and re-establish adequate contact after initial bone absorption. This process is known as “stress shielding” and will lead to separation of the bony fragments and significantly reduce the likelihood of bony fusion. Unsuccessful bone fusion may lead to construct failure and will frequently necessitate surgical revision with a second operative procedure. 
   Benzel (U.S. Pat. Nos. 5,681,312, and 5,713,900) and Foley (Pat. Applic. Pub. No. US2001/0047172A1) have independently proposed plating systems designed to accommodate bone settling. In either system, however, bony subsidence can be expected to cause one end of the plate to migrate towards an adjacent, normal disc space. This is highly undesirable since, with progressive subsidence, the plate may overlap the disc space immediately above or below the fused segments and un-necessarily limit movement across a normal disc space. Clearly, accommodation of bone settling at the plate&#39;s end is a sub-optimal solution. 
   Yuan et al described a multi-segmental plate consisting of two sliding parts in U.S. Pat. No. 5,616,142. While intended to be absorbable, this design may permit excessive play between the sliding segment and encourage bone screw loosening. In addition, this device does not permit application and maintenance of a compressive force across the bony construct. Baccelli noted these deficiencies in U.S. Pat. No. 6,306,136 and proposed a rigid plate capable of maintaining bony compression. However, the latter plate did not permit subsidence. 
   The implantation procedures of conventional plates in prior art practice have additional shortcomings. Distraction screws are used during disc removal and subsequent bone work and these screws are removed prior to bone plate placement. The empty bone holes created by removal of the distraction screws can interfere with proper placement of the bone screws used to anchor the plate and predispose to poor plate alignment along the long axis of the spine. This is especially problematic since the surgical steps that precede plate placement will distort the anatomical landmarks required to ensure proper plate alignment, leaving the surgeons with little guidance during plate implantation. For these reasons, bone plates are frequently placed “crooked” in the vertical plane and often predispose to improper bony alignment. Correct plate placement in the vertical plane is especially important in plates intended to accommodate bony subsidence, since the plate preferentially permits movement along its long axis. Thus, when the vertical axis of the plate and that of the spine are not properly aligned, the plate will further worsen the bony alignment as the vertebral bones subside. 
   The empty bone holes left by the removal of the distraction screws also act as stress concentration points within the vertebral bodies, as would any empty opening or crack within a rigid structural member, and predispose them to bone fracture and subsequent screw/plate migration. Improper plate placement and bony fractures can significantly increase the likelihood of construct failure and lead to severe chronic pain, neurological injury, and the need for surgical revision with a second procedure. 
   In view of the proceeding, it would be desirable to design an improved bone plating system and placement protocol. The new device should provide ease of use, reliable bone fixation, adjustable length, modular design, and the ability to accommodate and control bone settling. The design should also maximize the likelihood of proper plate placement and avoid maneuvers that weaken the vertebral bodies. No current plating system addresses all of these concerns. Therefore, what is needed is a system and method that overcomes these significant problems found in the conventional systems as described above. 
   SUMMARY 
   The present invention is that of a modular bone plate of adjustable length. The current invention provides a bone plate that permits maintenance of a compression force while also accommodating bony subsidence, among other features. A modular distraction screw is used for the bone work, including fusion, prior to plate placement. The distraction screw is placed as the first step of surgery when all relevant landmarks are still intact. After completion of the bone work, a proximal end of the distraction screw is detached, leaving a distal segment still implanted in the vertebral bodies above and below the newly fused disc space. The plate is guided to proper position along the upper and lower vertebra by the attached distal segments. The distal segments of the distraction screws are tightened onto the plate and the plate is held stationary while bone screws are placed. 
   The distal segments act as an anchor to guide the bone plate into the correct placement position and serve to hold the plate stationary while the plate&#39;s bone screws are placed. Since the distraction screws were placed with intact surgical landmarks, use of the distal segments to guide the plate significantly increases the likelihood of proper plate placement. In addition, the distal segments of the distraction screws serve as additional points of fixation for the plate and leave no empty bone holes which give rise to stress concentration points that further weaken the vertebral bodies. 
   After the plate is attached to the upper and lower vertebras, the plate is set to the desired length and the two segments are locked together. If application of a compressive force is desired, the plate can be used to maintain the force across the vertebral bodies by simply locking the plate segments after applying compression. Occasionally, surgeons are confronted with a grossly unstable spine from the patient&#39;s unique pathology and choose to forgo subsidence in favor of a more fixed and rigid construct. In these situations, plate placement is essentially complete and requires no further steps. More commonly, subsidence is desired and release of a second locking screw permits the plate to accommodate bony subsidence. Unlike current plating systems which provide either a rigid plate or one capable of subsidence, the current invention permits either option by the simple turn of one screw. Further, when subsidence is chosen, this plate will not overlap the adjacent disc space with bone movement, since subsidence is accommodated at the level of settling bone and not at the plate&#39;s end. 
   Extension of the fusion at a later date is easily accomplished without plate removal. An adapter is placed at either end of the plate that can couple with either a modified distraction screw or an additional bone plate. Fusion extension is started by connecting a modified distraction screw to the coupler at-the end of the plate immediately adjacent to the disc to be removed. A modular distraction screw is inserted into the adjacent vertebra and a discectomy and subsequent fusion are performed within the intervening disc space. After completion of the bone work, the modified distraction screw is removed leaving the bare coupler on the end of the plate. The proximal segment of the distraction screw is also removed leaving the distal segment attached to the adjacent vertebral body. An extension plate is used to span the space between the distal segment of the distraction screw on the adjacent vertebra and the end-coupler on the original plate. In this way, the fusion is extended and the newly fused segment is fixated without removal of the original plate. Further, the end-coupler can used to correct for any improper (“crooked”) placement of the original plate by rotating the extension plate into the true vertical. 
   The preceding discussion has focused on removal of one disc with fusion and plate fixation of the vertebral bodies above and below the evacuated disc space. However, “multi-level” procedures (that is, removal of two or more discs and fusion of three or more bones) can also be addressed with this system. Removal of two or more discs is accomplished by the step-wise removal of individual discs until all pathological levels have been addressed. Modular distraction screws may be used at each vertebral level if desired, but their use is required only at the upper and lower-most vertebras while conventional distraction screws can be used at all intervening levels. After completion of the bone work, the proximal segments of the distraction screws are removed leaving the distal segments attached to the upper and lower-most vertebral bodies. Regardless of the type of distraction screw used at the other levels, that screw is completely removed after the completion of the bone work. The empty bone holes left at these intervening level are far less important than those produced at the upper and lower-most vertebra, since the latter share a disproportionate share of the load. 
   In one embodiment of the present invention, plates used for multi-level procedures will have an expandable/subsidence mechanism overlying each disc space that is fused. The plate is guided to proper position along the upper-most and lower-most vertebra by the attached distal segments—as described above for single level procedures. The distal segments of the distraction screws are tightened onto the plate and the plate is held stationary while bone screws are placed into the upper and lower-most vertebras. In this way, the plate is fixed at each end. Depending on surgeon preference, fixation of the intervening vertebral levels may be started from either end of the plate. For illustration, fixation will be started inferiorly. The plate segment intended to fixate the vertebra immediately superior to the lower-most vertebra is moved into optimal position. The sliding mechanism between this segment and the plate segment attached to the lower-most vertebra is then locked, fixing these two segments together. Bone screws can then be easily and rapidly placed into the vertebra immediately superior to the lower-most vertebra. The process is repeated at each of the remaining vertebra. If compression is desired across the construct, it&#39;s applied across the upper and lower-most vertebras prior to placement of the bone screws into any of the intervening vertebra. Compression is maintained until all the vertebra have been fixed to the plate. Once all sliding mechanisms have been locked, the compression device may be released and the force will be maintained by the plate. If a rigid construct/plate is desired, then plate placement is complete. However, if subsidence is needed, the (subsidence) locking screw is opened at each level where bone subsidence is desired. In this way, this plate design permits the surgeon to choose the exact vertebral levels to fixed rigidly and those level that will be allowed to accommodate subsidence. Further, it permits the distance between the bone screws at different levels to be custom fit for the individual patient. These features are not shared by any currently available plating system. 
   In other embodiments, multi-level plates will be designed without a sliding/subsidence mechanism at every level. Instead, one or more sliding/subsidence mechanism(s) will be used to affect two or more levels by use of a slotted borehole configuration between levels. At each end, however, the plate will remain rigidly fixed to bone. In this way, subsidence continues to be accommodated at the level of bony movement and the plate remains stationary at each end. 
   All embodiments of the multi-level plates will preferentially, but not necessarily, contain central channels to accommodate the distal segment of the modular distraction screw and end-couplers so that extension of the fusion at a future date remains possible. 
   In other embodiments of the present invention, additional plate design, different locking mechanisms, and alternative end couplers are shown and described. Other embodiments, in addition to those illustrated, can also be used. 
   The plating systems described in the present invention provide ease of use, reliable bone fixation, adjustable length, modular design, and the ability to accommodate and control bone settling. These designs will also maximize the likelihood of proper plate placement, avoid maneuvers that weaken the vertebral bodies, and provide a significant advantage over the current and prior art. These and other features of the present invention will become more apparent from the following description of the embodiments and certain modifications thereof when taken with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The details of the present invention, both as to its structure and operation, may be gleaned in part by study of the accompanying drawings, in which like reference numerals refer to like parts, and in which: 
       FIG. 1  is a partial side view of a disassembled distraction screw according to an embodiment of the invention; 
       FIG. 2  is an assembled distraction screw and a cross sectional side view of the assembled distraction screw according to an embodiment of the invention; 
       FIGS. 3A-3B  are close up views of the connector portion of the elongated body of a distraction screw according to an embodiment of the invention; 
       FIGS. 4A-4B  are partial views of a distraction screw removal tool according to an embodiment of the invention; 
       FIGS. 5A-5B  are exploded perspective views of a bone plate according to alternative embodiments of the invention; 
       FIGS. 5C-5D  are perspective views of a mounted bone plate according to alternative embodiments of the invention; 
       FIGS. 6A-6F  are top, bottom, and side views of angled bracket plate components according to an embodiment of the invention; 
       FIG. 7A-7C  are top, bottom, and side views of square bracket plate components according to an embodiment of the invention; 
       FIGS. 8A-8B  are top views of a third plate component according to an embodiment of the invention; 
       FIGS. 9A-9B  are perspective views of a modified distraction screw attached to a bone plate according to an embodiment of the invention; 
       FIG. 10A  is a partial side view of a modified disassembled distraction screw according to an embodiment of the invention; 
       FIGS. 10B-10D  are partial side views of a modified assembled distraction screw according to an embodiment of the invention; 
       FIG. 10E  is a close up view of a modified distraction screw according to an embodiment of the invention; 
       FIG. 11A  is a perspective view of an offset, modified distraction screw according to an embodiment of the invention; 
       FIG. 11B  is a perspective view of an offset, modified distraction screw attached to a bone plate according to an embodiment of the invention; 
       FIG. 12A  is an exploded perspective view of a bone plate according to an embodiment of the invention; 
       FIG. 12B  is a top view of a first bone plate component according to an embodiment of the invention; 
       FIG. 12C  is a bottom view of a first bone plate component according to an embodiment of the invention; 
       FIG. 12D  is a top view of a second bone plate component according to an embodiment of the invention; 
       FIG. 12E  is a bottom view of a second bone plate component according to an embodiment of the invention; 
       FIG. 13A  is a sectional view of a bone plate according to an embodiment of the invention; 
       FIG. 13B  is a close up sectional view of the locking mechanism of a bone plate according to an embodiment of the invention; 
       FIG. 14A  is a perspective view of a mounted bone plate in an open position according to an embodiment of the invention; 
       FIG. 14B  is a perspective view of a mounted bone plate in a closed position according to an embodiment of the invention; 
       FIG. 15A  is an exploded perspective view of a jackscrew bone plate according to an embodiment of the invention; 
       FIG. 15B  is a sectional view of a jack screw bone plate according to an embodiment of the invention; 
       FIG. 15C  is a close up sectional view of the locking mechanism of a jack screw bone plate according to an embodiment of the invention; 
       FIGS. 16A-16B  are top and bottom views of a bone plate component with an open central channel and an alternative end coupler for a modified distraction screw according to an embodiment of the invention; and 
       FIGS. 17A-17B  are top views of combined bone plates with slotted screw holes and sliding mechanisms according to alternative embodiments of the invention. 
   

   DETAILED DESCRIPTION 
   Certain embodiments as disclosed herein provide for a modular bone distraction screw and a modular bone fixation plate with an adjustable length to accommodate bone settling. For example, one plating system disclosed herein allows for compression to be set during placement of the plate and also allows subsidence of the bone while maintaining the initial compression. 
   After reading this description it will become apparent to one skilled in the art how to implement the invention in various alternative embodiments and alternative applications. However, although various embodiments of the present invention will be described herein, it is understood that these embodiments are presented by way of example only, and not limitation. As such, this detailed description of various alternative embodiments should not be construed to limit the scope or breadth of the present invention as set forth in the appended claims. 
     FIG. 1  shows a modular distraction screw  110 , which comprises a distal segment  120  and a removable proximal  130  segment. The distal segment  120  has a head portion  122 , and a threaded shank portion  124 , which can be securely fastened unto bone. The proximal segment  130  comprises an elongated body  132  and deployable member  136 . Elongated body  132  has a smooth-walled internal bore  134  extending through its full length and houses the deployable member  136  within the bore. The deployable member  136  is adapted to be retractably deployed beyond the distal end of the internal bore  134 .  FIG. 2  shows the assembled distraction screw. 
     FIG. 3  illustrates distal segment  120 , which comprises a threaded shank portion  124  and a head portion  122 . Threads  126  of the shank portion  124  are preferably self-tapping and/or self-drilling. Depending on the particular application, the shank  124  can be of variable lengths and diameter. In one application, the outer diameter of the shank/threads is preferably equal to the widest point of head  122 . One of ordinary skill in the art would understand that the threads can be of any design that is well known to be applicable for screwing placement into mammalian bone. 
   Head  122  is circular with hollow central bore  1220 . The upper aspect  1222  of the circular head is of uniform diameter but the lower portion  1223  of the head is of progressively greater diameter such that the head has a sloping side wall below edge  1224 . Threads  1225  are located within bore  1220  and are complementary to threads  128  of deployable member  136 . Head  122  has a plurality of slots  1226  which are engageable by projections  1322  of the distal aspect of elongated body  132 , as shown in  FIG. 3A  and  FIG. 3B . Slots  1226  permit the head to collapse inward when centripetal force is applied to the outer wall of the head. 
   Deployable member  136  is advanced through bore  134  to engage distal segment  120  with the coupling of the complimentary threads  128  and  1225 . The proximal head  1362  of member  136  permits application of rotational force to deployable member  136  (as shown in  FIG. 2 ) further driving threads  128  and  1225  together and locking members  132 ,  136  and distal segment  120  together. While depicted as a hex configuration, any engagable configuration may be used to drive deployable member  136 . 
   The coupled proximal segment  130  and distal segment  120  employing the above-described means of engagement provide a modular distraction screw. When fully assembled, the screw functions as a unitary device. In a surgical application, a wrench (not shown) is attached to the tool attachment portion  180  of elongated member  132  ( FIG. 1 ), and the distraction screw is positioned at a site of a bone. A rotational force is applied to portion  180  causing the proximal and distal segments to rotate in unison so that thread  126  of the distal segment  120  engages the underlying bone and shank  124  is advanced into the bone. 
   After the distraction screw is used to perform the bone work, the proximal segment  130  is detached from distal segment  120 . The distraction screw is disassembled into its components by applying a rotational force to head  1362  of member  136  in a direction opposite (usually counter-clock wise) to that required for screw assembly (usually clock-wise). The distal segment is held stationary while threads  128  and  1225  are disengaged by applying a counter force to distal segment  120  using the proximal portion  180  of the elongated body  132 . In this way, the proximal segment  130  is removed leaving the distal segment  120  attached (implanted) to the bone structure. 
   As implanted, the distal segment  120  provides enhanced structural integrity of the bone by reducing the stress concentration generally expected of an empty opening in a structural member. In addition, leaving the distal segment  120  attached to bone eliminates the robust bone bleeding encountered after removal of current, commercially-available distraction screws and obviates the need to fill the empty hole with a hemostatic agent. 
   The distal segment  120  also provides a point of anchoring for a skeletal plate and help insure proper plate placement. Since placement of the distraction screws is performed as the first step in the surgical procedure, the anatomical landmarks required to ensure proper alignment of the plate in the desired anatomical plane are still intact. 
   Alternatively, a conventional one-piece distraction can be used to distract the vertebra during discectomy. After the bone work is finished, the conventional distraction screw is removed leaving an empty bone hole. A distal segment  120  is placed into the empty bone hole and provides an anchor point for the skeletal plate. 
     FIGS. 5A and 5B  show two vertebral bodies  2  and  4  and the plating system  8  of the present invention used to fixate them. The plating system includes sliding plate segments  10  and  20  and a coupler means or a coupler segment  30 , which couples the sliding segments  10  and  20  and controls their movements.  FIGS. 6-7  show the top and mid-sectional views of the embodiment of the bone fixation plate. 
   The plate segments  10  and  20  may be curved in either the vertical or horizontal plane in order to conform to the shape of the bone it is designed to fixate. For example, plates designed to attach onto the anterior aspect of the cervical spine are preferentially, but not necessarily, convex in both the vertical and horizontal planes. Further, the plate surface immediately adjacent to the bone surface may contain one or more horizontal indentations  1200  in order to permit the placement of additional curvature in the vertical plane. 
   The plating system or any of its components can be made of any biologically adaptable or compatible materials. Materials considered acceptable for biological implantation are well known and include, but are not limited to, stainless steel, titanium, combination metallic alloys, various plastics, resins, ceramics, biologically absorbable materials and the like. It would be understood by one of ordinary skill in the art that any system component can be made of any materials acceptable for biological implantation and capable of withstanding the torque required for insertion and the load encountered during use. Any components may be further coated/made with osteo-conductive (such as deminerized bone matrix, hydroxyapatite, and the like) and/or osteo-inductive (such as Transforming Growth Factor “TGF-B, ” Platelet-Derived Growth Factor “PDGF,” Bone-Morphogenic Protein “BMP,” and the like) bio-active materials that promote bone formation. Further, any instrument or device used in implant placement may be made from any non-toxic material capable of withstanding the load encountered during use. Materials used in these instruments need not be limited to those acceptable for implantation, since these devices function to deliver the implatable segments but are not, in themselves, implanted. 
   As shown in  FIGS. 5-7  sliding segment  10  has two boreholes  1110  which are formed through the plate to accommodate fastening elements, such as bone screw. Each borehole may be oriented in the true vertical plane or form an angle with the vertical. For use in the cervical spine, boreholes  1110  will preferentially, but not necessarily, be angled towards each other in the horizontal plane and away from the sliding end in the vertical plane. The top opening of the boreholes may be flush with the plate surface or may be recessed. The distance between the boreholes may also vary depending on the requirement of plate application and design. A depression  1120  is present between the boreholes with slot  1130  along the depression. The side walls  1132  of slot  1130  are preferentially, but not necessarily, angled with the true vertical such that the top opening of slot  1130  is slightly smaller than the bottom opening. Slot  1130  is adapted to accommodate or mate with screw head  122  of distal segment  120  of the distraction screw. While depicted as an elongated hole, slot  1130  may alternatively be a circular hole. 
   Plate segment  10  has three projections, consisting of two side projections  1140 ,  1160  and a central projection  1150 . Two indentations  1180  and  1190  are formed between these three projections. The inside wall of each projection  1140  and  1160  contain indentations  1142  and  1162 , respectively. While depicted as “V” shaped, these indentations may be made of any geometric configurations including, but not limited, square, oval, circular, and hybrid designs which are complimentary to the sliding portion of the other plate segment  20 . The central projection  1150  has a partial thickness middle segment  1152  and two full side walls  1154 . An opening  1156  with internal threads  1158  is provided on segment  1152 . The top surface of middle segment  1152  is preferentially textured so as to permit superior contact with the undersurface of the complementary plate component. 
   The other end portion of the plate segment  10  has a projection  1170 , which is preferentially, but not necessarily, position in the midline of the plate segment. The projection has a central hole  1172  with threads  1174 . Spines  1176  may be placed along the top of the projection to mate with complimentary spines on the add-on attachments, as shown in  FIG. 4 . These spines may be placed on any one or combination of surfaces adjacent projection  1170 . These surfaces may be textured or left smooth. 
     FIGS. 6D ,  6 E,  6 F &amp;  7  illustrate the complementary sliding plate segment  20  to sliding segment  10 . Again, two boreholes  210  are vertically formed through the plate to accommodate fastening elements. As with sliding plate segment  10 , these boreholes may be oriented in the true vertical plane or form an angle with it, may be flush with the plate surface or further recessed, and the distance between these holes may vary depending on the requirement of the plate application. A depression  220  is formed between the boreholes with a slot  230  whose side walls  232  are preferentially angled with the true vertical such that the top opening of the slot is slightly smaller than the bottom opening. Slot  230  is adapted to mate with and accommodate the distal segment of a distraction screw. 
   Sliding plate segment  20  has two projections  240 ,  260  and central connection  250 . Projection  240  has an extension  242  which is complementary to indentations  1142  of projections  1140 . Likewise, projection  260  has an extension  262  that is adapted to be received by indentations  1162  of projection  1160 . Projections  240  and  260  may be of any geometric configuration and cross-section including, but not limited, square, oval, circular, truncated triangular, modified rectangular and hybrid designs that are complimentary to the corresponding sliding portions of the segment  10 . Further, projections  240  and  260  may be of differing designs that are complimental to projections  1140  and  1160 . The central connection  250  has a partial thickness middle segment  252  and two side walls  254 . An opening  256  with internal threads  258  is located on segment  252 . Openings  256  and  1156  may be aligned with the direction of bone subsidence. 
   On its opposite end, plate segment  20  has a partial thickness projection  270  that is preferentially, but not necessarily, in the midline of the plate. Projection  270  has a central hole  272  with threads  274 . Spines  276  may be placed along the top of the projection to mate with complimentary spines of the add-on attachments. These spines may be placed on any one or combination of surfaces adjacent projection  270 . These surfaces may be textured or left smooth. 
     FIG. 8  illustrates top and oblique views of coupling means or segment  30 . Two full thickness channels  310  and  320  are formed within segment  30 . The channels are preferentially, but not necessarily, of different lengths and walls  312  and  322  of channels  310  and  320  are preferentially angled with the vertical plane. The top surface of coupling segment  30  is smooth while the bottom surface is preferentially textured in the portion of the segment with the larger channel  320 . The bottom of the segment with the smaller channel  310  is smooth. Coupling segment  30  couples plate segments  10  and  20  as depicted in  FIG. 5  with screws  40  and  41 . While not depicted, each screw has threads on which are complimentary to threads  158  of segment  10  and threads  258  of segment  20 . The screws have top depressions  414  and  404  for engagement by a screwdriver or other driving instrument. While both screws are depicted as being identical, each may be of any of the many well known fastener designs and may be inserted using any complimentary driver. 
   Projections  1150  and  250  of sliding plate segment  10  and  20  respectively may be of equal or different lengths. When unequal, central projection  1150  is made longer than projection  250  as a matter of preference. (Alternatively, the longer projection may be placed within segment  20 .) The longer channel  320  of segment  30  engages the longer central projection (element  1150  of segment  10 ) by screw  41  while the shorter channel  310  engages the shorter central projection (element  250  of segment  20 ) by screw  40 . 
   The bone screws and the screw for the coupler segment  30  may be of any of the many well known designs considered acceptable for implant attachment to the bony skeleton and made from any material intended for biological implantation. 
   As an option, any portion of the plating segments may be made of radiolucent materials (such as PEEK, PEAK, and the like) so that unfettered x-ray examination of the underlying bone can be performed in the post-operative period. Thus, projections  1150 ,  250  and segment  30  can be made from radiolucent materials so as to provide a window for x-ray examination of the bone without decreasing the overall strength of the plate. 
   After completion of the bone work and detachment of the proximal portions of the distraction screws, the distal segments are left attached to the vertebra above and below the newly fused disc space. The bone plate is fully assembled before implantation. Screw  40  is fully seated at the outside edge  312  of channel  310  so that plate segment  20  and coupler segment  30  are fixed relative to one another. However, screw  41  is partially seated on the outside edge  320  of channel  320  so that plate segment  10  and coupler segment  30  are free to slide relative to each other. Slot  1130  and  230  are aligned with the distal segments  120  which are implemented on the bone structure following bone work upon which the heads  122  of distal segments  120  are snapped into the slots. As the head  122  spring back, the plate segments are held between the screw heads  122  and the underlying bone  2  and  4 . 
   If the plate is poorly positioned because of bony irregularity, it can be removed to permit additional bone work.  FIGS. 4   a  &amp;  4   b  illustrate a screw head remover  300 , which can be used to remove the plate segments. When pushed onto head  122  of distal segment  120 , the screw head remover applies a centripetal force to the side walls, causing them to move inward, and permitting plate removal. Alternatively, if the plate is well positioned, the boreholes are moved into optimal position for bone screw placement. A screw driver is used to drive distal segment  120  further into the bone, thereby holding the plate stationary. The bone screws are then easily placed into the underlying bone. 
   Once the plate segments are set to the desired length, screw  41  is tightened. If desired, compression can be placed across the bony construct and maintained with closure of screw  41 . The inferior surface of segment  30  around the longer channel  320  and the superior surface of projection  1150  is preferentially, but not necessarily, textured so as to promote greater frictional contact between segments  10  and  30 . At this point, the plate is rigid. If accommodation of bony subsidence is desired, screw  40  is unlocked, permitting movement of segments  20  towards each other as bone settling occurs. The extent of subsidence permitted is governed by the length of channel  310 . 
   Extension of the fusion at a future date can be easily accomplished without plate removal. Incorporation of the vertebral body immediately above or below into the fusion mass is started by placement of a modular distraction screw  110  into that adjacent vertebra. A modified distraction screw is used to engage the end-coupler of the existing plate as shown in  FIG. 10 . As shown in  FIGS. 9 ,  10  and  11 , the modified distraction screw  500  comprises an elongated body  510  with an internal bore  512  extending through its entire length to distal end portion  516 . The elongated body  510  houses a deployable member  530 , which is disposed within the internal bore  512 . The deployable member  530  is adapted to be retractably deployed beyond the opening  516  of internal bore  512 . Threads  532  are located on one end of member  530  and head  534  is formed on the other end. Head  534  has diameter greater than that of the internal diameter of bore  512 . Depression  536  is formed within head  534  so as to permit engagement and rotation of deployable member  530  with a complimentary screwdriver. While depicted as a hexagonal depression intended to receive an Allen&#39;s wrench, any alternative means and arrangements for engaging and rotating the deployable member  530  can be employed including. Likewise, the engageable surface may be placed on the outer surface of head  534  or extend from it. 
   Adjacent to distal end  516  of elongated body  510 , spines are placed which are adaptable to compliment and engage with spines  270  and  1176  of end coupler  270  and  1170  respectively. The spines may be placed on any surfaces of the distal portion  516  of the elongated body  510  or both. Threads  532  of deployable member  530  are engageable to threads  1174  of end coupler  1170  or threads  274  of end coupler  270 , thus firmly affixing the modified distraction screw to the plate. The modified distraction screw and the modular distraction screw previously affixed to the adjacent vertebra are used to distract the vertebral bodies, permitting work on the intervening disc space. When the discectomy and subsequent bone work are finished, the modular distraction screw is separated leaving the distal segment attached to vertebral body. The modified distraction screw is removed leaving a bare end-coupler. A separate plate is used to span the distance between the distal segment and the end coupler. In this way, the fusion is readily extended to an adjacent level. 
   Occasionally, placement of the plating segments might result in the end coupler being too close to the adjacent disc space such that placement of the modified distraction screw onto the coupler could hinder surgical access to the disc space.  FIG. 11A  shows an offset modified distraction screw which may be used in this setting and  FIG. 11B  illustrates its placement. The screw components are similar to those described above and as shown in  FIG. 10 . 
   A further embodiment of the present invention is illustrated in  FIGS. 12-16 . As in the embodiments described above, the plating segments may be curved in either the vertical or horizontal plane, may contain one or more horizontal indentations in order to permit the placement of additional curvature in the vertical plane (not shown), and may be made of any biologically adaptable or compatible materials. 
   Each of the plate segments  140  and  150  possess two boreholes to accommodate bone fasteners, a central channel to couple with distal segment  120  of the modular distraction screw and an end-coupler. 
   A sliding end portion  80  of plate segment  140  is formed by two side projections  840 ,  860  and a central opening  850 . Projection  840  is an extension of the plate segment  80  with side indentation  842 . Indentation  842  may be made of any geometric configurations including, but not limited, square, oval, circular, and hybrid designs which is complimentary to wall  942  of projection  940  of plate segment  150 . Projection  860  has a top wall  862 , a side wall  864  and an inferior wall  866 . Preferably, both top and side walls are straight while the inferior wall is triangular. One of ordinary skill in the art would recognize that any geometric configurations may be used for the walls of projection  860  as long as they compliment the interacting surface of slide portion  90  of plate segment  50 . Top surface of wall  862  has opening  8620  which is key-hole shaped and composed of a larger, full thickness circular opening  8622  at one end and a partial thickness, slot  8624 . The inferior surface of wall  862  has a partial thickness channel with opening  8622  at one end and a channel  8626 . The latter is set beneath slot  8624 , is of the same length as slot  8624  and of the same width as the diameter of opening  8622 . 
   As shown in  FIG. 12 , the sliding end portion  90  of plate segment  50  is adapted to fit snuggly within central opening  850  and slidingly engages the inner walls of projections  840  and  860  of plate segment  140 . The sliding end portion  90  is formed by projection  940  which has side walls  942  and  946 . Wall  942  is depicted as projecting in a “&gt;” fashion but any geometric configuration may be used that compliments surface  842  of plate segment  80 . Likewise, wall  946  is configured to compliment  860  of plate segment  80 . Preferably, wall  946  has sloping surface  9462  and the partial thickness projection  9464  which has upper wall  9470  and lateral wall  9472 . The inferior aspect of wall  9472  is preferably slopped. Partial thickness projection  9464  has channel  9465  and a cross-sectional exploded view is shown in  FIG. 13B . The width of channel  9465  is preferably equal to the diameter of opening  8622  of the plate segment  140 . A central ridge  9466  is formed along the walls of channel  9465  which is preferably rectangular. Ridge  9466  does not extent to the bottom of channel  9465 , leaving channel  9467  beneath the ridge. Preferably, ridge  9466  does not extent to the top of channel  9465 , leaving another second channel  9468  above the ridge. The width of the opening formed at the level of ridge  9466  is less than the width of opening  8622 . 
   Plate segments  140  and  150  are coupled in assembly with channel  9465  and opening  8620  overlapping each other by a suitable coupler means. Coupler means incorporates a bolt element  96 , which comprises a screw  960  and locking nut  980 . Screw  960  has head  962  which is preferably square or hex shaped and fits snuggly beneath ridge  9466  and within channel  9465 . The thickness of head  962  is sufficiently thin so as not to extent beyond the inferior surface of projection  9464 . Shank  964  of screw  960  is circular and fits within the channel formed at the level of ridge  9466 . The shank has a flat end and total length greater than the thickness of projection  9464  but less than the combined thickness of projection  9464  and channel  8626 . Shank  964  also has threads  966  (not depicted) which engages nut  980 . Nut  980  has a central full thickness bore  982  with threads  984  (not depicted) adapted to compliment and engage threads  966 . The threads may be of any available and recognized thread design. Nut  980  fits snuggly within opening  8622  of segment  140 , but has diameter greater than that of channel  8624 . Preferably, the top surface of nut  980  has indentations  986  which can be engaged by the driving tool. 
     FIGS. 14A and 14B  illustrate the coupler means in the open and closed positions. When open, nut  980  of bolt element  96  is held within opening  8622  such that it cannot move relative to plate segment  140 . However, since the bolt element  96  is not fixed to plate segment  150 , the plate segments  140  and  150  can continue to move relative to one another in either direction. When the plate is set to the desired length, nut  980  is rotated until edge  9466  rests tightly between nut  980  and head  962 . The nut  980  also leaves opening  8622  and coming to rest within channel  8626 . In this way, bolt element  96  is fixed to plate segment  150  and freed from plate segment  140 . If desired, compression may be applied across the fused disc space prior to locking nut  980 . Since bolt element  96  rests at the far end of opening  8620 , any applied compressive force is maintained with closure of the locking mechanism. After closure, plate segments  140  and  150  can only move towards each other, thus accommodating subsidence. The length of opening  8620  determines the amount of subsidence permitted. 
   As shown in the drawings, each of the plate segments of the present invention have two boreholes to accommodate bone fasteners, a central channel to couple with distal segment  120  of the modular distraction screw and an end-coupler. These features have been described above and will not be illustrated further. 
   The plating system of the present invention can be applied, by way of a multilevel plating configuration to fixate three or more bones after the removal of two or more discs. As in the embodiments previously illustrated for single level plate, “multilevel” plates may be curved in either the vertical or horizontal plane, may contain one or more horizontal indentations in order to permit the placement of additional curvature in the vertical plane, and may be made of any biologically adaptable or compatible materials. Each of the upper and lower ends of the plates will contain two boreholes to accommodate bone fasteners, a central slot to anchor the distal segment  120  of the modular distraction screw and an end-coupler to accommodate possible modular extension of the fusion at a later date. 
     FIG. 17  shows an exemplary embodiment of the multi-level plates, where one of the number of sliding mechanisms can be used at each level such that the total number of sliding mechanisms is equal to the number of discs removed and fused. Longer plates can be made by the sequential addition of other levels. While the illustrated plate present only one exemplary embodiment of the sliding mechanism and coupler means, it is understood that any of the previously discussed embodiments may be used in any combination to produce these plates. Further, different sliding mechanism designs can be used at different levels, if desired. 
   With the exception of the two ends, a segment  300  with two full thickness bore holes is placed between each of the sliding portions. These boreholes may be oriented in the true vertical plane or form an angle with the vertical. The boreholes will be angled towards each other in the plate&#39;s short axis (horizontal plane) and form a right angle with the body of the plate in the long axis (vertical plane). The top opening of the boreholes may be flush with the plate surface or may be recessed. The distance between the boreholes may also vary depending on the requirement of plate application and design. 
   Removal of two or more discs is accomplished by the step-wise removal of individual discs until all pathological levels have been addressed. Modular distraction screws may be used at each vertebral level if desired, but their use is required only at the upper and lower-most vertebras while conventional distraction screws can be used at all intervening levels. After completion of the bone work, the proximal segments of the distraction screws are removed leaving the distal segments attached to the upper and lower-most vertebral bodies. At other disc levels, the distraction screw can be completely removed after the completion of the bone work. 
   The plate is guided to proper position along the upper-most and lower-most vertebra by the attached distal segments—as described above for single level procedures. The distal segments of the distraction screws are tightened onto the plate after selection of optimal bone screw position. In this way, the plate is held stationary while the bone screws are placed into the upper and lower-most vertebras and the plate is fixed at each end. Depending on surgeon preference, fixation of the intervening vertebral levels may be started from either end of the plate. For illustration, fixation will be started inferiorly. The plate segment intended to fixate the vertebra immediately superior to the lower-most vertebra is moved into a desired position. The sliding mechanism between this segment and the plate segment attached to the lower-most vertebra is then locked. Once these segments are immobilized, bone screws are placed into the vertebra immediately superior to the lower-most vertebra. The process is repeated at each of the remaining vertebra. If compression is desired across the construct, it&#39;s applied across the upper and lower-most vertebras prior to placement of the bone screws into any of the intervening vertebra. Compression is maintained until all the vertebras have been fixed to the plate. Once all sliding mechanisms have been locked, the compression device may be released and the force will be maintained by the plate. 
   Alternatively, one or more sliding mechanisms can be used to accommodate boney subsidence at two or more fused levels. This is accomplished by using a slotted borehole between levels.  FIG. 17A  illustrates this design feature in a two level plate in which only one sliding mechanism is employed. Again, the plate is placed after completion of the bone work and plate placement is started by fixation of the plate at each end using the distal segments of the distraction screws. The plate is set to the desired length and the sliding mechanism is locked. If desired, compression may be applied prior to closure of the mechanism. The bone screw is placed at the end of the slotted borehole immediately adjacent to the sliding mechanism and the subsidence screw is opened. In this way, the plate&#39;s adjustable length and subsidence can be accomplished using a single sliding mechanism. While the second embodiment of the sliding mechanism as well as the alternative embodiments of the end-coupler and central channel are illustrated, it is understood that any of the previously discussed embodiments may be used in any workable combination to produce these plates. 
     FIG. 17B  demonstrates the other potential designs that can be used for a three level plate. Other possible variations that can be used in creating a other multi-level plating system. Longer plates can be made by the sequential addition of other levels. 
   While the particular systems and methods herein shown and described in detail are fully capable of attaining the above described objects of this invention, it is to be understood that the description and drawings presented herein represent a presently preferred embodiment of the invention and are therefore representative of the subject matter which is broadly contemplated by the present invention. It is further understood that the scope of the present invention fully encompasses other embodiments that may become obvious to those skilled in the art and that the scope of the present invention is accordingly limited by nothing other than the appended claims.