DOWEL FOR FUSING BONES

Abstract: Existing bone dowels are improved through the use of rigid, preferably metallic end plates (306, 320). The plates are positioned relative to the anterior and posterior end surfaces of the bone dowel. The completed structure may be inserted, removed, and positioned into the vertebrae. The use of the rigid endplates (306, 320) facilitates load sharing which helps to prevent graft fracture. In the preferred embodiment, the end plates (306, 320) are positioned against the end surfaces of the bone section (400) through the use of a link member (308), which passes through the interior of the bone section (400) and connects the end plates (306, 320). One of the end plates (320) may include a threaded bore, and one end of the link member may be threaded to receive the threaded bore of that end plate to hold it in place.

Description: BONE REINFORCERS Cross Reference To Related Applications This patent application claims benefit from U.S. Patent Application Serial No. 09/580,231 , filed May 25, 2000, which application is incorporated herein by reference. Field of the Invention The present invention relates generally to orthopedic surgery and, in particular, to interbody cages and dowels of the type for use in interbody spinal fusions. Background of the Invention With respect to spinal surgery wherein one or more vertebrae are fused, the use of bone dowels have certain advantages over metal cages. First, allograft bone readily fuses to the vertebrae. Second, it is often impossible to determine if metal bone-filled cages have fused to adjacent vertebrae, because the metal obstructs x- ray imaging of the bone within the metal cages as well as the cage vertebra junction. Third, bone dowels have a modulus of elasticity closer to that vertebrae. Consequently, bone dowels stress shield less than metal cages. Bone dowels have certain disadvantages when compared to metal cages, however. Allograft bone incorporates into host bone through a process known as "creeping substitution." Host blood vessels grow into the allograft bone in the first stage of this process. Bone removing cells known as osteoclasts then invade the allograft bone. After sufficient bone is removed by the osteoclasts, bone building cells known as osteoblasts lay down new host bone on the allograft bone. This remodeling process may go on for years. As would be expected, the allograft is weakened by the channels formed by the blood vessels, as well as the bone removal by the osteoclasts. Although the allograft regains its strength once sufficient new bone is formed, allograft bone dowels are at risk of fracture during the period of time that they are weakened. Allograft bone dowel fracture is well known to those skilled in the art of spinal surgery. Bone dowels are also weaker than metal cages, even before they undergo creeping substitution. Consequently, bone dowels can fracture during surgical placement. Fractured dowels can be difficult to remove, and may lead to failure of a fusion to occur. The properties of bone also do not allow certain shapes or machining. Summary of the Invention This invention improves upon existing bone dowels through the use of rigid, preferably metallic end plates, thereby providing the advantages of bone dowels while eliminating the disadvantages, as discussed above. The plates are positioned relative to the anterior and posterior end surfaces of the bone dowel, which may be an existing, modified, or a specially fabricated bone section. The completed structure may be inserted, removed, and positioned into the vertebrae in the same manner as existing devices, that is, by way of a threaded or frictional fit. However, the use of the rigid endplates facilitates load sharing which helps to prevent graft fracture. In the preferred embodiment, the end plates are positioned against the end surfaces of the bone section through the use of a link member, which passes through the interior of the bone section and connects the end plates. One of the end plates may include a threaded bore, and one end of the link member may be threaded to receive the threaded bore of that end plate to hold it in place. The use of a link member allows a plurality of bone sections to be joumaled onto the link member, with a disk of rigid, preferably metallic material to be interposed between each bone section. Alternatively, the end surfaces of the bone section may each include an aperture having an inside diameter, with each end plate having an outside diameter corresponding to the inside diameter of the apertures, enabling the end plates to be positioned against the end surfaces of the bone section through a frictional fit. At least the bone section preferably includes an exterior threading, such that with a generally cylindrical outer shape the finished structure may be screwed into place. Alternative geometries, including rectangular, trapezoidal, and so forth may also be accommodated, with and without exterior threading or serrading. Brief Description of the Drawings FIGURE 1 is an isometric view of a prior-art threaded cylindrical metal cage; FIGURE 2 is an isometric view of a prior-art cylindrical, threaded bone dowel; FIGURE 3 is a perspective, exploded view drawing of certain components associated with one embodiment of the invention; FIGURE 4 is an isometric view of the embodiment of Figure 3 in an assembled form; FIGURE 5A is an exploded-view drawing depicting an alternative embodiment of the invention having threaded end sections; FIGURE 5B is a drawing of a completed assembly according to the invention of Figure 5A; FIGURE 6A is an end component associated with an alternative embodiment of the invention; FIGURE 6B is a progression of the embodiment of the invention introduced to with respect to Figure 6A; FIGURE 6C is a drawing of a completed assembly according to the invention of Figures 6A and 6B; FIGURE 7A begins a sequence of drawings showing how multiple bone dowels and plates may be stacked to provide a further alternative embodiment of the invention; FIGURE 7B shows the stacking of a first bone dowel and the receipt of a first end plate; FIGURE 7C shows the receipt of a second bone dowel; FIGURE 7D shows the receipt of an end plate; FIGURE 7E is a drawing which illustrates a completed stacked structure according to a further alternative embodiment of the invention; FIGURE 8A is a side-view drawing depicting an alternative method according to the invention wherein a bone reinforcer is assembled within an intervertebral disc space as opposed to being inserted as a finished component; FIGURE 8B is a drawing which shows a progression which began with reference to Figure 8A; FIGURE 8C continues the progression of Figures 8A and 8B, with the addition of an end-cap; FIGURE 8D shows the assembled reinforcer using the steps of Figures 8A- 8C; FIGURE 8E is a side-view drawing which shows how multiple disc spacers may be added during an in situ assembly; FIGURE 9A is a drawing of an extension post used in assembling a bone- reinforcer within a disc space; FIGURE 9B is a drawing which shows how an end-cap may be fastened to a central member such as a circular rod; FIGURE 9C is a drawing of an alternative embodiment wherein a reverse thread is used as opposed to a separate fastener with respect to an end plate; FIGURE 10A is a side-view drawing of an alternative impacted embodiment assembled within a disc space; FIGURE 10B is a side-view drawing of a completed structure according to the practice of Figure 10A, showing, in particular, the use of a retaining clip; FIGURE 11 is a side-view drawing of an alternative embodiment of the invention wherein a central member includes projections which holds loose bone graft material in position; FIGURE 12 is a side-view drawing of yet a further different alternative embodiment of the invention, wherein spikes of different length are used for the purpose of holding bone graft and for holding the completed structure within place within an intervertebral space; FIGURE 13A is a drawing which shows yet a further, different alternative embodiment of the invention, wherein a central member used to connect end plates is itself open through the use of multiple structural members; FIGURE 13B is a drawing which shows the device of Figure 13A with bone- graft material packed in and around the central member; FIGURE 14A illustrates a way in which multiple struts may be used, each terminating at both end plates; FIGURE 14B illustrates the use of multiple struts with one or more intermediate spacers; FIGURE 15A is a drawing which shows the use of multiple structural members between end sections in an impacted, as opposed to threaded, embodiment; FIGURE 15B is a drawing of an impacted embodiment wherein multiple struts are used lengthwise between the end sections; FIGURE 15C is a drawing which shows an impacted embodiment of the invention having an intermediate member, again interconnected with multiple struts; FIGURE 16A is a drawing of the invention inserted vertically in the disc space; FIGURE 16B is an isometric view of the embodiment of FIG. 16A; FIGURE 16C is a drawing of the invention inserted vertically between three vertebrae with the invention extending through the intermediate vertebra; FIGURE 16D is an isometric view of the embodiment of FIG. 16C; FIGURE17A is a cross-section drawing of an alternative, modular embodiment of FIGURE 16C; FIGURE 17B is an exploded-view drawing of FIGURE 17A; FIGURE 17C is a drawing of various sizes and shapes of the modular end pieces of FIGURE 17A; FIGURE 18A begins a sequence of drawings showing the insertion of bone graft material into the invention and surrounding the invention with a mesh screen to contain the graft material; FIGURE 18B shows insertion of the initial graft material; FIGURE 18C shows insertion of additional graft material and expansion of the mesh screen; FIGURE 18D shows the invention filled with bone graft material and surrounded with the mesh screen; FIGURE 19A begins a sequence of drawings showing a wire or other filament wrapped around the invention of FIG. 11 to contain the graft material; FIGURE 19B shows the invention surrounded by graft material and a wire or other filament; FIGURE 20A is a drawing of an alternative embodiment of FIGURE 16C with projections from the struts of the invention to help hold a wire or filament as described in FIGURE 19A; and FIGURE 20B is an enlarged view drawing of FIGURE 20A. Detailed Description of the Invention Figures 1 and 2 are perspective-view drawings of existing interbody fusion devices, with Figure 1 being rendered in the form of a metal cage, and Figure 2 being implemented in the form of a bone dowel. In both cases, the bodies 102 and 202 include respective apertures 104 and 204 to receive bone graft material to enhance fusing. The use of an all-metal component has its disadvantages, as does the use of an all-bone component as discussed above with respect to the background of the invention, such that those sufficiencies will not be repeated here. Broadly, the instant invention combines the judicious use of metal and bone components in reinforcers of this type, to gain the advantages of using both materials while avoiding the disadvantages. Figure 3 is an exploded view drawing of certain components of the invention, which are preferably fabricated from a biocompatible metal, metallic component or alternative material sufficient to impart strength to the finished article. The components in this case include an end plate 306 having attached thereto a rod of smaller diameter 308, terminating in a threaded end 310, preferably further including a recess 312 to receive a tool such as an alien wrench, screwdriver, and so forth, to be used for stabilization and/or tightening. Onto the threaded end 310, there is received a second outer plate 320 having threads 322 which mate with the threads 310. These are preferably reverse threads, so that they tighten rather than loosen when the device is installed. Although the rod 308 is shown preferably permanently connected to the end plate 306, it will be appreciated that a threaded, preferably reverse-threaded connection may be provided there as well. Figure 4 is a drawing which shows how the components of Figure 3 are assembled to produce a finished reinforcer according to the invention. Broadly, the end plates 306 and 320 are spaced apart when assembled to provide a spacer therebetween to receive a section of bone grating material 400 preferably including one or more apertures 402 to receive bone graft material sufficient to enhance fusion. Note that the threads 406 on the device overall are forwardly oriented, such that, by placing an appropriate tool into the aperture 322, the plug may be rotated into place without the threaded connection(s) of the end plates becoming loose. Figures 5A and 5B represent an alternative embodiment of the invention, wherein rigid discs 502, preferably of metal, having inner threads enabling them to be rotated onto a dowel 510 of bone material having threaded end sections 514 and one or more apertures 512. The end plates 502 are rotated onto the end sections 514 of the dowel 510, preferably until they become flush with the body of the dowel, as shown in Figure 5B. Note that since the end plates are preferably tightened against the ends of the dowel, forward or reverse threads may alternatively be used for such purpose. Figures 6A-6C illustrate a different embodiment of the invention, which may be used to produce finished articles of various shapes, including cylindrical, rectangular, trapezoidal, and other geometries. As shown in Figure 6A, the structure includes two end pieces 602 and 602', these being attached with a member 604 so that they are spaced apart from one another by an appropriate distance to receive the bone section 610 having one or more apertures 612 illustrated in Figure 6B. In ι this case, it is noted that, as opposed to a helical thread disposed on the outer body of the device, teeth are provided on one or more opposing surfaces, such that the device is tapped into place as opposed to being rotatably inserted, thereby enabling the shape to non-circular in cross-section. Figure 6C shows the completed structure, with the insert of Figure 6B being installed onto the supports shown in Figure 6A. Although the embodiments so far described generally illustrate two end plates separated from one another having a bone insert therebetween, the invention is not limited as to the number of plates or spacers, and may use intermediate discs or rings along the body of the device. Figure 7E is a drawing which shows such a finished article generally at 730, having at least one non-bone spacer along the length of the device. Figures 7A-7D show how such a device would preferably be assembled, namely beginning with a first plate 702 coupled to a rod 704 having a threaded end 706, a first section of bone material 710 would be journaled onto the rod 704, as shown in Figure 7A. A second non-bone ring 714 would then be added, as shown in Figure 7B, followed by a second piece of bone 720, as shown in Figure 7C. A final end plate 724, having an internal threaded bore which cooperates with the threaded end 706 of the rod 704 would then be added, as shown in Figure 7B, to achieve the finished structure shown in Figure 7E, generally at 730. It will be appreciated by one of skill in the art, that more than two or three non- bone spacers may be used in any of the embodiments shown herein, and that autograft or all graft bone may be used, that is, bone from the same patient or a different individual. For that matter, synthetic bone material may be used as opposed to naturally occurring bone and, in addition to threaded connections between the various components, alternative assembly techniques such as compression or force-fit interfaces may be used. Although the embodiments described thus far reside in completed reinforcers which are inserted into an intervertebral disc space, the invention is not limited to prefabricated structures, but, in fact, devices according to the invention may be assembled progressively within the disc space. Figures 8A-8D illustrate such a sequence of assembly, with Figure 8E being used to show that multiple spacer plates in addition to the end plates may also be assembled in situ. In these figures, a bone reinforcer is being assembled in the disc space between upper and lower vertebrae 802 and 804, respectively. The installation procedure may be carried out from an anterior or posterior approach. Assuming the latter, an anterior end plate 806 attached to a distal rod portion 808 is first installed, by pushing the end piece 806 into position using a detachable extender rod 810. To ensure that insertion progresses in a well-defined and controlled manner, an alignment sleeve 812 may temporarily be used as a guide. Having placed the anterior end piece, bone graft 820 is packed into the space around the rod 808, and a posterior end piece 822 is installed onto the assembly over extension 810. Figure 8C shows the posterior end plate in position, at which time the extension piece 810 is removed, as shown in Figure 8D, leaving only the bone graft material surrounding a central member connecting the two end plates 806 and 822. As with other embodiments described herein, the invention is not limited to the use of rigid end pieces, but rather, intermediate discs or rigid elements may be used, including embodiments wherein the device is assembled within the disc space. Once such configuration is shown in Figure 8E, wherein multiple plates 850 are used, with bone graft 852 being progressively added as each plate is installed. Figures 9A-9C illustrate ways in which the extension member 810 may be removed, and the posterior end plate installed. Figure 9A shows the anterior end plate 806, preferably rigidly attached to the central member 808, with extender 810 being attached thereto, along with the addition of normal or forward-oriented threads 820 located at the posterior terminating end of the member 808. Having removed the extender 810, the posterior end plate 822 may be installed through the use of a locking screw 916 having reversed threads from that of 820, to ensure that the last to install end plate does not become loose when the locking screw 916 is tightened down. As an alternative, the threads 820 may be reverse-oriented, as shown in Figure 9C, in which case the end piece may be screwed on without the need for an addition locking screw or other mechanism. Figure 10A illustrates a different alternative embodiment of the invention, wherein, as opposed to relatively thin end pieces, impactor plugs such as 1002 and 1020 are instead utilized. Figure 10A illustrates the initial steps associated with the introduction of this assembly, with the anterior plug 1002 being forced into place, and bone graft material being added around the central member 1004. Again, an alignment rod 1006 is preferably temporarily used for placement. As shown in Figure 10B, the second end plug 1020 is impacted into place over the central member 1004, at which time the alignment rod is removed. Since, in this embodiment, spiked or otherwise roughened superior and anterior surfaces are used on the plugs 1002 and 1020, a simplified retainer clip such as 1008 may be received by a corresponding groove in the central member 1004, as shown in the enlarged view. Although the plugs 1002 and 1020 may be circular in cross-section, in this particular embodiment they are preferably rectangular in cross-section, allowing a larger surface area for superior and inferior end plate engagement. Figures 11 and 12 illustrate other alternative embodiments of the invention, including the use of spikes or rod emanating from the central member between the end pieces or intermediate pieces. Such a configuration may be used with prefabricated components according to the invention or, alternatively, assembled in place between the disc space. In Figure 11 , two end plates 1102 and 1104 are used, though others may be added lengthwise along the central member 1106, but from the member 1106, protrusions 1120 are provided. Depending upon their composition, and strength, the protrusions 1120 may provide additional support along the length of the reinforcer, but in addition, the protrusions 1120 act to hold the bone graft material in place, thereby further enhancing fusion. Although the protrusions such as 1120 shown in Figure 11 may be uniform in length as measured from the central member connecting the end plates or intermediate pieces, as shown in Figure 12, protrusions could also be used which are collectively wider in cross-section than the intervertebral space, such that, during insertion, they are bent down and engage with the end plates, thereby preventing the completed structure from backing out while, at the same time, holding the bone graft material in position. These longer protrusions such as 1206, may be used in combination with shorter protrusions 1208, with the shorter protrusions being specifically intended to hold the bone graft material in place, while the longer protrusions act as barbs to hold the overall structure in position. As opposed to a solid central member connecting end plates, multiple structural members 1302 may be used for an open core, as shown in the threaded embodiment of Figure 13A. In conjunction with these cross-members, which may be arranged much like those found on a radio tower, optional spikes 1304 may be used to help hold bone graft into position, as shown in Figure 13B. As a further alternative arrangement, the multiple struts may go from end section to end section, as shown in the threaded embodiment of Figure 14A. Intermediate spacers may be used in all of these embodiments, including those which use multiple struts, as shown in Figure 14B. In addition, the use of multiple longitudinal and cross-braced struts may also be applied to impacted embodiments, with or without central spacers, as shown in Figures 15A-15C. The connecting struts may have an orientation that is wider in an anterior-to-posterior dimension than the superior-to-inferior dimension to keep the struts further from the end plate of the vertebrae, thereby allowing for an easier determination of fusion through x-ray analysis. The strut embodiments may also help to hold cancellous bone, which may be packed between and over the struts prior to insertion. Figures 16A-D illustrate other alternative embodiments of the invention inserted vertically between vertebrae. In these figures, bone reinforcers are being inserted in the disc space between upper and lower vertebrae 1602 and 1604, respectively. The bone reinforcers in this embodiment have plates 1606 of increased surface area to rest against the end plate of the vertebrae. The plates are perforated as at 1608 to allow bone and blood vessels to grow into the bone reinforcer from the adjacent vertebrae. The reinforcer extends through the central vertebra 1612. Protrusions 1616 from the surfaces of the plates 1606 and 1618 are used to help prevent migration of the inserted bone reinforcer. Figures 17A-C illustrate and alternative, modular embodiment of Figure 16C. Removable end pieces 1702 and 1704 are placed on the ends of the central strut structure 1706. The modular end pieces vary in size and shape as illustrated in Figure 17C. The modular embodiment allows the surgeon to "custom fit" the bone reinforcer to the patient. Figure 18A-D illustrate a sequence of filling the bone reinforcer with graft material. A mesh screen 1802, preferably radiolucent, surrounds the bone reinforcer to hold this graft material 1806 within the reinforcer. The screen is retracted as at 1804 to begin graft insertion. The screen is expanded as at 1808 to contain the graft material as graft material is added. Alternative means of holding the bone graft are shown in Figures 19A-B. A wire, mono filament, or multiple filament 1902 wrapped around the bone reinforcer to help hold the bone in position, as can be clearly seen in Figure 19B. Figure 20A illustrates and alternative embodiment with projections 2002 from the strut 2004 of the reinforcer to help hold the containable wire 2006 in position.

Claims: What is claimed is: 1. An improved bone plug reinforcer, comprising: a section of bone having an exterior, a partially hollow interior, and two end surfaces; and a pair of rigid, non-bone end plates, each positioned against a respective end surface of the bone section. 2. The apparatus of claim 1 , wherein the end plates and link member are at least partially metallic. 3. The improved bone plug reinforcer of claim 1 , wherein the end plates are positioned against the end surfaces of the bone section through the use of link member which passes through the interior of the bone section and connects to each end plate. 4. The apparatus of claim 3, wherein: one of the end plates includes a threaded bore; and one end of the link member is threaded to receive the threaded bore of that end plate. 5. The apparatus of claim 3, further including: a plurality of bone sections; and a disk of rigid material journaled on the link member between each bone section. 6. The apparatus of claim 1 , wherein the disks of rigid material are at least partially metallic. 7. The apparatus of claim 6, wherein one of the end plates includes a threaded bore; and one end of the link member is threaded to receive the threaded bore of that end plate. 8. The apparatus of claim 6, wherein the end plates are removable and press fit to the link member. 9. The improved bone plug reinforcer of claim 1 , wherein the end surfaces of the bone section each include an aperture having an inside diameter, each end plate has an outside diameter corresponding to the inside diameter of the apertures, and the end plates are positioned against the end surfaces of the bone section through a frictional fit between the inside and outside diameters. 10. The apparatus of claim 1 , wherein at least the bone section includes an exterior threading. 11. The apparatus of claim 1 , wherein the bone section in combination with the end plates assumes a generally cylindrical configuration. 12. The apparatus of claim 1 , wherein at least the bone section includes an exterior serrating. 13. The apparatus of claim 1 , wherein the end plates are rectangular. 14. The apparatus of claim 1 , wherein the link member or members have protrusions to hold bone graft material in place. 15. The apparatus of claim 1 , wherein the apparatus is surrounded by a mesh screen to hold bone graft material in place. 16. The apparatus of claim 1 , wherein the apparatus is surrounded by wire to hold the bone graft material in place. 17. The apparatus of claim 1 , wherein the apparatus is surrounded by a mono-filament wire to hold bone graft material in place. 18. The apparatus of claim 1 , wherein the apparatus is surrounded by a multi-filament member to hold bone graft in place. 19. Apparatus for reinforcing a bone plug defining a body between two end surfaces, comprising: a pair of rigid, non-bone end plates, each end plate being adapted for placement against respective end surfaces of the bone plug; and a link member passing through the body of the bone plug, the member having two ends, each fastened to on of the end plates. 20. The apparatus of claim 19, wherein the end plates and link member are at least partially metallic. 21. The apparatus of claim 19, wherein at least the bone plug includes an exterior threading. 22. The apparatus of claim 19, wherein one of the end plates includes a threaded bore and one end of the link member is threaded to receive said threaded bore of said end plate.

Inventors: FERREE BRET A, FERREE, BRET, A.
Applicants: FERREE BRET A, FERREE, BRET, A.
Application year: 2001
Main classification: A61F2
IPC codes: A61B17, A61F2