US20010032017A1 - Intervertebral implants - Google Patents

Abstract

A multipart intervertebral implant is provided which includes an implant portion and an implant extender portion. The implant portion and the implant extender portion can be fastened together using any known fastening means including pins, interlocking structure (e.g., dovetail, tongue and groove, etc.), adhesives, etc. The size of the implant extender portion can be selected during a surgical procedure to provide an implant suitable for a particular intervertebral receiving bed. An intervertebral implant is also provided which may be formed from a multiplicity of implant sections which are fastened together to provide an implant having a desired length. Implants having surface configurations which more closely correspond to the configuration of vertebral endplates are also provided.

Description

• This application claims priority from United States provisional application Ser. No. 60/173,973, filed Dec. 30, 1999, which is incorporated herein by reference.[0001]
BACKGROUND
• 1. Technical Field[0002]
• The present disclosure relates generally to biocompatible implants and, more particularly, to intervertebral implants suitable for implantation into the lumbar, thoracic and/or cervical regions of the spine during a spinal fusion procedure.[0003]
• 2. Background of Related Art[0004]
• Intervertebral implants for fusing together adjacent vertebrae of a spinal column are well known in the art. Such implants are formed in a variety of different shapes and sizes and are configured for insertion into receiving beds formed in the lumbar, thoracic and cervical regions of the spine. The implants may be formed from a variety of different biologically compatible materials including ceramics, polymers, human or animal bone, composites, etc. The implants may also be shaped to maintain the natural lordoses of the spine or to prevent the implant from backing out of an intervertebral receiving bed in which it will be implanted.[0005]
• Examples of known implants are disclosed in U.S. Pat. No. 4,877,020 to Vich and U.S. Pat. No. 4,878,915 to Brantigan. Vich and Brantigan each disclose cylindrical implants having an outer helical thread formed thereabout. The Vich implant is formed from autogenic bone taken from the iliac crest of a patient. The Brantigan implant is formed of an inert metal such as stainless steel, cobalt-chromium-molybdenum alloys and titanium.[0006]
• One problem associated with known implants is the difficulty in adapting an implant to meet the size requirements of a particular intervertebral receiving bed. For example, because anatomically all patients are different, the specific size of implant required for a surgical procedure will not be known to any certainty until a surgeon has prepared the intervertebral space for implantation. Thus, a surgeon must keep a variety of different size implants available for use or have means to alter the dimensions of the implant at his disposal.[0007]
• Another problem associated with known implants constructed from bone is that the anatomical limitations of donor bone limit the size of the implant which can be formed from bone. As a result, bone having satisfactory strength characteristics may not be available for use as an implant because of size limitations[0008]
• Finally, yet another problem associated with known implants is their inability to accurately maintain the natural lordoses of the spine. Because of the irregular shape of the vertebral endplates, wedge-shaped implants and cylindrical dowels are incapable of supporting adjacent vertebrae in their natural orientation without substantially altering the shape of the vertebral endplate(s).[0009]
• Accordingly, a continuing need exists for an intervertebral implant whose size may be easily altered by a surgeon during a surgical procedure to meet the size requirements of a particular implant receiving bed and for an implant capable of maintaining the natural lordoses of the spine without substantially altering the shape of the vertebral endplates.[0010]
SUMMARY
• In accordance with the present disclosure, intervertebral implants are provided which more precisely correspond in shape to the shape of the vertebral endplates. In one preferred embodiment, an implant is formed from a ring of material having a top surface and a bottom surface. The top and/or bottom surfaces of the implant include a series of annular stepped surfaces which together define a convex configuration which closely corresponds to the concave shape of the vertebral endplates. In an alternate embodiment, the annular stepped surfaces on the top and/or bottom surfaces of the implant can be replaced by a single helical pathway.[0011]
• In another preferred embodiment, a multipart intervertebral implant is provided which includes an implant portion and an implant extender portion. The implant portion and the implant extender portion can be fastened together using any known fastening means including pins, interlocking structure (e.g., dovetail, tongue and groove, etc.), adhesives, etc. The size of the implant extender portion can be selected during a surgical procedure to provide an implant suitable for a particular intervertebral receiving bed.[0012]
• In yet another preferred embodiment, an intervertebral implant may be formed from a multiplicity of implant sections which are fastened together to provide an implant having a desired length. These implant sections and the implants described above can be formed of any biocompatible material including bone.[0013]
BRIEF DESCRIPTION OF THE DRAWINGS
• Preferred embodiments of the presently disclosed intervertebral implants are described herein with reference to the drawings, wherein:[0014]
• FIG. 1 is a perspective view of one preferred embodiment of the presently disclosed intervertebral implant;[0015]
• FIG. 2 is a top view of the intervertebral implant shown in FIG. 1;[0016]
• FIG. 3 is a side view from a first side of the intervertebral implant shown in FIG. 1;[0017]
• FIG. 4 is a top perspective view of the intervertebral implant shown in FIG. 1;[0018]
• FIG. 5 is a side view from the other side of the intervertebral implant shown in FIG. 1;[0019]
• FIG. 6 is a perspective view of another preferred embodiment of the presently disclosed intervertebral implant;[0020]
• FIG. 6A is a perspective view of another preferred embodiment of the presently disclosed intervertebral implant;[0021]
• FIG. 7 is a top view of the intervertebral implant shown in FIG. 6;[0022]
• FIG. 8 is a side view of the intervertebral implant shown in FIG. 6;[0023]
• FIG. 9 is a top perspective view of the intervertebral implant shown in FIG. 6;[0024]
• FIG. 10 is another side view of the intervertebral implant shown in FIG. 6;[0025]
• FIG. 11 is an elevational view of the intervertebral implant shown in FIG. 6 positioned between adjacent vertebrae;[0026]
• FIG. 12 is a perspective view of another embodiment of the presently disclosed intervertebral implant;[0027]
• FIG. 13 is a side view of the intervertebral implant shown in FIG. 12;[0028]
• FIG. 14 is a top view of the intervertebral implant shown in FIG. 12;[0029]
• FIG. 15 is a rear end view of the intervertebral implant shown in FIG. 12;[0030]
• FIG. 16 is a perspective view of yet another embodiment of the presently disclosed intervertebral implant;[0031]
• FIG. 17 is a side view of the intervertebral implant shown in FIG. 16;[0032]
• FIG. 18 is a top view of the intervertebral implant shown in FIG. 16;[0033]
• FIG. 19 is a rear end view of the intervertebral implant shown in FIG. 16;[0034]
• FIG. 20 is a perspective view of yet another embodiment of the presently disclosed intervertebral implant.[0035]
• FIG. 21 is a side view of the intervertebral implant shown in FIG. 20;[0036]
• FIG. 22 is a top view of the intervertebral implant shown in FIG. 20;[0037]
• FIG. 23 is a rear end view of the intervertebral implant shown in FIG. 20;[0038]
• FIG. 24 is a top view of yet another embodiment of the presently disclosed intervertebral implant;[0039]
• FIG. 25 is yet another embodiment of the presently disclosed intervertebral implant;[0040]
• FIG. 26 is a side view of the intervertebral implant shown in FIG. 25 positioned between adjacent vertebrae;[0041]
• FIG. 27 is a side view of the presently disclosed intervertebral implant and implant extender positioned between adjacent vertebrae with the implant extender positioned adjacent the leading end of the intervertebral implant;[0042]
• FIG. 28 is a side view of the intervertebral implant and implant extender shown in FIG. 27 positioned between adjacent vertebrae with the implant extender positioned adjacent the trailing end of the intervertebral implant;[0043]
• FIG. 29 is a side view of an alternate embodiment of the presently disclosed intervertebral implant and implant extender in an interlocked configuration;[0044]
• FIG. 30 is a side view of the intervertebral implant and implant extender shown in FIG. 29 in a partially assembled configuration; and[0045]
• FIG. 31 is a perspective view of yet another embodiment of the presently disclosed intervertebral implant.[0046]
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS,
• Preferred embodiments of the presently disclosed intervertebral implant and implant extender will now be described in detail with reference to the drawings in which like reference numerals designate identical or corresponding elements in each of the several views.[0047]
• FIGS.[0048]1-5 illustrate a preferred embodiment of the presently disclosed intervertebral implant shown generally as10. Briefly,implant10 includes anupper surface12, alower surface14 and asidewall15 positioned between the upper and lower surfaces. Upper andlower surfaces12 and14 each include a series ofcircular steps16 which move upwardly from theouter periphery18 ofimplant10 to thecenter20 ofimplant10. Alternately, steps16 need not be centered about the center ofimplant10 nor do adjacent steps need be of the same height. Athroughbore22 extends between upper andlower surfaces12 and14 ofimplant10.Throughbore22 is dimensioned to receive growth factors including autograft, allograft, DBM, etc . . . , to stimulate bone growth.
• FIGS.[0049]6-10 also illustrate animplant100 having stepped upper andlower surfaces112 and114. In contrast to the implant shown in FIGS.1-5,implant100 includes a greater number ofsteps116 which define a more gradual taper thansteps16 ofimplant10. It is envisioned that any number of steps may be provided on the upper and/or lower surfaces of the implant to provide any desired surface curvature. For example, each stop may have a height of from about 0.15 mm to about 3 mm. Other step dimensions are also envisioned.
• FIG. 6A illustrates another alternate embodiment of the stepped implant shown in FIGS.[0050]1-5 shown generally as100′.Implant100′ includes ahelical pathway116′ which extends from theouter periphery118′ ofimplant100′ towards the center ofimplant100′. As discussed above, the helical pathway need not be centered about the central axis ofimplant100′ but rather it may be oriented to provide the desired curvature on the upper and/or lower surface ofimplant100′. Moreover, the height of the step defined by thehelical pathway116′ may vary along the length ofpathway116′.
• [0051]Implants10,100 and100′ may be formed from a variety of different biologically compatible materials including ceramics, polymers, human or animal bone, carbon fiber tantalum composites, etc. using a variety of known processes including molding, casting, machining, etc. Preferably,implant10 is formed from cadaveric human or animal bone by making a transverse cut through the diaphysis or metaphysis of a long bone, e.g., tibia, fibula, femur, ulna, radius, etc., to form a ring and thereafter machining the upper and/or lower surfaces of the implant, e.g., milling the stepped or helical configuration into the upper and lower surfaces of the ring. Alternately, only one of the upper and lower surfaces of the ring may be provided with a stepped configuration.
• The bone used to form[0052]implants10,100 and100′ may be partially or fully demineralized bone. Preferably the bone is surface demineralized. By surface demineralizing the bone, the osteoconductivity and the conformability of the outer surfaces of the implant are improved while the strength of the inner portion of the implant is retained.
• Referring to FIG. 11,[0053]implants10,100 and100′ may be positioned within a receiving bed formed betweenadjacent vertebrae32 and34. Because the upper and lower surfaces of the implants conform to the natural concavity of the vertebral endplates, only minimal preparation of the vertebrae is required by a surgeon prior to insertion of an implant to maintain the natural lordoses of the spine.
• FIGS.[0054]12-15 illustrate an alternate embodiment of the intervertebral implant shown generally as200.Intervertebral implant200 is preferably formed from a cortical ring allograft cut from the diaphysis or metaphysis of a long bone but may be formed from any biocompatible material having the requisite strength requirements.Implant200 includes a tapered, ring-shapedbody212 having flat top andbottom surfaces214 and216, respectively.Anterior end218 ofimplant200 has a height which is greater than the height ofposterior end220. The taper of the implant should be such as to conform to the vertebral end plates of adjacent vertebrae.Mating structure222 for engaging corresponding structure of an insertion tool is formed in asidewall224 ofimplant200 in theanterior end218 ofimplant200. A plurality ofconcentric rings226 are formed in top andbottom surfaces214 and216.Rings226 are preferably V-shaped, although other configurations are also envisioned, i.e., U-shaped, rectangular, etc. Athroughbore228 extends between top andbottom surfaces214 and216 ofimplant200. Ifimplant200 is formed from bone, throughbore228 may be defined by the intramedullary canal of the bone from which implant200 is cut.Implant200 is configured for anterior insertion into the intervertebral space. Growth factors including autograft, allograft, and demineralized bone particles may be positioned inthroughbore228 and/or rings226 to stimulate bone growth.
• FIGS.[0055]16-19 illustrate an alternate embodiment ofimplant200 shown generally as300.Implant300 is similar to implant200 but includes convex top andbottom surfaces314 and316 which are configured to engage the vertebral end plates of adjacent vertebrae. Top andbottom surfaces314 and316 also includeconcentric rings326 similar to those described above with respect toimplant200.
• FIGS.[0056]20-23 illustrate another alternate embodiment ofimplant200 shown generally as400.Implant400 is similar to implant200 except thatanterior end418 is approximately equal to the height ofposterior end420.
• FIG. 24 illustrates a partially threaded stepped implant shown generally as[0057]500.Implant500 is preferably formed from cortical bone by making a transverse cut through the diaphysis or metaphysis of a long bone to obtain a cylindrical bone plug and thereafter machining and threading the bone plug. Alternately,implant500 may be formed from any biocompatible material having the requisite strength requirements using any known process including machining, molding, etc.Implant500 includes acylindrical body510 having afirst end portion512 having a first outer diameter and asecond end portion513 having a second outer diameter larger than the first diameter. A variety of different diameter implants are envisioned. Athroughbore514 extends throughfirst end portion512 ofcylindrical body510.First end portion512 hasscrew threads516 at one end thereof, but does not include threads in the area aboutthroughbore514.Second end portion513 includesscrew threads518. Whenimplant500 is formed by cutting a bone plug from a long bone, the absence of screw threads in the area ofthroughbore514 facilitates the use of long bones having a thinner wall section, i.e., the bone wall between the intramedullary canal of a long bone and the outer surface of the bone plug cut therefrom can be thinner.
• When[0058]implant500 is inserted into intervertebral space between adjacent vertebrae,second end portion513 will sit in the vertebral wall and provide the majority of the retaining force.First end portion512 will also screw into adjacent endplates but thread engagement may be minimal, especially if the intervertebral space is very concave. A stepped reamer and tap can be used to prepare the intervertebral space.
• FIGS. 25 and 26 illustrate an alternate embodiment of[0059]implant500 shown generally as600.Implant600 is similar to implant500 except thatsecond end613 ofimplant600 is tapered from one end to the other.Implant600 may be used to vary the spine geometry. Referring to FIG. 26, during insertion ofimplant600 into the intervertebral space,implant600 will directly force adjacentvertebral surfaces630 and632 apart. The particular taper ofsecond end portion613 ofimplant600 can be chosen to provide the desired spacing of the adjacent vertebrae.
• Intervertebral implants in the form of threaded, cylindrical dowels formed of bone, specifically, human or animal cadaveric bone, are well known in the surgical arts. Typically, such implants are formed by making a transverse cut through the diaphysis or metaphysis of a long bone, i.e., the femur, tibia, fibula, ulna or radius, using a cylindrical drill bit. One problem associated with forming and using bone dowel implants is that anatomical limitations make it difficult to recover bone dowels having the desired length needed for intervertebral fusion procedures. This is especially true when performing procedures in the cervical region of the spine wherein small diameter dowels are required.[0060]
• In order to compensate for anatomical limitations, a dowel extender portion may be provided. Referring to FIGS. 27 and 28, a[0061]cylindrical dowel extender710 may be implanted within the intervertebral space prior to implantation of the main dowel portion D (FIG. 27), or alternately, after implantation of main dowel portion D (FIG. 28). Preferably,dowel extender portion710 includeshelical threads712 to engagevertebral end plates714 and716 and retain the dowel extender portion in place. However, non-threaded dowel extender portions are also envisioned. Eachdowel extender portion710 preferably includes engagement structure, such asslot718, for engaging an insertion tool (not shown).
• Generally, dowel extenders having a length of from about 4 to 8 mm are needed to supplement the main dowel, although other length dowel extenders may also be needed depending upon the particular surgical procedure being performed. A common thread pattern associated with intervertebral dowels is 10 threads per inch. Thus, a dowel extender having a length of 4 mm will only have about 1.57 threads and a dowel extender having a length of 6 mm will have only about 2.35 threads. Because of the limited number of threads and the short thread engagement length, it may be difficult to stabilize a dowel extender in the intervertebral space and problems may result. For example, if the dowel extender is not firmly seated in the intervertebral space between adjoining vertebrae when contacted by the main dowel, it may tip over.[0062]
• In order to provide greater stability, an alternate embodiment of the presently disclosed dowel extender is described herein. Referring to FIGS. 29 and 30,[0063]dowel extender800 includes acylindrical body810 having afirst end812 and asecond end814.Second end814 includes aprojection816 configured and dimensioned to be received within a correspondingly shaped slot formed on one end of main dowel D. Although illustrated as having a dove-tail configuration,projection816 may assume other configurations capable of interlocking with a correspondingly shaped slot. Referring to FIG. 29, locking pins820 may be provided to further securedowel extender portion800 to main dowel portion D. Alternately, a locking pin or pins may be used to entirely replaceprojection816 andsecure dowel extender800 to main dowel D.
• In an alternate embodiment, a bone dowel is constructed from multiple dowel segments which are secured together using interlocking structure. The interlocking structure may be formed integrally with each dowel segment, e.g., each dowel segment may have a slotted front end and a correspondingly shaped projection formed at a rear end. Each dowel segment has a predetermined length and is joined to one or more other dowel segments to form a dowel having a desired length. For example, dowel segments may be formed having lengths of 2, 4 and 6 mm. In order to form a dowel having a length of 20 mm, three 6 mm dowel segments and a 2 mm dowel segment can be joined together. The dowel segments are preferably formed from bone, although other biocompatible materials listed above are also envisioned. The dowel segments may be cylindrical, rectangular, wedge-shaped, etc. For example, FIG. 31 illustrates a wedge shaped[0064]intervertebral implant900 formed ofmultiple implant segments910 which are fastened together in the manner described above.
• It will be understood that various modifications may be made to the embodiments disclosed herein. For example, the configuration of the sidewall of any of the implants described above may be modified to better suit a particular procedure, i.e., the sidewalls can be formed to be rectangular, circular, triangular, semi-circular, etc. Moreover, the implants described above, although disclosed in the context of spinal implantation, may be suitable for other implantation procedures not specifically listed here but obvious to those of ordinary skill in the art. Therefore, the above description should not be construed as limiting, but merely as exemplifications of preferred embodiments. Those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto.[0065]

Claims (7)

1. An intervertebral implant comprising:

a first implant portion having a first end and a second end, the second end having engaging structure formed therein;

a second implant portion having a first end and a second end, the first end of the second implant portion including engaging structure formed thereon, the engaging structure formed on the first end of the second implant portion being configured to mate with the engaging structure formed on the second end of the first implant portion to fasten the second implant portion to the first implant portion.

2. An intervertebral implant according to

claim 1

, wherein one of the first and second engaging structures includes a dovetail connector.

3. An intervertebral implant according to

claim 1

, wherein the first and second implant portions are cylindrical.

4. An intervertebral implant according to

claim 1

, wherein the first and second implant portions are wedge-shaped.

5. An intervertebral implant according to

claim 1

, further including at least one pin extending between the first and second implant portions.

6. An intervertebral implant according to

claim 2

, wherein the other of the first and second engaging structures includes a slot configured to receive the dovetail connector.

7. An intervertebral implant according to

claim 1

farther including a third implant portion having first and second ends and third engaging structure.

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