CA2168835C

CA2168835C - Interbody spinal fusion implants

Info

Publication number: CA2168835C
Application number: CA002168835A
Authority: CA
Inventors: Gary Karlin Michelson
Original assignee: Warsaw Orthopedic Inc
Current assignee: Warsaw Orthopedic Inc
Priority date: 1992-10-29
Filing date: 1996-02-05
Publication date: 2007-04-24
Anticipated expiration: 2016-02-05
Also published as: US6224595B1; EP1525863A2; EP1525863A3; EP0732093B1; EP0732093A3; AU716409B2; CN1134810A; TW365158U; DK0732093T3; KR960030887A; PT732093E; DE69633756T2; TR199600134A2; US5593409A; JPH08266563A; US5785710A; DE69633756D1; CA2569778A1; AU4445196A; EP0732093B8

Abstract

The present invention discloses a spinal fusion implant that is at least partially cylindrical, made of material appropriate for human implantation and having preferably, but not necessarily, one closed end and one end capable of being closed, such that an internal chamber can be filled and hold any natural or artificial osteoconductive, osteoinductive, osteogenic, or other fusion enhancing material. The partially cylindrical implant directly participates and is incorporated in the ensuing fusion. In the preferred embodiment, the implant of the present invention relies on surface roughenings of the outer surface to enhance its stability and resist dislodgement from within the disc space between two adjacent vertebrae. The implant of the present invention incorporates at its rear end, an engagement means to facilitate insertion or extraction of the implant. The implant may be filled with, coated with, and/or composed of, fusion promoting substances. Finally, the implant of the present invention does not require rotation for its insertion and can be seated by linear advancement.

Description

INTERBODY SPINAL FUSION IMPLANTS
Field of the Invention The present invention relates to artificial spinal fusion implants to be placed across the intervertebral space left after the removal of a damaged spinal disc, and in particular to an improved, at least partially cylindrical, spinal fusion implant for implantation where two threaded cylindrical implants of requisite height would not fit within the transverse width of the spine.
Description of the Related Art In the past, Cloward, Wilterberger, Crock, Viche, Bagby, Brantigan, Michelson and others have taught various methods involving the drilling of holes across the disc space between two adjacent vertebrae of the spine for the purpose of causing an interbody spinal fusion. Cloward taught placing a dowel of bone within that drilled hole for the purpose of bridging the defect and to be incorporated into the fusion. Viche taught the threading of that bone dowel. Bagby taught the placing of ' 2168835 the bone graft into a metal buc)cet otherwise smooth on its surface, except for rows of radially placed holes communicative to the interior of the basket and to the bone graft. The Bagby device was disclosed as capable of being used in a luorse.
Brantigan taught the use of inert blocks preferably made of metal and having that metal at its external surface imitate the porosity of bone. Brantigan theorized that the bone dowel could be replaced entirely with a metal plug, that, while not itself active in the fusion, would nevertheless serve to S,~pport the vertebrae frorn within the disc space while allowing fusion to occur around it.
U.S. Patent No. 3,844,601 issued to Ma et al. on November 19, 1974, teaches a method and instrumentation for preparing rectangular spaces across the disc space into the adjacent vertebrae and for prepari.rrg a rectangular graft of tlae bone itself that is inserted in the rectangular spaces.
U.S. Patent No. 4,743,256 issued to Brantigan on May 10, 1988 teaches the use of an inert artificial spacer in the shape of a rectangle in place of using a rectangular borne graft as taught by Ma et al.
U.S. Patent No. 4,878,915 issued to Brantigan on November 7, 1989, teaches the use of fully cylindrical inert implants for use in interbody spinal fusion. Such implants do Trot participate in the bone fusion process but act as inert spacers acrd allow for Clue growth of bone to the outer surfaces of the implants.
U.S. Patent No. 4,834,757 issued to Brantigan on May 3U, 1989, teaches a rectangular sluaped, hollow spinal fusion implant for use in lieu of a rectangular bone graft or Brantigan~s earlier artificial inert spacer.
U.S. Patent No. 5,015,247 issued to Michelson on May 14, 1991, teaches the use of a thin-walled, higlrly perforated, threaded, hollow cylindrical implant closed or closable at both ends, so as to be compressably loaded with bone or other fusion promoting materials. Additionally, the Michelsorr device may 216~~35 then be coated with a bone production inducing chemical such as hydroxyapatite. The Miclielson patent also discloses an improved method of drilling boles across the disc space and into the two adjacent vertebrae and safely installing cylindrical implants such that the entire surgical procedure may be conducted through a hollow cylindrical tube. The hollow cylindrical tube may be left in place throughout the surgical procedure and serves to hold the adjacent vertebrae in place relative to each other, permits the guarded drilling of the holes across the disc space, and permits the insertion of the irnplant tlurough that same l:ube into the hole drilled across the disc space and into the adjacent vertebrae.
As regards this method of performing interbody spinal fusion using essentially cylindrical threaded implants, a special problem arises (see Figure 1) when an attempt is made to place two cylindrical implants (considered to be the preferred nurnber as it is a much more stable construct and has more surface area than a single implant placed centrally) side-by-side across a disc space and into the two adjacent vertebrae where tare lueight of the disc space is such that it reduires an implant of a diameter so large to penetrate into and significantly engage each of the adjacent vertebrae that it is no longer possible to place two such implants side-by-side and to still leave them contained within the transverse widtlu of the spine. If an attempt is made to remedy Llre problem by using smaller diameter implants placed side-by-side such drat both would then be able to fit within the transverse width of the spine, then tare implants would be of insufficient height to adequately engage the bone. If an attempt is made to remedy the problem by abandoning i.lie side-by-side double implant construct in favor of a single, centrally placed implant, irluen where the implant is sufficiently large enough to occupy a sufficient portion of the transverse width of tl.e disc space to pron«te firm stability, its vertical heiglut and excursion into the vertebrae would be so severe that if any two consecutive ,~

disc spaces were to be operated upon, the vertebrae in between would be cut in half.
U.S. Patent No, 5,055,104 issued to Ray on October 8, 1991 ("Ray Patent") discloses an implant comprising a helical coil without wall members that is assernbled after the coils are placed irr the disc space between tire vertebrae, which supposedly can then be removed after the vertebrae have become fused together. The Ray implant is defective and unworkable in that it is incapable of ueing used in the manner in which it is described as it is not possible to insert into hard bone an isolated helical coil without any wall members to support such a coil, which could would be analogous structurally to a slinky, (_See Ray Patent, Figures 1 and 7). Further, the Ray irnplant is unduly complex, because it would require the difficult, if not impossible, task of assembly wiLlrin the disc space. Figure 3 of the Ray Patent clearly reveals that Ray does not teach the use of a truncated cylindrical implant, but merely teaches the use of a truncated, helical coil appearing as a sharpened spring totally lacking any wall member which could be considered cylindrical. Therefore, Ray teaches only the use of an isolated thread which can only be inserted by rotation and cannot be linearly advanced.
If the overwhelming obstacles of the impossibility of inserting an isolated thread without wall members and flue Problem of the assembly within the disc space could be overcome, then the Ray implant would still be unsafe for its intended purpose as it would be at high risk of spontaneous disassembly and mechanical failure. Further, there would be insufficient room to safely rotate such a device for insertion as it is the very lack of such room that requires the use of a device having a decreased transverse width.
There is therefore, tyre need for a spinal fusion implarnt that is capable of being inserted into a hole drilled across flue disc space between two adjacent vertebrae and partially into tire two adjacent vertebrae such that the spinal fusion implant is capable of fitting within the transverse width of the spine wluen placed side-by-side next to a second of its kind.

SUMMARY OF THE PRESEIJ'r ILVVELdI-IU_N
The present invention is an irnproved interbody spinal fusion implant that is capable of being inserted into a hole drilled across the disc space between two adjacent vertebrae and into the two adjacent vertebrae such that tHre spinal fusion implant is capable of fitting within the transverse width of tire spine when placed side-by-side next to a second of its kind. The spinal fusion implant of the present invention comprises a thin-wall, multi-perforate, cylinder or partial cylinder, made of material appropriate for human implantation and leaving preferably, nut not necessarily, one closed end and one errd capable of being closed, such than an internal chamber can be filled and hold any natural or artificial osteoconductive, osteoinductive, osteogenic, or otluer fusion enhancing material. The spinal fusion implant of the present invention relies on rouglrenings of the outer surface to enhance its stability. Depending on the dimension of flue transverse width of the spine in whrich tire spinal fusion implant is being inserted, the spinal fusion implant of the present invention may have one or more flat sides to reduce the width of the spinal fusion implant. Tlne spinal fusion implant of tire present invention incorporates at il:s rear end, an engagement means to facilitate insertion or extraction of_ the irnp.lant, preferably at its rear end. The implant of the present invention may be made of., filled with and/or coated with fusion promoting substances. Further, the spinal fusion implant of the present invention does not require rotation for its insertion and can be seated by linear advancement.
The spinal fusion implant of tire present invention is generally effective, and is safer and more effective than i:lre cylindrical implants of tyre prior art for ttre special instance when it is desirable to insert two implants side-by-side into cylindrically prepared channels, and where the height of the . 21 ~~~3~
disc space between two adjacent vertebrae is so great relative to the transverse width of the spine, that two implants of the requisite height will not fit within the transverse width of the spine. Prior art has tauglut those knowledgeable in the art of spinal surgery, that the likelihood of obtaining a spinal fusion is proportionate to three factors: 1) the surface area of the implant 2) the quality and quantity of the graft material and 3) the stability of the fusion construct. The spinal fusion implant of the present invention increases each of these three factors by making it possible to use two implants side-by-side across a disc space that would otherwise lack sufficient width to accept more than one.
The spinal fusion implant of the present invention is more efficacious than the prior art on an individual implant basis for the following reasons:
1. Increased surface area. The spinal fusion implant of the present invention, because of its surface roughenings leas greater surface area for engaging tree adjacent vertebrae than an implant with smooth external surfaces. The presence or absence of holes does not materially affect this, so far as the holes are filled with material effectively contributing to the area of contact at the surface. The arced portions of the partially cylindrical implant of the present invention are in contact wil:lu Lhe adjacent vertebrae and provide a greater surface area than is possible with a flat portion from a non-cylindrical implant.
2. The guantity and guality of graft material presented. As the spinal fusion implant of the present invention is not screwed in, it need not be constructed to resist the torquing therewith associated. Thus, the implant of the present invention may be thinner walled and thereby, for a given diameter, have greater internal volume. The spinal fusion implant of the present invention has arced portions making the ' ' 21~~~~
_8_ implant stronger in compression than an implant lacking upper and lower curved supporting surfaces such that the wall of the implant can be relatively thinner than such implants. A
thinner wall is easier for bone to grow through. Also, the interpore bridges may be smaller allowing for greater porosity and thereby greater exposure to the internal graft material.
Further, the spinal fusion implant of the present invention may be constructed of and/or coated with, and/or loaded with a variety of materials and/or chemical substrates known t:o actively participate in the bone fusion process. As the spinal fusion implant of the present invention offers greater surface area, and greater internal volume for its outside diameter, it offers the opportunity for presenting a greater surface area and volume of these fusion materials.
3 ~ The imol~n~oF tire present inveption of fers grea -er st.auility than the prior aL_t imglanl.s. T'he least statrle implants are the implants lacking surface roughenings. Surface holes increase implant stability by increasing the interference of the implant to the opposed surfaces. The spinal fusion implant of the present invention is a further improvernent over flue prior art in that the surface roughenings of floe spinal fusion implant of the present invention resist motion in all directions. Further, all implants are subject to the possibility of backing out, by retracing the path by which they were inserted. fIowever, the spinal fusion irnplant of the present invention can have a surface configured to urge the spinal fusion implant forward as to offer increased resistance against such undesirable backward migration. Furtluer, the arced portions of the implant of tyre present invention provide a greater support area to better distribute the compression forces through tire vertebrae.
1'he spinal fusion implant. of the present invention is easier to use as it occupies less space, does not require pre-tapping, and can be inserted wittrout the need to rotate an instrument within the closed confines of the spinal wound.
Further, the spinal fusion implant of the present invention is easier to insert than implants lacking upper and lower curved supporting surfaces that are arcs of the same circle and which implants are to be inserted across the disc space and into the adjacent vertebrae as it is easier to prepare a round hole than a square hole, as a round hole can be drilled in a single step.
In accordance with the present invention, there is provided a spinal fusion implant made of a material appropriate for human implantation at least in part between two adjacent vertebral bodies, said implant comprising: a non-threaded cylindrical member having an exterior with opposed arcuate portions adapted to be oriented toward the adjacent endplates of the two adjacent vertebral bodies, each of said opposed arcuate portions having at least one opening passing therethrough to allow bone growth from adjacent vertebral body to adjacent vertebral body through said implant, said implant having a plurality of surface roughenings protruding from said exterior of said cylindrical member for engaging the two adjacent vertebral bodies to maintain said implant in place, said surface roughenings configured for linear insertion into the spine and to resist expulsion of said implant from between the two adjacent vertebral bodies.
In accordance with the present invention, there is further provided a spinal fusion implant made of a material appropriate for human implantation at least in part between two adjacent vertebral bodies, said implant comprising: a non-threaded cylindrical member having an exterior with opposed arcuate portions adapted to be oriented toward the adjacent endplates of the two adjacent vertebral bodies, each of said opposed arcuate portions having at least one opening passing therethrough to allow bone growth from adjacent vertebral body to adjacent vertebral body through said implant, said implant having a plurality of annular ratchetings defined around the circumference of said cylindrical member, said annular ratchetings being configured for linear insertion into the spine and to resist expulsion of said implant from between the adjacent vertebral bodies.
It is an object of embodiments of the present invention to provide an improved interbody spinal fusion implant such that it is possible to place two such implants side-by-side across a disc space and into two adjacent vertebrae in close approximation to each other and within the transverse width of the spine, where the transverse width of the spine would have otherwise been insufficient relative to the required implant height to have allowed for the accommodation of two prior art cylindrical threaded implants.
It is another object of embodiments of the present invention to provide a spinal fusion implant that is easier to insert, and does not require tapping prior to implantation.
It is yet another object of embodiments of the present invention to provide a spinal fusion implant that is safer, in which there is not need to run sharp threads near delicate structures.
It is still another object of embodiments of the present invention to provide a spinal fusion implant that is faster to implant between adjacent vertebrae via linear advancement as opposed to rotational advancement.

It is yet another object of embodiments of the present invention to provide a method for implanting partially cylindrical implants having at least one flat side.
These and other objects of embodiments of the present invention will be apparent from a review of the accompanying drawings and the following detailed description of the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is diagrammatic representation of a segment of the human spinal column comprising several vertebrae with various cylindrical threaded implants inserted across the disc space and into the two adjacent vertebrae to illustrate the problems encountered by those implants.
Figure 2 is a top plan view along lines 2--2 of Figure 1 with the top vertebrae removed, of two cylindrical threaded implants illustrating the minimum distance possible between the two threaded implants when placed beside each other across the disc space.
Figure 3 is a perspective side view of an embodiment of the spinal fusion implant of an embodiment of the present invention having surface roughenings in the form of ratchetings.
Figure 4 is a first side elevational view of the spinal fusion implant of Figure 3.
Figure 5 is a top plan view of two spinal fusion implants of Figure 3 illustrating the minimum distance possible between the two implants when placed beside each other across the disc space.

- lla -Figure 6 is a second side elevational view of the spinal fusion implant of Figure 3.
Figure 7 is a cross sectional view along lines 7--7 of the spinal fusion implant of Figure 6.
Figure 8 is a cross sectional view along lines 8--8 of the spinal fusion implant of Figure 6.
Figure 9 is a top end view of the spinal fusion implant of Figure 3.
Figure 10 is a bottom end view of the spinal fusion implant of Figure 3.
Figure 11 is a side perspective view of an alternative embodiment of the spinal fusion implant of the present invention.
Figure 12 is a first side elevational view of the spinal fusion implant of Figure 11.
Figure 13 is a second side elevational view of the spinal fusion implant of Figure 11.

21~~~~5 Figure 14 is a cross sectional view along lines 14--14 of floe spinal fusion implant of Figure 13.
Figure 15 is a perspective side view of an alternative embodiment of the spinal fusion implant of the present invention having surface roughenings in the form of )tnurlind.
Figure 16 is a first side elevational view of the spinal fusion implant of Figure 15.
Figure 17 is a top plan view of two spinal fusion implants of Figure 15 illustrating the minimum disl~ance possible between the two implant when placed beside each otluer across the disc space.
T'igure 18 is an enlarged fragmentary view along line 18 of Figure 16 showing the surface configuration of the implant of Figure 15.
Figure 19 is a second side elevational view of the spinal fusion implant of Figure 15.
Figure 20 is a cross sectional view along lines 20--20 of the spinal fusion implant of Figure 16.
Figure 21 is a top end view of tte spinal fusion implant of Figure 15.
Figure 22 is a bottom end view of the spinal fusion implant of Figure 15.
Figure 23 is a perspective side view of an alternative embodiment of flue spinal fusion implant of the present inventi.Oll having flat sides and surface roughenirig.s in the form of ratclretings.
Figure 24 is a first side elevational view of tire spinal fusion irnplant of Figure 23.
Figure 25 is a diagramrnatic representation of a segment of the Imman spinal column showing two implants of Figure 23 the preserrt invention inserted witluin flue spine.
Figure 26 is a top plan view along lines 26--26 of Figure 25 with the top vertebrae removed, illustrating the minimum distance possible between two spinal fusion implants of Figure 23 placed beside each other across the disc space.
Figure 27 is a top end view of the spinal fusion implant of Figure 23.

Figure 28 is a uottom end view of tire spinal fusion implant of Figure 23.

Figure 29 is a second side elevational view of the spinal fusion implant Figure 23.
of Figure 30 is a cross sectional view along lines 30--30 of the spinal fusion implant of Figure 29.

Figure 30A is a cross sectional view of an alternative embodiment of the spinal fusion implant of i:he present invention leaving only one flat side.

Figure 31 is a perspective side view of an alternative embodiment of the spinal fusion implant of the present invention having flat sides and surface roughenings in the form of ratcl-retings.

figure 32 is a first side elevational view of tire spinal fusion implant Figure 31.
of Figure 33 is a second side elevational view of the spinal fusion implant Figure 31.
of Figure 34 is a cross sectional view along lines 34--34 of the spinal fusion implant of Figure 33.

Figure 35 is a cross sectional view along lines 35--35 of tire spinal fusionimplant of Figure 33.

Figure 36 is a perspective side view of an alternative embodiment of tyrespinal fusion implant of the present invention having flat sides acrd having surface roughenings in the form of knurl ing.

Figure 37 is a first side elevational view of the spinal fusion implant figure 36.
of Fignrre 38 is a second side elevational view of the spinal fusion implant Figure 36.
of Figure 39 is a cross sectional view along lines 39--39 of the spinal fusionrimplant of Figure 38.

Figure 40 is an enlarged fragmentary view along line 40 of Figi.rre 37 showingthe surface configuration of tlne spinal fusion irnplant Figure 36.
of F'p.gure 41 is a perspective side view of an alternative ,~~.-embodiment of the spinal fusion implant of the present invention having surface roughenings comprising of a blasted external surface.
figure 42 is a perspective side view of an alternative embodiment of the spinal fusion implant of the present invention leaving flat sides and openings in the form of vertical and horizontal slots.
figure 43 is an elevational side view of a segment of the spinal column with an alternative embodiment of two spinal fusion implants of the present invention having corresponding concave and convex sides inserted across one disc space and an alternative embodiment of a single spinal fusion implant of the present invention having a two cylindrical portions inserted across one disc space.

v 2168835 Tlyrevious Devices Referring to Figure 1, a diagrammatic representation of a segment of the human spinal column generally referred to by tire letter S is shown. The segment of the spinal column S
comprises several vertebrae V and a disc space D between two adjacent vertebrae v. various cylindrical threaded spinal fusion implants, each having different diameters, are shown inserted across the disc space D.
When the height Hs of the disc space D is so large that two cylindrical implants, such as spinal fusion implants l0a and 10b, each having a sufficient diameter to cross tyre disc space D and sufficiently engage into the borne of adjacent verteUrae v, are placed across the disc space D, tlue combined overall widtlu of the two spvinal implants l0a arrd lOb exceeds tyre transverse width Ws of the spinal column S. As a result, a portion of each implant l0a and 1Ub protrudes from i.lre sides of the spinal column S and could cause severe and perhaps mortal damage to tire patient as delicate and vital structures lie adjacent to that area of the spinal column S such drat tire use of two cylindrical spinal Fusion implants l0a arrd lOb would not be desirable.
If instead of two spinal fusion implants l0a and lUb, a single implant, such as spinal fusion implant 12a were to be used having a sufficient diameter to provide for stability and fusion, then tyre implant would penetrate deeply into the adjacent vertebrae V. The spinal fusion implant 12a would have a diameter that is significantly greater than tyre heiglt HS
of the disc space D, such drat the verteurae V would have to be substantially bored out to accommodate Lhe large diameter oL
the spinal fusion ioplant 12a. As a result, a large part oC
the vertebrae V would ue removec9, and thus tire overall structural integrity of the vertebrae V would be substantially weakened. This is especially a problem when a second spinal fusion implant 12b identical to spinal fusion irnplant 12a is placed across disc space D on the other side of the same vertebrae V such that two spinal fusion implants 12a and 12b are placed across the disc spaces D on either side of the vertebrae V. As a result, the vertebra V is cleaved into a "butterfly" configuration as shown in Fiiure 1, acrd the structural integrity and strength of the vertebrae V is further diminished such that the effectiveness o.f the spinal fusion process is substantially reduced, and the vertebrae V are at High risk of devascularization acrd fracture.
Conversely, if two cylindrical irnplants such as spinal fusion implants 14 a and 14b, each having a sufficiently sized diameter such that when placed side-by-side in i:he disc space U, the combined overall width of the spinal fusion implants 14a arid 14b just fills the transverse width Ws of the spinal column S, the diameter of eaclu of the spinal fusion implants 14a and 14b will not be sufficient to cross the disc space v to engage the vertebrae V. Therefore, while the spinal fusion implants 14a and 14b will not protrude from the sides of the spinal column S, the spinal fusion implants 19a and 19b cannot reach and engage the bone of the vertebrae V and thus cannot function to stabilize the adjacent vertebrae V.
Referring to Figure 2, a top plan view, taken along line 2--2 of Figure 1 with the upper vertebrae V removed, of two cylindrical threaded implants l0a and lOb placed across the disc space D is sluown. Tlre threaded implants l0a and lOb have an external thread lJ.a and llb which trust have a minimum height that is proportional to the size of the threaded implant to be effecl.ive. The thread lla and llb of tl-re i~hreaded implants l0a and lOb converts torque to linear motion, such that flue threads lla and llb need to be of a sufficient lreiglrt to overcome tUe resistance of flue material, suclu as krone, in which the threaded implants l0a and lOb are being inserted, such resistance being proportional to tUre surface area and diameter of each of threaded implant l0a and lOb. Thus, the difference between tUe major diameter (including the threads) and the root diameter (minus the threads) of each threaded implant l0a and lOb is such that when two threaded implants l0a and lOb are implanted across the disc space D and into tyre adjacent vertebrae V, there must be a minimum distance between the two threaded implants l0a and lOb to allow for the height of the threads lla and llb. This would be true even if the threads lla and llb were interdigitated tile threaded implants l0a and lOb would still be offset by at least tyre height of the tluread of at least one of the threaded implants l0a and lOb. Such a minimum distance between flue two l.lrreaded implants l0a arrd lOb increases the combined overall widtlu of the two threaded implants l0a and lOb when inserted.
Therefore, in order for a cylindrical spinal fusion implant to be used in tire spinal fusion process wluere the height Hs of the disc space D between two adjacent vertebrae V is large relative to its width Ws, it is necessary to have an implant that can be implanted adjacent to a second of its kind in closer contact than is possible with threaded implants, while still providing for an implant surface that will provide mechanical stability in engagement to the adjacent vertebrae V. The use of a cylindrical irnplarrt is desirable as it is easy to prepare the recipient site by drilling a cylindrical hole across the disc space D and into the adjacent vertebrae V. Tlre curved surface of the cylindrical luoles drilled into tyre vertebrae V have increased surface area compared to a flat surface and also provides for the possibility of tight congruency when the cylindrical hole is fitted with an implant leaving corresponding cylindrical portions of matched diameter.
T-11~_P_~sen_~ lnv rr.~iQn Referring to Figures 3-10, an embodirnerrt of tire spinal fusion implant of the present invention, is stown and generally referred to by flue numeral 100. Tlre spinal fusion implant 100 has a substantially cylindrical configuration having a thin y outer wall 112 surrounding an internal chamber 114 and a longitudinal central axis L. The exterior of the spinal fusion implant 100 comprises surface roughenings l:hat provide a surface suitable for engaging tlue vertebrae V to stabilize the spinal fusion implant 100 across the disc space D and into flue adjacent vertebrae V once surgically implanted. In one embodiment of the spinal fusion implant 100, the surface roughenings comprise a plurality of ratcluetings 120 along l.he circumference of said spinal fusion implant. Each of the plurality of ratcheti.ngs 120 has a bone engaging edge 122 and an angled segment 124.
Each of the plurality of ratcltetings 120 ltas a lteigltt that is substantially less than the height of a requisite thread for a cylindrical threaded implant of tl~e sarne size. As a thread is a simple device for converting torque to linear advancement, the requisite heiglut of the thread is proportional to the surface area and diameter of the implant arid must be sufficient to pull a cylindrical implant having a diameter sufficient to cross the disc space D through a material as dense as bone. In contrast, the ratchetings 120 have a height that is significantly less than the requisite heigHt of a thread of a same sized threaded implant since the spinal fusion implant 100 is implanted across Llte disc space D and into the adjacent vertebrae V by linear advancement. The spinal fusion implant 100 may be pushed into the cylindrical disc space D by direct, linear advancement since it requires no thread to hull it forward through the spine. As no torque is required to advance the spinal fusion implant 100 there is no minimum requisite lteigltt of l,lte surface roughenings. 'flte only surface feature necessary is that wlticln gives the spinal fusion implant 100 stability once implanted.
Moreover, the ratchetings 120 may face in one direction, the direction in which tlue spinal fusion implant 100 is inserted, and function to prevent the spinal fusion implant 100 from backing out of the disc space D in a direction opposite to f Y

tyre direction of insertion once inserted between the two adjacent vertebrae V. J.'he ratcluetings 120 urge the spinal fusion implant 100 forward against flue unrernoved borne of tl~e vertebrae V. Since implants generally want to back out along the same path in which they are inserted, such as repeated movement of the patient's body over time and wluich would cause some other design of implant to come loose (e.g. cause a threaded cylindrical implant to possibly unscrew), the ratchetings 120 tend to urge tire spinal fusion implant 100 forward against the solid unre~noved bone further resisting dislodgement and controlling motion resulting in an exceedingly stable implantation.
Tlre borne engaging edges 122 of flue ratchetings 120 that brave a height at a highest point measured from the root diameter of the spinal fusion implant 100 that: is approximately 0.35 mm. In this manner the spinal fusion iuplant 100 rnay be placed beside a second of its kind at a distance of approximately 0.7 mm apart or if offset even closer, substantially reducing lure combined overall width of the two spinal fusion implants J.UO once surgically implanted. The ratchetings 120 may have a height in the range of 0.25 - 1.5 rnm, with the preferred height range being 0.35 - 0.75 min.
Referring to Figure 5, two spinal fusion implants 100a arrd lOUb are shown inserted across flue disc space U having the same dimensions of the disc space D shown in Figure 2. 'flee two spinal fusion implants 100a and 100b have a decreased overall combined width when compared to two threaded spinal fusion implants placed side by side previously described and illustrated in Figure 2. The decreased combined overall widl:in of the two spinal fusion irnpl.arrts 100a and lOUb is tUre difference between tyre root and major diameters of the spinal fusion implants 100a and 100b and is achieved by utilizing surface roughenings such as ratclretings 120 for stability. fhe surface roughenirrgs allow lire two spinal fusion implants 100a and 100b to come into considerably closer approximation to one another and require less total transverse width for their insertion than is possible for two threaded cylindrical irnplants having identical root diameters because of the requisite thread 1-reight of such threaded implants. Reducing the offset between implants allows for the uses of larger diameter implants which can then still fit within the transverse width Ws of tlue spinal column and achieve more substantial engagement into the adjacent vertebrae V.
Referring to Figure 7, a cross section of flue spinal fusion implant 100 is shown wherein the wall 112 leas openings 128 passing therethrough to communicate with the internal chamber 114. The internal chamber 114 rnay be filled with bone material or any natural or artificial bone growth material or fusion promoting material such that bone growth occurs from the ~eltebrae V througlu the openings 128 to tire rnaterial within internal chamber 114. While tire openings 128 have been shown in the drawings as being circular, it is appreciated that floe openings 128 may have any shape, size, or form suitable for use in a spinal fusion implant without departing from ttre scope of the present invention. Also, the number of openings rnay be varied or no openings rnay be present on the spinal fusion implant .
Referring to Figures 8 and 9, tlue spinal fusion implant 100 lras a cap 130 with a thread 132 that threadably attaches to one end of the spinal fusion implant 100. Once i;he cap 130 is attached to the spinal fusion implant 100, the edge 136 acts as an additional ratcheting 120 to further stabilize tyre spinal fusion implant 100 once it is implanted across the disc space D.
Thre cap 130 is removable to provide access to tlue internal chamber 114, suclu that flue internal chamber 114 can be filled and boll any natural or artificial osteoconductive, osteoinductive, osteogenic, or other fusion enhancing material. Some examples of such materials are bone harvested from the patient, or bone growtl-r inducing material such as, but not limited to, lrydroxyapatite, hydroxyapatite tricalciurn phosphate; or one morphogenic protein. The cap 130 and/or the spinal fusion implant 100 itself is made of material appropriate for human implantation such as titanium and/or may be made of, and/or filled and/or coated with a bone ingrowth inducing material such as, but not limited to, hydroxyapatite or hydroxyapatite tricalcium phosphate or any other osteoconductive, osteoinductive, osteogenic, or other fusion enhancing material.
Referring to Figure 4, alternatively the cap 130a may be "bullet"-shaped to facilitate insertion. The cap 130a has at its greatest diameter a diameter equal to the root diameter of the spinal fusion implant 100 such that no additional ratchetings 120 are formed.
Referring to Figure 10, the spinal fusion implant 100 has an engagement means at one end in the form of a rectangular slot 140 for engaging a driver instrument having a removable engagement means for intimately engaging the rectangular slot 140. A threaded portion of the driver instrument, which in one embodiment extends as a rod through a hollow tubular member and can be rotationally controlled, screws into a threaded aperture 142 in the slot 140 and binds the implant 100 and the driver instrument together.
Once affixed to the implant driver instrument, the spinal fusion implant 100 may be then introduced through a hollow cylindrical tube and driven into the cylindrical hole that has been drilled across the disc space D. The implant driver instrument may then be impacted by a mallet, or similar device, to linearly advance the spinal fusion implant 100 across the disc space D. Once the spinal fusion implant 100 is inserted across the disc space D, the ratchetings 120, engage the bone of the vertebrae V and the implant driver instrument is detached from the spinal fusion implant 100. The procedure for drilling the holes across - 21a -the disc space D and instrumentation pertaining thereto are described in U.S. Patent No. 5,484,437.

'r....

Referring to Figmres 11-19, an alternative enibodimerct of the spinal fusion implant of the present invention, generally referred to by the numeral 200 is shown. Tlre spinal fusion implant 200 is similar to the spinal fusion implant lUO except that the openings 228 are bisected by the bone engaging edge 222 of the plurality of ratclretings 220. In this manner, the bone engaging edges are interrupted by the openings 228 to provide a "tooth-like" edge that engages the bone of the vertebrae V and creates an interference fit to prevent the backing out of flue implant 200 once inserted. It is appreciated that the number of openings 228 and the number of bone engaging edges 222 may be varied and Lhat the opening 228 can be placed in any orientation uelative to tte ratcluetirm~s 220 or other surface roughening without departing from the scope of the present invention.
Referring to Figures 15-19, an alternative embodiment of the spinal fusion implant of the present invention generally referred to by the numeral 300 is shown. The spinal fusion implant 300 leas a substantially cylincJrical configuration laving surface rouglenings for stabilizing tile implant 300 within the intervertebral space D. As shown i.n Figure 18, the surface roughenings comprise a surface knurling 320 such as, but not limited to, the diamond-shaped bone engaging pattern shown. Tlue spinal fusion implant 300 may leave surface )tnurling 32U tlrrougluout the entire external surface of the spinal fusion incplant 300, throughout only a portion of the external surface, or any combination thereof, without departing from flue scope of the present invention. Tn i:hose circumstances where tluere is rco undrilled bone in the disc space D forward of the spinal fusion implant 300 to resist further forward advancement of the implant:, surface knurling 320 is preferred as it produces au exceedingly high interference fit wit:H ttce bone oL l:he vertebrae V and resists motion equally in all directions and without the tendency to urge itself forwarc7.
Referring to Figure 17, two spinal fusion implants 300a and 2~68~35 30Ub rnay be placed side-by-side across the disc space D having the same dimensions of tlue disc space D shown in Figure 2, such that tlue two spinal fusion implants 300a and 300b are touching each other and alms reducing the overall combined width of the two spinal implants 300a and 3UOb to the minimr_rm distance possible with a substantially cylindrical implant having a roughened surface. In this rrranner, two cylindrical spinal fusion implants 300a and 30Ub having a sr.rfficient diameter to cross the height HS of tlue disc space D can be placed across the disc space D witloout exceeding flue transverse width Ws of the spinal column S. The spinal fusion implants 300a and 300b are inserted by linear advancement as described move for spinal fusion implant 100. Therefore, as no threading is required for the insertion of srinal fusion implants 300a and 300b, little or no space creed be present b~tweeu the spinal fusion implants 300a and 30Ub, as compared to the space that would be required for a thrread wluen using threaded implants.
Tlrus, the spinal fusion implants 30Ua arid 300b may be placed closer togethrer to substantially reduce the overall combined width of two such implants.
Referring to Figures 23-30, an alternative enibodi.rnerrt of i:lne spinal fusion implant of the present invention is shown arid is generally referred to by the nurneral 400. T'he spinal fusion implant 400 has a similar configuration to that of the spinal fusion implant 200, except that it comprises a partially cylindrical member having arcuate portions 402 and 404 which are arcs of i:he same circle with portions ot_ its outer wall 4U5 that are flattened so as to present a first flat side 406 and a second flat side 408.
Referring to Figure 28, the spinal fusion irnpl.ant 400 has a major diameter M equal to the distance between two diametrically opposite non-flattened segments, such as arcuate portions 402 and 404 which are arcs of tire same circle. Tire width Wi of the spinal fusion implant 400 is equal to tire distance between a flattened segment and a point diametrically ~168~35 opposite the flattened segment, such as the distance between flue first and second flat sides 406 and 408.
Referring to Figure 25, a diagranunatic rei~resetttation of a segment of a spinal colurnn S comprising several vertebrae V is shown having two spinal fusion implants 400a and 400b inserted across the disc space U between the two adjacent vertebrae V.
Tlue spinal fusion implants 400a and 40Ub are identical and each lras a first arcuate portion 402a arid 4021,, respectively; a second arcuate portion 404a and 404b, respectively; a first flat side 406a and 40Gb, respectively; and a second flat side 408a and 408b, respectively. Tlue spinal fusion implants 400a and 400b are implanted across Llre disc space U with the second flat side 408a of spinal fusion implant 400a facinc3 and adjacent to the first flat side 408b of spinal fusion irnplart 400b suclu that the combined overall width of the two spinal frtsion implants 400a and 400b is less than twice the maximum diameter M of the implants. The spinal fusion implants 400a and 400b are inserted by linear advancement as described above for spinal fusion implant 100.
Prior to implantation, two partially overlappitm~
cylindrical holes are drilled across the disc space U arid info the adjacent vertebrae V. The holes are drilled sufficiently overlapping to allow tlue two spinal fusion implants 400a and 400b Lo be implanted with the flat sides perpendicular to l:lte plane of the disc space U, tUe disc space U being in a pla~ie perpendicular to the longitudinal vertical axis A of the spinal column S as shown in Figure 25.
Tlte spinal fusion implants 400a and 40Ub may be inserted separately such that once a first spinal fusion implant 400a is inserted across the disc space U, a second spinal fusion implant 400b is driven across the disc space U so tluat the flat side 402 or 404 of each spinal fusion implant 400 are adjacent to each other and are touching. In this manner, tl~e two spinal fusion implants 400a and 400b are implanted across the disc space U and engage the bore of the adjacent vertebrae v without exceeding tire transverse width WS of the spinal column S.
Alternatively, the two spinal fusion implants 400a and 400b rnay be implanted across the disc space D simultaneously by placing i.:lrem adjacent and facing each other, in the orientation described above, prior to implantation. The two spinal fusion implants 400a and 400b are then linearly advanced into the drilled holes across the disc space D.
Referring to Figure 28, tire effect of having first and second flat sides 406 and 408 is that the overall width w. of i the spinal fusion implant 4(10 is substantially reduced while the height of the spinal fusion implant 400 remains the maximum diameter M of the cylindrical portion of tlne spinal fusion implant 400.
Referring to Figures 25 arid 26, as tyre height of each spinal fusion implant 400a and 400b is sufficient to cross the disc space D and into the two adjacent vertebrae V, each spinal fusion irnplarrt 400a and 400b engages tire bone of the adjacent vertebrae V while the combined width of the two spinal fusion implant 100 does not exceed the transverse width Ws of the spinal column S. As a result, the advanl.:ages o)_ placing two cylindrical implants side by side across the disc space D rnay be obtained without exceeding flue width WS of the spinal column S. Thus, as shown in Figure 2G, the two spinal fusion implants 400a and 400b can be inserted across the disc space D, having the same dimensions a.~ the disc space D shown in Figure 2, and can be placed much closer together as a result of tire first flat side 408b placed adjacent to the second flat side 408a while continuing to engage the adjacent vertebrae V.
As shown in Figure 30, the spinal fusion irnplarrt 400 has a hollow internal central charnber 414 and has a series of openings 428 passing through tlue outer wall 405 and into the central chamber 414 of the spinal fllslorl llllplarrt 400. The openings 428 may also ue present on flue first anc7 second flat sides 406 and 408. Said openings 428 wlril.e shown as round holes for example, may be any other workable configuration v Z »~~~~

consistent with their purpose and may include, but is not limited to, ovals, slots, grooves and holes that are not round as is true for any of the cylindrical implants disclosed above.
Referring to Figure 30A, it is appreciated that it is also within the scope of the present invention that the spinal fusion implant 400' could leave only one flat side so as to provide only a first flat side 406'. This configuration is appropriate where the width Wi of the spinal fusion implant 400 need only be slightly reduced with respect to its maximum diameter M, to prevent the combined overall width of two such implants from exceeding the transverse width Ws of the spinal column S.
Referring to Figures 23, 24 and 29, the spinal fusion implant 400 of the present invention has a plurality of ratclretings 420 facing one direction, as described above for spinal fusion implant 100, along tyre outer surface of the cylindrical portion of the circumference of the spinal fusion implant 400. The ratclretings 420 have a bone engaging edges 422 acrd the angled configuration of the ratchetings 420 provide for a "one-way" insertion of the spinal fusion implant 400 as the movement of tire spinal fusion implant 400 in the opposite way is prevented by the etrgagement or the engaging edges 422 with the vertebrae V. However, the flat sides 4U2 and 404 are preferably SIIIOOtIu and lave a flat surface so as to allow placement itr the closest possible proximity of the two spinal fusion implants 4UOa and 400b. The bone engaging edge 422 of each ratcheting 420 bisects tl-re holes 428 to increase the stability of the spinal fusion irnplatrt 400 once implanted.
Tlue spinal fusion implants 100-G00 each have an overall lengtl-r in the range of 20mm to 30mm, with 25mm being preferred, and a maximum diameter M in the range of l4nun t;o 24nrm, with l8mm being preferred w(reu inserted in the lumbar spine from Llre posterior approacl-r, anrl 20nun being preferred when inserted in llre lumbar spine from l:he anterior approach. 1'he syirral fusion implant 400 is quite appropriate for use in the cervical and 21 ~8~~5 thoracic spine as well. In the cervical spine such implants would have a length in tyre range of 10-l8mm preferred 12 mm aria a maxirnum diameter M in the range of 12-20mm, with the preferred diameter being l6mm. Irr the thoracic spine such implants would have a length in the range of 1G-26mnr and a greatest diameter in tyre range of 14-20nrm, with tire preferred diameter being l6mm. In addition to the foregoing dimensions,' spinal fusion implants 400-600 leave a width Wi for use in the cervical spine in the range of 8-l6mm, with the preferred width W, being 10-l4mm; for use in the lurnbar spine in the range of >_ 18-26mm, with the preferred width Ws being 18-20mm; and for use in the lumLrar spine irr the range of 18-26rnm, with tire preferred width Wi being 20-24mm.
Referring to Figures 27 and 28, when viewed on end, the spinal fusion implant 400 of the present invention bras externally the geometrical configuration of a circle with a portion of each side tangentially arnputated vertically to form the first and second flat sides 406 and 408. The cap 430 extends beyond the narrowest diameter of the wall 412 along the first and second arcr.rate portions 402 and 4U4 at the end of the spinal fusion irnplarrt 400 and acts as an additional ratcheting 420 with an engaging edge 436. In this manner, the additional ratcheting 420 functions t:o further increase the stability of t,lre spinal fusion implant 400 once inserted between the adjacent vertebrae V and to further prevent the dislodgement of the spinal fusion implant 400 from the disc space D. 1'lre cap 430 is flush with the flat sides 406 and 408 to preserve the flat surfaces of flat sides 406 and 408. The cap 430 further tras a sloping sides 438a and 438b corresponding position wii:lr tire flat sides 40G and 408 to facilitate insertion of flue spinal fusion implant 400 and to permit for close side by side placement of two spinal fusion implants 400. Alternatively, tire cap 430 can Ue flush all the way around with tUre root diaeneter of the spinal fusion implant 400 to further facilitate insertion for a longer ramp length.
The spinal fusion implant 400 leas surface roughenings such as, but not limited to, ratchetings 420 suclu that the outer surface of the spinal fusion irnplant 400 may have a plurality of other surface roughenings to enhance to stability of the spinal fusion implant 400 and i:o resist dislodgement once implanted across the disc space D. For example, the spinal fusion implant 400 may have an irregular outer surface that may be created by blasting or rough casting and the like. Such arr irregular surface may be used alone or in combination with other surface roughenings such as ratchetings and/or knurling and as already discussed, the openings 428 may be holes, grooves, slots or other.
Referring to Figures 32-35, an alternative embodiment of the spinal fusion implant of the present invention is sluown and generally referred to by the numeral 500. Ttre spinal fusion implant 500 is substantially the same as the spinal fusion implant 400, except that the openings 528 are positioned on the ratcheting 520 such that the openings 528 are positioned between the bone engaging edges 522 and are not bisected by the bone engaging edges 522. In this manner the bone engaging edges 522 are continuous and uninterrupted to engage the bone of the vertebrae V and prevent the backing out of the implant 500 once inserted.
Referring to Figures 36-40, an alternative embodiment of the spinal fusion implant of the present invention is shown and generally referred to by the numeral 600. The spinal fusion implant 600 is substantially identical to the spinal fusion implant 400 described above except that in place of ratchetings 420, it has surface knurling 620 such as, but not limited to, the diamond-shaped bone engaging pattern shown in Figure 40.
The surface knurling 620 assists in the retaining of i:he spinal fusion implant 600 once it is inserted across the disc space D
between two adjacent vertebrae V. It is recognized that the surface knurling 620 of the implant 600 may be combined with any of a number of other surface roughenings such as, but'not limited to, ratchetings to assist in retaining i~lne spinal fusion implant 600 across the disc space D.

As shown in Figure 36, the cap 630 of the spinal fusion implant 600 has sloping sides 660 and 662 corresponding with the first and second flat sides 606 and 608 to faci).itate insertion of the spinal fusion implant 600 and to perrnit for close side by side placement of two spinal fusion implants 600.
It is appreciated that the implant invention may include arry and all surface roughening configuration that either increase the surface area or interference fit of i:he implant and the vertebrae v. It is appreciated that the ratchetings described above for the various embodiments of the spinal fusion irnplants of the present invention may also comprise a knurled or otluer surface roughenings in combination with the ratchetings to further enhance the retention of the spinal fusion implant across the disc space D once inserted.
Referring to Figure 41, an alternative embodiment of the spinal fusion implant of the present invention generally referred to by flue numeral 700 is shown. The spinal f1rS10I1 implant 700 has surface roughenings comprising of a blasted external surface 701 to provide an engagement surface for the vertebrae V when inserted across the disc space U. The spinal fusion implant has a plurality of openings 728, a removable cap 730 with a hex slot 734 for engaging a loex tool.
Referring to Figure 42, an alternative ernbodimerrt of the spinal fusion implant of the present invention generally referred to by the numeral 800 is shown. Tlue spinal fusion implant 800 is similar to spinal fusion implarnt 400 described above except that it has openings in the form of horizontal slots 828 on the flat side 806 and vertical slots 829 on the cylindrical portion of flue spinal fusion implant 800.
It is appreciated that flue spinal implants of the present invention may leave any configuration such that the combined overall width of the two such spinal fusion implants is less than twice the maximum diameter M of those implants without departing from the scope of the present invent_i.on.

Referring to Figure 43, a segment of the spinal column S is shown with an alternative embodiment of two spinal fusion implants 900a and 900b inserted across disc space D1 is shown. Spinal fusion implant 900a lras a concave surface 902 which is correspondingly shaped for receiving tine convex surface 904 of spinal fusion implant 900b. When tire two spinal fusion implants 900a and 900b are placed side by side, the concave surface 902 mates with the convex s~.rrface 904 such l:hat tire combined overall widtlo of the two spinal fusion implants is less than twice the maxirnurn diameter M of those implants. ~s a result, the advantages of placing two implants that are partially cylindrical, witlu respect to the portion enc3aging the vertebrae V, side by side across tire disc space U may be obtained without exceeding the width Ws of the spinal column S
Referring still to Figure 43, arr alternative eunodimerrt of tire spinal fusion implant of the present invention comprising a single spinal. fusion implant 1000 inserted across tie disc space D2 of the spinal column S is shown. The spinal fusion 2p implant 1000 comprises a first cylindrical portion 1010 and a second cylindrical porl:i.on 1012 and may have any of the sm:face roughenings described above in reference to tire embodiments set forth above. In the preferred embodiment, the spinal fusion implant 1000 is inserted by linear advancement into two overlapping cylindrical luoles drilled across flue disc space U2.
While the present invention leas been described in detail with regard to tyre ~~referred emboQirnents, it i s appreciated drat other variations of the present invention rnay be devised which do not depart from the inventive concept: anQ scope of the present invention.

Claims

1. A spinal fusion implant made of a material appropriate for human implantation at least in part between two adjacent vertebral bodies, said implant comprising:
a non-threaded cylindrical member having an exterior with opposed arcuate portions adapted to be oriented toward the adjacent endplates of the two adjacent vertebral bodies, each of said opposed arcuate portions having at least one opening passing therethrough to allow bone growth from adjacent vertebral body to adjacent vertebral body through said implant, said implant having a plurality of surface roughenings protruding from said exterior of said cylindrical member for engaging the two adjacent vertebral bodies to maintain said implant in place, said surface roughenings configured for linear insertion into the spine and to resist expulsion of said implant from between the two adjacent vertebral bodies.

2. A spinal fusion implant made of a material appropriate for human implantation at least in part between two adjacent vertebral bodies, said implant comprising:
a non-threaded cylindrical member having an exterior with opposed arcuate portions adapted to be oriented toward the adjacent endplates of the two adjacent vertebral bodies, each of said opposed arcuate portions having at least one opening passing therethrough to allow bone growth from adjacent vertebral body to adjacent vertebral body through said implant, said implant having a plurality of annular ratchetings defined around the circumference of said cylindrical member, said annular ratchetings being configured for linear insertion into the spine and to resist expulsion of said implant from between the adjacent vertebral bodies.

3. The spinal fusion implant of either claim 1 or 2, wherein said member includes a pair of opposed side portions between said opposed arcuate portions, at least one of said side portions being substantially flat.

4. The spinal fusion implant of claim 3, wherein at least one of said side portions has a curved exterior.

5. The spinal fusion implant of claim 4, wherein said exterior of at least one of said side portions is at least in part concave.

6. The spinal fusion implant of either claim 1 or 2, wherein said opposed arcuate portions are arcs of the same circle.

7. The spinal fusion implant of any one of claims 1-6, further comprising a plurality of cells in said exterior for retaining a fusion promoting substance.

8. The spinal fusion implant of any one of claims 1-6, wherein said spinal fusion implant is porous.

9. The spinal fusion implant of any one of claims 1-6, wherein said exterior has a bone ingrowth surface.

10. The spinal fusion implant of any one of claims 1-9, wherein said spinal fusion implant is treated with a fusion promoting substance.

11. The spinal fusion implant of any one of claims 1-9, wherein said spinal fusion implant is coated with a fusion promoting substance.

12. The spinal fusion implant of any one of claims 1-11, wherein said spinal fusion implant is stronger than bone.

13. The spinal fusion implant of any one of claims 1-12, further comprising a hollow interior in communication with said at least one opening.

14. The spinal fusion implant of claim 13, wherein said implant includes an end along the longitudinal central axis of said implant that is open to allow access to said hollow interior.

15. The spinal fusion implant of claim 14, wherein said end is adapted to be closed by a cap.

16. The spinal fusion implant of claim 15, in combination with a cap adapted to close said hollow interior.

17. The spinal fusion implant of any one of claims 1-16, in combination with a fusion promoting substance.

18. The spinal fusion implant of claim 17, wherein said fusion promoting substance includes at least one of bone, hydroxyapatite, hydroxyapatite tricalcium phosphate, and bone morphogenic protein.

19. The spinal fusion implant of any one of claims 1-18, in combination with an implant driver configured to insert said spinal fusion implant into the spine.

20. The spinal fusion implant of any one of claims 1-19, in combination with a guard having an opening for providing protected access to the disc space and the adjacent vertebral bodies and for guiding the insertion of said spinal fusion implant into the spine.

21. The spinal fusion implant of any one of claims 1-20, in combination with a drill for preparing an implantation space to receive said spinal fusion implant.