ZERO PROFILE SPINAL FUSION CAGE

Abstract

An interbody fusion cage having upper and lower canals for receiving the heads of bone screws that have been pre-installed in opposing vertebral body endplates. The proximal wall of the cage preferably has a vertical slot that communicates with each canal and is adapted to allow access by a screwdriver and tightening of the screws.

Claims

1. An intervertebral fusion system comprising: an intervertebral fusion cage including: i) a first component; and ii) a second component; and a threaded bone screw.

2. A method of inserting a fusion cage into an intervertebral disc space formed by superior and inferior vertebral bodies spaced from each other along an inferior-superior direction, the method comprising the steps of: inserting first and second components into the intervertebral disc space in an insertion direction, such that an outer surface of the first component faces the first vertebral body; and translating first and second ramped surfaces of the first component along third and fourth ramped surfaces of the second component, respectively, wherein the first and second ramped surfaces are planar and oblique to the outer surface, and the third and fourth ramped surfaces are planar and oblique to the outer surface; and inserting a superior bone screw through a superior hole of the fusion cage and into the superior vertebral body along a direction that is angled superiorly as it extends in the insertion direction; and inserting an inferior bone screw through an inferior hole of the fusion cage and into the inferior vertebral body along a direction that is angled inferiorly as it extends in the insertion direction.

3. An intervertebral fusion cage configured to be implanted in an intervertebral space defined by first and second vertebral bodies, the intervertebral fusion cage comprising: an outer component having an outer surface configured to engage the first vertebral body, the outer component presenting first, second, and third ramped surfaces that are oriented oblique to the outer surface; an inner component defining fourth, fifth, and sixth ramped surfaces that are oriented oblique to the outer surface; and wherein the inner component is translatable with respect to the outer component so as to cause 1) the first and fourth ramped surfaces to translate upon each other, 2) the second and fifth ramped surfaces to translate upon each other, and 3) the third and sixth ramped surfaces to translate upon each other.

Description

DESCRIPTION OF THE FIGURES

[0032] FIG. 1 discloses a first perspective view of a device of the present invention having a single screw and a single guide surface depression on each of the upper and lower bearing surfaces.

[0033] FIG. 2 discloses a second perspective view of the device of FIG. 1.

[0034] FIG. 3 discloses a device substantially similar to FIG. 1, but with teeth lining bearing surfaces bordering the guide surface depression.

[0035] FIG. 4 discloses a device substantially similar to FIG. 1, but with a discontinuous guide surface depression.

[0036] FIG. 5 discloses a device substantially similar to FIG. 4, but in a lateral cage configuration.

[0037] FIG. 6 discloses a first device substantially similar to FIG. 1, but with a cervical cage configuration.

[0038] FIG. 7 discloses a second device substantially similar to FIG. 1, but with a cervical cage configuration.

[0039] FIG. 8 discloses a first perspective view of a device substantially similar to FIG. 1, but with a pair of screws inserted into the proximal wall of the device and extending through the respective upper and lower bearing surfaces.

[0040] FIG. 9 discloses a side view of the cage of FIG. 8.

[0041] FIG. 10 discloses a first perspective view of a device substantially similar to the cage of FIG. 8, but with a continuous guide surface depression.

[0042] FIG. 11 discloses a first perspective view of the device of FIG. 10.

[0043] FIG. 12 discloses the assembly comprising the components of FIGS. 13 and 14.

[0044] FIG. 13 discloses a second component of the rail-based device of the present invention.

[0045] FIG. 14 discloses a first component of the rail-based device of the present invention.

[0046] FIGS. 15a-15b disclose another embodiment of the present invention.

[0047] FIG. 16 shows the device of the present invention being implanted into a disc space with a fusion cage inserter.

[0048] FIGS. 17a and 17b disclose a trial of the present invention.

[0049] FIGS. 18a-18d disclose various views of the inserter of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0050] Now referring to FIGS. 1 and 2, there is provided an intervertebral fusion cage comprising: [0051] a) a proximal wall 1 and a distal wall 3, [0052] b) first 5 and second 7 side walls connecting the proximal and distal walls, an upper bearing surface 9 adapted for gripping an upper vertebral endplate and a lower bearing surface 11 adapted for gripping a lower vertebral endplate, the upper bearing surface having at least one upper opening 13 therethrough adapted to promote bony fusion, the lower bearing surface having at least one lower opening (not shown) therethrough adapted to promote bony fusion, [0053] c) a first guide surface depression 15 formed in the upper bearing surface and extending substantially from the proximal wall to the distal wall, the depression adapted for distal reception of a first screw head 17, and [0054] d) a second guide surface depression 19 formed in the lower bearing surface and extending substantially from the proximal wall to the distal wall, the second depression adapted for distal reception of a second screw head.

[0055] Typically, the guide surface depression forms a longitudinal canal in each bearing surface. The distal portion 21 of the guide surface depression acts as a means for guiding the more proximal portion of the canal to the screw head. When the proximal portion 23 of the canal is slid over the screw head, it envelops the screw head, thereby locking the cage in place. Further tightening of the screw can be performed to further lock the cage in place.

[0056] In some embodiments, the cross-sectional profile of the depression or canal is substantially equivalent to the cross-sectional profile of the screw head, so that the first depression is well adapted for distal-to-proximal translation of the first canal towards the screw head. In some preferred embodiments thereof, the screw head is substantially spherical, while the transverse cross-section of the first canal substantially forms a portion of a circle, thereby providing a substantially matching fit of the canal and screw head.

[0057] In some embodiments, the first canal extends substantially along a centerline of the cage, thereby allowing the use of a single screw per bearing surface.

[0058] In some preferred embodiments, the first canal comprises a distal recess 25 and a proximal process 27. The proximal process effectively acts to lock the cage in place when it slides over and envelops the screw head.

[0059] In some embodiments, the first canal includes an outwardly extending (longitudinal) bump (not shown) adapted to limit translational movement of the interbody device with respect to the screw. This bump acts as an additional means for guiding the deeper portion of the canal to the screw head, at which the cage becomes locked.

[0060] Now referring to FIGS. 2 and 3, in some embodiments, additional strength is provided to the proximal portion 29 of the cage in order to withstand higher tensile forces and insertion forces. Preferably, this additional strength is achieved by using a stronger material in the proximal portion of the cage. In preferred embodiments thereof, the bearing surface 33 surrounding the proximal portion of the canal is formed from a metallic material.

[0061] In some embodiments, it is helpful to provide a final seating of the screw head once it becomes seated in the deeper proximal portion of the canal. In these embodiments, the proximal wall of the cage preferably has a vertical slot 35 that communicates with the horizontal guide surface canal and is adapted to allow access by a screwdriver. Thus, the surgeon has direct access to the screw head via a proximal route and can easily accomplish its final tightening.

[0062] In some embodiments (as in FIGS. 1-3), the guide surface canals are continuous longitudinal structures that guide the entry of the cage from the moment the canal contacts the screw head to the moment the screw seats in the proximal portion of the canal. However, in other embodiments (as in FIG. 4), the canal 45a and 45b may be discontinuous.

[0063] It is believed that the device of the present invention can be advantageous used in implanting lateral cages. Therefore, now referring to FIG. 5, there is provided a lateral cage 49 of the present invention, wherein the length L of the lateral cage (distal end 47 to proximal end 48 distance) is at least two times greater than the width W of the lateral cage (side wall 49 to side wall 50 length). Preferably, the length of the lateral cage is at least three times greater than the width of the lateral cage. Also in FIG. 5, the proximal portion 101 of the lateral cage is preferably made of a metal material such as titanium, while the distal portion 103 is made of a polymer-based material, such as CFRP.

[0064] It is believed that the device of the present invention can be advantageous used in implanting cervical cages. Therefore, now referring to FIGS. 6 and 7, there is provided a cervical cage of the present invention having a screw 51 extending from each guide surface depression 53.

[0065] In some embodiments, it is advantageous to add additional screws to the device in order to more completely secure the device to the vertebral endplates. Now referring to FIGS. 8 and 9, there is provided a first perspective view of a device having a pair of screws 55 inserted into the proximal wall 57 of the device and extending through the openings in the respective upper 59 and lower 61 bearing surfaces.

[0066] FIG. 10 discloses a distal perspective view of a device substantially similar to the cage of FIG. 8, but with a continuous guide surface depression 63. FIG. 11 discloses a proximal perspective view of the device of FIG. 10.

[0067] Various aspects of the present invention include an implant/instrument system, and a method of implantation. The present invention also includes a kit comprising: [0068] a) trial instruments comprising interbody spacing blocks having various sizes (height, angle, footprint), each with bone anchor placement guides. [0069] b) at least two bone fixation anchors to be placed into adjacent vertebral bodies while trialing with the aforementioned instrument, and [0070] c) an interbody implant configured for engaging with the heads of the implanted bone anchors after removing the trial instrument.

[0071] Now referring to FIGS. 17a and 17b, trial 101 has a handle attached to one side (not shown). Two pins are for the trial proper depth position. The vertebral body holes for anchoring the screws can be drilled through the guides.

[0072] The kit of the present invention allows the surgeon to fix the opposing vertebral bodies to one another through the interbody device without having the implant protrude outside of the disc space. Preferably, the heads of the bone anchors snap into proximal processes formed in canals located in the upper and lower surfaces of the interbody implant, thereby helping the implant resist migration. Preferably, the anchors can be inserted at various angles to accommodate anatomical differences as well as avoid any pre-existing hardware.

[0073] Preferably, the canals of the interbody implant sufficiently envelop the respective bone anchor heads so as to prevent back-out and pull-out of the anchor. In such situations, a secondary locking step/feature is not required. Preferably, the interbody implant allows passage therethrough of a driver to further seat and tighten the bone anchor into the bone after the implant has been placed. This is typically accomplished by a vertical slot 35 in the proximal wall that communicates with the canals. Preferably, the major diameter of the bone anchor is larger than the screw head diameter. In some embodiments, the major diameter is 5.5 mm). Larger major screw diameters can be used, as compared to conventional devices wherein the anchors are placed through the wall of the fixation device and limited by the height of the device. This is a major advantage.

[0074] The embodiments described herein are preferably designed for the cervical region, but also could be utilized for lumbar spine interbody fusion as well.

[0075] In a preferred embodiment of the invention shown in FIGS. 1 and 2, the bone anchor comprises a screw having a proximal head 17 that is substantially spherical such that it can accommodate variable angles and still engage with the interbody implant. Alternatively, other screw head shapes that match the shape of the canal recess and provide a reliable connection between two components may be used. Also, in preferred embodiments, the screws are positioned in the centerline so that they are less likely to interfere with an adjacent level plate with two screws in each vertebra, as some plates have screws positioned laterally so the screws of the present invention can go in between them. In use, the screws are first installed and then the interbody implant is slid between the vertebrae, using the insertion tool that aligns the central canals of the cage with the screw heads. In preferred embodiments, a small radial bump (not shown) located in the canal limits translational movement of the interbody device with respect to the screw. In another preferred embodiment, the screws are positioned such that upon sliding the interbody device into the disc space, the interbody device slides primarily over the screw heads (instead of primarily contacting the endplates—the screws are inserted slightly proud to allow larger distraction) to ease insertion. At the end of insertion, the screw heads “drop” into a larger proximal process of the canals, thereby ensuring that the upper and lower surfaces of the interbody device abut the bony endplates and providing the means of preventing the interbody device from backing out. The interbody device preferably has internal spaces opening outward through the upper and lower bearing surfaces and onto the adjacent bony endplates. These spaces may be packed with bone graft or bone graft substitute prior to implantation in order to promote bony fusion of the opposing endplates.

[0076] In some embodiments, the proximal wall of the cage has a small vertical slot 35 that provides access by a screwdriver shaft to the guide surface canals (and thereby the screw heads). After interbody device insertion into the disc space, the screws are preferably tightened through these slots to ensure construct stability. Screw backout is prevented by design of the cage and method of cage installation, as the screw heads are seated on the inner surfaces of the respective canals.

[0077] FIGS. 12-14 show another embodiment of this invention. In this embodiment, each screw 201 is inserted into its respective endplates along with an additional rail component 203. The rail component has a cross-section configured to slidingly engage a mating canal in the main interbody device. In this exemplary embodiment, both the rail and the guide surface canal 205 of the main interbody device 207 have dovetail engagement features, although other reliable locking configurations may be used. The advantage of the rail lies in its ability to accommodate variable screw trajectories without the need for a spherical head to ensure alignment with the interbody device. Generally, the rail must be implanted first to envelop the screws, with the screws partially tightened. Then, after sliding the interbody device over the rails, the screws are finally tightened, thereby making the whole construct rigid and properly attached to the vertebrae. The cage may be locked to the rails by the small springy or bump-like feature (not shown) incorporated into the rail and cage design. Preferably, the head of the screw seats on the inner wall of the dovetail canal, thereby preventing the screw from backing out.

[0078] Revision surgery can be performed by loosening the screws and removing the cage. In effect, the screws do not need to be removed during cage revision.

[0079] FIGS. 15a and 15b show another embodiment with a pair of screws 221 placed to either side of cage midline. The cage is implanted by sliding the screw heads into the canals of the cage. The final tightening of the screws may be accomplished through the holes 223 provided on the cage's proximal wall after the cage is put in place. The holes have a smaller diameter than the screw head and provide access for a screwdriver shaft while preventing screw backout. The cage is locked in place by the screws' tension, though an additional feature in the form of a small bump (not shown) can be added to prevent cage expulsion. An injectable bone graft or substitute can be injected through the cage side window after final cage assembly. Alternatively, a bone graft substitute such as TCP may be incorporated into the graft at the situs of manufacture.

[0080] FIG. 16 shows the device of the present invention being implanted into a disc space with a fusion cage inserter. The screws 225 are held in place on tynes distal of the device of the present invention, and are inserted into the vertebral endplate portion of the disc space so that substantially only their heads protrude from those endplates. Next, the device 227 is slid over the screw heads and is locked in place.

[0081] Now referring to FIGS. 18a-18d, a preferred inserter for inserting the present invention comprises a body 103, two blades 105 attached to the body, a pusher 107 attached to the shaft 109 and a handle 111 attached to the shaft.

[0082] To implant the cage, the surgeon has to spread the blades and insert the cage in between the blades, aligning the cage's proximal opening with the pusher pin 112. The cage has to be positioned along the inserter so that the pedals are reasonably collapsed in order to be inserted into intervertebral space. The central slots 113 of the blades need to be aligned with the already-implanted screw heads, and the inserter needs to be as vertical as possible (i.e., perpendicular to the anterior plane of the vertebrae) and inserted as deep as the blades' stop surfaces 114 will allow. At this point, the inserter handle is turned clockwise, thereby pushing the pusher and the cage forward. During insertion, pedals 115 become distracted, thereby making space for the cage. The cage is pushed into the disc space until the pusher “stops” contacting the vertebrae. The cage stops advancing forward and the blades withdraw from the disc space by continuing advance of the pusher until the blades are completely withdrawn.

[0083] The pusher blade 119 rides inside the pusher guiding slot 117, thereby preventing the pusher from spinning and aligning it properly to the blades. An impactor (not shown) is then used to advance the cage into the final position. At this point, the screw heads serve as stops for the impactor, and the cage cannot move any further distally. At this point, the anterior surface of the cage is flush with the screw heads' most protruding points. The final step is the tightening of the screws.

[0084] In some embodiments of the present invention, trialing occurs before implantation of the fusion cage. In particular, in accordance with the present invention, there is provided a method of inserting a fusion cage into an intervertebral disc space formed by upper and lower vertebral endplates, comprising the sequential steps of: [0085] a) inserting a trial having a guide surface canal into the disc space, [0086] b) drilling or awling the hole through the trial drill guides, [0087] c) inserting a first and the second screws into the upper and lower vertebral endplates, the first screw having a shaft and a screw head, [0088] d) removing the trial, [0089] e) inserting a fusion cage having a guide surface canal into the disc space so that the screw head is received in a distal portion of the canal, and [0090] f) distally translating the cage into the disc space so that the screw head becomes received in a proximal portion of the cage canal.

[0091] These cages of the present invention may be made from any non-resorbable material appropriate for human surgical implantation, including but not limited to, surgically appropriate metals, and non-metallic materials, such as carbon fiber composites, polymers and ceramics.

[0092] The interbody device and bone anchors are preferably made out of PEEK or CFRP or any other suitable material providing adequate strength and radiolucency. However, implantable metals such as titanium or stainless steel components may be required to ensure adequate strength for either the interbody device or bone anchors. In some cases the interbody device can be made as a combination of PEEK and metal. The metal component is preferably used for screw head retaining feature. In some cases, resorbable materials such as polylactide, polyglycolide, and magnesium are preferred.

[0093] In some embodiments, the cage material is selected from the group consisting of PEEK, ceramic and metallic. The cage material is preferably selected from the group consisting of metal and composite (such as PEEK/carbon fiber).

[0094] If a metal is chosen as the material of construction for a component, then the metal is preferably selected from the group consisting of titanium, titanium alloys (such as Ti-6Al-4V), chrome alloys (such as CrCo or Cr—Co—Mo) and stainless steel.

[0095] If a polymer is chosen as a material of construction for a component, then the polymer is preferably selected from the group consisting of polyesters, (particularly aromatic esters such as polyalkylene terephthalates, polyamides; polyalkenes; poly(vinyl fluoride); PTFE; polyarylethyl ketone PAEK; polyphenylene and mixtures thereof.

[0096] If a ceramic is chosen as the material of construction for a component, then the ceramic is preferably selected from the group consisting of alumina, zirconia and mixtures thereof. It is preferred to select an alumina-zirconia ceramic, such as BIOLOX Delta™, available from CeramTec of Plochingen, Germany. Depending on the material chosen, a smooth surface coating may be provided thereon to improve performance and reduce particulate wear debris.

[0097] In some embodiments, the cage member comprises PEEK. In others, it is a ceramic.

[0098] In some embodiments, the first component consists essentially of a metallic material, preferably a titanium alloy or a chrome-cobalt alloy. In some embodiments, the second component consists essentially of the same metallic material as the first plate.

[0099] In some embodiments, the components are made of a stainless steel alloy, preferably BioDur® CCM Plus® Alloy available from Carpenter Specialty Alloys, Carpenter Technology Corporation of Wyomissing, Pa. In some embodiments, the outer surfaces of the components are coated with a sintered beadcoating, preferably Porocoat™, available from DePuy Orthopaedics of Warsaw, Ind.

[0100] In some embodiments, the components are made from a composite comprising carbon fiber. Composites comprising carbon fiber are advantageous in that they typically have a strength and stiffness that is superior to neat polymer materials such as a polyarylethyl ketone PAEK. In some embodiments, each component is made from a polymer composite such as a PEKK-carbon fiber composite.

[0101] Preferably, the composite comprising carbon fiber further comprises a polymer. Preferably, the polymer is a polyarylethyl ketone (PAEK). More preferably, the PAEK is selected from the group consisting of polyetherether ketone (PEEK), polyether ketone ketone (PEKK) and polyether ketone (PEK). In preferred embodiments, the PAEK is PEEK.

[0102] In some embodiments, the carbon fiber comprises between 1 vol % and 60 vol % (more preferably, between 10 vol % and 50 vol %) of the composite. In some embodiments, the polymer and carbon fibers are homogeneously mixed. In others, the material is a laminate. In some embodiments, the carbon fiber is present in a chopped state. Preferably, the chopped carbon fibers have a median length of between 1 mm and 12 mm, more preferably between 4.5 mm and 7.5 mm. In some embodiments, the carbon fiber is present as continuous strands.

[0103] In especially preferred embodiments, the composite comprises: [0104] a) 40-99% (more preferably, 60-80 vol %) polyarylethyl ketone (PAEK), and [0105] b) 1-60% (more preferably, 20-40 vol %) carbon fiber, wherein the polyarylethyl ketone (PAEK) is selected from the group consisting of polyetherether ketone (PEEK), polyether ketone ketone (PEKK) and polyether ketone (PEK).

[0106] In some embodiments, the composite consists essentially of PAEK and carbon fiber. More preferably, the composite comprises 60-80 wt % PAEK and 20-40 wt % carbon fiber. Still more preferably the composite comprises 65-75 wt % PAEK and 25-35 wt % carbon fiber.

[0107] Although the present invention has been described with reference to its preferred embodiments, those skillful in the art will recognize changes that may be made in form and structure which do not depart from the spirit of the invention.

[0108] Alternatively, combinations of cage materials could be beneficial (i.e.,—a ceramic bottom half with a PEEK top half).

[0109] In other embodiments, the components are made from resorbable materials, such as Biocryl Rapide™, a PLA, PLG, TCP composite marketed by DePuy Mitek, located in Raynham, Mass.

[0110] When resorbable materials are selected, Preferred bioresorbable materials which can be used to make the sutures of the present invention include bioresorbable polymers or copolymers, preferably selected from the group consisting of hydroxy acids, (particularly lactic acids and glycolic acids; caprolactone; hydroxybutyrate; dioxanone; orthoesters; orthocarbonates; and aminocarbonates). Preferred bioresorbable materials also include natural materials such as chitosan, collagen, cellulose, fibrin, hyaluronic acid; fibronectin, and mixtures thereof. However, synthetic bioresorbable materials are preferred because they can be manufactured under process specifications which insure repeatable properties.

[0111] A variety of bioabsorbable polymers can be used to make the suture of the present invention. Examples of suitable biocompatible, bioabsorbable polymers include but are not limited to polymers selected from the group consisting of aliphatic polyesters, poly(amino acids), copoly(ether-esters), polyalkylenes oxalates, polyamides, tyrosine derived polycarbonates, poly(iminocarbonates), polyorthoesters, polyoxaesters, polyamidoesters, polyoxaesters containing amine groups, poly(anhydrides), polyphosphazenes, biomolecules (i.e., biopolymers such as collagen, elastin, bioabsorbable starches, etc.) and blends thereof. For the purpose of this invention aliphatic polyesters include, but are not limited to, homopolymers and copolymers of lactide (which includes lactic acid, D-,L- and meso lactide), glycolide (including glycolic acid), ε-caprolactone, p-dioxanone(1,4-dioxan-2-one), trimethylene carbonate(1,3-dioxan-2-one), alkyl derivatives of trimethylene carbonate, δ-valerolactone, β-butyrolactone, χ-butyrolactone, ε-decalactone, hydroxybutyrate, hydroxyvalerate, 1,4-dioxepan-2-one (including its dimer 1,5,8,12-tetraoxacyclotetradecane-7,14-dione), 1,5-dioxepan-2-one, 6,6-dimethyl-1,4-dioxan-2-one, 2,5-diketomorpholine, pivalolactone, χ, χ-diethylpropiolactone, ethylene carbonate, ethylene oxalate, 3-methyl-1,4-dioxane-2,5-dione, 3,3-diethyl-1,4-dioxan-2,5-dione, 6,8-dioxabicycloctane-7-one and polymer blends thereof. Poly(iminocarbonates), for the purpose of this invention, are understood to include those polymers as described by Kemnitzer and Kohn, in the Handbook of Biodegradable Polymers, edited by Domb, et. al., Hardwood Academic Press, pp. 251-272 (1997). Copoly(ether-esters), for the purpose of this invention, are understood to include those copolyester-ethers as described in the Journal of Biomaterials Research, Vol. 22, pages 993-1009, 1988 by Cohn and Younes, and in Polymer Preprints (ACS Division of Polymer Chemistry), Vol. 30(1), page 498, 1989 by Cohn (e.g. PEO/PLA). Polyalkylene oxalates, for the purpose of this invention, include those described in U.S. Pat. Nos. 4,208,511; 4,141,087; 4,130,639; 4,140,678; 4,105,034; and 4,205,399. Polyphosphazenes, co-, ter- and higher order mixed monomer-based polymers made from L-lactide, D,L-lactide, lactic acid, glycolide, glycolic acid, para-dioxanone, trimethylene carbonate and ε-caprolactone such as are described by Allcock in The Encyclopedia of Polymer Science, Vol. 13, pages 31-41, Wiley Intersciences, John Wiley & Sons, 1988 and by Vandorpe, et al in the Handbook of Biodegradable Polymers, edited by Domb, et al, Hardwood Academic Press, pp. 161-182 (1997). Polyanhydrides include those derived from diacids of the form HOOC—C.sub.6H.sub.4—O—(CH.sub.2).sub.m—O—C.sub.6H.sub.4—COOH, where m is an integer in the range of from 2 to 8, and copolymers thereof with aliphatic alpha-omega diacids of up to 12 carbons. Polyoxaesters, polyoxaamides and polyoxaesters containing amines and/or amido groups are described in one or more of the following U.S. Pat. Nos. 5,464,929; 5,595,751; 5,597,579; 5,607,687; 5,618,552; 5,620,698; 5,645,850; 5,648,088; 5,698,213; 5,700,583; and 5,859,150. Polyorthoesters such as those described by Heller in Handbook of Biodegradable Polymers, edited by Domb, et al, Hardwood Academic Press, pp. 99-118 (1997).

[0112] Advantages of the present invention include an ability to have a larger screw diameters. There is no need for a special screw locking mechanism. The screws are locked by the cage design, following a “screw first” concept. There is high reliability in the screw lock. The cage features screw dynamism, so that there is angle flexibility and no bending stress (tension only). The cage allows screw self-adjustment to the cage. The cage may have centrally aligned screws, so there is less risk to veins and arteries. The cage is strongly resistant to axial impact during insertion. The screws are closer to the anterior edge. Lastly, there is a possibility of reducing spondylothesis anteriorly.