Spinal fusion system and method for fusing spinal bones

This invention relates to a spinal fusion system and method for use as a prosthetic implant. The system and method includes a housing dimensioned to be situated between adjacent spinal bones, such as adjacent vertebrae. The housing cooperates with the spinal bones to define a graft area for receiving graft material, which may be inserted anteriorly into the housing during a surgical operation such as a vertebrectomy or discectomy. A housing may have various features such as migration preventers to prevent the housing from migrating posteriorly towards a spinal column and can be used with a cover that permits the housing to “float” relative thereto. Screws are provided in one embodiment and are dimensioned or configured to lock against each other to retain the screws and, consequently, the cover in place.

BACKGROUND OF THE INVENTION

1. Field Of The Invention

This invention relates to a veritable prosthetic system and device and a method for implanting the device and, more particularly, to a spinal fusion system and method for fusing spinal bones.

2. Description of the Related Art

Many types of prosthetic devices have been proposed in the past. For example, U.S. Pat. No. 5,192,327 to Brantagan concerns a surgical prosthetic modular implant used singularly or stacked together to support and fuse together adjacent vertebrae or to totally or partially replace one or more vertebrae in a vertebral column. Other surgical implant devices and methods are shown in U.S. Pat. Nos. 5,192,327; 5,261,911; 5,713,899; 5,776,196; 6,136,002; 6,159,245; 6,224,602; 6,258,089; 6,261,586; 6,264,655; 6,306,136; 6,328,738 and 6,592,586. Some or all of these devices have improved the success rate and have simplified the surgical techniques in inter-body veritable fusion.

Among some of the problems associated with the prior art devices is that after the device is inserted into a patient during a surgical procedure, there was a possibility of retropulsion of the inter-body device and graft material into the spinal cord or other neurological element.

Another problem with the prior art devices is that grafting material, which was inserted into the devices during the surgical procedure, could not easily be inserted from an anterior direction.

Moreover, in some of the prior art devices, the cover, if any, was typically fastened directly to the device and to spinal bones, which prevented the cover from being capable of moving relative to the device. In addition, in devices that used a cover, the cover did not function to both retain the grafting material in the device and simultaneously fix the spinal bones relative to each other.

Another problem with prior art cage systems is that the screws or fasteners which secured the cover onto the cages sometimes had a tendency to unscrew themselves which is undesirable because the graft material may exit the cage or because the cage itself may move. Another problem is that the screws may withdraw, causing injury to local structures by the screws themselves.

What is needed, therefore, is a system and method, which facilitates overcoming one or more of the aforementioned problems as well as other problems and to provide a device that has unique features that will facilitate reducing the risk associated with neurological surgeries and advance the present state of the art.

SUMMARY OF THE INVENTION

It is, therefore, one object of the invention to provide a spinal fusion system and method which utilizes a housing that can be inserted, but comprises features which, for example, enables the device to float relative to a cover, facilitates retaining any graft material within the device, facilitates fixing a relative relation among or between spinal bones, facilitates providing a cover for covering one or multiple devices, and/or includes locking features that facilitates preventing the screws which secure the cover to the spinal bones from the retracting.

Another object of one embodiment is to provide a plurality of screws that are capable of locking to facilitate preventing the fasteners to become unfastened or unscrewed.

Another object of the invention is to provide fasteners at least one of which has an eccentric to facilitate locking against an adjacent fastener in order to retain the fasteners and the cover in a locked position.

In one aspect, this invention comprises a prosthetic implant plate system comprising a plate member having at least one opening defined by at least one cut out, the plate member further comprising a first surface and a second surface, with a distance between the first surface and the second surface defining a thickness of the plate member, the at least one cut out extending through the thickness of the plate member, the plate member further comprising a seat or edge associated with the first surface and the at least one opening for receiving at least one screw after the at least one screw is received in the at least one opening, and a lock system for preventing withdrawal of the at least one screw after the at least one screw is received in an the at least one opening in the plate member and screwed into a spinal bone, the lock system comprising at least one resilient member having a detent resiliently biased in operative relationship with the at least one opening in order to capture the at least one screw between the detent and the seat or edge, the detent having a first portion adapted to engage or be engaged by an engaging surface of a screw head of the at least one screw and to be urged in a direction away from an axis of the at least one screw after a shank of the at least one screw is received in the at least one opening and in response to axial movement of the at least one screw, the first portion of the detent becoming resiliently biased or urged over at least a portion of the screw head after the engaging surface of the screw no longer engages the first portion of the detent and the screw head of the at least one screw is received by the seat or edge, thereby facilitating preventing the withdrawal of the at least one screw from the plate member, the plate member and the at least one resilient member providing a monolithic plate member, the at least one resilient member being resilient to cause the first portion of the detent to move in a first direction away from the axis of the at least one screw in response to the axial movement of the at least one screw and further enabling the first portion to move in a second direction generally opposite the first direction after the detent no longer engages the first portion of the screw head so that the first portion of said detent may become situated over the at least a portion of the screw head.

These and other objects and advantages of the invention will be apparent from the following description, the accompanying drawing and the appended claims.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now toFIG. 1, a partial side view of a patient or person P is shown having a spinal column S and a plurality of spinal bones, such as vertebrae,10,12,14and16. Note that a disc, such as discs18,20and22inFIG. 1, is located between adjacent pairs of spinal bones (e.g., between bones10and12,12and14, and14and16). During a spinal fusion procedure, such as a discectomy, the discs18,20and22may be removed so that adjacent vertebrae may be fused together

FIG. 2illustrates a fragmentary view of the spinal column S shown inFIG. 1, with the discs18,20and22removed. It should also be understood that during another surgical procedure, such as a vertebrectomy, it may be desired to remove part or all of one of the spinal bones10-16, as illustrated inFIG. 13. In this type of neurological procedure, it may also be desired to fuse adjacent spinal bones together for reasons that are conventionally known. This invention provides means for facilitating and performing such procedures. For ease of illustration,FIGS. 15-20provide corresponding anterior views to the side views shown inFIGS. 1-6, respectively.

In the embodiment being described, a spinal fusion system24is provided for use as a prosthetic implant during a neurological procedure such as the aforementioned vertebrectomy or discectomy. In general, after the discs18,20and22(FIG. 1) are removed, as illustrated inFIG. 2, a plurality of receiving areas26,28and30(FIGS. 2 and 17) are defined by the areas between the surfaces of adjacent spinal bones10,12,14and16. As illustrated inFIG. 2, the area26is bounded in part by the surface10aof spinal bone10and surface12aof spinal bone12. Likewise, area28is partially bounded by surface12bof spinal bone12and surface14aof spinal bone14, and area30is bounded by surface14bof spinal bone14and surface16aof spinal bone16.

As illustrated inFIGS. 3-7and11and as will be described in more detail later herein, the spinal fusion system24comprises a housing32dimensioned to be situated or received between adjacent spinal bones, such as bones10and12. A housing32is situated in each of the plurality of receiving areas26,28and30, as illustrated inFIGS. 3-4. Each housing32cooperates with adjacent spinal bones to define a graft area, such as areas34,35and36in the view illustrated inFIG. 17, for receiving graft material38(FIGS. 4 and 18). As illustrated inFIGS. 4 and 18, the graft material38is situated in the areas34,35and36after placement of the housing32.

As illustrated inFIG. 11, the housing32is generally U-shaped as shown. In the embodiment being described, the housing32comprises a well33defining multiple sides and comprising a predetermined shape selected to cause the graft material to be formed into a multi-sided fused coupling between adjacent spinal bones, such as bones10and12inFIG. 3. Although not shown, the housing32could define a shape other than rectangular, such as semi-circular, oval or other suitable shape as may be desired. Note that the housing32comprises a first wall32a, a second wall32band a third wall32cjoining the first wall32aand the second wall32b. One or more of the walls32a-32cmay comprise a plurality of holes or apertures40which facilitate the fusing process. The apertures40also permit visualization of graft material30on x-rays.

As mentioned later herein, the predetermined shape defined by the spinal fusion system24may provide a fused multi-sided plug of fusion material32having a height H (FIGS. 14 and 16) of at least two millimeters, but typically less than approximately 180 millimeters. This height H may vary depending on the vertical size or height H (as viewed inFIG. 16) of the areas26-30to be filled. For example, in the area26illustrated inFIGS. 2,14and16, the height H of the area26generally corresponds to a height H1(FIG. 1) of a disc, such as disc18. Thus, the fusion material38(FIG. 18) would resultantly have a fused height H2(FIG. 18) that generally corresponds to the height H (FIG. 16) and height H1(FIG. 1). If, for example, a housing32having a longer height is required, such as height H3inFIG. 14and height H4inFIG. 13, such as in the event of a vertebrectomy, then the fusion system24and housing32will define a height that generally corresponds to the dimension or height H (FIG. 9) to be traversed. Thus, it should be understood that the dimensions of the generally U-shaped housing32of the spinal fusion system24is selected depending on the size of the area26-30to be filled and the environment or application in which the spinal fusion system24is used. In general, the width and depth of the housing32will be approximately 9-20 millimeters and 7-20 millimeters, respectively.

As illustrated inFIGS. 5-7,11,14and21-22, the spinal fusion system24further comprises a cover42comprising a plurality of apertures44that receive a plurality of screws46, respectively, which are screwed directly into the spinal bones10and16, as illustrated, for example, inFIGS. 5-6.

As illustrated inFIG. 11, the housing32comprises a first rail, channel wall or wall portion48and a second rail, channel wall or wall portion50which cooperate to define a channel area52for receiving the cover42. It should be understood that when the cover42is received in the channel52, the sides42aand42bbecome associated with the sides48aand50a. It should be understood that the cover42is not permanently secured to the housing32after it is received in channel area52. This feature permits the housing32to migrate or float relative to the cover42even after the cover42is fixed to one or more of the spinal bones10-16as illustrated inFIGS. 6 and 20. As illustrated inFIG. 23, the edges42aand42bof cover42and sides48aand50amay be beveled and complementary to facilitate locating and mating engagement between the cover42and housing32.

As illustrated inFIGS. 3-6and16-20, after the graft material38is placed in the housing32and the graft areas35-36(FIG. 17) defined by the housing32and adjacent spinal bones, then the cover42is situated between the walls or rails48and50, as illustrated inFIGS. 6 and 19. The screws46may then be used to secure the cover42to one or more of the spinal bones10-16as illustrated inFIGS. 6 and 20. It should be understood that a feature of the invention is that the cover42facilitates aligning the housings32in a substantially co-lineal or relatively aligned position relative to each other and to the spinal bones10-16, as illustrated inFIGS. 6,19and20. In the setting of multiple level discectomy, the floating cover42allows limited, controlled settling of the cages or housings32in the vertical plane with respect to the cover42. As illustrated inFIGS. 6,8,10and20, the cover42also provides means for providing a mechanical fixation of the adjacent spinal bones10-16relative to each other. Thus, while the housing32cooperates with adjacent spinal bones, such as spinal bones10and12, to define a graft receiving area34, the cover is multi-functional in that it not only covers the opening of any graft areas, such as area34(FIG. 17), but it also secures and retains the spinal bones10-16in a fixed spatial relationship relative to each other and relative to the housings32. It should also be understood that the cover42may be fixed to one or more of the spinal bones10-16as may be desired to accomplish either of the aforementioned functions.

As illustrated inFIG. 11, note that the walls48and50further define projections48b,48c,50band50cas shown. As illustrated inFIGS. 3-6and17-20, the projections48b,48c,50band50cprovide a plurality of migration preventers for preventing the housing32from migrating posteriorly in the direction of arrow A (FIG. 3) toward the spinal cord S or other neurological elements after the housing32is situated between the adjacent spinal bones10-16as illustrated. Further, the migration preventers48b,48c,50band50cenable a surgeon to locate each housing32between adjacent spinal bones, such as spinal bones10-16inFIG. 1, and move the housing32in the direction of arrow A inFIG. 3until the migration preventers48b,48c,50band50cengage the surface10aof spinal bone10and migration preventers48b,48c,50band50cengage the surface12aof spinal bone12. As illustrated inFIG. 3, after the housings32are situated between the spinal bones10-16as shown, the migration preventers48b,48c,50band50cfacilitate preventing the wall32cfrom being over-inserted by the surgeon or from being over-inserted to a point where it engages the spinal cord S or other neurological elements.

The spinal fusion system24further comprises at least one migration stop or crossbar60as illustrated inFIGS. 11,12,29and30. The crossbar60may be either integrally formed in housing32, as shown inFIG. 26, or separate as illustrated inFIGS. 7,11,12,14,29, and30, for example. As illustrated in the exploded view inFIGS. 10 and 11, the surface60aof crossbar60engages and cooperates with surface42cof cover42to prevent anterior migration in the direction of arrow B. Thus, the spinal fusion system24of the embodiment being described provides means for preventing insertion of the housing32to a point where it might engage the spinal cord S (FIG. 3) or other neurological elements, such as dura mater, thecal sac, and also means for facilitating prevention of migration of the housing32in an anterior direction or in the direction of arrow B inFIG. 10after the housing32is situated as described herein and the cover42is mounted to one or more of the spinal bones10-16.

It should be understood that a plurality of the migration stops or cross bars60may be used alone or in combination with the migration preventers48b,48c,50band50c.It should be understood that the stops60could be detachable, as shown inFIG. 26, or they could be integrally formed in housing32(as shown inFIG. 26). Also, these cross bars60may be removably received in the notched receiving areas94(FIGS. 29-30). For example, in anatomy that provided limited space, the surgeon may elect not to use a housing with cross bars60or use a housing that does not have integrally formed cross bars.

The system24further comprises a system or means for preventing retraction or back out of the screws46after they are screwed into the spinal bones10-16in order to secure the cover42thereto. The spinal fusion system24of the present invention may be used with conventional screw lock devices or with a unique locking mechanism and system, which will now be described relative toFIGS. 21-23.

As illustrated inFIGS. 21-23, the spinal fusion system24and, more particularly, cover42may be provided with at least one or a plurality of resilient detents62which are generally L-shaped as shown and are resilient so that they can move laterally in the direction of double arrow C inFIGS. 21-22towards and away from a home position (FIG. 21) to permit the screws46first received in the apertures44, and, second, locked into the cover42. Thereafter, the screws46may be screwed into a spinal bone, such as spinal bone10, and when a screw head46aof the screw46engages a detent portion62aof the resilient lock62, the resilient lock62moves in a direction away from the apertures44until the screw head46aclears the portion62a. After a top surface46bof the screw head46ahas cleared the bottom surface62a1(as viewed inFIG. 23) of portion62a, the resilient lock62moves back toward aperture44to the home position until the portion62aand surface62a1are operatively positioned over surface46bof screw46, thereby retaining and preventing the screws46from backing out of the cover42and thereby preventing the screws46from backing out of the spinal bone10.

The plate comprises a first surface50d(FIG. 23) and a second surface50e. The plate member further comprises a seat or edge50f(FIG. 21) associated with the first surface50dand the detent62aassociated with the second surface50e.

In the embodiment being described, the components of the spinal fusion system24, such as the housing32, first channel wall portion48and second channel wall portion50, crossbar60, cover42and screws46may be made of any desired composition or material such as a polymer, composite polymer, titanium, stainless steel, carbon fiber or other suitable material.

A method for fusing spinal bones together will now be described relative toFIG. 22. It should be understood that this procedure may be used during a vertebrectomy or discectomy or other neurological procedure during which it is desired to fuse spinal bones together. For ease of illustration, the embodiment will be described as used during a discectomy procedure during which the discs18-22(FIG. 1) are removed so that spinal bones10-16may be fused together. The procedure begins by situating a patient P on an operating table (not shown) and providing an appropriate incision as conventionally known to expose the spinal bones such as the bones10-16illustrated in the side view shown inFIG. 1and in the anterior view illustrated inFIG. 15. (Block70inFIG. 22). At Block72, the vertebrae or discs, such as discs18-22inFIGS. 1 and 15, are surgically removed revealing the areas26-30inFIGS. 2 and 16. At Block74, the housings32are inserted in the direction of arrow A (FIG. 3) into the areas26,28and30until the migration preventers40b,48c,50cand50dengage the surfaces of the spinal bones10-16, such as the surfaces10aand12aillustrated inFIG. 3. (Block74inFIG. 22). As mentioned earlier herein, the migration preventers facilitate preventing inserting the housing32to a point which would cause the wall32cto engage the spinal column S.

As illustrated inFIGS. 3 and 17, the housing32cooperates with adjacent spinal bones, such as bones10and12to define the graft receiving area or cavity34in which the graft material38(FIG. 4) may be inserted. As mentioned earlier herein, these graft areas34-36may comprise a shape which is generally rectangular, as defined by the shape of the housing32, but it could comprise another shape by simply providing a housing32having a different predetermined shape. Thus, the housing32may be provided in a circular or arcuate shape in which case the graft area34would define a generally circular or arcuate area, which would cause the graft material to form a similar shape. Other curved or multi-sided shapes may be defined by providing an appropriately or correspondingly shaped housing32, depending on the selected or desired shape that the physician would like the fused graft material38to assume after it has fused to the adjacent spinal bones.

At Block76, the graft material48is inserted and at Block78, the cover42is situated in the slot or area52defined by the walls48and50. As mentioned earlier herein, the cover42facilitates covering the openings, such as openings34aand36aof the graft areas34and36, respectively. The surgeon secures the cover42to one or more of the bones, as illustrated inFIGS. 5-6and19-20and then closes the patient (Block80).

Again, and as mentioned earlier, a feature of the invention is that it provides a fixing system for fixing the location of the bones12-16relative to each other. Simultaneously, the system24permits the housing32to “float” between adjacent bones, such as bones10and12inFIGS. 3 and 6. This is advantageous for reasons mentioned earlier herein. Another advantage on this feature of the invention is that if it is necessary to operate on the same patient at a later time (Block82inFIG. 24) and, for example, add one or more housings32in order to fuse other spinal bones together, then the cover42can simply be removed at a later time, another discectomy or vertebrectomy performed and another housing32inserted. Another cover42, or perhaps a second cover may then be used to seal the additional housing32after it is situated in the manner described herein. Thus, this invention provides a system and method, which is flexible and will permit the addition or insertion of additional housings32of the same or different sizes during a second operating procedure as illustrated in Block82.

FIGS. 1-8and15-20illustrate the general procedure and use of the invention in an illustrative discectomy wherein three discs are removed, replaced with housing32, and graft material38inserted as described and cover42situated and mounted as described herein. In the illustration shown inFIGS. 1-8and15-20, three discs18-22are removed and the spinal bones12-16are fused together using the system and method as shown and described. It should be appreciated, however, that this system and method may be used with fewer or more housings32and with one or a plurality of covers42as may be desired or required. For example, if only one of the discs18-22needed to be excised and only two of the spinal bones10-16fused together, then only one housing32and cover42may be necessary. Likewise, as mentioned earlier herein, the housings32may comprise a different dimension or different height H (FIG. 14) to span a greater area, such as the area H4illustrated inFIGS. 13 and 14. For example,FIGS. 13 and 14illustrate a vertebrectomy wherein the spinal bone12has been removed along with the disc between spinal bones14and16. This provides areas80and81in which an elongated housing32′, such as the housing32′ illustrated inFIG. 14may be inserted. After the housings32and32′ are inserted between the spinal bones10-14and14-16as shown inFIG. 13, graft areas82and84are provided for receiving the graft material38. As illustrated inFIG. 13, the cover42would have a corresponding elongated shape for fixing the bones10and14together and for covering both openings82and84or housings32and32′.

It is also anticipated that the invention may be used in a multitude of procedures, such as a vertebrectomy (FIGS. 8 and 9), discectomy (FIGS. 1-7,13-20, or even a combination of a vertebrectomy and discectomy as illustrated inFIGS. 13-14. As mentioned and described earlier herein, a combination of different sizes of housings32and covers42may be used as shown. Although it is preferred that a single cover42be used, it may be desired in some applications to use multiple covers42, such as where the removed discs are not adjacent.

In the illustrations being described, the housings32comprise the crossbar60which cooperate with the cover42to prevent anterior migration of the housing after the screws46are secured to the spinal bones as illustrated inFIGS. 6,9and13.

FIGS. 25-30illustrate other embodiments of the invention. InFIG. 25, a generally U-shaped housing32is provided without the walls48and50or crossbar60. This embodiment may be useful. This may be useful if it were desired to insert housing32in local anatomy so that it could be loaded from the side or laterally, rather than anteriorly, as previously described.

InFIG. 26a housing32′″ is provided with the crossbars60, but without the walls48and50. In this embodiment it may be useful to use such a housing design when the local anatomy provides limited space.

FIG. 27illustrates yet another embodiment of the invention illustrating a housing32that is provided with a plurality of protrusion86,88,90and92that do not span completely between the walls32bor32atogether but yet provide the protrusions86-92which will engage the cover42if the housing attempts to migrate anteriorly as described earlier herein.FIGS. 1-24,29and30show embodiments of the invention where the crossbars60are not integrally formed with the housing32, but received in the notched areas90as shown. As mentioned earlier, the crossbars60may be separate or may be integrally provided with the housing32. Providing detachable crossbars60, such as is shown in the embodiments illustrated inFIGS. 25,28and29, enable the walls32aand32bto flex towards and away from each other. The housing32may be provided with a malleable material in which case the surgeon can change the general U-shape of the housing32to accommodate the size or shape of the areas34and36(FIG. 17). In the embodiment described, housing32and cover42may be made of titanium, polymer or a bioresorbable material.

FIG. 31illustrates the walls48and50having notched areas49and51for receiving the cover42which is dimensioned to fit, thereby eliminating the need for cross bars60.

FIGS. 32-36illustrate another embodiment of the invention. In this embodiment, those parts that are the same or similar to the parts illustrated inFIGS. 1-30are identified with the same part number except that the parts inFIGS. 31-36have an apostrophe (“'”) mark added thereto.

In this embodiment, the cage system10comprises a cover400for situating in the channel area52(FIG. 11) to facilitate preventing interior migration of the graft material38. In order to secure the cover400over the graft area38, a locking system, means and method are provided for retaining the cover400on the housing32. In the embodiment being described, the locking system402comprises a plurality of screws, fastening means or fasteners404,406,408and410that are received in openings, such as openings405in the cover plate400as shown. Note that the fasteners404-410comprise a plurality of female openings or slots404a,406a,408aand410afor receiving a tool, such as a hex wrench for tightening and loosening the fasteners404-410.

In the embodiment being described, the fasteners404and408comprise a head404band408bthat have a planar or flat portion404b1and408b1as shown. As best illustrated inFIGS. 34 and 35, note that the fasteners408and410each comprise threads or a threaded portion, such as threads or portions408cand410cof fasteners408and410, respectively. Note that a distance or small radius D1between center C1and edge,408b1inFIG. 34is smaller than the distance or large radius D2measured by the distance between center C1and edge408b1inFIG. 34. The difference in the distances D1and D2facilitates defining a cam surface or lobe on the wall408b2whose use and purpose will be defined later herein.

In the embodiment being described, one or more of the heads404b,406b,408band410bmay comprise an indicia, such as a grind mark or other indicator412and414(FIG. 32), to facilitate and assist a user, such as a doctor, to identify the small radius portion D1during a surgical procedure. Thus, the indicia412and414facilitate defining the surface associated with the flat portion, such as portion404b1.

It should be understood that when the pairs of fasteners404-406and408-410are aligned such that the surfaces404b1and408b1and short or small radius portion D1are situated in opposite or closest to wall406bof screw406and wall410bof screw410the adjacent fasteners406and410respectively, may be rotated and screwed into, for example, vertebrae10, which will secure and retain the cover400over the graft area38. Although not shown, the locking system of the present invention may comprise eccentric fasteners of screws having eccentric heads (i.e. where a head center is offset from a thread axis) and fasteners that are used with non-eccentric fasteners. For example, and as illustrated inFIG. 33, fasteners404and408may comprise the aforementioned eccentric, while adjacent fasteners406and410, respectively, may be non-eccentric fasteners or screws.

In any event, the small radius portion D1permits the adjacent fastener or screw such as screw410, whether it has an eccentric or not, to be turned when the small radius portion D1or flat portion408b1is situated in opposed relationship to the adjacent screw (as illustrated inFIGS. 32 and 34). For example,FIG. 34illustrates that when the fasteners are aligned such that the indicia414are aligned as illustrated inFIG. 32, a gap G exists between the portions408d1and wall410dof screw410as shown. The gap G permits either or both of the fasteners408and410to rotate in either a counter-clockwise or clockwise direction during fastening and unfastening of the fasteners to the vertebrae as described earlier herein with the prior embodiments.

When it is desired to secure the cover400over the housing32, the fasteners404-408are placed in the cover and aligned as illustrated inFIG. 32. The fasteners404-408are rotated and screwed into vertebrae10in a clockwise direction until it is seated. These fasteners404and408are then “backed out” less than a full turn until flat surface404b1and408b1are aligned as shown inFIG. 32. The surgeon may use the indicator412and414to perform this alignment. This alignment presents the gap G (FIG. 34), which permits the fastener410to be rotated in a clockwise direction until completely screwed into vertebrae10.

Next, the adjacent fastener (406for the404-406pair and410for the408-410pair) is inserted into opening405in cover400and inFIG. 32 and 34until they are fully seated into the vertebrae10. For example, in the illustration shown inFIGS. 32 and 34, the fastener408is rotated in a clockwise direction with a tool, such a hex wrench (not shown) until it is fully seated into the vertebrae10.

The fastener408is again rotated in the clockwise direction (as viewed) until the large radius portion D2and the wall portion408b2engages and comes against the wall410bof the fastener410.

It should be appreciated that when the fasteners404-410are secured in the locked position in the manner described, they facilitate retaining themselves in the locked position. For example, if fastener410begins to rotate in a counterclockwise direction (as viewed inFIG. 33) it will cause fastener408to rotate in a clockwise direction which, in turn, causes fastener408to tighten and resist the counterclockwise rotation of fastener410. If fastener410would rotate, fastener408would screw deeper into the vertebrae10.

FIG. 36is an illustration similar toFIG. 11showing the orientation and alignment of the cover400and fasteners404-410to the housing32.

Advantageously, the various embodiments of the invention illustrated inFIG. 1, provide a system and method for inserting graft material32into a graft area34and36(FIG. 17) to fuse a plurality of bones such as bones10-18together. The system and method also provide means for fixing the bones10-18relative to each other, while permitting the housing32to cooperate with adjacent bones10-18to define a graft area34and36(FIG. 17) and to also float relative to the cover42. The locking system illustrated inFIGS. 21-23further facilitates providing a locking system that does not require the use of any tools, yet prevents back out of the screws46.

While the apparatus and method described constitute preferred embodiments of this invention, it is to be understood that the invention is not limited to this precise apparatus and method, and that changes may be made in either without departing from the scope of the invention, which is defined in the appended claims.