US20020045945A1 - Expandable interbody fusion cage - Google Patents

Abstract

An expandable interbody fusion device in one embodiment includes a cylindrical body (10) defining a hollow interior (17) for receiving bone graft or bone substitute material. The body (10) is divided into a number of branches (24, 26, 40and41) connected to one another at a fixed end (20) and separated at an expandable end (18). The expandable cage may be inserted in its substantially cylindrical form and may be expanded by movement of an expansion member (50) to establish lordosis of the spine. The present invention provides an expansion member (50) that interacts with the interior surfaces of the device to maintain the cage in the expanded condition and provide a large internal chamber (17) for receiving bone in-growth material. Methods for insertion of the fusion device are also disclosed.

Description

BACKGROUND OF THE INVENTION
• The present invention relates to an implantable device for promoting fusion between two adjacent bony structures and a method of inserting the same. More particularly, the invention relates to an expandable fusion cage that may be inserted, in a reduced size configuration, into an intervertebral space and expanded after insertion to provide a desired size. While the device according to the present invention may have application in other areas of the body, the present invention is preferably utilized for vertebral interbody fusion.[0001]
• There have been an extensive number of attempts to develop an exceptional intradiscal implant that could be used to replace a damaged disc and yet maintain the stability of the disc interspace between adjacent vertebra, at least until complete arthrodesis is achieved. These “interbody fusion devices” have taken many forms. For example, one of the more prevalent designs takes the form of a cylindrical implant. These types of implants are presented by the patents to Bagby, U.S. Pat. No. 4,501,269; Brantigan, U.S. Pat. No. 4,878,915; Ray, U.S. Pat. Nos. 4,961,740 and 5,055,104; and Michelson, U.S. Pat. No. 5,015,247. In the cylindrical implants, the exterior portion of the cylinder can be threaded to facilitate insertion of the interbody fusion device, as represented by the Ray, Brantigan and Michelson patents. In the alternative, some of the fusion implants are designed to be pounded into the intradiscal space. This type of device is represented by the patent to Brantigan.[0002]
• Interbody fusion devices can be generally divided into two basic categories, namely solid implants and implants that are designed to permit bone in-growth. Solid implants are represented by U.S. Pat. Nos. 4,879,915; 4,743,256; 4,349,921; and 4,714,469. The remaining patents discussed above include some aspect that allows bone to grow across the implant. It has been found that the devices that promote natural bone in-growth achieve a more rapid and stable arthrodesis. The device depicted in the Michelson patent is representative of this type of hollow implant which is typically filled with a bone growth inducing substance to promote bone growth into and through the device. This implant includes a plurality of circular apertures which communicate with the hollow interior of the implant, thereby providing a path for tissue growth between the vertebral end plates and the bone growth material within the implant. In preparing the intradiscal space, the end plates are preferably reduced to bleeding bone to facilitate the tissue in-growth. During fusion, the metal structure provided by the Michelson implant helps maintain the patency and stability of the motion segment to be fused. In addition, once arthrodesis occurs, the implant itself serves as a sort of anchor for the solid bony mass.[0003]
• One problem that is not addressed by the above prior devices concerns maintaining and restoring the normal anatomy of the fused spinal segment. Naturally, once the disc is removed, the normal lordotic or kyphotic curvature of the spine is eliminated. With the prior devices, the need to restore this curvature is neglected. For example, adjacent vertebral bodies may be reamed with a cylindrical reamer that fits the particularly implant. In some cases, the normal curvature is established prior to reaming and then the implant inserted. However, this over-reaming of the posterior portion is generally not well accepted because of the removal of load bearing bone of the vertebrae and because it is typically difficult to ream through the posterior portion of the lower lumbar segment where the lordosis is the greatest. In most cases using implants of this type, no effort is made to restore the lordotic curvature so that the cylindrical implant is likely to cause a kyphotic deformity as the vertebrae settles around the implant. This phenomena can often lead to revision surgeries because the spine becomes imbalanced.[0004]
• In each of the above-listed patents, the transverse cross-section of the implant is substantially constant throughout its length and is typically in the form of a right circular cylinder. Other implants have been developed for interbody fusion that do not have a constant cross-section. For instance, the patent to McKenna, U.S. Pat. No. 4,714,469 shows a hemispherical implant with elongated protruberances that project into the vertebral end plate. Further, U.S. Pat. No. 5,669,909 to Zdeblick et al., shows a truncated conical implant adapted to be threadedly received in the intervertebral space. However, these devices require an opening at least as large as the largest segment of the device. The requirement for such a relatively large opening may limit the use of such devices, particularly where access to the spine is limited due to obstructing vessels and neurological structures.[0005]
• Still further implants have been developed that provide the ability to adjust the size of the implant after insertion. U.S. Pat. Nos. 5,665,122 to Kambin, 5,554,191 to LaHille et al., and 5,653,763 to Errico et al., disclose implants which provide at least some degree of adjustability of the height of the implant to restore lordosis. However, these implants do not allow the device to be easily and securely inserted into a disc space and the internal expansion mechanism limits the ability to pack the interior with a large amount of bone in-growth material.[0006]
• In view of the limitations of the prior devices, there remains a need for an interbody fusion cage capable of stabilizing the spine in a manner comparable to interbody implant designs presently in use, and at the same time providing a mechanism for restoring normal lordosis of the spine. After expansion, the implant should have an internal cavity adapted to receive bone graft or bone substitute to encourage bone growth through the expanded implant.[0007]
SUMMARY OF THE INVENTION
• In response to the needs still left unresolved by the prior devices, the present invention contemplates an expandable fusion cage adapted to be inserted between a pair of vertebral bodies to restore the normal angular relation between adjacent vertebrae. In particular, a device according to the present invention comprises a body having an outer bone engaging surface and an inner surface defining an internal chamber and a retaining mechanism. The body has a fixed portion and an expandable portion. The expandable portion is divided into at least a first branch and a second branch, interconnected at the fixed portion. The first branch is moveable with respect to the second branch in the expandable portion to expand the overall size of the fusion cage. An expansion member is sized to be at least partially received within the internal chamber and has an outer surface adapted to engage the retaining mechanism. Movement of the expansion member within the internal chamber urges the first branch to move with respect to the second branch thereby expanding the expandable portion. The expansion member is retained in position by engagement with the retaining mechanism of the body.[0008]
• In another aspect of the invention, an expandable intersomatic cage has a tubular body having an outer bone engaging surface and an inner surface defining an interior chamber. The inner surface further defines an inclined surface and a retaining mechanism. The body defines a fixed portion and a moveable portion, the inclined surface is disposed adjacent the moveable portion. In this aspect of the invention, an expansion member is included having an outer surface configured to engage the inclined surface and the retaining portion. Movement of the expansion member against the inclined surface moves the moveable portion to expand the cage. The expansion member is retained in position by engagement with the retaining mechanism.[0009]
• In another embodiment, the invention contemplates an insertion tool cooperable with an expandable fusion implant. The insertion tool is operable to insert the implant and move the expansion member to expand the cage. A cage according to the present invention is utilized with an insertion tool having an outer sleeve adapted to engage the body of the cage to transmit compressive force to the cage. The insertion tool further includes an inner shaft moveably disposed within the outer sleeve. The inner shaft is sized to be inserted into at least a portion of the interior chamber of the cage. The inner shaft has a distal end configured to removably engage the expansion member, whereby movement of the inner shaft with respect to the outer sleeve moves the expansion member in relation to the cage.[0010]
• In another aspect of the invention, methods are provided for insertion of an expandable intersomatic device. In one aspect a device according to the present invention is inserted into the disc space. The moveable portion of the device is positioned adjacent a portion of the vertebra requiring additional spacing. The expansion member is then moved within the internal chamber and against the moveable portion to expand the cage to the desired height. The expansion member may be engaged with the retaining mechanism to limit movement of the expansion member.[0011]
• One object of the present invention is to provide an implant that has a reduced size insertion configuration and is expandable from the insertion configuration to a larger configuration.[0012]
• Another object of the present invention is to provide an expandable implant that has a substantially unobstructed interior chamber to receive bone growth promoting material.[0013]
• Still another object of the present invention is to provide a fusion cage configured for easy insertion and expandable to a larger size to establish lordosis.[0014]
• Yet a further object of the present invention is to provide an improved method for inserting a fusion cage and restoring lordosis.[0015]
• Related objects and advantages of the present invention will be apparent from the following description.[0016]
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a top view of an expandable cage according to one embodiment of the present invention.[0017]
• FIG. 2[0018]ais a side cross-sectional view of the device of FIG. 1.
• FIG. 2[0019]bis the device of FIG. 2awith the inclusion of an expansion wedge according to one embodiment of the present invention.
• FIG. 3 is a partial cross-sectional perspective view of the expandable cage of FIG. 1 without an external thread pattern.[0020]
• FIG. 4 is a perspective view of the expansion wedge of FIG. 2[0021]b.
• FIG. 5 is an end view of the expansion wedge of FIG. 4.[0022]
• FIG. 6 is an elevational view of an insertion tool according to one embodiment of the present invention.[0023]
• FIG. 7[0024]ais a sagittal plane view showing a partial cross-sectional side view of the expandable cage of FIG. 2binserted between two adjacent vertebrae in an insertion configuration according to one embodiment of the present invention.
• FIG. 7[0025]bis the cage of FIG. 7ashown in an expanded position according to one embodiment of the present invention.
• FIG. 8 is a top view of an alternative embodiment of the expandable cage of FIG. 1.[0026]
• FIG. 9 is a side cross-sectional view of the expandable cage of FIG. 8.[0027]
• FIG. 10 is a partial cross-sectional perspective view of the expandable cage of FIG. 8 without an external thread pattern.[0028]
• FIG. 11 is a top view of a further embodiment of an expandable cage according to the present invention.[0029]
• FIG. 12 is a side partial cross-sectional view of the expandable cage of FIG. 11.[0030]
• FIG. 13 is a partial cross-sectional perspective view of the expandable cage of FIG. 11, without an external thread pattern.[0031]
• FIG. 14 is a side partial cross-sectional view of a further embodiment of the present invention.[0032]
• FIG. 15 is a side partial cross-sectional side view of yet a further embodiment of the present invention.[0033]
DESCRIPTION OF THE PREFERRED EMBODIMENTS
• For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated devices, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates.[0034]
• Referring now to FIGS. 1 through 3, there is shown a first embodiment of an[0035]expandable cage10 in accordance with the present invention. In this embodiment of the invention,expandable cage10 has a cylindricalouter surface11 defining an external thread pattern12 (not shown in FIG. 3) adapted to engage two adjacent vertebra (see FIGS. 7aand7b) and to advance the cage into the disk space ascage10 is rotated aboutlongitudinal axis13. As shown most clearly in FIG. 3,expandable cage10 is substantially hollow withinner surface21 defining aninternal cavity17.Expandable cage10 includes a bone in-growth window16 formed throughexpandable branch24 and an identical bone in-growth window19 formed throughexpandable branch26. These in-growth windows are adapted to permit communication between the vertebral bodies (FIG. 7) andinternal chamber17. In most application, bone growth promoting material will be placed withininternal chamber17 ofexpandable cage10 to encourage bone to grow into and throughfusion device10.
• As shown in the accompanying drawings,[0036]expandable cage10 is preferably composed of four separate branches, each separated atexpandable end18 by a channel extending longitudinally fromexpandable end18 toward fixedend20. Referring now to FIG. 2a, firstexpandable branch24 is separated from first fixedbranch40 bychannel14. In a similar manner, secondexpandable branch26 is separated from first fixedbranch40 bychannel22. Each ofchannels14 and22 extends fromexpandable end18 towards fixedend20. The channels terminate in a slightly larger diameter radiused opening which preferably acts as a hinge during expansion of the device to concentrate stress and deformation adjacent fixedend20. In a similar fashion, as shown in FIG. 3, secondfixed branch41 is separated from firstexpandable branch24 bychannel15 and from secondexpandable branch26 by a similar channel (not shown) Thus,expandable cage10 is formed by four branches, each separated from the other atexpandable end18 by channels extending fromouter surface11 tointernal chamber17. The branches are connected atfixed end20 by linkingarea44 such that each of the branches may move substantially independent from each other atexpandable end18 while remaining connected to the device by linkingarea44. Although the present embodiment utilizes integrally formed branches, it is contemplated that separate components may be joined to form the expandable cage without deviating from the invention.
• While four separate branches are shown in a preferred embodiment, it is contemplated that more or less branches could be utilized without deviating from the spirit or scope of the invention. Moreover, although in a preferred embodiment the channels extend from[0037]outer surface11 tointernal chamber17, it is contemplated that the channels may not extend to the interior chamber. Such a channel may be formed by an overlapping interface between two adjacent branches without creating an opening for bone in-growth intointernal chamber17. Further, although channels are shown as being preformed in the expandable cage, it is contemplated that the channels may be formed as the implanted is expanded. By way of example, and without limitation, this could occur by rupturing a frangible portion between adjacent branches or by deforming material disposed between adjacent branches.
• In one aspect of the invention,[0038]internal chamber17 comprises the majority of the volume of theentire cage10. Specifically referring to FIG. 2a,external surface11 defines adiameter63, excludingthreads12.Internal chamber17 has a diameter of at least64, that isdiameter63 less twice thebranch thickness62.
• In a preferred embodiment,[0039]branch thickness62 is selected such that the volume ofinternal chamber17, particularly with the inclusion of voids created by in-growth windows16 and19 and the various channels occupies the majority of the entire volume ofcage10. Thus, the present invention provides space for a large volume of bone growth promoting material to be inserted into the device to promote bone in-growth.
• [0040]Adjacent expansion end18, as shown in FIG. 2awith respect tobranches24,40 and26, each of the branches includesinclined surfaces28,34 and30, respectively. Adjacent the internal termination of each of theinclined surfaces28,34 and30, is formed aninternal shoulder36,42 and38, respectively. Referring to FIG. 3,branch41 includes a similarinclined surface35 andinternal shoulder43. As shown in FIG. 2a, arcuateinclined surfaces28 and30 are inclined atangle32 with respect to each other. In a preferred embodiment, this angle is approximately 96°, although it is understood that a variety of angles could be utilized depending on the amount of expansion desired, and the distance an expansion member will need to travel to create the expansion.
• Referring now to FIG. 2[0041]b,cage10 is shown with the inclusion ofexpansion wedge50 disposed adjacentexpandable end18.Expansion wedge50 is further illustrated in FIGS. 4 and 5.Expansion wedge50 includes first and second opposed expansion wedge inclines52 and54, which have a tapering arcuate surface that mates with and corresponds toinclined surfaces28 and30 ofbranches24 and26, respectively.Wedge50 includesside walls58 and60 with substantially planar surfaces adapted to engageinclined surfaces34 and35 of fixedbranches40 and41, respectively. Further,expansion wedge50 includes acentral opening56 which is adapted to receive a driving tool. In a preferred embodiment,central opening56 is threaded to receive a correspondingly threaded end of a driving tool.
• It will be understood that as[0042]wedge50 is advanced toward fixedend20, inclined surfaces52 and54 act uponinclined surfaces28 and30, respectively, to urgebranches24 and26 apart from each other atexpandable end18. Asexpansion wedge50 is inserted alonginclined surfaces28 and30,branches24 and26 will tend to expand atexpansion end18 as deformation occurs adjacent fixedend20. Substantiallycontinuous linking material44 links all of the branches and does not permit their expansion atfixed end20. Aswedge50 is further advanced toward fixedend20, the wedge is pushed beyondshoulders36 and38, such thatwedge50 is captured withincage10. As a result of the engagement ofback surface55 of the wedge againstshoulders36 and38,expansion wedge50 is prevented from being expelled from the cage, and the cage is maintained in an expanded condition with the wedge securely held in position. It will be understood that the engagement ofplanar surfaces58 and60 on fixedbranches40 and41, respectively, tends to deformably expand these branches little, if any, and therefore they remain substantially fixed in their original positions. Further, fixedbranches40 and41 each include shoulders42 and43 which engage theback surface55 ofwedge50 once it has passed beyond the shoulders. Thus, fixedbranches40 and41 also tend to hold the wedge in place and prevent its expulsion frominternal chamber17.
• Referring now to FIGS. 8 through 10, there is shown an alternative embodiment of the expandable cage of the present invention. In this embodiment,[0043]expandable cage310 has anouter surface311 and an external thread pattern312 (not shown in FIG. 10). As with the embodiment of FIG. 1,expandable cage310 includes two opposedexpandable branches324 and324, and two opposed fixedbranches340 and341 joined atfixed end320. Each of these branches define inclined surfaces and interior shoulders adjacentexpandable end318 adapted to engage and receiveexpansion wedge350. In contrast to the embodiment of FIG. 1,expandable branch324 includes two bone in-growth windows316 and317, separated byrib330. In a similar manner,expandable branch326 includes two bone in-growth windows322 and323 separated byrib332. The use of a plurality of bone in-growth windows in the expandable branches increases the overall strength of the branch which may be necessary for longer cages or cages constructed of relatively weak materials. While this embodiment has been shown with two windows per each expandable branch, it is contemplated that more than two bone in-growth windows may be utilized without deviating from the spirit and scope of the invention.
• Referring now to FIG. 6, there is shown an[0044]insertion tool70 suitable for use with an expandable cage according to the present invention.Insertion tool70 includes anouter sleeve72 having a drivingprojection73 adapted to engage drivinggroove46 ofexpandable cage10. Whileinsertion tool70 is illustrated withonly projection73, it will be understood that the device includes an opposing projection (not shown) for mating with a driving groove (not shown) disposed opposite drivinggroove46 oncage10.Insertion tool70 further includes handle71 adapted to transmit rotational force toouter sleeve72 to rotationally insertcage10.Outer sleeve72 includes an internal chamber, which is occupied by insertion toolinner shaft75. On thedistal end77, insertion toolinner shaft75 includes an externally threaded area adapted to engage the correspondingly internally threadedcentral opening56 ofexpansion member50. At the proximal end ofinner shaft75 there is ahandle76 for providing rotational force toinner shaft75. A series ofexternal threads78 are formed oninner shaft75adjacent handle76. A internally threadednut79 is disposed aboutinner shaft75 and is adapted to engagethreads78 to moveinner shaft75 with respect toouter sleeve72.
• In operation,[0045]insertion tool70 is engaged withexpandable cage10 such as shown in FIG. 7a. Drivingprojection73 ofouter tube72 engages drivinggroove46 ofexpandable cage10 and threadeddistal end77 ofinner shaft75 threadedly engages threadedopening56 ofexpansion wedge50. In this manner,expansion wedge50 is securely held in position adjacent theexpansion end18 while the threaded cylindrical cage is inserted into the intervertebral space. In a preferred embodiment, this device is utilized from a posterior approach to the spine with theexpansion wedge50 being positioned at the leading end of the device.
• Referring to FIG. 7[0046]a,expandable cage10 is threaded intointervertebral space92 withthread pattern12 engagingvertebra80 and82 to advance the cage into the disc space and securely hold it in position once it has reached a final position as shown in FIG. 7a. Since the expandable cage is preferably a cylinder having a uniform diameter, it may be inserted through an insertion tube having a diameter substantially equal to the thread diameter of thecage10. As shown in FIG. 7a, thesurface88 ofvertebrae80 is in contact withouter surface11 ofcage10. It will be understood that in many applications, a portion of the vertebral end plate will have been removed prior to cage insertion such thatcage10 engages the cancellous bone of the vertebrae. In a similar manner, thesurface90 ofvertebrae82 is in contact with theouter surface11 ofcage10. In its initial insertion position, thealignment84 ofvertebrae80 and thealignment86 ofvertebrae82 are in substantial parallel alignment withlongitudinal axis13 andexpandable branches24 and26 ofcage10.
• Referring now to FIG. 7[0047]b, with threadedend77 of the insertion device firmly engaged in threadedopening56 ofexpansion wedge50 and drivingprojection73 engaged in drivinggroove46, internally threadednut79 is rotated aboutexternal thread78 to drawshaft75 within outer tube72 (FIG. 6), thereby advancingexpansion wedge50 toward fixedend20. Asexpansion wedge50 is advanced toward fixedend20, the inclined surfaces ofexpansion wedge50 forceexpandable branches24 and26 apart adjacentexpandable end18. In a similar manner,vertebra80 and82 are forced apart adjacentexpandable end18 such that thealignment84 and86 remain substantially parallel to theexpandable branches24 and26, respectively, and not withlongitudinal axis13 ofcage10. In this manner, the lordotic curve of the spine may be established and maintained during the healing process. Moreover, the engagement ofexpansion wedge50 with the previously described shoulders of each of the branches prevents the expansion wedge from being expelled fromcage10. The insertion tool may be removed and the substantially unobstructed area withininterior chamber17 may be filled with bone in-growth material to encourage bone growth through the device. Such bone in-growth material may include autograft, allograft, bone morphogenic proteins in a carrier, or other known bone growth promoting materials. Insertion and expansion of the alternative embodiment shown in FIGS. 8 through 10 is accomplished in substantially the same manner.
• Referring now to FIGS. 11 through 13, there is shown a further embodiment of an expandable cage according to the present invention. As with the above-described embodiments of the invention,[0048]cage110 is a substantially cylindrical device having anouter surface111 defining anexternal thread pattern112.Cage110 defines a substantially cylindricalinternal chamber117.Cage110 includes a pair of opposingexpandable branches152 and154 separated by a pair of opposing fixedbranches148 and150.Fixed branch148 is separated fromexpandable branches152 and154 bychannels144 and146, respectively.Fixed branch150 is separated fromexpandable branches154 and152 bychannel147 and a similar channel (not shown), respectively.Cage110 further includes bone in-growth windows129 and120 formed throughexpandable branch152, and an identical pair of bone in-growth windows119 and121 formed throughexpandable branch154. Each of the bone in-growth windows extend fromouter surface111 tointernal chamber117. As with the above-described embodiments,cage110 includes a large unobstructedinternal chamber117 extending along thelongitudinal axis113 from adjacent theexpandable end118 toward thefixed end114. In the embodiment shown in FIGS. 11 through 13,windows120 and121 each include anotch122 and123 adjacentexpandable end118, respectively.
• [0049]Expander130 is sized to be received withininternal chamber117.Expander130 includes afirst portion125 having aprojection126 which extends intonotch122, and an oppositesecond portion127 havingprojection128 which extends intonotch123.Projections126 and128 work in conjunction with externally threadedplug124 in maintaining the position ofexpander130 withincage110.Expander130 further includes abend132. While a bend may be utilized in the preferred embodiment, it will be understood thatexpander130 may include a fold or a hinge betweenportions125 and127, that allows adaptation into the reduced sized configuration shown in FIG. 12.First portion125 includes alongitudinal axis72 andsecond portion127 includes alongitudinal axis70. In the reduced size insertion configuration shown in FIG. 12,longitudinal axis70 forms anangle162 with respect tolongitudinal axis72. In a preferred embodiment,angle162 is approximately 90°, although other angles are contemplated. In an expanded configuration (not shown), the angle betweenlongitudinal axes70 and72 may approach 180°, with a 180° angle providing the maximum expansion of the device.
• The[0050]internal chamber117 is defined by the four previously describedbranches148,150,152, and154, each defining a portion of thread pattern160 (only partially shown in FIG. 12).Plug124 includes a corresponding external thread adapted to engagethread pattern160. In a preferred embodiment, a connectingportion149 extends between fixedbranches148 and150 to limit splaying of the fixed branches as threadedplug124 is advanced towardexpandable end118. Threadedplug124 further includes acentral opening136 adapted to engage an insertion tool extension. In a preferred embodiment,central opening136 is formed in a hexagonal pattern to accept a similar hexagonally shaped insertion tool (not shown).Cage110 further includes a drivinggroove142 adjacentfixed end114, adapted for engagement with a driving tool projection to permit insertion ofcage110 between two adjacent bony structures. The driving tool of FIG. 6 may be utilized withcage110 with the exception that the driving toolinner shaft75 would include a hexagonally shaped portion atdistal end77. It will be understood that as threadedplug124 is threadedly advanced towardsexpandable end118, it urgesexpander130 into an expanded condition, thereby forcingbranches152 and154 apart. As shown in FIG. 13, the expansion mechanism of the present invention provides a large internal cavity to receive bone growth promoting material.
• A further embodiment according to the present invention is shown in FIG. 14.[0051]Cage180 is fashioned in a similar manner tocage110 with the exception that it includes a plurality of smaller bone in-growth windows rather than two large windows inexpandable branches192 and194. Variations of the number, size, shape and location of bone in-growth windows as may be dependent on the bone in-growth characteristics desired and the material properties of the cage is contemplated by the present invention. Further, the mechanism for expansion differs in thatexpander184 is a substantially planar device, i.e. no bends or hinge, having alongitudinal axis198.Expander184 is substantially planar and has afirst end188 engaged incorner186 formed betweenexpandable branch194 andend wall199. Theopposite end190 engages and moves alonginner surface193 ofexpandable branch192. Threadedplug182 threadedly engagesinternal thread pattern196 formed along the internal surfaces of the branches. It will be understood that in the unexpanded condition,axis198 is skewed with respect tolongitudinal axis197 ofcage180. However, as threadedplug182 advances towards expandable end181,expander184 moves towards an upright position withaxis198 moving towards a perpendicular arrangement withaxis197. The movement ofexpander184 towards an upright position expandscage180. In the expanded position, there is a large unobstructedinternal chamber189 extending fromplug182 to opening187 adjacent fixed end195. Thus, the largeinternal chamber189 may be packed with bone in-growth material to promote fusion between adjacent vertebra.
• Referring now to FIG. 15, there is shown still a further embodiment of the present invention.[0052]Cage210 includes anouter surface211 having athread pattern212 defined thereon.Cage210 includes a drivinggroove246 adapted to receive a driving tool such as that previously disclosed herein.Cage210 further includes a plurality ofwindows214 communicating fromexterior surface211 tointerior chamber217.Internal chamber217 is defined byinclined surfaces216 and215 (shown in dash), sloping from theexpandable end222 towards thefixed end224. The slope ofinclined surfaces216 and215 could also be reversed to allow expansion by movement of theplug218 in an opposite direction.Plug member218 includes an external thread pattern adapted to engage withthread pattern220 ofsurfaces216 and215. It will be understood that asplug member218 is threadedly advanced toward fixedend224,branches230 and231 are spread apart from one another. As shown in FIG. 15,branch230 includes an area of reducedwidth225, adapted to deform asplug member218 is advanced. As shown,plug member218 includes acentral opening237 adapted to receive an insertion tool extension to provide rotational force. Further, while drivinggroove246 is shown formed onexpandable end222, it will be understood that for some insertion techniques, it will be desirable to haveinsertion tool groove246 formed on fixedend224. Moreover, a central aperture may be formed through fixedend224 for passage of an insertion tool extension for engagement withplug218.
• While[0053]plugs124 of the embodiment of FIG. 11, plug182 of the embodiment of FIG. 14 and plug218 of the embodiment of FIG. 15 have been shown and described as having a series of external threads for engagement with a corresponding thread pattern defined on the branches of the device, it will be understood that all the branches, or only the fixed branches of each of the devices, may be formed to define a series of ratchets. With a ratchet configuration, each of theplugs124,182, and218 may be defined as having an outer surface adapted to advance over the ratchets to expand the device while having a trailing portion adapted to engage the ratchets to prevent expulsion. In this manner, the plugs may be advanced without threading. When utilizing this technique, the insertion tool may be adapted to securely hold the outer cage to prevent its further advancement as a result of the pushing or pulling force exerted on the plug members. In addition to modifications to the plug, the cage itself may be configured for push-in insertion and can be in shapes other than cylinders.
• Cages according to the present invention are preferably formed of a biocompatible material having sufficient strength to withstand the loads that will be placed upon them for a given application. Additionally, in the preferred embodiments the material should have sufficient flexibility to undergo at least a small amount of deformation as a result of the expansion process. Alternatively, for some devices, it may be desirable to provide hinge points rather than permit the material to undergo a deformation. Most preferably, the material utilized to form the cages of the present invention is a medical grade titanium alloy. However, the devices could be formed of stainless steel, various types of plastic, various composites including carbon fiber devices, and bone or bone substitutes.[0054]
• While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.[0055]

Claims (19)

What is claimed is:

1. An expandable fusion cage adapted to be inserted between a pair of vertebral bodies, comprising:

a body having an outer bone engaging surface and an inner surface defining an internal chamber and a retaining mechanism, said body defining at least a first branch, said branch having a fixed portion connected to said body and an expandable portion movable with respect to said body, said body defining at least a second branch connected to said body; and

an expansion mechanism sized to be at least partially received within said internal chamber and to engage said expandable portion, said expansion mechanism having an outer surface adapted to engage said retaining mechanism, whereby movement of said expansion mechanism within said internal chamber urges said first branch expandable portion to move with respect to said body thereby expanding said body, said expansion mechanism moving said first branch expandable portion at a greater rate than said second branch, said expansion mechanism retained in position by engagement with said retaining mechanism of the body.

2. The device ofclaim 1, wherein said bone engaging surface is an external thread.

3. The device ofclaim 1, wherein said retaining mechanism is a thread pattern defined on said inner surface and said expansion mechanism includes an externally threaded plug configured for engagement with said thread pattern.

4. The device ofclaim 1, wherein said retaining mechanism is an internal shoulder and said expansion mechanism is a wedge adapted to engage said shoulder.

5. The device ofclaim 1 wherein said body defines a third branch opposing said first branch, said third branch having a fixed portion connected to said body and an expandable portion, whereby movement of said expansion mechanism within said internal chamber urges both said first and third branches apart.

6. An expandable cage adapted to be inserted between a pair of vertebral bodies, comprising:

a tubular body having an outer bone engaging surface and an inner surface defining an interior chamber and a retaining mechanism, said body includes a fixed end and an expandable end and further including four longitudinal channels extending between said outer surface and said inner surface and from said expandable end toward said fixed end, said channels defining an opposing first and second branches separated by opposing third and fourth branches; and

an expansion mechanism having an outer surface configured to engage said inner surface and said retaining portion, whereby said expansion mechanism is moved against said inner surface to move said expandable end to expand said body, said expansion mechanism expanding said opposing first and second branches at a substantially greater rate than said opposing third and fourth branches, and said expansion mechanism retained in position by engagement with said retaining mechanism.

7. The device ofclaim 6, wherein said retaining mechanism includes at least one flange defined adjacent said inner surface and said expansion member includes a shoulder cooperable with said flange to maintain the position of said expansion member.

8. The device ofclaim 6, wherein said retaining mechanism is a thread pattern and said expansion member includes a corresponding external thread pattern.

9. The device ofclaim 6, wherein said bone engaging surface is an external thread.

10. An expandable cage adapted to be inserted between a pair of vertebral bodies, comprising:

a tubular body having an outer bone engaging surface and an inner surface defining an interior chamber having a threaded inclined surface, said body defining a fixed portion and a moveable portion, said inclined surface disposed adjacent at least said moveable portion; and

an expansion member having a threaded outer surface configured to engage said threaded inclined surface, whereby said expansion member is threadedly advanced along said threaded inclined surface to move said moveable portion to expand said body and said expansion member is retained in position by engagement with said threaded inclined surface.

11. The device of claims6 or10, wherein said body further defines bone in-growth openings between said inner surface and said outer surface.

12. The device of claims6 or10, wherein said interior chamber comprises at least 50% of the volume of the device, thereby allowing a substantial amount of bone growth promoting matter to be inserted into the interior chamber.

13. The device of claims6 or10, wherein said expansion member defines a central opening providing communication with said interior chamber.

14. An interbody fusion apparatus, comprising:

a hollow body having an outer bone engaging surface and an inner surface defining an internal chamber and a retaining mechanism, said body having a fixed portion and an expandable portion and defining at least a first branch and a second branch, said first branch and said second branch interconnected at said fixed portion, and said first branch moveable with respect to said second branch at said expandable portion;

an expansion member sized to be at least partially received within said internal chamber and having an outer surface adapted to engage said retaining mechanism, whereby movement of said expansion member within said internal chamber urges said first branch to move with respect to said second branch thereby expanding said expandable portion, said expansion member retained in position by engagement with said retaining mechanism of the body; and

an insertion tool having an outer sleeve adapted to engage said body to transmit compressive force and an inner shaft moveably disposed within said outer sleeve and sized to be inserted into at least a portion of said interior chamber, said inner shaft having a proximal end and a distal end, said distal end configured to removably engage said expansion member, whereby movement of said inner shaft with respect to said outer sleeve moves said expansion member in relation to said body.

15. The apparatus ofclaim 14, wherein said expansion member includes a central opening having a thread pattern and said distal end of said inner shaft includes a corresponding external thread pattern.

16. The apparatus ofclaim 14, wherein said expansion member includes a central hexagonal opening and said distal end of said inner shaft includes a corresponding external hexagonal shape.

17. The apparatus ofclaim 14, wherein said body further defines a driving groove and said outer sleeve defines a corresponding mating driving projection.

18. A method for insertion of an expandable interbody fusion device between a pair of vertebral bodies, comprising:

providing an expandable fusion cage having an external bone engaging surface, a movable portion and an internal cavity defining a retaining mechanism, and an expansion member sized to be at least partially received within the internal chamber and configured for engagement with the retaining mechanism and the movable portion;

inserting the expandable cage between two adjacent vertebrae;

positioning the moveable portion adjacent a vertebra;

moving the expansion member within the internal chamber and against the moveable portion to expand the cage; and

engaging the expansion member with the retaining mechanism to limit movement of the expansion member.

19. The method ofclaim 18, further including packing the internal cavity with material to promote gone in-growth.

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