Abstract:
An expandable intervertebral device including a body extending generally along a longitudinal axis and including at least two branch portions coupled together adjacent an end portion of the body. The device further includes an expansion member positioned between the at least two branch portions with at least a portion of the expansion member arranged at an angular orientation relative to the longitudinal axis whereby a change in the angular orientation relative to the longitudinal axis urges the at least two branch portions apart to expand the body.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application is a continuation of U.S. application Ser. No. 09/949,516, filed Sep. 7, 2001, now abandoned, which is a continuation of U.S. application Ser. No. 09/763,073, filed May 16, 2001 and issued as U.S. Pat. No. 6,436,140, which claims foreign priority benefits of International Patent Application Number PCT/1B99/01478, filed Aug. 26, 1999, and French Patent Application Number FR98/10832, filed on Aug. 28, 1998, the contents of each application hereby being incorporated by reference. 

   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. 
   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. 
   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. 
   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. 
   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. 
   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. 
   In view of the limitations of the prior devices, there remains a need for an expandable interbody device 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 device should have an internal cavity adapted to receive bone graft or bone substitute to encourage bone growth through the expanded device. 
   SUMMARY OF THE INVENTION 
   In response to the needs still left unresolved by the prior devices, the present invention contemplates an expandable intervertebral device adapted to be inserted between a pair of vertebral bodies to restore the normal angular relation between adjacent vertebrae. In particular, an expandable intervertebral device according to one form of the present invention includes a body extending generally along a longitudinal axis and including at least two branch portions coupled together adjacent an end portion of the body. The device further includes an expansion member positioned between the at least two branch portions with at least a portion of the expansion member arranged at an angular orientation relative to the longitudinal axis whereby a change in the angular orientation relative to the longitudinal axis urges the at least two branch portions apart to expand the body. 
   In another form of the present invention, an expandable intervertebral device is provided including a body extending generally along a longitudinal axis and including at least two branch portions coupled together adjacent an end portion of the body, with the body defining a series of ratchet elements positioned along the longitudinal axis. The device further includes an expansion member positioned between the at least two branch portions whereby axial movement of the expansion member generally along the longitudinal axis urges the at least two branch portions apart to expand the body with a portion of the expansion member engaged with at least one of the ratchet elements to maintain the expansion member in a select axial position along the longitudinal axis relative to the body. 
   In another form of the present invention, an expandable intervertebral device is provided including a body extending generally along a longitudinal axis and having a fixed end portion and a movable end portion, with the body including at least two branch portions coupled together adjacent the fixed end portion. The device further includes an expansion member positioned adjacent the movable end portion of the body whereby axial movement of the expansion member from the movable end portion toward the fixed end portion results in engagement between the expansion member and the at least two branch portions to urge the at least two branch portions apart to expand the body. 
   In other forms of the present invention, methods are provided for the insertion of an expandable intervertebral device between an adjacent pair of vertebrae. 
   One object of the present invention is to provide an expandable intervertebral device that has a reduced size insertion configuration and is expandable from the insertion configuration to a larger configuration. 
   Another object of the present invention is to provide an expandable intervertebral device that has a substantially unobstructed interior chamber to receive bone growth promoting material. 
   Still another object of the present invention is to provide an expandable intervertebral device configured for easy insertion and expandable to a larger size to establish lordosis. 
   Yet a further object of the present invention is to provide an improved method for inserting an expandable intervertebral device into a disc space to restore lordosis. 
   Related objects and advantages of the present invention will be apparent from the following description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a top view of an expandable cage according to one embodiment of the present invention. 
       FIG. 2   a  is a side cross-sectional view of the device of  FIG. 1 . 
       FIG. 2   b  is the device of  FIG. 2   a  with 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. 
       FIG. 4  is a perspective view of the expansion wedge of  FIG. 2   b.    
       FIG. 5  is an end view of the expansion wedge of  FIG. 4 . 
       FIG. 6  is an elevational view of an insertion tool according to one embodiment of the present invention. 
       FIG. 7   a  is a sagittal plane view showing a partial cross-sectional side view of the expandable cage of  FIG. 2   b  inserted between two adjacent vertebrae in an insertion configuration according to one embodiment of the present invention. 
       FIG. 7   b  is the cage of  FIG. 7   a  shown 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 . 
       FIG. 9  is a side cross-sectional view of the expandable cage of  FIG. 8 . 
       FIG. 10  is a partial cross-sectional perspective view of the expandable cage of  FIG. 8  without an external thread pattern. 
       FIG. 11  is a top view of a further embodiment of an expandable cage according to the present invention. 
       FIG. 12  is a side partial cross-sectional view of the expandable cage of  FIG. 11 . 
       FIG. 13  is a partial cross-sectional perspective view of the expandable cage of  FIG. 11 , without an external thread pattern. 
       FIG. 14  is a side partial cross-sectional view of a further embodiment of the present invention. 
       FIG. 15  is a side partial cross-sectional side view of yet a further embodiment of the present invention. 
   

   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. 
   Referring now to  FIGS. 1 through 3 , there is shown a first embodiment of an expandable cage  10  in accordance with the present invention. In this embodiment of the invention, expandable cage  10  has a cylindrical outer surface  11  defining an external thread pattern  12  (not shown in  FIG. 3 ) adapted to engage two adjacent vertebra (see  FIGS. 7   a  and  7   b ) and to advance the cage into the disk space as cage  10  is rotated about longitudinal axis  13 . As shown most clearly in  FIG. 3 , expandable cage  10  is substantially hollow with inner surface  21  defining an internal cavity  17 . Expandable cage  10  includes a bone in-growth window  16  formed through expandable branch  24  and an identical bone in-growth window  19  formed through expandable branch  26 . These in-growth windows are adapted to permit communication between the vertebral bodies ( FIG. 7 ) and internal chamber  17 . In most application, bone growth promoting material will be placed within internal chamber  17  of expandable cage  10  to encourage bone to grow into and through fusion device  10 . 
   As shown in the accompanying drawings, expandable cage  10  is preferably composed of four separate branches, each separated at expandable end  18  by a channel extending longitudinally from expandable end  18  toward fixed end  20 . Referring now to  FIG. 2   a , first expandable branch  24  is separated from first fixed branch  40  by channel  14 . In a similar manner, second expandable branch  26  is separated from first fixed branch  40  by channel  22 . Each of channels  14  and  22  extends from expandable end  18  towards fixed end  20 . 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 fixed end  20 . In a similar fashion, as shown in  FIG. 3 , second fixed branch  41  is separated from first expandable branch  24  by channel  15  and from second expandable branch  26  by a similar channel (not shown). Thus, expandable cage  10  is formed by four branches, each separated from the other at expandable end  18  by channels extending from outer surface  11  to internal chamber  17 . The branches are connected at fixed end  20  by linking area  44  such that each of the branches may move substantially independent from each other at expandable end  18  while remaining connected to the device by linking area  44 . 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 outer surface  11  to internal chamber  17 , 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 into internal chamber  17 . Further, although channels are shown as being preformed in the expandable cage, it is contemplated that the channels may be formed as the implant 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, internal chamber  17  comprises the majority of the volume of the entire cage  10 . Specifically referring to  FIG. 2   a , external surface  11  defines a diameter  63 , excluding threads  12 . Internal chamber  17  has a diameter of at least  64 , that is diameter  63  less twice the branch thickness  62 . 
   In a preferred embodiment, branch thickness  62  is selected such that the volume of internal chamber  17 , particularly with the inclusion of voids created by in-growth windows  16  and  19  and the various channels, occupies the majority of the entire volume of cage  10 . 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. 
   Adjacent expansion end  18 , as shown in  FIG. 2   a  with respect to branches  24 ,  40  and  26 , each of the branches includes inclined surfaces  28 ,  34  and  30 , respectively. Adjacent the internal termination of each of the inclined surfaces  28 ,  34  and  30 , is formed an internal shoulder  36 ,  42  and  38 , respectively. Referring to  FIG. 3 , branch  41  includes a similar inclined surface  35  and internal shoulder  43 . As shown in  FIG. 2   a , arcuate inclined surfaces  28  and  30  are inclined at angle  32  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   b , cage  10  is shown with the inclusion of expansion wedge  50  disposed adjacent expandable end  18 . Expansion wedge  50  is further illustrated in  FIGS. 4 and 5 . Expansion wedge  50  includes first and second opposed expansion wedge inclines  52  and  54 , which have a tapering arcuate surface that mates with and corresponds to inclined surfaces  28  and  30  of branches  24  and  26 , respectively. Wedge  50  includes side walls  58  and  60  with substantially planar surfaces adapted to engage inclined surfaces  34  and  35  of fixed branches  40  and  41 , respectively. Further, expansion wedge  50  includes a central opening  56  which is adapted to receive a driving tool. In a preferred embodiment, central opening  56  is threaded to receive a correspondingly threaded end of a driving tool. 
   It will be understood that as wedge  50  is advanced toward fixed end  20 , inclined surfaces  52  and  54  act upon inclined surfaces  28  and  30 , respectively, to urge branches  24  and  26  apart from each other at expandable end  18 . As expansion wedge  50  is inserted along inclined surfaces  28  and  30 , branches  24  and  26  will tend to expand at expansion end  18  as deformation occurs adjacent fixed end  20 . Substantially continuous linking material  44  links all of the branches and does not permit their expansion at fixed end  20 . As wedge  50  is further advanced toward fixed end  20 , the wedge is pushed beyond shoulders  36  and  38 , such that wedge  50  is captured within cage  10 . As a result of the engagement of back surface  55  of the wedge against shoulders  36  and  38 , expansion wedge  50  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 of planar surfaces  58  and  60  on fixed branches  40  and  41 , respectively, tends to deformably expand these branches little, if any, and therefore they remain substantially fixed in their original positions. Further, fixed branches  40  and  41  each include shoulders  42  and  43  which engage the back surface  55  of wedge  50  once it has passed beyond the shoulders. Thus, fixed branches  40  and  41  also tend to hold the wedge in place and prevent its expulsion from internal chamber  17 . 
   Referring now to  FIGS. 8 through 10 , there is shown an alternative embodiment of the expandable cage of the present invention. In this embodiment, expandable cage  310  has an outer surface  311  and an external thread pattern  312  (not shown in  FIG. 10 ). As with the embodiment of  FIG. 1 , expandable cage  310  includes two opposed expandable branches  324  and  324 , and two opposed fixed branches  340  and  341  joined at fixed end  320 . Each of these branches define inclined surfaces and interior shoulders adjacent expandable end  318  adapted to engage and receive expansion wedge  350 . In contrast to the embodiment of  FIG. 1 , expandable branch  324  includes two bone in-growth windows  316  and  317 , separated by rib  330 . In a similar manner, expandable branch  326  includes two bone in-growth windows  322  and  323  separated by rib  332 . 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 insertion tool  70  suitable for use with an expandable cage according to the present invention. Insertion tool  70  includes an outer sleeve  72  having a driving projection  73  adapted to engage driving groove  46  of expandable cage  10 . While insertion tool  70  is illustrated with only projection  73 , it will be understood that the device includes an opposing projection (not shown) for mating with a driving groove (not shown) disposed opposite driving groove  46  on cage  10 . Insertion tool  70  further includes handle  71  adapted to transmit rotational force to outer sleeve  72  to rotationally insert cage  10 . Outer sleeve  72  includes an internal chamber, which is occupied by insertion tool inner shaft  75 . On the distal end  77 , insertion tool inner shaft  75  includes an externally threaded area adapted to engage the correspondingly internally threaded central opening  56  of expansion member  50 . At the proximal end of inner shaft  75  there is a handle  76  for providing rotational force to inner shaft  75 . A series of external threads  78  are formed on inner shaft  75  adjacent handle  76 . An internally threaded nut  79  is disposed about inner shaft  75  and is adapted to engage threads  78  to move inner shaft  75  with respect to outer sleeve  72 . 
   In operation, insertion tool  70  is engaged with expandable cage  10  such as shown in  FIG. 7   a . Driving projection  73  of outer tube  72  engages driving groove  46  of expandable cage  10  and threaded distal end  77  of inner shaft  75  threadedly engages threaded opening  56  of expansion wedge  50 . In this manner, expansion wedge  50  is securely held in position adjacent the expansion end  18  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 the expansion wedge  50  being positioned at the leading end of the device. 
   Referring to  FIG. 7   a , expandable cage  10  is threaded into intervertebral space  92  with thread pattern  12  engaging vertebra  80  and  82  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. 7   a . 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 the cage  10 . As shown in  FIG. 7   a , the surface  88  of vertebrae  80  is in contact with outer surface  11  of cage  10 . It will be understood that in many applications, a portion of the vertebral end plate will have been removed prior to cage insertion such that cage  10  engages the cancellous bone of the vertebrae. In a similar manner, the surface  90  of vertebrae  82  is in contact with the outer surface  11  of cage  10 . In its initial insertion position, the alignment  84  of vertebrae  80  and the alignment  86  of vertebrae  82  are in substantial parallel alignment with longitudinal axis  13  and expandable branches  24  and  26  of cage  10 . 
   Referring now to  FIG. 7   b , with threaded end  77  of the insertion device firmly engaged in threaded opening  56  of expansion wedge  50  and driving projection  73  engaged in driving groove  46 , internally threaded nut  79  is rotated about external thread  78  to draw shaft  75  within outer tube  72  ( FIG. 6 ), thereby advancing expansion wedge  50  toward fixed end  20 . As expansion wedge  50  is advanced toward fixed end  20 , the inclined surfaces of expansion wedge  50  force expandable branches  24  and  26  apart adjacent expandable end  18 . In a similar manner, vertebra  80  and  82  are forced apart adjacent expandable end  18  such that the alignment  84  and  86  remain substantially parallel to the expandable branches  24  and  26 , respectively, and not with longitudinal axis  13  of cage  10 . In this manner, the lordotic curve of the spine may be established and maintained during the healing process. Moreover, the engagement of expansion wedge  50  with the previously described shoulders of each of the branches prevents the expansion wedge from being expelled from cage  10 . The insertion tool may be removed and the substantially unobstructed area within interior chamber  17  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, cage  110  is a substantially cylindrical device having an outer surface  111  defining an external thread pattern  112 . Cage  10  defines a substantially cylindrical internal chamber  117 . Cage  110  includes a pair of opposing expandable branches  152  and  154  separated by a pair of opposing fixed branches  148  and  150 . Fixed branch  148  is separated from expandable branches  152  and  154  by channels  144  and  146 , respectively. Fixed branch  150  is separated from expandable branches  154  and  152  by channel  147  and a similar channel (not shown), respectively. Cage  110  further includes bone in-growth windows  118  and  120  formed through expandable branch  152 , and an identical pair of bone in-growth windows  119  and  121  formed through expandable branch  154 . Each of the bone in-growth windows extend from outer surface  111  to internal chamber  117 . As with the above-described embodiments, cage  110  includes a large unobstructed internal chamber  117  extending along the longitudinal axis  113  from adjacent the expandable end  118  toward the fixed end  114 . In the embodiment shown in  FIGS. 11 through 13 , windows  120  and  121  each include a notch  122  and  123  adjacent expandable end  118 , respectively. 
   Expander  130  is sized to be received within internal chamber  117 . Expander  130  includes a first portion  125  having a projection  126  which extends into notch  122 , and an opposite second portion  127  having projection  128  which extends into notch  123 . Projections  126  and  128  work in conjunction with externally threaded plug  124  in maintaining the position of expander  130  within cage  110 . Expander  130  further includes a bend  132 . While a bend may be utilized in the preferred embodiment, it will be understood that expander  130  may include a fold or a hinge between portions  125  and  127 , that allows adaptation into the reduced sized configuration shown in  FIG. 12 . First portion  125  includes a longitudinal axis  72  and second portion  127  includes a longitudinal axis  70 . In the reduced size insertion configuration shown in  FIG. 12 , longitudinal axis  70  forms an angle  162  with respect to longitudinal axis  72 . In a preferred embodiment, angle  162  is approximately 90°, although other angles are contemplated. In an expanded configuration (not shown), the angle between longitudinal axes  70  and  72  may approach 180°, with a 180° angle providing the maximum expansion of the device. 
   The internal chamber  117  is defined by the four previously described branches  148 ,  150 ,  152 , and  154 , each defining a portion of thread pattern  160  (only partially shown in  FIG. 12 ). Plug  124  includes a corresponding external thread adapted to engage thread pattern  160 . In a preferred embodiment, a connecting portion  149  extends between fixed branches  148  and  150  to limit splaying of the fixed branches as threaded plug  124  is advanced toward expandable end  118 . Threaded plug  124  further includes a central opening  136  adapted to engage an insertion tool extension. In a preferred embodiment, central opening  136  is formed in a hexagonal pattern to accept a similar hexagonally shaped insertion tool (not shown). Cage  110  further includes a driving groove  142  adjacent fixed end  114 , adapted for engagement with a driving tool projection to permit insertion of cage  110  between two adjacent bony structures. The driving tool of  FIG. 6  may be utilized with cage  110  with the exception that the driving tool inner shaft  75  would include a hexagonally shaped portion at distal end  77 . It will be understood that as threaded plug  124  is threadedly advanced towards expandable end  118 , it urges expander  130  into an expanded condition, thereby forcing branches  152  and  154  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 . Cage  180  is fashioned in a similar manner to cage  110  with the exception that it includes a plurality of smaller bone in-growth windows rather than two large windows in expandable branches  192  and  194 . 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 that expander  184  is a substantially planar device, i.e. no bends or hinge, having a longitudinal axis  198 . Expander  184  is substantially planar and has a first end  188  engaged in corner  186  formed between expandable branch  194  and end wall  199 . The opposite end  190  engages and moves along inner surface  193  of expandable branch  192 . Threaded plug  182  threadedly engages internal thread pattern  196  formed along the internal surfaces of the branches. It will be understood that in the unexpanded condition, axis  198  is skewed with respect to longitudinal axis  197  of cage  180 . However, as threaded plug  182  advances towards expandable end  181 , expander  184  moves towards an upright position with axis  198  moving towards a perpendicular arrangement with axis  197 . The movement of expander  184  towards an upright position expands cage  180 . In the expanded position, there is a large unobstructed internal chamber  189  extending from plug  182  to opening  187  adjacent fixed end  195 . Thus, the large internal chamber  189  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. Cage  210  includes an outer surface  211  having a thread pattern  212  defined thereon. Cage  210  includes a driving groove  246  adapted to receive a driving tool such as that previously disclosed herein. Cage  210  further includes a plurality of windows  214  communicating from exterior surface  211  to interior chamber  217 . Internal chamber  217  is defined by inclined surfaces  216  and  215  (shown in dash), sloping from the expandable end  222  towards the fixed end  224 . The slope of inclined surfaces  216  and  215  could also be reversed to allow expansion by movement of the plug  218  in an opposite direction. Plug member  218  includes an external thread pattern adapted to engage with thread pattern  220  of surfaces  216  and  215 . It will be understood that as plug member  218  is threadedly advanced toward fixed end  224 , branches  230  and  231  are spread apart from one another. As shown in  FIG. 15 , branch  230  includes an area of reduced width  225 , adapted to deform as plug member  218  is advanced. As shown, plug member  218  includes a central opening  237  adapted to receive an insertion tool extension to provide rotational force. Further, while driving groove  246  is shown formed on expandable end  222 , it will be understood that for some insertion techniques, it will be desirable to have insertion tool groove  246  formed on fixed end  224 . Moreover, a central aperture may be formed through fixed end  224  for passage of an insertion tool extension for engagement with plug  218 . 
   While plugs  124  of the embodiment of  FIG. 11 , plug  182  of the embodiment of  FIG. 14  and plug  218  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 the plugs  124 ,  182 , and  218  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. 
   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.