Abstract:
An inter-vertebral cage with a wedge shape eccentrically located on the leading end of the cage. The ectopically shaped leading edge of this embodiment facilitates easier interbody spacer placement with less risk of nerve injury, as well as with likely decreased risk of destruction of the end plate during the distraction process when the interbody cage is impacted.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation in part of prior filed International Application Serial Number PCT/US2008/075010 filed Sep. 2, 2008, which claims priority to U.S. Provisional Patent Application Ser. No. 60/969,214 filed Aug. 31, 2007; which are hereby fully incorporated by reference into this disclosure. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates generally to spinal surgery and, in particular, to spinal cages of the type used in fusing adjacent vertebrae. 
         [0003]    Surgeons frequently insert interbody cages in the thoracic and lumbar spine from a variety of approaches including posterior, lateral, oblique, transverse and anterior. Frequently, the disc space is degenerated and collapsed in these situations. Surgeons will often use the cage to distract the disc space as the cage is impacted into the disc space. One design utilizes a ‘bulleted” cage, such as the Capstone™ cage by Medtronic™. This design has a rounded wedge symmetrically disposed on the leading edge of the cage. This allows the anterior aspect of the cage to distract the disc space as it is impacted. 
         [0004]    The problem is that such a cage has the distracting wedge shape directly in the medial aspect of the cage. With conditions such as listhesis in the setting of a collapsed disc, however, manipulation of the vertebrae from the medial aspect of the disc space is disadvantageous. 
         [0005]    What is needed, therefore, is a mechanism which allows easier insertion of an inter-vertebral cage which also decreases the rate of end plate fracture with distraction and lessens the risk of nerve root injury with cage manipulation during the insertion process. 
       SUMMARY OF INVENTION 
       [0006]    The invention, in a first embodiment, includes an inter-vertebral cage with a wedge shape eccentrically located on the leading end of the cage. The invention of this embodiment improves on wedge-shaped interbody spacers where the apex of the leading edge is in the center of the cage. The ectopically shaped leading edge of this embodiment facilitates easier interbody spacer placement with less risk of nerve injury, as well as with likely decreased risk of destruction of the end plate during the distraction process when the interbody cage is impacted. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    For a fuller understanding of the invention, reference should be made to the following detailed description, taken in connection with the accompanying drawings, in which: 
           [0008]      FIG. 1  is a perspective view of a general embodiment of the inventive interbody cage. 
           [0009]      FIG. 2  is a side plan view of a first embodiment of the invention wherein the longitudinal axis of the anterior portion of the cage is inferior to the longitudinal axis of the posterior portion and the inferior surface of the anterior portion is coplanar with the inferior surface of the posterior portion. 
           [0010]      FIG. 3A  is a side plan view of a second embodiment of the invention wherein the longitudinal axis of the anterior portion of the cage is superior to the longitudinal axis of the posterior portion and the superior surface of the anterior portion is coplanar with the superior surface of the posterior portion. 
           [0011]      FIG. 3B  is a side plan view of a third embodiment of the invention wherein the longitudinal axis of the anterior portion of the cage is inferior to the longitudinal axis of the posterior portion and the inferior surface of the anterior portion is superior to the inferior surface of the posterior portion. 
           [0012]      FIG. 3C  is a side plan view of a fourth embodiment of the invention wherein the longitudinal axis of the anterior portion of the cage is superior to the longitudinal axis of the posterior portion and the superior surface of the anterior portion is inferior to the inferior surface of the posterior portion. 
           [0013]      FIG. 4A  is a rear perspective view of a fifth embodiment of the invention wherein the superior surface of the posterior portion contains serrations. 
           [0014]      FIG. 4B  is a rear perspective view of a sixth embodiment of the invention wherein the opposing sides of the device contain serrations. 
           [0015]      FIG. 5A  is a perspective view of yet another embodiment wherein the superior surface of the posterior portion is apertured, thereby providing communication with the bone growth chamber. 
           [0016]      FIG. 5B  is a perspective view of alternate embodiment of the cage of  FIG. 5A  wherein the opposing sides of the posterior portion are apertured, thereby providing communication with the bone growth chamber. 
           [0017]      FIG. 6A  is a diagrammatic view of the leading edge of the device prior to insertion in the disc space. 
           [0018]      FIG. 6B  is a diagrammatic view of the leading edge of the device during insertion in the disc space. 
           [0019]      FIG. 6C  is a diagrammatic view of the device after insertion in the disc space. 
           [0020]      FIG. 6D  is a diagrammatic view of the device for application in Spondylolisthesis condition. 
           [0021]      FIG. 7A  is a rear perspective view of the device with associated insertion features such as screw holes. 
           [0022]      FIG. 7B  is a rear perspective view of an alternate embodiment of the device of  FIG. 7A  wherein the associated insertion features are recessed. 
           [0023]      FIG. 7C  is a rear perspective view of two connected devices. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0024]    In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings, which form a part hereof, and within which are shown by way of illustration specific embodiments by which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the invention. 
         [0025]    The invention includes, in a general embodiment, a self-distracting inter-vertebral cage adapted to facilitate fusion between adjacent upper and lower spinal vertebrae. The device of a preferred embodiment, therefore, restores an optimal inter-vertebral space height and a normal lordosis. In use, the inventive device is driven into an inter-vertebral space to distract the vertebrae, as is discussed in greater detail below. 
         [0026]    Many of the advantages provided by the inventive cage derive from its shape. For example, the unique anterior portion of the invention enables a practitioner to insert the cage into a collapsed-disc space, using the leading edge to selectively distract the adjacent vertebrae. Another advantage is the generally rectangular shape of the posterior portion of the invention, which enables the device to fit conveniently between adjacent vertebrae and further enhance fusion and distraction between the upper and lower vertebrae. The tight fit of the posterior portion in the inter-vertebral space also maintains the distraction of the disc space during fusion. 
         [0027]      FIG. 1  illustrates a general embodiment of the invention. Asymmetric disc-distracting device  10  is presented in the form of a generally rectangular cage having superior and inferior surfaces  12  and  14 , and first side  16  and second side  18 , respectively. 
         [0028]    Device  10 , as shown in  FIG. 2 , also comprises substantially rectangular posterior portion  20  (facing generally to the right of the page), and generally tapering (or wedge-shaped) anterior portion  30  (facing generally leftward on the page). Anterior portion  30 , in a preferred embodiment, extends integrally from posterior portion  20 . Transverse plane  19  defines an artificial boundary between posterior portion  20  and anterior portion  30 . Posterior portion  20  has a first longitudinal axis  22  and anterior portion  30  has a second longitudinal axis  32  relative to transverse plane  19 . First longitudinal axis  22  is an imaginary line running across the approximate center of trailing wall  21  of posterior portion  20  perpendicular to transverse plane  19 . Second longitudinal axis  32  extends from leading edge  31  of anterior portion  30  to transverse plane  19  and is parallel to first longitudinal axis  22 . It can furthermore be seen in  FIG. 2  that first longitudinal axis  22  and second longitudinal axis  32  are dis-planar with respect to transverse plane  19 . This dis-planar relationship causes device  10  to be asymmetrical along its combined longitudinal axis. 
         [0029]    The particular configuration of device  10 , including anterior portion  30 , in  FIGS. 1 and 2  is provided for representative purposes and other asymmetrical shapes are contemplated. For example, device  10  is shown in  FIGS. 1 and 2  as having parallel superior and inferior surfaces  12  and  14 . Device  10  could, however, be made with superior surface  12  and inferior surfaces  14  at a predetermined angle thereby enhancing lordosis. 
         [0030]    It is also contemplated that leading edge  31  can be disposed at any height which results in device  10  being asymmetrical along its combined longitudinal axis. Examples of alternate embodiments are shown in  FIGS. 3A through 3C . Anterior portion  30 , as shown, is substantially shorter than posterior portion  20 . However; in another embodiment, anterior portion  30  is the same length or longer than posterior portion  20 . A variety of other shapes and configurations are also contemplated. 
         [0031]    The following represents illustrative dimensions for use in construction of the inventive device. The following dimensions are provided by way of example only and the skilled artisan will appreciate such. Accordingly, it is contemplated that the following dimensions can be altered to fit the specific needs of the practitioner. In this illustrative embodiment, the leading edge  31  can be anywhere from three to six millimeters, and the height of the cage can range in sizes in height from six to seventeen millimeters. The width of the cage can vary from six to eighteen millimeters for cages utilized from a posterior approach. If cages are inserted using an anterior approach, up to 45 mm wide cages might be used. Such larger cages could also utilize internal cross beams to help distribute load and increase cage strength. Such heights are standard for similar cages used in the industry. The length of the cage can vary anywhere from 18-40 millimeters, similar to other cage lengths standard in the art. 
         [0032]    Referring again to  FIGS. 2 through 3C , anterior portion  30  may be disposed with variations of superior slope  34   a  and/or inferior slope  34   b . Accordingly, slopes  34   a ,  34   b  may be concave, parabolic, hyperbolic, elliptical, circular or any other shape as known in the art. Slopes  34   a ,  34   b  can also be linear or convex (hyperbolic, elliptical, circular or any other shape as known in the art). Additionally, superior surface  12  and/or inferior surface  14  may also be concave and/or convex, as needed, to better engage adjacent vertebral endplates. In still another embodiment, the trailing end of posterior portion  20  may be wider, relative to the lateral thickness of anterior portion  30 , to provide additional strength in compression of the posterior spine. 
         [0033]    Various materials may be used in the construction of device  10 . Illustrative materials include, but are not limited to, stainless steel, cobalt-chromium-molybdenum alloys, titanium, carbon reinforced polymers, shape memory alloys, materials commonly used for commercial and industrial springs, and nylon or other fiber or polymer materials used for the radiolucent screen or mesh sleeve. Further examples of carbon reinforced polymers include polyether ether ketone, (polyether ketone, ether ketone, and ketone), polycarbonate, polypropylenes, polyethylene and polysulfone type plastic materials with or without glass or carbon fibers. 
         [0034]    In yet another embodiment, shown in  FIG. 4A , superior surface  12  and/or inferior surface  14  may include serrations  15 , or alternative surfacing to better engage the endplates of the adjacent vertebrae. In an alternate embodiment, shown in  FIG. 4B , at least one of the opposing sides ( 16  and  18  respectively) has a plurality of serrations  15  thereon. Serrations  15  help device  10  remain in place once inserted between the vertebrae. Serrations  15  may be angled to allow ready insertion of device  10  in the distal, anterior direction and prevent retraction in the proximal, posterior direction. The implanted device therefore remains between adjacent vertebrae. 
         [0035]    In another embodiment, shown in  FIGS. 5A and 5B , at least one surface (i.e. superior surface  12 , inferior surface  14 , first side  16  or second side  18 ) are apertured to provide communication with interior chamber  40 . Chamber  40  is provided for the inclusion of an osteogenic material. The osteogenic material is advantageous in that device  10  fosters bone growth and permits the upper and lower vertebrae (as well as to adjacent bone fragments placed within the inter-vertebral space) to fuse to device  10 . The use of an osteogenic material in the fusion encourages bone growth and, as such, bone will grow from those adjacent bone sources having adequate blood supply. Over time, bone from the adjacent vertebrae grows into chamber  40 , and completely incorporates device  10 . 
         [0036]    Any suitable osteogenic material or composition is contemplated for placement within the chamber  40 . Illustrative osteogenic material include, for example, autograft, allograft, xenograft, demineralized bone, synthetic and natural bone graft substitutes, such as bio-ceramics and polymers, and osteo-inductive factors. Where bony material is placed within the chamber  40 , the material can be pre-packed before the device is implanted, or can be inserted through the apertures after the device is in position. A separate carrier to hold the materials within the chambers of the device can also be used. These carriers can include collagen-based carriers, bio-ceramic materials, hydroxyapatite and calcium phosphate compositions. The carrier material can be provided in the form of a sponge, a block, folded sheet, putty, paste, graft material or other suitable form. Moreover, the osteogenic compositions contained within the vertebral replacement device can comprise an effective amount of a bone morphogenetic protein, transforming growth factor .beta.1, insulin-like growth factor 1, platelet-derived growth factor, fibroblast growth factor, LIM mineralization protein (LMP), and combinations thereof or other therapeutic or infection resistant agent, held within a suitable carrier material. 
         [0037]    With reference now to  FIGS. 6A through 6C , an example of the placement of device  10  disc space  51  between upper vertebrae  50  and adjacent lower vertebrae  52  is shown. The example provided depicts the posterior lumbar interbody fusion (PLIF) method. Device  10 , however, has equal functionality in the anterior lumbar interbody fusion (ALIF) method. As shown in  FIG. 6A , leading edge  31  is positioned posteriorly between upper and lower vertebrae  50 ,  52  adjacent disc space  51 . In  FIG. 6B , device  10  is driven anteriorly (as indicated by arrow A) into disc space  51  between adjacent vertebrae  50 ,  52 .  FIG. 6C  shows device  10  in substantially locked engagement between adjacent vertebrae  50 ,  52 . Device  10  provides the desired level of distraction (as indicated by arrow B) of the anterior portions of adjacent vertebrae  50 ,  52 . 
         [0038]    With reference now to  FIG. 7A , an alternate embodiment is shown wherein trailing wall  21  includes manipulation holes  17   a  and  17   b , extending at least partially there through, which provide for temporarily coupling device  10  to an insertion device (not shown). Devices for inserting cages embody a variety of patents including threaded screws and paddles which hold the posterior edge. Impacters come in a variety of forms as well. Central hole  19  extends through the trailing wall  21  to chamber  40 . Central hole  19  is used to manipulate device  10  as well as provide communication chamber  40  and the surrounding vertebral structures. 
         [0039]    In a particular embodiment, shown in  FIG. 7B , screw holes  17   a  and  17   b  are formed in recess  23  which extends to one side of device  10  to accommodate a connecting bar, as shown in  FIG. 7C . In  FIG. 7C , first and second devices  10   a ,  10   b  are shown. In such an embodiment first and second devices are maintained between the upper and lower vertebrae such that the two devices simultaneously provide fusion potential and balance the vertebrae thereon.  FIG. 7C  illustrates one way two devices according to the invention may be interlocked using connector  60  with mechanical fasteners such as screws (not shown). Connector  60  fits into a recess  23  ( FIG. 7B ) provided on trailing walls  21   a  and  21   b  of the devices to prevent rotation. Note that devices  10   a  and  10   b  may be independent of one another in terms of the various embodiments discussed herein. Accordingly, they need not assume the same wedge size once implanted, but rather, may be different to accommodate spinal defects or other aspects of patient physiology. 
         [0040]    It will be seen that the advantages set forth above, and those made apparent from the foregoing description, are efficiently attained and since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matters contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. 
         [0041]    It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall there between. Now that the invention has been described,