Abstract:
An improved spine fusion cage is provided which is particularly useful with biological compounds which are utilized in place of or in combination with a patient&#39;s bone matter. In one embodiment there is provided a cage with preselected perforated and non-perforated zones to direct the growth of bone in desired directions. In another embodiment there is provided a cage having inner and outer perforated cage bodies separated by an annulus. An end closure with occluding surfaces suitable for introduction into the annulus serves to establish one or more desired zones or patterns of occluded apertures in the cage body. In still another embodiment an end closure having occluding surfaces is provided for use in connection with conventional perforated fusion cages so as to establish desired zones or patterns of occluded apertures.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field  
           [0002]    This invention is directed to improved devices for facilitating the fusion of vertebral bone structure, which devices can be inserted either anteriorly or posteriorly into the spine.  
           [0003]    2. Background  
           [0004]    Chronic back problems cause pain and disability for a large segment of the population. In many cases, such problems are attributable to relative movement between vertebrae in the spine. Spinal surgery includes procedures to stabilize adjacent vertebrae. Common stabilization methods often involve fusing adjacent vertebrae together.  
           [0005]    Fusion techniques include removing disc material which separates the vertebrae and impacting bone into the disc area. The impacted bone fuses with the bone material of the two adjacent vertebrae to thereby fuse the vertebrae together. In a further advance in the art, spinal implants have been developed to increase the probability of a successful fusion. Such devices generally comprise a hollow cylindrical cage into which bone growth inducing substances, such as bone chips or bone slurry, may be placed. The cage wall has holes extending radially therethrough, typically throughout the entire cage surface. The combination of the cage and bone growth inducing substance facilitates arthrodesis between the adjacent vertebral bone structures. Fusion cages in both a threaded and non-threaded form have come into wide use in the last several years. Such cages are inserted either anteriorly or posteriorly into the spine in the intervertebral disc space to fuse the adjacent vertebrae as aforedescribed and to decompress neural elements.  
           [0006]    With the continued development of techniques for achieving spinal fusion through the use of spine fusion cages, there has also been developed new materials to augment the fusion process. In the older method, the patient&#39;s own bone, or cadaver bone, was used in the cage to promote bony fusion. Newer biologic materials have now been discovered that greatly augment the fusion process and in some cases make using the patient&#39;s own bone unnecessary.  
           [0007]    However, with the utilization of the newer biologic materials there has arisen a significant problem. When bone growth accelerants, such as bone morphogenic proteins, are used in cages of existing design there is risk of inducing the growth of bone around and into sensitive neural tissues. This is especially the case when a posterior approach is utilized to implant the fusion cage, as bony overgrowth in this direction may impinge on spinal nerve roots. It accordingly should be appreciated that there is a need for a fusion cage designed to be used with such biologic materials to prevent bone growth from impinging on neural tissue.  
           [0008]    It is thus an object of the present invention to provide an improved spine fusion cage which prevents the overgrowth of bone around and into sensitive areas of neural tissue.  
           [0009]    It is another object of the invention to provide a spine fusion cage having a feature whereby a surgeon may selectively occlude holes in the cage wall to prevent bone growth therethrough.  
           [0010]    A further object of this invention is to provide a novel closure for spine fusion cages which can be used with presently available fusion cages in preventing bone growth into undesirable areas.  
         SUMMARY OF THE INVENTION  
         [0011]    In accordance with one embodiment of the present invention there is provided a novel spine fusion cage which can be inserted into an intervertebral disc space using either a posterior or anterior approach and which prevents overgrowth of bone around or into neural tissue. Growth of bone into sensitive areas is prohibited by providing the cage with various zones wherein the cage wall is either perforated or non-perforated. A cage body is provided having a posterior end and an anterior end and defining an internal cavity and a longitudinal axis. The cage body has an outer surface and a plurality of radial apertures extending through the outer surface in communication with the internal cavity in a preselected pattern. Preferably, there is a first non-perforated zone extending from the posterior end of the cage a preselected length toward its anterior end, second and third non-perforated zones on the lateral sides of the cage extending in opposing relation from the first zone further toward the anterior end, and two opposed perforated zones oriented so that upon insertion of the device the perforated zones will be adjacent the vertebral bodies to be fused to allow bone growth across the vertebral interspace. Each end of the cage is provided with a non-perforated closure. In this manner bone growth is prevented in areas adjacent the non-perforated zones when the fusion cage is in place.  
           [0012]    In accordance with another embodiment of the present invention there is provided a novel spine fusion cage which provides for the selective occlusion of apertures in the cage wall so as to prevent the growth of bone in undesired directions. As an example, there is provided an inventive cage having outer and inner cage elements. An outer cage body having a posterior end and an anterior end defines an internal cavity. A plurality of radial apertures extend through the outer surface of the outer cage body to communicate with the internal cavity in a pattern covering a substantial portion of the outer surface of the cage body. An inner cage body is disposed within the internal cavity of the outer cage body and is positioned as to form an annulus between the inner wall surface of the outer cage body and the outer wall surface of the inner cage body. The inner cage body likewise has a plurality of radial apertures extending through its outer surface so as to establish communication with the annulus and the outer surface of the outer cage. An end closure means having occluding surfaces suitable for introduction into the annulus between the outer and inner cages serves to establish one or more desired zones or patterns of occluded apertures amongst the plurality of apertures in the outer cage body, thereby obstructing bone growth in undesired directions.  
           [0013]    In still another embodiment there is provided an end closure means for effecting the closure of the posterior end of a fusion cage while establishing a desired occlusion pattern of apertures in the wall of the fusion cage. The closure means comprises a non-perforated sealing member to effect the closure of the posterior end of the internal cavity of the fusion cage and one or more occluding surfaces extending from the sealing member essentially parallel to the longitudinal axis of the fusion cage so as to establish one or more desired zones or patterns of occluded apertures amongst the plurality of apertures in the cage body.  
           [0014]    A better understanding of the present invention, its several aspects, and its advantages will become apparent to those skilled in the art from the following detailed description, taken in conjunction with the attached drawings, wherein there is shown and described the preferred embodiments of the invention, simply by way of illustration of the best mode contemplated for carrying out the invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 anatomically illustrates a bilateral posterior insertion of two inventive cylindrical spine fusion cages to achieve fusion across the L5/S1 disc space.  
         [0016]    [0016]FIG. 2 is an exploded perspective view of an embodiment of an inventive cage having preselected perforated and non-perforated zones on its outer surface.  
         [0017]    [0017]FIG. 3 is a perspective view taken along line  3 - 3  of FIG. 2.  
         [0018]    [0018]FIG. 4 is a sectional view taken along line  4 - 4  of FIG. 2.  
         [0019]    [0019]FIG. 5 is perspective view of an embodiment of an inventive cage having outer and inner cage elements.  
         [0020]    [0020]FIG. 6 is a sectional view taken along line  6 - 6  of FIG. 5.  
         [0021]    [0021]FIG. 7 is a perspective view of an end closure for use in connection with the cage of FIG. 5.  
         [0022]    [0022]FIG. 8 a top sectional view of the cage of FIG. 5 including the end closure of FIG. 7.  
         [0023]    [0023]FIG. 9 is an exploded side view of a conventional fusion cage modified to utilize an inventive end closure means to selectively occlude certain apertures in the outer surface of the cage.  
         [0024]    [0024]FIG. 10 depicts the partial insertion of the inventive closure means into the cage of FIG. 9.  
         [0025]    [0025]FIG. 11 depicts the full insertion of the inventive closure means into the cage of FIG. 9.  
         [0026]    [0026]FIG. 12 is top sectional view of a modified conventional cage including an inventive end closure means.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]    Several types of conventional spine fusion cages have been designed, such as those described by Bagby, Brantigan and Ray, respectively, in  Athrodesis by the Distraction - Compression Method Using a Stainless Steel Implant , Orthopaedics 1988, Vol. 11:931-4 ; A Carbon Fibre Implant to Aid Interbody Lumbar Fusion , Spine 1991, 16 (Suppl):S277-82 (with Steffee and Geiger); and  Threaded Titanium Cages for Lumbar Interbody Fusions , Spine 1997, 22:667-80; and as described in the patent art, for example, in U.S. Pat. Nos. 4,501,269; 5,055,104; 5,571,192; 5,702,449; 5,876,457; 5,906,616; 5,976,187; 5,980,522; 6,010,502; 6,015,436; and 6,039,762. Each of the foregoing publications and patents is incorporated herein by reference.  
         [0028]    Such devices provide for a relatively simple and effective technique for implementing lumbar interbody fusion by correcting any existing mechanical deformity of the spine while providing stability and a good environment until successful arthrodesis is obtained. These cage devices are hollow and are positioned between the articulating vertebrae, where they support and immobilize the joint as well as contain the growth of the bone graft that is packed into the internal cavity of the device.  
         [0029]    Anterior lumbar interbody fusion (ALIF) and posterior lumbar interbody fusion (PLIF) are two commonly adopted approaches for grafted lumbar interbody fusion with augmentation via a spine fusion cage. ALIF is performed through a retroperitoneal or transperitoneal approach with extensive discectomy followed by the placement of one or more cages in the vertebral interspace. In PLIF, partial or complete laminectomy and facetectomy is followed by posterior discectomy and the placement of one or more cages in the vertebral interspace. FIG. 1 is illustrative of a bilateral posterior insertion of two inventive cylindrical spine fusion cages  20  to achieve fusion across the L5/S1 disc space. The cages  20  are secured far enough apart from each other (by a few millimeters) to avoid contact and potential back-threading. It should be understood that the fusion cages of this invention can be installed in their operative positions via either the anterior or posterior approaches; however, the posterior approach is the most dangerous in regards for bony overgrowth impinging on neural tissue particularly when the cage is used along with bone growth inducing materials.  
         [0030]    The inventive cages  20  promote bony fusion by holding adjacent levels immobile and by allowing bone to grow only into the vertebral bodies an away from the spinal canal and nerve roots. Designs that do not control direction of growth are undesirable for use with biologic bone growth accelerants to the extent unchecked bony overgrowth may impinge upon neural tissues. Through the present invention there are provided designs for spine fusion cages which prevent bone growth around and into sensitive areas of neural tissue.  
         [0031]    Referring now to FIGS.  2 - 4 , and in accordance with one embodiment of the present invention, there is provided an inventive spine fusion cage  20  wherein growth of bone into sensitive areas is prohibited by providing the cage with various zones or areas wherein the cage wall is either perforated or non-perforated. A cage body  22  is provided having a posterior end  24  and an anterior end  26  and defining an internal cavity  28  and a longitudinal axis  30 . The cage body  22  is typically between 20-25 mm in length and may be of a variety of diameters (if cylindrical) and heights. The cage body  22  has an outer surface  32  and a plurality of radial apertures  34  extending through the outer surface  32  in communication with the internal cavity  28  in a preselected pattern. Preferably, there is a first non-perforated zone  36  extending from the posterior end  24  of the cage body  22  a preselected length, preferably 5-10 mm, toward its anterior end  26 , second and third non-perforated zones  38 ,  40  on the lateral sides of the cage body  22  extending in opposing relation from the first zone  36  further toward the anterior end  26 , and two opposed perforated zones  42 ,  44  oriented cephalad (or to the superior side) and caudad (or to the inferior side) so that upon insertion of the device the perforated zones  42 ,  44  will be adjacent the vertebral bodies to be fused to allow bone growth across the vertebral interspace. Each end  24 ,  26  of the cage body  22  is provided with a non-perforated closure. In the illustrated embodiment, the anterior end  26  is closed by an integral non-perforated end wall  46 , while there is provided a removable end cap  48  securable, by threaded attachment, friction fit or otherwise, to the posterior end  24  of the cage body  22 . The end cap  48  may be provided with a recess  50  for receiving an insertion tool, for example if the end cap is made to threadably connect to the cage body, and there is preferably provided on the top of the end cap  48  a line score  52  for aiding proper orientation of the device in the vertebral interspace.  
         [0032]    The cage body  22  may be provided with threads  54 , projections, ridges, protrusions, barbs, spurs or other insertion means to aid in placement of the cage within the interbody area. The anterior end  26  can be rounded in order to facilitate the insertion of the cage  20  relative to one or more bone structures. The cage  20  may be made of surgical steel, titanium or other acceptable implantable materials. Typically, the cage  20  is countersunk into the vertebral interspace with the end cap  48  in place by using an insertion tool (not shown) to screw the cage  20  into position. Once the cage is properly aligned, the end cap  48  is removed so that bone growth inducing material can be packed into the internal cavity  28  of the cage body  22 , whereupon the end cap  48  is tightly replaced.  
         [0033]    As can now be appreciated, the inventive cage  20  prevents bone growth into areas adjacent the non-perforated zones when the fusion cage is in place. Because the posterior 5-10 mm of the cage is non-perforated, including, importantly, the end cap, bony overgrowth is inhibited in areas immediately adjacent the posteriorly located neural tissues. In similar fashion, lateral overgrowth of bone is impeded by the second and third non-perforated zones. Desired growth through the vertebral interspace, however, is facilitated via the perforated zones.  
         [0034]    It should be understood to be within the ordinary skill of one in the art to modify the placement of the various perforated and non-perforated zones as warranted by orthopaedic considerations to achieve desired bone growth and preclude unwanted bone growth. It is also within the ordinary skill of one in the art to modify the aforedescribed device for anterior insertion procedures by providing a removable end cap on the anterior end of the cage body and reversing the thread direction on the outside surface of the cage body.  
         [0035]    As mentioned above, it is also advantageous for a surgeon to have the ability to selectively occlude apertures in the cage wall to prevent bone growth in undesired directions. Now referring to FIGS.  5 - 8 , to achieve this object, and in accordance with another embodiment of the present invention, there is provided a spine fusion cage  120  having an outer cage body  122  with a posterior end  124  and an anterior end  126  and defining an internal cavity  128  and a longitudinal axis  130 . The outer cage body  122  has an outer surface  132  and a plurality of radial apertures  134  extending through the outer surface  132  in communication with the internal cavity  128  in a pattern covering a substantial portion of the outer surface  132  of the cage body  122 . An inner cage body  136  into which is placed bone growth inducing substances is disposed within the internal cavity  128  of the outer cage body  122  and is positioned as to form an annulus  138  between the inner wall surface  140  of the outer cage body  122  and the outer wall surface  142  of the inner cage body  136 . The inner cage body  136  likewise has a plurality of radial apertures  144  extending through its outer surface  142  so as to establish communication with the annulus  138  and the outer surface  132  of the outer cage body  122 . A solid end closure  146  having opposed occluding surfaces  148 ,  150  suitable for introduction into the annulus  138  serves to establish one or more desired zones or patterns of occluded apertures amongst the plurality of apertures in the outer cage body  122 , thereby obstructing bone growth in undesired directions.  
         [0036]    More specifically, as shown in FIG. 7 end closure  146  is comprised of a non-perforated cap or closure means  152  having occluding surfaces  148  and  150  extending therefrom. Such surfaces may be of sufficient length to extend to the bottom of the cage member  120  as shown in FIG. 5 or may be of a more limited length so as to occlude only a portion of the apertures  134  in the outer cage body  122 . The end closure  146  may be constructed so as to provide a top circumferential crown portion  154  and between the occluding surfaces  148 ,  150  a shoulder  156  which may engage a rib means  158 ,  160  as shown in FIG. 8 to act as a longitudinal stop and to limit the degree of rotation which can be made by occluding surfaces  148 ,  150  so as to maintain the selected occlusion pattern. When positioned within the annulus  138  of the fusion cage  120 , the occluding surfaces  148 ,  150  serve to close openings in the posterior end of the cage  120  as well as to occlude openings which are in a lateral position so as to effect bone growth through the apertures in the caudal and cephalad directions when placed in the desired position between two vertebrae. Various interchangeable forms of end closures may be provided, for example having differently shaped and dimensioned occluding surfaces, so as to provide for the surgeon a selection which meets objectives according to various orthopaedic exigencies. It is also within the scope of this invention that the shape and dimensions of the occluding surfaces may be modifiable by the surgeon, such as if the occluding surfaces comprise a surgical plastic adapted to be cut or trimmed to achieve a desired configuration. In this manner, a cage possessing a full pattern of apertures can be used as a “universal” cage in combination with one of a wide selection of end closures or a modifiable end closure to achieve any desired patterned of perforation.  
         [0037]    The end closure  146  can be threaded or otherwise designed to effect the closure of the posterior end of the cage  120  and may be provided with securing means such as square or hex-shaped recess  162  which can be used with a socket wrench to tightly position the end closure  146  in the posterior end of the fusion cage  120 . In complementary fashion, threads may be provided at the posterior end of the cage  120  to receive a threaded end closure  146  or it can be so adapted that the end closure  146 , when not threaded, can be simply snapped into place to effect the desired closing of the fusion cage  120 .  
         [0038]    A thread  164  may be provided as part of the outer surface  132  of the fusion cage  120 . Such a thread can be replaced with a plurality of discrete threads or a plurality of projections, ridges, protrusions, barbs or spurs and be within the spirit and scope of the invention.  
         [0039]    In assembly of the fusion cage of this embodiment of the invention, following introduction of the selected biologic material into the internal cavity  128  within the inner cage body  136 , the annulus  138  remains clear so as to easily accept end closure  146  within the annulus  138  while the biologic materials are retained in the internal cavity  128 . Through the dimensioning, shaping and rotation of occluding surfaces  148 ,  150  there is achieved an occlusion of apertures so as to define the desired pattern of apertures through which bone growth is to be permitted.  
         [0040]    In keeping with the teachings of the present invention, there is further provided a novel closure for conventional spine fusion cages which can be used with little or no modification to presently available fusion cages in preventing bone growth into undesirable areas. This embodiment involves providing a means for the occlusion of selected apertures in currently available fusion cages, such as to those commonly referred to as Brantigan, BAK and Ray cages, so that bone growth is directed only toward the vertebral bodies and away from the spinal canal and nerve roots.  
         [0041]    Making reference now to FIGS.  9 - 11 , there is illustrated an end closure  220  for effecting the closure of the posterior end  222  of a conventional fusion cage body  224  while establishing a desired occlusion pattern of apertures in the wall of the cage body  224 , which cage possesses apertures  226  substantially entirely thereabout. The end closure  220  comprises a non-perforated sealing member  228  to effect the closure of the posterior end  222  of the cage body  224  and one or more occluding surfaces  230 ,  231  extending from the sealing member  228  essentially parallel to the longitudinal axis  230  of the cage body  224  so as to establish one or more desired zones or patterns of occluded apertures amongst the plurality of apertures in the cage body  224 . Reference is made to the disclosure provided above with respect to the aforedescribed end closure  146 , which disclosure is equally applicable to end closure  220  and further recitation is believed unnecessary. Suffice it to say that the prior described end closure  146  may be made adaptable to conventional fusion cages so as to achieve the objectives of the present invention.  
         [0042]    As depicted in FIG. 12, if desired the conventional type of fusion cage can be so modified as to provide ribs  232 ,  234  in association with the inner surface of the posterior end of the cage according to the teachings herein. FIG. 12 provides a top view of the fusion cage of FIG. 11 along the line  12 - 12  which shows the placement of the ribs  232  and  234  to accommodate occluding surfaces  230 ,  231  of the end closure  220 .  
         [0043]    While the invention has been described with a certain degree of particularity, it is understood that the invention is not limited to the embodiment(s) set for herein for purposes of exemplification, but is to be limited only by the scope of the attached claim or claims, including the full range of equivalency to which each element thereof is entitled.