Abstract:
A method for facilitating fusion of bone structures includes providing an implant member for positioning between opposed bone structures, the implant member having a first end and a second end, and a segmentable portion having an outer wall defining an internal cavity for reception of bone growth inducing substances. The outer wall has at least one groove which extends substantially continuously about the outer wall to define a plurality of discrete ring-like segments. Each ring-like segment includes a plurality of apertures extending therethrough in communication with the internal cavity to permit fusion of vertebral bone tissue. The vertebral space defined between adjacent vertebral bodies is accessed, and the desired implant member length for insertion into the space between adjacent vertebral bodies is determined. The implant member is cut to the desired length and advanced within the vertebral space between the adjacent vertebral bodies.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. application Ser. No. 09/815,504, filed on Mar. 23, 2001, now U.S. Pat. No. 6,899,734, the disclosure of which is hereby incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   The present disclosure generally relates to a surgical apparatus for fusing adjacent bone structures, and, more particularly, to a segmented and/or modular apparatus and method for fusing adjacent vertebrae. 
   The fusion of adjacent bone structures is commonly performed to provide for long-term replacement to compensate for vertebral subluxation typically caused by severe trauma to the spine, degenerative or deteriorated bone disorders, e.g., osteoporosis, abnormal curvature of the spine (scoliosis or kyphosis) and/or weak or unstable spine conditions typically caused by infections or tumors. In addition, an intervertebral disc, which is a ligamentous cushion disposed between adjacent vertebrae, may also undergo deterioration or degeneration as a result of injury, disease, tumor or other disorders. The disk shrinks or flattens leading to mechanical instability and painful disc translocations, commonly referred to as a “slipped disc” or “herniated disc”. 
   Conventional procedures for disc surgery include partial or total excision of the injured disc portion, e.g., discectomy, and replacement of the excised disc with biologically acceptable plugs or bone wedges. The plugs are driven between adjacent vertebrae to maintain normal intervertebral spacing and to achieve, over a period of time, bony ingrowth or “fusion” with the plug and opposed vertebrae. 
   Alternatively, a metallic fusion cage may be inserted within a tapped bore or channel formed in the intervertebral space thereby stabilizing the vertebrae and maintaining a pre-defined intervertebral space. A pair of fusion cages may also be implanted within the intervertebral space. After a period of time, the soft cancellous bone of the surrounding vertebral bone structures infiltrates the cage through a series of apertures in the cage wall and unites with bone growth inducing substances disposed within an internal cavity of the cage wall to eventually form a solid fusion of the adjacent vertebrae. 
   SUMMARY OF THE INVENTION 
   The present disclosure relates to a fusion implant apparatus for facilitating fusion of adjacent bone structures. The fusion apparatus includes a modular implant member for positioning between adjacent opposed bone structures and having a plurality of ring-like segments which engage one another in an end-to-end or stack-like manner. Each ring-like segment includes an outer wall which defines an internal cavity for the reception of bone growth inducing substances and includes a plurality of apertures which extend through the outer wall of the ring-like segments in communication with the internal cavity to permit fusion of vertebral bone tissue. Preferably, at least one ring-like segment includes first and second mechanical interfaces, the first mechanical interface being dimensioned to engage a corresponding mechanical interface disposed on another ring-like segment and the second mechanical interface being dimensioned to mechanically engage an end cap. At least one ring segment preferably incorporates a C-shaped or split ring configuration with semi-resilient characteristics to facilitate engagement with another ring-like segment. 
   The end cap preferably includes a plurality of detents or spike-like protrusions which project outwardly therefrom and which are designed to anchor the fusion cage to the underside of the vertebral bodies. The end cap may also include one or more flanges, retaining sleeves, locking pins, or other mechanically interfacing mechanism for securing the end cap to the body of the implant member. In one embodiment, the end cap has a C-shaped or split ring configuration to facilitate mounting the end cap to the implant member. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a modular fusion cage according to the present disclosure; 
       FIG. 2  is a side view with parts separated of an alternate embodiment of the modular fusion cage of the present disclosure; 
       FIG. 3A  is a bottom, perspective view of one embodiment of the end cap according to the present disclosure showing a plurality of locking pins projecting from an inner diameter of the end cap; 
       FIG. 3B  is a bottom, perspective view of an alternate embodiment of the end cap having a diametrically tapered inner diameter; 
       FIG. 3C  is a bottom, perspective view of an alternate embodiment of the end cap having a C-shaped or split-ring configuration; 
       FIG. 4A  is a top, perspective view of an alternate embodiment of an end cap having a central aperture located therethrough and a plurality of protrusions which project from the outer-facing surface thereof; 
       FIG. 4B  is a top, perspective view of an alternate embodiment of an end cap having an array of centrally-located apertures located therethrough and a plurality of protrusions which project from the outer-facing surface thereof; 
       FIG. 4C  is a top, perspective view of an alternate embodiment of an end cap having a central aperture located therethrough and a plurality of arcuately-shaped wedges which project from the outer-facing surface thereof; 
       FIG. 5A  is a schematic, lateral view showing the placement of the fusion cage of  FIG. 1  between two adjacent vertebrae; 
       FIG. 5B  is a schematic, top view showing a pair of fusion cages according to the present disclosure positioned within the intervertebral space for fusion of adjacent vertebrae; 
       FIG. 6A  is a rear, perspective view of an alternate embodiment of an end cap having a pair of retaining sleeves which secure the end cap to the fusion cage; and 
       FIG. 6B  is a front, perspective view of the embodiment of  FIG. 6A . 
   

   DETAILED DESCRIPTION 
   Referring now to the drawings in which like reference numerals identify similar or identical elements throughout the several views,  FIG. 1  illustrates one embodiment of a modular fusion cage implant  10  according to the present disclosure. Fusion implant  10  includes a generally elongated body  12  having a first end  14 , a second end  16  and an end cap  40  ( FIG. 3A-3C ) which is mountable to the body  12 . 
   Cage  10  is preferably fabricated from a suitable biocompatible rigid material such as titanium and/or alloys of titanium, stainless steel, ceramic materials or rigid polymeric materials. Moreover, it is envisioned that cage  10  is sufficient in strength to at least partially replace the supporting function of an intervertebral disc, i.e., to maintain adjacent vertebrae in desired spaced relation, during healing and fusion. 
   As best shown in  FIG. 1 , the body  12  of cage  10  includes an outer wall  15  which encloses an inner cavity  22  defined within the interior of the cage body  12 . Inner cavity  22  accommodates bone growth substances which induce the soft cancellous bone surrounding the vertebrae to grow inwardly towards the contact surfaces of the fusion cage  10  to stabilize the cage  10  between two adjacent vertebrae  202 ,  204  (See  FIGS. 5A and 5B ). It is envisioned that outer wall  15  can be designed in a variety of different shapes depending upon a specific purpose, e.g., oval-shaped, kidney-shaped, etc. It is also envisioned that the surface of outer wall  15  may be coated with a variety of different materials which facilitate insertion of the cage  10  and enhance retention of the cage  10  between opposing vertebrae  202 ,  204 . 
   As best shown in  FIG. 1 , a plurality of apertures  20  extend through outer wall  15  of cage body  12  and preferably promote immediate bone to bone contact between the vertebral bodies  202 ,  204  and the bone inducing substances packed within the internal cavity  22  of the cage body  12 . Such arrangement of apertures  20  is disclosed in commonly assigned U.S. Pat. Nos. 4,961,740 and 5,026,373, the contents of which are hereby incorporated by reference. Apertures  20  are preferably substantially the same in dimension although it is envisioned that the dimensions of the apertures  20  may vary to provide for more or less bone-to-bone contact depending upon a particular purpose. 
   Preferably, apertures  20  are oriented such that when the cage  10  is inserted between the upper and lower vertebral bone structures  202 ,  204 , the apertures  20  encourage bony ingrowth through cage body  12  from the vertebral bone structures  202 ,  204 . 
   As shown in  FIG. 1 , the body  12  of cage  10  includes seam or grooves  18  which encircle the outer periphery of body  12 . Grooves  18  segment the body  12  into discrete ring-like segments, e.g.,  13   a ,  13   b ,  13   c  and  13   d . Preferably, the grooves  18  act as cutting guides as well as measurement guides in determining the desired length of the fusion cage  10  needed. It is envisioned that the height of each ring-like segment, e.g.,  13   a , and therefore, the distance between each corresponding groove  18 , may be varied depending upon a particular purpose. 
   One embodiment of a fusion cage  100  according to the present disclosure is illustrated in  FIG. 2  and includes a plurality of ring-like segments  113   a  and  113   b  which mechanically engage one another in an end-to-end or stack-like manner to form elongated tube body  112 . Each ring segment,  113   a ,  113   b , includes a plurality of apertures  120  located therethrough which are appropriately sized to promote immediate bone to bone contact between the vertebral bodies  202 ,  204  ( FIG. 5A ) and the bone inducing substances as described above. The first end of ring-like segment  113   a , e.g., end  114   a , preferably includes a mechanical interface, e.g., flange  125 , which is dimensioned to mechanically engage a corresponding annular recess or lip  129  disposed within the opposed end  116   b  of ring-like segment  113   b . The ring segments  113   a ,  113   b  may be designed with other types of mechanically engaging interfaces depending upon a particular purpose, e.g., interlocking wedges, locking pins, etc. 
   As can be appreciated, groove  18  is formed as a result of the union of the two ring-like segments  113   a ,  113   b . The grooves  18  may project from body  12  and include sharp edges which eliminate the need for any internal rings in the sizing device. It is envisioned that the grooves will also promote subsidence into the vertebral bodies  202 ,  204 . 
   End  116   a  of ring segment,  113   a  includes a flange  125  which is designed to engage a third ring segment (not shown) to further the length of elongate body  112  depending upon a particular purpose or, alternatively, the flange  125  may engage an end cap  40  in a manner described below with respect to  FIGS. 3A-3C . It is envisioned that the end  114   b  of ring segment  113   b  may also include a mechanical interface (not shown) designed to engage another ring segment (not shown) or, alternatively, a second end cap  40  if needed. 
   As can be appreciated by the present disclosure, no screws are required to assemble the various components of the cage  100 , i.e., the ring segments  113   a ,  113   b , etc. and the end caps  40 ,  140 ,  240 ,  340 ,  440  and  540  as described below prior to and/or during implantation. Moreover, the size, number and shape (e.g., oval shape, kidney-shape, etc.) of the of ring-like segments  113   a ,  113   b  may vary enabling the surgeon to quickly and easily customize each fusion cage  100  according to the pre-defined intervertebral space “I” between a patient&#39;s opposing vertebrae  202 ,  204 . 
   As best shown in  FIG. 2 , ring segment  113   a  may also be designed in a C-shaped or split-ring configuration to facilitate engagement with ring segment  113   b . More particularly, ring segment  113   a  includes a pair of opposing edges  131   a ,  131   b , respectively, which define an open slit  130  in the outer wall  115  thereof which enables a surgeon to radially compress ring segment  113   a  inwardly enabling facile mechanical engagement of flange  125  with the lip  129  of ring segment  113   b . Once flange  125  and lip  129  are properly aligned or seated, i.e., engaged, the surgeon simply reduces inward compressive pressure to interlock the two ring segments  113   a  and  113   b  together. The surgeon simply compresses ring  113   a  again to disengage the two ring segments  113   a ,  113   b  if needed. As can be appreciated, a surgeon can quickly and easily change the size and shape of the cage  100  as needed to ensure proper positioning within the intervertebral space “I” between opposing vertebrae  202 ,  204 . 
   As stated above, it is also envisioned that cage  10 ,  100  can be dimensioned such that cage  10 ,  100  is generally symmetrical, i.e., end-to-end symmetry, which permits insertion of the cage  10  from either end  14 ,  16 . 
     FIGS. 3A-4C  disclose various configurations of the end cap  40  which mounts to the elongated body  12 ,  112 . As can be appreciated, the end cap  40  can be configured to mount to either or both ends  14 ,  16  depending upon a particular purpose. For example, and as best shown In  FIG. 1 , the end  14  of the body  12  may include an annular recess  23  which is configured to receive a plurality of diametrically opposing, semi-resilient detents or locking pins  48  which protrude from an inner diameter  44  of the end cap  40  of  FIG. 3A . As can be appreciated, the surgeon can easily grasp the outer periphery  42  of cap  40  and gently force the semi-resilient locking pins  48  towards body  12  until the pins  48  “snap” into recess  23 . Preferably, a plurality of apertures  46  extend through end cap  40  thereby permitting direct growth or passage of bone or bone growth inducing substances through apertures  46 . 
     FIG. 3B  shows an alternate end cap  140  which includes an outer peripheral portion  142 , an inner diameter  144  and a plurality of apertures  146  disposed therethrough. The inner diameter  144  generally tapers in an inward fashion, i.e., from an adjoining edge  145  which has a first diameter to a leading edge  147  which has a reduced diameter. The diameter of adjoining edge  145  is dimensioned slightly larger than the inner diameter  144  of elongated body  12  and the diameter of leading edge  147  is dimensioned slightly smaller than the inner diameter  144 . As can be appreciated, this diametrically-tapered configuration of the inner diameter  144  allows the surgeon to quickly and easily wedge end cap  140  within the end  14  of the fusion cage  10  in a secure, friction-fit manner. 
     FIG. 3C  shows another embodiment of an end cap  240  which includes an outer peripheral portion  242 , an inner diameter  244  and a single aperture  246  disposed therethrough. The end cap  240  is generally C-shaped and includes two opposing ends  247   a  and  247   b  which define a slit  249  therebetween giving the end cap  240  an overall split-ring appearance. 
   The inner diameter  244  of end cap  240  also includes a flange  243  disposed about the outer periphery thereof. Flange  243  is dimensioned to “snap” into and seat within annular recess  23  of body  12  (See  FIG. 1 ) after end cap  240  is inserted within body  12 . Preferably, slit  249  is communicated through both the outer peripheral portion  242  and the inner diameter  244  such that the entire end cap  240  has some degree of radial resiliency. As can be appreciated, this enables the surgeon to quickly and easily mount the end cap  240  onto body  12  upon the application and subsequent release of inward, radial pressure. After the end cap  240  is mounted to body  12 , bone or bone growth inducing substances can easily infiltrate cage  10 ,  100  through aperture  246  to promote fusion between the cage  10  and the vertebral bodies  202 ,  204 . 
     FIGS. 6A and 6B  show yet another embodiment of an end cap  640  according to the present disclosure which includes an outer portion  642  having an inner periphery  643  which defines a central aperture  646  therethrough. A pair of opposing, arcuately-shaped retaining sleeves  644   a  and  644   b  extend concentrically and distally along axis “A” with respect to inner periphery  643  and each include an outer rim-like flange  651   a  and  651   b , respectively, at the distal end thereof. Preferably, arcuate sleeves  644   a  and  644   b  are semi-resilient and are dimensioned to mechanically engage the inner annular recess  23  of body  12 . More particularly and upon insertion of end cap  640  into body  12 , sleeves  644   a  and  644   b  radially compress inwardly towards axis “A” when end cap  640  is inserted within body  12  and outer flanges  651   a  and  651   b  “snap” into annular recess  23  to secure end cap  640  to cage  10 . 
   As best seen in  FIGS. 6A and 6B , the face of the end cap  640  also includes a plurality of spike-like detents  652  which are designed to anchor the cage  12  (once assembled with end cap  640 ) to the underside of the opposing vertebral bodies  202 ,  204 . It is envisioned that these detents can be mounted or integrally incorporated with any one of the above-mentioned end cap configurations, e.g.,  40 ,  140 ,  240 ,  340 ,  440 , and  540 . 
   Moreover, various other configurations of detents are also envisioned. For example,  FIG. 4A  shows one arrangement of detents  352  which are integrally associated with a donut-like end cap  340 . In this particular embodiment, the detents  352  are arranged in an array-like fashion about a centrally disposed aperture  346  defined through end cap  340 . Preferably, detents  352  project from face  342  to engage the underside of the vertebral bodies  202 ,  204 . It is envisioned that the detents  352  may project from face  342  at varying angles relative to the face  342  which may facilitate insertion and/or improve subsidence of the cage  10  into the vertebral bodies  202 ,  204 . 
     FIG. 4B  shows another embodiment of an end cap  440  having a plurality of apertures  446  disposed through the face  450  thereof. Preferably, a plurality of spike-like detents  452  are arranged radially about the periphery of the face  450  proximate outer rim  442 . As best illustrated in a comparison of  FIGS. 4A and 4B , the detents  352 ,  452  may vary in size and dimension depending upon a particular purpose or to achieve a desired result. 
     FIG. 4C  shows yet another embodiment of a C-Shaped end cap  540  having two opposing edges  541   a  and  541   b  which define a slit  549  therebetween. The end cap  540  also includes a face  550  having outer and inner peripheries  542  and  547 , respectively. The inner periphery  547  defines a central aperture  546  therethrough which provides a passageway, for the bone growth inducing substances which are used to promote fusion as mentioned above. A plurality of arcuately-shaped wedges  552  having a generally triangular cross section extend outwardly from the proximal face  550  and serve to anchor the cage  10  (once assembled with the end cap  540 ) to the underside of the vertebral bodies  202 ,  204 . 
   It is envisioned that the slit  549  allows the end cap  540  to contract radially inwardly to facilitate mounting the end cap to the cage  10  as described above with respect to  FIG. 3C . 
   As can be appreciated, all of the above end cap embodiments snap into or mount to the cage  10  easily and readily without requiring screws or other retention devices. 
   The present disclosure also relates to a method of inserting the fusion cage  10  into an intervertebral space “I” defined between vertebrae  202 ,  204 . Initially, one lateral side of an intervertebral space “I” between the two vertebral bodies  202 ,  204  is accessed utilizing appropriate retractors (not shown) to expose the vertebral surface. Thereafter, a retractor is inserted within the intervertebral space “I” between vertebral bodies  202 ,  204  for distracting the vertebral bodies  202 ,  204  to a desired predetermined distance. A partial or full dissectomy may be performed. 
   The modular fusion cage  10  is then assembled and sized to determine the appropriate number of ring segments, e.g.,  13   a ,  13   b ,  13   c  and  13   d , needed to fit in the intervertebral space “I” and maintain the adjacent vertebral bodies in the predetermined space during the fusion process. As can be appreciated, no internal rings are required because of the sharp edges or grooves  18  produced in the sizing device which allow subsidence into the vertebral bodies  202 ,  204 . Moreover, the segmented design of the cage  10  eliminates the need for cutting tools and measuring guides. 
   The fusion cage  10  is then packed with bone growth inducing substances as in conventional in the art and one of the above-identified end caps is then mounted to the cage  10  in one of the above-identified manners. The cage  10  and end cap assembly is then mounted on an insertion instrument (not shown) and driven between the vertebral bodies  202 ,  204 . As mentioned above, the spike-like detents  352 ,  452  or  552  promote subsidence into the vertebral bodies  202 ,  204 . 
   Cage  10  is then released from the mounting instrument which is subsequently removed from the disc area. It is envisioned that a second end cap may be mounted to the distal end of cage  10  to retain the bone growth substances within cage  10 . 
   A second lateral side of the intervertebral space “I” may be accessed and the above-described process is repeated to insert a second cage  10  in side-by-side relation as shown in  FIG. 5B . Preferably, the cages  10 ,  10  are arranged such that the cages  10 ,  10  reside in adjacent side-by-side relation. 
   Once implanted, the fusion cages  10 ,  10  form struts across the intervertebral space “I” to maintain the vertebrae  202 ,  204  in appropriate spaced relation during the fusion process. Over a period of time, the vertebral tissue communicates through apertures  20  within cages  10 ,  10  to form a solid fusion. 
   From the foregoing and with reference to the various figure drawings, those skilled in the art will appreciate that certain modifications can also be made to the present disclosure without departing from the scope of the same. While the above description contains many specifics, these specifics should not be construed as limitations on the scope of the. disclosure, but merely as exemplifications of preferred embodiments thereof. Those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto.