Abstract:
A spinal implant is described in this disclosure. The implant includes first and second pieces separated by a controlled break location. Spinal implant kits having multiple spinal implant pieces derived from a common source also are disclosed.

Description:
[0001]     This application claims priority to U.S. Provisional application 60/325,804, filed Sep. 28, 2001. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to skeletal implants. More particularly, the present invention relates to implants for stabilizing intervertebral joints.  
       BACKGROUND OF THE INVENTION  
       [0003]     Chronic back problems cause pain and disability for a large segment of the population. In many cases, chronic back problems are caused by intervertebral disc disease. When an intervertebral disc is diseased, the vertebrae between which the disc is positioned may be inadequately supported, resulting in persistent pain. Stabilization and/or arthrodesis of the intervertebral joint can reduce the pain and debilitating effects associated with disc disease.  
         [0004]     Spinal stabilization systems and procedures have been developed to stabilize diseased intervertebral joints and, in some cases, to fuse the vertebrae that are adjacent the diseased joint space. Most fusion techniques include removing some or all of the disc material from the affected joint, and stabilizing the joint by inserting an implant (e.g., a bone graft or other material to facilitate fusion of the vertebrae) in the cleaned intervertebral space.  
         [0005]     Spinal implants can be inserted into the intervertebral space through an anterior approach, a posterior approach, or postero-lateral approach. The anterior approach involves a surgeon seeking access to the spine through the front (i.e., abdominal area) of the patient. The posterior approach involves a surgeon seeking access to the spine through the back of the patient. The postero-lateral approach is similar to the posterior approach with access coming more from either or both sides of the patient. A variety of different anterior, posterior and postero-lateral techniques are known.  
         [0006]     It is often an advantage to use the posterior approach because such an approach typically involves a smaller and less intrusive opening than those required by anterior approach techniques. Because a posterior approach involves a smaller opening, two or more implants are often used in this approach as compared to using a single larger implant. For example, in one technique, adjacent vertebral bodies are stabilized by implanting separate implants between the vertebral bodies on opposite sides of a sagittal plane passing through the midline of the vertebral bodies. When using multiple implants to support adjacent vertebrae, it is desirable for the implants to have similar or identical mechanical properties so that uniform support is provided on both sides of the sagittal plane. In some instances, it also is desirable for the implants to have similar or identical biologic properties (e.g., to reduce the risk of tissue rejection and to enhance the uniformity of creeping substitution).  
       SUMMARY OF THE INVENTION  
       [0007]     One aspect of the present invention relates to skeletal implants and skeletal implant kits adapted to ensure that multiple implants used to support opposing vertebrae have been derived from the same source.  
         [0008]     A variety of other aspects of the invention are set forth in part in the description that follows, and in part will be apparent from the description, or may be learned by practicing the invention. The aspects of the invention relate to individual features, as well as combinations of features. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a top, plan view of one embodiment of a spinal implant in accordance with the principles of the present invention;  
         [0010]      FIG. 2   a  is a front, top perspective view of the spinal implant of  FIG. 1 ;  
         [0011]      FIG. 2   b  is a rear, perspective view of the spinal implant of  FIG. 1 ;  
         [0012]      FIG. 2   c  is a front view of the spinal implant of  FIG. 1 ;  
         [0013]      FIG. 2   d  is a side view of the spinal implant of  FIG. 1 ;  
         [0014]      FIG. 3  shows the spinal implant of  FIG. 1  split into two pieces;  
         [0015]      FIG. 4  shows one piece of the spinal implant of  FIG. 1 ;  
         [0016]      FIG. 5   a  is a cross-sectional view taken along section line  5   a - 5   a  of  FIG. 1 ;  
         [0017]      FIG. 5   b  is a cross-sectional view taken along section line  5   b - 5   b  of  FIG. 1 ;  
         [0018]      FIG. 5   c  is a cross-sectional view taken along section line  5   c - 5   c  of  FIG. 1 ;  
         [0019]      FIG. 6   a - 6   e  show various views of an insertion tool suitable for inserting the spinal implant of  FIG. 1 ;  
         [0020]      FIG. 7  is a kit incorporating the spinal implant of  FIG. 1 ;  
         [0021]      FIG. 8  is a kit incorporating the spinal implant of  FIG. 1  with the spinal implant being separated into two pieces; and  
         [0022]      FIGS. 9   a  and  9   b  show the spinal implant of  FIG. 1  inserted into the intervertebral space between two vertebrae. 
     
    
     DETAILED DESCRIPTION  
       [0023]     The present invention is directed to skeletal implants, skeletal implant kits and methods for placing implants between bones desired to be fused. It is preferred for the implants to be used for vertebral/spinal applications such as fusing cervical, thoracic and/or lumbar intervertebral joints. In the case of fusing an intervertebral joint, implants in accordance with the principles of the present invention can be implanted using an anterior, posterior or postero-lateral approach to the patient&#39;s vertebrae.  
         [0024]     As used herein, an “implant” includes any implant suitable for facilitating fusion between adjacent bones and includes implants prepared from known implant materials including, non-bone material such as titanium, stainless steel, porous titanium, bio-glass, calcium phosphate, ceramic, carbon fiber-based polymers, biodegradable and polymers. However, it is preferred for implants in accordance with the principles of the present invention to be derived from natural bone tissue (e.g., allograft and xenograft bone). It is most preferred for implants in accordance with the principles of the present invention to be derived from natural bone such as from a cadaveric allograft bone source. For example, the implants can be derived by cross-sectioning cortical rings from cadaveric allograft bones such as femur, tibia or fibia bones. Alternatively, the implants can be formed/molded from ground, sintered or composite bone material. Bone tissue cut from a human femur bone is particularly suited for use in practicing the principles of the present invention. Xenograft bones (e.g., from a bovine source) also can be used.  
         [0025]     The term “allograft” will be understood to mean a bone implant from a donor transplanted to a genetically dissimilar recipient of the same species. The term “xenograft” will be understood to mean a bone implant from a donor transplanted to a recipient of a different species.  
         [0026]      FIG. 1  shows a spinal implant  20  that is an embodiment of the present invention. As shown in  FIG. 1 , the spinal implant  20  includes first and second pieces  22 ,  24  (i.e., legs). The first and second pieces  22 ,  24  include portions opposing one another so as to define an inner pocket  26 . The first and second pieces  22 ,  24  are integrally connected to one another at a central connection location  28 . In one embodiment, the implant member  20  has a reduced cross-sectional area at the central connection location  28 . The reduced cross-sectional area provides a controlled break location at the central connection location  28 . As best shown in  FIGS. 5   a - 5   c , the region of reduced cross-sectional area at the central connection location  28  is smaller than nominal cross-sectional areas (average cross-sectional areas) of each of the first and second pieces  22 ,  24  of the spinal implant member  20 .  
         [0027]     As shown in  FIG. 1 , the spinal implant  20  has a generally “C” or “U” shape. The implant member  20  includes a convex outer boundary  30  and an inner boundary  32  having a concave portion  33  and opposing straight portions  35 . As shown in  FIGS. 2   a  and  2   c , grooves  37  may be cut in the straight portions  35 . A fixture fits within the grooves  37  to secure the implant during manufacture of the implant  20 . The inner boundary  32  defines the pocket  26  of the implant  20 .  
         [0028]     Referring again to  FIG. 1 , a first notch  34  located at the outer boundary  30  of the implant  20  defines the reduced cross-sectional area at the controlled break location. A second notch  36  located at the inner boundary  32  of the spinal implant  20  also defines the reduced cross-sectional area. The first notch  34  is preferably larger than the second notch  36 . Both notches  34  and  36  are aligned along an axis of symmetry  38  of the spinal implant  20 .  
         [0029]     Preferably, the controlled break location is configured to allow the first and second pieces  22 ,  24  of the implant member  20  to be manually broken or “snapped” apart without requiring the use of a tool. The controlled break structure ensures that the implant  20  will break at a predetermined location (e.g., at the axis of symmetry  38  for the embodiment of  FIG. 1 ). The implant member  20  can be snapped by manually pulling the pieces  22 ,  24  apart by applying forces shown by arrows  25 . Alternatively, the implant  20  can be snapped by manually pressing the pieces together as shown by arrows  27 . Further, the implant member  20  can be broken by manually impacting the controlled break location against a relatively hard surface or edge such as the edge of a surgical instrument tray. In one embodiment, the reduced cross-sectional area provided at the controlled break location is at most 75 percent or, more preferably, about 50 percent of the nominal cross-sectional areas of each of the first and second pieces  22 ,  24 . The controlled break locations can be defined by a variety of techniques for generating a “weaker” region at a desired location. Such weakened region can be formed by techniques such as notching, scoring, etching, cutting, mechanically perforating, laser perforating, etc. Alternatively, the controlled break location can be “weakened” by altering the mechanical properties of the implant material at the controlled break location by techniques such as radiation, demineralization or other techniques.  
         [0030]      FIG. 3  shows the spinal implant  20  after the implant has been manually “snapped” at the controlled break location. While it is preferred for the spinal implant  20  to be manually broken, it will be appreciated that tools such as forceps, knives, scissors, saws, clamps or other devices could also be used to split the implant  20  into multiple separate pieces. Further, impact tools such as hammers, chisels or the like also could be used. If tools are desired to be used, a controlled break location may, but need not, be provided. Instead, a line or other demarcation can be used to define a predetermined break location that provides a guide for using the tool.  
         [0031]     Although the embodiment of  FIG. 1  shows the controlled break location located at the central axis of symmetry of the implant  20 , it will be appreciated that other embodiments can include controlled break locations offset from the center of the implant. Further, multiple controlled break locations can be provided to allow the implant to be broken into more than two pieces. Further, in another embodiment, an entire cortical ring is provided having two oppositely positioned break locations for allowing the implant to be snapped in half to form two separate implants.  
         [0032]     Referring again to  FIG. 1 , the first notch  34  is defined by first and second insertion force application surfaces  40 ,  42  aligned at an oblique angle relative to one another. The insertion force application surfaces  40 ,  42  are preferably aligned parallel to grooves  44  formed in top and bottom surfaces of the spinal implant  20 . During implantation of the first and second pieces  22 ,  24 , pins of an insertion tool (e.g., see insertion tool  52  of  FIGS. 6   a - 6   e ) are placed in openings  45  (shown in  FIGS. 2   b  and  6   e ) defined in the insertion force application surfaces  40 ,  42 . During insertion, insertion forces are applied to the surfaces  40 ,  42  via the tool  52  to individually push the pieces  22 ,  24  into the intervertebral space. Particularly for posterior approach techniques, it is desirable for the pieces  22 ,  24  to be inserted in a direction requiring the smallest possible opening to be defined through the patient&#39;s posterior region. For example, arrow  46  of  FIG. 4  shows a preferred direction of insertion. It is preferred for the insertion force surfaces  40 ,  42  to be perpendicularly aligned relative to the preferred insertion directions of their corresponding pieces  22 ,  24 .  
         [0033]     The grooves  44  of the implant  20  function to resist migration of the implant upon implantation between opposing bone surfaces. Other structures such as teeth, serrations, cross-cut serrations, notches, bumps, ridges, projections or other surface treatments could also be used.  
         [0034]     While the implant  20  can have a constant thickness, it is preferred for the implant  20  to be slightly tapered. In one embodiment, the spinal implant  20  can be tapered about 3 degrees such that a front end  48  of the implant  20  has a thickness T f  that is greater than a thickness T r  located at a rear end  50  of the implant  20 . The thicknesses T f  and T r  are labeled in  FIG. 2   d . In another embodiment, the front end  48  of the implant  20  may be chamfered to facilitate insertion.  
         [0035]      FIGS. 6   a - 6   e  show an insertion tool  52  suitable for individually implanting the first and second pieces  22 ,  24  of the spinal implant  20  into the intervertebral space of a patient. The insertion tool  52  includes an insertion end  55  having two parallel pins  57  adapted to fit within the openings  45  defined by the force application surfaces  40 ,  42  of the implant pieces  22 ,  24 . The tool  52  also includes a curved retaining surface  59  adapted to contact and complement a portion of the outer boundary  30  of the implant piece  22 ,  24  when the implant piece  22 ,  24  is mounted at the insertion end  55 .  
         [0036]     While other materials could be used, the spinal implant  20  is preferably derived from an allograft bone. In one embodiment, the implant  20  is a transverse cross-section from the femur of a cadaver, and includes a cortical ring. After the ring has been cross-sectioned, relatively soft bone tissue and marrow from the interior of the ring is preferably removed. Next, a portion of the outer cortical ring is removed (e.g., by a technique such as mechanically cutting with a blade or abrasion tool, laser cutting, etching, etc.) to provide the open end of the pocket  26  of the “C” shaped implant  20  (see  FIG. 1 ). Bone removal techniques are then also used to shape the outer and inner boundaries  30 ,  32  and to form the notches  34 ,  36 . While the particular shape depicted in  FIG. 1  is preferred, it will be appreciated that other shapes also could be used without departing from the principles of the present invention.  
         [0037]      FIG. 7  illustrates a kit  60  that is an embodiment of the present invention. The kit includes the spinal implant  20 , the insertion tool  52  and instructions of use. The components are contained within a sterile package  66  (e.g., a bag, plastic container or other sealed holding configuration). In other embodiments, the kit includes the spinal implant  20 , alone, within the sterile package.  
         [0038]      FIG. 8  shows another kit  60 ′ that is an embodiment of the present invention. Similar to the embodiment of  FIG. 7 , the kit  60 ′ includes the spinal implant  20 , the insertion tool  52  and the instructions of use  64 . Also similar to the embodiment of  FIG. 7 , the various parts are held within a sterile package  66 . However, in the embodiment of  FIG. 8 , the spinal implant  20  has been pre-broken into the first and second pieces  22 ,  24 . Preferably, both the first and second pieces  22 ,  24  were derived from the same source. For example, preferably the first and second pieces  22 ,  24  were provided from human bone tissue from the same cadaver. More preferably, the pieces  22 ,  24  were provided from the same cortical ring of the same bone. By packaging two or more implant pieces from the same source in one package, the surgeon that ultimately uses the implants will be assured that the pieces will exhibit similar or identical mechanical and biological properties. Further, by using bone pieces from the same donor, the risk of transferring disease to the patient is reduced by  50  percent as compared to using bone samples from two different donors. In other embodiments, the kit  60 ′ includes the first and second pieces  22 ,  24 , alone, within the sterile package.  
         [0039]     The configuration of the implant of  FIG. 1  provides similar advantages. For example, because the first and second implant pieces  22 ,  24  can be provided to a surgeon in an integrally connected configuration, the surgeon can be assured that the two pieces were derived from the same bone source. Further, the configuration of the controlled break location allows the surgeon to quickly and easily separate the two pieces without requiring a tool. In the event the implant is made of a non-bone material, the configuration ensures the surgeon that the implant pieces  22 ,  24  were manufactured in the same lot.  
         [0040]     To implant the spinal implant  20 , a diseased disc between two adjacent vertebrae  72 ,  74  is preferably removed using a conventional discectomy procedure (i.e., partial or complete discectomy). Opposing end plates  72 ′ and  74 ′ of the vertebrae  72 ,  74  are then preferably prepared to provide relatively flat contact surfaces. The end plates  72 ′,  74 ′ are then conditioned (e.g., with a rasp) to provide a more uniform and osteoconductive/osteoinductive site for the implant  20 . After the implant site has been prepared, the sterile package of the kit  60  is opened, allowing the surgeon to access the implant  20 . Preferably, the implant  20  is then manually “snapped” or broken into two pieces. One of the pieces  22  is then placed on the insertion tool  52 . With the insertion tool, the surgeon inserts the first piece  22  into the cleared intervertebral space between the vertebrae  72 ,  74 . Preferably, the first piece  22  is inserted using a posterior approach. As the first piece  22  is inserted, an insertion force is transferred through the insertion tool  52  to the insertion force surface  40  of the first implant piece  22 . As shown in  FIGS. 9   a  and  9   b , the first implant piece  22  is preferably positioned on one side of a sagittal plane  80  that passes through the midline of the vertebrae  72 ,  74 . Once the first implant piece  22  has been inserted, the tool  52  is withdrawn from the implant piece  22  and the second implant piece  24  is preferably inserted using the same procedure. However, the second implant piece  24  is preferably positioned on the opposite side of the sagittal plane  80 . As mounted in the intervertebral space, the front end  48  of the implant  20  is preferably located at an anterior position relative to the rear end  50 . To further promote fusion, additional bone material (e.g., cancellous allograft or autograft material) or other osteoconductive/osteoinductive material can be placed in the intervertebral space corresponding to the inner pocket  26  of the implant  20 . This material can be placed in the intevertebral space before insertion of the first implant piece  22 , after insertion of the first implant piece  22 , but before insertion of the second piece  24 , and/or after both implant pieces  22 ,  24  have been implanted.  
         [0041]     It will be appreciated that the kit  60 ′ can be used in essentially the same manner as the kit  60 , except the kit  60 ′ does not require the surgeon to manually break the spinal implant  20  into the separate first and second pieces  22 ,  24 . In both embodiments, the surgeon can be assured that both the first and second pieces  22 ,  24  of the spinal implant  20  were derived from the same donor source.  
         [0042]     With regard to the foregoing description, it is to be understood that changes may be made in detail without departing from the scope of the present invention. It is intended that the specification and depicted aspects of the invention may be considered exemplary, only, with a true scope and spirit of the invention being indicated by the broad meaning of the following claims.