Abstract:
An interbody spacer assembly includes a pair of end pieces spaced apart by a connector extending between them. The end pieces extend generally parallel to the end plates of adjoining vertebral bodies. Fasteners connect the end pieces to the vertebral bodies. Bone graft material or solid bone can be placed in the interior space defined by the end pieces and connector, which bone graft material or solid bone eventually fuses together and to the adjoining end plates through the end pieces. The spacer assembly has ratchets to connect the two end pieces. The ratchets near the fasteners are more closely spaced than the ratchets further from the fasteners, allowing the spacer assembly to more closely approximate the lordosis of the spine.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority of U.S. Provisional Patent Application No. 60/732,624, filed Nov. 2, 2005. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to cervical spine supports, and, in particular, to a device that acts as a spacer between cervical vertebral bodies so that bone graft material inserted within the device can fuse and replace pathological bone removed surgically.  
       BACKGROUND  
       [0003]     It is known in the prior art to use cage-like spacers made of titanium mesh in tube shapes between vertebrae to provide support to the cervical spine. Spacers are needed when either the vertebrae or disk are removed for pathological reasons due to injury or disease. The spacer maintains the granular bone tissue in place until the graft is complete. Some of the known prior art spacers, such as those described in application Ser. No. 10/293,843, which is incorporated by reference herein, may be difficult to install between existing vertebrae and difficult to satisfactorily fill with such bone tissue. Moreover, the cervical spacer as disclosed in application Ser. No. 10/293,843 may not correspond to the curvature of the cervical portion of the spine. In those locations along the spine where there is the most curvature, such as the neck and lower back, longitudinally straight spacers may fail or cause pain because they do not match or correspond to the natural curvature of the spine. This is particularly true when large sections of the vertebrae are replaced by a spacer because the curvature of a large section is greater than the curvature of a small section.  
         [0004]     Consequently, I have developed a curvilinear cervical interbody device that is easier to install between cervical vertebral bodies, adjusts to the curvature of the cervical portion of the spine, can be readily adjusted to account for the size of the vertebrae or disks that are removed, and results in a stronger and more reliable graft.  
       SUMMARY  
       [0005]     A spacer assembly is provided for use in spinal surgeries. The spacer assembly comprises two end pieces for interfacing with the end plates of adjacent vertebrae. Each end piece is generally disk-like in form and includes an inner surface facing the interior of the spacer and an outer surface facing the adjacent vertebrae. Each end piece has attached thereto a flange that extends longitudinally when installed (i.e., in the general direction of the length of the spine) and exteriorly of the end piece. The end pieces are spaced and reinforced by one or more connectors. The spacer assembly engages the adjacent vertebral disks by securing each flange with the adjacent vertebrae to couple the assembly and vertebrae together. The spacer assembly defines an interior region that is filled with morselized bone graft, structural bone graft, biologic fusion materials, or solid bone to fuse together and with the adjacent vertebrae, thereby replacing pathological bone or disk material removed surgically. The spacer assembly can be adjusted by ratcheted connectors, with the ratchets preferably being distributed so that the assembly&#39;s radius corresponds approximately to the radius of the spine in the area of the removed vertebrae or disks even as the assembly increases or decreases in average size.  
         [0006]     In one embodiment, the end pieces are contoured to conform to the cross-section shape of the spinal cord. The end pieces are further designed to promote bone growth into the adjacent areas by, for instance, including apertures or an opening between the interior region and the vertebrae.  
         [0007]     The inventive spacer assembly can be used to replace either a surgically removed disk (diskectomy) or vertebra (corpectomy). 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a side perspective view of the device in an extended position.  
         [0009]      FIG. 2  is a side perspective view of the device in a contracted position.  
         [0010]      FIG. 3  is a side perspective, cut-away view of the device implanted in the spine.  
         [0011]      FIG. 4  is a perspective view of the disassembled device.  
         [0012]      FIG. 5  is a perspective view of a second embodiment of the device which can be further extended than the first embodiment.  
         [0013]      FIG. 6  is a side perspective, cut-away view of the second embodiment implanted in the spine.  
         [0014]      FIGS. 7 and 7   a  are cross-sectional views showing the geometry of the intermeshing teeth of the ratchets. 
     
    
     DETAILED DESCRIPTION  
       [0015]     As seen in  FIGS. 1 and 2 , the spacer assembly  100  includes an upper end piece  110  and a lower end piece  112 . End piece  110  comprises an exterior surface  110   a  in first end plate  110   b , integrally formed flange  142  for attaching the assembly to a vertebral body, and stepped or ratcheting connectors  130   a ,  130   b ,  131 . End piece  112  comprises an exterior surface  112   a  on second end plate  112   b , integrally formed flange  144 , and stepped or ratcheting connectors  132   a ,  132   b ,  133 . End piece  112  is shown with an optional stabilizing piece  140  connecting ratcheting connectors  132   a ,  132   b  and  133 , for instance by going around the perimeter of the end piece  112 , providing additional structural integrity to the end piece. Thus, when the two end pieces are assembled, the tendency of the internal ratcheting connectors  130   a ,  130   b ,  131  to push out the external connectors  132   a ,  132   b ,  133  is minimized by the presence of the connecting piece  140 .  
         [0016]     In one embodiment, the ratcheting connectors  130 ,  132  are a pair of column-like parts, and the ratcheting connectors  131 ,  133  are wall-like, and extend the width of the spacer. While  FIGS. 1 and 2  show the end piece  110  as having ratcheting connectors  130 ,  131  internal to the connectors  132 ,  133  of end piece  112 , a variation in which the end piece  112  is internal to end piece  110  is also feasible. In such a variation, the connecting, stabilizing piece  140  would be on end piece  110 .  
         [0017]     As seen in  FIG. 3 , which features a cross-section of the spacer assembly  100  taken through lines  3 , the spacer assembly  100  in a collapsed state is located by a surgeon between the vertebral bodies of a spine  116 , from which one or more diseased or damaged vertebrae or disks were removed during surgery. The spacer assembly  100  is then expanded to maintain the vertebrae in a spaced-apart configuration. The spacer assembly  100  is placed from the front of the patient, using an anterior approach, to fill up the entire disc space or replace the entire vertebral body or bodies, both longitudinally and laterally.  
         [0018]     When the spacer assembly has been installed, the exterior surfaces  110   a  and  112   a  of the end pieces  110 ,  112  are substantially parallel to the adjoining surfaces  128   a ,  129   a  (often referred to as “end plates”) of the vertebral bodies  128 ,  129 . The end pieces  110 ,  112  preferably have a substantially flat or planar outer surface to provide a stable interface with the end plates, and the end pieces may be shaped and dimensioned to closely match the cross-sectional shape and dimensions of the end plates.  
         [0019]     The end pieces  110 ,  112  are adjustably connected to each other by their ratcheted connectors  130 ,  132  and  131 ,  133  so as to establish a desired length of the spacer assembly  100 . The ratcheted connectors allow the spacer assembly to be extended or shortened to conform most closely to the space between the vertebral bodies  128 ,  129 . By adjusting the ratcheting connectors for the desired spacing between the vertebral bodies, a surgeon can achieve optimal biomechanical strength in situ. The columnar ratcheted connectors  130   a  and  130   b  may be flexible enough to permit the surgeon to disengage them from their mating columnar ratcheted connectors  132   a ,  132   b.    
         [0020]     In addition, as described in more detail with respect to  FIGS. 7 and 7   a , the interdigitation of the teeth in the ratcheted connectors has been designed so that the ratcheted connectors  131 ,  133 , which are nearer the spine, are relatively shorter than the corresponding ratcheted connectors  130 ,  132 , which are further from the spine. As the spacer assembly is expanded, the lordosis or curvature of the assembly correspondingly increases. This results in a spacer assembly that more closely follows the lordosis of the spine in which it is placed.  
         [0021]     The ratcheted connectors may be of equal length or they may be of different lengths. It is the curvature of the connector which determines the degree of lordosis. As the device is expanded, the degree of lordosis increases.  
         [0022]     As shown in  FIGS. 7 and 7   a , the geometry of the intermeshing teeth of the ratchets may be established to account for the natural curvature as follows. With reference to  FIGS. 10 and 10   a , the end piece  112  includes forward teeth  14  formed on front wall  132  and rearward teeth  18  formed on rear columns  133   a ,  133   b . When viewed as arcs of circles, the front wall  132  and rear columns  133   a ,  133   b  extend concentrically about center  28 . The relative size between teeth  14  and teeth  18  corresponds to the sweep angle θ between adjacent teeth and the difference between R 1  and R 2 . More specifically, tooth height C 1  and C 2  can be found using the following:  
         C   ⁢           ⁢   1     =       2   ·   R     ⁢           ⁢     1   ·     sin   ⁡     (     θ   2     )               
         C   ⁢           ⁢   2     =       2   ·   R     ⁢           ⁢     2   ·     sin   ⁡     (     θ   2     )               
 
         [0023]     Thus, as the difference between R 1  and R 2  increases or decreases, the respective tooth heights will increase or decrease proportionally and according to the above formulae. The leading edge of each forward tooth  14  is thereby radially aligned with a corresponding leading edge of a rearward tooth  18 . The number of teeth formed in the end piece  112  is dictated by the height of the end piece and the sweep angle θ between teeth. In other words, the assembly is designed so that the exterior surfaces  110   a ,  112   a  of the endpieces  110 ,  112  become less parallel as the assembly expands, and more parallel as it collapses so that the spacer assembly has a curvature that is similar to the curvature or lordosis of the spine. The posterior ratchets are more closely spaced, i.e., the ratchets are smaller, than the anterior ratchets, and thus C 1 &gt;C 2 , so that as the device is lengthened, it does so in a curvilinear path or fashion.  
         [0024]     The end pieces  110 ,  112  may be squarish or approximately disk-shaped to conform to the cross-sectional shape of the end plates of the adjacent vertebrae. The exterior surfaces  110   a  and  112   a , respectively, of end pieces  110  and  112  interface with the end plates of adjacent vertebrae  128 ,  129 . The portion of the end pieces surrounding the spinal cord are preferably contoured to avoid compressing or otherwise affecting the spinal cord.  
         [0025]     The interior region  114  between end pieces  110 ,  112  is substantially open around its perimeter, and it can be easily filled with bone graft tissue to fuse to vertebral bodies  128 ,  129  of spine  116 . The end pieces  110 ,  112  contain apertures  126  extending through their thickness to allow the bone graft tissue to grow through the end pieces and into the adjacent vertebrae, and thereby providing direct contact between the bone graft tissue and the adjoining vertebrae. Multiple apertures  126  are preferred to permit the bone graft tissue in region  114  to fuse with the adjacent vertebrae.  
         [0026]     The end pieces  110 ,  112  have integrally formed flanges  142 ,  144  projecting approximately perpendicularly from the exterior surfaces  110   a ,  112   a , respectively, and the flanges  142 ,  144  are located around the perimeter of a portion of the exterior surfaces  110   a ,  112   a , respectively. The flanges act as stops to engage the assembly in proper position relative to the spine. They also prevent retropulsion or compression of the spinal cord, which can occur if the assembly were to slide too far into the spine toward the spinal cord  116  or otherwise shift out of place.  
         [0027]     The flanges have holes  150 ,  152  for receiving screws  136 ,  138  of the type customarily used in spine surgeries. These screws  136 ,  138  are screwed into the adjacent vertebral bodies  128 ,  129  respectively, preferably with commonly available locking mechanisms, to secure the spacer assembly in place relative to the spine. Alternatively, screws could be located through apertures in the end pieces and directly into the vertebrae. Preferably, the screws are inserted through the flange at an angle toward or away from the adjoining end piece, rather than parallel thereto, to increase the stability of the device and reduce the possibility of inadvertent displacement.  
         [0028]     As seen in  FIG. 4 , the wall  130  of end piece  110  comprises a step-like structure, and the columns  131   a ,  131   b  comprise step-like structures. Likewise, the wall  132  of end piece  112  comprises a step-like structure, and the columns  133   a ,  133   b  also comprise step-like structures. As shown, end piece  110  fits within end piece  112 , with the wall  130  and columns  131   a ,  131   b  interacting with wall  132 , and columns  133   a ,  133   b  in stepwise fashion. The columns  131   a ,  131   b ,  133   a ,  133   b  may be somewhat flexible laterally (i.e., perpendicular to the spine) to permit disengagement and contraction or expansion by the surgeon if that is necessary. This flexibility can be accomplished by appropriately thinning the wall and columns or by providing slits in them to allow bending. Additionally, flexible material or a spring-like mechanism could be used.  
         [0029]     A second embodiment of the spacer assembly is shown in  FIGS. 5-6 . This device is similar to the first embodiment except that it is sized sufficiently to allow it to replace two vertebral bodies.  
         [0030]     Not shown is a mesh, or retainer, that partially but does not entirely surround interior region  114  between the end pieces where the bone graft tissue is located and spans the distance between the end pieces and fills the interior region  114 . This mesh is preferably located at the anterior side of assembly  100  and helps retain the bone graft tissue and prevent it from dislodging during implantation of the assembly. The mesh is held in place relative to the rest of assembly  110  by screws extending through the mesh, through holes  150 ,  152  of flanges  142 ,  144 , and finally into the adjacent vertebrae. Thus, the mesh can be installed after the bone graft tissue is positioned.  
         [0031]     The remaining region  114  is not surrounded by mesh because a patient&#39;s muscle tissue along the spine will partially enclose the area  114 . Preferably, the mesh has an arcuate width that is slightly larger than the arcuate width of flanges  142 ,  144 . The connector is located at the posterior side of the assembly, closest to the spinal cord, where it protects the spinal cord from the bone graft tissue. This embodiment can be supplemented with anteriorly-located connectors in the form of posts, if desired for additional strength.  
         [0032]     Additionally, the exterior surfaces  110   a  and  112   a  of end pieces  110  and  112 , respectively, may be roughened or formed with alternating ridges and valleys (not shown). The ridges are angled relative to the planes of surfaces  110   a  and  112   a  so that the peak of each ridge is on the anterior side (i.e. farthest from the spinal cord) of the ridge. Stated differently, the ridges are slanted so that the anterior side of each ridge forms an angle less than 90 degrees with the plane of the exterior surface of the end piece (e.g.  110   a ), while the posterior side of each ridge forms an angle greater than 90 degrees with the plane (e.g.  110   a ) of the exterior surface of the end piece. This arrangement permits the assembly  100  to easily slide laterally between the spaced vertebrae  128 ,  129 , while also resisting lateral movement in the opposite direction away from the spaced vertebrae. This helps prevent inadvertent dislocation of the assembly away from the desired position between the vertebrae.  
         [0033]     The end pieces and flanges are desirably composed of titanium or a bioabsorbable material, but they may also be composed of other rigid materials such as other metals and plastics. There is no need for adjuvant fixation, such as with a plate or another device to stabilize the position of the assembly. An acceptable plastic would be polyetheretherketone. Resorbable plates may also be used.  
         [0034]     The present assembly has been described in connection with cervical vertebral bodies, but the same invention could be applied to the thoracic and lumbar spine by simply varying the shapes and dimensions of the components to correspond to the shapes and dimensions of the thoracic and lumbar vertebrae.  
         [0035]     It should be recognized that, while the spacer assembly has been described in relation to a preferred embodiment, those skilled in the art may develop a wide variation of structural details without departing from the principles described here. Accordingly, the appended claims are to be construed to cover all equivalents falling within the scope and spirit of the disclosure.