Abstract:
A secondary cover plate that contacts a stand-alone fusion cage without rigidly connecting to the cage, and instead only docks against the cage (or passes through a fixation cage) and is secured into the adjacent bone. The secondary cover plate is the last item that would be added to the fixation cage construct and would at least partially cover the head of one or more angled screws. The secondary cover plate could slidably engage the fixation cage without permanently snapping into any features of the cage itself. The secondary cover plate would then be final positioned by advancing into one or more adjacent vertebral bodies. The secondary cover plate advancing step could be achieved by tapping it into place (into bone) or rotating it into place (into bone) so that it is finally secured into the bone.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    A stand-alone fusion cage is a fusion cage that has at least one angled hole in its anterior face for receiving a bone screw that passes through the cage an into an adjacent vertebral body. There are numerous conventional embodiments of these cages that capture the angled bone-engaging screw so as to ensure that the screw does not back out of the cage. Some of the known anti-backout mechanisms include: assembled rotating cover plates, cams, bushings, expanding screws, set screws and secondary cover plates that either snap onto the cage itself or are docked to the cage and secured to the cage faceplate using additional hardware such as a screw. These secondary cover plates can fully or partially cover the most proximal (anterior) portion of the screw head and would in theory prevent any screw backouts. 
         [0002]    The following references are pertinent to the field of stand-alone cages: US 2008-0027550 (Link); U52010-0057206; U.S. Pat. No. 6,730,127; U52009-0088849; U52010-0145459; U.S. Pat. No. 7,662,182; U.S. Pat. No. 6,972,019; U52008-0249569; U52009-0105831; U.S. Pat. No. 7,306,605; U.S. Pat. No. 7,288,094; U52010-0312345; U52010-0286777; U.S. Pat. No. 6,945,973; U52010-0106249; U.S. Pat. No. 6,849,093; U.S. Pat. No. 6,984,234; U52009-0105830; U52009-0210062; U.S. Pat. No. 7,452,370; U.S. Pat. No. 6,558,423; U.S. Pat. No. 6,890,335; and U.S. Pat. No. 6,629,998. 
       SUMMARY OF THE INVENTION 
       [0003]    The present invention relates to a secondary cover plate that engages a stand-alone intervertebral fusion cage without rigidly connecting to the cage. Rather, the cover plate of the present invention only docks against or passes through the cage and is secured into the adjacent bone. 
         [0004]    The secondary cover plate of the present invention is typically the last component of the intervertebral fusion assembly that is inserted into the disc space. Typically, its primary function is to at least partially cover the head of one or more angled screws. 
         [0005]    In some embodiments, the secondary cover plate of the present invention slidably engages the fixation cage without permanently snapping into any features of the cage component. In other embodiments, the secondary cover plate threadably engages the cage without permanently snapping into any features of the cage. 
         [0006]    After this removable engagement, the secondary cover plate is then secured into its final position by advancing into one or more adjacent vertebral bodies. This advancing step may be achieved by tapping its bone-securement features into the adjacent bones or rotating these bone securement features into the adjacent bone, so that the cover plate is finally secured into the bone. 
         [0007]    Therefore, in accordance with the present invention, there is provided an assembly comprising:
       a) an intervertebral fusion cage positionable between adjacent vertebral bodies, the cage having an anterior face having a pair of anchor holes extending therethrough,   b) bone anchors received in the anchor holes, each anchor having a proximal head,   c) a cover plate having:
           i) a base portion having an anterior face and a posterior face,   ii) opposed bone-securing features extending from the base portion,
 
wherein the cover plate is removably connected to the cage.
   
               
 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0013]      FIG. 1A  discloses an exploded version of a first embodiment of the present invention (without screws) having a threaded post, wherein the bone-securing features are not in deployed position. 
           [0014]      FIG. 1B  discloses an exploded version of a first embodiment of the present invention (without screws) having a threaded post, wherein the bone-securing features are in deployed position. 
           [0015]      FIGS. 2A and 2B  are side and top views of the assembled first embodiment of the present invention. 
           [0016]      FIG. 3  discloses an exploded version of a second embodiment of the present invention (without screws) having a smooth post, wherein the bone-securing features are in deployed position. 
           [0017]      FIG. 4A  discloses an exploded version of a third embodiment of the present invention (without screws) having a threaded post and lateral flanges. 
           [0018]      FIGS. 4B and 4C  are side and top views of the assembled first embodiment of the present invention. 
           [0019]      FIG. 5  discloses an exploded version of a fourth embodiment of the present invention (without screws) having a smooth post and lateral flanges. 
           [0020]      FIG. 6  discloses an exploded version of a fifth embodiment of the present invention (without screws) having a smooth rectangular post and opposed bone-securing features. 
           [0021]      FIG. 7  discloses an exploded version of a sixth embodiment of the present invention (without screws) having a smooth rectangular post; opposed bone-securing features; and lateral flanges. 
           [0022]      FIG. 8  discloses an exploded version of a seventh embodiment of the present invention (without screws) having opposed bone-securing features and lateral flanges. 
           [0023]      FIG. 9  discloses an exploded version of an eighth embodiment of the present invention (without screws) having bone-securing features and lateral hooks. 
           [0024]      FIG. 10  discloses a cover plate in which the bone-securing features comprise barbs that provide both ease of insertion and expulsion resistance. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    Now referring to  FIGS. 1A-B ,  2 A-B,  3  and  6 , there is provided an assembly comprising:
       a) an intervertebral fusion cage  1  positionable between adjacent vertebral bodies, the cage having an anterior face  3  having a receptacle  5  and a pair of anchor holes  901 , and   b) a cover plate  7  having:
           i) a base portion  9  having an anterior face  11  and a posterior face (not shown),   ii) a post  13  extending from the posterior face of the base portion and removably engaged in the receptacle of the cage, and   iii) opposed bone-securing features  15  for engaging the adjacent vertebral bodies   
           c) bone anchors  903  received in the anchor holes, each anchor having a proximal head  905 , and       
 
         [0032]    In some embodiments, as in  FIG. 3 , the shaft and receptacle each have mating circular transverse cross-sections so that the shaft is rotatable with the receptacle. 
         [0033]    In some embodiments, as in  FIG. 1A , the shaft has a threadform  17  thereon, where in other, as in  FIG. 3 , the shaft is unthreaded. 
         [0034]    In some embodiments, as in  FIG. 3 , the bone-securing features are integral with the base portion. In others, the bone-securing features are rotatable about the base portion, as in  FIG. 9 . 
         [0035]    Typically, the cover plate further comprises a recess  19  opening upon the anterior face of the base portion. Preferably, this opening has a hexagonal transverse cross-section. 
         [0036]    In some embodiments, as in  FIGS. 1A and 3 , the bone-securing features extend laterally from the base portion and the shaft is rotatable with the receptacle. 
         [0037]    In some embodiments, as in  FIG. 6 , the bone-securing features extend posteriorly from the base portion. 
         [0038]    In some embodiments, as in  FIGS. 6 and 7 , the bone-securing features comprise an anti-backout feature  801 . 
         [0039]    The base portion of the cover plate acts as a hub for the other features of the cover plate. 
         [0040]    The recess that opens upon the anterior face of the base portion of the cover plate functions as a receptacle for a tool that is able to rotate the cover plate. In some preferred embodiments, this recess has a hexagonal transverse cross-section, so as to be useful with standard hexagonal screwdrivers. 
         [0041]    The purpose of the post to provide a removable engagement of the cover plate to the cage. Accordingly, the post does not preferably possess any features that would provide a permanent engagement between the cover plate and the cage, such as a snap feature. 
         [0042]    In some embodiments the post has a smooth outer surface, as in  FIG. 3 . This surface is usually adopted when the bone-securement features are tapped into the bone. 
         [0043]    In some embodiments the post has a threaded outer surface, as in  FIG. 1A . This threaded surface is usually adopted when the bone-securement features are rotated into the bone. 
         [0044]    In some embodiments the post has a circular transverse cross-section, as in  FIG. 1A . This threaded surface may be usefully adopted when the bone-securement features are rotated into the bone. 
         [0045]    In some embodiments the post has a rectangular transverse cross-section, as in  FIG. 6 . This surface is usually adopted when the bone-securement features are tapped into the bone. The non-circular cross section of the post provides for auto-alignment of the cover plate. 
         [0046]    The purpose of the flange is to provide a physical stop against the anterior movement of a screw expulsing from the cage. Because the flanges do not enter the bone, they need not have sharp features. In fact, because the implanted flanges are likely located near the aorta or vena cava of the patient&#39;s vascular system, it is preferred that the flanges consist of only smooth edges and surfaces. For example,  FIG. 4  shows a pair of flanges that have only smooth surfaces. 
         [0047]    Any feature that effectively penetrates the bone and locks the cover plate into an adjacent vertebral body can be considered a bone-securing feature. In some embodiments (as in  FIG. 1A ), the bone-securing feature  15  is a tooth, while in others (as in  FIG. 6 ), it is a knife-edge  203 . 
         [0048]    In some embodiments using cover plate rotation for bone securement, the tooth extends transversely from the longitudinal axis of the base portion of the cover plate and in the same plane as the base portion, as in  FIG. 1A . In some embodiments using cover plate tapping for bone securement, the knife-edge extends perpendicularly from the longitudinal axis of the base portion of the cover plate but normal to the plane of the base portion, as in  FIG. 6 . 
         [0049]    In some preferred embodiments, the bone-securing feature enters the bone by first placing the post of the cover plate into the corresponding cage receptacle so that the bone securement feature is against the anterior face of a vertebral body and simply tapping the base portion of the cover plate in a posterior direction until the cover plate contacts the anterior wall of the cage. In other embodiments, the cover plate need not contact the anterior wall of the cage. This method may be used for the securement of the assembly of  FIG. 6 . 
         [0050]    In other embodiments, the bone-securing feature enters the bone by first placing the post of the cover plate into the corresponding cage receptacle so that each bone securement feature is between the endplates of opposed vertebral bodies, and then rotating the cover plate until the cover plate contacts the anterior wall of the cage. This method may be used for securement of the assembly of  FIG. 1A . 
         [0051]    Now referring to  FIGS. 4A-C ,  5  and  7 , there is provided an assembly comprising:
       a) an intervertebral fusion cage  31  positionable between adjacent vertebral bodies, the cage having an anterior face  33  having a receptacle  35  and a pair of anchor holes  37 ,   b) bone anchors  38  received in the anchor holes, each anchor having a proximal head  40 , and   c) a cover plate  43  having:
           i) a base portion  45  having an anterior face  47  and a posterior face (not shown),   ii) a post  51  extending from the posterior face of the base portion and removably engaged in the receptacle of the cage, and   iii) opposed flanges  53  extending from the base portion, each flange located substantially anterior to a respective anchor head to prevent backout of the associated anchor.   In some embodiments, the cover plate further comprises:   iv) opposed bone-securing features  55  extending from the base portion, as in  FIG. 7 .   
               
 
         [0060]    Now referring to  FIG. 8 , there is provided an assembly comprising:
       a) an intervertebral fusion cage  61  positionable between adjacent vertebral bodies, the cage having an anterior face  63  having a pair of slots  65  and a pair of anchor holes  67 ,   b) bone anchors (not shown) received in the screw holes, each anchor having a proximal head,   c) a cover plate  73  having:
           i) a base portion  75  having an anterior face  77  and a posterior face (not shown),   ii) opposed flanges  81  extending laterally from the base portion, each flange located substantially anterior to a respective anchor head to prevent backout of the respective anchor head,   iii) opposed bone-securing features  83  extending from the base portion and passing through the respective slots.   
               
 
         [0067]    Now referring to  FIG. 9 , there is provided an assembly comprising:
       a) an intervertebral fusion cage  85  positionable between adjacent vertebral bodies, the cage having an anterior wall  87  having an anterior face  89 , the anterior face having a pair of pivoting features (preferably hooks  91  extending therefrom) and a pair of anchor holes  93  extending therethrough,   b) bone anchors  95  received in the anchor holes, each anchor having a proximal head,   c) a cover plate  97  having:
           i) a base portion  99  having an anterior face  101  and a posterior face (not shown),   ii) opposed flanges  103  extending laterally from the base portion, each flange located substantially anterior to a respective screw head to prevent backout of the respective anchor head,   iii) opposed bone-securing features  105  rotatably connected to the base portion,   iv) a bar  107  extending from the base portion and received in the hooks.   
               
 
         [0075]    In some embodiments, the secondary cover plate may include features that promote ease of insertion, but would also strongly resist expulsion forces. For example,  FIG. 10  discloses a cover plate in which the bone-securing features comprise barbs  951  that provide both ease of insertion and expulsion resistance. Likewise,  FIGS. 6 and 7  each disclose a bone engaging feature  801  on a surface of a knife-edge, wherein the tooth extends perpendicularly to the longitudinal axis of the knife-edge. The bone engaging feature can be a tooth (as shown), a pyramid, a ridge, a keel, or a spike. This tooth provides both ease of insertion and expulsion resistance. 
         [0076]    In addition to the anti-backout features that could be designed into the secondary cover plate, a combination of one or more of the following features could also be introduced to enhance performance: bone growth coatings (i.e., titanium calcium, phosphate, or hydroxyapatite; porous features in the bone-securing portions; anti-infection coatings; and tissue anti-adhesion coatings. 
         [0077]    In the embodiments shown (as in  FIG. 1A ), the advancement of the cover plate is coupled with rotation of the cover plate. However, in other embodiments (not shown), the advancement of the cover plate is de-coupled from rotation of the cover plate. This may be accomplished by making the cover plate from two different components having separate drive features. 
         [0078]    In general, the cage of the present invention is a stand-alone cage adapted for use in intervertebral fusions. These cages typically have screw holes through the anterior face for receiving bone screws. In some embodiments, and now referring to  FIG. 9 , the cage of the present invention comprises:
       a) an anterior wall  87  having an anterior surface  89 , an upper surface  401 , a lower surface (not shown), a first throughhole  93   a  extending upwards from the anterior surface and a second throughhole  93   b  extending downwards from the anterior surface,   b) a posterior wall  403 , and   c) first and second side walls  405  connecting the anterior and posterior walls.
 
Typically, the anterior, posterior and sidewalls of the cage define a central, vertical through-hole  407  that is adapted for promoting fusion between opposed vertebral bodies. Typically the sidewalls of such cages further comprise at least one throughhole to promote bone in-growth. Typically, the upper and lower surfaces of the cage have teeth or ridges  409  for rigidly gripping the opposed vertebral bodies. The posterior wall may have a tapered posterior surface adapted to ease insertion of the cage into the disc space. Generally, the cage may be used in either the lumbar, thoracic or cervical portions of the spine.
       
 
         [0082]    The bone anchors of the present invention are generally bone screws. 
         [0083]    In one method of using the present invention, the cover plate is intended to be inserted/driven perpendicular to the proximal face of the cage in-line with the inserter. It is either delivered through a separate device or through a multi-purpose inserter that delivers the cage and then selectively allows the user to engage the cover plate. After the cage is placed and positioned per surgeon preference, a cover plate could be inserted up against a proximal face of the cage and deployed. As taught, the plate or portions of the plate penetrates the adjacent vertebral bodies either through the anterior face or from within or partially within the disc space. 
         [0084]    In general, the cover plate, cage and bone anchors are made from metallic materials, ceramic or polymeric materials. 
         [0085]    If a metal is chosen as the material of construction, then the metal is preferably selected from the group consisting of nitinol, titanium, titanium alloys (such as Ti-6Al-4V), chrome alloys (such as CrCo or Cr—Co—Mo) and stainless steel. 
         [0086]    If a polymer is chosen as a material of construction, then the polymer is preferably selected from the group consisting of polycarbonates, polyesters, (particularly aromatic esters such as polyalkylene terephthalates, polyamides; polyalkenes; poly(vinyl fluoride); PTFE; polyarylethyl ketone PAEK; and mixtures thereof. 
         [0087]    In some embodiments, the bone screws are made of a stainless steel alloy, preferably BioDur® CCM Plus® Alloy available from Carpenter Specialty Alloys, Carpenter Technology Corporation of Wyomissing, Pa. In some embodiments, the cage is made from a composite comprising carbon fiber. Composites comprising carbon fiber are advantageous in that they typically have a strength and stiffness that is superior to neat polymer materials such as a polyarylethyl ketone PAEK. In some embodiments, the cage is made from a polymer composite such as a PEKK-carbon fiber composite. 
         [0088]    Preferably, the composite comprising carbon fiber further comprises a polymer. Preferably, the polymer is a polyarylethyl ketone (PAEK). More preferably, the PAEK is selected from the group consisting of polyetherether ketone (PEEK), polyether ketone ketone (PEKK) and polyether ketone (PEK). In preferred embodiments, the PAEK is PEEK. 
         [0089]    In some embodiments, the cage is made from a neat polymer without any carbon fiber additive. Preferably, the polymer is a polyarylethyl ketone (PAEK), more preferably PEEK. 
         [0090]    In some embodiments, the carbon fiber comprises between 1 vol % and 60 vol % (more preferably, between 10 vol % and 50 vol %) of the composite. In some embodiments, the polymer and carbon fibers are homogeneously mixed. In others, the material is a laminate. In some embodiments, the carbon fiber is present in a chopped state. Preferably, the chopped carbon fibers have a median length of between 1 mm and 12 mm, more preferably between 4.5 mm and 7.5 mm. In some embodiments, the carbon fiber is present as continuous strands. 
         [0091]    In especially preferred embodiments, the composite comprises: 
         [0000]    a) 40-99% (more preferably, 60-80 vol %) polyarylethyl ketone (PAEK), and
 
b) 1-60% (more preferably, 20-40 vol %) carbon fiber,
 
wherein the polyarylethyl ketone (PAEK) is selected from the group consisting of polyetherether ketone (PEEK), polyether ketone ketone (PEKK) and polyether ketone (PEK).
 
         [0092]    In some embodiments, the composite consists essentially of PAEK and carbon fiber. More preferably, the composite comprises 60-80 wt % PAEK and 20-40 wt % carbon fiber. Still more preferably the composite comprises 65-75 wt % PAEK and 25-35 wt % carbon fiber.