Abstract:
An apparatus for use with bone fasteners comprises a first fixation device having a slot configured to receive a bone fastener, and a second fixation device having another slot configured to receive another bone fastener. The first and second fixation devices are interconnected for rotation relative to each other when the bone fasteners are in the slots. Summarized differently, an apparatus for use with bone fasteners comprises a first fixation device having a first pair of openings defining bone fastener locations, and a second fixation device having a second pair of openings defining bone fastener locations. The first and second fixation devices are interconnected for movement relative to each other such that the bone fastener locations defined by the first and second pairs of openings are located at four corners of a rectangular array.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention is related to a fixation system. More particularly, the invention is related to a fixation system comprising a plurality of rotatably-connected plates each having at least one fixation hole.  
       BACKGROUND OF THE INVENTION  
       [0002]     Orthopedic fixation devices such as plates are frequently coupled to bone with fasteners inserted through plate holes. It is known that securing such fasteners to the bone plate, for example through the use of expansion-head screws, can decrease the incidence of loosening of the fixation assembly post-operatively. It is also known that a bushing may be disposed in each plate hole to receive the fastener to permit polyaxial movement so that the fastener may be angulated at a surgeon-selected angle. However, polyaxial movement of fasteners through set plate hole locations only increases attachment alternatives of the fasteners themselves. The plate holes remain fixed in relation to each other and to the longitudinal axis of the plate.  
         [0003]     Typically, a spinal fixation plate is applied to the anterior side of the affected vertebrae to span at least one affected disc space or vertebra (i.e. one in which at least a portion of the disc has been removed and a spinal fusion spacer has been inserted). The plate is fixed to the vertebrae using bone screws and acts to keep the vertebrae generally aligned during the initial period following fixation in which fusion of the spacer to the adjacent vertebrae occurs. The plate also acts to prevent the spacer from being expelled from the disc space during this initial period.  
         [0004]     Where a spinal fusion spacer is implanted between a pair of vertebrae to be fused, the spacer rests on the endplates of the vertebrae. The outer circumference of the end plates comprises hard cortical bone and thus provides a the best surface upon which to seat the spacer. The center portion of the endplates comprises a thin cortical bone shell overlying a core of softer cancellous bone. Most, if not all, of the spacer contact surface, however, may be located in this center portion.  
         [0005]     Subsequent to placement of the spacer, the surgeon typically compresses the disc space by pressing the adjacent vertebrae together. This compression ensures a good engagement between the spacer the endplates, increasing the chances that fusion will occur. Often in the period immediately following surgery, the spacer will subside slightly either into the under-portion of the endplates or due to graft resorption (in the case of allograft spacers).  
         [0006]     Where a rigid fixation plate is used to connect the vertebrae, this subsidence may tend to shift more of the spinal load to the plate than is desirable. Such load shifting can also occur due to inaccuracies in installing the plate to the vertebrae. In extreme circumstances, this load shifting can result in non-fusion of the spacer to the vertebra, since firm compression between the spacer and the vertebrae is one factor contributing to successful fusion.  
         [0007]     Accordingly, there exists a need for a fixation system which provides the desired support to the vertebrae to be fused, and which allows limited translation of the vertebrae with respect to at least a portion of the plate, thereby limiting the undesirable effects of load shielding by the plate due to graft subsidence caused by settling or normal forces experienced in the spinal column. Promoting fusion of the adjacent vertebrae is thus accomplished.  
         [0008]     The inventive translation plates compensate for this subsidence by providing the aforementioned benefits of a rigid fixation plate (general vertebral alignment, prevention of spacer expulsion), while allowing at least one vertebra to move with respect to the plate to compensate for the post-surgical subsidence. This compensation ensures that the majority of the spinal column load is borne by the spacer rather than the plate.  
       SUMMARY OF THE INVENTION  
       [0009]     An apparatus for use with bone fasteners comprises a first fixation device having a slot configured to receive a bone fastener, and a second fixation device having another slot configured to receive another bone fastener. The first and second fixation devices are interconnected for rotation relative to each other when the bone fasteners are in the slots.  
         [0010]     Summarized differently, an apparatus for use with bone fasteners comprises a first fixation device having a first pair of openings defining bone fastener locations, and a second fixation device having a second pair of openings defining bone fastener locations. The first and second fixation devices are interconnected for movement relative to each other such that the bone fastener locations defined by the first and second pairs of openings are located at four corners of a rectangular array. The bone fastener locations at opposite ends of the rectangular array are movable relatively toward each other lengthwise of the array while remaining uniformly spaced apart from each other across the array.  
         [0011]     A fixation assembly is described comprising a first fixation device having a first opening configured to receive a first bone fastener, and a second opening configured to receive a second bone fastener; and a second fixation device having a third opening configured to receive a third bone fastener, and a fourth opening configured to receive a fourth bone fastener; wherein the first and third openings form a first pair of openings and the second and fourth openings form a second pair of openings; the fixation devices arranged so that when first and third bone fasteners are at least partially inserted through the first pair of openings and into a first bone segment, and second and fourth bone fasteners are at least partially inserted through the second pair of openings and into a second bone segment, the first and second bone fixation devices are able to rotate relative to each other.  
         [0012]     The first fixation device may have a first substantially circular portion having a first midpoint, the second fixation device may have a second substantially circular portion having a second midpoint; and wherein the first and second midpoints may generally align when the first and second fixation devices are engaged. The first midpoint and the first and second openings may be generally collinear, and the second midpoint and the third and fourth openings may be generally collinear.  
         [0013]     The first fixation device may further comprise first and second extensions projecting radially from the first midpoint, and the second fixation device may further comprise third and fourth extensions projecting radially from the second midpoint. The first opening may be located at the first extension, the second opening may be located at the second extension, the third opening may be located at the third extension, and the fourth opening may be located at the fourth extension.  
         [0014]     At least one bone fastener may be allowed to translate within an opening in situ. The first and second bone segments may be adjacent vertebrae. The movement of either the first or second bone segments relative to one another may cause the attached bone fasteners to translate within their respective openings. The movement of either the first or second bone segments relative to one another may cause the first and second fixation devices to rotate relative to each other.  
         [0015]     The first fixation device may have an upper and lower surface, and the second fixation device may have an upper and lower surface, and wherein the lower surface of the first fixation device may be configured to slidably engage the upper surface of the second fixation device. The upper surface of the second fixation device may further comprise a groove, wherein at least a portion of the lower surface of the first fixation device may be configured to engage at least a portion of the groove.  
         [0016]     The assembly may further comprise a hub for connecting the first and second fixation devices. At least one opening may be a slot. The assembly may further comprise a third fixation device and a fourth fixation device.  
         [0017]     A fixation assembly is also described comprising a first and second pair of openings arranged in a generally rectangular array, wherein each opening is configured to receive a bone fastener; a first distance extending between the first pair of openings, and a second distance extending between the second pair of openings, wherein the translation of bone fasteners within the openings alters at least one of the first and second distances.  
         [0018]     The first and second pairs of openings may be slots. The slots may be centered on axes that extend between diagonally opposite corners of the rectangular array.  
         [0019]     A fixation assembly is further described, comprising a first fixation device having a first pair of openings defining bone fastener locations; and a second fixation device having a second pair of openings defining bone fastener locations; the first and second fixation devices being interconnected for movement relative to each other such that the bone fastener locations defined by the first and second pairs of openings are located at four corners of a rectangular array, and the bone fastener locations at opposite ends of the rectangular array are movable relatively toward each other lengthwise of rectangular array while remaining uniformly spaced apart from each other across the rectangular array.  
         [0020]     The first and second bone fixation devices may be interconnected for rotation relative to each other. The first and second pairs of openings may be slots. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     Preferred features of the present invention are disclosed in the accompanying drawings, wherein similar reference characters denote similar elements throughout the several views, and wherein:  
         [0022]      FIG. 1  is a top view of an embodiment of a one-level bone fixation assembly in an expanded position.  
         [0023]      FIG. 2  is a top view of the assembly of  FIG. 1 , in a compressed position.  
         [0024]      FIG. 3  is an enlarged cross-sectional view taken on line A-A of  FIG. 1 .  
         [0025]      FIG. 4  is a top view of an embodiment of a two-level bone fixation assembly in an expanded position.  
         [0026]      FIG. 5  is a cross-sectional view taken on line B-B of  FIG. 4 .  
         [0027]      FIG. 6  is a cross-sectional view taken on line C-C of  FIG. 4 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]     The plates described herein may be used in spinal fusion procedures in which a damaged or diseased disc (or part of a disc) is removed from between a pair of vertebrae and a spinal fusion spacer is placed between the vertebrae. The plates may be applied to an anterior portion of the affected vertebrae to span the affected disc space, and may be fixed to the vertebrae using bone screws. The plate may function to maintain the vertebrae aligned during the initial period following fixation in which fusion of the spacer to the adjacent vertebrae occurs. The plate may also function to share some of the axial spinal load applied to the fusion spacer to prevent extreme subsidence of the spacer into the vertebral body, such as where the patient has poor bone quality. The plates may also act to prevent the spacer from being expelled from the disc space during the initial post-operative period.  
         [0029]     The plates may be used for single-level (i.e. one-disc) or multiple-level (i.e. multiple disc) fusion procedures. Some embodiments may be used for corpectomy procedures, in which at least a portion of a vertebral body is removed. Single level plates generally may have two pairs of fastener holes, while the multi-level plates generally may have three or more pairs of holes.  
         [0030]      FIGS. 1-3  show an embodiment of a one-level bone fixation assembly. This embodiment includes first and second bone fixation plates  10  and  12 . The bone fixation plates  10  and  12  together define a bone fixation assembly  16  which, in this particular example, is a spinal fixation assembly.  
         [0031]     A first pair of fasteners  20  fasten the fixation assembly  16  to a first vertebra. A second pair of fasteners  22  fasten the fixation assembly  16  to a second vertebra adjacent to the first vertebra. The fixation plates  10  and  12  are interconnected for movement relative to each other such that the two pairs of fasteners  20  and  22  can move toward each other vertically while the fasteners  20  or  22  in each pair remain uniformly spaced apart from each other horizontally. This enables the fixation assembly  16  to shift as needed when compression of the spinal column causes the two fastened vertebra to move toward each other.  
         [0032]     In the embodiment of  FIG. 1 , the first bone fixation plate  10  is a plate with a circular central portion  30 . A pair of opposite end portions  32  of the plate  10  may project radially outward from the central portion  30  at diametrically opposite locations. The central portion  30  may be shaped as a ring with an annular inner edge surface  34  (see  FIG. 3 ). A pair of shoulders  38  may be located at the periphery of the central portion  30 . Each shoulder  38  may project axially upward, as shown in  FIG. 3 , and may extend in an arc past the location of a respective one of the outer end portions  32 , as shown in  FIG. 1 .  
         [0033]     The embodiment shown in  FIG. 1  also has a second bone fixation plate  12  with a ring-shaped central portion  50 , which may have a pair of opposite end portions  52  projecting radially outward from the central portion  50  at diametrically opposite locations. The ring-shaped central portion  50  of the second plate  12  may be received concentrically over the ring-shaped central portion  30  of the first plate  10 , and may fit closely between the shoulders  38  on the first plate  10 .  
         [0034]     Second plate  12  may similarly have a pair of shoulders  58 . As shown in  FIG. 3 , an annular inner edge surface  54  of the second plate  12  may be located adjacent to the annular inner edge surface  34  of the first plate  10 . A ring-shaped hub  60  may be crimped against the annular inner edge surfaces  34  and  54 . The hub  60  may hold the two plates  10  and  12  together, and may support them for sliding movement relative to each other rotationally about a common central axis  61 . The range of such movement may be defined by the circumferential length of the spaces  67  between the shoulders  38  and  58  when the two plates  10  and  12  are in the positions shown in  FIG. 1 . The plates  10  and  12  are thus rotatable from the initial, expanded positions of  FIG. 1  toward the rotated, compressed positions of  FIG. 2 . An elastomeric friction ring  64 , shown in  FIG. 3 , may be interposed between the plates  10  and  12  to prevent the plates  10  and  12  from rattling or rotating loosely.  
         [0035]     The opposite end portions  32  of the first plate  10  may be similarly dimensioned, shaped, and/or sized. Each end portion  32  may have an inner edge surface  70  defining a respective slot  71 . The slots  71  may be centered on a line  73  extending diametrically across the circular central portion  30  of the first plate  10 , and may be elongated radially. The opposite end portions  52  of the second plate  12  likewise may have inner edge surfaces  80  defining radially elongated slots  81  that may be centered on a diametrically extending line  83 , and may generally have all of the characteristics of the opposite end portions  32  of the first plate  10 . Slots  71  may also be fitted with captive clips (not shown) to allow fasteners  20  and  22  to move within the slots  71  and further prevent fastener  20  and  22  back-out, the details, materials, and methods of which are described in U.S. patent application Ser. No. 10/653,164 entitled “Bone Plate with Captive Clips”, by Duong, et al., filed Sep. 3, 2003, the entire disclosure of which application is expressly incorporated by reference herein.  
         [0036]     The fixation assembly  16  may be shiftable from the initial, expanded condition of  FIG. 1  to the shifted, compressed condition of  FIG. 2  upon rotation of the first and second plates  10  and  12  relative to each other about the axis  61 . When the spinal fixation assembly  16  is in the initial, expanded condition of  FIG. 1 , the slots  71  and  81  in the two plates  10  and  12  may define the locations at which the first and second pair of fasteners  20  and  22  are to be implanted into the first and second vertebra. Specifically, the first pair of fasteners  20  may be implanted into the first vertebra at the radially outer ends of the slots  71  and  81  in the upper end portions  32  and  52  of the plates  10  and  12 . The second pair of fasteners  22  may be implanted into the second vertebra at the radially outer ends of the slots  71  and  81  in the lower end portions  32  and  52  of the plates  10  and  12 . The first and second pairs of fasteners  20  and  22  may then be located at the four corners of a generally rectangular array in which the diametrically extending lines  73  and  83  may extend as diagonal axes between diagonally opposite corners of the array.  
         [0037]     Compression of the spine may cause the first and second vertebra to move toward each other. This may cause the first pair of fasteners  20  at the first vertebra to move vertically downward relative to the second pair of fasteners  22  at the second vertebra. The vertebral spacing between the first and second pair of fasteners  20  and  22  at the opposite ends of the generally rectangular array may thus be reduced from the expanded distance “Y 1 ” of  FIG. 1  either partially or fully to the compressed distance “Y 2 ” of  FIG. 2 . However, such shifting of the vertebra does not cause the first and second pair of fasteners  20  or  22  to move horizontally within the vertebra to which they are fastened. Therefore, the horizontal spacing “X” across the generally rectangular array of bone screws  20  and the  22  remains constant as the first and second vertebrae move toward each other. When the first pair of fasteners  20  move toward the second pair of fasteners  22  in this manner, their respective locations may therefore shift in the slots  71  and  81  such that each fasteners  20  and  22  moves radially inward of the respective slot  71  or  81 . The fasteners  20  and  22  may then slide against the inner edge surfaces  70  and  80  in the slots  71  and  81  so as to move the plates  10  and  12  rotationally about the axis  61  from the expanded positions of  FIG. 1  toward the compressed positions of  FIG. 2 . The fixation assembly  16  is thus shiftable to accommodate movement of the fastened vertebra while remaining fastened to those vertebra to provide alignment support for the spine.  
         [0038]      FIGS. 4-6  shows an embodiment of a two-level fixation assembly. This embodiment includes first and second shiftable bone fixation assemblies  110  and  112  that are together configured to be fastened to the spinal column in a connected series extending along the length of the spinal column. Generally, the fixation assemblies  110  and  112  and their components shown in  FIGS. 4-6  may have some or all of the characteristics of the fixation assembly  16  shown in  FIGS. 1-3 .  
         [0039]     Each fixation assembly  110  and  112  of  FIG. 4  is substantially the same as the bone fixation assembly  16  of  FIGS. 1-3 , and thus includes first and second fixation plates  114  and  116  that may be interconnected by a hub  118  for rotation relative to each other about a common central axis  119  as described above with reference to the fixation assembly  16 . However, the bone fixation assemblies  110  and  112  differ in that an end portion  120  of a plate  114  or  116  has a full thickness at a free end of the connected series, as shown in  FIG. 5 , or a reduced thickness where adjacent end portions  120  overlap in the connected series, as shown in  FIG. 6 .  
         [0040]     Generally, fixation plates  10 ,  12 ,  114 ,  116  may be of any suitable size, shape, and dimension, depending at least in part on the size of the vertebra the plates will be attached to, and the size of the intervertebral space to be spanned. Fixation plates  10 ,  12 ,  114 ,  116  may be substantially flat, to reduce the overall profile of the fixation assembly  16 ,  110 ,  112 .  
         [0041]     Each of the fasteners and fixation plates disclosed herein may be formed of a titanium alloy such as titanium-aluminum-niobium, which may be anodized. One material for use with each of the plates and screws described herein is Ti-6AI-7Nb, with a density of about 4.52gm/cc, a modulus of elasticity of about 105 GPa, an ultimate tensile strength of about 900 MPa, and a yield strength of about 800 MPa. Surfaces of the fasteners may also be burr free, with all sharp edges broken to a maximum of 0.1 mm.  
         [0042]     It should be noted that the aforementioned descriptions and illustrations have been provided as examples of the configurations of translation plates that may be designed and assembled using the principles of the invention. These examples will be understood to one of ordinary skill in the art as being non-limiting in that a fixation assembly employing one or more of the disclosed features may be produced as desired or required for a particular patient&#39;s need. Thus, the features disclosed are “modular” in nature.  
         [0043]     This written description sets forth the best mode of the claimed invention, and describes the claimed invention to enable a person of ordinary skill in the art to make and use it, by presenting examples of the elements recited in the claims. The patentable scope of the invention is defined by the claims themselves, and may include other examples that occur to those skilled in the art. Such other examples, which may be available either before or after the application filing date, are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.  
         [0044]     While the invention has been shown and described herein with reference to particular embodiments, it is to be understood that the various additions, substitutions, or modifications of form, structure, arrangement, proportions, materials, and components and otherwise, used in the practice and which are particularly adapted to specific environments and operative requirements, may be made to the described embodiments without departing from the spirit and scope of the present invention. Accordingly, it should be understood that the embodiments disclosed herein are merely illustrative of the principles of the invention. Various other modifications may be made by those skilled in the art which will embody the principles of the invention and fall within the spirit and the scope thereof.