Abstract:
An anterior spinal fixation system comprises a plurality of mounting constructs to hold one or more spinal rods or spinal plate-type braces. Each mounting construct includes a bottom plate and a top plate assembly. The bottom plate is attached to the vertebral body with a plurality of anchors, one or more of which may be polyaxial. In one embodiment, the top plate assembly engages the bottom plate through a quick-connect feature that provides simple and secure locking as well as tactile feedback. When the top plate assembly is engaged with the bottom plate, it compresses the rods or braces against the head of the screws, locking the angle of any polyaxial bone screws. In another embodiment, the top plate assembly threadably engages the bottom plate. For additional strength and rigidity, cross-connectors may be used between each pair of mounting constructs when two or more spinal rods are used.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims priority to and benefit under 35 USC 119(e) U.S. Provisional Application Ser. No. 60/550,477, filed on Mar. 5, 2004, the entire contents of which are herein incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to the correction of spinal deformities. Specifically, the present invention provides an improved apparatus and method for maintaining vertebrae in a desired spatial relationship. 
   2. Background 
   The human spinal column is composed of many vertebral bones stacked one upon the other, with an intervertebral disc between each pair of adjacent vertebral bones. The discs act as cartilaginous cushions and shock absorbers. The spinal cord runs in a bony canal formed by successive openings in these bones. Spinal nerves exit the spinal cord between pairs of vertebrae and supply nerve signals to and from other body structures. 
   Various problems with the human spine have been encountered that adversely affect its health. These problems include spinal column disorders such as scoliosis, kyphosis, spondylolisthesis, as well as traumatic events such as ruptured or slipped discs, broken or fractured spinal columns, and the like. Various forms of instrumentation and procedures are known for the surgical treatment of spinal disorders, for example, Harrington Spinal Instrumentation, Bobechko Hooks, Edwards Hooks and Rod Sleeves, Luque Segmental Spinal Instrumentation and Luque Rectangles, the Dunn Anterior Spinal System, and the Kostuik-Harrington Instrumentation. 
   The use of longitudinally extending surgical rods in the treatment of diseases or instability of the spine is well known in the medical arts. Such rods achieve rigid spinal fixation when mechanically coupled to bone anchors, such as hooks or screws. These surgical rods are used, generally, in pairs placed on the posterior surface of the left and right sides of the lamina of the human spine. 
   Some of the above systems utilize hook-type members, which are merely hooked over the laminae or on selected transverse processes of the spine. Other systems, such as the Luque Segmental Spinal Rectangles (used to stabilize spinal fractures and low back fusions), use Luque wires to secure the rectangle to the spine. In some of the prior art systems, screws are used to hold a single rod in place. In other systems, screws are used to hold a slotted plate in place, the location of the screws and slots being such that the plate is moved in order to align the plate apertures or slots with the end of the screw, a nut being used to hold the plate to the screw. With this latter arrangement there is little purchase between the plate and the screw and nut since only a small portion of the plate is engaged adjacent to the slots. Also, the plate cannot be configured to a fixed and stable curvature to follow the curvature desired by the surgeon. 
   Another known corrective device includes a plurality of plates. Each of the plates is secured over one end of a vertebra. Fasteners are connected to the vertebrae through the plates. A cable is then crimped in the head of the fastener to attach the cable to one vertebra. Tension is put on the cable while it is crimped to an adjacent vertebra until the desired correction is accomplished. This device can only put compressive forces on the spine so that the cables are always in tension. Once the cable is crimped in place, no further adjustment between the crimped fastener and cable is possible. 
   In devices utilizing rods, the corrective forces are generated by (usually) two rods that are wired around the spine. The rods may be bent to a desired curvature. The rods are not directly attached to all the vertebrae that the rods span; rather, they span numerous vertebrae and are connected to only a few vertebrae using anchors, generally hooks or screws. 
   One widely used anchor for rod systems is the conventional orthopedic hook having a block-shaped head portion with a central, cylindrical bore therethrough, and a hook portion. The bore of the conventional orthopedic hook is adapted to receive the surgical rod, and the head is slidably positioned over the surface of the surgical rod to the selected vertebra for attachment. The hook may have a variety of different shapes, lengths and openings to accommodate the specific vertebra to which it is to be anchored. With the hook portion properly anchored, the conventional orthopedic hook is locked to the surgical rod either by ratchet or by one or more set screws located within the block-shaped head. However, these systems do not provide polyaxial alignments of the anchors. Rather, the anchors are fixed in a given orientation with respect to the bone and allow no movement in vivo or in response to applied loads. 
   Another type of anchor is a special orthopedic screw having a block-shaped head with cylindrical bore therethrough. The screw, when its threaded end is attached to the selected anatomical site, is adapted for receiving and passing the elongated surgical rod through its cylindrical bore. Since the shank and threaded end of the screw extends perpendicularly with respect to the axis of the bore, once the screw has been anchored, the position of the head, with its cylindrical bore, is fixed with respect to the spine of the patient. 
   If the nature of the disease of the spine should require the attachment of a number of orthopedic screws at spaced-apart anatomical sites, it will be appreciated that manual insertion of an elongated surgical rod through the bores of the several spaced-apart orthopedic screws is surgically difficult. The alignment of the axis of the bore in the head of each screw must, of necessity, bear some relationship to a common axis related to the axis of the surgical rod, which rod must be inserted through the several bores. Since the nature of the surgical operation places the surgical rod under stress, as by resisting deforming forces of the spine, it will be appreciated that proper positioning of the heads and alignment of the bores of the several anchor attachment members is of paramount concern. 
   Some systems have attempted to provide bone screw/rod anchor devices that include polyaxial screws, with varying degrees of success. Most systems that attempt to provide for polyaxial capabilities employ a spherical head or ball-shaped head for the screw. While this allows angulation, it also provides an undesirable structure as the spherical head takes up too much space in the construct. Moreover, such systems rely on a locking screw to apply a compressive force between the lower surface of the rod and the upper surface of the ball to “lock” the angle of entry of the screw. 
   Other devices have used dual rods or an elongated plate-type brace held by a plurality of plates attached to the anterior portion of the vertebral bodies. A common problem with this type of system is the use of spikes extending from the surface of the plates that will be held against the vertebral body. These spikes present a variety of difficulties for the surgeon. First, the surgeon is unable to position the plate against the surface of the vertebra to check how well the plate will sit on the vertebra in the chosen location without driving the spikes into the bone. And, once the spikes are driven into the bone, the plate cannot be repositioned, for example, to relocate a screw hole away from a damaged portion of the vertebral body. Even if it is unnecessary to relocate the entire plate, these spikes also prevent the surgeon from being able to make small adjustments in orientation since the plate is firmly fixed by the spikes penetrating the vertebral body. 
   Another problem is the use of parts that require precise alignment to properly mate. One example of this is seen in U.S. Pat. No. 6,132,431. This device uses a C-shaped cover that the surgeon must fit over two flanges while holding an elongated plate-type brace in place between the flanges. With this particular device, the surgeon must then hold the brace and the cover in place while threading a set screw through the cover to compress the brace against the mounting plate. 
   Recognizing that the spinal fixation systems are installed during a surgical procedure while the patient is under anesthesia, it is important that the orthopedic surgeon have available for immediate use a fixation system that has mounting elements that are easily positioned and secured to the vertebrae. The system should also include a means for attaching surgical rods to the mounting elements that quickly and easily secures the rods. In a preferred embodiment, the device should provide the surgeon with a simple, effective lock that also provides the surgeon with tactile feedback that the lock is secure. And all of these features are needed in a low profile, space-efficient device. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention is directed to an anterior fixation system for the spine. The fixation system utilizes a quick connect, or alternatively a threaded mechanism, allowing spinal fixation rods to be quickly and securely connected to plates attached to two or more vertebral bodies without the use of small parts. As an added benefit, if polyaxial screws are used to attach the plates to the vertebral bodies, the system utilizes the compressive force locking the spinal fixation rods in place to lock in the angle of the screws relative to the plates. 
   The system comprises at least two mounting constructs, each attached to a different vertebral body. Each construct further comprises a bottom plate and a top cap assembly that engage to hold a pair of fixation rods or a plate-type brace in place. Each bottom plate is generally contoured to the anterior surface of a vertebral body and is attached with a plurality of bone screws. The top plate assembly can attach to the bottom plate with an integral quick-connect and quick-disconnect feature. 
   In a first embodiment of this invention, at least one polyaxial bone screw is used in attaching each bone plate to the vertebral body. This use of polyaxial bone screws and the absence of spikes on the back of the bottom plate provides flexibility to the surgeon in positioning the device in two ways. First, the surgeon can make subtle adjustments by rotating the device about the first screw. It also provides flexibility by allowing the screw to be driven at an angle in which it will seat in solid bone, even if the actual hole location is directly above a damaged spot in the bone. 
   In one embodiment, a cam lock provides a quick-connect feature of the device. Preferably, the bottom plate has a cam projection with two cam surfaces. The top plate assembly has a cam cap with mating cam surfaces. To assemble, the surgeon places the top cap assembly over the bottom plate, using the cam projection as a guide. Once the mating surfaces of the bottom plate and top plate assembly are touching, the cam cap is rotated using any of a number of standard driving mechanisms. This system requires the use of no small parts since the cam cap is pre-assembled to the top plate assembly before the surgery commences. Additionally, the surgeon knows that only a small turn, preferably 90 degrees, is required to make a secure and reliable connection. Also in this embodiment, there is no danger of cross threading or overthreading this connection. 
   In a second embodiment a threaded cap can be used in place of the cam cap to lock the bottom plate to the top plate. This embodiment can employ many of the same features as the first embodiment. In place of the cam projections, threaded projections can be employed. In place of the cam surfaces within the top cap, threads can be employed to engage the threads on the threaded projections. 
   In other arrangements, cross connectors can be used between mounting constructs to provide additional stability and rigidity to the system. These cross connectors are similar to the mounting constructs except the bottom plate in a cross connector does not have holes for bone screws. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a perspective view of a fixation assembly according to an embodiment of the present invention; 
       FIG. 2  is an exploded perspective view of a fixation assembly according to an embodiment of the present invention; 
       FIG. 3  is a perspective view of a bottom plate according to an embodiment of the present invention; 
       FIG. 4  is a top view of a bottom plate according to an embodiment of the present invention; 
       FIG. 5  is a front view of a bottom plate according to an embodiment of the present invention; 
       FIG. 6  is a cross-sectional front view of a bottom plate according to an embodiment of the present invention; 
       FIG. 7  is a perspective view of a top plate assembly according to an embodiment of the present invention; 
       FIG. 8  is a top view of a top plate assembly according to an embodiment of the present invention; 
       FIG. 9  is a front view of a top plate according to an embodiment of the present invention; 
       FIG. 10  is a cross-sectional front view of a top plate according to an embodiment of the present invention; 
       FIG. 11  is a perspective view of a cap according to an embodiment of the present invention; 
       FIG. 12  is a top view of a cap according to an embodiment of the present invention; 
       FIG. 13  is a front view of a cap according to an embodiment of the present invention; 
       FIG. 14  is a detail view of one of the cam surfaces from the cap shown in  FIG. 13 ; 
       FIG. 15  is a perspective view of a monoaxial screw for use with an embodiment of the present invention; 
       FIG. 16  is a side view of the monoaxial screw shown in  FIG. 15  according to an embodiment of the present invention; 
       FIG. 17  is a perspective view of a polyaxial screw according to an embodiment of the present invention; 
       FIG. 18  is a side view of the polyaxial screw shown in  FIG. 17  according to an embodiment of the present invention; 
       FIG. 19  is a cross-sectional front view of an embodiment of the present invention shown with one monoaxial screw and one polyaxial screw in use; 
       FIG. 20  is a perspective view of a fixation assembly according to a second embodiment of the present invention; 
       FIG. 21  is an exploded perspective view of a fixation assembly according to a second embodiment of the present invention; 
       FIG. 22  is a perspective view of a bottom plate according to a second embodiment of the present invention; 
       FIG. 23  is a top view of a bottom plate according to a second embodiment of the present invention; 
       FIG. 24  is a front view of a bottom plate according to a second embodiment of the present invention; 
       FIG. 25  is a cross-sectional front view of a bottom plate according to a second embodiment of the present invention; 
       FIG. 26  is a perspective view of a top plate assembly according to a second embodiment of the present invention; 
       FIG. 27  is a top view of a top plate assembly according to a second embodiment of the present invention; 
       FIG. 28  is a front view of a top plate assembly according to a second embodiment of the present invention; 
       FIG. 29  is a cross-sectional front view of a top plate assembly according to a second embodiment of the present invention; 
       FIG. 30  is a perspective view of a cap according to a second embodiment of the present invention; 
       FIG. 31  is a top view of a cap according to a second embodiment of the present invention; 
       FIG. 32  is a front view of a cap according to a second embodiment of the present invention; 
       FIG. 33  is a cross-sectional detail view of the threads of the cap shown in  FIG. 30 ; and 
       FIG. 34  is a cross-sectional front view of a second embodiment of the present invention shown with one monoaxial screw and one polyaxial screw in use. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   While the present invention will be described more fully hereinafter with reference to the accompanying drawings in which particular embodiments and methods are shown, it is to be understood from the outset that persons of ordinary skill in the art may modify the invention herein described while achieving the functions and results of this invention. Accordingly, the description that follows is to be understood as illustrative and exemplary of specific embodiments within the broad scope of the present invention and not as limiting the scope of the invention. In the following descriptions, like numbers refer to similar features or like elements throughout. 
   Ideally, a spinal fixation system will provide the surgeon with flexibility in attaching the apparatus to the vertebral bodies and a rapid and secure connection to the fixation rods that is safe and secure. The figures depict merely two of many embodiments of the invention: a cam embodiment ( FIGS. 1-19 ) and a threaded embodiment ( FIGS. 20-34 ).  FIG. 1  shows an apparatus  10  according to a first embodiment of the present invention. The apparatus  10  generally comprises two or more mounting constructs  20 ,  30 ; one or more cross connectors  40 , if necessary for added rigidity; two rods  12 ; two taper lock screws  50 ; and two polyaxial screws  60 . Obviously, however, other configurations are possible. 
   Referring now to  FIG. 2 , each mounting construct  20 ,  30  further comprises a bottom plate  70  and a top plate assembly  80 , which further comprises a top plate  90  and a cap  100 . 
   Referring now to  FIGS. 3-6 , the bottom plate  70  has a bottom surface  72  contoured to conform generally to the shape of the vertebral body. Opposite the bottom surface  72 , the bottom plate  70  has an upper surface  74  with one or more recesses  76  to accommodate the rods  12 . 
   The bottom plate  70  preferably further contains a first hole  120  and a second hole  130 . In the embodiment shown, the first hole  120  has a taper  122  from the upper surface  74  to the bottom surface  72  that acts to lock the angle of the taper lock screw  50  relative to the bottom plate  70 . Also in the embodiment shown, the second hole  130  has a spherical undercut  132  to allow the polyaxial screw  60  to be inserted into the bottom plate  70  at varying angles, preferably any angles between 0 and 30 degrees. However, other hole and screw configurations are possible so long as they are designed to allow the screw to seat firmly on the bottom plate. In monoaxial screw embodiments, the top of the head of the screw can be level with or slightly above or below the surface of the recess  76 . In polyaxial screw embodiments, the top of the head of the screw is preferably slightly above the surface of the recess  76  so that the rods  12  can lock the angle of the screw upon tightening of the cap  100  and top plate  90 . 
   Referring again to  FIGS. 3-6 , arising transversely from the upper surface  74  of the bottom plate  70  is a projection  110 . The projection  110  can take many forms. Its purpose is to provide a location for a means for coupling or engaging the top plate  90  to the bottom plate  70 . The engagement structure of the projection  110  can be viewed as a first engagement structure, while the engagement structure of the cap  100  (described below) may be viewed as a second engagement structure. The cam projection  110  shown in these figures may take many forms in alternate embodiments; it being necessary only to provide a quick lock capability. In the embodiment shown, two cam surfaces  112  are located toward the distal end of the cam projection  110 . However, various other arrangements are possible including, but not limited to, arrangements where the cam surfaces are located on the cap  100 . The cam surfaces  112  preferably are designed to tightly engage the cap  100  when the bottom plate  70  and the top plate assembly  80  are mated. 
   Referring now to  FIGS. 7-10 , the top plate assembly  80 , comprising the top plate  90  and the cap  100 , is shown. The top plate  90  has a lower surface  92  and an upper surface  96 . The lower surface  92  contains two recesses  94  to accommodate the rods  12 . The upper surface  96  has a cam cap recess  97 . Preferably the top plate  90  further comprises one or more projections  95  that interact with the projection  110  of the bottom plate  70 . A hole  98  runs through the cam cap recess  97  allowing the cam projection  110  to pass through the top plate  90  and mate with the cap  100 . Preferably, one or more projections  99  can be installed in an inner surface of the cam cap recess  97 . These projections  99  can mate with grooves  105  in the cap  100 , described below. 
   Referring now to  FIGS. 11-13 , the cap  100  preferably has a hexagonal upper portion  102 , a lower portion  104 , and a centrally located hole  106 . The hexagonal upper portion  102  is dimensioned to fit a standard socket to enable the surgeon to rotate the cam cap with a standard, surgical socket wrench. However, it is readily seen that any type of upper portion  102  is possible to engage the many types of driving devices that exist. The lower portion  104  has a surface that preferably is fitted with one or more grooves  105 , or alternatively detents. These grooves  105  or detents can be placed at certain locations about the perimeter of the lower portion  104  so as to correspond with certain known reference points of rotation. For example, three grooves  105  can be placed in the perimeter of the lower portion  104  to correspond to an open position, a closed position, and a midway position. Complementary features can be provided in the cam cap recess  97  to engage the grooves  105  or detents so as to provide a tactile feedback to the surgeon. 
   Referring now to  FIG. 14 , the hole  106  preferably has two cam surfaces  140  located toward the lower portion of the cap  100  that project inwardly. The hole  106  allows the cam projection  110  to pass through the cap  100  far enough that the cam surfaces  112  of the cam projection  110  of the bottom plate  70  are able to interact with the cam surfaces  140  when the cap  100  is rotated. 
   Referring now to  FIGS. 15 and 16 , an exemplary taper lock screw  50  is shown. It should be noted, however, that the invention is usable with various types of fasteners and the description herein of only two exemplary embodiments is not intended to limit the invention in any way. The taper lock screw  50  has a head  52  that is shown ( FIG. 19 ) having a hexagonal recess  54 , allowing it to be driven with a standard allen type wrench. It should be noted that many types of driving mechanisms, and therefore recesses  54 , are possible. The head  52  is tapered such that, when it is driven through the first hole  120  in the bottom plate  70  and into the vertebral body, it mates with the taper  122  to lock in the angle of the screw relative to the bone plate. Further, after the taper lock screw  50  is driven into the bone, the top surface of the head  52  will be level with the lowest point in the recess  76  such that the rod  12  compresses the head  52  when the cap  100  is rotated to engage the cam projection  110 . This relation is shown in detail in  FIG. 19 . 
   Referring now to  FIGS. 17 and 18 , an exemplary polyaxial screw  60  is shown. It should be noted, however, that the invention is usable with various types of fasteners and the description herein of only two exemplary embodiments is not intended to limit the invention in any way. The polyaxial screw  60  has a partially spherical head  62  shown with a hexagonal recess  64  allowing it to be driven with a standard allen type wrench. It should be noted that many types of driving mechanisms, and therefore recesses  64 , are possible. The spherical head  62  mates with the spherical undercut  132  in the second hole  130  in the bottom plate  70  to create a kind of ball and socket connection. The polyaxial screw  60  may be driven at one of many angles, ranging preferably from 0 to approximately 30 degrees, relative to the centerline of the hole  130  while still seating firmly in the bottom plate  70 . The spherical undercut  132  is preferably located such that, after the polyaxial screw  60  is driven into the bone, the highest point of the head  62  will be level with, or perhaps slightly above, the lowest point in the recess  76 . Therefore, when the cap  100  is rotated to engage the cam projection  110 , the rod  12  compresses the head  62  and locks in the angle of the polyaxial screw  60  relative to the bottom plate  70 . This relation is shown in detail in  FIG. 19 . 
   Referring again to  FIGS. 1 and 2 , cross connectors  40  located between the mounting constructs  20 ,  30  are also optionally provided. Two cross connectors are shown in the figures, but obviously any number (or none at all) of cross connectors may be used depending on the distance between the mounting constructs  20 ,  30  and the amount of additional torsional stability required. A cross connector  40  is preferably substantially identical to a mounting construct except the bottom plate  70  of the cross connector does not contain screw holes  120  and  130 . 
   In the preferred embodiment, the invention is formed of a material suitable for implantation in the human body, still more preferably a metal, with sufficient rigidity for the particular load to be applied. In this embodiment, the taper lock screws  50  have a diameter of approximately 7.0 mm, the polyaxial screws  60  have a diameter of approximately 7.0 mm, and the surgical rods  12  have a diameter of approximately 5.0 mm. One or more cross connectors may additionally be used as required for torsional stability. 
     FIGS. 20-34  depict a second, threaded embodiment of the invention. Many features and structures of this embodiment are similar to that of the first, cam embodiment described above and have been labeled accordingly. These similar features function in generally the same manner, so only the differences will be discussed below. 
     FIGS. 22-25  show the bottom plate  70  of the second embodiment. In this embodiment, a threaded projection  210  is disposed atop the upper surface  74 . Threads  212  are disposed on the threaded projection  210  to threadingly engage the cap  100  when the bottom plate  70  and the top plate assembly  80  are mated. 
   Referring now to  FIGS. 26-29 , the top plate assembly  80  comprises a top plate  90  and a cap  100 . The top plate  90  of the second embodiment typically does not (although it is possible to have) comprise one or more projections  99  to engage grooves  105 . Referring to  FIGS. 30-34 , the cap  100  of this embodiment comprises threads  150  to engage the threaded projection  210  to lock the construct together. 
   While there has been described and illustrated various features and particular embodiments of a novel thoracolumbar fixation system, it will be apparent to those skilled in the art that variations and modifications may be possible without deviating from the broad spirit and principle of the present invention, which shall be limited solely by the scope of the claims appended hereto.