Patent Abstract:
An anterior bone plate system is provided that promotes osseous fusion and allows subsidence while restricting extension. The bone plate system requires a minimum number of screws for securing the plate onto bone, thus reducing the amount of osseous tissue damage incurred by the bone structures to which they are attached. The system is also simple to use and provides for independent screw placement while incurring minimal soft tissue damage from lateral retraction. A method for implementing the system is also provided.

Full Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 10/108,249 filed Mar. 27, 2002, which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to fixation devices used in orthopedic and spinal surgery and particularly to bone fixation plates useful for positioning and immobilizing bone segments. 
     BACKGROUND OF THE INVENTION 
     For a number of known reasons, bone fixation devices are useful for promoting proper healing of injured or damaged vertebral bone segments caused by trauma, tumor growth, or degenerative disc disease. The external fixation devices immobilize the injured bone segments to ensure the proper growth of new osseous tissue between the damaged segments. These types of external bone fixation devices often include internal bracing and instrumentation to stabilize the spinal column to facilitate the efficient healing of the damaged area without deformity or instability, while minimizing any immobilization and post-operative care of the patient. 
     One such device is an osteosynthesis plate, more commonly referred to as a bone fixation plate, that can be used to immobilize adjacent skeletal parts such as bones. Typically, the fixation plate is a rigid metal or polymeric plate positioned to span bones or bone segments that require immobilization with respect to one another. The plate is fastened to the respective bones, usually with bone screws, so that the plate remains in contact with the bones and fixes them in a desired position. Bone plates can be useful in providing the mechanical support necessary to keep vertebral bodies in proper position and bridge a weakened or diseased area such as when a disc, vertebral body or fragment has been removed. 
     Such plates have been used to immobilize a variety of bones, including vertebral bodies of the spine. These bone plate systems usually include a rigid bone plate having a plurality of screw openings. The openings are either holes or slots to allow for freedom of screw movement. The bone plate is placed against the damaged vertebral bodies and bone screws are used to secure the bone plate to the spine, usually with the bone screws being driven into the vertebral bodies. Exemplary systems like the one just described can be found in U.S. Pat. No. 6,159,213 to Rogozinski, U.S. Pat. No. 6,017,345 to Richelsoph, U.S. Pat. No. 5,676,666 to Oxiand et al., U.S. Pat. No. 5,616,144 to Yapp et al., U.S. Pat. No. 5,549,612 to Yapp et al., U.S. Pat. No. 5,261,910 to Warden et al., and U.S. Pat. No. 4,696,290 to Steffee. 
     Despite the existence of these bone plate systems, there remains a need for an anterior bone plate system that minimizes any soft tissue and osseous tissue damage that would occur with its implementation and still be easy to use. The system must be able to provide effective fixation and immobilization of vertebral bodies, while also providing for the subsidence necessary for proper fusion and prevent axial extension of the plate. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention achieves the aforementioned goals by providing an anterior bone plate system that promotes osseous fusion and allows subsidence while restricting extension. The bone plate system further requires a minimum number of screws for securing the plate onto bone, thus reducing the amount of osseous tissue damage incurred by the bone structures to which they are attached. The system is also simple to use and provides for independent screw placement while incurring minimal soft tissue damage from lateral retraction. 
     These desired features are accomplished by providing a system comprising a bone plate having a first surface, a second bone-contacting surface opposed to the first surface, and a channel formed on the first surface extending lengthwise from one end of the plate to an opposite end. A plurality of apertures extend through the channel. Each aperture has a predefined shape and size. A plurality of screws capable of insertion into bone are also provided. Each screw has a lower threaded portion at one end and an open screw head at an opposite end. The screw head has a complementary shape and size sufficient to pass through the apertures of the bone plate. Also included is a locking mechanism for securing the bone plate onto the screws. 
     According to one aspect of the invention, the aperture and the screw head is oblong in shape. The bone plate can also include surface features such as cleats on the bone-contacting surface. The bone plate can further include a rigid flap or lip on each side of the plate extending lengthwise from one end of the plate to the opposite end. The flaps can extend over the channel. 
     In one exemplary embodiment of the present invention, each screw has a self-tapping end extending from the lower threaded portion. Further, each screw head can extend into an upper threaded portion configured to sit proud, i.e., not engaged in bone. A locking device can be provided to secure the bone plate onto the screws. The locking device can be a nut, cam, wedge, or a retaining ring. 
     In another exemplary embodiment of the present invention, each flap includes a notched region, and each screw head includes a pair of diametrically opposed ramps. Each of the ramps further includes a groove for engagement with the notched region of the flaps. In this system, the bone plate can be effectively locked onto the screws by placing the plate over the screws and rotating the screw heads 90 degrees so that the grooves of the ramps and the notched regions of the flaps form an interference fit. 
     Also provided is a method for stabilizing and rigidly fixing vertebral bodies in a patient, involving the steps of identifying a damaged region of the patient&#39;s spine, preparing a pilot hole in each of the vertebral bodies in the damaged region, removing osteophytes from the selected vertebral bodies, creating a smooth flat surface on the selected vertebral bodies, providing an anterior bone plate system as described above, inserting a screw in each of the pilot holes, placing the bone plate over the screws, and locking the bone plate to the screws. 
     The pilot holes can be prepared by inserting a distraction pin into each of the selected vertebral bodies. Each of the distraction pins should be inserted in the midline of the vertebral bodies, with only one pilot hole being made in each vertebral body. Hence, only one screw is inserted into any single vertebral body, reducing the osseous damage to the spine. 
     If necessary, the selected vertebral bodies can be distracted and a diseased disc removed from the damaged region. A graft can then be placed into the evacuated disc space. Prior to inserting the screws and plate, the vertebral bodies must be shaved to create a smooth flat surface that precisely matches the width of the bone plate. This ensures proper adhesion of the plate to the bony surface and also produces a low profile so that damage to surrounding soft tissue can be minimized. 
     Further features of the invention, its nature and various advantages, will be more apparent from the accompanying drawings and the following detailed description of the drawings and the preferred embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be more fully understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a perspective view of an anterior bone plate system of the present invention, wherein the screws are extending through the bone plate; 
         FIG. 2  is a side view of the system of  FIG. 1 ; 
         FIG. 3  is a perspective view showing the bottom of the system of  FIG. 1 ; 
         FIG. 4  is a perspective view of another embodiment of the anterior bone plate system of the present invention, wherein the screws are extending through the bone plate; 
         FIG. 5  is a side view of the system of  FIG. 4 ; 
         FIG. 6  is a detailed view of a portion of the screw of  FIGS. 4 and 5 ; 
         FIG. 7  is a perspective view of yet another embodiment of the anterior bone plate system of the present invention, wherein the screws are extending through the bone plate; 
         FIG. 8  is a perspective view showing the bottom of the system of  FIG. 7 ; 
         FIG. 9  is a cross-sectional view of the anterior bone plate system of  FIG. 7 ; and 
         FIG. 10  is a detailed view of the retaining ring present in the system of  FIGS. 7 and 9 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides an anterior bone plate system which is easy to use and allows independent screw placement. The system requires only one bone screw to be used per vertebral body level, thus reducing the amount of osseous tissue damage to the vertebral bodies. The bone plate system of the present invention also promotes good bone fusion by providing enough subsidence to facilitate proper bone growth, while at the same time preventing extension. Furthermore, the bone plate system of the present invention provides a good bone screw to plate interface locking mechanism. 
     Referring to  FIG. 1 , in one exemplary embodiment of the anterior bone plate system  100  of the present invention, a bone plate  110  with a plurality of screws  130  extending through apertures or slots  112  thereon are shown. The apertures  112  allow the plate  110  to be placed from above onto previously placed screws  130 , thereby enabling independent screw placement. Each screw  130  has a self-tapping distal end  132  extending into a lower threaded portion  134  with an aggressive thread pitch to facilitate its purchase into bone. As shown in  FIG. 2 , each screw  130  includes a screw head  136  at an opposite (proximal) end that is adapted to seat within a channel  118  extending lengthwise on a first side  114  of the plate  110  that is configured to face away from a bony surface. Channel  118  is machined parallel to the longitudinal axis of the plate  110  such that a rigid lip or flap  120  is formed lengthwise on both sides of the bone plate  110 . Each lip or flap  120  extends over a portion of the channel  118  as illustrated in  FIGS. 1 and 2 . Since the system  100  is a top loading plate system, this lip  120  ensures the proper placement of the bone plate  110  with respect to the screws  130  when the plate  110  is dropped down onto the screws  130 . 
     In the present invention, screws  130  can have open screw heads  136 . Each screw  130  can also include an upper threaded portion with threads set proud, i.e., not engaged in bone, for engaging a locking device such as a nut  150  as shown in  FIG. 2 . While a nut  150  has been illustrated, it is contemplated that other suitable locking devices such as a wedge, cam, or retaining ring can also be used. The major thread diameter of the lower threaded portion  134  of the screws  130  can be in the range of about 3.5 to 5.5 mm, while the cancellous thread pitch of the lower threaded region  134  can be approximately 1.5 or more to provide unicortical purchase into bone. The bone screws  130  are placed in the midline of the vertebral body, with only one bone screw  130  per vertebral body or level required to adequately stabilize the bone segments to the plate  110 . This advantageous feature of the present invention reduces the amount of bone damage that would occur to the spine when more than one screw is inserted within the same vertebral body. 
     Bone plate  110  can also include surface features  122  such as cleats or ridges on the second side  116  that is configured to contact bony surface as illustrated in  FIG. 3 . The surface features  122  help anchor the bone plate  110  onto the bony surface of the vertebral bodies yet still allow flexion or subsidence while preventing extension, which is undesirable for proper fusion and healing. As can be seen in  FIG. 3 , the apertures  112  of the bone plate  110  can be oblong in shape and extend in a lengthwise direction. Further, the screw heads  136  of each of the screws  130  can be oblong in shape, enabling them to pass through the apertures  112 . Thus, it may be necessary to align the screws  130  unidirectionally, i.e., the oblong-shaped screw heads  136  are aligned lengthwise, so that the plate  110  can be placed on top of the screws  130 . 
     The anterior bone plate system  100  can be configured such that the bone plate  110  is able to slide with respect to the screw heads  136  to allow for dynamic interaction with the bone segments. As is well understood and established by Wolff&#39;s Law, the ability of the plate  110  to distribute physiologic loads to the bone is important for the fusion process. Since osseous tissue grows along lines of stress, this translational characteristic acts to maintain compressive loads across the bone/graft interface to promote bony fusion. The ability of the bone plate  110  of the present invention to effect subsidence, particularly as a result of its oblong shaped apertures  212  that allow the plate  110  some micromotion relative to the screw heads  236 , provides for effective fusion of bony segments. 
     While the plate  110  has been illustrated as having three apertures  112 , it is contemplated that the bone plate  110  of the present invention should have at least two apertures  112  for immobilizing at least two bone segments. The plate  110  may also contain more than two or three apertures  112 . Further, while the apertures  112  and screw heads  136  have been described as having an oblong shape, it is understood that the apertures  112  and screw heads  136  can have any complementary size, shape or geometry. 
     In another exemplary embodiment of the present invention,  FIG. 4  illustrates an anterior bone plate system  200  which provides an additional benefit in that it does not require additional external components to secure the bone plate  210  to screws  230 . As shown in  FIG. 4 , bone plate  210  includes at least two apertures  212  extending from a first side  214  of the plate  210  that is configured to face away from bony structure to a second side  216  that is configured to contact bony surface. A channel  218  extends lengthwise down the first side  214  of the bone plate  210  and is machined parallel to the longitudinal axis of the plate  110  such that a rigid lip or flap  220  is created on both sides of the bone plate  210 . Each lip or flap  220  runs lengthwise and extends over at least a portion of channel  218  as illustrated in  FIGS. 4 and 5 . 
     Consistent with anterior bone plate system  100 , the apertures  212  of bone plate  210  allow the plate  210  to be placed on top of previously placed bone screws  230 . Each bone screw  230  can have a self-tapping distal end  232  extending into a lower threaded portion  234  with an aggressive thread pitch to facilitate its purchase into bone. As shown in  FIG. 6 , each screw  230  includes a screw head  236  at an opposite (proximal) end that is adapted to seat within the channel  218 . The major thread diameter of the lower threaded portion  234  of screw  230  can be in the range of about 3.5 to 5.5 mm, while the cancellous thread pitch of the lower threaded region  234  can be approximately 1.5 or more to provide unicortical purchase into bone. The bone screws  230  are placed in the midline of the vertebral body, with only one bone screw  230  per vertebral body or level required to adequately stabilize the bony segments to the plate  210 . This advantageous feature of the invention reduces the amount of bone damage that would occur to the spine when more than one screw is inserted within the same vertebral body. 
     Bone plate  210  can also include surface features  222  such as cleats or ridges on the second side  216  that is adapted to contact bony surface as illustrated in  FIG. 4 . The surface features  222  help anchor the bone plate  210  onto the bony surface of the vertebral bodies yet still allow flexion or subsidence while preventing extension, which is undesirable for proper fusion and healing. 
     As shown in  FIG. 6 , the screw heads  236  of each of the screws  230  can be oblong in shape, while the apertures  212  of bone plate  210  are also oblong in shape in a lengthwise direction so that the bone plate  210  can be placed on top of and pass through the screw heads  236  when the screws  230  are unidirectional, i.e., the oblong-shaped screw heads  236  are aligned lengthwise. Each screw  230  can also have an open head  236 , which can include a shaped bore  238  for attachment to an inserter tool or screwdriver (not shown). 
     Extending proximally from the screw head  236 , away from the self-tapping distal end  232 , are a pair of diametrically opposed ramps  240 . The ramps  240  are bi-level and include a cutaway portion, or groove  242  that is configured to frictionally engage with a notched region  222  on flaps  220  when the screws  230  are rotated 90°, i.e., rotated such that the major diameter MD of the screw head  236  is transversely oriented with respect to the long axis of plate  210 . The complementary surface features on the screw heads  236  and flaps  220  provide a simple and effective locking mechanism for securing the plate  210  to the bony surface, without the need for any additional locking devices. By simply rotating the screws  230  90° after mounting the plate  210  upon the screws  230 , the plate  210  is able to be locked onto the screw heads  236 , with the grooves  242  achieving an interference fit with the notched region  222  of the flaps  220 . 
     Anterior bone plate system  200  can be configured such that the bone screws  230  are capable of sliding with respect to the apertures  212  of the plate  210 , until the screws  230  are properly seated and locked. Once locked, the anterior bone plate system allows for subsidence and micromotion to promote healing and fusion, while preventing extension. In order for the locking system to be implemented, bone plate  210  and channel  218  should be sized and configured to allow the oblong screw heads  236  sufficient room to rotate within the channel  218 . 
     Bone plate  210  can contain at least two apertures  212  for allowing the screw heads  236  to pass through the plate  210 . The apertures  212  should be positioned on the bone plate  21   0  such that the plate can attach to an upper and lower vertebral body. According to one aspect of the invention, the bone plate  210  can include modified T-slots fabricated thereon extending from one end of the plate  21   0  and directed longitudinally to the opposite end. The T-slots can include one elliptical slot extending vertically, and another slot extending horizontally. Lobes can be featured on the T-slots to provide interference with the ramps  240 . By rotating the bone screw  230  90°, the screw head ramp interferes with the T-slots of the plate  210 . Continued rotation allows the screws  230  to cam until the interference is cleared on the other side and the bone plate  210  is locked onto the screws  230 . 
     In yet another exemplary embodiment of the present invention, anterior bone plate system  300  is shown in  FIGS. 7 and 8 . Bone plate system  300  includes bone plate  310  which has at least two apertures  312  extending from a first side  314  of the plate  310  that is configured to face away from bony structure to a second side  316  that is configured to contact bony surface. A channel  318  extends lengthwise down the first side  314  of the bone plate  310 . 
     Consistent with anterior bone plate systems  100  and  200 , the apertures  312  of bone plate  310  allow the plate  310  to be placed on top of previously positioned bone screws  330 . Each bone screw  330  can have a self-tapping distal end  332  and a lower threaded portion  334  with an aggressive thread pitch to facilitate its purchase into bone. As shown in  FIG. 8 , each screw  330  includes a screw head  336  at an opposite (proximal) end that is adapted to seat within the channel  218 . The major thread diameter of the lower threaded portion of the screws  330  can be in the range of about 3.5 to 5.5 mm, while the cancellous thread pitch of the lower threaded region  334  can be approximately 1.5 or more to provide unicortical purchase into bone. The bone screws  330  are placed in the midline of the vertebral body, with only one bone screw  330  per vertebral body or level required to adequately stabilize the bony segments to the plate  310  to reduce the amount of bone damage that would occur to the spine when more than one screw is inserted within the same vertebral body. 
     Bone plate  310  can also include surface features  322  such as cleats or ridges on the second side  316  that is adapted to contact bony surface as illustrated in  FIG. 7 . The surface features  322  help anchor the bone plate  310  onto the bony surface of the vertebral bodies yet still allow flexion or subsidence while preventing extension, which is undesirable for proper fusion and healing. 
     As shown in  FIG. 9 , each of the screws  330  can have an open head  336 , which can include a shaped bore  338  for attachment to an inserter tool or screwdriver (not shown). Each screw head  336  can also include circumferential grooves  344  machined on the outside diameter of the screw head  336 . The grooves  344  are set proud, i.e., not engaged in bone, for engaging a locking device such as retaining ring  346  as shown in  FIG. 10 . The channel  318  of the bone plate  310  is machined to provide a low profile that can accommodate the screw heads  336  and provide an overall consistently low profile, as illustrated in  FIG. 9 . Further, retaining rings  346  provide easy fastening between the bone screws  330  and the bone plate  310  without increasing the profile of the anterior bone plate system  300 . While a retaining ring  346  of the shape shown in  FIG. 10  is suitable, it is contemplated that the retaining ring can be configured in any number of geometries which would allow the retaining ring to be placed over the screw head  336  and against the bone plate  310 . 
     Bone plate  310  can have at least two apertures  312 . The apertures  312  can comprise two slots to enable the plate  310  to glide with respect to the screws  330 . Alternatively, the apertures  312  can be a slot and a hole, or any configuration of a slot and hole that enables the screw head  336  to be passed through the plate  310 , and which is configured to fix at least an upper and lower vertebral body. 
     In each of the anterior bone plate systems  100 ,  200 ,  300  described above, the bone plate  110 ,  210 ,  310  can be constructed so as to conform to the shape of the anterior surfaces of the vertebrae that it will be mounted upon. The plate can be curved along both its longitudinal and transverse axes such that the second side  116 ,  216 ,  316  is substantially concave to improve its conformity to the shape of the vertebral bodies. Further, one of ordinary skill in the art will appreciate that the bone plates  110 ,  210 ,  310  of the invention can be made of a variety of high strength, biologically compatible materials that are preferably compatible with MRI techniques. Useful materials include polymers, composite reinforced polymers, and metals such as stainless steel, titanium and titanium alloys. 
     In an exemplary method for implementing the anterior bone plate systems  100 ,  200 ,  300  described above, several steps are necessary to prepare the patient for surgery and before the plate system  100 ,  200 ,  300  can be installed. As an initial matter, the patient should be placed in a supine position, with the spine stabilized appropriately. Next, the patient is prepped and draped in the usual manner. Using radiographic imaging, the affected spinal level(s) or area(s) are identified. An incision is then made to optimize the exposure appropriate for the procedure. 
     Where necessary, decompression and grafting procedures are performed. To effect the decompression and grafting procedure, a distraction pin insertion instrument is used. The distraction pin insertion instrument allows distraction pins (also called Caspar pins) to be inserted perpendicular to the bone&#39;s anterior cortical surface or vertebral body, and perpendicular to the superior/inferior midline of the vertebral bodies. In the present invention, each vertebral body has only a single pin inserted. It is important to note that the pilot holes must be perfectly aligned, i.e., distraction pins must be inserted perfectly parallel. Routine distraction is then performed and a diskectomy, the surgical removal of a diseased disc, follows. A graft can then be placed into the evacuated disc space under gentle distraction and when the surgeon is comfortable with placement, the distraction instrument and Caspar pins are removed. 
     The bone screws  130 ,  230 ,  330  of the present invention are preferably inserted into pilot holes left in the vertebral bodies or bony segments that will be attached to the bone plate  110 ,  210 ,  310  by the Caspar pins. It is also possible to drill using about a 2.0 mm diameter drill bit prior to using the distraction pins. The distraction procedure provides each vertebral body with a pilot screw hole for placement of a bone screw  130 ,  230 ,  330 . It should be noted that, while the bone screws are preferably used with pilot holes, it is possible to use the anterior bone plate systems  100 ,  200 ,  300  without pilot holes as well, i.e., without first using a distraction pin instrument. 
     After the pilot holes are prepared, all anterior anomalies, i.e., osteophytes, that can impede bone plate  110 ,  210 ,  310  placement are removed. Using an osteophyte remover instrument, the endplate and anterior column of the vertebral body is prepared to allow the underside of the plate  110 ,  210 ,  310  to be sandwiched tightly against the vertebral bodies to promote osseous fusion. The osteophyte remover instrument is used to create a smooth flat surface to fit the plate  110 ,  210 ,  310  precisely and match the plate&#39;s width. By shaving the vertebral bodies in this manner, the bone plate  110 ,  210 ,  310  is able to be inserted flush against the bony surface and produce a low profile which reduces the amount of damage to surrounding soft tissue. 
     Self-tapping screws  130 ,  230 ,  330  of the present invention are inserted into Caspar pinholes and tightened down to the anterior cortex, and aligned such that the open screw heads  136 ,  236 ,  336  are pointed so the openings are in the sagittal plane. The appropriate length plate  110 ,  210 ,  310  is then chosen by the surgeon and dropped onto the screw heads  136 ,  236 ,  336 . The plate  110 ,  210 ,  310  is then secured down in the manner described above for bone plate systems  110 ,  210 ,  310  and finally the patient is closed in a standard manner. 
     It will be understood that the foregoing is only illustrative of the principles of the invention, and that various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention. All references cited herein are expressly incorporated by reference in their entirety.

Technology Classification (CPC): 0