Abstract:
A bone plate for use in anterior cervical spinal fixation has interlocking components to prevent dislodgement of the plate due to anatomical forces. The plate is formed in the shape of a block letter C and the exposed surface is smooth to prevent trauma to internal body tissue. The plate spans the intervertebral space with each end attached to an adjacent vertebrae by bone screws threadably engaged with the bone. The distal ends of the bone screws The plate has a screw lock in each end for preventing the locking screws from backing out.

Description:
FIELD OF THE INVENTION 
   This invention relates to the field of orthopedic surgery and, more particularly, to spinal fixation. 
   BACKGROUND OF THE INVENTION 
   The use of bone pins and plates for reducing fractures is well known in orthopedic medicine. The pins and plates extend across discontinuities in a bone to fix the broken ends in relation to each other to reduce pain and promote rapid healing without deformity. These devices are secured to the bone by bone screws or nails driven into the bone. More recently, pins, rods, plates and cages have been used to stabilize bone and joints that have deteriorated naturally or as a result of prior trauma. The bone plate of this invention is useful in all these situations. 
   The interface between the bone screws and the bone presents problems of stability and long term usage that have been addressed in different ways. One of the major problems is usually termed as back-out. This defines the condition in which the screws attaching the plate to the bone loosen over time, either relative to the bone or the plate or both. Severe back-out results in the bone screw working itself out of the bone and/or plate resulting in instability of the bone or joint. This situation results in increasing pain and danger from the instability, as well as, the movement of the screw. There may be several reasons for the back-out but anatomical stresses from body movements contributes greatly to the problem. 
   Prior art devices address the problem of back-out by use of secondary locking screws that hold the bone screws in the plate. The locking device engages the head of the bone screw and is tightened to fix the screw to the plate and, thus, the bone. Such devices are not particularly suited for deployment on the anterior aspect of the spine because of the close proximity of vital soft tissue organs which dictate a smooth, low profile, contoured surface. 
   Michelson, U.S. Pat. No. 6,454,771, discloses a bone plate for anterior cervical fixation. The plate has several holes for receiving bone screws. A locking screw mechanism is used to overlay the screw heads. 
   An expandable insert for placement between vertebrae is disclosed by Paes et al, U.S. Pat. No. 6,436,142. The device is in the nature of a lag screw and can expand with the insertion of an expansion screw. 
   U.S. Pat. No. 6,342,055 to Eisermann et al discloses a bone plate with bone screws having a snap-in retainer securing the heads to the plate. 
   Geisler, U.S. Pat. No. 6,231,610, discloses a bone plate with diverging bone screws and serrations on the plate to increase holding power. 
   U.S. Pat. No. 6,224,602 to Hayes discloses a bone plate with multiple bone screw holes which may be covered by a sliding locking plate. The bone plate has an undercut channel to hold the locking plate in contact with the screw heads. The locking plate is held to the plate by a locking screw once it is slid to the desired position. 
   Aust et al, U.S. Pat. No. 5,603,713, discloses an anterior lumbar plate attached by screws with various angular connections to the spine. 
   What is needed in the art is a bone plate that provides improved visualization of the vertebral interspace for graft placement, improved ability to align and position the plate, and tapped bone screws for better purchase. 
   SUMMARY OF THE INVENTION 
   Disclosed is a bone plate for use in anterior cervical spinal fixation. The bone plate has interlocking components to prevent dislodgement of the plate due to anatomical forces. The bone plate is C-shaped with a smooth exposed surface to prevent trauma to internal body tissue. The plate spans the intervertebral space with each end attached to an adjacent vertebrae by bone screws threadably engaged with tubular bone anchors. The plate has a screw lock in each end for preventing the bone screws from backing out. 
   It is an objective of this invention to provide a bone plate, suitable for anterior cervical fixation, having countersunk screw holes, a low profile in cross section allowing the bone plate to be countersunk into the bone and a smooth distal surface to reduce the possibility of traumatizing adjacent soft tissue during use. 
   Another objective of this invention is to provide a reduced width in the central portion of the bone plate for visualizing the intervertebral space below the proximal surface of the plate. 
   A further objective of the invention is to provide bone screws with a fluted leading end to provide a tapped bore in the bone for better purchase. 
   Another objective of the invention is to provide bone screws extending through the countersunk screw holes into the bone with the leading ends of the screws formed as a tap. 
   Yet another objective of the invention is to provide a bone plate with a sliding screw lock blocking back out of the bone screws. 
   Other objectives and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention. The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view, partially in section, of the plate of this invention implanted in the spine; 
       FIG. 2  is a perspective of assembled bone plate system of this invention; 
       FIG. 3  is a perspective of the bone screw of this invention; 
       FIG. 4  is a perspective of the screw lock; and 
       FIG. 5  is a top plan view of the screw lock. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The bone plate system  10  may be made from any materials having requisite strength and being suitable for use in the body. One complete bone plate system is shown in  FIG. 1  and  FIG. 2  though it is understood that several different sizes of interchangeable components may be supplied together as a kit for mixing and matching components to size a system for a particular patient. A kit may have several different sized bone screws  11  varying in diameter and length. The different sized locking screws may have the same sized heads  13  to be used in different sized bone plates  14  with screw holes and larger countersunk depressions  15  of a complementary size. There may be several different sized screw locks  16  to fit into the different sized screw lock slots  17 . The kit merely refers to the dissembled components that can be assembled to produce an integral whole which corresponds to the anatomical features of a particular patient. 
   The bone plate  14  is shaped in the form of a block letter C with a vertical staff  18  connected at each end to arms  19  and  20  extending normal to the staff. Each of the arms  19  and  20  contain two countersunk screw holes. The reduced width of the bone plate, in the area between the arms, permits a clear visualization of the intervetrebral space during the fitting and placement of the plate. 
   Each of the arms  19  and  20  contains a screw lock slot  17  located between the countersunk depressions  15  surrounding the screw holes. The screw lock slot  17  extends through each arm and communicates with the countersunk depressions  15  through slits  24  in the sidewalls of the countersunk depression. 
   The bone plate system  10  addresses the problem of back-out by providing several locking features in the connection of the plate  14  with the bone. These locking features all resist the biomechanical loads placed on the implanted system and result in an aggregate resistance to movement of the components. 
   The bone screws  11  are initially inserted into the bone through pilot holes drilled in the bone or by the use of guide wires. The plate  14  may be used as a guide to align the longitudinal axis of the bone screws with the longitudinal axis of the countersunk screw holes  15 ′ in the plate. The bone screws  11  are driven into the bone with a tool (not shown) that engages the slots  28  for rotation. The exterior screw threads  21  draw the screw into its seated position in the bone. However, the screw threads  21  produce a mirror image of the toroidal ramp at the bone interface which is a path of least resistence in the opposite direction. The leading or proximal end of the screw  11  has a number of flutes  25  in the side wall dividing the proximal circumference into segments  26 . Further, the leading end is tapered toward a smaller end. This tap structure of the leading end of the bone screws  11  produces a cutting or sweeping function in the bone-screw interface by collecting debris as the screw is inserted to provide a smooth bore in the bone for the following threads. 
   Once the bone screws  11  are extended through the screw holes of the bone plate  14  with the threads  21  threadably engaging the interior of the smooth bone bore, the locking screws are rotated by a tool (not shown) fitted into the receptacle  28 . As shown, the bone screw  11  has a smooth unthreaded shank near the head  13 . The leading end of the screw has a tapered portion  23  to engage the leading end of a pilot hole in the bone. The bone screw heads  13  are surrounded by the depressions  15  and compressively disposed against the bottoms of the depressions. Therefore, the screw  11  is locked in place by a smooth new bone-anchor interface and the screw threads engage the walls of the bore. Of course, other combinations of locking screws and anchor configurations may be used, such as, a constant taper of each. 
   The anterior surface of the vertebrae V and the plate  10  form a low profile interface, as shown in  FIG. 1A . This is especially important in reducing the possibility of internal trauma to adjacent soft tissue, e.g., in the anterior cervical spinal fixation. The plate is curved to further conform the plate to the anatomy and reduce the silhouette.  FIG. 1B  illustrates the plate set forth in  FIG. 1A  which is rotated 90 degrees and shown without a pictorial of the vertebrae. 
   After the screws  11  have been seated in the depressions  15 , the screw lock  16  is placed in the screw lock slot  17  formed in the plate  14 . The screw lock  16  has a substantially flat elongated body  30  with a narrow central extension  31  on one end. The extension  31  is located between two longitudinal slots  32  and  33  formed in the body  30 . The slots  32  and  33  provide some resilience to the shape of the body  30  to maintain the screw lock  16  in the screw lock slot  17 . The longitudinal edges of the body are preferably thinned at  34  and  35  to allow the screw lock  17  to slide over the screw heads  13 . In one embodiment, the screw lock  16  may be tapered from a larger thickness opposite the central extension  31  to wedge into slot  17  as the screw lock is inserted into the slot. 
   The assembly is completed by sliding the screw locks  16  through the arms  19  and  20  with the thinned edges  34  and  35  of the body  30  extending through the slits  24  to forcibly engage the screw heads  13 . The forces produced by the frictional contact of the screw lock with the screw lock slot and the screw heads firmly wedges the screw lock in place and prevents the lock screws from backing out. 
   The implanted bone plate system results in a positive lock at the proximal ends of the bone anchors and locking screws and an additional lock at the distal end of the locking screws. If the plate must be removed, at some later date, forceps may be applied to the extension  31  and the arms  19  and  20  to force the screw lock from the screw lock slot.