Abstract:
A dynamic cervical plate has a ratchet and pawl mechanism that allows the cervical plate to post operatively shorten the length of the plate and maintain compression between adjacent vertebrae. The plate has an elongated shaft with teeth on one surface and a groove along each longitudinal edge. A lateral plate is attached on one end of the shaft. The plate has screw holes for connecting with the head of a spinal screw. Another lateral plate is slidably engaged in the longitudinal grooves along the shaft and has a spring clip acting as a pawl with the teeth on the shaft. The sliding bar has screw holes on each side of the shaft. The clip is configured to span the screw holes to automatically engage and prevent screws from backing out of the holes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 11/624,575 filed Jan. 18, 2007 (issued Oct. 19, 2010 as U.S. Pat. No. 7,815,666) which is a continuation-in-part of U.S. patent application Ser. No. 10/776,369, filed Feb. 10, 2004, the contents of which is herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates to the field of orthopedic surgery and, particularly, to the area of spinal implants for stabilizing the spatial relationship of vertebrae. The device is designed for use in the cervical region of the spine though one skilled in the art may use the device in other regions of the spine and other skeletal fixations. 
       DESCRIPTION OF THE PRIOR ART 
       [0003]    Spinal plates are well known in the orthopedic art for fixing bones or bone fragments in a pre-selected spatial orientation. The plates are usually attached to the bones or bone fragments by screws designed to make a secure and long lasting connection not affected by the loads caused by normal activities of the host. Gertzbein et al, U.S. Pat. No. 5,620,443, teaches an adjustable cervical connector composed of dual rods spanning the distance between adjacent vertebrae. The rods carry at least two slidable transverse connectors which are attached to the vertebrae by spikes and pedicle screws thereby fixing the relationship of the bones. The connectors are immobilized on the rods by clamps. 
         [0004]    Richelsoph, U.S. Pat. No. 6,017,345, teaches a spinal plate spanning the distance between adjacent verrtebrae. The plate has screw holes in each end. The pedicle screws are inserted through the holes and allow for some movement. 
         [0005]    Shih et al, U.S. Pat. No. 6,136,002, teaches a similar device to that of Gertzbein with the clamps screwed onto the elongated rods. 
         [0006]    Published Patent Application US 2003/0060828 A1 to Michelson teaches a cervical plate with at least two plate elements slidably connected together and fixed by a set screw. The contacting surfaces of the plate elements are formed with ratcheting to provide added security. 
         [0007]    In all these prior art devices, the plate must be held in the selected position while the securing set screws or other fasteners are put in place and the final assembly is completed. 
         [0008]    What is needed in the art is a dynamic cervical plate that may be adjusted to length, locked in place to provide compression, and will automatically shorten its length to maintain compression. 
       SUMMARY OF THE PRESENT INVENTION 
       [0009]    Therefore, it is an objective of this invention to provide a cervical plate with an elongated shaft adapted to span the intervertebral space and having at least two screw receivers spaced along the length of the plate. The underside of the shaft includes integral serrations. The screw receivers each have countersunk apertures for accepting the heads of pedicle screws. Clips formed from a spring type material extend around the screw receivers and under the rod to cooperate with the serrations to allow the cervical plate to compress dynamically along the longitudinal axis of the rod when a compressive load is applied across the device. This load can be applied by the surgeon at the time of surgery and/or be produced during the healing phase by utilizing the compressive loads which occur during physical motion of the patient. The induction of a compressive load across the vertebral bodies to be fused, induces bone growth and when bone resorption occurs at the interface of the graft or implant and the vertebral bodies to be joined, those vertebral bodies are urged to move closer together, thus avoiding the formation of a gap therebetween and thereby acting to mitigate against pseudoarthrosis. The spring clips are also constructed to automatically engage the head portion of the pedicle screws upon complete insertion into the countersunk apertures. This construction prevents the screws from migrating out of the bone material during use of the device. In operation, as the screw head begins to enter the countersunk aperture the head forces a portion of the spring clip away from the aperture. As the head passes the clip, the clip returns to its original position to cover a portion of the screw head providing a positive lock for the screw. 
         [0010]    Thus one objective of this invention is to provide a cervical plate having locking mechanism that is manually operated simultaneously with the positioning of the screw receivers along the plate to provide compression across an intervertebral space. 
         [0011]    A further objective of this invention is to provide a cervical plate having an automatic locking mechanism having a retainer which extends over a portion of each screw hole to prevent back out migration of the screws. 
         [0012]    Yet another objective of this invention is to provide a cervical plate having a guide rail on the plate shaft cooperating with the screw receivers to permit sliding connection between the screw receivers and the plate shaft. 
         [0013]    Still another objective of this invention is to provide a cervical plate having a ratchet mechanism on the shaft and screw receivers to permit post-operative one-way movement shortening the distance between the screw receivers and maintaining compression across the intervertebral space. 
         [0014]    Still yet another objective of this invention is to provide a cervical plate which includes a spring clip constructed to automatically engage a portion of each pedicle screw upon insertion into the cervical plate to prevent the pedicle screw from backing out of engagement with a bone. 
         [0015]    An even further objective of this invention is to provide a cervical plate which includes a spring clip constructed to form part of the ratchet mechanism in addition to automatically engaging each pedicle screw upon insertion, whereby dynamic compression as well as screw migration is prevented. 
         [0016]    Still a further objective of this invention is to provide a cervical plate having the ability to shorten in response to compressive loads to allow for bone portions to be fused to move close together to maintain or restore contact therebetween. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a perspective view of the cervical plate and screw receivers of this invention; 
           [0018]      FIG. 2  is a bottom plan view of the cervical plate and screw receivers; 
           [0019]      FIG. 3  is a cross section of the cervical plate, along line  3 - 3  of  FIG. 1 , with the clip unseated; 
           [0020]      FIG. 4  is a cross section of the cervical plate, along line  4 - 4  of  FIG. 1 , with the clip seated in the ratchet; 
           [0021]      FIG. 5  is a perspective view of the cervical plate with screws having teeth formed in the head portion; 
           [0022]      FIG. 6  is a side elevational view of  FIG. 5 ; 
           [0023]      FIG. 7  is a top elevational view of  FIG. 5 ; 
           [0024]      FIG. 8  is a cross sectional view of  FIG. 7  taken along lines B-B; 
           [0025]      FIG. 9  is an enlarged perspective view illustrating the ratchet screw engaging the plate; 
           [0026]      FIG. 10  is a perspective view of a ratchet screw; 
           [0027]      FIG. 11  is a perspective view of one embodiment of the instant invention, illustrated with the pedicle screws in place; 
           [0028]      FIG. 12  is a top elevational view of the cervical plate shown in  FIG. 11 ; 
           [0029]      FIG. 13  is a side elevational view of the cervical plate shown in  FIG. 11 ; 
           [0030]      FIG. 14  is an exploded view of the cervical plate embodiment shown in  FIG. 11 ; 
           [0031]      FIG. 15  is a section view taken along line  9 - 9  of  FIG. 12 ; 
           [0032]      FIG. 16  is a section view taken along line  10 - 10  of  FIG. 12 ; 
           [0033]      FIG. 17  is a partial bottom perspective view illustrated with one of the end plates and pedicle screws omitted; 
           [0034]      FIG. 18  is a perspective view of the instant invention with ratchet pedicle screws; 
           [0035]      FIG. 19  is a top elevational view of  FIG. 18 ; 
           [0036]      FIG. 20  is an enlarged perspective view illustrating ratchet pedicle screw engagement; 
           [0037]      FIG. 21  is a cross-sectional view of  FIG. 20  illustrating ratchet pedicle screw engagement; and 
           [0038]      FIG. 22  is a perspective view of a ratchet pedicle screw. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0039]    Referring to  FIGS. 1-10 , the cervical plate  10  has an elongated flat shaft that is made in different lengths but must be of a length to span, at least, the distance between two vertebrae. The plate has a lateral plate  12  fixed to one end and a free end. The plate has at least one and more preferably two countersunk apertures  20 ,  20 ′ on each side of the plate for capturing the head of pedicle screws. Secured to the plate is a clip having ears  18 ,  18 ′. The clip is resilient and extends under the plate parallel but outside the periphery of the bar then rises vertically to the top of the plate and extends across the pedicle screw apertures  20 ,  20 ′. The portion that extends across the countersunk apertures  20 ,  20 ′ are the ears  18 ,  18 ′ for retaining the pedicle screws to prevent back-out. The clip is preferably constructed of a spring tempered metal to provide enough resiliency to allow flexing while the heads of the pedicle screws are seated in the aperture. Upon seating of the pedicle screw head in the countersunk aperture the ears of the clip automatically release on top of the screw heads. The clips may be constructed to apply a relatively constant pressure to the top portion of the screw head or alternatively there may be a small gap between the screw head and the ear. In either embodiment the screw is prevented from backing out of the bone. In one embodiment, the ears  18 ,  18 ′ have wedges  50  which engage the edges of the screw heads as the screws are tightened to further lock the screws in place. 
         [0040]    An alternative construction for retaining the screws is shown in  FIGS. 5 through 10 . In this embodiment, the ears  18 ,  18 ′ each have wedges  51  which engage teeth  53  formed in the head of the screw. The wedge  51  has a camming and locking surface. Likewise, the teeth  53  on the screw extend circumferentially about the head of each screw and each tooth of teeth  53  includes a camming and locking surface. The cooperation of wedge  51  and teeth  53  provide a ratcheting engagement between the wedge and the screw head when the screw is rotated in a first direction and a locking relationship between the each ear and its associated screw head when the screw is rotated in a direction counter to the first direction. 
         [0041]    The bottom of the shaft has a row of teeth or serrations  15  formed across the longitudinal axis of the plate. The teeth are angled to form a ratchet allowing one-way movement of a bar from the free end toward the lateral bar at one end of the plate. In some instances, the teeth may be cut normal to the shaft. Along each longitudinal side of the shaft is a groove  23 ,  23 ′ extending from the free end toward the lateral bar. 
         [0042]    Slidably attached to the free end of the shaft is at least one and preferably two movable plate(s)  13 ,  14  having the same general construction. Therefore, reference to elements of one plate is the same as the other. 
         [0043]    The plates  13 ,  14  have a distal surface which engages the vertebrae and are convexly curved to closely fit the curvature of the vertebrae. The plates have an aperture  21 ,  21 ′ near each end with a channel  25  extending through the plate. The channel is approximately the same depth and includes a substantially conjugate shape to that of the shaft to provide a low profile to the assembled cervical plate. The opposite edges of the channel have shoulders  26 ,  26 ′, shown in  FIG. 3 , that slide within the longitudinal grove  23 ,  23 ′ in the plate. In a most preferred embodiment the channel and shoulders form a dovetail arrangement. This provides a close association between the surface of the bar channel and the ratchet teeth of the plate and prevents any substantial unwanted angular movement between the shaft and the plate(s). 
         [0044]    Attached to bar  13  is a clip having a retainer  17 ,  17 ′. The clip has an elongated body with an oval shape when viewed from the top. The sides of the oval follow the edges of the depression so that the retainers  17 ,  17 ′ are on the proximal surface of the bar. At least one side of the clips is preferably welded  99  or otherwise permanently attached to the respective side of the bars. The rounded ends of the oval form the screw retainers. The pawl portion  27 ,  28  of the clips extend across the shaft engaging the teeth  15  to form the ratchet. The pawls are formed by a raised flange  24 , shown in  FIG. 3 . 
         [0045]    In the preferred embodiment, the clips  16  and  17  have a flange that extends above the surfaces of the bars to engage the teeth  15  of the ratchet on the shaft. Of course, the clips may have pawls  27 ,  27 ′ and  28 ,  28 ′ on both sides of the bar, shown in  FIG. 2 . By flexing the clip with an instrument, the flange  24  can be disengaged from the ratchet teeth  15 , as shown in  FIG. 3 , for initial adjustment. 
         [0046]    In operation, the vertebrae are manipulated into the desired position and grafting material placed as required to compensate for removal of bone and/or disk material. The plate is placed on the spine and adjusted to provide some compression on the site to assist in the grafting of the spine. As the bars are slid along the shaft, the shoulders of the bars and the grooves on the shaft maintain a close fit between the pawls and the teeth on the shaft requiring the pawls to be deflected by the teeth. Once the bars are in the desired location and the flanges are seated in the teeth, the ratchet prevents retrograde movement of the bars away from the head. The pedicle screws are driven into the spine. As the screw heads engage the apertures the retainers are flexed to permit the screw heads to seat in the apertures and are thereafter automatically released to block back-out or migration of the screw. 
         [0047]    It is well known that as the site heals and the adjacent vertebrae begin to graft together and as a result of the forces of gravity, there is some reduction in the span between the vertebrae. As this occurs the dynamic cervical plate can accommodate the reduction and maintain some compression because the shaft will move through the bars resulting in the clips moving from one ratchet tooth to the next automatically shortening the intervertebral distance. Of course, the pawls may be omitted, and the plate(s) may move in both directions along the shaft. 
         [0048]    The second bar may be added to the free end of the plate to add stability to the compressed site and to reduce and equalize the pressure. Of course, the pawls may be omitted, and the plate may move in both directions within the bars. 
         [0049]    Referring to  FIGS. 11-15 , an alternative embodiment of the cervical plate  100  is illustrated. The cervical plate  100  has an elongated flat shaft  102  that is made in different lengths but must be of a length sufficient to span, at least, the distance between two vertebrae. Slidably secured along the shaft  102  is a center lateral plate  104  and at least one and more preferably two end lateral plates(s)  106 . The center lateral plate  104  has countersunk apertures  20 ,  20 ′ on each side of the plate for capturing the head portion of pedicle screws  108 . Mounted to the plate is a spring clip  110  having locks  118 ,  118 ′. The clip is resilient in construction and extends parallel along the side of the bar and under the shaft  102 , then each end rises vertically to the top of the plate and extends across a portion of the pedicle screw apertures  20 ,  20 ′. The portion that extends across the countersunk apertures  20 ,  20 ′ are the locks  118 ,  118 ′ for retaining the pedicle screws to prevent loosening as well as back-out migration. The clip is preferably constructed of a spring tempered metal to provide enough resiliency to allow flexing while the heads of the pedicle screws are seated in the aperture. Upon seating of the pedicle screw head in the countersunk aperture the locks of the clip automatically release on top of the screw heads. The clips may be constructed to apply a relatively constant pressure to the top portion of the screw head. The screw is prevented from backing out of the bone. Each spring clip  110  has locks  118  and  118 ′. As best seen in  FIGS. 15 ,  20  and  21  locks  118  and  118 ′ each includes a wedge element  151  located upon the surface that will engage the screw head. Each wedge element includes at least one camming and one locking surface. The head of pedicle screw  108  has teeth  153  formed circumferentially about the head of the screw  108 . The teeth  153  include a plurality of camming and locking surfaces. The cooperation of wedge element  151  and teeth  153  provide a ratcheting and locking mechanism between the each lock  118  and  118 ′ and its associated screw head. The screw is thereby prevented from backing out of the bone. The cooperation of wedge  151  and teeth  153  provide a ratcheting engagement between the wedge and the screw head when the screw when rotated in a first direction and a locking relationship between the each lock and its associated screw head when the screw is rotated in a direction counter to the first direction. 
         [0050]    The bottom of the shaft has a row of teeth or serrations  15  formed across the longitudinal axis of the plate. The teeth are angled to form a ratchet, as shown in  FIG. 16 , allowing one-way movement of a plate along the bar. In the most preferred embodiment, the teeth are cut normal to the shaft. 
         [0051]    Slidably attached along the shaft is at least one and preferably two movable end plates  120  having the same general construction. Therefore, reference elements for each end element are the same. The slidable plates  120  have a distal surface  122  which engages the vertebrae and is convexly curved to closely fit the curvature of the vertebrae. Each plate includes an aperture  124  shaped and sized to approximate the size and shape of the shaft. This construction prevents unwanted movement between the plate(s) and the shaft and place the pawl portion  126  of the clip  110  in close approximation to the serrations on the shaft. The end plates may also include a contoured cut out portion  128  shaped to cooperate with a rib  130  formed integral to the shaft  102 . The rib provides additional strength to the shaft while the cut-out cooperates with the rib to provide a low profile to the assembled cervical plate. 
         [0052]    At least one side of the clips is preferably welded or otherwise permanently attached to the respective side of the plate. The pawl portion  126  of the clips extend across the shaft engaging the teeth  15  to form the ratchet. In one embodiment, the clips  110  have a radiused top edge ( FIG. 16 ) that cooperates with teeth  115  having at least one angled ramp surface  16  to define the ratchet assembly. By flexing the clip  110  with an instrument (not shown), the clip can be disengaged from the ratchet teeth  115  for initial adjustment or for controlled release of an engaged plate. 
         [0053]    In operation, the vertebrae are manipulated into the desired position and grafting material placed as required to compensate for removal of bone and/or disk material. The plate is placed on the spine and adjusted to provide some compression on the site to assist in the grafting of the spine. As the plates are slid along the shaft, the conjugate shape of the apertures  124  and the shaft  102  maintain a close fit between the pawls  126  and the teeth  115  on the shaft requiring the pawls to be deflected by the teeth. Once the bars are in the desired location and the pawls are seated in the teeth, the ratchet prevents retrograde movement of the plates away from the head. The pedicle screws are driven into the spine. As the screw heads engage the apertures the locks  118  and  118 ′ are flexed to permit the screw heads to seat in the apertures and are thereafter automatically released to block loosening and/or back out of the screw. 
         [0054]    It is well known that as the site heals and the adjacent vertebrae begin to graft together and as a result of the forces of gravity, there is some reduction in the span between the vertebrae. As this occurs the dynamic cervical plate can accommodate the reduction and maintain some compression because the shaft will move through the plates resulting in the pawl portion of the clip moving from one ratchet tooth to the next automatically shortening the intervertebral distance. Of course, the pawls may be omitted, and the plate(s) may move in both directions along the shaft. 
         [0055]    A number of embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the invention is not to be limited by the specific illustrated embodiment but only by the scope of the appended claims.