Abstract:
A bone plate for stabilizing adjacent vertebrae or ends of a bone having a span for extending across the discontinuity. The span has brackets for attaching to the bone. The brackets have countersunk apertures terminating through which bone screws are placed in the bone. An cam bore is located between the countersunk apertures and a cam with lobe surfaces is positioned in the cam bore. Upon rotation of the cam, the lobe surfaces engage an end of the wedge shoes and move them so that the other end of the wedge shoes move into the countersunk apertures and frictionally engage the heads of the bone screws. The wedge shoe is provided with means to lock the cam in position. The wedge shoe is also provided with means to indicate that the shoe has completely engaged the head of the bone screw in the countersunk aperture.

Description:
RELATED APPLICATIONS  
       [0001]     This application is a continuation in part of U.S. patent application Ser. No. 11/031,143 filed Feb. 4, 2005, and is related to U.S. patent application Ser. No. 11/124,455, filed May 6, 2005, which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to the field of orthopedic surgery and to bone plates which are affixed to bone by screws or other fastening devices including spinal plates for the cervical, thoracic and lumbar regions.  
       BACKGROUND OF THE INVENTION  
       [0003]     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.  
         [0004]     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 fastening devices 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.  
         [0005]     Spinal bone plates are usually attached to adjacent vertebrae to reduce pain due to injury or deterioration of the intermediate disk. The plate spans the intervertebral space to stabilize the vertebrae. Pedicle screws or bone screws are inserted through apertures in the opposite ends of the plate into the respective vertebrae or on opposite sides of a break. Due to anatomical forces on the skeleton, the screws sometimes back out of the bones and plates.  
         [0006]     What is needed in the art is a bone plate with an internal screw lock that rotates to wedge the bone screws to the plate. Also, an arrangement of the bone screws to permit the use of a narrow bone plate.  
       DESCRIPTION OF THE PRIOR ART  
       [0007]     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.  
         [0008]     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.  
         [0009]     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.  
         [0010]     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.  
         [0011]     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.  
         [0012]     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.  
         [0013]     Published application, US 2004/0102773 A1, to Morrison et al, uses the ends of the bone plate to cover the heads of the bone screws.  
         [0014]     U.S. Pat. No. 6,740,088 B1, to Kozak et al uses extra set screws to interfere with the heads of the bone screws.  
         [0015]     U.S. Pat. No. 6,730,127 B2 to Michelson attaches an overlay to the plate to partially cover the heads of the screws.  
         [0016]     What is needed in the art is a bone plate with an internal screw lock that rotates to wedge the bone screws to the plate. Also, an arrangement of the bone screws and screw lock to permit the use of a narrow bone plate.  
       SUMMARY OF THE INVENTION  
       [0017]     Disclosed is a bone plate for stabilizing adjacent vertebrae. The plate is formed from a span of rigid material for bridging intervertebral space, the span of material having a bone engaging surface and a distal surface. A first bracket is located at one end of the span and a second bracket is located at the other end of the span. The first bracket includes a first bone fastener aperture and a second bone fastener aperture therethrough with a cam bore between the first bone fastener aperture and the second bone fastener aperture. A first slot in the first bracket extends from the first bone fastener aperture to the cam bore. A second slot in the first bracket extends from the second bone fastener to the cam bore. An eccentric cam is mounted in the cam bore, the cam includes cam surfaces. The first and second bone fastener apertures and the cam bore are not in line with each other. The first and second bone fastener apertures are offset from the cam bore at an approximately 45° angle thereby forming a “V”. This arrangement permits the bone fasteners to be located closer to each other so that the width of the bone plate can be decreased.  
         [0018]     A first wedge shoe is slidably disposed in the slot between the cam and the first bone fastener aperture for contacting the cam surface. A second wedge shoe is slidably disposed in the slot between the cam and the second bone fastener aperture for contacting the cam surface. Rotating the cam slides the first and second wedge shoes partially into the first and second bone fastener apertures. A cam cover plate can be used to close the slots.  
         [0019]     Accordingly, it is a objective of the instant invention to provide a bone plate with an arrangement of bone fasteners which permit the use of an narrower than normal bone plate.  
         [0020]     It is a further objective of the instant invention to provide a bone plate with an integral screw lock.  
         [0021]     It is yet another objective of the instant invention to provide a bone plate with sliding wedge shoes for locking the bone screws.  
         [0022]     It is a still further objective of the instant invention to provide a low profile bone plate with countersunk bone screw apertures therethrough which also have wedge shoe openings.  
         [0023]     It is still yet another objective of the instant invention to provide a bone plate to span a plurality of discontinuities in the bone.  
         [0024]     Other objects and advantages of this invention will become apparent from the following description taken in conjunction with any accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention. Any drawings contained herein 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 FIGURES  
       [0025]      FIG. 1  is a perspective view of the distal side of the assembled bone plate and threaded screw fasteners of the instant invention;  
         [0026]      FIG. 2  is a perspective view of the bone engaging side of the assembled bone plate and threaded screw fasteners of the instant invention;  
         [0027]      FIG. 3  is a transverse cross section of a bone plate of the instant invention;  
         [0028]      FIG. 4  is a partial perspective view of the bone plate of the instant invention with the cam cover removed;  
         [0029]      FIG. 5  is a partial perspective view of the wedge shoe recess;  
         [0030]      FIG. 6  is a perspective view of a wedge shoe;  
         [0031]      FIG. 7  is a perspective view of the eccentric cam; and  
         [0032]      FIG. 8  is a perspective view of the cam cover plate. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]     While the present invention is susceptible of embodiment in various forms, there is shown in the accompanying drawings and will hereinafter be described a presently preferred embodiment with the understanding that the present disclosure is to be considered an exemplification of the invention and is not intended to limit the invention to the specific embodiments illustrated.  
         [0034]     The bone plate  10 , shown in  FIGS. 1, 2 , and  4  is based on an elongated span  11  having a first end and a second end with a first bracket  12  on the first end constructed and arranged to engage a first vertebrae (not shown) and a second bracket  13  on the second end constructed and arranged to engage a second vertebrae (not shown). The bone plate also has two major surfaces, a bone engaging surface  18  and a distal surface  19 . A first bracket  12  includes a first bone screw aperture  14  and a second bone screw aperture  15 . A cam  17  is located therebetween and offset toward the center portion of the bone plate. This offset enables the bone screw apertures  14  and  15  to be located closer to each other than if the cam  17  were positioned in between the apertures and in line with the apertures. This in turn permits the width of the bone plate  10  to be decreased.  
         [0035]     The composition of the bone of the vertebrae, the size of the vertebrae and other factors usually determine the size of the bone screw that will be employed to install the bone plate. Should a bone plate with a narrow width be required for a specific condition, the offset of the cam with respect to the bone screws will permit the use of a narrow bone plate. Each of the bone screw apertures  14  and  15  are provided with a countersunk portion or bore so that the head  101  of the bone screw  100  will be approximately level with the distal surface  19  of the bone plate  10 . Bone or Pedicle screws  100  are shown in the apertures with the screw heads  101  resting in the countersunk portions or bores of the apertures  14  and  15 . This construction contributes to the low profile of the installed bone plate thereby preventing undue trauma to the tissue on the anterior aspect of the cervical spine. Pins  200 , shown in  FIG. 4 , may be used in place of the bone screws. In addition, other known bone fasteners may also be employed. The size of the pedicle screw or other fastener will normally be a determining factor in the overall width of the bone plate  10 . A rotating eccentric cam  17 , shown in FIG.  7  is mounted in the cam bore  16 , as shown in  FIG. 2 . Its operation will be described later.  
         [0036]     The second bracket  13  has the same components as the first bracket  12 . An aperture  50  is located in the span  11  which connects the first and second brackets. The aperture  50  serves to promote boney ingrowth which leads to increased stability of the bone plate. As an alternate embodiment, the bone plate may be formed with a series of brackets spaced apart by multiple spans for use when several vertebrae are to be stabilized.  
         [0037]     The bone screw apertures  14  and  15  extend through the bone plate from the bone engaging surface to the distal surface. The cam bore  16  in the bone engaging surface is circular and serves as a guide and bearing surface for the end  31  of the actuator  20  of the eccentric cam  17 . The cam actuator  20  has a receptacle  21  located in a central axial portion thereof. A tool, such as an allen wrench (not shown), cooperates with the receptacle  21  to rotate the cam  17 . The top surface of the distal end of the cam  17  is approximately level with the distal surface of the bone plate. The cam  17  is also provided with eccentric cam lobe surfaces  22  and  23 . Teh cam lobe surfaces are formed approximately 45° apart in a preferred embodiment.  
         [0038]     As shown in  FIGS. 3 and 4 , the bone screw apertures  14  and  15  are connected by a V-shaped slot therebetween. Cam  17  is positioned at the midpoint of the “V”. Wedge shoe  25  is slidably positioned between the cam  17  and bone screw aperture  14 . Although a V-shaped slot is the preferred embodiment, any nonlineal shaped slot may be employed. A cam cover plate  27 , as shown in  FIG. 8 , closes each slot and forms a portion of the distal surface  19  of the bone plate. The cam cover plate extends transversely along the distal surface of the bone plate between the apertures. Each end  28  and  29  of the cover plate has an arcuate shape conforming to the shape of the countersunk apertures.  
         [0039]     As seen in  FIG. 6  the wedge shoes  25  and  26  are formed with a surface  32  which conforms to the head  101  of the screw  100  or pin  200 . They also include contact points  33  and  34  at the ends of the surface  32  and adjacent the sides of the shoes to insure more than a single positive pressure point of the shoe bearing against the head of the screw. This helps to prevent the screw from rotating and backing out once it has been installed. The opposite end of the shoes is provided with an indentation  36 . This indentation acts in conjunction with the cam lobe surfaces  22  and  23  to prevent the cam lobe surfaces from rotating and releasing pressure against the wedge shoes thereby disengaging them from the head of the bone screws.  
         [0040]     Wedge shoes  25  and  26  are also provided with a slot  37  which acts as a spring to maintain the shoe in contact with the head of the bone screw. When the cam is rotated such that the cam lobe surfaces  22  and  23  move the wedge shoes in contact with the head of the bone screws the width of the slot is decreased as it is placed under compression. This compressive force acts like a compressed spring to maintain the wedge shoe in contact with the head of the bone screw. Each wedge shoe  25  and  26  is also provided with a groove  38  which operates in conjunction with protrusion  40  on the side of each of the slots  42  and  44 . As the wedge shoe is moved toward the bone screw by the cam surfaces  22  and  23 , the protrusion moves from a forward portion to a rearward portion of the groove  38 . This movement imparts a “snapping” action to the wedge shoe which is translated to the cam and the cam actuator. This “snapping” action acts as an tactile indicator to the surgeon or whomever is installing the device that the cam has been rotated sufficiently to move the wedge shoes into complete engagement with the bone screw heads. Since the elements of the bone screw plate are small and the distance that the wedge shoe travels is very small it is difficult for a surgeon to determine when the cam has been rotated sufficiently to move the wedge shoe in complete engagement with the head of the bone screw. This “snapping” action eliminates this problem by providing a tactile feedback to the surgeon indicating that the shoe has moved sufficiently to properly engage the head of the bone screw.  
         [0041]     The heads of the bone screws have a spherical shape which matches the countersunk portions of the apertures. This allows for some flexibility in the placement of the bone plate and bone screws to compensate for anatomical considerations or to gain a better grip in the bone. After the bone screws or pins have been tightened, the cam is rotated by the actuator. The cam lobe surfaces  22  and  23  engage the wedge shoes and move then toward the apertures. When the wedge shoes are fully engaged with the heads of the bone screws, the surfaces  22  and  23  of the cam are disposed in indentations  36  of the wedge shoes and cannot freely move therefrom thereby locking the wedge shoe in engagement with the head of the bone screw.  
         [0042]     All patents and publications mentioned in this specification are indicative of the levels of those skilled in the art to which the invention pertains. All patents and publications are herein incorporated by reference to the same extent as if each individual publication was specifically and individually indicated to be incorporated by reference.  
         [0043]     It is to be understood that while a certain form of the invention is illustrated, it is not to be limited to the specific form or arrangement herein described and shown. It will be apparent to those skilled in the art that various changes may be made without departing from the scope of the invention and the invention is not to be considered limited to what is shown and described in the specification and any drawings/figures included herein.  
         [0044]     One skilled in the art will readily appreciate that the present invention is well adapted to carry out the objectives and obtain the ends and advantages mentioned, as well as those inherent therein. The embodiments, methods, procedures and techniques described herein are presently representative of the preferred embodiments, are intended to be exemplary and are not intended as limitations on the scope. Changes therein and other uses will occur to those skilled in the art which are encompassed within the spirit of the invention and are defined by the scope of the appended claims. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. Indeed, various modifications of the described modes for carrying out the invention which are obvious to those skilled in the art are intended to be within the scope of the following claims.