Patent Document

RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 10/932,272, filed Sep. 1, 2004, titled C ERVICAL  P LATE WITH  B ACKOUT  P ROTECTION , currently, which is a continuation-in-part of U.S. patent application Ser. No. 10/632,760, filed Aug. 1, 2003, titled C ERVICAL  P LATE , now U.S. Pat. No. 6,062,257, which is a continuation in part of continuation-in-part of U.S. patent applicaton Ser. No. 10/178,371, filed Jun. 24, 2002, titled C ERVICAL  P LATE , now U.S. Pat. No. 6,062,257. This application also is related to U.S. Pat. No. Ser. No. 10/178,656, filed Jun. 24, 2002, titled I MPACTOR FOR  U SE WITH  C ERVICAL  P LATE.    
    
    
     FIELD OF THE INVENTION 
     The present invention relates to apparatuses and methods for treating and correcting spinal abnormalities and, more particularly, to cervical plates having backout or reverse thread protection. 
     BACKGROUND OF THE INVENTION 
     The vertebrae of the human spine are arranged in a column with one vertebra on top of the next. Between each vertebra exists an intervertebral disc that transmits force between adjacent vertebrae and provides a cushion between the adjacent vertebrae. 
     Sometimes, back pain is caused by degeneration or other deformity of the intervertebral disc (“diseased disc”). Conventionally, surgeons treat diseased discs by surgically removing the diseased disc and inserting a bone graft in the space vacated by the diseased disc. The adjacent vertebrae are then immobilized relative to one another. Eventually, the vertebrae grow into one solid piece of bone. 
     Currently, it is difficult to insert the bone graft into the vacated space and fuse the adjacent vertebrae. The current process of inserting a bone graft and fusing the adjacent vertebrae will be explained with referring to  FIGS. 1 and 2 .  FIG. 1  shows two adjacent vertebrae  102  and  104 . Located between vertebrae  102  and  104  is an intervertebral space  106  partially filled by a bone graft  108 . When the bone graft  108  is first inserted into the intervertebral space  106 , the adjacent vertebrae  102  and  104  are manually kept apart by the surgeon using, for example, a retracting device (not shown). As shown in  FIG. 2 , once the bone graft  108  is placed, the surgeon releases the adjacent vertebrae  102  and  104  allowing them to squeeze the bone graft  108  and hold the bone graft  108  in place. 
     To immobilize the vertebrae  102  and  104  with the bone graft  108  in place, the surgeon next applies a cervical plate  202  over the adjacent vertebrae  102  and  104 . Cervical plate  202  may have a central viewing window  204  and one or more screw holes  206 , in this example four screw holes  206   a - 206   d  are shown. Four bone screws (which will be identified by reference numerals  208   a - 208   d ) would be screwed into the vertebrae using the screw holes  206  to anchor the cervical plate to the vertebrae and immobilize the vertebrae with respect to one another. 
     As can be appreciated, attaching the cervical plate  202  using the bone screws  208  is a difficult endeavor. Generally, a temporary screw (not shown) is placed in one of the screw holes, for example  206   a . Bone screw  208   c  would then be partially screwed into the bone at screw hole  206   c . The temporary screw in hole  206   a  would be replaced by bone screw  208   a , which would be tightened. Then the other bone screws  208  would be screwed into the bone in a cross point manner. The ability of the cervical plate to move freely in relation to the vertebrae  102  and  104  and the bone graft  108  until the bone screws anchor the plate causes difficult in attaching the cervical plate. This is made more difficult because, generally, only a portion of the cervical plate is visible to the surgeon at any given moment (due to space constraints and surgical tools). 
     Moreover, once threaded, bone screws  208   a - d  tend to backout or reverse thread. Current fusion plates have numerous backout devices, but those devices are less than satisfactory solutions in some instances. Thus, improved backout or reverse threading protection is desired. 
     SUMMARY OF THE INVENTION 
     To attain the advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, apparatuses to facilitate the insertion of a bone graft into an intervertebral space and positioning of a cervical plate are provided. In particular, a cervical plate having backout or reverse threading protection is provided. The cervical plate has one screw bores through which at least one screw extends to anchor the cervical plate to a vertebra. The plate further has at least one channel with at least one engaging edge. A cover plate has a cover plate that extends over a portion of a screw head of the screw. The cover plate also has a locking extension with at least one locking edge at a distal end. The locking extension is inserted into the channel such that the engaging edge and the locking edge engage and hold the cover plate such that the cover plate edge is substantially adjacent the portion of the screw head preventing the screw from backing out. 
     The foregoing and other features, utilities and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention as illustrated in the accompanying drawings. Further, the advantages and purpose of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present invention, and together with the description, serve to explain the principles thereof. Like items in the drawings are referred to using the same numerical reference. 
         FIG. 1  shows adjacent vertebrae with a bone graft; 
         FIG. 2  shows adjacent vertebrae with a bone graft and cervical plate; 
         FIG. 3  shows adjacent vertebrae with a bone graft and cervical plate having an attachment mechanism illustrative of the present invention; 
         FIG. 4  shows a cross-section of the bone graft and cervical plate of  FIG. 3 ; 
         FIG. 5  shows an alternative attachment mechanism illustrative of the present invention; 
         FIG. 6  is a flowchart  600  illustrating use of the present invention; 
         FIG. 7 ; shows an impactor illustrative of the present invention; 
         FIG. 8  shows the impactor of  FIG. 7  with a cervical plate illustrative of the present invention; 
         FIG. 9  is a flowchart illustrative of a use of the impactor consistent with the present invention; 
         FIGS.10A-10C  show exemplary embodiments of a screw back out prevention device consistent with the present invention; 
         FIG. 11  shows another embodiment of a screw back out prevention device consistent with the present invention; 
         FIG. 12  shows another embodiment of a screw back out prevention device consistent with the present invention; 
         FIG. 13  shows still another embodiment of a screw back out prevention device consistent with the present invention; and 
         FIG. 14  shows still another embodiment of a screw back out prevention device consistent with the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Some embodiments of the present invention are described with reference to  FIGS. 3 to 14 . In particular,  FIGS. 3 and 4  show a cervical plate  300  and bone graft  302 . Cervical plate  300  has a viewing window  304 , screw holes  306 , and an attachment mechanism  400  (shown in phantom in  FIG. 3 ). As best seen in  FIG. 4 , cervical plate  300  has attachment mechanism  400  attaching the bone graft  302  to the cervical plate  300 . While cervical plate  300  could be made of numerous biocompatible materials, it is believed cervical plate  300  should be made of bio absorbable material. Bio absorbable or resorbable devises are generally known in the art, see for example, U.S. Pat. No. 6,241,771, titled R ESORBABLE  I NTERBODY  S PINAL  F USION  D EVICES , issued to Gresser et al., on Jun. 5, 2001, incorporated herein by reference. Generally, the devices explained and contained herein may be constructed out or any number of biocompatible materials, including titanium, shaped memory allows, milled bone, resorbable material, PEEK, or the like. 
     Attachment mechanism  400  can be any of a number of different attachment mechanisms. For example, as shown in  FIG. 4 , attachment mechanism  400  comprises a pin or stud attached to the cervical plate inserted into a hole or detent in the bone graft  302 . Alternatively, attachment mechanism  400  could be a spike inserted into bone graft  302  without bone graft  302  having a corresponding hole or detent to receive the spike, similar to a thumb tack. Alternatively, attachment mechanism  400  could comprise a pin or stud attached to the bone graft  302  inserted into a hole or detent in cervical plate  300 . Also, attachment mechanism  400  could be any style snap lock or friction fitting, such as the cavity formed in  FIG. 5  between protrusions  504 , explained in more detail below. It would be possible to provide barbs and/or lips on attachment mechanism  400  to facilitate the connection. Moreover, while two attachment mechanisms  400  are shown, more or less attachment mechanisms could be used. Further, attachment mechanism  400  could be an adhesive layer between the cervical plate  300  and bone graft  302 . Still further, attachment mechanism  400  could be a screw device so that bone graft  302  and cervical plate  300  are attached using a screw mechanism. Finally, the cervical plates  300  could be made as a single integral unit with the bone graft  302 , although that would be difficult due to the numerous sizes and shapes of bone grafts and plates necessary to perform the surgery. 
       FIG. 5  shows cervical plate  500  attached to a bone graft  502  by prongs  504  on bone graft  502 . As shown, prongs  504  attached to the bone graft grasp cervical plate  500  forming a frictional engagement. Alternatively, but not shown, cervical plate  500  could have prongs that grasp bone graft  502 . 
     As one of ordinary skill in the art would recognize on reading this disclosure, the number of ways the bone grafts could be attached to the cervical plate is numerous. To the extent alternative attachment means are not expressly identify above, this description should not be limited to the embodiments identified and described above. Rather, the specific embodiments identified are for illustrative purposes. 
       FIG. 6  is a flowchart  600  illustrating a method of using the present invention. In particular, the surgeon fits a bone graft into the intervertebral space, step  602 . Then, with the bone graft in place and the adjacent discs holing the bone graft in place, the surgeon sizes a cervical plate, step  604 . Once the cervical plate and bone graft are sized, the surgeon removes the plate and graft from the patient, and attaches the bone graft to the cervical plate, step  606 . Next, the combination bone graft and plate device is placed in the intervertebral space such that the adjacent discs hold the bone graft and plate in place, step  608 . The cervical plate is then anchored to the adjacent vertebral bodies, step  610 . Because surgeon attached the cervical plate to the bone graft, and the adjacent vertebrae hold the bone graft in place, the cervical plate remains fixed in place while the surgeon anchors the plate to the vertebrae. 
     As one of ordinary skill in the art would recognize on reading the above disclosure, the same general device and procedure is used when inserting multiple bone grafts. For example, if fusing four vertebrae, a surgeon would need to place three bone grafts. Conventionally, the three bone grafts are sized and placed in the intervertebral space and a cervical plate is sized for the three grafts. Using the present invention, the three bone grafts could all be removed and attached to the cervical plate and then refitted into the patient. However, it is believed attaching multiple bone grafts to the cervical plate would make it difficult to fit the device into the patient. It is believed to be more efficient to insert bone grafts and size the bone grafts and cervical plate, then remove one of the bone grafts while leaving other bone grafts in the spine. The removed bone graft would be attached to the plate. The one bone graft with the cervical plate attached is refitted into the patient and screws can be used to anchor the entire device with the one bone graft providing the stability for the cervical plate. 
     As mentioned above, inserting the bone graft and cervical plate includes using a retracting device to hold the adjacent discs apart, inserting the bone grafts and sizing the cervical plate, removing cervical plate and the retracting device, allowing the adjacent discs to squeeze the bone graft, then replacing and anchoring the cervical plate. A difficulty arises using the present invention because the retracting devices need to be removed prior to placing and anchoring the cervical plate. Thus, when the bone graft is removed and attached to the cervical plate, the retracting devices need to be removed prior to replacing the combined bone graft and cervical plate. However, on removing the retracting devices, the adjacent discs move together making it difficult to insert the bone graft between the adjacent discs. 
       FIG. 7  shows an impactor  700  capable of opening the space between the discs to ease the insertion of the bone graft attached to the cervical plate. Impactor  700  has a handle  702 , a cervical plate holder  704 , and prongs  706  forming cavity  708 . Impactor  700  could have various numbers of prongs, but it is believed two prongs work well. Cervical plate holder  704  is designed to hold the cervical plate such that the prongs  706  extend downward beyond the cervical plate and bone graft attachment. Prongs  706  could be spaced to form cavity  708  such that placing the cervical plate in the cavity  708  would form a friction fitting releasably coupling the cervical plate to the impactor  700 . The prongs  706  would act as a wedge to separate the adjacent discs allowing placement of the bone graft in the intervertebral space. Once the bone graft is placed, the impactor  700  would be completely removed from the patient. Thus, the cervical plate and impactor would be releasably coupled prior to insertion.  FIG. 8  shows a top side elevation view of the impactor  700  holding a cervical plate  802 . Cervical plate  802  is shown without a viewing window, but one could be used if desired. 
       FIG. 9  is a flowchart  900  illustrating using the impactor  700  with the cervical plate  802 . For convenience, flowchart  900  is described for insertion of a single bone graft. One of skill in the art will recognize on reading the disclosure, however, that the device and procedure would be usable with insertion of multiple bone grafts. Initially, the surgeon fits a bone graft into the intervertebral space, step  902 . Then, with the bone graft in place and the adjacent discs holing the bone graft in place, the surgeon sizes a cervical plate, step  904 . Once the cervical plate and bone graft are sized, the surgeon removes the plate and graft from the patient, and attaches the bone graft and the cervical plate, step  906 . Next, the impactor is releasably attached to the cervical plate, step  908 . The impactor with the cervical plate and bone graft is used to separate the adjacent vertebrae, step  910 . With the impactor holding the adjacent vertebrae apart, the bone graft is inserted in the intervertebral space such that the cervical plate is placed to be anchored to the adjacent vertebrae, step  912 . The impactor is removed allowing the adjacent vertebrae to hold the bone graft and cervical plate in place, step  914 . Finally, the cervical plate is anchored to the vertebrae, step  916 . 
     Once surgically placed and anchored, cervical plate bone screws have a tendency to become loose, which is commonly referred to as backing out. Many devices have been devised to inhibit bone screws from backing out; however, none are particularly satisfactory. 
     Referring now to  FIGS. 10A and 10B , a cervical plate  1000  with a backout prevention device  1002  in accordance with one embodiment of the present invention is shown. While the respective vertebrae have not been specifically shown in  FIGS. 10A and 10B , cervical plate  1000  has been surgically implanted, and bone screws  1004  have been threaded into vertebral bodies through screw holes  1006  in plate  1000 . A surface  1008  of plate  1000  contains at least one channel  1010  or groove. Two grooves  1010  are shown, but channel  1010  could be a single channel extending the length of the plate as well. Backout prevention device  1002  comprises a cover  1012  and locking extension  1014  (best seen in  FIG. 10B ). Cover  1012  is sufficiently large to allow at least one perimeter edge  1016  of cover  1012  to extend over a head  1018  of screw  1004 . When installed, edge  1016  is substantially aligned with head  1018  and prevents backing out of screw  1004 . Locking extension  1014  comprises one or more elastic prongs  1020 , for example biocompatible plastic or spring metal. Prongs  1020  fit into channel  1010  to hold backout prevention device  1002  in place. To facilitate placement of prongs  1020  into channel  1010 , cover  1012  may have at least one access port  1022  through which a tool (not specifically shown) can be inserted. The tool would flex prong  1020  such that prong  1020  easily fits into channel  1010 . When the tool is removed, prong  1020  would tend to return to its original position and apply a frictional force to seat backout prevention device. Similarly, the tool could compress prongs  1020  to allow removal of backout prevention device  1002  as desired. 
     As shown by  FIG. 10B , one embodiment of backout prevention device  1002  comprises cover  1012  with multiple prongs  1020  forming an inverted V shape. While the inverted V shape is shown for convenience, prongs  1020  may be independent of each other and biased as explained below. Prongs  1020  have a distal end  1024  with a locking edge  1026 . In this case, locking edge  1026  comprises a shoulder or protrusion. As shown in  FIG. 10B , locking edge  1026  could have a wedge shape to facilitate insertion of prongs  1020  into channel  1010 . In this case, use of the tool and access port  1022  may be unnecessary. Correspondingly, channel  1010  includes an engaging edge  1028 . Prongs  1020  should be biased to force locking edge  1026  and engaging edge  1028 . In this case, prongs  1020  are biased to diverge, but in other arrangements they may be biased to converge. In this case, engaging edge  1028  comprises a lip or undercut. In another exemplary embodiment, locking edge  1026 A could comprise a hollow and engaging edge  1028 A could comprise a corresponding protrusion, etc. Similarly, surface  1008  could have a raised extension  1030  with protrusions  1028 A and cover  1012  could have a channel  1026 A with a lip or shoulder  1032 . The channel of cover  1012  would be aligned with raised extensions  1030  on surface  1008  forming a fitting, such as a snap lock or a frictional engagement. 
     While shown as a square or rectangular shape, cover  1012  can be formed of many shapes and/or configurations. For example, cover  1012  could be elliptical, circular, some other polygon, linear, an arc or curve, convex or concave, irregular, a zig-zag, a letter shape, or the like. Further, as shown head  1018  resides above plate  1000  and cover plate  1012  resides above head  1018 ; however, one of skill in the art would recognize on reading the disclosure, that head  1018  and cover plate  1012  could be countersunk such that the head  1018  and cover plate  1012  reside substantially in the same plane as plate  1000 . 
     Referring now to  FIG. 11 , another cervical plate  1100  with two backout prevention devices  1102   d  and  1102   u  is shown. Prevention device  1102   d  is shown deployed and prevention device  1102   u  is shown undeployed. Plate  1100  would be surgically implanted and bone screws  1104  threaded into the vertebral body through screw holes  1106 . 
     Prevention devices  1102  comprise a plurality of bars  1108  having a deployed state (shown by prevention device  1102   d ) and an undeployed state (shown by prevention device  1102   u ). In the deployed state, prevention device  1102   d  has a peripheral edge  1110  on bars  1108  that extend over a head  1112  of screws  1104 . In the undeployed state, prevention device  1102 u has bars  1108  relatively closer to each other where peripheral edge  1110  does not extend over head  1112 , which makes screws  1104  freely movable. When deployed, a locking device  1114 , such as a pin, stud, screw, clip, cam, or the like is inserted or rotated between bars  1108  to lock prevention device  1102 d in the deployed state. For example, after bars  1108  are deployed, a screw could be inserted to maintain the separation. Alternatively, a cam could be rotated to maintain separation. Locking device  1114  would be removed to undeploy the prevention device. While prevention device  1102  is shown sized similar to a single screw head, prevention device  1102  could have bars  1108  that extend the length of the cervical plate so only one prevention device is necessary for multiple sets of screws. 
       FIG. 12  shows a top plan view of another backout prevention device  1200  consistent with the present invention. Unlike prevention devices  1000  and  1100 , which generally prevent screws from backing out by engaging a screw head, prevention device  1200  engages, for example, a thread of a screw or a notch located, for example, in the screw head. Backout prevention device  1200  includes a bushing  1202  and at least one locking pin  1204 , in which three pins  1204  are shown in this case. Bushing  1202  forms a ring or thread through which a screw (not specifically shown) can be threaded. A spring  1206  or other elastic device pushes locking pin  1204  radially inward such that pins  1204  provide sufficient force on at least one thread or notch of the bone screws (not specifically shown in  FIG. 12 ). Spring  1206  should not be construed as a conventional helical spring, but could be a number of elastic devices including, for example, elastic plastics, air loaded dampers, magnetics, shaped memory alloys, or the like. Thus, spring or spring loaded should be used generically to mean a device with elastic movement ability. Basically, spring  1206  needs to provide sufficient outward force to seat pin  1204  against a thread, notch, or surface of the screw or screw head, but be sufficiently resilient to allow the screw to be threaded into bone. (Notice, spring  1206  and pin  1204  could be combined into a single unit using). Spring  1206  could be flush between the pin  1204  and bushing or, as shown, spring  1206  could reside in a channel  1208  in bushing  1202 . 
       FIGS. 13 and 14  show cross-sectional views of screw back out prevention device  1300  and  1400 , respectively.  FIG. 13  shows a portion of a cervical plate  1302  about a screw hole  1304 . Residing in screw hole  1304  would be a bushing  1306  that contains a notch  1308 , groove, or channel. A screw would be threaded into vertebral bodies (not specifically shown in  FIG. 13 ) through screw hole  1304  such that screw head  1310  resides as shown. Screw head  1310  has a notch  1312 , groove, or channel, corresponding to notch  1308 . Residing in notch  1308  and notch  1312  is a self-bonding material  1314 , such as high molecular weight polyethylene, nylons, and biopolymers and other self-bonding material commonly used in the aeronautical and automotive industries. Instead of self-bonding material  1314 , material  1314  could be replaced with heat fusible material, pressure sensitive material, radiation curing, or other adhesives that require activation to adhere. Self-bonding material  1314  provides back out prevention because when the screw is threaded into the vertebral bodies bonding material  1314  in notch  1308  engages bonding material  1314  in notch  1312  and bonds. Thus, screw head  1310  is held in place. Although shown in a notch, material  1314  could be layered, such as by a coating or spray, on bushing  1306  and screw head  1310 . 
     Similarly,  FIG. 14  shows a portion of a cervical plate  1402  about a screw hole  1404 . Residing in screw hole  1404  is a screw head  1406 . Screw hole  1404  has sidewalls  1408 . Residing on sidewalls  1408  exists at least one strip of self-bonding material  1410 . While the materials identified above for material  1314  could be used, in this particular example, self-bonding material  1410  is a spray on type of material. Screw head  1406  has exterior sidewalls  1412 . Residing on sidewalls  1412  exists at least one strip of self-bonding material  1410  corresponding to the strip of self-bonding material  1410  on sidewalls  1408 . Again, instead of a spray or coating, material  1410  could reside in a notch on sidewall  1408  and sidewall  1412 . 
     Alternatively to a self-bonding material, other materials may be used. For example material  1410  (or  1314 ) may be a type of heat fusion plastic. To use heat fusion, once the screw is threaded and the materials align, a heat activation signal would be applied to fuse the materials. Other bonding materials, such as epoxies, acrylics (such cyanoacrylates and anaerobics), silicones, pastes, tapes, and glues, and the like could be used for bonding material  1410  (or  1314 ), but would inhibit threading of the screw as well as be useful as a screw back out prevention device. 
     While the invention has been particularly shown and described with reference to some embodiment thereof, it will be understood by those skilled in the art that various other changes in the form and details may be made without departing from the spirit and scope of the invention.

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