Abstract:
A bone plate for use in anterior lumbar spinal fixation has interlocking components to prevent dislodgement of the plate due to anatomical forces. The exposed surface of the plate is smooth to prevent trauma to internal body tissue. The plate spans the intervertebral space with each end attached to an adjacent vertebrae by locking screws threadably engaged with tubular bone anchors. The bone anchors include helical members are threaded through the bone plate and inserted into holes within the bone. The plate has a countersunk cavity including portions overlapping the locking screws. A locking cap fits in the cavity to secure the other end of the locking screws from backing-out of the plate. The locking cap includes at least two chamfered portions that cooperate with mating recesses in the countersunk cavity of the bone plate.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 10/419,686, entitled “BONE FIXATION PLATE”, filed Apr. 21, 2003, now U.S. Pat. No. 7,481,829, the entire contents of which is hereby expressly incorporated by reference. 
    
    
     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 contribute 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 less complicated system with multiple locking components for added security. 
     SUMMARY OF THE PRESENT INVENTION 
     It is an objective of this invention to provide a bone plate, suitable for anterior lumbar 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. 
     A further objective of the invention is to provide threaded tubular bone anchors embedded in bone guided by the screw holes in the plate. The bone anchors being screwed into the bone by external threads. 
     Another objective of the invention is to provide locking screws extending through the countersunk screw holes into the bone anchors with the leading ends of the screws expanding the ends of the bone anchors to fix the bone screws to the bone anchors and the anchors to the bone. 
     Yet another objective of the invention is to provide a bone plate with a locking cap which extends over the area of the countersunk screw holes. The locking cap fits into a large countersunk area of the bone plate covering the countersunk screw holes allowing easy and positive assembly and preventing relative lateral movement between the locking cap and the plate. The locking cap has an aperture and the plate has a receptacle which align when the cap is placed in the countersunk area. A cap screw is threaded into the aligned aperture and receptacle to connect the plate and the cap. 
     A still further objective of the invention is to provide a kit of several interchangeable components including plates, locking screws, anchors, caps and cap screws to permit the assembly of matching components to fit the anatomy of the patient. 
     Another objective of the invention is to provide a bone plate system with double headed locking screws and a cam on the bone plate to wedge into the double headed screws. 
     Another objective of the invention is to provide a bone plate system with a bone plate that can be used as a template for drilling holes into the bone prior to insertion of the bone anchors. 
     Another objective of the invention is to provide a bone anchor that is designed to be implanted by threadably passing the bone anchor through the bone plate for insertion into a pre drilled hole. 
     Another objective of the invention is to provide a bone anchor in the form of a deformable helical member. 
     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 an exploded perspective view of the bone plate system of this invention; 
         FIG. 2  is a perspective of assembled bone plate system of this invention; 
         FIG. 2A  is a perspective of another embodiment of  FIG. 2 ; 
         FIG. 3  is a cross section of  FIG. 2  along line  3 - 3 ; 
         FIG. 4  is a cross section of the bone plate counter sunk into the bone; 
         FIG. 5  is a bottom view of the bone plate with the cam rotated into the double headed screws; 
         FIG. 6  is a perspective of a double headed locking screw; and 
         FIG. 7  is a cross section of a bone plate with a double headed screw in place. 
         FIGS. 8A through 8D  are perspective views showing the bone plate, the bone anchor and the locking screw in various states of assembly. 
         FIGS. 9A and 9B  are cross section views of showing the relationship of the bone anchor and locking screw in various states of assembly. 
         FIG. 10A  is a partially exploded side sectional view of the bone fixation plate with the locking cap unsecured to the bone plate. 
         FIG. 10B  is a partially exploded perspective sectional view of the bone fixation plate with the locking cap unsecured to the bone plate. 
         FIG. 11A  is a side sectional view of the fully assembled bone fixation plate. 
         FIG. 11B  is a perspective sectional view of the fully assembled bone fixation plate. 
     
    
    
     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 ,  FIG. 2  and  FIG. 2A  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 anchors  11  varying in diameter and length with complementary locking screws  12 . The different sized locking screws may have the same sized heads  13  to be used in different sized bone plates  14  with screw holes  15 ′ and countersunk depressions  15  of the same size. There may be several different sized locking caps  16 , as shown in  FIGS. 2 and 2A , to fit into the different complementary sized countersunk areas  19  of the plates while the aperture  17  and the threaded receptacle  18  are of the same size. The locking cap  16  is connected to the plate  14  by a cap screw  20 . The kit merely refers to the dissembled components, shown in  FIG. 1 , that can be assembled to produce an integral whole which corresponds to the anatomical features of a particular patient. 
     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 locking features are disposed over the length of the plate-bone connection such that the same biomechanical force may not act on all locks simultaneously. 
     The tubular bone anchors  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 anchors with the longitudinal axis of the countersunk screw holes  15 ′ in the plate. The bone anchors  11  are driven into the bone with a tool (not shown) that engages the slots  27  for rotation of the anchor. The exterior screw threads  21  of the anchor draw the anchor 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 resistance in the opposite direction. The leading or proximal end of the anchor  11  has a number of radial slits  25  through the side wall dividing the proximal circumference into segments  26 . Further, the leading end is tapered internally and externally toward a smaller end. This structure of the leading end of the bone anchors  11  creates a change in the bone-anchor interface by expanding, as the locking screws are inserted, to increase the resistance to reverse rotation. 
     Once the bone anchors  11  are in place, the locking screws  12  are extended through the screw holes  15 ′ of the bone plate  14  with the threads  22  and  23  threadably engaging the interior threads  24  of the bone anchors. The locking screws are rotated by a tool (not shown) fitted into the receptacle  28 . As shown, the locking screw  12  has a smooth unthreaded shank near the head  13 . An intermediate length of the locking screw has threads  22  to engage the interior threads of the bone anchor. The leading end of the locking screw has a tapered portion with threads  23  to engage the threaded tapered leading end of the bone anchor. The threaded engagement of the locking screws with the leading ends of the anchors and the resultant expansion of the anchors creates a difference in the threading along the interior length of the anchor which resists rotation in the opposite direction. The leading end of the locking screws may be unthreaded and act as a wedge. The locking screw heads  13  are completely enclosed by the depressions  15  and compressively disposed against the bottoms of the depressions. Therefore, the anchor  11  is locked in place by a new bone-anchor interface and the locking screw and anchor are locked together by compression and threading changes. Of course, other combinations of locking screws and anchor configurations may be used, such as, a constant taper of each. 
     After the locking screws  12  have been seated in the bone anchors  11  and depressions  15 , the locking cap  16  is placed in the complementary countersink  19  formed in the plate  14 . The size and thickness of the locking cap and the size and depth of the countersunk area allow ease of assembly of these components and a resulting low profile with a smooth outer or distal surface. The vertebrae V is shaped to form a counter sunk area in which the plate is placed, as shown in  FIG. 4 . This is especially important in reducing the possibility of internal trauma to adjacent soft tissue, e.g., in the anterior lumbar spinal fixation. 
     The assembly of the locking plate  16  and the bone plate  14  automatically aligns the receptacle  16  and the aperture  17  for connection by the cap screw  20 . The continuous side wall of the enlarged countersunk area and the periphery of the locking cap positively locate the components relative to each other. The cap screw  20  is tightened by a tool (not shown) that fits into receptacle  29 . Once the locking cap is secured in place, the heads of the locking screws are prevented from retraction. Because the cap screw  20  is centrally located in the locking cap  16 , any reverse rotation of the locking screws is opposed by a leveraging action between the locking cap and the cap screw. This action tends to jam the cap screw and locking cap tighter together. 
     As an alternative or additional attachment system is shown in  FIGS. 5 ,  6 , and  7 . To further secure the locking screws  12  to the plate  14  requires a slit  30  in the side of each of the depressions  15 . The slit in each adjacent depression faces the other. A cam  31  is rotatably mounted in the plate  14  between the adjacent depressions and has a flange formed with a blade portion  32  and a brake portion  33 . The blade portion  32  will register with the slit  30  in one position and be free of the slit in another position. The brake portion  33  will engage the curved plate when the blade portion is registered with the slit  30  to create a friction stop. In this embodiment, the locking screws have a double headed configuration  13 ,  13 ′ with a circumferential groove  34  between the two heads. When the locking screws are securely in place in the anchors  11  and plate  14 , the grooves  34  will register with the slits  30 . The cam  31  extends through the plate  14  into the countersunk portion  19  and is turned by a tool (not shown) to register the blade portion  32  with the slits  30  and the grooves  34 . In this position, the brake portion  33  of the flange engages the curved surface of the plate  14 . The locking cap  16  may then be applied, as described above. The cams may extend through the locking cap to be tightened along with the cap screw. 
     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. 
       FIGS. 8A through 8D  are perspective views showing an alternate embodiment of the bone fixation plate in various states of assembly. As shown in  FIG. 8A  each bone anchor  111  is threaded through the top surface of bone plate  114 . In this embodiment the bone plate  114  can serve as a template for drilling pilot holes into the bone. Bone plate  114  includes screw holes  115 , as shown in  FIG. 10B . Each of the bone anchors  111  are rotatably threaded through the upper surface of bone plate  114  via screw holes  115 . The upper end of each bone anchor  111  includes a collar portion  127 . A tool (not shown) is capable of engaging collar portion  127  to impart rotation to bone anchor  111  and thread each bone anchor through the bone plate  114 . As illustrated in  FIGS. 9A and 9B , the threaded portion of anchor  111  is formed as a helical member  126  presenting an exterior threaded surface  121 . Helical member  126  is configured as a deformable helical coil. Threaded surface  121  is threaded through screw hole  115 . Collar  127  is smaller in diameter than screw hole  115 . As initially positioned within the bone plate the outer diameter of the exterior threaded surface  121  has a constant outer diameter except for the first few lead-in threads which are somewhat reduced in diameter. The exterior threaded surface  121  of the helical member  126  is formed in profile as a buttress type thread. The interior circumferential surface of the helical member includes threads  124 . As initially positioned, the diameter of threads  124  along the interior surface of helical member  126  decreases in size as it progresses from the collar end to the leading end. The leading end of each anchor  111  has one or more notches or flutes  125  on the exterior threaded surface  121 .  FIG. 8D  illustrates the anchor member  111  threaded completely through bone plate  114 . 
     Once the bone anchors  111  are fully situated within bone plate  114  a locking screw  112  will be threaded into each bone anchor  111 .  FIG. 8C  shows a perspective of locking screw  112  partially inserted within bone anchor  111 . Likewise,  FIG. 9A  shows a sectional view of locking screw  112  partially inserted within bone anchor  111 . Each locking screw  112  includes a head portion  113 , a smooth upper shank portion, an intermediate threaded portion  122 , and a smooth lower shank portion. The locking screws are rotated by a tool (not shown) that engage head portion  113 . Head portion  113  has a diameter D- 1  greater than screw hole  115  in bone plate  114 . The collar  127  has a diameter D- 2  that is smaller than the diameter of the screw hole  115  in bone plate  114 . The threaded portion  122  has a constant outer diameter. As shown in  FIG. 9A , intermediate threaded portion  122  engages threads  124  on the interior circumferential surface of helical member  126 . As the locking screw  112  is threaded into the bone anchor  111  the smooth lower shank portion and the threaded  122  of locking screw  112  will increase the diameter of the interior circumferential surface of the helical member  126 . Likewise, as the locking screw  112  is thread into the bone anchor  111  the outer circumferential surface of the helical member  126  is expanded. Except for the lead-in threads, the outer diameter of helical member  126  is greater at the leading end of the anchor  111  than at the collar end  127 , as shown in  FIG. 9B . Expansion of thread member  126  will cause thread surface  121  to penetrate bone tissue and create a strong bone-anchor interface. 
     After the locking screws  112  have been seated within the bone anchors  111  a locking cap  116  is placed in a complimentary countersink area  119  in bone plate  114 . The size and thickness of the locking cap  116  and the size and depth of the countersink area  119  allow for ease of assembly and result in a low profile having a smooth outer surface. Prior to installation the locking cap  116  is generally concave in overall configuration, as shown in  FIGS. 10A and 10B . The cap  116  has an upper and lower surface and connecting side walls. The countersunk area  119  has a shape that generally conforms to the shape of the locking cap  116  with upstanding side walls. The locking cap  116  includes a pair of chamfered side walls  123  at opposite ends that cooperate with complimentary recesses  124  in the side walls of countersunk recess area  119 . The remaining side walls of the locking cap  116  and the walls of the countersink area  119  are substantially perpendicular to the upper and lower surfaces of the locking cap. Locking cap  116  includes an aperture  117 . When the locking cap  116  is placed within the countersunk area  119 , aperture  117  aligns with a threaded receptacle  118  located on bone plate  114 . A locking cap screw  120  is placed within aperture  117  and threaded into threaded receptacle  118 . As the cap screw  120  is threaded into its final position locking cap  116  is transformed from a generally concave configuration to a generally flat shape. In its fully assembled configuration, as shown in  FIGS. 11A and 11B , chamfered edges  123  will be wedged underneath respective recesses  124  formed in the side walls of the countersunk area  119 . Locking cap  116  may optionally includes slots, channels or grooves  130 , as shown in  FIGS. 11A and 11B , which will facilitate the flattening of locking cap  116  during assembly. The heads of the locking screws  112  are prevented from retraction once the locking cap is secured to the bone plate  114  by the cap screw  120 . The bone plate, bone anchors, locking screws, locking cap and locking cap screw are made of titanium alloy or any suitable biocompatible material. 
     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.