Abstract:
A spinal plate system and method for fixation of the human spine is provided. In an embodiment, the spinal fixation system includes a plate, a coupling member, a locking system for substantially locking the coupling member in a desired position, and an anchoring system to secure the coupling member in the locking system. The plate may have a hole that allows the coupling member to couple the plate with a bone. At least a portion of the coupling member may swivel in the hole so that a bottom end of the member may extend at a plurality of angles substantially oblique to the plate. The locking system may lock the coupling member in desired positions relative to the plate. The anchoring system may secure the coupling member in the locking system to inhibit the coupling system from detaching from the locking system when stressed. An assembly tool may be used to engage and disengage the anchoring system during the installation or removal of the spinal fixation system.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to spinal fixation systems and the like. The present invention also generally relates to a spinal plate system that includes a mechanism for fixably attaching heads of fasteners to a spinal plate. 
     2. Description of the Related Art 
     The use of spinal fixation plates for correction of spinal deformities and for fusion of vertebrae is well known. Typically, a rigid plate is positioned to span bones or bone segments that need to be immobilized with respect to one another. Bone screws may be used to fasten the plate to the bones. Spinal plating systems are commonly used to correct problems in the lumbar and cervical portions of the spine, and are often installed posterior or anterior to the spine. 
     Spinal plate fixation to the cervical portion of the spine may be risky because complications during surgery may cause injury to vital organs, such as the brain stem or the spinal cord. When attaching a fixation plate to a bone, bone screws are placed either bi-cortically (i.e., entirely through the vertebrae such that a portion of the fastener extends into the spinal cord region) or uni-cortically (i.e., the fastener extends into but not through the vertebrae). Uni-cortical positioning of bone screws has grown in popularity because it is generally safer to use. Bi-cortical fasteners are intended to breach the distal cortex for maximum anchorage into the bone; however, this placement of the fasteners may place distal soft tissue structures at risk. Fastener placement is particularly important in anterior cervical plate procedures because of the presence of the spinal cord opposite the distal cortex. Unfortunately, uni-cortical fasteners may move from their desired positions because of the soft texture of the bone marrow. In fact, the portion of the bone surrounding such fasteners may fail to maintain the fasteners in their proper positions. The result is backout of the fastener. 
     Backout of the fastener is particularly problematic when two fasteners are implanted perpendicular to the plate. When the fasteners are placed in such a manner, backout may occur as a result of bone failure over a region that is the size of the outer diameter of the fastener threads. To overcome this problem, two fasteners may be angled in converging or diverging directions with respect to each other within the bone. The amount of bone that is required to fail before backout may occur is increased by this configuration as compared to fasteners that are implanted in parallel. Although positioning convergent or divergent fasteners in a bone reduces the risk of backout, backout may still occur. 
     Backout may damage internal tissue structures and cause complications if the dislocated fastener penetrates the tissue structures. For example, if backout occurs, the fastener might breach the esophageal wall of the patient. Such a breach may permit bacterial contamination of surrounding tissues, including the critical nerves in and around the spinal cord. Such a breach could be fatal. 
     In an attempt to reduce the risk of damage to internal tissue structures, some cervical fastener plate systems have uni-cortical fasteners that attach both to the bone and to the plate. If a fastener does backout, the fastener remains connected to the plate so that it does not contact internal tissue structures. U.S. Pat. No. 5,364,399 to Lowery et al. describes one such system and is incorporated by reference as if fully set forth herein. The Lowery et al. plating system includes a locking fastener at each end of the plate. The locking fastener engages the head of the bone screw to trap the fastener within a recess of the plate. Since the locking fastener is positioned over portions of the bone screws, the locking fastener may extend above the upper surface of the plate. Thus, the locking fastener may come into contact with internal tissue structures, such as the esophagus. 
     Another plating system that includes a fastener-to-plate locking mechanism is the Aline™ Anterior Cervical Plating System sold by Smith &amp; Nephew Richards Inc. in Memphis, Tenn. A description of this system can be found in the Aline™ Anterior Cervical Plating System Surgical Technique Manual by Foley, K. T. et al., available from Smith &amp; Nephew Richards Inc., September 1996, pp. 1-16 and is incorporated by reference as if fully set forth herein. The bone screws of this system have openings within each bone screw head for receiving a lock fastener coaxially therein. Each bone screw may be inserted into a bone such that the head of the fastener is positioned within a hole of a plate placed adjacent to the bone. The head of each bone screw is slotted so that portions of the head are deflected toward the plate during insertion of the lock fastener within the opening of the bone screw. Positioning and inserting a lock fastener within the opening can be difficult due to the small size of the lock fastener. The surgeon may be unable to hold onto the lock fastener without dropping it. If a lock fastener falls into the surgical wound, it may be difficult to retrieve. In some instances, the lock fastener may be unretrievable. 
     SUMMARY OF THE INVENTION 
     An implant system may be used to immobilize a portion of a human spine. The implant system may include a plate comprising end holes, midline holes, fasteners, and expandable/contractible rings. The fasteners and rings may include mechanisms for anchoring or locking the fastener heads within the rings to inhibit backout of the fastener. 
     The end holes extend from an upper surface to a lower surface of the plate. The end holes may be disposed in pairs at opposite ends of the plate. Each end hole receives at least a portion of a head of a fastener. Herein, “fastener” means any elongated member, threaded or non-threaded, which is securable within a bone. Fasteners include, but are not limited to screws, nails, rivets, trocars, pins, and barbs. The fastener may be a bone screw. A fastener may have a fastener head. The fastener head typically includes an opening adapted to mate with a tool. The tool allows the insertion of the fastener into a bone. Each end hole may also be spherically contoured to permit the fastener to be “obliquely angulated” relative to the plate. Herein, “obliquely angulated” means that the fastener may be positioned throughout a wide range of angles relative to an axis that is perpendicular to the plate. Obliquely angulating a fastener into a bone may reduce the risk of backout of the fastener. 
     The expandable/contractible rings may be sized so that a ring seats within an end hole between the plate and the fastener. The inner surface of each ring may be shaped to mate with a fastener head while the outer surface may be shaped to mate with the inside surface of an end hole. The outer surface of each fastener head may be tapered so that an upper portion of the head is larger than a lower portion of the head. The inner surface of the ring may have a taper that generally corresponds to the taper of the fastener head. 
     Each ring may also have a gap that extends vertically through the ring to make the ring more readily expandable and contractible. During insertion of the fastener head into the ring, the fastener head exerts force against the ring to expand the ring against the inner surface of the hole. Expanding the ring against the inner surface of the hole may securely fix the fastener to the plate. 
     The fastener head and the ring may include a locking mechanism to attach the fastener head to the plate. The locking mechanism may inhibit backout of the fastener head from the ring if the fastener loosens in the bone. The locking mechanism may also inhibit the fastener head from contacting adjacent tissue structures since the locking mechanism attaches the fastener head to the plate. In some embodiments, there is tolerance for some freedom of movement in an axial direction between a locking mechanism and a fastener head. The availability of some axial movement may allow the fastener to back out or loosen slightly from the bone during a normal period of adjustment after implantation of a spinal fixation system. 
     Midline holes may be formed through the plate at various locations along a midline axis extending across the plate. The surface of the plate that surrounds each midline hole may be tapered. Further, the heads of fasteners that may be positioned within the plates may have tapered outer surfaces that generally correspond to the tapered surface of the plate. Thus, when such a fastener head is inserted into a midline hole, the shape of the plate causes the fastener to become fixably attached to the plate in a position that is substantially perpendicular to the plate. Midline holes may be used to attach a bone graft to the bore plate. Oblique angulation of fasteners positioned within the midline holes may not be required. 
     The bone plate may have one or more spikes located on the surface of the plate that faces the spinal column. Spikes may be disposed in pairs at opposite ends of the plate proximate the end holes. The spikes may be tapped into the bone to help inhibit the bone plate from slipping during surgical implantation. 
     Prior to surgical implantation of the spinal plate system, the expandable/contractible rings may be placed within the end holes of the plate. The plate may then be positioned adjacent to a portion of the spine that requires spinal fixation. Holes may be drilled and/or tapped at desired angles into portions of the bone underlying the end holes of a plate. Fasteners may be inserted through the end holes into the holes in the bone. The heads of the fasteners may be positioned within the end holes such that the rings surround at least a portion of the heads. The rings may lock the fasteners in place without occupying regions outside of the end holes. Further, since the rings are pre-positioned within the end holes, surgeons do not have to worry that they may drop the rings during insertion of the rings into the end holes of the plate. 
     In one embodiment, a locking mechanism secures a fastener head to a ring. A locking mechanism may have a top and one or more flexible arms that angle downwards and outwards from the top. The ends of the arms have prongs that are substantially parallel to the top of the locking mechanism. A locking mechanism in a compressed configuration, fits into an opening formed in the head of a fastener. The prongs of the locking mechanism fit within holes located near the bottom of the opening. The holes extend from the outer surface of the head to the opening. When the prongs are positioned in the holes, the prongs extend through the holes so that the locking mechanism is in an extended configuration. The prongs that extend out of the head of the fastener fit within a groove on the inner surface of the ring to enhance the connection between the fastener and the ring. 
     The locking mechanism may be inserted in the fastener head prior to the surgical procedure to avoid the risk of dropping the locking mechanism during the surgical procedure. An insertion and extraction tool retracts the prongs on the locking mechanism into the head during insertion or extraction of a fastener. The tool may include a handle, a shaft, and a hollow driver head shaped to match a drive section of the opening on the fastener head. Inserting the driver head into the opening slides the head over the locking mechanism and compresses the shafts of the locking mechanism. Compressing the shafts of the locking mechanism retracts the prongs into the fastener head. To insert a fastener into a bone, the user inserts the fastener into the bone until the head is fully inserted in the ring. Removing the driver head from the opening causes the shafts of the locking mechanism to expand outwards so that the prongs extend out of the holes into the fastener head. 
     To remove a fastener that has a locking mechanism, the user inserts the driver head of the insertion and extraction tool into the opening of the fastener head. The driver head compresses the shafts of the locking mechanism and causes the prongs to retract within the fastener head. The user may then remove the fastener from the bone. 
     In another embodiment, a tapered fastener head locks into a ring by one or more fingers on the ring that snap into grooves on the fastener head. L-shaped slots cut into the top of the ring may define the fingers. The fingers have springlike action so that the fingers snap into the grooves on the fastener head when a fastener head is inserted into the ring. As the fastener head passes into the ring, the tapered outer surface of the head expands the ring against the inner surface of the plate. When the groove on the fastener head reaches the fingers, the fingers snap into the groove, fixing the fastener in the ring and helping to inhibit backout. 
     In another embodiment, a fastener head locks to a ring by one or more ridges on the ring that snap into grooves on the fastener head. Notches cut into the top of the ring may form paddles. A ridge may extend along an inside surface of each paddle proximate the top of the ring. The paddles have a springlike action so that the ridges snap into the grooves on the fastener head during insertion of the fastener head into the ring. The ridges of the ring residing within a groove of the fastener head may fix the fastener in the ring and help inhibit backout of the fastener. 
     An extraction tool module fits over an insertion tool and allows the retraction of the ring ridges from the fastener head. The insertion tool includes a handle, a shaft, and a driver head shaped to match the opening on the fastener head. The extraction module slides over the shaft of the insertion tool. The extraction module may include a handle and an extraction head. The extraction head may include a tip that slides over the fastener head and contacts the ends of the paddles. The outer surface of the tip tapers. As the extraction module is pushed down, the tapered surface of the tip forces the paddles outwards and disengages the ridges on the paddles from the grooves on the fastener head. Disengaging the ridges on the paddles from the grooves on the fastener head allows the fastener to be backed out of the bone. 
     Using a locking mechanism between the fastener head and the ring may result in a strong connection between the fastener and the plate. Even if the shank of a fastener loosens within the bone, the fastener head will tend to remain within the hole of the plate so as not to protrude from the plate into surrounding body tissue. Allowing some axial freedom of movement for the fastener head in the ring may allow the fastener to back out slightly during an adjustment period after installation of the spinal fixation system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further advantages of the present invention will become apparent to those skilled in the art with the benefit of the following detailed description of embodiments and upon reference to the accompanying drawings in which: 
     FIG. 1 is a top view of an embodiment of a spinal plating system that may be used for fixation of the human spine. 
     FIG. 2 is a partial cross-sectional view of the spinal plating system taken substantially along line  2 — 2  of FIG.  1 . The fastener is not shown in section. 
     FIG. 3 is a partial cross-sectional view of the spinal plating system taken substantially along line  3 — 3  of FIG. 1, wherein the fasteners are in a converging orientation within end holes of a plate. The fasteners are not shown in section. 
     FIG. 4 is a partial cross-sectional view of the spinal plating system taken substantially along line  4 — 4  of FIG. 1, wherein the fasteners are in a diverging orientation within end holes of a plate  5 , the fasteners are not shown in section. 
     FIG. 5 depicts an embodiment of a fastener with grooves and holes to engage a locking mechanism. 
     FIG. 6 is a partial front view of a fastener with holes in the fastener head. 
     FIG. 7 is a top view of a fastener head having grooves for engaging a locking mechanism. 
     FIG. 8 is a top view of an embodiment of a fastener head with a hexagonal opening and holes to engage a locking mechanism. 
     FIG. 9 is a perspective view of a locking mechanism. 
     FIG. 10 is a perspective view of a ring that may be used with a fastener and a locking mechanism. 
     FIG. 11 is a partial sectional view of an insertion/extraction tool for fasteners with locking mechanisms. 
     FIG. 12 a  is a bottom view of a driver head of the insertion/extraction tool shown in FIG.  11 . The tool may be used with the fastener head shown in FIG.  7 . 
     FIG. 12 b  is a bottom view of the driver head of the insertion and removal tool shown in FIG. 11 which may be used with the fastener head shown in FIG.  8 . 
     FIG. 13 is a sectional view of a fastener head with an insertion/extraction tool and compressed locking mechanism during an insertion process. 
     FIG. 14 is a sectional view of a fastener head and locking mechanism inserted into a ring. 
     FIG. 15 is a perspective view of an embodiment of a fastener. 
     FIG. 16 is a front view of an embodiment of a fastener head. 
     FIG. 17 is a perspective view of an embodiment of a fastener. 
     FIG. 18 is a front view of an embodiment of a fastener head with a groove. 
     FIG. 19 is a perspective view of an embodiment of a ring. 
     FIG. 20 is a perspective view of an embodiment of a ring. 
     FIG. 21 is a perspective view of an embodiment of a ring. 
     FIG. 22 is front view of an insertion/extraction tool which may be used with the ring of FIG.  21 . 
     FIG. 23 a  is a partial cross sectional view of a fastener during insertion in the ring of FIG.  21 . The shaft of the insertion/extraction tool is not shown in cross section. 
     FIG. 23 b  is a partial cross sectional view of a fastener after insertion in the ring of FIG.  21 . The shaft of the insertion/extraction tool is not shown in cross section; and 
     FIG. 23 c  is a partial cross sectional view of a fastener during removal from the ring of FIG.  21 . The shaft of the insertion/extraction tool is not shown in cross section. 
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but to the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, and particularly to FIG. 1, a spinal plating system is designated generally as  20 . The spinal plating system  20  may be used to correct problems in the lumbar and cervical portions of the spine. For example, the plating system  20  may be implanted into the occiput bone that is located at the base of the skull. The plating system  20  may also be installed anterior to the spine. The spinal plating system  20  includes plate  22  that is placed adjacent to a portion of the spine and spans at least two vertebrae. Plate  22  may include four end holes  24 , located at corners of the plate. End holes  24  pass vertically through plate  22  such that the holes extend from an upper surface  26  to a lower surface  28  of the plate as depicted in FIG.  2 . End holes  24  are configured to receive rings  30 . Fasteners  32  fit within the rings  30 . Herein, “fastener” means any elongated member, threaded or non-threaded, which is securable within a bone. Fasteners include, but are not limited to screws, nails, rivets, trocars, pins, and barbs. The fastener may be a bone screw. Rings  30  fixedly attach fastener heads  34  of fasteners  32  to plate  22 . Gap  36  may exist in each of the rings  30 . A gap  36  allows for expansion and contraction of a ring  30 . Ring contraction allows a ring  30  to be easily inserted into an end hole  24  of the plate  22 . 
     The spinal plating system  20  may also include one or more central holes  38  that extend vertically through plate  22 . One of the central holes  38  may be located at about the mid-point of the plate  22 . Head  40  of fastener  42  is positioned within one of the central holes  38 . Multiple central holes  38  provide a surgeon with options as to the most desirable location for placement of a fastener  42 . Fastener  42  may be used to connect plate  22  to a bone graft (not shown). 
     FIG. 2 shows a fastener  42  within one of the central holes  38  of plate  22 . Fastener  42  may include head  40  and shank  44 . The shank  44  extends from the base of head  40 . In one embodiment, the inner surface of a central hole  38  tapers so that the hole is larger at upper plate surface  26  than at the lower plate surface  28 . The outer surface of the fastener head  40  has a taper that generally corresponds to the taper of the central hole  38 . During implantation of a fastener  42  into a bone graft, the shank  44  of the fastener  42  is inserted into a hole that has been formed in the bone graft under hole  38 . Because the lower portion of hole  38  is smaller than the upper portion of the fastener head  40 , fastener  42  may become locked into place within the central hole  38  once the fastener has been inserted to a desired depth within the bone graft. The bone plate  22  may have spikes  45  extending from the lower plate surface  28 . 
     As shown in FIG. 2, the plate  22  may have a curvature. The curvature may enhance fixation of the plate  22  to a bone. The bone plate  22  may have one or more spikes  45  located on the surface of the plate that faces the bone. The spikes  45  may be disposed in pairs at opposite ends of the plate proximate the end holes  24 . The spikes  45  may be tapped into the bone to help inhibit the bone plate  22  from slipping during surgical implantation. 
     FIG. 3 depicts a cross-sectional view of an embodiment of a final plating system  20  wherein a pair of fasteners  32  are in a converging configuration. FIG. 4 depicts a cross-sectional view of an embodiment of a spinal plating system wherein a pair of fasteners  32  are in a diverging configuration. Ring  30  fits into a hole  24  between plate  22  and fastener head  34 . Inner surfaces  46  of holes  24  may have arcuate or spherical contours. Outside surfaces  48  of rings  30  may have arcuate or spherical contours that substantially correspond to the contours of the inner surfaces  46  of the holes  24 . Having a contoured ring outer surface  48  that substantially corresponds to the contour of the inner hole surface  46  allows a ring  30  positioned in a hole  24  to be capable of polyaxial rotation within the end hole  24 . 
     The combination of ring  30  within end hole  24  functions like a ball and socket since the ring may be swiveled or polyaxially rotated within the end hole. The ability of the ring  30  to rotate polyaxially within the end hole  24  allows a fastener  32  to be positioned through the plate  22  at various angles with respect to an axis that is perpendicular to the plate. FIGS. 3 and 4 show angle A for particular fastener configurations. The angle A is defined between the longitudinal axis  50  of the fastener  32  and imaginary axis  52  that is perpendicular to the plate  22 . The angle A may range from 0 to about 45 degrees, preferably from about 0 to about 30 degrees, and more preferably from 0 to about 0 and 15 degrees. 
     Fasteners  32  may also be set in positions such that the fasteners are non-planar with respect to a latitudinal plane extending through plate  22 . For example, one fastener  32  may be positioned out of the page and another fastener  32  may be positioned into the page, as depicted in FIGS. 3 and 4. Fasteners  32  set in diverging or converging directions in the end holes  24  may reduce the possibility of backout. Further, the use of rings  30  to fixedly attach fasteners  32  to plate  22  may inhibit damage to tissue structures by any fasteners that do loosen within a bone, since such fasteners would remain attached to the plate  22 . Fasteners  32  may be placed in uni-cortical positions within a bone since the problem of fastener backout is reduced by having obliquely angulated fasteners in converging or diverging configurations. 
     Ring  30  may at least partially surrounds head  34  of fastener  32  positioned within end hole  24 . A shank  54  of fastener  32  may include threading  56  to allow the fastener to be inserted into a bone when fastener  32  is rotated. As depicted in FIG. 1, fastener head  34  may include a cavity  58  that extends from the top of the head to an inner portion of the head. Cavity  58  may be shaped to receive an end of a tool that inserts or removes the fastener  32  from a bone. The tool end may be in the form of a hex wrench, a star wrench or a screwdriver blade. 
     Inner surface  60  of ring  30  and outer surface  62  of head  34  may have mating tapered surfaces, as depicted in FIG.  3  and FIG.  4 . In one embodiment, the bottom portion of head  34  may be smaller than the upper portion of an unstressed ring  30 , while the upper portion of the head may be larger than the upper portion of the ring. As fastener  32  is inserted into a bone, head  34  applies a radial force to ring  30  which causes the ring to expand within the end hole  24 . Expanding the ring  30  increases the size of gap  36  and may cause the outside surface  46  of the ring to abut against inner surface  46  of the end hole  24 . An interference fit forms between fastener head  34 , ring  30 , and plate  22  in which these elements fit together such that each element obstructs the movement of the other elements. Hoop stress of ring  30  on head  34  fixedly attaches fastener  32  to plate  22 . 
     Ring  30  may be capable of swiveling within a hole  24  so that one portion of ring  30  is adjacent to the upper surface  26  of bone plate  22  while another portion of the ring lies adjacent to the lower surface  28  of the bone plate. In one embodiment, ring  30  may sufficiently thin to reside within end hole  24  without extending beyond the upper or lower surface  26 ,  28  of bone plate  22 . The ring  30  and fastener head  34  remain within end hole  24  so that the spinal plating system  20  may have a minimal profile width. Having rings  30  and the fastener heads  34  which do not extend above the upper surface  26  or below the lower surface  28  of plate  22  may prevent the rings and heads from contacting adjacent tissue structures. In other embodiments, however, fasteners  32  may be capable of being angulated relative to bone plate  22  such that the rings  30  extend from the end holes  24  beyond upper and/or lower surfaces of the bone plate. 
     In one embodiment, the spinal plating system  20  is prepared for surgical implantation by positioning rings  30  within end holes  24 . During the surgical procedure, holes may be drilled and tapped into the bones to which plate  22  is to be attached. Plate  22  may then positioned adjacent to the bones and over the holes in the bone. Fasteners  32  may be placed through a ring  30  and into the bone holes. Each fastener  32  may be obliquely angulated into the plate  22 . The fasteners  32  may be inserted into the bone until the fastener heads  34  expand the rings  30  against the inner surfaces  46  of the holes  24 ; thus fixing the fasteners to the rings, and the rings to the plate  22 . If necessary, a fastener  42  may be positioned in one of the central holes  38 . 
     In one embodiment, ring  30  has an outer width that is less than or about equal to the width of an end hole  24  in bone plate  22  at a location between an upper surface  26  and lower surface  28  of the bone plate. The width of each end hole  24  proximate the upper and lower surfaces  26 ,  28  of bone plate  22  is less than or about equal to an outer width of ring  30 . The width of the ring may inhibit a ring positioned in a hole from falling out of the hole. Prior to surgery, a ring  30  may be positioned within each end hole  24  of bone plate  22 . When seated within hole  24 , ring  30  may be capable of swiveling within the hole, but the ring is inhibited from falling out of the hole because of reduced width of the hole proximate the upper and lower surfaces  26 ,  28  of the plate  22 . A surgeon may use a bone plate  22  having rings  30  positioned within holes  24  prior to surgery. Alternatively, rings  30  may be manually positioned within holes  24  during surgery. 
     Texturing the outer surface  48  of a ring  30  or an inner surface  46  of a hole  24  may further inhibit movement of a fastener  32  with respect to a bone plate  22 . Both surfaces may be textured to more effectively inhibit movement of a fastener  32  with respect to a bone plate  22 . During manufacturing procedures, the outer surface  48  of ring  30  and the inner surface of end hole  24  are formed as relatively smooth surfaces. While the friction between these smooth surfaces tends to be sufficient to maintain fastener  32  in a fixed position with respect to plate  22 ; under stressful conditions ring  30  may rotate within hole  24 . By providing at least one textured surface, the coefficient of friction between hole  24  and ring  30  is increased. The increase in friction between hole  24  and ring  30  may help to inhibit fastener movement relative to plate  22 . 
     Several types of textured surfaces may be used to increase the coefficient of friction between ring  30  and hole  24 . In general, any process that transforms a relatively smooth surface into a textured surface having an increased coefficient of friction may be used. Methods for forming a textured surface include, but are not limited to: sanding, forming grooves within a surface, shot peening processes, electric discharge processes, and embedding of hard particles within a surface. 
     A shot peening process for forming a textured surface is described in U.S. Pat. No. 5,526,664 to Vetter which is incorporated by reference as if set forth herein. In general, a shot peening process involves propelling a stream of hardened balls, typically made of steel, at a relatively high velocity at a surface. To create a pattern upon an area of the surface the stream is typically moved about the surface. The speed by which the stream is moved about the surface determines the type of textured surface formed. 
     An electrical discharge process is based on the principle of removal of portions of a metal surface by spark discharges. Typically a spark is generated between the surface to be treated and an electrode by creating potential differential between the tool and the electrode. The spark produced tends to remove a portion of the surface disposed between the electrode and the surface. Typically, the electrode is relatively small such that only small portions of the surface are removed. By moving the electrode about the surface numerous cavities may be formed within the surface. Typically these cavities are somewhat pyramidal in shape. Various patterns may be formed within the surface depending on how the electrode is positioned during the discharge. Electric discharge machines are well known in the art. A method for forming a frictional surface within a metal surface using an electric discharge process is described in U.S. Pat. No. 4,964,641 to Miesch et al., which is incorporated by reference as if set forth herein. 
     Embedding hardened particles in a surface produces a textured surface. A method for embedding hardened particles in a metal surface is described in U.S. Pat. No. 4,768,787 to Shira, which is incorporated by reference as if set forth herein. The method of Shira involves using a laser or other high-energy source to heat the surface such that the surface melts in selected areas. Just before the molten area re-solidifies, a stream of abrasive particles is directed to the area. In this manner some of the particles tend to become embedded within the molten surface. The particles typically have a number of sharp edges that protrude from the surface after the particles have been embedded within the surface. 
     Any of the above methods of texturing may be used in combination with another method. For example, the inner surface  46  of hole  24  may be textured using a pattern of grooves. The outer surface  48  of ring  30 , however, may be textured using an electrical discharge method. When coupled together the textured surfaces of hole  24  and ring  30  may interact with each other to provide additional resistance to movement of the ring within the hole. 
     FIG. 5 illustrates an embodiment of fastener  100 . The fastener  100  may include fastener head  102 , opening  104 , optional grooves  106 , holes  108 , shank  110  and threads  112 . Opening  104  accepts a drive tool, such as drive tool  114 , which is described below. The opening  104 , grooves  106  and holes  108  accept locking mechanism  116 , as described below. Holes  108  extend from the outer surface  118  of head  102  to the opening  104 . In one embodiment, the outer surface  118  is substantially cylindrical. In another embodiment, the head  102  tapers from a widest portion near the upper surface of the head to a narrowest portion near the shank  110 . 
     FIG. 6 is a side view of the head  102  of an embodiment of fastener  100  showing holes  108  and optional rim  120 . Rim  120  may serve to limit the insertion of fastener  100  into a ring  30  during use. 
     FIG. 7 is a top view of the head  102  of a fastener  100  with optional grooves  106 . FIG. 8 shows an alternate embodiment of fastener  100  having a hexagonal shape opening  104  and no grooves. 
     FIG. 9 illustrates a locking mechanism  116  used with fastener  100 . Locking mechanism  116  includes top  122  with shafts  124  extending downwards and outwards from the top. Prongs  126  are located at ends of shafts  124 . Prongs  126  may be substantially parallel to each other and also may be substantially parallel to the locking mechanism top  122 . The shafts  124  have a spring-like action which allows the shafts  124  to be compressed. The spring-like action also allows the shafts to return to an original configuration when not compressed. 
     FIG. 10 illustrates an embodiment of a ring  130  that may be used in combination with fastener  100  and locking mechanism  116 . Ring  130  includes groove  132 . Groove  132  engages prongs  126  on locking mechanism  116  to secure fastener  100  in ring  130  after insertion. Gap  36  in ring  130  allows the ring to contract during insertion of the ring  130  into an end hole  24  of the bone plate  22 . Gap  36  also allows ring  130  to be expanded by the head  102  of fastener  100  in the ring  130  to abut the ring against the inner surface  46  of the end hole  24 . Abutting the ring  130  against the inner surface  46  of the end hole  24  may fix the position of fastener  32  relative to the bone plate  22 . 
     FIG. 11 illustrates tool  114 . The tool  114  may be used during the insertion and extraction of a fastener  100  and locking mechanism  116 . The insertion/extraction tool  114  includes a shaft  172 . One end of shaft  172  may include a handle  174  for turning the tool during insertion and removal of a fastener  100 . FIG. 11 shows a modified T-handle  174  coupled to the shaft  172 , but any type of handle that allows torque to be applied to the fastener during insertion and removal may be used. At an opposite end of shaft  172  from handle  174  is driver head  176 . The outer surface of driver head  176  may be shaped to complement the shape of opening  104  in the head of the fastener  100 . Driver head  176  may be inserted into the opening  104  of the fastener  100 . The fastener may be inserted in an end hole  24  of a bone plate  22  and into a bone by rotating insertion/extraction tool  114 . Driver head  176  includes cavity  178 . The inner surface of the cavity may slide over and compress the shafts  124  and prongs  126  of a locking mechanism  116 . 
     FIG. 12 a  shows a bottom view of an embodiment of a driver head  176  of an insertion/extraction tool  114 . The driver head of FIG. 12 a  may be used with the type of fastener head  102  shown in FIG.  7 . The driver head  176  has cavity  178  which allows the driver head to slide over and compress a locking mechanism  116 . The driver head  176  includes ridges  180  for engaging complementary grooves  106  in the opening  104  of a fastener head  102 . FIG. 12 b  shows an alternate embodiment of a driver head  176  of an insertion/extraction tool  114 . The driver head of FIG. 12 b  may be used with the type of fastener head  102  shown in FIG.  8 . The driver head  176  has cavity  178  which allows the driver head to slide over and compresses a locking mechanism  116 . The driver head  176  may be hexagonal shaped to mate with opening  104  of a fastener head  102 . 
     FIG. 13 shows a cross sectional view of an embodiment of a fastener  100 , locking mechanism  116 , and insertion/extraction tool  114  during the insertion process. Driver head  176  inserts into opening  104  of fastener head  102 . Shafts  124  of locking mechanism  116  are compressed within cavity  178  of driver head  176 . The compression of shafts  124  causes prongs  126  to retract in holes  108 , which allow fastener head  102  to be inserted into a ring  130  without interference by extended prongs  126 . When insertion/extraction tool  114  is removed, the shafts  124  uncompress, which causes the prongs  126  to extend out of holes  108 . 
     FIG. 14 shows a cross sectional view of an embodiment of a fastener  100 , locking mechanism  116 , and ring  130  after the fastener has been fully inserted in the ring and the insertion/extraction tool  114  has been removed. Shafts  124  of locking mechanism  116  are uncompressed, allowing prongs  126  to extend out of holes  108  in fastener head  102 . Prongs  126  extend into groove  132  on the ring  130 . 
     To remove a threaded fastener  100  from ring  130 , insertion/extraction tool  114  is inserted in the opening  104  in fastener head  102  to compress the shafts  124  of the locking mechanism  116 . Compressing the shafts  124  causes the prongs  126  to retract through holes  108  and removes the connection between the prongs and the ring  130 . The tool  114  may then be rotated to remove the fastener  100  from the bone. 
     After insertion of a fastener  100  and locking mechanism  116  into a bone, if the fastener  100  becomes loose within the bone, fastener backout from the bone plate may be resisted by the locking mechanism-groove connection between locking mechanism  116  and the ring  130 . Thus, even if fastener shank  110  loosens within the bone, the fastener head  102  will tend to remain within ring  130  in the hole  24  of the plate  22 . There may be some freedom of movement in the connection between the prongs  126  and the groove  130  to allow a fastener  100  to back out slightly from a bone after insertion. 
     During the surgical procedure for attaching a bone plate to bones using the devices depicted in FIGS. 5-14, holes may be drilled and tapped into the bones to which the bone plate  22  is to be attached. The bone plate  22  may be positioned adjacent to the bones. Rings  130  may be positioned within each end hole  24  before or during the surgical procedure. A fastener  100 , with a pre-inserted locking mechanism, may be positioned through a ring  130 . An insertion/extraction tool  114  may be inserted in the opening  104  of threaded fastener  100  to compress the locking mechanism  116  within the cavity of the driver head of the tool. Compressing the locking mechanism  116  retracts the prongs  126  of the locking mechanism within the fastener opening  104 . The fastener  100  may then be rotated to insert the fastener  100  into a bone. As the fastener  100  is rotated, fastener head  102  moves into the ring  130 . Movement of head  102  into ring  130  causes the ring to expand against the end hole  24  to fix the fastener  100  relative to the plate  22 . Once the fastener  100  is fully inserted, insertion/extraction tool  114  is removed. Removing the tool  114  causes the locking mechanism  116  to uncompress so that the prongs  126  extend through the holes  108  in the fastener head  102  and engage ring the groove  132  in the ring  130 . Fasteners  100  may be inserted through the remaining end holes  24  and into bone to securely attach the plate  22  to the bones. 
     FIG. 15 illustrates an embodiment of a fastener  200  with fastener head  202  having groove  204 . When a fastener  200  is inserted through a ring  230  positioned in a plate  22 , the groove  204  may engage fingers  232  on ring  230  (the ring shown in FIGS.  19  and  20 ) to secure the fastener  200  within the ring  230 . Fastener  200  may include the head  202  and shank  206  with threading  208 . Head  202  may include opening  210  configured to accept a driving tool. 
     The engagement of a finger  232  of a ring  230  on groove  204  may inhibit fastener  200  from backing out of the ring after insertion of the fastener into the plate  22 . In an embodiment, the outer surface of head  202  is substantially cylindrical. In another embodiment, as shown in FIG. 16, the head  202  may taper. The widest portion of the head  202  may be near the top surface of the head, and the narrowest portion may be near the shank  206 . 
     FIG. 17 illustrates an embodiment of a fastener  200  which has radial slots  212  extending from the outer surface of the head into the opening  210 . The radial slots  212  may allow a portion of head  202  to contract during insertion. The radial slots  212  may also be used to engage a portion of a drive head of an insertion/extraction tool (not shown). 
     FIGS. 15,  16 , and  17  illustrate fastener heads  202  wherein the grooves  204  are rims along top edges of the heads. FIG. 18 illustrates an embodiment of a fastener  200  wherein the groove  204  is located at a position along the side of the fastener head  202 . The groove  204  may be located at any position along the side of the fastener head  202 . When the fastener head  202  is driven through a ring  230 , the interaction of the fastener head, the ring, and the end hole  24  allows fingers  232  of the ring to snap into the groove  214 . The fingers  232  may secure the fastener head  202  to the ring  230 . 
     FIGS. 19 and 20 show perspective views of embodiments of ring  230  that may be used with fasteners having a groove. Ring  230  may include bottom  234 , top  238 , an outer surface  48 , an inner surface  60 , gap  36 , and slots  240  and notches  242 . The slots  240  and notches  240  may form the fingers  232 . Gap  36  may allow ring  230  to contract. Contraction of the ring  230  may facilitate the insertion of the ring into an end hole  24  in a bone plate  22 . Gap  36  may also allow the ring  230  to expand against the end hole  24  when a fastener head  202  passes into the ring. Expansion of the ring against the hole  24  fixes the fastener  200  relative to the bone plate  22 . 
     In some embodiments, outer surface  48  of the ring  230  may be textured to increase the coefficient of friction between ring  230  and the hole  24 . In some embodiments, inner surface  60  of the ring  230  may be tapered to match a tapered head of a fastener  200 . Having tapered surfaces may facilitate the expansion of ring in an end hole  24  during insertion of the fastener into the bone plate system  20 . 
     The shape of the end hole  24  may push the fingers  232  inwards past the edge of the groove  204  of a fastener  200  when the groove is inserted into a ring  230  so that the groove passes an upper edge of the ring slots  240 . The inward positioned fingers  232  may inhibit fastener  200  from backing out of the ring  230  and the hole  24 . When the fastener  200  is inserted into the ring  230 , the fastener head  202  may expand the outside surface  48  of the ring against the inner surface  46  of the end hole  24  to fix the fastener  200  to the ring  230 , and the ring to the plate  22 . 
     FIG. 21 is a perspective view of an embodiment of a ring that may be used with fasteners  200  that have rims  204 , such as the fasteners shown in FIGS. 15-17. Ring  430  may include bottom  432 , top  434 , outer surface  48 , inner surface  60 , gap  36 , notches  436 , and ridges  438 . Notches  436  divide the ring  430  into segments or paddles  440 . Notches  436  and gap  36  may allow ring  430  to contract, facilitating the insertion of the ring into a hole  24  of a bone plate  22 . Notches  436  and gap  36  may also allow ring  430  to expand when a fastener head  202  passes into the ring to fix the position of the fastener relative to the bone plate  22 . Notches  436  may also allow paddles  436  to bend outwards during insertion of a fastener  200 . The outer surface  48  and/or the inner surface  60  may be textured. The inner surface of the ring  430  may be tapered to correspond to the taper of a fastener head  202 . 
     FIG. 22 illustrates an embodiment of an insertion tool/extraction tool  450  for use with a fastener  200  and ring  430 . Insertion/extraction tool  450  may include shaft  452 , handle  454 , driver head  456 , and extraction member  458 . At one end of shaft  452  is handle  454  for turning the tool during insertion and removal of a fastener  200 . The illustration shows a T-handle, but any other type of handle that allows sufficient torque to be applied to the fastener  200  to allow for insertion or removal of the fastener may be used. At the opposite end of shaft  452  from handle  454  is driver head  456 . The outer surface of driver head  456  may be shaped complementary to the shape of the opening  210  in the head  202  of the fastener  200 . 
     The extraction member  458  shown in FIG. 22 may include grip  459 , passage  460 , (shown in FIG. 23 c ), tip  462 , and extraction head  464 . The passage  460  extends through the grip  459  and the extraction head  464 . During the insertion process, extraction member  458  may be removed from shaft  452 . To extract a fastener from a ring  430 , extraction member  458  may be slid back on to shaft  452 . Driver head  456  is inserted into the opening  210  of the fastener  200 . Extraction member  458  slides down shaft  452  until tip  462  of extraction head  464  contacts the top of the ring  430 . Downwards pressure on the extraction member  458  forces paddles  440  of the ring  430  outwards, and disengages the ridge  438  on the paddles  440  from the rim  204  on the fastener head  202 . The fastener  200  may then be backed out of the plate  22  by rotating the shaft  452  with the handle  454 . Preferably, rotating shaft  452  does not rotate the extraction member  458 . 
     FIG. 23 a,    23   b,  and  23   c  show partial cross sectional views of a threaded fastener  200 , ring  430 , and insertion/extraction tool  450  during the insertion and extraction processes. Referring to FIG. 23 a,  driver head  456  of insertion/extraction tool  450  is inserted in opening  210  of fastener head  202 . Ring  430  is positioned inside a hole  24  in a bone plate and the bone plate is positioned on a bone (bone and bone plate not shown). Fastener  200  is screwed into the bone until the outer surface of fastener head  202  contacts the surfaces of the paddles  440 . The tapering of the outer surface of fastener head  202  provides a ramping force on the surfaces of the paddles  440 , to bend the paddles outwards as fastener  200  is screwed farther into the bone. 
     In FIG. 23 b,  fastener  200  has been screwed in to the desired depth. Fastener head  202  penetrates ring  430  far enough to allow ridges  438  to snap onto rim  204  on fastener head  202 . Driver head  456  of insertion/extraction tool  450  is shown still inserted in opening  210  prior to removal from the opening. After insertion, if the fastener  200  becomes loose within the bone, fastener backout from the bone plate may be resisted by the ridge-rim connection between fastener head  202  and ring  430 . Thus, even if the fastener shank loosens within the bone, the fastener head  202  will tend to remain within ring  430  in the hole  24  of the plate  22  so as not to protrude from the plate into surrounding body tissue. In some embodiments, there may be some freedom of movement in the connection between the ridges  438  on the paddles  440  and the rim  204  to allow a fastener  200  to back out slightly from a bone after insertion. Typically, the freedom of movement is limited so that the fastener head  202  may not protrude from the plate  22 . 
     FIG. 23 c  shows insertion/extraction tool  250  being used to remove a fastener  200 . Driver head  256  is inserted in opening  210  of fastener  200 . Extraction head  464  is slid down shaft  452  of insertion/extraction tool  450  until the sloped surface of tip  462  applies a wedging force against the sloped upper surfaces of paddles  440 . The wedging force bends the paddles  440  outwards to disengage the ridges  438  from the rim  204 . Fastener  200  may then be backed out of the bone, the ring  430  and the plate  22 . 
     The plate, fasteners, and locking mechanisms may be made of steel (e.g, stainless steel), titanium, steel alloys or titanium alloys. These materials are generally nontoxic, bio-compatible, strong, and non-corrosive. Other materials that have these properties may also be used. The plate and the rings may be made of a number of bio-compatible materials including metals, plastics, and composites. 
     Any of the embodiments described above may be used individually or in combination with other embodiments described above. Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as examples of embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein without departing from the spirit and scope of the invention as described in the following claims.