PATENT ABSTRACT
A bone plating system invention is shown comprising a bone plate and bone fasteners. The bone plate has a top portion, a bottom portion, and an interior middle portion. The fastener-retaining passageway extends through the plate. The fastener-retaining passageway comprises an upper portion having an inwardly projecting capture lip. The capture lip has a first diameter. The fastener-retaining passageway also has a middle undercut portion that has a second diameter. The second diameter is larger than the first diameter. The plate also has at least one access channel extending through the capture lip so as to communicate with the interior middle portion of the plate. The fastener comprises a shaft, a fastener engager means extending from, disposed on or coupled with the shaft for engaging bone and a head mounted on the shaft having a radially elastic member.

PATENT DESCRIPTION
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    Not applicable.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. The Field of the Invention  
           [0003]    The present invention relates to the design and method of use of a bone plate and fastener implant and instrumentation system for stabilizing multiple bone segments. In one embodiment of this invention the system aligns and maintains adjacent human cervical vertebrae in a selected spatial relationship during spinal fusion of the cervical spine from the anterior aspect of the vertebrae.  
           [0004]    2. Related Technology  
           [0005]    The use of fixation plates and fastener systems for the treatment of spinal disorders for fusion of vertebrae has progressed considerably over the past twenty years. These systems usually include bone fasteners and plate systems that stabilize bone segments. The fasteners typically have a head, a shaft and threads that engage with the bone. The bone fasteners are placed by delivery mechanisms into corresponding openings in the plates and then into the bone itself. The fasteners are  
           [0006]    A common problem associated with the use of such fixation plates is the tendency of the bone fasteners to back out of the plate under the dynamics of human movement. As a result of backout, bone fasteners may loosen and eventually disengage from the bone plate resulting in poor fixation. Potentially, this loosening of the bone fastener at the bone plate interface may cause the fastener to ultimately work itself out of both the plate and the bone from which it was implanted. This problem is particularly of concern in areas such as the spine where a loose fastener may impinge or interfere with sensitive tissues and bone structures.  
           [0007]    Designers of such bone fixation systems have employed various techniques and developed different backout-preventing mechanism in an attempt to overcome the problem of fastener backout. These systems include secondary backout-preventing mechanisms and passive backout-preventing mechanisms. In secondary backout-preventing mechanisms, the bone fastener is first affixed into the bone through an opening in a bone plate. Once the fastener is in place, the secondary backout-preventing mechanism is then activated to secure the fastener to the plate. These secondary backout-preventing mechanisms comprise devices that are activated independently from the mechanism used to place the fastener. These mechanisms include secondary locking screws, locking collars, deformable tabs or other secondary locking devices that hold the bone fasteners in place after deployment within the plate and bone. The secondary backout-preventing mechanisms are typically independently activated in such ways that the mechanism limits the movement of the head of the bone fastener with the plate. This results in the fasteners being restrained by both the plate and the bone, thus lessening the likelihood of fastener backout.  
           [0008]    For example, some designs found in the related art disclose an anterior cervical plating system incorporating an independent locking screw that engages the head of a bone fastener to secure the cervical plate to the vertebra. The locking screw, positioned above the bone fastener after the bone fastener is placed, provides a rigid fixation of the fasteners to the plate.  
           [0009]    Other examples of designs found in the related art of secondary backout-preventing mechanisms include a threaded screw nut for use with a bone fixation system wherein the screw nut is partially insertable into an opening of the fixation plate, from the plate underside, and engages a portion of the bone fastener to thereby secure the bone fastener to the fixation plate after the fastener has been independently placed.  
           [0010]    Further examples of designs for secondary backout-preventing mechanisms found in the related art disclose a bone fixation system wherein the head of the bone fastener is hollow and expandable. After the fixation plate is secured to the underlying bone by the hollow head bone fastener, a setscrew is then advanced into the hollow head of the fastener to radially expand the head and thereby secure the head to the fixation plate.  
           [0011]    The successful use of such secondary backout-preventing mechanisms in the anterior cervical spine is particularly difficult because of the limited operating space available to the surgeon due to anatomic constraints. The above discussed secondary backout-preventing mechanisms require instrumentation to enter the surgical site and activate the backout-preventing mechanism. The instrumentation needed to activate these secondary backout-preventing mechanisms occupies space in the surgical site. In addition, the implementation of these mechanisms can be technically demanding and time consuming. To address the issues related to the limited space available for tools to activate secondary backout-preventing mechanisms and ease of use of the system, fastener and plate systems have been developed that incorporate passive backout-preventing mechanisms. These passive backout-preventing mechanisms are easier to activate since they typically deploy automatically while the surgeon drives the fastener into the opening in the plate and into the bone segment. Usually, no additional steps are required to fix the fastener to the plate. These systems include designs that lock the fastener to the plate by-either passively overcoming interference between the fastener and the plate or activating a passive spring like mechanism in the plate that locks the fastener to the plate.  
           [0012]    For example, a bone fixation system wherein the head of the bone fastener is frustoconical in shape and has a directionally corrugated outer surface, is found in the related art. Wherein each opening in the fixation plate has a complementarily corrugated inner surface and is similarly frustoconical in shape. As the fastener is advanced through the corrugated openings and into the underlying bone, the direction of corrugation in the head and in the plate opening permits the head to be received within the corresponding opening, while inhibiting rotation of the fastener in an opposite direction.  
           [0013]    Other passive mechanisms that are designed to prevent backout include a system in which a split ring is pre-mounted and attached to the plate. The split ring in the plate that retains the fastener to the plate by engaging the split ring with a groove in the fastener head, or the top of the fastener head. As the groove or the top of the fastener head aligns with the split ring, the split ring expands then snaps into the groove or over the top of the fastener, preventing the fastener from backing out.  
           [0014]    Due to the potentially high loads between the plates and the fasteners, the backout-preventing mechanism retaining force need be maximized. While the above described passive backout-preventing mechanisms found in the related art can restrain the fastener to the plate and limit backout, the force required to overcome these mechanisms is typically small, within the magnitude similar to the force needed to drive the fastener into the backout-preventing mechanism. This is because the backout-preventing mechanisms are deformed by the fasteners as the fasteners are driven into the openings in the plate and deformed again in the reverse direction when the fasteners are removed from the openings in the plate. Thus, the force required to remove the fastener from the plate is similar to the force initially used to insert the fastener. Unfortunately, the backout force that the passive backout-preventing mechanisms are capable of restraining may be less than is clinically required for specific high load conditions.  
         SUMMARY OF THE INVENTION  
         [0015]    It is desirable to have bone plating systems that accomplish one or more or a combination of the following features: a system allowing for easy fastener deployment while eliminating backout, retaining structural integrity, allowing fastener angulations, and improving the surgeon feedback when the fastener is deployed in the plate.  
           [0016]    One embodiment of the invention is an assembly comprised of a plate and fasteners sized to secure bone fragments. The plate has retaining passageways into which the fasteners pass. The assembly has a passive backout-preventing mechanism incorporated into the fastener that engages with the retaining passageway in the plate. This causes the fastener to be restrained by the retaining passageway. A secondary unloading mechanism is used to deactivate the backout-preventing mechanism allowing removal of the fastener from the plate.  
           [0017]    The fastener has a radially elastic compressible member on its head. This radially elastic compressible member becomes smaller in diameter as it is compressed radially, and larger in diameter as its radial compression is relaxed. The plate has a chamfer on the top surface of the retaining passageway to facilitate compression of the radially elastic compressible member as the fastener enters the plate.  
           [0018]    Retaining passageways are positioned through the plate in orientations that address specific orthopedic disorders. The functional diameter of the retaining passageway changes from the top of the plate to the bottom of the plate. Near the top of the plate, the functional diameter of the retianing opening is smaller than the uncompressed or relaxed diameter of the fastener head. In the middle portion of the plate, the functional diameter of the retaining passageway transitions to an undercut that is larger than the functional diameter of the retaining passageway near the top of the plate. This provides an area for the head to expand into. Near the bottom of the plate, the functional diameter of the retaining passageway is smaller than that of the functional diameter of the undercut in the middle portion of the plate. This prevents the head of the fastener from passing through the bottom of the plate.  
           [0019]    As the radially elastic compressible fastener head is driven into the opening, it is radially compressed by the chamfer on the top portion of the retaining passageway to a diameter small enough to clear the top of the opening. Because the fastener head is radially elastic- and compressible, it is designed to elastically decompress and expand radially once it is placed in the undercut. When the fastener head is positioned in the undercut portion of the retaining passageway, it expands and its movement is restricted by the geometry of the undercut.  
           [0020]    In a second embodiment, the fastener head has an incorporated retaining ring on its periphery that acts as the radially elastic compressible member. The retaining ring is incorporated into the fastener head and is positioned on the fastener in a circumferential groove that is also incorporated into the fastener head. The retaining ring is radially compressed as the fastener is driven into the opening in the plate. Once the retaining ring enters the undercut of the opening in the plate, it partially relaxes expanding and catching the underside of the undercut. This restrains the fastener from backing out of the plate.  
           [0021]    To remove the fastener from the plate, the retaining ring is radially compressed, by an independent, secondary removal tool, to a smaller diameter size that allows the fastener head to clear the functional diameter of the retaining passageway near the top of the plate. As the retaining ring is compressed, the fastener is removed from the retaining passageway in the plate. The plate has access channels positioned around the periphery of the retaining passageways to facilitate the use of a tool used to remove the restrained fasteners and to facilitate visualization of the locked retaining ring so that the surgeon has visual feedback indicating that the mechanism is activated. These access channels allow space for prongs on the distal end of the removal tool to enter through the top of the plate and engage with the fastener head and radially compress the retaining member while the retaining ring is still positioned in the undercut of the middle portion of the plate.  
           [0022]    In one alternate embodiment of the plate and fastener system, the circumferential groove on the periphery of the fastener head is substantially greater in height than the height of the retaining ring. This allows for a variable angle fastener in which the fastener head is retained from backout by the undercut in the retaining passageway, but is still able to toggle due to the clearance between the retaining ring height and height of the groove on the fastener head. This is to facilitate fastener angulation or toggle relative to the plate for variable angle fasteners. The variable angle fasteners also have variable angular position by having a shaft outside diameter that is smaller than functional diameter of the opening near the bottom of the plate. However to limit the load on the retaining ring, the shaft impinges the opening near the bottom of the plate before the retaining ring contacts the top of the undercut in the plate.  
           [0023]    In a further embodiment of the plate and fastener system, the circumferential groove on the periphery of the fastener head is closer to the height of the retaining ring than it is in the previously described first fastener embodiment. This allows for fasteners that are more fixed in angulation. The fastener toggles less due to the lessened clearance between the retaining ring height and height of the circumferential groove on the fastener head. The fixed fasteners also maintain angular position by having a shaft outside diameter that closely matches the functional diameter of the opening near the bottom of the plate.  
           [0024]    Thus, the type of fastener fixation, fixed or variable angled, can be determined by differences in the diameter of the shaft and the groove in the fastener head, and not differences in the diameters of the plate passageway design. All of the retaining passageways in the plate are similar and can potentially facilitate either a fixed fastener or a variable angle fastener with the only functional difference between the two types of fasteners being the geometry of the head and the shaft. Depending on the clinical situation, the surgeon can determine if a fixed or a variable fastener is required after the plate has been placed, and use the design of fastener that is most clinically appropriate. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    [0025]FIG. 1 is a perspective view of a bone plate and fastener system that shows three fasteners retained in the retaining passageways and three additional retaining passageways without fasteners retained;  
         [0026]    [0026]FIG. 2 is an anterior view of a bone plate and fastener system positioned in a cervical spine for use as a vertebra stabilization plate for spinal joint fusion showing six fasteners engaged with three vertebrae and one contiguous plate;  
         [0027]    [0027]FIG. 3 is a perspective view showing a bone plate and fasteners cut by a cross-sectional view plane that is aligned with the center of the fasteners;  
         [0028]    [0028]FIG. 4A is a cross-sectional view of the bone plate and fasteners shown from the perspective of the cross-sectional view plane of the of FIG. 3;  
         [0029]    [0029]FIG. 4B is a perspective view of the plate;  
         [0030]    [0030]FIG. 4C is a perspective view of an embodiment of the fastener showing wedge shaped slices in the fastener head functioning as the radial elastic member.  
         [0031]    [0031]FIG. 4D is a perspective view of an embodiment of the part of the fastener which couples with a retaining ring (not shown) in its groove to function as a radial elastic member;  
         [0032]    [0032]FIG. 4E is a perspective view of an embodiment of the fastener with an external protruding star shaped drive feature;  
         [0033]    [0033]FIG. 4F is a perspective view of one embodiment of a retaining ring;  
         [0034]    [0034]FIG. 5A is a detailed view of the retaining member from FIG. 3 showing the geometry of a variable angle fastener;  
         [0035]    [0035]FIG. 5B is a detailed view of the retaining member from FIG. 3 showing the geometry associated with that of a representative fixed angle fastener design;  
         [0036]    [0036]FIG. 6A is a close-up view showing the top of the bone plate, one fastener, and a retaining ring embodiment of the retaining member incorporated into the fastener head;  
         [0037]    [0037]FIG. 6B is the same close-up view of FIG. 6A showing, for clarity, the top of the fastener and the retaining ring without the plate;  
         [0038]    [0038]FIG. 7 is an isometric view of the plate and fastener system showing the removal tool positioned to radially compress the retaining ring and the fastener driver tool in place to remove the fastener;  
         [0039]    [0039]FIG. 8 is an isometric detail view of a distal end of one embodiment of the removal tool showing the prongs that radially compress the radially elastic compressive member of the fastener;  
         [0040]    [0040]FIG. 9 is an isometric detail view of the distal end of the removal tool prior to being positioned into the access channels in the retaining passageway in the plate before engagement with the fastener and radial compression of the radially elastic compressive member of the fastener;  
         [0041]    [0041]FIG. 10 is an isometric detail view of the removal tool engaged with the head of the fastener holding the radially elastic compressible member in radial compression while the driver tool is being moved into position to remove the fastener from the plate;  
         [0042]    [0042]FIG. 11A is an isometric detail view of a removal tool prior to engagement with the head of the fastener and radial compression of the retaining ring, the plate is removed from the view for clarity.  
         [0043]    [0043]FIG. 11B is an isometric detail view of a removal tool engaged with the head of the fastener holding the head in radial compression with the plate removed from the view for clarity.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0044]    Depicted in FIGS. 1 through 6B are different embodiments of a plate  100  and fastener  200  system for stabilization of sections of bone. As shown in FIG. 1, the plate  100  comprises a plurality of fastener-retaining passageway  110 , a plate top  130 , a plate bottom  160 , a plate outside periphery  170 , a plate interior portion  180 , and tissue-access openings  120 . The fastener-retaining passageway  110  is configured such that the axial movement of a fastener  200  is restricted-and the angular variability of the fastener  200  is limited when the fastener  200  is placed in the fastener-retaining passageway  110 . The number of fastener-retaining passageways  110 , their orientation and position are dependent upon the clinical indication for the plate  100  and fastener  200  system. As shown in the embodiment depicted in FIG. 1, the plate  100  may have multiple rows of fastener-retaining passageways  110  adapted to retain multiple fasteners  200 , and multiple tissue-access openings  120  adapted to access and visualize tissue through the plate  100 . Also shown in FIG. 1 is a plate holding feature  190 .  
         [0045]    As shown by example in FIG. 2, multiple rows of the fastener-retaining passageways  110  allow for the plate  100  and fastener  200  system to be adaptable for use in spinal vertebrae  10  stabilization. In this example, the plate  100  and fastener  200  system stabilizes the vertebrae  10  in rigid position relative to each other to allow fusion of a section of the cervical spinal column  5 . This specific example of the plate  100  and fastener  200  system, used to stabilize vertebrae  10  for spinal fusion, is only one of its many indications for use. Other indications may require the plate  100  to be configured differently but comprise of the same basic features. For stabilization of complex bone segments such as those found in the cervical spinal column  5 , multiple rows of the fastener-retaining passageways  110  and the fasteners  200  are preferred. However the plate  100  and fastener  200  system may be comprised of as few as one of the fastener-retaining passageways  110  and one fastener  200  if the plate  100  and the fastener  200  systems is used to stabilize a long bone fracture such as a femoral fracture or a tibial fracture. Likewise, the plate  100  and the fastener  200  systems may be comprised of two rows of the fastener-retaining passageways  110  and two rows of fasteners  200  to stabilize two bone sections. The versatility of the design of the plate  100  and the fastener  200  system allows the surgeon to select-the plate  100  that best fits the surgical need.  
         [0046]    As also shown in the embodiment in FIG. 1, the plate  100  may also have a plurality of the tissue-access openings  120 . The tissue-access opening  120  allows the surgeon to visualize the tissue or implant materials placed between, on or near the bone segments being stabilized. The tissue-access opening  120  also provides a place for the surgeon to access the tissue between bone segments to manipulate the tissue, or perform other procedures such as adding bone grafts, bioengineered materials or pharmaceuticals to stimulate bone healing. More than one of the tissue-access openings  120  may be positioned between the fastener-retaining passageways  110 . The shape of the tissue-access opening  120  is dependent on the clinical indication and the surgical instruments used through the opening. The tissue-access opening  120  may also be shaped such that it provides a feature or features to which an instrument (not shown) can be attached to hold onto the plate  100  during plate  100  insertion and manipulation.  
         [0047]    The plate  100  has a plate bottom  160  that is configured to approximate the surface of the bone that is being stabilized. The plate bottom  160  is typically concave in both its long axis and short axis. However, for bone fixation applications involving complex bone morphology, such as stabilization of pelvis fractures or skull bone fractures, the plate bottom  160  may be concave in one axis and convex in the other, or convex in both the long axis and the short axis, or twisted in either axis, or formed into any complex surface required for a specific procedure. For the embodiment shown in FIG. 2 in which the plate  100  is configured to stabilize vertebra in the cervical spinal column  5 , the curvature along the long axis of the plate corresponds to the natural lordotic curvature of the cervical spine while the curvature along the plate&#39;s short axis corresponds to the medial-lateral curvature of a vertebral body. In the case of the plate  100  configured to stabilize a cervical spinal column  5 , the plate  100  is typically formed with both the lordotic curvature and the medial-lateral curvature concaved to address the normal anatomy of the cervical spinal column  5 . However a surgeon can interoperatively bend the plate  100  with special instruments (not shown) to the shape that best fits the patient&#39;s anatomy.  
         [0048]    The embodiment of the plate holding feature  190  shown in FIG. 1 is a female threaded hole  191  that is dimensioned to receive a male threaded holder instrument (not shown). Other ways of holding on the plate with a holding instrument can be incorporated into the design. The plate holding member  190  can also be a protruding male thread that is adapted to receive a female threaded holder. It may also be non-threaded holding member such as an interference fit, a bayonet connection, a radially expanding collet connection or connection mechanisms commonly know in the art.  
         [0049]    [0049]FIGS. 3 and 4A show a cross-sectional view plane  600  cutting through the plate  100  and fastener  200  interfaces. The cross-sectional view plane  600  is for visualization purposes to show the details of the interface. FIG. 3 is a perspective view showing the plate  100  and two of the fasteners  200  cut by the cross-sectional view plane  600  that is aligned with the center of the fasteners  200 . FIG. 4A is a view of the plate  100  and the fastener  200  shown from the perspective of the cross-sectional view plane  600  of FIG. 3.  
         [0050]    As shown in FIG. 4A, the fastener  200  has a fastener proximal end  260  with a fastener head  250  having a fastener head diameter  253 , a fastener head topside  252 , a fastener head underside  251 , a fastener drive member  221  on the fastener proximal end  260  and a retaining ring  230  incorporated into the fastener head  250 . The fastener  200  also has a fastener shaft  277  extending distally from the fastener head  250 , with a fastener shaft diameter  275 , an elongated fastener engager  270  extending distally from the fastener shaft  277 , and a fastener distal tip  210  that is distal to the fastener engager  270 .  
         [0051]    As shown in FIGS. 3, 4A,  4 B,  5 A and  5 B, the plate  100  has a fastener-retaining passageway  110  that passes through the plate  100  from the plate top  130  through the plate interior portion  180  to through the plate bottom  160 . As shown in FIGS. 4B and 5A, the section of a fastener-retaining passageway  110  that passes near the plate top  130  is a fastener-retaining passageway capture lip  142  with a functional diameter smaller than that of the uncompressed fastener head  250 . The maximum diameter that can be passed through the fastener-retaining passageway capture lip  142  is a functional capture lip diameter  143 . The section of the fastener-retaining passageway  110  that passes through a plate interior portion  180  is a fastener-retaining passageway undercut  140  with a functional undercut diameter  144  that is larger than that of the functional capture lip diameter  143 . The functional undercut diameter  144  is the minimal diameter of the undercut. The section of the fastener-retaining passageway  110  that passes near the plate bottom  160  is the bottom retainer  146  and it has a functional bottom retainer diameter  147  that is smaller than that of the functional undercut diameter  144 . This bottom retainer  146  is the area of the plate  100  that restricts the fastener head  250  from passing through the plate  100 . The smallest functional diameter of the bottom retainer  146  is the functional bottom retainer diameter  147 .  
         [0052]    As shown in FIGS. 4B, 5A and  5 B, and discussed above, the smallest functional diameter of the fastener-retaining passageway  110  is that of the bottom retainer diameter  147 . The next largest functional diameter of the fastener-retaining passageway  110  is that of the capture lip diameter  143 . The largest functional diameter of the fastener-retaining passageway  110  is the functional undercut diameter  144 . Leading into the functional capture lip diameter  143  is a chamfer opening  148 . The chamfer opening  148  tapers in toward the functional capture lip diameter  143  allowing the fastener head  250  to compress as it is driven into the fastener-retaining passageway  110 . Thus the chamfer opening  148  provides a taper that constrains the retaining, ring  230  which elastically deforms as the fastener head  250  passes through the chamfer opening  148  from the top of the plate  130  through the fastener-retaining passageway capture lip  142  and into the functional undercut diameter  143 . The fastener head  250  is partially decompressed as it is retained by the fastener-retaining passageway undercut  140 . The elastic deformation and elastic recovery allows the fastener head  250  to lock into place and prevents the fastener  200  from backing out of the plate  100  after the fastener head  250  is retained by the fastener-retaining passageway undercut  140 .  
         [0053]    The amount that the fastener head  250  elastically recovers relative to the functional undercut diameter  144  is one factor that determines the fit between the plate  100  and the fastener  200 . The greater the elastic recovery, the tighter the fit between the fastener head  250  and the fastener-retaining passageway undercut  140 . To maintain a tight fit between the fastener head  250  and the fastener-retaining passageway undercut  140 , the fastener head  250  is dimensioned to partially elastically decompress. This assures constant contact and friction between the fastener head  250  and the fastener-retaining passageway undercut  140 .  
         [0054]    Other factors that affect the fit between the fastener  200  and the plate  100  include the relative difference between the functional bottom restrainer diameter  147  of the plate  100  and a fastener shaft diameter  275 . The closer that the dimension of the functional bottom restrainer diameter  147  is to the dimension of the fastener shaft diameter  275 , the less toggle between the plate  100  and the fastener  200 .  
         [0055]    The radial elastic compression and recovery of the fastener head  250  is a function of both the material properties of the fastener head  250  and the structural design of the fastener head  250 . The structural design and material properties of the fastener head  250  are variable depending on the radial elastic compression and recovery desired for the fastener head  250 . If the elastic deformation is more a function of the material properties of the fastener head  250 , the fastener head  250  can be fabricated from a biocompatible elastomeric polymer material such as polyurethane, delrin, polypropylene, PEEK or a biocompatible superelastic metallic alloy such as Nitinol. These highly elastic materials allow the fastener head  250  to elastically radially compress past the fastener-retaining passageway capture lip  142  and elastically recover to lock into place in the fastener-retaining passageway undercut  140 .  
         [0056]    If the fastener  200  is fabricated from a material that is not as highly elastic as those previously discussed, then the fastener head  250  geometry can be altered such that the required elastic radial deformation is still achieved. For example, the fastener head  250  can be designed to allow elastic radial deformation of the fastener head  250  by removing material to increase the bending displacement of the fastener head  250 . Examples of materials that the plate  100  or the fastener  200  are made from include titanium, titanium alloys, cobalt-chrome alloys, stainless steel alloys, zirconium alloys, other biocompatible metal materials, biocompatible ceramics, biocompatible composites, and biocompatible polymers. For example, the fastener head  250  can be manufactured in a helical spring or spiral spring fabrication that allows the radial-compression and radial recovery of the fastener head  250 . Or as shown in the embodiment of FIG. 4B, the fastener head  250  can be cut radially into wedge shaped slices  261  to allow each wedge shaped radial slice of the fastener head  250  to bend inward when radially compressed and elastically recover outward when the radial compression is removed. The wedge shaped slices  261  are formed by removing material in the shape of a flexion slot  262  between the wedge shaped slices  261 . To reduce stress concentrations at the bottom of the flexion slots  262 , and increase the flexibility of the wedge shaped slices  261 , stress concentration reducing radii  263  are cut in the bottom of the flexion slots  262 .  
         [0057]    In addition to fabricating the fastener head  250  from a highly elastic material or designing the shape of the fastener head  250  such that it allows for radial compression and decompression, the fastener head elastic deformation member  254  can be a combination of both a radially elastic fastener structural design and the fastener head  250  partially or fully fabricated from a highly elastic material. Different portions of the fastener can be fabricated from different materials with elastic properties tailored to the function of a particular fastener feature. For example, the fastener head elastic deformation member  254  can be fabricated from highly elastic materials, while the fastener engager  270  is fabricated from less elastic materials.  
         [0058]    In a second embodiment of the plate  100  and fastener  200  system, a retaining ring  230  is formed on the fastener head  250 . As shown in FIG. 4F, a retaining ring  230  has a retaining ring top  231 , a retaining ring underside  232 , a retaining ring inner diameter  233 , a retaining ring outer diameter  234 , and a retaining ring bottom  235 . To help facilitate radial elastic behavior, the retaining ring  230  has a retaining ring slot  237  with a retaining ring slot width  236  and a retaining ring slot wall  239  on both sides of the retaining ring slot  235 . The larger the retaining ring slot width  236 , the more the retaining ring  230  is able to radially compress before the retaining ring slot walls  239  interfere with each other, restricting further radial compression of the retaining ring  230 . The retaining ring  230  can be fabricated from highly elastic biocompatible metallic materials such as Nitinol or biocompatible polymers including Delrin, high molecular weight polyethylene, PEEK, polysulfone and nylons. It can also be fabricated from traditional orthopedic metallic materials such as titanium, titanium alloys, stainless steel alloys, cobalt chrome alloys and zirconium alloys.  
         [0059]    The fastener  200  has a fastener engager  270  that is adapted for fixation with the bone tissue by gripping onto and engaging the bone sections to be secured by the fastener  200  and plate  100  system. Although a screw type bone engaging member such as that of a engager thread  271  shown in FIG. 4 is the preferred embodiment of the fastener engager  270 , other configurations of the fastener engager  270  such as barbs, press fits, radial expansion fits, multiple lead threads, and combinations of these and other tissue engagement can be configured as the fastener engager  270  and used interchangeably with the fastener engager  270  shown.  
         [0060]    Referring to the embodiment of the fastener  200  shown in FIGS. 4C and 4D, the engager thread  271  that is shown represents one embodiment of the fastener engager  270 . The engager thread  271  has an engager root diameter  272 , an engager outside diameter  273  and an engager thread pitch  276 . The engager thread  271  can be a single lead thread or a multiple lead thread. The particular thread pitch illustrated in FIGS. 4C, 4D and  4 E are multiple lead threads. Multiple lead threads have multiple thread forms over a given thread engager length. This permits the surgeon to deliver the engager thread  271  with fewer turns than a traditional single lead thread. The engager thread  271  also has a fastener distal tip  210  positioned on the distal end of the fastener  200 . Incorporated in the engager thread  271  is a cutting flute  274  that is shaped to displace tissue as the fastener engager  270  is driven into the bone segment. The cutting flute  274  allows the fastener  200  to be driven into place without prior tapping of the thread profile by a thread-taping instrument (not shown). The number of cutting flutes  274  positioned along the circumference of the distal tip  210  that are cut into the distal tip  210  depends on the desired self-cutting ability needed for the particular clinical indication. For example, two cutting flutes  274  are typically needed for starting the engager thread  271  in hard bone, while one cutting flute  274  may be all that is needed to start the engager thread  271  in less hard bone.  
         [0061]    The embodiments of the fastener  200  shown in FIGS. 4A, 4C,  4 D and  4 E also have a fastener drive member  221  positioned on the fastener proximal end  260  that is on the opposite side of the fastener  200  from the fastener distal tip  210 . In the embodiment shown in FIGS. 4A, 4C, and  4 D the fastener non-circular drive member  223  is shown recessed into the fastener head  250 . In these embodiments, the fastener non-circular drive member  223  is configured to accept a similarly shaped driver drive feature  540  on a driver  500 . The driver drive feature  540  is used to drive the fastener  200 . In this embodiment in which the fastener engager  270  is the engager thread  271 , the driver  500  is rotated and screwing the fastener into bone. For other fasteners that use other engagement members such as barbs or radially expanding anchors, the fastener drive member  221  would be designed to accommodate the forces required to engage the fastener engager  270  into the bone and the plate  100 . The particular fastener non-circular drive member  223  shown is a fastener hexagonal drive slot  224 . However, the fastener non-circular drive member  223  slot can be shaped into other non-circular shapes such as a square, torques, star, or triangular.  
         [0062]    As shown in FIG. 4E, the non-circular drive member  223  can also be configured the shape of an external protrusion non-circular drive member  225  and used to drive the fastener  200  into the plate  100  and the engager thread  271  into the bone segments. The external protrusion non-circular drive member  225  shown in FIG. 4E is a five point star drive member  226 . However, the fastener non-circular drive member  223  protrusion can be shaped into other non-circular protrusions such as a hex, square, torques, pentagon, or triangular.  
         [0063]    Referring to FIG. 3 which is a perspective view showing a bone plate and fasteners cut in a cross-section aligned with the center of the fasteners. The fasteners can be angled such that the distal tips  210  point towards each other as shown, or way from each other. They can also be angled in and out of the cross-sectional plane shown. The neutral position of this fastener angle is indicated in FIG. 3 by a symbol α and is dependent on the orientation of a central longitudinal axis of the fastener-retaining passageway  132  relative to a line tangent to the top of the plate  131 . The angular play that the fastener  200  can rotate and toggle relative to the neutral position α is indicated in FIG. 3 by the symbol β. This angle β is three dimensional and conical passing into and out of the plane  600 .  
         [0064]    The configuration of the fastener-retaining passageway  110  in the plate  100  and the fastener head  250  allows for an angular play of β between the fastener  200  and the plate  100 . Once the fastener head  250  is engaged with the plate  100 , the fastener  200  can be oriented in a rotational position independently to any angle included in the angle β. The angular play β between the plate  100  and the fastener  200  is dependent upon the relative difference between the functional bottom retainer diameter  147  and the fastener shaft diameter  275 . The amount of angulation between the long axis of the fastener  200  and an axis through the center of the fastener-retaining passageway  110  is between 0° and 15°. Generally, the more play between the plate  100  and the fastener  200 , the more angular displacement.  
         [0065]    In the first embodiment of the fastener  200  and plate variable angle system shown in FIG. 5A, the angular play β between of the fastener  200  and the plate  100  is structurally limited by the relative spacing between the functional bottom retainer diameter  147  and the fastener shaft diameter  275 . The height of the undercut  140  is such that the retaining ring  230  stays within the undercut  140  regardless of where the fastener  200  is within its angular play of β. Consequently, the retaining ring  230  does not advance past the fastener-retaining passageway-undercut top  141  as the fastener is toggled.  
         [0066]    Similarly, in the second embodiment of the fastener  200  and plate  100  variable angle system shown in FIG. 5B, the fixed fastener system, the fastener  200  and the plate  100  are structurally constrained by the relative spacing between the functional bottom retainer diameter  147  and the fastener shaft diameter  275 . In the second embodiment, the retaining ring  230  is also in radial compression when it is within the fastener-retaining passageway undercut. Consequently, the retaining ring  230  in the second fixed angle embodiment of the fastener  200  does not advance past the fastener-retaining passageway undercut top  141  as the fastener is toggled. Also, because the retaining ring  230  is radially compressed and bias toward radial expansion, the retaining ring  230  is fully engaged in the undercut. This helps to better resist axial backout.  
         [0067]    Referring to FIG. 7, the plate  100  and fastener  200  implant system is shown with the associated instrumentation for removing the fastener  200  from the plate  100  and the bone segments. The driver  500  is shown attached to the fastener  200 . A removal tool  400  is shown attached to the head of the fastener  200 . The driver  500  has a driver handle  510  and a driver body  520  extending therefrom. The periphery of the driver handle  510  is shaped such to accept the surgeon&#39;s hand to facilitate driving and removal of the fastener  200  into and out of the plate  100  and bone. The driver body  520  is elongated to extend through the patients neck to the anterior cervical spine. Extending from the driver body  520  is a driver shaft  530 . The driver shaft  530  may be the same diameter as the driver body  520  or it may be a different diameter. Its diameter is dependent upon the application of use and the surgical site to which the drive is inserted. When adapted for use in minimally invasive surgery with a minimal incision, the driver shaft  530  is typically smaller in diameter that the driver body  520  to allow the minimal space to be occupied by the driver  500 .  
         [0068]    Protruding from the driver shaft  530  is a driver drive feature  540 . The driver drive feature  540  mates with the fastener drive member  221  in the fastener head  250  of the fastener  200 . Hence, the shapes of the fastener drive member  221  and the driver drive feature  540  are similar and sized such that the male portion fits into the female portion. In the embodiments shown in FIGS. 1-7, the fastener head  250  is a female portion and receives the male portion driver  500  by engagement of the driver drive feature  540  in the fastener drive member  221 . However, other embodiments of the driver drive feature  540  are internal female sockets that are designed to accept the male fastener drive member  221 . The geometry of the driver drive feature  540  is similar to that of the fastener drive member  221  on the fastener head  250 , but not necessarily exactly the same in shape. The shape needed to transmit the drive forces across mating surfaces need be present. In the embodiments of the driver drive feature  540  shown in FIGS. 1-7, the driver drive feature  540  is hexagonal shaped in geometry. However, other non-circular geometries such as a D shape, square, slotted circle, triangular, star, pentagon, or any other geometry suitable for transmitting the force or torque necessary to drive the fastener  200  are applicable shapes for the driver drive feature  540  and the fastener drive member  221 .  
         [0069]    The removal tool  400  shown in the embodiment depicted in FIGS. 7 through 11 has a handle  410  on the proximal end, a removal tool body  430  extending from the handle  410 , and a removal tool small diameter shaft  450  extending from the removal tool body  430 . The removal tool small diameter shaft  450  is dimensioned to fit into small incisions to access the plate  100  during the removal of the fastener  200  from the plate  100 . The removal tool small diameter shaft  450  has an internal diameter  451  dimensioned to receive the driver shaft  530  and an outer diameter dimensioned to fit within the fastener removal incision (not shown). Adjacently connected and distal to the small diameter shaft  450  of the removal tool  400  is a prong support  460 . The prong support  460  provides support for a set of prongs  440  that protrude from the prong support  460  in a pattern that is similar to the pattern of the retaining passageway access channels  115  in the plate  100 . In this embodiment, the pattern for the prongs  440  and the retaining passageway access channels  115  are four evenly spaced along the circumference of the retaining passageway  110 . Other patterns such as other multiples of prongs  440  and retaining passageway access channels  115  such as two, three, five, six or more can be incorporated into the design. In this embodiment, the prongs  440  and retaining passageway access channels  115  are approximately evenly spaced. This allows the removal tool  400  to fit through the plate  100  in multiple orientations around the periphery of the retaining passageway  110 . However in other embodiments, the distance between prongs  440  can be non-evenly spaced and the distance between the retaining passageway access channels  115  can be non-evenly spaced. This could result in specified orientations between the removal tool  400  and the plate  100 . Also the number of retaining passageway access channels  115  in the plate  100  may be more than the number of prongs  440  on the removal tool.  
         [0070]    As shown in FIG. 8, the prongs  440  comprise of a prong body  444  protruding from the prong support, a prong internal surface  442  facing the center of the removal tool  400 , a prong external surface  443  facing the outside of the removal tool  400 , and a prong distal tip  445  that faces the distal end of the removal tool  400 . A prong fillet  446  comprises transitional material between the prong support  460  and the prongs  440 . The prong fillet  446  also provides additional stability to the prong  440 .  
         [0071]    A prong lead in chamfer  441  is adjacent to the distal end of the internal surface  442 . The prong lead-in chamfer  441  slopes outwardly from the internal surface  442  to the distal tip  445 .  
         [0072]    The prongs  440  shown in FIGS. 7 through 11B are stationary prongs that do not articulate or move with respect to the prong support  460 . Another embodiment of the removal tool  400  is comprised of prongs that contain kinematic linkages that allow the prongs to move with respect to the prong support  460  in such a way as to radially compress the retaining ring  230  during fastener  200  disengagement from the plate  100 .  
         [0073]    Referring to FIG. 9, the prongs  440  of the removal tool  400  are being positioned into the fastener-retaining passageway access channels  115  in the plate  100 . As shown in FIG. 10, as the prongs  440  are positioned in the fastener-retaining passageway access channels  115 , they engage with the fastener head to radially compress the retaining ring  230 . This is shown more clearly in FIGS. 11A and 11B in which the plate  100  is removed from view for visual clarity. As shown in FIG. 11A, the chamfers  441  on the prongs  440  push against the retaining ring outer diameter  234  as the removal tool  400  is advanced longitudinally towards the fastener  200 . As shown in FIG. 11B, the retaining ring  230  is radially compressed by the chamfers  441  on the prongs  440  until the retaining ring outer diameter  234  is equal to or smaller than the diameter needed to clear the functional capture lip diameter  143 .  
         [0074]    While the present invention has been disclosed in its preferred form, the specific embodiments thereof as disclosed and illustrated herein are not to be considered in a limiting sense, as numerous variations are possible. The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments of the plate and fastener system are to be considered in all respects only as illustrative and not restrictive. No single feature, function, element or property of the disclosed embodiments is essential. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. The following claims define certain combinations and subcombinations that are regarded as novel and non-obvious. Other combinations and subcombinations of features, functions, elements and/or properties may be claimed through amendment of the present claims or presentation of new claims in this or related applications. Such claims, whether they are broader, narrower or equal in scope to the original claims, are also regarded as included within the subject matter of applicant&#39;s invention. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.