Patent Publication Number: US-7909859-B2

Title: Bone plate system and methods

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation-in-part of U.S. patent application Ser. No. 10/973,891, filed Oct. 26, 2004, and titled “Bone Plate System and Methods,” which claims priority to U.S. Provisional Application Ser. No. 60/548,140, filed Feb. 26, 2004, and titled “Bone Plate with Spring Retainers,” the specifications of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to bone plate systems and, more particularly, to a bone plate system including a retention system to retain the bone anchors against back from apertures of the plate, and to devices and methods for implanting and removing a bone plate system. 
     BACKGROUND OF THE INVENTION 
     There are presently many different types of plate and fixture systems for securing two or more bones or bone fragments in relative position so that the bones may fuse or heal, or so that tissue adjacent the bones may heal without disruption from the movement of the secured bones. As used herein, the term bone may refer to a bone, or a bone fragment or portion, and the term may refer to a portion of a bone that is covered with another material, such as the endplates covering the top and bottom surface of a vertebra. These systems have been used to secure spinal vertebrae and, more specifically, cervical vertebrae. 
     Bone plate systems are typically used to assist or direct spinal fusion or vertebral healing procedures. These procedures promote earlier post-operative patient mobility, decrease a need for post-operative collars, decrease the incidence of graft dislodgement if a graft is used, and improve success in correcting spinal deformities. 
     Furthermore, these systems have been found to assist in controlling and/or exerting a loading force applied to the surgical site. As used herein, the term fusion refers to the joining of materials, such as bone or graft material, and the fusion site is the entire region in which fusion may be desired. By applying a compressive load, it has been found that bone heals more optimally and with greater integrity, a principle known as Wolf&#39;s law. 
     A shortcoming with bone plates is the backing out or loosening of the screws. If the screws loosen, the bones are not properly secure and may move relative to each other. This may compromise the ability to achieve optimal bone fusion and bone alignment, or it may lead to loss of graft material, and damage or loss of bone. Furthermore, when the plate is a dynamic or dynamized plate, such that at least some screws may move relative to the plate, these issues may be further compounded or exacerbated by a screw backing out. 
     In order to increase the amount of loading or compressive force, a number of plate designs have been devised. For instance, compression slots have been formed in a plate whereby a screw receiving bore is in the form of a slot with tapered walls, and a screw with a tapered shank is driven against the tapered wall such that a force between the shank and the slot is directed transverse to the shank. Accordingly, that force compresses the screw and the bone to which the screw is connected towards another bone connected to the plate. Another manner for permitting compressive force between joined bones is to utilize a dynamic plate having at least one elongated screw aperture that allows settling of the vertebrae by gravity by allowing at least one secured bone to move slightly relative to the plate. However, heretofore known arrangements of fixed and dynamized apertures in such plates provide less than optimal capacity for controlling the movement and/or compression between more than two levels of secured vertebrae. 
     One example of a known dynamic bone plate is disclosed in U.S. Pat. No. 6,755,833, to Paul et al. The &#39;833 patent describes a bone plate assembly including a bone support plate having a plurality of paired apertures for receiving screws securing the plate to vertebrae and being curved about both a longitudinal and a lateral axis of the plate. For each vertical row of apertures, a single, long flexible band is secured by retainers within a recessed portion or channel that extends longitudinally for substantially the entire extent of the top surface of the plate. The plate is depicted having increasingly elongated slots extending from a first end, and each slot permits translation relative to adjacent vertebrae and relative to the first end. This lack of constraint between a screw and its respective slot allows a moment arm to extend from each slot to the hole at the first end. Accordingly, forces through the curved plate are magnified. As the channel reduces the effective plate thickness, the curved plate is more prone to deforming under these compressive forces and moment arms. The band itself must be secured with the plate by the retainers, which may result in a rough upper or top surface. Most cervical plates are secured to the anterior of the spine and may be felt by a patient along the esophagus. A rough surface, as well as significant plate thickness, can be uncomfortable or problematic for a patient during swallowing. 
     It should also be noted that it is common for the curve of a plate to be adjusted for a particular patient&#39;s anatomy, such as via bending of the plate by a plate bending tool. The ends of the bands of the &#39;833 patent are secured within end apertures so that, if the curve of the plate is decreased, the band ends may protrude from the plate. Alternatively, if the curve of the plate is increased, the band ends may pull out of the end apertures, and the bands or retainers may become loose from each other or the plate. For an increased curve, the bands may be stretched which reduces their ability to shift as desired to permit screw securement and to return to their natural position and which may impair their general integrity. Furthermore, an increase of the curvature of the plate draws the bands closer to and into the holes, reducing the ability to properly seat the screw underneath the band. 
     Another example of a known dynamic bone plate is shown in U.S. Pat. No. 6,533,786 to Needham, et al. The &#39;786 patent discloses a plate with a first pair of holes at a first end of the plate and a series of paired holes or slots extending from the first pair allowing for each vertebrae secured by the plate to translate toward the first plate end. To allow for subsidence between each vertebrae, or between each pair of holes or slots, the slots are increasingly large as they are arrayed away from the first hole pair. More specifically, the plate is fixedly secured to a first vertebra by the first pair of holes, and an adjacent pair of holes or slots is secured with a second, adjacent vertebra which is then permitted to translate toward the first vertebra. A third pair of slots is secured with a third vertebra, adjacent to the second vertebra, and is permitted to translate toward the second vertebra. 
     However, the third vertebra must translate at least as much as the second vertebra, else the second and third vertebrae will separate due to the translation of the second vertebra toward the first vertebra. Thus, the third pair of slots must provide for the translation expected or permitted by the second pair of slots, as well as the expected translation by the third vertebra relative to the second vertebra. Thus, according to this design, each pair of slots must be increasingly longer to permitted the cumulative amount of translation between the vertebra intermediate such pair of slots and the first pair of holes or slots. One problem with such a bone plate configuration is that forces on the plate and screws, such as moment arms from each level of screws to the lowest level, are greater. Another problem is that, though the screws may translate in the slots, this translation is not precisely controlled or predictable, and the greater translation that is required results in a greater ability for graft material to explant due to this lack of control, or be crushed by bearing a greater than desired load. 
     Accordingly, there is a need for improved bone plates, bone plate systems for retarding screw back-out, and improved tools and methods for utilizing bone plate systems. 
     SUMMARY 
     Bone plate systems are provided for securing a plurality of bones or bone fragments in a predetermined relative orientation. Each bone plate system is secured with the bones via bone anchors in the form of screws received into bores in the plate. 
     In accordance with one aspect, the screws are generally inhibited from backing out of the bone plate once secured by screw retainers. In some forms, the retainers include at least two side portions, preferably straight side portions, positioned above the head of a screw when the screw is secured with the bone and with the plate in the bores. This provides at lest two line contacts between the retainer and the screw should the screw attempt to back out from its secured position. 
     In accordance with another aspect, a single retainer is provided for each bore. In this manner, adjustment of the longitudinal curve of the plate about a transverse axis extending orthogonal to the longitudinal axis of the plate can be made by adjusting the curve of the plate through plate portions spaced from the bores, preferably plate portions that extend longitudinally between the bores and which do not have the retainers or bores associated therewith. Accordingly, the location and position of the retainer relative to the plate and its bore are not affected by adjustment of the plate curvature, thus avoiding a potential problem in properly locating the screw in the bore below the retainer. 
     In accordance with another aspect, the retainers are preferably pre-set during the manufacturing and assembly process. This helps to reduce surgeon or surgical technician error in properly installing the retainers, as well as reduce the ability for someone to alter the shape of the retainers themselves. Furthermore, the retainers require no additional steps to be properly positioned relative to the plate and, specifically, the screw openings or bores to limit screw back out. As the screw is driven through the bore and into the bone, the screw head forces the straight portions outward until the screw head has passed between and beyond the straight portions and the retainer in general. The retainer then returns to its original configuration so that the straight portions position themselves over the screw head. 
     As a further aspect, the retainers are preferably permanently secured with the bone plate. For instance, the retainers may have open ends while the bone plate may have recesses positioned closely adjacent and, preferably, contiguous with the bores. The retainers are assembled with the plate so that a portion of each retainer, such as the open end portions, may be received in the plate recesses. A portion of the plate may then be deformed over the open end portion in the plate recess to permanently secure the retainer to the plate. In a preferred form, the retainer end portion is captured in such a manner as to permit adjustment of the retainer&#39;s assembled position. 
     In an aspect, the retainers are received below a top surface of the bone plate. The retainers do not generally protrude beyond the surface of the plate so that they do not interfere with surrounding tissues. As described, the deformed portion of the plate for capturing a portion of the retainers is deformed inward from the top surface of the plate into a recess for the retainer end portion, again minimizing the ability of surface features of the bone plate system to interfere with surrounding tissues. 
     Though the retainers are positioned below the top surface of the plate, clearance may be provided between the retainer and a screw secured with the plate and located below the retainer. In some forms, screws which can assume polyaxial orientations relative to the bore require a certain amount of pivoting within the bore. This pivoting by the screw results in a shifting or pivoting of the screw head top surface so that, when not aligned with a central axis of the bore, a portion of the screw head is positioned closer to the top surface than if the screw is aligned with the bore axis. The retainer accommodates this shifting or pivoting by the screw head by being positioned with a small clearance above where the screw head top surface would be in a secured positioned and aligned with the axis. 
     In a preferred form, the recesses for the retainer open end portion include pocket portions into which free ends of the retainer are positioned. The pocket portions have tab portions of the plate body formed in overlying relation thereabove. These tab portions are bent downwardly into the recess to permanently capture the free ends of the retainer in the pocket portions of the recess. 
     So that the recesses are not along a bore-to-bore or screw-to-screw linear load path when the plate is secured with a plurality of bones, the recesses are preferably positioned laterally of the longitudinal axis of the plate; that is, to a lateral side of the bores from the longitudinally extending direction of the spine to which the plate is secured. Accordingly, stress risers created by recesses and possibly the deformed tabs are positioned outside of the load path. 
     In an additional aspect, a pair of bores is provided with a single recess for the respective retainers for the pair of bores. Specifically, a pair of bores is provided along a single level or tier of the plate for a multi-level or tiered bone plate so that screws located therein secure with a single vertebra or portions thereof. The recess for capturing the retainer ends is provided between the bores, and plate portions are deformed into the recess to capture the ends of both retainers. In a further aspect, the pair of bores may be provided with a single retainer which is secured in the recess and inhibits screw back out for the pair of screws located in the pair of bores. 
     In another aspect, the bone plate systems may include dynamized bone plates for allowing translation of one or more of the bones towards at least one other of the bones. This allows compressive loads along the spine to shift or translate the bones after securement with the bone plate. Compressive loads on healing or fusing bones is believed to promote healing, both in terms of strength and time. 
     In one aspect of the dynamized bone plate system, the plate includes paired bores which are dynamized to permit a screw secured therein and to a bone to translate within the bore as the bone itself translates. Toward this end, the bore is elongated to have straight portions in the direction of the longitudinal axis of the spine. Similarly, the retainers are elongated and generally have side portions, such as straight side portions, extending generally parallel to the bore straight portions and in the direction of the spinal axis. 
     The elongated, or dynamized, retainers include free ends which are captured in the plate recess by deforming a portion of the plate inward, as discussed above. Preferably, the free ends extend in the lateral direction relative to the longitudinal axis of the plate so that the recess of the plate for receiving the ends is not aligned with the linear load path that extends through the longitudinally aligned plate bores. In a more preferred form, the dynamized retainer includes first and second straight and parallel side portions where one of the side portions is shorter to provide a free end for being received in the recesses for capture with the plate. The retainer is configured so that the free end is located towards an end of the elongated bore and towards the end to which the screw head may translate. This minimizes the screw head being positioned adjacent or aligned with an opening between the free ends and with the recess for receiving the free ends, thereby maximizing coverage by the retainer side portions above a screw head located within the bore, the coverage being the amount of the retainer in an interference position to limit screw back out. 
     In another aspect of the bone plate systems, it is preferred that at least one level of bores for securing with a bone is non-dynamized. For instance, it may be desired to generally rigidly determine the relative position for two or more bones or vertebrae, in which case a plate having a level for securing with each bones may be provided where each level is secured without translation permitted. Alternatively, it may be desired to select a bone towards which the other bones are permitted to translate. For example, a two-level plate may be secured with a pair of bones, one of which is selected to translate relative to the plate and towards the other bone. In this example, a dynamized level or tier is provided for the translating bone and a non-dynamized level or tier is provided for the non-translating bone. This allows the greatest amount of control of the bones by the bone plate system, and minimizes undesirable movement of the plate, such as twisting. In some forms, the level or levels provided with non-dynamized bores may include surface features on a bottom side contacting the vertebra, the surface features embedding in or frictionally engaging with the vertebra to minimize the ability of the plate to shift or twist relative to that vertebra. 
     In some forms, the dynamized plate including the dynamized bores may span three or more bones, a level of bores provided for each bone, and control the translation of end bones towards an intermediate bone. Specifically, the plate may have a non-dynamized level secured with a bone that is intermediate at least two other bones, each of the other bones being secured with a dynamized level so that they may translate towards the non-dynamized level, which need not be centered between the other levels. For a three-level plate, this minimizes the amount of translation required of the elongated, dynamized slots to the amount of translation required for the bone to which it is secured. This also serves to reduce moment arms produced by the compression force on the plate through the screws in the non-dynamized, intermediate bore. 
     In another aspect, a tool for determining a plate size for a particular patient is provided. The tool is a sizing caliper having two legs which can be inserted together into a small incision and having a knob rotated to adjust the position of the legs relative to each other for determining a plate size. The sizing caliper may include a portion permitting the legs to be collapsed for withdrawal, and which then returns the legs to the measured position once withdrawn from the implant site so that the measurement is preserved. The sizing caliper may have a point or spike on one leg for securing in a selected position as the other leg is shifted relatively having a dull end or ball, or both legs may have ball shapes thereon for minimizing the risk of damage to surrounding tissues. 
     A further aspect is providing a plate bending tool for adjusting the longitudinal curve of the bone plate to more closely follow a curve of the patient&#39;s spine along the longitudinal axis. The bending tool includes a space between a fulcrum and bending members, and the bone plate is compressed therein for altering the plate curvature. In some forms, the plate may be inserted in a pre-determined position, and the fulcrum and bending members may be selectively positioned for increasing or decreasing the curvature of the plate. In some forms, plate bending tool may include a pair of fulcra, one each for either increasing or decreasing the curvature of the plate. The bending members may include a pair of bending members for increasing the curvature, utilized with a first of the fulcra, and a pair of bending members for decreasing the curvature which are used with the second fulcra. Operation of handles forces bending members and a fulcrum to shift toward each other, with the plate therebetween, for adjusting the plate curvature. 
     A tool for holding and positioning the bone plate system for implantation is provided in another aspect. The plate holding tool may have end tips received within recesses formed in sides and/or a bottom surface of the plate, and have opposed distally positioned portions, above the end tips, for positioning against a top surface of the plate so that the plate is held between the end tips and the opposed portions. The tool may have cooperating portions, such as ratcheting bars, so that the tool may be clamped with the plate. 
     In an aspect of the plate holding tool, the opposed portions positioned against the top surface of the plate may include throughbores for cooperation with other components or tools utilized in the implantation of the bone plate system. In some forms, the plate holder is constructed so that, when the end tips are secured with the plate, the throughbores are aligned with the plate bores. In some forms, the throughbores of the plate holding tool include structure for cooperatively engaging with a guide tool. 
     In an additional aspect, a guide tool is disclosed for use with the plate holding tool so that a drill, tap, or the like, may be used for creating a hole in the bone for receiving the screw. The guide tool has end structure for cooperatively engaging with the plate holder. In some forms, the end structure is generally a ball-shaped socket portion including resiliently deflectable fingers so that the socket portion may be compressed during reception into the throughbores of the plate holder. The guide tool and plate holder may cooperate to provide a pre-determined relative orientation, or may be pivotable so that an angle for the guide tool may be selected. In some forms, an outer sleeve may be provided on the guide tool, and the sleeve may be reciprocated to compress or release the deflectable fingers for engaging or disengaging the guide tool from the plate holder. 
     In some forms of the plate holding tool and guide tool, each tool may include a window or cut-out so that a surgeon has visual access into and through the bone plate bore with the plate holder and guide tool engaged. More specifically, the windows cooperate to allow a surgeon to see the surface of the vertebra in the area at which a screw is to be anchored. 
     In another aspect, a driver for driving screws into the bones for securing the plate therewith is provided. The driver includes a drive shaft with a terminal end thereon for engaging a screw head socket so that rotation of the drive shaft threadably forces the screw into a bone. The driver further may include an outer sleeve with external threads, which the screw head socket has an internally threaded portion. In this manner, the screw head may be threaded onto the sleeve to retain the screw therewith, either during implantation so that the screw does not separate and fall into the surgical field or during explantation when the screw is being removed from the bone. 
     In a further aspect, an extractor tool for use during extraction of the screw is provided. The extractor tool includes a terminal end including structure for forcing the retainer open so that the screw head may pass therethrough during removal. Preferably, the extractor includes a central cannula through which the driver is inserted for engaging and removing a screw. In some forms, the extractor may be frictionally around the driver during use, while in other forms the extractor may have a handle for manual control and manipulation. 
     In further aspects, methods of implanting bone plate systems are disclosed. For the plate, these methods may include selecting the numbers of levels on a plate, selecting the number of dynamized levels thereof, selecting the length of the plate such as with a sizing caliper, and adjusting the curvature of the plate such as with a plate bending tool. These methods may include selecting between screw tips such as self-tapping or self-drilling, and selecting among fixed or polyaxial screws. These methods may include preparing the implant site by holding the plate with a plate holder in a desired position so that the plate may be used as a template, by engaging a guide tool with the plate holder in a desired angle, and by directing a drill or tap or the like through the guide tool and plate holder to prepare a hole for receiving a screw. A driver may be used with the methods to engage and hold a screw thereon, the driver then driving the screw through a retainer located in the plate bore for preventing screw back-out. Should withdrawal of the screw be desired, the methods may include the use of an tool, such as an extractor tool cooperating with the driver, to open the retainer to permit the screw to be removed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of a bone plate system including features in accordance with the present invention and securing vertebrae in a particular orientation; 
         FIG. 2  is a perspective view of the bone plate of  FIG. 1 ; 
         FIG. 3  is a plan view of the bone plate of  FIG. 2 ; 
         FIG. 4  is a cross-sectional view of the bone plate system of  FIG. 1  taken along the line  4 - 4 ; 
         FIGS. 5A and 5B  are partial cross-sectional views of the bone plate with and without a retainer, respectively, located within a bore of the bone plate; 
         FIG. 6  is a plan view of a retainer; 
         FIG. 7  is a side elevational of the retainer of  FIG. 6  taken along the line  7 - 7 ; 
         FIG. 8  is a plan view of another retainer; 
         FIG. 9  is a cross-sectional view of the bone plate system and a screw indicating insertion into the bone plate and a bone to which the bone plate system is to be secured; 
         FIG. 10  is a cross-sectional view of the bone plate system and screw of  FIG. 9  with the screw partially inserted into the bone plate and bone; 
         FIG. 11  is a cross-sectional view of the bone plate system and screw of  FIG. 9  with the screw fully inserted into the bone plate and bone; 
         FIG. 12  is a cross-sectional view of another retainer; 
         FIG. 13  is a cross-sectional view of another retainer; 
         FIG. 14  is a plan view of another retainer; 
         FIG. 15  is a plan view of another retainer; 
         FIG. 16  is a plan view of another embodiment of a bore plate system including a bone plate having a bore and a retainer located therein; 
         FIG. 17  is a plan view of another embodiment of a bore plate system including a bone plate having a bore and a retainer located therein; 
         FIG. 18  is a plan view of another embodiment of a bone plate system including a bone plate having a bore and a retainer located proximate thereof; 
         FIG. 19  is a plan view of another embodiment of a bone plate system including a bone plate having bores and a retainer located proximate thereof; 
         FIG. 20  is a cross-sectional view of the bone plate and retainer of  FIG. 19  taken along the line  20 - 20 ; 
         FIG. 21  is a plan view of a further embodiment of a retainer; 
         FIG. 22  is a side elevational view of the retainer of  FIG. 21 ; 
         FIG. 23  is an end view of the retainer of  FIG. 21 ; 
         FIG. 24  is a second plan view of the retainer of  FIG. 21 ; 
         FIG. 25  is a plan view of a further embodiment of a retainer; 
         FIG. 26  is a side elevational view of the retainer of  FIG. 25 ; 
         FIG. 27  is a plan view of a second form of the retainer of  FIG. 25 ; 
         FIG. 28  is a plan view of a further embodiment of a bone plate; 
         FIG. 29  is a side elevational view of the bone plate of  FIG. 28 ; 
         FIG. 30  is a cross-sectional view of the bone plate of  FIG. 28  taken through the line  30 - 30 ; 
         FIG. 31  is an end view of the bone plate of  FIG. 28 ; 
         FIG. 32  is a cross-sectional view of the bone plate of  FIG. 28  taken through the line  32 - 32 ; 
         FIG. 33  is a fragmentary cross-sectional view of the bone plate of  FIG. 29  along the line  33 - 33 ; 
         FIG. 34  is a fragmentary cross-sectional view of a bore of the bone plate of  FIG. 30 ; 
         FIG. 35  is a fragmentary bottom plan view of the bone plate of  FIG. 28 ; 
         FIG. 36  is a fragmentary side elevation view of the bone plate of  FIG. 29 ; 
         FIG. 37  is a plan view of a further embodiment of a bone plate similar to the bone plate of  FIGS. 28-36 ; 
         FIG. 38  is a side elevational view of the bone plate of  FIG. 37 ; 
         FIG. 39  is a cross-sectional view of the bone plate of  FIG. 37  taken through the line  41 - 41 ; 
         FIG. 40  is an end view of the bone plate of  FIG. 37 ; 
         FIG. 41  is a side elevational view of a driver for use with bone plates, retainers, and screws; 
         FIG. 42  is a cross-sectional view of the driver of  FIG. 41 ; 
         FIG. 43  is a fragmentary view in partial cross-section of the driver of  FIG. 41 ; 
         FIG. 44  is a perspective view of a further embodiment of a bone plate; 
         FIG. 45  is a top plan view of the bone plate of  FIG. 44  showing three levels having non-dynamized bores and showing a retainer operating in a single hole and a retainer operating in a pair of holes of one of the levels; 
         FIG. 46  is an end elevational view of the bone plate of  FIG. 44  showing a curvature in the lateral direction; 
         FIG. 47  is a side elevational view of the bone plate of  FIG. 44  showing a curvature in the longitudinal direction; 
         FIG. 48  is a cross-sectional view of the bone plate taken through the line  48 - 48  of  FIG. 45 ; 
         FIG. 49  is a cross-sectional view of the bone plate taken through the line  49 - 49  of  FIG. 46 ; 
         FIG. 50  is a bottom plan view of the bone plate of  FIG. 44 ; 
         FIG. 51  is a top plan view of the retainer of  FIG. 45  for operating in a single hole; 
         FIG. 52  is a top plan view of the retainer of  FIG. 45  for operating in a pair of holes; 
         FIG. 53  is a side elevation view of a bone plate fastener in the form of a self-drilling bone screw; 
         FIG. 54  is a cross-sectional view of the bone screw of  FIG. 53 ; 
         FIG. 55  is a cross-sectional view of a bone plate fastener in the form of a self-tapping bone screw; 
         FIG. 56  is a side elevational view of a sizing tool for measuring portions of a spine; 
         FIG. 57  is a cross-sectional view of the sizing tool taken through the line  57 - 57  of  FIG. 56 ; 
         FIG. 58  is a fragmentary view of a distal end of the sizing tool; 
         FIG. 59  is an exploded view of the sizing tool; 
         FIG. 60  is a perspective view of a bending tool for adjusting the shape of a bone plate in accordance with the present invention; 
         FIG. 61  is a second perspective view of the bending tool; 
         FIG. 62  is a front plan view of the bending tool; 
         FIG. 63  is a perspective view generally of a front side of the bending tool taken from above thereof; 
         FIG. 64  is a perspective view generally of the front side of the bending tool taken from below thereof; 
         FIG. 65  is a rear plan view of the bending tool; 
         FIG. 66  is an exploded view of the bending tool; 
         FIG. 67  is a perspective view of a holding tool for positioning the plate during implantation; 
         FIG. 68  is a fragmentary view of a side of a distal end of the holding tool showing recess for a drill guide for implanting a bone plate; 
         FIG. 69  is a second fragmentary view of a back side of the distal end of the holding tool; 
         FIG. 70  is a front elevational view of the holding tool; 
         FIG. 71  is a third fragmentary view of a front side of the distal end of the holding tool showing the recess formed therein in phantom; 
         FIG. 72  is a side elevation view of a drill guide for directing cutting members; 
         FIG. 73  is a cross-sectional view of the drill guide; 
         FIG. 74  is a side elevational view of an extractor tool for assisting in removal of bone anchors from an implanted bone plate; 
         FIG. 75  is a rear elevational view of the extractor showing retainer-shifting tines; 
         FIG. 76  is a side elevational view of a holding pin for maintaining the position of a bone plate during implantation; 
         FIG. 77  is a side elevational view of an awl for creating a pilot hole for implanting a bone plate, shown without corresponding gripping handle; 
         FIG. 78  is a side elevational view of a drill for opening a hole for implanting a bone plate, shown without corresponding gripping handle; 
         FIG. 79  is a side elevational view of a tap for providing threads in a hole for implanting a bone plate, shown without corresponding gripping handle; 
         FIG. 80  is a partially exploded view of a variable depth drill, shown without corresponding gripping handle; 
         FIG. 81  is a top plan view of a position ratchet of the variable depth drill taken through the line  81 - 81  of  FIG. 80 ; 
         FIG. 82  is a top plan view of a further embodiment of a bone plate system similar to that of  FIG. 45  including a bone plate having retainers secured therewith by deformed tabs and having two pairs of non-dynamized holes; 
         FIG. 83  is a bottom plan view of the bone plate system of  FIG. 82  showing the retainers intersecting with throughbores provided for securing the plate with screws; 
         FIG. 84  is a perspective view of the bone plate system of  FIG. 82  showing the deformed tabs capturing free ends of the retainers; 
         FIG. 85  is a side elevational view of the bone plate system of  FIG. 82  showing recesses for being manipulated by a plate holder and showing a curvature in a longitudinal direction; 
         FIG. 86  is a plan view of the retainers of the bone plate system of  FIG. 82  in an installed configuration and the bone plate in phantom; 
         FIG. 87  is an exploded perspective top view of a bone plate system similar to that of  FIG. 82  including a non-dynamized pair of holes intermediate two pairs of dynamized holes or slots and showing tabs in an undeformed state for providing a recess in which retainers may be received for securing thereto and proximate the holes; 
         FIG. 88  is a bottom view corresponding to  FIG. 87  showing recesses for being manipulated by the plate holder; 
         FIG. 89  is a longitudinal cross-sectional view of a bone plate of the bone plate system of  FIG. 87  showing retainer ends positioned within the recesses defined by the tabs prior to being deformed and showing a curvature in a longitudinal direction; 
         FIG. 90  is a lateral cross-sectional view of the bone plate system of  FIG. 87  showing recesses for receiving the retainers, geometry of the holes for receiving the screws, and a curvature in the lateral direction; 
         FIG. 91  is a plan view of a retainer of the bone plate system of  FIG. 87  for use with a dynamized hole; 
         FIG. 92  is a side elevational view of an alternative bending tool for adjusting the shape of a bone plate by shifting an actuator handle relative to a handle of a body; 
         FIG. 93  is an enlarged perspective view of an upper portion of the bending tool of  FIG. 92  showing a positionable fulcrum forcable towards positionable bending assemblies by the actuator handle; 
         FIG. 94  is an enlarged perspective view of the upper portion of the bending tool similar to the view of  FIG. 93  with the fulcrum removed to show a support block for the fulcrum positioned within a guide of the body; 
         FIG. 95  is a perspective front view of the fulcrum showing a concave surface and a convex surface thereon; 
         FIG. 96  is a rear exploded view of the upper portion of the bending tool of  FIG. 93  showing a positioning assembly for the fulcrum, one of the bending assemblies, and a rail within the guide for the support block; 
         FIG. 97  is a front exploded view corresponding to  FIG. 96  showing the positioning assembly for the fulcrum, one of the bending assemblies, and a rail within guide slots for the bending assemblies; 
         FIG. 98  is a perspective view of an alternative embodiment of a sizing tool similar to that of  FIGS. 56-59  showing a pair of legs having ball-shaped tips; 
         FIG. 99  is a perspective view of an alternative embodiment of a holding tool similar to that of  FIG. 67  showing a cut-out or window for viewing the screw head during implantation; 
         FIG. 100  is a perspective view of a drill guide similar to that of  FIG. 72  showing an engaging end for securing with a holding tool; 
         FIG. 101  is a side elevational view of the engaging end of the drill guide of  FIG. 100  showing a socket portion for forming a ball-and-socket coupling with a holding tool; 
         FIG. 102  is a perspective view of the socket portion showing a window alignable with the window of the holding tool of  FIG. 99  for viewing the screw head during implantation; 
         FIG. 103  is a perspective view of the drill guide of  FIG. 100  showing a bore opening for receiving a driver therein and showing a surface flat for alignment of the window with the holding tool window of  FIG. 99 ; 
         FIG. 104  is a side elevational view of a driver similar to that of  FIG. 41  showing openings in an outer sleeve for promoting cleaning of the driver; 
         FIG. 105  is an exploded perspective view of an extractor tool similar to that of  FIG. 74  showing tines on a distal end for spreading a retainer to a position to permit removal of a screw from an implanted bone plate; and 
         FIG. 106  is a cross-sectional view of the extractor of  FIG. 105  showing a longitudinal bore therethrough for receiving a driver and showing a recess for receiving a friction member for retaining the extractor and driver in rotational engagement during removal of a screw. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     Referring to the FIGURES, exemplary bone plate systems for securing a plurality of bones  12  in a desired orientation or arrangement are illustrated including features in accordance with the present invention. In some forms, the bone plate system is a dynamized plate, or has at least one set of dynamic holes, so that bones  12  may compress and shift toward each other, such as the bone plate system  10  depicted in  FIG. 1 . In other forms, such as exemplified in  FIG. 28 , a bone plate system  700  is illustrated as non-dynamized. It should be noted that a bone plate system may be provided including a bone plate where each bore thereof is dynamic, as is described herein, or non-dynamic, also described herein. In addition, a dynamized bone plate may utilize a combination of dynamic and non-dynamic bores. 
     Referring now to  FIG. 1 , the bone plate system  10  assists in the healing and repair of damaged, fractured, or broken bones  12 . In the illustration of  FIG. 1 , the bones  12  are adjacently located vertebrae of a spine, each spaced by a spinal disc  14 . The bone plate system  10  may also be used, accordingly, to assist in the healing necessary after trauma has been experienced by the spinal disc  14 . For instance, the bone plate system  10  may be utilized for stabilization and securement when adjacent vertebrae are fused, with or without the assistance of a bone graft between the vertebrae. 
     In each of these examples, the bone plate system  10  is used to secure the bones  12  (and any prosthetic or graft) in a desired spatial relationship. Typically, the desired spatial relationship between the bones  12  (vertebrae) is generally vertical, such as the vertebrae would be in a normal, healthy spine when the person is standing. As discussed above, compression or loading of bones promotes healing of the bones or bone fragments and improves the integrity of the fusion therebetween. Particular to some bones in the human anatomy is that the weight of the person, due to gravity, compresses those bones, such as a femur. For spines, the fusion of adjacent vertebrae can similarly benefit from using the weight of the person to compress the adjacent vertebrae. 
     Accordingly, though the bones  12  are secured in a desired spatial relationship, the dynamized bone plate system  10  preferably allows the bones  12  to shift relative to each other. In other words, to capitalize on the compression imparted to the adjacent vertebrae by the weight of a person, the bone plate system  10  is designed to allow the bones  12  to compress in a manner dictated by the bone plate system  10 . 
     The bone plate system  10  includes a bone plate  20  secured to the bones  12  with bone anchors which are, in the preferred form, bone screws  22  each having a head  26  and a threaded shank  28 . The bone screws  22  are preferably polyaxial for being driven into the bones  12  at an angle in relation to the plate desired by the surgeon or dictated by the surgical site. However, fixed angle screws, as described herein, may also be used. 
     In use, a surgeon would secure the bone plate  20  by driving the bone screws  22  through bores  24  formed in the plate  20  for receiving the screws  22  and by driving the shank  28  of the screws  22  into the bones  12 . The plate  20  preferably has a pair of bores  24  forming a tier and being located at each level at which a bone  12  or bone cement is to be secured thereto. As depicted, the plate  20  has an uppermost tier  30 , an intermediary tier  32 , and a lowermost tier  34 , each respectively in general proximity to an uppermost vertebrae  12   a , an intermediary  12   b  vertebrae, and a lowermost vertebrae  12   c , where the plate  20  is utilized for securing the three vertebrae  12   a ,  12   b ,  12   c  in a spatial relationship. Although depicted as three tiers  30 ,  32 ,  34 , any number of tiers could be provided for securing a plurality of bones, bone segments, or implanted materials. 
     In order to permit the above-described compressive shifting of the bones  12  due to the weight of a person, the plate  20  is a dynamized or dynamic plate. In the presently depicted embodiments, the plate  20  allows the bones  12  to compress towards each other by allowing at least a portion of the bone anchors in the form of screws  22  to shift relative to the plate  20  in a manner defined by the plate  20 . To enable this compression, at least some of the bores  24  are dynamized bores  40 . 
     In the embodiments illustrated in  FIGS. 1-3 , the bores  24  of the uppermost and lowermost tiers  30 ,  34  are dynamized bores  40 , while the bores  24  of the intermediary tier  32  are non-dynamized bores  42  such that the non-dynamized bores  42  permit either no or minimal shifting of the screw head  26  and shank  28  within and/or relative to the non-dynamized bore  42 . In this manner, the uppermost bone  12   a  secured by the dynamized bores  40  of the uppermost tier  30  may translate toward the intermediary bone  12   b  secured by the non-dynamized bores  42  of the intermediary tier  32 . Likewise, the lowermost bone  12   c  secured by the dynamized bores  40  of the lowermost tier  34  and the intermediary bone  12   b  secured by the non-dynamized bores  42  of the intermediary tier  32  may translate relatively toward each other, in which case both the uppermost bone  12   a  and intermediary bone  12   b  are jointly translating towards the lowermost bone  12   c . It should be noted that the plate  20  may be equipped with two, or more, tiers  30 ,  32 ,  34  with each tier having non-dynamized bores  42 , or each tier having dynamized bores  40 , or any combination thereof, as desired. For instance, the plate  20  may have four tiers (not shown) including an uppermost tier, a lowermost tier, and superior and inferior intermediary tiers, preferably with one of the intermediary tiers having non-dynamized bores, most preferably the inferior intermediary tier having non-dynamized bores. 
     By orienting the dynamized bores  40  above and below the non-dynamized bores  42 , as depicted and described, the amount of translation by the three bones  12   a ,  12   b ,  12   c  is such that the uppermost and lowermost bones  12   a ,  12   c  translate relatively towards the intermediary bone  12   b  the proper amount. In contrast and by example, if the uppermost and intermediary tiers  30 ,  32  were provided with dynamized bores, the uppermost tier  30  would not only need to shift a distance towards the intermediary tier  32 , but would also need to shift more than the distance that the intermediary tier  32  shifts towards the lowermost tier  34 . In other words, the cumulative translation required for the uppermost tier  30  is minimized in the present arrangement, which improves effectiveness and minimizes explantation or crushing of a fusion graft. Furthermore, the present arrangement creates better fixation between the plate  20  and the bones  12  by reducing respective moment arms between the screws  22  of the intermediary tier  32  and screws  22  of the uppermost and lowermost tiers  30 ,  34 . 
     More specifically, and with reference to  FIG. 5A , the dynamized bore  40  has an interior surface  50  with a racetrack, or oval, shape for receiving the screw head  26  where a lowermost portion  50   a  of the interior surface  50  defines a racetrack-shaped throughbore  52  for receiving the screw shank  28 . As used herein, a racetrack shape refers to a shape with oppositely oriented arcuate portions joined by straight portions. Accordingly, when a screw  22  is located within the dynamized bore  40  and secured in a bone  12 , the screw  22  can shift towards the center (the intermediary tier  32 ) of the plate  20  due to compression of the bones  12 . Specifically, the screw head  26  can translate along the interior surface  50 , and the screw shank  28  can translate within the throughbore  52 , such translation being due to the weight of the erect person exerting a compression force along the spine. The interior surface  50  and throughbore  52  define the path of translation or shifting for the screw  22 . 
     The dynamized and non-dynamized bores  40 ,  42  have generally similar construction, with some notable differences. The non-dynamized bores  42  have an interior surface  60  for receiving the screw head  26 , and the interior surface  60  has a lowermost portion  60   a  defining a throughbore  62  for receiving the screw shank  28 . The interior surface  60  and lowermost portion  60   a  generally correspond to the interior surface  50  and lowermost portion  50   a  of the dynamized bore. However, in contrast, the interior surface  60 , lowermost portion  60   a , and throughbore  62  of the non-dynamized bore  42  are not racetrack-shaped, instead being generally circular, so that the screw  22  received therein is not permitted to shift or translate relative to the plate  20  due to compression force on the spine when the screw is secured to the plate  20  and the bone  12 . Simply stated, the non-dynamized bores  42  and features thereof for receiving a screw  22  are generally circular, while the dynamized bores  40  are elongated from a circle to have arcuate or circular ends with generally straight sections therebetween. Therefore, a lateral cross-section of a bore  24 , whether dynamized or non-dynamized, includes the same general features. 
     A plate  20  with representative pairs of bores  24  and screws  22  located therein is depicted in  FIG. 4 . Each bore  24  has an inner surface  70  and a lowermost portion  70   a  defining a throughbore  72  corresponding to the above described interior surfaces  50 ,  60 , lowermost portions  50   a ,  60   a , and throughbores  52 ,  62  of the dynamized and non-dynamized bores  40 ,  42 . When a screw  22  is secured within the bore  24 , the screw head  26  secures against the inner surface  70 , and the screw shank  28  extends from the throughbore  72  and is secured within the bone  12 . A bridge or neck  27  portion connects the head  26  to the shank  28 . Each portion of the screw  22 , other than helical threads of the shank  28 , is generally circular. 
     The preferred screw  22  is polyaxial so that the screw may be driven through the bore  24  at a selected angle. Accordingly, the screw head  26  is larger than the screw shank  28  and the throughbore  72 , and the screw shank  28  is smaller than the throughbore  72 . The inner surface  70  has a brace surface  74  within which the bridge  27  of the screw  22  is positioned, the bridge  27  being smaller in diameter than the bore  24  within the brace surface  74 . 
     The brace surface  74  transitions to a seat surface  78  at a shoulder position  80 . When the screw  22  is secured in a bone  12 , the screw head  26  rests against the shoulder position  80 , and the screw head  26  has an arcuate profile  82  that is able to seat properly against the shoulder  80  at the selected angle, as is depicted in  FIG. 4 . The neck or bridge  27  is sized relative to the bore  24  within the brace surface  74  to allow the proper amount of angulation for the screw  22  to be pivoted for a selected angle. That is, greater difference in diametral size of the neck  27  and the bore  24  within the brace surface  74  permits greater pivoting extent for the screw  22  located therein. 
     It should be noted that the shoulder  80  may have a number of constructions, such as simply an edge, or may be a chamfer, for instance. The seat surface  78  is preferably larger in diameter than the screw head  26  to provide clearance so that the screw  22  may be mounted in a selected angle. Alternatively, the seat surface  78  and screw head  26  may be spherical with closely matched diameters so that the screw head  26  may polyaxially slide against the seat surface  78  for driving the screw  22  in the bone  12  at the selected angle. 
     As discussed, screw back-out impairs the integrity of the securement of the bone plate system  10  to the bones  12 . Therefore, in order to prevent back-out of the screws  22 , the bone plate system  10  includes anchor retainers  100 , and preferably a retainer  100  is provided for each bore  24 . Above the seat surface  78  is a recess  90  extending around the inner periphery of the inner surface  70 . A portion of the retainers  100  may be secured or located in the recess  90 . The screw head  26  has a height H 1  that is less than a height H 2  of the seat surface  78 . The height differential between H 1  and H 2  allows the screw  22  to pivot a predetermined amount before the screw head top surface  29  interferes with the retainer  100  when it is pivoted. Accordingly, it is preferred that the top surface  29  of the screw  22  is generally positioned below a lower edge  90   a  of the recess  90  when the screw  22  is secured straight through the bore  24 , or is positioned below or generally coincident with the lower edge  90   a  when the screw is secured at a selected pivoted angle. 
     Above the recess  90 , the inner surface  70  has a receiving portion  92  that terminates at a top edge  21  meeting with a top surface  20   a  of the plate  20 . For a non-dynamized bore  42 , each described portion of the inner surface  70  is generally circular in shape. In addition, each described portion of the inner surface  70  for a dynamized bore  40  is generally circular at the arcuate ends E and is straight for the straight sides S (see  FIG. 3 ). 
     Once the screws  22  are located within the bores  24 , at least a portion of each retainer  100  is over top surface  29  of the screw head  26  such that the retainer  100  prevents the screw  22  from backing out through the bore  24 . The retainers  100  are preferably preset in the plate  20  during the assembly process such that a surgeon can handle the plate  20  and retainers  100  as a single unit. However, alternatively, the screws  22  may be driven into the bones  12  to secure the plate  20  thereto, and the retainers  100  may then be inserted to prevent back-out of the screws  22 . As depicted in  FIG. 1 , the plate  20  is shown secured to bones  12  with screws  22 , and the screws  22  are prevented from backing out by the retainers  100 . 
     In order to permit the screws  22  to be driven into the plate  20 , the preset retainers  100  expand while the screw  22  is being driven therethrough.  FIGS. 6-8  depict retainers  100  and, more particularly, depict a retainer  102  for a non-dynamized bore  42  (see  FIG. 6 ) and a retainer  104  for a dynamized bore  40  (see  FIG. 8 ). Each retainer  102 ,  104  has a closed end  110 , an open end  112  opposite the closed end  110 , and straights  114  located between the closed and open ends  110 ,  112 . The retainer  100  is held within the plate  20  by the recess  90 . 
     Prior to the screw  22  being inserted into the plate  20 , the straights  114  of the retainer  100  are positioned in a static position as is depicted in  FIG. 9 . More specifically, the straights  114  extend through the bore  24  to interfere with the path of the screw head  26  to contact the seat surface  78 . As the screw  22  is driven into the bone  12 , the screw head  26  contacts the straights  114 , as is depicted in  FIG. 10 . As the screw  22  continues into the bone  12  and plate  20 , the arcuate profile  82  of the screw head  26  cams against the straights  114  and forces, wedge-like, the straights  114  away from each other and into the recess  90 . Once the screw head  26  passes through and below the retainer  100  and its straights  114 , the retainer  100  generally returns to its static position such as that prior to insertion of the screw, as can be seen in  FIG. 11 . The screw  22  is, as discussed above, seated in the bone  12  and plate  20  such that the top surface  29  is generally below or approximately coincident with the lowermost edge  90   a  of the recess  90  so that the retainer  100  held within the recess  90  is over the top surface  29  of the screw head  26  to prevent screw back out. 
     As mentioned above, the recess  90  secures and holds the preset retainers  100  in the bores  24 . The closed end  110  of the retainer includes two arms  120  joined by an elbow  122  that is slightly arcuate, though preferably with a smaller radius of curvature than the bores  24 . The open end  112  includes two arms  121  each terminating with a leg  116  separated by a gap  118 . Each leg  116  has a straight portion in the form of a foot  117   a ,  117   b , aligned along an axis  117   c  generally orthogonal to an axis  114   a  of the straights  114 . 
     To insert the retainer  100  within the bore  24  of the plate  20 , the open end  112  is compressed so that the legs  116  are brought together and the gap  118  therebetween is reduced or eliminated. The retainers  100  are elastically resilient so that the closed end  110  may bend due to this compression, so that the open end  112  may be compressed and return to its natural shape when released, and so that the retainer  100  may expand and contract as the screw head  28  passes through and beyond the retainer  100 . The closed end  110  is then inserted into the recess  90 , and the open end  112  is then inserted into a tab-shaped recess  96 , as depicted in  FIGS. 5A and 5B . 
     Each bore  40 ,  42  includes the tab-shaped recess  96  extending from the top surface  20   a  of the plate  20  through the receiving portion  92  of the inner surface  70  of the bore  24 , and the tab recess  96  joins with the recess  90 . The tab recess  96  allows the compressed legs  116  to be received in a portion of the recess  90 . In addition, a retainer pilot  98  is provided as a bore for receiving the feet  117  of the retainer  100 . The retainer pilot  98  may be drilled from a lateral side L of the plate  20  so that it is coincident with and through the tab recess  96 . Accordingly, the edges of the retainer pilot  98 , the tab recess  96 , and the recess  90  that are outboard from the bore  24  are preferably aligned and coincident at a surface  99 . The left and right feet  117   a  and  117   b  are inserted in respective portions of the retainer pilot  98   a  and  98   b  for holding and securing the feet  117  therein so that the retainer  110  is secured within the bore  24 . It should be noted that the present embodiments of the retainers  102 ,  104  are described where the recess  90  extends around the entire periphery of the inner surface  70  of the bores  24 . However, it should be clear that much of the receiving portion of the inner surface  70  above such a recess  90  could be eliminated, provided that a receptacle (the recess  90  in the present embodiment) or other structure is provided for receiving or otherwise securing the closed end within the bore  24 , provided that a structure is provided for securing the feet  117  to the plate  20 , and provided there is enough clearance around the straights  114  to permit the straights  114  to be moved clear of the screw head  26  when the screw  22  is inserted into the bore  24 . 
     As discussed, the retainer pilot  98  may be drilled into the lateral side L of the plate  20  and, therefore, has a circular cross-section. Accordingly, the feet  117  of the retainer  100  have a cross-sectional shape so that the feet  117  fit securely within the retainer pilot  98 . That is, the feet  117  should be sized, in cross-section, to slide in and out of the retainer pilot  98  while not having a significant amount of play or looseness so that the retainer  100  rests firmly in positions over the top surface  29  of the screw head  26  for preventing back-out. 
     In one form of the retainer  100 , the entire cross-section is generally circular, as can be seen in  FIG. 7 , so that the cross-section is substantially similar to that of the drilled retainer pilot  98 . However, the retainer  100  need not have a uniform geometry such that the feet  117  and the straights  114  can have varying and/or different cross-sectional shapes. 
     In order to further retard the possibility of screw back-out, the retainer  100  may have an alternative cross-sectional geometry, as depicted in  FIGS. 12 and 13 , provided the feet  117  are properly fitted within the retainer pilot  98 . In  FIG. 12 , a retainer  130  is depicted having straights  132  with arcuate surfaces  134  facing inward towards the opposite straight  132 . The arcuate surfaces  134  allow the arcuate profile  82  of the screw head  26  to wedge the retainer  130  open when the screw  22  is being driven therethrough. The straights  132  further have a generally flat bottom surface  136  that contacts the top surface  29  of a screw head  26  and provides resistance against screw back-out if the normal clearance between the retainer  130  and screw head  26  is breached. 
     Similarly in  FIG. 13 , a retainer  140  is illustrated having straights  142  with a chamfer or cam surface  144  facing inward towards the opposite straight  142 . It should be noted that the entire retainer  140 , not simply the straights  142 , may have the chamfer surface  144 . The chamfers  144  allow the arcuate profile  82  of the screw head  26  to wedge the retainer  140  open when the screw  22  is being driven therethrough. Furthermore, the straights  142  may have a generally flat bottom surface, such as depicted in  FIG. 12 , or may have bottom surface  146  that rises from an inside edge  146   a  to an outside edge  146   b . Again, the bottom surface  146  is generally spaced above the top surface  29  of a screw head  26  when the retainer  140  and screw  22  are fully secured in a bone plate  20 . However, this bottom surface  146  provides further resistance against screw back-out since the displaced screw back-out force will tend not to open the retainer to allow the spring to escape. 
     Additionally, the screw head top surface  29  is preferably flat. A convex shape may promote or assist screw back out as the convex head may force the retainer  100  open. Alternatively, a concave shape may be employed for the top surface  29 , though such may decrease depth provided for a driver recess  26   a  and, therefore, may make the screw head  26  more fragile when being driven. 
     Additional retainer forms having a closed-loop form are provided in  FIGS. 14 and 15 . Closed-loop retainers may be formed as a closed loop, such as by stamping, or may be a single length of material where the two ends are then joined, such as with butt-welding or crimping. Closed-loop retainers, as described herein, provide an additional benefit of more uniform expansion than the retainers with an open end because a greater portion of the spring deflects when a screw  22  is inserted. 
     A closed-loop retainer  150  includes oppositely located concave sections  152 , two straights  154 , and asymmetrical lobes  156  joining the concave sections  152  to the straights  154 . As discussed above, the arcuate profile  82  of the screw head  26  may be driven against the straights  154  to force the retainer  150  open as the screw passes through, and the retainer  150  is resiliently elastic so that the retainer  150  generally returns to its undistorted shape after the screw head  26  has passed therethrough. The concave sections  152  allow the closed loop to be elastically compressed to reduce its overall size or footprint so that it may be inserted and seated within a bore  24 . For this embodiment, the bore  24  may include the recess  90 , as discussed above. Again, the recess  90  is provided to hold the retainer  150  in place, and to do so needs to provide a receptacle or structure for securing the lobes  156 . As depicted, the retainer  150  has a larger longitudinal dimension A than lateral dimension B, such as is used with a dynamized bore  40 . For a non-dynamized bore  42 , the straights  154  would be shorter so that the retainer  150  may be placed within a generally circular recess  90 . 
     As a further alternative, a retainer  160  may be provided having four concave sections  162  to allow for resilient compression of the retainer  160  for insertion, resilient expansion for allowing the screw head  26  to pass therethrough, and resilient contraction after the screw head  26  has passed therethrough so that the retainer rests on the top surface  29  of the screw head  26 . For the closed loop retainers  150 ,  160 , the tab recess  96  is not necessary. 
     It should be noted that, for retainers  150 ,  160  or other square, closed loop retainers used in non-dynamized bores, the bore may be generally square shaped. 
     In another form of the bone plate system  10  illustrated in  FIG. 16 , the plate  20  has a bore  170  and a retainer  172  secured therein for resiliently expanding to permit a screw head  26  to pass therethrough and contracting once the screw head  26  has passed therethrough, and for resting over the top surface  29  of the screw head  26 . The retainer  172  is generally V-shaped with portions  172   a ,  172   b , and is located within a recess  174  formed in the bore  170  in a manner similar to that described above. However, the recess  174  may also be a depression formed in the top surface  20   a  of the plate  20 . 
     The retainer  172  is staked or anchored by an anchor mount  176  located at the apex or bend  178  of the V-shape of the retainer  172 . The anchor mount  176  holds the retainer  170  to the plate  20 , and may be a peg or pin inserted through the plate  20 . 
     Like the previously discussed retainers, the retainer  172  is forced open by the arcuate profile  82  of the screw head  26  as the screw  22  is being driven between the portions  172   a ,  172   b  of the V-shape. Once the screw  22  has passed through, the portion  172   a ,  172   b  generally return to their previous position to rest on the top surface  29  of the screw head  26  and to prevent screw back out. As depicted, the bore  170  is a non-dynamized bore, and the retainer  172  is structured accordingly. For a dynamized bore, the bore  170  would be elongated and the retainer  172  would be structured to complement the dynamized bore as has been discussed above. 
     In another form of the bone plate system  10 , as illustrated in  FIG. 17 , the plate  20  has a bore  180  and a retainer  182  generally performing and being retained in the same manner as the bore  170  and retainer  172 . However, the bore  180  is a dynamized bore, and the retainer  182  is structured in a complementary fashion. The retainer  182  is provided with two sets of opposed zig-zag arms  184 . The screw  22  may be driven between these arms  184  in the same manner as for the straights of the retainers discussed above, or for the portions  172   a  and  172   b  for the V-shaped retainer  172 . Once the screw  22  is driven through, the arms  184  of the retainer  182  generally return to their previous position, as is discussed above. This retainer  182  may also be formed as a closed loop like the retainer shown in  FIG. 14 . 
     Another form of a bone plate system  250 , as illustrated in  FIG. 18 , includes a plate  252  having an uppermost tier  272 , an intermediary tier  274 , and a lowermost tier  276 , each tier having a pair of bores  24  and, in the preferred embodiment, the uppermost and lowermost tiers  272 ,  276  having dynamized bores  240 , and the intermediary tier having non-dynamized bores  242 . The recess  90  discussed above for other embodiments is unnecessary for the plate  252 , and, therefore, the bores  240 ,  242  have a continuous inner surface  280 ,  284  shaped for providing clearance for securing a polyaxial screw  22  at a selected angle (see  FIG. 4 ), or shaped for mating with a non-polyaxial screw. Accordingly, the bores  240  allow a screw  22  located therein to translate relative to the plate  252 , as described above, while the bores  242  do not permit such translation by a screw  22  located therein. 
     The plate  252  further include retainers in the form of a multi-bore retainer  290  which serves to impede screw back-out for more than one bore  240 ,  242  simultaneously. Alternatively, the plate  252  may include retainers in the form of a single-bore retainer  292  for impeding screw back-out for a single bore  24 , such as  240 ,  242 . Each retainer  290 ,  292  is generally a wire or generally straight member with a static position crossing through the path of a screw  22  to be located within one of the bores  240 ,  242 . Accordingly, as a screw  22  is driven into a bore  240 ,  242 , the arcuate profile  82  of the screw head  26  forces the retainer  290 ,  292  to move away from the center of the bore  240 ,  242  so that the screw head  26  may pass therethrough. Once the screw head  26  has passed by the deflected retainer  290 ,  292 , the retainer  290 ,  292 , returns to the static position so as to rest over the top surface  29  of the screw head  26  to prevent back-out thereof. 
     The retainers  290 ,  292  are elastically resilient with minimal or no plastic deformation due to being deflected. That is, the retainers  290 ,  292  elastically deform in order to be deflected from the static position, illustrated in  FIG. 18 , to permit the screw head  26  to pass therethrough. Once the screw head  28  passes by the retainers  290 ,  292 , the elasticity permits the retainers  290 ,  292  to contract or return generally to the static position. 
     The retainers  290 ,  292  may be a single filament wire with any cross-section. Alternatively, the retainers  290 ,  292  may be a multi-filament, wound wire or a biocompatible polymeric material. 
     Preferably, the retainers  290 ,  292  deflect more easily in a direction generally along a top surface  252   a  of the plate  252  than in a direction orthogonal to the top surface  252   a . A force applied by a screw  22  attempting to back out from the bore  240 ,  242  will force the retainers  290 ,  292  outwardly from the plate  252  for the portion proximate to the screw head  26 . A retainer  290 ,  292  of constant cross-section and constant material will deflect laterally due to the screw head  26  passing thereby, and the retainer  290 ,  292  will equally deflect outward from the plate  252  when under equal force. 
     Accordingly, the retainers  290 ,  292  may be provided with a structure so that the retainer  290 ,  292  may deflect elastically an appropriate amount when the screw head  26  passes therethrough and so that the retainer  290 ,  292  resists outward deflection when under stress. For example, the retainer  290 ,  292  may have a first dimension in a lateral direction, represented by arrow Δ, and a greater dimension in the direction orthogonal to the plate top surface  252   a , that is, outwardly from the plate  252 . Accordingly, a greater force is required to deflect the retainer  290 ,  292  outwardly, such as would happen from screw back-out, than is required to deflect the retainer  290 ,  292  to permit passage of a screw head  26 . 
     The retainers  290 ,  292  are generally located at or near the plate top surface  252   a . In one form, the retainers  290 ,  292  may be located on the plate top surface  252   a . However, as being located on the top surface  252   a  may interfere with or abrade living tissues located thereagainst, it is preferred that at least a portion of the retainer  290 ,  292  is located lower than a highest portion of the plate top surface  252   a.    
     In certain forms, the retainer  290 ,  292 , may be entirely positioned within and below the plate top surface  252   a , may be partially positioned within the top surface  252   a , or may be within yet flush with the plate top surface  252   a . Accordingly, the top surface  252   a  provides a excavated or depressed portion or region  252   b  located proximate to the bores  240 ,  242  such that the deflection of the retainer  290 ,  292  is permitted though generally localized by a wall  252   c . In other words, the top surface  252   a  of the plate  252  may have a height in a region  252   d  with respect to the top of the retainers  290 ,  292  such that the top surface  252   a  principally contacts the surrounding living tissues. In order to permit the retainers  290 ,  292  to deflect along but at least partially below the surface  252   a , the depression  252   b  is formed, and the wall  252   c  is formed between depression  252   b  and the region  252   d . The wall  252   c  localizes the deflection of the retainer  290 ,  292  while permitting the entire length of the multi-bore retainer  290  to stretch elastically to permit the screw  22  to be driven into the plate  252 . 
     The retainers  290 ,  292  may be connected to the plate  252  in a variety of manners. For instance, it may be possible to simply glue the retainers  290 ,  292  to the top surface  252   a . However, it is preferred that the retainers  290 ,  292  are connected in a more mechanical manner. 
     One manner for mounting the retainers  290 ,  292  is to provide a port or bore  300  in the plate for each end of the retainers  290 ,  292 . The retainers  290 ,  292  may be fed into the port  300  from the top surface  252   a  to a bottom surface (not shown) and tied or otherwise secured at the bottom surface. The retainers  290 ,  292  may be fed into the port  300  and soldered or welded into place, or crimped therein either by deformation directly at the port  300  or inward from a side of the plate, as represented by the arrow η. Furthermore, the plate  252  may be provided with a physical structure (not shown) located on the top surface  252   a , the retainers  290 ,  292  may be placed in the structure, and the retainers  290 ,  292  may be secured in the structure either by deforming the structure or by adding another securing member, such as a clip or crimp for clamping the retainer  290 ,  292  therein. 
     Referring now to  FIGS. 19 and 20 , a further embodiment of a retainer  400  and bone plate  402  is depicted. As can be seen, the plate  402  has dynamized bores  404  and non-dynamized bores  406 . The number and orientation of the dynamized and non-dynamized bores  404 ,  406  may vary in the manner described for the above-discussed embodiments, and the retainer  400  may be utilized singly or in tandem with another similar retainer in conjunction with one or more bores  404 ,  406 . The retainer  400  may be constructed similarly to the retainers  290 ,  292 , and may be a continuous loop such that the retainer passes over a bore, such as  404 , twice. 
     The retainer  400  is secured to the plate  402  by curved or undulating paths  410 . The paths  410  are inset into the top surface  402   a  of the plate  402  and are undercut at their lowest point  412 , as can be seen in  FIG. 20 . Each path has preferably a first apex  416 , a second apex  418 , and a third apex  420 , such that the retainer  400  strung therein is captured. Alternatively, the ends  430  of the retainer  400  may be captured or secured, as has been discussed above. 
     A further embodiment of an retainer  500 , depicted in  FIGS. 21-24 , may be used with the bone plate  20  similar to the retainer  100 , as described above. More specifically, the retainer  500  may be in a plurality of forms, wherein a first form, depicted in  FIGS. 21-24 , may be used with a dynamized bore  40 . The retainer  500  includes two side portions  514  corresponding to the straights  114  of the retainer  100 . In other forms, the length of the retainer  500  may be increased, such as by increasing the length of side portions  514 , to provide various retainers corresponding to longer dynamized bores, depending on the amount of subsidence desired. Excepting the length, the various forms of the retainer  500  are generally identical in all other respects. Each retainer  500  has a closed end portion  510  and an open end portion  512  opposite the closed end portion  510 . The retainer  500  is held within the plate  20  by the recess  90 , and closed end portion  510  is received directly within the recess  90 . By decreasing the length of retainer  500 , it may be used with a non-dynamized bore, such as bore  42 . 
     Prior to the screw  22  being inserted into the plate  20 , the straights  514  of the retainer  500  are shaped as is depicted and are in a static position. The position within the bore  24  of the retainer  500  is generally that as depicted in  FIG. 9  for the retainer  100 . The side portions  514  extend through the bore  24  to interfere with the path of the screw head  26  to contact the seat surface  78 . As the screw  22  is driven into the bone  12 , the screw head  26  contacts the side portions  514 . As the screw  22  continues into the bone  12  and plate  20 , the arcuate profile  82  of the screw head  26  cams against the side portions  514  and forces the side portions  514  away from each other and into the recess  90 . Once the screw head  26  passes through and below the retainer  500  and its side portions  514 , the retainer  500  generally returns to its static position, such as that prior to insertion of the screw. As discussed above, the screw  22  is seated in the bone  12  and plate  20  so that the retainer  500  is over the top surface  29  of the screw head  26  to prevent screw back out. 
     Like the retainer  100 , the retainer  500  is preset in the bores  24 . The closed end portion  510  of the retainer  500  includes an arcuate segment  520 , though it could include arms  120  joined by the elbow  122  described above for retainer  100 . The arcuate segment  520  provides a greater amount of the retainer  500  being received by the recess  90 , prospectively enhancing stability and securement to the retainer  500  within the plate  20 . 
     The arcuate segment  520  joins the side portions  514 . So that the side portions  514  are positioned to cross the bore  24 , ends  520   a  of the arcuate segment  520  meeting with the side portions  514  are directed such that the arcuate segment  520  curves greater than 180 degrees. In other words, the arcuate segment  520  curves inward so that the side portions  514  have a narrower overall width for crossing the bore  24 . Although the depiction of the retainer  500  presents the side portions  514  as generally straight and generally parallel to each other, the side portions  514  may be curved and/or skew to each other such that the distance between them increases as the side portions  514  extend away from the arcuate segment  520 , and, thus, the retainer  500  may be generally bowed-in. 
     The open end portion  512  includes two arms  521  each terminating with a leg  516  separated by a gap  518 . Each leg  516  has a straight portion in the form of a foot  517   a ,  517   b , aligned along an axis  517   c  generally orthogonal to an axis  500   a  of the retainer  500 . As for the retainer  100 , the retainer  500  may be inserted within the bore  24  by compressing the open end portion  512  to bring the legs  516  together and reduce or eliminate the gap  518 . The closed end portion  510  is then inserted into the recess  90 , and the open end portion  512  is then inserted into a tab-shaped recess  96 , as depicted in  FIGS. 5A and 5B . The retainers  500  are elastically resilient so that the retainer returns to its natural shape when released, and the retainer  500  may cooperate with the screw head  28 . Again, the securement and operation of the retainer  500  is similar to that of the retainer  100 , as described above. 
     In addition, the retainer  500  is depicted as generally circular in cross-section. Similar to the above discussion regarding, e.g.,  FIGS. 12 and 13 , the retainer  500  may have a non-uniform geometry, while the feet  517   a  and  517   b  are to be properly fitted within the retainer pilot  98 . For example, the retainer  500  may have upwardly facing arcuate surfaces corresponding to the arcuate surfaces  134  of retainer  130  in  FIG. 12  to facilitate a cam-wedge action between the screw and the retainer  500  when the screw  22  is being driven therethrough. The retainer  500  may also have downwardly facing flat surface corresponding to the bottom surface  136  of the retainer  130  for providing further resistance against screw back-out. Alternatively, the retainer  500  may have a upward, inwardly facing chamfer surface corresponding to the cam surface  144  of retainer  140  in  FIG. 13 . Again, the retainer may also have a bottom surface corresponding to the bottom surface  146 , rising from an inside edge  146   a  to an outside edge  146   b  to provide further resistance against screw back-out. 
     Referring now to  FIGS. 25-27 , forms of a further embodiment of a retainer  600  are depicted. In  FIGS. 25 and 26 , the retainer  600  is depicted in a first form  601  for use with a non-dynamized, non-dynamized bore  42 , while in  FIG. 27  the retainer  600  is depicted in a second form  602  for use with a dynamized bore  40 . The retainer  600  includes two side portions  614  corresponding to the straights  114  of the retainer  100 . In other forms, such as second form  602 , the length of the retainer  600  may be increased, such as by increasing the length of side portions  614  in the same manner side portions  514  of the retainer  500  may be increased, to provide retainers  600  with various lengths corresponding to dynamized bores providing for varying amounts of permitted subsidence. As with the retainer  500 , the various forms of the retainer  600  are generally identical in all respects other than the lengths. Each retainer  600  has a closed end portion  610  and an open end portion  612  opposite the closed end portion  610 . The retainer  600  is held within the plate  20  by the recess  90 , and closed end portion  610  is received within directly within the recess  90 . 
     In  FIGS. 25-27 , the retainer  600  is depicted in a shape prior to insertion in the recess  90 . When inserted into a plate  20 , the position within the bore  24  of the retainer  600  is generally that as depicted in  FIG. 9  for the retainer  100 . That is, the side portions  614  are compressed so as to be generally parallel and to extend through the bore  24  to interfere with the path of the screw head  26  to contact the seat surface  78 . As the screw  22  is driven into the bone  12 , the screw head  26  contacts the side portions  614 . Similar to that discussed above, the screw  22  continues into the bone  12  and plate  20 , the arcuate profile  82  of the screw head  26  cams against the side portions  614  and forces the side portions  614  away from each other and into the recess  90 . Once the screw head  26  passes through and below the retainer  600 , the side portions  614  return to the generally parallel position, and the screw  22  is seated so that the retainer  600  is over the top surface  29  of the screw head  26  to prevent screw back-out. 
     Like the retainers  100  and  500 , the retainer  600  is preset in the bores  24 . Like for retainer  500 , the closed end portion  610  of the retainer  600  includes an arcuate segment  620  providing a greater amount being received by the recess  90 , prospectively enhancing stability and securement to the retainer  600  within the plate  20 . 
     The arcuate segment  620  joins the side portions  614  such that the retainer  600  is configured similarly to retainer  500 . The arcuate segment  620  curves greater than 180 degrees, that is, curves inward, so that the side portions have a narrower overall width for crossing the bore  24  and for interfering with a seated screw  22 . 
     The retainer  600  has a geometry similar to retainers  100  and  500 . That is, the open end portion  612  has two arms  621  each terminating with a leg  616  separated by a gap  618 . Each leg  616  has a straight portion in the form of a foot  617   a ,  617   b . When positioned within a bore  24 , the feet  617   a ,  617   b  are aligned along an axis  617   c  generally orthogonal to an axis  600   a  of the retainer  600 , and, as described, the straight portions  614  are generally parallel. However, as depicted, the retainer  600  is not inserted and, therefore, the feet  617   a ,  617   b  are not aligned along the axis  617   c . The retainer  600  may be inserted in to the recess  90  in the same manner as described above for retainers  100  and  500 . 
     Though the retainer  600  is depicted as generally circular in cross-section in  FIG. 26 , the retainer  600  may have a non-uniform geometry, as described above for retainer  500 . 
     Referring now to  FIGS. 28-40 , forms of a further bone plate  700  are depicted. More specifically, a bone plate  701  is depicted in  FIGS. 28-36 , and a bone plate  702  is depicted in  FIGS. 37-40 . The bone plates  701  and  702  are generally identical in operation and features. However, the plate  701  is generally planar or flat, while plate  702  is curved, as can best be seen in comparing of  FIG. 31  with  FIG. 40  and in comparing  FIG. 29  with  FIG. 38 . The curvature of the plate  700  will be described in greater detail below. 
     As described above for plate  20 , the plate  700  is provided with representative pairs of bores  24  for receiving screws  22 . Each bore  24  has an inner surface  770  and a lowermost portion  770   a  defining a throughbore  772  corresponding to the above described interior surfaces  50 ,  60 ,  70 , lowermost portions  50   a ,  60   a ,  70   a , and throughbores  52 ,  62 ,  72  of the dynamized and non-dynamized bores  40 ,  42 . Though, that the plate  700  is depicted with non-dynamized bores  42  only, it should be noted that the plate  700  may also have dynamized bores  40 , with a geometry as described above for plate  10 . When a screw  22  is secured within the bore  24 , the screw head  26  secures against the inner surface  770 , and the screw shank  28  extends from the throughbore  772  and is secured within the bone  12 . 
     In one embodiment, the plate is provided with two tiers of paired bores in the form of non-dynamized circular bores at one end and a pair of elongated dynamized bores at the other end. A three tier embodiment having a pair of dynamized bores at each end and non-dynamized bores in the middle may also be provided. In another embodiment, a four tier plate having a pair of dynamized bores sized for about 2 millimeters of movement along the spine axis is followed by circular non-dynamized holes, followed by dynamized holes sized for about 1.25 millimeters of movement, followed by a pair of dynamized bores sized for about 2.5 millimeters of movement. Another possible embodiment consists of a five tier plate with a pair of dynamized bores at the ends sized for 2.5 millimeters of movement, with adjacent pairs of dynamized bores sized for 1.25 millimeters of movement, and a pair of non-dynamized holes in the center. 
     As stated above, the screw  22  is preferably polyaxial and may be driven through the bore  24  at a selected angle. Therefore, the screw head  26  is larger than the screw shank  28  and the throughbore  772 , and the screw shank  28  is smaller than the throughbore  772 . The inner surface  770  includes a brace surface  774  within which the bridge  27  of the screw  22  is positioned, the bridge  27  being smaller in diameter than the bore  24  within the brace surface  774 . 
     As for plate  20 , the brace surface  774  transitions to a seat surface  778  at a shoulder position  780 . When the screw  22  is secured in a bone  12 , the screw head  26  rests against the shoulder position  780 , and the arcuate profile  82  of the screw head  26  seats against the shoulder  780  at the selected angle. The neck or bridge  27  is sized relative to the bore  24  within the brace surface  774  to allow the proper amount of angulation for the screw  22  to be pivoted for a selected angle. 
     As for the shoulder  80 , the shoulder  780  may have a number of constructions, such as simply an edge, or may be a chamfer, or the seat surface  778  and screw head  26  may be spherical with closely matched diameters so that the screw head  26  may polyaxially slide against the seat surface  778  for driving the screw  22  in the bone  12  at the selected angle. 
     As discussed, a retainer is preferably provided for each bore  24 . In the same manner as for plate  20 , the plate  700  may include a recess  790  above the seat surface  778  and extending around the inner periphery of the inner surface  770 . A portion of the retainer may be secured or located in the recess  790 . The screw head  26  has height H 1  (see  FIG. 4 ) that is less than a height H 3  of the seat surface  78 . The height differential between H 1  and H 3  permits the screw  22  to pivot a predetermined amount before the screw head top surface  29  interferes with the retainer when it is pivoted. Accordingly, it is preferred that the top surface  29  of the screw  22  is generally positioned below a lower edge  790   a  of the recess  790  when the screw  22  is secured straight through the bore  24 , or is generally coincident with the lower edge  790   a  when the screw is secured at a selected pivoted angle. 
     Above the recess  790 , the inner surface  770  has a receiving portion  792  that terminates at a top edge  721  meeting with a top surface  700   a  of the plate  700 . For a non-dynamized bore  42 , each described portion of the inner surface  770  is generally circular in shape, while for a dynamized bore  40  the inner surface  770  has straight sides, as described above. 
     As can be seen in  FIGS. 31 and 32 , side edges  800  of the plate  700  are angled inward from the bottom surface  700   b  to the top surface  700   a . This reduces the profile of the plate and the likelihood of flesh becoming irritated from contact with the edges of the plate  700  in the lateral direction. 
     The plate  700  includes a tab shaped recess  796 , similar to tab recess  96  of plate  20 . However, instead of including retainer pilot  98 , the plate  700  may include access ports  798 , as best seen in  FIGS. 28 ,  33 - 35  and  37 . More specifically, the access port  798  may be cut from the bottom surface  700   b  towards the top surface  700   a  to a depth coincident with the recess  790 .  FIG. 33  illustrates a bore  24  of the plate  700  with a portion removed laterally through the recess  790  such that the access port  798  is visible from a top view of the plate  700 . In comparison,  FIG. 28  shows the plate  700  without the portion removed such that the access port  798  is partially obscured. A further comparison with  FIG. 35  shows the bottom side  700   b  of the plate  700  such that the access port  798  is fully illustrated, and an interior top surface  799  of the recess  790  proximal to the tab recess  796  can be seen through the access port  798  (see also  FIG. 34 ). 
     In use, the surgeon may initially locate the bone plate  20  against the bones  12 . The bone plate  20  includes windows  200  which permit viewing of a fusion site, such as a graft in place of a spinal disc  14 , located between the tiers  30 ,  32 ,  34 . The windows  200  are preferably square or diamond shaped and oriented so that corners  202  are aligned with the longitudinal and lateral directions of the bone plate  20 . In this manner, the windows  200  may extend to a height and a width such that the extent of the permitted view therethrough includes a portion directly between the various bores  24 . Alternatively, the window  200  may be an oval so that portions of the window  200  can provide a view located between the bores  24 , or any other shape. More specifically, the windows  200  extend so that ends  13  of the vertebrae  12  can be seen such that a surgeon can directly examine fusion sites at the ends  13 . By providing a window  200  as described, a surgeon may use radiography to view the fusion site without the plate  20  itself obscuring the view. The size of the window  200  is predetermined and is based on the structural necessities of the plate  20 , such as the size, strength and fatigue life of the plate. Although not shown, each window  200  may have a rib or extension spanning the window  200 . The rib may include an additional bore for receiving a screw which may secure to a bone or an implant such as a graft. These same features may, also, be present or utilized with bone plates  252 ,  700 . 
     As a further alternative, a window  200   a  may be provided as is shown with plate  700  in  FIG. 28 . In this form, the window  200   a  has an irregular shape including arcuate portions and straight portions such that the window  200   a  is configured such as to not impede the structural integrity of the plate  700  while further attempting to maximize the view available therethrough to a surgeon. 
     The bottom surface  20   b  of the bone plate  20  ( FIG. 4 ) may include spikes or protrusions (not shown) for securing the bone plate  20  to at least one bone  12 . In the depicted embodiments, the non-dynamized bores  42  are located on the intermediary tier  32 . Therefore, the bone plate  20  is not to shift relative to the middle bone  12   b  with non-dynamized bores  42 . In order to promote this, the protrusions are provided on the bottom surface  20   b  of the bone plate  20  in a region proximal to and contacting the middle bone  12   b . In various embodiments, the spikes or protrusions may be provided between the bone plate  20  and any bone to which the plate  20  is not to shift relatively, that is, any bone secured to the plate  20  with non-dynamized bores  42 . 
     Once the surgeon has positioned the plate  20  over the bones  12 , assisted by the windows  200 , the plate with spikes may be manually pushed or tapped into the bone to secure the plate  20  thereto. Alternatively, the fixing of the plate  20  to a bone with bone anchors, such as screws  22  in the non-dynamized bores  42 , may force the spikes into that bone. These same features may be present or utilized with each of the bone plates described herein. 
     The bone plate of the bone plate system, in its various embodiments, is often secured to bones or bone fragments  12  that have a curved surface facing the bone plates. As can be seen in  FIGS. 2 ,  4 , and  40 , for instance, the plates  20 ,  700  also have a curved or arcuate profile for following such a curved surface of the bone  12 . In addition, spinal vertebrae include characteristic bone protrusions (not shown) or randomly placed incongruities over which the bone plates would commonly be secured. 
     In order to provide greater accommodation for the spinal bone protrusions, the bone plates  20 ,  700 , for instance, have one or more grooves or valleys  20   c , on the bottom surface  20   b ,  700   b  of the respective bone plates  20 ,  700 . It should be noted that the bone plates may include grooves or valleys oriented in any direction along the bottom surface  20   b ,  700   b , for instance, of the plate. In other words, the bone  12  may have a distinctly uneven surface, and the valley  20   c  is provided so that localization of pressure at specific points between the bone  12  and the plate  20 ,  700  is reduced or minimized, thus reducing the likelihood of bone necrosis. In addition, the valleys  20   c  are preferably formed so that the principal points of contact between the plate  20 ,  700  and the bone  112  are controlled to be generally in the region of the bores  24 . Therefore, the positioned plate  20 ,  700  may rest in a balanced position against the bones  12  and may tend to avoid rocking caused by an unbalanced or uneven positioning. 
     Illustrated in  FIGS. 44-52  is a further form of a bone plate system  1000  including a bone plate  1002  and retainers  1004  for retarding the likelihood of backout by a bone screw  22 . The plate  1002 , as depicted, includes bores  24  in the form of non-dynamized bores  1042  similar to bores  42 , the bores  1042  being paired in three transverse tiers  1006   a ,  1006   b ,  1006   c  for securing to three vertebrae  12   a ,  12   b ,  12   c , as is depicted in  FIG. 1  and described above. Although depicted as non-dynamized, one or more of the bores  1042  could alternatively be dynamized, with geometry similar to that described above for plate  10 . Additionally, the plate  1002  may be provided with only two tiers of bores  24 , or more tiers than three. As depicted, the plate  1002  includes windows  200   a  as shown and described for plate  700  in  FIG. 28 . 
     Similarly to plates  20  and  700 , the bores  24  of plate  1002  have an inner surface  1010  and a lowermost portion  1012  defining a throughbore  1014  with a brace surface  1016  corresponding to the above described interior surface  60 , lowermost portion  60   a , throughbore  62 , and  74  of the non-dynamized bores  42 . When inserted into the bores  24  and secured therein, the screw head  26  secures against the inner surface  1010 , and the screw shank  28  extends from the throughbore  1014  for securing within the bone  12 . 
     The cooperation between the screws  22  and the bores  24  may be polyaxial or fixed, as has been described. A polyaxial screw  22  may be inserted into the bone  12  at a desired angle relative to the plate  1002 , and the bridge  27  of the screw  22  is smaller in diameter than the bore  24  within the brace surface  1016 . Alternatively, a fixed screw may be desired, which may include a screw shank  28  having a neck or bridge  27  having a substantially cylindrical shape that closely fits into and against the substantially cylindrical brace surface  1016  of the lowermost portion  1012 . As has been described above for other bone plate embodiments, the brace surface  1016  forms a shoulder  1022  with a seat surface  1024  corresponding to the seat surface  778  and shoulder  780  of plate  700 . 
     With specific reference to  FIGS. 45 ,  51 , and  52 , two similar forms of the retainer  1004  are depicted.  FIG. 51  depicts a preferred somewhat U-shaped retainer  1030  for being located within a single bore  24  to prevent backout of a single screw  22  located in the bore  24 . In comparison,  FIG. 52  illustrates a retainer  1032  for preventing backout of a pair of screws  22  located in adjacent bores  24 , either of the same tier  1006  or spanning across tiers  1006 . In  FIG. 45 , the plate  1002  presents the respective position of each retainer  1030 ,  1032 , though it is expected that the plate  1002  is provided with one form of the retainer  1004  or the other. The retainers  1004  are generally wire-like and have a cross-sectional geometry as described above. 
     As for previously described plates, recesses  1040  for receiving the retainers  1004  and extending around the inner periphery of the inner surface  1010  are included above the seat surfaces  1024 . Preferably, at least a portion of the retainer  1004  is positioned in the recess  1040  at a height greater than a height of the screw head  26  so that the difference between the heights permits the screw  22  to pivot a predetermined amount before the screw head top surface  29  interferes with the retainer  1004 , as has been described above. 
     The inner surface  1010  also has a receiving portion  1042  spanning between the recess  1040  and a top plate edge  1046 . Non-dynamized bores  42  may include a generally circular receiving portion  1042 , whereas the dynamized bores  40  may have generally straight sides, as described above. 
     Each bore  24  of the plate  1002  includes a recess  1050 , similar to the tab-shaped recess  796  described for plate  700 . However, the tab-shaped recesses  796  are oriented generally along the longitudinal axis of the plate  700 . For plate  1002 , recesses for adjacent bores  24  may be joined to form a single recess  1050 . Preferably, the recesses  1050  are laterally oriented so that a pair of bores  24  of the same tier  1006  are provided with the single recess  1050 . Above the recess  1050 , opposed tab walls  1052  are formed extending transverse to the recess  1050  and having a gap  1054  therebetween. 
     The gap  1054  allows the retainers  1004  to be located in the plate  1002 . Retainer  1030  includes a pair of arm portions  1060  and a bridge portion  1062 . In an unconstrained and natural position, the arms  1060  angle outward from the bridge  1062 . The retainer  1030  is located in the recess  1040  so that the bridge  1062  is positioned within an outboard portion  1064  of the recess  1040  and central portions  1060   a  of the arms  1060  extend across the bore  24 . To insert the retainer  1030  in the plate  1002 , terminal portions  1060   b  and the arms  1060  are elastically deflected inward and are passed through the gap  1054  between the tab walls  1052 . Once through the gap  1054 , the arms  1060  shift outward towards their natural position. Accordingly, the bridge  1062  may be positioned in the recess outboard portion  1064 , and the arm terminal portions  1060   b  are positioned below the tab walls  1052 . 
     With the retainers  1030  located in the plate  1002  in this position, the screw  22  may be driven through the retainers  1030  and secured with the plate  1002  and bone  12 . As the screw  22  passes through the retainer  1030 , the screw head  26  contacts the central portions  1060   a , thereby forcing or camming the central portions  1060   a  away from each other to permit the head  26  to pass between the arms  1060 . Once the head  26  is through, the arms  1060  shift back toward each so that at least a portion of the arms  1060  is positioned above the top surface  29  of the screw  22 , thereby being positioned to restrict screw back-out. 
     The retainer  1032  operates is manner similar to that of retainer  1030 . The retainer  1032  includes arms  1070 , two bridges  1072  connected to respective arms  1070 , and a connecting span  1074  between the bridges  1072 . To insert the retainer  1032 , the connecting span  1074  is inserted through the gap  1054  and the arms  1070  are elastically deflected or compressed inward to force the retainer  1032  into the bores  24 . In this manner, at least a portion of each bridge  1072  is located in recesses  1040  of respective bores  24 , and the arms  1070  are located underneath one tab wall  1052  while the span  1074  is located underneath the opposed tab wall  1052 . 
     Screws  22  may be secured with the plate  1002  by driving them through the retainers  1032  located therein. The screw head  26  being advanced into the plate  1002  contacts one of the arms  1070  and a portion of the span  1074 , elastically deflecting the arms  1070  and span portion  1074  outward to permit passing thereby. Once the head  26  has passed through, the arm  1070  and span portion  1074  return inwardly toward each so that at least a portion of the each is positioned above the top surface  29  of the screw  22 , thereby being positioned to restrict screw back-out. Once retainers  1030  and  1032  are inserted into the plate during assembly, the tab walls  1052  are bent down to prevent the retainers  1030  and  1032  from escaping recess  1040 . 
     Each of the bone plates is preferably made of biocompatible materials, such as titanium or titanium alloys, and stainless steels, or made of bioabsorbable materials, or made of a polymer, such as the polymer known as PEEK. In one embodiment, the plate is formed from anodized titanium. 
     As can be seen in FIGS.  4  and  53 - 55 , the screw head  26  includes a drive recess  300 . In the preferred embodiment, the drive recess  300  is a hexagonal recess for receiving a driver  900  (see  FIGS. 41-43 ) with, for example, a standard hex driving end  902 . With specific reference to  FIGS. 54 and 55 , a top portion  300   a  of the drive recess  300  is equipped with internal threads  302  with a large enough diameter that the driving end  902  may be inserted into the drive recess  300  with sufficient clearance of the internal threads  302  such that a sleeve  904  of the driver  900  may be threaded into the threads  302  with the driving end  902  located in the drive recess  300 . 
     The sleeve  904  may be utilized when a surgeon desires to remove the bone screw  22 . For instance, when installing the bone plate, the screws  22  may strip the threading made in the bone  12 . Therefore, the screw  22  must be removed and a new one inserted. In order to extract the screw  22  from the stripped bone portion, the driving end  902  may be inserted into the drive recess  300  and rotated, but the lack of purchase will prevent the driver  900  alone from removing the screw  22 . Therefore, the driving end  902  may be used to hold the screw  22  in a particular position, and the threaded sleeve  904  may be lowered and threaded into the top portion threads  302 , wherein the sleeve  904  may be used to extract the screw  22 . The sleeve  904  may also be used at any time that there may be a concern about the screw  22  becoming disconnected from the driver  900 , such as insuring that the removed screw is not dropped into the surgical site. 
     Turning now to  FIGS. 41-43 , the driver  900  is depicted. The driver  900  includes a handle  910  connected to a driving shaft  912  that terminates in the driving end  902  that is received by the drive recess  300 . Accordingly, turning of the handle  910  when the driving end  902  is seated within the drive recess  300  effects rotation of the screw  22 . The driver  900  also includes the sleeve  904  including external threads  920  at its distal end  922  for threading into the internal threads of the top portion  300   a  of the drive recess  300  of the screw  22 , as described above. The sleeve  904  is positioned closely about the driving shaft  912 . 
     The sleeve  904  and driving shaft  912  may shift relative to each other along the longitudinal axis  901 . That is, the sleeve  904  may be shifted relative to the driving shaft  912  in a distal direction such that the threads  920  of the sleeve  904  may be threaded into the drive recess  300 . When the sleeve  904  is being threaded into or out of the drive recess  300 , the driving end  902  will also be seated in the recess  300 . In order to permit this, the sleeve  904  and driving shaft  912  need to rotate relative to each other so that the driving shaft  912  is held stationary relative to the screw  22 . However, when the screw  22  is being inserted or extracted and the sleeve  904  is threaded into the recess  300 , the sleeve  904  and driving shaft  912  do not rotate relative to each other. Accordingly, the sleeve  904  is held in position around the driving shaft  912  with a bushing, such as a pair of rings  930 ,  932 . The rings  930 ,  932  provide a frictional fit so that the driving shaft  912  and sleeve  904  may be rotated together, and provide for an adjustable position longitudinally and rotationally, as manual force can overcome the friction between the driving shaft  912 , sleeve  904 , and rings  930 ,  932 . The sleeve  904  may further include a grip  918  for manually holding or operating the sleeve  904 . 
     A screw  22  to be implanted first receives the driving end  902  in the recess  300 . Next, the sleeve  904  is advanced to the recess  300 , and then screwed into the threaded portion  300   a . In this manner, the screw  22  is secured to the driver  900 . The screw  22  is then driven into the bone by clockwise rotation. To remove the driver  900 , the sleeve  904  is rotated counter-clockwise while the driving shaft  912  is held stationary. Once the sleeve  904  is freed from the recess  300 , the driving shaft end  902  is removed from the recess  300 . To remove the screw  22 , the steps are simply reversed. 
     As is shown, the region  924  proximal distal end  922  sleeve  904  is tapered. As such, the region  924  is not substantially larger than the threads  920  of the sleeve  904 . However, the driver  900  may further be used to shift the retainer to an open position such that the screw  22  may be extracted. Therefore, the region  924  may alternatively be sized to the diameter of the screw head  26  such the driver  900  may be used to shift the retainer to the open position for extracting the screw  22 . As a further alternative, a separate tool, such as in the form of an extractor  1900  ( FIGS. 74 ,  75 ), may be used to shift the retainer to an open position. 
     The extractor  1900  may be used in conjunction with the driver  900  for removing screws. As shown in  FIGS. 74 and 75 , the extractor  1900  includes a sleeve member  1902  with an elongated cavity  1904  therein, and a handle  1906  angled away from the sleeve member  1902  so that a surgeon may utilize the extractor  1900  without the handle  1906  obstructing the view. At a distal end  1910  of the extractor  1900 , a pair of prongs  1912  extends from the sleeve member  1902 . To remove the screws, the prongs  1912  may be placed within a retainer and then rotated so that each prong  1912  contacts a portion of the retainer to force the retainer to an open position. 
     While holding the retainer open with the extractor  1900 , the driver  900  may be inserted through the extractor cavity  1904 . The driving end  902  of the driver is received in the screw, and the external threads  902  of the driver sleeve  904  may be threaded into the screw head  26  so that the screw  22  may be easily and safely removed. The driver  900  is then rotated or pulled to withdraw the screw  22  from the bone, and is finally removed. 
     A self-drilling screw  1100  is depicted in  FIGS. 53 and 54 , and a self-tapping screw  1120  is depicted in  FIG. 55 . Each screw  1100 ,  1120  has a shank  28  including threads  28   a , a head  26 , and a neck  27  therebetween. As can be seen, the self-drilling screw  1100  includes a tip  1102  that is pointed and a cutting flute  1104  formed in the threads  28   a . Thus, the self-drilling screw  1100  may be placed against a bone  12 , or in a pilot hole of a bone  12 , and advanced by forcibly driving the screw  1100  into the bone  12 . The cutting flute  1104  forms a hole in the bone  12  as the screw  22  is advanced, and the threads  28   a  cut into the bone  12  to form cooperating threads in the bone  12 . Preferably, the self-drilling screws  1100  are provided in a length short enough to prevent accidental driving of these sharper screw tips into sensitive tissues, such as nerves or blood circulatory passages. 
     The self-tapping screw  1120  is provided with a tip  1122  that is rounded and substantially dull. The screw  1120  may be advanced into a pilot hole formed in the bone  12 , and the threads  28   a  form cooperating threads in the bone  12  as the screw  1120  is forcibly driven into the bone  12 . 
     As noted above, the bone screws  22  are preferably polyaxial for being driven into the bones  12  at an angle desired by the surgeon or dictated by the surgical site, or may be formed to provide a specific angle with the plate, these being referred to as fixed. As represented, the self-drilling screw  1100  is polyaxial, and the self-tapping screw  1120  is a fixed screw. More specifically, screw  1100  has a neck  1127   a  with a diameter D 1 , and screw  1120  has a neck  1127   b  with a diameter D 2 . The bone plates are provided with a brace surface surrounding a lower portion of the bore, and the bore lower portion has a specific diametral size or, for a dynamized bore, a transverse size or distance across the bore lower portion. These sizes may be uniform for an individual plate, or different bores may be provided with different sizes. 
     The screw necks  1127   a ,  1127   b  are, when secured in the plate, positioned within the bore lower portion  1012 . For the polyaxial screw  1100 , the diametral size of the bore  1012  is larger than the screw neck diameter D 1 . For the fixed screw  1120 , the diametral size of the bore  1012  is sized large enough only to permit the screw neck  1127   b  with the diameter D 2  to be inserted therein and fit against the brace surface  1016 , in a peg and hole fashion. Thus, the fixed screw  1120  is provided with an angle of insertion. 
     It should also be noted that, in the event the bone is stripped, rescue screws (not shown) may be provided for securing in the bone. A rescue screw is a screw that has a larger thread diameter, or a larger central or minor diameter, or both. The rescue screw is able to gain purchase in the stripped hole, treating the it as if it were merely a pilot hole, by virtue of its larger size. 
     As discussed, a surgeon may select from an array of bone plate systems. The number of bores and tiers in the plate may be chosen based on the number of vertebrae to be spanned. The bores may be selected to be dynamized or non-dynamized depending on the amount of post-implantation compression desired. The screws may be fixed or polyaxial depending on the ability or desire for having a variable or fixed orientation or angle for the screw to be driven. For each of these choices, the surgeon may also select the actual size or dimensions of the plate, bores, and screws, typically determined by examining the vertebral portions to which the plate is to be secured. 
     To determine the plate size for a particular patient, a plate sizer or sizing caliper  1200  is utilized, as is shown in  FIGS. 56-59 . In the present embodiment, the sizing caliper  1200  includes a pair  1202  of legs adjustably positioned to align with points on one or more vertebrae to determine the proper distance for the screws  22  or the proper length of the plate required for the patient&#39;s anatomy. 
     The pair  1202  is composed of a measuring leg  1204  and a reference leg  1206  that has a sharp tip  1208  for being placed on a first desired location point on a vertebra. The location point may be a location for a bone screw  22  to be inserted thereat, or it could be a location defining the extent of one end of the bone plate itself. A pilot hole may be made in the vertebra, and the sharp tip  1208  may be placed in the pilot hole so that the position of the reference leg  1206  is more easily retained in place on the vertebra. 
     The sizing caliper  1200  is then adjusted by rotating a knob  1220 . This rotation causes the measuring leg  1204  to move towards or away from the reference leg  1206  positioned at the first desired location point. The measuring leg  1204  includes a ball-shaped tip  1210  so that it may move across the surface of the vertebra, for instance, with minimal catching on the surface or other tissue attached thereto. The position of the measuring leg  1204  is adjusted until the ball tip  1210  is located at a second location point, which also may be a location for insertion of a bone screw  22  or a location defining the extent of a bone plate end. The measurement taken by the legs  1204 ,  1206  may then be compared directly to a plate, or may be compared to a scale. 
     The sizing caliper  1200  includes an elongate body  1230  with the knob  1220  located at a proximal end  1232  and the legs  1204 ,  1206  generally secured at a distal end  1234 . The legs  1204 ,  1206  are secured together at a pivot point  1240  through which a pin  1242  is inserted to retain the legs  1204 ,  1206  on the body  1230 . When the knob  1220  is rotated, the legs  1204 ,  1206  shift position, either inward or outward depending on the direction of knob rotation, by pivoting around the pivot point  1240 . 
     Both legs  1204 ,  1206  include a pivot arm  1256  that is outwardly angled in the proximal direction from the rest of the leg  1204 ,  1206  such that force on a terminal end of the pivot arm  1256  causes the legs to rotate around the pivot point  1240 . The respective pivot arms  1256   a ,  1256   b  of the legs  1204 ,  1206  are angled away from each other and, when force is applied to both pivot arms  1256   a ,  1256   b , the legs  1204 ,  1206  pivot in opposite directions. The legs  1204 ,  1206 , thus, operate in a scissors-like fashion. 
     To effect such movement with the knob  1220 , the pivot arms  1256  are pivotally connected to a central reciprocating member  1260 . Movement of the reciprocating member  1260  in one direction causes the legs  1204 ,  1206  to move towards each other, while movement in the other direction causes the legs  1204 ,  1206  to move apart. The path of the reciprocating member  1260  is defined by a channel  1209  in the body  1230  so that the path is generally linear. 
     The reciprocating member  1260  has a connection end  1261  pivotally attached to a proximal ends  1262   a  of links  1262 , which are then pivotally attached at distal ends  1262   b  to the outwardly angled pivot arms  1256 . Being attached to the connection end  1261  and to the outwardly angled pivot arms, the links  1262  are inwardly angled in the proximal direction. As the connection end  1261  moves towards the pivot point  1240  common to both legs  1204 ,  1206  and the distance therebetween is decreased, the links  1262  attached to the connection end  1261  are further spread outward relative to each other. Conversely, when the connection end  1261  is retracted along with the reciprocating member  1260 , the links  1262  are drawn together to draw the legs  1204 ,  1206  together. 
     The reciprocating member  1260  includes a proximal, drive end  1270  having a recess or cavity  1272  for receiving therein a drive end  1276  of a drive member  1274 . The cavity  1272  includes a distal wall  1278  generally facing an end surface  1280  of the drive end  1276 . A bias or compression member  1282  is located within the cavity  1272  in between the distal wall  1278  and the end surface  1280 . When the drive end  1270  is directed in a distal direction, the end surface  1280  of the drive end  1270  applies force to the compression member  1282 , which is translated to the distal wall  1278  and, hence, to the reciprocating member  1260 . In this manner, advancement of the drive member  1274  forces the reciprocating member to advance, which in turn spreads the legs  1204 ,  1206  towards an open position. 
     To retract the legs  1204 ,  1206 , the drive member  1274  is withdrawn, thereby retracting the reciprocating member  1260 . The reciprocating member cavity  1272  is provided with a transversely oriented opening  1273  or rail for guiding the motion of the drive member  1274 . The drive member  1274  is secured within the cavity  1272  by inserting a pin  1290  through the opening  1273  and through the drive member  1274 . When the drive member  1274  is retracted, the pin  1290  interferes with a rear wall  1275  in the opening  1273  so that the reciprocating member  1260  is also retracted. 
     The minimally invasive sizing caliper  1200  may be utilized over a span of vertebrae for which the surrounding tissue is not completely removed or resected. The minimally invasive sizing caliper  1200 , as well as other instruments, preferably may access the implant site without requiring an opening in the patient as large as the implant site. The sizing caliper  1200  may be directed into the patient opening, and the legs  1204 ,  1206  may then be opened. 
     When the legs  1204 ,  1206  are opened, they are often larger than the patient opening. So that the sizing caliper  1200  may be removed prior to comparing the caliper to, for instance, the distance between bores on a plate, the legs  1204 ,  1206  may pivot to a smaller position as the caliper  1200  is being withdrawn. Once clear of the patient opening, the legs  1204 ,  1206  return to the position they were in prior to removal and positioned at the vertebrae measuring points. This is achieved by use of the compression member  1282 . After the instrument is removed from the patient, it may be compared directly to the plates of different sizes or may be compared to a scale to determine the required plate size. 
     When the caliper  1200  is withdrawn, the patient&#39;s flesh may force the legs  1204 ,  1206  together, or a surgeon may alternatively force them closed to ease removal of the caliper  1200 . As can be seen, in order to force the legs  1204 ,  1206  together, the reciprocating member  1260  must retract towards the proximal end of the caliper  1200 . The compression member  1282  in the form of a spring permits such retraction. 
     As the legs  1204 ,  1206  are forced together, the reciprocating member  1260  compresses the spring  1282  against the drive member  1274 . The drive member  1274  remains stationary so that, once the caliper  1200  is removed and the force applied to the legs  1204 ,  1206  is relieved, the legs  1204 ,  1206  return to the position in which they were when measuring the implant site. An accurate measurement may then be taken from the legs  1204 ,  1206 . 
     To advance or retract the drive member  1274 , the knob  1220  is rotated in one direction or the other, as previously noted. The knob  1220  does not change position relative to the body  1230 , other than by rotating. The knob  1220  includes an enlarged grip portion  1219 , and a shaft portion  1221  extending therefrom in the distal direction and received within the body  1230 . More specifically, the shaft portion  1221  is received by a bushing  1223 , and a snap ring or C-ring  1225  is secured to a distal end  1227  of the shaft portion  1221  to retain the shaft portion  1221  within the bushing  1223 . The grip portion  1219  may be manually operated to rotate the knob  1220  relative to and within the bushing  1223 . 
     A nut  1231  with a threaded exterior portion  1233  and an internal bushing surface  1235  is located between a shoulder  1237  on the bushing  1223  and a shoulder  1239  on the grip portion  1219 . The threaded portion  1233  is threaded into a threaded opening  1241  in the proximal end of the body. Accordingly, the nut  1231  is retained with a generally stationary position within the body  1230 . Furthermore, the nut  1231  between the shoulders  1237 ,  1239  retains the knob  1220  in a position permitting rotation only. 
     The drive member  1274  is prevented from rotation by the pin  1290  received by the opening  1273  in the reciprocating member  1260 . The shaft portion  1221  of the knob  1220  includes an inner threaded cylindrical bore  1251  into which the drive member  1274  is threadably received. As the knob  1220  rotates relative to the body and reciprocating member, the drive member  1274  is unable to similarly rotate. Accordingly, the knob  1220  also rotates relative to the drive member  1274 . Because the drive member  1274  is threadably engaged with the knob  1220 , the threads therebetween cause the drive member  1274  to advance or retract relative to the knob  1220 , depending on the direction of rotation. Accordingly, the reciprocating member  1260  is advanced or retracted, and the legs  1204 ,  1206  are opened or closed. 
     Implantation of a bone plate is a relatively invasive procedure, and the bone plate residing in a living tissue environment contacts and interacts with that tissue. In fact, patients who have received bone plates on the anterior side of the cervical portion of the spine have been known to physically feel the presence of the plate, particularly when swallowing. Accordingly, it is desirable to minimize the degree to which the bone plate interferes with the surrounding tissues. Moreover, the structural performance of the bone plate benefits from conforming its shape to the natural curves of the spinal column. However, for each individual implant site, the curve of the spinal column and the lateral curve of each vertebrae is unique, though certainly within typical, somewhat predictable ranges. 
     As discussed herein, the cervical plate may be provided with a curvature in the longitudinal direction that is provided for the plate in order to conform the plate to the average natural curvature of the spine, as well as to reduce interference with surrounding tissues. It is often desirable to alter the standard shape of the plate to fit an individual patient&#39;s unique anatomy. This should be done in a manner so as not to scratch or mar the surfaces of the bone plate, which otherwise may negatively affect the long term fatigue performance of the bone plate. For this purpose, a plate bending instrument  1300  is provided for altering the curvature of the plate when necessary due to a unique anatomy, a preferred form of which is illustrated in  FIGS. 60-66 . 
     The plate bender  1300  is operated to either increase or decrease the radius of the lordotic curvature of the plate. In operation, the plate bender  1300  is selectively operable so that a pair of opposed actuator handles  1331 ,  1332  for bending the plate operate to impart a bending motion in a selected direction by directing the handles  1331 ,  1332  towards each other. The plate bender  1300  includes a pair of opposed arms  1310 ,  1320  extending laterally from the handles  1331 ,  1332 , and each of the handles  1331 ,  1332  and arms  1310 ,  1320  are operatively connected and pivotally secured at a pivot point  1375   a  by a pin  1375 . As will be described herein, the plate bender  1300  includes engaging pegs or keys  1372 ,  1382  that are selectively shiftable so that the each of the handles  1331 ,  1332  are operatively engageable with either of the arms  1310 ,  1320 . 
     With reference to  FIG. 63 , the opposed actuator handles  1331  and  1332  are pivotally joined at an upper portion. The opposed lower gripping portions of the handles  1331  and  1332  are biased outward from each other by a bias member in the form of attached springs  1335  and  1336 , which connect by insertion of tab  1337  into the mating slot  1338  of the spring  1336 . The springs  1335  and  1336  are connected to the inward facing sides of handles  1331  and  1332  by rivets or by any other suitable means of attachment. 
     Each handle  1331 ,  1332  respectively includes an upper shoulder portion including an extending flange  1333 ,  1334 , and a central aperture  1333   a ,  1334   a . The flanges  1333  and  1334  are connected to the cylindrical pin  1375  which extends through the apertures  1333   a  and  1334   a  to provide a central pivot  1375   a  for the handles  1331  and  1332 , and squeezing the handles  1331 ,  1332  against the bias provided by springs  1335 ,  1336  causes the flanges  1333 ,  1334  and handles  1331 ,  1332  to pivot about the pivot  1375   a.    
     With reference to  FIG. 63  and with particular reference to  FIGS. 63 and 66 , the first arm  1320  includes a portion extending generally laterally from the central pivot  1375   a  formed by opposed arm portions  1321   a  and  1321   b , which define a central horizontally extending race  1322 . The arm  1320  also includes an end portion  1323  which extends generally perpendicular to extending members  1321   a  and  1321   b . Extending members  1321   a  and  1321   b  are preferably curved to match the standard anterior and posterior curvature of the cervical plate. 
     The arm end portion  1323  includes opposed side walls  1324  and  1325  defining a channel or a race  1326  oriented transverse to the race  1322 . Similarly, the arm  1310  includes a laterally extending portion including extending members  1311   a  and  1311   b  which define a race  1312  extending in the opposite direction of the race  1322 . The arm  1310  includes an end portion  1313  formed from opposed sidewalls  1315  and  1314  which define the race  1316  extending generally perpendicular to the race  1312 . 
     Each arm  1310  and  1320  includes roller assemblies selectively positionable within races of the respective arms  1310 ,  1320 . Each arm  1310 ,  1320  is provided with a convex roller assembly  1340  and a concave roller assembly  1350 . Each roller assembly includes a roller having a central longitudinal aperture and a pin mounted through the aperture on one side and to a knob connected to the pin on the backside of the instrument. 
     Representatively, with reference to  FIG. 66 , convex roller assembly  1340   a  is comprised of convex roller  1341   a  which is positioned about its central aperture on the pin  1342   a  and connected to the pin by a screw  1345   a  through a washer  1346   a . A knob  1343   a  is attached to the opposite end of the pin  1342   a  by means of a connecting pin  1344   a , which extends through the knob  1343   a  in a direction traverse to the longitudinal direction of the pin  1342   a  and is retained by a shoulder at the end of the pin  1342   a . Loosening the knob  1343   a  permits the convex roller assembly  1340   a  to be positioned at any desired location within the race  1326  or the race  1322 . Tightening the knob  1343   a  secures the roller assembly  1340   a  at its desired position. As shown in  FIG. 66 , convex roller assemblies  1340   a ,  1340   b  and concave roller assemblies  1350   a  and  1350   b  are movably secured to the arms of the plate bending instrument  1300  in a corresponding manner. 
     The roller assemblies are used in conjunction with one of two fulcra to adjust the curvature of the plate. The plate bender instrument includes a lower fulcrum  1370  extending frontward in the general direction of the central pivot access  1375  and an upper fulcrum  1306  also extending in the same direction. The lower fulcrum  1370  has a convex bending surface  1370   a  directed upward, and the upper fulcrum  1306  has a concave bending surface  1306   a  directed downward. The fulcra  1306 ,  1370  are connected to tool mount  1360 , and the upper fulcrum  1306  is selectively positionable upward and downward along a slot  1361  in the tool mount  1360  by a knob  1307  that may be loosened to allow the movement therof and may be tightened to fix the selected position. The concave roller assemblies are used in combination with the lower fulcrum  1370  to increase the curvature of a plate, while the convex roller assemblies are used in with the upper fulcrum  1306 . 
     Representatively, by placing the plate above the fulcrum  1370  and positioning the concave roller assemblies  1350   a  and  1350   b  above the plate and spaced horizontally from the fulcrum  1370 , the concave rollers may apply a downward force to bend the plate about the fulcrum  1370 , as shown in  FIG. 65 , thereby increasing the curvature of the plate when the tool  1300  is operated. When used for this purpose, the convex roller assemblies  1340   a  and  1340   b  may be moved to the positions shown in  FIG. 63  so as to not interfere with the bending operation. On the other hand, a plate placed between the convex rollers  1340   a ,  1340   b  and against the upper fulcrum  1306  may be bent by the plate bender  1300  to decrease the curvature. 
     Each roller and fulcrum has a surface engageable with the plate that is covered by, and is preferably entirely formed of, a material that does not scratch or mar the plate during the bending operation, which otherwise may affect the fatigue life of the plate. Preferably, the rollers are made with the polymer known as PEEK. The curvature of the rollers and of the fulcra match the radius of the plate to minimize kinking, or incongruities in the plate bending, which also would affect the fatigue life of the plate. 
     As can be seen in  FIG. 66 , the arms  1320 ,  1310  include flange portions  1327 ,  1317  also connected by the pivot pin  1375  and pivotable about the pivot point  1375 . Thus, the arms  1320 ,  1310  and handles  1332 ,  1331  are joined at the common pivot point  1375 . More specifically, the flange portions  1317 ,  1327  are in a generally facing relationship. 
     Each flange portion  1317 ,  1327  has a pair of slots that are aligned with a counterpart of the other flange portion. That is, flange  1317  has a slot  1319  aligned with slot  1329  of the flange  1327 , and has a slot  1318  aligned with slot  1328  of the flange  1327 . When the arms  1320 ,  1310  and handles  1331 ,  1332  are joined at the pivot point  1375   a , the slots  1329  and  1319  are aligned with a key slot  1372   a  of the handle  1331 . Similarly, the slots  1328  and  1318  are aligned with a key slot (not shown) of the handle  1332 . More specifically, at least a portion of the key slot  1372   a  is aligned with the aligned arm slots  1329 ,  1319  so that a key  1372  may be received in the key slot  1372   a  and the arm slots  1329 ,  1319 . Moreover, the key  1372  is shiftable from a position where the key  1372  is received in the key slot  1372   a  and only one of the arm slots, such as  1329 , to a position where the key  1372  is received the key slot  1372   a  and only the other arm slot  1319 . 
     When the key  1372  is located in the key slot  1372   a  of the handle  1331  and, for instance, the arm slot  1329  of the arm  1320 , the handle  1331  and arm  1320  behave as a unitary arm around the pivot point  1375   a . Alternatively, when the key  1372  is located in the arm slot  1319 , the handle  1331  and arm  1310  pivot as a unit. The handle  1332  operates in the same way as described, cooperating with the arm slots  1318  and  1328 . 
     The position of the keys is shifted with selector buttons  1391 ,  1381 , respectively coupled with buttons  1393  and  1383 . Representatively, selector button  1393  is secured to a barrel  1394  located in a throughbore  1372   b  of the handle  1331 , and the barrel  1394  is then secured to selector button  1391 . The throughbore  1372   b  and the key slot  1372   a  are in communication so that a key recess  1394   a  of the barrel  1394  located in the throughbore  1372   b  may be accessed through the key slot  1372   a . The key  1372  is inserted into the key slot  1372   b  so that a portion is received in the barrel key recess  1394   a  and a portion extends out of the key slot  1372   b  for being received in one of the arm  1310 ,  1320 , as discussed above. 
     In this manner, movement of one of the selector buttons  1393 ,  1391  selects which of the arms  1310 ,  1320  with which the handle  1331  is joined. As an example, shifting the front selector button  1391  inward shifts the barrel  1394  connected thereto through the throughbore  1372   b  in the rearward direction, which causes the other button  1393  to also shift rearward. The shifting of the barrel  1394  also causes the key  1372  to shift in the key slot  1372   a  from the arm slots  1329  to the other arm slot  1319 . In this manner, the handle  1331  is disengaged from arm  1320 , and is engaged with arm  1310 . 
     In this manner, the handles are cooperatively attached by the described slot-key configuration to one of the arms to form bending levers. With the keys  1372 ,  1382  in a forward position, the handle  1331  and arm  1320  are connected to form a first opposed bending lever while the other handle  1332  is connected with the other arm  1310  to form a second opposed bending lever. On the other hand, when the keys  1372 ,  1382  are in a rearward position, the handle  1331  and arm  1310  are connected to form a first crossing bending lever while the other handle  1332  is connected with arm  1320  to form a second crossing bending lever. The bending levers, in any configuration, are connected by the pivot pin  1375  to pivot around pivot point  1375   a.    
     When in the configuration with the opposed bending levers (i.e., the keys  1372 ,  1382  in the forward position), the concave bending roller assemblies  1350   a ,  1350   b  are selectively positioned on the races towards the center of the plate bender  1300  in proximity to the lower fulcrum  1370 , as shown in  FIG. 63 . A plate is then placed between the rollers  1351   a  and  1351   b  and the lower fulcrum  1370  with the top plate surface against the rollers  1351   a ,  1351   b  and the bottom plate surface against the lower fulcrum  1370 . As the plate bender  1300  is operated by directing the handles  1331 ,  1332  inward, the arms  1310 ,  1320  move downward toward the lower fulcrum  1370  so that the concave rollers  1351   a ,  1351   b  press the plate between the rollers  1351   a ,  1351  and the lower fulcrum  1370 . In this manner, the plate is bent over the  1370  fulcrum to increase the curvature. 
     Conversely, when in the configuration with the crossing bending levers (i.e., the keys  1372 ,  1382  in the rearward position), the convex roller assemblies  1340   a ,  1340   b  are selectively positioned on the races and toward the center of the plate bender  1300  in proximity to the upper fulcrum  1306 . A plate is placed between the rollers  1341   a ,  1341   b  and upper fulcrum  1306  so that the top plate surface is against the upper fulcrum  1306  and the bottom plate surface is against the rollers  1341   a ,  1341   b . Operating the crossing bending levers in a scissors-like fashion by directing the handles  1331 ,  1332  inward causes the arms  1310 ,  1320  to move upward toward the upper fulcrum  1370 , thereby pressing plate between the convex rollers  1341   a ,  1341   b  and the upper fulcrum  1306 . Accordingly, the plate is bent to decrease its curvature. 
     Particular features of the bone plates themselves may assist in bending the bone plate. Though these same features may be present in each of the described bone plates, the valleys  20   c  of plate  20 , and plate  700  includes valleys  700   d , as can be seen in  FIGS. 28 and 37 , on the top surface  700   a  to assist in bending the plate. These are meant to be exemplary only. These valleys  20   c ,  700   d  create stress concentrations to control or predict where the greatest amount of bending takes place, particularly in the event a surgeon chooses to manually adjust the shape of the bone plate. 
     Once the proper bone plate has been selected and configured with the desired curvature to follow the lordotic curve of the section of the spine to which it is to be secured and to follow the curve of the vertebrae, a plate holder  1400  is used to hold the plate during implantation, and to hold a guide tool  1500  for positioning a pilot hole tool  1600  for creating a pilot hole in the vertebrae for the screws. 
     As can be seen in  FIGS. 67-70 , the plate holder  1400  is a scissors or forceps-type instrument having cooperating pivotable arms  1402 . The arms  1402  have proximal ends  1404  including finger grips  1405  allowing a surgeon or the like to manipulate the plate holder  1400 . The arms  1402  also have distal ends  1406  for connecting to the plate and for receiving the guide tool  1500 , as will be described below. The arms  1402  are connected at a pivot point  1403 , and, when the proximal ends  1404  are directed towards each other, the distal ends  1406  also move towards each other. The proximal ends  1404  also include opposed ratcheting bars  1408  so that, as the proximal ends  1404  move inwardly, the ratcheting bars  1408  catch each other. In this manner, once the plate holder  1400  is compressed on a plate, the ratcheting bars  1408  retain the position of the arms  1402  in the compressed position. To release the catch of the ratcheting bars  1408 , the arms  1402  are simply deformed or flexed slightly in a direction generally orthogonal to the plane of their pivotal movement. 
     Each of the distal ends  1406  of the arms  1402  includes a guide tool receptor  1420  for receiving the guide tool  1500 . The arms  1402  are bent at a point between the pivot point  1403  and the proximal ends  1404 , and the guide tool receptors  1420  are attached to a side  1422  of each of the arms  1402  that is opposite the direction of the bend of the arms  1402  so that a surgeon&#39;s hand that is manipulating the finger grips  1405  does not obstruct the surgeon&#39;s view of the distal ends  1406  or the guide tool receptors  1420 . 
     Each of the guide tool receptors  1420  has a bottom surface  1424 , and terminal portions  1430  of each arm  1402  include opposed barbs or prongs  1440 . As can be seen in  FIGS. 44 ,  47 , and  50 , the bone plate  1002 , as an example, includes recesses  1450  for receiving and cooperating with the prongs  1440 . The plate  1002  has a bottom surface  1452  and side edges  1454 , and a top surface  1456 . The recesses  1450  are formed in pairs, each pair corresponding to a point along the plate  1002  that may receive the opposed prongs  1440  of the plate holder  1400 . 
     The recesses  1450  are generally formed to open to the side edges  1454  and the bottom surface  1452 . Accordingly, the recess  1450  has a upper surface  1458  generally facing downward. Thus, a wall  1460  is formed between the upper surface  1458  of the recess  1450  and the top surface  1456  of the plate  1002 . The top surface  1456  may be provided with surface marks (not shown) that indicate where the recesses  1450  are located. 
     The recess  1450  has a structure and shape generally corresponding to the shape of the prongs  1440 . As can be seen best in  FIGS. 69 and 71 , each prong  1440  has a curved or arcuate top surface  1442  so as to be hemi-cylindrical. Each prong  1440  also has a tip surface  1444  that may have the edges  1444   a  rounded for ease of insertion into the recesses  1450 . 
     The plate holder arms  1402  may be pivoted to an open position such that the prongs  1440  are positioned at a distance greater than the lateral width of the plate  1002 . The prongs  1440  are then aligned with a selected pair of recesses  1450  corresponding to a pair of bores  24  in the plate  1002 . The proximal ends  1404  are then operated to direct the distal ends  1406  towards each other, and the prongs  1440  enter and fit closely within the recesses  1450  in the bone plate  1002 . 
     In order to ensure the plate holder  1400  holds the plate  1002  relatively tightly, portions  1462  of the arms  1402  proximate to the terminal portions  1430  are angled inwardly. This inward angle allows the portions  1462  to cooperate with the prongs  1440  so that the wall  1460  partially defining the recess  1450  is securedly held therebetween. Furthermore, the bottom surface  1424  of the guide tool receptors  1420  may contact the bone plate top surface  1456  to constrain any movement between the plate  1002  and the plate holder  1400 . 
     As noted, each guide tool receptor  1420  is configured to receive a guide tool  1500 . The guide tool  1500  is used to direct a pilot hole tool  1600  into contact with a point on the vertebrae for forming a pilot hole thereon. As will be discussed below, the pilot hole tool  1600  may be an awl  1620 , a drill  1630 , or a tap  1640 . It should be noted that the term pilot hole, as used herein, may mean a hole formed by compressing bone in a localized area as to make a recess in which a screw may find purchase, may be a hole formed by a rotating drill tip, or may be a hole formed by a tap such that the hole includes threads therein. The guide tool  1500  may be provided for cooperating with a single guide tool receptor  1420  and pilot hole tool  1600 , or may be provided for cooperating with a pair of guide tool receptors  1420  and pilot hole tools  1600 . 
     The guide tool receptor  1420  includes a throughbore  1470  therethrough for receiving the guide tool  1500  and the pilot hole tool  1600  therein. The throughbore  1470  has a central axis T, and the axes T 1 , T 2  of the pair of guide tool receptors  1420  for an included angle σ that is, in the preferred embodiment, 18 degrees. When the plate holder  1400  is secured to a plate, the guide tool receptors  1420  are positioned over and aligned with the bores  24  in the plate. 
     Each throughbore  1470  includes an upper portion  1480 , a lower portion  1482 , and a socket  1484  for cooperating with the guide tool  1500 . As can be seen in  FIGS. 72 and 73 , the guide tool  1500  includes a distal end  1502  including a socket portion  1504  for cooperating with the socket  1484  of the guide tool receptor  1420 . As will be described below, the guide tool socket portion  1504  is received in the throughbore  1470  so that, in a first form, the guide tool  1500  is fixed thereto and accepts a predetermined orientation so that its central longitudinal axis G is aligned and coincident with the axis T of the throughbore  1470 . In a second form, the guide tool  1500  may be received in the throughbore  1470  and be pivotable so that its axis G is offset from the throughbore axis T through a range, such as ±10 degrees in the direction of the longitudinal axis of the plate (cephalad/caudal direction) and ±5 degrees in the lateral direction. In this manner, a pilot hole tool  1600  is received within the guide tool  1500  and may be directed through a fixed guide tool  1500  along the longitudinal axis of the guide tool  1500  and the throughbore  1470 , or may be directed through a pivotable guide tool  1500  allowing the surgeon to make a selection as to the angle of entry by the pilot hole tool  1600  into the bone, and, thus, the angle of entry by a screw  22  subsequently inserted therein. The upper portion  1480  of the guide tool receptor socket  1484  is angled radially outward, such as to be frusto-conical, so that the guide tool  1500  may pivot therein, and the lower portion  1482  is also angled radially outward as a frusto-cone so that a pilot hole tool  1600  received within a pivoted guide tool  1500  may pass through the throughbore  1470  to engage the bone at the angle dictated by the guide tool  1500 . 
     The socket portion  1504  of the guide tool  1500  includes a number of finger-like projections  1520  extending in a circular array from a distal end  1522  of a cannula member  1524 . The projections  1520  include slot recesses  1526  therebetween so that the projections  1520  may be deflected inwardly. A terminal portion  1528  of each projection  1520  includes an arcuate outer surface portion  1521 , and the combination of the portions  1521  combine so that the socket portion  1504  is generally ball-like. The socket  1484  of the guide tool receptor  1420  has an inner surface  1484   a  that is generally partially spheroidal, and the socket  1484  of the guide tool receptor  1420  and the guide tool socket portion  1504  form a ball-and-socket type connection, though with limited movement, as described above. 
     To insert the guide tool socket portion  1504  in the socket  1484  of the guide tool receptor  1420 , the projections  1520  are compressed or deflected inwardly so that the ball-shape socket portion  1504  is received within the spheroidal socket  1484 . Once inserted in the spheroidal socket  1484 , the projections  1520  of the guide tool socket portion  1504  are then permitted to deflect outward towards their natural position so that the arcuate surface portions  1521  thereof may contact the spheroidal inner surface  1484   a  to form the ball-and-socket type joint. 
     The projections  1520  include an angled portion  1540  that angles radially outward from the cannula  1524 , increasing towards the distal end  1502 . The angled portion  1540  has an edge  1548  at the point of its maximum dimension, which serves to form a shoulder  1549  on the projections generally facing the distal end  1502 . A sleeve  1530  is utilized for camming against the angled portion  1540  to deflect the projections inward. The sleeve  1530  is generally cylindrical and has an inner surface  1532 , an outer surface  1534 , a distal end  1536 , and a proximal end  1538 . The sleeve  1530  may be reciprocated with respect to the guide tool cannula  1524  so that the inner surface  1532  cams against the angled portion  1540  between a first position in which the projections  1520  in a natural configuration, or are at least minimally deflected inwardly, and a second position in which the projections  1520  are deflected inwardly sufficient to allow the ball shape socket portion  1504  to be received within the socket  1484  of the guide tool receptor  1420 . For instance, in the first position the inner surface  1532  at the distal end  1532  may contact a lower portion  1542  of the angled portion  1540 . The sleeve  1530  may then be advanced so that the inner surface  1532  cams along the angled portion  1540  of the projections  1520 , thereby forcing the projections  1520  inward. 
     The sleeve proximal end  1538  includes a grip portion  1550  for manual manipulation of the sleeve  1530  relative to the cannula  1524 . Near the proximal end  1538 , the inner surface  1532  includes an annular shoulder  1552  oriented generally in the distal direction. The cannula  1524  has an inner surface  1554  that also includes an annular shoulder  1556 , though the shoulder  1556  is oriented generally towards the proximal end  1538  of the sleeve  1530 . The shoulders  1556 ,  1552  generally face each other, and a spring  1560  or other bias member is positioned between the shoulders  1556 ,  1552 . In this manner, the sleeve  1530  is biased to the first position wherein the projections  1520  are generally undeflected, or are minimally so. 
     When the sleeve  1530  is advanced, such as by applying manual force to the grip portion  1550  toward the distal end  1536  to deflect the projections  1520  inwardly, the spring  1560  is compressed against its bias. The cannula socket portion  1504  is then inserted and located into the guide tool receptor socket  1484 , whereupon the sleeve  1530  may be retracted, such as by releasing the sleeve  1530 , by the bias of the spring  1560 . The projections  1520  then shift outwardly toward their natural position within the socket  1484 . 
     Whether the guide tool  1500  is a fixed angle or variable angle guide tool  1500  is dependent on the construction of the guide tool  1500  itself. The projections  1520 , as stated, include the angled portion  1540  forming a shoulder  1549  generally facing the distal end  1502 . The movement of the guide tool  1500  relative to the socket  1484  is dependent on the shoulder  1549 . For the fixed guide tool  1500 , the projections  1520  assume a predetermined orientation within the socket  1484 , and, thus, the guide tool  1500  assumes a predetermined orientation. The variable angle guide tool  1500  may be pivoted with respect to the socket  1484  within a range, such as defined above. 
     The guide tool  1500  orients and directs a pilot hole tool  1600 , as noted above. More specifically, the cannula  1524  of the guide tool  1500  receives and guides the pilot hole tool  1600 , as well as limits the depth to which the pilot hole tool  1600  may advance therewithin. As can be seen in  FIGS. 77-79 , pilot hole tools  1600  in the form of an awl  1620 , a drill  1630 , and a tap  1640  are each formed as an elongate member having respective chuck or threaded ends  1650 , upper shoulders  1660 , shanks  1670 , lower shoulders  1680 , and driving end  1690 . 
     For each of the pilot hole tools  1600 , the threaded end  1650  is secured within an instrument for manual or powered operation. Once secured, the pilot hole tool  1600  is inserted into the cannula  1524 . As the pilot hole tool  1600  is directed into the bone, the tool  1600  advances further into the cannula  1524 . The cannula  1524  has an upper terminal edge  1570 , and the tool  1600  may be advanced until the upper shoulder  1660  contacts the terminal edge  1570 . Thus, the depth to which the tool  1600  may be driven into the bone is limited by the edge  1570  and the shoulder  1660 . In addition, the lower shoulder  1680  is positioned around the tip  1690  so that the tool  1600  may be driven into the bone only to a depth provided by the length of the tip  1690  below the lower shoulder  1680 . The lower shoulder  1680  is particularly provided in the event the tool  1600  is used without a guide tool  1500 . 
     Each of the tips  1690  of the respective awl  1620 , drill  1630 , and tap  1640  is provided with a construction particular to its operation. As depicted, the awl  1620  has a pyramidal-shaped tip  1622  having flat faces that meet in sharp edges, similar to a nail. However, the awl  1620  may, alternatively, have any pointed construction for being driven into bone to create a hole therein. For the awl  1620 , the bone may be compressed in the localized region of driving. 
     The drill  1630  includes a spirally fluted cutting bit  1632 , as is depicted. In this manner, the drill  1630  operates in the manner typical of drill bits or tips. As the bit  1632  rotates under pressure, a sharp terminal tip  1634  pierces the bone, and a fluted cutting edge  1636  widens the hole formed by the tip  1634 . The spiral configuration of the bit  1632  allows the drill  1630  to draw removed material out of the hole formed thereby. 
     The tap  1640  is inserted in a pilot hole that has already been made and is used for providing threads in the hole for receiving a screw therein. The tap  1640  has a tip  1642  having threads  1644  along its length to the lower shoulder  1680 . A spiral flute  1646  is provided across the threads along the length of the tip  1642  so that each thread  1644  is provided with a leading cutting edge. The threads  1644  may also include an outer sharp edge along their major profile for cutting bone. Accordingly, as the tap  1640  is rotated and advanced into the screw hole, the flute  1646  and threads  1644  cut threads into the bone for the screw. 
     Referring now to  FIG. 80 , a variable pilot hole tool  1700  is depicted. As illustrated, the tool  1700  is a variable depth drill, though it may also be a tap, for instance. The variable drill  1700  includes a threaded end  1702 , upper shoulder  1704 , a shank  1706 , a lower shoulder  1708 , and a driving end in the form of a fluted drill tip  1710  similar to the drill bit  1620  described above. The drill tip  1710  may be longer than the drill bit  1620  described above so that the lower shoulder  1708  limiting the drilling depth is positioned a greater distance from a drill tip end  1712 . The upper shoulder  1704  operates to contact the cannula upper edge  1570  when the drill  1700  has been advanced within the cannula  1524  to the desired depth. 
     However, for the variable drill tool  1700 , the upper shoulder  1704  may be selectively positioned along a portion of the shank  1706 . The upper shoulder  1704  is located on a housing  1720  that is, preferably, generally cylindrical. The housing  1720  has an inner cylindrical cavity  1722  including an upper portion  1724  and a lower portion  1726 . The lower portion  1726  has a reduced diameter in comparison to the diameter of the upper portion  1724 . Accordingly, a shoulder  1728  is formed between the upper and lower portions  1724 ,  1726 . The upper portion  1724  includes an interior surface having a threaded portion  1744 . Alternatively, a threaded nut  1746  may be inserted within the upper portion  1724  and have a threaded interior surface. 
     The shank  1706  is received within and through the housing  1720 . More specifically, the shank  1706  passes through a top opening  1760  and a bottom opening  1762  so that the threaded end  1702  is positioned above the top opening  1760  and the tip  1710  is positioned below bottom opening  1762 . The shank  1706  includes an externally threaded portion  1750  so that the shank  1706  may be threaded into the threads  1744  of the housing  1720 . In order to vary the depth of the variable drill tool  1700 , the shank  1706  is rotated relative to the housing  1720  so that the threads  1744 ,  1750  therebetween cooperate to advance or retract the shank  1706  with respect to the housing  1720 , thereby altering the amount of the shank  1706  extending below the bottom opening  1762 . As the bottom opening  1762  is aligned with the upper shoulder  1704 , the amount of shank  1706  that may be received in and through the guide tool  1500  is adjusted by the rotation of the shank  1706  along the threads  1744  of the housing  1720 . A set screw  1780  is provided for extending through the housing  1720  and into the lower cavity portion  1726  and, when advanced thereinto, the set screw  1780  contacts the shank  1706  to prevent movement of the shank  1706  relative to the housing  1720 . 
     The housing  1720  further includes a limiter  1790  for controlling the rotation of the shank  1706  relative to the housing  1720 . Specifically, the shank  1706  includes one or more grooves  1792  or recesses, and the limiter  1790  cooperates with the groove  1792  to regulate the relative rotation between the housing  1720  and the shank  1706 . The limiter  1790  is fixed within the housing  1720  and includes a bias member  1794  that biases a protrusion in the form of a V-tab  1796  into engagement with the spline recess  1972 . In the present embodiment, a single groove  1792  is provided, such that the V-tab  1796  engages the groove  1792  once for each rotation. In this manner, advancement of the shank  1706  relative to the housing  1720  is presented with discrete stops. 
     The V-tab  1796  is preferably biased into engagement with the groove  1792  by a bias member in the form of a spring  1794 . In the event the spring  1794  has a relatively low spring constant, rotation of the shank  1706  will cause the V-tab to cam out of engagement in the groove  1792 . If the spring  1794  is relatively stiff, the V-tab  1796  does not allow rotation of the shank  1706  relative to the housing  1720 . 
     To allow the shank  1706  to rotate relative to the housing  1720  when the spring  1794  is relatively stiff, the V-tab  1796  is manually released. The V-tab  1796  is secured on an inner surface  1798  of a limiter ring  1800 . The spring  1794  biases the limiter ring  1800  so that the V-tab  1796  engages the groove  1792 . To do so, the spring  1974  is located between an outer surface  1802  of the limiter ring  1800  and the interior  1719  of the housing  1720 . The V-tab  1796  is manually released by forcing the limiter ring  1800  against the bias of the spring  1794 . 
     An actuator button  1810  is provided for disengaging the groove  1792  and V-tab  1796 . The actuator button  1810  extending outward from the limiter ring  1800  and out through the housing  1720 . By depressing the actuator button  1810  into the housing  1720 , the limiter ring  1800  is shifted against the spring  1794 , thereby shifting the V-tab  1796  out of the groove  1792 . In this manner, the shank  1706  may then be rotated for being advanced or retracted relative to the housing  1720  and for adjusting the depth the drill permitted into the bone. The variable drill  1700  may be inserted into the guide tool  1500 , and a depth gage (not shown) may be used to confirm the proper drill depth. 
     As described, after resecting surrounding tissue from the implant site, the sizing caliper  1200  may be used to determine the proper plate size. The plate is selected based on dynamized bores, non-dynamized bores, or a combination. The plate bender  1300  is used if it is desired to alter the curvatures of the plate. The types of screws  22  are selected, whether they be fixed or variable angle (polyaxial) screws, and whether they be self-drilling, self-tapping, or neither so that a tap is minimally required. The plate is then grasped by the plate holder  1400  and positioned in the desired location on the vertebrae. A guide tool  1500  is selected to be either fixed or variable, and then inserted into the guide tool receptors  1420  of the plate holder  1400 . A self-drilling screw may be driven through the guide tool  1500  into the bone, or one or more pilot hole tools  1600  may be likewise used. 
     In the event a pilot hole tool  1600  is used, a temporary holding pin  1850  ( FIG. 76 ) may be inserted through the guide tool  1500  to secure a hole made in the bone, or to create a hole in the bone. The temporary holding pin  1850  has a threaded shank  1852 , and an enlarged head height  1854  so that the pin  1850  is clearly noticed as being temporary. In this manner, the plate may be secured so that the surgeon may address prepping other bone holes and driving screws through the other bores of the bone plate. 
     A preferred sequence of inserting screws in the bores of the plate is presented. Each screw should not be completely tightened until all are partially secured. After a first bore has received a screw, the bore diagonal thereto receives the next screw. Next, the third bore to receive a screw is the bore on the same vertebrae as the first bore receives a screw. The fourth bore is diagonal to the third bore. Any intermediate bores may then receive screws. Once all the screws are in place, they should be tightened in the same order. To remove the screws, the extractor  1900  may be used in conjunction with the driver  900 . 
     Referring now to  FIGS. 82-86 , a form of a two-tiered bone plate system  2000  similar to the bone plate system  1000  of  FIG. 44  is depicted, the bone plate system  2000  being non-dynamized. That is, the bone plate system  2000  includes a bone plate  2002  having bores  2004  in a first tier  2006  and a second tier  2008 , each tier being provided for rigidly securing the plate  2002  with respective superior and inferior vertebrae such as  12   a  and  12   b . A window  2010  is provided between the tiers  2006 ,  2008  for viewing an intervertebral site, such as would be desired for viewing an intervertebral graft or device or for viewing particular details of a damaged portion of the spine, such as through radiographic equipment. Each bore  2004  is non-dynamized and is generally constructed as the non-dynamized bores  1042  of the bone plate system  1000 . The plate  2002  is preferably pre-bent to have a curvature in a longitudinal direction, more preferably with a radius of curvature of approximately 200 millimeters shown in  FIG. 85 , and in a lateral direction, more preferably with a radius of curvature of approximately 20 millimeters. 
     The bores  2004  are constructed in the manner described above for bores  24  of plate  1002 . As such, each bore  2004  has an inner surface  2020  with a lowermost portion  2022  defining a throughbore  2024  with a brace surface  2026 . An implanted screw head  26  secures rigidly against the inner surface  2020  while the screw shank  28  extends from the throughbore  2024  to secure the plate  2002  with its respective vertebra  12 . As described above, the screws  22  and the bores  24  may be polyaxial or fixed. Like other forms described herein, the brace surface  2026  and a seat surface  2034  form a shoulder  2032 . 
     As viewed best in  FIGS. 82 and 83 , the plate system  2000  includes a retainer  2040  positioned within each bore  2004  for preventing screw back-out. The retainers  2040  are U-shaped and similar to the retainer  1004  depicted in  FIG. 51  so that, when assembled with the plate  2002 , two legs  2042  are positioned across the bore  2004 . When a screw  22  is driven into the bone  12 , the screw head  26  deflects the legs  2042  outward to permit its passage therebetween. Once the screw head  26  is secured against the seat surface  2020 , the legs  2042  shift and return to the position shown in  FIGS. 82 and 83 . Viewing the bone plate assembly  2000  from a bottom or bone-contacting side in  FIG. 83 , the screw head  26  has a larger diameter than the throughbore  2024  so that it is retained in the bore  2004 . As can be seen, the retainer legs  2042  are positioned well within the bore  2004  and across the throughbore  2024  itself to limit a screw  22  positioned between the seat surface  2020  and the retainer  2040  from backing out. In this manner, the retainer  2040  is positioned over a significant portion of the screw head  26 . 
     Assembly of the bone plate assembly  2000  is similar to the bone plate system  1000 , described above. An inner periphery of the inner surface  2020  includes retainer recesses  2060  extending therearound for receiving the retainers  2040 . It is preferred that the retainer  2040  is secured so that legs  2042  are positioned so that a screw head  26  may rest against the seat surface  2020  with an amount of clearance. In this manner, the clearance permits a polyaxial screw  22  to pivot an amount before the screw head top surface  29  comes into contact with the retainer  2040 , as described above. 
     The plate  2002  includes a retention recess  2070  similar to the recess  1050  of the plate  1002 . The retention recess  2070  receives free ends  2044  (see  FIG. 86 ) of the retainer legs  2042  and, more specifically, a single retention recess  2070  may be provided for a pair of bores  2004  and a pair of retainers  2040  so both retainers  2040  are captured therein, as will be described below. As for the recess  1050 , the retention recesses  2070  are preferably laterally and centrally oriented with respect to the bores  2004 . This allows compressive forces on the implanted plate  2002  to be transmitted therethrough along the longitudinal direction through the screws  22  with the retention recesses  2070  positioned laterally of the load path. 
     Furthermore, the retainers  2040  provide improved retention of the screw  22  within the plate  2002 . The retainers  2040 , as well as several other forms of retainers disclosed herein, provides line contact between the retainer  2040  and the screw head top surface  29 . In a preferred form, the retainers  2040  provide two line-contacts with each screw  22  for retaining the screw  22  with the plate  2002 , the retainer legs  2042  being positioned over a significant portion of the screw head  26 , as noted above. As can be seen in  FIG. 82 , the retainer legs  2042  are secured with the plate  2002  so that the legs  2042  are generally parallel to each other which serves to reduce the ability of a screw  22  secured at an angle from wedging the legs  2042  open. The entire retainer  2040 , including the legs  2042  extending between their free ends  2044  and a bridge  2046 , can be seen in  FIG. 86 . 
     As discussed at least for the plate assembly  1000 , the retainers  2040  are assembled with the plate  2002  by capturing the free ends  2044  within the retention recesses  2070 . Above each retention recess  2070  are opposed tab walls  2072  extending transverse to the retention recess  2070  with an opening  2078  therebetween. The tab walls  2072  form a gap  2074  between the tab wall  2072  and a surface  2076  on the bone plate  2002  proximate to and extending between adjacent retainer recesses  2060  within the bores  2004 . 
     To assemble the plate assembly  2000 , the legs  2042 , which would otherwise angle outward from each other, are compressed. The retainer  2040  is inserted in the bore  2004  with the bridge  2046  positioned in the retainer recess  2060  and the legs  2042  extending across the bore  2004 . The terminal free ends  2044  and the legs  2042  are deflected inwardly and passed through the opening  2078  between the tab walls  2072  and into the retention recess  2070 . Once clear of the tab walls  2072 , the legs  2042  splay outward towards their natural position. The tab walls  2072  are then deformed or swaged, such as with a peen, inward or downward toward the surface  2076 . The tab walls  2072 , when deformed, curve around the leg ends  2044  to generally prevent or restrict removal of the retainers  2040 , as can best be seen in  FIG. 84 . 
     Preferably, the leg ends  2044  are captured so that the retainer  2040  is permanently assembled with the plate  2002 , and removal requires prying or otherwise mechanically moving the tab walls  2072  away from the free ends  2044 . Preferably, the tab walls  2072  are deformed downward so that the ends  2044  are not rigidly fixed therewith, enabling the retainers  2040  to shift somewhat within the bores  2004 . The permanent assembly of the retainers  2040  with the plate  2002  reduces or minimizes the likelihood of improper insertion by a surgeon or surgical technician, as well as minimizes the likelihood of in-field rearranging or alteration of the configuration of the retainers  2040  with the plate  2002  or the configuration of the retainers  2040 . 
     When a screw  22  is driven through the retainer  2040  and into the bone  12 , the retainer  2040  is able to shift slightly to balance the deformation of the retainer  2040 , reducing the risk of a localized exceeding of the elastic limit of the retainer  2040 . This also permits greater polyaxial flexibility for the screw  22  without reducing the ability of the retainer  2040  to impede screw back-out. By deforming the tab walls  2072  downward, raised or rough surfaces on the plate  2002  are minimized, thereby reducing discomfort to a patient&#39;s esophagus or the likelihood of the plate  2002  catching on or irritating surrounding tissues. 
     The plate assembly  2000  may be held and manipulated with a plate holder such as the plate holder  1400 , discussed above, or a plate holder  2500 , discussed below. To facilitate this, the plate  2002  includes side recesses  2080  similar to the side recesses  1450  of the plate  1002 . The side recesses  2080  are positioned close to the bores  2004  so that the guide tool receptors  1420  of the plate holder  1400 , for instance, may be positioned over a pair of bores  2004  while the plate  2002  is being held, as discussed above. 
     A further form of a bone plate assembly  2100  is depicted in  FIGS. 87-91 . The bone plate assembly  2100  is a three-tiered dynamized plate assembly including a plate  2102  with a lower tier  2110 , an upper tier  2114 , and an intermediate tier  2112  of bores  2104  for receiving screws  22  for securing the plate  2102  with respective adjacent vertebrae  12   a ,  12   b , and  12   c . The intermediate tier  2112  includes non-dynamized bores  2116  so that the screw  22  received therein and its associated vertebra  12   b  are generally prevented from translating relative to the plate  2102 , while the upper and lower tiers  2114 ,  2110  include dynamized bores  2118  which permit relative translation of the superior and inferior vertebrae  12   a ,  12   c  towards the intermediate vertebra  12   b.    
     Utilizing the described configuration with the intermediate tier  2112  being fixed, the amount of translation that the dynamized bores  2118  of the upper and lower tiers  2114 ,  2110  need to permit is generally restricted to the amount of translation for the respective upper and lower vertebrae  12   a ,  12   c , in contrast to the prior art discussed above, and similarly to the bone plate assembly  10 , for instance. 
     The bone plate assembly  2100  is similar to the non-dynamized bone plate assembly  2000 . The bone plate assembly  2100  preferably is provided with initial curvature in the longitudinal and lateral directions, more preferably having respective radii of curvature of 200 millimeters and 20 millimeters. Recesses  2120  are provided along sides  2122  and a bottom surface  2124  of the plate  2102  for cooperating with the plate holder  1400 , for instance. The non-dynamized bores  2116  of the intermediate tier  2112  are substantially identical to the non-dynamized bores  2004  of the plate  2002 , and U-shaped retainers  2126  generally identical to the retainers  2040  are received therein. A recess  2140  is provided between the non-dynamized bores  2116 , and tab walls  2142  extend toward each other, separated by a gap  2144 , and above the recess  2140  to permit receipt of ends  2146  of legs  2148  of the retainers  2126  to be received therein. The tab walls  2142  are then deformed downward to secure the retainers  2126  with the plate  2102 , as has been described for plate  2002 . Windows  2130  are provided between the tiers  2110 ,  2112 ,  2114  for viewing the intervertebral site therebetween. 
     The major differences between the bone plate assembly  2100  and the bone plate assembly  2000  are in the dynamized bores  2118  and in retainers  2190  secured therein. The dynamized bores  2118  are similar to the non-dynamized bores  2116 , though elongated in the longitudinal direction of the plate  2102  to permit a screw  22  received therein and secured with a bone  12  to translate as the bone  12  itself translates towards the intermediate vertebra  12   b.    
     Towards this end, the features of the dynamized bores  2118  are racetrack or oval-shaped to have generally circular upper and lower ends  2150 ,  2152  and generally straight lateral sides  2154  ( FIG. 88 ), and each portion of the bores  2118  is correspondingly shaped in the same manner as the dynamized bores  40  of the bone plate  20  (see  FIG. 3 ). With reference to  FIG. 90 , the bores  2118  have a racetrack-shaped inner surface  2160  having a racetrack-shaped lowermost portion  2162  defining a racetrack-shaped throughbore  2164  with a racetrack-shaped brace surface  2166 . A racetrack-shaped shoulder  2168  is formed between the brace surface  2166  and a racetrack-shaped seat surface  2170 . An implanted screw head  26  secures against the inner surface  2160 , and the shank  28  of the screw  22  extends from the throughbore  2164  and is driven into its respective vertebra  12 . The inner surface  2160  permits the screw head  26  to translate therealong and within the bore  2118  as compression is applied to the vertebrae  12   a ,  12   c  such that the vertebrae  12   a ,  12   c  move toward the intermediate vertebrae  12   b . The screws  22  may be polyaxial or fixed in the manner described above, and both the dynamized and non-dynamized bores  2116 ,  2118  may be correspondingly structured such as described above for plate  1002 . 
     Each tier  2110 ,  2114  includes a recess  2180  for securing retainers  2190  with the plate  2102 . More specifically, each dynamized bore  2118  is provided with a retainer  2190  for preventing back-out of a screw secured therein. Each tier  2110 ,  2114  includes a pair of the dynamized bores  2118  and, hence, includes a pair of retainers  2190 . Each retainer  2190  includes a pair of free ends  2192 . The recess  2180  receives the pairs of free ends  2192  for both retainers  2190  of its respective tier  2110 ,  2114 , and the ends  2192  are secured therein, as will be discussed in greater detail below. This construction is similar to the recess  2140  of the non-dynamized bores  2116  securing the free ends  2146  of the U-shaped retainers  2126 . 
     The recess  2180  is positioned out of the bore-to-bore load path of the plate  2102 , as has been described for the retention recess  2070  of the plate  2002 . More specifically, the recess  2180  is offset of the load path so that it is lateral to the bores  2118 . As should be clear, the screws  22  are implanted through the bores  2118  so that they may translate inward and toward the intermediate tier  2112 . Accordingly, the screws  22  are implanted through the racetrack-shaped bores  2118  at a position towards the longitudinal ends of the plate  2102  so that the screws  22  may translate through the bores  2118  towards the middle of the plate  2102 . To minimize, if not eliminate, the ability of the screw head  26  to translate to a position in which it is aligned with the free ends  2192  of the retainer  2190 , the recess  2180  is positioned at a position of the bores  2118  towards the middle of the plate  2102 , as can be seen in  FIGS. 87 and 89 . 
     The recess  2180  is generally aligned with a racetrack-shaped retention recess  2200  for receiving the retainer  2190 . The retention recess  2200  is formed on the inner surface  2160  of the bore  2118 , and the retainer  2190  is positioned therein when assembled. The retention recess  2200  is preferably positioned so there is a small clearance between the retainer  2190  and a screw head  26  resting below the retainer  2190  and against the seat surface  2170 . As described above, this allows an amount of polyaxial positioning without the screw head top surface  29  coming into contact with the retainer  2190 . 
     The recess  2180  spans between the laterally-positioned and paired bores  2118  of its respective tier  2110 ,  2114 . A pair of tabs  2210  extend as shown in  FIG. 89  generally aligned with a top surface  2212  of the plate  2102 . The tabs  2210  are separated by the small gap  2214  and are positioned away from a surface  2216  defining one side of the recess  2180 . To secure the retainer  2190  in the recess  2180 , the retainer  2190  is positioned in the retention recess  2200 , the retainer free ends  2192  are compressed towards each other (described below) and inserted through the gap  2144 , and the free ends  2192  are released so that they expand and separate to be positioned within the recess  2180  and below the tabs  2210 . At this point, the retainer  2190  is generally restricted from separating from the plate  2102 . It is preferred then to swage or deform the tabs  2210  downward, toward the surface  2216 , so that the free ends  2192  are captured and generally permanently secured with the plate  2102 , as has been described above and is shown in  FIG. 84 . 
     The retainer  2190  for the dynamized bores  2118  is best viewed in  FIG. 91 . As can be seen, the retainer  2190  has an outer end  2220  that is, when assembled with the plate  2102 , generally proximate a longitudinal end of the plate  2102 . Opposite the outer end  2220  is an inner end  2222  positioned towards the middle of the plate  2102  when assembled. Accordingly, the retainer  2190  has a longitudinal length and direction aligned with the longitudinal length and direction of the plate  2102 , and aligned with the direction of travel by a translating screw  22  through the bore  2118 . As noted above, the free ends  2192  of the retainer  2190  are positioned lateral to the screw-to-screw load path of the plate  2102  and, hence, the free ends  2192  are positioned offset or lateral to the longitudinal direction of the retainer  2190 . It is also discussed above that the free ends  2192  are positioned away from the end of the retainer  2190  at which the screw  22  is initially implanted, prior to translation. This is so that, once the screw  22  has translated, the likelihood is minimized for the screw head to translate to a position in which it is aligned with the laterally positioned free ends  2192 . More specifically, the free ends  2192  are separated by a gap  2224 , and the described arrangement minimizes the likelihood that the screw head  26  will align with the gap  2224 . Nonetheless, in the event the screw head  26  were to translate sufficiently to become partially aligned with the gap  2224 , the screw head  26  would still be covered by portions of the retainer  2190 , as will be described in more detail. 
     As the recess  2180  receives free ends  2192  of a pair of retainers  2190 , the free ends  2192  of a first retainer  2190  are oriented inboard, towards the free ends  2192  of a second retainer  2190  of the same tier  2110 ,  2114 . The outer and inner ends  2220  and  2222  are connected by a straight portion  2230  oriented outboard of the free ends  2192 . The inner end  2222  connects the straight portion  2230  and extends to an upper free end  2192   a . The outer end  2220  connects with the outboard straight portion  2230  and extends to an inboard straight portion  2232  that is shorter than the outboard straight portion  2230 . The inboard straight portion  2232  in turn extends to and connects with a lower free end  2192   b.    
     The described geometry of the retainer  2190  provides improved coverage of the screw head  26  to prevent screw back-out. With the screw  22  implanted with the plate  2102  at the outer end  2220 , the screw  22  and its associated vertebra is permitted to translate toward the inner end  2222  and toward the intermediate bore  2116 . The outboard and inboard straight portions  2230 ,  2232  are generally parallel and are positioned above the screw head  26  to provide respective line-contacts with the top surface  29  thereof. As the screw  22  translates, the screw head  26  moves along the straight portions  2230 ,  2232  so that these portions of the retainer  2190  remain positioned above the screw  22  and in a position to block screw back-out. Were the screw  22  to translate to the inner end  2222 , the outboard straight portion  2230  remains positioned above the screw head  26 , as does at least the junctions between the free ends  2192  and the inboard straight portion  2232  and the inner end  2222 . 
     During implantation, the screw  22  is driven into the bone  12 . The head of the screw  26  wedges against the straight sections  2230 ,  2232  to force the retainer  2190  to open and permit the screw head  26  to pass therethrough. Once the screw head  26  has passed by, the retainer  2190  returns to its pre-insertion form so that the straight portions  2230 ,  2232  automatically position themselves over the screw head top surface  29 . Thus, the retainer  2190  prevents or minimizes the ability of the screw to back out. 
     The retainers, such as retainers  2040  and  2190 , operate in a manner similar to the other retainers described herein. Securing the bone plate, such as plate  2102 , and setting the retainers to prevent the screws  22  from backing out requires a minimum of steps. That is, the retainers are pre-set and assembled with the bone plate. The screw  22  need only be driven into the bone  12 , the action of which utilizes the screw head  26  to shift the retainer to an open position for permitting the screw head  26  to pass through and by the retainer. Once the screw head  26  has cleared, the retainer returns to its normal position. In one sense, the securing of the retainers is a no-step process for a surgeon implanting the plate. 
     In a preferred form, the retainers  2040  and  2190  are formed of a wire with a generally circular cross-section. The diameter of the wire is approximately 0.023 inches, and the material for the wire is preferably a biocompatible metal, such as titanium  64 , that is ductile enough to permit expansion of the retainer  2040 ,  2190  during screw insertion and with a stiffness or Young&#39;s modulus sufficiently high to restrict screw back out. 
     Referring now to  FIGS. 92-97 , an alternative form of a bending tool  2300  for adjusting the longitudinal curvature of a bone plate is depicted. The bending tool  2300  includes a body  2302  for carrying the components thereof. In the present form, the body  2302  includes an integrally formed body handle  2304 , though it may be a separate component that is preferably rigidly secured with the body  2302 . In operation, a bone plate is positioned between a fulcrum  2306  and a pair of bending assemblies  2308 , and an actuator handle  2310  is shifted toward the body handle  2304 . The actuator handle  2310  operates to force the fulcrum  2306  against the bone plate and towards bending members  2312  of the bending assemblies  2308 . The bending members  2312  and the fulcrum  2306  each have convex and concave surfaces and are selectively positionable so that operation of the actuator handle  2310  may increase or decrease the curvature of the bone plate. A pair of leaf springs  2314  are provided between the handles  2304 ,  2310  for biasing the actuator handle  2310  away from the body handle  2304 . 
     The actuator handle  2310  is pivotable on the body  2302  to shift the fulcrum  2306 . Towards this end, the actuator handle  2310  includes a throughbore  2316  generally aligned with the longitudinal direction of a grip portion  2320  of the actuator handle  2310  and positioned at an uppermost portion of the actuator handle  2310 . The body  2302  is provided with an accompanying throughbore (not shown), and the body  2302  and actuator handle  2310  are coupled by a pin  2318 . As the force applied to the actuator handle  2310  may be significant, and a portion of this force is resolved through the pin  2318 , a low-friction bushing (not shown) may be provided around the pin  2318 . 
     As it pivots towards the body handle  2304 , the actuator handle  2310  cams against the fulcrum  2306 . More specifically, the actuator handle  2310  further includes an actuating spur  2322  generally extending transversely away from the grip portion  2320 . The spur  2322  includes a roller  2324  secured thereto by a pin  2326 . The roller  2324  is free to rotate around the pin  2326  and relative to the spur  2322 . During pivoting of the grip portion  2320  towards the body handle  2304 , the spur  2322  and roller  2324  pivot upwardly and toward the fulcrum  2306 . The roller  2324  contacts the fulcrum  2306  to apply pressure thereagainst. As the roller  2324  is free to rotate, it is able to roll against the fulcrum  2306 . This allows for smooth and even relative moving, as opposed to sliding under pressure which may result in wear or frictional binding. 
     As noted above, the fulcrum  2306  is shiftable to apply pressure against a bone plate positioned between it and the bending members  2312 . The fulcrum  2306  has a bore  2330  for receiving and being supported by an axle  2332  (see  FIG. 96 ). As will be discussed below, the fulcrum  2306  is selectively positionable by rotation around the axle  2332 . The axle  2332  extends from the fulcrum bore  2330  and through a bore  2333  of a support block  2334 , which is itself received within an opening of the body forming a guide  2336  (see  FIG. 96 ). The support block  2334  includes a stepped block body  2338 . More specifically, the block body  2338  includes a portion  2338   a  with first lateral and longitudinal dimensions and a portion  2338   a  with greater lateral and longitudinal dimensions than those of the portion  2338   a , thus forming a step or shoulder  2340  between the portions  2338   a  and  2338   b.    
     The block body  2338  and guide  2336  cooperate to define the path of travel of the fulcrum  2306 . As shown, each of the block body portions  2338   a  and  2338   b  respectively have at least partially straight or flat lateral sides  2342   a ,  2342   b . The guide  2336  provides a complementary geometry to the block body  2338 . In particular, the guide  2336  has a portion  2336   a  with longitudinal and lateral dimensions and a portion  2336   b  with larger longitudinal and lateral dimensions than those of the portion  2336   a , thereby forming a step or shoulder  2344  between the portions  2336   a ,  2336   b . In the lateral directions, the guide  2336  has sides  2344   a ,  2344   b  which are at least partially straight and within and against which the flat portions of the block body sides  2342   a ,  2342   b  are positioned, as is best seen in  FIG. 94 . However, the vertical or longitudinal dimensions of the guide portions  2336   a ,  2336   b  are greater than the corresponding dimensions of the block body portions  2338   a ,  2338   b . This dimensional difference allows the support block  2334  to shift within the guide  2336 . The complementary flat side portions  2342   a ,  2342   b ,  2344   a , and  2344   b  allow the shifting or translation of the support block  2334  in the guide  2336 , as well as the fulcrum  2306  supported by the support block  2334 , while generally restricting or minimizing pivoting or rotation of the support block  2334  within the guide  2336 . 
     The bending assemblies  2308  include and support the bending members  2312  so that the bone plate may be positioned between the bending members  2312  and the fulcrum  2306  for adjusting the curvature of the plate. The bending assemblies  2308  are slidably supported by guide slots  2348  formed in the body  2302 . The guide slots  2348  are slightly arcuate to define an arcuate path, preferably with a radius of curvature of approximately 200 millimeters to coordinate with the pre-bent curvature of the plate. 
     Each bending assembly  2308  is selectively positionable along its respective guide slot  2348 . To select a position, the bending assembly  2308  includes a knob  2350  threadably cooperating with an axle  2352  on which the bending member  2312  is rotatably supported. To prevent the axle  2352  from rotating due to the rotation of the knob  2350 , the axle  2352  is provided with a generally square, central portion  2354 . The slightly arcuate guide slot  2348  has upper and lower surfaces  2356   a  and  2356   b  between which the central portion  2354  is located. To facilitate sliding and positioning of the axle  2352  along the slightly arcuate guide slot  2348 , corners  2357  of the central portion  2354  are rounded to minimize binding. The central portion  2354  forms a shoulder  2358  with the generally cylindrical axle  2352  which is positionable against a shoulder  2360  formed on the upper and slower surfaces  2356   a  and  2356   b.    
     Rotation of the knob  2350  in one direction extends the distance between the knob  2350  and the shoulder  2358  on the axle central portion  2354 . Accordingly, the knob  2350  moves away from the body  2302  surrounding the guide slots  2348 , and the axle shoulder  2358  moves away from the guide slot shoulder  2360 . This allows the bending assembly  2308  to then be slid along the guide slot  2348  to a desired position. Rotation of the knob  2350  in an opposite direction shortens the distance between the axle shoulder  2358  and the knob  2350  so that the knob  2350  moves toward and against the body  2302  while the axle shoulder  2358  moves towards and against the guide slot shoulder  2360 . Continued rotation of the knob  2350  tightens in a clamping fashion the body  2302  between the knob  2350  and the axle shoulder  2358  to fix the axle  2352  in the selected position. 
     As mentioned earlier, both the fulcrum  2306  and the bending members  2312  include convex and concave surfaces. Therefore, each of the fulcrum  2306  and bending members  2312  are selectively positioned for these surfaces to cooperate to bend the bone plate. More specifically and with reference to  FIG. 95 , the fulcrum  2306  has an convex extension  2360  with a convex surface  2362 , as well as a concave extension  2364  with concave surface  2366 . The bending members  2312  are generally block-shaped with opposed concave sides  2312   a  and convex sides  2312   b . Each of the bending members  2312  and the fulcrum  2306  may be rotated around their respective axles  2352  and  2322  to selectively position the convex or concave surface  2362 ,  2366  of the fulcrum  2306  in opposed relationship or orientation to the convex or concave sides  2312   a ,  2312   b  of the bending members  2312 . As can be seen in  FIG. 93 , the concave surface  2366  of the fulcrum  2306  is utilized with the convex sides  2312   b  of the bending members  2312 . When oriented as depicted, application of force against a bone plate positioned between the fulcrum  2306  and the bending members  2312  by forcing the fulcrum  2306  against the bone plate by pivoting the actuator handle  2310  decreases the curvature of the plate. When the fulcrum  2306  is pivoted so the convex extension  2364  is oriented towards the bending members  2312 , the bending members  2312  are also pivoted so that the convex sides  2312   b  are oriented towards the fulcrum  2306 . With the application of force on the plate in this configuration, the convex extension  2364  and the bending members  2312  serve to increase the curvature of the bone plate. 
     In operation, the fulcrum  2306  and the bending members  2312  are positioned and secured for either increasing or decreasing the curvature of a bone plate. The plate is then positioned between the fulcrum  2306  and the bending members  2312  so that the fulcrum  2306  is aligned with a desired bending area. It should be noted that it is preferred to bend the plate in an area aligned with the window, as opposed to the bores, so that the structural integrity of the bores and the retainers located therein is minimally affected, if at all. If it is desired to bend the plate along a second bending area, the plate is simply shifted to that second area after the first area has been bent. 
     The application of force on the bone plate by the fulcrum  2306  and the bending members  2312  has the potential for marring or otherwise galling the surface of the plate. This damage can create stress concentrators which may lead to failure of the implanted bone plate. To reduce this likelihood, it is preferred that the bone plate be made of a material that is harder than that of the bending members  2312  and the fulcrum  2306 . As examples of materials, the bone plate may be made of a biocompatible metal such as titanium  64 , the bending members  2312  are made of a polymeric material such as PEEK, and the fulcrum  2306  is made of stainless steel. It should also be noted that, when the knobs  2350  have been rotated to generally secure the bending assemblies  2308  in the desired position, the bending members  2312  are free to rotate around their respective axles  2352 . This allows the bending members  2312  to self-adjust with respect to the plate during the bending operation, thus reducing the likelihood that a misaligned convex side  2312   a  or concave side  2312   b  of the bending member  2312  may localize pressure on the bone plate. 
     The fulcrum  2306  is provided with distinct positions to align either the convex extension  2360  or the concave extension  2364  in a generally vertical direction for applying force to a bone plate positioned between the fulcrum  2306  and the bending members  2312 . As best seen in  FIGS. 95 and 96 , the convex and concave extensions  2360 ,  2364  are radially located around a fulcrum body  2368  and offset from each other by approximately ninety degrees. Thus, the fulcrum  2306  may be rotated approximately a quarter-turn between the distinct positions for aligning the convex and concave extensions  2360 ,  2364  in the operable orientation. 
     The fulcrum body  2368  cooperates with the support block  2334  to define the distinct positions. As can be seen in  FIGS. 94 and 96 , a rear side  2370  of the fulcrum body  2368  includes structure for cooperating with the support block  2334  for defining the positions of the fulcrum  2306 . More specifically, the rear side  2370  includes a pair of recesses in the form of blind holes  2372 , each being aligned between the fulcrum bore  2330  and a respective one of the convex and concave extensions  2360 ,  2364 . The support block  2334  includes a pin  2374  extending from the block body  2338  parallel to the fulcrum axle  2332 . To position the fulcrum  2306  in one of the distinct positions, the fulcrum  2306  is positioned so that the pin  2374  is aligned with and received within a blind hole  2372  selected for the convex or concave extension  2360 ,  2364  to which the blind hole  2372  corresponds. 
     To simplify the positioning and pivoting of the fulcrum  2306  as described, the fulcrum  2306  is spring-biased towards the support block  2334 . Reference is made herein to the fulcrum  2306  rotating or pivoting around its associated axle  2332 , and it should be noted that it is preferred for the axle  2332  to rotate with the fulcrum  2306 , thereby rotating within the support block bore  2333  and relative to the support block  2334 . To select or change its position, the fulcrum  2306  is drawn away from the support block  2334  so that the pin  2374  is withdrawn from the blind holes  2372 . Accordingly, the axle  2332  is shifted through the support block bore  2333 . As can be seen in  FIGS. 96 and 97 , a bias member in the form of a coil spring  2376  may be positioned within a spring recess  2378  formed in a rear portion of the support block  2334 . The axle  2332  extends through the spring  2376 , and the spring  2376  is secured between a shoulder  2380  formed within the spring recess  2378  and a washer  2382  secured on an end  2332   a  of the axle  2332  such as by a screw  2384 . As the fulcrum  2306  is pulled away from the support block  2334 , the spring  2376  is compressed between the washer  2382  secured with the fulcrum axle  2332  and the spring recess shoulder  2380 . Once the pin  2374  is aligned with the desired blind hole  2372  of the fulcrum body  2368 , the fulcrum  2306  may be released so that the spring  2376  shifts the pin  2374  into the blind hole  2372  and the fulcrum  2306  against the support block  2334 . The plate may then be positioned between the fulcrum  2306  and the bending members  2312  for adjusting the curvature of the bone plate. 
     A sizing caliper  2400  is shown in  FIG. 98  similar to the sizing caliper  1200  of  FIGS. 56-59 . The sizing caliper  2400  has a pair of legs  2402 ,  2406  which are adjustably positionable to determine a proper plate size for a particular patient and implant site. The sizing caliper  2400  is generally identical to the sizing caliper  1200 , with the difference that the legs  2402 ,  2406  of the sizing caliper  2400  each include a ball-shaped tip  2408 . This is in contrast to the measuring leg  1204  and the reference leg  1206  of the sizing caliper  1200 , where the reference leg  1206  has the sharp tip  1208 . In some instances, the use of the sharp tip  1208  in proximity to sensitive or fragile tissues may incur a greater potential for damage than is desirable. Accordingly, both legs  2402 ,  2406  have the ball-shaped tip  2408  allowing a surgeon to manipulate the sizing caliper  2400  at the implant with a more limited risk of damage to surrounding tissues. 
     With reference to  FIG. 99 , a plate holder  2500  for holding and positioning a bone plate during implantation. The plate holder  2500  is a forceps-type device having a pair cooperating pivotable arms  2502  secured by a pivot pin  2504 . A proximal end  2506  of the plate holder  2500  includes handles  2508  for manipulating the arms  2502 , and a distal end  2510  includes guide tool receptors  2514  for receiving a drill guide such as the drill guide  1500 , discussed above, or a drill guide  2600 , discussed below. In a more distal position from the drill guide receptors  2514  are barbs or prongs  2516  for being received in side recesses of the bone plate, such as the side recesses  2080  of the plate  2002 . 
     The plate holder  250  is generally identical to the plate holder  1400  of  FIGS. 67-71 , except for the construction of the drill guide receptors  2514 . As can be seen in  FIG. 102 , each of the drill guide receptors  2514  includes an opening or window  2520  formed by, in comparison with the plate holder  1400 , removing some material. The window  2520  is preferably formed on the side of the receptor  2514  opposite the connection between the receptor  2514  and its associated arm  2502 . During implantation of a screw  22 , the window  2520  allows a surgeon to see the head  26  of the screw  22 , thereby facilitating the surgeon&#39;s control of the screw  22  beyond the tactile feel imparted by driver, such as the drivers  900  and  2700  described herein. 
     Referring now to  FIGS. 100-103 , an alternative form of a drill guide  2600  is depicted. In comparison with the drill guide  1500  of  FIGS. 72 and 73 , the drill guide  2600  is a simpler device with a simpler operation. As described, the drill guide  1500  requires a two-handed operation to secure and release the socket portion  1504  and the plate holders  2500  and  1400 . In contrast, the drill guide  2600  requires merely a one-handed operation. 
     The drill guide  2600  includes a tubular body  2602  including a throughbore or cannula  2604  ( FIG. 103 ) for receiving the pilot hole tool  1600 , or the like, as described for the cannula  1524  of the drill guide  1500 . The tubular body  2602  has a proximal end  2606  into which the pilot hole tool  1600  is inserted into the cannula  2604 . The tubular body  2602  further has a distal end  2608  including a socket portion  2610  for engaging and removably securing with the plate holder guide receptors  2514 , for instance and as described above. 
     The socket portion  2610  forms a ball-and-socket type connection with the plate holder guide receptors  2514 . Towards this end, the socket portion  2610  includes a plurality of resiliently deflectable prongs  2612 , a portion of each of being positionable within the guide receptors  2514 . Each prong  2612  has an inner arcuate surface  2614  contiguous with the cannula  2604  for permitting the pilot hole tool  1600  to operate therewithin. Each prong  2612  further has an enlarged partial spheroidal portion  2616  so that the prongs  2612  together generally form a partial spheroidal structure for the socket portion  2610 . As noted above, the plate holder guide receptors, such as guide receptors  2514  of the plate holder  2500 , include an inner surface that is generally partially spheroidal. Between each prong  2612  is a gap  2620  so that each prong  2612  may be deflected inwardly by forcing the socket portion  2610  into the guide receptors  2514 . To remove the socket portion  2610  and the drill guide  2600  in general from the plate holder  2500  and the guide receptors  2514 , the drill guide  2600  is simply pulled away from the plate holder  2500 . Alternatively, the drill guide  2600  may be forced sideways out of alignment with the plate holder guide receptors  2514  so that the prongs  2612  deflect inwardly, and the drill guide  2600  releases from or pops out of the plate holder  2500 . 
     In comparison with the drill guide  1500 , the drill guide  2600  provides a view of the screw implantation or pilot hole site. As noted above, the plate holder  2500  provides a window  2520  for view the screw implantation site. The drill guide  2600  therefore has a window  2630  which may be aligned with the plate holder window  2520  for viewing the screw implantation site through the aligned windows  2630 ,  2520 . In simple terms, the window  2630  is a gap, like the gaps  2620 , between adjacent prongs  2612  though significantly larger than the gaps  2620 . The window  2630  may easily be formed by removing a prong  2612  during manufacturing. To facilitate the alignment of the drill guide window  2630  with the plate holder window  2520 , a surface feature such as a flat  2636  may be formed on a portion of the tubular body  2602 , as can be seen in  FIG. 103 . 
     The drill guide  2600  may be fixed or pivotable. That is, the drill guide  2600  may be constructed to assume a fixed orientation relative to the plate holder guide receptors  2514 . Alternatively, the drill guide  2600  may be pivotable to so that its orientation is selected by a user. For example, in making a pilot hole for a variable angle or polyaxial screw, the user may pivot the drill guide  2600  relative to the guide receptors  2514  to a desired angle for the pilot hole for the screw. 
     As shown, the drill guide  2600  is a fixed angle or orientation drill guide. Thus, the drill guide  2600  assumes a specific orientation with the plate holder guide receptors  2514 . To do so, the prongs  2612  each include an arcuate shoulder  2640  so that the shoulders  2640  together form a generally annular shoulder  2642 . When the drill guide socket portion  2610  is properly seated within the guide receptor  2514 , the annular shoulder  2642  contacts an upper surface of the guide receptors  2514  so as to form a stop therewith. The stop prevents the drill guide  2600  from pivoting relative to the guide receptor  2514 . It should be noted that a maximum amount of pivoting may be permitted by providing the prongs  2612  without the shoulders  2640  thereon. Pivoting may alternatively be provided in a pre-determined amount by providing the shoulders  2640  on the prongs  2612  and positioning the shoulders  2640  from the spheroidal portion  2616  a distance selected to permit the pre-determined pivoting prior to the shoulders  2640  shifting into contact with the guide receptors  2514 . 
     A driver  2700  similar to the driver  900  of  FIGS. 41-43  is shown in  FIG. 104 . In particular, the driver  2700  is substantially identical to the driver  900  in structure and operation, with the exception of a modified sleeve  2702  corresponding to the outer sleeve  904  of the driver  900 . The sleeve  2702  is threaded at its distal tip  2704  for threading into internal threads  302  of the screw head drive recess  300 . A driving shaft  2706  is received within the sleeve  2702  so that a driving end  2708  may protrude from the sleeve  2702 . The driving end  2708  is engagable with the hexagonal portion of the drive recess  300 . In order to promote cleaning of the driver  2700 , and in particular cleaning between the sleeve  2702  and the driving shaft  2706 , a plurality of holes or ports  2710  are formed in the sleeve  2702 . In this manner, the interior of the driver  2700  may be more easily flushed or otherwise cleaned, such as by autoclaving. 
     With reference to  FIGS. 105 and 106 , an extractor  2800  similar to the extractor  1900  of  FIGS. 74-75  is shown for assisting in opening the retainers disclosed herein for removal of a screw  22 . The extractor  2800  cooperates with a driver, such as driver  2700 , to open the retainers which the driver removes the screw  22 . Towards this end, the extractor  2800  includes a cylindrical, tubular body  2802  having a central cannula  2804  extending from a proximal end  2806  to a distal end  2808 . The driver may be inserted through the cannula  2804  so that the driving shaft  2704  and sleeve  2704  extend from the distal end  2808 . The driving shaft  2704  may be seated in the drive recess  300  of the screw head  26  while the sleeve  2704  is threaded into the drive recess  300 . 
     The distal end  2808  includes a pair of diametrially opposed prongs  2820  for shifting portions of a retainer, such as retainer  2192 , outward so that the screw  22  implanted below the retainer  2192  may be removed. To do so, the prongs  2820  are inserted between and in parallel alignment with portions of the retainer that are generally contacted by the screw head  26  as it is driven into the bone  12 . As an examplary operation with the retainer  2192 , the prongs  2820  may be aligned within each other between the outer and inner ends  2220 ,  2222 . The extractor  2800  is then rotated approximately a quarter-turn so that the prongs  2820  cam against and force the straight portions  2230 ,  2233  away from each. The extractor  2800  is then held generally in this position while the driver  2700  is utilized to remove the screw  22 . 
     To ease the joint operation of the extractor  2800  and driver  2700 , a friction fit is provided therebetween. While the extractor  2800  generally rotates approximately a quarter-turn, the driver  2700  rotates a number of revolutions. Therefore, it is desirable, if not necessary, for the extractor  2800  to permit the driver  2700  to rotate relative thereto. On another hand, operation of the extractor  2800  and driver  2700  is simplified if they jointly enter and exit the surgical field or implant site jointly. The friction fit permits the relative rotation by not securing in a specific orientation, while also enabling the driver  2700  and extractor  2800  to be jointly manipulated without separating. To achieve this friction fit, a preferred embodiment provides the cannula  2804  with an O-ring type member  2830  into which the driver sleeve  2702  is received. More specifically, the cannula  2804  includes an interior groove  2834  (see  FIG. 106 ) into which the O-ring  2830  is pre-set. 
     While the invention has been described with respect to specific examples including presently preferred modes of carrying out the invention, those skilled in the art will appreciate that there are numerous variations and permutations of the above described systems and techniques that fall within the spirit and scope of the invention.