Abstract:
A spinal fixation device combines an anchor member with an open receiver, such as a polyaxial bone screw or a hook, with a rotatable closure that operably clamps a spinal fixation rod to the anchor member. The anchor member has spaced apart arms forming a rod receiving channel. The arms have arm extensions or tabs connected to main portions of the arms by weakened regions to enable the extensions to be broken off or separated after the rod is clamped. The closure and inner surfaces of the arms and tabs have mating helical, anti-splay, reverse angle guide and advancement structure formed thereon that mechanically cooperate to prevent splaying of the arms and the extensions as the closure is advanced into the rod receiving channel. The increased length of the arms with the extensions enables the rod to be captured at a greater distance from the seat of the channel and allows the rod to be urged toward the seat by helical advancement of the closure into the channel, starting between the extensions. Separation of the break-off extensions results in an implant with a desirable low profile.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Application Ser. No. 60/627,000 filed Nov. 10, 2004, and is a continuation-in-part of U.S. patent application Ser. No. 10/986,377 filed Nov. 10, 2004, and also is a continuation-in-part of U.S. patent application Ser. No. 09/644,777 filed Aug. 23, 2000. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to improvements in interlocking or interconnecting helical guide and advancement structures such as reverse angle thread forms and helical flanges and, more particularly, to mating helical guide and advancement arrangements providing anti-splay interconnection when radial loading or engagement occurs. Such guide and advancement structures with anti-splay contours are particularly advantageous when used in combination with open headed bone screws formed with extended arms or tabs to facilitate the capture and reduction of spinal fixation rods, after which the arm extensions or tabs are broken off at weakened areas to form a low profile implant. In particular, in the present invention, the interlocking anti-splay components also are found on the extensions such that force can be applied to a closure and through the closure to a rod positioned between the extensions without splaying the extensions, as the closure holds them in fixed position relative to each other as the closure traverses between the extensions.  
         [0003]     Medical implants present a number of problems to both surgeons installing implants and to engineers designing them. It is always desirable to have an implant that is strong and unlikely to fail or break during usage. Further, if one of a set of cooperating components is likely to fail during an implant procedure, it is desirable to control which particular component fails and the manner in which it fails, to avoid injury and to minimize surgery to replace or repair the failed component. It is also desirable for the implant to be as small and lightweight as possible so that it is less intrusive to the patient. These are normally conflicting goals, and often difficult to resolve.  
         [0004]     One type of implant presents special problems. In particular, spinal anchor members such as bone screws, hooks, and the like are used in many types of back surgery for repair of problems and deformities of the spine due to injury, disease or congenital defect. For example, spinal bone screws typically have one end that threads into a vertebra and a receiver at an opposite end. The receiver is formed with an opening and a channel for receiving a rod or rod-like member that is then both captured in the channel and locked in the receiver to prevent relative movement between the various elements subsequent to installation.  
         [0005]     A particularly useful type of receiver for such bone screws is an open receiver or head wherein an open, generally U-shaped channel is formed in the receiver, and the rod is simply laid in the open channel. The channel is then closed with some type of a closure member that engages the walls or arms forming the receiver and clamps or secures the rod in place within the channel.  
         [0006]     While the open receiver devices are often necessary and preferred for usage, there is a significant problem associated with them. The open devices conventionally have two upstanding arms that are on opposite sides of the channel and receive the rod member. The top of the channel is closed by a closure member after the rod member is placed in the channel. Many open implants are closed by threaded plugs that screw into threads formed on internal surfaces between the arms, because such configurations have low profiles. However, such threaded plugs have encountered problems in that they produce radially outward forces that lead to splaying of the arms or at least do not prevent splaying that in turn may lead to loosening of parts and failure of the implant. In order to lock the rod member in place, a significant force must be exerted on the relatively small plug or on a set screw of some type. The forces are required to provide enough torque to insure that the rod member is clamped or locked securely in place relative to the bone screw, so that the rod does not move axially or rotationally therein. This typically requires torques on the order of 100 inch-pounds.  
         [0007]     Because implants with open receivers such as bone screws, hooks and the like are relatively small, the arms that extend upwardly at the receiver can be spread by radially outwardly directed forces in response to the application of the substantial torquing force required to clamp the rod member. Historically, early closures were simple plugs that were threaded with V-shaped threads and were screwed into mating threads on the inside of each of the arms. Outward flexure of the arms of the receiver was caused by mutual camming action of the V-shaped threads of the plug and receiver as advancement of the plug was resisted by clamping engagement with the rod while rotational urging of the plug continued. If the arms of such a receiver are sufficiently spread, they can allow the threads to loosen or disengage and the closure to fail. To counter this, various engineering techniques have been applied to the receiver to increase resistance to the spreading force. For example, in some receivers, the arms were significantly strengthened by increasing the width of the arms by many times. This leads to a larger profile implant, which is always undesirable and may limit the working space afforded to the surgeon during implant procedures. Alternatively, external caps have been devised that engage external surfaces of the receiver. In either case, the unfortunate outcome is a substantial increase in the bulk, size and profile of the implant, especially when external nuts have been used, that take up space along the rod, so as to leave too little space for placement of all of the implants needed for a particular procedure.  
         [0008]     The radial expansion problem of V-threads has been recognized in various other applications of threaded joints. To overcome this problem, so-called “buttress” thread forms have been developed. In a buttress thread, the trailing or thrust surface, also known as the load flank, is oriented perpendicular to the thread axis, while the leading or clearance surface, also known as the stab flank, remains angled. This results in a neutral radial reaction of a threaded receptacle to torque on the threaded member received. However, even buttress threaded closures may fail as such do not structurally resist splaying of the arms.  
         [0009]     Another challenge of medical implant design is the placement or capture of a rod or other structural member between the arms of an open receiver. Rods implanted in spinal fixation systems are typically bent or shaped to determine the shape of the corrected curvature of the spinal column and are anchored along their length by open receiver bone screws implanted into individual vertebrae. Because of the complex curvature that must be applied to the rods, it is often difficult to capture a portion of a straight or curved rod in a bone screw receiver and to clamp the rod within the receiver arms because such receiver arms are often minimized in length to reduce the profile thereof and minimize the impact of the implanted system on the patient. So although it is desirable, on the one hand, to form the arms of an open receiver as short as possible to result in a low profile implant, it is often difficult to urge a spinal fixation rod into the U-shaped channel between the arms of such a receiver.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention solves one or more problems previously described herein by combining a reverse angle structure for guiding and advancing a closure member into a receiver with the addition of arm extensions or tabs. Such extensions are disposed adjacent to main portions of the arms and connected thereto by weakened break-off regions.  
         [0011]     As compared to buttress and square thread forms that have a neutral radial effect on the screw receptacle, Applicant&#39;s reverse angle structure of the invention provides a thread form that positively draws threads of a receiver radially inwardly toward the thread axis when a closure member is rotated and torqued therein. In a reverse angle thread form, the trailing side of the external thread is angled toward the thread axis instead of away from the thread axis, as in conventional V-threads. The present invention utilizes such a thread form to provide an improved mating guide and advancement reverse angle structure for guiding and advancing a closure member between both the arm extensions and the receiver arms in response to relative rotation of the closure member and the receiver. The extended arms of the receiver provide ease in capturing a rod or other structural member therebetween. A closure member may then be more easily inserted and rotated to drive the rod downwardly into the receiver of the implant. Extensions according to the invention necessarily include weakened regions, providing a break-off location for removal of the extensions after the closure is fully seated in the implant, resulting in a desired low profile implant.  
         [0012]     The reverse angle guide and advancement structure of the present invention provides a distinct advantage over the use of conventional V-shaped threads in which the potential for outward flexure and splaying of the extensions, as well as the receiver arms, would be great, and might further result in the undesirable break off of the extensions prior to the closure member being disposed in the receiver, unless some sort of cap or sleeve would be used to keep the extensions from splaying. According to the invention, inner surfaces of the extensions have a helical reverse angle guide and advancement structure formed thereon to receive a closure with a complementary reverse angle guide and advancement structure thereon for rotation into the arms of the receiver. Stated in another way, the extensions have the same anti-splay structure thereon as is found on the arms of the receiver. Furthermore, the reverse angle structure on the extensions is aligned with that on the arms so as to provide a continuous helical path for the mating structure on the closure member to follow.  
         [0013]     The extensions or tabs enable the rod to be captured at a greater distance from the anchoring vertebra and urged toward the vertebra by advancement of the closure toward the open receiver. Just as the anti-splay guide and advancement structure on the closure member and the receiver arms cooperate to prevent splaying of the arms, the anti-splay structure on the extensions cooperates with the cooperating structure on the closure to prevent unwanted splaying of the extension and guides the closure to allow mating with the guide and advancement structure on the arms simply by rotating the closure. Thus, the guide and advancement structure on the closure does not have to be realigned with the cooperating structure on the arms. Furthermore, pressure applied to the rod while between the extensions is continued as the rod passes between the arms. The anti-splay reverse angle structure of the present invention makes the use of such extended arms or tabs possible, even when substantial force must be applied to the rod and even though the extensions include weakened regions so that when a rod has been seated in the rod receiving channel of the receiver and sufficiently clamped, the extensions or tabs can be broken off the main portions of the arms to provide the desired low profile implant. Because of the flimsy or weakened nature of such extensions, it would not even be feasible to successfully equip extensions with V-threads, not only because of the potential for outward splaying of the extensions as force is applied to the rod by the closure member, but also because of the potential that such splaying would cause premature break-off of such extensions.  
       Objects and Advantages of the Invention  
       [0014]     Objects and advantages of the present invention include: providing an improved helical guide and advancement structure for guiding and advancing an inner member into an outer member; providing a reverse angle structure wherein the outer member is subject to being splayed in reaction to advancement and torquing of the inner member within the outer member and wherein the inner member and the outer member are particularly configured to cooperate in such a manner as to radially resist such splaying while allowing rotation and axial advancement; providing such a reverse angle structure for cooperative radially overlapping surfaces between a closure and an implant with open receiver arms equipped with extensions for receiving a rod being passed between the extensions to receiver arms, the closure pressing against the rod by the rotation of the closure along the extension and the arms; providing such a reverse angle guide and advancement structure that is particularly well adapted for use in surgically implanted structure, such as spinal fixation hardware and, particularly, to receivers and cooperating closures that are used to receive and clamp spinal fixation rods; providing such a reverse angle guide and advancement structure that is particularly well adapted for use with bone screws having open receivers with extended arms for facilitating the capture and reduction of spinal fixation rods and that are afterwards separated from the screw receiver and related implants to provide low profile implants; and providing such improved reverse angle helical guide and advancement structure that is economical to manufacture, strong and effective in use, and is particularly well adapted for the intended purpose thereof.  
         [0015]     Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention.  
         [0016]     The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is an exploded perspective view of a polyaxial bone screw assembly according to the present invention having a shank, a receiver with arm extensions, and a shank retaining structure and further showing a rod and closure structure.  
         [0018]      FIG. 2  is an enlarged cross-sectional view of the shank taken along the line  2 - 2  of  FIG. 1 .  
         [0019]      FIG. 3  is an enlarged top plan view of the shank of  FIGS. 1 and 2 .  
         [0020]      FIG. 4  is an enlarged top plan view of the retaining structure of  FIG. 1 .  
         [0021]      FIG. 5  is an enlarged cross-sectional view taken along the line  5 - 5  of  FIG. 4 .  
         [0022]      FIG. 6  is a cross-sectional view of the receiver taken along the line  6 - 6  of  FIG. 1  and showing the retaining structure seated in the receiver, also in cross-section, and illustrating the retaining structure being inserted into the receiver in dashed lines.  
         [0023]      FIG. 7  is an enlarged and fragmentary side elevational view of the assembly of  FIG. 1  with portions broken away to show the detail thereof, illustrating the retaining structure mated with the shank and the closure structure pressing upon a rod disposed between the arm extensions and reducing the rod toward the receiver by rotation of a tool engaged with the closure structure.  
         [0024]      FIG. 8  is an enlarged and fragmentary view similar to  FIG. 7  showing the rod engaged with both the shank and the closure structure and with the arm extensions removed.  
         [0025]      FIG. 9  is a cross-sectional view taken along the line  9 - 9  of  FIG. 8 .  
         [0026]      FIG. 10 a  fragmentary and enlarged perspective view of the assembly of  FIG. 1  shown completely assembled with the rod and closure structure. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure.  
         [0028]     Referring to the drawings in more detail, the reference numeral  1  designates a receiver according to the invention having a component of a helical guide and advancement reverse angle structure, generally 3, in combination with upwardly extending break-off tabs or extensions  5  used in conjunction with a medical implant assembly, generally 7, that embodies the present invention. It is noted that any reference to the words top, bottom, up and down, and the like, in this application refers to the alignment shown in the various drawings, as well as the normal connotations applied to such devices, and is not intended to restrict positioning of the receiver  1  and the medical implant assembly  7  in actual use.  
         [0029]     The reverse angle guide and advancement structure  3  according to the invention includes a reverse angle thread form  10  extending helically on an inner member  16  and a complimentary reverse angle thread form  19  extending helically within an outer member  21  illustrated in the drawings as being a portion of the receiver  1 . The reverse angle thread forms  10  and  19  cooperate to helically guide the inner member  16  into the outer member  21  when the inner member  16  is rotated and advanced into the outer member  21 . The inner and outer thread forms  10  and  19  provide respective anti-splay surfaces  24  and  26  that cooperate to prevent splaying tendencies of the outer member  21  when the inner member  16  is strongly torqued therein.  
         [0030]     In the illustrated embodiment the medical implant assembly  7  includes the bone screw receiver  1  embodying the outer member  21 , and further includes a shank  34  having a body  36  integral with an upper portion or capture structure  38  and a retaining structure  42 . The shank  34 , the receiver  1  and the retaining structure  42  preferably are assembled prior to implantation of the shank body  36  into a vertebra  45 .  
         [0031]      FIG. 1  further shows the illustrated inner member  16  as part of a closure structure  48  that is helically advanced by rotation thereof into the receiver  1  and torqued against a longitudinal member, such as a rod  49 , to clamp the rod  49  within the receiver  1 . Although embodiments of the outer member  21  and the inner member  16  are illustrated herein as the receiver  1  and the closure  48 , the reverse angle structure  3  is not intended to be limited to such an application. It is especially noted that the implant assembly  7  may be a hook or other implant structure having a receiving channel for a rod or other structural member. Also, while the illustrated implant assembly  7  is shown as a polyaxial assembly, it is intended that the reverse angle structure  3  be adaptable for use with other types of polyaxial assemblies as well as mono-axial bone screws, hooks, and other types of implants.  
         [0032]     As will be described in greater detail below, the closure structure  48  biases the rod  49  or other longitudinal member against the upper portion or capture structure  38  of the shank  34  that in turn biases the retaining structure  42  into fixed frictional contact with the receiver  1 , so as to fix the rod  49  relative to the vertebra  45 . The receiver  1  and the shank  34  cooperate in such a manner that the receiver  1  and the shank  34  can be secured at any of a plurality of angles, articulations or rotational alignments relative to one another and within a selected range of angles both from side to side and from front to rear, to enable flexible or articulated engagement of the receiver  1  with the shank  34  until both are locked or fixed relative to each other near the end of an implantation procedure.  
         [0033]     The shank  34 , best illustrated in  FIGS. 1-3 , is elongate, with the shank body  36  having a helically wound bone implantable thread  54  extending from near a neck  56  located adjacent to the capture structure  38  to a tip  58  of the body  36  and extending radially outwardly therefrom. During use, the body  36  utilizing the thread  54  for gripping and advancement is implanted into the vertebra  45  leading with the tip  58  and driven down into the vertebra  45  with an installation or driving tool (not shown), so as to be implanted in the vertebra  45  to near the neck  56 , as shown in  FIGS. 8-10 , and as is described more fully in the paragraphs below. The shank  34  has an elongate axis of rotation generally identified by the reference letter A.  
         [0034]     The neck  56  extends axially outward and upward from the shank body  36 . The neck  56  is of reduced radius as compared to an adjacent top  62  of the body  36 . Further extending axially and outwardly from the neck  56  is the capture structure  38  that provides a connective or capture apparatus disposed at a distance from the body top  62  and thus at a distance from the vertebra  45  when the body  36  is implanted in the vertebra  45 .  
         [0035]     The capture structure  38  is configured for connecting the shank  34  to the receiver  1  and capturing the shank  34  in the receiver  1 . The capture structure  38  has an outer substantially cylindrical surface  64  having a helically wound advancement structure thereon which in the illustrated embodiment is a V-shaped thread  66  extending from near the neck  56  to adjacent to a seating surface  68 . Although a simple thread  66  is shown in the drawings, it is foreseen that other structures including other types of threads, such as buttress and reverse angle threads, and non threads, such as helically wound flanges with interlocking surfaces, may be alternatively used in alternative embodiments of the present invention.  
         [0036]     The shank  34  further includes a tool engagement structure  70  disposed near a top end surface or dome  72  thereof for engagement of a driving tool (not shown) that includes a driving structure in the form of a socket. The driving tool is configured to fit about the tool engagement structure  70  so as to form a socket and mating projection for both driving and rotating the shank body  36  into the vertebra  45 . Specifically in the embodiment shown in  FIGS. 1-10 , the tool engagement structure  70  is in the shape of a hexagonally shaped extension head coaxial with both the threaded shank body  36  and the threaded capture structure  38 .  
         [0037]     The top surface  72  of the shank  34  is preferably curved or dome-shaped as shown in the drawings, for positive engagement with the rod  49 , when the bone screw assembly  7  is assembled, as shown in  FIGS. 8-10  and in any alignment of the shank  34  relative to the receiver  1 . In certain embodiments, the surface  72  is smooth. While not required in accordance with practice of the invention, the surface  72  may be scored or knurled to further increase frictional engagement between the surface  72  and the rod  49 .  
         [0038]     The shank  34  shown in the drawings is cannulated, having a small central bore  74  extending an entire length of the shank  34  along the axis A. The bore  74  is defined by an inner cylindrical wall  75  of the shank  4  and has a first circular opening  76  at the shank tip  58  and a second circular opening  78  at the top surface  72 . The bore  74  is coaxial with the threaded body  36  and the capture structure outer surface  64 . The bore  74  provides a passage through the shank  34  interior for a length of wire (not shown) inserted into the vertebra  45  prior to the insertion of the shank body  36 , the wire providing a guide for insertion of the shank body  36  into the vertebra  45 .  
         [0039]     Referring to  FIGS. 1 and 6  through  10 , the receiver  1  has a generally U-shaped appearance with a partially cylindrical inner profile and a faceted outer profile. The receiver  1  includes a somewhat spherical base  80  integral with a pair of upstanding arms  82  forming a U-shaped cradle and defining a U-shaped channel  84  between the arms  82  with a lower seat  86  having substantially the same radius as the rod  49  for operably snugly receiving the rod  49 .  
         [0040]     Referring particularly to  FIGS. 1, 6  and  7 , the receiver  1  is provided with the break-off extensions or arm tabs  5  to increase the initial length of the arms  82  and, thus, forming a rod receiving passageway between the extensions  5  and thereby increasing the length of the rod receiving channel  84  by the length of the passageway. The purpose for the lengthened channel  84  is to enable capture of the rod  49  within the channel  84  at a greater distance from the vertebra  45 , whereby the rod  49  can be captured by the closure structure  48  and be “reduced” or urged toward a seated position within the channel  84  by advancement of the closure  48 . This provides effective leverage in reducing the position of the rod  49  or the vertebra itself. For this purpose, inner surfaces  88  of the extensions or tabs  5  are provided with the reverse angle thread  19  that extends continuously from main portions of the arms  82  and along the extensions  5  to form a continuous and uniform helical pathway therebetween.  
         [0041]     A pair of weakened regions  90  is disposed between the arm main portions  82  and the break-off extensions  5 . The weakened regions  90  may be regions adjacent v-shaped indentations or notches extending generally perpendicular to the axis A as illustrated in  FIGS. 1, 6  and  7 , or any other type of diminishing or lessening in the arm thickness to provide for ready separation of the extension  5  from the arms  82  by breaking the extensions  5  off of the arms  82  at the weakened regions  90 . The weakened regions  90  are strong enough to enable the rod  49  to be urged toward its seated position ( FIGS. 8 and 9 ). However, the extensions  5  can be broken off or separated from the main portions of the arms  82  by pivoting or bending the extensions  5  back and forth about the regions  90  while the main portions of the arms are held in place, after the closure structure  48  has passed between the extensions  5 . The resulting low-profile implanted structure is shown in  FIGS. 8-10 .  
         [0042]     The reverse angle thread form  19  is disposed about the inner surface  88  of the extensions  5  and the arms  82  in a discontinuous generally helical pattern or configuration, which is typical of threads and can have various pitches, be counterclockwise advanced or vary in most of the ways that conventional threads vary. The thread form  19  has a leading surface  92  and a trailing surface  94  that has also been identified previously herein as the anti-splay surface  26 . As used herein the terms leading and trailing refer to the direction of advancement with respect to mating engagement with the closure structure  48  when used to close the receiver  1  by moving the closure structure in a direction along a central axis of rotation B of the receiver  1  toward the base  80  of the receiver  1 . In the illustrated embodiment, advancement is produced by clockwise rotation of the closure structure  48 . As can be seen in  FIGS. 6 and 7 , the general shape of the cross section of the thread  19  is that of an obtuse triangle. It can also be seen that the intersection of the leading surface  92  and the trailing surface  94  with a plane passing through the axis of rotation B, shows that both surfaces  92  and  94  slope downwardly in a direction toward the base  80  of the receiver  1  from a root  96  to a crest  98  of the thread form  19 . As compared to a buttress thread wherein the trailing surface is disposed perpendicular to the axis of rotation, in a reverse angle thread form of the invention, the trailing surface is disposed at an angle with respect to the axis of rotation, the surface sloping in generally the same direction as the leading surface. This also contrasts with convention V-threads wherein the leading and trailing surfaces slope in opposed directions. The intersection of the trailing surface  94  with a plane passing through the axis of rotation B is typically at an angle of from about 1° to about 45° relative to a line perpendicular to the axis of rotation B. Further details regarding reverse angle threads of the invention are described in U.S. patent application Ser. No. 09/644,777, filed Aug. 23, 2000, incorporated by reference herein.  
         [0043]     Tool engaging apertures  104  are formed on outer surfaces or facets of the arms  82 . The apertures  104  may be used for holding the receiver  1  during assembly with the shank  34  and the retaining structure  42  and also during the implantation of the shank body  36  into the vertebra  45 . Communicating with the apertures  104  are upwardly projecting, hidden inner recesses  106 . A holding tool (not shown) is sized and shaped to have structure to mate with and to be received in the aperture  104  and locked into place by pulling the holding tool slightly axially upward relative to the base  80  and toward the arm extensions  5 . The holding tool and respective apertures  104  may be configured for a variety of engagement orientations, including, but not limited to, a twist on/twist off or a snap on/snap off engagement wherein the holding tool has legs that splay outwardly to position the tool for engagement in the apertures  104  and recesses  106 . It is noted that the apertures  104  and the cooperating holding tool may be configured to be of a variety of sizes and locations along any of the surfaces of the arms  82 .  
         [0044]     Communicating with and located beneath the U-shaped channel  84  of the receiver  1  is a chamber or cavity  108  substantially defined by an inner surface  110  of the base  80 , the cavity  108  opens upwardly into the U-shaped channel  84 . The inner surface  110  is substantially spherical, with at least a portion thereof forming a partial internal spherical seating surface  112 . The surface  112  is sized and shaped for mating with the retaining structure  42 , as described more fully below.  
         [0045]     The base  80  further includes a restrictive neck  113 , having a radius smaller than a radius of the spherical surface  110 . The neck  113  defines a bore  114  communicating with the cavity  108  and a lower exterior  116  of the base  80 . The bore  114  is coaxially aligned with respect to the rotational axis B of the receiver  1 . The neck  113  and associated bore  114  are sized and shaped to be smaller than a radial dimension of the retaining structure  42 , as will be discussed further below, so as to form a restriction at the location of the neck  113  relative to the retaining structure  42 , to prevent the retaining structure  42  from passing from the cavity  108  and out into the lower exterior  116  of the receiver  1  when the retaining structure  42  is seated within the receiver  1 . However, it is foreseen that the retaining structure could be compressible (such as where such structure has a missing section) and that the retaining structure could be loaded through the neck  113  and then allowed to expand and fully seat in the spherical seating surface of the receiver  1 .  
         [0046]     The retaining structure or ring  42  is used to retain the upper portion or capture structure  38  of the shank  34  within the receiver  1 . The retaining structure  42 , best illustrated by  FIGS. 4-5 , has an operational central axis that is the same as the elongate axis A associated with the shank  34 , but when the retaining structure  42  is separated from the shank  34 , the axis of rotation is identified as axis C, as shown in  FIG. 5 . The retaining structure  42  has a central bore  120  that passes entirely through the retaining structure  42  from a top surface  122  to a bottom surface  124  thereof. A first inner cylindrical surface  126  defines a substantial portion of the bore  120 , the surface  126  having a helically wound guide and advancement structure thereon as shown by a helical rib or thread  128  extending from adjacent the bottom surface  124  to adjacent a flat, seating surface  129  disposed perpendicular to the inner surface  126 .  
         [0047]     Although a simple helical rib  128  is shown in the drawings, it is foreseen that other helical structures including other types of threads, such as buttress and reverse angle threads, and non threads, such as helically wound flanges with interlocking surfaces, may be alternatively used in an alternative embodiment of the present invention. The inner cylindrical surface  126  with helical rib  128  are configured to mate under rotation with the capture structure outer surface  64  and helical guide and advancement structure or thread  66 , as described more fully below.  
         [0048]     The retaining structure  42  further includes a second inner wall or cylindrical surface  132 , coaxial with the first inner cylindrical surface  126 . The surface  132  is disposed between the seating surface  129  and the top surface  122  of the retaining structure  42  and has a diameter greater than that of the cylindrical surface  126 . As will be described more fully below, the cylindrical surface  132  in cooperation with the seating surface  129  and the surface  68  of the capture structure  38 , provide a recess about the base of the tool engagement structure  70  and a stable seating surface for the tool (not shown) used to drive the shank body  36  into bone. The surface or wall  132  which is the outer wall of the recess may be shaped to fit an outer surface of such a driving tool and may be faceted, for example, hexagonal in shape, to better grip the driving tool.  
         [0049]     The retaining structure or ring  12  has a radially outer partially spherically shaped surface  134  sized and shaped to mate with the partial spherically shaped seating surface  112  of the receiver  1  and having a radius approximately equal to the radius associated with the surface  112 . The retaining structure radius is larger than the radius of the neck  113  of the receiver  1 . Although not required, it is foreseen that the outer partially spherically shaped surface  134  may be a high friction surface such as a knurled surface or the like.  
         [0050]     The elongate rod or longitudinal member  49  that is utilized with the assembly  7  can be any of a variety of implants utilized in reconstructive spinal surgery, but is normally a cylindrical elongate structure having a cylindrical surface  136  of uniform diameter and having a generally smooth surface. The rod  49  is preferably sized and shaped to snugly seat near the bottom of the U-shaped channel  84  of the receiver  1  and, during normal operation, is positioned slightly above the bottom of the channel  84  at the lower seat  86 . In particular, the rod  49  normally directly or abutingly engages the shank top surface  72 , as shown in  FIGS. 8 and 9  and is biased against the dome shank top surface  72 , consequently biasing the shank  34  downwardly in a direction toward the base  80  of the receiver  1  when the assembly  7  is fully assembled. For this to occur, the shank top surface  72  must extend at least slightly into the space of the channel  84  when the retaining structure  42  is snugly seated in the lower part of the receiver cavity  108 . The shank  34  and the retaining structure  42  are thereby locked or held in position relative to the receiver  1  by the rod  49  firmly pushing downward on the shank top surface  72 .  
         [0051]     With reference to  FIGS. 1 and 6 - 10 , the closure structure or closure top  48  can be any of a variety of different types of closure structures for use in conjunction with the mating structure on the main portions of the upstanding arms  82  and the arm break-off extensions  5 . The illustrated closure top  48  is a cylindrically shaped plug having a generally cylindrical shaped radially outer surface  142 , a flat top  143  and a substantially flat bottom  144 . The closure structure  48  has an axis of rotation, generally indicated by the reference numeral D. The axis of rotation D is at the radial center of the closure structure  48 . An internal tool engagement structure in the form of an aperture or bore  147  that is co-axial with the axis of rotation D extends through the top  143  and partially through the closure structure  48 . The aperture  147  is poly-faceted so as to have a hexagonal cross section such that the closure structure  48  can be installed or removed with an allen type tool  148  that is engageable with the structure  48  at the aperture  147 . Although a hex-shaped aperture  147  is shown in the drawings, the tool engagement structure may take a variety of tool-engaging forms, such as multi-lobular drives sold under the trademark TORX, or may include more than one aperture of various shapes, such as a pair of spaced apertures, or the like. Although a particular closure structure  48  has been illustrated herein, it is foreseen that the invention can be used in conjunction with plugs and set screws of various types and configurations. For example, the closure structure may include a break off head for insertion.  
         [0052]     The closure structure  48  also includes structure to assist in engaging and securing the rod  49 , shown as a point  149  for penetrating the rod surface  136 . Although not shown, such a closure structure may further include a cutting rim and/or a roughened under surface.  
         [0053]     The closure structure  48  outer substantially cylindrical surface  142  embodies the inner member  16  having the reverse angle thread form  10 . The thread form  10  includes a leading surface  152  and a trailing surface  154  that has also been identified herein as the anti-splay surface  24 . As with the description herein with respect to the receiver  1 , the terms leading and trailing refer to the direction of advancement of the closure structure  48  into the receiver  1  by moving the closure structure  48  in a direction along the central axis of rotation B of the receiver  1  (also about the axis D of the structure  48 ) and toward the base  80  of the receiver  1 . The general shape of the cross section of the thread  10  is that of an obtuse triangle. It can be seen that at the intersection of the leading surface  152  and the trailing surface  154  with a plane passing through the axis of rotation D, both surfaces  152  and  154  slope upwardly or rearwardly in a direction away from the bottom surface  144  of the closure  48  from a root  156  to a crest  158  of the thread form  10 . Both surfaces  152  and  154  also slope upwardly or rearwardly in a direction away from the base  80  of the receiver  1  when the closure  48  is engaged with the receiver  1 .  
         [0054]     The reverse angle thread form is shaped and positioned so as to engage the discontinuous reverse angle thread form  19  that winds on the extensions  5  and the arms  82  to provide for rotating advancement of the closure structure  48  into the receiver  1  when rotated clockwise and, in particular, to cover the top or upwardly open portion of the U-shaped channel  84  to capture the rod  49 , without splaying of the extensions  5  or the arms  82 . The closure structure  48  also operably biases against the rod  49  by advancement and applies pressure to the rod  49  under torquing, so that the rod  49  is urged downwardly against the shank top end surface  72  that extends into the channel  84 . Downward biasing of the shank top surface  72  operably produces a frictional engagement between the rod  49  and the surface  72  and also urges the retaining structure  42  toward the base  80  of the receiver  1 , so as to frictionally seat the retaining structure external spherical surface  134  fixedly against the partial internal spherical seating surface  112  of the receiver  1 , also fixing the shank  34  and retaining structure  42  in a selected, rigid position relative to the receiver  1 .  
         [0055]     It is noted that as torque is applied to the closure  48  in a clockwise manner so as to advance the closure  48  in the receiver  1  the trailing surface  154  engages and pushes against the trailing surface  94  of the thread  19  of the receiver  1 . The force exerted on the closure  48  by this process is countered by a reactive force acting on the receiver  1  that has a first component that is axial, that is parallel to the axis of rotation D of the closure structure  48 , and a second component that has a radial inward vector, that is toward the axis of rotation D of the closure structure  48 .  
         [0056]     Prior to the polyaxial bone screw assembly  7  being placed in use according to the invention, the retaining structure  42  is typically first inserted or top-loaded, into the receiver U-shaped channel  84 , as is shown in dotted lines in  FIG. 6 , and then into the cavity  108  to dispose the structure  42  within the inner surface  110  of the receiver  1 . Then, the retaining structure  42  is rotated approximately 90 degrees so as to be coaxial with the receiver  1  and then seated in sliding engagement with the seating surface  112  of the receiver  1 , also shown in  FIG. 6 .  
         [0057]     With reference to  FIG. 7 , the shank capture structure  38  is pre-loaded, inserted or bottom-loaded into the receiver  1  through the bore  114  defined by the neck  113 . The retaining structure  42 , now disposed in the receiver  1  is coaxially aligned with the shank capture structure  38  so that the helical guide and advancement structure  66  rotatingly mates with the helical guide and advancement structure  128  of the retaining structure  42 .  
         [0058]     The shank  34  and or the retaining structure  42  are rotated to fully mate the structures  66  and  128  along the respective cylindrical surfaces  64  and  126 , fixing the capture structure  38  to the retaining structure  42 , until the seating surface  68  and the seating surface  129  are contiguous and disposed in the same plane as shown in  FIGS. 7-9 . Permanent, rigid engagement of the capture structure  38  to the retaining structure  42  may be further ensured and supported by the use of adhesive, a spot weld, deforming one or both threads with a punch or the like. At this time the shank  34  is in slidable and rotatable engagement with the receiver  1 , while the capture structure  38  and the lower aperture or neck  113  of the receiver  1  cooperate to maintain the shank body  36  in rotational relation with the receiver  1 . Only the retaining structure  42  is in slidable engagement with the head spherical seating surface  112 . Both the capture structure  38  and threaded portion of the shank body  36  are in spaced relation with the receiver  1 .  
         [0059]     The assembly  7  is then typically screwed into a bone, such as the vertebra  45 , by rotation of the shank  34  using a driving tool (not shown) that operably drives and rotates the shank  34  by engagement thereof with the hexagonally shaped extension head  70  of the shank  34 . Preferably, when the driving tool engages the tool engagement structure or head  70 , an end portion thereof is disposed in a recess defined by the structure  70 , the seating surface  68 , the contiguous seating surface  129  and the inner cylindrical surface  132 , with a bottom surface of the driving tool contacting and frictionally engaging both the seating surface  68  and the seating surface  129 . Some frictional engagement between an outer surface of the driving tool with the cylindrical surface  132  may also be achievable during rotation of the driving tool.  
         [0060]     Typically, the receiver  1  and the retaining structure  42  are assembled on the shank  34  before inserting the shank body  36  into the vertebra  45 , but in certain circumstances, the shank body  36  can be first partially implanted with the capture structure  38  extending proud to allow assembly with the receiver  1  utilizing the retaining structure  42 . Then the shank body  36  can be further driven into the vertebra  45 .  
         [0061]     The vertebra  45  may be pre-drilled to minimize stressing the bone and have a guide wire (not shown) that is shaped for the cannula  74  inserted to provide a guide for the placement and angle of the shank  34  with respect to the vertebra  45 . A further tap hole may be made using a tap with the guide wire as a guide. Then, the assembly  7  or the solitary shank  34 , is threaded onto the guide wire utilizing the cannulation bore  74  by first threading the wire into the bottom opening  76  and then out of the top opening  78 . The shank  34  is then driven into the vertebra  45 , using the wire as a placement guide.  
         [0062]     With reference to  FIGS. 7-10 , the rod  49  is eventually positioned between the break-off extensions  5  and the closure structure  48  is then inserted into and advanced between the extensions  5  by mating the thread  10  with the thread  19 , and rotating the structure  48  downwardly toward the base  80  so as to bias or push against the rod  49 . Pressure applied to the rod  49  by the structure  48  is continued as the rod  40  passes from the extensions  5  to a position disposed between the receiver arms  82  and near the seat  86 . The anti-splay reverse angle structure  3  of the cooperating closure structure  48 , the break-off extensions  5  and the arms  82 , bias the extensions  5  and arms  82  toward one another as the closure structure  48  travels downwardly toward the base  80  of the receiver  1 . Once both the rod and the closure structure  48  are disposed in the receiver  1  between the arms  82 , the break-off extensions  5  may be removed by bending the extensions  5 , causing the extensions  5  to break away from the arms  82  at the weakened regions  90 . The closure structure  48  may then be further tightened against the rod  49  as desired.  
         [0063]     The shank top end surface  72 , because it is rounded to approximately equally extend upward into the channel  84  approximately the same amount no matter what degree of rotation exists between the shank  34  and the receiver  1  and because the domed surface  72  is sized and shaped to extend upwardly into the U-shaped channel  84 , the surface  72  is engaged by the rod  49  and pushed downwardly toward the base  80  of the receiver  1  when the closure structure  48  biases downwardly toward and onto the rod  49 . The downward pressure on the shank  34  in turn urges the retaining structure  42  downward toward the receiver seating surface  112 , with the retaining structure seating surface  129  in frictional engagement with the receiver seating surface  112 . As the closure structure  48  presses against the rod  49 , the rod  49  presses against the shank and the retaining structure  42  that is now rigidly attached to the shank  34  which in turn becomes frictionally and rigidly attached to the receiver  1 , fixing the shank body  36  in a desired angular configuration with respect to the receiver  1  and the rod  49 .  
         [0064]      FIG. 10  illustrates the polyaxial bone screw assembly  7  and including the rod  49  and the closure structure  48  positioned in a vertebra  45 . The axis A of the bone shank  34  is illustrated as not being coaxial with the axis B of the receiver  1  and the shank  34  is fixed in this angular locked configuration. Other angular configurations can be achieved, as required during installation surgery due to positioning of the rod  49  or the like.  
         [0065]     If removal of the assembly  7  and associated rod  49  and closure structure  48  is necessary, disassembly is accomplished by using the driving tool  148  or other similarly sized tool of an Allen wrench type (not shown) mating with the aperture  147  and turned counterclockwise to rotate the closure structure  48  and reverse the advancement thereof in the receiver  1 . Then, disassembly of the assembly  7  is accomplished in reverse order to the procedure described previously herein for assembly.  
         [0066]     It is to be understood that while certain forms of the present invention have been illustrated and described herein, it is not to be limited to the specific forms or arrangement of parts described and shown.