Abstract:
A polyaxial bone screw assembly includes a threaded shank body having an upper capture structure, a head and a multi-piece retainer, articulation structure. The geometry of the retainer structure pieces corresponds and cooperates with the external geometry of the capture structure to frictionally envelope the retainer structure between the capture structure and an internal surface defining a cavity of the head. The head has a U-shaped cradle defining a channel for receiving a spinal fixation or stabilization longitudinal connecting member. The head channel communicates with the cavity and further with a restrictive opening that receives retainer pieces and the capture structure into the head but prevents passage of frictionally engaged retainer and capture structures out of the head. The retainer structure includes a substantially spherical surface that mates with the internal surface of the head, providing a ball joint, enabling the head to be disposed at an angle relative to the shank body. Methods of installation are also provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/630,478 filed Nov. 23, 2004. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention is directed to polyaxial bone screws for use in bone surgery, particularly spinal surgery. Such screws have a head that can swivel about a shank of the bone screw, allowing the head to be positioned in any of a number of angular configurations relative to the shank.  
         [0003]     Many spinal surgery procedures require securing various implants to bone and especially to vertebrae along the spine. For example, elongate rods are often utilized that extend along the spine to provide support to vertebrae that have been damaged or weakened due to injury or disease. Such rods must be supported by certain vertebrae and support other vertebrae.  
         [0004]     The most common mechanism for providing vertebral support is to implant bone screws into certain bones which then in turn support a rod or are supported thereby. Bone screws of this type may have a fixed head relative to a shank thereof. In the fixed bone screws, the head cannot be moved relative to the shank and the rod must be favorably positioned in order for it to be placed within the head. This is sometimes very difficult or impossible to do. Therefore, polyaxial bone screws are commonly preferred.  
         [0005]     Polyaxial bone screws allow rotation of the head about the shank until a desired rotational position of the head is achieved relative to the shank. Thereafter, a rod can be inserted into the head and eventually the head is locked or fixed in a particular position relative to the shank.  
         [0006]     A variety of polyaxial or swivel-head bone screw assemblies are available. One type of bone screw assembly includes an open head that allows for ease in placement of a rod within the head. A closure top or plug is then used to capture the rod in the head of the screw.  
         [0007]     Because such implants are for placement within the human body, it is desirable for the implant to have as little effect on the body as possible. Consequently, heavy, bulky implants are undesirable and lighter implants with a relatively small profile both in height and width are more desirable. However, a drawback to smaller, lighter implants is that they may be more difficult to rigidly fix to each other and into a desired position. Lack of bulk may also mean lack of strength, resulting in slippage under high loading.  
         [0008]     A drawback of some implants is that in an effort to provide an open, swivel head screw that is easy to assemble, the interior of the bone screw head is largely hollowed out, resulting in a cavity that may provide adequate space for the attachment of component parts within the cavity, but also may result in a head of reduced mass and thus possibly of reduced strength. If the head cavity size is reduced to maintain adequate bulk and strength of the head, the other bone screw components utilized to attach the shank to the head must be reduced in size or otherwise reconfigured, potentially resulting in bone screw shank components of lesser strength. Another potential difficulty is lack of working space for utilizing manipulating or fastening tools to attach the component parts to one another.  
         [0009]     Furthermore, certain part configurations and/or reduced sizing of the component parts may result in limitations on the frictional, load bearing surfaces within the screw head once the screw is implanted on a vertebra and the head is fixed in a set angle with respect to the shank body. For example, a select portion of the interior of the head may be removed to create a recess so that other component parts of adequate bulk may be inserted in the head. This may result in excessive local pressure on other surfaces of the head after shank installation, which in turn may lead to localized damage and eventual loosening or deformation of the component parts. It is therefore desirable to draw a balance between mass and strength of the individual bone screw component parts, number of component parts, the configuration thereof, and ease of installation of the implant component parts to each other and to the vertebrae.  
       SUMMARY OF THE INVENTION  
       [0010]     A polyaxial bone screw assembly according to the invention includes a shank with a capture structure loadable into a head, and a capture structure retaining and articulating structure that includes more than one discrete piece, each piece engageable with the capture structure and slidably engageable with the head.  
         [0011]     The shank has an elongate body, a driving tip and a capture structure, the body disposed between the driving tip and the capture structure. The shank body is configured for fixation to a bone. The capture structure is a polyhedral or inverted conical formation having an edge spaced from the shank body and an oblique surface disposed between the edge and the body, the surface sloping in a direction toward the driving tip. In a first embodiment, the capture structure is a polyhedron and includes front and back surfaces that are trapezoidal, and two oblique surfaces that are rectangular. In an alternative embodiment, the capture structure oblique surface is conical.  
         [0012]     The polyaxial bone screw head includes a top portion and a base. The head top portion defines an open channel. The base has a seating surface partially defining a cavity, the channel communicating with the cavity, and the cavity communicating with an exterior of the base through an opening sized and shaped to receive the capture structure therethrough.  
         [0013]     The retainer structure includes at least two and up to a plurality of discrete parts or pieces, each part or piece having an inner surface and an outer surface. Each inner surface of the retainer structure is configured to be in frictional engagement with an outer surface of the capture structure and each outer surface of the retainer structure is configured to be in slidable engagement with the seating surface in the base of the screw head. One illustrated embodiment of a retainer structure according to the invention is a discrete two-part or piece structure, with the pieces being substantial mirror images of one another. The two pieces preferably are in contact when fully installed in a bone screw head. In another illustrated embodiment according to the invention, first and second retainer structure pieces are connectable to one another, with a first piece forming a recess and the second piece having a projection that is receivable in the recess. The projection is slidable within the recess, allowing for telescoping of the structure during insertion into a bone screw head and also some rotation or jointed movement between the first and second pieces after insertion into the bone screw head, and particularly during subsequent insertion of the shank into the head. Upon full installment in the bone screw head, portions of the first and second pieces are in spaced relation with each other. In other embodiments according to the invention, two or more discrete retainer pieces remain un-attached to one another when fully installed and operational in the polyaxial bone screw head, and may be disposed in contact with or in spaced relation to one another.  
         [0014]     Both the multi-part retainer structure outer surface and the head seating surface are preferably substantially spherical. In one embodiment according to the invention, both the multi-part retainer structure inner surface and the capture structure oblique surface are planar. In another embodiment, each retainer structure part has an inner surface that is curved and the capture structure oblique surface is conical.  
         [0015]     In an embodiment according to the invention in which the capture structure oblique surface is planar, the capture structure also has a second oblique surface and front and rear parallel surfaces. First and second inner walls of a retainer structure part are frictionally engageable with the front and rear surfaces, respectively. Furthermore, in such an embodiment the retainer structure preferably includes first and second discrete parts, the second retainer structure part is a substantial mirror image of the first retainer structure part, with both retainer structure parts having first and second inner walls that frictionally cooperate with the front and rear parallel surfaces of the capture structure, as well as first and second sloping surfaces that frictionally engage the oblique surfaces of the capture structure.  
         [0016]     The capture structure may also include a tool engagement formation disposed thereon adapted for non-slip engagement by a tool for driving the shank body into bone. Furthermore, the tool engagement formation may project from the capture structure at a tool seating surface.  
         [0017]     Each retainer structure part or piece is sized and shaped to load into the head through either the open channel or the base opening of the head. The shank capture structure can also be sized and shape to be loadable into the head through the base opening and/or through the open channel.  
         [0018]     An assembly according to the invention further includes a closure structure insertable into the head for operably urging the shank in a direction to frictionally lock the position of the retainer structure outer surface relative to the head seating surface, thereby locking the shank body in a selected angle with respect to the head. Furthermore, the head may include upstanding spaced arms defining the open channel, the arms having guide and advancement structures on an inside surface thereof, with the closure structure being sized and shaped to be positionable between the arms for closing the channel. The closure structure has a closure guide and advancement structure for rotatably mating with the guide and advancement structures on the arms. Upon advancing rotation between the arms, the closure structure biases against a rod disposed in the channel.  
         [0019]     In a method according to the invention, retainer structure pieces are inserted into a polyaxial bone screw head cavity through either an upper rod-receiving channel or a lower shank receiving opening. A capture structure of a bone screw shank is then inserted into the head through the shank receiving opening of the head and into the cavity thereof. The capture structure is then moved toward the upper rod-receiving channel and the retainer structure pieces pivot about an edge of the capture structure while being moved toward the lower shank receiving opening until an inner surface of each of the retainer structure pieces is in frictional engagement with an oblique surface of the capture structure.  
         [0020]     Furthermore, according to a method of the invention, if the retainer structure is of two-part or piece construction, the method includes simultaneously pivoting both retainer pieces about the capture structure edge and then into position about an oblique surface or surfaces of the capture structure.  
         [0021]     It is foreseen that the retainer structure parts can be different in size and/or shape. It is foreseen that the capture structure can be cylindrical in shape with at least one inward groove or outward ridge for mating with the retainer structure. It is also foreseen that the capture structure can be spherical or elliptical in shape, with outer concave or convex surfaces, or have square upper and lower cross-sectional ends of same or different size.  
       OBJECTS AND ADVANTAGES OF THE INVENTION  
       [0022]     Therefore, it is an object of the present invention to overcome one or more of the problems with polyaxial bone screw assemblies described above. A further object of the invention is to provide apparatus and methods directed to a shank that is loadable into a cavity in a head of the screw and that utilizes a retainer structure that may be uploaded or downloaded into the cavity. Another object of the invention is to provide more than one retainer parts or segments configured to be engageable with the shank and slidably engageable with the head so as to articulate or fix the head relative to the shank once a desired configuration is acquired. Furthermore, it is an object of the invention to provide a lightweight, low profile polyaxial bone screw that assembles in such a manner that the components cooperate to create an overall structure that provides an even gripping of a shank capture structure to the head, avoiding excessive local pressure therebetween. Another object of the invention is to provide such components that do not require overly complicated fasteners or complicated methods of fastening within the bone screw head. Another object of the invention is to provide a polyaxial bone screw with features that present frictional or gripping surfaces for bone implantation tools and may be readily and securely fastened to each other as well as to the bone. Furthermore, it is an object of the invention to provide apparatus and methods that are easy to use and especially adapted for the intended use thereof and wherein the tools are comparatively inexpensive to produce.  
         [0023]     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.  
         [0024]     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  
       [0025]      FIG. 1  is an exploded perspective view of a polyaxial bone screw assembly according to the present invention having a shank with a capture structure at one end thereof, a head, and a two-piece retainer structure.  
         [0026]      FIG. 2  is an enlarged top plan view of the retainer structure of  FIG. 1 .  
         [0027]      FIG. 3  is an enlarged cross-sectional view of the retainer structure of  FIG. 2 , taken along the line  3 - 3  of  FIG. 2 .  
         [0028]      FIG. 4  is an enlarged top plan view of the shank of  FIG. 1 .  
         [0029]      FIG. 5  is an enlarged cross-sectional view of the head, taken along the line  5 - 5  of  FIG. 1  and shown with the retainer structure of  FIG. 3  in a method of assembly according to the invention.  
         [0030]      FIG. 6  is an enlarged cross-sectional view of the head, taken along the line  5 - 5  of  FIG. 1 , and shown with the retainer structure of  FIG. 3  in an alternative method of assembly according to the invention.  
         [0031]      FIG. 7  is an enlarged cross-sectional view of the head, taken along the line  5 - 5  of  FIG. 1 , an enlarged and partial cross-sectional view of the shank, taken along the line  7 - 7  of  FIG. 1 , and shown with the retainer structure of  FIG. 3  in an early assembly step according to the invention.  
         [0032]      FIG. 8  is an enlarged and fragmentary cross-sectional view of the head, shank and retainer structure, similar to  FIG. 7 , showing an intermediate assembly step.  
         [0033]      FIG. 9  is an enlarged and fragmentary cross-sectional view of the head, shank and retainer structure, similar to  FIG. 8 , showing a subsequent assembly step.  
         [0034]      FIG. 10  is an enlarged and fragmentary cross-sectional view of the head, shank and retainer structure, similar to  FIG. 9  showing a final assembly step and further showing a rotational extent of the shank in phantom.  
         [0035]      FIG. 11  is a cross-sectional view of a vertebra, and the assembled head and shank, similar to  FIG. 10 , showing the shank being implanted into the vertebra using a driving tool mounted on the shank capture structure.  
         [0036]      FIG. 12  is an enlarged, front plan view of the implanted head and shank of  FIG. 11 , shown in exploded view with a rod (in cross-section), break-off closure structure, and closure structure driving tool.  
         [0037]      FIG. 13  is an enlarged cross-sectional view taken along the line  13 - 13  of  FIG. 10 .  
         [0038]      FIG. 14  is a fragmentary front elevational view of a bone screw with attached break-away closure member, installed rod, and an anti-torque tool mounted on the rod with portions broken away to show a torque driver advancing toward the break-away closure member.  
         [0039]      FIG. 15  is a fragmentary front elevational view similar to  FIG. 14 , with portions broken away to show a fully installed rod and closure member with the break-away head removed by the torque driver.  
         [0040]      FIG. 16  is a fragmentary and enlarged perspective view of the assembly of  FIG. 12  shown completely assembled.  
         [0041]      FIG. 17  is an exploded perspective view of an alternative embodiment of a polyaxial bone screw assembly according to the present invention having a shank with an upper capture structure, a head, and a jointed two-part retainer structure.  
         [0042]      FIG. 18  is an enlarged top plan view of the retainer structure of  FIG. 17  shown in a compressed, loading orientation.  
         [0043]      FIG. 19  is an enlarged front elevational view of the retainer structure of  FIG. 18  in a compressed, loading orientation.  
         [0044]      FIG. 20  is an enlarged front elevational view of the retainer structure of  FIG. 18  shown in an expanded configuration.  
         [0045]      FIG. 21  is an enlarged top plan view of the retainer structure of  FIG. 18  shown in an expanded configuration.  
         [0046]      FIG. 22  is an enlarged top plan view of the shank of  FIG. 17 .  
         [0047]      FIG. 23  is an enlarged cross-sectional view of the head, taken along the line  23 - 23  of  FIG. 17 , shown with the compressed retainer structure of  FIGS. 18 and 19  (in side-elevational view) and illustrating a method of inserting the retainer structure into the head.  
         [0048]      FIG. 24  is an enlarged cross-sectional view of the head, taken along the line  23 - 23  of  FIG. 17 , shown with the compressed retainer structure of  FIGS. 18 and 19  fully inserted in the head.  
         [0049]      FIG. 25  is an enlarged cross-sectional view of the head, taken along the line  23 - 23  of  FIG. 17 , an enlarged and partial cross-sectional view of the shank, taken along the line  25 - 25  of  FIG. 17 , and shown with the expanded retainer structure of  FIGS. 20 and 21 , shown in cross-section and in an early assembly step according to the invention.  
         [0050]      FIG. 26  is an enlarged and fragmentary cross-sectional view of the head, shank and retainer structure, similar to  FIG. 25 , showing an intermediate assembly step.  
         [0051]      FIG. 27  is an enlarged and fragmentary cross-sectional view of the head, shank and retainer structure, similar to  FIG. 25 , showing a subsequent assembly step.  
         [0052]      FIG. 28  is an enlarged and fragmentary cross-sectional view of the head, shank and retainer structure, similar to  FIG. 25  showing a final assembly step.  
         [0053]      FIG. 29  is an exploded perspective view of another alternative embodiment of a polyaxial bone screw assembly according to the present invention having a shank with an upper capture structure, a head, and a two-piece retainer structure.  
         [0054]      FIG. 30  is an enlarged top plan view of the retainer structure of  FIG. 29 .  
         [0055]      FIG. 31  is an enlarged cross-sectional view of the retainer structure taken along the line  31 - 31  of  FIG. 30 .  
         [0056]      FIG. 32  is an enlarged top plan view of the shank of  FIG. 29 .  
         [0057]      FIG. 33  is an enlarged cross-sectional view of the head and shank, taken along the line  33 - 33  of  FIG. 29 , shown with the retainer structure of  FIG. 29  (also in cross-section), illustrating a method of inserting the retainer structure pieces into the head.  
         [0058]      FIG. 34  is an enlarged cross-sectional view of the head, shank and retainer structure pieces, similar to  FIG. 33 , showing an assembly step according to the invention.  
         [0059]      FIG. 35  is an enlarged cross-sectional view of the head, shank and retainer structure pieces, similar to  FIG. 34 , showing an intermediate assembly step according to the invention.  
         [0060]      FIG. 36  is an enlarged and fragmentary cross-sectional view of the head, shank and retainer structure, similar to  FIG. 35  showing a final assembly step. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0061]     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.  
         [0062]     In  FIGS. 1-16  the reference number  1  generally represents a first embodiment of a polyaxial bone screw apparatus or assembly according to the present invention. The assembly  1  includes a shank, generally  4 , that further includes a body  6  integral with an upwardly extending capture structure  8 ; a head  10 ; and a two-piece or part retainer structure  12 . The shank  4 , head  10  and retainer structure  12  preferably are assembled prior to implantation of the shank body  6  into a vertebra  15 , which procedure is shown in  FIG. 11 .  
         [0063]      FIG. 12  further shows a closure structure  18  of the invention for biasing a longitudinal member such as a rod  21  against the capture structure  8  which biases the retainer structure  12  into fixed frictional contact with the capture structure  8  and the head  10 , so as to fix the rod  21  relative to the vertebra  15 . The head  10  and the shank  4  cooperate in such a manner that the head  10  and the shank  4  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 head  10  with the shank  4  until both are locked or fixed relative to each other near the end of an implantation procedure.  
         [0064]     The shank  4 , best illustrated in  FIGS. 1, 4  and  7 , is elongate, with the shank body  6  having a helically wound bone implantable thread  24  extending from near a neck  26  located adjacent to the capture structure  8 , to a tip  28  of the body  6  and extending radially outwardly therefrom. During use, the body  6  utilizing the thread  24  for gripping and advancement is implanted into the vertebra  15  leading with the tip  28  and driven down into the vertebra  15  with an installation or driving tool  31 , so as to be implanted in the vertebra  15  to near the neck  26 , as shown in  FIG. 11 , and as is described more fully in the paragraphs below. The shank  4  has an elongate axis of rotation generally identified by the reference letter A. 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 assembly  1  in actual use.  
         [0065]     The neck  26  extends axially outward and upward from the shank body  6 . The neck  26  preferably is of slightly reduced radius as compared to an adjacent upper end or top  32  of the body  6  where the thread  24  terminates. Further extending axially and outwardly from the neck  26  is the capture structure  8  that provides a connective or capture apparatus disposed at a distance from the upper end  32  and thus at a distance from the vertebra  15  when the body  6  is implanted in the vertebra  15 .  
         [0066]     The capture structure  8  is configured for connecting the shank  4  to the head  10  and capturing the shank  4  in the head  10 . The structure  8  is a polyhedral formation, specifically a polyhedron-like structure, generally  38 , with front and rear parallel surfaces  40  and  41 . Each of the surfaces  40  an  41  is in the shape of an inverted isosceles trapezoid. The surfaces  40  and  41  are congruent and are also parallel to the axis A. The surfaces  40  and  41  are adjacent to a top surface  44  of the structure  38 , the top surface  44  being substantially planar and disposed perpendicular to the axis A. The surfaces  40  and  41  extend to an annular seating surface or ledge  45 , the ledge  45  being a substantially flat surface projecting radially from the axis A and disposed parallel to the top surface  44 . The top surface  44  has a width W that is greater than an outer diameter of the seating surface  45  and includes top edges  46  and  47  extending between the trapezoidal front and rear surfaces  40  and  41 . Finishing out the polyhedron-like structure  38  are oblique side surfaces or faces  48  and  49 , each of which are substantially rectangular in shape and slope inwardly from the top edges  46  and  47  respectively, to bottom edges  50  and  51  disposed adjacent to the annular seating surface  45 . The term oblique is used herein to describe the surfaces  48  and  49  that are slanted or inclined in direction or course or position neither parallel nor perpendicular nor right-angular, with respect to the shank body  6 , but otherwise may be disposed at a variety of angles with respect to the Axis A. The oblique surfaces  48  and  49  slope from the top surface  44  toward the Axis A in a direction toward the tip  28  of the shank body  6 . A width W 2  between the bottom edges  50  and  51  is smaller than the width W and also smaller than the outer diameter of the seating surface  45 .  
         [0067]     The shank  4  further includes a tool engagement structure  52  projecting axially from the top surface  44  to an end surface  53 . The tool engagement structure  52  functions to engage the driving tool  31  shown in  FIG. 11 . The tool  31  includes a driving structure in the form of a socket. The tool  31  is configured to fit about the tool engagement structure  52  so as to form a socket and mating projection for both driving and rotating the shank body  6  into the vertebra  15 . Specifically in the embodiment shown in  FIGS. 1-16 , the tool engagement structure  52  is in the shape of a hexagonally shaped extension head coaxial with the threaded shank body  6 , however, other shaped extensions for tool engagement are possible, such as grooved, multi-lobular, etc.  
         [0068]     The end surface  53  of the shank  4  is preferably curved or dome-shaped as shown in the drawings, for positive engagement with the rod  21 , when the bone screw assembly  1  is assembled, as shown in  FIG. 15  and in any alignment of the shank  4  relative to the head  10 . In certain embodiments, the surface  53  is smooth. While not required in accordance with practice of the invention, the surface  53  may be scored or knurled to further increase frictional engagement between the surface  53  and the rod  21 .  
         [0069]     The shank  4  shown in the drawings is cannulated, having a small central bore  54  extending an entire length of the shank  4  along the axis A. The bore  54  has a first circular opening  56  at the shank tip  28  and a second circular opening  58  at the domed surface  53 . The bore  54  is coaxial with the threaded body  6 . The bore  54  provides a passage through the shank  4  interior for a length of wire (not shown) inserted into the vertebra  15  prior to the insertion of the shank body  6 , the wire providing a guide for insertion of the shank body  6  into the vertebra  15 .  
         [0070]     With reference to  FIGS. 1 and 5 , the head  10  has a generally U-shaped appearance with a partially cylindrical inner profile and a substantially cylindrical outer profile. The head  10  includes a substantially cylindrical base portion  60  integral with a pair of upstanding arms  62  and  64  forming a U-shaped cradle and defining a U-shaped channel  66  between the arms  62  and  64  with an upper opening  67  and a lower seat  68  having substantially the same radius as the rod  21  for operably snugly receiving the rod  21 .  
         [0071]     Each of the arms  62  and  64  has an interior surface  70  that defines the inner cylindrical profile and includes a partial helically wound guide and advancement structure  72 . In the illustrated embodiment, the guide and advancement structure  72  is a partial helically wound flangeform configured to mate under rotation with a similar structure on the closure structure  18 , as described more fully below. However, it is foreseen that the guide and advancement structure  72  could alternatively be a V-shaped thread, a buttress thread, a reverse angle thread or other thread like or non-thread like helically wound advancement structures for operably guiding under rotation and advancing the closure top downward between the arms  62  and  64 .  
         [0072]     The head  10  includes external, closed end grip bores  74  and  75  disposed on the respective arms  62  and  64  for positive engagement by a holding tool (not shown)to facilitate secure gripping of the head  10  during assembly of the head  10  with the shank  4  and retainer structure  12 . Furthermore, the grip bores  74  and  75  may be utilized to hold the head  10  during the implantation of the shank body  6  into the vertebra  15 . The bores  74  and  75  are centrally located on the respective arms  62  and  64 . However, it is noted that the bores  74  and  75  may be configured to be of a variety of sizes and locations along outer surfaces of the arms  62  and  64 .  
         [0073]     Communicating with the U-shaped channel  66  of the head  10  is a chamber or cavity  78  substantially defined by an inner surface  80  of the base  60 . The cavity  78  opens upwardly into the U-shaped channel  66 . The inner surface  80  is substantially spherical, with at least a portion thereof forming a partial internal spherical seating surface  82  having a first radius R 1 , the surface  82  for mating with the retainer structure  12 , as described more fully below.  
         [0074]     The base  60  further includes a restrictive aperture, opening or neck  83 , having a second radius R 2  and partially defining a bore  84  communicating with the cavity  78  and a bottom exterior  86  of the base  60 . The bore  84  is coaxial with a rotational axis B of the head  10 . A bevel  88  extends between the neck  83  and the bottom exterior  86 . The neck  83  and associated bore  84  with radius R 2  are sized and shaped to be smaller than a radial dimension of the retainer structure  12  (radius R 1 ), when installed, as will be discussed further below, so as to form a restriction at the location of the neck  83  relative to the retainer structure  12 , to prevent the structure  12  from passing between the cavity  78  and the bottom exterior  86  of the head  10 , when fully seated in the head  10  and in operational engagement with the capture structure  8 . The bevel  88  widens the angular range of the shank  4  when assembled with the head  10 .  
         [0075]     The two-part retainer structure  12  is used to retain the capture structure  8  of the shank  4  within the head  10  and articulate the shank body  6  with respect to the head  10 . The retainer structure  12 , best illustrated by  FIGS. 1-3 , has an operational central axis that is the same as the elongate axis A associated with the shank  4 . The structure  12  includes a first piece or part  90  and a mirror image second piece or part  92 . The parts  90  and  92  provide a collar or collet about the capture structure  8  within the head  10 , when installed as will be discussed more fully below.  
         [0076]     The parts or pieces  90  and  92  slidably and closely grip both the capture structure  8  and the seating surface  82 , providing an even and uniform gripping surface between the shank  4  and the head  10  at the spherical seating surface  82  when force is directed onto the shank domed surface  53  by the rod  21  and closure structure  18 , or by other types of longitudinal members and closure structures.  
         [0077]     Although a two-piece retainer structure  12  is illustrated herein, it is foreseen that the retainer structure may be made up of a plurality of pieces, each slidably frictionally matable with both the capture structure  8  and the seating surface  82  of the head  10 . The pieces may also be of varying sizes and not necessarily mirror images of one another. Furthermore, although the illustrated embodiment shows the parts  90  and  92  in contact with each other when fully installed in the head  10  and in contact with the shank capture structure  8 , it is foreseen that the parts  90  and  92  may be sized and shaped so as to be in spaced relation to one another when fully installed with the capture structure  8  in the head  10 .  
         [0078]     Each retainer part  90  and  92  includes a substantially spherical outer surface,  94  and  95 , respectively, each having a radius substantially corresponding to the radius R 1  of the head seating surface  82 . The parts  90  and  92  further include respective planar top surfaces  97  and  98  and respective planar bottom surfaces  100  and  101 . The surface  97  and the surface  100  are parallel. The surface  98  and the surface  101  are parallel. The surfaces  100  and  101  abut and seat upon the annular seating surface  45  of the shank  4  when fully installed in the head  10  as shown in  FIG. 10 , with the top surfaces  97  and  98  disposed parallel to and substantially flush with the surface  44  of the capture structure  8 .  
         [0079]     With particular reference to  FIG. 2 , each of the retainer structure parts  90  and  92  have a squared-off U-shape or C-shape, when viewed from the top or bottom, formed about voids or through passages  103  and  104 , respectively, from respective top surfaces  97  and  98  to respective bottom surfaces  100  and  101 . The respective passages  103  and  104  are defined in part by angled or sloping surfaces  106  and  107 , respectively. The surface  106  has a top edge  110  and a bottom edge  111 . The surface  107  has a top edge  112  and a bottom edge  113 . When the retainer structure parts  90  and  92  are operationally disposed in the head  10  with the substantially spherical surfaces  94  and  95  in frictional contact with the spherical seating surface  82 , and the bottom surfaces  100  and  101  are seated on the annular seating surface  45  of the shank  4 , the surfaces  106  and  107  are disposed at a degree of inclination with respect to the bottom surfaces  100  and  101 , respectively, corresponding or congruent to a degree of inclination of the side surfaces  48  and  49  of the capture structure  8  with respect to the seating surface  45 , such that substantially full frictional contact is made between the surface  106  and the surface  48 ; and substantially full frictional contact is made between the surface  107  and the surface  49 .  
         [0080]     The retainer structure part  90  further includes parallel inner walls  116  and  117 , disposed perpendicular to the top and bottom surfaces  97  and  100 , respectively. The retainer structure part  92  includes parallel inner walls  119  and  120 , disposed perpendicular to the top and bottom surfaces  98  and  101 , respectively. The walls  116  and  119  are configured to frictionally mate with the rear trapezoidal surface  41  of the capture structure  8  when the sloped surface  106  is in contact with the side surface  48 ; and the walls  117  and  120  are configured to frictionally mate with the front trapezoidal surface  40  of the capture structure  8  when the sloped surface  107  is in contact with the side surface  49 .  
         [0081]     It is noted that because the parts  90  and  92  are mirror images of each other, the retainer structure functions equally well with the sloped surface  106  in contact with the side surface  49  and the sloped surface  107  in contact with the side surface  48 , with the respective alternative matching of the walls  116  and  119  with the front surface  40  and the walls  117  and  120  with the rear surface  41 . Although the illustrated wall surfaces  106 ,  107 ,  116 ,  117 ,  119 , and  120  are smooth and planar, it is foreseen that these surfaces may be roughened or abraded to provide enhanced frictional contact with the capture structure  8 . Additionally or alternatively the surfaces  40 ,  41 ,  48  and  49  of the capture structure  8  may be roughened or in some way abraded to provide enhanced frictional contact with the retainer structure  12 . Furthermore, the outer surfaces  94  and  95  of the retainer structure  12  that contact the substantially spherical seating surface  82  of the head may also be a high friction surface, such as a knurled surface.  
         [0082]     The retainer part or piece  90  further includes end walls  122  and  123 , extending from the outer surface  94  to the inner walls  116  and  117 , respectively. The end walls  122  and  123  are disposed substantially perpendicular to the top surface  97 . The wall  122  includes a top bevel  126  and the wall  123  includes a top bevel  127 . The retainer part  92  further includes end walls  130  and  131 , extending from the outer surface  95  to the inner walls  119  and  120 , respectively. The end walls  130  and  131  are disposed substantially perpendicular to the top surface  98 . The wall  130  includes a top bevel  134  and the wall  131  includes a top bevel  135 . The retainer parts  90  and  92  are configured such that, when operationally disposed in the head  10 , with the substantially spherical surfaces  94  and  95  in sliding frictional contact with the spherical seating surface  82 , and with the bottom surfaces  100  and  101  seated on the annular seating surface  45  of the shank  4 , the end walls  122  and  123  are in contact with the respective end walls  130  and  131 , as illustrated in  FIG. 13 . The bevels  126 ,  127 ,  134 , and  135  provide clearance space for installing the retainer structure parts  90  and  92  about the capture structure  8  within the head  10  in a method of the invention described subsequently herein. It is foreseen that also in accordance with the invention, to provide additional clearance during installation, the parts  90  and  92  may be configured such that the end walls  122  and  123  are in spaced, substantially parallel relation with the respective end walls  130  and  131 , when fully installed in the bone screw head  10 .  
         [0083]     The elongate rod or longitudinal connecting member  21  that is utilized with the assembly  1  can be any of a variety of implants utilized in reconstructive spinal surgery, but is normally a cylindrical, elongate structure having a substantially smooth, cylindrical surface  136  of uniform diameter. The rod  21  is preferably sized and shaped to snugly seat near the bottom of the U-shaped channel  66  of the head  10  and, during normal operation, is positioned slightly above the bottom of the channel  66  at the lower seat  68 . In particular, the rod  21  normally directly or abutingly engages the shank top end surface  53 , as shown in  FIG. 12  and is biased against the domed surface  53 , consequently biasing the shank  4  downwardly in a direction toward the base  60  of the head  10  when the assembly  1  is fully assembled. For this to occur, the shank top end surface  53  must extend at least slightly into the space of the channel  66  when the retainer structure  12  is snugly seated on the shank  4  in the lower part of the head cavity  78 . The shank  4  and retainer structure  12  are thereby locked or held in position relative to the head  10  by the rod  21  firmly pushing downward on the shank top end surface  53 . It is foreseen, however, that in other embodiments according to the invention, a top- or side-loadable insert may be positioned between the rod  21  and the top end surface  53  and cooperating pieces  90  and  92 , the insert initially engageable with the top surface  53  and one or both pieces  90  and  92 . In such an embodiment, it would not be necessary for the shank top end surface  53  to extend into the space of the channel  66 . Such an insert may be positioned in the bone screw head utilizing a variety of mechanisms, such as a ratchet or twist-and-lock system, and may be used to set the bone screw shank into position with respect to the head prior to insertion of the rod. After placement of the rod in the bone screw head, the insert would engage both the rod and the top surface  53  to secure the bone screw shank in a desired position.  
         [0084]     With reference to  FIGS. 12 and 14 - 16 , the closure structure or closure top  18  can be any of a variety of different types of closure structures for use in conjunction with the present invention with suitable mating structure on the upstanding arms  62  and  64 . The closure top  18  screws between the spaced arms  62  and  64 .  
         [0085]     The illustrated closure top  18  has a generally cylindrical shaped base  138  with a lower point or projection  139  for abrading or digging into the rod  21 , and an upwardly extending break-off head  140 . The base  138  includes a helically wound guide and advancement structure  141  that is sized, shaped and positioned so as to engage the guide and advancement structure  72  on the arms  62  and  64  to provide for rotating advancement of the closure structure  18  into the head  10  when rotated clockwise and, in particular, to cover the top or upwardly open portion  67  of the U-shaped channel  66  to capture the rod  21 , preferably without splaying of the arms  62  and  64 . The closure structure  18  also operably biases against the rod  21  by advancement and applies pressure to the rod  21  under torquing, so that the rod  21  is urged downwardly against the shank top end surface  53  that extends into the channel  66 . Downward biasing of the shank top surface  53  operably produces a frictional engagement between the rod  21  and the surface  53  and also urges the retainer structure  12  toward the base  60  of the head  10 , so as to frictionally seat the retainer structure  12  external spherical surfaces  94  and  95  fixedly against the partial internal spherical seating surface  82  of the head  10 , also fixing the shank  4  and retainer structure  12  in a selected, rigid position relative to the head  10 .  
         [0086]     The closure structure break-off head  140  is secured to the base  138  at a neck  144  that is sized and shaped so as to break away at a preselected torque that is designed to properly seat the retainer structure  12  in the head  10 . The break-off head  140  includes a central bore  145  and grooves  146  for operably receiving a driving and manipulating tool  147  having a projection  148  receivable in the bore  145  and stops  149  receivable in the grooves  146 . The break-off head  140  further includes an external faceted surface  150 , sized and shaped to receive a conventional mating socket type head of a torquing tool  152  to rotate and torque the closure structure  18 . An anti-torque tool  153  may also be provided as shown in  FIGS. 14 and 16  that extends about the head  10  and engages the rod  21  to hold the apparatus  1  stationary during rotation of the torquing tool  152 .  
         [0087]     The closure structure  18  also includes removal tool engagement structure which in the present embodiment is in the form of a hex-shaped and axially aligned aperture  154  disposed in the base  138 , as shown in  FIG. 16 . The hex aperture  154  is accessible after the break-off head  140  breaks away from the base  138 . The aperture  154  is coaxial with the helically wound guide and advancement structure  141  and is designed to receive a hex tool, of an Allen wrench type, into the aperture  154  for rotating the closure structure base  138  subsequent to installation so as to provide for removal thereof, if necessary. Although a hex-shaped aperture  154  is shown in the drawings, the tool engagement structure may take a variety of tool-engaging forms and may include more than one aperture of various shapes, such as a pair of spaced apertures, a left hand threaded bore, an easyout engageable step down bore or the like.  
         [0088]     Prior to the polyaxial bone screw assembly  1  being placed in use according to the invention, the retainer structure pieces  90  and  92  are typically first inserted or top-loaded into the head U-shaped channel  66 , as shown in  FIG. 5 , and then into the cavity  78  to dispose the structure  12  adjacent to the inner surface  80  of the head  10 . Alternatively, as shown in  FIG. 6 , one of the retainer structure pieces  90  is inserted or top-loaded into the channel  66 , while the other retainer structure piece  92 , is inserted or bottom-loaded into the cavity  78  through the bore  84 . Alternatively, both pieces  90  and  92  may be uploaded through the bore  84  (not shown).  
         [0089]     With reference to  FIG. 7 , after the retainer pieces  90  and  92  are disposed in the cavity  78 , the shank  4  is inserted or up-loaded into the head  10  through the bore  84  as indicated by an arrow  160 . With reference to  FIG. 8 , the top edges  46  and  47  of the trapezoidal capture structure  8  come into contact with the sloping inner surfaces  106  and  107  of the respective retainer pieces  90  and  92 . Initially all three components, the capture structure  8 , and the pieces  90  and  92  may move upwardly as illustrated by the arrow  160  in  FIG. 8 . With reference to  FIG. 9 , as the capture structure  8  continues to move upwardly and into the cavity  78  as shown by the arrow  160 , the retainer structure pieces  90  and  92  pivot about the edges  46  and  47  and begin to move downwardly toward the base  60  as shown by the arrows  162  and  163 .  
         [0090]     With reference to  FIG. 10 , the pieces  90  and  92  continue the downward movement until the bottom surfaces  100  and  101  abut and seat upon the annular seating surface  45  of the shank  4 . Once seated upon the annular surface  45 , the retainer structure sloping surface  106  frictionally engages the capture structure side surface  48  and the sloping retainer structure surface  107  frictionally engages the capture structure side surface  49 .  
         [0091]     Subsequent slight downward movement by the shank  4 , as well as the frictionally engaged retainer pieces  90  and  92 , shown by the arrow  166  in  FIG. 10 , seats the shank/retainer structure assembly in the head cavity  78 , with the retainer surfaces  94  and  95  in sliding engagement with the head seating surface  82 . The retainer structure  12 , now fully seated in the head  10  is coaxially aligned with the shank capture structure  8 . With reference to the shank  6  shown in phantom in  FIG. 10 , at this time, the capture structure  8 , the retainer structure  12 , the head seating surface  82  and the lower aperture or neck  83  cooperate to maintain the shank body  6  in rotational relation with the head  10 . According to the embodiment of the invention shown in  FIGS. 1-16 , only the retainer structure  12  is in slidable engagement with the head spherical seating surface  82 . Both the capture structure  8  and the threaded portion of the shank body  6  are in spaced relation with the head  10 . An extent of rotation is shown in  FIG. 10  where it is illustrated that the shank body  6  can be rotated through a substantial angular rotation relative to the head  10 , both from side to side and from front to rear so as to substantially provide a universal or ball joint wherein the angle of rotation is only restricted by engagement of the neck  26  of the shank body  6  with the restrictive neck  83  of the head  10 .  
         [0092]     With reference to  FIG. 11 , the assembly  1  is then typically screwed into a bone, such as the vertebra  15 , by rotation of the shank  4  using the driving tool  31  that operably drives and rotates the shank  4  by engagement thereof with the hexagonally shaped extension or tool engagement head  52  of the shank  4 . Preferably, when the driving tool  31  engages the head  52 , an end portion thereof abuts and frictionally engages the top surface  44  of the capture structure  8 .  
         [0093]     Typically, the head  10  and the retainer structure  12  are assembled on the shank  4  before implanting the shank body  6  into the vertebra  15 . Also, the vertebra  15  may be pre-drilled to minimize stressing the bone and have a guide wire (not shown) inserted therein to provide a guide for the placement and angle of the shank  4  with respect to the vertebra  15 . A further tap hole may be made using a tap with the guide wire as a guide. Then, the assembly  1  is threaded onto the guide wire by first threading the wire into the bottom opening  56  and then out of the top opening  58  of the cannulation bore  54 . The shank  4  is then driven into the vertebra  15 , using the wire as a placement guide.  
         [0094]     With reference to  FIGS. 12 and 14 - 16 , the rod  21  is eventually positioned within the head U-shaped channel  66 , and the closure structure or top  18  is then inserted into and advanced between the arms  62  and  64  with the driving and manipulation tool  147  so as to bias or push against the rod  21 . The anti-torque tool  153  is then placed in position about the head  10  and the rod  21  as shown in  FIGS. 13 and 14 , and the torquing tool or driver  152  is inserted about the break-off head  140 . The break-off head  140  is then twisted to a preselected torque utilizing the torque driver  152 , for example  90  to  120  inch pounds, to urge the rod  21  downwardly into a final desired position.  
         [0095]     The shank top end surface  53 , because it is rounded to approximately equally extend upward into the channel  66  approximately the same amount no matter what degree of rotation exists between the shank  4  and head  10  and because the surface  53  is sized to extend upwardly into the U-shaped channel  66 , the surface  53  is engaged by the rod  21  and pushed downwardly toward the base portion  60  of the head  10  when the closure structure  18  biases downwardly toward and onto the rod  21 . The downward pressure on the shank  4  in turn urges the retainer structure  12  pieces  90  and  92  downward and radially outward, toward the head seating surface  82 , with the retainer structure outer substantially spherical surfaces  94  and  95  in frictional engagement with the head seating surface  82 . The radially outward forces placed upon the pieces  90  and  92  by the capture structure inverted inclined side surfaces  48  and  49  also function to retain the pieces  90  and  92  within the head cavity  78 . As the closure structure  18  presses against the rod  21 , the rod  21  presses against the integral capture structure  8  and shank body  6 , and also the retainer structure  12  pieces  90  and  92  that are now frictionally engaged with the capture structure  8  and disposed or sandwiched between the capture structure trapezoidal structure  38  and the head  10 . Because of the location and configuration of the retainer structure pieces  90  and  92 , the shank  4  in turn becomes frictionally and rigidly attached to the head  10 , fixing the shank body  6  in a desired angular configuration with respect to the head  10  and the rod  21 .  
         [0096]      FIG. 16  illustrates the polyaxial bone screw assembly  1  and including the rod  21  and the closure structure  18 . The axis A of the bone shank  4  is illustrated as not being coaxial with the axis B of the head  10  and the shank  4  is fixed in this angular locked configuration. Other angular configurations can be achieved, as required during installation surgery due to positioning of the rod  21  or the like.  
         [0097]     If removal of the assembly  1  and associated rod  21  and closure structure  18  is necessary, disassembly is accomplished by using a driving tool of an Allen wrench type (not shown) mating with the aperture  154  and rotating the wrench counterclockwise to rotate the base  138  and reverse the advancement thereof in the head  10 . Then, disassembly of the assembly  1  is accomplished in reverse order to the procedure described previously herein for assembly.  
         [0098]     With reference to  FIGS. 17-28 , the reference number  201  generally represents a second or alternative embodiment of a polyaxial bone screw apparatus or assembly according to the present invention. The assembly  201  includes a jointed, telescoping, two-part or piece retainer structure  212 . The assembly  201  further includes the shank  4  and the head  10  according to the first embodiment of the apparatus according to the invention, described previously herein. Therefore, all of the reference numbers already identified herein with respect to the shank  4  and the head  10  are incorporated into the drawing  FIGS. 17-26  and the description thereof are incorporated by reference herein with respect to the assembly  201 .  
         [0099]     As with the first embodiment previously described herein, the shank  4 , head  10  and the retainer structure  212  preferably are assembled prior to implantation of the shank body  6  into the vertebra  15 , the procedure of which has been previously described herein as shown in  FIG. 11 . Furthermore, the closure structure  18  shown in  FIGS. 12 and 14 - 16 , as well as the rod  21 , and optionally an insert disposed below the rod, may be utilized with the assembly  201 , with the same functions and benefits, also as previously described herein.  
         [0100]     The two-part, jointed, slidably telescoping and detachable retainer structure  212  is used to retain the capture structure  8  of the shank  4  within the head  10  and articulate the shank body  6  with respect to the head  10 . The retainer structure  212 , best illustrated by  FIGS. 17-21 , has an operational central axis that is the same as the elongate axis A associated with the shank  4 . The structure  212  includes a first piece or part  220  and a substantially mirror image second piece or part  222 , with the exception that the part  220  has formed therein first and second narrow recesses  224  and  225  and the part  222  includes first and second extensions  228  and  229  that cooperate with the recesses  224  and  225 , respectively. The extensions  228  and  229  and cooperating recesses  224  and  225 , respectively, allow for the retainer structure  212  to be telescoped inwardly or otherwise slightly collapsed into an oval-like shape to fit into the restricted space of either the channel  66  or the bore  84  of the head  10 , providing for either top- or bottom-loading of the retainer structure  212  into the head  10 . After the structure  212  is disposed in the cavity  78  of the head  10 , the parts  220  and  222  telescope outwardly, with a portion of the extensions  228  and  229  disposed in the recesses  224  and  225 , respectively, the structure  212  providing a collar or collet about the capture structure  8  and frictionally attached thereto, while also seating fully and frictionally attaching to the seating surface  82  of the head  10 , when installed. As will be discussed more fully below, the parts or pieces  220  and  222  slidably and closely grip both the capture structure  8  and the seating surface  82 , providing an even and uniform gripping surface between the shank  4  and the head  10  at the spherical seating surface  82  when force is directed onto the shank domed surface  53  by the rod  21  and closure structure  18 , or by other types of longitudinal members and closure structures.  
         [0101]     Each retainer part  220  and  222  includes a substantially spherical outer surface,  234  and  235 , respectively, each having a radius substantially corresponding to the radius R 1  of the head seating surface  82 . The parts  220  and  222  further include respective planar top surfaces  237  and  238  and respective planar bottom surfaces  240  and  241 . The surface  237  and the surface  240  are substantially parallel. The surface  238  and the surface  241  are substantially parallel. The surfaces  240  and  241  abut and seat upon the annular seating surface  45  of the shank  4  when fully installed in the head  10  as shown in  FIG. 28 , with the top surfaces  237  and  238  disposed parallel to and substantially flush with the surface  44  of the capture structure  8 .  
         [0102]     With particular reference to  FIGS. 17 and 21 , each of the retainer structure parts  220  and  222  have a squared-off U-shape or C-shape, when viewed from the top or bottom, formed about an open through passage  243 , respectively, from the respective top surfaces  237  and  238  to the respective bottom surfaces  240  and  241 . The through passage  243  is defined in part by inclined or sloping surfaces  246  and  247  disposed on parts  220  and  222 , respectively. The surface  246  has a top edge  250  and a bottom edge  251 . The surface  247  has a top edge  252  and a bottom edge  253 . When the retainer structure parts  220  and  222  are operationally disposed in the head  10  with the substantially spherical surfaces  234  and  235  in frictional contact with the spherical seating surface  82 , and the bottom surfaces  240  and  241  are seated on the annular seating surface  45  of the shank  4 , the surfaces  246  and  247  are disposed at a degree of inclination with respect to the bottom surfaces  240  and  241 , respectively, corresponding or congruent to a degree of inclination of the side surfaces  48  and  49  of the capture structure  8  with respect to the seating surface  45 , such that substantially full frictional contact is made between the surface  246  and the surface  48 ; and substantially full frictional contact is made between the surface  247  and the surface  49 .  
         [0103]     The retainer structure part  220  further includes parallel inner walls  256  and  257 , disposed perpendicular to the top and bottom surfaces  237  and  240 . The retainer structure part  222  includes parallel inner walls  259  and  260 , disposed perpendicular to the top and bottom surfaces  238  and  241 . The walls  256  and  259  are configured to frictionally mate with the rear trapezoidal surface  41  of the capture structure  8  when the sloped surface  246  is in contact with the side surface  48 ; and the walls  257  and  260  are configured to frictionally mate with the front trapezoidal surface  40  of the capture structure  8  when the sloped surface  247  is in contact with the side surface  49 .  
         [0104]     It is noted that because the parts  220  and  222  are substantial mirror images of each other, the retainer structure functions equally well with the sloped surface  246  in contact with the side surface  49  and the sloped surface  247  in contact with the side surface  48 , with the respective alternative matching of the walls  256  and  259  with the front surface  40  and the walls  257  and  260  with the rear surface  41 . Although the illustrated wall surfaces  246 ,  247 ,  256 ,  257 ,  259 , and  260  are smooth and planar, it is foreseen that these surfaces may be roughened or abraded to provide enhanced frictional contact with the capture structure  8 . Additionally or alternatively the surfaces  40 ,  41 ,  48  and  49  of the capture structure  8  may be roughened or in some way abraded to provide enhanced frictional contact with the jointed retainer structure  212 . Furthermore, the outer surfaces  234  and  235  of the retainer structure  212  that contact the substantially spherical seating surface  82  of the head may also be a high friction surface, such as a knurled surface.  
         [0105]     The retainer part or piece  220  further includes lower end walls  262  and  263 , bounded by the outer surface  234  and the inner walls  256  and  257 , respectively. The end walls  262  and  263  are disposed substantially perpendicular to the top surface  237 . The retainer part  222  further includes lower end walls  266  and  267 , bounded by the outer surface  235  and the inner walls  259  and  260 , respectively. The end walls  266  and  267  are disposed substantially perpendicular to the top surface  238 .  
         [0106]     The recesses  224  and  225  formed in the part  220  open at the end walls  262  and  263 , respectively. The recesses  224  and  225  are elongate, extending substantially parallel to the top surface  237 . The recesses  224  and  225  are configured to slidably receive the joint extensions  228  and  229 , respectively. Furthermore, each of the recesses  224  and  225  is defined in part by an upper surface  268  and a lower surface  269 . The upper surface terminates at a side or end surface  270  and the lower surface terminates at an upwardly projecting lip  271 . Each of the extensions  228  and  229  is elongate and includes a knob-like protrusion  272  at an end thereof. The knob-like protrusion  272  is configured to be received in respective recesses  224  and  225  and during insertion abuts against a U-shaped surface  273  that connects the upper surface  268  with the lower surface  269 . As illustrated in  FIGS. 19, 20  and  25 , the lower lip  271  terminates at the end walls  262  and  263 , each of which extends from the lip  271  to the respective bottom  240  and  241 . After the knob-like protrusions  272  are received into the recesses  224  and  225 , a lower portion  274  of the protrusion  272  disposed adjacent to the extensions  228  and  229  is abutable against the lip  271 , the lip  271  partially containing the protrusion  272  in position within the recesses  224  and  225 . The lip  272  further functions as a pivot point for rotational movement of the retainer piece  222  with respect to the retainer piece  220  during insertion of the capture structure  8  into the head  10  as will be described more fully below, with such rotation limited by the abutment of the top surfaces of the joint extensions  228  and  229  against the upper end surface  270  of the retainer piece  220 .  
         [0107]     The recesses  224  and  255  each have a depth such that the retainer parts  220  and  222  may be slid together until the walls  262  and  266  abut, and the walls  263  and  267  abut, resulting in an oval-shaped structure  212  of a reduced width as shown in  FIGS. 18 and 19 , for down-loading or up-loading the structure  212  into the head  10  as will be described more fully below. Also, the retainer parts  220  and  222  are configured such that, when operationally disposed in the head  10 , with the substantially spherical surfaces  234  and  235  in sliding frictional contact with the spherical seating surface  82 , and with the bottom surfaces  240  and  241  seated on the annular seating surface  45  of the shank  4 , the respective end walls  262  and  263  are in spaced, substantially parallel relation with the respective end walls  266  and  267  as shown in  FIGS. 20 and 21 , with the joint extensions  228  and  229  partially disposed within the respective recesses  224  and  225 .  
         [0108]     Prior to the polyaxial bone screw assembly  201  being placed in use according to the invention, the retainer structure pieces  220  and  222  are assembled by inserting the knob-like protrusions  272  into the recesses  224  and  225  and then sliding the protrusions  272  into the recesses  224  and  225  until the retainer piece  222  abuts against the retainer piece  220 . The connected parts  220  and  222  are then inserted or top-loaded into the head U-shaped channel  66 , as shown in  FIG. 23 , rotated such that the oblong or longest width of the structure  212  is disposed vertically as illustrated in  FIG. 23 . The structure  212  is then inserted into the cavity  78  by rotation as illustrated in  FIG. 24 , to dispose the structure  12  adjacent to the inner surface  80  of the head  10 . Alternatively, the connected, abutting pieces  220  and  222  may be uploaded through the bore  84  (not shown).  
         [0109]     With reference to  FIG. 25 , after the retainer structure  212  is disposed in the cavity  78 , the pieces  220  and  222  are telescoped or spread apart outwardly, radially and the shank  4  is inserted or up-loaded into the head  10  through the bore  84 , the direction illustrated by an arrow  275 . With reference to  FIG. 26 , the top edges  46  and  47  of the trapezoidal capture structure  8  come into contact with the sloping inner surfaces  246  and  247  of the respective retainer pieces  220  and  222 . Initially all three components, the capture structure  8 , and the pieces  220  and  222  may move upwardly as illustrated by the arrow  175 , causing the retainer structure  212  to rotate or hinge in the direction of arrows  278  and  279  as shown in  FIG. 26 . With reference to  FIG. 27 , as the capture structure  8  continues to move upwardly and into the cavity  78  as shown by the arrow  275 , the retainer structure  212  hinges or pivots about the edges  46  and  47  and begin to move downwardly toward the base  60  as shown by the arrows  282  and  283 . The hinged or jointed pieces  220  and  222  continue the downward and radial movement until the bottom surfaces  240  and  241  abut and seat upon the annular seating surface  45  of the shank  4 . With reference to  FIG. 28 , once seated upon the annular surface  45 , the retainer structure sloping surface  246  frictionally engages the capture structure side surface  48  and the sloping retainer structure surface  247  frictionally engages the capture structure side surface  49 .  
         [0110]     Subsequent slight downward movement by the shank  4 , as well as the frictionally engaged retainer structure  212 , may be desired to fully seat the shank/retainer structure assembly in the head cavity  78 , with the retainer surfaces  234  and  235  in sliding engagement with the head seating surface  82 . The retainer structure  212 , now fully seated in the head  10  is coaxially aligned with the shank capture structure  8 . At this time, the capture structure  8 , the retainer structure  212 , the head seating surface  82  and the lower aperture or neck  83  cooperate to maintain the shank body  6  in rotational relation with the head  10 . According to the embodiment of the invention shown in  FIGS. 17-28 , only the retainer structure  212  is in slidable engagement with the head spherical seating surface  82 . Both the capture structure  8  and the threaded portion of the shank body  6  are in spaced relation with the head  10 . An extent of rotation similar to that shown in  FIG. 10  with respect to the assembly  1  is also possible with the assembly  201  of the invention. The shank body  6  can be rotated through a substantial angular rotation relative to the head  10 , both from side to side and from front to rear so as to substantially provide a universal or ball joint wherein the angle of rotation is only restricted by engagement of the neck  26  of the shank body  6  with the restrictive neck  83  of the head  10 .  
         [0111]     With reference to  FIGS. 12 and 14 - 16  described earlier herein with respect to the assembly  1 , the assembly  201  is similarly screwed into a bone, such as the vertebra  15 , by rotation of the shank  4  using the driving tool  31  that operably drives and rotates the shank  4  by engagement thereof with the hexagonally shaped extension or tool engagement head  52  of the shank  4 . Preferably, when the driving tool  31  engages the head  52 , an end portion thereof abuts and frictionally engages the top surface  44  of the capture structure  8 .  
         [0112]     Typically, the head  10  and the retainer structure  212  are assembled on the shank  4  before implanting the shank body  6  into the vertebra  15 . The steps of preparing the vertebra  15  for bone screw insertion, the bone screw implanting process, the rod reduction and closure top installment processes, and closure top removal process described herein with respect to the assembly  1  may also be performed with the assembly  201 . Such processes and above-described apparatus of the assembly  1  are incorporated by reference herein with respect to the assembly  201 .  
         [0113]     With reference to  FIGS. 29-36  the reference number  301  generally represents a third embodiment of a polyaxial bone screw apparatus or assembly according to the present invention. The assembly  301  includes a shank  304  that further includes a body  306  integral with an upwardly extending, substantially conical capture structure  308  and a two-piece or part retainer structure  312  configured to cooperate with the conical capture structure  308 . The assembly further includes the head  10  of the first assembly  1  described previously herein. Therefore, all of the reference numbers previously identified with respect to the head  10  are incorporated into the drawing  FIGS. 29-36  and the description thereof are incorporated by reference herein with respect to the assembly  301 .  
         [0114]     The shank  304 , the head  10  and the retainer structure  312  preferably are assembled prior to implantation of the shank body  306  into a vertebra (not shown), but similar to the vertebra  15  shown in  FIG. 11 , and the procedure described herein with respect to the assembly  1  and  FIG. 12 . Furthermore, the closure structure  18  shown in  FIGS. 12 and 14 - 16 , as well as the rod  21  shown therein may be utilized with the assembly  301 , as described with respect to the assembly  1  and incorporated by reference herein with respect to the assembly  301 . It is foreseen that an insert engageable with the rod  21  may also be utilized as described herein with respect to the assembly  1 . The head  10  and the shank  304  cooperate in such a manner that the head  10  and the shank  304  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 head  10  with the shank  304  until both are locked or fixed relative to each other near the end of an implantation procedure.  
         [0115]     The shank  304 , best illustrated in  FIGS. 29 and 33 , is elongate, with the shank body  306  having a helically wound bone implantable thread  324  extending from near a neck  326  located adjacent to the capture structure  308 , to a tip  328  of the body  306  and extending radially outwardly therefrom. During use, the body  306  utilizing the thread  324  for gripping and advancement is implanted into a vertebra leading with the tip  328  and driven down into the vertebra with an installation or driving tool, such as the tool  31  shown in  FIG. 11 , so as to be implanted in the vertebra to near the neck  326 . The shank  304  has an elongate axis of rotation generally identified by the reference letter A′.  
         [0116]     The neck  326  extends axially outward and upward from the shank body  306 . The neck  326  is of slightly reduced radius as compared to an adjacent upper end or top  332  of the body  306  where the thread  324  terminates. Further extending axially and outwardly from the neck  326  is the capture structure  308  that provides a connective or capture apparatus disposed at a distance from the upper end  332  and thus at a distance from a vertebra when the body  306  is implanted in the vertebra.  
         [0117]     The capture structure  308  is configured for connecting the shank  304  to the head  10  and capturing the shank  304  in the head  10 . The structure  308  is an inverted conical formation, with an outer conical surface or face  338 . The surface  338  extends between a substantially annular top surface  344  and a substantially annular seating surface or ledge  345  disposed adjacent to the neck  326 . Both the top surface  344  and the seating surface  345  are substantially planar, project radially from the axis A′ and are disposed perpendicular to the axis A′. The top surface  344  has an outer circular edge  348  that is also the outer edge of the conical surface  338 . The conical surface  338  has a lower, circular edge  350  that also defines an inner edge of the seating surface  345 . The outer edge  348  has a diameter D 1  that is greater than a diameter D 2  of the inner edge  340 . The diameter D 1  is also larger than an outer diameter of the seating surface  345 .  
         [0118]     The shank  304  further includes a tool engagement structure  352  projecting axially from the top surface  344  to a dome-shaped end surface  353 . The tool engagement structure  352  functions to engage the driving tool  31  shown in  FIG. 11 . The tool  31  includes a driving structure in the form of a socket. The tool  31  is configured to fit about the tool engagement structure  352  so as to form a socket and mating projection for both driving and rotating the shank body  306  into a vertebra. Specifically in the embodiment shown in  FIGS. 29 and 32 , the tool engagement structure  352  is in the shape of a hexagonally shaped extension head coaxial with the threaded shank body  306 .  
         [0119]     The end surface  353  of the shank  304  is preferably curved or dome-shaped as shown in the drawings, for positive engagement with the rod  21 , when the bone screw assembly  301  is assembled, and in any alignment of the shank  304  relative to the head  10 . In certain embodiments, the surface  353  is smooth. While not required in accordance with practice of the invention, the surface  353  may be scored or knurled to further increase frictional engagement between the surface  353  and the rod  21 .  
         [0120]     The shank  304  shown in the drawings is cannulated, having a small central bore  354  extending an entire length of the shank  304  along the axis A′. The bore  354  has a first circular opening  356  at the shank tip  328  and a second circular opening  358  at the domed surface  353 . The bore  354  is coaxial with the threaded body  306 . The bore  354  provides a passage through the shank  304  interior for a length of wire (not shown) inserted into a vertebra prior to the insertion of the shank body  306 , the wire providing a guide for insertion of the shank body  306  into the vertebra.  
         [0121]     The two-part or piece retainer structure  312  is used to retain the capture structure  308  of the shank  304  within the head  10  and articulate the shank body  306  with respect to the head  10 . The retainer structure  312 , best illustrated by  FIGS. 29-31 , has an operational central axis that is the same as the elongate axis A′ associated with the shank  304 . The structure  312  includes a discrete first piece or part  360  and a discrete mirror image second piece or part  362 . The parts  360  and  362  cooperate to provide a restraining and articulating discontinuous collar or collet about the capture structure  308  within the head  10 , when installed therein, as will be discussed more fully below.  
         [0122]     The retainer parts or pieces  360  and  362  slidably and closely grip both the capture structure  308  and the seating surface  82  of the head  10 , providing an even and uniform gripping surface between the shank  304  and the head  10  at the spherical seating surface  82  when force is directed onto the shank domed surface  353  by the rod  21  and closure structure  18 , or by other types of longitudinal members and closure structures.  
         [0123]     Although a two-piece retainer structure  312  is illustrated herein, it is foreseen that the retainer structure is more than one and up to a plurality of pieces, each slidably frictionally matable with both the capture structure  308  and the seating surface  82  of the head  10 . The pieces may also be of varying sizes and not necessarily mirror images of one another.  
         [0124]     Each retainer part  360  and  362  includes a substantially spherical outer surface,  364  and  365 , respectively, each having a radius substantially corresponding to the radius R 1  of the head seating surface  82 . The parts  360  and  362  further include respective planar top surfaces  367  and  368  and respective planar bottom surfaces  370  and  371 . The surface  367  and the surface  370  are substantially parallel. The surface  368  and the surface  371  are substantially parallel. The surfaces  370  and  371  abut and seat upon the annular seating surface  345  of the shank  304  when fully installed in the head  10 , as shown in  FIG. 32 , with the top surfaces  367  and  368  disposed parallel to and substantially flush with the surface  344  of the capture structure  308 .  
         [0125]     With particular reference to  FIG. 30 , each of the retainer structure parts  360  and  362  have a C-shape, when viewed from the top or bottom, formed about voids or through passages  373  and  374 , respectively, from respective top surfaces  367  and  368  to respective bottom surfaces  370  and  371 . The respective passages  373  and  374  are defined in part by inclined or sloping, inner conical surfaces  376  and  377 , respectively. The surface  376  has a semi-circular top edge  380  and a semi-circular bottom edge  381 . The surface  377  has a semi-circular top edge  382  and a semi-circular bottom edge  383 . When the retainer structure parts  360  and  362  are operationally disposed in the head  10  with the substantially spherical surfaces  364  and  365  in frictional contact with the spherical seating surface  82 , and the bottom surfaces  370  and  371  are seated on the annular seating surface  345  of the shank  304 , the inner conical surfaces  376  and  377  are disposed at a degree of inclination with respect to the bottom surfaces  370  and  371 , respectively, corresponding or congruent to a degree of inclination of the conical surface  338  of the capture structure  308  with respect to the seating surface  345 , such that substantially full frictional contact is made between the surface  338  and both the surfaces  376  and  377 .  
         [0126]     With reference to  FIG. 31 , although the inner conical surfaces  376  and  377  are shown in the drawing figures as smooth and planar, it is foreseen that these surfaces may be roughened or abraded to provide enhanced frictional contact with the capture structure  308 . Additionally or alternatively the conical surface  338  of the capture structure  308  may be roughened or in some way abraded to provide enhanced frictional contact with the retainer structure  312 . Furthermore, the outer surfaces  364  and  365  of the retainer structure  312  that contact the substantially spherical seating surface  82  of the head may also be a high friction surface, such as a knurled surface.  
         [0127]     The retainer structure part  360  further includes planar end walls  386  and  387 , disposed perpendicular to the top and bottom surfaces  367  and  370 , respectively. The retainer structure part  362  includes planar end walls  389  and  390 , disposed perpendicular to the top and bottom surfaces  368  and  371 , respectively. The walls  386 ,  387 ,  389  and  390  each include a top bevel  395 . The retainer parts  360  and  362  are configured such that, when operationally disposed in the head  10 , with the substantially spherical surfaces  364  and  365  in sliding frictional contact with the spherical seating surface  82 , and with the bottom surfaces  370  and  371  seated on the annular seating surface  345  of the shank  304 , the end walls  386  and  387  are in spaced, substantially parallel relation with the respective end walls  389  and  390 , but may also be in contact with one another. The bevels  395  provide clearance space for installing the retainer structure parts  360  and  362  about the capture structure  308  within the head  10  in a method of the invention described subsequently herein.  
         [0128]     With reference to  FIG. 33 , prior to the polyaxial bone screw assembly  301  being placed in use according to the invention, the retainer structure pieces  360  and  362  are typically first inserted or top-loaded into the head U-shaped channel  66 , as illustrated by an arrow  397 , and then into the cavity  78  to dispose the structure  312  adjacent to the inner surface  80  of the head  10 . Alternatively, one of the retainer structure pieces  360  is inserted or top-loaded into the channel  66 , while the other retainer structure piece  362 , is inserted or bottom-loaded into the cavity  78  through the bore  84  (not shown). Another alternative is to insert or upload both pieces  360  and  362  through the bore  84  (not shown).  
         [0129]     With reference to  FIGS. 33 and 34 , after the retainer pieces  360  and  362  are disposed in the cavity  78 , the shank  304  is inserted or up-loaded into the head  10  through the bore  84 . The outer circular edge  348  of the conical surface  338  of the capture structure  308  comes into contact with the sloping inner surfaces  376  and  377  of the respective retainer pieces  360  and  362 . Initially all three components: the capture structure  308 , and the pieces  360  and  362  may move upwardly as shown by an arrow  398 . Then, as the shank  304  continues to move upwardly and into the cavity  78  according to the arrow  398 , the retainer structure pieces  360  and  362  pivot about the edge  348  and begin to move downwardly toward the base  60  and outwardly about the capture structure  308  as illustrated by the arrows  399  and  400  in  FIGS. 34 and 35 .  
         [0130]     With reference to  FIG. 35 , the pieces  360  and  362  continue the downward movement until the bottom surfaces  370  and  371  abut and seat upon the annular seating surface  345  of the shank  304  as illustrated in  FIG. 36 . Once seated upon the annular surface  345 , the retainer structure sloping surfaces  376  and  377  frictionally engage the capture structure conical surface  338 .  
         [0131]     Subsequent slight downward movement by the shank  304  may be desirable to fully seat the shank/retainer structure assembly in the head cavity  78 , with the retainer surfaces  364  and  365  in sliding engagement with the head seating surface  82 . The retainer structure  312 , now fully seated in the head  10  is coaxially aligned with the shank capture structure  308 . At this time, the capture structure  308 , the retainer structure  312 , the head seating surface  82  and the restrictive neck  83  of the head  10  cooperate to maintain the shank body  306  in rotational relation with the head  10 . Only the retainer structure  312  is in slidable engagement with the head spherical seating surface  82 . Both the capture structure  308  and the threaded portion of the shank body  306  are in spaced relation with respect to the head  10 . An extent of rotation similar to that shown in  FIG. 10  with respect to the assembly  1  is also possible with the assembly  301  of the invention. The shank body  306  can be rotated through a substantial angular rotation relative to the head  10 , both from side to side and from front to rear so as to substantially provide a universal or ball joint wherein the angle of rotation is only restricted by engagement of the neck  326  of the shank body  306  with the restrictive neck  83  of the head  10 .  
         [0132]     With reference to  FIGS. 12 and 14 - 16  described earlier herein with respect to the assembly  1 , the assembly  301  is similarly screwed into a bone, such as the vertebra  15 , by rotation of the shank  304  using the driving tool  31  that operably drives and rotates the shank  304  by engagement thereof with the hexagonally shaped extension or tool engagement head  352  of the shank  304 . Preferably, when the driving tool  31  engages the head  352 , an end portion thereof abuts and frictionally engages the top surface  344  of the capture structure  308 .  
         [0133]     Typically, the head  10  and the retainer structure  312  are assembled on the shank  304  before implanting the shank body  306  into the vertebra  15 . The steps of preparing the vertebra  15  for bone screw insertion, the bone screw implanting process, the rod reduction and closure top installment processes, and closure top removal process described herein with respect to the assembly  1  may also be similarly performed with the assembly  301 . Such processes and above-described apparatus of the assembly  1  are incorporated by reference herein with respect to the assembly  301 .  
         [0134]     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.