Abstract:
An anchor assembly for use in spinal fixation to interconnect a longitudinal spinal rod with a patient&#39;s vertebra. The anchor assembly preferably includes a bone anchor, a body with a rod-receiving channel, an insert member (preferably a bushing), and a locking cap. The anchor assembly enables in-situ assembly where the bone anchor may be secured to the patient&#39;s vertebra prior to being received within the body of the bone anchor assembly. Accordingly, the anchor assembly enables a surgeon to implant the bone anchor without the body to maximize visibility and access around the anchoring site. Once the bone anchor has been secured to the patient&#39;s vertebra, the body may be snapped onto the bone anchor and a spinal rod may be inserted into the rod-receiving channel.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 14/081,117, filed Nov. 15, 2013, titled “Bone Fixation Assembly,” which claims the benefit of provisional U.S. Patent Application No. 61/727,290, filed Nov. 16, 2012, titled “Bone Fixation Assembly,” and provisional U.S. Patent Application No. 61/731,772, filed Nov. 30, 2012, titled “Reduction Tool for Use with Bone Fixation Assembly,” the contents of which are hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    As a result of various spinal disorders, it often is necessary to surgically correct and stabilize spinal curvatures, or to facilitate spinal fusion. Numerous systems for treating spinal disorders have been developed. For example, one example includes a bone fixation system that has a pair of elongated members, typically spinal rods, longitudinally placed on the posterior spine on either or both sides of the spinous processes of the vertebral column. Each rod is attached to various vertebrae along the length of the spine by way of bone fixation or bone anchor assemblies, e.g., pedicle screws. The body of the pedicle screw often has a rod-receiving channel and receives a locking cap to secure the spinal rod to the pedicle screw. To facilitate insertion of the spinal rod into the rod-receiving channels of the pedicle screws, pedicle screws have been developed wherein the body is separate from and pivotable with respect to the bone anchor (commonly known as polyaxial pedicle screws). 
       SUMMARY 
       [0003]    The present disclosure relates generally to orthopedics. In more particularity, the present disclosure is directed to a bone anchor assembly for use in a spinal fixation procedure that connects a support member (e.g., a spinal rod) to a vertebra. The anchor assembly preferably includes a bone anchor having a head portion (e.g., a bone screw), an insert member (e.g., a bushing), a body having a bore for receiving the insert member and a rod receiving channel, and a locking cap engageable with the body and for receiving the spinal rod. The bone anchor assembly preferably enables in-situ assembly. That is, the anchor assembly may be configured so that in use, the bone anchor may be secured to the patient&#39;s vertebra prior to being connected to the body. Accordingly, the anchor assembly preferably enables a surgeon to implant the bone anchor without the body and bushing to maximize visibility and access around the anchoring site. Once the bone anchor has been secured to the patient&#39;s vertebra, the body can “click-on” to the bone anchor. 
         [0004]    In some implementations, the anchor assembly includes bone anchor moveable with respect to a body subassembly prior to fixing the position of the spinal support member to the body subassembly. The body subassembly may be sized and configured to snap onto the head of the bone anchor and may include an insert member (e.g., a bushing), and receives a locking cap. The head portion preferably may include a first tool interface for engaging a first surgical instrument operatively associated with the bone anchor. The body preferably includes a longitudinal axis, an interior wall, an upper end with an upper opening, a lower end with a lower opening, a bore extending between the upper opening and the lower opening, and a rod-receiving channel. The rod-receiving channel may be configured and arranged to receive a spinal rod. 
         [0005]    The bushing may include an upper end and a lower portion that captures, and at least partially surrounds, the head portion of the bone anchor. The lower portion of the bushing includes at least one, preferably a plurality of, slot(s) extending from the lower end, the slots preferably defining a plurality of flexible arms, wherein each of the flexible arms have an outer surface. The bushing may be movably positionable within the bore of the body. 
         [0006]    Other systems, methods, features and/or advantages will be or may become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features and/or advantages be included within this description and be protected by the accompanying claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The foregoing summary, as well as the following detailed description of preferred embodiments of the disclosure, will be better understood when read in conjunction with the appended drawings. The preferred embodiments of a bone anchor system including a bone anchor assembly are shown in the drawings for the purposes of illustration. It should be understood, however, that the application is not limited to the precise arrangements, structures, features, embodiments, instrumentalities, and methods shown and described, and the arrangements, structures, features, embodiments, instrumentalities, and methods shown and described may be used singularly or in combination with other arrangements, structures, features, embodiments, instrumentalities, and methods. In the drawings: 
           [0008]      FIGS. 1A-1D  illustrate a side perspective view of a first embodiment of a bone anchor assembly in accordance with the present disclosure; 
           [0009]      FIGS. 2A-2F  illustrate a side perspective view of a second embodiment of a bone anchor assembly in accordance with the present disclosure; 
           [0010]      FIGS. 3A-3E  illustrate various configurations of a body of the bone anchor assembly; 
           [0011]      FIGS. 4A-4D  illustrate various configurations of a bushing of the bone anchor assembly; 
           [0012]      FIGS. 5A-5B  illustrate a front sectional view of a first embodiment of a polyaxial pedicle screw assembly of the present disclosure; 
           [0013]      FIGS. 6A-6B  illustrate a front sectional view of a second embodiment of a polyaxial pedicle screw assembly of the present disclosure; 
           [0014]      FIGS. 7A-7B  illustrate a front sectional view of a third embodiment of a polyaxial pedicle screw assembly of the present disclosure; 
           [0015]      FIGS. 8A-8B  illustrate a front sectional view of a fourth embodiment of a polyaxial pedicle screw assembly of the present disclosure; 
           [0016]      FIGS. 9A-9B  illustrate a front sectional view of a fifth embodiment of a polyaxial pedicle screw assembly of the present disclosure; 
           [0017]      FIGS. 10A-10B  illustrate a front sectional view of a sixth embodiment of a polyaxial pedicle screw assembly of the present disclosure; 
           [0018]      FIGS. 11A-11B  illustrate a front sectional view of a seventh embodiment of a polyaxial pedicle screw assembly of the present disclosure; 
           [0019]      FIGS. 12A-12B  illustrate a front sectional view of an eighth embodiment of a polyaxial pedicle screw assembly of the present disclosure; 
           [0020]      FIG. 13  illustrates a front sectional view of an eighth embodiment of a polyaxial pedicle screw assembly of the present disclosure; 
           [0021]      FIG. 14  illustrates a front sectional view of an ninth embodiment of a polyaxial pedicle screw assembly of the present disclosure; 
           [0022]      FIGS. 15A-15B  illustrate a front sectional view of an tenth embodiment of a polyaxial pedicle screw assembly of the present disclosure; 
           [0023]      FIGS. 16A-16B  illustrate a front sectional view of an eleventh embodiment of a polyaxial pedicle screw assembly of the present disclosure; 
           [0024]      FIGS. 17A and 18A  illustrate a third embodiment of bone anchor or bone fixation assembly; and 
           [0025]      FIGS. 17B and 18B  is illustrated a fourth embodiment of bone anchor or bone fixation assembly. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    Certain terminology is used in the following description for convenience only and is not limiting. The words “right”, “left”, “lower”, “upper”, “below”, “above”, “top”, and “bottom” designate directions in the drawings to which reference is made. The words “inwardly” or “distally” and “outwardly” or “proximally” refer to directions toward and away from, respectively, the geometric center of the bone anchor system and/or assembly, the described instruments and designated parts thereof. The words, “anterior”, “posterior”, “superior”, “inferior”, “medial”, and “lateral” and related words and/or phrases designate preferred positions and orientations in the human body to which reference is made and are not meant to be limiting. The terminology includes the above-listed words, derivatives thereof and words of similar import. 
         [0027]    Certain exemplary implementations of the disclosure will now be described with reference to the drawings. In general, such implementations relate to a polyaxial bone fixation element 
         [0028]    threaded ring  60  and set screw  90  may be supplied and assembled during the surgical implantation of the bone fixation assembly  100 . 
         [0029]    With reference to  FIG. 1C , to lock the bone anchor  10  once the rod is placed into the rod receiving channel  29 , the locking cap  92  may be placed into the upper opening  23  of the body  20 . The threaded ring  60  may then be threadably engaged with the threads  21  of the body  20  to connect the locking cap  92  to the body  20 . By engaging the locking cap  92  with the body  20 , the rod-receiving channel  29  is closed and the spinal rod is captured and retained in the bone fixation assembly  100 . To lock the movement of the spinal rod and the bone anchor  10  with respect to the body  20 , the threaded ring  60  is tightened and moves downward in the body  20 . As the threaded ring  60  is moved further downward in the body  20 , the threaded ring  60  pushes down on the upper end  47  and angled section  70  which pushes down on bushing  40 , causing the arms  45  of the bushing  40  to further collapse around the head  14  of the bone anchor  10 , thereby securing the bushing  40  in the locked position, thus securing the position of the bone anchor  10  with respect to the body  20 . As such, the threaded ring  60  controls the locking of the bone anchor  10 . 
         [0030]    In accordance with the above, to lock the rod in place, the setscrew  90  is tightened and as the setscrew  90  moves down within the bore of the threaded ring  60 , the bottom surface  95  of the setscrew  90  pushes down on the rod, thereby securing the position of the rod. This configuration provides the benefit of the anchor assembly  100  having a low profile when assembled. 
         [0031]    The bone fixation assembly  100  may be provided to a user in a kit including (1) bone anchors, (2) locking caps, (3) pre-assembled bushing body subassemblies, bushing/sleeve/body subassemblies, or fastener element body subassemblies, and (4) spinal rods. The pre-assembled bushing body subassemblies, bushing/sleeve/body subassemblies or fastener element/body subassemblies may be assembled during manufacture by inserting the bushing  40  into the axial bore  22  formed in the body  20  through the upper opening  23  formed in the body  20  until the bushing  40  is captured and retained in the body. The kit may be delivered to the user for use in, e.g., spinal surgery. During surgery, the surgeon may identify a level of the spine where the surgery will take place, makes an incision to expose the selected area and implants one or more bone anchors into the desired vertebrae. The subassembly may be clicked-on to the bone anchor  10  by urging the head  14  through the lower opening  24  in the body  20 . Accordingly, the body subassembly may be engaged with the head  14  of the bone anchor  10  in situ. The anchor assembly including the bone anchor  10 , the bushing  40 , the body  20 , and the locking cap  92  may be made from any biocompatible material including, but not limited to, metals such as, for example, titanium, titanium alloys, stainless steel, cobalt chromium, Nitinol, etc. Other materials such as, for example, composites, polymers, ceramics, and any other material may be used for the anchor assembly, its component parts, and spinal rods. 
         [0032]    Referring to  FIGS. 2A-2D , there is illustrated a second implementation of a bone anchor or bone fixation assembly  200  that generally includes a bone anchor  10  (e.g., a bone screw), a body  120 , a bushing  140 , and a locking cap  92 . The elements of the second implementation that are the same as the first implementation of  FIGS. 1A-1D  will not be described again below. The body  120  may generally be described as a cylindrical tubular body having a rod receiving channel  129 , a longitudinal axis  132 , an upper end  133  having an upper opening  123 , a lower end  134  having a lower opening  124 , and an axial bore  122  substantially coaxial with the longitudinal axis  132  of the body  120 . The axial bore  122  extends from the upper opening  123  to the lower opening  124  and has a lower chamber  136  having ledges  149 . 
         [0033]    The axial bore  122  at the upper opening  123  has a first diameter d 11  and, at the lower opening  24 , has a second diameter d 12 , which may be smaller than the first diameter d 11 . The second diameter d 12  may be sized and configured so that the head  14  of the bone anchor  10  may be passed through the lower opening  124  of the body  120 . An inner surface of the axial bore  122  includes a plurality of threads  121  in the upper end for engaging the locking cap  92 . The body  120  and the axial bore  122  may have nearly any mounting structure for engaging the locking cap  192  including, but not limited to, external threads, cam-lock, quarter lock, clamps, lugs, bayonets, etc. 
         [0034]    As shown in  FIG. 2B-2C , the bushing  140  is sized and configured such that it may be inserted into the body  120  through the upper opening  123 , but is prevented from exiting through the lower opening  124 . As shown in  FIG. 2B , the bushing  140  may be inserted into the body  120  in a rotated state, e.g., such that a longitudinal axis of the bushing  140  is perpendicular to the longitudinal axis  132  of the body  120 . Once a portion of the bushing  140  passes through the lower opening  124 , the bushing may be rotated such that the longitudinal axis of the bushing  140  is co-axial with the longitudinal axis  132  of the body  120  and such that the bushing  140  is positioned within the lower chamber  136  of the body  120  ( FIG. 2C ). 
         [0035]    To place and retain the bushing  140  in the body  20 , the saddle  169  preferably may be provided with structures, features, geometry and a configuration that interacts and interfaces with structures, features and geometry of the body  120  and the bushing  140 . In an example, the saddle  169  and body  20  may be provided with one or more ratchet teeth  141  as part of a locking mechanism  138  to prevent the bushing  140  from moving out of the body  120  through upper opening  123  and to lock the bushing  140  into a predetermined orientation within the body  120  when in the first (loading/unlocked) position ( FIGS. 2A and 2D ) and the second (loaded/locked) position ( FIGS. 2E-2F ). 
         [0036]    Referring again to  FIG. 2A , once the bushing  140  is placed and assembled into the body  120 , the bushing  140  may be retained within the body  120  by a saddle  169  such that the bushing  140  is generally prevented from (1) passing back up through the upper opening  123  formed in the body  120 ; and (2) passing through the lower opening  124  formed in the body  120 . For example, after the bushing  140  is rotated and positioned as shown in  FIG. 2C , the saddle  169  may be inserted into the upper opening  123  such that a lower surface  173  of the saddle  169  contacts an upper surface  172  of the bushing  140 . As such, the bushing  140  is retained within the lower chamber  136  of the body  120 . 
         [0037]    The bushing  140  may thus move within a portion of the axial bore  122  formed in the body  120  between a first (loading/unlocked) position ( FIGS. 2A and 2D ) and a second (loaded/locked) position ( FIGS. 2E-2F ). That is, the bushing  140  is moveable within the body  120  between a first position where the bone anchor  10  can be connected to or unconnected from the bushing  140 , and the second position where the bushing  140  is locked with respect to the bone anchor  10 . The lower end portion  136  of the bushing  140  preferably includes an interior cavity  151  that has a predetermined size to receive and secure the head  14  of the bone anchor  10  so that the bone anchor  10  can rotate polyaxially through a range of angles with respect to the bushing  140  and hence with respect to the body  120  when in an unlocked or loading/unloading position, as shown in  FIG. 2F . 
         [0038]    As with be described below with reference to  FIGS. 4A-4D , the interior cavity  151  formed in the bushing  140  may have a curvate or semi-spherical shape for receiving the curvate or semi-spherical head  14  of the bone anchor  10 . The interior cavity  151  formed in the bushing  140  may be constructed so that the bone anchor  10  can polyaxially rotate with respect to the bushing  140 , when the bushing is in an unlocked position, and hence, with respect to the body  20 . The bushing  140  preferably also includes one or more slots  144  extending from the lower end portion  146  thereof so that at least a portion of the bushing  140  is radially expandable so that the head  14  of the bone anchor  10  can be inserted through the lower opening  142  in the lower end portion  146  and into the interior cavity  151  of the bushing  140  and/or radially compressible to compress or lock against the head  14  of the bone anchor  10  when radial forces are applied thereto. In an implementation, the slots  144  define a plurality of flexible arms  145 . The slots  144  may extend from the lower end  146 , the upper end  147  or both ends  146 ,  147 . One slot  142  may extend the length of the bushing  140  creating a compressible spring clip. 
         [0039]    To interconnect or attach the bone anchor  10  to the body  120 , the body  120  may be provided with the bushing  140  pre-assembled and in the loading position, as shown in  FIG. 2A . A lower tooth  141   a  of the saddle  169  engages a tooth  41   c  of the body in the locking mechanism  138 . The lower surface  173  of the saddle  169  contacts the upper surface  172  of the bushing  140 . The head  14  of the bone anchor  10  is inserted into the lower opening  24  of the body  20  and into the interior cavity  51  of the bushing  40 . As shown in  FIG. 2D , the head  14  is further inserted into the interior cavity  151  of the bushing  140 , the head  14  engages the interior surfaces of flexible arms  145 . Thus, the head  14  is “snapped-in” to the bushing  140  as the flexible arms  145  frictionally retain the head  14  within the cavity  151 . 
         [0040]    As shown in  FIG. 2E , after the head  14  of the bone anchor  10  is fully inserted into the cavity  151  of the bushing  140 , a tool may push down on the saddle  169  to push the bushing  140  further within the lower chamber  136  of the body  120  to prevent the head  14  of the bone anchor  10  from becoming dislodged from bushing  140 . The downward movement causes the saddle  169  causes the tooth  141   b  to engage the tooth  141   c  of the body  120 . The downward movement also causes the bushing  140  to lock the head  14  of the bone anchor  10  within the cavity  151 . As the bushing  140  moves downward by force of the saddle  169 , the arms  145  of the bushing  140  come into contact with the one or more lower chamber surfaces  137  in the lower chamber  136  of the body  120 , which exert a force against the arms  145  of the bushing  140 , causing the arms  145  to be urged around the head  14  of the bone anchor  10  into a locking position, thereby locking the position of the bone anchor  10  relative to the body  120 . 
         [0041]    Referring to  FIG. 2F , when the bone anchor  10  is in the locked position the head  14  is rotatable within the cavity  151 . As illustrated, the bushing  140  of the second implementation, provides for approximately 41° of angulation in each direction with respect to the longitudinal axis  32 , as both the head  14  and the bushing  140  are rotatable within the lower chamber  136  of the body  120 . As illustrated, the neck portion  16  acts as a stop as it contacts the lower end  134  of the bushing  140 . Alternatively, the bushing  140  further includes wings  146  that contact ledges  149  of the saddle  169  that may act as a stop to limit the rotational movement of the bushing  140  within the interior cavity  151 . 
         [0042]    Referring to  FIG. 2F , the locking cap  92  is movable from an unlocked to a locked position to lock the bone anchor  10  and the rod (not shown) in place within the body  20 . The locking cap of  FIG. 2F  operates in substantially the same manner as the locking cap described with reference to  FIG. 1C . 
         [0043]    Thus, the above provides for implementations of a bone fixation assembly that provide for easy securing of the bone anchor within the assembly and for polyaxial rotation of the bone anchor within the bone fixation assembly. In the first and second embodiments, the interaction of the bone anchor, bushing and body have specifically designed sections thereof that come into contact to secure the bone anchor within the bushing. These interactions are further detailed with reference to  FIGS. 3-16 . 
         [0044]    With reference to  FIGS. 3A-3E , various configurations of the body  20 / 120  will now be described. As illustrated, the body may be have the lower end formed as having one of several shapes that may interact with the bushing  40 / 140 . Various configurations of the bushing  40 / 140  will be introduced in  FIGS. 4A-4D . The cooperative engagement of the several configurations of the body  20 / 120  and the bushing  40 / 140  are illustrated in  FIGS. 5-16 . 
         [0045]      FIG. 3A  illustrates a first embodiment of the body  20 / 120  having a convex lower end  234 . In particular, the chamber  236  may be formed as a convex region proximate to the lower opening  224  where the convex region  301  may have a radius of curvature r 1  from a center point c 1 . Thus, a surface  301 A of the circle defined by the radius of curvature r 1  defines the lower opening  224  as having the diameter d 2 . The center point c 1  may be selected such that the lower opening  224  may taper inwardly along the surface  301 A of the circumference, where the taper is toward the longitudinal axis  232 . 
         [0046]      FIG. 3B  illustrates a second embodiment of the body  20 / 120  having a spherical lower end  334 . As shown in  FIG. 3B , the spherical lower end  334  is formed having a radius r 2 , as measured from a center point c 2  at the longitudinal axis  232 . Thus, a spherical region  325  is created within the chamber  336  of the body  20 / 120  that has a center c 2 . The lower end  334  may include a tapered region  302  that tapers inwardly to a flat surface  303  that defines the diameter d 2  of the lower end  334 . 
         [0047]    As shown in  FIG. 3C , a third embodiment of the body  20 / 120  having a spherical lower end  334 . As shown in  FIG. 3C , the spherical lower end  334  is formed having the radius r 2 , as measured from a center point c 2  at the longitudinal axis  232 . However, the center point c 2  is at location that is shifted upwardly within the chamber  336  of the body  20 / 120  as compared with the center point c 2  in the body  320  of  FIG. 3B . Thus, the tapered region  302  that tapers inwardly to the flat surface  303  may be larger than that of  FIG. 3B . In accordance with  FIG. 3C , the center point c 2  may be shifted longitudinally anywhere along the longitudinal axis  232  within the body  20 / 120 . 
         [0048]    Alternatively or additionally, as shown in  FIGS. 3D and 3E , a non-spherical region  425  within a chamber  446  and having a center c 2 . The region  425  may be shifted laterally within the body  20 / 120  such that is at a point along a line that is perpendicular to the longitudinal axis  232 . For example, in  FIG. 3D  the center c 2  is shifted to the left of the longitudinal axis  232  (negative), whereas in  FIG. 3E , the center c 2  is shifted to the right of the longitudinal axis  232  (positive). 
         [0049]    Although the body has been explained as having a spherical or non-spherical shape, other shapes may be provided, such as, but not limited to, a conical shape, a torus-like shape, a concave shape or a convex shape. 
         [0050]    With reference to  FIGS. 4A-4D , there is illustrated various configurations of the bushing  40 / 140 .  FIG. 4A  illustrates a first embodiment of the bushing  240 . In the first embodiment, the bushing  240  is provided having a spherical exterior surface  255 , which may be sized and configured to contact the lower chamber surfaces of the body. As shown in  FIG. 4A , the spherical exterior surface  255  may be defined as portion of a sphere  401  having a radius r 4  extending from a center point c 4  of a lower end portion  246 . The lower end portion  246  includes a lower opening  242  and defines an interior cavity  251  for receiving and securing the head of the bone anchor so that the bone anchor can rotate polyaxially through a range of angles with respect to the bushing  240 . 
         [0051]      FIG. 4B  illustrates a second embodiment of the bushing  340  that includes a non-spherical exterior surface  355 , which may be sized and configured to contact the lower chamber surfaces of the body. In the embodiment of  FIG. 4B , the non-spherical surface  355  is defined by a non-spherical shape  402  having a center c 5  that may be shifted to the left of the longitudinal axis  32  (negative). A lower end portion  346  of the bushing  340  includes an interior cavity  351  for receiving and securing the head of the bone anchor through a lower opening  342  so that the bone anchor can rotate polyaxially through a range of angles with respect to the bushing  340 . 
         [0052]      FIG. 4C  illustrates a third embodiment of the bushing  440  that includes a non-spherical exterior surface  455 , which may be sized and configured to contact the lower chamber surfaces of the body. In the embodiment of  FIG. 4C , the center c 6  of the non-spherical exterior  455  is shifted to the right of the longitudinal axis  32  (positive). A lower end portion  446  of the bushing  440  includes an interior cavity  451  for receiving and securing the head of the bone anchor through a lower opening  442  so that the bone anchor can rotate polyaxially through a range of angles with respect to the bushing  440 . 
         [0053]      FIG. 4D  illustrates a fourth embodiment of the bushing  540  that includes a concave exterior surface  555 , which may be sized and configured to contact the lower chamber surfaces of the body. The concave exterior surface  555  may be formed having a radius of curvature r 7  as measured from a point c 7  outside the bushing  540 . A lower end portion  546  of the bushing  540  includes an interior cavity  551  for receiving and securing the head of the bone anchor through a lower opening  542  so that the bone anchor can rotate polyaxially through a range of angles with respect to the bushing  540 . 
         [0054]    For each of the embodiments of the body  20 / 120  and bushing  40 / 140  above, the interactions of the exterior surface of the bushing  40 / 140  and the lower chamber surfaces  37  of the body  20 / 120  is described below in greater detail. In particular, In accordance with the geometries disclosed in  FIGS. 3A-3E and 4A-4D , the various bushings and bodies may be assembled, as shown in  FIGS. 1 and 2  to provide interface geometries between the bushing lower exterior surface portions  255 ,  355 ,  455  and  555  and the lower chamber surfaces  37  that assume a partially spherical-to-partially spherical interface as well as a linear taper-to-linear taper interface in order to allow the compression of the bushing interior cavity  51  to thereby lock the position and angulation of the bone anchor is locked with respect to the body  20 / 120  and polyaxial bone fixation assembly  100  as a locking cap is advanced downward through the body  20 / 120 , urging the spinal rod and the bushing disposed therein downward through the body  120  as well. Table 1, below, sets for the example configurations of the body  20 / 120  and bushing  40 / 140 . 
         [0000]    
       
         
               
               
             
               
               
               
               
             
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Body 20/120 
               
             
          
           
               
                 Polyaxial Pedical 
                 Convex 
                 Concave 
                   
               
             
          
           
               
                 Screw Interface 
                 torus-like 
                 spherical 
                 torus-like 
               
             
          
           
               
                 Body/Bushing Matrix 
                 (FIG. 3A) 
                 (FIGS. 3B-3C) 
                 (FIGS. 3D-3E) 
                 Conical 
               
               
                   
               
             
          
           
               
                 Bushing 
                 convex 
                 spherical 
                 FIGS. 5A-5B 
                 FIGS. 9A-9B 
                 FIG. 14 
                 FIGS. 11A-11B 
               
               
                 40/140 
                   
                 (FIG. 4A) 
               
               
                   
                   
                 torus-like 
                 FIGS. 6A-6B 
                 FIGS. 10A-10B 
                 FIGS. 15A-15B 
                 FIGS. 12A-12B 
               
               
                   
                   
                 (FIGS. 4B-4C) 
               
               
                   
                 concave 
                 torus-like 
                 FIGS. 7A-7B 
                 FIG. 13 
                 FIGS. 16A-16B 
               
               
                   
                   
                 (FIG. 4D) 
               
             
          
           
               
                   
                 conical 
                 FIGS 8A-8B 
               
               
                   
                   
               
             
          
         
       
     
         [0055]      FIGS. 5A-5B  illustrate a front sectional view of a first embodiment of a polyaxial pedicle screw assembly of the present disclosure in which a torus-like convex body lower interior surface portion and a spherical-convex bushing lower exterior surface portion form the bearing surfaces that allow the assembly to be locked  FIGS. 5A-5B  illustrate a polyaxial bone fixation assembly  100 A/ 200 A that includes a body  20 / 120  having an axial bore characterized by a generally cylindrical interior geometry adjacent the axial bore  122  and characterized by a torus-like convex interior geometry along the lower chamber surfaces  37  adjacent the lower opening  124 . More specifically, as can best be seen taking into account the dotted circles of  FIG. 5A , the lower chamber surfaces  37  progresses from an essentially cylindrical surface adjacent the body axial bore  122  and transitions through a hemi-concave surface tracing out approximately 45 degrees and smoothly inflects to a hemi-convex surface tracing out approximately 45 degrees adjacent the body lower opening  124 . 
         [0056]    The bushing  40 / 140  is characterized by an exterior surface  55  that assumes a generally cylindrical geometry adjacent the upper end  47  and smoothly transitions to a spherical convex geometry or hemi-convex surface along the lower exterior surface portion  255  and adjacent to the lower end  241 . As the bushing  40 / 140  is urged downward through the polyaxial bone fixation assembly  100  and the lower exterior surface portion  255  bears against the lower chamber surfaces  37  of the body  120 , the interior cavity  51  is locked around the head of the bone anchor as the flexible arms  45  are drawn together and the position and angulation of the bone anchor is locked with respect to the body  120  and polyaxial bone fixation assembly  100 . The dark arrows show in  FIG. 5B  illustrate a point of contact between the body  20 / 120  and the bushing  40 / 140  when in the locked state. 
         [0057]      FIGS. 6A-6B  illustrate a front sectional view of a second embodiment of a polyaxial pedicle screw assembly of the present disclosure in which a torus-like convex body lower interior surface portion and a torus-convex bushing lower exterior surface portion form the bearing surfaces that allow the assembly to be locked.  FIGS. 6A-6B  illustrate a polyaxial bone fixation assembly  100 B/ 200 B that includes a body  20 / 120  having an axial bore characterized by a generally cylindrical interior geometry adjacent the axial bore  322  and characterized by a torus-like convex interior geometry at the lower chamber surfaces  37  adjacent the lower end  324 . More specifically, as can best be seen taking into account the dotted circles of  FIG. 6A , the body interior surface  126  progresses from an essentially cylindrical surface adjacent the body axial bore  222  and transitions through a hemi-concave surface tracing out approximately 45 degrees and smoothly inflects to a hemi-convex surface tracing out approximately 45 degrees adjacent the body lower opening  224 . 
         [0058]    The bushing  40 / 140  is characterized by an exterior surface  355  that assumes a generally cylindrical geometry adjacent the upper end  47  and smoothly transitions to a torus-like convex geometry or hemi-convex surface along the lower exterior surface portion  355  and adjacent to the lower end portion  346 . As the bushing is urged downward through the polyaxial bone fixation assembly  100 B/ 200 B and the lower exterior surface portion  355  bears against the lower chamber surfaces  37  of the body  20 / 120 , the interior cavity  351  is crush locked around the head of the bone anchor as the flexible arms  345  are drawn together and the position and angulation of the bone anchor is locked with respect to the body  20 / 120  and polyaxial bone fixation assembly  100 B/ 200 B. The dark arrows show in  FIG. 6B  illustrate a point of contact between the body  20 / 120  and the bushing  40 / 140  when in the locked state. 
         [0059]      FIGS. 7A-7B  illustrate a front sectional view of a third embodiment of a polyaxial pedicle screw assembly of the present disclosure in which a torus-like convex body lower interior surface portion and a torus-like concave bushing lower exterior surface portion form the bearing surfaces that allow the assembly to be locked,  FIGS. 7A-7B  illustrate a polyaxial bone fixation assembly  100 C/ 200 C that includes a body  20 / 120  having an axial bore characterized by a generally cylindrical interior geometry adjacent the axial bore  222  and characterized by a torus-like convex interior geometry at the lower chamber surfaces  37  adjacent the lower opening  224 . More specifically, as can best be seen taking into account the dotted circles of  FIG. 7A , the body interior surface  126  progresses from an essentially cylindrical surface adjacent the body axial bore  222  and transitions through a hemi-concave surface tracing out approximately 45 degrees and smoothly inflects to a partially-convex surface tracing out approximately 45 degrees adjacent the body lower opening  224 . 
         [0060]    The bushing  40  is characterized by an exterior surface  555  that assumes a generally cylindrical geometry adjacent the upper end  547  and smoothly transitions to a torus-like concave geometry along the lower exterior surface portion  555  and adjacent the lower end portion  546 . As the bushing  40 / 140  is urged downward through the polyaxial bone fixation assembly  100  and the lower exterior surface portion  555  bears against the lower chamber surfaces  37  of the body  20 / 120 , the interior cavity  551  is crush locked around the head of the bone anchor as the flexible arms  545  are drawn together and the position and angulation of the bone anchor is locked with respect to the body  20 / 120  and the polyaxial bone fixation assembly  100 C/ 200 C. The dark arrows show in  FIG. 7B  illustrate a point of contact between the body  20 / 120  and the bushing  40 / 140  when in the locked state. 
         [0061]      FIGS. 8A-8B  illustrate a front sectional view of a fourth embodiment of a polyaxial pedicle screw assembly of the present disclosure in which a torus-like convex body lower interior surface portion and a conical bushing lower exterior surface portion form the bearing surfaces that allow the assembly to be locked.  FIGS. 8A-8B  illustrate a polyaxial bone fixation assembly  100 D/ 200 D that includes a body  20 / 120  having an axial bore characterized by a generally cylindrical interior geometry adjacent the axial bore  222  and characterized by a torus-like convex interior geometry at the lower chamber surfaces  37  adjacent the lower opening  224 . More specifically, as can best be seen taking into account the dotted circles of FIG.  8 A, the body interior surface  126  progresses from an essentially cylindrical surface adjacent the body axial bore  222  and transitions through a hemi-concave surface tracing out approximately 45 degrees and smoothly inflects to a partially-convex surface tracing out approximately 45 degrees adjacent the body lower opening  224 . 
         [0062]    The bushing  40 / 140  is characterized by an exterior surface  55  that assumes a generally cylindrical geometry adjacent the upper end  47  and smoothly transitions through a conical taper along the bushing lower exterior surface portion  55  prior to terminating in a cylindrical geometry adjacent the bushing lower end portion  46 . As the bushing  40 / 140  is urged downward through the polyaxial bone fixation assembly  100 C/ 200 D and the conical taper disposed along the lower exterior surface portion  255 ,  355 ,  455  and  555  bears against the lower chamber surfaces  37  of the body  20 / 120 , the interior cavity  51  is crush locked around the head of the bone anchor as the flexible arms  45  are drawn together and the position and angulation of the bone anchor is locked with respect to the body  20 / 120  and the polyaxial bone fixation assembly  100 D/ 200 D. The dark arrows show in  FIG. 8B  illustrate a point of contact between the body  20 / 120  and the bushing  40 / 140  when in the locked state. 
         [0063]      FIGS. 9A-9B  illustrate a front sectional view of a fifth embodiment of a polyaxial pedicle screw assembly of the present disclosure in which a spherical-concave body lower interior surface portion and a spherical-convex bushing lower exterior surface portion form the bearing surfaces that allow the assembly to be locked.  FIGS. 9A-9B  illustrate a polyaxial bone fixation assembly  100 E/ 200 E that includes a body  20 / 120  having an axial bore characterized by a generally cylindrical interior geometry adjacent the axial bore  322  and terminating in a generally cylindrical interior geometry adjacent the lower opening  324  having a radius smaller than the radius characterizing the axial bore at the axial bore  322 . Prior to terminating in the cylindrical geometry at the lower opening  324 , the axial bore is characterized by a spherical concave surface geometry along the lower chamber surfaces  37 . More specifically, as can best be seen taking into account the dotted circles in  FIG. 9A , the body interior surface  126  progresses from an essentially cylindrical surface adjacent the body axial bore  322  and smoothly transitions through a hemi-concave surface along the body lower chamber surfaces  37  tracing out approximately 45 degrees and transitions to an essentially cylindrical surface adjacent the body lower opening  324 , the radius of the cylindrical surface adjacent the body lower opening  324  being smaller than the radius of the cylindrical surface adjacent the body axial bore  322 . 
         [0064]    The bushing  40 / 140  is characterized by an exterior surface  255  that assumes a generally cylindrical geometry adjacent the upper end  47  and smoothly transitions to a spherical-convex, or hemi-convex, surface along the bushing lower exterior surface portion  255  tracing out an angle of approximately 45 degrees that terminates at the bushing lower end portion  246 . As the bushing  40 / 140  is urged downward through the polyaxial bone fixation assembly  100  and the lower exterior surface portion  255  bears against the lower chamber surfaces  37  of the body  20 / 120 , the interior cavity  251  is crush locked around the head of the bone anchor as the flexible arms  245  are drawn together and the position and angulation of the bone anchor is locked with respect to the body  20 / 120  and the polyaxial bone fixation assembly  100 E/ 200 E. The dark arrows show in  FIG. 9B  illustrate a point of contact between the body  20 / 120  and the bushing  40 / 140  when in the locked state. 
         [0065]      FIGS. 10A-10B  illustrate a front sectional view of a sixth embodiment of a polyaxial pedicle screw assembly of the present disclosure in which a spherical-concave body lower interior surface portion and a torus-like convex bushing lower exterior surface portion form the bearing surfaces that allow the assembly to be locked.  FIGS. 10A-10B  illustrate a polyaxial bone fixation assembly  100 F/ 200 F that includes a body  20 / 120  having an axial bore characterized by a generally cylindrical interior geometry adjacent the axial bore  322  and terminating in a generally cylindrical interior geometry adjacent the lower opening  324  having a radius smaller than the radius characterizing the axial bore at the axial bore  322 . Prior to terminating in the cylindrical geometry at the lower opening  324 , the axial bore is characterized by a spherical concave surface geometry along the lower chamber surfaces  37 . More specifically, as can best be seen taking into account the dotted circles in  FIG. 10A , the body interior surface  126  progresses from an essentially cylindrical surface adjacent the body axial bore  322  and smoothly transitions through a hemi-concave surface along the body lower chamber surfaces  37  tracing out approximately 45 degrees and transitions to an essentially cylindrical surface adjacent the body lower opening  324 , the radius of the cylindrical surface adjacent the body lower opening  324  being smaller than the radius of the cylindrical surface adjacent the body axial bore  322 . 
         [0066]    The bushing  40 / 140  is characterized by an exterior surface  355  that assumes a generally cylindrical geometry adjacent the bushing upper end  347  and smoothly transitions to a torus-like convex, or hemi-convex, surface along the bushing lower exterior surface portion  355  tracing out an angle of approximately 45 degrees that terminates at the bushing lower end portion  346 . As the bushing is urged downward through the polyaxial bone fixation assembly  100 F/ 200 F and the lower exterior surface portion  355  bears against the lower chamber surfaces  37  of the body  20 / 120 , the interior cavity  351  is crush locked around the head of the bone anchor as the flexible arms  345  are drawn together and position and angulation of the bone anchor is locked with respect to the body  20 / 120  and the polyaxial bone fixation assembly  100 F/ 200 F. The dark arrows show in  FIG. 10B  illustrate a point of contact between the body  20 / 120  and the bushing  40 / 140  when in the locked state. 
         [0067]      FIGS. 11A-11B  illustrate a front sectional view of a seventh embodiment of a polyaxial pedicle screw assembly of the present disclosure in which a conical body lower interior surface portion and a spherical-convex bushing lower exterior surface portion form the bearing surfaces that allow the assembly to be locked.  FIGS. 11A-11B  illustrate a polyaxial bone fixation assembly  100 G/ 200 G that includes a body  20 / 120  having an axial bore characterized by a generally cylindrical interior geometry adjacent the axial bore  122  and terminating in a cylindrical interior geometry adjacent the lower opening  124  having a radius smaller than the radius characterizing the axial bore at the axial bore  122 . Prior to terminating in the cylindrical geometry at the lower opening  124 , the axial bore is characterized by a conical surface geometry that provides a linear taper along the lower chamber surfaces  37 . More specifically, as can best be seen taking into account the dotted circles in  FIG. 11A , the body interior surface  126  progresses from an essentially cylindrical surface adjacent the body axial bore  122  and transitions to a conical surface geometry along the body lower chamber surfaces  37  and transitions back to an essentially cylindrical surface adjacent the body lower opening  124 , the radius of the cylindrical surface adjacent the body lower opening  124  being smaller than the radius of the cylindrical surface adjacent the body axial bore  122 . 
         [0068]    The bushing  40 / 140  is characterized by an exterior surface  255  that assumes a generally cylindrical geometry adjacent the upper end  47  and smoothly transitions to a spherical-convex, or hemi-convex, surface along the bushing lower exterior surface portion  255  tracing out an angle of approximately 45 degrees that terminates at the bushing lower end portion  246 . As the bushing  40 / 140  is urged downward through the polyaxial bone fixation assembly  100 G/ 200 G and the bushing lower exterior surface portion  255  bears against the body lower chamber surfaces  37 , the interior cavity  251  is crush locked around the head of the bone anchor as the flexible arms  245  are drawn together and the position and angulation of the bone anchor is locked with respect to the body  20 / 120  and the polyaxial bone fixation assembly  100 G/ 200 G. The dark arrows show in  FIG. 11B  illustrate a point of contact between the body  20 / 120  and the bushing  40 / 140  when in the locked state. 
         [0069]      FIGS. 12A-12B  illustrate a front sectional view of an eighth embodiment of a polyaxial pedicle screw assembly of the present disclosure in which a conical body lower interior surface portion and a torus-like convex bushing lower exterior surface portion form the bearing surfaces that allow the assembly to be locked.  FIGS. 12A-12B  illustrate a polyaxial bone fixation assembly  100 H/ 200 H that includes a body  20 / 120  having an axial bore characterized by a generally cylindrical interior geometry adjacent the axial bore  122  and terminating in a cylindrical interior geometry adjacent the lower opening  124  having a radius smaller than the radius characterizing the axial bore at the axial bore  122 . Prior to terminating in the cylindrical geometry at the lower opening  124 , the axial bore is characterized by a conical surface geometry that provides a linear taper along the lower chamber surfaces  37 . More specifically, as can best be seen taking into account the dotted circles in  FIG. 12A , the body interior surface  126  progresses from an essentially cylindrical surface adjacent the body axial bore  122  and transitions to a conical surface geometry along the body lower chamber surfaces  37  and transitions back to an essentially cylindrical surface adjacent the body lower opening  124 , the radius of the cylindrical surface adjacent the body lower opening  124  being smaller than the radius of the cylindrical surface adjacent the body axial bore  122 . 
         [0070]    The bushing  40 / 140  is characterized by an exterior surface  355 ,  455  that assumes a generally cylindrical geometry adjacent the bushing upper end  47  and smoothly transitions to a torus-like convex, or hemi-convex, surface along the lower exterior surface portion  355  tracing out an angle of approximately 45 degrees that terminates at the bushing lower end portion  346 ,  446 . As the bushing  40 / 140  is urged downward through the polyaxial bone fixation assembly  100 H/ 200 H and the bushing lower exterior surface portion  355  bears against the lower chamber surfaces  37 , the interior cavity  351 ,  451  is crush locked around the head of the bone anchor as the flexible arms  345 ,  445  are drawn together and the position and angulation of the bone anchor is locked with respect to the body  20 / 120  and the polyaxial bone fixation assembly  100 H/ 200 H. The dark arrows show in  FIG. 12B  illustrate a point of contact between the body  20 / 120  and the bushing  40 / 140  when in the locked state. 
         [0071]      FIG. 13  illustrates a polyaxial bone fixation assembly  100 I/ 200 I that includes a body  20 / 120  having an axial bore characterized by a generally cylindrical interior geometry adjacent the axial bore  322  and terminating in a generally cylindrical interior geometry adjacent the lower opening  324  having a radius smaller than the radius characterizing the axial bore at the axial bore  322 . Prior to terminating in the cylindrical geometry at the lower opening  324 , the axial bore is characterized by a spherical concave surface geometry along the lower chamber surfaces  37 . The body interior surface  126  progresses from an essentially cylindrical surface adjacent the body axial bore  322  and smoothly transitions through a hemi-concave surface along the body lower chamber surfaces  37  tracing out approximately 45 degrees and transitions to an essentially cylindrical surface adjacent the body lower opening  324 , the radius of the cylindrical surface adjacent the body lower opening  324  being smaller than the radius of the cylindrical surface adjacent the body axial bore  322 . 
         [0072]    The bushing  40 / 140  is characterized by an exterior surface  555  that assumes a generally cylindrical geometry adjacent the upper end  547  and smoothly transitions to a torus-like concave geometry along the lower exterior surface portion  555  and adjacent the lower end portion  546 . As the bushing  40 / 140  is urged downward through the polyaxial bone fixation assembly  100 I/ 200 I and the lower exterior surface portion  555  bears against the lower chamber surfaces  37  of the body  20 / 120 , the interior cavity  551  is crush locked around the head of the bone anchor as the flexible arms  545  are drawn together and the position and angulation of the bone anchor is locked with respect to the body  20 / 120  and the polyaxial bone fixation assembly  100 I/ 200 I. The dark arrows show in  FIG. 13  illustrate a point of contact between the body  20 / 120  and the bushing  40 / 140  when in the locked state. 
         [0073]      FIG. 14  illustrates a polyaxial bone fixation assembly  100 J/ 200 J that includes a body  20 / 120  having an axial bore characterized by a generally cylindrical interior geometry adjacent the axial bore  422  and characterized by a torus-like concave interior geometry along the lower chamber surfaces  37  adjacent the lower opening  424 . More specifically, the lower chamber surfaces  37  progresses from an essentially cylindrical surface adjacent the body axial bore  422  and transitions through a hemi-concave surface tracing out approximately 45 degrees and smoothly inflects to a hemi-concave surface tracing out approximately 45 degrees adjacent the body lower opening  424 . 
         [0074]    The bushing  40 / 140  is characterized by an exterior surface  255  that assumes a generally cylindrical geometry adjacent the upper end  47  and smoothly transitions to a spherical concave geometry or hemi-concave surface along the lower exterior surface portion  255  and adjacent to the lower end portion  246 . As the bushing  40 / 140  is urged downward through the polyaxial bone fixation assembly  100  and the lower exterior surface portion  255  bears against the lower chamber surfaces  37  of the body  20 / 120 , the interior cavity  251  is crush locked around the head of the bone anchor as the flexible arms  245  are drawn together and the position and angulation of the bone anchor is locked with respect to the body  20 / 120  and polyaxial bone fixation assembly  100 J/ 200 J. The dark arrows show in  FIG. 14  illustrate a point of contact between the body  20 / 120  and the bushing  40 / 140  when in the locked state. 
         [0075]      FIGS. 15A-15B  illustrate a polyaxial bone fixation assembly  100 K/ 200 K that includes a body  20 / 120  having an axial bore characterized by a generally cylindrical interior geometry adjacent the axial bore  422  and characterized by a torus-like concave interior geometry at the lower chamber surfaces  37  adjacent the lower end  524 . More specifically, the body interior surface  126  progresses from an essentially cylindrical surface adjacent the body axial bore  422  and transitions through a hemi-concave surface tracing out approximately 45 degrees and smoothly inflects to a hemi-concave surface tracing out approximately 45 degrees adjacent the body lower opening  524 . 
         [0076]    The bushing  40 / 140  is characterized by an exterior surface  355  that assumes a generally cylindrical geometry adjacent the upper end  47  and smoothly transitions to a torus-like concave geometry or hemi-concave surface along the lower exterior surface portion  355  and adjacent to the lower end portion  346 . As the bushing is urged downward through the polyaxial bone fixation assembly  100 K/ 200 K and the lower exterior surface portion  355  bears against the lower chamber surfaces  37  of the body  20 / 120 , the interior cavity  351  is crush locked around the head of the bone anchor as the flexible arms  45  are drawn together and the position and angulation of the bone anchor is locked with respect to the body  20 / 120  and polyaxial bone fixation assembly  100 K/ 200 K. The dark arrows show in  FIGS. 15A and 15B  illustrate a point of contact between the body  20 / 120  and the bushing  40 / 140  when in the locked state. 
         [0077]      FIGS. 16A-16B  illustrate a polyaxial bone fixation assembly  100 L/ 200 L that includes a body  20 / 120  having an axial bore characterized by a generally cylindrical interior geometry adjacent the axial bore  422  and characterized by a torus-like concave interior geometry at the lower chamber surfaces  37  adjacent the lower opening  524 . More specifically, the body interior surface  126  progresses from an essentially cylindrical surface adjacent the body axial bore  422  and transitions through a hemi-concave surface tracing out approximately 45 degrees and smoothly inflects to a partially-concave surface tracing out approximately degrees adjacent the body lower opening  524 . 
         [0078]    The bushing  40 / 140  is characterized by an exterior surface  555  that assumes a generally cylindrical geometry adjacent the upper end  47  and smoothly transitions to a torus-like concave geometry along the lower exterior surface portion  555  and adjacent the lower end portion  546 . As the bushing  40 / 140  is urged downward through the polyaxial bone fixation assembly  100  and the lower exterior surface portion  555  bears against the lower chamber surfaces  37  of the body  20 / 120 , the interior cavity  551  is crush locked around the head of the bone anchor as the flexible arms  545  are drawn together and the position and angulation of the bone anchor is locked with respect to the body  20 / 120  and the polyaxial bone fixation assembly  100 . The dark arrows show in  FIGS. 16A and 16B  illustrate a point of contact between the body  20 / 120  and the bushing  40 / 140  when in the locked state. 
         [0079]    Thus, as described above, in the first and second embodiments of  FIGS. 1 and 2 , the interaction of the bone anchor, bushing and body have specifically designed sections thereof that come into contact to secure the bone anchor within the bushing, as described in  FIGS. 5-16 . In  FIGS. 17 and 18 , the interaction of the bone anchor and the bushing involves the head of the bone anchor deflecting the bushing as the head is inserted into the bushing. The arms of the bushing deflect outwardly as the head is inserted and then inwardly as the head is received within the bushing to secure the head therein. 
         [0080]    Referring now to  FIGS. 17A and 18A , there is illustrated a third embodiment of bone anchor or bone fixation assembly  1700  that generally includes a bone anchor  10  (e.g., a bone screw), a body  1720 , a bushing  1740 , and a locking cap  92 . In the implementation of  FIG. 17A , the bushing  1740  has a size and configuration to create an interference fit between the bushing  1740  and the bone anchor  10 , whereas in the implementation of  FIGS. 1A-1D  the bushing  40  include a cavity  51  that is shaped and sized to engage and secure the head  14  when pushing is locked into place. As in the above, the anchor assembly  1700  enables in-situ assembly of the bone anchor  10  to the body  1720  of the anchor assembly  1700  such that the bone anchor  10  may be secured to a patients vertebra prior to being received within the body  1720 . Aspects of the anchor assembly  1700  that are similar to the anchor assembly  100  are not repeated below. 
         [0081]    The bushing  1740  may be movably positionable within the body  20  between a first (unloaded/unlocked) position where the bone anchor  10  can be connected to or unconnected from the bushing  1740 , and a second (loaded/locked) position where the bone anchor  10  is locked with respect to the bushing  1740 . The bushing  1740  defines slots, as in  FIGS. 1 and 2 , that define a plurality of flexible arms  1745  that pivot about a point  1753  ( FIG. 18A ). The slots may extend from the lower end  1746 , the upper end  1747  or both ends  1746 ,  1747 . 
         [0082]    To interconnect or attach the bone anchor  10  to the body  1720 , the body  1720  may be provided with the bushing  1740  pre-assembled and in the loading position, in which a lower tooth  1741   a  of an upper portion of the bushing  1740  engages a tooth  1741   c  of the body in the locking mechanism  1738 . The head  14  of the bone anchor  10  is inserted into the lower opening  1724  of the body  1720  and into the interior cavity  1751  of the bushing  1740 . As the head  14  is further inserted into the interior cavity  1751  of the bushing  1740  such that the flexible arms  1745  initially pivot outwardly about the point  1753  and then back inwardly until it the head  14  engages the interior surfaces of flexible arms  1745  that pivot about the point  1753  of the bushing  1740  (see  FIG. 18A ). Thus, the head  14  is “clicked-in” to the bushing  40  as the flexible arms  45  retain the head  14  within the cavity  1751 . 
         [0083]    After the head  14  of the bone anchor  10  is fully inserted into the cavity  1751  of the bushing  1740 , the bushing  1740  is moved down into the lower chamber of the body  1720  to prevent the head  14  of the bone anchor  10  from becoming dislodged from bushing  1740 . The downward movement causes the upper portion of the bushing  1740  to be retained within the body  1720  by the interaction of the tooth  1741   b  engaging the tooth  1741   c  of the body  1720 . When the bone anchor  10  is in the locked position the head  14  is able to rotate polyaxially within the cavity  1751 , and thus about the body  20 . As illustrated, the bushing  40  of the first implementation, provides for approximately 25° of angulation in any direction with respect to the longitudinal axis  1732 . As illustrated, the neck portion  16  acts as a stop when the neck portion  16  contacts the lower and of the bushing  1740 . 
         [0084]    Referring to  FIGS. 17B and 18B , there is illustrated a fourth embodiment of a bone anchor or bone fixation assembly  1800  that generally includes a bone anchor  10  (e.g., a bone screw), a body  1820 , a bushing  1840 , and a locking cap  92 . In the implementation of  FIG. 17B , the bushing  1840  has a size and configuration to create an interference fit between the bushing  1840  and the bone anchor  10 , whereas in the implementation of  FIGS. 2A-2E  the bushing  140  include a cavity  151  that is shaped and sized to engage and secure the head  14  when pushing is locked into place. As shown in  FIGS. 17B and 18B , the bushing  1840  is sized and configured such that it may be inserted into the body  1820  through the upper opening, but is prevented from exiting through the lower opening. 
         [0085]    Referring again to  FIGS. 17B and 18B , once the bushing  1840  is placed and assembled into the body  1820 , the bushing  1840  may be retainable within the body  1820  by a saddle  1869 . For example, after the bushing  1840  is positioned, the saddle  1869  may be inserted into the upper opening  1823  such that a lower surface  1873  of the saddle  1869  contacts an upper surface  1872  of the bushing  1840 . As such, the bushing  1840  is retained within the lower chamber  1836  of the body  1820 . 
         [0086]    The bushing  1840  is movably positionable within the body  1820  between a first position where the bone anchor  10  can be connected to or unconnected from the bushing  1840 , and a second position where the bushing  1840  is locked with respect to the bone anchor  10 . The lower end portion  1836  of the bushing  1840  preferably includes an interior cavity  1851  for receiving and securing the head  14  of the bone anchor  10  so that the bone anchor  10  can rotate polyaxially through a range of angles with respect to the bushing  1840  and hence with respect to the body  1820  when in an unlocked or loading/unloading position. 
         [0087]    To interconnect or attach the bone anchor  10  to the body  1820 , the body  1820  may be provided with the bushing  1840  pre-assembled and in the loading position, in which a lower tooth  1841   a  of the saddle  1869  engages a tooth  1841   c  of the body in the locking mechanism  1838 . The lower surface  1873  of the saddle  1869  contacts the upper surface  1872  of the bushing  1840 . The head  14  of the bone anchor  10  is inserted into the lower opening  1824  of the body  1820  and into the interior cavity  1851  of the bushing  1840 . As shown in  FIG. 18B , the head  14  is further inserted into the interior cavity  1851  of the bushing  1840 , the flexible arms  1845  initially pivot outwardly about the point  1853  and then back inwardly until it the head  14  engages the interior surfaces of flexible arms  1845  that pivot about a point  1853  of the bushing  1840 . Thus, the head  14  is “snapped-in” to the bushing  1840  as the flexible arms  1845  frictionally retain the head  14  within the cavity  1851 . 
         [0088]    When the bone anchor  10  is in the locked position the head  14  is rotatable within the cavity  1851 . The bushing  140  of the fourth implementation, provides for approximately 41° of angulation in each direction with respect to the longitudinal axis  32 , as both the head  14  and the bushing  1841  are rotatable within the lower chamber  1836  of the body  1820 . 
         [0089]    While the foregoing description and drawings represent the preferred embodiment of the present disclosure, it will be understood that various additions, modifications, combinations and/or substitutions may be made therein without departing from the spirit and scope of the present disclosure as defined in the accompanying claims. In particular, it will be clear to those skilled in the art that the present disclosure may be embodied in other specific forms, structures, arrangements, proportions, and with other elements, materials, and components, without departing from the spirit or essential characteristics thereof. One skilled in the art will appreciate that the invention may be used with many modifications of structure, arrangement, proportions, materials, and components and otherwise, used in the practice of the disclosure, which are particularly adapted to specific environments and operative requirements without departing from the principles of the present disclosure. In addition, features described herein may be used singularly or in combination with other features. The presently disclosed embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, the scope of the disclosure being indicated by the appended claims and not limited to the foregoing description. 
         [0090]    It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present disclosure, as defined by the appended claims.