Abstract:
A bottom-loading multi-axial bone anchor apparatus is disclosed. The apparatus includes a receiver member, a crown member, a bone anchor and a retaining member. The receiver member defines an upper opening and a lower opening, which may form part of the same opening, a channel, and a groove. The crown member and bone anchor are loaded into the lower opening of the receiver member, and the retaining member fits around the bone anchor and into the groove in the receiver member. The bone anchor is capable of multi-axial positioning with respect to the receiver member. An elongated member is placed in the channel of the receiver member, contacting the crown member, and a compression member is applied via the upper opening. The compression member presses down on the elongated member, which presses down on the crown member and locks the bone anchor between the crown member and the retaining member.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to devices and implants used in osteosynthesis and other orthopedic surgical procedures. Specifically, the present invention contemplates a bottom loading bone anchor assembly capable of achieving multiple angular orientations with respect to an elongated member extending along bone tissue.  
         BACKGROUND OF THE INVENTION  
         [0002]    Several techniques and systems have been developed for correcting and stabilizing damage or malformation of bones, especially the long bones and the spine. In one type of system, an elongated member such as a bendable rod is disposed longitudinally along a length of the bone(s). In spinal applications, the rod is preferably bent to correspond to the normal curvature of the spine in the particular region being instrumented. For example, the rod can be bent to form a normal kyphotic curvature for the thoracic region of the spine, or a lordotic curvature for the lumbar region. In accordance with such a system, the rod is engaged to various vertebrae along a length of the spinal column by way of a number of fixation elements. A variety of fixation elements can be provided which are configured to engage specific portions of the vertebra and other bones. For instance, one such fixation element is a hook that is configured to engage the laminae of the vertebra. Another very prevalent fixation element is a screw that can be threaded into various parts of the vertebrae or other bones.  
           [0003]    In one typical spinal procedure utilizing a bendable rod, the rod is situated on opposite sides of the spine or spinous processes. A plurality of bone screws are threaded into a portion of several vertebral bodies, very frequently into the pedicles of these vertebrae. The rods are affixed to these plurality of bone screws to apply corrective and stabilizing forces to the spine.  
           [0004]    One example of a rod-type spinal fixation system is the TSRH® Spinal System sold by Medtronic Sofamor Danek, Inc. The TSRH® System includes elongated rods and a variety of hooks, screws and bolts all configured to create a segmental construct throughout the spine. In one aspect of the TSRH® System, the spinal rod is connected to the various vertebral fixation elements by way of an eyebolt. In this configuration, the fixation elements are engaged to the spinal rod laterally adjacent to the rod. In another aspect of the TSRH® System, a variable angle screw is engaged to the spinal rod by way of an eyebolt. The variable angle screw allows pivoting of the bone screw in a single plane parallel to the plane of the spinal rod. Details of this variable angle screw can be found in U.S. Pat. No. 5,261,909 to Sutterlin et al., owned by the Assignee of the present invention. One goal achieved by the TSRH® System is that the surgeon can apply vertebral fixation elements, such as a spinal hook or a bone screw, to the spine in appropriate anatomic positions. The TSRH® System also allows the surgeon to easily engage a bent spinal rod to each of the fixation elements for final tightening.  
           [0005]    Another rod-type fixation system is the Cotrel-Dubosset/CD® Spinal System sold by Medtronic Sofamor Danek, Inc. Like the TSRH® System, the CD® System provides a variety of fixation elements for engagement between an elongated rod and the spine. In one aspect of the CD® System, the fixation elements themselves include a body that defines a slot within which the spinal rod is received. The slot includes a threaded bore into which a threaded plug is engaged to clamp the rod within the body of the fixation element. The CD® System includes hooks and bone screws with this “open-back” configuration. Details of this technology can be found in U.S. Pat. No. 5,005,562 to Cotrel. One benefit of this feature of the CD® System is that the fixation element is positioned directly beneath the elongated rod. This helps reduce the overall bulkiness of the implant construct and minimizes the trauma to surrounding tissue.  
           [0006]    On the other hand, these fixation elements of the CD® System are capable only of pivoting about the spinal rod to achieve variable angular positions relative to the rod. While this limited range of relative angular positioning is acceptable for many spinal pathologies, many other cases require more creative orientation of a bone screw, for instance, relative to a spinal rod. Certain aspects of this problem are addressed by the variable angle screw of the TSRH® System, as discussed in the &#39;909 patent. However, there is a need for a bone screw that is capable of angular orientation in multiple planes relative to the spinal rod. Preferably, the bone screw is capable of various three-dimensional orientations with respect to the spinal rod. Screws of this type have been referred to as poly-axial or multi-axial bone screws.  
           [0007]    Others have approached the solution to this problem with various poly-axial screw designs. For example, in U.S. Pat. No. 5,466,237 to Byrd et al., a bone screw is described which includes a spherical projection on the top of the bone screw. An externally threaded receiver member supports the bone screw and a spinal rod on top of the spherical projection. An outer nut is tightened onto the receiver member to press the spinal rod against the spherical projection to accommodate various angular orientations of the bone screw relative to the rod. While this particular approach utilizes a minimum of components, the security of the fixation of the bone screw to the rod is lacking. In other words, the engagement or fixation between the small spherical projection on the bone screw and the spinal rod is readily disrupted when the instrumentation is subjected to the high loads of the spine, particularly in the lumbar region.  
           [0008]    In another approach shown in U.S. Pat. No. 4,946,458 to Harms et al., a spherical headed bone screw is supported within separate halves of a receiver member. The bottom of the halves are held together by a retaining ring. The top of the receiver halves are compressed about the bone screw by nuts threaded onto a threaded spinal rod. In another approach taken by Harms et al., in U.S. Pat. No. 5,207,678, a receiver member is flexibly connected about a partially spherical head of a bone screw. Conical nuts on opposite sides of the receiver member are threaded onto a threaded rod passing through the receiver. As the conical nuts are threaded toward each other, the receiver member flexibly compresses around the head of the bone screw to clamp the bone screw in its variable angular position. One detriment of the systems in the two Harms et al. patents is that the spinal rod must be threaded in order to accept the compression nuts. It is known that threading rods can tend to weaken the rods in the face of severe spinal loads. Moreover, the design of the bone screws in the &#39;458 and &#39;678 patents require a multiplicity of parts and are fairly complicated to achieve complete fixation of the bone screw.  
           [0009]    A further approach illustrated in U.S. Pat. No. 5,797,911 to Sherman et al., owned by the Assignee of the present invention, is to provide a U-shaped holder through the top of which a bone fastener topped with a crown member is loaded. The holder accommodates a rod in a channel above the crown member and a compression member above the rod. The compression member presses on the rod and crown member to lock the fastener against the holder in any of a number of angles in three dimensions with respect to the rod. This approach has proven to be quite effective in addressing the above-identified problems. However, it does not permit bottom-loading of the fastener. Additionally, the holder is somewhat bulky in order to accommodate the other structural components.  
           [0010]    Yet a further approach is shown in U.S. Pat. No. 5,733,285 to Errico et al., in which a holder is provided with a tapered and colletted portion at the bottom into which a bone fastener head is inserted. A sleeve is provided that slides down around the colletted portion to crush lock the colletted portion around the head of the bone fastener. This apparatus is believed to be relatively bulky and difficult to manipulate given the external sliding locking mechanism. It is further dependent on the fit of the external sleeve and the relative strength of the collet and its bending and crushing portions for secure locking of the bone fastener head.  
           [0011]    There is therefore a need remaining in the industry for a multi-axial bone anchor that can be readily and securely engaged to an elongated member of any configuration—i.e., smooth, roughened, knurled or even threaded—which achieves improved angulation of the bone anchor, improved strength, and reduced size, including profile and bulk, of the components used to engage the bone anchor to the elongated member in any of a variety of angular orientations.  
         SUMMARY OF THE INVENTION  
         [0012]    In one embodiment of the invention, a bone fixation assembly is provided that includes a receiver member defining an upper opening portion and a lower opening portion, a channel configured to receive an elongated member that communicates with the upper and lower opening portions, and a groove around a portion of the lower opening portion. The assembly further includes a crown member with an upper surface and a lower surface that is movably disposed in the lower opening portion, and a bone-engaging anchor having a lower portion configured to engage a bone and a head smaller than the lower opening portion, with the head being movably disposed in the lower opening portion adjacent to the concave lower surface of the crown member. The assembly also includes a retaining member defining an aperture smaller than the head of the bone anchor that is at least partially housed in the groove of the receiver member and positioned around the bone anchor and below its head.  
           [0013]    Additional embodiments, examples, advantages, and objects of the present invention will be apparent to those of ordinary skill in this art from the following specification.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a side elevational view of one embodiment of the multi-axial bone screw anchor assembly of the present invention.  
         [0015]    [0015]FIG. 2 is an exploded view of the embodiment of the invention depicted in FIG. 1.  
         [0016]    [0016]FIG. 3 a  is a side elevational view of an embodiment of the receiver member of the embodiment of the invention illustrated in FIG. 2.  
         [0017]    [0017]FIG. 3 b  is a front elevational view of the embodiment of the receiver member illustrated in FIG. 3 a.    
         [0018]    [0018]FIG. 3 c  is a sectional view, taken along the lines  3   c - 3   c  in FIG. 3 a , and viewed in the direction of the arrows, of the embodiment of the receiver member illustrated in FIG. 3 a.    
         [0019]    [0019]FIG. 3 d  is a sectional view, taken along the lines  3   d - 3   d  of FIG. 3 b  and viewed in the direction of the arrows, of the embodiment of the receiver member illustrated in FIG. 3 a.    
         [0020]    [0020]FIG. 4 a  is a side elevational view of an embodiment of a bone anchor used in the embodiment of the invention illustrated in FIG. 2.  
         [0021]    [0021]FIG. 4 b  is a sectional view, taken along the lines  4   b - 4   b  of FIG. 4 a  and viewed in the direction of the arrows, of the embodiment of the bone anchor illustrated in FIG. 4 a.    
         [0022]    [0022]FIG. 4 c  is a magnified view of one embodiment of the head of the embodiment of the bone anchor illustrated in FIG. 4 a.    
         [0023]    [0023]FIG. 5 a  is a top view of one embodiment of a crown member used in the embodiment of the present invention illustrated in FIG. 2.  
         [0024]    [0024]FIG. 5 b  is a sectional view, taken along the lines  5   b - 5   b  in FIG. 5 a  and viewed in the direction of the arrows, of the embodiment of the crown member illustrated in FIG. 5 a.    
         [0025]    [0025]FIG. 5 c  is a sectional view substantially similar to FIG. 5 b  of another embodiment of a crown member used in the embodiment of the invention illustrated in FIG. 2.  
         [0026]    [0026]FIG. 6 a  is a top view of one embodiment of a retaining member used in the embodiment of the invention illustrated in FIG. 2.  
         [0027]    [0027]FIG. 6 b  is a sectional view, taken along the lines of  6   b - 6   b  in FIG. 6 a  and viewed in the direction of the arrows, of the embodiment of the retaining member illustrated in FIG. 6 a.    
         [0028]    [0028]FIG. 7 is an enlarged sectional view of the embodiment of the present invention illustrated in FIG. 1.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0029]    For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the invention as illustrated therein, being contemplated as would normally occur to one skilled in the art to which the invention relates.  
         [0030]    Referring generally to FIGS. 1 and 2, there is shown one embodiment of a multi-axial bone anchor assembly  20  of the present invention. In the illustrated embodiment, assembly  20  includes a receiver member  30 , a bone anchor  50 , a crown member  70 , and a retaining member  90 . The assembly  20  of the present invention is designed for use with an elongated member R (FIG. 7) such as a spinal rod, bar or other orthopedic construct, as further described below.  
         [0031]    Referring now generally to FIGS. 3 a - 3   d , one embodiment of the receiver member  30  of the present invention is shown. Receiver member  30  defines an upper opening portion  31   a  and a lower opening portion  31   b , which in the illustrated embodiment form a single opening  32  extending through receiver member  30  from an upper aperture  33  in top end  34  to a lower aperture  35  in bottom end  36 . Lower opening portion  31   b  of opening  32 , in one specific embodiment, includes a chamber  38  defined by a chamber wall  39 . Alternatively, upper and lower opening portions  31   a ,  31   b  can have a variety of configurations, such as each having one or more sections of differing diameter.  
         [0032]    Opening  32  is partially surrounded by a chamfered or rounded edge  40   a  at top end  34  of receiver member  30 , and is surrounded by chamfered or rounded edge  40   b  at the bottom end  36  of receiver member  30 . Proximate to bottom end  36 , receiver member  30  defines a groove  41  and associated ledge  41   a  around opening  32 . In the illustrated embodiment, groove  41  extends around the entire perimeter of opening  32 , although it will be seen that groove  41  could extend only partially around the perimeter of opening  32 . Groove  41  has a groove depth A (FIG. 7) and a groove diameter B (FIG. 3 a ).  
         [0033]    Receiver member  30  in the illustrated embodiment includes a pair of upright branches  42 ,  43  through which opening  32  extends. Branches  42 ,  43  further define a U-shaped channel  45  transverse to opening  32  that communicates with upper portion  31   a  and lower portion  31   b  of opening  32 , and that accommodates an elongated member R (FIG. 7). In a specific embodiment, internal threads  44  are formed in branches  42 ,  43 , and branches  42 ,  43  are provided with indentations or holes  46 , which allow the surgeon to grip receiver member  30  with an appropriate tool (not shown). Internal thread  44  in a specific embodiment is a reverse angle thread, i.e. a thread in which the forward face points down and in toward receiver member  30 , as disclosed in commonly-owned U.S. patent application Ser. No. 09/188,825, filed Nov. 9, 1998, the disclosure of which is hereby incorporated by reference. Preferably, the top portion  47  of receiver member  30  (which includes branches  42 ,  43 ) is narrower than bottom portion  48  of receiver member  30 , thereby reducing the bulk and profile of receiver member  30 .  
         [0034]    Referring now generally to FIGS. 4 a - 4   c , an embodiment of a bone anchor  50  used in the present invention is shown. The illustrated bone anchor  50  is a bone screw, which in one embodiment is substantially like the bone screw disclosed in U.S. Pat. No. 5,885,286, the disclosure of which patent is hereby incorporated by reference. Bone anchor  50  includes an anchorage portion  52  and a head portion  54 . Anchorage portion  52  includes at least one thread  56 , which may be a cancellous self-tapping thread. Head portion  54  forms part of a sphere in the illustrated embodiment, though alternative curvate and other configurations may be employed. Head  54  in one particular embodiment includes a series of ridges  58  for improving purchase with the inside of crown member  70  (described below). Head  54  may have alternative friction-increasing surface configuration(s) such as roughening or knurling. Further, head  54  includes a tool-engaging print  60 , with which a tool (not shown) may be engaged to drive anchorage portion  52  into a bone. Tool-engaging print  60  is an interior print in the illustrated embodiment, although an exterior print could be used, and it may have any of a number of configurations, such as hexagonal, hexalobate, or other known torque-transferring configurations.  
         [0035]    Other embodiments of bone anchor  50  are contemplated as being within the scope of the present invention. For example, bone anchor  50  could be a bone-engaging hook rather than a screw. In that embodiment, anchorage portion  52  would be configured with a hook rather than an elongated section with thread  56 .  
         [0036]    Head  54  of bone anchor  50  is shaped and sized to fit within at least lower portion  31   b  of opening  32  and chamber  38  of receiver member  30 . Specifically, head  54  has a width that is smaller than the width of lower opening portion  31   b  and chamber  38 . As more fully described below, bone anchor  50  is inserted into receiver member  30 , with head  54  entering lower opening portion  31   b  and chamber  38  through bottom end  36  of receiver member  30 .  
         [0037]    Referring now to FIGS. 5 a - 5   b , there is shown one embodiment of crown member  70  of the present invention. In that embodiment, crown member  70  is in the shape of a circular disc, having an upper surface  72  with a beveled edge  74  and a lower surface  78 . Lower surface  78  is configured to accommodate head  54  of bone anchor  50 , and therefore the illustrated embodiment of lower surface  78  has the shape of part of a sphere. Alternatively or additionally, the lower surface of crown member  70  can have one or more other shapes, such as beveled or conical lower surface  78 ′ (FIG. 5 c ). Lower surface  78  can be provided with a friction- or purchase-enhancing surface configuration (e.g. roughening or knurling) for cooperation with head  54  of bone anchor  50 .  
         [0038]    The illustrated embodiment of crown member  70  also includes a hole  80 . Hole  80  is provided so that head  54 , and specifically tool-engaging print  60 , of bone anchor  50  may be accessed through crown member  70 . Crown member  70  is sized and shaped to fit within at least lower portion  31   b  of opening  32  and chamber  38  of receiver member  30 . The outer dimension of crown member  70  is preferably slightly smaller than the inner dimension of chamber  38  and lower portion  31   b  of opening  32  so that crown member  70  is slidably and rotatably movable within chamber  38  and opening  32 . Further, in the illustrated embodiment the outer dimension of crown member  70  is larger than the inner dimension of upper opening portion  31   a , so that crown member  70  cannot move into upper opening portion  31   a.    
         [0039]    Referring now to FIGS. 6 a - 6   b , there is shown one embodiment of retaining member  90  of the present invention. In the illustrated embodiment, retaining member  90  has the form of a C-shaped spring or clip defining a gap  91 . Retaining member  90  includes a top surface  92  and a bottom surface  94 . In the illustrated embodiment, retaining member  90  also includes internal surfaces  96 ,  98 ,  100  that substantially surround aperture  102 . In one specific embodiment, internal surface  96  forms a portion of a sphere of radius substantially identical to the radius of head  54  of bone anchor  50 , internal surface  98  is cylindrical, and internal surface  100  is conical and angled outward to allow a greater range of angular positioning of bone anchor  50 . In alternative embodiments, there may be single or multiple internal surfaces surrounding aperture  102 , which surface(s) may be cylindrical, conical, spherical or of other appropriate configuration. The diameter of aperture  102  is smaller than the diameter of head  54  of bone anchor  50  and the diameter of crown member  70 .  
         [0040]    Retaining member  90  has an unloaded or natural outer diameter D, i.e. a diameter measured when retaining member  90  is under no contractive (gap-closing) or expansive (gap-opening) stress. Diameter D of retaining member  90 , in one embodiment, is less than groove diameter B of groove  41 . Further, retaining member  90  has a body width W that is substantially constant throughout retaining member  90 . Body width W of retaining member  90  is greater than groove depth A of groove  41 .  
         [0041]    Generally referring to FIGS. 1, 2 and  7 , assembly  20  is assembled as follows: bone anchor  50 , crown member  70  and retaining member  90  are inserted into receiver member  30  through bottom end  36 , either individually or substantially in one step. For example, crown member  70  may be inserted first, followed by bone anchor  50  with retaining member  90  being inserted last. In one specific embodiment, retaining member  90  is fitted around bone anchor  50  just below head  54  prior to insertion of bone anchor  50  into receiver member  30 . Retaining member  90  can be placed around bone anchor  50  by inserting anchorage portion  52  of bone anchor  50  through aperture  102  of retaining member  90  and moving retaining member  90  over anchorage portion  52  toward head  54 . Alternatively, gap  91  of retaining member  90  may be pressed against the shank of bone anchor  50  below head  54 , so that gap  91  expands to allow placement of bone anchor  50  within aperture  102  of retaining member  90 , whereupon retaining member  90  returns to its original size and shape. By placing crown member  70  atop head  54  of bone anchor  50 , so that lower surface  78  of crown member  70  adjoins head  54 , and fitting bone anchor  50  and retaining member  90  together as described above, simultaneous insertion of bone anchor  50 , crown member  70  and retaining member  90  into receiver member  30  can be accomplished.  
         [0042]    Crown member  70  remains slideably and rotatably positioned in lower portion  31   b  of opening  32  and/or chamber  38  of receiving member  30 , and bone anchor  50  remains multi-axially moveable with respect to crown member  70  and receiving member  30 . Retaining member  90  is forced upward into lower portion  31   b  of opening  32 . Retaining member  90  contracts, making gap  91  smaller, as retaining member  90  is forced against chamfered edge  40   b  of receiving member  30 , until the outer diameter of retaining member  90  is the same as the diameter of lower portion  31   b  of opening  32 . Retaining member  90  is further advanced along opening  32  and into groove  41  so that retaining member  90  is fitted into at least a portion of groove  41 .  
         [0043]    As noted above, in one specific embodiment the groove diameter B of groove  41  is smaller than the outer diameter D of retaining member  90  in its natural (i.e., unloaded) condition. Thus, when retaining member  90  is within groove  41 , retaining member  90  presses against the walls of groove  41 . Alternatively, groove diameter B of groove  41  may be the same size or slightly larger than the natural outer diameter D of retaining member  90 . In this case, the lower surface  94  of retaining member  90  rests upon ledge  41   a  of groove  41 , and thereby holds retaining member  90  within groove  41 . Groove depth A of groove  41  is less than the body width W of retaining member  90 , so that when retaining member  90  is fitted in groove  41 , a portion of retaining member  90  projects into lower opening portion  31   b  of opening  32 .  
         [0044]    When retaining ring  90  is seated within groove  41 , bone anchor  50  and crown member  70  are retained within opening  32  of receiver member  30 . Crown member  70  is supported by head  54  of bone anchor  50 , and head  54  is supported by internal surface  96  of retaining member  90 . Retaining member  90  is held by groove  41  and/or ledge  41   a  of receiver member  30 , and thus bone anchor  50  and crown member  70  will not pass through retaining ring  90  and out of receiver member  30  when retaining ring  90  is within groove  41 .  
         [0045]    Preferably, assembly  20  is assembled (as described above) prior to use in a surgical procedure. In using the illustrated embodiment of assembly  20 , bone anchor  50  of assembly  20  is threaded into an appropriately prepared hole in a bone (not shown). It will be understood that in alternative embodiments of the invention, for example where bone anchor  50  is a bone hook, drilling a hole in bone and threading the anchor therein may not be necessary. Threaded anchoring portion  52  is inserted into the hole, and an appropriate screwing tool is used with tool-engaging print  60  of bone anchor  50  through hole  80  in crown member  70 , and bone anchor  50  is threaded into the bone. When bone anchor  50  has been threaded into the bone to the desired depth, receiver member  30  is positioned so that opening  32  forms a desired angle with bone anchor  50 , as depicted in FIG. 1. In the illustrated embodiment, the angle θ between bone anchor  50  and opening  32  can be any value up to 30 degrees in any direction. It will be seen that the maximum angle of bone anchor  50  relative to opening  32  can be changed in several ways, for example by thinning the portion of bone anchor  50  beneath head  54 , by providing steeper angulation of chamfered edge  40   b , and/or by placing groove  41  as close as possible to bottom end of  36  of receiver member  30 .  
         [0046]    As described above, receiver member  30  may be angled as the surgeon desires with respect to bone anchor  50 . An elongated member R such as a spinal rod, connector, or other orthopedic surgical implant is coupled with assembly  20 . Elongated member R is placed in channel  45  of receiver member  30 , and contacts top surface  72  of crown member  70 . A compression member  120 , such as a set screw or threaded plug, is threaded into threads  44  of receiver member  30  and down onto elongated member R. Compression member  120 , in one embodiment, is a set screw or plug having external threads  122  and a print  124  for applying torque, and in a specific embodiment is a break-off set screw as disclosed in U.S. Pat. No. 5,885,286 to Sherman et al., incorporated herein by reference. In a further embodiment, thread  122  is a reverse angle thread as disclosed in U.S. patent application Ser. No. 09/188,825, filed Nov. 9, 1998, incorporated herein by reference, which is compatible with the reverse angle embodiment of thread  44  of receiver member  30 , described above. Alternatively, where receiver member  30  is externally threaded, compression member  120  could be an internally-threaded nut.  
         [0047]    As compression member  120  is tightened, elongated member R is forced downward against crown member  70 , which pushes crown member  70  down onto head  54  of bone anchor  50 . Head  54  is thereby clamped between retaining member  90  and crown member  70 . In the embodiment of the invention in which head  54  includes ridges  58 , ridges  58  are pressed into lower surface  78  of crown member  70 . In this way, bone anchor  50  is locked into the desired angular position with respect to elongated member R and the remainder of assembly  20 .  
         [0048]    Alternatively, assembly  20  can be assembled during the surgical procedure. Bone anchor  50 , with retaining ring  90  already positioned beneath head  54 , is inserted into the bone. Crown member  70  is placed atop bone anchor  50  or in opening  32  in receiver member  30 . Receiver member  30  is then pressed down onto head  54  of bone anchor  50 , forcing retaining ring  90  to contract, to enter opening  32 , and to seat in groove  41  as described above. After assembly  20  is assembled in this fashion, an elongated member is loaded into receiver member  30  and locked as previously described.  
         [0049]    Preferred materials for the present invention include stainless steel and titanium. It will be recognized that any sturdy biocompatible material may be used to accomplish the osteosynthesis and other orthopedic surgical goals of the present invention. In one specific embodiment, crown member  70  may be made of a material somewhat softer than the material used for ridges  58  of head  54  of bone anchor  50 . Such construction will allow ridges  58  to penetrate somewhat more easily into interior surface  78  of crown member  70  during locking of assembly  20 , thereby providing a more definite purchase between ridges  58  and crown member  70 . In another specific embodiment, crown member  70  may be made of a material somewhat softer than the material used for elongated member R. Such construction will allow upper surface  72  of crown member  70  to deform to the shape of elongated member R during locking of assembly  20 , also providing a more secure locking of the implant.  
         [0050]    While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.