Abstract:
Assemblies, systems and components for a bi-polar bone anchor assembly. A receiver member includes a central aperture with upper and lower openings and a transverse channel. A bi-polar member and a bone anchor are loaded into the bottom of the receiver member and an internal threaded ring member fits over the outer lower threaded portion of the receiver member to retain the bi-polar member and the bone anchor therein. The bone anchor is capable of multi-axial and multi-polar positioning with respect to the receiver member. An elongated member may be placed in the channel of the receiver member in contact with the bone anchor member and a retaining member may be applied via the upper opening to press down on the elongated member thereby, locking the bone anchor member in place with the retaining member, bi-polar member, and receiver member.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of International Application PCT/US2006/009748 filed Mar. 17, 2006, which claims the benefit of U.S. Provisional Application No. 60/700469, filed Jul. 18, 2005. The disclosures of each of these related applications are incorporated herein by reference in their entirety. 
     
    
     TECHNICAL FIELD  
       [0002]     The present invention relates to devices and implants used in osteosynthesis and other orthopedic surgical procedures such as devices for use in spinal surgery, and, in particular, to a posterior pedicle screw, connector/rod assembly which is implantable within a patient for stabilization of the spine. Specifically, the present invention contemplates a top loading bone anchor assembly capable of achieving multiple angular, as well as multiple spherical axial orientations with respect to an elongated member extending along bone tissue.  
       BACKGROUND  
       [0003]     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.  
         [0004]     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 this plurality of bone screws to apply corrective and stabilizing forces to the spine.  
         [0005]     One example of a rod-type spinal fixation 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 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 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. One goal achieved by the 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 system also allows the surgeon to easily engage a bent spinal rod to each of the fixation elements for final tightening.  
         [0006]     Another rod-type fixation system provides a variety of fixation elements for engagement between an elongated rod and the spine. In one aspect of the 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 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.  
         [0007]     On the other hand, these fixation elements of the 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 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 as well as multiple spherical head orientations. Preferably, the bone screw axis is capable of various three dimensional orientations with respect to the spinal rod, as well as three dimensional spherical axis orientation to the receiving (head) element of the device&#39;s axial orientation of the bone engaging screw member. Screws of this type of angular orientation in multiple planes relative to the spinal rod have been referred to as poly-axial or multi-axial bone screws. One should note, as of yet, no known screw systems have employed both angular orientation in multiple planes relative to the spinal rod and three dimensional spherical axis orientation to the receiving (head) element of the device&#39;s axial orientation of the bone engaging screw member. The use of both angular orientation in multiple planes relative to the spinal rod and three dimensional spherical axis orientation to the receiving (head) element of the device&#39;s axial orientation of the bone engaging screw member technology allows for virtually unlimited axial angulations of the bone engaging screw member as well as an ultra-low profile of the said device utilizing a minimum of components without sacrificing the security of the interfaces of the invention components.  
         [0008]     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.  
         [0009]     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.  
         [0010]     A further approach illustrated in U.S. Pat. No. 5,797,911 to Sherman et al. 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 one 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.  
         [0011]     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.  
         [0012]     There is therefore a need remaining in the industry for an ultra-low profile, multi-axial/bi-polar 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 greatly improved angulations 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  
       [0013]     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 each having respective minimum widths, a channel configured to receive an elongated member (rod) and communicating with said upper opening portion and said lower opening portion, and a bi-polar member having an internal portion configured to engage a bone anchor head and an external portion configured to engage the internal geometry of the receiver member, said internal width of said bi-polar member being larger than said width of the head of the bone-anchor member and said external width of said bi-polar member larger than said minimum width of said lower opening portion of said internal threaded ring member, said head of the bone-anchor member being movably disposed in said lower opening portion adjacent to said internal surface of said bi-polar member; and a bone-engaging anchor having a lower portion configured to engage a bone and a head having a width, said width of said head being smaller than said minimum width of said lower opening portion, said head being movably disposed in said lower opening portion adjacent to said lower surface of said bi-polar member; and an ring member that fits around the bone anchor and over the outer lower portion of the receiver member to retain the bi-polar member and the bone anchor member.  
         [0014]     Once the bone anchor member and bi-polar member are retained in the lower opening of the receiving member, the bi-polar and the bone anchor member is capable of multi-axial positioning as well as multi-polar positioning with respect to the receiver member. A compression retaining member defining an aperture smaller than said width of said head, may be at least partially housed in said internally threaded portion of said receiver member and positioned over said elongated member and then tightened down against an inserted rod. Forces transmitted during tightening are imparted on the bone anchor member, bi-polar member, and the lower surface of the receiving member and the ring member to anchor all the components in any angular and/or axial configuration within design parameters.  
         [0015]     Additional embodiments, examples, advantages, and objects of the present invention will be apparent to those of ordinary skill in the art from the following specification. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0016]     It will be appreciated by those of ordinary skill in the art that the elements depicted in the various drawings are not to scale, but are for illustrative purposes only. The nature of the present invention, as well as other embodiments of the present invention may be more clearly understood by reference to the following detailed description of the invention, to the appended claims, and to the several drawings attached hereto.  
         [0017]      FIG. 1  is a side elevational view of one embodiment of the multi-axial bone screw anchor assembly of the present invention.  
         [0018]      FIG. 2  is an exploded view of the embodiment of the invention depicted in  FIG. 1 .  
         [0019]      FIG. 3A  is a side elevational view of an embodiment of the receiver member of the embodiment of the invention illustrated in  FIG. 2 .  
         [0020]      FIG. 3B  is a front elevational view of the embodiment of the receiver member illustrated in  FIG. 3A .  
         [0021]      FIG. 3C  is a sectional view, taken along the lines  3 C- 3 C in  FIG. 3B , and viewed in the direction of the arrows, of the embodiment of the receiver member illustrated in  FIG. 3A .  
         [0022]      FIG. 3D  is a sectional view, taken along the lines  3 D- 3 D of  FIG. 3B  and viewed in the direction of the arrows, of the embodiment of the receiver member illustrated in  FIG. 3A .  
         [0023]      FIG. 4A  is a side elevational view of an embodiment of a bone anchor used in the embodiment of the invention illustrated in  FIG. 2 .  
         [0024]      FIG. 4B  is a sectional view, taken along the lines  4 B-B of  FIG. 4A  and viewed in the direction of the arrows, of the embodiment of the bone anchor illustrated in  FIG. 4A .  
         [0025]      FIG. 4C  is a magnified view of one embodiment of the head of the embodiment of the bone anchor illustrated in  FIG. 4A .  
         [0026]      FIG. 5A  is a top view of one embodiment of a bi-polar member used in the embodiment of the present invention illustrated in  FIG. 2 .  
         [0027]      FIG. 5B  is a sectional view, taken along the lines  5 B- 5 B in  FIG. 5A  and viewed in the direction of the arrows, of the embodiment of the bi-polar member illustrated in  FIG. 5A .  
         [0028]      FIG. 5C  is a sectional view substantially similar to  FIG. 5B  of another embodiment of a bi-polar member used in the embodiment of the invention illustrated in  FIG. 2 .  
         [0029]      FIG. 6A  is a top view of one embodiment of an internal threaded ring member that fits around the bone anchor and over the outer lower threaded portion in the receiver member to retain the bi-polar member and the bone anchor member used in the embodiment of the invention illustrated in  FIG. 2 .  
         [0030]      FIG. 6B  is a sectional view, taken along the lines of  6 B-B in  FIG. 6A  and viewed in the direction of the arrows, of the embodiment of the internal threaded ring member illustrated in  FIG. 6A .  
         [0031]      FIG. 7A  is a top view of a retaining member for use with some embodiments of the present invention.  
         [0032]      FIG. 7B  is a side elevational view of the retaining member of  FIG. 7A .  
         [0033]      FIG. 8  is an enlarged sectional view of one illustrative embodiment of an assembled system in accordance with the present invention, including the components illustrated in  FIGS. 1, 2 ,  7 A, and  7 B. 
     
    
     DETAILED DESCRIPTION  
       [0034]     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.  
         [0035]     Referring generally to  FIGS. 1 and 2 , there is shown one embodiment of a multi-axial/bi-polar bone anchor assembly  20  in accordance with the principles of the present invention. In the illustrated embodiment, assembly  20  includes a receiver member  30 , a bone anchor  50 , a bi-polar member  70 , and an internal threaded ring member  90 . The assembly  20  of the present invention is designed for use with an elongated member R (depicted in  FIG. 8 ) such as a spinal rod, bar or other orthopedic construct, as further described below.  
         [0036]     Referring now generally to  FIGS. 3A-3D , additional details of one illustrative embodiment of a receiver member  30  in accordance with the present invention are shown. Receiver member  30  is formed as a generally circular member having at least one sidewall  33  surrounding a central aperture  32 . Sidewall  37  defines an upper portion  47  including top end  34  and a lower portion  48  including bottom end  36 . Central aperture  32  extends through receiver member  30  from an upper aperture  33  in top end  34  to a lower aperture  35  in bottom end  36 . Lower portion  31   b  of central aperture  32 , in one specific embodiment, includes a chamber/void  38  defined by a chamber wall  39  which is configured to form a spherical chamber. Alternatively, central aperture in upper and lower portions  31  a and  31  b can have a variety of configurations, such as each having one or more sections of differing diameter.  
         [0037]     Central aperture  32  includes a top portion  31   a  which may be partially surrounded by a chamfered or rounded edge  40   a  at top end  34  of receiver member  30 . Similarly, bottom portion  31   b  of central aperture  32  may be surrounded by a chamfered or rounded edge  40   b  at the bottom end  36  of receiver member  30 . Proximate to bottom end  36 , receiver member  30  may define external threads  41  and an associated ledge  41   a  ( FIG. 2C ). In the illustrated embodiment, threads  41  extends around the entire perimeter of lower surface  31   b,  although it will be seen that thread  41  could extend only partially around the perimeter of lower surface  31   b.    
         [0038]     Sidewall  33  of receiver member  30  may define one or more pairs of upright branches  42 ,  43  in upper portion  31   a  through which central aperture  32  extends. Branches  42 ,  43  further define one or more channels, such as U-shaped channel  45 , which extend transversely to central aperture  32 , and that may accommodate an elongated member R ( FIG. 8 ) therein. In one specific embodiment, internal threads  44  may be formed in branches  42  and  43  in the sidewall of central aperture  32 . These internal threads  44  may be a modified acme buttress thread or other suitable thread. In other embodiments, the branches  42 ,  43  may feature an external thread. The top portion  47  of receiver member  30  (which includes branches  42 ,  43 ) may be narrower than bottom portion  48  of receiver member  30  to thereby reduce the bulk and profile of receiver member  30 .  
         [0039]     Referring now generally to  FIGS. 4A-4C , one illustrative embodiment of a bone anchor  50  which may be used in the present invention is depicted. The illustrated bone anchor  50  is a bone screw. Bone anchor  50  includes an anchorage portion  52  and a head portion  54 . Anchorage portion  52  is formed as a shaft including at least one thread  56 , which may be a cancellous self-tapping thread. Head portion  54  is disposed at a proximal end of the anchorage portion  52  and forms part of a sphere in the illustrated embodiment, though alternative curvate and other configurations may be employed. In some embodiments, head  54  may include a series of ridges  58  for improving purchase with the inside of bi-polar 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, X-shaped, or other known torque-transferring configurations.  
         [0040]     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 such embodiments, anchorage portion  52  may be configured with a hook rather than an elongated section with thread  56 .  
         [0041]     Head  54  of bone anchor  50  is shaped and sized to fit within at least interior portion  78  of bi-polar member  70  (depicted in  FIGS. 5A-5C ) and chamber  38  of receiver member  30  ( FIG. 3C ). Specifically, head  54  has a width that is smaller than the width of bi-polar member  70  and chamber  38 . As more fully described below, bone anchor  50  is inserted into receiver member  30 , with anchorage portion  50  entering through opening  80  and interfacing with surface  78  of bi-polar member  70  ( FIG. 5A ).  
         [0042]     Referring now to  FIGS. 5A-5C , there is shown one illustrative embodiment of bi-polar member  70  in accordance with the principles of the present invention. In the depicted embodiment, bi-polar member  70  is formed as a circular disc, having an exterior surface  72  with a beveled edge  74  and an interior surface  78 . Interior surface  78  is configured to accommodate head  54  of bone anchor  50 . Accordingly, the illustrated embodiment of interior surface  78  has the shape of part of a sphere. It will be appreciated that in other embodiments, the shape may differ, in order to accommodate other head  54  shapes. For example, see the conical interior surface  78 ′ of  FIG. 5C . Interior 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 .  
         [0043]     Bi-polar member  70  also includes a hole  80  faced by interior surface  78 . Hole  80  is provided so that bone anchor  50  may be partially passed therethrough, allowing the bone engaging threads  56  of bone anchor  50  to be available through bi-polar member  70 , while head  54  is retained therein. The dimension of hole  80  of the bi-polar member  70  is preferably slightly larger than the outer dimension of bone anchor head  54  so that the bone anchor head  54  is slidably and rotatably movable within hole  80  and bipolar member  70 .  
         [0044]     Bi-polar member  70  is sized and shaped to fit within at least lower portion  31   b  of central aperture  32  and chamber  38  of receiver member  30 . The outer dimension of bi-polar member  70  is preferably slightly smaller than the inner dimension of chamber  38  and lower portion  31   b  of central aperture  32  so that bi-polar member  70  is slidably and rotatably movable within chamber  38  and central aperture  32 . Further, in the illustrated embodiment, the outer dimension of bi-polar member  70  is larger than the inner dimension of upper opening portion  31   a,  so that bi-polar member  70  cannot move into upper opening portion  31   a.    
         [0045]     Referring now to  FIGS. 6A-6B , there is depicted one illustrative embodiment of an internal threaded ring member  90  in accordance with the principles of the present invention. In the illustrated embodiment, internal threaded ring member  90  may be formed as a generally ring-shaped component including a bottom surface  92  and a top surface  94 . In the illustrated embodiment on one internal threaded ring member  90 , an internal surface  91  surrounds aperture  102  and includes a number of structures. The lower portion  96  of internal surface  91  forms a portion of a sphere of radius substantially identical to the radius of head  54  of bone anchor  50 , above which a medial portion  98  is generally cylindrical and an upper portion  100  is conical and angled outward to allow a greater range of angular positioning of an inserted bone anchor  50 . In alternative embodiments, the internal surface  91  may have single or multiple surface configurations, which 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 bi-polar member  70 .  
         [0046]     As depicted, the external surface  97  of the internal threaded ring member  90  may have a polygonal shape, such as rectangular or octagonal shape for interaction with a securing tool, such as a wrench.  
         [0047]      FIGS. 7A and 7B  depict one illustrative embodiment of a retaining member or compression member  120  in accordance with the principles of the present invention. As depicted, retaining member  120  may be a set screw or threaded plug having external threads  122  and a print  124  for interaction with a tool (not shown) for applying torque. In assembly, retaining member  120  may be threaded into threads  44  of receiver member  30  ( FIG. 3C ) and down onto an inserted elongated member R ( FIG. 8 ). In one alternative embodiment, where receiver member  30  is externally threaded, compression member  120  could be an internally-threaded nut.  
         [0048]     Generally referring to  FIGS. 1, 2  and  8 , assembly  20  may be assembled together by inserting a bone anchor  50  through a bi-polar member  70  and an internal threaded ring member  90 , then inserting the head  54  of the bone anchor and bi-polar member  70  into receiver member  30  through bottom end  36 . This may occur as a series of individual steps or may be substantially in one step as shown in ( FIG. 2 ). Internal threaded ring member  90  may then be rotated to secure the components to one another.  
         [0049]     Bi-polar member  70  remains slidably and rotatably positioned in lower portion  31   b  of central aperture  32  and/or chamber  38  of receiving member  30 , and bone anchor  50  remains multi-axially moveable with respect to bi-polar member  70  and receiving member  30 . Internal threaded ring member  90  is threaded upward into lower portion  48  of receiver member  30 .  
         [0050]     When internal threaded ring  90  is installed, bone anchor  50  and bi-polar member  70  are retained within central aperture  32  of receiver member  30 . The head  54  of bone anchor  50  is supported by bi-polar member  70 , and bi-polar member  70  is supported by the internal surface  96  of internal threaded ring member  90 . Thus bone anchor  50  and bi-polar member  70  will not pass through internal threaded ring  90  and out of receiver member  30  once the internal threaded ring  90  is installed.  
         [0051]     Assembly  20  may be assembled to this point prior to use in a surgical procedure. During installation, bone anchor  50  of assembly  20  is attached to an appropriately prepared bone (not shown). With the depicted embodiment, this may be by threading the bone anchor  50  into a predrilled hole in the bone. Threaded anchoring portion  52  is inserted into the hole, and an appropriate screwing tool may be used with tool-engaging print  60  of bone anchor  50 , 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 central aperture  32  forms a desired angle with bone anchor  50 , as depicted in  FIG. 1 . In alternative embodiments, for example where bone anchor  50  is a bone hook, drilling a hole in bone and threading the anchor therein may not be necessary.  
         [0052]     In the illustrated embodiment, the angle theta ( FIG. 1 ) between bone anchor  50  and central aperture  32  can be any value up to about 57 degrees in any direction (up to about 112 degrees total angulation). It will be seen that the angle of bone anchor  50  relative to opening  32  can be changed in two ways. First, the angle of bone anchor  50  with respect to the bi-polar component  70  may be adjusted. Second, the angle of the bipolar component  70  with respect to the receiver member  30  can be adjusted.  
         [0053]     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 may be coupled with assembly  20 . Elongated member R may be placed in channel  45  of receiver member  30  and contact interior surface  78  of bi-polar member  70 . A retaining member or compression member  120 , such as a set screw or threaded plug, may be threaded into threads  44  of receiver member  30  and down onto elongated member R. As compression member  120  is tightened, elongated member R is forced downward against bone anchor  50  and bi-polar member  70 , pushing bi-polar member  70  down onto head  54  of bone anchor  50 . Head  54  is thereby clamped between internal threaded ring member  90  and bi-polar member  70 . In the embodiment of the invention in which head  54  includes ridges  58 , ridges  58  are pressed into internal surface  78  of bi-polar member  70 . In this way, bone anchor  50  and bi-polar member  70  are locked into the desired angular position with respect to elongated member R and the remainder of assembly  20 .  
         [0054]     It will be appreciated that where appropriate and desired, the assembly  20  can be assembled during the surgical procedure.  
         [0055]     Components of assembly  20  may be constructed of any surgically acceptable material of sufficient strength to be used to retain elongated member R. For example, stainless steel, titanium, and their alloys can be used. It will be appreciated that any sturdy biocompatible material may be used to accomplish the osteosynthesis and other orthopedic surgical goals of the present invention.  
         [0056]     While the present invention has been shown and described in terms of preferred embodiments thereof, it will be understood that this invention is not limited to any particular embodiment and that changes and modifications may be made without departing from the true spirit and scope of the invention as defined and desired to be protected.