Abstract:
The present invention relates generally to medical devices and methods generally aimed at spinal surgery. In particular, the disclosed system and associated methods relate to performing spinal fixation with the use of a deformity system.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/945,821 (now U.S. Pat. No. 8,986,349), filed Nov. 12, 2010, which claims the benefit of priority from U.S. Provisional Patent Application Ser. No. 61/260,357, filed on Nov. 11, 2009, and U.S. Provisional Patent Application Ser. No. 61/390,561, filed on Oct. 6, 2010, the entire contents of which are hereby expressly incorporated by reference into this disclosure as if set forth in their entireties herein. 
    
    
     FIELD 
     The present invention relates generally to medical devices and methods generally aimed at spinal surgery. In particular, the disclosed system and associated methods relate to performing spinal fixation with the use of a deformity system. 
     BACKGROUND 
     The spine is formed of a column of vertebra that extends between the cranium and pelvis. The three major sections of the spine are known as the cervical, thoracic and lumbar regions. There are 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae, with each of the 24 vertebrae being separated from each other by an intervertebral disc. A series of about 9 fused vertebrae extend from the lumbar region of the spine and make up the sacral and coccygeal regions of the vertebral column. 
     The main functions of the spine are to provide skeletal support and protect the spinal cord. Even slight disruptions to either the intervertebral discs or vertebrae can result in serious discomfort due to compression of nerve fibers either within the spinal cord or extending from the spinal cord. If a disruption to the spine becomes severe enough, damage to a nerve or part of the spinal cord may occur and can result in partial to total loss of bodily functions (e.g. walking, talking, and breathing, etc. . . . ). Therefore, it is of great interest and concern to be able to both correct and prevent any ailments of the spine. 
     Fixation systems are often surgically implanted into a patient to aid in the stabilization of a damaged spine or to aid in the correction of other spinal geometric deformities. Spinal fixation systems are often constructed as framework stabilizing a particular section of the spine. Existing systems often use a combination of rods, plates, pedicle screws and bone hooks for fixing the framework to the affected vertebrae. The configuration required for each patient varies due to the patient&#39;s specific anatomical characteristics and ailments. As a result, there is a need for a modular spinal fixation system that allows for a large degree of custom configurations and that can assist the clinician in the corrective maneuvers often needed to rehabilitate severe deformities. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a spinal anchor assembly according to one example embodiment; 
         FIG. 2  is an exploded view of the spinal anchor assembly of  FIG. 1 ; 
         FIG. 3  is a partial cross section view of the spinal anchor assembly of  FIG. 1 ; 
         FIG. 4  is a perspective view of the receiver forming a part of the spinal anchor assembly of  FIG. 1 ; 
         FIG. 5  is a top view of one example of a closure structure; 
         FIG. 6  is a perspective view of the closure structure of  FIG. 5 ; 
         FIG. 7  is a perspective cross section view of the closure structure of  FIG. 5 ; 
         FIG. 8  is a perspective view of a collet forming part of the receiver assembly of  FIG. 1 ; 
         FIG. 9  is a top view of the collet forming part of the receiver assembly of  FIG. 1 ; 
         FIG. 10  is side view of the collet forming part of the receiver assembly of  FIG. 1 ; 
         FIG. 11  is a perspective view of another spinal anchor assembly according to a second example embodiment; 
         FIG. 12  is a partial cross section view of the spinal anchor assembly of  FIG. 11 ; 
         FIG. 13  is an exploded view of the spinal anchor assembly of  FIG. 11 ; 
         FIG. 14  is a perspective view of the receiver forming a part of the spinal anchor assembly of  FIG. 11 ; 
         FIG. 15  is a top view of the collar forming part of the receiver assembly of  FIG. 11 ; 
         FIG. 16  is a perspective view of the collar of  FIG. 15 ; 
         FIG. 17  a top view of the cradle forming part of the receiver assembly of  FIG. 11 ; 
         FIG. 18  is a perspective view of the cradle of  FIG. 17 ; 
         FIG. 19  is a perspective view of an another anchor assembly according to a third example embodiment; 
         FIG. 20  is an exploded view of the anchor assembly of  FIG. 19 ; 
         FIG. 21  is a perspective view of the bone screw forming part of an anchor assembly of  FIG. 19 ; 
         FIG. 22  is a perspective view of the bone screw of  FIG. 21  forming part of an anchor assembly of  FIG. 10 ; 
         FIG. 23  is a perspective view of a receiver forming a part of the anchor assembly of  FIG. 19 ; 
         FIG. 24  is a perspective view of one example collar forming part of the anchor assembly of  FIG. 19 ; 
         FIG. 25  is a perspective view of another example collar forming part of the anchor assembly of  FIG. 19 ; 
         FIG. 26  is a partial cross section view of the collar of  FIG. 24 ; 
         FIG. 27  is a partial cross section view of the collar of  FIG. 25 ; 
         FIG. 28  is a top view of the collar of  FIG. 24 ; 
         FIG. 29  is a top view of the collar of  FIG. 25 ; 
         FIG. 30  is a partial cross section view of the anchor assembly of  FIG. 19 ; 
         FIG. 31  is a perspective view of a partial cross section view of a spinal anchor assembly according to a fourth embodiment of the present invention 
         FIG. 31  is a perspective view of another spinal anchor assembly, according to a fourth example embodiment; 
         FIG. 32  is a partial cross section view of the spinal anchor assembly of  FIG. 31 ; 
         FIG. 33  is a partial cross section view of the spinal anchor assembly in an unlocked position, according to a fifth embodiment of the present invention; 
         FIG. 34  is a partial cross section view of the spinal anchor assembly of  FIG. 33  in the locked position; 
         FIG. 35  is a perspective view of the loading ring of  FIG. 33 ; 
         FIG. 36  is a perspective view of the collet of  FIG. 33 ; 
         FIG. 37  is a partial cross section view of a spinal anchor assembly in an unlocked position according to the sixth embodiment of the present invention; 
         FIG. 38  is a partial cross section view of the spinal anchor assembly of  FIG. 37  in the locked position; 
         FIG. 39  is a perspective view of the split ring of  FIG. 37 ; 
         FIG. 40  is a perspective view of the loading ring of  FIG. 37 ; 
         FIG. 41  is a perspective view of one example of a spinal anchor assembly, according to a seventh embodiment of the present invention; 
         FIG. 42  is a partial cross section view of the spinal anchor assembly of  FIG. 41 ; 
         FIG. 43  is a perspective view of one an arched transverse connector according to one example embodiment; 
         FIG. 44  is a top view of an arched transverse connector of  FIG. 43 ; 
         FIG. 45  is a side view of an arched transverse connector of  FIG. 43 ; 
         FIG. 46  is a partial section view of an arched transverse connector of  FIG. 43 ; 
         FIG. 47  is a perspective view of an eccentric pin of the arched transverse connector of  FIG. 43 ; 
         FIG. 48  is a side view of an eccentric pin of the arched transverse connector of  FIG. 43 ; 
         FIG. 49  is a perspective view of an arched transverse connector according to a second example embodiment; 
         FIG. 50  is a cross section view of the arched transverse connector of  FIG. 49 ; 
         FIG. 51  is a side view of an arched transverse connector according to a third example embodiment of the present invention; 
         FIG. 52  is a cross section view of arched transverse connector of  FIG. 51 ; 
         FIG. 53  is an exploded view of the arched transverse connector of  FIG. 51 ; 
         FIG. 54  is a perspective view of a slide arm of the arched transverse connector of  FIG. 51 ; 
         FIG. 55  is a perspective view of a securing block of the arched transverse connector of  FIG. 51 ; 
         FIG. 56  is a perspective view of a reduction tower, according to an example embodiment; 
         FIG. 57  is a cross section view of the reduction tower of  FIG. 56 ; 
         FIG. 58  is a perspective view of a locking tool according to one example embodiment for use with the spinal anchor assembly of  FIG. 1 ; 
         FIG. 59  is another perspective view of the locking tool of  FIG. 58 ; 
         FIG. 60  is a perspective view of one example of a tooling assembly, according to a first embodiment of the present invention; 
         FIG. 61  is a cross section view of a tooling assembly of  FIG. 60 ; 
         FIG. 62  is a partial cross section view of a tooling assembly of  FIG. 60 ; 
         FIG. 63  is a perspective section view of one example of a reduction tower grasping a spinal anchor assembly and reducing a rod; 
         FIG. 64  is a perspective partial section view of one example of a reduction tower grasping a spinal anchor assembly and reducing a rod; 
         FIG. 65  is a perspective view of one example of a reduction tower, according to a second embodiment of the present invention; 
         FIG. 66  is a cross section view of the reduction tower of  FIG. 65 ; 
         FIG. 67  is a perspective view of the reduction tower link in the open position; 
         FIG. 68  is an exploded perspective view of the reduction tower link of  FIG. 67 ; 
         FIG. 69  is a detailed view of the ratcheting mechanism of the reduction tower link of  FIG. 67 ; 
         FIG. 70  is a detailed view of the ratcheting mechanism with the outer and inner cylinders of  FIG. 67  removed; 
         FIG. 71  is a perspective view of the reduction tower link of  FIG. 67  with arrows indicating the direction of movement when the arms are squeezed together; and 
         FIG. 72  is an exemplary configuration of a deformity spinal fixation assembly. 
     
    
    
     DETAILED DESCRIPTION 
     Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as a compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. The spinal anchor assembly disclosed herein boasts a variety of inventive features and components that warrant patent protection, both individually and in combination. 
       FIG. 1  illustrates an example of a spinal anchor assembly  10  according to a first embodiment of the present invention. The spinal anchor assembly  10  includes a bone screw  11  and a receiver assembly  12 . A closure structure  13  (shown in  FIGS. 5-7 ) is used to capture a rod within the receiver assembly  12 . The spinal anchor assembly  10  and closure structure  13  are composed of a metal (e.g. titanium, stainless steel, etc.). 
     The bone screw  11  of the present invention is configured to attach securely within a bony structure (e.g. pedicle of a vertebra) and to allow the receiver assembly  12  to provisionally lock into position relative to the bone screw  11  after placement of the bone screw  11  within a bony structure. The receiver assembly  12  and bone screw  11  are configured to engage with full polyaxial motion. The receiver assembly  12  and bone screw  11  can also be provisionally locked (that is, fixed relative to each other prior to final capture and locking of a spinal rod into the receiver), as will be described in more detail below. This versatile engagement between the receiver assembly  12  and bone screw  11  provides both the ease of positioning and rod placement associated with polyaxial screws and the ability to leverage the bone anchor  10  to manipulate the vertebral body (e.g. parallel distraction and compression and/or vertebral body derotation) associated with fixed-axis anchors. 
     By way of example, the bone screw  11  of the spinal anchor assembly  10  may be engaged within a pedicle of a vertebra and aligned with a spinal rod connecting other anchors. A clinician may then engage an instrument to the receiver assembly  12  and provisionally lock the screw  11  and receiver  12 . With the screw locked, the clinician can utilize the instrument to apply a force upon the vertebra to correct the deformity prior to fixing the construct and thus the spinal column in a desired position. The ability to utilize the spinal anchor assembly  10  to reposition segments of the spine to correct deformities simplifies the procedure for the clinician by limiting the amount of tools and time required. 
     With reference to  FIGS. 1-3 , the bone screw  11  of the spinal anchor assembly  10  is comprised of a shank  17 , a body  8 , and a capture structure  16 . At least one helically-wound bone implantable thread  18  extends radially from the body  8  and functions to secure the placement of the bone screw  11  within a bony structure. The capture structure  16  includes at least one tool engaging feature  14  that can be used, for example, to engage and attach various tooling for aligning and advancing the bone screw  11  into a bony structure. The generally spherical shape of the capture structure  16  allows it, for example, to articulate within the collet  40  to achieve the polyaxial motion between the bone screw  11  and the receiver assembly  12 . The surface of the capture structure  16  may be textured (e.g. scored or knurled) for enhancing frictional engagement with the collet  40  that secures the positioning of the bone screw  11  relative to the receiver assembly  12 . 
     The receiver assembly  12  is configured to receive an elongate structure (e.g. a rod) and the closure structure  13  is designed to secure the rod within the receiver assembly  12 . Once the receiver assembly  12  and bone screw  11  are securely oriented in the desired orientation and the rod is captured in the receiver assembly  12 , the closure structure  13  can engaged to lock the rod in the receiver assembly  12 . 
     The receiver assembly  12  is typically provided in an assembled state (best shown in  FIG. 3 ) and includes a receiver  20  and a retaining and articulating structure or collet  40 . The receiver  20  has a generally U-shaped appearance with a generally cylindrical inner profile and a faceted outer profile. A base  26 , with a pair of upstanding arms  29  forms a U-shaped cradle which define U-shaped openings  27  through the faceted sides of the receiver  20 . Receivers may be provided in a variety of dimensions depending on the size and shape of the rod that it will be in secured frictional engagement with. 
     Both arms  29  have at least one helically-wound guide and advancement structure  25  at least partially situated along their internal walls beginning from the top surface  28  end of the receiver  20 . The guide and advancement structure  25  of the receiver  20  are configured to mate with at least one exterior helically-wound guide and advancement structure  72  of the closure structure  13 . When the internal and external guide and advancement structures  25 ,  72  of the closure structure  13  and receiver  20  are interlocked, their connection prevents the arms  29  of the receiver  20  from spreading open due to the mating features of the guide and advancement structures  25  and  72 . This interlocked configuration prevents splaying of the arms  29 . 
     As illustrated in  FIG. 4 , the outer surface of the receiver  20  includes tooling attachment features, such as grip bores  21 , on the outer surface of both arms  29  which function to allow a variety of tools to engage the receiver assembly  12  for subsequent positioning and implantation of the spinal anchor assembly  10 . Additional features of the receiver  20  include two sweeping steps  38  recessed inwardly from the inside walls of the arms  29  (with one sweeping step  38  situated on each arm  29 ). The sweeping steps  38  are utilized during the assembly of the receiver assembly  12  by allowing the locking ledges  55  of the collet  40  to be guided into position within the receiver  20 . Once the collet  40  is assembled within the receiver  20 , the collet  40  is allowed limited movement. By way of example, each sweeping step  38  includes a notch  39  that prevents the locking ledge  55  from backing out of the sweeping step  38  once it has traveled past the notch  39 . Additionally, the top and bottom walls of each sweeping step  38  restricts the longitudinal translation of the collet  40  relative to the receiver by restricting the longitudinal translation of the locking ledge to only between the top and bottom walls of the sweeping step  38 . By way of example only, each sweeping step  38  spans at least a portion of the inside wall of an arm  29  and are positioned generally 180 degrees apart from one another. 
     Located within the base  26  of the receiver  20  is a tapered cavity  34  that is sized and shaped for slidable mating and eventual frictional engagement with the tapered feature  48  of the collet  40 , as will be described in more detail below. By way of example only, the tapered cavity  34  may have a taper of approximately 2-3 degrees (shown as angle Y in  FIG. 3 ). The taper feature  48  of the collet  40  is shown as angle Z in  FIG. 10 . When the collet  40  is forced generally in the direction of the base  26  of the receiver  20  along its longitudinal axis, the tapered feature  48  of the collet will become frictionally secured (wedged) within the tapered cavity  34 . 
       FIGS. 8-10  illustrate an example of a collet  40  according to a first embodiment. The collet  40  includes a top surface  41 , a bottom surface  42 , an inner spherical surface  49 , a tapered feature  48 , locking ledges  55 , a saddle  46 , and a tooling engagement feature  52 . Notably, the collet  40  is not continuous, and instead includes a slot  44 . The slot  44  is dimensioned to be a distance X (best shown in  FIG. 9 ) and allows the collet  40  to be temporarily expanded or compressed to receive the capture structure  16  and to secure the capture structure  16  within the inner spherical surface  49 . 
     During assembly of the spinal anchor assembly  10 , the collet  40  receives, and permanently captures, the capture structure  16  within the inner spherical surface  49 . Once the capture structure  16  is captured within the inner spherical surface  49 , the collet  40  and associated bone screw  11  is assembled to the receiver  20 . This is accomplished by leading the distal end of the bone screw  11  through the center of the receiver until the locking ledges  55  are aligned with the sweeping step  38  of the receiver  20 . As described above, the locking ledges  55  travel along the sweeping steps  38  until they pass the notch  39 , where the collet  40  then becomes permanently limited in movement relative to the receiver  20 . At this point, the collet  40  is able to travel a limited distance along its longitudinal axis. Additionally, the bone screw  11  is able to articulate relative to the receiver assembly  12  achieve poly axial motion of the receiver assembly. By way of example, the bone screw  11  is able to articulate and form an angle between its longitudinal axis and the longitudinal axis of the receiver  20  of up to approximately 20 degrees in any direction. When the desired angular orientation is achieved, the receiver assembly  12  is locked into position relative to the bone screw  11 . For this to occur, the collet  40  is wedged into the receiver  20  which compresses the slot  44  and causes the inner spherical surface  49  to frictionally engage and secure the capture structure  16 . This permanently fixes the configuration of the receiver  20 , collet  40 , and bone screw  11 . 
     As discussed, the anchor assembly  10  can be both provisionally locked and finally locked. By way of a first example, the spinal anchor assembly  10  can be finally locked by driving a rod (e.g. rod  60 ) into the collet  40  and receiver  20  by engaging and advancing a closure structure  13  into the receiver  20 . As the closure structure  13  advances, the rod is forced down into the collet  40  and the collet  40  in turn is driven down and wedges into the tapered cavity  34 . At this point, the collet  40  is locked into position relative to the receiver  20  and the rod is securely locked between the closure structure  13  and collet  40 . 
     The spinal anchor assembly  10  can be provisionally locked by fully reducing the rod  60 ) into the collet  40  and receiver  20  with an instrument, such as the reduction tower  900  (described below). The reduction tower  900  releasably attaches to the receiver  20  and an arm directs the rod  60  into the receiver  20 , forcing the rod into the collet  40  which is driven down and wedges into the tapered cavity  34 . At this point, the collet  40 , receiver  20 , and bone screw  11  are locked into position relative to each other, however, the rod is not locked within the receiver assembly  12 , and the bone anchor can be used to adjust the position or orientation of the vertebra to which the anchor assembly  10  is attached (e.g. parallel distraction and compression or derotation). The closure structure  13  can be advanced in to the receiver  20  when it becomes desirable to secure the rod within the receiver assembly  12 . 
     By way of another example, the spinal anchor assembly  10  can be provisionally locked by driving a rod-like tooling feature into the rod into the collet  40 . Again, this provisional locking feature provides a platform for the clinician to utilize the screw to manipulate the position or orientation of the vertebra while still allowing the receiver to be adjusted for easier reception of the rod. 
     The tooling engagement features  52  of the collet  40  allow the user to unlock the anchor assembly  10  from the provisionally locked configuration, if necessary. A tool can engage the tooling engagement features  52  to, for example, compress and/or pull on the collet  40  in order to release the frictional engagement between the tapered feature  48  of the collet  40  and tapered cavity  34  of the receiver  20 . 
     The saddle  46  of the collet  40  provides a contouring surface for mating with a rod within the receiver assembly  12 . By way of example only (and best shown in  FIG. 8 ), the saddle  46  has two U-shaped surfaces that are generally shaped to receive a rod. The saddle  46  may be any number of shapes and sizes necessary to accommodate a particular rod, without departing from the scope of this invention. Furthermore, the shape and dimensions of the collet  40  and its features may be any number of shapes and dimensions without departing from the scope of this invention. 
       FIGS. 5-7  illustrate one example embodiment of a closure structure  13 . The closure structure  13  is shown by way of example to include a top surface  70 , a base  71 , and at least one exterior guide and advancement structure  72 . The top surface  70  includes at least one generally recessed tool engaging feature  73  which functions to engage a variety of tooling that assist in aligning and securing the closure structure  13  to the receiver assembly  12 . A recessed slot  75  on the top surface  70  functions to provide the clinician with an aligning mechanism for screwing the closure structure  13  into the receiver  20 . For example, the recessed slot  75  of the closure structure  13  should be aligned with the recessed slot  24  of the receiver  20  prior to advancing the closure structure  13  to facilitate proper engagement. Positioned centrally within the base  71  of the closure structure  13  is a point force feature  74  that applies a point force to secure a portion of an rod (e.g. rod  60 ). The point force feature  74  deforms upon final tightening of the screw and improves resistance to translation and centers the locking stress within the receiver  12 . It will be appreciated that while the closure structure  13  shown may be preferred, closure structures utilizing a number of other suitable structures and features may be utilized without departing from the scope of this invention. 
       FIG. 11  illustrates an example of a spinal anchor assembly  100  according to a second embodiment of the present invention. The spinal anchor assembly  100  includes a bone screw  111  and a receiver assembly  112 . By way of one example, a closure structure  13  (shown in  FIGS. 5-7 ) is used to capture a rod within the receiver assembly  112 . The spinal anchor assembly  100  is preferably composed of a metal (e.g. titanium, stainless steel, etc.). 
     The spinal anchor assembly  100  of the present invention is available to a clinician in a pre-assembled state such that the receiver assembly  112  is jointly attached to the capture structure  116  of the bone screw  111 . The receiver assembly  112  and bone screw  111  are able configured to engage with limited axial movement. More specifically, the receiver member  112  may articulate along a single plane (i.e. uniplanar movement), and can ultimately be secured at any number of angles within the single plane. Similar to the provisional locking anchor assembly  10 , the uniplanar engagement between the receiver assembly  112  and the bone screw  111  permits some flexibility for positioning the rod, while still providing the ability to leverage the anchor assembly to manipulate the vertebra to correct positioning and alignment of the vertebra. By way of example, the anchor assembly may be implanted such that the articulating plane is the sagittal plane (i.e. movement is cranial-caudal). Positioned as such, force may be applied to the screw in the transverse plane (i.e. medial/lateral direction) to derotate a vertebra. One advantage of limiting the angled articulation between the receiver assembly  112  and bone screw  111  to only along a single plane is so that force can be applied upon the spinal anchor assembly  100  in any direction that is along a non-articulating plane. By way of example, the bone screw  111  of the spinal anchor assembly  100  would be first secured within a pedicle of a vertebra. A clinician may engage an instrument to the receiver assembly  112  and to apply the correcting force. 
     With reference to  FIGS. 11-13 , the spinal anchor assembly  100  is comprised of a shank  117 , a body  108 , and a capture structure  116 . At least one helically-wound bone implantable thread  118  extends radially from the body  108  and functions to secure the placement of the bone screw  111  within a bony structure. Additionally, the capture structure  116  includes flat surfaces  119  on opposing sides of the capture structure  116 . The flat surfaces  119  restrict rotation between the bone screw  111  and the collar  140  to the plane parallel to the flat side  19 . Preferably, the articulating plane is aligned with the side openings  127  such the uniplanar movement is in line with the rod. 
     The capture structure  116  includes at least one tool engaging feature  114  that can be used, for example, to engage and attach various tooling for aligning and advancing the bone screw  111  into a bony structure. The generally spherical shape portions of the capture structure  116  allow the capture structure  116 , for example, to articulate within the collar  140  along a single plane. The surface of the capture structure  116  may be textured (e.g. scored or knurled) for enhancing frictional engagement with the collar  140 , which assists in securing the position of the bone screw  111  relative to the receiver assembly  112 , as will be discussed in more detail below. 
       FIG. 12  illustrates an example embodiment of a receiver assembly  112 . The receiver assembly  112  is typically provided in an assembled state (as shown in  FIGS. 11 and 12 ) and includes a receiver  120 , a retaining and articulating structure or collar  140 , and a cradle  160 . The receiver  120  has a generally U-shaped appearance with a generally cylindrical inner profile and a faceted outer profile. A base  126 , with a pair of upstanding arms  129  forms a U-shaped cradle which define U-shaped openings  127  through the faceted sides of the receiver  120 . Alternatively, receiver  120  may be provided with openings having any of a variety of shapes and dimensions depending, in part, on the size and shape of the rod to be received. 
     Both arms  129  have at least one helical wound guide and advancement structure  125  at least partially situated along their internal walls beginning from the top surface  128  end of the receiver  120 . The guide and advancement structure  125  of the receiver  120  are configured to mate with at least one exterior helically-wound guide and advancement structure of a closure structure (not shown in this embodiment), which assist in preventing the arms from spreading open. The closure structure  13  described in the anchor assembly  10  may be used with anchor assembly  110  to secure a portion of a rod within a receiver assembly  112 . Again, it should be appreciated that while the closure structure  13  shown may be preferred, closure structures utilizing a number of other suitable structures and features may be utilized without departing from the scope of this invention. 
     The outer surface of the receiver  120  includes tooling attachment features, such as grip bores  121 , on the outer surface of both arms  129 . Grip bores  121  function, for example, to allow a variety of tools to engage the receiver assembly  112  for subsequent implantation and positioning of the receiver assembly  112  and spinal anchor assembly  100 . Additional features of the receiver  120  include two steps  138  extending inwardly from the inside walls of the arms  129  (with one step  138  situated on each arm  129 ). By way of example only, each step  138  spans at least a portion of the inside wall of an arm  129  and is positioned generally 180 degrees apart from the other. Located within the base  126  end of the receiver  120  is a cavity that is defined by a generally spherical surface and is sized and shaped for slidable mating and eventual frictional engagement with the retaining and articulating structure or collar  140 , as described below. Along the walls of the cavity within the base  126  of the receiver is a pair of rounded pivot features  135 . The rounded pivot features  135  are located approximately 180 degrees apart from one another the wings  147  of the collar  140  and permit the collar  140  to articulate generally along a single plane. 
       FIGS. 15-16  illustrate an example of an embodiment of a retaining and articulating structure or collar  140 . The collar  140  is comprised of a top surface  141 , a bottom surface  149 , an outer convex surface  152 , an inner concave surface  145 , and a radial protrusion  148 . Notably, the collar  140  is not continuous, and instead includes a slot  144  extending from the top surface  141  to bottom surface  149 . The length of the slot  144  is dimensioned to be a distance X (best shown in  FIG. 16 ) and allows the collar  140  to be temporarily expanded or compressed to secure the collar  140  around the capture structure  160 , as described below. 
     Wings  147  protrude from opposing sides of the outer surface  152  the collar  140 . The wings  147  are sized and shaped to mate with the pivot feature  135  of the receiver  120 . By way of example, the wings  147  may be D-shaped, but may be any size and shape suitable for directing and limiting the pivot direction of the collar  140  (and associated bone screw  111 ). When mated, the wings  147  assist in both positioning the collar  140  within the receiver  120  and restricting the pivot directions of the collar  140  to along a single plane. The wings  147  also restrict relative rotation between the collar  140  and receiver  120  along their longitudinal axis. The bone screw  111  and associated collar  140  are able to pivot relative to the receiver  120  along a single plane for subsequent secure positioning and implantation. Interior protrusions  109  extend inwardly from opposing sides of the inner concave surface  145  of the collar  140 . The interior protrusions  109  function to mate with the flat surfaces  119  on a capture structure  116  to prevent the rotation of a bone screw  111  relative to the collar  140  along their longitudinal axis. 
       FIGS. 17-18  illustrate an example embodiment of a cradle  160 . The cradle  160  is comprised of a top surface  167 , spherical inner walls  166 , concave supports  165 , and a base  168 . Additional features of the cradle  160  include first outer diameter notches  163 , second outer diameter notches  191 , locking protuberances  190 , a central opening  164 , locking ledges  162 , and tool engaging features  161 . As previously noted, the receiver assembly  112  is typically acquired by a user in an assembled state. Furthermore, before the cradle  160  is assembled to the receiver  120 , the collar  140  is first assembled to the receiver  120 . 
     During assembly of the receiver assembly  112 , the collar  140  is positioned within the base of the receiver  120  so that the outer convex surface  152  rests generally along the spherical cavity located within the base  126  of the receiver  120 . The collar  140  is positioned within the receiver  120  such that the top surface  141  of the collar  140  is facing the top surface  128  of the receiver  120 . Even when the collar  140  is in its circumferentially compressed state, the collar  140  cannot exit the receiver  120  through its central opening  137 . Additionally, the pair of wings  147  protruding from the outer convex surface  152  of the collar  140  is generally mated with the pair of rounded pivot features  135  within the receiver. As mentioned above, the wings  147  mated with the pivot features  135  function to secure the positioning of the collar  140  and permit the collar  140  to articulate generally along a single plane relative to the receiver  120 . 
     Once the collar  140  is assembled to the receiver  120 , the cradle  160  can then be placed above the receiver  120  such that the bottom surface  26  of the cradle  160  is facing the top surface  28  of the receiver  120 . The cradle  160  is then dropped into the center of the receiver  120  (between the arms  129 ) until the cradle  160  rests generally circumferentially within the round inner walls of the receiver  120  and the base  168  of the cradle  160  sits on the steps  138 . The cradle  160  is then aligned (a tool may be engaged into the tool engaging features  161  to accomplish this) so that the first outer diameter notches  163  of the cradle  160  are aligned over the steps  138  of the receiver  120 . This allows the cradle  160  to travel past the steps  138  towards the base  126  of the receiver  120  until the base  168  of the locking ledges  162  rest against the inside wall of the receiver  120  and prevent the cradle  160  from traveling further down towards the bottom surface  126  end of the receiver  120 . At this point, the cradle  160  can be rotated along its central axis in the clockwise direction (again, a tool may be engaged into the tool engaging features  161  to accomplish this) so that the locking ledges  162  travel clockwise beneath the steps  138  of the receiver  120 . The cradle  160  is rotated clockwise until the steps  138  are forced past the locking protuberances  190  of the cradle  160  and the steps  138  are situated within the second outer diameter notches  191 . When the steps  138  of the receiver are situated within the second outer diameter notches  191 , the cradle  160  is permanently secured into place and the receiver assembly  112  is generally complete (and best shown in  FIG. 12 ). 
     The final step in assembling the spinal anchor assembly  100  during manufacturing and before being released for use is assembling the bone screw  111  to the receiver assembly  112 . To accomplish this, a capture structure  116  of a bone screw  111  is generally concentrically aligned with the central opening  137  of a receiver. The capture structure  116  is then passed through the central opening  137  of the receiver  120 , which has a larger diameter than the capture structure  116 . 
     As the capture structure  116  passes through the central opening  137  it pushes against the bottom surface  149  of the collar  140  until the collar  140  is pushing up against the base  168  of the cradle  160 . The capture structure  116  can then advance past the inner chamfer  46  of the collar  140  by forcing the collar  140  to increase circumferentially (by further lengthening the slot  144 ) until the largest diameter of the capture structure  116  passes through the inner protruding ring  151 . Once the capture structure  116  passes through the inner protruding ring  151 , the collar  140  begins to generally return to its original circumference and capture the capture structure  116  (best shown in  FIG. 12 ). Proper orientation is ensured such that the interior protrusions  109  are mated with the flat surfaces  119  on the capture structure  116 . 
     At this point, the connection between the bone screw  111  and receiver assembly  120  resembles a ball-and-socket joint, but with limited articulation to only along a single plane. More specifically, the capture structure  116  is free to articulate along a single plane relative to the collar  140  and the collar is able to articulate along a single plane relative to the receiver  120 . Thus, the collar  140 , bone screw  111  and receiver  120  are able to articulate relative to each other along a single plane until they are locked into position. By way of example, the bone screw is able to articulate and form an angle between its longitudinal axis and the longitudinal axis of the receiver  120  of approximately 30 degrees in either direction, for a total of 60 degrees of movement) in the articulating plane. 
     Therefore, a clinician may configure a spinal fixation system using at least one spinal anchor assembly  100 , at least one additional bone screw assembly (i.e. fixed, provisionally locking, polyaxial), and at least one rod. The clinician is able to easily align the rod with the receiver assembly  112  of the spinal anchor assembly  100 . Additionally, the clinician may leverage the uniplanar screw to direct a correcting force to the associated vertebra to correct positioning or alignment of the vertebra (e.g. derotation). Thereafter, the closure structure  13  can be engaged to press the rod against the cradle  160 , which in turn presses the capture structure  116  against collar  140 , and the collar  140  against the receiver  112 . Ultimately, the frictional engagement between the closure structure  13 , rod, cradle  160 , capture structure  116 , cradle  140 , and cavity of the receiver  120  are such that the bone screw  111  and receiver assembly  112  are secured in a desired final position relative to each other. 
       FIGS. 19-30  illustrate an example of a spinal anchor assembly  200  according another embodiment of a uniplanar spinal anchor. The spinal anchor assembly  200  includes a bone screw  211 , a receiver assembly  212 , and a closure structure  13  (shown in  FIGS. 5-7 ). The spinal anchor assembly  200  is preferably composed of a metal (e.g. titanium, stainless steel, etc.). 
     The bone screw  211  of the present invention is configured to securely engage within a bony structure (e.g. pedicle of a vertebra). The receiver assembly  212  is able to articulate relative to the bone screw  211  along a single plane. This uniplanar engagement between the receiver assembly  212  and the bone screw  211  permits some flexibility for positioning the rod, while still providing the ability to leverage the anchor assembly  200  to manipulate the vertebra to correct positioning and alignment of the vertebra. By way of example, the anchor assembly may be implanted such that the articulating plane is the sagittal plane (i.e. movement is cranial-caudal). Positioned as such, force may be applied to the screw in the transverse plane (i.e. medial/lateral direction) to derotate a vertebra. 
       FIG. 21  illustrates an example embodiment of the bone screw  211 . The bone screw  211  of the spinal anchor assembly  200  is comprised of a shank  217 , a body  208 , and a capture structure  216 . At least one helically-wound bone implantable thread  218  extends radially from the body  208  and functions to secure the placement of the bone screw  211  within a bony structure. Additionally, the capture structure  216  includes flat surfaces  219  on opposing sides of the capture structure  216 . The flat surfaces  219  function to assist in restricting the rotation between the bone screw  211  and the collar  240  along its longitudinal axis, as will be discussed in more detail below. The capture structure  216  may comprise a screw head. The screw head may comprise opposing fifth and sixth sides and opposing seventh and eight sides. 
     The capture structure  216  includes at least one tool engaging feature that can be used, for example, to engage and attach various tooling for aligning and advancing the bone screw  211  into a bony structure. The generally spherically-shaped portions  215  of the capture structure  216  allow the capture structure  216 , for example, to articulate within the collar  240  along a single plane. The surface of the capture structure  216  may be textured (e.g. scored or knurled) for enhancing frictional engagement with the collar  240 , which assists in securing the position of the bone screw  211  relative to the receiver assembly  212 , as will be discussed in more detail below. 
       FIGS. 23 and 30  illustrate an example embodiment of a receiver assembly  212 . The receiver assembly  212  is typically provided in an assembled state (as shown in  FIG. 19 ) and includes a receiver  220 , a retaining and articulating structure or collar  240 , and a cradle  160 . The receiver  220  has a generally U-shaped appearance with a generally cylindrical inner profile and a faceted outer profile. A base  226 , with a pair of upstanding arms  229  forms a U-shaped cradle which define U-shaped openings  227  through the faceted sides of the receiver  220 . It will be appreciated however, that receiver  220  may be provided having side openings selected from variety of suitable shapes and dimensions depending, in part, on the size and shape of the rod to be received. 
     Both arms  229  have at least one helical wound guide and advancement structure  225  at least partially situated along their internal walls beginning from the top surface  228  end of the receiver  220 . The guide and advancement structure  225  of the receiver  220  are configured to mate with at least one exterior helically-wound guide and advancement structure of a closure structure (not shown in this embodiment), which assist in preventing the arms from spreading open. The closure structure  13  described in the first embodiment may be used with this second embodiment of the present invention to achieve the securing of at least a portion of a rod within a receiver assembly  212 . Moreover, any variation of closure structures may be used to secure at least a portion of a rod within the receiver assembly  212 , without departing from the scope of this invention. 
     The outer surface of the receiver  220  includes tooling attachment features, such as grip bores  221 , on the outer surface of both arms  229 . Grip bores  221  function, for example, to allow a variety of tools to engage the receiver assembly  212  for subsequent implantation and positioning of the receiver assembly  212  and screw assembly  200 . Additional features of the receiver  220  include two steps  238  extending inwardly from the inside walls of the arms  229  (with one step  238  situated on each arm  229 ). By way of example only, each step  238  spans at least a portion of the inside wall of an arm  229  and are positioned generally 180 degrees apart from each other. Located within the base  126  end of the receiver  120  is a cavity that is defined by a generally spherical surface and is sized and shaped for slidable mating and eventual frictional engagement with collar  240 , as described below. Along the walls of the cavity within the base  226  of the receiver is a pair of rounded features  235 . The rounded features  235  are located approximately 180 degrees apart from each other and function to secure the positioning of the collar  240 . 
       FIGS. 24-29  illustrate example embodiments of a retaining and articulating structure or collar  240 . The collar  240  is comprised of a top surface  241 , a bottom surface  249 , an outer convex surface  252 , an inner concave surface  245 , an interior faceted surface  246 , and an exterior faceted surface  248 . An embodiment of the collar  240  shown in  FIGS. 24, 26, and 28  comprises a first side and a second side opposing the first side, wherein the first side and the second side are straight and parallel to each other, said collar having a third side and a fourth side opposing said third side, wherein the third side and the fourth side are rounded, and wherein the four sides define an inner cavity. In some embodiments of the collar  240 , interior surfaces of said opposing third side and fourth side are spherical. 
     In its preferred embodiment, the bone screw  211  may be assembled to the receiver assembly  212  by passing the distal end of the bone screw through the top opening of the receiver  212  and collar  240  (and before the cradle is assembled to the receiver  220 ) until the capture structure  216  is resting in the collar  240  (which is resting in the base of the receiver  220 ) forming collar assembly  280 . In one embodiment of collar  240 , the inner concave surfaces  245  has helical recesses  247  shaped into said inner concave surface  245 . Helical recesses  247  facilitate the placement of the bone screw  211  through the aperture  255  of the collar  240 . By way of example, helical recesses  247  allow for screws whose advancement structure  218  have diameters larger than aperture  255  to be used in bone screw assembly  200 . The helical thread  218  of a large diameter bone screw is threaded through helical recesses  247  until shank  217  has passed through the aperture  255  of collar  240 . Proper orientation is ensured such that the interior faceted surface  246  are mated with the flat surfaces  219  on the capture structure  216 . 
     Exterior faceted surfaces  248  are situated on opposing sides of the outer convex surface  252  of the collar  240 . Exterior faceted surface  248  is sized and shaped to mate with the rounded feature  235  of the receiver  220 . By way of example, the exterior faceted surfaces  248  may be D-shaped, but may be any size and shape suitable for limiting the movement of the collar  240  (and associated bone screw  211 ). When mated, the exterior faceted surface  248  assist in positioning the collar  240  within the receiver  220 . The bone screw  211  is able to pivot relative to the receiver  220  along a single plane for subsequent secure positioning and implantation, as described above and will be described in more detail below. Interior faceted surfaces  246  situated inwardly on opposing sides of the inner concave surface  245  of the collar  240 . By way of example, the interior faceted surfaces  246  may be D-shaped, but may be any size and shape suitable for directing and limiting the pivot direction of the capture structure  216  (and associated bone screw  211 ). In those embodiments in which the interior faceted surface  246  is D-shaped, it may include a straight edge portion and a curved portion forming the “D.” The interior faceted surfaces  246  function to mate with the flat surfaces  219  on a capture structure  216  to prevent the rotation of a bone screw  211  relative to the collar  240  along their longitudinal axis. 
     Furthermore, the cradle  260  in the present embodiment is generally identical in feature and function as the cradle  60  described in the second embodiment, and thus will not be repeated in detail again here. 
     At this point, the connection between the bone screw  211  and receiver assembly  220  resembles a ball-and-socket joint, but with limited articulation in only a single plane. More specifically, the capture structure  216  is free to articulate along a single plane relative to the collar  240 . The collar  240 , bone screw  211  and receiver  220  are able to articulate relative to each other along a single plane until they are locked into position, as will be described in detail below. By way of example, the bone screw is able to articulate and form an angle between its longitudinal axis and the longitudinal axis of the receiver  220  of approximately 30 degrees in either direction, for a total of 60 degrees, along the single articulating plane. 
     Therefore, a clinician may configure a spinal fixation system using at least one spinal anchor assembly  200 , at least one additional bone screw assembly of any variety of constraints (e. g. fixed, provisionally locking, polyaxial), and at least one rod. The clinician is able to easily align the rod with the receiver assembly  212  of the spinal anchor assembly  200 . Additionally, the clinician may leverage the uniplanar screw to direct a correcting force to the associated vertebra to correct positioning or alignment of the vertebra (e.g. derotation). Thereafter, the closure structure  13  can be engaged to press the rod against the cradle  160 , which in turn presses the capture structure  216  against collar  240 , and the collar  240  against the receiver  212 . Ultimately, the frictional engagement between the closure structure  13 , rod, cradle  260 , capture structure  216 , cradle  240 , and cavity of the receiver  220  are such that the bone screw  111  and receiver assembly  112  are secured in a desired final position relative to each other. 
       FIGS. 31 and 32  illustrate an example of a spinal anchor assembly  300  according to another embodiment of the present invention. The spinal anchor assembly  300  includes a bone screw  311 , and a receiver assembly  312 . The spinal anchor assembly  300  is preferably composed of a metal (e.g. titanium, stainless steel, etc.). The spinal anchor assembly  300  may be available to a clinician in a pre-assembled state such that the receiver assembly  312  is jointly attached to the capture structure  316  of the bone screw  311  and has full polyaxial motion. That is, the receiver assembly  312  and bone screw  311  are able to articulate in all directions and can ultimately be secured at any number of angles relative to each other and in any directions. 
     When the desired angular orientation is achieved to facilitate rod capture and the rod is received therein, the receiver assembly  312  is locked into position relative to the bone screw  311 . For this to occur, a closure structure  13  is engaged and presses down on the rod which presses down on the cradle  360  which presses down on the capture structure  316 . The capture structure presses down on the collet  340  and collet in turn presses into the receiver  320  which compresses the slot  344  and causes the inner spherical surface  349  to frictionally engage and secure the capture structure  316 . This permanently fixes the orientation of the receiver  320  relative to the bone screw  311 . 
     The bone screw  311  of the present invention is configured to attach securely within a bony structure (e.g. pedicle of a vertebra) with the receiver assembly  312  assembled to the capture structure  316  of the bone screw  311 . The receiver assembly and bone screw  311  are configured to engage in with full polyaxial motion. This polyaxial engagement between the receiver assembly  312  and bone screw  311  provides for simplified positioning and rod placement. The receiver assembly  312  is configured to receive a rod and a closure structure  13  secures the rod within the receiver assembly  312 . Once the rod is positioned in the receiver assembly  312  a closure structure  13  will lock the rod in the receiver assembly  312 , which also inhibits additional movement between the receiver assembly  312  and the bone screw  311 . 
     The bone screw  311  of the spinal anchor assembly  300  is comprised of a shank  317 , a body  308 , and a capture structure  316 . At least one helically-wound bone implantable thread  318  extends radially from the body  308  and functions to secure the placement of the bone screw  311  within a bony structure. The generally spherical shape of the capture structure  316  allows it, for example, to ultimately be frictionally engaged with the generally spherical features within the receiver assembly  312 . The surface of the capture structure  316  may be textured (e.g. scored or knurled) for enhancing frictional engagement with the retaining and articulating structure or collar  340 . 
     The receiver assembly  312  is typically provided in an assembled state and includes a receiver  320 , a retaining and articulating structure or collar  340 , and a cradle  360 . The receiver  320  has a generally U-shaped appearance with a generally cylindrical inner profile and a faceted outer profile. A base  326 , with a pair of upstanding arms  329  forms a U-shaped cradle which define U-shaped openings  327  through the faceted sides of the receiver  320 . It should be appreciated that side openings in the receiver may be provided in a variety of suitable shapes and dimensions depending on the size and shape of the rod to be received. Both arms  329  have at least one helically-wound guide and advancement structure  325  at least partially situated along their internal walls beginning from the top surface  328  end of the receiver  320 . The guide and advancement structure  325  of the receiver  320  are configured to mate with at least one exterior helically wound guide and advancement structure  72  of the closure structure  13 . Although an embodiment of a closure structure is described in detail herein, any number of closure structures may be used without departing from the scope of this invention. When the internal and external guide and advancement structures  325 ,  72  of the closure structure  13  and receiver  320  are interlocked, their connection prevents the arms  329  of the receiver  320  from spreading open due to the mating features of the guide and advancement structures  325  and  72 . This interlocked configuration prevents splaying of the arms  329 . 
     The outer surface of the receiver  320  includes tooling attachment features, such as grip bores  321  on the outer surface of both arms  329 . These tooling attachment features function, for example, to allow a variety of tools to engage the receiver assembly  312 . Additional features of the receiver  320  include two steps  338  extending inwardly from the inside walls of the arms  329  (with one step  338  situated on each arm  329 ). By way of example only, each step  338  spans at least a portion of the inside wall of an arm  329  and are positioned generally 180 degrees apart from each other. Located within the base  326  end of the receiver  320  is a cavity that is defined by a generally spherical surface which is sized and shaped for slidable mating and eventual frictional engagement with the retaining and articulating structure or collar  340 , as described below. 
     The collar  340  is comprised of a top surface  341 , a bottom surface  349 , an outer convex surface  352 , an inner concave surface  345 , and a radial protrusion  348 . Notably, the collar  340  is not continuous, and instead includes a slot (similar to the slot  44  discussed and illustrated in the second embodiment of the spinal anchor assembly  300 ) extending from the top surface  341  to bottom  342 . The slot is dimensioned to be a distance X and allows the collar  340  to be temporarily expanded or compressed to receive the capture structure  316  and to secure the collar  340  around the capture structure  316 , similar to the collar  40  described above, and thus will not be repeated in detail again here. Furthermore, the cradle  360  in the present embodiment is generally identical in feature and function as the cradle  60  described in the second embodiment, and thus will not be repeated in detail again here. 
     As discussed above, the connection between the bone screw  311  and receiver assembly  320  resembles a ball-and-socket joint before being locked into a configuration. This ball-and-socket characteristic enables the receiver assembly  320  to accommodate and capture a rod by rotating to achieve various angular positions relative to the fixed bone screw  311 . Therefore, as a clinician configures a spinal fixation system using at least one polyaxial bone screw assembly  300 , at least one additional bone screw assembly (i.e. fixed, provisional locking, polyaxial), and at least one rod, the clinician is able to easily align the rod with the receiver assembly  312  of the spinal anchor assembly  300 . The steps for locking in place of the spinal anchor assembly  300  in a desired position, in addition to the locking of the spinal anchor assembly  300  with the rod(s) are generally the same as the steps detailed in the second embodiment of the spinal anchor assembly  200 , and thus will not be repeated here. 
       FIGS. 33-36  illustrate an example of a spinal anchor assembly  1400  according to another embodiment of a provisional locking assembly. The spinal anchor assembly  1400  includes a bone screw  1410  and a receiver assembly  1412  similar in functions and features to the embodiment of the spinal anchor assembly  10  disclosed herein. Like features and functions will not be repeated. The spinal anchor assembly  1400  differs from the spinal anchor assembly  10  in that it includes a load ring  1414  engaged with the collet  1416  ( FIGS. 33-34 ). 
     As shown in  FIG. 35 , the load ring  1412  snaps into the collet  1416  and prevents the collet  1416  from compressing on the bone screw  1410  and or capture structure  13 . This allows the polyaxial motion of the bone screw  1410  relative to the receiver assembly  1412  to be maintained even after reduction of the collet  1416  into the receiver  1418 . Thus, the collet  1416  can become securely engaged (or wedged) into the receiver  1418  while the bone screw  1410  is able to still articulate within the collet  1416  to provide full polyaxial motion between the bone screw  1410  and receiver assembly  1412 . 
     When sufficient force is applied onto the load ring  1414 , the load ring  1414  disengages from the collet  1416  (as shown in  FIG. 35 ), thus enabling the collet  1416  to securely engage the capture structure  1420  of the bone screw  1410 , which inhibits additional movement between the receiver assembly  1412  and the bone screw  1411 . 
       FIGS. 37-40  illustrate an example of a spinal anchor assembly  1500  according still another embodiment of invention provisional locking assembly. The spinal anchor assembly  1500  includes a bone screw  1510  and a receiver assembly  1512  similar in functions and features to the embodiment of the spinal anchor assembly  1410 . Like features and functions will not be repeated here. The spinal anchor assembly  1500  differs from the spinal anchor assembly  10  in that it includes a load ring  1514  and a split ring  1516  (as shown in  FIGS. 39-40 ). 
     The loading ring  1514  snaps into the split ring  1516  and prevents the split ring  1516  from compressing on the bone screw  1510  and/or capture structure  1518  of the bone screw  1510 . This allows the polyaxial motion of the bone screw  1510  relative to the receiver assembly  1512  to be maintained even after reduction of the split ring  1516  into the receiver  1520 . Thus, the split ring  1516  can become securely engaged (or wedged) into the receiver  1520  while the bone screw  1510  is able to still articulate within the split ring  1516 . 
     To lock the receiver assembly  1512  relative to the bone screw  1511 , additional force is applied by engaging a capture structure  1518  (or lock screw) into the receiver assembly  1512 . As the capture structure  1518  is further engaged into the receiver  1514 , the bottom surface  71  of the closure structure  1518  is forced down upon the spring elements  1522  of the loading ring  1514  (best shown in  FIGS. 37-38 ). As the spring elements  1522  compress, they apply increasing force onto the split ring  1516 , thus permanently fixing the position of the bone screw  1510  relative to the receiver assembly  1512 . Furthermore, the bottom surface  71  of the capture structure  13  forces down upon the captured rod. Ultimately, the rod  60 , receiver assembly  1512 , and bone screw  1510  will be finally fixed relative to each other. 
       FIGS. 41 and 42  illustrate an example of a spinal anchor assembly  400  according to a seventh embodiment. The spinal anchor assembly  400  includes a bone screw  411 , a receiver assembly  412  having a collar  440  and a cradle  460 . The spinal anchor assembly  400  is largely similar to the spinal anchor assembly  300  and like features will not be further described herein. The spinal anchor assembly  400  differs from the spinal anchor assembly  300  in that it includes a close-topped receiver assembly  412 . The close topped receiver assembly  412  includes circular openings  427  to receive the rod. The rods slide through the circular shaped openings  427  and are locked with a closure structure. Any number of closure structures (including the closure structure  13  described herein) may be engaged into the receiver assembly  412  to secure the rod. Because the receiver assembly  412  is closed, the closure member may preferably be void of anti-splay features. 
       FIGS. 43-48  illustrate an example of an arched transverse connector  500  according to one example embodiment. The arched transverse connector  500  connects two rods that form a part of a spinal fixation assembly (for an example, refer to  FIG. 72 ) situated on either side of the spinal column. The arched transverse connector  500  includes a cross joint  502  with an eccentric pin  504 , and a pair of connector heads  506  located at the distal ends of connector arms  508 . The eccentric pin  504  in the cross joint  502  enables a user to simply lengthen and shorten the distance between the arched recesses  510  along the tubular joint collar  512 . Rotation of the eccentric pin  504  locks and unlocks the cross joint  502  to prohibit or allow translation of one connector arm relative to the other. 
     The transverse connector  500  is generally arched (as best viewed in  FIGS. 43 and 45 ) with the cross connector arms  508  following a radial arc. The generally arched shape of the transverse connector  500  avoids any unnecessary dural impingement when the arched transverse connector  500  is implanted. This is particularly important when the arched transverse connector  500  is assembled to a posterior spinal fixation assembly. By way of example only, the distance between the center of the arched recesses  510  may range approximately between 25-100 mm (and shown as dimension X in  FIG. 45 ). Preferably, the arched transverse connector  500  is composed of a metal (e.g. titanium, stainless steel, etc.), but may also be of a polymer (e.g. poly-ether-ether-ketone (PEEK)) or any other material suitable for the applications of the present invention. Additionally, the arched transverse connector  500  may be composed of a combination of both metal and polymer materials. 
     Each connector head  506  includes an eccentric pin  514  that is retained within a cavity  516  of the housing by a retaining c-ring  518 . The cavity  516  extends generally perpendicular from the front surface  520  of the connector head  506  to at least partially through the connector head  506 . The retaining c-ring  518  engages the circumferential step  522  on the outer surface of the eccentric pin  514  and the annular step  524  within the cavity  516 , which restricts longitudinal movement of the eccentric pin  514  relative to the cavity  516 . The engagement of the retaining c-ring  520  allows rotational movement of the eccentric pin  524  relative to the cavity  526 . A connector head  506  includes an arched recess  520  that is shaped and dimensioned to allow the secure placement of a rod (e.g. rod). By way of example, rotation of the eccentric pin  514  secures a portion of a rod within an arched recess, as will be described in greater detail below. 
     The surfaces within the arched recesses  510  provide frictional engagement to a rod when eccentric pins  514  engage the rod along their engagement surfaces  526 . Furthermore, the engagement surface  526  of the eccentric pins  514  and/or the surfaces within the arched recesses  510  may have surface features, or surface roughening, to enhance the frictional engagement between the arched transverse connector  500  and rods for enhanced security. By way of example, the eccentric pin  514  is shown as having a generally annular concavity to its engagement surface  526  (and best shown in  FIG. 48 ). The generally annular concavity allows for a greater surface area contact between the eccentric pin  514  and a generally cylindrical shaped rod. However, the engagement surface  526  of an eccentric pin  514  may be provided in a variety of shapes and dimensions necessary for providing optimal contact between a variety of shaped and sized rods, without departing from the scope of this invention. For example, pin  504  may be threadably received through an end of the connector arm  508 , such that as it is threaded into the connector arm, it squeezes the cross joint  502  to prohibit translation of the cross arms. 
     An eccentric pin  514  includes a top surface  528 , a bottom surface  530 , an engagement surface  526  and a positioning indicator  532 . A tooling engaging feature  534  is centrally positioned along the top surface  528 , which enables a variety of tools to engage the eccentric pin  514  and rotate it along its longitudinal axis (labeled as axis Y in  FIG. 48 ). At least one positioning indicator  532  indicates to the user the relative rotational positioning of the eccentric pin  514  relative to the connector head  506 . Additionally, at least one positioning indicator  536  may be on the front surface  520  of the connector head  506  to further assist the user in properly aligning the eccentric pin  514  relative to the connector head  506 , for example, to securely engage a rod. By way of example, if the user aligns the positioning indicators  532 ,  536  of the eccentric pin and the connector head  506  adjacent to each other, the eccentric pin  514  will be in the appropriate rotational position relative to the connector head  506  to securely engage an rod within the adjacent arched recess  510 . By way of further example, if the user rotates the eccentric pin  514  approximately 180 degrees from the previously described position (such that the positioning indicators  532  and  536  are aligned but not adjacent to each other), the eccentric pin  512  will be in the appropriate rotational position relative to the connector head  506  to release or accept an rod within the adjacent arched recess  510 . 
       FIGS. 46-48  further illustrate the eccentric pin  514 , which includes a positional stop  538  and a locking protuberance  540  that radially extend out along a portion of the circumference of the top surface  528 . The locking protuberance  540  locks the rotational positioning of the eccentric pin  514  once the locking protuberance  540  has passed a head notch  542  (and best shown in  FIG. 44 ). This helps prevent the eccentric pin  514  from unfavorably rotating and releasing the rod from the associated arched recess  510 . The positional stop  538  functions to limit the rotation of the eccentric pin  514  when the positional stop  538  comes into contact with the bumper  544 . 
     Furthermore, the eccentric pin  514  includes an annular recess  546  centrally located along the longitudinal axis (Y) and adjacent the bottom surface  530  of the eccentric pin  514 . The annular recess  546  functions to support the positioning of the bottom end of the eccentric pin  516  against the ledge  548  of the connector head  506 . Specifically, since the eccentric pin  506  is generally eccentric, the annular recess  546  is a non-eccentric feature which maintains the alignment of the eccentric pin  514  within the connector head  506 . Upon rotation of the eccentric pin  514  along its central axis within the connector head  506 , the eccentric body of the pin acts as a cam and forces the protruding side of the eccentric pin  515  (relative to its longitudinal axis Y) against an rod captured in the associated arched recess  510 . 
     Assembly of the arched transverse connector  500  to a pair of rods (i.e. rods) begins with a section of a first rod being captured within an unlocked first arched recess  510  and then a section of a second rod is captured within an unlocked second arched recess  510 . The arched transverse connector  500  is then operated by rotating the eccentric pins  514  into their locked positions to permanently secure the rods as described above. 
       FIGS. 49-50  illustrate another example embodiment of an arched transverse connector  600 . The arched transverse connector  600  is assembled during a surgical procedure to two rods and provides support between at least two rods that form a part of a spinal fixation assembly (for an example, refer to  FIG. 72 ). The arched transverse connector  600  includes an eccentric pin  602 , a pair of connector heads  604 , and connector bridge  606 . 
     The transverse connector  600  is generally arched (as best viewed in  FIG. 50 ) with the center axis of the connector bridge following a radial arc. The generally arched shape of the transverse connector  600  assists in avoiding any unnecessary dural impingement once the arched transverse connector  600  is implanted, particularly when the arched transverse connector  600  is assembled to a posterior spinal fixation assembly. Preferably, the arched transverse connector  600  is composed of a metal (e.g. titanium, stainless steel, etc.), but may also be of a polymer (e.g. poly-ether-ether-ketone (PEEK)) or any other material suitable for the applications of the present invention. Additionally, the arched transverse connector  600  may be composed of a combination of both metal and polymer materials. 
     This embodiment has minimal moving parts so that a clinician may optionally perform a minimal amount of adjustments to secure the arched transverse connector  600  to a pair of rods. Each connector head  604  includes an eccentric pin  602  that is retained within a cavity  608  of the housing by a retaining c-ring  610 . The cavity  610  extends generally perpendicular from the front surface  612  of the connector head  604  to at least partially through the connector head  604 . The retaining c-ring  610  and eccentric pin  623  have essentially the same features and functions as the retaining c-ring  518  and eccentric pin  514  of the first embodiment of the arched transverse connector  500 , such that their descriptions will not be repeated here. Similarly, the connector heads  604  (which include, for example, positioning indicators  614  and annular recess  616 ) have essentially the same features and functions as the connector heads  506  of the first embodiment of the arched transverse connector  500 , such that their descriptions will also not be repeated here. 
       FIGS. 51-55  illustrate another example embodiment of an arched transverse connector  700 . The arched transverse connector  700  is assembled during a surgical procedure to two rods (e.g. rods  60 ) and provides support between at least two rods that form a part of a spinal fixation assembly (for an example, refer to  FIG. 72 ). This embodiment of the arched transverse connector  700  enables the user to configure it in a multitude of configurations so that the arched transverse connector  700  may best accommodate the rods that it may attach to. The arched transverse connector  700  includes set screws  702 , a pair of housings  704 , connector arms  706 , and a set of securing blocks  708 . The arched transverse connector  700  has a number of adjustable connections, including; between the two connector arms  708 , and between the housings  706  and connector arms  708 . These adjustable connections enable various degrees of movement and linear translation of the arched transverse connector  700 , which will be discussed in more detail below. 
     The transverse connector  700  is generally arched (as best viewed in  FIGS. 51 and 52 ) with the center axis of the connector arms  706  following a radial arc. The generally arched shape of the transverse connector  700  assists in avoiding any unnecessary dural impingement once the arched transverse connector  700  is implanted. This is particularly the case when the arched transverse connector  700  is assembled to a posterior spinal fixation assembly. Preferably, the arched transverse connector  700  is composed of a metal (e.g. titanium, stainless steel, etc.), but may also be of a polymer (e.g. poly-ether-ether-ketone (PEEK)) or any other material suitable for the applications of the present invention. Additionally, the arched transverse connector  700  may be composed of a combination of both metal and polymer materials. 
     The housing  704  includes a partially threaded cavity  710  which extends generally at an angle from the front surface  712  of the housing  704  to at least partially through the housing  704 . The partially threaded cavity  710  provides at least one internal helical thread  714  for the engaging and advancing a set screw  702  into the housing  704 . The advancement of the set screw  702  into the housing  704  assists in binding the securing block  708  and a rod captured within the arched recess  716  of the housing  704 , as will be described in greater detail below. 
     Permanently securing the placement of rods within the arched recesses  716  involves further engagement of the set screws  702  into the housings  704  and associated securing blocks  708 . When a set screw  702  is further engaged into a housing  704  and associated securing block  708 , the securing block  708  binds a number of components within the arched transverse connector  700 . This ultimately results in the secure engagement of a rod within the arched recesses  716  and locking the configuration of the arched transverse connector  700 , which will be discussed in more detail below. Although the arched recesses  716  are shown as having an arched profile, any size and shaped recess suitable for securing any size and shaped rod can be implemented into the housing  704  without departing from the scope of the present invention. 
     At least one exterior helical thread  714  radially extends from the outer surface of the set screw  702  which allows the set screw  702  to engage and advance into the housing  704 . Upon advancement of the set screw  702  along its central axis into the housing  704  and associated securing block  708 , the securing block becomes bound into the housing  704 . The set screw  702  threadably engages the securing block  708  by way of the threaded through hole  718  of the securing block  708 . If a rod is captured within the arched recess  716 , the securing block also secures the rod within the arched recess  716 . The securing block includes a tongue  720  which at least partially functions to capture and securely engage a rod within the arched recess when the securing block  708  is bound to the housing  704 . Surface features on the engagement surfaces  722  of the securing blocks  708  and on the arched recesses  716  may be used to increase the frictional engagement between the securing blocks  708 , housing  704 , and rods. 
     Engagement of a set screw  702  into the housing  704  and associated securing block  708  also locks the configuration between the housing  704  and the adjacent connector arm  706 . The distal end of a connector arm  706  includes a spherical joint  724  and a keying feature  726 . The spherical joint  724  enables the connector arm  706  to articulate relative to the housing  704 . The keying feature  726  restricts the radial articulation of the housing  704  relative to the connector arm  706  so that there is not an unnecessary amount of articulation between the connector arm  706  and housing  704 . By way of example, the housing  704  is able to rotate approximately 20 degrees in both directions relative to the connector arm  706 . The keying feature  726  restricts the rotational movement by mating with the key slot  728  in the housing  706 , which provides limited space for the keying feature  726  to rotate. This provides a user with the ability to make relatively slight configuration adjustments between the housing  706  and connector arm  704 , while not making the adjustable connection too cumbersome for the user. 
     The connector arms  706  mate with each other at their medial ends, where a set screw  702  controls the locked and unlocked configuration between them. Both connector arms have features at their medial ends which function to enable the connector arms  706  to linearly translate relative to each other as well as slightly angle themselves relative to each other. The features which function to guide and limit the translation and angulation between the connector arms  706  may be any feature necessary to enable the configuration between the two connector arms  706  without departing from the scope of this invention. By way of example, a first connector arm may have a threaded hole  732  (shown best in  FIG. 53 ) that enables a connector arm set screw  702  to engage. The loosening and tightening of the set screw  702  would allow the second connector arm  706  to linearly translate to either expand or shorten the distance between the housings  704 . By way of example only, the distance between the center of the arched recesses  716  may range approximately between 40-75 mm (and shown as dimension X in  FIG. 51 ). 
     A slot  732  extending across at least part of the second connector arm  706  enables the relative linear translation between the two connector arms  706 , while also limiting their linear translation to the confines of the slot  732 . Additionally, the extruded guide  734  assists in maintaining alignment between the connector arms by mating with the slot  732 . The extruded guide  734  is not completely circular in cross section and, instead, includes flat extensions  736  (best shown in  FIG. 54 ) which limit the relative angulations between the two connector arms  706 . By way of example, the connector arms  706  are able to angle approximately 15 degrees in either direction about the center axis of the threaded hole  730 . This provides a user with the ability to make configuration adjustments between the housing  704  and connector arm  706 , while not making the adjustable connection too cumbersome for the user. Generally, the medial ends of both of the connector arms  706  have relatively flat surfaces in order to accommodate smooth linear translation between their mating features (i.e. extruded guide  734  and slot  732 ). 
     As best shown in  FIG. 53 , a split collar  738  is secured around the distal end of the connector arm  706 . The split collar  738  functions to enable the spherical joint  724  to articulate within the spherical socket  740  of the securing block  708  while maintaining a secure connection between the connector arm  706  and the housing  704 . Flanges  742  radially extending from the split collar  738  mate with an annular step  744  located in the inside walls of the side through hole  746  of the housing  704 . 
     As mentioned above, when the housing set screws  702  are engaged into their associated housings  704  and securing blocks  708 , the securing blocks  708  ultimately become fixed within the housings  704 . When a securing block  708  becomes fixed within a housing  704 , the securing block  708  also fixes the jointed configuration between the adjacent housing  704  and connector arm  706 . The further engagement of the set screw  702  into the securing block  708  causes the securing block  708  to bind into the housing cavity  748  which ultimately securely engages the distal end of the adjacent connector arm  706 . Ultimately, the securing block  708  forces against the distal end of the connector arm  706  (which pushes the connector arm  706  in a direction away from the housing  704 ) until the split collar  738  restricts any further movement of the connector arm  706 . The split collar  738  is confined to expanding only to the size of the annular step  744 , which is sized to restrict the split collar  738  from expanding enough to allow the distal end of the connector arm  706  from losing its connection with the housing  704 . By way of example, when the housing set screw  702  is generally fully engaged within the housing  704  and associated securing block  708 , the securing block  708  is forcing the permanent fixation of the connector arm  706  relative to the housing  704 . Furthermore, secure fixation of a securing block  708  within a housing  704  also secures a rod captured within the adjacent arched recess  716 , as described above. 
       FIGS. 56-64  illustrate an example of an embodiment of a reduction tower  900 . The reduction tower  900  comprises a proximal end  906  and a distal end  907  and includes a housing  901 , an interior shaft  902 , and a grasping element  910 . The housing  901  and interior shaft element  902  are both hollow to allow the passage of various tooling and/or parts through their centers, generally along their shared longitudinal axis. By way of example, a locking tool  1000  (as shown in  FIGS. 58 and 59 ) may be inserted through the center of the housing  901  and interior shaft  902  and attached at the proximal end of the housing  901  to form a tooling assembly  1100  (as shown by way of example in  FIGS. 60-62 ). This tooling assembly  1100  can be used to lock a bone screw assembly configuration (as described herein), and will be discussed in more detail below. A variety of tooling and parts may be combined with the reduction tower  900  to perform a variety of functions, as described below. Furthermore, the reduction tower  900  may also function to lock bone screw configurations without the addition of accessory tooling (i.e. locking tool  1000 ), which will also be discussed in more detail below. 
     The grasping element  910  of the reduction tower  900  includes a spring element  920 , a finger grip  911 , a latch  921 , and a grasping arm  922 . Grasping features  904  are located at the distal end of the grasping arm  922  and housing  901  and function to engage, for example, the receiver  20  (of receiver assembly  12 ) of a anchor assembly  10 . One advantageous use of the reduction tower  900  is the ability to lock any of the provisional locking screws without using a closure structure. 
     As described above, when the bone screw assembly  10  is secured to a bony structure in its unlocked position, the receiver assembly  12  can articulate freely relative to the bone screw  11 . After determining the necessary orientation of the receiver assembly  12  relative to the bone screw  11  to receive the rod and positioning the rod in the receiver, the clinician may use the reduction tower  900  to provisionally lock the bone screw assembly  10  orientation without using a closure member. 
     To provisionally lock the bone screw assembly  10 , for example, the clinician positions the open (unlocked) distal end  907  of the reduction tower  900  adjacent and generally concentric to the receiver assembly  12  of the spinal anchor assembly  10 . The user then advances the distal end  907  of the reduction tower  900  over at least a portion of the receiver assembly  12  and generally aligns the grasping features  904  of the reduction tower  900  with the tool engaging features (i.e. grip bores  21 ) of the receiver  20 . The clinician then locks the reduction tower  900  by compressing the grasping element  910  towards the housing  901 , for example, by pressing on the grasping arm  922  until the latch  921  engages the latch keeper  930 . When the latch  921  is fully engaged with the latch keeper  930 , the grasping features  904  can fully engage the tool engaging features of the receiver  20  (as illustrated in  FIGS. 58 and 59 ), thus securing the receiver  20  within the distal end  907  of the reduction tower  900 . 
     Once the reduction tower  900  is securely mated to the receiver assembly  12  of a spinal anchor  10 , the interior shaft  902  can be advanced in the direction of the distal end  907  of the reduction tower  900 . This can be accomplished in a number of different ways, such as, for example, by attaching a T-handle or a torquing tool (not shown) to the proximal end of the interior shaft  902  and forcing the interior shaft  902  to rotate along its longitudinal axis. At least a portion of the interior shaft has exterior threads  923  that engage interior threads  925  along at least a portion of the interior wall of the housing  901  (and best shown in  FIG. 62 ). This threaded engagement allows the interior shaft  902  to translate along its longitudinal axis relative to the housing  901  when the interior shaft  902  is rotated in either direction. 
     As depicted in  FIGS. 63-64 , in order to lock the configuration of the spinal anchor assembly  10 , the interior shaft  902  is advanced in the direction of the distal end  907  of the reduction tower  900 . The interior shaft  902  is advanced until the distal end of the interior shaft  902  is forcing down upon the rod (such as a rod  60 ) captured within the receiver assembly  12  (and best shown in  FIGS. 63 and 64 ). The distal end of the interior shaft  902  continues to force down upon the rod until the collet  40  has securely wedged itself into the receiver  20  such that the bone screw  11 , collet  40  and receiver  20  are no longer able to move independently of each other. As described above, when the collet  40  becomes securely wedged into the receiver  20 , the collet  40  compresses and secures the capture structure  16  of the bone screw  11  within its interior spherical surface  49 . By way of example, a feature within the reduction tower  900  may produce an audible indicator (i.e. a clicking sound) once the interior shaft  902  has advanced the necessary distance to lock the configuration of the spinal anchor assembly  10 . Any number of different mechanisms or features (i.e. visual markers, break-away torquing tool adapters) may be used to indicate to the user that the interior shaft  902  has advanced far enough so that the spinal anchor assembly  10  is now locked into its configuration, without departing from the scope of the present invention. 
     Once the interior shaft  902  has advanced the necessary distance to lock the configuration of the bone screw assembly  10 , the user may then advance the finger grip  911  to release the latch  921  from the latch keeper  930 , thus unlocking the grasping element  910  and releasing the engagement between the reduction tower  900  and receiver assembly  12 . The rod used to lock the configuration of the bone screw assembly  10  can then be removed from the receiver assembly  12 , as necessary, while the spinal anchor assembly  10  remains in the locked configuration. This allows the clinician to use the spinal anchor assembly  10 , for example, as a tool to assist in positioning the spine (i.e. de-rotation of the spine) and correcting spinal deformities. A rod may be secured within the receiver assembly  12  at a later time when the user is prepared to secure the positioning of the bone screw assembly  10  relative to an rod. 
     Optionally, and by way of example, a locking tool  1000  may be adapted to the reduction tower  900  to lock the spinal anchor assembly  10  into a desired configuration. The locking tool  1000  may be used instead of the distal end of the interior shaft  902  to lock the configuration of the bone screw assembly  10 . By way of example, the locking tool  1000  may be adapted to the reduction tower  900  by inserting its distal end into the proximal end  906  of the reduction tower  900  (through the hollow centers of the housing  901  and interior shaft  902 ). The distal end of the locking tool  1000  is advanced through the reduction tower  900  until the adapter  1002  of the locking tool  1000  is securely engaged to the proximal end  906  of the reduction tower  900 . The adapter  1002  includes two spring clips  1003  that allow the adapter  1002  to slide over the proximal end of the reduction tower  900  and engage the engaging ends  1006  of the spring clips  1003  into the tool locking features  912  at the proximal end of the housing  901 . The engaging ends  1006  of the spring clips  1003  mate with the tool locking features  912  of the housing  901  such that the locking tool  1000  is constrained from movement relative to the reduction tower  900 , both rotationally and along their shared longitudinal axis. 
     Once the locking tool  1000  is securely engaged at the proximal end  906  of the reduction tower  900 , a torquing tool (not shown), for example, can be adapted to the torque adapter  1004  at the proximal end of the tooling shaft  1005 . In order to lock the configuration of the spinal anchor assembly  10 , the tooling shaft  1005  is advanced in the direction of the distal end  907  of the reduction tower  900 . The tooling shaft  1005  is advanced by rotating the tooling shaft (i.e. by rotating the torque adapter  1004 ). The locking tool  1000  includes a threaded guide  1010  that functions to assist in the positioning of the tooling shaft  1005 . The tooling shaft  1005  also is partially threaded  1011  along a portion of its proximal end. Threaded engagement of the threaded guide  1010  with the tooling shaft  1005  enables the tooling shaft  1005  to linearly translate along its longitudinal axis when rotated along its longitudinal axis. 
     The tooling shaft  1005  is rotated and advanced toward the spinal anchor assembly  10  until at least a portion of the distal face  1007  is forcing down upon a rod captured within the receiver assembly  12 . The distal face  1007  of the tooling shaft  1005  continues to force down upon the rod (which subsequently forced down upon the collet  40 ) until the collet  40  has securely wedged itself into the receiver  20  such that the bone screw  11 , collet  40  and receiver  20  are no longer able to move independently of each other. Any number of different mechanisms or features (i.e. visual markers, break-away torquing tool adapters) may be used to indicate to the user that the tooling shaft  1005  has advanced far enough so that the bone screw assembly  10  is now locked into its configuration, without departing from the scope of the present invention. 
     Furthermore, the distal end of the locking tool  1000  is shaped and dimensioned such that it may advance into a receiver assembly  12  and lock the configuration of the associated bone screw assembly  10  without a rod captured within the receiver assembly  12 . This is accomplished generally similar to the steps for locking the configuration of the bone screw assembly  10 , as described above, but instead of the distal face  1007  of the tooling shaft  1005  forcing down on a rod, the distal face  1007  forces down upon generally the top surface  41  of the collet  40 . As described above, the tooling shaft  1005  is advanced until the collet  40  is securely wedged within the receiver  20 , thus locking the configuration of the bone screw assembly  10 . 
     The tooling assembly  1100  may be disassembled by compressing the spring clips  1003 , thus disengaging the engaging ends  1006  of the spring clips  1003  from the locking tool  1000 . The locking tool  1000  may be removed from the tool locking features  912  at the proximal end of the housing  901 . This releases the rotational and translational fixation between the locking tool  1000  and the reduction tower  900  so that the locking tool  1000  can slide out and away from the reduction tower  900 . The locking tool  1000  may be assembled to the reduction tower  900  before, during, or after the reduction tower  900  becomes securely engaged to a receiver assembly  12 . Furthermore, the reduction tower  900  may adapt and remove any number of various tooling throughout its use without departing from the scope of this invention. 
     When necessary, the receiver  12  may be released from the distal end  907  of the reduction tower  900  by advancing the finger grip  911  towards the distal end. A spring  931  is housed within the finger grip  911  which forces the finger grip  911  back to its original position once the user is no longer forcing it towards the distal end of the reduction tower  900 . Although shown as a spring  931 , any number of features may be associated with the finger grip  911  and/or latch  930  to allow the user to release the grasping element  910  from its locked position, without departing from the scope of this invention. 
     The grasping arm  922  is spring loaded by means of a cantilever spring  920 , but may be spring loaded using any number of elements that force the grasping arm  922  back to its original unlocked position. Although a cantilever spring  920  is shown in this example, any number of features or mechanisms may be used to assist in controlling the movement and placement of the grasping arm  922  without departing from the scope of this invention. 
       FIGS. 65-66  illustrate an example of a second embodiment of a reduction tower  1200 . The reduction tower  1200  includes a housing  1201 , an interior shaft  1202  and a grasping element  1210 . This second embodiment of a reduction tower  1200  is essentially the same in features and functions as the first embodiment of a reduction tower  900 , as described in detail above. Therefore, a repeated discussion of the similar features and functions of the reduction tower  1200  will not be repeated here. However, the reduction tower  1200  presented herein includes a release button  1203 . The release button  1203  functions to position a partially threaded  1204  feature relative to the interior shaft  1202 . The positioning of the partially threaded  1204  feature enables either a guided or free linear translation of the interior shaft  1202 . By way of example, engaging the release button  1203  disengages the partially threaded  1204  feature from its threaded engagement with the interior shaft  1202 . This enables the interior shaft  1202  to freely and quickly linearly translate along its longitudinal axis relative to the housing  1201 . This is desirable for quick positioning of the interior shaft  1202  relative to the housing  1201 . By way of further example, disengagement of the release button  1203  engages the partially threaded  1204  feature to the exterior threads on the outer wall of the interior shaft  1202 . This threaded engagement limits the linear translation of the interior shaft  1202  relative to the housing  1201  to only when the interior shaft  1202  is being rotated along its longitudinal axis. 
     For instances in which de-rotation of one or more vertebral bodies is desired, a reduction tower link  1300  is also provided. In accordance with a preferred embodiment of the present invention, engaging the reduction tower link  1300  to two or more reduction towers  900  allows derotation of all of the vertebrae together via a ratcheting mechanism. As shown in  FIGS. 67-71 , the reduction tower link  1300  includes a stationary arm  1302 , a moving arm  1304 , and a ratchet mechanism  1306 . Stationary arm  1302  further comprises a terminal groove  1308  and a receiving aperture  1310  at either end. The terminal grooves  1308  are sized and dimensioned to receive a ratchet pawl  1312 . Ratchet pawl  1310  is spring-loaded  1328  to engage the moving arm  1304  (or ratchet arm). The aperture  1310  is sized and dimensioned to receive the ratchet post  1322  as described below. Additionally, the inner face  1330  of the stationary arm  1302  is comprised of a softer, malleable material (including, but not limited to, silicone) to provide leeway as reduction tower link  1300  interacts with the reduction towers  900 . 
     The moving arm (or ratchet arm)  1304  also contains terminal grooves  1314  and a receiving aperture  1316  at either end as well as a malleable inner face  1318 . The terminal grooves  1314  which are sized and dimensioned to receive a final lock nut  1320 . As the final lock  1320  is turned, it draws the ratchet post  1322  through its receiving aperture  1316 . The inner face  1318  is also comprised of a softer, malleable material (including, but not limited to, silicone) to provide leeway as the reduction tower link  1300  interacts with the reduction towers  900 . 
     Ratcheting mechanism  1306  contains an inner ratchet post  1322  and an outer portion comprised of an outer cylinder  1324  and an inner cylinder  1326 . 
     As depicted in  FIGS. 69 and 71 , squeezing the arms  1302 ,  1304  together causes the ratchet  1306 , moving arm  1304 , and inner cylinder  1326  to move towards the stationary arm  1302  as indicated as arrow A. The spring-loaded ratchet pawl  1312  prevents release of the ratcheting mechanism  1306 . The final lock nuts  1320  may be tightened to achieve final positioning of the reduction tower link  1300 . Pushing the ratchet pawl  1312  disengages the spring-loaded mechanism and releases the arms  1302 ,  1304 . 
     According to a second embodiment, instead of a ratcheting mechanism, a cam may be used to bring the stationary arm  1302  and moving arm  1304  together around the reduction towers  900 . A knob on each arm  1302 ,  1304  may be used to adjust the distance between the bars  1302 ,  1304  in addition to the travel created by the cams. 
     An example surgical procedure is described below for use with, for example, the anchor systems and related tools described herein. The surgical procedure described herein is not intended to be exhaustive, such that additional steps that are not discussed herein may be incorporated to the procedure without departing from the intended scope. 
     The procedure begins, in pertinent part, by placing a spinal anchor (e.g. bone screw  11 ) into each of a plurality of pedicles. For each pedicle, the desired entry point is located and the cortex is perforated using an awl or burr. Next, a pilot hole is created by passing, for example, a narrow or lumbar gearshift prove through the pedicle and into the vertebral body. Care should be taken to ensure the instrument(s) do not breach the cortical wall of the pedicle, as the pilot hole will ultimately determine the final position of the screw. The pilot hole should be inspected for perforations by using a ball-tip probe to palpate the pedicle wall on all sides. Pedicle markers may also be placed into the pilot holes followed by lateral and anterior-posterior imaging to verify proper positioning. 
     In patients with dense bone or where tapping is preferred, depth gauging may be accomplished using the markings on the instrument shaft in conjunction with fluoroscopy. If depth gauging is performed, the ball-tip probe should again be used to inspect the pilot hole for perforation. After the appropriate screw length is determined, a screwdriver is used to drive the screw into the pilot hole and advance it until the desired depth is reached. A screw adjuster may be used if subsequent x-ray or fluoroscopy indicate that screw depth adjustment is necessary. 
     After the spinal anchors (e.g. bone screw  11 ) and receiver assemblies (e.g. receiver assembly  12 ) are secured to the desired pedicles, the rods  60  are prepared for placement. The systems described herein include an array of straight and pre-bent (lordosed) rods. Measurements are taken to determine the appropriate rod lengths using a rod template. The corresponding straight or pre-bent rod from the implant tray may then be selected and additional contouring may be performed as needed with any one of an array of rod benders: (e.g., French benders, in-situ sagittal benders, in-situ coronal benders, and plate style benders). 
     A rod holder may then be used to sequentially insert the rod  60  into each of the receiver assemblies (e.g. receiver assemblies  12 ) until the rod  60  is lying at the bottom of all of the receiver assemblies  12 . With a portion of a rod  60  fully seated in the receiver assemblies  12 , capture structures (e.g. capture structure  16 ) are engaged into the receiver assemblies  12 . By way of example, the clinician aligns the recessed slot  75  of the closure structure  13  with the recessed slot  24  of the receiver (e.g. receiver  20 ). The alignment of the recessed slots  24 ,  75  prevents incorrect engagement of the interior and exterior guide and advancement structures. Alternately, a lock screw starter guide may be used to capture the receiver assembly  20  (or closure structure  13 ) followed by introduction of the lock screw starter. 
     If the rod  60  is difficult to fully seat in the receiver assembly  120 , the rod  60  may be reduced using, for example, a rocker, persuader, or reduction tower  900 ,  1200 . When only a small amount of reduction is required, the rocker or the persuader is preferably utilized. Using the rocker, the receiver assembly  12  is grasped via the oval grip bores  21  on either side of the receiver assembly  20 . The rocker may then be deflected downward until the spinal anchor assembly  10  is levered up and the rod  60  is fully seated into position within the receiver assembly  12 . A closure structure  13  may then be inserted using a lock screw starter. Alternatively, a persuader may be used. To do so, the tip of the persuader is slid over the top of the receiver assembly  12 . To reduce the rod  60 , downward force is applied to the persuader while compressing the handle ratchet closed. Once the rod  60  is fully seated in the receiver assembly  12 , a lock screw starter may be used to place the closure structure  13 . The persuader may be disengaged from the receiver assembly  12  by releasing the ratchet and pulling up on the persuader. 
     A reduction tower  900  is preferably used whenever a large amount of reduction is required. Prior to using the reduction tower  900 , the grasping element  910  should be in the open position. To use the reduction tower  900 , the distal end  907  is placed over the rod  60  and around the receiver assembly  112  so that the grasping elements  910  rest on the capture structure  16  and the oval grip bores  21  are aligned. The receiver assembly  112  may be grasped via the grasping features  904  on the grasping element  910  by slowly closing the grasping element  910  towards the housing  901  as described above. With the reduction tower  900  securely engaged with the screw  11 , the T-handle attached to the proximal end  906  of the reduction tower  900  may be slowly turned until the rod  60  is fully seated in the capture structure  16 . A lock screw starter may be used to insert a closure structure  13  through the interior of the reduction tower  900  to hold the implant  10  in position. Following placement of the closure structure  13 , the lock screw starter may be removed, the reduction tower grasping arm  922  may be released and removed from the spinal anchor assembly  10 . 
     According to one aspect, rod rotation and vertebral derotation techniques may be utilized to correct coronal and rotary deformities in the spine. The system of the present invention offers a rod rotation wrench, a reduction tower  900 ,  1200 , and derotation guides (lock screw guides) to perform these operative techniques. In connection with the reduction tower  900 ,  1200  or a derotation guide, derotation of the spine may be achieved by using these instruments as an extended moment arm to rotate the vertebral bodies in the axial plane. 
     With the rod  60  fully reduced into the receiver assemblies  12  and the closure structures  13  are placed loosely, the rod  60  may be rotated into its desired position. A rod gripper may be placed over the rod  60  and its handle compressed to achieve rigid fixation. Two rod grippers may be used to transform the coronal deformity into kyophosis or lordosis within the sagittal plane. After the rod  60  has been rotated into the desired position, the closure structures  13  may be tightened. 
     Rods  60  may be rotated using a rod rotation wrench. The rod rotation wrench may be placed over the hex at the end of the rod  60  and rotated to the desired amount. Vertebral body rotation may be accomplished via the uniplanar, fixed, or provisional locking screws of the present invention. To apply rotational forces to the uniplanar or fixed screws, the lock screw guide is slid over the capture structure  16  of the screw  10 . The guide may then be moved in the medial-lateral direction to rotate the vertebral body in the axial plane. 
     To apply rotational forces to a provisional locking screw, the provisional locking screw must first be locked into a fixed position, preferably using the reduction tower  900 . To lock the provisional locking screw, the reduction tower  900  must be rigidly engaged to the capture structure  16  with the rod  60  fully reduced. To do so, a counter-torque instrument may be slid into the tool locking features  912  on the cranial/caudal sides of the reduction tower  900 . Next, the locking tool  1000  is inserted into the center of the reduction tower  900  The T-handle attached to the proximal end  906  of the reduction tower  900  may be turned in a clockwise direction until the breakaway torque is achieved. The provisional locking tool may be removed as described above. 
     With the provisional locking screw in a fixed position, the reduction tower  900  is leveraged in a medial/lateral direction to rotate the vertebral body in the axial plane. Once the amount of de-rotation is achieved, a closure structure  13  may be inserted (preferably using a lock screw starter) and provisionally tightened to hold the rod  60  in a fixed orientation. To minimize the chances of pedicle fracture during vertebral body de-rotation, it is preferable to spread the rotational forces over a series of adjacent pedicle screws. This is accomplished via the reduction tower link  1300  as described above with reference to  FIGS. 67-71 . 
     The reduction tower link  1300  is placed over two or more reduction towers  900  such that the reduction towers  900  are positioned between the inner face  1312  of the stationary arm  1302  and the inner face  1318  of the moving arm  1304 . Next, the arms  1302 ,  1304  are squeezed together via the ratcheting mechanism  1306  until the desired amount of vertebral body de-rotation has been achieved. The final lock nuts  1320  may then be tightened to achieve the final positioning of the reduction tower link  1300 . Pushing the ratchet pawls  1312  disengages the arms  1302 ,  1304  so that the reduction tower link  1300  may be removed from the reduction towers  900 . 
     If compression or distraction is desired, the closure structures  13  on one side of the motion segment should be tightened, leaving the other closure structure  13  loose to allow movement along the rod  60 . The compressor or distractor may be placed over the rod  60  and against the capture structures  16  of both spinal anchor assemblies  10 . With the compressor or distractor properly engaged, the desired amount of compression or distraction may be imparted upon the rods and the second closure structure  13  may be provisionally tightened to hold the construct in position prior to final tightening of the entire construct. 
     Once the necessary reduction, de-rotation, compression, and/or distraction is achieved, the entire construct is tightened. Beginning with the cephalad screw, the counter-torque is placed over the closure structure  13  until the slots at the distal end of the instrument are completely seated over the rod  60 . With a torque T-handle engaged, the lock screw driver is inserted through the counter-torque until it is securely seated in the closure structure  13 . Final tightening may then be delivered and these steps may be repeated on the remaining screws. 
     Next, fixed  600  and adjustable length  500 ,  700  transverse connectors may be placed to provide torsional stability to the construct. The appropriate length transverse connector is determined preferably by measuring the distance between the rods  60  using measurement calipers. If necessary, transverse connector benders can be used to make fine adjustments to the length of the fixed transverse connectors  600 . 
     According to the embodiment illustrated in  FIG. 23 , eccentric pins  514  (locking cams) are used to secure the transverse cross connector  500 . Prior to inserting the transverse cross connector  500 , the eccentric pins  514  should be in the fully open position. To open them, a transverse connector driver is used to rotate both eccentric pins  514  until the positioning indicators  532  are positioned toward the center of the transverse cross connector  500 . 
     With the transverse connector holder still attached, the transverse connector  500  is placed over the rods. Once the connector  500  is seated on both rods  60 , a transverse connector driver is used to turn the eccentric pins  514  180 degrees until the positioning indicators  532  on the eccentric pins  514  face laterally and align with the positioning indicator  536 . The eccentric pins  514  are then fully locked to the rods  60 . 
     A distractor may be placed over the transverse connector  500  such that it engages the medial aspect of the retaining c-ring  518 . Final locking is performed by distracting each ring  518  laterally until it is fully seated as described above. 
     While not specifically described above, it will be understood that various other steps may be performed in using and implanting the devices disclosed herein, including but not limited to creating an incision in a patient&#39;s skin, distracting and retracting tissue to establish an operative corridor to the surgical target site, advancing the implant through the operative corridor to the surgical target site, removing instrumentation from the operative corridor upon insertion of the implant, and closing the surgical wound. Furthermore, procedures described, for example only, may be applied to any region of the spine without departing from the scope of the present invention and dimensioning of the implant may be adjusted to accommodate any region. 
     While this invention has been described in terms of a best mode for achieving this invention&#39;s objectives, it will be appreciated by those skilled in the art that variations may be accomplished in view of these teachings without deviating from the spirit or scope of the invention. 
     Although described with respect to specific examples of the different embodiments, any feature of the spinal anchor system disclosed herein by way of example only may be applied to any of the embodiments without departing from the scope of the present invention. Furthermore, procedures described, for example only, involving specific regions of the spine (e.g. thoracic and lumbar) may be applied to another region of the spine without departing from the scope of the present invention and dimensioning of the implant may be adjusted to accommodate any region.