Abstract:
A method of using a spine stabilization system in one embodiment includes inserting a bone fastener shank through a receiver structure cavity and then through a distal opening of the receiver member, positioning a head of the fastener against a bearing surface of the receiver structure, positioning a bearing member on an upper portion of the head, positioning a first pivot bearing portion of a pivot member above the positioned bearing member, positioning a pivot portion of a connector assembly on the positioned first pivot bearing portion, positioning a second pivot bearing portion of the pivot member on an upper portion of the pivot portion, and threading a fixation screw into a threaded portion of the receiver structure, thereby (i) causing the pivot member to clamp the pivot portion, and (ii) clamping the head of the screw between the first bearing and the bearing surface.

Description:
[0001]    This application is a divisional of co-pending U.S. application Ser. No. 11/646,877 filed Dec. 28, 2006, the disclosure of which is hereby incorporated herein by reference in its entirety, and related to U.S. patent application Ser. No. 11/646,961, entitled “Spinal Anchoring Screw”, which was filed on Dec. 28, 2006. 
     
    
     FIELD 
       [0002]    This application relates to the field of spinal stabilization devices. In particular, this application relates to a method for using a posterior stabilization unit configured for use with a segmental unit of the spine. 
       BACKGROUND 
       [0003]    Spinal surgeries are commonly used in the medical profession to treat spinal conditions that result when functional segmental units of the spine are moved out of proper position or otherwise damaged. Examples of procedures used to treat spinal conditions include disc replacement, laminectomy, and spinal fusion. 
         [0004]    Following certain spinal procedures, such as spinal fusion, it is typically desirable to stabilize the spine by preventing movement between the vertebrae while the spine heals. This act of stabilizing the spine by holding bones in place during healing has greatly improved the success rate of spinal fusions and other procedures. 
         [0005]    With spinal stabilization procedures, a combination of metal screws and rods creates a solid “brace” that holds the vertebrae in place. These devices are intended to stop movement from occurring between the vertebrae. These metal devices give more stability to the fusion site and allow the patient to be out of bed much sooner. 
         [0006]    During the spinal stabilization procedure, pedicle screws are placed through the pedicles on the posterior portion of two or more vertebrae of the spinal column. The screws grab into the bone of the vertebral bodies, giving them a good solid hold on the vertebrae. Once the screws are placed on the vertebrae, they are attached to metal rods that connect all the screws together. When everything is bolted together and tightened, the assembly creates a stiff metal frame that holds the vertebrae still so that healing can occur. 
         [0007]    Posterior dynamic stabilization (PDS) generally refers to such a stabilization procedure where dynamic rods are positioned between the pedicle screws. These dynamic rods can generally bend, extend, compress, or otherwise deform in order to allow some limited movement between the pedicle screws. By allowing this limited movement between the pedicle screws and the associated vertebrae, less strain is placed on adjoining, non-stabilized functional segmental units during patient movements. In addition, the dynamic rod generally decreases the stresses on the screw shank, minimizing the possibility of screw backout or related screw failures. However, even with dynamic rods, stresses are experienced by the screw shank which could potentially result in screw backout or related failures under the appropriate circumstances. Accordingly, it would be desirable to provide a PDS system capable of further protecting the screw-bone interface and reducing the chances of screw backout. For example, it would be advantageous to provide a PDS system with a flexible stabilization element that offers different kinematics and loading requirements from those stabilization elements found in the prior art. Such a stabilization element would offer additional options to the surgeon when traditional PDS stabilization elements appear problematic. 
       SUMMARY 
       [0008]    Various embodiments of a dynamic screw for a spine stabilization system are disclosed herein. A dynamic screw for a spine stabilization system comprises at least one bone anchor assembly comprising a bone engaging member and a receiver member. The bone engaging member may comprise a bone screw including a screw head retained within the receiver member and a screw shank extending from the receiver member. The screw head may be pivotably retained within the receiver member. An elongated connecting member is pivotably connected to the bone engaging member. The elongated connecting member may be provided as a rod spanning between two or more bone anchor assemblies. The elongated connecting member is pivotably connected to the receiver member of the bone anchor assembly. 
         [0009]    In one embodiment, the pivotable connection between the elongated connection member and the receiver member is provided by a ball-shaped pivot member on the rod which engages a bearing surface provided within a cavity of the receiver member. Accordingly, the pivot point for the rod may be provided within the cavity in the receiver member. In one such embodiment, the rod may define an axis wherein the axis pivots about a pivot point on the axis when the rod pivots relative to the receiver member. In other embodiments, the pivot point of the rod is offset from the axis defined by the rod. 
         [0010]    The rod may be a fixed length or adjustable to accommodate different segmental units and patients of different sizes. In the adjustable embodiment, the rod comprises a shaft with a flexible central portion and at least one adjustable end. The adjustable end may be provided by various means. For example, the adjustable end may include a post configured to slide within the shaft of the rod. In one embodiment, the adjustable end is configured to threadedly engage the shaft. In another embodiment, the adjustable end is comprised of a shape memory alloy. 
         [0011]    When assembled, the spine stabilization system generally comprises at least two bone anchors with a rod extending between the two bone anchors. As mentioned above, each bone anchor includes a bone screw and a receiver member configured to retain the bone screw. The rod extends between the two receiver members. In one embodiment where the rod is fixed relative to the receiver members, the rod is adapted to bend when the receiver members move relative to one another. In another embodiment, the rod is pivotably connected to both the receiver members, and the rod is adapted to extend or compress when the receiver members move relative to one another. 
         [0012]    In an alternative embodiment, one or more bone anchors of the spine stabilization system include an insert in the form of a retention member that acts to lock a bearing for the bone screw within the receiver member. To this end, the receiver member includes a screw head cavity and a rod cavity with an insert positioned between the screw head cavity and the rod cavity. The screw head cavity is configured to receive a bearing that engages the head of the bone screw with the screw shank extending from the receiver member. In one embodiment, the bone screw bearing is a split bearing. The insert is positioned between the rod cavity and the bearing member and is configured to secure the split bearing within the receiver member. The insert may be provided to fit within a groove formed in an interior sidewall of the receiver member. In this embodiment, the insert comprises a retaining ring that secures the split bearing within the screw cavity. In another embodiment, the insert is comprised of a compressible material positioned between the bearing member and the rod cavity. When the rod is positioned in the rod cavity, the insert is compressed against the bearing member, thus locking the bearing member within the screw cavity. 
         [0013]    In yet another embodiment, the bone anchor assembly is configured with a low profile, wherein the rod is locked within the receiver member without the use of a fixation screw. In this embodiment, the bone anchor assembly includes a head and a screw shank extending from the head. The screw shank is pivotable with respect to the head. Furthermore, a rod cavity is formed within the head. The end of the rod includes features that lock the rod within the rod cavity when the rod is inserted into the rod cavity, thus connecting the rod to the head. For example, in one embodiment, the end of the rod comprises a plurality of fingers that may be flared to lock the rod within the rod cavity. The rod may also include a plurality of teeth that grasp or mesh with the rod cavity to further secure the rod within the cavity. 
         [0014]    In a further embodiment, a method of using a spine stabilization system in one embodiment includes inserting a bone fastener shank through a receiver structure cavity and then through a distal opening of the receiver member, positioning a head of the fastener against a bearing surface of the receiver structure, positioning a bearing member on an upper portion of the head, positioning a first pivot bearing portion of a pivot member above the positioned bearing member, positioning a pivot portion of a connector assembly on the positioned first pivot bearing portion, positioning a second pivot bearing portion of the pivot member on an upper portion of the pivot portion, and threading a fixation screw into a threaded portion of the receiver structure, thereby (i) causing the pivot member to clamp the pivot portion, and (ii) clamping the head of the screw between the first bearing and the bearing surface. 
         [0015]    The above described features and advantages, as well as others, will become more readily apparent to those of ordinary skill in the art by reference to the following detailed description and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  shows a posterior view of a spine stabilization system with a plurality of dynamic screws and dynamic rods connected between two vertebrae; 
           [0017]      FIG. 2  shows a side view of the spine stabilization system of  FIG. 1 ; 
           [0018]      FIG. 3A  shows a cross-sectional view of a bone anchor and rod which form part of the spine stabilization system of  FIG. 1 ; 
           [0019]      FIG. 3B  shows an exploded perspective view of the bone anchor and rod of  FIG. 3A ; 
           [0020]      FIG. 3C  shows a perspective view of a retainer insert of  FIG. 3B ; 
           [0021]      FIG. 3D  shows a top view of the retainer insert of  FIG. 3C ; 
           [0022]      FIG. 4  shows a cross-sectional view of an alternative embodiment of the bone anchor and rod of  FIG. 3 ; 
           [0023]      FIG. 5  shows a perspective view of an alternative embodiment of the bone anchor and rod of  FIG. 3  wherein the pivot point of the rod is offset from the central axis of the rod; 
           [0024]      FIG. 6  shows a cross-sectional view of the bone anchor and rod of  FIG. 5 ; 
           [0025]      FIG. 7  shows an alternative embodiment of the bone anchor and rod of  FIG. 5 ; 
           [0026]      FIG. 8  shows a cross-sectional view of an alternative embodiment of the bone anchor and rod of  FIG. 3  wherein the pivot point of the rod is provided on the central axis of the rod; 
           [0027]      FIG. 9  shows another cross-sectional view of the bone anchor and rod of  FIG. 8  rotated 90°; 
           [0028]      FIG. 10  shows a perspective view of an alternative embodiment of the bone anchor of  FIG. 8 ; 
           [0029]      FIG. 11  shows a perspective view of another alternative embodiment of the bone anchor of  FIG. 8 ; 
           [0030]      FIG. 12  shows a perspective view of another alternative embodiment of the bone anchor of  FIG. 8 ; 
           [0031]      FIG. 13  shows a cross-sectional view of the bone anchor of  FIG. 12 ; 
           [0032]      FIG. 14  shows a perspective view of yet another alternative embodiment of the bone anchor and rod of  FIG. 8 ; 
           [0033]      FIG. 15A  shows a dynamic rod for use with the bone anchor of  FIGS. 8-13 , wherein the dynamic rod includes ball shaped members on its ends; 
           [0034]      FIG. 15B  shows an alternative embodiment of the dynamic rod of  FIG. 15A  wherein the length of the rod is adjustable; 
           [0035]      FIG. 15C  shows another alternative embodiment of the dynamic rod of  FIG. 15A  wherein the length of the rod is adjustable; 
           [0036]      FIG. 15D  shows yet another alternative embodiment of the dynamic rod of  FIG. 15A  wherein the length of the rod is adjustable; 
           [0037]      FIG. 15E  shows another alternative embodiment of the dynamic rod of  FIG. 15A  wherein the length of the rod is adjustable; 
           [0038]      FIG. 15F  shows yet another alternative embodiment of the dynamic rod of  FIG. 15A  wherein the length of the rod is adjustable; 
           [0039]      FIG. 16  shows a perspective view of an alternative embodiment of a bone anchor and rod for use with the spine stabilization system of  FIG. 1  wherein the rod is secured to a cavity in the bone anchor without the use of a fixation screw; 
           [0040]      FIG. 17  shows a cross-sectional view of the bone anchor and rod of  FIG. 16 ; and 
           [0041]      FIG. 18  shows a cross-sectional view of the bone anchor and rod of  FIG. 17  rotated 90°. 
       
    
    
     DESCRIPTION 
       [0042]    With reference to  FIGS. 1 and 2 , an exemplary posterior dynamic stabilization (PDS) system  22  is shown arranged between two vertebrae  20 ,  21  of a spine. The PDS system  22  comprises a plurality of bone anchors  24  with a plurality of elongated connecting members  26  extending between the bone anchors  24 . The plurality of connecting members  26  may comprise rods, bars, or other elongated connecting members. Each bone anchor  24  is secured to the pedicle of one of the vertebrae  20  or  21 . Each elongated connecting member  26  extends between a first bone anchor fixed to an upper vertebra  20  and a second bone anchor fixed to a lower vertebra  21 . 
         [0043]    The bone anchor  24  is comprised of titanium, stainless steel, or other appropriate biocompatible material. As explained in further detail herein, each bone anchor  24  comprises a bone engaging member  34 , such as a bone screw (as shown in  FIG. 3 , for example). However, one of skill in the art will recognize that other bone engaging members  34  are possible, such as posts, pins, cemented surfaces, adhesive surfaces and other bone engaging members as are known in the art. 
         [0044]    In addition to the bone engaging member  34 , each bone anchor  24  also comprises a receiver member  40 . The receiver member  40  is configured to receive a bone engaging member  34  and/or an elongated connecting member  26 . If the bone engaging member  34  is a bone screw, the bone screw  34  includes a screw head  36  and a screw shank  38 . The screw head  36  is retained within the receiver member  40  and the screw shank  38  extends from the receiver member  40 . The screw shank  38  is configured to screw into the bone and secure the bone screw  34  to the pedicle or other portion of bone. The receiver member  40  may be rigidly or pivotably connected to the screw  34 . 
         [0045]    The receiver member  40  is also configured to receive an elongated connecting member, such as the rod  26 . The rod  26  includes two rigid ends  30 ,  32  with an elastic/resilient central portion  28  disposed between the rod ends. The elastic central portion  28  allows for some limited flexibility in the rod, while still allowing the rod to spring back to its original shape. Therefore, when opposing forces are applied to the ends  30 ,  32  of the rod  26 , the central portion flexes, allowing the rod to bend and/or elongate. When the opposing forces are removed, the rod returns to its original shape. With this configuration, the PDS system generally stabilizes two adjacent vertebrae, while still allowing for some limited movement between the vertebrae  20 ,  21 . However, one of skill in the art will recognize that other types of rods are possible, including rigid rods or other flexible rods comprised of elastomeric material, metal, or superelastic material, or other types of PDS rods as are known in the art. 
         [0046]    With reference now to  FIGS. 3A-3D , one embodiment of a bone anchor assembly  24  is shown. In this embodiment, each bone anchor assembly  24  comprises a bone engaging member  34  retained within a receiver member  40 . The bone engaging member is provided in the form of a bone screw  34  (which is also referred to herein as a “pedicle screw”). The bone screw  34  comprises a screw head  36  and a screw shank  38 . The screw head  36  is generally spherical in shape with a flat top  39 . A slot  37  is formed in the top of the screw head  36 . The slot  37  is configured to receive the tip of a screwdriver that may be used to drive the screw  34  into the bone. The screw shank  38  extends from the screw head  36 . The screw shank  38  is threaded to facilitate driving the screw into the bone. 
         [0047]    In the embodiment of  FIGS. 3A-3D , the receiver member  40  is a generally cup-shaped structure configured to hold both the screw  34  and the rod  26 . The receiver member  40  comprises cylindrical sidewalls  42  formed between a superior end  44  and an inferior end  46 . A bone screw cavity  48  is formed within the sidewalls  42  near the inferior end  46 . A fixation screw cavity  50  is formed within the sidewalls  42  near the superior end  44 . A rod cavity and passage  52  is formed in the receiver member between the fixation screw cavity  50  and the bone screw cavity  48 . 
         [0048]    The fixation screw cavity  50  is designed and dimensioned to receive a fixation screw  70  (also referred to herein as a setscrew). Accordingly, the cylindrical sidewalls  42  of the receiver member are threaded at the superior end  44 . These threads are configured to engage the threads on the fixation screw  70 . The fixation screw includes a slot  72  in the top that is adapted to receive the tip of a screwdriver, thus allowing the fixation screw  70  to be driven into the fixation screw cavity  50 . 
         [0049]    The rod passage  52  is provided directly below the fixation screw cavity  50 . The rod passage is designed and dimensioned to receive one of the dynamic rods  26  of the PDS system  22 . In particular, the rod passage  52  is designed to receive one of the rod ends  30 . In the embodiment of  FIG. 3 , the rod is loaded into the rod passage from the top of the receiver member by laying the rod within U-shaped dips formed in the superior end  44  of the receiver member  40 . After the rod  26  is positioned in the rod passage  52 , a fixation screw is driven into the fixation screw cavity until it contacts the rod. When the fixation screw it tightened, it locks the rod in place within the receiver member  40 . One of skill in the art will recognize that other appropriate locking features such as cam locks may be used to hold the rod in place. 
         [0050]    The bone screw cavity  48  is designed and dimensioned to retain the screw head  36  of the bone screw  34 , with the shank  38  of the bone screw extending from the receiver member  40 . An opening  56  is formed in the inferior end  46  of the receiver member  40 . In this disclosed embodiment, the diameter of the opening  56  is smaller than the diameter of the screw head  36 , but it is large enough to allow the screw shank  38  to pass through the opening  56 . Accordingly, the cylindrical wall  42  is slightly thicker at the inferior end  46  of the receiver member  40 . 
         [0051]    A bearing member  54  is positioned within the bone screw cavity  48  along with the screw head  36 . The bearing member  54  includes an inner bearing surface that generally conforms to the spherical shape of the screw head  36 . The screw head  36  is configured to rotate and pivot within the bearing member  54 . The outer bearing surface is designed and dimensioned to engage the interior portion of the cylindrical sidewalls  42  of the receiver member. 
         [0052]    In one embodiment, the bearing member  54  is a split bearing that includes a left side member  54   a  and a right side member  54   b.  The split bearing,  54   a,    54   b  provides for easier assembly by allowing the bearing surface to be assembled around the spherical screw head  36 . In addition, the split bearing members  54   a,    54   b  facilitate the use of different bearing materials. Appropriate bearing materials will be recognized by those of skill in the art. In the embodiment of  FIGS. 3A and 3B , the bearing members  54   a,    54   b  are comprised of ceramic. Examples of other types of appropriate bearing materials include cobalt chrome, UHMWPE, and other biocompatible materials. 
         [0053]    An insert  60  is provided in the receiver member. The insert  60  acts as a retention member to secure the bearing member  54  in place within the bone screw cavity  48  of the receiver member  40 . In the embodiment of  FIGS. 3A-3D , the insert  60  is C-shaped plate that serves as a retaining ring. As best seen in  FIGS. 3C and 3D , the insert  60  includes a semi-circular wall  64  with a void  65  formed in the wall. Two opposing ends  66   a  and  66   b  define the sides of the void  65 . The exterior perimeter  67  of the insert  60  is generally circular in shape, while the interior perimeter  68  is contoured to provide strength to the insert. In addition, the insert may include other structural features such as holes  69 . The insert  60  is generally comprised of a resilient biocompatible material, such as cobalt chrome or UHMWPE. The resilient features of the insert  60  allow the ends  66   a,    66   b  to be forced together, reducing the size of the void  65 , and then spring back to their original position. 
         [0054]    As shown in  FIG. 3A , the insert  60  is provided within a groove  62  formed in the cylindrical sidewalls  42  of the receiver member  40 . With reference to the exploded view of the anchor assembly  24  shown  FIG. 3B , it can be seen that the insert  60  is loaded into the retainer member  40  through a hole  50  in the top of the retainer member. First, the split bearing members  54   a,    54   b  are positioned about the head of the screw  38  and the screw is inserted into the receiver member  40 . Upon insertion, the split bearing members  54   a,    54   b  and screw head  36  are seated in the screw head cavity and the shank  38  extends through the hole in the bottom of the receiver member  40 . Next, the insert  60  is compressed and inserted into the receiver member  40 . When properly positioned, the resilient insert snaps into the groove  62  in the receiver member, thus locking the split bearing members  54   a,    54   b  in place within the retainer member. With the insert  60  locked in the groove  62 , the bearing member  54  is secured in place within the receiver member such that various stresses on the bone screw will not dislodge the bearing member within the anchor assembly  24 . After insertion of the insert  60 , the rod  26  is placed in the rod passage  53  of the receiver member and the fixation screw  70  is threaded in the fixation screw cavity  50  until it compresses against the rod, thus fixing the rod to the receiver member  40 . 
         [0055]      FIG. 4  shows an alternative embodiment of a bone anchor assembly including an insert for securing the bearing  54  within the receiver member  40 . In this embodiment, the insert  60  comprises a polyethylene disc positioned between the rod  26  and the bearing  54 . Before the fixation screw is tightened, the top surface of the polyethylene disc  60  is positioned within the rod cavity  52 . Thus, when the fixation screw  70  is tightened against the rod  26 , the polyethylene insert  60  is slightly compressed by the rod. The force of this compression is then transferred to the bearing member  54 , which is tightly compressed within the bone screw cavity  48 , thus securing the bearing in place. Although  FIGS. 3 and 4  show only two methods for holding the bearing  54  in place within the receiver member  40 , one of skill in the art will recognize that variations of the disclosed embodiments may be easily incorporated. For example, in one embodiment, a combination retaining ring and compression disc may be used. 
         [0056]    Rod Fixed to Receiver Member Providing With Pivot Point Offset From Rod Axis 
         [0057]    From  FIGS. 3 and 4 , it can be seen that an offset exists between the center axis of the rod and the pivot point of the rod  26  within the anchor assembly  24 . In particular, as shown in  FIG. 4 , the center axis  80  of the rod (shown by dotted line  80 ) is removed from the pivot point (shown by “X”  82 ) of the rod within the anchor assembly  24 . This offset provides one embodiment that may be used to help control the necessary kinematics and loading requirements of the rod. In these embodiments, the rod  26  is fixed to the anchor assembly, and is not allowed to pivot relative to the receiver member  40  which holds the bone screw  34 . 
         [0058]    An alternative embodiment of a bone anchor  24  where the center axis of the rod is offset from the pivot point of the rod within the anchor assembly is shown in  FIGS. 5 and 6 . In this embodiment, the anchor assembly includes a bone screw  34 , a U-shaped screw holder  86 , and a rod holder  88 . The bone screw includes a threaded shank  38 , but instead of a spherical head, the head  36  of the bone screw is flat and generally circular or disc-shaped. This flat screw head is designed and dimensioned to fit within a circular cavity formed in the base  90  of the U-shaped screw holder  86 . The circular cavity  87  allows the head  36  to rotate within the cavity  87  about the axis of the screw. A pivot pin  94  extends through the upright portions  92  of the U-shaped screw holder  86 . 
         [0059]    The rod holder  88  is pivotably mounted on the pivot pin  94 . The rod holder  88  is similar to the receiver member  40  described in  FIGS. 3 and 4 . However, in place of a screw cavity, the rod holder  88  of  FIGS. 5 and 6  includes a pin channel  95  configured to receive the pivot pin  94 . The rod holder  88  is allowed to rotate about the pivot pin  94 , thus allowing the rod holder  88  to pivot relative to the U-shaped screw holder  86 . A rod passage  52  is formed in the rod holder  88  above the pin channel  95 . A fixation screw  70  threadedly engages the interior threaded walls on the top of the rod holder  88 . When the fixation screw  70  is tightened against the rod, the rod is pinned in place within the rod holder  88 . 
         [0060]    In the embodiment of  FIGS. 5 and 6 , the rod is allowed only two degrees of freedom. First, the rod  26  is allowed to pivot by radial rotation around an axis defined by the screw shank  38  by virtue of the rotatable engagement between the screw head  36  and the circular cavity  87  of the U-shaped screw holder  86 . Second, the rod  26  is allowed to pivot about the pin  94  which is perpendicular to the screw shank. To facilitate rotation of the screw head  36  and the pin  94  within the U-shaped screw holder, the U-shaped screw holder may be comprised of ultra high molecular weight polyethylene (UHMWPE), cobalt chrome, titanium, stainless steel or other appropriate biocompatible bearing material as will be recognized by those of skill in the art. 
         [0061]    Another alternative embodiment of a bone anchor  24  where the center axis of the rod is offset from the pivot point of the rod is shown in  FIG. 7 . The bone anchor  24  of  FIG. 7  includes a receiver member in the form of a screw holding member  100  that is fixed to the shank  38  of the bone screw  34 . The rod  26  is secured to a rod holding member  102  which includes a cavity that receives the rod  26 . The rod holding member  102  includes a fixation screw  70  that clamps onto the rod in order to fix to the rod holding member  102  to the rod  26 . The rod holding member further includes a ball-shaped pivot member (shown by dotted lines  104  within the screw holding member  100 ). In this embodiment, the screw holding member  100  includes a cavity with a spherical bearing  106  and bearing surface that is also fixed relative to the screw shank  38 . The spherical bearing surface is configured to receive the pivot member  104  which is fixed to the rod  26 . Because the surface of the pivot member  104  is congruent with the bearing surface, the pivot member  104  is allowed to pivot within the screw holding member  100 . Accordingly, the rod  26  is configured to pivot relative to the shank  38 . The pivot point for the rod  26  is defined at the center of the pivot member  104  which is located within the center of the cavity in the screw holding member  100 . 
         [0062]    Rod Pivotably Connected To Receiver Member With Pivot Point on Rod Axis 
         [0063]    With reference now to  FIGS. 8-9 , an alternative embodiment of a bone anchor  24  for a PDS system is shown where the rod  26  is pivotably connected to the receiver member  40  of the bone anchor. The bone anchor  24  includes a bone screw  34  having a screw head  36  retained within the receiver member  40  with the screw shank  38  extending from the receiver member  40 . 
         [0064]    Two different bearings are retained within the receiver member  40 . In particular, a first bearing  110  provides a bearing surface for the screw head. The first bearing acts to stabilize the screw head  36  within the receiver member  40  while providing a surface upon which the screw head may pivot relative to the receiver member  40 . In one embodiment, the first bearing may be comprised of a metallic insert that acts to lock the bone screw  34  in place when a fixation screw is tightened, as discussed in further detail below. 
         [0065]    In addition to the first bearing  110 , a second bearing  112  is also provided within the receiver member  40  shown in  FIGS. 8 and 9 . The second bearing  112  provides spherical bearing surface for the rod  26 , allowing the rod  26  to pivot relative to the receiver member  40 . Accordingly, the rod  26  includes a pivot member  114  in the form of a spherical ball fixed on at least one end of the rod  26 . The spherical ball  114  engages the spherical bearing surface of the second bearing  112 , thus pivotably retaining the rod  26  within the receiver member  40  and facilitating smooth movement of the rod relative to the receiver member. In this embodiment, the pivot member  114  is fixed to the rod  26 , being integrally formed upon the rod. 
         [0066]    In the embodiment disclosed in  FIGS. 8 and 9 , the second bearing  112  is a split bearing that includes a superior bearing member  116  provided above the spherical ball  114  and an inferior bearing member  118  provided below the spherical ball  114 . In another alternative embodiment, the bearing is split into left and right halves such as the bearing shown in  FIG. 3B . The split bearing  112  is comprised of UHMWPE, ceramic, cobalt chrome, or any other biocompatible material. In one alternative embodiment, the first bearing  110  and the inferior bearing member  118  of the second bearing  112  may be provided as a single integral component. 
         [0067]    The components of the anchor assembly  24  may all be loaded into the receiver member  40  through a top hole. First, the bone screw  34  is inserted into the receiver member  40  with the screw head  36  seated in the screw head cavity and the shank  38  extending through the hole in the bottom of the receiver member  40 . Second, the first bearing  110  is placed over the screw head. Next, the inferior bearing member  118  of the second bearing  112  is placed on top of the first bearing  110 . The rod  26  is then placed in the receiver member with the spherical ball  114  engaging the bearing surface of the inferior bearing member  118 , and the cylindrical portion of the rod passing through the rod passage formed in the sidewalls of the receiver member. The superior bearing member  116  is then placed over the spherical ball  114 . This provides a superior bearing surface for the spherical ball. Finally, the fixation screw  70  is threaded into the top of the receiver member until it compresses against the second bearing member. Alternatively, the bearing components, screw, and rod may be pre-assembled and inserted into the receiver member as a unit. 
         [0068]    In the embodiment of  FIGS. 8 and 9 , the anchor  24  acts as polyaxial screw that can be locked down by the metal insert  110  that is tightened by the fixation screw  70  when the screw head  36  is in the desired position. The fixation screw  70  functions to lock the bone screw  34  and to slightly compress the second bearing  112 , thus keeping the second bearing in place within the receiver member  40 . 
         [0069]    In the embodiment of  FIGS. 8 and 9 , it can also be seen that the rod  26  is configured to pivot relative to the receiver member  40 . Accordingly, as shown in  FIG. 8 , the pivot point  82  which the rod  26  pivots about is located on an axis defined by the rod and extending along the rod, such as a central axis  80  or an axis extending axially through the rod or along the surface of the elongated rod  26 . In the case of  FIG. 8 , the axis is the central axis  80  of the rod. Because of this, the rod is constrained to motion in the axial direction. In other words, in this embodiment, the dynamic central portion of the rod is elongated or compressed, but is not bent when the receiver member  40  moves. Thus, for a given PDS assembly of two bone anchors and a rod, when the vertebrae move the bone screws  34 , the receiver members  40  also move along with the bone screws. Because the rod  26  is allowed to pivot relative to the receiver members  40  about pivot point  82 , movement of the receiver members  40  imparts axial forces on the rod  26  that cause the rod to either compress or elongate. Advantageously, this arrangement offers different kinematics and loading requirements from those stabilization elements where the pivot point is offset from an axis defined by the rod. These differing kinematics and loading requirements may be advantageous with certain materials and designs or with certain patients. 
         [0070]    One alternative embodiment to that of  FIGS. 8 and 9  involves the use of a setscrew nested in the fixation screw, allowing the polyaxial screw to be locked separate from the compression of the bearing surface. Furthermore, although there is a specific shape and locking of the bearing surface shown in  FIG. 8 , this could be altered based on materials used and the constraints of the rod. Of course one of skill in the art will recognize that numerous other adaptations of the embodiment of  FIGS. 8 and 9  are possible where the pivot point of the rod is located along the central or other axis of the rod. 
         [0071]    Another example of an alternative embodiment for the bone anchor of  FIGS. 8 and 9  is shown in  FIG. 10 .  FIG. 10  shows an embodiment of a bone anchor  24  which acts as a fixed screw instead of a polyaxial screw. In particular, in  FIG. 10 , the screw shank  38  is fixed to the receiver member  40 . In this embodiment, the screw shank  38  may be integrally formed with the receiver member  40  such that the receiver member  40  serves as the bone screw head. Alternatively, the screw shank  38  may be otherwise fixed to the receiver member  40  using some locking mechanism or other connection means. In the embodiment of  FIG. 10 , the inferior portion  118  of the bearing member  112  is first placed in the cavity  120  formed in the receiver member  40 . The ball shaped portion of the rod  26  is then loaded onto the inferior bearing surface and the superior bearing member  116  is placed on top of the rod within the cavity. Finally, the fixation screw  70  is used to secure the bearing  112  within the cavity  120  of the receiver member  40 . 
         [0072]      FIG. 11  shows another embodiment, similar to  FIG. 10 , where the screw shank is fixed to the receiver member  40 , and the screw head is formed as the receiver member  40 . In  FIG. 11 , the receiver member  40  is formed as a block  130  with a central cavity  132 . A bearing member  134  with a toroidal bearing surface  136  is positioned within the cavity  132  of the receiver member  40 . Because the receiver member  40  is fixed relative to the screw shank  38 , the bearing  134  is also fixed relative to the screw shank  38 . The toroidal bearing surface is configured to receive a spherical portion on the end of a rod, similar to the rod end in  FIGS. 8 and 9  that includes a spherical ball  114 . Engagement of the spherical ball  114  and the toroidal bearing surface  136  allows the rod  26  to pivot relative to the shank  38  of the bone screw  38 . In this embodiment, the bearing  134  is shown as being UHMWPE and as being held in place by a press fit. However, one of skill in the art will recognize that numerous other viable bearing materials and locking mechanisms may be used. Similar to the embodiments of  FIGS. 8-10 , the bone anchor disclosed in  FIG. 11  provides an arrangement where the pivot point of the rod is located along the central axis of the rod. 
         [0073]      FIGS. 12 and 13  show another alternative embodiment similar to  FIG. 11 . However, in the embodiment of  FIGS. 12 and 13 , the bearing  134  is not fixed relative to the screw shank  38 . Instead, the bone anchor  24  acts as a polyaxial screw, and the bone screw head  36  and shank  38  are connected to the block  130 /receiver member  40  in a pivotable relationship. Like the bone screw  34 , the bearing  134  is loaded in the top of the receiver member  40 , and a set screw or other locking member  71  holds the bearing  134  in place within the receiver member  40 . Although the bone anchor acts as a polyaxial screw, the bone screw  34  can be locked in place relative to the block  130  when the locking member  71  is tightened within the block. Accordingly, a metal insert  138  may be provided around the bearing  134 . When the locking member  71  is tightened, the metal insert  138  is locked into the screw head  36 , fixing the bone screw relative to the block  130 . One of skill in the art will recognize that various alternative versions of the embodiments of  FIGS. 11-13  are possible. For example, it will be recognized that a dual setscrew could be used and that although the bearing surface is shown as a solid piece, it could be split to allow for easier assembly and to facilitate the use of other materials. 
         [0074]    Yet another embodiment of a bone anchor  24  where the pivot point of the rod is located along the central axis of the rod is shown in  FIG. 14 . The embodiment of  FIG. 14  is very similar to that of  FIG. 11 , but in  FIG. 14  the block  130  and bearing  134  is provided on the rod  26  rather than the screw shank  38 . Likewise, a spherical ball  115  is provided on the screw shank  138  rather than on the rod  26 . The spherical ball  115  engages the bearing  134 , allowing the rod  26  to pivot relative to the bone screw  34 . In this embodiment, the bearing  134  is shown as being UHMWPE and as being held in place by a press fit. However, one of skill in the art will recognize that numerous other viable bearing materials and locking mechanisms may be used. 
         [0075]      FIGS. 15A-15F  show six possible designs for an adjustable length rod that could be used with the designs of  FIGS. 8-13  where the pivot point of the rod is provided along the center axis of the rod. As mentioned above, adjustable length rods are advantageous when providing a PDS system so that different sized systems may be constructed for segmental units of different sizes and patients of different sizes. Accordingly, the rods of  FIGS. 15A-15F  may be used to provide an adjustable PDS system comprising: a plurality of bone anchors; and at least one connecting member connected to and extending between the plurality of bone anchors, wherein the at least one connecting member is adjustable in length. In one embodiment, the at least one connecting member is fixedly connected to the plurality of bone anchors. In another embodiment, the at least one connecting member is pivotably connected to the plurality of bone anchors. In other embodiments, the adjustable connecting member is provided as a telescoping shaft with two or more portions that slide relative to one another and may be locked to one another. In another embodiment, the adjustable connecting member comprises a shaft with a threaded ball on the end that can be turned to effectively lengthen or shorten the connecting member. These and other embodiments are shown in  FIGS. 15A-15F . The embodiments of  FIGS. 15A-15F  show rods with helical dynamic portions provided in the center of the rod. However, it is intended that the embodiments disclosed herein could be used with any dynamic element, and not just helical dynamic portions. 
         [0076]      FIG. 15A  shows a basic rod  26  that generally comprises a shaft with a flexible elastic central portion  28 , a first end  30 , and a second end  32 . Ball-shaped members  114  are provided on the first end  30  and second end  32  of the rod  26 . The ball-shaped members are substantially spherical in the disclosed embodiment and are configured to engage the bearing surface of the rod bearing  112  retained within the bone anchor  24 . Exemplary bone anchors  24  configured to retain rod bearings for use with rods having ball-shaped ends are disclosed in  FIGS. 8-13 . In  FIG. 15A , the ball shaped members  114  are formed integral with the rod in  FIG. 15A . To this end, the ball-shaped members  114  may be molded as a single piece with the central dynamic portion  28  of the rod. Alternatively, the ball-shaped members  114  may be fixed to the dynamic portion  28  by other means, such as welding, adhesion, or other appropriate methods as will be recognized by those of skill in the art. In other alternative embodiments, the ball-shaped members  114  may be releasably connected to the dynamic portion  28 . For example, the ball shaped members  114  may be screwed, snapped, or friction fit onto the rod  26  at the rod ends  30 ,  32 . Those of skill in the art will recognize various other possibilities for securing the ball shaped members on the rod. In this embodiment, where the ball shaped members  114  are fixed relative to the dynamic portion  28 , the rod  26  may be provided in numerous discrete lengths to accommodate size differences between different patients and/or different segmental units of the spine. 
         [0077]    In an alternative embodiment, the ball shaped members  114  of the rod may be adjustably connected to the rod. With this arrangement, a single rod may be used to accommodate various size differences between patients and/or segmental units. Examples of rods  26  where the ball shaped member  114  is adjustable relative to the dynamic portion  28  are shown in  FIGS. 15B-15F . 
         [0078]    In  FIG. 15B , the ball-shaped members  114  are provided on posts  140 . The posts  140  fit within the rod shaft, and particularly within a mouth  142  formed on the rod ends  30 ,  32 . Each mouth  142  includes an upper jaw  144  and a lower jaw  146  that taper outwardly from the central axis of the rod. A passage is formed between the upper jaw  144  and the lower jaw that accepts one of the posts  140 . A locking ring  148  is provided on each rod end  30 ,  32 . When the locking ring  148  is moved over the mouth  142 , the upper jaw  144  and lower jaw  146  of the mouth are forced together, thus compressing the post  140  within the mouth  142  and locking the associated ball member  114  on the end of the rod  26 . Because the posts  140  and associated ball members  114  are slideable relative to the central dynamic portion  28  of the rod  26 , the size of the rod may be adjusted to various lengths to accommodate different segmental units of the spine and patients of different sizes. 
         [0079]    In  FIGS. 15C and 15D , the ball-shaped members  114  are taper-locked to a cap member  150  whose position can be adjusted to the desired length. In both of the embodiments of  15 C and  15 D, the cap member  150  includes a frusto-conical portion  152  that is inserted into a cavity  160  in the ball member  114  to taper-lock the ball member  114  to the cap member  150 . Of course, in the embodiments of  FIGS. 15C and 15D , another fastening means different from a taper-lock could be used to attach the ball member  114  to the cap member  150 . In both embodiments of  FIGS. 15C and 15D , the cap member  150  is secured to the rod using a setscrew. In the embodiment of  FIG. 15C , the cap member fits over the cylinder of the rod, and a screw hole  154  is formed in the cap member  150 . In the embodiment of  FIG. 15D , a post  156  is inserted within the rod cylinder and a screw hole  158  is formed in the rod  26 . Again, with both  FIGS. 15C and 15D , because the ball members  114  are slideable relative to the central dynamic portion  28  of the rod  26 , the size of the rod may be adjusted to various lengths to accommodate different segmental units of the spine and patients of different sizes. 
         [0080]    In the embodiment of  FIG. 15E  spaced teeth  160  are provided on the rod ends  30 ,  32 . Interlocking teeth  162  are also provided on the inside of the ball members  114 . The teeth  160 ,  162  are provided with slight tapers such that the teeth  160  on the rod cylinder interact with the teeth  162  on the inside of the ball members  114 . Depending on which set of spaced teeth  160 ,  162  are used, the length of the rod can be adjusted and fixed using a simple turn. 
         [0081]    In the embodiment of  FIG. 15F , the rod ends are comprised of a shape memory alloy (also referred to as “smart metals” or “memory metals”), such as nickel-titanium (NiTi), copper-zinc-aluminum, or copper-aluminum-nickel. Shape memory alloys exhibit temperature dependent memory properties which may be advantageously used to lock the ball members  114  on the ends  30 ,  32  of the rod  26 . In the embodiment of  FIG. 15F , the ends  30 ,  32  of the rod include nested cups  166  comprised of a shape memory alloy. A slit  164  formed through the cups  166  along the end of the rod. Associated grooves are provided on the inside of the ball members. The ball members  114  are free to slide on the cups  166  on the rod ends  30 ,  32  at room temperature. However, at body temperature, the cups  166  splay outward, thus locking the cups  166  into the grooves on the inside of the ball member and securing the ball members in place. 
         [0082]    Low Profile Design 
         [0083]      FIGS. 16-18  show an alternative embodiment configured for use with any of the above-described designs where the pivot point of the rod is offset from the central or other axis defined by the rod (e.g.,  FIGS. 3-7 ). The advantage addressed in the embodiment of  FIGS. 16-18  is that of a dynamic screw with a lower profile. In this embodiment, the lower portion of the bone anchor  24  is similar to that of  FIGS. 3 and 4 , and includes a bone screw cavity  48  configured to receive a bearing member  48  and the head  36  of a bone screw  34 . An insert  60  is provided above the bearing member  54  that locks the bearing member in the cavity  48 . Also similar to  FIGS. 3 and 4 , a rod cavity/passage  52  is provided above the insert. However, unlike  FIGS. 3 and 4 , no fixation screw is provided above the rod  26 . Instead, the rod  26  and bone anchor  24  include features that allow the rod  26  to be locked into the rod cavity  52  without the use of a fixation screw. 
         [0084]    In the exemplary embodiment of  FIGS. 16-18  the locking features provided on the rod  26  include fingers  170  provided on the rod ends with slits  172  cut into each rod end between the fingers  170 . Teeth  174  are also provided on the rod ends. 
         [0085]    The slits  172  allow the fingers  170  to contract toward each other as the end of the rod is forced into the rod cavity  52 . Once the rod is in the cavity  52 , the fingers  170  are flared back outwardly toward or past their original configuration. When the fingers are forced outwardly, they are pressed against the insert  60 , thus locking the rod in place within the rod cavity. Flaring of the fingers may be achieved through the use of a memory metal or by other means, such as a wedge forced into the slits at the end of the rod. With the rod in place within the rod cavity  52 , the rod presses against the insert  60  and receiver member  40 , which locks the bearing  54  in place within the bone anchor  24 . If the insert  60  is comprised of a relatively soft material, the teeth  174  may cut into the insert to assist in securing the rod within the bone anchor. In one alternative embodiment, the teeth  174  are designed to mate with complimentary teeth on insert  60  and receiver member  40  to assist in securing the rod within the bone anchor. 
         [0086]    In one embodiment, a cap is provided over the superior end  44  of the bone anchor. This may be desirable if the rod will be passed through tissue. The cap could either be permanent or temporary. As best seen in  FIG. 18 , the distance across the rod cavity  52  generally decreases when moving from the center of the rod cavity toward the superior end  44 . This decreased distance at the superior end  44  is less than the diameter of the rod  26 , and helps in preventing passage of the rod through the top of the receiver member  40 . However, if a cap were provided over the superior end  44  it could be used to further assist in retaining the rod  26  within the receiver member  40 . Furthermore, if this embodiment were used in an minimally-invasive surgery procedure, where it would be more difficult to assure that the receiver members  40  are aligned in the correct configuration to properly engage and lock down the rod, the cap could mate with a feature on the screw head in such a way that it would insure that the heads are placed correctly and that the receiver member is properly secured. 
         [0087]    Although the present invention has been described with respect to certain preferred embodiments, it will be appreciated by those of skill in the art that other implementations and adaptations are possible. For example, although the invention has been disclosed for use with reference to a single segmental spine unit, it could also be adapted for use with multi-level constructions. As another example, the dynamic rods disclosed herein include a helical flexible portion, but different dynamic rods may be used in other embodiments. As yet another example, the connection of the rod to the bone anchor may vary from those embodiments disclosed herein. Moreover, there are advantages to individual advancements described herein that may be obtained without incorporating other aspects described above. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred embodiments contained herein.