Abstract:
A posterior dynamic spinal stabilization device utilizing a spinal rod, a fixed anchor, a mobile anchor adapted for both translation and pivoting, a mobile anchor-limiting stop affixed to the rod and an optional bumper-type, a rod with at least one limiting stop. It allows predetermined and prescribed interpedicular motion. It allows the use of traditional pedicle screw-rod placement techniques in a flexible system possessing many treatment options using familiar components that are familiar to the surgeon, and can be used on multiple levels (especially to top off) and can transition loads by limiting displacements.

Description:
CONTINUING DATA 
     This patent application claims priority from pending U.S. Provisional Patent Application No. 61/220,341, filed Jun. 25, 2009, entitled “Posterior Dynamic Stabilization Device Having a Mobile Anchor” (DEP-6238USPSP), the specification of which is incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The vertebrae in a patient&#39;s spinal column are linked to one another by the disc and the facet joints, which control movement of the vertebrae relative to one another. Each vertebra has a pair of articulating surfaces located on the left side, and a pair of articulating surfaces located on the right side, and each pair includes a superior articular surface, which faces upward, and an inferior articular surface, which faces downward. Together the superior and inferior articular surfaces of adjacent vertebra form a facet joint. Facet joints are synovial joints, which means that each joint is surrounded by a capsule of connective tissue and produces a fluid to nourish and lubricate the joint. The joint surfaces are coated with cartilage allowing the joints to move or articulate relative to one another. 
     Diseased, degenerated, impaired, or otherwise painful facet joints and/or discs can require surgery to restore function to the three joint complex. Damaged, diseased levels in the spine were traditionally fused to one another. While such a technique may relieve pain, it effectively prevents motion between at least two vertebrae. As a result, additional stress may be applied to the adjoining levels, thereby potentially leading to further damage. 
     More recently, techniques have been developed to restore normal function to the facet joints. One such technique involves covering the facet joint with a cap to preserve the bony and articular structure. Capping techniques, however, are limited in use as they will not remove the source of the pain in osteoarthritic joints. Caps are also disadvantageous as they must be available in a variety of sizes and shapes to accommodate the wide variability in the anatomical morphology of the facets. Caps also have a tendency to loosen over time, potentially resulting in additional damage to the joint and/or the bone support structure containing the cap. 
     Other techniques for restoring the normal function to the posterior element involve arch replacement, in which superior and inferior prosthetic arches are implanted to extend across the vertebra typically between the spinous process. The arches can articulate relative to one another to replace the articulating function of the facet joints. One drawback of current articulating facet replacement devices, however, is that they require the facet joints to be resected. Moreover, alignment of the articulating surfaces with one another can be challenging. 
     Accordingly, there remains a need for improved systems and methods that are adapted to mimic the natural function of the facet joints. 
     Traditional spine fusion may result in early degeneration at adjacent spine levels due to increased loading and compensation. This may result in subsequent surgeries to fuse additional levels. Stabilization using more dynamic rods with traditional pedicle screw instrumentation may improve surgical outcomes and reduce additional surgeries for adjacent level degeneration. 
     PCT Patent Application WO02/17803 (Rivard I) and U.S. Pat. No. 6,554,831 (Rivard II) disclose a mobile dynamic implantable spinal apparatus designed with an aim towards treating scoliosis. Rivard I and II disclose a system including at least one correcting rod, at least one fixed bracket, at least one mobile carrier. The correcting rod is described as a scoliosis rod. Scoliosis rods are constructed of metallic materials, such as CoCr, Ti, and stainless steel. Such rods must be manufactured from very stiff materials in an effort to maintain correction. 
     U.S. Pat. No. 5,672,175 (Martin) discloses a dynamic spinal orthotic implant that preserves some of the physiologic spinal motion. The construct also includes a rod that is used to correct a lateral deviation or scoliosis curve. 
     US Patent Publication 2004/0049190 (Biedereman I) discloses a dynamic system utilizing two bone anchors, a rod, and a spring. One of the anchors is fixed to the rod, one anchor slides with respect to the rod, and the spring between them exerts a force from the fixed anchor to the sliding anchor. 
     US Patent Publication 2007/0233085 (Biedermann II) discloses a dynamic stabilization device utilizing a rod that is elastomeric and stretches rather than slides. 
     U.S. Pat. No. 7,326,210 (Jahng) discloses a dynamic stabilization device using a rod, elastomers, and a sliding sleeve. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a posterior dynamic spinal stabilization device utilizing a spinal rod, a fixed anchor, a mobile anchor adapted for both translation and pivoting, a mobile anchor-limiting stop affixed to the rod and an optional bumper-type, a rod with at least one limiting stop. 
     In accordance with the present invention, there is provided a posterior dynamic spinal stabilization system comprising:
         a) a rod having a first end and a second end,   b) a mobile anchor having i) a shank and ii) a rod-receiving portion mated to the first end of the rod, wherein the rod-receiving portion translates along the rod and pivots about the shank,   c) a fixed anchor having i) a shank and ii) a rod-receiving portion locked to the second end of the rod.       

     The present invention provides a PDS device that allows predetermined and prescribed interpedicular motion. It is the understanding of the present inventor that this is the first PDS device of its kind to limit such motion by displacement of a mobile element (and not by load upon a static element). The PDS device of the present invention can also be made to be compatible with minimally invasive surgery (MIS) techniques. It allows the use of traditional pedicle screw-rod placement techniques in a flexible system, thereby enabling many different treatment options, using components that are familiar to the surgeon. The PDS device of the present invention intends to preserve desirable amounts of physiologic motion and can be used on multiple levels (especially to top off), to transition loads by limiting displacements. 
     The present invention contemplates a PDS device for motion applications. In this sense, it is different than certain motion devices used in scoliosis applications, such as those of Martin and Rivard, in which the device allows growth and limited motion along the longitudinal axis of the rod. Whereas Martin and Rivard contemplate a correcting scoliosis rod, the present invention contemplates mobile anchor PDS systems for treating stenosis, degenerative spondylolisthesis, and degenerative disc disease. In topping off applications, the present invention can be used as a protection device adjacent to a fusion to prevent overloading of the healthy discs. In some instances the device may be used at more than one level, thereby allowing the device to absorb some of the load and prevent adjacent disc disease. 
     Therefore, in accordance with the present invention, there is provided a method of treating a spine of a patient having iatrogenic instability, comprising the steps of:
         i) attaching to the spine a posterior dynamic spinal stabilization system comprising:
           a) a rod having a first end and a second end,   b) a mobile anchor having i) a shank and ii) a rod-receiving portion mated to the first end of the rod, wherein the rod-receiving portion translates along the rod,   c) a fixed anchor having i) a shank and ii) a rod-receiving portion locked to the second end of the rod,   
               

     wherein the shank of the mobile anchor is attached to a first vertebra, and the shank of the fixed anchor is attached to the second vertebra. 
     The present invention allows desirable amounts of physiologic flexion, extension, lateral bending and axial rotation to be set by the surgeon after iatrogenic instability. The limited range of motion realized by the present invention is advantageous in certain PDS indications, such as topping-off fusions, stenosis, and degenerative spondylolisthesis. 
     Preferred embodiments of the present invention avoid the use of springs, and so allow motion to occur strictly within a desired window along the rod length. The extent of this limited motion is determined by a stop provided on the spinal rod component of the device, wherein the stop comprises a hard collar wrapped around the rod and an inferiorly-positioned soft bumper that prevents metal-on-metal contact between the anchor and the collar. Alternatively, a standard spinal rod having a conventional stop, furnished as either an integral or an attachable component, may be used. 
     One embodiment of the present invention allows the extent of anchor translation within the system to be dictated by the flexibility of the rod to which it is attached. The ability of the mobile anchor to slide on the rod will be dictated by the inherent flexibility of the rod. In these cases, a flexible rod will be expected to produce more translational motion, while a rigid rod would be expected to allow less translational motion. 
     The system of the present invention allows both the anchors and stops to be positioned at adjacent levels to limit motion, such that the loads can be effectively transitioned from one level to the next when topping off. This transitioning allows a motion gradient to be achieved to minimize overloads of adjacent levels. Because it beneficially provides physiologic displacements at adjacent levels, the present invention potentially beneficially reduces both the incidence of adjacent disc disease and the necessity of subsequent fusion procedures. 
     Although not necessarily preferred, the present invention may also utilize a spring rod or a polymer bumper to soften the flexion or extension limits. These soft stops would also allow motion similar to a neutral zone concept, wherein motion has relatively low resistance within the neutral zone but then experiences increased resistance after a prescribed limit. 
     Therefore, in accordance with the present invention, there is provided aposterior dynamic spinal stabilization system comprising:
         a) a rod having a first end and a second end,   b) a mobile anchor having i) a shank and ii) a rod-receiving portion mated to the first end of the rod, wherein the rod-receiving portion translates along the rod,   c) a fixed anchor having i) a shank and ii) a rod-receiving portion locked to the second end of the rod,   d) a first motion-limiting stop positioned on the rod at a location superior to the mobile anchor, and   e) a second motion-limiting stop positioned on the rod at a location inferior to the mobile anchor.       

     Also in accordance with the present invention, there is provided a posterior dynamic spinal stabilization system comprising:
         a) a rod having a first end and a second end,   b) a mobile anchor having i) a shank and ii) a rod-receiving portion mated to the first end of the rod, wherein the rod-receiving portion translates along the rod,   c) a fixed anchor having i) a shank and ii) a rod-receiving portion locked to the second end of the rod,   d) a first motion-limiting stop positioned on the rod at a location inferior to the mobile anchor and comprising an elastic bumper.       

     Also in accordance with the present invention, there is provided a dynamic spinal stabilization system, comprising:
         a) a polyaxial head housing having a throughbore;   b) a screw having a head and a shank, the shank passing through the throughbore of the housing;   c) a locking cap adapted to apply axial pressure to screw head, thereby locking the shank with respect to a rod, the cap comprising a low friction, lower bearing surface for bearing upon the rod, and   d) a set screw adapted to seat in the head housing to lock the rod thereto, the set screw comprising a low friction, upper bearing surface for bearing upon the rod.       

    
    
     
       DESCRIPTION OF THE FIGURES 
         FIGS. 1 a -1 c    disclose a posterior dynamic spinal stabilization system of the present invention. 
         FIG. 2  discloses a posterior dynamic spinal stabilization system of the present invention implanted in a spinal column. 
         FIG. 3  shows an illustration in partial section of an embodiment of the anchoring device in the unloaded state. 
         FIG. 4  shows an illustration in partial section of an anchoring device in the loaded state in the resting position. 
         FIG. 5  shows an illustration in partial section of an anchoring device in the loaded state during the action of a force upon the anchoring element. 
         FIG. 6 a    discloses a mobile anchor of the present invention that slides upon the rod without pivoting. 
         FIG. 6 b    discloses the anchor of  FIG. 6 a    as an exploded assembly. 
         FIG. 7 a    discloses a mobile anchor of the present invention in which the screw not only slides upon the rod, but it also pivots. 
         FIG. 7 b    discloses the anchor of  FIG. 7 a    as an exploded assembly. 
         FIG. 8  discloses a screw of the present invention attached to a rod, wherein the screw has a sleeve having an upper polymer portion and a lower metallic portion. 
         FIG. 9 a    is a cross section of a mobile screw-rod assembly wherein the screw has a spherical bearing surface. 
         FIG. 9 b    is a mobile screw having a spherical bearing surface. 
         FIG. 10  discloses an assembly having a polymer screw disposed between two screws. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Now referring to  FIGS. 1 a -1 c   , in one preferred embodiment of the invention, there is provided a posterior dynamic spinal stabilization system, comprising:
         a) a spinal rod  1  having a first end  3  used as a guide for a mobile anchor and a second end  5 ,   b) a mobile anchor  11  having i) a shank  13  and ii) a rod-receiving portion  15  containing a bearing insert  45 (or “bearing”) adapted to mate to the first end of the rod and the head of the shank, thereby enabling translation along the rod and pivoting about the rod,   c) a fixed anchor  21  having i) a shank  23  and ii) a rod-receiving portion  25  adapted to lock to the second end of the rod.   d) a first motion-limiting stop  61  positioned on the rod at a location superior to the mobile anchor, and   e) a second motion-limiting stop  71  positioned on the rod at a location inferior to the mobile anchor and comprising a polymeric bumper  73 .       

     Now referring to  FIG. 1 a   , gaps  75  are provided along the rod between the stops and the mobile anchor. The gaps define the extent of translation allowed to the mobile anchor. The length of each gap can be predetermined by the surgeon to limit translational anchor motion to a desired amount. 
     Polymer bearing insert  45  is shown through the polyaxial head in  FIG. 1 a   . The low friction nature of the insert allows the bearing to pivot about the rod and allows the rod-receiving portion to translate. The polymeric nature of the insert also reduces friction between the anchor and rod, thereby promoting limited translational motion of the anchor along the rod. 
     Mobile anchors can allow translational motion, pivoting motion, or both motions, depending on the materials used in the insert and the type of set screw used. A pivoting mobile anchor that is allowed to only pivot can be transformed into one that both translates and pivots by providing a new set screw. Whereas the set screw for the pivot-only anchor would be a purely metallic set screw (which produces a high friction contact with the rod and thereby prevents translation), the set screw  41  for the pivot-plus-translate anchor would be a metal set screw with a polymer or other low friction rod-contacting surface  43  (which produces a low friction contact with the rod and thereby promotes limited translation). 
     In some embodiments of the present invention, the anchor is adapted to pivot by providing a bearing element  45  that surrounds the head of the shank and provides a surface to receive the rod. Preferably, the articulating face of this bearing element can be made of any material typically used in articulating prosthetic implants, such as metals, ceramics and polymers such as UHMWPE. Preferably, the articulating face has a low coefficient of friction. 
     In some embodiments, the mobile head component of the present invention is substantially similar to the mobile head components described in US Published Patent Application Nos. 2004/0225289 (Biedermann III); 2005/0203516 (Biedermann IV); and 2005/0277919 (Slivka), the specifications of which are incorporated by reference in their entireties. 
     Now referring to  FIGS. 3-5 , in some embodiments of the present invention, flexible rods  300  of the present invention are combined with a pair of dynamic anchoring devices  201 . Preferably, each of these dynamic anchoring devices comprises i) a shank  203  for anchoring into a bone or a vertebra, ii) a head connected to the shank, iii) a receiving part (or “housing”)  205  for the head and iv) a motion-providing bearing element  220  acting on the head. The bearing element is formed and located in such a way that, upon a movement of the element from a first angular position of the shank relative to the receiving part into a second angular position, the rod stiffness provides a restoring force that forces the head to pivot. Accordingly, the present invention includes a dynamic stabilization device, in particular for vertebrae. In such a stabilization device, the flexible rod is connected to two anchoring devices. At least one of the anchoring devices is constructed as a dynamic anchoring element. 
     Therefore, the present invention provides an anchoring device comprising a screw element having a shank for anchoring in a bone or a vertebra and a head connected to the shank, a receiving part for receiving the head, and a motion-providing bearing element acting on the head, so that upon a movement of the motion providing bearing element from a first angular position of the shank relative to said receiving part into a second angular position, the rod flexibility urges the head to move. 
     Preferably, the motion providing bearing element acts on the side of the head facing away from the shank and is formed of a metal, ceramic or UHMWPE material. 
     Preferably, the head comprises a flat surface on the side facing away from the shank and the resilient pressure element comprises a flat surface cooperating therewith. The head may comprise a spherical segment-shaped section adjacent to the shank and a collar on the side facing away from the shank. Preferably, the head and the shank are separate parts, wherein the head has a central axis and the shank is connectable to the head at a predetermined angle α to the central axis. 
     Preferably, the receiving part comprises a support surface to support the head, the support surface and/or the head being polished or coated to reduce friction. 
     Preferably, the receiving part comprises a U-shaped recess for inserting a rod and the bearing element is arranged between the head and the rod when the rod is inserted into the receiving part. 
     The invention also provides a dynamic stabilization device for bones, in particular for vertebrae, having at least two anchoring devices connected to a flexible polymer rod, wherein one of the anchoring devices is formed as the anchoring device described above. 
     Additionally, the invention provides a method for using the dynamic anchoring device and a method for stabilizing bones, in particular for stabilizing vertebrae, wherein the anchoring device is as described above. 
     As can be seen in particular from  FIGS. 3-5 , in accord with one embodiment of the invention, the dynamic anchoring element  201  is formed as a polyaxial screw. It comprises a screw element  202  with a threaded shank part  203  and a head  204  formed in one piece therewith and a receiving part  205 . The head  204  is substantially formed in the shape of a segment of a sphere and has on its end opposite to the shank part  203  a widened edge or collar  206 , so that a flat front face  207  is formed which has a diameter which is larger than the diameter of the spherical segment-shaped section of the head. A recess for bringing into engagement with a screwing-in tool is further formed in the front face  207 . 
     The receiving part  205  is substantially formed as cylindrically symmetric and has on one of its ends a coaxial first bore  210  the diameter of which is larger than that of the threaded section of the shank  203  and smaller than the spherical diameter of the spherical segment-shaped section of the head  204 . It further has a coaxial second bore  211  which is open at the end opposite the first bore  210  and the diameter of which is large enough for the screw element  202  to be inserted through the open end with its threaded section through the first bore  210  and with the spherical segment-shaped section of the head  204  to the bottom of the second bore. In the receiving part, adjacent to the first bore  210  a section  212  is provided, shaped like a segment of a hollow sphere, the radius of which is substantially identical to the radius of the section of the spherical segment-shaped head  204 . The receiving part further has a U-shaped recess  213 , extending from the open end towards the first bore  210 , the bottom of which is directed towards the first bore  210  and by which two open legs  214  are formed, only one of which is illustrated in the figures. An inner thread  215  is formed in the receiving part adjacent to the open end of the legs  214 . The width of the U-shaped recess  213  is minimally larger than the diameter of a rod  300  to be received therein which connects several such polyaxial screws. The depth of the U-shaped recess is dimensioned in such a way that when the rod is inserted a fixing screw  216  can be screwed in between the legs. The set screw  216  is shown having a low friction rod contacting surface  43 . 
     The section  212  of the receiving part which is shaped like a segment of a hollow sphere is preferably polished smooth or coated with a material which increases the sliding capacity, so the head  204  can easily be swiveled in the section  212  of the receiving part. Alternatively, or additionally the head  204  is polished smooth or coated. 
     Between the inserted rod  300  and the head  204  of the screw element, a motion-providing bearing element  220  is provided. The bearing element  220  is formed in the shape of a cylinder and has a diameter which is smaller than the inner diameter of the second bore  211  of the receiving part and which is preferably identical to the diameter of the front face  207  of the head. The axial length of the bearing element  220  runs the full length of the head and sometimes therebeyond (i.e., is slightly larger than or identical to the distance between the front face  207  of the head  204  and the bottom of the U-shaped recess  213  in the inserted state). The upper surface of the bearing element is a low friction surface to allow for translation of the rod and/or pivoting of the head in response to physiologic motion. 
     Between the bearing element  220  and the inserted rod  300  a cap  221  is provided, which covers the bearing element on the side facing the rod and which is constructed from an inflexible material, for example a synthetic material or a body-compatible metal. The outer diameter of the cap  221  is dimensioned in such a way that the cap is displaceable by sliding in the second bore of the receiving part and the inner diameter of the cap substantially corresponds to the outer diameter of the bearing element  220  when this is in an unloaded state. 
       FIG. 3  shows the unloaded state in which the screw element  202 , the bearing element  220  and the cap  221  are inserted into the receiving part and the rod  300  is placed into the U-shaped recess  213 , but the inner screw has not yet been screwed down. In this state, the side  222  of the cap  221  facing away from the pressure element  213  is at a slightly higher position than the bottom of the U-shaped recess  213 , so that the rod rests with its lower side on the surface  222  of the cap and thus a gap  213  is formed between the lower side of the rod and the bottom of the U-shaped recess  213 . 
     In operation, as shown in  FIG. 3 , first the screw element  202  is inserted into the receiving part  205  from the open end thereof until the head rests against the section  212  of the receiving part shaped like a segment of a hollow sphere. The screw element is then screwed into the vertebra. Then, the bearing element  220  together with the cap  221  placed thereon is inserted into the receiving part, the receiving part is aligned and the rod inserted. Finally, the inner screw  216  is screwed into the receiving part. 
     As illustrated in  FIG. 4 , the inner screw is screwed in until it presses the rod against the bottom of the U-shaped recess and thus fixes the rod. At the same time, the rod presses on the cap  221 , which serves for even distribution of the force of pressure exerted by the rod on to the entire surface of the pressure element. In the state shown in  FIG. 4 , the bearing element  220  is under bias in respect of the screw head  204  and, due to the return force, it presses with its lower side evenly on the front face  207  of the head. In this way, the head is pressed against the section  212  of the receiving part. 
     The screw element  202  screwed into the vertebral body is moved out of its resting position by a self-movement of the vertebral column. When the vertebra moves towards the rod at an angle of 90° to the rod axis there is uniform compression of the pressure element and the angle of the shank relative to the receiving part does not change. When the vertebra moves at an angle other than 90° to the rod axis, as shown in  FIG. 5 , there is a swiveling of the head, which easily slides into section  212  of the receiving part. Thereby, the front face  207  of the screw head exerts a compression force upon the pressure element on one side which compresses it on one side near the edge. On the other hand, on the opposite side, the pressure element standing under pre-stress expands owing to the relief of pressure. Thus, the pressure element always remains in contact with the screw head. 
     The swivel range can also or additionally be set by the selection of the diameter of the collar  206  of the screw head relative to the diameter of the second bore  211  of the receiving part. When the collar  206  abuts on the wall of the receiving part in the swiveled position of the screw element  202 , no further swiveling is possible. 
       FIG. 6 a    discloses an additional anchor  111  of the present invention that slides upon the rod without pivoting.  FIG. 6 b    discloses the anchor of  FIG. 6 a    as an exploded assembly comprising:
         a) a conventional polyaxial head housing  111  having a throughbore (not shown),   b) a conventional screw  109  having a head and a shank, the shank passing through the throughbore of the housing so that the head sets in the housing,   c) a conventional locking cap  107  adapted to apply axial pressure to screw head, thereby locking the shank with respect to a rod,   d) lower bearing  106  seated within the locking cap and having a surface adapted for receiving a rod, the surface possessing a low friction face that reduces wear generation,   e) a set screw  103  adapted to seat in the head housing to lock the rod thereto, the set screw comprising a low friction, upper bearing surface  105  for bearing upon the rod and reducing wear generation.       

     The non-pivoting device of  FIGS. 6 a  and 6 b    can be suitably used as a PDS device for treating degenerative disc disease (DDD), stenosis, and degenerative spondylolisthesis. It may also be used in protective capacity for topping off applications in an effort to prevent adjacent disc disease. Therefore, in accordance with the present invention, there is provided a method of treating a spine while allowing physiologic or near-physiologic motions, comprising the steps of:
         i) attaching to the spine of a patient having a degenerated disc a posterior dynamic spinal stabilization system comprising:
           a) a rod having a first end and a second end,   b) a mobile anchor having i) a shank and ii) a rod-receiving portion mated to the first end of the rod, wherein the rod-receiving portion translates along the rod,   c) a fixed anchor having i) a shank and ii) a rod-receiving portion locked to the second end of the rod,
 
wherein the shank of the mobile anchor is attached to a first vertebra, and the shank of the fixed anchor is attached to the second vertebra.
   
               

     In preferred embodiments thereof, the rod receiving portion of the mobile anchor is adapted to pivot about the rod. 
       FIG. 7 a    discloses a mobile anchor  121  of the present invention in which the screw not only slides upon the rod, but it also pivots.  FIG. 7 b    discloses an exploded assembly comprising:
         a) standard polyaxial head  131  having a throughbore (not shown);   b) a conventional screw  129  having a head and a shank, the shank passing through the throughbore of the housing   c) a locking cap  127  adapted to apply axial pressure to screw head, thereby locking the shank with respect to the rod, the cap comprising a low friction, lower bearing surface  126  for the rod,   d) a set screw  123  adapted to seat in the head housing to lock the rod thereto, the set screw comprising a low friction, upper bearing surface  125  for bearing upon the rod.       

     In some embodiments of the present invention, the anchor is adapted to translate along the rod by providing the screw with low friction surfaces that contact the rod. Preferably, this would entail using both a low friction, motion-providing bearing element and a low friction set screw. In some embodiments, each of these low friction surfaces comprises a material selected from the group of a polymer, a ceramic and a metal. Preferably, it is a polymer. 
     The stops of the present invention may include any conventional component placed on the rod adjacent to the mobile anchor in order to limit the translation of the anchor along the rod. The stop may attach to a single rod, or attach to a pair of bilateral rods thereby acting as a cross-connector. The stop may be desirably spaced from the anchor so that the preferred amount of anchor translation along the rod is between 0.5 mm and 12 mm. In preferred embodiments, the stop is a bumper-stop and may be fabricated by using a metallic stop  71  with an anchor-limiting surface  73  made from a polymer or elastomer component. The polymer or elastomer may be slid onto the collar as a separate component or it may be overmolded. In addition, the elastomer component may have a geometry that would cause it to perform like a cushioning stop, i.e. a bumper with flexible struts, sponge-like. The stop may be integral to the rod, or may be a secondary component attached to the rod to create a stop. The surgeon may position the stops at distances from the mobile anchor that are considered to produce the desired limited amount of motion. 
     The spinal rod of the present invention may be a rod conventionally used in the spinal stabilization field. The present invention can be used with either a rigid rod (such as titanium or stainless steel) or a flexible rod (such as polymer, spring or composite) that allows bending and translation of the vertebral bodies relative to one another. The rod has a preferred diameter within the range of 1.0 to 8.0 mm, and may more preferably be about 5.5 mm. The rod may have a constant cross section or a tapered cross section. For example, it may form a ramp that makes motion increasingly difficult as the mobile anchor translates along the rod. In another embodiment, a plurality of polymer rods could be provided with varying flexibilities. In this case, the inherent stiffness of each rod will dictate the amount of bend it provides in vivo, and the bend will influence the anchor&#39;s ability to slide. In these cases, a relatively stiff rod may allow for less translation than a more flexible rod. In some embodiments, the metal or polymer rod carries a polymer sleeve that is free to slide along the rod, thereby limiting motion. In some preferred embodiments, the rod is a polymer rod. 
     Although many embodiments of the present invention use a polyaxial screw that is adapted to pivot, in some alternative embodiments, a conventional polyaxial screw may be provided that would allow the angle of the shank to be locked relative to the polyaxial head. The set screw component of the anchor contains two useful surfaces—a first that applies pressure radially to the cap (receiving part) and a second that applies pressure axially to the rod. Similarly, the cap contains a first surface that interfaces with the rod and a second that interfaces with the set screw. As the set screw is tightened, the rod is sandwiched between the polymeric surface on the set screw and the head of the shank. The polymeric inserts that are disposed between the spherical head of the shank and the rod can be made from any biocompatible low friction wear surface. The shank of either the mobile anchor or the fixed anchor may be adapted to allow pivoting motion. Alternatively, it may remain fixed. 
     In some embodiments (not shown), the rod is fitted with at least one polymeric sleeve. In these embodiments, the surgeon inserts the rod, passes the sleeve over the rod, and aligns the sleeves with the caps of adjacent bone anchors before tightening the sleeves within the caps. Such a sleeve not only provides a bearing surface within the bone anchor to prevent metal-on-metal contact, it also allows for some free sliding motion of the anchor along the rod. 
     In some embodiments, a construct may be fabricated using a plurality of anchors (along with the corresponding stops) sufficient to treat multiple levels in order to tailor and transition motion, such that a motion gradient is achieved. 
     One skilled in the art will appreciate that the rod of the device may be configured for use with any type of bone anchor, e.g., bone screw or hook; mono-axial or polyaxial. Typically, a bone anchor assembly includes a bone screw, such as a pedicle screw, having a proximal head and a distal bone-engaging portion, which may be an externally threaded screw shank. The bone screw assembly may also have a receiving member that is configured to receive and couple a spinal fixation element, such as a spinal rod or spinal plate, to the bone anchor assembly. 
     The receiving member may be coupled to the bone anchor in any well-known conventional manner. For example, the bone anchor assembly may be poly-axial, as in the present exemplary embodiment in which the bone anchor may be adjustable to multiple angles relative to the receiving member, or the bone anchor assembly may be mono-axial, e.g., the bone anchor is fixed relative to the receiving member. An exemplary poly-axial bone screw is described U.S. Pat. No. 5,672,176, the specification of which is incorporated herein by reference in its entirety. In mono-axial embodiments, the bone anchor and the receiving member may be coaxial or may be oriented at angle with respect to one another. In poly-axial embodiments, the bone anchor may be biased to a particular angle or range of angles to provide a favored angle to the bone anchor. Exemplary favored-angle bone screws are described in U.S. Patent Application Publication No. 2003/0055426 and U.S. Patent Application Publication No. 2002/0058942, the specifications of which are incorporated herein by reference in their entireties. 
     In some embodiments of the present invention, the rod-receiving portion of the device comprises inner and outer portions made of distinct materials. Each of these portions is designed to carry out one of the two primary functions of the rod-receiving portion: 1) attaching to the remainder of the mobile anchor and 2) sliding upon the rod. In these embodiments, the inner portion of the rod-receiving portion is made of a low friction material (preferably, a polymer) to allow limited sliding upon the rod therein. In contrast, the outer portion of the rod-receiving portion is made of a high stiffness material (such as a metal) in order to provide secure locking of the rod-receiving portion to the other components of the device. 
     In some embodiments, the lower part of the set screw functions as the upper bearing component of the rod-receiving portion. In these embodiments, the set screw can be made of two components having dissimilar materials. Preferably, the set screw can comprise: a) a lower polymer portion functioning as the low friction, upper bearing component of the rod-receiving portion, and b) an upper metallic or ceramic threaded portion whose high stiffness allows its locking to the mobile anchor housing. 
     Likewise, in some embodiments, the upper part of the locking cap may function as the lower bearing component of the rod-receiving portion. In these embodiments, the locking cap can be made of two components having dissimilar materials. Preferably, the locking cap can comprise a) an upper polymer portion functioning as the low friction, lower bearing component of the rod-receiving portion, and b) a lower metallic or ceramic backing that seats on the screw head. 
     In some embodiments, a separate component termed a “sliding bearing” can be used in the device of the present invention. The “sliding bearing” is a sleeve component that wraps around the rod within the mobile anchor housing. Accordingly, an upper portion of this sleeve is interpositioned between the set screw and the rod, and a lower portion of this sleeve is interpositioned between the locking cap and the rod. The sliding bearing can be composed of i) an inner polymer sleeve functioning as the low friction bearing surface and ii) an outer metal sleeve whose high stiffness allows a set screw or locking cap to clamp it. The metallic feature of this hybrid sleeve component desirably prevents compression of its polymer bearing feature, which may undesirably hinder motion. 
     In the embodiments described above, it is generally preferably that the spinal rod be made of a polymer (such as PEEK) in order to provide a certain amount of flexibility desirable for posterior dynamic stabilization and to accommodate limited sliding of the mobile anchor thereon. 
     Therefore, in accordance with the present invention, and now referring to  FIG. 8 , there is provided a posterior dynamic spinal stabilization system comprising:
         a) a spinal rod  311  having a first end portion  313  and a second end portion,   b) a mobile anchor  315  having i) a shank  317  and ii) a rod-receiving portion  319  mated to the first end portion of the rod, wherein the rod-receiving portion translates along the rod,   c) a fixed anchor having i) a shank and ii) a rod-receiving portion locked to the second end portion of the rod,
 
wherein an inner portion  321  of the rod-receiving portion of the mobile anchor comprises a polymer allowing limited motion of the mobile anchor on the rod, and
 
wherein an outer portion  323  of the rod-receiving portion of the mobile anchor comprises a high stiffness material (such as a metal) securing locking of the rod-receiving portion.
       

     Preferably, this system further comprises d) a set screw  325  housed within the mobile anchor and bearing against the rod, 
     wherein a lower part of the set screw comprises an upper bearing component of the rod-receiving portion. 
     In some embodiments, the set screw of this sliding bearing system comprises a) a polymer portion comprising the upper bearing component of the rod receiving portion, and b) a metallic or ceramic threaded portion securing the set screw to the mobile anchor. Preferably, the polymer portion comprises PEEK. 
     In other embodiments, this sliding bearing system further comprises:
         d) a set screw secured to the mobile anchor,
 
wherein the rod-receiving portion comprises a sliding bearing sleeve  329  wrapped around the rod, wherein the sliding bearing sleeve comprises an inner polymer component bearing against the rod and a metal outer surface contacting the set screw. Preferably, the inner polymer component comprises PEEK.
       

     In other embodiments, this sliding bearing system further comprises:
         d) a locking cap as a lower bearing component of the rod receiving portion.       

     Preferably, this locking cap comprises a) an upper polymer portion functioning as the lower bearing component, and b) a metallic or ceramic backing that seats on the shank. Preferably, the upper polymer portion comprises PEEK, and the rod comprises a polymer such as PEEK. 
     In some embodiments, the rod-receiving portion that acts as a sliding bearing has a substantially spherical outer surface. The spherical bearing is adapted to allow:
         a) sliding of the rod through the center of the spherical bearing, and   b) rotation of the spherical bearing within the device to allow toggling/pivoting while preventing torque transmission to screw-bone interface.       

     Therefore, in accordance with the present invention, and now referring to  FIGS. 9 a  and 9 b   , there is provided a posterior dynamic spinal stabilization system comprising:
         a) a rod  351  having a first end portion  353  and a second end portion,   b) a mobile anchor  361  having i) a shank and ii) a rod-receiving portion  363  mated to the first end portion of the rod, wherein the rod-receiving portion translates along the rod,   c) a fixed anchor having i) a shank and ii) a rod-receiving portion locked to the second end portion of the rod,
 
wherein the rod receiving portion of the mobile anchor has a substantially spherical outer surface  365 .
       

     In the spherical sliding bearing embodiments, the rod preferably comprises a polymer such as PEEK. 
     In some spherical sliding bearing embodiments, the rod-receiving portion of the mobile anchor has an inner surface adapted to contact the rod, wherein the inner surface comprises a polymer (such as PEEK). 
     In some spherical sliding bearing embodiments, the substantially spherical outer surface is adapted to allow pivoting of the mobile anchor about the rod. 
     In some embodiments, there is provided a polymeric bumper/stop component (or sleeve) located between adjacent pedicle screws that is adapted to float on the rod. The purpose of this polymeric sleeve is to provide a soft stop that limits the translation of the mobile anchor. The use of such a bumper obviates the need for employing fixed stops, thereby reducing inventory and simplifying the surgical procedure. The difference between the pedicle screw spacing and the length of the bumper of this embodiment defines a gap, which determines the amount of mobile anchor translation freely allowed along the rod. In these embodiments, the system provides a first region of free travel defined by the gap and then a second region of increasing resistance provided by the polymeric sleeve. 
     Therefore, in accordance with the present invention, and now referring to  FIG. 10 , there is provided a posterior dynamic spinal stabilization system, comprising:
         a) a rod  399  having a first end portion and a second end portion,   b) a mobile anchor  401  having i) a shank  403  and ii) a rod-receiving portion  405  mated to the first end portion of the rod, wherein the rod-receiving portion translates along the rod,   c) a fixed anchor  411  having i) a shank  413  and ii) a rod-receiving portion  415  locked to the second end portion of the rod,   d) a polymeric sleeve  417  received upon the rod at a location between the mobile anchor and the fixed anchor.       

     In some embodiments, thereof, the polymeric sleeve comprises PEEK. 
     In some polymer sleeve embodiments, the rod comprises a polymer (such as PEEK). 
     In some polymer sleeve embodiments, the location of the sleeve defines a first gap along the rod between the sleeve and the mobile anchor, and preferably the polymeric sleeve floats upon the rod. 
     In some embodiments, the locking cap of the present invention is a hybrid component comprising i) a lower metallic component adapted to apply axial pressure to the screw head, thereby locking the shank with respect to the housing, and ii) an upper polymer bearing surface for sliding upon the rod. 
     Therefore, in accordance with the present invention, there is provided a dynamic spinal stabilization system, comprising:
         a) a spinal rod having a first end portion and a second end portion,   b) a polyaxial head housing having a throughbore;   c) a screw having a head and a shank, the shank passing through the throughbore of the housing;   d) a locking cap comprising i) a lower metallic component adapted to apply axial pressure to the screw head, thereby locking the shank with respect to the housing, and ii) an upper polymer bearing surface for sliding upon the rod, and   e) a set screw seated in the head housing to bear against the rod, the set screw comprising a low friction, upper bearing surface for sliding upon the rod.
 
In some of these hybrid locking cap embodiments, the spinal rod comprises a polymer (such as PEEK).
       

     In some of the hybrid locking cap embodiments, the polymer bearing surface of the locking cap comprises PEEK. 
     In some embodiments, the set screw is a hybrid component comprising i) an upper metallic component seated in the head housing to lock the rod thereto, and ii) a lower polymeric upper bearing surface for sliding upon the rod. Therefore, in accordance with the present invention, there is provided a dynamic spinal stabilization system, comprising:
         a) a spinal rod having a first end portion and a second end portion,   b) a polyaxial head housing having a throughbore;   c) a screw having a head and a shank, the shank passing through the throughbore of the housing;   d) a locking cap adapted to apply axial pressure to the screw head, thereby locking the shank with respect to the rod, the cap comprising a low friction, lower bearing surface for sliding upon the rod, and   e) a set screw comprising i) an upper metallic component seated in the head housing to lock the rod thereto, and ii) a lower polymeric upper bearing surface bearing upon the rod.
 
In some of these hybrid set screw embodiments, the spinal rod comprises a polymer (such as PEEK).
       

     In some of the hybrid set screw embodiments, the polymeric bearing surface of the set screw comprises PEEK. 
     In some embodiments, the assembly may be implanted in accordance with the minimally invasive techniques and instruments disclosed in U.S. Pat. No. 7,179,261; and US Patent Publication Nos. US2005/0131421; US2005/0131422; US 2005/0215999; US2006/0149291; US2005/0154389; US2007/0233097; and US2005/0192589, the specifications of which are hereby incorporated by reference in their entireties. In  FIG. 1 c   , an MIS attachment feature located at an end of the rod is shown. 
     The material of construction for a rod used in accordance with the present invention may be selected from any biocompatible material, such as a polymer, titanium alloy, stainless steel, and a composite. The preferred surface finish on the contact surfaces of the metallic components will be within the range of Ra 0.02 to Ra 0.5 μm, preferably within the range of Ra 0.02 to Ra 0.15 μm. 
     The rod and stop components of the design may be made from biocompatible, implantable materials known in the art such as stainless steel, titanium, Nitinol, polyetheretherketone (PEEK) or alternative polyarylketones, carbon fiber reinforced polymers. The bumpers may be selected from high performance elastomers such as silicones, dimethylsiloxanes, silicone-urethanes, polyether-urethanes, silicone-polyether-urethanes, polycarbonate urethanes, and silicone-polycarbonate-urethanes. 
     Preferably, the rod, shank, cap and stop components are titanium alloy (Ti-6Al-4V) or cobalt-chrome alloy (e.g. Co—Cr—Mo). If a cobalt-chrome alloy is selected, the alloy is preferably in a work-hardened condition so as to resist deformation upon securing to the bone anchor (e.g with a set screw). Preferably, the solid rod component may be either titanium alloy or PEEK. 
     If a metal is chosen as a material of construction, then the metal is preferably selected from the group consisting of nitinol, titanium, titanium alloys (such as Ti-6Al-4V), cobalt-chrome alloys (such as CrCo or Cr—Co—Mo) and stainless steel. 
     If a polymer is chosen as a material of construction, then the polymer is preferably selected from the group consisting of CFRP, polycarbonates, polyesters, (particularly aromatic esters such as polyalkylene terephthalates, polyamides; polyalkenes; poly(vinyl fluoride); PTFE; polyarylethyl ketone PAEK; and mixtures thereof. 
     In some embodiments, the solid rod component is made from a composite comprising carbon fiber. Composites comprising carbon fiber are advantageous in that they typically have a strength and stiffness that is superior to neat polymer materials such as a polyarylethyl ketone PAEK. In some embodiments, the tube is made from a polymer composite such as a PEKK-carbon fiber composite. 
     Preferably, the composite comprising carbon fiber further comprises a polymer. Preferably, the polymer is a polyarylethyl ketone (PAEK). More preferably, the PAEK is selected from the group consisting of polyetherether ketone (PEEK), polyether ketone ketone (PEKK) and polyether ketone (PEK). In preferred embodiments, the PAEK is PEEK. 
     In some embodiments, the carbon fiber comprises between 1 vol % and 60 vol % (more preferably, between 10 vol % and 50 vol %) of the composite. In some embodiments, the polymer and carbon fibers are homogeneously mixed. In others, the material is a laminate. In some embodiments, the carbon fiber is present in a chopped state. Preferably, the chopped carbon fibers have a median length of between 1 mm and 12 mm, more preferably between 4.5 mm and 7.5 mm. In some embodiments, the carbon fiber is present as continuous strands. 
     In especially preferred embodiments, the composite comprises:
         a) 40-99% (more preferably, 60-80 vol %) polyarylethyl ketone (PAEK), and   b) 1-60% (more preferably, 20-40 vol %) carbon fiber,
 
wherein the polyarylethyl ketone (PAEK) is selected from the group consisting of polyetherether ketone (PEEK), polyether ketone ketone (PEKK) and polyether ketone (PEK).
       

     In some embodiments, the composite consists essentially of PAEK and carbon fiber. More preferably, the composite comprises 60-80 wt % PAEK and 20-40 wt % carbon fiber. Still more preferably the composite comprises 65-75 wt % PAEK and 25-35 wt % carbon fiber. 
     The elastomer bumper component is preferably made of a thermoplastic, biocompatible, high performance polycarbonate-urethance (PCU). The stiffness, or durometer of the PCU can be tailored to meet the specifications for the dynamic device. In preferred embodiments, the surface of the device components that will be attached to the elastomer bumper are treated prior to attaching the bumper using known surface treatment methods such as surface roughening (e.g. grit blasting), chemical functionalization (e.g. primers), and plasma treatments know in the art. Alternatively or in conjunction with using a surface treatment, an adhesive may be used to enhance bonding, e.g. using cyanoacrylates. In one preferred embodiment, the surfaces of the device components that will be attached to the elastomer bumper will first be roughened using grit blasting, then chemically functionalized using primer, then the elastomer will be overmolded onto the device components.