Patent Publication Number: US-2009234388-A1

Title: Spinal Stabilization Connecting Element and System

Description:
BACKGROUND 
     Elongated connecting elements, such as rods, plates, tethers, wires, cables, and other devices have been implanted along the spinal column and connected between two or more anchors engaged between one or more spinal motion segments. Such connecting elements can provide a rigid construct that resists movement of the spinal motion segment in response to spinal loading or movement of the spinal motion segment by the patient. Other connecting elements can resist loading or movement of the spinal motion segment that creates a tension force on the connecting element; however, the connecting element collapses in response to any compression loading and provides little or no resistance in response to such forces or movement. Still other connecting elements are flexible to permit at least limited spinal motion while providing resistance to loading and motion of the spinal motion segment in one of compression and tension. 
     SUMMARY 
     In one embodiment of the present disclosure, an elongated connecting element for use in a spinal stabilization system comprises a first section, a second section, a first elastomer disposed within the first section, and a second elastomer disposed between the first section and the second section. One of the first elastomer and the second elastomer resists movement of the first section and the second section toward each other and the other of the first elastomer and the second elastomer resists movement of the first section and the second section away from each other. 
     In another embodiment of the present disclosure, an elongated connecting element is used in a spinal stabilization system. The connecting element comprises first and second end anchors and an elastomeric bumper portion engaged between the first and second end anchors. The elastomeric bumper includes an outer radial surface. Movement of the first and second end anchors toward each other presses the outer radial surface of the bumper radially outward and movement of the first and second end anchors away from each other presses the outer radial surface radially inward. 
     In another embodiment of the present disclosure, an elongated connecting element is used in a spinal stabilization system. The connecting element comprises a first end anchor comprising a first elongated cylindrical section and an internal bore extending at least partially through the elongated cylindrical section. The connecting element further comprises a second anchor comprising a second elongated cylindrical section and a rod portion extending away from the second elongated cylindrical section. The connecting element further comprises a bumper between the first and second elongated cylindrical sections. The rod portion is sized to extend through the bumper and into the internal bore of the first end anchor. 
     In another embodiment of the present disclosure, a spinal stabilization system comprises first and second bone connecting assemblies, a flexible elongated connecting element extending between the first and second bone connecting assemblies, and an adjustable sleeve extending over at least a portion of the connecting element. 
     In another embodiment of the present disclosure, a method of stabilizing a spinal joint comprises inserting a first connecting assembly into a first vertebra and inserting a second connecting assembly into a second vertebra. The method further comprises extending an elongated connecting element between the first and second connecting assemblies and extending an adjustable sleeve over the elongated connecting element. The adjustable sleeve includes a first sleeve portion movably connected to the second sleeve portion. The method further comprises actuating a drive system to adjust a height of the adjustable sleeve by moving the first sleeve portion with respect to the second sleeve portion. 
     These and other aspects, forms, objects, features, and benefits of the present invention will become apparent from the following detailed drawings and description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings, which are incorporated in and constitute a part of the specification, embodiments of the invention are illustrated, which, together with a general description of the invention given above, and the detailed description given below, serve to exemplify the embodiments of this invention. 
         FIG. 1  is a perspective view of a vertebral joint with a spinal stabilization system according to one embodiment. 
         FIG. 2A  is a cross-sectional view of a spinal device according to one embodiment of the present disclosure. 
         FIG. 2B  is a perspective view of a spinal device according to one embodiment of the present disclosure. 
         FIG. 3  is a cross-sectional view of a spinal device showing a dampening assembly according to one embodiment of the present disclosure. 
         FIG. 4  is an exploded view of a spinal device according to one embodiment of the present disclosure. 
         FIG. 5  is a partial cross-sectional view of a spinal stabilization system according to one embodiment of the present disclosure. 
         FIG. 6  is a cross-sectional view of a spinal device according to another embodiment of the present disclosure. 
         FIG. 7  is another cross-sectional view of the embodiment of  FIG. 6 . 
         FIG. 7   a  is a cross-sectional view according to another embodiment of the present disclosure. 
         FIG. 8  is cross-sectional view of a spinal device according to an embodiment of the present disclosure illustrating resorbable components. 
         FIG. 9  is cross-sectional view of a spinal device according to another embodiment of the present disclosure illustrating resorbable components. 
         FIG. 10  is a view of a spinal device according to another embodiment of the present disclosure. 
         FIG. 11  is a cross-sectional view of a spinal stabilization system according to another embodiment of the present disclosure. 
         FIG. 12  is a view of spinal device according to another embodiment of the present disclosure. 
         FIG. 13  is a partial cross-sectional view of the spinal device of  FIG. 12 . 
         FIG. 14  is another partial cross-sectional view of the spinal device of  FIG. 12 . 
         FIGS. 15-22  are partial cross-sectional views of spinal device according to other embodiments of the present disclosure. 
         FIG. 23  is a cross-sectional view of a spinal device according to another embodiment of the present disclosure. 
         FIG. 24  is another cross-sectional view of the spinal device of  FIG. 23 . 
         FIGS. 25-26  are cross-sectional views of spinal devices according to other embodiments of the present disclosure. 
         FIG. 27  is a side view of a spinal stabilization system according to another embodiment of the present disclosure. 
         FIGS. 28-29  are a partial cross sectional views of different spinal stabilization systems according to other embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure relates generally to the field of orthopedic surgery, and more particularly to systems and methods for stabilizing a spinal joint or spinal motion segment. For the purposes of promoting an understanding of the principles of the invention, reference will now be made to embodiments or examples illustrated in the drawings, and specific language will be used to describe these examples. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Any alteration and further modifications in the described embodiments, and any further applications of the principles of the invention as described herein, are contemplated as would normally occur to one skilled in the art to which the disclosure relates. 
     Referring first to  FIG. 1 , a spinal stabilization system is indicated generally by the numeral  20 . Various specific embodiments of the spinal stabilization system will be described in detail below.  FIG. 1  shows a perspective view of first and second spinal stabilization systems  20  in which elongated connecting elements or spinal devices  10  are attached to vertebral members V 1  and V 2 . The spinal devices  10  are schematically depicted to indicate that that the spinal devices may be arranged in variety of different shapes and configurations as will be disclosed in the embodiments that follow. The present discussion describes the invention as implanted between two adjacent vertebrae for ease of description, but the invention is not limited to implantation between two adjacent vertebrae. A vertebral disc D extends between vertebral members V 1 , V 2  and together these structures define a vertebral joint. The system  20  may also be used if all or a portion of disc D has been removed and replaced with a fusion or motion preserving implant. In the example systems  20  shown, the devices  10  are positioned at a posterior side of the spine, on opposite sides of the spinous processes S. In alternative embodiments, spinal devices  10  may be attached to a spine at other locations, including lateral and anterior locations. Spinal devices  10  may also be attached at various sections of the spine, including the base of the skull and to vertebrae in the cervical, thoracic, lumbar, and sacral regions. Thus, the illustration in  FIG. 1  is provided merely as a representative example of one application of a spinal stabilization system  20 . 
     In the exemplary system  20 , the spinal devices  10  are secured to vertebral members V 1 , V 2  by connector assemblies  12  comprising a pedicle screw  14  and a retaining cap  16 . The outer surface of spinal device  10  is grasped, clamped, or otherwise secured between the pedicle screw  14  and retaining cap  16 . In alternative embodiments, the connector assemblies may allow sliding motion of the spinal device. Other mechanisms for securing spinal devices  10  to vertebral members V 1 , V 2  include hooks, cables, and other such devices. Further, examples of other types of retaining hardware include threaded caps, screws, and pins. Thus, the exemplary assemblies  20  shown in  FIG. 1  are merely representative of one type of attachment mechanism. 
     For the present discussion, an exemplary elongated connecting element is described as a rod assembly, but other elements and structures may be used, such as a plate, hollow cylinder, blocks, discs, etc., without departing from the spirit and scope of the invention. The invention is not limited to a rod and is limited only by the claims appended hereto. Moreover, if a rod is used, it is not limited to a circular cross section, but may have an oval, rectangular, hexagonal, or any other regular or irregular cross section shape without departing from the spirit and scope of the invention. The rod may be curved, non-curved, or capable of being curved, depending on the circumstances of each application. 
       FIG. 2A  illustrates a rod assembly  30  which may be used as the spinal device  10  of system  20 . The rod assembly  30  has a first section  32 , a second section  34 , and a dampening assembly  36 . As better illustrated in  FIG. 2 , the dampening assembly  36  includes a first elastomer  38 , a second elastomer  40 , and a connector  54 . The connector  42  has an anchor end  44  and a piston end  46 . As shown in  FIG. 1 , the anchor end  44  of the connector  42  is anchored in the second section  34  and the piston end  46  is disposed within the first section  32 . The piston end  46  may be connectable to the connector  42  for ease of assembly (See  FIG. 4 ). For example the piston end  46  may be threaded or fused to the connector  42 . In an alternative embodiment, the piston end may be integrally formed with the connector  42 . 
     A cavity  48  is defined by the first section  32  and the first elastomer, or flexion dampening elastomer,  38  is disposed within the cavity  48 . The cavity may be provided with a sleeve  49 . In one embodiment, the cavity is substantially cylindrical, but the cavity may be in the shape of a rectangular prism, a hexagonal prism, conical or frustoconical shape, or any other shape. 
     The second elastomer, or extension dampening elastomer,  40  is located between the first section  32  and the second section  34 . The connector  42  extends through the second elastomer  40 , through the first elastomer  38 , and terminates in the piston end  46 . The piston end  46  is outside of the first elastomer  38 , but still within the cavity  48 . As illustrated in  FIGS. 2A and 3 , the piston end  46  abuts the first elastomer  38 ; however, the piston end  46  may also be spaced away from the first elastomer  38  or embedded within first elastomer  38  without departing from the spirit and scope of the invention. It is within the spirit and scope of the invention for the first elastomer to be the extension dampening elastomer and the second elastomer to be the flexion dampening elastomer. 
     The first section  32  has a first end  50  and a second end  52  and the second section  34  has a first end  54  and a second end  56 . As illustrated in the embodiment in  FIG. 2A , the second elastomer  40  abuts the second section second end  56  and the first section first end  50 . The first elastomer  38  and the second elastomer  40  may not abut each other, but rather, may be separated by a portion of the first section  32 . In other words, the cavity  48  may not extend to the first section first end  50 . Alternatively, the cavity  48  could extend such that first elastomer  38  and second elastomer  40  abut each other. 
     The first section second end  52  may be open to the cavity  48 , as illustrated in  FIG. 2A , or closed without departing from the spirit and scope of the invention. 
       FIG. 4  illustrates an exploded view of embodiment of the rod assembly  30  showing first section  32 , second section  34 , first elastomer  38 , second elastomer  40 , connector  42 , anchor end  44 , and piston end  46 . One method of assembly is to insert anchor end  44  into the second section  34  and crimp or otherwise secure the anchor end  44  in the second section  34 . The securing, anchoring, or other mechanical retaining of anchor end  44  may be done in any conventional manner. 
     The second elastomer  40  is then placed onto the connector  42  and the first section  32  is placed onto connector  42  after the second elastomer  40 . First elastomer  38  is placed onto the connector  42  and into the cavity  48  within the first section  32 . Then the piston end  46  is secured in a conventional manner, such as by crimping, to the connector  42 . Either first elastomer  38  or second elastomer  40  can be pre-loaded by compression or tension, if desired, during the assembly process. 
     Other methods of assembly will be apparent to one of ordinary skill in the art without undue experimentation depending on the specific elements selected for assembly. 
       FIG. 2B  illustrates another embodiment of the present invention in which the first section  32  and the second section  34  are configured as plates, with holes  58  provided for attachment to vertebra, such as via pedicle screws. 
     The first section  32  and the second section  34  can be constructed of any suitable material, preferably a biocompatible material. Examples of material that can be used include cobalt-chromium alloys, titanium alloys, nickel titanium alloys, and/or stainless steel alloys, any member of the polyaryletherketone (PAEK) family such as polyetheretherketone (PEEK), carbon-reinforced PEEK, or polyetherketoneketone (PEKK); polysulfone; polyetherimide; polyimide; ultra-high molecular weight polyethylene (UHMWPE); and/or cross-linked UHMWPE. Any combination of these materials may also be suitable. For example, a suitable material may include a layer of carbon fiber reinforced PEEK inside an otherwise uniform PEEK material. The first and second sections may have a common base material, however the first section may have a different modulus of elasticity than the second elastomer through the use of molding techniques or material reinforcements. 
     The first elastomer  38  and the second elastomer  40  can be constructed of any suitable material, preferably a biocompatible material. The first elastomer and the second elastomer are, for example, flexible and resilient or elastic to permit motion of the spinal motion segment with which they are associated while providing a desired stabilization effect. The first elastomer  38  and the second elastomer  40  can be constructed such that one or both has a gradual or otherwise variable stiffness. Examples of material that can be used include any suitable biocompatible elastomer or polymer biomaterial, such as surgical latex, chloroprene, MIT&#39;s “biorubber” (glycerol and sebacic acid), polyethylene, polyester, polyurethane, urethane, polypropylene, silicone, or hydrogel, and combinations thereof. The first elastomer and the second elastomer can also be constructed in the form of a spring or any other shape exhibiting elastomeric properties from any suitable material. Examples of such material include cobalt-chromium alloys, titanium alloys, nickel titanium alloys, and/or stainless steel alloys. 
     In some embodiments, the first elastomer  38 , the second elastomer  40  or both are constructed at least partially of a resorbable material. The first elastomer  38  or second elastomer  40  will then gradually resorb into the body, allowing for gradually increasing movement over time. In some embodiments, the first elastomer  38  or second elastomer  40  has one or more components  38 A,  40 A, as discussed in more detail below, that are resorbable to selectively modify the amount of movement increase over time. See  FIGS. 8 and 9 . The resorbable component or components  38 A,  40 A are arranged in parallel or series with one or more non-resorbable, or permanent, components to provide additional adaptability of the stiffness, resiliency and elasticity of the first elastomer  38  or second elastomer  40  based on the circumstances of use. 
     The first elastomer  38  and the second elastomer  40  may have the same construction or may have different construction. The first elastomer and the second elastomer may have the same or different characteristics, such as shape, size, length, stiffness, elasticity, resiliency, etc. Either or both elastomers may be multi-durometer or have gradual or discrete changes in the stiffness, resiliency, or elasticity over the length, width, or diameter of the elastomer. 
     The connector  42  may be flexible or inflexible, elastic, inelastic, or semi-elastic and of any suitable form, such as a tether, suture, wire, band, cord, cable, rope, or a solid or hollow rod, for example. The connector  42  can be single strand, multiple strands, braided, or combinations thereof and constructed of any suitable material, preferably a biocompatible material. Examples of possible materials include but are not limited to woven or non-woven polymers, such as polyester, polyethylene, or any member of the polyaryletherketone (PAEK) family such as polyetheretherketone (PEEK), carbon-reinforced PEEK, or polyetherketoneketone (PEKK), polysulfone; polyetherimide, polyimide, ultra-high molecular weight polyethylene (UHMWPE), and/or cross-linked UHMWPE; superelastic metals, such as nitinol; shape memory alloy, such as nickel titanium; resorbable synthetic materials, such as suture material, metals, such as stainless steel and titanium; synthetic materials, allograft material; and bioelastomer material. 
     The sleeve  49  can be constructed of any suitable material, preferably a biocompatible material. Examples of material that can be used include cobalt-chromium alloys, titanium alloys, nickel titanium alloys, and/or stainless steel alloys, any member of the polyaryletherketone (PAEK) family such as polyetheretherketone (PEEK), carbon-reinforced PEEK, or polyetherketoneketone (PEKK); polysulfone; polyetherimide; polyimide; ultra-high molecular weight polyethylene (UHMWPE); and/or cross-linked UHMWPE. 
       FIG. 5  illustrates a spinal stabilization system in accordance with one embodiment of the present invention in which a first bone anchor assembly  60  is attachable to a first vertebra (not shown) and a second bone anchor assembly  64  is attachable to a second vertebra (not shown). The first section  32  of the elongated connecting element, or rod assembly,  30  is connected to the first bone anchor assembly  60  and the second section  34  of the elongated connecting element, or rod assembly,  30  is connected to the second bone anchor assembly  64 . The first bone anchor assembly  60  and the second bone anchor assembly  64  are attachable, for example, to the pedicles of adjacent vertebra. The attachment may be to non-adjacent vertebra, or other than to the pedicles of the vertebra, without departing from the spirit and scope of the invention. For convenience of description, reference will be made to adjacent vertebra and attachment to the pedicles of the vertebra. 
     The bone anchor assemblies  60 ,  64  are any conventional bone anchor assemblies capable of or designed for attachment to vertebrae in any conventional manner. The elongated connecting element  30  may be used with new bone anchor assemblies  60 ,  64  that are packaged or included with the elongated connecting element  30  as a system or the elongated connecting element  30  may be used for revision surgery with bone anchor assemblies  60 ,  64  that were implanted into vertebrae at a separate time. 
     The elongated connecting element first section  32  and the second section  34  may be securable directly to the first and second bone anchor assemblies  60 ,  64 , or there may be another element present to facilitate connection to the bone anchor assemblies  60 ,  64 . For example, the first section  32  or the second section  34  may be fitted with a collar made of any suitable material and the collar then is secured directly to the bone anchor assembly. The first section  32  and the second section  34  are directly or indirectly secured to the bone anchor assemblies  60 ,  64  in any conventional manner. 
     The first vertebra and the second vertebra may be adjacent vertebra or non-adjacent vertebra. In either case, the attachment of the elongated connecting element to the bone anchor assemblies will provide dynamic stabilization of the spine in the area of the first vertebra and the second vertebra to which the bone anchor assemblies are attached. This dynamic stabilization allows some motion of the spine between the first and second vertebra, but also dampens that motion. 
     If the patient bends backward, that exerts force on the connecting element/rod assembly  30  to move the first section  32  and the second section  34  toward each other to compress the rod assembly. This includes movement of the first section  32  toward the second section  34 , the movement of second section  34  toward the first section  32 , or both. 
     If the connecting element/rod assembly  30  were a rigid, inflexible rod, then the first and second vertebra would be unable to move relative to each other. However, the second elastomer  40  described herein is flexible and at least partially elastic and enables motion of the first section  32  and the second section  34  toward each other by compressing the second elastomer  40 . The first section  32  and the second section  34  each press the second elastomer  40  from substantially opposite directions, compressing the second elastomer  40  and enabling movement of the first section  32  and the second section  34  toward each other. This, in turn, enables the vertebra to which the first section  32  and the second section  34  are attached to move relative to each other. 
     The material of construction of the second elastomer  40  may be selected to provide the desired amount of allowed motion of the first section  32  and the second section  34  toward each other, depending on the elasticity and quantity of the material chosen as well as the specific configuration of the shape of the second elastomer  40 . Thus, the motion of the first section  32  and the second section  34  toward each other may be selectively limited, or dampened. 
     If the patient bends forward, that exerts force on the connecting element/rod assembly  30  to move the first section  32  and the second section  34  away from each other to expand the rod assembly. This includes movement of the first section  32  away from the second section  34 , the movement of second section  34  away from the first section  32 , or both. 
     If the connecting element/rod assembly  30  were a rigid, inflexible rod, then the first and second vertebra would be unable to move relative to each other. However, the first elastomer  38  described herein is flexible and at least partially elastic and enables motion of the first section  32  and the second section  34  away from each other by compressing the first elastomer  38 . It is surprising that the dampening of the flexion movement, and of movement of the first section  32  and the second section  34  away from each other, is accomplished by compression of the first elastomer  38  instead of by stretching an elastomer. 
     As can be seen in  FIG. 2A , the anchor end  44  of the connector  42  is anchored in the second section  34 . The connector  42  extends through the first elastomer  38  and terminates in the piston end  46  which is disposed at the opposite end of the first elastomer  38  from the anchor end  44 . When the forces are applied to first section  32  and second section  34  to move apart relative to each other, the piston end  46  exerts a compression force on the first elastomer  38 , which is confined within the cavity  48 . This enables the first section  32  and the second section  34  to move away from each other, but also limits, or dampens, the movement. This, in turn, enables the vertebra to which the first section  32  and the second section  34  are attached to move relative to each other. 
     The material of construction of the first elastomer  38  may be selected to provide the desired amount of allowed motion of the first section  32  and the second section  34  away from each other, depending on the elasticity and quantity of the material chosen as well as the specific configuration of the shape of the first elastomer  38 . Thus, the motion of the first section  32  and the second section  34  away from each other may be selectively limited, or dampened. 
     The material and details of construction of the first elastomer  38  and the second elastomer  40  are selected so that, for example, one elastomer is stiffer, or has a different durometer, than the other elastomer. Thus, the resistance of the each of the elastomers to pressure may be different to allow for more flexion movement than extension movement or more extension movement than flexion movement. This enables selected and customized dampening of flexion and extension. “Flexion” is the forward bending of the spine. “Extension” is the backward bending of the spine. 
     As one example, several different possible first elastomers  38  and second elastomers  40  having different stiffness, shape, etc., properties are provided to the surgeon, such as in a kit, to allow the surgeon to select a first elastomer  38  and a second elastomer  40  from a variety of components. Then the specific elongated connecting element can be assembled, such as described above, prior to surgery. As another example, the surgeon can assemble, or have assembled, an elongated connecting element in which the first elastomer  38  is assembled from several components. The surgeon selects, for example, a first component having a first stiffness and a second component having a second stiffness and those are threaded onto the connector  42  to form a single first elastomer  38 . Likewise, a second elastomer  40  may be assembled from one or more separate components having different properties. The components can be elastomers or non-elastomers, resorbable or non-resorbable, such that the resulting first elastomer  18  and second elastomer  40  are elastomers, as described above. 
     In some situations, there will only be a need for an elongated connecting element to enable and limit, or dampen, the movement of the first section and second section away from each other, such as during spinal flexion.  FIGS. 6 and 7  illustrate an embodiment of the connecting element  30  present invention in which the second elastomer  40  is not present. This embodiment includes the first section  32 , the second section  34 , the first elastomer, or flexion dampening elastomer  38 , the connector  42 , the anchor end  44 , the piston end  46 , and the cavity  48 . In this embodiment, extension of the spine will not result in dampened movement of the first section  32  and the second section  34  toward each other, because there is no compressible second elastomer  40  disposed between the first section  32  and the second section  34 . But upon flexion of the spine, the first section  32  and the second section  34  will move away from each other, as illustrated in  FIG. 7 , by the same mechanism described above. 
     Alternatively, as shown in  FIG. 7   a,  the elongated connecting element  30  includes a first section  32 , a second section  34 , and a first elastomer  38  disposed within first section  32 , as described above, that enables and limits, or dampens, both flexion and extension of the vertebra. In this embodiment, for example, connector  42  is not flexible and piston end  46  is disposed within the first elastomer  38  in the cavity  48 . The first elastomer  38  is bounded at both ends by the cavity  48 , or otherwise constrained within the first section  32 , such that exertion of force in any direction will result in compression of the first elastomer  38 . Thus, the non-flexible connector  42  will exert a compression force on the first elastomer  38  via the piston end  46  regardless of whether the force applied is to move first section  32  and second section  34  away from each other or toward each other. 
     As indicated above, the specific shape of first elastomer  38  and second elastomer  40  may be selected without departing from the spirit and scope of the invention. For convenience of description and illustration, the shape of the first elastomer  38  and the second elastomer  40  has been described and illustrated above as substantially cylindrical. As an alternate example,  FIG. 10  illustrates an embodiment of the present invention in which first elastomer  38  has a substantially frustoconical or conical shape.  FIG. 10  also illustrates resorbable components  38 A and  40 A, which may or may not be present, as discussed above. Other shapes for first elastomer  38 , second elastomer  40 , and resorbable components  38 A and  40 A are also contemplated and within the scope of the invention. 
       FIG. 11  illustrates another embodiment in which first bone anchor assembly  60 , second bone anchor assembly  64 , and a third bone anchor assembly  66  are included. This arrangement is used, for example, when the rod assembly  30  is to be attached across at least three vertebra with each bone anchor assembly attachable to a different vertebra.  FIG. 11  also illustrates a rod assembly  30  in which there are two first elastomers  38  and two second elastomers  40 . In the illustrated embodiment, the first elastomers  38  are substantially frustoconical, but may be of any shape. 
     As described, the elongated connecting element, or rod assembly,  30  has at least two regions. A first region, generally associated with first section  32  is configured to enable and to limit, or dampen, the expansion of the element, such as when the first vertebra and the second vertebra are in flexion. This has the result of enabling and limiting flexion of the vertebra to which the connecting element is attached. 
     In still another alternative example, the first region includes the anchored connector and the first section having a cavity similar to cavity  48 , but with a viscous fluid or gel disposed in the cavity and sealed in by the piston end. As another example, the cavity is substantially sealed with a fluid therein, and the piston end is provided with a valve arrangement to control the flow of fluid for dampening. As yet another example, the connector may also have some elasticity and the interaction between the selectively elastic connector and the selectively elastic first elastomer brings about the desired dampening effect. A further example includes the first elastomer being a spring element. In yet another example, the first section does not define a cavity, but the first elastomer is integral with the first section. Each of these examples, and their equivalents, are included as the first elastomer, flexion dampening elastomer, rebound elastomer, or rebound element. 
     As described, the elongated connecting element, or rod assembly,  30  has also a second region, generally between first section  32  and second section  34 , which is configured to enable and to limit, or dampen, the compression of the element, such as when the first vertebra and the second vertebra are in extension. This has the result of enabling and limiting extension of the vertebra to which the connecting element is attached. 
     As described above, in one embodiment, the second region includes the second elastomer between the first section and the second section. As another example, the second region includes a spring element between first section and second section. As a further example, the second region includes an integral portion of the connecting element that has a different modulus of elasticity (Young&#39;s modulus) than the first section and the second section, enabling some compression of the second region. Other means for enabling and limiting, or dampening, extension of the connected vertebra are within the spirit and scope of the invention 
     In some embodiments, the first region and the second region are separate and distinct from each other, although they may be connected and communicate with each other. 
     Referring now to  FIGS. 12-14 , in this embodiment a spinal device  70  may be used as the spinal device  10  of system  20 . The spinal device  70  includes end anchors  72 ,  74  which include extension portions  76 ,  78 , respectively. In this embodiment, extension portions  76 ,  78  may be generally cylindrical, but other shapes including curves or non-circular cross-sections may also be suitable. The end anchors  72 ,  74  also include endplates  80 ,  82 , respectively. A hollow passage  84  may end through end anchor  74 , and a corresponding hollow passage may pass through the end anchor  76 . Anchor plates  86 ,  88  may be threadedly engaged with the hollow passages of the end anchors  74 ,  76 , respectively. For example, the anchor plate  88  may be threaded into the hollow passage  84  until the anchor plate abuts the endplate  82 . 
     A bumper  90 , which in this embodiment has a generally toroidal shape with an outer radial surface  90   a  and an inner radial surface  90   b.  The bumper  90  extends between the anchor plates  86 ,  88 . In alternative embodiments, the bumper may be solid (i.e., lacking a center aperture), dome-shaped, frusto-conical, or other shapes that may be apparent to one skilled in the art. A sheath  92  may circumferentially surround the bumper  90  and be connected between the endplates  80 ,  82  by fasteners  94 ,  96 , respectively. In this embodiment, the fasteners  94 ,  96  may be, for example, wires recessed into a circumferential groove on the endplates  80 ,  82 . 
     The end anchors  72 ,  74  can be constructed of any suitable material, preferably a biocompatible material. Examples of materials that can be used include cobalt-chromium alloys, titanium alloys, nickel titanium alloys, and/or stainless steel alloys, any member of the polyaryletherketone (PAEK) family such as polyetheretherketone (PEEK), carbon-reinforced PEEK, or polyetherketoneketone (PEKK); polysulfone; polyetherimide; polyimide; ultra-high molecular weight polyethylene (UHMWPE); and/or cross-linked UHMWPE. Suitable ceramic materials may include carbon-based materials or alumina-based materials. 
     The bumper  90  may be formed of any suitable biocompatible elastomer or polymer biomaterial, such as surgical latex, chloroprene, MIT&#39;s “biorubber” (glycerol and sebacic acid), polyethylene, polyester, polyurethane, urethane, polypropylene, silicone, or hydrogel, and combinations thereof. The bumper may be formed of a single material having a single durometer measurement or modulus of elasticity. Alternatively the bumper may have regions of differing hardness. For example a core section of the bumper may be formed of a material having a higher modulus of elasticity than an outer portion. Such a construction would allow greater initial compression but eventually limit further compression. Alternatively, the bumper may have differences in durometer at different lateral locations along the bumper to permit, for example flexion-extension, but limit lateral bending. Multiple durometers would allow flexion and extension stiffness to be designed into the device. 
     The sheath  92  may be biocompatible and flexible materials such as a segmented polyurethane, BIOSPAN-S (aromatic polyetherurethaneurea with surface modified end groups from Polymer Technology Group), CHRONOFLEX AR/LT (aromatic polycarbonate polyurethane with low-tack properties from CardioTech International), CHRONOTHANE B (aromatic polyether polyurethane from CardioTech International), CARBOTHANE PC (aliphatic polycarbonate polyurethane from Thermedics). The sheath may be permeable or impermeable, and in some embodiments may be a woven textile. 
     The spinal device  70  may be installed at a vertebral joint using, for example, connector assemblies  12  to attach the end anchors  72 ,  74  to the vertebrae V 1 , V 2 . The device  70  may be used to control flexion and extension motion and to resist shear loads. During a flexion motion, the end anchors  72 ,  74  may transmit a tensile load to the sheath  92  that will compress the outer radial surface of the bumper  90  radially inward. The sheath  92  may further serve as a tether to limit excessive flexion motion. During extension, the end anchors  72 ,  74  may transmit a compressive load to the anchor plates  86 ,  88  which may apply a compressive force to the bumper, causing the outer radial surface  90   a  of the bumper to extend radially outward. The flexible nature of the sheath allows this radial movement of the bumper. 
     In an alternative embodiment, the anchor plates may be omitted and the endplates  80 ,  82  may directly engage the bumper. In another alternative embodiment, the anchor plates or endplates may be angled to control the transmission of shear forces. 
       FIGS. 15-22  depict partial cross-sectional views of spinal devices that may be used as the spinal device  10  of system  20 . The spinal devices each include end anchors  100 ,  102  which may be formed of metal, ceramic, or polymers such as those described above for end anchors  72 ,  74 . In  FIG. 15 , a bumper  104  may have a generally toroidal or cylindrical shape and may be formed of a material such as those described above for bumper  90 . A sheath  106  may connect between the end anchors  100 ,  102 , encapsulating the bumper. The sheath  106  may be formed of a flexible material such as those described above for sheath  92 . 
     In  FIG. 16 , a bumper  108  may have a generally toroidal or cylindrical shape and may be formed of a material such as those described above for bumper  90 . A sheath  110  may connect between the end anchors  100 ,  102  and surround the outer surface of the bumper  108 . The sheath  110  may be formed of a flexible material such as those described above for sheath  92 . A second sheath  112  may extend between the end anchors  100 ,  102  and surround the inner surface of the bumper  108 . Under flexion loading, both sheaths  110  and  112  will transmit a tensile load to compress or squeeze the bumper  108  radially inward from both the outer and inner surfaces. The sheaths  110 ,  112  may further serve to limit excessive flexion motion. 
     In  FIG. 17 , a bumper  116  may have a generally toroidal or cylindrical shape and may be formed of a material such as those described above for bumper  90 . A flexible material  116  may extend through the bumper  116  and connect between the end anchors  100 ,  102 . The material  116  may be formed of a flexible material such as those described above for sheath  92 . The material  116  may serve as a tether to limit excessive flexion motion and provide reinforcement responsive to shear loading. 
     In  FIG. 18 , an outer bumper  118  may have a generally toroidal or cylindrical shape and may be formed of a material such as those described above for bumper  90 . A sheath  120  may connect between the end anchors  100 ,  102 , encapsulating the bumper. The sheath  120  may be formed of a flexible material such as those described above for sheath  92 . An inner bumper  122  may extend inside the outer bumper  118  and between the end anchors  100 ,  102 . The inner bumper  122  may be formed of an elastomeric material such as those described for bumper  90 . In this embodiment, the inner bumper  122  may have a harder or softer modulus or elasticity or durometer measurement than the outer bumper. 
     In  FIG. 19 , a lower bumper  124  may have a generally inverted dome shape and may be formed of a material such as those described above for bumper  90 . An upper bumper  128  may be positioned on top of the lower bumper  124 . The upper bumper  122  may also be formed of an elastomeric material such as those described for bumper  90 . In this embodiment, the upper bumper  122  may have a harder or softer modulus or elasticity or durometer measurement than the lower bumper. The upper and lower bumpers  124 ,  128  may be affixed or integrally molded with one another or alternatively, may be allowed to float with respect to one another. A sheath  126  may connect between the end anchors  100 ,  102 , encapsulating the bumpers  124 ,  128 . The sheath  126  may be formed of a flexible material such as those described above for sheath  92 . 
     In  FIG. 20 , a bumper  129  comprises alternating layers of two elastomeric material  130  and  132 , each having different moduli of elasticity or durometer measurements. The bumper  129  is positioned between the end anchors  100 ,  102 . In this embodiment, a flexible material  134  may extend through the layered bumper  129  and connect between the end anchors  100 ,  102 . The material  134  may be formed of a flexible material such as those described above for sheath  92 . The material  134  may serve as a tether to limit excessive flexion motion and provide reinforcement responsive to shear loading. 
     In  FIG. 21 , a bumper  136  may have a generally disc shape and may be formed of a material such as those described above for bumper  90 . A flexible material  138  may extend linearly through the bumper  136  and connect between the end anchors  100 ,  102 . The material  138  may be formed of a flexible material such as those described above for sheath  92 . The material  138  may serve to limit excessive flexion motion and provide reinforcement responsive to shear loading. Flexible tethers  140  may extend through the bumper at an angle and connect between the end anchors  100 ,  102 . The angled tethers  140  may assist with torsion resistance as well as shear resistance and flexion resistance. 
     In  FIG. 22 , an outer bumper  142  may have a generally disc shape and may be formed of a material such as those described above for bumper  90 . An inner bumper  144  may be encapsulated within the outer bumper  142 . The inner bumper  144  may also be formed of an elastomeric material such as those described for bumper  90 . In this embodiment, the inner bumper  144  may have a harder or softer modulus or elasticity or durometer measurement than the outer bumper  142 . A sheath  146  may connect between the end anchors  100 ,  102 , encapsulating the bumper and at least a portion of the end anchors  100 ,  102 . The sheath  146  may be formed of a flexible material such as those described above for sheath  92 . The sheath  146  may provide resistance to excessive flexion motion and may also serve to contain wear debris. 
     Referring now to  FIGS. 23 and 24 , in this embodiment a spinal device  150  may be used as the spinal device  10  of system  20 . The spinal device  150  includes an end anchor  152  which includes an extension portion  154  and an endplate  156 . The end anchor  152  further includes an internal bore  158  extending through the endplate  156  and at least partially through the extension portion  154 . In alternative embodiments, the internal bore may pass entirely through the extension portion, resulting in the extension portion having a tubular configuration. The spinal device  150  further includes an end anchor  160  which includes an extension portion  161  and an endplate  162 . The end anchor  160  further includes a rod portion  164  extending from the endplate  162  in an opposite direction from the extension portion  161 . The spinal device  150  further includes a bumper  166 . In this embodiment, extension portions  154 ,  161  may be generally cylindrical, but other shapes including curves or non-circular cross-sections may also be suitable. In this embodiment, the extension portion  161 , the endplate  162 , and the rod portion  164  may be integrally formed. In alternative embodiments, the sections may be modular. For example, the extension portion could be threadably connected to the rod portion. 
     The end anchors  152 ,  160  can be constructed of any suitable material, preferably a biocompatible material. Examples of material that can be used include cobalt-chromium alloys, titanium alloys, nickel titanium alloys, and/or stainless steel alloys, any member of the polyaryletherketone (PAEK) family such as polyetheretherketone (PEEK), carbon-reinforced PEEK, or polyetherketoneketone (PEKK); polysulfone; polyetherimide; polyimide; ultra-high molecular weight polyethylene (UHMWPE); and/or cross-linked UHMWPE. Suitable ceramic materials may include carbon-based materials or alumina-based materials. 
     The bumper  166  may be formed of any suitable biocompatible elastomer or polymer biomaterial, such as surgical latex, chloroprene, MIT&#39;s “biorubber” (glycerol and sebacic acid), polyethylene, polyester, polyurethane, urethane, polypropylene, silicone, or hydrogel, and combinations thereof. The bumper may be formed of a single material having a single durometer measurement or modulus of elasticity. An example of a suitable durometer hardness may be between  50 A and  75 D. Alternatively the bumper may have regions of differing hardness. For example a core section of the bumper may be formed of a material having a higher modulus of elasticity than an outer portion. Such a construction would allow greater initial compression but eventually limit further compression. Alternatively, the bumper may have differences in durometer at different lateral locations along the bumper to permit, for example flexion-extension, but limit lateral bending. Multiple durometers would allow flexion and extension stiffness to be designed into the device. The bumper  166  may also be provided in different heights to allow a surgeon to select an appropriate distraction height. 
     The spinal device  150  may be assembled by sliding the bumper  166  over the rod portion  164  such that rod portion extends through the bumper and the bumper contacts the endplate  162 . The rod portion  164  may then be inserted into the internal bore  158  of the end anchor  152 . The internal bore  158  may be long enough that the endplate  156  contacts the bumper  166 . As assembled, the end anchor  152  may be allowed to slide or float on the rod portion  164 . The bumper  166  may also be allowed to slide or float on the rod portion  164 . Alternatively, the bumper may be affixed to either the endplate  162  or the endplate  156 . As shown in  FIG. 24 , to prevent the rod portion  164  from rotating within the internal bore  158 , the rod portion may include an outwardly extending key  167  configured to slide within a key channel  168  of the internal bore  158 . In alternative embodiments, the channel may be formed in the rod portion and the outwardly extending key formed on the internal bore. Other interdigitating features which may prevent rotation of the rod portion relative to the internal bore are also suitable. 
     The end anchors  152 ,  160  may be locked to the vertebrae V 1 , V 2  with connecting assemblies  12 . As implanted the device  150  may permit both flexion and extension motion. In extension, the bumper  166  may serve to block excessive extension. Depending upon the hardness of the bumper  166 , the bumper may provide either a hard stop or a dampened soft stop. 
     Referring now to  FIG. 25 , in this embodiment a spinal device  170  may be used as the spinal device  10  of system  20 . The spinal device  170  includes an end anchor  172  which includes an extension portion  174 , a rod portion  176 , and an endplate  178 . In this embodiment, the extension portion  174 , the endplate  178 , and the rod portion  176  may be integrally formed. In alternative embodiments, the sections may be modular. For example, the extension portion could be threadably connected to the extension portion. The end anchor  172  may be formed of a suitable biocompatible material such as those described for end anchor  160 . 
     The device  170  further includes a series of bumpers  180 ,  182 ,  184 ,  186 . The bumpers  180 - 186  may be formed of different materials or have different hardnesses. Fewer or more layers of bumpers may be used. The bumpers  180 - 186  may be formed of a suitable biocompatible material such as those described above for bumper  166 . 
     The device  170  may be installed by locking the end anchor extension portion  174  to the vertebra V 2  with a connecting assembly  12 , which may be a fixed connection. The rod portion  176  may be slidably attached to vertebra V 1  with a sliding connector that allows the rod portion  176  to slide within the sliding connector relative to the vertebra V 1 . In one alternative embodiment, the rod portion  176  may include a stop at an end opposite the endplate  178  to prevent the sliding connector from decoupling from the rod portion. As implanted, the device  170  may permit both flexion and extension motion. In extension, the series of bumpers  180 - 186  may serve to block excessive extension. Depending upon the hardness of the bumpers, the bumper may provide either a hard stop or a dampened soft stop. 
     Referring now to  FIG. 26 , in this embodiment a spinal device  190  may be used as the spinal device  10  of system  20 . The spinal device  190  includes an end anchor  192  which includes an extension portion  194 , an endplate  196 , and a rod portion  198 . A stop portion  199  may be located on the rod portion  198  at the end opposite the endplate  196 . The spinal device  190  further includes an end anchor  200  which includes an extension portion  201 , an endplate  204 , and a crimped section  202 . The end anchor  200  further includes an internal bore  203  extending through the endplate  204  and at least partially through the extension portion  201 . In alternative embodiments, the internal bore may pass entirely through the extension portion, resulting in the extension portion having a tubular configuration. The spinal device  190  further includes a bumper  206 . In this embodiment, the extension portion  194 , the endplate  196 , and the rod portion  198  may be integrally formed. In alternative embodiments, the sections may be modular. For example, the extension portion could be threadably connected to the rod portion. 
     The end anchor  192  may be formed of a suitable biocompatible material such as those described for end anchor  160 . The bumper  206  may be formed of a suitable biocompatible material such as those described above for bumper  166 . 
     The spinal device  190  may be assembled by sliding the bumper  206  over the rod portion  198  such that rod portion extends through the bumper and the bumper contacts the endplate  196 . The rod portion  164  may then be inserted into the internal bore  203  of the end anchor  200 . The internal bore  203  may be long enough that the endplate  204  contacts the bumper  206 . The stop  199  may be forced past the crimped section  202  so that the stop  199  becomes trapped within the internal bore  203  by the crimped section  202 . Alternatively, the crimped section may be formed after the stop  199  has been inserted fully into the internal bore  203 . Creating the crimped section after the stop has been inserted would eliminate the need to temporarily deform the end anchor  200  to force the stop past the crimped section. 
     As assembled, the end anchor  200  may be allowed to slide or float on the rod portion  198 . The bumper  206  may also be allowed to slide or float on the rod portion  198 . Alternatively, the bumper may be affixed to either the endplate  196  or the endplate  204 . To prevent the rod portion from rotating within the internal bore, the rod portion may include an outwardly extending key such as described above in  FIG. 24 . The end anchors  192 ,  200  may be locked to the vertebrae V 1 , V 2  with connecting assemblies  12 . As implanted the device  190  may permit both flexion and extension motion. In extension, the bumper  206  may serve to block excessive extension. Depending upon the hardness of the bumper  166 , the bumper may provide either a hard stop or a dampened soft stop. In this embodiment, the crimped portion  202  may prevent the end anchor portion  192  from dislocating from the end anchor portion  200 . 
     Referring now to  FIG. 27 , in this embodiment a spinal device  210  may be used as the spinal device  10  of system  20 . The spinal device  210  may be attached to vertebrae V 1 , V 2  with connecting assemblies which in this embodiment include head portions  212  and bone screw portions  214 . The spinal device  210  includes a rod portion  216  which extends along a longitudinal axis  218 . The spinal device  210  further includes a sleeve portion  220  which extends over the rod portion  216  and shares the central longitudinal axis  218 . 
     The rod portion  216  may be formed of a flexible biocompatible material including, for example, a material of the polyaryletherketone (PAEK) family such as polyetheretherketone (PEEK), carbon-reinforced PEEK, or polyetherketoneketone (PEKK); polysulfone; polyetherimide; polyimide; ultra-high molecular weight polyethylene (UHMWPE); and/or cross-linked UHMWPE. Flexible ceramic or metals may also be suitable. The sleeve portion may be formed of any of the materials described for the rod portion  216  or alternatively may be formed of a more resilient material such as the materials described above for bumper  166 . 
     As assembled and installed the spinal device  210  may permit both flexion and extension. In extension, the sleeve portion  220  may serve to limit extension by preventing further bending of the rod portion  216  when the upper head portion  212  of the connecting assembly contacts the sleeve portion. As can be readily understood, a shorter sleeve portion may permit more extension than would a longer sleeve portion. 
     As shown in  FIGS. 28 and 29 , the sleeve height may be adjustable in situ in situations where it may be difficult to determine pre- or intra-operatively the proper distraction height of the sleeve. In  FIG. 28 , a spinal device  230  may be used as the spinal device  10  of system  20 . The spinal device  230  may be attached to vertebrae V 1 , V 2  with connecting assemblies which in this embodiment include head portions  212  and bone screw portions  214 . The spinal device  230  includes the rod portion  216  which extends along the longitudinal axis  218 . In this embodiment, the spinal device  230  further includes a sleeve portion comprising sleeve section  232  and a sleeve section  236  both of which extend over the rod portion  216 . Sleeve section  232  may include an outer threaded section  234  which mates with an inner threaded section of the sleeve section  236 . The overall height of the combined sleeve portion may be controlled by adjusting the threaded connection between the section  232  and the section  236 . 
     In an alternative embodiment, one of the sections  232 ,  236  could be connected to a drive system which may include a receiver and a motor. The receiver may receive remote signals and control the action of the motor to turn one of the sections  232 ,  236  to increase or decrease the overall height of the sleeve portion. The drive system may be enclosed in a pacemaker style housing so that the height of the sleeve portion may be adjusted after the surgical implantation has occurred. Such a post-operative adjustment could be used to remove a segmental kyphosis if the height was too great upon insertion. Alternately, the distraction height may be increased post-operatively if, for example, subluxation of the facets occurred during extension. 
     In still another alternative embodiment, the overall height of the combined sleeve portion and/or the distance between the head portions  212  may be adjusted with a pump system. In this embodiment, the threaded connection may be omitted and replaced with a fluid pump to increase or decrease the overall height. To increase the overall height of the sleeve portions, a catheter may be inserted into the patient and into a connection on the spinal device. A syringe may be connected to the catheter. To increase the overall height, the syringe may deliver fluid to the pump via the catheter. To decrease the overall height, the syringe may remove fluid from the pump. 
     In still another alternative embodiment, the overall height of the combined sleeve portion and/or the distance between the head portions  212  may be adjusted by manipulating a mechanical driver such as a screw or a jack. For example, a cannula may be inserted into the patient to access a screw head of the screw. A screwdriver may be passed through the cannula to turn the screw head and thereby increase or decrease the overall height. 
     In  FIG. 29 , a spinal device  240  may be used as the spinal device  10  of system  20 . The spinal device  240  may be attached to vertebrae V 1 , V 2  with connecting assemblies which in this embodiment include head portions  212  and bone screw portions  214 . The spinal device  240  includes the rod portion  216  which extends along the longitudinal axis  218 . In this embodiment, the spinal device  240  further includes a sleeve portion comprising sleeve section  242 , a sleeve section  244 , and an intermediate sleeve section  246 , all of which extend over the rod portion  216 . Intermediate sleeve section  246  may include outer threads which mate with inner threaded sections of the sleeve sections  242 ,  244 . For example, sleeve section  244  may have left hand threads and the sleeve section  242  may have right hand threads. In this example, the intermediate section  246  may have a single thread pattern. The overall height of the combined sleeve portion may be controlled by adjusting the threaded connection between the sections  242 ,  244  and the intermediate section  246 . 
     In an alternative embodiment, the intermediate section  246  or one or both of the sections  242 ,  244  could be connected to a drive system which may include a receiver and a motor. The receiver may receive remote signals and control the action of the motor to turn one of the sections to increase or decrease the overall height of the sleeve portion. 
     These embodiments in which the height of the spinal device may be increased may be particularly useful for pediatric applications in which the patient&#39;s spine grows naturally and the spinal device should be adjusted to track the growth of the patient. 
     Specific Embodiments 
     A elongated connecting element for use in a spinal stabilization system comprises, a first section; a second section; a first elastomer disposed within the first section; and a second elastomer disposed between the first section and the second section. One of the first elastomer and the second elastomer resists movement of the first section and the second section toward each other and the other of the first elastomer and the second elastomer resists movement of the first section and the second section away from each other. 
     The connecting element further comprising a connector anchored in the second section, extending completely through the second elastomer, and extending at least partially through the first elastomer. 
     The connector comprises polymer braid, weave, or monofilament. 
     The first section defines a cavity in which the first elastomer is disposed, the cavity comprising a first end and a second end. 
     The first section comprises a liner disposed within the cavity. 
     The first elastomer and the second elastomer are not adjacent to each other. 
     The first section and the second section comprise the same or different material selected from the group consisting of cobalt-chromium alloy, titanium alloy, nickel titanium alloy, and/or stainless steel alloy, and any member of the polyaryletherketone family. 
     The first elastomer and the second elastomer comprise the same or different material selected from the group consisting of polyethylene, polyester, polyurethane, urethane, polypropylene, silicone, or hydrogel, and combinations thereof. 
     The first elastomer comprises a different material than the second elastomer. 
     The first elastomer has a different resiliency than the second elastomer. 
     The first elastomer or the second elastomer comprises a plurality of elastomeric components. 
     At least one of the first elastomer and the second elastomer comprises a resorbable component. 
     A rod assembly for use in a spinal stabilization system comprises a first region configured to enable dampened expansion of the rod assembly upon flexion of the spine. The rod assembly further includes a separate second region configured to enable dampened compression of the rod assembly upon extension of the spine. 
     The first region comprises a first elastomer disposed within a first section of the rod assembly. 
     The second region comprises a second elastomer disposed between a first section and a second section of the rod assembly. 
     A system for stabilization of a spine, comprises a first bone anchor assembly capable of attachment to a first vertebra; a second bone anchor assembly capable of attachment to a second vertebra; and an elongated connecting element. The elongated connecting element comprises a first section for attachment to the first bone anchor assembly; a second section for attachment to the second bone anchor assembly; a first region configured to enable dampened expansion of the connecting element upon flexion of the spine; and a separate second region configured to enable dampened compression of the connecting element upon extension of the spine. 
     The first region comprises a first elastomer disposed within the first section of the connecting element. 
     The second region comprises a second elastomer disposed between the first section and the second section of the connecting element. 
     The system further comprises a connector anchored in the second section, extending through the second elastomer and at least part of the first elastomer, terminating in an end in communication with the first elastomer. 
     A rod assembly for attachment to vertebrae in a spinal stabilization system, the rod comprising first means to enable and limit flexion of the vertebrae, and second means to enable and limit extension of the vertebra, wherein the first means is different from the second means. 
     The first means comprises a first elastomer disposed within a cavity defined within the rod assembly. 
     The second means comprises a second elastomer disposed between a first section and a second section of the rod assembly. 
     A rod assembly for use in a spinal stabilization system comprises a first section; a second section; and a rebound element disposed within the first section. The rebound element enables and dampens movement of the first section and the second section away from each other. 
     The rod assembly further comprises a connector anchored in the second section and extending at least partially through the rebound element. 
     The connector is in communication with the rebound element such that when the first section and the second section are moved away from each other, the connector exerts force on the rebound element and the resistance of the rebound element to the exerted force dampens the movement of the first section and the second section away from each other. 
     The rebound element enables and dampens movement of the first section and the second section toward each other. 
     A rod assembly for use in a spinal stabilization system comprises a first section defining a cavity; a second section; a connector anchored in the second section and connecting the first section to the second section; and a flexion dampening elastomer disposed within the cavity. The connector communicates with the flexion dampening elastomer to enable and dampen movement of the first section and the second section away from each other. 
     The rod assembly further comprises a dampening elastomer disposed between the first section and the second section, the dampening elastomer selected to enable and limit movement of the first section toward the second section. The connector extends through the dampening elastomer. 
     While the present invention has been illustrated by the above description of embodiments, and while the embodiments have been described in some detail, it is not the intention of the applicant to restrict or in any way limit the scope of the invention to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative apparatus and methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of the applicant&#39;s general or inventive concept. It is understood that all spatial references, such as “horizontal,” “vertical,”“top,” “upper,” “lower,” “bottom,” “left,” and “right,” are for illustrative purposes only and can be varied within the scope of the disclosure. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures.