Patent Publication Number: US-2020289164-A1

Title: Flexible spine stabilization system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/109,831 filed on Aug. 23, 2018, which is a continuation of U.S. patent application Ser. No. 14/876,883 filed on Oct. 7, 2015 which is a continuation-in-part application of U.S. patent application Ser. No. 13/894,903, which is a continuation of U.S. patent application Ser. No. 12/112,096 filed on Apr. 30, 2008, now issued as U.S. Pat. No. 8,465,526, which claims priority to U.S. Provisional Application Ser. No. 60/914,993 filed on Apr. 30, 2007, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present disclosure generally relates to flexible stabilization systems for spinal motion segment units. In particular, certain embodiments are directed to a soft stabilization system including at least two bone fasteners and a flexible portion conformable to the natural spinal movement. 
     BACKGROUND OF THE INVENTION 
     The spine includes a series of joints routinely called motion segment units, which is the smallest component of the spine that exhibits kinematic behavior characteristic of the entire spine. The motion segment unit is capable of flexion, extension, lateral bending and translation. The components of each motion segment unit include two adjacent vertebrae and their apophyseal joints, the intervertebral disc, and the connecting ligamentous tissue. Each component of the motion segment unit contributes to the mechanical stability of the joint. 
     Components of a motion segment that move out of position or become damaged can lead to serious pain and may lead to further injury to other components of the spine. Depending upon the severity of the structural changes that occur, treatment may include fusion, discectomy, or laminectomy. 
     Underlying causes of structural changes in the motion segment unit leading to instability include trauma, degeneration, aging, disease, surgery, and the like. Thus, rigid stabilization of one or more motion segment units may be an important element of a surgical procedure in certain cases (i.e., injuries, deformities, tumors, etc.), whereas it is a complementary element in others (i.e., fusion performed due to degeneration). The purpose of rigid stabilization is the immobilization of a motion segment unit. 
     As mentioned above, current surgical techniques typically involve fusing one or more unstable motion segment units and possibly, the removal of ligaments, bone, disc, or combinations thereof included in the unstable motion segment unit or units prior to fusing. There are several disadvantages to fusion, however. For example, the fusing process results in a permanent or rigid internal fixation of all or part of the intervertebral joints and usually involves metallic rods, plates, and the like for stabilization. In all cases, the systems are intended to rigidly immobilize the motion segment unit to promote fusion within that motion segment unit. 
     In addition to a loss of mobility, fusion also causes the mobility of the motion segment to be transferred to other motion segments of the spine. The added stresses transferred to motion segments neighboring or nearby the fused segment can cause or accelerate degeneration of those segments. One other disadvantage to fusion is that it is an irreversible procedure. In addition, it is believed that fusion of a motion segment has a clinical success of approximately 70 percent, and often does not alleviate pain experienced by the patient. 
     Thus, while such fusion systems have been used since the early 1960&#39;s, the intentionally rigid designs have often caused stress concentrations and have directly and indirectly contributed to the degeneration of the joints above and below the fusion site (as well as at the fusion site itself). In addition, rigid, linear bar-like elements eliminate the function of the motion segment unit. Finally, removal of portions of the motion segment unit reduces the amount of support available for the affected motion segment unit. 
     Fusion procedures can be improved by modifying the load sharing characteristics of the treated spine. Thus, it would be desirable to allow more of a physiologic loading between pedicular fixation and anterior column support. It would also be desirable to have a device that precludes or at least delays the need for fusion for all but the most advanced degeneration of a motion segment, particularly if such a device would allow close to normal motion and pain relief. 
     Thus, a need exists in the art for a soft spine stabilization system that replicates the physiologic response of a healthy motion segment. 
     SUMMARY OF THE INVENTION 
     According to one aspect, a flexible spinal stabilization system that can provide load sharing either as an enhancement to a fusion device or as a motion-preserving non-fusion device is provided. 
     According to another aspect, a flexible prosthesis for intervertebral or intersegmental stabilization designed to load share with a graft in the anterior column that allows for graft resorption while ensuring compressive loading on the graft for fusion procedures in the spine is provided. 
     Another embodiment is directed towards a device for intervertebral or intersegmental stabilization designed to ensure proper alignment and motion between vertebrae of the spinal column that helps partially unload the discs and facet joints to give pain relief. 
     According to another aspect, a flexible connection element may be used to as part of various components of a spine stabilization system. For instance, the flexible connection element may form all or part of one longitudinal stabilization members. In another aspect, the flexible connection element may also form at least part of a transconnector. Depending on what component of the spine stabilization system uses the invention, fasteners may also be connected to the component. For instance, in one embodiment the flexible connection element is connected to bone fasteners, such as pedicle screws or the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of one embodiment of a flexible connection element according to the invention; 
         FIG. 1A  is a perspective view of a portion of a flexible connection element according to the invention; 
         FIG. 2  is a perspective view of another embodiment of a flexible connection element; 
         FIG. 3  is a cross-sectional view of another embodiment of a flexible connection element; 
         FIG. 4  is a posterior view of one embodiment of a spine stabilization system of the invention; 
         FIG. 5  is an exploded view of one embodiment of a stabilization system according to the invention with an alternate embodiment of a flexible connection element; 
         FIGS. 5A-5C  are side views of the embodiment of  FIG. 5  in a neutral position and extension positions; 
         FIG. 6  is an exploded view of one embodiment of an end portion of the flexible connection element of  FIG. 5 ; 
         FIG. 7  is an assembled view of the end portion of  FIG. 6  shown in a first position; 
         FIG. 8  is an assembled view of the end portion of  FIG. 6  shown in a second position; 
         FIGS. 8A-8B  are exploded perspective and exploded cross-sectional views, respectively, of an embodiment of another end portion of the flexible connection element of  FIG. 5 ; 
         FIGS. 8C-8D  are assembled perspective and assembled cross-sectional views, respectively, of the embodiment of  FIGS. 8A-8B ; 
         FIGS. 9-10  are exploded views of an embodiment of another end portion of the flexible connection element of  FIG. 5 ; 
         FIG. 11  is a partial assembled view of the end portion of  FIGS. 9-10  shown in a second position; 
         FIG. 12  is a cross-sectional view of the end portion of  FIGS. 9-11  shown in a second position; 
         FIGS. 12A-12B  are exploded perspective and exploded cross-sectional views, respectively, of an embodiment of another end portion of the flexible connection element of  FIG. 5 ; 
         FIGS. 12C-12D  are assembled cross-sectional views of the embodiment of  FIGS. 12A-12B ; 
         FIG. 13  is a perspective view of another embodiment of a flexible connection element; 
         FIG. 14  is a side view of another embodiment of a flexible connection element; 
         FIGS. 15-16  are perspective views of alternate embodiments of stabilization systems according to the invention each with alternate embodiments of a flexible connection elements; 
         FIG. 17  is a perspective view of another embodiment of a stabilization system; 
         FIG. 18  is an exploded view of another embodiment of a flexible connection element; 
         FIG. 19  is an exploded view of another embodiment of a flexible connection element; 
         FIGS. 20-22  depict an alternate end portion of a flexible connection element according to the invention; 
         FIG. 23  is a perspective view of another flexible connection element; 
         FIGS. 24-25  are perspective and cross-sectional views, respectively, of another embodiment of a flexible connection element; 
         FIG. 26  is an exploded view of another embodiment of a flexible connection element; 
         FIG. 27  is an exploded view of another embodiment of a flexible connection element; 
         FIG. 28  is an exploded view of another embodiment of a flexible connection element; 
         FIGS. 29-30  are perspective and exploded views, respectively, of another embodiment of a flexible connection element; 
         FIGS. 31-32  are perspective and exploded views, respectively, of another embodiment of a flexible connection element; 
         FIGS. 33-34  are perspective and exploded views, respectively, of another embodiment of a flexible connection element; 
         FIG. 35  is an cross-sectional view of another embodiment of a flexible connection element; 
         FIGS. 36, 36A and 37  are perspective and exploded views, respectively, of another embodiment of a flexible connection element; 
         FIG. 38  is a perspective view of another embodiment of an end portion according to the invention; 
         FIGS. 39-40  are perspective and partial exploded views, respectively, of another embodiment of a flexible connection element; 
         FIG. 41  is a perspective view of another embodiment of a flexible connection element; 
         FIG. 42  is a perspective view of another embodiment of an end portion according to the invention; 
         FIGS. 43-45  are perspective, top, and cross-sectional views, respectively, of another embodiment of a flexible connection element; 
         FIG. 46  is a perspective view of another embodiment of a flexible connection element; 
         FIG. 47  is an exploded view of another embodiment of a flexible connection element; 
         FIG. 48  is a perspective view of another embodiment of a flexible connection element; 
         FIG. 49  is a perspective view of another embodiment of a flexible connection element; 
         FIGS. 50-52  are perspective views of additional embodiments of flexible connection elements; and 
         FIGS. 53-54  depict another embodiment of a flexible connection element; 
         FIG. 55  is a perspective view of another embodiment of a flexible connection assembly including modular rings. 
         FIG. 56  is a perspective view of a flexible connection assembly including just PEEK rings. 
         FIG. 57  is a perspective view of a flexible connection assembly including PEEK rings and a PCU ring. 
         FIG. 58  is a perspective view of a flexible connection assembly including alternating PEEK and metal rings. 
         FIG. 59  is a perspective view of the flexible connection assembly of  FIG. 58  in a flexed state. 
     
    
    
     BRIEF DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     Embodiments of the disclosure are generally directed to flexible stabilization systems for use with the anterior, antero-lateral, lateral, and/or posterior portions of at least one motion segment unit of the spine. The systems of the invention are designed to be conformable to the spinal anatomy, so as to be generally less intrusive to surrounding tissue and vasculature than existing rigid stabilization systems. 
     Certain embodiments may be used on the cervical, thoracic, lumbar, and/or sacral segments of the spine. For example, the size and mass increase of the vertebrae in the spine from the cervical to the lumbar portions is directly related to an increased capacity for supporting larger loads. This increase in load bearing capacity, however, is paralleled by a decrease in flexibility and an increase in susceptibility to strain. When rigid immobilization systems are used in the lumbar segment, the flexibility is decreased even further beyond the natural motion restriction of that segment. Replacing the conventional rigid immobilization systems with certain embodiments disclosed herein may generally restore a more natural movement and provide added support to the strain-susceptible area. 
     One embodiment of a spine stabilization system described herein includes at least two bone fasteners and at least one flexible connection element extending at least partially between the bone fasteners. In general, the flexible connection element may advantageously provide desirable properties for bending or twisting that allows the system to accommodate natural spine movement. According to some embodiments, the flexible connection element approximates or resembles a relatively circular cross-section tube or rod. In alternate embodiments, a flexible connection element may have other shapes as well. For instance the flexible connection element may have a cross-section that approximates or resembles a circle, an oval, an ellipse, or angular geometric shapes such as triangles, squares, rectangles, trapezoids, or the like. In many embodiments, the flexible connection element may be made from more than one component and the flexible connection element may have complex and varied cross-sections along its length. It should be understood that in these examples the different types of flexible connection elements described herein may be replaced or interchanged with a flexible connection element having different shapes or configurations, including the many variations described herein. 
     Embodiments of the present disclosure may also be used as a cross-brace or transconnector in communication with two rods along a portion of the length of the spine. It is well known that the strength and stability of a dual rod assembly can be increased by coupling the two rods with a transconnector that extends across the spine in a direction that is generally perpendicular to the longitudinal axes of the rods. When used as a transconnector, the disclosed embodiments may include a first fastener connecting the transconnector to a first rod and a second fastener connecting the transconnector to a second rod. Alternatively, the transconnector may be connected to one or more bone fasteners associated with a rod. Examples of transconnector designs that may be improved by the present disclosure are described in U.S. Pat. No. 5,743,911 to Cotrel, U.S. Pat. No. 5,651,789 to Cotrel, U.S. Pat. No. 6,139,548 to Errico, U.S. Pat. No. 6,306,137 to Troxell, U.S. Pat. No. 5,947,966 to Drewry, U.S. Pat. No. 5,624,442 to Mellinger, and U.S. Pat. No. 6,524,310 to Lombardo, all of which are incorporated herein in their entirety. 
     As explained in greater detail below, the flexible connection element can be configured in many different ways. For instance, the flexible connection element may be a relatively straight connection element, such as shown in  FIG. 1 . Alternatively, the flexible connection element may have a curved shape that corresponds approximately to the natural curvature of the portion of the spine that it supports. In each embodiment, the flexible connection element may be made of one or more components that are configured to allow the element to flex, bend, or twist. 
     The Flexible Connection Element 
     Embodiments of the flexible connection element generally provide stability, strength, flexibility, and resistance without the traditional rigidity of prior systems. While the flexible connection element may be designed in a variety of ways according to the invention, the types of design may differ depending on the final implementation of the system, i.e., lateral, posterior, etc. In a posterior application, for example, the flexible connection element may include a straight or curved profile along its length. 
     Referring to  FIG. 1 , one embodiment of a flexible connection element  10  is shown. Connection element  10  generally comprises first and second end members or portions  12 ,  14  and an intermediate portion or spacer  16  disposed therebetween. End portions  12 ,  14  and spacer  16  are disposed about a coupling member, such as a tether, cable, or cord  18  and extend along a longitudinal axis  20 . End portions  12 ,  14  are configured and dimensioned to be accepted and retained by a bone fastener or anchor such as a pedicle screw  34  or laminar hook. In general, end portions  12 ,  14  are made from a generally rigid material such as, for example, titanium or any other known biocompatible metal or rigid material. Intermediate portion  16  may be a flexible or resiliently deformable member that provides force absorbing effect in transmitting spinal column loads between the anchors to which flexible connection element  10  is engaged. Intermediate portion  16  may also permit relative movement between first and second end portions  12 ,  14 . 
     Various embodiments of flexible connection element  10  contemplate various alternative configurations of end portions  12 ,  14  intermediate portions  16 , and/or techniques for securing end portions  12 ,  14 . As best seen in  FIG. 1A , end portions  12 ,  14  may be in the form of spools and may have a generally barbell shaped body  22  with a middle body portion  24  extending between end plates or flanges  26 . The spacing between flanges  26  and the size of middle portion  24  may be dimensioned to fit within preexisting pedicle screw systems, such as those having an upright yoke or tulip-like receptacle. For instance, middle portion  24  may have a cylindrical shape and may be received in a pedicle screw similar to a cylindrical rod in other known stabilization systems. A channel or opening  27  may extend at least partially through body  22  for accommodating a coupling element or cord  18 . In alternate embodiments, such as those shown in  FIGS. 2 and 3 , spool members  12 ,  14  may have one end plate or flange  26  configured to engage intermediate portion  16  and the opposing end  28  may be cylindrical or rod shaped and may not have a flange. Cord  18  may extend entirely through the spools, as shown in  FIGS. 1 and 2 , or cord  18  may extend only partially within spools  12 ,  14 , as shown in  FIG. 3 . 
     According to the embodiment of  FIG. 1 , end portions or spools  12 ,  14  may be affixed to cord  18  and intermediate portion  16  may be slidable or moveable with respect to cord  18 . Any known means or method may be used to secure or affix cord  18  to spools  12 ,  14 . According to one variation, a mechanical clamping member such as a set screw may be used to affix spools  12 ,  14  to cord  18 . In the embodiment of  FIG. 3 , cord  18  may be crimped, glued, or otherwise secured to spools  12 ,  14 . In alternate embodiments discussed in more detail below, one or more spools  12 ,  14  may be slidable or moveable about cord  18 . 
     Intermediate portion or spacer  16  may be made from a flexible, soft, and/or elastically resilient or deformable biocompatible material such as for example, a biocompatible elastomer, silicone, polyurethane or polycarbonate urethane or any other known similar material. The intermediate portion may vary somewhat in shape, size, composition, and physical properties, depending upon the particular joint or level for which the implant is intended. The shape of the body of the intermediate portion should complement that of the adjacent end portion(s) or plates to which it engages to allow for a range of translational, flexural, extensional, and rotational motion, and lateral bending appropriate to the particular joint being replaced. The thickness and physical properties of the intermediate portion should provide for the desired degree of elasticity or damping. However, the intermediate portion should be sufficiently stiff to effectively cooperate with the end portions to limit motion beyond the allowable range. Polyurethane-containing elastomeric copolymers, such as polycarbonate-polyurethane elastomeric copolymers and polyether-polyurethane elastomeric copolymers, generally having durometer ranging from about shore  80 A to about shore  100 A and between about shore  30 D to about shore  65 D have been found to be particularly suitable for vertebral applications. If desired, these materials may be coated or impregnated with substances to increase their hardness or lubricity, or both. 
     In some embodiments, intermediate portion  16  has a generally cylindrical or tubular shaped body with a channel  30  extending longitudinally therethrough. Channel  30  may be appropriately sized and dimensioned for accommodating the coupling member or cord  18  therethrough. In the embodiment of  FIG. 1 , spacer  16  has a cylindrical profile and the external diameter  32  may be about the same as the diameter of flange  26  of end portions  12 ,  14 . Alternatively, spacer  16  may be smaller or larger in diameter, or may be variable in diameter. According to one embodiment, intermediate portion  16  may range in length depending on the application or surgeon preference. For instance, spacer  16  may be between about 4 mm and 38 mm, and in a kit a multitude of differing lengths and dimensions may be provided. One skilled in the art will appreciate that the flexibility of the connection element  10  may be changed by the selection of the intermediate portion material and/or varying its dimensions. 
     Coupling member or cord  18  may be made from polyethylene terephthalateor (PET), ultra high molecular weight (UHMW) polyethylene such as Dyneema® or any other known material. The cord may also be formed using a braided or stranded wire or synthetic or any combination as desired. The strands may be formed from identical materials or may differ from each other. For example, one strand may be wire, whereas other strands may be rubber-based. In the embodiment of  FIG. 1 , cord  18  may also be made from, or additionally contain, an elastic material selected to allow the cord to elastically deform along its longitudinal axis. In this regard, depending on the selected material, cord  18  may elastically stretch or elongate along axis  20 . In other embodiments, cord  18  may be designed to have a constant length so as to not stretch or elongate along its length. It will be clear to one skilled in the art that the structure, length and diameter of the coupling member will affect the flexibility of the connection element  10 . 
     Referring to  FIG. 4 , when end portions  12 ,  14  are retained by respective bone fasteners  34 , for example, and affixed to adjacent vertebrae, the connection element  10  provides stability while simultaneously permitting motion to the vertebrae in six degrees of freedom (i.e., x-axis, y-axis, z-axis, pitch, roll and yaw). Although the spacer  16  substantially limits the motion of the spools  12 ,  14  in the longitudinal axial direction, the compressibility of the spacer  16  and elasticity of cord  18  between the spools  12 ,  14  allows for stabilized motion of the spools  12 ,  14  in each of the six degrees of freedom while also providing a resistance and stability of motion in each of the six degrees of freedom. The intermediate portion  16  maintains the end portions  12 ,  14  in a substantially spaced relation, while allowing some relative movement of the spacer  16  when external forces cause the spacer body to bend or compress in any direction. 
     In some embodiments, the flexible connection element may be configured and adapted to exhibit preload forces even when the flexible portion is not undergoing externally applied torsional, axial, or bending loads. In this regard, the coupling member or cord  18  may be pre-tensioned so that the end portions  12 ,  14  are compressed against the intermediate portion  16  when engaged thereto. The amount of pre-tension can range from 0 to the tensile break strength of the coupling member of cord. The greater pre-tension loading of the cord generally results in a stiffer construct. This preloaded configuration may be beneficial for designing a preferential response to different types of external forces or loading. For instance, a preloaded flexible connection element may provide a greater resistance to torsional loads that would tend to further tighten the flexible connection element due to added frictional forces resisting sliding movement of the edges against each other. 
     Referring to  FIG. 5 , in another embodiment of a flexible connection element  40 , a bumper or other resiliently compressible member  42  may be disposed over cord  18  and positioned adjacent an outer end plate  44  of an end portion or spool  45 . A rigid stop, flange, or end member  46  may be fixedly attached or clamped to cord  18  on the opposite side of bumper  42  from the spool  45 . In this embodiment, spool  45  may be slidable, movable, or otherwise unconstrained with respect to cord  18 . In this regard, bumper  42  may be resiliently compressed between spool  45  and stop  46  when spools  45 ,  47  are separated or forced apart in the longitudinal direction of axis  20 . For example, referring to  FIGS. 5A-5B , in one embodiment when spools  45 ,  47  are retained by respective bone fasteners  34  and affixed to adjacent vertebrae, such a configuration facilitates the separating movement between spools  45 ,  47  and the respective bone fasteners to which they are attached. Referring to  FIG. 5A , showing connection element  40  in a first or neutral position with an overall length L 1 , spools  45 ,  47  may have a first separation distance L 2 . As shown in  FIG. 5B , in a second position, after a separating movement between spools  45 ,  47 , the second separation distance L 3  is greater than L 2  which replicates a change in the separation distance of the bone fasteners and the bone segments to which they are attached. Referring to  FIG. 5C , one may appreciate that such a feature may be desired to replicate the natural kinematics that a spinal motion segment undergoes under flexion wherein the elongation of the intrapedicular distance typically occurs. In one variation, the flexible element may accommodate up to 8 mm of a change ire intrapedicular distance under flexion in another variation, up to 4 mm of a change in intrapedicular distance may be accommodated. Such elongation may be accomplished independent from or, in addition to, any elongation in cord  18 . In this regard, the degree or extent to which flexible connection element  40  may elongate may be designed, preselected, or predicted with a greater degree of accuracy than reliance on elasticity or elongation in the cord alone. In one embodiment, bumper  42  may be made from the same material as intermediate portion  16 . In alternate embodiments, bumper  42  may be made from a different material than intermediate portion  16  or bumper may be made from the same material and have a different hardness or flexibility than intermediate portion  16 . 
     As shown in the embodiment of  FIG. 5 , an alternative end portion or spool  47  may be provided adjacent one end of flexible connection element  40 . As best seen in  FIGS. 6-7 , spool  47  generally comprises a middle portion  50  interposed between outer end plates or flange portions  52 . A central channel  54  extends axially through spool  47  and is generally configured and dimensioned to accommodate coupling member or cord  18 . Middle portion  50  generally comprises a lower clamp body  56  and an upper clamp body  58  selectably moveable with respect to lower clamp body  56  to clamp down and affix cord  18  with respect to spool  47 . In one variation, upper clamp body  58  has a pair of downwardly extending arms  60  having elongated openings  62  configured and dimensioned to receive protrusions or prongs  64 ,  66  extending outward from lower clamp body  56  so as to allow unidirectional one step clamping or locking of spool  47  with respect to cord  18 . Arms  60  are configured and dimensioned to deflect or bend outward slightly to move over protrusions  64 ,  66 . In this regard, protrusions  64 ,  66  may have a chamfer or angled outer surface  68  and arms  60  may have a chamfered, beveled, or angled inner lower surface  70  to facilitate arm deflection. Upper clamp body  58  may be first preassembled onto lower clamp body and positioned in a first position as shown in  FIG. 7 . In operation, as upper clamp body  58  is forced downward, the arms  60  may engage upper prongs  64  and deflect outward and over the upper prongs  64  such that the upper prongs extend through openings  60  and provisionally maintain upper clamp body  58  in the first position. As shown in  FIG. 7 , in the first position, upper clamp body  58  may be relatively loosely affixed to lower clamp body  56  such that a cord extending through middle portion  50  may slide or move with respect to spool  47 . To affix or clamp cord  18  with respect to spool  47  upper clamp body  58  may be forced downward further onto lower clamp body  56  and positioned in a second or locked position as shown in  FIG. 8  In operation, as upper clamp body  58  is forced downward, the arms  60  may engage lower prongs  66  and deflect outward and over the lower prongs  66  such that the lower prongs extend through openings  60  and maintain the upper clamp body  58  in the second, clamped, or locked position. As shown in  FIG. 8 , in the second position, upper clamp body  58  may be relatively rigidly affixed to lower clamp body  56  such that a cord extending through middle portion  50  may not slide or move with respect to spool  47 . One skilled in the art may appreciate that such a one step lock or damping feature may be desirable to allow for tensioning of cord  18  during installation in situ. Referring again to  FIG. 5 , one my also appreciate that with such a clamping feature integrated bun the middle portion  50  of spool  47 , the step of clamping or locking the cord may be accomplished by finally tightening down on a cap  35  or set screw  36  of a pedicle screw assembly  34 . In this regard, the tensioning and final clamping of cord  18  may be accomplished with a familiar procedure common to the installation of contemporary spinal stabilization systems. 
     Referring to  FIGS. 8A-8D , another embodiment of a spool  47  is disclosed which generally comprises a post or piercing means to affix cord  18  with respect to spool  47 . In one variation, upper clamp body  58  has a central finger or post  72  extending downwardly from the underside thereof in one variation, the post  72  may be configured and dimensioned to extend through the cord  18  so as to puncture or pierce through cord  18  and the distal tip  73  of post  72  may enter into a depression  74  provided on the interior of lower clamp body  56 . As with the above described embodiment, a pair of arms  76  extend downward from upper clamp  58  are configured and dimensioned to engage lower clamp body  56  so as to allow unidirectional one step clamping, piercing, and/or locking of spool  47  with respect to cord  18 . As shown in  FIGS. 8A-8B , in a first position, upper clamp body  58  may be spaced from or relatively loosely affixed to lower clamp body  56  such that a cord extending through middle portion  50  may slide or move with respect to spool  47 . To affix or clamp cord  18  with respect to spool  47  upper clamp body  58  may be forced downward further onto lower clamp body  56  and positioned in a second or locked position as shown in  FIGS. 8C-8D . As shown in  FIGS. 8C-8D , in the second position, upper clamp body  58  may be relatively rigidly affixed to lower clamp body  56  such that a cord extending through middle portion  50  may not slide or move with respect to spool  47 . 
     Referring to  FIGS. 9-12 , one embodiment of a clamp assembly  80  for clamping rigid stop, flange, or end portion  46  to cord  18  is shown. Clamp assembly  80  generally comprises an annular end body  82  having an end plate or flange  84  and a central cavity  86  configured and dimensioned to house a lower clamp body  88  and an upper clamp body  90 . Upper and lower damp bodies  90 ,  88  have a tapered or partially conically shaped outer surface  92  configured to engage, slide, mate, wedge, or otherwise contact a corresponding opposing tapered or shaped. interior wall surface  94  of cavity  86 . Upper clamp body  90  is movable with respect to lower clamp body  88  to clamp down and affix cord  18  with respect to end body  82 . In one variation, upper clamp body  90  has a pair of downwardly extending aims  96  having openings  98  configured and dimensioned to receive protrusions or prongs  100  extending outward from lower clamp body  88  so as to allow unidirectional clamping or locking of end  46  with respect to cord  18 . Arms  96  are configured and dimensioned to deflect or bend outward slightly to move over protrusions  100 . To affix or clamp cord  18  with respect to end  46 , upper clamp body  90  may be assembled over lower damp body  88  with cord  18  positioned therebetween. As shown in  FIG. 12 , cord  18  may be additionally cinched, clamped, or locked when the assembled upper and lower clamp bodies  90 ,  88  are positioned within cavity  86  and pulled or forced longitudinally against the tapered inner wall  94  such that the outer surface  92  engages, slides, mates, or wedges thereagainst to force the upper and lower damp bodies  90 ,  88  to contract upon cord  18  such that a cord extending through the clamp bodies  88 ,  90  may not slide or move with respect to end  46 . One skilled in the art may appreciate that such a tapered arrangement facilitates secure clamping during natural movement of flexible connection element  40  when installed. In one variation, a shoulder portion  102  of end body  82  may extend outward from flange  84  and may extend into a portion of bumper  42 . 
     Referring to  FIGS. 12A-12D , another embodiment of a clamp assembly  104  for clamping rigid stop, flange, or end portion  46  to cord  18  is shown. Clamp assembly  104  generally comprises an annular end body  82  having a central cavity  86  and an end plate or flange  84  configured and dimensioned to house an insertable clamp body  105 . Clamp assembly  104  generally comprises a post or piercing means to affix cord  18  with respect to end portion  46 . In one variation, insertable clamp body  105  has a central finger or post  106  extending downwardly from the underside thereof. In one variation, the post  106  may be configured and dimensioned to extend through the cord  18  so as to puncture or pierce through cord  18  and the distal tip  107  of post  106  may enter into a depression  108  provided on the interior of central cavity  86 . Insertable clamp body  105  is movable with respect to clamp body  82  to puncture, pierce and/or clamp down and affix cord  18  with respect to end body  82 . In one variation, insertable clamp body  105  has a pair of arms  109  configured and dimensioned to engage clamp body  82  so as to allow unidirectional one step clamping, piercing, and/or locking of end portion  46  with respect to cord  18 . As shown in  FIGS. 12A-12B , in a first position, insertable clamp body  105  may be spaced from or relatively loosely affixed to end body  82  such that a cord extending through cavity  86  may slide or move with respect to end body  82 . To affix or clamp cord  18  with respect to end portion  46 , insertable damp body  105  may be forced downward further onto end body  82  and positioned in a second or locked position as shown in  FIGS. 12C-12D . As shown in  FIGS. 12C-12D , in the second position, insertable clamp body  105  may be relatively rigidly affixed to end body  82  such that a cord extending through cavity  86  may not slide or move with respect to end portion  46 . 
     In general, the flexible connection elements described herein can be extended to stabilize two or more joints or spinal motion segments between three or more adjacent vertebrae, and affixed to respective vertebrae by three or more fasteners. Thus, in one exemplary embodiment, shown in  FIG. 13  a flexible connection element  110 , similar to connection element  40  of  FIG. 5  includes a plurality of spacers for providing flexible stabilization to a plurality of joints or spinal motion segments. In the embodiment of  FIG. 13 , a constrained spool  112  may be provided at a first end  114 , and unconstrained spools  116 ,  118  and spacers  120 ,  122  may be interposed between a bumper  124  and clamp assembly  126  disposed on a second end  128 . Additionally, the spacers  120 ,  122  may be alternated with various spool members (i.e. constrained or unconstrained) in any order or combination as needed by the surgeon. Further, an additional bumper may be positioned outside the first end such that a bumper would be provided at opposite ends of the construct. In this way, a hybrid multi-level or multi-spine segment connection unit may be designed, wherein each segment of the connection unit can provide a desired level of flexibility suited for each respective pair of inferior and superior vertebrae to be stabilized. For example, a first section of the connection unit that stabilizes a first pair of vertebrae may be very rigid, while a second section of the connection unit that stabilizes a second pair of vertebrae may be more flexible when compared to the first section. Numerous desired combinations of sections may be achieved to create a hybrid multi-level or multi-segment connection unit, in accordance with the present invention. 
     Referring to  FIG. 14 , in one aspect of the invention one or more angled or lordosed spools  130  may be provided to form a construct or flexible connection element  131  to conform to and/or restore the natural lordosis of the spine. Spools  130  may be similar to spools  45 ,  47  described above except the end plates or flanges  132  may have an angle  134  or be tapered with respect to the normal of longitudinal spool axis  136 . In one embodiment, the angle  134  of the end plate  132  is between about 3.5 degrees and about 5 degrees. In one variation, the end plate  132  may be angled about 4 degrees. 
     Referring to  FIGS. 15-16 , single and multi-level versions of another embodiment of a flexible connection element  140  are shown. Flexible connection element  140  is similar to connection element  131  of  FIG. 14  except the end plates or flanges  132  of spools  130  are configured and dimensioned to extend over at least a portion of the adjacent intermediate portion or spacer  16 . In this regard, end plates  132  of spools  130  may have a cylindrical internal portion  142  configured and dimensioned to house an end of the adjacent spacer  16 . One skilled in the art may appreciate that such a configuration may resist shear translational forces when implanted adjacent a motion segment of the spine. Such an end plate feature may be provided on spools or end portions with or without lordosis or in any other embodiments of end portions described herein. 
     Referring to  FIG. 17 , another embodiment of flexible connection element  150  is shown. Connection element  150  may be employed in a hybrid procedure employing fusion and dynamic stabilization. In this regard, an elongated end portion  152  may be provided and engaged between vertebrae to be fused and one or more adjacent vertebral levels can be dynamically stabilized with the intermediate portion  16  engaged between end portions  152 ,  154 . End portion  152  may have a rod portion  156  integrated into a spool portion  158  and may include a clamping means  160 , such as a set screw, to affix cord  18  to end portion  152 . In addition, a bumper  162  may be provided adjacent a second end  164  to facilitate elongation of the dynamically stabilized level. Connection elements are also contemplated that would provide for multiple spine levels stabilized by fusion and multiple levels dynamically stabilized. 
     Referring to  FIG. 18 , another embodiment of a flexible connection element  170  is shown. Flexible connection element  170  may have one or more cords  172  extending longitudinally between rigid end portions  174 ,  176  and the one or more cords  172  may be tied or crimped into holes  178  provided on end portions  174 ,  176 . A central protrusion, prong, or nub  180  may extend outward from the face of end plate or flange  182  and into flexible intermediate portion  184  to enhance the physical interconnection of the intermediate portion  184  to end members  174 ,  176 . 
     Referring to  FIG. 19 , an alternate embodiment of a flexible connection element  190  is shown wherein one or more cords  192  extend through intermediate portion  194  and may be rigidly attached to a first end portion  196  and a threaded member  198 . Threaded member  198  may be screwed or threadedly attached to a second end portion  200 . In this regard, threaded member  198  may be rotatably advanced to change the amount of tension in the cords and thus alter the stiffness of the construct of flexible connection element  190 . 
     Referring to  FIGS. 20-22 , another embodiment of an end member or portion  210  and intermediate portion  212  of a flexible connection element is shown. In this embodiment, end member  210  has a generally spherical seat or interface surface  214  that is configured to engage or contact intermediate portion  212 . In another aspect of the invention, a protrusion  216  may extend from interface surface  214  and extend into intermediate portion  212  to enhance the physical interconnection of the intermediate portion  212  to end member  210 . In a further aspect, the anterior portion or bottom  218  of end member  210  and intermediate portion  212  may be flat to facilitate a low profile once installed. It is also contemplated that such a flat bottom feature may be incorporated in the many alternate embodiments described throughout the specification. In a further aspect, a coupling member or cord  18  may extend eccentrically through intermediate portion  212 . For example, in the depicted embodiment, cord  18  may extend through intermediate portion adjacent the upper or posterior portion of spacer. In this regard, the flexible connection element constructed in such a fashion may be less rigid on one side as compared to the other. 
     Referring to  FIG. 23 , an alternate embodiment of a flexible connection element  220  is shown wherein the coupling member or cord  18  extends along the top or posterior side of intermediate portion  16  and may be secured or affixed to end members  224 ,  226  by a top mounted set screw lock  228 . As a result, like previously described embodiments the flexible connection element constructed in such a fashion may be less rigid on one side as compared to the other. 
     Various embodiments of flexible connection elements contemplate alternative end members or portions configured to engage alternative bone fasteners or anchors. In particular, the embodiments of  FIGS. 24-54 , discussed below, are generally configured to engage a post type anchor or bone screw. In general, these embodiments have at least one end portion comprising a hole or opening configured to receive the posted end of the bone anchor therethrough. However, one skilled in the art may appreciate that these embodiments may be modified to engage a top loading, yoke, or tulip type receiving member of an anchor. 
     Referring to  FIGS. 24-25 , another embodiment of a flexible connection element  230  is shown that is configured and dimensioned to engage a posted screw or bone fastener. According to one variation, the flexible connection element  230  may comprise an intermediate body portion  232  interposed between opposite end portions  236 ,  238 . Intermediate body portion  232  may be made from a similar resiliently deformable material as intermediate portions described above and may be molded over and between end portions  236 ,  238 . In one aspect of the embodiment, end portions  236 ,  238  may define a generally cylindrical opening  240  to accommodate a shaft therethrough, such as a shaft or post end of a posted screw fastener. In this regard, flexible connection element  230  is generally configured and dimensioned to be coupled to and to interconnect between two bone fasteners, one coupled to each end portion  236 ,  238 . In one variation, end portions  236 ,  238  may each comprises rigid sleeves or annular rings which may be encapsulated or molded into the material of the intermediate body portion. For example, if intermediate body portion is made from a polymer material, the polymer may be molded over annular rings  236 ,  238 . In another aspect, intermediate body portion  232  may have a rounded profile and may extend in the posterior direction a sufficient distance to cover or extend beyond a nut or other clamping member assembled upon the posted screw and engaging end portions  236 ,  238 . In general, when a nut or clamping member is assembled upon the end or post portion of anchor  234 , it sits down in a low profile position. In one variation, flexible connection element  230  may elongate and compress due to the elastic or resilient properties of the material of the intermediate portion without an integrated coupling member or cord. In alternate embodiments, one or more coupling members or cords may be provided extending about end portions  236 ,  238  and may or may not be molded into intermediate portion  232  to facilitate the flexible movement of connection element  230 . 
     Referring to  FIG. 26 , another embodiment of a flexible connection element  240  is shown wherein the intermediate portion or spacer (not shown) may be molded between end portions  244 ,  246 . In this embodiment, end portions  244 ,  246  generally have an opening  248  to house a mounting block  250  and one or more cords  252  may be fixed to the end portion  244  by mounting block  250 . Mounting block  250  may be pinned into the housing  248  by a post or pin member  255 . In one variation, one or more side holes  254  may be provided in the housing  248  to allow the spacer material to flow out through the openings during injection molding to mechanically lock the housing  248  to the intermediate portion. In one embodiment, the cord or cords  252  may be locked into block  250  by winding. The cord or cords  252  may be aligned in a medial/lateral or anterior/posterior direction. In this embodiment, the flexible connection element  240  may elongate due to the flexible properties of the cord itself. In one variation, the end portions  244 ,  246  may have a flat section  256  surrounding an opening  258  in the end portion to accommodate multi-level stacking or serial connection in the spine. In this regard, the flexible connection elements  240  may be flipped over or juxtaposed to facilitate face to face contact of flat sections  256  and nesting of each flexible connection element  240 . One skilled in the art may appreciate, that such a feature facilitates a low profile construction in addition to allowing for implantation over multiple levels. 
       FIG. 27  is a perspective view of another embodiment of a flexible connection element  270 . In this embodiment, a generally flattened band  272  may extend around end spools  274 ,  276  and about the periphery of the connection element  270 . A spacer body  278  may be made from a similar resiliently deformable material as intermediate portions described above and may be molded over and between end spools  274 ,  276  and band  272 . In one variation, band  272  may be made from a metal material such as titanium, spring steel, or other suitable material. According to one aspect, in this embodiment, band  272  may have one or more bends  278  or crimps along its length to allow for elastic deformation of the hand  272  and/or separation or retraction of end portions  274 ,  276  and facilitating the return to the default position or configuration. In another variation, cover or spacer body  278  may facilitate elastic deformation under compressive forces (i.e. when spools  274 ,  276  are forced closer together). In this regard, the cover body  278  may resiliently deform to block the compressive movement and after the compressive force dissipates the cover body  278  may restore itself to its original shape, thereby restoring the spacing between spools  274 ,  276  and the screws attached thereto. Like the embodiment of  FIG. 26 , described above, flexible connection element  270  may comprise a single segment in a multilevel construct. In this regard, the end portions  274 ,  276  may be juxtaposed to facilitate face to face contact of generally flat sections  279 . 
     Referring to  FIG. 28 , in a modification of the embodiment shown in  FIG. 26 , flexible connection element  280  may have one or more cords  282  extending longitudinally between end portions  284 ,  286  and the one or more cords may be tied or crimped into holes  288  provided on end members  284 ,  286 . The flexible intermediate portion  288  may be molded around pins  290  to enhance the physical interconnection of the intermediate portion  288  to end members  284 ,  286 . According to this embodiment, intermediate portion  288  may have a generally cylindrical shape with a generally circular cross-section. 
     Referring to  FIGS. 29-34 , various alternative cord connection mechanisms are shown. In the embodiment of  FIGS. 29-30 , at least three cords  301 ,  302 ,  304  are provided with at least two cord portions  302 ,  304  extending along the lower, bottom or anterior portion and at least one cord portion  300  along the upper, top, or posterior portion of intermediate section  306 . As with previous embodiments, intermediate section  306  may be made from an elastically resilient deformable material such as polycarbonate urethane or the like and the end members  308 ,  310  may be made from a suitable rigid material such as titanium or the like. Cord  301  provided along the upper portion of intermediate section  306  may be selectively lengthened or shortened prior to implantation to shape the flexible connection element  300  to accommodate lordosis. In this regard, if the upper cord portion  301  is shortened the flexible connection element  300  will bow or curve in the posterior direction. In another variation, the lower cord portions  302 ,  304  may be parts of a single loop of cord extending around the periphery of end members  308 ,  310  of the flexible connection element  300 . In addition, one may appreciate that such a configuration may provide different levels of stiffness in the anterior-posterior direction. This may be advantageous if it is desired to provide a greater level of stiffness when the flexible connection element  300  is flexed during spinal extension (e.g., when a patient bends backward) and a lesser level of stiffness when the flexible connection element  300  is flexed during spinal flexion (e.g., when a patient bends forward). Thus, flexible connection element  300  can provide different levels of stiffness in different directions of movement and, hence, varying levels of stability can be provided to different directions of movement of a vertebra secured thereto. 
     Referring to  FIGS. 31-32 , in a modification of the embodiment shown in  FIGS. 29-30 , upper cord  301  may be coupled or fixed to end members  308 ,  310  with a mechanical spring biased binding mechanism or member  320  similar to a karabiner. Referring to  FIGS. 33-34 , in another modification of the embodiment shown in  FIGS. 29-30 , cords  302 ,  304  may be moldably attached to end members  308 ,  310  and an upper cord  301  may be fixedly attached with one or more set screws  324  and hence adjusted or tensioned to create lordosis as explained above. Bottom or lower cords  302 ,  304  may have enlarged lead ends  326  configured and dimensioned to fit or key into corresponding eye holes  328  in end members  308 ,  310 . 
     Referring to  FIG. 35 , a saggital plane view shows a plurality of flexible connection elements  300  similar to the embodiment shown in  FIGS. 33-34  situated in a serial juxtaposed position to form an exemplary multilevel construct. In this regard the adjacent flexible connection elements are flipped, or inverted to facilitate a face to face positioning or contact of fiat sections  330  of end portions  308 ,  310 . One skilled in the art may appreciate that a post or shaft portion  336  of a bone fastener or screw may extend through two adjacent flexible connection elements. 
     Referring to  FIGS. 36, 36A and 37 , in a modification of the embodiment shown in  FIGS. 33-34 , end member  362  of flexible connection element  360  may have a flexible slit  364  that is compressible on a posted type screw or bone fastener. In this regard, the flexible slit  364  comprises a deflectable or deformable portion configured and dimensioned to deform, collapse, or compress to engage with a spherical or ball shaped feature that may be provided, for example, on a shaft of a post type screw. In operation, the end member  362  of this embodiment may be secured to a post type fastener without the need for more than one nut or clamping member when two end members are attached to a single post type screw. One skilled in the art may appreciate that such a configuration may facilitate the stacking or juxtaposition of flexible connection elements  360  in a multilevel construct as shown in  FIG. 35 . 
     Referring to  FIG. 38 , another embodiment of an end member  380  is shown. In this embodiment, a modified protrusion, rib, or key portion  382  extends from internal face  384  of end member  380 . Similar to previous described embodiments, protrusion  382  is configured and dimensioned to mate, extend into, or otherwise engage a correspondingly shaped indentation in an intermediate portion and to mechanically interface or connect therewith. In this variation, protrusion  382  has a generally arcuate or curved convex surface  386  extending in the anterior-posterior direction and has generally flat or planar side walls  388 . In operation, curved surface  386  generally facilitates rotational or pivotal relative movement in the anterior posterior direction between end member  380  and an intermediate portion. Side walls  388  meanwhile generally prohibit relative movement between the end member and the intermediate portion in a medial-lateral direction. 
     Referring to  FIGS. 39-41 , additional embodiments a flexible connection elements  390  are shown. As best seen in  FIG. 40  wherein one variation of a bottom portion of a clamp member is shown, clamp member  392  defining one or more generally spherical socket portions  394  may be provided to clamp or hold a ball shaped end member  396  of flexible connection element  390 . The ball shaped end member  396  allows selectably fixable angulation of flexible connection element  390  with respect to a post type screw as shown in  FIG. 39 . Once a desired angle is selected, the damp member  392  may be compressed by, for example, a nut  398  to clamp down and affix end member  396  within socket portion  394 . According to one embodiment, once the clamp member  392  is so affixed, no further movement or angulation between clamp member  392  and end member  396  is contemplated to occur without loosening or unclamping clamp member  392 . Referring to  FIG. 41 , in a modification of the embodiment of  FIG. 39 , clamping member  392  of flexible connection element  400  may have socket portions offset from the longitudinal axis  402 . 
       FIG. 42  depicts another embodiment of an end member  410 . In this embodiment, modified grooves, passageways, slots or indentations  412 ,  414  are provided to accommodate the extension of a coupling member or cord therethrough or thereabout. In this regard, a posterior groove  412  extends about the outer periphery of an upper portion  416  and is generally configured and dimensioned to accommodate, hold, or capture a posterior cord loop. An anterior groove  414  extends about the outer periphery of a lower portion  418  with a generally angled downward section  420  adjacent the lateral edges. Like posterior groove  412 , anterior groove  414  is generally configured and dimensioned to accommodate, hold, or capture an anterior cord loop. 
     Referring to  FIGS. 43-45 , another embodiment of flexible connection element  430  is shown wherein the coupling member comprises a looped cord  432  having an internal twist or crossed over portion. Intermediate portion  434  has an internal opening  436  configured and dimensioned to provide clearance or space to allow cord  432  to twist and tension. One skilled in the art may appreciate that the more cord  432  twists, the shorter the distance between end members  438 ,  440  may get, and hence the overall tension or stiffness of the construct may correspondingly increase. In this regard, the overall tension or stiffness of the construct may be controlled. 
     Referring to  FIG. 46 , the flexible connection element  460  may have an arcuate shaped interface  462  between end portions  464 ,  466  and intermediate portion or spacer  468 . In this embodiment, four coupling, members or cords may extend between end members  464 ,  466 . Clamping plates  470  may be provided on each end member adjacent the top and bottom of flange portion  472  to secure, clamp, or affix the cords to the end member. 
     Referring to  FIG. 47 , in a modification of the embodiment shown in  FIG. 46 , end members  464 ,  466  may have a laterally positioned opening  474  for side mounting to a post type screw. In this embodiment, an upper and lower coupling member or cord  476  may extend through intermediate portion  478  and clamping plates  480  may be provided on each end member adjacent the top and bottom of flange portion to secure, clamp or affix cords  476  to the end member. 
     Referring to  FIG. 48 , in a modification of the embodiment shown in  FIG. 47 , intermediate portion  490  and end members  492 ,  494  may have a trough, indentation, or groove  496  extending along the top and bottom of the construct and may be configured and dimensioned to accommodate a coupling member or cord therein. 
     Referring to  FIG. 49 , an alternate side mountable end portion  500  is shown. In this variation, a hole  502  may be provided to accommodate a set screw to secure cord  504  to end member  500 . A similar end portion  500  may be provided on an adjacent bone anchor and cord.  504  may couple them together with intermediate portion  506  disposed therebetween. 
     Referring to  FIGS. 50-51 , an alternate flexible connection element  510  may have an intermediate portion  512  with a generally ovoid or football shape and may have an indentation, groove, or trough  514  extending around the periphery and generally aligned and coextensive with an indentation, groove or trough  516 ,  518  extending about the periphery of end members  520 ,  522 . When assembled, troughs  514 ,  516  and  518  extend about the periphery of flexible connection element  510  and are configured and dimensioned to accommodate a coupling member or cord  524  in the shape of a continuous loop. In operation, when end members  520 ,  522  are compressed together, intermediate portion  512  may be resiliently compressed and/or deformed and when end members are separated, coupling member or cord  524  may be resiliently elastically elongated. As shown in  FIG. 51 , in one variation the embodiment of  FIG. 50  may be used in series with another flexible connection element  510  for spine stabilization over multi levels or motion segments. 
     Referring to  FIG. 52 , in a modification of the embodiment shown in  FIG. 50 , coupling member or cord  524  may extend internally through intermediate portion  512  and externally around the periphery of end portions  520 ,  522 . 
     Referring to  FIGS. 53-54 , in an alternate embodiment of a flexible connection element  540 , end members  542 ,  544  may have an angled end plate or flange  546  to interface with intermediate portion  548 . One skilled in the art may appreciate that such an angled flange feature saves space and facilitates installation of flexible connection element  540  in motion segments where space constraints dictate. For example, flexible connection element  540  may be utilized at the L 5 -S 1  level. As shown in  FIG. 54 , a multilevel construct may be provided with an end portion having angled flange portions  546 ,  547  on both sides of bone anchor  550  such that flanges  546 ,  547  may both engage intermediate portions or spacers  548 . 
       FIG. 55  is a perspective view of another embodiment of a flexible connection assembly including modular rings. The assembly  600  comprises a number of components as in prior embodiments, including one or more spools  47 , one or more bumpers  42  positioned adjacent to the one or more spools  47 , and a cord  18  that extends therethrough. The one or more spools  47  can include an upper clamp body and a lower clamp body (as shown in  FIG. 6 ), and are capable of being retained by bone fasteners  34  (as shown in  FIG. 5 ). In addition to these components, the assembly  600  comprises one or more modular spacers or rings  620  that are positioned between the spools  47 . These modular rings  620  advantageously provide a highly dynamic stabilization system that allows for enhanced motion, physiologic translation and axial compression. Depending on the needs of the patient, a surgeon can apply one or more modular rings  620  to adjust characteristics, such as flexibility and stiffness, of the assembly  600 . The modular rings  620  extend around the cord  18  in a similar fashion to the intermediate spacer  16  shown in  FIG. 1 . However, the modular rings  620  provide even more options for strength and flexibility than the sole intermediate spacer  16 , as they can be combined in multiple different ways by a surgeon as desired. 
     As shown in  FIG. 55 , the assembly  600  can accommodate one or more modular rings  620  that can be provided by a surgeon depending on the specific needs of a patient. It has been found that modular rings  620  of particular material and shape can provide benefits to the flexible connection assembly. In some embodiments, one or more of the modular rings  620  are formed of PEEK. The advantage of PEEK rings  622  is its biocompatibility. In some embodiments, one or more of the modular rings  620  are formed of polycarbonate-urethane, or PCU. The advantage of PCU rings  624  is that it provides compliance and compressibility. In some embodiments, one or more of the modular rings  620  are formed of a metal or metal alloy, such as titanium or cobalt-chrome (CoCr). The advantage of titanium or CoCr rings  626  is increased strength, with CoCr rings provided even greater strength than titanium if desired. 
     In the embodiment in  FIG. 55 , the one or more modular rings  620  are composed of a number of differently sized rings of different materials. For example, the PCU ring  624  is of a length greater than the adjacent PEEK ring  622  and the adjacent titanium ring  626 . In addition, in the present embodiment, the one or more modular rings  620  are composed of three different types of materials, including PEEK, PCU and titanium. In the present embodiment, the assembly  600  comprises at least five modular rings, positioned between a pair of spools  47 . 
       FIGS. 56-59  illustrate different embodiments of flexible connection assemblies including different combinations of modular rings  620 , in accordance with one or more embodiments. Each of the flexible connection assemblies includes their own advantages, as will be discussed in more detail below. 
       FIG. 56  is a perspective view of a flexible connection assembly including just PEEK rings. The flexible connection assembly  600  comprises a series of PEEK rings  622  stacked adjacent to one another between a pair of spools  47 . The PEEK rings  622  advantageously accommodate physiologic movement and translation, such as flexion and extension of the spine. When a patient bends, the PEEK rings  622  advantageously slide and translate over one another, thereby creating a proper bending form. 
       FIG. 57  is a perspective view of a flexible connection assembly including PEEK rings and a PCU ring. The flexible connection assembly  600  comprises a series of PEEK rings  622  stacked adjacent to one another between a pair of spools  47 . On one end of the stacked PEEK rings  622  is a PCU ring  624 , which advantageously increases the compressibility of the assembly. 
       FIG. 58  is a perspective view of a flexible connection assembly including alternating PEEK and metal rings. The flexible connection assembly  600  comprises a series of PEEK rings  622  alternating with metal rings  626 . By alternating the PEEK and metal rings, this construct advantageously prevents metal on metal contact, thereby reducing the risk of metal shavings in a patient, while still maintaining a construct of high strength. In addition to the PEEK and metal rings, the assembly  600  further comprises a PCU ring  624 , which increases the compressiblity of the assembly. 
       FIG. 59  is a perspective view of a flexible connection assembly having alternating PEEK and metal rings in a flexed state. From this view, one can see how providing a plurality of modular rings increases the ability of the construct to maintain better physiological translation in-line with a patient&#39;s movements. 
     Bone Fasteners 
     The bone fasteners included in the disclosed system include any type of fastener that may be attached to the flexible connection element of the invention, while remaining securely fastened onto the intended bone. Thus, the bone fasteners may include mono-axial screws, polyaxial screws, post-type screws, helical blades, expandable screws, such as Mollie bolt type fasteners, which are inserted or screwed into the bone and expand by way of some type of expansion mechanism, conventional screws, staples, sublaminar hooks, and the like. In one embodiment, the bone fasteners are coated with any number of suitable osteoinductive or osteoconductive materials to enhance fixation in the bone. In another embodiment, the bone fasteners are fenestrated to enhance bony ingrowth or to further anchor the fastener to the bone. 
     The bone fasteners may be made from a host of materials. For example, the fasteners may be formed from natural/biological materials, such as allograft, xenograft, and cortical bone. The fasteners may also be formed from synthetic bioresorbable materials, such as polyanhydride, polyactide, polyglycolide, polyorthoester, polyphosphazene, calcium phosphate, hydroxyapatite, bioactive glass, tyrosine-derived polycarbonate, and mixtures thereof. In another embodiment, the fasteners are formed from non-bioresorbable materials including, but not limited to, stainless steel, titanium, titanium alloys, cobalt chrome alloys, shape-memory alloys, and carbon-reinforced polymer composites. 
     In addition, the fasteners may include growth factors for bone ingrowth and bony attachment, or for soft tissue ingrowth. Non-limiting examples of growth factors include insulin-like growth factor 1, basic fibroblast growth factor, transforming growth factor 13-1, platelet-derived growth factor, bone-derived growth factors, arginine, bone morphogenetic protein, LIM mineralization protein, and combinations thereof 
     As mentioned previously, the flexible connection element also may be used in other component of a spinal fixation system. For instance, it may be used as part of a transconnector. In this embodiment, the flexible connection element may be disposed between two fasteners connected to rods positioned along the length of the spine. Any fastener that may be suitable for a conventional transconnector may be used with the present invention. Some examples of fasteners are described in U.S. Pat. No. 6,565,565 to Yuan, U.S. Pat. No. 6,562,040 to Wagner, U.S. Pat. No. 6,551,318 to Stahurski, and U.S. Pat. No. 6,540,749 to Schafer, all of which are incorporated herein in their entireties. 
     Assembly of the Systems 
     The flexible connection element may be connected to fasteners in a number of ways, i.e., so that the connection is constrained, unconstrained, articulated, or combinations thereof. For example, the end portions may be attached to bone anchors and inserted or installed adjacent a motion segment of the spine. The flexible connection element may be inserted into or onto anchor heads, which can be side-loading or top-loading in this aspect of the invention. Following the placement of the flexible connection element upon the anchor heads, clamping screws may be inserted into or upon the anchor heads and firmly screwed down securing all the connected elements in place. This design would generally allow flexibility between the two bone fasteners. 
     The stiffness of the disclosed systems may also be adjusted during the operation and post-operation using a set screw. This would allow surgeons and doctors to make adjustments depending on a specific scenario. 
     The system, once assembled, may serve a variety of functions in the motion segment unit. For example, the system may reduce the load on the degenerative disc and/or facet joints in the motion segment unit. In addition, the height of the adjacent vertebrae may be restored to eliminate crushing or slipping of the disc therebetween. Moreover, lordosis may be created/preserved using the disclosed systems in at least one motion segment unit of the spine. Furthermore, the stiffness of the motion segment unit may be restored with the implementation of the system of the invention. 
     In some embodiments, flexible connection elements may be disposed in combination with rods used to make a portion of the system rigid. For example, a motion segment neighboring a treated area that has been essentially immobilized with a rigid stabilization system may be supported with a flexible connection element. 
     While it is apparent that the invention disclosed herein is well calculated to fulfill the objects stated above, it will be appreciated that numerous modifications and embodiments may be devised by those skilled in the art.