Patent Publication Number: US-2011066187-A1

Title: Spinal stabilization system

Description:
TECHNICAL FIELD 
     The disclosure is directed to a spinal stabilization system for securing to a spinal column. More particularly, the disclosure is directed to spinal stabilization systems including elongate rods having desired stiffness and strength characteristics and/or elongate rods having regions for receiving a flexible member. 
     BACKGROUND 
     Commercially available spinal fixation systems for the lumbar region of the spinal column typically use fixed angle or polyaxial bone screws attached to adjacent vertebrae with a 5.5 or 6.0 millimeter diameter metallic rod extending between adjacent bone screws and secured thereto with a cap screw or other fastening member. Commonly used materials for the rods include stainless steel, commercially pure (CP) titanium, alpha-beta titanium alloy, such as Ti-6Al-4V and Ti-6Al-7Nb, or cobalt-chromium-molybdenum alloy (Co—Cr—Mo). Due to their relatively high rigidity compared to the natural spine, these systems have been referred to as a “rigid system”. 
     However, it has been found that the use of a rigid system may lead to adverse effects to the spinal column. For instance, it is believed that the high degree of stiffness of rigid systems may relate to increased stress on adjacent discs and facet joints. Over time, these increased stresses may lead to segment hypermobility, facet hypertrophy, osteophyte formation, and stenosis or so called adjacent level disease. 
     Recently, less rigid polymeric or carbon fiber rod systems have been introduced as an alternative to potentially reduce the stress on adjacent discs and facet joints and the incidences of adjacent level disease. These systems have been referred to as a “flexible system”. Commonly used materials for the polymer rod systems include polyether ether ketone (PEEK), PEEK composites, or other polymer materials. These systems, while more flexible than a rigid system, present their own shortcomings, such as the relatively low ultimate strength of the polymeric materials. 
     In view of the limitations of systems using these types of spinal rods, there is an ongoing need to provide alternative spinal stabilization systems for stabilization of spinal segments of the spinal column which include spinal rods having desired stiffness and/or strength characteristics and/or elongate rods having regions for receiving a flexible member. 
     SUMMARY 
     The disclosure is directed to several alternative designs, materials and methods of manufacturing medical device structures and assemblies. 
     Accordingly, one illustrative embodiment is a vertebral stabilization system including an elongate rod and a vertebral anchor for securement to a vertebra. The vertebral anchor includes a head portion for receiving a portion of the rod. The elongate rod may be formed of a material having a modulus of elasticity less than or equal to 110 GPa and an ultimate strength greater than 1 GPa. The elongate rod may have a structural bending stiffness in the range of about 500,000 N-mm 2  to about 2,000,000 N-mm 2  or about 1,250,000 N-mm 2 . In some instances, the elongate rod may be formed of a beta titanium alloy such as high strength Ti-15Mo-5Zr. In some instances the elongate rod has a diameter in the range of about 3.25 millimeters to about 4.5 millimeters. 
     Another illustrative embodiment is a vertebral stabilization system for a spinal column. The system includes a vertebral anchor for securement to a vertebra, an elongate rod, a flexible member, and a securing member to secure the elongate rod and the flexible member to the vertebral anchor. The vertebral anchor includes a head portion having first and second arms extending from a base of the head portion, where the head portion includes a channel defined between the first and second arms extending between a first side and a second side of the head portion. The elongate rod has a first region and a second region, wherein the first region of the elongate rod includes an outer surface having an engagement surface portion. The flexible member has a first region and a second region in which the first region of the flexible member is positionable adjacent the engagement surface portion of the first region of the elongate rod when the first region of the elongate rod and the first region of the flexible member are received in the channel of the head portion of the vertebral anchor. The securing member is configured to engage the first and second arms of the head portion of the vertebral anchor to secure both the elongate rod and the flexible member in the channel of the head portion of the vertebral anchor. 
     Another illustrative embodiment is a method of stabilizing the spinal column of a patient. The method includes securing first and second vertebral anchors to first and second vertebrae of the spinal column on a first, lateral side of the spinal column, and securing third and fourth vertebral anchors to the first and second vertebrae of the spinal column on a second, contra-lateral side of the spinal column. A first elongate rod is secured to the first and second vertebral anchors on the first, lateral side of the spinal column, and a second elongate rod is secured to the third and fourth vertebral anchors on the second, contra-lateral side of the spinal column. A spinal load is transferred between the first and second vertebrae such that between 17% to 19% of the spinal load is transferred through the posterior elements of the first and second vertebrae. 
     Yet another illustrative embodiment is a method of stabilizing a lumbar region of a spinal column. The method includes installing a first vertebral anchor on a first lumbar vertebra and a second vertebral anchor on a second lumbar vertebra. An elongate rod, having a diameter of less than 5.5 millimeters, may then be secured between the first vertebral anchor and the second vertebral anchor. In some instances, the elongate rod is formed of a material having a modulus of elasticity less than or equal to 110 GPa and an ultimate strength greater than 1 GPa. In some instances the elongate rod is formed of high strength Ti-15Mo-5Zr and has a diameter in the range of about 3.25 millimeters to about 4.5 millimeters. In some instances, the elongate rod has a structural bending stiffness in the range of about 500,000 N-mm 2  to about 2,000,000 N-mm 2 . 
     Still another illustrative embodiment is a vertebral stabilization system including an elongate rod, a vertebral anchor for securement to a vertebra, and a securing member configured for securement of the elongate rod to the vertebral anchor. The elongate rod has a diameter of 4.5 millimeters or less. The vertebral anchor includes a head portion having a first leg, a second leg and a channel extending between the first leg and the second leg for receiving the elongate rod. The securing member includes a first component rotatably coupled to a second component. The first component is configured for engagement with the first and second legs of the head portion of the vertebral anchor, and the second component is configured for engagement with the elongate rod. The elongate rod secured to the head portion of the vertebral anchor with the securing member has a fatigue strength greater than a spinal rod of a diameter of 5.5 millimeters formed of any of stainless steel, commercially pure (CP) titanium, Ti-6Al-4V alpha-beta titanium alloy, Ti-6Al-7Nb alpha-beta titanium alloy, or cobalt-chromium-molybdenum alloy (Co—Cr—Mo) secured to a bone screw with a set screw in direct contact with the spinal rod 
     The above summary of some example embodiments is not intended to describe each disclosed embodiment or every implementation of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be more completely understood in consideration of the following detailed description of various embodiments in connection with the accompanying drawings, in which: 
         FIG. 1  is a perspective view of an exemplary embodiment of a vertebral stabilization system; 
         FIG. 2  is an exploded view of a pair of vertebral anchors and an elongate connecting member of the vertebral stabilization system of  FIG. 1 ; 
         FIG. 3  is a perspective cross-sectional view of the securing member of the vertebral stabilization system of  FIG. 1 ; 
         FIG. 4  is a chart comparing the elastic modulus of these commercially available spinal rod materials; 
         FIG. 5  is a chart illustrating the relative structural bending stiffness of commercially available spinal rods of various materials as a percentage of structural bending stiffness of a 5.5 millimeter rod formed of Ti-6Al-4V; 
         FIG. 6  is a chart comparing the ultimate strength of commercially available spinal rod materials; 
         FIG. 7  is a graph illustrating the percent of load sharing of an axial spine compression load of 5.5 millimeter spinal rods formed of PEEK and Ti-6Al-4V relative to the structural bending stiffness of the spinal rod as a percentage of the structural bending stiffness of a 5.5 millimeter spinal rod formed of Ti-6Al-4V; 
         FIG. 8  is a chart illustrating the structural bending stiffness of connecting members formed of high strength beta titanium alloy Ti-15Mo-5Zr material and having diameters of 3.25 millimeters, 3.75 millimeters, 4 millimeters, and 4.5 millimeters; 
         FIG. 9  is a chart comparing the fatigue strength of a 4.25 millimeter diameter rod formed of Ti-15Mo-5Zr to the fatigue strength of a 5.5 millimeter rod formed of CP titanium; 
         FIGS. 10A and 10B  are perspective views of an exemplary transverse connector for use in the vertebral stabilization system of  FIG. 1 ; 
         FIG. 10C  is a perspective longitudinal cross-sectional view of the transverse connector of  FIGS. 10A and 10B ; 
         FIG. 11A  is a perspective view of an alternative embodiment of a transverse connector for use in the vertebral stabilization system of  FIG. 1 ; 
         FIG. 11B  is a longitudinal cross-sectional view of the transverse connector of  FIG. 11A ; 
         FIG. 12  is a perspective view of another exemplary vertebral stabilization system; 
         FIG. 13  is a perspective view of an elongate rod of the vertebral stabilization system of  FIG. 12 ; 
         FIG. 14  is a perspective view of an alternative elongate rod of the vertebral stabilization system of  FIG. 12 ; 
         FIG. 15  is a longitudinal cross-sectional view of the vertebral stabilization system of  FIG. 12 ; 
         FIG. 16  is a perspective view of another exemplary vertebral stabilization system; 
         FIG. 17  is an exploded view of the vertebral stabilization system of  FIG. 16 ; and 
         FIG. 18  is a perspective cross-sectional view of the vertebral stabilization system of  FIG. 16 . 
     
    
    
     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit aspects of the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention. 
     DETAILED DESCRIPTION 
     For the following defined terms, these definitions shall be applied, unless a different definition is given in the claims or elsewhere in this specification. 
     All numeric values are herein assumed to be modified by the term “about”, whether or not explicitly indicated. The term “about” generally refers to a range of numbers that one of skill in the art would consider equivalent to the recited value (i.e., having the same function or result). In many instances, the term “about” may be indicative as including numbers that are rounded to the nearest significant figure. 
     The recitation of numerical ranges by endpoints includes all numbers within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5). 
     Although some suitable dimensions, ranges and/or values pertaining to various components, features and/or specifications are disclosed, one of skill in the art, incited by the present disclosure, would understand desired dimensions, ranges and/or values may deviate from those expressly disclosed. 
     As used in this specification and the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the content clearly dictates otherwise. As used in this specification and the appended claims, the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise. 
     The following detailed description should be read with reference to the drawings in which similar elements in different drawings are numbered the same. The detailed description and the drawings, which are not necessarily to scale, depict illustrative embodiments and are not intended to limit the scope of the invention. The illustrative embodiments depicted are intended only as exemplary. Selected features of any illustrative embodiment may be incorporated into an additional embodiment unless clearly stated to the contrary. 
     Now referring to the drawings, an exemplary vertebral fixation system  10  for stabilizing a portion of a spinal column, such as one or more spinal segments of a spinal column, is illustrated in  FIGS. 1 and 2 . As used herein, a spinal segment is intended to refer to two or more vertebrae, the intervertebral disc(s) between the vertebrae and other anatomical elements between the vertebrae. For example, a spinal segment may include first and second adjacent vertebrae and the intervertebral disc located between the first and second vertebrae. The vertebral stabilization system  10  may provide support to the spinal segment and may help preserve the facet joints between adjacent vertebrae by providing facet offloading and/or may stabilize or reverse neural foraminal narrowing of the spinal column. 
     In some embodiments, the vertebral stabilization system  10  may be used to treat discogenic low back pain, degenerative spinal stenosis, disc herniations, facet syndrome, posterior element instability, adjacent level syndrome associated with spinal fusion, and/or other maladies associated with the spinal column. 
     The vertebral stabilization system  10  may include one or more or a plurality of vertebral anchors or fasteners  12 . Although the vertebral anchors  12  are depicted as threaded vertebral fasteners (e.g., pedicle screws, bone screws), in some embodiments the vertebral anchors  12  may be vertebral hooks (e.g., laminar hooks) or other types of fastening members for attachment to a bony structure such as a vertebra of the spinal column. Each of the vertebral anchors  12  may be configured to be secured to a vertebra of a spinal column. For instance, the first vertebral anchor  12   a  may be secured to a first vertebra and the second vertebral anchor  12   b  may be secured to a second vertebra. In a multi-lateral application, the third vertebral anchor  12   c  may be secured to the first vertebra and the fourth vertebral anchor  12   d  may be secured to the second vertebra on a contra-lateral side of the sagittal plane. Additional vertebral anchors  12  may be secured to additional vertebrae as desired. 
     The vertebral anchor  12  may include a head portion  14  and a bone engagement portion  16  extending from the head portion  14 . In some embodiments, the bone engagement portion  16  may be a shaft portion  18  of the vertebral anchor  12  extending from the head portion  14  along a longitudinal axis of the vertebral anchor  12 . In some embodiments, the vertebral anchor  12  may be a monoaxial screw in which the head portion  14  is stationary relative to the shaft portion  18 , and in other embodiments the vertebral anchor  12  may be a polyaxial screw in which the head portion  14  is actuatable (e.g., pivotable) relative to the shaft portion  18 . In some embodiments, the shaft portion  18  may be configured to be installed into a bony region of a vertebra of the spinal column. For example, the shaft portion  18  may be installed into a pedicle of a vertebra, or other region of a vertebra. In some embodiments, the shaft portion  18  may be a threaded region having helical threads configured to be screwed into a pedicle of a vertebra, or other bony region of a vertebra. 
     The head portion  14  may include a base portion  24 , from which the shaft portion  18  extends from, and first and second legs  26  extending from the base portion  24  on opposing sides of the head portion  14 . The first and second legs  26  may define an opening  28 , which may be a threaded opening in some instances, extending into the head portion  14  from an upper extent of the head portion  14  opposite the base portion  24 . In embodiments in which the opening  28  is threaded, each of the first and second legs  26  may include a threaded portion for threadedly engaging a threaded portion of a securing member  20 . In other embodiments, the first and second legs  26  may include other engagement features for engaging with a securing member positioned in the opening  28  between the first and second legs  26 . The head portion  14  may additionally include a channel  30 , such as a U-shaped channel, defined between the first and second legs  26 . The channel  30  may extend through the head portion  14  from a first side  32  of the head portion  14  to a second side  34  of the head portion  14 . The opening  28  may intersect the channel  30 . 
     The vertebral anchor  12  may include a securing member  20  configured to engage the head portion  14  to secure a stabilizing member or connecting member  22  (e.g., elongate rod or flexible cord) to the vertebral anchor  12 . For example, the securing member  20  may include a first component rotatably coupled to a second component. For instance, the securing member  20  may include an upper threaded screw  36  rotatably coupled to a lower, insert  38 . The threaded screw  36  may be rotated relative to the insert  38  about a pivot axis. The threads of the threaded screw  36  may mate with threads formed in the head portion  14 . For example, the threads of the threaded screw  36  may mate with threaded portions of the first and second legs  26  of the head portion  14 . In other embodiments, other securing members, such as threaded fasteners, may be used to secure a connecting member  22 , such as an elongate rod or flexible member, in the head portion  14  of the vertebral anchor  12 . 
     The threaded screw  36  may be formed of a first, rigid material such as metal, including stainless steel, titanium, titanium alloys or other metal, while the insert  38  may be formed of a different material. For instance, in some cases the insert  38  may be formed of a polymeric material, such as polyether ether ketone (PEEK), carbon fiber reinforced PEEK, ultra high molecular weight polyethylene (UHMWPE), or poly(methyl methacrylate) (PMMA). The insert  38  may be rotatably attached to the threaded screw  36  with a boss  40  that extends into an opening  42  of the threaded screw  36 , in some instances. For instance, the boss  40  may include an enlarged diameter portion that extends through the opening  42  and engages a rim  46  of the opening  42 . The boss  40  may be sufficiently deflectable or compressible such that the enlarged diameter portion of the boss  40  (which has a diameter or cross sectional distance greater than a diameter of the opening  42 ) may be urged through the opening  42 , but then retained in the opening  42  during usage. In some instances, the boss  40  may include one or more slots  47  dividing the boss  40  into a plurality of prongs  48  for allowing one or more of the prongs  48  of the boss  40  to deflect radially inward toward the pivot axis of the threaded screw  36  in order to allow the boss  40  to be urged through the opening  42 . Once inserted into the opening  42 , the interaction of the boss  40  with the rim  46  of the opening  42  may retain the insert  38  rotatably coupled to the threaded screw  36 . 
     The insert  38  may include a cylindrically concave lower surface  44  for contacting the cylindrical outer surface of a connecting member  22  when positioned in the head portion  14  of a vertebral anchor  12 . The insert  38  may be configured to allow a connecting member  22  having a diameter less than 5.5 millimeters to be secured in the channel  30  of the head portion  14  of a vertebral anchor  12  which is sized to receive a 5.5 millimeter or greater diameter rod (e.g., a vertebral anchor in which the channel of the head portion has a width measured between the first leg and the second leg of 5.5 millimeters or more). For instance, the lower surface  44  may have a radius of curvature approximating the radius of the connecting member  22 . For example, the radius of curvature of the lower surface  44  may be about 5.0 millimeters, about 4.5 millimeters, about 4.0 millimeters, about 3.75 millimeters, about 3.5 millimeters, or about 3.25 millimeters in some instances. 
     Furthermore, the insert  38  may more evenly distribute a securing force on the connecting member  22  in order to prevent notching of the connecting member  22  by a set screw. For instance, the presence of the insert  38  between the threaded screw  36  and the connecting member  22  prevents the threaded screw  36  from directly engaging the connecting member  22 . Thus, the insert  38  may provide a buffer to notching, pitting, fretting or galling of the connecting member  22  (which can reduce the fatigue strength of the connecting member  22 ) from direct contact with a set screw or other threaded fastener, while the threaded screw  36  may still be used to secure the connecting member  22  in the head portion  14  of the vertebral anchor  12 . The presence of the insert  38  between the threaded screw  36  and the connecting member  22  may provide a stress gradient between the threaded screw  36  and the connecting member  22 , distributing the forces exerted onto the connecting member  22  through tightening the threaded screw  36  in the head portion  14  of the vertebral anchor  12  over a larger portion of the exterior surface of the connecting member  22 . Therefore, the inclusion of the insert  38  may eliminate the notch sensitivity of the connecting member  22 , thereby increasing the fatigue strength of the connecting member  22  in the vertebral stabilization system  10  such that a connecting member  22  of a smaller diameter than conventional spinal rods (e.g., smaller than 5.5 millimeters) may be used in the vertebral stabilization system  10  while not compromising the fatigue strength of the connecting member  22  and the vertebral stabilization system  10 . In many instances, the inclusion of the insert  38  greatly increases the fatigue strength of the connecting member  22  and the vertebral stabilization system  10 . Thus, the vertebral stabilization system  10 , including an insert  38  contacting a connecting member  22  having a diameter less than 5.5 millimeters (e.g., about 5.0 millimeters or less, about 4.5 millimeters or less, or 4.0 millimeters or less) may have a fatigue strength greater than commercially available vertebral stabilization systems including a spinal rod of a diameter of 5.5 millimeters or greater formed of stainless steel, commercially pure (CP) titanium, alpha-beta titanium alloy (i.e., Ti-6Al-4V or Ti-6Al-7Nb), or cobalt-chromium-molybdenum alloy (Co—Cr—Mo) in direct contact with a threaded fastener (e.g., a set screw). For instance, it has been determined that the connecting member  22  of a diameter of 4.5 millimeters or less secured to the head portion  14  of the vertebral anchor  12  with the securing member  20  has a fatigue strength greater than a spinal rod of a diameter of 5.5 millimeters formed of stainless steel, commercially pure (CP) titanium, Ti-6Al-4V alpha-beta titanium alloy, Ti-6Al-7Nb alpha-beta titanium alloy, or cobalt-chromium-molybdenum alloy (Co—Cr—Mo) secured to a bone screw with a set screw in direct contact with the spinal rod. 
     The vertebral stabilization system  10  may also include one or more, or a plurality of stabilization members or connecting members  22  extending between vertebral anchors  12  of the vertebral stabilization system  10 . As an illustrative example, the vertebral stabilization system  10  shown in  FIGS. 1 and 2  includes a first connecting member  22   a  extending between and secured to the first vertebral anchor  12   a  and the second vertebral anchor  12   b , and a second connecting member  22   b  extending between and secured to the third vertebral anchor  12   c  and the fourth vertebral anchor  12   d.    
     As shown in  FIGS. 1 and 2 , in some embodiments the connecting member  22  may have a uniform cross-sectional dimension (e.g., diameter) along the entire length of the connecting member  22 . However, in other embodiments, the connecting member  22  may include one or more regions having a cross-sectional dimension (e.g., diameter) different from the cross-sectional dimension (e.g., diameter) of one or more other regions of the connecting member  22 . For instance, in some embodiments the connecting member  22  may include a first region, such as a first end region, configured to be received in the channel  30  of the head portion  14  of the first vertebral anchor  12   a  which has a first cross-sectional dimension (e.g., diameter), and the connecting member  22  may include a second region, such as a second end region, configured to be received in the channel  30  of the head portion  14  of the second vertebral anchor  12   b  which has a second cross-sectional dimension (e.g., diameter) which may be the same or different from the first cross-sectional dimension (e.g. diameter). The connecting member  22  may include a third region, such as an intermediate region between the first end region and the second end region, positionable between the head portions  14  of the first and second vertebral anchors  12   a / 12   b  which has a third cross-sectional dimension (e.g., diameter) which may be the same or different from the first cross-sectional dimension (e.g. diameter) and/or the second cross-sectional dimension (e.g., diameter). In some instances, the cross-section of the third region, or intermediate region, may be circular or non-circular. For instance, in some cases the third region, or intermediate region, may have a flattened, oval, elliptical, or rectangular cross-section having a cross-sectional dimension in a first direction which is greater than a cross-sectional dimension in a second direction perpendicular to the first direction. Such an embodiment may provide the connecting member  22  with preferential bending in a first plane relative to bending in a second plane perpendicular to the first plane. For instance, if the connecting member  22  were oriented with the larger cross-sectional dimension in a medial-lateral plane and the smaller cross-sectional dimension in an anterior-posterior plane, the connecting member  22  may more readily bend in the anterior-posterior plane than in the medial-lateral plane. Vise versa, if the connecting member  22  were oriented with the smaller cross-sectional dimension in a medial-lateral plane and the larger cross-sectional dimension in an anterior-posterior plane, the connecting member  22  may more readily bend in the medial-lateral plane than in the anterior-posterior plane. In some instances, the first and second end regions of the connecting member  22  may have a diameter of about 5.5 millimeters compatible with commercially available vertebral anchors, while the intermediate region may have a cross-sectional dimension (e.g., diameter) less than 5.5 millimeters, such as between about 4.5 millimeters to about  3 . 25  millimeters, or about 4.5 millimeters, about 4.0 millimeters, about 3.75 millimeter, or about 3.25 millimeters to increase the flexibility of the connecting member  22 . The connecting members  22  will be further discussed later herein. 
     The vertebral stabilization system  10  may desirably be used in the lumbar region of the spinal column. As used herein, the lumbar region includes the L5-S1 vertebral segment between the L5 lumbar vertebra and the S1 sacrum. However, in some instances the vertebral stabilization system  10  may be used in other regions of the spinal column, such as the cervical, thoracic and thoracolumbar regions. The vertebral stabilization system  10  may be installed multi-laterally on opposite sides of the sagittal plane of the spinal column, with the first and second anchors  12   a ,  12   b  and the first connecting member  22   a  positioned on one lateral side of the sagittal plane and the third and fourth vertebral anchors  12   c ,  12   d  and the second connecting member  22   b  positioned on the other lateral side (i.e., contra-lateral side) of the sagittal plane. However, in other instances the vertebral stabilization system  10 , including the first and second vertebral anchors  12   a ,  12   b  and the first connecting member  22   a , may be installed unilaterally (i.e., on a single side) on the spinal column. Additional vertebral anchors  12  and/or connecting members  22  may be used as desired to support vertebral segments of the spinal column as desired. 
     The connecting members  22  of the vertebral stabilization system  10  may have stiffness and strength characteristics different from commercially available spinal rods. Commonly used materials for commercially available rigid spinal rods for use in the lumbar region of the spinal column include stainless steel, commercially pure (CP) titanium, alpha-beta titanium alloy (i.e., Ti-6Al-4V or Ti-6Al-7Nb), and cobalt-chromium-molybdenum alloy (Co—Cr—Mo). Due to their relatively high rigidity compared to the natural spine, these systems have been referred to as a “rigid system”. 
     However, it has been found that the use of a rigid system may lead to adverse effects to the spinal column. For instance, it is believed that the high degree of stiffness of rigid systems may relate to increased stress on adjacent discs and facet joints. Over time, these increased stresses may lead to segment hypermobility, facet hypertrophy, osteophyte formation, and stenosis or so called adjacent level disease. 
     Recently, less rigid polymeric rod systems have been introduced as an alternative to potentially reduce the stress on adjacent discs and facet joints and the incidences of adjacent level disease. Commonly used materials for these polymer rod systems include polyether ether ketone (PEEK), PEEK composites, or other polymer materials. Due to their relatively high flexibility compared to a rigid system, these systems have been referred to as a “flexible system”. These systems, while more flexible than a rigid system, present their own shortcomings, such as the relatively low ultimate strength of the polymeric materials. 
     Table 1, below, compares the stiffness (in terms of the elastic modulus) of these commercially available spinal rod materials. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Elastic Modulus/Stiffness of Commonly Used Rod Materials 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                   
                 Short 
                 Continuous 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Carbon 
                 Carbon 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Fibre 
                 Fibre 
               
               
                   
                   
                   
                 CP 
                 Stainless 
                   
                   
                 Reinforced 
                 Reinforced 
               
               
                 Material 
                 Ti—6Al—4V 
                 Ti—6Al—7Nb 
                 Titanium 
                 steel 
                 Co—Cr—Mo 
                 PEEK 
                 PEEK 
                 PEEK 
               
               
                   
               
               
                 Elastic 
                 110 
                 110 
                 105 
                 220 
                 220 
                 4.1 
                 17 
                 65 
               
               
                 Modulus 
               
               
                 (GPa) 
               
               
                   
               
            
           
         
       
     
       FIG. 4  is a chart comparing the elastic modulus of these commercially available spinal rod materials. 
     The primary loading mode of spinal rods implanted in spinal fixation systems is caused by axial spine compression loads. Thus, a spinal rod with less structural bending stiffness will deform or bend more than a spinal rod with a greater structural bending stiffness, thus will shift a greater proportion of the spine compression load anteriorly to the vertebral bodies of the spinal segment. Additionally, the loading on the vertebral anchor (e.g., pedicle screw) will be distributed more evenly and reduce the stress at the bone/screw interface. The structural bending stiffness determines a spinal rod&#39;s bending flexibility and its load sharing characteristics. Regardless of the material of the spinal rod, a spinal rod having less structural bending stiffness will shift more of the spine compressive load anteriorly and reduce the stress at the interface between the bone and the vertebral anchor. 
     The structural bending stiffness (flexural strength) of a spinal rod having a circular cross-section can be calculated as: 
     
       
         
           
             
               
                 
                   K 
                   = 
                   
                     EI 
                     = 
                     
                       E 
                        
                       
                         
                           π 
                            
                           
                               
                           
                            
                           
                             d 
                             4 
                           
                         
                         64 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where, E is the elastic modulus of the material, I is the moment of the inertia, d is the diameter of the rod. Based on Equation (1), the structural bending stiffness of commercially available rods can be calculated and their values are listed below in Table 2. 
     
       
         
           
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Structural Bending Stiffness of Commonly Used Rods 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                 CP 
                 Stainless 
                   
               
               
                 Material 
                 Ti—6Al—4V 
                 Ti—6Al—4V 
                 Titanium 
                 Steel 
                 Co—Cr—Mo 
               
               
                   
               
               
                 Rod 
                 5.5 
                 5.5 
                 5.5 
                 5.5 
                 5.5 
               
               
                 diameter 
               
               
                 (mm) 
               
               
                 Elastic 
                 110 
                 110 
                 105 
                 220 
                 220 
               
               
                 Modulus 
               
               
                 (GPa) 
               
               
                 Bending 
                 4,938,478 
                 4,938,478 
                 4,714,002 
                 9,876,956 
                 9,876,956 
               
               
                 stiffness 
               
               
                 (N-mm 2 ) 
               
               
                 Percentage 
                 100% 
                 100% 
                 95.5% 
                 200% 
                 200% 
               
               
                 of 
               
               
                 Ti—6Al—4V 
               
               
                 5.5 mm 
               
               
                 rod 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                 Short 
                 Continuous 
               
               
                   
                   
                   
                   
                 Carbon 
                 Carbon 
               
               
                   
                   
                   
                   
                 Fibre 
                 Fibre 
               
               
                   
                   
                   
                   
                 Reinforced 
                 Reinforced 
               
               
                   
                 Material 
                 PEEK 
                 PEEK 
                 PEEK 
                 PEEK 
               
               
                   
                   
               
               
                   
                 Rod 
                 6.35 
                 5.5 
                 5.5 
                 5.5 
               
               
                   
                 diameter 
               
               
                   
                 (mm) 
               
               
                   
                 Elastic 
                 4.1 
                 4.1 
                 17 
                 65 
               
               
                   
                 Modulus 
               
               
                   
                 (GPa) 
               
               
                   
                 Bending 
                 327,061 
                 184,071 
                 763,219 
                 2,918,192 
               
               
                   
                 stiffness 
               
               
                   
                 (N-mm 2 ) 
               
               
                   
                 Percentage 
                 6.6% 
                 3.7% 
                 15.5% 
                 59.1% 
               
               
                   
                 of 
               
               
                   
                 Ti—6Al—4V 
               
               
                   
                 5.5 mm 
               
               
                   
                 rod 
               
               
                   
                   
               
            
           
         
       
     
       FIG. 5  is a chart illustrating the relative structural bending stiffness of these commercially available spinal rods of various materials as a percentage of structural bending stiffness of a 5.5 millimeter rod formed of Ti-6Al-4V. 
     As can be seen, the commercially available polymeric rod systems have a considerably reduced structural bending stiffness compared to the conventional rigid rod systems. However, the material strength of the polymeric rods may be deficient in at least some applications. The ultimate strength of the polymeric rod systems is significantly less than the ultimate strength of the conventional rigid rod systems. Table 3, below, compares the ultimate strength of these commercially available spinal rod materials. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Material Ultimate Strength 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Continuous 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Short Carbon 
                 Carbon 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Fibre 
                 Fibre 
               
               
                   
                   
                   
                 CP 
                 Stainless 
                   
                   
                 Reinforced 
                 Reinforced 
               
               
                 Material 
                 Ti—6Al—4V 
                 Ti—6Al—7Nb 
                 Titanium 
                 steel* 
                 Co—Cr—Mo 
                 PEEK 
                 PEEK 
                 PEEK 
               
               
                   
               
               
                 Ultimate 
                 896 
                 893 
                 550 
                 965 
                 1200 
                 110 
                 220 
                 710 
               
               
                 strength 
               
               
                 (MPa) 
               
               
                   
               
               
                 *Note: The ultimate strength of SS can vary from 90 to 190 ksi (620.5 to 1310 MPa) due to different cold work. The presented data is for SS with 40% cold work. 
               
            
           
         
       
     
       FIG. 6  is a chart comparing the ultimate strength of these commercially available spinal rod materials. 
     The use of polymeric rods presents additional concerns. For example, carbon debris generated from carbon fiber reinforced PEEK materials may cause biological concerns. Furthermore, unlike conventional metallic rods, polymeric rods can not be bent intra-operatively, preventing the use of polymeric rods in certain applications where rod manipulation (e.g., bending) may be desired or necessary. This may be especially true in multi-level procedures where spinal rods often need to be bent to follow the curvature of the spinal column. For these reasons, currently marketed polymeric rod systems are only indicated for single or two level procedures. In addition, transverse connectors typically can not be used in polymeric rod systems which may prevent the use of transverse connectors in applications in which additional torsion stiffness is needed or desired. 
     The major loading mode of spinal rods of spinal stabilization systems is bending caused by axial spine compression loads. As such, a spinal rod with less structural bending stiffness will deform more, and thus shift more load anteriorly to the vertebral bodies of the vertebrae. Research data on the load sharing characteristics of a natural spine has determined that the anterior elements carry about 82% of the total spinal compressive load, while the posterior elements carry about 18% of the total spinal compressive load. The anterior elements of the spinal column include the vertebral bodies and vertebral discs of the vertebral segment, while the posterior elements of the spinal column include the facet joints, the spinal cord and foremen of the spinal vertebral segment. 
     Data has indicated that at spinal segments having spinal stabilization systems utilizing 5.5 millimeter spinal rods formed of Ti-6Al-4V, the posterior elements carry about 30% of the total spinal compressive load, while the anterior elements carry about 70% of the total spinal compressive load. Additional data has indicated that at spinal segments having spinal stabilization systems utilizing 5.5 millimeter spinal rods formed of PEEK, the posterior elements carry about 15% of the total spinal compressive load, while the anterior elements carry about 85% of the total spinal compressive load. Thus, it can be seen that commercially available 5.5 millimeter rods formed of Ti-6Al-4V place too much of the spinal compressive load on the posterior elements, while commercially available 5.5 millimeter rods formed of PEEK shift too much of the spinal compressive loads to the anterior elements of the spinal column. 
     The chart at  FIG. 7  illustrates the percent of load sharing of an axial spine compression load of 5.5 millimeter spinal rods formed of PEEK and Ti-6Al-4V relative to the structural bending stiffness of the spinal rod as a percentage of the structural bending stiffness of a 5.5 millimeter spinal rod formed of Ti-6Al-4V. As can be seen from  FIG. 7 , the posterior elements carry about 30% of the total spinal compressive load at spinal segments having spinal stabilization systems utilizing 5.5 millimeter spinal rods formed of Ti-6Al-4V, whereas the posterior elements carry about 15% of the total spinal compressive load at spinal segments having spinal stabilization systems utilizing 5.5 millimeter spinal rods formed of PEEK. 
     Unlike the conventional spinal rod constructs described herein, the connecting member  22 , described herein, may have a structural bending stiffness (flexural strength) less than commercially available metallic rods, but greater than the structural bending stiffness (flexural strength) of commercially available polymeric rods. For example, the structural bending stiffness of the connecting member  22  may be between about 10% to about 40%, about 15% to about 30%, about 20% to about 25%, or about 25% of the structural bending stiffniess of a commercially available 5.5 millimeter Ti-6Al-4V spinal rod. In some instances, the structural bending stiffness of the connecting member  22  may be about 500,000 N-mm 2  to about 2,000,000 N-mm 2 , about 500,000 N-mm 2  to about 1,250,000 N-mm 2 , about 1,000,000 N-mm 2  to about 2,000,000 N-mm 2 , about 500,000 N-mm 2 , about 550,000, about 1,000,000 N-mm 2 , about 1,250,000 N-mm 2 , or about 2,000,000 N-mm 2 . 
     Thus, the use of a connecting member  22  having these characteristics in a spinal stabilization system at a spinal segment may more closely approximate the load sharing characteristics of a natural spine, such that the posterior elements carry about 18% of the total spinal compressive load, while the anterior elements carry about 82% of the total spinal compressive load. In some instances, between about 17% to about 19%, or about 18% of the total compressive load of a spinal segment having a spinal stabilization system  10  utilizing connecting members  22 , described herein, may be transferred through the posterior elements, while between about 81% to about 83%, or about 82% of the total compressive load may be transferred through the anterior elements of the spinal segment. 
     The distribution of the spine compressive load on the anterior and posterior elements of a spinal segment may be controlled by controlling the diameter of the connecting member  22  and/or the material of the connecting member  22 . Accordingly, the connecting member  22  may have a diameter less than 5.5 millimeters, for instance a diameter of about 5.0 millimeters or less or about 4.5 millimeters or less. For example, the connecting member  22  may have a diameter ranging from about 3.25 millimeters to about 4.5 millimeters. The connecting member  22  may be formed of a material, such as a metallic alloy, which has an ultimate strength greater than 1 GPa and an elastic modulus of less than or equal to 110 GPa. In some instances, the connecting member  22  may be formed of a material having an ultimate strength greater than 1 GPa and an elastic modulus of less than or equal to 100 GPa. One such material is a high strength beta titanium alloy, namely Ti-15Mo-5Zr beta titanium alloy having an elastic modulus of about 99 GPa and an ultimate strength of about 1.5 GPa. As can be seen from Table 4, below, high strength Ti-15Mo-5Zr beta titanium alloy has an elastic modulus less than that of conventional Ti-6Al-4V alpha-beta titanium alloy, but has a higher ultimate strength than Ti-6Al-4V. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Material Comparison 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                   
                 Short Carbon 
                 Continuous 
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                 Fibre 
                 Carbon Fibre 
               
               
                   
                   
                   
                   
                 Stainless 
                   
                   
                 Reinforced 
                 Reinforced 
                 High strength 
               
               
                 Material 
                 Ti—6Al—4V 
                 Ti—6Al—7Nb 
                 CP Titanium 
                 steel 
                 Co—Cr—Mo 
                 PEEK 
                 PEEK 
                 PEEK 
                 Ti—15Mo—5Zr 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Elastic 
                 110 
                 100 
                 105 
                 220 
                 220 
                 4.1 
                 17 
                 65 
                 99 
               
               
                 Modulus 
               
               
                 (GPa) 
               
               
                 Ultimate 
                 896 
                 893 
                 550 
                 965 
                 1200 
                 110 
                 220 
                 710 
                 1468 
               
               
                 Strength (MPa) 
               
               
                   
               
            
           
         
       
     
     Table 5, below, and  FIG. 8  illustrate the structural bending stiffness of connecting members  22  having diameters of 3.25 millimeters, 3.75 millimeters, 4 millimeters, and 4.5 millimeters formed of high strength beta titanium alloy Ti-15Mo-5Zr material. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Structural Bending Stiffness of Various 
               
               
                 Sized Rods formed of Ti—15Mo—5Zr 
               
            
           
           
               
               
            
               
                   
                 Rod diameter (mm) 
               
            
           
           
               
               
               
               
               
            
               
                   
                 3.25 
                 3.75 
                 4.0 
                 4.5 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 Elastic 
                 99 GPa 
                 99 GPa 
                 99 GPa 
                 99 GPa 
               
               
                 Modulus 
               
               
                 Bending 
                 541,899 
                 960,528 
                 1,243,440 
                 1,991,750 
               
               
                 stiffness 
               
               
                 (N-mm 2 ) 
               
               
                 Percentage of 
                 10.9% 
                 19.4% 
                 25.2% 
                 40.3% 
               
               
                 Ti—6Al—4V 
               
               
                 5.5 mm rod 
               
               
                   
               
            
           
         
       
     
     As can be seen from Table 5, the connecting member  22  formed of high strength beta titanium alloy Ti-15Mo-5Zr material and having a diameter of 4 millimeters has a structural bending stiffness of about 1,250,000 N-mm 2  or about 25% of the structural bending stiffness of a 5.5 millimeter spinal rod formed of Ti-6Al-4V. Thus, the use of a 4.0 millimeter diameter connecting member  22  formed of Ti-15Mo-5Zr in a spinal stabilization system at a spinal segment may more closely approximate the load sharing characteristics of a natural spine, such that the posterior elements carry about 18% of the total spinal compressive load, while the anterior elements carry about 82% of the total spinal compressive load. Furthermore, the connecting member  22  formed of high strength beta titanium alloy Ti-15Mo-5Zr material and having a diameter of 3.25 millimeters has a structural bending stiffness of about 500,000 N-mm 2  or about 10% of the structural bending stiffness of a 5.5 millimeter spinal rod formed of Ti-6Al-4V, the connecting member  22  formed of high strength beta titanium alloy Ti-15Mo-5Zr material and having a diameter of 3.75 millimeters has a structural bending stiffness of about 1,000,000 N-mm 2  or about 20% of the structural bending stiffness of a 5.5 millimeter spinal rod formed of Ti-6Al-4V, and the connecting member  22  formed of high strength beta titanium alloy Ti-15Mo-5Zr material and having a diameter of 4.5 millimeters has a structural bending stiffness of about 2,000,000 N-mm 2  or about 40% of the structural bending stiffness of a 5.5 millimeter spinal rod formed of Ti-6Al-4V. 
     Experimental testing following ASTM F1717-04 has been conducted for a 4.25 millimeter diameter rod formed of Ti-15Mo-5Zr. The test results indicated that the test rod withstood a 230 N load for 5,000,000 cycles. Earlier test results showed a commercially available 5.5 millimeter rod formed of CP titanium had a run-out load of 150 N. Thus, it was determined that the fatigue strength of the 4.25 millimeter diameter rod formed of Ti-15Mo-5Zr (230 N) was about a 50% greater than the fatigue strength of the 5.5 millimeter rod formed of CP titanium (150 N). A comparison of the fatigue strength of the 4.25 millimeter diameter rod formed of Ti-15Mo-5Zr to the fatigue strength of the 5.5 millimeter rod formed of CP titanium is shown at  FIG. 9 . 
     In some instances it may be necessary or desirable to install a first connecting member  22  of a first structural bending stiffness on a first, lateral side of the spinal column while a second connecting member  22  of a second structural bending stiffness different from the first structural bending stiffness is installed on a second, contra-lateral side of the spinal column. For instance, the first connecting member  22  may have a first diameter and the second connecting member  22  may have a second diameter different from the first diameter, while the material of the first and second connecting members  22  is the same or different. In other instances, the first connecting member  22  may be formed of a first material having a first elastic modulus and the second connecting member  22  may be formed of a second material having a second elastic modulus different from the first elastic modulus, while the diameter of the first and second connecting members  22  is the same or different. Such a selection may provide a surgeon a choice to match the desired bending stiffness and/or load distribution for a specific patient. 
     The connecting members  22 , as described herein, may be advantageous over a commercially available 5.5 millimeter diameter metallic rod system, as the connecting members  22  have a lower structural bending stiffness with an equal or greater fatigue strength than a commercially available 5.5 millimeter diameter metallic rod. Furthermore, the connecting members  22 , as described herein, may be advantageous over a commercially available 5.5 millimeter diameter polymeric rod system, as the connecting members  22  have a higher ultimate strength while maintaining a comparable structural bending stiffness. Thus, a connecting member  22 , as described herein, which has a diameter of less than 5.5 millimeters may be used in the lumbar region of a patient, where contemporary understanding indicates that commercially available 5.5 millimeter rods are required. 
     Additional benefits of the connecting member  22  having these characteristics include the ability of the connecting member  22  to be bent intra-operatively allowing the use of the connecting member  22  in applications where rod manipulation (e.g., bending) may be desired or necessary, such as lordosed regions of the lumbar region of the spinal column. Furthermore, transverse connectors, such as the transverse connector  60  shown in  FIG. 1 , can be used with the connecting member  22  having these characteristics in applications in which additional torsion stiffness is needed or desired, or otherwise where the use of transverse connectors may be desired. The connecting members  22  are also radio-opaque. 
     The transverse connector  60  is further illustrated at  FIGS. 10A ,  10 B and  10 C. The transverse connector  60 , in many respects, is similar to the transverse connector disclosed in U.S. Pat. No. 7,485,132, incorporated herein by reference. The transverse connector  60  may include a first section  61  and a second section  62  selectively coupled to the first section  61 . For instance, the first section  61  may be coupled to the second section  62  at any of a plurality of longitudinal and/or angular positions. For example, the first section  61  may include a housing  63  configured to receive a rod  64  of the second section  62  therein. The rod  64  may be secured in the housing  63  with a fastener  65  at any of a plurality of longitudinal and/or angular positions. Each of the first and second sections  61 ,  62  may include a rod coupling region  66  configured to surround a portion of an connecting member  22 . 
     An insert  68  may be positioned in the opening of the rod coupling region  66  for spacing the rod coupling region  66  of the transverse connector  60  from direct contact with the connecting member  22 . The insert  68 , which may be a C-shaped member or similarly shaped, may include a channel  67  extending therethrough into which a connecting member  22  may be positioned. The transverse connecter  60  may also include cam members  69  which may be rotated to secure the transverse connector  60  to the connecting member  22 . 
     The insert  68  may be formed of a material having a lower modulus of elasticity than the material forming the rod coupling regions  66  and the cam members  69  of the transverse connector  60 . For instance, the rod coupling regions  66  and/or the cam members  69  may be formed of a metallic material, such as stainless steel, CP titanium, titanium alloy, cobalt-chromium-molybdenum alloy (Co—Cr—Mo), or other biocompatible metallic material. The insert  68  may be formed of a polymeric material, such as polyether ether ketone (PEEK), carbon fiber reinforced PEEK, ultra high molecular weight polyethylene (UHMWPE), or poly(methyl methacrylate) (PMMA). 
     In order to secure the connecting member  22  to the transverse connector  60 , the cam members  69 , or other securing members, may be rotated with a driving tool, which exerts a force onto the connecting members  22  positioned in the channel  67  via the inserts  68 . The presence of the inserts  68  between the cam members  69  and the connecting members  22  may provide a stress gradient between the cam member  69  and the connecting members  22 , distributing the forces exerted onto the connecting members  22  through tightening the cam members  69  over a larger portion of the exterior surface of the connecting members  22 . Therefore, the inclusion of the inserts  68  may eliminate any notching of the connecting members  22 , thereby increasing the fatigue strength of the connecting members  22  in the vertebral stabilization system  10  such that a connecting member  22  of a smaller diameter than conventional spinal rods (e.g., smaller than 5.5 millimeters) may be used in the vertebral stabilization system  10  while not compromising the fatigue strength of the connecting member  22  and the vertebral stabilization system  10 . 
     An alternative embodiment of a transverse connector for use in the vertebral stabilization system  10  is illustrated in  FIGS. 11A and 11B . The transverse connector  80 , in many respects, is similar to the transverse connector disclosed in U.S. Pat. No. 6,328,740, incorporated herein by reference. The transverse connector  80  may include first and second housings  81 ,  82  coupled together with a linking portion  83 . Each of the first and second housings  81 ,  82  is configured to be coupled to a connecting member  22  of the vertebral stabilization system  10 . A rod coupling member  84  may be positioned in an opening of each of the first and second housings  81 ,  82 . The rod coupling member  84  may include first and second legs  85  which may be deflectable toward one another upon the application of force. 
     An insert  88  may be positioned in the opening of the rod coupling member  84  between the first and second legs  85  for spacing the rod coupling member  84  of the transverse connector  80  from direct contact with the connecting member  22 . The insert  88 , which may be a C-shaped member or similarly shaped, may include a channel  87  extending therethrough into which a connecting member  22  may be positioned. The transverse connecter  80  may also include threaded nuts  89  threadably engaging threaded shafts  86  of the rod coupling members  84  which may be rotated to draw the coupling members  84  into the first and second housings  81 ,  82  to secure the transverse connector  80  to the connecting member  22 . 
     The insert  88  may be formed of a material having a lower modulus of elasticity than the material forming the rod coupling members  84  or other components of the transverse connector  80 . For instance, the rod coupling members  84  and/or other components of the transverse connector  80  may be formed of a metallic material, such as stainless steel, CP titanium, titanium alloy, cobalt-chromium-molybdenum alloy (Co—Cr—Mo), or other biocompatible metallic material. The insert  88  may be formed of a polymeric material, such as polyether ether ketone (PEEK), carbon fiber reinforced PEEK, ultra high molecular weight polyethylene (UHMWPE), or poly(methyl methacrylate) (PMMA). 
     In order to secure the connecting member  22  to the transverse connector  80 , the threaded nuts  89  may be rotated with a driving tool, drawing the rod coupling members  84  into the housings  81 ,  82 . As the rod coupling members  84  are drawn into the housings  81 ,  82 , the legs  85  of the rod coupling members  84  are deflected toward one another due to the engagement between the rod coupling members  84  and the housings  81 ,  82 . As the legs  85  are deflected, a clamping force is exerted on the connecting member  22  via the insert  88 . The presence of the inserts  88  between the rod coupling members  84  and the connecting members  22  may provide a stress gradient between the rod coupling members  84  and the connecting members  22 , distributing the forces exerted onto the connecting members  22  through tightening the threaded nuts  89  over a larger portion of the exterior surface of the connecting members  22 . Therefore, the inclusion of the inserts  88  may eliminate any notching of the connecting members  22 , thereby increasing the fatigue strength of the connecting members  22  in the vertebral stabilization system  10  such that a connecting member  22  of a smaller diameter than conventional spinal rods (e.g., smaller than 5.5 millimeters) may be used in the vertebral stabilization system  10  while not compromising the fatigue strength of the connecting member  22  and the vertebral stabilization system  10 . 
     Another illustrative vertebral stabilization system  110  is illustrated at  FIG. 12 . The vertebral stabilization system  110  may include one or more or a plurality of vertebral anchors or fasteners  112 . Although the vertebral anchors  112  are depicted as threaded vertebral fasteners (e.g., pedicle screws, bone screws), in some embodiments the vertebral anchors  112  may be vertebral hooks (e.g., laminar hooks) or other types of fastening members for attachment to a bony structure such as a vertebra of the spinal column. Each of the vertebral anchors  112  may be configured to be secured to a vertebra of a spinal column. For instance, the first vertebral anchor  112   a  may be secured to a first vertebra, the second vertebral anchor  112   b  may be secured to a second vertebra, and the third vertebral anchor  112   c  may be secured to a third vertebra. 
     The vertebral anchor  112  may include a head portion  114  and a bone engagement portion  116  extending from the head portion  114 . In some embodiments, the bone engagement portion  116  may be a shaft portion  118  of the vertebral anchor  112  extending from the head portion  114  along a longitudinal axis of the vertebral anchor  112 . In some embodiments, the vertebral anchor  112  may be a monoaxial screw in which the head portion  114  is stationary relative to the shaft portion  118 , and in other embodiments the vertebral anchor  112  may be a polyaxial screw in which the head portion  114  is actuatable (e.g., pivotable) relative to the shaft portion  118 . In some embodiments, the shaft portion  118  may be configured to be installed into a bony region of a vertebra of the spinal column. For example, the shaft portion  118  may be installed into a pedicle of a vertebra, or other region of a vertebra. In some embodiments, the shaft portion  118  may be a threaded region having helical threads configured to be screwed into a pedicle of a vertebra, or other bony region of a vertebra. 
     The head portion  114  may include a base portion  124 , from which the shaft portion  118  extends from, and first and second legs  126  extending from the base portion  124  on opposing sides of the head portion  114 . The first and second legs  126  may define an opening  128 , which may be a threaded opening in some instances, extending into the head portion  114  from an upper extent of the head portion  114  opposite the base portion  124 . In embodiments in which the opening  128  is threaded, each of the first and second legs  126  may include a threaded portion for threadedly engaging a threaded portion of a securing member  120 . In other embodiments, the first and second legs  126  may include other engagement features for engaging with a securing member positioned in the opening  128  between the first and second legs  126 . The head portion  114  may additionally include a channel  130 , such as a U-shaped channel, defined between the first and second legs  126 . The channel  130  may extend through the head portion  114  from a first side of the head portion  114  to a second side of the head portion  114 . The opening  128  may intersect the channel  130 . 
     The vertebral anchor  112  may include a securing element, such as a threaded fastener  120  (e.g., a set screw, cap) configured to engage the head portion  114  to secure one or more elongate members to the vertebral anchor  112 . For example, the threaded fastener  120  may include threads which mate with threads formed in the head portion  114 . In other embodiments, other securing members, having engagement features, may be used to secure one or more elongate members, such as an elongate rod or flexible member, in the head portion  114  of the vertebral anchor  112 . 
     The vertebral stabilization system  110  may also include one or more, or a plurality of elongate connecting members extending between vertebral anchors  112  of the vertebral stabilization system  110 . As an illustrative example, the vertebral stabilization system  110  shown in  FIG. 12  includes a first elongate member, shown as an elongate rod  140 , extending between and secured to the first vertebral anchor  112   a  and the second vertebral anchor  112   b , and a second elongate member, shown as a flexible member  160  (e.g., a flexible cord), extending between and secured to the second vertebral anchor  112   b  and the third vertebral anchor  112   c.    
       FIG. 13  is a perspective view of the elongate rod  140  of the vertebral stabilization system  110 . The elongate rod  140  may have a first end  142 , a second end  144 , and a length between the first end  142  and the second end  144  sufficient to span the distance between first vertebral anchor  112   a  and the second vertebral anchor  112   b . The elongate rod  140  may be formed of any desired material, including those materials listed above such as stainless steel, commercially pure (CP) titanium, alpha-beta titanium alloy (e.g., Ti-6Al-4V), beta titanium alloy (e.g., Ti-15Mo-5Zr), other metals or metal alloys, polyether ether ketone (PEEK), PEEK composites, or other polymer materials. 
     The elongate rod  140  may include a first region  146  and a second region  148 . The first region  146  may have a circular cross-section having a desired diameter, such as a diameter of about 5.5 millimeters, about 5.0 millimeters, about 4.5 millimeters, about 4.25 millimeters, about 4.0 millimeters, about 3.75 millimeters, or about 3.5 millimeters, in some instances. It is contemplated that the first region  146  may also have a non-circular cross-section in some instances. 
     In some instances, the second region  148  of the elongate rod  140  may be of a reduced diameter relative to the first region  146 . For example, in some instances, the first region  146  may have a diameter of about 5.5 millimeters or more, while the second region  148  may have a diameter of less than 5.5 millimeters, such as about 5.0 millimeters, about 4.5 millimeters, about 4.25 millimeters, about 4.0 millimeters, about 3.75 millimeters, or about 3.5 millimeters. A transition region, such as a tapered region, or a step-wise transition may be located between the first region  146  and the second region  148 . 
     The second region  148  may include at least a portion having an exterior engagement surface  150  against which the flexible member  160  may be positioned adjacent to. In some instances the exterior engagement surface  150 , which is a portion of the exterior surface of the elongate rod  140 , may be a planar surface. In other instances, the exterior engagement surface  150  may be a slightly convexly curved surface having a radius of curvature different from the radius of curvature of the remainder of the outer surface of the second region  148  of the elongate rod  140 . For instance, the radius of curvature of the exterior engagement surface  150  may be greater than the radius of curvature of the outer surface around the circumference of the remainder of the second region  148 . Thus, the center of curvature of the exterior engagement surface  150  may be offset from the central longitudinal axis of the elongate rod  140 . In other embodiments, as shown in  FIG. 13 , the exterior engagement surface  150  may be a concave surface on the exterior of the elongate rod  140  forming an open channel along at least a portion of the second region  148  of the elongate rod  140  for placement of a portion of the flexible member  160  there along. Thus, the second region  148  may have a non-circular cross-section throughout at least a portion of the second region  148 . 
     The elongate rod  140  may also include a flange  152  at the second end  144  of the elongate rod  140 . The flange  152  may include a first side surface  154  and a second side surface  156  opposite the first side surface  154 . In some instances, the flange  152  may be generally circular with a center point coaxial with the central longitudinal axis of the elongate rod  140 , while in other instances, the center point of the flange  152  may be off-set from and non-coaxial with the central longitudinal axis of the elongate rod  140 . The flange  152  may include an opening, shown in the form of a notch  158 , extending toward the center of the flange  152  from the periphery of the flange  152 . Thus, the notch  158  may be open to the periphery of the flange  152 . The notch  158  may accommodate a portion of the flexible member  160  extending from one side of the flange  152  to the other side of the flange  152  when both the flexible member  160  and the elongate rod  140  are received and secured in the head portion  114  of the second vertebral anchor  112   b . Thus, the notch  158  may allow the flexible member  160 , extending along side of the elongate rod  140 , to remain closer to the central longitudinal axis of the elongate rod  140  as the flexible member  160  extends past the flange  152  toward the third vertebral anchor  112   c.    
       FIG. 14  is a perspective view of an alternate embodiment of an elongate rod  240 , similar to the elongate rod  140 , which may be used in the vertebral stabilization system  110 . For instance, the elongate rod  240  may include a first end  242 , a second end  244  and a length between the first end  242  and the second end  244 . Additionally, the elongate rod  240  may include a first region  246  and second region  248  which may include at least a portion having an exterior engagement surface  250  against which the flexible member  160  may be positioned adjacent to. In some instances the exterior engagement surface  250 , which may be a flattened exterior surface, may be a planar surface. In other instances, the exterior engagement surface  250  may be a slightly curved surface having a radius of curvature different from the radius of curvature of the remainder of the outer surface of the second region  248  of the elongate rod  240 . For instance, the radius of curvature of the exterior engagement surface  250  may be greater than the radius of curvature of the outer surface around the circumference of the remainder of the second region  248 . Thus, the center of curvature of the exterior engagement surface  250  may be offset from the central longitudinal axis of the elongate rod  240 . Thus, the second region  248  may have a non-circular cross-section throughout at least a portion of the second region  248 . 
     In some instances, the second region  248  of the elongate rod  240  may be of a reduced diameter relative to the first region  246 . For example, in some instances, the first region  246  may have a diameter of about 5.5 millimeters or more, while the second region  248  may have a diameter of less than 5.5 millimeters, such as about 5.0 millimeters, about 4.5 millimeters, about 4.25 millimeters, about 4.0 millimeters, about 3.75 millimeters, or about 3.5 millimeters. A transition region, such as a tapered region, or a step-wise transition may be located between the first region  246  and the second region  248 . 
     The elongate rod  240 , similar to the elongate rod  140 , may also include a flange  252  at the second end  244  of the elongate rod  240 . The flange  252  may include a first side surface  254  and a second side surface  256  opposite the first side surface  254 . In some instances, the flange  252  may be generally circular with a center point coaxial with the central longitudinal axis of the elongate rod  240 , while in other instances, the center point of the flange  252  may be off-set from and non-coaxial with the central longitudinal axis of the elongate rod  240 . The flange  252  may include an opening, shown in the form of a notch  258 , extending toward the center of the flange  252  from the periphery of the flange  252 . Thus, the notch  258  may be open to the periphery of the flange  252 . The notch  258  may accommodate a portion of the flexible member  160  extending from one side of the flange  252  to the other side of the flange  252  when both the flexible member  160  and the elongate rod  240  are received and secured in the head portion  114  of the second vertebral anchor  112   b . Thus, the notch  258  may allow the flexible member  160 , extending along side of the elongate rod  240 , to remain closer to the central longitudinal axis of the elongate rod  240  as the flexible member  160  extends past the flange  252  toward the third vertebral anchor  112   c.    
     The flexible member  160 , which in some instances may be a flexible cord, may be positioned adjacent to the elongate rod  140  in a side-by-side fashion in the head portion  114  of the second vertebral anchor  112   b , with a portion of the flexible member  160  overlapping a portion of the elongate rod  140  such that an exterior surface of the flexible member  160  is positioned adjacent to and in contact with an exterior surface of the elongate rod  140 . For example, a portion of the flexible member  160  may extend along and contact the exterior engagement surface  150  of the elongate rod  140 . With such a configuration, the central longitudinal axis of the elongate rod  140  may be offset from and non-coaxial with the central longitudinal axis of the flexible member  160 .  FIG. 15  is a longitudinal cross-sectional view of the vertebral stabilization system  110  shown in  FIG. 12  which further illustrates the overlapping positioning of the elongate rod  140  and the flexible member  160  in the head portion  114  of the second vertebral anchor  112   b.    
     The elongate rod  140  may be positioned in the channel  130  of the head portion  114  of the second vertebral anchor  112   b  such that the flange  152  is positioned facing the second side  134  of the head portion  114  with the elongate rod  140  extending from the head portion  114  of the second vertebral anchor  112   b  to the head portion  114  of the first vertebral anchor  112   a . Thus, at least the portion of the second region  148  of the elongate rod  140  including the exterior engagement surface  150  may be positioned within the channel  130  of the head portion  114  of the second vertebral anchor  112   b . Thus, a portion of the flexible member  160  overlapping and positioned adjacent the exterior engagement surface  150  of the elongate rod  140  may also be positioned within the channel  130  of the head portion  114  of the second vertebral anchor  112   b . The flexible member  160  may extend from the head portion  114  through the notch  158  of the flange  152  to the third vertebral anchor  112   c.    
     A spacer  162  may be disposed on the flexible member  160  and be positioned between the flange  152  and the head portion  114  of the third vertebral anchor  112   c . For instance, the spacer  162  may include a first end  164 , a second end  166  and a lumen  168  extending through the spacer  162  from the first end  164  to the second end  166 . The flexible member  160  may extend through the lumen  168  of the spacer  162 . When positioned between the flange  152  and the head portion  114  of the third vertebral anchor  112   c , the first end  164  of the spacer  162  may abut or otherwise contact the second side surface  156  of the flange  152  and the second end  166  of the spacer  162  may abut or otherwise contact a side surface of the head portion  114  of the third vertebral anchor  112   c.    
     The threaded fastener  120 , or other securing element, may be engaged (e.g., rotatably or threadably engaged) to the head portion  114  to exert a clamping force on the flexible member  160  and the elongate rod  140  to secure the flexible member  160  and the elongate rod  140  in the channel  130  of the head portion  114  of the second vertebral anchor  112   b.    
     As shown in  FIG. 15 , in some instances the flexible member  160  may be positioned above the elongate rod  140  such that the elongate rod  140  rests against the base portion  124  and the flexible member  160  is positioned between the elongate rod  140  and the threaded fastener  120 . Thus, in such instances the threaded fastener  120  may exert a force against the flexible member  160 , which in turn exerts a force against the elongate rod  140  to secure the flexible member  160  and the elongate rod  140  in the channel  130  of the head portion  114  of the second vertebral anchor  112   b . Positioning the flexible member  160  between the threaded fastener  120  and the elongate rod  140  may protect the elongate rod  140  from a notching effect (e.g., galling/fretting) attributed to direct contact between the threaded fastener  120  and the elongate rod  140  which may in turn increase the fatigue strength of the elongate rod  140 . In other instances, the elongate rod  140  may be positioned above the flexible member  160  such that the flexible member  160  rests against the base portion  124  and the elongate rod  140  is positioned between the flexible member  160  and the threaded fastener  120 . Thus, in such instances the threaded fastener  120  may exert a force against the elongated rod  140 , which in turn exerts a force against the flexible member  160  to secure the flexible member  160  and the elongate rod  140  in the channel  130  of the head portion  114  of the second vertebral anchor  112   b.    
     Another illustrative vertebral stabilization system  310  is illustrated at  FIGS. 16 and 17 . The vertebral stabilization system  310  may include one or more or a plurality of vertebral anchors or fasteners  312 , one of which is shown in  FIGS. 16 and 17 . Although the vertebral anchors  312  are depicted as threaded vertebral fasteners (e.g., pedicle screws, bone screws), in some embodiments the vertebral anchors  312  may be vertebral hooks (e.g., laminar hooks) or other types of fastening members for attachment to a bony structure such as a vertebra of the spinal column. Each of the vertebral anchors  312  may be configured to be secured to a vertebra of a spinal column. For instance, the vertebral anchor  312  shown may be secured to a first vertebra, while a second vertebral anchor may be secured to a second vertebra, and a third vertebral anchor may be secured to a third vertebra, as described above regarding the vertebral stabilization system  110 . 
     The vertebral anchor  312  may include a head portion  314  and a bone engagement portion  316  extending from the head portion  314 . In some embodiments, the bone engagement portion  316  may be a shaft portion  318  of the vertebral anchor  312  extending from the head portion  314  along a longitudinal axis of the vertebral anchor  312 . In some embodiments, the vertebral anchor  312  may be a monoaxial screw in which the head portion  314  is stationary relative to the shaft portion  318 , and in other embodiments the vertebral anchor  312  may be a polyaxial screw in which the head portion  314  is actuatable (e.g., pivotable) relative to the shaft portion  318 . In some embodiments, the shaft portion  318  may be configured to be installed into a bony region of a vertebra of the spinal column. For example, the shaft portion  318  may be installed into a pedicle of a vertebra, or other region of a vertebra. In some embodiments, the shaft portion  318  may be a threaded region having helical threads configured to be screwed into a pedicle of a vertebra, or other bony region of a vertebra. 
     The head portion  314  may include a base portion  324 , from which the shaft portion  318  extends from, and first and second legs  326  extending from the base portion  324  on opposing sides of the head portion  314 . The first and second legs  326  may define an opening  328 , which may be a threaded opening in some instances, extending into the head portion  314  from an upper extent of the head portion  314  opposite the base portion  324 . In embodiments in which the opening  328  is threaded, each of the first and second legs  326  may include a threaded portion for threadedly engaging a threaded portion of a securing member  320 . In other embodiments, the first and second legs  326  may include other engagement features for engaging with a securing member positioned in the opening  328  between the first and second legs  326 . The head portion  314  may additionally include a channel  330 , such as a U-shaped channel, defined between the first and second legs  326 . The channel  330  may extend through the head portion  314  from a first side of the head portion  314  to a second side of the head portion  314 . The opening  328  may intersect the channel  330 . 
     The vertebral anchor  312  may include a securing element, such as a threaded fastener  320  (e.g., a set screw, cap) configured to engage the head portion  314  to secure one or more elongate members to the vertebral anchor  312 . For example, the threaded fastener  320  may include threads which mate with threads formed in the head portion  314 . In other embodiments, other securing members, having engagement features, may be used to secure one or more elongate members, such as an elongate rod or flexible member, in the head portion  314  of the vertebral anchor  312 . 
     The vertebral stabilization system  310  may also include one or more, or a plurality of elongate connecting members extending between vertebral anchors  312  of the vertebral stabilization system  310 . As an illustrative example, the vertebral stabilization system  310  shown in  FIGS. 16 and 17  includes a first elongate member, shown as an elongate rod  340 , secured to the vertebral anchor  312 , and a second elongate member, shown as a flexible member  360  (e.g., a flexible cord), also secured to the vertebral anchor  312 . The elongate rod  340  may extend from the vertebral anchor  312  in a first direction to a second vertebral anchor while the flexible member  360  may extend from the vertebral anchor  312  in a second direction opposite the first direction to a third vertebral anchor. 
     As further illustrated in  FIG. 17 , the elongate rod  340  may have a first end  342 , a second end  344 , and a length between the first end  342  and the second end  344  sufficient to span the distance between the vertebral anchor  312  and a second vertebral anchor. The elongate rod  340  may be formed of any desired material, including those materials listed above such as stainless steel, commercially pure (CP) titanium, alpha-beta titanium alloy (e.g., Ti-6Al-4V), beta titanium alloy (e.g., Ti-15Mo-5Zr), other metals or metal alloys, polyether ether ketone (PEEK), PEEK composites, or other polymer materials. 
     The elongate rod  340  may include a first region  346  and a second region  348 . The first region  346  may have a circular cross-section having a desired diameter, such as a diameter of about 5.5 millimeters, about 5.0 millimeters, about 4.5 millimeters, about 4.25 millimeters, about 4.0 millimeters, about 3.75 millimeters, or about 3.5 millimeters, in some instances. It is contemplated that the first region  346  may also have a non-circular cross-section in some instances. 
     In some instances, the second region  348  of the elongate rod  340  may be of a reduced diameter relative to the first region  346 . For example, in some instances, the first region  346  may have a diameter of about 5.5 millimeters or more, while the second region  348  may have a diameter of less than 5.5 millimeters, such as about 5.0 millimeters, about 4.5 millimeters, about 4.25 millimeters, about 4.0 millimeters, about 3.75 millimeters, or about 3.5 millimeters. A transition region, such as a tapered region, or a step-wise transition may be located between the first region  346  and the second region  348 . 
     The second region  348  may include at least a portion having an exterior engagement surface  350  against which the flexible member  360  may be positioned adjacent to. In some instances the exterior engagement surface  350 , which is a portion of the exterior surface of the elongate rod  340 , may be a planar surface. In other instances, the exterior engagement surface  350  may be a slightly convexly curved surface having a radius of curvature different from the radius of curvature of the remainder of the outer surface of the second region  348  of the elongate rod  340 . For instance, the radius of curvature of the exterior engagement surface  350  may be greater than the radius of curvature of the outer surface around the circumference of the remainder of the second region  348 . Thus, the center of curvature of the exterior engagement surface  350  may be offset from the central longitudinal axis of the elongate rod  340 . In other embodiments, as shown in  FIG. 17 , the exterior engagement surface  350  may be a concave surface on the exterior of the elongate rod  340  forming an open channel along at least a portion of the second region  348  of the elongate rod  340  for placement of a portion of the flexible member  360  there along. Thus, the second region  348  may have a non-circular cross-section throughout at least a portion of the second region  348 . The exterior engagement surface  350  may include surface roughenings  370 . The surface roughenings  370  may help maintain the flexible member  360  from moving relative to the elongate rod  340  when the flexible member  460  and the elongate rod  340  are secured in the head portion  314  of the vertebral anchor  312 . The surface roughenings  370  may be comprised of any mechanical gripping means such as, but not limited to, one or more threads, ribs, projections, grooves, teeth, and/or serrations or combination thereof. 
     The elongate rod  340  may also include a first flange  352  at the second end  344  of the elongate rod  340 . In some instances, the first flange  352  may be generally circular with a center point coaxial with the central longitudinal axis of the elongate rod  340 , while in other instances, the center point of the first flange  352  may be off-set from and non-coaxial with the central longitudinal axis of the elongate rod  340 . The first flange  352  may include an opening  359  extending through the first flange  352 . 
     The elongate rod  340  may also include a second flange  353  spaced away from the first flange  352  toward the first end  342  of the elongate rod  340 . In some instances, the second flange  353  may be generally circular with a center point coaxial with the central longitudinal axis of the elongate rod  340 , while in other instances, the center point of the second flange  353  may be off-set from and non-coaxial with the central longitudinal axis of the elongate rod  340 . The second flange  354  may include a holding slot  355  for receiving the end portion of the flexible member  360 . In some instances the holding slot  355  may have a trapezoidal shape or other shape such that a width of the slot nearer the first flange  352  is less than a width of the slot further from the first flange  352 . In such an embodiment, an end portion of the flexible member  360  may be sized to have a cross-sectional dimension greater than a width of the slot  355  such that the end portion of the flexible member  360  may be urged into the slot  355  (e.g., in a direction perpendicular to the central longitudinal axis of the elongate rod  340 ) and retained in place by the interference fit between the end portion of the flexible member  360  and the side walls of the slot  355  such that the flexible member  360  may not be able to be readily removed from the slot  355  by pulling the flexible member  360  in a direction parallel to the central longitudinal axis of the elongate rod  340 . 
     In some instances, the flexible member  360  may be pre-assembled with the elongate rod  340  prior to the medical procedure. For instance, the flexible member  460  may be positioned through the opening  359  of the first flange  352  and into the open channel formed by the concave exterior surface of the exterior engagement surface  350  prior to the medical procedure. In some instances, the flexible member  360  may be crimped in the open channel by crimping the portion of the elongate rod  340  between the first and second flanges  352 ,  353  which partially surround the flexible member  360  to provisionally secure the flexible member  360  to the elongate rod  340  prior to the medical procedure. In some instances, the concave exterior surface of the exterior engagement surface  350 , while surrounding less than the entire circumference of the flexible member  360 , may surround and contact greater than 50% of the circumference of the flexible member  360 . Additionally and/or alternatively, the end portion of the flexible member  360  may be retained by the interference fit with the slot  355  to provisionally secure the flexible member  360  to the elongate rod  340  prior to the medical procedure. 
     The second flange  353  may be spaced from the first flange  352  such that when the elongate rod  340  is coupled to the vertebral anchor  312 , the head portion  314  of the vertebral anchor  312  is positioned between the first and second flanges  352 ,  353  with the first flange  352  positioned adjacent a first side of the head portion  314  and the second flange  353  positioned adjacent a second side of the head portion  314 . 
     The flexible member  360 , which in some instances may be a flexible cord, may be positioned adjacent to the elongate rod  340  in a side-by-side fashion in the head portion  314  of the vertebral anchor  312 , with a portion of the flexible member  360  overlapping a portion of the elongate rod  340  such that an exterior surface of the flexible member  360  is positioned adjacent to and in contact with an exterior surface of the elongate rod  340 . For example, a portion of the flexible member  360  may extend along and contact the exterior engagement surface  350  of the elongate rod  340 . With such a configuration, the central longitudinal axis of the elongate rod  340  may be offset from and non-coaxial with the central longitudinal axis of the flexible member  360 .  FIG. 18  is a longitudinal cross-sectional view of the vertebral stabilization system  310  shown in  FIG. 16  which further illustrates the overlapping positioning of the elongate rod  340  and the flexible member  360  in the head portion  314  of the vertebral anchor  312 . 
     The elongate rod  340  may be positioned in the channel  330  of the head portion  314  of the vertebral anchor  312  such that the head portion  314  of the vertebral anchor  312  is positioned between the first and second flanges  352 ,  353  with the elongate rod  340  extending from the head portion  314  of the vertebral anchor  312  to the head portion of a second vertebral anchor (not shown). Thus, at least the portion of the second region  348  of the elongate rod  340  including the exterior engagement surface  350  may be positioned within the channel  330  of the head portion  314  of the vertebral anchor  312 . Thus, a portion of the flexible member  360  overlapping and positioned adjacent the exterior engagement surface  350  of the elongate rod  340  may also be positioned within the channel  330  of the head portion of the vertebral anchor  312 . The flexible member  360  may extend from the head portion  314  through the opening  359  of the first flange  352  to a third vertebral anchor (not shown). 
     In some instances, a spacer (not shown) may be disposed on the flexible member  360  and be positioned between the first flange  352  and the head portion of the third vertebral anchor, as described above. For instance, a spacer, such as the spacer  162  of the vertebral stabilization system  110  shown above at  FIG. 12 , may be disposed around the flexible member  360  and have a first end abutting or otherwise in contact with the first flange  352  and a second end positioned proximate the head portion of a third vertebral anchor. 
     The threaded fastener  320 , or other securing element, may be engaged (e.g., rotatably or threadably engaged) to the head portion  314  to exert a clamping force on the flexible member  360  and the elongate rod  340  to secure the flexible member  360  and the elongate rod  340  in the channel  330  of the head portion  314  of the vertebral anchor  312 . In some instances, the threaded fastener  320  may include a retention feature, such as one or more protrusions which may project into and/or deform the flexible member  360  when the threaded fastener  320  is compressed against the flexible member  360 . 
     As shown in  FIG. 18 , in some instances the flexible member  360  may be positioned above the elongate rod  340  such that the elongate rod  340  rests against the base portion  324  and the flexible member  360  is positioned between the elongate rod  340  and the threaded fastener  320 . Thus, in such instances the threaded fastener  320  may exert a force against the flexible member  360 , which in turn exerts a force against the elongate rod  340  to secure the flexible member  360  and the elongate rod  340  in the channel  330  of the head portion  314  of the vertebral anchor  312 . Positioning the flexible member  360  between the threaded fastener  320  and the elongate rod  340  may protect the elongate rod  340  from a notching effect (e.g., galling/fretting) attributed to direct contact between the threaded fastener  320  and the elongate rod  340  which may in turn increase the fatigue strength of the elongate rod  340 . In other instances, the elongate rod  340  may be positioned above the flexible member  360  such that the flexible member  360  rests against the base portion  124  and the elongate rod  340  is positioned between the flexible member  360  and the threaded fastener  320 . Thus, in such instances the threaded fastener  320  may exert a force against the elongated rod  340 , which in turn exerts a force against the flexible member  360  to secure the flexible member  360  and the elongate rod  340  in the channel  330  of the head portion  314  of the vertebral anchor  312 . 
     Those skilled in the art will recognize that the present invention may be manifested in a variety of forms other than the specific embodiments described and contemplated herein. Accordingly, departure in form and detail may be made without departing from the scope and spirit of the present invention as described in the appended claims.