Abstract:
An elongated member forming a spinal support rod is implantable adjacent the spine of a patient, and includes an axial span or spans for spanning respective spinal levels to promote efficacious spinal support/stabilization. As with conventional spinal support rods used in connection with lumbar fusion and other related procedures, the elongated member extends in an axial direction, and is substantially dimensionally stable, both radially and axially. The elongated member is further capable of bending, flexing, and/or deflecting laterally (e.g., along any and/or substantially all transverse directions) to an extent that preserves at least some spinal motion. Such elongated members can include axial spans that manifest a radially segmented geometry relative to the axial direction, include a sleeve and a series of structural members or a coil spring enclosed within the sleeve, and/or include a coil spring and a restraining element passing at least partially through the coil spring.

Description:
BACKGROUND  
       [0001]     1. Technical Field  
         [0002]     The present disclosure relates to devices, systems and methods for spinal stabilization. More particularly, the present disclosure relates to devices, systems and methods for providing dynamic stabilization to the spine via the use of elongated members spanning one or more spinal levels.  
         [0003]     2. Background Art  
         [0004]     Each year, over 200,000 patients undergo lumbar fusion surgery in the United States. While fusion is a well-established procedure that is effective about seventy percent of the time, there are consequences even to successful fusion procedures, including a reduced range of motion and an increased load transfer to adjacent levels of the spine, which may accelerate degeneration at those levels. Further, a significant number of back-pain patients, estimated to exceed seven million in the U.S., simply endure chronic low-back pain, rather than risk procedures that may not be appropriate or effective in alleviating their symptoms.  
         [0005]     New treatment modalities, collectively called motion preservation devices, are currently being developed to address these limitations. Some promising therapies are in the form of nucleus, disc or facet replacements. Other motion preservation devices provide dynamic internal stabilization of the injured and/or degenerated spine, e.g., the Dynesis stabilization system (Zimmer, Inc.; Warsaw, Ind.) and the Graf Ligament. A major goal of this concept is the stabilization of the spine to prevent pain while preserving near normal spinal function.  
         [0006]     In general, while great strides are currently being made in the development of motion preservation devices, the use of such devices is not yet widespread. One reason that this is so is the experimental nature of most such devices. For example, to the extent that a given motion preservation device diverges, whether structurally or in its method of use or implementation, from well-established existing procedures such as lumbar fusion surgery, considerable experimentation and/or testing is often necessary before such a device is given official approval by governmental regulators, and/or is accepted by the medical community as a safe and efficacious surgical option.  
         [0007]     With the foregoing in mind, those skilled in the art will understand that a need exists for spinal stabilization devices, systems and methods that preserve spinal motion while at the same time exhibiting sufficient similarity to well-established existing spinal stabilization devices, systems and methods so as encourage quick adoption/approval of the new technology. These and other needs are satisfied by the disclosed devices, systems and methods that include elongated members for implantation across one or more levels of the spine.  
       SUMMARY OF THE PRESENT DISCLOSURE  
       [0008]     According to the present disclosure, advantageous devices, systems, kits for assembly, and/or methods for dynamic stabilization are provided. According to exemplary embodiments of the present disclosure, the disclosed devices, systems, kits and methods include an elongated member, e.g., a spinal support rod, that is configured and dimensioned for implantation adjacent the spine of a patient so as to promote efficacious spinal stabilization. The disclosed elongated member extends axially, e.g., as do spinal support rods used in connection with lumbar fusion and other related procedures. Among other similarities therewith, e.g., such as are described hereinbelow, the disclosed elongated member is substantially dimensionally stable, both radially and axially. Among some differences therewith, e.g., such as are described below, the disclosed elongate member is capable of bending, flexing, and/or deflecting laterally (e.g., along any and/or substantially all transverse directions) to an extent that preserves a degree of spinal motion.  
         [0009]     According to exemplary embodiments of the present disclosure, the elongated member includes an axial span that extends in an axial direction across a spinal level to promote efficacious spinal stabilization thereacross, and that manifests a radially segmented geometry relative to the axial direction. In some such embodiments, the elongated member is configured and dimensioned for implantation adjacent the spine such that at least two axial spans of the elongated member extend across respective spinal levels of the spine to promote efficacious spinal stabilization across both such spinal levels. In some such embodiments, the axial span has a rod-like profile and is adapted to be coupled to the spine of the patient via attachment to conventional spine attachment devices configured for coupling conventional support rods, such as solid, relatively inflexible spinal support rods used in conjunction with spinal fusion assemblies, to the spine. In alternative embodiments of the present disclosure, the axial span is adapted to be mounted with respect to a patient&#39;s spine using alternative mounting structures/members, e.g., mounting hooks, plates, cemented stems, or the like. Such rod-like profile can include a diameter in a range of from about 5.5 mm to 6.35 mm (although alternative dimensions are contemplated), and the axial span can be adapted to permit pedicle screws to be attached to the elongated member at multiple points along the length of the axial span so as to accommodate a range of different patient anatomies and intervertebral heights. Further with respect to some such exemplary embodiments, the axial span is axially substantially rigid as against axial forces arrayed in compression and/or tension.  
         [0010]     Still further with respect to some such exemplary embodiments, the radially segmented geometry manifested by the axial span permits the axial span to bend, flex or deflect along any and substantially all transverse directions while providing efficacious spinal stabilization across the spinal level during at least one of spinal flexion, spinal extension, spinal lateral bending, and spinal axial rotation. According to exemplary embodiments, the axial span provides efficacious spinal stabilization across the spinal level during: a) spinal flexion in which the spinal level defines an anterior bend of at least approximately five to seven degrees; b) spinal extension in which the spinal level defines a posterior bend of at least approximately three to seven degrees; and/or c) spinal bending in which said spinal level defines a lateral bend of at least approximately four to seven degrees. Yet further with respect to some such embodiments, the radially segmented geometry includes a rod of radially unitary construction and extending in the axial direction, and at least one sleeve extending in the axial direction and surrounding the rod. According to further exemplary embodiments, the rod can be fabricated, in whole or in part, from a superelastic material.  
         [0011]     According to further embodiments of the present disclosure, a surgically implantable spinal support rod is provided that includes an axial span that extends in an axial direction so as to span at least one spinal level, wherein the axial span manifests (at least in part) a radially segmented geometry relative to the axial direction. In some such embodiments, the radially segmented geometry manifested by the axial span includes at least one pair of axially-extending adjacent surfaces adapted to move relative to each other along the axial direction during a transverse deflection of the axial span. Such at least one pair of axially-extending adjacent surfaces can include first and second substantially cylindrically shaped surfaces, wherein each such surface faces radially outerward toward the other such surface, or wherein such surfaces are substantially aligned with respect to each other. In others of such embodiments, the axial span has a rod-like profile, and is adapted to be coupled to the spine of the patient via attachment to conventional spine attachment devices configured for coupling conventional support rods to the spine for purposes of spinal fusion. Such rod-like profile of the axial span can include a diameter in a range of from about 5.5 mm to 6.35 mm, although alternative dimensions and/or dimensional ranges may be employed.  
         [0012]     In accordance with still further embodiments of the present disclosure, a kit for assembling a dynamic spinal support system is provided. Such kit includes a spinal support rod having an axial span extending in an axial direction so as to span at least one spinal level, and manifesting a radially segmented geometry relative to said axial direction. Such kit also includes a plurality of spine attachment devices attachable to the axial span so as to couple the spinal support rod to the spine of the patient across the spinal level. In some such embodiments, at least one of such spine attachment devices includes a pedicle screw, hook, plate and/or cemented stem.  
         [0013]     In accordance with another embodiment of the present disclosure, the elongated member includes an axial span that extends in an axial direction across at least one spinal level to promote efficacious spinal stabilization thereacross, and that includes a sleeve and a series of structural members aligned along the axial direction, enclosed within the sleeve, and adapted to support the sleeve against lateral buckling, e.g., when the sleeve experiences a lateral bend and is supporting the spine across the at least one spinal level. In some such embodiments, the sleeve is adapted to generate an internal spring force in opposition to the lateral bend as the sleeve deflects so as to accommodate and moderate the lateral bend. In exemplary embodiments, the sleeve can be fabricated, at least in part, from a superelastic material, such as an alloy of nickel titanium. The structural members can be substantially spherical in shape and, in such embodiments, the sleeve can be substantially cylindrical in shape.  
         [0014]     In accordance with yet another embodiment of the present disclosure, the elongated member includes an axial span that extends in an axial direction across at least one spinal level to promote efficacious spinal stabilization thereacross, and further includes an axial sleeve and a coil spring disposed within the axial sleeve. In some such embodiments, the sleeve is fabricated from a superelastic material and/or an alloy of titanium. In some other such embodiments, the sleeve is fabricated from a polymeric material. In some other such embodiments, the coil spring is sized and oriented so as to support a peripheral shape of the axial sleeve against at least one of crushing and buckling during spinal stabilization.  
         [0015]     In accordance with another embodiment of the present disclosure, the elongated member includes an axial span that extends in an axial direction across at least one spinal level to promote efficacious spinal stabilization thereacross, and further includes an axially-extending coil spring and a restraining element disposed within the coil spring and extending at least partially through the coil spring in the axial direction so as to limit an axial extension of the elongated member. In some such embodiments, the restraining element includes a cable adapted to render the elongated member substantially rigid as against axial forces arrayed in compression. The cable can take the form of a wire rope cable.  
         [0016]     According to further embodiments of the present disclosure, a surgically implantable spinal support rod is provided that includes an axial span that extends in an axial direction so as to span at least one spinal level, wherein the disclosed spinal support bar is of unitary construction, both along the axial direction, and radially relative to the axial direction, and is further adapted to deflect laterally so as to permit at least three to seven degrees of bending in the spine across the at least one spinal level in at least one of spinal flexion, spinal extension, and spinal lateral bending. In some such embodiments, the spinal support bar manifests a substantially constant cross-sectional geometry across the at least one spinal level, e.g., a circular cross-sectional geometry. In some other such embodiments, the spinal support bar includes a central span extending in the axial direction, and channels formed in the central span so as to increase transverse flexibility of the central span. Such channels can extend in the axial direction, and/or such channels can extend transversely relative to the axial direction. In some other such embodiments, the spinal support bar includes a central span, a first end span, and a second end span disposed opposite the central span from the first end span. The central span may be associated with a reduced cross-sectional area relative to respective cross-sections of the first and second end spans, e.g., the central span can be associated with a circular cross section of a reduced diameter relative to respective circular cross-sections of the first and second end spans.  
         [0017]     The elongated members/spinal support rods of the present disclosure, and/or the spinal stabilization devices/systems of the present disclosure incorporating such elongated members/spinal support rods, advantageously include one or more of the following structural and/or functional attributes: 
        Spine surgery patients whose conditions indicate that they would benefit from retaining at least some spinal motion in flexion, extension, and/or axial rotation may be fitted with a dynamic spinal stabilization device/system as disclosed herein rather than undergo procedures involving substantial immobilization as between adjacent vertebrae;     The elongated members/spinal support rods in accordance with the present disclosure are compatible (e.g., by virtue of standard diameter sizing, substantial dimensional/diametrical stability, and/or rigidity in axial tension and axial compression, etc.) with most rod attachment hardware presently being implanted in conjunction with lumbar fusion surgery, enhancing the likelihood of quick adoption by the medical community and/or governmental regulatory approval;     The elongated members/spinal support rods disclosed herein are adaptable to pedicle screw, hook, plate and/or stem attachment, can be used across one or more spinal levels; permit at least approximately seven degrees of spinal extension, spinal flexion, and/or spinal lateral bending as between adjacent spinal vertebrae, and allow for adjustable attachment points along their axial lengths to accommodate differing patient anatomies.        
 
         [0021]     Advantageous spine stabilization devices, systems, kits for assembling such devices or systems, and methods may incorporate one or more of the foregoing structural or functional attributes. Thus, it is contemplated that a system, device, kit and/or method may utilize only one of the advantageous structures/functions set forth above, or all of the foregoing structures/functions, without departing from the spirit or scope of the present disclosure. Stated differently, each of the structures and functions described herein is believed to offer benefits, e.g., clinical advantages to clinicians or patients, whether used alone or in combination with others of the disclosed structures/functions.  
         [0022]     Additional advantageous features and functions associated with the devices, systems, kits and methods of the present disclosure will be apparent to persons skilled in the art from the detailed description which follows, particularly when read in conjunction with the figures appended hereto. Such additional features and functions, including the structural and mechanistic characteristics associated therewith, are expressly encompassed within the scope of the present invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     To assist those of ordinary skill in the art in making and using the disclosed devices and systems, reference is made to the appended figures, in which:  
         [0024]      FIGS. 1, 2  and  3  are respective side, top, and end views of a dynamic spinal stabilization device/system implanted into the spine of a patient, in accordance with a first embodiment of the present disclosure;  
         [0025]      FIG. 4  is a downward perspective view of an elongated member of the spinal stabilization device/system of  FIGS. 1-3 , at least a portion of the internal structure of which is illustrated via a partial cutaway;  
         [0026]      FIG. 5  is a side illustration of the elongated member of  FIG. 4 ;  
         [0027]      FIG. 6  is a cross-sectional view of the elongated member of  FIGS. 4 and 5 , taken along section line  6 - 6  in  FIG. 5 ;  
         [0028]      FIG. 7  is a side illustration of the spinal stabilization device/system of  FIGS. 1-3 , wherein the patient is in spinal flexion;  
         [0029]      FIG. 8  is a side illustration of the spinal stabilization device/system of  FIGS. 1-3 , wherein the patient is in spinal extension;  
         [0030]      FIGS. 9 and 10  are top views of the spinal stabilization device/system of  FIGS. 1-3 , wherein the spine of the patient is bending along the left and right lateral directions, respectively; and  
         [0031]      FIGS. 11 and 12  are end views of the spinal stabilization device/system of  FIGS. 1-3 , wherein the spine of the patient is subject to axial rotation to the right and to the left, respectively;  
         [0032]      FIGS. 13-20 ,  22  and  25  are downward perspective view of elongated members which may be substituted for the elongated member of  FIGS. 4-6  in accordance with respective modifications and/or alternative embodiments of the spinal stabilization/system of  FIGS. 1-3 ;  
         [0033]      FIG. 21  is a cross-sectional view of the elongated member of  FIG. 20 ;  
         [0034]      FIGS. 23-24  are cross-sectional views of the elongated member of  FIG. 22 ; and  
         [0035]      FIGS. 26-27  are cross-sectional views of the elongated member of  FIG. 25 . 
     
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0036]     The present disclosure provides advantageous devices, systems and methods for providing dynamic spinal stabilization. More particularly, the present disclosure provides elongated members in the form of rods that are suitable for surgical implantation across multiple spinal levels for purposes of support and stabilization in flexion, extension and/or axial rotation, and that are also laterally flexible so as to provide a range of motion in spinal flexion, extension and/or axial rotation.  
         [0037]     The exemplary embodiments disclosed herein are illustrative of the advantageous spinal stabilization devices/systems and surgical implants of the present disclosure, and of methods/techniques for implementation thereof It should be understood, however, that the disclosed embodiments are merely exemplary of the present invention, which may be embodied in various forms. Therefore, the details disclosed herein with reference to exemplary dynamic stabilization systems and associated methods/techniques of assembly and use are not to be interpreted as limiting, but merely as the basis for teaching one skilled in the art how to make and use the advantageous spinal stabilization systems and alternative surgical implants of the present disclosure.  
         [0038]     With reference to  FIGS. 1-3 , a dynamic spinal stabilization system  10  is shown implanted into and/or relative to the spine S of a patient, such spine S being rendered schematically in  FIGS. 1-3  (as well as in  FIGS. 7-12 , the details of which are described more fully hereinbelow) in the form of three adjacent sequential vertebrae V 1 , V 2  and V 3  separated by corresponding intervertebral gaps G 1  and G 2 . The dynamic stabilization system  10  is attached to the spine S along one lateral side thereof as defined by a bilateral axis of symmetry As thereof (another dynamic spine stabilization system  10  (not shown) can be attached to the spine S along the other lateral side thereof as desired and/or as necessary). The spinal stabilization system  10  includes three spine attachment elements  12 ,  14 ,  16 , and an elongated member  18  spanning all of the vertebrae V 1 , V 2 , V 3  (e.g., at least insofar as the gaps G 1 , G 2  therebetween).  
         [0039]     Each of the spine attachment elements  12 ,  14 ,  16  of the spinal stabilization system  10  includes an attachment extension  20  (depicted at least partially schematically) and an attachment member  22  (also depicted at least partially schematically). The spine attachment elements  12 ,  14 ,  16  are securely affixed to the respective vertebrae V 1 , V 2 , V 3  via respective ends of the attachment extensions  20  being embedded within corresponding voids in the tissue of the respective vertebrae V 1 , V 2 , V 2 , and being securely retained therein (i.e., so as to prevent the attachment extensions  20  from being pulled out of their respective voids, or rotated with respect thereto, whether axially or otherwise). The attachment extensions  20  are embedded into and/or retained within their respective vertebral voids via suitable conventional means, such as a helical thread and/or a helically-shaped inclined plane formed on the respective attachment extension  20 , a biocompatible adhesive, or other means of embedding and/or retention. The attachment extensions  20  form respective parts of and/or are mounted with respect to respective pedicle screws of conventional structure and function in accordance with at least some embodiments of the present disclosure. The attachment extensions  20  form parts of other types of structures than that of conventional pedicle screws in accordance with some other embodiments of the present disclosure, e.g., hooks, plates, stems or the like.  
         [0040]     The attachment extensions  20  and attachment members  22  of the spine attachment elements  12 ,  14 ,  16  are attached or coupled with respect to each other at respective ends of the attachment extensions  20  opposite the ends thereof that are embedded within the tissue of the respective vertebrae V 1 , V 2 , V 3 . Movable joints are advantageously formed at the points where the attachment extensions  20  and the attachment members  22  are attached/coupled. In at least some embodiments of the present disclosure, the ends of the attachment extensions  20  that are attached/coupled with respect to the respective attachment members  22  include respective pedicle screw heads of conventional structure and function. In some other embodiments of the present disclosure, such ends include types of structure other than that of conventional pedicle screw heads. The movable joints formed between the attachment extensions  20  and the attachment members  22  may advantageously permit relatively unconstrained relative rotation (e.g., global rotation) therebetween, as well as at least some rotation of each attachment member  22  about an axis defined by the corresponding attachment extension  20 . The structure and function of the movable joints between the attachment extensions  20  and the attachment members  22  of the respective spine attachment elements  12 ,  14 ,  16  will be described in greater detail hereinafter.  
         [0041]     The attachment members  22  of the spine attachment elements  12 ,  14 ,  16  are generally configured and dimensioned so as to be operatively coupled to known spinal support rods (not shown) such as spinal support rods of conventional structure and having a standard diameter (e.g., from about 5.5 mm to about 6.35 mm, although alternative dimensions may be employed) and that are commonly used in connection with lumbar fusion surgery and/or other spinal stabilization procedures. For example, in accordance with some embodiments of the present disclosure, each of the attachment members  22  is configured to couple to a conventional spinal support rod (not shown) so as to prevent relative movement between the attachment members  22  and the rod in a direction transverse (e.g., perpendicular) to the rod&#39;s axial direction of extension, and at least one of the attachment members  22  is further adapted to prevent relative movement between such attachment member  22  and the rod along the rod&#39;s axial direction of extension. The particular structures and characteristic functions of the attachment members  22  of the spine attachment elements  12 ,  14 ,  16  are discussed in greater detail hereinafter.  
         [0042]     Referring now to  FIGS. 4-6 , the exemplary elongated member  18  of the spinal stabilization system  10  ( FIG. 1 ) includes an axis  24  defined by an axial/longitudinal direction along which the elongated member  18  characteristically extends. As shown in  FIG. 6 , exemplary elongated member  18  has an outer perimeter  26  in end view that has a substantially circular shape. The circular outer perimeter  26  defines a basic diameter  28  of the elongated member  18  of an extent that is typically consistent with that of conventional spinal stabilization rods (e.g., an extent in a range of from about 5.5 mm to about 6.35 mm or alternative dimension) such that the elongated member  18  is compatible with hardware designed to couple to conventional spinal stabilization rods and associated anatomical features and criteria. Accordingly, and referring again to  FIGS. 1-3 , the elongated member  18  is generally compatible with the spine attachment elements  12 ,  14 ,  16 . More particularly, the elongated member  18  is coupled to the attachment members  22  of the spine attachment elements  12 ,  14 ,  16  such that transverse movement of the elongated member  18  relative to the respective attachment members  22  is substantially limited and/or prevented. This is consistent with the support and stabilization functions (described in greater detail hereinafter) of the elongated member  18  with respect to the spine S.  
         [0043]     With respect to at least one of the attachment members  22 , the elongated member  18  is coupled thereto such that motion/translation of the elongated member  18  in the axial direction (i.e., in the direction of the axis  24 ) relative to such attachment member(s) is substantially limited and/or prevented. This ensures that the elongated member  18  is prevented from freely and/or uncontrollably moving/translating in the axial direction with respect to the spine attachment elements  12 ,  14 ,  16  in the context of the overall spinal stabilization system  10 . Moreover, in accordance with the embodiment of the present disclosure illustrated in  FIGS. 1-6 , the global joints formed between the attachment members  22  and the attachment extensions  20  of the respective spine attachment elements  12 ,  14 ,  16  generally allow the attachment members  22  to rotate to some degree along with the elongated member  18  relative to the spine S. The significance of such aspects of the connection between the elongated member  18  and the spine attachment elements  12 ,  14 ,  16  is described more fully hereinafter.  
         [0044]     The elongated member  18  is also similar to conventional spinal stabilization rods in that it is substantially dimensionally stable in the radial direction (e.g., transversely/perpendicularly relative to the axial direction of extension of the elongated member  18  as represented by the axis  24 ). Accordingly, the elongated member  18  is capable of withstanding radially-directed compression forces imposed by any and/or all of the attachment members  22  either during the process of implanting the elongated member  18  along the spine S (e.g., in response to clamping forces imposed by any attachment member  22  on the elongated member  18 ) or during in situ use of the spinal stabilization system  10  (the details of such use being described more fully hereinafter). In accordance with at least some embodiments of the present disclosure, the material and structural aspects of the elongated member  18  described herein render the elongated member  18  substantially rigid in axial tension, as well as substantially incompressible when subjected to axially-directed compression forces.  
         [0045]     Still referring to  FIGS. 4-6 , exemplary elongated member  18  includes four axially-extending structures, to wit: a rod  30 , a first inner sleeve  32  surrounding the rod  30 , a second inner sleeve  34  surrounding the first inner sleeve  32 , and an outer sleeve  36  surrounding and/or enveloping the outer sleeve  34 . The rod  30  has a substantially circular cross-section defined by a basic diameter  38  that has an extent of approximately 2.0 to 3.0 mm, and that is substantially constant along the axial length of the elongated member  18  (e.g., along the axis  24 ). Accordingly, and at least when the rod  30  is in a straight and/or linear configuration, a peripheral outer surface  40  of the rod  30  is substantially cylindrical. The first inner sleeve  32  is also substantially circular in cross-section, being characterized by a substantially axially constant inner diameter  42  accommodative of the basic diameter  38  of the rod  30 , a radial thickness  44 , and a substantially axially constant outer diameter  46 . At least when the inner sleeve  32  is in a straight and/or linear configuration, both an inner surface  48  and a peripheral outer surface  50  of the first inner sleeve  32  are substantially cylindrical. The second inner sleeve  34  is also substantially circular in cross-section, being characterized by a substantially axially constant inner diameter  52  accommodative of the outer diameter  46  of the first inner sleeve  32 , a radial thickness  54 , and a substantially axially constant outer diameter  56 . At least when the inner sleeve  32  is in a straight and/or linear configuration, both an inner surface  58  and a peripheral outer surface  60  of the second inner sleeve  34  are substantially cylindrical.  
         [0046]     The outer sleeve  36  includes an axial portion  62  and two end caps  64  disposed on opposite ends  66 ,  68  of the axial portion  62  from each other. The axial portion  62  is substantially circular in cross-section, being characterized by a substantially axially constant inner diameter  70  accommodative of the outer diameter  56  of the second inner sleeve  34 , a radial thickness  72 , and the outer diameter  28 , which is further substantially axially constant. At least when the axial portion  62  is in a straight and/or linear configuration, both an inner surface  74  and a peripheral outer surface  76  of the axial portion  62  are substantially cylindrical. The end caps  64  are substantially hemispherical in shape, being characterized by a substantially constant inner radius  78 , a radial thickness  80 , and a substantially constant outer radius  82  that is of an extent complementary to that of the outer diameter  74  of the axial portion  62 .  
         [0047]     In at least some embodiments of the present disclosure, including the embodiment schematically depicted herein, the rod  30 , the first and second inner sleeves  32 ,  34 , and the outer sleeve  36  are each fabricated from a superelastic material, e.g., such as a nickel titanium alloy. The significance of such material compositions of these components is described more fully hereinbelow.  
         [0048]     The rod  30  extends substantially the entire length of the elongated member  18  along the axis  24 , beyond the ends  66 ,  68  of the axial portion  62  of the outer sleeve  36 , and into the interior spaces defined by the end caps  64  thereof, e.g., substantially as far as the inner wall surfaces thereof. The rod  30  is also of unitary construction throughout its length and cross-section. Combined with the inherently compact circular shape of the rod  30  in cross section, the superelastic material composition and unitary construction of the rod  30  render it substantially radially incompressible. The first and second inner sleeves  32 ,  34  extend substantially the full axial distance between the inner wall surfaces of the end caps  64 , being only slightly shorter than the rod  30  so as to accommodate the respective radiused geometries of the end caps  64 . The cumulative transverse extent of the diameter  38  of the rod  30 , the radial thickness  44  of the first inner sleeve  32 , and the radial thickness  54  of the second inner sleeve  34 , represents a substantial proportion of the transverse extent of the inner diameter  60  of the axial portion  62  of the outer sleeve  36 . More particularly, the radial/peripheral spaces between the rod  30  and the first inner sleeve  32 , and/or between the first and second inner sleeves  32 ,  34 , are relatively small. At the same time, the above-described coordination among the various diameters of the axially-extending structures of the elongated member  18  is also designed so as to reduce and/or eliminate any undue interference (e.g., via friction or otherwise) with the flexure-related functions of the elongated member  18 , which functions are described more fully hereinbelow.  
         [0049]     At least in part because of the closely matched diametrical dimensions of the rod  30  and the first inner sleeve  32 , the rod  30  substantially fully supports the first inner sleeve  32  against crushing, buckling, and/or plastic deformation during bending, flexure, and/or deflection of the overall elongated member  18  (e.g., during in situ use and/or during representative mechanical testing). For example, in accordance with at least some embodiments of the present disclosure, the attachment members  22  associated with the spine attachment elements  12 ,  14 ,  16  apply radial compression, radial impingement, and/or clamping forces to the elongated member  18  at their respective points of contact therewith, and the rod  30  provides structural and/or shape support to the first inner sleeve  32  at, along, and/or adjacent to such points of contact. The first inner sleeve  32 , being substantially fully supported against undue radial deflection or deformation (see above), provides similar structural and/or shape support to the second inner sleeve  34 . So, in turn, does the second inner sleeve  34  provide structural and/or shape support to the axial portion  62  of the outer sleeve  36 . Accordingly, the overall elongated member  18  is substantially radially incompressible along its entire axial length (e.g., along the axis  24 ), e.g., as against such bending stresses, radial impingement, and/or clamping or other transverse/radial forces as are applied to the elongated member  18 , whether by the attachment members  22 , or otherwise.  
         [0050]     In operation, e.g., when incorporated in the spinal stabilization system  10  adjacent the spine S of a patient as described hereinabove, the elongated member  18  is capable of supporting the spine S in any one or more, or all, of spinal flexion, spinal extension, and axial rotation. As may be seen by comparing  FIGS. 1 and 7 , the elongated member  18  of the spinal stabilization system  10  is sufficiently flexible to bend from a substantially linear configuration ( FIG. 1 ) to a configuration in which the elongated member  18  includes an anterior bend ( FIG. 7 ), while being also sufficiently stiff to provide ample support to the vertebrae V 1 , V 2 , V 3  of the spine S against undue spinal flexion, as determined by the anatomy and/or particular medical condition of the patient. In accordance with some embodiments of the present disclosure, the elongated member  18  is dimensioned and configured so as to permit such spinal flexion between adjacent vertebrae (e.g., between vertebrae V 1  and V 2 , or between vertebrae V 2  and V 3 ) to an extent of at least approximately three to seven degrees.  
         [0051]     As may be seen by comparing  FIGS. 1 and 8 , the elongated member  18  of the spinal stabilization system  10  is sufficiently flexible to bend from a substantially linear configuration ( FIG. 1 ) to a configuration in which the elongated member  18  includes a posterior bend ( FIG. 8 ), while being also sufficiently stiff to provide ample support to the vertebrae V 1 , V 2 , V 3  of the spine S against undue spinal extension, as determined by the anatomy and/or particular medical condition of the patient. In accordance with some embodiments of the present disclosure, the elongated member  18  is dimensioned and configured so as to permit such spinal extension between adjacent vertebrae (e.g., between vertebrae V 1  and V 2 , or between vertebrae V 2  and V 3 ) to an extent of at least approximately three to seven degrees.  
         [0052]     As may be seen by comparing  FIG. 2  to  FIGS. 9 and 10 , respectively, the elongated member  18  of the spinal stabilization system  10  is sufficiently flexible to bend from a substantially linear configuration ( FIG. 2 ) to a configuration in which the elongated member  18  includes a leftward lateral bend ( FIG. 9 ) or a rightward lateral bend ( FIG. 10 ), as reflected in the respective curves in the axis of symmetry A S  of the spine S, while being also sufficiently stiff to provide ample support to the vertebrae V 1 , V 2 , V 3  of the spine S against undue spinal lateral bending, as determined by the anatomy and/or particular medical condition of the patient. In accordance with some embodiments of the present disclosure, the elongated member  18  is dimensioned and configured so as to permit such spinal lateral bending between adjacent vertebrae (e.g., between vertebrae V 1  and V 2 , or between vertebrae V 2  and V 3 ) to an extent of at least approximately three to seven degrees.  
         [0053]     As may be seen by comparing  FIG. 3  to  FIGS. 11 and 12 , respectively, the elongated member  18  of the spinal stabilization system  10  is sufficiently flexible to bend from a substantially linear configuration ( FIG. 3 ) to a configuration in which the elongated member  18  includes a leftward helical bend ( FIG. 11 ) or a rightward helical bend ( FIG. 12 ) about the axis of symmetry A S  of the spine S, while being also sufficiently stiff to provide ample support to the vertebrae V 1 , V 2 , V 3  of the spine S against undue spinal twist/axial rotation, as determined by the anatomy and/or particular medical condition of the patient. In accordance with some embodiments of the present disclosure, the elongated member  18  is dimensioned and configured so as to permit such axial rotation between adjacent vertebrae (e.g., between vertebrae V 1  and V 2 , or between vertebrae V 2  and V 3 ) to an extent of at least approximately four (4) degrees. As is particularly evident in the illustrations provided in  FIGS. 11 and 12 , the joints between the attachment members  22  and the attachment extensions  20  of the spine attachment elements  12 ,  14 ,  16  permit the attachment members  22  ranges of motion relative to the respective attachment extensions  20 , and relative to each other, sufficient to track even a complex helical bend, free from undue friction and/or binding.  
         [0054]     Further with reference to each of  FIGS. 7-12 , the relationship between the attachment members  22  and the elongated member  18  during the formation and/or relaxation of bends in the elongated member  18  is such as to permit and/or restrict relative axial/longitudinal relative movement between the attachment members  22  and the elongated member  18  along the axial direction of extension of the elongated member  18  (e.g., along the axis  24 ), as needed or as desired (e.g., depending on the desired function or functions of the spinal stabilization system  10 , the needs of the particular patient, and/or the length of the elongated member  18 , among other considerations).  
         [0055]     Referring still further to  FIGS. 4-6 , the elongated member  18  is configured to permit relative movement as between respective adjacent surfaces of its axially-extending structures. More particularly, at least axially-directed relative movement is respectively permitted as between: 1) the peripheral outer surface  40  of the rod  30  and the inner surface  48  of the first inner sleeve  32 ; 2) the peripheral outer surface  50  of the first inner sleeve  32  and the inner surface  58  of the second inner sleeve  34 ; and 3) the peripheral outer surface  60  of the first inner sleeve  32  and the inner surface  74  of the axial portion  62  of the outer sleeve  36 . As relates to the operation of the spinal stabilization system  10  shown and described above with reference to  FIGS. 7-12 , transverse bending, flexure, and/or deflection of the elongated member  18  (e.g., as is produced during spinal flexion, extension and/or axial rotation) will generally result in at least some axially-directed relative movement as between the above-mentioned pairs of radially-adjacent, axially-extending surfaces. Such movement between internal surfaces tends to dissipate, reduce and/or prevent internal stresses from accumulating at corresponding radial intervals within the elongated member  18 . As those of skill in the art will recognize in light of the present disclosure, the dissipation and/or exclusion of such internal stresses via axially-directed relative motion between such pairs of radially-adjacent, axially extending surfaces renders the elongated member  18  more flexible, e.g., to at least a certain extent, than that which would otherwise be the case. For example, as compared to an elongated member (not shown) having the same outer diameter as the elongated member  18 , and being fabricated from the same superelastic material thereof, but having a unitary (e.g., rather than multicomponent) construction along the radial direction, the elongated member  18  offers less resistance, e.g., to at least a certain extent, to transverse bending, flexure and/or axial rotation.  
         [0056]     It should be appreciated that numerous advantages are provided by the elongated member  18  and/or by devices such as the spinal stabilization device  10  that incorporate the elongated member  18  in accordance with the foregoing description to provide dynamic stabilization to the spine of a patient. Spine surgery patients whose conditions indicate that they would benefit from retaining at least some spinal motion in flexion, extension and/or axial rotation may benefit through implantation of the dynamic spinal stabilization device  10  rather than undergoing procedures involving substantial immobilization as between adjacent vertebrae. The elongated member  18  (e.g., by virtue of its standard diameter sizing, substantial dimensional stability, and rigidity in tension and/or compression) is compatible with most rod attachment hardware presently being implanted in conjunction with lumbar fusion surgery and other spinal procedures, providing at least some basic similarity between the spinal stabilization device  10  and existing spinal stabilization devices. Such similarity is advantageous insofar as it tends to simplify the process of seeking widespread industry acceptance and/or regulatory approval. Exemplary embodiments of elongated member  18  are adaptable to pedicle screw attachment or other mounting systems (e.g., hooks, plates, stems and the like), allow for use across two or more spinal levels, permit at least approximately three to seven degrees of lateral flexibility in spinal extension, spinal flexion, and/or spinal lateral bending as between adjacent spinal vertebrae, and allow for adjustable pedicle screw attachment points along the elongated member  18  to accommodate differing patient anatomies.  
         [0057]     The axially symmetrical structure of the elongated member  18  affords an even, predictable level of bending flexibility (or, conversely, bending stiffness) in all lateral directions to facilitate smooth bending, and defines a substantial outer diameter compatible with the same conventional spine attachment hardware normally used in conjunction with solid, substantially laterally inflexible support rods. At the same time, the elongated member  18  is substantially radially incompressible, such that it maintains an adequate degree of rigidity against axial forces in compression (as well as in tension) for purposes of spinal support/stabilization. The peripheral outer surface  76  of the elongated member  18  has a regular cylindrical shape, facilitating secure coupling with hardware designed for coupling to cylindrically-shaped support rods of full diameter and substantially unitary structure. The superelastic material from which the different axially-extending components of the elongated member  18  may be fabricated (at least in part) resists buckling, distension, elastic deformation, and/or galling, and has excellent memory such that the bends produced in the elongated member  18  will be substantially fully removed in the event outside forces acting upon the elongated member are eliminated. Full encapsulation of all other axially-extending components of the elongated member  18  within the axial portion  62  and the end caps  64  of the outer sleeve  36  reduces and/or eliminates the risk that particulate matter, e.g., from metal-metal interaction, will be released in situ. The outer sleeve  36 , being fabricated from a superelastic material, includes an inherent degree of stiffness against bending, at least to the extent that its cylindrical shape is supported and/or preserved during bending, flexure, and/or deflection of the elongated member  18 . Accordingly, the radial thickness  72  of the axial portion  62  of the outer sleeve  36  can be pre-selected based on that proportion of the bending stiffness of the elongated member  18  which is intended to be supplied by the outer sleeve  36  itself.  
         [0058]     It should also be noted that the elongated member  18 , and/or the dynamic spinal stabilization device  10  of which the elongated member  18  forms a part, are subject to numerous modifications and/or variations. For example, the elongated member  18  can be attached in many different ways to the attachment members  22  of the respective spine attachment elements  12 ,  14 ,  16 , including embodiments wherein at least one of the attachment members  22  includes an axial hole through which the elongated member  18  either extends freely in the axial direction, or is clamped in place so as to prevent relative axial motion/translation, and embodiments wherein at least one of the attachment members  22  forms a hook-like structure that includes no clamping means and therefore does not limit axial relative motion/translation of the elongated member  18 . Many other variations in the spine attachment elements  12 ,  14 ,  16  are also possible, including the number of same provided in the context of the spinal stabilization device  10  (e.g., only two, four or more, etc.), as well as the method by which any or all are attached to their respective spinal vertebrae. The elongated member  18  can accordingly be shortened or lengthened, so as to be suitable for spanning a single pair of adjacent vertebrae, or more than three adjacent vertebrae. The number of inner sleeves can be only one, or more than two, and the diameters thereof, and/or of the rod  30 , can be changed as necessary, and/or as desired, e.g., so as to produce a particular (e.g., predefined) amount of bending stiffness in the elongated member  18 .  
         [0059]     The spinal stabilization system  10  of  FIGS. 1-3  and  7 - 12  is subject to further modification, e.g., via replacement therein of the elongated member  18  of  FIGS. 4-6  with elongated members exhibiting certain differences, such as differences in configurations, structures, materials, properties and/or features, relative to the elongated member  18 , as well as certain similarities with respect thereto. More particularly,  FIGS. 13-15  illustrate elongated members which are similar to the elongated member  18  at least insofar as they incorporate a similar outer sleeve and include more than one axially-extending component, but which also include differences at least as described below.  FIG. 16  illustrates an elongated member that is similar to the elongated member of  FIG. 15  at least insofar as it incorporates a geometrically similar outer sleeve, but which also includes differences in its outer sleeve at least as described below.  FIG. 17  illustrates an elongated member that is similar to the elongated member  18  at least insofar as it includes more than one axially-extending component, but which also includes differences at least as described below.  FIGS. 18-21  illustrate elongated members that are similar to the elongated member at least insofar as they include axially-extending bars fabricated (at least in part) from a superelastic material, but which also include differences at least as described below.  FIGS. 22-24  and  25 - 27  illustrate respective elongated members that are similar to the elongated member  18  at least insofar as they are flexible in more than one lateral/transverse direction, but which also include differences at least as described below. Other elongated members can similarly be substituted for the elongated member  18  in accordance with the present disclosure.  
         [0060]     Elements illustrated in  FIGS. 13-27  which correspond substantially to the elements described above with reference to  FIGS. 1-12 , and/or to elements illustrated previously with respect to another of  FIGS. 13-27 , have been designated with corresponding reference numerals increased by one or more increments of one thousand. The elongated members shown in  FIGS. 13-27  operate and are constructed in manners consistent with the foregoing description of the elongated member  18 , unless it is stated otherwise. In addition, the elongated members shown in  FIGS. 13-17  feature the same advantages as are described hereinabove with respect to the elongated members, and are subject to the same type and degree of variations and/or modifications, unless it is stated otherwise, or unless a contrary conclusion is required based on the corresponding descriptions and/or illustrations.  
         [0061]     Turning now to  FIG. 13 , an elongated member  1084  is illustrated that includes an outer sleeve  1036 , and an arrangement of rods  1030  (e.g., seven are shown of a common diameter, but more or fewer than seven may be employed, as may rods  1030  of differing diameters) disposed within and encapsulated by the outer sleeve  1036 . According to exemplary embodiments, the outer sleeve  1036  and the rods  1030  are all fabricated (at least in part) from a superelastic material, e.g., nickel titanium. The elongated member  1084  extends axially along an axis  1024 , and one of the rods  1030  is disposed along the axis  1024  and extends beyond the ends  1066 ,  1068  of the axial portion  1062  of the outer sleeve  1036  and into the hollow areas defined by the end portions  1064  of the outer sleeve  1036  to an extent of the inner walls thereof The remaining rods  1030  are disposed radially around the axially-disposed rod  1030 , and are shorter than the axially-disposed rod  1030  so as to accommodate the respective radial geometries of the end portions  1064 . The cumulative transverse extent of the rods  1030  represent a substantial proportion of the inner diameter of the outer sleeve  1036 , such that the shape and/or outer dimensions of the outer sleeve  1036  are substantially supported against crushing, plastic deformation, and/or galling, etc. At the same time, there exists radial space and/or spaces of a sufficient extent/s between and/or among the rods  1030 , and between the rods  1030  and the outer sleeve  1036 , so as to permit relative movement of such components relative to each other along the axial direction for purposes of allowing the elongated member  1084  to bend, flex and/or deflect along any and/or substantially all lateral/transverse directions.  
         [0062]     Referring now to  FIG. 14 , an elongated member  1086  is illustrated that includes an outer sleeve  1088 , and a series of structural elements  1090  disposed within and encapsulated by the outer sleeve  1036 . The shell  1088  is substantially similar to the shell  1036  described and illustrated hereinabove with reference to  FIG. 13 , at least except insofar as the outer sleeve  1088  includes end caps  1092  that are flattened as compared to the end caps  1064  of the outer sleeve  1036 , and/or do not necessarily exhibit the hemispheric-type shape thereof. The structural elements  1090  are 1) fabricated from a structurally rigid material, e.g., a steel that is compatible with (e.g., will not tend to induce galvanic corrosion in, and/or otherwise react with) the superelastic material of the outer sleeve  1088 , 2) spherically shaped (e.g., with substantially identical diameters corresponding to and/or matched with an inner diameter of the outer sleeve  1088  so as to provide cylindrical shape support thereto), and 3) relatively tightly packed between the end caps  1092 . The elongated member  1086  is substantially axially incompressible due to the tight packing of the structural elements  1090  between the end caps, and is substantially axially inextensible due to the tensile strength/rigidity of the outer sleeve  1088 . The structural elements  1090  have substantially smooth outer surfaces, generally remain in point contact with each other, and are adapted (e.g., by virtue of their spherical shape) to rotate relative to/around each other without offering substantial resistance to such motion. Accordingly, such bending stiffness as is present in the elongated member  1086  is substantially solely based on the material and structural properties of the outer sleeve  1088 . In this regard, it should be noted that without the shape/radial dimensional support provided to the outer sleeve  1088  by the structural elements  1090 , the capacity of the outer sleeve  1088  to supply such bending stiffness would be reduced and/or substantially degraded.  
         [0063]     Referring to  FIG. 15 , an elongated member  1094  is illustrated that includes an outer sleeve  1036  and a coil spring  1096  disposed within and encapsulated by the outer sleeve  1036 . The elongated member  1084  extends axially along the axis  1024 , and the coil spring  1096  is disposed along the axis  1024  and extends beyond the ends  1066 ,  1068  of the axial portion  1062  of the outer sleeve  1036  and into the hollow areas defined by the end portions  1064  of the outer sleeve  1036  to an extent of the inner walls thereof. The coil spring  1096  is fabricated from a structurally rigid material, e.g., a steel that is compatible with (e.g., will not tend to induce galvanic corrosion with, and/or otherwise react with) the superelastic material of the outer sleeve  1036 , and is cylindrically shaped (e.g., with a diameter corresponding to and/or matched with an inner diameter of the outer sleeve  1036  so as to provide cylindrical shape support thereto). Bending stiffness of the elongated member  1094  is an additive function of the individual bending stiffnesses of outer sleeve  1036  (e.g., as supported by the coil spring  1096 ) and coil spring  1096 .  
         [0064]     By comparison,  FIG. 16  illustrates an elongated member  1098  that is substantially similar to the elongated member  1094  of  FIG. 15 , at least except insofar as the outer sleeve  1100  thereof is fabricated, not from a superelastic material, but rather from a biocompatible polymer of suitable toughness and durability to permit the elongated member  1098  to interconnect with conventional spine attachment devices and/or the attachment members  22  (see  FIGS. 1-3 ) thereof. Accordingly, such bending stiffness as is present in the elongated member  1098  is substantially solely based on the material and structural properties of the coil spring  1096  thereof. Referring again to  FIG. 13 , an alternative version (not specifically shown) of the elongated member  1084  illustrated in  FIG. 13  and described hereinabove can be provided by substituting the outer sleeve  1100  of the elongated member  1094  of  FIG. 16  for the outer sleeve  1036  of the elongated member  1084 . In accordance with such construction, such bending stiffness as would be present in the alternative version of the elongated member  1084  would be substantially solely based on the number, material, and structural properties of the various rods  1030 . A similar substitution may be made for the outer sleeve  36   
         [0065]     Turning now to  FIG. 17 , an elongated member  2102  is illustrated that includes a coil spring  2104  and a cable  2106 . The elongated member  2102  extends in an axial direction (e.g., along an axis  2024 ), insofar as the coil spring  2104  is axially aligned with (e.g., defines) the axis  2024 , and the cable  2106  extends axially through the coil spring  2104 , and is also axially aligned with the axis  2024 . An outer diameter  2108  of the cable  2106  is of an extent compatible with an inner diameter  2110  of the coil spring  2104  such that an outer peripheral surface  2112  of the cable  2106  is substantially limited with respect to transverse movement relative to the coil spring  2104 , and/or is positively prevented from so moving. The coil spring  2104  is ordinarily in a fully compressed state (e.g., when the elongated member  2102  is in a substantially straight and/or linear configuration), and when so compressed, renders the elongated member  2102  substantially incompressible as against axial forces arrayed in compression. The cable  2106  is of conventional construction (e.g., steel wire rope), and as such renders the elongated member  2102  substantially inextensible as against axial forces arrayed in tension. Both the coil spring  2104  and the cable  2106  can extend substantially the entire length of the elongated member  2102  and are either attached to each other (e.g., at one or more locations along the length of the elongated member  2102 ) or are attached in common to a third element of structure (not shown) such that relative motion between the coil spring  2104  and the cable  2106  along the axial direction is substantially reduced and/or prevented. The elongated member  2102  can include an outer sleeve (not specifically shown) such as one of the outer sleeves  1036 ,  1088  of  FIGS. 13 and 14  respectively, or such as the outer sleeve  1100  of  FIG. 16 , wherein either or both the coil spring  2104  and the cable  2106  are partially and/or completely enveloped or encapsulated by such outer sleeve (not shown). In some such embodiments of the elongated member  2102 , the cable  2106  is affixed to and/or protrudes slightly out of either or both ends of such outer sleeve (not shown), so as to permit purchase to be gained on the cable  2106  (e.g., so as to permit one or more of the attachment member  22  ( FIGS. 1-3 ) to be attached directly thereto, thereby exploiting the substantial inextensibility of the cable  2106 ).  
         [0066]      FIG. 18  illustrates an elongated member  3114  that includes an axially-extending rod  3030  and, at least in the embodiment illustrated in  FIG. 18 , includes no further structure. The rod  3030  is fabricated from a superelastic material, e.g., a nickel titanium alloy, and includes a substantially constant transverse diameter scaled in size such that the elongated member  3114  offers a predetermined stiffness against lateral/transverse bending.  
         [0067]     In  FIG. 19  is shown another elongated member  3116  consisting solely of a rod, e.g., a rod  3118 . The rod  3118  is substantially similar to the rod  3030 , at least except insofar as it includes an axial portion  3120  along which the transverse diameter of the rod  3118  is reduced, e.g., from a substantially constant, relatively larger diameter at two spaced-apart axial locations  3122 ,  3124  at opposite ends of the axial portion  3120 , to a substantially constant, relatively smaller diameter along a substantial proportion of the axial length of the axial portion  3120 . In operation, the rod  3118  can connect to spine attachment elements along the axial locations  3122 ,  3124 . The overall bending stiffness of the elongated member  3116  can be tuned by selecting for the transverse diameter/dimension of the axial portion  3120  an appropriate/corresponding extent.  
         [0068]     In  FIGS. 20-21  is shown another elongated member  3126  consisting solely of a rod, e.g., a rod  3128 . The rod  3128  is substantially similar to the rod  3030  of  FIG. 18 , at least except insofar as it includes longitudinal channels  3130  cut into and/or formed in the material of a peripheral outer surface  3132  of the rod  3128 . The channels  3130  form a fluted configuration in which the channels  3130  are arranged in a regular array about the axial direction of extension of the rod (e.g., along the axis  3024 ). While four such channels  3130  are shown, more or fewer than four can be cut and/or formed in the rod  3128 . In operation, the rod  3128  can connect to spine attachment elements along axial locations  3132 ,  3134  disposed at opposite ends of an axial portion  3136  of the rod  3128  in which the channels  3130  are formed. The overall bending stiffness of the elongated member  3126  can be tuned by altering the number, shape, and/or size of the channels  3130  as necessary/as desired.  
         [0069]     Referring now to  FIGS. 22-24 , an elongated member  3134  is shown that includes a rod  3136 , and, at least in the embodiment illustrated in  FIGS. 22-24 , includes no further structure. The rod  3136  extends in an axial direction (e.g., along an axis  3024 ), and is fabricated from a relatively structurally stiff, biocompatible metallic material, e.g., stainless steel, titanium or the like, and has a basic diameter  3138  that is substantially cylindrical. Cut into and/or formed in a peripheral outer surface  3140  of the rod  3136  are a first, second, third, and fourth axially-extending series  3142 ,  3144 ,  3146 ,  3148  of facets or channels  3150 . The channels  3150  extend transversely straight across the material of the rod  3136  to a common radial depth or extent which is less than half that of the diameter  3138 . The channels  3150  of the first and second series  3142 ,  3144  are formed on diametrically opposite sides of the axis  3024  from each other. The channels  3150  of the third and fourth series  3146 ,  3148  are also formed on diametrically opposite sides of the axis  3024  from each other, the transverse direction of extension of the channels  3150  of the third and fourth series  3146 ,  3148  being rotated ninety degrees relative to the transverse direction of extension of the channels  3150  of the first and second series  3142 ,  3144 . Between each pair of opposing channels  3150  remains an axially-disposed extent  3152  of the material of the rod  3136  which is as wide as the rod  3136  in a first direction  3154 , but which is relatively slender compared thereto in a second direction  3156  perpendicular to the first direction.  
         [0070]     In operation, the rod  3136  can connect to spine attachment elements along axial locations  3158 ,  3160  disposed at opposite ends of an axial portion  3162  of the rod  3136  in which the channels  3150  are formed. Bending, flexure, and/or deflection of the rod  3136  is permitted substantially only at/along the numerous axially-disposed extents  3152  without risk of plastic deformation of the material of the rod  3136 . By cutting/forming channels  3150  of an appropriate number and to an appropriate depth in the rod  3136 , the overall bending stiffness of the rod  3136  can be reduced to a predetermined level. Because of the regular radial arrangement of the first, second, third, and fourth series  3142 ,  3144 ,  3146 ,  3148  of channels  3150 , the flexibility produced thereby in the rod  3136  is substantially even as to any and/or all transverse directions of bending, flexure, and/or deflection.  
         [0071]     Referring now to  FIGS. 25-27 , an elongated member  3164  is shown that includes a rod  3166 , and, at least in the embodiment illustrated in  FIGS. 25-27 , includes no further structure. The rod  3166  is substantially similar to the rod  3136  described above with reference to FIGS.  22 - 24 , with differences as described hereinbelow. The rod  3166  extends in an axial direction (e.g., along an axis  3024 ), and has a basic diameter  3168  that is substantially cylindrical. Cut into and/or formed in a peripheral outer surface  3170  of the rod  3166  are a first, second, third, and fourth axially-extending series  3172 ,  3174 ,  3176 ,  3178  of facets or channels  3180 . The channels  3180  extend transversely straight across the material of the rod  3166  to a common radial depth or extent which is less than half that of the diameter  3168 , and which is less deep than the channels  3150  associated with the rod  3136  illustrated in  FIGS. 22-24 . The channels  3180  of the first and second series  3172 ,  3174  are formed on diametrically opposite sides of the axis  3024  from each other. The channels  3180  of the third and fourth series  3176 ,  3178  are also formed on diametrically opposite sides of the axis  3024  from each other, the transverse direction of extension of the channels  3180  of the third and fourth series  3176 ,  3178  being rotated ninety degrees relative to the transverse direction of extension of the channels  3180  of the first and second series  3172 ,  3174 . Between each pair of opposing channels  3180  remains an axially-disposed extent  3182  of the material of the rod  3166  which is as wide as the rod  3166  along a first direction  3184 , but which is to a certain extent less wide than the rod  3166  along a second direction  3186  perpendicular to the first direction  3184 .  
         [0072]     The channels  3180  are relatively wider than the channels  3150  ( FIG. 22 ) and, as described above, shallower. Comers  3188  of all the channels  3180  are broken/beveled to a substantially greater extent than the channels  3150  ( FIGS. 23-24 ) such that a portion of the material of the rod  3166  is removed at opposite diametrical ends of the axially disposed extents  3182 . The extents  3182  are accordingly necked-down so as to be approximately as thick as the extents  3152  (FIGS.  23 - 24 ) at such diametrical ends.  
         [0073]     In operation, the rod  3166  can connect to spine attachment elements along axial locations  3190 ,  3192  disposed at opposite ends of an axial portion  3194  of the rod  3166  in which the channels  3180  are formed. Bending, flexure, and/or deflection of the rod  3166  is permitted substantially only at/along the numerous axially-disposed extents  3182  without risk of plastic deformation of the material of the rod  3166 . The relatively wider dimensions of the extents  3182  produce relatively less flexibility in the rod  3166  than the relatively narrower dimensions  3152  produce in the rod  3136  ( FIGS. 22-24 ), as may be desired and/or necessary in certain applications. The broken corners  3188  of the channels  3180  smooth the contours of the rod  3166  so as to ensure that the rod  3166  manifests substantially the same flexibility in any and/or substantially all transverse directions, and not just in the two perpendicular transverse directions defined by the four series of channels  3180 . (As will be apparent to those of skill in the art in light of the present disclosure, similarly large sized broken corners are not required in the context of the relatively more flexible rod  3136  ( FIG. 22 ).)  
         [0074]     It will be understood that the embodiments of the present disclosure are merely exemplary and that a person skilled in the art may make many variations and modifications without departing from the spirit and scope of the invention. All such variations and modifications, including those discussed above, are therefore intended to be included within the scope of the present invention as described by the following claims appended hereto.