Abstract:
The disclosure is directed to vertebral stabilization systems including polymeric components having improved selective visualization means, methods of improving the selective visualization of polymeric components of a vertebral stabilization system, and methods of using vertebral stabilization systems including polymeric components having improved selective visualization means to monitor the vertebral stabilization systems during their useful life.

Description:
TECHNICAL FIELD  
       [0001]    The disclosure is directed to a vertebral stabilization system. More particularly, the disclosure is directed to a dynamic stabilization system including one or more components including a readily visualized polymeric composition and a method of use thereof. 
       BACKGROUND  
       [0002]    The spinal column of a patient includes a plurality of vertebrae linked to one another by facet joints and an intervertebral disc located between adjacent vertebrae. The facet joints and intervertebral disc allow one vertebra to move relative to an adjacent vertebra, providing the spinal column a range of motion. Diseased, degenerated, damaged, or otherwise impaired facet joints and/or intervertebral discs may cause the patient to experience pain or discomfort and/or loss of motion, thus prompting surgery to alleviate the pain and/or restore motion of the spinal column. 
         [0003]    Accordingly, there is an ongoing need to provide alternative devices, assemblies, systems and/or methods that can function to alleviate pain or discomfort, provide stability, such as dynamic stability, and/or restore a range of motion to a spinal segment of a spinal column. It may be desirable that such apparatus or components thereof, exhibit a degree of radiopacity in order that the apparatus may be visualized with a fluoroscopy device or other visualization device during a medical procedure and/or during a post-operative reevaluation. 
       SUMMARY  
       [0004]    The disclosure is directed to several alternative designs, materials and methods of manufacturing medical device structures and assemblies and uses thereof. 
         [0005]    Accordingly, one illustrative embodiment is a vertebral stabilization system comprising a flexible elongate member, such as a cord, and a second elongate member, such as a spacer, at least partially surrounding the flexible elongate member (e.g., cord). At least one of the first elongate member and the second elongate member (e.g., the cord and/or the spacer) may comprise the polymerized residue of at least one monomer selected from the group consisting of halogen substituted (meth)acrylates, covalent salts of Group II elements other than beryllium, and chelates of Group II elements other than beryllium. In some instances, at least one of the cord and the spacer may be provided with markings suitable for use in a radiographic comparison method as described herein, said markings comprising the polymerized residue of at least one monomer selected from the group consisting of halogen substituted (meth)acrylates, covalent salts of Group II elements other than beryllium, and chelates of Group II elements other than beryllium. When assembled between first and second vertebral anchors, the cord may be in tension and the spacer may be in compression. 
         [0006]    Another illustrative embodiment is a method of improving the selective radiopacity of polymeric components of vertebral stabilization systems by providing a vertebral stabilization system comprising a flexible elongate member, such as a cord, extendable from a first vertebral anchor to a second vertebral anchor, and a second elongate member, such as a spacer, sized to surround a portion of the first elongate member (e.g., cord) between the first vertebral anchor and the second vertebral anchor. The method further comprises providing at least one monomer selected from the group consisting of halogen substituted (meth)acrylates, covalent salts of Group II elements other than beryllium, and chelates of Group II elements other than beryllium and applying the monomer to at least a portion of one of the first elongate member and the second elongate member (e.g. at least one of the cord and the spacer) and polymerizing the monomer. The vertebral stabilization system may be assembled by placing the spacer around the cord between a first vertebral anchor and a second vertebral anchor, securing the cord to the first vertebral anchor, and securing the cord to the second vertebral anchor such that the cord is in tension and the spacer is in compression. It will be appreciated that steps such as the monomer application and polymerization steps may be exchanged with the steps associated with assembling the vertebral stabilization system at the convenience of the operator without departing from the spirit of the invention. The order listed above is presented for illustrative purposes only. 
         [0007]    In yet another illustrative embodiment is a method of monitoring the stability of a vertebral stabilization system by providing a vertebral stabilization system described above, obtaining a first radiographic image of the vertebral stabilization system, waiting an interval of time, obtaining a second radiographic image of the vertebral stabilization system, comparing the first radiographic image to the second radiographic image, and noting differences between the first radiographic image and the second radiographic image. 
         [0008]    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  
         [0009]    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: 
           [0010]      FIG. 1  is a perspective view of an exemplary vertebral stabilization system; 
           [0011]      FIGS. 2A-D  are schematically illustrative of pattern-wise application of monomer(s); 
           [0012]      FIGS. 3A-D  are schematically illustrative of pattern-wise application of monomer(s); and 
           [0013]      FIGS. 4A-B  are schematically illustrative of first and second radiographic images. 
       
    
    
       [0014]    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  
       [0015]    For the following defined terms, these definitions shall be applied, unless a different definition is given in the claims or elsewhere in this specification. 
         [0016]    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. 
         [0017]    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). 
         [0018]    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. 
         [0019]    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. 
         [0020]    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. 
         [0021]    Referring now to  FIG. 1 , there is shown a vertebral fixation system  10  for stabilizing a portion of a spinal column, such as one or more spinal segments of a spinal column. 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 spinal stabilization system  10  may provide dynamic stabilization to a spinal segment, preserving and/or allowing for a range of motion of the spinal segment. 
         [0022]    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. 
         [0023]    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. Additional vertebral anchors  12  may be present in instances in which the vertebral stabilization system  10  spans three or more vertebra of the spinal column. 
         [0024]    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, and in other embodiments the vertebral anchor  12  may be a polyaxial screw. 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. 
         [0025]    The vertebral anchor  12  may include a securing element, such as a threaded fastener  20  (e.g., a set screw, cap) configured to engage the head portion  14  to secure a portion of a connecting member  22  to the vertebral anchor  12 . For example, the threaded fastener  20  may include threads which mate with threads formed in the head portion  14 . 
         [0026]    The vertebral stabilization system  10  may also include one or more, or a plurality of 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  FIG. 1  includes a connecting member  22  extending between the first vertebral anchor  12   a  and the second vertebral anchor  12   b.    
         [0027]    The connecting member  22  may be constructed of a plurality of components in some instances. For instance, the connector  22  may include a spacer  24 , and a cord  30  extending through the spacer  24 , as well as other components if desired. 
         [0028]    In some embodiments, the spacer  24  may be an annular spacer having a lumen (not shown) extending from a first end  26  to a second end  28  of the spacer  24 . For example, in some embodiments the spacer  24  may be a cylindrical member having a lumen extending therethrough. In other embodiments, the spacer  24  may be molded, extruded, or otherwise formed over and/or around the cord  30 . The spacer  24  may be positioned between the head portion  14  of the first vertebral anchor  12   a  and the head portion  14  of the second vertebral anchor  12   b.  For instance, when installed between the first and second vertebral anchors  12   a,    12   b,  the first end  26  of the spacer  24  may face, abut or otherwise contact a side surface of the head portion  14  of the first vertebral anchor  12   a,  and the second end  28  of the spacer  24  may face, abut or otherwise contact a side surface of the head portion  14  of the second vertebral anchor  12   b.    
         [0029]    The cord  30  may extend from the head portion  14  of the first vertebral anchor  12   a  to the head portion  14  of the second vertebral anchor  12   b.  In some embodiments, the cord  30  may extend into and/or extend through a channel, such as a U-shaped channel, extending through the head portion  14  of the first vertebral anchor  12   a,  and the cord  30  may extend into and/or extend through a channel, such as a U-shaped channel, extending through the head portion  14  of the second vertebral anchor  12   b.  In some embodiments, the threaded fastener  20  of the first vertebral anchor  12   a  may be tightened directly onto the cord  30  to retain the cord  30  in the channel of the head portion  14  of the first vertebral anchor  12   a,  and/or the threaded fastener  20  of the second vertebral anchor  12   b  may be tightened directly onto the cord  30  to retain the cord in the channel of the head portion  14  of the second vertebral anchor  12   b.  In other embodiments, the cord  30  may extend into, extend through, and/or be secured to another component which spaces the cord  30  from direct contact with the channel of the vertebral anchor  12   a,    12   b.  For example, the cord  30  may extend into, extend through, and/or be secured to a spindle, spool, sleeve, coupler, or other component, which in turn is secured in the channel of the head portion of the vertebral anchor  12   a,    12   b  with the threaded fastener  20  or other securing fastener. It is noted that during a medical procedure the portions of the cord  30  which are shown extending from the channels of the vertebral anchors  12   a,    12   b  may be trimmed as desired to reduce and/or eliminate the portion of the cord  30  extending from the vertebral anchors  12   a,    12   b.    
         [0030]    When implanted in a patient, the cord  30  of the vertebral stabilization system  10  may limit the range of flexion of the spinal segment, whereas the spacer  24  may limit the range of extension of the spinal segment. For instance, the cord  30  may be placed in tension and the spacer  24  may be placed in compression between the vertebral anchors  12   a,    12   b.    
         [0031]    At least one of the cord  30  and the spacer  24  may comprise a polymerized monomer which may be readily visualized when the monomer or monomers are selected from halogen substituted (meth)acrylates, covalent salts of Group II elements other than beryllium, and chelates of Group II elements other than beryllium. For instance, in some embodiments the cord  30  and/or the spacer  24  may comprise an iodine-containing monomer, such as an iodine-containing acrylate, or other halogen-containing monomer. In some instances, the halogen-containing monomer, or other radiolucent monomer, may be in a coating applied to the surface of the bulk material of the cord  30  and/or the spacer  24 . In other instances, the halogen-containing monomer, or other radiolucent monomer, may be polymerized in the bulk material of the cord  30  and/or the spacer  24 . In some embodiments, the bulk material of the cord  30  and/or the spacer  24  may include polyurethane, polycarbonate urethane, polyethylene, polyethylene terephthalate, polybutylene terephthalate, tepolymethyl methacrylate, polyaryl ether ketone, blends or copolymers with at least one of the above polymers as a component. 
         [0032]    In some embodiments, the cord  30  and/or the spacer  24  may include a marking comprising the at least one polymerized monomer selected from the group consisting of halogen substituted (meth)acrylates, covalent salts of Group II elements other than beryllium, and chelates of Group II elements other than beryllium. 
         [0033]      FIGS. 2A-2D  and  3 A- 3 D provide schematic illustrative examples of pattern-wise deposited and polymerized monomers which may be readily visualized when the monomer or monomers are selected from halogen substituted (meth)acrylates, covalent salts of Group II elements other than beryllium, and chelates of Group II elements other than beryllium. In these  FIGS. 2A-2D , the monomer has been applied to the cord  30  and polymerized prior to assembly of the vertebral stabilization system.  FIG. 2A  represents the marking in the form of one or more simple uniform width longitudinal stripes  42  applied to the cord  30  which could be used to determine if the cord  30  had been twisted during tensioning and clamping.  FIG. 2B  depicts a longitudinal tapered line which clearly indicates orientation of the cord  30  as well as any applied torque.  FIG. 2C  provides a uniformly spaced transverse bar pattern which may be applied to the cord  30  to indicate the spacing between the first and second head portions  14  of vertebral anchors  12 . The asymmetric pattern of  FIG. 2D  combines an orientation indication with transverse markings which may be used to judge the distance between the first and second vertebral anchors  12 . 
         [0034]    In  FIGS. 3A-3D , the monomer has been applied to spacer  24  in a pattern-wise manner and polymerized. In  FIG. 3A , the resulting markings  44  cooperate with mark  42  on the cord  30  to indicate the relative positioning of the cord  30  and spacer  24 . As in  FIG. 2B , the asymmetry in the markings applied to the spacer  24  may confirm the orientation of the vertebral stabilization system  10 . Although the mark  42  would generally be placed and polymerized on cord  30  prior to assembly of the stabilization system  10 , it may be desirable to apply the monomer(s) which will result in marks  44  to spacer  24  after the stabilization system  10  is in place to ensure that the markings are aligned in the desired relative relationship. The markings may be applied by any of the commonly employed methods, for example by rubber stamp. In such embodiments, the monomers may be, for example, photopolymerized in situ. 
         [0035]      FIG. 3B  illustrates an asymmetric mark  44  applied to spacer  24 , said mark indicating the orientation of the spacer  24 . The uniformly distributed marks  44  applied to spacer  24  of  FIG. 3C  may be used to judge the degree of compression of the spacer  24  following assembly of the vertebral stabilization system  10 . In  FIG. 3D , a combination of markings  42  applied to the cord  30  and marks  44  applied to spacer  24  may be used to judge the relative positions of the cord  30  and spacer  24  as well as any torques introduced into the system. 
         [0036]    In  FIG. 4A , the relative positions of spacer  24  and cord  30 , as indicated by markings  42  and  44 , have been captured as they might appear in a first reference radiographic image. At a later time, a second radiographic image  FIG. 4B  has been captured. A comparison of the two images indicates that the mark  42  on cord  30  has shifted relative to the marks  44  on spacer  24  during the interval between the two images, perhaps as the result of slippage between cord  30  and one of the vertebral anchors  12 . In this manner, the vertebral stabilization system  10  may be monitored over its useful life by periodically acquiring a new radiographic image for comparison. The detection of a change in alignment may indicate that the vertebral stabilization system  10  should be replaced, reconfigured, or otherwise adjusted. The evaluation may be performed without the need to expose the vertebral stabilization system  10  for direct observation. 
         [0037]    In some embodiments, the monomer is applied in a pattern-wise manner. The pattern may be applied to either or both of the cord  30  and the spacer  24 . The pattern or patterns may comprise uniform elements uniformly spaced or may comprise a combination of uniform or non-uniform elements as well as uniformly or non-uniformly spaced elements. In certain embodiments, patterned elements applied to the cord  24  will have a determined, or at least determinable, spatial relationship to a marking or markings on the spacer  24 . In some embodiments, the markings may include generally axially elongated elements such as one or more stripes, triangles, ellipses, broken line segments, a sequence of dots, or the like. In other embodiments, the markings may be oriented generally transversely relative to a long axis of the device. In yet other embodiments, the markings may include a combination of axial and transversely oriented elements. In certain embodiments, the markings applied to the cord  30  will resemble the markings applied to the spacer  24  while in other embodiments the markings will differ in at least one of form and orientation. 
         [0038]    In selecting the monomers to be used in marking the cord  30  and spacer  24 , it is desirable that the resulting polymer be readily visualized by conventional means both before and after the vertebral stabilization system  10  has been installed. In some embodiments, it is desirable to select monomers which contribute high electron density to the resulting polymer while maintaining biocompatibility and avoiding components which may release toxic or otherwise undesirable species. It has been found that polymers derived from halogenated monomers, particularly halogenated acrylic and methacrylic monomers, often referred to collectively as (meth)acrylates, are well suited for this purpose. Although various halogenated species may be useful in this regard, triiodo aromatic derivatives of (meth)acrylic may be employed, alone or in combination with other monomers, to provide a polymer which has relatively high electron density and low toxicity. Alternatively monomers comprising covalent salts or chelates of metals having high electron density may also be employed. Of the metals, it is believed that multivalent metals arc particularly well suited for this purpose in that they tend to form polydentate salts and/or chelation complexes which are resistant to undesirable release of the metal or metal ion. Of the metals, the higher atomic number Group II elements have been found to provide a good balance between high electron density and stability of the salt or complex. While halogenated (meth)acrylic monomers may readily be polymerized by addition polymerization, it will be appreciated that monomers which include covalent salts or chelates of Group II elements may be selected which polymerize by addition or condensation polymerization mechanisms. 
         [0039]    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.