Abstract:
Embodiments of the disclosure provide a spinal stabilization rod useful for connecting a set of bone fasteners that can anchor a spinal stabilization system onto vertebral bodies. The spinal stabilization rod comprises rigid sections for coupling with bone fastener assemblies engaged in vertebrae and a flexible band for coupling the rigid sections. A bumper may be disposed around the rod to encase the flexible band. The spinal stabilization system may include a dampener.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates generally to spinal implants, and more particularly to a pedicle screw-based stabilization system which addresses stability and range of motion in the anterior-posterior plane. 
       BACKGROUND 
       [0002]    The human spine consists of segments known as vertebrae linked by intervertebral disks and held together by ligaments. There are 24 movable vertebrae—7 cervical (neck) vertebrae, 12 thoracic (chest) vertebrae, and 5 lumbar (back) vertebrae. Each vertebra has a somewhat cylindrical bony body (centrum), a number of wing-like projections (processes), and a bony arch. The arches are positioned so that the space they enclose forms the vertebral canal. The vertebral canal houses and protects the spinal cord, and within it the spinal fluid circulates. Ligaments and muscles are attached to various projections of the vertebrae. The bodies of the vertebrae form the supporting column of the skeleton. Fused vertebra make up the sacrum and coccyx, the very bottom of the vertebral column. 
         [0003]    The spine is subjected to abnormal curvature, injury, infections, tumor formation, arthritic disorders, and puncture or slippage of the cartilage disks due to variety of reasons. Modern spine surgery often involves the use of spinal stabilization/fixation procedures such as a vertebral body fusion procedure to correct or treat various acute or chronic spine disorders and/or to support the spine. In conjunction with these procedures, some spinal implants may be utilized to help stabilize the spine, correct deformities of the spine such as spondylolisthesis or pseudoarthrosis, facilitate fusion, or treat spinal fractures. Some spinal implants such as a spinal fixation system may provide fused and/or rigid support for the affected regions of the spine. For example, a spinal fixation system may include a corrective spinal implant that is attached to selected vertebrae of the spine by screws, hooks, and clamps. The corrective spinal implant may include spinal rods or plates that are generally parallel to the patient&#39;s back. The corrective spinal implant may also include transverse connecting rods that extend between neighboring spinal rods. Spinal fixation systems can be used to correct problems in the cervical, thoracic, and lumbar portions of the spine, and are often installed posterior to the spine on opposite sides of the spinous process and adjacent to the transverse process. Spinal fixation systems when implanted inhibit movement in the affected regions in virtually all directions. 
         [0004]    Modern spine surgery often involves spinal fixation through the use of spinal implants or fixation systems to correct or treat various spine disorders or to support the spine. Spinal implants may help, for example, to stabilize the spine, correct deformities of the spine, facilitate fusion, or treat spinal fractures. A spinal fixation system typically includes corrective spinal instrumentation that is attached to selected vertebra of the spine by screws, hooks, and clamps. The corrective spinal instrumentation may include spinal rods or plates that are generally parallel to the patient&#39;s back. The corrective spinal instrumentation may also include transverse connecting rods that extend between neighboring spinal rods. Spinal fixation systems are used to correct problems in the cervical, thoracic, and lumbar portions of the spine, and are often installed posterior to the spine on opposite sides of the spinous process and adjacent to the transverse process. 
         [0005]    Various types of screws, hooks, and clamps have been used for attaching corrective spinal instrumentation to selected portions of a patient&#39;s spine. Examples of pedicle screws and other types of attachments are illustrated in U.S. Pat. Nos. 4,763,644; 4,805,602; 4,887,596; 4,950,269; and 5,129,388. Each of these patents is incorporated by reference as if fully set forth herein. 
         [0006]    Often, spinal stabilization systems include rods which can bear a portion of the forces that would otherwise be transmitted along the spine. These rods may be implanted in pairs or in other numbers along portions of the spine of interest. Some spinal stabilization systems may support a portion of the spine including only two vertebrae (and associated anatomical structures) while some spinal stabilization systems support portions of the spine extending beyond two vertebrae. Spinal stabilizations systems can be used to support various portions of the spine, including the lumbar portion of the spine and the thoracic portion of the spine. Regardless of the number of rods implanted, or the portion of the spine in which they may be implanted, the rods can be attached to one or more vertebrae of the spine to provide support and stabilize, align, or otherwise treat the region of the spine of interest. Surgical personnel may use one or more anchor systems to attach the rods to one or more vertebrae. One such anchor system includes pedicle screws constructs which define slots, keyways, grooves, apertures, or other features for accepting and retaining stabilization rods which may be static, dynamic, or a combination of both. In many pedicle screw constructs, pedicle screws are placed in vertebrae selected by surgical personnel. 
         [0007]    Often, spinal fixation may include rigid (i.e., in a fusion procedure) support for the affected regions of the spine. Such systems limit movement in the affected regions in virtually all directions (for example, in a fused region). More recently, so called “dynamic” systems have been introduced wherein the implants allow at least some movement of the affected regions in at least some directions, i.e., flexion, extension, lateral, or torsional. Dynamic spinal stabilization systems can better match a patient&#39;s anatomy than some spinal stabilization systems used to provide static support. When implanted in a patient, a dynamic spinal stabilization system can allow at least some movement (e.g., flexion, extension, lateral bending, or torsional rotation) of the affected regions of the spine in at least some of the directions, giving the patient a greater range of motion. Dynamic stabilization systems can be used in scenarios in which vertebral body fusion is not desired, in which vertebral body (re)alignment is desired, and in which it is desired to support or strengthen degraded, diseased, damaged, or otherwise weakened portions of the spine. 
       SUMMARY 
       [0008]    Embodiments disclosed herein can be used as part of a spinal fusion or non-fusion treatment to stabilize the spine and address the omnipresent back pain problem. Specifically, embodiments of a spinal stabilization system may be coupled to and disposed between two pedicle screws to provide flexion and limit extension. Rigid sections may be coupled to pedicle screws, and a flexible band may couple the rigid sections. The flexible band allows the spine to move in flexion, and the rigid sections contact each other to inhibit extension of the spine. 
         [0009]    Embodiments disclosed herein may include a rod for stabilizing a portion of a spine. Embodiments of the rod may include a first rigid section having a first selected end, a second rigid section having a second selected end and a flexible band for joining the first rigid section to the second rigid section. In some embodiments, each of the first rigid section and the second rigid section has a cylindrical body for insertion into a bone fastener assembly. In some embodiments, the flexible band limits the separation distance between the first rigid section and the second rigid section when the first rigid section and second rigid section are securely coupled to bone fastener assemblies engaged with vertebrae. In some embodiments, contact between the first rigid section and the second rigid section limits the motion of the first rigid section relative to the second rigid section. In some embodiments, the first rigid section further comprises an aperture for passage of a portion of the flexible band. In some embodiments, the flexible band comprises polyester. In some embodiments, the flexible band comprises Dacron® polyester. In some embodiments, the flexible band is attached to either the first rigid section or the second rigid section before advancement of the flexible band into the patient. In some embodiments, the rod further includes a cap on one or more of the first rigid section and the second rigid section. In some embodiments, one or more of the first rigid section and the second rigid section have an end with a shape formed for selected contact with an end of an adjacent rigid section, wherein the spine has a selected range of motion based on the contact between the end of the first or second rigid section and the end of the adjacent rigid section. In some embodiments, the rod further includes a dampener. In some embodiments, the first rigid section or the second rigid section comprises a cap and one or more spring elements. In some embodiments, the first rigid section or the second rigid section comprises a rigid outer cap bonded to a resilient inner cap, wherein motion of the first rigid section or the second rigid section relative to the bone fastener assembly is controlled by a property of the inner cap. In some embodiments, the first rigid section or the second rigid section comprises an outer polymer sleeve and an inner polymer disc. In some embodiments, the first rigid section or the second rigid section comprises an outer polymer cap defining a space with the first rigid section or the second rigid section and a hydrogel located inside the space. 
         [0010]    Embodiments of a spine stabilization system may include a first bone fastener assembly coupled to a first vertebra, a second bone fastener assembly coupled to a second vertebra, and a rod for coupling the first bone fastener assembly to the second bone fastener assembly. The rod may include a first rigid section having a first selected end, a second rigid section having a second selected end and a flexible band for joining the first rigid section to the second rigid section. In some embodiments, each of the first rigid section and the second rigid section has a cylindrical body for insertion into a bone fastener assembly. In some embodiments, the flexible band limits the separation distance between the first rigid section and the second rigid section when the first rigid section and second rigid section are securely coupled to bone fastener assemblies engaged with vertebrae. In some embodiments, contact between the first rigid section and the second rigid section limits the motion of the first rigid section relative to the second rigid section. In some embodiments, the flexible band comprises polyester. In some embodiments, the spine stabilization system includes a dampener coupled to the first rigid section or the second rigid section and coupled to the first bone fastener assembly or the second bone fastener assembly, wherein motion of the first rigid section or the second rigid section relative to the first bone fastener assembly or the second bone fastener assembly is dampened. In some embodiments, motion of the spine is controlled in one or more ranges of motion comprising flexion, extension, lateral bending and torsion. 
         [0011]    Embodiments may include a method of stabilizing a portion of the spine. The method may include coupling a first bone fastener assembly to a first vertebra, coupling a second bone fastener assembly to a second vertebra and coupling a rod to the first bone fastener assembly and to the second bone fastener assembly. In some embodiments, the rod has a first rigid section having a first selected end and a second rigid section having a second selected end, and coupling the rod to the first bone fastener assembly or the second bone fastener assembly includes joining the first rigid section to the second rigid section with a flexible band, wherein each of the first rigid section and the second rigid section has a cylindrical body for insertion into a bone fastener assembly. The flexible band limits the separation distance between the first rigid section and the second rigid section when the first rigid section and second rigid section are securely coupled to bone fastener assemblies engaged with vertebrae. Contact between the first rigid section and the second rigid section limits the motion of the first rigid section relative to the second rigid section. 
         [0012]    In some embodiments, the method may include comprising mechanically joining the first rigid section to the second rigid section using the flexible band. In some embodiments, the first rigid section or the second rigid section comprises a dampener, wherein coupling the first rigid section to the first bone fastener assembly or coupling the second rigid section to the second bone fastener assembly comprises joining the dampener to the first bone fastener assembly or the second bone fastener assembly. In some embodiments, the method may include coupling one or more cross-link devices to connect two or more rods. 
         [0013]    In one embodiment, a first rigid section of the rod may be coupled to a first pedicle screw implanted in a first vertebra and a second rigid section of the rod may be coupled to a second pedicle screw implanted in a second vertebra. The first and second rigid sections may each have a slot, opening or aperture formed therein. Either the first rigid section or the second rigid section may have a flexible band attached thereto, such as by threading the flexible band through an opening and joining two portions of the flexible band to inhibit withdrawing the flexible band from the aperture. Ends of the flexible band may be passed through the aperture in the other rigid section and connected to inhibit withdrawal of the flexible band from the aperture. Once the ends of the flexible band are connected, the spine may have a range of motion that is limited in flexion by the flexible band and that is limited in extension by the rigid sections contacting each other. 
         [0014]    Embodiments of a spine stabilization system may be used in a single-level stabilization system, or may be used in a multi-level spine stabilization system, which may provide advantages over multiple levels of the spine. 
         [0015]    Other features, advantages, and objects of the disclosure will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    A more complete understanding of the present disclosure and the advantages thereof may be acquired by referring to the following description, taken in conjunction with the accompanying drawings in which like reference numbers indicate like features and wherein: 
           [0017]      FIG. 1  depicts a simplified graphical representation of a side view showing a portion of a healthy spine and various types of movements of the spine; 
           [0018]      FIG. 2A  depicts a perspective view of one embodiment of a spinal rod; 
           [0019]      FIG. 2B  depicts a side view of a spinal stabilization system installed on vertebral bodies according to some embodiments of the disclosure; 
           [0020]      FIGS. 3-7  depict views of rigid sections of a spinal rod according to some embodiments of a spine stabilization system; 
           [0021]      FIG. 8  depicts a view of one embodiment of a spinal rod; 
           [0022]      FIGS. 9A and 9B  depict side and cross-section views of one embodiment of a spinal rod according to one embodiment; and 
           [0023]      FIGS. 10-13  depict simplified schematic representations of a spinal stabilization rod, according to some embodiments of the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    The spine stabilization system and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments detailed in the following description. Descriptions of well known starting materials, manufacturing techniques, components and equipment are omitted so as not to unnecessarily obscure the disclosure in detail. Skilled artisans should understand, however, that the detailed description and the specific examples, while disclosing preferred embodiments of the disclosure, are given by way of illustration only and not by way of limitation. Various substitutions, modifications, and additions within the scope of the underlying inventive concept(s) will become apparent to those skilled in the art after reading this disclosure. Skilled artisans can also appreciate that the drawings disclosed herein are not necessarily drawn to scale. 
         [0025]    A spine is composed of vertebral bones (i.e., vertebrae) stacked up on one another in a smooth alignment. The spinal canal sits within the spinal column and houses the spinal cord and spinal nerves that send signals to and from the brain. The linkages between the vertebrae are soft, with discs in the front and ligaments in the back, allowing displacement and adaptation to stress and load. Due to the unique and complex arrangements and configurations of vertebrae and intervertebral disc components, a healthy spine is strong and flexible and can function through the vigorous demands of daily living, work, and recreational activities. However, the spine is vulnerable to injury and to degeneration, a term which refers to the gradual failure of the spine&#39;s biomechanical functions due to aging and wear and tear. The soft tissues (e.g., the intervertebral discs, the ligaments and cartilage of the facet joints) are most vulnerable to degeneration. With normal aging, the discs would gradually collapse and ligaments lose their elasticity and stabilizing ability. Other factors such as those described below may also cause damage to the spine. 
         [0026]      FIG. 1  depicts a simplified graphical representation of a side view of a portion of healthy spine  12  and various types of movements thereof, i.e., flexion, extension and rotation. Various spine deformities, injuries, and the like may cause spine  12  to destabilize or require, resulting in changes to one or more ranges of motion. 
         [0027]    Table 1 below lists the ranges of motion for flexion, extension, lateral bending and torsion with normalized values for a normal spine, a destabilized spine, and a spine stabilized with rigid rods. 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Healthy 
                 Destabilized 
                 Stabilized with 
               
               
                   
                 Range of Motion 
                 Spine 
                 Spine 
                 Rigid Rod(s) 
               
               
                   
                   
               
             
             
               
                   
                 Flexion 
                 1.0 
                 1.40-2.80 
                 0.2-0.6 
               
               
                   
                 Extension 
                 1.0 
                 1.40-2.20 
                 0.2-0.6 
               
               
                   
                 Lateral Bending 
                 1.0 
                 1.40-2.40 
                 0.2-0.7 
               
               
                   
                 Torsion 
                 1.0 
                 1.40-2.40 
                 0.2-0.8 
               
               
                   
                   
               
             
          
         
       
     
         [0028]    As is known, the biomechanical functions of a healthy spine are very complex and difficult to replicate. There is a continuing need for better spinal implants and implantable devices that can stabilize a weakened/damaged spine and yet simultaneously allow some range of motion so that the patient can enjoy daily life and normal activities without constraints or restrictions. 
         [0029]      FIG. 2A  depicts a perspective view of one embodiment of spinal rod  30  having rigid section  30   a  and  30   b  connected by flexible band  40  passing through apertures  31  and further including ends  32  for contact between rigid sections  30   a  and  30   b.    
         [0030]    Rigid sections  30   a  and  30   b  may be manufactured from a biocompatible material. In some embodiments, rigid sections  30   a  or  30   b  are manufactured from PEEK, metal, carbon, composites, ceramics, or the like. Aperture  31  may be formed in rigid section  30   a  and  30   b.  In some embodiments, rigid section  30   a  or  30   b  may include bushing  33 . In some embodiments, bushing  33  may be formed first and placed into an injection molding machine, and material may be injected to form rigid section  30   a  or  30   b  around bushing  33 . Bushing  33  may be formed from a biocompatible material, steel, titanium, UHMWPE or some other material that can withstand the injection molding process. Bushing  33  may be formed from a material that allows flexible band  40  to move relative to rigid section  30   a  or  30   b.    
         [0031]    Rigid sections  30   a  or  30   b  may include ends  32  for contact with adjacent rigid sections  30   b  or  30   a.  The geometry of ends  32  of rigid sections  30   a  or  30   b  may determine the extension range of motion. In some embodiments, the geometry of ends  32  of rigid sections  30   a  and  30   b  may act as a mechanical stop or barrier after a desired range of motion is achieved. The geometry of ends  32  of rigid sections  30   a  and  30   b  can be designed such that rigid sections  30   a  and  30   b  will not affect flexion ROM, but will allow limited extension motion. Ends  32  of rigid sections  30   a  or  30   b  may be flat, rounded, or have other shapes. Ends  32  of rigid sections  30   a  or  30   b  may be symmetric or asymmetric. In some embodiments, ends  32  of rigid sections  30   a  or  30   b  may be designed and matched with one another in various ways to achieve a desired range of motion. 
         [0032]    Flexible band  40  may be used to restrict the amount of flexion in spine  12 . The ends of flexible band  40  may allow flexible band  40  to pass through apertures  31 , bushings  33  or otherwise couple rigid sections  30   a  and  30   b  to form rod  30 . In some embodiments, flexible band  40  may be manufactured from Dacron® polyester, carbon fiber, Ultra-High Molecular Weight Polyethylene (UHMWPE) fibers, or some other material that can withstand tension exerted on spine  12  by the patient. 
         [0033]      FIG. 2B  depicts a side view of one portion of spine  12  coupled to spine stabilization system  100 . In some embodiments, spine stabilization system  100  may include bone fastener assemblies  18  and rods  30 . 
         [0034]    Each bone fastener assembly lo may include a bone screw or other bone fastener, a collar, a ring, and other components for coupling rod  30  to a vertebra. Bone screws may have a head for engagement by a driver or other tool and a shank for advancement into a vertebra. A bone screw may be advanced into the patient via an extender sleeve. The bone screw may be positioned relative to the pedicle of a vertebra. A tool may be advanced into the patient and engage the head of the bone screw. For example, a driver may be advanced into the patient via the extender sleeve and engage the head of a bone screw. The driver may rotate the head of the bone screw to advance the bone screw into the vertebra. 
         [0035]    A collar, ring, or other component may be advanced into the patient and connected with the bone screw to form bone fastener assembly  18 . A collar may be used to couple rod  30  to a bone screw. Rod  30  may be positioned in the collar to couple rod  30  to bone fastener assembly  18 . Each rod  30  may include rigid sections  30   a  and  30   b,  flexible band  40 , apertures  31  and ends  32  having selected surfaces or geometries for contact with other ends  32 . In some embodiments, rigid sections  30   a  and  30   b  of rods  30  may be positioned in collars of bone fastener assemblies  18  prior to assembly of bone fastener assembly  18 . A ring may be used to provide a range of motion of a collar relative to the head of the bone screw. Closure members may be used to secure rod  30  in collars of bone fastener assemblies  18  coupled to spine  12 . 
         [0036]    Rigid sections  30   a  or  30   b  may be advanced into the patient and positioned in bone fastener assemblies  18 . Advancement of rigid sections  30   a  and  30   b  into a patient may be accomplished by attaching rigid sections  30   a  or  30   b  to a tool and advancing the end of the tool into the patient, by advancing a tool into the patient and then using the tool to guide the advancement of rigid sections  30   a  or  30   b,  or by engaging rigid section  30   a  or  30   b  with another component (e.g. bone fastener assembly  18 , a cross-link device, etc.) and then advancing the component and rigid section  30   a  or  30   b  into the patient. 
         [0037]    Rigid sections  30   a  and  30   b  may be connected using flexible band  40  to form part of spine stabilization system  100 . In some embodiments, rigid sections  30   a  and  30   b  may be coupled to bone fastener assemblies  18  and flexible band may be passed through one or more apertures  31  in rigid sections  30   a  or  30   b  and joined to inhibit withdrawal of flexible band from either rigid section  30   a  or  30   b.    
         [0038]    Surgical techniques to stabilizing the spine may include implanting bone screws in pedicles or otherwise coupling bone fastener assemblies  18  to vertebrae and coupling rods  30  to bone fastener assemblies  18  to provide a desired level of movement of spine  12 . 
         [0039]    The use of pedicle screws in spine stabilization is generally known in the art. Techniques of implanting the bone screws, both invasive and minimally-invasive (MIS) are familiar to most spine surgeons. Once bone screws are engaged with selected vertebrae, a rod may be coupled to the bone screws. One technique of coupling a rod to a spine involves implanting pedicle screws in pedicles and coupling a rod between the implanted pedicle screws. The patient may be positioned on a surgical table, implantation sites may be identified, bone screws may be advanced into the patient and implanted in the vertebrae, and a rod may be coupled to the bone screws. Tools, such as an awl, tap, sleeve, driver, and the like may be used to prepare a vertebra for implantation, to guide or position the pedicle screw at the implantation site, to implant the pedicle screw into the bone, and the like. U.S. Pat. No. 7,250,052 describes at least one method for implanting bone screws in vertebrae and is incorporated herein by reference. 
         [0040]    Once the bone screws are implanted in the vertebrae, other portions of spine stabilization system  100  may be advanced into the patient. One embodiment of a method for stabilizing the spine may include advancing first rigid section  30   a  of rod  30  into the patient and coupling first rigid section  30   a  to first bone fastener assembly  18 . In some embodiments, first rigid section  30   a  may be coupled to flexible band  40  prior to advancement into the patient. In some embodiments, first rigid section  30   a  may be positioned in a collar of bone fastener assembly  18  and secured using a closure member. In some embodiments, first rigid section  30   a  may be positioned in a collar and held in place by a tool coupled to the bone screw, the collar, both, or some other portion or component of bone fastener assembly  18 . Once first rigid section  30   a  is positioned in first bone fastener assembly  18 , second rigid section  30   b  may be advanced into the patient. In some embodiments, second rigid section  30   b  may be coupled to flexible band  40  prior to advancement into the patient. In some embodiments, second rigid section  30   b  may be positioned in bone fastener assembly  18  and secured using a closure member. In some embodiments, second rigid section  30   b  may be positioned in a collar in bone fastener assembly  18  and held in place by a tool coupled to the bone screw, the collar, both, or some other portion or component of bone fastener assembly  18 . Once first rigid section  30   a  and second rigid section  30   b  are positioned in bone fastener assemblies  18 , the ends of flexible band  40  may be passed through apertures  31  in one or both rigid sections  30   a  and  30   b.  After band  40  is passed through apertures  31  in rigid sections  30   a  and  30   b,  band  40  may be tightened or otherwise configured to have a desired length or tension. In some embodiments, after band  40  has been passed through apertures  31  in rigid sections  30   a  and  30   b,  the ends of band  40  may be joined to each other or rigid sections  30   a  or  30   b  to inhibit withdrawal of either end from first rigid section  30   a  or second rigid section  30   b.  In some embodiments, joining the ends of flexible band  40  may include mechanically, chemically, or thermally joining the ends. For example, mechanically joining the ends may include tying the ends together or to rigid sections  30   a  or  30   b,  crimping the ends together or to a feature of rigid sections  30   a  or  30   b,  or the like. Chemically configuring the ends may include using an adhesive to bond the ends together or to rigid sections  30   a  or  30   b.  In some embodiments, rigid section  30   a  or  30   b  may have a feature for connection to the ends of flexible band  40 . 
         [0041]    Once rod  30  has been assembled and rigid sections  30   a  and  30   b  have been secured to bone fastener assembly  18 , extender sleeves and other tools may be withdrawn from the patient. After implantation of spine stabilization system  100 , contact between ends  32  of rigid sections  30   a  and  30   b  may control or limit extension in the spine and flexible band  40  may control or limit flexion in the spine. The surgeon may decide the gap between rigid sections of rod  30 , depending upon the range of motion and patient&#39;s spine stability, by adjusting the length of flexible band  40  between rigid sections  30   a  and  30   b.    
         [0042]      FIGS. 3-7  depict views of embodiments of rigid sections  30   a  or  30   b.  In some embodiments, rigid section  30   a  or  30   b  may be symmetric to allow unbiased movement of the spine, may be asymmetric to bias the spine in a preferred orientation, may include features to restrict movement in a direction or about an axis, or may have some other feature or shape to promote a healthy spine. 
         [0043]      FIG. 8  depicts a side cross-section view of one embodiment of rod  30  having caps  34  on ends  32  of rigid sections  30   a  and  30   b.  Caps  34  may be useful to avoid hard contact between rigid sections  30   a  and  30   b.  In some embodiments, caps  34  may provide a desired cushion effect between adjacent rigid sections  30   a  and  30   b.    
         [0044]    In some embodiments, spinal stabilization system  100  may include other components for controlling motion of a spine.  FIGS. 9A and 9B  depict views of a portion of one embodiment of spine stabilization system  100  including rigid sections  30   a  and  30   b,  flexible band  40  and casing  60 . 
         [0045]    Casing  60  may surround flexible band  40  and isolate flexible band  40  from contact with other components of spine stabilization system  100 , anatomic structures, debris, or to contain wear particles of spine stabilization system  100 , or the like. In some embodiments, casing  60  may surround flexible band  40  and extend a desired distance along rigid sections  30   a  and  30   b  to isolate flexible band  40 . Casing  60  may allow stretching of band  40 . This casing may be manufactured from biocompatible materials such as polyurethane (PU) and its co-polymeric configurations. 
         [0046]    In some embodiments, spine stabilization system  100  may include bushing  70 . Bushing  70  may be manufactured from metals, metal alloys, ceramics, composites, polymers, or some other material. The presence of bushing  70  may reduce or minimize the friction between the rigid rod and flexible band. In some embodiments, bushing  70  may be manufactured from a material that can withstand an injection molding process. Bushing  70  may be inserted into an injection molding device and material may be injected into the device to surround bushing  70  and form casing  60 . Bushing  70  may reduce friction between flexible band  40  and aperture  31  in rigid section  30   a  or  30   b.    
         [0047]    Spine  12  may benefit from dampened motion. In some embodiments, rod  30  may include dampeners  75  to provide dampened motion to spine  12 .  FIGS. 10-13  depict cross-sectional views of embodiments of portions of rigid sections  30   a  or  30   b  including dampener  75 . As depicted in  FIG. 10 , in some embodiments, dampener  75  may include cap  80  and one or more spring elements  81 . Cap  80  may be able to move relative to rigid section  30   a  or  30   b.  Spring elements  81  may have a first end attached to cap  80  and a second end attached to a second end of rigid section  30   a  or  30   b.    
         [0048]    In some embodiments, rigid section  30   a  or  30   b  having dampener  75  may be coupled to bone fastener assembly  18 . In one embodiment, cap  80  may be coupled to bone fastener assembly  18  and rigid section  30   a  or  30   b  may be able to move with dampened motion relative to bone fastener assembly  18  such that motion of rigid section  30   a  or  30   b.  The range of motion or degree of dampening may be controlled by one or more of the spring stiffness properties, length of spring elements  81 , the number of spring elements  81 , the angle of attachment of spring elements  81  to cap  80  or rigid section  30   a  or  30   b,  and the like. 
         [0049]    As depicted in  FIG. 11 , in some embodiments, dampener  75  may include outer cap  80  and inner cap  82 . Outer cap  80  may be manufactured from a rigid material such as titanium, titanium alloys, stainless steel, or some other metal. Inner cap  82  may be manufactured from a resilient material such as a polymer. Inner cap  82  may be bonded to outer cap  80 . 
         [0050]    In some embodiments, rigid section  30   a  or  30   b  may be coupled to bone fastener assembly  18 . Cap  80  may be coupled to bone fastener assembly  18  such that rigid section  30   b  is able to move with dampened motion relative to bone fastener assembly  18 . Motion of rigid section  30   a  or  30   b  may be possible due to compression and strain on resilient material used to manufacture inner cap  82 . The range of motion or degree of dampening may be controlled by one or more of the viscosity of the polymer, elastic yield strength, and other properties or characteristics of polymer cap  82 . 
         [0051]    As depicted in  FIG. 12 , in some embodiments, dampener  75  may include polymer sleeve  84  and polymer disc  85 . Rigid section  30   a  or  30   b  may have an end formed with a tapered profile, stepped profile, or other narrowing profile such that rigid section  30   a  or  30   b  and polymer sleeve  84  may have the same outer profile as other portions of rod  30 . Polymer sleeve  84  and rigid section  30   a  or  30   b  may be coupled to bone fastener assembly  18  such that motion of rigid section  30   a  or  30   b  relative to bone fastener assembly  18  is dampened by polymer sleeve  84  or polymer disc  85  or both. 
         [0052]    In some embodiments, rigid section  30   a  formed with polymer sleeve  84  and polymer disc  85  may be inserted into bone fastener assembly  18 . The interaction between polymer sleeve  84  and polymer disc  85 , the material composition of polymer sleeve  84  or polymer disc  85 , or some combination may determine the dampening characteristics of rigid section  30   a  or  30   b.    
         [0053]    As depicted in  FIG. 13 , in some embodiments, rod  30  may include dampener  75  in which polymer cap  86  is coupled to rigid section  30   a  or  30   b  and forming a space therein and a polymer injected therein. Polymer cap  86  may be formed from polyurethane or some other soft polymer. In some embodiments, hydrogel  87  may be injected in liquid form at room temperature into the space formed by polymer cap  86  and rigid section  30   a  or  30   b,  and the hydrogel will solidify once inside the human body at elevated temperature of 37° C. Polymer cap  86  may be coupled to bone fastener assembly  18  such that motion of rigid section  30   a  or  30   b  relative to bone fastener assembly  18  is dampened by polymer cap  86  or hydrogel  87  or both. The range of motion or degree of dampening may be controlled by the properties of polymer cap  86  or hydrogel  87 , the interaction between polymer cap  86  and hydrogel  87 , or some combination. 
         [0054]    In some embodiments, polymer casing  60  and dampening element  75  may provide a vehicle for the delivery of drugs. A drug dosage may be disposed in spine stabilization system and be released after specific time intervals in a controlled manner. 
         [0055]    In some embodiments, an advantage to having rigid sections  30   a  or  30   b  with dampener  75  may be the extra cushioning effect, which may be particularly noticeable in discectomy patients. 
         [0056]    Another advantage to embodiments of spine stabilization system  100  is that a reasonable range of motion in either lateral or torsional motion may be possible based on one or more components of spine stabilization system  100 . In some embodiments, the anterior-posterior range of motion are a function of flexible band properties, properties of external polymer casing  60 , and stiffness of dampener element  75 . Lateral and torsional range of motion may primarily be a function of properties of lower rigid section  30   a  or  30   b.    
         [0057]    Another advantage to embodiments of spine stabilization system  100  may be the capability to be used in conjunction with other non-fusion devices, such as nucleus replacements and facet joint replacements. 
         [0058]    Another advantage may be the capability for embodiments of spine stabilization system  100  to be implanted using minimally invasive surgery (MIS) techniques and used with existing pedicle screw platforms. In the event that fusion between vertebrae is required, embodiments may be easily removed and replaced with other rods. 
         [0059]    In some embodiments, spinal stabilization system  100  may include additional rods positioned further superior or inferior along spine  12 , with the additional rods being anisotropic spinal stabilization rods, dynamic stabilization rods, non-dynamic rods, and/or rigid rods. Within this disclosure, the term “dynamic” refers to the flexing capability of a spinal rod. It should be understood that spinal stabilization system  100  may also include suitable transverse rods or cross-link devices that help protect the supported portion of spine  12  against torsional forces or movement. Some possible examples of suitable cross-link devices are shown in co-pending U.S. patent application Ser. No. 11/234,706, filed on Nov. 23, 2005 and naming Robert J. Jones and Charles R. Forton as inventors (the contents of this application are incorporated fully herein by reference). Other known cross-link devices or transverse rods may also be employed. 
         [0060]    In the foregoing specification, specific embodiments have been described with reference to the accompanying drawings. However, as one skilled in the art can appreciate, embodiments of the anisotropic spinal stabilization rod disclosed herein can be modified or otherwise implemented in many ways without departing from the spirit and scope of the disclosure. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the manner of making and using embodiments of an anisotropic spinal stabilization rod. It is to be understood that the embodiments shown and described herein are to be taken as exemplary. Equivalent elements or materials may be substituted for those illustrated and described herein. Moreover, certain features of the disclosure may be utilized independently of the use of other features, all as would be apparent to one skilled in the art after having the benefit of this description of the disclosure.