Patent Publication Number: US-2010114165-A1

Title: Posterior dynamic stabilization system with pivoting collars

Description:
TECHNICAL FIELD 
     This disclosure relates generally to spinal stabilization systems, and more particularly to spinal implants for dynamically stabilizing human spines. Even more particularly, this disclosure relates to embodiments of a pivoting collar and a posterior dynamic stabilization system utilizing the same. 
     BACKGROUND 
     The human spine consists of segments known as vertebrae separated by intervertebral disks  28  and held together by various ligaments. There are 24 movable vertebrae—7 cervical, 12 thoracic, and 5 lumbar. Each of the movable vertebrae has a somewhat cylindrical bony body (often referred to as the centrum), a number of winglike projections, and a bony arch. The bodies of the vertebrae form the supporting column of the skeleton. The arches of the vertebrae are positioned so that the spaces they enclose form a curvilinear passage which is often referred to as the vertebral canal. The vertebral canal houses and protects the spinal cord (which includes bundles of sensory and motor nerves for sensing conditions in or affecting the body and commanding movements of various muscles). Within the vertebral canal, spinal fluid can circulate to cushion the spinal cord and carry immunological cells to it, thereby protecting the sensory and motors nerves therein from mechanical damage and disease. Ligaments and muscles are attached to various projections of the vertebrae such as the superior-inferior, transverse, and spinal processes. Other projections, such as vertebral facets, join adjacent vertebrae to each other, in conjunction with various attached muscles, tendons, etc. while still allowing the vertebrae to move relative to each other. 
     Spines may be subject to abnormal curvature, injury, infections, tumor formation, arthritic disorders, punctures of the intervertebral disks, slippage of the intervertebral disks from between the vertebrae, or combinations thereof. Injury or illness, such as spinal stenosis and prolapsed disks may result in intervertebral disks having a reduced disk height, which may lead to pain, loss of functionality, reduced range of motion, disfigurement, and the like. Scoliosis is one relatively common disease which affects the spinal column. It involves moderate to severe lateral curvature of the spine and, if not treated, may lead to serious deformities later in life. Such deformities can cause discomfort and pain to the person affected by the deformity. In some cases, various deformities can interfere with normal bodily functions. For instance, some spinal deformities can cause the affected person&#39;s rib cage to interfere with movements of the respiratory diaphragm, thereby making respiration difficult. Additionally, some spinal deformities noticeably alter the posture, gate, appearance, etc. of the affected person, thereby causing both discomfort and embarrassment to those so affected. One treatment involves surgically implanting devices to correct such deformities, to prevent further degradation, and to mitigate symptoms associated with the conditions which may be affecting the spine. 
     Modern spine surgery often involves spinal stabilization through the use of spinal implants or stabilization systems to correct or treat various spine disorders and/or to support the spine. Spinal implants may help, for example, to stabilize the spine, correct deformities of the spine, facilitate fusion of vertebrae, or treat spinal fractures and other spinal injuries. Spinal implants can alleviate much of the discomfort, pain, physiological difficulties, embarrassment, etc. that may be associated with spinal deformities, diseases, injury, etc. 
     Spinal stabilization systems typically include corrective spinal instrumentation that is attached to selected vertebra of the spine by bone anchors, screws, hooks, clamps, and other implants hereinafter referred to as “bone anchors.” Some corrective spinal instrumentation includes spinal stabilization rods, spinal stabilization plates that are generally parallel to the patient&#39;s back, or combinations thereof. In some situations, corrective spinal instrumentation may also include superior-inferior connecting rods that extend between bone anchors (or other attachment instrumentation) attached to various vertebrae along the affected portion of the spine and, in some situations, adjacent vertebrae or adjacent boney structures (for instance, the occipital bone of the cranium or the coccyx). Spinal stabilization 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 superior-inferior process. Some implants can be implanted anterior to the spine and some implants can be implanted at other locations as selected by surgical personnel such as at posterior locations on the vertebra. 
     Often, spinal stabilization may include rigid support for the affected regions of the spine. Such systems can limit movement in the affected regions in virtually all directions. Such spinal stabilizations are often referred to as “static” stabilization systems and can be used in conjunction with techniques intended to promote fusion of adjacent vertebrae in which the boney tissue of the vertebrae grow together, merge, and assist with immobilizing one or more intervertebral joints. More recently, so called “dynamic” spinal stabilization systems have been introduced wherein the implants allow at least some movement (e.g., flexion or extension) of the affected regions of the spine in at least some directions. 
     Dynamic stabilization systems therefore allow the patient greater freedom of motion at the treated intervertebral joint(s) and, in some cases, improved quality of life over that offered by static stabilization systems. 
     SUMMARY 
     In one embodiment, a system for dynamically stabilizing a portion of a spine is provided. The system can include a spinal stabilization rod and a collar which can be attached to one of the vertebrae of the spine. The collar can define a bore, an internal surface of the bore, and a contact point on the internal surface. The bore can be shaped and dimensioned to accept the spinal stabilization rod and to allow the spinal stabilization rod to pivot about the contact point. In some embodiments, the spinal stabilization rod can be flexible so that it can bend about the contact point. 
     Regarding the bore, various embodiments include internal surfaces of differing shapes including, in some embodiments, generally semi-spherical internal surfaces. The internal surface can be further shaped and configured to limit the range through which the spinal stabilization rod pivots. For instance, the internal surface can limit the spinal stabilization rod to a range of about six degrees in any direction. In some embodiments, the range through which the spinal stabilization rod can pivot can differ for differing directions. In some embodiments, at least a portion of the bore can have an oval cross sectional shape. 
     In some embodiments, the system can include a second collar. Some second collars can define a slot for accepting the spinal stabilization rod. The slot of the second collar can have a diameter which is larger than the smallest diameter of the bore. Regarding the spinal stabilization rod, it can have two portions one of which has a diameter corresponding to that of the slot of the second collar. The other portion of the spinal stabilization rod can have a diameter corresponding to the smallest diameter of the bore. In some embodiments, the spinal stabilization rod can include a transition portion between the first and the second portions. 
     One embodiment provides a collar for dynamically stabilizing a portion of a spine in conjunction with a spinal stabilization rod. The collar can include a body which defines a bore, an internal surface of the bore, and a contact point on the internal surface. The bore can be shaped and dimensioned to accept the spinal stabilization rod and to allow the spinal stabilization rod to pivot about the contact point. In some embodiments, the spinal stabilization rod can be flexible so that it can bend about the contact point. 
     Regarding the bore, various embodiments include internal surfaces of differing shapes including, in some embodiments, generally semi-spherical internal surfaces. The internal surface can be further shaped and configured to limit the range through which the spinal stabilization rod pivots. For instance, the internal surface can limit the spinal stabilization rod to a range of about six degrees in any direction. In some embodiments, the range through which the spinal stabilization rod can pivot can differ for differing directions. In some embodiments, at least a portion of the bore can have an oval cross sectional shape. 
     Embodiments provide spinal stabilization systems which can statically stabilize two or more vertebrae while dynamically stabilizing one or more other vertebrae. 
     In some embodiments, spinal stabilization systems can allow rotation of certain vertebrae about one or more axes thereby allowing patients to flex/extend, rotate, or bend (or combinations thereof) various portions of their back. Thus, spinal stabilization systems of various embodiments can allow patients to bend or arch their backs, twist their torsos, bend side-to-side, and combinations thereof. Furthermore, embodiments provide dynamic stabilization systems which require no closure member or other components besides a spinal stabilization collar and rod. 
     These, and other, aspects will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. The following description, while indicating various embodiments and numerous specific details thereof, is given by way of illustration and not of limitation. Many substitutions, modifications, additions, or rearrangements may be made within the scope of the disclosure, and the disclosure includes all such substitutions, modifications, additions, or rearrangements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the 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 generally indicate like features. 
         FIG. 1  depicts a graphical representation of a spinal stabilization patient. 
         FIG. 2  depicts a graphical representation of a human spine. 
         FIG. 3  depicts a simplified top view of one embodiment of a posterior spinal dynamic stabilization system installed with a pair of pivoting collars. 
         FIG. 4  depicts a simplified side view of one embodiment of a posterior spinal dynamic stabilization system in which a pivoting collar is coupled to a flexible portion of a spinal stabilization rod. 
         FIG. 5  depicts a simplified cross-sectional view of one embodiment of a spinal stabilization system attached to a first vertebra via a first bone screw having a clamping collar and to a second vertebra via a second bone screw having a pivoting collar. 
         FIG. 6  depicts a simplified cross-sectional view of one embodiment of a spinal stabilization system, illustrating a kinematical interaction between one embodiment of a spinal stabilization rod and a pivoting collar. 
         FIG. 7  depicts a cross-sectional view of one embodiment of a pivoting collar. 
         FIG. 8  depicts one embodiment of a bore of a pivoting collar, providing selected limits to the range of motion of a spinal stabilization rod in differing directions. 
         FIG. 9  depicts a cross-sectional view of one embodiment of a pivoting collar. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosure 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. 
     As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having” or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, process, article, or apparatus that comprises a list of elements is not necessarily limited only those elements but may include other elements not expressly listed or inherent to such process, process, article, or apparatus. Further, unless expressly stated to the contrary, “or” refers to an inclusive or and not to an exclusive or. For example, a condition A or B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present). 
     Additionally, any examples or illustrations given herein are not to be regarded in any way as restrictions on, limits to, or express definitions of, any term or terms with which they are utilized. Instead, these examples or illustrations are to be regarded as being described with respect to one particular embodiment and as illustrative only. Those of ordinary skill in the art will appreciate that any term or terms with which these examples or illustrations are utilized will encompass other embodiments which may or may not be given therewith or elsewhere in the specification and all such embodiments are intended to be included within the scope of that term or terms. Language designating such nonlimiting examples and illustrations includes, but is not limited to: “for example”, “for instance”, “e.g.”, “in one embodiment”. 
       FIG. 1  depicts a graphical representation of spinal stabilization patient  10 . Patient  10  generally possesses the capability to move according to many degrees of freedom which are at least partially defined with reference to medial-lateral axis  12 , cranial-caudal axis  14 , and anterior-posterior axis  16 . More particularly, patients  10  may move such that portions of their backs (e.g., vertebrae) flex or extend (i.e., rotate about axes generally parallel to medial-lateral axis  12 ). For instance, patients  10  might lean forward or arch their backs. Patients  10  may also move such that portions of their backs rotate about axes generally parallel to cranial-caudal axis  14 . For instance, patients  10  might twist their torsos to look behind themselves. Furthermore, patients  10  might bend to one side (or the other) thereby causing portions of their backs to rotate about axes parallel to anterior-posterior axis  16 . Moreover, patients  10  might move such that portions of their backs translate relative to other portions of patients&#39;  10  backs along axes  12 ,  14 , or  16 . It is also likely that movements of patients  10  will involve various combinations of the aforementioned degrees of freedom. 
       FIG. 2  depicts a human axial skeleton including a skull (composed of numerous cranial bones (such as parietal bones, temporal bones, zygomatic bones, mastoid bones, maxilla bones, mandible bones, etc.) and spine  20  including numerous vertebrae  22 , intervertebral disks, etc. As discussed previously, spine  20  carries loads imposed on the patient&#39;s body and generated by patient  10 . Vertebrae  22  cooperate to allow spine  20  to extend, flex, rotate, etc. (as discussed with reference to  FIG. 1 ) under the influence of various muscles, tendons, ligaments, etc. attached to spine  20 . Spine  20  can also cooperate with various muscles, tendons, ligaments, etc. to cause other anatomical features of the patient&#39;s body to move. However, certain conditions can cause damage to spine  20 , vertebrae  22 , intervertebral disks, etc. and can impede the ability of spine  20  to move in various manners. These conditions include, but are not limited to abnormal curvature, injury, infections, tumor formation, arthritic disorders, puncture, or slippage of the intervertebral disks, and injuries or illness such as spinal stenosis and prolapsed disks As some of these conditions progress, or come into existence, various symptoms can indicate the desirability of stabilizing spine  20  or portions thereof. As a result of various conditions, the ability of patient  10  to move, with or without pain or discomfort, can be impeded. Based on such indications, medical personnel can recommend attaching one or more spinal stabilization systems to vertebrae  22  among other remedial actions such as physical therapy. 
     It may be helpful at this juncture to briefly describe portions of vertebrae  22 . Spinous processes and transverse processes allow tendons, muscles, etc. to attach to spine  20  for movement of spine  20  and various anatomical structures which are attached to spine  20  or affected thereby in various manners. These anatomical structures can include the patient&#39;s ribs, hips, shoulders, head, legs, etc. Spinous processes extend generally in a posterior and slightly inferior direction from vertebrae  22 . Transverse processes also extend generally laterally from vertebrae  22  and allow muscles and tendons to attach to vertebra  22 . Vertebral facets join adjacent vertebrae  22  to each other while allowing motion there between by being in sliding contact with corresponding vertebral facets of these adjacent vertebrae  22 . During certain types of motion of spine  20  (such as flexing and extending) caused (or resisted) by various muscles, vertebrae  22  tend to rotate relative to each other about axes of rotation generally in the vertebral bodies (and more particularly proximal to points about one third of the anterior-posterior length of the vertebral bodies away from the posterior surface of these vertebral bodies). Since vertebral facets allow vertebrae  22  to articulate about these axes of rotation, no, or little, reactionary forces or moments are generated by healthy spines  20  themselves during ordinary movements. 
     Previously available approaches to dynamically stabilizing spine  20  include attaching stabilization rods to spine  20  in manners causing the rods to lie posterior to the spinous processes and therefore anatomically distant from intravertebral areas in which the vertebral axes of rotation lie. Since such previously available stabilization rods are distant from the vertebral axes of rotation they tend to generate reaction forces which resist movement of spine  20 . Thus, as spine  20  extends or flexes, these previously available stabilization rods impede movement of spine  20 . More particularly, the distances between vertebral axes of rotation can act as moment arms thereby generating moments and forces on spine  20 . Therefore, spine  20  can cause reaction forces on the previously available spinal stabilization systems that can degrade the mechanical integrity and functioning of such spinal stabilization systems. Moreover, because such moments and forces (or their reactions) act on spine  20 , spine  20  and patient  10  comfort and health can be adversely affected. As a result, the range of motion and patient comfort could be adversely affected with previously available spinal stabilization approaches. In addition, the moments and forces generated due to the anatomically significant distances between the vertebral axes of rotation and the previously available spinal stabilization systems can degrade the mechanical integrity of and functioning of such spinal stabilization systems. 
       FIG. 3  depicts one embodiment of spinal stabilization system  100 . Spinal stabilization system  100  includes at least one spinal stabilization rod  102  and one or more clamping collars  104  and pivoting collars  106 . More particularly, as shown in  FIG. 3 , spinal stabilization system  100  can include a pair of spinal stabilization rods  102 , two pairs of clamping collars  104 , and a pair of pivoting collars  106 . Pairs of clamping collars  104  can be attached to vertebrae  22  on the opposite sides of spinous process by bone screws, anchors, wires, etc. as can pairs of pivoting collars  106 . Spinal stabilization rods  102  can be positioned on opposite sides of the vertebral spinous processes as shown. Furthermore, clamping collars  104  can securely clamp spinal stabilization rods  102  so that clamping collars  104  hold adjacent pairs of vertebrae  22  in fixed relationship to each other as shown by  FIG. 3 . Thus, spinal stabilization system  100  can statically stabilize these particular vertebrae  22 . Over time, these particular statically stabilized vertebrae  22  may grow together to form one boney mass thereby fusing and permanently stabilizing these vertebrae  22 . 
     However, as indicated by some patient  10  conditions, it may be desirable to dynamically stabilize some other particular vertebra  22 ′ with respect to other vertebrae  22 . For instance, medical personnel may deem it desirable to allow vertebra  22 ′ to translate relative to other vertebrae  22  while also allowing selected amounts of rotation of vertebra  22 ′. For instance, surgical personnel may deem it desirable that vertebra  22 ′ be allowed to rotate relative to one or more axis  12 ,  14 , or  16  (see  FIG. 1 ). In such situations, among others, medical personnel may attach pivoting collars  106  to vertebra  22 ′ and to engage pivoting collar  106  with spinal stabilization rod  102 . 
     More specifically, medical personnel may select spinal stabilization rod  102  which includes rigid portion  108  and flexible portion  110 . Rigid portion  108  can be of a material, shape, and dimension sufficient to withstand various loads (forces, moments, torques, etc.) expected to be applied to vertebrae  22 . Flexible portion  110  can be of a material, shape, and dimensions to withstand selected loads on vertebra  22 ′ (and adjacent vertebrae  22 ) while allowing relatively unrestricted motion in response to (or to generate) other loads. Flexible portions  110  of spinal stabilization rods  102  can pivotably and slidably engage pivoting collars  106  as discussed herein. 
     With reference now to  FIG. 4 , one embodiment of spinal stabilization system  100  is illustrated. More specifically,  FIG. 4  illustrates spine  20  with vertebra  22  and  22 ′ and intervertebral disks  28  along with the following components of spinal stabilization system  100 : spinal stabilization rod  102 , clamping collars  104 , pivoting collar  106 , and bone screws  112  and  114 . Bone screws  112  and  114  can attach collars  104  and  106  to vertebrae  22  and  22 ′ respectively. Bone screws  112  can be made of materials and have shapes and dimensions sufficient to withstand loads expected to be imposed thereon. In some embodiments, loads experienced by bone screws  114  can be less than those experienced by bone screws  112  since pivoting collars  106  might not constrain spinal stabilization rod  102  in as many directions as clamping collars  104 . Bone screws  114 , which attach pivoting collar  106  to vertebra  22 ′ can be made of materials and have shapes and dimensions sufficient to withstand loads expected to be imposed thereon. While  FIG. 4  illustrates bone screws  112  and  114  attaching collars  104  and  106  to vertebrae  22  and  22 ′ those skilled in the art will understand that other types of attachment devices can be used in conjunction with collars  104  and  106 . 
     Moreover,  FIG. 4  illustrates that rigid portion  108  of spinal stabilization rod  102 , can engage, and be securely clamped by, clamping collars  104 .  FIG. 4  also illustrates that flexible portion  110  of spinal stabilization rod  102  can pivotably and slidably engage pivoting collar  106 . Thus,  FIG. 4  illustrates that certain vertebrae  22  can be statically stabilized while other vertebrae  22  and  22 ′ can be dynamically stabilized. For illustrative purposes,  FIG. 4  also shows that spinal stabilization rod  102  can be in some reference position which, in  FIG. 4 , is shown as being generally straight through collars  104  and  106 . However, it is understood that other reference positions for spinal stabilization rod  102  are possible and within the scope of the disclosure. For instance, spinal stabilization rod  102  might have certain portions which are intentionally bent by surgical personnel during implantation or that flexible portion  110  might be curved due to the relative positions and orientations of vertebrae  22  and  22 ′. 
     Now with reference to  FIG. 5 , a cross sectional view of one embodiment of spinal stabilization system  100  is illustrated. More specifically,  FIG. 5  illustrates cranial-caudal axis  14 , anterior-posterior axis  16 , vertebrae  22 ,  22 ′, spinal stabilization system  100 , spinal stabilization rod  102 , clamping collar  104 , pivoting collar  106 , rigid portion  108  of spinal stabilization rod  102 , flexible portion  110  of spinal stabilization rod  102 , bone screws  112  and  114 , closure member  115 , transition portion  116  of spinal stabilization rod  102 , bore  117  of pivoting collar  106 , internal surfaces  118  and  119  and points of contact  120  and  122  of pivoting collar  106 . 
     Among other features of various embodiments,  FIG. 5  illustrates that closure member  115  can be used in conjunction with clamping collar  104  to securely clamp rigid portion  108  of spinal stabilization rod  102  in place.  FIG. 5  also illustrates that spinal stabilization rod  102  can include transition portion  116  between rigid portion  108  and flexible portion  110 . In some embodiments, the strengths of rigid portion  108  and flexible portion  110  can be determined by their respective diameters (or other dimensions, shapes, etc.) particularly when spinal stabilization rod  102  is formed from one continuous material such as polyetheretherketone (PEEK). 
     With regard to the engagement between spinal stabilization rod  102  and pivoting collar  106 ,  FIG. 5  illustrates that pivoting collar  106  can define a bore  117  or other cavity. Bore  117  can extend through the body of pivoting collar  106  generally in parallel with cranial-caudal axis  14 . Bore  117  can further define internal surfaces  118  and  119  which can be shaped and dimensioned to allow spinal stabilization rod  102  (or certain portions thereof) to pivot about contact points  120  and  122  within bore  117 . Spinal stabilization rod  102  can also slidably engage internal surfaces  118  and  119 . In some embodiments, pivoting collar  106  is made from titanium and internal surfaces  118  and  119  are polished to a finish sufficient to reduce sliding friction between internal surfaces  118  and  119  and spinal stabilization rod  102 . Furthermore, spinal stabilization rod  102  can be made of a material such as PEEK which has a low coefficient of friction with the material of pivoting collar  106 . Thus, spinal stabilization rod  102  can both translate and pivot relative to pivoting collar  106  thereby allowing vertebra  22 ′ to translate and rotate relative to vertebra  22 . By allowing vertebrae  22 ′ and  22  to translate and rotate relative to each other, vertebrae  22 ′ and  22  can rotate about their natural centers of rotation. Thus, loads imposed on, and generated by, spine  20  (see  FIG. 2 ) and spinal stabilization system  100  can be reduced if not eliminated by spinal stabilization rod  102  and pivoting collar  106 . 
     For instance, a comparison of  FIGS. 5 and 6  illustrates that some movement of patient  10  might cause pivoting collar  106  to rotate through angle “a” about medial-lateral axis  12  and away from anterior-posterior axis  16 . As pivoting collar  106  rotates through angle “a”, spinal stabilization rod  102  can slide along internal surfaces  118  and  119 . Additionally, points of contact  120  and  122  can move as a result of the kinematic interaction between spinal stabilization rod  102  and pivoting collar  106 . For instance, when pivoting collar  106  rotates clockwise (as shown in  FIGS. 5 and 6 ), upper contact point  120  can move to the right while lower contact point  122  can move to the left. Should pivoting collar  106  rotate counterclockwise, upper contact point  120  can move to the left while lower contact point  122  can move to the right. No matter which direction pivoting collar  106  rotates, spinal stabilization rod  102  can pivot about contact points  120  and  122  within bore  117 . 
       FIGS. 5 and 6  also illustrate flexible portion  110  of spinal stabilization rod  102 . Flexible portion  110  can be made of the same material as rigid portion  108  of spinal stabilization rod  102 . In some embodiments, spinal stabilization rod  102  can include transition portion  116  between rigid portion  108  and flexible portion  110 . In some embodiments, flexible portion  110  can be similar to rigid portion  108  except perhaps having different, and smaller, dimensions. For instance, spinal stabilization rod  102  can have a generally circular cross section and rigid portion  108  can have diameter “d 1 ” while flexible portion  110  can have another and smaller dimension “d 2 .” Thus, flexible portion  110  can be more flexible than rigid portion  108 . In some embodiments, the smaller dimensions of flexible portion  110  can facilitate the pivoting of spinal stabilization rod  102  in bore  117  of pivoting collar  106 . 
     With reference now to  FIG. 7 , internal surfaces  118  and  119  of pivoting collar  106  can be shaped and dimensioned so that internal surfaces  118  and  119  limit the range of motion of spinal stabilization rod  102 . In some embodiments, internal surfaces  118  and  119  can define lips  124  around the outer periphery of bore  117 . Lips  124  can be raised relative to the general contour of internal surfaces  118  and  119 . Thus, as spinal stabilization rod  102  pivots it comes into contact with at least one lip  124  and is therefore constrained from further motion. Moreover, it can be the case that spinal stabilization rod  102  comes into contact with one lip  124  on upper internal surface  118  and another lip  124  on lower surface  119  and on the opposite side of bore  117 . In some embodiments, lips  124  are shaped, dimensioned, and position to limit the range of motion of spinal stabilization rod  102  to the same limit (e.g., 6 degrees). In some embodiments, each lip  124  can be shaped, dimensioned, and positioned to limit the range of motion of spinal stabilization rod  102  to selected values. 
     With reference now to  FIG. 8 , in some embodiments, bore  217  of pivoting collar  106  can be shaped and dimensioned to limit the range of motion of spinal stabilization rod  102  to the same limit in all directions. In some embodiments, however, bore  217  can be shaped and dimensioned to provide selected limits to the range of motion of spinal stabilization rod  102  in differing directions. More specifically, bore  217  can have an oval cross sectional shape as shown with major diameter d 3  and minor diameter d 4 . Furthermore, bore  117  can define lateral surfaces and corresponding contact points thereon. Thus, spinal stabilization rod  102  can have one range of motion (defined by diameter d 3 ) to pivot about medial-lateral axis  12  and another range of motion (defined by diameter d 4 ) to pivot about anterior-posterior axis  16 . In some embodiments, Diameter d 3  or d 4  can correspond to diameter d 2  of flexible portion  110  of spinal stabilization rod  102 . Thus, pivoting collar  106  can provide some range of motion in one direction while limiting spinal stabilization rod  102  to less or no motion in another direction. 
     With reference now to  FIG. 9 , one embodiment of spinal stabilization system  100  is shown. More particularly, flexible portion  110  is shown engaged with pivoting collar  106 .  FIG. 9  further illustrates that upper internal surface  118  can have radius of curvature r 1  and lower internal surface  119  can have radius of curvature r 2 . Flexible portion  110  of spinal stabilization rod  102  is shown in  FIG. 9  as having pivoted about contact points  120  and  122  up to the range of motion permitted by internal surfaces  118  and  119 . Moreover, flexible portion  110  of spinal stabilization rod  102  is also shown as having flexed around internal surfaces  118  and  119  so that it follows radii of curvature r 1  and r 2  for at least some portion of its length. Because spinal stabilization rod  102  has flexed to some degree, spinal stabilization rod  102  can act as a spring and resist further motion. The amount of resistance of spinal stabilization rod  102  to further movement can be tailored by selected the material, shape, dimensions, etc. of flexible portion  110  to provide a desired spring constant. Thus, the resistance to movement provided by flexible portion  110  can change (e.g., increase or decrease) linearly (or otherwise) with that motion. 
     To attach embodiments of spinal stabilization system  100  to spine  20 , surgical personnel can prepare patient  10  for surgery and open an incision generally near spine  20 . In some embodiments, surgical personnel can use a posterior approach to spine  20  to attach spinal stabilization system  100  to spine  20 . Surgical personnel can attach one or more clamping collars  104  to selected vertebrae  22 . Surgical personnel can also attach one or more pivoting collars  106  to other selected vertebrae  22 . Surgical personnel may then engage pivoting collar  106  with flexible portion  110  of spinal stabilization rod  102 . More specifically, surgical personnel can insert flexible portion  110  through bore  117  of pivoting collar  106 . 
     Surgical personnel can place rigid portion  108  of spinal stabilization rod  102  in, or near, clamping collar  104 . If desired, surgical personnel can reduce spinal stabilization rod  102  into clamping collar  104 . With spinal stabilization rod  102  seated in clamping collar  104 , surgical personnel can advance closure member  115  (see  FIGS. 4  or  5 ) toward spinal stabilization rod  102 . Closure member  115  can clamp spinal stabilization rod  102  against the seat of clamping collar  104  thereby locking spinal stabilization rod  102  in place in clamping collar  104 . Surgical personnel can evaluate spinal stabilization system  100  and make adjustments as desired before closing the incision. 
     Thus, patients  10  treated with spinal stabilization system  100  (see  FIG. 1 ) may flex and extend their backs. Patients  10  may also rotate their torsos and bend side-to-side. Accordingly, patients  10  may enjoy greater ranges of motion while experiencing less discomfort than previously possible. Spinal stabilization systems  100  and, more particularly, pivoting collars  106  can be smaller and therefore less intrusive than clamping collars  104 . Spinal stabilization systems  100  of various embodiments can be simpler and have fewer parts (e.g., closure member  115 ) than previously available spinal stabilization systems. 
     Although embodiments have been described in detail herein, it should be understood that the description is by way of example only and is not to be construed in a limiting sense. It is to be further understood, therefore, that numerous changes in the details of the embodiments and additional embodiments will be apparent, and may be made by, persons of ordinary skill in the art having reference to this description. It is contemplated that all such changes and additional embodiments are within scope of the claims below and their legal equivalents.