Abstract:
Several aspects of alignment systems and methods used in spinal surgery are disclosed. For instance, in one aspect, there is disclosed an alignment instrument comprising a first alignment member for coupling to a spinal stabilization system at one location, a second alignment member for coupling to the spinal stabilization system at a second location, and a linkage assembly for coupling the first alignment member to the second alignment member.

Description:
CROSS-REFERENCES AND CLAIM OF PRIORITY  
       [0001]     This application claims priority to U.S. Provisional Patent Application Ser. No. 60/711,812, filed on Aug. 26, 2005, which is incorporated herein by reference.  
         [0002]     This application is related to commonly assigned U.S. Provisional Application Ser. No. 60/786,898, entitled “FULL MOTION SPHERICAL LINKAGE IMPLANT SYSTEM,” filed Mar. 29, 2006; U.S. Provisional Application Ser. No. 60/793,829, entitled “MICRO-MOTION IMPROVEMENTS,” filed on Mar. 29, 2006; U.S. Provisional Application Ser. No. 60/831,879, entitled “LOCKING ASSEMBLY,” filed on Jul. 19, 2006; U.S. Utility Application Serial No. 11/443,236, entitled, “SYSTEM AND METHOD FOR DYNAMICAL SKELETAL STABILIZATION,” filed on May 30, 2006; U.S. Provisional Application Ser. No. 60/692,943, entitled “SPHERICAL MOTION DYNAMIC SPINAL STABILIZATION DEVICE,” filed Jun. 22, 2005; International Patent Application No. PCT/US05/27996, entitled “SYSTEM AND METHOD FOR DYNAMIC SKELETAL STABILIZATION,” filed Aug. 8, 2005, and to commonly assigned U.S. patent application Ser. No. 10/690,211, entitled “SYSTEM AND METHOD FOR STABILIZING INTERNAL STRUCTURES,” filed Oct. 21, 2003, all of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0003]     This disclosure relates to skeletal stabilization and, more particularly, to aligning dynamic stabilization systems for the stabilization of human spines.  
       BACKGROUND  
       [0004]     The human spine is a complex structure designed to achieve a myriad of tasks, many of them of a complex kinematic nature. The spinal vertebrae allow the spine to flex in three axes of movement relative to the portion of the spine in motion. These axes include horizontal movement (bending either forward/anterior or aft/posterior), rolling movement (bending to either left or right side) and vertical movement (twisting of the shoulders relative to the pelvis).  
         [0005]     In flexing about the horizontal axis into flexion (bending forward or in an anterior direction) and extension (bending backward or in a posterior direction), vertebrae of the spine must rotate about the horizontal axis to various degrees. The sum of all such movement about the horizontal axis produces the overall flexion or extension of the spine. For example, the vertebrae that make up the lumbar region of the human spine move through roughly an arc of 15° relative to adjacent or neighboring vertebrae. Vertebrae of other regions of the human spine (e.g., the thoracic and cervical regions) have different ranges of movement. Thus, if one were to view the posterior edge of a healthy vertebra, one would observe that the edge moves through an arc of some degree (e.g., of about 15° in flexion and about 5° in extension if in the lumbar region) centered about a center of rotation. During such rotation, the anterior (front) edges of neighboring vertebrae move closer together, while the posterior edges move farther apart, compressing the anterior of the spine. Similarly, during extension, the posterior edges of neighboring vertebrae move closer together while the anterior edges move farther apart, thereby compressing the posterior of the spine. During flexion and extension the vertebrae move in horizontal relationship to each other providing up to 2-3 mm of translation.  
         [0006]     In a normal spine, the vertebrae also permit right and left lateral bending. Accordingly, right lateral bending indicates the ability of the spine to bend over to the right by compressing the right portions of the spine and reducing the spacing between the right edges of associated vertebrae. Similarly, left lateral bending indicates the ability of the spine to bend over to the left by compressing the left portions of the spine and reducing the spacing between the left edges of associated vertebrae. The side of the spine opposite that portion compressed is expanded, increasing the spacing between the edges of vertebrae comprising that portion of the spine. For example, the vertebrae that make up the lumbar region of the human spine rotate about an axis of roll, moving through an arc of around 10° relative to neighbor vertebrae throughout right and left lateral bending.  
         [0007]     Rotational movement about a vertical axis relative is also natural in the healthy spine. For example, rotational movement can be described as the clockwise or counter-clockwise twisting rotation of the vertebrae during a golf swing.  
         [0008]     In a healthy spine, the inter-vertebral spacing between neighboring vertebrae is maintained by a compressible and somewhat elastic disc. The disc serves to allow the spine to move about the various axes of rotation and through the various arcs and movements required for normal mobility. The elasticity of the disc maintains spacing between the vertebrae during flexion and lateral bending of the spine, thereby allowing room or clearance for compression of neighboring vertebrae. In addition, the disc allows relative rotation about the vertical axis of neighboring vertebrae, allowing twisting of the shoulders relative to the hips and pelvis. A healthy disc further maintains clearance between neighboring vertebrae, thereby enabling nerves from the spinal chord to extend out of the spine between neighboring vertebrae without being squeezed or impinged by the vertebrae.  
         [0009]     In situations where a disc is not functioning properly, the inter-vertebral disc tends to compress, thereby reducing inter-vertebral spacing and exerting pressure on nerves extending from the spinal cord. Various other types of nerve problems may be experienced in the spine, such as exiting nerve root compression in the neural foramen, passing nerve root compression, and enervated annulus (where nerves grow into a cracked/compromised annulus, causing pain every time the disc/annulus is compressed), as examples. Many medical procedures have been devised to alleviate such nerve compression and the pain that results from nerve pressure. Many of these procedures revolve around attempts to prevent the vertebrae from moving too close to one another in order to maintain space for the nerves to exit without being impinged upon by movements of the spine.  
         [0010]     In one such procedure, screws are embedded in adjacent vertebrae pedicles and rigid rods or plates are then secured between the screws. In such a situation, the pedicle screws press against the rigid spacer that serves to distract the degenerated disc space, thereby maintaining adequate separation between the neighboring vertebrae to prevent the vertebrae from compressing the nerves. Although the foregoing procedure prevents nerve pressure due to extension of the spine, when the patient then tries to bend forward (putting the spine in flexion), the posterior portions of at least two vertebrae are effectively held together. Furthermore, the lateral bending or rotational movement between the affected vertebrae is significantly reduced due to the rigid connection of the spacers. Overall movement of the spine is reduced as more vertebrae are distracted by such rigid spacers. This type of spacer not only limits the patient&#39;s movements, but also places additional stress on other portions of the spine, such as adjacent vertebrae without spacers, often leading to further complications at a later date.  
         [0011]     In other procedures, dynamic fixation devices are used. However, conventional dynamic fixation devices may not facilitate lateral bending and rotational movement with respect to the fixated discs. This can cause further pressure on the neighboring discs during these types of movements, which over time may cause additional problems in the neighboring discs. Furthermore, alignment of such dynamic fixation devices to enable a relatively natural range of motion while restricting undesirable motion is often difficult.  
         [0012]     Accordingly, improvements are needed in alignment instruments for aligning dynamic systems that approximate and enable a fuller range of motion while providing stabilization of a spine.  
       SUMMARY  
       [0013]     Several aspects of alignment systems and methods used in spinal surgery are disclosed. For instance, in one aspect, there is disclosed an alignment instrument comprising a first alignment member for coupling to a spinal stabilization system at one location, a second alignment member for coupling to the spinal stabilization system at a second location, and a linkage assembly for coupling the first alignment member to the second alignment member.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is emphasized that various features may not be drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.  
         [0015]      FIG. 1A  is an isometric view of a portion of a spine.  
         [0016]      FIG. 1B  is a perspective view of one embodiment of a dynamic stabilization system.  
         [0017]      FIG. 2  is a simplified diagrammatic perspective view of the dynamic stabilization system of  FIG. 1B .  
         [0018]      FIGS. 3A and 3B  are perspective views of the simplified dynamic stabilization system of  FIG. 2  in a generally neutral position and in flexion/extension, respectively.  
         [0019]      FIGS. 4A and 4B  are perspective views of the simplified dynamic stabilization system of  FIG. 2  in a generally neutral position and in lateral bending, respectively.  
         [0020]      FIGS. 5A and 5B  are perspective views of the simplified dynamic stabilization system of  FIG. 2  in a generally neutral position and in rotation, respectively.  
         [0021]      FIG. 6  is a perspective view of one embodiment of an alignment instrument in use with a dynamic stabilization system, such as the dynamic stabilization system of  FIG. 1B .  
         [0022]      FIGS. 7A and 7B  are perspective views of another embodiment of an alignment instrument in use with a dynamic stabilization system.  
         [0023]      FIG. 8  is a perspective view of yet another embodiment of an alignment instrument in use with a dynamic stabilization system.  
         [0024]      FIG. 9  is a flow chart of one embodiment of a method for substantially aligning a dynamic stabilization system.  
         [0025]      FIGS. 10A-10E  are photographs illustrating the method of  FIG. 9 .  
         [0026]      FIG. 11  is a flow chart of another embodiment of a method for substantially aligning a dynamic stabilization system.  
         [0027]      FIG. 12  is a perspective view of another embodiment of an alignment instrument for use with a dynamic stabilization system.  
         [0028]      FIG. 13  is a side view of the alignment instrument of  FIG. 12 .  
         [0029]      FIG. 14  is a flow chart of yet another embodiment of a method for substantially aligning a dynamic stabilization system. 
     
    
     DETAILED DESCRIPTION  
       [0030]     It is to be understood that the following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.  
         [0031]     Referring to  FIG. 1A , a portion of a spine  10  is shown in an isometric view. The spine portion  10  includes a vertebra  12  and a lower vertebra  14 . In an actual spine, an intervertebral disc (not shown) may be located above a vertebral plate  16  of the vertebra  12 , but is omitted for clarity. Furthermore, an upper adjacent vertebra (similar to vertebra  12 ) may be positioned above the intervertebral disc, but this upper adjacent vertebra is also omitted for clarity. In the present example, imaginary “x”, “y”, and “z” axes are superimposed upon the spine portion  10 . The intersection of the axes may be defined to be a center point “A” which, for purposes of this discussion, is positioned above the vertebral plate  16  within the intervertebral space.  
         [0032]     Natural spine motion may be modeled in relation to the x, y, and z axes. As previously discussed, flexion or extension movement may be modeled as a rotation of the vertebra about the x-axis. Lateral bending (bending towards the right or left) may be modeled as rotation about the z-axis. Rotation (twisting the torso in relation to the legs) may be modeled as rotation about the y-axis. Thus, the relative natural movement of the vertebrae of the spine  10  may occur in three dimensions with respect to the three illustrated axes.  
         [0033]     Generally, a dynamic stabilization system that may be used to stabilize the vertebra  12  with respect to an upper vertebra (not shown) facing the endplate  16  may be oriented so that various axes of the dynamic stabilization system are aligned with a center or centroid of rotation (i.e., the center point labeled “A”). To facilitate such alignment, an alignment system may be used that enables a surgeon to align the various axes of the dynamic stabilization system with the center point “A”.  
         [0034]     As will be described later in greater detail, the alignment instrument may be used to align the dynamic stabilization system so that the dynamic stabilization system moves along the surface of an imaginary three dimensional curved body, such as a sphere or ellipsoid. For discussion purposes, a sphere  18  is shown superimposed upon spine portion  10 . The center of the sphere  18  is at the center of rotation “A.” The dynamic stabilization system may be aligned so that a point on an upper vertebra (not shown) may move in relation to a corresponding point on the vertebra  12  by following a path that is generally restricted to the surface of the sphere  18  (or other three dimensional shape).  
         [0035]     For instance, using the sphere  18  as an example, assume a path has a starting point at point  20  which is on the surface of the sphere  18 . Further assume that the path has an ending point  22  which is also on the surface of the sphere  18 . Thus, it can be seen that the path between point  20  and point  22  that follows the surface of the sphere  18  has a vertical component  24  and a horizontal component  26 . Movement that is restricted to the vertical curved component  24  is considered to be two dimensional movement or rotation about the x-axis. Movement that is restricted to the horizontal component  26  is also two dimensional movement, but represents rotation about the y-axis. The combination of the vertical curved component and the horizontal curved component represents three-dimensional movement about the center of rotation “A”.  
         [0036]     If the path between points is restricted to the surface of a sphere, the path will have a constant radius of curvature “R” with respect to the center of rotation “A.” In some embodiments, the horizontal component  26  may have a radius of curvature R and the vertical component  24  may have a radius of curve R′. Thus, if the radii of curvature R equals R′ and they have the same center of rotation, the path would be on a sphere as illustrated in  FIG. 1A . However, if R′ does not equal R, then the imaginary three dimensional curved body may be an ellipsoid or another three dimensional surface.  
         [0037]     The desired location of the center point “A” with respect to the endplate  16  may vary depending on factors such as the patient&#39;s particular spinal structure and the desired result of the operation, and the surgeon may need to position the center point at a particular location within a range of possible locations. Such positioning may entail moving the center point “A” within a two or three dimensional space along any or all of the x-axis, y-axis, and z-axis. Accordingly, the alignment instrument may enable the surgeon to position the center point “A” where desired (within limitations imposed by the spinal structure and/or the alignment instrument itself) and to maintain the alignment of the dynamic stabilization system with the center point when the center point is repositioned.  
         [0038]     It is understood that, although the center point “A” may represent the center of rotation of a sphere as illustrated in  FIG. 1A , it is not limited to a single discrete point or a sphere. For example, the center of rotation may be a spherical or ellipsoidal area around which a dynamic stabilization system may rotate. Accordingly, the term “center of rotation” as used in the present disclosure is for purposes of illustration and is not limited to a single discrete point or to rotation around a spherical body.  
         [0039]     Referring to  FIGS. 1B and 2 , an embodiment of a dynamic stabilization system  100  having a spinal stabilization device  101  will now be described. It is understood that the particular dynamic stabilization system  100  and device  101  described herein are for purposes of example only, and that the various embodiments of alignment instruments disclosed in the present application may be used with spinal stabilization systems and devices other than those illustrated in  FIG. 1B . Furthermore, it is understood that the dynamic stabilization system  100  may include multiple spinal stabilization devices.  
         [0040]     The dynamic stabilization system  100  may be designed to permit a limited degree of movement between neighboring vertebrae in flexion/extension, lateral bending, and rotation directions, while restraining the degree of movement generally along an imaginary shell (e.g., a three dimensional shape) about a center of rotation “A”. In the present example, the shell is generally spherical and the center of rotation may lie at the origin of the spherical shell. However, it is understood that the shell may be another shape, such as an ellipsoid. Accordingly, the present disclosure is not limited to a center of rotation within a spherical shell.  
         [0041]     The dynamic stabilization system  100  may include bone anchors  102  and  104 , which may be coupled to polyaxial heads  106  and  108 , respectively. The polyaxial head  106  may include a slot  110  formed by sidewalls  112  and  114 , and the polyaxial head  108  may include a slot  116  formed by sidewalls  118  and  120 . An interior portion of each sidewall  112 ,  114 ,  116 , and  118  may be threaded to receive a locking cap  122  or  124 . The slots  110  and  116  may be configured to receive an extension (e.g., a rod)  126  and  128 , respectively, that may form part of the spinal stabilization device  101 . Each polyaxial head  106  and  108  may move relative to a longitudinal axis of their respective bone anchor until locked down by tightening the respective locking caps  122  and  124  against extensions  126  and  128 . The dynamic stabilization system  100  may also include the spinal stabilization device  101 , which may be coupled to the polyaxial heads  106  and  108 .  
         [0042]     The spinal stabilization device  101  may include extensions  126  and  128 . In the present embodiment, the extensions  126  and  128  may be coupled by a flexible support column  130 . The flexible support column  130  may include a collar  132  and a collar  134  with a resilient member  136 , such as a coil spring, positioned therebetween.  
         [0043]     The spinal stabilization device  101  may also include an elbow  138  having an upper member  140  and a lower member  142  pivotally interconnected at pivot  144 . The distal end  146  (relative to pivot  144 ) of upper member  140  may be pivotally connected to collar  132  at pivot  148  and the distal end  150  (relative to pivot  144 ) of lower member  142  may be pivotally connected to collar  134  at pivot pin  152 .  
         [0044]     In the present embodiment, the elbow  138  may be designed so that an axis passing longitudinally through each pivot pin  144 ,  148 , and  152  (e.g., axes  154 ,  156 , and  158 , respectively) intersects a center of rotation “A”. It is understood that factors such as the length of the upper member  140  and lower member  142 , the angle of the pivot pins  144 ,  148 , and  152 , and an amount of curvature in each of the upper and lower members, may alter the location of the center of rotation “A”. Due to the design of the elbow  138 , the spinal stabilization device  101  may allow flexion/extension, rotation, and/or lateral bending while the axes  154 ,  156 , and  158  maintain their intersection with the center of rotation “A”.  
         [0045]     In operation, bone anchors  102  and  104  may be attached to respective vertebrae (not shown) by screwing threaded portions of each anchor into the bone of the vertebrae. The polyaxial heads  106  and  108  may be coupled to their respective bone anchors  102  and  104  either before or after the bone anchors are inserted in the vertebrae. The first and second extensions  126  and  128  may be placed into the slots  110  and  116 . At this point, the polyaxial heads  106  and  108  may move with respect to the bone anchors  102  and  104 , respectively, to allow for proper positioning of the spinal stabilization device  101 . Once in position, locking caps  122  and  124  may be tightened to lock the extensions  126  and  128  into position. This may also force extensions  126  and  128  into bone anchors  102  and  104 , respectively, thereby locking polyaxial heads  106  and  108  into position.  
         [0046]     In certain embodiments, a damping element (e.g., the spring  136 ) may be installed between the vertebrae in a somewhat compressed condition to provide a vertical force for at least partially unloading an inter-vertebral disc, and to allow limited axial and bending movement between the neighboring vertebrae. In some embodiments, a partial disc replacement (PDR) element (not shown) may be used to provide interior support between the vertebrae.  
         [0047]     Referring to  FIG. 2 , one embodiment of a simplified spinal stabilization device, such as the spinal stabilization device  101  of  FIG. 1B , is illustrated. In the present example, the distal end  146  of upper member  140  is bent about an axis longitudinal to the upper member and about an axis perpendicular to the upper member, so that, when the elbow  138  is positioned in its approximately middle position (e.g., as depicted in  FIG. 1B ), the axis  156  of pivot  148  points downwardly and inwardly towards the center of rotation “A”. The distal end  150  of lower member  142  may be similarly bent about an axis longitudinal to the lower member and about an axis perpendicular to the lower member, so that the axis  158  of pivot  152  points upwardly and inwardly towards the same point “A.” Proximal ends  160  and  162  of upper and lower members  140  and  142 , respectively, may also be shaped so that the axis  154  of pivot  144  coupling the proximal ends also points inwardly towards the same point “A”.  
         [0048]     Bone anchors  102  and  104  may be installed in vertebral bodies (not shown) such that point “A” may be located as illustrated, for example, in  FIG. 1B . Because the axis of each of the pivots  144 ,  146 , and  152  point generally towards the same center of rotation “A”, the elbow  138  may restrict movement of the pivots about an imaginary spherical shell having a center of rotation at “A” as the vertebrae move relative to one another in flexion/extension, rotation, and/or lateral bending. This may restrict movement of the anchors  102  and  104 , and hence the vertebrae themselves, to movement about the center of rotation “A”. In some embodiments, this spherical movement about a center of rotation may mimic a natural motion of adjacent vertebrae as they move generally about the center of a healthy, natural disc when cushioned by the disc.  
         [0049]     Referring to  FIGS. 3A and 3B , an embodiment of the simplified spinal stabilization device  101  of  FIG. 2  diagrammatically illustrates the generally spherical movement of the pivots  144 ,  146 , and  152  about the center of rotation “A” during flexion/extension. More specifically,  FIG. 3A  illustrates the position of the upper and lower members  140  and  142  in a generally middle or “neutral” position and  FIG. 3B  illustrates the position of the upper and lower members after flexion/extension, as would occur when a person bends forward. As illustrated, the axes  154 ,  156 , and  158  intersect the center of rotation “A” in either position.  
         [0050]     Referring to  FIGS. 4A and 4B , an embodiment of the simplified spinal stabilization device  101  of  FIG. 2  diagrammatically illustrates the generally spherical movement of the pivots  144 ,  146 , and  152  about the center of rotation “A” during lateral bending. More specifically,  FIG. 4A  illustrates the position of the upper and lower members  140  and  142  in a generally middle or “neutral” position and  FIG. 4B  illustrates the position of the upper and lower members after bending to the right and slightly forward.  
         [0051]     Referring to  FIGS. 5A and 5B , an embodiment of the simplified spinal stabilization device  101  of  FIG. 2  diagrammatically illustrates the generally spherical movement of the pivots  144 ,  146 , and  152  about the center of rotation “A” during rotation. More specifically,  FIG. 5A  illustrates the position of the upper and lower members  140  and  142  in a generally middle or “neutral” position and  FIG. 5B  illustrates the position of the upper and lower members after clockwise rotation, as would occur when a person turns to the right.  
         [0052]     Referring to  FIG. 6 , in one embodiment, an alignment instrument  600  may be used to align one or more dynamic stabilization systems  100 A and  100 B (e.g., the dynamic stabilization system  100  of  FIG. 1B ) to a center of rotation “A”. As described previously, portions of the dynamic stabilization system  100  may be configured to rotate around the center of rotation “A”. As illustrated in  FIG. 6 , the alignment instrument  600  may attach to the dynamic stabilization systems  100 A and  100 B and may be used to alter the position of one or both of the dynamic stabilization systems prior to locking the dynamic stabilization systems into position.  
         [0053]     In the present example, the alignment instrument  600  may include alignment members  602  and  604  that may be coupled by a linkage assembly  606 . The alignment member  602  may include a shaft  608  having a collar  610  near a proximal end thereof and a coupler  612  at a distal end thereof. The alignment member  604  may include a shaft  614  having a collar  616  near a proximal end thereof and a coupler  618  at a distal end thereof. Each coupler  612  and  618  may be configured to removably couple to a portion of a spinal stabilization system, such as a polyaxial head. The alignment members  602  and  604  may each have a longitudinal axis  620  and  622 , respectively. In the present example, the longitudinal axes  620  and  622  may extend from the proximal end of each alignment member  602  and  604  to the distal ends, and may intersect the center of rotation “A”.  
         [0054]     The linkage assembly  606  may include arms  624  and  626  that may be pivotally coupled to one another at a proximal end of each arm. The arm  624  may be coupled to alignment member  602  and the arm  626  may be coupled to alignment member  604 . In the present example, a distal end of each arm  624  and  626  may include a bore therethrough for receiving the proximal ends of the alignment members  602  and  604 , respectively. More specifically, the bores may each have a longitudinal axis that may intersect the center of rotation “A”. For example, the axes of the bores may coincide with the axes  620  and  622  of the alignment members  602  and  604 . The collars  610  and  616  may prevent the arms  624  and  626 , respectively, from movement in the direction of the distal ends of the alignment members  602  and  604 .  
         [0055]     The linkage assembly  606  may also include a guide mechanism, such as guide pin assembly  628 . In this embodiment, the guide pin assembly  628  may include a shaft  630  having a foot  632  at a distal end thereof and a knob  634  at a proximal end thereof. The foot  632  may be placed proximal to or in contact with an outer tissue layer of a patient, and the shaft  630  may be used for alignment purposes using, for example, fluoroscopy techniques. The knob  634  may be used to adjust the relative positions of the arms  624  and  626  and, accordingly, the corresponding alignment members  602  and  604 .  
         [0056]     As described previously, linkage assembly  606  and corresponding alignment members  602  and  604  may be designed to point towards the common center of rotation “A”. It is understood that the common center of rotation “A” may not be a fixed point, but may be a point where the axes  620  and  622  intersect. Accordingly, by adjusting the relative positions of the arms  624  and  626 , the positions of the corresponding alignment members  602  and  604  may be altered. This movement may shift the common center of rotation “A”, but both axes  620  and  622  may continue to intersect the common center of rotation “A” as it is moved.  
         [0057]     For example, if the knob  634  is moved to the right, the common center of rotation “A” may move to the left. Similarly, if the knob  634  is moved to the left, the common center of rotation “A” may move to the right. If the linkage assembly  606  is moved toward the distal ends of the alignment members  602  and  604 , the common center of rotation “A” may shift towards the distal ends of the alignment members. If the linkage assembly  606  is moved toward the proximal ends of the alignment members  602  and  604 , the common center of rotation “A” may shift away from the distal ends of the alignment members. As the center of rotation “A” is shifted, the arms  624  and  626  may maintain the alignment of the alignment members  602  and  604  with the center of rotation “A”.  
         [0058]     In addition, the alignment instrument  600  may be designed to substantially align the dynamic stabilization systems  100 A and  100 B so that the axes of  154 ,  156 , and  158  of pivots  144 ,  148 , and  152 , respectively, of the corresponding spinal stabilization devices  101 A and  101 B ( FIG. 1B ) point generally towards the common center of rotation “A”. In the present example, the linkage assembly  606  may be adjustable to accommodate variations in a distance “d” between polyaxial heads of the spinal stabilization devices  101 A and  101 B to which extensions may be secured as described with respect to  FIG. 1B . Furthermore, the linkage assembly  606  may be adjusted to accommodate variations in an angle “α” between the two alignment members  602  and  604 . The angle “α” may be the angle between axis  620  and axis  622  with respect to the common center of rotation “A.” The guide pin assembly  628  may also be substantially aligned with the common center of rotation “A” such that an axis  636  passing longitudinally through shaft  630  also passes through the common center of rotation “A”. The guide pin assembly  628  may be used to position the common center of rotation “A” by substantially aligning with anatomical landmarks on the patient.  
         [0059]     In operation, the alignment members  602  and  604  may be coupled to the dynamic stabilization systems  100 A and  100 B while coupled to the linkage assembly  606 . Alternatively, the alignment members  602  and  604  may be coupled to the dynamic stabilization systems  100 A and  100 B separately and then coupled to the linkage assembly  606 .  
         [0060]     Referring to  FIGS. 7A and 7B , in another embodiment, an alignment instrument  700  may be used to align one or more dynamic stabilization systems  100 A and  100 B (e.g., the spinal stabilization system  100  of  FIG. 1B ) to a center of rotation “A” (not shown). As described previously, portions of the dynamic stabilization systems  100 A and  100 B may be configured to rotate around the center of rotation “A”.  
         [0061]     In the present example, the alignment instrument  700  may include adjustable gripping pliers  702  and  704  coupled by a linkage assembly  706 . The adjustable gripping pliers  702  may include opposing handle portions  708  and  710  that may include couplers (e.g., a gripping means such as opposing jaws)  712  and  714 , respectively, at a distal end thereof. The couplers  712  and  714  may be configured to couple to various features of the dynamic stabilization system  100 B, including polyaxial heads (e.g.,  106  and  108  of  FIG. 1B ), collars (e.g.,  132  and  134  of  FIG. 1B ) and/or extensions (e.g.,  126  and  128  of  FIG. 1B ). For example, the couplers  712  and  714  may include a yoke feature for snapping onto extensions  126  and  128 . An adjustment screw  716  may be used to vary a distance between the couplers  712  and  714 . The adjustable gripping pliers  704  may include opposing handle portions  718  and  720  that may include couplers (e.g., a gripping means such as opposing jaws)  722  and  724 , respectively, at a distal end thereof. An adjustment screw  726  may be used to vary a distance between the couplers  722  and  724 . The adjustable gripping pliers  702  and  704  may be designed to point to a common center of rotation (not shown), as has been described with respect to previous embodiments.  
         [0062]     The linkage assembly  706  may include arms  728  and  730  that may be pivotally coupled to one another. The arm  730  may be coupled to the handle portion  708  (as illustrated) and/or to the handle portion  710 , and arm  728  may be coupled to the handle portion  718  (as illustrated) and/or to the handle portion  720 . The arms  728  and  730  may be coupled to the handle portions in such a way as to enable the linkage assembly  706  to rotate with respect to the adjustable gripping pliers  702  and  704 . A guide pin assembly  732  may be used to adjust and align the linkage assembly  706 . In the present example, linkage assembly  706  may be similar or identical to the linkage assembly  606  of  FIG. 6  except for the manner in which the arms  728  and  730  are coupled to the handle portions.  
         [0063]     The linkage assembly  706  may be adjustable to accommodate variations in a distance “d” ( FIG. 7B ) between polyaxial heads of dynamic stabilization systems  100 A and  100 B and to accommodate variations in an arc “α” between the adjustable gripping pliers  702  and  704 . As described with respect to the linkage assembly  606  of  FIG. 6 , the linkage assembly  706  may enable the adjustable gripping pliers  702  and  704  to be moved while maintaining alignment of the adjustable gripping pliers and the corresponding dynamic stabilization systems  100 B and  100 A with the common center of rotation. The guide pin assembly  732  may also be substantially aligned with the common center of rotation and may be used to locate the common center of rotation by alignment of the guide pin assembly with anatomical landmarks using techniques such as fluoroscopy.  
         [0064]     Referring to  FIG. 8 , in another embodiment, a portion  800  of an alignment instrument is illustrated. The alignment instrument may be used to align one or more dynamic stabilization systems (e.g., the spinal stabilization system  100  of  FIG. 1B ) to a center of rotation “A” (not shown). As described previously, portions of the dynamic stabilization system  100  may be configured to rotate around the center of rotation “A”.  
         [0065]     The illustrated portion  800  of the alignment instrument may be coupled to the dynamic stabilization system  100  and another portion of the alignment instrument (not shown) may couple to another dynamic stabilization system. In the present example, the portion  800  may be configured to couple to various features of the dynamic stabilization system  100 , including polyaxial heads (e.g.,  106  and  108  of  FIG. 1B ), collars (e.g.,  132  and  134  of  FIG. 1B ) and/or extensions (e.g.,  126  and  128  of  FIG. 1B ).  
         [0066]     The portion  800  may include adjustable gripping pliers  802  formed by opposing members  804  and  806 . The opposing members  804  and  806  may include couplers  808  and  810 , respectively, such as gripping jaws forming a yoke feature for snapping onto a feature of the dynamic stabilization system  100 . The portion  800  may include a shaft  812  coupled to the gripping pliers  802 . An adjuster  814  may be provided for varying a distance between the opposing members  804  and  806 . For example, the adjuster  814  may be a knurled nut threaded onto a distal end of shaft  812  and coupled to the opposing members  804  and  806 . The shaft  812  may further include a collar  816  that may be used to position the shaft with respect to a linkage assembly  818 . The shaft  812  and/or opposing members  804  and  806  may be designed to point toward a common center of rotation that is also a common center of rotation for another portion (not shown) of the alignment instrument.  
         [0067]     A linkage assembly  818  may include arms  820  and  822  that may be pivotally coupled to one another at a proximal end of each arm. The arm  820  may be coupled to shaft  812  and the arm  822  may be coupled to a similar shaft (not shown) of the alignment instrument. In the present example, a distal end of arm  820  may include a bore therethrough for receiving the proximal end of the shaft  812 . The bore may have a longitudinal axis that may intersect the center of rotation.  
         [0068]     The linkage assembly  818  may also include a guide pin assembly  824 . The guide pin assembly  824  may include a shaft  826  having a foot  828  at a distal end thereof and a knob  830  at a proximal end thereof. The foot  828  may be placed proximal to or in contact with an outer tissue layer of a patient. The shaft  826  may be configured so that a longitudinal axis of the shaft may intersect the centre of rotation, and the shaft may be used for alignment purposes using, for example, fluoroscopy techniques. The knob  830  may be used to adjust the relative positions of the arms  820  and  822  and, accordingly, the corresponding shafts.  
         [0069]     Linkage assembly  818  and the coupled shafts may be designed to point towards a common center of rotation. It is understood that the common center of rotation may not be a fixed point, but may be a point to where the shaft  812  and/or opposing members  804  and  806  are directed, as are corresponding components (not shown) coupled to arm  822 . Accordingly, by adjusting the relative positions of the arms  820  and  822 , the orientation of the corresponding shafts may be altered. This movement may shift the common center of rotation, but the design of the alignment instrument may ensure that the coupled dynamic stabilization systems may continue to intersect the common center of rotation “A” as the movement occurs.  
         [0070]     Referring to  FIG. 9  and  FIGS. 10A-10E , one embodiment of a method  900  is illustrated for substantially aligning one or more dynamic spinal stabilization systems (e.g., the dynamic stabilization system  100  of  FIG. 1B ) with a desired common center of rotation. It is understood that a location of the common center of rotation may be selected prior to or during a surgical procedure and may vary depending on such factors as a patient&#39;s particular spinal structure.  
         [0071]     In step  902 , a surgeon may insert bone anchors into vertebral bodies, as shown in  FIG. 10A . The surgeon may then insert a spinal stabilization device (e.g., the spinal stabilization device  101  of  FIG. 1B ) into each pair of polyaxial heads corresponding to the bone anchors in step  904 , as shown in  FIG. 10B . In step  906 , the surgeon may determine a desired center of rotation between the adjacent vertebral bodies. In certain embodiments, the surgeon may use a guide pin assembly coupled to an alignment instrument (e.g., the guide pin assembly  628  of the alignment instrument  600  of  FIG. 6 ) to locate a midline of the patient or the sagittal plane, as shown in  FIG. 10C . For example, the center of rotation may be located on the sagittal plane at the top plate of the lower vertebral body within the intervertebral space. In some embodiments, an alignment rod may be coupled to the alignment instrument to aid in positioning the alignment instrument as shown in  FIG. 10C . In step  908 , the surgeon may substantially align the dynamic stabilization systems with the center of rotation using an alignment instrument such as the alignment instrument  600  of  FIG. 6 , as shown in  FIG. 10D . Subsequently, in step  910 , the surgeon may lock the polyaxial heads and remove the alignment instrument, as shown in  FIG. 10E .  
         [0072]     It is understood that it may be desirable to maintain all of the polyaxial heads in the same plane. However, if the pedicles of adjacent vertebrae are out of alignment, it may be difficult to maintain alignment of the polyaxial heads because the vertebral bodies into which the bone anchors are embedded are not themselves properly aligned. Accordingly, the alignment instrument may be used to aid in compensating for the lack of alignment of adjacent vertebral bodies and may enable a surgeon to make a substantially rectangular box configuration of the polyaxial heads. As previously described, the alignment instrument may aid in orienting the dynamic stabilization systems to point toward a common center of rotation.  
         [0073]     Referring to  FIG. 11 , in another embodiment, there is presented a method  1100  of substantially aligning one or more spherical motion dynamic spinal stabilization devices with an anatomical center of rotation is illustrated. In step  1102 , a surgeon may attach an embodiment of a gripping tool to a dynamic stabilization rod or another component of the spherical motion dynamic spinal stabilization device. In step  1104 , the surgeon may install one or more of the dynamic stabilization rods into polyaxial heads of pedicle anchor screws previously embedded into adjacent vertebral bodies. The surgeon, in step  1106 , may then start locking caps into the ends of the polyaxial heads to hold the dynamic stabilization rods in place.  
         [0074]     In step  1108 , the surgeon may attach an alignment instrument (e.g., the alignment instrument  600  of  FIG. 6 ) to the dynamic rod gripping tools. The surgeon may, in step  1110 , adjust the dynamic rods to optimize the location of the dynamic rods with respect to the end plates of the vertebral bodies. In step  1112 , the surgeon may rotate the alignment instrument to substantially match alignment features of the alignment instrument, such as a guide pin, with spinous processes on the vertebral bodies or other alignment indicators. In step  1114 , the surgeon may tighten the locking caps to secure the dynamic rods in substantially proper alignment with the center of rotation between the two vertebral bodies, after which the alignment instrument may be detached from the dynamic rods and removed.  
         [0075]     Referring to  FIGS. 12 and 13 , in another embodiment, an alignment instrument  1200  may be used to align one or more dynamic stabilization systems  1203 A and  1203 B to a center of rotation “A”. The dynamic stabilization systems  1203 A and  1203 B are conceptually similar to the dynamic stabilization systems  100 A and  100   b  discussed previously. Additional detail on these systems may be found in the commonly assigned U.S. Provisional Application Ser. No. 60/786,898, entitled “FULL MOTION SPHERICAL LINKAGE IMPLANT SYSTEM,” filed Mar. 29, 2006. As described previously, portions of the dynamic stabilization system  1203  may be configured to rotate around the center of rotation “A”, which may be positioned between vertebral bodies  1201 A and  1201 B. As illustrated in  FIGS. 12 and 13 , the alignment instrument  1200  may attach to the dynamic stabilization systems  1203 A and  1203 B and may be used to alter the position of one or both of the dynamic stabilization systems prior to locking the dynamic stabilization systems into position.  
         [0076]     In the present example, the alignment instrument  1200  may include alignment members  1202  and  1204  that may be coupled by a linkage assembly  1206 . The alignment member  1202  may include a body portion  1208  (e.g., a shaft) and the alignment member  1204  may include a body portion  1210  (e.g., a shaft). Although not shown, each alignment member  1202  and  1204  may include a collar or other adjustment mechanism at a proximal end (relative to the linkage assembly  1206 ) of the respective shafts  1208  and  1210  for adjusting a position of the shafts with respect to the linkage assembly  1206 .  
         [0077]     The shafts  1208  and  1210  may include couplers  1212  and  1214 , respectively, at the shafts&#39; distal ends. Each coupler  1212  and  1214  may be configured to removably couple to a portion of the dynamic stabilization systems  1203 A and  1203 B, such as a polyaxial head. The couplers  1212  and  1214  may be separate components that attach to the shafts  1208  and  1210 , or may be integrated into the distal ends of the shafts (e.g., the distal ends may be threaded to mate with the polyaxial heads or may be shaped to fit into a receptacle in a locking cap or other component that is coupled to the polyaxial head). The alignment members  1202  and  1204  may each have a longitudinal axis  1216  and  1218 , respectively. In the present example, the longitudinal axes  1216  and  1218  may extend from the proximal end of each alignment member  1202  and  1204  to the distal ends, and may intersect the center of rotation “A”.  
         [0078]     The linkage assembly  1206  may include arms  1220  and  1222  that may be coupled to a center portion  1224  of the linkage assembly at a proximal end of each arm. The arm  1220  may be coupled to alignment member  1202  and the arm  1222  may be coupled to alignment member  1204 . In the present example, a distal end of each arm  1220  and  1222  may include a bore  1226  and  1228 , respectively, for receiving the alignment members  1202  and  1204 . The bores  1226  and  1228  may each have a longitudinal axis that may intersect the center of rotation “A”. For example, the axes of the bores  1226  and  1228  may coincide with the axes  1216  and  1218  of the alignment members  1202  and  1204 . It is understood that although the bores  1226  and  1228  are illustrated in  FIG. 12  as being relatively wide compared to the shafts  1208  and  1210 , the bores may be sized to receive the shafts in a relatively tight fit while still allowing movement (e.g., rotation) of the shafts within the bores. The collars (not shown) or other adjustment mechanisms may prevent the arms  1220  and  1222  from movement in the direction of the distal ends of the alignment members  1202  and  1204 .  
         [0079]     In the present example, the arm  1222  may include a first portion  1230  and a second portion  1232 . Each portion  1230  and  1232  may be bent or curved to enable the bore  1228  to point towards the center of rotation “A”. The portions  1230  and  1232  may be coupled at an elbow  1234 . The elbow  1234  may include a bore  1236  positioned so that a longitudinal axis  1238  extending through the bore may intersect the center of rotation “A”. In some examples, the bore  1236  may be configured to receive an alignment member or other tool. It is understood that the bore  1236  may not be present in some embodiments. Although shown with the two portions  1230  and  1232 , it is understood that the arm  1222  may be formed as a single member having varying shapes (e.g., curved or bent). In other embodiments, the arm  1222  may be relatively straight, and bore  1228  may be formed to adjust for the lack of curvature of the arm. The arm  1220  may include a first portion  1240  and a second portion  1242 . Although not shown in detail in  FIG. 12 , the arm  1220  may include features that are similar or identical to features of the arm  1222  discussed previously.  
         [0080]     In the present example, the center portion  1224  of the linkage assembly  1206  may include a guide mechanism. In the illustrated embodiment, the guide mechanism may be a wheel-like member having an outer ring  1244 . The outer ring  1244  may serve as an alignment mechanism (e.g., an alignment cross) to aid in alignment using fluoroscopy or another suitable imaging process. It is understood that other shapes are possible and that the center portion  1224  is not limited to the shape illustrated in  FIG. 12 . The arms  1220  and  1222  may couple to the outer ring  1244 . The couplings may be fixed or movable (e.g., pivotal) relative to the linkage assembly depending on the particular configuration of the alignment instrument  1200 . In the present embodiment, the outer ring  1244  may include coupling points (e.g., bores or other attachment means)  1246 ,  1248 ,  1250 , and  1252 . In some embodiments, additional arms (not shown) may be coupled to the center portion  1224 . For example, arms may be coupled to the outer ring  1244  at connection points  1250  and  1252 . In such embodiments, alignment members (not shown) coupled to the arms may be used to align additional polyaxial heads. The arms may be coupled to the center portion  1224  at the same time as the arms  1220  and  1222 , or at different times (e.g., at an earlier or later time in a surgical procedure). Although the connection points  1246  and  1248  are illustrated as being equidistant from a vertical axis (not shown) dividing the outer ring  1244  into left and right halves, it is understood that the connection points may not be equidistant in some embodiments.  
         [0081]     An adjustment mechanism (e.g., a rod)  1254  may be used to manipulate the alignment instrument  1200 . For example, the rod  1254  may be used to adjust the center portion  1224  in the cephalad/caudal and/or anterior/posterior directions, thereby moving the center of rotation “A”.  
         [0082]     A shaft  1256  may be used to aid in alignment. For example, the shaft  1256  may include various markings  1258 . The markings  1258  may indicate a distance of the shaft&#39;s distal end from the center of rotation “A”. For example, if the distal end of the shaft  1256  is touching the patient&#39;s skin and if a marking  1258  labeled “10 cm” is adjacent to the proximal surface of the center portion  1224 , then the tip of the shaft  1256  may be ten centimeters from the point of rotation “A” (e.g., the center of rotation “A” is approximately ten centimeters under the patient&#39;s skin). Accordingly, the shaft  1256  may provide a visual guide to the depth of the center of rotation “A”. In the present example, the shaft  1256  may have a longitudinal axis  1260  that intersects the center of rotation “A”. Accordingly, the shaft  1256  may be used as an alignment guide using fluoroscopy or another suitable imaging technique.  
         [0083]     As described previously, linkage assembly  1206  and corresponding alignment members  1202  and  1204  may be designed to point towards the common center of rotation “A”. It is understood that the common center of rotation “A” may not be a fixed point, but may be a point where the axes  1216  and  1218  intersect. Accordingly, by adjusting the relative positions of the arms  1220  and  1222 , the positions of the corresponding alignment members  1202  and  1204  may be altered. This movement may shift the common center of rotation “A”, but both axes  1216  and  1218  may continue to intersect the common center of rotation “A” as it is moved.  
         [0084]     In the present example, if the rod  1254  is moved to the right, the common center of rotation “A” may move to the right. Similarly, if the rod  1254  is moved to the left, the common center of rotation “A” may move to the left. If the rod  1254  is moved inward (e.g., towards the polyaxial heads), the common center of rotation “A” may shift inward. If the rod  1254  is moved outward (e.g., away from the polyaxial heads), the common center of rotation “A” may shift outward. As the center of rotation “A” is shifted, the arms  1220  and  1222  may maintain the alignment of the alignment members  1202  and  1204  with the center of rotation “A”. Accordingly, the center of rotation “A” of the polyaxial heads may be aligned with a desired center of rotation using the alignment instrument  1200 .  
         [0085]     In operation, the alignment members  1202  and  1204  may be coupled to the dynamic stabilization systems  1203 A and  1203 B while coupled to the linkage assembly  1206 . Alternatively, the alignment members  1202  and  1204  may be coupled to the dynamic stabilization systems  1203 A and  1203 B separately and then coupled to the linkage assembly  1206 .  
         [0086]     It is understood that the alignment instrument  1200  may be modified to provide similar or identical functionality in a different configuration. For example, rather than being bores having a fixed axis, the bores  1226  and  1228  may be modified to provide an adjustable alignment mechanism (e.g., an adjustable housing and/or locking mechanism). Furthermore, the alignment members  1202  and  1206  may be integrated with the arms  1220  and  1222 , and/or the alignment members may couple to the arms using a different coupling mechanism than the illustrated bores  1226  and  1228 . It is also understood that the alignment members  1202  and  1204  need not be shafts, but may have other shapes.  
         [0087]     Referring to  FIG. 14 , in another embodiment, a method  1400  may be used to align a dynamic spinal stabilization system. The method  1400  may be used, for example, with the alignment instrument  600  of  FIG. 6  or the alignment instrument  1200  of  FIG. 12 .  
         [0088]     In step  1402 , first and second bone anchors may be inserted into a vertebral body. The first and second bone anchors may include first and second polyaxial heads, respectively, as described previously. It is understood that while a single vertebral body is used for purposes of example, the first and second bone anchors may be inserted into separate vertebral bodies. In step  1404 , first and second alignment members may be coupled to the first and second polyaxial heads, respectively, where the first and second alignment members are automatically centered on a first center of rotation. In step  1406 , a second center of rotation may be identified between the first vertebral body and a second vertebral body. It is understood that the second center of rotation may be identified prior to step  1402 . In step  1408 , a linkage assembly coupling the first and second alignment members may be manipulated to align the first center of rotation with the second center of rotation, where the first and second polyaxial heads are thereby aligned with the second center of rotation. In step  1410 , the first and second polyaxial heads may be locked with respect to the first and second bone anchors to maintain the alignment of the first and second polyaxial heads with the second center of rotation.  
         [0089]     Although only a few exemplary embodiments of this disclosure have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this disclosure. Also, features illustrated and discussed above with respect to some embodiments can be combined with features illustrated and discussed above with respect to other embodiments. Accordingly, all such modifications are intended to be included within the scope of this disclosure.