Patent Document

CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of U.S. Provisional Patent Application No. 60/732,265 filed Oct. 31, 2005, the disclosure of which is hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to orthopedic medicine, and more particularly to systems and methods for restricting relative motion between vertebrae. 
     Unfortunately millions of people experience back pain, and such is not only uncomfortable, but can be particularly debilitating. For example, many people who wish to participate in sports, manual labor, or even sedentary employment are unable to do so because of pains that arise from motion of or pressure on the spinal column. These pains are often caused by traumatic, inflammatory, metabolic, synovial, neoplastic and degenerative disorders of the spine. 
     In a normal spinal column, intervertebral discs that separate adjacent vertebrae from each other serve to provide stiffness that helps to restrain relative motion of the individual vertebrae in flexion, extension, axial rotation, and lateral bending. However, a damaged disc may provide inadequate stiffness along one or more modes of spinal motion. This inadequate stiffness may result in excessive relative vertebral motion when the spine is under a given load, as when the patient uses the muscles of the back. Such excessive relative motion may cause further damage to the disc, thereby causing back pain and ultimately, requiring replacement of the disc and/or other operations to decompress nerves affected by central, lateral or foraminal stenosis. 
     Heretofore, some stabilization devices have been proposed to restrict, but not entirely prevent, relative motion between adjacent vertebrae. These devices often contain linear springs that are too long to be easily positioned between adjacent vertebrae. Thus, they are often impossible to implant on motion segments where there is a short pedicle-to-pedicle displacement. Furthermore, known spinal implants typically have components that are either flexible, allowing limited relative motion between adjacent vertebrae, or rigid, providing fusion between vertebrae. Thus, they do not provide for interchangeability between flexible and rigid components. Accordingly, symptoms that would normally indicate stabilization and fusion of adjacent motion segments cannot be adequately treated, and vice versa. In other words, revision of an implant to provide fusion in place of stabilization is typically not feasible. Finally, many devices, when implanted in multiple levels along the spine, do not flexibly follow the natural curvature of the spine. Such devices may therefore cause discomfort, or restrict spinal motion in an unpredictable and unnatural manner. 
     Therefore, there exists a need for a system and method which corrects the above-noted shortcomings and allows for dynamic vertebral stabilization to restore normal movement and comfort to a patient. 
     SUMMARY OF THE INVENTION 
     A first aspect of the present invention is a stabilization system for controlling relative motion between a first vertebra and a second vertebra. In accordance with this first aspect, on embodiment stabilization system may include a first stabilizer having a first coupling adapted to be attached to a first anchoring member, a second coupling adapted to be attached to a second anchoring member and a resilient member configured to be coupled to the first and second couplings to transmit resilient force between the first and second couplings, the resilient member including a planar spring, wherein at least a portion of the planar spring flexes out-of-plane in response to relative motion between the vertebrae. 
     In other embodiments of the first aspect, the first stabilizer may further include a casing including a hollow first member and a hollow second member, wherein the resilient member is positioned within a cavity defined by engagement of the first and second hollow members. The resilient member is may also be positioned inside the casing such that the casing limits relative motion of the vertebrae by limiting deflection of the planar spring. The system may also include the first anchoring member and the second anchoring member, where the first and second anchoring members include a yoke polyaxially coupled to a fixation member implantable in a portion of either the first or second vertebra. The system may also include a first rigid connector including first and second couplings adapted to be attached to one of the first and second anchoring members, wherein the couplings are substantially rigidly connected together. In other embodiments, the path followed by the planar spring may be generally spiral-shaped, wherein the planar spring includes a central portion attached to the first coupling and a peripheral portion attached to the second coupling. The first stabilizer may further include a first articulation component configured to articulate to permit polyaxial relative rotation between one of the first or second couplings. The first articulation component may include a semispherical surface and a socket within which the semispherical surface is rotatable to permit polyaxial motion between the resilient member and the first anchoring member. The resilient member may be coupled to the first and second couplings such that the resilient member is able to urge the first and second couplings to move closer together and is also able to urge the couplings to move further apart. 
     The stabilization system may include a second component comprising a third coupling and a fourth coupling, wherein the third coupling is adapted to be attached to the first anchoring member such that the first anchoring member is capable of simultaneously retaining the first and third couplings. The second component may be a rigid connector, wherein the third and fourth couplings are substantially rigidly connected together, or the second component may be a second stabilizer comprising a second resilient member configured to exert resilient force between the third and fourth couplings. 
     Another aspect of the present invention is another stabilization system for controlling relative motion between a first vertebra and a second vertebra. In accordance with this second aspect, the stabilization system may include a first stabilizer having a first coupling adapted to rest within a yoke of a first anchoring member, a second coupling adapted to rest within a yoke of a second anchoring member, a resilient member coupled to the first and second couplings to transmit resilient force between the first and second couplings, the resilient member including a planar spring, wherein at least a portion of the planar spring flexes out-of-plane in response to relative motion between the vertebrae and a first articulation component configured to articulate to permit relative rotation between the first stabilizer and one of the first or second couplings. 
     Still another aspect of the present invention is a stabilization system for controlling relative motion between a first vertebra and a second vertebra. The stabilization system according to this aspect may include a first stabilizer having a first coupling adapted to be attached to a first anchoring member, a second coupling adapted to be attached to a second anchoring member, a resilient member configured to be coupled to the first and second couplings to transmit resilient force between the first and second couplings, the resilient member including a planar spring, wherein at least a portion of the planar spring flexes out-of-plane in response to relative motion between the vertebrae, a first articulation component configured to articulate to permit relative rotation between the first and second couplings and a first rigid connector including third and fourth couplings adapted to be attached to the first and second anchoring members, wherein the third and fourth couplings are substantially rigidly connected together. 
     Yet another aspect of the present invention is a method for controlling relative motion between a first vertebra and a second vertebra. In accordance with this aspect, the method may include the steps of positioning a planar spring of a first stabilizer attaching a first coupling of the first stabilizer to the first vertebra and attaching a second coupling of the first stabilizer to the second vertebra, wherein, after attachment of the couplings to the vertebrae, the planar spring is positioned to transmit resilient force between the vertebrae via flexure of at least a portion of the planar spring out-of-plane. 
     Yet another aspect of the present invention is another method for controlling relative motion between a first vertebra and a second vertebra. In accordance with this aspect, the method may include selecting a component selected from the group consisting of a first stabilizer and a first rigid connector, wherein the first stabilizer comprises a first coupling, a second coupling adapted to be attached to a second anchoring member secured to the second vertebra, a resilient member configured to transmit resilient force between the first and second couplings, and a first articulation component configured to articulate to permit relative rotation between the first and second couplings, wherein the first rigid connector comprises a first coupling and a second coupling substantially rigidly connected to the first coupling, attaching a first coupling of the selected component to a first anchoring member secured to the first vertebra and attaching a second coupling of the selected component to a second anchoring member secured to the second vertebra. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the subject matter of the present invention and the various advantages thereof can be realized by reference to the following detailed description in which reference is made to the accompanying drawings in which: 
         FIG. 1  is a perspective view of a dynamic stabilization assembly according to one embodiment of the invention. 
         FIG. 2  is an enlarged perspective view a stabilizer of the dynamic stabilization assembly of  FIG. 1 . 
         FIG. 3  is an exploded perspective view of the stabilizer of  FIG. 2 . 
         FIG. 4  is a further exploded perspective view of the stabilizer of  FIG. 2 . 
         FIG. 5  is a partially exploded perspective view of the stabilizer of  FIG. 2  having two end caps. 
         FIG. 6  is a perspective view of the stabilizer of  FIG. 2 , illustrating attachment of one end cap to an end coupling. 
         FIG. 7  is a perspective view of the stabilizer of  FIG. 2  with attached end caps. 
         FIG. 8  is a partially exploded perspective view of the dynamic stabilization assembly of  FIG. 1 . 
         FIG. 9  is a perspective view of two of the stabilizers of  FIG. 2 , placed end to end, with two end caps being detached therefrom. 
         FIG. 10  is a perspective view of two of the stabilizers of  FIG. 2 , placed end to end, with two end caps being attached thereto. 
         FIG. 11  is a perspective view of two stabilizers of  FIG. 2 , placed end to end, illustrating the coupling of the ends of the stabilizers to each other. 
         FIG. 12  is a perspective view of the stabilizer of  FIG. 2 , coupled end-to-end with a second stabilizer for multi-level vertebral stabilization. 
         FIG. 13  is a perspective view of the two stabilizers of  FIG. 12 , illustrating how the articulation components may be used to provide an overall curvature to the assembled modules. 
         FIG. 14  is a perspective view of the stabilizer of  FIG. 2 , coupled end-to-end with a rigid connector and an end cap for single level vertebral joint stabilization with joint immobilization at an adjacent level. 
         FIG. 15  is an exploded perspective view of the stabilizer and rigid connector of  FIG. 14 , illustrating the coupling of the stabilizer and the rigid connector to each other. 
         FIG. 16  is a perspective view of the stabilizer and rigid connector of  FIG. 14 , illustrating how the articulation components may be used to provide an overall curvature to the assembled modules. 
         FIG. 17  is a perspective view of another dynamic stabilization assembly according to an alternative embodiment of the invention. 
         FIG. 18  is an enlarged perspective view of a stabilizer and end couplings of the dynamic stabilization assembly of  FIG. 17 . 
         FIG. 19  is an exploded perspective view of the stabilizer of  FIG. 18 . 
         FIG. 20  is an exploded perspective view of the stabilizer and end couplings of  FIG. 18 . 
         FIG. 21  is a partially exploded perspective view of the dynamic stabilization assembly of  FIG. 17 . 
         FIG. 22  is a perspective view of an overhung stabilizer and articulating component of an overhung dynamic stabilization assembly designed for shorter pedicle-to-pedicle displacements. 
         FIG. 23  is an exploded perspective view of the overhung stabilizer of  FIG. 22 . 
         FIG. 24  is a partially exploded perspective view of an overhung dynamic stabilization assembly including the components of  FIG. 22 . 
         FIG. 25  is another partially exploded perspective view of the overhung dynamic stabilization assembly of  FIG. 24 . 
         FIG. 26  is a perspective view of a fully assembled overhung dynamic stabilization assembly of  FIG. 24 . 
         FIG. 27  is a perspective view of the dynamic stabilization assembly including the stabilizer of  FIG. 22 , along with the overhung stabilization assembly of  FIG. 24 . 
         FIG. 28  is an exploded perspective view of the dynamic stabilization assembly of  FIG. 27 . 
         FIG. 29  is a further exploded perspective view of the dynamic stabilization assembly of  FIG. 27 . 
     
    
    
     DETAILED DESCRIPTION 
     The present invention relates to systems and methods for stabilizing the relative motion of spinal vertebrae. Those of ordinary skill in the art will recognize that the following description is merely illustrative of the principles of the invention, which may be applied in various ways to provide many different alternative embodiments. This description is understandably set forth for the purpose of illustrating the general principles of this invention and is not meant to limit the inventive concepts in the appended claims. 
     Referring to  FIG. 1 , one embodiment of a single level dynamic stabilization system  10  is shown. The dynamic stabilization system  10  preferably includes a stabilizer  12 , a pair of fixation members  14 , a pair of yokes  16  securable to the fixation members  14 , and a pair of set screws  18 . The fixation members  14 , yokes  16 , and set screws  18  may be any of a variety of types known and available in the art, or may optionally be specially designed for operation with the stabilizer  12 . Each fixation member  14  with its corresponding yoke  16  and set screw  18  provides an anchoring member  19  designed to anchor the stabilizer  12  to a pedicle or other portion of a vertebra (not shown). In the embodiments described and illustrated herein, the fixation members  14  are represented as pedicle screws. However, they could also be other types of screws fixed to other parts of the vertebrae, pins, clips, clamps, adhesive members, or any other device capable of anchoring the stabilizer to the vertebrae. Additionally, each yoke  16  may be unitarily formed with a fixation member  14  as illustrated herein, or each yoke  16  may be a separate entity and be polyaxially securable to a fixation member  14 . 
     The stabilizer  12  is illustrated alone in  FIG. 2 . As shown in that figure, stabilizer  12  includes a central spring casing  22 , and a short arm  26  extending from the spring casing  22  on one side to an articulation component  24 . On the opposite side, a longer arm  27  extends from the spring casing  22  to another articulation component  25 . An end coupling  28  is also preferably located on the outside of each articulation component  24 ,  25 . It is noted that the particular construction of stabilizer  12  depicted in  FIG. 2  may vary. For example, the short arm  26  and longer arm  27  may be flipped to opposite sides. 
     Referring to  FIG. 3 , an exploded view of the stabilizer  12  is shown, thereby illustrating the inner components of the stabilizer. For example, a planar spring  20  is shown encased within the spring casing  22 . The planar spring  20  is preferably coiled in a planar spiral-like shape and has a threaded inner ring surface  30  and an outer ring surface  32 . In addition, the spring casing  22  is made up of two concentric hollow members, an inner hollow member  40  and an outer hollow member  42 , with the planar spring  20  being disposed within the inner hollow member  40 . A circular bore  44  occupies the center of the inner hollow member  40 , creating a round opening from an inside surface  46  to an outside surface  48 . A protruding circular lip  49  may also surround the bore  44  where it exits the outside surface  48 . An inner wall  52  of the lip  49  is preferably threaded. Similarly, a circular bore  54  occupies the center of the outer hollow member  42 , creating a round opening from an inside surface  56  to an outside surface  58 . A protruding circular lip  59  may also surround the bore  54  where it exits the outside surface  58 . 
     Shown adjacent to the inner hollow member  40  is the short arm  26 , which has a threaded outer surface  76  on the end closest to the inner hollow member  40 . This end terminates at a flat end  36 . Both surface  76  and flat end  36  are best shown in  FIG. 4 . On the opposite end of the short arm  26  is the articulation component  24 , which terminates at the end coupling  28 . Adjacent to the outer hollow member  42  is the long arm  27 , which has a threaded terminal segment  78  on the end closest to the outer hollow member  42 . The terminal segment terminates at a flat end  37  (best shown in  FIG. 4 ). On the opposite end of the long arm  27  is the articulation component  25 , which terminates at the end coupling  28 . 
     When assembled, the short arm  26  fits inside the bore  44  of the inner hollow member  40 . The threads on the outer surface  76  engage with the threads on the inner wall  52 , thereby securing the pieces together. As mentioned above, the planar spring  20  fits inside the inner hollow member  40 . In addition, the long arm  27  fits through the bore  54  of the outer hollow member  42 , with the threaded terminal segment  78  engaging the threaded inner ring surface  30  of the planar spring  20 . The inner hollow member  40  fits concentrically within the outer hollow member  42 , with the planar spring  20  also being disposed inside. Inside of the hollow members  40 ,  42 , the flat ends  36 ,  37  of the arms  26 ,  27  are preferably adjacent to one another but not touching. 
     When assembled with the hollow members  40 ,  42  and the arms  26 ,  27 , the planar spring  20  can, if acted upon, flex out of the plane within which it is coiled. When the longer arm  27 , to which the planar spring  20  is engaged, moves toward or away from the short arm  26 , the spiral-like shape of the planar spring  20  preferably extends out of its plane. When the longer arm  27  returns to its original position, the planar spring  20  also preferably recoils back to its plane. During this extension and recoil, the inside surface  46  of the inner hollow member  40 , and the inside surface  56  of the outer hollow member  42  act as barriers to limit the movement of the planar spring  20 . 
     Use of the planar spring  20 , as opposed to a longer helical spring, keeps the overall length of the stabilizer  12  relatively short. In alternative embodiments, a planar spring according to the invention need not have a spiral-like shape, but can rather be a cantilevered leaf spring, a flexible disc, or the like. Further, in other alternative embodiments, a planar spring need not be used; rather, a different type of spring or a conventional helical spring may be used. 
       FIG. 4  illustrates the articulation components  24 ,  25  in an exploded view. As is mentioned above, the articulation component  24  is located adjacent to and couples with the inner hollow member  40 , and the articulation component  25  is located adjacent to and couples with the outer hollow member  42 . Each articulation component  24 ,  25  preferably comprises a semispherical surface  60 , a cup  62 , which are both enclosed by the end coupling  28 . The cup  62  is preferably dish shaped, with a cylindrical support wall  64  and two ends. On one end of the cup  62  is a depression  66 , and on the opposite side of the cup  62  is a flat end  68 . The semispherical surface  60  preferably has a round side  70  which rotatably fits inside the depression  66 , so that each of the articulation components  24 ,  25  thus takes the form of a ball-and-socket joint. The opposite side of each semispherical surface  60  is a connecting side  72  which narrows into a neck  74 . The neck  74  preferably widens into either the short arm  26  or the long arm  27 , which extends away from the semispherical surface  60  on the opposite side from the round side  70 . As is discussed above, the outer wall  76  of the short arm  26  is threaded, as is the terminal segment  78  of the long arm  27 . In alternative embodiments, articulation components may be omitted, or may be formed by any other type of mechanical joints known in the art. 
     The end coupling  28  has a support wall  102  which forms the outer sides of the cup, and a base  104 . A circular hole  106  occupies the center of the base  104 , and where the edge of the hole  106  meets the base  104 , a circular rim  108  preferably surrounds the hole  106 . The inside diameter of the rim  108  is preferably less than the diameter of the semispherical surface  60  of the articulation components  24  and  25 , so that when assembled the semispherical surface  60  will fit into the end coupling  28  but not be capable of passing through the hole  106 . At the opposite end from the base  104 , the support wall  102  terminates in a flat edge  110 . Protruding from the edge  110  in the same plane as the support wall  102 , such that they form continuations of the support wall  102 , is a plurality of irregularly shaped teeth  112 . Between each tooth  112  and the adjacent tooth is a notch  114 . 
     When assembled, the round side  70  of each semispherical surface  60  rotatably rests in the depression  66  of the cup  62 , and the arm  26  or  27  extends away from the joining side  72  of the semispherical surface  60 . The generally cup-shaped end coupling  28  fits over each semispherical surface, arm and cup assembly. Each arm  26 ,  27  extends from its semispherical surface  60  through its respective hole  106 . As described above, the arms then extend into the spring casing  22 , the long arm  27  connecting to the planar spring  20  and the short arm  26  connecting to the inner hollow member  40 . Rotation of either semispherical surface  60  results in movement of its arm  26 ,  27 . When the short arm  26  moves, the flat end  37  of the opposite arm  27  may optionally contact the flat end  36  of the short arm  26  to acts as a stop to limit excessive movement. Similarly, when the long arm  27  moves, the flat end  36  of the opposite short arm  26  may stop excessive movement via contact with the flat end  37  of the long arm  27 . Thus the articulation components  24 ,  25  secure the arms  26 ,  27  in a rotatable manner to the spring casing  22  to permit the stabilizer  12  to obtain a variable curvature. 
     The assembled stabilizer  12  can be rotated into locking engagement with end caps or end couplings of other stabilizers for multi-level application. In fact,  FIG. 5  illustrates one coupled stabilizer  12 , having a coupled end cap  120  and an uncoupled end cap  120 . Each end cap  120  preferably has a general cup-shape, much like each end coupling  28 . Each end cap  120  preferably includes a support wall  122  which forms the outer sides of the cup, and a solid base  124  which forms the bottom of the cup. The inside diameter of the end cap  120  is sized to fit around either arm  26 ,  27 . At an opposite end from the base  124 , the support wall  122  terminates in a flat edge  130 . Protruding from the edge  130  in the same plane as the support wall  122 , such that they form continuations of the support wall  122 , are a plurality of irregularly shaped teeth  132 . Between each tooth  132  and the adjacent tooth is a notch  134 . 
     Referring to  FIG. 6 , an end cap  120  is illustrated in partial engagement to a stabilizer  12 . When an end cap  120  is to be attached to an end coupling  28 , the end cap  120  is preferably lined up with the end coupling  28  so that the teeth  112 ,  132  are pointed toward one another. The end cap  120  is then rotated and moved toward the end coupling  28  so that the teeth  132  fit into the notches  114 , while the teeth  112  fit into the notches  134 . When the teeth  112 ,  132  are fully seated in the notches  114 ,  134  such that the teeth  132  touch the edge  110  and the teeth  112  touch the edge  130 , the end cap  120  is further rotated until the teeth  112 ,  132  interlock with each other and the end cap  120  is locked in place. A stabilizer  12  with two end caps  120  each fully engaged on opposite ends of the stabilizer  12  is depicted in  FIG. 7 . In this depiction, the end caps  120  have been fully rotated so that the teeth  132  of the end caps  120  are interlocked with the teeth  112  of both end couplings  28 . 
       FIG. 8  shows an exploded view of the dynamic stabilization system  10  with a fully assembled stabilizer  12 , two anchoring members  19  with yokes  16  and fixation members  14 , and two set screws  18 . In this design, each fixation member  14  preferably has a pointed end  140  which aids in screwing the member into a corresponding vertebra when implanted. The opposite end of the fixation member  14  is preferably unitarily formed with a U-shaped yoke  16 , so that the bottom of the U is a head  142  of the fixation member  14 . Each yoke  16  has two curved opposing support walls  144 . Alternating between the support walls  144  are two opposing gaps  146 , which form a cavity  148  therebetween that occupies the interior of the yoke  16 . The inner surfaces  150  of the support walls  144  are also preferably threaded to engage a set screw  18 . 
     According to the embodiment depicted, in use, the stabilizer  12  is inserted into the yokes  16  of two anchoring members  19  whose fixation members  14  have previously been anchored in the pedicles, or other portion, of the corresponding vertebrae. The stabilizer  12  is laid lengthwise into the yokes  16  such that the long axis of the stabilizer  12  is perpendicular to the long axes of the fixation members  14 , and so that the spring casing  22  lies between the anchoring members  19 . Each end coupling  28 /end cap  120  pair preferably rests on the head  142  within the cavity  148 . Each end cap preferably occupies the gaps  146 , and the two articulation components  24 ,  25  lie adjacent to, but outside of, the two interior gaps  146 . 
     The end couplings  28  and attached end caps  120  are preferably secured within the yokes  16  of the anchoring members  19  through the use of the set screws  18 . One set screw  18  is screwed into the top of each yoke  16  so that its threads engage with the threaded inner surfaces  150  of the support walls  144 . The set screws  18  are then tightened to hold the stabilizer  12  in place. As described above, an alternative embodiment of the invention includes yokes  16  which are separate entities from the fixation members  14 , and are polyaxially securable to the fixation members  14 . If such separate polyaxially securable yokes  16  are included, tightening of the set screws  18  may also press the end couplings  28  and end caps  120  against the heads  142  of the fixation members  14 , thereby restricting further rotation of the polyaxially securable yokes  16  with respect to the fixation members  14  to secure the entire assembly. Those of ordinary skill in the art would readily recognize this operation. 
     Referring to  FIG. 9 , two assembled stabilizers  12  are illustrated positioned end-to end with two end caps  120  positioned at the outer ends of the stabilizers  12 . Two stabilizers  12  may be interlocked with each other end-to-end and implanted when it is desirable to stabilize the relative motion of three adjacent vertebrae.  FIG. 10  depicts a similar assembly, with two stabilizers  12  being illustrated end-to-end, and one end cap  120  being secured to each outer end coupling  28  in a similar fashion to that previously depicted in  FIG. 7 . On the inner ends of each stabilizer  12 , the teeth  112  of each end coupling  28  are aligned to fit into the notches  114  of the facing end coupling  28 .  FIG. 11  depicts the two stabilizers  12  in an end-to-end fashion and partially interlocked together. The teeth  112  of each facing end coupling  28  are in the notches  114  of the opposite end coupling  28 , and the stabilizers  12  have been partially turned so that the teeth  112  are partially interlocked. In  FIG. 12 , the two stabilizers  12  are shown completely interlocked end-to-end. The end couplings  28  of the two stabilizers  12  are rotated into locking engagement with each other and an end cap  120  is locked onto each unoccupied external end coupling  28 . The entire dynamic stabilization assembly has four articulation components  24 ,  25 , which will permit considerable differentiation in orientation between the three fixation members  14  that would be used to attach the stabilizers  12  to three adjacent vertebrae (not shown). In fact, in  FIG. 13 , two interlocked stabilizers  12  are illustrated with the articulation components  24 ,  25  in an articulated position so that the stabilizers  12  no longer lie in a straight line, but instead the multi-level dynamic stabilization assembly approximates a curve. This enables the assembly to conform to the desired lordotic curve of the lower spine or to other spinal curvatures, such as those caused by or used to correct scoliosis. Additional levels can be added if desired. 
     Referring to  FIG. 14 , a stabilizer  12  is depicted secured end-to-end to a rigid connector  160  to provide dynamic stabilization across one level, and posterior immobilization and/or fusion across the adjacent level. The rigid connector  160  has a rod  162  and an end coupling  164 . The end coupling  164  is toothed and notched so that it may engage the end coupling  28  on the stabilizer  12 . This is not unlike the other couplings discussed above. In addition, and like that discussed above, the rod  162  may be secured in the yoke  16  of a fixation member  14  with a set screw  18 . Similarly, the interlocked end coupling  164 /end coupling  28  combination may be secured in the yoke  16  of an anchoring member  19  in a manner similar to the previously described securing of the end couplings and end caps. Additional rigid connectors  160  or stabilizers  12  with associated anchoring members  19  can be added if additional levels are desired. 
       FIG. 15  depicts an exploded view of the system depicted in  FIG. 14 , having one stabilizer  12 , an end cap  120 , and one rigid connector  160 . The end coupling  164  has teeth  166  protruding from one end, and notches  167  between the teeth. When the rigid connector  160  is attached to the stabilizer  12 , the teeth  166  of the end coupling  164  fit into the notches  114  of the end coupling  28 . Simultaneously, the teeth  112  of the end coupling  28  fit into the notches  167  of the end coupling  164 . The stabilizer  12  and the rigid connector  160  are rotated in opposite directions so that the teeth  112 ,  166  interlock and the stabilizer  112  and the rigid connector  160  are locked together. The end cap  120  is interlocked onto the remaining open coupling  28  of the stabilizer  12  as previously described.  FIG. 16  depicts one stabilizer  12  interlocked with a rigid connector  160  and an end cap  120 , and in a position with components  24 ,  25  being articulated to allow the assembly to approximate a curve. 
     Thus, like the above described systems, dynamic stabilization across one level and posterior immobilization and/or fusion across the adjacent level may be accomplished while simultaneously following the desired curvature of the spine. In some cases, it may be desirable to allow immobilization and/or fusion across one level, and dynamic stabilization across the adjacent level on each end. In such a case, a rigid connector  160  with an end coupling  164  at each end could be used, allowing a stabilization module  12  to couple to each end of the rigid connector  160 . 
     Referring to  FIG. 17 , an alternative embodiment of a stabilization system  168  is depicted. In this system, a stabilizer  170  is secured to two anchoring members  19 . As in the previous embodiment, the anchoring members  19  each preferably include two yokes  16  connected with two fixation members  14 , and two set screws  18  are preferably used to hold the stabilizer  170  in place. 
     As seen in  FIG. 18 , the stabilizer  170  has a spring casing  172  and two articulation components  174 ,  175 . A two-piece end housing  178  also preferably extends from either articulation component  174 ,  175 . As shown in  FIG. 19 , the spring casing  172  preferably houses a planar spring  180 . The planar spring  180  has a first side  182  and a second side  183 . Extending from the first side  182  is an arm  184  which narrows into a neck  186  and terminates in a semispherical surface  188 . The spring casing  172  has an outer hollow member  190  and an inner hollow member  192 . The inner hollow member  192  is of a shallow dish shape, and has a circular plate  194  which forms the base of the hollow member, with a threaded outer rim  196  which encircles the outside of the plate  194 . An inner rim  198  encircles a round hole  200  in the center of the plate  194 . 
     Similarly, the outer hollow member  190  is of a deep dish shape with an interior cavity  202 . It has a circular plate  204  which forms the base of the hollow member, and a support member  206  which forms the side wall of the hollow member. An inner surface  208  of the support member  206  is threaded, but a neck  210  extends from the outside of the plate  204  and terminates in a semispherical surface  212 . This latter element is different from both inner hollow member  192  and that which is included in the above described embodiments of the present invention. 
     When assembled, the planar spring  180  preferably fits into the cavity  202  of the outer hollow member  190 , with the second side  183  adjacent to the plate  204  of the hollow member  190 . The inner hollow member  192  fits over the planar spring  180 , so that the arm  184  and the semispherical surface  188  extend through the hole  200  in the inner hollow member  192 . Thereafter, the threads on the outer rim  196  engage with the threads on the inner surface  208  of the outer hollow member  190 , joining the hollow members  190 ,  192  to form the casing  172 . The spring  180  is thusly captured inside the casing  172 , which prevents it from moving axially. When the arm  184  moves toward or away from the outer hollow member  190 , the planar spring  180  extends out of its plane. When the arm  184  returns to its original position, the planar spring  180  recoils back towards its plane. During this extension and recoiling, the plate  194  of the inner hollow member  192  and the plate  204  of the outer hollow member  190  act as barriers to limit the movement of the planar spring  180 . The arm  184  is encircled by the inner rim  198 , which acts as a bearing surface to prevent radial movement of the arm relative to the inferior hollow member  192 . 
     As seen in  FIG. 20 , a coupling in the form of a two-part end housing  178  fits over each semispherical surface  188 ,  212 . Each end housing  178  has a first wall  220  and a second wall  222 . The first wall  220  is shaped like a segment of a cylindrical body that is split lengthwise, and has an inner surface  224  and rounded outer surface  226 . At each lengthwise end of the first wall  220 , a rounded first hollow  228  is indented into the inner surface  224 . Indented into the inner surface  224 , between the hollows  228 , are two receiving holes  230 . The second wall  222  is also shaped like a segment of a cylindrical body and has an inner surface  234  and an outer surface  236 . Unlike the first wall  220 , the outer surface  236  is not rounded but is squared off so it is flat. The inner surface  234  has a rounded second hollow  238  indented into each lengthwise end. Each pair of rounded hollows  228 ,  238  cooperates to define a socket sized to receive the corresponding ball  188  or  212 . Two pin holes  240  extend from the outer surface  236  through the wall  222  to the inner surface  234 , such that two pins  242  can fit through the pin holes  240  and into the receiving holes  230  in the first wall  220 . The pins  242  and receiving holes  230  releasably hold the walls  220 ,  222  together around the semispherical surfaces  188 ,  212 , and prevent shearing of the walls. In other embodiments of the invention, the pins  242  and receiving holes  230  could be replaced by posts and brackets, or a snap mechanism or other mechanisms capable of releasably joining the walls  220 ,  222 . 
     The assembled stabilizer  170  fits into the yokes  16  of two anchoring members  19 , as is best shown in  FIG. 17  (shown disassembled in  FIG. 21 ). In the fully assembled state, the end housings  178  are preferably situated perpendicular to the fixation members  14 , so that the end housings  178  fit between support walls  144  of anchoring member  19 , and the rounded outer surface  226  is cradled on a curved floor  142  between walls  144 . Two set screws  18  are thereafter engaged in the threads  150  and tightened. The tightening of the set screws  18  creates pressure on the end housings  178 , holding the housings closed around the semispherical surfaces  188 ,  212 . As described in the previous embodiment, each anchoring member  19  may comprise a unitary piece which includes both the fixation member  14  and the yoke  16 , or the fixation member  14  and the yoke  16  may be separate pieces. In such an embodiment where the fixation member  14  and yokes  16  are separate pieces, tightening of the set screws  18  may also press the end housings  178  against the heads  142  of the fixation members  14 , thereby restricting further rotation of the yokes  16  with respect to the fixation members  14  to secure the entire assembly. 
     Like the above embodiment, two stabilizers  170  can be secured end-to-end in accordance with this latter embodiment. When two stabilizers  170  are to be used together, the stabilizers are partially assembled as shown in  FIG. 19  and described previously. The semispherical surface  212  or  188  from one stabilizer  170  is preferably placed in the empty hollow  228  of the first wall  220  of the second stabilizer  170  before the second wall  222  is joined to the first wall  220 . When the second wall  222  is joined to the first wall  220 , the semispherical surfaces  212 ,  188  are captured in the socket sections  228 ,  238  and the modules are joined. A stabilizer  170  can also be employed in combination with a rigid connector to provide dynamic stabilization across one level and posterior fusion across the adjacent level. Additional levels may be added as desired. Multiple stabilization/fusion levels can include two or more sequential rigid connectors, or rigid connectors sequentially interspersed with stabilizers. 
     Referring to  FIG. 22 , a portion of an “overhung” dynamic stabilization system is shown. This system can be used when an offset between adjacent fixation members is desired and/or when a short pedicle-to-pedicle displacement must be accommodated. In this embodiment, a stabilizer  250  includes a housing  252 , an articulation component  254  and an arm  256  which extends from the joint. A tunnel  258  provides an opening for placement of the stabilizer  250  over an anchoring member (best shown in  FIG. 26 ), and two set screws  259  are used to press a flexible stop  260  against the anchoring member, securing the stabilizer  250  in place. 
       FIG. 23  depicts an exploded view of the stabilizer  250  in more detail. As shown in that figure, the housing  252  has a chamber  262  which holds the articulation component  254 . A threaded cap  264  is screwed into the housing  252  closing off one end of the chamber  262 . A planar spring  266  with a threaded inner ring  268  is positioned within the cap  264 . Releasably screwed to the inner ring  268  is a socket  270  with a threaded end stud  272 . A cup  274  terminates the socket  270  at the end opposite the threaded end stud  272 . A semispherical surface  276  is connected to the arm  256 , and the semispherical surface  276  rotatably rests in the cup  274 . A tubular sleeve  278  surrounds the socket  270 , semispherical surface  276  and arm  256 . The sleeve  278  has a central bore  280  through which the arm  256  protrudes. The sleeve  278  also has two grooves  282  which run lengthwise down opposite outer sides of the sleeve. When the sleeve  278 , along with the enclosed socket  270 , semispherical surface  276  and arm  256  are in the chamber  262 , the sleeve is held in place by two pins  284 . The pins  284  are inserted through two pin holes  286  which perforate the outer wall of the housing  252 . The inserted pins  284  fit into the grooves  282 , and prevent the sleeve  278  and its enclosed contents from moving axially. 
     An unassembled stabilization system  248  is shown in  FIG. 24 . The system  248  includes the overhung stabilizer  250 , an anchoring member  19 , an anchoring member  288 , an articulation component  24 , an end coupling  28  and an end cap  120 . As described in previous embodiments, the anchoring member  19  has a fixation member  14 , a yoke  16  and a set screw  18 . The anchoring member  288  comprises a fixation member  14  and an extension post  290 . Once again, the fixation members  14  may comprise pedicle screws, screws fixed to other parts of the vertebrae, pins, clips, clamps, adhesive members, or any other device capable of anchoring the stabilizer to the vertebrae. Additionally, each yoke  16  may be unitarily formed with a fixation member  14  as illustrated herein, or each yoke  16  may be a separate entity and be polyaxially securable to a fixation member  14 . The articulation component  24  has a tubular joining arm  292  extending from an end coupling  28 . The joining arm  292  is shaped to fit over the end of the arm  256  which protrudes from the articulation component  254 . 
       FIG. 25  illustrates the stabilization system  248  in a partially assembled state. The stabilizer  250  is joined to the articulation component  24  and end coupling  28 , with the joining arm  292  fitting over the end of the arm  256  which protrudes from the articulation component  254  through the use of a press fit or other attachment mechanism. The end cap  120  fits on the opposite end of the end coupling  28 , in the manner previously described. The fully assembled stabilization system  248  is shown in  FIG. 26 . In this assembly, the end coupling  28  and end cap  120  fit in the yoke  16  of the anchoring member  19 , and are held in place by tightening the set screw  18 , in the same manner set forth previously. The assembled stabilizer  250  is placed over the anchoring member  288 , with the extension post  290  on the anchoring member  288  extending posteriorly through the tunnel  258 . The set screws  259  are engaged in the outer wall of the housing  252  adjacent to the extension post  290 . When the set screws  259  are tightened, they push against the flexible stop  260 , which in turn pushes against the post  290 , holding the stabilizer  250  in place on the extension post  290 . Finally, the joining arm  292  connects the articulation component  24  to the articulation component  254 , thus pivotably connecting the stabilizer  250 , secured to the anchoring member  288 , to the anchoring member  19 . 
     When the system  248  is fully assembled and anchored to two adjacent vertebrae, motion between the two vertebrae can cause the planar spring  266  to flex out of its plane. Referring back to  FIG. 23 , when the two adjacent vertebrae move closer together and the distance between them shortens, the planar spring  266  returns to its plane. When the two adjacent vertebrae move apart and the distance between them lengthens, the planar spring  266  flexes in the opposite direction along the spiral path, toward the sleeve  278 . As the planar spring  266  flexes, the sleeve  278  which holds the articulation component  254  slides along the chamber  262 . The grooves  282  allow the sleeve  278  to slide back and forth past the pins  284 , but the pins  284  restrict axial movement of the sleeve  278  and serve as stops to prevent the sleeve  278  from moving completely out of the chamber  262 . 
     Referring to  FIG. 27 , a multi-level dynamic stabilization system is shown which includes a stabilizer  12  as per  FIGS. 1-8 , and an overhung stabilizer  250  as per  FIGS. 22-26 . The stabilizer  12  is mounted on two anchoring members  19  and connected via the joining arm  292  to the overhung stabilizer  250  which is mounted an anchoring member  288 . The resulting dynamic stabilization system provides stabilization across two adjacent vertebral levels. The overhung stabilizer  250  allows one of the levels to have a relatively short pedicle-to-pedicle displacement.  FIG. 28  illustrates the stabilizers  12 ,  250 , two anchoring members  19  and one anchoring member  288  in an exploded view. Each anchoring members  19  includes a fixation member  14 , a yoke  16  and a set screw  18 , as set forth previously. The anchoring member  288  includes a fixation member  14  with an extension post  290 , as set forth previously. 
     Referring to  FIG. 29 , the stabilizers  12 ,  250  and the anchoring members  19 ,  288  are shown in a further exploded view. The stabilizer  12  has two end couplings  28 , one end coupling  28  connecting with one end cap  120  thereby forming a coupling mountable in a yoke  16 . The second end coupling  28  of the stabilizer  12  preferably couples with the end coupling  28  that connects to the joining arm  292 , forming a coupling mountable in another yoke  16 . The joining arm  292  fits over the arm  256  of the stabilizer  250 , thus connecting the stabilizer  250  to the stabilizer  12 . The stabilizer  250  is mountable on the anchoring member  288 , in the manner set forth previously. When assembled, this two level system has two articulation components  24 , one articulation component  25 , and one articulation component  254 , providing pivotability between the stabilized vertebrae. Additionally, an overhung stabilizer  250 , a stabilizer  12 , and/or a stabilizer  170  such as that depicted in  FIGS. 17-21  can be implanted in combination with a rigid connector  160  such as that depicted in  FIGS. 14-16 . 
     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.

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