Patent Publication Number: US-2016228260-A1

Title: Spinal Implant

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/726,816, now U.S. Pat. No. 9,314,346, which claims the benefit of and priority from U.S. Provisional Patent Application No. 61/210,572 filed on Mar. 19, 2009 and which was a continuation-in-part of U.S. patent application Ser. No. 12/029,046, now U.S. Pat. No. 8,308,801, which claimed the benefit of and priority from U.S. Provisional Patent Application No. 60/901,217 filed on Feb. 12, 2007, all of which are incorporated herein in their entirety for all purposes by this reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the present invention relate generally to spinal implants and, more particularly, to intervertebral disc prostheses. 
     2. Background and Related Art 
     The human spine functions through a complex interaction of several parts of the anatomy.  FIGS. 1 and 2  ( FIG. 2  being the cross-section A-A of  FIG. 1 ) illustrate a segment of the spine  4 , with vertebra  5 . The vertebra  5  includes the vertebral body  6 , the spinous process  8 , transverse process  10 , pedicle  12 , and laminae  14 . A functional spine, comprising several vertebra  5 , typically subcategorized as being part of the cervical, thoracic, lumbar, sacral, and coccygeal regions as known, provides support to the head, neck, trunk, transfers weight to lower limbs, protects the spinal cord  20 , from which peripheral nerves  32  extend, and maintains the body in an upright position while sitting or standing. 
     Also illustrated in  FIGS. 1 and 2 , the spinal segment  4  includes intervertebral discs  20  that separate adjacent vertebra  5 . The intervertebral discs  20  provide motion, load bearing and cushioning between adjacent vertebrae  5 . Intervertebral discs  20  are the largest avascular structure in the body, relying on diffusion for nutrition. The diffusion of nutrients is aided by the compression cycles that the intervertebral discs  20  undergo during the course of normal movement, which drives out waste products and cycles fluids. Lying down and resting reduces the load on the intervertebral discs  20  allowing nutrients to diffuse into the intervertebral discs  20 . 
     Also illustrated in  FIGS. 1 and 2 , the spinal segment includes spinal facet joints  16 . Spinal facet joints  16  join the adjacent vertebrae  6 . The spinal facet joints  16  are synovial joints that function much like those of the fingers. Together with the intervertebral disc  20 , the spinal facet joints  16  function to provide proper motion and stability to a spinal segment  4 . Thus, each spinal segment  4  includes three joints: the intervertebral disc  20  in the anterior aspect of the spinal segment  4  and the two spinal facet joints  16  in the posterior aspect of the spinal segment  4 . 
     For the spinal segment  4  to be healthy, each of the intervertebral disc  20  and the spinal facet joints  16  must be healthy. To remain healthy these joints require motion. The intervertebral disc  20  and the spinal facet joints  16  function together to provide both quality and quantity of motion. The quality of the motion is a exhibited by the non-linear energy storage (force-deflection, torque-rotation) behavior of the spinal segment  4 . The quantity of motion is the range of segmental rotation and translation. 
     Back pain due to diseased, damaged, and/or degraded intervertebral discs  20  and/or spinal facet joints  16  is a significant health problem in the United States and globally. A non-exhaustive and non-limiting illustration of examples of diseased and/or damaged intervertebral discs is shown in  FIG. 3 . While a healthy intervertebral disc  20  is illustrated at the top of the spine segment  18 , diseased and/or damaged discs are also illustrated. The diseased and/or damaged discs include a degenerated disc  22 , a bulging disc  24 , a herniated disc  25 , a thinning disc  26 , discs indicating symptoms of degeneration with osteophyte formation  28 , as well as hypertrophic spinal facets  29 . 
     A degenerating spinal segment  18  is believed to be the product of adverse changes to its biochemistry and biomechanics. These adverse changes create a degenerative cascade affecting the quality and/or quantity of motion and may ultimately lead to pain. For example, as the health of a spinal segment  18  degenerates and/or changes, the space through which the spinal cord  30  and peripheral nerves  32  ( FIGS. 1 and 2 ) pass can become constricted and thereby impinge a nerve, causing pain. For example, the spinal cord  30  or peripheral nerves  32  may be contacted by a bulging disc  24  or herniated disc  25  or hypertrophic spinal facet  29  as illustrated in  FIG. 3 . As another example, a change in the spinal segment  18 , such as by a thinning disc  26  may alter the way in which the disc functions, such that the disc and spinal facets may not provide the stability or motion required to reduce muscle, ligament, and tendon strain. In other words, the muscular system is required to compensate for the structural deficiency and/or instability of the diseased spinal segment  18 , resulting in muscle fatigue, tissue strain, and hypertrophy of the spinal facets, further causing back pain. The pain this causes often leads patients to limit the pain-causing motion. However, this limiting of motion, while offering temporary relief, may result in longer-term harm because the lack of motion limits the ability of the disc to expel waste and obtain nutrients as discussed above. 
     In many instances of degenerative disc disease, fusion of the vertebrae is the standard of care for surgical treatment, illustrated in  FIG. 4 . In the U.S. alone, approximately 349,000 spinal fusions are performed each year at an estimated cost of $20.2 billion. The number of lower back, or lumbar, fusions performed in the U.S. is expected to grow to approximately 5 million annually by the year 2030 as the population ages, an increase of 2,200%. 
     Spinal fusion aims to limit the movement of the vertebra that are unstable or causing a patient pain and/or other symptoms. Spinal fusion typically involves the removal of a diseased disc  50 , illustrated in outline in  FIG. 4 . The removed disc  50  is replaced by one or more fusion cages  52 , which are filled or surrounded by autograft bone that typically is harvested by excising one or more spinal facet joints  57 . Vertebral bodies  51  adjacent the removed disc  50  are stabilized with one or more posterior supports  58  that are fixedly connected to the vertebral bodies  51  with the use of pedicle screws  54  that are screwed—such as by use of a bolt-style head  56  to turn the pedicle screw  54 —into a hole drilled into the pedicle  12  of the vertebral bodies  51 . 
     Fusion, however, often fails to provide adequate or sufficient long-term relief in about one-half of the treatments, resulting in low patient satisfaction. Further, fusion, by definition, restricts the overall motion of the treated functional spine unit, imposing increased stresses and limiting range of motion on those portions of the spinal segment adjacent to the fused vertebral bodies  51 . Fusion of a spinal segment has been indicated as a potential cause of degeneration to segments adjacent to the fusion. The adjacent spinal facet joints  57  and adjacent discs  59  often have to bear a greater load as a result of the fusion than would typically be the case, leading to possible overloading and, in turn, degeneration. Thus, surgical fusion often provides short-term relief, but possibly greater long-term spinal degradation than would otherwise have occurred. 
     Thus, a challenge to alleviating the back pain associated with various ailments is to find an intervertebral disc prosthesis that provides sufficient freedom of movement to at least reduce the risk to the functional health of the adjacent spinal segments, and/or facet joints, and/or discs that are otherwise compromised or have their functional health degraded by spinal fusion, and, more preferably, maintain the functional health of the adjacent spinal segments and/or facet joints and/or discs. Further, an intervertebral prosthesis optionally provides sufficient stability to the diseased segment to alleviate pain and/or other symptoms. 
     A further challenge is simply the complex, multi-dimensional nature of movement associated with a functional spine unit. Illustrated in  FIG. 5  are the varying axes around which a functional spine unit moves. For example, a vertebra  5  is illustrated with an X-axis  60 , around which a forward bending motion, or flexion,  61  in the anterior direction occurs. Flexion  61  is the motion that occurs when a person bends forward, for example. A rearward bending motion, or extension,  62  is also illustrated. The Y-axis  63  is the axis around which lateral extension, or bending,  64 , left and right, occurs. The Z-axis  65  is the axis around which axial rotation  66 , left and right, occurs. Spinal fusion, as discussed above, limits or prevents flexion  61 -extension  62 , but also limits or prevents motion in lateral extension, or bending,  64  and axial rotation  66 . Thus, an improved alternative remedy to fusion preferably allows for movement with improved stability around each of the three axes,  60 ,  63 , and  65 . 
     Another difficulty associated with the complex motion of the spine is that the center-of-rotation for movement around each of the X-axis  60 , Y-axis  63 , and Z-axis  65  differs for each axis. This is illustrated in  FIG. 6 , in which the center-of-rotation for the flexion  61 -extension  62  motion around the X-axis  60  is located at flexion-extension center-of-rotation  70 . The center-of-rotation for the lateral extension, or bending,  64  motion around the Y-axis  63  is located at lateral extension, or bending, center-of-rotation  73 . The center-of-rotation for the axial rotation  66  around the Z-axis  65  is located at axial rotation center-of-rotation  75 . For more complex motion patterns (e.g., combined flexion, lateral extension/bending, etc.) a two-dimensional representation of the center-of-rotation is inadequate, but the three-dimensional equivalent called the helical axis of motion, or instantaneous screw axis can be employed. Intervertebral disc prostheses that force rotation of a spinal segment around any axis other than the natural helical axis impose additional stresses on the tissue structures at both the diseased spinal segments and the adjacent spinal segments. Compounding the issue for the centers-of-rotation is that they actually change location during the movement, i.e., the location of the centers-of-rotation are instantaneous, which is sometimes referred to as the helical axis. Thus, a preferable remedy to spinal problems would account for the helical axis throughout the range of motion. Stated differently, a preferable intervertebral disc prosthesis would allow the diseased spinal segment and adjacent spinal segments to undergo motion approximate that of the natural helical axis through the range of motions. 
     Many previous efforts have been made to solve at least some of the problems associated with spinal fusion, but with varying degrees of success. For example, U.S. Patent Publication No. 2008/0195213 filed on Feb. 11, 2008 to several of the present inventors, discloses an intervertebral disc prosthesis that provides for motion in two directions, typically flexion-extension and lateral extension/bending, but not for axial rotation. (U.S. Patent Publication No. 2008/0195213 is incorporated herein in its entirety for all purposes by this reference.) 
     Thus, there exists a need for an intervertebral disc prosthesis that provides for flexion-extension, lateral extension/bending, and axial rotation. 
     Further, there exists a need for an intervertebral spinal prosthesis that reduces the stress on a diseased and/or damaged spinal segment without overloading the adjacent discs and vertebrae that could initiate progressive degeneration or diseases in the adjacent discs and vertebrae. 
     A need also exists for a spinal implant that provides for proper force-deflection behavior of the spinal implant (kinetics)—as noted above in the discussion of  FIG. 5 —preferably to approximate those of a normal, functional spine unit to relieve the load and strain on the adjacent intervertebral discs, to protect the spinal facet joints, to reduce the risk of damage to segments of the spine adjacent to the diseased segment, to reduce muscle fatigue and reduce and/or eliminate subsequent pain. 
     A need also exists for a spinal implant that exhibits kinematics—such as the limits of the ranges-of-motion and the centers-of-rotation noted above in the discussion of  FIG. 6 —that, preferably, are maintained near those of a functional spine unit to maintain an effective range of motion for the intervertebral discs, spinal facet joints, muscles, ligaments, and the tendons around the spine and to reduce the amount of neural element strain, e.g., the strain on the spinal cord and/or other parts of the nervous system. 
     BRIEF SUMMARY OF THE INVENTION 
     Various features and embodiments of the invention disclosed herein provide robust and durable intervertebral disc prostheses that accommodate motion in three axes as compared to a single axis and/or double axes of motion of the prior art. 
     Embodiments of the invention include a spinal implant, such as an intervertebral disc prosthesis to replace an intervertebral disc that is removed from between two vertebra. Thus, embodiments of the spinal implant optionally are positioned between a first and a second vertebra. The spinal implant includes a first rolling-contact core that is operably coupled to the first vertebra. The rolling-contact core includes a convex surface having a first axis, the convex surface providing a rolling motion in a first direction to the vertebra coupled to the rolling-contact core relative to a second vertebra. At least one flexure optionally connected to the first rolling-contact core constrains, at least in part, the rolling motion of the first rolling-contact core. 
     Optionally, embodiments of the invention include a second rolling-contact core that is operably coupled to the first rolling-contact core. The second rolling-contact core also includes a second convex surface having a second axis rotated from the first axis, the second convex surface providing a rolling motion in a second direction to the first vertebra relative to the second vertebra. At least another flexure optionally connected to the second rolling-contact core constrains, at least in part, the rolling motion of the second rolling-contact core. 
     In various embodiments, at least one of the flexures and the rolling-contact cores are coupled or secured directly to the vertebra. Alternatively, embodiments of the invention include end plates, to which the flexures and rolling-contact cores are coupled. The end plates are secured to the first and second vertebra, thereby coupling the rolling-contact cores to the vertebrae. 
     Optionally, embodiments of the spinal implant include an axial-rotation core operably coupled to at least the first rolling-contact core. The axial-rotation core is configured to provide rotation to the first vertebra relative to the second vertebra around an axis orthogonal to the first axis and/or the second axis. The axial-rotation core optionally includes another flexure connected thereto that constrains, at least in part, the rotation. 
     Embodiments of the spinal implant include a geometry that, once implanted, is configured to allow flexion-extension, and/or lateral extension/bending, and/or axial rotation with an instantaneous or near-instantaneous centers-of-rotation for the diseased and/or damaged spinal segment and/or adjacent vertebrae and/or spinal facet joints and/or discs that are similar to that of a healthy spinal segment. Thus, the spinal implant restores, to a degree, close to normal movement of the diseased and/or damaged spinal segment and adjacent vertebrae and/or spinal facet joints and/or discs, which, in turn, aids in maintaining the health of adjacent vertebra and/or spinal facet joints and/or discs. 
     Other embodiments of the spinal implant provide protection to the spine, discs, spinal cord, and peripheral nerves by reducing the risk of harmful, damaging, and/or painful movements while still providing a sufficient range of motion to reduce the risk to adjacent vertebrae and/or spinal facet joints and/or intervertebral discs becoming diseased and/or damaged from lack of sufficient movement. Embodiments of the spinal implant do so by reducing the stresses on a diseased and/or damaged spinal segment from which an intervertebral disc is removed without overloading the adjacent spinal segments, including the adjacent intervertebral discs, spinal facet joints, and vertebrae, that could initiate progressive degeneration or diseases in the adjacent spinal segments. 
     Additionally, embodiments of the spinal implant preferably provide proper motion—such as the centers-of-rotation, whether instantaneous or otherwise, limits of the ranges-of-motion, and the types of motion—that are maintained near those of a functional spine unit to maintain an effective range of motion for the muscles and the tendons around the spine and to reduce the amount of spinal cord strain. 
     Embodiments of the spinal implant are preferably made of biocompatible materials, including, but not limited to, biocompatible polymers and plastics, stainless steel, titanium, nitinol, shape-memory materials and/or alloys, and other similar materials. 
     Embodiments of methods of using the spinal implant are also disclosed. 
     As used herein, “at least one,” “one or more,” and “and/or” are open-ended expressions that are both conjunctive and disjunctive in operation. For example, each of the expressions “at least one of A, B and C,” “at least one of A, B, or C,” “one or more of A, B, and C,” “one or more of A, B, or C” and “A, B, and/or C” means A alone, B alone, C alone, A and B together, A and C together, B and C together, or A, B and C together. 
     Various embodiments of the present inventions are set forth in the attached figures and in the Detailed Description as provided herein and as embodied by the claims. It should be understood, however, that this Summary does not contain all of the aspects and embodiments of the one or more present inventions, is not meant to be limiting or restrictive in any manner, and that the invention(s) as disclosed herein is/are and will be understood by those of ordinary skill in the art to encompass obvious improvements and modifications thereto. 
     Additional advantages of the present invention will become readily apparent from the following discussion, particularly when taken together with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       To further clarify the above and other advantages and features of the one or more present inventions, reference to specific embodiments thereof are illustrated in the appended drawings. The drawings depict only exemplary embodiments and are therefore not to be considered limiting. One or more embodiments will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
         FIG. 1  is a segment of a functional spine unit; 
         FIG. 2  is a cross-section of the segment of the functional spine unit illustrated in  FIG. 1 , taken along section A-A of  FIG. 1 ; 
         FIG. 3  is a segment of a spine illustrating various pathologies of intervertebral discs; 
         FIG. 4  is a prior art discectomy and spinal fusion; 
         FIG. 5  illustrates the three axes of motion around which a functional spine unit moves; 
         FIG. 6  illustrates the centers-of-motion of a functional spine unit; 
         FIG. 7  illustrates an embodiment of a spinal implant, shown from the lateral/side view implanted and coupled directly to a first vertebra and a second vertebra; 
         FIG. 8  illustrates an embodiment of a spinal implant, shown from the lateral/side view implanted and coupled to optional end plates, the end plates secured to a first vertebra and a second vertebra; 
         FIG. 9  illustrates lateral/side view of an embodiment of the spinal implant that optionally includes an axial-rotation core; 
         FIG. 10  is an exploded isometric view of the spinal implant of  FIG. 9 ; 
         FIG. 11  is an isometric view of a rolling-contact core of the spinal implant of  FIG. 9 ; 
         FIG. 12  is a top view of several embodiments of the axial-rotation core; 
         FIG. 13  is an isometric view of another embodiment, a pin-in-slot joint, of the axial-rotation core and an embodiment of a rolling-contact core; and, 
         FIG. 14  is a side view of several embodiments of rolling-contact cores and end plates. 
     
    
    
     The drawings are not necessarily to scale. 
     DETAILED DESCRIPTION OF THE INVENTION 
     As noted above, the kinetics and kinematics of the spine are quite complex, involving three separate axes around which motion occurs and three separate centers-of-rotation for the different motions. Applicants have recognized that previous spinal implants often address just one form of motion, typically flexion and extension, often through the use of springs of some type that flex and compress. Efforts to address more than one mode of rotation or motion typically tend to be complex, large, and often do not address each individual motion as effectively as devices dedicated to a single motion. 
     Turning to  FIGS. 7 and 8 , embodiments of a spinal implant  150  and  151 , such as an intervertebral disc prosthesis, are illustrated positioned between a first vertebra  106  and a second vertebra  108  of a spinal segment  100  in a space where an intervertebral disc (e.g., intervertebral disc  120 ) has been removed. 
     Referring to  FIGS. 7 and 8 , each of the embodiments of the spinal implant  150 ,  151  include a first rolling-contact core  152 . The first rolling-contact core  152  optionally includes at least one flexure  154 . The spinal implant  150 ,  151  optionally includes a second rolling-contact core  156  that optionally includes at least another flexure  158 . The spinal implant  151  in  FIG. 8  optionally includes a first end plate  162  and a second end plate  164 . 
     Further details of the embodiments of the spinal implant  150 ,  151  are illustrated in  FIGS. 9-11 . A first rolling-contact core  152  includes a first rolling surface  153  and a first chord surface  155 , as best illustrated in  FIG. 11 . The embodiment of the first rolling surface  153  as illustrated is a portion of a cylindrical segment defined by a first radius of curvature  182  with a first axis  180 , the first rolling surface  153  providing a first rolling motion in a direction  184 . The first chord surface  155  is a plane through the cylindrical segment. The width  185  of the first rolling-contact core  152  can equal the diameter of the cylinder, or twice the first radius of curvature  182 , in those embodiments in which the first chord surface  155  bisects the cylindrical segment through the first axis  180 . While the first rolling surface  153  is illustrated to be defined by the first radius of curvature  182  and, therefore, circular in shape, it is understood that the first rolling surface  153  can be defined by a parabola, ellipsoid, toroid, hyperbolic, or other curved surface. The first rolling surface  153  can optionally be of a shape engineered and selected to provide a desired range of motion, instantaneous axis of rotation, helical axis of motion, kinematic response, resistance to motion, and the like. 
     Optionally, the spinal implant  150 ,  151  includes a second rolling-contact core  156 , which includes a second rolling surface  157  and a second chord surface  159 , as best illustrated in  FIG. 10 . The embodiment of the second rolling surface  157  as illustrated is a cylindrical segment defined by a second radius of curvature  183  with a second axis  181  that provides a second rolling motion in a direction  186 . The second chord surface  159  is a plane through the cylindrical segment. While the second rolling surface  157  is illustrated to be defined by a second radius of curvature  183  and, therefore, circular in shape, it is understood that the second rolling surface  157  can be defined by a parabola, ellipsoid, toroid, hyperbolic, or other curved surface. The second rolling surface  157  can optionally be of a shape engineered and selected to provide a desired range of motion, instantaneous axis of rotation, helical axis of motion, kinematic response, resistance to motion, and the like. In addition, the second rolling surface  157  can be of a different geometry and have a different, second radius of curvature  183  (or other defining characteristic, such as the major and minor axis of an ellipsoid, or the focus of a parabola, as non-limiting examples) than the geometry and first radius of curvature  181  of the first rolling surface  155 . 
     The second rolling-contact core  156 , when included, is oriented such that the second axis  181  is rotated relative to the first axis  180  such that the second rolling motion occurs in a second, different direction  186  relative to the first rolling motion that occurs in the first direction  184 . The first axis  180  and second axis  181  can be rotated relative to each other from about 0 degrees to about 180 degrees and, more preferably, from about 30 degrees to about 150 degrees and, more preferably still, from about 70 degrees to about 110 degrees, as well as orthogonal to each other. For example, a spinal implant  150 ,  151  can be provided that allows rolling motion in flexion-extension (e.g., around the X-axis  60  in  FIG. 5 ), as well as lateral extension/bending (e.g., around the Y-axis  63  in  FIG. 5 ). 
     The spinal implant  150 ,  151  can be formed of biocompatible plastics, polymers, metals, metal alloys, laminates, shape-memory materials, and other similar materials, either wholly as one material or as a combination of materials—i.e., different components may be manufactured from different materials and/or a single component, such as a rolling-contact core, can be manufactured of two or more materials, such as have a softer or resilient outer surface over a more rigid inner material. Optionally, the materials can be resilient. That is, the materials can have a varying and selectable degree of elastic deformation to provide cushioning between the vertebra  106  and  108  in order to mimic, at least in part, the cushioning that intervertebral discs  120  provide to the spinal segment  100 . 
     Optionally, the first rolling-contact  152  core includes at least one flexure  154 , and the second rolling-contact core  156  optionally includes at least another flexure  158 . That is, one or more flexures  154 ,  158  can be used to create what may be referred to as a compliant mechanism or compliant spinal implant because its motion occurs, in part, through the flexible deflection of the flexures, as is described below. For example,  FIGS. 9 and 10  illustrate the use of three flexures  154 ,  158  on the respective rolling-contact cores  152 ,  156 . As illustrated, the flexures  154 ,  158  are disposed on the first rolling surface  153  and the second rolling surface  157 , respectively, although they can be positioned elsewhere. The flexures  154 ,  158  optionally can be made from a different material or the same material as the rolling-contact cores. The flexures  154 ,  158  optionally can be formed as flexible bands of a resilient or elastic material. That is, the flexures  154 ,  158  optionally exhibit elastic, spring-like behavior. The flexures  154 ,  158  optionally can be formed of biocompatible plastics, polymers, metals, metal alloys, laminates, shape-memory materials, and other similar materials, either wholly as one material or as a combination of materials—i.e., different components may be manufactured from different materials. 
     The flexures  154 ,  158  optionally are formed by separating a strip of material from the respective rolling-contact core  152 ,  156 . Alternatively, the flexures  154 ,  158  are coupled to the respective rolling surface  153 ,  157  by welding, adhesives, mechanical connectors, and the like at a first end of the flexure  154 ,  158 . At another end of the flexure spaced apart from the first end, the flexure  154 ,  158  is coupled to either an end plate or directly to a vertebra through the use of bio-compatible adhesives, mechanical connectors, such as screws, welding, and the like. 
     The flexures  154 ,  158  provide, in part, a spring-like constraint to the rolling motion in the directions  184  and  186 , respectively. That is, the further the rolling motion occurs, the greater the restoring force that the flexures  154 ,  158  impart to the rolling-contact core  152 ,  156  to return the rolling-contact core  152 ,  156  to a neutral or undeflected position. In addition, the flexures  154 ,  158 , maintain, in part, the relative position of the rolling-contact core  152 ,  156  to either the vertebrae  106 ,  108  and/or the end plates  162 ,  164 . That is, the flexures  154 ,  158  allow rolling motion, but limit, in part, the ability of the rolling-contact core  152 ,  156  to move laterally, posteriorly, or anteriorly out of position relative to the vertebrae  106 ,  108 . 
     The flexures  154 ,  158  as noted optionally couple the rolling-contact cores  152 ,  156  directly to the vertebrae  106 ,  108 , whether through adhesives or mechanical devices, such as screws. The flexures  154 ,  158  can be attached at the vertebral end plate, within the area of the vertebra bounded by the vertebral end plate, or elsewhere on the vertebra, including the pedicles and/or the spinous process, and the like. The rolling surfaces  153 ,  157  would then roll directly upon the vertebra  106 ,  108 . 
     Alternatively, the flexures  154 ,  158  can be coupled to the device end plates  162 ,  164  by mechanical devices, such as screws and the like, adhesives, welding, slots into which the ends of the flexures are retained, such as by clamping, and such other methods and systems. The rolling surfaces  153 ,  157  then roll upon a surface of the end plates  162 ,  164 . The end plates  162 ,  164  can be square, rectangular, shaped like the vertebra  106 ,  108 , as illustrated in  FIG. 10 , or other such shapes. 
     The end plates  162 ,  164  can be formed of biocompatible plastics, polymers, metals, metal alloys, laminates, shape-memory materials, and other similar materials, either wholly as one material or as a combination of materials, such as having a softer or resilient outer surface over a more rigid inner material. Optionally, the materials can be flexible and/or resilient. That is, the materials can have a varying and selectable degree of elastic deformation to provide cushioning between the vertebra  106  and  108  in order to mimic, at least in part, the cushioning that intervertebral discs  120  provide to the spinal segment  100 . Further, resilient end plates  162 ,  164  optionally distribute the compressive load borne by the spinal implant  150  across a larger percentage of the area within the vertebral end plates, which may reduce the degree or the risk of remodeling of the cancellous tissue of the vertebra. Alternatively, the end plates  162 ,  164  optionally distribute the compressive load to the vertebral end plates. 
     The end plates  162 ,  164  operably couple the flexures  154 ,  158  and the rolling-contact cores  152 ,  156 , respectively, to the first vertebra  106  and the second vertebra  108 . More preferably, the end plates  162 ,  164  are not just operably coupled the vertebra, but also secured to the vertebra which indicates a direct connection to the vertebra, whereas operably coupled can include either a direct or indirect connection to the vertebra. The end plates  162 ,  164  can be secured via adhesives and/or mechanical devices, such as bone screws that can be installed in the optional through-holes  163  ( FIG. 10 ). Optionally, threaded anchors can be screwed into the vertebra (and/or spinous process, and/or pedicles, and other such locations of the vertebra and spine), the threaded anchor then being threaded into a blind hole (not illustrated). Other similar examples of mechanical systems fall within the scope of the disclosure. 
     It is noted that the above embodiments describe rolling-contact cores  152 ,  156  that include a curved surface and end plates  162 ,  164  of substantially planar surfaces. Of course, other embodiments of rolling-contact cores and end plates fall within the scope of the disclosure. Non-limiting examples of such embodiments are illustrated in  FIG. 14  and include: a rolling-contact core  452   a  that includes a convex surface and a substantially planar end plate  462   a ; a substantially planar rolling-contact core  452   b  and a convex end plate  462   b ; a convex rolling-contact core  452   c  and a convex end plate  462   c ; a convex rolling-contact core  452   d  and a concave end plate  462   d ; and a concave rolling-contact core  452   e  and a convex end plate  462   e . Of course combinations, such as rolling-contact cores and end plates of different configurations and combinations fall within the scope of the disclosure. Further, these other embodiments of rolling-contact cores and end plates optionally use the flexures described above to constrain, at least in part, the motion of the rolling-contact cores. In addition, these other embodiments of rolling-contact cores and end plates optionally use an axial rotation core as described below. 
     Optionally, the spinal implant  150 ,  151  includes an axial-rotation core  160  configured to provide axial rotation in a direction  188  ( FIG. 10 ) of a first vertebra relative to a second vertebra, such as vertebra  106 ,  108 , respectively. The axial rotation can, for example, occur around the Z-axis  65  as illustrated in  FIG. 5 , i.e., orthogonal to the first axis  180  and the second axis  181 . The axial-rotation core  160  can optionally be of a shape engineered and selected to provide a desired range of motion, instantaneous axis of rotation, helical axis of motion, kinematic response, resistance to motion, and the like. For example, while  FIGS. 9 and 10  illustrate an embodiment of an axial-rotation core  160  that is a cross or cruciform in shape, other non-limiting examples of embodiments include those illustrated in  FIG. 12 , such as  260   a  (a split-ring);  260   b  (a split-V);  260   c  (another split ring, in three portions);  260   d  (cross or cruciform); and  260   e  (one-half of split-ring). Other shapes fall within the scope of the disclosure. 
     The axial-rotation core  160  can be formed of biocompatible plastics, polymers, metals, metal alloys, laminates, shape-memory materials, and other similar materials, either wholly as one material or as a combination of materials. Optionally, the materials can be resilient. That is, the materials can have a varying and selectable degree of elastic deformation to provide cushioning between the vertebra  106  and  108  in order to mimic, at least in part, the cushioning that intervertebral discs  120  provide to the spinal segment  100 . The axial-rotation core  160  can be formed to be an integral part of one or more of the rolling-contact cores  152 ,  156 , or it can be a separate component coupled, either directly or indirectly, to the rolling-contact cores  152 ,  156 , such as through the use of adhesives and mechanical connecting devices, such as screws, welding, and the like. 
     Embodiments of the axial-rotation core  160  include those that are positioned between a rolling-contact core and a vertebra (not illustrated) and/or an end plate  162 ,  164 . Other embodiments include positioning the axial-rotation core  160  between two rolling-contact cores  152 ,  156  as illustrated in  FIGS. 9 and 10 . Other positions of the axial-rotation core  160  relative to the vertebra and the spinal implant  150 ,  151  and its components fall within the scope of the disclosure. 
     Optionally, the axial-rotation core  160  includes at least one axial or third flexure  161  and, optionally, more flexures  161 . The axial flexure(s)  161  can be coupled, directly or indirectly, to various parts of the axial-rotation core  160 , as illustrated in  FIG. 10 . Alternatively, the axial flexure  161  optionally can couple, in part, the axial-rotation core  160  to at least one of the rolling-contact cores, such as the first rolling-contact core  152  as illustrated in  FIG. 9 . In yet other embodiments, the axial flexure(s)  161  couple the axial-rotation core  160  to one or more of the end plates  162 ,  164  and/or the vertebra itself, such as the vertebra  106 ,  108 , and/or its vertebral endplates, and/or the pedicles, and/or the spinous process, and the like. The coupling of the axial flexure(s)  161  optionally can be achieved through the use of mechanical devices, such as screws and the like, adhesives, welding, slots into which the flexures are retained, such as by clamping, and such other methods and systems. 
     The axial flexure(s)  161  optionally can be made from a different material or the same material as the axial-rotation cores  160 . The axial flexure(s)  161  optionally can be formed as flexible bands of a resilient or elastic material. That is, the axial flexure(s)  161  optionally exhibit elastic, spring-like behavior. The axial flexure(s)  161  optionally can be formed of biocompatible plastics, polymers, metals, metal alloys, laminates, shape-memory materials, and other similar materials, either wholly as one material or as a combination of materials—i.e., different components may be manufactured from different materials. 
     The axial flexure(s)  161  provide, in part, a spring-like constraint to axial rotation in the direction  188 . That is, the greater the axial rotation, the greater the restoring force that the axial flexure(s)  161  impart to the axial-rotation core  160  to return the axial-rotation core  160  to a neutral or undeflected position. In addition, the axial flexure(s)  161  maintain, in part, the relative position of the axial-rotation core  160  to either the vertebrae  106 ,  108  and/or the end plates  162 ,  164 . That is, the axial flexure(s)  161  allow axial rotation, but limit, in part, the ability of the axial-rotation core  160  to move laterally, posteriorly, or anteriorly out of position relative to the vertebrae  106 ,  108 . 
     Another embodiment of the axial-rotation core is illustrated in  FIG. 13 . The axial-rotation core  390  is a pin-in-slot joint. That is, the axial-rotation core  390  includes a slot  392  formed within the chord surface  355  of a rolling-contact core  352 . The slot  392  is oriented to provide lateral movement in a direction  399  that, for example, may correspond to the Y-axis  63  in  FIG. 5 . A pin  394  is configured to be received and retained at a first end within the slot  392 . The pin  392  is coupled at the connection  396  to, for example, a rolling-contact core  356 , of which only a small portion is illustrated for clarity. The pin  394  is configured to rotate in a direction  398  around, for example, the Z-axis  65  in  FIG. 5 , thereby imparting a relative axial rotation between the rolling-contact cores  352 ,  356  and, consequently, the vertebrae coupled thereto. Thus, the axial-rotation core  390  provides a center-of-rotation that is capable of translation in a lateral direction while also providing axial rotation. 
     Optionally, the axial-rotation core  390  includes axial flexures (not illustrated in  FIG. 13 ), such as those axial flexure(s)  161  discussed above. 
     Embodiments of the spinal implant disclosed herein provide additional benefits, such as: 
     Kinetics similar to a healthy spine: Embodiments of the spinal implant provide relative motion to vertebra in the three axes discussed above regarding  FIG. 5  similar to that of a healthy spine. One result of this benefit is that the patient&#39;s muscles and ligaments do not have to compensate for an unnatural motion of the spinal implant, unlike the case with prior art devices. In other words, the spinal implant provides more natural motion, which would encourage patients to move more with less attendant pain as their muscles would not be compensating or overworking for a prior art spinal implant that does not provide such natural motion around all three axes. 
     Kinematics similar to a healthy spine: Related to the kinetics are the natural kinematics of embodiments of the spinal implants. As discussed above, the centers-of-rotation for flexion-extension, lateral extension/bending, and axial rotation, are each located in different places. Prior art devices cannot accommodate these separate centers-of-rotation around more than one axis, if even that; nor can they provide for the instantaneous or near instantaneous change in the location of the centers-of-motion as a spinal segment moves; nor can they provide for motion approximate the motion of a natural helical axis. Stated differently, the center-of-rotation of prior art devices is often in a different location than the natural center-of-rotation of the spine for a given movement. To compensate, patients with prior art devices suffered strain upon the spinal cord and peripheral nerves, muscle strain caused by the muscles overworking and compensating for the two different centers-of-rotation (that of the prior art device and that of the spine), ligament strain, and, consequently, pain. In contrast, embodiments of the present spinal implant provide centers-of-rotation in each of the three axes that are the same, or nearly the same, as a patient&#39;s natural centers-of-rotation for the spine. Thus, patients typically have less pain and, consequently, greater movement, to the benefit of the discs and the spine in general. 
     Adjust to the individual spine: As noted, embodiments of the spinal implant can be designed and/or selected preoperatively for an individual patient in order to provide implants that restore the diseased spine to near healthy function. That is, the particular geometry of the spinal implant and its components can be individually tailored to a particular patient and the particular location within the patient&#39;s spine at which the spinal implant is to be implanted. 
     Thus, disclosed above, in addition to the embodiments of the spinal implant are methods of treating a spine with a spinal implant, such as an intervertebral disc prosthesis, configured to provide motion in three axes and that provides kinetics and kinematics similar to that of a functional spine, as well as other methods that will be recognized by one of skill in the art. 
     As alluded to above, embodiments of methods of using the spinal implant are disclosed. While the spinal implants disclosed herein can be positioned within a spinal segment by using an anterior, posterior, or lateral approach in the patient, a preferred method is to use a posterior approach. Further, it is preferred that a minimally invasive procedure be used, such as by laparoscopy in which only one or a few, small incisions are made and the surgery is conducted with laparoscopic tools. The methods include making an incision; providing an embodiment of the spinal implant disclosed herein; positioning the spinal implant between a first vertebra and a second vertebra; and coupling the spinal implant to at least the first vertebra. Securing the spinal implant to the vertebrae may be done by applying straps, applying biocompatible adhesives, installing pedicle screws, and the like, as known in the art. 
     The present invention, in various embodiments, includes providing devices and processes in the absence of items not depicted and/or described herein or in various embodiments hereof, including in the absence of such items as may have been used in previous devices or processes, e.g., for improving performance, achieving ease and/or reducing cost of implementation. 
     The foregoing discussion of the invention has been presented for purposes of illustration and description. The foregoing is not intended to limit the invention to the form or forms disclosed herein. In the foregoing Detailed Description for example, various features of the invention are grouped together in one or more embodiments for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the following claims are hereby incorporated into this Detailed Description, with each claim standing on its own as a separate preferred embodiment of the invention. 
     Moreover, though the description of the invention has included description of one or more embodiments and certain variations and modifications, other variations and modifications are within the scope of the invention, e.g., as may be within the skill and knowledge of those in the art, after understanding the present disclosure. It is intended to obtain rights which include alternative embodiments to the extent permitted, including alternate, interchangeable and/or equivalent structures, functions, ranges or steps to those claimed, whether or not such alternate, interchangeable and/or equivalent structures, functions, ranges or steps are disclosed herein, and without intending to publicly dedicate any patentable subject matter.