Patent Publication Number: US-7585324-B2

Title: Cervical intervertebral stabilizer

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No.: 60/658,345, filed Mar. 3, 2005, the entire disclosure of which is hereby incorporated by reference. 
    
    
     BACKGROUND 
     The present disclosure generally relates to apparatus and methods for treatment of spinal disorders using an intervertebral prosthesis which is disposed in an intervertebral disc space following removal of a damaged or diseased intervertebral disc. 
     The objective in intervertebral disc replacement or intervertebral stabilization is to provide a prosthetic disc that combines both stability to support the high loads of the patient&#39;s vertebrae and flexibility to provide the patient with sufficient mobility and proper spinal column load distribution. 
     Numerous artificial intervertebral discs for replacing a part or all of a removed disc have been developed, namely, elastomer discs, ball and socket discs, mechanical spring discs and hybrid discs. Elastomer discs typically include an elastomer cushion which is sandwiched between lower and upper rigid endplates. The elastomer discs are advantageous in that the elastomer cushion functions similar in mechanical behavior to the removed intervertebral disc tissue. However, a disadvantage of this disc type is that the elastomer cushion experiences long term in-vivo problems stemming from microcracking, which detracts from its usefulness as a replacement option. Furthermore, attachment of the flexible elastomer cushion to rigid endplates presents additional difficulties. Examples of elastomer discs are disclosed in U.S. Pat. Nos. 5,702,450; 5,035,716; 4,874,389; and 4,863,477. 
     Ball and socket discs typically incorporate two plate members having cooperating inner ball and socket portions which permit articulating motion of the members during movement of the spine. The ball and socket arrangement is adept in restoring “motion” of the spine, but, is poor in replicating the natural stiffness of the intervertebral disc. Dislocation and wear are other concerns with this disc type. Examples of ball and socket discs are disclosed in U.S. Pat. Nos.: 5,507,816; and 5,258,031. 
     Mechanical spring discs usually incorporate one or more coiled springs disposed between metal endplates. The coiled springs preferably define a cumulative spring constant sufficient to maintain the spaced arrangement of the adjacent vertebrae and to allow normal movement of the vertebrae during flexion and extension of the spring in any direction. Examples of mechanical spring discs are disclosed in U.S. Pat. Nos. 5,458,642; and 4,309,777. 
     The hybrid artificial intervertebral disc incorporates two or more principles of any of the aforementioned disc types. For example, one known hybrid disc arrangement includes a ball and socket set surrounded by an elastomer ring. This hybrid disc provides several advantages with respect to load carrying ability, but, is generally complex requiring a number of individual components. Furthermore, long term in vivo difficulties with the elastomer cushion remain a concern as well as wear of the ball and socket arrangement. 
     Another type of intervertebral disc prosthesis is disclosed in U.S. Pat. No. 5,320,644. With reference to  FIGS. 1-3 , the &#39;644 patent discloses a unitary intervertebral disc member  1  made from an elastically deformable material. The disc member  1  has parallel slits  5  each arranged at a right angle to the axis of the disc member. The parallel slits  5  partially overlap one another to define overlapping regions  6  between adjacent slits. The overlapping regions  6  create leaf springs  7  for the transmission of forces from one vertebral attachment surface to the other. In regions of adjacent slits  5  where they do not overlap the spring action on the leaf springs  7  is interrupted by fixation zones  9  of solid prosthesis material. The forces acting on the intervertebral disc are transmitted from one leaf spring plane to the next leaf spring plane via the fixation zones  9 . The load paths are inherently abrupt with highly localized transfer of load through the sparsely placed fixation zones  9 . There are even instances where the entire load is carried through a single fixation zone  9  in the center of the disc. The abrupt load paths can lead to high stress regions, which can detract from the appropriate biomechanical performance, i.e., strength, flexibility, and range-of-motion, of the prosthesis. 
     U.S. Pat. No.: 6,296,664 discloses an intervertebral prosthesis having a disc member defining a longitudinal axis extending the height of the disc member and a lateral axis transverse to the longitudinal axis. The disc member includes an exterior wall which has a slit defined therein. The slit defines a longitudinal component of direction and a lateral component of direction. Preferably, the exterior wall includes a plurality of helical slits, adjacent slits being disposed in at least partial overlapping relation to define an overlapping region. Upon insertion of the disc member within the intervertebral space with the support surfaces in contacting engagement with respective vertebral portions of the adjacent vertebrae, forces exerted by the vertebral portions on the support surfaces are transferred along the exterior wall through the overlapping region. 
     All of the above intervertebral devices suffer from common problems, for example, they are limited in the reaction forces that they produce in response to compressive forces. For instance, once mechanical spring discs bottom out, there is no further articulation provided. This is undesirable in some applications. Further, the above described devices are not suitable for posterior implantation. Still further the above described devices are difficult to implant, reposition, or remove. 
     Thus, there has been discovered a need for a new intervertebral stabilizer. 
     SUMMARY OF THE INVENTION 
     In accordance with one or more embodiments of the present invention, a cervical intervertebral stabilizer for a cervical region of a spine includes: a first surface operable to engage an endplate of a first vertebral bone of a spine; a second surface spaced apart from the first surface and operable to engage an endplate of an adjacent second vertebral bone of the spine; a spring element including at least one of: (i) a helical wound spring; and (ii) a hollow body having at least one slit forming a plurality of annular circumferential helical coils, the spring element being disposed between the first and second surfaces and being operable to provide reactive force in response to compression loads from the first and second vertebral bones, wherein at least some diameters of respective turns of the helical coils differ. 
     Those of the turns having larger diameters are preferably disposed towards the first and second surfaces and those of the turns having smaller diameters are centrally located between the turns having larger diameters. Alternatively or in addition, the cross-sectional profile taken through the spring element is preferably at least partially hourglass shaped. Alternatively, the cross-sectional profile taken through the spring element is a multiple hourglass shape. 
     Other aspects, features, advantages, etc. will become apparent to one skilled in the art when the description of the preferred embodiments of the invention herein is taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For the purposes of illustrating the various aspects of the invention, there are shown in the drawings forms that are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
       It is noted that the numerous figures herein are drawn substantially to scale at least in terms of the relationships among the elements of the particular views shown. 
         FIGS. 1-2  illustrate perspective and side (or lateral) views, respectively, of an intervertebral stabilizer in accordance with one or more embodiments of the present invention; 
         FIGS. 3-4  illustrate perspective views of certain spring features of the intervertebral stabilizer of  FIGS. 1-2 ; 
         FIG. 5  is a side view of an intervertebral stabilizer in accordance with one or more further embodiments of the present invention; 
         FIGS. 6-8  illustrate perspective, side (or lateral), and anterior views, respectively, of an intervertebral stabilizer in accordance with one or more further embodiments of the present invention; 
         FIGS. 9-10  illustrate perspective and anterior views, respectively, of the intervertebral stabilizer of  FIGS. 6-8  in use; 
         FIGS. 11-13  illustrate perspective, side, and anterior views, respectively, of an intervertebral stabilizer in accordance with one or more further embodiments of the present invention; 
         FIG. 14  is a sectional view of an intervertebral stabilizer in accordance with one or more further embodiments of the present invention; 
         FIGS. 15-17  illustrate perspective, anterior, and side views, respectively, of an intervertebral stabilizer element in accordance with one or more further embodiments of the present invention; 
         FIGS. 18-20  illustrate perspective, side, and cross-sectional views, respectively, of an intervertebral stabilizer in accordance with one or more further embodiments of the present invention; 
         FIGS. 21-22  illustrate perspective and side views, respectively, of an intervertebral stabilizer in accordance with one or more further embodiments of the present invention; 
         FIGS. 23-24  illustrate perspective and side views, respectively, of an intervertebral stabilizer in accordance with one or more further embodiments of the present invention; 
         FIGS. 25-26  illustrate perspective and side views, respectively, of an intervertebral stabilizer in accordance with one or more further embodiments of the present invention; 
         FIG. 27  is a perspective view of an intervertebral trial in accordance with one or more embodiments of the present invention; 
         FIG. 28  is a perspective view of an alternative configuration of a spacer for the intervertebral trial of  FIG. 27 ; 
         FIGS. 29-31  illustrate perspective, top, and lateral cross-sectional views, respectively, of an insertion tool suitable for implanting one or more of the intervertebral stabilizers herein; 
         FIG. 32  is a perspective view of a wedge ramp insertion tool suitable for assisting in the implantation of one or more of the intervertebral stabilizers discussed herein; 
         FIGS. 33-37  are perspective views illustrating an implantation process utilizing the wedge ramps and insertion tools of  FIGS. 29-32 ; and 
         FIGS. 38-39  illustrate perspective and side views of an extraction tool suitable for repositioning and/or extracting one or more of the intervertebral stabilizers discussed herein. 
     
    
    
     DETAILS OF THE EMBODIMENTS OF THE INVENTION 
       FIGS. 1-2  illustrate an embodiment of a spinal intervertebral stabilizer  50  in accordance with one or more aspects of the present invention. The stabilizer  50  is sized and shaped to fit in the intervertebral space between adjacent vertebral bones of the spine. It is understood that the size and shape of the stabilizer  50  may be adapted to fit in an intervertebral space at any level of the spine, such as the cervical spine, thoracic spine, or lumbar spine. The stabilizer  50  is sized and shaped to be inserted into the inter-vertebral space from an anterior direction. The stabilizer  50  includes an upper surface  52  of a first member  53  and a lower surface  54  of a second member  55  that are operable to engage end plates of the respective vertebral bones. A spring element in the form of a helical coil  56  is interposed between the upper and lower surfaces  52 ,  54  of the first and second members  53 ,  55 . 
     The helical coil  56  includes at least one first segment  56 A having a first diameter and at least one second segment  56 B having a second diameter. In the embodiment shown, two second segments  56 B are disposed axially with respect to a single first segment  56 A, which is interposed between the second segments  56 B. As shown, the stabilizer  50  may provide some movement in compressive and/or expansion directions due to the spaces between the respective turns of the second segments  56 B of the helical coil  56 . These spaces between the turns may be adjusted to provide differing amounts of compressive or expansion movement of the stabilizer  50 . The compressive and expansion movement may also be adjusted by varying material properties of the segments  56 B. The stabilizer  50  may also provide some movement in bending due to the first segment  56 A of the helical coil  56 . More particularly, the first and second segments  56 A,  56 B of the stabilizer  50  permits movement as to displacement, rotation, subluxation, flexion, extension, bending, or any combination thereof. 
     Among the movements permitted by the stabilizer  50  is flexing by collapsing one side of the stabilizer  50  and expanding the other side. The degree of collapsing and expanding of the stabilizer  50  may be varied depending on the spring properties of the first and second segments  56 A,  56 B of the helical coil  56 . In this regard, reference is now made to  FIGS. 3-4 , which are conceptual illustrations of the spring properties of the first and second segments  56 A,  56 B, respectively, of the helical coil  56  as if employed separately. As shown in  FIG. 3 , the relatively small spring diameter of the first segment  56 A promotes bending (deflection f) because the deflection f is inversely proportional to an outside diameter D of the turns of the first segment  56 A. This may be expressed as follows: f=k/D, where k is the spring constant of the spring. Thus, the smaller the diameter D of the first segment  56 A, the more deflection f is achieved and vice verse. Notably, the first segment  56 A simultaneously prohibits compression and expansion (B) because such movement is directly proportional to the diameter D. This may be expressed as follows: B=k·D. In contrast, as the second segment  56 B has a relatively larger diameter spring, it promotes compression and expansion, and inhibits bending. 
     The resultant functionality of the helical coil  56  is that one or more of the segments of the coil  56  permit compression/expansion and inhibit flexion (such as segments  56 B), while one or more other of the segments permit flexion and inhibit compression/expansion (such as segment  56 A). Thus, the first and second segments  56 A,  56 B of the stabilizer  50  permit movement as to displacement, rotation, subluxation, flexion, extension, bending, or any combination thereof. 
     The functionality of the varying diameter coil segments  56 A,  56 B may be adapted in an embodiment with a spring element having a more gradually changing diameter as is illustrated in  FIG. 5 . This embodiment includes a spinal intervertebral stabilizer  80  in accordance with one or more aspects of the present invention. The stabilizer  80  is sized and shaped to fit in the intervertebral space between adjacent vertebral bones of the spine, and as with one or more other embodiments herein, it is understood that the size and shape of the stabilizer  80  may be adapted to fit in an intervertebral space at any level of the spine, such as the cervical spine, thoracic spine, or lumbar spine. As with the stabilizer  50 , the stabilizer  80  includes a spring element having a plurality of segments, some of which promote compression/expansion, while others promote flexion. The spring element is in the form of a helical coil of hourglass cross section. These and other properties of the stabilizer  80  will be discussed in more detail herein with reference to several specific examples. 
       FIGS. 6-8  illustrate an embodiment of a spinal intervertebral stabilizer  100 . As best seen in  FIGS. 9-10 , the stabilizer  100  is sized and shaped to fit in the intervertebral space between adjacent vertebral bones  10 ,  12  of the spine. It is understood that the size and shape of the stabilizer  100  may be adapted to fit in an intervertebral space at any level of the spine, such as the cervical spine, thoracic spine, or lumbar spine. The stabilizer  100  is sized and shaped to be inserted into the inter-vertebral space from an anterior direction. 
     The stabilizer  100  includes an upper surface  102  and a lower surface  104  that are operable to engage the end plates  14 ,  16  of the respective vertebral bones  10 ,  12 . The body  106  of the stabilizer  100  is of generally cylindrical construction. As best seen in  FIGS. 7-8 , a cross-sectional profile of the body  106  is hourglass shaped. The body  106  also includes a spring element in the form of a helical coil in which a continuous or substantially continuous slot  108  extends helically from a terminal end  108 A adjacent the first surface  102  to a terminal end  108 B adjacent the second surface  104 . The slot  108  provides the stabilizer  100  with a spring capability by creating respective turns or coils of the helical coil. In alternative embodiments, the spring feature of the body  106  may be formed from a helical wound spring, such as of circular, rectangular, or other shape cross-sectional configuration. In a preferred embodiment, the body  106  is formed of a substantially solid cylindrical hollow body in which the helical coils are formed from the substantially continuous slot  108  that is cut into the body  106  through to the hollow portion  120  thereof. 
     In a preferred embodiment, the upper surface  102 , the lower surface  104 , and the body  106  are formed as an integral element, e.g., of single-piece construction. 
     At rest, the stabilizer  100  preferably takes the orientation shown. The spring features of the body  106  are preferably designed such that the stabilizer  100  maintains a minimum distance between the vertebral bones  10 ,  12  inasmuch as the surfaces  102 ,  104  may not be compressed towards one another beyond a minimum distance. As shown, the stabilizer  100  provides some movement in the compressive direction because the slot  108  provides some distance between the “coils” of the spring feature. This distance or space between the coils may be adjusted to provide differing amounts of compressive movement of the stabilizer  100 . For example, the space may be at a minimum, such as zero, which would inhibit any compressive movement of the stabilizer  100  and also the vertebral bodies. 
     Although the stabilizer  100  limits the distance between the vertebral bodies, it permits some movement as to displacement, rotation, subluxation, flexion, extension, bending, or any combination thereof. For example, the designed permits longitudinal or flexing by collapsing one side of the stabilizer  100  and expanding the other side. Depending on the amount of space provided between the coils of the spring feature of the body  106 , the center of rotation associated with flexing may be well outside the inter-vertebral space, potentially one to five inches or more outside the inter-vertebral space. 
     As best seen in  FIG. 7 , the upper surface  102  includes a peripheral edge  112  that overhangs at least one coil of the body  106 , and preferably overhangs all of the coils of the body  106 . Similarly, the lower surface  104  includes a peripheral edge  114  that overhangs at least one coil of the body  106 , and preferably all of the coils of the body  106 . In this regard, a moment arm Ma is defined by a lateral distance between an outer surface of the at least one coil  110  of the body  106  and the peripheral edge  112  of the upper surface  102 . The same or another moment arm may also be defined in terms of the peripheral edge  114  of the lower surface  104  and the outer surface of the coil  110 . Those skilled in the art will appreciate that the moment arm Ma may also be defined in terms of the point at which the slot  108  collapses and respective adjacent coils engage one another. Irrespective of how the moment arm is defined, a compressive force Fc acting on, for example, at least a portion of the peripheral edge  112  and any portion of the lower surface  104  tends to collapse the spring of the body  106  and full compression of the spring results in closure of the slot  108  in the vicinity of the force Fc such that adjacent coils engage one another. Further compressive force Fc will work in conjunction with the moment arm Ma such that portions of the coils on an opposite side of the spring of the body  106  from the engaged coils tends to expand. 
     A surgeon is preferably provided with a plurality of different sized intervertebral stabilizers  100  that he or she may utilize to fit the particular physiology of the patient. In general, relatively larger intervertebral stabilizers  100  will be useful in the lumbar region of the spine, smaller sized intervertebral stabilizers  100  will be useful in the thoracic region of the spine, and still smaller sized intervertebral stabilizers  100  will be useful in the cervical spine. By way of example, it is preferred that a height H of the intervertebral stabilizer  100  (e.g., measured between the upper and lower surface  102 ,  104 ) is between about 8.0 mm to about 18.0 mm for use in the lumbar region of the spine. More particularly, a number of different sized intervertebral stabilizers  100  are preferably available to the surgeon, such as having a height of between (i) about 8.0 mm to 10.0 mm; (ii) about 10.0 mm to about 14.0 mm; and (iii) about 14.0 mm to about 18.0 mm. 
     The spring feature of the body  106  is preferably formed utilizing about 1.0 slot or 2.0 coils when the height of the intervertebral stabilizer  100  is about 8.0 mm to about 10.0 mm. In this context, about two turns or coils are created by one slot traversing at least partially around the body  106 . The spring feature of the body  106  is preferably formed from about 2.0 slots or 3.0 coils when the height of the intervertebral stabilizer  100  is about 10.0 mm to about 14.0 mm. Additionally, the spring feature of the body  106  is preferably formed from about 3.0 slots or 4.0 coils when the height of the intervertebral stabilizer  100  is about 14.0 mm to about 18.0 mm. 
     With reference to  FIGS. 11 ,  12 , and  13 , perspective, side, and anterior views, respectively, of an intervertebral stabilizer  100 A are illustrated. The intervertebral stabilizer  100 A is preferably used in the cervical spine. In this regard, the height of the intervertebral stabilizer  100 A is preferably between about 6.0 mm to about 9.0 mm. The surgeon is preferably provided with a plurality of different sized intervertebral stabilizers  100 A for the cervical region of the spine. In particular, an intervertebral stabilizer  100 A having a height of about 5.0 mm to about 7.0 mm is formed in which the spring feature of the body  106  is formed utilizing about 3.0 slots or 4.0 coils. In a further embodiment of the intervertebral stabilizer  100 A, the height is about 7.0 mm to about 9.0 mm and the spring feature of the body  106  is formed from about 4.0 slots or 5.0 coils. 
     As best seen in  FIGS. 6 and 11 , the intervertebral stabilizers  100 ,  100 A may include one or more bone adhesion facilitating elements  121  operable to promote bone adhesion to at least one of the upper and lower surface  102 ,  104 . As shown, the bone adhesion facilitating elements  121  may include one or more spikes oriented in any number of directions and being of generally triangular cross-section. Other embodiments of the invention contemplate that the bone adhesion facilitating elements are formed from one or more keels extending from the upper and/or lower surface  102 ,  104 ; and/or from one or more roughening elements (such as dimpling or knurling) on one or both of the upper and lower surfaces  102 ,  104 . 
     In one or more embodiments, the hollow portion  120  of the body  106  may extend from the upper surface  102  to the lower surface  104  unimpeded. With reference to  FIG. 14 , in one or more further embodiments, the hollow portion  120  may include a membrane  122  disposed in the passage and substantially closing off the passage to inhibit bone growth therethrough. Preferably, the membrane  122  is formed as an integral element of the body  106 . For example, the hollow portion  120  may be formed from first and second hollow portions  120 A,  120 B that do not pass all the way through the body  106 . With or without the membrane  122 , the hollow portion  120  is preferably of hourglass shape. 
     As best seen in  FIG. 14 , with or without the membrane  122 , the shape of the hollow portion  120  is preferably also hourglass shaped. Although the present invention is not limited to any particular theory of operation, it is believed that such an hourglass shaped hole or hollow portion  120  maximizes or at least significantly increases the moment arm Ma, the offset between the compressive load application point at the peripheral edge (e.g. at  112 A in  FIG. 1 ) and the outer surface of the spring element (e.g., at  110 ). 
     Reference is now made to  FIGS. 15-17 , which illustrate perspective, anterior, and side views, respectively, of an alternative embodiment of a spinal inter-vertebral stabilizer element  100 B. The stabilizer element  100 B may include some or all of the features discussed hereinabove with respect to the stabilizers  100  and/or  100 A. Respective peripheral edges  142  and  144  of the upper and lower surfaces  132 ,  134  circumscribe a kidney shape. In use, two of the stabilizer elements  100 B (mirror images of one another) are inserted into a single intervertebral space such that an overall envelope created by at least portions of the peripheral edges  142  and/or  144  of the two stabilizer element  100 B approximate the shape of the intervertebral space. 
     The intervertebral stabilizer  100 B is preferably sized and shaped to be inserted posteriorly or transversely into the intervertebral space. In this regard, the stabilizer element  100 B preferably includes a length L measured along an anterior-to-posterior direction of the spine and a width W along a lateral direction of the spine. The width of the stabilizer element  100 B is preferably smaller than the length thereof such that the stabilizer  100 B may be implanted from the posterior or transverse-posterior direction into the intervertebral space. 
     As with the intervertebral stabilizer  100  of  FIGS. 6-8 , the intervertebral stabilizer element  100 B is preferably provided to the surgeon in a number of different sizes (for each mirror image thereof) to accommodate different levels in the spine and/or different physiology of a given patient. It is preferred that the height H of the stabilizer element  100 B (e.g., measured between the upper and lower surface  132 ,  134 ) adheres to the various dimensions discussed hereinabove with respect to the stabilizer  100 . Further, the height of the stabilizer element  100 B is preferably characterized as being one of: (i) about 7.0 mm to about 15.0 mm when the spring element of the body  136  includes about 1.0 slot or 2.0 coils; (ii) about 11.0 mm to about 20.0 mm when the spring element of the body  136  includes about 2.0 slots or 3.0 coils; and (iii) about 13.0 mm to about 26.0 mm when the spring element of the body  136  includes about 3.0 slots or 4.0 coils. 
     Reference is now made to  FIGS. 18-20 , which are perspective, side and cross-sectional views, respectively, of a further embodiment  100 C of the present invention. In many ways, the stabilizer  100 C is substantially the same as the stabilizer  100  of  FIGS. 6-8 . Indeed, the stabilizer  100 C includes an upper surface  102  and a lower surface  104  that are operable to engage the end plates  14 ,  16  of the respective vertebral bones  10 ,  12 . The body  106  of the stabilizer  100 C is of generally cylindrical construction and includes a spring element in the form of a helical coil in which a continuous or substantially continuous slot  108  extends helically from a terminal end adjacent the first surface  102  to a terminal end adjacent the second surface  104 . Unlike the prior embodiments, the cross-sectional profile of the body  106  is only partially hourglass shaped. Indeed, the body  106  includes a substantially flat portion  106 ′ that does not have an hourglass contour. The detailed discussion above regarding the full hourglass-shaped body  106  applies with equal weight here, although those skilled in the art will appreciate that the principles of operation of the full hourglass-shaped body  106  may also be inherently extended. A hollow portion or aperture  120  preferably extends from the surface  102  to the surface  104 . The hollow portion  102  may also include the membrane  122  ( FIG. 14 ), and/or may also be hourglass shaped. 
     Reference is now made to  FIGS. 21-22 , which are perspective and side views, respectively, of a further embodiment of the present invention. The stabilizer  100 D includes an upper surface  102  and a lower surface  104  that are operable to engage the end plates of respective vertebral bones. The body  106  of the stabilizer  100 D includes a plurality of hourglass shaped segments  106 A,  106 B, etc. (two such segments being shown for illustration). As with some of the prior embodiments, the cross-sectional profile of the body  106  is hourglass shaped; however, this embodiment of the invention include multiple hourglass shapes in axial alignment. A hollow portion or aperture  120  preferably extends from the surface  102  to the surface  104 . The hollow portion  102  may also include the membrane  122  ( FIG. 14 ), and/or may also be hourglass shaped. The detailed discussion above regarding a single hourglass-shaped body  106  applies with equal weight here, although those skilled in the art will appreciate that the principles of operation of the single hourglass-shaped body  106  may also be inherently extended. 
     As best seen in  FIG. 22 , the intervertebral stabilizer  100 D may include one or more bone adhesion facilitating elements  121  operable to promote bone adhesion to at least one of the upper and lower surface  102 ,  104 . The bone adhesion facilitating elements  121  may include one or more spikes oriented in any number of directions and being of generally triangular cross-section. Other embodiments of the invention contemplate that the bone adhesion facilitating elements are formed from one or more keels extending from the upper and/or lower surface  102 ,  104 ; and/or from one or more roughening elements (such as dimpling or knurling) on one or both of the upper and lower surfaces  102 ,  104 . Alternatively or in addition, the stabilizer  100 D may include a flange  160  of generally transverse orientation with respect to the end surface (e.g., surface  102 ) and operable to engage a sidewall of the vertebral bone by driving one or more screws through aperture (s)  162  of the flange  160  into the vertebral bone. As shown, the stabilizer  100 D includes one or more bone adhesion facilitating elements  121  (e.g., spikes) on the surface  104  and a flange  160  extending from the surface  102 . Other combinations may be employed without departing from the spirit and scope of the invention. 
     Reference is now made to  FIGS. 23-24 , which are perspective and side views, respectively, of a further embodiment of the present invention. While the embodiments of the invention discussed above may be used to stabilize a single pair of vertebral bones, the stabilizer  100 E is operable to accommodate a larger space for multiple levels of intervertebral bones. The stabilizer  100 E includes an upper surface  102  and a lower surface  104  that are operable to engage the end plates of respective vertebral bones. The vertebral bones, however, need not be adjacent to one another; rather, a vertebral bone of an intervening level may be removed and the remaining vertebral bones may be stabilized using the stabilizer  100 E. The body  106  of the stabilizer  100 E includes a plurality of axially aligned, hourglass shaped segments  106 A,  106 B (two such segments being shown for illustration). A further segment  106 C is interposed between the hourglass segments  106 A,  106 B, where the segment  106 C does not include a spring feature. The segment  106 C accounts for the removed vertebral bone. As shown, the stabilizer  100 E includes one or more bone adhesion facilitating elements  121  (e.g., spikes) on the surface  104  and a flange  160  extending from the surface  102 . Other combinations may be employed without departing from the spirit and scope of the invention. 
     Reference is now made to  FIGS. 25-26 , which are perspective and side views, respectively, of a further embodiment  100 F of the present invention. As with the stabilizer  100 E of  FIGS. 23-24 , the stabilizer  100 F is operable to accommodate multi-level stabilization. The stabilizer  100 F includes an upper surface  102  and a lower surface  104  that are operable to engage the end plates of respective vertebral bones. Again, the vertebral bones are not adjacent to one another; rather, two or more vertebral bones of intervening level(s) may be removed and the remaining vertebral bones may be stabilized using the stabilizer  100 F. The body  106  of the stabilizer  100 F includes a plurality of axially aligned, hourglass shaped segments  106 A,  106 B,  106 E (three such segments being shown for illustration). Further segments  106 C and  106 D are interposed between the hourglass segments  106 A,  106 B and between  106 B,  106 E, respectively. The segments  106 C and  106 D do not include spring features as they accounts for the removed vertebral bones. As shown, the stabilizer  100 E includes a flange  160  on the surfaces  102 ,  104  to secure the stabilizer  100 F. It is noted that further bone adhesion promoting elements may also be employed without departing from the spirit and scope of the invention. 
     Reference is now made to  FIGS. 27-39 , which illustrate various instrumentations for implanting, for example, the intervertebral stabilizer  100  into a patient.  FIG. 27  is a perspective view of an intervertebral trial  200  which is preferably used to prepare the end plates  14 ,  16  of the intervertebral bones  10 ,  12 , respectively, prior to implantation of the stabilizer  100 . More particularly, after anterior incision and access to the intervertebral bones  10 ,  12  is obtained, the intervertebral space between the end plates  14 ,  16  is evacuated by removing the disk, some connecting tissue, etc. Next, the trial  200  is utilized to abrade the end plates  14 ,  16  of the vertebral bones  10 ,  12 . The trial  200  includes a handle  202  and at least one spacer element  204 . In a preferred embodiment, another spacer  206  (preferably of different size or character) is included at an opposite end of the handle from the spacer  204 . For purposes of brevity, reference will now be made only to spacer element  204 , it being understood that the description of spacer  204  may be applied to spacer  206  with equal force. 
     The spacer element  204  depends from the handle  202  and is preferably sized and shaped to fit in the intervertebral space between the respective end plates  14 ,  16 . The spacer element  204  includes an upper surface  208  and a lower surface  210  that are spaced apart by a height dimension. Preferably, the height is of a sufficient magnitude to at least slightly expand the intervertebral space when the spacer element  204  is urged between the end plates  14 ,  16 . More particularly, the upper surface  208  preferably engages the end plate  14 , while the lower surface  210  engages the end plate  16 . 
     At least one of the upper and lower surfaces  208 ,  210  preferably includes a roughening element  212 , such that insertion of the spacer element  204  into the intervertebral space abrades the associated end plate in preparation for implantation of the intervertebral stabilizer  100 . In a preferred embodiment, both the upper and lower surface  208 ,  210  include a roughening element  212  such that insertion of the spacer element  204  into the intervertebral space simultaneously abrades both end plates  14 ,  16 . Preferably, the roughening element is formed from substantially sharp knurling disposed on the respective surfaces  208 ,  210 . 
     With reference to  FIG. 28  the shape of the spacer element  204  may be kidney shaped for posterior or lateral implantation. 
     In a preferred embodiment, the surgeon is provided with a plurality of trials  200 , each with differing sized spacer elements  204 ,  206 , such that the surgeon may choose an appropriate sized trial  200  in order to prepare the intervertebral space for implantation. In addition, the plurality of trials  200  may include differing levels of roughness, for example, by adjusting the sharpness and magnitude of the knurling  212 . The abrasion of the end plates  14 ,  16  facilitates bone growth and secure engagement of the upper and lower surfaces  102 ,  104  of the stabilizer  100  upon implantation into the intervertebral space. 
     With reference to  FIGS. 29 ,  30  and  31 , an insertion tool  250  is preferably utilized to implant an intervertebral stabilizer, such as one or more of the intervertebral stabilizers discussed above into the intervertebral space. For purposes of discussion, reference to the stabilizer  100  of  FIGS. 6-8  will be made, it being understood that the description may be applied to the other stabilizer embodiments contemplated herein. 
     The insertion tool  250  includes a handle  252  and a head  254 . The head  254  is operable to releasably engage the intervertebral stabilizer  100  such that the surgeon may manipulate the position of the stabilizer  100  by way of the handle  252  in order to urge the stabilizer  100  into the intervertebral space. As best seen in  FIG. 31 , the head  254  includes at least a pair of spaced apart pins  256 ,  258  that facilitate the engagement between the head  254  and the intervertebral stabilizer  100 . 
     As best seen in  FIGS. 6 and 8 , the intervertebral stabilizer  100  (or any of the other embodiments herein) may include at least a pair of spaced apart apertures  150 ,  152  operable to receive the pins  256 ,  258  of the insertion tool  250 . As shown, a longitudinal axis A of the stabilizer  100  is normal to the first and second surface  102 ,  104  and the apertures  150 ,  152  extend transversely with respect to the longitudinal axis A. The apertures  150 ,  152  may extend at least partially into the body  106 , although it is preferred that the apertures  150 ,  152  extend all the way through the body  106  into the hollow portion  120 . As the stabilizer  100  is intended for anterior implantation, the apertures  150 ,  152  are preferably disposed on an anteriorly directed side of the body  106 . Thus, as best seen in  FIGS. 29 and 30 , the insertion tool  250  engages the intervertebral stabilizer  100  from the anterior direction such that the surgeon may urge the stabilizer  100  into the intervertebral space from the anterior direction. 
     Further, the apertures  150 ,  152  are preferably positioned longitudinal with respect to one another, parallel to the longitudinal axis A. For example, the aperture  150  is disposed toward the first surface  102  and the second aperture  152  is disposed toward the second surface  104 . As shown in  FIG. 8 , the apertures  150 ,  152  are entirely within the body  106  such that they form a closed interior surface. In a preferred embodiment, the apertures  150 ,  152  are disposed at terminal ends  108 A,  108 B of the slit  108  such that the slit  108  communicates with the interior of the respective apertures  150 ,  152 . 
     As shown in  FIGS. 11 and 13 , an alternative embodiment may provide a pair of apertures  154 ,  156  that extend only partially into the body  106  such that respective slots are formed at the first and second surfaces  102 ,  104 . In a preferred embodiment, one or more spikes, keels, or roughening elements  121  are disposed at least partially along the slot. For example, when a pair of spikes  121  are provided along the slot, a dual function may be enjoyed, namely: (i) the spikes assist in forming the slot, thereby facilitating engagement between the insertion tool  250  and stabilizer  100 A; and (ii) once implanted, the stabilizer  100 A is encouraged to remain in the implanted position owing to the spikes  121  engaging the respective end plates  14 ,  16 . 
     As best seen in  FIGS. 15 and 16 , one or more apertures  158  (one aperture being shown for simplicity) may be provided in the body  136  of the stabilizer element  100 B in order to assist in the implantation of the element  100 B into the intervertebral space from a posterior or transverse posterior direction. Thus, the aperture  158  is posteriorly directed. 
     Turning again to  FIGS. 29-31 , the pins  256 ,  258  are preferably flexible in a direction parallel to the longitudinal axis A ( FIG. 6 ) of the stabilizer  100 . Thus, assuming that the pins are spaced apart at an appropriate distance, the apertures  150 ,  152  urge the pins  254 ,  258  apart when the head  254  engages the stabilizer  100 . Similarly, depending on the spring constant of the spring feature of the stabilizer  100  as compared with the flexibility of the pins  254 ,  258 , the pins  254 ,  258  may urge the upper and lower surface  102 ,  104  together when the head  254  engages the stabilizer  100 . 
     As best seen in  FIG. 31 , a height of the head  254  (measured parallel to the longitudinal axis A) is preferably less than the height of the stabilizer  100 . This insures that the head  254  does not interfere with the implantation of the stabilizer  100  in the intervertebral space. As the cross-sectional profile of the body  106  is hourglass shaped, the head  254  is preferably sized such that it at least partially enters into the depression defined by the hourglass shape of the body  106 . It is preferred that the contour of the head  254  matches the curvature of the body  106  as is best seen in  FIG. 31 . Further, the head  254  preferably flairs out in a transverse (e.g., perpendicular) direction to the longitudinal axis A and terminates at respective prongs  260 ,  262  that provide lateral engagement with the body  106  of the stabilizer  100 . This advantageously assists in the lateral stability of the engaged insertion tool and stabilizer  100  as the stabilizer  100  is implanted into the intervertebral space. 
     Reference is now made to  FIGS. 32 and 33 , which illustrate a further insertion tool for implantation of the intervertebral stabilizer  100  from an anterior direction. Again, for purposes of discussion, reference to the stabilizer  100  of  FIGS. 6-8  will be made, it being understood that the description may be applied to the other stabilizer embodiments contemplated herein. The insertion tool preferably includes first and second elongate ramps  300 ,  302  that cooperate to assist in the implantation of the stabilizer  100  into the intervertebral space. A single ramp  300  is shown in  FIG. 32 , it being understood that the ramp  302  is substantially similar as will be evident to one of ordinary skill in the art after reviewing this specification. Each elongate ramp  300 ,  302  includes a proximal end  304  and a distal end  306 . The distal end  306  is sized and shaped for insertion into the intervertebral space in order to engage one of the end plates. The end  306  preferably includes a stop member  308  that is operable to engage the associated intervertebral bone and to limit a distance that the distal end  306  may enter into the intervertebral space. As best seen in  FIG. 33 , the stop  308  abuts the vertebral bone  10 . 
     In use, the first and second ramps  300 ,  302  are disposed opposite to one another to define upper and lower surfaces  320 ,  322  when the distal ends  306  thereof are inserted into the intervertebral space. The proximal ends  304  of the ramps  300 ,  302  are preferably fixed positionally with respect to one another by a clamp member  310 . More particularly, each of the distal ends  304  includes a bore  312  into which an end of the clamp  310  may be inserted. The clamp  310  is preferably of a U-shape in order to fix the relative positions of the proximal ends  304  with respect to one another. It is noted that the surgeon may omit use of the clamp if he or she insures that the proximal ends  304  of the ramps  300 ,  302  are fixed with respect to one another by clamping same with his or her hand. 
     With reference to  FIGS. 34 and 35 , the intervertebral stabilizer  100  is preferably slid along the surfaces  320 ,  322  from the proximal end  304  toward the distal end  306 . In a preferred embodiment, respective protrusions, such as spikes  121  are spaced apart on at least one of the upper and lower surface  102 ,  104  of the stabilizer  100  at a distance to accommodate a width of the ramps  300 ,  302 . Advantageously, slideable engagement of the spikes  121  with respective lateral edges  324 ,  326  of the ramps  300 ,  302  insure that the stabilizer  100  slides properly along the surfaces  320 ,  322  and remains between the ramps  300 ,  302 . Preferably, the lateral edges  324 ,  326  are chamfered in an appropriate way to complement the contour of the spikes  121  to improve slideability and/or stability of the intervertebral stabilizer  100  as it slides along the ramps  300 ,  302 . 
     As best seen in  FIG. 35 , the substantially fixed positions of the distal ends  304  of the ramps  300 ,  302  and the sliding intervertebral stabilizer  100  at least opens the intervertebral space owing to the lever action of the ramps  300 ,  302 . It is also preferred that simultaneously with the opening of the intervertebral space, the intervertebral stabilizer  100  is compressed. It is noted that if the surgeon chooses to manually urge the distal ends  304  of the ramps  300 ,  302  together while the intervertebral stabilizer  100  is interposed between the proximal and distal ends  304 ,  306  of the ramps  300 ,  302 , then additional opening of the intervertebral space and/or compression of the intervertebral stabilizer  100  may be obtained. Of course, the surgeon would have to perform this manual operation without the clamp  310 . 
     As best seen in  FIG. 36 , appropriate sliding of the intervertebral stabilizer  100  along the ramps  300 ,  302  utilizing the handle  252  of the insertion tool  250  results in proper positioning of the intervertebral stabilizer  100  within the intervertebral space. Thereafter, the ramps  300 ,  302  may be removed such that the intervertebral stabilizer  100  engages the respective end plates  14 ,  16  of the adjacent vertebral bones  10 ,  12 . Thereafter, the surgeon may remove the insertion tool  250  thereby completing the implantation of the intervertebral stabilizer  100  into the intervertebral space. Appropriate closure procedures may then be carried out. 
     With reference to  FIGS. 38 and 39 , the surgeon may extract and/or reposition the intervertebral stabilizer  100  after implantation into the intervertebral space. In this regard, an extractor  350  may be utilized. The extractor  350  preferably includes a handle  352  having proximal and distal ends  354 ,  356 , respectively. The extraction tool  350  preferably further includes an engagement element  358  depending from the proximal end  356 . The engagement element  358  is preferably operable to releaseably engage the stabilizer  100  after it has been positioned within the intervertebral space. More particularly, the engagement element includes a longitudinally extending member  360  and a transversely extending member  362 . The longitudinally extending member  360  is preferably sized such that is may pass through one or more of the apertures  150 ,  152  (or any of the other aperture embodiments herein). The transversely extending member  362  is preferably sized to pass through the slit  108  as the longitudinally extending member  360  is inserted into the aperture, for example, aperture  150 . The longitudinally extending member  360  is preferably of sufficient length to cause the transversely extending member  362  to enter the hollow portion  120  of the body  106 . Thereafter, the surgeon preferably rotates the handle  352  of the extraction tool  350  such that the transversely extending member  362  engages an interior surface of the hollow portion  120  such that the stabilizer  100  may be pulled by the handle  352  of the extraction tool  350 . 
     In an alternative embodiment, the engagement element  358  includes a threaded portion (not shown) that may be screwed into a threaded aperture in order to engage the stabilizer. For example, the aperture  158  ( FIG. 15 ) of the stabilizer element  100 B may be threaded and the engagement element  358  of the extraction tool  350  may be threaded into the aperture  158  to permit the surgeon to reposition and/or extract the stabilizer element  100 B. 
     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.