US20120010660A1 - System and method for segmentally modular spinal plating - Google Patents

Abstract

A modular implant for stabilizing the relative motion of spinal vertebrae comprises at least two pairs of plates which connect to two adjacent spinous processes, a spacer configured to be positioned between the spinous processes, and a fastener which pivotably connects to the plates and the spacer. The spacer is interchangeable and may comprise a variety of materials, each providing a different level of elasticity to the spinous processes. Relative motion between vertebrae can also be controlled by varying the surface configuration of the plates and by varying threading of the fastener. Several implants may be linked to provide stabilization across multiple vertebral levels, and the relative motion provided at each vertebral level may differ. A method for revising the implant is provided which comprises accessing the implant and replacing the spacer.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS
• This application is a continuation of:
• pending U.S. patent application Ser. No. 11/809,602, filed May 31, 2007, which carries Applicant's docket no. MLI-57 NPROV and is entitled SYSTEM AND METHOD FOR SEGMENTALLY MODULAR SPINAL PLATING, which claims the benefit of:
• prior U.S. Provisional Patent Application No. 60/803,594, filed May 31, 2006 by T. Wade Fallin et al., which carries Applicants' docket no. MLI-57 PROV, and is entitled SYSTEM AND METHOD FOR SEGMENTALLY MODULAR SPINAL PLATING.
• The above-mentioned documents are herein incorporated by reference in their entirety.
BACKGROUND OF THE INVENTION
• 1. The Field of the Invention
• The present invention relates generally to spinal orthopedics, and more specifically, to posterior implants designed to dynamically stabilize or immobilize one or more spinal motion segments.
• 2. The Relevant Technology
• Orthopedic medicine provides a wide array of implants that can be attached to bone to alleviate various pathologies. Due to the degeneration of spinal tissues, it can be desirable to dynamically stabilize, or even immobilize, adjacent vertebral levels. Unfortunately, currently available implants are often usable only to treat a very narrow range of pathologies. Many such devices are also bulky, difficult to implant, or difficult to revise. There is a need in the art for posterior spinal implants capable of providing dynamic stabilization at a desired level of stiffness so that a variety of pathologies can be treated via first implantation or revision.
BRIEF DESCRIPTION OF THE DRAWINGS
• Various embodiments of the present invention will now be discussed with reference to the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope.
• FIG. 1 is a posterior view of a segmentally modular spinal plating system fixed to a portion of the spine.
• FIG. 2 is a posterior view of the segmentally modular spinal plating system ofFIG. 1, which includes a spacer assembly, a pair of straight plates, a pair of jogged plates, and three fasteners.
• FIG. 3 is a perspective view of the spacer assembly ofFIG. 2.
• FIG. 4 is a perspective view of the straight plates ofFIG. 2.
• FIG. 5 is a perspective view of the jogged plates ofFIG. 2.
• FIG. 6 is a posterior view of a partial assembly of one straight plate ofFIG. 4 joined by a fastener to one jogged plate ofFIG. 5.
• FIG. 7 is a posterior view of the partial assembly ofFIG. 6, joined to the spacer assembly ofFIG. 3.
• FIG. 8 is a posterior view of the partial assembly ofFIG. 7, joined to an additional straight plate and an additional jogged plate.
• FIG. 9 is a posterior perspective view of the segmentally modular spinal plating system ofFIG. 1.
• FIG. 10 is a posterior perspective view of a two-level segmentally modular spinal plating system fixed to a portion of the spine.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
• The present invention relates to systems and methods for stabilizing the relative motion of spinal vertebrae. Those of skill in the art will recognize that the following description is merely illustrative of the principles of the invention, which may be applied in various ways to provide many different alternative embodiments.
• Referring toFIG. 1, a posterior view illustrates a segmentally modularspinal plating system10 affixed to a portion of the lumbar spine. Thesystem10 comprises a non-threadedstraight plate12, a singly-threadedstraight plate13, a non-threaded joggedplate14, a doubly-threaded joggedplate15, and aspacer assembly16. Thestraight plates12,13 lie on both lateral sides of thespinous process2 of the lower vertebra, and are connected by afastener18 which passes through the non-threadedstraight plate12, through thespinous process2, and through the singly-threadedstraight plate13. The joggedplates14,15 lie on both lateral sides of thespinous process4 of the adjacent vertebra above and are connected by afastener20 which passes through the non-threaded joggedplate14, through thespinous process4 and through the doubly-threaded joggedplate15. Between the twospinous processes2,4, thespacer assembly16 is connected to both of thestraight plates12,13 and both of the joggedplates14,15 by afastener22. Thefastener22 lies transverse to the spinal column on an axis about which the plates can pivot; this axis extends along the medial/lateral direction (i.e., left-to-right when viewingFIG. 1).
• Referring toFIG. 2, an enlarged posterior view illustrates the segmentally modularspinal plating system10 in more detail. The twostraight plates12,13 are secured by afastener18. The two joggedplates14,15 are secured by afastener20. “Jogged plates” refers to plates which are not planar between two ends, but are non-planar or bent between two ends. Thestraight plates12,13 and the joggedplates14,15 are linked together by thespacer assembly16. In the embodiment depicted inFIG. 2, thespacer assembly16 includes aspacing member40 and arim cap42. Thespacer assembly16 is threaded on thefastener22. In alternative embodiments, thespacer assembly16 is a one-piece unitary construct. In the embodiment depicted, thefasteners18,20,22 are threaded bolts. In alternative embodiments, thefasteners18,20,22 need not be threaded bolts but could be non-threaded bolts, bolts with nuts, screws, pins, or rivets, among others.
• FIG. 3 illustrates thespacer assembly16 in an enlarged perspective view. Thespacing member40 is generally tubular in form; however, the shape of thespacing member40 is such that it best conforms to the morphology of the area on thespinous processes2,4 which it contacts and can be other non-circular, more organic, bone conforming shapes. Thespacing member40 has anouter end44 and aninner end46. Theouter end44 is wider in diameter than the rest of thespacing member40, and forms a flange-like shape along arim48. Aradial spline50 occupies an outer interface surface49 of therim48. Abore52 extends through the longitudinal center of thespacing member40. The outer cylindrical wall of the spacing member is abearing surface54.
• Therim cap42 is generally flat and circular, and has aninner side56 and anouter interface side58. Depressed into theinner side56 is aconcavity64. A diameter of theconcavity64 is sized to hold theinner end46 of thespacing member40. A radial spline60 (not visible inFIG. 3) occupies an outer interface surface59 of theouter side58. Abore62 runs through the center of therim cap42.
• When implanted as part of the segmentally modularspinal plating device10, the spacing member40 (particularly the bearing surface54) and therim cap42 come in contact with the sides and outer edge of thespinous processes2,4 as seen inFIG. 1. To serve different stabilization purposes, thespacing member40 andrim cap42 may be composed of a variety of materials. If dynamic stabilization is desired, the spacingmember40 andrim cap42 can be composed of a semi-rigid, elastically compliant biocompatible polymer such as polyurethane or the spacingmember40 may be designed with spring elements (not shown) that allow flexibility and compressibility of thespacer assembly16 when it is loaded by thespinous processes2 and4. If stabilization is desired, the spacingmember40 andrim cap42 can be composed of a substantially rigid biocompatible materials including metals such as titanium, cobalt chromium alloys, stainless steel alloys or the like or other substantially rigid materials such as PEEK, Ultra High Molecular weight polyethylene (UHMWPE), Delrin, ceramics, or other biocompatible structural engineering polymers or ceramics. If fusion is desired, the spacingmember40 andrim cap42 may be composed of natural or synthetic bone material. Finally, if completely dynamic movement is desired, thedevice10 may be implanted without aspacer40 andrim cap42, and with a non-threaded rod replacing thefastener22.
• Referring toFIG. 4, an enlarged view shows the non-threadedstraight plate12 and the singly-threadedstraight plate13. The twostraight plates12,13 are substantially identical to each other in shape and are symmetrical from side to side. The only difference between thestraight plates12,13 is whether or not the bores in the plates are threaded. Eachstraight plate12,13 has a substantially planar, elongated elliptical shape with an outer facingside72 and an inner facingside74. The center of eachstraight plate12,13 is aplanar member76, which is terminated at one longitudinal end by afirst annulus78. Anon-threaded bore80 perforates thefirst annulus78. An outer facingradial spline82 is an outer interface surface79 of thefirst annulus78, and an inner facingradial spline84 is on an inner interface surface81 of thefirst annulus78.
• The opposite longitudinal end of theplanar member76 is terminated by asecond annulus88. On the non-threadedstraight plate12, anon-threaded bore90 perforates thesecond annulus88. On the singly-threadedstraight plate13, a threadedbore96 perforates thesecond annulus88. An outer facingradial spline92 is on an outer interface surface89 of thesecond annulus88, and an inner facingradial spline94 is an inner interface surface91 of thesecond annulus88. Because the non-threadedstraight plate12 has twonon-threaded bores80,90, and is otherwise symmetrical, it may be turned side to side or end to end prior to assembly in thedevice10. However, the singly-threadedstraight plate13 has one non-threaded bore80 and one threadedbore96. Thus, it may be turned side to side but not end to end to be properly assembled in thedevice10.
• FIG. 5 displays the two joggedplates14,15. As with thestraight plates12,13, the joggedplates14,15 are identical to each other except for the threading of the bores. Each joggedplate14,15 has an outer facingside102 and aninner facing side104. Acentral jogged member106 terminates at one longitudinal end at afirst annulus108. On the non-threaded joggedplate14, anon-threaded bore110 perforates thefirst annulus108. On the doubly-threadedjogged plate15, a threadedbore116 perforates thefirst annulus108. On both joggedplates14,15 an outer facingradial spline112 is on an outer interface surface109 of thefirst annulus108, and an inner facingradial spline114 is on an inner interface surface111 of thefirst annulus108.
• The opposite longitudinal end of the joggedmember106 is terminated at asecond annulus118. On the non-threaded joggedplate14, anon-threaded bore120 perforates thesecond annulus118. On the doubly-threadedjogged plate15, a threadedbore126 perforates thesecond annulus118. On each joggedplate14,15, an outer facingradial spline122 is on an outer interface surface119 of thesecond annulus118, and an inner facingradial spline124 is on an inner interface surface121 of thesecond annulus118. Because of the non-planar configuration of the jogged plates, the joggedplates14,15 cannot be turned side to side or end to end but must be specifically oriented to be properly assembled in thesystem10.
• FIGS. 6 through 9 show the steps in assembly of thedevice10.FIG. 6 depicts thefastener22 with the non-threadedstraight plate12 and the non-threaded joggedplate14. During assembly, first the non-threaded joggedplate14 is placed onto thefastener22. A threadedshaft32 of thefastener22 is placed through thebore120 of thesecond annulus118, from the outer facingside102 to the inner facingside104. The non-threaded joggedplate14 is slid along theshaft32 until thesecond annulus118 contacts a fastener head34.
• Following placement of the non-threaded joggedplate14 as described above, the non-threadedstraight plate12 is put onto thefastener22. Thebore80 on thefirst annulus78 is slid along the threadedshaft32, until thefirst annulus78 contacts the non-threaded joggedplate14. At this point, theinner radial spline124 of the non-threaded joggedplate14 engages with theouter radial spline82 of the non-threadedstraight plate12, causing theplates12,14 to releasably lock into place relative to one another. The interaction of theradial splines124,82 allows for very precise adjustment to obtain the desired length of thedevice10. During the implantation process, the angle at which theplates12,14 engage may be adjusted by pulling the plates apart, rotating one plate or another around theshaft32 until the desired position of the plates about the axis is found, then pushing theplates12,14 back together so that the radial splines82,124 engage. Because thebores80,120 of theplates12,14 are not threaded, they can freely rotate around theshaft32 until locked into place by meshing the splines. Adjusting the angle of theplates12,14 about the axis adjusts the length of thedevice10. The axis is defined by interaction between the bores of the plates and the outer surface of thefastener shaft32.
• FIG. 7 illustrates the addition of thespacer assembly16 to the partial assembly depicted inFIG. 6. Thebore52 of thespacer member40 is slid onto the threadedshaft32 of thefastener22. Thespacer member40 is slid onto thefastener22 until theouter radial spline50 of thespacer member40 engages with theinner radial spline84 of the non-threadedstraight plate12. Therim cap42 is then slid onto thefastener22. Thebore62 of therim cap42 is slid onto the threadedshaft32 of thefastener22, until theconcavity64 fits over theinner end46 of thespacer member40. Alternatively, thespacer member40 and therim cap42 may first be press fit or threaded together, with theinner end46 fitting into theconcavity64, and then slid as a single piece onto thefastener22.
• Referring toFIG. 8, the singly-threadedstraight plate13 and the doubly-threadedjogged plate15 are shown as added to the partial assembly depicted inFIG. 7. The non-threaded bore80 of thefirst annulus78 on the singly-threadedstraight plate13 is slid onto the threaded shaft32 (not visible) of thefastener22. When thefirst annulus78 contacts therim cap42, theinner radial spline84 of the singly-threadedstraight plate13 engages with theouter radial spline60 on therim cap42. In the manner described above, during implantation the singly-threadedstraight plate13 may be rotated around the threadedshaft32 until the desired position about the axis is achieved, before the meshing of thesplines60,84 releasably locks the singly-threadedplate13 into place.
• The doubly-threadedjogged plate15 is the last plate to be added to the assembly. The threaded bore126 on itssecond annulus118 is screwed onto the threadedshaft32, until theinner radial spline124 engages with theouter radial spline82 on the singly-threadedstraight plate13. As withplates12 and14,plates13 and15 may be adjusted about axis until the desired angle and length for thedevice10 is found.
• FIG. 9 depicts thesystem10 as fully assembled, with thefasteners18 and20 in place. Before thefasteners18,20 are added, the partially assembledsystem10 is placed on the spine so that the spacingmember40 is between thespinous processes2,4 of the vertebrae. The twofirst annuli108 of the joggedplates14,15 extend in a cephalad direction on either side of the upperspinous process4. The twosecond annuli88 of thestraight plates12,13 extend in a caudal direction on either side of the lowerspinous process2. The threadedshaft24 of thefastener18 passes through the non-threaded bore90 of the non-threadedstraight plate12, through thespinous process2 of the lower vertebra, then screws into the threaded bore96 of the singly-threadedstraight plate13. The engagement of the threads on the threadedshaft24 with the threads in the threaded bore96 tightens thefastener18 in place. Similarly, the threadedshaft26 of thefastener20 passes through thenon-threaded bore110 of the non-threaded joggedplate14, through thespinous process4 of the upper vertebra, then screws into the threaded bore116 of the doubly-threadedjogged plate15. The engagement of threads on the threadedshaft26 with the threads in the threadedbore116 tightens thefastener20 into place. In the embodiment depicted,fasteners18 and20 pass through thespinous processes2 and4 to fasten the plates to the spinous processes. However, in other embodiments, other means of attachment may be used to fasten the plates to the spinous processes, such as bands, clamps, cables, wire, and sutures, among others.
• FIG. 10 illustrates a two-level segmentally modularspinal plating system210 fixed in place in a portion of the spine. Thesystem210 comprises asingle level system10, plus two additional non-threadedstraight plates12, an additional singly-threadedstraight plate13, a doubly-threadedstraight plate17, anadditional spacer assembly16, and two additional fasteners. A non-threadedstraight plate12 and the doubly-threadedstraight plate17 are added to the upper level of thesingle level system10, and connect thesingle level system10 to anfastener22. Thefastener22 retains the non-threadedstraight plate12, a second non-threadedstraight plate12, aspacer assembly16, a singly-threadedstraight plate13, and the doubly-threadedstraight plate17. Thespacer assembly16 fits between thespinous processes4 and6. Afinal fastener18 retains one non-threadedstraight plate12 and the singly-threadedstraight plate13. When a twolevel system210 is utilized, the twospacer assemblies16 may be composed of like material to provide similar stabilization between all involved vertebrae, or the twospacer assemblies16 may be of different materials to provide different types of stabilization between the different vertebrae. The twolevel system210 enables stabilization between three vertebrae; additional levels may be added if desired, again withinterchangeable spacer assemblies16.
• The embodiments depicted inFIGS. 1 through 10 utilize a threaded bolt for each of thefasteners18,20,22, andplates12,13,14,15 with bores that are threaded or not threaded. An alternative embodiment could have bolt fasteners that are shallowly threaded along most of the shaft, and deeply threaded at the end of the shaft. In this alternative, all the bores of the plate could be threaded, but only the plates that connect at the end of the shaft (the outermost plates) would threadably engage the shaft (where the shaft is deeply threaded) and tighten onto the fastener. In yet another alternative, the plates need not be threaded at all but a threaded nut could be added on the end of the threaded fastener shaft to tighten all the components together.
• Another alternative embodiment (not shown) does not have radial splines on the outer or inner interface surfaces of the plates or other means of regulating movement between the plates. This allows for dynamic movement between the plates since the interface between the plates is not restricted by any mechanism that prevent rotation or sliding between the plates. Plates according to this alternative embodiment, when used in conjunction with a more)) elastically compliant spacer, allow for more dynamic movement between vertebrae.
• Another embodiment (not shown) permits dynamic movement between the plates, as in the embodiment of the preceding paragraph, and also provides resilient force, for example, via the addition of springs, to stabilize such dynamic movement. For example, torsional springs (not shown) may be registered on theplates12,14, on theplates13,15, or at the junctions between both pairs ofplates12,14,13,15. In the alternative, damping force may be applied to the motion between theplates12,14,13,15, for example, through the use of a frictional yet movable interface (not shown) such as frictional coatings on the various interface surfaces of theplates12,14,13,15, or a sealed chamber containing a viscous fluid and a damper that moves through the fluid in response to relative rotation between theplates12,14,13,15 (not shown). These are merely examples; those of skill in the art will recognize that many other mechanisms may be used to provide resilient force, damping force, or some combination of the two between theplates12,14,13,15 to control relative motion between thespinous processes2,4.
• The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. It is appreciated that various features of the above-described examples can be mixed and matched to form a variety of other alternatives, each of which may have different plates, spacer assembly or threading system according to the invention. As such, the described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.

Claims (20)

1. An implantable modular device for stabilizing relative motion of spinal vertebrae, comprising:

a first pair of plates attachable to a first spinous process;

a second pair of plates attachable to a second spinous process;

a first spacing member comprising an inner end and an outer end, wherein the first spacing member is positionable to fit between the first and second spinous processes to provide a selected level of stiffness between the spinous processes, the first spacing member comprising a first bearing surface and a second bearing surface orthogonal to the first bearing surface, the first bearing surface positionable to contact an inferior surface of the first spinous process and a superior surface of the second spinous process, the second bearing surface shaped as a continuous annulus, the second bearing surface positionable to directly contact a lateral surface of the first spinous process and an ipsilateral surface of the second spinous process;

a cap member configured to engage the inner end of the first spacing member, wherein the cap member comprises a third bearing surface shaped as a continuous annulus, the third bearing surface positionable to directly contact a lateral surface of the first spinous process and an ipsilateral surface of the second spinous process, the third bearing surface on the alternate side of the first spinous process from the second bearing surface; and

a fastener joining the first spacing member and the first and second pairs of plates, the fastener comprising a head and a shaft, the shaft having a first end and a second end and a longitudinal axis extending therebetween,

wherein the first and second pairs of plates are rotatably attachable together at any of a variety of relative orientations in response to securement of the fastener to the first and second pairs of plates to permit further relative rotation between the pairs of plates, thereby permitting dynamic relative motion between the first and second spinous processes.

2. The device ofclaim 1, wherein the first spacing member is positioned transverse to the first and second pair of plates, the first spacing member having an axis extending in a medial lateral direction, wherein the first spacing member is interchangeable with a different spacing member to permit variation in the stiffness between the spinous processes, wherein each spacing member comprises a material chosen from the group consisting of biocompatible polymers, metals, natural tissues, and synthetic tissues.

3. The device ofclaim 1, further comprising:

a third pair of plates attachable to the second spinous process via an attachment mechanism that also attaches the second pair of plates to the second spinous process;

a fourth pair of plates attachable to a third spinous process;

a second spacing member configured to fit between the second and third spinous processes to provide a selected level of stiffness between the spinous processes; and

at least one additional fastener joining the second spacing member to the third and fourth pairs of plates.

4. The device ofclaim 1, wherein the cap member further comprises a concavity configured to engage the inner end of the first spacing member.

5. The device ofclaim 4, wherein the cap member further comprises a cap bore extending through the cap member, the cap bore configured to slidably receive the fastener.

6. The device ofclaim 1, wherein the first and second pairs of plates are further configured to be rigidly attached together at any of the variety of relative orientations in response to securement of the fastener to the first and second pairs of plates to substantially prevent further relative rotation between the pairs of plates, thereby keeping the first and second spinous processes at a substantially fixed relative displacement.

7. The device ofclaim 1, wherein the first pair of plates are substantially planar and the second pair of plates are substantially non-planar.

8. The device ofclaim 1, wherein the fastener comprises a longitudinal axis extending in a medial lateral direction and positioned transverse to the first and second pair of plates, wherein the fastener shaft is threaded and the outermost one of the second pair of plates comprises a threaded bore, the threaded fastener shaft threadibly engaging the threaded bore to axially draw the first pair of plates, the second pair of plates and the spacer together along the longitudinal axis of the fastener shaft.

9. An implantable modular device for stabilizing relative motion of spinal vertebrae, comprising:

a first pair of plates attachable to a first spinous process immediately adjacent a second spinous process, the first pair of plates comprising a first plate and a second plate;

a second pair of plates attachable to the second spinous process, the second pair of plates comprising a third plate and a fourth plate;

a first fastener having a shaft with a longitudinal axis, the shaft extending through the first and second pairs of plates;

and a spacing member positionable to fit between the first and second spinous processes wherein the spacing member comprises an inner end and an outer end and a first flange positioned toward the outer end, the first flange shaped as a continuous annulus, the first flange positionable to directly contact a lateral surface of the first spinous process and an ipsilateral surface of the second spinous process.

10. The device ofclaim 9, further comprising a second fastener comprising a shaft, the shaft configured to pass through the first and second plates, wherein the shaft comprises a threaded portion and the second plate comprises a bore with threads, wherein the threaded portion of the shaft is directly engageable with the second plate bore threads to axially draw the first and second plates together.

11. The device ofclaim 10, further comprising a third fastener, the third fastener configured to pass through the third and fourth plates.

12. The device ofclaim 9, further comprising a spacing member bore, wherein the first fastener passes through the spacing member bore.

13. The device ofclaim 12, further comprising a cap member configured to engage the inner end of the first spacing member, wherein the cap member comprises a second flange shaped as a continuous annulus, the second flange positionable to directly contact a lateral surface of the first spinous process and an ipsilateral surface of the second spinous process, the second flange on the alternate side of the first spinous process from the first flange.

14. The device ofclaim 9, wherein at least one of the first and second pairs of plates is substantially non-planar.

15. The device ofclaim 9, wherein at least one of the first and second plates further comprises a first interface surface comprising a plurality of ridges, and at least one of the third and fourth plates further comprises a second interface surface comprising a plurality of ridges, wherein the ridges of the first interface surface are directly engageable with the ridges of the second interface surface to lock the first and second pairs of plates together at any of the variety of relative orientations.

16. An implantable modular device for providing dynamic relative motion of spinal vertebrae, comprising:

a first pair of plates attachable to a first spinous process, the first pair of plates comprising a first plate and a second plate;

a second pair of plates attachable to a second spinous process, the second pair of plates comprising a third plate and a fourth plate;

a first spacing member positioned transverse to the first and second pair of plates, the first spacing member with an axis extending in a medial lateral direction, the first spacing member comprising an inner end and an outer end, the first spacing member further comprising a first bearing surface, the first spacing member configured to fit between the first and second spinous processes to provide a selected level of stiffness between the spinous processes, and

a fastener formed separately from the first spacing member, the fastener having a shaft and a fastener axis and extending through the first plate, the third plate, the spacing member, the fourth plate, and the second plate in sequence,

wherein the first spacing member further comprises a locking feature, wherein the locking feature is lockable with one of the first and second pairs of plates to lock the first spacing member to one of the first and second pairs of plates.

17. The device ofclaim 16, wherein at least one of the first and second plates further comprises a first interface surface, and at least one of the third and fourth plates further comprises a second interface surface, wherein the first interface surface is in slidable contact with the second interface surface when the first pair of plates is in direct engagement with the second pair of plates.

18. The device ofclaim 16, wherein the first bearing surface is configured to contact an inferior surface of the first spinous process and a superior surface of the second spinous process; and wherein the first spacing member further comprises a second bearing surface, the second bearing surface shaped as a continuous annulus, the second bearing surface configured to directly contact a lateral surface of the first spinous process and an ipsilateral surface of the second spinous process.

19. The device ofclaim 18, wherein the first spacing member further comprises a cap member configured to engage the inner end of the first spacing member, wherein the cap member comprises a third bearing surface, the third bearing surface shaped as a continuous annulus, the third bearing surface positionable to directly contact a lateral surface of the first spinous process and an ipsilateral surface of the second spinous process, the third bearing surface on the alternate side of the first spinous process from the second bearing surface.

20. The device ofclaim 16, wherein the first spacing member is elastically compliant and configured to provide resilient force to stabilize the dynamic relative motion.

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