US20120109212A1 - Collar bore configuration for dynamic spinal stabilization assembly - Google Patents

Abstract

A dynamic spinal stabilization assembly includes at least one mounting collar with a bore therethrough along a longitudinal axis, and a spinal rod slidably extending through the bore. The bore includes a medially disposed first section of reduced size that tapers both inwardly and outwardly relative to the axis, and respective end sections of relatively larger size. The bore may be defined by an interior wall that convexly curves toward the axis in the first section, advantageously with a constant non-zero radius of curvature. The bore profile helps minimize potential binding that may occur between the collar and the rod. The rod is coupled to bone anchoring elements, with at least one such connection being via the collar.

Description

BACKGROUND
• The present invention relates to spinal stabilization, and more particularly to dynamic spinal stabilization.
• Numerous systems have been developed for stabilizing the vertebral column so as to promote healing, reduce pain, and/or allow for spinal fusion. Typical systems involve anchor members (e.g., polyaxial screws) secured to consecutive vertebrae, with a spinal rod rigidly fixed to the anchor members. The anchor members are typically screwed into the posterior portions of the vertebrae and pass through the pedicles and a substantial portion of the vertebral bodies and therefore provide a fixed and durable connection. The spinal rods are then clamped to the anchor members in a conventional fashion, creating a rigid stabilization structure. In most situations, one such structure is provided on each lateral side of the spine.
• While such structures hold the vertebrae correctly positioned relative to each other, they tend to considerably stiffen the spine. This may significantly limit the patient's post-operative freedom of movement and/or may lead to undesirable loadings on nearby vertebrae. Accordingly, efforts have been made to develop stabilization approaches that can tolerate some movement, with the resulting systems typically referred to as dynamic spinal stabilization systems. Examples of dynamic stabilization systems are shown in U.S. Pat. No. 5,672,175 to Martin and U.S. Patent Application Publication No. 2005/0171540 to Lim et al.
• While the prior art dynamic spinal stabilization systems, such as the Martin and Lim et al. systems, allow for dynamic spinal stabilization, they may not be entirely satisfactory in some situations. Thus, there remains a need for alternative approaches to dynamic spinal stabilization, advantageously approaches that allow for easy installation while remaining robust in use.
SUMMARY
• A dynamic spinal stabilization assembly according to one embodiment comprises a rod assembly having a rod slidably extending through a bore of a mounting collar. The rod assembly may be mounted to a suitable bone anchoring element (e.g., polyaxial pedicle bone screw) by fixedly mating the collar to the anchoring element. Such an arrangement allows the rod to move relative to the anchoring element by sliding within the mounting collar. The bore in the collar has a profile shaped to help minimize potential binding that may occur between the collar and the rod that might otherwise inhibit the desired sliding motion.
• In one illustrative embodiment, an assembly for dynamic stabilization of a spine comprises at least one mounting collar comprising a bore therethrough along a longitudinal axis; a spinal rod slidably extending through the bore; wherein the bore comprises a medially disposed first section of reduced size that tapers both inwardly and outwardly relative to the axis and respective end sections of relatively larger size. The bore may be defined by an interior wall that convexly curves toward the axis in the first section, advantageously with a constant non-zero radius of curvature. The rod may comprise a first larger size section and an adjacent second smaller size section, with the second section extending through the collar's bore. The assembly may further comprise first and second bone anchoring elements disposed in spaced relation; the first bone anchoring element coupled to the rod, optionally fixedly; the second bone anchoring element slidably coupled to the rod via the collar. If desired, the rod may slidingly extend through more than one mounting collar, and/or at least one elastic element may be disposed on each longitudinal side of the collar(s).
• Other aspects of various embodiments of the inventive apparatus and related methods are also disclosed in the following description. The various aspects may be used alone or in any combination, as is desired.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 shows one embodiment of a dynamic spinal stabilization assembly secured to a spinal column, with the spinal column in the neutral position.
• FIG. 2 shows a perspective partially exploded view of the dynamic spinal stabilization assembly ofFIG. 1.
• FIG. 3 shows a top view of the dynamic spinal stabilization assembly ofFIG. 2 with locking elements removed for clarity.
• FIG. 4 shows a longitudinal cross-sectional view of the rod assembly ofFIG. 1.
• FIGS. 5A-5E show longitudinal cross-sectional views of various embodiments of a sliding collar.
• FIG. 6 shows the dynamic spinal stabilization assembly ofFIG. 1 with the spinal column undergoing extension.
• FIG. 7 shows the dynamic spinal stabilization assembly ofFIG. 1 with the spinal column undergoing flexion.
• FIG. 8 shows a longitudinal cross-sectional view of a rod assembly of another embodiment.
• FIG. 9 shows a longitudinal cross-sectional view of a rod assembly of another embodiment.
• FIG. 10 shows a partially exploded view of another rod assembly embodiment.
DETAILED DESCRIPTION
• A dynamic spinal stabilization assembly according to one embodiment is shown inFIG. 1, and generally indicated at20. For simplicity,FIG. 1 shows the dynamic spinal stabilization assembly being used to dynamically stabilize two adjacent vertebrae, asuperior vertebra12 and aninferior vertebra16, in aspinal column10. The dynamicspinal stabilization assembly20 ofFIG. 1 includes two or morebone anchoring elements30 and arod assembly50. For simplicity, thebone anchoring elements30 ofFIG. 1 take the form of monolithic monoaxial pedicle bone screws, and are therefore sometimes referred to herein as bone screws. However, it should be understood that other forms of anchoringelements30 may be used, such as pedicle hooks, more complex polyaxial pedicle screws, closed-headed bone screw assemblies, offset connectors, or the like, or combinations thereof. Referring toFIG. 2, eachbone screw30 includes a boneengaging section32, ahead section38, and alocking element48. Thebone engaging section32 engages therelevant vertebra12,16 in a fashion well known in the art of pedicle screws. For example, thebone engaging section32 is typically formed as a straight shank that extends alongaxis34, with suitableexternal threads36 for engaging bone. Thehead section38 is joined toshank32 and receives and supports therod assembly50. Thehead section38 typically includes abase section39 proximate theshank32 and twoupstanding arms40 that together help define an open-toppedchannel42 having a channel axis44 oriented transverse toshank axis34. When the dynamic spinal stabilization assembly is assembled, therod assembly50 rests in thischannel42. Accordingly, thechannel42 may, if desired, include ribs, protrusions, or other alignment features to aid in keeping the collars60 (discussed below) properly aligned. The interior of the upper portion ofarms40 advantageously includesthreads46 or other means for engaging thelocking member48. Thelocking member48 may take any form known in the art, but typically takes the form of a simple exteriorly threaded setscrew. Advancing thelocking member48 toward theshank32 allows therod assembly50 to be clamped to thebone screw30 between thelocking member48 and thebase portion39 ofhead section38. If desired, optional suitable press plates or similar structures (not shown) may be disposed both above and below therod assembly50 when it is inchannel42; these press plates may be associated with thehead section38, thelocking element48, or distinct therefrom.
• Referring toFIGS. 3-4, therod assembly50 includes aspinal rod52, amounting collar60, a pair ofelastic elements80, and anend cap84. Thespinal rod52 inFIGS. 3-4 is generally straight alongrod axis54, and can be conceptually divided into aprimary section56 and asecondary section58. Theprimary section56 may be generally cylindrical, with a larger diameter than thesecondary section58, and typically extends to the approximate midpoint ofrod52. Theprimary section56 is intended to be fixedly mounted to acorresponding bone screw30. Thesecondary section58 is likewise generally cylindrical, but is of smaller diameter. Thus, ashoulder57 is formed where theprimary section56 andsecondary section58 meet. The distal end of thesecondary section58, away from theprimary section56, may include suitable threads (either internal or external) or other means for releasably mating withend cap84. Becauserod50 is expected to carry significant loads, therod52 may be made from a suitably strong rigid material known in the art, such as titanium, or from a semi-rigid material such as PEEK, polyurethane, polypropylene, or polyethylene. And, the rod may have other cross-sectional shapes (e.g., square or otherwise faceted, with longitudinal ribs/channels) and/or may be non-linear, as is desired.
• Referring toFIGS. 4-5B, thecollar60 may take the form of a hollow cylindrical body that is slidably mounted onrod52. Thecollar60 comprises anexterior surface61, and aninterior surface68 defining acentral bore62. Theexterior surface61 is advantageously generally uniform, and is generally concentric about borelongitudinal axis64, with a diameter that matches that of rodprimary section56. Thecentral bore62 extends alongaxis64 from one end ofcollar60 to the other for an overall length of L. The bore62 shown inFIGS. 5A-5B is non-cylindrical in that theinterior surface68 does not trace a perfect cylinder. Instead, thebore62 tapers outward from itsmidpoint66. Referring toFIGS. 5A-5B, the profile of thebore68 may be longitudinally divided for ease of reference into anmedial section70 and respective outboard or endsections74, with themedial section70 comprising the longitudinal middle ofbore62 and extending for a length X of at least 80% of the length L ofbore62. As seen inFIGS. 5A-5B, themedial section70 tapers both inward toward, and outward away from,axis64, such thatinterior surface68 is disposed closer toaxis64 inmedial section70 thanend sections74. Such a profile is contrasted with a profile where the inboard section is substantially cylindrical (with a boundary wall that is flat and parallel to the axis), even if the entries to the bore are radiused and/or linearly tapered in the end sections. Due tomedial section70 being closer toaxis64 thanend sections74 for the embodiment ofFIGS. 5A-5B, the collar's wall thickness T is greater nearmidpoint66 than toward therespective end sections74. For the embodiments ofFIGS. 5A-5B, at least themedial section70 advantageously bows inward toward, or is convexly curved toward,axis64, advantageously with a constant radius of curvature R. Thus, the wall thickness T may vary continuously across themedial section70. For the embodiment ofFIG. 5A, the longitudinal profile ofbore62 is curving across substantially the entire profile, thereby allowing the collar wall thickness T to vary continuously across substantially the entire length L ofcollar60. For the embodiment ofFIG. 5B, the longitudinal profile ofbore62 is relatively straight (e.g., cylindrical) inend sections74, but bowed towardaxis64 inmedial section70.
• Other exemplary embodiments ofcollar60 are shown inFIGS. 5C-5E. The embodiment ofFIG. 5C has a profile ofbore62 such thatinterior surface68 approaches most closely toaxis64 at a point that is longitudinally off-center. The embodiment ofFIG. 5D has a profile ofbore62 such thatinterior surface68 approaches most closely toaxis64 at two spaced apart points, creating two necked-down regions. The embodiment ofFIG. 5E has a profile ofbore62 which is formed by aninterior surface68 with longitudinally running channels; creating a bore that circumferentially varies in size at a given longitudinal point. Thus, theinterior surface68 is closest toaxis64 at a midpoint ofbore62 in some embodiments (e.g.,FIGS. 5A,5B,5E); in other embodiments, this closest point may be asymmetrically located along bore62 (seeFIG. 5C) or may be multiple points spaced from one another (seeFIG. 5D). Thus, the medial section need not be centered on the exact middle of the profile ofbore62, but instead need only be disposed generally toward the middle of the profile ofbore62. In some embodiments, the longitudinal profile ofbore62 may have multiple “humps” that extend toward axis62 (seeFIG. 5D), rather than a single one. Further, in some embodiments, thebore62 may circumferentially vary in size at a given longitudinal point, such as by having a circumferentially segmented “hump” or “humps” divided by longitudinally running channels (seeFIG. 5E). These various aspects may be combined as appropriate for different circumstances.
• Thecollar60 should be of sufficient strength to withstand the expected clamping forces required to mate therod assembly50 to thebone anchoring elements30. Therefore, thecollar60 should be formed of a suitably strong material such as titanium, stainless steel, cobalt chromium, ceramics, or the like. Further, theexterior surface61 of thecollar60 should be relatively hard, and thecollar60 should have sufficient wall thickness to withstand the expected loadings.
• As seen inFIG. 4,elastic elements80 may be disposed between thecollar60 andshoulder57 and betweencollar60 andend cap84 respectively. In some embodiments, theelastic elements80 may take the form of simple coil springs disposed aboutrod52. Advantageously, however, theelastic elements80 may take the form of annular bodies of elastomeric material, such as polycarbonate urethane, as shown inFIGS. 3-4. Theseelastic elements80, or bumpers, should be able to undergo compression and resiliently return to their natural state upon removal of the corresponding load. Thebumpers80 may, if desired, be advantageously sized to be radially slightly smaller thanprimary section56 ofrod52. The endfaces of thebumpers80 are advantageously complementary in shape to the surfaces they abut against. Thus, if thecollars60 have longitudinal end faces that are concave, the endfaces of thebumpers80 are advantageously complementarily convex, and vice versa. Further, while only onebumper80 is shown disposed on each side ofcollar60, it should be understood that there may be one ormore bumpers80 on each side ofcollar60.
• Theend cap84 is secured to, or may be formed by, the corresponding end of rodsecondary section58. Theend cap84 may take any form known in the art, such as a simple enlarged cap that is threaded onto the respective rod end. Theend cap84 functions to prevent thecollar60 andbumpers80 from longitudinally moving off the rodsecondary section58. In addition, theend cap84 helps limit the overall movement of the spinal segment being stabilized.
• When the dynamicspinal stabilization assembly20 as shown inFIG. 1,rod52 is fixedly secured to one vertebra via a correspondingbone screw30, and slidably coupled to the other vertebra via anotherbone screw30. For example, the rodprimary section56 is disposed inchannel42 of thebone screw30 associated withinferior vertebrae16, and secured therein by tightening the correspondingsetscrew48. Therod assembly50 also extends throughchannel42 of thebone screw30 associated withsuperior vertebrae12, and slidably secured thereto by clampingcollar60 to thebone screw30 via the correspondingsetscrew48. While thecollar60 is advantageously fixedly clamped to thebone screw30, therod52 is only slidingly coupled to thatbone screw30 due to the sliding fit betweencollar60 androd52.
• Because therod52 is slidably coupled to honescrew30, via slidingcollar60, the bone screws30 are allowed to move longitudinally toward or away from each other along therod52, rather than being held in a fixed relative relationship. For example, the bone screws inFIG. 1 are spaced from one another by distance H. When thespinal column10 undergoes extension, the bone screws30 will have a tendency to move toward each other, shortening the distance to H′ as shown inFIG. 6. Such movement is allowed by the sliding coupling between therod52 andbone screw30 associated with thesuperior vertebra12, and will tend to compress thebumper80 located between thecollar60 andshoulder57. Thus, thatbumper80 provides a resistance to, and dampening of, the relative compression between the bone screws30. When thespinal column10 is subsequently returned to its normal position, thebumper80 expands back to its “normal” state. Likewise, the bone screws30 have a tendency to move away from each other when thespinal column10 undergoes flexion, lengthening the distance to H″ as shown inFIG. 7. As can be seen inFIG. 7, thebumper80 disposed superiorly to thecollar60 is compressed between thecollar60 andend cap84 when thespinal column10 is undergoes flexion. Thus, thebumpers80 help to elastically resist/dampen movement of therod52 relative to the bone screws30.
• As can be appreciated, the size, shape, materials, and configuration of thecollar60, and to a greater extent thebumpers80, help determine the kinematic response of therod assembly50. For example, increasing the length ofbumpers80 relative tocollar60 may help make therod assembly50 have a softer response to longitudinal loadings. Depending on where the increasedlength bumpers80 are located, this may result in decreased resistance to flexion or extension. On the other hand, increasing the relative length of thecollar60 may tend to make therod assembly50 act stiffer. Also, if gaps are present between all or some of thebumpers80 and theadjacent collar60 and/orend cap84, this may allow some relatively unrestricted motion before the dampening of thebumpers80 starts. Conversely, having thebumpers80 under a preloading may increase the dampening effect. Thus, the kinematic response of therod assembly50, and thus the entire dynamicspinal stabilization assembly20, may be adjusted as desired by changing the size, shape, materials, and configuration of thecollar60 and/orbumpers80.
• The profile of the collar bore62 is designed to help facilitate the desired sliding motion betweencollar60 androd52. More particularly, the profile is designed to help discourage undesirable binding of thecollar60 against the outer surface ofrod52 in thesecondary section58. It is believed that the profile of the various embodiments allows thecollar60 to slide easily against therod52 without binding. Further, the profile, in some embodiments, provides more material proximate the middle ofcollar60, where clamping to thebone screw30 is most likely to occur, while reducing the material required in other areas. To further help facilitate the desired sliding motion, theinterior surface68 may be coated with, or otherwise formed with, a suitable friction reducing material. For example, theinterior surface68 may be coated with a low friction material (e.g., a ceramic or low friction polymer), and/or finished in a suitable manner, to reduce any friction between thecollar60 and the exterior surface ofrod52. Alternatively, or additionally, the exterior surface ofrod52 may likewise be coated and/or finished. Further, thecollars60 of most embodiments are able to handlerods52 that are bent, rather than only being able to function with straight rods.
• The dynamicspinal stabilization assembly10 may be installed during a surgical procedure. The surgical site is prepared in a conventional fashion, and thespinal column10 is approached via a posterior and/or lateral approach. If desired, a minimally invasive technique may be used, such as that discussed in U.S. Patent Application Publication No. 2005/0171540, which is incorporated herein by reference. Once the bone screws30 are installed into therespective vertebrae12,16, therod assembly50 may be inserted into thechannels42 such thatcollar60 is aligned with one of thechannels42. If the surgeon is assembling therod assembly50, the surgeon may adjust the rigidness of theassembly20, or a section thereof, before installation by changing the configuration of thecollar60 and/orbumpers80, such as by using astiffer bumper80 in one location and asofter bumper80 in another. The lockingelements48 are tightened so as to fixedly secure therod assembly50 to onebone screw30 and slidably secure therod52 to theother bone screw30. The surgical procedure then proceeds in a conventional fashion.
• The discussion above has assumed a cylindrical exterior shape for thecollars60 andbumpers80; however, such is not required in all embodiments. Indeed, the exterior of thecollar60 andbumpers80 may alternatively be faceted, such as square, rectangular, or hexagonal, if desired. Or, if desired, thecollars60 andbumpers80 may have any other desired exterior shape or combination of shapes. And, it should be noted that thebumpers70 need not be of a uniform longitudinal length.
• Further, it may be advantageous for the exterior of thecollars60 to include outwardly extending flanges. Such flanges may aid in properly aligning thecollar60 in thechannel42 ofbone anchoring element30. And, it may be further advantageous for theend cap84, and/or therod52 atshoulder57, to include outwardly extending flanges as well. The presence of such flanges may allow thebumpers80 to be larger in size, while still being retained in the proper position.
• In some embodiments, thecollar60 may be freely rotatable about the rodlongitudinal axis54. In other embodiments, thecollar60 may be constrained against such rotation. For example, therod52 may have a non-circular cross section, with thebore62 of thecollar60 having a corresponding shape. The non-circular cross-section may be any appropriate shape (e.g., square or otherwise faceted, D-shaped, etc.) and/or may include longitudinally running ribs/channels, as is desired.
• As can be appreciated, therod52 need not be straight; indeed, a pre-bent rod may be used. If the amount of rod bending is significant, it may be advantageous for thebore62 to be tapered to accommodate the bend in therod52. For such situations, thelongitudinal axis54 of therod52 is not a straight line.
• The discussion above has assumed that therod assembly50 has a single slidingcollar60; however, various embodiments may have multiple slidingcollars60. For example, therod assembly50 ofFIG. 8 has a larger diameter, centrally locatedprimary section56, with smaller diametersecondary sections58 disposed on each side thereof. Thisrod assembly50 further comprises a slidingcollar60 disposed toward each end ofrod52, with suitably disposedelastic elements80 andend caps84. As can be appreciated, such arod assembly50 may be used to stabilize three or more vertebral levels. For the embodiment ofFIG. 9, the centrally locatedprimary section56 andadjacent bumpers80 ofFIG. 8 is replaced with a suitably sized central bumper80a(or stack of bumpers). In another embodiment (not shown), the centrally locatedprimary section56 ofFIG. 8 may be replaced with a slidingcollar60, so that there are three slidingcollars60 in therod assembly50, such as one for each of three different vertebral levels. And, these ideas could be extended to additional vertebral levels.
• In other embodiments, therod assembly50 may comprise a plurality ofcollars60 arranged so that a givenbone screw30 clamps multiple slidingcollars60 in order to slidingly mount therod assembly50. SeeFIG. 10. For such embodiments, thecollars60 are spaced closer together in their “normal” state than the length ofchannel42 of the relevant bone screws30. For additional information, attention is directed to pending U.S. patent application entitled “Dynamic Spinal Stabilization Assembly with Sliding Collars,” identified by attorney docket number 4906-221 and filed on the same day hereas, the disclosure of which is incorporated herein by reference.
• Finally, as discussed above, the dynamicspinal stabilization assembly10 may include a variety ofbone anchoring elements30, including monoaxial and polyaxial pedicle bone screws. When used with polyaxial bone screws, care should be taken to ensure that the configuration of thecollar60 allows the polyaxial motion to be locked down, if desired. Further, for some embodiments, it may be desirable for the polyaxial bone screw to include the press plates or similar structures discussed above so that the clamping force for holding therod assembly50 may be transmitted, where appropriate, to the polyaxial locking mechanism.
• The present invention may be carried out in other specific ways than those herein set forth without departing from the scope and essential characteristics of the invention. Further, the various aspects of the disclosed device and method may be used alone or in any combination, as is desired. The disclosed embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.

Claims (12)

1. An assembly for dynamic stabilization of a spine, comprising:

at least one mounting collar comprising a bore therethrough along a longitudinal axis;

a spinal rod slidably extending through said bore;

wherein the bore comprises a medial first section of reduced size that tapers both inwardly and outwardly relative to the axis and respective end sections of relatively larger size.

2-7. (canceled)

8. The assembly ofclaim 1 wherein said mounting collar comprises a first collar and wherein the assembly further comprises a second mounting collar comprising a bore therethrough along a longitudinal axis thereof; wherein the spinal rod slidably extends through the bore of the second collar.

9-10. (canceled)

11. The assembly ofclaim 1 further comprising first and second bone anchoring elements disposed in spaced relation:

wherein said mounting collar comprises a first collar having its bore is defined by an interior wall that convexly curves toward said axis in said first section;

wherein the assembly further comprises a second mounting collar comprising a bore therethrough along a longitudinal axis thereof; wherein the spinal rod slidably extends through the bore of the second collar;

wherein said rod comprises a first larger size section and an adjacent second smaller size section; wherein the second section extends through the bore of the first collar;

wherein the first bone anchoring element couples to the rod; and

wherein the second bone anchoring element slidably couples to the rod via the first collar.

12-26. (canceled)

27. A method of dynamically stabilizing a spinal column, comprising:

anchoring first and second bone anchoring elements to respective vertebra;

providing an elongate rod assembly comprising:

a mounting collar having a bore therethrough along a longitudinal axis, said bore comprising a centrally disposed first section of reduced size that tapers both inwardly and outwardly relative to the axis and respective end sections of relatively larger size; and

an elongate rod slidably extending though the bore;

slidably mounting said rod to said first bone anchoring element by affixing the collar to the first bone anchoring element;

supporting said rod with said second bone anchoring element.

28. The method ofclaim 27 wherein said supporting said rod with said second bone anchoring element comprises slidably mounting said rod to said second bone anchoring element.

29. The method ofclaim 27 wherein said rod assembly further comprises first and second elastic elements disposed about said rod on respective longitudinal sides of the mounting collar; the method further comprising compressing at least one of the elastic elements.

30. The method ofclaim 29 wherein said first and second elastic elements are substantially identical.

31. A method of dynamically stabilizing a spinal column, comprising:

anchoring first and second bone anchoring elements to respective vertebra;

providing ah elongate rod assembly comprising:

at least one mounting collar having a bore therethrough along a longitudinal axis, said collar having an outer surface that is longitudinally non-curving and the collar having a first and second longitudinally-end faces facing in generally opposite directions;

an spinal rod slidably extending though the bore;

wherein the bore is defined by an interior wall with a continuously convexly curved profile with a medial first section of reduced size that tapers both inwardly and outwardly relative to the axis and respective end sections of relatively larger size;

providing a first elastic element disposed on a first longitudinal side of the mounting collar and disposed beyond the first longitudinally-end face with respect to the medial first section;

providing a second elastic element disposed on a second longitudinal side of the mounting collar opposite the first longitudinal side and disposed beyond the second longitudinally-end face with respect to the medial first section;

wherein the mounting collar, first elastic element, and second elastic element are disposed such that longitudinal displacement of the mounting collar relative to the spinal rod causes the first or second elastic element to be longitudinally compressed,

slidably mounting said rod to said first bone anchoring element by affixing the collar to the first bone anchoring element; and

supporting said rod with said second bone anchoring element.

32. The method ofclaim 32 further comprising:

spacing first and second bone anchors in spaced relation;

coupling first bone anchoring element to the spinal rod; and

slidably coupling the second bone anchoring element to the spinal rod via the collar.

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