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1. (WO2009155360) SYSTEMS AND METHODS FOR POSTERIOR DYNAMIC STABILIZATION
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SYSTEMS AND METHODS FOR POSTERIOR DYNAMIC STABILIZATION

BACKGROUND OF THE INVENTION

1. The Field of the Invention

[0001] The invention relates to orthopaedics, and more particularly, to systems and

methods for treatment for the cervical or thoracolumbar spine that embody both principles of

providing motion restoration as well as balance control.

2. The Relevant Technology

[0002] Cervical spondylosis is an almost universal concomitant of human aging. More

than half of middle-age populations have radiographic or pathologic evidence of cervical

spondylosis. Spondylosis with resulting cord compression is the pathogenic factor in 55% of

cervical myelopathy cases. The exact pathophysiology of cervical spondylotic myelopathy

(CSM) remains unclear. Some proposed mechanisms include direct mechanical compression, microtrauma and ischemia to the cervical spinal cord.

[0003] A variety of factors have been implicated as predictors of clinical outcome

following surgery. These include age, duration of symptoms prior to surgery, severity of

myelopathy before surgery, multiplicity of involvement, anteroposterior canal diameter,

transverse area of the spinal cord and high-signal intensity area on T2- weighted imaging. [0004] Surgery is reserved for patients with a progressive history of worsening signs or

symptoms, severe spinal cord compression found on imaging studies and failure to respond to

non-operative treatment. Operative treatment is directed at relieving the spinal cord

compression by expanding the spinal canal diameter. Surgical options include anterior discectomy and fusion (ACDF), corpectomy, laminectomy with or without fusion, and

laminoplasty. The choice of an anterior or posterior approach to decompression is influenced by several factors: the degree of disc herniation, osteophyte formation, ligamentous

hypertrophy, facet degeneration, number of levels involved, spinal alignment and mobility must all be taken into consideration. A relative indication for an anterior approach, including

corpectomy or cervical discectomy and fusion, is the pre-operative presence of cervical

kyphosis or straightening of cervical spine. In such circumstances, an anterior single or

multilevel approach restores the alignment of the anterior and middle columns, avoiding post- laminectomy progression of kyphosis with worsening deficit. However, multi-level anterior procedures may be associated with significant risks and potential complications. In the

setting of myelopathy secondary to multilevel posterior disease, particularly in the elderly, a

posterior approach may be more appropriate.

[0005] For patients with a neutral to lordotic cervical alignment, laminoplasty has been advocated as an alternative to laminectomy and fusion or multi-level corpectomy.

Laminoplasty has the theoretical advantage of preserving spinal motion. Unfortunately,

laminoplasty is not indicated in the setting of preoperative cervical straightening or kyphosis.

In the setting of straightening, pre-operative kyphotic deformity or degenerative spondylosis

in the subaxial spine, laminectomy alone has been implicated in the development iatrogenic post-laminectomy kyphosis. Removal of the interspinous ligaments, ligamentum flavum

along with devascularization of the paravertebral muscles has been implicated in the loss of

the "posterior tension band" in decompression cases. Unfortunately, multi-level

decompression and fusion can be associated with significant loss of range of motion for the subaxial cervical spine. In addition, multilevel fusion can be associated with significant risks

for adjacent segment degeneration.

[0006] Laminectomy remains a mainstay of surgical decompression for multi-level CSM.

However, drawbacks include the risks of post-laminectomy kyphosis, instability, accelerated

spondylotic changes, and late neurological deficit. Post-laminectomy kyphosis is twice as likely to develop if there is preoperative loss of the normal cervical lordosis. Laminectomy

with concomitant posterolateral fusion has been advocated as a means of attaining neural decompression while avoiding iatrogenic kyphosis. Fusion has, however, the disadvantage of

converting a functionally mobile, mechanically stable spinal unit into a fixed, nonfunctional

one. Analysis of strain distribution in intervertebral discs following fusion has shown an

increase in longitudinal strain, most commonly at levels immediately adjacent to the fused

segments. The resultant increase in stress on discs adjacent to the fused levels is thought to lead to accelerated disc degeneration and/or mechanical instability at adjacent levels.

Radiographic changes of spondylosis and instability at levels above and below cervical

fusions have been described by several authors. No motion sparing surgical solution currently

exists for these patients. Therefore, a need exists for technology that allows reconstitution of the posterior tension band following decompression with laminectomy.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] 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. [0008] Figure 1 provides a posterior perspective view of a dynamic stabilization system

according to one embodiment of the invention, secured to a portion of a spine;

[0009] Figure 2A provides a posterior perspective view of a bridge element of the

dynamic stabilization system of Figure 1 ;

[0010] Figure 2B provides an anterior view of the bridge element of Figure 2 A;

[0011] Figure 2C provides a caudal view of the bridge element of Figure 2A;

[0012] Figure 2D provides a medial perspective view of the bridge element of Figure 2A;

[0013] Figure 3 A provides a caudal view of a bridge element and two anchoring

members of the dynamic stabilization system of Figure 1 ;

[0014] Figure 3B provides an exploded perspective view of an anchoring member of

Figure 3A;

[0015] Figure 4 provides an enlarged caudal cross-sectional view of a portion of the

bridge element and one anchoring member of Figure 3 A, anchored in a vertebra;

[0016] Figure 5A provides an enlarged perspective view of an elastically deformable bias

element of Figure 1 ;

[0017] Figure 5B provides an enlarged perspective view of an alternate embodiment of an elastically deformable bias element;

[0018] Figure 5C provides an enlarged perspective view of an alternate embodiment of an

elastically deformable bias element;

[0019] Figure 5D provides an enlarged perspective view of an alternate embodiment of an elastically deformable bias element;

[0020] Figure 6 provides a partially exploded perspective view of a bridge element, two

bias elements, two attachment mechanisms and two anchoring members of the dynamic

stabilization system of Figure 1;

[0021] Figure 7 provides a posterior perspective view of an alternate embodiment of a dynamic stabilization system, comprising two elastically deformable bias elements and two

rigid bias elements, anchored to a portion of a spine;

[0022] Figure 8 provides a posterior perspective view of an alternate embodiment of a

stabilization system, comprising four bias elements, anchored to a portion of a spine;

[0023] Figure 9 provides a posterior perspective view of an alternate embodiment of a

dynamic stabilization system, comprising two bias elements attached to bridge elements at

the vertebral midline, anchored to a portion of a spme;

[0024] Figure 10 provides a posterior perspective view of an alternate embodiment of a

dynamic stabilization system, comprising two elastically deformable bias elements attached to cross each other at one vertebral level, and two rigid bias elements at a second vertebral

level, anchored to a portion of a spine;

[0025] Figure 11 provides a posterior perspective view of an alternate embodiment of a

dynamic stabilization system comprising a bias element extending across two vertebral

levels, anchored to a portion of a spine;

[0026] Figure 12A provides a posterior perspective view of a bridge element of the

dynamic stabilization system of Figure 11 ;

[0027] Figure 12B provides a caudal perspective view of the bridge element of Figure

12A;

[0028] Figure 12C a perspective view of a clamp of the dynamic stabilization system of

Figure 11 ;

[0029] Figure 12D provides a perspective view of the clamp of Figure 12C;

[0030] Figure 13 provides a perspective view of an alternate embodiment of a dynamic

stabilization system comprising two elastically deformable bias elements extending across

two vertebral levels, anchored to a portion of a spine, and a tensioning tool;

[0031] Figure 14 provides a posterior perspective view of an alternate embodiment of a dynamic stabilization system, anchored to a portion of a spine;

[0032] Figure 15A provides a posterior perspective view of a bridge element of the

dynamic stabilization system of Figure 14;

[0033] Figure 15B provides a perspective view of a portion of the bridge element of

Figure 15A;

[0034] Figure 15C a perspective view of a clamp of the dynamic stabilization system of

Figure 14; and

[0035] Figure 15D provides a perspective view of the clamp of Figure 15C.

(THIS PAGE WAS FOUND NUMBERED, BUT BLANK, BY THE RO/US)

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [0036] The present invention relates to systems and methods for providing dynamic stabilization between spmal segments. 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 This description

is made for the purpose of illustrating the general principles of this invention and is not meant

to limit the inventive concepts in the appended claims.

[0037] Disclosed herein is a novel technology that allows reconstitution of the posterior

tension band following decompression with laminectomy. The system allows semi¬

constrained motion between the spinal segments, preserving the normal mobility of the spine,

while providing a restoring force that prevents post-operative kyphosis as well as allows correction of pre-existing deformity in the sagittal and/or coronal planes. Thus the system

provides motion restoration with the addition of balance control. In an implantation

procedure, lateral mass screws may be placed with a modified Magerl technique using

anatomic landmarks. Laminectomy may be performed following placement of the screws and

the decompression is achieved. Alternately, screws may be placed first, followed by laminectomy. Finally, the novel posterior cervical dynamic stabilization system is affixed to

the cervical spine In the lumbar or thoracic spine, the novel posterior dynamic stabilization

system could be affixed via pedicle screws. The following novel embodiments could be

easily adapted for the lumbar and thoracic spines.

[0038] Referring to Figure 1, one embodiment of a posterior dynamic stabilization system

10 is shown secured to a portion of a spine. In the embodiment depicted, laminectomy has

been performed on the C3, C4 and C5 cervical vertebrae. The system 10 comprises at least

two bridge elements 100. each of which is sized and shaped to span medial-laterally across

the resected vertebra between the lateral masses. Each bridge element 100 comprises a first anchoring feature 106, a second anchoring feature 108, and a bridge body 110 extending

between and connecting the anchoring features. Each bridge element 100 may be secured to a vertebra by at least one anchoring member 150. Each anchoring member 150 may

comprise an anchor or screw sized and shaped to be received in an anchoring feature 106 or

108, and may interface with the anchoring feature to secure the bridge element 100 to the

bone. At least one bias element 200 is attached at one end to one bridge element 100, and at

another end to a second bridge element 100, via attachment mechanisms 300. Each bias element 200 may be elastically deformable to provide dynamic stabilization between the

vertebrae involved. In alternative embodiments, one or more bias elements may comprise a

more rigid material to provide a stiff er degree of stabilization. The system 10 depicted in

Figure 1 includes four elastically deformable bias elements, allowing semi-constrained motion between the spinal segments, preserving the normal mobility of the spine. The

distribution of the bias elements may be symmetrical as seen in Figure 1, while alternative

embodiments include asymmetrically arranged bias elements.

[0039] Referring to Figures 2A - 2D, views of bridge element 100 are seen from several

perspectives. Bridge element 100 may be referred to as a laminar bridge, and is essentially a prosthetic lamina to replace the lamina that has been removed during the laminectomy for

decompression. The laminar bridge preferably includes bone ingrowth contact areas that

contact the bone and encourage long-term bony fixation, areas which are ideally located on

the anterior faces near the bone anchor attachment location, in order to contact the posterior aspects of the lateral masses. The laminar bridge is shaped to be situated well above the

thecal sac to prevent any contact with the dura. Bridge element 100 may comprise titanium,

stainless steel, aluminum, cobalt chrome, Nitinol, PEEK (poly ether ether ketone), UHMWPE

(ultra high molecular weight polyethylene), or other suitable sufficiently rigid biocompatible materials. In alternate embodiments, a bridge element may comprise an elastically

deformable material.

[0040] The bridge elements may take many forms other than those depicted here to

accomplish the same function. For example, a bridge element may comprise two or more

parts instead of the monolithic version shown. The bridge may be tubular in form or include

hollow portions to improve radiographic visualization if needed. In addition, the size of each

bridge element can vary as needed. For example, in some cases, such as when a greater clearance of the dural sac is required on one side of the vertebra, a wider and/or longer

anchoring feature may be required on such one side. Also, the length, width, thickness and/or

height of the bridge body may vary as required by patient anatomy or as needed for a desired

correction. A bridge element may be medial-laterally symmetrical as depicted in Figures 2A-2D, or asymmetrical as needed.

[0041] Each bridge element 100 comprises a first end 102 having an anchoring feature

106, and a second end 104 having an anchoring feature 108. The bridge element 100 further

comprises a posterior side 112 and a generally opposite anterior side 114. A plurality of

individual discretely located attachment features 118 are distributed along the bridge body 110. The attachment features, which may be threaded to engage corresponding threads on an

attachment mechanism, may be distributed evenly or unevenly along the bridge body.

[0042] Referring to Figures 2A and 2B, anchoring feature 106 includes an aperture 116

extending from the posterior side to the anterior side of the bridge element. The aperture 116 depicted is substantially cylindrical; however in other embodiments the aperture may be

tapered to accommodate polyaxial adjustability of an anchoring member received in the

aperture. On the posterior side, a concave cutout 120 encircles the aperture opening. The

cutout 120 allows for polyaxial adjustment of the anchoring member, and is faceted to

interface with a correspondingly faceted surface of the anchoring member. In addition to or in place of faceting, the cutout may include surface features such as divots, splines, knurling,

longitudinal grooves, circumferential grooves, facets, nubs, and combinations thereof, and/or include surface treatments, roughening or excoriation to promote gripping contact between

the anchoring feature and the anchoring member, and to prevent unintended backout of the

anchoring member.

[0043] The anterior surface of the anchoring feature 106 includes a bone apposition

portion 122. The bone apposition portion 122 may be knurled as depicted in Figures 2B and 2C, and/or may include features such as roughening, excoriation, porous structures or

treatments such as porous titanium coating, plasma-sprayed titanium, hydroxylapatite

coating, tricalcium phosphate coating, to promote gripping contact and to promote bony

ingrowth for long-term fixation. As seen in Figures 2C and 2D, the anterior surface of the anchoring feature 106 may be angled relative to the posterior surface of the bridge element, to

optimally correspond to the natural or resected bone surface to which it is secured during

implantation. Anchoring feature 108, found at the second end 104 of the bridge element 100,

includes aperture 124 and bone apposition portion 126, which correspond to those of

anchoring feature 106.

[0044] As seen in Figures 2A-2D, bridge body 110 extends medial-laterally between

anchoring feature 106 and anchoring feature 108. Bridge body 110 is curved or arched to

avoid contact with the dura when implanted, and a posterior height h of the curve or arch may

exceed the height of the removed natural lamina. The attachment features 118 depicted are holes, which may include threads for engagement with threaded attachment members. Other

embodiments may include attachment features which are at continuous non-discrete locations

along the bridge body. Other embodiments may also include attachment features configured

to engage various attachment mechanisms such as clamps, threaded fasteners, locking nuts,

posts, holes, press-fits, quick-release and quick- attachment connections, !4-turn connections, t-slots, dovetail joints, living hinges, and flanges, among others. All of the attachment

features 118 may be medially offset from the anchoring features 106, 108 when the bridge element 100 is properly secured to a vertebra in the manner set forth herein, that is, in a

medial-lateral orientation so as to span the vertebra.

[0045] Referring to Figures 3A and 3B, a single bridge element 100 may be secured by

two anchoring members 150. In the embodiment depicted, anchoring member 150 comprises

bone anchor 152 and nut 154, which may be a locking nut. Bone anchor 152 comprises distal threaded portion 156. proximal threaded shank 158, and drive feature 160. In the depicted

embodiment, drive feature 160 is an external hex drive and is positioned between the distal

and proximal threaded portions; however in other embodiments the drive feature may be

internal and/or comprise a different shape or location. For example, an alternate anchoring member may comprise a screw with a proximally located internal drive feature having a

rectangular, triangular or pentagonal shape. Other suitable screw-type bone anchors may

include lateral mass screws, monoaxial bone screws, polyaxial bone screws, screws with

spherical heads, pedicle screws, screws with trilobular or lobular heads, tulip heads, proximal

shanks, nuts, slots, serrations, or grooves, among others. Yet other anchoring members may be substituted for bone screws, such as staples, wires, cable, clamps, or hooks, among others.

Anchoring members may include structures to assist in long-term fixation, including but not

limited to porous titanium coating, plasma-sprayed titanium, hydroxylapatite coating,

tricalcium phosphate coating, porous structures and/or rough surface treatments. [0046] Nut 154 comprises an internal lumen 162 shaped to engage with the bone anchor

152. In the embodiment depicted, internal lumen 162 is threaded such that it may threadedly

engage the proximal threaded portion 158 of the bone anchor 152. Nut 154 further comprises

a drive portion 164 and an interface portion 166. The interface portion 166 is convex and

comprises surface facets which correspond to the faceting of concave cutout 120 on the bridge element. Other embodiments of the interface portion 166 may comprise facets, and/or

other surface features such as divots, splines, knurling, longitudinal grooves, circumferential grooves, facets, nubs, and combinations thereof, and/or include surface treatments,

roughening or excoriation.

[0047] Referring to Figure 4, a cross-sectional view of an anchoring member 150 in

engagement with a bridge member 100 is shown. Distal threaded portion 156 of bone anchor

152 is engaged in a bone, and anchoring feature 106 is placed over the bone screw such that aperture 116 surrounds drive feature 160 and bone apposition portion 122 contacts the

surface of the bone. Nut 154 is threaded onto the proximal threaded shank 158 to secure

bridge member 100 to the bone, and convex interface portion 166 engages with concave

cutout 120. The inner diameter of concave cutout 120 is greater than the outer diameter of convex interface portion 166, to allow for polyaxial positioning of anchoring member 150.

As set forth previously, aperture 116 may also be tapered at its distal, or bone-engaging, end

to also allow polyaxial placement of anchoring member 150.

[0048] As seen in Figure 1, multiple bias elements 200 are each coupled at a cephalad

end to a first bridge element 100, and coupled at a caudal end to a second bridge element 100. In the embodiment shown, bias element 200 comprises a compliant, elastically deformable

material which allows constrained motion between the first and second bridge elements. Such

compliant, elastically deformable materials may include elastomers, silicones, urethanes, bio- absorbable materials, woven textile structures, knit textile structures, braided textile structures, molded thermoplastic polymers, ethylene-vinyl acetate, PEEK, or UHMWPE; and

materials such as Nitinol, titanium, and stainless steel formed into elastically deformable

structures such as springs.

[0049] The bias element is intended to replicate or partially simulate the natural posterior

tension band in order to place physiologic constraints to motion and balance once these natural structures have been compromised after surgery. The preferred embodiment includes

a compliant material which is suited for tension / extension, such as a silicone or elastomer. The bias element is preferably configured with two attachment ends to be secured to the

laminar bridges, as well as a central portion which may be bowed posteriorly in order to

encourage buckling in posterior direction during patient extension. During patient flexion,

the bias element incurs tensile forces and the bias element resists those partially incurring

deflection and allowing the flexion to occur. The bias element may allow all anatomic range of motions seen in the spine including flexion, extension, lateral bending and rotation. All

coupled motions may be possible. The bias element may have a restoring force, preventing

the development of post-laminectomy deformity. The geometry of the bias element may be

configured to provide a correcting force in all planes for correction of sagittal and coronal deformities.

[0050] Referring to Figures 5A-5D, different configurations of elastically deformable

bias elements are shown. Bias element 200 comprises a first fixation portion 202, a second

fixation 204 and a bias body 206 extending between the first and second fixation portions. In

the embodiment shown, the first and second fixation portions 202, 204 are each formed from a rigid material which is substantially more rigid and less compliant than the elastically

deformable material. Such rigid materials may include titanium, stainless steel, aluminum,

cobalt chromium, Nitinol, PEEK, and UHMWPE, among others. Each fixation portion 202,

204 comprises a joining feature 208, which in the embodiment shown, is a hole. The joining feature 208 is configured to cooperate with an attachment mechanism to join or attach the

bias element to a bridge element. The bias body 206 of bias element 200 comprises five

separate strands of an elastically deformable material. Other embodiments of the bias element

may include more or fewer strands, and/or strands which are woven, braided, knit, or

otherwise intertwined.

[0051] Referring to Figure 5B, bias element 210 comprises a bias body 212 having

accordion-type folds or pleats. Figure 5C illustrates a bias element 220 with a substantially flat bias body 222, and Figure 5D illustrates a bias element 230 having a bias body 232 with a

flexure 234. It is appreciated that each material used, and/or configurations of the bias body,

may be mixed and matched to provide bias elements with varying degrees of elasticity as

necessary for the amount of motion and/or correction desired. In addition to the bias elements

depicted, other bias elements within in the scope of the invention may have different cross-sectional geometries as well, such as circular, rectangular, ovoid, annular, or any freeform

shape, as well as being solid, hollow, or porous. Also, bias elements may vary in length

and/or width to provide varying degrees of elasticity or compliance.

[0052] An alternative embodiment of the invention may include at least one bias element which is formed entirely of rigid materials, in order to provide additional motion or balance

control constraints on the functional spinal unit, or motion segment, involved. Rigid materials

suitable for such a more rigid, less compliant bias element include titanium, stainless steel,

aluminum, cobalt chromium, Nitinol, PEEK, and UHMWPE, among others. A rigid bias

member may be monolithically formed as one piece, or may include a body portion and fixation portions which are rigidly joined together. A system comprising rigid bias elements

coupled between bridge elements may provide a rigid stabilizing force between the bridge

elements. It is appreciated that compliant and rigid bias elements may be mixed and matched

to achieve the customized needs of the patient in a multi-level procedure. The bias element(s) may be configured to specifically introduce sagittal (lordosis or kyphosis) or coronal balance.

Alternatively, or in addition, the bias element(s) may introduce anterior or posterior

translation.

[0053] Referring to Figure 6, a partially exploded view shows a bridge element 200 with

a bias member 200 attached to the bridge, and an unattached bias member 200 and attachment mechanism 300 In the embodiment depicted, attachment mechanism 300

comprises a screw 310. Screw 310 includes a head portion 312 and a threaded shaft 314. The head portion 312 includes a drive feature 316 which in the example shown is internal; other

embodiment may include an external drive feature. To attach the bias member 200 to the

bridge element 100, the first fixation portion 202 may be placed adjacent the desired

attachment feature 118 such that their respective holes are aligned, then shaft 314 inserted

through joining feature 208 and engaged in attachment feature 118. Alternately, the shaft 314 may be inserted through joining feature 208, then screw 310 and bias element 200 are moved

together toward the bridge element and the screw 310 engaged with the attachment feature

118 to attach the bias element to the bridge element. The joining feature 208 of the bias

element 200 may not be threaded, to allow angular adjustment of the bias element relative to the bridge elements before the position of the bias element is fixed by engaging the

attachment mechanism 300 with the attachment feature 118.

[0054] Attachment mechanism 300 may be a self-locking screw, or may comprise a

locking washer, or backup nut to ensure locking engagement with the bias member and the

bridge element, and to prevent unintended backout or removal of the attachment mechanism. It is appreciated that attachment mechanism 300 is removable to provide for revision or

adjustment of the bias element relative to the bridge elements. It is also appreciated that other

attachment mechanisms exist to secure the bias element to the laminar bridge, including but

not limited to clamps, clips, threaded fasteners, posts, holes, press-fits, quick-release and quick-attachment connections, ^-turn connections, t-slots, dovetail joints, living hinges, and

flanged connections.

[0055] Referring again to Figures 1, 3A and 3B, and 6, system 10 may be implanted as

follows. The cervical vertebrae are exposed, and bone anchors 152 are placed with a modified

Magerl technique using anatomic landmarks. Laminectomy is then performed following placement of the anchors, and decompression is achieved. Alternatively, laminectomy may

first be performed followed by placement of the anchors. Bridge elements 100 are placed over the bone anchors and secured to the resected vertebrae by nuts 154. Two bridge

elements 100 may be secured to adjacent vertebrae for single level stabilization, or three or

more bridge elements may be used to provide stabilization across multiple levels. Bias

elements 200 are attached to the bridge elements via attachment mechanisms 150. Each bias

element may be attached to the bridge elements such that it extends essentially perpendicular to the bridge elements, as in Figure 1, or may be attached in a non-perpendicular position. As

bias elements are attached, tension may be applied manually or with a tensioning tool to

achieve a desired tension between the bridge elements. Of course, the implantation methods

set forth herein may be applied to any of the posterior dynamic stabilization systems or variations disclosed.

[0056] Figures 7 -14 show alternative embodiments of posterior dynamic stabilization

systems. In Figure 7, system 12 comprises three bridge elements 100 secured to three

respective vertebrae. Elastically deformable bias elements 200 are secured bilaterally

between the first and second bridge elements, to provide dynamic stabilization at that vertebral level. Rigid bias elements 400 are secured bilaterally to the second and third bridge

elements, to provide rigid stabilization at that vertebral level. Another alternative

embodiment could include elastically deformable bias elements secured to bridges at one

vertebral level, and different elastically deformable bias elements with a lower or higher degree of elasticity, at a second vertebral level.

[0057] Referring to Figure 8, system 14 comprises three bridge elements 100 secured to

three respective vertebrae. Rigid bias elements 400 are secured bilaterally to the first and

second bridge elements and to the second and third bridge elements, to provide rigid

stabilization at both vertebral levels. In an alternative embodiment of system 14, the bridge elements may comprise elastically deformable material while the bias elements compnse

rigid material, to provide dynamic stabilization at both vertebral levels. [0058] Referring to Figure 9, system 16 comprises three bridge elements 100 secured to

three respective vertebrae. An elastically deformable bias element 200 is attached to the first

and second bridge elements, aligned with the midline or sagittal plane of the vertebrae and

the bridge elements. A second elastically deformable bias element 200 is attached to the

second and third bridge elements, and is also aligned with the midline or sagittal plane of the vertebrae and the bridge elements. It is appreciated that the second deformable bias element

may have the same, or different, elasticity as the first bias element. Another alternative

embodiment could include rigid bias elements 400 aligned along the midline or sagittal plane

at one or both vertebral levels.

[0059] Referring to Figure 10, system 18 comprises three bridge elements 100 secured to

three respective vertebrae. A cephalad end of an elastically deformable bias element 200 is

attached to the first bridge element, and a caudal end is attached to the second bridge element at a

location medial-laterally offset from the first location. A second elastically deformable bias

element 200 is attached in an opposite manner, so that the second bias element crosses over the first bias element. Such a configuration may aid in maintaining patient balance and/or

correcting deformities in the sagittal and/or coronal planes. Two rigid bias elements extend

between the second and third bridge elements to provide rigid stabilization at that level.

[0060] Figures 11-14 depict posterior dynamic stabilization systems which each include a continuous length of elastically deformable material which is attached across two vertebral

levels. Referring to Figure 11, system 20 comprises three bridge elements 130, each of which

is oriented medial-laterally across a vertebra and secured to the vertebra via two anchoring

members. A bias element 240 extends across all three bridge elements at a midline or sagittal

position, and is coupled to each bridge element by an attachment mechanism 320.

Attachment mechanism 320 comprises a clamp 322 and a screw 324. The screw engages the

clamp and the bridge element to both attach the clamp to the bridge element and attach the bias element to the clamp at a desired tension.

[0061] Bridge element 130 is shown in Figures 12A and 12B. Bridge 130 comprises a

first end having an anchoring feature 136, and a second end having an anchoring feature 138.

A plurality of individual discretely located attachment features 148 are distributed along a

bridge body 140. The attachment features, which may be threaded to engage corresponding threads on an attachment mechanism, may be distributed evenly or unevenly along the bridge

body. In comparison with bridge element 100 as seen in Figure 2C, it can be seen that the

bridge body 140 of bridge element 130 is flatter than bridge body 110 of bridge element 100.

This flatter configuration helps to compensate for the anterior-posterior dimension of the clamp in order to provide a suitably low profile implant.

[0062] Figure 12C depicts a medial perspective view of clamp 322, and Figure 12D

depicts a caudal view of the clamp. The clamp 322 comprises an open loop portion 325 sized

to receive a portion of bias material. A first tab 326 and a second tab 328 are continuations of

the loop portion 325. Tab 326 has an opening 330, and tab 328 has an opening 332, and the tabs are positioned relative to one another such that the openings are concentrically aligned.

Two stops 334, 336 extend from tab 328. When the clamp is positioned on a bridge element

body 140 as in Figure 11, the stops 334, 336 project on either side of the bridge body and

may prevent rotation of the clamp relative to the bridge body. Openings 330, 332 are sized and shaped to receive a shaft of screw 324. As seen in Figure 11, clamp 322 is positionable

on a bridge element at an attachment feature 148, with stops 334, 336 positioned on either

cephalad/caudal side of the bridge element. Bias element 240 is insertable through the loop

portion 325. Screw 324 is insertable through clamp openings 330, 332 and into the

attachment feature 148. When screw 324 is tightened, loop portion 325 closes around bias element 240, and clamp 322 is rigidly attached to the bridge element, unable to rotate or

translate. It is also appreciated that other attachment mechanisms exist to secure the bias element to the bridge, including but not limited to clamps, clips, threaded fasteners, posts,

holes, press-fits, quick-release and quick- attachment connections, ^-turn connections, t-slots,

dovetail joints, living hinges, and flanged connections.

[0063] Bias element 240 may comprise a single piece or multiple pieces of an elastically

deformable material. The material composition, length, width, and/or elasticity of bias element 240 may vary as needed to attain the desired tension for balance control, deformity

correction or other desired outcome.

[0064] Referring to Figure 13, posterior dynamic stabilization system 22 comprises three

bridge elements 131, 132, and 133. bias elements 240 and 242, assembled with a plurality of anchoring members and attachment mechanisms Attachment mechanisms 341, 342, 343,

344, 345 and 346 may each comprise an attachment mechanism 320. Bias element 240 has

been tensioned and attached to the three bridge elements by attachment mechanisms 341,

342, 343 to provide tension at two vertebral levels. Bias element 242 comprises a length of

elastically deformable material. During an implantation process, bias element 242 may be inserted through the loop portions of attachment mechanisms 344, 345, 346. The attachment

mechanism 344 on bridge element 131 may be tightened to firmly hold the bias element 242.

A tensioning tool 450 may be coupled to a portion of the bias element, and actuated to

provide tension to the bias element. The tensioning tool 450 comprises a spring 452, the tool configured such that deflection of the spring provides tension to the bias element. The spring

deflection may be viewed through a window or slot in the tool and a measurement scale may

be present on the tool to indicate the magnitude of tension on the bias element. When a

desired tension is attained, the attachment mechanism 345 on bridge element 132 may be

tightened to lock down the bias element 242 at the desired tension. Finally, the tension on bias element 242 may be adjusted between bridge element 132 and 133 by actuation of the

tensioning tool, and attachment mechanism 346 tightened to lock down the bias element 242 at the desired tension. Bias element 242 may then be severed between attachment mechanism

346 and the tensioning tool. It is appreciated that bias element may be attached in the same

manner as bias element 242, and that the bias elements 240, 242 can be inserted, tensioned

and locked down in a cephalad-to-caudal order, or vice versa. It is also appreciated that a

third bias element may be added to the system to provide additional dynamic support if desired.

[0065] Another alternative embodiment of a posterior dynamic stabilization system is

shown in Figure 14. System 24 comprises three bridge elements 180, each secured in a

medial-lateral orientation to a vertebra by anchoring members 150. A single bias element 250 is attached to first and second bridge elements 180 in a midline position. Two additional bias

elements 250 are attached to the second and third bridge elements in a bilateral arrangement.

Attachment mechanisms 350, which comprise clamp 352 and screw 354, attach the bias

elements 250 to the bridge elements 180. Each bias element 250 comprises a first fixation

portion 252, a second fixation portion 254, and a bias body 256, and each fixation portion comprises a joining feature 258.

[0066] Figures 15A-15D show the details of bridge element 180 and clamp 352. Bridge

element 180 comprises a first end 182 having an anchoring feature 184, and a second end 186

having an anchoring feature 188. Bridge body 190 has a circular cross-section, and an outer connection surface 192 In the embodiment depicted, the connection surface 192 is ridged in

order to promote a non-slipping grip connection between the bridge body and the attachment

mechanism(s) 340. In alternative embodiments, connection surface 192 may comprise other

gripping features such as knurling, longitudinal grooves, facets, nubs, and combinations

thereof, and/or include surface treatments, roughening or excoriation. The attachment mechanisms 350 can be attached to the bridge element 180 at any location along the bridge

body 190, thus providing a plurality of continuous non-discrete attachment locations. This configuration allows the practitioner to select the precise attachment location needed to

produce the desired result, whether it is balance control, deformity correction or a

combination.

[0067] As seen in Figures 15C and 15D, clamp 352 comprises an open loop portion 356

sized to surround a portion of a bridge body 190. A first tab 358 and a second tab 360 are continuations of the loop portion 356. Tab 358 has an opening 362, and tab 360 has an

opening 364, and the tabs are positioned relative to one another such that the openings are

axially aligned. Openings 362, 364 are sized and shaped to receive a shaft of screw 354. As

seen in Figure 14, clamp 352 is positionable on a bridge body 180 at any non-discrete location along the body. One joining feature 258 of a bias element 250 may be placed

adjacent the openings 362, 364 of the clamp 352. Screw 354 is insertable through the joining

feature 258 and the clamp openings 362, 364. When screw 354 is tightened, loop portion 356

closes around the bridge body 190, and clamp 352 is rigidly attached to the bridge element.

Prior to tightening, clamp 352 may be rotated about the bridge body 190 to a desired position. This rotation allows for tension adjustment of the bias element 250 between the two bridge

elements 180. Of course, during the implantation process, the positions of the attachment

mechanisms along the lengths of bridge bodies may be adjusted, as can the rotational position

of the attachment mechanism, by loosening the screw 354, making the desired adjustment(s), and re-tightening the screw. The second joining feature of bias element 250 is attached to a

second bridge 180 in a similar fashion. It is also appreciated that other attachment

mechanisms exist to secure the bias element to the bridge, including but not limited to

clamps, clips, threaded fasteners, locking nuts, posts, holes, press-fits, quick-release and quick-attachment connections, *4-turn connections, t-slots, dovetail joints, living hinges, and

flanged connections.

[0068] The present invention may be embodied in other specific forms without departing

from its spirit or essential characteristics. For example, above are described various

alternative examples of systems for providing posterior dynamic stabilization. It is

appreciated that various features of the above-described examples can be mixed and matched

to form a variety of other alternatives. For example, elastically deformable and rigid bias elements may be used in combination or separately. The bias elements may be placed parallel

to one another and perpendicular to the bridging elements, or non-parallel and/or non- perpendicular. It is also appreciated that this system should not be limited to the cervical

spine, and may be used on any portion of the spine. 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.