Apparatus and method for dynamic vertebral stabilization

A posterior vertebral stabilizer has a resilient member such as a linear spring, which operates in tension and compression. The resilient member may be kept straight by a stabilization rod extending through the spring, or by a telescoping assembly that encases the resilient member. The ends of the stabilizer are attachable to pedicles of adjacent vertebrae so that the stabilizer adds stiffness to control flexion and extension of the vertebrae. Two such stabilizers may be used, and may be connected together by a crosslink designed to limit relative rotation of the stabilizers. Thus, the stabilizers may restrict axial rotation and lateral bending between the vertebrae, while permitting stiffened flexion and extension. Such stabilizers help provide the stiffness of a healthy intervertebral disc. In the event that fusion of the joint becomes necessary, a set screw or other component may be used to further restrict flexion and extension.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates generally to orthopedic medicine, and more precisely, to systems and methods for restricting relative motion between vertebrae.

2. The Relevant Technology

Many people experience back pain. Back pain is not only uncomfortable, but can be particularly debilitating. Many people who wish to participate in sports, manual labor, or even sedentary employment are unable to do so because of pains that arise from motion of or pressure on the spinal column. Such pains are often caused by traumatic, inflammatory, metabolic, synovial, neoplastic and degenerative disorders of the spine.

The intervertebral discs that separate adjacent vertebrae from each other serve to provide stiffness that helps to restrain relative motion of the vertebrae in flexion, extension, axial rotation, and lateral bending. However, a damaged disc may provide inadequate stiffness along one or more modes of spinal motion. Inadequate stiffness may result in excessive relative vertebral motion when the spine is under a given load, as when the patient uses the muscles of the back. Such excessive relative motion may cause further damage to the disc, thereby causing back pain and ultimately, requiring replacement of the disc and/or other operations to decompress nerves affected by central, lateral or foraminal stenosis.

Some stabilization devices have been proposed to restrict, but not entirely prevent, relative motion between adjacent vertebrae. Such devices are often somewhat complex and/or bulky. Many such devices cannot be tailored to limit the types of motion (i.e., flexion/extension, axial rotation, or lateral bending) that are most painful. Additionally, in the event that stabilization ultimately becomes insufficient, most known stabilization devices do not provide any mechanism that can be used to more fully secure the spinal motion segment.

DETAILED DESCRIPTION

The present invention advances the state of the art by providing systems and methods that can be used to stabilize relative motion between two vertebrae. The present invention can be used as an alternative to spinal fusion to alleviate back pain resulting from traumatic, inflammatory, metabolic, synovial, neoplastic and degenerative spinal disorders. The configuration and operation of at least one embodiment of the invention will be shown and described in greater detail with reference toFIGS. 1 and 2, as follows.

In this application, the phrase “telescopic engagement” and variations thereof refer to two members, wherein a portion of one hollow member fits around a portion of a second member to permit relative linear motion of the two members. “Locking” of two members refers to substantially preventing relative translation or rotation between the members along at least one axis. “Generally symmetrical” refers to items that are arranged in a manner that is symmetrical or nearly symmetrical to each other, with no requirement of precise symmetry. For example, the left and right sides of the spinal column may be considered to be generally symmetrical, despite the fact that anatomical differences and asymmetries will exist between them. Two components that are “integrally formed” with each other are formed as a single piece.

Referring toFIG. 1, a perspective view illustrates a portion of a spine10.FIG. 1illustrates only the bony structures; accordingly, ligaments, cartilage, and other soft tissues are omitted for clarity. The spine10has a cephalad direction12, a caudal direction14, an anterior direction16, a posterior direction18, and a medial/lateral axis20, all of which are oriented as shown by the arrows bearing the same reference numerals. In this application, “left” and “right” are used with reference to a posterior view, i.e., a view from behind the spine10. “Medial” refers to a position or orientation toward a sagittal plane (i.e., plane of symmetry that separates left and right sides from each other) of the spine10, and “lateral” refers to a position or orientation relatively further from the sagittal plane.

As shown, the portion of the spine10illustrated inFIG. 1includes a first vertebra24, which may be the L5 (Fifth Lumbar) vertebra of a patient, and a second vertebra26, which may be the L4 (Fourth Lumbar) vertebra of the patient. The systems and methods may be applicable to any vertebra or vertebrae of the spine10and/or the sacrum (not shown). In this application, the term “vertebra” may be broadly interpreted to include the sacrum.

As shown, the first vertebra24has a body28with a generally disc-like shape and two pedicles30that extend posteriorly from the body28. A posterior arch, or lamina32, extends between the posterior ends of the pedicles30to couple the pedicles30together. The first vertebra24also has a pair of transverse processes34that extend laterally from the pedicles30generally along the medial/lateral axis20, and a spinous process36that extends from the lamina32along the posterior direction18.

The first vertebra24also has a pair of superior facets38, which are positioned toward the top of the first vertebra24and face generally medially. Additionally, the first vertebra24has inferior facets40, which are positioned toward the bottom of the first vertebra24and face generally laterally. Each of the pedicles30of the first vertebra24has a saddle point42, which is positioned generally at the center of the juncture of each superior facet38with the adjacent transverse process34.

Similarly, the second vertebra26has a body48from which two pedicles50extend posteriorly. A posterior arch, or lamina52, extends between the posterior ends of the pedicles50to couple the pedicles50together. The second vertebra26also has a pair of transverse processes54, each of which extends from the corresponding pedicle50generally along the medial/lateral axis20, and a spinous process56that extends from the lamina52along the posterior direction18.

The second vertebra26also has a pair of superior facets58, which are positioned toward the top of the second vertebra26and face generally inward. Additionally, the second vertebra26has inferior facets60, which are positioned toward the bottom of the second vertebra26and face generally outward. Each of the pedicles60of the second vertebra26has a saddle point62, which is positioned generally at the center of the juncture of each superior facet58with the adjacent transverse process54.

The superior facets38of the first vertebra24articulate (i.e., slide and/or press) with the inferior facets60of the second vertebra26to limit relative motion between the first and second vertebrae24,26. Thus, the combination of each superior facet38with the adjacent inferior facet60provides a facet joint64. The first and second vertebrae24,26thus define two facet joints64that span the distance between the first and second vertebrae24,26. The inferior facets40of the first vertebra40and the superior facets58of the second vertebra26are part of other facet joints that control motion between the first and second vertebrae24,26and adjacent vertebrae (not shown) and/or the sacrum (also not shown). The vertebrae24,26are separated from each other by an intervertebral disc66.

As shown inFIG. 1, an apparatus70according to one embodiment of the invention is coupled to the vertebrae24,26on either side of the sagittal plane to provide dynamic stabilization. In this application, “dynamic stabilization” refers to selectively limiting, but not entirely preventing, the relative motion of two objects. The apparatus70may be termed a “stabilizer.”

As embodied inFIG. 1, the apparatus70is designed to preserve relatively free relative motion between the saddle points42,62of the vertebrae24,26along the cephalad and caudal directions12,14, thereby permitting flexion, extension, and lateral bending of the spine10with little restriction. However, the apparatus70is also designed to significantly restrict relative motion between the saddle points42,62along the anterior direction16, the posterior direction18, and the medial/lateral axis20. Accordingly, rotation of the spine10and relative anterior/posterior or medial/lateral motion of the vertebrae24,26under shear are restricted.

As shown, each apparatus70has a bridge72, a stabilization rod74(not visible inFIG. 1), a pair of pins76, a pair of castle nuts78, and a pair of fixation members80. The fixation members70are implanted in the pedicles30,50of the vertebrae24,26, respectively. More precisely, each of the fixation members70has a distal end (not shown) implanted in the pedicle30or50and a proximal end84that is exposed to protrude from the corresponding saddle point42or62. Each proximal end84has threads86that enable threaded attachment of the corresponding castle nut78.

The remainder of the apparatus70is secured to the saddle points42,62via the castle nuts78. The bridge72spans the distance between the saddle points42,62in a manner that enables relative cephalad/caudal motion with resilient support. The stabilization rod74is movably secured within the bridge72via the pins76to limit relative motion between the saddle points42,62along the anterior direction16, the posterior direction18, and the medial/lateral axis20. These functions and relationships will be described in greater detail in the discussion ofFIG. 2, as follows.

Referring toFIG. 2, an exploded, perspective view illustrates one of the apparatus70ofFIG. 1in isolation. As shown, the bridge72has a first end92, a second end94, and a central portion96between the first and second ends92,94. The first end92may be coupled to the first vertebra24, and the second end94may be coupled to the second vertebra26, so that upon implantation, the first end92is generally cephalad and the second end94is generally caudal.

Each of the first and second ends92,94has a mounting interface100that facilitates attachment of the first or second end92or94to the corresponding saddle point42or62. Each of the first and second ends92,94also has a mounting aperture102from which the corresponding mounting interface100extends. The mounting interfaces100and the mounting apertures102may each be sized to permit passage of the corresponding proximal end84therethrough. Moreover, the mounting interfaces100and mounting apertures102are sufficiently large that the proximal end84may pass therethrough at a variety of angles nonparallel to the axis of the mounting interface100and mounting aperture102. Thus, the apparatus70accommodates spinal morphologies in which the pedicles30,50are not perpendicular to the desired orientation of the bridge72by permitting the fixation members80to extend non-perpendicular to the bridge72.

Each mounting interface100has a generally concave, semispherical shape that is designed to receive and compress the corresponding castle nut78to substantially prevent relative rotation between the bridge72and the corresponding fixation member80. Therefore, the orientation of the bridge72with respect to the fixation members80may be fixed in any of a variety of orientations to accommodate differing spinal morphologies. The manner in which the castle nuts78cooperate with the mounting interfaces100will be described in greater detail subsequently.

As shown, each of the mounting interfaces100has an interior orifice106and an exterior orifice108. The interior orifices106provide communication with a bore112of the central portion96of the bridge112, and the exterior orifices108provide access to the interior orifices106. Thus, the stabilization rod74may easily be installed in the bore112by inserting the stabilization rod74through one of the exterior orifices108, and then through the adjacent interior orifice106.

The central portion96has a pin registration slot114adjacent to the first end92, and a pin registration orifice116adjacent to the second end94. The pin registration slot114and the pin registration orifice116communicate with the bore112, and are designed to receive the pins76. More precisely, the pin registration orifice116receives the corresponding pin76such that the pin76is unable to move with respect to the bridge72along the cephalad, caudal, anterior, and posterior directions12,14,16,18. The pin registration slot118receives the other pin76such that the pin76is unable to move with respect to the bridge72along the anterior and posterior directions16,18, but may move along the pin registration slot118in the cephalad and caudal directions12,14.

In addition to the pin registration slot114and the pin registration orifice116, the central portion96has a supplemental orifice118, which may be used to carry out various functions. According to one example, a set screw (not shown inFIG. 1) or other implement may be seated in the supplemental orifice118to restrict sliding of the stabilization rod74within the bore112, thereby converting the apparatus70from a stabilization device to a fixation, or fusion device.

The central portion96also has a resilient section120, which may take the form of a linear spring integrally formed with the remainder of the bridge72. The resilient section120permits the first and second ends92,94to move toward or away from each other to enable relative cephalad/caudal motion of the saddle points42,62of the vertebrae24,26, respectively. The resilient section120also provides resilient force tending to push or pull the ends92,94into a relative position in which the resilient section120is substantially undeflected. Such a position may correspond to a spinal disposition in which the vertebrae24,26are neither flexed nor extended with respect to each other.

InFIG. 2, the resilient section120is integrally formed with the first and second ends92,94of the bridge72. In alternative embodiments (not shown), a resilient section may be separately formed from ends to which the resilient section is permanently or removably attached. For example, if the resilient section120were a separate piece from the ends92,94, the stabilization rod74would act to hold the resilient section120and the ends92,94together after the bridge72and the stabilization rod74had been assembled.

Returning to the embodiment ofFIG. 2, the stabilization rod74has a first end124, a second end126, and a central portion128between the first and second ends124,126. Each of the first and second ends124,126has a pin registration orifice132sized to receive the corresponding pin76. More specifically, the pin registration orifices132may be sized to receive the pins76with some interference to provide a press fit so that, once inserted into the orifices132, the pins76remain in place until deliberately removed.

The ends124,126may each be sized to fit into the bore112of the bridge72with relatively little clearance to maintain coaxiality between the bridge72and the stabilization rod74. Alternatively, if desired, coaxiality may be maintained by providing relatively small clearance between the pins76and the pin registration slot114and the pin registration orifice116. Maintaining coaxiality between the bridge72and the stabilization rod74restricts relative motion of the first and second ends92,94of the bridge72to motion along the axis of the bridge72, thereby permitting significant relative motion between the saddle points42,62only along the cephalad and caudal directions12,14.

The central portion128has a stepped down region136with a diameter slightly smaller than that of the first and second ends124,126. Thus, clearance exists between the stepped down region136and the inward-facing surfaces of the resilient section120so that the resilient section120will not bind on the central portion128as the ends92,94of the bridge72move together or apart.

Each of the castle nuts78has a torquing end140and a compression end142. The torquing end140is designed to receive torque from a tool (not shown) with an end that meshes with the torquing end140. The compression end142has a generally semispherical shape and is compressible to lock the orientation of the castle nut78with respect to the corresponding mounting interface100. This permits locking of the orientation of the bridge72with respect to the fixation members80to prevent shear slippage of the vertebrae24,26with respect to each other and to generally restrict relative anterior/posterior and medial/lateral motion between the vertebrae24,26.

Each castle nut78also has a bore144that passes through the torquing end140and the compression end142. The bore144has threads (not shown) that mate with the threads86of the corresponding fixation member80. The torquing end140has a plurality of crenelations146that enable the torquing tool (not shown) to interlock with the torquing end140without interfering with positioning of the proximal end84of the fixation member80in the bore144.

The compression end142of each castle nut78has a plurality of fingers148arrayed in radially symmetrical fashion about the axis of the castle nut78. The fingers148are separated from each other by slots150so that the fingers148are able to deflect inward upon engagement with the corresponding mounting interface100. The fingers148are deflected inward in response to tightening of the castle nut78into the mounting interface100as the castle nut78is rotated to advance it along the proximal end84of the corresponding fixation member80.

Deflection of the fingers148increases the contacting surface area between the compression end142and the mounting interface, thereby enhancing frictional engagement of the castle nut78with the mounting interface100. The resulting frictional forces are generally adequate to maintain the relative orientations of the bridge72and the fixation members80during normal motion of the spine10. The mating semispherical shapes of the compression ends142and the mounting interfaces100allow such frictional locking to occur in any of a variety of orientations of the bridge72with respect to the fixation members80, thereby permitting usage of the apparatus70with a variety of spinal morphologies.

Referring toFIG. 3, a partially exploded view illustrates the apparatus70ofFIGS. 1 and 2, with extra components to help lock the apparatus70to substantially prevent elongation, contraction, and/or rotation of the apparatus70. As shown, each of the exterior orifices108may have a plurality of threads154. Similarly, the supplemental orifice118may have a plurality of threads156. The extra components, shown exploded from the apparatus70inFIG. 3, include a pair of end plugs158that may be received by the exterior orifices108, and a locking component, which may take the form of a set screw160, which may be received by the supplemental orifice118.

As shown, each of the end plugs158has threads162designed to interface with the threads154of the corresponding exterior orifice108. Furthermore, each of the end plugs158has a torquing feature164, such as a hexagonal recess, that facilitates rotation of the end plug158through the use of a suitable too such as a hex-head driver. Thus, each end plug158can be rotated into engagement with the corresponding exterior orifice108.

Similarly, the set screw160has threads166that interface with the threads156of the supplemental orifice118. The set screw160also has a torquing feature168, such as a hexagonal recess, that operates in a manner similar to that of the torquing features164of the end plugs158to facilitate rotation of the set screw160into engagement with the supplemental orifice118.

Referring toFIG. 4, a perspective view illustrates the apparatus70in fully assembled form, with the end plugs158and the set screw160in place. The end plugs158may be sufficiently actuated to cause the leading end of each end plug158to press against the side of the corresponding castle nut78. Pressure against the castle nut78further restricts rotation of the castle nut78within the corresponding mounting interface100, thereby further securing the ends92,94against rotation with respect to the corresponding pedicles30,50. This tends to restrict flexion, extension, lateral bending, and axial rotation of the vertebrae24,26.

Although the ends92,94are substantially secured against rotation with respect to the pedicles30,50via engagement of the castle nuts78with the mounting interfaces100, usage of the end plugs158provides additional securement. In alternative embodiments, the ends of a stabilizer may be allowed to dynamically rotate polyaxially with respect to vertebral attachment points. The apparatus70may easily modified to provide such polyaxiality. End plugs158may then be used to selectively restrict relative polyaxial motion.

The set screw160may be sufficiently actuated to cause the leading end of the set screw106to press against the first end124of the stabilization rod74. Pressure against the first end124tends to arrest sliding of the first end124with respect to the first end92of the bridge72, thereby keeping the apparatus70from elongating or contracting.

When the apparatus70is unable to elongate or contract, the vertebrae24,26are substantially unable to move relative to each other in flexion, extension, lateral bending, and axial rotation. Accordingly, usage of the set screw160, with or without the end plugs158, may amount to fusion of the vertebrae24,26. If stabilization via the apparatus70is unsuccessful in preventing further damage to the intervertebral disc66or to the vertebrae24,26, the set screw160may easily be applied to fuse the vertebrae24,26without requiring removal of the apparatus70or further removal of bone tissue.

It may be desirable to provide some structure to limit the ability of the vertebrae24,26to move in axial rotation and/or lateral bending, without significantly limiting flexion or extension. This may be particularly desirable for a stabilizer with end points that are attached to the vertebrae in such a manner that polyaxial rotation between the end points and the vertebrae is permitted. Such polyaxial rotation may permit a pair of stabilizers to “windshield wiper,” or rotate in tandem to permit relatively unrestricted axial rotation. Similarly, relative rotation of stabilizers of a bilateral pair may enable lateral bending.

Referring toFIG. 5, a perspective view illustrates left and right apparatus70that are linked together via a crosslink180. The crosslink180may operate to restrict relative rotation between the apparatus70on the left-hand side and the apparatus70on the right-hand side, thereby restricting relative axial rotation and/or lateral bending of a pair of vertebrae, as described above.

As shown, the crosslink180includes a rod182, a pair of brackets184, and a pair of fasteners, which may take the form of screws186, that hold the brackets184to the rod182and the left and right apparatus70. The rod182may have a generally cylindrical shape, and may pass generally underneath the spinous process36of the first vertebra24(shown inFIG. 1). The rod182has a first end190attached to one of the apparatus70and a second end192attached to the other apparatus70.

Each screw186has a head200, a shank (not shown), and a torquing feature202extending into the head. The torquing feature202may take the form of a hexagonal recess like those of the end plugs158and the set screw160, as described previously. The shank may be threaded to interface with corresponding threads (not shown) of the brackets184.

Each of the brackets184has a first grip210and a second grip212. The first grip210is designed to secure each bracket184to the corresponding end190,192of the rod182. The second grip212secures each bracket184to the corresponding apparatus70. The first and second grips210,212are designed to be energized by the corresponding screw186to retain the rod182and the corresponding apparatus70. For example, each of the brackets184may have a bore (not shown) extending through both of the grips210,212, with threads only on the end of the bore furthest from the end at which the corresponding head200will be positioned. Accordingly, tightening of each screw186may cause axial compression of the bore of the corresponding bracket184.

The first grip210has a slot220with a compression portion222and a gripping portion224. At the compression portion222, the slot220is relatively narrow. At the gripping portion224, the slot220widens to provide a generally cylindrical interior surface shaped to receive the corresponding end190or192of the rod182. The sides of the compression portion222are drawn toward each other by tightening the corresponding screw186. As a result, the sides of the gripping portion224press inward against the corresponding end190or192for secure retention.

The second grip212similarly has a slot230with a compression portion232and a gripping portion234. At the compression portion232, the slot230is relatively narrow. At the gripping portion234, the slot230widens to provide a generally cylindrical interior surface shaped to receive the first end92of the bridge72of the corresponding apparatus70. The sides of the compression portion232are drawn toward each other by tightening the corresponding screw186. As a result, the sides of the gripping portion234press inward against the end92of the bridge72of the corresponding apparatus70for secure retention.

The brackets184enable efficient installation because tightening the screws186causes the brackets184to simultaneously retain the rod182and the left and right apparatus70. According to one installation method, after the left and right apparatus70have been attached to the vertebrae24,26, the crosslink180can be easily inserted into loose engagement with the left and right apparatus70, such that the rod182is not securely retained. With the vertebrae24,26at the desired relative orientation in axial rotation and lateral bending (presumably a neutral orientation), the screws186can be tightened to restrict further relative rotation between the left and right apparatus70, thereby restricting further axial rotation and/or lateral bending.

According to alternative embodiments, a crosslink need not extend between two stabilizers. For example, a crosslink (not shown) may have a first end attached to one apparatus70, and a second end attached directly to one of the vertebrae24,26. The second end may be attached to any desirable feature such as a pedicle30or50or a spinous process36or56. Such a crosslink would inhibit rotation of the apparatus70with respect to the vertebrae24,26in a manner similar to that of the crosslink180. Such a crosslink may be particularly desirable if only one stabilizer is used. An end of a crosslink that is “substantially secured” with respect to a vertebra may be attached to a stabilizer such as the apparatus70coupled to the vertebra, attached directly to the vertebra, or indirectly attached to the vertebra through the use of a different element such as a fastener or another type of spinal prosthesis.

Additionally, a wide variety of other crosslink embodiments may be used. For example, in place of the brackets184, retention members (not shown) may be attached to the apparatus70or to the rod182via adhesives, set screws, clips, or other devices. Furthermore, if desired, a crosslink may be made from fewer pieces. For example, two telescoping rod segments may each have an integrated end capable of being attached to one apparatus70. As another example, a crosslink may be designed to provide locking as well as crosslinking, thereby making it unnecessary to install a separate locking component. Such a crosslink may have a built-in set screw or other locking component, or may otherwise retain the corresponding stabilizers in such a manner that they are unable to elongate or contract when the crosslink is in place. Those of skill in the art will recognize that a wide range of alternatives may be used within the scope of the present invention.

Usage of the apparatus70may beneficially add stiffness in flexion, extension, axial rotation, and lateral bending, whether used with or without the crosslink180. The crosslink180may help to add additional stiffness in axial rotation and lateral bending. The manner in which the apparatus70and/or the crosslink180may help to restore natural spinal biomechanics will be shown and described with reference toFIGS. 6 and 7, as follows.

Referring toFIG. 6, a chart illustrates the manner in which the flexion, extension, axial rotation and/or lateral bending of a damaged or diseased joint motion segment may be adjusted according to many prior art methods. According to traditional thinking, a corrected displacement curve236shows the magnitude of flexion, extension, axial rotation, and/or lateral bending of two vertebrae separated by a healthy intervertebral disc as a function of moment loading. A pathological displacement curve238shows the magnitude of axial rotation or lateral bending of two vertebrae separated by a diseased or damaged intervertebral disc as a function of moment loading according to some traditional analysis methods.

When applied to a joint motion segment having the pathological displacement curve238, a stabilizer adds stiffness in flexion, extension, axial rotation, and/or lateral bending across substantially the entire range of motion of the joint. Known stabilizers often have resilient members that provide a single spring constant across the entire range of motion, thereby applying a proportionate increase in stiffness along the range of motion of the joint. The result is to move a spinal motion segment from the motion characteristics of the pathological displacement curve238toward those of the corrected displacement curve236. Since such a stabilizer may not provide any mechanical stops, the corrected displacement curve236has a substantially constant slope, which does not accurately replicate natural biomechanics.

Referring toFIG. 7, a chart illustrates the manner in which the flexion and extension of a damaged or diseased joint motion segment can be enhanced through the use of the apparatus70, or any other stabilizer according to the invention. A natural displacement curve240shows the natural magnitude of relative rotation as a function of moment loading of two vertebrae separated by a healthy intervertebral disc, healthy facet joints, and connected by healthy ligaments. A pathological displacement curve242shows the magnitude of relative rotation as a function of moment loading of two vertebrae separated by one or more of: diseased or damaged intervertebral disc, diseased or damaged ligaments, and diseased or damaged facet joints. The natural displacement curve240also represents an ideal displacement curve after the application of the apparatus70to a pathological joint motion segment, where restoration of natural biomechanics has been achieved.

As shown, a pair of boundaries250illustrates the limits of a neutral zone252of the natural displacement curve240. Within the neutral zone252, relatively large displacement occurs because the stiffness of the intervertebral disc, ligaments, facet joint capsules and other adjacent tissues is relatively low. Outside the boundaries250, the natural displacement curve240has motion limited zones254within which the stiffness of these members is greater due to the fact that they are under higher deflection. Additionally, within the motion limited zones254, it abutment of bone structures such as facet joints may contribute a relative larger stiffness so that relatively small displacement occurs with the incremental addition of moments.

Boundaries260similarly illustrate the limits of a neutral zone262of the pathological displacement curve242. Outside the boundaries260, the pathological displacement curve242has motion limited zones264within which motion in response to incremental addition of moments is generally more limited than within the neutral zone262. Generally, the pathological displacement curve242exhibits far more motion for any given input moment than the natural displacement curve240. The slope of the neutral zone262is lower than that of the neutral zone252, and the boundaries260are not reached until a higher moment is applied. The slopes of the motion limited zones264may even be higher than those of the motion limited zones254. As mentioned previously, such a condition may accelerate deterioration of, and necessary surgical intervention for, the intervertebral disc due to excessive intervertebral motion.

When applied to a joint motion segment having the pathological displacement curve242, the apparatus70ofFIGS. 1 through 5beneficially adds stiffness in flexion and extension across substantially the entire range of motion of the joint. When the crosslink180is also in place, even more stiffness in axial rotation and lateral bending may be added, without significantly inhibiting motion in flexion and extension. The result is to move a spinal motion segment from the motion characteristics of the pathological displacement curve242back toward those of the natural displacement curve240. It may be desirable to stiffen the spinal motion segment even beyond the level of stiffness provided by a natural, healthy spinal motion segment to protect a diseased or damaged intervertebral disc from further damage.

More precisely, the resilient section120of the central portion96of the bridge72adds stiffness that increases the slope of the neutral zone262to approximate that of the neutral zone252of the natural displacement curve240. The boundaries260are thus brought inward proximate the locations of the boundaries250. Within the motion limited zones264of the pathological displacement curve242, the apparatus70provides mechanical stops that limit motion by providing additional stiffness to approximate the motion limited zones254of the natural displacement curve240. Such mechanical stops may include, but are not limited to, the ends of the pin registration slot114of the central portion96of the bridge72because the ends of the pin registration slot114limit extension and contraction of the apparatus70.

It has been discovered that the natural and pathological displacement curves240,242ofFIG. 7more accurately characterize the stiffness of a joint than the corrected and pathological displacement curves236,238ofFIG. 6. The present invention is more closely tuned to correcting the actual pathology, and to providing a displacement curve that more closely approximates the natural displacement curve of a joint.

The apparatus70ofFIGS. 1 through 5is only one of many different designs that can provide dynamic stabilization according to the invention. The apparatus70utilizes stabilization, as provided by the stabilization rod74, in conjunction with a resilient member, i.e., the resilient section120of the central portion96of the bridge72, to provide motion characteristics that provide the needed stabilization while more closely replicating natural kinematics. In the apparatus70, the stabilization rod74passes through the resilient section120. However, in selected alternative embodiments, a stabilization assembly may extend around the outside of a resilient member. Such an embodiment will be shown and described in connection withFIGS. 8 through 10, as follows.

Referring toFIG. 8, an exploded, perspective view illustrates an apparatus270according to one alternative embodiment of the invention. The apparatus270includes castle nuts (not shown), each of which has a threaded bore and a torquing interface such as the crenelations146of the castle nuts78of the previous embodiment. However, the castle nuts of the current embodiment do not have a compression end because they are not designed to lock the apparatus270to prevent rotation with respect to the vertebrae24,26(shown inFIG. 1). Rather, the castle nuts have flat ends that hold the ends of the apparatus270against the pedicles30,50, while permitting limited polyaxial relative rotation due to the structure of the ends of the apparatus270, as will be described subsequently. The castle nuts may cooperate with fixation members80like those of the previous embodiment to attach the apparatus270to the vertebrae24,26.

In addition to the castle nuts and fixation members80, the apparatus270includes a bridge272, a resilient rod274, a pair of pins76, and a pair of split spheres282. The bridge272does not provide resiliency, but rather, acts as a stabilization assembly. The resilient rod274provides resiliency. Thus, the bridge272and the rod274cooperate to perform a function similar to that of the bridge72and the stabilization rod74of the previous embodiment. The pins76may be identical to those of the previous embodiment.

Each of the split spheres282may be formed of a relatively pliable material such as a polymer. Each split sphere282may have a semispherical surface284with an open portion286that permits the split sphere282to flex to enlarge or contract the semispherical surface284. Furthermore, each split sphere282has a pair of end rings288. Each end ring288has a generally tubular configuration that protrudes beyond the adjacent semispherical surface284. The split spheres282operate to enable polyaxial rotation of the apparatus270with respect to the vertebrae24,26in a manner that will be described subsequently. The polyaxial rotation is “dynamic,” which means that it is able to occur after the apparatus270has been securely attached to the pedicles30,50.

As shown, the bridge272ofFIG. 3has a first containment member292and a second containment member294. The containment members292,294cooperate to substantially contain the resilient rod274, as will be described in greater detail subsequently. Each of the first and second containment members292,294has an end296. Additionally, the first containment member292has a telescoping portion298, and the second containment member294has a telescoping member300designed to telescopically engage the telescoping portion298of the first containment member292.

Each end296has a mounting interface302with a generally semispherical shape that converges to a pair of generally symmetrical mounting apertures102, only one of which is visible on each mounting interface302inFIG. 8. Like the mounting interface100of the previous embodiment, each mounting interface302has an interior orifice106and an exterior orifice108. The interior and exterior orifices106,108cooperate to facilitate installation of the resilient rod274within the bridge272. Furthermore, the exterior orifices108may receive end plugs158like those of the previous embodiment to facilitate locking of the apparatus270to optionally prevent rotation with respect to the vertebrae24,26after attachment. Additionally, the telescoping portion298of the first containment member292has a supplemental orifice304with threads306to facilitate locking, as will be discussed subsequently.

The first telescoping portion298has an interior surface308with a generally cylindrical shape. The second telescoping portion300is designed to slide within the first telescoping portion298, and therefore has an exterior surface310that fits within the interior surface308with clearance. The second telescoping portion300also has an interior surface312within which the resilient rod274is generally positionable.

The first containment member292has a pin registration orifice314positioned generally at the juncture of the corresponding end296with the telescoping portion298. The pin registration orifice314is sized to receive the corresponding pin76with either clearance or interference, as desired. The second containment member294similarly has a pin registration orifice316positioned generally at the juncture of the corresponding end296with the telescoping portion300to receive the corresponding pin76with either clearance or interference. The telescoping portion300of the second containment member294has a stepped down interior surface (not visible inFIG. 8) that is sized to fit with relatively small clearance around the corresponding portion of the resilient rod274.

The resilient rod274has a first end324, a second end326, and a central portion328between the first and second ends324,326. The first end324has a pin registration orifice332designed to receive the corresponding pin76in concert with the pin registration orifice314of the first containment member292. Similarly, the second end326has a pin registration orifice334designed to receive the corresponding pin76in concert with the pin registration interface316of the second containment member294.

The central portion328has a stepped down region336designed to reside within the stepped down interior surface350of the telescoping portion300of the second containment member294. The stepped down region336may fit into the stepped down interior surface350with relatively small clearance so that the engagement of the stepped down region336with the stepped down interior surface (not visible inFIG. 8) helps to maintain coaxiality of the bridge272with the resilient rod274. The central portion328also has a resilient section338, which may be a linear spring like that of the resilient section120of the previous embodiment.

As in the previous embodiment, the resilient section338is integrally formed with the remainder of the resilient rod274. However, in alternative embodiments (not shown), a resilient section may be a separate piece with the remainder of a resilient rod, and may be attached to the other resilient rod components or may remain coupled thereto by virtue of assembly with the corresponding bridge.

Returning to the apparatus270ofFIG. 3, a locking component may optionally be provided. The locking component may take the form of a set screw340configured somewhat similarly to the set screw160of the previous embodiment, in that the set screw340has threads342and a torquing feature344. The threads342are shaped to mate with the threads306of the supplemental orifice304so that the set screw340can be rotated into engagement with the supplemental orifice304.

Referring toFIG. 9, a fully assembled, partially cut away view illustrates the apparatus270in a fully assembled state, without the end plugs158and the set screw340. As described previously, the telescoping portion300of the second containment member294has a stepped down interior surface350that fits around the stepped down region336of the central portion328of the resilient rod274with relatively little clearance. The stepped down interior surface350may slide relatively freely around the stepped down region336, but the clearance between the two may be small enough to inhibit relative rotation between the containment members292,294, except about the axis of the containment members292,294. The split spheres282have been inserted into the corresponding mounting interfaces302.

The bridge272and the resilient rod274may be relatively easily assembled by sliding the stepped down region336of the resilient rod274through the exterior orifice108, the interior orifice106, and then into the stepped down interior surface350of the second containment member294. The second end326of the resilient rod274may be fixed with respect to the end296of the second containment member294by sliding one of the pins76through the pin registration orifice316of the second containment member294, and through the pin registration orifice334of the second end326of the resilient rod. The first end324of the resilient rod274may then be fixed with respect to the end296of the first containment member292by sliding the other pin76through the pin registration orifice314of the first containment member292, and through the pin registration orifice332of the first end324of the resilient rod.

By virtue of the pins76, the engagement of the interior surface308with the exterior surface310, and/or the engagement of the stepped down region336with the stepped down interior surface350, the first and second containment members292,294may be constrained to remain substantially coaxial with each other and with the resilient rod274. The resilient section338provides resilient force to urge the saddle points42,62to a displacement in which the resilient section338is substantially undeflected. Thus, the apparatus270performs a function similar to that of the apparatus70ofFIG. 1. In alternative embodiments, an apparatus like the apparatus270may be tuned to provide slight distraction of the vertebrae24,26, i.e., urge the posterior elements of the vertebrae24,26to move apart from each other more than in a normal neutral position of the spinal motion segment to further protect the intervertebral disc66from damage.

Referring toFIG. 10, a perspective view illustrates the apparatus270in a fully assembled state, with the end plugs158and the set screw340in place. Prior to installation of the end plugs158, the ends296of the containment members292,294are able to rotate polyaxially with respect to the corresponding saddle points42,62. The proximal ends84of the fixation members80(shown inFIG. 1) pass through the split spheres282, and the castle nuts (not shown) are rotated into place to press against the exposed end rings288of the split spheres282to hold the split spheres282relatively securely to the fixation members80.

The semispherical surfaces284of the split spheres282articulate with the mounting interfaces302to permit triaxial rotation of each end296relative to the fixation member80that passes through it. Each of the end rings288may serve as a motion stop by contacting the adjacent mounting aperture102of the corresponding mounting interface302when the end296reaches a pre-established orientation with respect to the corresponding vertebra24or26. If desired, alternative embodiments (not shown) may utilize end rings with non-circular peripheries to provide tighter control over the polyaxiality provided by the corresponding split sphere. For example, an oval-shaped, squared, or otherwise deliberately shaped end ring may be used as a cam to permit a higher degree of rotation about one axis than about another.

The end plugs158are rotated into the exterior orifices108to abut against the split spheres282, thereby restricting, or even preventing, rotation of the ends296relative to the vertebrae24,26. More precisely, end interior ends of the end plugs158engage the semispherical surfaces284of the split spheres282, thereby restricting rotation of the split spheres282within the mounting interfaces302. Thus, the apparatus270is then constrained to remain at a fixed orientation with respect to the vertebrae24,26.

As the set screw340is tightened into abutment with the exterior surface310of the telescoping portion300of the second containment member294, pressure of the set screw340against the exterior surface310prevents further relative motion between the telescoping portions298,300. Thus, the apparatus270is unable to elongate or contract, and as with usage of the set screw160of the previous embodiment, flexion, extension axial rotation, and lateral bending are substantially prevented. As in the previous embodiments, the set screw340and the end plugs158may cooperate to lock the apparatus270to substantially fuse the vertebrae24,26together. However, as in the previous embodiment, the set screw340and the end plugs158may be used independently of each other.

Set screws provide only one of many different locking components that may be used to lock an apparatus according to the invention. In alternative embodiments, clips may be used. Such clips may have prongs or other features that are insertable into aligned holes of the two telescoping portions298,300. If desired, the telescoping portions298,300may have multiple hole combinations that can be aligned at different relative positions of the telescoping members298,300to permit locking of the telescoping portions298,300at any of the relative positions.

According to another alternative embodiment, a locking component may include a rod (not shown) with ends that have rings or other features that can engage fixation members80independently. Such a rod may be attached to the two engagement members80parallel to the apparatus270to provide intervertebral fusion, or the apparatus270may even be removed to permit attachment of the rod in its place.

According to yet another alternative embodiment, a locking component may take the form of a curable resin, bone graft, or the like. Such a material may be injected into an apparatus270and allowed to harden to provide locking. Those of skill in the art will recognize that a variety of other locking components may be used. Similarly, many different structures may be used to lock the ends of an apparatus such as the apparatus270to restrict or prevent rotation of the ends with respect to the vertebrae24,26.

Returning toFIG. 10, in one specific example, the telescoping portion298of the first containment member292has an outside diameter of about 8 millimeters, and the telescoping portion300of the second containment member294has an outside diameter of about 7 millimeters. Upon assembly of the bridge272and the resilient rod274, the centers of the mounting apertures102may be about 35 millimeters apart when the resilient section338is substantially undeflected. In use, the resilient section338may be expected to deflect by plus or minus about five millimeters.

The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. 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 it changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.