US20060015100A1 - Spinal stabilization devices coupled by torsional member - Google Patents

Abstract

Spine stabilization devices, systems and methods are provided in which torsion member extends between first and second stabilization devices that are oppositely positioned relative to the vertebrae. The torsion member includes a (i) a first segment that is substantially co-planar with and perpendicular to the axis of at least one of the stabilization devices; (ii) a second segment that extends from the first segment and that is angularly and upwardly oriented relative to the first segment; (iii) a third segment that extends from the second segment, is substantially perpendicular to at least one of the stabilization devices, and is oriented in a plane that is elevated with respect to, but substantially parallel to, the plane of at least one of the stabilization devices, (iv) a fourth segment that extends from the third segment and that is a substantial mirror image of the second segment; and (v) a fifth segment that extends from the fourth segment and that is a substantial mirror image of the first segment.. Methods for mounting the torsion member with respect to attachment members associated with opposed pedicle screws are also disclosed.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS
• The present application claims the benefit of a co-pending provisional patent application entitled “Dynamic Spine Stabilizer,” filed on Jun. 23, 2004 and assigned Ser. No. 60/581,716. The entire contents of the foregoing provisional patent application are incorporated by reference herein.
BACKGROUND OF THE DISCLOSURE
• 1. Technical Field
• The present disclosure is directed to a dynamic stabilization device and system for spinal implantation and, more particularly, to a dynamic stabilization device and system that is adapted to be positioned/mounted relative to first and second laterally-spaced pedicle screws and that includes at least one dynamic stabilization member that is positioned beyond the region defined between the pedicle screws, e.g., in an “overhanging” orientation.
• 2. Background Art
• Low back pain is one of the most expensive diseases afflicting industrialized societies. With the exception of the common cold, it accounts for more doctor visits than any other ailment. The spectrum of low back pain is wide, ranging from periods of intense disabling pain which resolve, to varying degrees of chronic pain. The conservative treatments available for lower back pain include: cold packs, physical therapy, narcotics, steroids and chiropractic maneuvers. Once a patient has exhausted all conservative therapy, the surgical options range from micro discectomy, a relatively minor procedure to relieve pressure on the nerve root and spinal cord, to fusion, which takes away spinal motion at the level of pain.
• Each year, over 200,000 patients undergo lumbar fusion surgery in the United States. While fusion is effective about seventy percent of the time, there are consequences even to these successful procedures, including a reduced range of motion and an increased load transfer to adjacent levels of the spine, which accelerates degeneration at those levels. Further, a significant number of back-pain patients, estimated to exceed seven million in the U.S., simply endure chronic low-back pain, rather than risk procedures that may not be appropriate or effective in alleviating their symptoms.
• New treatment modalities, collectively called motion preservation devices, are currently being developed to address these limitations. Some promising therapies are in the form of nucleus, disc or facet replacements. Other motion preservation devices provide dynamic internal stabilization of the injured and/or degenerated spine, without removing any spinal tissues. A major goal of this concept is the stabilization of the spine to prevent pain while preserving near normal spinal function. The primary difference in the two types of motion preservation devices is that replacement devices are utilized with the goal of replacing degenerated anatomical structures which facilitates motion while dynamic internal stabilization devices are utilized with the goal of stabilizing and controlling abnormal spinal motion.
• Over ten years ago a hypothesis of low back pain was presented in which the spinal system was conceptualized as consisting of the spinal column (vertebrae, discs and ligaments), the muscles surrounding the spinal column, and a neuromuscular control unit which helps stabilize the spine during various activities of daily living. Panjabi M M. “The stabilizing system of the spine. Part I. Function, dysfunction, adaptation, and enhancement.”J Spinal Disord5 (4): 383-389, 1992a. A corollary of this hypothesis was that strong spinal muscles are needed when a spine is injured or degenerated. This was especially true while standing in neutral posture. Panjabi M M. “The stabilizing system of the spine. Part II. Neutral zone and instability hypothesis.”J Spinal Disord5 (4): 390-397, 1992b. In other words, a low-back patient needs to have sufficient well-coordinated muscle forces, strengthening and training the muscles where necessary, so they provide maximum protection while standing in neutral posture.
• Dynamic stabilization (non-fusion) devices need certain functionality in order to assist the compromised (injured or degenerated with diminished mechanical integrity) spine of a back patient. Specifically, the devices must provide mechanical assistance to the compromised spine, especially in the neutral zone where it is needed most. The “neutral zone” refers to a region of low spinal stiffness or the toe-region of the Moment-Rotation curve of the spinal segment (seeFIG. 1). Panjabi M M, Goel V K, Takata K. 1981 Volvo Award in Biomechanics. “Physiological Strains in Lumbar Spinal Ligaments, an in vitro Biomechanical Study.”Spine7 (3): 192-203, 1982. The neutral zone is commonly defined as the central part of the range of motion around the neutral posture where the soft tissues of the spine and the facet joints provide least resistance to spinal motion. This concept is nicely visualized on a load-displacement or moment-rotation curve of an intact and injured spine as shown inFIG. 1. Notice that the curves are non-linear; that is, the spine mechanical properties change with the amount of angulations and/or rotation. If we consider curves on the positive and negative sides to represent spinal behavior in flexion and extension respectively, then the slope of the curve at each point represents spinal stiffness. As seen inFIG. 1, the neutral zone is the low stiffness region of the range of motion.
• Experiments have shown that after an injury of the spinal column or due to degeneration, neutral zones, as well as ranges of motion, increase (seeFIG. 1). However, the neutral zone increases to a greater extent than does the range of motion, when described as a percentage of the corresponding intact values. This implies that the neutral zone is a better measure of spinal injury and instability than the range of motion. Clinical studies have also found that the range of motion increase does not correlate well with low back pain. Therefore, the unstable spine needs to be stabilized especially in the neutral zone. Dynamic internal stabilization devices must be flexible so as to move with the spine, thus allowing the disc, the facet joints, and the ligaments normal physiological motion and loads necessary for maintaining their nutritional well-being. The devices must also accommodate the different physical characteristics of individual patients and anatomies to achieve a desired posture for each individual patient.
• With the foregoing in mind, those skilled in the art will understand that a need exists for a spinal stabilization device which overcomes the shortcoming of prior art devices. The present invention provides such an apparatus and method for spinal stabilization.
SUMMARY OF THE DISCLOSURE
• The present disclosure provides advantageous apparatus and methods for stabilizing adjacent spinal vertebrae in spinal axial rotation and spinal lateral bending. According to an exemplary embodiment, first and second stabilization members of a spine stabilization system are configured for coupling with respect to first and second pedicle screw pairs. The pedicle screw pairs are typically implanted in adjacent vertebrae, and the stabilization devices that extend therebetween contribute spinal stability. According to exemplary embodiments of the present disclosure, the stabilization devices are dynamic stabilization devices that include elastic elements (e.g., springs) that deliver desirable force profiles in situ.
• Coupling of the first and second pedicle screw pairs is advantageously achieved through a torsion member that spans the spinal region from the first pedicle screw pair to the second pedicle screw pair. According to exemplary embodiments of the present disclosure, mounting or attachment members are provided with respect to the respective pedicle screw pairs that facilitate mounting of both the stabilization devices and the torsion member. The pedicle screw pairs and the associated spine stabilization devices are typically arranged in parallel (or substantially in parallel, as dictated by the anatomy), and the disclosed torsion member acts to further stabilize the spine by acting on the respective pedicle screw pairs.
• The torsion member is typically mounted with respect to a mounting/attachment member associated with each of the pedicle screw pairs. Stabilization is achieved through the torsion member's interaction with the respective first and second attachment members. In this way, the torsion member is able to further stabilize the adjacent spinal vertebrae with respect to spinal axial rotation and/or spinal lateral bending. The torsion member is configured and dimensioned to accommodate interaction with the respective first and second attachment members and includes a geometry that is characterized as follows: (i) a first segment that is substantially co-planar with and perpendicular to the axis of the first stabilization member; (ii) a second segment that extends from the first segment and that is angularly and upwardly oriented relative to the first segment; (iii) a third segment that extends from the second segment, substantially perpendicular to the first stabilization device and oriented in a plane that is elevated with respect to but parallel to the plane of the first stabilization device, (iv) a fourth segment that extends from the third segment and that is a substantial mirror image of the second segment; and (v) a fifth segment that extends from the fourth segment and that is a substantial mirror image of the first segment. Thus, exemplary embodiments of the disclosed torsion member are substantially symmetrical relative to a mid-point of the third segment.
• The first through fifth segments of the disclosed torsion member are typically integrally formed, e.g., through metal forming techniques employed with respect to bar stock. The bar stock is typically of circular, rectangular, elliptical and/or square cross-section and may be of constant or variable cross-sectional dimension. In addition, the disclosed torsion member is advantageously adapted to be mounted with respect to first and second attachment members at the exposed ends of the first and fifth segments or, alternatively, may be integrally formed with respect to one or both attachment members, e.g., through conventional welding techniques.
• The disclosed torsion member offers advantageous functionality in spine applications through the disclosed multi-planar design, thereby accommodating limited amounts of axial and/or lateral bending of the spine, while imparting desired levels of spinal stabilization. As noted herein, the disclosed torsion members are susceptible to a variety of variations without departing from the spirit or scope of the present disclosure. Thus, for example, asymmetric geometries and/or the inclusion of additional jogs in the torsion member may be advantageously employed to affect the system stiffness and/or result in the torsion member manifesting differing degrees of internal bending stiffness with respect to bending in different spatial planes.
• According to other exemplary embodiments of the present disclosure, a spine stabilization system is provided by which a spine stabilization system incorporating the disclosed torsion member in combination with laterally-spaced spine stabilization devices to provide spine stabilization in spinal flexion, extension, axial rotation, and lateral bending, thereby providing comprehensive spinal support to those who require it. A method of stabilizing a pair of spinal vertebrae is further provided that involves: (i) mounting spine stabilization devices with respect to pedicle screws positioned on opposite lateral sides of a pair of spinal vertebrae so as to stabilize the same in spinal flexion and/or extension, and (ii) securing a torsion member between the pedicle screws so as to provide further stabilization in spinal axial rotation and spinal lateral bending, the torsion member having the advantageous multi-planar properties and attributes described herein.
• The disclosed spine stabilization systems and methods have a variety of applications and implementations, as will be readily apparent from the disclosure provided herein. Additional advantageous features and functionalities associated with the present disclosure will be apparent from the detailed description which follows, particularly when read in conjunction with the figures appended hereto.
BRIEF DESCRIPTION OF THE DRAWINGS
• To assist those of ordinary skill in the art in making and using the disclosed spinal stabilization device/system, reference is made to the accompanying figures, wherein:
• FIG. 1 is Moment-Rotation curve for a spinal segment (intact and injured), showing low spinal stiffness within the neutral zone.
• FIG. 2 is a schematic representation of a spinal segment in conjunction with a Moment-Rotation curve for a spinal segment, showing low spinal stiffness within the neutral zone.
• FIG. 3ais a schematic of a spinal stabilization device in conjunction with a Force-Displacement curve, demonstrating the increased resistance provided within the central zone according to spinal stabilization systems wherein a dynamic element is positioned between laterally-spaced pedicle screws.
• FIG. 3bis a Force-Displacement curve demonstrating the change in profile achieved through spring replacement.
• FIG. 3cis a dorsal view of the spine with a pair of dynamic stabilization devices secured thereto.
• FIG. 3dis a side view showing the exemplary dynamic stabilization device in tension.
• FIG. 3eis a side view showing the exemplary dynamic stabilization device in compression.
• FIG. 4 is a schematic of a dynamic spine stabilization device that is adapted to position dynamic elements between laterally-spaced pedicle screws.
• FIG. 5 is a schematic of an alternate dynamic spine stabilization device that is adapted to position dynamic elements between laterally-spaced pedicle screws.
• FIG. 6 is a Moment-Rotation curve demonstrating the manner in which dynamic stabilization devices using the principles of the present disclosure assist in spinal stabilization.
• FIGS. 7ais a free body diagram of a dynamic stabilization device in which dynamic elements are positioned between laterally-spaced pedicle screws.
• FIG. 7bis a diagram representing the central zone of a spine and the forces associated therewith for dynamic stabilization according to the present disclosure.
• FIG. 8 is a perspective view of an exemplary dynamic stabilization device in accordance with the present disclosure.
• FIG. 9 is an exploded view of the dynamic stabilization device shown inFIG. 8.
• FIG. 10 is a detailed perspective view of the distal end of a first pedicle screw for use in exemplary implementations of the present disclosure; according to exemplary embodiments of the present disclosure, the second pedicle screw is identical.
• FIG. 11 is a detailed perspective view of a first pedicle screw secured to an exemplary attachment member according to the present disclosure.
• FIG. 12 is a perspective view of the exemplary dynamic stabilization device shown inFIG. 8 as seen from the opposite side.
• FIG. 13 is a perspective view of a dynamic stabilization device of the type depicted inFIG. 8 with a transverse torsion bar stabilizing member.
DESCRIPTION OF EXEMPLARY EMBODIMENTS
• Exemplary embodiments of the disclosed dynamic stabilization system/device are presented herein. It should be understood, however, that the disclosed embodiments are merely exemplary of the present invention, which may be embodied in various forms. Therefore, the details disclosed herein are not to be interpreted as limiting, but merely as the basis for teaching one skilled in the art how to make and/or use the devices and systems of the present disclosure.
• With reference toFIGS. 2, 3a-eand4, a method and apparatus are disclosed for spinal stabilization. In accordance with a preferred embodiment of the present disclosure, the spinal stabilization method is achieved by securing an internal dynamicspine stabilization device10 betweenadjacent vertebrae12,14 and providing mechanical assistance in the form of elastic resistance to the region of the spine to which the dynamicspine stabilization device10 is attached. The elastic resistance is applied as a function of displacement such that greater mechanical assistance is provided while the spine is in its neutral zone and lesser mechanical assistance is provided while the spine bends beyond its neutral zone. Although the term elastic resistance is used throughout the body of the present specification, other forms of resistance may be employed without departing from the spirit or scope of the present disclosure.
• As those skilled in the art will certainly appreciate, and as mentioned above, the “neutral zone” is understood to refer to a region of low spinal stiffness or the toe-region of the Moment-Rotation curve of the spinal segment (seeFIG. 2). That is, the neutral zone may be considered to refer to a region of laxity around the neutral resting position of a spinal segment where there is minimal resistance to intervertebral motion. The range of the neutral zone is considered to be of major significance in determining spinal stability. Panjabi, M M. “The stabilizing system of the spine. Part II. Neutral zone and instability hypothesis.”J Spinal Disorders1992; 5 (4): 390-397.
• In fact, the inventor has previously described the load displacement curve associated with spinal stability through the use of a “ball in a bowl” analogy. According to this analogy, the shape of the bowl indicates spinal stability. A deeper bowl represents a more stable spine, while a more shallow bowl represents a less stable spine. The inventor previously hypothesized that for someone without spinal injury there is a normal neutral zone (that part of the range of motion where there is minimal resistance to intervertebral motion) with a normal range of motion, and in turn, no spinal pain. In this instance, the bowl is not too deep nor too shallow. However, when an injury occurs to an anatomical structure, the neutral zone of the spinal column increases and the ball moves freely over a larger distance. By this analogy, the bowl would be more shallow and the ball less stable, and consequently, pain results from this enlarged neutral zone.
• In general, pedicle screws16,18 attach the dynamicspine stabilization device10 to thevertebrae12,14 of the spine using well-tolerated and familiar surgical procedures known to those skilled in the art. In accordance with a preferred embodiment, and as those skilled in the art will certainly appreciate, a pair of opposed stabilization devices are commonly used to balance the loads applied to the spine (seeFIG. 3c). The dynamicspine stabilization device10 assists the compromised (injured and/or degenerated) spine of a back pain patient, and helps her/him perform daily activities. The dynamicspine stabilization device10 does so by providing controlled resistance to spinal motion particularly around neutral posture in the region of neutral zone. As the spine bends forward (flexion) thestabilization device10 is tensioned (seeFIG. 3d) and when the spine bends backward (extension) thestabilization device10 is compressed (seeFIG. 3e).
• The resistance to displacement provided by the dynamicspine stabilization device10 is non-linear, being greatest in its central zone so as to correspond to the individual's neutral zone; that is, the central zone of thestabilization device10 provides a high level of mechanical assistance in supporting the spine. As the individual moves beyond the neutral zone, the increase in resistance decreases to a more moderate level. As a result, the individual encounters greater resistance to movement (or greater incremental resistance) while moving within the neutral zone.
• The central zone of the dynamicspine stabilization device10, that is, the range of motion in which thespine stabilization device10 provides the greatest resistance to movement, may be adjustable at the time of surgery to suit the neutral zone of each individual patient. In such exemplary embodiments, the resistance to movement provided by the dynamicspine stabilization device10 is adjustable pre-operatively and intra-operatively. This helps to tailor the mechanical properties of the dynamicspine stabilization device10 to suit the compromised spine of the individual patient. The length of the dynamicspine stabilization device10 may also be adjustable intra-operatively to suit individual patient anatomy and to achieve desired spinal posture. The dynamicspine stabilization device10 can be re-adjusted post-operatively with a surgical procedure to adjust its central zone to accommodate a patient's altered needs.
• According to exemplary embodiments of the present disclosure, ball joints20,22 link the dynamicspine stabilization device10 with the pedicle screws16,18. The junction of the dynamicspine stabilization device10 and pedicle screws16,18 is free and rotationally unconstrained. Therefore, first of all, the spine is allowed all physiological motions of bending and twisting and second, the dynamicspine stabilization device10 and the pedicle screws16,18 are protected from harmful bending and torsional forces, or moments. While ball joints are disclosed in accordance with a preferred/exemplary embodiment of the present disclosure, other linking structures may be utilized without departing from the spirit or scope of the present disclosure.
• As there areball joints20,22 at each end of thestabilization device10, no bending moments can be transferred from the spine to thestabilization device10. Further, it is important to recognize the only forces that act on thestabilization device10 are those due to the forces of thesprings30,32 within it. These forces are solely dependent upon the tension and compression of thestabilizer10 as determined by the spinal motion. In summary, thestabilization device10 sees only the spring forces. Irrespective of the large loads on the spine, such as when a person carries or lifts a heavy load, the loads coming to thestabilization device10 are only the forces developed within thestabilization device10, which are the result of spinal motion and not the result of the spinal load. Thestabilization device10 is, therefore, uniquely able to assist the spine without enduring the high loads of the spine, allowing a wide range of design options.
• The loading of the pedicle screws16,18 in thepresent stabilization device10 is also quite different from that in prior art pedicle screw fixation devices. The only load the stabilizer pedicle screws16,18 see is the force from thestabilization device10. This translates into pure axial force at the ball joint-screw interface. This mechanism greatly reduces the bending moment placed onto the pedicle screws16,18 as compared to prior art pedicle screw fusion systems. Due to the ball joints20,22, the bending moment within the pedicle screws16,18 is essentially zero at the ball joints20,22 and it increases toward the tip of the pedicle screws16,18. The area of pedicle screw-bone interface which often is the failure site in a typical prior art pedicle screw fixation device, is the least stressed site, and is therefore not likely to fail. In sum, the pedicle screws16,18, when used in conjunction with the present invention, carry significantly less load and are placed under significantly less stress than typical pedicle screws.
• InFIG. 2, the Moment-Rotation curve for a healthy spine is shown in configurations withstabilization device10. This curve shows the low resistance to movement encountered in the neutral zone of a healthy spine. However, when the spine is injured, this curve changes and the spine becomes unstable, as evidenced by the expansion of the neutral zone (seeFIG. 1).
• In accordance with a preferred embodiment of the present invention, people suffering from spinal injuries are best treated through the application of increased mechanical assistance in the neutral zone. As the spine moves beyond the neutral zone, the necessary mechanical assistance decreases and becomes more moderate. In particular, and with reference toFIG. 3a,the support profile contemplated in accordance with the present invention is disclosed.
• Three different profiles are shown inFIG. 3a.The disclosed profiles are merely exemplary and demonstrate the possible support requirements within the neutral zone.Profile1 is exemplary of an individual requiring great assistance in the neutral zone and the central zone of the stabilizer is therefore increased providing a high level of resistance over a great displacement;Profile2 is exemplary of an individual where less assistance is required in the neutral zone and the central zone of the stabilizer is therefore more moderate providing increased resistance over a more limited range of displacement; andProfile3 is exemplary of situations where only slightly greater assistance is required in the neutral zone and the central zone of the stabilizer may therefore be decreased to provide increased resistance over even a smaller range of displacement.
• As those skilled in the art will certainly appreciate, the mechanical assistance required and the range of the neutral zone will vary from individual to individual. However, the basic tenet of the present invention remains; that is, greater mechanical assistance for those individuals suffering from spinal instability is required within the individual's neutral zone. This assistance is provided in the form of greater resistance to movement provided within the neutral zone of the individual and the central zone of thedynamic spine stabilizer10.
• The dynamicspine stabilization device10 provides mechanical assistance in accordance with the disclosed support profile. Further, thestabilization device10 may advantageously provide for adjustability via a concentric spring design.
• More specifically, the dynamicspine stabilization device10 provides assistance to the compromised spine in the form of increased resistance to movement (provided by springs in accordance with a preferred embodiment) as the spine moves from the neutral posture, in any physiological direction. As mentioned above, the Force-Displacement relationship provided by the dynamicspine stabilization device10 is non-linear, with greater incremental resistance around the neutral zone of the spine and central zone of thestabilization device10, and decreasing incremental resistance beyond the central zone of the dynamicspine stabilization device10 as the individual moves beyond the neutral zone (seeFIG. 3a).
• The relationship ofstabilization device10 to forces applied during tension and compression is further shown with reference toFIG. 3a.As discussed above, the behavior of thestabilization device10 is non-linear. The Load-Displacement curve has three zones: tension, central and compression. If K1 and K2 define the stiffness values in the tension and compression zones respectively, the present stabilizer is designed such that the high stiffness in the central zone is “K1+K2”. Depending upon the preload of thestabilization device10 as will be discussed below in greater detail, the width of the central zone and, therefore, the region of high stiffness can be adjusted.
• With reference toFIG. 4, a dynamicspine stabilization device10 in accordance with one aspect of the present disclosure is schematically depicted. The dynamicspine stabilization device10 includes a support assembly in the form of ahousing20 composed of afirst housing member22 and asecond housing member24. Thefirst housing member22 and thesecond housing member24 are telescopically connected via external threads formed upon theopen end26 of thefirst housing member22 and internal threads formed upon theopen end28 of thesecond housing member24. In this way, thehousing20 is completed by screwing thefirst housing member22 into thesecond housing member24. As such, and as will be discussed below in greater detail, the relative distance between thefirst housing member22 and thesecond housing member24 can be readily adjusted for the purpose of adjusting the compression of thefirst spring30 andsecond spring32 contained within thehousing20. Although springs are employed in accordance with a preferred embodiment of the present disclosure, other elastic members may be employed without departing from the spirit or scope of the present disclosure. Apiston assembly34 links thefirst spring30 and thesecond spring32 to first and second ball joints36,38. The first and second ball joints36,38 are in turn shaped and designed for selective attachment to pediclescrews16,18 extending from therespective vertebrae12,14.
• The first ball joint36 is secured to theclosed end38 of thefirst housing member20 via a threadedengagement member40 shaped and dimensioned for coupling, with threads formed within anaperture42 formed in theclosed end38 of thefirst housing member22. In this way, the first ball joint36 substantially closes off theclosed end38 of thefirst housing member22. The length of the dynamicspine stabilization device10 may be readily adjusted by rotating the first ball joint36 to adjust the extent of overlap between thefirst housing member22 and theengagement member40 of the first ball joint36. As those skilled in the art will certainly appreciate, a threaded engagement between thefirst housing member22 and theengagement member40 of the first ball joint36 is disclosed in accordance with a preferred embodiment, although other coupling structures may be employed without departing from the spirit or scope of the present disclosure.
• Theclosed end44 of thesecond housing member24 is provided with acap46 having anaperture48 formed therein. As will be discussed below in greater detail, theaperture48 is shaped and dimensioned for the passage of apiston rod50 from thepiston assembly34 therethrough.
• Thepiston assembly34 includes apiston rod50; first andsecond springs30,32; and retainingrods52. Thepiston rod50 includes astop nut54 and anenlarged head56 at itsfirst end58. Theenlarged head56 is rigidly connected to thepiston rod50 and includes guide holes60 through which the retainingrods52 extend during operation of dynamicspine stabilization device10. As such, theenlarged head56 is guided along the retainingrods52 while the second ball joint38 is moved toward and away from the first ball joint36. As will be discussed below in greater detail, theenlarged head56 interacts with thefirst spring30 to create resistance as the dynamicspine stabilization device10 is extended and the spine is moved in flexion.
• Astop nut54 is fit over thepiston rod50 for free movement relative thereto. However, movement of thestop nut54 toward the first ball joint36 is prevented by the retainingrods52 that support thestop nut54 and prevent thestop nut54 from moving toward the first ball joint36. As will be discussed below in greater detail, thestop nut54 interacts with thesecond spring32 to create resistance as thedynamic spine stabilizer10 is compressed and the spine is moved in extension.
• Thesecond end62 of thepiston rod50 extends from theaperture48 at theclosed end44 of thesecond housing member24, and is attached to anengagement member64 of the second ball joint38. Thesecond end62 of thepiston rod50 is coupled to theengagement member64 of the second ball joint38 via a threaded engagement. As those skilled in the art will certainly appreciate, a threaded engagement between thesecond end62 of thepiston rod50 and theengagement member64 of the second ball joint38 is disclosed in accordance with a preferred embodiment, although other coupling structures may be employed without departing from the spirit of the present invention.
• As briefly mentioned above, the first andsecond springs30,32 are held within thehousing20. In particular, thefirst spring30 extends between theenlarged head56 of thepiston rod50 and thecap46 of thesecond housing member24. Thesecond spring32 extends between the distal end of theengagement member64 of the second ball joint38 and thestop nut54 of thepiston rod50. The preloaded force applied by the first andsecond springs30,32 holds the piston rod in a static position within thehousing20, such that the piston rod is able to move during either extension or flexion of the spine.
• In use, when thevertebrae12,14 are moved in flexion and the first ball joint36 is drawn away from the second ball joint38, thepiston rod50 is pulled within thehousing24 against the force being applied by thefirst spring30. In particular, theenlarged head56 of thepiston rod50 is moved toward theclosed end44 of thesecond housing member24. This movement causes compression of thefirst spring30, creating resistance to the movement of the spine. With regard to thesecond spring32, thesecond spring32 moves with thepiston rod50 away from second ball joint38. As the vertebrae move in flexion within the neutral zone, the height of thesecond spring32 is increased, reducing the distractive force, and in effect increasing the resistance of the device to movement. Through this mechanism, as the spine moves in flexion from the initial position bothspring30 andspring32 resist the distraction of the device directly, either by increasing the load within the spring (i.e. first spring30) or by decreasing the load assisting the motion (i.e. second spring32).
• However, when the spine is in extension, and the second ball joint38 is moved toward the first ball joint36, theengagement member64 of the second ball joint38 moves toward thestop nut54, which is held is place by the retainingrods52 as thepiston rod50 moves toward the first ball joint36. This movement causes compression of thesecond spring32 held between theengagement member64 of the second ball joint38 and thestop nut54, to create resistance to the movement of the dynamicspine stabilization device10. With regard to thefirst spring30, thefirst spring30 is supported between thecap46 and theenlarged head56, and as the vertebrae move in extension within the neutral zone, the height of thesecond spring30 is increased, reducing the compressive force, and in effect increasing the resistance of the device to movement. Through this mechanism, as the spine moves in extension from the initial position bothspring32 andspring30 resist the compression of the device directly, either by increasing the load within the spring (i.e. second spring32) or by decreasing the load assisting the motion (i.e. first spring30).
• Based upon the use of two concentrically positionedelastic springs30,32 as disclosed in accordance with the present disclosure, an assistance (force) profile as shown inFIG. 2 is provided by the presentdynamic spine stabilizer10. That is, the first andsecond springs30,32 work in conjunction to provide a large elastic force when the dynamicspine stabilization device10 is displaced within the central zone. However, once displacement between the first ball joint36 and the second ball joint38 extends beyond the central zone of thestabilization device10 and the neutral zone of the individual's spinal movement, the incremental resistance to motion is substantially reduced as the individual no longer requires the substantial assistance needed within the neutral zone. This is accomplished by setting the central zone of the device disclosed herein. The central zone of the force displacement curve is the area of the curve which represents when both springs are acting in the device as described above. When the motion of the spine is outside the neutral zone and the correlating device elongation or compression is outside the set central zone, the spring which is elongating reaches its free length. Free length, as anybody skilled in the art will appreciate, is the length of a spring when no force is applied. In this mechanism the resistance to movement of the device outside the central zone (where both springs are acting to resist motion) is only reliant on the resistance of one spring: eitherspring30 in flexion orspring32 in extension.
• As briefly discussed above, dynamicspine stabilization device10 may be adjusted by rotation of thefirst housing member22 relative to thesecond housing member24. This movement changes the distance between thefirst housing member22 and thesecond housing member24 in a manner which ultimately changes the preload placed across the first andsecond springs30,32. This change in preload alters the resistance profile of the present dynamicspine stabilization device10 from that shown inProfile2 ofFIG. 3ato an increase in preload (seeProfile1 ofFIG. 3a) which enlarges the effective range in which the first andsecond springs30,32 act in unison. This increased width of the central zone of thestabilization device10 correlates to higher stiffness over a larger range of motion of the spine. This effect can be reversed as evident inProfile3 ofFIG. 3a.
• The dynamicspine stabilization device10 is attached to pedicle screws16,18 extending from the vertebral section requiring support. During surgical attachment of the dynamicspine stabilization device10, the magnitude of the stabilizer's central zone can be adjusted for each individual patient, as judged by the surgeon and/or quantified by an instability measurement device. This optional adjustable feature of dynamicspine stabilization device10 is exemplified in the three explanatory profiles that have been generated in accordance with the present disclosure (seeFIG. 2; note the width of the device central zones).
• Pre-operatively, the first and secondelastic springs30,32 of the dynamicspine stabilization device10 can be replaced by a different set to accommodate a wider range of spinal instabilities. As expressed inFIG. 3b,Profile2bdemonstrates the force displacement curve generated with a stiffer set of springs when compared with the curve shown inProfile2aofFIG. 3b.
• Intra-operatively, the length of the dynamicspine stabilization device10 is adjustable by turning theengagement member40 of the first ball joint36 to lengthen thestabilization device10 in order to accommodate different patient anatomies and desired spinal posture. Pre-operatively, thepiston rod50 may be replaced to accommodate an even wider range of anatomic variation.
• The dynamicspine stabilization device10 has been tested alone for its load-displacement relationship. When applying tension, the dynamicspine stabilization device10 demonstrated increasing resistance up to a pre-defined displacement, followed by a reduced rate of increasing resistance until the device reached its fully elongated position. When subjected to compression, the dynamicspine stabilization device10 demonstrated increasing resistance up to a pre-defined displacement, followed by a reduced rate of increasing resistance until the device reached its fully compressed position. Therefore, the dynamicspine stabilization device10 exhibits a load-displacement curve that is non-linear with the greatest resistance to displacement offered around the neutral posture. This behavior helps to normalize the load-displacement curve of a compromised spine.
• In another embodiment of an aspect of the disclosed design and with reference toFIG. 5, the stabilization device110 may be constructed with an in-line spring arrangement. In accordance with this embodiment, the housing120 is composed of first andsecond housing members122,124 which are coupled with threads allowing for adjustability. A first ball joint136 extends from thefirst housing member122. Thesecond housing member124 is provided with anaperture148 through which thesecond end162 ofpiston rod150 extends. Thesecond end162 of thepiston rod150 is attached to the second ball joint138. The second ball joint138 is screwed onto thepiston rod150.
• Thepiston rod150 includes anenlarged head156 at itsfirst end158. The first andsecond springs130,132 are respectively secured between theenlarged head156 and the closed ends138,144 of the first andsecond housing members122,124. In this way, the stabilization device110 provides resistance to both expansion and compression using the same mechanical principles described for the previous embodiment.
• Adjustment of the resistance profile in accordance with this alternate embodiment is achieved by rotating thefirst housing member122 relative to thesecond housing member124. Rotation in this way alters the central zone of high resistance provided by the stabilization device110. As previously described one or both springs may also be exchanged to change the slope of the force-displacement curve in two or three zones respectively.
• To explain how thestabilization device10,110 assists a compromised spine (increased neutral zone), reference is made to the moment-rotation curves (FIG. 6). Four curves are shown: 1. Intact, 2. Injured, 3. Stabilizer and, 4. Injured+Stabilizer. These are, respectively, the Moment-Rotation curves of the intact spine, injured spine, stabilizer alone, and stabilizer plus injured spine Notice that this curve is close to the intact curve. Thus, the stabilization device, which provides greater resistance to movement around the neutral posture, is ideally suited to compensate for the instability of the spine.
• With reference to FIGS.8 to13, astabilization device210 according to the present disclosure is schematically depicted. This embodiment positions the first andsecond springs230,232 on opposite sides of apedicle screw218. As with the earlier embodiments, thestabilization device210 includes ahousing220 having afirst attachment member260 with a first ball joint262 extending from afirst end264 of thehousing220 and asecond attachment member266 with second ball joint268 extending through a central portion of thestabilizer220. Each of the ball joints262,268 is composed of asocket270a,270bwith aball272a,272bsecured therein.
• More particularly, each of the pedicle screws216,218 includes aproximal end274 and a distal end276 (as the first and second pedicle screws216,218 are identical, similar numerals will be used in describing them). Theproximal end274 includestraditional threading278 adapted for secure attachment along the spinal column of an individual. Thedistal end276 of thepedicle screw216,218 is provided with acollet278 adapted for engagement within a receivingaperture280a,280bformed within theball272a,272bof the first andsecond attachment members260,266 of thestabilization device210.
• Thecollet278 at thedistal end276 of thepedicle screw216,218 is formed with the ability to expand and contract under the control of the medical practitioner installing thepresent stabilizer210. Thecollet278 is composed of a plurality offlexible segments282 with acentral aperture284 therebetween. As will be explained below in greater detail, theflexible segments282 are adapted for movement between an expanded state used to lock thecollet278 within the receivingaperture280a,280bof theball272a,272band an unexpanded state wherein thecollet278 may be selectively inserted or removed from the receivingaperture280a,280bof theball272a,272b.
• The receivingapertures280a,280bof therespective balls272a,272bare shaped and dimensioned for receiving thecollet278 of thepedicle screw216,218 while it is in its unexpanded state. Retention of thecollet278 is further enhanced by the provision of alip286 at thedistal end276 of thecollet278. Thelip286 is shaped and dimensioned to grip the receivingaperture280a,280bfor retaining thecollet278 therein.
• Expansion of thecollet278 ofpedicle screw216,218 is achieved by the insertion of aset screw288 within thecentral aperture284 formed between thevarious segments282 of thepedicle screw collet278. As theset screw288 is positioned within thecentral aperture284, thesegments282 are forced outwardly. This increases the effective diameter of thecollet278 and ultimately brings the outer surface of thecollet278 into contact with the receivingaperture280a,280b,securely locking thecollet278, that is, thedistal end276 of thepedicle screw216,218 within the receivingaperture280a,280bof theball272a,272b.
• Access for the insertion of theset screw288 within thecentral aperture284 of thecollet278 is provided by extending the receivingaperture280a,280bthe entire way through theball272a,272b.In this way, thecollet278 is placed within the receivingaperture280a,280bof theball272a,272bwhile in its unexpanded state, theset screw288 is inserted within thecentral aperture284 between thevarious segments282 to cause thesegments282 to expand outwardly and lock thecollet278 within the receivingaperture280a,280b.In accordance with a preferred embodiment, theset screw288 is secured within thecentral aperture284 via mating threads formed along the inner surface along of the central aperture and the outer surface of theset screw288.
• Although the present ball joint/pedicle screw structure has been disclosed with reference to a particle stabilizer structure, those skilled in the art will appreciate that the ball joint/pedicle screw structure may be employed with various stabilizer structures without departing from the spirit of the present invention. In fact, it is contemplated the disclosed connection structure may be employed in a variety of environments without departing from the spirit of the present invention.
• With reference to thestabilization device210, analignment pin250 extends from thefirst attachment member260 through a bearingaperture290 within thesecond attachment member266. Thealignment pin250 includes anabutment member256 at itsfree end258. First andsecond springs230,232 are concentrically positioned about thealignment pin250. Thefirst spring230 is positioned to extend between thefirst attachment member260 and thesecond attachment member266, while thesecond spring232 is positioned to extend between thesecond attachment member266 and theabutment member256 at thefree end258 of thealignment pin250. The arrangement of thealignment pin250, first andsecond attachment members260,266 and first andsecond springs230,232 allows for resistive translation of thealignment pin250 relative to the vertebrae. In practice, thealignment pin250, springs230,232 andattachment members260,266 are arranged to create a compressive preload across the system.
• This design allows for an axial configuration which generates the desired Force-Displacement curves as shown with reference toFIG. 3, while allowing for a much shorter distance between the first and second attachment members. The stabilization device disclosed above may also be used in the stabilization of multiple level systems. It is contemplated that stabilization on multiple levels may be achieved through the implementation of multiple alignment pins coupled via multiple spring sets and pedicle screws.
• Thealignment pin250 also provides tensile force for achieving the preload utilized in conjunction with thesprings230,232. In accordance with an exemplary embodiment, thealignment pin250 is flexible and provides flexible guidance for thesprings230,232 without debris causing bearing surfaces, provides tensile for the preload, provides a low friction, straight bearing surface as it moves through thesecond attachment member266 and functions at times as a straight member and at other times as a flexible guide forsprings230,232.
• As mentioned above, thealignment pin250 is cable of functioning as both a straight guide member and as a flexible guide member. The determination as to whether thealignment pin250 functions as a straight guide member or a flexible guide member for thesprings230,232 is generally based upon location of thealignment pin250 relative to the remainingstabilization device210 components as the spine moves. This functionality is especially important during flexion. In accordance with an exemplary embodiment, thealignment pin250 has a uniform cross sectional shape capable of performing as both a straight guide member and a flexed guide member.
• In accordance with yet a further embodiment, and with reference toFIG. 13, thestabilization device210 may be used in conjunction with a torsion member ortorsion bar292 connecting thestabilization device210 to adjacent stabilizers as shown inFIG. 3c.In accordance with an exemplary embodiment, thetorsion bar292 is connected to theattachment members260,266 of adjacent stabilization devices with conventional connection structures. The use of thetorsion bar292 increases stability in axial rotation or lateral bending. Thetorsion bar292 generally has a uniform cross section for purposes where uniform torsion is required. However, and in accordance with exemplary embodiments of the present disclosure, it is contemplated that thetorsion bar292 may have an asymmetric cross section so as to provide for flexibility of stiffness in two planes. In such instances, the asymmetric crosssectional torsion bar292 will affect the system stiffness in lateral bending and axial rotation independently. Further, thetorsion bar292 may be utilized to tune the systems stabilization in all three planes.
• With more particular reference toFIG. 13,torsion member292 is typically mounted with respect to a mounting/attachment member260 associated with each of the pedicle screw pairs. Stabilization is achieved through the torsion member's interaction with the respective first and second attachment members. In this way, thetorsion member292 is able to further stabilize the adjacent spinal vertebrae with respect to spinal axial rotation and/or spinal lateral bending. Thetorsion member292 is configured and dimensioned to accommodate interaction with the respective first andsecond attachment members262. As shown inFIG. 13,exemplary torsion member292 includes: (i) afirst segment300 that is substantially co-planar with and perpendicular to the axis of thefirst stabilization member210; (ii) asecond segment302 that extends from thefirst segment300 and that is angularly and upwardly oriented relative to thefirst segment300; (iii) athird segment304 that extends from thesecond segment302, is substantially perpendicular to thefirst stabilization device210 and oriented in a plane that is elevated with respect to, but parallel to, the plane of thefirst stabilization device210, (iv) afourth segment306 that extends from thethird segment304 and that is a substantial mirror image of thesecond segment302; and (v) afifth segment306 that extends from thefourth segment304 and that is a substantial mirror image of thefirst segment300. Thus, exemplary embodiments of the disclosedtorsion member292 are substantially symmetrical relative to a mid-point of thethird segment304.
• The first through fifth segments of the disclosed torsion member typically define a stabilizing element and are typically integrally formed, e.g., through metal forming techniques employed with respect to bar stock. The bar stock is typically of circular, rectangular, elliptical and/or square cross-section and may be of constant or variable cross-sectional dimension. In addition, the disclosedtorsion member292 is advantageously adapted to be mounted with respect to first andsecond attachment members260 at the exposed ends of the first andfifth segments300,308, respectively, or, alternatively, may be integrally formed with respect to one or bothattachment members260, e.g., through conventional welding techniques.
• The disclosed torsion member offers advantageous functionality in spine applications through the disclosed multi-planar design, thereby accommodating limited amounts of axial and/or lateral bending of the spine, while imparting desired levels of spinal stabilization. As noted herein, the disclosed torsion members are susceptible to a variety of variations without departing from the spirit or scope of the present disclosure. Thus, for example, asymmetric geometries and/or the inclusion of additional jogs in the torsion member may be advantageously employed to affect the system stiffness and/or result in the torsion member manifesting differing degrees of internal bending stiffness with respect to bending in different spatial planes.
• In addition to the dynamic spine stabilization device described above, other complementary devices are contemplated. For example, a link-device may be provided for joining the left- and right-stabilizer units to help provide additional stability in axial rotation and lateral bending. This link-device would be a supplement to the dynamic spine stabilization device and would be applied as needed on an individual patient basis. In addition, a spinal stability measurement device may be utilized. The measurement device would be used to quantify the stability of each spinal level at the time of surgery. This device would attach intra-operatively to a pair of adjacent spinal components at compromised and uncompromised spinal levels to measure the stability of each level. The stability measurements of the adjacent uninjured levels relative to the injured level(s) can be used to determine the appropriate adjustment of the device. Additionally, the stability measurements of the injured spinal level(s) can be used to adjust the device by referring to a tabulated database of normal uninjured spinal stabilities. The device will be simple and robust, so that the surgeon is provided with the information in the simplest possible manner under operative conditions.
• The choice of spring(s) to be used in accordance with the present disclosure to achieve the desired force profile curve is governed by the basic physical laws governing the force produced by springs. In particular, the force profile described above and shown inFIG. 3ais achieved through the unique design of the present stabilizer.
• First, the stabilization device functions both in compression and tension, even through the two springs within the stabilizer are both of compression type. Second, the higher stiffness (K₁+K₂) provided by the stabilization device in the central zone is due to the presence of a preload. Both springs are made to work together, when the preload is present. As the stabilization device is either tensioned or compressed, the force increases in one spring and decreases in the other. When the decreasing force reaches the zero value, the spring corresponding to this force no longer functions, thus decreasing the stabilization device function, an engineering analysis, including the diagrams shown inFIGS. 7aand7b,is presented below (the analysis specifically relates to the embodiment disclosed inFIG. 5, although those skilled in the art will appreciate the way in which it applies to all embodiments disclosed in accordance with the present invention).
• • F₀is the preload within the stabilization device, introduced by shortening the body length of the housing as discussed above.
  • K₁and K₂are stiffness coefficients of the compression springs, active during stabilization device tensioning and compression, respectively.
  • F and D are respectively the force and displacement of the disc of the stabilization device with respect to the body of the stabilizer.
  • The sum of forces on the disc must equal zero. Therefore,
    F+(F₀−D×K₂)−(F₀+D×K₁)=0,
    and
    F=D×(K₁+K₂).
  • With regard to the central zone (CZ) width (seeFIG. 3a):
    • On Tension side CZ_Tis:
      CZ_T=F₀/K₂.
    • On Compression side CZc is:
      CZ_c=F₀/K₁.
• As those skilled in the art will certainly appreciate, the concepts underlying the present disclosure may be applied to other medical procedures. As such, these concepts may be utilized beyond spinal treatments without departing from the spirit or scope of the present invention.
• While exemplary embodiments have been shown and described, it will be understood that there is no intent to limit the invention by such disclosure, but rather, is intended to cover all modifications and alternate constructions falling within the spirit and scope of the invention.

Claims (8)

1. A torsion member for use as part of a stabilization system that includes at least one stabilization member, comprising:

(i) a first segment that is substantially co-planar with and perpendicular to the axis of the at least one stabilization member;

(ii) a second segment that extends from the first segment and that is angularly and upwardly oriented relative to the first segment;

(iii) a third segment that extends from the second segment, is substantially perpendicular to the at least one stabilization device, and is oriented in a plane that is elevated with respect to, but substantially parallel to, the plane of the at least one stabilization device,

(iv) a fourth segment that extends from the third segment and that is a substantial mirror image of the second segment; and

(v) a fifth segment that extends from the fourth segment and that is a substantial mirror image of the first segment.

2. A torsion member according toclaim 1, wherein said first, second, third, fourth and fifth segments collectively define a stabilizing element, and wherein said stabilizing element has a substantially constant cross-section.

3. A torsion member according toclaim 1, wherein said first, second, third, fourth and fifth segments collectively define a stabilizing element, and wherein said stabilizing element has a variable cross-section.

4. A torsion member according toclaim 1, wherein said first, second, third, fourth and fifth segments collectively define a stabilizing element, and wherein the cross-section of said stabilizing element is selected from the group consisting of circular, rectangular, elliptical and square.

5. A torsion member according toclaim 1, wherein said first and fifth segments are adapted to be mounted with respect to an attachment member.

6. A torsion member according toclaim 1, wherein at least one of said first and fifth segments is integrally formed with respect to an attachment member.

7. A method for providing spinal stabilization, comprising:

(a) mounting stabilization devices with respect to first and second pedicle screw pairs, the stabilization devices being positioned on opposite lateral sides of a pair of spinal vertebrae, and

(b) securing a torsion member between the first and second pairs of pedicle screws, the torsion member including:

(i) a first segment that is substantially co-planar with and perpendicular to the axis of at least one of the stabilization devices;

(ii) a second segment that extends from the first segment and that is angularly and upwardly oriented relative to the first segment;

(iii) a third segment that extends from the second segment, is substantially perpendicular to at least one of the stabilization devices, and is oriented in a plane that is elevated with respect to, but substantially parallel to, the plane of the at least one of the stabilization devices,

(iv) a fourth segment that extends from the third segment and that is a substantial mirror image of the second segment; and

(v) a fifth segment that extends from the fourth segment and that is a substantial mirror image of the first segment.

8. A method according toclaim 7, wherein the torsion member is mounted with respect to first and second attachment members that are associated with the first and second pairs of pedicle screws, respectively.

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