Abstract:
Spine stabilization devices, systems and methods are provided in which a single resilient member or spring is disposed on an elongate element that spans two attachment members attached to different spinal vertebrae. The elongate element passes through at least one of the two attachment members, permitting relative motion therebetween, and terminates in a stop or abutment. A second resilient member is disposed on the elongate element on an opposite side of the sliding attachment member, e.g., in an overhanging orientation. The two resilient members are capable of applying mutually opposing urging forces, and a compressive preload can be applied to one or both of the resilient members.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority benefit to a co-pending non-provisional application entitled “Method for Stabilizing a Spine,” which was filed on Apr. 19, 2011 and assigned Ser. No. 12/089,878, and which claims priority benefit to a non-provisional patent application entitled “Dynamic Stabilization Device Including Overhanging Stabilizing Member,” which was filed on Jun. 23, 2005 and assigned Ser. No. 11/159,471 (now U.S. Pat. No. 7,931,675). The foregoing non-provisional patent applications claim priority benefit to a provisional patent application entitled “Dynamic Spine Stabilizer,” filed on Jun. 23, 2004 and assigned Ser. No. 60/581,716. The entire content of the foregoing provisional patent application is incorporated by reference herein. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    This invention was made with government support under Grant No. AR045452 awarded by the National Institute of Health. The government has certain rights in the invention. 
     
    
     BACKGROUND OF THE DISCLOSURE 
       [0003]    1. Technical Field 
         [0004]    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. 
         [0005]    2. Background Art 
         [0006]    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. 
         [0007]    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. 
         [0008]    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. 
         [0009]    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 Disord  5 (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 Disord  5 (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. 
         [0010]    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 (see  FIG. 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.”  Spine  7 (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 in  FIG. 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 in  FIG. 1 , the neutral zone is the low stiffness region of the range of motion. 
         [0011]    Experiments have shown that after an injury of the spinal column or due to degeneration, neutral zones, as well as ranges of motion, increase (see  FIG. 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. 
         [0012]    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 
       [0013]    The present disclosure provides advantageous apparatus and methods for stabilizing adjacent spinal vertebrae in spinal axial rotation and spinal lateral bending. The disclosed stabilization devices and systems are adapted to be disposed between laterally-spaced pedicle screws attached to the same spinal vertebra. The disclosed spinal stabilization devices/systems are advantageously adapted to include at least first and second stabilizing elements which function, according to exemplary embodiments, in concert for stabilization in spinal flexion and spinal extension. Thus, according to exemplary embodiments of the present disclosure, the spinal stabilization devices/systems provide stabilizing functionality with laterally-spaced pedicle screws, but in a manner that is not confined within the region defined between such laterally-spaced pedicle screws. Such spinal stabilization designs offer several clinical advantages, including a reduced spatial requirement between the laterally-spaced pedicle screws since a portion of the stabilization functionality is achieved through structures positioned beyond such laterally-spaced region, e.g., in an overhanging orientation relative to one of the laterally-spaced pedicle screws. 
         [0014]    According to an exemplary embodiment of the present disclosure, a dynamic spine stabilization device/system is provided that is adapted to span adjacent spinal vertebrae. Attachment members are provided to mount/position the dynamic spine stabilization device/system with respect to laterally-spaced pedicle screws that are mounted into the adjacent spinal vertebrae. The attachment members of the spine stabilization device/system are generally coupled/mounted with respect to the laterally-spaced pedicle screws and are adapted to couple to the disclosed spinal stabilization device/system. Of note, the spinal stabilization device/system includes a dynamic element that is positioned between the first and second pedicle screws, and at least one additional dynamic element that is positioned beyond or external to the region defined by the laterally-spaced pedicle screws. 
         [0015]    According to an exemplary embodiment of the present disclosure, first and second dynamic elements are associated with the disclosed dynamic stabilization device/system. A first dynamic element is positioned on a first side of an attachment member and a second dynamic element is positioned on the opposite side of such attachment member. Relative motion between the pedicle screws, which is based upon and responsive to spinal motion (i.e., in flexion or extension), is stabilized through the combined contributions of the first and second dynamic elements. Each dynamic element includes one or more components that contribute to the dynamic response thereof, e.g., one or more springs. According to exemplary embodiments, a pair of springs are associated with each of the dynamic elements, e.g., in a nested configuration. In a further exemplary embodiment, each of the dynamic elements includes a single spring, and the single springs are adapted to operate in concert to provide an advantageous stabilizing response to spinal motion. 
         [0016]    According to other exemplary embodiments of the present disclosure, a dynamic stabilization device/system is provided in which an elongate rod/pin extends from both sides of an attachment member. The dynamic stabilization device/system may be equipped with one or more stops associated with the elongate rod/pin. First and second resilient members may be disposed on the pin (e.g., springs), the first resilient member being located in the region defined between the first attachment member and the second attachment member, and the second resilient member being located between either the first or the second attachment member and the stop. According to further exemplary embodiments of the present disclosure, a compressive preload may be established with respect to a first resilient member, a second resilient member, or both, to provide a desired stabilizing force profile, as described in greater detail herein. 
         [0017]    Exemplary methods of use of the disclosed dynamic stabilization devices and systems are also provided in accordance with the present disclosure. The disclosed dynamic stabilization devices, systems and methods of use 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 
         [0018]    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: 
           [0019]      FIG. 1  is Moment-Rotation curve for a spinal segment (intact and injured), showing low spinal stiffness within the neutral zone. 
           [0020]      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. 
           [0021]      FIG. 3   a  is 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. 
           [0022]      FIG. 3   b  is a Force-Displacement curve demonstrating the change in profile achieved through spring replacement. 
           [0023]      FIG. 3   c  is a dorsal view of the spine with a pair of dynamic stabilization devices secured thereto. 
           [0024]      FIG. 3   d  is a side view showing the exemplary dynamic stabilization device in tension. 
           [0025]      FIG. 3   e  is a side view showing the exemplary dynamic stabilization device in compression. 
           [0026]      FIG. 4  is a schematic of a dynamic spine stabilization device that is adapted to position dynamic elements between laterally-spaced pedicle screws. 
           [0027]      FIG. 5  is a schematic of an alternate dynamic spine stabilization device that is adapted to position dynamic elements between laterally-spaced pedicle screws. 
           [0028]      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. 
           [0029]      FIG. 7   a  is a free body diagram of a dynamic stabilization device in which dynamic elements are positioned between laterally-spaced pedicle screws. 
           [0030]      FIG. 7   b  is a diagram representing the central zone of a spine and the forces associated therewith for dynamic stabilization according to the present disclosure. 
           [0031]      FIG. 8  is a perspective view of an exemplary dynamic stabilization device in accordance with the present disclosure. 
           [0032]      FIG. 9  is an exploded view of the dynamic stabilization device shown in  FIG. 8 . 
           [0033]      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. 
           [0034]      FIG. 11  is a detailed perspective view of a first pedicle screw secured to an exemplary attachment member according to the present disclosure. 
           [0035]      FIG. 12  is a perspective view of the exemplary dynamic stabilization device shown in  FIG. 8  as seen from the opposite side. 
           [0036]      FIG. 13  is a perspective view of a dynamic stabilization device of the type depicted in  FIG. 8  with a transverse torsion bar stabilizing member. 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0037]    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. 
         [0038]    With reference to  FIGS. 2 ,  3   a - e  and  4 , 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 dynamic spine stabilization device  10  between adjacent vertebrae  12 ,  14  and providing mechanical assistance in the form of elastic resistance to the region of the spine to which the dynamic spine stabilization device  10  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. 
         [0039]    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 (see  FIG. 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 Disorders  1992; 5(4): 390-397. 
         [0040]    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. 
         [0041]    In general, pedicle screws  16 ,  18  attach the dynamic spine stabilization device  10  to the vertebrae  12 ,  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 (see  FIG. 3   c ). The dynamic spine stabilization device  10  assists the compromised (injured and/or degenerated) spine of a back pain patient, and helps her/him perform daily activities. The dynamic spine stabilization device  10  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) the stabilization device  10  is tensioned (see  FIG. 3   d ) and when the spine bends backward (extension) the stabilization device  10  is compressed (see  FIG. 3   e ). 
         [0042]    The resistance to displacement provided by the dynamic spine stabilization device  10  is non-linear, being greatest in its central zone so as to correspond to the individual&#39;s neutral zone; that is, the central zone of the stabilization device  10  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. 
         [0043]    The central zone of the dynamic spine stabilization device  10 , that is, the range of motion in which the spine stabilization device  10  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 dynamic spine stabilization device  10  is adjustable pre-operatively and intra-operatively. This helps to tailor the mechanical properties of the dynamic spine stabilization device  10  to suit the compromised spine of the individual patient. The length of the dynamic spine stabilization device  10  may also be adjustable intra-operatively to suit individual patient anatomy and to achieve desired spinal posture. The dynamic spine stabilization device  10  can be re-adjusted post-operatively with a surgical procedure to adjust its central zone to accommodate a patient&#39;s altered needs. 
         [0044]    According to exemplary embodiments of the present disclosure, ball joints  20 ,  22  link the dynamic spine stabilization device  10  with the pedicle screws  16 ,  18 . The junction of the dynamic spine stabilization device  10  and pedicle screws  16 ,  18  is free and rotationally unconstrained. Therefore, first of all, the spine is allowed all physiological motions of bending and twisting and second, the dynamic spine stabilization device  10  and the pedicle screws  16 ,  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. 
         [0045]    As there are ball joints  20 ,  22  at each end of the stabilization device  10 , no bending moments can be transferred from the spine to the stabilization device  10 . Further, it is important to recognize the only forces that act on the stabilization device  10  are those due to the forces of the springs  30 ,  32  within it. These forces are solely dependent upon the tension and compression of the stabilizer  10  as determined by the spinal motion. In summary, the stabilization device  10  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 the stabilization device  10  are only the forces developed within the stabilization device  10 , which are the result of spinal motion and not the result of the spinal load. The stabilization device  10  is, therefore, uniquely able to assist the spine without enduring the high loads of the spine, allowing a wide range of design options. 
         [0046]    The loading of the pedicle screws  16 ,  18  in the present stabilization device  10  is also quite different from that in prior art pedicle screw fixation devices. The only load the stabilizer pedicle screws  16 ,  18  see is the force from the stabilization device  10 . This translates into pure axial force at the ball joint-screw interface. This mechanism greatly reduces the bending moment placed onto the pedicle screws  16 ,  18  as compared to prior art pedicle screw fusion systems. Due to the ball joints  20 ,  22 , the bending moment within the pedicle screws  16 ,  18  is essentially zero at the ball joints  20 ,  22  and it increases toward the tip of the pedicle screws  16 ,  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 screws  16 ,  18 , when used in conjunction with the present invention, carry significantly less load and are placed under significantly less stress than typical pedicle screws. 
         [0047]    In  FIG. 2 , the Moment-Rotation curve for a healthy spine is shown in configurations with stabilization device  10 . 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 (see  FIG. 1 ). 
         [0048]    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 to  FIG. 3   a , the support profile contemplated in accordance with the present invention is disclosed. 
         [0049]    Three different profiles are shown in  FIG. 3   a . The disclosed profiles are merely exemplary and demonstrate the possible support requirements within the neutral zone. Profile  1  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; Profile  2  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; and Profile  3  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. 
         [0050]    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&#39;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 the dynamic spine stabilizer  10 . 
         [0051]    The dynamic spine stabilization device  10  provides mechanical assistance in accordance with the disclosed support profile. Further, the stabilization device  10  may advantageously provide for adjustability via a concentric spring design. 
         [0052]    More specifically, the dynamic spine stabilization device  10  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 dynamic spine stabilization device  10  is non-linear, with greater incremental resistance around the neutral zone of the spine and central zone of the stabilization device  10 , and decreasing incremental resistance beyond the central zone of the dynamic spine stabilization device  10  as the individual moves beyond the neutral zone (see  FIG. 3   a ). 
         [0053]    The relationship of stabilization device  10  to forces applied during tension and compression is further shown with reference to  FIG. 3   a . As discussed above, the behavior of the stabilization device  10  is non-linear. The Load-Displacement curve has three zones: tension, central and compression. If K 1  and K 2  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 “K 1 +K 2 ”. Depending upon the preload of the stabilization device  10  as will be discussed below in greater detail, the width of the central zone and, therefore, the region of high stiffness can be adjusted. 
         [0054]    With reference to  FIG. 4 , a dynamic spine stabilization device  10  in accordance with one aspect of the present disclosure is schematically depicted. The dynamic spine stabilization device  10  includes a support assembly in the form of a housing  20  composed of a first housing member  22  and a second housing member  24 . The first housing member  22  and the second housing member  24  are telescopically connected via external threads formed upon the open end  26  of the first housing member  22  and internal threads formed upon the open end  28  of the second housing member  24 . In this way, the housing  20  is completed by screwing the first housing member  22  into the second housing member  24 . As such, and as will be discussed below in greater detail, the relative distance between the first housing member  22  and the second housing member  24  can be readily adjusted for the purpose of adjusting the compression of the first spring  30  and second spring  32  contained within the housing  20 . 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. A piston assembly  34  links the first spring  30  and the second spring  32  to first and second ball joints  36 ,  38 . The first and second ball joints  36 ,  38  are in turn shaped and designed for selective attachment to pedicle screws  16 ,  18  extending from the respective vertebrae  12 ,  14 . 
         [0055]    The first ball joint  36  is secured to the closed end  38  of the first housing member  20  via a threaded engagement member  40  shaped and dimensioned for coupling, with threads formed within an aperture  42  formed in the closed end  38  of the first housing member  22 . In this way, the first ball joint  36  substantially closes off the closed end  38  of the first housing member  22 . The length of the dynamic spine stabilization device  10  may be readily adjusted by rotating the first ball joint  36  to adjust the extent of overlap between the first housing member  22  and the engagement member  40  of the first ball joint  36 . As those skilled in the art will certainly appreciate, a threaded engagement between the first housing member  22  and the engagement member  40  of the first ball joint  36  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. 
         [0056]    The closed end  44  of the second housing member  24  is provided with a cap  46  having an aperture  48  formed therein. As will be discussed below in greater detail, the aperture  48  is shaped and dimensioned for the passage of a piston rod  50  from the piston assembly  34  therethrough. 
         [0057]    The piston assembly  34  includes a piston rod  50 ; first and second springs  30 ,  32 ; and retaining rods  52 . The piston rod  50  includes a stop nut  54  and an enlarged head  56  at its first end  58 . The enlarged head  56  is rigidly connected to the piston rod  50  and includes guide holes  60  through which the retaining rods  52  extend during operation of dynamic spine stabilization device  10 . As such, the enlarged head  56  is guided along the retaining rods  52  while the second ball joint  38  is moved toward and away from the first ball joint  36 . As will be discussed below in greater detail, the enlarged head  56  interacts with the first spring  30  to create resistance as the dynamic spine stabilization device  10  is extended and the spine is moved in flexion. 
         [0058]    A stop nut  54  is fit over the piston rod  50  for free movement relative thereto. However, movement of the stop nut  54  toward the first ball joint  36  is prevented by the retaining rods  52  that support the stop nut  54  and prevent the stop nut  54  from moving toward the first ball joint  36 . As will be discussed below in greater detail, the stop nut  54  interacts with the second spring  32  to create resistance as the dynamic spine stabilizer  10  is compressed and the spine is moved in extension. 
         [0059]    The second end  62  of the piston rod  50  extends from the aperture  48  at the closed end  44  of the second housing member  24 , and is attached to an engagement member  64  of the second ball joint  38 . The second end  62  of the piston rod  50  is coupled to the engagement member  64  of the second ball joint  38  via a threaded engagement. As those skilled in the art will certainly appreciate, a threaded engagement between the second end  62  of the piston rod  50  and the engagement member  64  of the second ball joint  38  is disclosed in accordance with a preferred embodiment, although other coupling structures may be employed without departing from the spirit of the present invention. 
         [0060]    As briefly mentioned above, the first and second springs  30 ,  32  are held within the housing  20 . In particular, the first spring  30  extends between the enlarged head  56  of the piston rod  50  and the cap  46  of the second housing member  24 . The second spring  32  extends between the distal end of the engagement member  64  of the second ball joint  38  and the stop nut  54  of the piston rod  50 . The preloaded force applied by the first and second springs  30 ,  32  holds the piston rod in a static position within the housing  20 , such that the piston rod is able to move during either extension or flexion of the spine. 
         [0061]    In use, when the vertebrae  12 ,  14  are moved in flexion and the first ball joint  36  is drawn away from the second ball joint  38 , the piston rod  50  is pulled within the housing  24  against the force being applied by the first spring  30 . In particular, the enlarged head  56  of the piston rod  50  is moved toward the closed end  44  of the second housing member  24 . This movement causes compression of the first spring  30 , creating resistance to the movement of the spine. With regard to the second spring  32 , the second spring  32  moves with the piston rod  50  away from second ball joint  38 . As the vertebrae move in flexion within the neutral zone, the height of the second spring  32  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 both spring  30  and spring  32  resist the distraction of the device directly, either by increasing the load within the spring (i.e. first spring  30 ) or by decreasing the load assisting the motion (i.e. second spring  32 ). 
         [0062]    However, when the spine is in extension, and the second ball joint  38  is moved toward the first ball joint  36 , the engagement member  64  of the second ball joint  38  moves toward the stop nut  54 , which is held is place by the retaining rods  52  as the piston rod  50  moves toward the first ball joint  36 . This movement causes compression of the second spring  32  held between the engagement member  64  of the second ball joint  38  and the stop nut  54 , to create resistance to the movement of the dynamic spine stabilization device  10 . With regard to the first spring  30 , the first spring  30  is supported between the cap  46  and the enlarged head  56 , and as the vertebrae move in extension within the neutral zone, the height of the second spring  30  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 both spring  32  and spring  30  resist the compression of the device directly, either by increasing the load within the spring (i.e. second spring  32 ) or by decreasing the load assisting the motion (i.e. first spring  30 ). 
         [0063]    Based upon the use of two concentrically positioned elastic springs  30 ,  32  as disclosed in accordance with the present disclosure, an assistance (force) profile as shown in  FIG. 2  is provided by the present dynamic spine stabilizer  10 . That is, the first and second springs  30 ,  32  work in conjunction to provide a large elastic force when the dynamic spine stabilization device  10  is displaced within the central zone. However, once displacement between the first ball joint  36  and the second ball joint  38  extends beyond the central zone of the stabilization device  10  and the neutral zone of the individual&#39;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: either spring  30  in flexion or spring  32  in extension. 
         [0064]    As briefly discussed above, dynamic spine stabilization device  10  may be adjusted by rotation of the first housing member  22  relative to the second housing member  24 . This movement changes the distance between the first housing member  22  and the second housing member  24  in a manner which ultimately changes the preload placed across the first and second springs  30 ,  32 . This change in preload alters the resistance profile of the present dynamic spine stabilization device  10  from that shown in Profile  2  of  FIG. 3   a  to an increase in preload (see Profile  1  of  FIG. 3   a ) which enlarges the effective range in which the first and second springs  30 ,  32  act in unison. This increased width of the central zone of the stabilization device  10  correlates to higher stiffness over a larger range of motion of the spine. This effect can be reversed as evident in Profile  3  of  FIG. 3   a.    
         [0065]    The dynamic spine stabilization device  10  is attached to pedicle screws  16 ,  18  extending from the vertebral section requiring support. During surgical attachment of the dynamic spine stabilization device  10 , the magnitude of the stabilizer&#39;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 dynamic spine stabilization device  10  is exemplified in the three explanatory profiles that have been generated in accordance with the present disclosure (see  FIG. 2 ; note the width of the device central zones). 
         [0066]    Pre-operatively, the first and second elastic springs  30 ,  32  of the dynamic spine stabilization device  10  can be replaced by a different set to accommodate a wider range of spinal instabilities. As expressed in  FIG. 3   b , Profile  2   b  demonstrates the force displacement curve generated with a stiffer set of springs when compared with the curve shown in Profile  2   a  of  FIG. 3   b.    
         [0067]    Intra-operatively, the length of the dynamic spine stabilization device  10  is adjustable by turning the engagement member  40  of the first ball joint  36  to lengthen the stabilization device  10  in order to accommodate different patient anatomies and desired spinal posture. Pre-operatively; the piston rod  50  may be replaced to accommodate an even wider range of anatomic variation. 
         [0068]    The dynamic spine stabilization device  10  has been tested alone for its load-displacement relationship. When applying tension, the dynamic spine stabilization device  10  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 dynamic spine stabilization device  10  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 dynamic spine stabilization device  10  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. 
         [0069]    In another embodiment of an aspect of the disclosed design and with reference to  FIG. 5 , the stabilization device  110  may be constructed with an in-line spring arrangement. In accordance with this embodiment, the housing  120  is composed of first and second housing members  122 ,  124  which are coupled with threads allowing for adjustability. A first ball joint  136  extends from the first housing member  122 . The second housing member  124  is provided with an aperture  148  through which the second end  162  of piston rod  150  extends. The second end  162  of the piston rod  150  is attached to the second ball joint  138 . The second ball joint  138  is screwed onto the piston rod  150 . 
         [0070]    The piston rod  150  includes an enlarged head  156  at its first end  158 . The first and second springs  130 ,  132  are respectively secured between the enlarged head  156  and the closed ends  138 ,  144  of the first and second housing members  122 ,  124 . In this way, the stabilization device  110  provides resistance to both expansion and compression using the same mechanical principles described for the previous embodiment. 
         [0071]    Adjustment of the resistance profile in accordance with this alternate embodiment is achieved by rotating the first housing member  122  relative to the second housing member  124 . Rotation in this way alters the central zone of high resistance provided by the stabilization device  110 . 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. 
         [0072]    To explain how the stabilization device  10 ,  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. 
         [0073]    With reference to  FIGS. 8 to 13 , a stabilization device  210  according to the present disclosure is schematically depicted. This embodiment positions the first and second springs  230 ,  232  on opposite sides of a pedicle screw  218 . As with the earlier embodiments, the stabilization device  210  includes a housing  220  having a first attachment member  260  with a first ball joint  262  extending from a first end  264  of the housing  220  and a second attachment member  266  with second ball joint  268  extending through a central portion of the stabilizer  220 . Each of the ball joints  262 ,  268  is composed of a socket  270   a,    270   b  with a ball  272   a,    272   b  secured therein. 
         [0074]    More particularly, each of the pedicle screws  216 ,  218  includes a proximal end  274  and a distal end  276  (as the first and second pedicle screws  216 ,  218  are identical, similar numerals will be used in describing them). The proximal end  274  includes traditional threading  278  adapted for secure attachment along the spinal column of an individual. The distal end  276  of the pedicle screw  216 ,  218  is provided with a collet  278  adapted for engagement within a receiving aperture  280   a,    280   b  formed within the ball  272   a,    272   b  of the first and second attachment members  260 ,  266  of the stabilization device  210 . 
         [0075]    The collet  278  at the distal end  276  of the pedicle screw  216 ,  218  is formed with the ability to expand and contract under the control of the medical practitioner installing the present stabilizer  210 . The collet  278  is composed of a plurality of flexible segments  282  with a central aperture  284  therebetween. As will be explained below in greater detail, the flexible segments  282  are adapted for movement between an expanded state used to lock the collet  278  within the receiving aperture  280   a,    280   b  of the ball  272   a,    272   b  and an unexpanded state wherein the collet  278  may be selectively inserted or removed from the receiving aperture  280   a,    280   b  of the ball  272   a,    272   b.    
         [0076]    The receiving apertures  280   a,    280   b  of the respective balls  272   a,    272   b  are shaped and dimensioned for receiving the collet  278  of the pedicle screw  216 ,  218  while it is in its unexpanded state. Retention of the collet  278  is further enhanced by the provision of a lip  286  at the distal end  276  of the collet  278 . The lip  286  is shaped and dimensioned to grip the receiving aperture  280   a,    280   b  for retaining the collet  278  therein. 
         [0077]    Expansion of the collet  278  of pedicle screw  216 ,  218  is achieved by the insertion of a set screw  288  within the central aperture  284  formed between the various segments  282  of the pedicle screw collet  278 . As the set screw  288  is positioned within the central aperture  284 , the segments  282  are forced outwardly. This increases the effective diameter of the collet  278  and ultimately brings the outer surface of the collet  278  into contact with the receiving aperture  280   a,    280   b,  securely locking the collet  278 , that is, the distal end  276  of the pedicle screw  216 ,  218  within the receiving aperture  280   a,    280   b  of the ball  272   a,    272   b.    
         [0078]    Access for the insertion of the set screw  288  within the central aperture  284  of the collet  278  is provided by extending the receiving aperture  280   a,    280   b  the entire way through the ball  272   a,    272   b.  In this way, the collet  278  is placed within the receiving aperture  280   a,    280   b  of the ball  272   a,    272   b  while in its unexpanded state, the set screw  288  is inserted within the central aperture  284  between the various segments  282  to cause the segments  282  to expand outwardly and lock the collet  278  within the receiving aperture  280   a,    280   b.  In accordance with a preferred embodiment, the set screw  288  is secured within the central aperture  284  via mating threads formed along the inner surface along of the central aperture and the outer surface of the set screw  288 . 
         [0079]    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. 
         [0080]    With reference to the stabilization device  210 , an alignment pin  250  extends from the first attachment member  260  through a bearing aperture  290  within the second attachment member  266 . The alignment pin  250  includes an abutment member  256  at its free end  258 . First and second springs  230 ,  232  are concentrically positioned about the alignment pin  250 . The first spring  230  is positioned to extend between the first attachment member  260  and the second attachment member  266 , while the second spring  232  is positioned to extend between the second attachment member  266  and the abutment member  256  at the free end  258  of the alignment pin  250 . The arrangement of the alignment pin  250 , first and second attachment members  260 ,  266  and first and second springs  230 ,  232  allows for resistive translation of the alignment pin  250  relative to the vertebrae. In practice, the alignment pin  250 , springs  230 ,  232  and attachment members  260 ,  266  are arranged to create a compressive preload across the system. 
         [0081]    This design allows for an axial configuration which generates the desired Force-Displacement curves as shown with reference to  FIG. 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. 
         [0082]    The alignment pin  250  also provides tensile force for achieving the preload utilized in conjunction with the springs  230 ,  232 . In accordance with an exemplary embodiment, the alignment pin  250  is flexible and provides flexible guidance for the springs  230 ,  232  without debris causing bearing surfaces, provides tensile for the preload, provides a low friction, straight bearing surface as it moves through the second attachment member  266  and functions at times as a straight member and at other times as a flexible guide for springs  230 ,  232 . 
         [0083]    As mentioned above, the alignment pin  250  is cable of functioning as both a straight guide member and as a flexible guide member. The determination as to whether the alignment pin  250  functions as a straight guide member or a flexible guide member for the springs  230 ,  232  is generally based upon location of the alignment pin  250  relative to the remaining stabilization device  210  components as the spine moves. This functionality is especially important during flexion. In accordance with an exemplary embodiment, the alignment pin  250  has a uniform cross sectional shape capable of performing as both a straight guide member and a flexed guide member. 
         [0084]    In accordance with yet a further embodiment, and with reference to  FIG. 13 , the stabilization device  210  may be used in conjunction with a torsion bar  292  connecting the stabilization device  210  to adjacent stabilizers as shown in  FIG. 3   c . In accordance with an exemplary embodiment, the torsion bar  292  is connected to the attachment members  260 ,  266  of adjacent stabilization devices with conventional connection structures. The use of the torsion bar  292  increases stability in axial rotation or lateral bending. The torsion bar  292  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 the torsion bar  292  may have an asymmetric cross section so as to provide for flexibility of stiffness in two planes. In such instances, the asymmetric cross sectional torsion bar  292  will affect the system stiffness in lateral bending and axial rotation independently. Further, the torsion bar  292  may be utilized to tune the systems stabilization in all three planes. 
         [0085]    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. 
         [0086]    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 in  FIG. 3   a  is achieved through the unique design of the present stabilizer. 
         [0087]    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 (K 1 +K 2 ) 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 in  FIGS. 7   a  and  7   b , is presented below (the analysis specifically relates to the embodiment disclosed in  FIG. 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).
       F 0  is the preload within the stabilization device, introduced by shortening the body length of the housing as discussed above.   K 1  and K 2  are 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,       
 
         [0000]        F +( F   0   −D×K   2 )−( F   0   +D×K   1 )=0, and
 
         [0000]        F=D ×( K   1   +K   2 ).
       With regard to the central zone (CZ) width (see  FIG. 3   a ):
           On Tension side CZ T  is:   
               
 
         [0000]        CZ   T   =F   0   /K   2 .           On Compression side CZc is:             
         [0000]        CZ   c   =F   0   /K   1 . 
         [0095]    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. 
         [0096]    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.