Abstract:
Devices, systems and methods for dynamically stabilizing the spine are provided. The devices include an expandable spacer having an undeployed configuration and a deployed configuration. The spacer has axial and radial dimensions for positioning between the spinous processes of adjacent vertebrae. The systems include one or more spacers and a mechanical actuation device for delivering and deploying the spacer. The methods involve the implantation of one or more spacers within the interspinous space.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/305,820, filed on Dec. 15, 2005; which is a continuation-in-part of U.S. patent application Ser. No. 11/190,496, filed on Jul. 26, 2005; which is a continuation-in-part of U.S. patent application Ser. No. 11/079,006, filed on Mar. 10, 2005; which is a continuation-in-part of U.S. patent application Ser. No. 11/052,002 filed on Feb. 4, 2005; which is a continuation-in-part of U.S. patent application Ser. No. 11/006,502 filed on Dec. 6, 2004; which is a continuation-in-part of U.S. patent application Ser. No. 10/970,843 filed on Oct. 20, 2004; all of the above are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed towards the treatment of spinal disorders and pain. More particularly, the present invention is directed to systems and methods of treating the spine, which eliminate pain and enable spinal motion, which effectively mimics that of a normally functioning spine. 
     BACKGROUND OF THE INVENTION 
       FIG. 1  illustrates a portion of the human spine having a superior vertebra  2  and an inferior vertebra  4 , with an intervertebral disc  6  located in between the two vertebral bodies. The superior vertebra  2  has superior facet joints  8   a  and  8   b , inferior facet joints  13   a  and  13   b , and spinous process  25 . Pedicles  3   a  and  3   b  interconnect the respective superior facet joints  8   a ,  8   b  to the vertebral body  2 . Extending laterally from superior facet joints  8   a ,  8   b  are transverse processes  7   a  and  7   b , respectively. Extending between each inferior facet joints  13   a  and  13   b  and the spinous process  25  are laminal zones  5   a  and  5   b , respectively. Similarly, inferior vertebra  4  has superior facet joints  15   a  and  15   b , superior pedicles  9   a  and  9   b , transverse processes  17   a  and  17   b , inferior facet joints  19   a  and  19   b , laminal zones  21   a  and  21   b , and spinous process  27 . 
     The superior vertebra with its inferior facets, the inferior vertebra with its superior facet joints, the intervertebral disc, and seven spinal ligaments (not shown) extending between the superior and inferior vertebrae together comprise a spinal motion segment or functional spine unit. Each spinal motion segment enables motion along three orthogonal axes, both in rotation and in translation. The various spinal motions are illustrated in  FIGS. 2A-2C . In particular,  FIG. 2A  illustrates flexion and extension motions and axial loading,  FIG. 2B  illustrates lateral bending motion and  FIG. 2C  illustrated axial rotational motion. A normally functioning spinal motion segment provides physiological limits and stiffness in each rotational and translational direction to create a stable and strong column structure to support physiological loads. 
     Traumatic, inflammatory, metabolic, synovial, neoplastic and degenerative disorders of the spine can produce debilitating pain that can affect a spinal motion segment&#39;s ability to properly function. The specific location or source of spinal pain is most often an affected intervertebral disc or facet joint. Often, a disorder in one location or spinal component can lead to eventual deterioration or disorder, and ultimately, pain in the other. 
     Spine fusion (arthrodesis) is a procedure in which two or more adjacent vertebral bodies are fused together. It is one of the most common approaches to alleviating various types of spinal pain, particularly pain associated with one or more affected intervertebral discs. While spine fusion generally helps to eliminate certain types of pain, it has been shown to decrease function by limiting the range of motion for patients in flexion, extension, rotation and lateral bending. Furthermore, the fusion creates increased stresses on adjacent non-fused motion segments and accelerated degeneration of the motion segments. Additionally, pseudarthrosis (resulting from an incomplete or ineffective fusion) may not provide the expected pain-relief for the patient. Also, the device(s) used for fusion, whether artificial or biological, may migrate out of the fusion site creating significant new problems for the patient. 
     Various technologies and approaches have been developed to treat spinal pain without fusion in order to maintain or recreate the natural biomechanics of the spine. To this end, significant efforts are being made in the use of implantable artificial intervertebral discs. Artificial discs are intended to restore articulation between vertebral bodies so as to recreate the full range of motion normally allowed by the elastic properties of the natural disc. Unfortunately, the currently available artificial discs do not adequately address all of the mechanics of motion for the spinal column. 
     It has been found that the facet joints can also be a significant source of spinal disorders and debilitating pain. For example, a patient may suffer from arthritic facet joints, severe facet joint tropism, otherwise deformed facet joints, facet joint injuries, etc. These disorders lead to spinal stenosis, degenerative spondylolithesis, and/or isthmic spondylotlisthesis, pinching the nerves that extend between the affected vertebrae. 
     Current interventions for the treatment of facet joint disorders have not been found to provide completely successful results. Facetectomy (removal of the facet joints) may provide some pain relief; but as the facet joints help to support axial, torsional, and shear loads that act on the spinal column in addition to providing a sliding articulation and mechanism for load transmission, their removal inhibits natural spinal function. Laminectomy (removal of the lamina, including the spinal arch and the spinous process) may also provide pain relief associated with facet joint disorders; however, the spine is made less stable and subject to hypermobility. Problems with the facet joints can also complicate treatments associated with other portions of the spine. In fact, contraindications for disc replacement include arthritic facet joints, absent facet joints, severe facet joint tropism, or otherwise deformed facet joints due to the inability of the artificial disc (when used with compromised or missing facet joints) to properly restore the natural biomechanics of the spinal motion segment. 
     While various attempts have been made at facet joint replacement, they have been inadequate. This is due to the fact that prosthetic facet joints preserve existing bony structures and therefore do not address pathologies that affect facet joints themselves. Certain facet joint prostheses, such as those disclosed in U.S. Pat. No. 6,132,464, are intended to be supported on the lamina or the posterior arch. As the lamina is a very complex and highly variable anatomical structure, it is very difficult to design a prosthesis that provides reproducible positioning against the lamina to correctly locate the prosthetic facet joints. In addition, when facet joint replacement involves complete removal and replacement of the natural facet joint, as disclosed in U.S. Pat. No. 6,579,319, the prosthesis is unlikely to endure the loads and cycling experienced by the vertebra. Thus, the facet joint replacement may be subject to long-term displacement. Furthermore, when facet joint disorders are accompanied by disease or trauma to other structures of a vertebra (such as the lamina, spinous process, and/or transverse processes) facet joint replacement is insufficient to treat the problem(s). 
     Most recently, surgical-based technologies, referred to as “dynamic posterior stabilization,” have been developed to address spinal pain resulting from more than one disorder, when more than one structure of the spine have been compromised. An objective of such technologies is to provide the support of fusion-based implants while maximizing the natural biomechanics of the spine. Dynamic posterior stabilization systems typically fall into one of two general categories: posterior pedicle screw-based systems and interspinous spacers. 
     Examples of pedicle screw-based systems are disclosed in U.S. Pat. Nos. 5,015,247, 5,484,437, 5,489,308, 5,609,636 and 5,658,337, 5,741,253, 6,080,155, 6,096,038, 6,264,656 and 6,270,498. These types of systems involve the use of screws that are positioned in the vertebral body through the pedicle. Certain types of these pedicle screw-based systems may be used to augment compromised facet joints, while others require removal of the spinous process and/or the facet joints for implantation. One such system, the Zimmer Spine Dynesys® employs a cord which is extended between the pedicle screws and a fairly rigid spacer which is passed over the cord and positioned between the screws. While this system is able to provide load sharing and restoration of disc height, because it is so rigid, it does not effective in preserving the natural motion of the spinal segment into which it is implanted. Other pedicle screw-based systems employ articulating joints between the pedicle screws. Because these types of systems require the use of pedicle screws, implantation of the systems are often more invasive to implant than interspinous spacers. 
     Where the level of disability or pain to the affected spinal motion segments is not that severe or where the condition, such as an injury, is not chronic, the use of interspinous spacers are preferred over pedicle based systems as they require a less invasive implantation approach and less dissection of the surrounding tissue and ligaments. Examples of interspinous spacers are disclosed in U.S. Pat. Nos. Re. 36,211, 5,645,599, 6,149,642, 6,500,178, 6,695,842, 6,716,245 and 6,761,720. The spacers, which are made of either a hard or compliant material, are placed in between adjacent spinous processes. The harder material spacers are fixed in place by means of the opposing force caused by distracting the affected spinal segment and/or by use of keels or screws that anchor into the spinous process. While slightly less invasive than the procedures required for implanting a pedicle screw-based dynamic stabilization system, implantation of hard or solid interspinous spacers still requires dissection of muscle tissue and of the supraspinous and interspinous ligaments. Additionally, these tend to facilitate spinal motion that is less analogous to the natural spinal motion than do the more compliant and flexible interspinous spacers. Another advantage of the compliant/flexible interspinous spacers is the ability to deliver them somewhat less invasively than those that are not compliant or flexible; however, their compliancy makes them more susceptible to displacement or migration over time. To obviate this risk, many of these spacers employ straps or the like that are wrapped around the spinous processes of the vertebrae above and below the level where the spacer is implanted. Of course, this requires some additional tissue and ligament dissection superior and inferior to the implant site, i.e., at least within the adjacent interspinous spaces. 
     With the limitations of current spine stabilization technologies, there is clearly a need for an improved means and method for dynamic posterior stabilization of the spine that address the drawbacks of prior devices. In particular, it would be highly beneficial to have a dynamic stabilization system that involves a minimally invasive implantation procedure, where the extent of distraction between the affected vertebrae is adjustable upon implantation and at a later time if necessary. It would be additionally advantageous if the system or device was also removable in a minimally invasive manner. 
     SUMMARY OF THE INVENTION 
     The present invention provides devices, systems and methods for stabilizing at least one spinal motion segment. The stabilizing devices include an expandable spacer or member having an unexpanded configuration and an expanded configuration, wherein the expandable member in an expanded configuration has a size, volume, diameter, length, cross-section and/or shape configured for positioning between the spinous processes of adjacent vertebrae in order to distract the vertebrae relative to each other. 
     In certain embodiments, the expandable member is a helical body having a varying cross-section along its longitudinal axis, such that compression, squeezing, or other longitudinal translation of the helical body causes the helical body to expand in at least one direction. When placed between two spinous processes, the expansion allows support and stabilization of the processes relative to each other. 
     The stabilizing devices may be configured such that the transformation from the low-profile state to the high-profile state is immediate or gradual, where the extent of expansion is controllable. The transformation may occur in one-step or evolve in continuous fashion where at least one of volume, shape, size, diameter, length, etc. is continually changing until the desired expansion end point is achieved. This transformation may be reversible such that after implantation, the stabilizing device may be partially or completely unexpanded, collapsed, deflated or at least reduced in size, volume, etc. in order to facilitate removal of the member from the implant site or to facilitate adjustment or repositioning of the member in vivo. 
     The stabilizing devices may be configured to stay stationary in the implant site on their own (or “float”) or may be further fixed or anchored to surrounding tissue, e.g., bone (e.g., spinous processes, vertebrae), muscle, ligaments or other soft tissue, to ensure against migration of the implant. In their final deployed state, the stabilizing devices may be flexible to allow some degree of extension of the spine or may otherwise be rigid so as prevent extension altogether. Optionally, the devices may include one or more markers on a surface of the expandable member to facilitate fluoroscopic imaging. 
     The invention further includes methods for stabilizing at least one spinal motion segment which involve the implantation of one or more devices or expandable spacers of the present invention, in which the expandable member is positioned between the spinous processes of adjacent vertebrae in an unexpanded or undeployed condition and then subsequently expanded or deployed to a size and/or shape for selectively distracting the adjacent vertebrae. The invention also contemplates the temporary implantation of the subject devices which may be subsequently removed from the patient once the intended treatment is complete. The methods may also include adjustment of the implants in vivo. 
     Many of the methods involve the percutaneous implantation of the subject devices from either an ipsolateral approach or a mid-line approach into the interspinous space. Certain methods involve the delivery of certain components by a lateral approach and other components by a mid-line approach. The implantation methods may involve the use of cannulas through which the stabilizing devices are delivered into an implant site, however, such may not be required, with the stabilizing devices be configured to pass directly through an incision. 
     These and other objects, advantages, and features of the invention will become apparent to those persons skilled in the art upon reading the details of the invention as more fully described below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is best understood from the following detailed description when read in conjunction with the accompanying drawings. It is emphasized that, according to common practice, the various features of the drawings are not to-scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawings are the following figures: 
         FIG. 1  illustrates a perspective view of a portion of the human spine having two vertebral segments. 
         FIGS. 2A ,  2 B and  2 C illustrate left side, dorsal and top views, respectively, of the spinal segments of  FIG. 1A  under going various motions. 
         FIG. 3A  illustrates a side view of a spacer device according to an embodiment of the present invention in an unexpanded or collapsed state coupled to a cannula of the delivery system of the present invention. 
         FIG. 3B  is a cross-sectional view of a spacer device consistent with  FIG. 3A . 
         FIG. 3C  is a cross-sectional view of the spacer device of  FIG. 3A  in an expanded configuration. 
         FIG. 3D  is a detailed view of the cross-section of an spacer device consistent with  FIG. 3A , having oppositely-angled helical turns compared to the embodiment of  FIG. 3C , also shown in an expanded configuration, showing the varying cross-section of the continuous helical segment. 
         FIG. 3E  is a more detailed view of certain of the varying cross-sections of the helical segment. 
         FIG. 3F  is a side view illustrating the longitudinal and rotational movements employed in deploying certain embodiments of the invention. 
         FIG. 4  is a cross-sectional view of a spacer device according to an embodiment of the present invention employing a threaded screw to deploy the device. 
         FIG. 5  is a cross-sectional view of a spacer device according to an embodiment of the present invention employing a ratchet to deploy the device. 
         FIG. 6  is a cross-sectional view of a spacer device according to an embodiment of the present invention employing a groove to engage a segment of the vertebrae. 
         FIGS. 7(A)  and (B) illustrate stages of deployment of a spacer device according to an embodiment of the present invention employing cooperating segments. 
         FIG. 8  shows a spacer device according to an embodiment of the present invention employing a drive module. 
         FIG. 9  shows a portion of a spacer device employing a generic set of elements that move radially upon application of a force along an axis. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Before the subject devices, systems and methods are described, it is to be understood that this invention is not limited to particular embodiments described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims. 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. 
     It must be noted that as used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a spinal segment” may include a plurality of such spinal segments and reference to “the screw” includes reference to one or more screw and equivalents thereof known to those skilled in the art, and so forth. 
     Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limits of that range is also specifically disclosed. Each smaller range between any stated value or intervening value in a stated range and any other stated or intervening value in that stated range is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included or excluded in the range, and each range where either, neither or both limits are included in the smaller ranges is also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the invention. 
     All publications mentioned herein are incorporated herein by reference to disclose and describe the methods and/or materials in connection with which the publications are cited. The publications discussed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided may be different from the actual publication dates which may need to be independently confirmed. 
     The present invention will now be described in greater detail by way of the following description of exemplary embodiments and variations of the devices and methods of the present invention. The invention generally includes an interspinous spacer device as well as instruments for the percutaneous implantation of the interspinous spacer. A key feature of the interspinous spacer device is that it is expandable from a low profile configuration to a higher profile or operative configuration. This design allows the device, when in the low profile condition, to be delivered by percutaneous means without requiring the removal of any portion of the spinal motion segment into which the device is implanted. 
     Referring now to the drawings and to  FIGS. 3A-F  in particular, an exemplary interspinous spacer device  10  of the present invention is illustrated in collapsed and expanded configurations, respectively. Interspinous device  10  includes an expandable spacer body  14  that has a size and shape when in the expanded condition for operative positioning between the spinous processes of adjacent superior and inferior vertebrae of the spinal motion segment being treated. It should be noted that embodiments of the current invention may be employed, e.g., as spacers, void creators, etc., and may be particularly useful between the interspinous processes or in other sections of the spine. However, other embodiments may be employed in any other location where a void is desired to be created or filled. 
     The interspinous device  10  includes a distal end  11  and a proximal end (not shown). At the distal end  11  is disposed a tip  12 , which may be made of a biocompatible material such as polymers, metals, nitinol, etc. The expandable member  14  has a distal end  22  and a proximal end  18 , with the distal end  22  adjacent the tip  12  and the proximal end  18  adjacent a delivery shaft  16 . Interior of the delivery shaft  16  and the expandable member  14 , and coupled to tip  12 , is a central shaft  24 . 
     The expandable spacer body  14  is generally formed by a helical body having a cross-sectional shape that varies along at least a portion of a longitudinal axis such that longitudinal translation of the proximal end  18  towards the distal end  22 , or vice-versa or both, effectively squeezing the expandable spacer body  14  in a longitudinal fashion, causes portions of the expandable spacer body  14  to expand to provide a spacer support between the spinous processes. In particular, various portions of the expandable spacer body  14  expand in a radial dimension from a value r 0  to a value r&gt;r 0 , up to a maximum value of r max . In certain embodiments, the axial dimension of the expandable spacer body in the undeployed configuration is greater than that in the deployed configuration. 
     The distal end  22  of the expandable spacer body may be affixed to the tip  12  and to the central shaft  24  such that a sliding longitudinal translation of the proximal end  18  over the central shaft leads to expansion. Various other combinations of affixations and sliding members may also be employed. In general, the longitudinal translation over a distance x leads to expansion of the expandable spacer body  14 . 
     The expandable spacer body  14  is made of a biocompatible material such as a non-porous material, e.g., nitinol, polymers, or titanium. The shaft  16  may be made of various biocompatible materials, including titanium, stainless steel, etc. The spacer body and/or the shaft may be coated with a lubricious coating or other such treatment to facilitate sliding. These may include, e.g., Teflon®, silicone, surface energy treatments, etc. 
     As noted above, translation of the proximal and distal ends towards each other leads to radial expansion of portions of the expandable spacer body  14 . In more detail, the expandable spacer body  14  is formed of a helical body. The cross-section of the helical body changes to cause the expansion upon longitudinal translation. Referring in particular to  FIGS. 3B-3E , cross-sections of various helical segments  14   a ,  14   b ,  14   c , . . . ,  14 max, . . . ,  14   n  are shown for a helical body having n turns. 
     Of course, it is noted that these elements reflect cross-sections that in an actual device may be continuously changing along the helix. For clarity, these two-dimensional cross-sections are discussed here, with the understanding that they refer to a three-dimensional helical structure that may be continuously changing in cross-section. For example, portions of the helical body between segments  14   b  and  14   c  emerge above the plane of the page and descend below the plane of the page. These portions may have cross-sectional shapes that are the same as or different than that of segments  14   b  and  14   c . In general, they may be close to the same, and may be shaped in a fashion intermediate to that of  14   b  and  14   c.    
     In an unexpanded configuration, all segments  14   a - 14   n  may have substantially the same radius r 0 . In an expanded configuration, as shown in  FIGS. 3B and 3C , the radius generally varies from r 0  to an r&gt;r 0  to r max  back to r&gt;r 0  and finally back to r 0 . The segment at r max  is denoted  14 max. The variation of r with respect to the longitudinal translation x depends on the way the cross-sections of  14   a - n  vary. As may be seen, the cross-sections of  14   a - n  may vary not haphazardly but in a regular fashion. Certain typical segment cross-sections may be seen in  FIG. 3E . 
     While the general system of variation of cross-sections depends on the usage and geometry of the desired spacer, some general rules may apply in certain embodiments, but it should be especially noted that these do not apply to all systems. First,  14   i  may be closer or more similar in shape to  14   j  than the same is to  14   a  or  14   n . Next, if  14   a  has a distal edge that is at an angle θ a1  and a proximal edge that is at an angle θ a2 , then θ a1  and θ n2  may be equal to zero. If θ b1  up to θ max1 , i.e., angles distal of segment  14 max, are considered to have a positive value of angle, then θ max2 up  up to θ n1 , i.e., angles proximal to segment  14 max, may be considered to have a negative value of angle. For the segment with the maximal radial dimension,  14 max, its distal edge may have an angle with opposite sign to that of its proximal edge. It is noted again that these are general statements that hold for certain embodiments but do not hold for others. 
     It is additionally noted that the embodiment of  FIGS. 3B and 3C  differs from that of  FIG. 3D , in that the angles of the segments are the opposite. While the embodiment of  FIG. 3D  may be easier to implement, either system may be employed. 
     Referring to  FIG. 3F , the way in which the longitudinal translation A may occur can vary. In one embodiment, the device may be inserted between the spinous processes in a manner disclosed in, e.g., U.S. patent application Ser. No. 11/190,496, filed Jul. 26, 2005, entitled “SYSTEMS AND METHODS FOR POSTERIOR DYNAMIC STABILIZATION OF THE SPINE”, which is incorporated by reference herein in its entirety. One or both of the central shaft  24  or the delivery shaft  16  may be rotated as shown by B to cause the relative motion of the distal end and the proximal end towards each other via a screw, ratchet, or thread arrangement. In general, any deployment arrangement may be employed that causes the relative movement of the distal and proximal ends towards each other. Further details of deployment, arrangements that may be used with embodiments of the current invention are described in the patent application just incorporated by reference above. 
     As one example, referring to  FIG. 4 , a method and device for causing translation of the proximal and/or distal ends of the spacer body  50  is shown as employing a threaded shaft assembly  34  having a distal end  28  which is fixed to the distal tip of the device, a proximal end  32 , a threaded section  24 , and a non-threaded section  22 . The threaded shaft assembly may be part of the implanted device or may be removable. In the case of a removable threaded shaft assembly, the assembly  34  is in two parts or is otherwise detachable, e.g., at release element  23 . Release element  23  may employ a magnet to releasably hold the two segments together, a fusible link, a “pull-out” or threaded shaft, and so on. 
     A fixed segment or threaded module  26  is provided which the threaded segment may be rotated against to, e.g., pull the distal tip toward the proximal end to deploy the spacer. In an alternative embodiment, a balloon or other such expandable member within the device may be employed to expand the same in the absence of a compressional force. A “filler” material can be disposed within the helical spacer body to maintain the expansion. This may include compressible materials such as elastomers or uncompressible materials such as cements. A locking mechanism may be similarly employed, and the locking may be permanent or reversible. In a related embodiment, the locking may allow a limited range of translation, including translation after the device is implanted, to accommodate movement, loads, etc. 
     The device may be afforded a capability to reposition the same following implantation, and this reposition may be done in a minimally-invasive manner. For example, a tool may be percutaneously placed to engage the compression assembly, e.g., to turn the screw, or to provide additional compression or tension which corresponds to additional radial expansion or contraction. 
     In an alternative embodiment, as shown in  FIG. 5 , a ratchet assembly  100  is shown with an expandable body  14 , a distal ratchet shaft  122  with a proximal catch  125 , a one-way ratchet  126 , a proximal ratchet lumen  134 , and a deployment pull string  136 . In use, by pulling pull string  136 , catch  126  moves in a one-way manner into ratchet  126 , compressing body  14  and expanding the spacer body  14 . A release mechanism as described can be employed to detach the spacer body from the deployment device. 
     Referring to  FIG. 6 , for any of the embodiments, the spacer body  14  may include a void or recess  20  which may be employed to capture or engage a vertebral segment such as an interspinous process. 
     Referring to  FIG. 7 , the spacer segments  14   i  may be provided with tracks, tongues, or grooves, etc., between the translating elements to facilitate sliding, limit travel, prevent undesired rotation, torquing, stabilize the elements, force directionality of motion, etc. In  FIG. 7 , the same are shown as projections  14   i ′ on each side of segment  14   i . Segment  14   i  is intended to generally refer to a generic segment. In  FIG. 7 , only segments  14   a - 14   d  and  14 max are shown.  FIG. 7(A)  shows an undeployed configuration, and  FIG. 7(B)  shows a deployed configuration. 
       FIG. 8  shows a perspective cross-sectional segment of an alternative embodiment of a spacer body according to the principles of the invention. Similar elements have similar reference numerals as the figures above, and are not described again. The embodiment of  FIG. 8  further includes a covering  138  and a drive module  146 . The covering  138  may be, e.g., a mesh bag, a balloon, etc. The drive module  146  is shown coupled to rods  142  and  144 , and allow deployment of the device in an automatic fashion. The drive module may include a power supply such as an integral battery, may be controlled wirelessly, and may have one or two (as shown) motorized lead screws. One rod  144  may be coupled to distal end  154 , and the other rod  142  may be coupled to proximal end  148 . Alternatively, proximal end  152  and distal end  156  may be employed. 
     It should be noted that while a helical body is shown, the same is not required in certain embodiments of the invention. For example, as shown in  FIG. 9 , any system with two or more elements  14   i ,  14   j , may be employed, where compression causes one element or both to move radially away from a central axis  158 . In this system, the first element has a first surface that mates with a second surface of a second element, and the mating surfaces lie at an angle not equal to 90 degrees from the central axis  158 . The compressive force exerted between the first and second elements, acting along axis  158 , causes either or both to move radially away from the central axis. 
     In certain embodiments, the expandable body is made of a non-compliant or semi-compliant material so as to maintain a substantially fixed shape or configuration and ensure proper, long-term retention within the implant site. In other embodiments, the expandable member may be made of a compliant material. In any embodiment, the compressibility and flexibility of can be selected to address the indications being treated. 
     In certain embodiments of present invention, either during the implant procedure or in a subsequent procedure, the size or volume of the implanted expandable spacer may be selectively adjusted or varied. For example, after an initial assessment upon implant, it may be necessary to adjust, either reduce or increase, the size or volume of the spacer to optimize the intended treatment. Further, it may be intended to only temporarily implant the spacer for the purpose of treating a temporary condition, e.g., an injured or bulging or herniated disk. Once the repair is achieved or the treatment completed, the spacer may be removed, either with or without substantially reducing the size or volume of the spacer. In other embodiments, the spacer may be made of biodegradable materials wherein the spacer degrades after a time in which the injury is healed or the treatment completed. 
     When unexpanded or deflated, as shown in  FIGS. 3A and 3B , the expandable spacer body  14  has a low profile, such as a narrow, cylindrical, elongated shape, to be easily translated through a delivery cannula. 
     The device may further include radiopaque markers on the surface of the expandable body  14  visible under fluoroscopic imaging to facilitate positioning of the expandable body. Any number of markers may be employed anywhere on the expandable body  14 , or the helical body itself may be radiopaque. Other markers may also be employed, including ultrasound markers. Any of the markers described, or other such markers, may be employed to determine the level of deployment or the sufficiency of deployment. For example, two markers may be disposed on the device such that if the markers are seen to be in a particular alignment, the device is considered to be fully deployed. One of ordinary skill in the art given this teaching will see numerous other ways in which the use of markers can provide significant information about the position, orientation, and deployment of the device. 
     Once installed, the interspinous device may be further secured to the spinous processes  18 ,  22  to ensure that the expandable body does not slip or migrate from its implanted position. Any type of anchoring means, such as screws, tacks, staples, adhesive, etc. may be employed. The delivery shaft  16  may be removed from the expandable spacer body  14  using devices and techniques disclosed in the patent application incorporated by reference above. 
     The subject devices and systems may be provided in the form of a kit which includes at least one interspinous device of the present invention. A plurality of such devices may be provided where the devices have the same or varying sizes and shapes and are made of the same or varying biocompatible materials. Possible biocompatible materials include polymers, plastics, ceramic, metals, e.g., titanium, stainless steel, tantalum, chrome cobalt alloys, etc. The kits may further include instruments and tools for implanting the subject devices, including but not limited to, a cannula, a trocar, a scope, a device delivery/inflation/expansion lumen, a cutting instrument, a screw driver, etc., as well as a selection of screws or other devices for anchoring the spacer to the spinous processes. Instructions for implanting the interspinous spacers and using the above-described instrumentation may also be provided with the kits. 
     The preceding merely illustrates the principles of the invention. It will be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. Furthermore, all examples and conditional language recited herein are principally intended to aid the reader in understanding the principles of the invention and the concepts contributed by the inventors to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents and equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure. The scope of the present invention, therefore, is not intended to be limited to the exemplary embodiments shown and described herein. Rather, the scope and spirit of present invention is embodied by the appended claims.