Patent Publication Number: US-2011071568-A1

Title: Spacer Devices Having Retainers And Systems For The Treatment Of Spinal Stenosis And Methods For Using The Same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application Ser. No. 61/245,568, bearing the same title and filed Sep. 24, 2009, the specification and claims of which are fully incorporated by reference herein for all purposes. 
    
    
     FIELD OF THE INVENTION 
     The subject matter described herein relates generally to the treatment of spinal stenosis and more particularly, to interspinous spacer devices and systems for the implantation of those devices and methods for using both. 
     BACKGROUND 
     Spinal stenosis is a condition in which a narrowing of the spinal canal and/or neural foramen leads to compression of the surrounding spinal tissue which can include the spinal cord or spinal nerves. Spinal stenosis can be caused by a number of factors, but is most commonly attributed to the natural process of spinal degeneration that occurs with aging. It has also been attributed to causes such as spinal disc herniation, osteoporosis or the presence of a tumor. 
     Spinal stenosis can occur locally or globally anywhere along the spinal column. When limited to a local region, spinal stenosis is most commonly found in the lumbar region and, to a lesser extent, in the cervical region. Spinal stenosis can result in numerous symptoms that are generally dependent upon the location along the spine in which the stenosis occurs. For instance, cervical spinal stenosis can result in spastic gait, numbness or weakness in upper and/or lower extremities, radicular pain in the upper limbs as well as various other muscular, intestinal and/or nervous system abnormalities. Lumbar spinal stenosis typically results in lower back pain as well as pain or abnormal sensations in the legs, thighs or feet, as well as some intestinal and/or nervous system abnormalities. 
     Treatment for spinal stenosis generally seeks to create additional space for the affected nerves by removing surrounding tissue or bone and/or distracting the adjacent vertebral bodies, thereby relieving the nerve compression causing the patient&#39;s symptoms. Treatment can vary from complicated surgical procedures (e.g., laminectomy and/or foraminotomy in the lumbar region, and laminectomy, hemilaminectomy and/or decompression in the cervical region), to the rigid fixation of adjacent vertebral bodies in relation to each other (e.g., spinal fusion), to the implantation of interspinous spacer devices that distract affected vertebrae without rigid fixation. 
     Of these, the implantation of an interspinous spacer is an attractive option for the patient since the surgical implantation procedure is relatively less invasive than spinal fusion and the patient retains more freedom in movement. Many spacer devices proposed or offered to date suffer from an over-invasive implantation procedure requiring large incisions in the back and the creation of a wide access opening to allow significant manipulations of the device to occur on the lateral side of the spinal column, or they suffer from a complicated design that does not lend itself to ease of implantation. 
     Furthermore, some spacer devices require dissection of the supraspinous ligament to grant access to the interspinous space and then total resection of the interspinous ligament and any spinous process overgrowth to create a cavity in which the device can be implanted. This is further to the displacement and modification of surrounding soft tissue. 
     Accordingly, improved interspinous spacer devices that can avoid these and other deficiencies are needed. 
     SUMMARY 
     Example embodiments of interspinous spacer devices, delivery devices, and methods for using the same are described herein. In brief, these spacer devices generally include a spacer portion configured for placement over or through the interspinous ligament, and an attachable retainer having a bail-like configuration that encompasses and accommodates the intervening supraspinous ligament, as well as other tissue. The spacer portion can have single or multi-piece constructions. The multi-piece spacer construction can have separate elements for applying against the interspinous ligament on opposite sides, held together by the clamping force of the retainer. These elements can also pierce through the interspinous ligament and join with the opposing element, to provide a spacer with increased stability and resistance to spinal compression. Planar stabilizers can be placed on the spacer portion and/or the retainer, to stabilize the device against the superiorly and/or inferiorly located spinous processes. 
     Other systems, methods, features and advantages will be or will become apparent to one with skill in the art upon examination of the description herein. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the subject matter described herein, and be protected by the accompanying claims. In no way should the features of the example embodiments be construed as limiting the appended claims absent express recitation of those features in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The details of the systems, devices and methods may be gleaned in part by study of the accompanying figures, in which like reference numerals refer to like parts. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the relevant principles. Moreover, all illustrations are intended to convey concepts, where relative sizes, shapes and other detailed attributes may be illustrated schematically rather than literally or precisely. 
         FIG. 1A  is a perspective side view of a spinal column. 
         FIG. 1B  is a side view of three lumbar vertebrae of a spinal column. 
         FIG. 1C  is a top down view of a lumbar vertebral body. 
         FIGS. 2A-C  are a perspective, top and side view, respectively, depicting an example embodiment of an interspinous spacer in an unassembled state. 
         FIGS. 2D-F  are a perspective, top and side view, respectively, depicting the same embodiment of the interspinous spacer in an assembled state. 
         FIG. 2G  is a side view depicting the same embodiment of the interspinous spacer implanted along a patient&#39;s spinal column. 
         FIG. 3A  is an exploded perspective view depicting another example embodiment of an interspinous spacer in an unassembled state. 
         FIG. 3B  is a perspective view depicting the example embodiment of the interspinous spacer in a partially assembled state. 
         FIG. 3C  is a perspective view depicting another example embodiment of an interspinous spacer in an unassembled state. 
         FIGS. 3D-E  are cross-sectional top views depicting additional example embodiments of an interspinous spacer in assembled states. 
         FIG. 3F  is a perspective view depicting another example embodiment of spacer elements in an unassembled state. 
         FIG. 3G  is a perspective view depicting another example embodiment of spacer elements in an unassembled state. 
         FIGS. 3H-I  are top and perspective views, respectively, depicting the example embodiment of the interspinous spacer in a partially assembled state without a retainer. 
         FIG. 3J  is a perspective view depicting another example embodiment of an interspinous spacer in an unassembled state. 
         FIG. 4A  is a perspective view depicting another example embodiment of an interspinous spacer in an unassembled state. 
         FIGS. 4B-C  are top views depicting the example embodiment of the interspinous spacer in various states of assembly. 
         FIGS. 5A-B  are top and perspective views, respectively, depicting an example embodiment of a retainer. 
         FIG. 5C  is a cross-sectional top view depicting another example embodiment of an interspinous spacer in an assembled state. 
         FIG. 5D  is a perspective view depicting another example embodiment of an interspinous spacer in an assembled state. 
         FIGS. 6A-C  are top, side and perspective views, respectively, depicting an example embodiment of a retainer in an at-rest state. 
         FIG. 6D  is a perspective view depicting another example embodiment of a retainer in an open state. 
         FIGS. 7A-B  are perspective views depicting an example embodiment of a delivery device. 
     
    
    
     DETAILED DESCRIPTION 
     The present application is related to U.S. provisional patent application Ser. Nos. 61/045,169, filed Apr. 15, 2008 and 61/144,070, filed Jan. 12, 2009, and U.S. patent application Ser. No. 12/352,796, filed Jan. 13, 2009, the disclosures of which are fully incorporated by reference herein for all purposes. For example, the descriptions of the U-shaped and multi-piece spacer devices in those applications can be relevant to the spacer devices described herein, as can the description of the corresponding delivery devices and related tools, as well as the methods for using each (e.g., implantation, delivery, etc.). 
     The interspinous spacer devices described herein include a spacer portion that is configured to receive and couple with a retainer. The spacer portion can be configured for placement in a location between adjacent spinous processes, preferably over or through the interspinous ligament that typically exists in the span between these processes. The spacer portion is a rigid, or substantially rigid, device that can maintain a minimal spacing between adjacent spinous processes, which in turn maintains a minimum spacing for the spinal nerves thereby avoiding compression of those nerves, which can cause pain or discomfort to the patient. 
     The retainer preferably accommodates the presence of the supraspinous ligament and is preferably configured with a linear/curved U-shape that extends posteriorly from the spacer portion along both sides of the interspinous ligament and around the entirety of the supraspinous ligament. The retainer maintains the spacer portion in the proper orientation and position with respect to the superior and inferior spinous processes and can prevent the spacer portion from moving anteriorly towards the ligamentum flavum and spinal nerves. Implantation of the interspinous spacer devices can therefore avoiding substantial irritation or trauma to the supraspinous ligament and the anteriorly located ligamentum flavum. With a multi-piece spacer portion, the retainer can further apply a clamping force to hold the separate pieces together between the adjacent processes. 
     Also described herein are systems for the delivery of interspinous spacer devices for use by the administering physician or medical professional. In addition, methods for the use of the spacer devices and delivery systems are provided. These devices, systems and methods will be described herein the context of treatment of spinal stenosis in the lumbar region of the spine, although, it should be noted that these devices, systems and methods can be used to treat spinal stenosis at any location (e.g., cervical, thoracic) along the spinal column. 
     To better illustrate these devices, systems and methods, a description of the basic spinal anatomy will first be set forth.  FIG. 1A  is a perspective side view of a spinal column  10  showing five vertebral bodies  11 , each separated by an intervertebral disc  19 . More specifically, this region is the lumbar region of the spine and the five vertebral bodies  11  are lumbar vertebrae L1-L5. Each vertebral body  11  includes a posterior portion  12  having numerous bony features. The most prominent feature is spinous process  14 , which is an elongate, fin-shaped feature that is situated the furthest posteriorly from each vertebral body  11 . Located adjacent to spinous process  14  are left and right transverse processes  15  and left and right mamillary processes  16  (only the left side is shown here). These processes  14 - 16  are connected to each vertebral body  11  by way of left and right pedicles  17  (only left side shown). 
       FIG. 1B  is a side view of three lumbar vertebrae of spinal column  10  with the left side pedicles  17  and processes  15 - 16  omitted to allow depiction of the interspinous tissue  20 . Located adjacent to each vertebral body  11  and generally anterior to spinous process  14  (indicated as being obscured by dashed lines) is ligamentum flavum  21 , which is immediately adjacent to the vertebral forman  25  and intervertebral foramen  26 . Posterior to ligamentum flavum  21 , is the wider interspinous ligament  22 , which extends alongside each spinous process  14 . Posterior to interspinous ligament  22  is supraspinous ligament  23 , which generally extends along the posterior edge of the interspinous tissue  20 . 
       FIG. 1C  is a top down view of a lumbar vertebral body  11 . Here, left and right pedicles  17 - 1  and  17 - 2  can be seen in greater detail extending away from vertebral body  11 . Also shown is spinous process  14 , left and right transverse processes  15 - 1  and  15 - 2 , mamillary processes  16 - 1  and  16 - 2  and left and right lamina  18 - 1  and  18 - 2 . Between features  14 - 18  and the bulk of vertebral body  11  is a space referred to as the vertebral foramen  25 . It is through the vertebral foramen  25  and intervertebral foramen  26  (shown in  FIGS. 1A-B ) that the spinal cord and other spinal nerves (not shown) are routed. Spinal stenosis is generally a narrowing or reduction in size of either or both of forarnen  25 - 26  that results in the undesired compression of the nerves located therein. 
     Turning now to the example embodiments,  FIGS. 2A-F  depict an example embodiment of a interspinous spacer device  100  configured for implantation within a patient.  FIG. 2A  is a perspective view depicting device  100  in an unassembled state, while  FIG. 2B  is a top view and  FIG. 2C  is a side view of device  100  in the same state. Here, device  100  includes spacer portion  101  and retainer  110 , which are configured to releasably couple together. Spacer portion  101  can be configured in numerous ways, and here it is a one-piece, generally cylindrical body  102 . Spacer body  102  has a generally conical end (e.g., a bullet nose)  103 . Nose  103  facilitates insertion of spacer body  102  into the interspinous space (as described later). Spacer body  101  also includes openings or slots  104  and  105 , which provide access to interior spaces, or channels,  106  and  107 , respectively. Channels  106  and  107  are configured to receive retainer  110 . 
     Retainer  110  can also be configured in numerous ways, and is here configured as a one-piece, generally U-shaped, or bail-like body  111  having a distal end  115  and a proximal end  116 . Retainer  110  includes two elongate struts  112  and  113  connected together by a curved intermediate connective portion  114  located at proximal end  116 . Retainer  110  can also be configured with more than two struts interfacing with spacer portion  101 . Also, spacer device  100  can include multiple retainers  110  for interfacing with any number of spacer portions  101 , or sub-bodies of spacer portion  101  (such as spacer elements  131  and  132  described later). 
     The free ends  193  and  194  of elongate struts  112  and  113  are tapered and configured for insertion into channels  106  and  107 , respectively, to adjustably lock spacer portion  101  with retainer  110 . Struts  112  and  113  can include one or more locking features  122  and  123 , which are here configured as ratchet-like teeth, or abutments, respectively. These preferably each interface with locking features  108  and  109 , positioned within channels  106  and  107 , respectively. Here, locking features  108  and  109  are configured as catches. The distal face of each tooth  122  and  123  is preferably at approximately 45 degrees and matches the angle of the proximal face of respective catches  108  and  109 . The proximal face of each tooth  122  and  123  is preferably at approximately 90 degrees and matches the angle of the distal face of respective catches  108  and  109 , to lock or secure retainer  110  once engaged. Also, teeth  122  and  123  can be placed in the same positions along the length of struts  112  and  113 , respectively, or can be offset. 
     In one example embodiment of assembly, continued advancement of retainer  110  into channels  106  and  107  causes struts  112  and  113  to deflect outwards as each successive tooth  122  and  123  transitions along the respective catch  108  and  109 . Once the tooth passes the respective catch, struts  112  and  113  deflect back towards one another and engage the catch, thereby locking retainer  110  in place in the desired position. In another example embodiment, struts  112  and  113  can be deflected apart, then advanced into position and released, to allow engagement between the teeth and the respective catches. 
     This multi-tooth configuration allows several retainer depths for varying anatomy. If channels  106  and  107  enclose (or surround) struts  112  and  113 , then adequate space should be left to allow struts  112  and  113  to deflect during advancement. Channels  106  and  107  can also have an open side along their length, to provide room for the deflection of struts  112  and  113 , respectively, and also to facilitate release should it be desired. Alternatively, catches  108  and  109  can be spring-loaded so that deflection of struts  112  and  113  is not required. Interspinous spacer device  100  is shown in the assembled and locked state in corresponding  FIGS. 2D-F . 
     One of skill in the art will readily recognize, based on this disclosure, that many other types of suitable locking devices can be used, not limited to the ratchet-type mechanism and locking features described here. For instance, clip-based, screw-based, snap-based, and high friction-based interfaces can also be used, as well as magnetic elements. Also, when spacer body  102  is singular, a locking mechanism can be provided between only one strut and the spacer body. 
     Struts  112  and  113  of retainer  110  also include opposing stabilizer members, which are configured here as planar lobes. Strut  112  includes opposing lobes  118 - 1  and  118 - 2 , and strut  113  includes opposing lobes  119 - 1  and  119 - 2 . The opposing lobes each project away from the other in an orientation that allows them to lie alongside the interspinous tissue (e.g., the interspinous ligament) and spinous processes such as depicted in  FIG. 2G , and thereby provide stabilization to the device. The struts  112  and  113  of retainer  110  are generally co-planar, i.e., they reside in the same plane. Lobes  118  generally lie in the same plane, which is transverse, and preferably perpendicular to, the plane of struts  112  and  113 . The same applies to lobes  119 . 
     Lobes  118  are preferably integrally formed with body  111 , but can also be attachable. Each lobe  118  includes a shaped edge  120  complementary to the surface of spacer body  102 , to allow the lobe to be positioned directly adjacent spacer body  101 . Lobes  119  have similar complementary shaped edges  121 . Here, the shaped edges are curved to match the generally elliptical cross-profile of spacer portion  101 . Lobes  118  and  119  can be included with any embodiment described herein, and can be also or alternatively located on spacing portion  101 , if desired. 
     Struts  112  and  113  of retainer  110  also include lateral projections  125  and  126 , each having an aperture, or hole,  127  and  128 , respectively, for interfacing with a removal tool that can grasp projections  125  and  126  through holes  127  and  128 , respectively, and use this leverage to pull struts  112  and  113  apart to release from spacer body  101 . 
       FIG. 2G  is a side view depicting this embodiment of spacer device  100  implanted within a patient&#39;s spinal column. Spacer portion  101  is positioned between the interspinous processes of the L4 and L5 vertebrae, with retainer  110  extending posteriorly along the sides of interspinous ligament  22  and around the posterior edge of supraspinous ligament  23 . Spacer portion  101  is located through the interspinous ligament  22  preferably such that it does not contact the ligamentum flavum  21 . Contact with the supraspinous ligament  23  can also be minimized or avoided if desired. 
     To implant device  100 , the medical professional preferably makes one or more incisions in the back to allow access to the tissue surrounding the spinous processes. The desired interspinous space between adjacent spinous processes is then located. An incision (or other access opening) is made through the interspinous ligament, and spacer portion  101  is inserted through the incision and into position between the spinous processes. Retainer  110  is then coupled with spacer portion  101  and locked in the desired position, such that device  100  resembles that shown in  FIG. 2G . 
       FIG. 3A  is an exploded perspective view depicting another example embodiment of interspinous spacer device  100 , sharing certain similarities to that of  FIGS. 2A-F , although here, stabilizer members  118  and  119  are positioned on spacer portion  101 , which has a multi-piece construction. The multi-piece spacer portion  101  includes inner bodies  133  and  134 , each of which are configured to receive outer sleeves  135  and  136 , respectively.  FIG. 3B  depicts sleeves  135  and  136  coupled with the inner bodies  133  and  134 , respectively, to form first and second opposing spacer elements  131  and  132 . More than two spacer elements can also be used, with more than two struts of retainer  110  or multiple retainers  110 . Sleeve  135  has first and second openings, or slots,  139 - 1  and  139 - 2 , to allow for the passage of strut  112  through channel  106 . Similar slots  140  are present in sleeve  136 . Sleeves  135  and  136  are generally atraumatic and formed from a softer, less rigid material to lessen any friction or impact with the adjacent tissue and bone. Sleeves  135  and  136  can be formed from a polymeric material, such as PEEK (polyetheretherketone), and the like. 
     Inner body  134  includes a smaller diameter cylindrical end, or nose,  138  which opposes the end  137  on inner body  133 . End pieces  137  and  138  can each include opposing projecting faces, or a recessed portion, such as a cup, can be present within end  137  to receive nose  138  during implantation (i.e., to integrate or mate the space elements  131  and  132 ).  FIG. 3C  depicts another configuration without sleeves  135  and  136 . Here, end  138  includes sidewall  141  and a blunt projection  142 . Blunt projection  142  is configured to be received within recessed portion  145  of end  137 , as depicted in the cross-sectional top view of  FIG. 3D .  FIG. 3D  depicts retainer  110  after insertion into both of spacer elements  131  and  132 . Retainer  110  is preferably made deflectable and biased towards the closed state depicted here, where spacer elements  131  and  132  are in close proximity, preferably contacting (if no intervening tissue is present, as described below). The force generated by retainer  110  is preferably sufficient to maintain spacer elements  131  and  132  in the proper position on the spinal column, as well as to resist the compressive forces generated by the superiorly and inferiorly located spinous processes, such as would occur during extension of the spine. Mating and interlocking features for the two spacer elements are described herein, and the inclusion of those features add further resistance to these compressive forces. 
     A blunt shape of nose  138  can aid in locating the interspinous space against which the spacer elements  131  and  132  are positioned. The medical professional can pass blunt nose  138  of spacer element  132  over the tissue and use the tactile feedback to ascertain where the adjacent spinous processes are located in relation to the interspinous space therebetween. Once the desired interspinous space is identified, spacer element  131  is placed in a position opposing spacer element  132  (if not already done so, for instance, by the delivery device). Struts  112  and  113  are deflected apart so that retainer  110  is in an open state. This allows struts  112  and  113  to then be inserted into spacer elements  131  and  132 , which are separated by the interspinous tissue. Upon the locking of retainer  110  with spacer elements  131  and  132 , retainer  110  is released to allow it to transition back to the closed state. Retainer  110  can also be forced anteriorly via the curved connector  114  and struts  112  and  113  will separate and return to the closed state as they pass over the catches  108  and  109 . This draws or brings spacer elements  131  and  132  together into the configuration shown in  FIG. 3D . 
     When retainer  110  closes, the interspinous tissue, which can be very thin and distensible, can be trapped between spacer elements  131  and  132 . Over a period of time, this intervening trapped tissue preferably becomes necrosed and is eventually removed by the patient&#39;s own bodily processes. Apertures in the spacer elements can facilitate access to this tissue (e.g., by macrophages) to speed its removal. 
     If desired, these spacer elements  131  and  132  can also be configured to cut or core this intervening tissue. As shown in  FIG. 3D , a close fit exists between the sidewall  141  of spacer element  132  and the inner wall  144  of spacer element  131 . The leading tapered surface  143  of spacer element  131  acts as an annular, ring-like blade that incises through, or cores out a section of the interspinous ligament upon closing of the device  100 . 
       FIG. 3E  is a cross-sectional top view of another example embodiment configured to incise the intervening interspinous ligament. Here, nose  137  of spacer element  131  has a tapered outer sidewall  148  and a recessed portion  147 , while nose  138  of spacer element  132  has a tapered inner sidewall  149  and a recessed portion  150 . These opposing tapered edges again act to incise the intervening tissue, and trap it within recessed portions  147  and  150 . This embodiment is also shown with surrounding atraumatic sleeves  135  and  136 , which provide a substantially continuous surface across spacer elements  131  and  132  and cover any gaps present in the junction between the joined spacer elements  131  and  132 . Although, negligible gaps may still exist, these gaps are not substantial in that they are not large enough to readily allow the adjacent spinous processes (or surrounding tissue) to begin to force the spacer elements apart during compression. 
       FIGS. 3F-I  depict additional example embodiments of spacer elements  131  and  132  configured to cooperate to shear the intervening tissue. In the perspective view of  FIG. 3F , spacer element  131  includes multiple blades  180 , each having an upper flat surface  181  and a lower sloped surface  182  that meet to form a sharp edge. Spacer element  132  includes multiple blades  183 , each having an upper sloped surface  184  and a lower flat surface  185  that likewise come together to form a sharp edge. The blades  180  are located in positions offset from the blades  183  of the opposing spacer element, such that the two spacer elements  131  and  132  can be brought together with the flat surfaces  181  and  185  in close proximity, or contact. These blades thus have a shearing or guillotine type effect that cuts through the intervening tissue. Sheared tissue can be displaced into recessed gaps  191  and  192  that remain present after closure, at which point the tissue can be processed naturally by the body. 
       FIG. 3G  is a perspective view of another example embodiment of spacer elements  131  and  132  with a different blade configuration. Here, spacer element  131  includes top-most and bottom-most beveled blades  188  and  189 , respectively. Beveled blade  188  has a leading, sharp end-tip and a beveled upper surface. One or more (in this case three) intervening V-shaped blades  187  are present between blades  188  and  189 . V-shaped blades  187  have a leading, sharp end-tip at the junction of the upper and lower sloped surfaces. Likewise, spacer element  132  also includes V-shaped blades  187  (four) with an inverted beveled blade  190  in the top-most position. The inverted beveled blade has a surface corresponding to that of beveled blade  188 , so as to receive that blade in a close fit.  FIGS. 3H-I  are top and perspective views, respectively, of these spacer elements joined together in a close fit. 
       FIG. 3J  depicts another example embodiment of spacer  100  where spacer element  131  is pre-connected to strut  112  and spacer element  132  is separate. Retainer  110  is spread apart and spacer element  132  can then be connected to strut  113  during the implantation procedure. Spacer  131  and retainer  110  can be placed in the desired implantation location first with spacer element  132  attached thereafter, or conversely, spacer element  132  can be placed in the desired position first spacer element  131  and retainer  110  connected thereafter. 
     Turning now to  FIGS. 4A-C , another example embodiment is depicted where spacer portion  101  includes two spacer elements  151  and  152  configured to interlock together and with retainer  110 .  FIG. 4A  is a perspective view in an unassembled state, and  FIGS. 4B-C  are top views in various states of assembly. Spacer element  132  includes an extension, or hub,  156  that is insertable into a matching recess  155  in spacer element  131 . Any intervening interspinous tissue is preferably removed beforehand. Spacer elements  131  and  132  include enclosed channels  153  and  154  for receiving struts  112  and  113 , respectively. Hub  156  includes a sub-channel  157  that aligns with channel  153  of spacer element  131  after spacer elements  131  and  132  are inserted together. As retainer  110  is advanced into channels, teeth  122  and  123  pass over and lock with catches  158  and  159  (as seen in  FIGS. 4B-C ) in the selected position.  FIG. 4C  depicts device  100  in the first locked position and it can be seen that strut  112  extends into sub-channel  157  of extension  156  and prevents separation of spacer elements  131  and  132 . The interlocking of the spacer elements  131  and  132  together, along with the retainer  110 , acts to resist any tendency that those elements will separate during extension of the spine, when compressive force is imparted onto the joint between spacer elements  131  and  132  by the superior and inferior spinous processes. 
       FIGS. 5A-C  depict another example embodiment of spacer device  100  having a modified manner of interlocking between retainer  110  and spacer elements  131  and  132 . Here, abutments (or teeth)  162  are configured as rectangular bosses, as opposed to having a sloped or non-parallel upper and lower sides. This configuration prevents movement in both the anterior and posterior directions once engaged. Any adjustment of the position of the retainer  110  requires teeth  162  to first be disengaged. 
       FIGS. 5A-B  are top and perspective views, respectively, of retainer  110  in an at-rest state, while  FIG. 5C  is a cross-sectional top view depicting struts  112  and  113  of retainer  110  deflected outwards, or opened, so as to allow engagement with spacer elements  131  and  132 . Spacer elements  131  and  132  have open channels  168  and  169  to allow struts  112  and  113 , respectively, to be inserted such that teeth  162  engage the desired recesses  167 , which preferably also have a rectangular shape. One of skill in the art will readily recognize that other shapes for teeth  162  and recesses  167  can be used that will still increase resistance to movement in both anterior and posterior directions. The bias of retainer  110  towards the at-rest state holds it in place against spacer elements  131  and  132  (even if retainer  110  is in an intermediate state and not fully transitioned into the at-rest state). 
     As shown in  FIG. 5A , strut ends are closer to each other than in the more open state of  FIG. 5C . Generally, the closer the strut ends are in the at-rest state, the more force that can be generated when in the state of  FIG. 5C , so long as significant plastic deformation does not occur when spreading the strut ends. Furthermore, more closure force is generally applied when only the distal-most tooth is engaged, as opposed to the proximal-most tooth (e.g., when retainer  110  is fully advanced). Another embodiment of retainer  110  capable of applying relatively greater force at each tooth position as compared with this embodiment, is described with respect to  FIGS. 6A-D  below. 
     This embodiment of  FIGS. 5A-C  also includes engagement features  163  and  164  on struts  112  and  113 , respectively. These features are provided to allow a delivery device to more readily grasp the struts and retract them, or maintain them in an “open” retracted state during delivery. Once in the desired position, struts  112  and  113  can be slowly released to allow retainer  110  to transition back to the at-rest state and interlock with spacer elements  131  and  132 . Here, engagement features  163  and  164  are shaped in a dovetail fashion, with a narrow base, or neck,  166  and a relatively wider fan-out portion  165 . This configuration can also be referred to as T-shaped. 
       FIG. 5D  depicts another example embodiment where spacer elements  131  and  132  have multiple channels  171  and  172 . The presence of multiple channels can allow customization for different anatomies. The medical professional can use different retainers  110  having varying widths W to accommodate different thicknesses in the posterior region of the patients spine. Multiple channels can also allow for interfacing with delivery or removal instrumentation. For instance, outer channels  171 - 1  and  172 - 2  can receive a spanner wrench-type instruement that can open and close spacer elements  131  and  132 . Any number of channels can be included in each spacer element. Here, channels  171  and  172  are spaced along the lateral X axis, but the channels can also be spaced along the Y axis, in which case the width would remain constant but the position of retainer  110  could vary superiorly or inferiorly. Also, any combination of channels can be provided along both X and Y axes, to provide further adaptability. 
     Spacer elements  131  and  132  also have a tapered configuration (rounded triangular cross-sectional profile), such that sloped faces  173  and  174  come together at the anterior end of the spacer elements  131  and  132 . This demonstrates the adaptability of the spacer elements to account for anatomical variations. Spacer elements  131  and  132  can have other cross-sectional profiles, such as egg-shaped, elliptical, oval, and circular, or rounded polygonal profiles such as rectangular, square, pentagonal, hexagonal, octogonal, and the like. 
       FIGS. 6A-D  depict another example embodiment of retainer  110  configured to provide relatively greater closure force.  FIG. 6A  is a top view of retainer  110  in the at-rest state, while  FIG. 6B  is a side view and  FIG. 6C  is a perspective view of the same. The ratchet teeth are not shown.  FIG. 6D  is a perspective view showing retainer  110  in an outwardly deflected, or open, state. Each strut  112  and  113  includes a curved posterior (proximal) portion  175  and a relatively straight anterior (distal) portion  176 . The width of retainer in the posterior section is relatively greater than the width in the anterior section, as the posterior section of each strut flares outwards away from the opposite strut. It should be noted that this configuration of retainer  110  can be used with any embodiment of spacer  100  described herein. 
       FIG. 7A  is a perspective view depicting an example embodiment of delivery device  200 . Here, delivery device  200  is used with an embodiment of spacer device  100  similar to that depicted in  FIG. 3C . Delivery device  200  includes a main handle  201  which is connected to a housing  207  and a device shaft  206 , which is in turn connected to a spacer interfacing device  212 .  FIG. 7B  depicts spacer interfacing device  212  in greater detail, and while interfacing with another embodiment of spacer device  100 , similar to that of  FIG. 7A  but having a projecting nipple-like feature  170  on spacer element  132  and a corresponding recess in spacer element  131 , the sloped surfaces of which aid in self-alignment of elements  131  and  132  during delivery.  FIG. 7B  depicts spacer elements  131  and  132  (fully apart) and retainer  110  prior to engagement.  FIG. 7A  depicts spacer elements  131  and  132  after having been brought together and retainer  110  after engagement with spacer elements  131  and  132 , i.e., near the completion of the delivery procedure. 
     Interfacing device  212  includes distal seats on which spacer elements  131  and  132  are placed. These seats can be configured as one or more pins  214  and  215 , which are insertable into corresponding apertures  129  and  130 , respectively, in spacer elements  131  and  132 . Spacer elements  131  and  132  are held in place by a locking mechanism, which are slidable bars  210 . Bars  210  slide within channels in the sidewalls of interfacing device  212 . The position of these bars  210  is controlled by actuators  204 - 1  and  204 - 2 , respectively, which are configured here as hexagonal bolts that reside within threaded lumens inside interfacing device  212 . Advancement of bolts  204  cause the bolt shafts  216  to depress bars  210  and lock spacer elements  131  and  132  in place (shown in  FIG. 7B ). Device  200  is preferably used, in this configuration, to position spacer elements  131  and  132  appropriately, for example, by using tactile feedback provided by projection  170 . 
     Once in the desired position, spacer elements  131  and  132  are brought together across the interspinous space. This can be accomplished with actuator  203 , which is also configured as a hexagonal bolt. Tightening of actuator  203  causes threaded bolt shaft  209  to draw the right side portion  219  of interfacing device  212  towards the left side portion  218 . Relative motion of side portions  218  and  219  is guided by alignment pins  220  and  221 . 
     Delivery device  200  also includes an actuator  202 , configured here as a handle, for controlling the position of retainer  110  with respect to spacer elements  131  and  132 . Actuator  202  is coupled with a shaft  205 , which is threaded through an axial nut within housing  207  (and thus not shown). The distal end of shaft  205  is coupled with a retainer interface  208 , which is configured here as a sled. The distal end of sled  208  has a curved surface corresponding to the shape of the proximal portion of retainer  110 . In this embodiment, retainer  110  is biased towards a closed configuration, but the curved receptacle of sled  208  compresses retainer  110  beyond the closed configuration such that retainer  110  is biased to expand from the configuration shown. This compression holds retainer  110  to sled  208  passively, without the need for an active (i.e., capable of opening and/or closing) retaining mechanism, although one can be provided if desired. 
     Rotation of actuator  202  causes sled  208  to advance distally and drive retainer  110  downwards into spacer elements  131  and  132  after closure. Side portions  218  and  219  each include a guide slot  222  and  223 , respectively, for guiding the advancement of retainer  110 . Once retainer  110  is engaged with spacer elements  131  and  132 , actuator  203  can be reversed to spread side portions  218  and  219  apart again and release spacer device  100 , as depicted in  FIG. 7A . 
     One of skill in the art will readily recognize that the actuators of delivery device  200  can be manually controlled or electrically controlled, such as with an electronic interface. Furthermore, device  200  can include visual guides that instruct the medical professional as to the position of the spacer components and the proper delivery sequence. These guides can be printed or can be provided through an electronic display. 
     The components of spacer device  100  can be formed from any number or types of materials that are suitable for the needs of the individual application. Each of spacer body  102 , the main (core) portions of spacer elements  131  and  132 , and retainer  110  can be formed from metallic or polymeric materials. Retainer  110  is preferably (but not necessarily) formed from elastic (or superelastic) shape memory materials, i.e., materials that can exhibit a bias to revert towards a predetermined shape or state, such as nickel-titanium alloys (e.g., nitinol) and the like. This bias can be present before and after implantation or can be configured to initiate once a predetermined temperature is reached (e.g., slightly below human body temperature). Other suitable materials include titanium, stainless steel, Elgiloy and various polymers such as polyetheretherketones (PEEK), polycarbonate urethane (PCU), ultra high molecular weight polyethylene (UHMWPE), and the like. Materials that are not magnetic can allow compatibility with magnetic resonance imaging (MRI) systems. Materials that approximate bone density, such as PEEK, can minimize trauma to the adjacent spinous processes and are especially suitable for sleeves  135  and  136 . Each of spacer elements  131  and  132  can also be formed from the same or different materials. Any portion or body of spacer  100  can itself be formed from any number of one (monolithic) or more (multi-body) separate pieces. For example, struts  112  and  113  can be formed from a rigid (i.e., inflexible) material and connective portion  104  can be formed from a more flexible material, for instance, to ease bending in that region or to minimize irritation to the supraspinous ligament. Alternatively, body  111  can be monolithic, as shown in the figures. Likewise, the stabilizers can be made integral with retainer  110 , or spacer elements  131  and  132 , or can be attached separately. 
     Furthermore, any portion of spacer device  100  can be coated with any desired material, such as bio-compatible substances, substances to alter the surface friction (either increase or decrease) between the device and any surrounding tissue, substances to promote healing, atraumatic and conformable substances as described earlier, absorbable and other substances to promote the growth of scar tissue or other tissue (e.g., poly-L-lactide (PLLA), polyglycolide (PGA), sheep intestinal submucosa, etc.), and the like. 
     While the embodiments are susceptible to various modifications and alternative forms, specific examples thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that these embodiments are not to be limited to the particular form disclosed, but to the contrary, these embodiments are to cover all modifications, equivalents, and alternatives falling within the spirit of the disclosure. Statements expressly indicating that certain features are not limited in a particular manner should not be interpreted as implying that the absence of such statements with regard to other features implies that those other features are in any way limited to the disclosed embodiment.