Patent Publication Number: US-2009234457-A1

Title: Systems, devices and methods for treatment of intervertebral disorders

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 11/292,335, filed on Dec. 1, 2005, incorporated herein by reference in its entirety, which claims priority from U.S. provisional application Ser. No. 60/632,396 filed on Dec. 1, 2004, incorporated herein by reference in its entirety. 
     This application is a continuation-in-part of U.S. application Ser. No. 11/505,783, filed on Aug. 16, 2006, incorporated herein by reference in its entirety, which is a divisional of application Ser. No. 10/154,857, filed on May 24, 2002, now U.S. Pat. No. 7,156,877, incorporated herein by reference in its entirety, which claims priority from U.S. provisional application Ser. No. 60/310,882, filed on Jun. 29, 2001, incorporated herein by reference in its entirety. 
     This application is related to PCT Publication No. WO 2006/060482, published Jun. 8, 2006, incorporated herein by reference in its entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC 
     Not Applicable 
     NOTICE OF MATERIAL SUBJECT TO COPYRIGHT PROTECTION 
     A portion of the material in this patent document is subject to copyright protection under the copyright laws of the United States and of other countries. The owner of the copyright rights has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the United States Patent and Trademark Office publicly available file or records, but otherwise reserves all copyright rights whatsoever. The copyright owner does not hereby waive any of its rights to have this patent document maintained in secrecy, including without limitation its rights pursuant to 37 C.F.R. § 1.14. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention pertains generally to repairing intervertebral disc disorders, and more particularly to implants and surgical procedures for repairing a degenerated intervertebral disc. 
     2. Description of the Background Art 
     An estimated 4.1 million Americans annually report intervertebral disc disorders, with a significant portion of them adding to the nearly 5.2 million low-back disabled. Though the origin of low-back pain is varied, the intervertebral disc is thought to be a primary source in many cases, and is an initiating factor in others where a degenerated disc has led to altered spinal mechanics and non-physiologic stress in surrounding tissues. 
     The intervertebral disc is a complex structure consisting of three distinct parts: the nucleus pulposus; the annulus fibrosus; and the cartilaginous end-plates. The nucleus pulposus is a viscous, mucoprotein gel that is approximately centrally located within the disc. It consists of abundant sulfated glycosaxninoglycans in a loose network of type II collagen, with a water content that is highest at birth (approximately 80%) and decreases with age. The annulus fibrosus is that portion of the disc which becomes differentiated from the periphery of the nucleus and forms the outer boundary of the disc. The transition between the nucleus and the annulus is progressively more indefinite with age. The annulus is made up of coarse type I collagen fibers oriented obliquely and arranged in lamellae which attach the adjacent vertebral bodies. The fibers run the same direction within a given lamella but opposite to those in adjacent lamellae. The collagen content of the disc steadily increases from the center of the nucleus to the outer layers of the annulus, where collagen reaches 70% or more of the dry weight. Type I and II collagen are distributed radially in opposing concentration gradients. The cartilaginous end-plates cover the end surfaces of the vertebral bodies and serve as the cranial and caudal surfaces of the intervertebral disc. They are composed predominately of hyaline cartilage. 
     The disc derives its structural properties largely through its ability to attract and retain water. The proteoglycans of the nucleus attract water osmotically, exerting a swelling pressure that enables the disc to support spinal compressive loads. The pressurized nucleus also creates tensile pre-stress within the annulus and ligamentous structures surrounding the disc. In other words, although the disc principally supports compressive loads, the fibers of the annulus experience significant tension. As a result, the annular architecture is consistent with current remodeling theories, where the ˜60° orientation of the collagen fibers, relative to the longitudinal axis of the spine, is optimally arranged to support the tensile stresses developed within a pressurized cylinder. This tissue pre-stress contributes significantly to the normal kinematics and mechanical response of the spine. 
     When the physical stress placed on the spine exceeds the nuclear swelling pressure, water is expressed from the disc, principally through the semipermeable cartilaginous end-plates. Consequently, significant disc water loss can occur over the course of a day due to activities of daily living. For example, the average diurnal variation in human stature is about 19 mm, which is mostly attributable to changes in disc height. This change in stature corresponds to a change of about 1.5 mm in the height of each lumbar disc. Using cadaveric spines, researchers have demonstrated that under sustained loading, intervertebral discs lose height, bulge more, and become stiffer in compression and more flexible in bending. Loss of nuclear water also dramatically affects the load distribution internal to the disc. In a healthy disc under compressive loading, compressive stress is created mainly within the nucleus pulposus, with the annulus acting primarily in tension. Studies show that, after three hours of compressive loading, there is a significant change in the pressure distribution, with the highest compressive stress occurring in the posterior annulus. Similar pressure distributions have been noted in degenerated and denucleated discs as well. This reversal in the state of annular stress, from physiologic tension due to circumferential hoop stress, to non-physiologic axial compression, is also noted in other experimental, analytic and anatomic studies, and clearly demonstrates that nuclear dehydration significantly alters stress distributions within the disc as well as its biomechanical response to loading. 
     The most consistent chemical change observed with degeneration is loss of proteoglycan and concomitant loss of water. This dehydration of the disc leads to loss of disc height. In addition, in humans there is an increase in the ratio of keratan sulphate to chondroitin sulphate, an increase in proteoglycan extractability, and a decrease in proteoglycan aggregation through interaction with hyaluronic acid (although the hyaluronic acid content is typically in excess of that needed for maximum aggregation). Structural studies suggest that the non-aggregable proteoglycans lack a hyaluronate binding site, presumably because of enzytruitic scission of the core protein by stromelysin, an enzyme which is thought to play a major role in extracellular matrix degeneration. These proteoglycan changes are thought to precede the morphological reorganization usually attributed to degeneration. Secondary changes in the annulus include fibrocartilage production with disorganization of the lamellar architecture and increases in type II collagen. 
     Currently, there are few clinical options to offer to patients suffering from these conditions. These clinical options are all empirically based and include (1) conservative therapy with physical rehabilitation and (2) surgical intervention with possible disc removal and spinal fusion. In contrast to other joints, such as the hip and knee, very few methods of repair with restoration of function are not available for the spine. 
     Therefore, there is a need for a minimally invasive treatment for degenerated discs which can repair and regenerate the disc. The present invention satisfies that need, as well as others, and overcomes the deficiencies associated with conventional implants and treatment methods. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention comprises an implant and minimally invasive method of treating degenerated discs which can repair and regenerate the disc. More particularly, the present invention comprises a bioactive/biodegradable nucleus implant and method of use. The implant is inflated inside the nucleus space after the degenerated nucleus has been removed to re-pressurize the nuclear space within the intervertebral disc. Nuclear pressure produces tension in the annular ligament that increases biomechanical stability and diminishes hydrostatic tissue pressure that can stimulate fibro-chondrocytes to produce inflammatory factors. The device will also increase disc height, separate the vertebral bodies and open the spinal foramina. 
     By way of example, and not of limitation, an implant according to the invention comprises a collapsible, textured or smooth membrane that forms an inflatable balloon or sack. To inflate the implant, the implant is filled with a high molecular weight fluid, gel or combination of fluid and elastomer, preferably an under-hydrated HA hydrogel/growth factor mixture with or without host cells. Integral to the membrane is a self-sealing valve that allows one-way filling of the implant after it is placed within the disc. The implant membrane is made from a material that allows fibrous in-growth thereby stabilizing the implant. A variety of substances can be incorporated into the device to promote healing, prevent infection, or arrest pain. The implant is inserted utilizing known microinvasive technology. Following partial or total nucleotomy with a small incision, typically annular, the deflated implant is inserted into the nuclear space through a cannula. The implant is then filled through a stem attached to the self-sealing valve. Once the implant is filled to the proper size and pressure, the cannula is removed and the annular defect is sealed. 
     One of the main difficulties in repairing the degenerated disc is increasing the disc height. The disc and surrounding tissues such as ligaments provide a great deal of resistance to disc heightening. For this reason it is unlikely that placing a hydrogel alone into the nuclear space will be able to generate enough swelling pressure to regain significant disc height. The present invention, however, addresses this problem by allowing initial high pressures to be generated when the implant is inflated in the nuclear space. The initial high pressure is sufficient to initiate the restoration of the original disc height. This initial boost in disc height facilitates the later regeneration stages of this treatment. 
     In the long term, having a permanent pressurized implant is not likely to be ideal because it may not be able to mimic the essential biomechanical properties of the normal disc. However, the invention also addresses this issue by using a biodegradable sack. The initially impermeable membrane permits high pressurization. When the membrane biodegrades, it allows the hydrogel mixture to take action in playing the role of the normal nucleus pulposus with its inherent swelling pressure and similar mechanical properties. 
     A variety of growth factors or other bioactive agents can be attached to the surface of the implant or included in the hydrogel mixture that is injected inside the implant. The membrane could be reinforced or not reinforced with a variety of fiber meshes if necessary. Furthermore, a variety of materials could be used for the membrane; the only requirement is that they be biodegradable such that the membrane is impermeable when initially implanted and until it biodegrades. A variety of materials could be injected into the sack such as cartilage cells, alginate gel, and growth factors. 
     The present invention comprises systems, devices and methods, which can be employed alone or in any combination with each other or in any combination with systems, methods and devices known in the art, in connection with treatment of intervertebral disorders. 
     Another aspect of the invention is a stent for facilitating regeneration of an intervertebral nucleus and/or retention of a bladder-type implant, wherein the intervertebral nucleus is bounded at its upper and lower extremities by opposing vertebral endplates of adjacent vertebrae, and at its periphery by annulus fibrosus. The stent has top and bottom portions comprising metal hoops having a footprint adapted to engage with peripheral regions of the opposing vertebral endplates while leaving a central region of the vertebral endplates open. The stent also includes a plurality of lateral members connecting said top and bottom portions. The lateral members and top and bottom portions are configured to allow the stent to collapse for insertion into the nuclear cavity via an annulus port and then expand upon placement in the nuclear cavity. 
     In some embodiments, where the stent is configured to be installed in between adjacent lumbar vertebrae, the top and bottom hoops may have an increased ring gauge to accommodate higher compressive loads. 
     In an alternative embodiment, the stent is configured to be installed in between adjacent cervical endplates. Accordingly, the stent may extend across the majority of the vertebral endplates outward through the region normally occupied by the annulus. In this configuration the upper and lower hoops are preferably elliptical to match the contours of the vertebral bodies. Furthermore, the upper and lower hoops may have a series of serrations to engage the vertebral bodies. The hoops may also have one or more flanges that extend to the anterior portions of the outside wall of the vertebral body, thereby allowing fixation to the anterior surfaces of the vertebra. 
     In some modes of the present aspect, the stent is configured to support at least a portion of compression loads generated between the opposing vertebral endplates to facilitate regeneration of the intervertebral nucleus. In some embodiments, the stent functions as a flexible cage to allow movement of the vertebral endplates while at the same time keeping the nuclear cavity open for tissue regeneration. The footprint of the top and bottom portions may be circular, or somewhat elliptical to match the anatomy of the intervertebral nucleus. 
     Preferably, the metal hoops and lateral members comprise a memory material, such as nitinol. The hoops may also be textured and/or a growth factor to promote bony in growth, or an anti-inflammatory factor to treat discogenic pain. 
     In an alternative embodiment, the stent is configured to be expanded around an inflatable membrane. In this case, the inflated membrane supports intervertebral compression, while the stent prevents membrane lateral expansion or lateral migration. 
     Yet another aspect of the invention is a method for facilitating regeneration of the intervertebral disc, comprising inserting a collapsed stent into a nuclear cavity in the nucleus pulposus tissue, and expanding the stent to support a portion of intervertebral compression loads and thereby facilitate nuclear regeneration. 
     In a preferred mode, inserting the collapsed stent is done by creating an annular portal annulus fibrosus to access the nucleus pulposus, removing the nucleus pulposus tissue to create the nuclear cavity, and inserting the collapsed stent through the annular portal and into the nuclear cavity. In the cervical spine, most of the anterior and posterior annulus is removed prior to stent placement, and in this case, implant retention is facilitated by anterior flanges. 
     Generally, the upper and lower metal hoops to are expanded to engage the vertebral endplates, and generate an axial force on the vertebral endplates via a loading from the plurality of lateral members to separate the upper and lower hoops against the endplates. 
     In an another embodiment, an inflatable membrane may be first inserted into a nuclear cavity in the nucleus pulposus tissue, and then the inflatable membrane is expanded to further support a portion of intervertebral compression loads and thereby facilitate nuclear regeneration. Alternatively, the stent is inserted into a nuclear cavity while in a collapsed configuration over the inflatable membrane, and inflation of the inflatable membrane releases the stent from the collapsed configuration. 
     Yet another aspect of the invention is an implant for repairing an intervertebral disc. The implant has an inflatable membrane with an inner layer configured to withstand compressive forces generated in the intervertebral disc, and a textured external layer that to promotes fibrous tissue in growth in the intervertebral disc. 
     In some embodiments, the textured layer is formed from a foamed, uncured polyurethane. An exemplary textured layer may have an average pore size ranging from approximately 400 microns to approximately 800 microns, and a volume porosity in the range of approximately 75% to approximately 80%. 
     The implant may also have an internal self-sealing fill valve for filling the membrane. In some embodiments, the valve comprises internal opposing walls that collapse as a result of a compressive load disposed on said internal chamber. 
     A further aspect of the invention is a method for creating a textured inflatable implant by forming an inflatable membrane, and dipping the inflatable membrane into a solution of foamed, uncured polyurethane to form a final textured surface layer. 
     Yet another aspect of the invention is an implant having an inflatable membrane, a filler material comprising a first fluid for inflating the membrane, and a plurality of microspheres dispersed in said filler material, each of said microspheres holding a second fluid. The microspheres may be filled with gas, or with a liquid to help maintain hydration of the first fluid over a period of time. 
     The microspheres may also be configured to promote movement of fluid between the microspheres and the first fluid based on pressure exerted on the first fluid. For example, the microspheres may transfer the second fluid to the first fluid at rate that increases with increased pressure. The second fluid inside the microspheres may be water, therapeutic agent, or other solution beneficial in promoting healing. 
     Yet a further aspect of the invention is an implant for repairing an intervertebral disc disposed between opposing vertebral endplates of adjacent vertebrae. The implant has membrane having upper and lower walls configured to engage said vertebral endplates, and reinforced peripheral walls joining the upper and lower walls. The peripherally reinforced walls may have a variety of beneficial attributes, including prevent bulging of the membrane a result of compressive forces imposed on said membrane from the vertebral endplates, increasing fatigue resistance, or providing stiffness in an under inflation condition. Additionally, the reinforced peripheral wall may create a nonlinearity in overall device stiffness during bending or compression to improve overall intervertebral stability 
     In one embodiment, the peripheral walls are thicker than the upper and lower walls have to provide localized stiffness. As an alternative or addition, the peripheral walls may also be reinforced with a fiber matrix. For example, the fiber matrix comprises a plurality of woven fibers oriented at an angle of approximately 60 degrees relative to vertical. 
     Yet another aspect is an implant comprising membrane with a plurality of inner chambers for holding an inflation medium. 
     In one embodiment, the membrane has a first chamber with a different stiffness than the second chamber. For example, the first chamber may be filled with a gel having a first stiffness, and the second chamber may be filled with a gel having a second stiffness that is stiffer than the first gel. The second chamber may also surround the periphery of the first chamber. 
     Preferably, the first chamber and the second chamber have independent, concentrically oriented valves. 
     In another embodiment, wherein the first chamber is configured to hold a gel to mechanically support the opposing vertebral endplates, with the second chamber holding a therapeutic agent to promote tissue in growth. 
     Another aspect is a method of treating a region of annulus fibrosus disposed between adjacent vertebral bodies. The method includes the steps of installing one or more sutures into a vertebral body rim adjacent to the annulus fibrosus region, attaching the one or more sutures to a netting, and securing the netting across the annulus fibrosus region. 
     Preferably, the netting is secured across an annulus defect, such as hole in the annulus or annulus degeneration. In addition the netting may have one side (the side away from the annulus) with an anti-adhesion film to prevent connective tissue attachment. Accordingly, the side adjacent to the annulus would have an adhesion promoting surface that may consist of texture plus growth factor. 
     Preferably, at least two sutures are installed into the vertebral rim. The sutures may be installed simultaneously with use of a specially modified tool. 
     In one embodiment, the suture anchors are placed with a pliers-type tool with a plurality of tangs on each side, wherein each tang is adapted to attach to a suture anchor. 
     The sutures may be attached directly to the vertebral rim, or attached via installing suture anchors in the vertebral endplate adjacent to the annulus fibrosus region. 
     Yet a further aspect is a system for treating a region of annulus fibrosus having one or more anchors configured to be installed in the rim of each vertebral body, a netting configured to disposed across the annulus fibrosus region, and one or more sutures configured to attach the netting to the anchors. The netting preferably comprises a woven mesh. In some embodiments, woven mesh has a cross-ply matching the annulus fibrosus architecture. Additionally, one side of the mesh may have a polymer configured to promote tissue in growth, and an opposing side configured to prevent adhesion. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
       The invention will be more fully understood by reference to the following drawings which are for illustrative purposes only: 
         FIG. 1  is a side view of an implant according to the present invention, shown in a collapsed state. 
         FIG. 2  is a side view of the implant of  FIG. 1 , shown in an inflated state, with a portion of the membrane cut away to show the internal filler material. 
         FIG. 3  is cross-sectional side view of the implant of  FIG. 1 , shown in the inflated state and showing the integral, internal fill valve. 
         FIG. 4  is a side view of a mandrel for molding an implant according to the present invention. 
         FIG. 5  is a side view of an implant membrane according to the present invention as it would be seen after being dip molded on the mandrel shown in  FIG. 4  but before removal from the mandrel. 
         FIG. 6  is an end view of the implant shown in  FIG. 5  prior to heat-sealing the open end. 
         FIG. 7  is an end view of the implant shown in  FIG. 5  after heat-sealing the open end. 
         FIG. 8  is an exploded view of a delivery system for placement of an implant according to the invention shown in relation to the implant. 
         FIG. 9  is an assembled view of the delivery stem shown in  FIG. 8  with the implant attached. 
         FIG. 10  is a side schematic view of a degenerated intervertebral disc prior to repair using an implant according to the present invention. 
         FIG. 11A  through  FIG. 11G  is a flow diagram showing a surgical procedure for placement of an implant according to the present invention. 
         FIG. 12  is a perspective view of an introducer sheath according to the invention with a trocar inserted and positioned in the nuclear space of an intervertebral disc so as to create an annular opening in the disc. 
         FIG. 13  is a perspective view of a Crawford needle and Spine Wand inserted in the introducer sheath shown in  FIG. 12  and positioned for ablation of the nuclear pulposus in an intervertebral disc. 
         FIG. 14  is a detail view of the implant end portion of the assembly of  FIG. 13 . 
         FIG. 15  is a perspective view of an implant launcher and fill assembly according to the present invention shown with an introducer sheath, launcher sheath, fill tube positioned prior to deployment of an implant in the nuclear space of an intervertebral disc and with the proximal end portions of the introducer sheath and launcher sheath partially cut away to expose the implant and buttress. 
         FIG. 16  is a detail view of the implant end portion of the assembly of  FIG. 15 . 
         FIG. 17  is a perspective view of the assembly of  FIG. 15  after insertion of the implant in the nuclear space of the intervertebral disc. 
         FIG. 18  is a detail view of the implant end portion of the assembly of  FIG. 17 . 
         FIG. 19  a perspective view of the assembly of  FIG. 15  after deployment of the implant in the nuclear space and prior to retraction of the implant and inner annular buttress. 
         FIG. 20  is a detail view of the implant end portion of the assembly of  FIG. 19 . 
         FIG. 21  is a perspective view of the assembly of  FIG. 15  after partial retraction of the implant and inner annular buttress with the inner annular buttress shown engaging and plugging the annular opening in the intervertebral disc. 
         FIG. 22  is a detail view of the implant end portion of the assembly of  FIG. 21 . 
         FIG. 23  is a perspective view of the assembly of  FIG. 15  after the implant is inflated. 
         FIG. 24  is a detail view of the implant end portion of the assembly of  FIG. 23   
         FIG. 25  is a perspective view of an intervertebral stent in accordance with the present invention. 
         FIG. 26  illustrates the stent of  FIG. 25  in a collapsed configuration. 
         FIG. 27  is a schematic diagram of the stent of  FIG. 25  installed in a nuclear cavity in between two adjacent lumbar vertebrae in accordance with the present invention. 
         FIG. 28A  illustrates a perspective view of an alternative intervertebral stent in accordance with the present invention. 
         FIG. 28B  illustrates a top view of the stent of  FIG. 28A . 
         FIG. 28C  illustrates a lateral view of the stent of  FIG. 28A  implanted between two adjacent cervical vertebrae. 
         FIG. 28D  illustrates an anterior view of the stent of  FIG. 28A  implanted between two adjacent cervical vertebrae. 
         FIG. 28E  illustrates a superior view of the stent of  FIG. 28A  in an exemplary orientation with respect to a cervical vertebrae. 
         FIG. 29  illustrates the stent of  FIG. 25  collapsed around bladder-type implant in accordance with the present invention. 
         FIG. 30  illustrates a cross-sectional view of an implant having an inflatable bladder with a textured surface in accordance with the present invention. 
         FIG. 31  illustrates a cross-sectional view of an implant having filler material comprising microspheres in accordance with the present invention. 
         FIG. 32  illustrates a cross-sectional view of an implant having reinforced peripheral walls in accordance with the present invention. 
         FIG. 33  illustrates a cross-sectional view of an implant having multiple chambers in accordance with the present invention. 
         FIG. 34  illustrates a top cross-sectional view of the implant of  FIG. 33 . 
         FIG. 35  illustrates a cross-sectional view of an alternative implant with a suspended chamber. 
         FIG. 36  shows a schematic view of a system for repairing an annular defect. 
         FIG. 37A  illustrates an anchor of the system of  FIG. 36  installed in the vertebral body. 
         FIG. 37B  illustrates a close-up view of the mesh used in the system of  FIG. 36 . 
         FIG. 37C  illustrates an exemplary cable tie that may be used in the system of  FIG. 36 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following descriptive material, various aspects and embodiments of the invention are described as systems, devices or methods. It will be appreciated that these aspects and embodiments can be used in a stand-alone manner, and further that any aspect or embodiment can be used in combination with one or more of the aspects or embodiments described herein. In addition, those skilled in the art will appreciate that any of the aspects or embodiments of the invention described herein can be used in combination with other devices, systems and methods known in the art. 
     Referring more specifically to the drawings, for illustrative purposes the present invention is embodied in the apparatus generally shown in  FIG. 1  through  FIG. 37C . It will be appreciated that the apparatus may vary as to configuration and as to details of the parts, and that the method may vary as to the specific steps and sequence, without departing from the basic concepts as disclosed herein. 
     1. Nuclear Disc Implant 
     Referring first to  FIG. 1  through  FIG. 3 , an implant  10  according to the present invention comprises a collapsible membrane  12  that is formed into a inflatable balloon or sack that will conform to the shape of the nucleus pulposus when inflated. Membrane  12  preferably comprises an inert material such as silicone or a similar elastomer, or a biodegradable and biocompatible material such as poly (DL-lactic-co-glycolic acid; PLGA). Since the implant will serve as an artificial inner annulus, and its internal chamber will contain a pressurized nuclear filler material  14  used for inflation, the membrane material should be relatively impermeable while possessing the necessary compliance and strength. In addition, the membrane material should be sufficiently flexible so that the implant can easily be passed through a surgical catheter or cannula for insertion. 
     Table 1 compares certain characteristics of the inner annulus to a number of commercially-available elastomers that were considered for the membrane material. Key design requirements were biocompatibility, stiffness, and elongation-to-failure. While any of these materials, as well as other materials, can be used, our preferred material was aliphatic polycarbonate polyurethane (HT-4) which has a stiffness that closely approximates that of the inner annulus, can be fabricated into complex shapes using dip molding, possess significant failure properties, and has a track-record for in vivo use. 
     The peripheral surface of the implant is preferably coated with one or more bioactive substances that will promote healing of the inner annulus and integration of the implant with the surrounding annular tissue. Also, the top and bottom surfaces of the implant are preferably coated with one or more bioactive substances that will promote healing of the cartilaginous endplates and integration of the implant with the endplates. 
     To limit the amount of lateral bulging when the implant is axially compressed, the peripheral surface of the implant can be reinforced with a fiber matrix if desired. In that event, the angle of the fibers relative to the vertical axis of placement should be approximately ±60° to closely approximate that of the native collagen fibers in the inner annulus. 
     Implant  10  includes an integral, internal, self-sealing, one-way valve  16  that will allow the implant to be inserted in a deflated state and then be inflated in situ without risk of deflation. Valve  16  functions as a flapper valve to prevent leakage and maintain pressurization of the implant when pressurized with the nuclear filler material. Because valve  16  is internal to the implant, compression of implant  10  will place internal pressure on valve  16  to keep it in a closed position. Due to the self-sealing nature of valve  16 , the same pressure that might be sufficient to allow the nuclear filler material to escape will cause valve  16  to remain closed so as to create a barrier to extrusion. 
     To better understand the operation and configuration of valve  16 , reference is now made to  FIG. 4  which shows the preferred embodiment of a mandrel  18  for fabricating the implant. Mandrel  18  preferably comprises a planar stem portion  20 , a first cylindrical base portion  22 , a mold portion  24 , a second cylindrical base portion  26 , and a shank  28 . To fabricate an implant, distal end  30  of the mandrel is dipped in a bath of membrane material to a defined depth which is generally at a point along second base portion  26  and molded to a thickness between approximately 5 mils and 7 mils. 
       FIG. 5  generally depicts the configuration of the implant after it has dried on the mandrel. However, the mandrel is not shown in  FIG. 5  so that the implant can be more clearly seen. After the membrane material dries on the mandrel, it is drawn off of the mandrel by rolling it toward distal end  30 . As a result, the membrane is turned inside-out. By inverting the membrane in this manner, the portion of membrane material that coated stem portion  20  becomes valve  16  which is now located inside the implant as shown in  FIG. 3 . The portion of membrane material that coated first base portion  22  becomes an entrance port  32  into valve  16 . Note that the distal end  34  of valve  16  was sealed during molding, while the distal end  36  of the implant is still open as shown in  FIG. 5  and  FIG. 6 . Accordingly, to finish the fabrication process, distal end  36  of the implant is heat-sealed to close it off as shown in  FIG. 7 . 
     To inflate the implant, a needle-like fill stem is inserted through entrance port  32  so as to puncture the distal end  34  of valve  16  and extend into the interior chamber of the implant. The implant is then filled with a fluid material, such as a high molecular weight fluid, gel or combination of fluid and elastomer which has a viscosity that will permit its introduction into the implant through, for example, an 18-gauge needle. The specific properties of filler material  14  should allow the material to achieve and maintain the desired osmotic pressure. The filling takes place after the implant is placed within the disc. Preferably filler material is a cross-linkable polyethylene glycol (PEG) hydrogel with chondroitin sulfate (CS) and hyaluronic acid (HA) with or without host cells as will now be described. 
     Table 2 shows the characteristics of a number of commercially-available hydrogels that were considered for filler material  14 . While any of these materials, as well as other materials, can be used, we selected an in situ cross-linkable polyethylene glycol (PEG) gel because of its bio-compatibility and physical properties. The PEG gel is a two component formulation that becomes a low-viscosity fluid when first mixed and which cross-links to a firm gel after insertion. The cross-link time depends on the formulation. A key feature of the gel is its osmotic pressure. We sought to formulate a gel that would possess an osmotic pressure of near 0.2 MPa which is that of the native nucleus pulposus. 
     The preferred PEG gel comprises a nucleophilic “8-arm” octomer (PEG-NH2, MW 20 kDa) and a “2-arm” amine-specific electrophilic dimer (SPA-PEG-SPA, MW 3.4 kDa), and is available from Shearwater Corporation, Huntsville, Ala. The addition-elimination polymerization reaction culminates in a nitrogen-carbon peptide-like linkage, resulting in a stable polymer whose rate of polymerization increases with pH and gel concentration. The range of pH (approximately 10 for the unmodified gel) and concentration (approximately 0.036 g/mL to 0.100 g/mL) investigated resulted in a polymerization time of approximately 10 minutes to 20 minutes. To fortify the hydrogel&#39;s inherent swelling due to hydrogen bonding, high molecular weight additives chondroitin sulfate (CS) and hyaluronic acid (HA) with established fixed charged densities were incorporated into the gel matrix. 
     The swelling pressures of the hydrogel filler (cross-linked polyethylene glycol (PEG) hydrogels and derivatives incorporating HA and CS) were measured by equilibrium dialysis as a function of gel and additive concentration. Polyethylene glycol (Molecular Weight 20 kDa available from Sigma-Aldrich Corporation) was also used as the osmotic stressing agent, while molecularporous membrane tubing was used to separate sample gels from the dialysate. Gels were formed over a broad concentration range (0.036 to 0.100 g/mL), weighed, placed in dialysis tubing (Spectra/Por Membrane, Molecular Weight Cut Off of 3.5 kDa available from Spectrum Medical Industries), and allowed to equilibrate for 40 to 50 hours in the osmotic stressing solution, weighed again to determine hydration, then oven dried (at 60 degrees Celsius) and weighed once again. Hydration values taken at various osmotic pressures allowed the construction of osmotic pressure curves. By adjusting the concentrations of CS or HA we were able to meet our design criteria, successfully achieving swelling pressures above 0.2 MPa. A potential deleterious interaction between the elastomer and hydrogel was noted. One PEG-CS specimen aged in saline demonstrated breakdown of the elastomer shell. This may have been due to the relatively low-molecular weight CS penetrating into the membrane material (polyurethane) leading to an increased rate of hydrolysis. 
     Referring now to  FIG. 8  and  FIG. 9 , the invention includes an implant delivery system comprising a hollow implant fill stem  38 , a hollow buttress positioner  40 , and an inner annular buttress  42 . Implant fill stem  38  is configured for inflating implant  10  after insertion, and inner annular buttress  42  is configured to extend into and block a hole  66  (see  FIG. 11A ) that is made in the annulus for insertion of implant  10 . Once inserted, inner annular buttress  42  prevents extrusion of the implant during spinal loading. Inner annular buttress  42  preferably comprises a polymer head portion  44  of suitable diameter for plugging hole  66 , a smaller diameter polymer body portion  46  extending from head portion  44 , and metal barbs or pins  48  having ends  50  that extend outward in relation to body portion  46  such that they will engage the annulus to prevent expulsion of inner annular buttress  42  (and implant  10 ) during spinal loading. Pins  48 , which can be formed of stainless steel, Nitinol®, or the like, can be molded or otherwise inserted into head portion  44  for retention therein. 
     An inner passage  52  extends through inner annular buttress  42  for attachment to buttress positioner  40  and insertion of fill stem  38  through inner annular buttress  42  into implant  10 . Inner passage  52 , head portion  44  and body portion  46  are preferably coaxial. Buttress positioner  40  and inner annular buttress  42  are coupled together using mating threads  54   a ,  54   b  or another form of detachable coupling that allows buttress positioner  40  to be easily removed from inner annular buttress  42  after placement. Note that inner annular buttress  42  can be attached to implant  10  using adhesives, ultrasonic welding or the like, or can be separate and unattached from implant  10 . 
     Fill stem  38  includes a collar  56  for attachment to a syringe  58  or other device to be used for inflating the implant with the filler material. Fill stem  38  and syringe  58  are coupled together using threads (not shown) or another form of detachable coupling. Preferably, syringe  58  includes a pressure gauge (not shown) for determining the proper inflation pressure. The implant and delivery system would be deployed into the nucleus pulposus space by being inserted into a conventional catheter, cannula or the like (not shown) having a retractable cover (not shown) that protects the implant during insertion. 
       FIG. 10  depicts the vertebral bodies  60 , cartilage endplates  62 , degenerated nucleus  64 , and degenerated annulus  66  in the spine. The indications for use of the implant are a patient with back pain or radiating pain down the leg where the cause of the pain has been determined to be a herniated disc which is impinging on the surrounding spinal nerves. Deployment of the implant is preferably according to the following surgical procedure shown in  FIG. 11A  through  FIG. 11G  which is minimally invasive. 
     As shown in  FIG. 11A , the first step in the surgical procedure is to perform a minimally invasive postero-lateral percutaneous discectomy. This is executed by making a small hole  68  through the annulus fibrosus of the intervertebral disc and removing the nucleus pulposus tissue through that hole. Several technologies were considered to facilitate removing degenerated nuclear material through a small opening made through the annulus fibrosus. The most promising technology is the ArthroCare Coblation probe (ArthroCare Spine, Sunnyvale, Calif.). This device vaporizes the nucleus in situ. Because of density differences that exist between the nucleus and annulus, the Coblation probe removes the less-dense nuclear material more easily than the annulus. This allows the surgeon to remove the nuclear material while minimizing damage to the remaining annulus or adjacent vertebral body. 
     The referred protocol for creating a nuclear space for the implant comprises making a small puncture within the annulus with a pointed, 3 mm diameter probe. This pointed probe serves to separate annular fibers and minimize damage to the annulus. Next, a portion of the nucleus is removed using standard surgical instruments. The Coblation probe is then inserted. Suction and saline delivery are available with the probe, although we have found that suction through another portal using, for example, a 16-gage needle, may be required. A critical feature of device success is the method of creating a nuclear space while minimizing trauma to the outer annulus fibrosus. The outer annulus should be preserved, as it is responsible for supporting the implant when pressurized. 
     Next, as shown in  FIG. 11B , the deflated implant  10  is inserted into the empty nuclear space  70 . This is accomplished by inserting the implant through a conventional insertion catheter (cannula)  72 . Note that fill stem  38 , buttress positioner  40  and inner annular buttress  42  are also inserted through catheter  72 , which also results in compression of pins  48 . The cover  74  on the insertion catheter  72  is then retracted to expose the implant as shown in  FIG. 11C . Next, as shown in  FIG. 11D , the implant is inflated with the filler material  14 , until it completely fills the nuclear space  70 .  FIG. 11E  shows the implant fully inflated. Note the resultant increase in disc height and restoration of tensile stresses in the annulus. The pressurized implant initiates the restoration of the original biomechanics of the healthy disc by increasing the disc height, relieving the annulus of the compressive load, and restoring the normal tensile stress environment to the annulus. The restoration of the normal tensile stress environment in the annulus will promote the annular cells to regenerate the normal annulus matrix. 
     The catheter and delivery system (e.g., fill stem  38  and buttress positioner  40 ) are then removed, leaving inner annular buttress  42  in place and implant  10  sealed in position as shown in  FIG. 11F . Note that inner annular buttress  42  not only serves to align and place the implant, but prevents extrusion during spinal loading. In addition, the one-way valve  16  in the implant prevents the hydrogel/growth factor mixture from leaking back out of the nucleus implant. Therapeutic agents on the peripheral and top/bottom surfaces of the implant stimulate healing of the inner annulus and cartilage endplates. In addition the surface growth factors will also promote integration of the implant with the surrounding tissue. 
     Finally,  FIG. 11G  depicts the implant biodegrading after a predetermined time so as to allow the hydrogel/growth factor mixture to play its bioactive role. The hydrogel is hydrophilic and thereby attracts water into the disc. Much like the healthy nucleus pulposus, the hydrogel creates a swelling pressure which is essential in normal disc biomechanics. The growth factor which is included in the hydrogel stimulates cell migration, and proliferation. We expect the environment provided for these cells to stimulate the synthesis of healthy nucleus pulposus extracellular matrix components (ECM). These cells will thereby complete the regeneration of the nucleus pulposus. 
     It will be appreciated that the implant can be inserted using other procedures as well. For example, instead of performing a discectomy (posterolateral or otherwise), the implant could be inserted into a preexisting void within the annulus that arises from atrophy or other form of non-device-induced evacuation of the nucleus pulposus, such as for, example, by leakage or dehydration over time. 
     Example 1 
     Prototype implant shells were fabricated by Apex Biomedical (San Diego, Calif.). The fabrication process included dip molding using a custom-fabricated mandrel. The mandrel was dipped so that the elastomer thickness was between 5 and 7 mils (0.13-0.17 mm). After dipping, the implant was removed from the mandrel, inverted (so that the stem was inside the implant) and heat-sealed at the open end. This process resulted in a prototype that could be filled with the PEG gel, which when cross-linked could not exit through the implant stem. The stem effectively sealed the implant by functioning as a “flapper valve”. This means that by being placed within the implant, internal pressures (that might serve to extrude the gel) compress and seal the stem, creating a barrier to extrusion. This sealing mechanism was verified by in vitro testing. 
     Example 2 
     Elastomer bags filled with PEG were compressed to failure between two parallel platens. The implants failed at the heat seal at approximately 250 Newtons force. These experiments demonstrated that under hyper-pressurization, the failure mechanism was rupture at the sealed edge, rather than extrusion of gel through the insertion stem. When the device is placed within the intervertebral disc, support by the annulus and vertebral body results in a significantly increased failure load and altered construct failure mechanism. 
     Example 3 
     Ex vivo mechanical testing were performed with human cadaveric spines to characterize the performance of the device under expected extreme in vivo conditions. We conducted a series of experiments that consisted of placing the device in human cadaveric discs using the developed surgical protocols and then testing the construct to failure under compressive loading. The objective of these experiments was to characterize the failure load and failure mechanism. The target failure load was to exceed five times body weight (anticipated extremes of in vivo loading). Importantly, the failure mode was to be endplate fracture and extrusion of the implant into the adjacent vertebra. This is the mode of disc injury in healthy spines. We did not want the construct to fail by extrusion through the annulus, particularly through the insertion hole, since this would place the hydrogel in close proximity to sensitive neural structures. 
     Load-to-failure experiments demonstrated that the implant may sustain in excess of 5000 N (approximately seven times body weight) before failure, and that the failure mode was endplate fracture. These preliminary experiments demonstrate that the implant can sustain extremes in spinal compression acutely. 
     Referring now to  FIG. 12 , the nuclear space can be prepared for receiving the implant by removing degenerated nuclear material using a coblation probe or the like as described above. Upon exposing the targeted disc  100 , the nuclear space  102  can be accessed via a trocar  104 , such as a stainless steel, 7 Fr. OD, trocar with a small Ultem handle  106 . Preferably, a corresponding 7 Fr introducer sheath  108  also having a small Ultem handle  110 , is used for insertion of the trocar. An example of a suitable introducer sheath is a 7 Fr plastic sheath with 0.003 inch walls and a 1.5 inch working length, such as a modified Cook or equivalent. The trocar is then removed upon access leaving a patient access point. Use of an introducer tends to minimize wear and tear on the hole, thus maximizing engagement of inner annular buttress  42 . In the embodiment shown, inner annular buttress  42  would typically have a 0.071 OD and a length of 0.070 inches, and carry three pins  48  having a diameter of approximately 0.008 inches and a length of approximately 0.065 inches. 
     Referring to  FIG. 13  and  FIG. 14 , a Crawford needle  112  (e.g., 17 gage×6 inch, included with the ArthroCare Convenience Pack Catalog No. K7913-010) and ArthroCare Perc-DLE Spine Wand  114  (ArthroCare catalog number K7813-01) are introduced into the nucleus through the introducer sheath  108  and the nucleus pulposus is ablated. By moving the Wand in and out of the needle, the degree of articulation of the distal tip can be controlled. The Crawford needle also provides added rigidity for improved manipulation of the device. 
     Referring now to  FIG. 15  and  FIG. 16 , an alternative embodiment of the delivery system shown in  FIG. 8  and  FIG. 9  is illustrated. In this embodiment, introducer sheath  108  is used as a port into the nuclear space  102 . In  FIG. 15 , the end portion of introducer sheath  18  has been cutaway for clarity. A plastic launcher sheath  116  (e.g., 0.084 inch×0.090 inch×3 inch) is slidably insertable into the introducer sheath is provided. Note that the end portion of launch sheath  116  has also been cutaway for clarity. Preferably, launcher sheath  116  includes a small plastic handle  118 , and all or a portion of the launcher sheath is preferably flexible to assist with deployment of the implant as described below. A fill tube  120  (e.g., 14 XT×3.9 inch long) is provided that is slidably insertable into launcher sheath  116 . Fill tube  120  also preferably includes a small plastic handle  122 . The fill tube preferably terminates at its proximal end with a female leur lock  124  having a 0-80 UNF thread to which the assembly of buttress  42  (carrying implant  10 ) is threadably attached. It will be appreciated that buttress  42  can be attached to leur lock  124  after fill tube  120  has been inserted into launcher sheath  116  and extended therethrough such that leur lock  124  extends through the end of launcher sheath  116 . At this point pins  48  can be manually depressed and the un-deployed implant/buttress assembly pulled into the launcher sheath. Alternatively, buttress  42  can be attached to leur lock  124  and fill tube  120  then inserted into launcher sheath  116 . With either approach, the assembly of implant  10 , buttress  42 , launcher sheath  116  and fill tube  120  can then be inserted into introducer sheath  108  and pushed into the nuclear space  102 . A small c-clip style spacer or the like (not shown) can be used to maintain separation between handles  118  and  122  to prevent premature deployment of the implant as will be more fully appreciated from the discussion below. 
     As can be seen from  FIG. 17  and  FIG. 18 , implant  10  can then be advanced into the nuclear space  102  by pushing launcher sheath  116  through introducer sheath  108  until handle  118  contacts handle  110 . Note that the flexibility of launcher sheath  106  allows it to deflect if necessary to fit the contour of the nuclear space.  FIG. 19  and  FIG. 20  then show the implant being deployed by retracting both the introducer sheath  108  and the launcher sheath  116  by pulling handles  110  and  118  back toward handle  122  on fill tube  120  until they are in contact with handle  122 . From  FIG. 20  it can be seen that pins  48  will then spring outward into the nuclear space and into a position that is ready for engagement with the annulus. Then, as can be seen in  FIG. 21  and  FIG. 22 , pulling back on fill tube  120  will cause the pins  48  on buttress  42  to engage the annulus  68 . With inner annular buttress  42  secured in place, implant  10  can then be filled as shown in  FIG. 23  and  FIG. 24 . Once implant  10  is filled, fill tube  120  can be unscrewed from buttress  42  and removed. 
     2. Intervertebral Stent 
       FIG. 25  illustrates an embodiment of the present invention comprising an internuclear stent  200 . The stent  200  is configured to keep the nuclear space  70  (shown in  FIG. 27  between adjacent lumber vertebra) open by supporting a portion of the intervertebral compression loads and thereby facilitate nuclear regeneration. The stent comprises a top hoop  202  and bottom hoop  204  that are separated by a plurality of lateral members  206 . The lateral members  206  and hoops  202 ,  204  may comprise a memory material or metal, such as a nitinol. The hoops  202 ,  204  may also be textured to promote bony in growth. The hoops  202 ,  204  may also have a relatively large gauge to accommodate the higher compressive forces generated in the lumbar spine. The footprint (e.g. diameter D) of the hoops  202 ,  204  is preferably configured such that the hoops  202 ,  204  engage with the stiffer peripheral regions of the vertebral endplate  62  while leaving the central endplate open for diffusion into the nucleus. The footprint of hoops  202 ,  204  may be circular, or elliptical in shape to match virtual cavity  70  produced after nucleus removal. 
     The sides, or lateral members  206  of the implant  200  are preferably made of flexible nitinol wires that allow the implant to collapse as shown in  FIG. 26  to allow for a minimal profile for installation of the stent  200  into the nuclear cavity. 
     The stent  200  is preferably inserted through an annular portal  68 , as shown in  FIG. 11A , then expand once in the nuclear cavity  70 . Prior to insertion of the stent  200 , a minimally invasive postero-lateral percutaneous discectomy removing the nucleus pulposus tissue  64  to create the nuclear cavity  70  as described in the above text associated with  FIG. 10A . 
     The axial stiffness of the stent  200  is preferably only sufficient to partially unload the disc. Thus, the stent  200  is generally not configured to act like a rigid interbody fusion cage, but rather a flexible cage to allow movement while at the same time keeping the nuclear space  70  open for tissue regeneration. 
     In another embodiment illustrated in perspective view  FIG. 28A , stent  210  may be specifically configured to be implanted between adjacent cervical vertebra. As shown in a top view in  FIG. 28B , stent  210  is preferably elliptical in shape to match the perimeters of the vertebral bodies. Because treatment of cervical vertebrae often involves removal of much or all of the annulus, the stent  210  preferably has a larger footprint to extend to the perimeter of the vertebral bodies. To help retain the stent  210  from moving with respect to the vertebra, the top hoop  212  and bottom hoop  214  may have serrations  215  to catch the bony vertebral endplate surfaces. Serrations  215  may be in the form of grooves, hook-like protrusions, or a roughed (e.g. bead-blasted) surface to increase friction between the stent  210  and the vertebral bodies  60 . For additional retention, the top hoop  212 , and/or the bottom hoop  214  may have a flange  216  that extends to the anterior, exterior wall of the vertebral body  60 . The flange  216  may have a mounting hole  218  to allow for screw fixation into the anterior wall of the vertebral body  60 . 
     The size, stiffness, and geometry of stents  200 ,  210  may also be varied to accommodate different patients, or to produce different therapeutic effect. The stents  200 ,  210  may also be coated with appropriate bioactive factors to facilitate healing, such as TGF-b, FGF, GDF-5, OP-1, or factors that reduce inflammation. 
     The stent  200 ,  210 , may be a stand-alone device that is used to enhance disc stability while facilitating nuclear regeneration. For example, this stent  200  could be placed after discectomy to facilitate disc repair in a physiologic configuration. The stent may also be used in conjunction with stem cells and polymer carriers to regenerate the nucleus 
     In an alternative embodiment, the stent  200 ,  210  may be used to provide additional mechanical support for the biodegradable membrane  10  described in  FIGS. 1-24 , also described in PCT Application WO 2003/002021, published on Jan. 9, 2003, incorporated herein by reference in its entirety.  FIG. 28E  illustrates the membrane  10  disposed within stent  210  with respect to the cervical vertebrae body. The stent  210  supports the peripheral expansion of the bladder  10  and holds it in place. This is particularly beneficial in cervical vertebrae implants where most of the host annulus (which would otherwise provide lateral support for the bladder  10 ) is removed. Thus the membrane  10  generally supports spine compressive loads, while the stent  210  prevents membrane  10  migration or lateral expansion. 
     As shown in  FIG. 29 , the stent  200 ,  210  may be placed in a collapsed position over deflated membrane  10 , and then inserted into the nuclear cavity via insertion catheter (cannula)  72 . Once the target region is reached, the membrane  10  may be inflated with filler material, thereby releasing the stent  200 ,  210  from its collapsed state into its expanded state. 
     Alternatively, the stent  200 ,  210  may be placed into the nuclear space  70  in its collapsed state by itself, as shown in  FIG. 27 . Subsequently, after the stent  200 ,  210  is expanded in the nuclear space, the membrane may be inserted (as shown in  FIGS. 11A-11C ) into the nuclear space  70  between the upper and lower loops  202 ,  204  of the stent  200 . 
     The stent of the present invention is particularly advantageous, since no interdiscal stent exists that could work synergictically with surrounding tissues while providing space and the appropriate mechanical environment to facilitate disc regeneration. 
     3. Surface Texturing as a Means to Stabilize a Nuclear Implant 
     In a further embodiment of the invention, the surface of the nuclear implant  10  described in  FIGS. 1-24  could be textured using a foaming agent along with a lower viscosity formulation of the polyurethane to formulate an enhanced implant  220 , as illustrated in  FIG. 30 . 
     As a final stage of dip manufacturing, the implant may be dipped into a foamed, uncured polyurethane, forming a final textured surface finish, or layer  224  outside of membrane  222 . The final surface texture of the outside layer  224  would typically have an average pore size in the range of approximately 400 microns to approximately 800 microns, volume porosity in the range of approximately 75% to approximately 80%, and thickness of approximately 1 mm to approximately 2 mm. This texturing would facilitate fibrous tissue ingrowth. 
     The above process may be used to augment mechanisms to stabilize the nuclear implant described above in  FIGS. 1-24 . The texturing may also be a means to provide growth factors to encourage tissue encapsulation. For example, the implant  220  could be dipped into a growth factor solution prior to implantation. Alternatively, the growth factor could be bound to the textured surface  224 . 
     It is further appreciated that the above described texturing could be also used in combination with other implants known in the art, both in spinal applications, and in other anatomical locations where promoting in growth with surrounding tissues is desirable. 
     4. Nuclear Implant Filler with Microbubbles/Microspheres 
     Referring now to  FIG. 31 , small microbubbles or microspheres  234  could be incorporated into the gel filler  232  of implant  230  (or implant  10  shown in  FIGS. 1-24 ). The microspheres  234  may be gas filled to provide a measure of compliance. The microspheres  234  may also be liquid filled to serve as a reservoir of hydration to help maintain gel hydration over the long term. The chemistry and/or geometry of the bubbles microspheres are configured such that the movement of fluid between microspheres  234  and hydrogel  232  is ‘dynamic’ and dependent on factors such as hydrogel pressure or hydration. For example, it may be of benefit for microspheres  234  to give off water when the hydrogel pressure is high, as a means to maintain implant volume (since high pressure may tend to cause hydrogel to give off water to the external environment). 
     In an alternative embodiment, the microspheres  234  may serve as a reservoir for drugs having appropriate bioactive factors to facilitate healing to further enhance the performance of the gel filler  232 . 
     It is further appreciated that the microspheres  234  may be used for any inflatable implant currently used in the art. 
     5. Nuclear Implant Bladder with Peripheral Reinforcement 
       FIG. 32  illustrates an implant  240  in accordance with the present invention having peripheral reinforcement. For example, top and bottom walls  242  may have the same thickness T 1  as bladder  10  shown in  FIGS. 1-24 . Accordingly, side, or peripheral walls  244  may have a different thickness T 2  around the circumference of the bladder. The periphery, or lateral margins  244  of the bladder  240  may be fabricated with a thickened region T 2  to provide localized stiffness. 
     This increased peripheral thickness may have several beneficial effects, including preventing extrusion, or increasing fatigue resistance. This thickened peripheral edge  244  may also serve to provide device stiffness in an “under inflation situation”. The peripheral thickening may further be configured to cause nonlinearity in overall device stiffness, such as during extreme bending or compression, that would improve overall intervertebral stability. It will be appreciated that an advantage of this aspect of the invention is that peripheral stiffness will enhance mechanical performance. 
     This dual thickness construction may be incorporated in bladders having the self-sealing internal valve  16  of the present invention, as well as other implant bladders known in the art. 
     6. Nuclear Implant Bladder with Multiple Chambers 
     Referring now to  FIGS. 33 and 34 , implant  250  may be manufactured to have multiple chambers instead of a single bladder. For example, implant  250  may have an internal chamber  256  positioned at the center of the implant, and a peripheral chamber  258  surrounding internal chamber  256 , as shown in side cross-section view in  FIG. 33 , and top cross-section view in  FIG. 34 . 
     To facilitate filling of the chambers, implant  250  may have a peripheral valve  252  allowing access to the peripheral chamber  258 , and a central valve  254  allowing access to internal chamber  256 . Valves  252  and  254  are preferably concentric located with respect to each other, as shown in  FIGS. 33 and 34 . This facilitates delivery of the inflation medium to both chambers via the same annular portal  68  (shown in  FIG. 11A ) without having to reposition the implant  250 . Alternatively, the valves may be placed at differing locations 
     Valves  252  and  254  are also preferably integrated, internal, self-sealing valves as shown and described in  FIGS. 1-24 . However, a 2-piece valve bladder system, or any other bladder/valve configuration known in the art, may be used for the multi-chamber implant of the present invention. 
     In an alternative embodiment, either or both of the internal and peripheral chambers of implant  250  may also further be divided into a plurality of smaller chambers. 
     The bladders of implant  250  may also be configured to have differing stiffness. For example, the internal chamber  256  may be filled at a different pressure than the peripheral chamber  258 . Additionally, the central chamber  256  may be filled with a softer gel, while the peripheral  258  chamber is filled with a stiffer gel. External walls  262  encasing the peripheral chamber may also have differing or larger thickness than the internal walls  260  of the internal chamber  260 . Any of these configurations may be used to advantageously prevent occurrence of implant extrusion through an annular defect. 
     Finally, the implant  250  could be configured to have an inner mechanical support bladder in chamber  256 , and an outer drug delivery bladder in peripheral chamber  258 . Thus, the internal chamber  256  may be filled first with a hydrogel having properties that allow the chamber to reach the desired osmotic or swelling pressure, and then the outer chamber  258  is then filled with a liquid or gel carrying therapeutic agents. Potential drugs for delivery include tgf-b and gdf-5 to encourage tissue ingrowth and implant stability. Other choices include anti-inflammatory drugs to specifically target pain, such as Remicade (anti-tnf-alpha), or glucosamine. 
     In an alternative version shown in  FIG. 35 , implant  270  comprises an internal chamber  274  suspended inside peripheral chamber  272 . To maintain the central position of the internal chamber  274  with respect to the peripheral chamber  272 , supports  276  may connect the two chambers while still allowing the filler material to occupy the internal chambers of the implant. 
     The multiple bladder approach shown above also has the additional advantage of providing redundancy to the system. Separate chambers may act as a failsafe mechanism in the event that a single bladder fails. In this situation, the multiple bladders would prevent catastrophic failure, with the remaining bladder or bladders maintaining implant performance. 
     7. Method of Sealing or Repairing the Annulus Fibrosus 
       FIG. 36  illustrates a system  280  and method for annular repair (e.g. such as a annular portal  68  generated from an implant as described in the embodiments above, or a region of degenerated annulus) in accordance with the present invention. As illustrated in  FIG. 36 , one or more suture anchors  282  are first placed into vertebral rims  62  of opposing vertebral bodies  60  (also shown in cross-section view in  FIG. 37A ). The number of anchors may vary depending on the size of the repair to the annulus  66 . The anchors may be installed using a tool (not shown) that allows them to be placed simultaneously. For example, for 3 anchors on each vertebral rim, a pliers-type tool may be used with three tangs on each side (one for each suture anchor  282 ), each tang having a suture anchor  282  attached. The surgeon could open or close the pliers to accommodate different disc heights. 
     Once the anchors  282  are set, netting  282  (such as the cargo net  288  shown in  FIG. 37B ) is attached to the anchors  282  via sutures  284 . The cargo-net  288  is made of a woven mesh or fabric, which has a cross-ply that matches the annular architecture. One side of mesh (that is placed against the annulus tissue  66  may comprise a woven polymer such as polyethylene or polypropylene to promote tissue ingrowth (e.g. 800 micron pore size). Correspondingly, the opposite side (placed facing away from the annulus  66 ), may comprise a woven Teflon, or similar lubricant, to prevent adhesion. 
     The netting  288  is then stretched over the annulus defect  290 , and the free-ends of the sutures  284  are pulled to adjust the fit of the netting  288 . This may be facilitated using a ‘cable-tie’ type fastener  286  (in addition to, or in lieu of sutures  284 ), illustrated in further detail in  FIG. 37C . The system  280  allows the netting  288  to give during intervertebral movement, thus not unduly constraining the patients natural range of motion, nor unduly stressing the anchor points. 
     In one embodiment, one of several surgical sealants known in the art may be placed between the mesh  288  and the outer annulus  66 . 
     As an alternative using suture anchors, the surgeon may instead suture directly through and around the vertebral rims  282 . 
     In some instances, the vertebral bodies  60  may be avoided altogether, and sutures  284  may be installed directly through the annulus  66 . This may be facilitated using minimally-invasive suturing techniques similar to those currently employed for rotator cuff repair. For example, Opus Medical (www.opusmedical.com) describes an ‘AutoCuff System’ that includes a tool and technique for automated tissue suturing through a narrow/deep tissue channel (this constraint will likely accompany most disc repair surgical techniques). A similar device may be configured for suturing the annulus fibrosus, having customized tips and implant anchors that optimize the repair strength for the disc. 
     It is appreciated that system and methods illustrated in FIGS.  36 A and  37 A-C may be used as a stand-alone technology to seal an annular defect after discectomy. Alternatively, the system may be used to “finish up” insertion of a nuclear implant by sealing the annular defect. 
     It is appreciated existing annular repair approaches attempt to attach to annulus only. Since the quality of the annulus in many cases may be poor, these methods have a high possibility of failure. With the present invention, repair is facilitated by attaching to the vertebral margins in a manner similar to the natural annulus. The approach of the present invention is expected to provide better sealing ability, particularly in situations when the annulus is weakened. 
     Although the description above contains many details, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. For example, collagen could be used instead of polymer, and polylysine or type 2 collagen with a cross-linking agent could be used instead of hydrogel. Therefore, it will be appreciated that the scope of the present invention fully encompasses other embodiments which may become obvious to those skilled in the art. In the appended claims, reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more.” All structural, chemical, and functional equivalents to the elements of the above-described preferred embodiment that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Moreover, it is not necessary for a device or method to address each and every problem sought to be solved by the present invention, for it to be encompassed by the present claims. Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element herein is to be construed under the provisions of 35 U.S.C. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for.” 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Elastomer Properties 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 Tensile 
                 Tensile 
                   
               
               
                   
                   
                   
                 Modulus 
                 Modulus 
                 strength 
                 Strength 
                 Elongation 
               
               
                 Material 
                 Description 
                 Supplier 
                 (psi) 
                 (MPa) 
                 (psi) 
                 (MPa) 
                 (%) 
               
               
                   
               
               
                 Inner 
                   
                   
                   
                 5 to 10 
                   
                 1 to 3 
                 10 to 20 
               
               
                 Annulus 
               
               
                 HT-3 
                 aliphatic 
                 Apex 
                 295.00 
                 2.03 
                 5300.00 
                 36.54 
                 470.00 
               
               
                   
                 polycarbonate 
                 Medical 
               
               
                   
                 polyurethane 
               
               
                 HT-4 
                 aliphatic 
                 Apex 
                 990.00 
                 6.83 
                 7100.00 
                 48.95 
                 375.00 
               
               
                   
                 polycarbonate 
                 Medical 
               
               
                   
                 polyurethane 
               
               
                 HT-6 
                 polycarpralactone 
                 Apex 
                 290.00 
                 2.00 
                 5800.00 
                 39.99 
                 850.00 
               
               
                   
                 copolyester 
                 Medical 
               
               
                   
                 polyurethane 
               
               
                 HT-7 
                 aromatic polyester 
                 Apex 
                 340.00 
                 2.34 
                 9000.00 
                 62.06 
                 550.00 
               
               
                   
                 polyurethane 
                 Medical 
               
               
                 HT-8 
                 aliphatic polyether 
                 Apex 
                 290.00 
                 2.00 
                 5500.00 
                 37.92 
                 710.00 
               
               
                   
                 polyurethane 
                 Medical 
               
               
                 HT-9 
                 aromatic polyester 
                 Apex 
                 550.00 
                 3.79 
                 7000.00 
                 48.27 
                 550.00 
               
               
                   
                 polyurethane 
                 Medical 
               
               
                   
               
            
           
         
       
     
                     TABLE 2                  Osmotic Pressure as a Function of Gel Formulation                                 Gel Formulation   [PEG]   [HA]   [CS]   Π (MPa)                                         1   3.6%   0.11%     —   0.011       2   5.0%   —   —   0.025       3   5.0%   —   0.68%     0.028       4   6.0%   —   —   0.033       5   7.5%   —   —   0.052       6   7.5%   2%   —   0.080       7   7.5%   —    6%   0.130       8   7.5%   3%   —   0.155       9   7.5%   —   11%   0.220       10     9%   —   13%   0.310       11    10%   —   15%   0.332                    
The additives in formulation #8 consisted of a pre-swollen HA-PEG gel that was dried then finely cut and incorporated into a new PEG gel.