Abstract:
An expandable implant for augmenting a damaged or otherwise torn posterior annulus fibrosus. The implant is deployable into an evacuated posterior disc space following a subtotal nuclectomy. The implant includes an inflatable balloon which is fixed to the annulus fibrosus to protect against migration. When inflated with a curable polymer, the posterior wall of the implant is intimately disposed along the inner surface of the annulus fibrosus to provide a substantially fluid-tight seal of the annular tear or defect and reinforce the weakened posterior annulus. The anterior wall of the implant sequesters the nuclear remnant to guard against disc remnant herniation. The implant can restore intradiscal pressure and disc height. An integrated posterior reinforcement band creates a strong, yet flexible and resilient structure, resistant to penetration of the implant through the annular tear or defect.

Description:
BACKGROUND 
     1. Field of the Disclosure 
     The present disclosure relates generally to spinal surgery and more particularly to a method and apparatus for reinforcing a torn or damaged annulus fibrosus. 
     2. Description of Related Art 
     Spinal discs typically have an intact annulus fibrosus throughout life that confers normal resistance to applied forces. Degenerating discs, however, exhibit mechanical failure of the disc. A common mechanical failure is radial tears in the annulus fibrosus. 
     Radial tears of the annulus fibrosus are characteristic of all degenerating intervertebral discs and are often associated with loss of disc height, segmental instability and pain. By definition, a radial tear is a defect that extends through all layers of the annulus fibrosus, from its interior margin to the periphery of the disc. 
     Radial tears are found most commonly in the lumbar spine and are most prevalent at the L4-L5 and L5-S1, disc levels. Most radial tears involve the posterior annulus and occur without disc herniation, even in patients with clinical findings typical of a herniated disc. In the lumbar region, pain is often referred to a lower extremity. The referred pain may be difficult to distinguish clinically from radiculopathy of a herniated nucleus pulposus compressing a spinal nerve. 
     Breakdown of the nucleus pulposus generates fluids and tissues that may leak outward through the radial tear, causing inflammation of adjacent meninges and innervated granulation tissue along the radial tear. 
     The disruption of the annulus fibers by a radial tear is associated with loss of intradiscal pressure and disc height resulting in hypermobility or instability of the motion segment at the level of the affected disc. It is postulated that the spinal instability permitted by a radial tear may cause pain by straining the facet joints and connective tissues that extend across the degenerating disc. 
     The associated increased mobility may produce intermittent nerve root compression at spinal levels containing a radial tear. Intermittent occult spinal stenosis, lateral recess stenosis and neural foraminal stenosis may occur during changes in posture and physiologic loading of the spine. 
     Functional tests such as discography may demonstrate radial tears at multiple levels and identification of the symptomatic radial tear may be difficult to identify. The value of discography for identifying the symptomatic disc level and for predicting the outcome from spinal fusion has been debated. 
     Physicians sometimes make iatrogenic holes or annulotomies in various sizes and shapes and locations into the annulus as part of surgical procedures to address contained disc herniation or symptomatic radial tears. 
     In recent years, there has been increasing interest in repair, regeneration and support of the torn annulus fibrosus to restore the normal function of the disc. There is also interest in repairing radial tears or postsurgical annular defects to prevent recurrence of disc herniations and to slow the degenerative process after nuclectomy, in the context of successful treatment of different types of painful spinal conditions. 
     Annular repair, whether biologic or surgical, is a developing technology to address symptomatic annular tears, with or without disc herniations, or in surgical treatments of spinal conditions. 
     A number of surgical techniques and implants are currently under development and additional clinical data are expected in the next several years. Absorbable gelatin sponge (i.e., gel foam) appears to have promise. However, clinical testing has only been done on small size defects, and it is difficult to extrapolate this model to a nuclectomy. Sutures with anchors (for example, Xclose and Inclose, Anulex Technologies, Inc., Minnetonka, Minn.) have been introduced commercially and are currently undergoing a U.S. Food and Drug Administration (FDA) trial. Barricaid (Intrinsic Therapeutics, Inc., Woburn, Mass.) is a commercially available implant that anchors into the vertebral body and supports a woven mesh barrier inserted into the defect. 
     In an effort to reduce tissue damage associated with surgical intervention, there is a need to utilize minimally invasive surgical access or percutaneous access. In younger patients with an annular tear or disc herniation who have an otherwise undamaged or minimally degenerated annulus fibrosus and nucleus pulposus, a partial or targeted nuclectomy may be preferable to a total nuclectomy. 
     Current surgical procedure approaches, access and tools limit the surgeon&#39;s ability to determine the position, size and shape of the cleared disc space. This lack of precision presents difficulty in designing, sizing, placement and securement of a nuclear implant. Improper nuclear implant placement may interfere with biomechanical behavior of the spinal segment. This also increases the likelihood of migration and expulsion of the implant. 
     In sum, an annular tear or defect is clinically significant and requires treatment. However, commercially available treatments for an annular tear or other damage to an annulus fibrosus have many problems and/or disadvantages. Various embodiments of the present disclosure may mitigate or solve one or more of the problems and/or disadvantages. 
     SUMMARY 
     According to an exemplary aspect of the present disclosure, an apparatus for reinforcing a damaged annulus fibrosus comprises an inflatable balloon having a first end and a second end, wherein a posterior portion of the inflatable balloon is adapted to seal against an annulus fibrosus; a first anchor coupled to the first end of the inflatable balloon, the first anchor being adapted to anchor the first end of the inflatable balloon to a first location of the annulus fibrosus at a first lateral side of the annulus fibrosus; and a second anchor coupled to the second end of the inflatable balloon, the second anchor being adapted to anchor the second end of the inflatable balloon to a second lateral side of the annulus fibrosus. 
     The posterior portion of the inflatable implant may comprise a reinforcing layer coupled to a posterior side of the balloon so that the posterior portion is substantially non-compliant, and an anterior portion of the balloon may comprise a silicone material so that the anterior portion is substantially compliant. 
     The inflatable balloon may comprise any elastomeric biocompatible material suitable for long-term human implantation, such as silicone or PTFE. The inflatable balloon may comprise an electrospun polymeric material. The inflatable balloon may further comprise a carbon layer disposed on an inner surface of the inflatable balloon. The PTFE material may be porous. 
     The first anchor may comprise at least one inflation port for injecting an inflation material into the inflatable balloon. 
     The second anchor may further comprise at least one inflation port for injecting an inflation material into the inflatable balloon. 
     A curable material may be disposed in the inflatable balloon. A gas may be disposed in the inflatable balloon. 
     The inflatable balloon may comprise an anterior portion with different properties than the posterior portion. The posterior portion may be less compliant than the anterior portion. The posterior portion may comprise multiple layers. 
     The inflatable balloon may comprise multiple layers. The multiple layers may be adapted to provide different properties to the inflatable balloon. 
     An outer layer of the posterior portion may comprises an opening for allowing an inner layer of the posterior portion to protrude therethrough upon inflation. 
     The first anchor may comprise an expanded portion for preventing the first anchor from being pulled through the annulus fibrosus thereby mitigating against migration or expulsion. 
     The second anchor may comprise a retaining member with an expanded portion for helping to prevent the second anchor from being pulled through the annulus fibrosus. The retaining member may comprise a retaining ring with at least one opening for slidably receiving a slidable plate with at least one aperture for retaining a string extending from the second anchor. 
     The inflatable balloon may comprises multiple layers. The multiple layers may be adapted to provide different properties to the inflatable balloon. 
     The inflatable balloon may comprise a first chamber and a second chamber disposed entirely within the first chamber. 
     The second anchor may comprise first and second loops of material extending from the second anchor. A retaining member for engaging one of the first and second loops of material may be provided. 
     According to another exemplary aspect, the present disclosure is directed to a method of reinforcing the posterior of an annulus fibrosus, comprising inserting first and second cannulas through first and second annulotomies located on first and second sides of the posterior of an annulus fibrosus to gain access to a nuclear disc space; performing a subtotal nuclectomy to create a cavity extending between the first and second cannula, wherein a posterior side of the cavity is formed by the annulus fibrosus and the anterior side of the cavity is formed by a nuclear remnant; inserting an inflatable implant into the cavity formed by the subtotal nuclectomy so that a first end of the implant is located at the first annulotomy and a second end of the implant is located at the second annulotomy; inflating the implant with a curable medium so that implant substantially fills the cavity and presses against the annulus fibrosus to form a substantially fluid tight seal with the annulus fibrosus; anchoring the first end of the implant to the first annulotomy; and anchoring the second end of the implant to the second annulotomy. 
     The posterior portion of the annulus fibrosus may have a defect, and the first and second annulotomies may be spaced away from the defect on first and second sides of the defect. 
     The first and second annulotomies may be located on the posterolateral sides of the annulus fibrosus. 
     The steps of anchoring the first and second ends of the implant to the first and second annulutomies may comprise removing the first and second cannulas so that the first and second annulotomies engage the first and second ends of the implant. 
     The step of inserting the inflatable implant into the cavity may comprise deploying the implant into the cavity through the first cannula and may further comprise snaring the end of the implant through the second cannula and maneuvering the balloon into position. 
     The step of anchoring the second end of the implant may comprise attaching a retaining member to the second end of the implant, wherein the retaining member is larger than the second annulotomy. 
     The step of expanding the implant may further comprise inflating an interior chamber with a gaseous medium. 
     According to another exemplary aspect, the present disclosure is directed to a method of reinforcing a posterior annulus fibrosus comprises accessing a nuclear space formed by the annulus fibrosus; performing a partial nuclectomy to create a cavity extending from a first lateral side of the annulus to a second lateral side of the annulus, wherein a posterior side of the cavity is formed by an inner wall of the annulus fibrosus and an anterior side of the cavity is formed by a nucleus remnant; inserting an inflatable balloon with a first end and a second end into the cavity formed by the partial nuclectomy; inflating the inflatable balloon with a curable medium; anchoring the first end of the inflatable balloon to the annulus fibrosus on the first lateral side of the annulus fibrosus; and anchoring the second end of the inflatable balloon to the annulus fibrosus on the second lateral side of the annulus fibrosus. 
     The step of accessing the nuclear space may comprise obtaining bilateral posterolateral percutaneous access with first and second cannulas at the first and second sides of the annulus fibrosus. 
     The inflatable balloon may extends from the first cannula to the second cannula. 
     The method may further comprise withdrawing the first and second cannulas, so that first and second annulotomies engage the first and second ends of the balloon to anchor the first and second ends of the balloon to the annulus fibrosus. 
     The inflatable balloon may be deployed through the first cannula into the cavity. The balloon may be snared through the second cannula and maneuvered into position. 
     A portion of the inflatable balloon which is adapted to protrude from the inflatable balloon may be aligned with an annular defect, and the inflatable balloon may be inflated so that the balloon protrudes into the annular defect. The annular defect may comprise a defect caused by one of a herniated nucleus pulposus or an iatrogenic defect. 
     The balloon may form a substantially fluid tight seal with the annulus fibrosus. 
     The balloon may substantially prevent leakage of fluids out of the posterior of the annulus fibrosus. 
     In another aspect of the present disclosure, an anchor for anchoring a nuclear implant to an annulus fibrosus through an annulotomy may comprise an anchor member sized to fit through the annulotomy; at least one loop of material extending from the anchor member; and a retaining member with an aperture for receiving and engaging the at least one loop of material, the retaining member being larger than the annulotomy. 
     The retaining member may comprise a retaining ring and at least one movable member which is movable between an open position and a closed position, wherein the movable member has an opening for receiving the at least one loop of material so that loop of material is movable when the plate is in an open position and immovable when the plate is in a closed position. 
     The at least one loop of material may comprise an anchor loop. The at least one loop of material may further comprise a second loop of material for allowing a user to snare the second loop of material. 
     The term “coupled” is defined as connected, although not necessarily directly. The terms “a” and “an” are defined as one or more unless this disclosure explicitly requires otherwise. The terms “substantially,” “approximately,” and “about” are defined as largely but not necessarily wholly what is specified (and includes what is specified; e.g., substantially 90 degrees includes 90 degrees and substantially parallel includes parallel), as understood by a person of ordinary skill in the art. In any disclosed embodiment, the terms “substantially,” “approximately,” and “about” may be substituted with “within [a percentage] of” what is specified, where the percentage includes 0.1, 1, 5, and 10 percent. 
     The terms “comprise” (and any form of comprise, such as “comprises” and “comprising”), “have” (and any form of have, such as “has” and “having”), “include” (and any form of include, such as “includes” and “including”) and “contain” (and any form of contain, such as “contains” and “containing”) are open-ended linking verbs. As a result, a system, or a component of a system, that “comprises,” “has,” “includes” or “contains” one or more elements or features possesses those one or more elements or features, but is not limited to possessing only those elements or features. Likewise, a method that “comprises,” “has,” “includes” or “contains” one or more steps possesses those one or more steps, but is not limited to possessing only those one or more steps. Additionally, terms such as “first” and “second” are used only to differentiate structures or features, and not to limit the different structures or features to a particular order. 
     A device, system, or component of either that is configured in a certain way is configured in at least that way, but it can also be configured in other ways than those specifically described. 
     Any embodiment of any of the systems and methods can consist of or consist essentially of—rather than comprise/include/contain/have—any of the described elements, features, and/or steps. Thus, in any of the claims, the term “consisting of” or “consisting essentially of” can be substituted for any of the open-ended linking verbs recited above, in order to change the scope of a given claim from what it would otherwise be using the open-ended linking verb. 
     The feature or features of one embodiment may be applied to other embodiments, even though not described or illustrated, unless expressly prohibited by this disclosure or the nature of the embodiments. 
     Details associated with the embodiments described above and others are presented below. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a sectional view of an annulus fibrosus with a defect and a disc herniation; 
         FIG. 2  is a sectional view of the annulus fibrosus of  FIG. 1  with the disc herniation removed; 
         FIG. 3  is a sectional view of the annulus fibrosus of  FIGS. 1 and 2  with a posterior portion of the disc removed to form a cavity for receiving an implant; 
         FIG. 4  is a sectional view of a step in delivering an implant into the disc space for receiving an implant; 
         FIG. 5  is a sectional view of another step in delivering an implant into the disc space for receiving an implant; 
         FIG. 6  is a sectional view of an implant after insertion; 
         FIG. 7  is a sectional view of an implant loaded into a delivery cannula; 
         FIG. 8  is a sectional view of an anchoring mechanism for anchoring the implant of  FIG. 4 ; 
         FIG. 9  is a sectional view of the anchoring mechanism of  FIG. 8 , in a compressed position in the delivery cannula; 
         FIG. 10  is a sectional view of another anchoring mechanism; 
         FIG. 11  is a plan view of a retaining member; 
         FIG. 12  is a plan view of the retaining member of  FIG. 11 , in an open position; 
         FIG. 13  is a plan view of the retaining clip of  FIG. 11 , in a closed position; 
         FIG. 14  is a plan view of an alternative embodiment of a retaining member; 
         FIG. 15  is a sectional view of a retaining member in accordance with another embodiment, in an open position; 
         FIG. 16  is a sectional view of the retaining member of  FIG. 15 , in an open position; 
         FIG. 17  is a sectional view of two retaining members for use in the retaining member of  FIG. 15 ; 
         FIG. 18  is a sectional view of the retaining member of  FIG. 15  taken along line  18 - 18 ; 
         FIG. 19  is a top view of another embodiment of an anchoring mechanism; 
         FIG. 20  is a side view of the anchoring mechanism of  FIG. 19 ; 
         FIG. 21  is a sectional view of another embodiment of an implant for repairing an annulus fibrosus, before inflation; 
         FIG. 22  is another sectional view of the implant of  FIG. 21  taken along line  21 - 21 , before inflation; 
         FIG. 23  is a sectional of the implant of  FIG. 21 , after inflation; 
         FIG. 24  is another sectional of the implant of  FIG. 21  taken along line  24 - 24 , after inflation; 
         FIG. 25  is a sectional view of the implant of  FIG. 21 , after implantation and inflation; 
         FIG. 26  is a sectional view of another embodiment of an implant for repairing an annulus fibrosus, before inflation; 
         FIG. 27  is a sectional view of the implant of  FIG. 23 , after inflation; and 
         FIG. 28  is a sectional view of the implant of  FIG. 23 , after implantation and inflation. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings, in which are shown exemplary but non-limiting and non-exhaustive embodiments of the invention. These embodiments are described in sufficient detail to enable those having skill in the art to practice the invention, and it is understood that other embodiments may be used, and other changes may be made, without departing from the spirit or scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the invention is defined only by the appended claims. In the accompanying drawings, like reference numerals refer to like parts throughout the various figures unless otherwise specified. 
       FIGS. 1-3  illustrate a process for preparing a spinal segment with a defect in the annulus for receiving an implant according to an embodiment of the present disclosure. Referring to  FIG. 1 , annulus fibrosus  100  has a defect  102  in the posterior portion  104  of annulus fibrosus  100 .  FIG. 1  illustrates a defect  102  with a herniated nucleus pulposus; that is nucleus pulposus  106  has extruded through the annular defect  102 , forming a disc herniation  108 . While a herniated nucleus pulposus has been illustrated here, it should be understood that defect  102  can be any defect such as a radial tear, a fissure, an opening or any other type of flaw in an annulus fibrosus, and includes those resulting from degenerative disc disease, trauma, or other conditions. 
     A first access cannula  110  and a second access cannula  112  are inserted through annulotomies in annulus fibrosus  100  to provide bilateral posterolateral percutaneous access to the disc cavity from a first posterolateral side of annulus  100  and a second posterolateral side of annulus  100 . First and second access cannulas  110 ,  112  are preferably inserted through the safe zone defined by Kambin&#39;s triangle. First and second access cannulas  110 ,  112  are spaced away from annular defect  102  so that they are not immediately adjacent to defect  102 . The access cannulas  110 ,  112  may be inserted by using sequentially dilating cannulas, by minimally invasive surgical approaches, or by other techniques known to those skilled in the art. The use of sequentially dilating cannulas to create annulotomies spreads the fibers of the annulus fibrosus without severing them. This allows the fibers to close when the cannulas are removed. 
     Referring to  FIGS. 2 and 3 , an initial step is to remove disc herniation  108  (if present). This may be accomplished using conventional surgical techniques known to those skilled in the art. Next, a partial nuclectomy is performed to remove the posterior portion  114  of nucleus pulposus  106 , thereby leaving nucleus remnant  162 . In some embodiments, the partial nuclectomy is performed using percutaneous techniques through first and second access cannulas  110 ,  112 . This allows the controlled selective nuclectomy of the posterior nucleus, sparing the otherwise healthy nucleus pulposus  106  and nuclear remnant  162 . 
     The partial nuclectomy creates a cavity  116  for receiving an implant  118 . For clarity, cavity  116  is illustrated as a relatively large cavity in the accompanying figures; however, it should be understood that the cavity  116  is preferably a small cavity (for example, it may be approximately 1-2 mm wide). The posterior side of cavity  116  is formed by the inner wall of annulus fibrosus  100 , while the anterior side of cavity  116  is formed by nucleus remnant  162 . A physician may use a sizing balloon (i.e., a highly compliant balloon) with a contrast solution to measure the size and shape of cavity  116  while performing the nuclectomy to help ensure cavity  116  is properly formed, and to help determine the amount of curable material to be used in the implant. 
     Referring to  FIGS. 4-5 , an implant  118  is delivered to the disc cavity  116  through first and second access cannulas  110 ,  112 . As shown in  FIG. 4 , deflated implant  118  is inserted through first access cannula  110  into disc cavity  118  so that second end  122  of implant  118  extends out of first access cannula  110  and into disc cavity  116 . Second end  122  of implant  118  is then snared or otherwise captured using tools inserted through second access cannula  112  using techniques known to those skilled in the art, and implant  118  is the pulled through disc cavity  116  into the desired position. Second end  122  may have a loop of string or another retaining member to assist in snaring second end  122  and pulling it into the distal end of the second access cannula. One or more radiopaque markers may be provided on implant  118  to assist in placing implant  118  at a desired location. 
     Implant  118  comprises an elastomeric balloon  124  extending from first end  120  to second end  122 . A first anchor  126  is coupled to first end  120  of balloon  124  and a second anchor  128  is coupled to second end  122  of balloon  124 . First anchor  126  includes an inflation port  130  (see  FIG. 7 ) for inflating balloon  124 . First and second anchors  126 ,  128  may be formed integrally with balloon  124  or may be formed separately and then coupled to balloon  124 . 
     After implant  118  is placed in the desired position, an inflation stylus  144  which is inserted into inflation port  130  is used to deliver inflation material  132  to inflate balloon  124 . Preferably, inflation stylus  144  is inserted into inflation port  130  before delivery through the catheter (i.e., it is pre-assembled, as shown in  FIG. 7 ). In some embodiments, inflation material  132  is a curable silicone material. Inflated balloon  118  entirely fills cavity  116  so that it presses against the inner wall of annulus fibrosus  100 , against the posterior side of nucleus remnant  162 , and against the superior and inferior endplates. Preferably, the inflation takes place under observation (e.g., fluoroscopy), so that the balloon  124  may be inflated in an amount and pressure sufficient to substantially restore normal intradiscal pressure, normal disc height (i.e., the distance between adjacent vertebral endplates) and angulation, and normal biomechanical function. A second inflation port (not shown) may be utilized for inflating gas into a second balloon chamber (which is described in further detail below). 
     The implant  118  presses against and forms a substantially fluid tight seal with the inner wall of annulus fibrosus  100  to reinforce the posterior annulus fibrosus  100 . If a defect  102  is present, implant  118  closes the defect  102 . Preferably, implant  118  stretches across substantially all of the posterior portion of the annulus fibrous so that implant  188  reinforces the entire posterior of annulus fibrosus  100 . This helps to reduce the risk of recurrent disc herniation. Sealing the nuclear space helps preserve intradiscal pressure and minimize seepage of nucleus pulposus breakdown products through the annular tear or defect, which can cause patient discomfort. 
     After inflation material  132  is cured, first and second access cannulas  110 ,  112  are removed, and as shown in  FIG. 6 , first and second anchors  126 ,  128 , are anchored to first and second openings (or annulotomies)  134 ,  136  formed by the first and second access cannulas  110 ,  112 . When annulotomies  134 ,  136  are formed by spreading the fibers of annulus fibrosus  100 , the fibers tighten and contract when cannulas  110 ,  112  are removed due to the elasticity of the fibers of annulus fibrosus  100 , and thereby engage first and second ends  120 ,  122  of balloon  124 . The widening of the disc space (i.e., the restoration of normal disc height) contributes to the tightening of the fibers of annulus fibrosus  100 . This fixates ends  120 ,  122  to annulus fibrosus  100  on opposite sides of defect  102  at annulotomies  134 ,  136  which are spaced away from defect  102  towards the lateral sides of annulus  100 . Furthermore, in the case of a herniated nucleus pulposus, anchors  126 ,  128  are spaced away from the surgical site used to remove disc herniation  108 . Anchors  126 , and  128  help reduce the risk of implant  118  from being expelled from disc cavity  116  or migrating within disc cavity  116 . 
     Referring to  FIGS. 8-10 , first anchor  126  and second anchor  128  may have additional features to minimize the possibility of the anchors being pulled through annulus fibrosus  100 . First anchor  126  may comprise a cylindrical stem portion  142  with a flange  140 . During insertion, cylindrical stem portion  142  is stretched over inflation stylus  144  to open inflation port  130  to allow curable material to flow into balloon  124 . When inflation stylus  144  is retracted, cylindrical stem portion  142  collapses to act as a valve to close inflation port  130  to seal balloon  124  and prevent inflation material  132  from escaping balloon  124 . The inflation stylus and stem portion  142  may have features to prevent inadvertent disconnection or leakage (such as an enlarged discharge end or corresponding groove and slot features). Flange  140  is compressed by the delivery cannula and access cannula  110  during delivery. Upon removal of inflation stylus  144  and access cannula  110 , flange  140  expands to form an expanded portion  146  which help prevent cylindrical stem portion  142  from being pulled through annulus fibrosus  100 . 
     Referring to  FIGS. 10-14 , second anchor  128  is coupled to second end  122  of balloon  124 . Second anchor  128  may be a cylinder or any other suitable shape. Second anchor  128  is delivered through the disc space  116  and into the distal end of the second cannula. Second anchor  128  may have a flange or expanded portion such as first anchor  126 . Alternatively, second anchor  128  may use a different anchoring mechanism. In one embodiment, second anchor  128  comprises a loop of material with first and second sides  170 ,  172  which can be formed of a surgical suture material. A physician may snare the loop and use the loop to pull the second anchor into the second cannula during the implantation procedure. A retaining member (or clip)  174  can be attached to the strings and drawn tight against annulus fibrosus  100  to help hold second anchor  128  in place. In some embodiments, retaining member  174  comprises a first member  176  with a sliding plate  178  slidably connected to it. First member  176  has at least one and preferably two apertures  180 . Sliding plate  178  has at least one and preferably two apertures  182 . A biasing member  184  biases sliding plate  178  outwardly (as shown in  FIG. 13 ). When sliding plate  178  is squeezed close, apertures  180  and  182  are aligned, and first and second sides  170 ,  172  of the loop of surgical suture material can be placed through the apertures. A user must cut the loop into two separate strings  170 ,  172  so that they can be placed through the apertures. When released, plate  178  is biased outward so that strings  170 ,  172  are trapped by apertures  180 ,  182  (see  FIG. 12 ). A user may slide retaining member  174  down strings  170 ,  172  until it is tight against the outer margin of annulus fibrosus  100 . In some embodiments, the outer diameter of retaining member  174  in the open position is substantially the same size as the inner diameter of second access cannula  112  so that sliding plate  178  is held into the open position until it exits the distal end of second access cannula  112 . Although two strings are shown in the illustrated embodiment, any number of strings may be used, from one or more. Similarly, retaining members with a different number of sliding plates may be used, as shown in  FIG. 14 , which illustrates a retaining member  190  with two sliding plates  192 . 
       FIGS. 15-20  illustrate another embodiment of a retaining member and an anchoring mechanism. Retaining member  194  comprises an outer retaining ring  196  with first and second opening  198 ,  200  for receiving first and second sliding members  202 ,  204 , respectively. First and second sliding members  202 , and  204  slide in and out (i.e., toward and away from the center of retaining ring  198 ), and are biased outward by biasing members such as springs (which are not illustrated for clarity). The outer diameter of retaining ring  196  is substantially the same as the inner diameter of a delivery cannula  206 . First and second sliding members  202 ,  204  are held in the open position (i.e., squeezed together so that slot  208  is open) by the delivery cannula  206 . Sliding members  202 ,  204  are preferably the same shape and configuration. Each sliding member comprises a flat member  210  with a protruding u-shaped member  212  which forms an aperture  214 . A tongue  216  extends from u-shaped member  212 . The u-shaped member  212  is offset from the centerline of the flat member  210  so that the sliding members have be inserted into retaining ring  196  in different orientations. This way, u-shaped members  212  form complementary sliding members (referring to  FIG. 18 ) and do not interfere with one another when placed in the open position. Retaining member  194  may be formed of any biocompatible material, such as stainless steel or polymeric material. 
     To use retaining member  194 , the sliding plates are squeezed together so that apertures  214  of the two plates are aligned and form slot  208 . String  170 ,  172  are placed through slot  208 , and slot  208  is loaded into cannula  206 . Retaining member  194  is then delivered down delivery cannula  206  by a pusher (not illustrated) until it is adjacent the second anchor  216 . The pusher can be a cannula with an outer diameter which fits snugly within delivery cannula  206 . While holding retaining member  194  firmly against second anchor  218 , the delivery cannula is removed, allowing the biasing members to close sliding members  202 ,  204  (i.e., press the sliding members outward to close slot  208 , as shown in  FIGS. 16 and 18 ). In some embodiments, retaining ring  196  and sliding plates  202 ,  204  may have locking features to lock the sliding plates in the closed position once the retaining member  194  is in place. Once sliding plates  202 ,  204  expand outward from retaining ring  196 , they form an enlarged portion which helps anchor the end of the implant and prevent the anchor from being pulled through the annulus fibrosus. 
       FIGS. 19 and 20  illustrate an anchor  218  which is particularly suitable for use with the retaining member of  FIGS. 15-18 . Anchor  218  comprises a cylindrical member  220  with an embedded loop of wire or string  222 . Loop  222  is preferably formed of a material which retains its shape, such as wire or stiff nylon. Loop  222  can be used to snare anchor  218  and pull it into the proper position within the distal segment of the second cannula, as described previously. Preferably, loop  222  is long enough that it can be pulled out the end of second delivery cannula  212 , although it is shown shorter in the illustrations for clarity. A second anchoring loop  224  is placed near the anchor and fastened to loop  222 . When anchor  218  is used with retaining member  194 , the tongues  216  extend into anchoring loop  224  when tongues  216  are closed to form a positive connection. Thus, instead of relying solely on friction between loop  222  and sliding plates  202 ,  204 , anchoring loop  224  is positively locked to retaining member  194 . 
     Referring to  FIGS. 21-25 , in some embodiments balloon  124  of implant  118  may comprise a multi-layered balloon. For example, balloon  124  may have a reinforcement layer  150  on the posterior portion  156  of the implant  118  to strengthen the implant and aid incorporation of the implant  118  into the annulus fibrosus. Reinforcement layer  150  may be located on the interior or exterior of balloon  124 . Reinforcement layer  150  may be any biocompatible material with a high strength, such as a woven, knitted or braided textile or an ePTFE material. Other fiber deposition techniques such as electrospinning may be used to form reinforcement layer  150 . After balloon  124  is inflated and the curable material is cured, the implant  118  comprises a flexible and resilient structure which is resistant to migration through annular defect  102 . 
     In some embodiments, balloon  124  comprises a silicone material. In some embodiments, balloon  124  comprises a PTFE material (such as an expanded PTFE material). A layer of carbon material may be included to enhance the bond between the PTFE material and the curable material. The porosity of the PTFE material may be controlled to allow partial penetration of the curable material while it is in the liquid state to create an integrated structure after curing which minimizes the possibility of delamination. Balloon  124  may be porous to allow the escape of air in the system or air trapped in the liquid silicone during mixing. 
     In some embodiments, balloon  124  may have differential and directional expandability to control the direction and amount of expansion of balloon  124 . In some embodiments, balloon  124  has segments of increased base and dimensional stability. In some embodiments, the surface of balloon  124  may have a unique surface topography to enhance desired characteristics. For example, the surface may have increased lubricity to reduce friction against vertebral endplate surfaces and enhance tissue in-growth in the annulus fibrosus. Balloon  124  may be formed in various shapes to fit different cavity configurations. 
     In some embodiments, balloon  124  is adapted to fill an annular defect. Outer layer  150  of balloon  124  may have at least one opening  152  through which inner layer  160  protrudes when the implant  118  is inflated within cavity  116 . By aligning opening  152  in outer layer  150  with defect  102  in annular fibrosis  100 , inner layer  160  protrudes out opening  152  to form a protrusion  154 . Protrusion  154  acts as a plug to fill the annular defect  102  and the associated cavity, which assists in treating annular defect  102 . In some embodiments, inner layer  160  of balloon  124  comprises a compliant material, such as a silicone membrane; and outer layer  150  comprises a material which is less compliant than inner layer  160 . In some embodiments, outer layer  150  is fiber reinforced. The opening  152  in less compliant outer layer  150  allows compliant inner layer  160  to expand therethrough. 
     In some embodiments, anterior wall  158  of balloon  124  is relatively thinner or more compliant than posterior wall  156  of balloon  124 . As balloon  124  expands, differential expansion occurs with more compliant anterior wall  158  expanding more than that of posterior wall  156 . In some embodiments, less compliant posterior wall  156  comprises one or more layers of a matrix of spun fibers. Multi-layered designs may be configured to strengthen or otherwise affect certain properties of the balloon, including mechanical properties such as burst strength and puncture resistance. 
     Use of multilayered and differential balloon walls may facilitate various aspects of manufacturing, deployment and therapies involving nuclear implants. For example, as discussed above, a multi-layered design may be thinner and/or more compliant along its anterior wall  156 . This may enable implant balloon  124  to be folded or otherwise packed into a smaller delivery configuration. Additionally, smaller profiles may require access cannulas having a smaller diameter and smaller annulotomy puncture, which decreases the possibility of implant extrusion. 
     Posterior wall  156  of a multilayer nuclear implant balloon  124  may be formed of multiple layers of the same material, or multiple layers of different materials. In some embodiments, layers may be formed of materials with similar compliance, providing an additive effect. In other embodiments, layers may be formed of different materials and formed independently, allowing for properties of each layer to be individually optimized. 
     Balloon  124  may be designed to expand symmetrically or asymmetrically. The balloon wall may comprise at least one radially asymmetrically dilatable portion, wherein the cross-section has one or more regions that differ in terms of compliance, strength and other characteristics. Such balloons provide enhanced performance though use of differential and directional expandability of the implant increased dimensional stability, improved cavity contour and improved tissue incorporation. 
     In some embodiments, balloon  124  has areas of electrospun polymeric material in combination with a silicone material, and other areas of uncovered silicone material. The electrospun polymeric material may comprise expanded PTFE (ePTFE). Application of PTFE to posterior wall  156  of implant  118  at least partially constrains inflation along the inner annulus and provides enhanced performance through use of differential and direction expandability. Furthermore, PTFE enhances soft tissue purchase by encouraging fibrocyte migration and incorporation of the implant with the posterior annulus. This helps guard against further disc herniation. 
     Balloon  124  may comprise flexible materials which accommodate a low-profile delivery without compromising strength and elasticity. In some embodiments, balloon  124  comprises an ultra-high molecular weight polyethylene (UHMwPE) material, such as DYNEEMA®, available from Royal DSM of Herleen, Netherlands. In some embodiments, balloon  124  may be coated with a hydrophilic material to increase lubricity. 
     In some embodiments, anterior wall  158  is relativity thinner and more compliant than posterior wall  156  so that it expands more than posterior wall  156 , sequestering the remaining portion of nucleus pulposus  106  and providing disc space widening and angle restoration. 
     In one embodiment the posterior portion of the nuclear implant consists of a flexible tubular three-dimensionally braided structure of metal or polymeric monofilaments, and polymeric multifilament yarns. The metallic thread elements or strands are favored for applications requiring additional reinforcement and effective protection against migration of the implant through a large annulus defect or a very weakened posterior annulus. 
     Referring to  FIGS. 26-28 , in some embodiments, an implant  230  for a partial nuclectomy comprises a proximal anchor  232 , a distal anchor  234 , and an inflatable balloon  236  extending therebetween. The inflatable balloon  236  is a multi-chambered balloon, having at least a first chamber  238  and a second chamber  240 . In one embodiment, second chamber  240  is disposed inside first chamber  238  and is completely surrounded by first chamber  238 . The inner and outer chamber may be filled with different materials to provide implant  230  with desired characteristics. It should be understood to one of ordinary skill in the art that any device, apparatus and/or system suitable for injecting fluid can be used to inflate first and second chambers  238 ,  240 . In some embodiments, first chamber  238  is filled with a curable elastomeric material  242 , while second chamber  240  is filled with a compressible material  244  (i.e., a gaseous material). This allows the implant  230  to absorb sudden increases in intra-discal pressure, acting effectively as a shock absorber. Like the other embodiments, the balloon  236  can comprise multiple layers, and a reinforcing layer  246  can be provided. 
     The above specification and examples provide a complete description of the structure and use of exemplary embodiments. Although certain embodiments have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the scope of this invention. As such, the various illustrative embodiments of the present devices are not intended to be limited to the particular forms disclosed. Rather, they include all modifications and alternatives falling within the scope of the claims, and embodiments other than the one shown may include some or all of the features of the depicted embodiment. For example, components may be combined as a unitary structure, and/or connections may be substituted. Further, where appropriate, aspects of any of the examples described above may be combined with aspects of any of the other examples described to form further examples having comparable or different properties and addressing the same or different problems. Similarly, it will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. 
     The claims are not intended to include, and should not be interpreted to include, means-plus- or step-plus-function limitations, unless such a limitation is explicitly recited in a given claim using the phrase(s) “means for” or “step for,” respectively.