Patent Publication Number: US-2009222098-A1

Title: Spinal nucleus replacement with varying modulus

Description:
BACKGROUND 
     Within the spine, the intervertebral disc functions to stabilize and distribute forces between vertebral bodies. The intervertebral disc comprises a nucleus pulposus which is surrounded and confined by the annulus fibrosis. 
     Intervertebral discs are prone to injury and degeneration. For example, herniated discs typically occur when normal wear, or exceptional strain, causes a disc to rupture. Degenerative disc disease typically results from the normal aging process, in which the tissue gradually loses its natural water and elasticity, causing the degenerated disc to shrink and possibly rupture. 
     Intervertebral disc injuries and degeneration may be treated by fusion of adjacent vertebral bodies or by replacing the intervertebral disc with a prosthetic. To maintain as much of the natural tissue as possible, the nucleus pulposus may be supplemented or replaced while maintaining all or a portion of the annulus. A need exists for nucleus replacement and supplementation implants that will reduce the potential for implant migration within the annulus and/or expulsion from the annulus. 
     SUMMARY 
     In one embodiment, an intervertebral disc augmentation implant comprises an implant body adapted for implantation within an annulus fibrosis the intervertebral disc, adjacent to an at least partially intact nucleus pulposus of the intervertebral disc, and comprising a core area, a peripheral wall area, a top face area, and a bottom face area. The implant body is formed from at least one material and the implant body has a modulus of elasticity gradient that gradually changes from the core area of the implant body to the peripheral wall area of the implant body. 
     In another embodiment, a method of augmenting a nucleus pulposus comprises accessing an annulus surrounding the nucleus pulposus and forming an opening in the annulus. The method further comprises inserting an intervertebral nucleus augmentation implant. The implant comprises an implant body including a core area, a peripheral wall area, a top face, and a bottom face. The implant body is formed from at least one material and the implant body has a modulus of elasticity gradation that gradually changes from the core area of the implant body to the peripheral wall area of the implant body. 
     In another embodiment, an implant for augmenting or replacing the natural nucleus pulposus within an intervertebral space comprises a central region and an outer region extending between the central region and an outer wall. The outer region has a modulus of elasticity that becomes progressively higher from the central region to the outer wall. The implant is sized for insertion through an opening in an annulus fibrosis surrounding the nucleus pulposus. 
     Additional embodiments are included in the attached drawings and the description provided below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sagittal view of a section of a vertebral column. 
         FIGS. 2 and 3  are a cross-sectional side view and a top view, respectively, of a nucleus augmentation implant having regions of different moduli of elasticity. 
         FIGS. 4 and 5  are a cross-sectional side view and a top view, respectively, of another nucleus augmentation implant having regions of different moduli of elasticity. 
         FIGS. 6 and 7  are a cross-sectional side view and a top view, respectively, of another nucleus augmentation implant having regions of different moduli of elasticity. 
         FIGS. 8 and 9  are a cross-sectional side view and a top view, respectively, of another nucleus augmentation implant having regions of different moduli of elasticity. 
         FIGS. 10-15  are cross-sectional side views of nucleus augmentation implants with gradual gradation in the modulus of elasticity from the center of the implant toward the periphery of the implant. 
         FIG. 16  is a top view of the nucleus augmentation implant of  FIG. 14 . 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure relates generally to devices and methods for relieving disc degeneration or injury, and more particularly, to devices and methods for augmenting a nucleus pulposus. For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments, or examples, illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Any alterations and further modifications in the described embodiments, and any further applications of the principles of the invention as described herein are contemplated as would normally occur to one skilled in the art to which the invention relates. 
     Referring first to  FIG. 1 , the reference numeral  10  refers to a vertebral joint section or a motion segment of a vertebral column. The joint section  10  includes adjacent vertebral bodies  12 ,  14 . The vertebral bodies  12 ,  14  include endplates  16 ,  18 , respectively. An intervertebral disc space  20  is located between the endplates  16 ,  18 , and an annulus fibrosis  22  surrounds the space  20 . In a healthy joint, the space  20  contains a nucleus pulposus  21 . The nucleus pulposus  21  may degenerate with age, disease, or trauma. 
     Referring now to  FIGS. 2 and 3 , a nucleus augmentation implant  30  may be used to supplement the function of an existing nucleus pulposus or replace all or a portion of the nucleus pulposus. Thus, the implant  30  may fill all or a portion of the disc space  20  within the annulus  22 . The implant  30  comprises two regions, a central region  32  and a peripheral region  34 . The implant has a top face  35 , a bottom face  36 , and a side wall  37 . Each region  32 ,  34  has a different modulus of elasticity. For example, central region  32  may have a lower modulus of elasticity than the peripheral region  34 , such that the implant  30  has a softer center and a stiffer peripheral area. In alternative embodiments, the inverse may be true with the center stiffer than the peripheral area. In this embodiment, at least a portion of the central region  32  extends from the top face  35  of the implant  30  to the bottom face  36  with the modulus of elasticity varying primarily from the center of the implant toward the side wall  37 . 
     Referring now to  FIGS. 4 and 5 , a nucleus augmentation implant  40  may be used to supplement the function of an existing nucleus pulposus or replace all or a portion of the nucleus pulposus. Thus, the implant  40  may fill all or a portion of the disc space  20  within the annulus  22 . The implant  40  comprises three regions, a central region  42 , a middle region  43 , and a peripheral region  44 . The implant has a top face  45 , a bottom face  46 , and a side wall  47 . Each region  42 ,  43 ,  44  has a different modulus of elasticity. For example, central region  42  may have a lower modulus of elasticity than regions  43 ,  44 , and the middle region  43  may have a lower modulus of elasticity than the region  44 , such that the implant  40  has a softer center and an increasingly stiff outer area. In alternative embodiments, the inverse may be true with the center stiffer than the peripheral area. In this embodiment, at least a portion of the central region  42  extends from the top face  45  of the implant  40  to the bottom face  46  with the modulus of elasticity varying primarily from the center of the implant toward the side wall  47 . 
     Referring now to  FIGS. 6 and 7 , a nucleus augmentation implant  50  may be used to supplement the function of an existing nucleus pulposus or replace all or a portion of the nucleus pulposus. Thus, the implant  50  may fill all or a portion of the disc space  20  within the annulus  22 . The implant  50  comprises two regions, a central region  52  and a peripheral region  54 . The implant has a top face  55 , a bottom face  56 , and a side wall  57 . Each region  52 ,  54  has a different modulus of elasticity. For example, central region  52  may have a lower modulus of elasticity than region  54 , such that the implant  50  has a softer center and a stiffer peripheral area. In alternative embodiments, the inverse may be true with the center stiffer than the peripheral area. In this embodiment, the central region  52  is encased within the peripheral region  54  with the region  54  extending between the central region  52  and the top and bottom faces  55 ,  56 . The modulus of elasticity varies both from the center of the implant toward the side wall  57  and from the center toward the top and bottom faces  55 ,  56 . 
     Referring now to  FIGS. 8 and 9 , a nucleus augmentation implant  60  may be used to supplement the function of an existing nucleus pulposus or replace all or a portion of the nucleus pulposus. Thus, the implant  60  may fill all or a portion of the disc space  20  within the annulus  22 . The implant  60  comprises three regions, a central region  62 , middle region  63 , and a peripheral region  64 . The implant has a top face  65 , a bottom face  66 , and a side wall  67 . Each region  62 ,  63 ,  64  has a different modulus of elasticity. For example, central region  62  may have a lower modulus of elasticity than regions  63 ,  64  and middle region  63  may have a lower modulus of elasticity than region  64 . The resulting implant  60  has a softer center and a stiffer peripheral area. In alternative embodiments, the inverse may be true with the center stiffer than the peripheral area. In this embodiment, the central region  62  is encased within the middle region  63  and the middle region  63  encased within the peripheral region  64 . The middle region  63  and the peripheral region  64  extend between the central region  62  and the top and bottom faces  65 ,  66 . The modulus of elasticity varies both from the center of the implant toward the side wall  67  and from the center toward the top and bottom faces  65 ,  66 . 
     Referring now to  FIG. 10 , an implant  70  includes a series of thin layers  72  emanating from a central area  73  and extending outward toward a side wall  74 . The implant  70  has a top face  75  and a bottom face  76 . The layers  72  may be concentric about the central area  73  and each layer may extend substantially from the top face  75  to the bottom face  76 . The layers  72  may have different moduli of elasticity. For example, the modulus of elasticity of the central area  73  may be relatively low, with the moduli of the layers growing increasingly larger toward the outer layers near the side wall  74 . In this configuration, the implant  70  would have a soft center and an increasingly stiff periphery. In one embodiment, five or more thin layers may be used. 
     When the implant  70  is subjected to an asymmetric load, the variance in modulus from the central area  73  and across the layers  72  may allow the central area to shift away from the load, thereby reducing the likelihood that the entire implant will migrate or become expelled from the annulus  22 . The thin layers  72  may also affect a relatively smooth, less abrupt, change in moduli which may permit an even, continuous motion of the joint. Further, the sequence of thin layers  72  may advance the integrity of the implant as the thin layers may have better cohesion with less likelihood of separation under flexion and lateral bending. 
     Referring now to  FIG. 11 , an implant  80  includes a series of thin layers  82  emanating from a central area  83  and extending outward toward a side wall  84 , a top face  85 , and a bottom face  86 . The layers  82  may be concentric about the nuclear central area  83  and extend generally radially outward toward the side wall  84 , top face  85 , and bottom face  86 . In this embodiment, each successive layer may surround or encapsulate the inner layer such that the implant  80  is layered in multiple directions as compared to the embodiment in  FIG. 10  in which the implant  70  is generally laterally layered only in the direction of the side wall  74 . The layers  82  may have different moduli of elasticity. For example, the modulus of elasticity of the central area  83  may be relatively low with the moduli of the layers growing increasingly larger toward the outer layers near the side wall  84 , the top face  85 , and the bottom face  86 . In this configuration, the implant  80  would have a soft center and an increasingly stiff periphery. In one embodiment, five or more thin layers may be used. 
     When the implant  80  is subjected to an asymmetric load, the variance in modulus from the central area  83  and across the layers  82  may allow the central area to shift away from the load, thereby reducing the likelihood that the entire implant will migrate or become expelled from the annulus  22 . The thin layers  82  may also affect a relatively smooth, less abrupt, change in moduli which may permit an even, continuous motion of the joint. Further, the sequence of thin layers  82  may advance the integrity of the implant as the thin layers may have better cohesion with less likelihood of separation under flexion and lateral bending. 
     Referring now to  FIG. 12 , an implant  90  includes a continuous body  92  having a side wall  94 , a top face  95 , and a bottom face  96 . The body  92  may be formed of a continuous material having a gradual modulus of elasticity gradient which changes from the center of the body  92  outward toward the side wall  94 . For example, the modulus of elasticity of the center of the body  92  may be relatively low with the moduli growing increasingly larger toward the side wall  94 . In this configuration, the implant  90  would have a soft center and an increasingly stiff periphery. 
     The body  92  may be formed of an integrally molded elastomeric material with modulus gradation incorporated into the body during the molding process. In this embodiment, the modulus at a given lateral distance between the center and the side wall may be relatively consistent across the span of the body  92  from top face  95  to bottom face  96 . Thus, the centers of the top and bottom faces  95 ,  96  may be relatively soft compared to the portions of the top and bottom faces near the side wall  94 . 
     When the implant  90  is subjected to an asymmetric load, the variance in modulus from the central area  92  to the side wall  94  may allow the central area to shift away from the load, thereby reducing the likelihood that the entire implant will migrate or become expelled from the annulus  22 . As compared to the larger regions in the embodiments of  FIG. 2  or  4 , the gradation of the modulus of elasticity in the body  92  may affect a continuous, less abrupt, change in moduli which may permit smoother, more continuous motion. Further, the continuous body  92  may advance the integrity of the implant  90  as the integrally formed body may be less likely to break apart under flexion and lateral bending. 
     Referring now to  FIG. 13 , an implant  100  includes a continuous body  102  having a side wall  104 , a top face  105 , and a bottom face  106 . The body  102  may be formed of a continuous material having a gradual modulus of elasticity gradient which changes from the center of the body  102  outward toward the side wall  104 . In this example, the modulus of elasticity of the center of the body  102  may be relatively high with the moduli decreasing toward the side wall  104 . In this configuration, the implant  100  would have a stiff center and an increasingly soft periphery. 
     The body  102  may be formed of an integrally molded elastomeric material with modulus gradation incorporated into the body during the molding process. In this embodiment, the modulus at a given lateral distance between the center and the side wall may be relatively consistent across the span of the body  102  from top face  105  to bottom face  106 . As compared to the larger regions in the embodiments of  FIG. 2  or  4  or even the layers of  FIG. 10 , the gradation of the modulus of elasticity in the body  92  may affect a continuous, less abrupt, change in moduli which may permit smoother, more continuous motion. Further, the continuous body  92  may advance the integrity of the implant  90  as the integrally formed body may be less likely to break apart under flexion and lateral bending. 
     Referring now to  FIG. 14 , an implant  110  includes a continuous body  112  having a side wall  114 , a top face  115 , and a bottom face  116 . The body  112  may be formed of a continuous material having a gradual modulus of elasticity gradient which changes from the center of the body  112  outward toward the side wall  114 , the top face  115 , and the bottom face  116 . In this example, the modulus of elasticity of the center of the body  112  may be relatively low with the modulus increasing radially outward toward the side wall  104 , the top face  115 , and the bottom face  116 . In this configuration, the implant  110  would have a relatively soft center with an increasingly stiff top, bottom and side periphery. 
     The body  112  may be formed of an integrally molded elastomeric material with modulus gradation incorporated into the body during the molding process. As compared to the body  92  of  FIG. 12 , the body  112  may be relatively stiff across the top and bottom faces  115 ,  116  as well as along the side walls  114 . 
     When the implant  110  is subjected to an asymmetric load, the variance in modulus from the central area  112  to the side wall  114  may allow the central area to shift away from the load, thereby reducing the likelihood that the entire implant will migrate or become expelled from the annulus  22 . As compared to the larger regions in the embodiments of  FIG. 2  or  4 , the gradation of the modulus of elasticity in the body  112  may affect a continuous, less abrupt, change in moduli which may permit smoother, more continuous motion. Further, the continuous body  112  may advance the integrity of the implant  110  as the integrally formed body may be less likely to break apart under flexion and lateral bending. 
     As shown in  FIG. 16 , the implant  110  is positioned within the nucleus  21  and within the annulus  22 . A portion of the annulus  22  may be removed or otherwise opened to allow entry of the implant  110 . This opening may later be sutured, blocked, or otherwise repaired to limit expulsion of the implant  110 . 
     Referring now to  FIG. 15 , an implant  120  includes a continuous body  122  having a side wall  124 , a top face  125 , and a bottom face  126 . The body  122  may be formed of a continuous material having a gradual modulus of elasticity gradient which changes from the center of the body  122  outward toward the side wall  124 , the top face  125 , and the bottom face  126 . In this example, the modulus of elasticity of the center of the body  122  may be relatively low with the modulus increasing radially outward toward the side wall  124 , the top face  125 , and the bottom face  126 . In this configuration, the implant  120  would have a relatively soft center with an increasingly stiff periphery. 
     The body  122  may be formed of an integrally molded elastomeric material with modulus gradation incorporated into the body during the molding process. As compared to the body  102  of  FIG. 13 , the body  122  may be relatively stiff across the top and bottom faces  125 ,  126  as well as along the side walls  124 . As compared to the larger material regions in the embodiments of  FIG. 6  or  8  or even the layers of  FIG. 11 , the gradation of the modulus of elasticity in the body  122  may affect a continuous, less abrupt, change in moduli which may permit smoother, more continuous motion. Further, the continuous body  122  may advance the integrity of the implant  120  as the integrally formed body may be less likely to break apart under flexion and lateral bending. 
     The gradual changes in gradient may be achieved through molding methods, including injection molding methods. Within an implant formed of an otherwise homogeneous material, the modulus of elasticity may be varied by varying the amount and type of chemical crosslinking. The gradient changes may also result from combining or dispersing additional materials in varying amounts throughout an otherwise homogeneous material to achieve a desired combined or blended modulus. Modulus gradation can also result from the use of reinforcing materials. The implants may be formed of solid materials, for example, molded silicone, hydrogel, or polyurethane. In other embodiments, implants may be more porous, formed, for example, of a woven fabric made of ultra high molecular weight polyethylene (UHMWPE) fibers, polyethylene terephthalate (PET) fibers, polyester fibers, or metallic fibers. The fabric content or weave density may be varied to achieve the gradient change. A woven fabric may also be embedded in a solid polymer material to form an implant having a varied modulus due to fabric concentration, content, or weave density. Furthermore, variations in gradation may be achieved through physical features such as changes in implant thickness, surface patterns, material porosity, or material voids. 
     The implants described above may be formed of elastomeric materials such as polyurethane, silicone, silicone polyurethane copolymers, polyolefins, such as polyisobutylene rubber and polyisoprene rubber, neoprene rubber, nitrile rubber, vulcanized rubber and combinations thereof. Non-elastic polymers such as polyethylene, polyester, and polyetheretherketone (PEEK) may also be suitable. The non-elastic polymers may be incorporated in the form of fibers, non-woven mesh, woven fabric, or braided structure. 
     Certain portions of the implant, such as lower modulus regions, layer, or areas may be formed of more deformable or compliant materials including soft elastomers and polymeric gels. Suitable hydrogels may include poly(vinyl alcohol), poly(acrylic acids), poly(methacrylic acids), copolymers of acrylic acid and methacrylic acid, poly(acrylonitrile-acrylic acid), polyacrylamides, poly(N-vinyl-2-pyrrolidone), polyethylene glycol, polyethyleneoxide, polyacrylates, poly(2-hydroxy ethyl methacrylate), copolymers of acrylates with N-vinyl pyrrolidone, N-vinyl lactams, polyurethanes, polyphosphazenes, poly(oxyethylene)-poly(oxypropylene) block polymers, poly(oxyethylene)-poly(oxypropylene) block polymers of ethylene diamine, poly(vinyl acetate), and sulfonated polymers, polysaccharides, proteins, and combinations thereof. 
     Materials may be selected to achieve a desired performance. For example, in the embodiments of  FIGS. 2 ,  3 ,  6 ,  7 , the central region may be formed of ultra high molecular weight polyethylene (UHMWPE) and the peripheral region may be formed of polyurethane to achieve a centrally stiffer device. Alternatively, the central region may be formed of silicone with a polyurethane peripheral region. Other suitable material combinations may include a hydrogel central region with silicone peripheral region, a hydrogel central region with polyurethane peripheral region, or a silicone central region with polyurethane peripheral region 
     In the three region embodiments of  FIGS. 4 ,  5 ,  8 , and  9 , the central region may be formed of a gel, the middle region formed of a silicone, and the peripheral region formed of a polyurethane to create a centrally soft and outwardly stiffer device. Alternatively, the central region may be formed of a gel, the middle region formed of a polyurethane, and the peripheral region formed of a silicone to create a device having a stiffer middle region than either the central or peripheral regions. Another suitable material combination may be a hydrogel central region, silicone middle region, and polyurethane peripheral region. Another suitable material combination may be a hydrogel central region, polyurethane middle region, and a woven fabric peripheral region. Another suitable material combination may be a silicone central region, a polyurethane middle region, and a woven fabric peripheral region. 
     In other alternative embodiments, the implant may have more than two or three regions. For example, a hydrogel center with silicone middle layer, a polyurethane middle layer, and a woven fabric peripheral region. 
     In the gradient embodiment of  FIG. 12 , the center of the implant may be formed of 50 Shore A polyurethane with the hardness of the polyurethane gradually increasing to 80 Shore A. In the gradient embodiment of  FIG. 14 , the center of the implant may be a polyurethane gel that changes from a gelantinous consistency to a harder consistency material such as an 80 Shore A polyurethane. In other alternative gradient embodiments, the hardness may gradually change from 20-100 Shore A or from 40-90 Shore A. 
     In embodiments in which the central region has a lower modulus of elasticity than the other regions, the higher modulus regions may support and/or contain the softer core. Additionally, under flexion or lateral bending motions, the softer and more deformable central region will deform more than the middle or peripheral regions. Further, as the implant is loaded unevenly or off center, the softer central region may deform and shift away from the load to reduce the potential for the whole implant to displace. This ability to compensate for load shifts may reduce the potential for implant migration or expulsion. 
     The implants described above may assume any of a variety of shapes including spherical, elliptoid, boomerang, Saturn-like, disc, capsule, kidney, oval, rectangular, or cylindrical. The center regions, layers, or areas of the implants may have a similar or different shape than the overall shape of the implant. 
     Prior to positioning any of the implants described above in the intervertebral disc space  20 , an incision may be made in the annulus fibrosis or an existing annulus defect may be identified. The annulus  22  may be accessed through a posterior, lateral, anterior, or any other suitable approach. In one embodiment, a guide wire or other small instrument may be used to make the initial hole. If necessary, successively larger holes are cut from an initially small puncture. The hole (also called an aperture, an opening, or a portal, for example) may be as small as possible to minimize expulsion of the material through the hole after the surgery is complete. 
     Also if necessary, a dilator may be used to dilate the hole, making it large enough to deliver the implant to replace or augment the disc nucleus. The dilator may stretch the hole temporarily and avoid tearing so that the hole can return back to its undilated size after the instrument is removed. Although some tearing or permanent stretching may occur, the dilation may be accomplished in a manner that allows the hole to return to a size smaller than its dilated size after the surgery is complete. 
     Through this annulus opening, all or a portion of the natural nucleus pulposus may be removed. Any of a variety of tools may be used to prepare the disc space, including specialized pituitary rongeurs and curettes for reaching the margins of the nucleus pulposus. Ring curettes may be used to space abrasions from the vertebral endplates as necessary. Using these instruments, a centralized, symmetrical space large enough to accept the implant footprint may be prepared in the disc space. It is understood that the natural nucleus pulposus need not be removed, but rather, as shown in  FIG. 16 , an implant with modulus variations may be used in cooperation with existing nucleus tissue to compensate for deficiencies in the existing tissue. The disc space may then be distracted to a desired level by distractors or other devices known to the skilled artisan for such purposes. After preparing the disc space  20  and/or annulus  22  for receiving the implant, the implant may be delivered into the intervertebral disc space using any of a variety of techniques known in the art. 
     As used throughout this description, the terms “modulus” and “modulus of elasticity” are broadly used to refer to physical material properties such as hardness or elasticity. High modulus materials are relatively hard or stiff, and low modulus materials are relatively soft and resilient. 
     Although only a few exemplary embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this disclosure. Accordingly, all such modifications and alternative are intended to be included within the scope of the invention as defined in the following claims. Those skilled in the art should also realize that such modifications and equivalent constructions or methods do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. It is understood that all spatial references, such as “horizontal,” “vertical,” “top,” “upper,” “lower,” “bottom,” “left,” “right,” “anterior,” “posterior,” “superior,” “inferior,” “upper,” and “lower” are for illustrative purposes only and can be varied within the scope of the disclosure. In the claims, means-plus-function clauses are intended to cover the elements described herein as performing the recited function and not only structural equivalents, but also equivalent elements.