Patent Publication Number: US-2009222099-A1

Title: Self Centering Nucleus Implant

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 augmentation 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 for implantation between a pair of vertebral bodies comprises an elastically deformable outer casing having at least one thickness dimension and a core member having isotropic material properties. The core member is entirely encased within the outer casing and has a height dimension along an axis defined by the pair of vertebral bodies. The modulus of elasticity of the core member is greater than a modulus of elasticity of the outer casing, and the height dimension of the core member is greater than the at least one thickness dimension of the outer casing. 
     In another embodiment, a method of replacing a nucleus of an intervertebral disc located between a pair of vertebral bodies comprises accessing an annulus surrounding the nucleus and forming an opening in the annulus. The method further comprises inserting an intervertebral nucleus replacement implant. The implant comprises an elastically deformable outer casing having at least one thickness dimension and an isotropic core member entirely encased within the outer casing. The core member comprises a height dimension along an axis defined by the pair of vertebral bodies. A modulus of elasticity of the core member is greater than a modulus of elasticity of the outer casing. The height dimension of the core member is greater than the at least one thickness dimension of the outer casing. 
     In another embodiment, an implant for replacing at least a portion of a nucleus of an intervertebral disc between a pair of vertebral bodies comprises an elastically deformable outer casing having at least one thickness dimension and a non-composite core member having a height dimension along an axis defined through the pair of vertebral bodies. All surfaces of the core member are encased within and in direct contact with the outer casing, and a modulus of elasticity of the core member is greater than a modulus of elasticity of the outer casing. 
     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. 
         FIG. 2  is a side cross sectional view of an implant with a core portion having a capsule shaped cross section. 
         FIG. 3  is a side cross sectional view of an implant with a core portion having an oval cross section. 
         FIG. 4  is a side cross sectional view of an implant with a core portion having an outer flange. 
         FIG. 5  is a side cross sectional view of an implant with a core portion having a circular cross section. 
         FIG. 6  is a top cross sectional view of an implant with a circular implant cross section and a circular core portion cross section. 
         FIG. 7  is a top cross sectional view of an implant with a capsule shaped cross section and a capsule shaped core portion cross section. 
         FIG. 8  is a top cross sectional view of an implant with a kidney shaped cross section and a kidney shaped core portion cross section. 
         FIG. 9  is a top cross sectional view of an implant with an oval shaped cross section and a circular core portion cross section. 
         FIG. 10  is a side cross sectional view of an implant under axial loading. 
         FIG. 11  is a side cross sectional view of an implant under offset loading. 
     
    
    
     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. A central longitudinal axis  24  may extend through the vertebral joint  10 . 
     Referring now to  FIG. 2 , a nucleus implant  30  may be used to augment the function and the existing tissue of the nucleus  21  or may be used to replace all or a portion of the nucleus  21 . Thus, the implant  30  may fill all or a portion of the disc space  20  within the annulus  22 . The implant  30  comprises a core portion  32  encapsulated within an outer casing  34 . 
     The outer casing  34  is a skin-like layer which is softer and more elastically deformable than the core portion  32 . Specifically, the outer casing  34  has a modulus of elasticity less than the modulus of elasticity of the core portion  32 . 
     The outer casing  34  has a top thickness dimension  36  and a side thickness dimension  38 . The thickness dimensions  36 ,  38  may be between 1 mm and 5 mm. The volume of the outer casing  34  may be between 5% and 50% of the total volume of the implant  30 . Specifically, an outer casing volume between 20% and 30% of the total volume of the implant may be suitable. 
     The core portion  32  is harder and less elastically deformable than the outer casing  34 . The core portion  32  may have a height  40  as measured along the axis  24 . The height  40  may be greater than the thickness  36  and may even be greater than twice the thickness  36 . The implant  30  may have an overall height  42  as measured along the axis  24 . The thickness dimension  36  may be less than 25% of the of the implant height  42 . 
     The core portion  32  has an upper surface  44 , a lower surface  46 , and outwardly radiused corners  48 . In this embodiment, the upper and lower surfaces  44 ,  46  are generally flat, such that in the cross sectional side view, the core portion  32  has a capsule shaped profile. 
     Referring now to  FIG. 3 , a nucleus implant  50  may be used to augment the function and the existing tissue of the nucleus  21  or may be used to replace all or a portion of the nucleus  21 . Thus, the implant  50  may fill all or a portion of the disc space  20  within the annulus  22 . The implant  30  comprises a core portion  52  encapsulated within an outer casing  54 . 
     The outer casing  54  is a skin-like layer which is softer and more elastically deformable than the core portion  52 . Specifically, the outer casing  54  has a modulus of elasticity less than the modulus of elasticity of the core portion  52 . 
     The outer casing  54  has a minimum top thickness dimension  56  and a minimum side thickness dimension  58 . The thickness dimensions  56 ,  58  may be between 1 mm and 5 mm. The volume of the outer casing  54  may be between 5% and 50% of the total volume of the implant  50 . Specifically, an outer casing volume between 20% and 30% of the total volume of the implant may be suitable. 
     The core portion  52  is harder and less elastically deformable than the outer casing  54 . The core portion  52  may have a maximum height  60  as measured along the axis  24 . The maximum height  60  may be greater than the minimum thickness  56  and may even be greater than twice the minimum thickness  56 . The implant  50  may have an overall height  62  as measured along the axis  24 . The minimum thickness dimension  56  may be less than 25% of the of the implant height  62 . 
     The core portion  52  has an upper surface  64  and a lower surface  66 . In this embodiment, the upper and lower surfaces  64 ,  66  are generally curved, such that in the cross sectional side view, the core portion  52  has an oval shaped profile. 
     Referring now to  FIG. 4 , a nucleus implant  70  may be used to augment the function and the existing tissue of the nucleus  21  or may be used to replace all or a portion of the nucleus  21 . Thus, the implant  70  may fill all or a portion of the disc space  20  within the annulus  22 . The implant  70  comprises a core portion  72  encapsulated within an outer casing  74 . 
     The outer casing  74  is a skin-like layer which is softer and more elastically deformable than the core portion  72 . Specifically, the outer casing  74  has a modulus of elasticity less than the modulus of elasticity of the core portion  72 . 
     The outer casing  74  has a minimum top thickness dimension  76  and a minimum side thickness dimension  78 . The thickness dimensions  76 ,  88  may be between 1 mm and 5 mm. The volume of the outer casing  74  may be between 5% and 50% of the total volume of the implant  70 . Specifically, an outer casing volume between 20% and 30% of the total volume of the implant may be suitable. 
     The core portion  72  is harder and less elastically deformable than the outer casing  74 . The core portion  72  may have a maximum height  80  as measured along the axis  24 . The maximum height  80  may be greater than the minimum thickness  76  and may even be greater than twice the minimum thickness  76 . The implant  70  may have an overall height  82  as measured along the axis  24 . The minimum thickness dimension  76  may be less than 25% of the of the implant height  82 . 
     The core portion  72  has an upper surface  84 , a lower surface  86 , inwardly radiused corners  88 , and a perimeter flange  89 . In this embodiment, the upper and lower surfaces  84 ,  86  are generally flat and intersect with the flange  89  at the inwardly radiused corners  88 . 
     Referring now to  FIG. 5 , a nucleus implant  90  may be used to augment the function and the existing tissue of the nucleus  21  or may be used to replace all or a portion of the nucleus  21 . Thus, the implant  90  may fill all or a portion of the disc space  20  within the annulus  22 . The implant  90  comprises a core portion  92  encapsulated within an outer casing  94 . 
     The outer casing  94  is a skin-like layer which is softer and more elastically deformable than the core portion  92 . Specifically, the outer casing  94  has a modulus of elasticity less than the modulus of elasticity of the core portion  92 . 
     The outer casing  94  has a minimum top thickness dimension  96  and a minimum side thickness dimension  98 . The thickness dimension  96  may be between 1 mm and 5 mm. The volume of the outer casing  94  may be between 5% and 50% of the total volume of the implant  90 . Specifically, an outer casing volume between 20% and 30% of the total volume of the implant may be suitable. 
     The core portion  92  is harder and less elastically deformable than the outer casing  94 . The core portion  92  may have a maximum height  100  as measured along the axis  24 . The maximum height  100  may be greater than the minimum thickness  96  and may even be greater than twice the minimum thickness  96 . The implant  90  may have an overall height  102  as measured along the axis  24 . The minimum thickness dimension  96  may be less than 25% of the of the implant height  102 . 
     The core portion  92  has an upper surface  104  and a lower surface  106 . In this embodiment, the upper and lower surfaces  104 ,  106  are generally curved, such that in the cross sectional side view, the core portion  92  has a circular profile. 
     Referring now to  FIG. 6 , a nucleus implant  110  may have a side cross-sectional view the same as or similar to any of the implants  30 ,  50 ,  70 ,  90  described above. The implant  110  has an outer casing  112  surrounding a core portion  114 . In this embodiment, a top cross- sectional view of the implant  110  is circular with a circular core portion  114 . 
     Referring now to  FIG. 7 , a nucleus implant  116  may have a side cross-sectional view the same as or similar to any of the implants  30 ,  50 ,  70 ,  90  described above. The implant  116  has an outer casing  118  surrounding a core portion  120 . In this embodiment, a top cross-sectional view of the implant  116  is capsule shaped with a capsule shaped core portion  120 . 
     Referring now to  FIG. 8 , a nucleus implant  122  may have a side cross-sectional view the same as or similar to any of the implants  30 ,  50 ,  70 ,  90  described above. The implant  122  has an outer casing  124  surrounding a core portion  126 . In this embodiment, a top cross-sectional view of the implant  122  is kidney shaped with a kidney shaped core portion  120 . 
     Referring now to  FIG. 9 , a nucleus implant  128  may have a side cross-sectional view the same as or similar to any of the implants  30 ,  50 ,  70 ,  90  described above. The implant  128  has an outer casing  130  surrounding a core portion  132 . In this embodiment, a top cross-sectional view of the implant  128  is oval shaped with a circular core portion  132 . Thus, a core portion may have a different shape than the overall implant. 
     The overall implants and the core portions described above may assume any of a variety of three-dimensional shapes including spherical, elliptoid, boomerang, Saturn-like, disc, capsule, kidney, or cylindrical. 
     Any of the core portions in the embodiments described above may be uniform, non-composite structures and may have isotropic material properties throughout the core portion. Composite structures, such as layered structures, having anisotropic material properties may also be suitable. All surfaces of the core portion may be in direct contact with the outer casing. However, in composite structures, only outer edges of the inner layers may be in contact with the casing. The core portions described above may be formed of polymers such as ultra-high molecular weight polyethylene (UHMWPE), polyurethane, silicone-polyurethane copolymers, polyetheretherketone, or polymethylmethacrylate. Suitable metals may include cobalt-chrome alloys, titanium, titanium alloys, stainless steel, or titanium nickel alloys. Suitable ceramics may include alumina, zirconia, polycrystalline diamond compact, or pyrolitic carbon. In embodiments in which the core portion is formed from radiolucent materials, a radiocontrast marker or material such as barium sulfate, tungsten, tantalum, or titanium may be added to the core portion for purposes of viewing the implant with imaging equipment. 
     The outer casings may be formed of polyurethane, silicone, silicone polyurethane copolymers, polyolefins, such as polyisobutylene rubber and polyisoprene rubber, neoprene rubber, nitrile rubber, vulcanized rubber and combinations thereof. Any of the outer casings in the embodiments described above may be uniform, non-uniform or varying in thickness. For example in  FIG. 4  above, the thickness of the casing  74  is greater in the peripheral area near the radiused corners  88  than in the more central region over the upper surface  84 . The casings described in the embodiments above may reduce the contact stress between the core portion of the implant and the adjacent tissue as the spinal joint undergoes flexion-extension and lateral bending motion. The deformable properties of the casings may also serve to reduce the potential for implant migration or expulsion through an opening in the annulus. 
     In one exemplary embodiment, the core portion may be formed of UHMWPE with the outer casing formed of silicone having a durometer hardness of 60 Shore A. In another exemplary embodiment, the core portion may be formed of 80 Shore A BIONATE® polycarbonate-urethane with the outer casing formed of 50 Shore A silicone. In another exemplary embodiment, the core portion may be formed of 80 Shore A PURSIL silicone-polyetherurethane with the outer casing formed of 50 Shore A elastomeric polyurethane. All durometer hardness values are approximate. The core portion, for example, may have a hardness greater than the exemplary values. The outer casing, for example, may have a hardness lower than the exemplary values. 
     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. In alternative embodiments, portions of the annulus  22  may be resected to allow passage of the implants. 
     Through the 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 scape 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, an implant of the type described above 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. 
     Referring now to  FIG. 10 , the implant  70  may be installed within the disc space  20  using a technique such as that described above. In this embodiment, the implant  70  is subjected to an axial load  140  equally distributed about a center of mass  142 . The relatively thin outer casing  74  above and below the center of mass  142  may increase the axial load bearing capability of the implant  70 . The thin outer casing  74  allows the load  140  to be transmitted almost directly to the more rigid core portion  72  which is able to provide greater support. Under axial loading  140 , the outer casing  74  may deform, expanding radially as shown by arrows  144 . 
     Referring now to  FIG. 11 , the implant  70  may be subjected to a off-set loads  146  under flexion-extension or lateral bending motions. Under these types of motions, the thicker outer casing  74  near the peripheral portion of the implant  70  may reduce the contact stress between the vertebral bodies  12 ,  14  and the core portion  72 , increasing the stress distribution. Under the off-set load  146 , the center of mass  142  of the core portion  72  may shift away from the load  146 . When the spinal joint  10  is returned to alignment, the elastic outer casing  74  may return to its original configuration and thereby cause the core portion  72  to return to its original position within the casing  74 . In this way the implant  70  may have self-centering qualities. The deformable nature of the casing  74  and the self-centering nature of the implant  70  may reduce the likelihood that the entire implant will migrate or become expelled from the annulus  22 . 
     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.