Abstract:
Spinal disk including a shell, first endplate, second endplate and core. The shell includes sidewalls, back wall, front wall, top wall and bottom wall defining a compartment. The first endplate includes a first base, first top and first attachment. The first base is retained in the compartment. The first top is disposed in a first opening in the top wall forming a contact surface continuous with a surface of the top wall. The first attachment extends from the first top portion. The second endplate includes a second base, second top and second attachment. The second base is retained in the compartment. The second top is disposed in a second opening in the bottom wall forming a contact surface continuous with a surface of the bottom wall. The second attachment extends from the second top portion. The core is disposed in the compartment between the first endplate and the second endplate.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 11/939,991 filed on Nov. 14, 2007, which claims priority to U.S. Provisional Patent Application No. 60/859,990 filed on Nov. 20, 2006, the disclosures of which are hereby incorporated in their entireties by reference herein. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    Example embodiments relate generally to implantable spinal devices. More particularly, example embodiments are directed to an implantable intervertebral spinal disk to reconstruct a damaged spinal disk of a spinal motion segment of the vertebrae and to restore movement thereto, as well as method for assembly of the implantable intervertebral spinal disk. 
         [0004]    2. Brief Discussion of Related Art 
         [0005]    A normal spinal disk is a cylindrical weight-bearing fibrous structure with a non-compressible viscous center. Due to its ability to deform, the spinal disk not only supports normal functional loads of the human body (e.g., load bearing) but also cushions and evenly distributes the pressures or stresses applied with body movement and positioning (e.g., load sharing). The spinal disk articulates between two bony vertebrae, one vertebra above the disk and one vertebra below the disk, through large surface area interfaces known as endplates. An endplate is a thin (e.g., 1 mm to 3 mm) approximately round plate (e.g., 2 cm to 4 cm in diameter) of dense bone and cartilage accounting for a majority of the vertebral weight-bearing capacity. 
         [0006]    The spinal disk represents just one of the components defining motion or articulation between vertebrae. The other components are two symmetric facet joints that form a triangular arrangement with the spinal disk being disposed in front. The spinal disk functions as a substantial hydraulic spacer between the vertebrae. Vertical loads with flexion, extension, lateral bending or rotation movements applied to the spine cause the spinal disk to deform and create secondary movement between the vertebrae. Movement across the spinal disk is coupled to the movement of the symmetric facet joints, which function similarly to classical joints with relative translation between two opposing surfaces. 
         [0007]    The articulations between the vertebrae, including the foregoing spinal disk and facet joints, frequently deteriorate with age or trauma and become a source of pain. Spinal disk deterioration causes the spinal disk to lose its normal consistency and volume, facilitating the spinal disk to collapse and causing abnormally painful motion within the anterior spinal column. Furthermore, the abnormal motion across the spinal disk increases the stresses on the facet joints and accelerates degeneration of the facet joints. 
         [0008]    Historically, surgical treatment of spinal disk disorders required fusion or elimination of movement across an abnormal spinal disk. This has been accomplished by allowing bone to grow between adjacent vertebrae and through the disk space of the abnormal spinal disk. Although fusion generally relieved the source of pain, fusion however did not restore normal movement of the fused spinal motion segment. Invariably, fusion eliminates a range of motion in the fused spinal motion segment, limits overall spinal range of motion and places abnormal pressures or stresses on other non-fused normal spinal motion segments with body movement and positioning. Thus, the abnormal pressures or stresses caused by fusion may further accelerate the degeneration of the foregoing articulations between normal vertebrae. 
         [0009]    A new class of restorative or motion preserving spinal devices has been introduced to overcome the foregoing limitations of fusion. These motion preserving spinal devices aim to restore and maintain spinal disk height while approximating a range of motion and function of the normal spinal disk. The motion preserving spinal devices include artificial spinal disks that generally have rigid movably coupled components and ball-socket articulation. 
         [0010]    More specifically, the artificial spinal disks function through direct contact and movement between two opposing surfaces, usually metal or plastic. The ball-socket articulation (among other mechanical contact points) produces hazardous debris and cannot reproduce adequately normal spinal disk deformation or its load sharing capacity. The mechanical contact points (including ball-socket articulation) of the artificial spinal disk components wear with repetitive motion and produce debris which may induce scarring, toxicity and bone absorption. The scarring may be extensive with the potential for neural injury and bone loss. Certain debris (e.g., nickel) accumulates in the body and may cause systemic toxicity. The mechanical wear further may cause breakdown of artificial spinal disk components and resultant painful malfunction of the artificial spinal disk. Furthermore, non-constrained components may extrude into the abdomen with disastrous consequences. 
         [0011]    One way of approximating the motion of the normal spinal disk has been to implement a floating center of movement. However, computer simulations using finite element analysis of currently available artificial spinal disks have shown excessive or abnormal motion at spinal disk interfaces, particularly in extension, when compared to the normal spinal disk. These data have been confirmed by biomechanical testing of the artificial spinal disks in cadavers. The abnormal motion at the artificial disk interfaces wears artificial spinal disk components and puts abnormal strain on the facet joints of the vertebrae, significantly accelerating painful and debilitating degeneration of the vertebrae. 
         [0012]    While the new class of restorative or motion preserving spinal devices aims to solve the limitations of fusion, the foregoing abnormal strain on the facet joints, the wear of the artificial spinal disk with resultant debris and possible failure of the artificial spinal disk increase painful and debilitating degeneration of the vertebrae and may further in the case of extrusion present real dangers one&#39;s health. 
       SUMMARY 
       [0013]    An implantable spinal disk is disclosed. The implantable spinal disk includes an external shell, first endplate, second endplate and core internal component. 
         [0014]    The external shell of the implantable spinal disk includes a pair of sidewalls, back wall, front wall, top wall and bottom wall which define a compartment that extends therebetween in the shell. The top wall has a first opening from exterior of the shell into the compartment. The bottom wall has a second opening from exterior of the shell into the compartment. 
         [0015]    The first endplate of the implantable spinal disk includes a first base portion, first top portion and at least one first attachment device. The first base portion is retained in the compartment by the top wall and extends from a first sidewall to a second sidewall of the pair of sidewalls. The first top portion is disposed in the first opening and extends about the first opening to form an exterior contact surface continuous with a top surface of the top wall. The at least one first attachment device extends from the first top portion and is configured to penetrate into a first vertebra. 
         [0016]    The second endplate of the implantable spinal disk includes a second base portion, second top portion and at least one second attachment device. The second base portion is retained in the compartment by the bottom wall and extends from the first sidewall to the second sidewall. The second top portion is disposed in the second opening and extends about the second opening to form an exterior contact surface continuous with a bottom surface of the bottom wall. The at least one second attachment device extends from the second top portion and is configured to penetrate into a second vertebra. 
         [0017]    The core internal component of the implantable spinal disk is disposed in the compartment and extends from the first sidewall to the second sidewall between the first endplate and the second endplate. 
         [0018]    These and other purposes, goals and advantages of the present application will become apparent from the following detailed description of example embodiments read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    Some embodiments are illustrated by way of example and not limitation in the figures of the accompanying drawings in which: 
           [0020]      FIG. 1  is a perspective view of an implantable intervertebral spinal disk; 
           [0021]      FIG. 2  is a semi-transparent perspective view of the implantable intervertebral spinal disk in accordance with  FIG. 1 ; 
           [0022]      FIG. 3  is a semi-transparent side view of the implantable intervertebral spinal disk in accordance with  FIG. 1 ; 
           [0023]      FIG. 4  is a perspective view of an outer shell of the implantable intervertebral spinal disk in accordance with  FIG. 1 ; 
           [0024]      FIG. 5  is a top view of the an endplate of the implantable intervertebral spinal disk in accordance with  FIG. 1 ; 
           [0025]      FIG. 6  is a perspective view of a pair of endplates of the implantable intervertebral spinal disk in accordance with  FIGS. 1 and 5 ; and 
           [0026]      FIG. 7  is a perspective view of a core internal component of the intervertebral spinal disk in accordance with  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    An example implantable intervertebral spinal disk and method for assembling the implantable intervertebral spinal disk are described. In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of example embodiments. It will be evident, however, to one skilled in the art that an example embodiment may be practiced without these specific details. 
         [0028]      FIG. 1  is a perspective view  100  of an example implantable intervertebral spinal disk  102  adapted to restore spinal disk height and to approximate closely the biomechanics of a normal spinal disk, mitigating abnormal strain on the facet joints, debris and possible failure caused by the conventional implantable spinal devices. The implantable intervertebral spinal disk  102  is suitable for the arthroplasty of the cervical, the thoracic and the lumbar spine. The implantable intervertebral spinal disk  102  comprises an external shell  118  illustrated in detail in  FIG. 4  and described hereinbelow with reference to the same, and plural internal components illustrated in detail in  FIGS. 2-7  and described hereinbelow with reference to the same. The internal components include a pair of endplates (illustrated in detail in  FIGS. 5 and 6  and described hereinbelow) and a core internal component (illustrated in detail in  FIG. 7  and described hereinbelow) sandwiched between the pair of endplates. The external shell  118  is viscoelastic, a flexible non-compressible solid that is deformable and yet sufficiently resilient to stress or pressure of body movement and positioning. The external shell  118  includes plural openings, one on a top surface thereof and one on a bottom surface thereof (e.g., opening  112 ), through which the respective endplates protrude. The plural openings are generally oval for ease of manufacturing, assembly and the like. However, the shapes of the openings are not limited and a variety of other shapes may easily be implemented. The openings expose respective contact surfaces (e.g., contact surface  114  exposed through opening  112 ) for the integration between the endplates and the bony vertebrae. The bone of each vertebrae (e.g., above and below the implantable intervertebral spinal disk  102 ) will grow and bind to the respective contact surface (e.g., contact surface  114 ) of each endplate but not to the external shell  118  as will be described below. The openings should be as large as possible to provide the largest endplate-bone contact surfaces, yet also function to retain the endplates within the exterior shell  118  as will also be described below. To improve the retention of the endplates within the exterior shell  118 , a groove may be provided on the upper covered surfaces of the endplates with corresponding ridges on the interior of each exterior shell  118 . This provides a securing mechanism to mitigate the possibility of the external shell  118  rolling off the endplates with the operation of the implantable intervertebral spinal disk  102 . 
         [0029]    Further with reference to  FIG. 1 , the external shell  118  includes side surfaces  104 ,  108 , a back surface  106  and a front surface  110 . The side surfaces  104 ,  108  and the back surface  106  are generally flat surfaces, while the front surface  110  is a generally curve-shaped or arcuate surface. Additionally, the side surfaces  104 ,  108  gradually increase in height from the back surface  106  to the front surface  110 . More specifically, the implantable intervertebral spinal disk  102  is taller in front than in the back to provide for the natural curvature of the cervical or lumbar segments of the spine into which the implantable intervertebral spinal disk  102  will most often be implanted. The degree of the implantable intervertebral spinal disk  102  triangulation varies between different levels of the spine (e.g., cervical, lumbar, thoracic) and between different people. The endplates will generally provide between a zero (0) and a six (6) degree angle with a horizontal plane through the center of the implantable intervertebral spinal disk  102  (e.g., through the center of the core internal component). The combined angle of the plural endplates will most commonly be between three (3) and six (6) degrees. The height of the implantable intervertebral spinal disk  102  may vary from about 12 mm to about 20 mm in the front, and from about 2 mm to about 3 mm shorter in the back (e.g., about 9 mm to about 17 mm). The implantable intervertebral spinal disk  102  is about 30 mm to about 35 mm wide (between side surfaces  104 ,  108 ), about 20 mm to about 25 mm deep (between back surface  106  and front surface  1  lo), and about 10 mm to about 15 mm high (from contact surface to contact surface). The attachment devices  116  on each contact surface are about 4 mm high. The foregoing overall dimensions of the implantable intervertebral spinal disk  102  (as well as the dimensions of internal components that are described below) may be varied for the different levels of the spine (e.g., cervical, lumbar, thoracic) and for different people. 
         [0030]    Still further with reference to  FIG. 1 , each of the endplates includes a base portion that is wider than the respective opening to be contained by the external shell  118 , an upper portion that approximates the size, shape and height of the opening and that fits in the opening, and plural attachment devices  116  that protrude through the opening. The base portion of the endplates is bounded by the external shell  118 . The upper portion of the endplates forms a continuous surface (e.g., contact surface  114 ) with the exterior shell  118 . The contact surface (e.g., contact surface  114 ) may be planar or convex (e.g., contact surface  114  rounded outward) to fit the geometry of the vertebrae. However, the contact surface is not limited and may be variable or even custom shaped to fit a particular disk space between the vertebrae. The plural attachment devices  116  of each endplate are projections in a geometric arrangement adapted to penetrate into a respective vertebra and anchor the vertebra, inducing bony ingrowths to integrate or fixate the example implantable intervertebral spinal disk  102  between vertebrae. Each of the plural attachment devices  116  is generally of a trapezoidal shape with a triangular cross-section (wedge shape) in the vertical dimension to achieve easier penetration into the vertebrae and is curve-shaped or arcuate in the horizontal dimension. The plural attachment devices  116  may be disposed in a generally oval or circular arrangement on the upper portion  114  of the endplates. The shape of the plural attachment devices  116  and the arrangement of the plural attachment devices  116  on the contact surfaces are adapted to further mitigate translation (side-to-side) movement and rotational movement. 
         [0031]    Yet further with reference to  FIG. 1 , other arrangements of the plural attachment devices  116  are of course possible. Each of the plural attachment devices  116  may be similarly or differently shaped. Furthermore, the plural attachment devices  116  may include spikes, keels, flanges and the like to fixate the intervertebral spinal disk  102  to the vertebrae. Still further, the plural attachment devices  116  may be irregularities on the contact surfaces to increase friction, small teeth, or ridges running in the same or different directions. The shape of ridges can be symmetrically triangular and may point back like shark teeth. Yet further, a single ridge or several ridges may be disposed on the contact surfaces from front to back of the implantable intervertebral spinal disk  102  that may be impacted into the vertebra. The plural attachment devices  116  may include a tag like extension wrapping in front of the vertebra with holes for screws that fixate the implantable intervertebral spinal disk  102  to the vertebrae. Still further, spikes, hooks or other fixation devices may be concealed within a center of the endplates and engaged when the implantable intervertebral spinal disk  102  is inserted into the disk space between the vertebrae. 
         [0032]      FIG. 2  is a semi-transparent perspective view  200  of the implantable intervertebral spinal disk  102  in accordance with  FIG. 1 . The external shell  118  is shown to be transparent to illustrate internal components of the implantable intervertebral spinal disk  102 . As described in reference to  FIG. 1  above and shown in greater detail in  FIG. 2 , the internal components include a pair of opposing endplates  202 ,  204 , and a core internal component  208  sandwiched between the pair of endplates  202 ,  204 . Endplate  202  includes a contact surface  114  with attachment devices  116  arranged thereon in a first arrangement and opposing endplate  204  includes a contact surface  206  with attachment devices  116  arranged thereon in a second arrangement. The attachment devices  116  of the first and second arrangements may be similarly or differently shaped, and further may be arranged similarly or differently on the respective contact surfaces  114 ,  206 . 
         [0033]    Further with reference to  FIG. 2 , the core internal component  208  will be described in greater detail with reference to  FIG. 7  herein below. At this point it is sufficient to note that the core internal component  208  is viscoelastic, a flexible non-compressible solid or liquid that is deformable and yet sufficiently resilient to stress or pressure of body movement and positioning. It is also noted here and will be described in greater detail hereinafter that the external shell  118  is generally stiffer then the core internal component  208 . The external shell  118  is adapted not only to contain or bound the internal components  202 ,  204  and  208 , but also to provide sufficient inward vertical pressure during operation between the internal components,  202 ,  204 , and  208  and inward lateral pressure on the internal components  202 ,  204  and  208 . During operation the pressures on the implantable intervertebral spinal disk  102  may be substantial and may change or shift about the implantable intervertebral spinal disk  102  with different body movements (e.g., sitting, standing, bending, and/or twisting movements). Under these pressures the implantable intervertebral spinal disk  102  deforms vertically (generally insignificantly) with range of angulation of about 4 degrees to about 5 degrees in flexion and about 2 degrees to about 3 degrees in extension. The external shell  118  is resilient to mitigate movement or shifting (side to side and/or back to front) between the internal components  202 ,  204  and  208 . 
         [0034]    In addition, the core internal component  208  is preloaded in the external shell  119  to produce outward pressure, expanding or stretching the external shell  118  about internal components  202 ,  204  and  208 , and further pressing the endplates  202 ,  204  respectively up and down against the external shell  118  and the adjacent vertebrae. Thus, the external shell  118  and the core component  208  provide for sufficient pressure between internal components  202 ,  204  and  208  to mitigate the movement or shifting between the internal components  202 ,  204 , and  208  under varying stresses applied to the spine (e.g., flexion, extension, lateral bending or rotation movements) causing the core internal component  208  to deform during the operation of the intervertebral spinal disk  102 . In addition to the foregoing inward and outward pressures, because the endplates  202 ,  204  protrude through the external shell  118  and have a tight fit therethrough (e.g., through opening  112 ), movement or shifting of the these internal components  202 ,  204  is further restrained during the operation of the intervertebral spinal disk  102 . 
         [0035]      FIG. 3  is a semi-transparent side view  300  of the implantable intervertebral spinal disk  102  in accordance with  FIG. 1 . As illustrated in the side view  300 , the height  302  at the rear  306  of the implantable intervertebral spinal disk  102  is greater than the height  304  at the front  308  of the implantable intervertebral spinal disk  102 . The rise between the rear  306  and the front of the implantable intervertebral spinal disk  102  is adapted to approximate the disk space between vertebrae into which the implantable intervertebral spinal disk  102  is to be implanted, aligning the vertebrae in their natural position in relation to each other. As noted hereinbefore, contact surfaces  114 ,  206  may be of approximately parallel configuration for thoracic uses, may have varying rise configurations for lumbar and cervical uses, and/or may accommodate individual anatomy for any of the foregoing segments of the spine (e.g., thoracic, lumbar, cervical) into which the implantable intervertebral spinal disk  102  is to be implanted. The height  304  of the implantable intervertebral spinal disk  102  may vary from about 12 mm to about 20 mm at the front  308 , and from about 2 mm to about 3 mm shorter at the back  306  (e.g., about 9 mm to about 17 mm). As described with reference to  FIG. 1  above, the implantable intervertebral spinal disk  102  is about 30 mm to about 35 mm wide, about 20 mm to about 25 mm deep, and about 10 mm to about 15 mm high (from contact surface  114  to contact surface  206 ). The attachment devices  116  on each contact surface  114 ,  206  are about 4 mm high. The overall size of the implantable intervertebral spinal disk  102 , as well as the differences in heights  302 ,  304  to achieve a different rise, may be configured or adjusted based on a particular patient&#39;s vertebral dimensions (anatomy) and configuration of the vertebral space between vertebrae into which the implantable intervertebral spinal disk  102  is to be implanted. As illustrated in the side view  300 , the height of the core internal component  208  is approximately the same throughout the component. 
         [0036]    Further with reference to  FIG. 3 , the height of the endplates  202 ,  204  may be varied from the rear  306  to the front  308  to achieve the desired heights  302 ,  304  at the respective rear  302  and front  304  of the implantable intervertebral spinal disk  102 . More specifically, the height of base portion of the endplates  202 ,  204  (described in greater detail in reference to  FIGS. 5 and 6  hereinbelow) may be varied similarly for each endplate  202 ,  204  from the rear  306  to the front  308  to achieve the desired heights  302 ,  304  of the implantable intervertebral spinal disk  102 . It is noted, however, that the heights of the front and rear of each of the endplates  202 ,  204  may be adjusted independently to maintain the same overall shape of the implantable intervertebral spinal disk  102 , yet achieve a relatively different angle of the core internal component  208  relative to the configuration of the vertebral space between vertebrae into which the implantable intervertebral spinal disk  102  is to be implanted. It is further noted that the overall heights  302 ,  304  may also be achieved by varying the height between the rear and the front of the core internal component  208 , or by the combination of the core internal component  208  and the endplate  202 ,  204 . 
         [0037]      FIG. 4  is a perspective view  400  of an exterior shell  118  of the implantable intervertebral spinal disk  102  in accordance with  FIG. 1 . The exterior shell  118  is adapted to contain the internal components  202 ,  204  and  208  as described hereinabove, while providing inward pressure on the internal components when loaded into the exterior shell  118 . The external shell  118  is made of a biocompatible and bio-stable (non-biodegradable) material that is viscoelastic, a flexible non-compressible solid that is deformable and yet sufficiently resilient to stress or pressure of body movement and positioning. The exterior shell  118  should have a Youngs modulus of elasticity of about 7 MPa to about 13 MPa. A variety of elastomers, such as polyurethanes, silicones, hydrogels, collagens, hyalurons, proteins and other synthetic polymers can be used to achieve the foregoing biocompatibility, bio-stability and the range of viscoelastic properties. 
         [0038]    Further with reference to  FIG. 4 , the exterior shell  118  includes a first oval opening  112  and a second oval opening  404  to secure respective endplates  202 ,  204 , yet facilitate the protrusion of plural attachment devices  116  through openings  112 ,  404 . As already mentioned hereinabove in reference to  FIG. 1 , the openings may be of a different shape. The dimensions of the openings  112 ,  404  may be from about 15 mm to about 25 mm. The exterior shell  118  has a thickness  406  from about 1 mm to about 2 mm to provide deformation and elasticity (e.g., about 7 MPa to about 13 MPa) and an interior space (or compartment)  408  to contain internal components  202 ,  204  and  208  as described herein. 
         [0039]      FIG. 5  is a top view  500  of an endplate  202 ,  204  of the implantable intervertebral spinal disk  102  in accordance with  FIG. 1 . The endplates  202 ,  204  are made of a biocompatible metal, such as titanium, stainless steel or other biocompatible metal. The endplate  202 ,  204  has a base portion  510  that is defined by approximately straight sides  502 ,  506  (e.g., from about 17 mm to about 22 mm), straight back  504  (e.g., about 25 mm to about 30 mm wide) that is approximately perpendicular to the sides  502 ,  506 , and a curve-shaped or arcuate front  508 . Atop the base portion and approximately centrally disposed thereon is a top portion  512  that is oval-shaped to fit the openings of the exterior shell  118  (e.g., openings  112 ,  404 ). The top portion  512  may be shaped differently (e.g., other than oval) to fit differently-shaped exterior shell  118  openings. What is important to note is that the top portion  512  should be as large as possible to provide the largest endplate-bone contact surface  518  (e.g., contact surfaces  114 ,  206  of  FIGS. 1-3 ) for integration between the endplate  202 ,  204  and the bony vertebrae, yet provide a sufficient base portion  510  to be retained securely within the exterior shell  118 . Furthermore, atop the top portion  512  on the contact surface  518  there are disposed attachment devices  116  in approximately oval or circular arrangement. As described hereinabove with reference to  FIG. 1 , the attachment devices  116  are adapted to penetrate into respective vertebra and induce bony ingrowths. 
         [0040]    Further with reference to  FIG. 5 , the contact surface  518  may further be coated with calcium phosphate, hydroxyapatite, ceramics, biomimetic appetites, bioactive glass, covalently attached bioactive conjugates such as osteopontin, bone sialoprotein, bone acidic glycoprotein-75, osteocalcin, bone morphogenic proteins, transforming growth factors, laminin, type IV collagen, type VIII collagen, enamel proteins, cell adhesion peptides, prostaglandins, serum proteins, glucocorticosteroids, phosphoserine, pyrophosphates, phosphothreonine, phosvitin, phosphophoryn, phosphonates, phosphotases and bone and epithelial proteoglycans, as well as other osteoinductive and osteoconductive materials. 
         [0041]      FIG. 6  is a perspective view  600  of a pair of endplates  202 ,  204  of the implantable intervertebral spinal disk  102  in accordance with  FIGS. 1 and 5 . The endplates  202 ,  204  include respective base portions  510 , top portions  512  and attachment devices  116 . The base portions  510  of endplates  202 ,  204  have respective planar surfaces  602 ,  604  for mating with a planar surface of the core internal component  208  shown in greater detail in  FIG. 6 . The base portions  510  of endplates  202 ,  204  further have varying heights  606  and  608  described hereinabove in greater detail in reference to  FIG. 3 . Although the heights  606 ,  608  may be varied to achieve various angulations as described herein in relation to  FIG. 3 , the height  606  may be about 1 mm to 1.5 mm, while height  608  may be about 2 mm to about 2.5 mm. 
         [0042]      FIG. 7  is a perspective view  700  of a core internal component  208  of the intervertebral spinal disk  102  in accordance with  FIG. 1 . The core internal component  208  has similar dimensions to the base portions  510  of endplates  202 ,  204 . More specifically, the core internal component  208  is defined by approximately straight sides  702 ,  706  (e.g., from about 17 mm to about 22 mm), straight back  704  (e.g., about 25 mm to about 30 mm wide) that is approximately perpendicular to the sides  702 ,  706 , and a curve-shaped or arcuate front  708 . The core internal component  208  has a height  712  of about 6 mm to about 8 mm. 
         [0043]    Further with reference to  FIG. 7 , the core internal component  208  is made of a biocompatible and bio-stable (non-biodegradable) material that is viscoelastic, a flexible non-compressible solid that is deformable and yet sufficiently resilient to stress or pressure of body movement and positioning. The core internal component  208  should have a Youngs modulus of elasticity of about 3 MPa to about 8 MPa. A variety of elastomers, such as polyurethanes (e.g., polycarbonated urethane), silicones, hydrogels, collagens, hyalurons, proteins and other synthetic polymers (e.g., injectable polymers) can be used to achieve the foregoing biocompatibility, bio-stability and the range of viscoelastic properties. The core internal component  208  may be pre-fabricated into a solid and assembled into the implantable intervertebral spinal disk  102  before implantation or may be injected into a constrained space between the endplates  202 ,  204 , such as via a balloon between the endplates  202 ,  204 , after the implantable intervertebral spinal disk  102  is implanted between the vertebrae. The core internal component  208  may, but need not, include vertical reinforcement columns  710  made of a stiffer viscoelastic material. For example, if the implantable intervertebral spinal disk  102  is narrow (which may present a placement advantage because side to side exposure requires getting around big blood vessels), a pair of vertical reinforcement columns  710  may be inserted into sides  702 ,  706  of the core internal component  208  through similarly shaped vertical cut-outs in the core internal component  208 . The vertical reinforcement columns  710  may have a Youngs modulus of elasticity between 6 MPa to about 13 MPa (e.g., greater than the Youngs modulus of elasticity of the core internal component itself). 
         [0044]    In accordance with  FIGS. 1-7 , the implantable intervertebral spinal disk  102  may be made of pre-fabricated exterior shell  118  and internal components  202 ,  204  and  208 , assembled into implantable intervertebral spinal disk  102 , which is then inserted into the disk space between vertebrae. In the assembly process, one of the endplates  202 ,  204  is inserted into interior space  408  of exterior shell  118  through one of the openings  112 ,  404  by stretching the viscoelastic exterior shell  118  around the base portion  510  of the endplate  202 ,  204 . A metal bar or like assist instrument (e.g., screwdriver) may be used to assist in stretching the exterior shell over the endplate  202 ,  204 . A portion of the endplate  202 ,  204  is placed in the opening  112 ,  404 . The assist instrument may then be placed over that portion of the endplate  202 ,  204  and under the exterior shell  208  through the opening  112 ,  404 . The assist instrument is then mechanically traced about the opening  112 ,  404  until the exterior shell covers the base portion  510  of endplate  202 ,  204  and the top portion  512  is in the opening  112 ,  404 . Once the first endplate  202 ,  204  is inserted into the interior space  408  of the exterior shell  118 , the core internal component  208  may be inserted into the interior space  408  of the exterior shell  118  through the opposite opening  112 ,  404  in a similar fashion. The other of the endplates  202 ,  204  is then inserted into interior space  408  of exterior shell  118  through the same opening  112 ,  404  as the core internal component  208  in a similar fashion. The assembled implantable intervertebral spinal disk  102  is thereafter inserted in the disk space between the vertebrae. 
         [0045]    Further in accordance with  FIGS. 1-7 , the implantable intervertebral spinal disk  102  may also partially pre-fabricated and assembled into implantable intervertebral spinal disk  102 , which is then inserted into the disk space between vertebrae and completed after insertion. More specifically, one of the endplates  202 ,  204  is inserted into interior space  408  of exterior shell  118  through one of the openings  112 ,  404 . An empty balloon that will form the core internal component  208  once completed may be inserted into the interior space  408  of the exterior shell  118  through the opposite opening  112 ,  404 . The other of the endplates  202 ,  204  is then inserted into interior space  408  of exterior shell  118  through the same opening  112 ,  404  as the balloon that will form the core internal component  208 . The implantable intervertebral spinal disk  102  is then inserted into the disk space between vertebrae. The balloon is thereafter filled with a viscoelastic injectable polymer to complete the fabrication and implantation of the implantable intervertebral spinal disk  102 . 
         [0046]    In operation in accordance with reference to  FIGS. 1-7 , the implantable intervertebral spinal disk  102  restores spinal disk height and approximates closely the biomechanics of a normal spinal disk, mitigating abnormal strain on the facet joints and debris caused by the conventional implantable spinal devices, as well as the likelihood of failure or extrusion of the components of the implantable intervertebral spinal disk  102 . 
         [0047]    Thus, an example implantable intervertebral spinal disk and method for assembling the implantable intervertebral spinal disk have been described. Although specific example embodiments have been described, it will be evident that various modifications and changes may be made to these embodiments without departing from the broader spirit and scope of the invention. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense. The accompanying drawings that form a part hereof, show by way of illustration, and not of limitation, specific embodiments in which the subject matter may be practiced. The embodiments illustrated are described in sufficient detail to enable those skilled in the art to practice the teachings disclosed herein. Other embodiments may be utilized and derived therefrom, such that structural and logical substitutions and changes may be made without departing from the scope of this disclosure. This Detailed Description, therefore, is not to be taken in a limiting sense, and the scope of various embodiments is defined only by the appended claims, along with the full range of equivalents to which such claims are entitled. 
         [0048]    Such embodiments of the inventive subject matter may be referred to herein, individually and/or collectively, by the term “invention” merely for convenience and without intending to voluntarily limit the scope of this application to any single invention or inventive concept if more than one is in fact disclosed. Thus, although specific embodiments have been illustrated and described herein, it should be appreciated that any arrangement calculated to achieve the same purpose may be substituted for the specific embodiments shown. This disclosure is intended to cover any and all adaptations or variations of various embodiments. Combinations of the above embodiments, and other embodiments not specifically described herein, will be apparent to those of skill in the art upon reviewing the above description. 
         [0049]    The Abstract is provided to comply with 37 C.F.R. §1.72(b) and will allow the reader to quickly ascertain the nature and gist of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. 
         [0050]    In the foregoing description of the embodiments, various features are grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting that the claimed embodiments have more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus the following claims are hereby incorporated into the Description of the Embodiments, with each claim standing on its own as a separate example embodiment.