Abstract:
A segmented disc nucleus replacement prosthesis and system for its implantation wherein the segments are made of a compliant, homogeneous material throughout. The prosthesis comprises a plurality of modular segments that mate together in a rail-and-slot arrangement. The rails and slots are configured to interlock and hold together under load despite being formed of compliant materials. In one embodiment, insertion tools and stabilizers are utilized for manipulation of the modular segments, the insertion tools and stabilizers being designed to accommodate for handling the compliant modular segments.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/685,383, filed Mar. 16, 2012, the disclosure of which is incorporated herein in its entirety except for express definitions contained therein. 
    
    
     BACKGROUND 
     The use of segmented spinal implants where the segments are implanted sequentially using insertion guides for so-called “minimally invasive” surgical techniques is known. United States Patent Application No. 2008/0133017 to Beyar, et al. (hereinafter “Beyar”) discloses a two-level, motion preserving total disc replacement system using dual-level segments, each segment comprising a top “slice” and a bottom “slice” in order to construct both levels of the motion-preserving device. 
     U.S. Pat. No. 7,591,853 to Felt, et al. (hereinafter “Felt”) discloses a segmented disc nucleus implant that includes a hard inner core surrounded by a compliant outer shell. While the compliant outer shell facilitates motion preserving aspects, the inner core can be constructed of harder materials such as polyetheretherketone (PEEK) to facilitate structures for reliably interlocking the segments. 
     A system that, like the device of Felt but unlike the device of Beyar, replaces only the disc nucleus for enhanced reduced invasiveness, and that further enhances the motion preserving aspects of the device of Felt would be welcomed. 
     SUMMARY 
     Various embodiments of the invention include a segmented nucleus disc prosthesis made of a single compliant material homogeneously throughout the prosthesis. The compliant material provides a disc nucleus implant that more closely mimics the motion of a natural disc nucleus than the other segmented motion-preserving devices available. The segmented aspect enables implantation of the device with reduced invasiveness, as well as a modular design that enables prostheses of varying dimension to be constructed from modular segment “building blocks.” 
     Heretofore, the presence of harder materials was thought necessary to facilitate the interlocking. Thus, the segmented implants of Beyar and Felt have included interlocking structures comprising hard materials such as metals and PEEK. The presence of these hard materials can affect the complexity of the design of motion preserving devices, or limit the motion of the spine relative to a natural disc nucleus. The disclosed invention differs from Beyar and Felt in that the interlocking modular segments do not include hard materials to facilitate the interlocking aspects. Rather, the interlocking modular segments of various embodiments of the invention are made of a single compliant material. Furthermore, Beyar teaches a total disc replacement, rather than a disc nucleus replacement, and is therefore generally more invasive than the present invention. 
     Structurally, various aspects of the embodiments of the invention are directed to enable affirmative interlocking of the segments despite the use of the compliant material. In certain embodiments, a plurality of modular segments is included, each including a superior side, an inferior side, a proximal end, and a distal end opposite the proximal end. The superior and inferior sides are disposed on opposing faces of a transverse plane of the respective modular segment, the transverse plane being orthogonal to a superior/inferior coordinate of the respective modular segment when in an implanted configuration. In some embodiments, the transverse plane corresponds to a central transverse plane. Each of the plurality of modular segments are adapted to interlock with an adjacent one of the plurality of modular segments in a side-by-side arrangement on the transverse plane when in the implanted configuration. The plurality of modular segments can comprise a first end modular segment including a first end body portion and a first end rail portion, the first end rail portion extending from a flanking face of the first end body portion the first end rail portion defining a first end rail axis that passes through the proximal end and the distal end of the first end modular segment. The first end rail axis lies on the transverse plane of the first end modular segment. The first end rail portion includes a plurality of diametrically opposed barbs that extend radially outward relative to the first end rail axis and parallel to the superior/inferior coordinate of the first end modular segment, the first end rail portion having a first end rail cross-section that is normal to the first end rail axis. In one embodiment, the first end rail portion includes a web and a rail head, the web being disposed between the rail head and the flanking face and extending along the transverse plane. The rail head can include planar faces that intersect the web at one of a right angle and an acute angle. 
     The prosthesis can further comprise an opposing end modular segment including a body portion that defines an opposing end elongate slot having an interior surface, the opposing end elongate slot passing through the body portion of the opposing end modular segment to define an opposing end slot axis, the opposing end slot axis lying on a transverse plane that is normal to the superior/inferior coordinate of the opposing end modular segment. The body portion of the opposing end modular segment defines an opposing end body cross-section normal to the opposing end slot axis, the body portion of the opposing end modular segment further defining a plurality of recesses that are recessed from the interior surface of the opposing end elongate slot. Each of the plurality of recesses can extend radially outward relative to the opposing end slot axis and parallel to the superior/inferior coordinate of the opposing end modular segment. 
     The body portion of the opposing end modular segment can include a superior lip portion and an inferior lip portion, each of the lip portions being adjacent the opposing end elongate slot and extending parallel to the opposing end slot axis and each protruding toward the transverse plane of the opposing end modular segment. Each of the superior lip portion and the inferior lip portion of the opposing end modular segment can define an interior face that complements the planar faces of the rail head. In one embodiment, the first end rail portion defines a first mounting port accessible from the proximal end of the first end modular segment. The first end segment can further comprise a stop portion at the distal end of the first segment. 
     In one embodiment, the opposing end body cross-section can be complementary to the first end rail cross-section of the first end rail portion for sliding engagement between the first end modular segment and the opposing end modular segment along the first end rail axis. Each of the plurality of recesses of the opposing end modular segment can be positioned and dimensioned complementary to a corresponding one of the plurality of barbs of the first end modular segment. The diametrically opposed barbs of the first end rail portion are adapted for capture within the plurality of recesses of the body portion of the opposing end modular segment when in the implanted configuration. The first end modular segment and the opposing end modular segment can be adapted to interlock with each other to define an implanted configuration presenting a generally continuous periphery that generally corresponds to the evacuated nucleus disc space. 
     In another embodiment the plurality of modular segments that make up the prosthesis includes an intermediate modular segment having an intermediate body portion and an intermediate rail portion. The intermediate rail portion extends from a flanking face of the intermediate body portion, the intermediate rail portion defining an intermediate rail axis that passes through the proximal end and the distal end of the intermediate modular segment. The intermediate rail portion can include a plurality of diametrically opposed barbs that extend radially outward relative to the intermediate rail axis and parallel to the superior/inferior coordinate of the intermediate modular segment. The intermediate rail portion defines an intermediate cross-section normal to the intermediate rail axis. 
     The intermediate body portion defines an intermediate elongate slot having an interior surface, the intermediate elongate slot passing through the intermediate body portion to define an intermediate slot axis. The intermediate rail axis and the intermediate slot axis can be substantially parallel to each other and lying on the transverse plane of the intermediate modular segment, the intermediate body portion defining an intermediate body cross-section normal to the intermediate slot axis. The body portion of the intermediate modular segment can further defining a plurality of recesses that are recessed from the interior surface of the intermediate elongate slot and extend radially outward relative to the intermediate slot axis and parallel to the superior/inferior coordinate of the intermediate modular segment. In one embodiment, the intermediate body portion includes a superior lip portion and an inferior lip portion, each being adjacent the intermediate elongate slot and extending parallel to the intermediate slot axis and each protruding toward the transverse plane of the intermediate modular segment. 
     In one embodiment of the invention, the intermediate body cross-section is complementary to the first end rail cross-section of the first end rail portion for sliding engagement between the first end modular segment and the intermediate modular segment along the first end rail axis. Each of the plurality of recesses of the intermediate modular segment can be positioned and dimensioned complementary to a corresponding one of the plurality of diametrically opposed barbs of the first end modular segment. The diametrically opposed barbs of the first end rail portion can be configured for capture within the plurality of recesses of the intermediate modular segment when in the implanted configuration. In this embodiment, the opposing end body cross-section is complementary to the intermediate rail cross-section for sliding engagement between the opposing end modular segment and the intermediate modular segment along the intermediate rail axis. Each of the plurality of recesses of the opposing end modular segment can be positioned and dimensioned complementary to a corresponding one of the plurality of diametrically opposed barbs of the intermediate modular segment. The diametrically opposed barbs of the intermediate rail portion can be adapted for capture within the plurality of recesses of the body portion of the opposing end modular segment when in the implanted configuration. The first end modular segment can be adapted to interlock with the intermediate modular segment and the intermediate modular segment being adapted to interlock with the opposing end modular segment to define an implanted configuration presenting a generally continuous periphery that generally corresponds to the evacuated nucleus disc space. 
     In various embodiments, each modular segment is of a homogenous material having a compressive modulus between about 2 and about 100 MPa. 
     In various embodiments, each of the plurality of modular segments includes structure defining a mounting port disposed on and accessible from the proximal end, the mounting port including an interior surface and a plurality of detents that extend from a first side of the interior surface, wherein a second side opposite the first side defines a cylindrical surface. 
     In another embodiment of the invention, a system for configuring the modular disc nucleus prosthesis includes a plurality of insertion tools, one for each of the plurality of segments and each including a tip portion having a plurality of notches formed on one side thereof, the tip portion extending along a rotation axis and being dimensioned for insertion into the mounting ports of the modular segments, the notches being configured to mate with the detents within the mounting port. The tip portion can be selectively releasable from the corresponding one of the mounting ports by rotating the insertion tool about the central axis. 
     Various embodiments of the invention are suitable for implantation from any direction relative to the superior/inferior coordinate (i.e., a posterior, anterior or lateral approach, or any approach in between). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts various components of a disc nucleus replacement system in an embodiment of the invention; 
         FIGS. 2A through 2D  are perspective views of various modular disc nucleus prostheses embodiments of the invention; 
         FIGS. 3A through 3C  are perspective views of individual modular segments of the prostheses of  FIGS. 2A through 2D  in embodiments of the invention; 
         FIG. 4  is a sectional view of a first end modular segment (“A-segment”) in an embodiment of the invention; 
         FIG. 5  is a sectional view of an intermediate modular segment (“B-segment”) in an embodiment of the invention; 
         FIG. 6  is a sectional view of an elongate slot portion of the modular segments of  FIGS. 4 and 5  in an embodiment of the invention; 
         FIG. 7  is a sectional view of an opposing end modular segment (“C-segment”) in an embodiment of the invention; 
         FIG. 8  is a partial sectional view of an alternative rail portion and body portion configuration in an embodiment of the invention; 
         FIG. 9  is a side view of an A-segment or a B-segment in an embodiment of the invention; 
         FIG. 10  is a sectional view of a B-segment at the transverse plane in an embodiment of the invention; 
         FIG. 11  is an exploded view of an A/B insertion tool in an embodiment of the invention; 
         FIG. 11A  is an enlarged, elevation view of a tip portion of the A/B insertion tool of  FIG. 11  in an embodiment of the invention; 
         FIG. 11B  is a sectional view of the “D-shaped” shaft portion of the A/B insertion tool of  FIG. 11  in an embodiment of the invention; 
         FIG. 12  is a perspective view of an assembly of an A- or B-segment/insertion tool assembly in an embodiment of the invention; 
         FIG. 12A  is an elevational view of the assembly of  FIG. 12 ; 
         FIGS. 13A through 13H  depict the assembly of a modular nuclear disc prosthesis in an embodiment of the invention; 
         FIG. 14  is a side view of a removal tool in an embodiment of the invention; 
         FIG. 14A  is a sectional view of the removal tool of  FIG. 14 ; 
         FIGS. 15A through 15D  depict operation of the removal tool of  FIG. 14  in an embodiment of the invention; 
         FIG. 16  is a perspective view of an A-segment stabilizer in an embodiment of the invention; 
         FIG. 16A  is a side view of the A-segment stabilizer of  FIG. 16  in an embodiment of the invention; 
         FIG. 17  is a perspective view of a B-segment stabilizer in an embodiment of the invention; 
         FIG. 17A  is a sectional view of the B-segment stabilizer of  FIG. 17  in an embodiment of the invention; 
         FIG. 17B  is a partial plan view of the B-segment stabilizer of  FIG. 17  in an embodiment of the invention; 
         FIGS. 18A and 18B  are plan views depicting the coupling of the A-segment stabilizer to an A-segment/insertion tool assembly in an embodiment of the invention; 
         FIG. 19  is a perspective view of a C-segment insertion tool in an embodiment of the invention; 
         FIG. 19A  is an end view of the C-segment insertion tool in an embodiment of the invention; 
         FIG. 19B  is a partial elevational view of the tip portion of the C-segment insertion tool of  FIG. 19  in an embodiment of the invention; 
         FIG. 20  is a perspective view of a C-segment stabilizer C-segment insertion tool in an embodiment of the invention; 
         FIG. 20A  is an elevational view of the C-segment stabilizer C-segment insertion tool of  FIG. 20  in an embodiment of the invention; and 
         FIG. 20 b    is an end view of the C-segment stabilizer C-segment insertion tool of  FIG. 20  in an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a disc nucleus replacement system  30  is depicted in an embodiment of the invention. The disc nucleus replacement system  30  includes modular segments  32   a ,  32   b  and  32   c  for assembly of modular disc nucleus prostheses  34   a  through  34   d  ( FIG. 2 ) (hereinafter “the prosthesis” or “prostheses”)”, insertion tools  36   a  and  36   c  for installing the modular segments  32  of the prosthesis, and various tools to assist in the installation of the prosthesis, including a loading platform  42 , a removal tool  44 , an A-segment stabilizer  46 , a B-segment stabilizer  48  and a C-segment stabilizer  52 . 
     Throughout this disclosure, it is noted that certain components have numerical references consisting of a number followed by a letter suffix (e.g., modular segments  32   a ,  32   b  and  32   c  and insertion tools  36   a  and  36   c  above). Where this numbering convention is utilized, the number refers to the item generically or collectively, and the letter to the item in particular. Following this convention, when the number is referred to alone, the reference is to the item generically or collectively (e.g., modular segment(s)  32  or insertion rod(s)  36 ). 
     Referring to  FIGS. 2A through 2D  (referred to collectively as  FIG. 2 ) and  FIGS. 3A through 3C  (referred to collectively as  FIG. 3 ), various prostheses and the modular segments from which they are constructed are depicted in embodiments of the invention. Each prosthesis  32   a  through  32   d  includes a plurality of modular segments  32  (i.e., at least two segments) interlocked with each other. Each modular segment  32  includes a superior side  62 , an inferior side  64 , a proximal end  66 , and a distal end  68  opposite the proximal end  66 . The superior and inferior sides  62  and  64  of each modular segment  32  are disposed on opposing faces of a transverse plane  72   a ,  72   b  and  72   c  of the respective modular segment  32 , the transverse planes  72   a ,  72   b  and  72   c  being orthogonal to a superior/inferior coordinate  74   a ,  74   b  and  74   c  of the respective modular segment  32   a ,  32   b  and  32   c  when in an implanted configuration. Proximal/distal coordinates  76   a ,  76   b  and  76   c  are also defined as orthogonal to the superior inferior coordinates  74   a ,  74   b  and  74   c  and extending in a direction from the proximal ends  66  to the distal ends  68 . Each of the plurality of modular segments  32  are adapted to interlock with an adjacent one of the plurality of modular segments in a side-by-side arrangement relative to the respective transverse planes  72  when in the implanted configuration. 
     The superior and inferior sides  62  and  64  are so named to correspond with their orientation along a respective superior/inferior coordinate  74  or superior/inferior direction when the prosthesis  34  is installed in an implanted configuration within the human body. In one embodiment, the modular segments  32  of the prosthesis  34  can be symmetric about the transverse plane  72 ; that is, for this embodiment, the superior side  62  of each modular segment  32  is a mirror image of the inferior side  64  about the transverse plane  72 , thus establishing the transverse plane  72  as a central transverse plane  78 . 
     The prostheses depicted in  FIG. 2  include up to three modular segment types, depicted in  FIG. 3 : the first end modular segment  32   a , alternatively referred to herein as an “A” segment ( FIG. 3A ); the intermediate modular segment  32   b , alternatively referred to herein as a “B” segment ( FIG. 3B ); and the opposing end modular segment  32   c , alternatively referred to herein as a “C” segment ( FIG. 3C ). In various embodiments of the invention, prostheses  34  can include one or more B-segments  32   b , or can have no B-segments  32   b  (i.e., the A-segment  32   a  is coupled directly to the C-segment  32   c , as depicted in  FIG. 2A ). While the length, width and thickness dimensions of individual B-segments  32   b  can vary, the general characteristics are the same, as described below. In some embodiments employing multiple B-segments  32   b , the B-segments  32   b  are identical. 
     Referring to  FIG. 4  and again to  FIG. 3A , the first end modular segment  32   a  (A-segment) is depicted in an embodiment of the invention. The first end modular segment  32   a  includes a body portion  84   a  and a rail portion  82   a , the rail portion  82   a  extending from a flanking face  86   a  of the body portion  84   a . In one embodiment, the rail portion  82   a  includes a web  88   a  and a rail head  92   a , the flanking face  86   a  and the rail head  92   a  being separated by the web  88   a . The rail head  92   a  can define a rail axis  94   a  that passes through the proximal and distal ends  66   a  and  68   a  of the modular segment  32   a . The rail portion  82   a  can also include a plurality of diametrically opposed barb portions  96   a  that extend radially outward relative to the rail axis  94   a  and parallel to the superior/inferior coordinate  74   a  of the first end modular segment  32   a . In one embodiment, the first end modular segment  32   a  includes a stop portion  98   a  located at the distal end  68   a . Cross-sections  102   a  and  104   a  of the rail portion  82   a  and the body portion  84   a , respectively, of the first end modular segment  32   a  are depicted at  FIG. 4 , the cross-sections  102   a ,  104   a  being normal to the rail axis  94   a.    
     Herein, the rail portion  82   a , rail axis  94   a , rail portion cross-section  102   a , body portion  82   a  and body portion cross-section  104   a  of the first end modular segment  32   a  are alternatively referred to as the first end rail portion  82   a , first end rail axis  94   a , first end rail cross-section  102   a , first end body portion  82   a , and first end body portion cross-section  104   a , respectively, to clarify association with the first end modular segment  32   a.    
     Referring to  FIGS. 5 and 6  and again to  FIG. 3B , the intermediate modular segment  32   b  (B-segment) is depicted in an embodiment of the invention. The intermediate modular segment  32   b  includes a body portion  82   b  and a rail portion  82   b , the rail portion  82   b  extending from a flanking face  86   b  of the body portion  82   b . In one embodiment, the rail portion  82   b  includes a web  88   b  and a rail head  92   b , the flanking face  86   b  and the rail head  92   b  being separated by the web  88   b . The rail head  92   b  can define a rail axis  94   b  that passes through the proximal and distal ends  66   b  and  68   b  of the modular segment  32   b . The rail portion  82   b  can also include a plurality of diametrically opposed barb portions  96   b  that extend radially outward relative to the rail axis  94   b  and parallel to the superior/inferior coordinate  74   b  of the intermediate modular segment  32   b . In one embodiment, the intermediate modular segment  32   b  includes a stop portion  98   b  located at its distal end. Cross-sections  102   b  and  104   b  of the rail portion  82   b  and the body portion  84   b , respectively, of the intermediate modular segment  32   b  are depicted at  FIG. 5 , the cross-sections  102   b ,  104   b  being normal to the rail axis  94   b.    
     The body portion  84   b  of the intermediate modular segment  32   b  defines an elongate slot  112   b  that passes through the body portion  84   b  and includes an interior surface  114   b . The elongate slot  112   b  further defines a slot axis  116   b  that is substantially parallel to the intermediate rail axis  94   b  of the intermediate modular segment  32   b . In one embodiment, the rail axis  94   b  and the slot axis  116   b  lie on the transverse plane  72   b  of the intermediate modular segment  32   b . In the depicted embodiment, the transverse plane  72   b  corresponds to the central transverse plane  78   b . The body portion  84   b  can also include a plurality of recesses  118   b  that extend parallel to the superior/inferior coordinate  74   b  in both the superior and inferior directions relative to the slot axis  116   b  ( FIG. 6 ). 
     Herein, the rail portion  82   b , rail axis  94   b , rail portion cross-section  102   b , body portion  84   b , elongate slot  112   b , slot axis  116   b  and body portion cross-section  104   b  of the intermediate modular segment  32   b  are alternatively referred to as the intermediate rail portion  82   b , intermediate rail axis  94   b , intermediate rail portion cross-section  102   b , intermediate body portion  84   b , intermediate elongate slot  112   b , intermediate slot axis  116   b  and intermediate body portion cross-section  104   b , to clarify association with the intermediate modular segment  32   b.    
     The intermediate body portion  84   b  can also be characterized as having a superior lip portion  122   b  and an inferior lip portion  124   b , each being named for their location along the superior/inferior coordinate  74   b  relative to the transverse plane  72   b . The lip portions  122   b ,  124   b  are adjacent to and partially define the intermediate elongate slot  112   b , and protrude toward each other. A gap  126   b  is defined between the superior lip portion  122   b  and the inferior lip portion  124   b , defining an open side  128   b  of the elongate slot  112   b . In the depicted embodiment, each lip portion  122   b ,  124   b  protrudes toward the central transverse plane  78   b . The lip portions  122   b  and  124   b  can also define an opposing flanking face  130   b  that faces in a direction opposite the flanking face  86   b  of the intermediate body portion  84   b.    
     Referring to  FIG. 7  and again to  FIG. 6 , the opposing end modular segment  32   c  (C-segment) is depicted in an embodiment of the invention. (It is noted that the cross-section depicted in  FIG. 6  applies to both  FIGS. 5 and 7 ). The opposing end modular segment  32   c  includes a body portion  84   c  that defines an elongate slot  112   c , the elongate slot  112   c  further defining a slot axis  116   c  that lies on the transverse plane  72   c . The elongate slot  112   c  includes an interior surface  114   c  and passes through the body portion  84   c  of the opposing end modular segment  32   c . The body portion  84   c  includes a body portion cross-section  104   c  that is normal to the slot axis  116   c . The body portion  84   c  of the opposing end modular segment  32   c  can also include a superior lip portion  122   c  and an inferior lip portion  124   c  having the same characteristics as the superior and inferior lip portions  122   b  and  124   b  of the intermediate modular segment  32   b . The body portion  84   c  of the opposing end modular segment  32   c  can further define a plurality of recesses  118   c  that are recessed from the interior surface  114   c  of the elongate slot  112   c  of the body portion  84   c . The recesses  118   c  can extend radially outward relative to the slot axis  116   c  and are parallel to the superior/inferior coordinate  74   c  of the opposing end modular segment  32   c.    
     Herein, the body portion  84   c , elongate slot  112   c , slot axis  116   c  and body portion cross-section  104   c  of the opposing end modular segment  32   c  are alternatively referred to as the opposing end body portion  84   c , opposing end elongate slot  112   c , opposing end slot axis  116   c  and opposing end body portion cross-section  104   c , to clarify association with the opposing end modular segment  32   c.    
     The rail heads  92  can each include faces  132  that are substantially planar and substantially parallel to the respective superior/inferior coordinate  74 , the faces  132  thereby being at a right angle relative to the respective web portion  88 . The body portion  84  of the adjacent, mating modular segment  32 , being complementary to the rail portion  82 , can include the superior and inferior lip portions  122  and  124  that also include interior faces  134  that are substantially planar and substantially parallel to the superior/inferior coordinate  74  (e.g.,  FIGS. 5 and 7 ). 
     Referring to  FIG. 8 , an alternative rail cross-section  102   d  and mating body portion cross section  104   d  is presented in an embodiment of the invention. For these embodiments, lip portions  122   d ,  124   d  also include faces  132   d  that are each substantially planar, but each being oblique relative to the superior/inferior coordinate  74   d  so as to define a “dovetail” profile. That is, the planar faces  132   d  of the rail head  92   d  that are adjacent the web  88   d  for the rail portion  82   d  intersect web  88   d  at an acute angle θ. The body portion  84   d  of the adjacent, mating segment, being complementary, also defines an acute angle θ relative the web portion  88   d.    
     Functionally, the right angle or acute angle configurations between the faces  132  and the web portion  88  enhance the mechanical coupling between adjacent segments in a direction that is normal to both the superior/inferior coordinate  74  and rail axis  94 . These configurations rely primarily on compressive contact between the engaged segments and less on friction between the segments, thereby providing for a positive mechanical coupling therebetween. The enhanced coupling is particularly advantageous when the segments comprise a soft or compliant material having a relatively low hardness. A non-limiting example of a soft or compliant material is a polymer such a biocompatible polyurethane. A non-limiting example of a hardness of a soft or compliant material is a material with a durometer hardness ranging from about Shore 18 A to about Shore 55D. A further and non-limiting example of a soft or compliant material is a material with a compressive modulus between about 2 and about 100 MPa. In a preferred embodiment, the compressive modulus is between about 6 and about 20 MPa. 
     The cross-sections  102  and  104  of the various rail portions  82  and the various body portions  84  can be configured to be complementary to itself and the other modular segments  32 . That is, the various rail portion cross-sections  102  can be shaped and dimensioned to mate with the various body portion cross-sections  104 . Likewise, the various recesses  118  can be positioned and dimensioned to accept (i.e., to be complementary with) the barb portions  96  on the various rail portions  82  of the modular segments  32 . 
     In this way, a given A-segment  32   a  can be coupled to either a given B-segment  32   b  or a given C-segment  32   c , a given C-segment  32   c  can be coupled with either a given A-segment  32   a  or a given B-segment  32   b , and a given B-segment  32   b  can be coupled with another B-segment  32   b . The modularity of the system enables the construction of a variety of prosthesis sizes by interlocking the various segments together in a side-by-side manner, the A, B and C-segments  32   a ,  32   b  and  32   c  constituting the building blocks of the modular system. 
     In certain embodiments, the flanking faces  86  of the various segments are oblique relative to the rail axes  94  (i.e., are not parallel to the rail axes  94 ). Instead, the flanking faces  86  slope slightly towards the rail axes  94  at an angle α from the proximal end  66  to the distal end  68 , as best seen in  FIG. 18A . That is, the flanking faces  86  are spaced further from the rail axes  94  at the proximal ends  66  than at the distal ends  68 . Thus, for embodiments that include this aspect, the rail axis  94  of a given modular segment  32  will intersect plane of the respective flanking face  86  at a point distal to the modular segment  32 . 
     To accommodate the oblique flanking face configuration, the lip portions  122 ,  124  of the modular segments  32   b  and  32   c  can be of varying thickness from the proximal end  66  to the distal end  68  of the respective body portion  84   b ,  84   c . While the interior face  134  of a given lip portion  122 ,  124  is parallel to the respective slot axis  116 , the thickness of the lip portions  122 ,  124  (i.e., the dimension normal to the slot axis  116 ) can decrease from the proximal end  66  to the distal end  68 , so that the lip portions  122 ,  124  themselves form a complementary oblique interface with the oblique flanking face  86  of the adjacent modular segment  32   a  or  32   b.    
     Referring to  FIG. 9 , a side view of a modular segment  32   a  or  32   b  is presented in an embodiment of the invention. The barb portions  96  can each define an inclined profile  142 . The inclined profile  142  intersects an outer surface of the rail portion  82  at an intersection point  144  on the proximal end of the barb portion  96 . From the intersection point  144 , the dimension of the barb portion increases toward a distal end  146  of the barb portion  96 . In the depicted embodiments, the distal ends  146  of the barb portions  96  are parallel to the superior/inferior coordinate  74  of the respective modular segment  32 . Thus, in this embodiment, the barb portions  96  each define a right-triangular profile in a plane that is parallel to both the superior/inferior coordinate  74  and the rail axis  94  of a given segment  32   a ,  32   b.    
     In one embodiment, the corresponding recesses  118  of the body portion  84  of the adjacent modular segment  32   b  or  32   c  can define a similar, triangular shape that is complementary to the triangular shape of the barb portion  96  ( FIG. 6 ). In other embodiments, the recesses  118  can be, for example, rectangular, so long as a distal boundary  148  of the recesses  118  are complementary to the distal ends  146  of the barb portions  96 . 
     For assembly of the implant of, for example,  FIG. 2 b   , the B-segment  32   b  is positioned proximal to the proximal end of the A-segment  32   a , so that the slot axis  116   b  of the body portion  84   b  of the B-segment  32   b  is substantially concentric with the rail axis  94   a  of the rail portion  82   a  of the A-segment  32   a . The body portion  84   b  of the B-segment  32   b  is then slid over the rail portion  82   a  of the A-segment  32   a  in the distal direction along the rail axis  94   a  until the barb portions  96   a  of the rail portion  82   a  are captured within the recesses  118   b  of the body portion  84   b  of the B-segment  32   b . The distal end  68   b  of the body portion  84   b  of the B-segment  32   b  can be substantially registered against the stop portion  98   a  of the A-segment  32   a  when the barb portions  96   a  of the A-segment  32   a  are secured within the recesses  118   b  of the B-segment  32   b.    
     As the body portion  84   b  of the B-segment  32   b  is slid over the rail portion  82   a  of the A-segment  32   a , the interior surface  114   b  of the elongate slot  112   b  of the B-segment  32   b  rides over the protruding barb portions  96   a  of the A-segment  32   a . This interaction causes the barb portions  96   a  of the A-segment  32   a  to be compressed and the wall of the body portion of the B-segment  32   b  to deflect upwards. However, once the barb portions  96   a  are registered within the respective recess  118   b , there is essentially no deformation of the components. 
     After the B-segment  32   b  is secured to the A-segment  32   a , the C-segment  32   c  is positioned proximal to the proximal end of the B-segment  32   b , so that the slot axis  116   c  of the body portion of the C-segment  32   c  is substantially concentric with the rail axis  94   b  of the B-segment  32   b . The body portion  82   c  of the C-segment  32   c  is then slid over the rail portion  82   b  of the B-segment  32   b  in the distal direction along the rail axis  94   b  until the barb portions  96   b  of the rail portion  82   b  are captured within the recesses  118   c  of the body portion  84   c  of the C-segment  32   c . The distal end  68   c  of the body portion  84   c  of the C-segment  32   c  can be substantially registered against the stop portion  98   b  of the B-segment  32   b  when the barb portions  96   b  of the B-segment  32   b  are secured within the recesses  118   b  of the C-segment  32   c.    
     For a 2-segment implant ( FIG. 2A ), the C-segment  32   c  is interlocked directly to the A-segment  32   b  in similar fashion. Likewise, for an implant having four or more segments, additional intermediate B-segments are interlocked in similar fashion. As a non-limiting example, embodiments can have as many as 8 modular segments (one A-segment  32   a , one C-segment  32   c , and six B-segments  32   b ). 
     Functionally, the various structural aspects of the rail and slot portions  82 ,  112  of the modular segments  32  prevent relative motion between the modular segments  32  in all directions, even where a relatively soft or compliant material is utilized for the modular segments  32 . The engagement of a given rail portion  82  with an adjacent body portion  84  prevents relative motion between the engaged segments along the superior/inferior coordinates  74 . Engagement between the barb portions  96  and stop portions  98  of a given modular segment  32 , when engaged with an adjacent segment  32 , prevent relative motion between the engaged segments  32  along the proximal/distal coordinates  76 . Both the lip portions  122  and  124  and the barb portions  96  provide shear resistance to movement parallel to the transverse plane  72 . The superior and inferior lip portions  122  and  124  of a given modular segment  32 , along with the barb portions  96  of an adjacent, engaged modular segment  32 , prevent separation of the modular segments  32 . 
     The inclined profile  142  of the barb portions  96  enable the body portion  84  of an adjacent segment  32  to be more easily slid over the barb portions  96  as the adjacent segment  32  is moved in the distal direction relative to the given segment  32 . However, once the barb portions  96  are registered within their corresponding recesses  118 , the distal ends  146  of the barb portions  96  interact with the distal boundaries  148  of the recesses  118  to prevent the adjacent segment from moving along the proximal/distal coordinate  76 . 
     For embodiments utilizing oblique flanking faces  86 , there is little or no sliding interference between the flanking faces  86  and the superior and inferior lip portions  122  and  124  of adjacent segments until the adjacent segments are at or near the implanted position. This helps limit the frictional load during assembly. 
     Referring to  FIG. 10 , a cross-section of a B-segment  32   b  that cuts through the transverse plane  72   b  is presented depicting a mounting port  152   b  in an embodiment of the invention. The modular segments  32  can each include such a mounting port  152  for mounting the respective modular segment  32  to an insertion tool. While the discussion below is directed to the mounting port  152   b , the general aspects apply to all mounting ports  152 . 
     In one embodiment, the mounting port  152   b  defines a substantially cylindrical cavity  154   b  that is concentric about the rail axis  94   b  of the modular segment  32   b  and is accessible from the proximal end  66   b  of the modular segment  32   b . The mounting port  152   b  can further include internal detents  156  that extend from one side of an internal wall  158   b  of the mounting port  152   b . In one embodiment, the detents  156  can each define a triangular or right triangular profile  162 , wherein a proximal face  164  of each detent  156  is inclined relative to the rail axis  94   b  and a distal face  166  of the detent  156  is orthogonal to or only slightly acute relative to the rail axis  94   b.    
     Referring to  FIGS. 11, 11A and 11B , the A/B insertion tool  36   a , used to augment insertion of both the A- and B-modular segments  32   a  and  32   b  is depicted in an embodiment of the invention. The A/B insertion tool  36   a  includes a shaft portion  172   a  with a flag  174   a  extending from a proximal end  176   a  and a tip portion  178   a  extending from a distal end  182   a . The tip portion  178   a  defines a rotation axis  184   a  and further defines notches  186  formed on one side that are shaped and positioned complementary to the detents  156  of the mounting ports  152 . In certain embodiments, the shaft portion  172   a  includes a cross-section  188   a  that has the same profile as the rail head  92  of the modular segments  32 . Accordingly, when in the proper rotational orientation about the rotation axis  184   a , the shaft portion  172   a  effectively provides a proximal extension of the rail head  92 . In the depicted embodiment, the shaft portion  172   a  of the A/B insertion tool  36   a  defines a “D-shaped” profile  190   a  having an arcuate portion  192   a  and a flat face portion  194   a . The flag  174   a  of the A/B insertion tool  36   a  can be “L-shaped” as depicted in  FIG. 11 , with a short leg  196   a  of the flag  174   a  extending from the flat face portion  194   a  of the D-shaped shaft portion  172   a.    
     Referring to  FIGS. 12 and 12A , assembly of the A/B insertion tool  36   a  and one of the A- and B-segments  32   a  and  32   b  is depicted in an embodiment of the invention. In one embodiment, the assembly can be augmented by a segment loading platform  200 . In one embodiment, the segment loading platform  200  includes a segment bay  202  that is aligned with a “U-shaped” channel  204  having an arcuate portion  206  concentric about a loading axis  208 . The segment bay  202  is configured with a bottom portion  212  configured to accept and register the rail portion of the modular segment  32   a  or  32   b . The U-shaped channel  204  is dimensioned for sliding engagement with the D-shaped profile  190   a  of the shaft portion  172   a.    
     One of the A- or B-segments  32   a  or  32   b  is placed in the segment bay  202  so that the rail portion  82  of the segment  32  is properly registered within the bottom portion of the segment bay. The shaft portion  172   a  of the A/B insertion tool  36   a  is placed within the U-shaped channel  204  of the segment loading platform  200  so that the arcuate portion  192   a  of the D-shaped profile  190   a  registers against the arcuate portion  206  of the U-shaped channel  204 . The registrations of the modular segment  32   a  or  32   b  and the shaft portion  172   a  of the A/B insertion tool  36   a  aligns the rotation axis  184   a  of the tip portion  178   a  and the rail axis  94   a  (and therefore the mounting port  152   a  or  152   b ) of the corresponding modular segment  32   a  or  32   b . The registrations also rotationally orient the tip portion  178   a  of the A/B insertion tool  36   a  and the mounting port  152   a  or  152   b  of the modular segment  32   a  or  32   b  so that the notches of the tip portion  178   a  are aligned with the detents  156   a  or  156   b  of the mounting port  152   a  or  152   b . The tip portion  178   a  is slid into the mounting port  152   a  or  152   b  until each of the plurality of detents  156   a  or  156   b  of the mounting port  152   a  or  152   b  occupies a corresponding one of the notches  186   a  on the tip portion  178   a.    
     It is noted that the C-segment  32   c  does not include a mounting rail, and therefore cannot include a mounting port that is concentric with a rail portion. Accordingly, the C-segment includes a mounting port  152   c  formed in the body portion  84   c , the mounting port  152   c  defining an axis  214  that is parallel with and on the same transverse plane  72   c  as the slot axis  116   c  and having the same aspects as the mounting ports  152   a  and  152   b  of the A- and B-segments  32   a  and  32   b.    
     Referring to  FIGS. 13A through 13H , an assembly sequence is depicted for the three-segment prosthesis  34   b  of  FIG. 2B . An A-segment/insertion tool assembly  220   a  comprising the A-segment  32   a  and the A/B insertion tool  36   a  is first placed in an evacuated disc nucleus space ( FIG. 13A ; evacuated disc nucleus space not depicted). A B-segment/insertion tool assembly  220   b  comprising the B-segment  32   b  and another A/B insertion tool  36   b  is then slid over a proximal end  222   a  of the A-segment/insertion tool assembly  220   a  and translated along the shaft  172   a  of the A/B insertion tool  36   a  of the A-segment/insertion tool assembly  220   a  ( FIG. 13B ). During this step, the open side  128   b  of the elongate slot  112   b  of the B-segment  32   b  is aligned to pass over the short leg  196   a  of the L-shaped flag  174   a  of the A-segment insertion tool  36   a  of the A-segment/insertion tool assembly  220   a , which also places the elongate slot  112   b  of the B-segment  32   b  in proper orientation for translation along the D-shaped shaft  172   a  of the A/B insertion tool  36   b  of the B-segment/insertion tool assembly  220   b.    
     The B-segment  32   b  is then slid over the rail portion  82   a  of the A-segment  32   a  until the B-segment  32   b  registers against the stop portion  98   a  of the A-segment  32   a  ( FIG. 13C ). The open side  128   b  of the elongate slot  112   b  slides over the web  88   b  of the rail portion  82   b , the open side  128   b  having been properly aligned when slid over the short leg  196   a  of the L-shaped flag  174   a . The user can determine that the B-segment  32   b  is in place when the flags  174   a  and  174   b  of the A/B insertion tools  36   a  and  36   b  of the A- and B-segment insertion tool assemblies  220   a  and  220   b  are aligned. Upon registration of the B-segment  32   b  against the stop portion  98   a  of the A-segment  32   a , the barb portions  96   a  on the rail portion  82   a  of the A-segment  32   a  should be registered within the recesses  118   b  of the B-segment  32   b . However, the user can tug the B-segment/insertion tool assembly  220   b  in the proximal direction relative to the A-segment/insertion tool assembly  220   a  to assure that the barb portions  96   a  are set within the recesses  118   b.    
     The A/B insertion tool  36   a  of the A-segment/insertion tool assembly  220   a  is then removed. Removal is accomplished by rotation the A/B insertion tool  36   a  of the A-segment/insertion tool assembly  220   a  180° about the rotation axis  184   a  ( FIG. 13D ). This action causes the notches  186   a  of the tip portion  178   a  of the A/B insertion tool  36   a  of the A-segment/insertion tool assembly  220   a  to rotate away from the detents  156   a  in the mounting port of the A-segment  32   a , thus enabling the A/B insertion tool  36   a  of the A-segment/insertion tool assembly  220   a  to be removed from the mounting port  156   a  with reduced interference from the detents  156   a . The A/B insertion tool  36   a  of the A-segment/insertion tool assembly  220   a  is then removed from the mounting port  152   a , leaving only the A- and B-segments  32   a  and  32   b  coupled to the B-segment/insertion tool assembly  220   b  ( FIG. 13E ). 
     A C-segment/insertion tool assembly  220   c  comprising the C-segment  32   c  and the C insertion tool  36   c  is then aligned so that the slot portion  112   c  of the C-segment  32   c  is slid over a proximal end  222   b  of the B-segment/insertion tool assembly  220   b , and the C-segment/insertion tool assembly  220   b  being translated along the shaft  172   b  of the insertion tool  36   b  of the B-segment/insertion tool assembly  220   b  ( FIG. 13F ). The C insertion tool  36   c  is described in more detail below in the discussion attendant to  FIG. 19 . The C-segment  32   c  is then slid over the rail portion  82   b  of the B-segment  32   b  until the C-segment  32   c  registers against the stop portion  98   b  of the B-segment  32   b  ( FIG. 13G ). The insertion tool  36   b  of the B-segment/insertion tool assembly  220   b  is then removed ( FIG. 13H ). The steps depicted at  FIGS. 13F through 13H  are conducted in the same manner as the steps depicted at  FIGS. 13B through 13E . 
     The insertion tool of the C-segment/insertion tool  36   c  is removed by rotating the insertion tool  36   c  180° ( FIG. 13H ) and removing it from the mounting port  152   c , thereby leaving the prosthesis fully assembled an in place ( FIG. 2B ). 
     In certain embodiments, supplemental tools can be included and utilized in for enhanced manipulation of the modular segments. The supplemental tools are of particular utility when handling modular segments that are of a homogeneous, compliant material. The supplemental tools can include the removal tool  44 , the A-segment stabilizer  46 , the B-segment stabilizer  48  and the C-segment stabilizer  52  ( FIG. 1 ). 
     Referring to  FIGS. 14 and 14A , the removal tool  44  is depicted in an embodiment of the invention. The removal tool  44  includes a handle portion  232  and a shaft portion  234 . The shaft portion  234  defines a bore  236  having an inner diameter  238  concentric about a central axis  242 . The inner diameter  238  of the bore  236  is dimensioned large enough to slide over the D-shaped profile  190   a  or  190   b  of the A- and B-segment insertion tool  36   a  or  36   b , as well as the round profile of the C-segment insertion tool  36   c . The removal tool  44  includes a slot  244  on one side thereof, the slot  244  extending on one side of the removal tool and from a location  246  proximate a proximal end  248  of the removal tool  44  through a distal end  252  of the removal tool  44 . 
     Referring to  FIGS. 15A through 15D , operation of the removal tool  44  is depicted in an embodiment of the invention. Typically after the insertion tool has been rotated 180° to disengage the detents  156  and notches  186  ( FIGS. 15A and 15B ), the central axis  242  of the removal tool  44  is aligned with the rotation axis  184  of the insertion tool  36  and slid over the proximal end  176  of the insertion tool  36 , the slot  244  being aligned to pass over the short leg  196  of the L-shaped flag  174 . The shaft portion  234  of the removal tool  44  is slid over the shaft portion  172  of the insertion tool  36  until the distal end  252  of the removal tool  44  is brought into contact with the modular segment  32  ( FIG. 15C ). The insertion tool  44  is then pulled out of the mounting port  152  by application of a clamping force F between the flag portion  174  of the insertion tool  36  and the handle portion  232  of the removal tool  44  ( FIG. 15D ). The operator typically applies the clamping force F by squeezing the flag portion  174  and the handle portion  232  between the index finger and the thumb or palm of the hand. 
     Functionally, while the act of rotating a given insertion tool  36  180° makes removal of the insertion tool  36  from the mounting port  152  easier, the friction between the tip portion  178  of the insertion tool  36  and the modular segment  32  can still be substantial, in part because the detents  156  are compressed against the cylindrical surface of the tip portion  178  after the 180° rotation. The removal tool  44  provides a controlled, mechanically leveraged way to remove insertion tools  36  in situ while maintaining a low profile. 
     Referring to  FIGS. 16, 16A, 17, 17A and 17B , the A- and B-segment stabilizers  46  and  48  are depicted in embodiments of the invention. The A- and B-segment stabilizers  46  and  48  include many common aspects, which are indicated in the figures with like-numbered numerical references. The A-stabilizer  46  includes a shaft portion  262  having a handle  264  attached at a proximal end  266 . The shaft portion  262  includes what is effectively a channel structure  268  defining a channel  272  on one side thereof, the channel structure  268  including opposed flanges  274  and  276  separated by a flat portion  278 . The channel  272  thus defined is dimensioned to enable insertion tools  36  to slidably translate therein, with the flat portion  278  of the D-shaped shaft  172  engaged with the flat portion  278  of the A-segment stabilizer  46 . At a distal end  282 , the flat portion  278  includes an extension portion  284  that extends beyond the opposed flanges  274 ,  276 , the extension portion  284  including a slot  286  formed thereon. The slot  286  is formed along an elongate axis  292  and is accessible from the distal end  282 . 
     The B-segment stabilizer  48  also includes the channel structure  268  extending from the proximal end  266  to near the distal end  282 . At the proximal end  266 , the B-segment stabilizer includes a ramp portion  294  formed within the channel  272 . In the absence of a handle, the B-segment stabilizer includes a grip portion  296  formed on the proximal end  266 . Near the distal end  282 , the B-segment stabilizer  48  includes an additional guide structure  302  that effectively defines an asymmetric H-beam profile  304 . The channel structure  272  and guide  302  structure define the channel  272  continuously along the length of the B-segment stabilizer  48 . The guide structure  302  includes opposed flanges  274  and  276  that extend normal to the flat portion  278  in both directions. The guide structure  302  also includes opposed lip portions  306  that extend toward each other to define a gap  308  therebetween. 
     Referring to  FIGS. 18A and 18B , operation of the A-segment stabilizer  46  is depicted in an embodiment of the invention. Prior to insertion of the A-segment  32   a  into the evacuated disc nucleus space, the A-segment/insertion tool assembly  220   a  is loaded into the channel  272  of the A-segment stabilizer  46  ( FIG. 18A ). After or simultaneously with the loading, the A-segment stabilizer  46  is translated toward the A-segment  32   a  until the web  88   a  of the rail portion  82   a  registers within the slot  286  ( FIG. 18B ). 
     For the B-segment stabilizer  48 , the guide structure  302  is slid over the distal end  176   a  of the A insertion tool  36   a  to capture the D-shaped shaft portion  172   a  of the A insertion tool  36   a  (shown in phantom in  FIG. 17A ) between the flange portions  274 ,  276  and lip portions  306  of the guide structure  302  of the B-segment stabilizer  48 . The channel  272  of the B-segment stabilizer  48  is translated over the B-segment/insertion tool assembly  220   b  until the web  88   b  of the rail portion  82   b  is registered in the slot  286 . In the depicted embodiment, the B-segment stabilizer  48  does not include a handle akin to the A-segment stabilizer  46  because such a handle would create clutter and interference amongst the flags  178   a  and  178   b  of the A- and B-insertion tools  36   a  and  36   b . In this way, additional guidance and control for coupling the B-segment  32   b  to the A-segment  32   a  is provided in situ. 
     The ramp portion  294  guides the flags  174   a  and  174   b  at the proximal ends of the insertion tools  36   a  and  36   b  away from each other during assembly of the prosthesis  34 . This prevents the flag  174   b  of the A/B insertion tool  36 B from catching on the flag  174   a  of the adjacent A/B insertion tool  36   a.    
     Removal of the A- and B-segment stabilizers  46  and  48  is accomplished by disengaging them from the web  88   a ,  88   b  of the respective rail portion  82   a ,  82   b  in the proximal direction. 
     Referring to  FIGS. 19, 19A and 19B  (referred to collectively as  FIG. 19 ), the C insertion tool  36   c  is depicted in an embodiment of the invention. The C insertion tool  36   c  includes a shaft portion  172   c  having a tip portion  178   c  at a distal end  182   c  and a flag portion  174   c  at a proximal end  176   c . In one embodiment, the flag portion  174   c  extends in an opposite direction from the flag portions  174   a ,  174   b  of the A/B insertion tools  36   a ,  36   b . The tip portion  178   c  defines a tip portion axis  310 . The notch aspects  186   c  of the tip portion  178   c  for the C insertion tool  36   c  can the same as for the A/B insertion tool  36   a . The shaft portion  172   c  of the C insertion tool  36   c  is essentially cylindrical about a cylindrical axis  312 . In the depicted embodiment, the cylindrical axis  312  of the shaft portion  172   c  and the tip portion axis  310  of the tip portion  178   c  are eccentric ( FIGS. 19A and 19B ). 
     Referring to  FIGS. 20, 20A and 20B , the C-segment stabilizer  52  is depicted in an embodiment of the invention. The C-segment stabilizer  52  includes a hollow shaft portion  322  having a handle  324  on a proximal end  326  and an guide structure  328  near a distal end  332 . The hollow shaft portion  322  includes structure defining a slot  334  extending on a first side  336  of thereof and from a location  338  proximate the proximal end  326  and through the distal end  332  of the C-segment stabilizer  52 . The guide structure  328  comprises two opposing flanges  342  and  344  that extend from a second side  346  of the hollow shaft portion  322 , the second side  346  being opposite the first side  336 . The opposing flanges  342 ,  344  each include lip portions  352  that extend toward each other to define a gap  356  therebetween. The A/B insertion tool  36   b  and the C insertion tool  36   c  are depicted in phantom in  FIG. 20B . 
     In operation, the hollow shaft portion  322  of the C-segment stabilizer  52  is aligned with the cylindrical axis  312  of the C insertion tool  36   c  and with the slot  334  aligned to pass over the flag portion  174   c . The C-segment stabilizer  52  is then translated over the C insertion tool  36   c  until the distal end  332  engages the C-segment  32   c.    
     Functionally, the guide structure  328  captures the D-shaped shaft  172   c  of the adjacent A/B insertion tool  36   b  between the flanges  324  and  344  of the guide structure  328 , to further assist the user in guiding the B-segment  32   b  into the evacuated disc nucleus space. The slot  334  of the C-segment stabilizer enables passage of the hollow shaft portion  322  over the shaft flag portion  174   c  of the C insertion tool  36   c . Likewise, the gap  356  enables passage of the guide structure  328  over the flag portion  174   b  of the A/B insertion tool  36   b . The inner diameter of the hollow shaft  322  is dimensioned so that the A/B insertion tool  36   b  cannot be inserted in the C-segment stabilizer. Thus, the round hollow shaft  322  of the C-segment stabilizer  52  serves as a key to prevent insertion of the A/B insertion tool  36   a  therein. The eccentricity of the tip portion  178   c  relative to the shaft portion  172   c  allows room for the structure of the hollow shaft portion  322  between the insertion tools  36   b  and  36   c . The distal end  332  of the C-segment stabilizer  52  provides a bearing surface that spreads the force of the insertion operation over a larger area, thus preventing deformation of the C-segment  32   c  during insertion of the C-segment  32   c.    
     A purpose of the A-, B-, and C-segment stabilizers  46 ,  48  and  52  generally is to enable manipulation the respective A-, B- and C-segments  32   a ,  32   b  and  32   c  during implantation, as well as maneuvering the prosthesis  34  within the evacuated disc nucleus space while the prosthesis  34  is at various stages of assembly. The stabilizers  46 ,  48 ,  52  reduce the risk of the tip portion  178  of the various insertion tools  36  becoming dislodged from the respective mounting port  152  during positioning of the partially or fully assembled prosthesis  34 . 
     In certain embodiments, various of the components discussed above are included as a kit. The kit can include some or all of the components presented in  FIG. 1 . The kit can also include operating instructions on a tangible medium such as a paper document, a compact disc (CD), a digital video disc (DVD), or a central computer accessed, for example, over the internet. The operating instructions can include various of the instructions and sequences described above.