Patent Publication Number: US-11648037-B2

Title: Extension-ready spinal support system with vascular-safe pedicle screw

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of International Patent Application No. PCT/US2018/030928, filed May 3, 2018, and is a continuation-in-part of U.S. patent application Ser. No. 15/970,368, filed May 3, 2018, which claims the benefit of U.S. Provisional Patent Application No. 62/500,820, filed on May 3, 2017, and of U.S. Provisional Patent Application No. 62/500,719, filed May 3, 2017, the disclosures of which are hereby incorporated by reference herein in their entirety. This application also claims the benefit of U.S. Provisional Patent Application No. 62/867,616, filed Jun. 27, 2019, the disclosure of which is hereby incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure relates to spinal support systems and more particularly to spinal support systems that are readily extensive with pedicle screws that inhibit cutting or slicing of soft tissues. 
     BACKGROUND OF THE DISCLOSURE 
     Implementation of various spinal surgical techniques often requires the use of spinal support rods that are anchored to the vertebrae through the use of pedicle screws to provide stabilization of the spine during healing or correction. Examples include maintaining adjacent vertebrae stationary so that bone growth tissue can bridge the vertebrae in a spinal fusion process. Another example is the use of spinal support rods to apply a coercive force to the spine for corrective purposes (e.g., correction of scoliosis). 
     The pedicle screws are tapped into the vertebrae of the spine for the anchoring. The anatomy of the human body is such that the spine includes and is proximate to numerous vascular vessels. A concern is that the pedicle screw could contact and damage the vascular vessels during the tapping process. 
     Also, the spinal rods are typically disposed in rod receptacles at the head of the pedicle screw. The rod receptacles typically define diametrically opposed slots, with the spinal rod extending through the slots. The formation of the slots further define opposed wall segments that extend proximally on lateral sides of the spinal rod. The spinal rod is typically held within the slots with a set screw that threadably engages the wall segments. Over time, creep stresses may cause the wall segments to plastically deform radially outward and away from the set screw, with an accompanying decrease the clamping force of the set screw against the spinal rod due to loss of elasticity of the wall segments. This can cause loosening of the assembly and slippage of the resident spinal rod within the spinal rod receptacle. In some instances, torque requirements, particularly where the spinal rods are subject to high tension forces, can cause the set screw to slip within the spinal rod receptacle during implantation. 
     In some cases, surgery is later required to treat other vertebrae of the same patient. “Extension” systems have been developed which enable additional spinal support rods to be coupled to existing spinal rods or pedicle screws of the previous surgery, thereby reducing surgical trauma and recovery times. Such an extension system is found, for example, at U.S. Pat. No. 9,655,654 to Abbasi entitled “Spinal Rod Support Structure with Clamp,” owned by the owner of the current application, the disclosure of which is incorporated by reference herein in its entirety except for patent claims and express definitions contained therein. 
     The present application identifies shortcomings and limitations in the art of spinal rod extension systems and provides improvements to remedy such shortcomings and limitations. 
     SUMMARY OF THE DISCLOSURE 
     Various embodiments of the disclosure are directed to a “vascular-safe” pedicle screw. The vascular-safe pedicle screw includes one or more self-tapping flutes that interrupt or terminate the threads at several points along the threads. These terminations are configured to push soft tissue aside as opposed to slicing or tearing through the soft tissue, so that the self-tapping flute is less likely to slice into vascular vessels, while still effectively tapping into bone tissue. The distal portion of the pedicle screw may also include features that reduce the circumferential contact area of the pedicle screw in the direction of rotation, to increase the applied pressure to the soft tissue for a given applied rotational force for augmenting penetration of the pedicle screw without resort to sharp cutting edges. Some embodiments of the disclosed pedicle screw are not only vascular-safe, but also self-tapping, self-steering, self-centering, or a combination thereof. 
     Various embodiments of the disclosure also provide an “extension ready” spinal support system that enables the extension to be accomplished with reduced disturbance to an existing spinal support structure relative to conventional extension systems. In some embodiments, existing connections between the existing spinal support rod and pedicle screws can remain intact while extension rod receptacles are mounted directly to the existing base rod receptacles. In some embodiments, the extension receptacle may be polyaxial with respect to the base receptacle. In some embodiments, the extension receptacle is provided with a low profile (i.e., shortened axial projection length from the base rod receptacle) relative to the polyaxial embodiment by eliminating polyaxial structure between the base receptacle and the extension receptacle and the additional axial lengths associated therewith and providing a monoaxial structure that rotates about but does not pitch relative to the extension axis. 
     In various embodiments of the disclosure, a spinal support system is disclosed where a unitary reinforcement cap threadably engages both the internal and the external surfaces of the wall segments of the rod receptacle. By doubling the number of threads that are engaged, additional strength and structural integrity are realized over standard set screw arrangements. Also, because of the unitary construction, wherein an internal set screw portion and an external skirt portion are integral to the reinforcement cap, lateral movement between the set screw portion and the skirt portion is limited. 
     In some embodiments, the unitary reinforcement cap does not threadably engage the external surface, but instead slides over the external surface to capture and support to the rod receptacle. Accordingly, such embodiments do not require threads on the external surface of the rod receptacle, and can be configured for a retrofit of conventional rod receptacles. 
     In some embodiments, the threads are of a canted cantilever construction, wherein the interfaces of threadably engaged components are tailored to generate reaction force vectors that prevent slippage at the interfaces. 
     Conventional spinal rod extension systems exist where extension spinal support rod receptacles (also referred to as extension “tulips” in the parlance of the spinal support arts) can be mounted “piggyback” to an existing or “base” spinal support rod receptacle. However, conventional systems require that the connection between the base spinal support rod receptacle be disturbed or modified. For example, in some systems, the set screw securing the base spinal rod to the base receptacle must be removed so that the extension receptacle can utilize the interior threads of the base receptacle. This releases the base spinal support rod from the base receptacle, thus requiring that the base spinal support rod be reset before the surgeon can proceed with implantation of the extension system. Often, the required structure that extends from the base receptacle is of undesirable axial length, causing concealment issues. 
     Another drawback of conventional rod receptacle arrangements is the separation or “splaying” of the tulip. When a conventional set screw is seated within a conventional tulip, forces are generated on the tulip wall portions that force the tulip wall sections away from each other. This separation of the tulip walls is referred to as “splaying.” 
     Various embodiments of the disclosure alleviate one or more of these shortcomings and limitations. The disclosed embodiments enable extension of the spinal support structure without need for removing the tulips or pedicle screws of the already-implanted spinal support structure. Certain embodiments alleviate the need to remove the set screw from the interior threads of the base receptacle. As with conventional rod receptacles, the disclosed base rod receptacle includes interior threads to which a set screw is coupled for securing the base spinal support rod. In addition, the disclosed system further includes exterior threads to which an optional extension assembly can be mounted at a later time. By utilizing the exterior threads for mounting the extension system, the set screw that secures the base spinal support rod to the base rod receptacle does not have to be removed from the interior threads of the base rod receptacle. In this way, the base spinal support system can remain intact and undisturbed as the extension system is coupled thereto. 
     Other embodiments may involve removal of the set screw from the base receptacle, but utilize a reinforced, dual threaded or capped base that is stronger and more robust than standard set screw arrangements and prevents splaying of the tulip. 
     Structurally, the vascular-safe pedicle screw comprises a head portion, a threaded shaft portion distal to the head portion and including threads that extend to a distal end portion of the threaded shaft portion, and a self-tapping flute at the distal end portion of the threaded shaft portion. The self-tapping flute is recessed into the threaded shaft to define at least one terminated thread at a face of the self-tapping flute. In some embodiments, the terminated threads are curved to define a convex profile that extends in a tangential direction from proximate a root of the at least one terminated thread at the face of the self-tapping flute to a crest of the at least one terminated thread. The distal end portion of the threaded shaft portion may also define a depression at a leading end of the thread. In some embodiments, the head portion and the threaded shaft portion define a central passage that passes therethrough. The threaded portion may define a plurality of fenestrations that are in fluid communication with the central passage. In some embodiments, the threads are double threads. The threaded shaft portion may include dulled edges, for example polished or roughened edges. In some embodiments, the threaded shaft portion defines a surface roughness having arithmetical mean deviation that is in a range of 5 micrometers to 15 micrometers inclusive. 
     In various embodiments of the disclosure, a spinal rod support system is disclosed, comprising a pedicle screw, including a head portion, a threaded shaft portion distal to the head portion and including threads that extend to a distal end portion of the threaded shaft portion, and a self-tapping flute at the distal end portion of the threaded shaft portion, the self-tapping flute being recessed into the threaded shaft to define terminations of the threads at a face of the self-tapping flute, the terminations being defining a convex profile that extends in a tangential direction from a root to a crest of the threads at the face of the self-tapping flute. In some embodiments, the spinal rod support system includes a base rod receptacle coupled to the pedicle screw, and an extension rod receptacle configured to couple to the base rod receptacle. The threaded shaft portion may include dulled edges. In some embodiments, a base portion is configured to threadably couple to the base rod receptacle, the base portion including a skirt portion that extends from a mounting platform, the mounting platform defining a center hole accessible from a proximal face of the mounting platform, the extension rod receptacle extending into the center hole. The base rod receptacle may define exterior threads, with the skirt portion of the base defining interior threads configured to mate with the exterior threads of the base rod receptacle. In some embodiments, the spinal rod support system comprises a reinforcement cap configured to couple to one of the base rod receptacle and the extension rod receptacle, the reinforcement cap including a set screw portion and a skirt portion, wherein the base rod receptacle defines interior threads, the extension rod receptacle defines interior threads, and the set screw portion of the reinforcement cap defines exterior threads configured to mate with either of the interior threads of the base rod receptacle or the interior threads of the extension rod receptacle. In some embodiments, the extension rod receptacle defines exterior threads. The skirt portion of the reinforcement cap may define interior threads configured to mate with either of the exterior threads of the base rod receptacle or the exterior threads of the extension rod receptacle. 
     In some embodiments, a “reverse canted cantilever” arrangement is disclosed, wherein the interior threads of the base rod receptacle and the interior threads of the extension rod receptacle each define a first canted cantilever profile that extends radially inward and in a distal direction, and the exterior threads of the base rod receptacle and the exterior threads of the extension rod receptacle each define a second canted cantilever profile that extends radially outward and in a proximal direction. 
     In various embodiments of the disclosure, a spinal support system comprises a first rod receptacle for mounting to a pedicle screw, the first rod receptacle having a first side wall that includes a first interior surface and a first exterior surface. The first side wall defines a first pair of diametrically opposed slots that extend axially along the first side wall and are open at a proximal end of the first rod receptacle, the first interior surface defining first interior threads. A first set screw includes threads configured to mate with the first interior threads of the first rod receptacle. The first exterior surface defines first exterior threads. An extension assembly including a base portion mounted to a second rod receptacle, the base portion including a mounting platform and a skirt portion that extends from the mounting platform, the skirt portion including interior threads for mating with the first exterior threads of the first rod receptacle, the second rod receptacle having a second side wall that includes a second interior surface and a second exterior surface. The second exterior surface of the second rod receptacle may include second exterior threads. The second side wall defines a second pair of diametrically opposed slots that extend axially along the second side wall and are open at a proximal end of the second rod receptacle, the second interior surface defining second interior threads. A second set screw includes threads configured to mate with the second interior threads of the second rod receptacle. 
     The extension assembly may include a pivot member that attaches the second rod receptacle to the base portion, the pivot member including a head portion and a shaft portion, the shaft portion including shaft threads formed on an exterior surface thereof, the shaft portion defining a pivot axis. In some embodiments, the second rod receptacle defines an opening at a distal end thereof, the opening sized to accommodate passage of the shaft portion of the pivot member. The mounting platform of the base portion may define a center hole for receiving the pivot member, and may include interior threads for mating with the external shaft threads. In some embodiments, the center hole is a through hole. 
     In some embodiments, the second rod receptacle includes an internal flange having an interior face, the head portion of the pivot member being dimensioned to register against the interior face of the internal flange to secure the second rod receptacle to the base portion. The head portion of the pivot member may be one of a flat head and a countersink head, and the interior face of the internal flange of the second rod receptacle may conforms to the head portion to enable selective monoaxial rotation about the pivot axis. 
     In some embodiments, a pedicle screw coupled to a distal end portion of the first rod receptacle. The pedicle screw and the first rod receptacle may be configured for polyaxial rotation of the first rod receptacle about a head of the pedicle screw. In some embodiments, a lock ring configured to engage a spinal support rod, the lock ring including a distal face that conforms to the pedicle screw and a proximal face that includes one or more malleable features for engaging the spinal support rod. The one or more malleable features may be plastically deformable. In some embodiments, the one or more malleable features includes a raised ridge. In some embodiments, the raised ridge is annular ring. 
     Some embodiments include a cap including a skirt portion having interior threads for mating with the second exterior threads of the second rod receptacle. In some embodiments, the skirt portion is not threaded, but instead slides over the exterior of the base rod receptacle to prevent splaying. An exterior surface of the skirt portion may define a plurality of flats, each of said plurality of flats being parallel to the pivot axis. In some embodiments, one or more of the first interior threads, the first exterior threads, the second interior threads, the second exterior threads, and the internal threads of the center hole define a canted cantilever profile. The canted cantilever profile may extend radially and variously in a distal direction or a proximal direction. 
     In various embodiments of the disclosure, a spinal rod support system comprises an extension rod receptacle, a base portion including a mounting platform that defines a center hole accessible from a proximal face of the mounting platform, and means for coupling the extension rod receptacle to the base portion. In some embodiments, the means for coupling the extension rod receptacle to the base portion includes a pivot member threadably engaged with the center hole. In some embodiments, the means for coupling the extension rod receptacle to the base portion includes a pivot member swaged to the center hole. In some embodiments, the means for coupling the extension rod receptacle to the base portion includes a pivot member fused to the center hole. The pivot member may be welded to the center hole at a distal face of the base portion, for example, a distal face of the mounting platform. 
     The spinal support system may further comprise a base rod receptacle, and means for coupling the base portion to the base rod receptacle. In some embodiments, the base portion includes a skirt portion, and the means for coupling the base portion to the base rod receptacle includes threaded engagement of the skirt portion to the base rod receptacle. In some embodiments, the base portion includes a set screw portion that is unitary with and extends from a distal face of the mounting platform, and the means for coupling the base portion to the base rod receptacle includes threaded engagement of the set screw portion to the base rod receptacle. In some embodiments, the set screw portion and the base portion include mating threads that define canted cantilever profiles for the threaded engagement. The canted cantilever profiles may extend radially and in a distal direction. In some embodiments, the spinal support system comprises a base spinal support rod disposed in the base rod receptacle, the set screw portion being configured to clamp the base spinal support rod within the base rod receptacle. In some embodiments, the base portion includes a set screw portion that is unitary with and extends from a distal face of the mounting platform, and the means for coupling the base portion to the base rod receptacle includes threaded engagement of the set screw portion to the base rod receptacle. In some embodiments, the means for coupling the extension rod receptacle to the base portion enables only monoaxial rotation of the extension rod receptacle about a pivot axis. 
     In various embodiments of the disclosure, a method of fabricating an extension assembly for a spinal support system comprises: inserting a pivot member into a rod receptacle so that a shaft portion of the pivot member extends from a distal end of the rod receptacle; disposing the shaft portion in a center hole of a base portion so that an internal flange of the rod receptacle is captured between a head portion of the pivot member and the base portion so that an axial gap dimension defined between the head portion and the base member is greater than an axial thickness of the internal flange; and securing the pivot member to the base portion. In some embodiments, the step of disposing and the step of securing includes threadably engaging the shaft portion with the center hole. In some embodiments, the step of disposing includes registering a stop on the pivot member against the base portion to define the axial gap dimension. In some embodiments, the step of securing includes one of fusing and swaging the shaft member to the base portion. 
     In various embodiments of the disclosure, a reinforcement cap is disclosed for a spinal support system, comprising: a platform portion including a distal face and a proximal face separated by a perimeter portion; a skirt portion that extends from the distal face of the platform portion, the skirt portion including an interior surface, at least a portion of the interior surface including interior threads formed thereon; and a set screw portion that extends from the distal face of the platform portion, the set screw portion being surrounded by the skirt portion and including an exterior surface that faces radially outward, at least a portion of the exterior surface including exterior threads formed thereon, the set screw portion defining a rotation axis, the set screw portion and the skirt portion being concentric about the rotation axis to define an annular gap between the exterior threads of the set screw portion and the interior threads of the skirt portion. The skirt portion may include an exterior surface that is tangential with the perimeter portion. In some embodiments, the set screw portion extends distally beyond the skirt portion. The skirt portion may include an exterior surface that defines a plurality of flats, each of the plurality of flats being parallel to the rotation axis. 
     In some embodiments, the exterior threads of the set screw portion define a canted cantilever profile. Likewise, in some embodiments, the interior threads of the skirt portion define a canted cantilever profile. The canted cantilever profile of the set screw portion may slope in a distal direction and toward the rotation axis. The canted cantilever profile of the set screw portion may slope in a distal direction and away from the rotation axis. 
     In some embodiments, the platform portion defines a socket accessible from the proximal face, the socket being concentric with and extending along the rotation axis. The platform portion may also define a tapped center hole accessible from the proximal face, the tapped center hole being concentric with and extending along the rotation axis, the socket extending distally from the tapped center hole. The reinforcement cap may further define a center passage concentric with the rotation axis and extending from the socket through a distal end of the set screw portion. 
     In various embodiments of the disclosure, a spinal support system comprises the reinforcement cap as described above; and a spinal rod receptacle having a side wall that includes an interior surface and an exterior surface, the side wall defining a pair of diametrically opposed slots that extend axially along the side wall and are open at a proximal end of the spinal rod receptacle, the interior surface of the spinal rod receptacle defining interior threads, the exterior surface of the spinal rod receptacle defining first exterior threads, wherein the interior threads of the spinal rod receptacle are configured to mate with the exterior threads of the set screw portion of the reinforcement cap, and the exterior threads of the spinal rod receptacle are configured to mate with the interior threads of the skirt portion of the reinforcement cap. In various embodiments, a spinal rod is configured for insertion into the diametrically opposed slots. A pedicle screw may be disposed within the spinal rod receptacle, a shaft of the pedicle screw extending distally from the spinal rod receptacle. 
     In various embodiments of the disclosure, a method for securing a spinal support rod to a spinal rod receptacle is disclosed, comprising: (a) disposing a spinal support rod through diametrically opposed slots of a spinal rod receptacle; (b) threadably engaging interior threads of a skirt portion of a reinforcement cap with exterior threads of the spinal rod receptacle; (c) simultaneously with step (b), threadably engaging exterior threads of a set screw portion of the reinforcement cap with interior threads of the spinal rod receptacle; and (d) tightening the reinforcement cap against the spinal support rod. In some embodiments, step (d) includes driving the reinforcement cap with a tool that mates with a socket formed on the reinforcement cap. In some embodiments, step (d) includes driving the reinforcement cap with a tool that engages flats formed on the skirt portion of the reinforcement cap. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view of an extension ready base assembly according to an embodiment of the disclosure; 
         FIG.  1 A  is an upper perspective view of a base rod receptacle of  FIG.  1    in isolation according to an embodiment of the disclosure; 
         FIG.  2    is a sectional view of an extensible spinal support system in full assembly with the extension ready base assembly of  FIG.  1    according an embodiment of the disclosure; 
         FIG.  3    is an enlarged, partial sectional view of a lock ring in the assembly of  FIG.  2    according to an embodiment of the disclosure; 
         FIG.  4    is an enlarged, partial sectional view of a threaded wall portion of a base rod receptacle of the assembly of  FIG.  2    according to an embodiment of the disclosure; 
         FIG.  5    is an enlarged, partial sectional view of a threaded wall portion of an extension rod receptacle of the assembly of  FIG.  2    according to an embodiment of the disclosure; 
         FIGS.  6  through  8    are sectional views of alternative extensible spinal support systems in full assembly with the extension ready base assembly of  FIG.  1    according embodiments of the disclosure; 
         FIG.  9    is a perspective view of a platform of an extensible spinal support system with exterior wrench flats according to an embodiment of the disclosure; 
         FIGS.  10  and  11    are sectional views depicting assembly of an extension sub-assembly having a threaded pivot member according to an embodiment of the disclosure; 
         FIGS.  12  and  13    are sectional views depicting assembly of an extension sub-assembly having a welded pivot member according to an embodiment of the disclosure; 
         FIGS.  14  and  15    are sectional views depicting assembly of an extension sub-assembly having a riveted pivot member according to an embodiment of the disclosure; 
         FIGS.  16  and  17    are sectional views of extension sub-assemblies with base portions that have integral set screw portions according to an embodiment of the disclosure; 
         FIG.  18    is a perspective view of the pedicle screw of  FIGS.  1  and  2    according to an embodiment of the disclosure; 
         FIG.  18 A  is a top axial view of the pedicle screw of  FIG.  18    according to an embodiment of the disclosure; 
         FIG.  18 B  is a sectional view of the pedicle screw at plane B-B of  FIG.  18    according to an embodiment of the disclosure; 
         FIG.  19    is a perspective view of an alternative pedicle screw according to an embodiment of the disclosure; 
         FIG.  20 A  is an enlarged, distal end view of the pedicle screw of  FIG.  19    according to an embodiment of the disclosure; 
         FIGS.  20 B and  20 C  are an enlarged, partial perspective views of the pedicle screw of  FIG.  19    according to an embodiment of the disclosure; 
         FIG.  21    is a top axial view of a set screw according to an embodiment of the disclosure; 
         FIG.  22    is a section view of the set screw XXII-XXII of  FIG.  21    at plane according to an embodiment of the disclosure; 
         FIG.  23    is a perspective view of the set screw of  FIG.  21    according to an embodiment of the disclosure; 
         FIG.  24    is a perspective view of a lock ring according to an embodiment of the disclosure; 
         FIG.  25    is an upper perspective view of a spinal support system according to an embodiment of the disclosure; 
         FIG.  26    is a partial sectional view of the spinal support system of  FIG.  25    according to an embodiment of the disclosure; 
         FIG.  27    is a lower perspective view of a reinforcement cap according to an embodiment of the disclosure; 
         FIG.  28    is a sectional view of the reinforcement cap of  FIG.  27    according to an embodiment of the disclosure; 
         FIG.  29    is a sectional view of a reinforcement cap having a tapped center hole according to an embodiment of the disclosure; 
         FIG.  30    is a lower perspective view of a reinforcement cap according to an embodiment of the disclosure; 
         FIG.  31    is a sectional view of the reinforcement cap of  FIG.  30    according to an embodiment of the disclosure; 
         FIG.  32    is an upper perspective view of a reinforcement cap with flats according to an embodiment of the disclosure; 
         FIG.  33    is a sectional view of a conventional rod receptacle in assembly; 
         FIG.  33 A  is an enlarged, partial view of  FIG.  33   ; 
         FIG.  34    is a sectional view of a rod receptacle in assembly having threads that define a canted cantilever profile according to an embodiment of the disclosure; 
         FIG.  34 A  is an enlarged, partial view of  FIG.  34    according to an embodiment of the disclosure; 
         FIG.  35    is a partial sectional view of a rod receptacle in assembly having reversed canted cantilever threads according to an embodiment of the disclosure; and 
         FIG.  35 A  is an enlarged, partial view of  FIG.  35    according to an embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE FIGURES 
     Referring to  FIGS.  1  through  5   , an extensible spinal support system  20  is depicted according to an embodiment of the disclosure. The extensible spinal support system  20  as depicted includes an extension ready base assembly  21  including a pedicle screw  22 , a first or base rod receptacle  24 , a first or base set screw  42 , and a first spinal support rod  36  ( FIG.  1   ). The extensible spinal support system  20  further includes an extension subassembly  26  and a cap  28  ( FIG.  2   ). The extension subassembly  26  includes a base portion  32  and a second or extension rod receptacle  34 . The base rod receptacle  24  is configured to receive the first or base spinal support rod  36 , and the extension rod receptacle  34  is configured to receive a second or extension spinal support rod  38 . In the depicted embodiments, the base spinal support rod  36  is retained within the base rod receptacle  24  with the base set screw  42 . Likewise, for the extensible spinal support system  20 , the extension spinal support rod  38  is retained within the extension rod receptacle  34  with a second or extension set screw  44 . 
     The pedicle screw  22  includes a head portion  64  and a threaded shaft portion  65  centered about a central axis  67 . The pedicle screw  22  extends from a distal end  62  of the base rod receptacle  24 , the head portion  64  being captured within the distal end  62  of the base rod receptacle  24 . In some embodiments, a lock ring  66  is captured between the base spinal support rod  36  and the head portion  64  of the pedicle screw  22 . In the depicted embodiment, the head portion  64  of the pedicle screw  22  defines a spherical surface portion  68 , with the base rod receptacle  24  including a complementary mating surface  72  that engages and conforms to the spherical surface portion  68  of the head portion  64  of the pedicle screw  22 . 
     Functionally, the spherical surface portion  68  of the head portion  64  of the pedicle screw  22  and the complementary mating surface  72  of the base rod receptacle  24  enable the base rod receptacle  24  to pitch about the head portion  64  of the pedicle screw  22  relative to the central axis  67 , and to rotate about the head portion  64  in the pitched orientations. By this arrangement, the pedicle screw  22  and the base rod receptacle  24  are configured for polyaxial rotation of the base rod receptacle  24  about the head portion  64  of the pedicle screw  22 . 
     Herein, “proximal” refers to a direction  76  that is toward a surgeon during operation or implantation and away from a bone or patient. “Distal” refers to a direction  74  that is away from the surgeon during operation or implantation and toward the bone or patient to which the extensible spinal support system  20  is implanted (i.e., a direction opposite the proximal direction  76 ). 
     The base rod receptacle  24  includes a side wall  82  having an interior surface  84  and an exterior surface  86  ( FIG.  1 A ). A pair of diametrically oppose slots  88  are defined on the side wall  82 , the slots  88  extending axially along the side wall  82  and being open at a proximal end  92  of the base rod receptacle  24 . By formation of the diametrically opposed slots  88 , the side wall  82  defines diametrically opposed wall segments  90  on opposing sides of the diametrically opposed slots  88 . The interior surface  84  extends axially along the side wall  82  and includes interior threads  94  formed thereon. The exterior surface  86  also extends axially and includes exterior threads  96  formed thereon. The base portion  32  of the extension subassembly  26  includes a mounting platform  122  and a skirt portion  124  that extends from the mounting platform  122  in the distal direction  74 . The skirt portion  124  includes an interior surface  123  and an exterior surface  125 . 
     The extension rod receptacle  34  includes a side wall  142  having an interior surface  144  and an exterior surface  146 . A pair of diametrically opposed slots  148  are defined on the side wall  142 , the slots  148  extending axially along the side wall  142  and being open at a proximal end  152  of the extension rod receptacle  34 . By formation of the diametrically opposed slots  148 , the side wall  142  defines diametrically opposed wall segments  150  on opposing sides of the diametrically opposed slots  148 . The interior surface  144  extends axially along the side wall  142  and includes interior threads  154  formed thereon ( FIG.  5   ). The exterior surface  146  also extends axially and includes exterior threads  156  formed thereon. 
     In the depicted embodiment, the extension subassembly  26  includes a pivot member  170  that attaches the extension rod receptacle  34  to the base portion  32 , the pivot member  170  including a head portion  172  and a shaft portion  174  and defining a through passage  175 . In assembly, the shaft portion  174  is concentric with a pivot axis  182  about which the extension rod receptacle  34  is rotatable. The extension rod receptacle  34  defines an opening  184  at a distal end  186  thereof. The opening  184  is sized to accommodate passage of the shaft portion  174  of the pivot member  170 . 
     In the depicted embodiment, the mounting platform  122  of the base portion  32  defines a center hole  188  for receiving the pivot member  170 . The extension rod receptacle  34  may include an internal flange  194  having an interior face  196 , the head portion  172  of the pivot member  170  being dimensioned to extend radially over the interior face  196  of the internal flange  194  to loosely secure the extension rod receptacle  34  to the base portion  32 . The head portion  172  of the pivot member  170  may be one of several head styles available to the artisan, for example a flat head (depicted), a socket head, a countersink head, or a spherical head. The interior face  196  of the internal flange  194  may be configured to conform to the head portion  172 . In some embodiments, a collet  198  is disposed interstitially between the extension spinal support rod  38  and the head portion  172 . The collet  198  may be a disc spring (depicted), lock washer, or other that exerts a bias force parallel to the pivot axis  182  when compressed between the support rod  38  and the head portion  172 . The cap  28  includes a top portion  222  and a skirt portion  224  that extends from the mounting platform  222 . The skirt portion  224  includes interior threads  226  for mating with the exterior threads  156  of the extension rod receptacle  34 . 
     In operation, the pedicle screw  22  is inserted into the base rod receptacle  24  so that the head portion  64  of the pedicle screw  22  can be registered against the mating surface  72  of the base rod receptacle  24 . The pedicle screw  22  is set into the bone of a vertebrae. The lock ring  66 , if utilized, is disposed within the base rod receptacle  24  and arranged for contact with the head portion  64  of the pedicle screw  22 . The base rod receptacle  24  is arranged in a desired orientation on the head portion  64  of the pedicle screw  22  and the base spinal support rod  36  disposed in the base rod receptacle  24 . The lock ring  66  is thereby disposed between the base spinal support rod  36  and the head portion  64  of the pedicle screw  22 . The base set screw  42  is threadably engaged with the interior threads  94  of the side wall  82  and tightened so that the base spinal support rod  36  is clamped between the base set screw  42  and the lock ring  66  (if utilized) or, alternatively, the head portion  64  of the pedicle screw  22 . The tightening of the base set screw  42  also seats the head portion  64  of the pedicle screw  22  against the mating surface  72  of the base rod receptacle  24  to secure the base rod receptacle  24  in the desired orientation relative to the head portion  64 . 
     Referring to  FIGS.  6  and  7    and again to  FIG.  2   , extensible spinal support systems  20  having alternative embodiments for the base portion  32  are presented according to embodiments of the disclosure. In  FIGS.  2 ,  6 , and  7   , the extensible spinal support systems and their associated base portions are referred to collectively and generically by reference characters  20  and  32 , respectively, and specifically by the reference characters  20  and  32  followed by a letter suffix (e.g., extensible spinal support system  20   a  and the associated base portion  32   a ). Several components and attributes are common to all extensible spinal support systems  20 , which are indicated with same numbered reference characters. 
     For the extensible spinal support system  20   a  ( FIG.  2   ), the base portion  32   a  and the base set screw  42  are separate components that are installed independently of each other. For the extensible spinal support systems  20   a  and  20   b , the skirt portion  124  includes interior threads  126  for mating with the exterior threads  96  of the base rod receptacle  24 . For the extensible support system  20   b  ( FIG.  6   ), the base portion  32   b  additionally includes a set screw portion  228  that is integral therewith, the set screw portion  228  having external threads  232  that mate with the interior threads  94  of the side wall  82  of the base rod receptacle  24 . Accordingly, for the extensible spinal support system  20   b , the external threads  232  of the set screw portion  228  and the interior threads  226  of the skirt portion  124  of the base portion  32   b  are threaded simultaneously. For the extensible spinal support system  20   c  ( FIG.  7   ), the base portion  32   c  also includes the set screw portion  228  that is integral therewith, and having external threads  232  that mate with the interior threads  94  of the side wall  82  of the base rod receptacle  24 . However, for the extensible spinal support system  20   c , an inner surface  234  of the skirt portion  124  is smooth (does not include interior threads), such that the skirt portion  124  rotates about and slides over but does not threadably engage the exterior threads  96  of the base rod receptacle  24 . Accordingly, the base portion  32   c  mates only with the interior threads  94  of the side wall  82 . 
     By integrating the set screw portion  228  with the mounting platform  122  as in subassemblies  26   b  and  26   c , and skirt portion  124  the structural integrity of the extensible spinal support system  20  is enhanced. For example, lateral forces applied to the extensible spinal support system  20  will incur greater resistance because the set screw portion  228  is integrated with (e.g., unitary with) the mounting platform  122 , establishing a shear stress at the junction of the integrated set screw portion  228  and the mounting platform  122  that provides additional resistance to deformation relative to the extension subassemblies  26  of  FIGS.  10  through  15   . 
     Functionally, each of the extensible support systems  20  provide a unique advantage. Extensible spinal support system  20   a  enables mounting of the extension assembly  26  without disturbing the base set screw  42 , eliminating the need to reset the base spinal support rod  36 . The extensible spinal support system  20   a  may find application where disturbance of the base spinal support system is not necessary or is ill advised. The extensible spinal support system  20   b  provides a dual threaded arrangement that enhances structural integrity of the extension assembly  26  to the base rod receptacle  24 . Such enhancement of the structural integrity may be advantageous for high torque and high stress applications, such as scoliosis correction. Extensible spinal support system  20   c  also includes the integrated set screw portion  228  and attendant benefit while the smooth, sliding fit of the skirt portion  124  enables easier installation where the dual threaded arrangement of the extensible spinal support system  20   b  may be unnecessary or difficult. The sliding fit of the skirt portion  124  for the extensible spinal support system  20   c  effectively captures the side walls  82  of the base rod receptacle  24  to limit splaying. The extensible spinal support system  20   c  can also be configured for retrofitting with tulips of conventional spinal support systems that are not “extension ready.” 
     Referring to  FIG.  8   , an extensible spinal support system  20   d  is depicted according to an embodiment of the disclosure. As depicted the extensible spinal support system  20   d  includes many of the same components and attributes as the extensible spinal support system  20   a , which are indicated with same numbered referenced characters. While components and attributes of the extensible spinal support system  20   a  are presented in the extensible spinal support system  20   d , such components and attributes are not limiting. That is, the extensible spinal support system  20   d  may implement various aspects of the extensible spinal support systems  20   b  and  20   c  as well, which one of skill in the spinal support arts recognizes in view of this disclosure. 
     For the extensible spinal support system  20   d , the head portion  172  of the pivot member  170   d  is spherical, and the extension rod receptacle  34  is configured as discussed above for the base rod receptacle  24 . By this arrangement, the pivot member  170  and the extension rod receptacle  34  are configured for polyaxial rotation of the extension rod receptacle  34  about the head portion  172  of the pivot member  170 . 
     Referring to  FIG.  9   , the skirt portion  124  of the base portion  34  is depicted with flats  240  according to an embodiment of the disclosure. The flats  240  are parallel to the pivot axis  182  and may be implemented with any of the depicted or contemplated embodiments. The embodiment of  FIG.  9    depicts a total of six flats  240 . By way of non-limiting example, the number of flats  240  may range from four to twelve. In some embodiments, the skirt portion  124  defines two flats  240  that are diametrically opposed. In some embodiments, the skirt portion  124  defines four flats  240  that are distributed as two diametrically opposed pairs that are rotationally offset at 90 degrees with respect to each other. 
     Referring to  FIGS.  10  through  17   , assembly of various extension subassemblies  26  are depicted according to embodiments of the disclosure. In  FIGS.  10  through  17   , the extension subassemblies and their associated pivot members are referred to collectively and generically by reference characters  26  and  170 , respectively, and specifically by the reference characters  26  and  170  followed by a letter suffix (e.g., extension subassembly  26   a  and the associated pivot member  170   a ). Several components and attributes are common to all extension subassemblies  26 , which are indicated with same numbered reference characters. 
     For the extension subassembly  26   a  ( FIGS.  10  and  11   ), the pivot member  170   a  defines exterior shaft threads  176  formed on an exterior surface  178  of the shaft portion  174 . The center hole  188  of the mounting platform  122  includes internal threads  192  for mating with the exterior shaft threads  176 . In some embodiments, the pivot member  170   a  includes a stop  242 , such as a shoulder  244  having a diameter  246  that is greater than the diameter of the exterior shaft threads  176 . The stop  242  cooperates with the head portion  172  to define an axial gap dimension  248  that is greater than an axial thickness  250  of the internal flange  194 . 
     In assembly, the pivot member  170   a  is inserted through the extension rod receptacle  34  and into the opening  184  so that the shaft portion  174  extends from the distal end  186  of the extension rod receptacle  34  and the head portion  172  of the pivot member  170   a  is within the extension rod receptacle  34 . The shaft portion  174  of the pivot member  170   a  and the center hole  188  of the mounting platform  122  of the base portion  32   a  are then aligned and the exterior shaft threads  176  of the shaft portion  174  threaded into the internal threads  192  of the center hole  188 . In some embodiments, the pivot member  170   a  is threaded into the center hole  188  until the stop  242  is firmly seated against the platform  122  over the mouth of the center hole  188  to define the axial gap dimension  248 . Alternatively, the pivot member  170   a  may otherwise engage the base portion  32   a  in a manner that causes the pivot member  170   a  to stop within the center hole  188 ; for example, the threads  176  may cease at a point on the pivot shaft  174  that provides the desired axial gap dimension  248 . Having secured the pivot member  170   a  to the mounting platform  122 , the extension rod receptacle  34  is coupled to the base portion  32 . 
     For the extension subassembly  26   b  ( FIGS.  12  and  13   ), the shaft portion  174  of the pivot member  170   b  is a right cylinder  252 , defining a smooth exterior surface  178 . The center hole  188  of the mounting platform  122  is also right cylindrical, and may be dimensioned to provide a close sliding fit with the shaft portion  174 . In some embodiments, the pivot member  170   b  includes a stop (not depicted) akin to pivot member  170   a.    
     In assembly, the pivot member  170   b  is inserted through the extension rod receptacle  34  and into the opening  184  so that the shaft portion  174  extends from the distal end  186  of the extension rod receptacle  34  and the head portion  172  of the pivot member  170   b  is within the extension rod receptacle  34 . The shaft portion  174  of the pivot member  170   b  and the center hole  188  of the mounting platform  122  of the base portion  32  are then aligned and the shaft portion  174  positioned within the center hole  188  to so that the axial gap dimension  248  is defined between the head portion  172  and the mounting platform  122 . In some embodiments, the pivot member  170   a  is threaded into the center hole  188  until the stop  242  is firmly seated against the platform  122  over the mouth of the center hole  188 , thereby defining the axial gap dimension  248 . In some embodiments, the length of the shaft portion  174  is dimensioned to provide the desired axial gap dimension  248  when a distal end  253  of the shaft portion  174  is flush with a distal face  256  of the platform  122 . 
     With the pivot member  170   b  positioned in the center hole  188  to define the axial gap dimension  248  greater than the axial thickness  250  of the internal flange  194 , the pivot member  170   b  is secured to the platform  122 . In the depicted embodiment, the distal end  253  of the pivot member  170   b  is welded to the distal face  256  of the mounting platform  122  to form a weld  254  at the perimeter of the center hole  188 . The weld  254  may be continuous, a stitch weld, or a tack weld. The welding operation may be performed with welding techniques available to the artisan, including but not limited to electron beam welding. Alternatively, instead of welding, the pivot member  170   b  may be secured by other bonding or fusion techniques, such as brazing, soldering, or gluing. Upon securing the pivot member  170   b  to the base portion  32 , the internal flange  294  of the extension rod receptacle  34 , being captured between the head portion  172  and the mounting platform  122 , is coupled to the base portion  32 . 
     For the extension subassembly  26   c  ( FIGS.  14  and  15   ), the shaft portion  174  of the pivot member  170   c  is also the right cylinder  252 , defining the smooth exterior surface  178 . The center hole  188  of the mounting platform  122  is also right cylindrical, and may be dimensioned to provide a close sliding fit with the shaft portion  174 . Similar to pivot members  170   a  and  170   c , the pivot member  170   c  may include the stop  242  such as the shoulder  244  with diameter  246 , the diameter  246  being greater than the diameter of the center hole  188 . As with the subassemblies  26   a  and  26   b , the stop  242  of the subassembly  26   c  cooperates with the head portion  172  to define an axial gap dimension  248  that is greater than an axial thickness  250  of the internal flange  194 . The distal face  256  of the mounting platform  122  may define a recessed lead in  258  that surrounds the center hole  188 . 
     In assembly, the pivot member  170   c  is inserted through the extension rod receptacle  34  and into the opening  184  so that the shaft portion  174  extends from the distal end  186  of the extension rod receptacle  34  and the head portion  172  of the pivot member  170   c  is within the extension rod receptacle  34 . The shaft portion  174  of the pivot member  170   c  and the center hole  188  of the mounting platform  122  of the base portion  32  are then aligned and the shaft portion  174  positioned within the center hole  188  so that the axial gap dimension  248  is defined between the head portion  172  and the mounting platform  122 . In some embodiments, the pivot member  170   c  is inserted into the center hole  188  until the stop  242  registers against the platform  122  over the mouth of the center hole  188 , thereby defining the axial gap dimension  248 . In some embodiments, the length of the shaft portion  174  is dimensioned to provide the desired axial gap dimension  248  when the distal end  253  is flush with the distal face  256  of the platform  122 . 
     With the pivot member  170   c  positioned in the center hole  188  to define the axial gap dimension  248  greater than the axial thickness  250  of the internal flange  194 , the pivot member  170   c  is secured to the platform  122  by a swaging process. The swaging process deforms the distal end  253  of the shaft portion  174  into the recessed lead in  258 . In this way, the mounting platform  122  is swaged between the stop  242  and the deformed distal end  253  of the pivot member  170   c , akin to a rivet. The internal flange  294  is captured between the head portion  172  of the pivot member  170   c  and the mounting platform  122  of the base portion  32 , thereby coupling the extension rod receptacle  34  to the base portion  32 . 
     The depictions of  FIGS.  10  through  15    present base portions  32  akin to base portion  32   a , i.e., without an integral set screw portion. However, the assembly techniques for the subassemblies  26  described above are readily implemented for base portions  32  that include the set screw portion  228  integral therewith, akin to base portions  32   b  and  32   c . The embodiments of  FIGS.  6  and  7    depict the threaded pivot member  170   a  in combination with the integral set screw portion  228 . The embodiments of  FIGS.  16  and  17    depict the pivot members  170   b  and  170   c  with longer shaft portions  174  having distal ends  253  that reach the distal faces  257  of the integral set screw portions  228 . The weld  254  ( FIG.  16   ) and the recessed lead in  258  ( FIG.  17   ) are formed on the distal faces  257  of the integral set screw portion  228 . Accordingly, the subassemblies  26  may be fabricated with any of the base portions  32  described and depicted herein. 
     For the various subassemblies  26 , because the axial gap dimension  248  is greater than the axial thickness  250  of the internal flange  194 , monoaxial rotation of the extension rod receptacle  34  about the pivot axis  182  is achieved. That is, the internal flange  194 , though effectively captured between the head portion  172  of the pivot member  170  and the base portion  32  of the extension subassembly  26 , can be rotated about the pivot axis  182 . In the depicted embodiment, movement of the of the extension rod receptacle  34  relative to the base portion  32  is effectively limited substantially to rotation about the pivot axis  182 , i.e., a “monoaxial” rotation. 
     For the subassemblies  26  of  FIGS.  10 - 15   , because of the exterior threads  96  on the side wall  82  of the base rod receptacle  24 , the extension rod receptacle  34  can be mounted to the base rod receptacle  24 . That is, the extension subassemblies  26  of  FIGS.  10 - 15    can be mounted directly to the exterior threads  96  of the base rod receptacle  24 . For the subassemblies  26  of  FIGS.  16  and  17   , the extension rod receptacle  34  can be mounted to the interior threads  94  of the base rod receptacle  24 , without need for exterior threads on the sidewall  82 . Accordingly, because of the various subassemblies  26 , the base rod receptacle  24  is referred to as “extension ready.” 
     For the pre-assembled extension subassemblies  26  of  FIGS.  10 - 15   , the base portion  32  is aligned with the base rod receptacle  24  and the interior threads  126  of the skirt portion  124  of the base portion  32  threaded over the exterior threads  96  of the base rod receptacle  24 . For the pre-assembled extension subassemblies  26  of  FIGS.  16  and  17   , the base portion  32  is aligned with the base rod receptacle  24  and both the interior threads  126  of the skirt portion  124  and the exterior threads external threads  232  of the set screw portion  228  are threadably engage with the exterior threads  96  of the base rod receptacle  24  and the internal threads  192  of the center hole  188 , respectively. For the subassemblies  26 , because the pivot member  170  is in fixed relationship with the base portion  32 , the base portion  32  may be drawn tight against the proximal end  92  of the base rod receptacle  24  by with a driver inserted in the socket of the pivot member  170 . The extension rod receptacle  34  may be rotated to a desired angular orientation about the pivot axis  182 . The extension spinal support rod  38  is inserted into the extension rod receptacle  34 , extending laterally through the diametrically opposed slots  148 . The extension set screw  44  or, alternatively, the set screw portion  228 , is threadably engaged with the interior threads  154  of the side wall  142  and tightened. When the set screw  44  is tightened against the extension spinal support rod  38 , the extension rod receptacle  34  is drawn in the proximal direction (upward in  FIG.  2   ) so that the internal flange  194  is drawn tight against the head portion  172 , thereby locking the extension rod receptacle  34  in place and in a fixed rotational orientation relative to the head portion  172 . 
     In some embodiments, in the absence of the extension subassembly  26 , the cap  28  can be mounted to the exterior threads  96  of the base rod receptacle  24 . Functionally, this arrangement provides support against outward lateral deflections (splaying) of the wall segments  90  would otherwise be provided by the skirt portion  124  of the mounting platform  122  of the extension subassembly  26 . Various reinforcement caps  28  that can be implemented with either the base rod receptacle  24  or the extension rod receptacle  34  are described below attendant to  FIGS.  25 - 32   . 
     Similar to the extensible spinal support system  20   a  of  FIG.  2   , the extensible spinal support systems  20   b  and  20   c  of  FIGS.  6  and  7    may be mounted to a previously implanted extension ready base assembly without need for removing or otherwise releasing the base spinal support rod  36 . The base portions  32   b  and  32   c  of the extension subassemblies  26   b  and  26   c  may be mounted to the extension ready base assembly  21  of  FIG.  1    instead of the set screw  42 , the base portions  32   b  and  32   c  serving as a cap for securing the base spinal rod  36  to the base rod receptacle  24 . The extension rod receptacle  34  can later be mounted directly to the center hole  188  of the mounting platform  122  of the base portion  32   b  with the pivot member  170   a , thereby leaving the arrangement of the base rod receptacle  24  the base portions  32   b ,  32   c  and the base spinal support rod  36  intact. Accordingly, the base rod receptacle  24  in combination with the base portions  32   b  or  32   c  is also referred to as “extension ready.” In such an embodiment, the extension subassemblies  26   b  and  26   c  are not pre-assembled, but instead assembled on the existing base portion  32   b ,  32   c , being built up from the mounting platform  122  as described above. 
     Referring to  FIGS.  18  through  18 B , the pedicle screw  22  is depicted in isolation in an embodiment of the disclosure. In addition to the components and attributes discussed above, the pedicle screw  22  may define a socket  259  in the head portion  64 , accessible from a proximal end  260  of the pedicle screw  22 . The threaded shaft portion  65  may be double threaded as depicted. In some embodiments, a center passage  261  extends through the head portion  64  and threaded shaft portion  65 . In the depicted embodiment, the socket  259  is hexagonal, but other geometries, such as a square, rectangle, cross, or star pattern may be utilized. 
     Functionally, the socket  259  accommodates driving of the pedicle screw  22  with an appropriate mating wrench (e.g., hexagonal wrench for the depicted embodiment, or a square bit, rectangular bit, cross (PHILLIPS) bit, or star (TORR®) bit as appropriate). The center passage  261  may be sized, for example, to accommodate sliding passage of a KIRSCHNER wire or a larger diameter rod. 
     Referring to  FIGS.  19  and  20 A through  20 C , a pedicle screw  22   a  is depicted according to an embodiment of the disclosure. The pedicle screw  22   a  includes some of the same components and attributes as pedicle screw  22 , some of which are identified with same-labeled reference characters. In some embodiments, the pedicle screw  22   a  includes double threads  262   a  and  262   b , referred to collectively and generically as threads or double threads  262 . The pedicle screw  22   a  may define a self-tapping flute  264  at a distal end portion  265 . In some embodiments, the threaded portion  65  of the pedicle screw  22   a  defines fenestrations  271  that are in fluid communication with the center passage  261 . The pedicle screw  22   a  may define a depression  268  at a leading end  269  of each double thread  262   a  and  262   b.    
     In some embodiments, the threads  262  that are terminated at the self-tapping flute  264  and include radiused or curved terminations  266  at the faces  267  of the self-tapping flute  264 , thereby defining a convex profile that extends tangentially (i.e., in the θ-direction of the r-θ-z coordinate of  FIG.  19   ) from proximate a root of the threads  262  at the faces  267  of the self-tapping flute  264  to a crest of the threads  262 . In some embodiments, the curved terminations  266  define radii R. By way of non-limiting example, the radii may be within a range of: 100 to 400 micrometers inclusive; 100 to 300 micrometers inclusive; 150 to 300 micrometers inclusive; or 150 to 250 micrometers inclusive. Herein, a range that is said to be “inclusive” includes the endpoint values of the range as well as all values therebetween. 
     In some embodiments, a leading edge  263  of the flute  264  is rounded or otherwise dulled to blunt an otherwise sharp edge. The edges of the threaded shaft portion  65 , such as corners at the crest of the threads  262 , may be dulled to blunt an otherwise sharp edge. Techniques for producing the dulled edges include, for example, anodizing, surface polishing, or, in contrast, by surface roughening. Polishing techniques include tumbling the pedicle screw  22  in a granular or powder ceramic. Roughening techniques include sandblasting, wire brushing, laser-induced roughening, and etching. In some embodiments, the roughening techniques provide a surface roughness having an arithmetical mean deviation over the surfaces of the pedicle screw  22  that is in a range of 5 micrometers to 15 micrometers inclusive. The arithmetical mean deviation parameter, commonly referred to as “Ra” in the texturing arts, is described, for example, at Degarmo, et al., “Materials and Processes in Manufacturing,” (9th ed.), p. 223, John Wiley &amp; Sons (2003), ISBN 0-471-65653-4, the disclosure of which is hereby incorporated by reference herein except for express definitions contained therein. 
     Functionally, the curved terminations  266  reduce the sharpness of the threads  262  at the faces  267  of the self-tapping flute  264 . With the curved terminations  266 , the self-tapping flute  264  is still effective in tapping into bone tissue, but will not tend to cut soft tissue. Rather than cutting soft tissues, the curved terminations  266 , as well as the dulled corners of the threads  262 , tend to push soft tissue aside as opposed to slicing or tearing through the soft tissue. Such soft tissue includes blood vessels, which will tend to be deflected (instead of sliced) by the curved terminations  266 . The fenestrations  271  promote the ingrowth of tissue into the pedicle screw  22   a  for a more secure mooring of the pedicle screw  22   a  within the bone over time. 
     The depression(s)  268  help compensate for the lack of a sharp cutting edge at the leading end  269  of the threads  262 . In some embodiments, one depression  268  is formed at the leading end of each of the double threads  262   a  and  262   b  (depicted). The depressions  268  reduce the circumferential area of the pedicle screw  22   a  in the direction of rotation. For a given applied rotational force, the reduction in area produces an increase in the pressure applied by the pedicle screw  22   a . The increased pressure augments penetration of the distal end portion  265  of the pedicle screw  22   a  through tissue without resort to sharp cutting edges, and also provides a relief that enables the tissue to flow over and around the leading end  269  of the thread  262 . 
     Referring to  FIGS.  21  through  23   , the base or extension set screw  42 ,  44  is depicted in isolation in an embodiment of the disclosure. In the depicted embodiments, the base and extension set screws  42  and  44  are identical and are referred to collectively and generically as the “set screw  42 ,  44 ”. The set screw  42 ,  44  includes exterior threads  270  that mate with the interior threads  94 ,  154  of the base or extension rod receptacle  24 ,  34 . The set screw  42 ,  44  may define a socket  272 , accessible from a proximal end  274  of the set screw  42 ,  44 . In some embodiments, a center passage  276  extends from the socket  272  through a distal end  278  of the set screw  42 ,  44 . In the depicted embodiment, the socket  272  is hexagonal, but other geometries, such as a square, rectangle, cross, or star pattern may be utilized. 
     Functionally, the socket  272  accommodates driving of the set screw  42 ,  44  with an appropriate mating wrench (e.g., hexagonal wrench for the depicted embodiment, or a square bit, rectangular bit, cross (PHILLIPS) bit, or star (TORX®) bit as appropriate). The center passage  276  may be sized, for example, to accommodate sliding passage of a KIRSCHNER wire or larger diameter rod. 
     Referring to  FIG.  24   , the lock ring  66  is depicted in an embodiment of the disclosure. In some embodiments, the lock ring  66  includes a distal face  282  and a proximal face  284  separated by a perimeter portion  286 . The distal face  282  may be convex and define a spherical profile  288 . The proximal face  284  may include one or more malleable features  292  for engaging the base spinal support rod  36 . The distal face  282  terminates at a distal edge  294  of the perimeter portion  286 . In the depicted embodiment, a plurality of relief slots  296  are defined that are open at the distal edge  294  and extend axially into the perimeter portion  286 . In some embodiments, the one or more malleable features  292  are plastically deformable. The one or more malleable features  292  may define a raised ridge  298 , for example an annular ring as depicted in  FIG.  24   . In some embodiments, the lock ring  66  includes a radial detent  299  that extends radially outward from the perimeter portion  286 . 
     Functionally, the convexity of the distal face  282  that accommodates and can slide over the spherical profile of the head portion  64  of the pedicle screw  22 , thereby enabling the polyaxial movement of the base rod receptacle  24  relative to the head portion  64 . The one or more malleable features  292  conform to the shape of the base spinal support rod  36  when the set screw  42  is tightened to secure the base spinal support rod  36  in place. The conformance of the malleable feature(s)  292  acts to grip the base spinal support rod  36 , thereby inhibiting the base support rod  36  from rotating or sliding within the diametrically opposed slots  88  of the base rod receptacle  24 . Upon tightening of the base set screw  42 , the relief slots  296  enable the perimeter portion  286  to conform to the head portion  64  at the distal edge  294  for more effective gripping of the head portion  64  of the pedicle screw  22 . The conformance of the malleable feature(s)  292  and perimeter portion  286  act to secure and inhibit movement between the head portion  64 , the base spinal support rod  36 , and the base rod receptacle  24 . The radial detent  299  may interface with internal features  297  ( FIG.  3   ) within the base rod receptacle  24 , such as an internal inset flange (depicted) or optionally a groove (not depicted). The internal features  297  restrains the proximal face  284  from deflecting proximally (upward in  FIGS.  2  and  3   ) when the lock ring  66  is deformed under the clamping force between the base spinal support rod  36  and the head portion  64 , thereby maintaining gripping contact between the head portion  64  and the proximal face  284  of the lock ring  66 . 
     Referring to  FIGS.  25  and  26   , the extension ready base assembly  21  is depicted with the reinforcement cap  28  coupled to the base rod receptacle  24 . Referring to  FIGS.  27  and  28   , a reinforcement cap  28   a  is depicted according to an embodiment of the disclosure. Herein, various reinforcement caps are presented and referred to collectively and generically by reference character  28 , and specifically by the reference character  28  followed by a letter suffix (e.g., reinforcement cap  28   a ). The reinforcement cap  28   a  includes a platform portion  322  including a distal face  324  and a proximal face  326  separated by a perimeter portion  328  ( FIGS.  26 ,  27 , and  28   ). A skirt portion  332  extends from the distal face  324  of the platform portion  322  portion, the skirt portion  332  including an interior surface  334 , at least a portion of which includes interior threads  336  formed thereon. A set screw portion  342  extends from the distal face  324  of the platform portion  322 , the set screw portion  342  being surrounded by the skirt portion  332  and including an exterior surface  338  that faces radially outward. At least a portion of the exterior surface  338  includes exterior threads  344  formed thereon. The set screw portion  342  defines a rotation axis  346  about which the set screw portion  342  and the skirt portion  332  are concentric. An annular gap  348  is defined between the exterior threads  344  of the set screw portion  342  and the interior threads  336  of the skirt portion  332 . 
     The reinforcement cap  28   a  may define a socket  272  accessible from the proximal face  326  of the cap  28   a . In the depicted embodiment, the socket  272  is hexagonal, but other geometries, such as a square, rectangle, octagon, cross, or star pattern may be utilized. In some embodiments, a center passage  276  extends from the socket  272  through a distal end  278  of the set screw portion  342 . 
     Referring to  FIG.  29   , a reinforcement cap  28   b  modified to define a tapped center hole  360  is depicted according to an embodiment of the disclosure. The reinforcement cap  28   b  includes many of the same components and attributes as the reinforcement cap  28   a , which are indicated with same-numbered numerical characters. The tapped center hole  360  is characterized as having an inner wall  362  on which threads  364  are formed, and a bottom surface  366 . The socket  272  extends distally from the bottom surface  366 . 
     Referring to  FIGS.  30  and  31   , a reinforcement cap  28   c  is depicted according to an embodiment of the disclosure. The reinforcement cap  28   c  includes many of the same components and attributes as the reinforcement cap  28   a  and  28   b , which are indicated with same-numbered reference characters. A distinction of the reinforcement cap  28   c  is that the skirt portion  332  does not include interior threads, such that the skirt portion  332  slidingly engages and rotates about the exterior threads  96  of the base rod receptacle  24  but does not threadably engage the exterior threads  96 . Accordingly, the reinforcement cap  28   c  threadably engages only with the interior threads  94  of the side wall  82 . In some embodiments, the skirt portion  332  may define an inner diameter  372  that slides over the exterior threads  96  with a close, sliding fit. 
     Referring to  FIG.  32   , the skirt portion  332  of the reinforcement cap  28  is depicted with flats  374  according to an embodiment of the disclosure. The flats  374  extend parallel to the rotation axis  346  and may be implemented with any of the depicted or contemplated reinforcement caps  28 . The embodiment of  FIG.  32    depicts a total of six flats  374  as a non-limiting example. In some embodiments the number of flats  374  is in a range of six to twelve inclusive. In some embodiments, the skirt portion  332  defines two flats  374  that are diametrically opposed. In some embodiments, the skirt portion  332  defines four flats  374  that are distributed as two diametrically opposed pairs that are rotationally offset at 90 degrees with respect to each other. 
     Functionally, the interior threads  94  of the base rod receptacle  24  are configured to mate with the exterior threads  344  of the set screw portion  342  of the reinforcement cap  28 . For the reinforcement caps  28   a  and  28   b , the exterior threads  96  of the base rod receptacle  24  are configured to mate with the interior threads  336  of the skirt portion  332  of the reinforcement cap  28 ,  28   b . The socket  272  accommodates driving of the reinforcement cap  28 ,  28   b  with an appropriate mating wrench (e.g., hexagonal wrench for the depicted embodiment, or a square bit, rectangular bit, cross (PHILLIPS) bit, or star (TORX®) bit as appropriate). The flats  374 , when implemented, provide an alternative way to apply torsion to the reinforcement cap  28 , for example by use of socket tool that slides over and engages the flats  374 . The flats  374  can also be used in so-called rescue situations, providing alternative gripping surfaces for removal of components of the spinal support system  20 . The center passage  276  may be sized, for example, to accommodate sliding passage of a KIRSCHNER wire or a guide rod. 
     The unitary or otherwise integral structure of the set screw portion  342  with the platform portion  322  and skirt portion  332  provides additional structural strength and integrity relative to a separate cap and set screw arrangement. For example, the spinal support system  20  provides greater resistance to lateral forces because the set screw portions  342  is integrated with the platform portion  322 , establishing a shear stress at a junction  368  of the screw portion  342  and the platform portion  322  that provides additional resistance to deformation relative to an assembly where the cap and set screw are separate components. The added strength and structural integrity provided by the integrated arrangement of the reinforcement cap  28  may be advantageous for high torque and high stress applications, such as scoliosis correction. 
     Referring to  FIGS.  33 ,  33 A,  34 , and  34 A , threads defining a canted cantilever profile  450  and the advantage provided over conventional threaded arrangements are depicted and described according to embodiments of the disclosure. Conventional threaded arrangements  400 , schematically depicted at  FIGS.  33  and  33 A , may include, for example, exterior threads  402  of a set screw  404  that are engaged with interior threads  406  of a wall segment  408  of a spinal rod receptacle  410  (akin to opposed wall segments  90  and  150  of the rod receptacles  24  and  34  of the extensible spinal support system  20 ). Both the set screw  404  and the wall segment  408  are concentric about a central axis  412  that defines the z-axis of a right-cylindrical coordinate system  424  having an axial coordinate z and a radial coordinate r. When the set screw  404  is tightened in the first direction  416  to set against a spinal support rod  415 , a clamping force vector FC is generated, for which there is an equal and opposite force vector FC′ in a second direction  418  that is opposite the first direction  416 . The force vector FC′ in turn generates reaction force vectors FR generated at contact interfaces  422  between the proximal faces of the exterior threads  402  of the set screw  404  and the distal faces of the interior threads  406  of the wall segment  408 . The reaction forces FR generate an axial component FRZ and radial component FRR. Because of the standard shape of the threads  402  and  406 , the radial components FRR generate a radial outward force FRO, i.e., away from the central axis  412 . 
     For configurations such as the depicted extensible spinal support system  20 , the wall segment  408  (e.g., wall segment  90  of the extensible spinal support system  20 ) is, in some embodiments, not supported by any structure other than resistance to bending at the base of the wall segment. In such embodiments, the wall segment  408  will tend to cause deflections δo that deflect radially outward in response to the radial outward force FRO. As the wall segment  408  deflects radially outward, the overlap between the threads  406  and  408  at the interfaces  422  is reduced, thereby weakening the coupling between the set screw  404  and the wall segment  408 . The tighter the draw on the set screw  404 , the greater the radial outward force FRO and the greater the deflection of the wall segment  408 , further decreasing the overlap at the interfaces  422 . Accordingly, as the torque requirements of the conventional set screw  404  are increased, the coupling between the set screw  404  and the wall segment  408  becomes more tenuous. Over time, creep stresses may cause the deflection of the wall segment  408  (splaying) and the attendant decrease in the overlap at the interfaces  422 , causing the clamping force FC to reduce. This can cause loosening of the assembly and slippage of the resident spinal rod within the spinal rod receptacle  410 . In some instances, torque requirements can cause the set screw  404  to slip within the spinal rod receptacle  410  during implantation. 
     A remedy for the splaying and attendant slippage of the spinal rod is a threaded arrangement utilizing threads having the canted cantilever profile arrangement  450 , as depicted at  FIGS.  34  and  34 A . The various threads  94 ,  96 ,  126 ,  154 ,  156 , and  270  ( FIGS.  2 ,  4 , and  5   ) of the extensible spinal support system  20  may utilize a canted cantilever profile arrangement. The canted cantilever profile arrangement  450 , schematically depicted at  FIGS.  34  and  34 A , may include, for example, exterior threads  452  of a set screw  454  that are engaged with interior threads  456  of a wall segment  458  of a spinal rod receptacle  460  (akin to opposed wall segments  90  and  150  of the rod receptacles  24  and  34  of the extensible spinal support system  20 ). Both the set screw  454  and the wall segment  458  are concentric about a central axis  462  that defines the z-axis of a right-cylindrical coordinate system  464  having an axial coordinate z and a radial coordinate r. When the set screw  454  is tightened a first direction  466  to set against a spinal support rod  465 , the clamping force vector FC is generated, for which there is the equal and opposite force vector FC′ in a second direction  468  that is opposite the first direction  466 . The force vector FC′ in turn generates reaction force vectors FR generated at contact interfaces  472  between the proximal faces of the exterior threads  452  of the set screw  454  and the distal faces of the interior threads  456  of the wall segment  458 . The reaction forces FR generate an axial component FRZ and radial component FRR. 
     However, unlike the conventional threaded arrangements  400 , the contact interfaces  472  of the canted cantilever profiles  450  are sloped radially inward (i.e., toward the central axis  462 ) in the first direction  466 . By this arrangement, the radial component FRR is vectored inward, toward the central axis  462 . The forces so generated will tend to cause deflections δi of the wall segment  458  that is radially inward in response to the radial inward force FRI. Because of the radial inward deflections δi, the wall segments  458  tend to be supported by the set screw  454 . Accordingly, the coupling between the set screw  454  and the spinal rod receptacle  460  provided by the canted cantilever profile arrangement  450  is stronger and can provide a greater clamping force FC than can the conventional threaded arrangement  400  of spinal rod receptacle  410 . 
     For the extensible spinal support system  20 , the interior threads  94 ,  154  of the base and extension rod receptacle  24 ,  34  interact with the set screws  42 ,  44  in the manner described attendant to the canted cantilever profile arrangement  450  of  FIGS.  34  and  34 A . The exterior threads  96 ,  156  of the base and extension rod receptacle  24 ,  34  may also implement a canted cantilever arrangement (see, e.g.,  FIGS.  2 ,  4 , and  5   ), but may be configured to generate different forces and deflections. For example, the exterior threads  96 ,  156  of the base and extension rod receptacle  24 ,  34  are sloped radially outward (i.e., away from the central axis  462 ) in the distal direction  74 . By this arrangement, the radial components of the reaction forces at the interface of the exterior threads  96 ,  156  and the interior threads  126 ,  226  of the skirt portions  124 ,  224  is vectored outward, away from the center axis  67 . The forces so generated will tend to cause the wall segments  90 ,  150  to deflect radially outward. Because of the radial outward deflections, the wall segments  90 ,  150  tend to be supported by the skirt portions  124 ,  224 . The skirt portions  124 ,  224 , being tangentially continuous, incurs a hoop stress that further counters the outward radial forces and limits splaying. The outward radial forces at the exterior threads  96 ,  156  also tend to counter and can be tailored to balance the inward radial forces FRI to further reduce overall radial deflection and deformation of the wall segments  90 ,  150 . 
     Alternatively, the exterior threads  96 ,  156  of the receptacles  24 ,  34  and the interior threads  126 ,  226  of the skirt portions  124 ,  224  may be of a conventional arrangement. Conventional threads, as described attendant to  FIGS.  33  and  33 A , provide radial outward forces that are subsequently supported by the skirt portions  124 ,  224  and as a counter to the radial inward force FRI, to prevent splaying. 
     Referring to  FIGS.  35  and  35 A , a partial view of the extensible spinal support system  20  having a reinforcement cap  28   d  and rod receptacle  24 ,  34  that define a reverse canted cantilever profile  480  is depicted according to an embodiment of the disclosure. The reinforcement cap  28   d  includes many of the same components and attributes as the reinforcement caps  28   a  and  28   b , as well as the reaction vectors of  FIGS.  34  and  34 A , all of which are indicated with same-numbered reference characters. A distinction of the  FIGS.  35  and  35 A  embodiment is that the exterior threads  96 ,  156  of the receptacles  24 ,  34  are sloped radially outward (i.e., away from the rotation axis  346 ) in the proximal direction  76  instead of the distal direction  74 , with the interior threads  336  of the skirt portion  332  of the reinforcement cap  28   d  being configured to mate therewith. 
     In  FIG.  35 A , the reaction force vector FR at the contact interface  472  and the components FRR and FRZ thereof are superimposed from  FIG.  34 A  at the exterior threads  344  of the set screw portion  342  and the interior threads  94 ,  154  of the base or extension rod receptacle  24 ,  34 . A reaction force vector FR′ at a contact interface  472 ′ and the components FRR′ and FRZ′ thereof are depicted at the interior threads  336  of the skirt portion  332  and the exterior threads  96 ,  156  of the base or extension rod receptacle  24 ,  34 . 
     Functionally, by this arrangement, not only do the reaction force vectors FR at the contact interface  472  generate the radial component FRR vectored inward (i.e., toward the center axis  466 ), but so do the reaction force vectors FR′ at the contact interfaces  472 ′. Accordingly, both the contact interfaces  472  and  472 ′ contribute to the radial inward force FRI that opposes the splaying that may result from the setting of the set screw portion  342 . 
     The depictions of  FIGS.  35  and  35 A  illustrate the reinforcement cap  28   d  in assembly on the base or extension rod receptacle  24 ,  34 . The same arrangement for the reverse canted cantilever profile  480  may be employed mutatis mutandis with the various base portions  32  in assembly with the rod receptacle  24 , which one of ordinary skill in the pedicle screw and tulip arts understands in view of this disclosure. 
     Each of the additional figures and methods disclosed herein can be used separately, or in conjunction with other features and methods, to provide improved devices and methods for making and using the same. Therefore, combinations of features and methods disclosed herein may not be necessary to practice the disclosure in its broadest sense and are instead disclosed merely to particularly describe representative and preferred embodiments. 
     Various modifications to the embodiments may be apparent to one of skill in the art upon reading this disclosure. For example, persons of ordinary skill in the relevant arts will recognize that the various features described for the different embodiments can be suitably combined, un-combined, and re-combined with other features, alone, or in different combinations. Likewise, the various features described above should all be regarded as example embodiments, rather than limitations to the scope or spirit of the disclosure. 
     Persons of ordinary skill in the relevant arts will recognize that various embodiments can comprise fewer features than illustrated in any individual embodiment described above. The embodiments described herein are not meant to be an exhaustive presentation of the ways in which the various features may be combined. Accordingly, the embodiments are not mutually exclusive combinations of features; rather, the claims can comprise a combination of different individual features selected from different individual embodiments, as understood by persons of ordinary skill in the art. 
     Any incorporation by reference of documents above is limited such that no subject matter is incorporated that is contrary to the explicit disclosure herein. Any incorporation by reference of documents above is further limited such that no claims included in the documents are incorporated by reference herein. Any incorporation by reference of documents above is yet further limited such that any definitions provided in the documents are not incorporated by reference herein unless expressly included herein. 
     Unless indicated otherwise, references to “embodiment(s)”, “disclosure”, “present disclosure”, “embodiment(s) of the disclosure”, “disclosed embodiment(s)”, and the like contained herein refer to the specification (text, including the claims, and figures) of this patent application that are not admitted prior art. 
     For purposes of interpreting the claims, it is expressly intended that the provisions of 35 U.S.C. 112(f) are not to be invoked unless the specific terms “means for” or “step for” are recited in the respective claim.