Patent Publication Number: US-9402652-B1

Title: Spinal fusion system for osteoporotic vertebrae

Description:
RELATED APPLICATIONS 
     This patent application is a divisional of U.S. application Ser. No. 13/671,287, filed Nov. 7, 2012 which claims the benefit of U.S. Provisional Application No. 61/594,838, filed on Feb. 3, 2012. 
    
    
     TECHNICAL FIELD 
     This invention relates to spinal appliances and more specifically to a spinal fusion system which is especially applicable to osteoporotic bone. 
     BACKGROUND 
     Spinal surgery often requires different appliances to be attached to the spine in order to achieve a certain surgical goal. For example, in the area of spinal fusion, the spinal disc is removed between adjacent vertebrae and an appliance is attached which distracts and compresses adjacent vertebrae together. The resulting bone-on-bone contact of adjacent vertebrae results in the vertebrae fusing together, provided that the adjacent vertebrae are held in relative stationary contact over time. A variety of appliances have been devised which attach to the spinal vertebrae to keep them stationary so that fusion can occur. 
     One type of appliance relates to a system of pedicle screws comprising screws having head segments wherein fusion rods are inserted into the head segments, Exemplary of this type of system are those manufactured by Medtronic, Inc., Synthes Inc., and Depuy Inc. A number of pedicle screws are installed on the spine, appropriate to the number of vertebrae desiring to be fused. The head segments pivot on the screw portion and are aligned so that their open sides are parallel with the vertical axis of the spine and a fusion rod of appropriate length is placed in the heads of adjacent pedicle screws, the rod subsequently being locked down inside the head segment with lock nuts. 
     The use of pedicle screws to anchor a spinal appliance works well with healthy, young bone; however where the bone has become osteoporotic, as in the case with older patients, the screws often become loose or back out. One solution to this problem has been to attach the fusion rods to the spine using nylon sub-laminar straps. This procedure is described in O&#39;Brien et al., “ Nylon Sublaminar Straps in Segmental Instrumentation for Spinal Disorders ,” Clinical Orthopedics and Related Research, Number 23 Feb. 1986 pg. 168. The nylon sub-laminar straps are non-invasive, and can be wrapped around the vertebrae. However, these sub-laminar straps do not attach the fusion rods in a locked-down fashion, and therefore, do not lock the vertebrae in a sufficiently stationary manner. Therefore, the vertebrae cannot be sufficiently distracted and compressed together for proper fusion using these types of sub-laminar straps. 
     United States Patent Application No. 20090105761 (Robie) represents a system for anchoring adjacent spinal vertebrae without using a screw system. This invention uses a type of strap that is anchored by staples to an inferior vertebral body wherein the strap is further threaded through the spinous process of the superior vertebral body thus tying together two adjacent vertebrae. This patent application does not address the issue of its performance in osteoporotic bone. The dependence of this system on staples may be a drawback, as stapling to a foundation of osteoporotic bone may not result in a long-term anchoring of the strap if the staple eventually works free. 
     The foregoing reflects the state of the art of which the inventor is aware, and is tendered with a view toward discharging the inventor&#39;s acknowledged duty of candor, which may be pertinent to the patentability of the present invention. It is respectfully stipulated, however, that the foregoing discussion does not teach or render obvious, singly or when considered in combination, the inventor&#39;s claimed invention. 
     SUMMARY OF THE INVENTION 
     The invention is a spinal fusion system for application especially to osteoporotic bone, although its application is not limited as such. The system relies on attaching at least two rod anchors on respective adjacent vertebrae. The rod anchors have head segments adapted for receiving a fusion rod and a base that sits tightly against the vertebral surface. The head segment and base could be one solid piece or could be attached to each other in a rotatable fashion. The base could contact the vertebral surface at a plurality of contact points, such as a base having a plurality of individual legs. The base could also contact the vertebral surface over a larger surface area, such as with a relatively planar, malleable base which would be appropriately shaped to accommodate complex vertebral anatomy. 
     The base is held tightly against the vertebral surface through the use of flexible threadable segments. The threadable segments can be made of any flexible material that is nonreactive inside the body. Examples of materials which could comprise the threadable segments could be flexible nonreactive materials such: plastics, nylon, carbon fiber, or metal. Exemplary embodiments illustrated herein employ Mersilene® tape or Dyneema® thread as threadable segments which accommodate the requirements of the invention. In addition a “zip-tie” or sublaminar strap system could operate as a threadable segment, provided that these could be tensioned sufficiently to render the rod anchors stationary against the vertebrae. 
     Once the rod anchors are tied down and the flexible segment is tensioned, the fusion rod can be inserted into the head segment and locked down to distract and compress the adjacent vertebrae so fusion can occur. This system does not rely on screws or staples which require invading the bone surface to obtain a secure hold. Instead, this system&#39;s reliance on flexible threadable segments allows the system to be attached to the exterior of the bone which is believed to result in a more reliable and longer lasting fusion system for patients suffering from osteoporosis. 
     Because this device is capable of attaching to vertebrae rigidly at first and then accepts the fusion rod, it possesses all the capabilities of pedicle screws without their downfall; which downfall is a high failure rate in osteoporotic bone, Pedicle screws rely on cancellous bone in the core of vertebrae for their grip, which is significantly affected in osteoporosis. This invention relies on the outer cortical bone for its grip which is affected a lot less in osteoporosis. 
     Accordingly, the following objects and advantages of the invention apply: 
     It is an object of this invention to provide a reliable system for attaching a spinal fusion appliance to osteoporotic bone. 
     It is another object of this invention to provide a spinal fusion appliance which does not rely on screws, staples, hooks, claws or other bone-invasive means to attach the appliance to the spine in a rigid, stable fashion. 
     It is still another object of this invention to provide a spinal fusion appliance which does not experience the failure rate of bone anchoring systems (in osteoporotic bone) that depend on screws, staples, hooks, claws or other bone-invasive anchoring means. 
     Further objects and advantages of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing the preferred embodiments of the invention, without placing limitations thereon. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be more fully understood by reference to the following drawings which are for illustrative purposes only: 
         FIG. 1  is a perspective view of a first embodiment rod anchor of the inventive spinal fusion system. 
         FIG. 2  is a plan view of the first embodiment of the rod anchor of the inventive spinal fusion system, showing vertical channels extending through the base of the rod anchor for threading Mersilene® tape. 
         FIG. 3  is a front view of the rod anchor of  FIG. 1  shown with Mersilene® tape suture material threaded through the vertical channels in the base of the anchor. 
         FIG. 4  is a front view of the inventive spinal fusion system employing the rod anchor of  FIG. 1  secured to a patient&#39;s vertebrae; this view showing the rod secured with a lock-nut. 
         FIG. 5  is a perspective view of a second embodiment rod anchor of the inventive spinal fusion system. 
         FIG. 6  is a side view of the second embodiment rod anchor of  FIG. 5 , illustrating a channel opening for threading Mersilene® tape suture material. 
         FIG. 7  is a front view of the rod anchor of  FIG. 5  shown with Mersilene® tape suture material threaded through the channel opening. 
         FIG. 8  is a front view of the inventive spinal fusion system employing the rod anchor of  FIG. 5  secured to a patient&#39;s vertebra. 
         FIG. 9  is a plan view of a section of the spine showing the inventive spinal fusion system attached to the individual vertebra with Mersilene® tape. 
         FIG. 10  is a close-up side perspective view of a section of the spine showing the inventive spinal fusion system attached to individual vertebra with Mersilene® tape. 
         FIG. 11  is a cross-section of lumbar vertebra showing the rod anchors attached to the vertebra with Mersilene® tape. 
         FIG. 12  is a bottom perspective view of a third embodiment of the rod anchor showing a plurality of anchor spikes located on the planar bottom surface of the base of the rod anchor. 
         FIG. 13  is a front view of a fourth embodiment of the rod anchor having a cavity in the base of the rod anchor for inserting a compression member. 
         FIG. 14  is a partially exploded front view of the rod anchor introduced in  FIG. 13  being positioned on the vertebra and showing the compression member, fusion rod and locknut. 
         FIG. 15  is a front view of the assembled system incorporating the fourth embodiment of the rod anchor shown positioned on the vertebra. 
         FIG. 16  is a partially exploded front view of a fifth embodiment of the rod anchor; this embodiment having a threaded cavity in its base and a threaded cavity lock nut. 
         FIG. 17  is a front view of the assembled system incorporating the fifth embodiment of the rod anchor introduced in  FIG. 16 . 
         FIG. 18  is a front view of a sixth embodiment of the rod anchor, this embodiment employing a threaded cap. 
         FIG. 19  is a bottom perspective view of the cap introduced in  FIG. 18 . 
         FIG. 20  is a top perspective view of the cap introduced in  FIG. 18 . 
         FIG. 21  is a front view of a seventh embodiment of the rod anchor shown with a base having offset vertical channels for threading Mersilene® tape. 
         FIG. 22  is a front view of an eighth embodiment of the rod anchor shown with a base having vertical channels and lock-down screws threaded into the base and intruding into the channels to secure the Mersilene® tape threaded into the channels. 
         FIG. 23  is a front view of a ninth embodiment of the rod anchor shown with the base having channels which end in a crimping edge for securing the Mersilene® tape threaded into the channels. 
         FIG. 24  is a close-up view of the crimping edge introduced in  FIG. 23 . 
         FIG. 25  is a front view of a base embodiment wherein a plurality of through-channels are arranged in the base to allow the Mersilene® tape to be threaded and tied off as shown. 
         FIG. 26  is a side view of a tenth embodiment of the rod anchor, this embodiment presenting a base that is malleable. 
         FIG. 27  is an elevated perspective view of the rod anchor embodiment introduced in  FIG. 26  incorporated into a spinal fusion system and mounted to a section of lumbar vertebrae. 
         FIG. 28  is a perspective view of an eleventh embodiment of the rod anchor, this embodiment comprising a male coupling member that locks down a threadable segment and couples to a head segment. 
         FIG. 29  is a close-up cutaway view of the male coupling member of the eleventh embodiment. 
         FIG. 30A  is a perspective view of the eleventh embodiment showing the head segment detached from the male coupling member. 
         FIG. 30B  is a perspective view of the eleventh embodiment showing the head segment attached to the male coupling member. 
         FIG. 31  is a perspective view of the eleventh embodiment shown with a side-loading head segment. 
         FIG. 32  is a perspective view of the base and male coupling member of the eleventh embodiment shown with a threadable segment attached. 
         FIG. 33A  is a side elevated perspective view of the rod anchor of the eleventh embodiment mounted upon the lamina of a vertebra, having the threadable segment passing through a channel of the male coupling member, the threadable segment being secured at its end with knots. 
         FIG. 33B  is a side elevated perspective view of the rod anchor of the eleventh embodiment mounted upon the lamina of a vertebra, having the threadable segment passing through a third through-hole located in the base. 
         FIG. 34  is a plan view of rod anchors of the eleventh embodiment mounted on a section of vertebra as part of a fusion system including fusion rods loaded into top-loading head segments. 
         FIG. 35  is a bottom view of the base of the eleventh embodiment, showing a textured bottom surface. 
         FIG. 36  is a perspective view of a twelfth embodiment of the rod anchor, this embodiment employing a lockdown member mounted on a planar base that is separated from the head segment. 
         FIG. 37  is a perspective view of the twelfth embodiment mounted on a lamina of a vertebra. 
         FIG. 38  is a side view of a thirteenth embodiment of the rod anchor. This embodiment employs a plurality of legs which attach to the head segment, the legs distributing force through multiple points of contact upon the vertebral surface. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In one exemplary prior art system, fusion rods were anchored with pedicle screws, wherein the pedicle screws had a top head segment adapted for receiving a fusion rod and a lower screw portion pivotally attached to the head segment. The head segment of a pedicle screw had u-shaped aligned entry and exit openings imparted through the sidewalls of the head segment to allow a fusion rod to pass through and be secured within the head segment, using a threaded lock nut. The u-shaped openings provided a rounded bottom for resting the circular fusion rod therein. The head segment of the reference prior art system has a hollow vertical core surrounded by sidewalls, the sidewalls having an interior threaded surface for engaging with the threaded lock nut. The screw portion depending downward from the head segment is driven into the spine of a patient to secure the pedicle screw. 
     By way of exemplary embodiments, the inventive spinal fusion system for osteoporotic vertebrae allows the securement of rod anchors to a patient&#39;s spine without using screws or staples. The head segments may be of a top loading or side loading variety as will be further shown herein. Referring to  FIG. 1 , a first embodiment of the rod anchor  12  shows that it employs the head segment  14  (in this case a top loading head segment) of the prior art in that it has u-shaped openings  16 ,  18  for inserting a fusion rod  20 . Referring to  FIG. 4  and a second embodiment shown in  FIG. 9 , the fusion rod  20  connects adjacent rod anchors  12  together when they are placed on a patient&#39;s spine. The fusion rod  20  is secured with a lock nut  22  and the rod anchor  12  is secured to the spine with Mersilene® tape to form the spinal fusing system  10 . The interior sidewall of the rod anchor  12  is threaded  24  to receive the lock nut  22 . As further seen in  FIG. 2 , the head segment  14  of the rod anchor  12  is pivotally attached (see arrows indicating a pivoting motion in  FIG. 1 ) to a base  26  having a planar bottom surface  28 , the base  28  of this first embodiment having two vertical channels  30 , each having an entry and exit for threading a threadable segment, such as Mersilene® tape  32  and tying the rod anchor  12  securely to the spine. The threadable segment  32 , can be a segment cut from a roll of flexible material, such as a roll of Mersilene® tape, or the threadable segment may be a stand-alone flexible appliance that is purpose built for threading into the base  26  and locking down the rod anchor  12  securely. Once the rod anchor is secured, the pivoting head segment  14  can be aligned in the proper direction for attaching the fusion rod  20  as will be further described herein. Referring also to  FIG. 3  the Mersilene® tape  32  is shown being threaded through the channels  30  which extend through the bottom of the base. The Mersilene® tape  32  is then tied off so that the knot extends between the two channels  30  shown in  FIG. 2  on the inside of the head segment  14 . Once, the Mersilene® tape  32  is knotted and secured to a vertebra  34 , a fusion rod  20  is inserted in U-shaped openings  16 ,  18  of the head segment  14 , on top of the knotted Mersilene® tape  32 , the rod  20  being secured with a threaded lock nut  22  which engages with threads  24  as shown in  FIG. 4 . 
     Referring now to  FIGS. 5-11 , the second embodiment of the rod anchor  12  is shown. In this embodiment, the head segment  14  of the rod anchor  12  is positioned atop base  26  which has a flattened bottom surface  28 , wherein the base has a horizontal through-channel with opening  36  for inserting the Mersilene® tape  32 . In  FIG. 5 , the base  26  is shown with the head segment  14  pivoting (see arrows in  FIG. 5 ) in relation to the base  26 . In  FIG. 6-8 , it is shown that the base  26  includes a channel opening  36  in its side for threading Mersilene® tape  32  which secures the rod anchor  12  to a patient&#39;s vertebra  34 . In  FIG. 7 , the threading of the Mersilene® tape  32  through the base  26  is shown with a crimp ring  38 , which is used to tension and tie off the tape around the patient&#39;s vertebra  34 . In  FIG. 8 , the flat bottom surface  28  of the base  26  is positioned on a patient&#39;s vertebra  34  and Mersilene® tape  32  is passed through the channel openings  36  on each side of base  26  and tied off by threading the tape  32  through the crimp ring  38  and knotting it. 
     When the inventive spinal fusion system  10  is fully assembled upon a patient&#39;s vertebrae  34 , the assembly appears as that shown in  FIG. 9 ; this view employing rod anchors of the second embodiment as presented in  FIGS. 5-8 . A section of adjacent vertebrae is shown prepared for fusion by employing the instant invention. As shown, adjacent vertebrae have a rod anchor  12  attached with Mersilene® tape  32  and a fusion rod  20  is positioned in the rod anchors  12 . Each rod anchor has a lock nut  22  which locks the rod  20  immovably within the rod anchor  12 . When left so immobilized for a period of time by the inventive fusion system  10 , the section of spine to which it is applied fuses fully. 
       FIG. 10  is a close-up side view of the assembled system  10 . In this view the inventive spinal fusion system  10  is shown attached to a section of lumbar vertebrae  40 . The rod anchors  12  (as represented by the second embodiment shown in  FIGS. 5-9 ) are positioned on the lamina, which offers a relatively flattened surface for placing the flat bottom surface  28  of the base  26 . The Mersilene® tape  32  is looped through the rod anchor  12  in the manner previously described and then looped around the lamina and through the spinal foramen and tied off, thus securing the rod anchor  12  to the lumbar vertebrae  40 . In  FIG. 11 , this shows a cross section through a lumbar vertebra  40  showing the Mersilene® tape  32  looping around the lamina  44  and through the spinal foramen  46  to ultimately secure the rod anchors  12  to the lumbar vertebra. 
       FIG. 12  introduces a third embodiment of the rod anchor  12 . This embodiment has a plurality of anchor points  48  protruding from the planar bottom surface  28  of the base  26  of the rod anchor  12 . These anchor points  48  can take the form of a plurality of pointed spikes which “bite” into the surface of the vertebra, thus aiding in the attachment of the rod anchor  12  solidly to the vertebra. The Mersilene® tape is then threaded through the channel in the base  26  of the rod anchor  12  in the same ways as described previously herein. During surgery, the surgeon would set the base  26  on the vertebra  34 , thus making contact between the anchor points  48  and the spinal surface. The surgeon could then gently tap the top of the head segment  14  of the rod anchor  12 , thus lightly driving the anchor points  48  into the surface of the vertebra until the bottom  28  of the base  26  is flush with the vertebral surface. The rod anchor  12  could then be attached to the vertebra with Mersilene® tape as previously described and shown. 
       FIG. 13  illustrates a further fourth embodiment of the rod anchor  12  which has a modification to the base  26  that involves imparting a cavity  50  (shown in phantom) into the middle of the base  26 . The cavity  50  is preferably cylindrical and is intended to receive a cylindrical compression member  51  therein. The compression member  51  includes a top surface  53  which contacts the fusion rod. The compression member  51  is intended to compress the Mersilene® tape  32  resting on the bottom of the cavity  50  and hold it fast. 
       FIG. 14  shows the system  10  employing the rod anchor  12  introduced in  FIG. 13 , The compression member  51  is shown descending through the hollow vertical core  52  of the head segment  14  to eventually rest in the cavity  50 , which is substantially aligned with vertical core  52 . Here it is shown how the Mersilene® tape  32  is threaded through the openings  36  (not visible in this view) and interior channel  56  (shown in phantom) of the base  26 , the channel  56  opens into the cavity  50 . The Mersilene® tape  32  crosses the cavity  50  and when the compression member  51  is inserted into the cavity  50  it rests directly on the tape  32 . 
     In  FIG. 15 , the complete assembly of the invention is shown for the embodiment introduced in  FIG. 13 . The fusion rod  20  rests on the top surface  53  of the compression member  51 . A lock nut  22  exerts downward force upon the fusion rod  20  and compression member  51 . The downward force of the rod  20  on the top surface  53  applies the bottom surface  58  of the compression member  51  tightly against the Mersilene® tape  32  lying on the bottom of the cavity  50 . This downward applied force locks the Mersilene® tape  32  into an immovable position. Although it is not shown, the Mersilene® tape  32  would be tensioned with a mechanical tensioner prior to locking down the compression member  51  against the Mersilene® tape  32 . The lock nut  22  is threaded into the rod anchor  12  until it contacts the fusion rod  20  and applies downward pressure upon the fusion rod  20 . The fusion rod  20  is tightened downward until it rests tightly against the lowest point  60  of U-shaped openings  16 ,  18 . At this point, the fusion rod  20  applies massive downward pressure against the compression member  51 , which likewise exerts massive pressure on the bottom surface  58  of compression member  51 . Beneath bottom surface  58  is the Mersilene® tape  32  which is compressed and held fast between the bottom of cavity  50  and bottom surface  58 . This pressure on the Mersilene tape  32  is sufficient to render it immovable with the cavity  50   
     In  FIG. 16  an exploded view of the system employing a fifth embodiment of the rod anchor  12  is shown, this embodiment employing a central cavity  70  with threaded side walls; the cavity  70  extending into the base  26  and aligning with the hollow vertical core  52  of the head segment  14 . Referring also to  FIG. 17 , the threaded cavity  70  receives a secondary lock nut  72  which is threaded into the cavity  70  until it contacts the Mersilene® tape  32  lying on the bottom of cavity  70 . The Mersilene® tape  32  extends through horizontal channels  56  in base  26  and across the bottom of cavity  70 . By bottoming the secondary lock nut tightly against Mersilene® tape  32 , the tape can be held immovably. Therefore the Mersilene® tape  32  can be wrapped around the vertebra and then tightened with a tensioning tool and held fast in the cavity  70  by the secondary lock nut  72 . 
       FIG. 18  shows a sixth embodiment of the rod anchor  12  which employs a threaded cap  74  which engages with threads  76  on the exterior sidewalls of head segment  14 . The cap  74  is an alternative to a lock nut  22  which has been shown in the previous drawings.  FIGS. 19 and 20  show the cap  74 . Cap  74  has threads  78  along its interior side wall which engage with the threads  76  on the exterior sidewall of the head segment  14 . Cap  74  has a lower circumferential bottom edge  80 . As cap  74  is screwed down, bottom edge  80  contacts fusion rod  20  and tightens down against fusion rod, thus holding it in place, as shown in  FIG. 18 . Alternatively, instead of being screwed down, cap  74  could be press fit upon head segment by means well known in the art. 
       FIG. 21  shows a seventh embodiment of the rod anchor  12  with offset vertical channels  82  (shown in phantom) imparted laterally in the base  26 . As shown the Mersilene® tape  32  is threaded through the vertical channels  82  and can be tied off or crimped by means already described herein. 
       FIG. 22  shows an eighth embodiment of the rod anchor  12 , again with lateral vertical channels  84  (shown in phantom) imparted in the base  26 . Mersilene® tape  32  is threaded through the channels  84 . In this embodiment, however, lock screws  86  are threaded into the base  26  so that they protrudingly engage into the vertical channels  84  so as to be tightened against the Mersilene® tape  32 , thus holding the tape fast, as shown. 
       FIG. 23  shows a ninth embodiment of the rod anchor  12 , this embodiment employing channels  88  in the base  26  which taper outward toward the edge  90  of the base  26 , thus allowing the Mersilene® tape to be threaded and exit at the edges  90  of the base  26 . Where the channels  88  end, the metallic base material is made sufficiently malleable so that the edges  90  of the base  26  can be crimped together, thus immovably trapping the Mersilene® tape  32  in a tightened relation. With this embodiment, one side of the Mersilene® tape  32  can be crimped at one edge  90  while the Mersilene® tape  32  is pulled tight at the other edge  90  of the base  26  and crimped.  FIG. 24  shows a close-up of the crimping edge  90  of the base showing the thread  32  entering and exiting the channel  88  at the crimp edge  90 . 
       FIG. 25  illustrates another embodiment of the base  26  of the invention. The base  26  is constructed from a malleable material, such as a malleable metal which can be formed to fit a particular vertebral surface. This view also shows another method of tying off the base using the Mersilene® tape  32 . A single channel  94  occupies one side of the base  26  into which the tape  32  is threaded and the opposite side of the base included two channels  96   a ,  96   b  for threading the two separate ends of the tape  32  through, and tying the whole arrangement off. 
       FIG. 26  illustrates a tenth embodiment of the rod anchor  12  incorporating a malleable base  26 ; the malleable base  26  being formable to the irregular surface  98  of a vertebra  34 . By form-fitting to a vertebral surface  98 , this embodiment achieves better purchase upon the vertebra. The malleable base  26  retains the channel and channel opening  36  for threading the Mersilene® tape, as described in prior embodiments. The form fitting function is achieved by the surgeon using tools to form the base  26 , as needed, to better fit upon the various surface anomalies encountered on the surface  98  of the vertebra  34 . 
       FIG. 27  shows a plan view of the system  10  applying the rod anchor embodiment  12  introduced in  FIG. 26  and mounted on a vertebra  34 . The various shapings  100  of the malleable bases  26  as determined by the surgeon are readily apparent to best mate to the changing surfaces  98  of the vertebra  34 . 
       FIG. 28  shows an eleventh embodiment of the rod anchor  12  which employs a malleable base  26  and a separate head segment  14  which can be coupled upon the base  26  via a male coupling member  106  attached to the base  26 . The base represented here is planar, square in shape, and has two through-holes  108   a ,  108   b  near one side of the base  26  and a third through-hole  110  located at an opposite side of the base from the first two through-holes. While the base  26  shown here is square, the shape of the base can differ to meet the needs of the surgical application; also the arrangement of the through-holes may also vary. The base and remainder of this embodiment, is preferably made of titanium or any other solid, bio-compatible material. The base can be malleable in quality so it can easily be worked into a preferred shape by a surgeon using pliers and other tools know in the art. The male coupling member  106  protrudes upward from the top surface  112  of the base  26 , the coupling member  106  here shown as being centered on the base; however it can be arranged in an off-set location on the base, if desired.  FIG. 29  shows how male coupling member  106  has a threaded center  114  which engages a threaded lock screw  116 , here being represented by a hex screw. The lock screw  116  protrudingly engages into a channel  118  extending transversely through the body of male coupling member  106 , the channel  118  having two opposite openings  120 ,  122 . In channel  118  is threaded a threadable segment  32  comprised of Mersilene® tape or Dyneema® thread (an additional example of a threadable segment). The end  124  of the lock screw  116  protrudes into the channel  118  a length sufficient to compress the threadable segment  32  against the walls of the channel  118  and hold it fast. 
       FIGS. 30A and 30B  show a head segment  14  being coupled to the male coupling member  106  and base  26  of the eleventh embodiment. The head segment  14  has a cavity  126 , (female portion) into which is inserted the male coupling member  106 . The head segment  14  is then depressed down upon the male coupling member  106  until it snaps into place upon the male coupling member  106 . In one permutation, the “snap” engagement is accomplished by the male coupling member  106  engaging with an expandable ring. The male coupling member  106  comprises a wide shoulder section  128  relative to a narrowed circumference beneath the shoulder section. The head segment  14  has an expanding ring (not shown) seated within cavity  126 , which expands over the shoulder section  128  of the male coupling member  106  and then the ring compresses when pushed farther down to meet the narrowed circumference  130  of the male coupling member  106 . Once the ring is compressed over the narrowed circumference, the head segment  14  remains tightly coupled to the male coupling member  106 , but pivots, as already noted in prior embodiments. The advantage of presenting the eleventh embodiment with the head segment  14  detached is that the threading of the threadable segment through channel  118  of the male coupling member  106  is less obstructed with the head segment  14  detached. However, the eleventh embodiment could be presented with the head segment  14  already attached, prior to mounting on the lamina wherein the head segment  14  would seat high enough on male coupling member so as not to obstruct the channel  118 . The head segment  14  which accepts a top-loading fusion rod  20  has a vertical core  52  which allows the top of the lock screw  116  to be accessed. Alternately, a side-loading head segment  14  could be adapted to snap fit onto the male coupling member  106  as shown in  FIG. 31 . While other embodiments disclosed herein could also be modified to accept a side-loading head segment, the eleventh embodiment&#39;s snap fit feature allows a surgeon to select whether to use a top-loading or a side-loading head segment for a particular application. 
       FIG. 32  shows the base  26  and male coupling member  106  prepared for surgery, wherein the ends  132   a ,  132   b  of the threadable segment  32  are knotted to prevent them from slipping through the through-holes  108   a ,  108   b  as shown. A needle  134  is threaded along the threadable segment  32  which serves to guide the threadable segment  32  around the lamina  44  and through the channel  118  located in the male coupling member  106 .  FIG. 33 a    shows the needle having passed through channel  118  after completing the circuit around the lamina  44 , the lamina being shown in cutaway. Ends  132   a  and  132   b  of threadable segment  32  are knotted and held fast in holes  108   a ,  108   b . The threadable segment  32  is subjected to a tensioning device (not shown), and tensioned to a level of. Once the proper tension is reached, the lock screw  116  is tightened upon the threadable segment  32  located in the channel  118 , thus securing the eleventh embodiment immovably on the lamina  44 . Any excess portion of the threadable segment  32  is then cut and tied off. Alternately, the threadable segment can be passed through the additional through-hole  110  prior to threading through channel  118  and tightening down as shown in  FIG. 33   b.    
       FIG. 34  is a plan view of a fusion system  10  employing the eleventh embodiment mounted on a section of vertebra  34 , including fusion rods  20  loaded into top-loading head segments  14 . The various shapings  100  of the malleable bases  26  as determined by the surgeon are readily apparent to best mate to the changing surfaces  98  of the vertebra  34 . 
     To further aid in holding fast to a laminar surface,  FIG. 35  shows the bottom surface  136  of the base of the eleventh embodiment. The bottom surface preferably has a roughened texture  138  to enhance grip when placed upon the laminar surface. 
       FIG. 36  introduces yet a twelfth embodiment of the rod anchor  12 , wherein the base  26  is similarly planar as with the eleventh embodiment, yet the head segment  14  and coupling member  106  are separate from a lockdown member  140  which secures the threadable segment  32 . The base  26 , having a malleable quality, can be formed to the lamina  44  as already described. The separate lockdown member  140  has a channel  142  for passing the threadable segment  32  after threading it around the lamina  44 . A lock screw  144  of the lockdown member  140  protrudingly engages into the channel  142  for tightening the threadable segment  32  against the walls of the channel. A tensioning device (not shown) can be used to achieve the appropriate tension on the threadable segment to keep the rod anchor  12  immovably attached to the lamina  44 . Head segment  14  is adjacent to the lockdown member  140  and pivots upon the coupling member  106 . This embodiment has the advantage of not having to tie knots in through-holes as with the eleventh embodiment, as the threadable segment  32  is completely secured by the lockdown member  140 , as both ends of the threadable segment  32  thread through the channel  142  from opposite directions after being threaded around the lamina  44  as shown in  FIG. 37 . Also, the separate lockdown member  140  is unobstructed by the head segment  14  in this embodiment, which allows for unobstructed threading of the threadable segment  32 . 
       FIG. 38  represents a thirteenth embodiment of the rod anchor  12 , shown with a three-footed base  146 , wherein each foot includes a channel with a channel opening  1484  defining an entry or exit for securing each leg with a threadable segment. 
     Finally, although the description above contains much specificity, this should not be construed as limiting the scope of the invention, but as merely providing illustrations of some of the presently preferred embodiments of this invention. This invention may be altered and rearranged in numerous ways by one skilled in the art without departing from the coverage of any patent claims, which are supported by this specification.