Abstract:
A spinal implant system for stabilization of the spine is disclosed comprising a pair of bone anchors, an elongate stabilization device received in the bone anchors, the stabilization device having an elongate inner stabilizing member and an outer stabilizing member disposed about the inner member and wherein said anchors are configured to inhibit translation of the outer member and to permit translation of the inner member.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The subject application is a utility application stemming from U.S. provisional application Ser. No. 61/043,880 filed Apr. 10, 2008 the disclosure of which is herein incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The spinal stabilization implant system disclosed herein is designed to provide a predetermined stabilization constraint to the natural spine within beneficial motion and flexibility limits. 
       BACKGROUND OF THE INVENTION 
       [0003]    A human spine comprises a number of joints often referred to motion segments. These segments exhibit kinematics characteristic of the entire spine. The motion segments are capable of flexion, extension, lateral bending and translation. The components of each motion segment are important for the stability of the joint and each unit include two adjacent vertebrae and their apophyseal joints, the intervertebral disc, and the connecting ligamentous tissue. 
         [0004]    Components of a motion segment that move out of position or become damaged can lead to serious pain and may lead to further injury to other components of the spine. Depending upon the severity of the structural changes that occur, treatment may include fusion, discectomy, and laminectomy. 
         [0005]    Underlying causes of structural changes in the motion segment unit leading to instability include trauma, degeneration, aging, disease, surgery, and the like. Thus, rigid stabilization of the motion segment unit may be the most important element of a surgical procedure in certain cases (i.e., injuries, deformities, tumors, etc.), whereas it is a complementary element in others (i.e., fusion performed due to degeneration). The purpose of rigid stabilization is the immobilization of a motion segment unit. 
         [0006]    The rigid design of systems common in the prior art typically cause stress concentrations and contribute to the degeneration of the joints above and below the fusion site. In addition, rigid, bar-like elements eliminate the function of the motion segment unit. 
         [0007]    Fusion procedures can be improved by modifying the load sharing characteristics of the treated spine. A need exists in the art for a soft spine stabilization system that replicates the physiologic response of a healthy motion segment. 
       SUMMARY OF THE INVENTION 
       [0008]    This disclosure encompasses stabilization systems for spinal motion segments. In particular, the present invention is directed to various embodiments of a soft stabilization system comprising a specialized elongated fixation member having an outer elongated member surrounding an inner elongated member. The system further comprises at least two specialized bone anchors designed typically in the form of pedicle screws to restrain the outer elongated member without compressing the inner elongated member thereby causing undesired wear of components. 
         [0009]    The system described herein has many benefits over earlier soft fixation systems. This system can easily span multiple vertebral levels since multiple pedicle screws can be attached to one elongated fixation member thereby providing multi-level soft stabilization even during a minimally invasive surgery. Competitive systems by their design do not allow multiple level soft fixation. The elongated member in this system can be contoured or bent anywhere along the rod whereas other soft stabilization systems have limited or no ability to create an even bend unless it is built into the system initially. There are no stress concentrations on the elongated fixation member since this member is a combination of continuous materials vs. the multiple components of rods in the prior art which are assembled and have combinations of stiff and elastic combinations along the rod. 
         [0010]    Other benefits include: consistent stiffness along the length of the elongated fixation member thereby providing flexibility in fixing screws anywhere along this member with no required distance between the screws. Also, various outer member sleeve sizes can accommodate to various sizes of yolks making it potentially compatible with many different pedicle screw systems. Further, the elongated fixation member can be inserted in a minimally invasive fashion—pericutaneously. All other systems have to be inserted into the yolk of a pedicle screw at specific points, usually under direct vision. Since the rod is made of the combination of the same materials continuously along its length, it can be blindly inserted into a yolk of a pedicle screw. Additionally the stiffness of this soft fixation system can be adjusted to the relative size, weight and functional demands of the patient by selecting different inner stabilization member materials and elastic outer stabilization member materials. 
         [0011]    Additional benefits include the system would be the only one that could be assembled intra-operatively based on testing of the patients relative flexibility or stiffness measured intra-operatively. The diameter of the elongated fixation member would not be needed to be changed to increase or decrease stiffness which currently is required of systems in the prior art. Stated otherwise, the prior art systems attempt to vary the size or length of elastic and rigid components to increase or decrease stiffness. The system disclosed herein is capable of easy exchange of components of various materials or the relative thicknesses of the inner rigid member and outer elastic components. The system can be pre-assembled by the manufacturer or assembled by the surgeon to meet specific physical demands of a patient or other surgical goals. A family of products that vary in both ability to bend in the saggital and coronal planes, as well as an ability to elongate with flexion and extension is contemplated. 
         [0012]    Finally, this dynamic rod concept has less risk of fatigue fracture due to the uniformity along the rod and lack of stress risers which have plagued other systems. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a partially exploded view of a preferred embodiment of the spinal implant system. 
           [0014]      FIG. 2  is a perspective view of the spinal implant system. 
           [0015]      FIG. 3  is a partially exploded view of a preferred embodiment of the elongated stabilization assembly. 
           [0016]      FIG. 3A  is an alternative perspective view of an outer elongated member in the form of a coiled spring. 
           [0017]      FIG. 3B  is an alternative perspective view of an inner elongated member. 
           [0018]      FIG. 3C  is an alternative perspective view of the lead tip. 
           [0019]      FIG. 4  is a perspective view of a preferred embodiment of the elongated stabilization assembly. 
           [0020]      FIG. 5  is a perspective view of an alternative outer elongated member having a ribbed outer surface. 
           [0021]      FIG. 6  is a section view of the ribbed alternative outer elongated member shown in  FIG. 5 . 
           [0022]      FIG. 7  is a detail view of the ribbed alternative outer elongated member shown in  FIG. 5 . 
           [0023]      FIG. 8  is a perspective view of a second alternative outer elongated member. 
           [0024]      FIG. 9  is a section view of the alternative outer elongated member shown in  FIG. 8 . 
           [0025]      FIG. 10  is a perspective view of a third alternative outer elongated member. 
           [0026]      FIG. 11  is a detail view of recesses in the surface and wall of the outer elongated member illustrated in  FIG. 10 . 
           [0027]      FIG. 12  is a section view of the third alternative outer elongated member shown in  FIG. 10 . 
           [0028]      FIGS. 13A &amp; 13B  is a perspective and detailed view of the outer elongated member in  FIG. 10  with the addition of a grooved outer surface. 
           [0029]      FIGS. 14A &amp; 14B  is a perspective and detailed view of an embodiment of an outer elongated member having a shaped or knurled outer surface portion. 
           [0030]      FIG. 15  is a perspective view of a polyaxial pedicle screw having a threaded cap and configured to restrain the elongated stabilization assemblies shown in  FIG. 4  and elsewhere. 
           [0031]      FIG. 16  is an exploded perspective view of a polyaxial pedicle screw having a threaded cap and configured to restrain the elongated stabilization assemblies shown in  FIG. 4  and elsewhere. 
           [0032]      FIG. 17A-D  illustrates various views of a threaded pedicle screw cap assembly configured to restrain the elongated stabilization assemblies shown in  FIG. 4  and elsewhere. 
           [0033]      FIG. 18A-C  illustrates various views of the upper cap portion of the threaded pedicle screw cap assembly illustrated in  FIG. 17 . 
           [0034]      FIG. 19A-E  illustrates various views of the upper saddle portion of the threaded pedicle screw cap assembly illustrated in  FIG. 17 . 
           [0035]      FIG. 20A-C  illustrates various views of the polyaxial pedicle screw portion illustrated  FIG. 16  and elsewhere. 
           [0036]      FIG. 21A-D  illustrates various views of the lower saddle portion of the polyaxial pedicle screw illustrated in  FIG. 16  and elsewhere. 
           [0037]      FIG. 22A-D  illustrates various views of the polyaxial pedicle screw yoke shown in  FIG. 16  and elsewhere. 
           [0038]      FIG. 23A-B  illustrates an exploded and assembled perspective view of a fixed pedicle screw with a lower saddle machined into the yoke. 
           [0039]      FIG. 24A-B  illustrates an exploded and assembled perspective view of a fixed pedicle screw comprising a removable lower saddle. 
           [0040]      FIG. 25A-C  illustrates various views of a lower saddle for a fixed pedicle screw configured to restrain an elongated stabilization assembly as disclosed herein. 
           [0041]      FIG. 26A-B  illustrates an exploded and assembled perspective view of a polyaxial pedicle screw utilizing a non-threaded insertion cap. 
           [0042]      FIG. 27A-B  illustrates an exploded and assembled perspective view of a non-threaded insertion cap configured to restrain the elongated stabilization assemblies shown in  FIG. 4  and elsewhere. 
           [0043]      FIG. 28A-D  illustrates various views of the upper cap portion of the non-threaded pedicle screw cap assembly illustrated in  FIG. 26 . 
           [0044]      FIG. 29A-C  illustrates various views of the polyaxial pedicle screw yoke shown in  FIG. 26 . 
       
    
    
     DETAILED DESCRIPTION 
       [0045]    This disclosure describes a spinal stabilization system comprising a specialized elongated fixation member and at least two specialized bone fasteners designed to restrain the elongated fixation member thereby softly stabilizing the associated spinal segments. The elongated fixation member comprises an outer elongated member surrounding an inner elongated member. The specialized bone fasteners/anchors restrain the outer elongated member without substantial compression on the inner member and without inhibiting translatory motion of the inner elongated member with respect to the outer member. 
         [0046]      FIG. 1  illustrates a preferred embodiment of the spinal stabilization system  100  disclosed herein in a partially exploded form. The system  100  comprises an elongated stabilization member  120  and at least two fixed or polyaxial bone anchors in the form of pedicle screws  140  configured to restrain the elongated stabilization member  120 . This embodiment of the fully assembled system illustrated in  FIG. 2  as would be implanted in a spine spanning 2 vertebral levels. The overall length of the stabilization member can be adjusted and more screws  140  can be utilized to span a single vertebral level or multiple levels. 
         [0047]    As seen in  FIGS. 3 and 4 , the elongated stabilization member  120  comprises an elastic elongated outer member  121  represented as a coiled spring in this embodiment. Member  120  also comprises an elongated inner member  130 , preferably in the form of a solid rod to control the bendability of the construct. The elastic outer member controls the torsion and elongation of the elongated stabilization member construct. 
         [0048]    Inner member  130  is preferably made from carbon fiber, PEEK or similar polymers, titanium, or titanium alloys, cobalt chrome, stainless steels, but may also be manufactured from other biocompatible materials. The inner member  130  comprises an inner member surface portion  137  which may have a low wear coating  138  to improve wear and decrease friction between the inner member surface portion  137  and the outer member  121  as the two members  121  and  130  move with respect to each other. It is preferred that the inner member  130  has a circular cross section, although not required, and is smooth across its surface to further ease movement of the outer member  121  across the inner member surface  137 . 
         [0049]    Similarly, the outer member also comprises an outer member surface portion  139 . Alternatively, surface portion  139  may have a low wear coating  138 . Depending on the materials chosen for each member  121 ,  130 , the surface portions may not require a low wear coating, have only one of the surfaces  137 ,  139  coated, or both surfaces may be coated. For example, the inner member  130  may be manufactured from cobalt chrome and coated in PEEK while the outer member  121  is manufactured from nitinol. Alternatively as example, the inner member  130  may be manufactured of PEEK and coated with titanium or cobalt chrome. 
         [0050]    The inside cannulation profile  122  of outer member  121  preferably matches the outside profile of the inner member  130  with adequate gapping between the surfaces  137   139  for smooth gliding movement therebetween. Although the inner member  130  embodiment shown in  FIG. 3  has a preferred circular cross section, it is recognized that the cross section could be oval or other non-circular shape provided the outer member  121  and screws  140  are adapted to accommodate the non-circular profile. 
         [0051]    The outer member  121  functions as a flexible elastic housing preferably in the form of a tube, a cannulated rod, or spring. As seen in the preferred embodiment in  FIG. 3A , the outer member  121  is in the form of a coiled spring wherein the round spring coils  123  form a circular cannulation through the center of the outer member  121 . The spring may comprise compression gaps  124  which will provide for a gradual increased spring resistance between the screws  140  as the spring undergoes compression due to spinal extension forces exerted by the screws  140 . Once the screws  140  move in relation to a predetermined amount of spinal extension, these compression gaps  124  will close to prevent further spinal extension. 
         [0052]    Similarly in spinal flexion, the screws  140  will move apart and the outer member  121  will become stiffer as the member  121  is extended past its neutral point. As the screws  140  approximate the lead tip  131  and the instrument tip  135 , the screws  140  and thus spinal flexion will eventually be stopped as the outer member  121  compresses against stops  132  and  136 . If the compression gaps  124  directly adjacent stops  132  and  136  are closed, the spine will be prevented from further flexion. Also limiting flexion is the portion of the spring situated between the screws  140 . During spinal flexion, this portion of the spring is pulled into spring extension and become stiffer thereby also assisting in limiting flexion motion. 
         [0053]    The above paragraphs describe the outer member  121  bias action for a stabilization system  100  applied to a spine in a neutral position. However, components of this system  100  have several means for creating a variety of affects. For example, if the system  100  is implanted in the neutral spine with the outer member  121  intermediate the screws  140  in slight compression, the system  100  may be used to open the gaps between the vertebral bodies and relieve compression and pain that may be exerted on nerves exiting the spinal canal. 
         [0054]    There are a multitude of other adjustments that can be made to the elongated stabilization member  120 . For example, material choices for the outer member  121  and for the inner member  130  will greatly influence the stiffness of the member  120 . As will be described later, the stabilization member  120  may be assembled according to the surgeon&#39;s specifications inside or outside the operating room. Therefore it is foreseen that the surgeon may make choices for an inner member  130  such as diameter, material stiffness, and overall length. Likewise the surgeon may also make choices for an outer member  121  such as coils/inch, material stiffness, coil inner/outer diameter, spring constant, inner/outer member length ratio, etc. The variety of choices for each of these variables will provide the skilled surgeon ample opportunity to adjust the elongated stabilization member  120 . The system  100  is therefore adaptable to a spectrum of patients of various sizes, shapes, weights, and spinal conditions. In this manner the system  100  may come in the form of a kit with a variety of parts to be assembled to the surgeon&#39;s preference. As such the lead tip  131  and/or the instrument tip  135  may be removable from the inner member  130  for mounting various outer members  121  therebetween. If only one tip  131   135  is removable, the other may be integral to the manufacture of the inner member  130 . Otherwise, the tips may be restrained to the inner member  130  by common connections such as machine threads, bayonet connection, welding, pinning, mohr&#39;s taper, press fit, chemical bonding, or other similar fastening mechanisms.  FIG. 3  illustrates the lead end of the inner member  130  having an inner member connection portion  125  in the form of threads in this embodiment. Complimenting this is a tip connection portion  126  also in the form of threads in this embodiment. 
         [0055]    For convenience sake, the elongated stabilization member  120  may come preassembled wherein the surgeon only has to choose a preassembled member  120  meeting his or her predetermined requirements. The elongated stabilization member  120  may also come pre-bent, as seen in  FIGS. 3 and 4  typically to match the natural curvature of the neutral spine. However the stabilization member  120  may be manufactured straight. In either case, the surgeon has the option of bending the stabilization member  120  with a bender specially designed for this purpose and further designed not to damage the outer member  121 . 
         [0056]    The instrument tip  135  comprises structure for connection to a rod inserter instrument. As seen in  FIG. 4 , it is preferable if the instrument tip  135  and the lead tip  131  are generally no larger than the diameter of the outer member  121 . This streamlined profile of the elongated stabilization member  120  is a particular benefit when used in a minimally invasive surgery as the member  120  can be passed down a tube through the tissues of the skin, fascia, and muscle and into the screws  140  for final fixation. Since the outer member  121  extends substantially the length of the inner member  130 , the stabilization member  120  typically does not require precise visual placement within the screws  140  which ultimately means less surgical incision is required. The instrument tip  135  preferably comprises an instrument connector portion  127 . In this embodiment, the instrument connector portion  127  comprises a face portion  128  and a mounting pin or recess  129 A for grasping by an inserter instrument. A contemplated inserter for this service comprises a complementary face on the instrument to mate with the face portion  128 , as well as a complimentary pin or recess to mate with pin or recess  129 A. Once the elongated inserter instrument is mounted to the complimentary structure, a sleeve is slid down the shaft of the instrument over the outer instrument tip body  129 B to securely hold the elongated stabilization member  120  to the instrument. 
         [0057]    The lead tip  131  has a nose  133  configured for entry through the soft tissues normally encountered in a spine surgery. A particular benefit of this spinal system is that it is configured for use minimally invasively if so desired wherein the nose  133  may be shaped to have a bullet shaped tip for easy movement through tissue. In addition, the pedicle screws  140  of this system may be fixed on infinite points of the outer member  121  thereby requiring far less invasive viewing for precise placement of the elongated stabilization member  120  compared to soft fixation systems of competitors. 
         [0058]    The lead tip  131  and the instrument tip  135  are configured as generally flat stops against the ends of the outer member  121 . Unlike that shown in  FIGS. 3 and 3A , the ends  110  of outer member  121  are preferably finished to be flatted to create a low wear interface between the ends  110  and the stops  132  and  136 . In addition, a low wear polymer washer or coating may be utilized. Although flattened ends  110  and stops  132  and  136  are preferred, it is apparent that other non-flattened interfaces will also work well as long as they provide for a low stress low wear interface. 
         [0059]    For increased torsion resistance, tips  131  and  135  may be modified to include restraining structure (i.e. clamping bands, set screws, pinning) to restrain one or more ends of the outer member  121  thereby minimizing rotational or torsional movement of the outer member  121  about the inner member  130 . 
         [0060]    Alternative embodiments of the outer member  121  are illustrated in  FIGS. 5-14 . The embodiments are preferably manufactured in the form of an elastomeric polymer such as a polyurethane or similar material. Certain biocompatible metals such as nitinol with elastomeric properties may also be appropriate. In each of these embodiments, the outer member  121  comprises an outer member surface portion  139 . The inside cannulation profile  122  of outer member  121  preferably matches the outside profile of the inner member  130  with adequate gapping between the adjacent surfaces  137  &amp;  139  for smooth gliding low wear movement therebetween. These outer member embodiments are absent the coiled structure illustrated in  FIG. 3A  as they tend to rely on the greater elastomeric properties of the material to provide similar functional benefits. 
         [0061]    The outer member  121  embodiment illustrated in  FIGS. 5-7  comprises an outer surface portion  200  configured for restraint by a screw  140 . In this embodiment, the outer surface portion  200  is configured with a restraint surface structure  201  in the form of radial ribs or grooves  210  complementing the screw  140  restraint structure to be described later. The restraint surface structure  201  provides a physical engagement structure, as opposed to a smooth level surface, for secure restraint by screws  140 . The ribs  210  are configured to a predetermined depth so to not significantly weaken the wall of the outer member  121 . 
         [0062]    The outer member  121  embodiment illustrated in  FIGS. 8 &amp; 9  comprises an outer surface portion  200  configured for restraint by a screw  140 . In this embodiment, the outer surface portion  200  is configured with restraint wall structure  202  complementing the screw  140  restraint structure to be described later. This restraint wall structure  202 , implemented here in the form of recesses  203 , provide a physical engagement structure, as opposed to a smooth level surface, for secure restraint by screws  140 . The recesses  203  are configured to a predetermined depth so to not significantly weaken the wall of the outer member  121 . In this embodiment the recesses  203  are in the form of a rectangle extending through the wall of outer member  121 . Alternatively, the recesses  202  may extend only partially through the outer member  121  to a predeterminded depth suitable for adequate restraint engagement by the screws  140 . 
         [0063]    A preferred implementation of the restraint wall structure  202  is illustrated in the embodiment of  FIG. 10-12 . In this embodiment the recesses  203  in the outer member  121  have the shape similar to the number 8 in a radial pattern about the surface of the outer member  121 . Unlike recesses  203  in  FIG. 8 , the recesses  203  in  FIG. 11  have radiused corners  204  thereby reducing stress concentrations at these points and reducing the likelihood of outer member  121  material failure. In addition, each row of the radial 8 shaped recesses  203  are offset thereby dispersing stress more evenly through the material. In addition, the number 8 profile is preferred over a simple oval profile since the 8 profile will better tolerate stresses due to extension of the outer member  121  as well as serving as bumper stops  205  if outer member undergoes compression. 
         [0064]    The outer member  121  embodiment illustrated in  FIGS. 13A and 13B  is similar to the embodiment in  FIG. 10  except that outer surface portion  200  also comprises restraint surface structure  201  implemented as a series of longitudinal ribs in this embodiment. This restraint surface structure  201  provides a physical engagement structure, as opposed to a smooth level surface, for secure restraint by screws  140 . The recesses  203  are configured to a predetermined depth so to not significantly weaken the wall of the outer member  121 . 
         [0065]    In yet another example,  FIGS. 14A &amp; 14B  illustrate an outer member  121  having a shaped or knurled outer surface portion  200 . The patterns may be varied. In this embodiment ribs or grooves  210 A are formed in a longitudinal pattern with crossing ribs or grooves  210 B formed in a radial pattern. This pattern creates a multitude of surface bosses  211  which together create a restraint surface structure  201  providing a physical engagement structure, as opposed to a smooth level surface, for secure restraint by screws  140 . Recesses  203 , such as those illustrated in  FIG. 11 , may be added if so desired for further restraint or to vary the overall stiffness or extendability of the outer member  121 . 
         [0066]    In a final example, an outer member  121  manufactured from a polymer may include an integral metallic spring member, preferably coiled, (not shown) molded within the polymer. This integral spring member may add beneficial spring characteristics that a polymer outer member  121  could not achieve alone. 
         [0067]    Now described in detail are several embodiments of fixed and variable angle pedicle screws illustrating modifications to make them suited to restrain the outer member  121  of the elongated stabilization member  120  thereby creating a functioning spinal stabilization system  100  as disclosed herein. 
         [0068]    In the preferred embodiment, a pedicle screw  140  of the threaded poly-axial variety is illustrated in  FIG. 15 . This screw comprises a locking cap assembly  310 , a poly-axial yoke  320 , a lower saddle  330 , and a poly-axial bone screw  340 . 
         [0069]    The locking cap assembly  310  illustrated in  FIGS. 17A-D  is a threaded embodiment. The assembly  310  comprises a drive member  311  (threaded in this embodiment) which when advanced drives the upper saddle  312  and lower saddle  330  together thereby restraining the outer member  121  while also driving the lower saddle  330  down to pinch and thereby lock the poly-axial bone screw  340  in a predetermined position with respect to the yoke  320 . A restrainer  313  prevents separation of the lower saddle  330  from the drive member  311 . 
         [0070]    The drive member  311  further illustrated in  FIG. 18A-C  comprises a thread portion  315 , a driving surface  314  for driving against the upper saddle  312 , an aperture  316  for receiving the restrainer  313 , and a drive recess  317  for advancing the drive member  311  utilizing an appropriate driver tool. The locking cap assembly  310  preferably includes a cap stop  318  shown here in the form of a rim on the cap to provide tactile feedback to the user to indicate the cap is fully advanced into the yoke  320 . 
         [0071]    The upper saddle  312  of this embodiment is further illustrated in  FIGS. 19  A-D. This component comprises an advancement face  402  driven down by the driving surface  314  when the drive member  311  is advanced. An aperture  316  is provided for receiving a portion of the restrainer  313  to keep the upper saddle  312  tethered to the drive member  311 . The upper saddle  312  comprises a broad outer member restraint surface  321  intended to mate with outer surface portion  200  of the outer member  121  thereby preventing motion and accompanying wear from occurring therebetween. The perimeter of the saddle  312  is shaped to fit down the center of the yoke. 
         [0072]    The upper saddle  312  further comprises saddle drive surfaces  322 . These surfaces  322  will mate against opposing drive surfaces  322  on the lower saddle  330  to continue the transmission of compression forces when the drive member  311  is advanced to create screw  340  locking. These surfaces  322  also define the spacing between the upper saddle  312  and lower saddle  330  to create a predefined diameter outer member aperture  323  assuring the outer member  121  is restrained but doesn&#39;t overly compress against the inner member  130  causing undesired wear debris therebetween. Therefore, relatively even stress distribution about the outer member  121  is important for long term performance of this system  100 . Pedicle screw designs which impart point contact on the outer sleeve are less desirable. 
         [0073]    Again, the outer member restraint surface  321  is configured to mate with the outer surface portion  200  of the outer member  121  as described above. In this embodiment of  FIG. 19D , the restraint surface  321  is configured with a helical groove  325  of geometry similar to the coiled outer member  121  illustrated in  FIG. 3A . Further, coatings may be used between these surfaces to prevent undesired slippage therebetween. An anti-torsion element  324 , preferably in the form of one or more ridges, grooves, or bosses may be mated with complementary elements on the outer surface of the coiled outer member  121  for torsion prevention. As yet another example illustrated in  FIG. 19E , restraint surface  321  is configured with fixation elements  326  of predetermined dimension to carefully interlock with the ribs or grooves  210 A and  210 B of the outer member illustrated in  FIGS. 14A  &amp; B. 
         [0074]    The bone screw  340  shown in  FIG. 20  capable of poly-axial movement. This means that the shaft  360  of the screw  340  is capable of locking at multiple degrees of orientation with respect to the yoke  320 . Bone screws that are non-polyaxial or fixed, most commonly have a shaft that is integral to the yoke  320  as illustrated in  FIG. 23 . The poly-axial bone screw  340  show in  FIG. 20  comprises a spherical shaped head  361 . The head  361  sits in the seat of the yoke  362  and its spherical shape assures that it will maintain continuous contact between the lower saddle  330  and the yoke  320  regardless of the angle of the screw. At the top of the head is a drive recess  317  for receiving a drive instrument for advancing the screw  340  into the vertebrae. The screw  340  may or may not have a cannula  362  per the preference of the surgeon. Such a cannula is generally used to advance the screw down a guidewire for minimally invasive placement. Bone screw threads  366  hold the screw in the vertebral body. 
         [0075]      FIG. 21A-D  illustrates a preferred embodiment of the lower saddle  330  for accommodating a poly-axial screw. This saddle  330  comprises a screw head recess  370  configured to mate with the screw head  361  primarily to transmit compression forces from advancing the drive member  311  therein locking the head  361  in a predetermined orientation with the yoke  320 . The perimeter of the lower saddle  330  is sized to fit snug in the inner bore of the poly-axial yoke  320 . The lower saddle  330  also comprises drive surfaces  322  to which mate with those on the upper saddle for the functions explained previously. Similar to the upper saddle, an outer member restraint surface  321  is configured to mate with the outer surface portion  200  of the outer member  121 . In this embodiment it is configured with a helical groove  325  to carry the spring coils  123  of the outer member in  FIG. 3A , but as discussed earlier, it is best configured to cooperate with the outer member restraint surface  321 . A central aperture  371  provides access for instruments to advance the bone screw  140 . 
         [0076]    The yoke  320  is utilized to hold the primary components of the spinal stabilization system  100  together. Illustrated in  FIG. 22A-D  is an example of one embodiment of a yoke  320  suited for a poly-axial screw  340  as described in  20 A and a threaded style locking cap assembly  310  as described in  17 A. The poly-axial style yoke comprises a seat  362  for seating of the screw head  361 , an inner chamber  363  for the head  361  to reside, internal or external threads or grooves  364  for advancement of the locking cap assembly  310 , and an elongate member canal  365  configured to receive the elongated stabilization member  120 . Yoke stop  367  interferes with cap stop  318  when the drive member  311  is fully deployed to the predetermined position. 
         [0077]      FIG. 23A  and  FIG. 23B  illustrate an example of a threaded fixed pedicle screw  140  configured for this spinal stabilization system  100 . Fixed screws are known to be more reliable than poly-axial screws since the shaft  360  is typically machined integral to the yoke  320  eliminating any chance for slippage between the yoke  320  and screw head  361 . This embodiment utilizes the same locking cap assembly  310  illustrated previously in  FIG. 17A . A differentiator for this embodiment is the outer member restraint surface  321  is machined integral to the floor of the yoke  320  with a helical groove  325  of geometry similar to the coiled outer member  121  illustrated in  FIG. 3A . This integral restraint surface  321  eliminates the need for a lower saddle  330 . However, manufacturing difficulties may warrant a fixed screw having a separate lower saddle  330  as illustrated in  FIGS. 24A and 24B . 
         [0078]    The lower saddle  330  in  24 A is further illustrated in  FIGS. 25A-C . This lower saddle  330  shares many of the same features of the saddle illustrated in  FIGS. 21A-D . However, the saddle  330  in  FIGS. 25A-C  is configured for a fixed screw wherein the shaft  360  is integrated to the yoke  320 . There is no screw head  361  for the yoke  320  to seat, therefore this saddle  330  is absent a screw head recess  370 . The saddle base  372  in  FIG. 25A  rests on the floor  373  of the inner chamber  363 . The saddle base  372  or perimeter wall  375  of the  FIGS. 21 and 25  may further comprise an anti-torsion element  374  in the form of a notch, ridge, boss, recess or other form to cooperate with a complementary element  374  on the floor  373  or inner chamber  363  side wall to prevent the saddle  330  from unintentionally falling out of the yoke  320  and for prevention of rotation between lower saddle  330  and yoke  320 . The yoke  320  of the fixed variety also comprises a drive recess  317  to drive the implant into bone. 
         [0079]    As an alternative embodiment to pedicle screws  140  described above, a poly-axial screw  140  with locking cap assembly  310  of the flanged variety may be implemented as illustrated in  FIGS. 26A-B ,  27 A-B, and  28 A-D. The upper saddle  312  and restrainer  313  in this embodiment mirror those described earlier. The drive member  311  comprises one or more flanges  400  that is substantially flattened and configured to reside in the groove  364  formed in the yoke  320  wall. The driving surfaces  314  formed on the bottom side of the drive member  311  are sloped and cooperate with the advancement face  402  on upper saddle  312  to advance saddle  312  toward the outer member  121  therein locking the construct. Alternatively, the flanges  400  could be inclined, much like a single thread, provided the groove  364  formed in the yoke  320  is correspondingly inclined. In such a configuration, inclined driving surfaces  314  that are sloped may be unnecessary. 
         [0080]    A yoke  320  of the poly-axial variety, configured to operate with the cap described in  FIGS. 28A-D  is illustrated in  FIGS. 29A-C . This yoke  320  shares common features of the yoke illustrated in  FIG. 22A-D  with the exception that the recess in the wall is a groove  364  as opposed to a thread. The screw and thread arrangement could be reversed such that the groove resides on the cap and the flange resides on the yoke. 
         [0081]    The pedicle screws  140  described here are only a few examples of screws  140  that could be utilized with this stabilization system  100 . Clearly, pedicle screws of other varieties such as those that are side loading, lock through sliding an inner member over an outer member, utilize snap in caps, have caps engaging the outside of the yoke, and other functional designs, could easily implement similar features described herein to cooperate with specialized elongated stabilization member  120  to produce similar results. 
         [0082]    Although the apparatus disclosed herein has been described with respect to preferred embodiments, it is apparent that modifications and changes can be made thereto without departing from the spirit and scope of the invention as defined by the claims.