Patent Publication Number: US-2022226023-A1

Title: Spinal tethering devices, systems, and methods

Description:
CLAIM OF PRIORITY 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 63/138,678, filed on Jan. 18, 2021, the benefit of priority of which is claimed hereby, and which is incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     The present document relates to surgical techniques and devices for use in correcting spinal deformities, such as scoliosis. More specifically, this document discusses various vertebral implants for use in securing a flexible elongate member (e.g., tether or cord) between vertebral bodies and techniques for correction of spinal deformities using the disclosed implants. 
     BACKGROUND 
     Dynamic stabilization techniques, such as vertebral body tethering, are used in spinal treatment procedures for juveniles to permit enhanced mobility of the spine while also providing sufficient counter loading of a spinal curvature to effect treatment through bone growth modulation, particularly during times of rapid growth. Such dynamic stabilization systems may include fixed, uniaxial, or polyaxial bone anchors installed in adjacent or nearby vertebrae of the spine, various cord clamping devices, and a flexible cord secured to the bone anchors, with the cord tensioned between the bone anchors. 
     Current techniques and implants suffer from various deficiencies such as screw plow (migration), cord/tether failure, and difficulties in implantation among other things. The following disclosure discusses various implants and surgical procedures to address these and other short comings with traditional approaches to spinal tethering. 
     SUMMARY 
     The present inventors have recognized, among other things, that improving various aspects of spinal tethering systems can involve improvements such as implant (e.g., bone screw and/or anchor) revisions to mitigate screw migration, cord securing mechanisms (e.g., cord clamping) to reduce stress on the cord or reduce implantation time, construct strengthening to reduce or eliminate construct failure, and construct changes to prevent overcorrection, among others. Details of various concepts are provided below. 
     This overview is intended to provide an overview of subject matter of the present patent application. It is not intended to provide an exclusive or exhaustive explanation of the invention. The detailed description is included to provide further information about the present patent application. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A-1L  are various views of a saddle insert cord washer and set screw combination in accordance with the present disclosure. 
         FIGS. 2A-2F  are various views of a cap-style set screw and cord recess head bone screw in accordance with the present disclosure. 
         FIGS. 3A-D  are various views of a spherical bottom set screw in accordance with the present disclosure. 
         FIGS. 4A-4D  are various views of cap-style set screw and bone screw combinations in accordance with the present disclosure. 
         FIGS. 5A-5F  are various views of a dynamic poly-axial head bone screw in accordance with the present disclosure. 
         FIGS. 6A-6C  are illustrations of a dynamic construct using cord slip tension sharing in accordance with the present disclosure. 
         FIGS. 7A-7C  are illustrations of a dynamic construct using head tilt tension sharing in accordance with the present disclosure. 
         FIGS. 8A-8J  are various views of bone anchor concepts in accordance with the present disclosure. 
         FIGS. 9A-9F  are various views of a threaded bone anchor in accordance with the present disclosure. 
         FIGS. 10A-10H  are various views of an offset single cord clamp in accordance with the present disclosure. 
         FIGS. 11A-11G  are various views of an offset single cord clamp with dual bone screws in accordance with the present disclosure. 
         FIGS. 12A-12D  are various views of an offset dual cord clamp in accordance with the present disclosure. 
         FIGS. 13A-13F  are various views of a symmetric offset dual cord clamp in accordance with the present disclosure. 
         FIGS. 14A-14F  are various views of a top loading modular cord clamp implant in accordance with the present disclosure. 
         FIGS. 15A-15F  are various views of a posted screw modular cord clamp implant in accordance with the present disclosure. 
         FIGS. 16A-16H  are various views of a top loading self-locking cord clamp implant in accordance with the present disclosure. 
         FIGS. 17A-17F  are various views of a cord clamping set screw in accordance with the present disclosure. 
         FIGS. 18A-18H  are various views of another cord clamping set screw in accordance with the present disclosure. 
         FIG. 19  is a flowchart illustrating a technique for using partially assembled implants in a spinal tethering procedure in accordance with the present disclosure. 
         FIG. 20  is a flowchart illustrating a technique for using fully assembled implants in a spinal tethering procedure in accordance with the present disclosure. 
         FIGS. 21A-21E  are various views of a ploy-axial screw and bone anchor assembly in accordance with the present disclosure. 
         FIGS. 22A-22E  are various views of an expanding bone anchor in accordance with the present disclosure. 
         FIGS. 23A-23G  are various views of an offset single cord clamp with dual bone screws in accordance with the present disclosure. 
     
    
    
     In the drawings, which are not necessarily drawn to scale, like numerals may describe similar components in different views. Like numerals having different letter suffixes may represent different instances of similar components. The drawings illustrate generally, by way of example, but not by way of limitation, various embodiments discussed in the present document. 
     DETAILED DESCRIPTION 
     Exemplary embodiments will now be described more fully with reference to the accompanying drawings. The present disclosure provides details on concepts for improving various aspects of a spinal tethering system, such as a spinal tethering system detailed U.S. Pat. No. 10,653,453 titled “DYNAMIC STABILIZATION SYSTEMS AND ASSOCIATED METHODS”, which is hereby incorporated by reference in its entirety. The improvements discussed also relate to aspects of the spinal tethering instruments discussed in U.S. Pat. No. 10,905,474 titled “SURGICAL CORD TENSIONING DEVICES, SYSTEMS, AND METHODS” and U.S. Pat. No. 10,939,941 titled “SURGICAL CORD TENSIONING DEVICES, SYSTEMS, AND METHODS”, which are both hereby incorporated by reference in their entirety. 
     The following description discusses various implants used in a spinal tethering system, such as bone screws and anchors (staples). Most spinal tethering systems utilize bone screw designs from traditional spinal fixation systems targeting similar portions of the spine, such as fixed head pedicle screws. These traditional bone screws were originally designed to couple solid spinal rods between vertebral bodies, and can induce unwanted stress on a flexible cord or tether when used in a spinal tethering system. Spinal fixation systems often include fixed, uni-axial, and multi-axial bone screws, where the screw head (e.g., tulip or saddle) is fixed or allowed to move (uni-axial or multi-axial) in reference to the shaft of the bone screw. Bone screws from spinal fixation systems were typically designed to receive spinal fusion rods rather than flexible elongate members, such as a cord or tether. The improvements outlined below are discussed in reference to these commonly available bone screw designs. The anchors (staples) referenced below involve an implant that is designed to surround at least a portion of a bone screw and engage surrounding bone surface to distribute loads from the bone screw over a greater area of cortical bone. Again, improvements outlined below to anchors are discussed in reference to these anchor or staple designs. 
     Bone screw and anchor designs discussed below can provide numerous benefits over traditional pedicle screws for use in spinal tethering. The benefits can include providing a stronger construct, reducing screw plow (e.g., movement/migration of the screw in the vertebral body over time), enhancing cord clamping, reducing cord breakage, reducing surgical procedure steps (eliminate multiple steps for separate anchor and screw placement), and providing for dual cords to increase strength and provide redundancy. 
     Other devices discussed below include cord clamping mechanisms that deviate from use of set screws and typical tulip head pedicle screws. The cord clamping mechanisms discussed below provide benefits such as reducing stress points on the flexible cord, increasing holding strength, and allowing for controlled distribution of tension across multiple implant levels, among others. Additional benefits of each different implant is discussed below in reference to the illustrations of the specific designs. 
       FIGS. 1A-1L  are various views of a saddle insert cord washer  120 ,  140 ,  150  and set screw  110  combination.  FIG. 1A  is an isometric view of an implant  100  including fixed tulip head (screw head)  130 , a large cord washer  120 , a set screw  110 , and a screw shaft  105  extending from the screw head  130 . The implant  100  is designed to enhance the strength of the grip on a cord extending through screw head  130  between the threaded sidewalls  132 . The cord washers, large cord washer  120 , medium cord washer  140  and small cord washer  150  (collectively referenced as cord washer  120 ,  140 ,  150  or simply cord washer  120 ), are designed to allow for dynamic cord slip at certain cord tension levels. Most of the embodiments are illustrated with the large cord washer  120 , but any of the other cord washers can be used within the disclosed embodiments. The cord tension where slip occurs will be above the tension possible with a tensioning instrument during implantation, but below the cord&#39;s working tension limit (and well below the breaking strength of the cord). In some examples, the cord tension where slip occurs will be above the tension possible with the tensioning instrument and below the level of tension where screw plow occurs for a particular bone screw design. Dynamic constructs using cord slip are discussed below in more detail in reference to  FIGS. 6A-6C . 
     In this example, the implant  100  includes a set screw  110 . The set screw  110  can include a washer interface  112  that is a donut-shaped flat surface designed to mate with a similar surface, the set screw recess  124 , on the superior side of the cord washer  120 . The large surface area allows for distribution of the force generated by tightening the set screw  110  to push the cord washer  120  down on the cord. The washer interface  112  further includes a washer peg  118  extending from the center of the donut-nut shaped flat surface. The washer peg  118  is designed to engage a set screw bore  128  in the center of the cord washer  120 . In some examples, the washer peg  118  includes in small ridge protruding from the distal end to generate a friction fit with the set screw bore  128  that keeps the cord washer  120  attached to set screw  110  during implantation. The set screw  110  also includes external threads  116  and a driver interface  114 . The threads  116  engage with the threaded sidewalls  132  of the screw head  130 . The driver interface  114  is designed to receive a torquing instrument to enable tightening the set screw  110  into screw head  130 . In this example, the driver interface  114  is a HEX shaped recess. In some examples, the implantation technique will include a torquing instrument with a torque limiter set to put a pre-defined amount of pressure on the cord, which results in cord slip within a defined range of cord tension. 
     In this example, the implant  100  includes a tulip-head style screw head  130  with two opposing threaded sidewalls  132 . The threaded sidewalls  132  include internal threads designed to receive the set screw  110 . The screw head  130  also includes various implant instrument interfaces, such as instrument interface recesses  134  and instrument interface grooves  136 . The instrument interface recesses  134  are oval shaped recesses on opposing sides of the screw head  130  within an outer surface of the threaded sidewalls  132 . The instrument interface recesses  134  can be used to attach access instrumentation to the sides of screw head  130 . The instrument interface grooves  136  are longitudinal grooves cut into each corner of the screw head  130 . The instrument interface grooves  136  can be used to attached access instrumentations or instruments to implant the screw (torquing instruments, such as a specialized screwdriver). 
       FIGS. 1D-1E  are cross-sectional views of the implant  100  with each of the different cord washers.  FIG. 1D  is a cross-sectional view of the implant  100  with the large cord washer  120 . The cross-sectional views include an illustration of the cord axis  160 , which is representative of the path the tether or cord will take through the implant  100 .  FIG. 1D  illustrates the set screw  110  fully engaged with the cord washer  120 , while  FIGS. 1E and 1F  illustrate the set screw  110  just prior to full engagement.  FIG. 1E  is a cross-sectional view of the implant  100  with a medium cord washer  140 , and  FIG. 1F  is a cross-sectional view of the implant  100  with a small cord washer  150 . 
       FIGS. 1J-1L  are isometric views of the various sizes of cord washer in accordance with the present disclosure.  FIG. 1J  is an isometric view of the large cord washer  120 . The large cord washer  120  includes cord grips  122  extending inferiorly from outer lower edges of the clamp extensions  129 . The large cord washer  120  also includes a set screw recess  124  and a set screw bore  128  to receive the set screw  110 . The large cord washer  120  includes an inferior surface designed to engage the cord when assembled into the implant  100 . In this example, the cord grips  122  extend more inferiorly than the majority of the inferior surface  126 , which creates additional friction on the cord to increase the level of tension prior to cord slip.  FIG. 1K  is an isometric view of the medium cord washer  140  that includes a similar structure as the other cord washers but does not include cord grips  122 . The other difference with the medium cord washer  140  is mid-sized clamp extensions  149 . The clamp extensions  149  increase the friction on the cord but create a lower cord slip tension as compared to the large cord washer  120 .  FIG. 1L  is an isometric view of the small cord washer  150 , which also has a similar structure to the other cord washers. The small cord washer  150  includes even smaller clamp extensions  159 , which generate yet lower level of slip tension. In practice, each of the cord washers will generate a pre-defined level of slip tension on the cord when the set screw is torqued to a pre-defined level during implantation. 
       FIGS. 2A-2F  are various views of an implant  200  including a cap-style set screw  230  and a cord recess within the head bone screw  210 . The cap-style set screw  230  provide additional cord interface or gripping locations in comparison to a standard set screw. For example, the cap-style screw illustrated includes a central cord engagement member  234  and an outer cylindrical body that both operate to engage (e.g., compress) the cord within the cord recess  224  of the screw head  220 . The implant  200  also includes features to enhance the strength of the implant, such as anchor  240  that can be implanted prior to implantation of bone screw  210 . 
     In this example, the bone screw  210  includes a saddle or tulip style screw head  220  with integrated threaded arms  222  designed to receive the cap screw  230 . The bone screw  210  includes screw threads  212  along the screw shaft. The screw threads  212  include cutting flutes  214  at the distal end of the screw shaft. The bone screw  210  can also include a fenestration  216  to enable implantation using K-wire guides. The screw head  220  includes threaded arms  222  separated by a cord recess  224 . On the exterior of the screw head  220  there are features to assist with implantation including instrument interface grooves  226  and instrument interface threads  228 . 
     The illustrated example implant  200  includes a cap screw  230  to secure a cord within the cord recess  224  of the bone screw  210 . The cap screw  230  includes cap threads  232  on the internal sidewalls of a cylindrical body  236 . The cap screw  230  also includes a central cord engagement member  234  extending inferiorly from the center inside the cylindrical body  236 . Opposite the central cord engagement member  234  is a driver interface  238  recessed into the superior side of the cap screw  230 . In this example, the central engagement member  234  includes a spherical tip, in other examples the tip could be a flat cylindrical surface or a spherical tip with a larger radius, among other shapes. The only limitation on the surface shape for the end of the central cord engagement member  234  is that it be symmetrical and allow for rotation of the cap screw  230  during tightening without adverse engagement with the cord. 
     Finally, implant  200  can include an anchor  240  to assist in preventing screw plow (migration) and strengthen the overall construct. In this example, the anchor  240  can include anchor spikes  242 , a screw passage  244 , a screw interface  246 , and an instrument groove  248 . The anchor  230  is illustrated with three anchor spikes  242  that each include a triangular shape body with a chisel tip. Other anchor spike configurations are envisioned, including two or four anchor spikes  242  as well as different body shapes. Different compatible anchor concepts are discussed below in reference to  FIGS. 8A-8J and 9A-9F . In this example, the anchor  240  includes a chaffered internal screw interface  246  that is designed to mate with an external lower surface of the screw head  220  as illustrated in cross-section in  FIG. 2F . In some examples, the screw interface  246  can be a concave radiused cylindrical ring that mates with a similar convex radiused surface on the inferior side of screw head  220  that allows for axial angulation of the bone screw  210  in reference to anchor  240  upon implantation. The instrument groove  248  provides a gripping location for an implantation instrument. 
       FIGS. 3A-D  are various views of a spherical bottom set screw  300 . The spherical bottom set screw  300  is designed distribute cord contact over a larger surface area and prevent points of failure while securing a cord within a bone screw housing. The spherical bottom can also allow for dynamic cord slip at tensions above a pre-defined value. The set screw  300  includes a driver interface  310 , threads  320 , and the spherical engagement surface  330 . The set screw  300  is designed for use within any saddle or tulip style screw head. 
       FIGS. 4A-4D  are various views of cap-style set screw and bone screw combinations that are alternatives to the design discussed above in reference to  FIGS. 2A-2F . These examples include two different bone screws, implant  400 A includes bone screw  410 A that has threaded arms  422  integrated into the bone screw head  420 A, while implant  400 B includes bone screw  410 B that utilizes an anchor  440 B that includes threaded anchor arms  441  to receive cap screw  450 . 
     In this example, implant  400 A includes a bone screw  410 A that is structured similarly to the bone screw  210  discussed above. The bone screw  410 A includes screw head  420 A that includes integrated threaded arms  422  with a cord recess  424  in between. The cord recess  424  is a smooth U-shaped space between the threaded arms  422 . In this example, the cord recess  424  includes a bottom portion that aligns with a cord recess  435  cut out of a body of anchor  430 . The screw head  420 A also includes a driver interface  426  recessed into the center distal of the cord recess  424 . The bone screw  410 A also can include a fenestration  428 . 
     Implant  400 A also includes anchor  430  designed to encircle a distal portion of screw head  420 A at the screw interface surface  446 . In this example, the anchor  430  is designed to be implanted after the bone screw  410 A, as the screw interface surface  446  includes a proximal angled surface that abuts a corresponding angled surface along the outer periphery of the base of the screw head  420 A where threaded arms  422  adjoin. In these examples, the anchor  430  includes a series of anchor teeth  442  distributed around the circumference of the inferior side of the anchor  430 . The anchor  430  also includes an instrument groove to assist in securing the anchor  430  in an implant instrument while the screw head  420 A is inserted through the screw passage  434  of the anchor  430 . The anchor  430  could be modified in accordance with other anchors discussed herein to include different anchor spikes or even accommodate dual bone screws. 
       FIGS. 4C and 4D  illustrate implant  400 B that includes an anchor  440  with threaded anchor arms  441  integrated into the anchor (instead of the screw head, as in implant  400 A). The screw head  420 B includes a cylindrical disc shaped head that mates with a screw interface surface  446  within the screw passage  444  of anchor  440 . In this example, the screw interface surface  446  is a circumferential lip extending from the inner sidewall of anchor  440  to capture screw head  420 B. The anchor  440  includes anchor teeth  442  that are similar to those described above in reference to anchor  430 . Also similar to anchor  430 , anchor  440  includes a cord recess  445 , but the cord recess  445  is formed in the space between threaded anchor arms  441  and includes a U-shaped notch in the cylindrical body of anchor  440 . Anchor  440  also includes an instrument groove  448  for use in gripping the anchor with an implant instrument. Anchor  440  is designed to be implanted ahead of bone screw  410 B or in conjunction with the bone screw  410 B. 
     Both implants  400 A and  400 B utilize a cap-style set screw, such as cap screw  450 . In these examples, the cap screw  450  includes a cylindrical body  456  with internal cap threads  452  designed to engage threaded arms  422  or threaded anchor arms  441 . The cap screw  450  grips a cord (or tether) via cylindrical engagement surface  454 . The cylindrical engagement surface  454  create two engagement points on a cord running through the implant. Implants  400 A and  400 B can also utilize a cap-style set screw similar to the one discussed above in reference to  FIGS. 2A-2E  that includes a central cord engagement member  234  in addition to the cylindrical engagement surface  454 . 
       FIGS. 5A-5F  are various views of an implant  500  in the form of a dynamic poly-axial headed bone screw  510 . In this example, the implant includes a bone screw  510  includes a set screw  520 , a cord washer  530 , a saddle body  540  and a friction insert  550 . The bone screw  510  includes a spherical screw head  512  connected to a screw shaft  516  via a neck  514 . The neck  514  has a reduced diameter in comparison to the spherical screw head  512 , which allows for greater angulation of the screw shaft  516  in relationship to the saddle body  540 . The bone screw  510  is coupled to the saddle body  540  via a friction insert  550 . The friction insert  550  includes a body interface disc  552  that sits in an insert groove  545  recessed into an internal cylindrical sidewall of the saddle body  540 . The friction insert  550  includes a series of head interface fingers  554  separated by compression gaps  558 . The head interface fingers  554  and compression gaps  558  work in conjunction with the internal surface of the saddle body  540  to generate a predictable friction fit on the spherical screw head  512  that enables dynamic cord tension sharing (as discussed further below in reference to  FIGS. 6A-7C ). 
     Implant  500  includes a set screw  520  with external threads  522 , and engagement extension  524  and a cylindrical mating surface  526 . The engagement extension  524  is designed to mate with the set screw bore  538  of the cord washer  530 . The cord washer  530  also includes a set screw recess  534  designed to receive the cylindrical mating surface  526  of the set screw  520 . Similar to other cord washers discussed herein, the cord washer  530  includes cord grips  532  on either outbound end to extend the surface area (e.g., inferior surface  536 ) of the cord washer  530  that interfaces with a cord running in the cord recess  544  between the threaded saddle arms  542  of the saddle body  540 . The set screw  520  with cord washer  530  attached is threaded into the saddle body  540  via the threaded saddle arms  542  after the friction insert  550  and bone screw  510  are assembled into the screw passage  546  of the saddle body  540 . The entire implant  500  can be implanted using the instrument interfaces  548 . In this example, the instrument interface  548  includes a vertical groove running from a superior surface of the threaded saddle arms  542  inferiorly to a circular recess. 
     When set screw  520  compresses cord washer  530  into saddle body  540 , the entire implant  500  becomes a portion of a dynamic construct. The fiction insert  550  and spherical screw head  512  generate an interface that moves upon application of sufficient tension on the cord captured by the cord washer  530 . The spherical screw head  512  will pivot upon application of a tension on the cord that exceeds the tension applicable by a tensioning instrument, but beyond the yield strength of the cord. 
       FIGS. 6A-6C  are illustrations of a dynamic construct  600  using cord slip tension sharing provided by implants, such as implant  200  discussed above. A concern with tether constructs used to correct adolescent scoliosis involves cord failure at the apex of the construct (or another stress point caused by growth or physical activity). It has been determined that tether constructs can develop stress points, typically at the apex of the construct as growth occurs. The present inventors determined that creating implants, such as implant  200 , that can allow for cord slippage at pre-determined tension levels can alleviate some forms of cord stress and avoid potential failures in a tether construct. 
     In an example, the dynamic sharing implants are designed to share tension loading at tension levels above the level attainable with a tensioning instrument and below the cord yield strength. Dynamic tension sharing systems will also take into consideration the tension level where screw plow is known to occur for a given implant design. Screw plow involves the bone screw portion of an implant migrating within a vertebral body in response to tension generated by the tether (cord). In an example tethering system, a tensioning instrument can generate approximately 500 Newtons (N) of tension between implants (e.g., between each vertebral body). In this example, screw plow may occur around 800N of tension and the cord yield strength may be as high as 3000N of tension. Implants within a tethering construction will also often induce stresses onto the cord that reduces the cord yield strength to as little as ⅓ the actual yield strength. In other words, the effective construct strength may be as little as 1000N of tension before cord failure is likely. Many of the implant designs outlined within this disclosure are designed to increase the overall construct strength by increasing the effective strength of the construct, in part by reducing stress levels induced on the cord. Designs such as the cord washers shown in  FIGS. 1A-1K and 5A-5F  are examples of implant designs that seek to reduce stresses induced on the cord. An ideal implant design would enable the construct to utilize the full yield strength of the cord (e.g., 3000N). Realistically, the designs discussed herein may reach levels approaching 2000N between improvements in cord clamping and holding strength within the vertebral body. 
     In this example, the dynamic tension sharing construct implants are designed to load share at a tension below the 800N screw plow level of standard pedicle-style bone screws and above the 500N tensioning instrument level. In another example, the implant may be designed with a screw plow tension level of 1000N, and the dynamic tension sharing target may be in a range of 600N to 900N, which is well above initial tensioning level of ˜500N and below the screw plow tension level of this implant of 1000N. Most of the implant designs discussed herein that allow for dynamic tension sharing can have the tension level target for sharing adjusted by set screw torque or other similar implant parameters. 
       FIG. 6A  illustrates a standard tether construct  600  with implants  610  in each vertebra  605  connected by cord  630 . In  FIG. 6A , the implants  610  include static heads  615  that are not designed for dynamic cord slip behavior. As shown in  FIG. 6A , the cord  630  can include areas with higher stress, such as the cord at the apex  632  and cord sections  634  adjacent to the apex  632 . In  FIG. 6B , the tether construct  600  is illustrated with the cord  630  experiencing critical tension at the apex  632 . Replacing static heads  615  with dynamic heads  620  that will allow for cord slippage above a pre-defined tension level can result in the critical cord stress at the apex  632  being distributed across adjacent cord sections  634 , as shown in  FIG. 6C . Allowing the cord to slip through certain screw heads, can result in distributing the cord stresses across multiple levels and reducing the opportunity for cord failure at any one level. 
     In these examples, the implants  600  are configured with dynamic heads  620  that allow for cord slip once the cord has reached a load that is above the tensioning load but below the screw plow and cord failure loads. When the cord  630  reaches a critical level between any two screws, the dynamic heads  620  release to distribute the tension across the two adjacent levels. If the tension is high enough, the dynamic heads  620  for these additional adjacent levels may also release to further distribute the higher tension load across more of the tether construct  600 . 
       FIGS. 7A-7C  are illustrations of a dynamic construct  700  using head tilt tension sharing implants, such implant  500  discussed above. Dynamic construct  700  operates in a manner similar to dynamic construct  600  discussed above, but with dynamic heads  720  that tilt in response to tension loads above a pre-defined level (e.g., above initial tension level but below screw plow or cord failure strength).  FIG. 7A  illustrates dynamic construct  700  with implants  710  coupling cord  730  across seven spinal levels (e.g., connecting eight vertebral bodies).  FIG. 7A  illustrates the cord at the apex  732  at risk of failure.  FIG. 7B  illustrates the two implants  710  responding to the cord at risk (apex  732 ) by tilting inward towards each other, which operates to shorten the cord segment at the apex  732  and lengthen the adjacent segments  734 . The head tilt operation effectively shares the tension load across two additional levels as shown in  FIG. 7C , with the critical tension load spread across the apex  732  and two adjacent segments  734 . In certain examples, the dynamic heads  720  can utilize a uniaxial design that only allows for tilting within a single plane, and that plane of tilt would be aligned with the cord in these examples. Uniaxial bone screw designs known in the industry could be adapted for use in such a dynamic construct. 
       FIGS. 8A-8J  are various views of bone anchors  800  used to strengthen bone screw connection to a vertebral body for spinal tethering techniques. In this example, the bone anchor  800  is designed to be implanted ahead of a bone screw, which is then inserted through screw passage  840 . In some examples, the bone anchor  800  is designed with anchor spikes  810  that expand radially outward as a bone screw  850  with a tapered neck  854  is implanted into the vertebral body. The bone anchor  840  can include a cylindrical anchor body  805  that forms screw passage  840 . Around the outer circumference of cylindrical anchor body  805  is an instrument groove  820 . Extending from an inferior rim of the cylindrical anchor body  805  are multiple anchor spikes  810 . The superior surface of the cylindrical anchor body  805  forms a screw engagement surface  830 , which can include a radiused surface slopping into the screw passage  840 . 
     In an example, the anchor spikes  810  can include a screw neck interface  812 , a chisel tip  814  and a retention feature  816 . The retention feature  816  extends radially outward from the main body of the anchor spike  810  with a concave radius. The retention feature  816  is designed to grip cortical bone as the bone screw  850  expands the anchor spikes  810  through interaction with the screw engagement surface  830  (as illustrated in  FIGS. 8F-8G ). Bone screw  850  is designed for use with bone anchor  800 . The tapered neck  854  is designed to cause the anchor spikes  810  to expand radially outward as the bone screw  850  is implanted through the bone anchor  800 . Bone screw  850  includes a tulip head  852  attached to a tapered neck  854  that extends into threaded shafter  856 . The bone screw  850  can also include a fenestration  858  to enable implantation with K-wire guides. The tulip head  852  is fixed to the tapered neck  854  and designed to receive a cord for use in a tethering construct. The bone screw  850  can be combined with cord washers and various set screw designs discussed above to enhance cord/tether gripping interface, among other things. 
       FIG. 8J  illustrates a variation of bone anchor  800  that includes barbed anchor spikes  811 . In this example, the barded anchor spikes  811  form a triangular spike with bards on the outer radiused surface. In this example, the outer radiused anchor spike surface match the overall radius of the cylindrical body  820 . The bone anchor  800  illustrated in  FIG. 8J  can include all of the other features discussed in reference to  FIGS. 8A-8I   
       FIGS. 9A-9F  are various views of a threaded bone anchor  900  for use with various bone screws discussed herein. The threaded bone anchor  900  is another bone anchor design to enhance the overall strength of an implant used for spinal tethering. The threaded bone anchor  900  can reduce screw plow and is designed to be implanted ahead of a corresponding bone screw. In this example, the threaded bone anchor  900  includes a cylindrical body  910  forming a screw passage  914 . The cylindrical anchor body  910  includes anchor threads  930  disposed on an outer sidewall surface. The anchor threads  930  can include cutting flutes  920  and terminate along a chisel edge  922 . The anchor threads  930  extend distally from an upper rim of the cylindrical anchor body  910  that includes an instrument groove  940  around the outer circumference and a screw engagement surface  912  around an inner circumference. The screw engagement surface  912  can be a chaffered or radiused surface designed to receive an outer surface of a distal portion of a bone screw head. In some examples (not illustrate), the instrument groove  940  can include vertical grooves or interruptions to assist with rotational grip for an implant instrument. 
       FIGS. 10A-10H  are various views of an implant  1000  that includes an offset single cord clamp  1010  for use in creating a spinal tether construct. The offset cord clamp  1010  enables full or partial assembly of the implant  1000  outside the body prior to implantation. In certain examples, the offset cord clamp  1010  is assembled onto the cord outside the body as discussed in greater detail below in reference to  FIGS. 19 and 20 . All of the implants discussed in reference to  FIGS. 10A-18H  are designed for use in either a partially assembly implant procedure or a fully assembled implant procedure discussed below. The implants assist in eliminating the challenges of threading a spinal cord into implants already placed in the vertebral bodies. 
     In this example, the implant  1000  includes an offset cord clamp  1010  and bone screw  1060 . The offset cord clamp  1010  includes a set screw  1015 , an upper clamp body  1020 , a lower clamp body  1030  and a cord clamp washer  1050 . The set screw  1015  is received into the screw passage  1024  in the upper clamp body  1020 , passes through the body ring  1054  of the cord clamp washer  1050  and engages the internal threads  1034  of the receiver cylinder  1032  in the lower clamp body  1030 . The cord  1070  is received within cord passage  1022  of the upper clamp body and held in place by the lower cord interface  1052  of the cord clamp washer  1050 . The upper clamp body  1020  includes a set screw lip  1026  extending around an inner circumference of the screw passage  1024  that interfaces with the inferior circumferential edge of the set screw head  1017 . The interface between the set screw lip  1026  and the set screw head  1017  generates a compressive force on the offset cord clamp  1010 . In this example, the lower clamp body  1030  includes a washer interface  1040  to receive the body ring  1054  of the cord clamp washer  1050 , so that when the set screw  1015  is tightened into the lower clamp body  1030 , the cord  1070  is compressed between the cord passage  1022  of the upper clamp body  1020  and the lower cord interface  1052  of the cord clamp washer. The set screw  1015  can include a driver interface  1018  and an instrument bore  1019  within the driver interface  1018 . The driver interface  1018  enables engagement of a driver instrument to tighten the set screw  1015 , while the instrument bore  1019  allows passage of a second driver instrument to engage a driver interface  1064  on the bone screw  1060 . The through bore design on the set screw  1015  is one of the features that enables the implant  1000  to be fully assembled outside the patient&#39;s body (ex situ). 
     In this example, the lower clamp body  1030  includes an integrated anchor  1042 . The integrated anchor  1042  includes anchor spikes  1044  extending distally for implantation into a vertebral body. The lower clamp body  1030  also includes the receiver cylinder  1032  that includes internal threads  1034 , screw head seat  1036 , and cord groove  1038 . The internal threads  1034  extend helically downward from the upper lip of the receiver cylinder  1032  and terminate at the screw head seat  1036 . The screw head seat  1036  is adapted to receive a portion of a screw head  1062  of the bone screw  1060 . In this example, the bone screw  1060  includes a head flare  1068  that includes a radiused outer surface, and the screw head seat  1036  includes a corresponding concave radiused surface. The screw head seat  1036  is designed to allow for some poly-axial movement of the bone screw  1060  within the lower clamp body  1030 . The outer sidewall of the receiver cylinder  1032  includes a cord groove  1038  that when assembled onto the cord  1070  engages a side of the cord  1070 . In this example, the cord groove  1038  has a concave radius that approximates the diameter of the cord  1070 . 
     Finally, implant  1000  includes a bone screw  1060  that is used to secure the implant  1000  to a vertebral body. The bone screw  1060  includes a screw head  1062 , a driver interface  1064  and a fenestration  1066 . The fenestration  1066  allows for use of K-wire guides during implantation. As noted above, the screw head  1062  includes a head flare  1068  portion that engages with the lower clamp body  1030 . 
       FIGS. 11A-11G  are various views of an implant  1100  that includes an offset single cord clamp  1110  with dual bone screws. In this example, an offset cord clamp  1100  with characteristics similar to offset cord clamp  1000  discussed above but including dual bone screws to further enhance the strength of a spinal tether construct. The implant  1100  is illustrated with a standard and a small screw (e.g.,  FIG. 11A ) as well as with two standard diameter screws (e.g.,  FIG. 11B ). As the overall design of implant  1100  is not dramatically affected by different screw sizes, the description applies equally to the different screw sizes. 
     In these examples, the implant  1100  includes the offset cord clamp  1110 , a first bone screw  1160  and a second bone screw  1180 . The first bone screw  1160  is structured similar to bone screw  1060  discussed above and also engages the offset cord clamp  1110  in a manner similar to that discussed above. The first bone screw  1160  includes a screw head  1162 , a driver interface  1164 , and a fenestration  1166 . The screw head  1162  includes a head flare  1169  that engages with a first screw head seat  1136  in a lower clamp body  1130 . The second bone screw  1180  also includes a screw head  1182 , a driver interface  1184 , and a fenestration  1186  (but only in the larger diameter versions). The screw head  1182  also includes a head flare  1188  that engages a second screw seat  1149  in the lower clamp body  1130 . 
     In this example, the offset cord clamp  1110  can include a set screw  1115 , an upper clamp body  1120 , a lower clamp body  1130 , and a cord clamp washer  1150 . The set screw  1115  includes set screw threads  1116 , a set screw head  1117 , a driver interface  1118 , and an instrument bore  1119  (structure is similar to set screw  1015  discussed above). The upper clamp body  1120  is also similar to upper clamp body  1020  discussed above. The upper clamp body  1120  includes a cord passage  1122  to receive cord  1170 . The upper clamp body  1120  also includes a screw passage  1124  to receive the first bone screw  1160 . The upper clamp body  1120  further includes a set screw lip  1126  extending from an inner circumference of the screw passage  1124 , which is formed in a cylindrical portion of the upper clamp body  1120 . 
     The lower clamp body  1130  includes a receiver cylinder  1132  forming a first screw passage  1135  with internal threads  1134  along the inner sidewall. The receiver cylinder  1132  also includes a first screw head seat  1136  formed along an internal distal circumference of the receiver cylinder  1132 . The receiver cylinder  1132  also includes a cord groove  1138  disposed on an external sidewall. The base of the lower clamp body  1130  includes a washer seat  1140  to receive the cord clamp washer  1150 . The lower clamp body  1130  further includes an integrated anchor  1142  with anchor spies  1144 . In this example, the lower clamp body  1130  extends laterally to include a second screw passage  1148  with a second screw seat  1149  formed around an inner circumference of the screw passage  1148 . The cord clamp washer  1150  includes a lower cord interface  1152  and a body ring  1154 . The cord clamp washer  1150  is assembled over the receiver cylinder  1132  and down onto the washer seat  1140 . 
       FIGS. 12A-12D  are various views of implant  1200  including an offset dual cord clamp  1210 . The implant  1200  is another variation on implant  1000 , this variation adds the ability to incorporate a second cord to a spinal tether construct. Various structures discussed in reference to implant  1200  are similar to corresponding structures in implant  1000 , and much of the descriptions are interchangeable. For example, the implant  1200  includes a lower clamp body  1230  that integrates the cord washer  1050  discussed above but is otherwise a very similar structure. The implant  1200  is designed for both partial and fully assembled implantation as described below. 
     In this example, the implant  1200  includes an offset cord clamp  1210 , a bone screw  1260 , an inner cord  1270  and an outer cord  1272 . The offset cord clamp  1210  includes an upper clamp body  1220  and a lower clamp body  1230  held together with a set screw  1215 . The upper clamp body  1220  includes an inner cord passage  1222 , a screw passage  1224 , and an outer cord passage  1228 . In certain examples, the upper clamp body  1220  and the lower clamp body  1230  can include engagement features that allow these structures to snap together allowing the set screw to be inserted later in the procedure. The inner cord passage  1222  and outer cord passage  1228  are designed to receive a particular diameter cord and include large clamping surface areas to enhance the ability of the implant  1200  to grip the inner cord  1270  and the outer cord  1272 . In an example, the inner cord passage  1222  and the outer cord passage  1228  can include cord relief cutouts  1229 . The screw passage  1224  forms a large cylinder that includes a set screw lip  1226  around an upper circumference. The set screw lip  1226  engages the set screw head  1217  to secure the upper clamp body  1220  to the lower clamp body  1230  and capture the inner cord  1270  and the outer cord  1272  within the respective cord passages. 
     The lower clamp body  1230  includes receiver cylinder  1232  that receives the upper clamp body  1220  on an outer surface and includes internal threads  1234  to receive the set screw  1215  on an inner surface. The receiver cylinder  1232  also includes a screw head seat  1236  around a lower inner circumference to receive a screw head  1262  of bone screw  1260 . The receiver cylinder  1232  can also include a cord groove  1238  around the outer circumference, the cord groove  1238  can include a concave radius similar to the outer diameter of the inner cord  1270 . In certain examples, the outer surface of the receiver cylinder  1232  is designed to generate a friction fit with the inner surface of the screw passage  1224  of the upper clamp body  1220 , which allows for temporary assembly without the set screw  1215 . Finally, the lower clamp body  1230  includes a cord interface surface  1240  that extends laterally from the receiver cylinder  1232  to function in conjunction with the inner cord passage  1222  and the outer cord passage  1228  to capture the inner cord  1270  and the outer cord  1272 . In this example, the lower clamp body  1230  is illustrated without an integrated anchor, but the lower clamp body  1230  can include an integrated anchor or the implant  1200  can be used with any of the standalone anchors discussed herein. An integrated anchor version of implant  1200  would resemble implant  1000  discussed above. 
       FIGS. 13A-13F  are various views of an implant  1300  including a symmetric offset dual cord clamp  1310  designed to secure an anterior cord  1370  and a posterior cord  1372 . The implant  1300  is similar in concept and construction as compared to implants  1000 ,  1100 , and  1200  discussed above. Like implant  1200 , implant  1300  is designed to secure dual cords. Implant  1200  secures dual cords in an offset configuration, while implant  1300  is symmetric around the bone screw  1360 . Both implants  1200  and implants  1300  could be used together in a single spinal tether construct at different levels. For example, an offset implant  1200  might be used at a level where some alternative loading is desirable on a particular vertebral body. Similar to implant  1000 , implant  1300  includes a lower clamp body  1330  within an integrated anchor  1342 . 
     In this example, the symmetric offset cord clamp  1310  can include a set screw securing an upper clamp body  1320  onto a lower clamp body  1330  with a cord clamp washer  1350  in between. In another example, the cord clamp washer  1350  may be integrated into the lower clamp body  1330  similar to implant  1200  discussed above. In this example, the set screw  1315  includes set screw threads  1316 , a set screw head  1317 , a driver interface  1318 , and an instrument bore  1319  through the driver interface  1318 . The upper clamp body  1330  can include an anterior cord passage  1322 , a lower body passage  1324 , and a posterior cord passage  1328  on the opposite side of the lower body passage  1324 . In this example, the lower body passage  1324  forms a cylindrical bore that includes a set screw lip  1326  around an upper periphery to receive an outer edge of the set screw head  1317  once assembled. 
     In an example, the lower clamp body  1330  includes a receiver cylinder  1332  that receives cord clamp washer  1350  and the upper clamp body  1320  via the lower body passage  1324 . The receiver cylinder  1332  includes internal threads  1334  extending from a superior edge down to a screw seat similar to screw head seat  1236  or screw head seat  1036  discussed above. The receiver cylinder  1332  also includes an outer sidewall with a cord groove  1338  designed to receive the anterior cord  1370  and the posterior cord  1372  once the implant  1300  is assembled onto both cords. The lower clamp body  1330  also includes a washer seat  1340  extending from a lower periphery of a washer interface  1346  formed in distal end of the receiver cylinder  1332 . The washer interface  1346  is designed to receive a ring body of the cord clamp washer  1350 . Below the washer seat  1340  extends an integrated anchor  1342  including a plurality of anchor spikes  1344 . The offset cord clamp  1310  is completed by the cord clamp washer  1350  that includes an anterior cord extension  1352  and a posterior cord extension  1354  which form the base of the anterior cord passage  1322  and the posterior cord passage  1328 , respectively. 
     The offset cord clamp  1310  is secured to a vertebral body with bone screw  1360 . The bone screw  1360  includes a screw head  1362 , a driver interface  1364 , and a fenestration  1366 . Similar to bone screw  1060 , the screw head  1362  includes a head flare  1368  that interfaces with the screw head set around the distal interior surface of the receiver cylinder  1332 . 
     Similar to implant  1200  discussed above, portions of the lower clamp body  1330 , such as the receiver cylinder  1332 , can be designed to generate a friction fit with portions of the upper cord clamp  1320 . For example, the inner surface of the lower body passage  1324  can generate a friction fit with a portion of the outer surface of the receiver cylinder  1332 . 
       FIGS. 14A-14F  are various views of an implant  1400  that includes a top loading modular cord clamp assembly  1450 . The implant  1400  is another spinal tethering implant that can be partially for fully assembled ex situ to ease overall implantation process. Ex site assembly of the implants can shorten procedure time and ease the overall process by avoiding manipulation of implants and the cord in situ. Partial or full ex situ assembly also enables distraction-based tensioning between the completed implants in each vertebra. In this example, the implant  1400  can include bone screw  1410 , saddle body  1420 , anchor  1430 , cord assembly  1450  and set screw  1460 . In some examples, the cord assembly  1450  and set screw  1460  can be provided as a pre-assembled component, such as is shown in  FIG. 14D . Additionally, in some examples, the saddle body  1420  and the anchor  1430  can be integrated together or delivered as a pre-assembled component (also as illustrated in  FIG. 14D ). 
       FIG. 14D  is an example of how implant  1400  may be used in a partially assembled procedure. In this example, the surgeon first implants the combined saddle body  1420  and anchor  1430 . Next, the bone screw  1410  is implanted through screw passages  1426 ,  1434  in the saddle body  1420  and anchor  1430  respectively. Finally, the cord assembly  1450  with set screw  1460  attached is assembled onto cord  1470  ex situ and inserted into saddle body  1420  in situ. Each of the implant components discussed can be implanted via an all-through-one (ATO) port as discussed further below. 
     In this example, the bone screw  1410  includes a screw head  1412 , a driver interface  1414 , and a screw shaft  1416  with screw threads  1418 . The screw threads can include cutting flutes  1419  on the distal end of the bone screw  1410 . The saddle body  1420  can include threaded saddle arms  1422 , a cord recess  1424 , a screw passage  1426 , and a screw seat  1428 . The threaded saddle arms  142  and cord recess  1424  for a U-shaped structure with the screw passage  1426  extending through a longitudinal axis. The screw seat  1428  can be a concave surface around a circumference of the screw passage  1426  and is designed to receive a lower surface of the screw head  1412 . The anchor  1430  includes anchor spikes  1432  extending inferiorly around the screw passage  1434 . The anchor  1430  also includes a saddle interface surface  1436  that is a chaffered or radiused surface around an interior circumference of the anchor  1430 . Around an exterior circumference of the anchor  1430  is an instrument groove  1438 . 
     The cord assembly  1450  includes a clamp body  1451  to receive the cord  1470  with a cord passage  1456  and the set screw  1460 . The clamp body  1451  can also include cord clamp extensions  1458  to extend the clamping surface of the clamp body  1451 . The cord clamp body  1451  also include a set screw cylinder  1452  surrounded by a set screw interface surface  1454 . The set screw cylinder  1452  operates to rotatably secure the set screw  1460  to the clamp body  1451  to ease implantation of the cord assembly  1450 . The set screw interface surface is a donut-shaped flat surface designed to receive the end of the set screw  1460  to distribute forces created by tightening the set screw  1460  into saddle body  1420  onto the cord  1470 . The cord assembly  1450  is designed to be coupled to the cord ex situ and then inserted into the saddle body  1420  in situ after the saddle body  1420 , anchor  1430 , and bone screw  1410  are already implanted. The clamp body  1451  is designed to slide into the cord recess  1424  of the saddle body  1420  and be tightened down with the set screw  1460 . The interaction between the clamp body  1451 , the saddle body  1420  and the set screw  1460  operate to compress the cord  1470  from all sides and create a distributed cord grip that avoids inducing stresses on to the cord  1470  (as compared to a typical set screw directly on the cord, as an example). 
       FIGS. 15A-15F  are various views of an implant  1500  including a posted screw  1510  and a modular cord clamp (cord assembly  1530 ). The implant  1500  is design for partially assembled implantation procedure where the posted screw is implanted, and the cord assembly is assembled onto the cord ex situ and then inserted onto the implanted screw.  FIG. 15B  illustrates the two implantable components of implant  1500 . 
     In this example, the implant  1500  includes a bone screw  1510 , an anchor  1520 , and a cord assembly  1530 . As noted above, in some examples the anchor  1520  is integrated into the cord assembly, or at least implanted in the step of the implantation technique. In an example, the bone screw  1510  is a posted bone screw with a threaded screw head  1512 , a driver interface  1514 , a screw shoulder  1515 , a screw shaft  1516 , screw threads  1518 , and cutting flutes  1519 . The screw shoulder  1515  is designed to receive/engage an inferior surface around an inner circumference of the screw passage  1524  of the anchor  1520 . The anchor  1520  can include anchor spikes  1522 , the screw passage  1524 , and a cord assembly interface surface  1526 . 
     In this example, the cord assembly  1530  includes a screw head passage  1532 , a head nut  1534 , a nut seat  1536 , a set screw passage  1540 , an interior threaded sidewall  1542 , and a set screw  1544 . The screw head passage  1532  is designed to receive the threaded screw head  1512  through a distal bore created by the nut seat  1536  and the head nut  1534  through the proximal opening. The head nut  1534  is designed to lock the cord assembly  1530  onto the bone screw  1510  via the threaded screw head  1512 . The cord passage  1538  is designed to receive a cord or tether for the spinal tether construct and includes a superior open formed by the set screw passage  1540 . The cord passage  1538  includes a section of interior threaded sidewall on opposing sides of the set screw passage  1540  to engage the set screw threads  1546  as the set screw  1544  is threaded into the passage to compress the cord and lock it in place. The set screw  1544  is tightened via a set screwdriver interface  1548 . In this example, the cord assembly  1530  includes a distal anchor engagement cylinder that fits into the screw passage  1524  of the anchor  1520 . In some examples, the distal anchor engagement cylinder can create a fiction fit with the anchor  1520  to enable the anchor  1520  to function as an integral part of the cord assembly  1530 . 
     As is the case with most of the implants discussed herein, implant  1500  includes numerous components that could be interchanged with similar components discussed in reference to other example implants. For example, anchor  1520  could be exchanged for any of the various anchors discussed herein. 
       FIGS. 16A-16H  are various views of a top loading self-locking cord clamp implant  1600 . The implant  1600  illustrated in these figures includes two basic components saddle body  1620  and cord assembly  1630 . The saddle body  1620  represents any tulip style pedicle screw head and is shown in these figures without a bone screw for clarity. The saddle body  1620  in this example includes threaded saddle arms  1622  to receive a set screw, such as set screw  1640 . The saddle body  1620  also includes a cord recess to receive the cord assembly  1630  and screw passage  1626  with a screw seat  1628  around the circumference of the opening forming the screw passage  1626 . The cord assembly  1630  can be used with any tulip-style head bone screw. 
     The focus of implant  1600  is the cord assembly  1630  that is designed to enable partially assembled implant techniques where the cord assembly is put on the cord  1610  ex situ and then the cord  1610  and cord assembly are implanted into a bone screw with saddle body  1620  and optionally an anchor. The cord assembly  1630  in this example includes an outer cord clamp housing  1632  coupled to a set screw  1640 . The outer cord clamp housing  1632  includes a set screw passage  1633 , a cord passage  1634 , cord clamp extensions  1636 , an inner cord clamp  1638 , and a bias member  1639 . In an example, the inner cord clamp  1638  is spring biased (by bias member  1639 ) to automatically lock the cord assembly onto the cord  1610 . In an example, the bias member  1639  is a coil spring or similar bias device positioned around the base of the inner cord clamp  1638 . The set screw  1640  includes a clamp interface  1642  in the form of a cylindrical extension that couples into the set screw passage  1633  and allows for manual displacement of the inner cord clamp  1638  to release the cord  1610  (or facilitate inserting the cord assembly  1630  onto the cord  1610 ). 
       FIGS. 16D and 16E  are illustrations of the cord assembly  1630  and set screw  1640  with the inner cord clamp  1638  biased in a close or locked position on the cord  1610 . In  FIG. 16E  the bias member  1639  is illustrated as elongated boxes to indicate that the spring or similar structure is pushing the inner cord clamp  1638  against the cord  1610 .  FIGS. 16F and 16G  illustrate the cord assembly and set screw  1640  with the inner cord clamp  1638  in an open or unlock position to allow for free movement of the cord  1610  through the cord assembly  1630 .  FIG. 16G  illustrates the bias member  1639  in a compressed position as the inner cord clamp  1638  is being pushed away from engagement with the cord. In an example, the inner cord clamp  1638  can be disengaged from the cord  1610  by application of a force downward on the set screw counteracting the bias member  1639 .  FIG. 16H  is a cross-sectional view of the cord assembly  1630  inserted into the saddle body  1620  and the set screw  1640  fully tightened down. In the fully tightened down position, the set screw  1640  compresses the cord  1610  to fully lock it into the implant  1600 . In this example, the head  1644  of the set screw  1640  limits how much compression of the cord  1610  is possible within implant  1600 . In some examples, the set screw  1640  and cord assembly  1630  are designed to allow for dynamic cord slip above a pre-define cord tension. 
       FIGS. 17A-17F  are various views of a cord clamping set screw assembly (cord assembly  1720 ). The implant  1700  includes a bone screw with a saddle body  1710  and a cord assembly  1720  that clamps a cord within the saddle body  1710 . The bone screw including the saddle body  1710  further includes a pair of threaded saddle arms  1712 , instrument interfaces  1714 , a cord recess  1716 , and instrument grooves  1718  in each corner of the saddle body  1710 . The saddle body  1710  is representative of any tulip-style pedicle screw head and the cord assembly  1720  can be adapted for use within any similar spinal bone screw assembly. 
     The cord assembly  1720  includes set screw  1722  and cord clamp  1730 . The cord clamp  1730  attaches to the set screw  1722  via clamp peg  1726  extending from a distal surface of the set screw  1722 . The clamp peg  1726  is received into a central bore in the cord clamp  1730  and the set screw  1722  distributes clamping forces onto the cord clamp  1730  via a cylindrical mating surface  1728 . The clamp peg  1726  enables the set screw  1722  to rotate relative to the cord clamp  1730  to facilitate engaging the cord assembly with the saddle body  1710 . The cord clamp  1730  further includes clamp arms  1732 . The clamp arms  1732  include flex grooves  1734  and terminate in clamping tips  1736 . The cord clamp  1730  can also include clamp washer extensions  1738  to extend the cord clamping surface area of the cord clamp  1730 . The clamp arms  1732  are designed to be compressed onto a cord when the cord assembly is inserted into the saddle body  1710  and the set screw  1722  tightened via threads  1724  engaging with the threaded saddle arms  1712 . The flex grooves  1734  allow the clamp arms  1732  to flex outward to accept a cord in between the clamp arms  1732 . The cord assembly  1720  is designed to be attached to a cord ex situ and then the cord and cord assembly  1720  implanted into a bone screw with saddle body  1710  already implanted into the vertebral body. 
       FIGS. 18A-18H  are various views of another cord clamping set screw type implant  1800 . In this example, the implant  1800  includes a saddle body  1810  and a cord assembly  1820 . Again, the saddle body  1810  is representative of any tulip-style pedicle screw that might be used in a typical spinal fusion procedure. In this example, the saddle body  1810  does include a couple structures that might be specific to implant  1800 , such as the oval clamp recess  1818  and fenestration  1819 . The saddle body  1810  in this example also includes a pair of threaded saddle arms  1812 , instrument interfaces  1814 , a cord recess  1816 , and instrument grooves  1817 . 
     In this example, the cord assembly  1820  includes a set screw  1822  and a cord clamp  1830 . The set screw  1822  can couple to the cord clamp  1830  via a spherical concave clamping surface  1826  (as shown in  FIGS. 18B and 18D ). The set screw  1822  also includes external threads  1824  and a cylindrical mating surface  1828  around an outer distal edge of the set screw  1822  below the external threads  1824 . The cord clamp  1830  can include clamp arms  1832 , oval alignment extension  1838 , and a cylindrical guide peg  1836 . The clamp arms  1832  are designed with sufficient flex due to the flex grooves  1834  to receive a cord and hold position on the cord. The cord clamp  1830  is designed to be fabricated from an at least somewhat flexible material, such as Teflon, polyetheretherketone (PEEK) or similar dimensionally stable thermoplastic with characteristic acceptable for implantation into the human body. In other examples, the cord clamp  1830  can be fabricated from metallic materials such as nitinol or titanium. The cylindrical guide peg  1836  and oval alignment extension  1838  features assist in implanting the cord assembly  1820  into the saddle body  1810 . The cylindrical guide peg  1836  can first find the oval clamp recess  1818  and then be guided into fenestration  1819  while the oval alignment extension  1838  fit snuggly into the oval clamp recess  1818 . Once the cord assembly  1820  is aligned into position in the saddle body  1810 , the set screw  1822  is used to tighten the implant  1800  with the concave clamping surface  1826  compressing the clamp arms  1832  around the cord to secure it in position relative to the saddle body  1810 . 
       FIG. 19  is a flowchart illustrating a technique  1900  for using partially assembled implants in a spinal tethering procedure. Most of the implants discussed above are useable in the technique  1900 , such as implants  1000 ,  1100 ,  1200 ,  1300 ,  1400 ,  1500 ,  1600 ,  1700 ,  1800 , and  2300  (discussed in reference to  FIGS. 23A-23G  below). The technique  1900  is discussed below in reference to implant  1600 , and more specifically in reference to cord assembly  1630 . The partially assembled implant technique involves first implanting all of the bone screws or similar implants into the target vertebral bodies that will be part of the spinal tether construct. After the bone screws are implanted, the cord assembly implants are assembled onto the cord ex situ, then the cord with the cord assembly implants are inserted into the already implanted bone screws. The cord assembly implants may all be put on the cord at once or may be assembled onto the cord one by one as they are implanted into the bone screws. 
     In this example, the technique  1900  can being at  1910  with the surgeon implanting bone screws with or without anchors through one or more all-through-one (ATO) ports. The bone screws implanted at operation  1910  can include the bone screw  850  with expanding anchor  810  for example. The bone screw  850  includes a saddle body comparable to saddle body  1620  and would be compatible with cord assembly  1630 . At  1920 , the technique  1900  continues with the surgeon attaching at least two cord assemblies  1630  to a cord  1610  (note, this could be done one at a time, but this particular version of the technique  1900  does it two at a time). The technique  1900  continues at  1930  with the surgeon inserting the first cord assembly  1630  attached to cord  1610  through the ATO and into saddle body  1620  in the first bone screw  850  in the tether construct. Next, the technique  1900  continues at  1940  with the surgeon positioning the second cord assembly  1630  attached to the cord  1610  into the second saddle body  1620  of the second bone screw  850 . At  1950 , the technique  1900  continues with the surgeon tensioning the cord  1610  between the first bone screw and the second bone screw. Tensioning can include tightening the set screws associated with each of the first and second cord assemblies  1630 . 
     Once the first set of two cord assemblies  1630  are implanted and tensioned, the technique  1900  continues at  1960  with the surgeon determining whether the last bone screw in the construct has been reached. If the last bone screw has not had a cord assembly inserted, then the technique  1900  loops back through operations  1920  to  1960  until the cord has been implanted into each bone screw with a cord assembly and at least initially tensioned. Note, after the initial pass-through operations  1920  through  1960 , one cord assembly at a time can be assembled onto the cord and the surgeon can progress through the tether construct one bone screw at a time. Back at  1960 , the technique  1900  continues on after the last bone screw in the construct is coupled to the cord  1610  to operation  1970 . At  1970 , the technique  1900  optionally continues with final tensioning and tightening of set screws in the implanted cord assemblies. Final tensioning can involve loosening and re-tightening each set screw while applying distraction pressure between adjacent bone screws. Finally, once the construct has been sufficiently tensioned, the technique  1900  completes at  1980  with removal of all instruments and ATOs from the patient. 
       FIG. 20  is a flowchart illustrating a technique  2000  for using fully assembled implants in a spinal tethering procedure. Several of the implants discussed above are useable in the technique  2000 , such as implants  1000 ,  1200 , and  1300  in particular. The technique  2000  is discussed below in reference to implant  1000 , and more specifically in reference to offset cord clamp  1010 . The fully assembled implant technique involves fully assembling the implant onto the cord ex situ, then the cord with the implant(s) are inserted through an ATO and implanted in the target vertebral body. The fully assembled implants may all be put on the cord all at once or may be assembled onto the cord one by one as they are implanted. 
     In this example, the technique  2000  can begin at  2010  with the implants  1000  assembled ex-situ. At  2020 , the technique  2000  continues with the surgeon attaching an assembled implant  1000  onto cord  1070 . At this operation, a number of implant assemblies  1000  can be attached to the cord  1070 , or they can be attached to the cord one-by-one as they are implanted. At  2030 , the technique  2000  continues with the surgeon passing the implant  1000  and the cord  1070  through an ATO and implanting the implant  1000  into a target vertebral body. Next, at  2040 , the technique  2000  continues with the next implant  1000  and attached cord  1070  being passed into the patient through the ATO and implanted into an adjacent vertebral body. At  2050 , the technique  2000  continues with the surgeon using a distraction instrument (or similar cord tensioning device) to tension the cord between the adjacent vertebral bodies. The technique  2000  continues at  2060  with the surgeon locking set screw(s) on the implants  1000  to retain cord tension induced by the tensioning instrument. At  2070 , the technique  2000  continues with the surgeon determining whether the last implant in the construct has been implanted. If there are more implants, the technique  2000  continues back through operations  2020  through  2060  on the next level in the spinal tether construct. Once all implants  1000  have been implanted and tensioned at least initially, the technique  2000  continues with optional operation  2080  where the surgeon can adjust the cord tension with final tensioning and tightening of set screws. At  2090 , the technique  2000  completes with all instruments and ATOs being removed from the patient. 
       FIGS. 21A-21E  are various views of implant  2100  that includes a ploy-axial bone screw  2110 , a saddle body  2120 , and an anchor  2140 . The implant  2100  is designed to be implanted as a complete assembly and then have a cord or tether implanted afterward. The bone screw  2110  can include a screw head  2112  and a screw shaft  2116 . The screw head  2112  includes a driver interface  2114 , while the screw shaft  2116  includes screw threads  2118  and cutting flutes  2119 . The saddle body  2120  can include opposing threaded saddle arms  2122  with a cord recess  2124  in between. The saddle body  2120  can also include a screw passage  2126  formed in a lower cylindrical body portion of the saddle body  2120 . The lower cylindrical body can also include an inferior cylindrical recess  2127  and an inferior ring gear  2128  around a distal circumference. Within the screw passage  2126 , the saddle body  2120  also includes a snap ring groove  2129  to receive a snap ring  2130  that retains the bone screw  2110  once assembled. The anchor  2140  can include anchor spikes  2142  extending inferiorly opposite a superior toothed surface  2146  designed to interface with the inferior ring gear  2129  on the saddle body  2120 . Around the superior toothed surface  2146  is a superior cylindrical sidewall  1247  that fits into the inferior cylindrical recess  2127  on the saddle body  2120 . Around a distal circumference edge of the anchor  2140  is a chamfered interior edge  2148  that allows for greater angulation of the bone screw  2110 . The anchor  2140  may also include other structural features discussed above in reference to other similar anchors. 
     The implant  2100  can be assembled by inserting the screw head  2112  of the bone screw  2110  into the distal end of the screw passage  2126  of the saddle body  2120 . With the screw head  2112  pushed into the saddle body  2120 , the snap ring  2130  is inserted into the snap ring groove  2129  to retain the bone screw  2110  within the saddle body  2120 . The anchor  2140  can be slid over the distal end of the bone screw  2120  and positioned around the distal end of the saddle body  2120  prior to implantation. Alternatively, the anchor  2140  can be implanted in the target vertebral body, and then the bone screw and saddle body assembly can be implanted through the anchor  2140 . 
       FIGS. 22A-22E  are various views of implant  2200  that includes an expanding bone anchor  2210 . In this example, the tapered bone anchor  2210  is implanted into a target vertebral body and then saddle insert  2230  is inserted into the tapered bone anchor  2210  to cause expansion of the external threads  2212  along the expansion slots  2214 . The purpose of implant  2200  is to enhance the holding strength of a uni-cortical bone screw construct. In currently available spinal tethering systems bi-cortical bone screws are typically used to ensure sufficient holding and avoid screw plow from cord tension. Bi-cortical screws require additional caution during implantation to avoid disturbing soft tissues on the far side of the target vertebral body. Bi-cortical screws are passed through the far cortical bone surface blindly, which can result in negative outcomes in rare instances. Accordingly, the implant  2200  is designed for uni-cortical implantation with the implant only being implanted through the near cortical bone surface and into the cancellous bone within the vertebral body. The expansion of the tapered bone anchor  2210  provides the additional strength instead of passing through the far layer of cortical bone. 
     In this example, the implant  2200  can include a tapered bone anchor  2210  and a saddle insert  2230 . The tapered bone anchor  2210  can include external threads  2212  broken by expansion slots  2214  distributed around the circumference. The bone anchor  2210  can also include an instrument interface  2216  to assist with implanting the bone anchor  2210 . Internal features of the bone anchor  2210  can include a saddle funnel  2218  to receive a lower surface of the saddle insert  2230 , a central threaded bore  2220 , and a tapered internal bore  2222 . The saddle insert  2230  can include a saddle body  2232  with opposing threaded saddle arms  2234 . The saddle body  2232  can include instrument interfaces  2236 , instrument grooves as well as a cord recess  2238  from by the opposing threaded saddle arms  2234 . Below the saddle body  2232 , the saddle insert  2230  includes a central bore  2240 , a threaded anchor interface  2244 , and an expansion peg  2246 . 
     Once the bone anchor  2210  is implanted into a vertebral body, the saddle insert  2230  can be inserted into the central threaded bore  2220 . As the threaded anchor interface  2244  engages the central threaded bore  2220 , the expansion peg  2246  extends into the tapered internal bore  2222  and pushes the lower portion of the external threads outward expanding the expansion slots  2214 .  FIG. 22E  illustrates the point at which, in this example, the expansion peg  2246  begins to expand the bone anchor  2210 . In some examples, the expansion peg  2246  is long enough to extend pass the end of the tapered internal bore  2222  to cause maximum expansion. 
     The implant  2200  is designed for potential use with cord assemblies discussed above, such as cord assemblies  1450 ,  1630  or  1720 . The implant  220  includes a tulip-style saddle body  2232  that could be adapted for use with any of the noted cord assemblies. In another example, the saddle body  2232  of saddle insert  2230  could be replaced with a threaded post head to allow for a cord assembly, such as cord assembly  1530 , to be used with the expanding bone anchor  2210 . 
       FIGS. 23A-23G  are various views of an offset single cord clamp  2310  with dual bone screws in accordance with the present disclosure. The cord clamp illustrated in these figures is similar to those discussed above in reference to  FIGS. 10A-13F , but includes a two-piece clamp housing (e.g., upper clamp member  2320  and lower clamp member  2330 ). The two-piece clamp housing enables the cord clamp to be affixed onto the cord prior to implantation by snapping the upper clamp member  2320  into the lower clamp member  2330  and threading the cord  2380  through the cord passages  2322  and  2336 —no other structures are required to hold the offset cord clamp  2310  onto the cord  2380  prior to implantation. The offset cord clamp  2310  is illustrated as including dual bone screws (e.g., first bone screw  2360  and second bone screw  2370 ), but could be modified to use a single bone screw (similar to offset cord clamp  1010 ). Additionally, the offset cord clamp  2310  attaches to a single cord, but could be modified to clamp two cords (offset or symmetric configurations illustrated above). Accordingly, any of the cord clamp designs illustrated in  FIGS. 10A-13F  above can be modified to use a similar two-piece housing. 
     In this example, the implant  2300  includes an offset cord clamp  2310  couplable to a cord  2380  and anchor  2340 . The anchor  2340  is designed to receive a first bone screw  2360  and a second bone screw  2370 . As noted above, the offset cord clamp  2310  includes an upper clamp member  2320  couplable to a lower clamp member  2330 . The upper clamp member  2320  includes a cord passage  2322 , a screw passage  2324 , a set screw lip  2326  and a cylindrical body  2328 . The lower clamp member  2330  includes a receiver cylinder  2332  that receives the cylindrical body  2328 . The lower clamp member  2330  also includes an internal lip  2334  and a cord passage  2336 . The cord passage  2322  and the cord passage  2336  operate together to couple the offset cord clamp  2310  onto the cord  2380 . Once fully assembled in-situ, the set screw  2315  engages the set screw lip  2326  and internal threads  2350  of the anchor  2340  to compress the upper clamp member  2320  into the lower clamp member  2330 . Compressing the upper clamp member  2320  and lower clamp member  2330  generates a clamping force throughout the cord passage  2322  and cord passage  2336  to lock the cord in position relative to the offset cord clamp  2310 . 
     In this example, the set screw  2315  includes set screw threads  2316 , a set screw head  2317 , a driver interface  2318 , and an instrument bore  2319 . The set screw threads  2316  engage with the internal threads  2350  within the cylindrical body  2346  of the anchor  2340 . The set screw head  2317  engages with the set screw lip  2326  within the cylindrical body  2328  of the upper clamp member  2320 . The instrument bore  2319  is designed to allow a bone screwdriver to engage with the driver interface  2364  in the first screw head  2362  of the first bone screw  2360 . 
     The anchor in this example, anchor  2340 , includes a first screw passage  2342  and a second screw passage  2354  to accommodate dual bone screws. The dual bone screws are intended to increase the holding strength while avoiding bi-cortical screws. The anchor  2340  also includes a first screw seat  2344  and a second screw seat  2356  that are designed to engage with the first screw head  2362  and the second screw head  2372 , respectively. The anchor  2340  also includes a cylindrical body  2346  with an external cord groove  2348  and internal threads  2350 . The cord groove  2348  is designed to abut a portion of the cord  2380  when the entire implant  2300  is assembled. Anchor spikes  2352  extend inferiorly from the bottom side of the anchor  2340 . The anchor spikes  2352  are designed to be embedded into the near cortical bone of the vertebral body to further enhance the strength of implant  2300 . 
     Each of the dual bone screws include screw heads with a flared outer surface that interfaces with the screw seats ( 2344 ,  2356 ) in the anchor  2340 . The first bone screw  2360  includes a first screw head  2362 , a driver interface  2364 , and threads  2366 . The second bone screw  2370  includes a second screw head  2372 , a driver interface  2374 , and threads  2376 . In this example, the second bone screw  2370  is smaller than the first bone screw  2360 , but the anchor  2340  could be adapted for use with any combination of bone screw sizes. 
     The above detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show, by way of illustration, specific embodiments in which the invention can be practiced. These embodiments are also referred to herein as “examples.” Such examples can include elements in addition to those shown or described. However, the present inventors also contemplate examples in which only those elements shown or described are provided. Moreover, the present inventors also contemplate examples using any combination or permutation of those elements shown or described (or one or more aspects thereof), either with respect to a particular example (or one or more aspects thereof), or with respect to other examples (or one or more aspects thereof) shown or described herein. 
     In the event of inconsistent usages between this document and any documents so incorporated by reference, the usage in this document controls. 
     In this document, the terms “a” or “an” are used, as is common in patent documents, to include one or more than one, independent of any other instances or usages of “at least one” or “one or more.” In this document, the term “or” is used to refer to a nonexclusive or, such that “A or B” includes “A but not B,” “B but not A,” and “A and B,” unless otherwise indicated. In this document, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Also, in the following claims, the terms “including” and “comprising” are open-ended, that is, a system, device, article, composition, formulation, or process that includes elements in addition to those listed after such a term in a claim are still deemed to fall within the scope of that claim. Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. 
     The above description is intended to be illustrative, and not restrictive. For example, the above-described examples (or one or more aspects thereof) may be used in combination with each other. Other embodiments can be used, such as by one of ordinary skill in the art upon reviewing the above description. The Abstract is provided to comply with 37 C.F.R. § 1.72(b) at the time of filing this application, to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. Also, in the above Detailed Description, various features may be grouped together to streamline the disclosure. This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Rather, inventive subject matter may lie in less than all features of a particular disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description as examples or embodiments, with each claim standing on its own as a separate embodiment, and it is contemplated that such embodiments can be combined with each other in various combinations or permutations. The scope of the invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. 
     The above detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show, by way of illustration, specific embodiments in which the invention can be practiced. These embodiments are also referred to herein as “examples.” Such examples can include elements in addition to those shown or described. However, the present inventors also contemplate examples in which only those elements shown or described are provided. Moreover, the present inventors also contemplate examples using any combination or permutation of those elements shown or described (or one or more aspects thereof), either with respect to a particular example (or one or more aspects thereof), or with respect to other examples (or one or more aspects thereof) shown or described herein. The Abstract is provided to comply with 37 C.F.R. § 1.72(b), to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of a claim. Also, in the above Detailed Description, various features may be grouped together to streamline the disclosure. This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Rather, inventive subject matter may lie in less than all features of a particular disclosed embodiment.