Patent Publication Number: US-2022211518-A1

Title: Device and method for deployment of an anchoring device for intervertebral spinal fusion

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is a continuation of U.S. patent application Ser. No. 16/458,909 filed on Jul. 1, 2019, which is a divisional of U.S. patent application Ser. No. 14/718,514 filed on May 21, 2015, which is incorporated in its entirety herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to intervertebral spacers for fusing adjacent vertebras, and more particularly to a device and methods for doing so. 
     BACKGROUND 
     Intervertebral spinal fusion is well known in the art. In the prior art, an intervertebral spacer is implanted between two adjacent intervertebral bodies. The spacer allows a surgeon to deposit bone graft between the problem vertebras in order to fuse the vertebras together. To achieve proper fusion, the implanted spacer must be securely anchored between the vertebras such that there is little to no movement once implanted. Protrusions arranged on the superior and inferior surfaces of the spacer provides a means to stabilize the spacer between the vertebras. However, it has been discovered that spacers stabilized in this way may still move due to the stress exerted on the implanted spacer when the patient moves. Other commonly employed stabilizing techniques include pedicle screws and rods. In this technique, pedicle screws are independently screwed into two or three spine segments. A short rod is then used to connect the pedicle screws to prevent motion at the segments that are being fused. However, this technique is time consuming because the pedicle screws need to be independently screwed into the vertebras. It also requires the surgeon to make large/numerous incisions in the patient to insert the pedicle screws. Because of these deficiencies in the prior art, there exists a need to provide a more effective and efficient way of stabilizing adjacent vertebras in the field of intervertebral spinal fusion. 
     SUMMARY 
     For the purpose of the following description and the appended claims, “proximal” and its inflected forms are defined as the part, portion, section, etc., of an object that is closest to the person using that object. 
     For the purpose of the following description and the appended claims, “distal” and its inflected forms are defined as the part, portion, section, etc., of an object that is furthest away to the person using that object. 
     The present invention provides a way to stabilize adjacent vertebras without some of the deficiencies of the prior art discussed above. In the illustrative embodiment of the present invention, a spacer is provide with an upper guide and a lower guide. The upper and lower guides are adapted to guide the simultaneous deployment of a respective upper anchor and lower anchor of an anchoring device when force is applied thereto. More precisely, force is simultaneously applied to a proximal portion of the upper and lower anchors. The force simultaneously deploys the upper and lower anchors into their respective intervertebral bodies. The upper and lower anchors are constructed and dimensioned in such a way to pierce and penetrate into their respective vertebras. The combination of the anchors and the protrusions arranged on the surfaces of the spacer provides additional stabilization of the implanted spacer. These advantages of the present invention will be apparent from the following disclosure and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  depicts a perspective view of an intervertebral spacer in accordance with an illustrative embodiment of the present invention; 
         FIG. 1B  depicts another perspective view of the intervertebral spacer of  FIG. 1A ; 
         FIG. 2A  depicts a top view of the intervertebral spacer of  FIGS. 1A and 1B ; 
         FIG. 2B  depicts a side view of the intervertebral spacer of  FIGS. 1A and 1B ; 
         FIG. 3A  depicts one side of an anchor in accordance with an illustrative embodiment of the present invention; 
         FIG. 3B  depicts the other side of the anchor of  FIG. 3A ; 
         FIG. 4A  depicts two anchors being loaded into the intervertebral spacer of  FIGS. 1A and 1B ; 
         FIG. 4B  depicts the two anchors of  FIG. 4A  loaded into the intervertebral spacer of  FIGS. 1A and 1B , the two anchors being in an undeployed state; 
         FIG. 5A  depicts a perspective view of an implantation instrument in accordance with an illustrative embodiment of the present invention; 
         FIG. 5B  depicts a cross-sectional view of the implantation instrument of  FIG. 5A , the cross-sectional view depicting a narrower section and a wider section of the implantation instrument; 
         FIG. 5C  depicts an exploded, cross-sectional view of the wider section of the implantation instrument of  FIG. 5A ; 
         FIG. 5D  depicts a cross-sectional view of the implantation instrument gripping the lateral surfaces of the intervertebral spacer of  FIGS. 1A and 1B ; 
         FIG. 6A  depicts the implantation instrument of  FIG. 5A  having deployed the anchors of  FIG. 4A ; 
         FIG. 6B  depicts an exploded, top view of the deployed anchors of  FIG. 6A ; 
         FIGS. 6C and 6D  depict an exploded, perspective view of the deployed anchors of  FIG. 6A ; 
         FIG. 7A-7C  depict a spacer and anchor in accordance with an alternative embodiment of the present invention, wherein the upper and lower anchors of the anchoring device form a single, unitary piece; 
         FIG. 8A-8C  depict a spacer and anchor in accordance with an alternative embodiment of the present invention, wherein the upper and lower anchors of the anchoring device are disposed entirely within the spacer; 
         FIG. 9A-9H  depict an upper anchor and a lower anchor arranged on a drive plate in accordance with an alternative embodiment of the present invention; and 
         FIG. 10  depicts a spacer having worm gear for deploying one or more anchors in accordance with an alternative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1A and 1B  depict perspective views of intervertebral spacer  100  in accordance with an illustrative embodiment of the present invention. Spacer  100  generally has a rectangular shape, but the present invention is not limited to such a shape. Spacer  100  can have any shape, size, or combination thereof to meet the needs of a spinal fusion candidate. 
     As depicted in  FIGS. 1A and 1B , spacer  100  comprises superior surface  102 , inferior surface  104 , lateral surfaces  106  and  108 , distal portion  110 , and proximal portion  112 . Inferior surface  104  is a mirror image of superior surface  102  and lateral surface  108  is a mirror image of lateral surface  106 . Spacer  100  is preferably formed from titanium alloy but other biocompatible materials (e.g., polyetheretherketone (PEEK), other surgical grade metals, alloys, or a combination thereof) can also be used to form spacer  100 . 
     Beginning at distal portion  110 , spacer  100  is constructed to have a tapered end that narrows towards the distal most end. This design helps facilitate easier entry of spacer  100  into the narrow disc space arranged between two adjacent vertebral bodies. 
     To fuse the adjacent vertebras together, bone graft is used. For this purpose, the body of spacer  100  is provided with through-hole  114 . The through-hole extends through the center of surfaces  102 ,  104 ,  106 , and  108  and is adapted to receive the bone graft for fusing the adjacent vertebras. In the illustrative embodiment, through-hole  114  generally has a rectangular shape. However, those skilled in the art will appreciate after reading this disclosure that through-hole  114  can have any shape, size, or a combination thereof. As further depicted in  FIGS. 1A and 1B , surfaces  102  and  104  are provided with a plurality of protrusions or teeth  116  to help prevent spacer  100  from expulsion after being implanted between the adjacent vertebras. It will be appreciated by those skilled in the art, after reading this disclosure, that teeth  116  can be angled in any number of degrees (e.g., 45°, 90°, etc.) and can have any number of orientations without departing from the scope of the present invention. Through-hole  114  and teeth  116  can be seen more clearly in  FIGS. 2A and 2B . 
     Turning now to proximal portion  112 , upper and lower guides are provided to respectively guide the deployment of upper anchor  118  and lower anchor  120  into their respective vertebral bodies. The upper and lower anchors will be discussed in more detail below, with respect to  FIGS. 3A and 3B . In the illustrative embodiment, the upper guide is characterized by an upper inclined surface  122  (e.g., a curvilinear surface, etc.) and an upper pair of oppositely positioned lateral recesses  124 . Because the lower guide is a mirror image of the upper guide, the lower guide is also characterized by a lower inclined surface  126  and a lower pair of oppositely positioned lateral recesses  128 . The upper and lower pair of lateral recesses  124  and  128  are dimensioned to respectively complement the arc, curvature, etc., of the upper and lower anchors. An advantage of recesses  124  and  128  is that they ensure that their respective anchors maintain a desired trajectory when impacted by an anchor driver. The recesses  124  and  128  also prevent their respective anchors from egressing out of spacer  100  when impacted by the anchor driver. These features and their advantages will be discussed in more detail below, with reference to  FIGS. 4A and 4B . 
     Proximal portion  112  also comprises a pair of oppositely positioned lateral chamfers  130  and  132 . Each of the lateral chamfers has a sloping edge and is positioned proximally to their respective locking recesses  134 ,  136 ,  138 , and  140 . As will be described in more detail below, with reference to  FIGS. 6A-6D , the chamfer-recess combination is a mechanism that allows upper anchor  118  and lower anchor  120  to be locked to spacer  100  after deployment. It will be appreciated by those skilled in the art, after reading this disclosure, that locking recesses  134 ,  136 ,  138 ,  140  could be detents in some embodiments and through-holes in other embodiments. 
     Proximal portion  112  further comprises lateral surfaces  142  and  144  that are respectively constructed with gripper recesses  146  and  148 . The gripper recesses are dimensioned and arranged to receive corresponding ribs of an implantation instrument employed by a surgeon. The ribs are adapted to fit squarely into their corresponding recesses so that spacer  100  can be securely gripped by the surgeon. It should be noted that gripping the spacer with an implantation instrument serves at least two purposes. First, it enables the surgeon to more easily orient spacer  100  in a desired position within the narrow disc space of the adjacent vertebras. Secondly, it prevents spacer  100  from coming free from the implantation instrument while the surgeon is impacting the upper and lower anchors with an anchor driver. Although each of the lateral surfaces is depicted as having three gripping recesses, it will be appreciated by those skilled in the art that each of the lateral surfaces can have more or less gripper recesses than depicted. This feature of the present invention will be described in more detail below, with reference to  FIGS. 5A-5D . 
       FIGS. 3A and 3B  are perspective views of an anchor in accordance with an illustrative embodiment of the present invention. Since upper anchor  118  and lower anchor  120  have substantially the same physical and functional characteristics, thus being interchangeable, the following discussion of  FIGS. 3A and 3B  will use the word “anchor” to describe both the upper and lower anchors. Further, it should be noted that upper anchor  118  and lower anchor  120  (whether formed as independent pieces or as a single unitary piece) collectively define an anchoring device. 
       FIG. 3A  depicts the surface of an anchor that is adapted to slide along an inclined surface of a guide (e.g., upper inclined surface  122  or lower inclined surface  126 ). In the illustrative embodiment, the anchor is constructed to have a curved or semi-curved surface that is contoured to be substantially the same as the inclined surface of the guide it slides on. The surface of the anchor is preferably smooth throughout its length in order to reduce the amount of friction drag produced when the surface slides along the inclined surface. 
     The anchor also comprises a pair of oppositely positioned lateral sides  302  and  304 , which are adapted to slide into their respective lateral recesses (e.g., upper lateral recesses  124  or lower lateral recesses  128 ). The anchor is also constructed with a pair of flexible prongs  306  and  308 , which respectively comprises lateral projections  310  and  312 . The flexible prongs and lateral projections work in cooperation to lock the anchor to spacer  100  in a deployed position. The lateral sides, flexible prongs, and lateral projections of the anchor are also depicted in  FIG. 3B . 
     To enable the anchor to penetrate a vertebral body, distal portion  314  of the anchor is tapered to form an edge. Since the anchor is made of titanium alloy, the distal portion of the anchor is sufficiently strong to pierce and penetrate through the endplate of the vertebral body. Although the anchor is preferably formed from titanium alloy, other biocompatible materials (e.g., polyetheretherketone (PEEK), other surgical grade metals, alloys, or a combination thereof) can be used to form the anchor. 
     It will be clear to those skilled in the art that the foregoing discussion of  FIGS. 3A and 3B  applies to both upper anchor  118  and lower anchor  120 . 
       FIG. 4A  depicts upper anchor  118  and lower anchor  120  being loaded into spacer  100 . As discussed above, the upper guide of spacer  100  has an upper pair of oppositely positioned lateral recesses  124 . Each lateral recess  124  is adapted to receive a respective one of lateral sides  302  and  304  of upper anchor  118 . Similarly, the lower guide of spacer  100  has a lower pair of oppositely positioned lateral recesses  128  (shown more clearly in  FIG. 1B ). Each lateral recess  128  is adapted to receive a respective one of lateral sides  302  and  304  of lower anchor  120 . Turning now to  FIG. 4B , this figure depicts spacer  100  loaded with the upper and lower anchors. In  FIG. 4B , upper anchor  118  and lower anchor  120  are in an undeployed state and are disposed entirely within spacer  100 . That is, no part of upper anchor  118  and lower anchor  120  extend beyond the profile of teeth  116  arranged on spacer  100 . In the loaded/undeployed state, spacer  100  is ready to be gripped by an implantation instrument for simultaneous deployment into their respective intervertebral bodies. 
       FIG. 5A  is a perspective view of implantation instrument  500 , which comprises, inter alia, housing  502 , anchor driver  504 , handle  506 , and a pair of oppositely positioned grippers  508  and  510 . As will be discussed in more detail below, with reference to  FIGS. 5B-5D , anchor driver  504  can be advanced forwards or retracted backwards via handle  506  to respectively grip or release spacer  100 . 
       FIG. 5B  is a cross-sectional view of the implantation instrument of  FIG. 5A . As shown in this view, housing  502  is divided into two sections—namely, a narrower section  512  and a wider section  514 . Anchor driver  504  is constructed to fit squarely into narrower section  512  with little or no lateral and radial movement, while the area of wider section  514  is dimensioned to accommodate the width of anchor driver  504  and a pair of adjacently positioned, oppositely bowed leaf springs  516  and  518 . 
     In the configuration depicted in  FIG. 5B , anchor driver  504  can be advanced forwards towards leaf springs  516  and  518  via handle  506 . As the forward advancement causes anchor driver  504  to be wedged between leaf springs  516  and  518 , their respective grippers  508  and  510  will begin to simultaneously pivot inward to clamp onto the lateral surfaces of spacer  100 . 
     More precisely, and with reference to  FIG. 5C , the forward advancement of anchor driver  504  causes gripper  508  to pivot inwardly about pivot point  520 . This pivot action is a result of leaf spring  516  being compressed outwards towards the wall of housing  502  as anchor driver  504  engages the bowed portion of leaf spring  516 . As gripper  508  pivots inwards, ribs  524  engage their respective gripper recess  146  (depicted in  FIG. 1A ) arranged on spacer  100 . Likewise, gripper  510  will pivot inwardly about pivot point  522  in response to the forward advancement of the driver, resulting in ribs  526  engaging their respective gripper recess  148  (depicted in  FIG. 1B ). By means of the foregoing, spacer  100  can be securely gripped by implantation instrument  500 , as depicted in  FIG. 5D . 
     As depicted in  FIG. 5D , the head of anchor driver  504  stops at or slightly before the distal end of housing  502  after gripping spacer  100 . While spacer  100  is being gripped by implantation instrument  500 , spacer  100  is positioned within the narrow disc space between adjacent vertebras. Continuing to grip spacer  100  with implantation instrument  500 , the surgeon removes cap  530  and is now ready to impact handle  506  with a weighted object (e.g., hammer, mallet, etc.). In accordance with the illustrative embodiment, cap  530  has two functionalities. First, cap  530  when attached to handle  506  disallows forward movement of anchor driver  504  past a certain point—namely, the distal end of housing  502 . Secondly, cap  530  prevents inadvertent deployment of upper anchor  118  and lower anchor  120  during positioning of spacer  100  within the adjacent vertebral bodies. 
     When the surgeon impacts handle  506  with a weighted object, anchor driver  504  is driven forwards into the proximal portion of upper anchor  118  and lower anchor  120 , thereby simultaneously deploying the anchors into their respective vertebras. The surgeon may impact handle  506  one or more times so that the anchors reach a desired depth within their vertebras, and so that the anchors engage the locking feature of the present invention described in more detail below. Once upper anchor  118  and lower anchor  120  is locked to spacer  100  in the deployed position, the surgeon can retract anchor driver  502  so that leaf springs  516  and  518  can return to their relaxed state. While returning to their relaxed state, grippers  508  and  510  will begin to pivot outwardly to disengage from their gripper recesses, thereby releasing spacer  100 . 
       FIG. 6A  depicts a perspective view of implantation instrument  500  in which driver anchor  504  has simultaneously deployed upper anchor  118  and lower anchor  120 . As discussed above, the head of anchor driver  504  is simultaneously driven into the proximal portion of upper anchor  118  and lower anchor  120  as the surgeon impacts handle  506 . This causes both the upper anchor  118  and lower anchor  120  to independently slide along the upper inclined surface  122  and lower inclined surface  126 , respectively. The upper and lower inclined surfaces respectively press against the surface of the upper and lower anchors (i.e., the surface depicted in  FIG. 3A ) to deploy the anchors into their respective vertebral bodies.  FIGS. 6B-6D  depict upper anchor  118  and lower anchor  120  simultaneously deployed after being impacted by anchor driver  504 . As shown in these figures, the distal ends of upper anchor  118  and lower anchor  120  in the deployed state are radially extended outside of spacer  100 . That is, the distal ends of upper anchor  118  and lower anchor  120  extend past the height of teeth  116  of spacer  100  after being deployed. 
     From the foregoing discussion, it will be clear to those skilled in the art that upper anchor  118  and lower anchor  120  are separate elements that slide independently of each other along their respective upper and lower guides. It will also be clear from the foregoing discussion that an advantage of using the upper and lower anchors of the present invention is that they provide additional anchorage for stabilizing a spacer. In other words, not only is the spacer anchored to the intervertebral bodies via its teeth, the spacer is also provided with additional anchorage by the upper and lower anchors, since they extend past the profile of the teeth and therefore penetrating deeper into the intervertebral bodies. 
     Returning to  FIGS. 6C and 6D , these figures depict upper anchor  118  and lower anchor  120  locked to spacer  100  in a deployed position. Since upper anchor  118  and lower anchor  120  are locked to spacer  100  in substantially the same way, the following discussion of  FIGS. 6C and 6D  will use the word “anchor” to describe both the upper and lower anchors. 
     As the anchor is impacted by driver  504 , lateral projections  310  and  312  will respectively engage the sloping edge of lateral chamfers  130  and  132 . Lateral chamfers  130  and  132  are depicted in the figures as being arranged proximally to locking recesses  134 ,  136 ,  138 , and  140  of spacer  100 . The pressure and force of the impact causes flexible prongs  306  and  308  to flex laterally inwardly. As lateral projections  310  and  312  past their respective lateral chamfers, flexible prongs  306  and  308  will return to a relaxed state, thereby causing lateral projections  310  and  312  to laterally extend into their corresponding locking recess  134 ,  136 ,  138 , and  140 . This locking feature of the present invention prevents the anchors from disengaging from spacer  100  after being deployed into the vertebral bodies. 
     It will be clear to those skilled in the art, after reading this disclosure that numerous modification can be made to the illustrative embodiment without departing from the scope of the invention. For example, in one alternative embodiment, upper anchor  118  and lower anchor  120  can be constructed as a single unitary piece.  FIGS. 7A-7C  depict such an anchoring device. 
     As depicted in  FIG. 7A , upper anchor  702  of anchoring device  700  comprises underside  704  that is adapted to press against upper inclined surface  706  of the upper guide arranged on spacer  100 . Similarly, lower anchor  708  of anchoring device  700  comprises underside  710  that is adapted to press against lower inclined surface  712  of the lower guide arranged on spacer  100 . As anchoring device  700  is advanced forwards, pressure causes the undersides to press against their respective inclined surfaces, which guides upper anchor  702  and lower anchor  708  to radially and simultaneously deploy into their respective vertebral bodies. As depicted in  FIGS. 7B and 7C , upper anchor  702  and lower anchor  708  extend past the profile of teeth  714  to provide additional anchorage. Once the upper and lower anchors have been simultaneously deployed into their vertebra, locking cap  716  can be used to lock the anchors in their deployed position. Specifically, locking cap  716  is adapted to press the proximal end of anchoring device  700  to lock the anchoring device to spacer  100 . 
     In another embodiment, as depicted in  FIGS. 8A-8C , spacer  100  houses both upper anchor  802  and lower anchor  804 . In other words, both the upper and lower anchors are disposed entirely within spacer  100  when the anchors are in a relaxed state. As shown in  FIG. 8B , an internal drive screw  806  (i.e., an anchor drive) can be turned so that wedge  812  can be advanced forwards towards the bowed portion of both upper anchor  802  and lower anchor  804 . Wedge  812  is forcibly advanced towards the bowed portion to simultaneously force upper anchor  802  and lower anchor  804  to extend through an opening arranged on superior surface  808  and inferior surface  810  of spacer  100 . More precisely, as drive screw  806  is turned, wedge  812  abuts against the bowed portion of upper anchor  802  and lower anchor  804 . As wedge  812  abuts against the bowed portion of the anchors, the inclined surface of wedge  810  slides along the surface of upper anchor  802  and lower anchor  804 . The sliding motion applies pressure to the surfaces of the anchors, thereby forcing both upper anchor  802  and lower anchor  804  to radially extend outside of the openings of spacer  100  and into their respective intervertebral bodies. 
     In a further embodiment, as depicted in  FIGS. 9A-9H , the anchoring device has a drive plate  906  from which upper anchor  902  and lower anchor  904  extend. 
     The drive plate of  FIG. 9A  includes through-hole  908  arranged at its central axis. The drive plate can be divided into four quadrants, with through-hole  908  being the origin point, like in a two-dimensional Cartesian plane. Upper anchor  902  extends from a first one of the quadrants (e.g., Quadrant I in a two-dimensional Cartesian plane), while lower anchor  904  extends from a second one of the quadrants (e.g., Quadrant III in the two-dimensional Cartesian plane), wherein the first and second quadrants are diagonally located from each other on drive plate  906 . Although the anchors have been described as having a specific arrangement on drive plate  906 , it will be clear to those skilled in the art after reading this disclosure that upper anchor  902  and lower anchor  904  can be arranged anywhere on the drive plate without departing from the scope of the present invention. 
     As further depicted in  FIG. 9A , each of upper anchor  902  and lower anchor  906  has a pointed tip and a plurality of projections arranged on their lateral surfaces. The plurality of projections can be, for example, and without limitation, barbs that are angled away from the point in which the anchors penetrate into their respective vertebras. The barbs are advantageous because they make it difficult for the anchors to come loose, thus ensuring that the spacer is securely stabilized between the vertebras after implantation.  FIG. 9A  also depicts a pair of oppositely positioned grippers of holder  910  gripping onto the lateral surfaces of drive plate  906 . 
     Turning now to  FIG. 9B , while drive plate  906  is gripped by holder  910 , a surgeon can position the grippers of holder  910  to also grip onto endplate  912  of spacer  900 . Once endplate  912  is gripped by the surgeon, a driver  914  can be inserted into holder  910 , which passes through through-hole  908  of drive plate  906 . The driver engages one end of drive screw  916  (shown in  FIG. 9C ) housed within spacer  900 . Once the driver has engaged the drive screw, the surgeon can turn driver  914  so that drive screw  916  can be threaded into the body of wedge  918 . This causes wedge  918  to move backwards towards the proximal end of spacer  900 , which in turn causes superior surface  920  and inferior surface  922  of the spacer to slide along the inclined surface of wedge  918 . This can be seen more clearly in  FIGS. 9C and 9D . As superior surface  920  and inferior surface  922  radially extend in opposite directions of each other, upper anchor  902  and lower anchor  904  engage upper guide  924  and lower guide  926  of spacer  900 . As shown in  FIG. 9D , the tips of upper anchor  902  and lower anchor  904  do not extend past the profile of teeth  928  of spacer  900 , even after superior surface  920  and inferior surface  922  have been fully extended. 
     Once the superior and inferior surfaces of spacer  900  have been fully extended, the surgeon can now retract driver  914  and insert pull screw  930  (i.e., anchor driver) as shown in  FIG. 9E . Pull screw  930  is physically adapted to be inserted through through-hole  908  and into the threaded hole of drive screw  916 . Pull screw  930  can now be threaded to advance drive plate  906  towards the proximal end of spacer  900 , which causes upper anchor  902  and lower anchor  904  to respectively slide along upper guide  924  and lower guide  926  as the drive plate is advanced towards the proximal end of the spacer. As upper anchor  902  and lower anchor  904  slide along their respective guides, the anchors simultaneously and radially extend away from spacer  900  and into their respective intervertebral bodies. Pull screw  930  is threaded by the surgeon until drive plate  906  is fully seated against endplate  912 . Not only does threading pull screw  930  in this way fully deploy the anchors into their respective intervertebral bodies, it also locks the anchors to spacer  900  in a deployed position, as shown in  FIGS. 9F-9H . 
       FIG. 10  depicts a spacer-anchor combination in accordance with an alternative embodiment of the present invention. More specifically, the figure depicts spacer  1000 , a plurality of upper anchors  1002 , worm  1004 , and gear  1006 . In accordance with this embodiment, the worm is physically adapted to turn the gear, but the gear cannot turn the worm. This is because the angle on the worm is so shallow that, when the gear tries to spin it, the friction between the gear and the worm holds the worm in place. With this in mind, a surgeon can implant spacer  1000  in the disc space of adjacent vertebras. The surgeon can then use a tool to turn worm  1004  in order to rotate gear  1006  in a particular direction. As the gear rotates, upper anchors  1002  are simultaneously deployed into an intervertebral body. Once deployed, pressure from adjacent vertebras compressing down onto gear  1006  will not cause the gear to rotate. This is because, as discussed above, the angle on the worm is so shallow that the friction between the gear and the worm essentially locks the worm in place. Accordingly, upper anchors  1002  will be locked in their deployed position until worm  1004  is operated. 
     It is to be understood that the disclosure describes a few embodiments and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure and that the scope of the present invention is to be determined by the following claims.