Patent Publication Number: US-2015066042-A1

Title: Minimally invasive devices and methods for delivering fixation devices and implants into a spine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority benefit of U.S. Provisional Application Nos. 61/590,789, 61/591,248, and 61/653,853, entitled “MINIMALLY INVASIVE DEVICES AND METHODS FOR DELIVERING FIXATION DEVICES AND IMPLANTS INTO A SPINE,” filed Jan. 25, 2012, Jan. 26, 2012, and May 31, 2012, respectively, the entireties of all of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     1. Field 
     The present application relates to devices and methods for providing spinal stabilization. In particular, the present application relates to minimally invasive devices and methods for delivering fixation devices and implants into a spine. 
     2. Description of the Related Art 
     Spinal bone degeneration can occur due to trauma, disease or aging. Such degeneration can cause abnormal positioning and motion of the vertebrae, which can subject nerves that pass between vertebral bodies to pressure, thereby causing pain and possible nerve damage to a patient. In order to alleviate the pain caused by bone degeneration, it is often helpful to maintain the natural spacing between vertebrae to reduce the pressure applied to nerves that pass between vertebral bodies. 
     To maintain the natural spacing between vertebrae, spinal stabilization devices are often provided to promote spinal stability. These spinal stabilization devices can include fixation devices, such as spinal screws, which are implanted into vertebral bone. The fixation devices work in conjunction with other implanted members, such as rod members, to form stabilization systems. 
     Conventional stabilization systems often require open surgeries and other invasive procedures in order to deliver the implants into the body. These invasive procedures often cause a great deal of pain and trauma to the patient, and require a substantial recovery time. Thus, there exists a need for minimally invasive devices and methods that can assist in providing spinal stabilization. 
     SUMMARY 
     In some embodiments, a minimally invasive spine surgery access device includes an elongate body having a proximal end, a distal end, and an inner lumen extending there through. The access device includes a threaded portion proximate the proximal end, two grasping elements at the distal end, and a longitudinal slot along at least one side of the elongate body. In some embodiments, the access device includes a spring latch attached to the elongate body. The spring latch has a tab that extends into the threaded portion and is moveable between an elevated position and a depressed position. The access device further includes a hollow and internally threaded lock nut configured to thread onto the threaded portion. An inner perimeter of a distal end of the lock nut includes a series of ramps configured to engage the spring latch tab to inhibit counter rotation of the lock nut. Advancing the lock nut distally on the threaded portion causes compression of the grasping elements, and movement of the spring latch tab to the depressed position disengages the tab from the lock nut ramps to permit counter rotation of the lock nut. 
     In some embodiments, a minimally invasive spine surgery access device includes an elongate inner sleeve and elongate outer sleeve. The inner sleeve includes an externally threaded portion at a proximal end and a pair of distal slots located on opposite sides of the inner sleeve extending proximally from a distal end at least partially along a length of the inner sleeve. The outer sleeve slidably receives the inner sleeve, and the outer sleeve includes a pair of distal slots located on opposite sides of the outer sleeve and extending proximally from a distal end at least partially along a length of the outer sleeve. When assembled, the distal slots of the outer sleeve are substantially aligned with the distal slots of the inner sleeve. The outer sleeve also includes a proximal slot extending from a proximal end of the outer sleeve at least partially along a length of the outer sleeve. The distal end of the inner sleeve includes two grasping elements that extend distally past the distal end of the outer sleeve and include protrusions or pins configured to grasp a spinal implant. Proximal movement of the inner sleeve relative to the outer sleeve causes compression of the grasping elements, and distal movement of the inner sleeve relative to the outer sleeve causes expansion of the grasping elements. The access device includes a spring latch attached to the inner sleeve and positioned within the proximal slot of the outer sleeve. A distal portion of the spring latch is attached to the inner sleeve, and a proximal end of the spring latch includes a tab and a button distal to the tab. The proximal end of the spring latch is moveable between and an elevated position and a depressed position. The access device further includes a hollow and internally threaded lock nut configured to thread onto the threaded portion of the inner sleeve. Threading of the lock nut onto the threaded portion causes the lock nut to engage the proximal end of the outer sleeve and move the inner sleeve proximally relative to the outer sleeve to cause compression of the grasping elements. An inner perimeter of a distal end of the lock nut includes a series of ramps having shoulders of about 90°, and the ramp shoulders are configured to engage the spring latch tab to inhibit counter rotation of the lock nut. The button on the spring latch is moveable to its depressed position to disengage the spring latch tab from the lock nut ramps to permit counter rotation of the lock nut and to permit distal movement of the inner sleeve relative to the outer sleeve. The relative positions of the inner and outer sleeves are in a locked configuration when the lock nut is engaged with the spring latch tab and the grasping elements are in a compressed configuration, and the relative positions of the inner and outer sleeves are in an unlocked configuration when the ramps of the lock nut are not engaged with the spring latch tab and the inner and outer sleeves are moveable relative to one another to permit compression and expansion of the grasping elements. 
     In some embodiments, a screw delivery device includes a shaft, a grip, a tip configured to engage a portion of a bone screw, and a saddle configured to engage another portion of a screw assembly to help distribute load during tightening of the bone screw. 
     In some embodiments, a screw delivery device includes an inner elongate shaft and an outer elongate shaft. The inner elongate shaft has a proximal end and a distal end, the distal end of the inner shaft including a hexalobe tip configured to engage a first portion of a spinal screw and a saddle proximal to the hexalobe tip configured to engage a second portion of the spinal screw. The outer elongate shaft is provided over the inner elongate shaft and is axially and rotationally moveable relative the inner shaft. The outer shaft has a proximal end and a distal end, the proximal end of the outer shaft including a grip and the distal end of the outer shaft including an externally threaded portion configured to engage an internally threaded third portion of the spinal screw. 
     In some embodiments, a system includes a screw delivery device and a screw assembly. The screw assembly includes, a housing having a lower opening and internal threads configured to receive an externally threaded set screw, a threaded shaft having an enlarged head, the threaded shaft extending through the lower opening and the enlarged head sitting adjacent the lower opening, and a rod receiving surface within the housing having a concave shape configured to receive a rod. The threaded shaft is rotatable relative to the housing prior to insertion of the set screw. The hexalobe tip of the screw delivery device is configured to engage a corresponding opening in the enlarged head of the screw assembly, the saddle is configured to engage the rod receiving surface, and the externally threaded portion at the distal end of the outer shaft is configured to engage the internal threads of the housing. 
     In some embodiments, a method of delivering a screw assembly to a spinal location includes positioning a screw assembly at a desired spinal location. The screw assembly includes a housing having a lower opening and internal threads configured to receive an externally threaded set screw, a threaded shaft having an enlarged head, the threaded shaft extending through the lower opening and the enlarged head sitting adjacent the lower opening, and a rod receiving surface within the housing having a concave shape configured to receive a rod. The threaded shaft is rotatable relative to the housing prior to insertion of the set screw. The method includes engaging a screw delivery device with the screw assembly at the spinal location. The screw delivery device includes an inner elongate shaft having a proximal end and a distal end, the distal end of the inner shaft comprising a hexalobe tip configured to engage a corresponding opening in the enlarged head of the screw assembly and a saddle proximal to the hexalobe tip configured to engage the rod receiving surface and an outer elongate shaft provided over the inner elongate shaft, the outer shaft being axially and rotationally moveable relative the inner shaft, the outer shaft having a proximal end and a distal end, the proximal end of the outer shaft comprising a grip and the distal end of the outer shaft comprising an externally threaded portion configured to engaged the internal threads of the housing. The method further includes rotating the outer shaft relative to the inner shaft while the hexalobe tip engages the enlarged head of the screw assembly to cause the externally threaded portion at the distal end of the outer shaft to engage the internal threads of the housing and rotating the screw delivery device while the inner and outer shafts remain relatively locked in position to drive the threaded shaft into the spinal location, said rotation rotating the hexalobe tip against the opening in the enlarged head of the screw assembly with the saddle distributing load to the rod receiving surface during said rotation. 
     In some embodiments, a rod insertion device includes an elongate, generally cylindrical shaft and a gripping end at a distal end of the shaft. The gripping end includes two or more gripping elements configured to grip a rod member. The device further includes a handle at a proximal end of the shaft and an actuator member configured to cause relative movement between the shaft and gripping end. Distal movement of the shaft or proximal movement of the gripping end causes compression of the gripping elements. In some embodiments, the gripping elements include serrations configured to mate with serrations on the rod member. 
     In some embodiments, a rod for use for spinal fixation includes an elongate cylindrical member having a first end and a second end, wherein one of the ends comprises a plurality of serrations. 
     In some embodiments, a method of delivering a rod to a spinal location includes providing a rod having a serrated portion and providing a rod insertion device that includes an elongate, generally cylindrical shaft having a gripping end at a distal end of the shaft and a handle at a proximal end of the shaft. In some embodiments, the gripping end includes two or more gripping elements configured to grip the rod member and the gripping elements include serrations. The method further includes grasping the rod with the rod insertion device by engaging the serrations of the gripping elements and serrated portion of the rod and delivering the rod to the spinal location with the rod insertion device. 
     In some embodiments, a rod reducer includes a cannula having a proximal end and a distal end and a slot at the distal end configured to interact with a rod member. An internally threaded portion is coupled to the proximal end of the cannula so that the cannula can remain substantially rotationally fixed while the threaded portion rotates. 
     In some embodiments, a method of positioning a rod member within a screw assembly at a spinal location includes providing a rod reducer that includes a cannula having a proximal end and a distal end, an internally threaded portion coupled to the proximal end, and a slot at the distal end configured to interact with a rod member. The method includes advancing the rod reducer over a tower access device having a distal end positioned at the spinal location and a proximal end positioned outside of the patient, the proximal end of the tower access device having an externally threaded portion, engaging a rod member positioned within the screw assembly with the slot at the distal end of the rod reducer, and distally threading the internally threaded portion of the rod reducer to the externally threaded portion of the tower access device by rotating the internally threaded portion, wherein said distal threading urges the rod into engagement with the screw assembly, wherein said cannula and the slot remain rotationally fixed relative to the rod member as the internally threaded portion is rotated. 
     In some embodiments, a hinge for connecting two or more spinal access tower devices includes at least two clamps configured to engage two tower access devices. Each clamp includes a pivot having a spherical outer surface and a cylindrical bore therethrough, the cylindrical bore configured to receive a tower device. Each clamp also includes a clamshell clamp having a cylindrical outer surface and spherical inner surface circumferentially surrounding the pivot and a clamp tightener configured to tighten the clamshell clamp around the pivot. In use, the spherical outer surfaces of the pivots allow for pivotal movement relative to the clamshell clamps. The hinge further includes a ratchet bar coupling the two clamps and a tightening knob configured to lock the ratchet bar to fix the lateral spacing between the clamps. 
     In some embodiments, a method for connecting two or more spinal access tower devices includes positioning at least two clamps around at least two adjacent tower devices, respectively. In some embodiments, each clamp includes a pivot having a spherical outer surface and a cylindrical bore therethrough, the cylindrical bore configured to receive a tower device and a clamshell clamp circumferentially surrounding the pivot, the clamshell clamp having a cylindrical outer surface and a spherical inner surface. The method further includes tightening the clamshell clamps around each of the pivots to secure the at least two adjacent tower devices, wherein the spherical outer surfaces of the pivots allow for pivotal movement of the tower devices relative to the clamshell clamps, and adjusting a lateral spacing between the clamps by moving one or more of the clamps along a ratchet bar coupling the two clamps. 
     In some embodiments, a system for fixation of the spine includes: one or more tower access devices having a proximal end configured to extend outside of a patient and a distal end configured to engage a spinal screw; a screw delivery device configured to be delivered through the one or more tower access devices and configured to tighten the screw, wherein the screw delivery device comprises a saddle configured to engage a portion of the screw to distribute load during tightening; a rod inserter configured to deliver a rod through the one or more tower access devices to the screw; and a threaded reducer configured to engage a tower access device, the threaded reducer having a slot configured to engage the rod while the tower access device engages the screw. In further embodiments, one or more dilation tubes configured to provide access to a portion of a patient&#39;s spine may be provided. In further embodiments, an awl and/or a tap configured to prepare the portion of the patient&#39;s spine to receive a screw may be provided. Further embodiments comprise a compressor, a distractor, and/or other devices described herein. Such devices can be provided in a single kit or a single tray. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an exploded view of an embodiment of a spinal stabilization system; 
         FIG. 2  shows a partially assembled view of the spinal stabilization system of  FIG. 1 ; 
         FIG. 3  shows a perspective view of an embodiment of a minimally invasive tower access device; 
         FIG. 4A  shows a side view of the minimally invasive tower access device of  FIG. 3 ; 
         FIG. 4B  shows an exploded view of the minimally invasive tower access device of  FIGS. 3 and 4A ; 
         FIG. 5  shows an embodiment of a spring latch of the minimally invasive tower access device of  FIG. 3 ; 
         FIG. 6  shows an embodiment of a lock nut of the minimally invasive tower access device of  FIG. 3 ; 
         FIG. 7  shows the spinal stabilization system of  FIG. 1  coupled to the minimally invasive tower access device of  FIG. 3 ; 
         FIGS. 8A-8D  show dilators having increasing dimensions according to embodiments of the present disclosure; 
         FIG. 9  shows an awl according to embodiments of the present disclosure; 
         FIG. 10  shows a tap according to embodiments of the present disclosure; 
         FIGS. 11A-11B  show perspective views of an embodiment of a screw delivery device; 
         FIG. 11C  shows an exploded view of the screw delivery device of  FIG. 11A ; 
         FIG. 11D  shows a detail view of a distal end of the screw delivery device of  FIG. 11A ; 
         FIG. 12A  shows a side view of an embodiment of a rod insertion device; 
         FIG. 12B  shows a perspective view of an outer shaft and handle of the rod insertion device of  FIG. 12A ; 
         FIG. 12C  shows a perspective view of an inner shaft of the rod insertion device of  FIG. 12A ; 
         FIG. 12D  shows a detailed view of a distal end of the inner shaft of  FIG. 12C ; 
         FIGS. 12E-12F  show views of a serrated rod; 
         FIG. 12G  shows a perspective view of the rod insertion device of  FIG. 12A  engaging the rod of  FIG. 12E ; 
         FIG. 12H  shows a detail view of a distal end of the rod insertion device of  FIG. 12A  engaging the rod; 
         FIGS. 13A-13B  show front perspective views of a threaded rod reducer according to embodiments of the present disclosure; 
         FIG. 13C  shows a front perspective view of a top portion of the threaded rod reducer of  FIGS. 13A-13B ; 
         FIG. 14  shows a perspective view of an embodiment of a set screw inserter; 
         FIG. 15  shows a perspective view of an embodiment of a hinge; 
         FIG. 16  shows a front view of an example caliper; 
         FIG. 17  shows a perspective view of an example compressor; 
         FIG. 18  shows a perspective view of an example distracter; 
         FIG. 19  shows a perspective view of an example final tightener; 
         FIG. 20  shows a dilation tube inserted over a K-wire; 
         FIG. 21  shows an awl inserted through a dilation tube and over a K-wire; 
         FIG. 22  shows a tap inserted through a dilation tube and over a K-wire; 
         FIG. 23  shows the minimally invasive tower access device of  FIG. 3  coupled to the spinal stabilization system of  FIG. 1  and inserted through a dilation tube over a K-wire; 
         FIG. 24  shows the screw delivery device of  FIGS. 11A and 11B  inserted through the assembly of  FIG. 23 ; 
         FIG. 25  shows the system of  FIG. 25  with the dilation tube removed; 
         FIG. 26  shows the rod insertion device of  FIG. 12A  coupled to a rod with the tower access device of  FIG. 3  coupled to the spinal stabilization system of  FIG. 1 ; 
         FIGS. 27-28  show rod reduction; 
         FIG. 29  shows set screw insertion; 
         FIG. 30  shows a pair of spinal stabilization system and tower access device assemblies coupled by a rod and hinge; 
         FIG. 31  shows compression; 
         FIG. 32  shows distraction; 
         FIG. 33  shows final tightening of the spinal stabilization systems. 
     
    
    
     DETAILED DESCRIPTION 
     The present application relates to minimally invasive devices and methods for assisting in the delivery of fixation devices and other implants to a target location in a patient. While the minimally invasive devices described herein can be used to assist various procedures, in some embodiments, they are used to assist in delivering fixation devices and other implants to help stabilize the spine. For example, a spinal stabilization system (e.g., including a bone fastening assembly such as a pedicle screw and an elongated connecting member or a rod) can be used to provide stability to two or more adjacent vertebrae. The bone fastener assembly is placed in each of the vertebrae to be stabilized, and the elongated connecting member or rod is coupled to the assemblies. 
     Spinal Stabilization System 
     As shown in  FIG. 1 , an example embodiment of a spinal stabilization system  100  can include a screw  110 , a housing  112 , a set screw  116 , and a rod  118 . The system  100  can further include one or more clamps  114 . The screw  110  can have a threaded shaft  120  configured to be secured to a vertebra and an enlarged semi-spherical head  122 . The screw  110  can be cannulated and configured to receive a guidewire or k-wire as discussed herein. In some embodiments, the housing  112  can be tulip-shaped and can have a U-shaped seat for receiving the rod  118 . The housing  112  can also include apertures or holes  124  on its outer surface for engaging a surgical access device as described in greater detail herein. The housing  112  can further include internal threads  126  at its upper end configured to receive the externally threaded set screw  116 . 
     To assemble the system  100 , the screw  110  and one or more clamps  114 , if present, are loaded into the housing  112 . When the system  100  is assembled, as shown in  FIG. 2 , the threaded shaft  120  extends through a lower opening in the housing  112 , with the enlarged head sitting adjacent the lower opening. In some embodiments, the enlarged head sits directly on a lower internal surface of the housing. An inner surface of the one or more clamps  114  engages the enlarged head  122  of the screw  110 , and an outer surface of the one or more clamps  114  engages an inner surface of the housing  112 . Upper surfaces of the one or more clamps  114  are concave in shape and may act as a rod receiving surface to engage the rod  118 . In some embodiments, the screw  110  can polyaxially rotate relative to the housing  112  prior to being fixed. Once the screw  110  has been inserted into the patient&#39;s vertebra, the rod  118  is inserted in the U-shaped seat of the housing  112  and engages an upper surface of the one or more clamps  114 , if present. The set screw  116  is inserted into the housing  112  to secure the rod  118  and fix the screw  110  relative to the housing  112 . 
     Additional details regarding example spinal screws are described in U.S. Patent Publication No. 2010/0241175, filed Jul. 28, 2009, entitled “Pedicle Screws and Methods of Using the Same,” the entirety of which is hereby incorporated by reference. Other types of screws, including various pedicle screws, can also be used with the devices and methods described herein. It will be appreciated that the clamps  114  can be replaced with a washer or other intervening member which may have a concave rod receiving surface configured to receive the rod as described above. 
     Minimally Invasive Tower Access Device 
     In some embodiments, a minimally invasive tower access device is provided. The access device includes an outer sleeve and an inner sleeve that telescopingly or slidably engage with one another. The inner sleeve includes one or more grasping elements that can grasp a fixation device (e.g., a spinal screw such as a pedicle screw) for delivery into a bone member of a spine. Once the access device is coupled to the spinal screw, the access device and spinal screw can be delivered either through an incision in an open surgery, or minimally invasively through a relatively smaller incision, such as percutaneously. Once through the incision, the spinal screw can be brought to a location proximate to a bone member where it can be inserted. 
     The access device can serve as a portal or opening that extends from the bone member to outside of the patient. Instruments can be delivered through the access device. For example, a screw delivery device can be provided through the access device to secure the spinal screw to the bone member. A number of additional instruments can be used with the access device to provide spinal stabilization, for example, a rod inserter, a threaded rod reducer, and a set screw inserter. In addition, implants can be delivered adjacent the side of the access device. For example, a rod implant can be delivered along the side of the access device which can connect in between the implanted screws. By using one or more access devices to deliver screws or other implants as described herein, a spinal stabilization system can be formed. The one or more access devices advantageously allow screws and other implants to be inserted in a specific location with ease, and allow for a surgeon to comfortably maintain external control of the screw from outside of a patient&#39;s body. 
     An example embodiment of a minimally invasive tower access device  200  is shown in  FIGS. 3 and 4A . As shown, the access device  200  includes an elongated outer sleeve  220  that slidably receives an elongated inner sleeve  210 . The inner sleeve  210  includes a spring latch  230 , shown in  FIG. 5 . A lock nut  240  engages a portion of the inner sleeve  210 . When all of the components are assembled as shown in  FIGS. 3 and 4A , they form an access device  200  that can be used to deliver fixation devices, for example, a spinal stabilization system  100  as shown and described herein, to a target location of a patient&#39;s spine via minimally invasive procedures. 
       FIG. 4B  illustrates an exploded view of various components (e.g., the outer sleeve, inner sleeve, and lock nut) of the tower access device of  FIGS. 3 and 4A . The outer sleeve  220  is a hollow cylindrical body extending from a proximal end (i.e., the end exposed during surgery) to a distal end (i.e., the end inside the patient&#39;s body during surgery) and is configured to receive the inner sleeve  210  therein. As shown, the outer sleeve  220  includes three slots, a proximal slot  222  formed on the proximal end of the outer sleeve  220  and a pair of distal slots  221  formed on the distal end of the outer sleeve  220 . In other embodiments, the outer sleeve  220  can include a single distal slot  221 . In the illustrated embodiment, the proximal slot  222  and distal slots  221  extend to the proximal and distal ends, respectively, of the outer sleeve  220 , and partially along the length of the outer sleeve  220 . The pair of distal slots  221  are located on opposite sides of the outer sleeve  220  In other embodiments, the proximal  222  and/or distal  221  slot can be formed within the body of the outer sleeve  220  rather than extending to the proximal or distal end. 
     In some embodiments, the proximal slot  222  opens along one side of the outer sleeve  220 , while the distal slots  221  opens along two sides of the outer sleeve  220  (as shown in  FIGS. 3-4B ). The proximal slot  222  can be located in between the two openings that form the distal slots  221 . While a center longitudinal axis of the proximal slot  222  is shown at about 90 degrees away from a center longitudinal axis of either of the distal slots  221  (as measured along the circumference of the outer sleeve  220 ), the proximal slot  222  can be located at any angle relative to the openings of the distal slots  221 , such as between 0 and 90 degrees. While the proximal slot  222  is smaller in both width and length than the distal slots  221  in the illustrated embodiment, the slots need not be limited to these relative dimensions. 
     Both the proximal slot  222  and the distal slots  221  of the outer sleeve  220  can serve particular functions. In some embodiments, the proximal slot  222  can serve to receive the spring latch  230 , which can be coupled to the inner sleeve  210 . The proximal slot  222  can work in conjunction with the spring latch  230  to identify the current mode of operation of the access device  200  (e.g., “locked” or “unlocked” mode) as best shown in  FIG. 3 . The spring latch  230  can include a marker that can help identify the particular mode of operation. With the spring latch  230 , the mode of operation of the access device  200  can be easily visible to the surgeon. In some embodiments, the distal slots  221  can serve to receive one or more stabilization implants therethrough. For example, in some embodiments, a stabilizing rod member, such as rod  118 , can be delivered along the side of the access device  220  and angled through one or both of the distal slots  221 . 
     The distal slots  221  of the outer sleeve  220  can have a length between 4 cm and 8 cm, or a length between 6 cm and 7 cm. In some embodiments, the lengths of the distal slots  221  are much longer (e.g., at least 5.5 cm) than slots in conventional access devices. In some embodiments, the lengths of the distal slots  221  of the outer sleeve  220  are between ⅓ and ¾, or approximately ½ in some instances, the length of the entire body of the outer sleeve. In some embodiments the distal slots  221  of the outer sleeve can be even longer, and can extend almost the entire length of the outer sleeve  220 . A longer slot advantageously allows a rod implant can be more easily delivered through the slot to provide spinal stabilization. In addition, providing a longer slot length makes the instrument lighter by removing material from the system. A challenge, however, is that with the longer slot, the sidewalls that form the slot may need to be stronger in order to withstand forces on the sidewalls in some embodiments. Accordingly, in some embodiments, the thickness of the sidewalls that form the longer distal slots  221  of the outer sleeve  220  are preferably increased relative to conventional sleeves to withstand forces on the sidewalls. In some embodiments, the thickness of the sidewalls that form the longer distal slots  221  are between about 0.05 cm and 0.4 cm, or between about 0.2 cm and 0.3 cm. 
     As shown in  FIG. 4B , the inner sleeve  210  is also a hollow cylindrical body extending from a proximal end to a distal end. The inner sleeve  210  can be slidably received within the outer sleeve  220 , and in some embodiments, can be secured in a position relative to the outer sleeve  220  by using the lock nut  240 . In some embodiments, a proximal end of the inner sleeve  210  includes an externally threaded portion  214 , and the lock nut  240  includes internal threads configured to threadingly engage the threaded portion  214 . In some embodiments, the inner sleeve  210  has an interior diameter of between about 0.5 cm and 2 cm, or between about 1.0 cm and 1.1 cm. 
     A distal section of the inner sleeve  210  includes distal slots  211  and a pair of compressible grasping elements  212 . Like the distal slots  221  of the outer sleeve  220 , the distal slots  211  of the inner sleeve  210  can open on two sides of the inner sleeve  210 . In some embodiments, the distal slots  211  of the inner sleeve  210  are approximately the same size (e.g., similar width and height) of the distal slots  221  of the outer sleeve  220 . One skilled in the art will appreciate that the dimensions of both the distal slots  221  of the outer sleeve  220  and slots  211  of the inner sleeve  210  can vary with respect to one another. The distal slots  211  of the inner sleeve  210  can be placed in part or in complete alignment with the distal slots  221  of the outer sleeve  220 . In some embodiments, when the distal slots  211  of the inner sleeve  210  is aligned with the distal slots  221  of the outer sleeve  220 , a rod implant that is delivered into the patient can pass through both of the slots. The rod implant can be angled through the slots such that each end of the rod implant makes contact with a screw head within the access device  200 . 
     The distal slots  211  of the inner sleeve  210  can have a length between about 4.0 cm and 8.0 cm, or between about 6.0 cm and 7.0 cm. In some embodiments, the length of the distal slots  211  is much longer (e.g., at least 5.5 cm) than slots in conventional access devices. In some embodiments, the length of the distal slots  211  of the inner sleeve  210  is between ⅓ and ¾, or approximately ½ in some instances, the length of a non-threaded body of the inner sleeve  210 . 
     In some embodiments, the pair of grasping elements  212  comprises a pair of compressible arms or tines for receiving a portion of a spinal stabilization system, such as housing  112  described herein. One skilled in the art will appreciate that the shape of the grasping elements  212  need not be limited to the description described herein. In some embodiments, the distance from one grasping element to another is slightly greater than the diameter of the hollow interior of the outer sleeve  220  in an uncompressed state. In these embodiments, in order for the inner sleeve  210  to be received through the proximal end of the outer sleeve  220 , the grasping elements  212  should be slightly compressed. When the grasping elements  212  exit the distal end of the outer sleeve  220 , the grasping elements  212  can return to their uncompressed state, thereby advantageously helping to secure the inner sleeve  220  to the outer sleeve  210  by limiting the inner sleeve  220  from unintentionally backing out of the outer sleeve  210 . 
     In some embodiments, the grasping elements  212  are flat, while in other embodiments (as shown in  FIG. 4B ), the grasping elements  212  can include some curvature so as to accommodate a screw head of a particular shape. In some embodiments, the grasping elements  212  include protruding members  216  that can be received in apertures  124  of the housing  112  to secure the housing  112  to the inner sleeve  210 . The protruding members  216  can be rigid or somewhat flexible, and are configured to be inserted into two or more holes or apertures  124  formed on the housing  112  upon compression of the grasping elements  212  of the inner sleeve  210 . While the protruding members  216  can have a smooth surface finish, in some embodiments, the protruding members  216  have a roughened surface finish that can provide a frictional force between the protruding members  216  and surfaces of the housing  112  that form the receiving apertures  124 . The protruding members  216  can have a cross-sectional area that is circular, rectangular, trapezoidal or any other shape, so long as they are securely receivable in a corresponding aperture of the housing  112 . In some embodiments, in addition to or as an alternative to the protruding members  216  configured to be received in apertures  124  of the housing  112 , the grasping elements  212  include one ore more pins  218  configured to be received in one or more holes  124  in the housing  112 , for example as shown in  FIG. 7 . In some embodiments, rather than have protruding members that resemble pins, the inner sleeve  210  can include flanges that extend from a bottom surface of the distal end of the inner sleeve  210 . The flanges can be compressible such that when compressed, the flanges surround and secure a portion of the housing  112  (such as a bottom portion), thereby coupling the inner sleeve  210  to the housing  112 . 
     The inner sleeve  210  can be slidably received in the outer sleeve  220  such that the grasping elements  212  of the inner sleeve  210  can extend beyond the distal end of the outer sleeve  220 . In some embodiments, the inner sleeve  210  can be slidably received in the outer sleeve  220  such that in a first position, the grasping elements  212  are uncompressed. Sliding the inner sleeve  210  relative to the outer sleeve  220  in a second position can result in compression of the grasping elements  212 . For example, the outer sleeve  220  can be slid down the inner sleeve  210  such that the distal end of the outer sleeve  220  helps to compress the grasping elements  212 . In some embodiments, the outer sleeve  220  can completely cover the grasping elements  212  to compress the grasping elements, while in other embodiments, the outer sleeve  220  only covers a portion of the grasping elements  212  to cause compression. The compression mechanism provided by the outer sleeve  220  sliding over the compressible grasping elements  212  of the inner sleeve  210  is advantageous over conventional screw delivery devices, as the body of the outer sleeve  220  helps to reduce the risk of the protruding members  212  becoming accidentally loose from the housing  112 . Moreover, having a slidably engaged outer sleeve  220  and inner sleeve  210  reduces the need for extra tools that might be used in conventional screw delivery devices for securing an access device to a screw member. 
     The inner sleeve  210  can also include a spring latch member  230 . The spring latch  230  can include a button  232  and tab member  234  as shown in  FIG. 5 . In the illustrated embodiment, the tab member  234  is shaped as a ramp having a shoulder of about 90 degrees. The button  232  may include the marker described above for indicating a locked or unlocked position. The spring latch  230  can also include holes  236  for receiving one or more fixation members (e.g., screws) for attaching the spring latch  230  to the inner sleeve  210 . As illustrated in  FIG. 4B , the spring latch is fixed to the inner sleeve at or near a distal end of the spring latch with a pair of screws. At the proximal end of the spring latch, the button  232  and the tab member  234  are moveable between an elevated position with respect to the outer wall of the inner sleeve  210 , and a depressed position where the button  232  and tab  234  come closer to the outer wall. This movement may be accommodated by providing a recess in the outer wall where the proximal end of the spring latch is located. The tab member  234  extends at least partially into the threaded portion  214 . The button  232  can be sized and shaped to be easily depressed by a user using a thumb or other finger. 
     The spring latch  230  can serve multiple functions. In some embodiments, the spring latch  230  (when fixed to the inner sleeve  210 ) can fit within the proximal slot  222  of the outer sleeve  220  and can serve to identify the current mode of operation of the access device  220  when the inner sleeve  210  and outer sleeve  220  are slid relative to one another. For example, the spring latch  230  can include a marker on the button  232  that can identify when the outer sleeve and inner sleeve are in an “unlocked” position in which the two sleeves remain slidable relative to one another. Additionally, alignment of the proximal slot  222  with the spring latch  230  can advantageously promote proper placement of the outer sleeve  220  relative to the inner sleeve  210  so that the distal slot  221  of the outer sleeve  221  aligns with the distal slot  211  of the inner sleeve  210 . 
     In some embodiments, the tab member  234  of the spring latch can interact with lock nut  240  when the lock nut  240  is rotated to a distal section of the external threaded portion  214  of the inner sleeve  210 . For example, the tab member  234  can interact with an inner engagement surface  244  of the lock nut  240  to advantageously limit unintentional counter or back rotation of the lock nut  240  when in use. As discussed above, the button  232  and tab member  234  of the spring latch are moveable between an elevated position and a depressed position. In the elevated position, the tab member  234  engages the inner engagement surface  244  of the lock nut  240 . In the depressed position, the tab member  234  releases from the inner engagement surface  244 . In some embodiments, for example as shown in  FIG. 6 , the inner engagement surface  244  of the lock nut  240  is shaped as a ramp having a shoulder of about 90 degrees to correspond to the shape of the tab member  234 . 
     In use, the lock nut  240  is used to secure the inner sleeve  210  and outer sleeve  220  in a locked mode by threading the lock nut  240  onto the threaded portion  214  of the inner sleeve  210  until the inner engagement surface  244  engages the tab member  234 . The lock nut  240  engages the proximal end of the outer sleeve, such that further rotation of the lock nut  240  moves the inner sleeve proximally relative to the outer sleeve. This causes compression of the grasping elements. In the locked mode, the inner sleeve  210  is secured in position relative to the outer sleeve  220 , and the grasping elements  212  of the inner sleeve  210  are compressed about a fixation device, for example as shown in  FIG. 7 . The two sleeves are locked together because the interaction between the inner engagement surface  244  and tab member  234  advantageously limits unintentional counter or back rotation of the lock nut  240  in use. To release the lock nut  240 , the surgeon or other user depresses the button  232  which disengages the tab member  234  from the ramps of the inner engagement surface  244 . This permits counter-rotation of the lock nut to permit distal movement of the inner sleeve relative to the outer sleeve. When the ramps of the lock nut and the spring latch tab are not engaged, the inner and outer sleeves are moveable relative to one another to permit compression and expansion of the grasping elements. 
     Additional details regarding example tower access devices having features that may be incorporated in the devices above, and their methods of manufacture and use, can be found in U.S. Patent Publication No. 2012/0022594, filed Jul. 26, 2010, entitled “Minimally Invasive Surgical Tower Access Devices and Related Methods,” the entirety of which is hereby incorporated by reference. 
     Dilation Tubes 
     Various instruments can be used to prepare an insertion site in a patient&#39;s spine for implants such as spinal stabilization system  100  as described herein. For example, minimally invasive surgical procedures often make use of dilators to gradually enlarge a working channel.  FIGS. 8A-8D  show example embodiments of improved dilation tubes  300  having increasing dimensions.  FIGS. 8A-8D  show an  8 mm dilation tube  300   a,  13 mm dilation tube  300   b,  18 mm dilation tube  300   c , and 24 mm dilation tube  300   d , respectively. The dilation tubes  300  include a shaft  302  and two load bearing portions  304   a, b . In some embodiments, the load bearing portions  304   a, b  are integrally formed with the shaft  302 . A dilation tube  300  can be made by starting with a hollow cylindrical tube having an inner diameter that is the desired inner diameter of the final tube  300  and an outer diameter approximately the same or greater than the final outer diameter desired for the load bearing portions  304   a, b . The tube can then be cut, shaved, or otherwise reduced in diameter to achieve the final desired dimensions. Alternatively, in some embodiments, the load bearing portions  304   a, b  are collars removably or permanently coupled to the shaft  302 . In some embodiments, the load bearing portions  304   a, b  are contoured, for example as shown in  FIGS. 8A-8D , to enhance the user&#39;s grip. As illustrated, the load bearing portions  304   a ,  304   b  may have a plurality of parallel, longitudinal grooves extending along their lengths. In the illustrated embodiment, a distal end of the distalmost load bearing portion  304   b  transitions to a more narrow distal tip, which can facilitate insertion of the dilation tube into the working channel. 
     The shaft  302  has a smaller outer diameter than the load bearing portions  304   a, b  to advantageously reduce the overall weight of the dilation tube. For example, the 8 mm dilation tube can have a length of about 7.550 inches (in.), an inner diameter of about 0.069 in., an outer shaft  302  diameter of about 0.229 in., and an outer load bearing portion  304   a, b  diameter of about 0.335 in. The 13 mm dilation tube can have a length of about 6.550 in., an inner diameter of about 0.345 in., an outer shaft diameter of about 0.405 in., and an outer load bearing portion diameter of about 0.539 in. The 18 mm dilation tube can have a length of about 5.550 in., an inner diameter of about 0.549 in., an outer shaft diameter of about 0.609 in., and an outer load bearing portion diameter of about 0.740 in. The 24 mm dilation tube can have a length of about 4.550 in., an inner diameter of about 0.750 in., an outer shaft diameter of about 0.810 in., and an outer load bearing portion diameter of about 0.975 in. The dilation tubes can also have an enlarged portion  306  at a proximal end. The enlarged proximal portion can have an outer diameter of about 0.249 in. for the 8mm dilation tube, about 0.445 in. for the 13 mm dilation tube, about 0.649 in. for the 18mm dilation tube, and about 0.850 in. for the 24 mm dilation tube. In some embodiments, the enlarged portion includes a knurled surface to improve grip. 
     Awl and Tap 
     As shown in  FIGS. 9 and 10 , an improved awl  320  and tap  330  can each have two load bearing portions  324   a, b ,  334   a, b  and a reduced diameter shaft  322 ,  332  similar to the dilation tube  300  to advantageously reduce the weight of the instruments. The load bearing portions  324   a, b ,  334   a, b  of the awl  320  and tap  330  can also be integrally formed or coupled to the shaft  322 ,  332  as described herein with respect to dilation tube  300 , and can be similarly contoured to improve grip. 
     As shown, a distal end of the awl  320  includes a trocar tip  328  having sharp edges to allow the surgeon or other user to create a pilot hole for an implant. A distal end of the tap  338  includes external threading having a cutting edge to create female threads in the pilot hole. A portion  337  of the awl shaft  332  distal and adjacent to the distalmost load bearing portion  334   b  can taper (i.e., decrease in diameter) toward the distal end so that the shaft  332  has a smaller diameter adjacent the distal end  338 . The awl  320 , tap  330 , and/or other instruments described herein can include various types of handles. As illustrated, the awl  320  includes a palm handle  326  at a proximal end of the shaft  322 , and the tap  330  includes a T-handle  336  at a proximal end of the shaft  332 . In some embodiments, the T-handle  336  can be coupled to the shaft  322  via a hex adapter. Other types of handles (e.g., axial, trilobe, etc.) and coupling mechanisms are also possible. 
     Screw and Rod Delivery Devices 
     Once the insertion site(s) have been prepared, the tower access device  200  or another access device can be coupled to a spinal stabilization system  100  or another implant and inserted into the working channel to the target site, as described further below. Various instruments according to the present disclosure can be used, alone or in combination with tower access device  200  or another access device, to deliver fixation devices and/or other implants, such as spinal stabilization system  100 , to a patient&#39;s spine. 
     For example, screw delivery device  400  shown in  FIGS. 11A-11D  is configured to be inserted through the inner sleeve  210  of the access device  200 . In some embodiments, screw delivery device  400  has inner  410  and outer  420  shafts that are axially and rotationally moveable relative to each other. In some embodiments, the inner  410  and/or outer  420  shafts have one or more slots  422  to advantageously reduce the weight of screw delivery device  400 . In the illustrated embodiment, the outer shaft  420  includes four longitudinal rows of four slots  422  each, for a total of sixteen slots  422 . Adjacent rows of slots  422  can be staggered as shown in  FIG. 11A . The slots have a width of about 0.150 in. and a length L (as illustrated in  FIG. 11A ) of about 0.750 in. There is a spacing of about 0.750 in. between adjacent slots (measurement S illustrated in  FIG. 11A ). The total length of the outer shaft  420  can be about 7.000 in to about 7.020 in. 
     As shown in  FIG. 11C , a distal end of the inner shaft  410  can have a hexalobe tip  412  to engage a corresponding recess on a bone screw, for example, screw  110 . Other screw-screw delivery device engagement configurations are also possible. The inner shaft  410  can also include a saddle  414  near the distal end proximal to the hexalobe tip  412 . The saddle  414  is configured to contact another portion of the implant, for example, the one or more clamps  114  or any other rod receiving surface of the bone fixation device in use. The saddle  414  can have a shape similar to a half cylinder to matingly engage the shape of the rod receiving surface. A proximal end  418  of the inner shaft  410  can be configured to be coupled to a handle, e.g., a palm, axial, tri-lobe, T, or other handle. In the illustrated embodiment, a proximal end of the outer shaft  420  includes a grip  424 , and a distal end of the outer shaft  420  includes an externally threaded portion  426 . 
     In use, the surgeon or other user inserts the screw delivery device  400  into the working channel, for example, through the tower access device  200 , so that the hexalobe tip  412  engages the corresponding recess on the spinal screw and the saddle  414  contacts the rod receiving surface. The user can then use grip  424  to rotate the outer shaft  420  relative to the inner shaft  410  to thread the externally threaded portion  426  into the internal threads  126  of the housing  112  (shown in  FIG. 1 ). As shown in  FIGS. 11A and 11C , there may initially be a gap  416  between the saddle  414  and externally threaded portion  426 . In some embodiments, the gap  416  closes as the externally threaded portion  426  of the outer shaft  420  is threaded down into the housing  112 , for example as shown in  FIG. 11B . In other embodiments; depending on the dimensions of the housing  112 , a portion of the gap  416  may remain once the externally threaded portion  426  is threaded into the housing  112 . The user can then rotate the inner  410  and outer  420  shafts together to insert the spinal screw  110  into the patient&#39;s spine. In some embodiments, the inner  410  and outer  420  shafts become substantially locked relative to one another by virtue of the hexalobe tip  412  engaging the recess on the spinal screw, the saddle  414  contacting the rod receiving surface, and the externally threaded portion  426  engaging the internal threads  126  of the housing  112  so that the inner  410  and outer  420  shafts rotate together when inserting the spinal screw  110  into the patient&#39;s spine. The spinal screw  110 , rod receiving surface such as clamps  114 , and housing  112  rotate together during insertion of the spinal screw  110  into the patient&#39;s spine. In some embodiments, rotation of the screw delivery device  400  causes rotation of the spinal stabilization system  100  as well as the access device  200  to which it is coupled. The saddle  414  contacting the one or more clamps  114  advantageously helps distribute the load during tightening of the spinal screw  110  to reduce the amount of torque on the hexalobe tip  412 . 
     A rod insertion device  500  can be used to insert a spinal rod  118  into the U-shaped seat of housing  112 . As shown in  FIG. 12A , the rod insertion device  500  can include an elongate, generally cylindrical shaft  510  having a gripping end  520  at a distal end of the shaft  510  and a handle  530  at a proximal end of the shaft  510 . The device  500  can further include an actuator knob  540 . In the illustrated embodiment, the gripping end  520  includes two gripping elements configured to grip a rod  118 , for example as shown in  FIGS. 12G and 12H . In some embodiments, the gripping end  520  includes serrations  522  configured to mate with corresponding serrations  523  on the rod  118  as shown in  FIG. 12C . In some embodiments, the rod  118  is substantially smooth over most of its length and has serrations over a portion of one or both ends. The serrated portion of the rod can also be flattened and more narrow than the remainder of the rod  118  as shown in  FIGS. 12E and 12F . In the illustrated embodiment, the serrated portion has a thickness T of about 0.094 in. and a length SL of about 0.332 in. The serrations can advantageously provide a higher mechanical resistance to force as compared to a friction lock to enhance the grip of the gripping end  520  on the rod member  118 . In some embodiments, each gripping element of the gripping end  520  includes a central pin  521  (shown in  FIG. 12D ) configured to engage a corresponding hole  525  through rod  118  (shown in  FIG. 12F ) at the end of the rod with the serrations. 
     The rod insertion device  500  can act as a collet to grip the rod  118 . For example, the gripping end  520  can be coupled to an inner shaft  512  surrounded by shaft  510 . As shown in  FIG. 12B , the handle  530  is coupled to the proximal end of the shaft  510 . As shown in  FIG. 12C , the actuator knob  540  is coupled to the inner shaft  512 . In some embodiments, the gripping end  520  is integrally formed with the inner shaft  512 . Alternatively, the gripping end  520  can be coupled to the inner shaft  512  via a threaded connection  513  or any other suitable connection. As shown, the gripping end  520  can be coupled to the shaft  510  by two dowel pins  516 . The pins  516  sit in troughs  517  on opposite sides of the gripping end  520  and engage the shaft  510  through a pair of openings  526  on either side of the shaft  510 . The pins  516  can slide or roll in the troughs  517  to allow for relative movement between the shaft  510  and the gripping end  520  and inner shaft  512 . In some embodiments, two additional dowel pins  532  couple the handle  530  to the shaft  510  and inner shaft  512 . The pins  532  extend alongside the inner shaft  512 , through the shaft  510  and engage the handle  530  through a pair of openings  536  on either side of the handle  530 . Other arrangements of dowel pins and other coupling mechanisms are also possible. 
     In some embodiments, the shaft  510  can be advanced distally with respect to the inner shaft  512  to partially cover and compress the gripping end  520  so that the gripping end  520  tightens on the rod  118 . In some embodiments, the inner shaft can be retracted proximally so that the gripping end  520  is partially pulled into shaft  510  to compress the gripping end  520 . In some embodiments, the actuator knob  540  can be used to move the shaft  510  and the gripping end  520  relative to one another. The gripping end  520  can be locked and unlocked as needed during the procedure if the surgeon wants to adjust the angulation of rod insertion. In some embodiments, the surgeon can release the actuator knob  540  partway to release the grip of the serrations  522  on the rod  118  while pins on the gripping elements remain in place to hold the rod  118 . The surgeon can then adjust the angle of the rod  118  and re-engage the actuator knob  540  to securely grip the rod  118 . 
     A threaded rod reducer  600 , for example as shown in  FIGS. 13A-13C , can be used to fully seat the rod  118  in the housing  112 . In some embodiments, the rod reducer  600  is configured to be placed over the tower access device  200 . The example threaded rod reducers  600  shown in the two different versions of  FIGS. 13A and 13B  include a cannula  610  and an internally threaded top portion  620 . The threaded rod reducer  600  does not include a handle, which can advantageously reduce the size of the instrument and allow for easier operation during use. A distal end of the cannula  610  includes a slot  612  configured to engage the rod  118 . In some embodiments, the slot  612  forms a half-circle having a radius of between about 0.1 cm and about 0.5 cm, or between about 0.3 cm and about 0.4 cm. At the distal end of the cannula  610 , near the slot  612 , the cannula  610  may have an enlarged diameter as compared to the rest of the cannula  610  body. A proximal end  614  of the cannula  610  can also have an enlarged diameter. The threaded top portion  620  is coupled to the proximal end  614  of the cannula  610 . The proximal end  614  of the rod reducer of  FIG. 13B  has a contoured outer surface compared to the cylindrical proximal end  614  of the rod reducer of  FIG. 13A . As shown in  FIG. 13C , the threaded top portion  620  can include a reduced diameter portion  622  configured to sit within the proximal end  614  of the cannula  610 . Two dowel pins  630  sit in the reduced diameter portion  622  and engage the proximal end  614  of the cannula  610  through a pair of openings on each side of the proximal end  614 . The dowel pins  630  hold the top portion  620  and cannula  610  together while allowing the top portion  620  to rotate relative to the cannula  610 . 
     In use, the threaded top portion  620  threads onto the proximal threaded portion  214  of the tower access device  200  and the slot  612  engages the rod  118 . As the top portion  620  moves distally on the proximal threaded portion  214 , the slot  612  engages the rod  118  so that the cannula  610  urges the rod  118  into position with respect to the housing  112 . 
     As described herein, a spinal stabilization system  100  can include a set screw  116  to fix the rod  118  and spinal screw  110 . A set screw inserter  700  can be provided and used to thread the set screw  116  into the internal threads  126  of the housing  112 . As shown in  FIG. 14 , the set screw inserter  700  can have two load bearing portions  724   a,b  and a reduced diameter shaft  722  similar to the dilation tubes  300 , awl  320 , and tap  330  described herein to advantageously reduce the weight of the inserter  700 . 
     Hinge For Tower Access Devices 
     In some spinal stabilization procedures, two or more spinal stabilization systems  100  are implanted into adjacent vertebrae and connected by one or more rods  118 . Two or more tower access devices  200  can be used to insert the systems  100 . A hinge  800  as shown in  FIG. 15  can be used to couple two tower access devices  200 , for example, to hold them in place relative to each or to allow for compression or distraction of the vertebrae. In the illustrated embodiment, the hinge  800  includes two pivots  810 , two clamshell clamps  820 , two clamp tighteners  830 , a ratchet bar  840 , and a tightening knob  842 . The pivots  810  have spherical outer surfaces and cylindrical bores therethrough. The pivots  810  are sized and configured to slide onto the tower access devices  200 . The clamshell clamps  820  circumferentially surround and are configured to be tightened around the pivots  810 . The clamshell clamps  820  have spherical inner surfaces configured to engage the spherical outer surfaces of the pivots  810 . The spherical surfaces allow the pivots  810  to pivot and angulate relative to the clamshell clamps  820  with multiple degrees of freedom. The clamp tighteners  830  can be tightened to secure the clamshell clamps  820  around the pivots  810  and adjust the amount of movement allowed by the pivots  810 . The ratchet bar  840  can be used to adjust the lateral spacing between the clamshell clamps  820  to match the spacing of the towers  200 . When the desired spacing is achieved, the ratchet bar  840  can be locked via the tightening knob  842  to help hold the towers  200  in place. The hinge  800  can also advantageously serve as a fulcrum for compression or distraction. In some embodiments, a hinge having more than two pivots  810  and clamshell clamps  820  can be used to couple more than two access devices  200 . 
     Other Instruments 
     Various other instruments can also be provided. For example, a caliper  900 , as shown in  FIG. 16 , can be used to measure the length needed for the rod  118 . A compressor  910  and/or distractor  920 , as shown in  FIGS. 17 and 18 , respectively, allow for adjustment of the lateral spacing of the towers and therefore allow for compression or distraction of the vertebrae. A final tightener  930 , shown in  FIG. 19  and similar to set screw inserter  700 , can be provided for final tightening of the set screw  116  once the desired spacing of the spinal stabilization systems  100  has been achieved. The final tightener  930  can have a T-handle as shown in  FIG. 19  or another type of handle, e.g., a palm, trilobe, axial, or other handle. 
     Systems or Kits 
     In some embodiments, one or more of the instruments described herein can be packaged together in a kit or system, for example by packaging one or more instruments in a single tray. For example, a kit or system may include one or more of: dilation tubes  300 , an awl  320 , a tap  330 , a screw delivery device  400 , a rod inserter  500 , a threaded reducer  600 , and tower access devices  200 . A kit or system can further include a set screw inserter  700  and/or final tightener  930 . In some embodiments, a kit or system can include a compressor  910  and/or distracter  920 . Furthermore, in some embodiments, a kit or system can include a Jamshidi needle and/or a K-wire. In some embodiments, the kit or system may further include implants such as screws and rods and may include a caliper  900 . 
     Method for Delivering Fixation Device or Implant into a Spine 
       FIGS. 20-33  show an example embodiment of a method for delivering a fixation device or implant into a spine for spinal stabilization using various instruments as described herein. To begin, the surgeon makes a small incision through the patient&#39;s skin at the desired insertion site and inserts a Jamshidi needle through the skin to the bone surface. Once the Jamshidi needle is docked at the desired insertion location, for example, the junction of the transverse process and facet joint for a pedicle screw insertion procedure, the inner stylet is removed and a K-wire is inserted into the vertebral body. The K-wire is then used to guide a series of dilation tubes of increasing diameter that sequentially enlarge the working channel.  FIG. 20  illustrates a dilation tube  300  being delivered over a K-wire  950 . 
     The working channel to the patient&#39;s spine can be dilated to the desired size by using progressively larger dilation tubes. In some embodiments, the surgeon can use traditional dilator tubes to gradually enlarge the working channel and then insert a dilation tube  300  having the improved shape described herein as the final dilation tube  300  through which other instruments can be inserted. Alternatively, multiple dilation tubes  300  of different sizes can be used. The surgeon can use dilation tubes  300  to introduce instruments such as an awl  320 , drill, and/or tap  330  to the insertion site to prepare the bone for the implant, for example as shown in  FIGS. 21 and 22 . Different sized dilation tubes  300  can be used to introduce different instruments. 
     Once the insertion site has been prepared, the surgeon can couple an implant, for example, the screw and housing of a spinal stabilization system  100 , to the tower access device  200  by engaging the protruding members  216  of the grasping elements  212  with the apertures  124  on the housing  112 . The lock nut  240  is rotated clockwise on the threaded portion  214  of the inner sleeve  210  to move the outer sleeve  220  relative to the inner sleeve  210 , compress the grasping elements  212  about the housing  112 , and lock the assembly. The surgeon can then introduce the assembly to the insertion site through the dilation tube  300  as shown in  FIG. 23 . Alternatively, the dilation tube  300  can be removed prior to introducing the tower access device  200 , and the tower access device  200  can be tracked over the K-wire. 
     The screw  110  can be inserted into the patient&#39;s vertebra using the screw delivery device  400  introduced through the tower access device  200 , which in turn can be introduced through the dilation tube  300  as shown in  FIG. 24 . In  FIG. 24 , the screw delivery device  400  is illustrated including a trilobe handle  419 . Other types of handles are also possible as described herein. Alternatively, the dilation tube  300  can be removed prior to introducing the screw delivery device  400 , for example as shown in  FIG. 25 . 
     One or more screws can be fixed into bone using the devices and methods described above. For example, in one embodiment, a first screw attached to a first access device can be delivered into a first vertebrae, while a second screw attached to a second access device can be delivered into a second vertebrae. 
     Once the desired number of screws and access devices are delivered, the screw delivery device  400  and dilation tube  300  can be removed and the rod  118  can be inserted using the rod insertion device  500  as shown in  FIG. 26 . As described herein, the actuator knob  540  can be pulled proximally to cause the gripping end  520  of the rod insertion device  500  to grip the rod  118 . The pins and serrations  522  can enhance the grip. In some embodiments, the rod  118  is delivered via a mini-open procedure, in which an incision is made between the first and second screw. In other embodiments, the rod  118  is delivered percutaneously along the side of either of the first and second access devices. The rod  118  can be delivered along the outer sidewall of a first access device. The rod  118  can be delivered at an angle such that a first end is received into a distal slot  211 ,  221  of the first access device  200 . The rod  118  can then be directed such that its second end is received into a distal slot  211 ,  221  of the second access device  200 . One end of the rod  118  can be fixed to the first screw, while the opposite end of the rod member can be fixed to the second screw, thereby forming a stabilizing connection between the screws. The surgeon can advantageously release the actuator knob  540  halfway to loosen the grip of the gripping end  520  on the rod  118  while leaving the pins in place to assist in adjusting the rod  118  to the desired position. This allows the surgeon to manipulate and change an angle of the rod insertion device  500  relative to the rod  118  and/or the rod  118  relative to the access device  200  as needed to deliver the rod  118  to the desired position. 
     The threaded reducer  600  can then be introduced over the tower access device  200  so that the slot  612  engages the rod  118  as shown in  FIG. 27 . As the threaded reducer  600  is threaded onto the inner sleeve  210  of the tower access device  200 , the rod  118  is forced into contact with the one or more clamps  114  of the system  100  as shown in  FIG. 28 . The rod insertion device  500  may be left in place during reduction with the threaded reducer  600  to help the surgeon maintain the rod  118  in the desired location. 
     Once the rod  118  is in place, the rod insertion device  500  can be removed, and a set screw inserter  700  can be inserted through the tower access device  200  to thread the set screw  116  into the housing  112  to secure the screw  110  rod  118  as shown in  FIG. 29 . The set screw  116  can provide a downward force on the rod  118 , and this downward force can be transferred from the rod  118  to the clamps  114  and housing  112 , thereby providing a secure locking mechanism for the system. 
     As described herein, multiple tower access devices  200  can be used to insert multiple systems  100  into the patient&#39;s spine. For example, additional tower access devices  200  and spinal stabilization systems  100  can be placed in adjacent locations such as the pedicles of adjacent vertebra. The rod  118  then spans the systems  100  to stabilize the spine as shown in  FIG. 30 . The hinge  800  can be used to secure the towers  200  relative to each other while allowing some pivotal movement. If desired, a compressor  910  or distracter  920  can be used to force adjacent tower access devices  200  closer to or farther apart from one another and in turn compress or distract the screws  110  and the patient&#39;s spine as shown in  FIGS. 31 and 32 , respectively. The hinge  800  can advantageously act as a fulcrum for such distraction or compression. Finally, the surgeon can perform final tightening with a final tightener  930  or screwdriver as needed, as shown in  FIG. 33 . 
     Once the implants are in place, the surgeon can release and remove the hinge  800 . The surgeon can then release the grasping elements  212  of the access devices  200  from the housings  112  by depressing the spring latch  230  button  232  to release the engagement of the spring latch  230  tab  234  and lock nut  240  engagement surface  244 . The lock nut  240  can then be threaded off of the access device  200  so the access device(s)  200  can be removed. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the present embodiments without departing from the scope or spirit of the advantages of the present application. Thus, it is intended that the present application cover the modifications and variations of these embodiments and their equivalents.