Patent Publication Number: US-11642158-B2

Title: Methods and devices for spinal screw insertion

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. application Ser. No. 15/848,609, filed Dec. 20, 2017 and entitled “Methods and Devices for Spinal Screw Insertion,” which is a divisional of U.S. application Ser. No. 14/450,952, filed on Aug. 4, 2014 and entitled “Methods and Devices for Spinal Screw Insertion,” which are hereby incorporated by reference in their entireties. 
    
    
     FIELD 
     Bone anchors and associated instrumentation and methods are disclosed herein. 
     BACKGROUND 
     Bone anchors can be used in orthopedic surgery to fix bone during healing, fusion, or other processes. In spinal surgery, for example, bone anchors can be used to secure a spinal fixation element to one or more vertebrae to rigidly or dynamically stabilize the spine. 
     In a conventional procedure for coupling a bone anchor to bone, access to the bone is obtained, for example by forming a skin incision and resecting soft tissue disposed over the bone or by using a minimally-invasive technique. An insertion needle with a stylet disposed therein, sometimes referred to as a Jamshidi needle, is then driven into the bone to establish the trajectory for a bone opening. Next, the stylet is removed and a guidewire is inserted through the needle. The needle is then withdrawn over the guidewire, leaving the guidewire in place. A cannulated tap is then advanced over the guidewire and driven into the bone to enlarge the bone opening into a pilot hole for the bone anchor. Thereafter, the tap is withdrawn over the guidewire, again leaving the guidewire in place within the bone opening. A cannulated bone anchor is then advanced over the guidewire and driven into the bone opening. Finally, the guidewire is removed and one or more fixation elements are coupled to the bone anchor. 
     The conventional procedure detailed above suffers from a number of disadvantages. For example, the process involves several steps which can be time-consuming and cumbersome, particularly where a number of bone anchors are being installed. In addition, many of these steps (e.g., advancing the needle, advancing the guidewire, advancing the tap, and advancing the bone anchor) are done with fluoroscopic guidance to confirm the correct trajectory and insertion depth. With each additional step, the radiation exposure to the patient and surgical team increases, potentially causing dangerous complications or negative long-term health effects. The steps of removing the needle and removing the tap can also cause the guidewire to dislodge from the bone opening, requiring the process to be started anew. Further still, advancing the anchor or advancing the tap can inadvertently cause the guidewire to advance within the bone opening, potentially damaging delicate anatomical structures disposed in proximity to the bone. Advancing the anchor or advancing the tap can also cause the guidewire to become kinked, making removal of the guidewire very difficult. Accordingly, a need exists for improved bone anchors and associated instrumentation and methods. 
     SUMMARY 
     Various surgical instruments and methods are disclosed herein for implanting a bone anchor into bone. In one embodiment, a surgical instrument for driving a bone anchor assembly into bone is provided and includes an elongate shaft having a distal tip configured to couple to a bone anchor assembly, a handle assembly coupled to a proximal end of the shaft, and a stylet extending through an inner lumen of the handle assembly and the elongate shaft. The surgical instrument can further include a carrier disposed within the handle assembly, with the carrier being configured to non-rotatably translate relative to the elongate shaft in a proximal direction through the handle assembly to move the stylet proximally when the elongate shaft is rotated in order to drive a bone anchor assembly into bone. 
     The handle assembly can include a first and a second handles rotatably coupled to one another. Rotation of the first handle while the second handle is held stationary can cause the carrier to non-rotatably translate axially relative to the elongate shaft. Rotation of the second handle while the first handle is held stationary can cause the carrier to rotatably translate relative to the first handle. In some embodiments, the first handle can be positioned distal to the second handle. The surgical instrument can further include a stylet holder coupled to the carrier and releasably engaged to the stylet. The stylet holder can be releasably engaged with the carrier and can allow axial translation of the stylet when the stylet holder is in a first position relative to the carrier, and can prevent axial translation of the stylet when the stylet holder is in a second position relative to the carrier. The surgical instrument can further include a bone anchor assembly matable to the distal tip of the elongate shaft and having threads formed thereon that define an insertion rate of the bone anchor assembly into bone. The carrier can be configured to translate within the handle assembly at a rate that is equal to the insertion rate of the bone anchor assembly. 
     In some embodiments, a surgical instrument for driving a bone anchor assembly into bone can include an elongate shaft having proximal and distal ends, a mating feature formed on the distal end that is configured to mate to a bone anchor assembly, and an inner lumen extending through the elongate shaft. A stylet can extend through the inner lumen of the elongate shaft. The surgical instrument can also include a proximal handle configured to rotate the elongate shaft to drive a bone anchor assembly coupled to the mating feature on the distal end of the elongate body into bone. In addition, the surgical instrument can include a distal handle rotatable relative to the proximal handle. The surgical instrument can further include a carrier threadably disposed within the handle assembly and configured to move axially relative to the elongate shaft in response to rotation of one of the proximal and distal handles relative to the other one of the proximal and distal handles in order to cause the stylet to translate axially relative to the elongate shaft. 
     In one embodiment, rotation of the proximal handle while the distal handle is held stationary can cause rotation of the elongate shaft, and rotation and axial translation of the carrier within the handle assembly. Conversely, rotation of the distal handle while the proximal handle is held stationary can cause the carrier to translate axially within the handle assembly without rotating the elongate shaft. In certain embodiments, the proximal handle can be mated to the proximal end of the elongate shaft, and the distal handle can be rotatably disposed about a proximal portion of the elongate shaft at a location distal to the proximal handle. The carrier can be threadably coupled to threads formed within the distal handle and the stylet can be integrally formed on and can extend distally from the carrier. Alternatively or in addition, the stylet can be removably mated to the carrier by a mating element. 
     In certain embodiments, the surgical instrument can include a stylet holder disposed within the handle assembly that can allow the stylet to extend therethrough. The stylet holder can have a first position, in which the stylet is freely slidable relative to the stylet holder, and a second position in which the stylet holder rigidly engages the stylet to prevent movement of the stylet relative to the stylet holder. The stylet holder can be configured to be advanced into the carrier to cause the stylet holder to move from the first position to the second position. The stylet holder can include a clamping feature that is compressed by the carrier in the second position to cause the clamping feature to rigidly engage the stylet. 
     In other aspects, the elongate shaft of the surgical instrument can include a proximal portion with one or more slots, with the proximal portion extending through the distal handle and having a proximal end mated to the proximal handle. The carrier can be slidably disposed within the proximal portion, and the carrier can be non-rotatable relative to the proximal portion such that the carrier and the elongate shaft rotate together. The carrier can include one or more threaded features that extend through the one or more slots. The one or more slots can allow translation while preventing rotation of the carrier within the elongate shaft. 
     The surgical instrument can include other components, such as a protective sleeve removably disposed around the elongate shaft, and/or a positioning handle coupled to a proximal end of the stylet that is configured to adjust a length of stylet that extends from a bone anchor assembly coupled to the distal end of the elongate shaft. The positioning handle can include a tool feature for engaging a stylet holder disposed within the handle assembly and having the stylet extending therethrough. The tool feature can be configured to advance the stylet holder into the carrier to cause the stylet holder to move from a first position to a second position, wherein the second position locks the stylet to the stylet holder. The positioning handle can include an outer housing encompassing at least a part of a positioning feature that secures the proximal end of the stylet to the positioning handle. The positioning feature can include a push button that releasably engages an engagement position along the outer housing. The positioning feature can be configured to move with the stylet relative to the outer housing when the push button is disengaged from the engagement position. 
     In some embodiments, a surgical instrument for driving a bone anchor assembly into bone can include an elongate shaft having proximal and distal ends, a mating feature formed on the distal end and configured to mate to a bone anchor assembly, an inner lumen extending through the elongate shaft, and a handle assembly coupled to the elongate shaft. The handle assembly can include a distal handle coupled to the proximal end of the elongate shaft and it can be configured to rotate the elongate shaft to drive a bone anchor assembly coupled to the mating feature on the distal end of the elongate body into bone. The handle assembly can further include a proximal handle threadably coupled to the distal handle and coupled to a proximal end of a stylet that extends through the inner lumen of the elongate shaft, with the proximal handle being rotatable relative to the distal handle in order to cause the stylet to axially translate relative to the elongate shaft. 
     In some embodiments, a method of delivering a bone anchor assembly is provided. The method can include coupling an engagement portion on a shaft of a driver to a corresponding engagement portion on a bone anchor assembly. The method can also include rotating a second handle of the driver while holding a first handle of the driver stationary to cause a carrier disposed within the driver to translate a stylet relative to the bone anchor assembly. The method can further include rotating a first handle of the driver while holding a second handle of the driver stationary to cause the shaft to rotate and advance the bone anchor assembly along the stylet. When the first handle is rotated while the second handle is held stationary, the carrier can be caused to translate axially within the driver to move the stylet axially relative to the elongate shaft. 
     The present disclosure further provides devices and methods as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG.  1 A  is an exploded perspective view of a prior art bone anchor assembly; 
         FIG.  1 B  is a sectional view of the prior art bone anchor assembly of  FIG.  1 A ; 
         FIG.  2 A  is an exploded perspective view of one embodiment of a surgical instrument having a stylet that can be coupled to a carrier, the carrier and stylet can be axially translated as a result of rotating proximal and distal handles relative to one another; 
         FIG.  2 B  is a cross-sectional view of the surgical instrument of  FIG.  2 A  with a bone anchor assembly coupled to a distal end of an elongate shaft of the surgical instrument; 
         FIG.  3    is a cross-sectional view of another embodiment of a surgical instrument having a stylet integrated with and extending distally from a carrier; 
         FIG.  4    is a cross-sectional view of yet another embodiment of a surgical instrument having a stylet threadably engaged with a carrier; 
         FIG.  5 A  is an exploded perspective view of a surgical instrument having proximal and distal handles and a stylet holder for carrying a stylet, according to another embodiment; 
         FIG.  5 B  is a side view of the surgical instrument of  FIG.  5 A  with a bone anchor assembly coupled to a distal end of an elongate shaft of the surgical instrument; 
         FIG.  5 C  is a cross-sectional view of the surgical instrument and bone anchor assembly of  FIG.  5 B ; 
         FIG.  5 D  is an exploded perspective view of the surgical instrument shown in  FIGS.  5 A- 5 C  having a stylet with a positioning handle; 
         FIG.  5 E  is a side view of the positioning handle of  FIG.  5 D  showing a window in an outer body of the positioning handle revealing a marking; 
         FIG.  5 F  is a top view of the positioning handle of  FIG.  5 D  showing a push button for adjusting a position of a stylet; 
         FIG.  5 G  is a cross-sectional view of the positioning handle of  FIG.  5 D  showing a positioning feature within the outer body of the positioning handle. 
         FIG.  6 A  is a cross-sectional view of another embodiment of a surgical instrument in a retracted position having a stylet coupled to a proximal handle, which can allow the stylet to be axially translated as a result of rotating the proximal handle relative to a distal handle of the surgical instrument; 
         FIG.  6 B  is a cross-sectional view of the surgical instrument of  FIG.  6 A  in an extended position; 
         FIG.  7    is a cross-sectional view of a protective sleeve positioned over a part of the elongate shaft of the surgical instrument shown in  FIGS.  6 A and  6 B ; 
         FIG.  8    is a cross-sectional view of a protective sleeve positioned over a part of the elongate shaft of surgical instrument shown in  FIG.  4   ; 
         FIGS.  9 A- 9 C  schematically illustrate a method of using the surgical instrument of  FIGS.  2 A and  2 B  to drive a bone anchor assembly into bone; and 
         FIG.  10 A- 10 D  schematically illustrate a method of using the positioning handle and surgical instrument of  FIGS.  5 A- 5 G  to drive a bone anchor assembly into bone. 
     
    
    
     DETAILED DESCRIPTION 
     Certain exemplary embodiments will now be described to provide an overall understanding of the principles of the structure, function, manufacture, and use of the devices and methods disclosed herein. One or more examples of these embodiments are illustrated in the accompanying drawings. Those skilled in the art will understand that the devices and methods specifically described herein and illustrated in the accompanying drawings are non-limiting exemplary embodiments and that the scope of the present invention is defined solely by the claims. The features illustrated or described in connection with one exemplary embodiment may be combined with the features of other embodiments. Such modifications and variations are intended to be included within the scope of the present invention. 
     Methods and devices for inserting bone anchor assemblies into bone are provided herein. Use of these anchors or instruments can eliminate one or more of the steps in a conventional bone anchor installation procedure, improving surgical efficiency and safety. In general, surgical insertion devices are provided that include a proximal handle and a distal handle, with the proximal handle configured to control the movement of an elongate shaft of the device and the distal handle configured to control the movement of a guidewire or stylet extending through the device. For example, rotation of the distal handle relative to the proximal handle can cause the stylet to axially translate in a proximal or distal direction relative to the elongate shaft. In addition, rotation of the proximal handle relative to the distal handle can cause the elongate shaft to rotate, which can assist with driving a bone anchor assembly coupled to a distal end of the elongate shaft into bone. We note that the terms guidewire and stylet are used interchangeably herein, and any configuration of a guidewire or stylet can be used with the various instruments and methods disclosed herein. 
     Prior Art Bone Anchor Assembly 
       FIGS.  1 A- 1 B  illustrate one embodiment a prior art bone anchor assembly  10  that includes a bone anchor  12 , a receiver member  14  for receiving a spinal fixation element, such as a spinal rod  22 , to be coupled to the bone anchor  12 , and a closure mechanism  16  to capture a spinal fixation element within the receiver member  14  and fix the spinal fixation element with respect to the receiver member  14 . The bone anchor  12  includes a proximal head  18  and a distal shaft  20  configured to engage bone. The receiver member  14  has a proximal end  26  having a pair of spaced apart arms  28 A,  28 B defining a recess  30  therebetween and a distal end  32  having a distal end surface  34  defining an opening through which at least a portion of the bone anchor  12  extends. The closure mechanism  16  can be positionable between and can engage the arms  28 A,  28 B to capture a spinal fixation element, e.g., a spinal rod  22 , within the receiver member  14  and fix the spinal fixation element with respect to the receiver member  14 . 
     The proximal head  18  of the bone anchor  12  is generally in the shape of a truncated sphere having a planar proximal surface  36  and an approximately spherically-shaped distal surface  38 . The illustrated bone anchor assembly is a polyaxial bone screw designed for posterior implantation in the pedicle or lateral mass of a vertebra. The proximal head  18  of the bone anchor  12  engages the distal end  32  of the receiver member  14  in a ball and socket like arrangement in which the proximal head  18  and the distal shaft  20  can pivot relative to the receiver member  14 . The distal surface  38  of the proximal head  18  of the bone anchor  12  and a mating surface within the distal end  32  of the receiver member  14  can have any shape that facilitates this arrangement, including, for example, spherical (as illustrated), toroidal, conical, frustoconical, and any combinations of these shapes. 
     The distal shaft  20  of the bone anchor  12  can be configured to engage bone and, in the illustrated embodiment, includes an external bone engaging thread  40 . The thread form for the distal shaft  20 , including the number of threads, the pitch, the major and minor diameters, and the thread shape, can be selected to facilitate connection with bone. Exemplary thread forms are disclosed in U.S. Patent Application Publication No. 2011/0288599, filed on May 18, 2011, and in U.S. Patent Application Publication No. 2013/0053901, filed on Aug. 22, 2012, both of which are hereby incorporated by reference herein. While a threaded distal shaft  20  is shown, the distal shaft can have other structures for engaging bone, including a hook. The distal shaft  20  of the bone anchor  12  can be cannulated, having a central passage or inner lumen  72  extending the length of the bone anchor to facilitate delivery of the bone anchor over a guidewire or stylet in, for example, minimally-invasive procedures. Other components of the bone anchor assembly  10 , including, for example, the closure mechanism  16 , the receiver member  14 , and the compression member  60  (discussed below), can be cannulated or otherwise have an opening to permit delivery over a guidewire or stylet. The distal shaft  20  can also include one or more sidewall openings or fenestrations that communicate with the inner lumen  72  to permit bone in-growth or to permit the dispensing of bone cement or other materials through the bone anchor  12 . The sidewall openings can extend radially from the inner lumen  72  through the sidewall of the distal shaft  20 . Exemplary systems for delivering bone cement to the bone anchor assembly  10  and alternative bone anchor configurations for facilitating cement delivery are described in U.S. Patent Application Publication No. 2010/0114174, filed on Oct. 29, 2009, which is hereby incorporated by reference herein. The distal shaft  20  of the bone anchor  12  can also be coated with materials to permit bone growth, such as, for example, hydroxyapatite, and the bone anchor assembly  10  can be coated partially or entirely with anti-infective materials, such as, for example, tryclosan. 
     The receiver member  14 , which couples to the bone anchor  12 , includes a pair of spaced apart arms  28 A,  28 B at the proximal end  26  defining a U-shaped recess  30  therebetween for receiving a spinal fixation element, e.g., a spinal rod  22 . Each of the arms  28 A,  28 B can extend from the distal end  32  of the receiver member  14  to a free end. The outer surfaces of each of the arms  28 A,  28 B can include a feature, such as a recess, dimple, notch, projection, or the like, to facilitate connection of the receiver member  14  to instruments. For example, the outer surface of each arm  28 A,  28 B can include an arcuate groove at the respective free end of the arms. Such grooves are described in more detail in U.S. Pat. No. 7,179,261, issued on Feb. 20, 2007, which is hereby incorporated by reference herein. 
     The distal end  32  of the receiver member  14  includes a distal end surface  34  which is generally annular in shape defining a circular opening through which at least a portion of the bone anchor  12  extends. For example, the distal shaft  20  of the bone anchor  12  can extend through the opening. 
     The bone anchor  12  can be selectively fixed relative to the receiver member  14 . Prior to fixation, the bone anchor  12  is movable relative to the receiver member  14  within a cone of angulation generally defined by the geometry of the distal end  32  of the receiver member and the proximal head  18  of the bone anchor  12 . The bone anchor  10  can be a favored angle screw, for example as disclosed in U.S. Pat. No. 6,974,460, issued on Dec. 13, 2005, and in U.S. Pat. No. 6,736,820, issued on May 18, 2004, both of which are hereby incorporated by reference herein. Alternatively, the bone anchor assembly can be a conventional (non-biased) polyaxial screw in which the bone anchor pivots in the same amount in every direction. 
     The spinal fixation element, e.g., the spinal rod  22 , can either directly contact the proximal head  18  of the bone anchor  12  or can contact an intermediate element, e.g., a compression member  60 . The compression member  60  can be positioned within the receiver member  14  and interposed between the spinal rod  22  and the proximal head  18  of the bone anchor  12  to compress the distal outer surface  38  of the proximal head  18  into direct, fixed engagement with the distal inner surface of the receiver member  14 . The compression member  60  can include a pair of spaced apart arms  62 A and  62 B defining a U-shaped seat  64  for receiving the spinal rod  22  and a distal surface  66  for engaging the proximal head  18  of the bone anchor  12 . 
     The proximal end  26  of the receiver member  14  can be configured to receive a closure mechanism  16  positionable between and engaging the arms  28 A,  28 B of the receiver member  14 . The closure mechanism  16  can be configured to capture a spinal fixation element, e.g., a spinal rod  22 , within the receiver member  14 , to fix the spinal rod  22  relative to the receiver member  14 , and to fix the bone anchor  12  relative to the receiver member  14 . The closure mechanism  16  can be a single set screw having an outer thread for engaging an inner thread  42  provided on the arms  28 A,  28 B of the receiver member  14 . In other embodiments, however, the closure mechanism  16  can include an outer set screw operable to act on the compression member  60  and an inner set screw operable to act on the rod  22 . 
     The bone anchor assembly  10  can be used with a spinal fixation element such as rigid spinal rod  22 . Alternatively, the spinal fixation element can be a dynamic stabilization member, such as a flexible or selectively flexible member, that allows controlled mobility between the instrumented vertebrae. 
     In use, the bone anchor assembly  10  can be assembled such that the distal shaft  20  extends through the opening in the distal end  32  of the receiver member  14  and the proximal head  18  of the bone anchor  12  is received in the distal end  32  of the receiver member  14 . The compression member  60  can be positioned within the receiver member  14  such that the arms  62 A,  62 B of the compression member are aligned with the arms  28 A,  28 B of the receiver member  14  and the lower surface of the compression member  14  is in contact with the proximal head  18  of the bone anchor  12 . A driver tool can extend through the compression member  60  and can be fitted with the bone anchor  12  to drive the bone anchor  12  into bone. A spinal fixation element, e.g., the spinal rod  22 , can be located in the recess  30  of the receiver member  14 . The closure mechanism  16  can be engaged with the inner thread  42  provided on the arms  28 A,  28 B of the receiver member  14 . A torsional force can be applied to the closure mechanism  16  to move it within the recess  30  so as to force the spinal rod  22  into engagement with the compression member  60  and to in turn force the compression member  60  onto the proximal head  18  of the bone anchor  12 , thereby fixing the spinal rod  22  relative to the receiver member  14  and locking the angular position of the bone anchor  12  relative to the receiver member  14 . 
     The surgical instruments disclosed herein can be configured to operate in conjunction with bone anchor assemblies of the type described above or other types known in the art. As indicated above, it will be appreciated that the bone anchor assembly  200  can be a monoaxial screw, a polyaxial screw, a uniplanar screw, a bone hook, a favored-angle screw, and/or any of a variety of other bone anchor types known in the art. Further information on favored-angle screws can be found in U.S. patent application Ser. No. 13/648,184, filed on Oct. 9, 2012, which is hereby incorporated by reference herein. 
     Insertion Instruments 
     In general, various insertion instruments are provided for driving a bone anchor assembly into bone. The insertion instruments generally include a handle assembly and an elongate shaft extending distally therefrom for coupling to a bone anchor assembly. The instruments are configured to receive a stylet therethrough and the handle assembly is configured to control positioning of the stylet. In particular, the handle assembly can be configured to allow for adjustment of an axial position of the stylet relative to a bone anchor assembly coupled to the elongate shaft. The handle assembly can also be configured to move the stylet proximally relative to a bone anchor assembly during insertion of the bone anchor assembly into bone. Such movement of the stylet can occur automatically, in response to rotation of a portion of the handle to drive the bone anchor assembly into bone. Such a configuration is particularly advantageous as it will prevent further insertion of the stylet into bone during advancement of the bone anchor assembly. A person skilled in the art will appreciate that the instruments disclosed herein can have a variety of configurations, and that the various features disclosed in the various embodiments are interchangeable. 
       FIGS.  2 A and  2 B  illustrate one exemplary embodiment of a surgical instrument  100  for driving a bone anchor assembly into bone. The surgical instrument can include a handle assembly  106  having an elongate shaft  102  extending distally therefrom, and a stylet  104  extending through the handle assembly  106  and the elongate shaft  102 . The elongate shaft  102  can be configured to mate to a bone anchor assembly, and the handle assembly  106  can be configured to both drive a bone anchor assembly into bone, and manipulate the stylet both before and during insertion of a bone anchor assembly into bone. 
     The elongate shaft  102  can have a variety of configurations, but generally the shaft  102  includes a proximal end  108  for coupling to the handle assembly and a distal end  110  for mating to a bone anchor assembly. A length of the shaft  102  can vary, but the shaft preferably has a length sufficient to allow the handle assembly to be positioned outside of a patient&#39;s body while the distal end  110  is positioned into a patient&#39;s body adjacent to bone. To facilitate mating to a bone anchor assembly, the distal end  110  of the elongate shaft  102  can include a mating feature  112  formed thereon. The mating feature  112  can be formed anywhere along the elongate shaft  102 , such as the distal end  110 , and it can be configured to engage a bone anchor assembly (e.g., bone anchor assemblies of the type described above with respect to  FIGS.  1 A- 1 B ). The mating feature  112  can include a threaded portion  113  configured to engage corresponding threads formed in the receiver member of the bone anchor assembly. The mating feature  112  can also include a tip  124  disposed distally of the threaded portion  113  and configured to engage a drive socket or a proximal surface of the bone anchor disposed within the receiver member. The tip  124  can have a diameter that is less than the diameter of the threaded portion  113 . The mating feature  112  can also be configured to engage a bone tap, or a bone tap can be formed integrally with the elongate shaft  102 . One or more bulges  126  or areas of increased diameter can be formed along the length of the elongate shaft  102  to engage and stabilize extension or protective sleeves that can be coupled to the bone anchor assembly. As shown in  FIG.  2 B , the bone anchor assembly can include break-off extensions  125 , which can act as a delivery cannula during insertion of the set screw and can be broken off at the end of the procedure. 
     The elongate shaft  102  can include a cannulated proximal portion having a bore extending at least partially therethrough and one or more slots formed therein that define opposed tabs  136 . The slot(s) can extend through only a portion of the proximal portion such that the opposed tabs  136  are connected at their proximal ends, or the slots extend through the entire length of the proximal portion as shown in the illustrated embodiment. A person skilled in the art will appreciate that the slots can have any length as may be required to allow for translation of the carrier, discussed below. In the illustrated embodiment, the tabs  136  can have a generally cylindrical configuration and define a hollow generally cylindrical interior lumen for receiving a carrier. A distal end of each tab  136  can be mounted on and can extend from a mounting surface defined by a proximal flange  143  formed on the elongate shaft, and a proximal end of each of the opposed tabs  136  can mate with a distal end of the proximal handle  120  such that rotation of the proximal handle  120  is effective to rotate the elongate shaft  102 . In particular, the proximal end of each tab  136  can be sized to be received within a distal end of the proximal handle  120 , as discussed below, and can be fixedly mated thereto, e.g., using an adhesive, welding, threads, or any other mating feature. In an exemplary embodiment, threads (not shown) are formed on an outer surface of the proximal end of the opposed tabs  136  for mating with corresponding threads formed within the proximal handle  120 . Where the tabs are not connected, a support collar  138  can optionally be disposed within a proximal portion of the opposed tabs  136  to prevent inward radial movement of the opposed tabs and to maintain the threaded connection between the opposed tabs  136  and the proximal handle  120 , as shown in  FIG.  2 B ; however, one or more of a variety of features can be used to connect any part of the elongate shaft  102 , such as the opposed tabs  136 , to the proximal handle  120 . In addition, although described herein as opposed tabs  136 , the proximal end of the elongate shaft  102  can have a variety of configurations including various shapes and sizes. Preferably, the proximal end is in the form of a body having a lumen and at least one slot, with the shape of the body varying as may be determined based on the shape of the handle disposed therearound. Any number of a variety of features and configurations can be included at the proximal end of the elongate shaft  102  and are not limited to opposed tabs  136 . 
     As indicated above, the handle assembly  106  can be located adjacent the proximal end  108  of the elongate shaft  102  and can include a proximal handle  120  and a distal handle  122 . The handle assembly  106  can be positioned and sized to allow a user, such as a surgeon, to grasp a part of the handle assembly  106  and operate the surgical instrument  100 . While the proximal and distal handles  120 ,  122  can each have a variety of configurations, in the illustrated embodiment each handle  120 ,  122  has a generally elongate cylindrical configuration and is cannulated with an inner lumen extending therethrough, as shown in  FIG.  2 B . Each handle  120 ,  122  can include gripping features, such as knurling or other surface features, formed thereon to facilitate grasping of the device. The proximal handle  120  can be coupled to the proximal end  108  of the elongate shaft  102  and can be configured to rotate the elongate shaft  102 , such as to drive a bone anchor assembly coupled to the mating feature  112  into bone. In particular, as described above, threads or other mating features on a proximal end of the opposed tabs  136  can mate to threads  147  formed within a distal portion of the inner lumen of the proximal handle. 
     As shown in  FIG.  2 B , the inner lumen in the proximal handle  120  includes an enlarged diameter region along its distal portion for receiving a proximal-most end or extension  145  of the opposed tabs  136  at the proximal end of the elongate shaft  102 . The distal handle  122  can be positioned just distal to the proximal handle  120  and can be moveably disposed about a proximal portion of the elongate shaft  102 . In particular, as shown in  FIG.  2 B , the distal handle  122  can be disposed about the opposed tabs  136  of the elongate shaft  102  and can be configured to freely rotate relative to at least the elongate shaft  102 . The proximal flange  143  on the elongate shaft  102  can assist in preventing translational movement of the distal handle  122  relative to the proximal handle  120  and elongate shaft  102 . In order to facilitate mounting of the distal handle  122  about the opposed tabs  136 , the inner lumen extending through the distal handle can have a diameter that is slightly larger than an outer diameter of the opposed tabs  136 . As further shown in  FIG.  2 B , the distal handle  122  can include interior threads  134  formed therein and extending along at least a portion or an entire length of the inner lumen extending through the distal handle  122 . The threads  134  can mate with threads on the carrier, as discussed below. In use, each of the proximal and distal handles  120 ,  122  can be rotated relative to one other while the other handle is held stationary. Rotation of the proximal handle  120  will rotate the shaft  102  to drive a bone anchor assembly into bone, and rotation of the distal handle  122  can assist with positioning the stylet  104  relative to the elongate shaft  102 , such as to allow a length of stylet  104  to extend from the distal end  110  of the elongate shaft  102 , as will be discussed in detail below. 
     Although described as the proximal handle  120  being configured to rotate the elongate shaft  102  and the distal handle  122  being configured to axially translate the stylet  104  relative to the elongate shaft  102 , the proximal handle  120  can be configured to axially translate the stylet  104  relative to the elongate shaft  102  and the distal handle  122  can be configured to rotate the elongate shaft  102 . 
     Continuing to refer to  FIGS.  2 A and  2 B , the surgical instrument  100  can further include a carrier  130  movably disposed within the handle assembly  106 . The carrier  130  can be configured to couple to a stylet, as discussed below, and in use the carrier can facilitate positioning of the stylet relative to a bone anchor assembly coupled to the elongate shaft  102 . 
     While the carrier  130  can have a variety of configurations, in the illustrated embodiment the carrier  130  has a generally cylindrical configuration and is cannulated with an inner lumen extending therethrough. As shown in  FIG.  2 B , the carrier  130  can be slidably disposed within the opposed tabs  136 . The carrier  130  can thus have an outer diameter that is less than an inner diameter of the lumen defined by the opposed tabs  136  to allow the carrier to be disposed therein. The carrier  130  can include one or more threaded features  132  formed on an outer surface of the carrier  130  that engage the interior threads  134  located on an inner surface of the distal handle  122 . As shown in  FIG.  2 B , the carrier  130  includes first and second threaded features formed on opposed sides thereof and not extending fully circumferentially around the carrier, with the opposed threaded features being received within the opposed slots  140  in the opposed tabs  136 . Thus a portion of the outer surface of the carrier  130  can be non-threaded and configured to slide along or adjacent the inner walls of the opposed tabs  136 , such as when the carrier  130  translates and moves the stylet  104  in a proximal or distal direction. The one or more slots  140  can allow the one or more threaded features  132  formed on opposed sides of the carrier to extend therethrough and to engage the threaded features  134  of the distal handle  122 . Such a configuration will allow the threaded features  134 , and thus the carrier  130 , to translate axially along the opposed tabs  136 , yet will prevent rotation of the carrier relative to the opposed tabs  136 , and thus the elongate shaft  102 . As a result, when the proximal handle  120  is be held by the user and the distal handle  122  is rotated, the threads  134  in the distal handle  122  will interact with the threaded features  132  on the carrier  130  to cause the carrier  130  to translate axially without rotating along the opposed tabs  136  and relative to the elongate shaft  102 . In addition, if the distal handle  122  is held and the proximal handle  120  is rotated, the opposed tabs  136 , which rotate in coordination with the proximal handle  120 , will force the carrier  130  to rotate. The interaction between the threads in the distal handle  122  and the threaded features  132  on the carrier will thus cause the carrier  130  to translate axially along the opposed tabs  136  and thus relative to the elongate shaft. 
     In some implementations, the thread pitch of the carrier  130  and distal handle  122  can be the same as the thread pitch of the bone screw. The direction of the threads in the carrier  130  and distal handle  122 , however, are preferably reversed as compared to a direction of the threads on a bone-screw. Such a configuration can allow the bone screw to advance into the bone at approximately the same rate as the stylet is retracted, as discussed below. Reversal of the thread pitch also results in a configuration in which the proximal handle  120  can be rotated in a first direction, e.g., clockwise, to drive a bone screw into bone, while rotation of the distal handle in a second opposite direction, e.g. counter-clockwise, is effective to advance the carrier  130  and stylet distally through distal handle  122 . In other embodiments, the thread pitch of the carrier  130  and distal handle  122  can differ from the thread pitch of the bone screw so as to result in movement of the stylet at a rate that is greater or less than a rate of insertion of the bone screw. In addition, the slots  140  can extend approximately 30 millimeters to approximately 60 millimeters. Thus, this can allow the carrier  130  and stylet  104  to translate a distance of approximately 30 millimeters to approximately 60 millimeters. However, the slots  140  can extend a variety of lengths and are not limited to the examples described herein. 
     Movement of the carrier  130  within the distal handle  122 , as described above, can cause corresponding movement of a stylet coupled thereto.  FIGS.  2 A and  2 B  illustrate a stylet  104  having a generally elongate configuration with a pointed distal tip to facilitate insertion into bone. As noted above, the stylet  104  or guidewire can have a variety of configurations. As shown in  FIGS.  2 A and  2 B , the carrier  130  can include an inner lumen  144  that extends along the length of the carrier  130  and is configured to allow the stylet  104  to extend through. The stylet  104  can be integrally formed on and can extend distally from the carrier  130 , or it can be removably mated to the carrier, such as with the assistance of a mating element (e.g., set screw). For example, the mating element can allow the length of stylet  104  extending in a direction from the carrier  130  to be altered. In the illustrated embodiment, the carrier  130  can include a threaded thru-hole  146  extending radially through a sidewall thereof that can accept a set screw  142  that can be advanced into the carrier  130  in order to engage and secure the stylet  140  within the inner lumen  144  of the carrier. Additionally, the handle assembly  106 , such as the distal handle  122 , can include a thru-hole  148  that can allow a tool to be inserted through the thru-hole and to access the set screw for adjusting the axial position of the stylet  104  relative to the carrier  130 . For example, adjusting the position of the stylet  104  relative to the carrier  130  can affect the length of stylet  104  that can extend from the distal end  110  of a bone anchor assembly coupled to the elongate shaft  102 . For example, distal translation of the stylet  104  can allow the length of stylet extending from the distal end  110  of the elongate shaft  102  to increase and proximal translation of the stylet  104  can allow the length of stylet  104  extending from the distal end  110  of the elongate shaft  102  to decrease. 
       FIG.  3    illustrates an embodiment of a surgical instrument  200  for driving a bone anchor assembly into bone that is identical to that of  FIGS.  2 A and  2 B , except that the stylet  204  is integrally formed with the carrier  230 . As shown in  FIG.  3   , the stylet  204  is unitary or monolithic with the carrier  230  and extends distally from a distal side of the carrier  230 . In this embodiment, the stylet  204  can translate relative to the elongate shaft  202 , but the stylet  204  cannot translate relative to the carrier  230 . As such, the maximum length of stylet  204  that can extend from the distal end  210  of the elongate shaft  202  when the carrier  230  is in a most distal position relative to the distal handle  222  cannot be adjusted (i.e., lengthened or shortened). 
       FIG.  4    illustrates another embodiment of a surgical instrument  300  for driving a bone anchor assembly into bone. The instrument  300  of  FIG.  4    is identical to that of  FIGS.  2 A and  2 B , except that the stylet  304  is threadably engaged with the carrier  330 . For example, the stylet  304  can include threads  350  along a proximal portion of the stylet  304 . In addition, the carrier  330  can include a thru-hole  352 , with at least a part of the thru-hole including threads that can threadably engage the threads  350  along the stylet  304 . Furthermore, the thru-hole  350  can either have more than one diameter (e.g., a recessed bore) or not be threaded all the way through, such as in order to prevent the stylet  304  from advancing too far through the carrier  330  or becoming disengaged. As such, the proximal end of the stylet  304  can be passed through the thru-hole  352  of the carrier  330  and the threaded portion  350  of the stylet  304  can be threadably engaged with the carrier  330  until the stylet  304  is secured to the carrier  330 . Once the stylet  304  is secured to the carrier  330 , such as during manufacturing of the surgical instrument, the stylet  304  can translate relative to the elongate shaft  302 , but the stylet  304  cannot translate relative to the carrier  330 . As such, the maximum length of stylet  304  that can extend from the distal end  310  of the elongate shaft  302  when the carrier  330  is in a most distal position relative to the distal handle  322  cannot be adjusted (i.e., lengthened or shortened). 
       FIGS.  5 A- 5 G  illustrate yet another embodiment of a surgical instrument  400  for driving a bone anchor into bone. The instrument  400  of  FIGS.  5 A- 5 G  is identical to that of  FIGS.  2 A and  2 B , except that the surgical instrument  400  includes a stylet holder  460  that can assist with coupling a stylet  404  to a carrier  430 . The stylet holder  460  can include a thru-hole  462  along the length of the stylet holder  460  and the stylet  404  can extend through the thru-hole  462  in order to slidably engage with or secure to a part of the thru-hole  462 . The stylet holder  460  can be loaded and/or removed from either the distal end or proximal end of the stylet holder. The engagement of the stylet  404  with the stylet holder  460  can depend on a positioning of the stylet holder  460  relative to the carrier  430 , as will be described in greater detail below. 
     The stylet holder  460  can have a generally cylindrical configuration, although an outer diameter can vary along external portions thereof. As shown, the stylet holder  460  includes a proximal non-threaded cylindrical portion that is sized to be received within the support collar  438 , and a distal portion having a threaded member  462  and a clamping feature  464 . The distal portion is configured to be received within the carrier  430  such that the threaded member  462  formed along an outer surface of the stylet holder  460  is threadably engaged with a threaded bore  432  of the carrier  430 . The clamping feature  464  is in the form of a slotted tapered nose defining opposed arms that are compressible radially inward to engage the stylet. The arms taper radially inward toward the distal-most end. 
     In use, the stylet holder  460  can be movable between a first position (unlocked) and a second position (locked) relative to the carrier  430 . For example, in the first position, the stylet holder  460  can be, at most, partially threadably engaged with the carrier  430 . In addition, when the stylet holder  460  is in the first position the clamping feature  464  of the stylet holder  460  is not engaged with the carrier. When the clamping feature  464  of the stylet holder  460  is not engaged with the carrier  430 , the clamping feature  464  does not compress around the stylet  404  and allows the stylet  404  to be axially slidably moved within the stylet holder  460 . 
     In the second position, the stylet holder  460  is fully threaded into the threaded bore  432  of the carrier  430  such that the clamping feature  464  of the stylet holder  460  is received within a tapered bore  434  formed in the carrier  430 . When the clamping feature  464  of the stylet holder  460  is engaged with the tapered bore  434  of the carrier  430 , the tapered bore  434  causes the arms of the clamping feature  464  to compress toward one another and around the stylet  404  such that the stylet holder  460  rigidly engages the stylet  404 , thereby preventing axial translation of the stylet  404  independent of the stylet holder  460 . 
     The stylet holder  460  can be advanced into the carrier  430  by the threaded engagement between the stylet holder  460  and the carrier  430 . In addition, the stylet holder  460  can include a tool-engaging feature  466  (e.g., a recessed hex feature) that can allow a tool (e.g., a protruding hex feature) to engage and force the stylet holder  460  to rotate, such as relative to the carrier  430 . For example, the stylet holder  460  can be forced to rotate in a first direction (e.g., clockwise) relative to the carrier  430  in order to move the stylet holder  460  to the second position. In addition, stylet holder  460  can be forced to rotate in a second direction (e.g., counterclockwise) relative to the carrier  430  in order to move the stylet holder  460  to the first position. 
       FIGS.  5 A- 5 C  further illustrate a ring  463  disposed between the proximal and distal handles  420 ,  422 . The ring can be utilized with any of the embodiments disclosed herein, and acts as a holder for a positioning device, such as a three-dimensional sensor array for use in facilitating navigation during a surgical procedure. As show, the ring  463  is generally annular in shape and includes a bore  465 , e.g., a threaded bore, formed therein for mating with a positioning device. 
       FIG.  5 D  illustrates an embodiment of tool that can be used to adjust a position of the stylet and to move the stylet holder between the first and second positions. A person skilled in the art will appreciate that the illustrated tool can be used with any of the devices described herein. As shown in  FIG.  5 D , the tool can include a positioning handle  480  that is configured to mate to a proximal end of the stylet  404 . The positioning handle  480  can allow a user to advance the distal end of the stylet  404  through the stylet holder  460 , such as when the stylet holder  460  is in the first position (i.e., the stylet  404  can translate relative to the stylet holder  460 ). In addition, the distal end of the positioning handle  480  can have a tool feature  482  (e.g., a protruding hex feature) that can engage the tool-engaging feature  466  of the stylet holder  460 . As such, after advancing the stylet  404  through at least the stylet holder  460 , the positioning handle  480  can engage the tool-engaging feature  466  of the stylet holder  460  and force the stylet holder  460  to rotate relative to the carrier  430  and form the second position with the carrier  430  (i.e., the stylet holder  460  rigidly engages the stylet  404 ). 
     As shown in  FIGS.  5 E- 5 G , the positioning handle  480  can include an outer body  484  encompassing at least a part of a positioning feature  486 . The stylet  404  can extend from a distal end of the positioning feature  486  and out through a distal end of the outer body  484 . The positioning feature  486  can include a push button  488  that can be depressed in order to disengage the positioning feature  486  from the outer body  484  and allow the positioning feature  486  and stylet  404  to translate relative to the outer body  484 . As such, the positioning feature  486  can vary the length that the stylet  404  extends from the positioning handle  480 , which can also vary the length of stylet  404  that can extend distally from the elongate shaft  402  (e.g., after the positioning handle  480  has coupled the stylet  404  to the stylet holder  460 ). Although described herein as a push button  488 , any number of a variety of features for disengaging the positioning feature  486  from the outer body  484  can be used, such as, for example, a sliding or threaded feature. 
     In some embodiments, the push button  488  can engage the outer body  484  of the positioning handle  480  in a number of engagement positions  490 . In addition, either the engagement positions  490  or the positioning feature  486  can include markings  492  that can inform a user as to the approximate length the stylet  404  extends from the elongate shaft  402  based on the type (e.g., length) of bone anchor attached (or to be attached) to the elongate shaft  402 . For example, the outer body  484  can include a window  494  that reveals one of a plurality of markings (e.g., numbers) formed on the positioning feature  486 . Each marking can correspond to a length of stylet  404  extending distally beyond a distal-most end of the elongate shaft  402 . In addition, the markings can correspond to various bone anchor length to be used with the instrument to allow a user to select an appropriate bone anchor and adjust the stylet  404  relative to the elongate shaft  402  based on the selected bone anchor. For example, the user can select a 45 millimeter bone anchor to be implanted in a patient. The user can then set the positioning handle  480  (e.g., by pushing the push button  488  and moving the positioning feature  486 ) such that a marking (e.g., shown in the window  494 ) indicates that the stylet  404  is appropriately positioned relative to the elongate shaft  402  or bone anchor for a 45 millimeter bone anchor attached to the surgical instrument  400 . In this position, a predetermined length (e.g., approximately 1 millimeter to approximately 30 millimeters) of the tip of the stylet  404  can extend distally beyond a distal-most end of the bone anchor mounted onto the elongate shaft  402 . 
     In addition, after the stylet  404  has been initially positioned relative to the elongate shaft  402 , the user can continue to observe the positioning of the stylet  404  relative to the elongate shaft  402  and bone anchor mounted thereon. For example, while not shown, in some embodiments, the stylet holder  460  can include a proximal extension that can include markings corresponding to the length of stylet  404  extending distally beyond the elongate shaft  402  or bone anchor. Additionally, the proximal handle  420  can include one or more viewing windows  423  that can allow a user to view the markings along the proximal extension of the stylet holder  460  in order to determine the length of stylet  404  extending from the distal end of the elongate shaft  402  or bone anchor. Although described herein as using markings, such as numbers, to indicate the stylet length to the user, any number of indicia, such as colors, pictures, etc., can be used. 
     In use, when the stylet holder  460  is in the first position (unlocked), the user can manipulate the positioning feature  486  to cause the stylet  404  to translate relative to the stylet holder  460  and along the elongate shaft  402 , thereby adjusting a length of the stylet extending from a bone anchor assembly coupled to a distal end of the elongate shaft  402 . Rotation of the positioning handle  480  can thread the stylet holder  460  into the carrier  430 , thereby moving the stylet holder  460  to the second position (locked). The positioning handle  480  can be removed and the stylet  404  can be released from the positioning handle  480 , such as due to a sliding or snap fit between the positioning handle  480  and stylet  404 . 
       FIGS.  6 A and  6 B  illustrate another embodiment of a surgical instrument  500  for driving a bone anchor assembly into bone. The surgical instrument  500  can include an elongate shaft  502 , a stylet  504 , and a handle assembly  506 . The elongate shaft  502  can have a proximal end  508  and a distal end  510 , with a mating feature  512  formed on the distal end  510  and configured to mate to a bone anchor assembly. The elongate shaft  502  can be hollow with an inner lumen that can extend through and along the length of the elongate shaft  502 . The stylet  504  can extend through at least a part of the elongate shaft  502 , such as the inner lumen. 
     The handle assembly  506  can be coupled to a proximal end  508  of the elongate shaft  502  and can include a proximal handle  520  and a distal handle  522 . The handle assembly  506  can be positioned and sized to allow a user, such as a surgeon, to grasp a part of the handle assembly  506  and operate the surgical instrument  500 . In this embodiment, the handles are reversed as compared to prior embodiments. In particular, the proximal handle  520  controls a position of the stylet, whereas the distal handle  522  is used for driving a bone anchor assembly into bone. 
     As shown, the distal handle  522  is positioned between the proximal handle  520  and the proximal end  508  of the elongate shaft  502 . The distal handle  522  has a proximal portion with a reduced diameter region that allows the proximal portion to be received within a bore  525  formed in the proximal handle  520 . The proximal portion can include threads  592  formed on an external surface thereof that engage with corresponding threads  590  formed within the bore  525  extending proximally into a distal end of the proximal handle  520 . The distal portion of the distal handle  522  is enlarged to facilitate grasping, and includes a bore  527  formed in a distal-most end thereof for receiving a proximal end  508  of the elongate shaft  502 . As a result, rotation of the distal handle  520  relative to the proximal handle  520  will rotate the elongate shaft  502 . As further shown, the bore  525  formed in proximal handle  520  includes a reduced diameter region  529  at a proximal-most end thereof for mating with a stylet  504 . The stylet  504  can be threadably mated within the reduced diameter region  529 , or otherwise fixedly mated thereto. As such, rotation of the proximal handle  520  relative to the distal handle  522  can translate the stylet  504  in a proximal or distal direction relative to the elongate shaft  502 . Furthermore, rotation of the distal handle  522  can rotate the elongate shaft  502 . 
     A mating feature  512  can be formed at the distal end  510  of the elongate shaft  502  and can be configured to engage a bone anchor assembly (e.g., bone anchor assemblies of the type described above with respect to  FIGS.  1 A- 1 B ). The mating feature  512  can include a threaded surface configured to engage corresponding threads formed in the receiver membrane of the bone anchor assembly. The mating feature  512  can also include a tip  524  disposed distally of the threaded surface configured to engage a drive socket or a proximal surface of the bone anchor or a compression cap (not shown) disposed within the receiver member. The tip  524  can have a diameter that is less than the diameter of the threaded portion. The mating feature  512  can also be configured to engage a bone tap, or a bone tap can be formed integrally with the elongate shaft  502 . One or more bulges  526  or areas of increase diameter can be formed along the length of the elongate body to engage and stabilize extension or protective sleeves that can be coupled to the bone anchor assembly. 
     In use, the stylet  504  can translate along a length of the inner lumen of the elongate shaft  502  and can extend out from the distal end  510  of the elongate shaft  502 . In addition, translation of the stylet  504  along the inner lumen can allow the length of stylet  504  that extends from the distal end  510  of the elongate shaft  502 , and in particular from a distal end of a bone anchor assembly coupled to the elongate shaft, to vary. For example, distal translation of the stylet  504  can allow the length of stylet extending from a bone anchor assembly to increase and proximal translation of the stylet  504  can allow the length of stylet  504  extending from a bone anchor assembly to decrease. When the proximal handle  522  is fully threadably engaged, as shown in  FIG.  6 B , the stylet is fully extended from the bone anchor assembly and is at its maximum length. 
       FIG.  7    illustrates an embodiment of a protective sleeve  600  positioned over a part of an elongate shaft of a surgical instrument, such as the elongate shaft  502  of surgical instrument  500  described above and shown in  FIGS.  6 A and  6 B . The protective sleeve  600  can assist with neurotransmitting procedures, such as neuromonitoring navigation. The protective sleeve can act as an insulator that can insulate at least the stylet. By way of non-limiting example the protective sleeve can be formed from one or more of a radio-opaque and radiolucent material, and can include radiopaque markers. 
       FIG.  8    illustrates another embodiment of a protective sleeve  600  positioned over a part of an elongate shaft of a surgical instrument, such as the elongate shaft  302  of surgical instrument  300  described above and shown in  FIG.  4   . Although the protective sleeve  600  is shown and described as being coupled to the surgical instruments  300  and  500  shown in  FIGS.  7  and  8   , any number of surgical instruments, including any disclosed herein, can be coupled with the protective sleeve  600 . 
     Methods 
     The various instruments disclosed herein can be used to perform a variety of surgical procedures. While exemplary methods are discussed below for delivering a bone screw to a vertebra, a person skilled in the art will appreciate that the instruments can be used to deliver a variety of implants in various surgical procedures. By way of non-limiting example, the instruments can be used to deliver screws to soft tissue or bone throughout a patient&#39;s body, in minimally invasive, arthroscope, endoscopic, open, or other surgical procedures. 
       FIGS.  9 A- 9 C  schematically illustrate one exemplary methods of using a surgical instrument having a stylet to drive a bone anchor assembly into bone  824 . The method detailed below can be used with any of the instruments disclosed above (e.g., the instruments  100 ,  200 ,  300 ,  400 ), with any necessary modifications being apparent to one skilled in the art having read the above disclosure. By way of example, the method is described in connection with instrument  100  of  FIGS.  2 A- 2 B . 
     To begin with, an incision can be made to access the bone  824  (e.g., a vertebra) to which the bone anchor assembly  900  (e.g., a pedicle screw) is to be coupled. The bone anchor assembly  900  can be coupled to the instrument  100  and advanced through the incision to position the bone anchor assembly in proximity to the bone surface. Prior to, during, or after insertion into through the incision, the stylet  104  can be indexed to an initial position based on various parameters such as the length of the bone anchor assembly. This can be accomplished, for example, by rotating the distal handle  122  while holding the proximal handle  120  fixed, to cause the carrier  130  and corresponding stylet  104  to translate axially. In other embodiments, for example using the instrument of  FIGS.  4 - 5 G , the stylet can be axially slid relative to the stylet holder prior to locking the stylet to the stylet holder. In some embodiments, the stylet  104  can be initially positioned such that the stylet  104  protrudes from the distal end of the bone anchor assembly  900  by a desired amount. This can be achieved, for example, using the positioning handle  480  (e.g., by pushing the push button  488  and moving the positioning feature  486 ) such that a marking (e.g., shown in the window  494 ) indicates that the stylet  404  is appropriately positioned relative to the elongate shaft  402 . For example, the positioning handle  480  can be set to display a marking that corresponds to a length of the selected bone anchor, such that a predetermined portion of the tip of the stylet extending distally beyond a distal end of the bone anchor attached to the surgical instrument  400 . The length can be further adjusted by adjusting the stylet relative to the carrier as may be desired. It will be appreciated that the stylet  104  can be initially positioned such that the stylet  104  does not protrude from the distal end of the bone anchor assembly  900 . 
     As shown in  FIG.  9 A , the protruding stylet  104  can be docked into the pedicle  822  by tapping or urging the instrument distally towards the bone surface. The distal handle  122  can be rotated relative to the proximal handle  120 , e.g., in a clockwise direction, in order to cause the stylet  104  to mechanically advance into the bone, as shown in  FIG.  9 B . Alternatively, or in addition, an impact force can be applied to the stylet  104  in the distal direction to advance the stylet  104  into the bone. The proper trajectory and depth can be confirmed with fluoroscopy. The insertion depth can also be inferred by the surgeon (e.g., based on the number of rotations of the distal handle  122 , audible or tactile feedback, visual feedback provided by graduations or markings, such as shown through the viewing window  423  of the proximal handle  420 , or based on the carrier hitting a stop disposed in or on the surgical instrument). 
     Once the stylet  104  is advanced to the desired depth, the proximal handle  120  can be rotated, e.g., in a clockwise direction, relative to the distal handle  122  (i.e., the distal handle  122  is held fixed) in order to drive the bone anchor assembly  900  along the path created by the stylet  104 , as shown in  FIG.  9 C . Referring back to  FIGS.  2 A and  2 B , rotation of the proximal handle  120  while holding the distal handle fixed  122  will cause corresponding rotation of the opposed tabs  136  on the proximal end of the elongate shaft  102 . Since the carrier  130  is keyed to the tabs  136 , the carrier  130  will be caused to rotate in coordination with the tabs  136 . As a result, the threaded features  132  on the carrier will rotate relative to the threads  134  formed in the distal handle  122 , which is held stationary. The carrier  130  will thus be forced to translate along the opposed tabs  136 . Since the threads are reversed as compared to the threads on the bone screw, the carrier  130  will move proximally within the distal handle  122 , thus moving the stylet  104  in a proximal direction relative to the bone screw  900 . Since the stylet  104  is held fixed against the bone, the stylet  104  can be maintained at a constant depth within the bone as the bone anchor assembly  900  is advanced distally over the stylet  104 . The handle assembly will move distally along the stylet  104  in coordination with distal advancement of the bone anchor assembly  900 . In embodiments in which the threaded features of the carrier  130  and distal handle  122  have the same pitch as the threaded portion of the bone anchor assembly, retraction of the stylet  104  into the bone anchor assembly  900  can occur at the same rate as the advancement of the bone anchor assembly  900 , such that the stylet  104  remains at a substantially fixed depth within the bone. In embodiments in which the threaded features of the carrier  130  and distal handle  122  have a smaller pitch than the threaded portion of the bone anchor assembly, retraction of the stylet  104  into the bone anchor assembly  900  can occur at a slight faster rate than advancement of the bone anchor assembly  900 , such that the stylet  104  is at least partially retracted in a proximal direction relative to the bone as the bone anchor is driven into bone. 
     When the bone anchor assembly  900  is driven to the desired depth, the stylet  104  and the elongate body  102  can be detached from the bone anchor assembly and removed from the incision. Subsequent steps, such as affixing a spinal rod or other component to a receiver member of the bone anchor assembly can then be performed. 
     The bone anchor assembly can include various self-tapping features to facilitate insertion into the bone and to prevent the bone from fracturing during anchor insertion. In some instances, patient anatomy or surgeon preferences can require the bone to be tapped before inserting the bone anchor assembly. In such instances, the above method can be modified to use embodiments of the surgical instrument that include an integral bone tap or which are coupled to a bone tap via the engagement portion. 
     As discussed above, a tool can be used to adjust the position of a stylet of an instrument with respect to either an elongate shaft of the instrument or a bone anchor assembly coupled to the elongate shaft. As shown in  FIGS.  5 D- 5 G , the tool can include a positioning handle  480 , which can be used to adjust the position of the stylet  404  relative to an elongate shaft, such as the elongate shaft  402  of the instrument  400  embodiment shown in  FIGS.  5 A- 5 D . Although the positioning handle  480  is described herein with respect to the instrument  400  embodiment shown in  FIGS.  5 A- 5 D , the positioning handle  480  can be used with any of the instrument embodiments. 
       FIGS.  10 A- 10 D  schematically illustrate another exemplary method of using a tool or positioning handle  480  with the surgical instrument  400  having a stylet  404  to drive a bone anchor assembly  900  into bone  824 . To begin with, a distal end of the positioning handle  480  can be removably coupled to a proximal end of the stylet  404 , as shown in  FIG.  10 A . The length of stylet  404  extending from the elongate shaft  402  of the instrument  40  can be adjusted by pressing the push button  488  and sliding the positioning feature  486  in a distal direction (e.g., to increase the length) or proximal direction (e.g., to decrease the length). Once the stylet  404  length has been set (i.e., the push button is released and engages an engagement position  490 ), the user can engage the tool feature  482  at the distal end of the positioning handle  480  into the tool-engaging feature  466  of the stylet holder  460  in order to rotate the stylet holder  460 . The stylet holder  460  can be rotated relative to the carrier  430  until the stylet holder  460  forms the second position with the carrier  430  where the stylet holder  460  rigidly engages the stylet  404  and locks the position of the stylet  404  relative to at least the stylet holder  460 . The user can then remove the positioning handle  480  from the stylet  404  (e.g., pull the positioning handle  480  off the proximal end of the stylet  404 ). 
     The distal end of the instrument  400  can then be inserted into the incision and the distal end of the stylet  404  can be docked against the bone. The user can then apply a distally directed force on the proximal end of the instrument  400  in order to force the stylet  404  into the bone. Alternatively or in addition, the user can hold the proximal handle  420  and rotate the distal handle  422  in order to force the stylet  404  in the distal direction and into the bone. Once a desired length of stylet  404  has engaged the bone, the user can then rotate the proximal handle  420  in order to drive the bone anchor assembly  900  into the bone. 
     Alternatively, the bone anchor assembly  900  may not have been attached to the distal end of the elongate member  402  prior to insertion of the distal end of the instrument into the incision. A tool (e.g., having a protruding hex feature) can be inserted into the tool-engaging feature  466  (e.g., having a recessed hex feature) of the stylet holder  460  in order to rotate the stylet holder  460  relative to the carrier  430  and position the stylet holder  460  in the first position. As described above, when the stylet holder  460  is in the first position, the stylet  404  can move relative to the stylet holder  460 . Therefore, the stylet  404  can remain in place (e.g., inserted into the bone) and the remainder of the surgical instrument  400  can be slid off the proximal end of the stylet  404 , as shown in  FIG.  10 B . Such a configuration allows various other procedures to be performed at the surgical site without interference from the instrument  400 , while at the same time maintaining the position of the stylet. Once the site is ready for anchor implantation, a user can attach a bone anchor assembly  900  onto the distal end  410  of the elongate member  402 . Once the bone anchor assembly  900  is attached, the user can lead the proximal end of the stylet  404  through the distal end of the bone anchor assembly  900  and continue to advance the surgical instrument along the stylet  404  until the distal end of the bone anchor assembly  900  is in contact with the bone, as shown in  FIGS.  10 C and  10 D . The stylet holder  460  can then be re-positioned into the second position (e.g., using the tool) in order to secure the stylet  404  to the stylet holder  460 . The user can then proceed with driving the bone anchor assembly  900  into the bone over the stylet  404  (e.g., rotating the proximal handle  420 ), as described above. 
     When the bone anchor assembly is driven to the desired depth, the stylet  404  and the elongate body  402  can be detached from the bone anchor assembly  900  and removed from the incision. Subsequent steps, such as affixing a spinal rod or other component to a receiver member of the bone anchor assembly  900  can then be performed. 
     It should be noted that any ordering of method steps implied by the drawings or description herein is not to be construed as limiting the disclosed methods to performing the steps in that order. Rather, the various steps of each of the methods disclosed herein can be performed in any of a variety of sequences. In addition, as the described methods are merely exemplary embodiments, various other methods that include additional steps or include fewer steps are also within the scope of the present invention. 
     The stylet of the various embodiments disclosed herein can be rigid or flexible. The stylet can be formed from a radiopaque material to facilitate visualization under fluoroscopy and other imaging techniques. Other components of the devices disclosed herein (e.g., elongate body portions, handle portions, and the like) can be formed from a radiolucent material so as not to interfere with visualization of the guide projection. Exemplary radiolucent materials include carbon fiber and high-strength polymers. The devices disclosed herein can also be compatible with image-guide surgical systems and with stimulation systems (e.g., neuromonitoring systems typically used to monitor for pedicle breach and to confirm screw or instrument placement). 
     The methods and devices disclosed herein can provide a number of advantages. For example, in some embodiments, the time required to target and place the bone anchor assembly can be reduced, the radiation exposure to the patient and to the surgical staff can be reduced, and procedural steps such as needle placement, guidewire insertion and removal, and tapping can be eliminated. By way of further example, in some embodiments, inadvertent advancement of instrumentation can be eliminated by controlling the guide projection depth throughout the procedure, risk of removing a guidewire during removal of a needle or tap can be eliminated, and bending or kinking of a guidewire can be prevented. 
     The devices and methods disclosed herein can be used in minimally-invasive surgery and/or open surgery. While the devices and methods disclosed herein are generally described in the context of advancing a bone anchor into a pedicle, it will be appreciated that the methods and devices disclosed herein can be used with any human or animal bone, implant, non-living object, and so forth. 
     Although the invention has been described by reference to specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but that it have the full scope defined by the language of the following claims.