Patent Publication Number: US-10765466-B2

Title: Bone anchor driver and methods

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of, and claims priority to, U.S. patent application Ser. No. 14/570,559, filed Dec. 15, 2014, and entitled BONE ANCHOR DRIVER AND METHODS, the entirety of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     Methods and devices are provided for applying a closure device to a bone anchor. 
     BACKGROUND OF THE INVENTION 
     Spinal fixation devices are used in orthopedic surgery to align and/or fix a desired relationship between adjacent vertebral bodies. Such devices typically include a spinal fixation element, such as a relatively rigid fixation rod or plate that is coupled to adjacent vertebrae by attaching the element to various anchoring devices, such as hooks, bolts, wires, or screws. The fixation elements can have a predetermined contour that has been designed according to the properties of the target implantation site, and once installed, the fixation element holds the vertebrae in a desired spatial relationship, either until desired healing or spinal fusion has taken place, or for some longer period of time. 
     Spinal rods can be mated to a number of anchoring devices fixed to or engaged with the vertebrae along a segment of the spinal column. Since each vertebra varies in shape and size, a variety of anchoring devices have been developed to facilitate engagement of a particular portion of the bone. Pedicle screw assemblies, for example, have a shape and size that is configured to engage pedicle bone. Such screws typically include a threaded shank that is adapted to be threaded into a vertebra, and a head portion having a rod-receiving element. A set screw, a plug, or other fastening elements can be used to lock the fixation element, e.g., a spinal rod, into the rod-receiving head of the pedicle screw. In use, the shank portion of each screw is threaded into a vertebra, and once properly positioned, a rod is seated through the rod-receiving member of each screw and the rod is locked in place by tightening a cap or other closure mechanism to securely interconnect each screw and the fixation rod. 
     The process of placing a rod within or adjacent to an implanted bone anchor so that they are interconnected is referred to as “reducing” the rod. Rod reduction is typically performed using suitable instruments that can create appropriate forces on the implanted bone anchor and the rod. Furthermore, after the rod is seated in the rod-receiving member and captured in a rod-receiving portion of the head of a bone anchor (e.g., a pedicle screw), a final tightening is typically performed on the screw using fastening instruments, for stabilization of the rod. The tightening of the screw can also be performed for maintenance of a surgical correction. 
     To complete rod reduction and perform final tightening, surgeons use multiple instruments (e.g., screw drivers and torque wrenches) which often need to be applied in multiple steps. This can be a time-consuming and complicated procedure, particularly when multiple bone anchors are used to fixate a rod and/or when the surgical procedure is performed to correct a complex deformity or injury. Multiple steps that require application of manual force induce fatigue so that surgeon&#39;s performance during surgery can decrease, which can compromise the outcome of the surgery. As another drawback of existing approaches, torque created by the instruments during the final tightening can be difficult to control. 
     Accordingly, there is a need for improved methods and devices for applying a closure device to a spinal anchoring device. 
     SUMMARY OF THE INVENTION 
     Methods and devices are provided for applying a closure mechanism to a spinal anchor. In general, a bone anchor driver is provided that is configured to drive a first closure mechanism into a bone anchor, and that has a torque limiting mechanism that causes the driver to automatically switch to drive a second closure mechanism into the bone anchor. 
     In one aspect, a bone anchor driver for driving inner and outer closure mechanisms onto a bone anchor assembly is provided. In some embodiments, the bone anchor driver includes an outer shaft configured to engage an outer closure mechanism coupled to a bone anchor assembly, and an inner shaft disposed within the outer shaft and configured to engage an inner closure mechanism coupled to a bone anchor assembly. In one embodiment, the bone anchor driver can be used to couple the inner closure mechanism with the bone anchor assembly. In such an embodiment, the closure mechanism is not in contact with the bone anchor assembly prior to operation of the bone anchor driver. 
     The bone anchor driver further includes a torque limiting mechanism configured to automatically switch between a first position in which the torque limiting mechanism is disengaged from the inner shaft and is engaged with the outer shaft, and a second position in which the torque limiting mechanism is disengaged from the outer shaft and is engaged with the inner shaft. The bone anchor driver can also include an actuator configured to apply a rotational force to the torque limiting mechanism to selectively drive the inner and outer shafts. 
     The torque limiting mechanism of the bone anchor driver, which can be disposed between the inner and outer shafts, can vary in a number of ways. In one embodiment, the torque limiting mechanism can be configured to switch between the first and second positions when a torque applied to the torque limiting mechanism exceeds a threshold torque. In one embodiment, the inner and outer shafts can define a longitudinal axis, and the torque limiting mechanism can translate axially along the longitudinal axis to move between the first and second positions. 
     In one embodiment, at least one of the inner and outer shafts includes a ramp that causes the torque limiting mechanism to translate axially when the torque limiting mechanism moves between the first and second positions. 
     The torque limiting mechanism can include a first engaging member configured to engage with a first complementary engaging member of the outer shaft and a second engaging member configured to engage with a second complementary engaging member of the inner shaft. 
     The first and second engaging members can vary in a number of ways. For example, the first engaging member can be positioned in a first plane and the second engaging member can be positioned in a second plane that is spaced a distance apart from the first plane. The first engaging member and the first complementary engaging member can be positioned in the same plane when the torque limiting mechanism is in the first position, and the first engaging member and the first complementary engaging member can be positioned in different planes when the torque limiting mechanism is in the second position. 
     In one embodiment, the first engaging member can be disposed within a first bore extending through the torque limiting mechanism, and the second engaging member can be disposed within a second bore extending through the torque limiting mechanism. The first and second bores can vary in a number of ways. For example, an axis of the first bore can extend transverse to an axis of the second bore. 
     In one embodiment, the torque limiting mechanism can be in the first position when a torque applied thereto is less than or equal to a threshold torque and the first engaging member is biased into engagement with the first complementary engaging member so that the torque limiting mechanism is disengaged from the inner shaft and is engaged with the outer shaft to drive the outer shaft. When the torque exceeds the threshold torque, the first engaging member can be configured to disengage from the first complementary engaging member and to switch from the first position to the second position in which the second engaging member is biased into engagement with the second complementary engaging member in the inner shaft so that the torque limiting mechanism is disengaged from the outer shaft and is engaged with the inner shaft to drive the inner shaft. 
     In another aspect, a bone anchor and driver assembly is provided that includes a bone screw and a bone anchor driver. The bone anchor can have a bone engaging member configured to be implanted in bone, a receiver member polyaxially coupled to the bone engaging member and configured to receive a spinal fixation element therein, an outer closure mechanism configured to mate to the receiver member for locking a polyaxial position of the receiver member with respect to the bone engaging member, and an inner closure mechanism configured to be received within the outer closure mechanism and to lock a spinal fixation element within the receiver member. The bone anchor driver can have an outer shaft configured to engage and drive the outer closure mechanism into the receiver member, an inner shaft configured to engage and drive the inner closure mechanism into the receiver member, and an actuator movable between a first position in which the actuator applies a driving force to the outer shaft and a second position in which the actuator applies a driving force to the inner shaft, the actuator being configured to automatically move from the first position to the second position in response to a torque applied thereto. 
     The bone anchor and driver assembly can vary in a number of ways. For example, the actuator can move from the first position to the second position when a torque applied to the actuator exceeds a threshold torque of a torque limiting mechanism coupled between the actuator and the inner and outer shafts. In one embodiment, the threshold torque can cause the actuator to move from the first position to the second position when the outer closure mechanism is fully engaged with the receiver member to lock the polyaxial position of the receiver member with respect to the bone engaging member. 
     In another aspect, a method for operating a bone anchor driver is provided that in some embodiments includes actuating a driver to rotate a first shaft to drive a first closure mechanism onto a bone anchor. When a torque applied to the driver exceeds a threshold torque, the driver automatically moves from a first position decoupled from the first shaft and into a second position coupled with a second shaft to drive a second closure mechanism into the bone anchor. 
     The method for operating the bone anchor driver can vary in a number of ways. For example, driving the first closure mechanism into the bone anchor can lock a receiver member of the bone anchor in a fixed angular orientation with respect to a bone engaging member of the bone anchor, and driving the second closure mechanism into the bone anchor can lock a spinal fixation element in a fixed position within the receiver member of the bone anchor. In one embodiment, a torque limiter can cause the driver to move from the first position to the second position when a torque applied to the driver exceeds the threshold torque. The torque limiter can translate axially along a longitudinal axis of the first and second shafts when the driver moves between the first position and the second position. In other aspects, actuating the driver can include activating an external power source to cause the driver to rotate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments described above will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings. The drawings are not intended to be drawn to scale. For purposes of clarity, not every component may be labeled in every drawing. In the drawings: 
         FIG. 1  is a schematic view of one exemplary embodiment of a bone anchor driver and a bone screw; 
         FIG. 2A  is a schematic view of another exemplary embodiment of a bone anchor driver; 
         FIG. 2B  is a schematic view of the bone anchor driver of  FIG. 2A  equipped with a counter-torque sleeve; 
         FIG. 3A  is an exploded perspective view of an exemplary embodiment of a gear box of the bone anchor drivers of  FIGS. 1, 2A, and 2B ; 
         FIG. 3B  is an exploded transparent, perspective view of the gear box of  FIG. 3A ; 
         FIG. 4A  is a cross-sectional view of a longitudinal cross-section of the gear box of  FIG. 3A ; 
         FIG. 4B  is another cross-sectional view of a longitudinal cross-section of the gear box of  FIG. 3A ; 
         FIG. 4C  is another cross-sectional view of a longitudinal cross-section of the gear box of  FIG. 3A ; 
         FIG. 4D  is another cross-sectional view of a longitudinal cross-section of the assembly  FIG. 3A ; 
         FIG. 5A  is a transparent, longitudinal cross-sectional perspective view of the gear box of  FIG. 3A ; 
         FIG. 5B  is a transparent, perspective view of a torque limiting mechanism of the gear box of  FIG. 5A ; 
         FIG. 5C  is a schematic view of the torque limiting mechanism of  FIG. 5B ; 
         FIG. 6  is a schematic view of an exemplary embodiment of an inner shaft of the gear box of  FIG. 3A ; 
         FIG. 7  is a schematic top, cross-sectional view illustrating a groove of an inner shaft of the gear box of  FIG. 3A ; 
         FIG. 8A  is a schematic top, cross-sectional view illustrating a complementary engaging member of an outer shaft of the gear box of  FIG. 3A ; and 
         FIG. 8B  is a schematic top, cross-sectional view illustrating a complementary engaging member of an inner shaft of the gear box of  FIG. 3A . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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. 
     Various exemplary methods and devices are provided to apply a closure mechanism to a bone anchor. In one embodiment, a bone anchor drive is provided and includes an outer shaft configured to engage and drive an outer closure mechanism onto a receiver member of a bone anchor to lock a position of the receiver member with respect to a bone engaging member. The bone anchor driver can also include an inner shaft configured to engage and drive an inner closure mechanism into the receiver member to lock a spinal fixation element within the receiver member. The driver can be configured to automatically switch between driving the outer shaft and driving the inner shaft. For example, the driver can include a torque limiting mechanism that automatically switches between driving the outer shaft and the inner shaft. In this way, a single instrument can be used to both lock the angular orientation of the receiver member and lock (or “reduce”) the spinal fixation element. The automatic switching can allow a two-piece closure assembly to be applied in a single step. The torque limiting mechanism can have a threshold force that causes the automatic switching and prevents an excess force from being applied to the closure assembly. 
     Accordingly, the methods and devices described herein allow performing a spinal surgical procedure in a simplified and time-saving manner. Because the bone anchor driver can be used to selectively drive both the outer and inner shafts to apply first and second closure mechanisms, a need for multiple instruments is reduced or eliminated, and surgeon&#39;s fatigue typically associated with repetitive motions can be decreased. 
       FIG. 1  illustrates one embodiment of a bone anchor driver  100  used to apply a closure mechanism to a bone anchor assembly  102 . In the illustrated embodiment, the bone anchor assembly  102  is in a form of a polyaxial bone screw having a bone engaging member  104  with a proximal head  106  and a distal bone shank  108 . The bone anchor assembly  102  also includes a receiver member  110  that polyaxially seats the head  106  of the bone engaging member  104 , and that is configured to receive a spinal fixation element  112 , such as a spinal rod, to couple the spinal fixation element  112  to the bone anchor assembly  102 . The receiver member  110  shown in  FIG. 1  has a proximal end having a pair of spaced apart arms  111 A,  111 B that define a recess or a slot therebetween for receiving the spinal fixation element  112  (e.g., a spinal rod). 
     The illustrated bone anchor assembly  102  also includes a compression member or cap  114  that is received within a central passage in the receiver member  110  and that has a distal surface that rests on the proximal head  106  of the bone engaging member  104 , and a proximal surface that seats the spinal fixation element  112 . The bone anchor assembly  102  further includes a closure assembly having an outer closure mechanism or set screw  124  and an inner closure mechanism or set screw  126 . The outer set screw  124  is configured to be disposed within and to threadably mate with the arms  111 A and  111 B of the receiver member  110 , and the inner set screw  126  is configured to be threadably disposed within the outer set screw  124 . 
     The outer set screw  124  is operable to act on the compression member  114  to fix the bone anchor  104  relative to the receiver member  110 . The inner set screw  126  is operable to act on the spinal fixation element or rod  112  to fix the spinal rod  112  relative to the receiver member  110 . In this way, the closure assembly permits the bone anchor  104  to be fixed relative to the receiver member  110  independently of the spinal rod  112  being fixed to the receiver member  110 . 
     A person skilled in the art will appreciate that the bone anchor driver and methods described herein can be used with various bone anchor assemblies, including various hooks and screws known in the art. The bone anchor assembly of  FIG. 1  is described in more detail in U.S. Patent Application Publication No. 2014/0094849, entitled “Bone Anchor Assemblies,” filed Sep. 17, 2013, the contents of which are incorporated herein by reference in their entirety. It should be appreciated that the embodiments described herein are not limited to any particular type of the bone anchor  104  and that the receiver member  110  can be coupled to the bone anchor  104  in any suitable manner. For example, the bone anchor  104  can be a polyaxial screw such that, prior to fixation, it can be adjustable to multiple angles relative to the receiver member  110 . An exemplary polyaxial bone screw is described U.S. Pat. No. 8,313,516, which is hereby incorporated herein by reference in its entirety. 
     An exemplary bone anchor can also be a favored-angle polyaxial screw in which a cone of angulation is biased in one direction. The favored angle can aid in rod capture during a spinal procedure as the receiver member  110  can have additional range of motion in one direction, e.g., laterally away from the spinal column. Exemplary favored angle bone screws are described in U.S. Patent Application Publication No. 2014/0277159, which is hereby incorporated herein by reference in its entirety. Any other suitable types of bone anchors can be used additionally or alternatively. 
     Furthermore, in some embodiments, an exemplary bone anchor can be in a form of a monoaxial screw. In such embodiments, the compression member or cap  114  can be example, a typhoon cap, such as the Monarch Typhoon Cap available from DePuy Spine, Inc. of Raynham, Mass., and described in U.S. Pat. No. 6,755,829, which is incorporated herein by reference. An outer closure mechanism (e.g., outer set screw  124 ) can be used to cause the typhoon cap to capture a spinal fixation element (e.g., spinal fixation element  112 ) in a manner that allows movement of the bone anchor with respect to the spinal fixation element. An inner closure mechanism (e.g., inner set screw  126 ) can then be used to lock the position of the bone anchor with respect to the spinal fixation element. 
     As further shown in  FIG. 1 , the bone anchor driver  100  can be configured to apply the inner and outer set screws to the bone anchor  104 . The bone anchor driver  100  can have a variety of configurations. In the embodiment illustrated in  FIG. 1 , the bone anchor driver  100  includes an outer shaft  134 , an inner shaft  136  disposed within the outer shaft  134 , a gear box  140  configured to deliver rotational force to selectively drive the outer and inner shafts  134 ,  136 , and an activation handle  142  configured to be held and moved (e.g., rotated) by a surgeon. As shown in  FIG. 1 , the bone anchor driver  100  can optionally include an outer sleeve  138  positioned around the outer and inner shafts  134 ,  136  and having a proximal cylindrical portion and a distal flange having a distal increased diameter portion that receives therein the receiver member  110 . The outer sleeve  138  can have any suitable configuration and, in some embodiments, can be operable as a counter-torque component. 
     The inner and outer shafts can have a variety of configurations to allow for mating with the closure assembly. In the illustrated embodiment, the distal end  134   d  of the outer shaft  134  can engage the outer set screw  124  seated in the receiver member  110  so that the outer shaft  134  can drive the outer set screw  124 . Similarly, a distal end  136   d  of the inner shaft  136  can engage the inner set screw  126  seated within the outer set screw  124  in the receiver member  110  to drive the inner set screw  126 . The inner and outer shafts  136 ,  134  can be configured to engage the inner and outer set screws  126 ,  124  in any suitable manner. For example, each shaft can include a distal drive tip, such as a hex tip, hex sock, or any other features for engaging a set screw as known in the art. The outer and inner shafts  134 ,  136  can drive the outer and inner set screws  124 ,  126 , respectively, by applying a rotational force thereto. In this way, the outer and inner screws  124 ,  126  can be independently tightened to lock the orientation of the receiver member  110  with respect to the bone anchor  104  and to reduce the spinal fixation element  112 . 
     The gear box  140  can have a variety of configurations, various exemplary embodiments of which will be disclosed in more detail below. As also discussed in more detail below, the gear box  140  can include a torque limiting mechanism configured to selectively drive one of the outer and inner shafts  134 ,  136  while the other remains stationary, and to automatically shift when a threshold torque is met to drive the other one of the outer and inner shafts. Although not shown herein, a person skilled in the art will appreciate that the bone anchor driver  100  can include various features that facilitate rotation of the outer and inner shafts  134 ,  136  with respect to each other. 
     The handle  142  can also have a variety of configurations. For example, the handle  142  can be a “T-handle,” an “L-handle,” or a handle having any other suitable configuration such that it is ergonomic and has a grip portion allowing it to be conveniently held by a surgeon. In the illustrated embodiment, the handle  142  extending proximally from the gear box  140  is rotatable by a surgeon to apply a rotational force to the outer and inner shafts  134 ,  136  via the gear box  140 . 
       FIGS. 2A and 2B  illustrate another embodiment of a bone anchor driver  200  that includes gear box  240  and a handle  242 . In this embodiment, the handle  242  is configured to be operated using an external power source, unlike the manual handle of  FIG. 1 . As schematically shown in  FIG. 2A , the handle  242  can be coupled to a powered driver  246  which can be connected to a main power supply via a wire and/or it can be battery-powered. The bone anchor driver  200  can be activated in any suitable manner. For example, the surgeon can activate a button, lever, or any other trigger mechanism for operating the screw driver  200  to apply a tightening torque to the outer set screw or inner set screw. 
     The gear box  240  of bone anchor driver  200  can be similar to gear box  140  of bone anchor driver  100 . A gear box in accordance with the described embodiments, which can be implemented in a manually and/or power-driven bone anchor driver, will be discussed in more detail below. As further shown in  FIG. 2A , the bone anchor driver  200  includes an outer shaft  234  and an inner shaft  236  disposed within a lumen of the outer shaft  234 , only a distal portion of which is shown in  FIG. 2A . 
     As shown in  FIG. 2B , the bone anchor driver  200  can optionally include a counter-torque sleeve  244 . The counter-torque sleeve  244  can be disposed around the outer shaft  234  (not shown in  FIG. 2B ) such that a proximal end of the counter-torque sleeve  244  is coupled to the handle  242 . The counter-torque sleeve  244  can help maintain the receiver member of the bone anchor in a fixed position while the driver rotates the inner and outer set screws into the receiver member. While not shown, a person skilled in the art will appreciate that an inner portion of the counter-torque sleeve can be configured to engage non-rotatably an outer surface of the receiver member. The counter-torque sleeve  244  can be a component of the bone anchor driver  200  formed integrally with one or more components thereof. Alternatively, the counter-torque sleeve  244  can be a separate sleeve. It should be appreciated that any suitable type of a counter-torque sleeve can be utilized as embodiments are not limited in this respect. 
     As indicated above, the bone anchor drivers of  FIGS. 1, 2A, and 2B  each include a gear box for transferring a rotational force from the handle to the inner and outer shafts.  FIGS. 3A and 3B  illustrate one exemplary embodiment of a gear box. As shown, the gear box  300  (e.g., gear box  140  of the bone anchor driver  100  or gear box  240  of the bone anchor driver  200 ) includes a drive transfer member or actuator  302 , an outer drive shaft or outer shaft  304 , an inner drive shaft or inner shaft  306 , and a torque limiting mechanism  308  disposed between the outer shaft  304  and the inner shaft  306 . The drive transfer member  302 , outer and inner shafts  304 ,  306 , and torque limiting mechanism  308  can have a common central longitudinal axis B extending between proximal and distal ends  300   a ,  300   b  of the gear box  300 , as shown in  FIG. 3B . 
     Although not shown in  FIGS. 3A and 3B , a distal end  304   b  of the outer shaft  304  can be coupled to or can be integrally formed with a proximal end  304   a  of the outer shaft  134 ,  234  of the bone anchor drivers of  FIGS. 1 and 2A-2B . Similarly, a distal end  306   b  of the inner shaft  306  can be coupled to or can be integrally formed with a proximal end the inner shaft  136 ,  236  of the bone anchor drivers of  FIGS. 1 and 2A-2B . 
     The drive transfer member  302 , which is configured to transfer a rotational force from the handle to the torque limiting mechanism  308  to selectively drive the outer and inner shafts  304 ,  306 , can have a variety of configurations. As shown, for example, in  FIG. 4A , the drive transfer member  302  is generally configured as an elongate shaft and is disposed within a proximal end  300   a  of the gear box  300 , and in particular extends through the outer shaft  304  and extends into the torque limiting mechanism  308  such that a distal end  302   b  of the drive transfer member  302  non-rotatably engages a proximal end  308   a  of the torque limiting mechanism  308 . As a result, rotation of the drive transfer member will cause corresponding rotation of the torque limiting mechanism. 
     The distal end  302   b  of the drive transfer member  302  that engages the torque limiting mechanism  308  can have a cross-sectional shape that fits a complementary bore  309  formed in the proximal end  308   a  of the torque limiting mechanism  308 . By way of a non-limiting example, the distal end  302   b  of the drive transfer member  302  can have a rectangular cross-sectional shape. However, it should be appreciated that the drive transfer member  302  can have a hexagonal, square, or any other cross-sectional shape that complements a shape of the bore  309  in the torque limiting mechanism  308 , as embodiments are not limited to any particular configuration of the drive transfer member  302 . 
     The proximal portion of the drive transfer member  302  that is positioned within a lumen  310  extending longitudinally through the outer shaft  304  can include any suitable features that facilitate rotation of the drive transfer member  302  with respect to the outer shaft  304 . For example, in the illustrated embodiment, a bearing  312  can be interposed between the drive transfer member  302  and the outer shaft  304  to facilitate sliding and rotation of the drive transfer member  302  with respect to the outer shaft  304 . 
     As shown in  FIGS. 3A and 3B , the bearing  312  can be generally ring-shaped and can be disposed within a suitable bearing retainer, such as a bearing cage  314 . The bearing cage  314  can be seated on an annular support ring  316  formed around the outer surface of the drive transfer member  302  approximately mid-way between the proximal and distal ends  302   a ,  302   b  thereof. In one embodiment, the bearing cage  314  can include cylindrical inner and outer washers  315 ,  317 , as shown in  FIGS. 3A and 3B , that seat the bearing cage  314  therebetween. One skilled in the art will appreciate that a bearing of any suitable configuration can be interposed in any manner between the drive transfer member  302  and the outer shaft  304 . Further, it should be appreciated that any other features that facilitate interaction between the drive transfer member  302  and the outer shaft  304  can be used additionally or alternatively. 
     The outer shaft  304  of the gear box  300  can also have a variety of configurations. In the illustrated exemplary embodiment, the outer shaft  304  is a tubular elongate member having a lumen  310  formed therein that extends between the proximal and distal ends  304   a ,  304   b  thereof. An inner diameter of the lumen in the outer shaft  304  can be such that the inner shaft  306  and the torque limiting mechanism  308  fit within the inner lumen  310  of the outer shaft  304 . As shown in  FIG. 3B , for example, the lumen  310  includes four regions of different diameters. In particular, the lumen  310  includes a proximal portion  310   a  that is sized to slidably and rotatably receive the bearing assembly and a proximal portion of the drive transfer member  302 ; a proximal-mid portion  310   b  that is sized to slidably receive a proximal portion of the torque limiting mechanism  308  and a distal portion of the drive transfer member  302 ; a distal-mid portion  310   c  sized to slidably and rotatably receive a distal portion of the torque limiting mechanism  308  and a portion of the inner shaft  306 ; and a distal portion  310   d  that is sized to slidably and rotatably receive a distal portion of the inner shaft  306  and bearings  322 ,  324 . The proximal-mid, distal-mid, and distal portions  310   b ,  310   c ,  310   d  can have successively increasing diameters that increase distally, and the proximal portion  310   a  can have a diameter that is greater than a diameter of the proximal-mid portion  310   b  and less than a diameter of the distal-mid portion  310   c.    
     The inner surfaces of the proximal, proximal-mid, distal-mid, and distal portions  310   a ,  310   b ,  310   c ,  310   d  can have any suitable features that facilitate interaction between the outer shaft  304 , the torque limiting mechanism  308 , and the inner shaft  306 . For example, as shown in  FIG. 3B , a proximal end of the distal-mid portion  310   c  can have a ramp  311  having a contour that is complementary in shape to a contour of a ramp formed on the torque limiting mechanism  308  to facilitate movement of the torque limiting mechanism  308  within the outer shaft  304 , as will be discussed in more detail below. A person skilled in the art will appreciate that the outer shaft  304  can have any suitable configuration, as embodiments are not limited in this respect. 
     As shown in  FIG. 4A , the inner lumen  310  (e.g., the distal-mid portion  310   c  thereof) of the outer shaft  304  can also include first complementary engaging members  412 ,  414  for engaging with respective first engaging members  402 ,  404  of the torque limiting mechanism  308  that are discussed in more detail below. In the illustrated exemplary embodiment, the first complementary engaging members  412 ,  414  can be in the form of recesses or detents formed in the inner sidewall of the outer shaft  304 . The first complementary engaging members  412 ,  414  can be formed on opposite sides of the distal-mid portion  310   c  of the inner lumen  310  and they can be axially aligned. As shown in  FIG. 4A , the first complementary engaging members  412 ,  414  are disposed at a first longitudinal position along a length of the outer shaft  304 . The first complementary engaging members  412 ,  414  can have a configuration and size appropriate to engage with the first engaging members  402 ,  404  of the torque limiting mechanism  308  (e.g., balls or other engaging members), as also discussed in more detail below. 
     The inner shaft  306  can also have various configurations. As shown in  FIGS. 3A and 3B , the inner shaft  306  can have proximal, middle, and distal portions  318 ,  319 ,  320  that can be separate components or that can be integrally formed with each other. The proximal, middle, and distal portions  318 ,  319 ,  320  have a common longitudinal axis B. As shown in  FIGS. 3A and 3B , and additionally shown in  FIG. 6 , which schematically illustrates an overall shape of the inner shaft  306  (some details are omitted), the proximal portion  318  can have a smallest outer diameter among the proximal, middle, and distal portions  318 ,  319 ,  320 , and the middle portion  319  can have an outer diameter that is greater than the outer diameter of the proximal portion  318  and less than an outer diameter of the distal portion  320 . 
     The proximal and middle portions  318 ,  319  of the inner shaft  306  can be at least partially inserted into the torque limiting mechanism  308 , as shown in  FIGS. 4A-4D . As shown in  FIGS. 3A and 3B , a bearing cap  323  can be inserted over the proximal portion  318  to facilitate interaction between the inner shaft  306  and the torque limiting mechanism  308 . Any other suitable component can additionally or alternatively be used for this purpose. 
     One or more portions of the inner shaft  306  can be shaped and sized so as to engage with the torque limiting mechanism  308  which is configured to drive the inner shaft. For example, as shown in  FIGS. 3A, 3B , and  FIG. 6 , the inner shaft  306  can have an outer collar  321  formed between the middle and distal portions  319 ,  320  thereof. As shown in  FIG. 6 , the outer collar  321  can include a ramp  602  formed thereon and having a contour that is configured to cause the torque limiting mechanism  308  to translate axially between a first position in which it is engaged with the outer shaft  304  and disengaged from the inner shaft  306 , and a second position in which it is engaged with the inner shaft  306  and disengaged from the outer shaft  304 . 
     In the illustrated embodiment, the ramp  602  includes a first portion  602 A and a second portion  602 B axially formed around the inner shaft  306  and interconnecting at first and second junctions  603 ,  605  located at opposite sides of a longitudinal axis B of the inner shaft  306 . In one embodiment, the first and second portions  602 A,  602 B can have equal or substantially equal circumferences and they can be shaped and sized so that they mirror each other. For example, as shown in  FIG. 6 , the first portion  602 A can have an upper surface that gradually declines (decreases) in height in a counterclockwise direction from the first junction  603  to the second junction  605 . In a similar manner, the second portion  602 B can have an upper surface that gradually declines (decreases) in height in a counterclockwise direction from the second junction  605  to the first junction  603 . As a result, the ramp  602  will have first and second stepped portions formed at the first and second junctions  603 ,  605 . A person skilled in the art will appreciate that the incline angle of each portion  602 A,  602 B can vary, but preferably that the incline angle is configured to cause the inner shaft  306  to move a sufficient distance axially so that the torque limiting mechanism  308  will move between the first and second portions  602 A,  602 B, as will be discussed in more detail below. 
     In some embodiments, the first and second stepped portions can have the same or substantially the same heights. It should be appreciated that the surfaces of the first and second portions  602 Aa,  602 B can have any suitable contours. For example, in some embodiments, the gradually inclined surfaces of the first and second portions  602 A,  602 B can be transverse to the longitudinal axis B of the inner shaft  306  throughout (i.e., the surfaces are flat), or they can be at least partially or entirely inwardly inclined. Furthermore, in some embodiments, one or more portions of the first and second portions  602 A,  602 B can be at least partially flat around a radial portion thereof such that the first and second portions  602 A,  602 B are not inclined through their entire surfaces. 
     As shown in  FIGS. 4B and 5A , the middle portion  319  of the inner shaft  306  can include second complementary engaging members  416 ,  418  formed in opposite sides of the inner shaft  306  for engaging with respective second engaging members  406 ,  408  of the torque limiting mechanism  308  that are discussed in more detail below. In the illustrated exemplary embodiment, the second complementary engaging members  416 ,  418  can be recesses or detents formed in the outer surface of the inner shaft  306 . The second complementary engaging members  416 ,  418  can have a configuration and size appropriate to receive the second engaging members  406 ,  408  of the torque limiting mechanism  308  (e.g., balls or other engaging members), as also discussed in more detail below. 
     In some embodiments, the distal portion  320  of the inner shaft  306  can be associated with axially spaced apart bearings  322 ,  324  interposed between the distal portion  320  of the inner shaft  306  and the outer shaft  304  to facilitate rotation of the inner shaft  306  and the outer shaft  304  with respect to each other. In the illustrated exemplary embodiment, as shown in  FIGS. 3A-4B , the bearings  322 ,  324  can be disposed in respective cages  326 ,  328 , and inner and outer washers or retaining members  329 ,  331 , which can be semicircular or circular, hold the bearings  322 ,  324  in place. As also shown, the bearings  322 ,  324  can be spaced a distance apart along the outer surface of the inner shaft  306  so that an intermediate member  330  (e.g., circular or having other shape) is positioned therebetween around the inner shaft  306 . The intermediate member  330  can include an inner portion  330   a  and an outer portion  330   b  that is configured to fit over the inner portion  330   a  such that the outer diameter of the intermediate member  330  matches the outer diameter of the bearing cages  326 ,  328 . 
     It should be appreciated that any suitable number of bearings (e.g., one, two, three, or more), which can have any configuration (e.g., ring, ball, etc.) can be interposed between the inner and outer shafts  306 ,  304 , as the described embodiments are not limited to any particular features that reduce rotational friction between the inner and outer shafts  306 ,  304 . 
     The torque limiting mechanism  308  can have a number of various configurations. As shown in  FIGS. 4A and 4B , the torque limiting mechanism  308  extends between the inner and outer shafts  306 ,  304 . The torque limiting mechanism  308  generally includes proximal, middle, and distal portions  332 ,  334 ,  336 , as shown in  FIGS. 5B-5C . The proximal portion  332  can have an outer diameter that is less than outer diameters of the middle and distal portions  334 ,  336 , and the outer diameters of the middle and distal portions  334 ,  336  can be the same or approximately the same. One skilled in the art will appreciate that the proximal, middle, and distal portions  332 ,  334 ,  336  of the torque limiting mechanism  308  can have any suitable sizes and configurations. 
     The torque limiting mechanism  308  can receive a rotational force applied thereto by the drive transfer member  302 . As discussed above, the torque limiting mechanism  308  can have a bore  309  extending distally from its proximal end  308   a  along a longitudinal axis C (shown in  FIGS. 5B and 5C ) of the torque limiting mechanism  308 . The bore  309  can have a shape such that it is keyed to slidably and matably receive therein the drive transfer member  302 . By way of a non-limiting example, the bore  309  can have a square or hexagonal shape to match a shape of the drive transfer member  302 . The bore  309  can terminate proximally of the middle portion  334 . 
     The rotational force applied to the torque limiting mechanism  308  by the drive transfer member  302  can be transferred to the outer shaft  304  or the inner shaft  306 . As shown in  FIG. 5B , the middle and distal portions  334 ,  336  of the torque limiting mechanism  308  include a bore  313  extending proximally along the longitudinal axis C thereof from the distal end  308   b  to approximately the proximal end of the middle portion  334  such that the bore  313  is not in communication with the drive bore  309 . The bore  313  can receive the proximal portion  318  and at least part of the middle portion  319  of the inner shaft  306 . 
     The torque limiting mechanism  308  can be disposed within the outer shaft  304  such that it can translate axially along the longitudinal axis of the outer shaft  304  and the longitudinal axis of the inner shaft  306 . In the illustrated embodiment, the longitudinal axes B of the inner and outer shafts  306 ,  304  and the longitudinal axis C of torque limiting mechanism  308  coincide with each other. 
     To facilitate the axial movement of the torque limiting mechanism  308 , a proximal facing surface of the middle portion  334  can be configured as a ramp  610  circumferentially formed around the outer surface of the middle portion  334  as shown in  FIGS. 5B and 5C . The ramp  610  can be configured to engage a complementary surface formed in the outer shaft  304  such as, for example, ramp  311  formed at the distal-mid portion  310   c  of the lumen  310  extending through the outer shaft  304 . The ramp  610  can have any suitable configuration. As shown in  FIGS. 5B and 5C , the ramp  610  can have portions each having a surface that extends circumferentially around the outer surface of the torque limiting mechanism  308  from a raised stepped portion, such as a portion  612  (also shown in  FIGS. 3A and 3B ), while gradually decreasing in height in a clockwise direction. In some embodiments, the gradually declining surface of the ramp  610  can have one or more portions that can be inwardly inclined. Furthermore, one or more portions of the ramp  610  can be flat such that the ramp  610  may not be gradually declined throughout the entire surface thereof. It should be appreciated that the torque limiting mechanism  308  can have any other features that help it to move within the outer shaft  304 . 
     The torque limiting mechanism  308  can further include features that facilitate its movement with respect to the inner shaft  306 . Thus, a distal facing surface of the distal portion  336 , which is a bottom of the torque limiting mechanism  308 , can be shaped as a ramp  616  configured to couple to the surface of a ramp formed on the inner shaft  306 , e.g., ramp  602  in  FIG. 6  formed on the outer collar  321 . The ramp  616 , shown in  FIGS. 3B, 5B and 5C , can be complementary to the ramp  602 . For example, the ramp  616  can have portions that are complementary to the first and second portions  602 A,  602 B of the ramp  602 . 
     The configuration of the ramp  616  allows the torque limiting mechanism  308  to engage the inner shaft  306  such that, when a certain threshold torque is reached, the torque limiting mechanism  308  rotates independent of the inner shaft  306  and translates axially upward or downward. In the illustrated embodiment, the ramp  616  can have formed thereon stepped surfaces complementary to the stepped surfaces of the ramp  602  on the inner shaft  306 . For example, as shown in  FIG. 5C , the ramp  616  can have a stepped edge  618  configured to mate with the first or second stepped portions formed at the first and second junctions  603 ,  605  of the ramp  602  formed on the outer collar  321  of the inner shaft  306 . 
     The torque limiting mechanism  308  can be disposed between the outer and inner shafts  304 ,  306  so that it can automatically switch between being engaged with and rotatable with the outer shaft  304  while being disengaged from the inner shaft  306 , and being engaged with and rotatable with the inner shaft  306  while being disengaged from the outer shaft  304 . To selectively drive the outer and inner shafts  304 ,  306  in this manner, the torque limiting mechanism  308  includes engaging members configured to engage with respective complementary engaging members of the outer and inner shafts  304 ,  306 . As shown in  FIGS. 3A-5B , the middle portion  334  of the torque limiting mechanism  308  can include two first engaging members  402 ,  404  configured to engage with the first complementary engaging members  412 ,  414  of the outer shaft  304 , and the distal portion  336  of the torque limiting mechanism  308  can include two second engaging members  406 ,  408  configured to engage with second complementary engaging members  416 ,  418  of the inner shaft  306 . 
     In the illustrated exemplary embodiment, the first engaging members  402 ,  404  are positioned in a first plane and the second engaging members  406 ,  408  are positioned in a second plane that is spaced a distance apart from the first plane. As shown in  FIGS. 4A and 5B , the first engaging members  402 ,  404  can be disposed within first bores  502 ,  504 , respectively, extending across the middle portion  334  transverse to the longitudinal bore  313  in the torque limiting mechanism  308 . The second engaging members  406 ,  408  can be disposed within second bores  506 ,  508 , respectively, extending across the distal portion  336  transverse to the longitudinal bore  313 . As shown in  FIG. 4A , the first bores  502 ,  504  can extend approximately through a mid-portion of the middle portion  334  along an axis that is substantially perpendicular to a longitudinal axis C of the torque limiting mechanism  308 . In a similar manner, as shown in  FIG. 4B , the second bores  506 ,  508  can extend approximately through a mid-portion of the distal portion  336  along an axis that is substantially perpendicular to the longitudinal axis C of the torque limiting mechanism  308 . In some embodiments, however, the first bores  502 ,  504  and/or the second bores  506 ,  508  can extend through the torque limiting mechanism  308  at an angle with respect to the longitudinal axis C. 
     The first bores  502 ,  504  and the second bores  506 ,  508  can extend through the torque limiting mechanism  308  such that they each form openings on the outer surface thereof. The first engaging members  402 ,  404  and the second engaging members  406 ,  408  can engage with respective first complementary engaging members  412 ,  414  and the second complementary engaging members  416 ,  418  through the openings of the first and second bores  502 ,  504 ,  506 ,  508 . 
     In the illustrated embodiments, the second bores  506 ,  508  can be spaced radially apart from the first bores  502 ,  504  such that an axis of the second bores  506 ,  508  extend transverse, e.g., perpendicular, to an axis of the first bores  502 ,  504 . It should be appreciated that although the first bores  502 ,  504  are shown to extend along the same axis, in some embodiments, the first bore  502  can extend at an angle to the first bore  504 . Similarly, the second bore  506  can extend at an angle to the second bore  508 . 
     In some embodiments, as discussed above, the first complementary engaging members  412 ,  414  of the outer shaft  304 , which are configured to engage with the first engaging members  402 ,  404 , can be recesses, detents, or any other suitable features formed in the surface of the lumen  310  of the outer shaft  304 . As shown, for example, in  FIG. 4A , the first recesses  412 ,  414  can be formed in the same plane. As shown in  FIG. 4B , the second complementary engaging members  416 ,  418  of the inner shaft  306 , which are configured to engage with the second engaging members  406 ,  408 , can also be recesses, detents, or any other suitable features formed in the outer surface of the middle portion  319  of the inner shaft  306 . The second recesses  416 ,  418  can be formed in the same plane, which is distally spaced apart from the plane in which the first recesses  412 ,  414  are formed. 
     The first engaging members  402 ,  404  and the second engaging members  406 ,  408  can have any suitable configurations, including configurations that differ between the first engaging members  402 ,  404  and the second engaging members  406 ,  408 . In the illustrated embodiment, each of the first and second engaging members  402 ,  404 ,  406 ,  408  is in the form of a ball or other retaining element. Each engaging member can be biased into engagement with a respective complementary engaging member. As shown in  FIG. 4A , the first engaging member or ball  402  is biased into engagement with the first complementary engaging member or recess  412  in the outer shaft  304  by a first spring  402 A, and the first engaging member or ball  404  is biased into engagement with the first complementary engaging member or recess  414  formed in the outer shaft  304  by a first spring  404 A. Second springs  406 A,  408 A can bias the second engaging members or balls  406 ,  408 , respectively, into engagement with the second complementary engaging members or recesses  416 ,  418  formed in the inner shaft  306 . 
     When the torque limiting mechanism  308  is in the first position in which it is engaged with the outer shaft  304  and is disengaged from the inner shaft  306 , the first balls  402 ,  404  can protrude from the first bores  502 ,  504  and into the first recesses  412 ,  414 . The second balls  406 ,  408  will not be aligned with the second recesses  416 ,  418 , and thus the second balls  406 ,  408  will be disposed within the second bores  506 ,  508 . In a similar manner, when the torque limiting mechanism  308  is in the second position in which it is engaged with the inner shaft  306  and is disengaged from the outer shaft  304 , the second balls  406 ,  408  can protrude from the second bores  506 ,  508  and into the second recesses  416 ,  418 . The first balls  402 ,  404  will not be aligned with the first recesses  412 ,  414 , and thus the first balls  402 ,  404  will be disposed within the first bores  502 ,  504 .  FIG. 4D  illustrates the torque limiting mechanism  308  turned 90 degrees along its longitudinal axis C from the second position shown in  FIG. 4B  such that the first engaging members  402 ,  404  for engaging with the first complementary engaging member (recesses, in this example)  412 ,  414  formed in the lumen  310  of the outer shaft  304  are visible. 
     It should be appreciated that the first engaging members  402 ,  404  and the second engaging members  406 ,  408  are shown as balls that are spring-biased by way of example only, as any other engaging members configured to engage complementary engaging members of the inner and outer shafts can be used additionally or alternatively. For example, any suitable biasing elements other than a spring can be used. As another variation, any suitable object can be used that can be biased by a biasing element. Furthermore, the first engaging members  402 ,  404  and the second engaging members  406 ,  408  can be other types of engaging members, as embodiments are not limited to any specific configuration of the first and second engaging members. 
     As shown, for example, in  FIG. 3A , in one embodiment, the torque limiting mechanism  308  can include a closure member  340  that can be disposed axially around the distal portion  336  of the torque limiting mechanism  308  to enclose the openings of the second bores  506 ,  508  in the torque limiting mechanism  308 . 
     The recesses  412 ,  414 ,  416 ,  418  can be shaped to retain the engaging members, e.g., balls, so as to prevent the balls from being rotated out of the recesses until a predetermined torque is applied thereto. The recesses can thus have a configuration that results in a desired threshold force. By way of non-limiting example, each recess can have a generally hemi-cylindrical shape, however one or more of the sidewalls can be angled so as to allow the ball to be rolled out of the recess in a circumferential direction. The angle of the sidewall can be adjusted as desired to achieve the necessary threshold torque. Exemplary embodiments of recess configurations will be discussed in more detail below. A person skilled in the art will appreciate that various techniques can be used to retain an engaging member in an engaged position until a desired threshold force is applied to cause the engaging member to become disengaged. Furthermore, in embodiments in which engaging members other than balls biased by respective springs are utilized additionally or alternatively, the recesses can be configured accordingly, to receive and retain those engaging members. 
     The described devices and methods allow automatically switching between driving the inner shaft and driving the outer shaft. With reference to  FIGS. 4A-4D , the bone anchor driver having a torque limiting mechanism  308  can be used to selectively drive an outer shaft to tighten an outer closure mechanism or an inner shaft to tighten an inner closure mechanism. The torque limiting mechanism  308 , driven (manually or electrically) via the drive transfer member  302 , can automatically switch between driving the outer and inner shafts  304 ,  306 , which can be done repeatedly. For example, a surgeon can actuate a handle, such as handle  142  ( FIG. 1 ) or handle  242  ( FIGS. 2A and 2B ), to deliver a rotational force to the drive transfer member  302  that, in turn, applies that force to the torque limiting mechanism  308 . 
     As shown in  FIG. 4A , when the rotational force is applied to the torque limiting mechanism  308 , the torque limiting mechanism  308  can be in a first position in which it is engaged with the outer shaft  304  and is disengaged from the inner shaft  306 . In the first position, the first engaging members  402 ,  404  are engaged with and disposed in the same plane as the first complementary engaging members  412 ,  414 . For example, in one embodiment, as shown in  FIG. 4A , the first balls  402 ,  404  are biased by the springs  402 A,  404 A into engagement with the complementary recesses  412 ,  414  formed in the outer shaft  304 . In this way, the rotational force applied to the torque limiting mechanism is transmitted to the outer shaft  304  to drive the outer shaft  304 . As discussed above, the distal end  304   b  of the outer shaft  304  can be coupled to or can be integrally formed with outer shaft  134  such that the rotational force applied to the outer shaft  304  causes an outer closure mechanism (e.g., outer set screw  124  in  FIG. 1 ) to rotate into the receiver member  110 . 
       FIG. 4C  illustrates the torque limiting mechanism  308  turned 90 degrees along its longitudinal axis C from the first position shown in  FIG. 4A  such that only the second engaging members  406 ,  408  for engaging with the second complementary engaging member  416 ,  418  (recesses, in this example) of the inner shaft  306  are visible. As shown in  FIG. 4C , in the first position, the second balls  406 ,  408  are misaligned axially with respect to the second complementary recesses  416 ,  418  formed in the inner shaft  306  and the torque limiting mechanism  308  is thus not engaged with the inner shaft  306 . 
     The torque limiting mechanism  308  as shown in  FIG. 4A  will continue to rotate the outer shaft  304  as long as the torque applied thereto is less than or equal to a first threshold torque. The first threshold torque can have any suitable value(s), as the described embodiments are not limited in this respect. 
     When the outer set screw  124  is fully engaged with the receiver member (e.g., receiver member  110  in  FIG. 1 ) to lock the polyaxial position of the receiver member with respect to the bone engaging member (e.g., bone engaging member  104  in  FIG. 1 ), the torque required to rotate the outer set screw  124  will increase and thus exceeds the first threshold torque. For example, in the illustrated embodiment, when resistance from the outer set screw  124  becomes such that the torque applied to the torque limiting mechanism  308  exceeds the first threshold torque, the bias of each of the springs  402 A,  404 A can be overcome to cause the balls  402 ,  404  to disengage from the recesses  412 ,  414  such that the torque limiting mechanism  308  becomes disengaged from the outer shaft  304 . When it becomes disengaged from the outer shaft  304 , the torque limiting mechanism  308  can rotate independent of the outer shaft  304 . 
     A torque that exceeds the first threshold torque can cause the torque limiting mechanism  308 , driven by the drive transfer member  302 , to automatically switch from the first position to the second position in which it is disengaged from the outer shaft  304  and becomes engaged with the inner shaft  306 . Because the switch from the first position to the second position (and vice versa) occurs automatically, without an action by an operator (e.g., surgeon), it can be perceived by the surgeon as a single operation, which improves overall experience of the surgeon and simplifies the surgery. 
     To transition from the first position to the second position, the torque limiting mechanism  308  can move distally to the position as shown in  FIG. 4B . As the torque limiting mechanism  308  continues to rotate independent of the outer shaft, it moves distally due to the declining ramp  602 . The second engaging members  406 ,  408  will move into alignment and engagement with the complementary second engaging members  416 ,  418  formed in the inner shaft  306 . For example, in the illustrated exemplary embodiment, the balls  406 ,  408  can be biased by the springs  406 A,  408 A to engage with the recesses  416 ,  418  formed in the inner shaft  306 , as shown in  FIG. 4B . In the second position, the balls  402 ,  404  are disposed distal of the recesses  412 ,  414  formed in the outer shaft  304 . In this way, the balls  402 ,  404  move with the torque limiting mechanism  308  while remaining disengaged from the outer shaft  304 . 
     Once the torque limiting mechanism  308 , while being disengaged from the outer shaft  304 , engages with the inner shaft  306  in this manner, the force applied to the torque limiting mechanism  308  is transmitted to the inner shaft  306  to thus drive the inner shaft  306 . 
     Driving the inner shaft  306  locks a spinal fixation element, such as the spinal fixation element or rod  112  shown in  FIG. 1 , in a fixed position within the receiver member  110  of the bone anchor assembly  102 . The spinal fixation element  112  can be said to be “reduced.” As discussed above, the distal end  306   b  of the inner shaft  306  can be coupled to or can be integrally formed with inner shaft  136  or  236 . Thus, the rotational force applied to the inner shaft  306  by the torque limiting mechanism  308  in the second position can cause an inner closure mechanism (e.g., inner set screw  126  in  FIG. 1 ) to tighten to thereby lock the spinal fixation element  112  in a fixed position within the receiver member  110 . 
     In some embodiments, the inner shaft  306  engaged with the torque limiting mechanism  308  as shown in  FIG. 4B  can continue rotating until the inner set screw  126  is tightened and a torque applied to the torque limiting mechanism  308  exceeds a second threshold torque. For example, when resistance from the inner set screw  126  becomes such that the torque applied to the torque limiting mechanism  308  exceeds the second threshold torque, the bias of each of the springs  406 A,  408 A can be overcome to cause the balls  406 ,  408  to disengage from the recesses  416 ,  418  such that the torque limiting mechanism  308  becomes disengaged from the inner shaft  306 . The second threshold torque can have any suitable value, which can be the same or different from a value of the first threshold torque. 
     When the torque applied to the torque limiting mechanism  308  exceeds the second threshold torque, the torque limiting mechanism  308  can switch from the second position to the first position in which it is engaged with the outer shaft  304  and is disengaged from the inner shaft  306 . Thus, the bone anchor driver can be operated such that the torque limiting mechanism  308  can automatically and repeatedly switch between driving the outer and inner shafts  304 ,  306  when the respective first and second threshold torques are exceeded. 
     With reference to  FIGS. 4A-4D, 5B, 5C and 6 , the configuration of complementary ramps formed on the inner and outer shafts  306 ,  304  and the torque limiting mechanism  308  will cause the torque limiting mechanism  308  to translate axially between the first and second positions for driving the outer and inner shafts, respectively. For example, when the torque limiting mechanism  308  is in the first position for driving the outer shaft, as shown in  FIGS. 4A and 4C , the more proximal ramp  610  on the torque limiting mechanism  308  will be in close mating engagement with the complementary ramp  311  formed at the distal-mid portion  310   c  of the lumen  310  extending through the outer shaft  304 . The more distal ramp  616  on the torque limiting mechanism  308  will be spaced apart from ramp  602  formed in the inner shaft  306 , as shown in  FIG. 4A . 
     When the torque limiting mechanism moves from the first position to the second position for driving the inner shaft, the torque limiting mechanism  308  will rotate independent of and relative to the inner and outer shafts  306 ,  304 . Such clockwise rotation of the torque limiting mechanism will cause ramp  610  to rotate relative to ramp  311 . As ramp  610  is rotated in a clockwise direction relative to ramp  311 , the increasing height of ramp  610  and of ramp  311  will cause the torque limiting mechanism  308  to move distally away from ramp  610  on the outer shaft  304  and to move toward ramp  602  on the inner shaft  306 . In particular, the stepped portions of the ramp  610  (only portion  612  is shown in  FIG. 5C ) on the torque limiting mechanism will move out of engagement with the stepped portions on the ramp  311  on the outer shaft, and the ramps  610 ,  311  will ride against one another, increasing in height through their relative rotation to cause distal movement of the torque limiting mechanism. Once the torque limiting mechanism is moved to a distal-most position, the torque limiting mechanism will be in the second position for driving the inner shaft  306 . The more distal ramp  616  on the torque limiting mechanism will be in mating engagement with the ramp  602  on the inner shaft. In particular, the declining portions of the ramps can slide into engagement with one another until the stepped portions of ramp  616  engage with the stepped portions formed at the first and second junctions  603 ,  605  of ramp  602 . Thus, in the second position, the ramp  616  of the torque limiting mechanism  308  is in full engagement with and contacts the surface of the ramp  602  formed in the inner shaft  306 , as shown in  FIGS. 4B and 4D . 
     When switching from driving the inner shaft  306  in the second position to driving the outer shaft  304  in the first position, the geometry of the ramp  602  on the inner shaft will similarly cause the torque limiting mechanism to ride up the ramp  602  and move proximally toward the first position. As a result, ramp  616  of the torque limiting mechanism  308  will no longer sit in contact with the surface of the ramp  602 . 
     As one skilled in the art will appreciate, a torque applied to the torque limiting mechanism can be measured in any suitable manner. For example, a suitable component (e.g., a handle) of the bone anchor driver can include one or more sensors configured to measure the torque. A torque applied to both inner and outer shafts can be measured. Any other parameters, such as, for example, a driving speed of the driver (e.g., in RPM (revolutions per minute)) can be measured as well. In some embodiments, the torque and/or the driving speed can be measured continuously when the bone anchor driver is in use. Data connected by the sensor(s) and/or any other measuring elements can be visually displayed on a suitable display such that a surgeon can monitor it during a surgery. 
     In some embodiments, the first and second threshold torques can be adjustable, which can be performed manually or automatically. Thus, a torque limiting mechanism of a bone anchor driver implemented using the described embodiments can be adjusted to switch between driving outer and inner shafts depending on different threshold torque levels. 
     In some embodiments, a torque can be adjusted by changing one or more characteristics of first and second engaging members and/or first and second complementary engaging members. For example, the torque can depend on angle(s) of bores  502 ,  504 ,  506 ,  508  holding engaging members  402 ,  404 ,  406 ,  408  with respect to a longitudinal axis of the torque limiting mechanism. As an example, a more acute angle of a bore can require less torque to switch between the first and second positions of the torque limiting mechanism. 
     Furthermore, the features of first and second engaging members such as a force applied by a biasing element (e.g., a spring), configuration of the biasing element and the retaining element (e.g., a ball), and their position with respect to each other can affect the torque to be used to switch between driving the outer and inner shafts of the bone anchor driver.  FIG. 7  illustrates an example of an angle α between a longitudinal axis  702  of a bore  700 , e.g., second bore  506  or  508  for engagement with the inner shaft, (and therefore a longitudinal axis of a spring or other biasing member with coincides with the longitudinal axis of the bore) and an axis  704  that is transverse to the longitudinal axis of the torque limiting mechanism that can be adjusted to adjust the torque. As the angle α decreases, the greater torque value(s) can be required to switch between engaging the outer and inner shafts. 
     Additionally or alternatively, in some embodiments, the contour of complementary engaging members, such as recesses or detents formed in the inner and outer shafts, can be modified to adjust the first and second threshold torques. The recesses or detents can have a ramp formed on inner surfaces thereof, and an angle of the ramp can be modified to adjust the first and second threshold torques. The complementary engaging members can be shaped and sized so that to adjust an angle at which an engaging member, such as a ball or other retention element, engages with the respective complementary engaging. In addition to illustrating that an angle alignment of the engaging member can affect the torque limits,  FIG. 7  illustrates schematically, by an arrow  706 , that different configurations of a ramp  708  formed in the complementary engaging member can affect the torque limits. 
       FIG. 8A  illustrates the geometry of a first complementary engaging member  802  (e.g., first complementary engaging member  412  or  414 ) of the outer shaft  304  for engaging with a first engaging member of the torque limiting mechanism  308  that can affect at least one of the first and second threshold torques. As shown, the geometry of the first complementary engaging member  802  can be characterized by various parameters such as, for example, a first radius R 1  measured from a center O of outer shaft  304  (which coincides with the center of the torque limiting mechanism  308 ) to a plane extending through an outer surface of the engaging member  802  parallel to the longitudinal axis of the outer shaft  304 , a second radius R 2  of the inner surface of the engaging member  802 , a third radius R 3  measured from the center O of outer shaft  304  to an inner surface of the engaging member  802 , and an angle θ between the third radius R 3  and an axis  810  that is transverse to the common central longitudinal axis of the outer shaft  304  and the torque limiting mechanism  308 . In one example, the first radius R 1  is about 12.5 mm, the second radius R 2  is about 2 mm, the third radius R 3  is about 14.5 mm, and the angle θ is about 15°. 
     As another example,  FIG. 8B  illustrates the geometry of a second complementary engaging member  820  (e.g., second complementary engaging member  416  or  418 ) of the inner shaft  306  for engaging with a second engaging member of the torque limiting mechanism  308  that can affect at least one of the first and second threshold torques. As shown, the geometry of the second complementary engaging member  820  can be characterized by various parameters such as, for example, a first radius R 1  measured from the outer surface of the torque limiting mechanism  308  to a plane extending through an outer surface of the engaging member  820  parallel to the longitudinal axis of the inner shaft  306 , a second radius R 2  of the inner surface of the engaging member  820 , and a third radius R 3  measured from the outer surface of the torque limiting mechanism  308  to an inner surface of the engaging member  820 . In the illustrated embodiment, the threshold torque primarily depends on the radius R 2 . The radius R 2  can have any suitable value defined by a shape of the inner surface of the second complementary engaging member  820  (e.g., linear, curved, or any other shape). When the shape of the inner surface of the engaging member  820  is such that the inner surface is aligned with the angle of entry of an engaging member (e.g., second engaging member  406  or  408 ), a threshold torque required to switch the torque limiting mechanism  308  from driving the outer shaft  304  to driving the inner shaft  306  can decrease. In other words, if the inner surface of the second complementary engaging member  820  is such that it is easier for the engaging member to escape the recessed surface of the complementary engaging member  820  as the outer shaft  304  is driven, a smaller threshold torque can be required for the torque limiting mechanism  308  to switch from driving the outer shaft  304  to driving the inner shaft  306 . 
     It should be appreciated that the first and second complementary engaging members  802 ,  820  can have any suitable contours and various features of the contours can be adjusted to thereby adjust at least one of the first and second threshold torques used to switch the bone anchor driver from driving the inner and outer shaft. 
     Having thus described at least one illustrative embodiment, various alterations, modifications and improvements will readily occur to those skilled in the art. 
     For example, in one embodiment, a bone anchor driver can be provided that includes a torque limiting mechanism configured to automatically switch between a first position in which the torque limiting mechanism is disengaged from the outer shaft and is engaged with the inner shaft, and a second position in which the torque limiting mechanism is disengaged from the inner shaft and is engaged with the outer shaft. 
     Further, although in the illustrated embodiments, an outer closure mechanism is configured to mate to a receiver member for locking a polyaxial position of the receiver member with respect to the bone engaging member, and an inner closure mechanism is configured to lock a spinal fixation element within the receiver member, in one embodiment, the outer closure mechanism can be configured to lock a spinal fixation element within the receiver member, and the inner closure mechanism can be configured to lock a polyaxial position of the receiver member with respect to the bone engaging member. 
     As another variation, while in the illustrated embodiment a bone anchor assembly is in a form of a polyaxial bone screw, as mentioned above, the bone anchor assembly can be in a form of a monoaxial bone screw. In such an embodiment, a two-piece locking cap can be used. For example, an outer closure mechanism can coupled with a typhoon cap and it can be driven to capture a spinal fixation element (e.g., a spinal rod) while allowing for translation of the monoaxial bone screw along the spinal fixation element. An inner closure mechanism can then be driven to lock the bone anchor assembly with respect to the spinal fixation element. 
     The devices discussed herein can be designed to be disposed of after a single use, or they can be designed to be used multiple times. In either case, however, the device can be reconditioned for reuse after at least one use. Reconditioning can include any combination of the steps of disassembly of the device, followed by cleaning or replacement of particular pieces, and subsequent reassembly. In particular, the device can be disassembled, and any number of the particular pieces or parts of the device can be selectively replaced or removed in any combination. Upon cleaning and/or replacement of particular parts, the device can be reassembled for subsequent use either at a reconditioning facility, or by a surgical team immediately prior to a surgical procedure. Those skilled in the art will appreciate that reconditioning of a device can utilize a variety of techniques for disassembly, cleaning/replacement, and reassembly. Use of such techniques, and the resulting reconditioned device, are all within the scope of the present application. 
     Preferably, the embodiments described herein will be processed before use. First, a new or used instrument is obtained and if necessary cleaned. The instrument can then be sterilized. In one sterilization technique, the instrument is placed in a closed and sealed container, such as a plastic or TYVEK bag. The container and instrument are then placed in a field of radiation that can penetrate the container, such as gamma radiation, x-rays, or high-energy electrons. The radiation kills bacteria on the instrument and in the container. The sterilized instrument can then be stored in the sterile container. The sealed container keeps the instrument sterile until it is opened in the medical facility. 
     It is preferred that device is sterilized. This can be done by any number of ways known to those skilled in the art including beta or gamma radiation, ethylene oxide, steam, and a liquid bath (e.g., cold soak). 
     One skilled in the art will appreciate further features and advantages of the methods and devices based on the above-described embodiments. Accordingly, the methods and devices are not to be limited by what has been particularly shown and described, except as indicated by the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety.