BACKGROUND 
     The present disclosure contemplates instrumentation and procedures for achieving spinal fixation or more particularly for percutaneously introducing a spinal fixation system into a patient. 
     A typical spinal fixation system  10  as shown in  FIG. 1  spans between successive vertebrae V of the spine. An elongated member, such as rod  12 , extends along the length of the spine and provides an anchor point for connecting each vertebra to the rod. The rod is typically contoured to approximate the normal curvature of the spine for the particular instrumented spinal segments, which may include lordosis or kyphosis. Anchor devices  15  are provided for connecting the vertebral segments to the elongated member. These anchor devices may include hooks, bolts, screws or other means for engaging a vertebra. For the purposes of the present discussion, the anchor device  15  is a bone screw assembly, such as the screw assembly shown in  FIG. 2 . However, it should be appreciated that the instrumentation and procedures disclosed herein may be implemented with other types of anchor devices, such as a hook engaged to the lamina of a vertebra for instance. 
     The bone engaging fastener or screw assembly  15  includes a shank  16  that carries threads configured to engage vertebral bone. For instance, the fastener is a pedicle screw with a shank that is threaded for engagement within the pedicle of the vertebra. The screw assembly further includes a head  16   a  by which the screw, and ultimately the vertebra, is fastened to the spinal rod  12 . In particular, the head  16   a  supports a yoke  17  that is generally U-shaped to receive the spinal rod therethrough, as depicted in  FIG. 2 . The rod  12  may be supported in part by a collar  18  mounted over the head  16   a  of the bone screw. A cap  19  carries a set screw  20  that locks the rod within the yoke  17  and thus fastens the rod  12  to the bone screw. 
     One embodiment of a bone screw assembly  15  is disclosed in co-pending U.S. application Ser. No. 11/762,898 (the &#39;898 application), entitled “Multi-Axial Fixation Assembly”, field on Jun. 14, 2007 and published as No. 2008/0119858, the disclosure of which is incorporated herein by reference. For the purposes of the present disclosure, the bone screw  15  may be constructed as disclosed in the &#39;898 application, although it is understood that other bone screw or multi-axial fastener configurations may be implanted using the instruments and procedures disclosed herein. In the multi-axial bone screw assembly  15  the yoke  17  is articulatingly attached to the threaded bone screw  16 , and more specifically to the head  16   a  of the bone screw, so that the yoke  17  can adopt a range of spherical angles relative to the bone screw. Thus, the yoke can articulate relative to the bone screw fastened in the vertebra so that the slot  42  can be aligned to receive the connecting rod  25 . 
     While in the past spinal fixation systems have been implanted in open procedures involving relatively large incisions through the patient&#39;s tissue with significant muscle retraction, more recent procedures have been developed to percutaneously introduce spinal fixation systems in a minimally invasive manner. One technique known as the Sextant® System is described in U.S. Pat. No. 6,530,929, issued to Justis, et al. In the &#39;929 patent, separate incisions are made for introducing respective pedicle screws each attached to a tubular extension extending outwardly from the patient through each incision. A pivot arm coupled to the extensions introduces an elongate rod through another separate incision remote from the incisions receiving the extensions. The pivot arm urges the rod beneath the skin and into the pedicle screws for fixation. Other percutaneous systems such as that shown in U.S. Pat. No. 7,306,603 issued to Boehm, Jr. et al. utilize tubular pedicle screw extensions to place a rod longitudinally through the extension into one of the pedicle screws. The rod is then pivoted about the pedicle screw through an incision between the pedicle screws to the second pedicle screw. Others still employ systems such as that shown in U.S. Pat. No. 7,250,052 issued to Landry et al. wherein slots in the screw extensions are used to guide a rod between the extensions through a single incision into position in two or more pedicle screws. 
     Nevertheless, there is current desire for minimally invasive instruments and procedures for the percutaneous placement of spinal fixation systems that are relatively simple and easy to use and that provide for enhanced assurance of rod introduction and connection to the spinal implants. 
     SUMMARY 
     A method of attaching an extension to a bone anchor for use in percutaneous spinal surgery is provided that comprises the steps of: providing a bone anchor including a first portion for attachment to a vertebra and a second portion for attachment to a connecting element, the bone anchor including an extension coupling surface; providing an elongate hollow extension having a distal end and a proximal end, the extension having a rotationally movable anchor engaging member projecting radially from the distal end thereof; placing the distal end of the extension adjacent the bone anchor; and rotating the anchor engaging member to engage the extension coupling surface on the anchor and couple the extension and the bone anchor. 
     A further method is provided for attaching a bone screw and an extension to a vertebra of a spine that comprises the steps of: providing a multi-axial bone screw including an elongate shaft having a threaded screw portion at the distal end and a screw head at the proximal end, and a yoke articulatingly attached to the screw head, the yoke having a slot therethrough for receiving a connecting rod and including a lower surface, the screw head having a bone engaging surface between and spaced from the threaded screw portion and the yoke lower surface, the yoke having an opening communicating with the screw head; attaching an elongate hollow screw extension to the yoke for articulating movement therewith; extending a tip of a screw driver through the extension and into the opening of the yoke to releasably engage the head of the bone screw for joint rotational movement therewith; and threadably driving by the screw driver the threaded portion of the bone screw into a vertebra until the bone engaging surface of the screw head seats in the vertebra with the lower surface of the yoke spaced proximally of and not in contact with the vertebra, thereby allowing unhindered articulation of the yoke and the screw extension attached thereto. 
     Another method of percutaneously fixing a connecting rod to vertebrae of a spine, comprises the steps of: providing a multi-axial bone screw including a shaft having a threaded screw portion and a yoke articulatingly attached to the shaft, the yoke having an exterior surface and a slot therethrough for receiving a connecting rod, the slot defined by a pair of upstanding arms having opposed interior surfaces, the yoke including an extension coupling surface; and providing a screw extension including; (a) an elongate hollow outer sleeve having a distal end and a proximal end the outer sleeve including a perimetric sidewall defining a hollow interior, the sidewall having a pair of opposing slots extending through the sidewall and opening at the distal end of the outer sleeve; (b) an elongate hollow inner sleeve disposed within the outer sleeve and having a distal end and a proximal end, the outer sleeve and the inner sleeve being movable rotationally relative to each other, the screw extension having a pair of opposing slots through the sidewall and opening at the distal end, the inner sleeve including a perimetric sidewall defining a hollow interior, the sidewall having a pair of opposing slots extending through the sidewall and opening at the distal end of the outer sleeve, the slots of the inner sleeve being alignable with the slots of the outer sleeve, the inner sleeve and the outer sleeve being coupled to allow rotational but not axial movement; and (c) a screw engaging member projecting radially from the distal end of one of the outer sleeve and the inner sleeve and configured for locking engagement with the yoke extension coupling surface upon relative rotation of the inner sleeve and the outer sleeve. 
     The method may further comprise: forming a bone screw extension assembly by placing the bone screw and the screw engaging member in juxtaposition and rotating the inner sleeve and the outer sleeve relative to each other to rotate the screw engaging member into cooperative releasable locking engagement with the yoke extension coupling surface, the opposing slots of the inner sleeve and the opposing slots of the outer sleeve being aligned with the slot in the yoke; attaching the bone screw extension assembly to the vertebra by threading the threaded portion of the bone screw into the vertebra; connecting the proximal end of an elongate connecting rod to a rod introducer, the distal end of the rod being free; and introducing from a position exteriorly of the bone screw extension assembly the distal end of the rod into the aligned slots of the inner sleeve and the outer sleeve and displacing the rod along the slots into the slot of the yoke. 
     In a further aspect, a screw driver is provided for driving a multi-axial bone screw into a vertebra of a spine, the bone screw including an elongate shaft having a threaded screw portion at the distal end and a screw head at the proximal end, and a yoke articulatingly attached to the screw head, the yoke having a slot therethrough for receiving a connecting rod and including a lower surface, the yoke having an opening communicating with the screw head, in which the screw driver comprises: an elongate inner shaft having a distal end and a proximal end, the inner shaft defining a driving tip at the distal end configured to engage the screw head for rotation thereof; and an outer retention sleeve disposed about the inner shaft and having a distal end and a proximal end, the inner shaft being freely rotatable within the outer retention sleeve and including the tip at the distal end thereof, the distal end of the outer retention sleeve configured to releasably engage the yoke for joint rotational movement therewith. 
    
    
     
       DESCRIPTION OF THE FIGURES 
         FIG. 1  is a representation of a portion of a patient&#39;s spine instrumented with a multi-level fixation system. 
         FIG. 2  is a perspective view of a bone engaging fastener in the form of a pedicle screw suitable for use with the instrumentation and procedures disclosed herein. 
         FIG. 3  is a perspective view of instrumentation disclosed herein used to introduce an elongated connecting element to a fixation assembly. 
         FIG. 4  is an exploded perspective view of a bone screw and a screw extension assembly disclosed herein. 
         FIG. 5  is an enlarged view of the bone screw and the distal end of the screw extension assembly shown in  FIG. 4 . 
         FIG. 6  is a cross-sectional view of the bone screw and screw extension assembly shown in  FIG. 5  with the screw extension assembly in a first position. 
         FIG. 7  is a cross-sectional view of the bone screw and screw extension assembly shown in  FIG. 6  with the screw extension assembly mounted on the bone screw in the first position. 
         FIG. 8  is a perspective view of the bone screw and screw extension assembly shown in  FIG. 7 . 
         FIG. 9  is an enlarged cross-sectional view of the bone screw and screw extension assembly shown in  FIG. 4  with the screw extension assembly mounted on the bone screw in a second position. 
         FIG. 10  is a perspective view of the bone screw and screw extension assembly shown in  FIG. 9 . 
         FIG. 11  is an enlarged cross-sectional view of the bone screw and screw extension assembly shown in  FIG. 4  with the screw extension assembly mounted on the bone screw in a third position. 
         FIG. 12  is an enlarged exploded view of the proximal end of the screw extension assembly and the socket driver shown in  FIG. 4   
         FIG. 13  is an enlarged perspective view of the distal end of the screw extension assembly shown in  FIG. 4  with the assembly in a first loading position. 
         FIG. 14  is a cross-sectional view of the socket driver mounted to the distal end of the screw extension assembly in a first position. 
         FIG. 15  is an enlarged perspective view of the distal end of the screw extension assembly shown in  FIG. 4  with the assembly in a second loading position. 
         FIG. 16  is an enlarged perspective view of the distal end of the screw extension assembly shown in  FIG. 4  with the assembly in a locked position. 
         FIG. 17  is an enlarged cross-sectional view of one embodiment of the distal end of the screw extension assembly shown in  FIG. 4 . 
         FIG. 18  is a perspective view of the bone screw and screw extension assembly with a screw driver mounted thereon. 
         FIG. 19  is a cross-sectional view of the bone screw, screw extension assembly and screw driver shown in  FIG. 18 . 
         FIG. 20  is a perspective view of the rod introducer assembly and connecting rod shown in  FIG. 3 . 
         FIG. 21  is a cut-away view of the rod introducer assembly shown in  FIG. 20  with the connecting rod engaged thereto and the assembly in a locked position. 
         FIG. 22  is a cut-away view of the rod introducer assembly and rod shown in  FIG. 21  with the assembly with a first locking mechanism released. 
         FIG. 23  is a cut-away view of the rod introducer assembly and rod shown in  FIG. 21  with the assembly with a second locking mechanism released and the rod disengaged from the assembly. 
         FIG. 24  is an enlarged perspective view of the distal end of the rod introducer assembly shown in  FIG. 20  with the rod disengaged from the assembly. 
         FIG. 25  is an enlarged perspective view of the distal end of the rod introducer assembly shown in  FIG. 20  with the rod engaged to the assembly. 
         FIG. 26  is an enlarged cross-sectional view of the rod engaged to the assembly as shown in  FIG. 25 . 
         FIG. 27  is an enlarged view of the second locking mechanism of the rod introducer assembly shown in  FIG. 20  with the mechanism in a locking position. 
         FIG. 28  is an enlarged view of the second locking mechanism of the rod introducer assembly shown in  FIG. 20  with the mechanism in a release position. 
         FIG. 29  is a cross-sectional view of a rod detector assembly for use with the instruments and procedures disclosed herein, shown with the detector flag in a first position. 
         FIG. 30  is an enlarged cross-sectional view of the distal end of the rod detector assembly shown in  FIG. 29 . 
         FIG. 31  is a cross-sectional view of the rod detector assembly shown in  FIG. 29  with the detector flag in a second indicator position. 
         FIG. 32  is an enlarged cross-sectional view of the proximal end of the rod detector assembly shown in  FIG. 29 . 
         FIG. 33  is a perspective view of the screw extension assembly, rod introducer assembly and rod detector assembly in one position during a procedure disclosed herein. 
         FIG. 34  is a view of a rod introducer assembly and screw extension assembly with a rod disposed therein, prior to mounting the introducer assembly on the extension assembly. 
         FIG. 35  is an enlarged cross-sectional view of the proximal end of the rod introducer assembly mounted on the screw extension assembly with the introducer assembly in a first position. 
         FIG. 36  is an enlarged view of the distal end of the rod introducer assembly in the first position mounted on the screw extension assembly. 
         FIG. 37  is an enlarged cross-sectional view of the proximal end of the rod introducer assembly shown in  FIG. 35  with the introducer assembly in a second position. 
         FIG. 38  is an enlarged view of the distal end of the rod introducer assembly in the second position mounted on the screw extension assembly. 
         FIG. 39  is an enlarged cut-away view of the advancement mechanism of the rod introducer assembly shown in  FIG. 34 . 
         FIG. 40  is an enlarged cut-away view of the advancement mechanism shown in  FIG. 39 . 
         FIG. 41  is a perspective view of a compression/distraction device as disclosed herein, shown with the jaws open and the fulcrum in a first position. 
         FIG. 42  is a top view of the compression/distraction device shown in  FIG. 41 . 
         FIG. 43  is a perspective view of the compression/distraction device shown in  FIG. 41 , shown with the jaws closed. 
         FIG. 44  is a top view of the compression/distraction device shown in  FIG. 43 , shown with the jaws closed. 
         FIG. 45  is a perspective view of a compression/distraction device as disclosed herein, shown with the jaws open and the fulcrum in a second position. 
         FIGS. 46   a - c  are perspective views of alternative fulcrums for use with the compression/distraction device shown in  FIG. 41 . 
         FIG. 47  is a perspective view of the compression/distraction device of  FIG. 41  used in a compression procedure. 
         FIG. 48  is a perspective view of the compression/distraction device of  FIG. 41  used in a distraction procedure. 
         FIG. 49  is a perspective view of one step of one procedure disclosed herein. 
         FIG. 50  is a perspective view of a further step of the procedure. 
         FIG. 51  is a perspective view of an additional step of the procedure. 
         FIG. 52  is a perspective view of one step of another procedure disclosed herein. 
         FIG. 53  is a perspective view of a further step of the procedure. 
         FIG. 54  is a perspective view of an additional step of the procedure. 
         FIG. 55  is a perspective view of one step of yet another procedure disclosed herein. 
         FIG. 56  is a perspective view of a further step of the procedure. 
         FIG. 57  is a perspective view of an additional step of the procedure. 
     
    
    
     DETAILED DESCRIPTION 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the invention is thereby intended. It is further understood that the present invention includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the invention as would normally occur to one skilled in the art to which this invention pertains. 
     Referring to  FIGS. 3 and 4 , certain components of the instrumentation disclosed herein are depicted as used according to certain procedures disclosed herein. In particular, three bone screw assemblies  15  are engaged to three vertebrae V in preparation for a multi-level fixation of the spine. An elongate connecting member, such as connecting rod  25 , is configured to be received within the yokes  17  of each of the screw assemblies to connect each of the vertebral levels in a conventional manner. When the construct is complete, the rod will be locked to each of the screw assemblies, such as by the cap  19  and set screw  20  illustrated in  FIG. 2 . As shown in  FIG. 3 , each of the screw assemblies  15  carries a screw extension assembly  32  that is sized to be accessible outside the patient&#39;s skin. The patient&#39;s skin or fascia is depicted as a phantom line S for illustrative purposes only, with the understanding that the level of the fascia relative to the fixation location on the vertebral bodies will vary from patient to patient. The instruments further include a rod introducer assembly  34  that is used to introduce the connecting rod  25  through and into the yokes  17  of each of the bone screw assemblies  15 . Once the rod is situated within the bone screw yokes, a rod persuader assembly  36  may be used to fully seat the rod therein for final tightening. The nature and manner of operation of these and other instruments are described herein. 
     Screw Extension Assembly 
     Details of the screw extension assembly  32  and its interface with the bone screw assembly  15  will be explained with reference to  FIGS. 4-17 . Looking first at the bone screw assembly  15 , and particularly at  FIGS. 4-7 , the yoke  17  includes opposed upstanding arms  40  that are separated to define a slot  42  therebetween. The slot  42  is sized and configured to relatively snugly receive the connecting rod  25  therein. For some bone screw assemblies, the connecting rod may be seated within a U-shaped base of the slot  42 . For the present disclosure, the connecting rod is seated on the sleeve  18  rather than at the base of the yoke slot, all in accordance with the bone screw assembly disclosed in the &#39;898 application incorporated by reference above. 
     The arms  40  of the yoke  17  include facing interior surfaces  44  which define internal threads  48 , as best seen in  FIG. 6 . The threads  48  are configured to mate with the set screw  20  to clamp the connecting rod  25  within the yoke and for final fixation of the bone screw assembly, as described in the &#39;898 application. The upstanding arms  40  further include an exterior surface  46  that is partially cylindrical and flat side surfaces  47  on opposite sides of the slot  42 . The yoke further defines a tool bore  49  aligned with a tool recess  22  at the base of the bone screw head  16   a  that is used to drive the bone screw  16  into the vertebral bone. 
     As thus far described, the yoke  17  is generally similar to the yokes of other bone screw assemblies, including the bone screw described in the &#39;898 application. In the embodiment disclosed herein, the interior surface  44  of the yoke  17  defines an undercut  50  that forms a coupling surface  51  at the mouth of the slot  42 , as best seen in  FIG. 6 . The coupling surface  51  provides an interface for coupling to the screw extension assembly  32 . 
     The pedicle screw extension assembly  32  includes an elongate hollow outer sleeve  55  having a perimetric sidewall that defines a bore  56  extending from a proximal end  55   a  to a distal end  55   b . A lower bore portion  56   a  of the bore adjacent the distal end  55   b  is sized to be relatively snugly received about the exterior surface  46  of the yoke  17  as shown in  FIG. 7 . The outer sleeve further defines a slot  59  through the sleeve sidewall adjacent the distal end  55   b  of the sleeve and extending across the diameter of the sleeve, as shown in  FIG. 4 . The slot  59  is sized to receive a connecting rod  25  therethrough as depicted in  FIG. 3 . The slot  59 , which opens through the distal end  55   b , may be long enough proximally in certain embodiments to extend above the fascia S so that the connecting rod  25  may be introduced into the screw extension assembly  32  outside the patient, as explained in more detail herein. 
     Returning to  FIGS. 5 ,  6 , the extension assembly also includes an elongate hollow inner sleeve  57  concentrically and rotatably disposed within the bore  56  of the outer sleeve  55 . The inner sleeve has a perimetric sidewall that defines a central bore  58  from a proximal end  57   a  (see  FIG. 13 ) to a distal end  57   b  that is configured for passage of other instruments as described herein. The inner sleeve further defines a slot  67  opening through the sleeve sidewall at the distal end  57   b  of the inner sleeve that is generally coincident in length and width with the slot  59  of the outer sleeve. The inner sleeve  57  is rotatable relative to the outer sleeve  55  between a first position shown in  FIG. 5  in which the inner sleeve  57  essentially covers or closes the slot  59  in the outer sleeve, and a second position illustrated in  FIG. 8  in which the two slots  59  and  67  are aligned so that a connecting rod can be pass through the screw extension assembly  32 . 
       FIGS. 5-11  show the screw extension assembly  32  in various stages of relative movement between the outer and inner sleeves  55 ,  57  to engage the yoke  17  of the bone screw assembly  15 . In  FIGS. 5 and 6 , the screw assembly  15  is shown just prior to contact with the screw extension assembly. The outer and inner sleeves are in the first position described above in which the inner sleeve  57  covers or closes the slot  59  in the outer sleeve  55 . The yoke  17  of the bone screw assembly is aligned so that the upstanding arms  40  are aligned with the slot  58  in the inner sleeve  57 . The flat side faces  47  are thus aligned to pass into the slot  58  in a close fit. 
     In  FIG. 7  the yoke  17  is fully seated within the screw extension assembly  32 . More specifically, the proximal end  17   a  of the yoke is seated against the yoke mating surface  76  at the base of the lower bore portion  56   a  of the bore  56  in the outer sleeve  55 . This lower portion  56   a  may further define flat surfaces  72  to align the flat side faces  47  of the yoke  17  as the yoke advances into the lower bore portion  56   a . It can thus be appreciated that once the yoke  17  is fully seated within the lower bore portion  56   a  of the outer sleeve the yoke and outer sleeve will rotate and pivot together. More importantly, the outer sleeve will hold the yoke while the inner sleeve rotates relative to both components to firmly engage and lock the yoke to the screw extension assembly. 
     In order to effect this engagement, the inner sleeve  57  is provided with a yoke engagement member  60  at the distal end  57   b  of the sleeve. The yoke engagement member  60  includes generally radially outwardly directed flanges  61  that interface with coupling surfaces  51  defined by undercuts  50  at the proximal end  17   a  of the yoke, as seen in  FIG. 6 . As shown in  FIG. 7 , when the yoke is seated within the outer sleeve, the yoke engagement member  60  of the inner sleeve  57  is aligned with the coupling surfaces  51  of the yoke  17 . The radial flanges  61  are initially situated within flange recesses  71  defined in the outer sleeve  55 . From this position the inner sleeve  57  may be rotated relative to the outer sleeve  55  and to the yoke  17  connected to the outer sleeve. The effect of this relative rotation is illustrated in  FIGS. 8-11 . In  FIGS. 8-9  the inner sleeve  57  is shown at the beginning of this relative rotation. As best seen in  FIG. 9 , as the inner sleeve rotates the radial flanges  61  are guided by the flange recesses  71  beneath the undercuts  50  and into engagement with the coupling surfaces  51  of the yoke proximal end  17   a . The radial flanges  61  and the undercuts  50  are configured so that continued rotation of the inner sleeve relative to the yoke tends to pull the yoke upward or proximally toward the yoke mating surface  76  of the outer sleeve, as shown in  FIGS. 10-11 . In this position the rod slots  59  and  67  are aligned and the screw extension assembly is essentially supported by the bone screw assembly, which is itself subsequently anchored to the vertebra. The yoke engaging flange  61  and undercut  50  may be configured to provide a tighter fit as the inner sleeve is rotated relative to the outer sleeve. This may be accomplished, for instance, by increasing the thickness of the radial flange  61  radially outwardly in an upward angle and forming the undercut  50  to have a complementary configuration to accommodate the increased thickness of the radial flange  61  around the circumference of the coupling surface. 
     The screw extension assembly  32  may incorporate additional features to ensure a tight engagement between the extension assembly and the bone screw assembly  15  or yoke  17 . Referring to  FIGS. 5 and 9 , the inner sleeve  57  may incorporate a securement member  64  that is configured to engage a securement recess  73  in the outer sleeve  55 . The securement member may include a downwardly or distally projecting securement flange  65  that is received within an upwardly opening flange groove  74 , as best seen in  FIG. 9 . Like the interface between the yoke engaging flange  61  and undercut  50 , the securement flange  65  and flange groove  74  may be configured to provide a tighter fit as the inner sleeve  57  is rotated relative to the outer sleeve  55 . Thus, the width of the flange  65  may be increased along the circumference or the width of the groove  74  decreased along the circumference so that the fit becomes tighter as the inner sleeve approaches the second position shown in  FIG. 11 . Upon rotation of the outer and inner sleeves  55 ,  57 , securement flange  65  extending into flange groove  74  also serves to minimize or prevent outward radial splaying of the outer and inner sleeves  55 ,  57 . 
     The combination of the yoke engaging member  60  and the securement member  64  of the inner sleeve and the interface of these elements to the yoke and outer sleeve, respectively, allows the screw extension assembly  32  to be firmly fastened to the yoke  17  and screw assembly  15  when the bone screw  16  is threaded into a vertebra. The screw extension assembly  32  may be manipulated or articulated relative to the bone screw  16 . The rod slots  59  and  67  will thus always be aligned with the slot  42  in the yoke  17  of the bone screw assembly to facilitate placement of the connecting rod  25 , as described herein. 
     In the illustrated embodiment, the yoke engagement member  60  incorporates a radially inwardly directed flange  61  while the yoke  17  incorporates a radially formed coupling surface  51  and undercut  50 . Alternatively, these features may be reversed between the inner sleeve and yoke so that the yoke  17  incorporates a radially outwardly directed flange that mates with a radially inwardly formed groove in the distal end  55   b  of the inner sleeve  55 . Similarly, the securement member  64  of the inner sleeve  57  and the securement recess  73  of the outer sleeve  55  may be reversed or re-oriented. 
     As thus far described it can be seen that the operation of the screw extension assembly  32  relies upon rotation of the inner sleeve relative to the outer sleeve. In one aspect of the assembly  32 , the proximal end  55   a  of the assembly is configured to accept a socket driver  38 , as shown in  FIG. 4 . The structure and operation of the socket driver is shown in more detail in  FIGS. 12-17 . The socket driver  38  includes a generally cylindrical socket  80  with a driver socket  82  formed in the base of the rectangular socket and a generally rectangular rim  81  formed at the distal opening of the cylindrical socket  80 . A spindle  84  is provided for connection to a driving tool for rotating the socket driver  38  or to provide a gripping interface to manually rotate the socket driver. The rectangular rim  81  is configured to engage the generally rectangular outer surface  86  at the proximal end  55   a  of the outer sleeve  55 . When the rim  81  is in contact with the outer surface  86  the socket driver  38  cannot be rotated relative to the outer sleeve  55 . In the illustrated embodiment the mating surfaces of the rim and outer surface are generally rectangular, although other configurations are contemplated that prevent relative rotation between the socket driver and the outer sleeve. 
     However, the outer sleeve  55  further defines a radially inward groove  87  defined below or distal to the rectangular surface  86 . This groove  87  is arranged to be aligned with the rectangular rim  81  when the socket driver  38  is fully seated on the proximal end  55   a  of the outer sleeve  55 , as depicted in  FIG. 14 . Thus, when the end of the outer sleeve is adjacent the end of the cylindrical socket  80  the rim  81  is aligned with the groove  87 . In this position, there is no surface of the outer sleeve that bears against the rectangular surface of the rim  81  so the socket driver  38  is free to rotate relative to the outer sleeve  55 . 
     The driver socket  82  is configured to engage the proximal end  57   a  of the inner sleeve  57 . In particular, the proximal end  57   a  includes a mating end  88  that is complementary to the driver socket  82 . In one embodiment, the driver socket and mating end have a hex configuration so that the socket driver  38  can be used to rotate the inner sleeve  57  when the mating end  88  is disposed within the driver socket  82 , as shown in  FIG. 14 . In a particular configuration the driver socket  82  may define a 12-point contact socket so that in combination with the rectangular outer surface  86  at the proximal end  55   a  of the outer sleeve  55 , the socket driver  38  may be engaged every ninety degrees. 
     The screw extension assembly  32  may incorporate features to prevent relative rotation between the inner and outer sleeves. For instance, when the screw extension assembly is engaged to a bone screw assembly it is desirable to ensure that the two assemblies are locked and cannot be inadvertently disengaged. Since engagement or disengagement occurs with relative rotation between the inner and outer sleeves, preventing inadvertent rotation of the inner sleeve can prevent inadvertent disengagement from the screw assembly  15 . Accordingly, the screw extension assembly includes a displaceable retention ring  90  that initially engages the mating end  88  of the inner sleeve  57 . The retention ring  90  may include a hex interface  90   a  for engaging the hex features of the mating end. The retention ring  90  is held against rotation relative to the outer sleeve, while permitting axial movement of the ring within the outer sleeve. Thus, the retention ring may define one or more longitudinally extending capture slots  91  that receive a corresponding capture pin  92  that is embedded in the outer sleeve as shown in  FIG. 12 . The retention ring  90  is thus permitted to slide axially or longitudinally within a bore  94  at the proximal end  55   a  of the outer sleeve  55  from the extended position shown in  FIG. 12  to a depressed position shown in  FIG. 14 . A biasing spring  93  is disposed within the bore  94  to bias the retention ring  90  to the extended position in which the retention ring engages the hex end  88  of the inner sleeve  57 , as described above. 
     As shown in  FIGS. 14-15 , the retention ring  90  can be moved to its depressed position by pressing the socket drive  38  downward or toward the proximal end  55   a  of the outer sleeve. The base of the cylindrical socket  80  contacts the retention ring  90  pushing it down with the socket driver until the cylindrical socket bottoms on the top of the outer sleeve. In this position the retention ring  90  is clear of the hex end  88  so that the hex end is free to be rotated by the hex socket  82 . (As explained above, in this position shown in  FIG. 14  the rectangular rim  81  is also clear of the rectangular outer surface  86  of the outer sleeve). 
     The screw extension assembly  32  further includes an indicator  95  that indicates to the surgeon the relative position of the inner and outer sleeves. Thus, when the screw extension assembly  32  is in its initial orientation (i.e., with the inner sleeve in the position shown in  FIG. 5  to accept a bone screw yoke) the indicator includes the indicia  95   a  “LOAD” viewable in the window  95   c  formed in the outer sleeve. The indicia  95   a  is affixed or applied in a suitable manner to the outer surface of the inner sleeve. When the screw extension assembly  32  has been coupled to the yoke  17  of the bone screw assembly (as shown in  FIGS. 10-11 ) the indicia  95   b  “LOCKED” is visible through the window  95   c , as illustrated in  FIG. 16 . As shown in  FIG. 4 , this indicator  95  is at the proximal end  55   a  of the outer sleeve so that it is readily visible to the surgeon outside the surgical site. 
     In an alternative embodiment a modified retention ring  90 ′ is operable to free the inner sleeve for rotation relative to the outer sleeve, as illustrated in  FIG. 17 . In this embodiment, a number of retention balls  92 ′ are situated between a locking bore  97  defined in the outer sleeve  55  and a corresponding number of ball recesses  98  defined in the inner sleeve  57 . The retention ring  90 ′ is initially positioned as shown in  FIG. 17 . When the ring is pushed in the direction of the arrow R a lower cam surface  96  contacts and bears against the retention balls  92 ′. This contact gradually pushes the retention balls  92 ′ radially inward in the direction of the arrow B to a release position in which the balls are seated within the corresponding recesses  98 . In this position the inner sleeve  57  is free to rotate relative to the outer sleeve  55 . A biasing spring  93 ′ may be provided to bias the retention ring  90 ′ away from the release position and to the locked position in which relative rotation is prevented. 
     In one embodiment, the socket driver  38  may be provided with a stepped shaft  89 ′ extending from the socket hex  82  ( FIG. 14 ) and projecting through the inner sleeve  57  as shown in  FIG. 17 . The stepped shaft  89  includes a stepped distal end  89 ′ b  that is sized to be retained by the capture balls  92 ′ when the socket driver  38  is fully seated on the inner sleeve and has fully depressed the retention ring  90 ′. The capture balls  92 ′ thus prevent removal of the socket driver as long as they are in the inboard position denoted by the arrow B. 
     Screw Driver Instrument 
     The screw extension assembly  32  is configured to accept additional tools for access to the bone screw assembly. For instance, the bore  58  of the inner sleeve  57  is sized to receive a screw driver  100  as shown in  FIGS. 18-19 . The screw driver  100  includes at the proximal end a handle  101  connected to a shaft  102  to permit manual rotation of the shaft. The shaft  102  includes at the distal end a tip defining an engagement end  103  that is configured to engage a drive tool recess  22  in the base of the bone screw head  16   a . The engagement end and drive tool recess can be configured in a conventional manner, such as with a hex or Torx feature. The shaft  102  is sized so that the engagement end  103  can be received within the recess  22  while the handle  101  is accessible at the proximal end of the screw extension assembly  32 . 
     The screw driver tool  100  includes an outer retention sleeve  104  having an interior bore  104   a  through which the shaft  102  extends. The shaft  102  and retention sleeve  104  are coupled to each other to allow free relative axial and rotational movement therebetween. The distal end  105  of the retention sleeve  104  is provided with exterior threads to match the internal threads  48  on the interior surfaces  44  of yoke  17 . The retention sleeve  104  is connected to a knob  106  ( FIG. 18 ) situated on or adjacent the proximal end of the screw extension assembly  32  that is configured to facilitate manual rotation of the retention sleeve to thread the distal end  105  into the yoke. A stop  108  is rotatably mounted on the shaft  102  and is configured to seat within the slot  42  of the yoke  17  to support the shaft and retain the sleeve. Upon threaded connection of the outer retention sleeve  104  to the yoke  17 , the retention sleeve  104  bears against the stop  108  and the stop bears against the yoke to provide joint rotational movement of the retention sleeve, stop and yoke. Prior to such threaded connection, the engagement end  103  of the inner shaft is guided into the drive tool recess  22  in the base of the bone screw head  16   a . The stop  108  may be sized to prevent threading of the retention sleeve into the yoke unless and until the end  103  of the shaft is engaged within the tool recess of the bone screw. Once the tool  100  is properly seated, rotation of the handle  101  that is connected to the shaft  102  will rotate the bone screw shank  16 . With the screw extension assembly  32  and the retention sleeve  104  attached to the yoke for joint movement, and with the yoke  17  being able to freely articulate with respect to screw shank  16 , the screw extension assembly  32  may be manually held while the handle  101  is rotated to drive the screw shank  16  into a pedicle of a vertebra. 
     The screw extension assembly thus provides an avenue for guiding the screw driver instrument  100  into engagement with the bone screw. Even if the screw extension assembly is articulated relative to the bone screw, a minor manipulation of the assembly will automatically align the screw driver instrument with the drive tool recess. Once engaged the screw driver can be used to thread the bone screw  16  into the vertebra in a known manner and then removed from the screw extension assembly. The shaft  102  of the screw driver  100  may be provided with a guide wire lumen  107  to allow introduction of the tool over a previously positioned guide wire. 
     Rod Introducer Assembly 
     With the bone screw assemblies anchored in the vertebrae with the screw extension assemblies engaged to the screw assemblies, the connecting rod  25  can be introduced through the rod slots  59 ,  67  in the extension assemblies using a rod introducer assembly  34 , as shown in  FIG. 3 . Details of the rod introducer assembly and its operation are shown in  FIGS. 20-28 . The introducer assembly  34  includes a handle  110  configured to be manually grasped to manipulate the connecting rod  25  attached to the introducer assembly. The handle is also configured for easy access to the actuation mechanism  112  used to enable grabbing and locking a connecting rod to the assembly, as well as to push buttons  145  and  152  used to release the actuation mechanism in various stages of operation, as explained herein. The handle  110  and lever  113  of the actuation mechanism  112  may be particularly configured to permit one-handed operation of the lever during its stages of actuation. 
     Looking first at  FIG. 26 , the connecting rod  25  includes an introduction end  27  that may be tapered to facilitate introduction of the rod through tissue, an incision, and/or the rod slots  59  and  67  in the screw extension assemblies. The elongated body  26  of the rod is sized to span the distance between the instrumented vertebrae and may have a curvature calibrated to accommodate or correct the orientation of the instrumented vertebral levels in a known manner, such as for lordosis and kyphosis. The connecting rod further includes an engagement end  28  that defines an opening  29  and a series of flats  30   a - 30   e . These features of the engagement end  28  provide the interface with the rod insertion assembly  34 . 
     Turning to  FIGS. 20-21 , the rod insertion assembly  34  includes an outer sleeve  114  extending from the handle  110 . As shown in  FIG. 3  the outer sleeve has a length approximating the height of the extension assemblies  32  above the bone screws mounted in the vertebrae. The outer sleeve has a length sufficient for the surgeon to manipulate the handle  110  outside the patient while the connecting rod  25  carried by the instrument is fully seated within the yokes of the bone screw assemblies. The outer sleeve  114  is at least generally tubular along a portion of its length and it thus hollow to slidably receive an inner actuator shaft  116  for translational movement within. The distal portion of the outer sleeve, which in the illustrated embodiment may constitute about half the length of the sleeve, branches into opposed flexible legs  118  separated by an expandable slot  121  through the top and bottom of the sleeve  114 . The legs are capable of flexing outwardly relative to each other to form an expandable opening  119  into which the engagement end  28  of the connecting rod  25  is introduced (see also  FIGS. 24-25 ). The legs  118  may be configured to be initially biased together or toward each other, the biasing force being provided by the natural resilience of the legs. 
     The expandable slot  121  of the legs defines opposing cam elements  128 . The cam elements  128  are configured to provide a reduced slot width with a cam surface  129  ( FIG. 25 ) leading to that reduced width. The inner actuator shaft  116  includes an actuator pin  122  that projects diametrically across the outer sleeve  114  ( FIG. 21 ) and is arranged to contact the cam elements  128  as shown in  FIGS. 24 and 25 . In the configuration shown in  FIG. 24  the actuator pin  122  is disposed directly between both cam elements  122 , widening the gap between the elements, which in turn forcibly deflects the flexible legs  118  apart, and which ultimately increases the size of the expandable opening  119 . On the other hand, when the actuator pin  122  is in the position shown in  FIG. 25 , the pin  122  is beyond or distal of the cam elements  128  so that the flexible legs are biased toward each other, thereby decreasing the size of the expandable opening  119 . In this position, a locking pin  124  projecting from the inner actuator shaft  116  engages a recess  127  formed by locking hooks  126   a ,  126   b  adjacent the distal end  126  of the outer sleeve  114 . It is noted that each leg includes a locking hook. Thus, when the locking pin  124  is disposed within the recess  127  defined by each locking hook  126   a ,  126   b , the pin prevents separation of the locking hooks, and consequently separation of the flexible legs  118 . Each leg defines a notch, such as notch  118   b  to receive the locking hook of the opposing leg, such as hook  126   a , as shown in  FIG. 25 . 
     The flexible legs  118  of the outer sleeve  114  include inwardly directed posts  120  that are sized to be received within the opening  29  in the engagement end  28  of the connecting rod  25 , as shown in  FIG. 24 . The posts  120  are disposed generally perpendicular to the longitudinal axis of the outer sleeve  114  and are particularly sized so that they provide adequate space between them when the expandable opening  119  is at its largest extent so that the engagement end  28  of the rod can fit between the posts. The posts are also sized so that they do not contact each other when disposed within the opening  119 , as shown in  FIG. 25 . (Alternatively the posts may be half cylinders that overlap each other within the opening  119 ). The posts  120  thus provide structure for engaging the connecting rod  25  and holding it to the rod introducer assembly  34  when the flexible legs  118  are in their closed position. The locking pin  124  and locking hooks  126   a, b  hold the legs together so that the connecting rod cannot be removed from the rod introducer assembly  34 , at least not without disengaging the locking pin and hooks. 
     The locking pin  124  and the actuator pin  122  are advanced or retracted by axial movement of the inner actuation shaft  116  within the outer sleeve  114 . This movement is accomplished by the actuation mechanism  112 . The lever  113  of the actuation mechanism is coupled to the inner shaft  116  by a linkage  133 . The lever itself is pivotably mounted to the handle  110  at pivot pin  132  so that the lever can pivot from the first position shown in  FIG. 21 , to a second position shown in  FIG. 22  to a third position shown in  FIG. 23 . The pivoting movement of the lever about pivot pin  132  is translated to linear movement of the inner shaft  116  through the linkage  133 . In the first position shown in  FIG. 21 , the lever is locked within the handle  110  and the inner shaft has moved to its farthest distal extent. In this farthest distal position, the locking pin  124  is engaged in the locking recesses  127  and the legs  118  are locked together to grip the connecting rod, as shown in  FIG. 25 . In the third position shown in  FIG. 23 , the lever is fully unlocked from the handle and the inner shaft  116  has moved to its nearest proximal extent. In this proximal position, the actuator pin  122  has separated the legs  118  so that the connecting rod is automatically disengaged from the rod introducer assembly  34 , as shown in  FIG. 24 . In the second position, also indicated as a neutral position, shown in  FIG. 22 , the posts  120  are disposed within the opening in the engagement end  28  of the connecting rod so the rod is still held by the rod introducer assembly, but in this position the rod can be pivoted about the posts to vary the angle of the connecting rod relative to the outer sleeve  114  as the rod is introduced into the surgical site, as explained more fully herein. 
     As shown in  FIG. 26 , the distal end of the inner actuator shaft  116  defines a rod engaging end  156 . This end  156  defines a series of flat portions  157   a, b, c  that generally correspond to the flat surfaces  30   a - 30   e  of the engagement end  28  of the rod. The flat portions of the rod engaging end  156  define a partial polygonal socket which is configured to complementarily mate with the flat surfaces of the engagement end of the rod so that when the rod engaging end  156  is held directly against the engagement end  28  of the rod the rod cannot pivot about the posts  120 . Flat surface  157   b  lies transverse, and preferably generally perpendicular, to and crosses the longitudinal axis of the inner shaft  116 , with adjacent flat surfaces  157   a  and  157   c  lying angularly with respect to surface  157   b  and defining the socket therewith. In a preferred arrangement, the socket defined by the flat portions  157   a, b, c  of the rod engaging end  156  are the mirror image of the flat surfaces  30   a - 30   e  of the engagement end  28  of the rod. Thus, in the first position of the lever  113  shown in  FIG. 21  the rod engaging end  156  is in flush contact with the engagement end  28  of the rod. However, in the second position of the lever shown in  FIG. 22 , the inner shaft  116  is backed off slightly from the engagement end  28  of the rod, as shown in  FIG. 26 . The gap between the flat surfaces of the engagement end, such as surfaces  30   c ,  30   d  and  30   e , and the flat connecting surfaces  157   a - c  of the actuator shaft, allows the rod to be pivoted about the posts  120 . The proximity of the rod engaging end  156  to the engagement end  28  of the rod provides resistance to this movement so that the rod can be moved to a particular angle and held there without any outside force. The resistance provided by the corners of the polygonal socket at the rod engaging end  156  acts as detent and is readily overcome by slight manual pressure which creates tactile feedback to the surgeon and an audible snapping sound. 
     The array of flat surfaces  30   a - e  at the engagement end  28  of the rod  25  allow the rod to be positioned and locked in five angular orientations relative to the rod introducer assembly  34 . Thus, when the flat surface  30   d  is aligned with the rod engaging end  156  of the actuator shaft  116 , the rod is oriented at a 45 degree angle relative to the outer sleeve  114  of the introducer assembly, as shown in  FIG. 26 . When flat surface  30   e  is aligned with flat engagement surface  157   b , the rod is at an angle of 90 degrees. When the flat surface  30   c  is aligned the rod is generally collinear with the outer sleeve, or at an angle of 0 degrees. The rod  25  can also be pivoted downward from the position shown in  FIG. 26  to the 45 and 90 degree angles. It is noted that when the rod is pivoted to one of the “upward” positions, such as shown in  FIG. 26 , the curvature of the rod is generally toward the rod introducer assembly and is used typically for lordotic applications. On the other hand, if the rod is reversed to one of the “downward” positions opposite that shown in  FIG. 26 , the curvature of the rod faces away from the introducer assembly and is used typically in kyphotic applications. This feature provides flexibility for the surgeon to apply different corrections using the same rod and introducer assembly. 
     It should be understood that the angular configurations of the flat portions  157   a, b, c  of the rod engaging end  156  and the corresponding flat surfaces  30   a - 30   e  of the engagement end  28  of the rod  25  may be varied to obtain other desired angles, which may, for example be in thirty degree or other increments. In addition, the rod engaging end  156  of the inner shaft  116  may be formed to have only a single flat surface, such as surface  157   b  to hold tightly against one of the flat surfaces  30   a - 30   e  of the engagement end  28  of the rod  25 . It should also be appreciated that the socket defined at the rod engaging end  156  of the inner shaft  116  may be formed of a curved surface to mate frictionally with a like curved surface formed on the engagement end  28  of the rod  25 . As such, in the second position as shown in  FIG. 22  the gap would allow free non-detected pivotal movement of the rod  25  about the posts  120  until the shaft  116  is moved axially more distally to cause the rod engaging end  156  to tightly frictionally engage and hold the engagement end  28  of the rod  25  in a position as shown in  FIG. 21 . 
     The rod introducer assembly  34  incorporates two locks used to hold the actuation mechanism  112 , and particularly the lever  113 , in the first position ( FIG. 21 ) and in the second position ( FIG. 22 ). As shown in  FIG. 21 , the first locking mechanism  140  includes a first locking surface  141  on the lever  113 . A first locking element  143  is slidably mounted within the handle  110  and may be integrated onto a push button  145  that is biased outward by spring  146 . The first locking surface  141  and the first locking element  143  may be in the form of engaging hooks, as illustrated in  FIG. 21 . The spring  146  biases the push button outward so that the two hooks remain engaged until the push button  145  is depressed against the spring. 
     When the push button  145  is depressed to release the first locking mechanism  140 , the lever  113  pivots slightly upward to the second position shown in  FIG. 22 . The lever is pushed upward, or pivoted about the pivot pin  132  by the post  135 . The post  135  is biased toward the lever  113  by a spring  136  and in a direction to cause the lever to pivot about the pivot pin. It can be appreciated that the lever can be returned to the first position shown in  FIG. 21  by depressing the lever downward toward the handle, thereby pushing the lever back against the post  135  and spring  136 . 
     The lever  113  is held in the second position shown in  FIG. 22  by the second locking mechanism  150 , which is shown in more detail in  FIGS. 23 ,  27  and  28 . The second locking mechanism  150  includes a second locking surface  151  integrated into the lever  113 . It is noted that  FIG. 23  only shows one such surface  151  since this figure is a cross-sectional view through the lever. Thus, an additional locking surface  151  is a mirror image to the surface depicted in  FIG. 23 . The second locking surfaces bear against the second locking element  153  mounted within the handle  110 . As shown in the detail view of  FIG. 27 , the second locking element  153  includes two plates  155  forced apart by a spring  154  interposed therebetween. Each locking surface  151  of the lever  113  thus fit between a respective plate  155  and a side wall  111  of the handle  110 , as generally depicted in  FIG. 21 . It can be appreciated that the plates  155  thus exert a friction force against the second locking surfaces  151  to prevent the lever  113  from pivoting upward. 
     The two plates  155  are carried by respective release buttons  152  that project laterally outward from the handle  110  when the second locking mechanism  150  is in its locked position illustrated in  FIG. 27 . The release buttons  152  may be depressed inward toward each other, as shown in  FIG. 28 , to push the plates  155  towards each other against the force of the spring  154 . When the plates are in the position shown in  FIG. 28 , the contact between the plates and the second locking surfaces  151  of the lever  113  is reduced or eliminated so that the lever  113  is free to pivot upward to the third position shown in  FIG. 23 . 
     Rod Detector 
     The rod detector assembly  160  is used to detect the presence of an elongated connecting rod  25  into the bone screw assemblies  15  engaged to the vertebrae, as shown in  FIG. 3 . However, in certain procedures the point of entry of the rod  25  into the rod slots  59 ,  67  of the screw extension assemblies  32  (as described above) may not be visible to the surgeon. More particularly, since the rod may percutaneously enter the screw extension assemblies beneath the fascia S the surgeon may not be able to visually verify that the rod is properly positioned within the rod slots  59 ,  67 , and ultimately within the slots  42  in the yokes  17  of the bone screw assemblies. A rod detector  160  is provided that can provide a readily seen and easily discernable visual indicator to the surgeon above the surgical site, as shown in  FIG. 32 . Details of the rod detector are shown in  FIGS. 29-31 . 
     The rod detector  160  includes a generally tubular body  161  that is sized to fit within the bore  58  of the inner sleeve  57  of the screw extension assembly  32 , as best seen in  FIG. 31 . In certain embodiments, the tubular body  161  is open at a slot  162  along a substantial portion of the length of the body, as seen in  FIG. 30 . At the distal end, the body forms two diametrically opposed branches  163  that are coincident with the slot  162  from one side of the body and that form a diametrically opposite slot. The detector includes a cap  166  affixed to the tubular body that is sized to seat on top of a screw extension assembly, as depicted in  FIG. 32 . The tubular body is sized to extend along a substantial portion of the length of the inner sleeve  57  but not so far as to interfere with the introduction of the rod  25  into the rod slot  67  of the sleeve. 
     The rod detector  160  includes a flag  164  that projects upward from the cap  166  as shown in the figures. The flag  164  may be connected to or integral with a strip  165  that spans the length of the tubular body to a base  172  at the top of the opposed branches  163 , as seen in  FIG. 29 . The strip  165  is connected to a tip  169  that projects from the bottom of the tubular body  161 . The tip  169  has a length or projects outward from the tubular body a sufficient distance to extend substantially into the slot  42  of the yoke  17  in a bone screw assembly mounted to the screw extension assembly  32  that the rod detector passes through. The tip  169  thus has a length sufficient so that it will be contacted by a rod  25  as it enters the slot  42  in the yoke. 
     The flag  154 , strip  165  and tip  169  thus form a generally continuous indicator  168  that is pivotably connected to the tubular body  161  at a pivot mount  173  as shown in  FIG. 30 . The pivot mount thus permits the indicator  168  to rock back and forth about the mount  173  from the position shown in  FIG. 29  in which the flag  164  is to the right of the cap  166 , and the position shown in  FIG. 31  in which the flag  164 ′ is to the left of the cap. A slot  167  in the cap  166  accommodates this movement of the flag. A bias spring  174  bears against the strip  165  to push the strip and flag  164  to the position shown in  FIG. 29 . This position is the neutral position of the rod detector  160 , indicative of the absence of a rod within the screw extension assembly  32 . 
     The flag is moved from the neutral position  164  to the positive indication position  164 ′ in response to deflection of the tip  169 . Movement of the tip to the position  169 ′ in  FIG. 31  is sufficient to cause the flag to shift to the position  164 ′. The slight movement of the tip is magnified by the pivot mount  174  operating as a fulcrum and the length of the strip  165  terminating in the flag  164 . In one embodiment the tip  169  is formed as a thin flexible strip of material, such as Nitinol, that is capable of bending to the position  169 ″. The tip deflects in response to pressure from a connecting rod  25  as shown in  FIG. 33 . The additional flexibility of the tip  169  allows the tip to be long enough to enter the slot  42  of the yoke  17  and still allow passage of the connecting rod through the screw extension assembly and/or bone screw yoke. 
     The rod detector  160  may incorporate elements to enforce proper positioning of the detector relative to the screw extension assembly and to temporarily restrain the detector from removal. Thus, the detector may include a guide post  170  extending through the slot  162  in the tubular body to pass through a hole  165   a  in the strip  165  when the strip is deflected, as shown in  FIG. 31 . The guide post  170  carries a spring biased positioning ball  171  that extends outward from the tubular body  161  opposite the slot  162 . This positioning ball  171  is configured to seat within a positioning groove  176  (see also  FIG. 12 ) defined in the bore  58  of the inner sleeve  57 . This feature provides resistance to removal of the rod detector  160  from the screw extension assembly. The base  166   a  of the cap  166  may be configured complementarily to the proximal end  88  of the inner sleeve, such as in a hex configuration. This feature prevents relative rotation between the rod detector and the screw extension assembly once the detector has been seated within the inner sleeve. 
     Rod Persuader Assembly 
     Once a connecting rod  25  is situated at least within the screw extension assemblies  32  at each instrumented vertebral level, the rod must be nestled or seated within the slot  42  of the yoke  17  of each bone screw assembly  15 . In the procedures described herein, the rod may be fully seated in the yoke slot by manipulation of the rod introducer assembly  34 . This approach is often challenging in part because the rod introduction site is not readily visible or because there are no suitable tactile indicators that the rod is properly seated in every bone screw assembly. In order to ensure proper placement of the rod, a rod persuader assembly  36  may be mounted on one or more of the screw extension assemblies  32  as illustrated in  FIG. 3 . Details of the rod persuader assembly  36  and its operation can be understood from  FIGS. 34-40 . 
     The persuader assembly includes an outer tube  180  defining a bore  181  sized to pass over the outer sleeve  55  of the screw extension assembly  32 , as shown in  FIG. 3 . The distal end of the outer tube defines diametrically opposed scallops  182  that are configured to seat on the outer surface of the connecting rod  25  when the persuader is in operation. The persuader includes an advancement mechanism  184  driven by a lever  185 . The lever  185  may be connected to or integral with a coupling element  186 . The coupling element is arranged and configured to engage a persuader coupling member  78  defined on the outer sleeve  55  of the screw extension assembly  32 . In one embodiment the coupling element  186  and coupling member  78  form a rack and pinion arrangement. Thus, as shown in  FIG. 35  the coupling element  186  of the rod persuader assembly  36  is a pinion gear while the coupling member  78  of the outer sleeve is the rack. It can thus be appreciated that as the coupling element  186  is pivoted about the pivot hub  188  the pinion gear travels up or down the rack, depending upon the direction of rotation. It is noted that the screw extension assembly includes a coupling member  78  on opposite sides of the outer sleeve  55 . The coupling members are arranged at 90 degrees to the rod slot  59 . With this arrangement the scallops  182  of the outer tube  180  will contact the connecting rod  25  and the persuader assembly  36  may be coupled to the screw extension assembly  32  on either opposite side. 
     The advancement mechanism  180  may be provided with a release lever  189  that releases a locking mechanism  190  operable to lock the coupling element  186  and coupling member  78 , or rack and pinion, in the position shown in  FIGS. 38-39 . In this position, the outer tube  180  has been advanced the full length of its travel along the outer sleeve  55  so that the scallops  182  contact the connecting rod  25  and force the rod into the slot  42  of the yoke  17  and/or into the sleeve  18  of the screw assembly  15 . As shown in  FIG. 39 , the release lever  189  is connected to a release element  193  by a linkage  194 . A pawl  191  is pivotably mounted to the outer tube  180  to engage the pinion gear or coupling element  186  to prevent rotation in one direction while permitting rotation in the opposite direction. The pawl  191  thus prevents rotation of the lever  185  upward to the position shown in  FIG. 34  but permits rotation downward from the position in  FIG. 35  to the position in  FIG. 39 . The release element  193  includes a prong  195  that is arranged to push or rotate the pawl  191  away from the coupling element  186 , thereby allowing the element (pinion gear) and lever  185  to rotate freely in either direction. Thus, depressing the release lever  189  toward the advancement lever  185  actuates the linkage  194  to push the release element  193  toward the pawl  191 . The release element  193  may be spring biased outward from the hub  188 , which in turn biases the locking mechanism  190  to the locked position with the pawl  191  in contact with the coupling element  186 . 
     The rod persuader assembly  36  may include a feature to temporarily hold the advancement lever  185  in the upward position shown in  FIG. 40 . In this position the rod persuader assembly and especially the scallops  182  of the outer tube  180  are offset from the yoke  17  and the rod  25 . This arrangement may be beneficial in procedures in which the rod persuader assembly is mounted on a screw extension assembly prior to introduction of a connecting rod. This temporary holding feature may be implemented by a spring-biased ball  196  biased toward a detent  197  in the pivot hub  188 . The detent  197  is arranged to receive the ball  196  only when the lever  185  is in its upright position. Otherwise the ball simply rolls or slides along the remainder of the pivot hub  188 . 
     Distraction/Compression Instrument 
     A distraction/compression instrument  200  is illustrated in  FIGS. 41-46 . The assembly is configured to particularly operate on screw extension assemblies of bone screws engaged within adjacent vertebrae as shown in  FIGS. 47-48 . The instrument  200  includes a pair of opposed jaws  202 R and  202 L defining a contractible workspace  203  therebetween. (The designation R and L is arbitrary and merely indicative of like components on opposite sides of a midplane M passing along the longitudinal axis of the instrument  200  and between the jaws. (For clarity the R and L designation may not be used when referring to both jaws  202  together). The jaws  202 R, L are generally elongate and parallel to each other and define a plane T extending through the jaws  202 R, L and perpendicular to the midplane M, as shown in  FIG. 41 . The jaws may be provided with pads  202   a  that may be formed of a material adapted to contact the outer sleeve of a screw extension assembly  32  without damaging the sleeve. The pads  202   a  may also be resilient and/or compressible to modestly embrace the outer sleeves as the jaws  202  are drawn together. 
     The jaws are linked to a corresponding pair of handles  204 R, L in a scissors-type configuration—i.e., the handle  204 R is on the opposite side of the mid-plane of the apparatus from the corresponding jaw  202 R. The handles are pivotably connected at a pivot  205  and with a corresponding linkage arm  206 R, L extending beyond the pivot. The two linkage arms  206  are connected to the corresponding jaws  202  by a linkage mechanism  208  that is configured to allow the jaws  202 R, L to be drawn together with the facing surface of the jaws or the pads  202   a  remaining generally parallel to each other and to the mid-plane M of the instrument. The linkage mechanism  208  includes a cross arm  210 R, L connecting each linkage arm  206 R, L to the corresponding jaw  202 R, L, as best seen in  FIG. 42 . The cross arms  210  are pivotably connected at a pivot  211 . The ends of the cross arms  210  are slidably engaged to a guide channel member  213 R, L attached to or integral with a corresponding jaw  204 R, L. The linkage mechanism  208  is thus configured to that as the handles  204 R, L are squeezed together the cross arms  210 R, L slide to the end of the guide channel members  213 R, L, as shown in  FIG. 43 . The cross arms  210  also pivot together, thereby drawing the jaws  202  together and closing the workspace to the reduced configuration  203 ′ shown in  FIGS. 43 ,  44 . 
     An adjustable ratchet mechanism  215  is connected between the ends of the handles  204 R, L. The ratchet mechanism is operable to hold the handles in a plurality of positions ranging from the fully open position shown in  FIG. 42  to the fully closed position shown in  FIG. 43 . A leaf spring assembly  116  is disposed between the handles and configured to bias the handles apart. An adjustable stop  217  may be provided on the ratchet mechanism  215  to adjust the span of the fully open position of the handles when they are biased outward by the leaf spring assembly  217 . Other mechanisms for biasing the handles and/or holding the handles in a particular position are contemplated. 
     The compression/distraction instrument  200  includes a fulcrum  218  that provides leverage for the compression or distraction of the vertebrae. The fulcrum includes a base  219  that is mounted on a support  220 . One leg  220 R of the support is connected to the jaw  202 R while another leg  220 L is connected to the other jaw  202 L. A cross beam  221  is supported by the two legs  220 R, L generally parallel to the plane of movement P of the jaws  202 , as shown in  FIG. 41 . The base  219  of the fulcrum  218  defines a bore  219   a  configured to be slidably mounted on the cross beam  221 . The beam  221  may be provided with a guide or anti-rotation slot  222  that receives one or more pins  223  extending from the fulcrum base  219  into the slot. The fulcrum  218  is thus supported in the instrument  200  to allow slidable movement of the axis of the fulcrum  218  in a plane that is spaced above and substantially parallel to the plane T. A plunger or friction pin  226  may be provided in the base  219  that is adapted to frictionally engage or apply pressure to the cross beam  221  in a manner sufficient to hold the fulcrum against shifting or wobbling while still allowing the fulcrum to slide along the beam. The plunger  226  may be adjustable to vary the pressure applied to the cross beam. 
     The cross beam  221  is affixed or attached to one of the legs, leg  220 L for instance. The other leg, leg  220 R in this example, includes a collar  224  defining an opening  225  to slidably receive the cross beam  221 . Thus, as the jaws  202  move together the collar  224  and more specifically the leg  220 R slides along the cross beam, as seen by comparing  FIGS. 41 and 43 . 
     As shown in  FIGS. 41-45 , the fulcrum  218  is in the form of a generally elongate cylindrical rod having an effective width W 1 . The width of the fulcrum impacts the manner in which the vertebrae are distracted/compressed. Thus, in one aspect, the instrument  200  may be provided with additional fulcrums having different configurations and widths. For instance, the fulcrum  227  shown in  FIG. 46   a  is also cylindrical but has an effective width W 2  that is less than the width W 1  of the fulcrum  218 . Alternatively the fulcrum can be generally rectangular with rounded sides, like the fulcrum  228  and  229  in  FIGS. 46   b  and  46   c , respectively. The two fulcrums may have differing widths W 3  and W 4  that may also differ from the widths W 1  and W 2 . In each of the illustrated embodiments the fulcrums present a rounded surface to contact the screw extension assemblies. The rounded surface facilitates pivoting of the extension assemblies about the fulcrum as described below. 
     As its name suggests, the compression/distraction instrument  200  may be used to selectively compress or distract adjacent vertebrae that are instrumented with the bone screw assemblies  15  and connecting member/rod  25 . Whether the instrument is used to compress or distract depends upon the orientation of the fulcrum, such as fulcrum  228 , relative to the jaws  202 . Thus, as shown in  FIG. 47 , the instrument  200  is arranged for compression with the fulcrum  228  above the jaws  202  or, in other words, with the jaws  202  disposed between the fulcrum and the connecting rod  25 . In this orientation, when the handles are manually squeezed together the jaws  202  pivot toward each other in the direction of the arrows P. Since the jaws bear against the screw extension assemblies  32  below the fulcrum  228  the extension assemblies pivot about the fulcrum toward each other in the direction of the arrows C. This movement draws the screw assemblies  15  together along the connecting member  25 , thereby compressing the adjacent vertebrae to which the screw assemblies are engaged. 
     When distraction is desired the instrument  200  is inverted—i.e., turned over—so that the fulcrum  228  is between the jaws  202  and the screw assemblies  15 , as shown in  FIG. 48 . In this orientation when the jaws are moved toward each other in the direction P the extension assemblies  32  pivot about the fulcrum so that the distal or lower portion of the assemblies flare outward in the direction of the arrows D. This movement of the extension assemblies slides the screw assemblies  15  along the rod  25 , thereby distracting the adjacent vertebrae. 
     It can be appreciated that the amount of distraction or compression is limited by the angle through which the screw extension assemblies may pivot before contacting each other. For instance, in the compression mode of  FIG. 47 , the screw extension assemblies  32  will contact each other around the middle of the connecting rod  25 . In the distraction mode of  FIG. 48 , the proximal ends of the assemblies, such as the proximal ends  55   a  of the outer sleeves of the assemblies, will contact each other when the extension assemblies have pivoted far enough outward in the direction D. The amount of angular movement of the screw extension assemblies that occurs before this contact is affected by the width of the fulcrum. Increasing the width of the fulcrum increases the amount of angular pivoting, and conversely decreasing the fulcrum width decreases the range of extension assembly pivoting. 
     In addition, the location of the fulcrum along the length of the extension assemblies  32  will also affect the maximum available pivot angle. In the compression mode of  FIG. 47 , the closer the fulcrum is moved to the screw assemblies  15  or to the surgical incision S the greater the angular range of motion. Conversely, in the distraction mode of  FIG. 48 , the angular range of motion increases as the fulcrum is moved farther from the screw assemblies or incision. 
     As shown in  FIG. 45  the fulcrum, such as fulcrum  218 , may be slidably offset from the mid-plane M of the instrument  200 . Upon actuation of the instrument the fulcrum will slide along the cross beam  220  as the fulcrum successively contacts the extension assemblies. The sliding of the fulcrum  218  and the use of fulcrums having different widths allow the surgeon more flexibility in handling different sized and spaced vertebrae in patients. 
     Percutaneous Surgical Procedures 
     The instruments disclosed herein may be used to percutaneously introduce pedicle screws and a connecting member for multiple level fixation of the spine. The instruments may be used in several different approaches as described with reference to  FIGS. 49-57 . In each approach the pedicle of the patient is accessed according to known techniques. Guide wires may be used to locate the pedicle of each vertebra to be instrument and to facilitate the subsequent introduction of tools, instruments and implants. Once the guide wires are properly positioned a series of separate incisions I are created to provide a pathway to each pedicle. Thus, in one approach a series of tissue dilators and/or tissue retractors may be introduced over each guide wire to create the pathway to the each pedicle. A final dilator or tissue retractor may remain in position to create the working channel for introduction of the pedicle screw assembly  15  into the corresponding pedicle. The size or diameter of the working channel may be larger if the bone screw assembly is to be introduced with a rod persuader assembly mounted to a screw extension assembly. 
     Once the working channel pathway has been created the pedicle is prepared in a known manner for introduction of a bone screw. Thus, the pedicle may be cannulated by a safety awl and then tapped to a suitable depth. A bone screw assembly  15  is engaged to a screw extension assembly  32  as described above and as shown in  FIGS. 10-11 . With the extension assembly in its locked configuration (see  FIG. 16 ) the yoke  17  of the bone screw assembly  15  is tightly held by the screw extension assembly  32 . In some procedures it may be desirable to also mount a rod persuader assembly  36  onto the screw extension assembly  32  as shown in  FIGS. 3 and 36 . In this instance, the persuader assembly may be locked onto the extension assembly with the advancement mechanism  184  and lever  185  in the position shown in  FIG. 37 . At this point the instruments are not in a position to receive a connecting rod so there is no need to retract the persuader assembly distal end from the vicinity of the yoke. 
     Once the screw extension assembly (and alternatively the rod persuader assembly) is engaged to the bone screw assembly  15  the screw driver assembly  100  may be advanced through the bore  58  of the inner sleeve  57  of the extension assembly  32 , as depicted in  FIG. 19 . The shaft  102  may be advanced entirely through the extension assembly and into the screw assembly until the engagement end  103  is seated within the tool engagement recess  22  of the bone screw. The entire assembly, bone screw first, is then advanced along the previously placed guide wire until the bone screw  16  of the screw assembly arrives at the tapped opening in the pedicle. The screw driver assembly  100  may then be used to drive the bone screw into the pedicle until seated. The screw extension assembly  32  may be held by the surgeon while the pedicle screw assembly  15  is driven by the screw driver assembly  100  into the pedicle. When the lower surface of the head  16   a  of the bone screw seats in the pedicle, the lower portion of the yoke is spaced above the surface of the vertebra allowing unhindered articulation of the yoke and pedicle screw extension assembly  32  which is tightly affixed to the yoke. The depth and positioning of the bone can be verified in a known manner. 
     Once the pedicle screw position has been verified the screw driver assembly and guide wire may be removed. In a procedure that does not utilize an initially placed rod persuader assembly, the bone screw assemblies  15  and screw extension assemblies  32  will appear as shown in  FIG. 49  with each assembly extending through its own incision I. In a procedure in which a persuader is initially placed on at least one screw extension assembly  32  the surgical site will appear as in  FIG. 52 . The screw extension assemblies  32  may be used to gage the size of the connecting member or rod  25  required to span the instrumented vertebrae. Thus, a known caliper instrument (not shown) may be seated on the outermost extension assemblies to indicate the desired rod length. The proper length rod is selected and contoured as desired. As previously discussed, the pre-bent rod  25  disclosed herein may be used in one orientation for correcting or creating lordosis and in the opposite orientation for correcting or creating kyphosis. More complex bends may be introduced into the connecting rod using a suitable rod bender. 
     The selected rod is grasped by the rod introducer assembly  34  as shown in  FIG. 25  and as described above. As to the proper positioning of the curvature of the rod, for a lordotic curve the rod should curve toward the handle  110  of the introducer assembly, as shown in  FIG. 49 . For a kyphotic curve the rod should curve in the opposite direction away from the handle. Once the rod orientation has been verified the second locking mechanism  150  of the introducer assembly  34  may be engaged to hold the rod in its “neutral” position, as depicted in  FIG. 22 . In the neutral position the rod is retained by the introducer assembly but may pivot about the engagement posts  120 , as described above in relation to  FIGS. 22 and 26 . The desired angle of the rod  25  relative to the outer sleeve  114  of the rod introducer assembly  34  may be set and the rod locked by engaging the first lock  140  upon fully depressing the lever  113  (see  FIG. 21 ). In many procedures the rod is initially situated at a 45 degree angle to the outer sleeve  114 , as shown in  FIG. 49 . If it is found during the procedure that a different rod angle is needed, the push button  145  may be depressed to release the first lock  140  and placing the introducer assembly in the neutral position to permit adjustment of the rod angle. Once the new rod angle has been set the lever  113  may be depressed to engage the first lock and tightly grip the rod again. 
     With the screw assemblies threaded into the pedicles with the screw extensions attached the connecting rod can then be introduced. In certain procedures one or more rod detectors  160  may be placed within one or more screw extension assemblies  32 , as illustrated in  FIG. 33 . In one procedure the rod is introduced through the incision at an extreme cephalad or caudal one of the screw extension assemblies, as depicted in  FIG. 49 . In this depiction the rod  25  is oriented at a 45 degree angle to the outer sleeve  114  of the rod introducer  34 . The introducer is manipulated so that the leading end  27  of the rod  25  passes through the slot  59  of the extension assembly and subsequently or simultaneously through incision I. The slot  59  may act as a guide to slide the distal end  27  of the rod downward through the incision. If necessary the angle of the rod may be adjusted as described above to facilitate entry of the rod through the incision. 
     Once below the fascia S the rod can be advanced subcutaneously beneath the fascia toward the other screw assemblies. The sides of the rod slots  59  and  67  in the outer and inner sleeves, respectively, of the extreme screw extension assembly can further act as a guide to keep the rod  25  aligned with the rod slots in the other extension assemblies. As the rod enters the rod slots of the successive extension assemblies the indicator flag  164  ( FIG. 33 ) of the associated rod detector will shift positions to indicate that the rod is within the respective slot. When the rod is fully positioned within each of the screw extension assemblies  32  the outer sleeve  114  of the rod introducer  34  may abut the outer sleeve  55  of the extreme extension assembly, as shown in  FIG. 50 . The rod detectors may then be removed. 
     At this point it is desirable that the rod be oriented at a 90 degree angle to the outer sleeve  114  of the introducer, as illustrated in  FIG. 51 . Adjustment of the rod angle can be accomplished by depressing the pushbutton  145  to release the second lock and allow the rod to be pivoted relative to the outer sleeve  114 . It can be pointed out that due to the construction of the rod introducer the surgeon will receive a tactile indication produced by the rod introducer when the rod has dislodged from the current angular position and re-seated in the new position. In accordance with this particular procedure the rod introducer  34  is the only tool required to seat the rod within the yokes in anticipation of locking the screw assembly with a set screw or a clamping mechanism in accordance with the design of the screw assembly. Consequently, once the rod is fully seated the rod introducer  34  may be disconnected from the rod  25  by depressing the pushbutton  145  to release the second lock and then depressing the release buttons  152  to release the first lock. The actuation lever  113  may then be pivoted outward from the handle  110 , as shown in  FIG. 51  to spread the flexible legs of the outer sleeve and release the legs from the engagement end  28  of the rod (see  FIG. 20 ). The rod persuader is then withdrawn through the incision. 
     In an alternative procedure, one or more rod persuaders  36  may be selectively used to seat the rod  25  within the bone screw assemblies  15 , as shown in  FIGS. 52-54 . The rod persuader  36  may be introduced through the incision I with the screw extension assembly  32 , as described above, or at the discretion of the surgeon after the screw assemblies and extension assemblies have been engaged to the vertebrae. One or more rod detectors may be positioned as described above. Prior to introducing the rod  25  the rod persuader(s) must be in the retracted position shown in  FIG. 52  to avoid interfering with the rod as it enters the rod slots in the screw extension assemblies. Thus, the advancement lever  185  of the assembly  36  is in its upward position. The spring biased ball and detent structure discussed above ( FIG. 40 ) will hold the lever and thus the outer tube  180  in the retracted position. 
     As shown in  FIG. 52  the rod  25  is introduced through the incision I and rod slot  59  at the extreme cephalad or caudal screw assembly  15  and extension assembly  32 . Once the rod has been fully advanced through each of the extension assemblies (with the outer sleeve  114  of the introducer  34  abutting the outer sleeve  55 ) the rod introducer assembly  34  may be moved to the neutral position and the rod persuader assembly  36  can be actuated. The lever  185  is pivoted downward, which drives the outer sleeve  180  downward so that the rod scallops  182  seat on the rod  25 , as illustrated in  FIG. 53 . The advancement lever  185  is pivoted to its lowermost position to drive the outer sleeve  180  fully downward, as shown in  FIG. 54 . In this position the outer sleeve has pushed the rod  25  to fully seat within the yokes of the screw assemblies. The rod introducer  34  may remain engaged to the rod  25  during this process. Once the rod is fully seated the introducer  34  may be disengaged from the rod as explained above. 
     In the procedures just described the rod is introduced exteriorly of the extension assemblies through an incision common with an outermost screw and extension assembly. In an alternative procedure the rod is introduced into the surgical site through a separate incision I rod , as illustrated in  FIGS. 55-56 . This separate incision I rod  may be oriented at a 45 degree trajectory with respect to the extreme cephalad or caudal screw extension assembly through which the rod is first introduced. The rod  25  is preferably at the 45 degree orientation relative to the rod introducer  34  as shown in  FIG. 55 . As shown in  FIG. 55  the outer sleeve  114  will pass through the separate incision I rod  to guide the rod subcutaneously through each successive extension assembly. With this procedure it may be desirable to position a rod detector within each screw extension assembly to provide a visual indication when the rod enters each assembly. 
     As shown in  FIGS. 55-56  each extension assembly may be provided with a rod persuader assembly  36 . Prior to introducing the rod  25  the advancement levers  185  of all the rod persuader assemblies are in their fully retracted positions. Once the rod is in place the levers are pivoted downward to the respective outer sleeves  180  downward to seat the rod in the corresponding yoke. It can be appreciated that the advancement levers may be pivoted simultaneously or sequentially or partially rotated in steps, all with the goal of smoothly seating the rod within each screw assembly  15 . 
     Another procedure approach is shown in  FIG. 57 . In this approach a common incision I C  is formed between the separate incisions through which the screw and extension assemblies have been advanced. The rod  25  may be introduced through the rod slots  59  of each extension assembly  32  above the fascia S under direct vision, so that rod detectors are not required. In this approach the rod persuader(s) are not mounted on the screw extension assemblies until after the rod has been properly positioned within the assemblies. 
     Once the rod has been positioned above the fascia the rod introducer assembly  34  can be manipulated to push the rod through the incision I C  to the position shown in  FIG. 57 . If necessary the rod angle relative to the outer sleeve  114  may be adjusted, as described above. The rod may be fully seated within the screw assemblies  15  with or without the rod persuader assemblies. 
     In each approach, once the rod has been fully seated within the screw assemblies the set screw or locking element may be advanced through each screw extension assembly to engage the respective bone screw assembly. In some instances the screw assemblies are finally tightened onto the rod. In other instances compression or distraction may be necessary. In these instances the bone screw assemblies may be provisionally tightened in a manner that permits one or more of the screw assemblies to slide along the connecting rod. The compression/distraction device  200  may be used as described above to perform the necessary adjustments to the screw assemblies, after which the assemblies may be finally tightened. After the rod and screw fixation construct is complete the screw extension assemblies can be removed and the incisions closed. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the invention are desired to be protected.
A method of attaching an extension to a bone anchor for use in percutaneous spinal surgery is provided that comprises the steps of: providing a bone anchor including a first portion for attachment to a vertebra and a second portion for attachment to a connecting element, the bone anchor including an extension coupling surface; providing an elongate hollow extension having a distal end and a proximal end, the extension having a rotationally movable anchor engaging member projecting radially from the distal end thereof; placing the distal end of the extension adjacent the bone anchor; and rotating the anchor engaging member to engage the extension coupling surface on the anchor and couple the extension and the bone anchor.