Patent Publication Number: US-2010131016-A1

Title: Less invasive surgical system and methods

Description:
FIELD OF THE INVENTION 
     The present invention is directed to surgical instruments and, in particular, surgical instruments for less invasive procedures for spinal fixation and methods for using the same. Specifically, the less invasive surgical instruments enable an operator to affix a plurality of screws to the spine and introduce a spinal fixation rod therebetween. 
     BACKGROUND OF THE INVENTION 
     Spinal fixation systems used to correct spinal deformities generally consist of a series of bone fasteners anchored to the pedicles, lamina or transverse process of the vertebrae. The bone fasteners are interconnected to one another by one or more elongated spinal rods or plates. In order to access the spinal area for implantation of these spinal fixation systems and their individual components, open approach surgical techniques have historically been employed. These open procedures generally involve large skin incisions and extensive tissue retraction and resection, all which may result in considerable post-operative pain and prolonged hospital stays. 
     More recently, surgeons have used minimally invasive techniques to reduce the post-operative effects of spinal fixation procedures. A paraspinal approach is one form of minimally invasive technique and involves muscle splitting or muscle sparing in order to gain access to the posterior elements of the spine. Such a technique minimizes trauma to tissues adjacent the spine. Unlike open approaches where muscles and other soft tissue are cut, split, stripped and dissected, the paraspinal approach involves separation or splitting of the muscles along their fibers. 
     To perform a paraspinal surgical procedure, a midline skin incision is made and followed by bi/unilateral fascia incisions. The muscles are then separated to allow bilateral access to the spine via a single skin incision. Additionally, one or more off-midline skin incisions may be made to allow for a more direct approach. 
     Implanting a spinal rod fixation system generally involves at least two steps: (i) placing implants (e.g., screws) into the spine and (ii) inserting a rod between the implants. Proper placement of the implants requires correctly positioning the implants in the spine. The starting insertion point, the trajectory of the implants and the implants&#39; size are crucial to implant placement. 
     The spinal implant generally comprises a screw portion and a body portion. The screw portion is inserted into the spine. And, the body portion generally has a channel into which a spinal rod is inserted and secured. The rod insertion procedure requires insertion of the rod through an incision in the skin, which may be separate and distinct from the incision through which the implant(s) is placed. In other embodiments, the rod is inserted through the same incision as the implant(s). The rod connects the implants together. 
     There exists a need for a less invasive spinal implant and rod introduction system that improves direct visualization, enables a rod to be connected to an implant anchored at varying depths in the body and is generally simple to use. 
     SUMMARY OF THE INVENTION 
     The present invention generally relates to instruments for less invasive surgical procedures and, in particular, a less invasive system that may be used for inserting bone screws into the vertebrae and connecting a fixation rod therebetween. The present invention also relates to methods of performing less invasive surgical procedures using these instruments. 
     The less invasive system may comprise dilation tools, one or more insertion/working cannulas, a plurality of screws, at least one rod for connecting the screws, and a rod inserter. After a surgeon determines an insertion location, an incision may be made in a patient. Dilation tools may then be inserted into the incision to enlarged the incision so that tools may be inserted therethrough. In one embodiment, a guide wire may be inserted down into a vertebra. In an embodiment incorporating a guide wire, the tools used to perform the procedure may be cannulated to receive the guide wire. A series of sequentially larger dilators may be positioned over the guide wire until the incision has been dilated a desirable amount to form an opening in the patient. A retractor may then be inserted over the dilators in a closed position. The dilators may be removed and the retractor may be opened to enlarge the incision to form an opening. Such a method may expose a plurality of vertebrae for fixation. With the incision enlarged, insertion cannulas, bone screws, fixation rod(s) and various surgical tools may be positioned within the opening formed by the retractor. 
     In one embodiment, a dilation mechanism may be used for increasing the size of an incision to form an opening. The dilation mechanism may comprise at least one dilator having a elongated cylindrical shape with a channel passing therethrough. In addition, the dilation mechanism may comprises a retractor having at least two blades for being inserted through the incision. The at least two blades may have an opened position and a closed position. The blades may be configured to be inserted over the at least one dilator in the closed position and may be configured to move to the opened position to create the opening. Moreover, the dilation mechanism may comprises an inserter, which may have an elongated portion, a proximal end, a distal end and an enlarged portion on the distal end of the elongated portion. The at least one dilator may be configured to receive the inserter therein. 
     In another embodiment, one or more incisions may be made in a patient and multiple dilators of increasing size may be used to expand the incisions. A working cannula may be positioned over the largest dilator. In one embodiment, the working cannula may have a proximal end, a distal end, and a channel extending from the proximal end to the distal end. The channel of the working cannula may be sized and configured to receive at least one dilator. Once the working cannula(s) are in place, the dilators may be removed. An insertion cannula, bone screws, fixation rod(s) and various surgical tools may be positioned within the working cannula. In an embodiment where the working cannula may be large enough (e.g., where two or more vertebrae may be exposed), multiple insertion cannulas may be inserted through the same working cannula. 
     In other embodiments, sequential dilators and/or a retractor may be unnecessary. An insertion cannula may be operatively connected to an inserter having a bullet-shaped head and inserted as a single unit into an incision. The inserter may be removed after insertion, leaving the insertion cannula remaining in a patient, through which a procedure may be performed. In one embodiment, the inserter may have an elongated portion, a proximal end, a distal end and an enlarged portion on the distal end of the elongated portion. The inserter may be sized and configured to be received within the passageway of an insertion cannula. 
     A cavity forming device, such as a drill may be used to form a cavity within each vertebra involved in the procedure. The drill may be passed through the working cannula, retractor and/or the insertion cannula. In other procedures, an awl, probe and/or tap may be used to create a cavity in the vertebrae. However, any means of creating a cavity is envisioned. Once a cavity has been made in a vertebra, screw(s) may then be inserted into the vertebrae. The screws may be polyaxial screws having a shank portion and a head portion. The head portion may have a channel therethrough for receiving a fixation rod and may be connected to the shank portion so that the head portion may pivot about the shank portion. Other procedures may use screws where the shank and head portions may be one piece and fixed with respect to each other. 
     The screws may be attached to the insertion cannula and inserted as a single unit into the working cannula and/or retractor. With the insertion cannula and screw positioned in the working cannula and/or retractor, an implantation mechanism such as a screwdriver may be inserted in the insertion cannula and engage the screw to drive the screw into bone. In another embodiment, a screwdriver may be inserted in the insertion cannula and engage the screw prior to insertion into a patient. These devices may then be inserted as a single unit into the working cannula and/or retractor. In other embodiments, the insertion cannula may be inserted into the working cannula and/or retractor and, subsequently, a screw and screwdriver may be inserted down into the insertion cannula. In all embodiments, the insertion cannula may be used to manipulate the head portion for enabling insertion of a fixation rod. The implantation mechanism may be used to insert the screw into bone. 
     In one embodiment, the insertion cannula may have a proximal end, a distal end, a passageway from the proximal end to the distal end, and at least one slot intersecting the passageway. In such an embodiment, the insertion cannula may also comprise a surface and the distal end may comprise a threaded portion on the surface for engaging a spinal fixation device. 
     In another embodiment, the insertion cannula may comprise a proximal end, a distal end, a passageway from the proximal end to the distal end, at least one slot intersecting the passageway and a flexible portion for engaging a spinal fixation device. The at least one slot may be sized and configured to receive an elongated fixation device. The flexible portion may be a pair of arms defined by two diametrically opposed slots. In such an embodiment, the pair of arms may be configured to snap onto the spinal fixation device. In another embodiment, the flexible portion may comprise at least one flexible member having a first end portion and a second end portion. The first end portion may be operably connected to the insertion cannula and the second end portion may be freely moveable with respect the insertion cannula. The second end portion may be sized and configured to engage the spinal fixation device. 
     In yet another embodiment, the insertion cannula may comprise a proximal end, a distal end and at least one sidewall, which may define a passageway from the proximal end to the distal end. The insertion cannula may have at least one slot in the at least one sidewall communicating with the passageway and a flexible portion for engaging the spinal fixation device. The flexible portion may comprise a first end portion and a second end portion. The first end portion may be operably connected to the at least one sidewall of the cannula and the second end portion may be freely moveable into and out of the passageway of the cannula. The second end portion may be engagable with a spinal fixation device. 
     In another embodiment, the insertion cannula may comprise an inner cannulated shaft having a proximal end, a distal end and a bore therethrough. In addition, the insertion cannula may comprise an outer cannulated shaft having a proximal end, a distal end, and a bore therethrough. The bore of the outer cannulated shaft may be sized and configured to receive the inner cannulated shaft. In this embodiment, the flexible portion may comprise at least one flexible member having a first end portion and a second end portion. The first end portion may be operably connected to the inner cannulated shaft and the second end portion may be freely moveable with respect the inner cannulated shaft. The outer cannulated shaft may be sized and configured to travel along the inner cannulated shaft from a first position to a second position and move the second end portion of the at least one flexible member towards the spinal fixation device. Moreover, the inner cannulated shaft may comprise a slot and the outer cannulated shaft comprises a protrusion, which may be engagable with the slot. The slot may have at least one notch and may be sized and configured for positioning the outer cannulated shaft at least one location on the inner cannulated shaft. 
     In yet another embodiment, the cannula may comprise a cannulated shaft having a longitudinal recess and an elongated member positionable within the recess. In this embodiment, the flexible portion may comprise at least one flexible member having a first end portion and a second end portion. The first end portion may be operably connected to the cannulated shaft and the second end portion may be freely moveable with respect the cannulated shaft. The elongated member may be sized and configured to engage the at least one flexible member such that the at least one flexible member may be moved towards the spinal fixation device. 
     Furthermore, in one embodiment, an implantation mechanism may be sized and configured to be inserted into the passageway of an insertion cannula. The implantation mechanism may comprise a shaft having a proximal end, a distal end, and an engagement portion on the distal end sized and configured to engage the spinal fixation device. Furthermore, the implantation mechanism may comprise a protrusion on the engagement portion, which may engage a longitudinal recess of an inner cannulated shaft. The protrusion may be moveable along the recess. The protrusion and longitudinal recess may be configured to align the at least one slot of the cannula relative to the spinal fixation device. 
     Moreover, in other embodiments, the implantation mechanism may further comprises a first sleeve having a proximal end and a distal end. The first sleeve may be positionable around the shaft. The engagement portion may comprise a protruding portion and at least one shoulder portion. The protruding portion may engage the shank portion of an implant and the at least one shoulder portion may engage the channel of the head portion of the implant. The head portion of the spinal fixation device may have internal threads and the distal end of the first sleeve may have a threaded portion, the threaded portion of the first sleeve may be sized and configured to engage the internal threads of the head portion of the spinal fixation device. 
     In another embodiment, an implant positioner may be sized and configured to be inserted into the passageway of the cannula. The implant positioner may have an elongated shaft, a proximal end, a distal end and an engaging portion on the distal end sized and configured to engage a spinal fixation device and manipulate the spinal fixation device relative to the cannula. 
     A fixation device inserter may be used to insert a fixation rod into the head portions of the screws. In one embodiment, a separate incision may be made in a patient at a distance from the incision(s) used to insert the screws. The fixation device inserter may be coupled to a fixation rod and may be used to insert the fixation rod through the separate incision and into the side of the head portion of the screws. In one such embodiment, the a fixation device inserter may comprise an elongated shaft and a moveable member having a proximal end, a distal end and an engaging portion at the distal end for engaging an elongated fixation rod. The moveable member may be positioned within the elongated shaft. Moreover, the fixation device inserter may comprise an actuation mechanism operably associated with the moveable member. The actuation mechanism may be configured to move the moveable member between a first position and a second position. 
     In another embodiment, the rod inserter may be used to position a fixation rod down through the insertion cannula into the head portion through the top of the screw. Once the fixation rod is in place, a locking cap may be positioned down the working cannula, retractor and/or insertion cannula and engaged to the head portion of the screws such that the fixation rod may be fixed therein. 
     The method for performing a less invasive procedure, in one embodiment, may comprise providing a first cannula having a flexible portion, providing a second cannula and providing a first and second implant. The first and second implant may comprise a shank portion, a head portion and a channel passing through the head portion for receiving an elongated fixation device. The method may further comprise attaching the flexible portion of the first cannula to the first implant and attaching the second cannula to the second implant. Furthermore, the first implant may be inserted into a first vertebrae and the second implant may be inserted into a second vertebrae. Additionally, an elongated fixation device may be inserted into the channel of the head portion of the first and second implants. The elongated fixation device may be locking in the head portion of the first and second implants. In an embodiment where the flexible portion may be a pair of arms, the method may further comprise snapping the head portion of the first implant between the pair of arm. Moreover, an implant positioner may be inserted into at least one of the first and second cannulas to manipulate the head portion of at least one of the first and second implants relative to at least one of the first and second cannulas. 
     In another embodiment, the method may further comprise providing a fixation device inserter having an elongated shaft, a moveable member positioned within the elongated shaft for engaging the elongated fixation rod, and an actuation mechanism operably associated with the moveable member. The actuation mechanism may be configured to move the moveable member between a first position and a second position. The method may additionally comprise inserting a first implant and first cannula through at least one of a first opening and a first incision, and inserting a second implant and second cannula through at least one of the first opening, the first incision and a second opening. Moreover, the method may comprise creating at least one of a third incision and a third opening and using the fixation device inserter to position the elongated fixation device through one of the third incision and third opening, and into the channel of the head portion of at least one of the first and second implants. 
     In another embodiment, the method for performing a less invasive procedure may comprise providing a first cannula having a threaded portion, providing a second cannula and providing a first and second implant. The first and second implant may comprise a shank portion, a head portion and a channel passing through the head portion for receiving an elongated fixation device. The head portion of the first implant may also have threads. The method may further comprise engaging the threaded portion of the first cannula to the threads of the first implant and attaching the second cannula to the second implant. Furthermore, the first implant may be inserted into a first vertebrae and the second implant may be inserted into a second vertebrae. Additionally, an elongated fixation device may be inserted into the channel of the head portion of the first and second implants. The elongated fixation device may be locking in the head portion of the first and second implants. 
     In yet another embodiment, the method may comprise making an incision and inserting a shank portion of an implant into a vertebrae. The method may further comprise engaging the shank portion with an implantation mechanism. The head portion may be engaged with a cannula to form a unit, which may be inserted into the incision. Additionally, the method may comprise engaging the head portion with the shank of the implant after the shank portion of the implant has been inserted into the vertebrae. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention can be better understood by reference to the following drawings, wherein like references numerals represent like elements. The drawings are merely exemplary to illustrate certain features that may be used singularly or in combination with other features and the present invention should not be limited to the embodiments shown. 
         FIG. 1  is a side view of an exemplary embodiment of a trocar and an exemplary embodiment of a guide wire positioned in the spine; 
         FIG. 2  is a partial cross-sectional view of an exemplary embodiment of an inserter; 
         FIG. 3A  is a side view of an exemplary embodiment of a dilator; 
         FIG. 3B  is a top view of the dilator of  FIG. 3A  along A-A; 
         FIG. 4A  is a partial cross-sectional view of an exemplary embodiment of a working cannula; 
         FIG. 4B  is a top view of the working cannula of  FIG. 4A   
         FIGS. 5A and 5B  are perspective views of an exemplary embodiment of a retractor in a closed position ( FIG. 5A ) and an opened position ( FIG. 5B ); 
         FIG. 6A  is a cross-sectional view of an exemplary embodiment of a screw; 
         FIG. 6B  is a top view of an exemplary embodiment of the screw of  FIG. 6A ; 
         FIG. 7A  is a perspective view of an exemplary embodiment of an insertion cannula; 
         FIG. 7B  is a perspective view of an exemplary embodiment of an inner portion of the insertion cannula of  FIG. 7A ; 
         FIG. 7C  is a top view of the inner portion of  FIG. 7B ; 
         FIG. 7D  is a partial cross-sectional view of an exemplary embodiment of an outer portion of the insertion cannula of  FIG. 7A ; 
         FIG. 7E  is a perspective view of the outer portion of  FIG. 7D ; 
         FIG. 7F  is a perspective view of the insertion cannula of  FIG. 7A  in a first position engaging the screw of  FIG. 6A ; 
         FIG. 7G  is a perspective view of the insertion cannula of  FIG. 7A  in a second position engaging the screw of  FIG. 6A ; 
         FIG. 7H  is another perspective view of the insertion cannula of  FIG. 7A ; 
         FIG. 8A  is a perspective view of another exemplary embodiment of an insertion cannula; 
         FIG. 8B  is a perspective view of an exemplary embodiment of an inner portion of the insertion cannula of  FIG. 8A ; 
         FIG. 8C  is a top view of the inner portion of  FIG. 8B ; 
         FIG. 8D  is a partial cross-sectional view of an exemplary embodiment of an outer portion of the insertion cannula of  FIG. 8A ; 
         FIG. 8E  is a perspective view of the outer portion of  FIG. 8D ; 
         FIG. 9A  is a perspective view of another exemplary embodiment of an insertion cannula; 
         FIG. 9B  is a perspective view of an exemplary embodiment of a portion of the insertion cannula of  FIG. 9A ; 
         FIG. 9C  is a top view of the portion of  FIG. 9B ; 
         FIG. 9D  is a perspective view of an exemplary embodiment an elongated piece for insertion in the portion of  FIG. 9B ; 
         FIG. 10A  is a perspective view of another exemplary embodiment of an insertion cannula; 
         FIG. 10B  is a cross-sectional view of an exemplary embodiment of an outer portion of the insertion cannula of  FIG. 10A ; 
         FIG. 10C  is a cross-sectional view of an exemplary embodiment of an inner portion of the insertion cannula of  FIG. 10A ; 
         FIG. 10D  is a cross-sectional view of the insertion cannula of  FIG. 10A ; 
         FIG. 11A  is a perspective view of another exemplary embodiment of an insertion cannula; 
         FIG. 11B  is a perspective view of the insertion cannula of  FIG. 11A  engaging the screw of  FIG. 6A ; 
         FIG. 12A  is a perspective view of another exemplary embodiment of an insertion cannula; 
         FIG. 12B  is a side view of the insertion cannula of  FIG. 12A  engaging the screw of  FIG. 6A ; 
         FIG. 13A  is a side view of an exemplary embodiment of a drill; 
         FIG. 13B  is a side view of the drill of  FIG. 13A  being drilled into the spine through a cannula; 
         FIGS. 13C and 13D  are side views of an exemplary embodiment of a drill bit of the drill of  FIG. 13A ; 
         FIG. 14  is a side view of an exemplary embodiment of an implant positioner; 
         FIG. 15A  is a side view of an exemplary embodiment of a screwdriver; 
         FIG. 15B  is a side view of an exemplary embodiment of an inner shaft of the screwdriver of  FIG. 15A ; 
         FIG. 15C  is a partial cross-sectional view of an exemplary embodiment of a first sleeve of the screwdriver of  FIG. 15A ; 
         FIG. 15D  is a cross-sectional view of an exemplary embodiment of a second sleeve of the screwdriver of  FIG. 15A ; 
         FIG. 15E  is a side view of the screwdriver of  FIG. 15A  with an exemplary handle portion; 
         FIG. 15F  is a side view of the screwdriver of  FIG. 15A  positioned in the insertion cannula of  FIG. 12A  and engaging the screw of  FIG. 6A ; 
         FIG. 15G  is a side view of another exemplary embodiment of a screwdriver; 
         FIG. 16A  is a perspective view of an exemplary embodiment of a rod inserter; 
         FIG. 16B  is a partial cross-sectional view of the rod inserter of  FIG. 16A ; 
         FIG. 16C  is a side view of an exemplary embodiment of a moveable member of the rod inserter of  FIG. 16A ; 
         FIG. 17  is a side view of another exemplary embodiment of a rod inserter engaging insertion cannulas and having a rod positioned through the screws of  FIG. 6A ; 
         FIG. 18  is a side view of another exemplary embodiment of a rod inserter; 
         FIG. 19A  is a perspective view of an exemplary embodiment of a rod being inserted into the insertion cannulas of  FIG. 11A ; 
         FIG. 19B  is a perspective view of another exemplary embodiment of a rod being inserted into the insertion cannulas of  FIG. 12A ; 
         FIG. 20A  is a side view of an exemplary embodiment of a pusher; 
         FIG. 20B  is a side view of an exemplary embodiment of the pusher of  FIG. 20A  positioned in the insertion cannula of  FIG. 12A ; 
         FIG. 20C  is a side view of an exemplary embodiment of a shank portion of the screw of  FIG. 6A ; 
         FIG. 21  is a perspective view of an exemplary embodiment of a locking cap screwdriver; and 
         FIG. 22  is a perspective view of an exemplary embodiment of a fixation system attached to the spine. 
     
    
    
     DETAILED DESCRIPTION 
     The less invasive system of the present invention may comprise a means for dilating an incision in a patient (e.g., sequential dilator, retractor), at least one insertion/working cannula, a plurality of screws, at least one rod for connecting the screws, and a rod inserter. It should, however, be understood that those of ordinary skill in the art will recognize many modifications and substitutions which may be made to various elements of the present invention. And, while the instruments and implants may be described with reference to implanting in the vertebrae, they may be used in other surgeries in other locations in a patient. 
     A. Dilation Tools 
     A radiographic image may be taken of the spine, including the vertebrae which are to receive implants. From the radiographic image, an insertion point may be located on a patient&#39;s back. An incision may then be made into the patient&#39;s back to form an opening through which the less invasive system may be used. The opening may then be dilated. It will be understood by those skilled in the art that dilation of an opening in a patient may be performed using any number of devices, including the devices described in further detail below. 
     1. Trocar and Guide Wire 
     As shown in  FIG. 1 , a trocar  100  may be an elongated member  110  having a proximal end  120  and a distal end  130 . The trocar  100  may be inserted through the incision in the patient—for example, using fluoroscopic guidance—and may engage a vertebra. The distal end  130  of the trocar  100  may have a point  135  for puncturing and/or forming a hole through the cortex of the vertebra. Moreover, the trocar  100  may have an enlarged front portion  102  on the distal end  130  of the trocar  100 . The enlarged front portion  102  may assist in increasing the size of the opening and creating a percutaneous passageway to a vertebra. And, in an alternative embodiment, the distal end  130  of the trocar  100  may have threads (not shown) for engaging a vertebra to hold the trocar  100  in place on a vertebra. 
     Additionally, the proximal end  120  of the trocar  100  may have a handle  125 . A handle  125  may make it easier for an operator to use/manipulate the trocar  100 . The handle  125  may be integrally formed with the elongated member  110  or it may be mechanically joined at the proximal end  120 . The handle may be radiolucent so that it may be invisible under fluoroscopic observation or when using X-rays. Moreover, the handle  125  may be removable. In one embodiment, the handle  125  may be T-shaped. In another embodiment, the handle  125  may be spherical in shape. Further, the trocar  100  may comprise a channel  140  therethrough which may extend from the proximal end  120  to the distal end  130 . 
     A guide wire or rod  150  may be inserted through the channel  140  at the proximal end  120  of the trocar  100  and may be positioned in the hole formed in the pedicle by the trocar  100 . In an embodiment having a handle  125 , the guide wire  150  may also be inserted through the handle  125 . A guide wire  150 , such as that shown in  FIG. 1 , may be used to guide various devices and/or implants into a patient and towards the spine. For example, the guide wire  150  may be used to guide dilators, insertion/working cannulas, a drill, a screwdriver, and implants (e.g., bone screws) to a location on the spine. It should be noted that any device described herein may be inserted into a patient without the use of the guide wire  150 . The guide wire  150  may be inserted down through the channel  140  until it engaged the vertebra. Thereafter, a surgical mallet or other striking instrument (not shown) may be used to hammer the guide wire  150  into the hole formed by the trocar  100 . In this way, the guide wire  150  may be anchored to the vertebra. 
     To assist an operator to hammer the guide wire  150  into a vertebra, the guide wire  150  may comprise a cap  152 , which may be located at a proximal end  154  of the guide wire  150 . Such a construction may provide an operator with an enlarged surface for striking a mallet or other instrument against the guide wire  150 . The cap  152  may be made of metal, plastic, rubber or any other material that may withstand repeated impact. Moreover, the cap  152  may be any shape (e.g., circular, polygonal, spherical) or size. In addition, the cap  152  may be engagable with another component (e.g., a slap hammer), which may be used to implant the guide wire  150  into bone. 
     The step of inserting the trocar  100 /guide wire  150  may be repeated any number of times through separate incisions or the same incision, depending on how many vertebrae and/or the number of implants that may be involved in the procedure being performed. 
     2. Dilator 
     Once the guide wire  150  is in place, the trocar  100  may be removed from the patient&#39;s body. A sequential dilator system such as disclosed in U.S. patent application Ser. No. 10/884,705 filed Jul. 2, 2004, entitled Sequential Dilator System, the entire content of which is hereby incorporated by reference, may be used to enlarge the opening in the patient. It should be understood, however, that any dilator system may be used with the less invasive system. 
     A dilator inserter  200 , such as the one shown in  FIG. 2 , may be inserted over the guide wire  150  down towards the surgical site proximate a vertebra. The dilator inserter  200  may comprise an elongated shaft  210 , which may have a central bore  212  in which the guide wire  150  may be received. The dilator inserter  200  may have a handle  216  at its proximal end  214  and an bullet-shaped tip  220  at its distal end  218 . The handle  216  may be removable to allow one or more dilator tubes  350  ( FIG. 3A ) to be inserted over the dilator inserter  200 . Moreover, the bullet-shaped tip  220  may assist in enlarging the opening and created an enlarged percutaneous pathway to a vertebra. The tip  220 , however, may be any shape and may or may not be enlarged. 
     Once the dilator inserter  200  is in place, one or more different sized dilator tubes  350  such as shown in  FIG. 3A  may be inserted over the dilator inserter  200 . It should be noted that a dilator inserter  200  may not be used and one or more dilator tubes  350  may be inserted directly over the guide wire  150 . The dilator tubes  350  may have an elongated shaft  352  with a channel  354  therethrough dimensioned and configured for receiving the dilator inserter  200  and/or other dilators  350 . In addition, the dilator tube  350  may have markings  355 , which may provide an operator with a visual indication of the depth of the dilator  350  within the body of a patient. 
     A first dilator  350 , which may have an inner diameter D ( FIG. 3B ), may be positioned over the dilator inserter  200  and down to a surgical site proximate a vertebra. It should be understood, however, that the dilator inserter  200  and a first dilator  350  may be attached to each other and inserted into the body as a single unit. The inserter  200  may then be removed from the first dilator  350 . Thereafter, a second dilator  350  having an inner diameter greater than the outer diameter D′ of the first dilator  350  may be inserted over the first dilator  350 . This process may be repeated numerous times with sequentially larger dilators  350  until the opening has been dilated to a size desired by the surgeon and appropriate for the procedure to be performed (e.g., large enough to receive implants and/or instruments). And, in an embodiment where multiple openings may be used for a procedure, the dilation process may be repeated for each opening. 
     In one embodiment, after the largest dilator  350  is in place, a working cannula  475  ( FIGS. 4A and 4B ) may be positioned over the dilators  350  and down to the surgical site proximate the vertebra to be operated on. The working cannula  475  may have a proximal end  476 , which may remain accessible to the surgeon, and a distal end  478  which, upon insertion of the working cannula  475 , may be located adjacent the surgical site. In addition, the working cannula  475  may have a channel  480  extending from an opening  497  at the proximal end  476  to an opening  498  at the distal end  478  through which an operator may insert implants and surgical instruments. The working cannula  475  may have a constant diameter from the proximal end  476  to the distal end  478 . Moreover, the working cannula  475  may have a handle  482  at its proximal end  476  to allow a surgeon to grab the working cannula  475  during surgery and/or enable the working cannula  475  to be attached to an operating table. In this way, the working cannula  475  may be held stationary during surgery. 
     Once the working cannula  475  is in place, all dilators  350  and/or the guide wire  150  may be removed from the body. In one embodiment, the guide wire  150  may remain in place to guide implants and instruments to a surgical site. Each dilator  350 , however, may be removed after a larger dilator  350  is positioned thereover. Different parts of a procedure may be performed through different working cannulas  475 . If the working cannula  475  is large enough and expose multiple vertebrae, an entire procedure may be performed through a single working cannula  475  (i.e., without making another opening in the patient). Nevertheless, other means of dilating an opening may also be used in addition to or in place of dilators  350  such as, for example, a retractor. 
     3. Retractor 
     A retractor  500 , such as disclosed in U.S. patent application Ser. No. ______ (attorney docket no. 8932-804-999 (708716-999781)), filed Aug. 13, 2004, entitled Multiple-Blade Retractor, the entire content of which is hereby incorporated by reference, may also be used to enlarge an opening in a patient. In one embodiment, the blades  502  of the retractor  500  may be positioned over at least one dilator  350  into the opening in a closed position ( FIG. 5A ). In another embodiment, the retractor  500  may be inserted directly into an opening in a patient without the use of a dilator  350 . Once the retractor  500  is in the opening in a patient, the handles  504  of the retractor  500  may then be squeezed together and the blades  502  may be spread apart ( FIG. 5B ). The blades  502  may be locked in the opened position using a locking mechanism  506 . The locking mechanism  506  may comprise a threaded rod  508  and a nut  510 . The nut  510  may be turned on the threaded rod  508  until it engages a handle  504 . However, any means of locking the retractor  500  may be used to keep the retractor in an opening position. All subsequent surgical procedures may be performed inside the enlarged opening created by the blades  502  of the retractor  500  (i.e., between the blades  502 ). It will be appreciated by those skill in the art that any other retractor known in the surgical art may also be used to enlarge an opening in a patient. 
     B. Implantation Tools 
     1. Bone Screw 
     In one embodiment of the present invention as shown in  FIGS. 6A and 6B , the implant to be inserted into a vertebra may be a polyaxial screw  650 . It is contemplated, however, that any screw may be used with the less invasive system so long as the screw incorporates or may be attached to a rod receiving channel sized and configured to receive a spinal rod. The bone screw  650  may comprise a shank portion  654  and a head portion  652  operably connected to the shank portion  654 . The shank portion  654  may be threaded and the threads may be self tapping. The bone screw  650  may be polyaxial such that the head portion  652  articulates and is rotatable with respect to the shank portion  654 . The shank portion  654  may be a separate piece from the head portion  652  and may engage the head portion  652 . The shank portion  654  may also be integral with the head portion  652  so that there is no movement between the two portions. 
     The shank portion  654  may have a proximal end  658  which may be received within the head portion  652 . In one embodiment, the shank portion  654  may be snapped into the head portion  652 . The proximal end  658  of the shank portion  654  may also have a surgical tool engaging recess  674  such as, for example, in the form of hexagon for receiving a corresponding hexagonal portion of a surgical tool. Moreover, the shank portion  654  may have a central axial channel  660 , which may receive a guide wire  150  so that the screw  650  may be guided to the surgical site proximate a vertebra. Other shapes and configurations of the shank portion  654  and head portion  652  may be utilized to obtain polyaxial rotation between the head portion  652  and the shank portion  654 . 
     As shown in  FIGS. 6A and 6B , the head portion  652  may be cylindrical and may comprise a base portion  662  and two spaced apart arms  664 . The arms  664  may have chamfered edges  668  and recesses  670  to engage surgical tools (e.g., various insertion cannulas discussed below). In one embodiment, the arms  664  may have external threads (not shown) for engaging various insertion cannulas and/or other surgical tools. The head portion  652  may also have a central bore  667 . The central bore  667  may pass through the head portion  652  such that the proximal end  658  of the shank portion  654  may extend through a distal opening  663  in the distal end  665  of the head portion  652 . The arms  664  may form a channel  666  that may intersect the central bore  667 . The channel  666  may be any shape (e.g., U-shaped) and may receive a fixation rod. Further, the head portion  652  may have internal threads  672  on the walls of the bore  667  for engaging corresponding external threads  680  on a cap  681  and/or a surgical instrument (e.g., screwdriver, insertion cannula, etc.). The cap  681  may be received within the bore  667 . Other caps interacting with different mechanisms at the proximal end  671  of the head portion  652  may be used to retain the fixation rod within the bone screw. 
     2. Insertion Cannula 
     An insertion cannula may be used for insertion of an implant (e.g., screw, fixation rod). It should, however, be understood by those skilled in the art that it may not be necessary to use a insertion cannula for all procedures. The insertion cannula may be connected to a screw  650  and/or other tool(s) (e.g., a screwdriver) and, as one unit, may be inserted through the working cannula  475  and/or the opening created by the retractor  500 . In another embodiment, the screw  650  and/or other tool(s) may be inserted into the insertion cannula after the insertion cannula has been positioned into a patient. In an embodiment using a guide wire  150 , the insertion cannula, screw  650 , and/or other tool (e.g. screwdriver) may be inserted down over the guide wire  150  to the vertebrae. 
     The type of insertion cannula used may depend on, for example, the preference of the surgeon, the anatomy of the body and/or the requirements of the surgical procedure. In particular, the insertion cannula chosen may be a factor of the method by which the fixation rod may be inserted into a bone screw. In some embodiments, the insertion cannula may be designed for insertion of the fixation rod from the side of the bone screw. In these embodiment, the fixation rod may be inserted through an incision which may be separate from the incision through which the bone screws were inserted into a patient. In other embodiments, the insertion cannula may be designed for insertion of the spinal rod through the top of a bone screw. And, a spinal rod may be inserted through the same incision which the bone screws were inserted through. In other embodiments, the insertion cannula may be configured to enable a rod to be inserted from either the top or side of a bone screw. 
     In an embodiment where no working cannula  475  or retractor  500  is used, the insertion cannula may be connected to inserter  200  (which may be specially designed to fit within the insertion cannula) and may be inserted as a single unit with inserter  200  through the incision in the patient. A guide wire  150  may be used to guide the inserter/cannula construction down to a vertebra. The inserter  200  and/or guide wire  150  (if used) may be withdrawn from the insertion cannula and all steps of a procedure may be performed through the insertion cannula. For example, a drill, bone screw, screwdriver, fixation rod and other surgical tools may be inserted through the insertion cannula. In other embodiments, an insertion cannula may be inserted directly into an opening in a patient without the use of any additional instruments. Thus, the insertion cannula may perform the function of dilating/retracting an opening. 
     It should be noted, however, that any combination of instruments (e.g., trocar  100 , guide wire  150 , inserter  200 , dilator(s)  350 , working cannula  475  and/or retractor  500 ) may be used to assist in inserting an insertion cannula into the body of a patient. 
     In performing a less invasive procedure, more than one insertion cannula, such as those discussed below, may be used, for example, to insert bone screws  650  into adjacent vertebrae and hold and manipulate the bone screws  650 . The insertion cannulas may be inserted through separate incisions in a patient and/or may be inserted through the same incision. For example, in an embodiment where a working cannula  475  may be used, separate incisions may have separate working cannulas  475  and separate insertion cannulas may be inserted through each working cannula  475  down to the vertebrae. It should be appreciated that if a working cannula  475  is large enough, multiple insertion cannulas may be inserted into the working cannula  475 . Moreover, in an embodiment where a retractor  500  may be used, multiple insertion cannulas may also be inserted through the opening created by the retractor  500 . In such embodiments, the entire procedure may be performed through the working cannula(s)  475  or retractor  500 . 
     One advantage of the insertion cannulas may be that the orientation of the head portion  652  may be visible through the proximal ends of each cannula. As such, a surgeon may be able to verify the location of a fixation rod in the channel  666 . To further enhance a surgeon&#39;s ability to see down into a surgical site, any of the insertion cannulas described below may have a light source. In addition, the insertion cannulas may comprise a suction-irrigation system to remove fluid and tissue that may be obstructing a surgeon&#39;s view of a surgical site, thereby improving the surgeon&#39;s view of the surgical site. Moreover, a microscope or endoscope (not shown) may be attached to the insertion cannulas to provide a magnified view of a surgical site. And, where a procedure requires stabilization of the insertion cannulas, the insertion cannulas may be connected to one of any number of attachments such as, for example, the SYNTHES® Spine Synframe Access and Retractor System. These attachments may be attached, for example, to an operating table and may hold the insertion cannulas in place with respect to the patient, thereby eliminating the need for a surgeon or a nurse to hold the cannulas during surgery. 
     Furthermore, the components of any insertion cannula discussed herein may be made, for example, of metal, plastic, rubber, a combination of materials or a composite material. For example, the components may be made from stainless steel, titanium, aluminum, an alloy, carbon fiber composite, or a polymer (e.g., polyvinyl chloride (PVC), polyethylene, polyesters of various sorts, polycarbonate, Teflon coated metal, polyetherether ketone (PEEK), ultra high molecular weight polyethylene (UHMWPE)). And, the components of the insertion cannulas may have a non-glare coating and/or may be radiolucent. In addition, the components of the insertion cannula may be made, for example, by casting, extrusion, injection molding, compression molding, forging, machining, or transfer molding. 
     Various factors can be considered when determining the material used to make the various components of the insertion cannulas, including the ability to withstand sterilization/cleaning (e.g., using an autoclave; cleaning products used for sterilization in hospitals), weight, durability, resistance to staining (e.g., from blood or substances used in surgery) and the ability to grip the components, particularly with latex gloves which are generally used during surgery. 
     Furthermore, while the cannulated shafts of the insertion cannulas discussed below may be illustrated having circular cross-sections, the cross-sections may be any shape, for example, oval, square, rectangular, triangular, or otherwise polygonal. 
     a. Side Insertion Cannula 
     As shown in  FIG. 7A , an insertion cannula  700  may comprise an inner cannulated shaft  710  received within an outer cannulated shaft  720 . Furthermore, as illustrated in  FIGS. 7B and 7C , the inner cannulated shaft  710  may have a proximal end  712 , a distal end  714  and a bore  716  extending from the proximal end  712  to the distal end  714 . The bore  716  may define a central axis  718  and may be dimensioned and configured for receiving a spinal implant (e.g., bone screw) and/or surgical instruments. The bore  716  may also be configured to be positioned over the guide wire  150 , inserter  200  and/or at least one dilator  350 . A channel or slot  722  may be located at the distal end  714  of the inner cannulated shaft  710  and may extend at an angle (e.g., perpendicular) to the central axis  718 . The channel or slot  722  may be U-shaped (although other shapes are contemplated) and may accommodate at least a portion of a fixation rod which may be inserted therethrough as will be described in greater detail below. 
     The inner cannulated shaft  710  may have an inner diameter, for example, between about 3 mm and about 20 mm and, more preferably, between about 12 mm and about 15 mm. However, the inner diameter may be any size so long as a screw  650  and/or tools may be positioned therethrough. The inner cannulated shaft  710  may have an outer diameter, for example, between about 3 mm and about 20 mm and, more preferably, between about 14 mm and about 17 mm. Moreover, the inner cannulated shaft  710  may have a length, for example, between about 40 mm and about 160 mm and, more preferably, between about 110 mm and about 130 mm. The channel or slot  722  may have a width, for example, between about 3 mm and about 10 mm and, more preferably between about 4 mm and about 7 mm. The channel or slot  722  may also have a height, for example, between about 3 mm and about 20 mm and, more preferably, between about 10 mm and about 14 mm. It should be noted that these dimensions may also be applicable to the inner cannulated shaft of  FIG. 8B  discussed below. 
       FIGS. 7D and 7E  illustrate an embodiment of the outer cannulated shaft  720 . The outer cannulated shaft  720  may have a proximal end  724 , a distal end  726  and a bore  728  extending from the proximal end  724  to the distal end  726 . The bore  728  may define a central axis  735  and may be dimensioned and configured to receive the inner cannulated shaft  710 . The outer cannulated shaft  720  may be shorter in length than the inner cannulated shaft  710  so as to permit the full length of the outer cannulated shaft  720  to slide along the exterior of the inner cannulated shaft  710  for reasons that will become apparent below. And, the distal end  726  of the outer cannulated shaft  720  may taper inwardly for close engagement with the distal end  714  the inner cannulated shaft  710 . 
     The outer cannulated shaft  720  may have an inner diameter, which may be larger than the outer diameter of the inner cannulated shaft  710 , and an outer diameter, for example, between about 3 mm and about 20 mm and, more preferably, between about 16 mm and about 19 mm. Moreover, the outer cannulated shaft  720  may have a length, for example, between about 20 mm and about 140 mm and, more preferably, between about 70 mm and about 80 mm. It should be noted that these dimensions may also be applicable to the outer cannulated shaft of  FIG. 8E  discussed below. 
     In order to prevent rotation of the outer cannulated shaft  720  with respect to the inner cannulated shaft  710  and/or keep the two components aligned, the outer cannulated shaft  720  may have at least one projection  740  ( FIG. 7D ) located on an interior wall of the bore  728  for engagement with at least one slot  750  ( FIG. 7B ) in the wall of the inner cannulated shaft  710 . In one embodiment, the projection  740  may be a screw, pin, or bolt which passes through opening  760  ( FIG. 7F ) to engage the spring detent  752  to the outer cannulated shaft  720 . Alternatively, the inner cannulated shaft  710  may have the projection and the outer cannulated shaft  720  may have the slot. The projection  740  may move within the slot  750 . Other methods of preventing rotation and/or keeping the shafts  710 ,  720  aligned are also envisioned. For example, the inner and outer shafts  710 ,  720  may have corresponding flat wall portions. 
     Additionally, the proximal end  724  of the outer cannulated shaft  720  may have at least one handle portion to enable an operator to move the outer cannulated shaft  720  along the inner cannulated shaft  710 . In use, an operator can wrap his/her fingers around the handle portions. As shown in the embodiment of  FIG. 7E , the shaft  720  may have a pair of diametrically opposed handles  738 . The handle  738  may be integrally formed with the proximal end  724  or may be attached thereto, for example, by welding, gluing or by mechanical means (e.g., screws, bolts). Moreover, similar to the inner cannulated shaft  710 , the outer cannulated shaft  720  may have a channel or slot  727 , which may pass through its distal end  726 . The channel or slot  727  may be aligned with the channel or slot  722  of the inner cannulated shaft  710  such that at least a portion of a fixation rod may be inserted therethrough. The channel or slot  727  may have a width, for example, between about 3 mm and about 10 mm and, more preferably between about 5 mm and about 8 mm. The channel or slot  727  may also have a height, for example, between about 3 mm and about 10 mm and, more preferably, between about 5 mm and about 8 mm. It should be noted that the channel or slot  727  dimension may also be applicable to the channel or slot  827  of  FIG. 8E  discussed below. 
       FIGS. 7F and 7G  illustrate the positioning of a bone screw  650  in the insertion cannula  700 . The bone screw  650  may be positioned in the bore  716  at the distal end  714  of the inner cannulated shaft  710 , and the screw  650  and cannula  700  may be inserted into the patient as a single unit. In another procedure where the cannula  700  has already been positioned in a patient, the screw  650  may be subsequently inserted down the bore  716  from the proximal end  712  to the distal end  714 . To fix the bone screw  650  and, in particular, the head portion  652  with respect to the cannula  700 , the outer cannulated shaft  720  may be moved from a first position ( FIG. 7F ), where the distal end  726  of the outer cannulated shaft  720  may be positioned closer to proximal end  712  of the inner cannulated shaft  710 , to a second position ( FIG. 7G ), where the distal end  726  of the outer cannulated shaft  720  may be positioned closer to the distal end  714  of the inner cannulated shaft  710 . Prior to fixing the screw  650  with respect to the insertion cannula  700 , an operator may align the channels or slots  722 ,  727  with the channel  666  of head portion  652  of the bone screw  650 . 
     The outer cannulated shaft  720  may be provided with a spring detent  752  ( FIG. 7F ), which may flex and which may engage a groove  756  in the first position and a groove  758  ( FIG. 7B ) in the second position. In particular, a protrusion (not shown) of spring detent  752  may pass through opening  758  ( FIG. 7E ) of handle  738  to engage grooves  756 ,  758 . Such a construction may provide a tactile and/or audible indication to an operator that the outer cannulated shaft  720  is in the first or second position relative to the inner cannulated shaft  710 . In one embodiment, the engagement of the protrusion of the spring detent  752  to the grooves  756 ,  758  may lock the outer cannulated shaft  720  in the first and/or second position on the inner cannulated shaft  710 . 
     In the second position shown in  FIG. 7G , the distal end  726  of the outer cannulated shaft  720  may push against one or more flexible members  736 . This, in turn, may result in the flexible member  736  engaging the recesses  670  of the head portion  652  of the bone screw  650 . Such an engagement may prevent axial movement of the screw  650  relative to the insertion cannula  700 . The outer cannulated shaft  720  may also push against an arm  737  ( FIGS. 7A and 7H ). The arm  737  may engage the head portion  652  of the screw  650  and, in particular, the arm  737  may fit inside the channel  666  of the head portion  652 . In such a position, the arm  737  may prevent rotational movement of the head portion  652  of the screw  650  with respect to the cannula  700 . Thus, the head portion  652  of the bone screw  650  may be fixed axially and rotationally with respect to the cannula  700 , while still permitting the head portion  652  to pivot around the shank portion  654 . It should be noted that, in some embodiments, the flexible member(s)  736  and/or arm  737  may fix axial and/or rotational movement of the screw  650  with respect to the cannula  700  prior to moving the outer cannulated shaft  720  to the second position. The cannula  700  may be used by an operator to manipulate the head portion  652  of one bone screw  650  so that the channel  666  may be aligned with the channel  666  of another bone screw  650 . Once the channels  666  are aligned, an implant (e.g., fixation rod) may be inserted through the channels  666 . 
     Additionally, in order to help an operator align the head portion  652 , the inner cannulated shaft  710  may be provide with a flat surface  713 , or mark or score (not shown). The flat surface  713  may be positioned outside of the body during a procedure and may provide a visual indicator to an operator of the orientation of the channels or slots  722 ,  727  and/or channel  666 , which may be positioned within the body. The flat surface  713  may also help an operator align the channels or slots  722 ,  727  with channel  666  within the body. 
       FIG. 8A  shows another embodiment of an insertion cannula—cannula  800 . Similar to cannula  700 , the insertion cannula  800  may comprise an inner cannulated shaft  810  received within an outer cannulated shaft  820 . As shown in  FIGS. 8B and 8C , the inner cannulated shaft  810  may have a proximal end  812 , a distal end  814  and a bore  816  extending from the proximal end  812  to the distal end  814 . The bore  816  may define a central axis  818  and may be dimensioned and configured for receiving a spinal implant (e.g., bone screw) or surgical instruments. A channel or slot  822  may be located at the distal end  814  of the inner cannulated shaft  810  and may extend at an angle (e.g., perpendicular) to the central axis  818 . The channel or slot  822  may be U-shaped (although other shapes are contemplated) and may accommodate at least a portion of a fixation rod. 
     The proximal end  812  of the inner cannulated shaft  810  may have one or more holes  815 . The holes  815  may be used to connect a mechanism (not shown) between adjacent cannulas  800  such that the cannulas may be in a fixed position relative to one another or may pivot relative to each other. In one embodiment, the holes  815  may be used to attach a guidance mechanism for guiding a fixation rod into the body. When the attached cannulas are manipulated, the cannulas  800  may impart a compressive or distraction force on the vertebrae. The proximal end  812  may be configured to cradle or secure a light source, which may be used to illuminate a surgical site through the bore  816 . The proximal end  812  may also be configured to engage other devices. 
       FIGS. 8D and 8E  illustrate an embodiment of the outer cannulated shaft  820 . The outer cannulated shaft  820  may have a proximal end  822 , a distal end  824  and a bore  826  extending from the proximal end  822  to the distal end  824 . The bore  826  may define a central axis  828  and may be dimensioned and configured to receive the inner cannulated shaft  810 . The outer cannulated shaft  820  may be shorter in length than the inner cannulated shaft  810  so as to permit the full length of the outer cannulated shaft  820  to slide along the exterior of the inner cannulated shaft  810  for reasons that will be detailed below. The distal end  824  of the outer cannulated shaft  820  may taper inwardly for close engagement with the distal end  814  of the inner cannulated shaft  810 . 
     The outer cannulated shaft  820  may also be provided with at least one engaging portion  830 . The engaging portion  830  may have at least one protrusion  836  (i.e., within the bore  816 ) for engaging a slot  838  in the wall of the inner cannulated shaft  810 . In one embodiment having two engaging portions  830 , such as  FIG. 8E , one portion  830  may be flexible and the other portion  830  may be fixed with respect to the outer cannulated shaft  820 . The engaging portion  830  may be flexible so that protrusion  836  may releasably snap into and out of the notches  840 ,  842 ,  844 . Such a construction may prevent rotation of the outer cannulated shaft  820  with respect to the inner cannulated shaft  810  and/or keep the inner and outer shafts  810 ,  820  in a particular orientation (e.g., keep channels or slots  822  and  827  in alignment). As with cannula  700 , others means of aligning the shafts  810 ,  820  are also envisioned. And, since the slot  838  may have one or more notches  840 ,  842 ,  844  for receiving the protrusion  836 , such a construction may also allow the outer cannulated shaft  820  to be fixed with respect to the inner cannulated shaft  810  at set intervals. 
     The bone screw  650  may be positioned in the bore  816  at the distal end  814  of the inner cannulated shaft  810 , and the screw  650  and cannula  800  may be inserted into the patient as a single unit. In another procedure where the cannula  800  has already been positioned in a patient, the screw  650  may be subsequently inserted down the bore  816  from the proximal end  812  to the distal end  814 . 
     The bone screw  650  may be attached to the cannula  800  similar to the way it is attached to cannula  700  above. The outer cannulated shaft  820  may be moved from a first position, where the protrusion  836  of the engaging portion  830  engages the notch  840 , to a second position, where the protrusion  836  of the engaging portion  830  engages the notch  842  or  844 . Prior to moving the outer cannulated shaft  820  to the second position, an operator may align the channels or slots  822 ,  827  with the channel  666  of head portion  652  of the bone screw  650 . 
     In the second position (not shown), the distal end  824  of the outer cannulated shaft  820  may push against at least one flexible member  832 . However, it should be noted that multiple flexible members may be used. This, in turn, may result in the flexible member  832  and, consequently, the protrusion  834  engaging the recesses  670  of the head portion  652  of the bone screw  650 . The flexible member  832  may prevent axial movement of the screw  650  with respect to the insertion cannula  800 . Additionally, the outer cannulated shaft  820  may push against an arm  837  ( FIG. 8A ). The arm  837  may engage the head portion  652  of the screw  650  and, in particular, the arm  837  may fit inside the channel  666  of the head portion  652 . In such a position, the arm  837  may prevent rotational movement of the head portion  652  of the screw  650  with respect to the cannula  800 . Thus, the head portion  652  of the bone screw  650  may be fixed axially and rotationally with respect to the cannula  800 . At the same time, the head portion  652  may be able to pivot about the proximal end  658  of the shank portion  654 . It should be noted that, in some embodiments, the flexible member(s)  832  and/or arm  837  may fix axial and/or rotational movement of the screw  650  with respect to the cannula  800  prior to moving the outer cannulated shaft  820  to the second position. 
     The insertion cannula  800  may be used by an operator to manipulate the head portion  652  of one bone screw  650  so that the channel  666  may be aligned with a channel  666  of another bone screw  650  and that an implant (e.g., fixation rod) may be inserted through the channels  666 . The inner cannulated shaft  810  may have a channel  813 , which may be oriented in the same direction as the channels or slots  822  and/or  827 . Moreover, the inner cannulated shaft  810  may be provided with flattened surface  812   a , which may be located on the side of the inner cannulated shaft  810  that the channel or slot  822  passes through. The channel  813  and/or flattened surface  812   a  may be positioned outside the body during a procedure and may provide the operator of a visual indicator of the orientation of the channels or slots  822 ,  827  and/or channel  666  inside the body. Such a construction may also help an operator align the channels or slots  822 ,  827  with channel  666  in the body. 
       FIG. 9A  illustrates yet another embodiment of an insertion cannula—cannula  900 . The system  900  may comprise a cannulated shaft  910  and an elongated piece  920 . Shown in  FIGS. 9B and 9C , the cannulated shaft  910  may have a proximal end  912 , a distal end  914 , and a bore  916  with an axis  918  extending from the proximal end  912  to the distal end  914 . A channel or slot  928  may pass through the distal end  914  of the cannulated shaft  910  such that at least a portion of a fixation rod may pass therethrough. The cannulated shaft  910  may also have a recess  922 , which may extend between the proximal end  912  and the distal end  914  and which may hold an elongated piece  920  ( FIG. 9D ) therein. The shape of the recess  922  as shown in  FIG. 9C  may be designed to receive a corresponding shape of the elongated piece  920  such that the elongated piece  920  may move up and down in the recess  922 , but may not be pulled out of the recess in a direction transverse to the axis  918 . Moreover, the recess  922  may have a slot  924  for engaging a protrusion (not shown) of the elongated piece  920 . This construction may prevent the elongated piece  920  from separating or twisting relative to the cannulated shaft  910  and may guide the movement of the elongated piece  920  within the recess  922 . 
     The cannulated shaft  910  may have an inner diameter, for example, between about 3 mm and about 20 mm and, more preferably, between about 12 mm and about 15 mm. However, the inner diameter may be any size so long as a screw  650  and/or tools may be positioned therethrough. The shaft  910  may have an outer diameter, for example, between about 3 mm and about 20 mm and, more preferably, between about 15 mm and about 18 mm. Moreover, the cannulated shaft  910  may have a length, for example, between about 40 mm and about 160 mm and, more preferably, between about 110 mm and about 130 mm. The channel or slot  928  may have a width, for example, between about 3 mm and about 10 mm and, more preferably between about 5 mm and about 8 mm. The channel or slot  928  may also have a height, for example, between about 3 mm and about 20 mm and, more preferably, between about 12 mm and about 16 mm. 
     The elongated piece  920 , as shown in  FIG. 9D , may comprise a proximal end  930  and a distal end  932 . The elongated piece  920  may have a length, for example, between about 20 mm and about 140 mm and, more preferably, between about 110 mm and about 130 mm. Moreover, the elongated piece  920  may have a enlarged portion  934 , which may provide an operator with a portion to actuate the elongated piece  920 . The enlarged portion  934  may have a surface treatment, such as a knurling, for enhancing an operator&#39;s grasp on the piece  920 . 
     The bone screw  650  may be positioned in the bore  916  at the distal end  914  of the cannulated shaft  910 , and the screw  650  and cannula  900  may be inserted into the patient as a single unit. In another procedure where the cannula  900  has already been positioned in a patient, the screw  650  may be subsequently inserted down the bore  916  from the proximal end  912  to the distal end  914 . An operator may orient the channel or slot  928  with the channel  666  of head portion  652  of the bone screw  650 . To fix the bone screw  650  with respect to the cannula  900 , the distal end  932  of the elongated piece  920  may engage a flexible member  927 . In such a position, the elongated piece  920  may push the flexible member  927  towards the head portion  652  of the bone screw  650  so that the flexible member  927  engages the recesses  670  of the head portion  652 . Thus, the head portion  652  of the bone screw  650  may be fixed with respect to the cannula  900 . It should be noted that in some embodiments, the flexible member  927  may fix axial movement of the screw  650  with respect to the cannula  900  prior to the elongated piece  920  engaging the member  927 . The head portion  652  may pivot with respect to the proximal end  658  of the shank portion  654 . This may enable an operator to manipulate the head portion  652  in preparation for insertion of a fixation rod. 
       FIG. 10A  illustrates a cannula  1000  which, similar to the cannulas described above, may be used to hold a screw  650 . As shown in  FIGS. 10A-10D , the cannula  1000  may comprise an inner cannulated shaft  1010  and an outer cannulated shaft  1020 . The inner cannulated shaft  1010  may have a proximal end  1012 , a distal end  1014  and a bore  1016  extending from the proximal end  1012  to the distal end  1014 . The distal end  1014  of inner cannulated shaft  1010  may have an external threaded portion  1015  for engaging the internal threads  672  of the head portion  652  of the screw  650 . The proximal end  1012  may have a surface treatment, for example knurling, to facilitate gripping of the cannula  1000 . Additionally, the proximal end  1022  may have a flattened portion, score or marking (not shown), which may indicate the orientation of channel  666  of the bone screw  650 . 
     The inner cannulated shaft  1010  may have an inner diameter, for example, between about 3 mm and about 20 mm and, more preferably, between about 7 mm and about 11 mm. The inner shaft  1010  may have an outer diameter, for example, between about 3 mm and about 20 mm and, more preferably, between about 10 mm and about 12 mm. Moreover, the inner cannulated shaft  1010  may have a length, for example, between about 40 mm and about 160 mm and, more preferably, between about 110 mm and about 130 mm. 
     As shown in  FIG. 10D , an insert  1032  may be positioned within the inner cannulated shaft  1010 . The insert  1032  may comprise fingers  1034  for engaging a surgical instrument (e.g., screwdriver) inserted within the bore  1016  of the inner cannulated shaft  1010 . As such, a surgical instrument may be snap-fit into the proximal end  1012  of the inner cannulated shaft  1010  while, at the same time, being able to rotated therein. The insert  1032  may also have protrusion  1036  for engaging a portion of a surgical instrument. 
     The outer cannulated shaft  1020  may be positioned over the inner cannulated shaft  1010  and may be moveable thereon. The outer cannulated shaft  1020  may have a proximal end  1022 , a distal end  1024  and a channel  1025  therethrough extending from the proximal end  1022  to the distal end  1024 . The outer cannulated shaft  1020  may have one or more protrusions  1038  for engaging one or more recesses  1040  of the inner cannulated shaft  1010 . In one embodiment, the protrusion  1038  may be an annular protrusion extending around the entire interior surface of the outer cannulated shaft  1020 . Moreover, the recess  1040  may be an annular recess in the outer periphery of the inner cannulated shaft. Such a construction may allow the outer cannulated shaft  1020  to move axially with respect to the inner cannulated shaft  1010 . 
     In addition, the outer diameter of the outer cannulated shaft  1020  may be equal to the outer diameter of the head portion  652  of the screw  650 . In a Construction where the diameter of the outer cannulated shaft  1020  may be the same as the diameter of the head portion  652 , the distal portion  1024  of the outer cannulated shaft  1020  may rest against the chamfered edges  668  of the head portion  652  of the screw  650 . In another embodiment, the outer cannulated shaft  1020  may have a diameter, which may be larger than the diameter of the head portion  652  of the screw  650 . 
     The outer cannulated shaft  1020  may have an inner diameter, which may be larger than the outer diameter of the inner cannulated shaft  1010 , and an outer diameter, for example, between about 3 mm and about 20 mm and, more preferably, between about 12 mm and about 15 mm. Moreover, the outer cannulated shaft  1020  may have a length, for example, between about 20 mm and about 140 mm and, more preferably, between about 100 mm and about 110 mm. 
     To fix the bone screw  650  with respect to the cannula  1000 , the external thread portion  1015  of the inner cannulated shaft  1010  may engage the inner threaded portion  672  of the head portion  652  of the screw  650  ( FIG. 6A ). It should be noted, however, that any means of engaging the inner cannulated shaft  1010  to the bone screw  650  is also envisioned. The outer cannulated shaft  1020  may be in its rear position with the proximal end  1022  of the outer cannulated shaft  1020  engaging the proximal end  1012  of the inner cannulated shaft  1010 , such as shown in  FIG. 10D . Once the inner cannulated shaft  1010  has engaged the head portion  652 , the outer cannulated  1020  shaft may be moved down the inner cannulated shaft  1010  such that the protrusion  1038  moves within the recess  1040 . And, the distal end  1024  of the outer cannulated shaft  1020  engages the head portion  652  of the screw  650 . In particular, as shown in  FIG. 10A , the outer cannulated shaft  1020  may have extended portions  1026 , which may be inserted into the space between the parallel arms  664  of the head portion  652  (i.e., into the top of the channel  666 ). The bone screw  650  may then be fixed axially and rotationally with respect to the cannula  1000 . 
     b. Side and/or Top Insertion Cannula 
       FIGS. 11A and 11B  illustrates an insertion cannula  1100 , which may be used for inserting a rod from the top or side of a screw  650  (i.e., the screw  650  may be side or top loaded). The cannula  1100  may comprise an elongated member  1110  having a proximal end  1112 , a distal end  1114  and a channel  1116  extending from the proximal end  1112  to the distal end  1114 . Moreover, the cannula  1100  may have a first longitudinal slot  1122 , which may intersect the channel  1116  and which may extend from the proximal end  1112  to the distal end  1114 . The slot  1122 , however, may cover any length of the elongated member  1110  and may end a distance from the proximal end  1112 . In addition, the cannula  1100  may have a second longitudinal slot  1123 , which may also intersect the channel  1116 . The second slot  1123  may be shorter, longer or the same length as the first slot  1122 . It should be appreciated that the second slot  1123  may be unnecessary. The slot(s)  1122 ,  1123  may enable a rod to be inserted down through the channel  1116  and into a screw  650 . 
     The distal end  1114  of the insertion cannula  1100  may engage the head portion  652  of the screw  650  ( FIG. 11B ). For example, the distal end  1114  may have protrusions (not shown) on inner walls  1125  of the elongated member  1110  for engaging the recesses  670  of the head portion  652 . Alternatively, the distal end  1114  may loosely engage the head portion  652 . In general, the cannula  1100  may be used to manipulate the head portion  652  of the screw  650  so that a fixation device may be inserted and attached to the head portion  652 . Furthermore, the proximal end  1112  may have a handle  1124  for handling and manipulating cannula  1100 . And, the handle  1124  may have a attachment portion  1129  for attached the cannula  1100  to, for example, an operating table. 
       FIG. 12A  illustrates another embodiment of an insertion cannula, which may be used to insert a spinal rod from the top or side of a bone screw  650 . The insertion cannula  1200  may comprise a proximal end  1202 , a distal end  1204 , and a channel  1206  extending through the cannula  1200  from the proximal end  1202  to the distal end  1204 . Moreover, the cannula  1200  may comprise at least one slot  1208  intersecting the channel  1206 . The slot(s)  1208  may enable a rod to be inserted down through the channel  1206  and into the screw  650 . As shown in  FIG. 12A , in an embodiment with more than one slot  1208 , the slots  1208  may define two arms  1210  and  1212 . Such a construction may results in the arm  1210  and  1212  being flexible such that a screw  650  and, specifically, the head portion  652  may be clipped/snapped between the arms  1210  and  1212  ( FIG. 12B ). Alternatively, the slots  1208  may enable a rod to be inserted from the side of a screw  650 . The slots  1208  may extend from the distal end  1204  of the cannula  1200  to a position a distance away from the proximal end  1202 . In an embodiment with more than one slot  1208 , the slots  1208  may be the same length or different lengths. 
     Moreover, the proximal end  1202  may have a surface treatment, such as a knurling, or may have a grip to enhance an operator&#39;s grasp of the cannula  1200 . In addition, the arms  1210  and/or  1212  may have protrusions  1214  for engaging the recess  670  of the head portion  652  of the screw  650 . When the screw  650  is engaged by the cannula  1200  as shown in  FIG. 12B , the screw  650  may be fixed such that the head portion  652  may move rotationally (i.e., may rotate within the cannula  1200 ), but not axially with respect to cannula  1200 . In one embodiment, the screw  650  may be fixed with respect to insertion cannula  1200  so that rotational movement may also be prevented. The cannula  1200  may be used to pivot the head portion  652  relative to the shank portion  654 . 
     The insertion cannula  1100 ,  1200  may have an inner diameter, for example, between about 3 mm and about 20 mm and, more preferably, between about 12 mm and about 16 mm. It should be noted that the inner diameter of cannulas  1100 ,  1200  may be any size so long as a screw  650  and/or tools may be positioned therethrough. The insertion cannula  1100 ,  1200  may have an outer diameter, for example, between about 3 mm and about 20 mm and, more preferably, between about 14 mm and about 17 mm. Moreover, the cannulas  1100 ,  1200  may have a length, for example, between about 40 mm and about 200 mm and, more preferably, between about 140 mm and about 160 mm. The slots  1208  may have a width, for example, between about 3 mm and about 10 mm and, more preferably between about 5 mm and about 8 mm. The slots  1208  may also have a height, for example, between about 30 mm and about 160 mm and, more preferably, between about 110 mm and about 130 mm. 
     3. Drill 
     A drill  1350  ( FIG. 13A ) may be used to create a cavity in the vertebrae into which a screw  650  may be inserted. The drill  1350  may be positioned into working cannula  475 , retractor  500 , and/or an insertion cannula and moved down towards a surgical site. For example, in a technique using a guide wire  150 , such as shown in  FIG. 13B , the drill  1350  may be cannulated and may be guided by the guide wire  150  down an insertion cannula. The drill  1350  may also be guided by a guidewire  150  down a working cannula  475  or retractor  500 . 
     As illustrated in  FIG. 13A , the drill  1350  may comprise a drill bit  1352  and a handle  1362 . The drill bit  1352  and handle  1362  may be one integral piece or two pieces connected together. For example, the drill bit  1352  may be connected to the handle  1362  by a coupling  1364 . The handle  1362  may be used to manually drill a hole in a vertebra. The handle  1362  may be T-shaped, spherical, elliptical or any other shape. It should, however, be noted that the drill  1350  may not have a handle  1362  but, instead, the drill bit  1352  may be attached to a power drill. Thus, the drill bit  1352  may be operated electrically or pneumatically. 
     As shown in  FIGS. 13C and 13D , the drill bit  1352  may have a shaft  1361 , a connecting portion  1354  on the proximal end  1355  of the shaft  1361  and a tip  1356  at the distal end  1357  of the shaft  1361 . In addition, the drill bit  1352  may have a central bore  1358  for those procedures where an operator may desire to use a guide wire  150 . The shaft  1361  may have a uniform diameter or may have a portion having a larger diameter. Further, the connecting portion  1354  may be attached to a handle such as described above or may be connected to a power drill. The tip  1356  of the drill bit  1352  may be configured similar to other bone cutting drill bits known by those of skill in the art. 
     Moreover, in one embodiment, a window  1364  may be located between the proximate end  1354  and the distal end  1356  and may permit an operator to see, for example, the guide wire  150  or bone tissue moving through the central bore  1358  of the drill bit  1352 . In addition, the shaft  1361  may comprise markings  1362  spaced apart at set intervals. Such markings  1362  may enable an operator to monitor the depth of the drill bit  1352  into bone tissue. 
     In use, after the drill  1350  is inserted into a working cannula  475 , retractor  500  and/or insertion cannula and positioned proximate a vertebra, the drill  1350  may be rotated manually by the handle  1362  or electrically/pneumatically by a power drill to form a cavity in which an implant, such as the screw  650 , may be anchored. 
     It should, however, be understood by those skilled in the art that any means of creating a cavity in bone is envisioned. For example, an operator may use an awl, probe and/or tap instead of or in addition to the drill  1350  to create a cavity in the vertebrae. Moreover, in some embodiments, a screw  650  may be inserted directly into bone without first using a drill or other cavity creating tool to create a cavity. 
     4. Implant Positioner 
       FIG. 14  illustrates a positioner  1450 , which may be used to orient a bone screw  650  within a working cannula  475 , a retractor  500  and/or an insertion cannula. The positioner  1450  may comprise a shaft  1452  and a forward portion  1454  operatively connected to the shaft  1452 . The forward portion  1454  may be configured to engage the head portion  652  of the screw  650  and/or the proximal end  658  of the shank portion  654 . In particular, the front portion  1454  may have a projection  1456  for engaging the recess  674  and a portion  1458  for engaging the channel  666  of the head portion  652 . The positioner  1450  may also have a handle  1460 , which may be attached on the shaft  1452  to match the orientation of the portion  1458  in channel  666 . Therefore, when the portion  1458  engages the channel  666  of the screw  650 , the handle  1460  may be aligned with the direction of the channel  666 . Such a construction may provide an operator with a visual indicator outside of the body of the orientation of the channel  666  within the body. The handle  1460  may be aligned with visual indicators (e.g., flat surface  713 , flattened surface  812   a ) on the insertion cannulas to align the channel(s) of the insertion cannulas with the channel  666 . 
     In use, the positioner  1450  may engage the bone screw  650  after the screw  650  has been positioned within a working cannula  475 , retractor  500  and/or insertion cannula. Alternatively, the positioner  1450  may be used to insert a screw  650  down a working cannula  475 , retractor  500  and/or insertion cannula. An operator may manipulate the positioner  1450  to rotate the head portion  652  of the screw  650 . For example, the positioner  1450  may be used to orient the channel  666  with the corresponding channels of an insertion cannula prior to fixing the orientation of the head portion  652  with respect to an insertion cannula and/or inserting a fixation rod. Alternatively, the positioner  1450  may be used to manipulate the head portion  652  of the screw  650  after the head portion  652  has already been fixed with respect to the insertion cannula. In this way, the positioner  1450  may be used to align channels  666  of multiple screws  650  so that a fixation rod may be inserted therethrough. 
     5. Screwdriver 
     A screwdriver  1500  may be positioned within the working cannula  475 , retractor  500 , and/or an insertion cannula  700 ,  800 ,  900 ,  1100 ,  1200  and may engage a screw  650 . As shown in  FIGS. 15A through 15D , the screwdriver  1500  may comprise a shaft  1510 , a locking sleeve  1520  and a holding sleeve  1530 . The shaft  1510  may have an engagement portion  1516  at a distal end  1514  and coupling portion  1513  at a proximal end  1512 . Moreover, the shaft  1510  may be cannulated (i.e., having a channel (not shown) therethrough for accepting, for example, a guide wire  150 ). As shown in  FIG. 15B , the engagement portion  1516  may be in the shape of a hexagon to engage the recess  674  of the screw  650 . And, the distal end  1514  may also have at least one shoulder  1518  for engaging the U-shaped channel of the head portion  652  of the screw  650 . 
     Moreover, the coupling portion  1513  may be coupled to a handle portion, such as handle  1515  ( FIG. 15E ) for manual operation of the screwdriver  1500 . The handle  1515  may be T-shaped or any other shape. In an embodiment where the screwdriver  1500  may be cannulated, the handle  1515  may also have a channel (not shown) passing therethrough that may align with the channel through the shaft  1510  so that a guide wire  150  may be inserted therein. Alternatively, the coupling portion  1513  may be connected to a device having a motor for rotating the screwdriver  1500 , for example, a power drill. The handle  1515  may be positioned on the shaft  1510  to match the orientation of the shoulders  1518 . Therefore, when the shoulders  1518  engage the channel  666  of the screw  650 , the handle  1515  may be aligned with the direction of the channel  666 . Such a construction may provide an operator with a visual indicator outside of the body of the orientation of the channel  666  within the body. The handle  1515  may be aligned with visual indicators (e.g., flat surface  713 , flattened surface  812   a ) on the insertion cannulas to align the channel(s) of the insertion cannulas with the channel  666 . 
       FIG. 15C  shows the locking sleeve  1520 . The shaft  1510  may be positioned within a channel  1528  of the locking sleeve  1520  such that the locking sleeve  1520  may slide along the shaft  1510 . The locking sleeve  1520  may comprise a gripping end  1522  and a screw engaging end  1524 . The gripping end  1522  may have a surface  1523 , which may have a treatment (e.g., knurling) or a grip for allowing an operator to firmly grasp the locking sleeve  1520 . In another embodiment, the gripping end  1522  may have at least one indentation or groove (not shown) for grasping. The screw engaging end  1524  may have an external threaded portion  1526  for engaging the internal threaded portion  672  of the head portion  652 . 
     As shown in  FIGS. 15A and 15D , the holding sleeve  1530  may be sized to fit over the locking sleeve  1520 . The holding sleeve may extend out of an insertion cannula  700 ,  800 ,  900 ,  1100 ,  1200  so that an operator may hold the screwdriver  1500  while, at the same time, rotate the screwdriver  1500  to insert a screw  650  into a vertebra. The holding sleeve  1530  may be connected to the locking sleeve  1520  such that the holding sleeve  1530  may rotate, but not move axially, with respect to the locking sleeve  1520 . In one embodiment, the holding sleeve  1530  may have one or more longitudinal slots  1534  and a protrusion  1536 , which may extend around the inner periphery of the holding sleeve  1530 . The slots  1534  may allow a proximal end  1532  of the holding sleeve  1530  to flex, thereby enabling the protrusion  1536  to be snapped into and/or out of engagement with the groove  1525  ( FIG. 15C ) of the locking sleeve  1520 . The groove  1525  may extend around the periphery of the locking sleeve  1520 . In another embodiment, the holding sleeve  1530  may have one or more protrusions (not shown), which may engage one or more grooves (not shown) of the locking sleeve  1520 . 
     Additionally, in one embodiment having two shoulders  1518 , one shoulder  1518  may have a protrusion  1518   a  ( FIG. 15B ) extending therefrom. It should be noted that, in some embodiments, there may be no protrusion  1518   a . The protrusion  1518   a  may be received in and move along recess  730  of cannula  700  ( FIG. 7C ), recess  860  of cannula  800  ( FIG. 8C ) and/or recess  938  of cannula  900  ( FIG. 9C ). When the screwdriver  1500  is inserted in an insertion cannula, the screwdriver  1500  may move down the insertion cannula, guided by the protrusion  1518   a  in the recess  730 ,  860 ,  938 . Thereafter, the screwdriver  1500  may be rotated to drive the screw  650  into a vertebra. The insertion cannula may rotate with the rotation of the screwdriver  1500 . In one embodiment where the protrusion  1518  may be disengaged from the recess  730 ,  860 ,  938  during insertion of the screw  650 , the screwdriver  1500  may not be withdrawn from the screw  650  until the protrusion is re-aligned with the recess  730 ,  860 ,  938 . And, because of the position of the screwdriver  1500  within the screw  650 , upon alignment of the protrusion  1518   a  with the recess  730 ,  860 ,  938 , the channel  666  of the screw  650  may be aligned with the channels or slots  722  and/or  727 ,  822  and/or  827 , or  928 . The screwdriver  1500  may then be withdrawn from the screw  650  and the insertion cannula, and the screw  650  may be subsequently fixed with respect to the insertion cannula as described above. 
     In use, the screwdriver  1500  may be inserted into the screw  650 . As shown in  FIG. 15F , the engagement portion  1516  may engage the recess  674  and the shoulder  1518  may engage the U-shaped channel  666 . The locking sleeve  1520  may be moved from a first position where the screw engaging end  1524  of the locking sleeve  1520  may be positioned away from shoulder  1518  ( FIG. 15E ) to a second position where the screw engaging end  1524  of the locking sleeve  1520  may be positioned proximate the shoulder  1518  ( FIG. 15F ). The external threaded portion  1526  of the locking sleeve  1520  may engage the internal threaded portion  672  of the head portion  652  of the screw  650 . Thus, the screwdriver  1500  may be axially and rotationally fixed with respect to the screw  650 . The head portion  652  and the shank portion  654  may be rigidly fixed together, thereby allowing for implantation of the bone screw  650 . The screwdriver  1500 , screw  650  and insertion cannula  700 ,  800 ,  900 ,  1100 ,  1200  may be connected together and inserted into the patient at the same time. For example, as shown in  FIG. 15F , the screwdriver  1500 , screw  650 , and cannula  1200  may be inserted into a patient as a single unit. Alternatively, the screwdriver  1500  may be inserted along with the screw  650  into the working cannula  475 , retractor  500  and/or insertion cannula  700 ,  800 ,  900 ,  1100 ,  1200  after the working cannula  475 , retractor  500  and/or insertion cannula  700 ,  800 ,  900 ,  1100 ,  1200  have already been inserted into a patient. In another embodiment, the screw  650  and working cannula  475 , retractor  500 , and/or insertion cannula  700 ,  800 ,  900 ,  1100 ,  1200  may be positioned in the body and the screwdriver  1500  may be subsequently inserted therein. 
     Once the screw  650  is positioned near a vertebra, the screwdriver  1500  may be rotated until the screw  650  is inserted a desirable distance into a vertebra. Thereafter, the screwdriver  1500  may be removed from the working cannula  475 , retractor  500  and/or insertion cannula  700 ,  800 ,  900 ,  1100 ,  1200 . 
     In a procedure using the cannula  1000 , a screwdriver, such as screwdriver  1550  ( FIG. 15G ), may be used to insert a screw  650 . The screwdriver  1550  may have a shaft  1551  with a distal end  1552  and a proximal end  1553 , a hex portion  1554  at the distal end  1552 , and a handle  1555  at the proximal end  1553 . The hex portion  1554  may engage recess  674  of screw  650 . Moreover, the handle  1555  may be integral with the shaft  1551  or may be a separate piece attachable to the shaft  1551 . 
     To engage the screwdriver  1550  with the cannula  1000 , cannula  1000  may comprise an external threaded portion (not shown) on proximal end  1012 , which may engage an internal thread portion (not shown) within an enlarged portion  1556  of screwdriver  1550 . In such a construction the enlarged portion  1556  may be mounted so that the enlarged portion  1556  may move axially and/or rotationally about the shaft  1551  and the handle  1555 . Thus, upon fixing the enlarged portion  1556  to the cannula  1000 , the shaft  1551  and handle  1555  may be rotated and/or moved axially with respect to the cannula  1000 . In other embodiments, the enlarged portion  1556  may be fixed with respect to the shaft  1551  and/or handle  1555 . In another embodiment, the screwdriver  1550  may be held within bore  1016  by at least one finger  1034 , which may flex to snugly engage the screwdriver  1550 . Furthermore, each finger  1034  may be provided with at least one protrusions  1036 , which may engage the screwdriver  1550 . It should, however, be understood that screwdriver  1550  may be used with any other insertion cannula, such as cannula  700 ,  800 ,  900 ,  1100 ,  1200 . 
     Similar to screwdriver  1500 , screwdriver  1550  may be inserted into the body as a single unit with an insertion cannula and screw  650 . Alternatively, the screwdriver  1550  may inserted into an insertion cannula and/or screw  650  after the insertion cannula and/or screw  650  have been inserted into the body. 
     6. Rod Inserter 
     Once the screw  650  has been inserted, the positioner  1450  and/or insertion cannulas may be used to move the head portion  652  of the screw. In a procedure involving the use of multiple screws  650  and a fixation rod, the ability to move the head portion  652  may be desirable. In particular, using the positioner  1450  and/or an insertion cannula to move the head portions  652  may enable an operator to align adjacent head portions  652  so that a fixation rod may easily be inserted through the channels  666  of all the head portions  652 . 
     The type of inserter used may depend on the type of insertion cannula, which has been used by an operator in performing a surgical procedure. For instance, in procedures using an insertion cannula  700 ,  800 ,  900 ,  1000 ,  1100  or  1200  an inserter such as inserter  1600  ( FIG. 16A ) or inserter  1700  ( FIG. 17 ) may be used to insert a fixation rod from the side of a screw  650 . Moreover, in procedures using devices such as insertion cannula  1100  and/or cannula  1200 , a rod inserter similar to inserter  1800  ( FIG. 18 ) may be used to insert a fixation rod through the top of a screw  650 . In a procedure using the positioner  1450 , the positioner  1450  may be removed prior to insertion of a fixation rod. 
     a. Side Inserter 
       FIGS. 16A and 16B  show a rod inserter  1600 , which may be used to engage and insert a fixation rod  1675  into a patient and through the side of a screw  650 . It should, however, be appreciated that inserter  1600  may be used to insert a rod from the top of a screw  650 . The rod inserter  1600  may comprise an elongated member  1602 , a moveable member  1650 , and an actuating member  1620  operably attached to the moveable member  1650 . While the moveable member  1650  may be positioned inside the elongated member  1602 , the moveable member  1650  may also be positioned on the outside of the elongated member  1602 . Moreover, the inserter  1600  may have a handle portion. It should be understood that any portion of the inserter  1600  that may be grasped by an operator may be considered to be a handle portion. A handle portion  1610  may be connected to the proximal end  1604  of the elongated member  1602 . Alternatively, the handle portion  1610  may surround the elongated member  1602 , which may extend through the handle  1610  and which may connect directly to the actuating member  1620 . The handle portion  1610  may have a grip or a surface treatment, such as knurling, to allow an operator to grasp the inserter  1600 . In one embodiment, the handle  1610  may have a diameter, which may be larger than the diameter of the elongated member  1602 . 
     The components of the inserter  1600  may be made, for example, of metal, plastic, rubber, a composite material, or a combination of materials. For example, the components may be made from stainless steel, titanium, aluminum, an alloy, carbon fiber composite, or a polymer (e.g., polyvinyl chloride (PVC), polyethylene, polyesters of various sorts, polycarbonate, Teflon coated metal, polyetherether ketone (PEEK), ultra high molecular weight polyethylene (UHMWPE)). In addition, various methods may be used to make the components of the inserter  1600 , including casting, extrusion, injection molding, compression molding, forging, machining, or transfer molding. 
     Various factors may be considered when determining the material used to make the various components of the inserter  1600 , including ability to withstand sterilization/cleaning (e.g., using an autoclave; cleaning products used for sterilization in hospitals), weight, durability, resistance to staining (e.g., from blood or substances used in surgery) and the ability to grip the components, particularly with latex gloves which are generally used during surgery. The handle  1610  may be made of the same or different material as the other components. 
     As shown in  FIGS. 16A and 16B , the elongated member  1602  may comprise a proximal end  1604 , a distal end  1606 , and a passageway  1608  extending from the proximal end  1604  to the distal end  1606 . The proximal end  1604  of the elongated member  1602  may be positioned adjacent a handle portion  1610 . Alternatively, the proximal end  1604  of the elongated member  1602  may be located proximate an actuating member  1620 . The elongated member  1602  may be have a curved shape, such as shown in  FIG. 16A , or may be straight. The radius of curvature of the elongated member  1602  may be, for example, between about 80 mm and about 120 mm and, more preferably, between about 90 mm and about 110 mm and, most preferably, between about 95 mm and about 105 mm. The fixation rod  1675  may also be curved and may have the same or different radius of curvature. In other embodiments, the fixation rod  1675  may be straight. 
     The handle portion  1610  may have a proximal end  1612 , a distal end  1614 , and a passageway  1616  therethrough. In an embodiment where the handle portion  1610  and the elongated member  1602  may be separate pieces, the handle portion  1610  and the elongated member  1602  may be joined together by, for example, a screw, nut, bolt, threads, adhesive or welding. In an embodiment where the elongated member  1602  may engage the actuating member  1620 , the elongated member  1602  may extend through the passageway  1616  of the handle portion  1610  and may engage a channel  1618  of the actuating member  1620 . 
     The actuating member  1620  may be positioned proximate the proximal end  1612  of the handle portion  1610 . The actuating member  1620  may be rotatable with respect to the handle portion  1610  and/or the elongated member  1602 . Rotation of the actuating member  1620  may cause the moveable member  1650  to move within the passageway  1608  of the elongated member  1602 . 
     The moveable member  1650  may be positioned within the passageway  1608  of the elongated member  1602 , may extend through the handle portion  1610  and may be operably connected to the actuating member  1620 . The moveable member  1650  may be flexible or rigid. In one embodiment, the moveable member  1650  may be a cable. As shown in  FIG. 16C , the moveable member  1650  may have a proximal end  1652  and a distal end  1654 . The proximal end  1652  of the moveable member  1650  may be received in the channel  1618  of the actuating member  1620 . The moveable member  1650  may be connected to the actuating member  1620  such that the moveable member  1650  moves within the channel  1608  of the elongated member  1602  when the actuating member  1620  is rotated. The distal end of the moveable member  1650  may be, for example, hook-like. In one embodiment, a protrusion  1656  may be positioned on the distal end  1654  of the moveable member  1650  for engaging and securely holding the fixation rod  1675  ( FIG. 16A ) in the passageway  1608  of the inserter  1600 . It will be appreciated that the moveable member  1650  may be a single piece or separate pieces attached together. As a separate piece, for example, the moveable member  1650  may be made of a cable connected to a hook-like portion, which may engage a fixation rod. 
     Upon rotation of the actuating member  1620 , the moveable member  1650  may be moved from a first position where a distal end  1654  of the moveable member  1650  extends out of the distal end  1606  of the elongated member  1602  to a second position where the entire moveable member  1650  may be positioned within the elongated portion  1602 . When the actuating member  1620  is rotated in the opposite direction, the moveable member  1650  may move from the second position to the first position. 
     To engage a fixation rod  1675 , the actuating member  1620  may be rotated so that the distal end  1654  of the moveable member  1650  may move outside of the elongated member  1602 . The protrusion  1656  may engage a receiving portion (not shown) of the fixation rod  1675 . One skilled in the art would appreciated that any means of connecting the fixation rod  1675  to the elongated member  1602  is envisioned (e.g., threads) so long as the rod  1675  may be disengaged from the inserter  1600 . The actuating member  1620  may then be rotated in the opposite direction so that the moveable member  1650  may be drawn back into the elongated member  1602  with the fixation rod  1675 . In this position, the fixation rod  1675  may be fixed securely with respect to the inserter  1600 . 
     The fixation rod  1675  may then be inserted into a patient and into a head portion  652  of a screw  650 . The rod  1675  may have a tip  1676 , which may be sharp and/or pointed to facilitate the rod&#39;s movement through tissue. Once the rod  1675  is in position, the actuating member  1620  may once again be rotated to extend the moveable member  1650  from the elongated member  1602 . The rod  1675  may then be disengaged from the inserter  1600  and the inserter  1600  may be removed from the patient, leaving the rod  1675  in place. In an embodiment where, for example, two screws  650  may be inserted into the vertebrae through two separate insertion sites spaced apart from one another, the inserter  1600  (or any side inserter, such as  1700 ) may provide the advantage of allowing a fixation rod to be inserted into the screws  650  underneath the skin and muscle, without the need to make an additional incision through skin and muscle between the insertion sites of the screws  650 . Such a procedure may minimize trauma to the body. It should be noted that any means of inserting a rod  1675  into a patient and through the head portion  652  of a screw  650  is envisioned preferably where the rod  1675  may be disengaged from an inserter. 
     As an alternative to the inserter  1600 , an inserter  1700  may be used to insert a rod  1750  into a patient and through a head portion  652  of a screw  650 . The inserter  1700  may comprise a body portion  1702  and an engagement portion  1704 . As shown in  FIG. 17 , the inserter  1700  may be connected to the top portion of an insertion cannula such as those describe above. In other procedures, the inserter  1700  may be connected to the top of a working cannula  475  or retractor  500 . Further, the inserter  1700  may be attached directly or indirectly (through another component) to the working cannula  475 , retractor  500 , and/or insertion cannula at any position along the length of the working cannula  475 , retractor  500 , and/or insertion cannula. 
     The engagement portion  1704  may comprise, for example, threads (not shown) for engaging corresponding threads (not shown) on the proximal end of the working cannula  475 , retractor  500 , and/or insertion cannula. Alternatively, the engagement portion  1704  may comprise a clip (not shown) for engaging a clip engaging portion (not shown) on the working cannula  475 , retractor  500 , and/or insertion cannula. However, any method of connecting two components known to those of skill in the art is envisioned. 
     The rod  1750  may be rotatable with respect to the inserter  1700 . For example, a portion of the rod  1750  may be inserted in opening  1710  of inserter  1700  and fixed to inserter  1700  such that the rod  1750  may rotate within the opening  1710 . The rod  1750  may be formed to define a path of travel, which may be arcuate in shape. Where more than one insertion cannula may be used, the insertion cannulas may be fixed with respect to each other. Consequently, the head portions  652  of the screws  650  attached to the insertion cannula may also be in a fixed orientation. The orientation of the insertion cannula and the head portion  652  may be a factor that sets the path of travel of the rod  1750 . In order to insert the rod  1750  into the head portion, the rod  1750 , which starts from a position outside of a patient&#39;s body, may be swung through an arc into a patients body such that the rod  1750  passes through the head portions  652  of the screws  650 . A portion of the rod  1750  may then be disengaged from the remainder of the rod  1750  and may remain in the screws  650  as the remainder of the rod  1750  may be removed from the body. It should be appreciated that any device which connects to a working cannula  475 , retractor  500 , and/or insertion cannula and may move a fixation rod through an arcuate path into a screw  650  is envisioned. 
     b. Top Inserter 
       FIG. 18  illustrates an embodiment of a rod inserter which may be used to insert a fixation rod from the top of a screw  650 . The rod inserter  1800  may comprise a distal end  1802  and a proximal end  1804 . The inserter  1800  may also comprise a first elongated member  1806  and a second elongated member  1808  which may be rotatably connected to each other. The first elongated member  1806  may have a first jaw portion  1803  and the second elongated member  1808  may have a second jaw portion  1805 . The jaw portions  1803 ,  1805  may define an engagement portion  1807  therebetween into which a fixation rod may be inserted. Both the first and second elongated member  1806 ,  1808  may have a gripping portion  1810 ,  1812 , respectively, such that an operator may be able to hold the portion  1810 ,  1812  and move the elongated member  1806 ,  1808  relative to each other. In one embodiment, the gripping portions  1810 ,  1812  may be scissor-type griping portions into which an operator may be able to position at least one finger. Moreover, the gripping portions  1810 ,  1812  may have a locking mechanism  1814  to hold the elongated members  1806 ,  1808  in a fixed position relative to each other. In one embodiment, the locking mechanism  1814  may comprise mating teeth  1815  to fix the elongated portion  1806 ,  1808  at fixed intervals relative to each other. 
     In operation, an operator may separate the gripping portions  1810  and  1812 . This, in turn, may result in the jaw portions  1803 ,  1805  moving apart from one another, and the engagement portion  1807  may increase in size to receive a fixation rod. Once a rod is inserted in the engagement portion  1807 , an operator may move the gripping portions  1810 ,  1812  back together so that the rod may be fixed within the engagement portion  1807  and between jaw portions  1803 ,  1805 . The elongated member  1806 ,  1808  may be held in position by locking mechanism  1814 . An operator may then use the inserter  1800  to insert a rod, such as rod  1900 , down insertion cannulas  1100  ( FIG. 19A ) and/or cannulas  1200  ( FIG. 19B ) and through the top of screws  650 . It should be understood that other instruments that may be attached to a fixation rod may be used to move the rod down into the screws. 
     C. Methods for Less Invasive Surgery 
     To perform a spinal fixation procedure, a surgeon may use a radiographic image of the spine to determine one or more insertion points on a patient&#39;s back. One or more incisions may then be made depending on the procedure to be performed and the instruments which may be used. An incision may have a length, for example, between about 1 cm and about 10 cm and, more preferably, between about 2 cm and about 5 cm. A trocar  100  may be inserted into the incision(s), followed by a guide wire  150 , which may be positioned adjacent to or into a vertebra. To insert the guide wire  150  into the vertebrae, a hammer or other surgical tool may be used to strike the guide wire  150  or cap  152 . With the guide wire  150  in place, the trocar  100  may be removed. It should be noted that a trocar  100  may be unnecessary and the guide wire  150  may be positioned directly through the incision and adjacent or into a vertebra. 
     An inserter  200  may then be positioned down over the guide wire  150  towards the vertebrae. The incision may be dilating by inserting sequentially larger dilators  350  over the inserter  200 . However, in one embodiment, the dilators  350  may be inserted directly over the guide wire  150  without the use of an inserter  200 . When the incision(s) has been dilated to a size appropriate for the procedure to be conducted, in one embodiment, a working cannula  475  or retractor  500  may be inserted over the largest dilator  350 . It should be understood that in some embodiments, a screw  650  and/or an insertion cannula may be inserted directly into the body without using a working cannula  475  and/or retractor  500  to dilate an incision. In one embodiment, an insertion cannula may be attached to an inserter, similar to inserter  200 . The insertion cannula and inserter may be inserted into an incision as one unit, down to a vertebra. Once the working cannula  475 , retractor  500  and/or insertion cannula are in position, the inserter  200  and/or dilator(s)  350  may be removed from the patient. The guide wire  150  may remain in place or may be removed. It should be noted that the working cannula  475 , retractor  500  or insertion cannula may be inserted directly into the incision(s) (e.g., without the use of a trocar  100 , guide wire  150 , inserter  200  and/or dilators  350 ). The dilation process may be repeated for each incision. 
     An opening may be created by the working cannula  475 , retractor  500  and/or insertion cannula. In an embodiment where a single insertion cannula may be inserted into a single opening, the opening may have a diameter, for example, between about 1 cm and about 4 cm and, more preferably, between about 1.5 cm and about 3 cm. Furthermore, in an embodiment where more than one insertion cannula may be inserted through a single opening, the opening may have a diameter, for example, between about 2 cm and about 10 cm and, more preferably, between about 2 cm and about 5 cm. It should be noted, however, that the opening may be any shape, for example, oval, circular, egg-shaped, square, rectangular, or otherwise polygonal. 
     In an embodiment using working cannula(s)  475 , the drill  1350  may be inserted down a guide wire  150  (if present) and into the working cannula(s)  475  and may be rotated to create one or more cavities in the vertebrae into which one or more screws  650  may be positioned. When a working cannula  475  is large enough, an entire procedure may be performed through the working cannula  475 . Moreover, in an embodiment where a retractor  500  may be used, the blades  502  of the retractor  500  may be spread apart so that the entire surgical procedure may be performed within the opening created by the blades  502  of the retractor  500 . The drill  1350  may be inserted down a guide wire  150  (if present) and may be used to drill one or more holes in the vertebrae. Alternatively, the drill  1350  may be used (without the use of a guide wire  150 ) by an operator to create holes in any vertebrae, which may be accessed through the working cannula  475  and/or retractor  500 . In some embodiments, multiple guide wires  150  may be used for creating multiple cavities and guide multiple tools to bone. It should also be understood that a drill may be positioned through an insertion cannula for drilling holes. 
     In one embodiment, after the drilling step, one or more shank portions  654  ( FIG. 20C ) may inserted into vertebrae using a screwdriver  1500 ,  1550 . In such an embodiment, a pusher  2000  ( FIG. 20A ) may be used to subsequently connect the head portions  652  and the shank portions  654 . As illustrated in  FIG. 20B , the head portion  652  may be connected to a distal end of insertion cannula  1200  and the pusher  2000  may be inserted into insertion cannula  1200 . The entire assembly may then be inserted into a working cannula  475  or retractor  500 . When the head portion  652  engages the shank portion  654 , a translational force may be asserted by an operator on the pusher  2000  such that the head portion  652  snaps onto the shank portion  654 . Thereafter, the pusher  2000  may be removed from the insertion cannula  1200  while the insertion cannula  1200  remains attached to the assembled screw  650 . The same method may be performed using insertion cannulas  700 ,  800 ,  900 ,  1000  or  1100 . In some embodiments, the pusher mechanism may be the insertion cannula itself and a pusher  2000  may be unnecessary. An operator may attach the head portion  652  to an insertion cannula  700 ,  800 ,  900 ,  1000 ,  1100  or  1200  and may assert a translational force to the insertion cannula to insert the head portion  652  on the shank portion  654 . 
     Furthermore, in one embodiment where the shank portion  654  and head portion  652  may be inserted separately into the body, there may be no dilation mechanism used to created an opening in the patient. In such an embodiment an incision may be created through a patient&#39;s skin. A guide wire  150  may be inserted through the incision, down to a vertebra. The shank portion  654  may be inserted down the guide wire  150  and driven into the vertebra. The head portion  652 , insertion cannula and pusher  2000  (if used) may be connected together and, as one unit, inserted down over the guide wire  150  until the head portion  652  engages the shank portion  654 . The head portion  652  may be attached to the shank portion  654  and the pusher  2000  (if used) may subsequently be removed from the body, leaving the assembled screw  650  and the insertion cannula in the body. 
     In other embodiments, the bone screw  650  may be connected and/or fixed to insertion cannula  700 ,  800 ,  900 ,  1000 ,  1100  and/or  1200  as discussed above. One or more insertion cannulas  700 ,  800 ,  900 ,  1000 ,  1100  and/or  1200  (with screws  650  attached) may then be inserted into the working cannula  475  or the retractor  500  and may be guided by a guide wire (if present). In one embodiment, the insertion cannula  700 ,  800 ,  900 ,  1100  and/or  1200  may be positioned into the incision, working cannula  475  or retractor  500  without the screw  650 . Thereafter, the screw  650  may be inserted down the insertion cannula  700 ,  800 ,  900 ,  1100  and/or  1200 . In other embodiments, the bone screw  650  may be inserted through the working cannula  475  or the retractor  500  and driven into the cavity created by the drill  1350 . The insertion cannula  700 ,  800 ,  900 ,  1000 ,  1100  and/or  1200  may then be inserted through the working cannula  475  or retractor  500  and may be connected and/or fixed to the screw  650 . 
     An implantation mechanism such as a screwdriver  1500 ,  1550  may be used to drive a screw  650  into a vertebra. The screwdriver  1500 ,  1550  may be inserted into an insertion cannula  700 ,  800 ,  900 ,  1100  and/or  1200  before or after the insertion cannula is positioned in the body. In an embodiment where the screwdriver  1500 ,  1550  and screw  650  may be inserted into an insertion cannula after the insertion cannula is in the body, the screwdriver  1500 ,  1550  may be used to move a screw  650  down towards the distal end of an insertion cannula  700 ,  800 ,  900 ,  1100  and/or  1200 . In an embodiment where a cannula  1000  may be used, the screwdriver  1550  may be inserted into insertion cannula  1000  before or after the cannula  1000  and screw  650  may be positioned in the body. Moreover, it will be appreciated by those skilled in the art that the insertion cannula may be the implantation mechanism and may be used to drive a screw  650  into bone. In such an embodiment, a screwdriver  1500 ,  1550  may be unnecessary. For example, in an embodiment where the head portion  652  and shank portion  654  may be one integral piece (e.g., where the head and shank may have a fixed orientation), upon fixing the insertion cannula with respect to the screw  650 , the insertion cannula may be rotated to insert the shank portion  654  into a vertebra. 
     With the bone screw  650  properly located above the pedicle and the hole created by the drill  1350 , the screwdriver  1500 ,  1550  may be rotated to drive the shank  654  of the bone screw  650  into the pedicle of a vertebra. In an embodiment where the threaded shank  654  is self-tapping, the shank  654  may be anchored to the vertebra upon rotation of the screw  650 . Moreover, in an embodiment where a screw may be inserted down through an insertion cannula  700 ,  800 ,  900 ,  1100  and/or  1200 , as the screw  650  is rotated, the screw  650  may move closer to the distal end of the insertion cannula  700 ,  800 ,  900 ,  1100  and/or  1200 . Once the screw  650  is in the vertebra to a desirable extent, the screwdriver  1500 ,  1550  may be disengaged from the screw  650  and removed from the patient. 
     In an embodiment using cannula  700 ,  800 ,  900 ,  1100  and/or  1200 , a positioner  1450  may be positioned through the insertion cannula  700 ,  800 ,  900 ,  1100  and/or  1200  and guided down a guide wire (if present). The positioner  1450  may be used to rotate the head portion  652  of the screw  650  so that the channel  666  aligns with the corresponding channel of the insertion cannula  700 ,  800 ,  900 ,  1100  and/or  1200 . The insertion cannula  700 ,  800 ,  900 ,  1100  and/or  1200  may then be fixed to the screw  650  as described above. The positioner  1450  may be removed before or after the insertion cannula  700 ,  800 ,  900 ,  1100  and/or  1200  is fixed to the screw  650 . 
     It should be noted that in some embodiments, the insertion cannula  700 ,  800 ,  900 ,  1000 ,  1100  and/or  1200  may be fixed with respect to the screw  650  prior to the screw  650  being inserted into the body and/or driven into bone. In such an embodiment, the insertion cannula  700 ,  800 ,  900 ,  1000 ,  1100  and/or  1200  may rotate with the screwdriver  1500 ,  1550  as the screw is being inserted into bone. Such a construction may provide the advantage of enabling an operator to align the channels of the respective insertion cannulas with channel  666  of the screw  650  outside the body, thus eliminating the need for alignment of the channels inside the body without direct visualization by a surgeon. 
     In a fixed state, the head portion  652  may be prevented from moving axially and/or rotationally with respect to the insertion cannula  700 ,  800 ,  900 ,  1000 ,  1100 ,  1200 . At the same time, the head portion  652  may pivot about the shank portion  654 . Thus, an operator may use the insertion cannulas to orient the head portion  652  of the screws  650  so that the channel  666  aligns with the channel  666  of an adjacent screw  650 . Thereafter, a fixation rod may be inserted in the head portions  652  of the screws  650 . 
     The fixation rod may be inserted from the top or the side of the screw  650  depending upon the insertion cannula and/or rod inserter used by a surgeon. In a procedure where a fixation rod may be inserted from the side, an incision (separate from the incision through which the screw  650  is positioned) may be made in the patient at a distance from the incision containing the insertion cannulas. In a procedure where the rod is inserted from the top of the screw  650  and the screws  650  may be inserted through separate insertion cannulas (in separate openings), an incision may be made through the skin and muscle between the insertion cannulas so that a rod may be positioned down to the vertebrae. In an embodiment where a working cannula  475  or retractor  500  may be used, there may be no need to create a separate incision when the rod is inserted from the top of the screw  650  because there may be no tissue between the insertion cannulas. 
     The fixation rod may be guided into position by feel or by fluoroscopic guidance using inserter  1600 ,  1700 , or  1800 . In an embodiment using inserter  1600 , a separate incision may be made a distance from the incision(s) used to insert the working cannula  475 , retractor  500 , and/or the insertion cannula  700 ,  800 ,  900 ,  1000 ,  1100 ,  1200 . The inserter  1600  may be fixed to a rod  1675  as discussed above. The inserter  1600  may be used to guide the rod  1675  through the separate incision, underneath the skin and muscle located between adjacent screws  650 , and into the channels  666  of the screws  650  from the side of the screw  650 . The inserter  1600  may be disengaged from the rod  1675  and removed from the patient. The disengagement step may occur before or after the rod has been fixed to the screws  650 . The inserter  1600  may be used in a way such that the inserter  1600  may not contact or engage any portion of an insertion cannula that may be located outside of the body (e.g., the inserter  1600  may contact a portion of an insertion cannula located only within the body). 
     In another embodiment, the engagement portion  1704  of the inserter  1700  may engaged the working cannula  475 , retractor  500 , and/or the insertion cannula. Adjacent insertion cannulas may be connected to one another such that the insertion cannulas may be in a fixed orientation relative to each other. Similar to insertion using the inserter  1600  above, a separate incision may be created in the patient. The rod  1750  may be rotated through the separate incision, underneath the skin and muscle located between adjacent screws  650 , and into the channels  666  of the screws  650  from the side of the screw  650 . A portion of the rod  1750  may be disengaged from the inserter  1700  and the remainder of the rod  1750  may be removed from the patient. 
     In an embodiment using the inserter  1800 , a fixation rod may be inserted between the jaw portions  1803  and  1805  into the engagement portion  1807 . The jaws  1803  and  1805  may be locked in position onto the fixation rod by engaging the mating teeth  1815  of locking mechanism  1814 . As shown in  FIGS. 19A and 19B , the inserter  1800  may then be used to position the fixation rod down through cannula  1100  or  1200  and into the head portion  652  of the screw  650 . The inserter  1800  may be disengaged from the fixation rod and removed from the patient before or after the rod has been fixed to the screws  650 . 
     In one embodiment, where two or more cannulas  1100 ,  1200  may be inserted into the body through separate openings in a patient, a fixation rod may be inserted down one cannula  1100 ,  1200  and rotated, underneath the skin, into at least one other cannula  1100 ,  1200 . Similar to a side insertion technique, such a technique may provide the advantage of allowing a fixation rod to be inserted into the screws  650  underneath the skin and muscle, without the need to make an additional incision through skin and muscle between two or more cannulas  1100 ,  1200 . 
     A surgeon may be able to directly visualize the rod being inserted through the screw  650  by looking down into the working cannula  475 , retractor  500 , and/or insertion cannulas. It should be understood, however, that in some techniques, an operator may insert a fixation rod down and/or into the insertion cannulas without the use of any device by using his/her hands and/or fingers. As the fixation rod is being inserted, the insertion cannula may be manipulated to further facilitate introduction of the fixation rod. 
     In order to secure the fixation rod in the channel  666  of bone screw  650 , a locking cap  681 , such as the one shown in  FIG. 6A , may be threaded into the head portion  652 . A locking cap screwdriver  2100  ( FIG. 21 ) may be used to move the locking cap  681  down the insertion cannula. The prongs  2102  of the locking cap screwdriver  2100  may engage receiving portions (not shown) on the locking cap  681 . The screwdriver  2100  may then be rotated such that the external threaded portion  680  of the locking cap  681  engages the internal threaded portion  672  of the head portion  652 . This step may be repeated for each screw  650 . It will be appreciated by those skilled in the art that any locking cap (e.g., nut, clip, etc.) may be used so long as it may hold a fixation rod to a screw  650 . In some embodiments, for example, where the locking cap may be a nut (not shown) which may be positioned around the head portion  652 , the insertion cannulas may be removed from the body prior to engaging the locking cap with the screw  650 . Moreover, it will also be appreciated that the locking cap screwdriver may have different orientations to correspond to different shaped locking caps (e.g., the locking cap screwdriver may have a hex portion to engage a corresponding hex recess of a locking cap). In other embodiments, a locking cap screwdriver may be unnecessary (e.g., where the locking cap may be a nut that treads onto the head portion  652 ). Once all locking caps  681  are positioned in the screws  650 , as shown in  FIG. 22 , the fixation rod may be fixed with respect to the screws  650 . 
     The working cannula  475 , retractor  500 , and/or insertion cannulas may be disengaged from the screws  650  and removed from the patient. The opening(s) may be closed by methods known by those skilled in the art. 
     While the foregoing description and drawings represent the preferred embodiments of the present invention, it will be understood that various additions, modifications and substitutions may be made therein without departing from the spirit and scope of the present invention as defined in the accompanying claims. In particular, it will be clear to those skilled in the art that the present invention may be embodied in other specific forms, structures, arrangements, proportions, and with other elements, materials, and components, without departing from the spirit or essential characteristics thereof. One skilled in the art will appreciate that the invention may be used with many modifications of structure, arrangement, proportions, materials, and components and otherwise, used in the practice of the invention, which are particularly adapted to specific environments and operative requirements without departing from the principles of the present invention. The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and not limited to the foregoing description.