before the subject devices , systems and methods are described , it is to be understood that this invention is not limited to particular embodiments described , as such may , of course , vary . it is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only , and is not intended to be limiting , since the scope of the present invention will be limited only by the appended claims . unless defined otherwise , all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs . it must be noted that as used herein and in the appended claims , the singular forms “ a ”, “ an ”, and “ the ” include plural referents unless the context clearly dictates otherwise . thus , for example , reference to “ a spinal segment ” may include a plurality of such spinal segments and reference to “ the screw ” includes reference to one or more screws and equivalents thereof known to those skilled in the art , and so forth . all publications mentioned herein are incorporated herein by reference to disclose and describe the methods and / or materials in connection with which the publications are cited . the publications discussed herein are provided solely for their disclosure prior to the filing date of the present application . nothing herein is to be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention . further , the dates of publication provided may be different from the actual publication dates which may need to be independently confirmed . the present invention will now be described in detail by way of the following description of exemplary embodiments and variations of the systems and methods of the present invention . while more fully described in the context of the description of the subject methods of implanting the subject systems , it should be initially noted that in certain applications where the natural facet joints are compromised , inferior facets , lamina , posterior arch and spinous process of superior vertebra may be resected for purposes of implantation of certain of the dynamic stabilization systems of the present invention . in other applications , where possible , the natural facet joints , lamina and / or spinous processes are spared and left intact for implantation of other dynamic stabilization systems of the present invention . it should also be understood that the term “ system ”, when referring to a system of the present invention , most typically refers to a set of components which includes multiple bone stabilization components such as a superior , cephalad or rostral ( towards the head ) component configured for implantation into a superior vertebra of a vertebral motion segment and an inferior or caudal ( towards the feet ) component configured for implantation into an inferior vertebra of a vertebral motion segment . a pair of such component sets may include one set of components configured for implantation into and stabilization of the left side of a vertebral segment and another set configured for the implantation into and stabilization of the right side of a vertebral segment . where multiple bone segments such as spinal segments or units are being treated , the term “ system ” may refer to two or more pairs of component sets , i . e ., two or more left sets and / or two or more right sets of components . such a multilevel system involves stacking of component sets in which each set includes a superior component , an inferior component , and one or more medial components therebetween . the superior and inferior components ( and any medial components therebetween ), when operatively implanted , may be engaged or interface with each other in a manner that enables the treated spinal motion segment to mimic the function and movement of a healthy segment , or may simply fuse the segments such as to eliminate pain and / or promote or enhance healing . the interconnecting or interface means include one or more structures or members that enables , limits and / or otherwise selectively controls spinal or other body motion . the structures may perform such functions by exerting various forces on the system components , and thus on the target vertebrae . the manner of coupling , interfacing , engagement or interconnection between the subject system components may involve compression , distraction , rotation or torsion , or a combination thereof . in certain embodiments , the extent or degree of these forces or motions between the components may be intraoperatively selected and / or adjusted to address the condition being treated , to accommodate the particular spinal anatomy into which the system is implanted , and to achieve the desired therapeutic result . in certain embodiments , the multiple components , such as superior and inferior spinal components , are mechanically coupled to each other by one or more interconnecting or interfacing means . in other embodiments , components interface in a manner that constrains their relative movement and enables the treated segment to mimic the function or partial function and / or movement or partial movement of a healthy segment . typically , spinal interconnecting means is a dorsally positioned component , i . e ., positioned posteriorly of the superior and inferior components , or may be a laterally positioned component , i . e ., positioned to the outer side of the posterior and inferior components . the structures may include one or more struts and / or joints that provide for stabilized spinal motion . the various system embodiments may further include a band , interchangeably referred to as a ligament , which provides a tensioned relationship between the superior and inferior components and helps to maintain the proper relationship between the components . reference will now be made in detail to the present embodiments of the invention , examples of which are illustrated in the accompanying drawings . wherever possible , the same reference numbers will be used throughout the drawings to refer to the same or like parts . in addition , each of the inventive embodiments described herein may be employed in a percutaneous procedure , a mini - open procedure or an open procedure . utilization of minimally invasive techniques can shorten the procedure &# 39 ; s time and speed recovery by the patient . however , the application of these inventions in a minimally invasive manner is not a requirement . fig1 shows a perspective view of a rod reduction instrument 10 according to the present invention . the rod reducer 10 includes an inner cannula assembly 12 , a locking shaft ( or middle cannula ) 14 , a plunger ( or outer cannula ) 16 , a body assembly 18 , a pinion driver 20 , a handle 22 , and a handle connector 24 . the cannula assembly 12 is connected to the locking shaft 14 so that one of the cannula assembly 12 and locking shaft 14 is movable with respect to the other ; the other one of the cannula assembly 12 and locking shaft 14 is connected to the body assembly 18 . the cannula assembly 12 is configured such that it substantially fits inside the locking shaft 14 . the plunger 16 is connected to the body assembly 18 such that it is movable with respect to cannula assembly 12 and the locking shaft 14 . the handle 22 is connected to the handle connector 24 which is connected to the body assembly 18 . the pinion driver 20 is removably inserted into the body assembly 18 . fig2 is a side elevational view of the rod reducer 10 without the pinion driver 20 . in normal use , the rod reducer instrument 10 is oriented so that the handle 22 is located proximally and accessible by the surgeon and the opposite end of the instrument 10 is oriented distally away from the surgeon and towards the operative site . the operative site is generally the spinal column of a patient and in particular , a vertebral body in which a bone screw system is located . a typical bone screw system is described in u . s . patent application ser . no . 11 / 362 , 366 entitled “ systems and methods for stabilization of bone structures ” filed on feb . 23 , 2006 , u . s . patent application ser . no . 11 / 586 , 849 entitled “ systems and methods for stabilization of bone structures ” filed on oct . 25 , 2006 and u . s . patent application ser . no . 11 / 726 , 093 entitled “ screw systems and methods for use in stabilization of bone structures ” filed on mar . 20 , 2007 all incorporated herein by reference in their entirety . there is shown in fig3 a multi - level application of three bone screw systems 42 a , 42 b , 42 c installed into vertebrae v 1 , v 2 and v 3 , respectively , along one side of a patient &# 39 ; s spine . each bone screw system 42 a , 42 b , 42 c includes a seat 44 a , 44 b , 44 c that allows a rod 46 to be positioned within a seat rod channels 47 a , 47 b , 47 c and secured to bone screws 48 a , 48 b , 48 c . in one variation of the seat , the seat includes parallel flat portions on the outer surface of the seat . in order to facilitate the surgeon &# 39 ; s positioning of the rod 46 in all of the seats 44 a , 44 b , 44 c of screw systems 42 a , 42 b , 42 c , the rod reducer instrument 10 is engageable to the seat and as shown in fig3 in particular , the rod reducer instrument 10 is engageable to middle seat 44 b to reduce the distance between the seat 44 b and the rod 46 as the condition of the spine has slightly displaced the middle vertebra v 2 making it difficult seat the 46 into the middle seat 44 b . hence , the rod reducer instrument is positioned against the rod 46 and thereafter operable to move the rod 46 and the seat 44 b into closer proximity to each other in a manner that draws vertebra v 2 in the direction of the arrow in fig3 such that the rod 46 is positioned within the channel 47 b , inside the seat 44 b and capable of being secured thereto with a locking mechanism such as a cap and / or set screw combination ( not shown ). various bone screw systems that employ polyaxial or uni - axial screws are within the scope of the present invention as well as non - screw fastening systems such as hooks and other bone or tissue engaging devices . the rod 46 is typically an elongated substantially cylindrical member that is straight or curved , however the invention is not so limited and a rod having any size or shape is within the scope of the invention so long as it can be secured to the seat 44 of the bone screw system 42 and serves its stabilizing function . the cannula assembly 12 will now be described . with reference to fig4 , there is shown a side elevational view of the cannula assembly 12 . the cannula assembly comprises an inner cannula 26 connected to a seat clamp 28 . a top view of the inner cannula 26 is shown in fig5 . the cannula 26 is generally cylindrically shaped and has a first end 52 and a second end 54 . a central inner bore 60 ( visible in fig6 a ) is formed between the first end 52 and the second end 54 . the cannula 26 includes a retaining slot 30 , a pin hole 32 and a “ finger ” slot 34 all formed in the cannula 26 along a central longitudinal cross - sectional plane . a second “ finger ” slot 34 that is not visible in fig5 is included on the cannula 26 in a location directly opposite from the first finger slot 34 . referring back to fig4 , the cannula also includes a cannula rod channel 50 formed at the distal end of the cannula 26 . the cannula rod channel 50 is formed in a longitudinal plane that is substantially perpendicular to the longitudinal plane along which the retaining slot 30 and pin hole 32 are formed . a second cannula rod channel 50 that is not visible in fig4 is formed in a location directly opposite from the first cannula rod channel 50 . turning back to fig5 , the cannula 26 further includes a shoulder 36 , a neck 38 and head 40 . the neck 38 is interconnected between the shoulder 36 and the head 40 and has a reduced outer cross - sectional diameter along at least a portion of the neck 38 relative to the shoulder 36 and head 40 . the shoulder 36 forms an abutment 56 at a proximal end of the shoulder 36 and the head 40 is located distally at the second end 54 of the cannula 26 . now referring to fig6 a , 6 b and 6 c , there is shown a cross - sectional view and two end views , respectively , of the cannula 26 according to the present invention with the bore 60 of the cannula 26 being clearly visible . in one variation , the bore 60 includes two longitudinally extending bore channels 62 located directly opposite to each other extending along the length of the cannula 26 between the first and second ends 52 , 54 . the cannula bore channels 62 are parallel to and in substantial alignment with the rod channels 50 . as seen in fig6 a , the inner diameter of the cannula 26 is substantially the same along the neck 38 , shoulder 36 and proximal portions of the cannula 26 . also , as seen in fig6 , the interior diameter along at least a portion of the head 40 of the cannula 26 is greater relative to the rest of the interior of the cannula 26 . this portion along the head 40 with a greater inner diameter defines a seat receiving portion 58 . generally , the seat receiving portion 58 is interconnected with and opens to the second end 54 of the cannula 26 . a ledge 66 on the interior surface serves as a stop against which the inserted seat 44 abuts . the seat receiving portion 58 is configured to conform to the shape and size of a seat 44 of a bone screw system 42 and to receive at least a portion of the seat 44 inside the seat receiving portion 58 . fig7 shows a cross - sectional area of the seat receiving portion 58 . as seen in fig7 , the seat receiving portion 58 includes parallel flat portions 64 formed on the inner surface of the seat receiving portion 58 . these flat portions 64 match flat portions 49 on a seat 44 of a bone screw system 42 . when the seat receiving portion 58 engages a seat 44 , the flat portions 64 of the seat receiving portion 58 line up with the flat portions 49 of the seat 44 , thereby , properly orienting the seat 44 to the cannula 26 so that the seat 44 can be inserted into the seat receiving portion 58 of the cannula . in this orientation , the seat rod channels 47 advantageously line up with the rod channels 50 in the cannula 26 interconnecting the seat rod channels 47 with the cannula rod channels 50 for placement of the rod 46 into the seat rod channel 47 . also , in this orientation , the cannula bore channels 62 are advantageously lined up and interconnected with the seat rod channels 47 such as for advantageously translating a cap or set screw down the length of the cannula 26 in an orientation that is in alignment with the seat 44 for connecting thereto to lock the rod 46 inside the seat rod channel 47 . turning now to fig8 , there is shown a seat clamp 28 having a proximal end 72 and a distal end 74 according to the present invention . the seat clamp 28 includes a base 68 and two substantially parallel fingers 70 extending longitudinally from the base 68 . the base 68 is ring - shaped having an inner diameter that is slightly larger than the outer diameter of the proximal portion of the cannula 26 yet smaller than the outer diameter of the shoulder portion 36 such that seat clamp 28 contacts the abutment 56 at the ring 68 . at the distal end 74 of each finger 70 , an inwardly extending nib 76 is formed . each nib 76 is configured to mate with corresponding recesses formed on the seat 44 of a bone screw system . other mating configurations are discussed in reference to fig2 a , 24 b , 24 c and 24 d . fig9 illustrates a cross - sectional view taken longitudinally through the fingers 70 of the seat clamp 28 . as seen in fig9 , the fingers 70 are angled slightly outwardly to create a leaf spring exerting an outward spring force when the distal ends 74 are pushed together as will be evident and described hereinbelow . to assemble the cannula assembly 12 , the cannula 26 and seat clamp 28 are oriented such that the distal end 74 of the seat clamp 28 and the distal end 54 of the cannula 26 are lined up and the ring - shaped base 68 of the seat clamp 28 is passed over the proximal end 52 of the cannula 26 until the base 68 contacts the shoulder abutment 56 . the seat clamp 28 is rotated to align the two fingers 70 with the two finger slots 34 such that the fingers 70 are allowed to flex into the finger slots 34 . with reference to fig1 and 11 , the locking shaft or middle cannula 14 will now be described . fig1 shows a perspective view of the locking shaft 14 and fig1 shows a cross - sectional view . the locking shaft 14 is generally cylindrically shaped and has a proximal end 78 and a distal end 80 and a central inner bore 82 extending between the proximal end 78 and the distal end 80 . a pair of rod channels 84 opposite from one another is formed in the locking shaft 14 at the distal end 80 . each rod channel 84 extends longitudinally and opens at the distal end 80 . a pin through slot 86 is formed near the proximal end 78 of the locking shaft 14 . the proximal end 78 includes a flat portion 88 on the outer surface of the locking shaft 14 for insertion into the body assembly 18 and for preventing rotation with respect to the body assembly 18 . still referencing fig1 and 11 and with particular reference to fig1 , the locking shaft 14 includes a head portion 90 and a shaft portion 92 . as seen in fig1 , the head portion 90 has an outer diameter that is larger than the outer diameter of the shaft portion 92 with an outer locking shaft abutment 94 formed at the intersection of the head portion 90 and shaft portion 92 for biasing a first spring ( not shown ). turning now to the inside of the locking shaft 14 , the inner diameter of the shaft portion 92 is smaller than the inner diameter of the head portion 90 . an inner locking shaft abutment 96 is formed inside at the intersection of the shaft portion 92 and the head portion 90 for biasing a second spring ( not shown ). the inner locking shaft abutment 96 is located distally relative to the outer locking shaft abutment 94 . the head portion 90 further includes a cannula head receiving portion 98 that has an inner diameter that is larger than the inner diameter of the rest of the head portion 90 . an inner cannula head abutment 100 is formed inside at the intersection of the cannula head receiving portion 98 of the head portion 90 and the rest of the head portion 90 . with reference to fig1 and 13 , the plunger or outer cannula 16 will now be described . the plunger 16 is generally cylindrically shaped and has a proximal end 102 and a distal end 104 . a central plunger bore 106 extends between the proximal end 102 and the distal end 104 . the pair of corresponding rod engaging surfaces 108 are formed at the distal end 104 and positioned opposite from each other . also formed at the distal end 104 , are a pair of scalloped portions 10 ( fig1 ). the scalloped portions 110 interconnect with the pair of rod engaging surfaces 108 such that the side profile of the plunger 16 is angled towards the plunger &# 39 ; s central longitudinal axis 112 as seen in fig1 . the plunger 16 further includes a rack connecting portion 114 integrally formed or connected to the outer surface of the plunger 16 for connecting to a rack as will be described below . the rack connecting portion 114 includes a rack receiving channel 116 . turning now to the inside of the plunger 16 and referencing fig1 in particular , the inside of the plunger 16 is divided into a proximal section 118 and a distal section 120 . the proximal section 118 is characterized by an inner diameter that is smaller than the inner diameter of the distal section 120 of the plunger 16 . the intersection of the proximal section 118 with the distal section 120 forms an inner plunger abutment 122 for biasing the first spring ( not shown ). the body assembly 18 will now be discussed . the body assembly 18 includes a body 124 , a plunger driver 126 connected to the body 124 and a cannula spring lock system 128 connected to the body 124 . referring now to fig1 , there is shown a cross - sectional view of the body 124 having a proximal end 142 and a distal end 144 . the body 124 includes a plunger driver receiving portion 130 , a handle connecting portion 132 , a locking shaft receiving portion 134 and a cannula spring lock system receiving portion 136 . still referencing fig1 , the plunger driver receiving portion 130 is configured to receive a plunger driver 126 . the plunger driver 126 can be selected from any number of mechanical driving systems . in one variation of the invention , the plunger driver 126 is a rack and pinion system . therefore , the plunger driver receiving portion 130 of the body 124 is configured to receive a rack and pinion system . to that end , the plunger driver receiving portion 130 includes a rack passageway 138 and a pinion receiving aperture 140 . the rack passageway 138 extends between the proximal end 142 and the distal end 144 . the pinion receiving aperture 140 is interconnected with the rack passageway 138 as shown in fig1 . in one variation , the body 124 includes a rack viewing window 146 for viewing the position of the rack with respect to the body that will be discussed in greater detail below with respect to fig2 . in one variation of the invention , the rack and pinion system includes a rack lock to arrest retraction of a spring biased rack . to that end , the body 124 further includes a rack lock receiving portion 148 and a rack lock spring receiving portion 150 that houses a spring to bias a trigger . a pin hole 156 for connecting a trigger of the rack lock is formed in the body 124 and interconnected with the rack lock receiving portion 148 . still referencing fig1 , the handle connecting portion 132 of the body 124 includes a reduced cross - sectional portion of the body at the proximal end 142 for connecting with the handle 22 via a handle connector 24 . in one variation , the handle 22 is integrally formed with the body 124 and there is no need for a reduced cross - sectional portion of the body . still referencing fig1 , the locking shaft receiving portion 134 of the body includes a bore 152 sized slightly larger than the outer diameter of the locking shaft 14 to provide for a tight compression fit engagement with the body 124 . in one variation , the bore 152 extends from one end of the body to the other forming an opening 151 towards proximal end of the body . the opening 151 of the bore 152 opens at the surface of the body 124 to provide access to inside the bore 152 . the body 124 also includes a locking pin aperture 154 ( visible in fig1 ) that is interconnected with the bore 152 for receiving a pin to help lock the locking shaft 14 and cannula assembly 12 in place . a flat portion ( not shown ) is formed inside the bore 152 such that it corresponds to the flat portion 88 of the locking shaft 14 for aligning the locking shaft 14 inside the bore 152 and for preventing rotation of the locking shaft 14 inside the bore 152 relative to the body . in one variation , locking mechanism inserter notches 158 are formed in the body at the proximal end of the bore 152 for connecting with and orienting a locking mechanism inserter instrument with respect to the body . still referencing fig1 , a cannula spring lock system receiving portion 136 includes an aperture 160 extending between the bore 152 and the outer surface of the body 124 for receiving a cannula spring lock pin . turning now to fig1 , there is shown a side - elevational view of a rack 162 of the rack and pinion system 126 . the rack 162 includes an elongated element having teeth 164 and a connecting portion 166 configured to connect to the rack connecting portion 114 of the plunger . the rack 162 is configured to be located in the rack passageway 138 . in one variation , the rack 162 includes an indicator 168 that can be a mark , protrusion or any kind of indicator located and configured to be visible through the rack viewing window 146 . referring now to fig1 and 17 , there is shown a perspective view of a pinion 170 of the rack and pinion system 126 . the pinion 170 includes an axle 172 and a geared portion 174 . in one variation , the gear portion 174 includes a plurality of teeth and is centrally located along the pinion 170 as shown in fig1 and 17 . in general , the pinion is substantially cylindrical in shape and is configured to be inserted into the pinion receiving aperture 140 of the body 124 . in one variation , the pinion 170 includes at least one pinion bore 176 formed inside the pinion 170 . in one variation , two pinion bores 176 , one at either end of the pinion 170 , are formed such that each pinion bore 176 opens at the surface of each end of the pinion 170 as shown in fig1 . alternatively , a single pinion bore 176 extends end - to - end inside the pinion 170 . as seen in fig1 , the inner surface of the at least one pinion bore 176 is substantially hexagonal in shape configured to receive therein a hexagonal member . although the inner surface is hexagonal , the invention is not so limited and the inner surface can be any shape . the rim of the pinion bore 176 includes lead - in ramps 178 for guiding and facilitating insertion of a hexagonal member into the pinion bore 176 . the inner surface of the pinion bore 176 includes a locking groove 180 configured to receive a locking member therein . referring now to fig1 , there is shown a cross - sectional view of a trigger 182 of a rack lock . the trigger 182 is angled and includes a rack engaging end 184 configured to engage the teeth of the rack 162 . the trigger 182 also includes a finger receiving portion 186 . the trigger 182 further includes a spring receiving portion 188 configured to receive a spring therein . in one variation , the spring receiving portion 188 is a well located at the proximal end of the finger receiving portion 186 . the trigger 182 further includes a pin hole 190 configured to receive a pin , attach the trigger 182 to the body 124 and serve as a pivot point for the trigger 182 . referring now to fig1 , the cannula spring lock system 128 will now be described . fig1 is a partial cross - sectional view of the body with a cross - sectional view of the cannula spring lock system 128 disposed inside the cannula spring lock system receiving portion 136 of the body 124 . the cannula spring lock system 128 includes a pin 204 , spring 206 , plug 208 and knob 210 . the pin 204 includes a flange stop 212 extending outwardly and configured to abut a ledge inside the cannula spring lock system receiving portion 136 of the body 124 . the pin 204 is disposed inside the receiving portion 136 . the spring 206 is inserted onto the pin 204 and the plug 208 is threaded into the receiving portion 136 to contain the spring 206 between the flange stop 212 and the plug 208 such that a spring force is generated to bias the pin 204 into the pin hole 32 of the cannula 26 . the pin 204 is connected to the knob 210 via threaded portions on the pin 204 and knob 210 . with reference back to fig1 , the handle connector 24 will now be described . in one variation , the handle connector 24 includes a body engaging portion 192 , a handle engaging portion 194 and an interconnecting stem 196 connected between the body engaging portion 192 and the handle engaging portion 194 . in one variation , the body engaging portion 192 is configured to mate with a corresponding portion of the body 124 . for example , a ring - shaped body engaging portion 192 mates with a body portion having a circular cross - section with an outer diameter that is smaller than the inner diameter of the ring - shaped body engaging portion 192 as shown in fig1 . the handle engaging portion 194 , in one variation , includes a projection having a circular cross - section that mates with a bore in the handle 22 having a circular shape . although circular cross - sections and shapes have been discussed , the invention is not so limited and cross - sections and shapes of any kind are within the scope of the present invention . in another variation , the handle 22 and handle connector 24 may be integrally formed and in another , the handle 22 , handle connector 24 and body 124 may be integrally formed . still referring back to fig1 , the handle 22 will now be described . the handle 22 is large enough for comfortable gripping by a user and may include surface features that aid in handling of the rod reducer . in one variation , the handle 22 includes a bore inside the handle 22 that opens at one end of the handle and has a shape that is complementary for receiving a handle engaging portion of the handle connector or body . referring now to fig2 and fig2 , the pinion driver 20 will now be described . fig2 is a perspective view of the pinion driver 20 and fig2 is an end - elevational view of the pinion driver 20 . the pinion driver 20 includes a handle portion 198 and a pinion engaging member 200 . the pinion driver 20 is generally t - shaped and the handle portion 198 is configured to allow a user to comfortably and firmly grip the pinion driver 20 . the pinion engaging member 200 is configured to engage the pinion 170 . in one variation , the pinion engaging member 200 is substantially hexagonal in shape to complementarily match a hexagonal - shaped pinion bore 176 . the pinion engaging member 200 may be tapered to facilitate insertion into the pinion 170 . although hexagonal - shaped members are described , the invention is not so limited and any functional shape is within the scope of the invention . in one variation of the invention , a ball plunger 202 is contained inside a recess in the pinion engaging member 200 . as seen in fig2 , the ball plunger 202 extends outwardly from at least one surface of the pinion engaging member 200 and is configured to engage the locking groove 180 inside the pinion bore 176 . referring now to fig2 , there is shown a cross - sectional view of the rod reducer instrument 10 according to the present invention . the assembly of the instrument 10 will now be discussed in reference to fig2 . a first spring 214 is passed over the proximal end 78 of the locking shaft 14 until it is seated against the outer locking shaft abutment 94 . the plunger 16 is then passed over the locking shaft 14 such that the first spring 214 is disposed between outer locking shaft abutment 94 and the inner plunger abutment 122 . the locking shaft 14 , first spring 214 and plunger 16 combination is then connected to the body 124 by first aligning the flat portion 88 of the locking shaft 14 with the flat portion of the bore 152 of the body , inserting the proximal end 78 of the locking shaft 14 into the bore 152 and press - fitting it into the locking shaft receiving portion 134 of the body 124 . additional adhesive material may be employed to secure the locking shaft 14 inside the body 124 . the plunger 16 is permitted to move with respect to the locking shaft 14 and is spring biased such that the plunger 16 is forced towards the proximal end of the locking shaft 78 or towards the body 124 . next , a second spring 216 is inserted in through the distal end 80 of the locking shaft 14 . the cannula assembly 12 is then inserted in through the distal end 80 of the locking shaft 14 . a pin 218 is passed through the locking pin aperture 154 of the body 124 and through the pin through slot 86 of the locking shaft 14 and into the retaining slot 30 of the cannula 26 to help retain the locking shaft 14 . the second spring 216 is retained between the seat clamp 28 and the inner locking shaft abutment 96 such that the second spring 216 is compressed to exert a spring force onto the cannula assembly 12 pushing the cannula assembly 12 in a direction away from the body 124 . with the plunger 16 secured , as the cannula 26 is pushed in towards the body from the distal end , the pin 204 of the cannula spring lock system 128 pops , as it is spring biased , into the pin hole 32 of the cannula 26 to lock the cannula assembly 12 into position . the knob 210 is pulled to withdraw the pin 204 and release the cannula 26 and the second spring 216 pushes the cannula assembly 12 such that the head 40 protrudes out from the distal end 80 of the locking shaft 14 . still referencing fig2 , the assembly of the plunger driver 126 will now be discussed . in general , the plunger driver 126 is inserted into the plunger driver receiving portion 130 of the body 124 . in one variation of the invention in which the plunger driver 126 is a rack and pinion system , the rack 162 is inserted into the rack passageway 138 of the body 124 and connected to the rack connecting portion 114 of the plunger 16 via a pin . the pinion 170 is inserted into the pinion receiving aperture 140 and the geared portion 174 of the pinion 170 is engaged with the teeth 163 of the rack 162 . a bushing ( not shown ) is employed to press fit the pinion 170 inside the body . a third spring 220 is inserted into the rack lock spring receiving portion 150 of the body and into the spring receiving portion 188 of the trigger 182 . the trigger 182 is then secured to the body 124 via a pin ( not shown ) passed through the pinhole 156 of the body 124 and into the pinhole 190 of the trigger 182 such that the rack engaging end 184 of the trigger 182 can pivot and contact the rack 162 . the handle 22 is then connected to the body 124 . if a handle connector 24 is employed , the handle connecting portion 132 of the body 124 is passed into the body engaging portion 192 of the connector 24 and the handle engaging portion 194 is inserted into a bore of the handle 22 . in one variation , a dynamic handle connector is provided in which the handle connector 24 permits various orientations of the handle 22 to be locked with respect to the body 124 which provides greater comfort and ease of operation for the surgeon . referring now to fig2 a - 23c , the function of the rod reducer system 10 will now be discussed . the rod reducer 10 is typically employed in open or mini - open or percutaneous minimally invasive operating procedures in which two or more bone screws 222 or fasteners are threaded into a pedicle or other portion of two or more adjacent vertebral bodies . a single bone screw system 222 having a seat 226 with a rod channel 228 deployed in a single vertebral body 224 is shown in fig2 a - 23c . additional bone screw systems 22 deployed in adjacent vertebral bodies are not shown for clarity and these bodies and systems would be located above or below the page . a rod 230 is provided and disposed substantially longitudinally along the length of the spine . a cross - section of a single rod 230 is shown in fig2 a - 23c . the rod may be straight or bent to correspond to the normal curvature of the spine in the particular region being instrumented or to such other curvature as the surgeon may deem appropriate to correct the defect . for example , the rod 230 can be bent to form a kyphotic curvature for the thoracic region of the spine , or to form a lordotic curvature for the lumbar region . the rod 230 is engaged to a number of fixation elements fixed to or engaged with the vertebrae along the segment of the spinal column . a variety of fixation elements can be provided that are configured to engage the vertebrae . the bone screw system 222 includes a seat 226 and a rod channel 228 for receiving the rod 230 therein . affixing the rod 230 to the bone screw system 222 requires the rod 230 to be in close proximity to the bone screw system 222 , as shown in fig2 a - 23c , or inside the rod channel 228 . in some cases , such as in patients with spondylolisthesis where there is an anteroposterior translatory movement of two spinal vertebrae in relation to each other due to instability between the two involved vertebrae , a rod 230 and an implanted screw system 222 must be moved with respect to each other so that the rod 230 is juxtaposed with the seat 266 or occupies the space within the rod channel 228 or other opening in the seat 226 attached to the screw 222 so that the rod 230 can be coupled to the screw 222 . first , as shown in fig2 a , the rod reducer 10 is held by the handle 22 and is oriented such that the rod 230 is disposed inside the cannula rod channels 50 . the instrument 10 is then advanced such that the rod 230 slides with respect to the rod channels 50 and the distal second end 54 engages the seat 226 such that the seat 226 is received inside the seat receiving portion 58 of the cannula head 40 . flat portions 64 inside the seat receiving portion 58 assist in aligning the instrument 10 with the seat 226 . the top 232 of the seat 226 abuts the ledge 66 ( shown in fig6 a ) inside the cannula head 40 . this position of the instrument 10 with respect to the seat 226 is shown in fig2 b wherein a part of the seat 226 is inside the cannula 26 and hidden from view . in a single action , without the surgeon removing his hand from the handle 22 , the surgeon continues to advance the instrument 10 in a direction substantially parallel to the screw 222 into the patient . the continued advancement of the instrument 10 results in the second spring 216 to be further compressed and the distal end 80 of the locking shaft 14 contacts the outer surface of the seat clamp fingers 70 pushing the nibs 76 of the seat clamp 28 into corresponding recesses 234 formed on the outer surface of the seat 226 to lock the seat 226 inside the seat receiving portion 58 of the instrument 10 . turning briefly now to fig2 a , there is shown the seat clamp fingers 70 pushing the nibs 76 of the seat clamp into corresponding recesses 234 formed in the outer surface of the seat 226 . in another variation shown in fig2 b , the nibs 76 are clamped below a lower flange 236 to lock the seat 226 inside the seat receiving portion 58 . in yet another variation shown in fig2 c and 24d , the nibs 76 of the fingers 70 are clamped inside recesses as shown in fig2 c or below the lower flange 236 as shown in fig2 d . additionally , in fig2 c and 24d , a second portion 238 is configured to slide with respect to the fingers 70 and contact the upper surface 232 of the seat 226 to lock the seat 226 to the instrument 10 . referring back to fig2 a , 23 b and 23 c , the pin 204 pops into the pin hole 32 of the cannula 26 to lock the cannula assembly 12 into position and lock the seat 226 inside the cannula 26 . the seat 226 is easily released from the cannula 26 by pulling on the knob 210 . as a result of pulling the knob 210 , the second spring 216 bias forces the seat clamp and the cannula 26 out from the distal end 80 of the locking shaft 14 . if the seat 226 is not released , the resulting position of the instrument is as shown in fig2 c and an unlocked seat 226 is as shown in fig2 b . from the position of the instrument 10 shown in fig2 c , the surgeon places the pinion driver 20 into the pinion bore 176 lining up the hexagonal shapes and inserting until the ball plunger 202 of the pinion driver 20 pops into the locking groove 180 of the pinion 170 . it should be noted that the pinion driver 20 can be inserted on either side of the instrument , thereby , further increasing the ease of installation for the surgeon . the surgeon grabs the handle portion 198 and turns the pinion driver 20 to advance the rack 162 and connected plunger 16 . the plunger 16 moves toward the distal end of the instrument 10 with respect to the body 124 . with further advancement of the plunger 16 , the rod engaging surfaces 108 contact the rod 230 and further advancement of the plunger 16 via the pinion driver 20 draws the rod 230 and bone screw system closer together until the rod 230 is substantially seated inside the rod channel 228 . the degree of advancement of the plunger 16 is conveniently visible through the rack viewing window 146 in which the indicator 168 of the rack 162 is visible to the surgeon as shown in fig2 . the indicator 168 and window 146 advantageously provide a gauge for rod advancement . if the indicator 168 is all the way in a distal position as shown in fig2 , the rod 230 is fully seated . if the indicator 168 is not all the way in a distal position , the indicator 168 shows the surgeon how much more advancement of the plunger 16 is necessary to seat the rod 230 . the surgeon is not required to look inside the patient to check to see if the rod is completely seated . since sometimes the view inside the patient is obstructed by flesh and instruments , the instrument window and indicator facilitate installation for the surgeon and reduce overall installation time . if the rod 230 is not fully seated , the surgeon can continue to advance the pinion driver 20 and simply view the indicator 168 to see if seating is complete . referring now to fig2 , after seating of the rod 230 is completed with the plunger , according to one variation of the invention , a secondary reduction instrument 240 is inserted into the proximal opening 151 of the bore 152 . in one variation of the secondary reduction instrument 240 , the secondary reduction instrument 240 includes a proximal threaded section 242 configured to engage a threaded bore section 244 . the proximal threaded section 242 is connected to a distal piston section 246 configured to enter the seat 226 and rod channel 228 to push the rod 230 deeper into the seat by threaded advancement of the proximal threaded section 242 . this variation highlights the utility and advantage of a configuration with a proximal opening 151 in the body 124 that connects with the bore 60 of the cannula 26 that provides access all the way to the bone screw system . referring now to fig2 a and 27b , after the rod is completely seated , a locking mechanism 248 in the form of a cap or set screw is inserted through the opening 151 in the bore 152 and located on the seat 226 to lock the rod 230 inside the rod channel . in one variation the locking mechanism 248 is in a form of a cap . the locking mechanism 248 is connected to a locking mechanism inserter instrument 250 which is inserted into the opening 151 of the bore 152 of the body 124 as shown in fig2 a . the locking mechanism inserter notches 158 orientate the locking mechanism inserter instrument 250 with respect to the rod reducer 10 . once the locking mechanism 248 is delivered and inserted into the seat 226 , the locking mechanism inserter instrument 250 is turned clockwise , for example , to lock the locking mechanism to the seat 226 . a second portion 252 of the locking mechanism inserter instrument 250 is turned to release the locking mechanism 248 after which the inserter instrument 250 is removed from the bore 152 through opening 151 . referring now to fig2 a and 28b , the trigger 182 is pressed to pivot the rack engaging end 184 away from the rack 162 to retract the plunger 16 via the spring force generated by the first spring 214 . the pinion driver 20 is preferably removed prior to depressing the trigger 182 to avoid rapid de - rotation of the pinion driver 20 as the plunger 16 retracts to a position shown in fig2 b . the plunger 16 is advantageously retracted independent of the release of the seat 226 via pulling the knob 210 of the cannula spring lock system 128 . while keeping the seat 226 locked to the rod reducer instrument 10 , the retraction of the plunger 16 permits the adjustment of the vertebral body via leverage placed on the handle 22 and then followed by another advancement of the plunger to seat the rod via the plunger driver 126 . this process can be repeated to help seat the rod into position in difficult anatomical situations . the preceding merely illustrates the principles of the invention . it will be appreciated that those skilled in the art will be able to devise various arrangements which , although not explicitly described or shown herein , embody the principles of the invention and are included within its spirit and scope . furthermore , all examples and conditional language recited herein are principally intended to aid the reader in understanding the principles of the invention and the concepts contributed by the inventors to furthering the art , and are to be construed as being without limitation to such specifically recited examples and conditions . moreover , all statements herein reciting principles , aspects , and embodiments of the invention as well as specific examples thereof , are intended to encompass both structural and functional equivalents thereof . additionally , it is intended that such equivalents include both currently known equivalents and equivalents developed in the future , i . e ., any elements developed that perform the same function , regardless of structure . the scope of the present invention , therefore , is not intended to be limited to the exemplary embodiments shown and described herein . rather , the scope and spirit of present invention is embodied by the appended claims .
a rod reduction instrument for position a rod relative to a seat of a bone anchor in a spinal implant system is provided . the instrument includes three concentric cannulas with circumferentially aligned rod receiving portions formed therein . one cannula is movable with respect to another to lock and unlock the seat of a bone anchor to the rod reduction instrument . the rod to be reduced is positioned inside at least one of the rod receiving portion . one cannula is moved with respect to another to lock the seat of the bone anchor to the rod reduction instrument . once locked to the bone anchor , the remaining cannula is moved to reduce the distance between the rod and the seat within at least one of the rod receiving portions . the distance between the rod and the seat is reduced until the rod is position inside the seat . a secondary instrument is inserted through a central bore of the rod reduction instrument to introduce a cap and lock the cap to the seat securing the rod to the bone anchor .