Abstract:
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.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and is a continuation-in-part of U.S. Provisional Patent Application Ser. No. 60/919,198 entitled “Rod reducer” filed on Mar. 20, 2007, hereby incorporated by reference in its entirety. 
    
    
     FIELD 
     The present invention generally relates to surgical instruments and methods for using these instruments. More particularly, but not exclusively, instruments and methods for correcting the positioning of and stabilizing one or more bone structures of the spine are disclosed. 
     BACKGROUND 
     In the field of orthopedic spinal surgery, it is well known to correct the positioning of and stabilization of the spine or to facilitate fusion at various levels of the spine after an injury, malformation, or other defect by use of implanted rod systems affixed to vertebral bodies of the spine. In one such system, one or more rods are disposed longitudinally along a length of the spine spanning two or more vertebral bodies. The rod is sometimes bent, either prior to or during surgery, 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 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 is engaged to a number of fixation elements fixed to or engaged with the vertebrae along the spinal column. 
     A variety of fixation elements that are configured to engage the vertebrae can be utilized. For instance, one such fixation element is a polyaxial bone screw with a head or collar which can be threaded into a pedicle or other portion of the vertebral body. Typically, for single level fusion, two bone screws are implanted into adjacent vertebral bodies on one side of the spine and two bone screws are implanted in the same vertebral bodies on the other side of the spine. A rod is provided and coupled to the two bone screws along one side and another rod is provided and coupled to the two bone screws along the other side of the spine. The secured rods provide corrective and stabilizing forces to the spine. 
     Generally, affixing a rod to a bone screw requires the rod to be properly seated in the collar of the anchor assembly. 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 and an implanted screw must be moved with respect to each other so that the rod occupies the space within a channel or other opening in a collar attached to the screw so that the rod can be coupled to the screw. The rod is then coupled to the implanted bone screw using a set screw, plug or other appropriate fastener that is inserted into an opening or channel of the seat. The process of placing a rod within or adjacent to an implanted fixation element so that they can be coupled together is termed “reducing” the rod because the rod and implanted fixation element are drawn together or caused to converge. 
     Rod reduction is commonly performed by a surgeon using his or her hands and/or rigid tools as pliers, levers or other instrumentation adaptable to create the necessary pushing and/or pulling forces on the implanted screw and rod in an open or mini-open surgical procedure. Such procedures generally require the surgeon to place the rod directly over the implanted fixation element and intersect the longitudinal axis of the fixation element. Consequently, access to the rod and the implanted fixation element along that axis and directly above the opening in the fixation element into which the rod is to be placed, is necessary. However, such access can be difficult depending on such factors as the degree to which the patient anatomy needs to be corrected and the overall physiology of the patient, and can be very difficult in procedures in which surgical invasiveness is to be minimized as a result of the small ports or incisions of such procedures. Additionally, the physiology of the patient can require that the screw be placed at an angle such that the surgeon would have difficulty accessing and exerting force in the necessary orientation on the rod and/or fixation element, especially if used with certain screw systems having seats with a limited range of motion. Also, screw systems that have an unsecured rod-receiving portion relative to the bone screw can make reduction a task in minimally invasive surgical procedures. 
     Hence, there is a need for rod reducing instruments that can be used efficiently, safely and securely in rod reduction procedures and there is a need for rod reduction instruments that can be used in both minimally invasive and open surgical approaches. 
     SUMMARY 
     According to one aspect of the invention a rod reduction instrument for positioning a rod relative to a seat of a bone anchor in a spinal implant system is disclosed. The instrument includes a body having a handle at a proximal end of the instrument. The instrument further includes an inner cannula having a first proximal end, a first distal end configured to receive a seat and a first longitudinal axis defining a first bore from the first proximal end to the first distal end. The inner cannula is connected to the body at the first proximal end such that it is movable with respect to the body. The inner cannula has a first rod channel opening at the first distal end and extending towards the first proximal end. The inner cannula has a seat clamp at the first distal end configured to lock the inner cannula to the seat of a bone anchor. The seat clamp comprises at least one deflectable member. The instrument further includes a middle cannula. The middle cannula has a second proximal end, a second distal end and a second longitudinal axis defining a second bore from the second proximal end to the second distal end. The middle cannula is connected to the body at the second proximal end. The middle cannula is positioned at least partially over the inner cannula and movable with respect to the inner cannula. The middle cannula has a second rod channel opening at the second distal end and extending towards the second proximal end. The second rod channel is circumferentially aligned with the first rod channel. The instrument further includes an outer cannula having a third proximal end, a third distal end and a third longitudinal axis defining a third bore from the third proximal end to the third distal end. The outer cannula is positioned at least partially over the middle cannula and movable with respect to the middle cannula. The outer cannula has a third rod channel opening at the third distal end and extending towards the third proximal end. The third rod channel is circumferentially aligned with the first and second rod channels. The instrument is configured to receive the seat of a bone anchor at the first distal end and is configured to lock to the seat at the first distal end with movement of the instrument in a direction towards the seat such that such movement slides the middle cannula distally with respect to the inner cannula and over the at least one deflectable member to thereby deflect the at least one deflectable member inwardly towards the first longitudinal axis and lock the at least one deflectable member to the seat. Movement of the middle cannula proximally with respect to the inner cannula uncovers the at least one deflectable member allowing the deflectable member to spring away from the seat and the first longitudinal axis to thereby unlock the seat from the instrument. With the rod to be reduced located in one of the first or second rod channels and the seat received in the distal end, movement of the outer cannula distally with respect to the middle cannula slides the outer cannula over the middle cannula to engage the third rod channel with the rod and reduce the distance between the rod and the seat. 
     According to another aspect of the invention, a method for positioning a rod relative to a seat of a bone anchor in a spinal implant system is disclosed. The method includes the step of implanting the bone anchor having a seat to a vertebra of a spinal column. The rod is positioned adjacent to the bone anchor and a rod reduction instrument is provided. The rod reduction instrument includes a body having a handle at a proximal end of the instrument. The instrument further includes an inner cannula having a first proximal end, a first distal end configured to receive the seat and a first longitudinal axis defining a first bore from the first proximal end to the first distal end. The inner cannula is connected to the body at the first proximal end such that it is movable with respect to the body. The inner cannula has a first rod channel opening at the first distal end and extending towards the first proximal end. The inner cannula has a seat clamp at the first distal end that is configured to lock the inner cannula to the seat of a bone anchor. The seat clamp has at least one deflectable member. The instrument further includes a middle cannula having a second proximal end, a second distal end and a second longitudinal axis defining a second bore from the second proximal end to the second distal end. The middle cannula is connected to the body at the second proximal end. The middle cannula is positioned at least partially over the inner cannula and movable with respect to the inner cannula. The middle cannula has a second rod channel opening at the second distal end and extending towards the second proximal end. The second rod channel is substantially circumferentially aligned with the first rod channel. The rod reduction instrument further includes an outer cannula having a third proximal end, a third distal end and a third longitudinal axis defining a third bore from the third proximal end to the third distal end. The outer cannula is positioned at least partially over the middle cannula and movable with respect to the middle cannula. The outer cannula has a third rod channel opening at the third distal end and extending towards the third proximal end. The third rod channel is substantially circumferentially aligned with the first and second rod channels. The rod reduction instrument is positioned such that the rod is located in the first, second or third rod channel. The rod reduction instrument is moved towards the bone anchor. The first distal end of the rod reduction instrument is contacted to the seat such that the seat is aligned with at least one of the first, second or third rod channel. After contacting the first distal end of the rod reduction instrument to the seat, the rod reduction instrument is advanced towards the seat to lock the rod reduction instrument to the seat. The outer cannula is advanced over the middle cannula towards the seat to reduce the distance between the rod and the seat. 
     According to another aspect of the present invention, a rod reduction instrument for positioning a rod relative to a seat of a bone anchor in a spinal implant system is disclosed. The rod reduction instrument includes a body having a handle at the proximal end of the instrument, a seat receiving portion at the distal end of the instrument and a rod receiving portion opening at the distal end and extending towards the proximal end. The instrument further includes a seat locking feature configured to lock the rod reduction instrument to a seat in a single push down action by the user. 
     According to another aspect of the present invention, a rod reduction instrument for positioning a rod relative to a seat of a bone anchor in a spinal implant system is disclosed. The instrument includes a body having a handle at the proximal end of the instrument and a seat receiving portion at the distal end of the instrument. The instrument further includes a rod receiving portion opening at the distal end and extending towards the proximal end and a seat locking feature configured to releasably lock the rod reduction instrument to a seat. The instrument further includes a cannula movable with respect to the seat to reduce the distance between the seat and the rod and a cannula locking feature configured to releasably lock the position of the cannula wherein the cannula locking feature is independent from the seat locking feature. 
     According to yet another aspect of the present invention, rod reduction instrument for positioning a rod relative to a seat of a bone anchor in a spinal implant system is disclosed. The instrument includes a body having a handle at the proximal end of the instrument and a seat receiving portion at the distal end of the instrument configured to connect to the seat. The instrument includes a rod receiving portion opening at the distal end and extending at least partially towards the proximal end configured to receive a rod therein. The instrument further includes a cannula movable with respect to the body to reduce the distance between the seat and the rod. The instrument further includes a driver configured to translate the cannula. The driver has a removable driver handle. The removable handle is insertable into the driver on the left side of the instrument or on the right side of the instrument. 
     According to another aspect of the present invention, a rod reduction instrument for positioning a rod relative to a seat of a bone anchor in a spinal implant system is disclosed. The instrument includes a body having a handle at the proximal end of the instrument and a seat receiving portion at the distal end of the instrument configured to connect to the seat. The instrument includes a rod receiving portion opening at the distal end and extending towards the proximal end configured to receive a rod therein. The instrument further includes a cannula movable with respect to the body to reduce the distance between the seat and the rod. The instrument further includes a driver configured to move the cannula to reduce the distance between the seat and the rod. The driver has an indicator providing indication of the degree of reduction to the user. 
     According to another aspect of the invention, a rod reduction instrument for positioning a rod relative to a seat of a bone anchor in a spinal implant system is provided. The rod reduction instrument includes a body having a handle at the proximal end of the instrument and a seat receiving portion at the distal end of the instrument configured to connect to the seat. The instrument includes a rod receiving portion opening at the distal end and extending towards the proximal end configured to receive a rod therein. The instrument further includes a cannula movable with respect to the body to reduce the distance between the seat and the rod. The cannula is configured to engage the rod at the outer edges of the seat. 
     According to another aspect of the invention, a rod reduction instrument for positioning a rod relative to a seat of a bone anchor in a spinal implant system is disclosed. The instrument includes a body having a handle at the proximal end of the instrument and a seat receiving portion at the distal end of the instrument configured to connect to the seat. The instrument includes an elongated rod receiving portion opening at the distal end and extending at least partially towards the proximal end configured to receive a rod therein. The instrument further includes a cannula movable with respect to the body to reduce the distance between the seat and the rod and a bore opening at the proximal end of the instrument and extending to the distal end of the instrument. The central bore is in communication with the seat when connected thereto. The instrument further includes a secondary instrument insertable through the central bore of the instrument without removal of the rod reduction instrument from the seat. 
     According to another aspect of the invention, a method for positioning a rod relative to a seat of a bone anchor in a spinal implant system is disclosed. The method includes the step of engaging the bone anchor having a seat to a vertebra of a spinal column. A rod is positioned adjacent to the bone anchor and a rod reduction instrument is provided. The rod reduction instrument includes a body having a handle at the proximal end of the instrument and a seat receiving portion at the distal end of the instrument configured to connect to the seat. The instrument further includes an elongated rod receiving portion opening at the distal end and extending towards the proximal end configured to receive a rod therein and a cannula movable with respect to the body to reduce the distance between the seat and the rod. The instrument includes a bore opening at the proximal end of the instrument and extending to the distal end of the instrument. The bore is in communication with the seat when connected thereto. The rod reduction instrument is positioned such that the rod is located in the rod receiving portion. The rod reduction instrument is moved towards the bone anchor. The distal end of the rod reduction instrument is connected to the seat. The cannula is moved towards the seat to reduce the distance between the rod and the seat. A cap inserter having a cap attached thereto is inserted into proximal opening of the bore. The cap inserter is turned attaching the cap to the seat and the cap inserter is removed leaving the cap attached to the seat. 
     According to another aspect of the invention, 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 portions. 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 with respect to the other two cannulas 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 positioned 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, thereby, securing the rod to the bone anchor. 
     Other advantages will be apparent from the description that follows, including the drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is best understood from the following detailed description when read in conjunction with the accompanying drawings. It is emphasized that, according to common practice, the various features of the drawings are not to-scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawings are the following figures: 
         FIG. 1  is a perspective view of a rod reduction instrument according to an embodiment of the present invention. 
         FIG. 2  is a side elevational view of a rod reduction instrument without the pinion driver attached to the instrument according to an embodiment of the present invention. 
         FIG. 3  is a side view of three bone screw systems implanted into three respective vertebral bodies, a rod interconnecting the three bone screw systems and rod reduction instrument connected to the middle bone screw system. 
         FIG. 4  is a side elevational view of the cannula assembly of the rod reduction instrument according to the present invention. 
         FIG. 5  is a top view of the cannula according to the present invention. 
         FIG. 6A  is a cross-sectional view of the cannula according to the present invention. 
         FIG. 6B  is a distal end view of the cannula according to the present invention. 
         FIG. 6C  is a proximal end view of the cannula according to the present invention. 
         FIG. 7  is a cross sectional view of the seat receiving portion of the cannula according to the present invention. 
         FIG. 8  is perspective view of the seat clamp of the cannula assembly according to the present invention. 
         FIG. 9  is a cross-sectional view the seat clamp of the present invention. 
         FIG. 10  is a perspective view of the locking shaft according to the present invention. 
         FIG. 11  is a cross-sectional view of the locking shaft according to the present invention. 
         FIG. 12  is a perspective view of the plunger according to the present invention. 
         FIG. 13  is a side elevational view of the plunger according to the present invention. 
         FIG. 14  is a cross-sectional view of the body according to the present invention. 
         FIG. 15  is a side elevational view of the rack of the rack and pinion system according to the present invention. 
         FIG. 16  is a perspective view of the pinion of the rack and pinion system according to the present invention. 
         FIG. 17  is a cross-sectional view of the pinion of  FIG. 16  according to the present invention. 
         FIG. 18  is a cross-sectional view of a trigger according to the present invention. 
         FIG. 19  is a partial cross-sectional view of the body and cross-sectional view of the cannula spring lock system according to the present invention. 
         FIG. 20  is a perspective view of a pinion driver according to the present invention. 
         FIG. 21  is an end elevational view of the pinion driver of  FIG. 19  according to the present invention. 
         FIG. 22  is a cross-sectional view of the rod reduction instrument of  FIG. 2  according to the present invention. 
         FIG. 23A  is a side view of the rod reduction instrument according to the present invention and a bone screw system. 
         FIG. 23B  is a side view of the rod reduction instrument according to the present invention engaging the seat of a bone screw system. 
         FIG. 23C  is a side view of the rod reduction instrument according to the present invention locked onto the seat of a bone screw system. 
         FIG. 24A  is a side view of a seat with gripping fingers inserted into the recesses of the seat according to the present invention. 
         FIG. 24B  is a side view of a seat with gripping fingers clamped below the lower flange of the seat according to the present invention. 
         FIG. 24C  is a side view of a seat with gripping fingers inserted into the recesses of the seat and a second portion of the instrument contacting the top surface of the seat according to the present invention. 
         FIG. 24D  is a side view of a seat with gripping fingers clamped below the lower flange of the seat and a second portion of the instrument contacting the top surface of the seat according to the present invention. 
         FIG. 25  is a partial top view showing the rack window and rack indicator according to the present invention. 
         FIG. 26  is a partial view of a secondary reduction instrument connected to the seat according to the present invention. 
         FIG. 27A  is a partial view of the reduction instrument and locking mechanism inserter according to the present invention. 
         FIG. 27B  is a partial view of the locking mechanism inserted into the bore of the rod reduction instrument according to the present invention. 
         FIG. 28A  is a partial view of the reduction instrument with the plunger in an advanced position according to the present invention. 
         FIG. 28B  is a partial view of the reduction instrument with the plunger in a retracted position according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     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. 
       FIG. 1  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 .  FIG. 2  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  FIG. 3  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  FIG. 3  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  FIG. 3  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  FIG. 4 , 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  FIG. 5 . 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  FIG. 6A ) 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  FIG. 5  is included on the cannula  26  in a location directly opposite from the first finger slot  34 . 
     Referring back to  FIG. 4 , 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  FIG. 4  is formed in a location directly opposite from the first cannula rod channel  50 . 
     Turning back to  FIG. 5 , 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  FIGS. 6A ,  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  FIG. 6A , 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  FIG. 6 , 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 . 
       FIG. 7  shows a cross-sectional area of the seat receiving portion  58 . As seen in  FIG. 7 , 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  FIG. 8 , 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  FIGS. 24A ,  24 B,  24 C and  24 D. 
       FIG. 9  illustrates a cross-sectional view taken longitudinally through the fingers  70  of the seat clamp  28 . As seen in  FIG. 9 , 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  FIGS. 10 and 11 , the locking shaft or middle cannula  14  will now be described.  FIG. 10  shows a perspective view of the locking shaft  14  and  FIG. 11  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  FIGS. 10 and 11  and with particular reference to  FIG. 11 , the locking shaft  14  includes a head portion  90  and a shaft portion  92 . As seen in  FIG. 11 , 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  FIGS. 12 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  ( FIG. 12 ). 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  FIG. 13 . 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  FIG. 13  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  FIG. 14 , 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  FIG. 14 , 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  FIG. 14 . 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  FIG. 25 . 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  FIG. 14 , 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  FIG. 14 , 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  FIG. 1 ) 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  FIG. 14 , 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  FIG. 15 , 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  FIGS. 16 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  FIGS. 16 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  FIG. 17 . Alternatively, a single pinion bore  176  extends end-to-end inside the pinion  170 . As seen in  FIG. 16 , 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  FIG. 18 , 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  FIG. 19 , the cannula spring lock system  128  will now be described.  FIG. 19  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  FIG. 1 , 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  FIG. 1 . 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  FIG. 1 , 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  FIG. 20  and  FIG. 21 , the pinion driver  20  will now be described.  FIG. 20  is a perspective view of the pinion driver  20  and  FIG. 21  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  FIG. 21 , 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  FIG. 22 , 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  FIG. 22 . 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  FIG. 22 , 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  FIGS. 23A-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  FIGS. 23A-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  FIGS. 23A-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  FIGS. 23A-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  FIG. 23A , 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  FIG. 6A ) inside the cannula head  40 . This position of the instrument  10  with respect to the seat  226  is shown in  FIG. 23B  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  FIG. 24A , 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  FIG. 24B , 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  FIGS. 24C and 24D , the nibs  76  of the fingers  70  are clamped inside recesses as shown in  FIG. 24C  or below the lower flange  236  as shown in  FIG. 24D . Additionally, in  FIGS. 24C 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  FIGS. 23A ,  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  FIG. 23C  and an unlocked seat  226  is as shown in  FIG. 23B . 
     From the position of the instrument  10  shown in  FIG. 23C , 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  FIG. 25 . 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  FIG. 25 , 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  FIG. 26 , 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  FIGS. 27A 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  FIG. 27A . 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  FIGS. 28A 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  FIG. 28B . 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.