Abstract:
Minimally invasive methods and instruments for reducing spinal stabilization rods into pedicle screws. In some embodiments, a method comprises positioning an instrument over a pedicle screw where a rod remains proud. A distal end of a first extension of the instrument is engaged with a collar of the pedicle screw. A second extension of the instrument is moved in translation relative to the first extension until a distal end of the second extension contacts the rod. This translation is accomplished by manipulating a pair of handles of the instrument. The rod is urged into the collar of the pedicle screw by application of force on the rod through the distal end of the second extension. The rod can be locked in the collar of the pedicle screw using a set screw passed through a passage in the first extension of the instrument.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of and claims priority to U.S. patent application Ser. No. 12/117,484, filed May 8, 2008, pending, entitled “AN INSTRUMENT FOR THE REDUCTION OF A ROD INTO POSITION IN A PEDICLE SCREW,” which is hereby fully incorporated by reference herein. 
     
    
     TECHNICAL FIELD 
       [0002]    Embodiments of the disclosure relate generally to instruments for spinal surgery and more particularly to instruments for reducing rods of spinal stabilization systems into position in pedicle screws. 
       BACKGROUND 
       [0003]    Modern spine surgery often involves the use of spinal stabilization procedures to correct or treat various acute or chronic spine disorders or to support the spine. Spinal stabilization systems may help, for example, to stabilize the spine, correct deformities of the spine such as spondylolisthesis or pseudarthrosis, facilitate fusion, or treat spinal fractures. Some spinal stabilization systems may provide rigid support for the affected regions of the spine such as when they are used in conjunction with a vertebral body fusion procedure. Some spinal stabilization systems can limit movement in the affected regions in virtually all directions, again, such as when used in conjunction with a vertebral fusion procedure. Dynamic spinal stabilization systems can be provided which can allow the patient a greater range of motion (in terms of flexion, extension, or both) and can better match the patient&#39;s anatomy than some spinal stabilization systems used to provide static support. Dynamic stabilization systems can be used in scenarios in which vertebral body fusion is not desired, in which vertebral body (re)alignment is desired, in which it is desired to support or strengthen degraded, diseased, damaged, or otherwise weakened portions of the spine. 
         [0004]    Often, spinal stabilization systems include rods which can bear a portion of the forces that would otherwise be transmitted along the spine. These rods may be implanted in pairs or in other numbers along portions of the spine of interest. Some stabilization systems support a portion of the spine including only two vertebrae (and associated anatomical structures) while some stabilization systems support portions of the spine extending beyond two vertebrae. Stabilizations systems can be used to support portions of the lumbar spine although stabilization systems can be used to support other portions of the spine such as the thoracic spine. Regardless of the number of rods implanted, or the portion of the spine in which they may be implanted, the rods can be attached to one or more vertebrae of the spine to provide support to, stabilize, align, or otherwise treat the region of the spine of interest. Many times, surgical personnel use one or more anchor systems to attach the rods to one or more vertebrae. One such anchor system includes pedicle screws constructs which define slots, keyways, grooves, apertures, or other features for accepting and retaining stabilization rods (static, dynamic, or both). In many pedicle screw constructs, pedicle screws are placed in vertebrae selected by surgical personnel. 
         [0005]    Sometimes it happens that a rod (or more than one rod) remains proud of its desired or final position in the rod slot of the screw head by some height or distance. Such scenarios include surgical procedures in which it is desired to anchor a rod to more than one vertebra. One such scenario can occur when pedicle screws have been implanted in two vertebrae and it is desired to anchor a rod to a third vertebra lying between the two vertebrae. In this, and other scenarios, a rod reduction instrument can be navigated to the implant site by surgical personnel to correct this situation by urging the rod into position in the pedicle screw. When surgical personnel are using previously available rod reduction instruments, their view of the surgical site can be blocked by the body of the instrument. In other situations, the actuation handles of the instrument rotate into the line of sight of the surgical personnel. As they attempt to reduce the rod into its desired position and lock the rod in place, surgical personnel sometimes cannot see portions of the surgical site or spinal stabilization system. In some scenarios, reduced visibility of the implant site can result in slower, less efficient, and less accurate surgical results than desired. Yet, with previously available rod reduction instruments, little can be done to aid surgical personnel in this situation. The situation can be aggravated when the patient is abnormally large. 
       SUMMARY 
       [0006]    Embodiments of the present disclosure provide rod reduction instruments for spinal stabilization systems that eliminate, or at least substantially reduce, the shortcomings of prior art rod reduction instruments. 
         [0007]    Various embodiments provide rod reduction instruments for use with pedicle screws and other bone anchors to improve visibility of the implant site during implant procedures. In some embodiments, the handles of the extension can have a parallel action, with extensions perpendicular to the parallel action of the handles. When squeezed, the handles can push one extension through the other. One of the extensions can have geometry corresponding to that of the pedicle screws and symmetric collapsible slots associated with the mating geometry to aid in grasping the pedicle screw. 
         [0008]    The other extension can form a cylinder surrounding the first extension and can include features for collapsing the end of the first extension as the handles are compressed. The end of the second extension can have geometry corresponding to a rod to be seated in the pedicle screw. Both extensions can define cannulas. The cannula of the first extension can accept a setscrew and locking driver whereas the cannula of the second extension can accept the first extension. 
         [0009]    Various embodiments provide instruments, instrument kits, and methods for reducing rods of spinal stabilization systems into place. Embodiments include an instrument for the reduction of a rod into position in a pedicle screw. The instrument can comprise an inner extension, an outer extension, a first handle and a second handle. The inner extension can have proximal and distal ends with the distal end of the inner extension adapted to engage the pedicle screw. The outer extension can be positioned about the inner extension and have proximal and distal ends and a longitudinal axis. The distal end of the outer extension can be adapted to engage the rod. The first handle can have proximal and distal ends, with the distal end of the first handle being coupled with the proximal end of the inner extension. The second handle can have proximal and distal ends with the distal end of the second handle being coupled with the proximal end of the outer extension. The handles can extend generally perpendicular from the respective extensions and be operatively coupled to form an actuator for urging the extensions in a direction relative to each other along the longitudinal axis. The handles can be offset from the proximal end of at least one of the extensions. 
         [0010]    The offset can be greater than about half an inch in a direction parallel to the longitudinal axis. The offset can be about two and one half inches in a direction perpendicular to the longitudinal axis. The instrument can have a locking mechanism extending between the handles. The locking mechanism can include a ratchet with a toothed member extending between the first and second handles. The locking mechanism can include a pawl operatively coupled to one of the handles to engage the toothed member to prevent the handles from moving apart from one another. The instrument can include a biasing member coupled to the handles to urge the handles apart. The biasing member can be a leaf spring positioned to urge the pawl to engage the toothed member. The leaf spring can extend beyond the end of one of the handles to allow a user to disengage the pawl from the toothed member. The inner extension can define a passage to allow a compression member to be inserted through the passage. The instrument can include a parallel action compressor. 
         [0011]    Embodiments include a kit for the reduction of a rod into position in a pedicle screw. The kit can include a rod reduction instrument, a set of pedicle screws, and a set of rods. The kit can include a set of setscrews for locking a rod in position in a pedicle screw. The inner extension of the rod reduction instrument can have a cannulation for accepting the setscrew. The kit can include a locking driver for locking the rod in position in the pedicle screw using a setscrew. The instrument can include an offset in a direction parallel to a longitudinal axis of the instrument, perpendicular to the longitudinal axis, or a combination thereof. 
         [0012]    Embodiments include an instrument for the reduction of a rod into position in a pedicle screw. The instrument can comprise a female extension, a male extension, a first handle and a second handle. The female extension can have proximal and distal ends with the distal end of the female extension adapted to engage the pedicle screw. The male extension can be positioned about the female extension and have proximal and distal ends and a longitudinal axis. The distal end of the male extension can be adapted to engage the rod. The first handle can have proximal and distal ends, with the distal end of the first handle being coupled with the proximal end of the female extension. The second handle can have proximal and distal ends with the distal end of the second handle being coupled with the proximal end of the male extension. The handles can extend generally perpendicular from the respective extensions and be operatively coupled to form an actuator for urging the extensions in a direction relative to each other along the longitudinal axis. The handles can be offset from the proximal end of at least one of the extensions. 
         [0013]    These, and other, aspects will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. The following description, while indicating various embodiments and numerous specific details thereof, is given by way of illustration and not of limitation. Many substitutions, modifications, additions, or rearrangements may be made within the scope of the disclosure, and the disclosure includes all such substitutions, modifications, additions, or rearrangements. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    A more complete understanding of the disclosure and the advantages thereof may be acquired by referring to the following description, taken in conjunction with the accompanying drawings in which like reference numbers generally indicate like features and wherein: 
           [0015]      FIG. 1  illustrates a spinal stabilization system including rods and pedicle screws according to some embodiments. 
           [0016]      FIG. 2  illustrates a rod reduction instrument being used in a rod reduction procedure according to some embodiments. 
           [0017]      FIG. 3  illustrates a rod reduction instrument according to some embodiments. 
           [0018]      FIG. 4  illustrates a handle of a rod reduction instrument of some embodiments. 
           [0019]      FIG. 5  illustrates a portion of a handle of a rod reduction instrument of some embodiments. 
           [0020]      FIG. 6  illustrates a second handle of a rod reduction instrument of some embodiments. 
           [0021]      FIG. 7  illustrates a portion of a second handle of a rod reduction instrument of some embodiments. 
           [0022]      FIG. 8  illustrates a cross sectional view of a locking mechanism for a pair of handles of a rod reduction instrument of some embodiments. 
           [0023]      FIG. 9  illustrates a female bar of a rod reduction instrument of some embodiments. 
           [0024]      FIG. 10  illustrates a male bar of a rod reduction instrument of some embodiments. 
           [0025]      FIG. 11  illustrates a biasing member of a rod reduction instrument of some embodiments. 
           [0026]      FIG. 12  illustrates a portion of a biasing member of a rod reduction instrument of some embodiments. 
           [0027]      FIG. 13  illustrates a second biasing member of a rod reduction instrument of some embodiments. 
           [0028]      FIG. 14  illustrates a portion of a second biasing member of a rod reduction instrument of some embodiments. 
           [0029]      FIG. 15  illustrates another portion of a biasing member of a rod reduction instrument of some embodiments. 
           [0030]      FIG. 16  illustrates an offset arm of a rod reduction instrument of some embodiments. 
           [0031]      FIG. 17  illustrates another offset arm of a rod reduction instrument of some embodiments. 
           [0032]      FIG. 18  illustrates an inner extension distal end of a rod reduction instrument of some embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0033]    Various embodiments of the disclosure are illustrated in the FIGURES, like numerals being used to refer to like and corresponding parts of the various drawings. Embodiments of the disclosure provide spinal stabilization system rod reduction instruments and methods. 
         [0034]    As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having” or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, process, article, or apparatus that comprises a list of elements is not necessarily limited only those elements but may include other elements not expressly listed or inherent to such process, process, article, or apparatus. Further, unless expressly stated to the contrary, “or” refers to an inclusive or and not to an exclusive or. For example, a condition A or B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present). 
         [0035]    Additionally, any examples or illustrations given herein are not to be regarded in any way as restrictions on, limits to, or express definitions of, any term or terms with which they are utilized. Instead, these examples or illustrations are to be regarded as being described with respect to one particular embodiment and as illustrative only. Those of ordinary skill in the art will appreciate that any term or terms with which these examples or illustrations are utilized will encompass other embodiments which may or may not be given therewith or elsewhere in the specification and all such embodiments are intended to be included within the scope of that term or terms. Language designating such nonlimiting examples and illustrations includes, but is not limited to: “for example”, “for instance”, “e.g.”, “in one embodiment”. 
         [0036]      FIG. 1  illustrates a spinal stabilization system secured to several vertebrae  10  with an anchor system. The spinal stabilization system shown in  FIG. 1  can be implanted with a posterior approach in which an incision is made in the patient. An anchor system can then be implanted in the patient&#39;s spine and spinal stabilization systems can be placed in one or more of the anchor systems. The rods may then be secured to the anchor systems and, if necessary, reduced into their final positions in the anchor systems. The rods may then be locked to the anchor systems and any instruments used to implant the spinal stabilization system can be removed from the patient. The surgical site may then be closed. 
         [0037]    In various embodiments, pedicle screws  12  can secure rods  14  in place to support the spine. Only one pair of stabilization rods  14  is shown in  FIG. 1 . However, one skilled in the art will appreciate that different numbers of rods  14  may be utilized in various spinal procedures. As illustrated in  FIG. 1 , rods  14  can be fixed to selected vertebrae  10  of the spine laterally on opposite sides of the spine utilizing pedicle screws  12 . 
         [0038]    In some embodiments, any type of anchor system could be used instead of, or in addition to, pedicle screws  12 . Anchor systems which can be used include pedicle screws  12 , hooks, wires, etc. Pedicle screws  12  and rods  14  can be made from biocompatible material(s). Examples of biocompatible materials include titanium, stainless steel, and any suitable metallic, ceramic, polymeric, and composite materials. 
         [0039]    The spinal stabilization system illustrated in  FIG. 1  can be installed posterior to the spine, typically with the rods  14  extending parallel to the longitudinal axis of the spine lying in the mid-sagittal plane. According to some embodiments, the system can include additional rods  14  positioned further superior or inferior along the spine, with additional rods  14  being dynamic stabilization rods such as rods  14 , or other types of non-dynamic, or rigid, rods  14 . It should be understood that various spinal stabilization system may also include suitable transverse rods or cross-link devices that help protect the supported portion of the spine against torsional forces or movement. Some possible examples of suitable cross-link devices are shown in co-pending U.S. patent application Ser. No. 11/234,706, entitled “Apparatus And Methods For Spinal stabilization system With Variable Link Mechanism”, filed on Nov. 23, 2005 by Robert J. Jones et al., the entire contents of which are incorporated herein as if set forth in full. Other cross-link devices or transverse rods may also be employed. According to some embodiments, rods  14  can be configured to possess sufficient column strength and rigidity to protect the supported portion of the spine against lateral and torsional forces or movement. In some cases the vertebrae Rod reduction instruments of various embodiments can be used in scenarios involving the spinal stabilization system illustrated by  FIG. 1  as well as other spinal stabilization systems to move a pedicle screw and hence vertebra relative to a rod to seat the rod in the pedicle screw. 
         [0040]    With reference now to  FIG. 2 ,  FIG. 2  illustrates rod reduction instrument  100  in use during a rod reduction procedure.  FIG. 2  illustrates vertebrae  10  with pedicle screw  12  implanted therein and rod  14  proud of its intended position in pedicle screw  12  by height h 1  (the distance of rod  14  from a position at rest in pedicle screw  12 , not labeled). By the term “proud” it can be meant that rod  14  is some distance away from the bottom (as viewed in  FIG. 2 ) of pedicle screw  12 . Rod reduction instrument  100  can include inner extension  102 , outer extension  104 , handles  106 , parallel action compressor  108 , etc. Inner extension  102  and outer extension  104  can be annular (or have other cross sectional shapes), coaxial, and can have distal ends adapted to engage, respectively, pedicle screw  12  and rod  14  such that inner and outer extensions  102  and  104  can urge pedicle screw  12  and rod  14  together during rod reduction procedures. Outer extension  104 , of some embodiments, can include features to radially compress the distal end of inner extension  102 , thereby assisting inner extension  102  in grasping pedicle screw  12  and retaining pedicle screw  12  while rod  14  is reduced into position. Inner and outer extensions  102  and  104  can be operatively coupled to an actuator such as parallel action compressor  108 . Parallel action compressor  108  can be operatively coupled to handles  106 . However, those skilled in the art will appreciate that many types of actuation systems can be employed in lieu of parallel action compressor  108 . 
         [0041]    Longitudinal axis  110 , along which inner and outer extensions  102  and  104  can be oriented, is also shown in  FIG. 2 . During rod reduction procedures, surgical personnel can align inner extension  102  with rod  14  so that features such as a slot, recess, keyway, groove, etc. on inner extension  102  will receive rod  14  when surgical personnel advance instrument  100  along longitudinal axis  110 . Surgical personnel can then advance inner and outer extensions  102  and  104  along longitudinal axis  110  toward pedicle screw  12  and rod  14  until the distal end of inner extension  102  contacts pedicle screw  12 . 
         [0042]    Surgical personnel can urge handles  106  together to cause handles  106  and parallel action compressor  108  to operate to urge inner and outer extensions  102  and  104  in directions opposite each other along longitudinal axis  10 . Surgical personnel can urge handles  106  together while maintaining contact between pedicle screw  12  and inner extension  102 . As surgical personnel urge handles  106  together, the distal end of outer extension  104  can begin radially compressing the distal end of inner extension  102  thereby causing inner extension  102  to grasp pedicle screw  12 . As outer extension  104  continues translating along longitudinal axis  110 , the distal end of outer extension  104  can contact rod  14  and urge it into its desired position in pedicle screw  12 . A setscrew or locking or compression member may then be slid through a longitudinal cannula defined by the body of inner extension  102 . The compression member can then be used to lock rod  14  into position in pedicle screw  12 . Surgical personnel may then urge handles  106  apart (or allow a biasing member to do so) causing outer extension  104  to translate back along longitudinal axis  110  relative to inner extension  102 , thereby releasing the grasp which inner extension  102  had on pedicle screw  12 . Surgical personnel may then translate instrument  100  away from pedicle screw  12 . 
         [0043]      FIG. 2  also illustrates offset distances d 1  and d 2  which can extend the overall distance between handles  106  and inner and outer extensions  102  and  104  thereby allowing surgical personnel improved visibility of the distal end of instrument  100 , pedicle screw  12 , rod  14 , and the overall surgical site during all phases of rod reduction procedures. Offset distance d 1  can be along a direction parallel to longitudinal axis  110  while offset distance d 2  can be along a direction perpendicular to longitudinal axis  110 . Those skilled in the art will understand that offsets d 1  and d 2  can be provided by a variety of differently shaped components of instrument  100  and, when used in conjunction with each other, can provide an offset in a direction diagonal to longitudinal axis  110 . Offsets d 1  and d 2  each individually, and taken together, can allow surgical personnel who may be grasping handles  106  to peer around, over, or beyond the proximal ends  123  and  125  of inner and outer extensions  102  and  104  to view pedicle screw  12  and rod  14  even when inner and outer extensions  102  and  104  engage the same as when surgical personnel are reducing rod  14  into position. With improved visibility of the surgical site, according to some embodiments, surgical personnel can operate more quickly, efficiently, and accurately. 
         [0044]    With reference now to  FIG. 3 ,  FIG. 3  illustrates a number of features of instrument  100 . Among other features,  FIG. 3  illustrates inner extension  102 , outer extension  104 , handles  106 , parallel action compressor  108 , female bar  109 A, male bar  109 B, longitudinal axis  110 , offset arms  112 , diagonal portions  114 , biasing members  116 , toothed member  118 , ratchet assembly  120 , distal end  122  of inner extension  102 , proximal end  123  of inner extension  102 , distal end  124  of outer extension  104 , proximal end  125  of outer extension  104 , outer ratchet holder  832 , inner ratchet holder  834 , and ratchet pawl  836 . Ratchet components  832 ,  834 , and  836  will be discussed further with reference to  FIG. 8 . 
         [0045]    Inner extension  102  can be elongated along longitudinal axis  110  with distal end  122  being adapted to engage pedicle screw  12  (of  FIG. 1 ) or another bone anchor system. Distal end  122  can be made of a pliant material or otherwise be configured to be pliant so that distal end  124  of outer extension  104  can radially compress distal end  122  to cause it to grasp pedicle screw  12 . Distal end  122  can be adapted to receive rod  14  in a proud position relative to pedicle screw  12  via a slot or other feature. Inner extension  102  can define a cannula through which a setscrew or compression or locking member, etc. can be passed to lock rod  14  in place in pedicle screw  12 . Inner extension can have a user selected length which allows surgical personnel to maneuver distal ends  122  and  124  to a surgical site where pedicle screw  12  and rod  14  may have been previously implanted. 
         [0046]    Outer extension  104  can be elongated along longitudinal axis  110  and can define a cannula into which inner extension  102  can fit. Outer extension  104  can be coaxial with inner extension  102 . The side of outer extension  104  toward parallel action compressor  108  can be open so that, when inner and outer extensions  102  and  104  translate relative to each other, offset arm  112 A can move without interference from outer extension  104 . Distal end  124  may also include features to compress distal end  122  of inner extension  102  to cause it to grasp pedicle screw  12  as outer extension  104  moves toward rod  14 . Distal end  124  of outer extension  104  can have a groove, slot, etc. with which it can engage rod  14 . In some embodiments, inner extension  102  (which can receive rod  14 ) can be referred to as female extension  102 . Distal end  124  of outer extension  104  can be configured to urge rod  14  into position in pedicle screw  12 . In some embodiments, outer extension  104  can be referred to as male extension  104 . When assembled with inner extension  102 , distal end  124  of outer extension  104  can be positioned generally adjacent features on inner extension  102  for receiving rod  14  as illustrated by  FIG. 3 . Proximal end  125  of outer extension  104  can extend beyond proximal end  123  of inner extension  102  a distance sufficient so that when surgical personnel operate instrument  100 , outer extension  104  can translate relative to inner extension  102  to reduce rod  14  into position. 
         [0047]    Proximal ends  123  and  125  of inner and outer extensions  102  and  104  can couple to offset arms  112 B and  112 A respectively. The coupling of proximal ends  123  and  125  to offset arms  112 B and  112 A can be by way of interference fits, mechanical couplings, fasteners, welds, brazes, solder, etc. Offset arms  112  can extend from inner and outer extensions  102  and  104  a distance d 2  in a direction perpendicular to longitudinal axis  110 . In some embodiments, offset arms  112  can include diagonal portions  114  extending in a direction which is diagonal relative to longitudinal axis  110  thereby providing an offset of distance d 1  in the direction parallel to longitudinal axis  110 . In some embodiments, offset distances may be defined by members having other shapes. The proximal ends of offset arms  112  can operatively couple with parallel action compressor  108  in such a manner that parallel action compressor  108  can actuate offset arms  112 A and  112 B. Being coupled with female (inner) extension  102 , offset arm  112 A can be referred to as female offset arm  112 A in some embodiments. Being coupled with male (outer) extension  104 , offset arm  1128  can be referred to as male offset arm  112 B in some embodiments. The proximal ends of offset arms  112 A and  112 B can couple to the distal ends of bars  109 A and  109 B of parallel action compressor  108 . 
         [0048]    Bars  109 A and  109 B can form the parallel linkages of parallel action compressor  108 . Bars  109 A and  109 B can extend from offset arms  112 A and  112 B in a direction perpendicular to longitudinal axis  110  thereby further offsetting handles  106  from inner and outer extensions  102  and  104 . Bar  109 A can be coupled with female offset arm  112 A and can be referred to as female bar  109 A in some embodiments. Bar  109 B can be coupled with male offset arm  112 B and can be referred to as male bar  109 B in some embodiments. As viewed from the side of instrument  100 , offset arm  112 A and bar  109 A can have shapes corresponding to the shapes of offset arm  112 B and bar  112 B respectively and can be parallel thereto. While offset arms  112 A and  112 B are shown with a portion oriented diagonal to longitudinal axis  110 , other configurations of offset arms  112  are envisioned. In some embodiments, offset arms  112  can be curved, be “S” shaped, have a stair-step profile when viewed from the side, etc. 
         [0049]    Handles  106 A and  106 B can pivotably couple to bars  109 A and  109 B at the distal ends of bars  109 A and  109 B; slidably engage bars  109 A and  109 B at the proximal ends of bars  109 A and  109 B; and pivotably couple to each other between bars  109 A and  109 B (as shown in  FIG. 3 ) to form the input linkages of parallel action compressor  108 . Handles  106  can extend perpendicularly from longitudinal axis  110  a distance beyond parallel action compressor  108 . Handles  106  may be curved, have knurls, and other ergonomic features to facilitate operation of instrument  100  by surgical personnel. Biasing members  116  may be coupled to handles  106  to bias handles  106  apart thereby biasing the distal end  124  of outer extension  104  away from distal end  122  of inner extension  102  via parallel action compressor  108  and offset arms  112 A and  112 B. Biasing members  116  can be leaf springs, coil springs, torsional springs, etc. In some embodiments, biasing members can curve an appropriate amount (or otherwise be configured) to yield a user selected biasing force over the travel span of handles  106 . In some embodiments, biasing member(s) can be supplied within parallel action compressor  108  or between offset arms  112 A and  112 B. 
         [0050]    Instrument  100  of  FIG. 3  can also include ratchet assembly  120  including toothed member  118  and ratchet components  832 ,  834 , and  836 . Ratchet assembly can be statically coupled to one handle  106 A and operatively coupled to the other handle  106 B so that as surgical personnel urge handles  106 A and  106 B together, ratchet assembly  120  prevents handles  106  from moving apart. Instrument  100  can include features, as discussed herein, to release ratchet assembly  120  to allow handles  106  to move together. 
         [0051]    To operate instrument  100  of  FIG. 3 , surgical personnel may grasp handles  106  which can be biased apart by biasing members  116 . Surgical personnel may navigate distal ends  122  and  124  of inner and outer extensions  102  and  104  to a spinal stabilization system such as the one illustrated in  FIG. 1 . During the navigation of distal ends  122  and  124  to the surgical site, and at other times, surgical personnel can peer over, around, or beyond proximal ends  123  and  125  of inner and outer extensions  102  and  104  (due in part to offset distances d 1  and d 2 ) to view the surgical site, the spinal stabilization system, vertebrae  10 , and distal ends  122  and  124  of inner and outer extensions  102  and  104 . Such improved visibility provided by various embodiments can aid surgical personnel in operating more swiftly, efficiently, and accurately than here to for possible with previously available instruments. 
         [0052]    In some scenarios, surgical personnel may have previously placed the spinal stabilization system at a surgical site on a patient&#39;s spine through a posterior incision. More particularly, surgical personnel may have implanted a number of pedicle screws  12  in vertebrae  10  and placed rods  14  in one or more pedicle screws  12  (as illustrated in  FIG. 1  and  FIG. 2 ). Surgical personnel may align distal end  122  of inner extension  102  to accept rod  14  and advance instrument  100  until rod  14  is within distal end  122  of inner extension  102 . When desired, surgical personnel can peer over or around proximal ends  123  and  125  while aligning distal end  122  with rod  14 . Surgical personnel may advance distal end  122  to pedicle screw  12 , place distal end  122  thereon, and begin pressing handles  106  together. Again, surgical personnel can view the surgical site all the while if desired. 
         [0053]    Distal end  124  of outer extension  104  may advance toward distal end  122  of inner extension  102  via the action of parallel action compressor  108  as transmitted to inner and outer extensions  102  and  104  by offset arms  112 . As distal end  124  of outer extension  104  advances toward and over distal end  122  of inner extension  102 , features on distal end  124  can radially compress distal end  122  causing it to grip pedicle screw  12 . Features of distal end  124  can engage rod  14  pushing it toward and into pedicle screw  12 . Ratchet pawl  836  of ratchet assembly  120  can slidably engage teeth of toothed member  118  as handles  106  move toward each other. If surgical personnel release pressure on handles  106 , biasing members  116  can urge handles  106  apart until a tooth on toothed member  118  engages ratchet pawl  836  thereby maintaining handles  106  and distal ends  122  and  124  of inner and outer extensions  102  and  104  in substantially the same position they were in when the pressure on handles  106  was released. 
         [0054]    Surgical personnel can resume (or continue, as the case may be) pressing handles  106  together, advancing distal end  124  over distal end  122 , and seating rod  14  within pedicle screw  12 . As stated previously, surgical personnel can view the surgical site and visually confirm that rod  14  is seated in pedicle screw  12 . If for some reason, rod  14  has not seated properly in pedicle screw  12 , surgical personnel can release ratchet assembly  120  by pressing on biasing member extension  1546  (discussed with reference to  FIG. 15 ), translate distal end  124  of outer extension  104  away from pedicle screw  12  and again attempt to seat rod  14  in pedicel screw  12 . 
         [0055]    With previously available instruments engaged with pedicle screw  12  and rod  14 , it could happen that surgical personnel might not have been able to verify proper seating of rod  14  in pedicle screw  12  because handles, extensions, or other portions of these instruments obstructed the surgical personnel&#39;s view of the surgical site. In some scenarios, surgical personnel could only confirm the seating of rod  14  in pedicle screw  12  indirectly such as be tactile sensations transmitted from pedicle screw  12  and rod  14  through the instrument. As a result, in some scenarios, rod  14  might have been left in an improper position such as being proud of its intended position in pedicle screw  12  by some distance. In some scenarios, surgical personnel had to disengage previously available instruments from pedicle screw  12  and rod  14  to verify proper alignment therebetween. 
         [0056]    If surgical personnel determined that pedicle screw  12  and rod  14  were misaligned, little could be done to correct the situation other than manually disengaging rod  14  from pedicle screw  12 . Such manual intervention sometimes necessitated enlarging the incision to gain greater access to the surgical site and, more particularly, pedicle screw  12  and rod  14 . Enlarging the incision and prying rod  14  from pedicle screw  14 , in some scenarios, could damage pedicle screw  12  and/or rod  14 , increase trauma to the surgical site, prolong surgery (and the need for local and/or general anesthesia, cause greater patient discomfort, and prolong recovery time. In some scenarios, surgical personnel might desire instead to leave pedicle screw  12  and rod  14  misaligned resulting in less than optimal performance of the spinal stabilization system and a diminishment of the relief from the condition being treated which had been sought by the patient. These results can be avoided if surgical personnel use rod reduction instruments  100  constructed in accordance with various embodiments. 
         [0057]    With rod  14  confirmed as being seated in pedicle screw  12 , surgical personnel can release pressure on handles  106 A and  106 B of instrument  100  thereby allowing ratchet assembly  120  to maintain distal ends  122  and  124  of inner and outer extensions  102  and  104 , and therefore pedicle screw  12  and rod  14 , in position relative to one another for as long as it takes for surgical personnel to confirm proper alignment of pedicle screw  12  and rod  14  or as long as desired. When desired, surgical personnel can then advance a set screw or compression or locking member through the cannula of inner extension  102 . Surgical personnel can use the compression member to lock rod  14  in its desired and perhaps confirmed position in pedicel screw  12 . Surgical personnel can then remove any instrument or locking driver (with which they locked the compression member in place) from the cannula of inner extension  102 . If desired, surgical personnel can release ratchet assembly  120  and draw handles  106  apart, thereby withdrawing distal end  124  of outer extension  104  from distal end  122  of inner extension  102 . As distal end  124  withdraws, it allows distal end  122  to expand radially, thereby releasing the grasp of distal end  122  from pedicle screw  12 . Surgical personnel may then withdraw instrument  100  from the surgical site and close the same. 
         [0058]    With reference now to  FIGS. 4-7 , handles  106 A and  106 B can include pairs of coupling points  426  and  626  on the portions of handles  106 A and  106 B, respectively, which can pivotably couple with bars  109 A and  109 B or slidably engage bars  109 A and  109 B as the case may be (handles  106 A and  106 B are illustrated pivotably coupled to bars  109 A and  109 D in  FIG. 3 ). Handles  106 A and  106 B can also include coupling points  428  and  628  in the portions of handles  106 A and  106 B where handles  106 A and  106 B pivotably couple to each other. Coupling points  426 ,  428 ,  626 , and  628  can be adapted to receive pins, bolts, tabs, projections, etc. as may be appropriate for the type of engagement (pivotable coupling, sliding engagement, etc.) which coupling points  426 ,  428 ,  626 , and  628  facilitate between handles  106 A and  106 B and bars  109 A and  109 B and between handles  106 A and  106 B themselves. With particular reference to  FIG. 4 , handle  106 A can have a thumb grip  432  or other ergonomic features to facilitate surgical personnel grasping and actuating instrument  100 . Handle  106 A can also include features  434  to fixedly attached toothed member  118  (see  FIG. 3 ) to handle  106 A. Features  434  can be fasteners, detents, etc. 
         [0059]    Handles  106 A and  106 B can also include slots  430  and  630  as shown in  FIGS. 5 and 7 . Slots  430  and  630  can be shaped to accommodate the movements of handles  106 A and  106 B and bars  109 A and  109 B as surgical personnel move handles  106 A and  106 B to operate instrument  100 . Slot  630  of handle  106 A can have a generally uniform cross section (as seen from above in  FIGS. 1-3 ) to allow one a portion of handle  106 A to pass partially through handle  106 B as handles  106 A and  106 B are actuated. Slot  430  of handle  106 A can allow a portion of handle  106 B to pass partially through handle  106 A as handles  106 A and  106 B are actuated. Slot  430  can have an enlarged portion through which a portion of bars  109 A and  109 B can pass as handles  106 A and  106 B are actuated. 
         [0060]    Ratchet assembly  120  is further illustrated in  FIG. 8 .  FIG. 8  shows the proximal end of handle  106 B to which ratchet assembly  120  can be operatively coupled in some embodiments. Ratchet assembly  120  can include outer ratchet holder  832 , inner ratchet holder  834 , and ratchet pawl  836 . Ratchet pawl  836  can be a pin, small rod, tab, etc. As will be discussed herein, ratchet assembly  120  can cooperate with toothed member  118  and extension  1546  (see  FIG. 15 ) of biasing member  116 B to allow handles  106  to ratchet apart from each other. Biasing member extension  1546  can extend beyond the proximal end of handle  106 B and engage ratchet pawl  836 . Biasing member  116 B can include pawl release feature  1548  on extension  1546  (see  FIG. 15 ). Toothed member  118  can extend to, or beyond, handle  106 B and ratchet assembly  120 . Toothed member  118  may have a radius of curvature corresponding to the location of ratchet pawl  836  relative to coupling points  428  and  628  and on handle  106 B (see  FIG. 3 ) so that when handles  106  pivot relative to each other, toothed member  118  remains engaged with ratchet pawl  836 . Biasing members  116 , by biasing handles  106 A and  106 B apart in some embodiments, can assist with keeping ratchet pawl  836  engaged with various teeth of toothed member  118 . 
         [0061]    Outer ratchet holder  832  can couple ratchet assembly  120  to handle  106 B. Inner ratchet assembly  834  can couple to outer ratchet holder  832  and provide guides for toothed member  118  and extension  1346 . In operation, as surgical personnel urge handles  106 A and  106 B together, ratchet pawl  836  slidably engages various teeth of toothed member  118 . When handles  106 A and  106 B begin to move apart, ratchet pawl  836  engages the tooth upon which it rests in such a manner that ratchet pawl  836  prevents movement of toothed member  118  and, hence, handles  106 A and  106 B. When handles  106 A and  106 B again move toward each other, ratchet pawl  836  can again slidably engage various teeth of toothed member  118 . When desired, surgical personnel may urge pawl release feature  1548  into contact with the portion of toothed member currently beside ratchet assembly  120 . Doing so can cause pawl release feature  1548  to urge toothed member  118  away from ratchet assembly  120 , thereby releasing ratchet pawl  836  from engagement with toothed member  118 . 
         [0062]      FIGS. 9 and 10  illustrate bars  109 A and  109 B which can form parallel linkages of parallel action compressor  108 . Bars  109 A and  109 B can include slots  938  and  1048  for slidably engaging handles  106 A and  106 B with any of various pins, bolts, tabs, projections, etc. In some embodiments, bars  109 A and  10 B include attachments points  940  and  1040  for coupling handles  106 A and  106 B to bars  109 A and  109 B. Attachment points  942  and  1042  of  FIGS. 9 and 10  can be adapted to receive pins, bolts, tabs, projections, etc. to pivotably couple handles  106 A and  106 B and bars  109 A and  109 B. Since bars  109 A and  109 B couple indirectly with female (inner) extension  102  and male (outer) extension  104  through offset arms  112 A and  112 B, bars  109 A and  109 B can be referred to as female bar  109 A and male bar  109 B. Bars  109 A and  109 B can be straight with generally rectangular cross sections generally free of features which might interfere with handles  106 A and  106 B as handles  106 A and  106 B are actuated. 
         [0063]      FIGS. 11-15  illustrate biasing members  116 . Biasing member  116 A can couple to handle  106 A at one end and to biasing member  116 B at the other end. Biasing member can couple to handle  106 B at one end and to biasing member  116 A at the other end. The coupling of biasing members  116  to handles  106 A and  106 B can be by any number of coupling techniques including, but not limited to, screws, bolts, pop rivets, brazes, welds, clamps, etc. In some embodiments, biasing members  116 A and  116 B can include couplings  1244  and  1444  (of  FIGS. 12 and 14 ) at their ends opposite the coupling to handles  106 A and  106 B. Couplings  1244  and  1444  can be used to couple biasing members  116 A and  116 B to each other. In some embodiments, couplings  1244  and  1444  can disengage from each other to release the biasing force on handles  106 A and  106 B caused by biasing members  116 . 
         [0064]    With more particular reference to  FIGS. 13 and 15 , biasing member  116 B can include extension  1546 . Extension  1546  can be a straight portion of biasing member  116 B which, when assembled into instrument  100 , can extend beyond ratchet assembly  120  (see  FIG. 8 ) and beyond the proximal end of handle  106 B. Extension  1546  can include pawl release feature  1548  (see  FIG. 15 ) so that by pressing on extension  1546 , surgical personnel can urge biasing member  116  away from the proximal end of handle  106 B and ratchet pawl  836  thereby releasing ratchet assembly  120  (see  FIG. 8 ) and allowing handles  106 A and  106 B to be drawn apart if desired. Surgical personnel can remove pressure from ratchet release feature  1548  to allow toothed member  118  to re-engage ratchet assembly  120  and ratchet pawl  836 . With toothed member  118  re-engaged with ratchet pawl  836 , surgical personnel can compress handles  106 A and  106 B toward each other to actuate inner and outer extensions  102  and  104  but cannot draw handles  106 A and  106 B apart to withdraw outer extension  104  from inner extension  102 . 
         [0065]    With reference now to  FIGS. 16 and 17 , offset arms  112 A and  112 B are further illustrated by  FIGS. 16 and 17 . Offset arms  112 A and  112 B can be similar to each other except for features to accommodate the differences between inner and outer extensions  102  and  104 . Offset arms  112 A and  112 B can include couplings  1650  and  1750  for coupling with corresponding features on bars  109 A and  109 B. Offset arms  112 A and  112 B can define apertures  1648  and  1748 . Apertures  1648  and  1748  can be shaped to cause an interference fit with inner and outer extensions  102  and  104  thereby coupling inner and outer extensions  104  and  102  to offset arms  112 B and  112 A respectively. Offset arms  112 A and  112 B can, in some embodiments, define gussets, ridges flanges, etc. adjacent to, or within, apertures  1648  and  1748  for retaining inner and outer extensions  102  and  104  particularly when surgical personnel exert force on handles  106  to urge rod  14  into pedicle screw  12  (see  FIG. 2 ). Aperture  1648  can, in conjunction with the cannula of inner extension  102 , allow surgical personnel to place a set screw or locking or compression member on pedicle screw  12  and rod  14  (see  FIG. 2 ). Surgical personnel can translate a locking driver through aperture  1648  and inner extension  102  cannula to lock rod  14  in its desired position in pedicle screw  12 . 
         [0066]    With reference now to  FIG. 18 ,  FIG. 18  illustrates one embodiment of distal end  122  of inner extension  102 . Distal end  122  can include resilient fingers  1802  which can be adapted to receive rod  14  (of  FIG. 2 ). Resilient fingers  1802  can be sized and configured such that, as outer extension  104  translates toward and over distal end  122 , outer extension  104  can compress resilient fingers  1802  in toward pedicle screw  12  (see  FIG. 2 ). Resilient fingers  1802  can be biased to expand back to the position shown in  FIG. 18  as outer extension withdraws from distal end  122 . Resilient fingers  1802  can therefore grasp pedicle screw  12  and release it depending on the position of outer extension  104 . While  FIG. 18  shows resilient fingers  1802 , some embodiments include other features for holding pedicle screw  12  in position relative to distal end  122 . Such features can include threads, detents, bayonet type fittings, etc. without departing from the scope of the disclosure. 
         [0067]    Although embodiments have been described in detail herein, it should be understood that the description is by way of example only and is not to be construed in a limiting sense. It is to be further understood, therefore, that numerous changes in the details of the embodiments and additional embodiments will be apparent, and may be made by, persons of ordinary skill in the art having reference to this description. It is contemplated that all such changes and additional embodiments are within scope of the claims below and their legal equivalents.