Abstract:
This disclosure describes example embodiments of rod reduction instrumentation. The rod reducers can be used during the installation of a rod based surgical fixation system to help urge the rod into the fixation anchors. The reducers described provide various configurations delivering large reduction distance capabilities, strong controlled reduction coupled with an ability to quickly advance the reducer if desired, and reduction of bulk through the surgical corridor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a utility patent application that claims priority to U.S. Provisional Application Ser. No. 61/802,046, filed on Mar. 15, 2013, the entire contents of which are hereby expressly incorporated by reference into this disclosure as if set forth fully herein. 
    
    
     FIELD 
     The present application relates to the field of spinal surgery and spinal fixation devices, including instruments and associated methods for seating or reducing a spinal fixation rod into a fixation anchor during the installation of a spinal fixation construct. 
     BACKGROUND 
     Spinal fixation constructs are utilized to provide stability to the spine. Most often the fixation construct is used as an adjunct to fusion surgery during which adjacent vertebrae are prepared to facilitate bone growth between them. Because motion between the vertebrae tends to inhibit bone growth, the fixation constructs are employed to prevent motion so that bone can grow and achieve a solid fusion. When the position of one or more vertebrae must be adjusted to restore a more natural alignment of the spinal column, the fixation construct also serves to maintain the new alignment until fusion is achieved. 
     Fixation constructs of various forms are known in the art, of which, rod based fixation constructs are one of the most common. Typically in a rod based construct multiple anchors are coupled to a portion (e.g. the posterior elements) of two or more vertebrae and then connected by a fixation rod. The anchors further include a rod housing in which the fixation rod is captured and locked. The rod housing may be fixed or rotatably coupled to the anchor portion and generally includes a pair of upstanding arms separated by a rod channel. When constructing the fixation construct the surgeon must align and seat the rod in the rod channel of each anchor, an undertaking that is generally referred to as “reduction”. Reduction can be a challenge, particularly when one or more of the vertebrae to be connected are out of alignment with other vertebrae, and the reduction distance and force requirements can vary greatly from anchor to anchor. Known rod reduction instruments or reducers, can be bulky, time consuming or frustrating to employ, limited in achievable reduction depth, and other issues that can make them less than desirable. 
     The various reduction instruments described herein are directed towards facilitating simple and efficient rod reduction during installation of a fixation construct. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Many advantages of the present invention will be apparent to those skilled in the art with a reading of this specification in conjunction with the attached drawings, wherein like reference numerals are applied to like elements and wherein: 
         FIG. 1  is a perspective view of a rod reducer for urging a spinal rod to an anchor, according to a first example embodiment; 
         FIG. 2  is a perspective of the example rod reducer of  FIG. 1  with the rod in a fully reduced position within the anchor; 
         FIG. 3  is a side view of the example rod reducer of  FIG. 1 ; 
         FIG. 4  is a backside view of the example rod reducer of  FIG. 1 ; 
         FIG. 5  is a front side view of the example rod reducer of  FIG. 1 ; 
         FIG. 6  is a perspective view of a coupling unit of the example rod reducer of  FIG. 1 ; 
         FIG. 7  is a perspective view of a translating unit of the example rod reducer of  FIG. 1 ; 
         FIG. 8  is another perspective view of the distal end of the translating unit of  FIG. 7 ; 
         FIG. 9  is a perspective view of a translating coupler of the coupling unit of  FIG. 6 ; 
         FIGS. 10A-10C  are side views of the example reducer of  FIG. 1  depicting a sequence for reducing a rod, according to one example embodiment; 
         FIGS. 11-12  are lateral and perspective views illustrating the final reduction position shown in  10 C in connection with a whole fixation construct fixing two adjacent vertebrae; 
         FIG. 13  is a perspective view of a rod reducer according to a second example embodiment; 
         FIG. 14  is a different perspective view of the rod reducer of  FIG. 13 ; 
         FIG. 15  is a back view of the example rod reducer of  FIG. 13 ; 
         FIG. 16  is a side view of the example rod reducer of  FIG. 13 ; 
         FIG. 17  is a cross-section view of the example rod reducer of  FIG. 13  as viewed in  FIG. 16 ; 
         FIG. 18  is a perspective view of an example embodiment of a locking cap for use with the reducers of  FIGS. 1, 13, and 20 ; 
         FIG. 19  is a cross-section view of a the example locking cap of  FIG. 18  preloaded onto the reducer of  FIG. 13 , according to one example embodiment; 
         FIG. 20  is a perspective view of a rod reducer according to a third example embodiment; 
         FIG. 21  is an exploded view illustrating a the coupling unit and translation unit of the example reducer of  FIG. 20 ; 
         FIG. 22  is a side view of the example rod reducer of  FIG. 20 ; 
         FIG. 23  is a front side view of the example rod reducer of  FIG. 20 ; 
         FIG. 24  is a backside view of the example rod reducer of  FIG. 20 ; 
         FIG. 25  is a cross-section view of the example rod reducer of  FIG. 20  viewed in  FIG. 22 ; 
         FIG. 26  is a cross-section view of the example rod reducer of  FIG. 20  as viewed in  FIG. 24 ; 
         FIG. 27  is a perspective view of a translating unit and translating coupler of the example rod reducer of  FIG. 20 ; 
         FIG. 28  is a perspective view of the translating unit of  FIG. 27 ; 
         FIG. 29  is a perspective view of the translating coupler of  FIG. 27 ; and 
         FIGS. 30A-30C  are side views of the example reducer of  FIG. 20  depicting a sequence for reducing a rod, according to one example embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Various example embodiments of devices and techniques for rod reduction during spinal instrumentation procedures are described herein. In the interest of clarity, not all features of an actual implementation are necessarily described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. The rod reduction instruments and related implants, instruments and methods described herein boast a variety of inventive features and components that warrant patent protection, both individually and in combination. 
     The example reduction assembly, or reducer, embodiments described herein are used during the installation of a fixation construct  10  onto the spine of a patient. The fixation construct  10  includes anchor members  12  connected by a fixation rod  14  locked to each anchor  12 . An anchor  12  is implanted in each vertebra to be fixed by the construct  10 . For example, two anchors  12  may be used to fix two vertebrae together; three may be used to fix three vertebrae together; four may be used to fix four vertebrae together; and so on. The anchor  12  includes a bone anchor  18  and a housing  20  for capturing and locking a fixation rod  14 . The bone anchor  18  may be a bone screw suitable for stable fixation to vertebral bone (e.g. pedicle or vertebral body), as shown. The bone anchor  18  may also include other fixation devices (e.g. hooks, staples, clamps, etc. . . . ). The housing  20  has a base that attaches with the bone anchor and a pair of upstanding arms that together form a rod channel  22 . The housing also includes a mechanism  24  to lock the fixation rod  14  in position in the rod channel  22 . For example, the mechanism  24  may include a locking cap guide and advancement feature disposed on the interior face of each arm that interacts with a complementary feature on a locking cap  16 . The base may be fixed to the anchor  18  or may be coupled such that the housing can rotate in one or more directions (e.g. polyaxial). The housing also includes one or more instrument engagement features  26  for releasably coupling to one or more instruments during implantation. One example of an anchor configured for use with the reducers described herein is shown and described in U.S. patent application Ser. No. 13/456,210, filed Apr. 25, 2012, the entire contents of which are incorporated herein by reference. The reducers described herein can be engaged to one or more of the anchors  12  of the fixation construct  10  to facilitate alignment and advancement of the rod  14  into the rod channel  22  of each anchor. 
     With reference to  FIGS. 1-12 , a reducer  100  according to a first example embodiment is illustrated. As depicted in  FIG. 1  the reducer  100  is configured to couple to a single side or arm of anchor  12  (advantageously reducing bulk in the surgical corridor) and impart a downward force on the rod  14 . The downward force on the rod acts to draw the rod and anchor housing  20  together until the rod  14  fully seats in the rod channel  22 , as shown in  FIG. 2 . A locking mechanism, such as locking cap  16  (see  FIG. 18 ), may then be at least partially engaged to capture the rod  14  in the housing  20  prior to decoupling the reducer  100  from the anchor  12 . The reducer  100  includes a coupling unit  102  unit that connects to the anchor  12  and a translation unit  104  that translates relative to the coupling unit  102  to urge the rod  14  towards the anchor. 
     With reference to  FIG. 6 , the coupling unit  102  includes a single anchor coupling arm  106  and a translation coupler  108 . The anchor coupling arm  106  has a partially cylindrical profile with an inner face  110  and outer face  112 . A cavity  114  at the distal end of the coupling arm  106  is dimensioned to snugly receive an arm of the anchor housing  20  therein. Included in the cavity is an engagement feature (not shown) that mates with the instrument engagement features  26  on the anchor to releasably fix the anchor housing  20  to the coupling arm  106 . By way of example, the engagement feature may be the same or similar to the engagement feature  216  of reducer  200  described below. Above the cavity  114 , the inner face  110  has a lower elongated concave recess  116  with a central slot  118  extending deeper still towards the outer face  112 . A lower slot  120  generally coinciding with the length of the recess  116  extends through the coupling arm  106 , opening near each edge of the outer face  112  and intersecting the central slot  118 . The inner face  110  also includes an upper elongated concave recess  122  with a central slot  124  opening through the outer face. An upper slot  126  generally coinciding with the length of the recess  122  extends through the coupling arm  106 , opening near each edge of the outer face  112  and intersecting the central slot  124 . The outer face  112  includes a ridge track  128 , having a series of downward pointing ridges  130 . That is, the ridges  130  have an upper surface that slopes aggressively away and down from the outer face  112 . The lower surface of ridges  130  may be perpendicular to the outer face, or preferably, may slope mildly also away and down from the outer face  112 . 
     The translation coupler  108 , shown in  FIG. 9 , includes an internally threaded ring  132 . A wing  134  extends from the ring  132  through the central slot  124  and is pivotally connected to a switch  136  via pin  138  or other suitable mechanism. A stabilizing bar  140  situated in slot  126  passes through an aperture  142  in the wing  134  to fix the translation coupler  108  to the coupling arm  106  while allowing the translation coupler to translate up and down along the slot  126  and upper recess  122 . A pawl  144  at the distal end of the switch  136  engages the ridge track  128  to prevent upward or proximal translation without disengaging the pawl  144 . In a preferred example, the switch  136  is spring biased to the engaged pivot position. In this configuration, the application of downward force causes the pawl  144  to slide down the slope of the upper surface of each ridge  130  and automatically return to the engaged position when the pawl  144  passes the lower surface of the ridge  130 . Thus, the translation coupler  108  can be advanced distally without manipulating the switch  136  but requires a user to manipulate the switch  136  to disengage the pawl  144  and allow proximal translation. 
     With reference to  FIGS. 7-8 , the translation unit  104  includes a threaded shaft  146  capped with a drive nut  148  at the proximal end and a foot  150  configured to engage and drive the rod  14  at the distal end. The threaded shaft  146  engages the internal threading of the ring  132  to translate the translation unit  104  relative to the coupling unit  102  upon rotation of the shaft  146 . The drive nut  148  can be engaged by a handle (not shown) to facilitate rotation. The foot  150  includes a cylindrical body  152  that complements the lower recess  116  and that is coupled to the shaft  146  in such a way that the foot  150  and shaft  146  are fixed longitudinally but freely rotatable relative to each other. This can be accomplished, for example, with an expansion ring situated in complementary internal and external grooves in the cylindrical body  152  and shaft  146 , respectively. Or in one alternative, the distal end of the shaft  146  can include flexible fingers having a ridge that is received in the internal groove of the cylindrical body  152 . A wing  154  extends from the cylindrical body and is situated in the central slot  118 . A stabilizing bar  156  situated in slot  120  passes through an aperture  158  in the wing  154  to stabilize the foot and eliminate any movement of the foot other than translation up and down along the slot  120  and lower recess  116 . A brim  160  extends out and down from the portion of the cylindrical body  152  not in contact with the inner recess  116 . An inner cavity  162  enclosed by the brim  160  is configured to receive a portion of the anchor housing  20  therein. The front of brim  160  descends lower than the sides such that a rod recess  164  is formed between the coupling arm  102  and the brim front to help capture and guide the rod into the rod channel  22 . Passage  166  extends through the translation unit  104  from the drive nut  148  to the foot  150  to receive locking cap  16  and a driver therethrough to engage the locking cap  16  to the housing  20  prior to removing the reducer  100 . The translation unit  104  may further be configured to carry a preloaded locking cap, as illustrated below with respect to reducer  200 . 
     Turning to  FIGS. 10A-10C , use of the reducer  100  is illustrated by way of example. Anchors  12  are implanted in each of the vertebra to be fixed, including anchor  12  in vertebraV 1  which is the anchor to be reduced in this example, and the rod is inserted to the anchor housings. As seen in  FIG. 10A , the rod rests above the housing  20 . The distal end of the coupling arm  102  is advanced onto an arm of the anchor housing  20  until the engagement features on the coupling arm  102  engage with the engagement feature  26  of the housing  20 . The coupling arm  102  is coupled to the housing with the foot  150  of the translation unit  104  spaced proximal to the housing  20  and the rod  14 . The user can then direct force distally onto the translation unit  102  such that the translation coupler  108  translates distally along the coupling arm  102  (translating the translating unit  104  distally along with it). Thus, the translation coupler  108  acts as quick-advance mechanism to advance the translation unit  104  without requiring the added effort and time required to threadingly advance the shaft  146  through the threaded ring  132 . As the translation coupler  108  and translation unit  104  are advanced, the pawl  144  engages each ridge  130  on the track  128  in turn to prevent unwanted proximal translation. The translation coupler  108  and translation unit  104  can be advanced this way until the translation unit bottoms out on the slots  124  and/or  126 , the foot  150  reaches the rod  14  ( FIG. 10B ), or beyond that, the force required to further move the rod becomes too great. With the foot  150  in contact with the rod  14 , the threaded shaft  146  is rotated to advance the threading through the threaded ring  132  until the rod is fully seated in the anchor housing  20 , as shown in  FIG. 10C .  FIGS. 11 and 12  illustrate this final position shown in  10 C with the additional anchor  12  of fixation construct  10  implanted in adjacent vertebra V 2 . Though shown as a two level construct, additional anchors  12  can be implanted in additional vertebrae to extend the construct  10  over multiple levels. The construct  10  may also be implanted bilaterally with additional anchors  12  and another rod  14  implanted on the contralateral side of the vertebrae V 1  and V 2 . The reducer  100  may be used on any or all of the anchors  12  in the construct. After the rod  14  is fully seated in housing  20  a locking cap  16  can be advanced through the passage  166  and engaged with the locking engagement feature  24  to capture and lock the rod  14  to the anchor  12 . The switch  136  can then be manipulated to disengage the pawl  144  from the track  128  retract the translating unit  104  if desired, and the coupling arm  102  is disengaged from the housing  20 . 
     Turning to  FIGS. 13-19 , a reducer  200  according to a second example embodiment is illustrated. Reducer  200  is similar to reducer  100  and is also configured to couple to a single side or arm of anchor  12 . The reducer  200  includes a coupling unit  202  that connects to the anchor  12  and a translation unit  204  that translates relative to the coupling unit  202  to urge the rod  14  towards the anchor. The coupling unit  202  includes a single anchor coupling arm  206 . The anchor coupling arm  206  has a partially cylindrical profile with an inner face  208  and an outer face  210 . A cavity  212  at the distal end of the coupling arm  206  is dimensioned to snugly receive an arm of the anchor housing  20  therein. An engagement feature  214  includes a flexible finger  216  formed in the coupling arm and having a distal ridge  218  that projects into the cavity  14  to engage the engagement features  26  of the housing  20 . The distal surface of the ridge  218  is tapered to automatically deflect the finger  216  outward as the arm of housing  20  is advanced into cavity  212 , permitting the ridge  218  to pass the top of the housing until it engages the feature  26 . The inner face  208  includes a central slot  220  and a slot  222  that extends through the coupling arm  206  opening near each edge of the outer face  210  and intersecting the central slot  220 . A fixed coupling body  224  projects inward from the proximal end of the coupling arm  206  and encloses a threaded passage  226 . 
     The translation unit  204  includes a threaded shaft  228  capped with a drive nut  230  at the proximal end and a foot  232  configured to engage and drive the rod  14  at the distal end. The threaded shaft  228  engages the internal threading of the passage  226 . The drive nut  230  can be engaged by a handle (not shown) to facilitate rotation that translates the translation unit  204  relative to the coupling unit  202 . The foot  232  includes a generally cylindrical body  234  that complements the inner face  208  and that is coupled to the shaft  228  in such a way that the foot  232  and shaft  228  are fixed translationally but freely rotatable relative to each other. This can be accomplished, for example, with an expansion ring  237  situated in complementary internal and external grooves in the cylindrical body  234  and shaft  228 , respectively. Or in one alternative example, the distal end of the shaft  228  can include flexible fingers having a ridge that is received in the internal groove of the cylindrical body  234 . A wing  236  extends from the cylindrical body  234  and is situated in the central slot  220 . A stabilizing bar  238  situated in slot  222  passes through an aperture in the wing  236  to stabilize the foot and eliminate any movement of the foot other than translation up and down. A brim  240  extends out and down from the portion of the cylindrical body  152  not in contact with the inner face  208 . An inner cavity  242  enclosed by the brim  240  is configured to receive a portion of the anchor housing  20  therein. The front of brim  240  descends lower than the sides such that a rod recess  244  is formed between the coupling arm  202  and the brim front to help capture and guide the rod into the rod channel  22 . An opening  246  in the front of the foot  232  provides a view into the foot to permit viewing of a preloaded locking cap  16  (not shown). 
     With reference to  FIG. 17 , the translation unit  204  further includes a passage  248  extending from the drive nut  230  to the foot  232  in which a drive shaft  250  having a distal drive feature  252  is situated. A rim  254  above the drive feature maintains the drive feature within the foot  232 . The drive shaft is permitted to freely translate a limited distance within the passage  248  such that the locking cap may be engaged and fully advanced into the anchor housing  20 . 
     An expansion ring  256  situated in a groove just below the drive feature  252  maintains the locking cap  16  on the drive feature  252  ( FIG. 19 ) until the locking cap  16  is engaged in the housing, after which the drive feature  252  may be removed from the locking cap by pulling up on the drive shaft. 
     In use the reducer  200  is used similarly to the reducer  100  but without the-quick advance translation. Again, anchors  12  are implanted in each of the vertebra to be fixed and the rod is inserted to the anchor housings. The distal end of the coupling arm  202  is advanced onto an arm of the anchor housing  20  until the engagement features  216  on the coupling arm  202  engage with the engagement feature  26  of the housing  20 . The coupling arm  202  is coupled to the housing with the foot  232  of the translation unit  204  spaced proximal to the housing  20  and the rod  14 . A handle may be coupled to the drive nut  230  and the threaded shaft  228  rotated to advance the threading through the threaded passage  226  translating the foot  232  distally until the rod  14  is fully seated in the anchor housing  20 . Again, though shown as a two level construct, additional anchors can be implanted to extend the construct  10  over multiple levels and/or bi-laterally. After the rod  14  is fully seated in housing  20 , the drive shaft  250  can be pressed downward and rotated to engage the locking cap  16  with the locking engagement feature  24  to capture and lock the rod  14  to the anchor  12 . 
     With reference to  FIGS. 20-30 , a reducer  300  according to a third example embodiment is illustrated. The reducer  300  is configured to couple to an implanted anchor housing  20  and impart a downward force on the rod  14 . The downward force on the rod acts to draw the rod and housing  20  together until the rod  14  fully seats in the rod channel  22 . A locking mechanism, such as locking cap  16 , may then be at least partially engaged to capture the rod  14  in the housing  20  prior to decoupling the reducer  100  from the anchor  12 . As illustrated in  FIGS. 20-21 , the reducer  300  includes a coupling unit  302  unit that connects to the anchor  12  and a translation unit  304  that translates relative to the coupling unit  302  to urge the rod  14  towards the anchor. With reference to  FIG. 20 , the coupling unit  302  includes an outer sleeve  306  and a translation coupler  308 . The outer sleeve  306  has a generally centralized and cylindrical body  310 . A connector mast  312  extends proximally from the body  310  and a pair of anchor coupling arms  316  extend distally from the body. The connector mast  312  is capped by a head  314  that is configured to engage with additional instruments if desired. By way of example, the head is configured to mimic the proximal end of the minimally invasive guides described in U.S. patent application Ser. No. 13/456,210 such that any instruments that engage or couple with the guides may also engage or couple with the reducer  300  (for example, vertebral body derotation assemblies, counter torques, etc. . . . ). The anchor coupling arms  316  are separated by a channel  318  that aligns with the anchor rod channel  22  when the reducer  300  is coupled to the anchor  12 . To couple to the anchor  12 , a cavity  320  at the distal end of the coupling arms  316  is dimensioned to snugly receive the arms of the anchor housing  20  therein. An engagement feature  322  is included on each coupling arm. By way of example, the engagement feature  322  includes a flexible finger  324  formed in the coupling arm and having a distal ridge  326  that projects into the cavity  320  to engage the engagement features  26  of the housing  20 . The distal surface of the ridges  326  are tapered to automatically deflect the finger  324  outward as the arms of housing  20  are advanced into cavity  320 , permitting the ridges  326  to pass the tops of the housing arms until they engages the anchor features  26 . This way, the reducer  300  can be positioned over the rod and quickly snapped onto and secured to the anchor with the simple application of downward pressure. To later disengage the reducer  300  from the housing  20  an instrument may be advanced into the channel and manipulated to apply outward pressure to each of the fingers  324 . 
     The body  310  is split into four quadrants including a front wall  328 , a back wall  330 , and two sidewalls  332 . Four elongated side slots  334  separate each wall from the next. An elongated front slot  336  also runs through the middle of the front wall  328 . The back wall  330  includes a ridge track  338 , having a series of downward pointing ridges  340 . That is, the ridges  340  have an upper surface that slopes aggressively away and down from the back wall  330 . The lower surface of ridges  340  may be perpendicular to the outer face, or preferably, may slope mildly also away and down from the back wall  330 . 
     The translation coupler  308 , shown in  FIG. 29 , includes an internally threaded ring  342  surrounded by a front plate  344  and a back plate  346 . Bars  348  couple the front plate  344  and back plate  346  together and engage cutouts in the outer surface of threaded ring  342  to longitudinally and rotationally fix the ring  342  in position relative to the front and back plates. The bars  348  can slide along the ring  342  cutouts such that the front plate  344  can be moved towards the ring  342  causing the back plate  344  to move farther away from the ring  342 , and vice versa. The bars  348  couple the front and back plates through the side slots  334  such that the translation coupler  308  is coupled to body  310  and can translate up and down along the body  310  with the front plate  344  moving along the front wall  328  and the back plate  346  moving along the back wall  330 . The inner surface of the back plate  346  includes a row of ridges  350  that are complementary to the ridges  340  of ridge track  338  and thus, inhibit upward or proximal translation of the translation coupler  308  when the ridges  350  and  340  are engaged. The inner surface of the front plate  344  includes a pair of cylindrical spring housings  352  situated centrally one on top of the other. The cylindrical spring housings  352  are dimensioned to pass through the front slot  336 . The springs  354  fitted in the housings  352  engage the threaded ring  342  to bias the front plate  344  away from the front wall  328  and the back plate  346  into contact with the back wall  330  and hence the ridge track  338 . In this configuration, the application of downward force causes ridges  350  to slide down the sloped upper surfaces of each ridge  340  and automatically return to the engaged position when the ridge  350  passes the lower surface of the ridge  340 . Thus, the translation coupler  308  can be advanced distally by the application of downward force, but requires a user to manipulate front plate  344  to disengage the back plate  346  from the back wall  330  and allow proximal translation. Other configurations for lockingly engaging the back plate  346  to the back wall are also contemplated. For example only, instead of the ridge track  338 , the side slots  334  adjacent the back wall can have circular cutouts along the slot length. A pair of cylindrical bars can be used on each side to connect the front and back plates instead of the single flat bar  348 , the cylindrical bars passing through cylindrical cutouts in the threaded ring  342 , and connecting to the back plate  346  via enlarged cylindrical discs that are dimensioned to slide laterally into the circular cutouts but cannot pass from one cutout to the next along the slot  334 . 
     With reference to  FIG. 28 , the translation unit  304  includes a shaft  356  capped with a drive nut  358  at the proximal end and a pusher member  360  ending in a pair of reduction arms  362  at the distal end. The reduction arms  362  are situated between the coupling arms  316  and align with the channel  318  on each side. The distal ends of reduction arms  362  are preferably concave in shape to contour to the rod. Protrusions  364  just above each reduction arm  360  on the pusher member  360  slide along the channel  318  between the coupling arms  316  to prevent rotation of the pusher member  360 . Along the shaft  356  between the drive nut  358  and pusher member  360  is a threaded region with threading complementary to the threaded ring  342  to translate the translation unit  304  relative to the coupling unit  302  upon rotation of the shaft  356 . The drive nut  148  can be engaged by a handle (not shown) to facilitate rotation. The pusher member  360  is coupled to the threaded shaft  356  in such a way that the pusher member and shaft are fixed longitudinally but freely rotatable relative to each other. To accomplish this, by way of example, the distal end of the threaded portion includes flexible fingers  366  each having a ridge  370  that is received in an internal groove  368  of the pusher member  360 . A passage  372  extends through the translation unit  104  from the drive nut  148  to reduction arms  360  to receive locking cap  16  and a driver therethrough to engage the locking cap  16  to the housing  20  prior to removing the reducer  300 . Alternatively, the translation unit  304  may further be configured to carry a preloaded locking cap, for example, as described and illustrated with respect to reducer  200 . 
     Turning to  FIGS. 30A-30C , use of the reducer  300  is illustrated by way of example. Anchors  12  are implanted in each of the vertebra to be fixed, including anchor  12  in vertebraV 1  which is the anchor to be reduced in this example, and the rod is inserted to the anchor housings. As seen in  FIG. 30A , the distal ends of the coupling arms  316  are advanced over the rod such that the rod  14  is captured in the channel  318  and onto the anchor housing  20  until the engagement features  324  engage the features  26  on the housing. The user can then direct force distally onto the translation unit  304  such that the translation coupler  308  translates distally along the body  310  (translating the translating unit  304  distally along with it). Thus, the translation coupler  308  acts as quick-advance mechanism to advance the translation unit  304  without requiring the added effort and time required to threadingly advance the shaft  356  through the threaded ring  342 . As the translation coupler  308  and translation unit  304  are advanced, the back plate ridges  350  engage each ridge  340  on the track  338  in turn to prevent unwanted proximal translation. The translation coupler  308  and translation unit  304  can be advanced this way until the translation unit bottoms out on the slots  334 , the reduction arms  360  reach the rod  14  ( FIG. 30B ), or beyond that, the force required to further move the rod becomes too great or a more controlled and precise reduction is desired. With the reduction arms  360  in contact with the rod  14 , the threaded shaft  356  is rotated to advance the threading through the threaded ring  342  until the rod is fully seated in the anchor housing  20 , as shown in  FIG. 30C . Though shown as a two level construct, additional anchors  12  can be implanted in additional vertebrae to extend the construct  10  over multiple levels and/or bilaterally with additional anchors  12  and another rod  14  implanted on the contralateral side of the vertebrae. The reducer  300  may be used on any or all of the anchors  12  in the construct. After the rod  14  is fully seated in housing  20  a locking cap  16  can be engaged with the locking engagement feature  24  to capture and lock the rod  14  to the anchor  12 . The front plate  344  can then be depressed to disengage the back plate from the track  338  to retract the translating unit  104  if desired, and the reducer  300  disengaged from the housing  20 . 
     While specific embodiments have been shown by way of example in the drawings and described herein in detail, it will be appreciated that the invention is susceptible to various modifications and alternative forms (beyond combining features disclosed herein). The description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention.