Abstract:
An instrument for obtaining spinal rod measurements in situ includes a measurement member, a first indicating member, and a second indicating member. The measurement member measures a length between two spinal implants. The first indicating member couples with the measurement member and includes a first measurement scale coupled with a first shaft for engaging a first spinal implant of the two spinal implants. The second indicating member couples with the measurement member and includes a second measurement scale coupled with a second shaft for engaging a second spinal implant of the two spinal implants.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation of U.S. Ser. No. 13/835,938 entitled “Spondylolisthesis Reduction System” which was filed on Mar. 15, 2013 and which claims priority to U.S. Provisional Application Ser. No. 61/612,919 which was filed on Mar. 19, 2012 and is incorporated herein by reference in its entirety. 
     
    
     FIELD 
       [0002]    The present disclosure generally relates to the field of spinal orthopedics, and more particularly to instruments for reducing spondylolisthesis. 
       BACKGROUND 
       [0003]    The spine is a flexible column formed of a plurality of bones called vertebrae. The vertebrae are hollow and piled one upon the other, forming a strong hollow column for support of the cranium and trunk. The hollow core of the spine houses and protects the nerves of the spinal cord. The different vertebrae are connected to one another by means of articular processes and intervertebral, fibrocartilaginous bodies. Various spinal disorders may cause the spine to become misaligned, curved, and/or twisted or result in fractured and/or compressed vertebrae. It is often necessary to surgically correct these spinal disorders. 
         [0004]    The spine includes seven cervical (neck) vertebrae, twelve thoracic (chest) vertebrae, five lumbar (lower back) vertebrae, and the fused vertebrae in the sacrum and coccyx that help to form the hip region. While the shapes of individual vertebrae differ among these regions, each is essentially a short hollow shaft containing the bundle of nerves known as the spinal cord. Individual nerves, such as those carrying messages to the arms or legs, enter and exit the spinal cord through gaps between vertebrae. 
         [0005]    Spondylolisthesis is the anterior or posterior displacement of a vertebra of the vertebral column in relation to the vertebra below. In the lower region of the back where the lumbar vertebrae meet the sacrum, spondylolisthesis may occur more frequently. For example, at the L 5 -S 1  level, the fifth lumbar vertebra may slip forward or in the anterior direction relative to the first level of the sacrum. Treatment for spondylolisthesis depends on the severity of the slippage. For severe cases, surgical correction is required. 
         [0006]    Various systems and methods are known to alleviate and correct spondylolisthesis. For example, German Patent 41 27 303, filed Aug. 17, 1991 (also disclosed in European Patent No. 0528177, filed Jul. 16, 1992) to Aesculap AG, discloses such a device. Other devices include U.S. Pat. No. 6,565,568, filed Sep. 28, 2000 to Rogozinski and U.S. Pat. Pub. No. 2009/0216237, filed Jun. 30, 2006 to Frezal et al. However, some of these systems may be difficult to maneuver, attach, and remove from screw heads. Some of these systems may make it difficult to insert and secure fixation rods after correcting the slippage without removing portions of the systems. 
       SUMMARY 
       [0007]    A system for reducing deformities in the spine includes a first tower assembly and a second tower assembly. The first tower assembly includes a first tower that couples to a first screw in a first vertebral level, a load transfer ring rotatable coupled to the first tower, and a load transfer link rotatably coupled to the load transfer ring. The second tower assembly includes a second tower that couples to a second screw in a second vertebral level, a load applicator secured to the second tower, and a load transfer member rotatably coupled to the second tower and linked to the load transfer link. The load applicator applies force to the load transfer member to position the first tower assembly relative to the second tower assembly. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1A  is a perspective view of a system for reducing deformities in the spine according the principles of the present disclosure; and  FIG. 1B  is a perspective view of alternative embodiment of the first and second mount. 
           [0009]      FIG. 2  is top view of a portion of the spine and associated bone screws for use with the system according to the principles of the present disclosure. 
           [0010]      FIG. 3  is a side view of the portion of the spine and associated bone screws of  FIG. 2  according to the principles of the present disclosure. 
           [0011]      FIGS. 4A-5B  are cross-sectional views of a tower assembly of the system and one of the bone screws according to the principles of the present disclosure. 
           [0012]      FIG. 6A  is a perspective view of the second mount;  FIG. 6B  is perspective exploded of the second mount;  FIG. 6C  is perspective view of the locking tube;  FIG. 6D  is a side view of the load reduction arm;  FIG. 6E  is an enlarged view of section  6 E from  FIG. 6D  showing the ratcheted portion of the load reduction arm;  FIG. 6F  is a perspective view of the first mount;  FIG. 6G  is an exploded perspective view of the first mount. 
           [0013]      FIG. 7A  is a perspective view of a first tower assembly of the system according to the principles of the present disclosure; and  FIG. 7B  is a perspective view of a second tower assembly of the system according to the principles of the present disclosure. 
           [0014]      FIG. 8A  is a side view of the first and second tower assemblies mounted with the first and second mounts in the unreduced state;  FIG. 8B  is enlarged side view of the of the first and second tower assemblies mounted with the first and second mounts in the unlocked state;  FIG. 8C  is a side view of the first and second tower assemblies mounted with the first and second mounts in the reduced state; and  FIG. 8D  is an enlarged side view of the first and second tower assemblies mounted with the first and second mounts in the locked state. 
           [0015]      FIG. 9  is a perspective view of a right side of the system including the first and the second tower assemblies of  FIGS. 6-7  according to the principles of the present disclosure. 
           [0016]      FIG. 10A  is a side view of the right side of the system prior to positioning a L 5  vertebra of the spine relative to a S 1  level of the sacrum according to the principles of the present disclosure. 
           [0017]      FIG. 10B  is a side view of the right side of the system after positioning the L 5  vertebra of the spine relative to the S 1  level of the sacrum according to the principles of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    For a mid to high grade spondylolisthesis at the L 5 -S 1  level, the anatomy is exposed and bone screws are placed in the pedicles of the L 5  lumbar vertebra bilaterally near the cephalad end of the sacrum. Bone screws are also placed in the sacrum. A system of instruments may be used to reposition the L 5  vertebra relative to the S 1  level. The system may comprise a set of mirrored tower assemblies which attach to the tops of the screw heads. The tower assemblies which attach to the lumbar bone screws comprise a tower, a ring attached to the tower which may rotate about the longitudinal axis of the tower, and a link attached to the ring which may rotate about an axis at right angles to the tower&#39;s longitudinal axis. The tower assemblies which attach to the sacral bone screws each comprise a tower, a pivoting level, and a drive screw. 
         [0019]    The sacral towers may provide a relative ground reference for the reduction apparatus while the lumbar towers may act as load transfer structures. A drive apparatus mounted to the sacral towers provides forced to produce the necessary anatomical correction. The tower assemblies transmit the leverage generated by the drive apparatus into a posterior load on the L 5  vertebral body. The system applies a generally posteriorly directed force to the vertebral body while allowing the vertebral body to travel posteriorly along a path of least resistance. The system may not dictate an exact path the vertebral body takes during the reduction procedure. Rods can then be placed in the heads of the bone screws and secured in place without removing the system. 
         [0020]    Benefits of the present invention include the ability to attach the tower assemblies and remove them from the screw head in a single action. The present invention also provides the ability to insert fixation rods and secure them with set screws without removing the towers. The present invention also allows the vertebral body to travel along a path of least resistance rather than dictating a path that may include interference from the sacrum, vertebral disc, or other tissues. 
         [0021]    Embodiments of the invention will now be described with reference to the Figures, wherein like numerals reflect like elements throughout. The terminology used in the description presented herein is not intended to be interpreted in any limited or restrictive way, simply because it is being utilized in conjunction with detailed description of certain specific embodiments of the invention. Furthermore, embodiments of the invention may include several novel features, no single one of which is solely responsible for its desirable attributes or which is essential to practicing the invention described herein. The words proximal and distal are applied herein to denote specific ends of components of the instrument described herein. A proximal end refers to the end of an instrument nearer to an operator of the instrument when the instrument is being used. A distal end refers to the end of a component further from the operator and extending towards the surgical area of a patient and/or the implant. 
         [0022]    Reference to the invention may also be described with respect to coronal, sagittal, and transverse axes of the body. The coronal axis refers to an axis running substantially from front (anterior) to back (posterior) of the body and extending through the mid-section. The sagittal axis refers to an axis running substantially from left to right of the body and extending through the mid-section to intersect the coronal axis at a right angle. The transverse axis refers to an axis running substantially from head to toe of the body and crossing the point where the coronal and sagittal axes intersect at a right angle. Furthermore, the coronal, sagittal, and transverse planes refer to the standard definitions associated with each term. Namely, the coronal plane being a plane perpendicular to the coronal axis and formed by the transverse and sagittal axes, the sagittal plane being perpendicular to the sagittal axis and formed by the coronal and transverse axes, and the transverse plane being perpendicular to the transverse axis and formed by the sagittal and coronal axes. 
         [0023]    Referring to  FIGS. 1-10 , a system  100  for correcting spondylolisthesis is shown in conjunction with two sets of bone screws inserted into two vertebrae of a spinal column. As shown in  FIG. 1 a   , the system  100  may include first tower assemblies  102  and second tower assemblies  104 . The tower assemblies  102  and  104  may be attached to bone screws at the distal end of the tower assemblies  102  and  104 . For example, in  FIGS. 2 and 3 , a first set of bone screws  10  has been inserted into a fifth lumbar vertebra L 5  and a second set of bone screws  12  has been inserted into the first level of the sacrum  51  through a minimally invasive surgery (MIS) technique. The system  100  may be used in conjunction with a spinal fixation system that includes one or more fixation rods  30  disposed through a lumen  103  of the tower assemblies  102  and  104  and setscrews (not shown) to permanently align and rigidly fix two or more levels of the spinal column such as the L 5  and  51  levels. Exemplary bone screws and fixation systems may be found in U.S. Pub. No. 2010/0036443 and U.S. Pub. No. 2009/0171391 both of which are incorporated herein by reference in their entirety. 
         [0024]    Although the system  100  of the present disclosure is described herein with reference to the L 5  and S 1  levels, the system  100  may be used in other regions of the spine where spondylolisthesis or other slippage of vertebral bodies may occur. As shown in  FIG. 1 a   , the tower assemblies  102  and  104  may removably couple with sets of bones screws  10  and  12  respectively via MIS procedures. The tower assemblies  102  and  104  include a load transfer member  144  on the proximal end  104  of the tower assemblies  102  and  104  to transmit a leverage generated by the drive apparatus to cause relative movement of the tower assembly  102  from tower assembly  104 . The first tower assembly  102  may be referred to as a lumbar tower assembly. The second tower assembly  104  may be referred to as a sacral tower assembly. The tower assemblies  102  and  104  may couple to the bone screws  10  and  12  respectively and in substantially similar fashion. 
         [0025]    An alternative system  200  is shown in  FIG. 1 b   , where the first tower assembly  102  includes a first mount  222  and the second tower assembly  104  includes a second mount  224 . The first mount  222  operably coupled to the proximal end of the first tower assembly  102 , and the second mount  224  is operably coupled to the proximal end of the second tower assembly  104 . A load reduction arm  280  is operably coupled to the first mount  222  and second mount  224  to transmit a leverage and cause relative movement of the first tower assembly  102  from second tower assembly  104  to a locked state and a reduced state. 
         [0026]    Anatomy and the degree of severity of the spondylolisthesis will vary from patient to patient. Thus, after placement of the bone screws  10 ,  12 , longitudinal axes of the bone screws  10 ,  12  may not be co-planer when observed from a viewpoint normal to the transverse plane as shown by  FIG. 2 . For example, an angle A may be formed by the axes. Additionally, an angle between the longitudinal axes may vary when observed from a viewpoint normal to the sagittal plane as shown by  FIG. 3 . For example, an angle B may be formed by the axes. To accommodate for the variations between patients and severity of the spondylolisthesis, various interconnecting elements of the system  100  provide sufficient degrees of freedom to allow for variations in placement and actuation of the system  100 . Each tower assembly  102  and  104  may include additional features that enable positioning and alignment of the L 5  vertebra relative to the S 1  level of the sacrum prior to fixation with the rods  30 . Because each set of tower assemblies includes mirrored components, references throughout this description may refer to left sides and right sides of the system  100  interchangeably. Left and right may indicate the left side and right side from the viewpoint of a patient. Furthermore, each left and right tower assembly  102  and  104  may couple with the bone screws in substantially similar fashion. 
         [0027]    As shown in  FIGS. 4A-5B , a portion of one tower assembly is shown in conjunction with one of the bone screws. For ease of discussion, the description herein will refer to one of the lumbar tower assemblies  102  and one of the L 5  vertebra bone screws  12 . The lumbar tower assembly  102  may include features to enable single-action coupling with and removal from a receiving portion  20  of the bone screw  10 . For example, the lumbar tower assembly  102  may include clips  106  in sidewalls  108  of the tower assembly  102 . The clips  106  may extend along the length of the sidewall and may pivot on pins  110 . Each clip  106  may include a proximal end with grips or pads  112  which may be depressed by the surgeon to actuate the clip  106 . Each clip  106  may include a distal end with a projection  114 , such as a boss or protrusion that extends radially inward from the clip  106 . The projection  114  may engages with a recessed portion  22 , such as a bore, pocket, or indentation, of the receiver portion  20  of the bone screw  10 . A bias mechanism  116 , such as a coil spring, leaf spring, or other elastic mechanism, may position the clip  106  into an engaged or closed position with the receiver portion  20 . The surgeon may apply force via the pads  112  to position the clip  106  into a disengaged or open position, wherein the projection  114  disengages the receiver portion  20 , thus permitting removal of the tower assembly  102  from the screw  10 . The projection  114  may include a ramped surface  118  or taper to facilitate coupling with the receiver portion  20  without actuating the clips  116  to the open position. The proximal end of the tower assemblies  102  and  104  may also include a mating feature  109  as to allow the load transfer member  144  to be operably coupled to the proximal end of the tower assemblies  102  and  104 . The mating feature  109  may protrude into the surface of the sidewalls  108  and may also protrude outwards from the surface of the sidewalls  108 , as to provide a lipped and indented mating feature  109  with a space therebetween. 
         [0028]    Referring to  FIGS. 6A-6B , the second mount  224  includes a load transfer ring  230   a  for mounting the second mount  222  on the proximal end of the second tower assembly  104 . A mounting trigger  232   a  may be operably coupled with the load transfer ring  230   a  as to lock the proximal end of the tower assembly  104  in place. The mounting trigger  232   a  may be rotatably coupled to the proximal end of the second mount  224  by way of an opening  233   a,  a pin  234   a,  and a spring  235   a.  The mounting trigger  232   a  may include a locking feature  236  as to mate with the mating feature  109  on the proximal end of the sidewalls  108  of the first and second tower assemblies  102 ,  104 . The locking feature  236  and the load transfer ring  230   a  may secure the second mount  224  to the second tower assembly  104 . The second mount  224  also includes a reduction drive shaft  240  that is operably coupled with a drive shaft bore  242  mounted on the proximal end of the second mount  224 . The reduction drive shaft  240  includes a substantially threaded portion  241  along the exterior of the reduction drive shaft  240  to operably couple with a threaded portion along the interior of the drive shaft bore  242 . The reduction drive shaft  240  is substantially parallel with the load transfer ring  230   a  and the tower assembly  104 . The distal end  244  of the reduction drive shaft  240  operably engages a distraction slide lock  250  on the distal end of the second mount  224 . The reduction drive shaft  240  may be advanced along its longitudinal axis by operation of the threaded engagement as to displace a second end of the distraction slide lock  250 . The first end of the distraction slide lock  250  is rotatably engaged with the distal end of the second mount  224  by way of a pin  252   a . The distal end of the second mount  224  includes a pin opening  228   a  and the distraction slide lock  250  includes a pin opening  254  to permit rotation movement of the distraction slide lock  250  about its first end. A top portion  258  of the distraction slide lock  250  engages the distal end  244  of the reduction drive shaft  240  as to permit the distal end  244  to slide along the top portion  258  and displace the second end of the distraction slide lock  250 . In one embodiment, the top portion  258  is a curved portion as to engage the distal end  244 . 
         [0029]    A distraction trigger  260  is operably coupled on the second end of the distraction slide lock  250  by way of a trigger pin  262  and a trigger opening  264 . The distraction trigger  260  includes a locking feature  265  as to lock the distraction slide lock  250  in place by a locking tube  270  (as shown in  FIG. 6C ) displaced within the lock bore  268  that traverses the bottom portion of the distraction slide lock  250 . The locking tube  270  includes a central lumen  272  traversing the length of the locking tube  270 , a recess opening  274  traversing the surface and communicating with the central lumen  272 , a recess  276  displaced into the surface of the locking tube  270 , and a stop feature  278  on the second end. The stop feature  278  on the second end of the locking tube  270  includes a circumference that is greater than the lock bore  268 , as to prevent the locking tube  270  from moving towards the first end of the distraction slide lock  250 . The locking feature  265  operably engages the recess opening  274  and traverses the thickness of the locking tube  270  as to mate with the load reduction arm  280 . 
         [0030]    Referring to  FIG. 6D , the load reduction arm  280  includes a central shaft with a diameter D 1  that is less than the diameter of the locking tube  270 , such that the central shaft may longitudinally move through the locking tube  270 . The load reduction arm  280  includes a ratcheted portion  282  along the central shaft to the second end of the load reduction arm  280 . As shown in  FIG. 6E , the ratcheted portion  282  includes stepped features  283  as to mate with the locking feature  265 . In one embodiment, the stepped features  283  include a height H 1  that mates with at least a portion of the locking feature  265 . The load reduction arm  280  includes a length L 1 , which may be determined by the length between vertebrae to be displaced. The first end of the load reduction arm  280  includes raised features  287  that are a greater diameter than D 1 . The first end of the load reduction arm  280  includes an opening  286  to be rotatably coupled with the first mount  222 . 
         [0031]    Referring to  FIGS. 6F-6G , the first mount  222  includes the load reduction arm  280  rotatably coupled to the distal end of the first mount  222  by way of an opening  228  b and a pin  252   b  operably coupled through the opening  286  of the load reduction arm  280 . Similar to the second mount  224 , the first mount  222  includes a load transfer ring  230  on the proximal end of the first mount  222 . The proximal end of the first mount  222  includes a trigger  232   b  that may be operably coupled with the load transfer ring  230   b  as to lock the proximal end of the tower assembly  102  in place. The mounting trigger  232   b  may be rotatably coupled to the proximal end of the first mount  222  by way of openings  233   b,  pins  234   b,  a spring  235   b,  and tabs  237 . The mounting trigger  232   a  may include a locking feature as to mate with the mating feature  109  on the proximal end of the sidewalls  108  of the first and second tower assemblies  102 ,  104 . The locking feature and the load transfer ring  230   b  may secure the first mount  222  to the first tower assembly  102 . The distal end of the first mount  222  also includes at least one profiled tab  223 , which permit the raised features  287  of the load reduction arm  280  to rotate. The load reduction arm  280  may displace the first mount  222  along its longitudinal axis. 
         [0032]    Continuing now with  FIG. 7A , the lumbar tower assembly  102  includes additional features that link to the sacral tower assembly  104 . Once linked, the sacral tower assembly  104  may be used to apply forces on the lumbar tower assembly  102  to reposition the L 5  vertebra. For example, the lumbar tower assembly  102  may include a lumbar tower  120 , a load transfer ring  122 , and a load transfer link  124 . The transfer ring  122  may be rotatably coupled to the proximal end of the lumbar tower  120 . The transfer ring  122  may rotate about a longitudinal axis of the lumbar tower  120 . A transfer post  126  may extend radially from the transfer ring  122  and perpendicular to the longitudinal axis of the lumbar tower  120 . The transfer link  124  may be rotatably coupled to the lumbar tower  120  by the transfer post  126  of the transfer ring  122 . For example, the transfer link  124  may include a first bore  128  at its proximal end that slides over the transfer post  126 . The transfer link  124  may rotate about the longitudinal axis of the transfer post  126 . A second bore  130  may extend through a distal end of the transfer link  124  and perpendicular to the transfer post  126 . The second bore  130  may be configured to receive additional features of the system that connect with the sacral tower assembly  104  as described herein. 
         [0033]    Referring now to  FIG. 7B , the sacral tower assembly  104  includes additional features that link to the lumbar tower assembly  102 . For example, the sacral tower assembly  104  may include a sacral tower  140 , load applicator  142 , and a load transfer member  144 . The load applicator  142  may include a load ring  146  that rotatably couples to a proximal end of the second tower  140 . The load ring  146  may rotate about the longitudinal axis of the second tower  140 . The load ring  146  may be secured to the second tower  140  via a set screw (not shown.) An extension member  148  may extend proximally from the load ring  146  and parallel to the longitudinal axis of the second tower  140 . The extension member  148  may include a thru-bore  150  at its proximal end configured to receive the load applicator  142 . For example, the load applicator  142  may include a threaded shaft  152  and a handle  154 . The thru-bore  150  may include threads configured to engage the threaded shaft  152  on the load applicator  142 . The surgeon may rotate the handle  154  to advance the threaded shaft  152  through the thru-bore  150 . 
         [0034]    A second post  156  may extend radially or tangentially from the load ring  146  and connect with features of a load transfer member  144  as described below. For example, the load transfer member  146  may include a receiving portion  158  and a linking portion  160  that connect the first tower assembly  102  with the second tower assembly  104 . The receiving portion  158  engages with the threaded shaft  152  of the load applicator  142 . For example, the load applicator  142  may extend through the thru-bore  150  to engage with a proximal end of the receiving portion  158 . At a distal end of the receiving portion  158 , a thru-bore  162  provides pivotal connection with the second post  156 . The linking portion  160  extends away from the distal end of the receiving portion  158  to form a substantially “L” shaped load transfer member  146 . The linking portion  160  may rotatably couple to the receiving portion  158  at a first end  164 . A second end  166  of the linking portion  160  may cantilever away from the receiving portion  158  to engage with the transfer link  124 . 
         [0035]    Referring now to  FIGS. 8A-8B , the tower assemblies  102  and  104  are attached to the first mount  222  and the second mount  224  for the alternative system  200 . The screws are placed on the towers and are inserted in a minimally invasive fashion, and then the first and second mounts  102 ,  102  are attached to the top or proximal ends of the tower assemblies  102  and  102  by the mounting triggers. The displaced vertebrae may be separated by a height H 2  and a length L 2 .  FIG. 8A  shows the alternative system  200  in the unreduced state where the load reduction arm  280  can move freely during the reduction until the distraction slide lock  250  is locked via the locking tube  270  and the distraction trigger  260 . The L 5  vertebrae may be reduced into position by levering off of the placed screw in the  51  vertebrae, and the reduced state of the alternative system is shown in  FIG. 8C . Distraction can be applied before or after reduction of the L 5  vertebrae and the distraction may also be held in place by the reduction drive shaft  240  and operation of a handle  290  operably coupled to the proximal end of the reduction drive shaft  240 . As shown in  FIG. 8B , the alternative system  200  is in the unlocked state where the first end of the locking tube  270  sticks out at least a portion beyond the distraction slide lock  250 , and the reduction arm  280  can slide freely while the distraction trigger  260  and locking feature  265  remains engaged in the recess  276  behind the recess opening  274 . A hand held distractor (not shown) is placed at position  300  between the raised feature  287  of the load reduction arm  280  and the first end of the locking tube  270  and a distraction load is applied, which causes the locking tube  270  to move until the stopping feature  278  abuts or bottoms out against the face of the second end of the distraction slide lock  250 , as shown in  FIGS. 8C-8D . Once the stopping feature  278  of the locking tube  270  abuts the second end of the distraction slide lock  250 , the distraction trigger  260  drops into the recess opening  274  of the locking tube  270 , and the locking feature  265  engages the ratcheted portion  282  to hold the distraction in the locked state until released, as shown in  FIG. 8D . 
         [0036]    Referring now to  FIG. 9 , the two tower assemblies  102  and  104  of the right side of the system  100  are shown. Various features of the tower assemblies  102  and  104  provide multiple degrees of freedom that allow for rotational and translational movement in multiple planes. For example, the transfer ring  122  may allow for a rotation  123  of the second end  166  of the transfer member  144  in a first plane substantially parallel to the coronal plane. The transfer link  124  and post  126  may allow for a rotation  127  of the second end  166  of the transfer member  144  in a second plane substantially parallel to the sagittal plane. The bore  130  may allow for a rotation  125  of the second end  166  of the transfer member  144  in a third plane substantially parallel to the transverse plane. At the opposite end of the transfer member  144 , the rotatable coupling at the first end  164  allows for a rotation  163  of the first end  164  in the plane substantially parallel to the coronal plane. The thru-bore  162  and second post  156  allow for a rotation  167  of the first end  164  in the plane substantially parallel to the sagittal plane. Sliding engagement between the transfer member  144  and the bore  130  allows for a translational movement  169  of the lumbar tower assembly  102  relative to the sacral tower assembly  104  in a plane substantially parallel to the sagittal plane, as shown in  FIG. 10A . The lumbar tower assembly  102  may also be positioned anteriorly-posteriorly as the second end  166  of the transfer member transfers force applied by the load applicator  142 . 
         [0037]    In  FIGS. 10A and 10B , the system  100  may be used to correct spondylolisthesis at the L 5 -S 1  level of the spine. For example, in  FIG. 10A , the L 5  vertebra has slipped forward or anteriorly from the  51  level of the sacrum. The slippage may occur due to degeneration of disc material between the L 5  and  51  levels. The slippage may occur from a fracture of degeneration of the vertebral body and/or from fracturing of the L 5  vertebra. In some cases, bone growth may occur on an upper surface of the  51  level due to rubbing from the L 5  vertebra. The system  100  may be used to reposition the L 5  vertebra into proper alignment with the  51  level and hold the L 5  and  51  levels in place while permanent fixation is added in the form of fixation rods and set screws. 
         [0038]    Continuing with  FIG. 10A , the surgeon may rotate the load applicator  142  to apply a force on the receiving portion  158  of the transfer member  144 . The load applicator  142  may advance  171  towards the receiving portion  158  in a plane substantially parallel to the sagittal plane as shown in  FIG. 10A . As the load applicator  142  advances, the transfer member  144  rotates  173  about the post  156  also substantially in the plane parallel to the sagittal plane. Because the L 5  vertebra may not be aligned with the  51  level of the sacrum in the sagittal plane, the linking portion  160  may also rotate  127  relative to the receiving portion  158  via the first end  164  as described above. As the force increases, the second end  166  of the linking portion  160  rotates proximally  175 , thus pulling  177  the first tower assembly posteriorly  102  in line with the second tower assembly  104 . The second end  166  slidably engages with the transfer link  124  to permit free translational movement  169  of the L 5  vertebra in the sagittal plane. The transfer link  124  also rotates freely to accommodate the movement of the L 5  vertebra. 
         [0039]    Referring now to  FIG. 10B , the load applicator  142  continues to advance  171  and apply force  173  to pull the L 5  level into proper alignment with the S 1  level. As the L 5  level is positioned posteriorly, the sliding engagement  169  of the linking portion  160  with the transfer link  124  and rotatable connection  127  at the transfer link  124  and post  126  allow the L 5  vertebra to follow a path of least resistance. Once the vertebra L 5  is properly aligned with the S 1  level of the sacrum, the rods  30  may be inserted into the receiving portions  20  of the screws  10  and  12  as shown in  FIG. 1 . Each tower assembly  102  and  104  may also be cannulated to permit insertion of setscrews within the receiving portions  20  of the screw  10  and  12  to permanently secure the L 5 -S 1  level. Additionally, a spacer or other interbody device may be secured between the L 5  vertebra and S 1  level of the sacrum. Bone material may be inserted with the spacer or interbody device to promote bone fusion and bone growth to permanently fuse the L 5 -S 1  level. 
         [0040]    Example embodiments of the methods and systems of the present invention have been described herein. As noted elsewhere, these example embodiments have been described for illustrative purposes only, and are not limiting. Other embodiments are possible and are covered by the invention. Such embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.