Abstract:
A method for fusing vertebral bodies comprising the steps of: mounting a first engagement member of a fixation member to a pedicle of a first vertebra and a second engagement member of the spinal fixation member to a pedicle of a second vertebra, the first engagement member having a first extension engaged with an expansion ring, the second engagement member having a second extension engaged with the expansion ring; inserting screws through the first and second engagement members and into the pedicles of the respective first and second vertebrae; rotating the expansion ring in a first direction, thereby distracting the first and second vertebrae; implanting an intervertebral implant between the first and second vertebrae after rotating the expansion ring in the first direction; and rotating the expansion ring in a second direction after implanting the intervertebral implant, thereby compressing the first and second vertebrae.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 13/792,810, filed on Mar. 11, 2013, the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to systems and methods for use in spinal surgery, and in particular, relates to an implant system that allows for distraction, compression and fixation of vertebral bodies. 
     An example of spinal surgery is spinal fusion. Spinal fusion is a procedure that may be a solution to severe or debilitating back pain, instability stemming from spinal disorders such as herniated or torn intervertebral discs, spondylolisthesis (e.g., where the vertebrae move out of position), or degenerative disc disease. Interbody fusion, specifically in the lumbar region, can be performed using a variety of approaches including posterior lumbar interbody fusion (“PLIF”), transforaminal lumbar interbody fusion (“TLIF”), lateral lumbar interbody fusion (“LLIF”), or anterior lumbar interbody fusion (“ALIF”). 
     These lumbar interbody fusion procedures involve the removal of at least a portion of an intervertebral disc from the lumbar section of the spinal column, followed by fusion of the vertebrae that sit above and below the intervertebral disc. Typically, such fusion procedures involve the use of interbody implants (i.e., interbody fusion) and connected posterior structures. Fusion is achieved by distracting the vertebrae and inserting an intervertebral implant typically containing bone graft material into the space created by the removal of the intervertebral disc. Once the intervertebral implant is properly placed, the vertebrae may be compressed against it. Pedicle screws and rods may be used as an adjunct to interbody fusion to restrict the motion of the vertebrae while fusion takes place, to aid in setting the desired angle of lordosis, and to provide further fixation of the vertebrae. 
     Certain instruments exist that are used for distracting the vertebrae such as a pedicle or lamina distractors, for example. Other instruments exist that are used for compression of the vertebral bodies. Some instruments both distract and compress. However, these currently existing distraction and compression instruments are external instruments that are separate and apart from supplemental fixation devices, such as the above-noted pedicle screws and rods. Thus, multiple sets of instruments must be used for one procedure increasing the possibility of operating room confusion. Further, multiple instruments to achieve compression, distraction, and fixation increases the length of the interbody fusion procedure, which increases the probability for complications and may increase recovery time. 
     Therefore, there is a need for an internal fixation system that also acts as a distraction and compression device while utilizing only one set of instruments. 
     BRIEF SUMMARY OF THE INVENTION 
     According to a first embodiment of the present invention, a spinal fixation device is disclosed herein. The spinal fixation device includes a first engagement member. The first engagement member includes a first body and at least one extension. The first body includes a first aperture extending therethrough for receipt of a fastener. The at least one extension extends outwardly from the first body. The spinal fixation device further includes a second engagement member. The second engagement member includes a second body and at least one threaded extension. The at least one threaded extension extends outwardly from the second body, and the second body includes a second aperture extending therethrough for receipt of a fastener. Also included in the spinal fixation device, an expansion ring having a first end and a second end. The first end includes a threaded internal surface in threaded communication with the at least one threaded extension. The second end is rotatable with the at least one extension of the first engagement member such that a torque applied to the expansion ring moves the second engagement member with respect to the first engagement member. 
     Further, the at least one extension may include a first interlocking feature. Also, the second end of the expansion ring may include an internal surface that may include a second interlocking feature. The first interlocking feature and second interlocking feature may communicate rotatably. Additionally, the at least one extension of the first engagement member may be at least two arms elastically cantilevered to the first body. Also, the first interlocking feature of the at least one extension may be a flange, and the second interlocking feature may be a groove. 
     Continuing with this embodiment, a torque applied to the expansion ring may move the first and second engagement members towards and away from each other. Further, the spinal fixation device may include a driver. The driver may include a manipulation end and an engagement end. The engagement end may include a first array of teeth. Moreover, the second end of the expansion ring may include a second array of teeth configured to orthogonally engage the first array of teeth such that applying a torque to the manipulation end rotates the expansion ring, thereby moving the second engagement member with respect to the first engagement member. 
     Additionally, the spinal fixation device may include a plurality of pedicle engagement extensions extending from the first body and the second body. Further, at least one of the plurality of pedicle engagement extensions may include a window extending therethrough for viewing a pedicle bone. Even further, the first engagement member and second engagement member may include a plurality of serrations disposed between the plurality of pedicle engagement extensions. 
     The spinal fixation device may further include at least one first locking ring for locking a fastener. The at least one first locking ring may be disposed within the first aperture of the first engagement member. The spinal fixation device may also include at least one second locking ring that may be disposed within the second aperture of the second engagement member. The locking rings may be split rings. Additionally, the at least one threaded extension may be polyaxially mounted to the second engagement member. 
     According to another embodiment of the present invention, a spinal fixation device including a first screw capture member. The first screw capture member includes a first body and at least one extension. The first body includes a first aperture extending therethrough. The at least one extension extends outwardly from the first body. Also included in the spinal fixation devices is a second screw capture member. The second screw capture member includes a second body and at least one threaded extension, which extends outwardly from the body. The second body includes a second aperture extending therethrough. Additionally, the device includes a first pedicle engagement member. The first pedicle engagement member includes a first passageway extending therethrough and is rotatably mounted with the first screw capture member such that the first passageway of the first pedicle engagement member is in communication with the first aperture of the first screw capture member. Further included in the device, a second pedicle engagement member including a second passageway, which extends therethrough. The second pedicle engagement member is rotatably mounted to the second screw capture member such that the second passageway of the second pedicle engagement member is in communication with the second aperture of the second screw capture member. Also included in the spinal fixation device is an expansion ring. The expansion ring includes a first end and a second end. The first end includes a threaded internal surface, which is in threaded communication with the at least one threaded extension. The second end is in rotatable communication with the at least one extension of the first screw capture member. Where a torque is applied to the expansion ring, the second screw capture member and second pedicle engagement member move with respect to the first screw capture member and first pedicle engagement member. 
     Additionally, the at least one extension may include a first interlocking feature, and the second end of the expansion ring may include an internal surface. The internal surface may include a second interlocking feature such that the first interlocking feature and second interlocking feature may communicate rotatably. Further, the at least one extension of the first screw capture member may be at least two arms elastically cantilevered to the first body. Also, the first interlocking feature of the at least one extension may be a flange, and the second interlocking feature may be a groove. Moreover, where a torque may be applied to the expansion ring, the second screw capture member and second pedicle engagement member may move toward and away from the first screw capture member and first pedicle engagement member. 
     Continuing with this embodiment, the spinal fixation device may include a driver. The driver may include a manipulation end and an engagement end. The engagement end may have a first array of teeth. Further, the second end of the expansion ring may include a second array of teeth configured to orthogonally engage the first array of teeth. This configuration may be such that applying a torque to the manipulation end rotates the expansion ring, thereby moving the second screw capture member and second pedicle engagement member with respect to the first screw capture member and first pedicle engagement member. Moreover, the spinal fixation device may additionally include a plurality of pedicle engagement extensions. The plurality of pedicle engagement extensions may extend from the first and second pedicle engagement members. 
     Additionally, the at least one of the plurality of pedicle engagement extensions may include a window extending therethrough for viewing a pedicle bone. Further, the first and second pedicle engagement members may include a plurality of serrations disposed between the plurality of pedicle engagement extensions. Even further, the spinal fixation device may include at least one first locking ring and at least one second locking ring for locking a fastener. The at least one first locking ring may be disposed within the first passageway of the first pedicle engagement member. The at least one second locking ring may be disposed within the second passageway of the second pedicle engagement member. The locking rings may be split rings. Moreover, the at least one threaded extension may be polyaxially mounted to the second screw capture member. 
     In another embodiment of the present invention, a method for fusing vertebral bodies including a step of mounting a first engagement member of a fixation member to a pedicle of a first vertebra, and a second engagement member of the spinal fixation member to a pedicle of a second vertebra. The first engagement member includes a first extension engaged with an expansion ring. The second engagement member includes a second extension engaged with the expansion ring. The method further includes a step of inserting screws through the first and second engagement members and into the pedicles of the respective first and second vertebrae. Additionally, the method includes the step of rotating the expansion ring in a first direction, thereby distracting the first and second vertebrae. Further included is the step of rotating the expansion ring in a second direction, thereby compressing the first and second vertebrae. 
     Additionally, the method may include the step of implanting an intervertebral implant between the first and second vertebrae, whereby the step of rotating to distract occurs before implanting and the step to compress occurs after this step. Further, the method may include the steps of engaging the expansion ring with a tool and actuating the tool to rotate the expansion ring. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspect, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
         FIG. 1  is a front perspective view of a posterior fixation assembly. 
         FIG. 2  is an exploded perspective view of the posterior fixation assembly of  FIG. 1 . 
         FIG. 3  is a side view of the posterior fixation assembly of  FIG. 1 . 
         FIG. 4  is rear perspective view of the posterior fixation assembly of  FIG. 1 . 
         FIG. 5  is a cross-sectional view of the posterior fixation assembly of  FIG. 1  along line A-A. 
         FIG. 6  is a perspective view of a driver engaging the posterior fixation assembly. 
         FIG. 7 a    is a rear view of two posterior fixation assemblies fixed onto the pedicles of vertebral bodies. 
         FIG. 7B  is a side view of a posterior fixation assembly fixed onto the pedicles of vertebral bodies. 
         FIG. 7C  is a top view of posterior fixation assemblies fixed onto the pedicles of vertebral bodies. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1-4  depict a posterior fixation assembly  10  in accordance with an embodiment of the present invention. The posterior fixation assembly  10  generally includes a first screw capture element  40 , a second screw capture element  50 , and an expansion ring  60 . The posterior fixation assembly  10  may be made from any biocompatible material, such as titanium and stainless steel. 
     As shown, the first screw capture element  40  includes a cylindrical portion  43 , a linking extension  41 , and at least two spring arm extensions  42   a ,  42   b . The cylindrical portion  43  has an internal passageway  46  for receiving a screw to allow connection to a portion of a vertebral body, such as a pedicle. The internal passageway  46  may also be adapted to rotatably receive a portion of a pedicle engagement device  20  as described below. The linking extension  41  extends outwardly from the cylindrical portion  43  and generally orthogonally with respect to the longitudinal axis thereof. The linking extension  41  may be an annulus-type structure with a hollow interior and a sidewall. The at least two spring arm extensions  42   a ,  42   b  may extend from the linking extension  41  and may be curved to receive a portion of the threaded extension  51  as described below. The spring arm extensions  42   a ,  42   b  are generally separated by a distance that allows for movement towards each other. One end of each of the spring arm extensions  42   a ,  42   b  may include a flange  45   a ,  45   b . Further, the spring-arm extensions  42   a ,  42   b  may be tapered to facilitate connection to the expansion ring and to facilitated receipt of the threaded extension  51  as described below. In one embodiment, the spring arm extensions  42   a ,  42   b  and the linking extension  41  may be manufactured as a monolithic component, which cantilevers the spring arm extensions  42   a ,  42   b  with the linking extension  41 . This cantilever feature and the spaces separating the spring arm extensions  42   a ,  42   b  allow the spring arm extensions  42   a ,  42   b  to have a spring-like action. 
     In one embodiment (not shown), the first screw capture element  40  may be configured for multilevel spinal fixation. In other words, the posterior fixation assembly  10  may be configured to allow for the distraction, compression, and fixation of more than a single level of vertebrae utilizing at least one interconnected device. In this embodiment, the first screw capture element  40  may include two linking extensions  41  extending outwardly from the cylindrical portion  43  generally in opposite directions from each other. Further, each linking extension  41  may include at least two spring arm extensions  42   a ,  42   b  extending from each linking extension  41 . This configuration provides the operator flexibility in building the device  10  to accommodate the patient&#39;s needs. 
     The second screw capture element  50  generally includes a cylindrical portion  52  and a threaded extension  51 . The cylindrical portion has an internal passageway  53  for receiving a screw to allow connection to a pedicle. The internal passageway  53  may be adapted to receive a portion of a pedicle engagement device  20  as described below. The threaded extension  51  extends outwardly from the cylindrical portion  52 . 
     In one embodiment, the threaded portion  51  may extend orthogonally with respect to the longitudinal axis of the cylindrical portion. Another embodiment may include an overall system that may be provided with various options for the angle of the threaded extension  51 . For example, a kit may be provided with several second screw capture elements  52  with threaded extensions  51  that are angled anywhere from 0 to 20 degrees with respect to the longitudinal axis of the cylindrical portion to provide a desired angle of lordosis when fully implanted. Further, this kit may include several first screw capture elements  40  that include one linking extension  41  and several first screw capture elements  40  that include two linking extensions  41 . In another embodiment, the threaded extension  51  may be removably connected to the cylindrical portion allowing a polyaxial-type movement, for example a lockable ball and socket configuration. These multi-angle options provide the surgeon with flexibility in setting the desired angle of lordosis. 
     In another embodiment (not shown), the second screw capture element  50  may also be configured for multilevel spinal fixation. In this embodiment, the second screw capture element  50  may include two threaded extensions  51  extending outwardly from the cylindrical portion  52  generally in opposite directions from each other. Further, this embodiment may be provided with the multi-angle options provided the second screw capture element  52  that includes a single threaded extension  51  and may also be included in the kit, as described above. This configuration provides the operator flexibility in building the device to accommodate the patient&#39;s needs. 
     In a further embodiment (not shown), either or both of the first and second screw capture feature  40 ,  50  may have an offset feature. This offset feature may allow either the first or second screw capture feature  40 ,  50  to sit more anteriorly, medially, or laterally with respect to the other when fixed to the spine in order to more readily conform to the patient&#39;s anatomy. 
     Both the first screw capture element  40  and second screw capture element  50  may also include a pedicle engagement device  20 . The pedicle engagement device  20  has an internal passageway  25  extending therethrough and may also include a long extension  21 , a short extension  22 , serrations  24  and a locking ring  70 . The internal passageway  25  is generally cylindrical. However, a portion of the internal passageway  25  may have a non-uniform cross-section adapted to receive the locking ring  70 . One end of the pedicle engagement devices  20  is configured to rotatably engage the first and second screw capture elements  40 ,  50  as described below. The long extension and short extension  21 ,  22  extend from the other end of the pedicle engagement device  20  and generally sit opposite each other forming a mouth-like structure for engaging a portion of a vertebra, such as a pedicle. The long extension  21  is configured to overlap and engage a portion of the superior side of a pedicle, while the short extension  22  is configured to overlap and engage a portion of the inferior side of the pedicle. The serrations  24  are disposed along the distance between the long and short extension for aiding in the fixed engagement with the vertebra. In one embodiment, the long extension  21  may include a window  23 , which provides enhanced visibility to ensure the pedicle engagement devices  20  are properly seated onto the pedicle. 
     The pedicle engagement device  20  may have a different size and shape depending on the location along the spine the procedure is being performed. For example, the long and short extensions  21 ,  22  may be different sizes and lengths for L3 and L4 than for L5 and S1. Further, a system could include multiple geometric variations to provide substantial conformity to the patient population. Additionally, pedicle engagement devices  20  may even be customized to the individual patient&#39;s anatomy by using data collected from an MRI or CT scan. 
     The locking ring  70  is a ring that may be split at a portion along its circumference to provide for expansion and contraction. A series of grooves may be arranged along the internal surface of the ring for engagement with a torque applying device. As shown, the locking ring  70  is rotatably disposed within the internal passageway  25  of the pedicle engagement device  20  within the non-uniform cross-sectional portion. The non-uniformity of the cross-section allows for the locking ring  70  to contract as it is rotated, thereby closing the locking ring  70  and restricting its further rotation. 
     In one embodiment, a portion of the pedicle engagement devices  20  is disposed within the first and second screw capture elements  40 ,  50 . This rotational ability of the pedicle engagement devices  20  facilitates optimal seating onto the pedicles. This rotatable connection may be achieved by a ridge that runs along the outer surface of the pedicle engagement device  20  that engages a groove within the internal passageway  46 ,  53  of the first and second screw capture elements  40 ,  50 . 
     While the pedicle engagement devices  20  and first and second screw capture elements  40 ,  50  are described herein as being discrete elements that are in a rotational configuration, it is recognized that these elements may be monolithically integrated. In such an embodiment the pedicle engagement device  20  and second screw capture element  50  would be one monolithic device, and another pedicle engagement device  20  and the first screw capture element  40  would be another monolithic device. 
     In accordance with one embodiment of the current invention, the expansion ring  60  may include one end that is internally threaded and another end that includes an array of teeth along the circumference. The expansion ring may also have geometry along the inner surface for engagement with flanges  45   a ,  45   b , as seen in  FIG. 5 . 
     In assembling device  10 , the spring arm extensions  42   a ,  42   b  of the first screw capture element  40  are inserted into the teeth-end of the expansion ring  60  until the flanges  45   a ,  45   b  engage with the internal geometry of the expansion ring  60 . The spring-like action firmly seats the flanges  45   a ,  45   b  into place. This allows for rotation of the expansion ring  60  with respect to the first screw capture element  40 , but prevents translational movement. The linking extension  41  preferably has an opening  44  in the sidewall to expose the teeth of the expansion ring. This exposure allows for engagement of a torque applying device, such as driver  80  as seen in  FIG. 6 , to the teeth  61  in order to apply a torque to the expansion ring  60 . 
     The threaded extension  51  is screwed into the threaded-end of the expansion ring  60 . Once the threaded extension  51  has traveled a certain distance into the expansion ring  60 , the threaded extension  51  moves into the open area between the spring arm extensions  42   a ,  42   b , thereby preventing the flanges  45   a ,  45   b  from disengaging the internal geometry of the expansion ring  60 , as shown in  FIG. 5 . This configuration between the threaded extension  51 , expansion ring  60 , and spring arm extensions  42   a ,  42   b  allows for a torque applied to the expansion ring  60  via the teeth  61  to move the second screw capture element  50  closer or further away (depending upon the direction of the torque) from the first screw capture element  40 , thus providing for compression and distraction of the vertebrae  90  attached to assembly  10 . 
     In one exemplary embodiment of the posterior fixation assembly  10 , where multilevel spinal distraction, compression and fixation is desired, the first screw capture element  40  that includes a single linking extension  41  is substituted by a first screw capture element  40  that includes two linking extensions  51 . In this embodiment, an expansion ring  60  and first screw capture element  40  are assembled to each linking extension  41  as described above. This configuration would provide for two levels of fixation, or three vertebrae. Where more than two levels are desired, one of the second screw capture elements  50  that include a single threaded extension  51  may also be substituted for by a second screw capture element  50  that includes two threaded extensions  51 . Thus, depending on the number of levels of vertebrae sought to be fused, there may be a first or second screw capture element  40 ,  50  that includes a single linking extension  41  or threaded extension  51 , respectively, at the ends of the posterior fixation device  10 , and a series of alternating first and second screw capture elements  40 ,  50  that include two linking extensions  41  and two threaded extensions  51 , respectively, disposed therebetween. 
       FIGS. 7A-C  illustrate the posterior fixation assembly  10  in fixed engagement with pedicles of vertebral bodies, which may be achieved in any number of spinal interbody procedures including PLIF, TLIF, LLIF, and ALIF. As an example, where a transforaminal approach is taken and single level of vertebrae  90  is identified for fusion. An awl is hammered into the pedicles of the superior vertebra  91  and inferior vertebra  92  on one side of the spinous process. A hole is then drilled into the indentation created by the awl. A wire is then placed into the drilled holes and the first and second screw capture elements  40 ,  50  are routed over the respective wires. The pedicle engagement devices  20  of the first and second screw capture elements  40 ,  50  are initially seated onto the pedicles with the long extensions  21  oriented on the superior side of their respective pedicles. The pedicle engagement devices  20  may be rotated as needed in order to properly orient the long and short extensions  21 ,  22  with respect to the geometry of the pedicles. A cannulated screw may be passed over each wire through the respective first and second screw capture elements  40 ,  50 . The cannulated screws may be fully seated by screwing the cannulated screws into the pedicles to the desired torque. As the screws move into a fully seated position, the serrations  24  of the pedicle engagement devices  20  are brought into engagement with bone to provide a secure attachment to the respective vertebra  91 ,  92 . Once the screws are fully seated, a torque may be applied to the locking rings  70  via a torque applying device. As each locking ring  70  rotates, the non-uniform geometry of the pedicle engagement device forces the locking ring  70  to clamp down on the cannulated screw, thereby locking it into position to prohibit back-out. The driver  80  is then placed into interlocking engagement with the teeth  61  as seen in  FIG. 6  and rotated, thus rotating the expansion ring  60  resulting in the distraction of the vertebral bodies. 
     The surgeon then prepares the disc space on the opposite side of the spinous process and inserts an intervertebral implant with bone graft into the space. Bone graft may be autologous, allograph, demineralized bone matrix (DBM), bone morphogenic protein (BMP) or any known combination of biologics. The driver  80  is then rotated in the opposite direction, thereby turning the expansion ring  60  in the opposite direction resulting in compression of the intervertebral implant and biologics. This compression also allows the surgeon to accurately set the desired angle of lordosis for the individual level of the spine. Achieving this angle may be further facilitated by use of a second screw capture element  50  that has an angled threaded extension  51 , as described above. Further, the compression stimulates bone growth in accordance with Wolff&#39;s law. 
     Once the angle of lordosis is set, the surgeon then fixates another posterior fixation assembly  10  on the pedicles on the same side of the spinous process as the disc preparation was performed. This is achieved through the same steps as the first fixation by indenting with the awl, drilling the indentation, wiring the drilled hole, guiding the first and second screw captured devices  40 ,  50  and cannulated screws over the wire onto the pedicles, and seating the first and second screw capture features  40 ,  50 ; thereby achieving internal fixation and implantation. 
     In an example of a method of use in a PLIF procedure, the surgeon may fully seat a posterior fixation assembly  10  on both sides of the spinous process prior to preparation of the intervertebral space, as seen in  FIG. 7A . Thus, after using a posterior approach and identifying the level for fusion, the surgeon hammers an awl into the pedicles of the superior vertebra  91  and inferior vertebra  92  on both sides of the spinous process. The surgeon then drills a hole at each indentation and wires the holes. The posterior fixation assemblies are routed over the wires via the first and second screw capture elements  40 ,  50 . A cannulated screw is guided over the wires and screwed into the drill holes, thereby fixing the posterior fixation assemblies  10  onto the vertebrae  90 . The locking rings  70  are clamped onto the screws. Screwdriver  80  may then be used is used to engage the teeth  61  of each posterior fixation assembly  10  and to rotate the expansion rings  60  resulting in distraction of the vertebral bodies, as seen in  FIG. 7B . The surgeon then prepares the disc space between the respective posterior fixation assemblies  10  and the spinous processes. After implantation of the intervertebral implant, the screwdrivers  80  rotate the expansion rings  60  to provide for compression. Once implantation and supplemental fixation have been achieved, the patient is closed up. 
     In an ALIF procedure, the disc space is prepared and implanted first through an anterior approach. The patient is then rotated and a posterior fixation assembly  10  is then attached to the pedicles on each side of the spinous process with the same steps using the awl, drill, wires, and seating of the first and second capture features. 
     The posterior fixation assembly  10  may also be used for other applications such as corrective surgery for scoliosis. In a method of use for scoliosis, an initial surgery would be performed installing the posterior fixation assemblies  10 , and then subsequent procedures would be performed wherein the surgeon may use the driver  80  to turn the expansion rings  60  to further expand or contract the disc spaces. Thus, over time, the incorrect curvature of the spine can be corrected. 
     In the above described methods of use, the posterior fixation assembly  10  may be fixed to the vertebrae  90  such that the first screw capture element  40  may be oriented superior or inferior to the second screw capture element  50 . In other words, the first screw capture element  40  may be secured to the superior vertebra  91  while the second screw capture element  50  may be secured to the inferior vertebra  92 , as shown in  FIGS. 7A and 7B . Inversely, the second screw capture feature  50  may be secured to the superior vertebra  91  while the first screw capture element  40  may be secured to the inferior vertebra  92  (orientation not shown). Thus, the orientation of the posterior fixation assembly  10  may be flipped 180 degrees without altering its distraction, compression, and fixation capabilities. 
     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.