Patent Publication Number: US-2007123989-A1

Title: Method and instruments to treat spondylolisthesis by an anterior minimally invasive approach of the spine

Description:
TECHNICAL FIELD  
      This application is related to U.S. Provisional Patent Application No. 60/728,919, entitled “METHOD AND INSTRUMENTS TO TREAT SPONDYLOLISTHESIS BY AN ANTERIOR MINIMALLY INVASIVE APPROACH OF THE SPINE”, filed Oct. 21, 2005, which is incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD  
      The invention relates to methods and instruments that may be used for intra-operative surgical treatment of spondylolisthesis by an anterior minimally invasive approach of the spine.  
     BACKGROUND OF THE INVENTION  
      Spondylolisthesis is a term used to describe when one vertebra slips forward on the vertebra below it ( FIG. 1 ). This usually occurs because there is a spondylolysis in the superior vertebra. There are two main parts of the spine that keep the vertebrae aligned, the disc and the facet joints. When spondylolysis occurs, the facet joint can no longer hold the vertebra back. The intervertebral disc may slowly stretch under the increased stress and allow the upper vertebra to slide forward. In the vast majority of cases, stretching of the intervertebral disc only allows for a small amount of forward slip.  
      Surgical treatment for spondylolisthesis needs to address both the mechanical symptoms and the compressive symptoms if they are present. Usually this means that the nerves exiting the spine should be freed of pressure and irritation. Performing a complete laminectomy (removing the lamina) usually accomplishes relieving the pressure and irritation on the nerves exiting the spine. Removing the lamina allows more room for the nerves. It also enables the surgeon to remove the lump of tissue surrounding the spondylolysis defect. The result is reduced irritation and inflammation on the nerves. Once the nerves are freed, a spinal fusion is usually performed to control the segmental instability. (source: www.spineuniversity.com)  
      The goals of surgery are to remove pressure on spinal nerves (i.e., decompression) and to provide stability to the lumbar spine. In most cases of spondylolisthesis, lumbar decompression should be accompanied by uniting one spinal vertebra to the next (i.e. spinal fusion) with spinal instrumentation (i.e., implants that are often used to help aid the healing process). Surgery can be performed from the back of or posterior approach to the spine (i.e., distraction and reduction can be achieved before tightening the posterior fixation) and/or from the front or an anterior approach to of the spine (i.e., anterior fusion). Such methods negatively affect the vital posterior muscular structures.  
     SUMMARY OF THE INVENTION  
      The present invention provides a a method of performing spondylolisthesis reduction. Preferably the method, instruments and implants preserve the vital posterior muscular structures, thus reducing the surgical morbidity associated with fusion surgery, preferably including lumbar fusion surgery.  
      The method includes the steps of inserting an interbody spacer between two vertebrae, and attaching an anatomically designed reduction plate to the two vertebrae by two screws. The reduction plate includes upper and lower boreholes, where at least one screw, using a stable plate-screw connection, is fixed into the vertebra which is more anteriorly positioned than the other vertebra, and the other screw, a non-locking screw, is fixed to the other vertebra. The method further includes driving the non-locking screw to reduce the vertebral slippage distance.  
      In another embodiment, the method includes the steps of inserting an interbody spacer between two vertebrae, attaching an anatomically designed reduction plate to at least one of the two vertebrae using at least one non-locking screw, and attaching the interbody spacer to the reduction plate by a fastening means through a borehole, preferably a central borehole, of the reduction plate and the interbody spacer. The interbody spacer may be attached to the anteriorly positioned vertebra by at least one bone screw. The method further includes rotating the non-locking screw to reduce the vertebral slippage distance.  
      In still another embodiment, the method includes inserting an interbody spacer between two vertebrae, where the interbody spacer may be attached to the vertebrae by locking screws. The method further includes inserting a locking screw mechanism, and adjusting the locking screw mechanism such that the vertebrae are aligned vertically, wherein the superior or upper vertebra is moved in relation to the inferior or lower vertebra.  
      In a further embodiment, the method includes attaching pedicle screws to vertebrae surrounding a vertebra exhibiting a spondylolisthesis condition, attaching preassembled pedicle screws into the vertebra exhibiting the spondylolisthesis condition, and attaching rods to the pedicle screws. The method further includes repositioning the vertebra exhibiting the spondylolisthesis condition using a reduction instrument such that the head of the preassembled pedicle screws coincide with the rods, and affixing the preassembled pedicle screws to the rods using locking caps and a screwdriver.  
      In still a further embodiment, the method includes inserting a screw into the anterior area of adjacent vertebrae, where one of the adjacent vertebrae exhibits a spondylolisthesis condition, and fixing the screw attached to the vertebra not exhibiting the spondylolisthesis condition to an external rigid element. The method further includes using an adjustable mechanism to adjust the screw inserted into the vertebra exhibiting the spondylolisthesis condition until slippage distance of that vertebra is reduced. This method may be performed externally from an incision area.  
      Other objectives and advantages in addition to those discussed above will become apparent to those skilled in the art during the course of the description of a preferred embodiment of the invention which follows. In the description, reference is made to accompanying drawings, which form a part thereof, and which illustrate an example of the invention. Such example, however, is not exhaustive of the various embodiments of the invention, and the claims that follow should not be limited to the examples shown. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The spondylolisthesis reduction methods and instrumentation are explained in even greater detail in the following exemplary drawings, wherein the instrumentation and methods of operation may be better understood and wherein like references numerals represent like elements. The drawings are merely exemplary to illustrate the structure, operation and method of treating spondylolisthesis and certain features that may be used singularly or in combination with other features and the invention should not be limited to the embodiments shown.  
       FIG. 1  depicts a segment of a spine where one vertebra disc has moved or slipped forward of the other vertebrae (spondylolisthesis);  
       FIG. 2  is a side view of an embodiment of the present invention;  
       FIG. 3  is a side view of a modification of the embodiment depicted in  FIG. 2 ;  
       FIG. 4A  is a side view of another embodiment of the present invention;  
       FIGS. 4B and 4C  are side views of the before and after positions of the implant surfaces of the embodiment depicted in  FIG. 4A ;  
       FIGS. 5A and 5B  are perspective views of different embodiments of the implant of FIGS.  4 A-C;  
       FIGS. 6A  and B are side views of another embodiment of the present invention;  
      FIGS.  7 A-D are side views of a reduction instrument;  
       FIGS. 8A  and B are side views of another embodiment;  
      FIGS.  9 A-D are side views of another embodiment; and  
       FIGS. 10A and 10B  are views of another embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Spondylolisthesis reduction can be performed either within the wound site (in situ) or outside the wound site (ex situ), where the wound site refers to the area of incision.  
      The in-situ methods allow for spondylolisthesis reduction by a minimally invasive approach, preferably using an implant as a reduction device.  
      In a preferred embodiment, as depicted in  FIG. 2 , an interbody spacer  10  (e.g. metallic, allograft or polymeric cage) may be placed between the two vertebrae  1 ,  2 . An anatomically designed reduction plate  20  with upper and lower borehole(s) may be placed in front of the treatment segment (site) and attached to the vertebrae by at least one upper screw  24  and at least one lower screw  25 . The reduction plate  20  may either be straight or curved (pre-stressed). By fixing one screw  24  into the vertebra positioned more anteriorly by a stable or locking plate-screw connection  26  (LCP locking screw concept), the other non-locking screw  25  may be used to reduce the vertebral slipping distance B by driving the screw in a direction A, as depicted in  FIG. 2 . This is accomplished by a “lagging feature” which occurs when the head of the screw  25  comes in contact with the reduction plate  20 , further turning of screw  25  causes the displaced vertebra to move posteriorly and align vertically with the other vertebrae.  
      In an alternative configuration ( FIG. 3 ) of the embodiment depicted in  FIG. 2 , the reduction plate  20  may be affixed by at least one screw  25  to the vertebra  2  that has slipped forward, and optionally a second screw  24  may be connected to the other vertebra  1 . Further, the interbody spacer  10  may preferably be connected to the reduction plate  20 , and is preferably expandable in height. The reduction plate  20  may have a central borehole  23  provided, preferably, with an internal thread (not shown), and preferably parallel to the upper and lower boreholes  21 ,  22 . Correspondingly thereto, the interbody spacer  10  may have a central borehole  23  with an internal thread (not shown) for accommodating a fastening means  27 , for example a screw, for fastening the reduction plate  20  to the interbody spacer  10 . The interbody spacer  10  may have additional boreholes  13  such that the axes of these boreholes  13  are not parallel to each other or the central borehole  23 . From the front surface of the interbody spacer  10 , the additional boreholes may diverge. At least one longitudinal fixation element  12 , for example a bone screw, may be used to further connect the interbody spacer  10  to the vertebra  1 , thereby increasing the rotational stability of the reduced segment. The non-locking screw  25  may be used to reduce the vertebral slippage B by rotating the screw in a similar manner as in the embodiment depicted in  FIG. 2 .  
      In another preferred embodiment, an interbody spacer  30 , (e.g. SynCage, SynFix) is depicted in FIGS.  4 A-C. The interbody spacer  30  may comprise two horizontal halves consisting of an upper half  31  and a lower half  32 . The interbody spacer  30  may be placed between two vertebrae  1 ,  2 , so that the contact surface  33  of the upper half  31  and the contact surface  34  of the lower half  32  fit the curvature of the upper  3  and lower  4  endplates of the vertebrae, respectively. The upper half  31  and lower half  32  of the interbody spacer  30  are fixed to the adjacent vertebrae with a locking screw mechanism  40 . The interbody spacer  30  may further be attached to the adjoining upper and lower vertebrae  1 ,  2  by corresponding locking screws  35 .  
      The slipping distance B of one vertebra can then be reduced by the locking screwing mechanism  40  that brings the interbody spacer halves  31 ,  32  into vertical alignment with each other, and thus realign the spine, as shown in  FIG. 4C . The locking screwing mechanism  40  preferably moves the lower half  32  of the interbody spacer  30  with respect to the upper half  31  of the interbody spacer  30 .  
      The locking screwing mechanism  40  of the interbody spacer  30  may comprise a central screw ( FIG. 5A ) that upon rotation may move one of the halves  31 ,  32  either forward or backward. In  FIG. 5B , the screwing mechanism  40  may be a central rail that allows forward and backwards movement of the upper and lower halves  31 ,  32  and a lateral pin/rod or ratchet  41  to secure the two halves  31 ,  32  in position. The lateral pin  41  may project through the lateral sides  36 ,  37  of one of the halves  31 ,  32 . As shown in  FIG. 5B , the lateral pin is inserted in the upper halve  31  of the interbody spacer  30 .  
      In another embodiment, as depicted in  FIGS. 6A and 6B , spondylolisthesis reduction may be accomplished using pedicle screws, spondylo screws or similar  70 , rods  71 , repositioning instruments  50 , and preassembled pedicle screws  72 . In this embodiment, spondylolisthesis reduction is accomplished from the posterior.  
      The reduction instrument  50  (FIGS.  7 A-D) may include three main assemblies, an inner tube  51 , a reduction sleeve  55 , and a guiding tube  61 . The inner tube  51  may include a linear shaft  52  having a slot  54  at the distal end  64  and a perpendicular handle  53  at its proximal end  62 . The reduction sleeve  55  may also have a linear shaft  56  with a slot  60  at its distal end  65 . The linear shaft  56  may have external threads  57  at the proximal end  63  about which a nut  59  is attached. The nut  59  may be used to pull a preassembled pedicle screw  72  towards a rod  71 . Also attached to the proximal end  63  of the reduction sleeve  55  is a handle  58 , perpendicular to the linear shaft  56 . The linear shaft  56  of the reduction sleeve  55  is hollow, allowing the inner tube  51  to be inserted into the proximal end  63  of the reduction sleeve  55 . The third main assembly of the reduction instrument  50  is a guiding tube  61  which fits over the reduction sleeve  55  and which tightens the instrument securely to the implant.  
      Pedicle screws  70  are attached to the vertebrae on either side of the displaced vertebra. One or more preassembled pedicle screws  72  are attached to the displaced vertebra. Rods  71  are inserted and locked onto the pedicle screws  70  attached to either side of the displaced vertebra. Reduction instrument  50  is placed over each preassembled pedicle screw  72 . The nut  59  and reduction sleeve  55  on the reduction instrument  50  are simultaneously rotated to gradually pull the preassembled pedicle screws  72  to the rod  71  which moves the displaced vertebra. More specifically, the guiding tube  61  is moved distally as the nut  59  is rotated so that the distal end  66  of the guiding tube  61  contacts the rod which is arranged in the slot  60  of the reduction sleeve  55 . Further rotation moves the reduction sleeve  55  relative to the guiding tube  61  which pulls the preassembled pedicle screw  72  and hence the vertebra upward. Once the preassembled pedicle screw  72  coincide with the rod  71 , so that the rod is within a channel (not shown) in the top of the preassembled pedicle screw  72 , the inner tube  51  of the reduction instrument  50  is removed and a long screwdriver with a locking cap (not shown) is inserted in the proximal end  63  of the reduction sleeve  55 . The locking cap may be affixed onto the head of the preassembled pedicle screw  72 , thereby securing the rod  71  to the preassembled pedicle screw  72 .  
      The ex-situ methods for spondylolisthesis reduction allows for a minimally invasive procedure outside the wound site using adequate instruments.  
      In one embodiment of the ex-situ method, depicted in  FIGS. 8A and 8B , a screw  80  is inserted into the anterior part of each vertebral body at the levels to be reduced. One of the screws  80  is fixed to an external rigid element  90  (e.g. SynFrame) attached to, for example, a surgical table. The second screw  80 , which is not fixed to the surgical table, is attached to an adjustable mechanism  83  (e.g. a thread member). The second screw  80  may be displaced by the adjustable mechanism  83  until the slipping distance is reduced. As noted, the adjustable mechanism  83  may be a thread member such that the thread of the second screw  80  corresponds to the thread of the adjustable mechanism forming a screw-in-screw type configuration, such that when the adjustable mechanism is rotated it pulls the displaced vertebra upwards (posterior direction).  
      In another embodiment, as depicted in FIGS.  9 A-D, spondylolisthesis reduction may be accomplished using a replacement support system  100 . The replacement support system  100  may include an outer support  110 , one or more bone screws  120 , an inner support  130 , and one or more translation screws  140 .  
      The outer support  110  may have, for example, a U-shape with two sides  111 ,  112  and a connecting piece  113 . One side  111  may have at least two holes  114 ,  115 . The wall of hole  115  may have threads for engaging the threads of a screw. Whereas, the wall of hole(s)  114  is preferably smooth. The other side  112  may have at least one hole  116  whose wall is also preferably smooth.  
      The inner support  130  may also be U-shaped, similar to the outer support  110 , with sides  131  and  132  and connecting piece  133 . Both sides  131 ,  132  may each have at least one hole  134 ,  136 . The wall of hole  134  is preferably smooth, whereas the wall of hole  136  preferably has threads. The inner support  130  may be smaller than the outer support  110  such that it may be positioned between sides  111  and  112 .  
      The following describes the assembly and method of using the replacement support system  100 . After having mobilized/distracted a spinal segment, a spacer  90  may be inserted between two vertebrae. The spacer  90  may be fixed to first vertebra  200  by a locking screw mechanism  300 . The replacement support system  100  may be assembled such that the outer support  110  is attached to the spacer  90  by a screw  101  or other fixation device through hole  115 . One or more bone screws  120  or similar fixation means may be screwed into the second vertebra  400 . The one or more bone screws  120 , extending through holes  114  and  134 , are supported by but not affixed to the outer support  110  and inner support  130 . Sides  112  and  132  are coupled through one or more translation screws  140 , such that the one or more translation screws are supported by the outer support  110  but connected to the inner support  130  by corresponding threads on the screw and wall of hole  136 . Rotation of the one or more translation screws  140  allow for movement of the inner support  130  with respect to the outer support  110 . Movement can consist of either pulling or pushing back one of the second vertebra.  
      After the replacement support system  100  has been installed onto the vertebrae, the second vertebra  400  can be pulled or pushed back by rotating the one or more translation screws  140  until the first and second vertebrae are aligned such that the spacer  90  may be fixed onto the second vertebra  400 . Following the repositioning procedure, one or more screws  300  may be inserted into the spacer  90  and second vertebra  400 , fixing the spacer  90  to the second vertebra  400  ( FIGS. 9B and 9C ). Once the one or more screws  300  fixing the spacer  90  and second vertebra  400  are in place, the replacement support system  100  can be removed ( FIG. 9D ).  
      In another embodiment, a spacer  500  may be expanded allowing for repositioning and distracting of vertebrae. The spacer  500  may comprise an upper and lower spacer plates  510 ,  520 . The spacer plates  510 ,  520  may have the shape and footprint similar to existing interbody fusion implant geometries. The spacer plates  510 ,  520  may be connected by two or more bars  530 ,  540 . The bars  530 ,  540  may be connected to the spacer plates  510 ,  520  by a hinge, joint, or some similar connecting means  531 ,  532 ,  541 ,  542 . As shown in  FIG. 10A , the spacer  500  is in an unexpanded form, in which the bars  530 ,  540  are substantially parallel with the spacer plates  510 ,  520 . In  FIG. 10B , the spacer  500  is in an expanded form, where the spacer plates  510 ,  520  are positioned further apart from one another and the bars  530 ,  540  are substantially perpendicular to the spacer plates  510 ,  520 . The angle of the bars  530 ,  540  with respect to the spacer plates  510 ,  520  may be determined/chosen according to the amount of repositioning and distraction needed. A fixation mechanism (not shown) within the joints/hinges maintain the angle of the bars  530 ,  540  with respect to the spacer plates  510 ,  520 , stabilizing the structure of the spacer  500  and ensuring that the reposition of the vertebrae does not move subsequently.  
      The spacer  500 , in its unexpanded form, may be inserted between two vertebrae (not shown) exhibiting spondylolisthesis. The upper and lower spacer plates  510 ,  520  may be fixed to the vertebrae with screws (not shown). After the upper and lower spacer plates  510 ,  520  have been fixed to the vertebrae, the spinal segment is repositioned and distracted by expanding the spacer  500  such that the space between the vertebrae is increased and simultaneously repositioning the vertebrae until they are aligned. The bars  530 ,  540  and fixation mechanisms ensure the spacer  500  maintains its expanded form, thereby stabilizing the spinal segment. The void created between the spacer plates  510 ,  520  may be filled with autologous bone or bone substitute to allow for fusion between the upper and lower vertebrae. The lateral and posterior parts of the spacer  500  may be surrounded by a membrane, initially fixed to the spacer plates  510 ,  520 , to avoid the autologous bone or bone substitute from escaping.  
      Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.  
      It will be appreciated by those skilled in the art that various modifications and alterations of the invention can be made without departing from the broad scope of the appended claims. Some of these have been discussed above and others will be apparent to those skilled in the art.