Patent Publication Number: US-2005131407-A1

Title: Flexible spinal fixation elements

Description:
FIELD OF THE INVENTION  
      This application relates to tools for use in spinal surgery, and in particular to a spinal fixation element that is flexible prior to locking, and methods for implanting the same.  
     BACKGROUND OF THE INVENTION  
      Spinal fusion is a procedure that involves joining two or more adjacent vertebrae with a bone fixation device so that they no longer are able to move relative to each other. For a number of known reasons, spinal fixation devices are used in orthopedic surgery to align and/or fix a desired relationship between adjacent vertebral bodies. Such devices typically include a spinal fixation element, such as a relatively rigid fixation rod, that is coupled to adjacent vertebrae by attaching the element to various anchoring devices, such as hooks, bolts, wires, or screws. The fixation elements can have a predetermined contour that has been designed according to the properties of the target implantation site, and once installed, the instrument holds the vertebrae in a desired spatial relationship, either until desired healing or spinal fusion has taken place, or for some longer period of time.  
      Recently, the trend in spinal surgery has been moving toward providing minimally invasive devices and methods for implanting spinal fixation devices. The use of rigid, generally elongate spinal fixation elements, however, can be difficult to implant using minimally invasive techniques. One such method, for example, is disclosed in U.S. Pat. No. 6,530,929 of Justis et al., which utilizes two percutaneous access tubes for introducing an anchoring device, such as a spinal screw, into adjacent vertebrae. A spinal rod is then introduced through a third incision a distance apart from the percutaneous access sites, and the rod is transversely moved into the rod-engaging portion of each spinal screw. The percutaneous access tubes can then be used to apply closure mechanisms to the rod-engaging heads to lock the rod therein. While this procedure offers advantages over prior art invasive techniques, the transverse introduction of the rod can cause significant damage to surrounding tissue and muscle. Moreover, the use of three separate access sites can undesirably lengthen the surgical procedure.  
      Accordingly, there remains a need for improved minimally invasive devices and methods for introducing a spinal fixation element into a patient&#39;s spine.  
     SUMMARY OF THE INVENTION  
      The present invention generally provides a spinal fixation element that is formed from an elongate, bioimplantable member having at least two segments that are selectively movable with respect to one another. As a result, the elongate member is configurable in a first, flexible position, in which the segments are adapted to be angularly manipulated with respect to one another, and a second, locked position, in which the segments are aligned in a desired orientation and are immovable with respect to one another. Each segment preferably has a shape that is adapted to prevent movement between the segments when the segments are in the second, locked position.  
      The segments can have a variety of configurations, and in one embodiment, each segment can include a female end and an opposed male end such that the female end of each segment is adapted to nest the male end of an adjacent segment. In another embodiment, each segment has a substantially tubular shape with a concave end and an opposed convex end such that the concave end of each segment is adapted to nest the convex end of an adjacent segment. In yet another embodiment, every other segment preferably has a substantially spherical shape and intervening segments have a substantially tubular shape with opposed ends that are adapted to seat the spherical segments.  
      In other aspects of the invention, the elongate body can include at least two elongate segments that are mated to one another at an end thereof by a hinge. A sleeve member can be disposed around the hinge to maintain the elongate body in the second, locked position. Alternatively, or in addition, the device can include a locking mechanism that is adapted to mate to the hinge to maintain the elongate body in the second, locked position.  
      The present invention also provides a spinal fixation element that is formed from an elongate body that includes first and second separate segments. Each segment can be in the form of a generally elongate, hemi-spherical rod having two portions connected to one another at an end thereof by a hinge, and the hinge on each of the first and second separate segments is preferably configured to maintain the elongate body in the second, locked position when the first and second separate segments are placed together to form a cylinder.  
      In another embodiment, a spinal fixation element is provided having a flexible elongate cable, and a bioimplantable, generally elongate member slidably disposed around the cable. The elongate member is configurable in a first, flexible position, in which the member is adapted to be manipulated in multiple angular orientations, and a second, locked position, in which the member is fully compressed and it is immovably aligned in a desired orientation. In exemplary embodiment, the generally elongate member is a bellows, and more preferably opposed terminal ends of the bellows are adapted to seat a portion of a spinal anchor.  
      The present invention also provides a spinal implant kit that includes a percutaneous access tube having an inner lumen extending between proximal and distal ends, and a selectively flexible spinal fixation element that is configurable in a bendable position, in which the flexible spinal fixation element can be inserted through the lumen in the percutaneous access tube and angularly manipulated as it exits from the percutaneous access tube, and a locked position, in which the flexible spinal fixation element is compressed to be immovably aligned in a desired orientation.  
      Methods for implanting a flexible spinal fixation element are also provided. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a side perspective view of one embodiment of a flexible spinal fixation element, in the expanded position, coupled to two spinal screws;  
       FIG. 2  is a side perspective view of the spinal fixation element and spinal screws of  FIG. 1  with the spinal fixation element in a locked position;  
       FIG. 3  is a top perspective view of the spinal fixation element and spinal screws shown in  FIG. 2  in a curved configuration;  
       FIG. 4A  is a side perspective view of a flexible spinal fixation element disposed over a cable in accordance with another embodiment of the present invention;  
       FIG. 4B  is a side perspective view of the flexible spinal fixation element of  FIG. 4A  in the locked position;  
       FIG. 5  is a cross-sectional view of yet another embodiment of a flexible spinal fixation element in accordance with the present invention;  
       FIG. 6A  is a side perspective view of another embodiment of a flexible spinal fixation element in accordance with the present invention;  
       FIG. 6B  is a side perspective view of the flexible spinal fixation element of  FIG. 6A  and a sleeve adapted to be disposed over the fixation element to maintain the fixation element in a locked position;  
       FIG. 7A  is a side perspective view of yet another embodiment of a flexible spinal fixation element according to the present invention;  
       FIG. 7B  is a side perspective view of the flexible spinal fixation element of  FIG. 7A  in the locked position;  
       FIG. 8A  is a side perspective view of a bellows-type flexible spinal fixation element in accordance with yet another embodiment of the present invention;  
       FIG. 8B  is a side perspective view of the flexible spinal fixation element of  FIG. 8A  in a locked configuration;  
       FIG. 9A  is a side perspective view of a first percutaneous access device mated to a first spinal screw, and a cut-away view of a second percutaneous access device mated to a second spinal screw and having a flexible spinal fixation element extending therethrough;  
       FIG. 9B  illustrates the flexible spinal fixation element of  FIG. 9A  extending distally through the percutaneous access device;  
       FIG. 9C  illustrates the flexible spinal fixation element of  FIG. 9B  extending between the adjacent spinal screws; and  
       FIG. 9D  is a cross-sectional view of a portion of the spinal screws shown in  FIG. 9C  having the spinal fixation element extending therebetween and having a cable mated thereto. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention generally provides a spinal fixation element that is movable between a first position, in which the spinal fixation element is adapted to be angularly manipulated, and a second, locked position, in which the spinal fixation element is aligned in a desired orientation and is immovable. The configuration of the spinal fixation element can vary, but the fixation element is preferably formed from a bioimplantable member having segments or a bellows configuration that allows the fixation element to be selectively configurable between the first and second positions. In use, the flexibility of the spinal fixation element allows the fixation element to be introduced through a percutaneous access device, thereby advantageously allowing the fixation element to be implanted using minimally invasive techniques.  
      In one embodiment of the present invention, shown in  FIGS. 1-5 , the spinal fixation element can be formed from two or more segments that are slidably disposed around a cable. The cable, which serves as a guide wire for receiving and percutaneously delivering the segments to adjacent spinal anchors, allows the segments to be individually introduced into the surgical site, or to be angularly manipulated with respect to one another as they are implanted. Once the segments are positioned between adjacent spinal anchors, they can then be compressed or otherwise brought together to form a rigid spinal fixation element. The configuration, shape, and/or size of each segment is preferably selected to allow the segments to be locked into a desired configuration with respect to one another.  
      In the embodiment illustrated in  FIGS. 1-3 , the spinal fixation element  10  includes several segments  12   a - 12   f , each of which is substantially cup-shaped and is slidably disposed around a cable  30 . The cup-shape of the segments  12   a - 12   f  is such that each segment  12   a - 12   f  includes a first end  14   a - 14   f  having a substantially hollow, concave shape, and a second end  16   a - 16   f  having a substantially convex shape. This configuration allows the segments  12   a - 12   f  to be aligned along the cable  30  in the same direction so that the hollow, concave end  14   a - 14   f  of each segment receives or nests the convex end  16   a - 16   f  of the adjacent segment  12   a - 12   f . The concave and convex configuration of the segments  12   a - 12   f  is particularly advantageous in that it allows the desired orientation of the fixation element  10  to be selectively adjusted, for example, to have a curved configuration, as shown in  FIG. 3 .  
      In use, the segments  12   a - 12   f  can be compressed between adjacent spinal anchors, such as spinal screws  50   a  and  50   b , to lock the segments  12   a - 12   f  with respect to one another, thereby forming a rigid spinal fixation element  10 , as shown in  FIG. 2 . In an exemplary embodiment, the terminal segments, i.e., segments  12   a  and  12   f , are adapted to receive, or be received by, the head  52   a ,  52   b  of each screw  50   a ,  50   b . In the embodiment shown in  FIGS. 1-3 , the screw heads  52   a ,  52   b  each have a shape that substantially corresponds to the shape of the segments  12   a - 12   f  so that the heads  52   a ,  52   b  form the terminal ends of the spinal fixation element  10  when the segments  12   a - 12   b  are compressed therebetween. Compression of the segments  12   a - 12   f  can be achieved by forcing the spinal screws  50   a ,  50   b  toward one another, as will be discussed in more detail below. Once the segments  12   a - 12   f  are formed into a spinal fixation element  10  and positioned in the desired configuration, the ends of the cable  30 , which extend through the head  52   a ,  52   b  formed on each adjacent spinal screw  50   a ,  50   b , can be locked into the head  52   a ,  52   b  using a closure mechanism, such as, for example, a set screw  51   a ,  51   b  ( FIG. 3 ), that is threaded into each head  52   a ,  52   b.    
       FIG. 4A  illustrates another embodiment of a spinal fixation element  20  having segments  22   a - 22   d ,  24   a - 24   c  that are slidably disposed along a cable  30   a , and in use, as shown in  FIG. 4B , the segments  22   a - 22   e ,  24   a - 24   d  ( FIG. 4B  illustrates two additional segments) are adapted to lock together to form a rigid spinal fixation element  20 . In this embodiment, segments  22   a - 22   e  have a substantially tubular shape with opposed first and second concave ends  26   a   1 - 26   e   1 ,  26   a   2 - 26   e   2 , and the intervening segments  24   a - 24   d  are substantially spherical. As a result, the concave ends  26   a   1 - 26   e   1 ,  26   a   2 - 26   e   2  of the tubular segments  22   a - 22   e  will seat or nest the spherical segments  24   a - 24   d  to form a rigid spinal fixation element  20  when the segments  22   a - 22   d ,  24   a - 24   e  are compressed between adjacent spinal anchors. As previously stated with respect to  FIGS. 1-3 , the anchors and/or the terminal end segments, i.e., segments  22   a  and  22   e  in  FIG. 4B , should have complementary configurations such that the receiver heads on the adjacent anchors form the terminal end segments of the fixation element  20 . Thus, in the embodiment shown in  FIGS. 4A-4B , for example, the receiver head of each anchor (not shown) should have a substantially spherical shape. Each head should also be adapted to receive the cable  30   a  and to receive a closure mechanism that is effective to lock the cable  30   a  in each head.  
      In yet another embodiment, shown in  FIG. 5 , the segments that form the spinal fixation element can include complementary male and female ends that are adapted to receive and/or mate to one another. As shown, each segment  42   a - 42   e , which is slidably disposed around a cable  30   b , includes a first, leading male end  42   a   1 - 42   e   1  and a second, trailing female end  42   a   2 - 42   e   2 . The segments  42   a - 42   e  are aligned along the cable  30   b  in the same direction so that the trailing female end  42   a   2 - 42   e   2  of each segment  42   a - 42   e  receives the leading male end  42   a   1 - 42   e   1  of the next adjacent segment  42   a - 42   e . The size of the male and female ends  42   a   1 - 42   e   1 ,  42   a   2 - 42   e   2  of the segments  42   a - 42   e  is preferably adapted to form a tight fit, e.g., a press-fit, therebetween, thus allowing the segments  42   a - 42   e  to be locked with respect to one another.  
      In order to lock the segments  42   a - 42   e  between the receiver heads of adjacent spinal anchors, the heads of the anchors can optionally include a male or female component for mating with the segments  42   a - 42   e , or alternatively the terminal segments, e.g., segments  44   a ,  44   b  can be adapted to be positioned between the heads of the anchors. As shown in  FIG. 5 , the terminal segments  44   a ,  44   b  each include a substantially flattened terminal end surface  44   a   1 ,  44   b   1 . While not shown, this surface  44   a   1 ,  44   b   1  can, however, have a shape that corresponds to an outer surface of the heads of the adjacent anchors. Again, the anchor receiver heads should be configured to receive a closure mechanism to secure the cable therein, thus locking the segments  42   a - 42   e  therebetween.  
      While the segments shown in  FIGS. 1-5  can be locked together by a press-fit that is formed from compression of the segments between the heads of adjacent spinal anchors, the segments can optionally include features to facilitate the locking engagement therebetween. The concave ends  26   a   1 - 26   e   1 ,  26   a   2 - 26   e   2  of the tubular segments  22   a - 22   e  and/or the a portion or all of the spherical segments  24   a - 24   d  shown in  FIGS. 4A-4B , for example, can include surface features formed thereon to prevent slippage between the segments  22   a - 22   d ,  24   a - 24   e . The surface features (not shown) can be formed from a knurled surface, surface protrusions, a coating (e.g., a polymeric coating), or any other technique that will facilitate engagement between the segments  22   a - 22   d ,  24   a - 24   e . In another embodiment, the segments can be configured to removably engage one another using, for example, a snap-fit. A person skilled in the art will appreciate that a variety of techniques can be used to provide a locking engagement between the segments.  
       FIGS. 6A-8B  illustrate additional embodiments of spinal fixation elements in accordance with the present invention. As with the fixation elements shown in  FIGS. 1-5 , each of the spinal fixation elements illustrated in  FIGS. 6A-8B  is configurable between a first, flexible position, and a second position in which the fixation element can be locked into a desired configuration.  
      Referring now to  FIGS. 6A-6B , the spinal fixation element  60  includes first and second segments  62   a ,  62   b  that are mated to one another by a hinge  64 . Each segment  62   a ,  62   b  can have any shape and size, but preferably each segment  62   a ,  62   b  has a generally cylindrical, elongate shape that allows the fixation element  60  to be used in place of traditional spinal rods. The hinge  64  is disposed between terminal ends  62   a   2 ,  62   b   2  of the segments  62   a ,  62   b , and it allows the segments  62   a ,  62   b  to pivot with respect to one another. This is particularly advantageous in that the fixation element  60  can be introduced into adjacent spinal anchors through a percutaneous access tube, as the hinge  64  allows the segments  62   a ,  62   b  to bend with respect to one another. A person skilled in that art will appreciate that, in order to introduce the fixation element  60  through a percutaneous access device, each segment should have a length Is that is small enough to permit percutaneous access.  
      Once the fixation element  60  is positioned between adjacent spinal anchors, with terminal ends  62   a   1 ,  62   b   1  disposed within receiver heads of the adjacent anchors, a sleeve  66  or similar device can be disposed over the hinge  64  to prevent further bending of the segments  62   a ,  62   b , thereby locking the segments  62   a ,  62   b  with respect to one another. Alternatively, or in addition, a screw of other locking mechanism can be applied to the hinge  64  to prevent further bending of the hinge  64 . In another embodiment, where three spinal anchors are used, the hinge  64  can be positioned and locked within a receiver head of the middle spinal anchor, and the terminal ends  62   a   1 ,  62   b   1  can be disposed within adjacent spinal anchors. While only one hinge  64  is shown, a person skilled in the art will appreciate that the fixation element  60  can include any number of segments and hinges.  
      In yet another embodiment, shown in  FIGS. 7A-7B , the spinal fixation element  70  can be formed from two separate segments  72 ,  74 , each of which includes two portions  72   a ,  72   b ,  74   a ,  74   b  that are mated to one another by a hinge  72   c ,  74   c . The segments  72 ,  74  are preferably configured such that the hinges  72   c ,  74   c  prevent one another from bending when the segments  72 ,  74  are joined and locked at opposed ends to form a spinal rod  70 . In the illustrated embodiment, for example, segment  72  is formed from two portions  72   a ,  72   b , each having an elongate, hemi-spherical shape. The hinge  72   c  is configured to allow the segments  72   a ,  72   b  to bend only uni-directionally. Segment  74  is similarly formed from two portions  74   a ,  74   b , each having an elongate, hemi-spherical shape. The hinge  74   c  between portions  74   a ,  74   b , however, is configured to allow the segments  72   a ,  72   b  to bend toward one another in a direction that is opposite to the direction that segments  72   a ,  72   b  bend. As noted above with respect to fixation element  60 , the segments  72 ,  74  also preferably have a length L s  that allows the fixation element  70  to be percutaneously implanted.  
      In use, each segment  72 ,  74  can be introduced, preferably percutaneously, into a surgical site and positioned to extend between adjacent spinal anchors. The segments  72 ,  74  are positioned so that the hemi-spherical segments  72 ,  74 , when placed together, form a single, cylindrical elongate rod  70 . As a result, the hinges  72   c ,  74   c  prevent one another from bending, thus forming a rigid spinal rod  70 . The terminal ends of the fixation element  70  can be locked into receiver heads of adjacent spinal anchors using techniques known in the art.  
      In another embodiment of the present invention, the spinal fixation element can be in the form of a bellows  80 , as shown in  FIGS. 8A and 8B . The bellows configuration of the fixation element  80  allows the fixation element  80  to be angularly manipulated as it is introduced into a surgical site and positioned between adjacent spinal anchors. The terminal ends  82   a ,  82   b  of the fixation element  80  are preferably adapted to seat the head of a spinal anchor, and thus they should have a shape that conforms to the shape of an outer surface of a spinal anchor head. Once positioned between adjacent anchors, the fixation element  80  can be locked at a desired orientation by compressing the bellows, as shown in  FIG. 8B , and locking the cable  30   c , which extends through the bellows  80 , to the adjacent anchors.  
      A person skilled in the art will appreciate that the spinal fixation element of the present invention can have a variety of other configurations to allow the fixation element to be movable between a first position, in which the fixation element can be angularly manipulated, and a second position, in which the fixation element can be locked into a desired orientation.  
       FIGS. 9A-9D  illustrate an exemplary method of implanting a spinal fixation element using minimally invasive surgical techniques in accordance with the present invention. Fixation element  10  shown in  FIGS. 1-3  is shown for illustration purposes only, and a person skilled in the art will appreciate that the method can be performed using any suitable spinal fixation element.  
      Referring to  FIGS. 9A and 9B , two or more spinal anchors, e.g., spinal screws  50   a ,  50   b , are implanted in adjacent vertebrae (not shown). While spinal screws  50   a ,  50   b  are shown, a variety of spinal anchors can be used with the present invention. As is further shown, each anchor has a percutaneous access tube  100   a ,  100   b  mated thereto. The spinal fixation element  10 , tubes  100   a ,  100   b , and/or anchors  50   a ,  50   b  can optionally be provided as part of a spinal kit. The anchors  50   a ,  50   b , percutaneous access tubes  100   a ,  100   b , and methods for implanting the same are described in more detail in a patent application filed concurrently herewith and entitled “Methods and Devices for Minimally Invasive Spinal Fixation Element Placement,” which is incorporated by reference herein in its entirety.  
      Once the spinal screws  50   a ,  50   b  are implanted with the tubes  100   a ,  100   b  attached thereto, the spinal fixation element  10  is introduced into one of the tubes, e.g., tube  100   b , and it is advanced distally toward spinal screw  50   a . A pusher shaft  90  can optionally be used to advance the fixation element  10  toward the anchor  50 . In this embodiment, the spinal fixation element  10  is disposed around a cable  30 . Thus, while not shown, the cable  30  is preferably advanced through the percutaneous access tube  100   b  and positioned to extend between the heads  52   a ,  52   b  of the adjacent anchors  50   a ,  50   b  prior to advancing the spinal fixation element  10  toward the anchor  50 . The leading end of the cable  30  can optionally be locked into head  52   b  of anchor  50   b , and the remaining portion of the cable  30  can serve as a guide cable. The fixation element  10  can then be passed along the cable  30 , either as a whole or as individual segments, until the fixation element  10  is positioned between the heads  52   a ,  52   b  of the adjacent anchors  50   a ,  50   b , as shown in  FIG. 9C .  
      Once properly positioned, the percutaneous access tubes  100   a ,  100   b  can optionally be compressed toward one another using, for example, medical pliers, to compress the fixation element  10  between the adjacent anchors  50   a ,  50   b . A closure device, such as a set screw, can then be introduced into the head  52   a ,  52   b  of each anchor  50   a ,  50   b , or into the head of anchor  50   a  if anchor  50   b  already includes a closure mechanism, to lock the cable  30  thereto, as shown in  FIG. 9D . The locking of the cable  30  between the adjacent anchors  50   a ,  50   b  will advantageously counteract tensile forces, thus preventing the anchors  50   a ,  50   b  from separating with respect to one another. And conversely, the fixation element  10 , which is fully compressed between the anchors  50   a ,  50   b , will advantageously counteract compressive forces, thus preventing the anchors  50   a ,  50   b  from moving toward one another.  
      One skilled in the art will appreciate further features and advantages of the invention based on the above-described embodiments. Accordingly, the invention is not to be limited by what has been particularly shown and described, except as indicated by the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety.