Abstract:
An anti-torsion spine fixation device includes an elongated member spanning from one vertebra to another and connected to each vertebra. The anti-torsion spine fixation device may span more than one vertebral level, but is fixed bilaterally to the most cephalad and caudad vertebrae.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to, and the benefit of, U.S. Provisional Patent Application Ser. No. 61/188,090 filed Aug. 6, 2008. The entire contents of the aforementioned application are incorporated by reference herein. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to orthopedic spine surgery, and more particularly, to apparatuses and methods for stabilizing and fixing the spine. 
       BACKGROUND AND RELATED ART 
       [0003]    Correction of a spinal deformity typically requires stabilization and fixation of vertebrae in a particular spatial relationship. Surgical spinal correction procedures involve the placement of a plurality of bone pins, anchors, cables, hooks, or screws placed in adjacent vertebrae and using spinal rods to maintain a predetermined spatial relationship between the vertebrae. Such devices may be permanently implanted in the subject. However, in other cases, the devices may be subsequently removed when no longer needed. 
         [0004]    In an effort to maintain normal growth or height while correcting a younger patient&#39;s abnormally curved spine, unilateral constructs may be implanted with the purpose of maintaining height on one side of the spine, the convex side of the curve, while the concave side continues to grow. Over time, this method of instrumentation may, on the concave side of the scoliotic curve, grow the spine straight. 
         [0005]    Spinal instrumentation such as pedicle screws and rods may be used to achieve this type of correction. Some traditional rod and screw constructs are subject to becoming misaligned over time. 
       SUMMARY 
       [0006]    An anti-torsion spine fixation device includes a plurality of anchors disposed on opposing pedicles of at least two vertebrae disposed adjacent to a scoliotic curve with a connecting rod traversing the anchors such that the path of the rod approximates a “C”. The anchors closest to the convex portion of the scoliotic curve are coupled by the rod. There is no corresponding coupling structure near the concave portion of the scoliotic curve, thereby defining a gap or “corrective opening” in the rod&#39;s path which corresponds to the concave portion of the scoliotic curve. 
         [0007]    The anti-torsion spine fixation device so configured allows for corrective growth at the corrective opening while restricting growth near the convex portion of the scoliotic curve. Additionally, the anti-torsion spine fixation device inhibits further rotation of a non-scoliotic spine. Further, because the rod is joined to opposing anchors on a single vertebra, the anti-torsion spine fixation device limits torsional motion of the spine by requiring the torsional motion to be acted on the uni-lateral length of rod which is fixed to bilateral anchors of at least one vertebra. 
         [0008]    According to another aspect of the present disclosure, an anti-torsion spine fixation device includes a plurality of anchors and rod segments coupled to vertebrae configured to define multiple opposing corrective openings. The path of the correcting rod is configured such that the device both allows growth at each corrective opening and restricts torsion along its length. 
         [0009]    According to another aspect of the present disclosure, rod segments may be retained in each anchor by a setscrew. According to another aspect of the present disclosure, rod segments may be retained in each anchor by a clamp. According to another aspect of the present disclosure, anchors may be secured to their respective locations upon a vertebra by a pedicle screw. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The foregoing and other features of the present disclosure will become apparent to one skilled in the art to which the present disclosure relates upon consideration of the following description of the disclosure with reference to the accompanying drawings, wherein: 
           [0011]      FIG. 1  is an anterior plan view of an anti-torsion spine fixation device coupled to two vertebrae; 
           [0012]      FIG. 2  is a perspective view of an anti-torsion spine fixation device of  FIG. 1 ; 
           [0013]      FIG. 3  is an anterior plan view of the anti-torsion spine fixation device configured in a bi-directional construct; 
           [0014]      FIG. 4  is an isometric view of the anti-torsion spine fixation device of  FIG. 3 ; 
           [0015]      FIG. 5  is an anterior plan view of an anti-torsion spine fixation device having an expanding member configured to accommodate growth of a patient; 
           [0016]      FIG. 6  is a perspective view of the anti-torsion spine fixation device of  FIG. 5 ; 
           [0017]      FIG. 7   a  is a perspective view of a first polyaxial bone screw; and 
           [0018]      FIG. 7   b  is a perspective view of a second polyaxial bone screw. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0019]    Embodiments of the presently disclosed apparatuses and methods for spinal surgery will now be described in detail with reference to the appended drawings, in which like reference numerals designate identical or corresponding elements in each of the several views. Throughout the following description, the term “proximal,” will refer to the end of a device or system that is closest to the operator, while the term “distal” will refer to the end of the device or system that is farthest from the operator. In addition, the “long axis of the spine” runs approximately in the direction from the head to the tailbone, with the direction toward the head referred to as being “cephalad” and the direction toward the tailbone referred to as being “caudad.” Further still, for the purposes of this application, the term “medial” indicates a direction toward the middle of the body of the patient while the term “lateral” indicates a direction away from the middle of the body of the patient. 
         [0020]    A spinal fixation device  1  will now be described with reference to  FIGS. 1 and 2 . The spinal fixation device  1  includes a rod  50 , having rod segments  51 ,  52 ,  53 ,  54 , and  55 . Rod  50  is coupled to anchors  100   a,    200   a,    200   b,  and  100   b.  Although rod  50  is illustrated as including a plurality of rod segments for ease of explaining the disclosed features, it is contemplated that rod  50  may be a single continuous rod that is shaped to fit the desired anchor locations or may be a number of rod segments coupled together that form rod  50 . Anchors  100   a  and  100   b  arc coupled to caudad vertebra  4  at respective locations  150  and  153 . Anchors  200   a,  and  200   a  are coupled to cephalad vertebrae  3  at respective locations  151  and  152 . 
         [0021]    In the present disclosure, the term “anchor” refers to devices suitable for coupling one or more rods to one or more bone structures such as a vertebral body. For example, with reference to  FIG. 1 , anchors  200   a  and  200   b  disposed on cephalad vertebrae  3  are shown as taper lock style polyaxial screws. One example of a taper lock style polyaxial screw is disclosed in commonly assigned International Patent Application Publication No. PCT/US2008180682, filed on Oct. 22, 2008, and shown in  FIG. 7   b  of the present disclosure as anchor  200 . Similarly, anchors  100   a  and  100   b  disposed on caudad vertebrae  4  are shown as polyaxial style screws such as those disclosed in commonly assigned International Patent Application Publication No. PCT/US2008/80668, filed on Oct. 22, 2008, and shown in  FIG. 7a  of the present disclosure as anchor  100 . Both aforementioned applications are incorporated by reference herein in their entirety. Either of these bone anchor types may be used to couple portions of rod  50  to vertebrae. 
         [0022]    With reference to  FIG. 7   a,  a bone anchor  100  is shown having, an elongated shaft  110  defining a longitudinal axis having a distal end portion and a proximal end portion, a helical thread  120  disposed thereupon, a substantially conical distal tip  130 , and a proximal head assembly  140 . Proximal head assembly  140  and elongated shaft  110  are pivotably coupled to allow angular displacement of proximal head assembly  140  relative to the longitudinal axis. Further, proximal head assembly  140  has a generally U-shaped cross-section defining a channel  141  configured to retain a rod such as spinal fixation rod  50  shown in  FIG. 1 . Further still, there are opposing threads  142  disposed on opposing faces of channel  141  configured to receive a set-screw (not shown) capable of retaining a rod. 
         [0023]    With reference to  FIG. 7   b,  a bone anchor  200  is shown having, an elongated shaft  210  defining a longitudinal axis having a distal end portion and a proximal end portion, a helical thread  220  disposed thereupon, a substantially conical distal tip  230 , and a proximal head assembly  240 . Proximal head assembly  240  and elongated shaft  210  are pivotably coupled to allow angular displacement of proximal head assembly  240  relative to the longitudinal axis. 
         [0024]    Further, proximal head assembly  240  includes a collet member  242  and a saddle member  243 . Saddle member  243  has a generally U-shaped cross-section defining a channel  241 . Further still, saddle member  243  has a slot  244  extending from the nadir of the channel  241  towards the bottom of saddle member  243  which essentially bisects the saddle member  241  along a central axis. It is contemplated that slot  244  may not extend all the way through the body portion. Proximal head assembly  240  is configured to retain a rod within channel  241  by the reducing the width of slot  244 . 
         [0025]    With reference to  FIGS. 1 and 2 , spinal fixation device  1  is configured to be disposed upon a patient&#39;s spine such that the convex portion of a scoliotic curve corresponds to rod segment  52  which spans anchors  100   a  and  200   a  while the concave portion of the scoliotic curve disposed between anchors  200   b  and  100   b  has no such corresponding connecting structure defining corrective gap  161  therebetween. Alternatively, the presently disclosed spinal fixation device  1  is adaptable for use in a patient where a uni-lateral rod is desired and the possibility of “crankshafting” is a concern. As a patient&#39;s spine grows, such an arrangement of rod segments and anchors allows the concave portion of the curve disposed within corrective gap  161  to grow while maintaining a substantially constant distance at the convex portion of the curve between anchors  100   a  and  200   a,  thereby helping to correct the scoliotic deformity. 
         [0026]    Traditional unilateral spinal constructs may require additional stabilization to prevent or inhibit torsion about the long axis of the spine in addition to correction of the convex and concave portions of the scoliotic curve. 
         [0027]    As shown in  FIGS. 1 and 2 , a rod segment  53  disposed on the cephalad vertebrae  3  approximates an arcuate path from rod segment  52  to  54  such that the apex of the arc is directed towards the patient&#39;s head. Similarly, a rod segment  51  disposed on caudad vertebrae  4  approximates an arcuate path from rod segment  52  to rod segment  55  such that the apex of the arc is directed towards the patient&#39;s feet. 
         [0028]    Segments  51  and  53  provide additional coupling between vertebrae  4  and  3  beyond the clamping pressure exerted on segment  52  at anchors  100   a  and  200   a.  In such a configuration, rotation of anchors  100   a  and  200   a  relative to one another about the long axis of the spine is impossible without a corresponding translation of anchors  200   b  and  100   b  and consequently, a deformation of the rod segments between those anchors. Therefore, the resistance to torsional deformation of the anti-torsion spine fixation device may be defined by the torsional yield strength of the material from which the rod segments are made. 
         [0029]    Where multiple scoliotic curves are present, additional anchors and rods may be configured in a curve which approximates multiple anti-torsion spine fixation devices whose corrective action is directed toward the multiple scoliotic curves while maintaining torsional rigidity about the long axis of the spine. 
         [0030]    As shown in  FIGS. 3 and 4 , bi-lateral spinal fixation device  2  includes the constructs present in spinal fixation device  1  with the addition of constructs coupled to intermediate vertebrae  5  optionally disposed between cephalad vertebrae  3  and caudad vertebrae  4 . The constructs disposed on vertebrae  3  and  4  are shown in  FIGS. 1 and 2  and described hereinabove. The differences between spinal fixation device  1  and spinal fixation device  2  are described hereinbelow. 
         [0031]    In this configuration, spine fixation device  2  has opposing anchors  200   c  and  200   d  at locations  154  and  155  on an intermediate vertebra  4 . Additionally, spine fixation device  2  includes rod  60 , which includes the rod segments present in rod  50  described hereinabove with the additional rod segments being discussed hereinafter. Rod segment  52  joins the cephalad portion of anchor  200 f to the caudad portion of anchor  200   c,  rod segment  57  joins cephalad portion of anchor  200   c  to the caudad portion of anchor  200   d,  and rod segment  58  joins the cephalad portion of anchor  200   d  to the caudad portion of anchor  200   b  such that the curve approximated by adjoining rod segments defines opposing corrective gaps  162  and  163 . Rod segments  52 ,  58  maintain the torsional rigidity of the device established by the curved paths of rods  53  and  51  in the manner described above with regards to fixation device  1 . Specifically, rotation of anchors  200   c  and  200   f  relatively to one another creates a corresponding displacement of anchors  200   d  and  200   g  which is resisted by the rod segments interconnecting the aforementioned anchors. Rod  60  further includes rod segments  62 ,  63 ,  66 , and  67  as shown in  FIG. 4 . Rod segments  62 ,  63  connect rod segment  57  with rod segments  58  and  52 . Rod segment  66  includes rod segments  66   a,    66   b,  and  66   c,  while rod segment  67  includes rod segments  67   a,    67   b,  and  67   c.  Similar to rod  50  ( FIG. 1 ), rod  60  is illustrated as including a plurality of rod segments for ease of explaining the disclosed features, it is contemplated that rod  60  may be a single continuous rod that is shaped to fit the desired anchor locations or may be a number of rod segments coupled together that form rod  60 . 
         [0032]    As shown in  FIGS. 5 and 6 , an additional stabilization device  300  such as a coupled rod device, a sliding rod device, and anchors may be disposed within corrective gap  161  without coming in contact with the anti-torsion spine fixation device. The additional stabilization device may include, for example, an automatically lengthening spine device such as that disclosed by commonly assigned PCT application PCT/US2009/33553 filed on Feb. 9, 2009, the disclosure of which is herein incorporated by reference in its entirety. Such devices are generally referred to as “growing spine devices.” Other known growing spine devices include, for example, distraction rods such as those disclosed by Bumpus in U.S. Pat. No. 4,931,055 and implantable spinal distraction splints such as those disclosed by Ulrich in U.S. Pat. No. 4,658,809. 
         [0033]    The use and function of spinal fixation device  1  will be discussed during the course of a typical installation procedure and as part of the treatment of one or more scoliotic deformities. Initially, the location, orientation, and breadth of one or more scoliotic curves on a patient&#39;s spine will be determined using methods known in the art. Next, an operator identifies at least one caudad vertebrae  4  and cephalad vertebrae  3  for each curve such that a substantial portion of the curve is disposed between the aforementioned caudad and cephalad vertebrae. Next, an operator will secure at least two anchors to each selected vertebrae using methods commonly known in the art such that the anchors are disposed on opposing pedicles of their respective vertebrae. 
         [0034]    A configuration of anchors and screws corresponding to the preceding paragraph is shown in  FIGS. 1 and 2 . 
         [0035]    Next, in the event that only one pair of caudad and cephalad vertebrae have been selected, an operator will couple spinal fixation rod  50 ,  60  to each anchor using a set screw as shown respectively in  FIGS. 1 and 3 , a cam/clamp as is known in the art, or any other combination of rod coupling devices known in the art. Spinal fixation rod  50  includes a plurality of rod segments configured in a shape approximating a “C” such that the fixation rod spans the convex portion of the curve while there is no corresponding structure on the concave portion, defining a corrective opening. Further, the path of the rod segments defines arcuate caudad and cephalad rod portions which join the opposing anchors disposed on their respective caudad and cephalad vertebrae. 
         [0036]    A configuration of anchors and fixation rod segments corresponding to the preceding paragraph is shown in  FIGS. 1 and 2 . 
         [0037]    As a patient grows, the spacing of the vertebrae at the joined convex side of the scoliotic curve remains relatively constant, while the spacing of the vertebrae at the corrective gap corresponding to the convex portion is allowed to expand with the patient&#39;s growth. Further, the long segments of the spinal fixation rod provide improved torsional coupling for the device thereby reducing the tendency of the spine to develop new torsional deformities. 
         [0038]    Finally, all or part of the device may be surgically removed or altered at the conclusion of modification of treatment. 
         [0039]    It will be understood that various modifications may be made to the embodiments of the presently disclosed spinal fixation systems. Therefore, the above description should not be construed as limiting, but merely as exemplifications of embodiments. Those skilled in the art will envision other modifications within the scope and spirit of the present disclosure.