Abstract:
An implantable device includes a spacing member and an elongate member. The elongate member includes a body portion having a first end coupled to the spacing member and a second end having a retention member. In a system for maintaining disc space between adjacent vertebrae, the implantable device is used in conjunction with a spine fixation member. The spine fixation member includes bone anchors and a spinal rod for releasably securing the retention member of the elongate member thereto in order to prevent expulsion of the spacing member from the disc space. Methods of using the system include unilaterally and bilaterally replacing diseased or damaged intervertebral discs from a posterior or transformational approach.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/995,534, filed Sep. 27, 2007, and U.S. Provisional Application No. 60/999,310, filed Oct. 17, 2007, the entire contents of which are incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates generally to orthopedic spine surgery and in particular to devices, systems, and methods for vertebral body spacing using a posterior or transformational surgical approach. 
       DESCRIPTION OF THE RELATED ART 
       [0003]    The human spine is comprised of thirty-three vertebrae at birth and twenty-four as a mature adult. The vertebra is made up of the vertebral body and posterior elements, including the spinous process, transverse processes, facet joints, laminae, and pedicles. The vertebral body consists of a cortical shell which surrounds the cancellous center. Between each pair of vertebrae is an intervertebral disc, which maintains the space between adjacent vertebrae and acts as a cushion under compressive, bending and rotational loads and motions. A healthy intervertebral disc has a great deal of water in the nucleus pulposus; the center portion of the disc. The water content gives the nucleus a spongy quality and allows it to absorb spinal stress. Excessive pressure or injuries to the disc can cause injury to the annulus; the outer ring that holds the disc together. 
         [0004]    Generally, the annulus is the first portion of the disc that seems to be injured. These injuries are typically in the form of small tears. These tears heal by scar tissue. The scar tissue is not as strong as normal annulus tissue. Over time, as more scar tissue forms, the annulus becomes weaker. Eventually this can lead to damage of the nucleus pulposus. The nucleus begins to lose its water content due to the damage; it begins to dry up. Because of water loss, the discs lose some of their ability to act as a cushion. This can lead to even more stress on the annulus and still more tears as the cycle repeats. As the nucleus loses its water content it collapses, allowing the vertebrae above and below the disc space to move closer to one another. This results in a narrowing of the disc space between the two vertebrae and often times impingement of the nerves branching off the spinal cord. As this shift occurs, the facet joints located at the back of the spine are forced to shift. This shift changes the load distribution and balance at the facet joints affecting the way the facet joints work together and can lead to problems in the facet joints or to a premature breakdown of the joint. 
         [0005]    When a disc or vertebrae is damaged due to disease or injury standard practice is to remove part or all of the intervertebral disc, insert a natural or artificial disc spacer and construct an artificial structure to hold the affected vertebrae in place to achieve a spinal fusion. In doing so, while the diseased or injured anatomy is addressed and the accompanying pain is significantly reduced, the natural biomechanics of the spine are affected in a unique and unpredictable way and, more often than not, the patient will develop complicating spinal issues in the future. 
         [0006]    To that end, it would be advantageous to treat the disease or injury while maintaining or preserving the natural spine biomechanics. Normal spine anatomy, specifically intervertebral disc anatomy, allows one vertebrae to rotate, with respect to its adjacent vertebrae, about all three axes of the spine. Similarly, the intervertebral disc also allows adjacent vertebrae to translate along all three axes, with respect to one another. 
         [0007]    For the above stated reasons, an implantable device which may be used to maintain the disc space between adjacent vertebrae, allow rotation about at least one axis and allow translation about at least one axis, and have a means to prevent expulsion while complementing a posterior stabilizing spinal construct would be helpful. The implantable device would be capable of being introduced into the body using a posterior approach, similar to a PLIF, T-PLIF, or X-PLIF spinal fusion device and may provide a prolonged life span in the body that can withstand early implantation, as is often indicated for younger patients, and will have a limited amount of particulate debris so as to reduce complications over the useful life of the device. 
       SUMMARY 
       [0008]    An implantable device includes a spacing member and an elongate member. The elongate member includes a body portion having a first end coupled to the spacing member and a second end having a retention member. The spacing member is compressible and may be configured to include dual directional radii along a longitudinal axis and a lateral axis. In embodiments, the spacing member is oblong. The spacing member is configured to translate along at least one axis and rotate about at least one axis. 
         [0009]    In a system for maintaining disc space between adjacent vertebrae, the implantable device is used in conjunction with a spine fixation member. The spacing member of the implantable device is adapted for insertion into a disc space between a pair of vertebrae and a spine fixation member is disposed between the pair of vertebrae. In embodiments, the spine fixation member includes bone anchors adapted to be affixed to the vertebrae and a spinal rod which is placed across and secured to the bone anchors. The elongate member of the implantable device is affixed to the spacing member on a first end which is configured to extend out of the disc space. A second end of the elongate member is adapted to be releasably connectable with the spine fixation member in order to prevent expulsion of the spacing member from the disc space. 
         [0010]    Methods of using the system, including unilaterally and bilaterally replacing diseased or damaged intervertebral discs from a posterior or transformational approach, are also disclosed. In accordance with the present disclosure, bone anchors are placed in adjacent vertebrae of a patient. A spacing member operably connected to an elongate member is introduced into a disc space opening formed between the adjacent vertebrae. A spinal rod is placed across and secured to the bone anchors and the elongate member is secured thereto. In embodiments utilizing a unilateral vertebral body spacing device, a single spacing member is placed centrally between the adjacent vertebrae. In embodiments utilizing a bilateral vertebral body spacing device, two spacing members are bilaterally placed between the adjacent vertebrae. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    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 with reference to the accompanying drawings, wherein: 
           [0012]      FIG. 1A  is a top view of a single level construct with the vertebral body spacing device, placed bilaterally, in accordance with the present disclosure; 
           [0013]      FIG. 1B  is an end view of the single level construct with the vertebral body spacing device of  FIG. 1A ; 
           [0014]      FIG. 1C  is a side view of the single level construct with the vertebral body spacing device of  FIG. 1A ; 
           [0015]      FIG. 2A  is a top view of the vertebral body spacing device of  FIGS. 1A-1B ; 
           [0016]      FIG. 2B  is an isometric view of the vertebral body spacing device of  FIGS. 1A-1B ; 
           [0017]      FIG. 2C  is a cross-sectional view of the vertebral body spacing device of  FIGS. 1A-1B ; 
           [0018]      FIG. 2D  is a side view of the vertebral body spacing device of  FIGS. 1A-1B ; 
           [0019]      FIG. 3A  is a top view of a single level construct with a vertebral body spacing device, placed centrally, in accordance with the present disclosure; 
           [0020]      FIG. 3B  is an end view of the single level construct with the vertebral body spacing device of  FIG. 3A ; 
           [0021]      FIG. 3C  is a side view of the single level construct with the vertebral body spacing device of  FIG. 3A ; 
           [0022]      FIG. 4A  is a top view of the vertebral body spacing device of  FIGS. 3A-3B ; 
           [0023]      FIG. 4B  is an isometric view of the vertebral body spacing device of  FIGS. 3A-3B ; 
           [0024]      FIG. 4C  is a side view of the vertebral body spacing device of  FIGS. 3A-3B ; 
           [0025]      FIG. 4D  is a cross-sectional side view of the vertebral body spacing device of  FIGS. 3A-3B ; 
           [0026]      FIG. 5A  is a top view of an alternate embodiment of the spacing member of the vertebral body spacing device in accordance with the present disclosure; 
           [0027]      FIG. 5B  is a front view of the spacing member of  FIG. 5A ; 
           [0028]      FIG. 5C  is a cross-sectional view of the spacing member of  FIG. 5B  taken along line C-C; 
           [0029]      FIG. 6A  is a front view of another embodiment of the spacing member of the vertebral body spacing device in accordance with the present disclosure; 
           [0030]      FIG. 6B  is a front view of the spacing member of  FIG. 6A ; and 
           [0031]      FIG. 6C  is a cross-sectional view of the spacing member of  FIG. 6B  taken along line B-B. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0032]    The vertebral body spacing device of the present disclosure is used in orthopedic spine surgery. A spacing member for maintaining disc space between adjacent vertebrae is coupled with an elongated member which is used to prevent the device from expelling out of the disc space. The device is contemplated as a single, central vertebral body spacing device attached to one spinal rod or as a bilateral device attached to bilaterally placed spinal rods. Either orientation will facilitate a fusion or dynamic stabilization procedure. The former takes advantage of Wolff&#39;s Law when a bone graft is introduced. The latter may be used with a posterior dynamic rod system providing anterior column support thereby helping to unload the facet joints and restore balance to the spine. 
         [0033]    Referring now to the drawings, in which like reference numerals identify identical or substantially similar parts throughout the several views,  FIGS. 1A-2D  illustrate views of an embodiment of the vertebral body spacing device, placed laterally, in accordance with the principles of the present disclosure. Vertebral body spacing device  10  includes spacing member  20  and elongate member  30 . 
         [0034]    Spacing member  20  is dimensioned to be positioned between two adjacent vertebrae. Spacing member  20  has sufficient contact surface area with a vertebral body endplate such that minimal subsidence into the endplate occurs and rotation about the longitudinal axis of the spine is possible. As illustrated in the current embodiment, spacing member  20  defines a longitudinal axis “A” and a lateral axis “B” ( FIGS. 2B and 2D ). Spacing member  20  is elongated along longitudinal axis “A” in the shape of an oblong. The oblong shape provides dual directional radii, one in the longitudinal axis “A” and the other in the lateral axis “B”, along the outer surface of spacing member  20 . The dual directional radii of the oblong outer surface of spacing member  20  provides an articulating surface whereby rotation of the vertebrae about the other two axes is achieved. Other shapes are within the purview of those skilled in the art for providing rotational movement to the vertebrae, such as other spherical shapes having two radii of curvature. 
         [0035]    Spacing member  20  is also compressible. Material selection may also affect compressibility of spacing member  20 . Spacing member  20  may be made of biocompatible materials, including polymeric, metallic, and composites thereof. Polymeric materials include, for example, polyethylene, polypropylene, polyurethane, and polyetheretherketone. Metallic materials may include metals such as surgical grade stainless steel and titanium alloys. In the current embodiment, spacing member  20  may be a composite of a first material  21  within a second material  23 . In some embodiments, the first material  21  may provide a soft segment and the second material  23  may provide a hard segment. 
         [0036]    The degree of translation, therefore, is affected by the material properties and the geometry of the spacing member  20  as well as by other means within the purview of those skilled in the art. For example, translation along the other two axes may be achieved by sizing spacing member  20  appropriately for the disc space such that the working area of spacing member  20  is within the softer, interior area of the endplate, as it is the cortical rim of the endplate which prevents further translation of one vertebral body to the other. 
         [0037]    As illustrated in  FIGS. 5A-6C  spacing member  320  may include voids  322  to allow flexure of the implant in predefined directions. Voids  322  may be generally cylindrical shaped and extend at least partially through spacing member  320 . Voids  322  may be radially spaced about spacing member  320  and may be equally or unevenly spaced thereabout. Further, the voids may be any shape within the purview of those skilled in the art. The placement, number, and size of voids  322  within spacing member  320  may be controlled as needed to provide flexibility along at least one predefined axis. 
         [0038]    Referring again to  FIGS. 1A-2D , spacing member  20  includes opening  24  for coupling with elongate member  30 . Opening  24  extends at least partially into spacing member  20  and is configured and adapted to securely receive elongate member  30 . 
         [0039]    Elongate member  30  is connected to spacing member  20  in order to prevent expulsion of spacing member  20  when positioned in a disc space. Elongate member  30  includes a body portion  32  having a first end  34  and a second end  36 . First end  34  of elongate member  30  is coupled with opening  24  of spacing member  20 . First end  34  is configured and adapted to fit within opening  24  of spacing member  20 . As illustrated in the current embodiment, first end  34  and opening  24  are press fit to couple elongate member  30  and spacing member  20 . First end  34  of elongate member  30  may be connected with opening  24  of spacing member  20  through any mechanical and chemical means within the purview of those skilled in the art, such as, for example, interference fitting, press fitting, friction fitting, welding, and adhesive binding. 
         [0040]    Second end  36  of elongate member  30  includes retention member  38  for releasably coupling elongate member  30 , and thus spacing member  20 , to spine fixation member  40 . Spine fixation member  40  may be any posterior stabilizing longitudinal member within the purview of those skilled in the art, such a solid spinal rod  42  spanning at least two bone anchors  44 . Retention member  38  may be any member capable of securing elongate member  30  to spine fixation member  40 . As illustrated in the current embodiment, retention member  38  is a hook  39   a  and screw  39   b  set. Other comparable mechanical means for attached the elongate member  30  to spine fixation member  40  are envisioned and within the purview of those skilled in the art. 
         [0041]    Elongate member  30  is a semi-rigid device capable of being bent in such a way as to allow implantation and alignment of the device using standard posterior or transforminal approaches and preventing interference with anatomy, such as the exiting nerve root, once implanted, while maintaining the formed shape. 
         [0042]    A system  50  for maintaining disc space between adjacent vertebrae utilizes the vertebral body spacing device  10  of the present disclosure in conjunction with a spine fixation device  40  including pedicle screws  44  and rod  42 . Any spine fixation member  60 , however, adapted to span across adjacent vertebrae and capable of being coupled to vertebral body spacing device  10  may be utilized. 
         [0043]    Pedicle screws  44  are bilaterally placed at adjacent vertebrae such that each set of pedicle screws  44  are aligned on one side of the vertebrae. The surgeon modifies the appropriate anatomy in order to access the disc space between the vertebrae for accepting spacing members  20 . Spacing members  20  are fitted to the disc space. Elongate members  30 , which may be fastened with its respective spacing member  20  via first end  34  prior to implantation or subsequently coupled to spacing member  20  after placement in the disc space, extends out of the disc space and is manipulated for optimal placement of the device. Elongate member  30  may be bent, twisted, curved, straightened, or otherwise shaped to ensure proper securement of spacing member  20  within the disc space, to prevent interference with the existing anatomy of the patient, and to properly align the retention member  38  of the elongate member  30  with spine fixation member  40 . Rods  42  are placed across and secured to its respective set of pedicle screws  44  and retention member  38  of the elongate member is secured thereto. In embodiments in which retention member  38  is a hook  39   a  and screw  39   b  set, the hook  39   a  is positioned around rod  42  and screw  39   b  is placed therethrough to fasten the hook  39   a  to rod  42 . As illustrated the vertebral body spacing devices  10  are placed bilaterally within the vertebrae such that each spacing device  10  is on the same side as the spine fixation member  40  to which it will be fastened. 
         [0044]    In some embodiments, before implantation of spacing member  20 , a trial device may be attached to a spinal rod and introduced into the disc space for appropriate sizing of the spacing member  20  and to determine if there will be any soft tissue interference by the introduction of the spacing member  20  into the disc space. The trial device may be used to ascertain the amount of contacting surface area of spacing member  20  with the vertebral body endplate to ensure rotational movement of spacing member  20 . 
         [0045]    The vertebral body spacing device of the present disclosure is also contemplated to be used as a single, central vertebral body spacing device as illustrated in an alternate embodiment shown in  FIGS. 3A-4D . Vertebral body spacing device, shown generally as  110 , includes spacing member  120  having opening  124  and elongate member  130  including body portion  132  having first end  134  and second end  136 . First end  134  is configured for coupling with opening  124  of spacing member  120  and the second end  136  includes retention member  138  which is adapted to be mechanically fastened to a spine fixation member  140 . 
         [0046]    In this embodiment, spacing body  120  is elongated along lateral axis “B” to ensure a central fitting and sufficient contact of the working area of the spacing member  120  within the softer, interior area of the endplate between adjacent vertebrae. By providing more contacting surface area of spacing member  120  with the vertebral body endplate, a system  150  for maintaining disc space between adjacent vertebrae utilizing a unilateral vertebral body spacing device  110  may be achieved. 
         [0047]    System  150  includes spine fixation device  140  including pedicle screws  144  and rod  142  similar to that described above in  FIGS. 1A-2D . 
         [0048]    Pedicle screws  144  may be bilaterally placed at adjacent vertebrae in order to provide proper bone fixation between the adjacent vertebrae. Alternatively, a single set of pedicle screws  144  may be placed at adjacent vertebrae. The surgeon modifies the appropriate anatomy in order to access the disc space between the vertebrae for accepting a single spacing member  120 . A trial device may be introduced into the disc space for appropriate sizing of spacing member  120  or the spacing member  120  may be directly fitted to the disc space. Elongate member  130 , which may be fastened with spacing member  120  via first end  134  prior to implantation or subsequently coupled to spacing member  120  after placement in the disc space, is manipulated for optimal placement of the device. Rods  142  are placed across and secured to its respective set of pedicle screws  44 . Retention member  138  of elongate member  130  may be affixed and secured to either rod  142 . 
         [0049]    Vertebral body spacing device  10  may be fabricated as a single unit or multiple pieces designed for assembly by the surgeon at the time of use. Further, individual components of the vertebral body spacing device  10 , i.e. the spacing member  20  and elongate member  30 , may likewise be built as a single unit or include more than one piece for assembly. As a single unit, the device or component may be monolithically formed or pre-formed as a composite of multiple pieces. The vertebral body spacing device  10  and unassembled components thereof may be available in a range of sizes to better fit the patient&#39;s anatomy and offer greater surgical flexibility. 
         [0050]    It will be understood that various modifications may be made to the embodiments disclosed herein. Therefore, the above description should not be construed as limiting, but merely as an exemplification of the embodiments. Those skilled in the art will envision other modifications within the scope and spirit of the present disclosure. Such modifications and variations are intended to come within the scope of the following claims.