Abstract:
An articulated modular spinal fusion cage is implanted in the intervertebral space and adjusted in situ from an anterior access position to support adjacent vertebrae in normal curved alignment. The cage includes a first leg having a cylindrical pivot member and a second leg having a socket. The socket permits pivotal movement of the first leg with respect to the second leg to an anteriorly open, wedge-shaped orientation which may be selectively angularly adjusted. The laterally elongated socket and pivot member form a fulcrum that is positioned anteriorly from the posterior leg ends to enhance torsional stability and increase anterior preload. A driver is inserted through a bore in the socket and corresponding groove in the flange and is operable to engage a sloped interior surface of the first leg and to urge the anterior end upwardly by rotating the pivot member within the socket.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present application is a continuation of U.S. patent application Ser. No. 10/292,723 entitled ARTICULATED ANTERIOR EXPANDABLE FUSION CAGE SYSTEM, now U.S. Pat. No. 6,685,742. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention is broadly concerned with a spinal fusion cage system. More particularly, it is directed to an articulated implant which can be installed between a pair of adjacent vertebrae and selectively expanded in situ to form a wedge with an adjustable angle of inclination for supporting and stabilizing the vertebrae in normal curved alignment in order to promote fusion of the aligned vertebrae.  
           [0003]    The spine is a column of stacked vertebrae which are normally axially aligned along the median plane. When viewed from the front or back, the spine appears to be straight. When viewed from a lateral perspective, however, it is shown to be comprised of four distinct curves. Each vertebra is angularly displaced in the coronal plane in accordance with its position along one of the respective curves.  
           [0004]    The structure of each vertebra includes a rounded, weight bearing anterior element, or vertebral body, which is separated from the adjacent superior and inferior vertebral bodies and cushioned by fibrocartilage pads or discs. These intervertebral discs support the adjacent vertebrae in an appropriate angular orientation within a respective spinal curve and impart flexibility to the spine so that it can flex and bend yet return to its original compound curvate configuration.  
           [0005]    Aging, injury or disease may cause damage to the discs or to the vertebrae themselves. When this occurs, it may be necessary to surgically remove a disc and fuse the adjacent vertebral bodies into a single unit. Such surgical arthrodesis is generally accomplished by implanting a cage-like device in the intervertebral or disc space. The cages are apertured, and include a hollow interior chamber which is packed with live bone chips, one or more gene therapy products, such as bone morphogenic protein, cells that have undergone transduction to produce such a protein, or other suitable bone substitute material. Following implantation, bone from the adjacent vertebrae above and below the cage eventually grows through the apertures, fusing with the bone of the adjacent vertebral bodies and fixing the adjacent vertebrae as well as the cage in position.  
           [0006]    Once the disc has been removed from the intervertebral space, the angular orientation of the adjacent vertebrae is established and stabilized by the three dimensional geometry of the implanted fusion cage, and the vertebrae will eventually fuse in this orientation. The lumbar curve presents a region of normal anterior convexity and posterior concavity or lordosis. There is a need for an anterior implant for use in this region which can be adjusted in situ to achieve and maintain normal lordosis of the vertebrae.  
           [0007]    Previous attempts to achieve normal spinal curvature with fusion cages have involved trial insertion of cages of various different sizes into the intervertebral space. The cage is repeatedly removed and replaced with another unit of a slightly different size until an optimal angular incline is achieved. There is a need for a modular and articulated implant which can be installed in a first configuration, and adjusted in situ into a wedge configuration from an anterior access position.  
           [0008]    Once installed in an intervertebral space, spinal implants are subject to compressive forces exerted by gravity and movement of the spinal column. Normal forward bending activity exerts substantially greater compressive force on the vertebrae than backward bending. Consequently, there is a need for an implant which will accept an increased anterior preload to withstand anterior compressive forces and to maintain the disc space height.  
           [0009]    Spinal implants are also subject to twisting forces caused by unequal lateral distribution of weight on the adjacent vertebral bodies. This may occur, for example, during normal sideward bending and reaching activity. There is also a need for an implant which will provide torsional stability to resist such twisting forces. In particular, in order to withstand the greater compressive forces associated with forward bending movements, there is a need for an implant that will provide enhanced anterior torsional stability.  
           [0010]    The apparatus of the present invention is specifically designed to provide a modular intervertebral implant which can be both installed and selectively expanded in situ from an anterior access position to form a wedge which stabilizes the adjacent vertebrae in normal curved alignment while providing lateral stability, increased anterior preload and enhanced anterior torsional stability.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention is directed to an articulated modular cage system for implantation in the intervertebral space and adjustment in situ from an anterior access position to support the adjacent vertebrae in a normal curved alignment while permitting fusion of the adjacent bones. The fusion cage system of the present invention includes a first leg having a pivot member, a second leg having a socket for receiving the pivot member and a driver. The socket permits movement of the first leg about an axis of pivotal rotation from a closed, parallel insertion position to an anteriorly open, wedge-shaped orientation which may be selectively adjusted to provide appropriate angular support. The socket and pivot member are laterally elongated to provide lateral support. The pivot member includes a cylindrical notch or aperture, and the socket includes a threaded bore which are aligned for receiving a driver.  
           [0012]    The driver is operable to engage a sloped interior surface of the first leg and to urge the anterior end of the first leg apart from the anterior end of the second leg while causing the pivot member to rotate within the socket. Registry of the driver within both the bore and the aperture serves to prevent lateral displacement of the pivot member within the socket. The pivot member and socket are inset or positioned anteriorly of the posterior ends of the respective legs in order to enhance torsional stability and to optimize the anterior preload. This is achieved by decreasing a moment arm length between an effective area of engagement of the adjacent vertebrae and the location of the connection between the legs of the cage. Positioning the pivot axis anterior of the posterior ends of the legs also helps to optimize the intervertebral spacing and angular alignment of the adjacent vertebrae to avoid undesirably stressing the next vertebrae beyond the vertebrae engaged by the fusion cage.  
           [0013]    Objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of the invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a partially exploded perspective view of an articulated expandable spinal fusion cage system in accordance with the present invention, illustrating a threaded driver.  
         [0015]    [0015]FIG. 2 is a fragmentary side elevational view of the cage of FIG. 1 installed between adjacent vertebrae, showing a bore and interior surfaces in phantom to illustrate the path of installation of the driver.  
         [0016]    [0016]FIG. 3 is a view similar to FIG. 2, illustrating the driver installed between the bearing surfaces of the legs and the anterior portions of adjacent vertebrae displaced to achieve lordosis.  
         [0017]    [0017]FIG. 4 is a front elevational view taken along line  4 - 4  of FIG. 3 showing an anterior end of the top leg with the driver omitted.  
         [0018]    [0018]FIG. 5 is a rear elevational view taken along line  5 - 5  of FIG. 3 showing a posterior end of the top leg with the driver omitted.  
         [0019]    [0019]FIG. 6 is a bottom plan view taken along line  6 - 6  of FIG. 2, showing a bottom side of the top leg of the cage with the driver omitted.  
         [0020]    [0020]FIG. 7 is a front elevational view taken along line  7 - 7  of FIG. 3 showing an anterior end of the bottom leg with the driver omitted.  
         [0021]    [0021]FIG. 8 is a top plan view taken along line  8 - 8  of FIG. 2 showing a top side view of the bottom leg with the driver omitted.  
     
    
       [0022]    The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0023]    As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure.  
         [0024]    Certain terminology will be used in the following description for convenience in reference only and is not intended to be limiting. For example, the words “anterior”, “posterior”, “superior” and “inferior” and “lateral” and their derivatives will refer to the device as it may be installed in anatomical position as depicted in FIGS. 2-3.  
         [0025]    Referring now to the drawings, an articulated anterior expandable spinal fusion cage system in accordance with the invention is generally indicated by the reference numeral  1  and is depicted in FIGS. 1-8. FIGS. 2 and 3 illustrate a partial side view of a human spine showing an intervertebral region  2 , which is the functional location of implantation of the fusion cage system  1 , between the vertebral bodies of selected upper and lower adjacent vertebrae  3  and  4 .  
         [0026]    Referring again to FIG. 1, the fusion cage system  1  broadly includes a fusion cage  10  and a driver  11 . The cage  10  includes a first leg  12  depicted and normally installed in a superior orientation and adjustably coupled with a second, normally inferior leg  13  by a pivot joint or bearing  14  positioned posteriorly of a centerline C, passing midway between the ends. The first leg  12  has an anterior end  15 , a posterior end  16  and a pair of opposed sides  20  interconnected by a central web portion  21 . The sides of the cage  20  are depicted as generally planar and orthogonal to the web portion  21 , although they may also be of curvate, angular or compound curvate or angular construction. The legs  12  and  13  are normally of the same width and length, but it is also foreseen that either of the legs  12  and  13  may be of somewhat broader construction in order to selectively enhance the superior or inferior bone-supporting surface area.  
         [0027]    The first leg  12  includes an outer, bone-supporting surface  22  and an inner surface  23 , best shown in FIG. 6. The web  21  is apertured by one or more ports or windows  24 , which extend between the outer and inner surfaces  22  and  23 .  
         [0028]    The leg inner surface  23  includes an anterior portion  30  and a posterior portion  31 . The anterior portion  30  has a linear cam or bearing surface  32  that terminates posteriorly in a first abutment surface or stop  33  that is generally orthogonal to the outer surface  22 . When viewed from the side (FIGS. 1-3), the bearing surface  32  slopes downwardly at an angle as it approaches the second end  16  in the configuration of a ramp or wedge having the abutment surface  33  as its base. The posterior portion  31  extends in generally parallel relationship with the web outer surface  22  except for a dependent, generally cylindrical pivot member  34  having a pivot axis P. The pivot member  34  depends the entire width of the cage  10  between the sides  20  and includes a central notch, aperture or groove  35  (FIGS. 4-6) for receiving the driver  11 .  
         [0029]    The leg outer surface  22  includes a series of serrations or teeth  40  for engaging the surface of a respective adjacent vertebra  3  against slippage along an anterior-posterior axis within the intervertebral joint  2 . The leg inner surface  23  is generally smooth. The anterior bearing surface  32  is axially grooved to form a channel  41  (FIG. 6) adapted for sliding reception of the driver  11 .  
         [0030]    The second leg  13  has an anterior first end  42 , a posterior second end  43  and a pair of opposed sides  44  interconnected by a central web portion  45 . The leg  13  also includes an outer, bone-supporting surface  50  and an inner surface  51 , best shown in FIG. 8. The web  45  is apertured by one or more ports or windows  52 , which extend between the outer and inner surfaces  50  and  51 .  
         [0031]    The leg inner surface  51  includes an anterior portion  53  and a posterior portion  54 . The anterior portion  53  has a support surface  55  that extends in generally parallel relationship with the leg outer surface  50 . The posterior portion  54  also extends in generally parallel relationship with the leg outer surface  50 , except for an upstanding, approximately rectangular knuckle  60 . The knuckle  60  is elongated laterally, so that it extends the full width of the cage  10  between the sides  44 .  
         [0032]    The knuckle  60  includes anterior, posterior, and upper or superior surfaces  61 ,  62  and  63  (FIG. 3). The superior surface  63  includes a laterally extending, generally cylindrical channel which serves as a socket  64  for receiving the cylindrical pivot member  34  of the first leg  12  in pivoting relationship to form the pivot bearing  14 . The laterally elongated pivot member  34  and socket  64  cooperatively provide lateral support to the cage  10  against sideward bending stresses which may be brought to bear following installation.  
         [0033]    The anterior and posterior knuckle surfaces  61  and  62  are generally orthogonal to the outer surface  50 , and the anterior surface  61  serves as an abutment surface or stop for the first leg abutment surface  33 . The upper knuckle surface  63  is generally parallel with the outer surface  50 , except that the posterior aspect is somewhat relieved so that it does not serve as a stop when the first leg  12  pivots in the socket  64  of the second leg  13 . As shown in FIGS.  7  and  8 , the knuckle  60  includes a central bore  65  having flighting or threads  70  for receiving and engaging the driver  11 .  
         [0034]    Like the first leg  12 , the second leg outer surface  50  includes a series of bone-engaging serrations or teeth  71  (FIG. 2). The leg inner surface  51  is generally smooth. The first and second leg anterior portions  15  and  42  cooperatively define an open-sided chamber  72  when the cage  10  is assembled as depicted in FIGS. 1-3.  
         [0035]    The driver  11  is depicted in FIG. 1 to include a radially expanded head  73  and a shaft  74  terminating in a generally flattened driving end  75 . The shaft  74  is sized and shaped for reception within channel  41 , and preferably includes threads  80  for operable reception within matingly threaded bore  65 , with the radially expanded driver head  73  engaging the angled bearing surface  32  of the upper leg  12 . It is also foreseen that in certain applications the shaft  74  could be smooth and unthreaded. The driver head  73  is coupled with the shaft  74  by a generally frustoconical shank portion  81 , and terminates in a narrow, generally cylindrical bearing surface  82 . The head  73  also includes a non-round socket or receiver  83  configured for non-slip reception of a driving tool such as a wrench (not shown). While the receiver  83  is depicted as being generally hexagonal in shape, it is understood that it may be configured as a square, slot, multi-lobular or any other shape corresponding to a preselected driving tool.  
         [0036]    The diameter of the driver head  73  and the length of the shaft  74  are sized so that the driver  11  extends posteriorly through the channel  41  of the upper leg  12  for driving registry of the shaft  74  within the groove  35  of the first leg and central bore  65  of the lower leg  13  and engagement of the driver head bearing surface  82  with the angled bearing surface  32  of the upper leg  12 . In this manner, the channel  41 , groove  35  and bore  65  cooperate with the shaft  74  of the driver  11  to effectively lock the legs  12  and  13  against lateral displacement.  
         [0037]    The legs  12  and  13  and driver  11  may be constructed of a non-metallic material such as carbon fiber reinforced composite or tissue-derived polymer material, or of a strong, inert material having a modulus of elasticity such as a metal, like stainless steel or titanium alloy, or of porous tantalum or any other biocompatible material or combination of materials. It is foreseen that it may be desirable in certain applications to employ a radiolucent material such as carbon fiber reinforced composite which will not block post operative radiographic images of bridging bone growth.  
         [0038]    It is also foreseen that the fusion cage system  1  may also include a pair of independently adjustable cages  10 , installed in generally side-by-side relationship within a single intervertebral space  2 , as set forth more fully in U.S. Pat. No. 6,454,807 and incorporated herein by reference.  
         [0039]    In use, the anterior surface of a selected intervertebral region  2  of the spine of a patient is surgically exposed. The soft tissues are separated, the disc space is distracted and the disc is removed, along with any bone spurs which may be present. The disc space is distracted to a predetermined height which serves to decompress any affected nerve roots and to permit preparation of the intervertebral region  2 .  
         [0040]    The fusion cage system  1  is assembled by a surgeon or assistant by laterally aligning the cylindrical pivot member  34  of the first leg  12  with the socket  64  of the second leg  13  and sliding the pivot member  34  laterally into engagement with the socket  64  until the groove  35  is aligned with the bore  65 . The driver  11  is next grasped and the threaded end  75  is introduced into the bore  65  and rotated by hand or with the use of an insertion tool until the threads  80  of the driver shaft  74  engage the threads  70  of the bore  65 . Registry of the driver  11  within both the groove  35  of the first leg  12  and the bore  65  of the second leg serves to prevent any lateral movement or play of the pivot member  34  within the socket  64 . The driver  11  may be rotated a few additional turns in order to secure against disengagement from the bore  65  during insertion. However, unless the intervertebral space  2  is substantially larger than the cage  10 , rotation is generally stopped when the conical shank  81  engages the bearing surface  32  of the first leg  12 , so that the cage  10  can be inserted in its smallest, or closed configuration.  
         [0041]    The first and second leg anterior first ends  15  and  42  are next grasped and pressed together until the first leg abutment surface  33  comes to rest against the second leg abutment surface or stop  61  and the cage  10  is maximally compressed. The assembled fusion cage  10  presents a closed, overall rectangular configuration, with the outer surfaces  22  and  50  of the legs  12  and  13  in a generally parallel orientation, as depicted in FIGS. 1 and 2 and the driver  11  projecting slightly anteriorly from the cage  10 .  
         [0042]    The cage  10  may be press-fit directly into the distracted intervertebral region  2 , or the vertebrae  3  and  4  may be predrilled to receive the cage system  1 . Although an anterior approach is preferred, it is foreseen that a posterior or even lateral approach could also be employed.  
         [0043]    The surgeon next positions a tool (not shown) in the driver head  73  and rotates the tool in a clockwise or posteriorly advancing direction to drive or pull the threaded shaft  74  further into the bore  52  and advance the head  73  in a posterior direction. Continued rotation of the driver  11  simultaneously causes the end  75  to advance posteriorly, the pivot member  34  to rotate within the socket  64 , and the bearing surface  32  of the first leg  12  to ride up over the beveled shank  81  until the bearing surface  82  of the driver  11  engages the bearing surface  32  of the first leg  12 . In this manner, rotational advancement of the driver  11  causes it to progressively wedge the bearing surface  32  apart from the support surface  55  of the second leg  13  until the cage  10  begins to assume a generally wedge shape when viewed from the side.  
         [0044]    In this manner, the angle formed by the outer, bone supporting surfaces  22  and  50  of the legs  12  and  13 , is determined by the displacement of the bearing surfaces  32  of the first leg  12  away from the support surface  55  of the second leg  13 , which in turn is determined by the posterior advancement of the driver bearing surface  82  along the first leg bearing surface  32 . The driver  11  is of a preselected size to cause displacement of the first leg  12  to form the cage  10  into an appropriate wedge shape which will support the adjacent vertebrae  3  and  4  at the proper height as well as the desired angular alignment to achieve normal curvature of the respective spinal region.  
         [0045]    Advantageously, the laterally elongate cylindrical configuration and anteriorly inset or forward positioning of the pivot bearing  14  cooperatively formed by the pivot member  34  and socket  64 , relative to the posterior ends  16  and  43  of the legs  12  and  13 , enhance both the lateral and torsional stability of the cage system  1  as well as its anterior preload. The configuration of the channel  41  for receiving the driver shaft  74  and the anterior preload also cooperate to enhance torsional stability.  
         [0046]    The surgeon next transplants a quantity of packed bone cells or a suitable bone substitute material into the chamber  72  by a lateral approach through the open area between the first and second legs  12  and  13 . Alternatively, the bone cells may be introduced into the chamber  72  by a posterior approach through the bore  65  prior to installation of the driver  11  or by any combination of these methods. Bone for use in the graft may be harvested from the patient as live bone, from a bone bank or from a cadaver. Demineralized bone matrix, bone morphogenic protein or any other suitable material may also be employed.  
         [0047]    Following implantation, the bone grows between vertebrae  3  and  4 , through the windows  24  and  52  with the bone in the chamber  72  and around the cage system  1  to fuse the bodies of vertebrae  3  and  4  together.  
         [0048]    Those skilled in the art will appreciate that the fusion cage  10  may also be assembled and installed into the intervertebral space  2  prior to insertion of the driver  11  into the bore  65 . In addition, while a single exemplary driver  11  and cage  10  having a wedge-shaped first leg  12  is depicted, a variety of drivers  11  and cages  10 , having variously configured bearing surfaces  32  of different shapes, each producing a different degree of displacement of the first leg  12  may be incorporated in a set to allow the surgeon to preselect a cage system  1  to achieve a desired angle of displacement and consequent positioning of the vertebrae  3 ,  4  relative to each other. It is foreseen that various other configurations of the pivot bearing  14  could be advantageously employed in the cage system  1 .  
         [0049]    The cage system  1  of the invention is designed to permit adjustment by rotation of the driver  11  in situ until the desired alignment between the vertebra  3  and  4  is achieved. However, if necessary, the cage system  1  may also be removed and the installation repeated using a cage  10  and driver  11  having different configurations until the desired angular alignment is achieved.  
         [0050]    It is to be understood that while certain forms of the present invention have been illustrated and described herein, it is not to be limited to the specific forms or arrangement of parts described and shown.