Abstract:
Replaces anterior spinal column with an expandable lattice implant made of supportive material that includes use of fusion augmenting material. Allows ease of use while promoting both immediate spinal stability and eventual arthrodesis. May be utilized during anterior or retroperitoneal approaches to the spinal column, primarily addressing anterior spinal column pathology. May utilize fusion augmenting material of limited biomechanical strength compared to the strength of its rigid components. May be accompanied by additional anterior or posterior spinal instrumentation and fixation. Uses a pair of circular endplate discs that are distracted from each other by ribs of rigid support rods. Extension may be performed using an expansion tool once device is placed within intervertebral space. Hollow portions of device may be packed with bone or other materials to enhance eventual fusion. Shape of discs at each end may be manipulated prior to surgery to adapt to the specific spinal curvature desired.

Description:
[0001]    This application claims benefit of U.S. Provisional Patent Application Ser. No. 60/809,810 filed Jun. 1, 2006, the specification of which is hereby incorporated herein by reference. 
     
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    Embodiments of the invention described herein pertain to the field of medical devices and methods. More particularly, but not by way of limitation, one or more embodiments of the invention enable an expandable intervertebral implant and method. 
         [0004]    2. Description of the Related Art 
         [0005]    The ideal implant is one that restores the normal alignment of the spinal column and provides biomechanical stability. This is provided in the short term by immediate spinal fixation utilizing metallic implants such as titanium or stainless steel. The difficulty with such implants in the long term is that a lack of solid bony fusion (pseudoarthrosis) may lead eventually to hardware failure and subsidence of the implant. The utilization of bone only as an implant is complicated by its difficulty in accurately sizing the graft and subsidence of the material. Because of the sometimes narrow corridors through which surgical access to the implant site is afforded, the utilization of expandable interbody devices has intuitive advantages. Excessive machining of bone-only implants is avoided as well, and a single implant size may be utilized for many different applications. Partial correction of the deformity may also be achieved. 
         [0006]    Prior patents have described expandable intervertebral devices utilizing a variety of different mechanisms and proposing numerous different advantages (U.S. Pat. No. 5,665,122, No. 5,505,732, No. 5,653,762, No. 6,183,517, No. 6,193,757, No. 7,044,971, No. 5,059,193, No. 6,102,950, No. 6,126,689, No. 5,749,916). These devices, and others described in published patent applications, are limited in many ways. Most are designed for use in situations arising solely from degenerative conditions of the intervertebral disc. They are thus not designed to expand beyond a relatively small size and cannot replace a vertebral body following a corpectomy. Devices made from titanium or other metallic substances may not allow for eventual bony fusion even though they provide immediate stability and may be prone to eventual pseudoarthrosis and implant subsidence. This may be true even if the device is designed to be hollow to allow for the placement of bone graft material, because only large openings and large contact surfaces at the endplates above and below the implant will allow for eventual solid bony fusion (arthrodesis). 
         [0007]    For at least the limitations described above there is a need for an expandable intervertebral implant and method. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    Embodiments of the invention are designed to replace disc material or bone utilized during surgical stabilization of the spine. Specifically, embodiments of the invention incorporate a rigid external frame designed to expand to fit the desired space along with additional fusion augmenting material leading to the enhancement of arthrodesis. 
         [0009]    The combination of intervetebral disc material and the solid bone of the vertebral body together form a stable support structure for the mammalian spine. In humans, several types of disease processes may result in partial or complete destruction of these support structures. These may include but are not limited to arthritic degeneration, infection, traumatic disruption, or neoplasm. During the course of treating these conditions surgically it is sometimes necessary to remove all or part of both the discs (discectomies) and/or the vertebral bodies (corpectomies). This results in biomechanical destabilization of the spine necessitating replacement with either bone or an implanted device. 
         [0010]    Embodiments of the implant are designed to replace the anterior spinal column by an expandable lattice implant made of solid supportive material. This cage can be sheathed with fusion material both around its cylindrical shaft and at its ends. This allows for maximal ease of use in a variety of clinical applications while promoting both immediate spinal stability and eventual arthrodesis. Embodiments may be utilized during anterior or retroperitoneal approaches to the spinal column, primarily addressing anterior spinal column pathology. Since the device may include incorporation of fusion augmenting material of limited biomechanical strength when compared to the strength of its rigid components, it may be accompanied by additional anterior or posterior spinal instrumentation and fixation. This is normally performed in cases of anterior intervertebral implant placement in such applications and is not a drawback to this device design. 
         [0011]    To achieve these objectives, a pair of circular endplate discs are distracted from each other by ribs of rigid support rods. This may be performed using an expansion tool once the device is placed within the intervertebral space. The hollow portions of the device may be packed with bone or other materials to enhance eventual fusion. The shape of the discs at each end may be manipulated prior to manufacture so that a variety of shapes are available to the operating surgeon thus maintaining adaptability to the specific spinal curvature desired. The use of fusion augmenting material in the interstices of the metallic lattice provides additional biomechanical strength following initial implantation while maximizing the probability of eventual arthrodesis. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The above and other aspects, features and advantages of the invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein: 
           [0013]      FIG. 1  is an end on view of an embodiment of the device. 
           [0014]      FIG. 2  is a side view of an embodiment of the device comprising the saucer like endplate engagers and the intervening cylindrical metallic-bioabsorbable lattice. 
           [0015]      FIG. 3  is a side of an embodiment of the device that incorporates views in the compressed (a) and expanded (b) states. 
           [0016]      FIG. 4  is an exploded view of an embodiment of the expandable design elements of the cylindrical shaft lattice. 
           [0017]      FIG. 5  is a view from the side of an embodiment of the device demonstrating the use of the insertion tool in expanding the device. 
           [0018]      FIG. 6  is an end view of an embodiment of the device showing the placement of the insertion tool. 
           [0019]      FIG. 7  illustrates a flow chart for manufacturing an expandable intervertebral fusion implant in accordance with at least one embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    An expandable intervertebral implant and method will now be described. In the following exemplary description numerous specific details are set forth in order to provide a more thorough understanding of embodiments of the invention. It will be apparent, however, to an artisan of ordinary skill that the present invention may be practiced without incorporating all aspects of the specific details described herein. In other instances, specific features, quantities, or measurements well known to those of ordinary skill in the art have not been described in detail so as not to obscure the invention. Readers should note that although examples of the invention are set forth herein, the claims, and the full scope of any equivalents, are what define the metes and bounds of the invention. 
         [0021]    An embodiment of the expandable intervertebral fusion implant appears in  FIGS. 1-6 . A flowchart for manufacturing an embodiment of the device appears in  FIG. 7 .  FIG. 1  is an end on view of an embodiment of the device. An embodiment of the implant includes endplate discs  10  from which protrude perpendicular struts  30 . Protruding from the vertebral body-contacting surface of the endplate discs are multiple teeth or ridges  18  (see  FIG. 3 ) which prevent migration of the implant by keeping teeth/ridges firmly embedded within the vertebral body. 
         [0022]      FIG. 2  is a side view of an embodiment of the device comprising the saucer like endplate engagers and the intervening cylindrical metallic-bioabsorbable lattice. The body of the implant, or cylindrical component  38 , includes a lattice of rigid support rods  16  (pointing into the page as shown in  FIG. 1  or shown as a side view in  FIG. 4 ) and is designed as a separate structure for insertion into a space between adjacent vertebral bodies in the spine. Two or more of cylindrical component  38  may be utilized to form the body when at least one cylindrical component  38  is sized to fit within another cylindrical component. This allows for extension of the body. A variety of sizes of the cylindrical component  38  may be made available depending on the application following but not limited to discectomy or corpectomy. Fusion augmenting material  20  (see FIGS.  1 - 3 , 5 ) may make up large volumes between the rigid structures of the device and is denoted as dark grey shading. The body of the device is configured to securely hold fusion augmenting material and any means or method of holding the fusion augmenting material within the device is in keeping with the spirit of the invention. 
         [0023]    As used herein, the term fusion augmenting material defines one or more bioabsorbable, porous, and/or graft materials that promote the growth of bone tissue from one vertebral body across a disc space to an adjacent vertebral body to thereby substantially eliminate relative motion between those vertebrae. The graft material refers to bone materials, autologous or any other material that may be utilized to graft, including cancellous bone for example. 
         [0024]    A typical endplate disc  10  is shaped to allow the placement against the vertebral body around the circumference and has a hollow center to allow for the placement of the fusion augmenting material  20  to contact the vertebral body. The perpendicular struts  30  acts as load sharing structures and may provide scaffolds for attaching the fusion augmenting material  20 . Any other arrangement that provides a manner of placement of endplate discs onto vertebral body and provide hardware to couple and secure the cylindrical component  38  is keeping the spirit of the invention. For example, an endplate discs may be configured with swing-arms to wrap around the vertebral body or may be a pair of two U-shaped pieces that interlock with each other after assembly on the vertebral body. 
         [0025]    Open spaces  12  are provided through which additional fusion material may be placed into the interstices of the device. (See also  FIG. 3 ) The placement of the additional fusion material may be performed after the assembly and expansion of the cylindrical component  38 . This is also to encourage bone growth through the center of the device leading to a solid fusion. The additional fusion material may include autologous bone material, such as cancellous bone for example. 
         [0026]    The rigid portions of the device (light grey) such as endplate disc  10  and perpendicular struts  30  for example may be made of biocompatible materials such as stainless steel, titanium, graphite, ceramic, or various plastics or composites. The portions promoting bony fusion (dark grey) may be made of a fusion augmenting material through which bony fusion may occur. The exact material chosen depends on the specific application. 
         [0027]      FIG. 2  shows the manner in which the two main components of the device are designed to fit together from a side perspective. Stylized models of the vertebral bodies above and below the implant are shown, and removal of the intervening anatomic structures has taken place. The endplate discs  10  have been placed against the vertebral bodies. Additional modification to the curvature, thickness, or shape of the endplate discs  14  may be made in order to alter the degree of lordotic or kyphotic angle desired. This may be done at the time of manufacture and a variety of sizes and shapes may then be made available at the time of implantation. Placement of the cylindrical component  38  may be performed along sunken rails  40  on the endplate discs  10  which have been machined to fit the radius and depth of piece  38  precisely. This may be done either before or after device implantation. The surgical approach may be done through an anterior or retroperitoneal approach, and the endplates of the vertebral bodies above and below the device may be prepared to promote bone fusion. 
         [0028]      FIG. 3  is a side of an embodiment of the device that incorporates views in the compressed (a) and expanded (b) states. Expansion of the device occurs along the long axis of the cylindrical component using an external expansion tool. Following adequate distraction and expansion, the locking plate  36  is placed in a groove along the cylindrical component  38  and into the endplate disc  10 . This assembly is secured by screw  32  which must be designed to be self-locking in order to prevent inadvertent loosening. (See also  FIG. 2  for exploded view with locking plate  36  and screw  32  shown to the left of cylindrical component  38  in anticipation of insertion to the right to couple and lock cylindrical component  38  to endplate disc  10 ). The screw and locking plate are not shown on the bottom endplate disc in  FIGS. 3 and 5  are optional if a biomechanical test indicates that the cylindrical component can be secured by coupling through the sunken rails of the endplate discs without additional locking. Additional expansion of the implant may be carried out at any time following assembly. Assembly may also precede implantation, so that sunken rails  40  of the endplate discs  10  are substantially parallel to allow coupling of the cylindrical component  38  to the endplate discs  10 . 
         [0029]      FIG. 4  shows a magnified view of the interface between the support rods  16  running along the long axis of cylindrical component  38 . Interdigitating teeth  22  are integrated in the rods and oriented so that expansion in the direction of the arrows may occur but compression may not. Such teeth may be utilized or arranged along all potential mating surfaces of the longitudinal rods at each of the four corners of the device as shown in  FIG. 1 . Any number of support rods or any other arrangement that provides a manner of providing longitudinal expansion between the endplate discs  10  is in keeping with the spirit of the invention. 
         [0030]    Removal of the device may be performed after removal of screw  32  and locking plate  36  as per  FIG. 3 . Because the rails  40  are designed to fit the cylindrical component  3 , this portion may be then removed using a removal tool without compression of the device. The endplate discs may then be pried from their positions and removed. Any other arrangement that provides a manner of securing and the subsequent removal of cylindrical component  38  to/from endplate discs  10  is in keeping with the spirit of the invention. 
         [0031]    Further elaboration of the process of expansion for the device is shown in  FIG. 5 . From a side view, the expansion tool  42  is handled at ends  43  and slid forward to engage the circular supports  10  (also see  FIG. 3 ). This may be done either at the endplates or at the mid-position of the cylindrical component. The position of the footplates  48  of the expansion tool are shown from a lateral view in  FIG. 5  as well as in an end on view in  FIG. 6 . This is shown both before (a) and after (b) insertion. The footplates  48  are placed medial to the support rods  16  and above the perpendicular struts  30 . After engagement of the tool in the device, expansion is performed by moving the grips  43  away from each other around the pivot points  44 . 
         [0032]      FIG. 7  illustrates a flow chart for manufacturing an expandable intervertebral fusion implant in accordance with at least one embodiment of the invention. Embodiments of the invention may be utilized after preparing the spinal column and removing damaged vertebral materials, including but not limited to damaged disc nucleus, between two intact vertebrae. However, at any time preceding use of the device, the device may be manufactured according to  FIG. 7 . 
         [0033]    The process of manufacturing an embodiment of the invention starts at  701  by configuring two endplate discs so that they may be utilized on opposing sides of two adjacent intact vertebrae. This for example may include the addition of teeth to the endplate discs so that they are configured to remain stationary when placed against the vertebrae. The cylindrical component is then configured to be coupled to the two embedded endplate discs at  702 . The step for example may include the machining of lips on the ends of the cylindrical components and/or rails into the endplate discs so that a cylindrical body may be inserted into the discs at a later time. The device is configured to allow for an expansion tool to be inserted into the device at  703 . For example, machining of spaces or leaving gaps in the device that allow for the insertion of the tool are in keeping with the spirit of this step. The expansion tool is used to expand the cylindrical component so that the endplate discs bears against the vertebral bodies after time of manufacture. At time of manufacture however, the expansion tool may be inserted to test for proper expansion of the device before time of surgery. The device is configured for fixing the length of the cylindrical component at  704 . This may include for example the inclusion of interdigitating teeth  22  on support rods  16  or any other mechanism which allows the device to be expanded to a fixed length. The device is configured to allow the placement of additional fusion material into embodiments of the device at  705 . This may include for example the addition of struts or spaces or gaps that allow for any type of fusion augmenting material to be added at the time of manufacture or at the time of surgery. 
         [0034]    While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims.