Abstract:
Disclosed are devices and methods for the re-construction of the vertebral lamina after partial or complete laminectomy. Pursuant to an exemplary method, a first bone fastener is anchored to a vertebral bone. A second bone fastener is anchored to the same vertebral bone, wherein the first and second fasteners are located on opposite sides of the vertebral midline. A connector member is used to inter-connect the two bone fasteners and produce a prosthetic lamina. 
     Multiple embodiments of the lamina prosthesis are illustrated. These devices reconstruct the spinal canal, mark the position of the nerves at re-operation and provide a stable platform for the placement of additional spinal stabilization implants.

Description:
REFERENCE TO PRIORITY DOCUMENT 
     This application claims priority of co-pending U.S. Provisional Patent Application Ser. No. 60/724,386, filed Oct. 6, 2005 and U.S. Provisional Patent Application Ser. No. 60/751,772, filed Dec. 19, 2005. Priority of the aforementioned filing dates is hereby claimed and the disclosures of the Provisional Patent Applications are hereby incorporated by reference in their entirety. 
     This application also is related to International application Serial No. PCT/US2006/39822, filed the same day herewith. 
     Where permitted, the subject matter of each of the above noted provisional application and international application is incorporated by reference in its entirety by reference thereto. 
    
    
     BACKGROUND 
     The central hole within a vertebra is termed the spinal canal and it houses and protects the spinal cord and/or spinal nerves. Whether from degenerative disease, traumatic disruption, infection or neoplastic invasion, the spinal canal may become narrowed over one or more vertebral levels and lead to compression of the indwelling neural tissues. Narrowing of the spinal canal is termed spinal stenosis and this condition can produce significant pain, neurologic dysfunction and disability. In addition, mal-alignment of adjacent spinal vertebrae can further narrow the spinal canal and cause additional pain and disability. 
     The current surgical treatment of spinal stenosis is decompression of the neural tissues by removal of the bone and ligament elements that produce nerve compression. Laminectomy, the removal of the lamina segment of the vertebrae, is the most common way to achieve decompression of the spinal canal and hundreds of thousands of patients undergo this operation every year in the United States alone. Although the operation provides nerve decompression, it also has negative and permanent side-effects upon the spinal segments. 
     Laminectomy removes one side of the nerve&#39;s natural bony housing and leaves the posterior aspect of the neural tissues exposed and unprotected. The exposed nerves are vulnerable to injury and this vulnerability is especially problematic if future surgery is required. At the time of re-operation, the nerves have no posterior bony covering that can be used as a marker of nerve location. In addition, the dural sac that naturally encases the nerves will invariably scar onto the surrounding soft tissues thereby obscuring the tissue layers and making the exact position of the nerves unknown to the operating surgeon. This makes the dural sac and contained nerves particularly vulnerable to inadvertent injury during subsequent surgery and a dural injury rate of 10-20% rate has been reported at the time of re-operation. These dural violations can lead to permanent nerve injury, disability and chronic pain. 
     Laminectomy produces a defect in the bone and ligament structures that ensure the longitudinal alignment of adjacent vertebra and weakens the structural integrity of the spine. Many patents that undergo laminectomy will subsequently develop spinal instability and disabling pain. To treat the instability, various devices have been developed to support the spine. While some of these devices permit motion, others promote fusion and complete immobilization of the unstable spinal segments. Regardless of the specifics of design or function, these devices are anchored onto the vertebral bodies with bone screws or similar fasteners and will require a stable attachment platform onto which they may be affixed. 
     The stability of the attachment platform is critical to the proper function of the implant and those devices that are poorly anchored to the underlying bone will inevitably loosen with repeated vertebral movement. Solid device attachment is especially important in the implantation of devices that preserve vertebral motion. While fusion devices bear load until the bone has fused, motion preservation implants must provide indefinite support of the vertebral movement. As motion preservation implants are used with increasing frequency, there is a growing need for an improved method of attachment onto the underlying vertebral bone. 
     SUMMARY 
     Devices and methods are disclosed to address the above-described shortcomings of spinal canal decompression. The illustrated embodiments reconstruct the vertebrae after partial or complete removal of the lamina bone. They restore the integrity of the vertebral arch and provide a stable platform onto which additional implants may be attached. Those additional implants permit re-stabilization of the spinal segments that have been rendered unstable by disease or as a consequence of prior surgery. They include devices that promote bony fusion and complete spinal immobilization as well as devices that preserve motion between different spinal segments. 
     In one embodiment, a rod is used to connect two or more bone screws that are placed within the same vertebra but on opposite sides of the vertebral midline. This reconstructs the vertebral ring and marks the position of the nerves at re-operation. It also provides a stable platform for the subsequent attachment of additional devices. Since the rod connects two screws within the same vertebra, the rod is prevented from rotating relative to the anchoring vertebra. This method provides exceptional rotational stability and it is a significant improvement over the current techniques. Further, since the screws are affixed to the vertebrae using non-parallel trajectories, the screws can not be dislodged without the avulsion of the large bone wedge contained between them. These two factors synergistically increase the pull-out resistance of the screw/rod complex and significantly increase the stability of the attachment platform. 
     Additional embodiments of the rod are illustrated and some of those embodiments contain additional points of articulation. The latter minimizes the need for rod contouring at the time of surgery and expedites the procedure. Embodiments of a rigid inter-connecting rod with a mobile segment are also disclosed. These embodiments are particularly applicable in anchoring devices that stabilize the spinal segments while preserving spinal. 
     The varied embodiments disclosed in this application provide devices and methods that reconstruct the posterior ring of the vertebrae after complete or partial laminectomy. They cover and protect the underlying neural tissues and provide a reliable marker of nerve position during re-operation. The disclosed screw/rod arrangements also provide a very stable platform onto which additional spinal stabilization implants may be affixed. Finally, a method for the stabilization and preservation of spinal motion in even grossly unstable spinal segments is also presented. 
     In one aspect, there is disclosed a device for the reconstruction of vertebral lamina after at least partial laminectomy, comprising: a first fastener and a second fastener attached at one end onto a posterior aspect of a vertebra wherein the fasteners are positioned on opposite sides of the vertebral midline; a connector attached to another end of each fastener and adapted to transition between a first and second state, wherein the connector and fastener are freely movable relative to one another in a first state and immobilized relative to one another in a second state; and a rod that is adapted to attach onto one connector and fastener at one end and a second connector and fastener at another end, wherein the body of the rod crosses the vertebral midline. 
     In another aspect, there is disclosed a device for the reconstruction of vertebral lamina after at least partial laminectomy, comprising: a first fastener and a second fastener attached at one end onto a posterior aspect of a vertebra wherein the fasteners are positioned on opposite sides of the vertebral midline; a connector attached to another end of each fastener and adapted to transition between a first and second state, wherein the connector and fastener are freely movable relative to one another in a first state and immobilized relative to one another in a second state; a first rod that is adapted to attach onto a first connector and fastener at one end and a housing at another end; a second rod that is adapted to attach onto a second connector and fastener at one end and the housing at another end; wherein the housing is adapted to transition between a first state and a second state, wherein the end of each rod that interacts with the housing is freely movable relative to the housing in the first state and immobilized relative to the housing in the second state. 
     In another aspect, there is disclosed a method for stabilizing a first bone fastener is attached to the posterior aspect of a vertebral body that has undergone a laminectomy, comprising attaching the first bone fastener to at least one additional bone fastener that is anchored onto the same vertebral body, wherein at least one of the additional anchors is on an opposite side of the vertebral midline with respect to the first bone fastener. 
     In another aspect, there is disclosed a method of connecting adjacent vertebral bodies, comprising: positioning an interconnecting member so that the interconnecting member crosses the vertebral midline; and attaching the interconnecting member to first and second bone fasteners, wherein the first and second bone fasteners are anchored into the posterior aspect of vertebral bone that have had a partial or complete lamenectomy. 
     Other features and advantages will be apparent from the following description of various embodiments, which illustrate, by way of example, the principles of the disclosed devices and methods. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows two intact vertebral bodies V 1  and V 2 . 
         FIG. 2  illustrates the same vertebral bodies V 1  and V 2  after surgical resection of the lamina. 
         FIG. 3A  shows a pedicle screw being inserted into a pedicle segment P of the vertebral body V 2 . 
         FIG. 3B  shows an exemplary embodiment of a pedicle screw. 
         FIG. 4  shows a pedicle screw anchored into each pedicle segment P of the vertebral bodies V 1  and V 2 . 
         FIG. 5  shows a rod that is used to interconnect the screws on each side of the midline of the same vertebral body V 2 . 
         FIG. 6  shows a conventional method of vertebral immobilization 
         FIG. 7A  shows a cross sectional view of the screw/rod configuration of  FIG. 5 . 
         FIG. 7B  shows a cross sectional view of the screw/rod configuration of  FIG. 6 . 
         FIGS. 8A and 8B  show alternative embodiments of a rod that interconnects a set of screws on opposite sides of the midline of the same vertebral body. 
         FIG. 9  shows an alternative embodiment of an articulating rod that interconnects screws on either side of the midline on the same vertebral body. 
         FIG. 10A  shows a perspective, exploded view of the articulating rod. 
         FIG. 10B  shows a cross-sectional view of the articulating rod. 
         FIG. 10C  shows a cross-sectional view of the articulating rod attached to screws on the vertebral body V 2 . 
         FIG. 11  shows another embodiment of an articulating rod in an exploded state. 
         FIGS. 12A and 12B  show an articulating member of the rod. 
         FIG. 13  shows another embodiment of an articulating rod. 
         FIG. 14  shows an articulating member of the rod. 
         FIGS. 15A and 15B  shows articulating rods of  FIGS. 11 and 13  being used to interconnect the screws on each side of the midline of the same vertebral bodies 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows two intact vertebral bodies V 1  and V 2 . For clarity of illustration, the vertebral bodies are represented schematically and those skilled in the art will appreciate that actual vertebral bodies include anatomical details not shown in  FIG. 1 . For clarity of illustration, certain anatomical details, such as the patient&#39;s skin, are not shown in at least some of the figures. The vertebral arch is comprised of two pedicles, the short stout processes that extend from the sides of the vertebral body and two laminae, the broad flat plates that project from the pedicles and join in a triangle to form a hollow archway (the foramen). 
       FIG. 2  illustrates the same vertebral bodies V 1  and V 2  after surgical resection of the lamina. The negative effects of laminectomy can be countered by the reconstruction of the lamina. With reference to  FIG. 3   a , a pedicle screw  305  is inserted into a pedicle segment P of the vertebral body V 2 . The pedicle screw  305  is shown in  FIG. 3   b  and generally includes a shank  310  with a head  315  that is removably mounted in a housing or receiver  320  in a polyaxial configuration. The receiver  320  includes means, such as slots  325 , adapted to receive an elongate stabilizer, or interconnecting member, such as a rod. It should be appreciated that the structure and type of pedicle screw can vary. As shown in  FIG. 4 , a pedicle screw  305  can be inserted into each pedicle segment P of the vertebral bodies V 1  and V 2 . 
       FIG. 5  shows a rod  505  that is used to interconnect the screws  305   a  and  305   b  on opposite sides of the vertebral midline M of the same vertebral body V 2 . The rod  505  is lowered into the receiver  320  of each screw. A locking means, such a locking nut  605  secures the rod to the receiver  320 . The screws  305   c  and  305   d  on each side of the midline of the same vertebral body V 1  can also be interconnected by another rod  505 . A rod  505  connects the screws  305   a  and  305   b  on the vertebral body V 2  and a rod  505  connects the screws  305   c  and  305   d  on the vertebral body V 1 . As demonstrated, the device reforms the posterior border of the neural canal and protects the nerves within it. It also provides a marker of nerve location at re-operation. 
     In the illustrated construct, the two rods  505  can also serve as an attachment platform for devices that realign and stabilize the spine. Depending on the specifics of the design, these devices may function to preserve spinal motion or immobilize the two vertebral bodies.  FIG. 6  shows a conventional method of vertebral immobilization. In the construct of  FIG. 6 , a rod  805   a  is used to connect the screws  305   a  and  305   c  of the two vertebral bodies, the rod  805   a  being positioned on a single side of the midline. A rod  805   b  is used to connect the screws  305   b  and  305   d  of the two vertebral bodies, the rod  805   b  being positioned on a single side of the midline. Using the method construct of  FIG. 6 , the screws  305  have no rotational stability since each screw is anchored in a different vertebral body and each screw may rotate freely relative to its anchor site. 
       FIG. 7A  shows a cross sectional view of the screw/rod configuration of  FIG. 5 , while  FIG. 7B  shows a cross sectional view of the screw/rod configuration of  FIG. 6 . As shown in  FIG. 7A , interconnection of the screws on each or opposite sides of the vertebral midline M reconstructs the posterior aspect of the spinal canal and produces a platform with exceptional rotational stability. Further, the pull-out resistance of the screws  305  is also enhanced since the interconnected screws  305  capture a wedge W of bone between them. The wedge W must be avulsed before the screws can dislodge thereby providing greater pull-out resistance for the screws than the configuration of  FIG. 7B . By contrast, the interconnected screws connected by the method of  FIGS. 6 and 7B  do not capture a wedge of bone and are completely dependent on thread purchase to resist pull-out. 
       FIG. 8A  shows an alternative embodiment of a rod  1005  that can be used to interconnect a set of screws  305   c  and  305   d  on opposite sides of the midline of the same vertebral body. In  FIG. 8A , rod  1005  interconnects screws  305   c  and  305   d  while a straight rod  505  connects the screws  305   a  and  305   b . The rod  1005  includes a central section  1010  that passes over the midline, a pair of bends  1015 , and a pair of coupling sections  1020  transverse to the central section  1010  that couple to the receiver members  320  of the screws  305 . Use of the rod  1005  provides increased space between the upper rod  505  and the lower rod  1005  and permits use of a longer/larger device to connect them.  FIG. 8B  shows a first rod  1005   a  interconnecting screws  305   a / 305   b  and a second rod  1005   b  interconnecting screws  305   c / 305   d . This configuration shown allows the center point of a device used to connect the two rods  1005  to substantially match the position of the disc space. This is particularly useful when using a device that preserves segmental motion since the device may now be centered on the axis of rotation of the disc space and vertebral bodies. 
       FIG. 9  shows an embodiment of an inter-connecting rod  1705  that contains additional points of articulation mechanism. The articulation mechanism  1710  permits the first rod section  1315  of the rod  1705  to articulate and rotate relative to the second rod section  1320 .  FIG. 10A  shows component members of the device while  FIG. 10B  shows a cross-sectional view of the rod  1705 .  FIG. 10C  shows a perspective, cross-sectional view of the assembled construct. The articulation member  1710  includes a housing  1905  that couples to the first rod section  1315  and the second rod section  1320  in a ball-and-socket configuration. That is, the ends  1325 ,  1327  of the first and second rod sections each have ball shapes that are rotatingly positioned inside the housing  1905 . A locking member  1910  can be compressed downward onto the ball-ends  1325 ,  1327  of the first and second rod sections to lock the position and orientation of the first and second rod sections  1315 ,  1320  relative to the housing  1905 . A set screw  1920  has threads that mate with threads on the housing  1905 . The set screw  1920  is tightened downward to compress and lock the locking member  1910 . 
       FIG. 11  shows another embodiment of an articulating rod  1105  in an exploded state. The rod  1105  includes five sections that are interconnected to one another and that can articulate relative to one another. The rod  1105  includes a pair of articulating members  1110  that interconnect the rigid rod sections. With reference to  FIGS. 12A and 12B , each of the articulating members  1110  is formed of a plurality of sections  1210  and  1220 . The articulating member permits the attached rod components to rotate along their axes. In one embodiment, the first and second fastener attachment segments (rod components) can rotate relative to one another and where the rotational range is +45 to −45 degrees. The articulating member is a flexure based bearing, utilizing internal flat crossed springs, capsuled in a cylindrical housing, to provide precise rotation with low hysteresis and no frictional losses. The bearing is stiction-free, requires no lubrication, and is self-returning. The articulating member can resist rotational movement away from a neutral state and the extent of resistance to rotation is directly related to the extent of rotation. The extent of resistance to rotation can be a pre-determined property of the device. In one embodiment, the articulation member is has high radial stiffness, high axial stiffness and is frictionless (hence, no particle wear debris). An exemplary articulating member of the type shown in  FIGS. 12A and 12B  is distributed by Riverhawk Company of N.Y. under the name FREE FLEX PIVOT.  FIG. 13  shows another embodiment of the articulating rod having an articulating member  1305  that is constructed in a manner similar to the articulating member  1110 . The articulating member permits the rod sections to rotate along their axes.  FIG. 14  shows a cross-sectional view of the articulating member  1305 , which comprises several sections formed of a plurality of internal, interconnected structures that are adapted to move and/or deform relative to one another. 
       FIG. 15A  shows articulating rods of  FIGS. 11 and 13  being used to interconnect the screws on each side of the midline of the same vertebral bodies.  FIG. 15A  shows an articulating rod in combination with a rigid screw and rod. The rigid screw and rod form a cantilever framework that is attached to the stable segment. Dynamic screws are then anchored into the vertebral bodies with abnormal alignment and/or motion and attached to the rigid rod. In this way, the degenerated segments are stabilized while motion is preserved. In the embodiments of  FIG. 15 , there is shown a first fastener and a second fastener attached at one end onto a posterior aspect of a first vertebra, wherein the fasteners are positioned on opposite sides of the vertebral midline. A housing is attached to another end of each fastener and adapted to transition between a first and second state, wherein the housing and fastener are freely movable relative to one another in a first state and immobilized relative to one another in a second state. A device of  FIG. 11  is adapted to attach onto a housing and fastener at one end and a second housing and fastener at another end, wherein the body of the device forms a first platform P 1  that crosses the midline of vertebra V 1 . A rigid device as shown above can be attached to a second vertebra V 2  and forms a second platform P 2  that crosses the midline of vertebral V 2 . An inter-connector L can be rigidly affixed onto each of platforms P 1  and P 2  and form a cantilever support structure, wherein vertebra V 2  forms the construct&#39;s stable base of support and upon which a mobile vertebra V 1  is anchored. 
     The preceding disclosure provides a method through which alignment may be corrected and motion may be preserved even in those degenerated segments that currently require fusion and complete immobilization. In this method, a rigid screw and rod are used as a cantilever framework onto which other vertebral segments can be attached using dynamic connectors. Depending on the anchor site, the dynamic connectors can be attached on one side of the rigid anchor or on both sides of it. In the cervical spine, for example, stability can be provided to a large segment of the neck by placement of a rigid bone screw in an intermediate level (usually C 5 ) and then connecting it to a rigid rod. This forms a cantilever framework onto which dynamic anchors can be attached. The dynamic screws are attached to an upper level (usually C 2 ) and a lower level (usually C 7  or T 1 ) and, collectively, the construct provides effective stabilization of the neck while preserving motion. 
     This method can be alternatively applied using a rod capable of movement along its long axis, such as a rod with an articulating member. When employed, the rod would retain the cantilever framework needed for stabilization but provide an extended range of motion during movement. It should be appreciated that the rigid and dynamic screws disclosed are illustrative and that the method itself can be used with any rigid and dynamic fasteners. 
     Although embodiments of various methods and devices are described herein in detail with reference to certain versions, it should be appreciated that other versions, embodiments, methods of use, and combinations thereof are also possible. Therefore the spirit and scope of the appended claims should not be limited to the description of the embodiments contained herein.