Patent Publication Number: US-2006009845-A1

Title: Method and device for kinematic retaining cervical plating

Description:
CROSS REFERENCE TO RELATED CO-PENDING APPLICATIONS  
      This application claims the benefit of U.S. provisional application Ser. No. 60/586,761 filed on Jul. 8, 2004 and entitled METHODS AND DEVICES FOR KINEMATIC RETAINING CERVICAL (KRC) PLATING which is commonly assigned and the contents of which are expressly incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates to an apparatus and a method for connecting and stabilizing spinal vertebrae, and more particularly to an apparatus and a method that connects spinal vertebrae while preserving spinal stability and mobility.  
     BACKGROUND OF THE INVENTION  
      The human spine  29  comprises individual vertebras  30  (segments) that are connected to each other to form a spinal column, shown in  FIG. 1A . The vertebras  30  are separated and cushioned by thin pads of tough, resilient fiber known as inter-vertebral discs  40 , shown in  FIG. 1B . Inter-vertebral discs  40  provide flexibility to the spine  29  and act as shock absorbers during activity. The function of the spine  29  is to protect the neural structures  44  and to allow us to stand erect, bear axial loads, and be flexible for bending and rotation. Disorders of the spine occur when one or more of the individual vertebras  30  and/or the inter-vertebral discs  40  are abnormal. In these pathologic circumstances, surgery may be tried to restore the function of the spine to normal, achieve stability, protect the neural structures  44 , or to relief the patient of discomfort. The goal of spine surgery for a multitude of spinal disorders especially those causing compression of the neural structures  44  is often decompression of the neural elements and/or fusion of adjacent vertebral segments. Fusion works well because it stops pain due to movement at the facet joints or intervertebral discs, holds the spine in place after correcting deformity, and prevents instability and or deformity of the spine after spine procedures such as laminectomies or corpectomies. Laminectomy involves the removal of part of the lamina  47 , i.e., the bony roof of the spinal canal, shown in  FIG. 1C . Corpectomy involves removal of the vertebral body  32  as well as the adjacent disc spaces  40 . Laminectomy is often used to directly decompress the posterior neural elements  42  and to relieve pain or neurologic compromise caused by posterior compressive structures. In some cases laminectomy may also achieve indirect decompression of anterior compressive structures.  
      In contrast, anterior decompression directly removes anterior compressive structures and is known to have improved results in these cases over indirect decompression afforded by laminectomies. Anterior discectomy, i.e., removal of the inter-vertebral discs  40 , and fusion or anterior corpectomy and fusion are most commonly performed in the cervical spine but there is increasing application in the thoracic and lumbar spine.  
      In recent years, there is an increase in the use of plate fixation  27  to stabilize the cervical spine  28  after anterior decompression and fusion, shown in  FIG. 1D . Plate fixation  27  provides increased stability and may allow for less reliance on rigid external orthosis such as hard cervical collars and halos for stability. Plates  27  may also increase the rate of fusion and may decrease the incidence of graft complications such as graft extrusions and subsidence. Although, current plating systems offer these advantages, there is a growing body of data that document significant failure rates for reconstruction with plates after multilevel anterior corpectomy and fusion. It is believed that the long lever arm of the plate especially across two or more vertebral corpectomies leads to pullout of the screws  25  and dislodgement of the plate  27  which can result in esophageal erosion and death. Furthermore, current anterior cervical plates  27  do not provide graft subsidence and continuous graft loading which is believed to be advantageous for fusion. It is also technically challenging to place a plate  27  across two or more disc spaces while maintaining the correct length and avoiding placing the screws  25  in the graft or the adjacent disc space. It is also difficult to place the plate  27  in a straight line longitudinally between adjacent vertebras. These technical difficulties often lead to a higher rate of complications including plate failures.  
      A modification of the standard cervical plate has been tried to function as a buttress plate. In the buttress plate design, the plate is attached to one vertebral body and extends across the endplate and partially over the graft to act solely as a block to graft dislodgement. However, this design has been abandoned since it was demonstrated that these buttress plates would dislodge when the graft shifted anteriorly against the plate and would themselves cause catastrophic problems such as esophageal erosions. Part of the reason for this failure is the fact that the plates were designed to overhang the disc space, which created a lever arm that made it easier to dislodge the anchor screws  25  in the vertebral body.  
      More recently, plates have been designed to allow motion between the fused segments either at the fixation points between the plate  27  and the screws  25  or as a sliding mechanism within the plate with the ends of the plate fixed to screws in the vertebral body. Examples of these “dynamic” plating systems include the Ant-Cer system offered by Spinal Concepts of Texas, and the ABC system offered by Aesculap, of Germany. These new “dynamic” plating systems are believed to offer superior fusion rates since they allow continuous graft loading and natural graft subsidence while acting as a block to anterior graft displacement. However, these new “dynamic” plating systems still do not remove the technical difficulties in placing the plate across the entire length of the fused segments.  
      Accordingly, there is a need for a plating system that removes the difficulties in placing the plate across the entire length of the fused segments, while providing stability and allowing motion between the fused segments.  
     SUMMARY OF THE INVENTION  
      In general, in one aspect, the invention features a spinal implant assembly for replacing intervertebral elements between a first spinal vertebra and an adjacent second spinal vertebra. The spinal implant assembly includes an intervertebral implant for inserting between the first and second spinal vertebrae and a first kinematic retaining plate. The intervertebral implant comprises a body having a top surface, a bottom surface, and a first appendage extending from the top surface of the intervertebral implant. The first appendage is adapted to fit within and form a tongue and groove attachment with a first opening formed in the first spinal vertebra. The first kinematic retaining plate is attached to the first spinal vertebra so that it secures the first appendage in the first opening. The intervertebral implant further comprises a second appendage extending from the bottom surface and the second appendage is adapted to fit within and form a tongue and groove attachment with a second opening formed in the second spinal vertebra. A second kinematic retaining plate is attached to the second spinal vertebra so that it secures the second appendage in the second opening.  
      Implementations of this aspect of the invention may include one or more of the following features. The first kinematic plate has on or more holes and is attached to the first spinal vertebra via one or more screws going through the one or more holes, respectively. The first appendage comprises side surfaces that are straight, curved, serrated, spiked, or angled relative to the top surface of the intervertebral implant, and the first opening comprises corresponding side surfaces that are straight, curved, serrated, spiked or angled relative to the top surface of the intervertebral implant, respectively. The vertebrae are cervical vertebrae, thoracic vertebra, or lumbar vertebrae. The intervertebral implant is made of bone, polyetheretherketone (PEEK), Nitinol, metals, titanium, steel, metal composites, biodegradable materials, collagen matrices, synthetic polymers, polysaccharides, calcium minerals, calcium salts, or composites containing calcium or phosphorous naturally or man made. The kinematic retaining plate is made of bone, polyetheretherketone (PEEK), Nitinol, metals, titanium, steel, metal composites, biodegradable materials, or composites containing calcium or phosphorous naturally or man made. The intervertebral implant comprises more than one appendages extending from the top surface, and the more than one appendages are adapted to fit within and form tongue and groove attachments with more than one openings formed in the first spinal vertebra. The intervertebral implant further comprises one or more cavities or one or more fenestrations. The intervertebral implant comprises an elastic structure. The intervertebral implant is inserted between the first and second spinal vertebrae for providing either anterior spinal fusion or posterior spinal fusion.  
      In general, in another aspect, the invention features a spinal implant assembly for replacing intervertebral elements between a first spinal vertebra and an adjacent second spinal vertebra. The spinal implant assembly comprises an intervertebral implant for inserting between the first and second spinal vertebrae, the intervertebral implant comprising a body having a top surface, a bottom surface, and first and second appendages extending from the top surface and the bottom surface, respectively. The first and the second appendages are adapted to fit within and form a tongue and groove attachment with first and second openings formed in the first and second spinal vertebrae, respectively. The first and the second appendages comprise first and second holes, respectively, and are attached to the first and second spinal vertebrae via first and second screws going through the first and second holes, respectively.  
      In general, in another aspect, the invention features a spinal implant assembly for replacing intervertebral elements between a first spinal vertebra and an adjacent second spinal vertebra. The spinal implant assembly comprises first and second intervertebral implants for inserting between the first and second spinal vertebrae. The first intervertebral implant comprises a body having a top surface, a bottom surface, and a first appendage extending from the top surface. The first appendage is adapted to fit within and form a tongue and groove attachment with a first opening formed in the first spinal vertebra. The second intervertebral implant comprises a body having a top surface, a bottom surface, and a second appendage extending from the bottom surface. The second appendage is adapted to fit within and form a tongue and groove attachment with a second opening formed in the second spinal vertebra. The first and the second appendages comprise first and second holes, respectively, and are further attached to the first and second spinal vertebra via first and second screws going through the first and second holes, respectively.  
      Implementations of this aspect of the invention may include one or more of the following features. The bottom surface of the first intervertebral implant comprises a first articulating structure and the top surface of the second intervertebral implant comprises a second articulating structure configured to articulate with the first articulating structure. The first intervertebral implant is articulately connected to the second intervertebral implant by articulating the first and the second articulating structures. The bottom surface of the first intervertebral implant and the top surface of the second intervertebral implant comprise coatings made of titanium, tantalum, stainless steel, polyethylene, diamond, chrome, cobalt, biodegradable materials, metal alloys, ceramic, or composites.  
      In general, in another aspect, the invention features method of replacing intervertebral elements between a first spinal vertebra and an adjacent second spinal vertebra. The method includes inserting an intervertebral implant between the first and second spinal vertebrae. The intervertebral implant comprises a body having a top surface, a bottom surface, and a first appendage extending from the top surface of the intervertebral implant. The first appendage is adapted to fit within and form a tongue and groove attachment with a first opening formed in the first spinal vertebra. The method also includes attaching a first kinematic retaining plate to the first spinal vertebra so that it secures the first appendage in the first opening.  
      Among the advantages of this invention may be one or more of the following. The implantable graft and kinematic retaining plates stabilize the spine, while allowing the patient to retain spinal flexibility by preserving motion between adjacent vertebras. The design of the plates allows for easy placement of the plates and screws because the plates are attached to only one vertebral body. The tongue and groove attachment configuration between the graft and the vertebral bodies provides more surfaces for better fusion between the graft and the endplates of the vertebras and greater stability for rotation. Furthermore, because the plates are confined to the vertebral bodies, this design allows for stability of the ends of the graft while allowing for natural graft subsidence and dynamic graft loading of the remainder of he graft and while preventing graft dislodgement.  
      The details of one or more embodiments of the invention are set forth in the accompanying drawings and description below. Other features, objects and advantages of the invention will be apparent from the following description of the preferred embodiments, the drawings and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Referring to the figures, wherein like numerals represent like parts throughout the several views:  
       FIG. 1A  is a side view of the human spinal column;  
       FIG. 1B  is an enlarged view of area A of  FIG. 1A ;  
       FIG. 1C  is an axial cross-sectional view of a vertebra;  
       FIG. 1D  is a radiographic side view of a cervical plating system;  
       FIG. 2  is a schematic view of the process of removing an intervertebral disc and inserting a graft between two vertebras;  
       FIG. 3A  is a schematic view of the process of securing the graft of  FIG. 2  by attaching two kinematic retaining plates;  
       FIG. 3B  is a side cross-sectional view (along axis AA′) of the spinal implant assembly of  FIG. 3A ;  
       FIG. 4  is a perspective schematic view of a vertebra with resected vertebral body;  
       FIG. 5  depicts schematic diagrams of various graft shapes;  
       FIG. 6A  depicts another embodiment of the spinal implant assembly;  
       FIG. 6B  is a side cross-sectional view (along axis AA′) of the embodiment of  FIG. 6A ;  
       FIG. 7A  depicts another embodiment of the spinal implant assembly;  
       FIG. 7B  is a side cross-sectional view (along axis AA′) of the embodiment of  FIG. 7A ;  
       FIG. 8A  depicts another embodiment of the spinal implant assembly; and  
       FIG. 8B  is a side cross-sectional view (along axis AA′) of the embodiment of  FIG. 8A . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Referring to  FIG. 2 , a new grafting technique for replacing an intervertebral disc  40  includes first removing the intervertebral disc  40  form the space between two adjacent vertebras  30   a ,  30   b , then forming grooves  32   a ,  32   b  in vertebras  30   a ,  30   b , respectively, then preparing a graft  90  and inserting the graft in the space between the vertebras  30   a ,  30   b . The graft  90  is either an autograft or an allograft and includes tongue extensions  92   a ,  92   b  extending from the top  91   a  and bottom  91   b  of the graft  90 , respectively. The tongue extensions  92   a ,  92   b  are designed to fit closely in grooves  32   a ,  32   b , respectively, in a tongue and groove or “dovetail” attachment configuration. The tongue and groove attachment configuration provides multidirectional stability and allows immediate range of motion of the spine without the need for external bracing. In one example, shown in  FIG. 4 , the groove  32   a , has dimensions  33   a ,  33   b ,  33   c  of 3 mm, 10 mm, 5 mm, respectively. The dimension  33   b  is usually less than the dimension  34   a  of the vertebra  30   a . The tongue extension  92   a  has a similar three dimensional configuration as the groove  32   a  and is dimensioned to fit closely within the groove  32   a . Grooves  32   a ,  32   b  are formed within the vertebras  30   a ,  30   b , respectively, with a special instrument. In one example, this special instrument is a burr with a stop that allows the formation of a groove with a predetermined depth. In another example, this special instrument is a cutting device with a stop that allows the formation of a groove with a predetermined depth and shape.  
      Referring to  FIGS. 3A and 3B , kinematic retaining plates  94  and  96  are placed over and attached to the vertebras  30   a ,  30   b , respectively. Plates  94 ,  96  prevent the dislodgment of the graft  90  while allowing dynamization of the graft, since they do not restrict vertical motion. In one example, plates  94 ,  96  have rectangular shape and have dimensions  94   a  of 14 mm and  94   b  of 5 mm. Plate  94  includes two screw holes  95   a ,  95   b , and plate  96  includes three screw holes  97   a ,  97   b ,  97   c . Holes  95   a ,  95   b  and  97   a ,  97   b ,  97   c  allow fixed or variable angled screws to be inserted into the vertebral bodies of vertebras  30   a ,  30   b , respectively for attaching the plates to the vertebras.  
      Other embodiments are within the scope of the following claims. Retaining plates  94 ,  96  may be circular, oblong, have rounded edges, or have multiple screw holes. One or more screws may go through the plate and any part of the graft in order to attach the graft to the plate. The graft and plate may be one-piece such that the plate acts as a stop against the vertebral body. Referring to  FIG. 6A  and  FIG. 6B , the tongue extensions  92   a ,  92   b , may include holes  98   a ,  98   b  respectively, that receive screws for attaching the graft directly to the vertebras  30   a ,  30   b . In this embodiment there is no need for a plate to further secure the graft to the vertebral bodies. This configuration allows even more stability to the construct. In another embodiment, the graft tongue extensions  92   a ,  92   b , may have front surfaces (not shown), that overhang and extend to cover the front of the vertebral openings  32   a ,  32   b . Also, by graft we mean any one-piece interbody structure that has a design that interdigitates with the vertebras in a tongue and groove attachment form, as described above. The graft may be made of bone, polyetheretherketone (PEEK), Nitinol, metal such as titanium, steel, or metal composites, biodegradable material, composites containing calcium or phosphorous naturally or man made. The graft may be solid or have one or more cavities that are enclosed or open or one or more fenestrations. Referring, to  FIG. 5 , there may be one or more tongue extensions extending from the top or bottom surfaces of the graft to interdigitate with the vertebral endplates either straight or angled from 0 to 90 degrees with the surface of the vertebral endplates. The surface of the tongue may comprise of straight sides with or without serrations or “spikes”. The shape of the tongue extensions may also vary to have angled or curved surfaces. The graft may be expandable or compressible either through the material properties such as Nitinol or mechanically. The tongue and groove relationship between the graft and the vertebral endplate may be with one or both vertebral endplates. The graft can be one piece connecting between the two adjacent vertebral endplates or two separate pieces  90   a ,  90   b  with a space between the ends opposite to the ends connected to the vertebral endplates, as shown in  FIG. 7A , and  FIG. 7B . The space between the grafts  90   a ,  90   b  allows for multidirectional motion. The ends of the graft may be covered with materials of varying properties and durability that include but not limited to titanium, stainless steel, polyethylene, diamond, chrome, cobalt, biodegradable materials, metal alloys. These surface coverings may be capped or coated on the ends of the graft. The adjacent ends  93   a ,  93   b  of the two separate intervertebral pieces  90   a ,  90   b , respectively, may include articulating structures, as shown in  FIG. 9A  and  FIG. 9B . The articulating structures may have varying configuration from a flat on flat design to a ball and socket design, as shown in  FIG. 9A , and  FIG. 9B . This design is the first to combine a graft material that may fuse to the endplate and that is contained within the endplate by a plate as described in this application and also having a different -material covering the opposite end that allows for articulation between vertebral endplates secondarily to articulation between the ends of the graft. Other motion preserving designs such as disc replacements have a modular polyethylene core between two connecting end pieces or have two articulating pieces that are also connected to the endplates as a single piece.  
      Several embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.