Abstract:
Individual plate sections of a dynamic bone fixation plate can be internally interlocked to maintain the assembled plate and to limit relative motion between the sections. A dynamic bone fixation plate can include a first plate section, a second plate section, and a compressible interlock member. The first plate section includes a first joint structure and the second plate section includes a second joint structure, where the second joint structure can be dynamically mated with the first joint structure. The compressible interlock member can be disposed within the first joint structure and the second joint structure to limit relative motion of the first joint structure and the second joint structure.

Description:
RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/886,916 entitled “Dynamic Cervical Plate”, filed on Jan. 26, 2007, the entire teachings of which are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    Various types of implantable devices are useful in fixing bones in the body. The structure of the device is typically dependent on the bone or bone sections being fused. One type of fixation device is a cervical plate, which are implanted to increase neck stability and to promote fusion of adjacent vertebrae following surgery to remove a diseased or damaged disc in the spine. The cervical plates are available as either static or dynamic structures. 
         [0003]    A typical static cervical plate is a single metal piece that uses screws to attach the plate to two (or more) adjacent vertebrae. Because these plates are one piece of metal, they are relatively rigid, allowing little of no movement between the connected vertebrae. 
         [0004]    More recently, a “dynamic” plate technology has been developed, whereby two or more individual plate sections or pieces are joined together to form the implanted cervical plate. The union between the sections allows for some movement between the individual pieces, while still providing stability and promoting bone fusion. Therefore, when it is installed, the vertebrae will have a small level of movement. Typically, dynamic plate sections are mated together using a male/female dovetail design, but other mating designs are possible. 
         [0005]    In both cases, the cervical plate is typically made from a rigid biocompatible material, such as Titanium or stainless steel. 
       SUMMARY 
       [0006]    One challenge to constructing dynamic fixation plates is in connecting the individual parts in a manner that allows movement of the plates, but does not allow the plates to become unintentionally disassembled. In accordance with particular embodiments of the invention, individual plate sections can be internally interlocked to maintain the assembled plate and to limit relative motion between the sections. 
         [0007]    In accordance with a particular embodiment of the invention, a dynamic bone fixation plate can include a first plate section, a second plate section, and a compressible interlock member. The first plate section includes a first joint structure and the second plate section includes a second joint structure, where the second joint structure can be dynamically mated with the first joint structure. The compressible interlock member can be disposed within the first joint structure and the second joint structure to limit relative motion of the first joint structure and the second joint structure. 
         [0008]    More particularly, the first joint structure and the second joint structure can mate as a dovetail joint. When joined, the first and second plate sections can form a dynamic cervical plate. 
         [0009]    The first joint structure can include a slot and the second joint structure can include a channel, the slot and channel being aligned. Furthermore, the interlock member can be disposed within the slot and channel. The first plate section can be moveable relative to the second plate section by a distance based on the dimensions of the channel. In addition, an access port can extend from the channel to the outside of the second joint structure. 
         [0010]    The interlock member can comprise a superelastic material, which can be machined. More particularly, the material can be a Nickle-Titanium alloy, such as Nitinol materials. 
         [0011]    In accordance with another particular embodiment, a dynamic cervical plate can comprise a first plate section, a second plate section, and a compressible interlock member. The first plate section can include a male dovetail structure and the second plate section can include a female dovetail cavity, where the male dovetail structure is slidably mated with the female dovetail cavity. The compressible interlock member is disposed within the male dovetail structure and the female dovetail cavity to limit relative motion of the male dovetail structure within the female dovetail cavity. 
         [0012]    In accordance with another particular embodiment, a dynamic cervical plate can comprise a first plate section, a second plate section, and a superelastic interlock member. The first plate section can include a male dovetail structure and the second plate section can include a female dovetail cavity, where the male dovetail structure is slidably matable with the female dovetail cavity. The superelastic interlock member can be disposed within the male dovetail structure and the female dovetail cavity to limit relative motion of the male dovetail structure within the female dovetail cavity. 
         [0013]    Embodiments of the invention can also include methods of manufacturing and using the described plates. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of particular embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
           [0015]      FIG. 1  is an exploded perspective view of an exemplary prior art dynamic cervical plate. 
           [0016]      FIG. 2  is an exploded perspective view of a particular dynamic cervical plate having an interlock system in accordance with the invention. 
           [0017]      FIG. 3  is a cross-sectional view of a preassembled dovetail joint for the fixation plate assembly of  FIG. 2 . 
           [0018]      FIG. 4  is a cross-sectional view of an assembled dovetail joint for the fixation plate assembly of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    Multi-section dynamic bone fixation devices are known in the art and are commercially available from various manufacturers. In general, each manufacturer incorporates its own specific solution for mating the sections. For cervical plates, dovetail mating is typical. For illustration purposes, the concepts of the invention are described with reference to a specific dynamic cervical plate. The invention, however, is not limited to the described cervical plate its specific mating solution. 
         [0020]      FIG. 1  is an exploded perspective view of an exemplary prior art dynamic cervical plate  10 . The plate  10  includes a first section  100  and a second section  200 , which, when assembled, define a window or void  15 . The plate  10  is generally fabricated from a rigid biocompatible material, such as Titanium alloys. 
         [0021]    As shown, the first section  100  includes a first main body  110  and the second section includes a second main body  210 . Mounting holes  115   a ,  115   b ,  215   a ,  215   b  extend through the main body  110 ,  210  for receiving screws that mount the assembled plate  10  to the desired vertebrae. Also shown are interior contours  112 ,  212  and exterior contours  114 ,  214 . 
         [0022]    The plate sections  100 ,  200  mate using a male/female dovetail interconnect. Each section  100 ,  200  is generally U-shaped having leg structures  120  and  220 , respectively. The leg structures  120 ,  220  slidably mate to provide dynamization when attached to the bone. The first section  100  includes legs  120   a ,  120   b  that are fabricated as male dovetails. The second section  200  includes legs  218   a ,  218   b  that have respective female cavities  220   a ,  220   b  dimensioned to receive the male legs  120   a ,  120   b.    
         [0023]    Once assembled, the plate sections  100 ,  200  are secured to the bone, but the plate sections  100 ,  200  are free to move relative to each other because the leg structures  120 ,  218  can slide relative to each other. The sections  100 ,  200  can slide apart, especially during surgery. One challenge to constructing dynamic fixation plates is in connecting the individual parts in a manner that allows movement of the plates, but does not allow the plates to become unintentionally disassembled. 
         [0024]      FIG. 2  is an exploded perspective view of a particular dynamic fixation plate having an interlock system in accordance with the invention. As shown, the male dovetails  120   a ,  120   b  include an interlock slot  125   a ,  125   b . The female dovetail cavities  220   a ,  220   b  include an additional interlock channel  225   a ,  225   b.    
         [0025]    During assembly, a compressible interlock member  300  is seated in the interlock slots  125 . The interlock member  300  is then compressed into the slot  125  and the male dovetail leg  120  is slid into the female dovetail cavity  220 . As shown, the interlock member  300  is an arch shaped member resembling a miniature leaf spring. 
         [0026]      FIG. 3  is a cross-sectional view of a preassembled dovetail joint for the fixation plate assembly of  FIG. 2 . As shown, the interconnect slot  125  is rectangular in cross section and the male dovetail leg  120  is positioned inside the female dovetail cavity  220 . The interlock member  300  is compressed within the space of the interconnect slot  125 . 
         [0027]    Returning to  FIG. 2 , once the interlock member  300  is registered with the interlock channel  225 , the interlock member  300  expands to be received by the interlock channel  225 . The interlock member  300  is then within both the interlock slot  125  and the interlock channels  225  such that the interlock member  300  is essentially incompressible in the direction of sliding. Thus, the male dovetail leg  120  cannot be slid out of the female dovetail cavity  220 . 
         [0028]      FIG. 4  is a cross-sectional view of an assembled dovetail joint for the fixation plate of  FIG. 2 . As shown, the interconnect channel  225  has an arch shaped cross section and the interconnect slot  125  is aligned with the interconnect channel  225 . In addition, the interlock member  300  has expanded to occupy both the interconnect slot  125  and the interconnect channel  225 . Note that the arch shape of the interconnect channel  225  complements the arch shape of the expanded interlock member  300 , but that complementary shape for the interconnect channel  225  is not required. As shown, the interlock member  300  is under some compression. 
         [0029]    Also shown is an access port  230 , which extends from the female dovetail cavity  225  to the exterior of the female leg  218 . To remove the male dovetail leg  125  from the female dovetail cavity  220 , the interlock member  300  must be compressed into the interconnect slot  125  or channel  225 . To that end, a tool such as pin or needle can then be inserted into the access port  230  to engage and compress the interlock member  300 . In the particular embodiment of  FIG. 2 , the legs are separated to about their maximum extension so that the interconnect slot  125  is aligned with the access port  230 . Once compressed into the interconnect slot  125 , the male dovetail legs  120  are disengaged from the female leg  218  and can slid out of the female dovetail cavity  220 . 
         [0030]    As shown in  FIG. 2 , the amount of relative motion between the two sections  100 ,  200  is defined by the dimensions of the interlock slots  125  and interlock channels  225 , in particular the longitudinal length LC of the interlock channels  225  minus the length LM of the interlock member  300 . 
         [0031]    In a particular embodiment, the compressible interlock member  300  is fabricated from a malleable biocompatible material. In a particular embodiment, the malleable material is a Nickle-Titanium alloy, such as Nitinol, which has shape memory and superelastic properties at body temperatures. In use, the Nitinol interlock member  300  deforms under compression, but because of the superelastic effect, the spring will return to its original shape. 
         [0032]    More particularly, the interlock member  300  can be machined from a Nitinol bar, which can provide improved performance over similarly shaped springs that are stamped from Nitinol sheet material. In a specific embodiment, the interlock member  300  is machined from a 0.250 inch (nominal) diameter bar of SE-510 Nitinol, commercially available from Nitinol Devices and Components, Inc. of Fremont, Calif. A particular alloy bar is superelastic straight, centerless ground, with an Aƒ at about 10° C. Any Nitinol alloy having an Aƒ at between about 10° C. and 25° C. would be acceptable, with 18° C. being a target temperature. Depending on the design specifics, other Nitinol alloys, or other superelastic materials, with varying characteristics can also be used for the interlock member  300 . Because the interlock member  300  can be machined, its shape is not constrained by limitations inherent in wire or sheet materials. 
         [0033]    As shown, the interlock member  300  is an arch-shaped member similar to a miniature leaf spring. The dimensional constraints on the interlock member  300  are that it should fit within the interconnect slot  125  when compressed (such as being flattened) and that its expanded free height should be higher than the interconnect slot  125 . In a particular embodiment, the expanded free height of the interlock member is at least as high as the combined heights of the interconnect slot  125  and the interconnect channel  225 . Consequently, the dimensions of the interlock member  300 , the interconnect slot  125  and the interconnect channel  225  are interrelated. 
         [0034]    It should be understood that other interlock members forms can be employed with corresponding modifications to the interconnect slots and interconnect channels. While not limiting, examples of such other forms are disclosed in the incorporated provision application. The concepts of the invention are not limited to the disclosed forms, as one of ordinary skill in the art can readily appreciate other useable forms. In addition, the concepts of the invention are not limited to cervical plates and can be applied to other dynamic plate systems beyond that shown in  FIGS. 1-4 . 
         [0035]    While this invention has been particularly shown and described with references to particular embodiments, it will be understood by those skilled in the art that various changes in form and details may be made to the embodiments without departing from the scope of the invention encompassed by the appended claims. For example, various features of the embodiments described and shown can be omitted or combined with each other.