Abstract:
An improved polyaxial mechanism is disclosed. The mechanism preferably works in conjunction with a bone plate and fixation mechanism, such as a screw, rod, or the like. The improved polyaxial mechanism increases the strength of the locking among the locking mechanism, fixation mechanism, and bone plate. In addition, the improved polyaxial mechanism allows for easy locking among the elements, as well as more difficulty in unlocking the elements. A method for utilizing the improved polyaxial mechanism is also disclosed.

Description:
BACKGROUND OF THE INVENTION 
     Screw-and-plate osteosynthesis systems must allow immobilizing of one or more bone fragments in reference to others. It is known to use spherical-head screws cooperating with a spherical aperture housed in a plate and bringing the plate into compression over the bone until the friction of the plate on the bone stabilizes the assembly. These assemblies allow choice of the angle of implantation of screws during the operation and causing a return movement and a compression of a detached bone fragment. Certain of these systems allow, due to the oblong shape of the aperture made in the plate, compression of one bone fragment on another. The shortcoming of these systems is their low resistance to compression stresses exerted parallel to the plane of the plate. 
     Use is also known of a second generation of screw-and-plate systems called monoaxial-locking and polyaxial-locking systems in which the strength of the assembly no longer depends on compression of the plate on the bone but on a fixation of the screw in the plate. These systems allow achieving assembly away from the bone with, for the more elaborate ones, the possibility of choosing the angle of implantation of screws during the operation while achieving strength sufficient for postoperative stresses. 
     Commonly owned U.S. Patent Publication No. 2005/0143742 (“the &#39;742 publication”), the disclosure of which is hereby incorporated by reference herein, teaches an improvement to the above-noted and already useful devices. More particularly, the &#39;742 publication teaches a device which is used for solidly connecting a part such as a plate to an underlying support (e.g., a bone) using at least one fixing element such as screw. According to the invention of the &#39;742 publication, the fixing element takes the form of a threaded rod or screw which passes through a hole housing a ring belonging to the part, such that it is screwed into the support material. The ring is preferably a constriction ring including a non-circular outer profile which co-operates with the non-circular inner profile of the hole formed in the part. Upon rotation of the ring, such is wedged in place and constricted and thereby blocking the threaded rod or screw against movement with respect to the plate. 
     Even in view of the innovative and highly useful device taught in the &#39;742 publication, which is itself an improvement upon well-known and useful technology, there still exists room for improved functionality and design in fixation of bone plates. For example, there exists a need for improved bending strength between the threaded rod or screw and the plate, among other improvements. 
     SUMMARY OF THE INVENTION 
     A first aspect of the present invention is a polyaxial locking mechanism for locking a fixation mechanism with respect to a bone plate. The polyaxial locking mechanism preferably includes a body disposed within an aperture formed in the bone plate. The body further includes a central opening for receiving the fixation mechanism, a slot formed through the body and into the central opening, and a circumferentially extending interior groove formed on an interior surface of the body. Preferably, in a loosened position, the body is capable of polyaxial movement with respect to the bone plate, and upon rotation of the body to a tightened position, the body, the fixation mechanism, and the bone plate are fixed with respect to one another. 
     A second aspect of the present invention is a polyaxial locking mechanism for locking a bone screw with respect to a bone plate. The polyaxial locking mechanism preferably includes a circular ring shaped body disposed within an aperture formed in the bone plate. The body further includes a central opening for receiving the bone screw, a slot formed through the body and into the central opening, a circumferentially extending interior groove formed on an interior surface of the body, and a plurality of exterior grooves formed on an exterior surface of the body, the exterior grooves being situated in different sections. Preferably, in a loosened position, the body is capable of polyaxial movement with respect to the bone plate, and upon rotation of the body to a tightened position, the body, the bone screw, and the bone plate are fixed with respect to one another. 
     A third aspect is a polyaxial locking mechanism for locking a bone screw with respect to a bone plate. The polyaxial locking mechanism preferably includes a circular ring shaped body disposed within an aperture formed in the bone plate. The body further includes a central opening for receiving the bone screw, a slot formed through the body and into the central opening, a circumferentially extending interior groove formed on an interior surface of the body, the interior groove adapted to engage a portion of the bone screw, a plurality of exterior grooves formed on an exterior surface of the body, the exterior grooves being situated in different sections, a cut out formed on the interior surface of the body, two peaks separated by a valley for use in engagement with a driver for rotating the body, and a circumferential wall for engagement with a head portion of the bone screw. Preferably, in a loosened position, the body is capable of polyaxial movement with respect to the bone plate, and upon rotation of the body to a tightened position, the body, the bone screw, and the bone plate are fixed with respect to one another. 
     A fourth aspect of the present invention is a fracture fixation system. In accordance with one embodiment of this fourth aspect, the fracture fixation system includes a bone plate having a non-circular aperture formed therein, a body disposed within the aperture, and a bone fastener received within the central opening of the body. The body preferably includes a central opening, a slot formed through the body and into the central opening, and a circumferentially extending interior groove formed on an interior surface of the body, the bone fastener having a head engaged with the circumferentially extending interior groove. Preferably, when in a loosened position, the body is capable of polyaxial movement with respect to the bone plate, and upon rotation of the body to a tightened position, the body, the bone fastener, and the bone plate are fixed with respect to one another. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the subject matter of the present invention and the various advantages thereof can be realized by reference to the following detailed description in which reference is made to the accompanying drawings in which: 
         FIG. 1  is a perspective view of a polyaxial locking mechanism or ring in accordance with one embodiment of the present invention. 
         FIG. 2  is a top view of the polyaxial locking mechanism or ring shown in  FIG. 1 . 
         FIG. 3  is a cross sectional side view taken along line A-A of  FIG. 2  of the polyaxial locking mechanism or ring shown in  FIG. 1 . 
         FIG. 4  is a cross sectional side view taken along line B-B of  FIG. 2  of the polyaxial locking mechanism or ring shown in  FIG. 1 . 
         FIG. 5  is a cross sectional side view taken along line C-C of  FIG. 2  of the polyaxial locking mechanism or ring shown in  FIG. 1 . 
         FIG. 6  is a partial cross sectional side view of the polyaxial locking mechanism or ring of  FIG. 1 , illustrating its engagement with a screw. 
         FIG. 7  is a top view of the polyaxial locking mechanism or ring of  FIG. 1  in engagement with a screw and a bone plate. 
         FIG. 8  is a cross sectional perspective view of the polyaxial locking mechanism of  FIG. 7  with a ring driver attached thereto. 
         FIG. 9  is a cross sectional side view of the polyaxial locking mechanism of  FIG. 7  with a ring driver attached thereto. 
         FIG. 10  is a perspective view of the ring driver of  FIGS. 8 and 9 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings, wherein like reference numerals refer to like elements, there is shown in  FIGS. 1-5 , a polyaxial locking mechanism or ring designated generally by reference numeral  10 . Ring  10  is preferably designed so as to be received within an aperture formed in a bone plate designed for use with the ring. This will be discussed more fully below. The figures depict a preferred embodiment ring  10 . However, it is to be understood that ring  10  may vary in size and/or shaped depending upon the particular plate it is designed for. Likewise, differently configured rings fall within the scope of the present invention. For example, certain of the below discussed elements of ring  10  may vary according to the ultimate use of the ring within a particular patient. 
     As is best shown in the perspective view of  FIG. 1 , ring  10  essentially consists of a body  12  which forms a ring-shape save for a slot  14 . In the preferred embodiment, ring  10  is circular with a central axis  13 . Body  12  includes several distinct elements and/or sections. More particularly, body  12  includes three peaks  16   a - c  separated by three valleys  18   a - c , two cut outs  20   a - b  in the form of axial grooves ( 20   b  is best shown in  FIG. 2 ), a circumferentially extending interior groove  22 , a lower circumferential wall  24  and a plurality of exterior grooves separated into three distinct exterior grooved areas  26   a - c . The ring shape of body  12  also forms a central opening  28  that is capable of receiving the head of a screw, a rod or other fixation mechanism. Furthermore, the cooperation between peaks  16   a - c  and adjacent valleys  18   a - c  forms tapered flanks  30   a - f . The use of each of these elements will be discussed in detail below, as will their cooperation with other components in a bone plate system. 
     As is mentioned above, ring  10  is designed so as to be received within an aperture  52  of a bone plate  50  (see the fully constructed top view of  FIG. 7 ). The cooperation between ring  10  and aperture  52  is such that polyaxial movement of ring  10  is allowed, prior to tightening of the ring with respect to the plate. Initially, ring  10  is capable of rotating (to a certain extent) about several different axis with respect to aperture  52 . Aperture  52  is preferably non-circular, such that a tightening rotation of ring  10  about an axis normal to aperture  52  (i.e., in the direction of central axis  13 ) causes constriction of ring  10 . More particularly, rotation of ring  10  in this fashion causes a reduction in the size of slot  14  and central opening  28 . This constriction thereby locks or retains ring  10  on the fastening element and in position with respect to plate  50  and also retains any screw, rod or other fixation mechanism disposed within central opening  28 . It is to be understood that cut outs  20   a  and  20   b  are provided to weaken the overall structure of body  12  and therefore make the rotation and constriction of ring  10  easier. Because cut outs  20   a  and  20   b  are disposed on the interior of body  12  their inclusion essentially removes material in body  12  which thereby provides deflectable weakened wall areas which lessens the force provided by the structure against constriction. Thus, normal tightening rotation of ring  10  is preferably made much easier. 
     The aforementioned constriction capabilities of ring  10  require that body  12  be constructed of a material suitable for the movement required. In addition, ring  10  is preferably designed so that upon a loosening rotation of ring  10  with respect to plate  50 , the constriction force is reduced or completely removed. Hence, body  12  of ring  10  must also be resilient enough to allow this opposite spring back motion of its material. Suitable materials include, but are not limited to Ti6Al4V ELI, stainless steel, polymers, absorbable polymers, titanium and any other suitable material. Furthermore, it is contemplated that ring  10  may, in certain sections, be coated with a hard surface layer to ensure proper cooperation with plate  50 . For example, in embodiments where plate  50  is also constructed of Ti6Al4V ELI, ring  10  may be coated in certain areas or over its entire surface with TiNbN. This difference in materials preferably creates a difference in hardness level between the two parts, with the coating of ring  10  being harder than the material of plate  10 . Thus, ring  10  or its surface may be coated with any material that is harder than that of plate  50 . Some other materials, may include titanium and the like. This will be discussed more fully below. 
       FIG. 6  depicts the cooperation between ring  10  and a bone screw  32 . Of course, it is to be understood that ring  10  may be utilized in conjunction with other fixation mechanisms, such as threaded or unthreaded rods, pins, or the like. Bone screw  32  as shown includes a head portion  34  and a threaded shank  36 . While threaded shank  36  is typical of a standard shank of a bone screw (either self tapping or not), head portion  34  includes several elements tailored towards cooperation with ring  10 . First, head portion  34  is sized and configured so that upon insertion of shank  36  through central opening  28  a bottom surface  40  of the head portion engages circumferential wall  24  of body  12 . This engagement prevents screw  32  from being completely pushed through central opening  28 . A top surface  38  of head portion  34  preferably includes a depression and/or extension  42  (best shown in  FIG. 7 ) capable of engaging an insertion tool, such as a screw driver. Head portion  34  is also provided with an axial groove  44  which essentially divides head portion  34  into a top section  46  and a bottom section  48 . Upon the above-discussed constriction of ring  10 , bottom section  48  is preferably engaged within interior groove  22  formed in body  12 . This engagement or cooperation preferably creates a stronger connection between ring  10  and screw  32  to ensure that they lock rotationally. 
       FIG. 7  depicts the cooperation among ring  10 , screw  32 , and bone plate  50 . As is discussed above, ring  10  is disposed within a properly sized and shaped aperture  52  formed in plate  50 . This may be done prior to a surgical procedure, such that plate  50  is packaged with ring  10  already disposed therein. During an operation, a surgeon or other medical professional first preferably positions plate  50  with respect to a bone surface. This step may involve manipulating two bones or bone fragments with respect to one another. With plate  50  positioned, screw  32  is then inserted through central opening  28 . During or subsequent this step, screw  32  may be polyaxially moved, as is allowed by the cooperation of ring  10  and aperture  52 . Thus, screw  32  may be positioned about several different axis so as to create the necessary force to hold two bones or bone fragments together, so as to engage a certain portion of the bone surface, and/or for any other reason determined by the surgeon or medical professional. 
     Once properly positioned, screw  32  may be tightened into the bone surface. It is noted that for self tapping screws, a simple tightening of same may suffice. Otherwise, for other types of screws, a drilling step may be required prior to insertion of the screw in the bone. This drilling step is preferably done prior to introduction of screw  32  within central opening  28 , and may or may not involve the use of a drill guide. Upon fully tightening of screw  32  into the bone, bottom surface  40  of head portion  34  engages circumferential wall  24  of body  12  (this position is depicted in  FIG. 6 ). Ring  10  and screw  32  may now be locked with respect to plate  50 . 
       FIGS. 8 and 9  depict the tightening of ring  10  with respect to plate  50 , as well as a ring driver  60  utilized in this tightening step. It is to be understood that this tightening step requires the use of ring driver  60  (or another suitable tool) to rotate ring  10  to create the aforementioned constriction and locking.  FIG. 10  depicts one suitable ring driver  60  for use with the above-described ring  10  and plate  50 . Ring driver  60  preferably includes an elongate portion  62  connected to a driver head  64 . It is noted that elongate portion  62  may comprise a handle, or, as is shown in  FIGS. 8-10 , an end of the portion opposite to head  64  may include an interface  66  for connection to another tool like a manual or motorized handpiece, a drill, or the like. Driver head  64  preferably includes at least one, and preferably three fingers  68   a - c  sized and shaped so as to be capable of being disposed between peaks  16   a - c , within valleys  18   a - c . More particularly, fingers  68   a - c  are preferably sized so that they may easily fit within valleys  18   a - c  respectively, and preferably include surfaces  70   a - f  which are essentially mirror images of tapered flanks  30   a - f . This ensures a snug fit between the components. It is to be understood that there may exist some play between the structure of fingers  68   a - c  and peaks  16   a - c . Thus, rotation of driver  60  in either direction may first involve a slight movement of fingers  68   a - c  with respect to ring  10 . However, the ultimate snug fit between driver  60  and ring  10  preferably allows for a better grip and fluid force transmission during tightening/loosening of ring  10  with respect to plate  50 . 
     During tightening of ring  10  within aperture  52  of plate  50 , the plurality of grooves formed on exterior grooved areas  26   a - c  preferably cut into the inside surface of aperture  52 . This creates a strong, form fit contact. In preferred embodiments, the grooves on areas  26   a - c  are designed so that different friction forces are exhibited on ring  10  upon tightening. For instance, the friction between ring  10  and plate  50  is increased perpendicular to the grooves formed on exterior grooved areas  26   a - c  after tightening. This allows for less of a forced to be required for tightening, than for loosening, thereby preventing the unwanted loosening of ring  10  with respect to plate  50 . Hence, rotation of ring  10  in one direction is easier than in the other. In addition, the grooves on areas  26   a - c  may be tapered off so that full polyaxial movement of ring  10  with respect to plate  50  may be allowed when ring  10  is in the loosened position, but, upon tightening, the grooves properly cut into the inside of aperture  52 . It is to be understood that the above-described difference in material hardness on certain sections of ring  10  with regard to plate  50  preferably applies to areas  26   a - c . In other words, areas  26   a - c  are preferably coated with a material which exhibits harder properties than that of at least the inside surface of aperture  52 . This ensures easy and more solid cutting of ring  10  into plate  50 , which in turn provides a more solid locking between the two components. 
     When ring  10  is fully rotated in the tightening direction, ring  10 , screw  32 , and plate  50  are all preferably locked with respect to one another. Of course, rotation in the opposite, or loosening direction, back to the original position preferably allows polyaxial movement of ring  10  and screw  32  with respect to plate  50 . Thus, if for any reason, the surgeon or medical professional determines that screw  32  has been improperly positioned after tightening, ring  10  may be loosened and the screw may be repositioned. The step may also be performed through the use of driver  60 . 
     The above described improved polyaxial locking mechanism or ring  10  essentially strengthens the locking among ring  10 , screw  32 , and plate  50 . Cut outs  20   a  and  20   b , and tapered flanks  30   a - f  (and their cooperation with fingers  68   a - c  of driver  60 ) preferably make for an easier constriction of the ring, and hence easier tightening of ring  10  with respect to plate  50 . The cooperation between interior groove  22  of ring  10  and bottom section  48  of screw  32  preferably ensures that the screw remains within the ring, even upon the application of push-out forces and bending forces caused by rotational movement of screw  32  with respect to plate  50 . These forces may be created upon the shifting of plate  50  or the bone surfaces being treated. Additionally, the fact that the grooves of areas  26   a - c  cut into the interior of aperture  52  creates a solid interface between ring  10  and plate  50  in the tightened position. All of this creates a stronger polyaxial locking between any bone plate employing ring  10  and suitable fixation mechanism, such as a screw, rod or the like. 
     It is to be understood that the polyaxial locking mechanism of the present invention may take on many different forms. For example, although shown as being a circular ring, it is contemplated to provide a differently shaped structure, such as an oblong structure or a shape including one or more straight sides. In addition, it is to be understood that the present invention may be applied to any known bone plate, with such plate including an aperture or the like suitable for reception of the polyaxial locking mechanism therein. Finally, it is to be understood that in accordance with the present invention, the polyaxial locking mechanism may be designed so as to be retained within an aperture of a bone even without locking between the two. For example, in the above-described embodiment of the present invention, aperture  52  of plate  50  is preferably designed so as to include a curvature which tends to accept and retain ring  10  therein. In such a case, polyaxial movement is still allowed when ring  10  is in the initial position, and prevented upon tightening. 
     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.