Abstract:
A memory metal bone staple having a metal grain which runs longitudinally along the legs and bridge of the staple. The staple has barbed retaining features and no sharp or abrupt corners on its outer edges.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Application No. 61/784,463 filed on Mar. 14, 2013, the disclosure of which is incorporated herein by reference for all purposes. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to orthopedic staples used for bone fixation. In particular, the present invention relates to orthopedic staples comprised of nickel/titanium alloy. 
     BACKGROUND OF THE INVENTION 
     The use of orthopedic staples, or bone staples, is a common method of bone fixation in orthopedic surgery. A particular type of bone staple is known as the memory staple. Memory staples are comprised of a nickel/titanium alloy, also known as Nitinol. Nickel/titanium alloy is unique in that it undergoes a phase transformation in its crystal structure, changing between the stronger austenite form to the weaker martensite form, when exposed to certain environmental stimuli, e.g., heat or stress. Memory metal made from nickel/titanium alloy is categorized as either shape memory or super elastic. Shape memory metal responds to changes in temperature whereas super elastic metal responds to stress or force applied to the metal. When a shape memory alloy is at cooler temperatures, it is in its martensitic form. The martensitic form is easily deformed to a new shape. However, when the alloy is heated through its transformation temperatures, it reverts to austenite and recovers its previous shape with great force. 
     Bone staples are generally U-shaped, having a bridge and first and second legs extending from respective sides of the bridge. Bone staples formed from shape memory nickel/titanium alloy are formed at high temperatures with the legs of the staple angled inwardly. The staples are then cooled and stored with the staple legs maintained in an open position, roughly perpendicular to the staple bridge. When the shape memory staples are inserted into the bone of the patient, the bridge spanning the bone fracture, the body temperature of the patient warms the alloy causing it to return to its austensitic form and thus its original shape, with the legs angled inwardly. The staple legs thus exert a compressive force to urge the portions of the bone on opposite sides of the fracture toward each other, which ensures better bone fusion and retards backing out of the staple. 
     Memory metal can also be super elastic. This unique alloy shows super elastic behavior if deformed at a temperature which is slightly above its transformation temperature. Super elastic bone staples are produced with the staple legs angled inwardly. During insertion of the staple, the staple legs are opened using a staple spreader, and the staple is then inserted into the patient&#39;s bone. As soon as the force of the staple spreader is removed from the staple legs, the super elastic metal returns the staple to its original shape with the staple legs angled inward. Again, this results in compressive force on the bone by the staple legs. 
     Bone stapes today are produced both in shape memory and super elastic forms. In particular, when manufacturing prior art super elastic staples, an ingot of nickel/titanium alloy is first rolled into a flat sheet. The rolling of the metal into a flat sheet results in the formation of grain lines in the metal, much like the grain lines found in wood. The staples cut from the sheets thus have grain lines as well. The shape of the staples cut from the sheet is such that the grain lines of the metal will run generally transverse to the longitudinal axes of the staple&#39;s bridge, legs, or both. Grain lines which run transverse or perpendicular to the staple at the intersections between the longitudinal axes of the bridge and/or legs magnify stresses at these intersections in the super elastic staples which can lead to failures or at least reduced ability of the staple to be manipulated freely without fear of failure. 
     SUMMARY OF THE INVENTION 
     In one aspect, the present invention provides a super elastic staple having a metal grain which runs along the longitudinal axes of both the legs and bridge of the staple. 
     In another aspect, the present invention provides a super elastic staple having a metal grain which runs along the longitudinal axes of both the legs and bridge and has barbed retaining features. 
     In still another aspect, the present invention provides a super elastic staple having no sharp or abrupt corners or edges on its outer surfaces. 
     These and further features and advantages of the present invention will become apparent from the following detailed description, wherein reference is made to the figures in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an isometric view of one embodiment of the staple of the present invention. 
         FIG. 2  is a front, elevational view of the staple shown in  FIG. 1 . 
         FIG. 3  is an environmental view of the staple of the present invention being opened prior to insertion in a fractured bone. 
         FIG. 4  is a view similar to  FIG. 3  but showing the staple inserted in a bone. 
         FIG. 5  is a cross-sectional view taken along the lines  5 - 5  of  FIG. 1 . 
         FIG. 6  is a view similar to that shown in  FIG. 5 , showing an alternate cross-section of the staple of the present invention. 
         FIG. 7  is a view similar to that shown in  FIG. 5 , showing an alternate cross-section of the staple of the present invention. 
         FIG. 8  is a view similar to that shown in  FIG. 5 , showing an alternate cross-section of the staple of the present invention 
         FIG. 9  is a side, elevational view of a staple blank used for forming the staple shown in  FIG. 1 , and showing how the grain lines of the metal run along the length of the blank. 
         FIG. 10  is a view, similar to  FIG. 2 , but showing schematically the grain lines of the metal running longitudinally along the bridge and legs of the staple. 
         FIG. 11  is a view similar to  FIG. 10 , but showing how the grain lines of a prior art staple run. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     As explained above memory metal comprises shape memory metal and super elastic memory metal. Although described with respect to super elastic metal, it will be understood that the staple of the present invention can also be formed from shape memory metal. 
     Turning to  FIGS. 1 and 2 , there is shown a staple, shown generally as  10 , having a bridge  20 , and legs  30 . Bridge  20  comprises inner surface  22  and outer surface  24 . It will be understood, for reasons explained more fully hereafter, that the distance between surfaces  22  and  24  constitutes the maximum thickness of the staple  10 . Legs  30  comprise inner surface  32 , outer surface  34 , and terminate in feet  40 , having laterally inwardly directed barbs  41 . At least a portion of each inner surface  32  is tapered to form tapered surfaces  35  In the relaxed position, legs  30  of staple  10  are angled generally inwardly, as shown in  FIGS. 1 and 2 . In a preferred embodiment, bridge  20  is slightly arcuate (see  FIG. 2 ). 
     As seen in  FIG. 3 , during insertion into a bone B having a fracture F, the surgeon uses a spreader S to open legs  30  of staple  10  to an angle substantially perpendicular to bridge  20 . The opening of legs  30  of staple  10 , flattens arcuate bridge  20 . Thus, the normally arcuate shape of bridge  20  is preferred. If bridge  20  were simply flat, pulling legs  30  open could cause bridge  20  to bow inwardly and press against bone B. As seen in  FIG. 4 , after insertion into bone B such that staple  10  bridges fracture F, staple  10  is released from the spreader. The legs  30  then move back toward their original, relaxed position. The force of the legs  30  returning to their inwardly angled position exerts a compressive force on both sides of fracture F. As well, the barbs  41  of feet  40  are pressed into bone B and thus provide resistance to the staple backing out. 
     Turning to  FIG. 5 , a transverse cross-section of one embodiment of the staple bridge can be seen. Because staple  10  is formed from extruded wire, it can be easily formed with virtually any desired cross-sectional configuration. Thus the outer surface edges can be rounded without the necessity for further working of the staple. In contrast, prior art staples which are typically cut from sheets have straight edges, forming 90° angles when viewed in cross-section. This can be a problem as patients will often rub the staples through the skin during the healing process. Staples having sharp edges on the outer surfaces cause irritation to the patient&#39;s skin as a result of this rubbing. Accordingly, many manufacturers of prior art stamped or laser cut staples will round the edges of the staples by placing the staples in a tumbling media. This tumbling step abrades and polishes the edges of the staples to provide more rounded edges. Unfortunately, the process impacts the entire surface area of the staple, thus rounding off any barbs and inner surfaces which perform better when sharp. In contrast, in the present invention the shape of the outer and inner surfaces of staple  10  can be customized simply by changing the shape of the die through which the wire is extruded. 
       FIGS. 6-8  are similar to  FIG. 5  but show alternative cross-sectional shapes of staple  10 . As seen in  FIGS. 5-8 , the transverse cross-sectional shape can vary widely.  FIG. 6  shows a transverse cross-section which is generally rectangular with rounded corners.  FIG. 7  shows a more rounded transverse cross-section and  FIG. 8  shows a transverse cross-section which is substantially circular but has a circular segment removed forming a flat inner surface  22 . Each of these possible configurations have a flat inner surface, which grips the bone well, and a radiused, contoured outer surface which mitigates the irritation to the skin of the patient. It will be understood that other transverse cross-sectional shapes are contemplated by the present invention provided they have no abrupt surface changes, e.g., sharp corners or edges, on the outer surfaces. 
     Another advantage of the staple of the present invention resides in the fact that the surgeon can employ smaller diameter holes in the bone to implant the staple. In this regard, when repairing a bone fracture using staples of the type of the present invention, the surgeon will drill two holes on opposite sides of the fracture (see  FIG. 3  for example). Desirably the holes will have as small a diameter as possible. In the case of prior art staples cut from sheets, the barbs, protrusions or the like extend laterally from the surfaces of the legs. Such barbs or protrusions may extend from the inner surface of the legs, the outer surface of the legs, or both. The result of such barbs, protrusions or the like extending laterally from the surface of the leg is that, at that location of the barb, the width of the leg, as measured from the laterally outermost point to the laterally innermost point, is substantially wider than the width of the leg without the barb. This necessitates that a larger hole be drilled in the bone by the surgeon. 
     This is to be contrasted with the staple of the present invention, wherein the thickness of the staple leg measured from the outermost surface, to the innermost tip of the barb, is not greater than the thickness of the wire from which the staple is formed. This is accomplished by grinding or otherwise removing material from the blank segment of the wire after it is cut to the desired length such that it provides a staple blank  10 A, as shown in  FIG. 9 . As can be seen, when the staple blank  10 A is formed into the staple  10 , the legs  30  of the staples have tapered portions  35  occasioned by removal of the metal as discussed above. Thus over and above reducing the footprint of the staple at its point of insertion into the holes drilled into the bone, the taper facilitates insertion of the staple into the holes in the bone. It will be understood that blank  10 A can be worked to form a plurality of barbs such that staple  10  would have a plurality of barbs on each leg  30  or a single barb on one leg and a plurality of barbs on the other leg  30 . 
     Turning to  FIG. 10 , the grain lines G of the metal of staple  10  run along the length of the staple  10  from one foot  40  to the other foot  40  by virtue of the fact that staple  10  is formed from an extruded nickel/titanium alloy wire. In forming the staples  10 , the extruded nickel/titanium alloy wire is cut into appropriate sized segments or blanks. In this regard, bone staples generally range from 8 mm to 25 mm in bridge length. The side of the wire blank which is to be the inner surface, i.e., the side which is not smooth or rounded, is then worked in a suitable manner to remove material from two portions of the blank to form tapered portions  35  (see  FIG. 1 ), and feet  40  having barbs  41 . The wire is then formed into the desired staple shape. Finally, to ensure the desired performance of the super elastic metal, the staples are thermally set at temperatures ranging from 400-600° C. The result is a super elastic staple having grain lines that run along the pathway of staple  10  from one foot  40  to the other foot  40 . This pathway is indicated generally by lines G in  FIG. 10 . 
       FIG. 11  shows schematically how the grain lines of the metal run in a prior art staple  110 , which has been laser cut from a sheet of nickel/titanium alloy. It can be seen that grain lines G 1  running through staple  110  are transverse to the pathway G of  FIG. 10 , particularly at the intersections of the bridge  112  and the legs  114 . Generally speaking, grain lines which are transverse to this intersection tend to set up stress risers which can compromise the integrity of the staple  110 . In general, when cutting staples from a sheet, one obtains staples wherein the grain lines of the metal will be transverse to some part of the staple, including the intersections of the bridge and the legs. It will be appreciated that the prior art staples can be cut from the sheet such that the grain lines run in virtually any direction. However, no matter what the orientation of the staple cut from the sheet, at some point, the grain lines will be transverse to the pathway of the staple and more particularly to the intersection of the bridge and the leg. This is to be contrasted with the staples of the present invention and shown in  FIG. 10  wherein the grain lines G are all running in such a fashion that they follow the pathway of the staple as opposed to being transverse to the pathway at any point. 
     Although specific embodiments of the invention have been described herein in some detail, this has been done solely for the purposes of explaining the various aspects of the invention, and is not intended to limit the scope of the invention as defined in the claims which follow. Those skilled in the art will understand that the embodiment shown and described is exemplary, and various other substitutions, alterations and modifications, including but not limited to those design alternatives specifically discussed herein, may be made in the practice of the invention without departing from its scope.