Abstract:
An orthopedic fastener is disclosed, with a cylindrical shank having pre-selected diameter, the shank also providing adjacent unthreaded and threaded portions. The threaded portion is configured to secure the orthopedic fastener into cortical bone. The shank further has a thread root transition zone providing a tapered thread root profile between the unthreaded and threaded portions. The thread root transition zone is positioned on the shank such that, when the orthopedic fastener is operably secured into a specimen of cortical bone, the thread root transition zone is located outside said specimen of cortical bone.

Description:
RELATED APPLICATIONS 
       [0001]    None. 
       FIELD OF THE INVENTION 
       [0002]    This invention relates generally to orthopedic fasteners configured to secure into cortical bone, and more particularly, in a preferred embodiment, to an orthopedic fastener with a tapered thread root transition between threaded and unthreaded portions that is operably located outside the bone. 
       BACKGROUND OF THE INVENTION 
       [0003]    Designs of threaded orthopedic fasteners have historically struggled with a transition from unthreaded to threaded portions. The manufacturing process of cutting a thread on a shank typically leaves an abrupt discontinuity between threaded and unthreaded portions, where the thread root diameter is less than the unthreaded shank diameter. Problems occur when such orthopedic fasteners are secured into bone. When repetitive loads are exerted on the secured fastener (such as are likely to be experienced in exemplary orthopedic procedures such as bone distraction or fracture repair), such fasteners become inherently susceptible to acute and/or fatigue failure at the abrupt threaded/unthreaded transition, since repetitive loads become more focused at the threaded/unthreaded discontinuity when the fastener is held rigidly. 
         [0004]    This tendency towards failure is especially pronounced when such orthopedic fasteners are secured into cortical bone. Cortical bone, typically found on the outside of a bone, is harder than cancellous bone, typically found on the inside of a bone. This means that threads on orthopedic fasteners can typically be fastened tighter into cortical bone than cancellous bone. This in turn means that when the fastener is secured into cortical bone, it can be held more rigidly than when secured into cancellous bone, causing repetitive loads on the fastener to become yet more focused at the threaded/unthreaded discontinuity. 
         [0005]    The orthopedic fasteners of the present invention are designed and claimed for use in cortical bone. That is not to say that they may not have serviceable applications in certain types of cancellous bone. However, the focus is on cortical bone. 
         [0006]    The references in the previous paragraphs to cortical and cancellous bone deserve further discussion. Orthopedic fasteners are typically designed to fasten either cortical bone or cancellous bone. As noted above, cortical bone, typically found on the outside of a bone, is tougher and harder than cancellous bone, which is typically found on the inside of a bone. Cancellous bone has soft and malleable characteristics, making it useful in, for example, bone grafts. In comparison, cortical bone is considerably harder. When over-stressed, cortical bone will typically crack (at the higher stresses), whereas cancellous bone will typically deform and “pack” (at the lower stresses). As a result, these and other differences in the material properties of cortical and cancellous bone compel different thread designs for orthopedic fasteners. For, example, the thread profile provided on some types of cancellous bone fasteners is a thinner and sharper thread, designed to hold an unimpaired thickness of cancellous bone between the threads, thereby reinforcing the security of the fastener as placed in the cancellous bone. In this way, cancellous bone fasteners of this design will secure into the cancellous bone without “packing” or otherwise deforming the malleable cancellous bone between threads. In contrast, thread profiles provided on other types of cancellous bone fasteners are configured to do the opposite. Such designs are configured to intentionally “pack” the surrounding cancellous bone in order to afford better friction grip on the threads. 
         [0007]    Published U.S. Patent Application No. 2003/0158555 (Sanders et al.) discloses conventional orthopedic fastener for use in either cortical or cancellous bone, depending on the type of accompanying threads selected, such as has been described in the previous few paragraphs. More specifically, FIG. 1 a  of Sanders et al. discloses an abrupt discontinuity between unthreaded and threaded portions. As noted above, the abrupt discontinuity is likely to make the fastener increasingly susceptible to repetitive load failure at the discontinuity once the fastener is secured into bone. 
         [0008]    Some prior art designs have attempted to overcome the problems with abrupt discontinuities by providing threads whose root diameter is the same as the shank diameter. While serviceable, such designs are not preferable because of the more complicated fastener manufacturing required to provide threads whose root diameter is the same as the shank diameter upon which they are deployed. Some designs of this type are also disadvantageous in that the bending resistance of the shank is diminished by wholesale removal of metal down to a thread root diameter. 
         [0009]    Other prior art designs have attempted to overcome the problems with abrupt discontinuities by providing transitional thread roots between unthreaded to threaded portions to smooth out the transition. Typically, such prior art thread root transitions are conical in shape. For example, U.S. Pat. No. 5,242,447 (Borzone) discloses, in FIG. 2, a threaded pin with a tapered thread root over the entire length of the threaded portion. Such an arrangement is considered disadvantageous, in that a specimen of cortical bone receiving the pin must inevitably also receive at least a portion of tapered thread root transition. In such an arrangement, the tapered thread root transition, especially near the unthreaded portion of the pin, is likely to cause unwanted radial stress on the zone of cortical bone immediately surrounding the point of entry of the pin, resulting in almost inevitable damage to the bone during insertion of the pin. Such unwanted radial stress is also likely to lead to cracking of the bone over time, especially in the presence of operational repetitive loading of the pin. Typically this repetitive-load cracking condition begins from radial stress micro-cracks caused by insertion of the pin. The micro-cracks enlarge and degenerate into substantial cracks as repetitive operational loads are placed upon the pin. 
         [0010]    U.S. Pat. No. 6,949,100 (Venturini) discloses, in FIGS. 1 and 2, a bone screw with a tapered thread root over the entire length of the threaded portion. Such an arrangement is disadvantageous for the same reasons as discussed immediately above with respect to U.S. Pat. No. 5,242,447 (Borzone). Indeed, one disclosed embodiment in Venturini teaches a conical taper of approximately 2 mm in reduced thread root diameter over the length of the taper. Those of ordinary skill in this art will understand the radial stress exerted on cortical bone by a thread root diameter change of 2 mm to be inoperable on cortical bone, almost inevitably causing cracking of the bone at the point of entry while attempting to secure the pin. Such a thread root diameter change, coupled with the thread shape and profile disclosed by Venturini, are clearly understood by those of ordinary skill in this art to be indicative of a bone screw better suited for fixation in cancellous bone, not cortical bone. 
         [0011]    U.S. Patent Application No. 2007/0053765 (Warnick et al.) discloses numerous styles of threads for bone screws, at least one of which (for example, in FIG. 3) provides a tapered thread root transition zone. However, those of ordinary skill in the art will understand that the tapered thread root transition zone disclosed by Warnick et al. is intended to be inserted at least partially into the bone, noting that removal of the tapered root transition from inside the bone is likely to cause re-tightening issues. Additionally, as noted above with respect to Borzone and Venturini, the considerable radial stress exerted on cortical bone at the point of entry of a tapered root transition zone into the bone renders such tapered thread root transitions highly disadvantageous when located inside cortical bone. 
         [0012]    U.S. Pat. No. 7,198,488 (Lang et al.) discloses a dental implant suitable for receiving crowns and other false teeth, at least one embodiment of which provides a tapered thread root transition zone. However, those of ordinary skill in this art will understand that the tapered root transition zone disclosed by Lang et al. is intended to be inserted entirely into the bone. 
         [0013]    U.S. Pat. No. 5,665,087 (Huebner) discloses, in FIG. 1, a bone screw with a tapered thread root. However, those of ordinary skill in this art will understand that the tapered thread root disclosed by Huebner is intended to be inserted entirely into the bone. Moreover, it will be seen from the disclosure of Huebner that the bone screw is designed for use in cancellous bone. 
         [0014]    Therefore, there exists a need for an improved threaded orthopedic fastener whose discontinuity between threaded and unthreaded portions has increased fatigue life performance (i.e., is resistant to repetitive loads) when secured into cortical bone. Other pitfalls encountered by the prior art also should be avoided, such as deploying a tapered thread root transition inside the cortical bone. 
       SUMMARY OF THE INVENTION 
       [0015]    The present invention addresses one or more of the above-described drawbacks of the prior art. One aspect of this invention includes an orthopedic fastener with a cylindrical shank having pre-selected diameter, the shank also providing adjacent unthreaded and threaded portions. In this aspect of the invention, the threaded portion is configured to secure the orthopedic fastener into cortical bone. The shank further has a thread root transition zone providing a tapered thread root profile between the unthreaded and threaded portions. The tapered thread root profile transitions from zero to full thread cut over a predetermined number of thread turns. The thread root transition zone is positioned on the shank such that, when the orthopedic fastener is operably secured into a specimen of cortical bone, the thread root transition zone is located outside said specimen of cortical bone. 
         [0016]    It will be understood that in locating the thread root transition zone outside the specimen of cortical bone, the invention is expressly not limited to a location completely outside the bone. For example, without limitation, the thread root transition zone could be located in a zone of cancellous bone or fatty bone marrow (such as is frequently found between two thicknesses of cortical bone) while the fastener is secured into cortical bone. 
         [0017]    It is therefore a technical advantage of the invention to provide a tapered thread root transition between unthreaded and threaded portions on an orthopedic fastener, such that when the tapered root transition is located outside cortical bone into which the orthopedic fastener is operably secured, the discontinuation between the unthreaded and threaded portions will be more resistant to failure from repetitive loading. 
         [0018]    A further technical advantage of the invention is that the tapered thread root transition is relatively straightforward to manufacture. 
         [0019]    A further technical advantage of the invention is that since the tapered thread root transition is advantageously located outside of cortical bone into which the orthopedic fastener may be secured, the taper on the thread root transition does not add any radial stress to the cortical bone immediately around the fastener. 
         [0020]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should be also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
           [0022]      FIGS. 1 and 1A  depict a prior art fastener  100  having an abrupt discontinuity  115  between adjacent threaded and unthreaded portions; 
           [0023]      FIGS. 2 and 2A  depict a prior art fastener  200  in which an abrupt discontinuity between adjacent threaded and unthreaded portions is avoided by providing thread roots  235  with a diameter substantially the same as the diameter of shank  225 ; 
           [0024]      FIGS. 3 and 3A  depict a fastener  300  generally in accordance with the present invention, including a thread root transition zone  315 ; 
           [0025]      FIG. 4A  is a cross-section view of a bone specimen  405 , depicting an exemplary deployment of the fastener  300  of  FIGS. 3 and 3A  operably secured in cortical bone  405 , where thread root transition zone  315  is located completely outside the bone; 
           [0026]      FIG. 4B  a cross-section view of bone specimen  415 , depicting another exemplary deployment of the fastener  300  of  FIGS. 3 and 3A  operably secured in cortical bone  420 , where thread root transition zone  315  is located inside the bone, in interior region  425 ; and 
           [0027]      FIG. 4C  is a cross-section view of bone specimen  435 , depicting another exemplary deployment of the fastener  300  of  FIGS. 3 and 3A  operably secured in cortical bone  440 , where thread root transition zone  315  is located outside the bone (as also illustrated in  FIG. 4A ), but where threaded portion  310  engages both regions of cortical bone  440  either side of interior region  445 . 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    As has been described above more generally with respect to the prior art,  FIGS. 1 and 1A  depict a prior art fastener  100  with unthreaded portion  105  immediately adjacent to threaded portion  110 .  FIG. 1A  is an enlarged detail view as shown on  FIG. 1 . As is typical in the prior art, an abrupt discontinuity  115  occurs at the transition from unthreaded portion  105  to threaded portion  110 . It will be appreciated that in the typical manufacture of prior art fastener  100 , threads  120  are cut on to shank  125  such that thread crests  130  are substantially the same diameter as shank  125 , while thread roots  135  have a smaller diameter than the diameter of shank  125 . An abrupt change in fastener core diameter thus occurs at discontinuity  115 , enhancing the susceptibility of fastener  100  to failure at discontinuity  115  when fastener  100  is subjected to operational loads. 
         [0029]    A further example of the prior art is depicted in  FIGS. 2 and 2A .  FIG. 2A  is an enlarged detail view as shown on  FIG. 2 . Prior art fastener  200  again provides unthreaded portion  205  immediately adjacent to threaded portion  210 . However, in  FIGS. 2 and 2A , and in comparison to  FIGS. 1 and 1A , threads  220  are provided on shank  225  such that thread roots  235  are substantially the same diameter as shank  225 , while thread crests  230  have a larger diameter than the diameter of shank  225 . As a result, prior art fastener  200  does not have an abrupt change in fastener core diameter as may be seen on  FIG. 1A . However, as described above in more detail in the “Background” section of this disclosure, prior art fasteners of the style depicted in  FIGS. 2 and 2A  have proven to be disadvantageous, in that wholesale removal of metal on shank  225  down to the diameter of thread roots  235  significantly reduces the operational bending resistance of shank  225 , and further significantly increases overall manufacturing costs. 
         [0030]      FIGS. 3 and 3A  illustrate an exemplary embodiment generally in accordance with the present invention.  FIG. 3A  is an enlarged detail view as shown on  FIG. 3 . Fastener  300  provides unthreaded portion  305  adjacent to threaded portion  310 . In the illustrated exemplary embodiment, thread crests  330  on threaded portion  310  have a diameter substantially the same as the diameter as shank  325  on unthreaded portion  305 . In this way, the disadvantages of prior art fasteners such as described above with reference to  FIGS. 2 and 2A  are avoided. However, although the exemplary embodiment of  FIGS. 3 and 3A  illustrates thread crests  330  having a diameter substantially the same as the diameter on shank  325 , it will be appreciated that the invention is not limited in this regard. Other embodiments of the invention (not illustrated) may, for example, provide thread crests  330  with a varying diameter along threaded portion  310 , or with a uniform thread crest diameter that is different from the diameter of shank  325 . 
         [0031]    With further reference to  FIG. 3A , however, and in distinction to prior art fasteners such as described above with respect to  FIG. 1A , a thread root transition zone  315  is provided on fastener  300 . In thread root transition zone  315 , thread roots  325  are deployed to decrease in diameter in tapered fashion from the diameter of shank  325  to a constant diameter that is less than the diameter of shank  325 . In this way, the disadvantages of an abrupt discontinuity between unthreaded and threaded portions are avoided (such as discontinuity  115  as described above with reference to  FIGS. 1 and 1A ). 
         [0032]    Embodiments of the invention have been illustrated and described herein using fixation pins as exemplary orthopedic fasteners. It will be appreciated, however, that the invention is not limited in this regard, and that it may be embodied on any orthopedic fastener, such as, without limitation, pedicle screws, orthopedic nails, bone screws, locking screws, hip screws, or even dental implants. 
         [0033]    It will be further appreciated, with further reference to  FIG. 3A , that although the exemplary embodiment of thread root transition zone  315  has been illustrated as a conically tapered root, the invention is not limited in this regard. It will be appreciated that the scope of the invention includes other root tapers, such as, for example, a curved taper. 
         [0034]    In a preferred embodiment, the thread root transition zone may provides any number in a range from about two (2) to about five (5) thread turns for the tapered thread root profile to transition from zero threaded profile to full threaded profile. It will be appreciated, however, that the invention is not limited in this regard, and that a thread root transition zone providing any number of thread turns to accomplish the transition is within the scope of the invention. 
         [0035]    Those of ordinary skill in this art will further appreciate that the invention is not limited to any particular size of orthopedic fastener on which the thread root transition zone may be deployed. 
         [0036]    Turning now to  FIGS. 4A ,  4 B and  4 C, exemplary deployments of fastener  300  (as illustrated on  FIGS. 3 and 3A ) are depicted. In  FIGS. 4A ,  4 B and  4 C, fasteners  300  are shown fully secured into specimens of cortical bone. Bone specimen  405  on  FIG. 4A , bone specimen  415  on  FIG. 4B , and bone specimen  435  on  FIG. 4C  are shown in cross-section. It will be appreciated that fasteners of the present invention are described and claimed wherein thread root transition zone  315  (as illustrated on  FIG. 3A ) is located outside of cortical bone when the fastener is operably secured into a specimen of cortical bone. In  FIG. 4A , it will be seen that fastener  300  is operably secured into cortical bone  410 , and that thread root transition zone  315  is located completely outside of bone specimen  405 . By contrast, however, in  FIG. 4B , it will be seen that although thread root transition zone  315  is located outside of cortical bone  420 , it is still located within bone specimen  415  in which fastener  300  is operably secured. In the exemplary deployment illustrated in  FIG. 4B , thread root transition zone  315  is located in interior region  425  of bone specimen  415  It will be appreciated that, depending on the specific bone from which exemplary bone specimen  415  on  FIG. 4B  may be taken, interior region  425  may comprise cancellous bone and/or fatty bone marrow.  FIG. 4C  is similar to  FIG. 4A  in that  FIG. 4C  shows thread root transition zone  315  is located completely outside of bone specimen  435 . However,  FIG. 4C  shows threaded portion  310  on fastener  300  to be secured into both regions of cortical bone  440  located either side of interior region  445 . 
         [0037]    It will be further appreciated that orthopedic fasteners according to the present invention are not limited to any particular type of orthopedic application or discipline. For example, without limitation, it is envisaged that orthopedic fasteners according to the present invention will have applications in both human and veterinary orthopedics. 
         [0038]    Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alternations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.