Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. patent application Ser. No. 12/535,816 titled “Angulated Locking Screw/Plate Interface” and filed Aug. 5, 2009, which depends on provisional patent application No. 61/106,511 titled “Angulated Locking Screw/Plate Interface” and filed Oct. 17, 2008, which are both incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present disclosure relates generally to repairing bone fractures, and more particularly, to an angulated locking plate/screw interface. 
       BACKGROUND OF THE INVENTION 
       [0003]    When repairing a broken, fractured, or shattered bone, a physician may often be faced with the task of affixing a fixation plate to the bone in order to align the bone, and possibly, to hold bone fragments together. In order to affix the fixation plate to the bone, a surgeon may insert a locking bone screw through one of a plurality of threaded screw holes in the fixation plate and into a predrilled hole in the bone. Alternatively, self-drilling screws may be used. Since numerous threaded screw holes may be spread out across the entirety of the fixation plate, the surgeon may affix virtually any portion of the fixation plate to the bone by inserting a suitable number of locking bone screws through the plate and into the bone. 
         [0004]    The trend in fixation for many medical practice areas such as the small bone orthopedic market and the craniofacial market is to use locking screws and plates that prevent the locking bone screws from backing out of the fixation plate once inserted. To achieve this lockable engagement, the inner surface of each threaded screw hole may be threaded to engage a corresponding set of locking threads on the head of each locking bone screw. Consequently, as a locking bone screw is screwed into one of the threaded screw holes in the fixation plate, the locking threads in the screw hole and the locking threads on the head of the locking bone screw may deform against each other to lock the locking bone screw into the fixation plate. 
         [0005]    In certain cases, proper placement and positioning of the fixation plate may call for inserting a locking screw into a threaded screw hole at an angle other than perpendicular to the central axis of the threaded screw hole. For example, if the underlying bone beneath a particular screw hole is weak due to its proximity to a fracture line, the surgeon may wish to angle the bone screw away from the fracture line so as to anchor the screw into a more solid bony mass. Consequently, the ability to lockably engage a bone screw into a fixation plate at an angle off of perpendicular from the plate maybe a desirable feature for a surgeon repairing a broken, fractured, or shattered bone. 
       SUMMARY OF THE INVENTION 
       [0006]    The present disclosure provides for a system and method for lockably engaging bone screws into a fixation plate. In particular embodiments, the system may include a locking fixation plate including a threaded screw hole defined by an inner surface surrounding the threaded screw hole. The inner surface may include an upper countersink and a threaded portion, and the threaded portion may include a pair of threads arranged in a double helix configuration. In particular embodiments, the system may further include a screw comprising a generally conical head tapering into a generally cylindrical shaft that ends at a tip and double helix threads beginning near the tip and extending along the generally cylindrical shaft and onto the generally conical head. In particular embodiments, a thread height of each of the double helix threads may be constant over a majority of the generally cylindrical shaft and taper as the threads extend onto the head. 
         [0007]    In particular embodiments, the portion of the said threads disposed on the generally conical head may be configured to interfere with the pair of threads disposed on the inner surface of the screw hole to lock the screw into the screw hole once the screw is screwed into the screw hole. 
         [0008]    In particular embodiments, each thread of the double helix threads of the screw may include a thread root, and the thread height of a portion of each thread disposed on the head may be shallow enough to enable the portion of the root disposed on the head to contact the pair of threads disposed on the inner surface of the screw hole when the screw is screwed into the screw hole. Depending upon design, a pitch of each thread on the head of the screw may be different than the pitch of the same thread on the body of the screw. 
         [0009]    Depending upon design, the inner surface of the plate may further include a lower countersink, and the threaded portion may be disposed between the upper countersink and the lower countersink. 
         [0010]    In particular embodiments, the threaded screw hole may be surrounded by a rim. Furthermore, the upper countersink may include a non-locking portion having a first countersink angle and a locking portion having a second countersink angle, and the non-locking portion may be disposed between the rim and the locking portion. 
         [0011]    In particular embodiments, each thread of the pair of threads on the plate may have an included angle, and the second countersink angle may be equal to the included angle of each thread of the pair of threads. 
         [0012]    Depending upon design, the screw may include a first material and the plate include a second material and the first material may be harder than the second material. 
         [0013]    In particular embodiments, a method of using a screw and a locking fixation plate may include inserting a screw into a bone through a locking fixation plate. The locking fixation plate includes a threaded screw hole defined by an inner surface surrounding the threaded screw hole and the inner surface including an upper countersink and a threaded portion. Furthermore, the threaded portion may include a pair of threads arranged in a double helix configuration. The screw may include a generally conical head tapering into a generally cylindrical shaft that ends at a tip and a pair of threads beginning near the tip and extending along the generally cylindrical shaft and onto the generally conical head. Furthermore, a thread height on the screw may be constant over a majority of the generally cylindrical shaft and taper as the thread extends onto the head. 
         [0014]    In particular embodiments, the method may further include rotating the screw in the screw hole such that the portion of the threads disposed on the generally conical head of the screw interfere with the helical pair of threads disposed on the inner surface of the screw hole to lock the screw into the screw hole. 
         [0015]    In particular embodiments, the method may further include locking the screw into the screw hole at an angle other than parallel to a central axis of the screw hole. 
         [0016]    Technical advantages of particular embodiments of the present disclosure include a double lead thread formed on the inside of the screw holes in the locking fixation plate that, as compared to a single lead thread, may enable a locking screw to engage the plate in half as many turns and engage the plate at an angle other than parallel to the central axis of the screw hole. Furthermore, each screw hole may include upper and lower countersinks that facilitate angled insertion of the bone screw through the plate by preventing the threading inside the screw hole from dictating the angle of insertion, and by providing clearance for the screw to tilt within the screw hole, yet another technical advantage. Also, the upper countersink may include both a locking portion and a non-locking portion. This feature may enable the threaded screw hole to accommodate either a locking screw or a non-locking screw, yet another technical advantage. Another technical advantage of the present invention relates to the double helix screw. Such a design permits the engagement of the screw within the plate and half as many turns. This feature, combined with the double-threaded feature of the fixation plate, accelerates the engagement and minimizes the amount of time the surgeon needs to install the fixation plates with the screws. Other technical advantages of the present disclosure will be readily apparent to one skilled in the art from the following figures, descriptions, and claims. Moreover, while specific advantages have been enumerated above, various embodiments may include all, some, or none of the enumerated advantages. 
     
    
     
       DETAILED DESCRIPTION OF THE INVENTION 
         [0017]    For a more complete understanding of the present disclosure and its advantages, reference is now made to the following descriptions, taken in conjunction with the accompanying drawings, in which: 
           [0018]      FIG. 1  illustrates an example embodiment of a system for attaching bone segments together including a locking plate and a plurality of locking screws according to the present disclosure; 
           [0019]      FIGS. 2A and 2B  illustrate enlarged views of one of the locking screws of  FIG. 1 ; 
           [0020]      FIGS. 3A and 3B  illustrate enlarged views of a locking screw hole that may be included in the locking plate of  FIG. 1  according to the present disclosure; 
           [0021]      FIGS. 4A ,  4 B and  4 C illustrate enlarge views of double helix locking screws; 
           [0022]      FIGS. 4D and 4E  illustrate multihelix locking screws; and 
           [0023]      FIG. 5  illustrates an example embodiment of a system for attaching bone segments together, including a locking plate and a plurality of double helix locking screws according to the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]      FIG. 1  illustrates an example system  100  for attaching together bone segments according to an example embodiment of the present disclosure. In the pictured embodiment, system  100  is being used relative to a single fractured bone  102 . However, particular embodiments of system  100  may be applied equally as well to virtually any bone or group of bones in the body. For example, system  100  may be used to attach bone  102  and another bone, or bone  102  and a synthetic element such as a surgical implant. 
         [0025]    In particular embodiments, system  100  may include one or more locking screws  200  that may be used to secure a fixation plate  300  to bone  102 . For reference purposes, fixation plate  300  and other components of system  100  may be referred to as having a top or upper or side intended to face away from bone  102  and a lower or bottom side intended to face toward bone  102  (e.g., to be placed upon bone  102 ). Though particular features of those components may be explained using such intended placement as a point of reference, this method of explanation is not meant to limit the scope of the present disclosure to any particular configuration of fixation plate  300 , its features, or any other components, or to any particular placement or orientation of fixation plate  300  relative to bone  102 . 
         [0026]    Fixation plate  300  may generally include a body  301  comprising a plurality of threaded screw holes  302  connected to each other in a web-like distribution by a plurality of ribs  304 , although any suitable geometry of plate  301  is contemplated. In particular embodiments, ribs  304  may be thinned down relative to threaded screw holes  302  to facilitate bending of ribs  304  rather than threaded screw holes  302  when fixation plate  300  is contoured, for example to match the contour of bone  102 . 
         [0027]    Depending upon design, one or more ribs  304  may comprise one or more positioning holes  306  that may be used to position fixation plate  300  relative to bone  102 . As an example, to position fixation plate  300  relative to bone  102  using a positioning hole  306 , a surgeon may insert one end of a Kirschner wire (“K-wire”) into bone  102  near the desired location for fixation plate  300 . The surgeon may then insert the free end of the K-wire through one of positioning holes  306  and slide fixation plate  300  down onto bone  102  using the K-wire as a guide. Additionally, the surgeon may rotate fixation plate  300  about the K-wire to achieve a desired orientation of fixation plate  300  relative to bone  102 . Once fixation plate  300  has been properly positioned on bone  102 , the surgeon may secure fixation plate  300  to bone  102  using, for example, one or more locking screws  200 . The surgeon may then remove the K-wire from bone  102 . 
         [0028]    To secure fixation plate  300  to bone  102  using a locking screw  200 , the surgeon may insert locking screw  200  through one of threaded screw holes  302  and into bone  102 . Locking screw  200  may be inserted either parallel to the central axis  303  of screw hole  302 , or at an angle θ relative to central axis  303 . In certain procedures, the surgeon may pre-drill a pilot hole in bone  102  to establish the trajectory for locking screw  200 , or, depending on design, locking screw  200  may be self-drilling in nature, making the pre-drilled hole in bone  102  unnecessary. In either case, once the tip  206  of locking screw  200  is contact with bone  102 , the surgeon may use a screw driver or other suitable instrument to screw locking screw  200  into bone  102  until the head  202  of locking screw  200  comes to bear on the inner surface of screw hole  302 . In particular embodiments, both the underside of head  202  and the inside of screw hole  302  may be threaded to permit locking screw  200  to lockably engage screw hole  302 . In that case, further rotation of locking screw  200  at this point may cause the threaded portion of head  202  to interfere with the threading inside screw hole  302  and lock screw  200  into screw hole  302 . The above-described process may be repeated for any suitable number of locking screw until locking plate  300  is firmly attached to bone  102 . Once bone plate  300  has been secured to bone  102 , the incision above bone  102  may be closed, leaving the patient to heal. 
         [0029]    One of ordinary skill in the art will appreciate that the above-described embodiment and use of system  100  was presented for the sake of explanatory simplicity and will further appreciate that the present disclosure contemplates using any suitable number of locking screws  200  in combination with any suitable configuration of bone plate  300  to repair bone  102 . 
         [0030]      FIGS. 2A and 2B  illustrate a more detailed view of one of the locking screws  200  of  FIG. 1 . In particular,  FIG. 2A  illustrates a side view of locking screw  200  taken perpendicular to the length of shaft  204 , and  FIG. 2B  illustrates a top view of locking screw  200 , looking down at the top side of head  202 . 
         [0031]    As illustrated in  FIG. 2A , locking screw  200  generally includes a body  201  having a generally conical head  202  that tapers into a generally cylindrical shaft  204  ending at a tip  206 . Screw  200  may further include a single generally continuous thread  208  formed around body  201  extending over a majority of locking screw  200  from tip  206  along the length of shaft  204  and onto head  202 . In particular embodiments, screw  200  maybe formed by carving thread  208  out of a blank. This may enable head  202  to be low profile so as to minimize the profile thickness of the plate/screw interface when screw  200  is engaged with fixation plate  300 . 
         [0032]    Thread  208  may generally be defined by a leading flank  210 , a trailing flank  212 , a crest  214  connecting the outer edge of leading flank  210  to the outer edge of trailing flank  212 , and a root  216  connecting the inner edge of leading flank  210  to the inner edge of trailing flank  212 . The dimensions of thread  208  may generally be described by one or more of a thread height  218 , a leading flank thread angle  220 , a trailing flank thread angle  222 , a pitch  224 , a crest width  226 , and a thread diameter  236 . The dimensions of body  201  may generally be described by one or more of a length  228 , a head taper angle  230 , a head diameter  232 , and a shaft diameter  234 . Although screw  200  may be configured to any suitable size or shape, in particular embodiments, length  228  may range, for example, from 18 millimeters to 55 millimeters, thread height  218  may be about 0.030 in. along the length of shaft  204  and may decrease from 0.030 in to about 0.000 in. according to taper angle  230  over the length of head  202 , taper angle  230  may be about 26 degrees, leading thread flank angle  220  may be about 30 degrees, trailing thread flank angle  222  may be about 10 degrees, pitch  224  may be about 0.054 in. along the length of shaft  204  and about 0.048 in. along the length of head  202 , crest width  226  may be about 0.001 in. to 0.004 in. flat (e.g., wide), thread diameter  236  may be about 0.138 in., head taper angle  130  may be about 26 degrees, head diameter  232  may be about 0.176 in., and shaft diameter  234  may be about 0.078 in. 
         [0033]    In particular embodiments, different portions of thread  208  may be configured to perform different functions. For example, the thread height  218  of the portion of thread  208  disposed on shaft  204  may be relatively large to enable thread  208  to bite into bone  102  along the length of shaft  204  while the thread height  218  of the portion of thread  208  disposed on head  202  may be relatively small along the length of head  202  to enable thread  208  and root  216  to interact with the threading inside screw hole  302  to lock screw  200  into plate  300 . Depending upon design of screw  200 , the pitch  224  of the portion of thread  208  disposed on shaft  204  may be the same as or different from the pitch  224  of the portion of thread  208  disposed on shaft  204  in order to enable screw  200  to lockably engage the threading on the inside of screw hole  302 . As an example, pitch  224  may be constant along the entire length of screw  200 . 
         [0034]    As mentioned above, thread height  218  may taper as thread  208  extends onto head  202 . This tapering may ensure that root  216  is brought into contact with the crest of the threading inside screw hole  302  when the threaded portion of head  202  comes to bear on the inner surface of screw hole  302 . In particular, thread height  218  may be shallow enough on head  202  to enable the threading inside screw hole  302  to interact with (e.g., deform against or bite into) root  216  as well as thread  208 . This dual interaction may increase the contact surface area between head  202  and the inner surface of screw hole  302  and provide a stable point of connection between plate  300  and screw  200 , for example, when screw  200  is screwed into screw hole  302  at an angle other than perpendicular to the surface of screw hole  302 . The desired locking effect may be caused by the threading inside screw hole  302  digging into thread  208  and root  216 . If thread height  218  was not tapered on head  202  to enable root  216  to interact with the threading on the inner surface of screw hole  302 , the threading inside screw hole  302  might only dig into thread  208 , providing for a relatively weak point of connection. 
         [0035]    The deformation process described above may be aided by forming screw  200  and plate  300  out of two materials having unequal hardness. For example, screw  200  may be formed of a material that is relatively softer than plate  300  to enable the threading inside screw hole  302  to dig into thread  208  and root  216 . Alternatively, screw  200  may be formed of a material that is relatively harder than plate  300 , in which case, the threading inside screw hole  302  may deform against thread  208  and root  216 . In any case, the desired locking effect may be caused by threadable interface between the threading on head  202  and the threading in screw hole  302 . 
         [0036]    Depending upon design, screw  200  and plate  300  may be formed from any one or more materials suitable for forming medical implants, such as materials that have high strength-to-weight ratios and that are inert to human body fluids. In certain embodiments, screw  200  or plate  300  may be formed from one or more titanium alloys, which provide several benefits. For example, titanium alloys are relatively lightweight, provide adequate strength for withstanding forces typically experienced by a medical implant, are inert to human body fluids, and are visible in radiographs of the implant region. In a particular embodiment, screw  200  may be formed from the titanium based alloy Ti6Al4V ELI (per ASTM F136), and plate  300  may be formed from grade 2 or grade 3 titanium (per ASTM F67). In certain other embodiments, screw  200  or plate  300  may be formed from one or more resorbable polymers, such as polylactides, polyglycolide, glycolide/lactide copolymers or other copolymers, or one or more implantable plastics, such as polyethylene or acetal copolymers for example. 
         [0037]    Since the desired locking effect described above between plate  300  and screw  200  may depend primarily upon the interface between the threading inside screw hole  302  and the threading on head  202 , head  202  may lockably engage screw hole  302  independent of the size of shaft  204 . Thus, shaft  204  may have virtually any configuration (e.g., fully threaded, partially threaded, self-threaded, unthreaded, long, or short) while still maintaining the ability to lockably engage plate  300  by virtue of its connection to head  202 . 
         [0038]    As shown in  FIG. 2B , an engagement  238  is formed in head  202 . In particular embodiments, engagement  238  may be adapted to receive an implantation tool such as a driver that may be used to rotate screw  200  about a longitudinal axis  240  in order to screw locking screw  200  into bone  102  and to lock locking screw  200  into screw hole  302 . As an example, engagement  238  may comprises a cruciform-shaped recess adapted to mate with the screw driver; however, in other embodiments, engagement  238  may comprise any other suitable type of recess or engagement adapted to receive or mate with any suitable driver tool. For example, engagement  238  may comprise a recess having a hexagonal, rectangular, octagonal, or other shape. One of ordinary skill in the art will appreciate that the above described embodiments of locking screw  200  were presented for the sake of explanatory simplicity and will further appreciate that the present disclosure contemplates locking screw  200  having any suitable dimensions and configuration, being formed from any suitable materials, and being used for any suitable purpose. 
         [0039]      FIGS. 3A and 3B  illustrate enlarged views of an example embodiment of a threaded screw hole  302  that may be included in locking plate  300  in accordance with the present disclosure. In particular,  FIG. 3A  depicts an isometric view of threaded screw hole  302  and  FIG. 3B  depicts a cross-sectional view of threaded screw hole  302  taken along line A of  FIG. 3A . 
         [0040]    Referring to  FIG. 3A , threaded screw hole  302  may generally be defined by one or more of an upper countersink  310   a , lower countersink  310   b , and threaded portion  307  disposed between countersinks  310 . Threaded screw hole  302  may further be surrounded by arum  314  that comprises generally flat surface encircling threaded screw hole  302 . 
         [0041]    Depending upon design, threaded portion  307  may include double lead threads  308  comprising a first thread  308   a  arranged with a second thread  308   b  in a double helix configuration. As an example and not by way of limitation, threads  308   a  and  308   b  may be identical to one another in all respects (e.g., size, length, and included thread angle α), except that thread  308   a  may be opposed from thread  308   b  by 180 degrees. As compared to a single lead thread, double lead threads  308  may enable screw  200  to lockably engage plate  300  in half as many rotations, enable screw  200  to engage screw hole  302  at an angle other than parallel to the central axis  303  of threaded screw hole  302 , and provide a greater amount of surface area to engage the threading on head  202 , thereby increasing the force needed to disengage screw  200  from locking fixation plate  300 . 
         [0042]    Referring to  FIG. 3B , in particular embodiments, upper counter sink  310   a  may include two portions, a locking portion  312  configured to lockably engage locking screw  200 , and a non-locking portion  313  configured to seat a traditional screw having a non-locking (e.g., unthreaded) head. Locking portion  312  may be distinguished from non-locking portion  313  by the fact that threads  308  do not extend into non-locking portion  313 . Including non-locking portion  313  on top of locking portion  312  may enable threaded screw hole  302  to accommodate either locking screw  200  or a traditional non-locking screw having a smooth under surface configured to bear against non-locking portion  313  when screwed into abode  102 . 
         [0043]    Depending upon design, locking portion  312  may be defined by a locking countersink angle Φ while, non-locking portion may  313  may be defined by a non-locking countersink angle γ. Lower counter sink  310   b  may also be defined by a lower countersink angle Ψ. Though countersinks  310  may have any suitable configuration, in particular embodiments, locking countersink angle Φ may be about 60 degrees, non-locking countersink angle γ may be about 90 degrees, and lower countersink angle Ψ may be about 60 degrees. In particular embodiments, the included angle α of threads  308   a  and  308   b  may be equal to locking countersink angle Φ. 
         [0044]    Countersinks  310  may facilitate the ability of screw  200  to be inserted through screw hole  302  at an angle other than co-axial with central axis  303 . For example, lower countersink  310   b  may provide clearance on the underside of plate  300  which enables shaft  204  to tilt within screw hole  302  up to a predefined angle before thread  208  (e.g., the threading on shaft  204 ) contacts the bottom surface of plate  300 . Upper countersink  310   a  may enable screw  200  to lock into screw hole  302  at an angle other than perpendicular to the surface of screw hole  302  by preventing the threaded portion  307  from dictating the angle of insertion. When locking screw  200  is engaged with threaded screw hole  302 , the portion of head  202  that is not engaged with threaded portion  307  may bear against upper countersink  310   a  to provide additional support for screw  200 . In particular embodiments, upper countersink  310   a  may be deep enough to take in the entirety of head  202 , even when head  202  is screwed into screw hole  302  at an angle other than parallel to the central axis  303  of threaded screw hole  302 . 
         [0045]    Referring back to threaded portion  307 , threaded portion  307  may further be defined by one or more of a minor diameter  316 , a major diameter  318 , and a thread pitch  320 . Although threaded portion  307  may be configured to any suitable size or shape, in particular embodiments, threaded portion  307  may include a double lead thread having a minor diameter  316  of 0.161+/−0.001 in., a major diameter  318  of 0.192+/−0.001 in. and a thread pitch  320  of 0.028 in. 
         [0046]    Furthermore, in particular embodiments, the outer edges of head  202  may be beveled to enable the edges of head  202  to remain below the plane of rim  314 , even when inserted into screw hole  302  at an angle. Thus, the low profile of head  202  in combination with the custom size of countersinks  310  may provide a low plate/screw profile and reduce patient palpation of the implant (e.g., plate  300  and screw  200 ) by enabling head  202  to sink below the plane of rim  314  while still maintaining the desired angular locking interface. 
         [0047]      FIGS. 4A and 4B  illustrate a more detailed view of the double helix locking screws  400 . In particular,  FIG. 4A  illustrates a side view of locking screw  400  taken perpendicular to the length of shaft  404 , and  FIG. 4B  illustrates a top view of locking screw  400 , looking down at the top side of head  402 . 
         [0048]    As illustrated in  FIG. 4A , locking screw  400  generally includes a body  401  and a generally conical head  402  that tapers into a generally cylindrical shaft  404  ending at a tip  406 . Screw  400  further includes two generally continuous threads  1001  and  1002  intertwined in a double helix format. Threads  1001  and  1002  extend over the majority of locking screw  400  from tip  406  along the length of shaft  404  onto head  402 . In particular embodiments, screw  400  may be formed by carving threads  1001  and  1002  out of a blank. This may enable head  402  to be low-profile so as to minimize the profile thickness of the plate/screw interface when screw  400  is engaged within a fixation plate  500 , as shown in  FIG. 5 . 
         [0049]    Each double helix thread  1001  and  1002  may generally be defined by a leading flank  410 / 411 , a trailing flank  412 / 413 , and a crest  414 / 415  connecting the outer edge of respective leading flank  410 / 411  to the corresponding outer edge of trailing flank  412 / 413 . Each thread includes a root  416 / 417 . It will be understood by one skilled in the art that since threads  1001  and  1002  are overlapping and helical in nature, the descriptions previously provided with respect to the characteristics of a single thread, as discussed above in  FIGS. 2A and 2B , apply in a similar manner with respect to  FIGS. 4A and 4B , except that there are two threads interspaced and helical in characteristics. The use of a helical threads accelerates the advancement of the screw into the anchoring material due to the multiple use of threads. It will be understood by one skilled in the art that the use of a double helix screw  400  may include more than two interspaced threads, such as three or four interspaced threads, as shown in  FIGS. 5 ,  4 D and  4 E as multiple threads  1001 / 1002 / 1003  in the case of  FIG. 4D  and threads  1001 / 1003 / 1003 / 1004  in the case of  FIG. 4E . Such designs will advance the screw quicker due to the triple or quadruple nature of the screw threads. It will be understood by one skilled in the art that the disclosure herein is intended to cover the use of triple, quadruple or multiple threaded screws and is not limited to a double helix screw. 
         [0050]    As discussed in  FIG. 2A  above, the dimensions of threads  1001  and  1002  in  FIG. 4  may generally be described by one or more of a corresponding thread height  418 / 419 . Similarly, the dimensions of thread  1001  and  1002  may be described by a corresponding leading flank thread angle  420 / 421 , a corresponding trailing flank thread angle  422 / 423 , a corresponding pitch  424 / 425 , a corresponding crest width  426 / 427 , and thread diameter  436 . Additionally, the thread diameters of each thread  1001 / 1002  may vary, which provides additional engagement and threading characteristics. 
         [0051]    The dimensions of body  401  may generally be described by one or more of a length  428 , a head angle  430 , a head diameter  432 , and a shaft diameter  434 . Although screw  400  may be configured to any suitable size or shape, in particular embodiments, length  428  may range, for example, from 10 mm to 70 mm, thread height  418 / 419  may be about 0.030 inches along the length of shaft  404 , and may decrease from 0.030 inches to about 0.000 inches according to the taper of angle  430  over the length of head  402 . Head angle  430  defines the general tapered angle of the conical head  402  and may be between about 15 and 20 degrees and preferably about 18 degrees. Leading thread flank angle  420 / 421  may be about 30 degrees. Trailing flank angle  422 / 423  may be about 10 degrees. Pitch  424 / 425  may be about 0.049 inches along the length of shaft  404  and about 0.044 inches along the length of head  402 . Crest width  426 / 427  may be about 0.001 inches to about 0.004 inches flat (e.g., wide). Thread diameter  436  may be about 0.138 inches and thread diameter  437  may be about 0.138 inches. Diameter  432  may be about 0.196 inches, and shaft diameter  434  may be about 0.094 inches. Referring still to  FIG. 4A , crests  426 / 427  gradually terminate into the end of the conical portion  402 , distal from the tip  406 . This provides for a smooth transition and across the conical portion  402  and a flat top when seated within screw hole  502  as discussed further below. 
         [0052]    In particular embodiments, different portions of thread  1001 / 1002  may be configured to perform different functions. For example, to enable thread  1001 / 1002  to bite into bone  102 , the thread height  418  and/or  419  may vary along the length of shaft  404  along the length of head  402  to interact with the double helix threading inside screw hole  502  ( FIG. 5 ) to lock screw  400  into plate  500 . Depending upon the design of screw  400 , the corresponding pitch  424  of thread  1001  or pitch  425  of thread  1002  may be the same as, or different from, the pitch of the threads located within fixation plate  500 . 
         [0053]      FIG. 5  illustrates a system  500 , which is identical to system  100 , as noted above, other than its preparation for inclusion of screws  400 , rather than  200 . That is, system  500  is intended to be employed with the double helix screws  400 . The description of system  500 , and in particular fixation plate  500  is identical to fixation plate  300 , as noted in  FIG. 1 . Its configuration is similar to that, as shown in  FIG. 1 , comprising fixation plate  300 . Fixation plate  500  may generally include a body  501  comprising a plurality of threaded screw holes  502  connected to each other in a web-like distribution by a plurality of ribs  504 , although any suitable geometry of plate  501  is contemplated. In particular embodiments, ribs  504  may be thinned down relative to threaded screw holes  502  to facilitate bending of ribs  504  rather than threaded screw holes  502  when fixation plate  300  is contoured, for example to match the contour of bone  102 . Its implementation would be identical to that with respect to fixation plate  300 . That is, securing fixation plate  500  to bone  102  uses the locking screw  400 , as discussed above. As in the case of plate  300 , the surgeon may insert locking screw  400  through one or more threads screw holes  502  and into bone  102 . Locking screw  400  may be inserted either parallel to the central axis  503  of screw hole  502 , or at an angle θ relative to central angle  503 . In certain procedures, the surgeon may pre-drill a pilot hole in bone  102  to establish a trajectory for locking screw  400 , or, depending on design, locking screw  400  may be self-drilling in nature, making the pre-drilled hole in bone  102  unnecessary. In either case, once the tip  406  of locking screw  400  is in contact with bone  102 , the surgeon may use a screwdriver or other suitable instrument to screw locking screw  400  into bone  102  until the head  402  of locking screw  200  come to bear against the end of the surface of screw hole  502 . In particular embodiments, both the underside of head  402  and the inside of screw hole  502  may be threaded to commit locking screw  400  to lockably engage screw hole  502 . In that case, further rotation of locking screw  400  may cause the threaded portion of head  402  to interfere with the threading inside screw hole  502  and lock screw  400  into screw hole  502 . The above-described process may be repeated for any suitable number of locking screws until plate  500  is firmly attached to bone  102 . Once the fixation plate  500  has been secured to bone  102 , the incision about  102  may be closed, leaving the patient to heal. 
         [0054]    In this manner, the double helix threaded screw, as shown in  FIG. 4A , may interact with the double helix thread shown within plate  500 . In particular, thread height  418 / 419  may be shallow enough on head  402  to enable the threading inside screw hole  502  to interact with (e.g., deform against or bite into) the threads within hole  502 . And, as noted above, since crests  426 / 427  gradually terminate into the end of the conical portion  402 , distal from the tip  406 , the top of conical portion  402  is flat with the top of screw hole  502 . Further, the present invention provides that the configuration and design of the threads located within screw hole  502  would be identical to that shown and described above with respect to  FIGS. 3A and 3B . In this manner, screw  400  is inserted within screw hole  502  which has the tapered and threaded configurations as noted above with respect to  FIGS. 2A and 2B  within  FIGS. 3A and 3B  except that a double helix screw is used as noted above and described in  FIGS. 4A ,  4 B, and  4 C. Thus, the dual interaction between head  402  and the inner surface of screw hole  502  provides a stable point of connection between plate  500  and screw  400 . Such would occur once screw  400  is screwed into screw hole  502  at an angle other than perpendicular to the surface of screw hole  502 . The desired locking effect may be caused by the threading inside screw hole  502  digging into threads  1001  and  1002  and their corresponding roots  416  and  417 . 
         [0055]    The deformation process described above may be aided by forming screw  400  into plate  500  out of two materials having unequal hardness. For example, screw  400  may be formed of a material that is relatively softer than plate  500  to enable the threading inside screw hole  502  to dig into threads  1001  and  1002  in corresponding root  416 / 417 . 
         [0056]    Alternatively, screw  400  may be formed of a material that is relatively harder than plate  500 , in which case the threading inside screw hole  502  may deform against threads  1001  and  1002  in corresponding root  416 / 417 . In any case, the desired locking effect may be caused by threadable interface between the threading of head  402  and the threading in screw hole  502 . 
         [0057]    Depending upon design, screw  400  and plate  500  may be formed from any one or more materials suitable for forming medical implants, such as materials that have high strength-to-weight ratios and that are inert to human body fluids. In certain embodiments, screw  400  or plate  500  may be formed from one or more titanium alloys, which provide several benefits. For example, titanium alloys are relatively lightweight, provide adequate strength for withstanding forces typically experienced by a medical implant, are inert to human body fluids, and are visible in radiographs of the implant region. In a particular embodiment, screw  400  may be formed from the titanium based alloy Ti6Al4V ELI (per ASTM F136), and plate  300  may be formed from grade 2 or grade 3 titanium (per ASTM F67). In certain other embodiments, screw  400  or plate  500  may be formed from one or more resorbable polymers, such as polylactides, polyglycolide, glycolide/lactide copolymers or other copolymers, or one or more implantable plastics, such as polyethylene or acetal copolymers for example. 
         [0058]    Since the desired locking effect described above between plate  500  and screw  400  may depend primarily upon the interface between the threading inside screw hole  502  and the threading on head  402 , head  402  may lockably engage screw hole  502  independent of the size of shaft  404 . Thus, shaft  404  may have virtually any configuration (e.g., fully threaded, partially threaded, self-threaded, unthreaded, long, or short) while still maintaining the ability to lockably engage plate  500  by virtue of its connection to head  402 . 
         [0059]    As shown in  FIG. 4B , an engagement  438  is formed in head  402 . In particular embodiments, engagement  438  may be adapted to receive an implantation tool such as a driver that may be used to rotate screw  400  about a longitudinal axis  440  in order to screw locking screw  400  into bone  102  and to lock locking screw  400  into screw hole  502 . As an example, engagement  438  may comprises a cruciform-shaped recess adapted to mate with the screw driver or, as shown in  FIG. 4C , the engagement  439  may comprise a hexalobe or star-shaped recess. However, engagement  438 / 439  may comprise any other suitable type of recess or engagement adapted to receive or mate with any suitable driver tool. For example, engagement  438 / 439  may comprise a recess having a hexagonal, rectangular, octagonal, or other shape. One of ordinary skill in the art will appreciate that the above described embodiments of locking screw  400  were presented for the sake of explanatory simplicity and will further appreciate that the present disclosure contemplates locking screw  400  having any suitable dimensions and configuration, being formed from any suitable materials, and being used for any suitable purpose. 
         [0060]    One of ordinary skill in the art will appreciate that the above-described embodiments were presented for the sake of explanatory simplicity and will further appreciate that the present disclosure contemplates any suitable configuration and number of screw holes  302 , ribs  304 , and positioning holes  306  in fixation plate  300 ; and screw holes  502 , ribs  504 , and positioning holes  506  in fixation plate  500 . Moreover, although the present disclosure, including the fixation system and the screw, collectively and individually, has been described in several embodiments, a myriad of changes, substitutions and modifications may be suggested to one skilled in the art, and it is intended that the present disclosure encompasses such changes, substitutions and modifications as fall within the scope of the present appended claims.

Technology Category: a