Abstract:
Bone screws have heads which are provided with a hex socket as well as a threaded portion. A screw driver system includes a hex driver and a threaded driver. The screw driver system permits driving the screw through a plate system into a bone until desired compression is obtained. Then the screw is locked to the plate while maintaining the imparted compression.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates broadly to orthopedic devices. More particularly, this invention relates to systems for locking bone screws relative to bone plates. 
         [0003]    2. State of the Art 
         [0004]    For various fractures of bones of the body, plating is a well known technique to impart the stabilization desirable for proper healing. In plating, a rigid, typically metal plate is placed on the outer surface of the bone across the fracture, and screws are secured through the plate and into the bone on either side of the fracture in a manner which permits the rigid plate to offer support to the bone during healing. 
         [0005]    The screws include threads along a shaft adapted to engage cortical bone. Most commonly, the head portion of the screw is generally a standard screw head which provides a compressive force about a corresponding round screw hole of the plate as the screw is thread into the bone, thereby causing compression of the plate against the bone. 
         [0006]    U.S. Pat. No. Re. 28,841 to Allgower describes a plate that is used with generally standard screws having heads with a convex undersurface. The plate includes oblong screw holes which each define at one end an upper ramped portion and a generally smaller radius of curvature about the ramped portion. In use, a hole is drilled into the bone through the screw hole adjacent the ramp and a screw is inserted through the screw hole adjacent the ramp into the drilled hole and rotated until the head of the screw contacts the ramp. Upon such engagement, there is displacement of the bone plate in a direction to move the ramped portion away from the screw and the cause the plate to apply pressure to maintain the bone parts together about the fracture in tight engagement. 
         [0007]    More recently, particularly at the metaphysis of long bones though not limited thereto, there have been desirable results with screws having exteriorly threaded heads which threadably engage threads in the plate to lock the screws relative to the plate and thereby limit compression of the plate relative to the bone. However, such systems do not provide the necessary control of compression between the plate and bone. Control over compressive forces is lost as soon as the threads of the head of the screw lock relative to the plate. Therefore, such a system provides sub-optimal stability. 
         [0008]    Certain plates sold by Synthes are designed with a hole called a COMBI-HOLE™. The COMBI-HOLE™ is an elongated screw hole including two joined circular sections, each extending through approximately 250°. One of the circular sections is threaded and thus adapted to received a screw with a threaded head at a fixed angle. When used as such, the system has the same lack of control over compression as discussed above. 
       SUMMARY OF THE INVENTION 
       [0009]    It is therefore an object of the invention to provide a plate and screw system whereby the amount of compression between the plate and bone can be controlled completely by the surgeon. 
         [0010]    It is an additional object of the invention to provide a plate and screw system which permits complete control over compression prior to and during locking the screw to the plate. 
         [0011]    In accord with these objects, which will be discussed in detail below, bone screws according to the invention have heads which are provided with a hex socket as well as a threaded portion. A screw driver system according to the invention includes a hex driver and a threaded driver. The threaded driver is provided with an axial through bore which is dimensioned to receive the hex driver. The screw driver system is coupled to a screw by threadably engaging the threaded driver with the threaded portion of the screw head, and delivering the hex driver through the bore in the threaded driver to engage the hex socket in the screw head. Optionally, a set screw is used to secure the hex driver to the threaded driver. The end of the threaded driver is dimensioned to have a diameter larger than the diameter of the head of the screw with which it is used. According to a first embodiment, the screw head is frustroconical with a smooth side surface and the screw head includes internal threads coaxial with the hex socket. The first embodiment is used with a bone plate having a frustroconical screw hole. 
         [0012]    When the screw is attached to the driver system and is passed through the hole in the bone plate, the threaded driver alone is used (or both drivers coupled together are used) to rotate the screw to engage the bone. After the screw is advanced into the bone, the frustroconical screw head enters the frustroconical screw hole in the bone plate, but is prevented by the threaded driver from contacting the inner wall of the screw hole. The larger diameter end of the threaded driver prevents the screw head from advancing further. Moreover, the end of the threaded driver contacts the area of the bone plate surrounding the screw hole and exerts compression on the plate as the screw is further advanced into the bone. When the desired compression is reached, turning of the threaded driver is stopped (and if the drivers where engaged, they are now disengaged) and the hex driver is used to further advance the screw. Turning the hex driver while holding the threaded driver advances the screw while starting to decouple the threaded portion of the screw head from the threaded driver. In order to maintain compression until the screw head is locked in the screw hole of the bone plate, the length of the threads on the screw head and the threaded driver prevent the screw head from being decoupled before the screw head is locked. When the frustroconical screw head lockingly engages the frustroconical screw hole, the threaded driver is backed off the screw head by turning it in the direction opposite to driving. 
         [0013]    A second embodiment of a bone screw according to the invention is identical to the first embodiment but has a threaded exterior surface on the wall of the frustroconical head. The threaded surface creates high localized pressure between the thread crests and the tapered screw hole wall of the plate. It also causes some scoring and a better mechanical hold. 
         [0014]    A third embodiment of a bone screw has a cylindrical head with two unequal diameter circumferential edges, one near the top and one near the bottom of the head, that make simultaneous contact with the tapered screw hole wall of the plate when the head is locked to the plate. 
         [0015]    A fourth embodiment of a bone screw has a cylindrical head with external threads and an internal hex socket. This embodiment of the screw is used with the second embodiment of the threaded driver which has internal threads which engage the exterior of the screw head and is also used with a bone plate that has a cylindrical threaded screw hole. The threads on the head of the screw preferably advance at a similar rate to the threads on the shaft so as to maintain the compression achieved while the head is being locked to the plate. Alternatively, the pitch of the screw head threads may be decreased slightly with respect to the screw shaft threads so as to increase compression slightly as the screw is locked to the plate. 
         [0016]    Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a side elevational view of a hex screw driver according to the invention; 
           [0018]      FIG. 2  is a transparent side elevational view of a threaded screw driver according to the invention; 
           [0019]      FIG. 3  is a transparent side elevational view of the hex screw driver coaxially inserted through the threaded screw driver; 
           [0020]      FIG. 4  is a longitudinal sectional view of a first embodiment of a bone screw according to the invention; 
           [0021]      FIG. 5  is a partially transparent side elevational view of the screw drivers of  FIGS. 1 and 2  coupled to the screw of  FIG. 4 ; 
           [0022]      FIG. 6  is a broken partially transparent side elevational view of the screw drivers and screw of  FIGS. 1-5  and a bone plate showing the screw unlocked from the plate during plate compression; 
           [0023]      FIG. 7  is a view similar to  FIG. 6  but showing the screw locked to the plate; 
           [0024]      FIG. 8  is a view similar to  FIG. 6  but showing a second embodiment of a screw according to the invention; 
           [0025]      FIG. 9  is a view similar to  FIG. 7  but showing the second embodiment of the screw; 
           [0026]      FIG. 10  is a view similar to  FIG. 8  but showing a second embodiment of a threaded screw driver according to the invention and a bone plate having a threaded screw hole; 
           [0027]      FIG. 11  is a view similar to  FIG. 9  but showing the second embodiment of a threaded screw driver and the bone plate having a threaded screw hole; 
           [0028]      FIG. 12  is a view similar to  FIG. 8  but showing another embodiment of a bone screw according to the invention; and 
           [0029]      FIG. 13  is a view similar to  FIG. 9  but showing another embodiment of a bone screw according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0030]    Turning now to  FIGS. 1-3  a first embodiment of a screw driver system  10  according to the invention includes a hex driver  12  and a threaded driver  14 . The hex driver  12  includes a cylindrical handle  16 , a smaller diameter shank  18 , and a male hex end  20 . The threaded driver  14  includes a cylindrical handle  22  with a tapered end  24 . An exteriorly threaded cylinder  26  extends from the tapered end  24 . An axial through bore  28  extends through the handle  22  and through the cylinder  26 . The through bore  28  exhibits a tapered wall  30  just prior to the cylinder  26 . Optionally, a set screw hole  32  is arranged orthogonal to the bore  28 . The hex driver  12  is dimensioned to fit through the bore  28  as shown in  FIG. 3  with the male hex end  20  extending beyond the threaded cylinder  26 . The tapered wall  30  in the bore  28  prevents the hex driver  12  from passing through the bore. The screw driver system  10  is designed for use with a bone screw of the type shown in  FIG. 4 . 
         [0031]    Turning now to  FIG. 4 , a bone screw  40  includes a threaded shaft  42  having a cutting flute  44  at one end and a screw head  46  at the other end. The head  46  is substantially frustroconical and houses a hex socket  48  and a threaded inner wall  50  which is coaxial with the hex socket. Referring now to  FIGS. 3-5 , those skilled in the art will appreciate that, when assembled, the threaded cylinder  26  of the driver  14  engages the threaded inner wall  50  of the screw head  46  and the male hex end  20  of the driver  12  engages the hex socket  48 . The screw and driver assembly is shown in  FIG. 5 . When the screw and drivers are assembled as shown, the screw is ready to be inserted through a bone plate and threaded into bone. 
         [0032]      FIGS. 6 and 7  show two stages of applying the screw  40  to a bone plate  50  having a frustroconical screw hole  52 . After the screw and drivers are assembled as shown in  FIG. 5  and the drivers are optionally fastened to each other with a set screw  34  through the set screw hole  32 , the handle  22  of the driver  14  is turned while the screw  40  is inserted through the plate  50  and into a pre-drilled hole in the bone (not shown). As the screw advances into the bone, the head  46  of the screw advances toward the plate  50  and ultimately enters the frustroconical hole  52  as shown in  FIG. 6 . Before the head  46  can frictionally engage the hole  53 , the tapered end  24  of the driver  14  abuts the plate  50 . Since the head  46  of the screw  40  is threadably coupled to the end of the driver  14 , it is prevented from advancing away from the driver  14  into the hole  52 . When the screw, driver and plate are arranged as shown in  FIG. 6 , rotating the driver  14  advances the screw into the bone while the tapered end  24  of the driver  14  compresses the plate  50  against the bone. When the surgeon feels that the proper amount of compression has been achieved, rotation of the driver  14  is stopped. If necessary, the set screw is  34  released from the hole  32  ( FIG. 5 ) and the hex driver  12  is rotated further in the same direction while the threaded driver  14  is held stationary. This causes the screw  40  to advance while the head  46  rotates out of engagement with the driver  14  until the head  46  frictionally engages the hole  52  as shown in  FIG. 7 . The driver  12  may then be removed by lifting it out of the socket in the screw head and the bore in the driver  14 . The driver  14 , if not decoupled from the screw by the advancement, can then be uncoupled by reverse rotation. 
         [0033]    From the foregoing, those skilled in the art will appreciate that the system of the invention allows the surgeon to carefully apply a controlled amount of compression to the plate prior to locking the screw to the plate. It also maintains the selected compression while the screw is being locked to the plate. The tapered end of the threaded driver serves two functions, it reduces friction between the driver and the plate, making it easier to rotate the driver under compression. It also provides a clearer view of the screw and screw hole during insertion of the screw into the hole. However, it is not necessary that the driver be tapered. 
         [0034]      FIGS. 8 and 9  illustrate an alternate embodiment of a screw  140  which is substantially identical to the screw  40  having a threaded shaft  142 , a cutting flute at one end (not shown) and a frustroconical head  146  at the other end. The difference between this screw  140  and the screw  40  is that the outer surface of the head  146  is threaded. The screw  140  works with the same driver assembly  10  and bone plate  50  as the screw  40  in the same manner as described above but the threaded surface  146  creates high localized pressure between the thread crests and the tapered screw hole wall  52  of the plate. It also causes some scoring and a better mechanical hold. The pitch of the threads  146  should be the same as the pitch of the threads  142  to help maintain the same level of compression and create one scoring track. 
         [0035]      FIGS. 10 and 11  illustrate another embodiment of a bone screw according to the invention. The screw  340  has a threaded shaft  342  and a cylindrical head  346  with two unequal diameter circumferential edges  346   a ,  346   b . The interior of the head  346  is the same as the head  46  of the screw  40  ( FIG. 4 ) and the screw is used with the driver assembly  10  ( FIG. 3 ) and the bone plate  50  described above. When the screw  340  is locked into the tapered hole  52  as shown in  FIG. 13 , the two circumferential edges  346   a ,  346   b  make simultaneous contact with the tapered hole, one near the top and one near the bottom. Such circumferential ridges can also be used on screws with frustroconical heads or any shaped head which will rotate within the screw hole. 
         [0036]      FIGS. 12 and 13  illustrate another embodiment of a bone screw  240 , a second embodiment of a bone plate  150 , and a second embodiment of a driver system  110 . The driver system  110  uses the same hex driver  12  as the driver system  10  but uses a different threaded driver  114 . The driver  114  has an interiorly threaded cylindrical recess inside its tapered end  124  as opposed to the exteriorly threaded cylinder  26  of the driver  14  ( FIGS. 2 and 3 ). The screw  240  has a cylindrical head  246  which is provided with threads that match the threads in the recess  126  of the driver  114 . The screw head  246  also has an interior hex socket like the hex socket  48  in the screw  40  ( FIG. 4 ) but it does not have interior threads like the threads  50  in the screw  40 . The threads on the shank  242  of the screw  240  have generally the same pitch as the threads on the head  246 . The plate  150  has a screw hole  152  with interior threads which match the threads of the screw head  126 . As can be seen in  FIGS. 10 and 11 , the threads  152  terminate before reaching the bottom of the plate  150 . The screw, driver, and plate of this arrangement operate in a manner similar to the systems described above. The screw  240  is attached to the driver  114  by threading its head  246  into the threaded recess  126  of the driver. The driver  114  drives the screw into the bone and advances the head  246  to the top of the plate  150 , but prevents it from entering the threaded hole  152 . The tapered end  124  of the driver abuts the top of the plate and compresses it. When the surgeon feels the appropriate amount of compression, the hex driver  12  is rotated while keeping the threaded driver  114  stationary. This rotates the screw head  246  out of engagement with the threaded recess  126  and into engagement with the threaded hole  152 . The screw head bottoms out on the threads in the hole  152  before passing through the plate  150 . It is desirable that multiple thread starts be provided on the screw head  246  to avoid compression when the plate threads start to engage and to avoid cross-threading. It is important to note that the external threads on the head  246  are free to engage both the driver and the plate and are not blocked from engaging either by any other part of the head. 
         [0037]    There have been described and illustrated herein embodiments of a bone plating system and methods of using the same. While particular embodiments of the invention have been described, it is not intended that the invention be limited thereto, as it is intended that the invention be as broad in scope as the art will allow and that the specification be read likewise. For example, while the screws have been described as having a hex socket which is driven by a hex driver, it will be appreciated that other types of sockets and drivers can obtain substantially the same results as a hex socket and driver. It will therefore be appreciated by those skilled in the art that yet other modifications could be made to the provided invention without deviating from its scope as claimed.