Abstract:
A device and method for fixation of bone fractures has a bone screw comprising a shank with a threaded end portion, on the outer surface. The screw has a through bore with two bore portions differing in diameter. A step in the diameter is formed between these bore portions and is located within the end of the screw having the thread. This step in diameter can support a metal insert which in turn supports a polymer pin when the latter if pressurized with a sonotrode in the bone screw. Together with an applied ultrasonic vibration the pressure fluidizes the polymer pin and presses the material through holes configured in the wall of the bone screw and into surrounding bone.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application is a divisional of U.S. patent application Ser. No. 11/879,045, filed on Jul. 13, 2007, the disclosure of which is hereby incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The invention relates in general to sonic fusion technology, it relating more particularly to a device and a method for the fixation of bone fractures, with a bone screw for augmenting within a bone. 
         [0003]    Known from U.S. Pat. No. 4,653,489 is a system wherein a fixation cement is introduced through a bone screw into a portion of a bone afflicted by osteoporosis. Femoral neck fractures as well as distal femoral fractures can be fixated by means of this device. 
         [0004]    The system in accordance with prior art comprises a bone screw having a flow cavity, i.e. an axial through bore through which bone cement can be introduced into the portion at the tip of the screw. The bone cement is advanced by a device which is releasably attached to the trailing end of the screw. This device is similar to a commercially available syringe in comprising substantially a cylindrical barrel and a plunger. The barrel forms a cavity in which the plunger is movable to and fro. 
         [0005]    In use of this prior art device the fixation cement is filled into the barrel, after which the plunger is urged against the cement. By applying manual compression force the fixation cement is jetted into the axial through bore of the bone screw. Due to the pressure the fixation cement is adequately fluidized so that it can pass through the proximal end of the bone screw into the bone, as a result of which the bone screw is augmented in the bone. 
         [0006]    This system has the drawback that the manual pressure applied to the fixation cement varies, not only basically from application to application but also during the application itself so that the distribution of the fixation cement within the portion of the bone at the tip of the bone screw is neither reliable nor even. 
       SUMMARY OF THE INVENTION 
       [0007]    An object of the invention is to define a device and a method by means of which a reliable and even augmentation of a bone screw at an implantation site in the bone can now be assured. 
         [0008]    An aspect is achieved in accordance with the invention by the bone screw for fixation of a bone fracture, having a bone screw comprising a shank having a first threaded end and having along its longitudinal center line an axial through bore having a first bore portion with a first diameter and a second bore portion with a second diameter, wherein the first diameter is larger than the second diameter and the second bore portion is adjacent the first shank end, and a step in the bore between the first and the second bore portions. Another aspect is achieved by a method for fixation of a bone fracture comprising the steps of screwing a bone screw having an axial bore therein with transverse passageways, connecting the bore with an outer surface of the bone screw into a bone across a fracture site, combining a polymer pin with a metal insert, inserting the polymer pin together with the metal insert into the axial through bore in the bone screw, pressuring and vibrating the polymer pin, wherein the polymer pin is supported by the metal insert which is in turn supported by a step in the diameter in the through bore, resulting in the polymer pin being fluidized at its tip, the fluidized polymer material being pressed out of the bone screw. 
         [0009]    The invention will now be detailed by way of a preferred embodiment with reference to the attached drawings in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a section view of a bone screw in accordance with one embodiment of the invention; 
           [0011]      FIG. 2  is a detail view of the tip of the bone screw as shown in  FIG. 1 ; 
           [0012]      FIG. 3  is a view of a polymer pin in accordance with one embodiment of the invention; 
           [0013]      FIGS. 4   a  and  4   b  are a side view and respectively a plan view of a metal insert in accordance with one embodiment of the invention; 
           [0014]      FIG. 5  is a section view of a device for fixation of a bone fracture in accordance with one embodiment of the invention, with the bone screw as shown in  FIG. 1  in which the polymer pin as shown in  FIG. 3  and the metal insert as shown in  FIGS. 4   a ,  4   b  are inserted; and 
           [0015]      FIG. 6  is a detail view of the tip of the device as shown in  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Referring now to  FIG. 1  there is illustrated a bone screw  10  in accordance with the invention. The bone screw comprises a shank and a thread  12  machined in an end portion of the shank, although the thread may also cover the shank of the screw full length. In addition, the bone screw  10  is cannulated. The cannulation is provided along the longitudinal center line of the screw as a through bore  11  composed of two bore portions  16 ,  17 . Bore portion  16  comprises a first diameter and bore portion  17  a second diameter, the first diameter being larger than the second diameter. In addition the bore portion  16  forms the main portion of through bore  11 . In the preferred embodiment, just a small portion adjoining the end of the shank of the bone screw in which the thread  12  is machined is formed by the bore portion  17 . The transition from the bore portion  16  to the bore portion  17  is formed by a step  18  in the bore  11 . The step  18  in the bore forms an annular ridge having substantially right-angled edges at the wall of the through bore within the bone screw. The edges of the step  18  in the diameter may be machined flat or rounded. The screw features transverse holes  14  which extend through the wall of the bore portion  16  to allow polymeric material within the bore  11  to flow out of bore  11  and into the adjacent bore. Furthermore the position of the step in diameter together with the holes in the wall can be positioned optionally along the longitudinal center line and thus the location of the polymeric augmentation can be determined in accordance with the particular application and the desired effect. 
         [0017]    In the preferred embodiment the holes  14  may be configured in differing directions perpendicular to the longitudinal center line of the bone screw and arranged in the end portion with the thread  12 . Preferably the holes  14  are arranged in a region of the end portion which also features the bore portion  16 . In the embodiment as shown in  FIG. 1  two holes  14  each are configured axially juxtaposed in the bore portion  16  and through the thread  12 . Furthermore, four such pairs of holes are evenly distributed about the circumference of the bone screw, in other words, circumferentially spaced by 90°. It is, however, just as possible that three, four, five or more holes may be provided circumferentially and it is not necessary that the holes circumferentially distributed are all at same axial level. Apart from this, transverse or longitudinal oblong holes, slots, or the like may be provided. 
         [0018]    Referring now to  FIG. 2  there is illustrated the tip of the bone screw as shown in  FIG. 1  but on a magnified scale, the step  18  in the diameter between the bore portion  16  and bore portion  17  now being particularly evident. Apart from this, a few of the holes  14  are shown which are configured passing through the thread  12  in the bore portion  16 . 
         [0019]    Referring now to  FIG. 3  there is illustrated a polymer pin  20  elongated in shape and slightly tapered at a conically tapered end  22 . Provided in the conically tapered end  22  of the polymer pin  20  is a concavity or counterbore  24  in the end face. The polymer pin  20  may also be made of other materials such as for instance a thermoplastic material suitable for augmenting a bone screw, both resorptive and non-resorptive materials being useful. 
         [0020]    Referring now to  FIGS. 4   a  and  4   b  a metal insert is shown in a side view and in a plan view. The metal insert features a substantially disk-shaped end  32  and a substantially pin-shaped end  34 . The disk-shaped end  32  has an outer diameter somewhat smaller than the diameter of the bore portion  16  and somewhat larger than the diameter of the bore portion  17 . The pin-shaped end  34  is configured so that it can be inserted into the counterbore in the polymer pin. 
         [0021]    In another embodiment (not shown) the metal insert features instead of the pin-shaped end  34  a protruding end suitable for snap mounting, the polymer pin in this case having a snap mounting end corresponding to the protruding end. When the metal insert is snap mounted with the polymer pin, both elements can be inserted together into the bone screw, it being of advantage when the snap mount comprises a slight clearance when connected. This clearance has the advantage that when the polymer pin is pressurized it can be better fluidized at the joint with the metal insert to thus easier jet from the bone screw into the bone. 
         [0022]    It is furthermore possible that the metal insert instead of featuring a protruding or pin-shaped end has a through bore into which a corresponding end of the polymer pin can engage. In this embodiment the polymer material is jetted axially from the bone screw not only through the holes  14  but also out the leading end of the screw through the hole in the metal insert. the proportion of the polymer material emerging from the holes and bores can be varied by the size thereof. 
         [0023]    Depending on the aspect concerned, a snap mount may also be provided in combination with axial and/or radial holes, it being just as possible, however, to configure the metal insert integrally with the bone screw. In this arrangement the step in the diameter between two portions of the bore is configured by a larger difference in diameter; indeed, even an axial blind hole may be used on the cannulation instead of the full axial through bore in the bone screw. 
         [0024]    The following details inserting the bone screw into a bone. Firstly a K wire is powered up to the site in a bone at which the bone screw is to be located. Via the K wire the bone screw is then advanced and ultimately screwed into place until it is sited as desired. After insertion of the bone screw in the bone the K wire is removed. This procedure makes it necessary that the bone screw features a full length through bore. This is a popular operation technique because the operator can best check the position of the screw. The K wire is also used to measure the necessary screw length. 
         [0025]    After removal of the K wire the passageway or through bore  11  along the longitudinal center line of the bone screw is free to receive polymer pin  20  together with metal insert  30 . Tip  34  of the pin shaped end of metal  30  insert is inserted into counterbore  24  of polymer pin  20 . Referring now to  FIG. 5  there is illustrated how metal insert  30  rests on the step formed by the step  18  in the bore when polymer pin  20  with the metal insert  30  has been inserted facing the direction of the tip. It is in this way that the step  18  in the diameter forms within the bone screw a counterhold for the metal insert which in turn supports the polymer pin when the polymer pin is pressurized and vibrated by an ultrasonic handpiece/sonotrode which, for this purpose, is mounted on the free end of the bone screw. The vibration and pressure generated by the ultrasonic handpiece and applied to the polymer pin fluidizes the polymer pin so that the material of the polymer pin emerges from the radially arranged holes  14  into the bone. It is in this way that the polymer pin furnishes the material for augmenting the bone screw in the bone. 
         [0026]    It is to be noted that the present invention is not just limited to the indications as recited above. In other words, all screw applications which can be supplied by cannulated screws can be potentially supplied with the option of polymeric fastening and thus with the device in accordance with the invention. 
         [0027]    Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.