Abstract:
A system and method are for bone fixation. A radially expandable sleeve is inserted over an elongated shaft of a bone screw. The bone screw and radially expandable sleeve are inserted into a transverse opening extending through a bone fixation device positioned in a bone until the radially expandable sleeve is fully housed within the transverse opening. The bone screw is further screwed into the bone relative to the radially expandable sleeve to cause expansion of the radially expandable sleeve within the bone fixation device, thereby locking the sleeve to the bone fixation device.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    The present application is a continuation of U.S. patent application Ser. No. 11/259,537 filed on Oct. 25, 2005 which is a continuation of, and claims priority under 35 U.S.C. § 120 to, International Application No. PCT/CH2004/000095, filed Feb. 23, 2004. The entire content of both applications is expressly incorporated herein by reference thereto. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention generally relates to a device for fixing bones. 
       BACKGROUND OF THE INVENTION 
       [0003]    It is known in the art to use bone screws for fixing bones. German document DE-A 35 38 238 discusses a bone screw which interacts with a dowel. The dowel is anchored first separately in the bone and then a bone screw is screwed into the dowel. This may connecting two bone fragments in accordance with the dowel/screw principle, which is also known from general fastening techniques. 
         [0004]    Also known in the art are transverse locking screws used in conjunction with medullary pin. However, such locking screws have some amount of clearance with respect to the transverse borehole of the medullary pin. 
       SUMMARY OF THE INVENTION 
       [0005]    In the case of the present invention, a fixation of two bone fragments is not the primary objective and is realized only secondarily by means of a medullary pin. Instead, a primary objective of the present invention is to anchor a bone screw stably in a transverse borehole of a medullary pin. The present invention provides a remedy for the above disadvantage/problem. It is an object of the invention to provide a bone screw, which can be brought into the transverse borehole of a medullary pin without any clearance. 
         [0006]    The objective is accomplished with a bone screw having a distal end and a proximal end, which has the distinguishing features of a shaft with a core diameter “k” and having an external thread with an external diameter “d”. The external diameter “d” of the thread or the core diameter “k” of the shaft tapers in a middle region of the shaft from a larger dimension “d 2 ” or “k 2 ” respectively towards the distal end to a smaller dimension “d 1 ” or “k 1 ”, respectively, where “d 1 ” and “k 1 ” do not equal zero. Further, the shaft is provided with a concentrically disposed, radially expandable sleeve, and the sleeve being positioned on the part of the shaft which has the smaller external diameter “d 1 ”. 
         [0007]    In one embodiment, the thread has a constant depth and the core diameter “k” decreases in the middle region in the direction of the distal end from a larger value “k 2 ” to a smaller value “k 1 ”, where and “k 1 ” do not equal zero.’ 
         [0008]    In another embodiment, the sleeve has an internal thread. 
         [0009]    In still a further embodiment, the sleeve has at least one longitudinal slot. The at least one longitudinal slot may extend over the whole length of the sleeve. Alternatively, the at least one longitudinal slot may extend over a portion of the length of the sleeve. 
         [0010]    In a further embodiment, the sleeve may have several longitudinal slots. The longitudinal slots may extend over a portion of the length of the sleeve where openings of the longitudinal slots positioned alternatingly at the top and bottom of the sleeve. 
         [0011]    In still a further embodiment, the sleeve may have a diameter D H  has a peripheral, encircling flange with an enlarged diameter D F , where D F  is greater than D H  at one of its ends. 
         [0012]    In still a further embodiment, the sleeve consists of a non-absorbable plastic, preferably of PEEK. Alternatively, the sleeve may consist of a metallic material. Furthermore, the sleeve may consist of a bio-absorbable plastic. 
         [0013]    In a further embodiment, a part of the shaft disposed between the proximal end of the bone screw and the region of the shaft having an external thread diameter of “d 2 ” has a diameter of “d 3 ” such that d 3 &gt;d 2 , where the part of the shaft having a diameter of “d 3 ” has an external thread. 
         [0014]    In another embodiment, a device for bone fixation includes one or more bone screws with a sleeve, and one or more medullary pins. The sleeve may have a continuous diameter D H  and a length H L . The the one or more medullary pins may have a longitudinal axis, a diameter D N , a transverse borehole extending transversely to its longitudinal axis, where the transverse borehole has a diameter D B  which is larger than d 2  as well as larger than d B-L5 . 
         [0015]    In one embodiment, the diameter D H  of the sleeve fulfills the condition that D H ≦1.2D B . 
         [0016]    In another embodiment, the length H L  of the sleeve is more than 1.5D N . 
         [0017]    In still a further embodiment, the length H L  of the sleeve is greater than 0.2D N . 
         [0018]    The medullary pin may have a continuous cannulation and a wall thickness W N . 
         [0019]    In a further embodiment, the length H L  of the sleeve corresponds to the wall thickness W N  of the medullary pin. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0020]    The present invention can be better understood by reference to the following drawings, wherein like reference numerals represent like elements. The drawings are merely exemplary to illustrate certain features that may be used singularly or in combination with other features and the present invention should not be limited to the embodiments shown. 
           [0021]      FIG. 1  shows a longitudinal cross-section of a bone screw with a sleeve mounted thereon; 
           [0022]      FIG. 2  shows a longitudinal cross-section of the bone screw of  FIG. 1  inserted in a medullary pin with a transverse borehole; 
           [0023]      FIG. 3  shows a longitudinal cross-section of the bone screw of  FIG. 1 ; 
           [0024]      FIG. 4  shows a perspective view of the sleeve of  FIG. 1 ; and 
           [0025]      FIG. 5  shows a perspective view of an alternate embodiment of the sleeve of  FIG. 1 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]    An embodiment of the bone screw of the present invention is depicted in  FIGS. 1 and 3 . The bone screw  1 , having a core  7  with a diameter “d”, includes a shaft  3 , a distal end  4  and a proximal end  5  with a screw head  6 . The shaft  3  may include external threads  2 . The external threads  2  may be configured so that the thread depths t 1  and t 2  on the segments with lengths L 5  and L 3  are identical. The bone screw  1  has five segments, which are disposed axially behind one another, these segments being formed as follows. 
         [0027]    The screw head  6  extends a length L 1 , measured from the proximal end  5 . Adjoining thereon, a shaft part  8  with a constant external diameter d 3  and an external thread  9  having a constant core diameter k 3  extends a length L 2 . Adjoining onto the shaft part  8  is a segment of the shaft  3  with a constant external diameter d 2 , where d 2  is less than k 3 , and a thread  2  having a constant core diameter k 2  extending a length L 3 . Adjoining thereon, is another segment of the shaft  3 , tapering at an angle α towards the distal end  4 , and extending a length L 4  so that the external diameter d 2  of the thread decreases in this segment to an external diameter d 1 . Likewise, the core diameter k 2  decreases in this segment to a core diameter k 1 . A segment of the shaft  3  with the one constant external diameter d 1  and also a thread  2  having a constant core diameter k 1  extends between the segment of the length L 4  and the distal end  4 . 
         [0028]    A radially expandable sleeve  10  may fit over part of the shaft  3  of the bone screw  1 . The sleeve as depicted in  FIG. 4  may have a length H L , and a continuous diameter D H . The proximal end of the sleeve  10  may have a flange  12  with a diameter D F . 
         [0029]    The outer casing of the sleeve  10  may be formed with a continuous longitudinal slot  11 , from which the sleeve  10  gains its elasticity. Furthermore, the cavity of the sleeve  10  may be provided with an internal thread  13 . The axial movement, which produces the radial expansion of the sleeve  10  due to the particular geometry of the conical section of the core in the middle part L 4  is generated by the pitch of the internal thread. Rotational or screwing movement is required for introducing the screw into the transverse borehole of a medullary pin  20 . 
         [0030]    As shown in  FIG. 1 , the sleeve  10  may be screwed from the distal end  4  of the screw so far over the shaft  3 , that the sleeve&#39;s proximal end, which is provided with the flange  12 , extends into the conical segment of length L 6 . Due to the conicity, the sleeve  10  is expanded somewhat and has a diameter d B-L5  which is greater than D H  ( FIG. 1 ). If the sleeve  10  is further screwed onto the shaft  3 , the sleeve  10  (shown by a broken line) may be expanded further by the conical segment of length L 4 , until it reaches a diameter d B-L3  on the segment of length L 3  with the external diameter d 3 . 
         [0031]    At one end of the sleeve  10 , the flange  12  with external diameter D F  being greater than D H  is disposed for axial contact with a suitable supporting surface. The flange  12  functions as a stop or as a positioning means for the sleeve  10  at the medullary pin  20 , so that the bone screw  1 , with its comically expanding middle part, must be screwed through the sleeve  10 , expanding the latter radially, while the sleeve  10  itself is in contact with the middle part. 
         [0032]    The sleeve  10  may, for example, consist of a non-absorbable plastic, preferably of PEEK. The advantages of such a sleeve are biocompatibility, ready produceability, simple installation, and the fact that, if the pin is pulled out, the sleeve can be compressed in the event that it remains hanging in the hole of the pin. 
         [0033]    The sleeve may, however, also consist of a metallic material. The advantages here are higher strength, biocompatibility, and the fact that the sleeve may consist of the same material as the medullary pin and the bone screw. However, the metallic sleeve is also more ductile, so that it can flow somewhat, if it is inserted at an angle. Finally, the metallic sleeve has the advantage that fragments are not formed. 
         [0034]    The sleeve may also consist of an absorbable plastic. A first advantage lies therein that the angularly stable locking decreases with time. A second advantage lies therein that, in the event that such a sleeve is lost, it is not a serious disadvantage that the sleeve is absorbed over time. 
         [0035]    The embodiment of the bone screw  1  together with sleeve  10  shown in  FIG. 1 , may be introduced into a transverse borehole  22  of a medullary pin  20  ( FIG. 2 ). The medullary pin  20  is provided with a continuous cannulation  23 , which extends coaxially with the longitudinal axis  21  of the medullary pin  20  and has an external diameter D N  and a wall thickness W N . The transverse borehole  22  may have a diameter D B , which is larger than d 2  as well as larger than d B-L5 . In one embodiment, diameter D B  may be approximately 90% of the diameter d B-L3  of the sleeve  10 . The sleeve  10  may be inserted completely into the transverse borehole  22  and, with the flange  12 , is in contact with the wall of a facet  24  of the transverse borehole  22 . 
         [0036]    In one embodiment, the length H L  of the sleeve  10  advantageously corresponds to the wall thickness W N  of the medullary pin  20 . 
         [0037]    In another embodiment, the diameter D H  of the sleeve  10  advantageously satisfies the condition that D H ≦1.2 D B , and the length H L  of the sleeve should be less than 1.5 D N  and more than 0.2 D N . 
         [0038]    In another embodiment of the invention, the thread  2  of the bone screw  1  has a constant depth, the diameter of the core of the shaft, in the middle region L 4  decreases towards the distal end  4  from the larger value k 2  to the smaller value k 1 , where k 1  is larger than zero. The radial expansion accordingly takes place by way of the expansion of the core diameter. This guarantees a certain clamping force in the transverse borehole of the medullary pin  20 . In comparison to an expansion of the external diameter of the thread, the expansion of the core diameter results in a larger mass, which is available for locking the screw. 
         [0039]    In another embodiment, the sleeve  10  may have one or more longitudinal slots  11  resulting in the advantage that the radial expansion of the sleeve  10  can be achieved with a lesser expenditure of force or with a smaller torque. Pre-installation of the sleeve  10  on the bone screw  1  equalizes shear forces applied to the sleeve  10 , thus subjecting the sleeve  10  to a lower internal stress. If a single longitudinal slot  11  is provided, as shown in  FIG. 4 , the slot  11  may extend over the whole height of the sleeve  10 . However, the longitudinal slot  11  may also extend only over a portion of the height of the sleeve  10 . In the case of this embodiment, the slot  11  may be open at one side or closed at both sides. For example, as shown in  FIG. 5 , several partial slots  11 ′ may also be provided over a circumference of the sleeve  10 , the partial slots  11 ′ being open alternately at the top and bottom of the sleeve  10 . 
         [0040]    In the case of a further embodiment, a shaft part is disposed in the region L 2  with an enlarged external diameter d 3 &gt;d 2  between the proximal end  5  of the bone screw  1  and that part of the bone screw with an enlarged external diameter d 2 . This arrangement has the advantage that it closes off the hole drilled in the front corticalis of the bone, so that the flange  12  of the sleeve  10  can be brought through the hole in the first corticalis without expending too much force. Moreover, the shaft part may be provided in the region L 2  with an external diameter d 3 &gt;d 2  with an external thread. The external thread with the external diameter d 3  results once again in a screw-like anchorage in the bone. 
         [0041]    Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. 
         [0042]    It will be appreciated by those skilled in the art that various modifications and alterations of the invention can be made without departing from the scope of the appended claims. Some of these have been discussed above and others will be apparent to those skilled in the art.