Abstract:
A bone screw includes a screw shaft extending longitudinally along a screw axis and a screw head extending from a proximal end of the screw shaft and configured to be releasably coupled to a surgical tool. The bone screw also includes a through hole defining a through hole axis and extending through the screw head, the through hole axis intersecting the screw axis at an acute angle α, the through hole being adapted to receive therein a second screw and tapering from a first end at a proximal end of the screw head to a second end opening to an outer surface of the screw head.

Description:
PRIORITY CLAIM 
       [0001]    The present invention claims priority to U.S. Provisional Application Ser. No. 61/334,234 filed on May 13, 2010 and entitled, “Bone Screw Assembly and Implantation of the Same,” the entire disclosure of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention generally relates to bone screw assemblies and instruments for implantation of the same as well as to an associated method for implantation of the bone screw assembly using the instruments. More particularly, the invention relates to a bone screw including a second screw and insertion instruments for implantation of the same as well as to a method for implantation of the bone screw and the second screw by using the insertion instruments. 
       BACKGROUND 
       [0003]    In the field of orthopedic surgery bone fixation devices using bone screws are commonly used. These bone fixation devices include bone plates, intervertebral implants or intramedullary nails by means of which two or more bones or bone fragments are fixed relative to each other. Typically, the bone fixation devices comprise bone anchors mostly in the form of bone screws, pins or nails by means of which the bones or bone fragments are fixed to the bone plate, intervertebral implant or intramedullary nail and consequently fixed relative to each other. One problem that can arise in case of the above mentioned bone fixation devices is that the bone screws, pins or nails can for instance become dislodged in the bone or in the bone plate, intervertebral implant or intramedullary nail during normal movements of the patient. 
         [0004]    Thus, there remains a need for an improved bone anchor device for use in bone fixation that allows to drill a hole for a securing screw into a bone under a particular angle with respect to the axis of a bone fastener and to insert the securing screw in a guided manner. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention relates to a bone screw with a screw head configured to be releasably coupled to a surgical instrument or tool such that the surgical instrument or tool is coaxially supported in a first position and can be pivoted in a guided manner about an axis extending diametrically to the screw head into a second position. 
         [0006]    According to an exemplary embodiment of the present invention, the bone screw comprises a screw axis, a shaft to be anchored in a bone and screw head including a through hole with a through hole axis cutting the axis of the bone screw at an acute angle wherein the through hole is adapted to receive a second screw that can be anchored in the bone as well. The screw head comprises a transverse channel with a channel axis extending diametrically across the screw head and wherein the transverse channel is open at the rear end of the bone screw. The screw head of the bone screw further includes a recess which has—in at least a cross-section orthogonal to the channel axis—a circularly curved edge with a centre located at the point where the through hole axis cuts the screw axis. 
         [0007]    One of the advantages of the bone screw according to the invention is that the configuration of the transverse channel and the recess permits an aiming guide with a complementarily configured tip to be attached to the screw head and rotated about a rotation axis from a first position aligned with the screw axis into a second position aligned with the through hole axis. Another advantage of the bone screw is the pivotable joining of the screw head and the aiming guide. The aiming guide can be inserted into the incision coaxially to the screw axis of the bone screw by using a tissue protection tube and/or a guide wire and attached to the screw head. After removing the tissue protection tube and/or the guide wire the aiming guide can be rotated about the rotation axis. In addition, the aiming guide can then be rotated into a second position in which the longitudinal axis of the aiming guide is aligned with the through hole axis of the through hole in the screw head so that a bore hole for the second screw can be drilled into the bone which is exactly aligned with the through hole in the screw head; the bore hole for the second screw can be drilled in a completely guided manner and the second screw can be positioned in a guided manner by means of the aiming guide. Another advantage is that the transverse channel and the recess allow to attach a complementarily formed tip of a screwdriver to the bone screw in such a manner that the screwdriver is positioned coaxially to the screw axis of the bone screw. 
         [0008]    In an exemplary embodiment of the bone screw, the through hole axis cuts the screw axis at a depth T&gt;0 measured from the rear end of the bone screw towards the screw shaft. 
         [0009]    In another exemplary embodiment of the bone screw, the recess has a constriction at the rear end of the bone screw. Thus, the recess forms a female connector for a snap-lock connection with a respective male connector arranged at a surgical instrument or tool. 
         [0010]    In a further exemplary embodiment of the bone screw, the through hole comprises an internal thread, preferably a conical internal thread. This allows the advantage that the second screw can be firmly connected to the screw head of the bone screw. 
         [0011]    In another exemplary embodiment of the bone screw, the internal thread has a thread pitch P and a threaded length L T  and wherein the ratio L T /P is minimum 2.0, preferably minimum 2.3. This configuration of the internal thread allows a rigid and angularly stable anchorage of the screw head of the second screw in the screw head of the bone screw. 
         [0012]    In yet another exemplary embodiment of the bone screw, the recess has a spherical shape with a radius of the sphere R. This configuration of the recess allows a surgical instrument or tool to be pivoted about an axis which extends through the point where the through hole axis and the screw axis intersect so that the instrument or tool can be pivoted from a first position aligned with the screw axis to a second position aligned with the through hole axis. 
         [0013]    In a further exemplary embodiment, the bone screw further comprises a second screw insertable into the through hole coaxially to the through hole axis. 
         [0014]    In another exemplary embodiment of the bone screw, the second screw has a conically threaded head engagable with the conical internal thread in the through hole. 
         [0015]    In yet a further exemplary embodiment of the bone screw, the recess includes a depression traversing the constriction and forming a wall portion with the shape of a surface section of a cylinder, cone or prism the axis of which coincides with the through hole axis. By means of the depression a stop for the rotation of an instrument or tool inserted in the recess in the screw head of the bone screw is provided so that the instrument or tool can be exactly aligned with the through hole for the second screw. 
         [0016]    In another exemplary embodiment of the bone screw, the angle α amounts to minimum 10°, preferably to minimum 20°. 
         [0017]    In yet another exemplary embodiment of the bone screw, the angle α amounts to maximum 70°, preferably to maximum 35°. 
         [0018]    In again another exemplary embodiment of the bone screw, the transverse channel has a U-shape in a cross-section orthogonal to the channel axis. The U-shaped channel can have a semicircular bottom with a radius of curvature r C , wherein the centre of the semicircular edge of the transverse channel is located on the channel axis. The channel axis can be located at a depth T C  measured from the rear end of the bone screw towards the screw shaft, wherein the depth T C  is equal or greater than the depth T of the point where the through hole axis cuts the screw axis. In a particular configuration of the transverse channel the channel axis cuts the screw axis through the point where the through hole axis and the screw axis intersect, i.e. T C =T. In this case the semicircular bottom defines a seat coaxially to the recess for rotatably receiving cylindrical pins of an aiming guide which have a pin diameter equal to twice the radius of curvature r C  of the semicircular bottom of the transverse channel. In case of a spherical recess the rotatable movement of the aiming guide is limited to a uniaxial pivot movement due to the pins engaging the transverse channel. 
         [0019]    In a further exemplary embodiment of the bone screw, the channel axis cuts the screw axis through the point at which the screw axis and the through hole axis intersect. 
         [0020]    In yet another exemplary embodiment of the bone screw, the screw head of the bone screw comprises an external thread designed in such a manner that the bone screw can be counter-sunk in a bone. The external thread on the screw head is preferably_conical so that it allows to countersink the screw head in the bone. This configuration is particularly useful if the bone screw is used as a locking screw for an intramedullary nail. 
         [0021]    In another exemplary embodiment of the bone screw, the screw head of the bone screw has a longitudinal slot so that the screw head is radially elastically expandable. The screw head can have the shape of a segment of a sphere so that the bone screw can be inserted into a complementarily shaped hole in a bone plate or other implant under a surgeon desired angle. Once the bone screw is correctly positioned the second screw can be inserted until the head of the second screw expands the screw head of the bone screw in the hole so allowing to secure the bone screw in a surgeon selected angle relative to a bone plate or other implant. 
         [0022]    In accordance with another aspect of the present invention, a screwdriver is provided for screwing the above bone screw into a bone. The screwdriver essentially comprises a male connector terminally arranged at the front end which is suitable to be coupled to the recess in the screw head of the bone screw. Further, the connector includes a tip constricting towards the front end of the screwdriver and two driving protrusions diametrically projecting over the tip in either direction and defining a central axis which extends orthogonal to the longitudinal axis of the screwdriver. The driving protrusions fit in the transverse channel in the screw head of the bone screw. In at least a cross-section orthogonal to the central axis the tip has a circularly curved periphery with a radius R and a centre located on the longitudinal axis. The driving protrusions can have the shape of pins or blades. In case of blade-shaped driving protrusions the tips of the blades define the central axis. In case of pin-shaped driving protrusions the axes of the pins define the central axis. 
         [0023]    In an exemplary embodiment, the screwdriver further comprises a longitudinal slot extending parallel to the longitudinal axis and which is open at the front end so that the connector is radially elastically compressible. Further, the tip has a constriction towards the shaft which forms at least in a cross-section orthogonal to the central axis a curved contact shoulder. Thus, the connector forms a male connector for a snap-lock connection with a respective female connector arranged at the bone screw. 
         [0024]    In a further exemplary embodiment of the screwdriver, the tip has a spherical shape with a radius of the sphere R and with a centre located on the longitudinal axis. 
         [0025]    In a further exemplary embodiment of the screwdriver, the two driving protrusions are circular-cylindrically shaped wherein the central axis orthogonally cuts the longitudinal axis through the centre of the spherical tip. 
         [0026]    In another exemplary embodiment of the screwdriver, the male connector further comprises an axial stop located at a distance T measured from the central axis towards the shaft so that the stop contacts the rear end of the bone screw when the connector is coupled to the recess in the screw head. Thus, the screwdriver is kept exactly coaxially to the screw axis of the bone screw when the stop abuts the rear end of the bone screw. 
         [0027]    In yet another exemplary embodiment of the screwdriver, the male connector further includes a nose projecting over the tip in a direction towards the front end and at an acute angle with respect to the longitudinal axis of the screwdriver. This configuration allows the advantage that the screwdriver can only be inserted in one orientation into the seat in the screw head of the bone screw. 
         [0028]    In again another exemplary embodiment, the screwdriver further comprises a coaxial through bore penetrating through the shaft and the male connector and having an internal thread for engaging an external thread arranged on a locking pin which is insertable in the through bore in such a manner that the locking pin can be advanced towards the front end of the screwdriver to prevent the tip from radially collapsing. 
         [0029]    In accordance with a further aspect of the present invention, an aiming guide is provided for drilling a hole in the bone the axis of which coincides with the through hole axis of the through hole in the screw head of the bone screw. The aiming guide essentially comprises a guide sleeve, a coaxial through bore and a male connector terminally arranged at the front end which is suitable to be coupled to the recess in the screw head of the bone screw. The connector includes a tip a width of which decreases towards the front end of the aiming guide and two pins diametrically projecting over the tip in either direction and coaxially arranged on a central axis which extends orthogonal to the longitudinal axis. In at least a cross-section orthogonal to the central axis the tip has a circularly curved periphery with a radius R and a centre located on the longitudinal axis. 
         [0030]    In an exemplary embodiment, the aiming guide further comprises a longitudinal slot extending parallel to the longitudinal axis and which is open at the front end so that the connector is radially elastically compressible. Additionally, a curved contact shoulder is formed at the proximal end of the tip. Specifically, the curved contact shoulder is formed by a constriction at the proximal end of the tip (adjacent the guide sleeve) where a cross-sectional area of the tip in a plane orthogonal to the central axis of the tip is reduced relative to a maximum diameter portion of the tip distal thereto. This configuration allows the connector to operate as a male connector for a snap-lock connection with a respective female connector arranged in the bone screw. 
         [0031]    In another exemplary embodiment of the aiming guide, the tip has a spherical shape with a radius of the sphere R and with a center located on the longitudinal axis. 
         [0032]    In a further exemplary embodiment of the aiming guide, the two pins are circular-cylindrically shaped and wherein the central axis orthogonally cuts the longitudinal axis through the centre of the spherical tip. The circular cylindrical pivot pins are coaxially and rotatably insertable in the transverse channel. 
         [0033]    In yet another exemplary embodiment of the aiming guide, a cylindrical or conical collar is arranged between the tip and the guide sleeve coaxially to the longitudinal axis of the aiming guide. 
         [0034]    In again another exemplary embodiment of the aiming guide, the collar has a radius r≦R. 
         [0035]    In a further exemplary embodiment, the aiming guide further comprises a drill guide which can be inserted into the through hole. 
         [0036]    In another exemplary embodiment of the aiming guide, the drill guide has a conical tip. The conical tip is shaped in such a manner that it fits into the tapered through hole in the bone screw. This allow to lock the drill guide aligned to the through hole axis of the through hole for the second screw. 
         [0037]    In accordance with yet another aspect of the present invention, an assembly including a bone screw, a screwdriver and an aiming guide is provided. This assembly can be used if a standard locking screw which is commonly available can be inserted into the through hole in the bone screw. 
         [0038]    In accordance with again another aspect of the present invention, an assembly including a bone screw, a tissue protection tube, an aiming guide and a screwdriver is provided. 
         [0039]    In accordance with still another aspect of the present invention, a method for bone fixation using an intramedullary nail including a number of proximal and distal locking holes and a number of bone screws with a second screw each is provided. The method essentially comprises the steps of:
       a) performing an incision into the tissue surrounding a bone to be treated;   b) positioning an intramedullary nail in the bone;   c) coupling an aiming device to the intramedullary nail;   d) inserting a tissue protection tube into a selected guide bore in the aiming device coaxially to one of the locking holes;   e) drilling a first bore hole into a bone for insertion a bone screw by using the aiming device, wherein the first bore hole is aligned with the selected proximal or distal locking hole;   f) coupling a bone screw to the connector of the screwdriver;   g) advancing the bone screw through the tissue protection tube;   h) screwing the bone screw into the bone using the screwdriver;   i) removing the screwdriver;   j) inserting the aiming guide through the tissue protection tube;   k) attaching the aiming guide to the bone screw in such a manner that the longitudinal axis of the aiming guide is aligned with the screw axis of the bone screw;   l) removing the tissue protection tube;   m) repeating steps d) to l) until a bone screw each is inserted in all or in the selected proximal and/or distal locking holes of the intramedullary nail;   n) removing the aiming device from the intramedullary nail;   o) pivoting the aiming guide about the central axis until the collar of the aiming guide abuts a stop in the recess in the screw head of the bone screw so that the longitudinal axis of the aiming guide is aligned with the through hole axis of the through hole in the bone screw;   p) inserting the drill guide into the through bore in the aiming guide;   q) drilling a second bore hole into the bone using the drill guide as a guide for the drill bit;   r) removing the drill guide;   s) inserting the second screw through the through bore in the aiming guide;   t) advancing the second screw into the bone;   u) removing the aiming guide;   v) repeating steps o) to u) until a second screw each is anchored in the bone passing through the through hole of each of the bone screws; and   w) closing the incision.       
 
         [0063]    Instead of subsequently performing steps d) to l) for one bone screw and repeating the sequence for each bone screw to be inserted each step can be repeated for all or for the selected number of bone screws to be inserted. Similarly, each step for inserting the second screw can be repeated for all or for the selected number of bone screws instead of subsequently performing steps o) to u) for one bone screw and repeating the sequence for each bone screw to be inserted. 
         [0064]    In an exemplary embodiment, the stop is formed by the wall portion of the depression in the recess in the screw head of the bone screw. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0065]    An exemplary embodiment of the present invention will be described in the following by way of example and with reference to the accompanying drawings in which: 
           [0066]      FIG. 1  illustrates a longitudinal section of an embodiment of the bone screw according to the invention; 
           [0067]      FIG. 2  illustrates a lateral view of the embodiment of the bone screw of  FIG. 1 ; 
           [0068]      FIG. 3  illustrates a perspective view of an embodiment of the screwdriver according to the invention; 
           [0069]      FIG. 4  illustrates a longitudinal section of the embodiment of the screwdriver of  FIG. 3 ; 
           [0070]      FIG. 5  illustrates a lateral view of the embodiment of the screwdriver of  FIG. 3 ; 
           [0071]      FIG. 6  illustrates a lateral view of the embodiment of the screwdriver of  FIG. 3  which is orthogonal to the lateral view of  FIG. 5 ; 
           [0072]      FIG. 7  illustrates a lateral view of another embodiment of the screwdriver according to the invention; 
           [0073]      FIG. 8  illustrates a lateral view of the embodiment of the screwdriver of  FIG. 7  which is orthogonal to the lateral view of  FIG. 7 ; 
           [0074]      FIG. 9  illustrates a partial section through the embodiment of the screwdriver of  FIGS. 7 and 8  and a bone screw attached thereto; 
           [0075]      FIG. 10  illustrates a perspective view of an embodiment of the aiming guide according to the invention; 
           [0076]      FIG. 11  illustrates a longitudinal section through the embodiment of the aiming guide of  FIG. 10  and a bone screw coaxially attached thereto; 
           [0077]      FIG. 12  illustrates a longitudinal section through the embodiment of the aiming guide of  FIG. 10  and a bone screw attached thereto under the angle α; 
           [0078]      FIG. 13  illustrates a longitudinal section through the embodiment of the aiming guide of  FIG. 10  and a bone screw attached thereto under the angle α and together with a drill guide inserted in the aiming guide and a drill bit; 
           [0079]      FIG. 14  illustrates a section through the aiming device and the drill guide of  FIG. 13 ; 
           [0080]      FIG. 15  illustrates a longitudinal section through the embodiment of the aiming guide of  FIG. 10  and a bone screw attached thereto under the angle α and together with a second screw inserted in the aiming guide; 
           [0081]      FIG. 16  illustrates a longitudinal section through the embodiment of the aiming guide of  FIG. 10  and a bone screw attached thereto under the angle α and together with a second screw firmly secured in the through hole in the screw head of the bone screw; and 
           [0082]      FIG. 17  illustrates an intramedullary nail together with a bone screw and a second screw according to an embodiment of the method for bone fixation according the invention. 
           [0083]      FIG. 18  illustrates a lateral view of a system according to an alternate embodiment of the present invention, in a first configuration. 
           [0084]      FIG. 19  illustrates a lateral view of the system of  FIG. 18 , in a second configuration. 
           [0085]      FIG. 20  illustrates a cross-sectional lateral view of the system of  FIG. 18 , in the second configuration. 
           [0086]      FIG. 21  illustrates an enlarged cross-sectional lateral view of a portion of the system of  FIG. 18 . 
       
    
    
     DETAILED DESCRIPTION 
       [0087]    The present invention may be further understood with reference to the following description and the appended drawings, wherein like elements are referred to with the same reference numerals. The present invention relates to bone screw assemblies and instruments for implantation of the same as well as to an associated method for implantation of the bone screw assembly using the instruments. In particular, the invention relates to a system and method facilitating implantation of a first bone screw, including a through hole extending through a head portion thereof along a through hole axis, and a second screw inserted into the through hole along the through hole axis such that the first and second bone screws are implanted into a bone in a stable configuration. 
         [0088]      FIGS. 1 and 2  illustrate an embodiment of the bone screw  1  with a screw head  2  comprising a conical external thread  29 . The bone screw  1  includes a screw axis  6 , a threaded shaft  10 , a screw head  2  and a rear end  8  at a proximal end thereof. The screw head  2  comprises a through hole  9  penetrating through the screw head  2  and having a through hole axis  7  cutting the screw axis  6  under an acute angle α. The through hole axis  7  cuts the screw axis  2  at a depth T measured from the rear end  8  of the bone screw  1 . The through hole  9  has a conical internal thread  11 . A second screw  50  ( FIGS. 16 and 17 ) can be inserted into the through hole  9  coaxially to the through hole axis  7 . The second screw  50  has a conically threaded head  51  which is engagable with the conical internal thread  11  in the through hole  9 . The screw head  2  includes a concave seat  14  for releasably coupling a surgical instrument or tool to the bone screw  1 . The concave seat  14  comprises a transverse channel  5  and a centrally located recess  3 . The transverse channel  5  comprises a channel axis  101  located at a depth T C  measured from the rear end  8  of the bone screw  1  and diametrically extending across the screw head  2 . The channel axis  101  cuts the screw axis  6  through the point where the through hole axis  7  cuts the screw axis  6 . Further, the transverse channel  5  is open at the rear end  8  of the bone screw  1  and the transverse channel  5  has a U-shaped cross-section with a semicircular bottom  100  orthogonal to the channel axis  101 . The semicircular bottom  100  has a radius of curvature r C  wherein a center of an edge the semicircular bottom  100  of the transverse channel  5  is located on the channel axis  101 . The depth T C  is equal to the depth T of the point where the through hole axis  7  cuts the screw axis  1 . The semicircular bottom  100  defines a seat coaxially to the recess  3  for rotatably receiving cylindrical pins  25  of an aiming guide  23  ( FIG. 10 ) which have a pin diameter equal to twice the radius of curvature r c  of the semicircular bottom  100  of the transverse channel  5 . 
         [0089]    The recess  3  has a spherical shape with a radius of the sphere R and a centre  4  coinciding with the point at which the screw axis  6  and the through hole axis  7  intersect. So the recess  3  forms a pivot bearing for rotatably supporting and guiding a complementarily spherically shaped male connector  232  of an aiming guide  23  ( FIG. 10 ). Due to the facts that the channel axis  101  cuts the screw axis  6  at the point where the centre  4  of the spherically shaped recess  3  is located on the screw axis  6  and that the pivot pins  25  of the aiming guide  23  fit in the transverse channel  5  rotatably about the channel axis  101  the polyaxial pivot bearing formed by the ball-and-socket joint is limited to an uniaxial pivot bearing. When the aiming guide  23  is coupled to the bone screw  1  the aiming guide  23  can only pivot about the channel axis  101  which is orthogonal to a plane defined by the screw axis  6  and the through hole axis  7  of the through hole  9  for the second screw  50 . This allows to position the aiming guide  23  in a first position coaxial to the screw axis  6  of the bone screw  1  and in a second position coaxial to the through hole axis  7  of the through hole  9  for the second screw  50 . Thus, it will be understood by those of skill in the art, that the second screw  50  may be precisely inserted into the through hole  9  along through hole axis  7 , increasing a stability of the screws  1 ,  50  in situ. 
         [0090]    Furthermore, the recess  3  has a constriction  31  at the rear end  8  of the bone screw  1  so that the recess  3  forms a female connector for a snap-lock connection. Additionally, the recess  3  includes a depression  12  which forms a wall portion  121  with the shape of a surface section of a circular cylinder with a radius r≦R. The axis of the circular cylinder coincides with the through hole axis  7  of the through hole  9 . The depression  12  forms a stop for the rotation of an aiming guide  23  about the channel axis  101  when the aiming guide  23  is coupled to the screw head  2  of the bone screw  1 . By means of the stop the aiming guide  23  can be exactly aligned with the through hole  9 . 
         [0091]      FIGS. 3 to 6  illustrate an embodiment of the screwdriver  13  to be used with the bone screw  1  according to  FIGS. 1 and 2 . The screwdriver  13  comprises a longitudinal axis  130 , a shaft  131 , a front end  135  at a distal end thereof and a male connector  132  which can be coupled to the above embodiment of the bone screw  1 . In order to releasably couple the screwdriver  13  to the bone screw  1  the connector  132  is essentially complementarily formed to the concave seat  14  in the screw head  2  of the bone screw  1 . The connector  132  includes a partially spherical tip  17  the cross-sectional area of which in a plane perpendicular to the longitudinal axis decreases toward the shaft  131  (i.e., toward the front end  135 ). Thus, a cross-sectional area of the spherical tip  17  at an end adjacent to the shaft  131  and at the front end  135  is smaller than a cross-sectional area of a mid-section of the spherical tip  17 . The tapering of the spherical tip  17  towards the shaft  131  forms a curved contact shoulder  138  which abuts the constriction  31  of the recess  3  at the rear end  8  of the bone screw  1 . The screwdriver  13  further comprises a longitudinal slot  20  open at the front end  135  to form the tip  17  as an elastic male connector for a snap-lock connection between the tip  17  and the recess  3  in the screw head  2  of the bone screw  1 . The longitudinal slot  20  is arranged orthogonal to a plane defined by the longitudinal axis  130  and the central axis  136  and penetrates through the shaft  131 . The connector  132  further includes two driving protrusions  18  extending laterally from the spherical tip  17  in either direction and which are coaxially arranged on a central axis  136 . The driving protrusions  18  are circular-cylindrically shaped with a cylinder axis coinciding with the central axis  136 . The connector  132  additionally comprises an axial stop  21  which is located between the shaft  131  and the connector  132  at a distance T measured from the central axis  136  towards the shaft  131 . The axial stop  21  abuts the rear end  8  of the bone screw  1  allowing to keep the screwdriver  13  exactly coaxially to the screw axis  6  of the bone screw  1 . The shaft  131  and the connector  132  comprise a coaxial through bore  134  with an internal thread  137  for engaging an external thread arranged on a locking pin  35  ( FIG. 4 ) which is insertable in the through bore  134  in such a manner that the locking pin  35  can be advanced towards the front end  135  of the screwdriver  13  in order to prevent the tip  17  from radially collapsing so that it can be firmly kept in the recess  3  in the screw head  2  of the bone screw  1 . 
         [0092]    The embodiment of the screwdriver  13  illustrated in  FIGS. 7 to 9  differs from the embodiment of  FIGS. 3 to 6  only therein that the male connector  132  further includes a nose  34  extending from the tip  17  in a direction towards the front end  135 . The nose  34  has a nose axis  38  extending under the angle α with respect to the longitudinal axis  130  of the screwdriver  13 . Thus, the screwdriver  13  can be coupled to the bone screw  1  in only one rotative position, namely the one position where the nose  34  engages the through hole  9  in the bone screw  1  in such a manner that the nose axis  38  coincides with the through hole axis  7 . 
         [0093]      FIG. 10  illustrates an embodiment of the aiming guide  23  to be used with the bone screw  1  according to  FIGS. 1 and 2 . The aiming guide  23  comprises a longitudinal axis  230 , a coaxial through bore  28 , a guide sleeve  26 , a front end  235  and a male connector  232  terminally arranged at the front end  235 . To releasably couple the aiming guide  23  to the bone screw  1  the connector  232  is essentially complementarily formed to the seat  14  in the screw head  2  of the bone screw  1 . The connector  232  includes a spherically shaped tip  231  which tapers inward toward the front end  235  and toward the guide sleeve  26  forming a ball-and-socket joint with the recess  3  of the above described embodiment of the bone screw  1 . The spherically shaped tip  231  has a radius of the sphere R and a centre  233  located on the longitudinal axis  230 . Additionally, the connector  232  includes two pins  25  diametrically projecting over the tip  231  in either direction and which are coaxially arranged on a central axis  234  which extends orthogonal to the longitudinal axis  230  and which cuts the longitudinal axis  230  through the centre  233  of the spherically shaped tip  231 . The pins  25  are circular-cylindrically shaped with a cylinder axis coinciding with the central axis  234  so as to form axles coaxially and rotatably insertable in the transverse channel  5  in the screw head  2  of the bone screw  1 . The aiming guide  23  further comprises a longitudinal slot  27  open at the front end  235  to form the tip  231  as an elastic male connector for a snap-lock connection between the aiming guide  23  and a the spherical recess  3  in the screw head  2  of the bone screw  1 . The longitudinal slot  27  is arranged orthogonal to a plane defined by the longitudinal axis  230  and the central axis  234  and penetrates through the guide sleeve  26 . Between the tip  231  and the guide sleeve  26  a cylindrical collar  33  is arranged coaxially to the longitudinal axis  230  and which has a radius r≦R. 
         [0094]    As illustrated in  FIGS. 13 and 14  a drill guide  36  can be inserted in the through bore  28  in the aiming guide  23 . The drill guide  36  has a conical tip  37  which fits into the tapered through hole  9  in such a manner that the drill guide  36  is exactly aligned with the through hole axis  7  of the through hole  9  in the bone screw  1 . 
         [0095]      FIGS. 11 to 17  show an embodiment of the method for bone fixation by using an intramedullary nail  300  and bone screws  1  along with second screws  50 , which are briefly described in the following section. The intramedullary nail  300  comprises a nail axis  303 , a proximal end  305 , a peripheral surface  304 , a number of proximal locking holes  301  with a most proximal locking hole  302  and a number of distal locking holes  306 . The proximal and distal locking holes  301 ,  306  extend transverse to the nail axis  303 . The intramedullary nail  300  is inserted into the intramedullary canal of a long bone in such a manner that the portion of the intramedullary nail  300  containing the distal locking holes  306  is located in a distal bone fragment and the portion containing the proximal locking holes  301  is located in the proximal bone fragment. In order to lock the intramedullary nail  300  in the bone a bone screw  1  each is driven through all or a number of selected proximal and distal locking holes  301 ,  306 . Using the bone screw  1  according to the invention the bone screws  1  can be driven through all or the selected proximal and distal locking holes  301 ,  306  and the second screws  50  can be anchored in the bone. It should be appreciated that instead of driving the bone screws  1  into the proximal and distal locking holes  301 ,  306  the second screws  50  could be driven into the proximal and distal locking holes  301 ,  306  and the bone screws  1  could be anchored in the bone. 
         [0096]    The method for inserting the bone screws  1  into the nail  300  and anchoring the second screws  50  into the bone comprises the steps of making an incision into the tissue surrounding a bone to be treated and positioning an intramedullary nail  300  in the bone. An aiming device (not shown) to the proximal end  305  of the intramedullary nail  300 , wherein the aiming device has guide bores for inserting guide sleeves and/or tissue protection tubes  40  coaxially to each of all or of a number of selected proximal and/or distal locking holes  301 ,  306 . A tissue protection tube  40  ( FIGS. 9 and 11 ) is inserted into a selected guide bore in the aiming device coaxially to one of the proximal and distal locking holes  301 ,  306  until the front end of the tissue protection tube  40  contacts the surface of the bone. A first bore hole is then drilled into a bone for insertion of a bone screw  1  through the selected proximal or distal locking hole  301 ,  306  by using the aiming device, wherein the first bore hole is aligned with the selected proximal or distal locking hole  301 ,  306 . Further, the bore hole extends on either side of the intramedullary nail  300  in such a manner that a bone screw  1  can penetrate through the selected proximal or distal locking hole  301 ,  306  of the intramedullary nail  300  when the bone screw  1  is anchored in the bone. The bone screw  1  is coupled to the connector  132  of the screwdriver  13  by using the snap-lock connection between the bone screw  1  and the screwdriver  13  and advancing the bone screw  1  through the tissue protection tube  40 . The bone screw  1  is then screwed into the bone using the screwdriver  13 . Once the bone screw  1  is screwed into the bone, the screwdriver  13  may be removed. The above-described steps may be repeated until a bone screw  1  has been inserted into all of the desired proximal and/or distal locking holes  301 ,  302 . 
         [0097]    The aiming guide  23  is then inserted through the protection tube  40 , as shown in  FIG. 11 , to attach the aiming guide  23  to the bone screw  1  using the snap-lock connection between the tip  231  of the aiming device  23  and the recess  3  in the screw head  2  of the bone screw  1  such that the longitudinal axis  230  of the aiming guide  23  is aligned with the screw axis  6  of the bone screw  1 . The tissue protection tube  40  and the aiming device may be removed and the and the aiming guide  23  pivoted about the central axis  234  defined by the pins  25  arranged at the connector  232  of the aiming guide  23  until the collar  33  of the aiming guide  23  abuts a stop in the recess  3  in the screw head  2  of the bone screw  1  so that the longitudinal axis  230  of the aiming guide  23  is aligned with the through hole axis  7  of the through hole  9  in the bone screw  1  ( FIG. 12 ). The stop is formed by the wall portion  121  of the depression  12  in the recess  3  in the screw head  2  of the bone screw  1 . The drill guide  36  is then inserted into the through bore  28  in the aiming guide  23  ( FIG. 13 ) and a second bore hole is drilled into the bone using the drill guide  36  as a guide for the drill bit  39 . Once the second bore hole has been drilled, the drill guide  36  is removed and the second screw  50  is inserted through the through bore  28  in the aiming guide  23  ( FIG. 15 ) and advanced through the second screw  50  into the bone ( FIG. 16 ). The aiming guide  23  may then be removed and the steps described above repeated until a second screw  50  each is anchored in the bone passing through the through hole  9  of each of the bone screws  1 . Once all of the desired bone screws  1  and second screws  50  have been inserted into the bone, the incision may be closed. 
         [0098]    As shown in  FIGS. 18-21 , an alternate embodiment of the assembly of the present invention is substantially similar to the assembly described above in regard to  FIGS. 1-17 , comprising a first bone screw  1 ′, a second bone screw  50 ′ and a screwdriver  13 ′. The screwdriver  13 ′, however, combines elements of the screwdriver  13  and the aiming guide  23 , as described above, such that two separate devices are not required for insertion of the first bone screw  1 ′ and for aiming the second bone screw  50 ′. In addition, the screwdriver  13 ′ includes a connector  132 ′ at a distal end  135 ′ thereof, which extends around a head  2 ′ of the first bone screw  1 ′ rather than within a recess thereof. 
         [0099]    The first bone screw  1 ′ extends along a first axis  6 ′ and includes a head  2 ′, which has an exterior surface that is at least partially spherical. The exterior surface may also include portions that are substantially planar permitting a torsional force to be applied thereto via the screwdriver  13 ′. Similarly to the bone screw  1 , the first bone screw  1 ′ includes a through hole  9 ′ extending along a second axis  7 ′ to receive the second screw  50 ′ therein. The second screw  50 ′ is substantially similar to the second screw  50 ′ described above. 
         [0100]    The screwdriver  13 ′ includes a shaft  131 ′ extending along a longitudinal axis  130 ′ with a connector  132 ′ formed at the distal end  135 ′ thereof. The screwdriver  13 ′ also includes a channel  28 ′ extending therethrough along the longitudinal axis  130 ′ sized and shaped to permit the second bone screw  50 ′ to be inserted therethrough. The connector  132 ′ includes a partially spherical interior surface  231 ′ sized and shaped to receive the head  2 ′ of the first bone screw  1 ′ therein. In one embodiment, the connector  132 ′ may be keyed (e.g., include planar portions corresponding to the planar portions of the head  2 ′) permitting the screwdriver  13 ′ to apply torsional forces to the bone screw  1 ′ while also permitting the first bone screw  1 ′ to pivot with respect to the screwdriver  13 ′ via the partially spherical surfaces of the connector  132 ′ and the head  2 ′. The interior surface  231 ′ may receive the head  2 ′ via, for example, a snap fit. 
         [0101]    In an alternative embodiment, the head  2 ′ may include pins extending radially outward therefrom, which are substantially similar to the pins  25  of the connector  232  of the aiming guide  23 , and the connector  132 ′ may include a transverse channel diametrically extending thereacross similarly to the channel  5  of the bone screw  1 , as described above. It will be understood by those of skill in the art that such a configuration also permits the first bone screw  1 ′ to be rotated via the screwdriver  13 ′ while also permitting the first bone screw  1 ′ to be pivoted relative thereto. 
         [0102]    The first and second bone screws  1 ′,  50 ′ and the screwdriver  13 ′ may be used in a manner substantially similar to the method described above. In particular, the first bone screw  1 ′ may be inserted into a desired one of the proximal and/or distal locking holes  301 ,  306  of an intramedullary nail  300  inserted into the bone. A first bore hole may be drilled through the desired one of the first and second holes  301 ,  306  to accommodate the first bone screw  1 ′. As described above, the screwdriver  13 ′ is coupled to the first bone screw  1 ′ by receiving the head  2 ′ within the connector  132 ′. In an initial configuration, the longitudinal axis  130 ′ of the screwdriver  13 ′ is coaxially aligned with the first axis  6 ′ of the bone screw  1 ′. The first bone screw  1 ′ is screwed into the desired one of the holes  301 ,  306  and the first bore hole via the screwdriver  13 ′. Once the first bone screw  1 ′ has been inserted, as desired, the screwdriver  13 ′ is pivoted with respect to the first bone screw  1 ′ about the head  2 ′ until the channel  28 ′ thereof is coaxially aligned with the second axis  7 ′ of the through bore  9 ′. A second bore hole may be drilled into the bone through the channel  28 ′ and through bore  9 ′ to accommodate the second bone screw  50 ′. The second bone screw  50 ′ may then be guided through the channel  28 ′ and into through bore  9 ′ to be advanced into the second bore hole in the bone. It will be understood by those of skill in the art that the above-described steps may be repeated, as desired, until a desired number of first and second bone screws  1 ′,  50 ′ have been inserted into the bone. 
         [0103]    Although the 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, composition 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. 
         [0104]    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 broad scope of the appended claims. Some of these have been discussed above and others will be apparent to those skilled in the art.