for convenience , the same or equivalent elements in the various embodiments of the invention illustrated in the drawings have been identified with the same reference numerals . further , in the description that follows , any reference to either orientation or direction is intended primarily for the convenience of description and is not intended in any way to limit the scope of the present invention thereto . referring to fig1 a two - part bone screw 14 is shown according to one embodiment of the invention . in this embodiment , the bone screw 14 comprises , concentric to the longitudinal axis 3 of the screw , a screw shank 1 and a screw head 2 which is in two parts separable in the axial direction and which consists of a bottom part 4 on the screw shank side and a top part 5 at the end . the screw shank 1 serves to anchor the bone screw in a bone or bone part . the bottom part 4 is of convex design toward the screw shank and has a bearing surface 26 in the form of a segment of a sphere . bottom part 4 and screw shank 1 are in one piece . in this embodiment , the connection between top part 5 and screw shank / bottom part 4 is a press - fit connection . a peg 24 is arranged on the top part 5 , concentric to the longitudinal axis 3 of the screw , and peg 24 can be introduced into a bore 25 in the screw shank 1 , which bore is concentric to the longitudinal axis 3 of the screw . to turn the screw shank 1 into the bone or the bone part , a hexagon socket 16 is arranged on the bottom part 4 , concentric to the longitudinal axis 3 . the wrench width of the hexagon socket 16 can be selected such that the side surfaces 17 of the hexagon socket 16 do not touch the peg 24 . the top part 5 is of a spherical design spaced axially at one end of the screw , so that the assembled screw head 2 is made convex at the end by the top part 5 . referring to fig2 a further embodiment of the two - part bone screw 14 according to the invention is shown . the bone screw 14 comprises , concentric to the longitudinal axis 3 of the screw , a screw shank 1 and a screw head 2 which is in two parts separable in the axial direction and which consists of a bottom part 4 on the screw shank side and a top part 5 at the end . the screw shank 1 serves to anchor the bone screw in a bone or bone part . the bottom part 4 is stepped and has , toward the screw shank , a plane bearing surface 7 with a lower edge 6 . bottom part 4 and screw shank 1 are in one piece . in this embodiment , the connection between top part 5 and screw shank / bottom part 4 is a cone connection . arranged on the top part 5 , concentric to the longitudinal axis 3 of the screw , there is a conical peg 8 which can be secured in a bore 9 concentric to the longitudinal axis 3 of the screw , with an inner cone 10 in the screw shank 1 . to turn the screw shank 1 into the bone or the bone part , a hexagon socket 16 is arranged on the bottom part 4 , concentric to the longitudinal axis 3 . the wrench width of the hexagon socket 16 can be selected such that the side surfaces 17 of the hexagon socket 16 do not touch the conical peg 8 . referring to fig3 another embodiment of the two - part bone screw 14 according to the invention is shown . the bone screw 14 of fig3 differs from the embodiment shown in fig2 only in that the connection between top part 5 and screw shank 1 is a screw connection . a threaded peg 11 is arranged on the top part 5 , concentric to the longitudinal axis 3 of the screw , and the threaded peg 11 can be screwed into a bore 12 , concentric to the longitudinal axis 3 of the screw . bore 12 has an internal thread 13 on the screw shank 1 . in this embodiment , bottom part 4 and screw shank 1 are in one piece . the hexagon socket 16 passes axially through the bottom part 4 , while the bore 12 with the internal thread 13 begins only at that end of the screw shank 1 toward the bottom part and extends into the latter . also arranged on the top part 5 there are two or more surfaces 15 oriented parallel to the longitudinal axis 3 of the screw and acting as external two edges for turning the screw shank 1 by means of a suitable screwdriver . in the alternative , it is also possible to have an external square or external hexagon , instead of the external two edges . referring to fig4 a further embodiment of the two - part bone screw 14 according to the invention is shown . the embodiment of fig4 differs from the embodiment shown in fig3 only in that the connection between top part 5 and screw shank 1 is a bayonet lock . a peg 18 is arranged on the top part 5 , concentric to the longitudinal axis 3 of the screw . peg 18 includes a radially protruding pin 19 which can be snapped into a bore 20 , with groove 21 , concentric to the longitudinal axis 3 of the screw . the groove 21 has a part 22 extending parallel to the longitudinal axis 3 of the screw and a part 23 extending peripherally into the bore 20 . referring to fig5 and 6 , an embodiment of the bone fixation device according to the invention is shown which serves to connect a longitudinal support 27 to a pedicle screw 28 within a spinal column fixation system . this device comprises a pedicle screw 28 which has , concentric to its longitudinal axis 3 , a screw shank 1 to be anchored in the bone , a screw head 2 , a receiving head 29 with the central axis 30 , which serves to connect a longitudinal support 27 to the pedicle screw 28 , and clamping means 31 . these clamping means 31 are generally in the form of a nut , and can be screwed by means of an internal thread 32 over an external thread 34 adjoining the upper side 33 of the receiving head 29 toward the screw head , and serve to fix the longitudinal support 27 and the pedicle screw 28 within the receiving head 29 . the screw head 2 is in two parts and has a bottom part 4 and a top part 5 , their connection being designed in accordance with one of the embodiments shown in fig1 through 4 and described above . the top part 5 is designed as a segment of a sphere axially at one end , the zenith 35 of the sphere segment lying on the longitudinal axis 3 of the screw and forming the screw - head end of the pedicle screw 28 . the receiving head 29 has an upper side 33 toward the screw head , an underside 36 toward the screw shank , a through - bore 37 passing through the receiving head 29 , coaxial to the central axis 30 , for receiving the pedicle screw 28 and , additionally , a channel 38 extending transverse to the central axis 30 and open toward the upper side 33 in order to receive a longitudinal support 27 . in this way , the longitudinal support 27 can be inserted from the upper side 33 into the open channel 38 and can be fixed therein in a releasable manner by the clamping means 31 . the through - bore 37 comprises a concave portion 39 which tapers toward the underside 36 and which , in the embodiment of the device according to the invention shown here , is designed as a segment of a sphere . the two - part screw head 2 has a concentric circular - disk - shaped bottom part 4 with a diameter d . the concave portion 39 is spherical with a radius of curvature x and , toward the upper side 33 , opens into a cylindrical portion with the diameter d . in the embodiment of the device according to the invention shown here , the radius of curvature x corresponds to the radius of the cylindrical portion x = d / 2 . likewise , in the embodiment shown here , the diameter d of the bottom part 4 is smaller than the diameter d of the cylindrical portion d & lt ; d . this dimensioning of the bottom part 4 and of the concave portion 39 ensures that the bottom part 4 , in the concave portion 39 of the though - bore 37 , can be made to bear on the wall 40 of the through - bore 37 at different angles between the longitudinal axis 3 of the screw and the central axis 30 . fig1 - 6 illustrate illustrate a bone screw 14 ; 28 for osteosynthetic bone fixation , with a screw shank 1 to be anchored concentric to the longitudinal axis 3 of the screw in a bone or bone part , and with a screw head 2 . the bone screw 14 ; 28 is in more than one part axially , at least the screw head 2 being in two parts in the axial direction and comprising a bottom part 4 toward the screw shank and a top part 5 toward the end which can be connected to the bottom part 4 and / or to the screw shank 1 . in a preferred embodiment the bottom part 4 and the screw shank 1 are in one piece . in addition , the top part 5 of the bone screw 14 ; 28 can be connected in a releasable manner to the bottom part 4 and / or to the screw shank 1 . in one variation , the top part 5 can be connected to the screw shank 1 or bottom part 4 by means of a cone connection . in another alternative , the top part 5 can be connected to the screw shank 1 or bottom part 4 by means of a screw connection . in another embodiment , the top part 5 can be connected to the screw shank 1 or bottom part 4 by means of a bayonet lock . the bottom part 4 may be of convex design toward the screw shank . for example , in one variation the bottom part 4 of the bone screw 14 ; 28 is designed as a segment of a sphere toward the screw shank . in another embodiment , the bottom part 4 is designed as a circular disk . in addition , the rim of the bottom part 4 may be stepped and have a lower edge 6 . the top part 5 may be of convex design , with the zenith of the top part 5 intersecting the longitudinal axis 3 of the screw at one end . in one embodiment , the top part 22 is of spherical design . in another embodiment , the top part 5 is of semispherical design . in addition , the diameter d of the bottom part 4 of the bone screw 14 ; 28 is preferably between 8 and 10 mm . also , the bottom part ( 4 ) of the bone screw 14 ; 28 preferably has a thickness of 1 to 2 mm . also , preferably the external diameter of the screw shank 1 is 5 to 6 mm . in another exemplary embodiment , the diameter d of the bottom part 4 is preferably between 4 and 6 mm . in another embodiment , the bottom part 4 preferably has a thickness of 0 . 5 to 1 mm . in another embodiment , the external diameter of the screw shank 1 is preferably 3 to 5 mm . in use , the bone screw 14 ; 28 is characterized in that it serves for the fixation of bones or bone parts in an osteosynthesis fixation device . in particular , the bone screw 14 ; 28 , is particularly well - suited for the fixation of bones or bone parts on a bone plate . in one exemplary embodiment , the bone screw is a pedicle screw and serves for the fixation of vertebrae in a spinal column fixation device . a device for osteosynthetic bone fixation according to one embodiment comprises at least one bone screw 14 ; 28 as described above , characterized in that it additionally comprises at least one plate - shaped , prismatic or cylindrical fixation body 41 which has at least one bore 37 with a central axis 30 for receiving the bone screw 14 ; 28 , an underside 36 toward the screw shank , and an upper side 33 toward the screw head , with the bore 37 comprising a concave portion 39 tapering toward the underside 36 , and the diameter d of the bottom part 4 being dimensioned such that the bottom part 4 , in the concave portion 39 of the bore 37 , can be made to bear on the wall 40 of the bore 37 at different angles between the longitudinal axis 3 of the screw and the central axis 30 . in one embodiment , the bone screw 14 ; 28 used in the device is a pedicle screw 14 ; 28 with a convex screw head 2 ; and the fixation body 41 is a receiving head 29 with central axis 30 , and additionally has a channel 38 extending transverse to the central axis 30 and open toward the upper side 33 in order to receive a longitudinal support 27 ; and the device additionally comprises in order to receive a longitudinal support 27 ; and the device additionally comprises clamping means 31 which can be connected to the receiving head 29 in a releasable manner from the direction of the upper side 33 and serve for fixing a longitudinal support 27 and the pedicle screw 14 ; 28 within the receiving head 29 . in one embodiment , the bottom part 4 has the diameter d , and the concave portion 39 is of spherical design and has the diameter d , where d = d . in another embodiment , the bottom part 4 has the diameter d , and the concave portion 39 is of spherical design and has the diameter d , where d & gt ; d . in one variation , the ratio d : d is between 0 . 5 and 1 . 0 and in another variation the ratio d : d is between 0 . 85 and 0 . 95 . in one embodiment , the concave portion 39 is designed as a segment of a sphere , where the spherical segment has a radius x , and x = d / 2 . in another embodiment , the ratio of d / 2 to x is between 0 . 5 and 1 . 0 . in another embodiment , the ratio of d / 2 to x is between 0 . 85 and 0 . 95 . in one embodiment , the diameter d of the concave portion 39 and the diameter d of the bottom part 4 are between 8 and 10 mm . in one embodiment , the fixation body 41 is a bone plate with at least one through - bore for a bone screw 14 . while various descriptions of the present invention are described above , it should be understood that the various features can be used singly or in any combination thereof . therefore , this invention is not to be limited to only the specifically preferred embodiments depicted herein . further , it should be understood that variations and modifications within the spirit and scope of the invention may occur to those skilled in the art to which the invention pertains . accordingly , all expedient modifications readily attainable by one versed in the art from the disclosure set forth herein that are within the scope and spirit of the present invention are to be included as further embodiments of the present invention . the scope of the present invention is accordingly defined as set forth in the appended claims .
the invention is related to a fastener and a bone fixation assembly for the internal fixation of vertebral bodies . the bone fastener includes a shank having a longitudinal axis , a head positioned adjacent the top end of the shank and having at least two portions separable in the axial direction . the head includes a bottom portion connected to the shank and a top portion connectable to the bottom portion . the bottom portion defines a socket situated coaxially to the longitudinal axis to accept a driver to rotate the fastener , and the top portion has an exterior with a convex design .