in fig1 and 2 , a bone anchoring device according to a first embodiment includes a bone anchoring element 1 in the form of a bone screw having a shank 2 that is at least partially provided with a bone thread and a head 3 . the head 3 has a spherical segment - shaped outer surface portion including a greatest outer diameter e of the sphere and a flat free end with a recess 4 for engagement with a screwing - in tool . the bone anchoring device further includes a coupling assembly 5 for receiving a stabilization rod 100 and for coupling the stabilization rod 100 to the bone anchoring element 1 . the coupling assembly 5 includes a receiving part 6 for receiving the head 3 of the bone anchoring element 1 and for receiving the rod 100 , a pressure element 7 configured to be arranged in the receiving part 6 and a rod receiving element 8 also configured to be arranged in the receiving part 6 . the pressure element 7 serves for locking the head 3 in the receiving part 6 . the rod receiving element 8 serves for receiving the rod 100 and for acting onto the pressure element 7 to lock the head 3 . at least one pin 9 may also be provided for cooperating with the rod receiving element 8 . further , a locking element 10 in the form of an inner screw is provided for securing the rod 100 in the receiving part 6 and in the rod receiving element 8 and for exerting a force via the rod receiving element onto the pressure element 7 to lock the head 3 in the receiving part 6 . the receiving part 6 will now be explained with reference to fig1 to 8 . the receiving part 6 has a first end 6 a that is a top end and an opposite second end 6 b that is a bottom end , a central axis of symmetry c passing through the first end 6 a and the second end 6 b . a bore 61 is provided that is coaxial to the central axis c . in a first region adjacent to the first end 6 a , the receiving part 6 has as substantially u - shaped recess 62 with a bottom directed towards the second end 6 b and two free lateral legs 62 a , 62 b extending towards the first end 6 a . in the region of the legs 62 a , 62 b an internal thread 63 is provided that cooperates with the locking element 10 . the channel formed by the u - shaped recess 62 is sized so as to receive the rod 100 therein for connecting a plurality of bone anchoring devices . in the region of the legs 62 a , 62 b up to substantially a height in axial direction that is defined by the bottom of the u - shaped recess 62 , the bore 61 has a first inner diameter . in a region below the legs 62 a , 62 b , the bore 61 has a widened portion 61 a with a diameter greater than the first inner diameter of the first portion . between the second end 6 b and the widened portion 61 a , the bore 61 has a narrowing portion 61 b that tapers towards the second end 6 b with a cone angle . an opening 64 is provided at a second end 6 b , the diameter of the opening 64 being larger than the largest diameter e of the head 3 to allow the insertion of the head 3 from the second end 6 b of the receiving part 6 . the widened portion 61 a and the narrowing portion 6 lb define an accommodation space for the head 3 of the bone anchoring element 1 . at a distance from the first end 6 a below the internal thread 63 a circumferentially extending first groove 66 is provided in the inner surface of each of the legs 62 a , 62 b . the first groove 66 serves for engagement with a retaining portion of the rod receiving element 8 as described below . a circumferentially extending second groove 67 may be provided at the lower end of the internal thread 63 that serves also for engagement with the retaining portion of the rod receiving elements . at one of the legs 62 a , 62 b a transverse bore 68 is provided that extends through the leg , for example through the leg 62 b , in a direction substantially perpendicular to the central axis m for receiving the pin 9 . the bore 68 is located approximately at a center of the leg . the pin 9 preferably has such a length that once inserted into the bore 68 , the pin 9 extends a short distance into the bore 61 to provide a stop for the rod receiving element 8 as described below . the pin 9 may be flush with the outer surface of the receiving part 6 when inserted . as can be seen in particular in fig1 and 9 to 12 , the pressure element 7 has a first end with a free end surface 7 a and a second end 7 b . the second end 7 b of the pressure element 7 is configured to be closer than the first end of the pressure element 7 to the second end 6 b of the receiving part 6 when the pressure element 7 is arranged in the receiving part 6 . adjacent to the second end 7 b the pressure element 7 has a cap - like flexible portion 71 that has a hollow interior chamber 72 that is substantially spherical segment - shaped . the cap - like portion 71 is open at the second end 7 b to allow the insertion of the head 3 of the bone anchoring element 1 . an outer surface of the cap - like portion 71 has a narrowing portion 71 a adjacent to the second end 7 b that tapers in a direction towards the second end 7 b . the taper corresponds substantially to a taper of the narrowing portion 61 b of the receiving part 6 . the narrowing portion goes over in a substantially spherical segment - shaped outer surface portion 71 b . at one circumferential position there is an internal recessed portion 73 adjacent to the second end so that the wall of the cap - like portion 71 is thinner at the recessed portion 73 than at the remaining portions . a width and height of the recessed portion 73 is such that the shank 2 of the bone anchoring element can extend therein . this allows the shank 2 of the bone anchoring element 1 to be pivoted to a greater angle in the direction of the recessed portion 73 than in the opposite direction when the bone anchoring element 1 is inserted . hence , the pivot angle is enlarged to one side . the enlargement can be , for example , approximately 10 °, however , other enlarged pivot angles may be contemplated , depending on the wall thickness and geometry . at a distance from the second end 7 b a circumferentially extending slit 74 is provided . the slit 74 extends circumferentially around the central axis c of the receiving part 7 along a plane substantially perpendicular to the central axis c when the pressure element 7 is arranged in the receiving part 6 . further , the slit 74 extends around more than 180 °, preferably more than 270 ° and further preferably more than 340 ° around the central axis c . therefore , by means of the slit 74 , a ring - shaped portion at the second end is provided that is integrally connected to the rest of the pressure element 7 by a wall portion forming a connecting strip 76 . the connecting strip 76 has such a length in the circumferential direction that it provides a stable connection of the ring - shaped portion to the rest of the pressure element 7 . at one end of the circumferentially extending slit 74 , there is a substantially vertical slit 77 that extends from the second end fully through the ring - shaped portion 75 into the slit 74 . by means of this , the ring - shaped portion is cut through or split in a circumferential direction and forms a slit ring 75 that can be expanded and compressed in a radial direction . a width of the vertical slit 77 is preferably smaller than the width of the circumferential slit 74 , the outer surface of the slit ring 75 is tapered and forms the narrowing outer surface portion 71 a towards the second end of the pressure element 7 . a position and size of the slit ring 75 is such that when the head 3 of the bone anchoring element 1 is inserted from the open end of the cap - like portion 71 , the slit ring 75 expands so that the width of the vertical slit 77 becomes larger , and when the head 3 has been fully inserted into the hollow interior chamber 72 , the slit ring 75 encompasses the head 3 at the largest diameter e and below the largest diameter e of the head 3 in a direction towards the shank 2 . a maximum outer diameter of the cap - like portion 71 is slightly smaller than the inner diameter of the first portion of the bore 61 of the receiving part 6 and is also smaller than the diameter of the bore in the widened portion 61 a . hence , the slit ring 75 can expand in the widened portion 61 a of the receiving part 6 . at the side of the cap - like portion 71 that is opposite to the second end 7 b a conically - tapered portion 78 is provided that tapers towards the first end surface 7 a and that has a conical bore 78 a that allows access to the recess 4 of the head 3 of the anchoring element 1 . the conical portion 78 is of such a height and outer dimension that it can pass through a bottom opening of the rod receiving element 8 described below . at a distance from the first end surface 7 a an annular projection 78 b is formed at the conical segment - shaped portion 78 that has a flat upper surface and an inclined lower surface . an outer circumferential surface 78 c of the projection 78 b is substantially cylindrical . at the transition of the cap - like portion 71 and the conical segment - shaped portion 78 there is a conically widening collar 79 that widens towards the first end surface 7 a and that has a flat upper surface 79 a forming an abutment for cooperating with a portion of the rod receiving element 8 . the conical collar 79 extends in a radial direction beyond the first end surface 7 a and also beyond the projection 78 b . the flat upper surface 79 a is located in axial direction below the first end surface 7 a of the pressure element 7 . the structure and / or size of the collar 79 is such that the collar 79 is at least slightly deformable in axial direction when an axial load acts upon the upper flat surface 79 a . in this case , the collar is spread outwards . the deformation of the collar may be a resilient deformation . fig1 shows a modified embodiment of the pressure element . the pressure element 7 ′ differs only with respect to the transverse slit in the cap - like portion . the cap - like portion 71 ′ has a slit 74 ′ that extends from a substantially vertical slit 77 ′ in a helix - shaped manner at least partially around the central axis . in a further modification , the slit 77 ′ is not substantially vertical but is just a portion of the helical slit 74 ′. another modified embodiment of the pressure element is shown in fig1 . the pressure element 7 ″ differs from the pressure element according to the embodiment shown in fig9 to 12 by the shape of a conical collar . the conical collar is interrupted at regular distances by slits 79 b to form circumferential collar portions 79 ′, in the embodiment shown , four collar portions 79 ′. with the number of slits and the size and thickness of the collar portions a specific flexibility of the collar 79 ′ can be achieved . a further modified embodiment of the pressure element is depicted in fig1 . the pressure element 7 ′″ differs from the pressure element shown in fig9 to 12 by the design of the cap - like portion . the cap - like portion 71 ″ comprises a plurality of vertical slits 77 ″ that are spaced equidistantly . the vertical slits 77 ″ are open towards the second end 7 b and are closed by a widened circular portion 77 a in the direction towards the first end of the pressure element 7 ′″. the number and size of the slits is selected to achieve a specific flexibility of the cap - like portion 71 ″. furthermore , the pressure element 7 ′″ according to this embodiment does not have the annular projection 78 b at the conical segment shaped portion 78 . it shall be understood that many different combinations of features of the various embodiments of the pressure element can be combined to generate various other embodiments of the pressure element . referring now to fig1 and fig1 to 19 , the rod receiving element 8 is a part that is separate from the pressure element 7 . it is a substantially cylindrical part that comprises a first end with a free end surface 8 a and an opposite second end with a free end surface 8 b . adjacent to the first end with the first end surface 8 a there is a substantially rectangular recess 81 that cuts out such a portion of the cylinder that two free upstanding legs 81 a , 81 b remain , the free end surface of which forms the first end surface 8 a . on the second end 8 b , there is an opening 82 , preferably a circular opening , that extends through the rod receiving element 8 into the recess 81 . an inner diameter of the opening 82 is smaller than an outer diameter of the first end surface 7 a of the pressure element so that the first end surface 7 a of the pressure element can extend through the opening 82 into the rod receiving element 8 . furthermore , the inner diameter of the opening 82 is substantially the same as or has a slight undersize compared to the outer diameter of the cylindrical outer surface portion 78 c of the annular projection 78 b provided at the conical segment - shaped portion 78 of the pressure element 7 . hence , the pressure element 7 can be held together with the rod receiving element 8 when the conical portion 78 extends through the opening 82 of the rod receiving element 8 and the cylindrical outer surface portion 78 c of the projection 78 contacts the inner surrounding surface of the rod receiving element 8 at the opening 82 in an interference - fit manner . on the bottom of the recess 81 between the legs 81 a and 81 b two concave cylinder - segment shaped rod supporting projections 83 a , 83 b are provided on both sides of the bore forming the opening 82 . the radius of the concave portion 83 a , 83 b is preferably selected such that it is adapted to the radius of the specific rod to be used . the rod supporting projections 83 a , 83 b may be separated from the legs 83 a , 83 b by a groove 84 on each side of the projection . the width of the recess 81 is such that the rod 100 can be received in the recess 81 . when the rod 100 is inserted into the recess 81 of the rod receiving element 8 , it can rest on the rod supporting projections 83 a , 83 b . a height of the legs 81 a , 81 b is such that the first end surface 8 a is located slightly above the upper rod surface when the rod 100 is placed into the channel 81 and rests on the rod supporting projections 83 a , 83 b . an outer diameter of the rod receiving element 8 is only slightly smaller than an inner diameter of the bore 61 of the receiving part 6 so that the rod receiving element 8 can move in the bore 61 and is guided therein . the rod receiving element 8 further comprises at each leg 81 a , 81 b an elongate recess 85 that extends substantially from the bottom of the recess 81 to a distance from the first end surface 8 a . the recesses 85 extend completely through each leg 81 a , 81 b in a radial direction . the recesses 85 have a width so that the pin 9 can be accommodated therein . a bottom end 85 a of each elongate recess 85 forms a stop for the pin 9 that extends into the respective recess . the stop prevents the rod receiving element 8 from escaping through the first end 6 a of the receiving part 6 when the pressure element 7 is in an inserting position . furthermore , the recesses 85 may serve together with the at least one pin 9 as a securing device to maintain the alignment between the recess 62 of the receiving part 6 and the channel 61 of the rod receiving element 8 . the rod receiving element 8 also has on each leg at approximately the center of each leg in an axial direction , a circumferentially extending projection 86 with a flat upper surface 86 a and an inclined lower surface 86 b the inclination of which is such that the projection 86 can move out of the grooves 66 , 67 when the rod receiving element is pushed downwards towards the second end 6 b of the receiving part 6 . the shape of the projection 86 is such that as can be seen in fig2 a and 25 , the projection 86 fits into the groove 66 , 67 , respectively and can abut with the upper flat surface 86 a against the upper edge of the groove 66 , 67 . furthermore , on each leg a longitudinally extending cylinder segment - shaped recess 87 is provided that extends from the bottom of the recess 81 to the first end surface 8 a . the bone anchoring device as a whole or in parts is made of a bio - compatible material , such as a bio - compatible metal or metal alloy , for example titanium , stainless steel , a nickel - titanium alloy , for example , nitinol , or of bio - compatible plastic materials , such as , for example , polyetheretherketone ( peek ). referring to fig2 a to 20 c , the coupling assembly may be pre - assembled in the following manner . the rod receiving element 8 and the pressure element 7 are inserted into the receiving part 6 . thereby the pressure element may be inserted through the lower opening 64 of the receiving part 6 . the rod receiving element 8 may be inserted from the first end 6 a of the receiving part by slightly pressing the legs 81 a , 81 b together until the projection 86 snaps into the second groove 67 . the pressure element 7 is in the first embodiment shown in fig2 a and 20 b temporarily held in the rod receiving element 8 by an interference fit . this is achieved by the cylindrical surface portion 78 c of the projection 78 b on the pressure element 7 that has slight oversize with respect to the inner surface of the rod receiving element 8 at the opening 82 . the rod receiving element 8 is prevented from escaping through the first end 6 a of the receiving part during insertion of the pressure element because the flat upper surface 86 a abuts against the upper edge of the second groove 67 in the receiving part 6 . the pressure element 7 is also in its uppermost position , where the flat surface 79 a of the collar 79 abuts against a second end surface 8 a of the rod receiving element 8 . by means of this arrangement , the cap - like portion 71 and in particular the slit ring 75 is located at a position where the coaxial bore 61 has the widened portion 61 a with the greatest inner diameter . as can be seen in fig2 a , the slit ring 75 has space to expand in this position . in an alternative embodiment , as shown in fig2 c in which the pressure element does not have the annular projection 78 b , for example with the pressure element 7 ′″ as depicted in fig1 , the pressure element 7 is not held in the insertion position by interference fit with the rod receiving element 8 but via snap connection of the outermost edge of the collar 79 that engages a third groove 69 at the inner side of the receiving part . the third groove 69 may be small and shallow as it is only necessary for holding the pressure element in the insertion position . referring to fig2 to 26 b , the use of the coupling assembly 5 together with a bone anchoring element 1 and a rod 100 will be explained . as depicted in fig2 , first , a suitable bone anchoring element 1 is implanted into the bone . in fig2 , the reference numeral 200 depicts the bone surface . because the bone anchoring element 1 can be placed into the implantation site without the coupling assembly being connected thereto , the step of implanting the bone anchoring element 1 is facilitated . thereafter , the coupling assembly 5 with the pressure element 7 and the rod receiving element 8 in the insertion position is placed onto the head 3 of the bone anchoring element i . as depicted in fig2 , the head 3 enters the receiving part 6 through the lower opening 64 and enters the cap - like portion 71 of the pressure element 7 through the open second end 7 b of the pressure element 7 . when the head 3 touches the slit ring 75 of the pressure element 7 , the pressure element 7 presses with the collar 79 against the second end surface 8 b of the rod receiving element and shifts the rod receiving element upward until the bottom 85 a of the elongate recess 85 of the leg 81 b abuts against the pin 9 extending through the leg 62 b of the receiving part 6 . hence , the rod receiving element 8 is prevented from being pushed out through the first end 6 a of the receiving part 6 , when the head is inserted . the pressure element 7 is rotatable in the receiving part . it may be placed such , that the recessed portion 73 that provides the enlarged pivot angle is at a specific position . to achieve this , an alignment feature ( not shown ) can be provided that allows to rotate the pressure element such that the recessed portion 73 assumes the desired position . further introduction of the head 3 into the cap - like portion 71 expands the slit ring 75 within the widened portion 61 a of the bore 61 of the receiving part 6 . the head 3 can then be completely inserted . because the slit ring 75 does not expand at the connection strip 76 , the insertion of the head 3 may not be precisely coaxial with the central axis c but instead may be slightly out of or misaligned with the central axis c . by the further introduction of the head 3 , the slit ring 75 may be expanded to a maximum extent and allows the head 3 to enter the upper portion of the hollow interior chamber 72 until the head 3 abuts against the inner wall of the chamber 72 . here , the slit ring 75 can elastically contract around the head 3 as shown in fig2 . when the head 3 abuts against the inner wall of the cap - like portion 71 in the upper region , the slit ring 75 encompasses a portion of the head 3 below the region with the largest diameter b as can be seen in fig2 . in this condition , the slit ring 75 frictionally clamps the head 3 , so that the bone anchoring element 1 can be pivoted to a desired angular position relative to the receiving part 6 and can be held there by the friction fit between the pressure element 7 and the head 3 . finally , pulling the receiving pan 6 and pressing down the rod receiving element 8 with an instrument ( not shown ) as depicted in fig2 , presses the slit ring 75 into the narrowing portion 61 b , so that the tapered outer surface 71 a of the slit ring 75 engages the narrowing portion 61 b of the receiving part 6 . simultaneously , the projection 86 snaps into the first groove 66 of the receiving part 6 . in this condition , as depicted in fig2 , the rod receiving element 8 is . prevented from moving upward because of the stop provided by the upper edge of the groove 66 against which the flat upper surface 86 a of the projection 86 of the rod receiving element 8 abuts . the head 3 is already clamped by the cap - like portion 71 of the pressure element 7 . because the slit ring 75 is located between the head 3 and the narrowing portion 61 b of the receiving part , the lower opening 64 of the receiving part 6 is narrowed and the head 3 is prevented from falling out or being pushed out through the lower opening 64 . this is the pre - locking condition . in clinical use , usually at least two bone anchoring devices are inserted into the bone and the receiving parts are aligned . because the head 3 of the bone anchoring element 1 is held in the pressure element by a frictional force , the receiving part 6 can be easily aligned manually and its angular position is maintained by the frictional force between the head and the pressure element 7 . then , as can be seen in fig2 , the rod 100 is inserted into the receiving part 6 and the locking element 10 is screwed between the legs 62 a , 62 b of the receiving part 6 . the rod 100 first is supported by the support surfaces 83 a , 83 b of the rod receiving element 8 . tightening of the locking element moves the rod receiving element 8 slightly downward while the locking element 10 only contacts the first end surface 8 a of the rod receiving element 8 and exerts a force on the first end surface 8 a . the force f is transferred via the second end surface 8 b onto the upper flat end surface 79 a of the collar 79 of the pressure element 7 onto the head 3 and from the head 3 onto the slit ring 75 and the receiving part as depicted in fig2 . by means of this , the head 3 is locked in its angular position with respect to the receiving part while the rod 100 is still movable in the direction of the rod axis . referring now to fig2 a and fig2 b , a further advancement of the locking element 10 results in the second end surface 8 b exerting a larger force f onto the upper surface 79 a of the collar 79 of the pressure element 7 so that the collar 79 is slightly deformed , as can be seen in fig2 b . by means of this , the rod receiving element 8 can move slightly further downwards , so that the locking element 10 presses onto the rod . thereby , the rod is fixed . in this condition , the force is transmitted through the rod and the first end surface 7 a of the pressure element 7 onto the head 3 and the receiving part 6 . in the locked condition , as shown in fig3 and 4 , the anchoring element 1 may assume a greater pivot angle when it is abutting the recessed portion 73 of the pressure element with the shank than in an opposite direction . if the position of the rod 100 relative to the receiving part 6 has to be corrected , the locking element 10 is rotated backwards until the pressure onto the rod is relieved and the rod becomes freely movable again . because the slit ring 75 is held in the narrowing portion of the receiving part , the locking of the head 3 is maintained . fig2 shows a kit of a bone anchoring device with a coupling assembly comprising at least two rod receiving elements 8 , 8 ′. the coupling assembly 5 comprising the receiving part 6 , the pressure element 7 and the rod receiving element 8 is the same as previously described and can have all the modifications as previously described . the rod receiving element 8 is a first rod receiving element and is sized and configured to receive the rod 100 that is a first rod having a first outer diameter . the rod receiving element 8 ′ is a second rod receiving element that is sized and configured to receive a second rod 100 ′ that has a second diameter smaller than the first diameter of the first rod 100 . consequently , rod support surfaces 83 a ′, 83 b ′ of the second rod receiving element 8 ′ may have a curvature that corresponds to the second rod 100 ′ having the second diameter . furthermore , a height of the upstanding legs 81 a ′, 81 b ′ of the second rod receiving element 8 ′ is smaller than a height of the legs 81 a , 81 b of the first rod receiving element 8 , so as to project only slightly over the upper rod surface of the second rod 100 ′ when the second rod 100 ′ is inserted into the rod receiving element 8 ′. an outer diameter in size and an inner diameter is the same as for the first rod receiving element 8 . fig2 shows the bone anchoring device which has the second rod receiving element 8 ′ and which is coupled to the second rod 100 ′. fig2 shows the first bone anchoring device which has the first rod receiving element 8 and is coupled to the first rod 100 . because the height of the second rod receiving element 8 ′ is smaller than the height of the first rod receiving element 8 , the locking element 10 must be screwed deeper in between the legs as depicted in fig2 compared to the case in which the first diameter rod 100 is used as depicted in fig2 . by means of the kit , with a single bone anchoring device , various rods with various diameters can be used by selecting the appropriate rod receiving element . it is even possible to use a rod with sections of different diameters , for example , a rod that has a first diameter in one section and a second diameter smaller than the first diameter in another section . in this case , the first bone anchoring device has a first rod receiving element with a greater height and a second bone anchoring device has a second rod receiving element with a smaller height . the kit opens a variety of applications and provides the advantage of a modular system . for example , for the bone anchoring element , all kinds of anchoring elements can be used and combined with the coupling assembly . these anchoring elements are e . g . screws of different lengths , with different diameters , cannulated screws , screws with different thread forms , nails , hooks , etc . the head and the shank may also be separate parts that are connectable to each other . additionally , a kit of the coupling assembly with at least one further pressure element can be provided . the pressure element can be designed without a recess for an enlarged pivot angle or can have several recesses , for example two or more equidistantly spaced recesses for having enlarged pivot angles in two or more directions . by combining the elements like the pressure element and the rod receiving element with a receiver , the most suitable bone anchoring device for a specific clinical application can be easily generated . a second embodiment of the coupling assembly is described with reference to fig3 . fig3 shows the bone anchoring device with the anchoring element 1 , the rod 100 and the locking element 10 as in the previous embodiments . the pressure element 700 differs from the pressure element of the previous embodiments in that it is free from a deformable portion that cooperates with the rod receiving element and does not have the annular projection for connecting it in an interference - fit manner to the rod receiving element . instead of this , the pressure element 700 comprises a non - deformable annular projection 790 with a flat upper surface 790 a that cooperates with a collar - like conically widening projection 890 at the second end of the rod receiving element 800 . the collar like projection 890 has a substantially inverse shape compared to the collar 79 as described in the previous embodiments . it is slightly flexible when it engages the flat upper surface 790 a of the pressure element 700 when a load acts onto the rod receiving element 800 via the locking element 10 . furthermore , the rod receiving element 800 comprises at its second end 8 b an inner projection 880 with a conical outer surface that is configured to engage the conical outer surface of the conical portion 78 of the pressure element 700 in order to temporarily hold the pressure element 700 . in this embodiment , the deformation takes place at the rod receiving element 800 . the effect is the same as in the previously described embodiments , namely that the rod receiving element can move slightly downward when the locking element 10 is tightened so that the first end surface 7 a of the pressure element moves upward to contact the rod and the locking element 10 can engage the rod . various modifications of the embodiments described before may be contemplated . the receiving part is not limited to the exact shape as shown . the recess 61 does not have to have an exact u - shape . the bore 61 can have several sections with different widths , as long as the enlarged portion that provides space for expansion of the pressure element is provided . the narrowing portion at the bottom of the receiving part is shown to be tapered , but can also be rounded . also , the external surface of the cap - like portion at the bottom end of the pressure element can be rounded , combinations of the surfaces of receiving part and pressure element that cooperate to clamp the head can be tapered and tapered , tapered and rounded and vice - versa rounded and rounded . the pressure element may have a cylindrical portion instead of the conical portion 78 . the head and correspondingly the interior chamber and the exterior surface of the cap - like portion of the pressure element can have another shape . for example two opposite flattened portions may be present that render the pivot connection monoplanar . all kinds of rods can be used . while rods with a smooth surface are shown , roughened rods or rods having a structure may be used . the rods may also be flexible rods . while a number of different embodiments are disclosed herein , it is appreciated that different components from the different embodiments can be mixed and matched to produce a variety of still other different embodiments . while the present invention has been described in connection with certain exemplary embodiments , it is to be understood that the invention is not limited to the disclosed embodiments , but is instead intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims , and equivalents thereof .
an instrument for inserting a bone anchoring element into a bone is provided , the instrument including a shaft having an end portion for engaging the bone anchoring element and for transferring torque to the bone anchoring element and a longitudinal axis defining an axis of rotation ; a cutting member connected to the shaft with cutting portions that are configured to cut bone material ; wherein the cutting member has an outer diameter defined by the cutting portions that is substantially the same or larger than an outer diameter of a receiving part of the polyaxial bone anchor in a region at a bottom end of the receiving part .