as shown in fig1 and 2 , a bone anchoring device according to a first embodiment comprises a bone anchoring element 1 in the form of a bone screw having a threaded shaft 2 and a head 3 , which is in this embodiment a spherical segment - shaped head . the head 3 has a recess 4 for engagement with a screwing - in tool . the bone anchoring device further comprises a receiving part body 5 for receiving a rod 6 to connect the rod 6 to the bone anchoring element 1 . further , a closure element 7 in the form of an inner screw is provided for securing the rod 6 in the receiving part body 5 . in addition , the bone anchoring device includes a pressure element 8 for locking the head 3 in the receiving part body 5 . the pressure element 8 is held in the receiving part body 5 , for example via pins 9 a , 9 b . the receiving part body 5 is now explained with reference to fig1 and 8 to 14 . the receiving part body 5 comprises a first end 10 and a second end 11 opposite the first end and an axis of symmetry m passing through the first end and the second end . a bore 12 is provided which is coaxial with the axis of symmetry m . in a first region adjacent to the first end 10 , the receiving part body 5 has a u - shaped recess 13 , which is symmetric with respect to the axis of symmetry m , the recess 13 having a bottom directed towards the second end 11 and two free lateral legs 14 a , 14 b extending towards the first end 10 . in the region of the legs 14 a , 14 b an internal thread 15 , which cooperates with the inner screw 7 , is provided . a channel formed by the u - shaped recess 13 is sized so as to receive the rod 6 therein , the rod to connect a plurality of anchoring devices . at a second region near the second end 11 , a cylindrical portion of the bore 12 is followed by a first tapered portion 16 tapering towards the second end 11 with a cone angle . the first tapered portion 16 is followed by a second tapered portion 17 tapering towards the second end 11 with a cone angle smaller than the cone angle of the first tapered portion . an opening 18 is provided at the second end 11 . a diameter of the opening 18 is larger than a diameter of the head 3 and smaller than the inner diameter of the bore 12 . the coaxial bore 12 provides an accommodation space 19 for the screw head 3 . on each leg 14 a , 14 b , bores 20 a , 20 b extending through the legs 14 a , 14 b are provided for receiving the pins 9 a , 9 b . the bores 20 a , 20 b are located approximately at a center of each leg . as can be seen in fig1 and 3 to 7 , the pressure element 8 includes a first portion 81 , which is substantially cylindrical and which has an outer diameter which is slightly smaller than the inner diameter of the bore 12 , so that the pressure element 8 is movable in the bore 12 , and a second portion 82 which is recessed with respect to the first portion and which has a maximum outer diameter which is smaller than the inner diameter of the bore 12 . the first portion 81 has at its end opposite to the second portion 82 a u - shaped recess 83 for receiving the rod 6 therein when the pressure element 8 is situated in the receiving part body 5 . at the lateral sides of the u - shaped recess the pressure element comprises two elongate recesses 84 in the outer wall which are arranged opposite to each other , and which are aligned such that a longer side of the elongate recesses 84 is parallel to the axis of symmetry m of the receiving part body 5 . as can be seen in fig4 , the width of the lateral sides of the first portion 81 decrease towards the end opposite to the second portion 82 for at least a part of the first portion 81 . the second portion 82 is formed similar to a collar , and comprises a first end at the side of the first portion 81 which is recessed with respect to the first portion 81 and a second end , and a hollow interior 85 which is substantially spherically shaped to clamp the spherical head 3 therein . the second end of the second portion 82 has an opening 86 for introduction of the head 3 . an outer wall of the second portion 82 comprises a first portion 87 which is spherical and a second portion 88 adjacent to the opening 86 which is tapered . the tapered portion 88 of the pressure element cooperates with the second tapered portion 17 of the receiving part body . the second portion 82 of the pressure element further comprises a plurality of slits 89 extending from an edge of the opening 86 through the second portion 82 . the number and dimensions of the slits 89 are such that the wall of the second portion is flexible enough to snap onto the head 3 when the head 3 is being inserted . further , the pressure element comprises a coaxial bore 90 for providing access to the screw head 3 by a tool . as can be seen in fig8 to 14 , since the pressure element has a maximum outer diameter which is slightly smaller than the inner diameter of the bore 12 , it can be inserted from the first end 10 of the receiving part body 5 into the receiving part body 5 . once the pressure element 8 is inserted , the pins 9 a , 9 b are introduced in the bores 20 a , 20 b so that they engage in the elongate recesses 84 , thereby holding the pressure element 8 in the receiving part body 5 and with the u - shaped recess 83 aligned with the u - shaped recess 13 of the receiving part body 5 , and preventing escaping of the pressure element through the first end 10 . since the second portion 82 of the pressure element 8 has an outer diameter which is smaller than the inner diameter of the bore 12 , and since the second portion 82 of the pressure element has a flexible outer wall , the second portion can expand within the accommodation space 19 when the screw head 3 is introduced . all parts described before may be made of a bio - compatible material , such as a bio - compatible metal like stainless steel or titanium , or a bio - compatible metal alloy , for example , nitinol , or are made from a bio - compatible plastic material , for example , polyetheretherketone ( peek ). the steps of assembling the bone anchoring device are now explained with respect to fig8 to 14 . first , when the pressure element 8 is introduced into the receiving part body 5 and held by the pins 9 a , 9 b , it is oriented such that the opening 86 is directed towards the second end 11 of the receiving part body 5 . as shown in fig8 , the pressure element 8 is in its uppermost position which is an insertion position for allowing the screw head 3 to be inserted . in the insertion position , the flexible second portion of the pressure element is positioned freely in the accommodation space 19 . the insertion position is limited against upward movement by an abutment which is in this embodiment provided by the engagement of the pins 9 a , 9 b with the lower end of the recesses 84 . as shown in fig9 , the screw head 3 is then inserted through the opening 18 into the accommodation space 19 , where the second portion 82 of the pressure element 8 is located . the screw head 3 is introduced into the hollow interior 85 of the second portion 82 of the pressure element through the opening 86 . due to the flexibility of the wall provided by the slits 89 , the second portion 82 expands in the accommodation space 19 as shown in fig9 . the second portion 82 snaps onto the screw head 3 until it encompasses the screw head 3 as shown in fig1 . the pins also prevent the pressure element 8 from escaping through the open first end 10 when the screw head 3 is inserted . in the insertion position the screw head 3 is pivotable in the receiving part and also removable if the pressure element remains in this position . there may be means ( not shown ) for holding the pressure element temporarily in the insertion position . alternatively , the pressure element can be held in the insertion position by means of a tool ( not shown ). fig1 and 12 show a pre - locking position of the pressure element . when the screw head 3 is inserted and the pressure element 8 is pressed down slightly , the outer wall of the second portion 82 of the pressure element is guided by the first tapered portion 16 , so that it contacts the second tapered portion 17 of the receiving part body . the tapered portion 88 of the outer wall of the pressure element 8 and the tapered portion 17 of the receiving part body 5 come into engagement , whereby a pre - stress is exerted by the second portion 82 of the pressure element onto the screw head 3 , which pre - locks the head 3 . pre - locking means that under conditions arising during surgery , an angular position of the bone anchoring element 1 with respect to the receiving part body 5 is maintained and can be loosened only by exerting an additional force onto the receiving part body or the bone anchoring element . the pre - locking position of the pressure element can be achieved either manually by pushing down the pressure element , or by inserting the rod and screwing in the inner screw 7 so that the rod presses onto the pressure element thereby moving it slightly downwards . fig1 a and 13 b show the engagement of the outer surface portion of the pressure element and the inner surface portion of the receiving part body in the pre - locking condition . as described before , the surfaces 17 , 88 can be tapered as shown in fig1 a . alternatively to , or in addition to , tapered surfaces , portions of the surfaces 88 ′, 17 ′ can be rounded as shown in fig1 b . the first tapered portion 16 at the receiving part body 5 serves only for guidance in some embodiments and can be omitted . fig1 c to 13 h schematically show various examples of engagements of the outer surface portion of the pressure element and the inner surface portion of the receiving part body . in fig1 c the two tapered portions 17 , 88 are tapered at substantially a same angle . this provides a substantially even pressure distribution ( approximated by arrows ) between the second portion 88 of the pressure element and the tapered portion 17 of the receiving part body . fig1 d and 13 e show two different variations where the portions are tapered at different angles . fig1 d shows a main contact area at the bottom of portion 17 ″ and fig1 e shows a main contact area at the top of portion 17 ′″. fig1 f shows a tapered portion 88 ″″ of the pressure element and a rounded or curved portion 17 ″″ of the receiving part body , wherein the curvature is directed towards the center of the receiving part body . with such a configuration , a focused contact area at the rounded portion can be achieved . in fig1 g and 13 h the narrowing portion 170 of a modified example of the receiving part body 5 has a double spherical radius formed by two curved portions 170 a and 170 b adjacent to each other with a groove 170 c therebetween . the curvatures are directed to a central axis m . a modified pressure element 180 has at its lower end two correspondingly curved portions 180 a and 180 b which correspond to the curved portions 170 a and 170 b , with a crest 180 c therebetween , and an outer crest 180 d at an outer edge of the pressure element 180 . as shown in fig1 g , when the pressure element 180 moves downwards , its lowermost edge 180 d engages in the groove 170 c . in this position , there is a frictional clamping of the head 3 , which still allows the head 3 to be pivoted upon exertion of a force which is greater than the force needed to pivot the head 3 when the head 3 is introduced in the insertion position . this is an example of the pre - locking condition . as shown in fig1 h , by pressing the pressure element 180 further downward , the respective curvatures 180 a , 180 b of the pressure element 180 engage with the corresponding curvatures 170 a , 170 b of the narrowing portion 170 to lock the head 3 . the third position of , for example , the embodiment of fig8 , which is a locking position , is shown in fig1 . the locking position is defined as the position in which the screw head 3 is locked in the receiving part body 5 . in this position , the tapered outer surface portion 88 of the pressure element engages the second tapered portion 17 of the receiving part body over a larger area . thereby , the flexible wall of the second portion 82 of the pressure element 8 is pressed onto the head 3 so that the head 3 is locked . this position is achieved by further tightening the inner screw 7 , thereby further pressing down the rod 6 . in use , the bone anchoring device can be , for example , pre - assembled in such a way that a suitable bone anchoring element 1 is selected and introduced into the receiving part body 5 with an inserted pressure element 8 . the bone anchoring device can be brought into the pre - locking condition by slightly pushing down the pressure element 8 . then , the screw element 1 is screwed into the bone . the recess 4 of the head 3 can be accessed with a screw tool through the coaxial bore 12 and the bore 90 . to correctly align the receiving part with respect to the rod to which it shall be connected , an additional force is exerted onto the receiving part either manually or by application of an instrument . once a desired position of the rod with respect to other bone anchoring devices is achieved , the inner screw 7 is tightened , thereby shifting down the pressure element into the locking position . a final tightening of the inner screw locks the rod and the head substantially simultaneously . the bone anchoring device can be pre - assembled either by the manufacturer or in the course or preparation of surgery , or at any other time . in one embodiment , the surgeon can select the desired receiving parts and bone anchoring elements prior to surgery according to specific details of the particular clinical application . the design of the bone anchoring device allows selection of an appropriate bone anchoring element in terms of diameter , length , and other features . hence , a modular system is provided which includes various receiving parts and several bone anchoring elements , which then individually can be chosen and adapted . a second embodiment of the bone anchoring device is now described with respect to fig1 to 26 . parts which are similar to those of the first embodiment have the same reference numerals , and the descriptions thereof will not be repeated . the second embodiment of the bone anchoring device differs from the first embodiment in the design of the pressure element and the pins . the first portion 81 ′ of the pressure element 8 ′ differs from the first portion 81 of pressure element 8 . the first portion 81 ′ of the pressure element 8 ′ has a shallow recess 83 ′ which is curved in a substantially cylinder segment shape for guiding the rod 6 . it further includes two upstanding resilient fingers 91 a , 91 b extending from respective lateral sides of the recess 83 ′ in the direction of the first end 10 of the receiving part body . the fingers 91 a , 91 b have outwardly directed portions 92 at their free ends , respectively . an outer diameter of the first portion 81 ′ at least in the region of the fingers , is smaller than the inner diameter of the bore of the receiving part body , such that the resilient fingers 91 a , 91 b can move . pins 9 a ′, 9 b ′ each have a flattened portion 93 , a width of which is substantially the same as a width of the outwardly extending portions 92 of the resilient fingers 91 a , 91 b . when the pins 9 a ′, 9 b ′ are inserted into the bores 20 a , 20 b , the portions 93 extend into the coaxial bore 12 of the receiving part body 5 as shown in fig2 . in this manner , an abutment is provided for the receiving part 8 ′. the assembly of the bone anchoring device according to the second embodiment is explained with respect to fig2 to 25 . in fig2 , first , the pressure element 8 ′ is inserted from the first end 10 into the receiving part body 5 . then the pins 9 a ′, 9 b ′ are inserted into the holes in the legs . the pressure element 8 ′ is in such a position that the outwardly extending portions 92 of the resilient fingers 91 a , 91 b are located above the flattened portions 93 of the pins 9 a ′, 9 b ′. in this position the pressure element 8 ′ is in an insertion position . then , the screw head is inserted through the openings 18 and 86 until the second portion 82 of the pressure element 8 ′ is around the screw head 3 . then , as shown in fig2 , the pressure element 8 ′ is moved slightly downward to enter a pre - locking position . here , the resilient fingers 91 a , 91 b are first pressed inwards for the outwardly extending portion 92 to move past the flattened portion 93 of the pins . as shown in fig2 , the outwardly extending portions 92 then move outward and the flattened portions 93 form an abutment for the pressure element . in this pre - locking position , the tapered outer surface portion of the second portion 82 engages the tapered portion 17 of the receiving part body . since the pins 9 a ′, 9 b ′ act as a stop , the pre - locking position is secured ( i . e ., the pressure element 8 ′ does not move back to the insertion position ). inserting the rod and screwing in the inner screw 7 moves the pressure element further down until it achieves a final locking position as shown in fig2 . the clinical use of the bone anchoring device is similar to that of the first embodiment . further modifications of the embodiments shown are possible . for example , the head of the bone anchoring element can have any other shape , for example , a cylindrical shape whereby a monoaxial bone screw is provided allowing rotation of the screw element with respect to the receiving part around a single axis . the head 3 can also be , for example , conically shaped , where the hollow interior portion 85 of the pressure element is adapted to this shape . in a further modification , the receiving part body 5 , or at least the second portion 82 of the pressure element , is made of a bio - compatible plastic material which provides elasticity to a certain degree . in this case , the slits may be omitted . alternatively to the second embodiment a spring portion can be provided , for example in the receiving part body , which may cooperates with a portion of the pressure element . generally , the pins are only one example of achieving an abutment for the pressure element . other constructions such as snap rings or various other features may also be utilized . in one embodiment , the receiving part body can have an inclined open end to allow a greater angulation of the head in one direction . in one embodiment , the outer surface of the second portion 82 of the pressure element can be tapered , and the second tapered portion 17 of the receiving part body can be inwardly curved . 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 .
a receiving part for receiving a rod for coupling the rod to a bone anchoring element , the receiving part including : a receiving part body having a channel for receiving a rod and defining an accommodation space for accommodating a head of a bone anchoring element , the accommodation space having an opening for inserting the head ; and a pressure element at least partially in the accommodation space , the pressure element including a first portion having a recess to receive the rod , and a second portion having a flexible portion to clamp the head , wherein the pressure element is moveable along a longitudinal axis of the receiving part body from an inserting position to insert the head , to a pre - locking position to clamp the head in the receiving part by a pre - stress exerted by the pressure element , to a locking position to lock the head in the receiving part .