Coupling device for coupling a rod to a bone anchoring element and bone anchoring device with such a coupling device

Abstract

A coupling device is provided for coupling a rod to a bone anchoring element. The coupling device includes a receiving part having a first end, a second end, a central axis extending through the first end and the second end, and a channel for receiving a rod. The receiving part defines an accommodation space for accommodating a head of a bone anchoring element, the accommodation space having an opening for inserting the head. The coupling device further includes a pressure element arranged at least partially in the accommodation space, the pressure element having a flexible portion to clamp an inserted head, and a clamping element extending at least partially around the flexible portion of the pressure element. The clamping element is configured to move from a first position to a second position in which the clamping element exerts a clamping force onto the pressure element, where the movement includes rotating the clamping element around the central axis.

Claims

The invention claimed is: 1. A coupling device for coupling a rod to a bone anchoring element, the coupling device comprising: a receiving part having a first end and a second end, a central axis extending through the first end and the second end, a channel at the first end for receiving a rod, and an accommodation space for accommodating a head of a bone anchoring element, the accommodation space defining an opening for inserting the head; a pressure element configured to be located at least partially in the accommodation space, the pressure element comprising a flexible portion configured to expand to receive the head of the bone anchoring element, and a clamping element configured to extend at least partially around the flexible portion of the pressure element and exert a clamping force onto the pressure element, wherein when the pressure element and clamping element are located inside the receiving part, the clamping element is configured to rotate around the central axis to exert a clamping force onto the pressure element. 2. The coupling device of claim 1 , wherein the clamping force exerted onto the pressure element by the clamping element is greater in a second position of the clamping element than in a first position of the clamping element. 3. The coupling device of claim 2 , wherein the clamping element comprises an engagement portion for engagement with a tool to move from the first position to the second position to adjust the clamping force exerted onto the pressure element. 4. The coupling device of claim 1 , wherein the flexible portion of the pressure element has a cap-like shape and is configured to surround an inserted head. 5. The coupling device of claim 1 , wherein an advancement structure is provided that is configured to permit an axial advancement of the clamping element relative to the receiving part when the clamping element is rotated about the central axis. 6. The coupling device of claim 5 , wherein the advancement structure comprises a helical groove at an outer wall of the clamping element and an engagement member at an inner wall of the receiving part for engaging the helical groove. 7. The coupling device of claim 6 , wherein the engagement member comprises at least one pin configured to engage the helical groove. 8. The coupling device of claim 5 , wherein the advancement structure comprises a thread at an outer wall of the clamping element configured to cooperate with a thread at an inner wall of the receiving part. 9. The coupling device of claim 1 , wherein the pressure element and the clamping element are separate parts. 10. The coupling device of claim 1 , wherein the pressure element, the clamping element, and the receiving part are manufactured as a monolithic piece and are configured to be separated from each other at predetermined breaking points. 11. The coupling device of claim 1 , wherein the clamping element is shaped as a slotted ring configured to extend at least partially around the pressure element. 12. The coupling device of claim 11 , wherein the clamping element comprises at least one protrusion and/or at least one recess at a side facing the pressure element, the at least one protrusion and/or the at least one recess configured to cooperate with at least one recess and/or at least one protrusion of the pressure element to secure the clamping element in a first position. 13. The coupling device of claim 1 , wherein the pressure element comprises at least one axial slot to provide flexibility to the flexible portion. 14. The coupling device of claim 13 , wherein the pressure element further comprises at least one horizontal slot to provide flexibility to the flexible portion. 15. A bone anchoring device comprising: a bone anchoring element having a shaft and a head; and a coupling assembly for coupling a rod to the bone anchoring element, the coupling assembly comprising: a receiving part having a first end and a second end, a central axis extending through the first end and the second end, a channel at the first end for receiving the rod, and an accommodation space for accommodating the head, the accommodation space defining an opening for inserting the head; a pressure element configured to be located at least partially in the accommodation space, the pressure element comprising a flexible portion configured to expand to receive the head of the bone anchoring element, and a clamping element configured to extend at least partially around the flexible portion of the pressure element and exert a clamping force onto the pressure element, wherein when the pressure element and clamping element are located inside the receiving part, the clamping element is configured to rotate around the central axis from a first position where the flexible portion of the pressure element is expandable in the accommodation space to receive the head, to a second position where the head is held in the flexible portion of the pressure element and the clamping element exerts a clamping force onto the pressure element. 16. The bone anchoring device of claim 15 , wherein in the second position, the head is pivotable in the receiving part. 17. The bone anchoring device of claim 15 , wherein the flexible portion of the pressure element has a cap-like shape and is configured to surround an inserted head. 18. The bone anchoring device of claim 15 , wherein in the second position, a largest width of the head of the bone anchoring element is positioned inside the pressure element and is located between the second end of the receiving part and the clamping element. 19. The bone anchoring device of claim 15 , wherein in the second position, an end of the pressure element is between the clamping element and the first end of the receiving part. 20. A method of using a bone anchoring device, the bone anchoring device comprising a bone anchoring element having a shaft and a head, and a coupling assembly comprising a receiving part having a first end and a second end, a central axis extending through the first end and the second end, a channel at the first end for receiving a rod, and an accommodation space for accommodating the head, the accommodation space defining an opening for inserting the head, a pressure element configured to be located at least partially in the accommodation space, the pressure element comprising a flexible portion configured to expand to receive the head, and a clamping element configured to extend at least partially around the flexible portion of the pressure element and exert a clamping force onto the pressure element, the method comprising: inserting the head of the bone anchoring element into the receiving part through the opening when the flexible portion of the pressure element is expandable in the accommodation space; inserting the head into the flexible portion of the pressure element; and rotating the clamping element to exert a clamping force onto the pressure element.
CROSS-REFERENCE TO RELATED APPLICATIONS The present application claims the benefit of U.S. Provisional Application Ser. No. 62/160,479, filed May 12, 2015, the contents of which are hereby incorporated by reference in their entirety, and claims priority from European Patent Application EP 15 167 435.5, filed May 12, 2015, the contents of which are hereby incorporated by reference in their entirety. BACKGROUND Field The present disclosure relates to a coupling device for coupling a rod to a bone anchoring element. The coupling device includes a receiving part for receiving a rod and for coupling the rod to a bone anchoring element. The receiving part is configured to accommodate a head of a bone anchoring element such that the head can pivot with respect to the receiving part. The coupling device further includes a pressure element with a flexible portion to clamp a head inserted therein and a clamping element configured to exert a compression force onto the pressure element to increase a friction force between the pressure element and the head. Description of the Related Art U.S. Pat. No. 8,926,671 describes a receiving part for receiving a rod and for coupling the rod to a bone anchoring element. The receiving part includes a receiving part body for accommodating a head of the bone anchoring element and a pressure element with a flexible portion to clamp an inserted head. The pressure element is movable along a longitudinal axis of the receiving part body from an insertion position where the head is insertable into the receiving part body to a pre-locking position where the head is clamped in the receiving part body by a pre-stress exerted by the pressure element. The pressure element is further movable to a locking position where the head is locked in the receiving part body. The pre-stress exerted by the pressure element allows a desired angular position of the bone anchoring element to be maintained relative to the receiving part by friction before the head of the bone anchoring element is finally locked. U.S. Pat. No. 6,248,105 B1 describes a device for connecting a longitudinal rod with a bone anchoring element, such as a pedicle screw, thereby forming a fixation system for the spine. The device includes a connecting member accepting a longitudinal rod. A radially compressible spring chuck is arranged within the connecting member that has a cavity for receiving a head of the bone anchoring element. An insert slides into a bore hole of the connecting member and has a recess with a complementarily conical shape to a conical shape of the spring chuck. The insert radially compresses the spring chuck at an interface between the conical shapes and therewith fastens the head of the pedicle screw. U.S. Pat. No. 8,951,294 B2 describes a spinal implant with an anchoring part and a mounting part with an internal axial housing to transversely receive a connecting rod. The spinal implant includes retaining elements that are situated near the bottom of the axial housing of the mounting part. The retaining elements are adapted to fasten a locking member in a stationary locking position in which a ball joint connection, formed by first and second connecting elements, is locked to fasten the anchoring part and the mounting part in rotation relative to each other about at least two orthogonal axes. US 2013/0096622 A1 describes a polyaxial bone anchoring device having a pressure element that exerts pressure onto a head of an anchoring element. The pressure element maintains the head in an adjustable angular position relative to a receiving part by friction using threaded set screws. A desired friction force can be achieved by controlling the thread turning and advancement of the set screws. SUMMARY Embodiments of the invention provide an improved coupling device and an improved bone anchoring device that facilitates handling during surgery. The coupling device allows adjusting a friction force exerted on a head of a bone anchoring element in an easy manner during surgery. The coupling device may be particularly useful for a polyaxial bone anchoring element of the bottom loading type, where the head of the bone anchoring element is inserted into a receiving part from a bottom end thereof. More particularly, the coupling device allows the receiving part to be placed onto a head of a bone anchoring element in-situ, after a shaft of the bone anchoring element has already been inserted into a bone. Hence, in one or more embodiments, an in-situ type bottom loading bone anchoring element is provided, where a friction force acting onto the head can be increased by actuating a clamping element of the coupling device. By frictional clamping of the head before final locking of the bone anchoring device, the procedure of aligning a plurality of receiving parts for inserting a rod is considerably simplified. In one embodiment, the clamping element cooperates with a portion of the receiving part according to a bayonet-like locking advancement structure, where an engagement member, such as a pin, engages a helical groove that allows the clamping element to advance in an axial direction while rotating. The advancement is stepless, thereby providing a stepless adjustment of the clamping force onto the head. The interaction between the engagement member and the helical groove may be self-locking such that a desired position of the clamping element relative to the receiving part can be selected and secured against inadvertent movement. In another embodiment, an advancement structure that permits an axial advancement of the clamping element is formed by a threaded connection between the clamping element and the receiving part. The advancement structure also allows the clamping element to advance relative to the receiving part in a stepless manner and to adjust the friction force acting on the pressure element in a stepless manner. A pitch of the threaded connection may be the same as a pitch of a threaded connection between the receiving part and a locking element such that the clamping element can be screwed into the receiving part in the same manner as the locking element. In a further embodiment, the clamping element is an open ring similar to a snap ring that can exert a compression force onto the pressure element when the clamping element is placed around the pressure element. The clamping element has protrusions at a side of the clamping element facing the pressure element. In a first condition, the protrusions rest in recesses of the pressure element. In a second condition, the protrusions are moved out of the recesses thereby enhancing the clamping force on the pressure element. Hence, the clamping force can be increased with only a minimal movement of the clamping element. The pressure element may have at least one longitudinal slot providing a flexible portion for clamping the head. In one embodiment, the pressure element may have an additional horizontal slot to reduce an insertion force of the head into the pressure element. In a still further embodiment, the pressure element, the clamping element, and preferably also the receiving part are manufactured as a monolithic piece. In this embodiment, the connection between the pressure element and the clamping element and preferably also the connection between the clamping element and the receiving part each have at least one predetermined breaking point. By exerting a force, for example by rotating the clamping element, the connection breaks at the predetermined breaking point. Thereafter, the pieces are separate parts. An additive manufacturing method can be used to manufacture the monolithic pieces, such as laser sintering or electron beam melting. Thereby, sophisticated structures may be produced. BRIEF DESCRIPTION OF THE DRAWINGS Further features and advantages will become apparent from the description of embodiments by means of the accompanying drawings. In the drawings: FIG. 1 shows an exploded perspective view of a first embodiment of a bone anchoring device with a first embodiment of a coupling device; FIG. 2 shows a cross-sectional view of the bone anchoring device of FIG. 1 in an assembled state, the cross-section taken in a plane perpendicular to a rod axis; FIG. 3 shows a perspective view from above a receiving part of the coupling device according to the first embodiment of FIGS. 1 and 2 ; FIG. 4 shows a perspective view from below the receiving part of FIGS. 1 to 3 ; FIG. 5 shows a top view of the receiving part of FIGS. 3 and 4 ; FIG. 6 shows a cross-sectional view of the receiving part of FIGS. 3 to 5 , the cross-section taken along line A-A in FIG. 5 ; FIG. 7 shows a perspective view from above a pressure element of the coupling device according to the first embodiment of FIGS. 1 and 2 ; FIG. 8 shows a perspective view from below the pressure element of FIG. 7 ; FIG. 9 shows a top view of the pressure element of FIGS. 7 and 8 ; FIG. 10 shows a cross-sectional view of the pressure element of FIGS. 7 to 9 , the cross-section taken along line B-B in FIG. 9 ; FIG. 11 shows a perspective view from above a clamping element of the coupling device according to the first embodiment of FIGS. 1 and 2 ; FIG. 12 shows another perspective view of the clamping element of FIG. 11 ; FIG. 13 shows a top view of the clamping element of FIGS. 11 and 12 ; FIG. 14 shows a cross-sectional view of the clamping element of FIGS. 11 to 13 , the cross-section taken along line CC-CC in FIG. 13 ; FIGS. 15 a to 15 c show cross sectional views of steps of placing the coupling device according to the first embodiment of FIGS. 1 to 14 onto a bone anchoring element; FIGS. 15 d to 15 f show cross-sectional views of steps of adjusting a clamping force onto an inserted head via the clamping element according to the first embodiment of FIGS. 1 to 14 ; FIG. 16 shows an exploded perspective view of a second embodiment of a bone anchoring device with a second embodiment of a coupling device; FIG. 17 shows a cross-sectional view of the bone anchoring device of FIG. 16 , the cross-section taken in a plane perpendicular to a rod axis; FIG. 18 shows a partial cross-sectional view of the bone anchoring device of FIGS. 16 and 17 without an inserted rod; FIG. 19 shows a perspective view from above a receiving part of the coupling device according to the second embodiment of FIGS. 16 to 18 ; FIG. 20 shows a cross-sectional view of the receiving part of FIG. 19 , the cross-section taken in a plane perpendicular to the rod axis; FIG. 21 shows a perspective view from above a clamping element of the coupling device according to the second embodiment of FIGS. 16 to 18 ; FIG. 22 shows a perspective view from below the clamping element of FIG. 21 ; FIG. 23 shows a side view of the clamping element of FIGS. 21 and 22 ; FIG. 24 shows an exploded perspective view of a third embodiment of a bone anchoring device with a third embodiment of a coupling device; FIG. 25 shows a cross-sectional view of the bone anchoring device of FIG. 24 , the cross-section taken in a plane perpendicular to the rod axis; FIG. 26 shows a partial cross-sectional view of the bone anchoring device of FIGS. 24 and 25 ; FIG. 27 shows a perspective view from above a pressure element of the coupling device according to the third embodiment of FIGS. 24 to 26 ; FIG. 28 shows a perspective view from below the pressure element of FIG. 27 ; FIG. 29 shows a top view of the pressure element of FIGS. 27 and 28 ; FIG. 30 shows a cross-sectional view of the pressure element of FIGS. 27 to 29 , the cross-section taken along line D-D in FIG. 29 ; FIG. 31 shows an exploded perspective view of a fourth embodiment of a bone anchoring device with a fourth embodiment of a coupling device; FIG. 32 shows a cross-sectional view of the bone anchoring device of FIG. 31 , the cross-section taken in a plane perpendicular to a rod axis; FIG. 33 shows a partial cross-sectional view of the bone anchoring device of FIGS. 31 and 32 without an inserted rod; FIG. 34 shows a perspective view from above a receiving part of the coupling device of FIGS. 31 to 33 ; FIG. 35 shows a perspective view from below the receiving part of FIG. 34 ; FIG. 36 shows a top view of the receiving part of FIGS. 34 and 35 ; FIG. 37 shows a cross-sectional view of the receiving part of FIGS. 34 to 36 , the cross-section taken along line E-E in FIG. 36 ; FIG. 38 shows a perspective view from above a pressure element of the fourth embodiment of FIGS. 31 to 33 ; FIG. 39 shows a perspective view from below the pressure element of FIG. 38 ; FIG. 40 shows a top view of the pressure element of FIGS. 38 and 39 ; FIG. 41 shows a cross-sectional view of the pressure element of FIGS. 38 to 40 , the cross-section taken along line F-F in FIG. 40 ; FIG. 42 shows a perspective view from above a clamping element of the fourth embodiment of FIGS. 31 to 33 ; FIG. 43 shows a perspective view from below the clamping element of FIG. 42 ; FIG. 44 shows a top view of the clamping element of FIGS. 42 and 43 ; FIG. 45 shows a side view of the clamping element of FIGS. 42 to 44 ; FIGS. 46 a to 46 d show steps of mounting the pressure element and the clamping element to the receiving part according to the fourth embodiment of FIGS. 31 to 45 ; FIGS. 46 e to 46 h show steps of mounting the coupling device according to the fourth embodiment of FIGS. 31 to 45 onto a bone anchoring element and adjusting a friction force onto an inserted head with the clamping element; FIG. 47 a shows a perspective view from above a fifth embodiment of a coupling device; FIG. 47 b shows an enlarged view of a detail of FIG. 47 a; FIG. 48 shows a cross-sectional view of the coupling device of the fifth embodiment of FIGS. 47 a and 47 b; FIG. 49 shows a top view of the coupling device of FIGS. 47 a to 48 ; and FIG. 50 shows a cross-sectional view of the coupling device of FIGS. 47 to 49 , the cross-section taken along line G-G in FIG. 49 . DETAILED DESCRIPTION As shown in FIGS. 1 and 2 , a bone anchoring device according to a first embodiment includes a bone anchoring element 1 . The bone anchoring element 1 may be a bone screw having a threaded shaft 2 and a head 3 that may be spherically segment-shaped. The head 3 has a recess 3 a for engagement with a screwing-in tool. The bone anchoring device further includes a coupling device for receiving a rod 100 to connect the rod 100 to the bone anchoring element 1 . The coupling device includes a receiving part 5 for receiving the rod 100 and the head 3 of the bone anchoring element 1 . The coupling device also has a pressure element 6 for exerting a pressure on the head 3 of the bone anchoring element 1 that has been inserted therein to clamp and finally lock the head 3 in the receiving part 5 . Additionally, a clamping element 7 is provided for exerting a compression force onto the pressure element 6 to increase the pressure force on the inserted head 3 . The bone anchoring device further includes a locking element 8 for securing the rod 100 in the receiving part 5 and for exerting a force to lock the head 3 in the receiving part 5 . The locking element 8 may be, for example, a set screw. The receiving part 5 is now explained with reference to FIGS. 1 to 6 . The receiving part 5 includes a first end or proximal end 5 a , a second end or distal end 5 b opposite the first end 5 a , and an axis of symmetry C passing through the first end 5 a and the second end 5 b . A passage 51 is provided that extends from the first end 5 a to the second end 5 b and is substantially rotationally symmetric about the axis of symmetry C. In a first region adjacent to the first end 5 a , the receiving part 5 has a substantially U-shaped recess 52 that is symmetric with respect to the axis C, the recess 52 having a bottom directed towards the second end 5 b . The recess 52 forms two free lateral legs 53 a , 53 b that extend towards the first end 5 a . An internal thread 54 is provided in each of the legs 53 a , 53 b that cooperates with the locking element 8 . The internal thread 54 may be a flat thread (as shown in FIG. 2 ) to eliminate spreading of the legs 53 a , 53 b when tightening the locking element 8 . The internal thread 54 may, however, have any other suitable thread form. An undercut 54 a may be provided at a bottom end of the internal thread 54 (i.e., at an end of the internal thread 54 closest to the second end 5 b of the receiving part 5 ). The undercut 54 a may have a depth in a radial direction of the receiving part 5 that is substantially the same as the depth of the thread 54 in the radial direction. A channel formed by the substantially U-shaped recess 52 is sized to receive the rod 100 therein, where the rod 100 is configured to connect a plurality of anchoring devices. As can be seen in particular in FIGS. 2 and 6 , a portion of the passage 51 in an upper section of the receiving part 5 between the first end 5 a and approximately the bottom of the U-shaped recess 52 is substantially cylindrical. At approximately the bottom of the U-shaped recess 52 , the passage 51 widens, for example conically, to form an accommodation space 55 that has an inner diameter greater than an inner diameter of the passage 51 located in the upper section of the receiving part 5 . The accommodation space 55 narrows towards the second end 5 b with a narrowing portion 55 a extending from a projecting edge 55 b to the second end 5 b . A size of the accommodation space 55 is such that the head 3 of the bone anchoring element 1 and a lower portion of the pressure element 6 can be accommodated therein. The passage 51 forms an opening 56 at the second end 5 b that opens into the accommodation space 55 . A diameter of the opening 56 is greater than a greatest diameter of the head 3 such that the head 3 is insertable into the receiving part 5 through the opening 56 at the second end 5 b . It should be noted that the narrowing portion 55 a can narrow in several shapes, such as a tapered shape as shown in FIGS. 2 and 6 , a spherical shape, or can narrow otherwise. At a distance from the first end 5 a , circumferentially extending notches 58 a , 58 b with downwardly inclined upper and lower surfaces may be provided for engagement with a tool. On each of the legs 53 a , 53 b , bores 59 a , 59 b , extending through the legs 53 a , 53 b , respectively, are provided for receiving pins 9 a , 9 b . The bores 59 a , 59 b are located approximately at a center of each leg 53 a , 53 b in a circumferential direction of the receiving part 5 and are offset from each other by approximately 180°. Referring to FIGS. 7 to 10 , the pressure element 6 has a first end 6 a and an opposite second end 6 b . The pressure element 6 is configured to be mounted to the receiving part 5 such that the second end 6 b of the pressure element 6 is directed towards the second end 5 b of the receiving part 5 . The pressure element has a first portion 61 adjacent to the first end 6 a that is substantially cylindrical and that has an outer diameter that is smaller than an inner diameter of the passage 51 of the receiving part 5 . In the cylindrical first portion 61 , a recess with a substantially V-shaped cross-section is formed that provides a rod support surface 62 for supporting an inserted rod 100 . The V-shape permits the pressure element 6 to support rods of different diameters. Inserted rods to be supported contact the rod support surface 62 with at least two contact lines extending perpendicular to the cylinder axis of the first portion 61 . When the pressure element 6 is in a mounted state in the receiving part 5 , the cylinder axis of the first portion 61 coincides with the central axis of symmetry C of the receiving part 5 . A second portion 63 of the pressure element 6 is between the cylindrical first portion 61 and the second end 6 b with a hollow interior 64 having a shape adapted to clamp the head 3 therein. In particular, the hollow interior 64 of the second portion 63 has a spherical shape with a length in an axial direction of the pressure element 6 sufficient to accommodate a greatest diameter e of the head 3 therein (see FIG. 2 ). As described in more detail below, the second portion 63 is open at the second end 6 b to allow insertion of the head 3 of the bone anchoring element 1 into the hollow interior 64 from the second end 6 b . An outer wall of the second portion 63 includes a first outer surface portion 65 that is substantially spherical and a second outer surface portion 66 adjacent to the second end 6 b that narrows towards the second end 6 b . In particular, in the embodiment shown, the second outer surface portion 66 is tapered. As can be seen in FIG. 2 , a largest outer diameter of the first outer surface portion 65 of the pressure element 6 is smaller than a greatest inner diameter of the accommodation space 55 and greater than an inner diameter of the opening 56 of the receiving part 5 . The second portion 63 of the pressure element 6 further has at least one vertical slit 67 , preferably a plurality of slits 67 , that are open to the second end 6 b and extend from a bottom of the second portion 63 almost up to the first portion 61 . The at least one vertical slit 67 may widen into a substantially circular enlarged end portion 68 forming a closed end of the slit 67 . The enlarged end portion 68 of the slits 67 can have another shape, for example, an oval shape, or any other shape that is different from the slits 67 . In some embodiments, the enlarged end portion 68 can be omitted such that each slit 67 has the same width at its closed end as at its respective open end. The number and dimensions of the slits 67 are such that the wall of the second portion 63 is flexible enough to snap onto the head 3 when the head 3 is inserted into the hollow interior 64 . The second portion 63 of the pressure element 6 is a flexible portion that is adapted to exert a pressure onto an inserted head 3 and to hold the head 3 by a frictional force between an inner surface of the hollow interior 64 of the second portion 63 and an outer surface of the head 3 . The flexible portion 63 is a cap-like portion that fits tightly onto the head 3 . In addition, it shall be noted that the second outer surface portion 66 of the pressure element 6 may have another shape, for example, a rounded shape or any other shape suitable to cooperate with the narrowing portion 55 a of the receiving part 5 such that the outer surface portion 66 can be compressed when it enters into the narrowing portion 55 a of the receiving part 5 . The pressure element 6 is configured to be inserted into the receiving part 5 through the lower opening 56 , whereby the second portion 63 is compressed during insertion. Alternatively, the pressure element 6 may be inserted into the first end 5 a of the receiving part 5 and moved downwardly or distally through the passage 51 . The outer diameter of the cylindrical first portion 61 of the pressure element 6 is smaller than the inner diameter of the passage 51 such that the clamping element 7 fits there-between. Because the second portion 63 of the pressure element 6 has an outer diameter that is smaller than an inner diameter of the accommodation space 55 and has a flexible wall, the second portion 63 can expand within the accommodation space 55 when the head 3 is inserted into the hollow interior 64 of the pressure element 6 . Further, the pressure element 6 includes a coaxial bore 69 providing access to the head 3 by a tool configured to engage the engagement recess 3 a of the head 3 . Referring to FIGS. 11 to 14 , the clamping element 7 is a ring-shaped part. The ring is closed and is not flexible. The clamping element 7 has a first end 7 a , an opposite second end 7 b , and a substantially cylindrical outer shape. As depicted in FIG. 2 , the clamping element 7 is configured to be mounted to the receiving part 5 such that the second end 7 b of the clamping element 7 is directed towards the second end 5 b of the receiving part 5 . When the clamping element 7 is in a mounted state in the receiving part 5 , the cylinder axis of the clamping element 7 coincides with the central axis of symmetry C of the receiving part 5 . As depicted in FIG. 2 , an outer diameter of the clamping element 7 is slightly smaller than an inner diameter of the passage 51 in the upper section of the receiving part 5 . As such, the clamping element 7 can be inserted into the first end 5 a of the receiving part 5 and moved downwardly or distally through the passage 51 . As can be seen, for example, in FIGS. 2 and 14 , an inner surface 71 of the clamping element adjacent to the first end 7 a is cylindrical with an inner diameter slightly larger than an outer diameter of the first portion 61 of the pressure element 6 . The cylindrical surface 71 of the ring-shaped clamping element 7 encompasses the outer cylindrical surface of the first portion 61 of the pressure element 6 in the mounted state. In addition, the clamping element 7 has a conically widening inner surface portion 72 between the cylindrical inner surface 71 and the second end 7 b . The size of the conically widening inner surface portion 72 is such that the clamping element 7 can be mounted around the pressure element 6 with the inner surface portion 72 encompassing an upper region of the flexible second portion 63 of the pressure element 6 . Thereby, the inner surface portion 72 contacts the first outer surface portion 65 of the flexible second portion 63 of the pressure element 6 . By a downward movement of the clamping element 7 relative to the receiving part 5 , the flexible second portion 63 of the pressure element 6 is compressed. Further, the inner surface portion 72 may have another shape that includes an increase in inner diameter towards the second end 7 b. Referring in particular to FIGS. 11 and 12 , two helical grooves 73 a , 73 b having the same pitch are provided on the outer surface of the clamping element 7 . The helical grooves 73 a , 73 b are open towards the first end 7 a and have closed ends 74 a , 74 b towards the second end 7 b . The closed ends 74 a , 74 b may have rounded contours. A width of the grooves 73 a , 73 b is only slightly larger than the diameter of the pins 9 a , 9 b . In particular, the width of the grooves 73 a , 73 b in relation to the diameter of the pins 9 a , 9 b and the pitch may be such that a frictional force between the pins 9 a , 9 b and the grooves 73 a , 73 b results in a self-locking connection whereby the clamping element 7 can advance relative to the pins 9 a , 9 b only if a force is applied that exceeds the self-locking force. An inclination of the grooves 73 a , 73 b and a length of the grooves 73 a , 73 b between the open ends and the closed ends 74 a , 74 b is such that a desired advancement of the clamping element 7 in relation to an inserted pin 9 a , 9 b is possible. In the embodiment depicted in FIGS. 11 and 12 , the grooves 73 a , 73 b each extend around the central axis C by more than around 90° and less than around 180°. However, any other inclination and length of the grooves 73 a , 73 b may be selected such that the clamping element 7 can be moved relative to the inserted pins 9 a , 9 b with a predetermined increment. The grooves 73 a , 73 b and the pins 9 a , 9 b form an advancement structure that permits the clamping element 7 to advance in the receiving part 5 by rotating the clamping element 7 around the axis of symmetry C. When the pressure element 6 and the clamping element 7 are inserted into the receiving part 5 , the closed ends 74 a , 74 b of the helical grooves 73 a , 73 b prevent the clamping element 7 from escaping through the first end 5 a . As such, the closed ends 74 a , 74 b form a securing structure. As further depicted in FIGS. 11 to 14 , the clamping element 7 includes a plurality of engagement portions or recesses 75 in the surface of the first end 7 a that are configured to be engaged with a tool used for rotating the clamping element 7 . The parts of the bone anchoring device can be made of a bio-compatible material, such as a bio-compatible metal or a bio-compatible metal alloy, for example stainless steel, titanium, NiTi-alloys, such as Nitinol, magnesium or magnesium alloys or from a bio-compatible plastic material, such as, for example, polyether ether ketone (PEEK) or poly-l-lactide acid (PLLA). The parts of the bone anchoring device can be made of the same or of different materials. Referring to FIGS. 15 a to 15 c , steps of assembling the bone anchoring device of FIGS. 1 to 14 are explained. In FIG. 15 a the pressure element 6 and the clamping element 7 are inserted into the receiving part 5 . The pressure element 6 is in a position in which the second outer surface portion 66 of the pressure element abuts against the narrowing portion 55 a of the receiving part 5 . The pressure element 6 may be inserted from the lower opening 56 at the second end 5 b or may be inserted through the first end 5 a and moved downwardly or distally through the passage 51 . The clamping element 7 is mounted such that the pins 9 a , 9 b rest in the closed end portion 74 a , 74 b of the helical grooves 73 a , 73 b , respectively, in an uppermost or first position of the clamping element 7 . In this position, the clamping element 7 is arranged around the cylindrical first portion 61 of the pressure element 6 and does not contact the flexible second portion 63 . Next, as depicted in FIG. 15 b , the head 3 of the bone anchoring element is inserted into the pressure element 6 and the receiving part 5 through the lower opening 56 of the receiving part 5 . In a first alternative, the bone anchoring device is assembled in-situ such that the bone anchoring element 1 has been already inserted into the bone and the coupling device, including the receiving part 5 with the pressure element 6 and the clamping element 7 , is thereafter mounted onto the head 3 (arrow al). In a second alternative, the bone anchoring device is assembled outside the human body and the head 3 is inserted manually into the receiving part 5 through the lower opening 56 before the bone anchoring element 1 is anchored to the bone. Next, as depicted in FIG. 15 c , the head 3 moves the pressure element 6 upward and the head 3 snaps into the hollow interior 64 of the flexible portion 63 of the pressure element 6 . When the greatest diameter e of the head 3 passes the second end 6 b of the pressure element and moves into the hollow interior 64 , the flexible portion 63 can expand in the accommodation space 55 of the receiving part 5 . During this step, the clamping element 7 stays in a fixed axial position and cannot be pushed upward or proximally as the clamping element 7 is held by the pins 9 a , 9 b. FIGS. 15 d to 15 f illustrate steps of achieving and adjusting a frictional clamping of the head 3 . As depicted in FIG. 15 d , when the head 3 has been fully inserted into the pressure element 6 , the receiving part 5 may be pulled upwards (arrow b 1 ), whereby the pressure element 6 together with the inserted head 3 is moved downward relative to the receiving part 5 . The second outer surface portion 66 of the pressure element 6 passes the edge 55 b protruding into the accommodation space 55 and enters the narrowing portion 55 a whereby the second outer surface portion 66 is compressed and the head 3 is clamped by friction. In addition, the head 3 can no longer be removed through the lower opening 56 due to the force of the pressure element 6 . This position is a pre-locking position of the pressure element 6 . In order to allow in-situ mounting of the receiving part 5 onto the head 3 , a required insertion force for inserting the head 3 into the pressure element 6 should not be too high. However, if the insertion force is too low, resultant frictional clamping of the pressure element 6 onto the head 3 after insertion of the head 3 might not be strong enough for convenient handling. To enhance frictional clamping of the head 3 , the clamping element 7 is actuated in a next step using a tool 150 as shown in FIGS. 15 e and 15 f . The tool 150 may be formed as a tubular rod with engagement portions at a front or distal end that are arranged and sized to engage (e.g., fit in) the engagement portions 75 of the clamping element 7 . The clamping element 7 is rotated by rotation of the tool 150 such that the pins 9 a , 9 b are guided in the helical grooves 73 a , 73 b . Thereby, the clamping element 7 advances downward until the inner surface portion 72 of the clamping element 7 contacts the first outer surface portion 65 of the pressure element 6 . Further rotation of the tool 150 advances the clamping element 7 such that the clamping element 7 exerts a compression force onto the flexible second portion 63 of the pressure element 6 which increases the pressure of the pressure element 6 onto the inserted head 3 . As a result thereof, the frictional force that holds the head 3 in a desired angular position relative to the receiving part 5 before final locking is increased. Due to the bayonet-like advancement structure of the clamping element 7 , a stepless advancement of the clamping element 7 relative to the receiving part 5 is possible, which in turn provides a stepless adjustment of the compression force onto the head 3 . FIG. 15 f depicts a partial cross-sectional view which illustrates how the groove 73 b has moved relative to the mounted pin 9 b . With a self-locking connection between the groove 73 b and the pin 9 b , a desired position of the clamping element 7 in the receiving part 5 can be selected and maintained. Finally, after alignment of the receiving part 5 of several bone anchoring devices, the rod 100 is inserted and the locking element 8 is tightened. Thereby, the pressure element 6 is further pressed against the narrowing portion 55 a to lock the bone anchoring devices. A second embodiment of the bone anchoring device and of the coupling device will be described with reference to FIGS. 16 to 23 . Parts, portions, and elements that are identical or similar to those of the first embodiment are marked with the same reference numerals, and the descriptions therefore will not be repeated. The second embodiment differs from the first embodiment in the design of the clamping element and the advancement structure. All other parts are the same as in the first embodiment. As depicted in FIGS. 16 to 20 , the receiving part 5 ′ includes an internal thread 90 that extends from the lower end of the internal thread 54 to an axial height of approximately slightly below the bottom of the U-shaped recess 52 . The internal thread 90 may be separated from the internal thread 54 by the undercut 54 a . A thread pitch of the internal thread 90 is substantially the same as a thread pitch of the internal thread 54 and a major diameter of the internal thread 90 is larger than a minor diameter of the internal thread 54 . The shape of the internal thread 90 may be different from the shape of the internal thread 54 . For example, the thread flanks of the internal thread 90 may have a triangular cross-section or any other cross-section. However, the dimensions of the threads of the internal thread 90 should be such that the clamping element 7 can be screwed through the upper portion of the passage 51 having the internal thread 54 and into the portion of the passage 51 having the internal thread 90 . It should be noted, that the receiving part 5 ′ does not need to have the pins 9 a , 9 b in this embodiment. As shown in FIGS. 21 to 23 , the clamping element 7 ′ includes an external thread 73 ′ provided on its outer cylindrical surface that is configured to cooperate with the internal thread 90 of the receiving part 5 ′. At the first end 7 a , the clamping element 7 ′ may have a plurality of engagement recesses 75 ′. In particular, the number of engagement recesses 75 ′ may be greater than in the first embodiment. The advancement structure in the form of the cooperating threads 73 ′, 90 provides a stepless advancement of the clamping element 7 ′ relative to the receiving part 5 ′. The threaded connection between the clamping element 7 ′ and the receiving part 5 ′ acts as a securing structure that inhibits inadvertent movement of the clamping element 7 ′ relative to the receiving part 5 ′. In use, after the pressure element 6 has been inserted into the receiving part 5 ′, the clamping element 7 ′ is mounted to the receiving part 5 ′ by screwing the clamping element 7 ′ downward from the first end 5 a . The clamping element 7 ′ is moved downward to an axial position that still allows the pressure element 6 to move upward or proximally to insert the head 3 . Screwing-in the clamping element 7 ′ renders the assembly simple and safe, as jamming of the clamping element 7 ′ in the passage 51 is prevented. When the head 3 has been inserted into the pressure element 6 and the pressure element 6 has reached the pre-locking position shown in FIG. 17 , the clamping element 7 ′ can be further screwed downward with a tool to exert a compression force onto the pressure element 6 to enhance the clamping force acting onto the head 3 by friction. After the rod 100 has been inserted and the locking element 8 has been tightened, the pressure element 6 is further pressed against the narrowing portion 55 a to lock the bone anchoring device. A third embodiment of the bone anchoring device will be described with reference to FIGS. 24 to 30 . Parts, portions and elements of the third embodiment that are identical or similar to those of the first embodiment are marked with the same reference numerals and the descriptions thereof will not be repeated. The bone anchoring device differs from the first embodiment in the design of the coupling device and in particular in the design of the pressure element. The coupling device of the third embodiment includes the receiving part 5 and the clamping element 7 of the first embodiment and a modified pressure element 6 ′. As shown, for example, in FIGS. 27-30 , the flexible second portion 63 ′ of the pressure element 6 ′ includes one single vertical slit or recess 67 ′ that is provided at an angle of substantially 90° in a circumferential direction relative to a longitudinal axis of the V-shaped rod support surface 62 . The vertical recess 67 ′ is open towards the second end 6 b of the pressure element 6 ′. At a distance from the second end 6 b , the vertical recess 67 ′ opens into a horizontal recess 68 ′ that extends in a circumferential direction from both sides from the vertical recess 67 ′. The horizontal recess 68 ′ extends to ends that are spaced apart from each other. By means of the vertical recess 67 ′ and the horizontal recess 68 ′, a ring 65 a is formed that is connected to the other portions of the pressure element 6 ′. The ring 65 a is flexible and can be expanded and/or compressed. An upper portion of the pressure element 6 ′ between the horizontal recess 68 ′ and the cylindrical portion 61 has a substantially spherical outer surface that is slightly recessed with respect to the ring 65 a . In other words, the ring 65 a has a slightly greater outer diameter forming a circumferential edge 65 b. As depicted in FIGS. 25 and 26 , in the assembled state when the clamping element 7 has been moved downward to contact the pressure element 6 ′, the clamping element 7 presses the pressure element 6 ′ with the inner surface portion 72 onto the edge of the ring 65 a that has the slightly greater diameter. The use of the bone anchoring device according to the third embodiment is analogous to the use of the bone anchoring devices according to the previous embodiments. However, the horizontal slit 68 ′ in connection with the vertical slit 67 ′ renders the second portion 63 ′ of the pressure element 6 ′ more flexible than the previous embodiments. Therefore, the amount of insertion force required to insert the head 3 into the pressure element 6 ′ may be decreased relative to previous embodiments. In turn, the amount of frictional force needed to hold the inserted head 3 in the pressure element 6 ′ and the receiving part 5 may be smaller. By means of the clamping element 7 , the frictional force can be increased relative to previous embodiments. A fourth embodiment of the bone anchoring device will be explained with reference to FIGS. 31 to 45 . Parts, portions and elements that are identical or similar to the parts, portions and elements of the previous embodiments are indicated with the same reference numerals and the descriptions thereof will not be repeated. The bone anchoring device of the fourth embodiment differs from the previous embodiments in the structure of the receiving part, the pressure element and the clamping element. Referring to FIGS. 31 to 37 , the receiving part 5 ″ is adapted to accommodate the clamping element 7 ″. The receiving part 5 ″ includes opposite horizontal slits 501 a , 501 b on each leg 53 a , 53 b . The opposite horizontal slits 501 a , 501 b are located at one side of the U-shaped recess 52 . The slits 501 a , 501 b extend substantially perpendicular to the central axis C and are open towards the outer surface of the legs 53 a , 53 b . At their outer region, the slits 501 a , 501 b may have a substantially square or rectangular cross-section with a size adapted to accommodate at least a portion of the clamping element 7 ″ therein. The slits 501 a , 501 b are located at an axial position that is approximately at the lower end of the internal thread 54 , which allows mounting the clamping element 7 ″ to the receiving part 5 ″ from the side through the U-shaped recess 52 . At the side of the slits 501 a , 501 b , a substantially vertically extending shallow recess 502 is provided that extends from the bottom of the substantially U-shaped recess 52 towards the second end 5 b . A width of the shallow vertical recess 502 in a circumferential direction is slightly larger than half of the distance between the legs 53 a , 53 b , as can be seen in particular in FIG. 36 . The shallow recess 502 serves as a space for a portion of the clamping element 7 ″ to permit the clamping element 7 ″ to rotate between a first position and a second position and to limit the movement of the clamping element 7 ″ between these positions. Moreover, two opposite bores 59 a ′, 59 b ′ are provided at approximately the center of the legs 53 a , 53 b and at an axial position slightly below the axial position of the horizontal slits 501 a , 501 b . The bores 59 a ′, 59 b ′ are configured to accommodate pins 9 a ′, 9 b ′, respectively therein. The pins 9 a ′, 9 b ′ are configured to engage the pressure element 6 ″ as explained below. Turning now to FIGS. 38 to 41 , the pressure element 6 ″ differs from the pressure element of the first and second embodiments by the shape of the upper portion adjacent to the flexible second portion 63 . The flexible second portion 63 is identical to the flexible second portion 63 of the first embodiment. The cylindrical first portion 61 ′ has an outer diameter that is greater than an outer diameter of the flexible second portion 63 in the upper region, hence, the flexible second portion 63 is recessed with respect to the first portion 61 ′. The first portion 61 ′ includes two opposite upstanding legs 61 a , 61 b . Grooves 62 a , 62 b respectively separate the upstanding legs 61 a , 61 b from an inner portion having the rod supporting surface 62 . The grooves 62 a , 62 b may have a widened bottom to render the legs 61 a , 61 b slightly flexible to aid with insertion. The grooves 62 a , 62 b extend substantially parallel to the longitudinal axis of the rod support surface 62 . An inner surface of the legs 61 a , 61 b is substantially flat. At an upper end of the legs 61 a , 61 b , the legs 61 a , 61 b each include collar portions 600 a , 600 b . The collar portions 600 a , 600 b are shaped to fit in a corresponding cutout in the receiving part 5 ″ in the region of the horizontal slits 501 a , 501 b , as shown in particular in FIG. 33 . The legs 61 a , 61 b each include an elongate through-hole 601 a , 601 b , the longitudinal axes of which are substantially parallel to the central axis C. The through-holes 601 a , 601 b are configured to receive the pins 9 a ′, 9 b ′, respectively. When the pins 9 a ′, 9 b ′ engage the through-holes 601 a , 601 b , the pressure element 6 ″ is restricted to move only a predetermined distance in the axial direction and is limited by the abutment of the pins 9 a ′, 9 b ′ against the respective ends of the through-holes 601 a , 601 b in the axial direction. The clamping element 7 ″ of the fourth embodiment will be explained with reference to FIGS. 31 to 33 and 42 to 45 . The clamping element 7 ″ is shaped as an open ring having free ends 70 a , 70 b and a slot 700 therebetween such that the clamping element 7 ″ can act as a snap ring. More in detail, the clamping element 7 ″ extends slightly more than 180° in a circumferential direction around the central axis C. The slot 700 may be smaller or may be larger than as shown. Further, the clamping element 7 ″ has a substantially cylindrically shaped outer wall with an axial length such that the clamping element 7 ″ fits approximately into a recessed region between the first portion 61 ′ and the second portion 63 of the pressure element 6 ″, as shown in FIG. 32 . As further depicted in FIG. 32 , the clamping element 7 ″ has a substantially conically widening inner surface portion 720 that is adapted to extend around the upper section of the flexible second portion 63 of the pressure element 6 ″. Between the widening inner surface portion 720 and the first end 7 a of the clamping element 7 ″, there may be a portion 710 that can be rounded to avoid jamming of the clamping element 7 ″ during mounting. A plurality of rounded protrusions 721 are provided in the widening inner surface portion 720 that are configured to engage the widened end portions 68 of the slits 67 of the flexible portion 63 of the pressure element 6 ″. It shall be noted that a modification of the design of the pressure element 6 ″ and the clamping element 7 ″ may be conceivable. Instead of the widened portions 68 of the slit 67 in the pressure element 6 ″ and the rounded protrusions 721 on the inner surface of the clamping element 7 ″, protrusions could be present at the end of the slits 67 of the pressure element 6 ″ and corresponding dimples could be present in the inner surface of the clamping element 7 ″. At approximately the center of the clamping element 7 ″ in a circumferential direction, a protrusion 730 , with a substantially rectangular cross-section, is provided at the outer surface of the clamping element 7 ″ opposite to the protrusions 721 . In the axial direction, the protrusion 730 extends above the first end 7 a . The protrusion 730 serves for engagement with a tool for moving the clamping element 7 ″ relative to the pressure element 6 ″. Mounting of the pressure element 6 ″ and the clamping element 7 ″ into the receiving part 5 ″ according to the fourth embodiment will be explained with reference to FIGS. 46 a to 46 d . In a first step, the pressure element 6 ″ is inserted into the receiving part 5 ″ from the first end 5 a . Upon insertion into the receiving part 5 ″, the legs 61 a , 61 b may be slightly flexed towards each other such that the pressure element 6 ″ can be maintained at a desired axial position. When the region of the pressure element 6 ″ between the cylindrical first portion 61 ′ and the flexible second portion 63 is located at an axial height of approximately the horizontal slits 501 a , 501 b , the clamping element 7 ″ is inserted into the slits 501 a , 501 b of the receiving part 5 ″ until the clamping element 7 ″ extends around the pressure element 6 ″ just beneath the cylindrical first portion 61 ′. In this configuration, the protrusion 730 of the clamping element 7 ″ is aligned with the center of the substantially U-shaped recess of the receiving part 5 ″, and the rod support surface 62 of the pressure element 6 ″ is also aligned with the U-shaped recess 52 of the receiving part 5 ″. Further, one of the slits 67 is located at a circumferential position corresponding to the center of the rod support surface 62 of the pressure element 6 ″. A corresponding protrusion 721 at the widening inner surface portion 720 of the clamping element 7 ″ can engage the widened portion 68 of the slit 67 as depicted in FIG. 46 c . Thereafter, as shown in FIG. 46 d , the pressure element 6 ″ and the clamping element 7 ″ are moved downward together as indicated by the arrow al. Thereby, the elongate through-holes 601 a , 601 b overlap with the bores 59 a ′, 59 b ′ such that the pins 9 a ′, 9 b ′ can be inserted into the bores 59 a ′, 59 b ′ and extend into the through-holes 601 a , 601 b. Use of the bone anchoring device of the fourth embodiment will be explained with reference to FIGS. 46 e to 46 h . First, as depicted in FIG. 46 e , the receiving part 5 ″ with the mounted pressure element 6 ″ and clamping element 7 ″ is placed onto the head 3 of the bone anchoring element 1 . This may be performed in-situ after the bone anchoring element 1 has been already inserted into the bone. As with the previous embodiments, the pressure element 6 ″ snaps onto the head 3 such that the head 3 is accommodated in the pressure element 6 ″ and in the accommodation space 55 of the receiving part 5 ″ as shown in FIG. 46 f . Thereafter, as illustrated in FIG. 46 g , the receiving part 5 ″ is pulled upward or proximally according to the arrow b 1 . The pins 9 a ′, 9 b ′ respectively move upward in the elongate through-holes 601 a , 601 b of the pressure element 6 ″. The protrusion 730 of the clamping element 7 ″ moves at least partially into the shallow recess 502 of the receiving part 5 ″. In this position, the pressure element 6 ″ has reached the pre-locking position where it is no longer possible to remove the head 3 through the lower opening 56 . Thereafter, as shown in FIG. 46 h , the clamping element 7 ″ is rotated in the clockwise direction around the central axis C (arrow c 1 ). The clamping element 7 ″ may be rotated using a tool that engages the protrusion 730 of the clamping element 7 ″. Thereby, the rounded protrusions 721 of the clamping element 7 ″ move out of the widening portions 68 of the pressure element 6 ″ and press against the flexible second portion 63 of the pressure element 6 ″. The compression of the rounded protrusions 721 against the flexible second portion 63 of the pressure element 6 ″ increases the clamping force acting on the head 3 . The rotational movement of the clamping element 7 ″ relative to the receiving part 5 ″ is limited by the abutment of the protrusion 730 against the sidewall of the shallow recess 502 . Moreover, the pins 9 a ′, 9 b ′ prevent the pressure element 6 ″ from escaping out of the receiving part 5 ″. The steps of inserting the rod 100 and the locking element 8 , and tightening the locking element 8 to lock the bone anchoring device are identical to the previous embodiments. A fifth embodiment of the polyaxial bone anchoring device and the coupling device will be described with reference to FIGS. 47 a to 50 . The bone anchoring element 1 of the fifth embodiment is identical to the previous embodiments. The receiving part 5 ′″, the clamping element 7 ′″, and the pressure element 6 ′″ are manufactured as a monolithic piece and are separated after manufacturing. The pressure element 6 ′″ of the fifth embodiment has a shape similar to the pressure element in the first embodiment. The clamping element 7 ′″ of the fifth embodiment is similar to the clamping element of the first and second embodiments, however, instead of the two grooves 73 a , 73 b , the clamping element 7 ′″ has two helical projections 7000 a , 7000 b on its outer surface. The helical projections 7000 a , 7000 b are configured to engage corresponding helical grooves 9000 a , 9000 b in the receiving part 5 ′″. As shown in more detail in FIGS. 47 a and 47 b , the clamping element 7 ′″ is monolithically connected to the pressure element 6 ′″ at predetermined breaking points P. The predetermined breaking points P are configured to break such that the clamping element 7 ′″ and the pressure element 6 ′″ become separated. The predetermined breaking points P have such a size that they break when a tool engages the engagement portions 75 of the clamping element 7 ′″ and rotates the clamping element 7 ′″. Similarly, the clamping element 7 ′″ is monolithically connected to the receiving part 5 ′″ at predetermined breaking points Q as shown in FIGS. 48 and 50 . The size of the breaking points Q is such that the connection between the receiving part 5 ′″ and the clamping element 7 ′″ breaks at the predetermined breaking points Q by rotation of the clamping element 7 ′″ with a tool. The receiving part 5 ′″, the pressure element 6 ′″, and the clamping element 7 ′″ may be separated before mounting the coupling device to a bone anchoring element 1 . In particular, the pressure element 6 ′″ and the clamping element 7 ′″ may be manufactured such that the position of the pressure element 6 ′″ is an inserting position for inserting the head 3 . The separation can take place during mounting of the receiving part 5 ′″ onto the head 3 of the bone anchoring element 1 in-situ. A method for manufacturing the coupling device according to the fifth embodiment may be performed by an additive manufacturing method, such as selective laser sintering, selective laser melting, electron beam-sintering, and/or electron beam-melting. Modifications of the above described embodiments are possible. For the bone anchoring element all kinds of bone anchors can be used, such as screws, nails with or without barbs, cannulated bone anchors, two-part bone anchors where head and shaft are separate parts that can be assembled, and other bone anchoring elements. The head of the bone anchoring element may have a design that allows the bone anchoring element with a pressure element adapted thereto to be pivoted only in a single plane. For example, the head may have at least one flat surface portion extending substantially parallel to the shaft axis and the pressure element may have a cooperating portion to limit the pivoting to a single plane. Any design for providing an enlarged pivot angle may be used also. For the locking element all kinds of locking devices can be used, such as bayonet-type locking devices, two-part locking devices that allow clamping the rod and the head independently with two locking elements, outer locking nuts, and the like. While some of the embodiments include two pins, it shall be understood that one pin is sufficient. The features of the above described embodiments can be combined among each other to provide a variety of still further 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.

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