Patent Publication Number: US-11638597-B2

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

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/356,293, filed Mar. 18, 2019, which is a continuation of U.S. patent application Ser. No. 15/818,278, filed Nov. 20, 2017, now U.S. Pat. No. 10,271,877, which is a continuation of U.S. patent application Ser. No. 15/152,044, filed May 11, 2016, now U.S. Pat. No. 9,839,446, which 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 a 1 ). 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 a 1 . 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   m , the clamping element  7   m , and the pressure element  6 ′″ are manufactured as a monolithic piece and are separated after manufacturing. The pressure element  6   m  of the fifth embodiment has a shape similar to the pressure element in the first embodiment. The clamping element  7   m  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   m . As shown in more detail in  FIGS.  47   a  and  47   b   , the clamping element  7   m  is monolithically connected to the pressure element  6   m  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   m  and rotates the clamping element  7 ″. Similarly, the clamping element  7   m  is monolithically connected to the receiving part  5   m  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   m , 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   m  and the clamping element  7   m  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   m  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.