Abstract:
An instrument for inserting a bone anchoring element into a bone is provided, the instrument including a shaft ( 2 ) having an end portion ( 23, 23′ ) for engaging the bone anchoring element and for transferring torque to the bone anchoring element ( 1, 1′ ) and a longitudinal axis defining an axis of rotation (R);
       a cutting member ( 4, 4′ ) connected to the shaft ( 2 ) with cutting portions ( 45 ) that are configured to cut bone material;   wherein the cutting member ( 4, 4′ ) has an outer diameter (D) defined by the cutting portions ( 45 ) that is substantially the same or larger than an outer diameter (dd) of a receiving part ( 500 ) of the polyaxial bone anchor in a region at a bottom end ( 500   b ) of the receiving part.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    The present application claims priority to and the benefit of U.S. Provisional Patent Application Ser. No. 61/858,026, filed Jul. 24, 2013, the contents of which are hereby incorporated by reference in their entirety, and claims priority from European Patent Application EP 13 177 919.1, filed Jul. 24, 2013, the contents of which are hereby incorporated by reference in their entirety. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    The invention relates to a coupling assembly for coupling a rod to a bone anchoring element, wherein the coupling assembly comprises a receiving part with a recess for receiving the rod and an accommodation space for accommodating a head of the bone anchoring element and further with a pressure element to clamp the head and with a rod receiving element that cooperates with the pressure element. The pressure element and the rod receiving element are configured to cooperate in such a way that when the anchoring element is inserted into the coupling assembly, a head of the bone anchoring element can be clamped first without fixing the rod and subsequently the rod can be fixed using a single drive locking element. The invention further relates to a bone anchoring device comprising such a coupling assembly and a bone anchoring element. In addition, the invention relates to a kit of such a coupling assembly and different rod receiving elements to allow the use of rods with different diameters. 
         [0004]    2. Description of Related Art 
         [0005]    US 2010/0160980 A1 describes a locking mechanism and a method of fixation of a bone screw and a rod to the spine. The locking mechanism includes a body, an insert, a rod seat and a set screw. The body includes a bottom portion configured to receive the bone screw and the insert, but prevents the insert and the bone screw from passing therethrough once the insert and the bone screw are engaged. The rod seat is between the rod and the insert. 
         [0006]    US 2013/0018428 A1 describes an orthopedic fixation device that comprises a coupling element and a bone fastener, whereby the bone fastener can be loaded into the coupling element through the bottom of a bore in the coupling element. 
       SUMMARY 
       [0007]    There is still a need to provide a coupling assembly for coupling a rod to a bone anchoring element and a bottom loading polyaxial bone anchoring device that is improved regarding various aspects. 
         [0008]    In particular, it is an object of the invention to provide an improved coupling assembly for coupling a rod to a bone anchoring element that provides an improved or easier handling during surgery and that has an enlarged field of applications. 
         [0009]    The object is solved by a coupling assembly according to claim  1 , by a bone anchoring device according to claim  16  and by kit of a coupling assembly and different rod receiving elements of claim  19 . Further developments are given in the dependent claims. 
         [0010]    The coupling assembly has a structure that allows a sequential locking of the head of the bone anchoring element and the rod with a locking element using a single drive. By tightening the locking element, first the head is clamped while the rod is still movable. Upon deformation between cooperating portions of the pressure element and the rod receiving element, the rod is fixed subsequently to the locking of the head. 
         [0011]    The pressure element may have a recess or a recessed portion at its inner wall that allows pivoting the bone anchoring element to a larger angle in the direction of the recess or the inner recessed portion compared to the opposite direction. Because the pressure element is rotatable in the coupling element as long as the head is not locked, a position of the enlarged pivot angle can be selected within 360° around a central axis. 
         [0012]    A rod receiving element engages with the receiving part in such a manner that the pressure element is held in a pre-locking position that prevents removal of the head of the bone anchoring element once the head is inserted. In the pre-locking position, the head of the bone anchoring element may be held by a frictional force exerted by the pressure element onto the head. The frictional force may be such that pivoting of the head is still possible by applying a force to overcome the frictional force. 
         [0013]    According to embodiments of the invention, the pressure element has a slit ring at its bottom end. The slit ring can expand in a radial direction to allow for insertion of the head of the bone anchoring element. The force necessary for introducing the head into such a flexible portion of the pressure element is reduced compared to pressure elements that have only longitudinal or coaxial slits. This further simplifies the handling during surgery. 
         [0014]    The coupling assembly can be assembled in situ with a bone anchoring element that has been already inserted into a bone or a vertebra. The rod receiving element may be designed to receive a rod of a specific diameter or diameter range. By providing a kit comprising the coupling assembly and at least an additional rod receiving element that is configured to receive a rod of a different diameter, the coupling assembly can be used with rods of different diameter. This enlarges the field of application of the bone anchoring device. 
         [0015]    The bone anchoring device can be part of a modular system, which includes several bone anchoring elements that may differ in regard of their length of the shank, anchoring features of the shank, such as different thread types, pitches, different diameter of the shank cannulated or non-cannulated shanks and one coupling assembly. The modularity can be further increased by using the different rod receiving elements to couple the coupling assembly to rods of different diameters. This opens for the surgeon the choice between a large variety of implants. In addition, the costs for stock-keeping may be reduced. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    Further features and advantages of the invention will become more apparent from the description of various embodiments using the accompanying drawings. In the drawings: 
           [0017]      FIG. 1  shows a perspective exploded view of a first embodiment of a bone anchoring device; 
           [0018]      FIG. 2  shows a perspective view of the bone anchoring device of  FIG. 1  in an assembled state; 
           [0019]      FIG. 3  shows a cross-sectional view of the bone anchoring device according to the first embodiment of  FIGS. 1 and 2 , the section taken perpendicular to an axis of an inserted rod; 
           [0020]      FIG. 4  shows a cross-sectional view of the bone anchoring device according to the first embodiment of  FIGS. 1 and 2 , the section taken in a plane containing the central axis and a rod axis of an inserted rod; 
           [0021]      FIG. 5  shows a perspective view from above of a receiving part according to the first embodiment; 
           [0022]      FIG. 6  shows a perspective view from the bottom of a receiving part shown in  FIG. 5 ; 
           [0023]      FIG. 7  shows a top view of the receiving part shown in  FIGS. 5 and 6 ; 
           [0024]      FIG. 8  shows a cross-sectional view of the receiving part shown in  FIGS. 5 and 6  along line A-A in  FIG. 7 ; 
           [0025]      FIG. 9  shows a perspective view from above of a pressure element according to a first embodiment; 
           [0026]      FIG. 10  shows a perspective view from the bottom of the pressure element shown in  FIG. 9 ; 
           [0027]      FIG. 11  shows a top view of the pressure element shown in  FIG. 9 ; 
           [0028]      FIG. 12  shows a cross-sectional view of the pressure element of  FIGS. 9 and 10  along line B-B in  FIG. 11 ; 
           [0029]      FIG. 13  shows a side view of a second embodiment of a pressure element; 
           [0030]      FIG. 14  shows a perspective view from above of a third embodiment of the pressure element; 
           [0031]      FIG. 15  shows a perspective view from above of a fourth embodiment of the pressure element; 
           [0032]      FIG. 16  shows a perspective view from above of a rod receiving element according to a first embodiment; 
           [0033]      FIG. 17  shows a perspective view from the bottom of the rod receiving element of  FIG. 16 ; 
           [0034]      FIG. 18  shows a top view of the rod receiving element shown in  FIGS. 16 and 17 ; 
           [0035]      FIG. 19  shows a cross-sectional view of the rod receiving element of  FIGS. 16 and 17  along line D-D in  FIG. 18 ; 
           [0036]      FIG. 20   a  shows a cross-sectional view of the coupling assembly of the bone anchoring device according to the first embodiment wherein the pressure element and the rod receiving element are preassembled with the receiving part and wherein the section is taken in a plane containing the central axis and that is perpendicular to a rod axis; 
           [0037]      FIG. 20   b  shows an enlarged portion of  FIG. 20   a;    
           [0038]      FIG. 20   c  shows an enlarged cross-sectional view of the cooperation between the receiving part, the pressure element and the rod receiving element in a modified example; 
           [0039]      FIG. 21  shows a cross-sectional view of a first step of assembling the bone anchoring device according to the first embodiment; 
           [0040]      FIG. 22  shows a cross-sectional view of a second step of assembling the bone anchoring device according to the first embodiment; 
           [0041]      FIG. 23  shows a cross-sectional view of a third step of assembling the bone anchoring device according to the first embodiment; 
           [0042]      FIG. 24  shows a cross-sectional view of a step of clamping the head of the bone anchoring element in the coupling assembly according to the first embodiment; 
           [0043]      FIG. 25  shows a cross-sectional view of the bone anchoring device with inserted rod and locking element in a step of locking the head without locking the rod; 
           [0044]      FIG. 26   a  shows a cross-sectional view of the bone anchoring device with inserted rod and locking element in a step of locking the rod; 
           [0045]      FIG. 26   b  shows an enlarged portion of a detail of  FIG. 26   a;    
           [0046]      FIG. 27  shows a perspective exploded view of a kit of a coupling assembly with different rod receiving elements for using rods with different diameters; 
           [0047]      FIG. 28  shows a cross-sectional view of the bone anchoring device with inserted rod of a first diameter and with a rod receiving element adapted to receive a rod of the first diameter; 
           [0048]      FIG. 29  shows a cross-sectional view of the bone anchoring device with inserted rod and locking element and a rod receiving element adapted for use with a rod of a second diameter different from the first diameter and such a rod having the second diameter; and 
           [0049]      FIG. 30  shows a cross-sectional view of a bone anchoring device according to a second embodiment with inserted rod and locking element, wherein the section is taken in a plane containing the central axis of the coupling assembly and perpendicular to the rod axis. 
       
    
    
     DETAILED DESCRIPTION 
       [0050]    In  FIGS. 1 and 2 , a bone anchoring device according to a first embodiment includes a bone anchoring element  1  in the form of a bone screw having a shank  2  that is at least partially provided with a bone thread and a head  3 . The head  3  has a spherical segment-shaped outer surface portion including a greatest outer diameter E of the sphere and a flat free end with a recess  4  for engagement with a screwing-in tool. 
         [0051]    The bone anchoring device further includes a coupling assembly  5  for receiving a stabilization rod  100  and for coupling the stabilization rod  100  to the bone anchoring element  1 . The coupling assembly  5  includes a receiving part  6  for receiving the head  3  of the bone anchoring element  1  and for receiving the rod  100 , a pressure element  7  configured to be arranged in the receiving part  6  and a rod receiving element  8  also configured to be arranged in the receiving part  6 . The pressure element  7  serves for locking the head  3  in the receiving part  6 . The rod receiving element  8  serves for receiving the rod  100  and for acting onto the pressure element  7  to lock the head  3 . At least one pin  9  may also be provided for cooperating with the rod receiving element  8 . 
         [0052]    Further, a locking element  10  in the form of an inner screw is provided for securing the rod  100  in the receiving part  6  and in the rod receiving element  8  and for exerting a force via the rod receiving element onto the pressure element  7  to lock the head  3  in the receiving part  6 . 
         [0053]    The receiving part  6  will now be explained with reference to  FIGS. 1 to 8 . The receiving part  6  has a first end  6   a  that is a top end and an opposite second end  6   b  that is a bottom end, a central axis of symmetry C passing through the first end  6   a  and the second end  6   b.  A bore  61  is provided that is coaxial to the central axis C. In a first region adjacent to the first end  6   a,  the receiving part  6  has as substantially U-shaped recess  62  with a bottom directed towards the second end  6   b  and two free lateral legs  62   a,    62   b  extending towards the first end  6   a.  In the region of the legs  62   a,    62   b  an internal thread  63  is provided that cooperates with the locking element  10 . The channel formed by the U-shaped recess  62  is sized so as to receive the rod  100  therein for connecting a plurality of bone anchoring devices. In the region of the legs  62   a,    62   b  up to substantially a height in axial direction that is defined by the bottom of the U-shaped recess  62 , the bore  61  has a first inner diameter. In a region below the legs  62   a,    62   b,  the bore  61  has a widened portion  61  a with a diameter greater than the first inner diameter of the first portion. Between the second end  6   b  and the widened portion  61   a,  the bore  61  has a narrowing portion  61   b  that tapers towards the second end  6   b  with a cone angle. An opening  64  is provided at a second end  6   b,  the diameter of the opening  64  being larger than the largest diameter E of the head  3  to allow the insertion of the head  3  from the second end  6   b  of the receiving part  6 . The widened portion  61   a  and the narrowing portion  6  lb define an accommodation space for the head  3  of the bone anchoring element  1 . 
         [0054]    At a distance from the first end  6   a  below the internal thread  63  a circumferentially extending first groove  66  is provided in the inner surface of each of the legs  62   a,    62   b.  The first groove  66  serves for engagement with a retaining portion of the rod receiving element  8  as described below. A circumferentially extending second groove  67  may be provided at the lower end of the internal thread  63  that serves also for engagement with the retaining portion of the rod receiving elements. 
         [0055]    At one of the legs  62   a,    62   b  a transverse bore  68  is provided that extends through the leg, for example through the leg  62   b,  in a direction substantially perpendicular to the central axis M for receiving the pin  9 . The bore  68  is located approximately at a center of the leg. The pin  9  preferably has such a length that once inserted into the bore  68 , the pin  9  extends a short distance into the bore  61  to provide a stop for the rod receiving element  8  as described below. The pin  9  may be flush with the outer surface of the receiving part  6  when inserted. 
         [0056]    As can be seen in particular in  FIGS. 1 and 9  to  12 , the pressure element  7  has a first end with a free end surface  7   a  and a second end  7   b.  The second end  7   b  of the pressure element  7  is configured to be closer than the first end of the pressure element  7  to the second end  6   b  of the receiving part  6  when the pressure element  7  is arranged in the receiving part  6 . Adjacent to the second end  7   b  the pressure element  7  has a cap-like flexible portion  71  that has a hollow interior chamber  72  that is substantially spherical segment-shaped. The cap-like portion  71  is open at the second end  7   b  to allow the insertion of the head  3  of the bone anchoring element  1 . An outer surface of the cap-like portion  71  has a narrowing portion  71   a  adjacent to the second end  7   b  that tapers in a direction towards the second end  7   b.  The taper corresponds substantially to a taper of the narrowing portion  61   b  of the receiving part  6 . The narrowing portion goes over in a substantially spherical segment-shaped outer surface portion  71   b.  At one circumferential position there is an internal recessed portion  73  adjacent to the second end so that the wall of the cap-like portion  71  is thinner at the recessed portion  73  than at the remaining portions. A width and height of the recessed portion  73  is such that the shank  2  of the bone anchoring element can extend therein. This allows the shank  2  of the bone anchoring element  1  to be pivoted to a greater angle in the direction of the recessed portion  73  than in the opposite direction when the bone anchoring element  1  is inserted. Hence, the pivot angle is enlarged to one side. The enlargement can be, for example, approximately 10°, however, other enlarged pivot angles may be contemplated, depending on the wall thickness and geometry. 
         [0057]    At a distance from the second end  7   b  a circumferentially extending slit  74  is provided. The slit  74  extends circumferentially around the central axis C of the receiving part  7  along a plane substantially perpendicular to the central axis C when the pressure element  7  is arranged in the receiving part  6 . Further, the slit  74  extends around more than 180°, preferably more than 270° and further preferably more than 340° around the central axis C. Therefore, by means of the slit  74 , a ring-shaped portion at the second end is provided that is integrally connected to the rest of the pressure element  7  by a wall portion forming a connecting strip  76 . The connecting strip  76  has such a length in the circumferential direction that it provides a stable connection of the ring-shaped portion to the rest of the pressure element  7 . At one end of the circumferentially extending slit  74 , there is a substantially vertical slit  77  that extends from the second end fully through the ring-shaped portion  75  into the slit  74 . By means of this, the ring-shaped portion is cut through or split in a circumferential direction and forms a slit ring  75  that can be expanded and compressed in a radial direction. A width of the vertical slit  77  is preferably smaller than the width of the circumferential slit  74 , The outer surface of the slit ring  75  is tapered and forms the narrowing outer surface portion  71   a  towards the second end of the pressure element  7 . A position and size of the slit ring  75  is such that when the head  3  of the bone anchoring element  1  is inserted from the open end of the cap-like portion  71 , the slit ring  75  expands so that the width of the vertical slit  77  becomes larger, and when the head  3  has been fully inserted into the hollow interior chamber  72 , the slit ring  75  encompasses the head  3  at the largest diameter E and below the largest diameter E of the head  3  in a direction towards the shank  2 . 
         [0058]    A maximum outer diameter of the cap-like portion  71  is slightly smaller than the inner diameter of the first portion of the bore  61  of the receiving part  6  and is also smaller than the diameter of the bore in the widened portion  61   a . Hence, the slit ring  75  can expand in the widened portion  61   a  of the receiving part  6 . 
         [0059]    At the side of the cap-like portion  71  that is opposite to the second end  7   b  a conically-tapered portion  78  is provided that tapers towards the first end surface  7   a  and that has a conical bore  78   a  that allows access to the recess  4  of the head  3  of the anchoring element  1 . The conical portion  78  is of such a height and outer dimension that it can pass through a bottom opening of the rod receiving element  8  described below. At a distance from the first end surface  7   a  an annular projection  78   b  is formed at the conical segment-shaped portion  78  that has a flat upper surface and an inclined lower surface. An outer circumferential surface  78   c  of the projection  78   b  is substantially cylindrical. 
         [0060]    At the transition of the cap-like portion  71  and the conical segment-shaped portion  78  there is a conically widening collar  79  that widens towards the first end surface  7   a  and that has a flat upper surface  79   a  forming an abutment for cooperating with a portion of the rod receiving element  8 . The conical collar  79  extends in a radial direction beyond the first end surface  7   a  and also beyond the projection  78   b.  The flat upper surface  79   a  is located in axial direction below the first end surface  7   a  of the pressure element  7 . The structure and/or size of the collar  79  is such that the collar  79  is at least slightly deformable in axial direction when an axial load acts upon the upper flat surface  79   a.  In this case, the collar is spread outwards. The deformation of the collar may be a resilient deformation. 
         [0061]      FIG. 13  shows a modified embodiment of the pressure element. The pressure element  7 ′ differs only with respect to the transverse slit in the cap-like portion. The cap-like portion  71 ′ has a slit  74 ′ that extends from a substantially vertical slit  77 ′ in a helix-shaped manner at least partially around the central axis. In a further modification, the slit  77 ′ is not substantially vertical but is just a portion of the helical slit  74 ′. 
         [0062]    Another modified embodiment of the pressure element is shown in  FIG. 14 . The pressure element  7 ″ differs from the pressure element according to the embodiment shown in  FIGS. 9 to 12  by the shape of a conical collar. The conical collar is interrupted at regular distances by slits  79   b  to form circumferential collar portions  79 ′, in the embodiment shown, four collar portions  79 ′. With the number of slits and the size and thickness of the collar portions a specific flexibility of the collar  79 ′ can be achieved. 
         [0063]    A further modified embodiment of the pressure element is depicted in  FIG. 15 . The pressure element  7 ′″ differs from the pressure element shown in  FIGS. 9 to 12  by the design of the cap-like portion. The cap-like portion  71 ″ comprises a plurality of vertical slits  77 ″ that are spaced equidistantly. The vertical slits  77 ″ are open towards the second end  7   b  and are closed by a widened circular portion  77   a  in the direction towards the first end of the pressure element  7 ′″. The number and size of the slits is selected to achieve a specific flexibility of the cap-like portion  71 ″. Furthermore, the pressure element  7 ′″ according to this embodiment does not have the annular projection  78   b  at the conical segment shaped portion  78 . 
         [0064]    It shall be understood that many different combinations of features of the various embodiments of the pressure element can be combined to generate various other embodiments of the pressure element. 
         [0065]    Referring now to  FIG. 1  and  FIGS. 16 to 19 , the rod receiving element  8  is a part that is separate from the pressure element  7 . It is a substantially cylindrical part that comprises a first end with a free end surface  8   a  and an opposite second end with a free end surface  8   b.  Adjacent to the first end with the first end surface  8   a  there is a substantially rectangular recess  81  that cuts out such a portion of the cylinder that two free upstanding legs  81   a,    81   b  remain, the free end surface of which forms the first end surface  8   a.  On the second end  8   b,  there is an opening  82 , preferably a circular opening, that extends through the rod receiving element  8  into the recess  81 . An inner diameter of the opening  82  is smaller than an outer diameter of the first end surface  7   a  of the pressure element so that the first end surface  7   a  of the pressure element can extend through the opening  82  into the rod receiving element  8 . Furthermore, the inner diameter of the opening  82  is substantially the same as or has a slight undersize compared to the outer diameter of the cylindrical outer surface portion  78   c  of the annular projection  78   b  provided at the conical segment-shaped portion  78  of the pressure element  7 . Hence, the pressure element  7  can be held together with the rod receiving element  8  when the conical portion  78  extends through the opening  82  of the rod receiving element  8  and the cylindrical outer surface portion  78   c  of the projection  78  contacts the inner surrounding surface of the rod receiving element  8  at the opening  82  in an interference-fit manner. 
         [0066]    On the bottom of the recess  81  between the legs  81   a  and  81   b  two concave cylinder-segment shaped rod supporting projections  83   a,    83   b  are provided on both sides of the bore forming the opening  82 . The radius of the concave portion  83   a,    83   b  is preferably selected such that it is adapted to the radius of the specific rod to be used. The rod supporting projections  83   a,    83   b  may be separated from the legs  83   a,    83   b  by a groove  84  on each side of the projection. The width of the recess  81  is such that the rod  100  can be received in the recess  81 . When the rod  100  is inserted into the recess  81  of the rod receiving element  8 , it can rest on the rod supporting projections  83   a ,  83   b.  A height of the legs  81   a ,  81   b  is such that the first end surface  8   a  is located slightly above the upper rod surface when the rod  100  is placed into the channel  81  and rests on the rod supporting projections  83   a,    83   b.    
         [0067]    An outer diameter of the rod receiving element  8  is only slightly smaller than an inner diameter of the bore  61  of the receiving part  6  so that the rod receiving element  8  can move in the bore  61  and is guided therein. 
         [0068]    The rod receiving element  8  further comprises at each leg  81   a,    81   b  an elongate recess  85  that extends substantially from the bottom of the recess  81  to a distance from the first end surface  8   a.  The recesses  85  extend completely through each leg  81   a,    81   b  in a radial direction. The recesses  85  have a width so that the pin  9  can be accommodated therein. A bottom end  85   a  of each elongate recess  85  forms a stop for the pin  9  that extends into the respective recess. The stop prevents the rod receiving element  8  from escaping through the first end  6   a  of the receiving part  6  when the pressure element  7  is in an inserting position. Furthermore, the recesses  85  may serve together with the at least one pin  9  as a securing device to maintain the alignment between the recess  62  of the receiving part  6  and the channel  61  of the rod receiving element  8 . 
         [0069]    The rod receiving element  8  also has on each leg at approximately the center of each leg in an axial direction, a circumferentially extending projection  86  with a flat upper surface  86   a  and an inclined lower surface  86   b  the inclination of which is such that the projection  86  can move out of the grooves  66 ,  67  when the rod receiving element is pushed downwards towards the second end  6   b  of the receiving part  6 . The shape of the projection  86  is such that as can be seen in  FIGS. 20   a  and  25 , the projection  86  fits into the groove  66 ,  67 , respectively and can abut with the upper flat surface  86   a  against the upper edge of the groove  66 ,  67 . 
         [0070]    Furthermore, on each leg a longitudinally extending cylinder segment-shaped recess  87  is provided that extends from the bottom of the recess  81  to the first end surface  8   a.    
         [0071]    The bone anchoring device as a whole or in parts is made of a bio-compatible material, such as a bio-compatible metal or metal alloy, for example titanium, stainless steel, a nickel-titanium alloy, for example, Nitinol, or of bio-compatible plastic materials, such as, for example, polyetheretherketone (PEEK). 
         [0072]    Referring to  FIGS. 20   a  to  20   c , the coupling assembly may be pre-assembled in the following manner. The rod receiving element  8  and the pressure element  7  are inserted into the receiving part  6 . Thereby the pressure element may be inserted through the lower opening  64  of the receiving part  6 . The rod receiving element  8  may be inserted from the first end  6   a  of the receiving part by slightly pressing the legs  81   a,    81   b  together until the projection  86  snaps into the second groove  67 . The pressure element  7  is in the first embodiment shown in  FIGS. 20   a  and  20   b  temporarily held in the rod receiving element  8  by an interference fit. This is achieved by the cylindrical surface portion  78   c  of the projection  78   b  on the pressure element  7  that has slight oversize with respect to the inner surface of the rod receiving element  8  at the opening  82 . The rod receiving element  8  is prevented from escaping through the first end  6   a  of the receiving part during insertion of the pressure element because the flat upper surface  86   a  abuts against the upper edge of the second groove  67  in the receiving part  6 . The pressure element  7  is also in its uppermost position, where the flat surface  79   a  of the collar  79  abuts against a second end surface  8   a  of the rod receiving element  8 . 
         [0073]    By means of this arrangement, the cap-like portion  71  and in particular the slit ring  75  is located at a position where the coaxial bore  61  has the widened portion  61   a  with the greatest inner diameter. As can be seen in  FIG. 20   a , the slit ring  75  has space to expand in this position. 
         [0074]    In an alternative embodiment, as shown in  FIG. 20   c  in which the pressure element does not have the annular projection  78   b,  for example with the pressure element  7 ′″ as depicted in  FIG. 15 , the pressure element  7  is not held in the insertion position by interference fit with the rod receiving element  8  but via snap connection of the outermost edge of the collar  79  that engages a third groove  69  at the inner side of the receiving part. The third groove  69  may be small and shallow as it is only necessary for holding the pressure element in the insertion position. 
         [0075]    Referring to  FIGS. 21 to 26   b , the use of the coupling assembly  5  together with a bone anchoring element  1  and a rod  100  will be explained. As depicted in  FIG. 21 , first, a suitable bone anchoring element  1  is implanted into the bone. In  FIG. 21 , the reference numeral  200  depicts the bone surface. Because the bone anchoring element  1  can be placed into the implantation site without the coupling assembly being connected thereto, the step of implanting the bone anchoring element  1  is facilitated. Thereafter, the coupling assembly  5  with the pressure element  7  and the rod receiving element  8  in the insertion position is placed onto the head  3  of the bone anchoring element I. As depicted in  FIG. 23 , the head  3  enters the receiving part  6  through the lower opening  64  and enters the cap-like portion  71  of the pressure element  7  through the open second end  7   b  of the pressure element  7 . When the head  3  touches the slit ring  75  of the pressure element  7 , the pressure element  7  presses with the collar  79  against the second end surface  8   b  of the rod receiving element and shifts the rod receiving element upward until the bottom  85   a  of the elongate recess  85  of the leg  81   b  abuts against the pin  9  extending through the leg  62   b  of the receiving part  6 . Hence, the rod receiving element  8  is prevented from being pushed out through the first end  6   a  of the receiving part  6 , when the head is inserted. 
         [0076]    The pressure element  7  is rotatable in the receiving part. It may be placed such, that the recessed portion  73  that provides the enlarged pivot angle is at a specific position. To achieve this, an alignment feature (not shown) can be provided that allows to rotate the pressure element such that the recessed portion  73  assumes the desired position. 
         [0077]    Further introduction of the head  3  into the cap-like portion  71  expands the slit ring  75  within the widened portion  61   a  of the bore  61  of the receiving part  6 . The head  3  can then be completely inserted. Because the slit ring  75  does not expand at the connection strip  76 , the insertion of the head  3  may not be precisely coaxial with the central axis C but instead may be slightly out of or misaligned with the central axis C. By the further introduction of the head  3 , the slit ring  75  may be expanded to a maximum extent and allows the head  3  to enter the upper portion of the hollow interior chamber  72  until the head  3  abuts against the inner wall of the chamber  72 . Here, the slit ring  75  can elastically contract around the head  3  as shown in  FIG. 23 . 
         [0078]    When the head  3  abuts against the inner wall of the cap-like portion  71  in the upper region, the slit ring  75  encompasses a portion of the head  3  below the region with the largest diameter B as can be seen in  FIG. 23 . In this condition, the slit ring  75  frictionally clamps the head  3 , so that the bone anchoring element  1  can be pivoted to a desired angular position relative to the receiving part  6  and can be held there by the friction fit between the pressure element  7  and the head  3 . 
         [0079]    Finally, pulling the receiving pan  6  and pressing down the rod receiving element  8  with an instrument (not shown) as depicted in  FIG. 24 , presses the slit ring  75  into the narrowing portion  61   b,  so that the tapered outer surface  71   a  of the slit ring  75  engages the narrowing portion  61   b  of the receiving part  6 . Simultaneously, the projection  86  snaps into the first groove  66  of the receiving part  6 . In this condition, as depicted in  FIG. 24 , the rod receiving element  8  is. prevented from moving upward because of the stop provided by the upper edge of the groove  66  against which the flat upper surface  86   a  of the projection  86  of the rod receiving element  8  abuts. The head  3  is already clamped by the cap-like portion  71  of the pressure element  7 . Because the slit ring  75  is located between the head  3  and the narrowing portion  61   b  of the receiving part, the lower opening  64  of the receiving part  6  is narrowed and the head  3  is prevented from falling out or being pushed out through the lower opening  64 . This is the pre-locking condition. 
         [0080]    In clinical use, usually at least two bone anchoring devices are inserted into the bone and the receiving parts are aligned. Because the head  3  of the bone anchoring element  1  is held in the pressure element by a frictional force, the receiving part  6  can be easily aligned manually and its angular position is maintained by the frictional force between the head and the pressure element  7 . Then, as can be seen in  FIG. 25 , the rod  100  is inserted into the receiving part  6  and the locking element  10  is screwed between the legs  62   a,    62   b  of the receiving part  6 . The rod  100  first is supported by the support surfaces  83   a,    83   b  of the rod receiving element  8 . Tightening of the locking element moves the rod receiving element  8  slightly downward while the locking element  10  only contacts the first end surface  8   a  of the rod receiving element  8  and exerts a force on the first end surface  8   a.  The force F is transferred via the second end surface  8   b  onto the upper flat end surface  79   a  of the collar  79  of the pressure element  7  onto the head  3  and from the head  3  onto the slit ring  75  and the receiving part as depicted in  FIG. 25 . By means of this, the head  3  is locked in its angular position with respect to the receiving part while the rod  100  is still movable in the direction of the rod axis. 
         [0081]    Referring now to  FIGS. 26   a  and  FIGS. 26   b , a further advancement of the locking element  10  results in the second end surface  8   b  exerting a larger force F onto the upper surface  79   a  of the collar  79  of the pressure element  7  so that the collar  79  is slightly deformed, as can be seen in  FIG. 26   b . By means of this, the rod receiving element  8  can move slightly further downwards, so that the locking element  10  presses onto the rod. Thereby, the rod is fixed. In this condition, the force is transmitted through the rod and the first end surface  7   a  of the pressure element  7  onto the head  3  and the receiving part  6 . 
         [0082]    In the locked condition, as shown in  FIGS. 3 and 4 , the anchoring element  1  may assume a greater pivot angle when it is abutting the recessed portion  73  of the pressure element with the shank than in an opposite direction. 
         [0083]    If the position of the rod  100  relative to the receiving part  6  has to be corrected, the locking element  10  is rotated backwards until the pressure onto the rod is relieved and the rod becomes freely movable again. Because the slit ring  75  is held in the narrowing portion of the receiving part, the locking of the head  3  is maintained. 
         [0084]      FIG. 27  shows a kit of a bone anchoring device with a coupling assembly comprising at least two rod receiving elements  8 ,  8 ′. The coupling assembly  5  comprising the receiving part  6 , the pressure element  7  and the rod receiving element  8  is the same as previously described and can have all the modifications as previously described. The rod receiving element  8  is a first rod receiving element and is sized and configured to receive the rod  100  that is a first rod having a first outer diameter. The rod receiving element  8 ′ is a second rod receiving element that is sized and configured to receive a second rod  100 ′ that has a second diameter smaller than the first diameter of the first rod  100 . Consequently, rod support surfaces  83   a ′,  83   b ′ of the second rod receiving element  8 ′ may have a curvature that corresponds to the second rod  100 ′ having the second diameter. Furthermore, a height of the upstanding legs  81   a ′,  81   b ′ of the second rod receiving element  8 ′ is smaller than a height of the legs  81   a,    81   b  of the first rod receiving element  8 , so as to project only slightly over the upper rod surface of the second rod  100 ′ when the second rod  100 ′ is inserted into the rod receiving element  8 ′. An outer diameter in size and an inner diameter is the same as for the first rod receiving element  8 . 
         [0085]      FIG. 28  shows the bone anchoring device which has the second rod receiving element  8 ′ and which is coupled to the second rod  100 ′.  FIG. 29  shows the first bone anchoring device which has the first rod receiving element  8  and is coupled to the first rod  100 . Because the height of the second rod receiving element  8 ′ is smaller than the height of the first rod receiving element  8 , the locking element  10  must be screwed deeper in between the legs as depicted in  FIG. 27  compared to the case in which the first diameter rod  100  is used as depicted in  FIG. 27 . 
         [0086]    By means of the kit, with a single bone anchoring device, various rods with various diameters can be used by selecting the appropriate rod receiving element. It is even possible to use a rod with sections of different diameters, for example, a rod that has a first diameter in one section and a second diameter smaller than the first diameter in another section. In this case, the first bone anchoring device has a first rod receiving element with a greater height and a second bone anchoring device has a second rod receiving element with a smaller height. The kit opens a variety of applications and provides the advantage of a modular system. For example, for the bone anchoring element, all kinds of anchoring elements can be used and combined with the coupling assembly. These anchoring elements are e.g. screws of different lengths, with different diameters, cannulated screws, screws with different thread forms, nails, hooks, etc. The head and the shank may also be separate parts that are connectable to each other. 
         [0087]    Additionally, a kit of the coupling assembly with at least one further pressure element can be provided. The pressure element can be designed without a recess for an enlarged pivot angle or can have several recesses, for example two or more equidistantly spaced recesses for having enlarged pivot angles in two or more directions. By combining the elements like the pressure element and the rod receiving element with a receiver, the most suitable bone anchoring device for a specific clinical application can be easily generated. 
         [0088]    A second embodiment of the coupling assembly is described with reference to  FIG. 30 .  FIG. 30  shows the bone anchoring device with the anchoring element  1 , the rod  100  and the locking element  10  as in the previous embodiments. The pressure element  700  differs from the pressure element of the previous embodiments in that it is free from a deformable portion that cooperates with the rod receiving element and does not have the annular projection for connecting it in an interference-fit manner to the rod receiving element. Instead of this, the pressure element  700  comprises a non-deformable annular projection  790  with a flat upper surface  790   a  that cooperates with a collar-like conically widening projection  890  at the second end of the rod receiving element  800 . The collar like projection  890  has a substantially inverse shape compared to the collar  79  as described in the previous embodiments. It is slightly flexible when it engages the flat upper surface  790   a  of the pressure element  700  when a load acts onto the rod receiving element  800  via the locking element  10 . Furthermore, the rod receiving element  800  comprises at its second end  8   b  an inner projection  880  with a conical outer surface that is configured to engage the conical outer surface of the conical portion  78  of the pressure element  700  in order to temporarily hold the pressure element  700 . In this embodiment, the deformation takes place at the rod receiving element  800 . The effect is the same as in the previously described embodiments, namely that the rod receiving element can move slightly downward when the locking element  10  is tightened so that the first end surface  7   a  of the pressure element moves upward to contact the rod and the locking element  10  can engage the rod. 
         [0089]    Various modifications of the embodiments described before may be contemplated. The receiving part is not limited to the exact shape as shown. The recess  61  does not have to have an exact U-shape. The bore  61  can have several sections with different widths, as long as the enlarged portion that provides space for expansion of the pressure element is provided. The narrowing portion at the bottom of the receiving part is shown to be tapered, but can also be rounded. Also, the external surface of the cap-like portion at the bottom end of the pressure element can be rounded, Combinations of the surfaces of receiving part and pressure element that cooperate to clamp the head can be tapered and tapered, tapered and rounded and vice-versa rounded and rounded. The pressure element may have a cylindrical portion instead of the conical portion  78 . 
         [0090]    The head and correspondingly the interior chamber and the exterior surface of the cap-like portion of the pressure element can have another shape. For example two opposite flattened portions may be present that render the pivot connection monoplanar. 
         [0091]    All kinds of rods can be used. While rods with a smooth surface are shown, roughened rods or rods having a structure may be used. The rods may also be flexible rods. 
         [0092]    While a number of different embodiments are disclosed herein, it is appreciated that different components from the different embodiments can be mixed and matched to produce a variety of still other different embodiments. 
         [0093]    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.