Abstract:
An instrument for holding and inserting a bone anchor into the bone is provided, wherein the bone anchor comprises a shank for anchoring in the bone and a head; the instrument comprising: a holding member for holding the head of the bone anchor and comprising at least two arms configured to at least partially encompass the head, wherein the at least two arms comprise an inner surface forming a seat for the head; a drive shaft for engaging the head of the bone anchor and defining a longitudinal axis of the instrument; and a displacement member configured to act onto the holding member such that the holding member can assume a first configuration in which it is configured to permit the head to enter the seat and a second configuration in which it is configured to hold the head in the seat.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present disclosure claims the benefit of U.S. Provisional Patent Application Ser. No. 61/953,548, filed on Mar. 14, 2014, the contents of which are hereby incorporated by reference in their entirety, and claims priority to European Patent Application EP 14160107.0, filed on Mar. 14, 2014, the contents of which are hereby incorporated by reference in their entirety. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to an instrument for holding and inserting a bone anchor into the bone. The instrument comprises a holding member including at least two arms that are configured to hold a head of the bone anchor between them and a displacement member that acts onto the holding member so that the holding member can assume a first configuration in which it is configured to receive the head and a second configuration in which it holds an inserted head. Further, the instrument comprises a drive shaft that is configured to engage the head of the bone anchor. A particular application of the instrument is the use with a polyaxial screw of the bottom loading type that comprises a bone anchor and a receiving part to receive a rod to connect the rod to the bone anchor. In this application, the bone anchor is first inserted into the bone using the instrument and the receiving part is mounted thereafter in situ onto the bone anchor. 
         [0004]    2. Description of the Related Art 
         [0005]    An instrument for holding and inserting of bone screws, in particular of pedicle screws, is known from DE 20 2005 012 781 U1. The instrument comprises a holding member with at least two gripping arms for a bone screw and a displacement member that is movable relative to the gripping arms. By means of the displacement member the gripping arms can assume a configuration which allows the insertion of the bone anchor and a configuration in which the bone anchor is held between the arms. The gripping arms comprise projections that engage recesses on the bone screw in order to allow the screwing-in of the bone screw via the gripping arms. 
         [0006]    US 2012/0296171 A1 describes an inserter for a bone anchor comprising a driver for engaging the head of a bone screw and a receiver member with a spherical inner surface portion that becomes engaged with the head and/or the neck of the bone anchor. 
       SUMMARY 
       [0007]    It is the object of the invention to provide an instrument for holding and inserting a bone anchor that provides a safe and robust connection between the bone anchor and the instrument and an automatic alignment of the bone screw and the instrument. 
         [0008]    The object is solved by an instrument according to claim  1  and by a system of an instrument in a bone anchor according to claim  14 . Further developments are given in the dependent claims. 
         [0009]    The instrument includes a holding member with a seat for the head of a bone anchor, a displacement member for acting onto the holding member such that the holding member can assume a first configuration in which it is configured to permit the head to enter the seat and a second configuration in which it is configured to hold the inserted head, and a drive shaft that is configured to engage the head of the bone anchor. A center of the seat is offset from a center of the head in such a manner that the head is pressed against an abutment surface. Thereby, a play between an engagement portion of the drive shaft and the head is eliminated. This allows a precise insertion of the bone anchor without experiencing a play between the drive shaft and the bone anchor. Also, by means of this design, the connection between the instrument and the bone anchor is robust. 
         [0010]    The head of the bone anchor may be held in the seat in a form-fit or positive-fit manner. Furthermore, the drive shaft of the instrument may comprise an engagement portion with a structure that engages a corresponding engagement portion of the head of the bone anchor in a form-fit manner. Therefore, the bone anchor is automatically aligned correctly with the drive shaft. 
         [0011]    The size and position of the seat may be designed such that a lower end of the holding member acts as an abutment for the bone surface. Hence, when the bone anchor is inserted with the instrument, the free lower end of the holding member abuts against the bone surface and indicates that a correct insertion of the bone anchor has been reached that allows to mount the receiving part thereafter in situ. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Further features and advantages of the invention will become apparent from the description of embodiments by means of the accompanying drawings. In the drawings: 
           [0013]      FIG. 1  shows a perspective exploded view of the instrument according to an embodiment. 
           [0014]      FIG. 2  shows a perspective view of the instrument of  FIG. 1  in an assembled state in a first configuration wherein it is configured to receive a head of a bone anchor. 
           [0015]      FIG. 3  shows a perspective view of the instrument of  FIG. 3  in a second configuration wherein the head of the bone anchor is received and clamped by the instrument. 
           [0016]      FIG. 4  shows a cross-sectional view of a main portion of the instrument, the cross-section taken in a plane containing the longitudinal axis of the instrument. 
           [0017]      FIG. 5  shows an enlarged cross-sectional view of an upper portion of the instrument of  FIG. 4 . 
           [0018]      FIG. 6  shows an enlarged cross-sectional view of a lower portion of the instrument of  FIG. 4 . 
           [0019]      FIG. 7  shows a perspective view of the drive shaft of the instrument of  FIGS. 1 to 6 . 
           [0020]      FIG. 8  shows an enlarged view of portions of the drive shaft of  FIG. 7 . 
           [0021]      FIG. 9  shows a perspective view from the front end side of a holding member of the instrument of  FIGS. 1 to 6 . 
           [0022]      FIG. 10  shows a perspective view from the rear end side of the holding member shown in  FIG. 9 . 
           [0023]      FIG. 11  shows a top view onto the rear end of the holding member of  FIGS. 9 and 10 . 
           [0024]      FIG. 12  shows a cross-sectional view of the holding member of  FIGS. 9 to 10  along line A-A in  FIG. 11 . 
           [0025]      FIG. 13  shows a perspective view from the rear end side of a displacement member of the instrument according to  FIGS. 1 to 6 . 
           [0026]      FIG. 14  shows a perspective view from the front end side onto the displacement member of  FIG. 13 . 
           [0027]      FIG. 15  shows a top view onto the rear end of the displacement member of  FIGS. 13 and 14 . 
           [0028]      FIG. 16  shows a cross-sectional view of the displacement member of  FIGS. 13 and 14 . 
           [0029]      FIG. 17  shows a perspective view from the rear end side of an actuator of the instrument according to  FIGS. 1 to 6 . 
           [0030]      FIG. 18  shows a perspective view from the front end side onto the actuator of  FIG. 17 . 
           [0031]      FIG. 19  shows a top view from the rear end of the actuator of  FIGS. 17 and 18 . 
           [0032]      FIG. 20  shows a cross-sectional view of the actuator of  FIGS. 17 and 18  along line C-C in  FIG. 19 . 
           [0033]      FIG. 21  shows a cross-sectional view of a portion of the instrument in a first step, of inserting a head of the bone anchor when the instrument is in the first configuration. 
           [0034]      FIG. 22  shows an enlarged cross-sectional view of a portion of the instrument in the first configuration with the inserted head of the bone anchor. 
           [0035]      FIG. 23   a  shows a cross-sectional view of the instrument in the second configuration with the inserted head of the bone anchor. 
           [0036]      FIG. 23   b  shows an enlarged cross-sectional view of a detail of  FIG. 23   a.    
           [0037]      FIGS. 24 to 26  show schematic enlarged cross-sectional views of a lower portion of the instrument with inserted head. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    Referring to  FIGS. 1 to 3 , an instrument for holding and inserting a bone anchor into the bone according to an embodiment includes a drive shaft  1  that is configured to engage a head portion of the bone anchor and to transmit torque to the bone anchor, a holding member  2  for receiving and holding the head of the bone anchor, and a displacement member  3  that is configured to act onto the holding member  2 . The holding member can assume a first configuration as depicted in  FIG. 2  wherein the head of the bone anchor can be inserted into the holding member, and a second configuration as depicted in  FIG. 3 , wherein the head of the bone anchor is firmly held in the holding member  2 . The instrument further includes an actuator  4  that cooperates with the drive shaft  1  and the displacement member  3  such as to move the displacement member  3  into a first and second position corresponding to the first and second configuration of the holding member  2 . Finally, the instrument includes a handle  5  that is connectable to the drive shaft  1 . 
         [0039]    Referring more in detail to  FIGS. 2 and 3 , a bone anchor  100  in this embodiment comprises a threaded shank  101  and a head  102  at one end of the threaded shank  101 . The head  102  typically has an surface portion that is spherically-shaped and a substantially flat free end surface  103 . The spherical surface portion may include a region of the greatest diameter of the sphere. Such bone anchors are commonly used in polyaxial bone screws that serve for stabilization of the spinal column by means of stabilizing rods. The polyaxial bone screws typically have a receiving part (not shown) that is configured to pivotably receive the bone anchor  100  and to receive the spinal stabilization rod (not shown) to couple the rod to the bone anchor. The instrument is particularly applicable to bottom loading type polyaxial bone screws wherein the head  102  of the bone anchor  100  is inserted into the receiving part from the bottom end thereof, i.e. from the end that faces the bone surface. 
         [0040]    With reference to  FIGS. 4 to 8 , the drive shaft  1  may be a monolithic substantially rod-shaped member that comprises a first end  11 , that is in use the proximal end, and an opposite second end  12 , that is in use a distal end. At the first end  11  there is an engagement projection  13  is provided that is configured to engage a corresponding recess in the head of head  102  of the bone anchor to transmit torque to the head  102 . The engagement projection  13  has a torx-shape. However, any other shape is possible that allows to connect the engagement projection  13  in a form-fit or positive-fit manner to the head  102  of the bone anchor  100 . Following the engagement projection  13 , the drive shaft  1  comprises a first shaft portion  14  with a first diameter that is greater than that of the engagement projection  13 . At the front end of the first shaft portion  14  an abutment surface  14   a  is formed that extends circumferentially around the engagement projection  13  and that is substantially flat. 
         [0041]    At the side opposite to the engagement projection  13 , the first shaft portion  14  comprises a threaded section  15  with an outer thread that serves for cooperating with a corresponding inner thread of the holding member  2  and that allows to mount in the holding member  2  onto the shaft portion  14 . At a distance from the threaded section  15  there is a section  16  with a larger outer diameter so that an annular abutment surface  16   a  facing towards the first end  11  is provided. This abutment surface  16   a  functions as a stop for the holding member  2  when the holding member is mounted to the first shaft portion  14 . An intermediate portion  16   b  with a slightly smaller diameter than the portion  16  may be present to improve the stability and or the alignment of the connection between the holding member  2  and the drive shaft  1 . 
         [0042]    Between the distal end  12  and the larger diameter section  16  there is a second shaft portion  17  that comprises approximately at the middle section a portion  18  with a greater outer diameter. The portion  18  is threaded and cooperates with the actuator  4 . Preferably, the thread of the portion  18  is a fine pitch thread and more particularly, it may be a multi-start fine pitch thread, such as a double-start fine pitch thread. This allows to obtain a greater displacement in the axial direction of the actuator  4  when the actuator  4  is screwed back and forth along the threaded section  18 . Hence, the displacement of the displacement member  3  can be achieved with less turns or in shorter time. An end section  19  adjacent to the second end  12  comprises one or more longitudinal flats  19   a  compared to a single-start thread to provide a form-fit connection to the handle  5  such that the handle  5  can not rotate relative to the drive shaft  1  once it is mounted to the drive shaft  1 . An annular groove  19   b  on the end section  19  may be provided that cooperates with a corresponding projection in the handle  5  to prevent inadvertent disassembling of the handle. 
         [0043]    By means of the cylindrical drive shaft  1  a longitudinal axis L of the instrument is defined. 
         [0044]    Next, the holding member  2  will be explained referring to  FIGS. 9 to 12 . The holding member  2  is sleeve-shaped and has a first or proximal end  21  and an opposite second or distal end  22 . At a distance from the second end  22  there is an internally threaded section  23  with a slightly smaller diameter than that of the sleeve at the second end  22 . The internally threaded section  23  cooperates with the threaded section  15  on the drive shaft  1 . By the reduction of the diameter a shoulder  23   a  is formed that faces towards the second end  22  and that cooperates with the portion  16   b  on the drive shaft  1 . The holding member is slotted by a slot  24  that is open to the first end  21  and that extends through the holding member  2  in a plane including the longitudinal axis L so that two resilient arms  25   a,    25   b  are formed by the slot  24 . The slot  24  widens into a substantially elongate opening  24   a  and is that closed towards the second end  22 . The opening  24   a  provides greater flexibility to the arms  25   a,    25   b.    
         [0045]    At a distance from the first end  21 , there is an inner spherical segment-shaped section that provides a seat  26  for the head  102  of the bone anchor. The radius of the inner spherical segment-shaped section  26  is such that it substantially matches the shape of an outer surface portion of the head  102 . As can be seen more in detail in  FIG. 23   b , the spherical segment-shaped section  26  is designed such that a region with a largest diameter of the sphere defined by the section  26  is between an upper edge  26   a  and a lower edge  26   b  of the seat. Hence, when the head  102  is received in the seat  26  and the arms  25   a,    25   b  are pressed together, the head  102  is held in a form-fit manner in the seat  26 . Adjacent to the seat  26 , there is a cylindrical section  27  with an inner diameter that is smaller than that of the lower edge  26   b  of the seat  26 . Between the cylindrical section  27  and the first end  21 , there is a tapered section  28  that tapers towards the seat defined by the seat  26 . When the head  102  is received in the seat  26  and the arms  25   a ,  25   b  are pressed together, the head  102  is held in the holding member such that it can not be removed through the first end  21 . 
         [0046]    Referring more in detail to  FIGS. 6 and 23   b , the center of the sphere C 1  of the seat  26  is offset in an axial direction from the center of the sphere C 2  of the head  102  when the head  102  is placed into the seat  26 . More specifically, in the second configuration, the distance d 1  of the center of the sphere of the seat  26  from the abutment surface  14   a  of the drive shaft is smaller than the distance d 2  of the center of the sphere C 2  of the head  102  from the abutment surface  14   a  when the head  102  is held in the seat  26  and the two arms  25   a,    25   b  are pressed together. By means of this, when the head  102  is held in the seat  26  it experiences a force that tends to move the head  102  towards the abutment surface  14   a.    
         [0047]    The dimensions of the lower portion of the holding member that includes the seat  26 , the cylindrical portion  27  and the tapered portion  28  correspond substantially to a receiving part of a polyaxial bone screw or are slightly larger in an axial direction. Therefore, when the head  3  is held in the holding member  2  and inserted into the bone, the first end  21  of the holding member forms an abutment for the bone surface so that the bone anchor  100  is inhibited from further insertion into the bone. The head  102  protrudes above the bone surface to such an extent that the receiving part can be easily mounted thereon. 
         [0048]    The length of the holding member  2  is such that when the holding member  2  is screwed onto the threaded portion  15  of the drive shaft until its second end  22  abuts against the abutment surface  16   a,  the first end  21  extends beyond the first end  11  of the drive shaft by a distance. The distance is such that the offset between the centers of the spheres C 1  and C 2  of the seat  26  and the head  102 , respectively, causes the free end surface  103  of the head  102  to be pressed against the abutment surface  14   a  of the drive shaft  1 . An inner diameter of the holding member  2  is such that the portion  14  of the drive shaft can pass there through and is guided therein. 
         [0049]    Furthermore, as depicted in particular in  FIG. 12 , an outer surface portion  29  adjacent to the first end  21  of the holding member  2  is tapered such that it narrows towards the second end  22 . Hence, the holding member  2  resembles a collet chuck and functions like a collet chuck. 
         [0050]    The displacement member  3  will be described with reference to  FIGS. 4 to 6  and  13  to  16 . The displacement member  3  is formed as a substantially cylindrical sleeve with a first or proximal end  31  and an opposite second or distal end  32  of the sleeve. An inner diameter of the displacement member  3  is such that, as can be seen in  FIGS. 4 to 6 , the holding member  2  can extend through the displacement member  3 . Adjacent to the first end  31  an inner surface  33  of the displacement member is slightly tapered in a direction such that it widens towards the first end  31 . This slightly tapered section  33  is followed towards the second end by a second slightly tapered section  34  with an angle of the taper that is even smaller than the angle of the taper of the section  33 . The remaining part of the displacement member  3  has a cylindrical inner surface  35 . The function of the slightly tapered surfaces  34  and  33  is to cooperate with the tapered outer surface  29  of the holding member  2  when the displacement member  3  is moved from its first position in the first configuration to the second position in the second configuration. During this displacement, the tapered inner surfaces  34  and  33  slide along the tapered outer surface  29  thereby generating an increasing pressure onto the arms  25   a,    25   b  that presses the arms together to clamp the inserted head  102 . 
         [0051]    The second end  32  has a free annular end surface  32   a  that acts as an abutment for the actuator  4  as described below. From the end surface  32   a,  a short circular rim  32   b  with a smaller inner diameter protrudes upward. Moreover, a flap-like extension or flap  36  projects above the rim  32   b.  The flap  36  has a substantially cylindrical inner surface and a substantially cylindrical outer surface and a trapezoidal contour when viewed in a side front view, hence, its width on the basis adjacent to the rim  32   b  is larger than its free end. Furthermore, at the free end, the flap  36  comprises an outwardly extending circumferential projection  37  that cooperates with a portion of the actuator  4  to be described below. The height of the flap  36  between the shoulder  32   a  and the outward projection  37  corresponds substantially to the height of an inner portion of the actuator  4  in an axial direction. The flap  36  has the function of a follower that rotatably connects the displacement member  3  to the actuator but prevents an axial movement of the displacement member  3  relative to the actuator  4 . An axial advancement of the actuator results in a following axial advancement of the displacement member  3 . 
         [0052]    The displacement member  3  may further have a plurality of elongated openings  38  that extend completely through the cylindrical wall portion. The openings  38  serve for facilitating the cleaning of the parts. 
         [0053]    A total axial length of the displacement member  3  is such that in a position of the actuator  4  that corresponds to the first configuration, the first end  31  has a distance from the first end  21  of the holding member that allows the holding member  2  to spread outward from the first end  31  of the displacement member  3  to allow the insertion of the head. Furthermore, the length is such that in a second position of the actuator  4  that corresponds to the second configuration, the tapered inner sections  33 ,  34  cooperate with the tapered outer surface  29  of the holding member to clamp the head  102 . 
         [0054]    The actuator  4  will be described with reference to  FIGS. 4 to 6  and  17  to  20 . The actuator  4  comprises a first end or proximal end  41  and an opposite second or distal end  42 . At a small distance from the second end  42  a bore  43  with an internal thread is provided that cooperates with the external thread of the section  18  of the drive shaft  1 . Therefore, the internal thread is also preferably a multi-start fine thread. Adjacent to the threaded bore  43 , there is an inner substantially cylindrical section  44  that has a diameter greater than an outer diameter of the threaded section  18  of the drive shaft  1  such that the drive shaft  1  can extend therethrough with the threaded section  18 . Between the cylindrical inner section  44  and the first end  41 , there is a second cylindrical section  45  with a smaller diameter than the section  44  such that an annular shoulder  45   a  is provided within the actuator  4 . An inner diameter of the cylindrical section  45  is such that the portion  17  of the drive shaft  1  and in addition the flap  36  of the displacement member  3  can extend therethrough, as can be seen in particular in  FIG. 5 . A distance between the shoulder  45   a  and the first end  41  is such that the flap  36  can abut with the lower side of its circumferential outward projection  37  against the shoulder  45   a  and the first end  41  of the actuator can abut against the free end surface  32   a  of the displacement member  3 . Hence, the displacement member  3  is configured to be connected via the flap  36  to the actuator  4  such that the actuator  4  can rotate with respect to the displacement member  3  but the displacement member  3  is fixed in an axial direction with respect to the actuator  4 . 
         [0055]    The actuator  4  further comprises an upper substantially cylindrical outer section  46  adjacent to the second end  42  with a plurality of flats  46   a  that facilitate gripping. The flats  46   a  are shown to be oval-shaped, but can have any other shape. The number can be any number. 
         [0056]    Instead of the flats  46   a  other means can be used for facilitate gripping, such as, for example, longitudinal ripples or grooves or a roughened surface. 
         [0057]    The actuator further comprises a second outer cylindrical section  47  adjacent to the first end  41  with an outer diameter that substantially corresponds to an outer diameter of the displacement member  3  so that the actuator  4  is flush with the displacement member  3  as depicted in  FIGS. 3 to 5 . 
         [0058]    Referring further to  FIG. 1 , the handle  5  comprises a coaxial bore  51  that has an internal flat portion that cooperates with the flat  19   a  of the drive shaft, so that the handle  5  can be mounted onto the rear end portion  19  of the drive shaft  1  in a rotationally fixed manner. The bore  51  of the handle  5  may have an internal annular projection that engages the groove  19   b  of the drive shaft  1  so that the handle  5  may be clicked onto the drive shaft  1 . 
         [0059]    All parts of the instrument that may have contact with tissue or bone may be made of a body-compatible material, such as, for example, stainless steel. 
         [0060]    The instrument is easy to assemble and disassemble. This provides the advantage that after use it can be easily cleaned. The instrument is mounted as follows. First the holding member  2  is screwed onto the drive shaft so that the first portion  14  of the drive shaft extends through the holding member  2 . The position of the holding member  2  on the drive shaft is limited in an axial direction by the abutment of the second end  22  of the holding member against the abutment surface  16   a.  Then the displacement member  3  is connected to the actuator  4  in that the flap  36  is inserted into the actuator  4  until its projection  37  rests on the shoulder  45   a  and the first end  41  of the actuator rests on the free end surface  32   a  of the displacement member  3 . Then, the pre-assembled displacement member  3  and actuator  4  are placed onto the drive shaft  1  from the second end  12  of the drive shaft so that the internal threaded portion  43  of the actuator is screwed onto the externally threaded portion  18  of the drive shaft  1 . Finally, the handle  5  is clicked onto the rearward end  19  of the drive shaft  1 . 
         [0061]    The use of the instrument with a bone anchor will be described with reference to  FIGS. 21 to 23   b . In the first configuration, the actuator  4  is in a first position closer to the second end  12  of the drive shaft  1  than in the second configuration. As shown in  FIG. 2 , to reach the first configuration which is the “open configuration”, the actuator  4  is rotated by hands in the counter clockwise direction, so that the displacement member  3  follows this movement and assumes a retracted position. The arms  25   a,    25   b  of the holding member project out of the displacement member  3  to such an extent that they are configured to be resiliently pressed apart from each other. As depicted in  FIG. 21 , in the first configuration, the head  102  presses the two arms  25   a,    25   b  away until it snaps into the seat  26 . During the insertion, the engagement projection  13  of the drive shaft enters into a corresponding engagement recess  104  provided at the free end surface of the head  102 . The engagement projection  13  can enter into the recess  104  until the free end surface  103  of the head  102  abuts against the abutment surface  14   a  of the drive shaft. During this insertion step, the shank  101  is aligned with the drive shaft  1 . The head  102  is held in a form-fit manner in the axial direction and also in the seat  26  so that it cannot be removed easily. 
         [0062]    By rotating the actuator  4  in the clockwise direction, the displacement member  3  is moved towards the first end of the holding member  2  as depicted in  FIG. 3 . When the displacement member  3  moves toward the first end  21  of the holding member  2 , the tapered inner surfaces  33 ,  34  of the displacement member  3  come into contact with the outer tapered surface portion  29  of the holding member  2  as depicted in  FIG. 23   a . Thereby, the two arms  25   a,    25   b  are pressed together. Simultaneously, the pressure is transmitted to the head  102  as depicted through the horizontal arrow F r  in  FIG. 23   b . Because the center of the sphere C 1  of the spherical seat  26  is offset with respect to the center of the sphere C 2  of the head  102  in a direction toward the abutment surface  14   a  of the drive shaft  1 , a force F u  acts onto the head  102  that has a component directed in an axial direction towards the abutment surface  14   a.  As a result thereof the head  102  is firmly pressed onto the abutment surface  14   a.  A possible play between the engagement portion  13  and the engagement recess  104  is eliminated. Therefore, the surgeon experiences an insertion of the screw without any play. 
         [0063]    In the second configuration which is the “closed configuration”, the head  102  is firmly held in the holding member. In this configuration it can be inserted into the bone. The first end  21  of the holding member  2  serves as an abutment for the bone that limits the insertion and ensures that a sufficient portion of the bone anchor projects over the bone surface so that the receiving part can be mounted onto the head  102  afterwards. 
         [0064]    An embodiment of a system of the instrument and a bone anchor comprises the above described instrument and a bone anchor having a head, wherein the seat and the dimensions of the holding member is adapted to the dimension of the head. 
         [0065]    Modifications of the above described embodiments are possible. 
         [0066]    Referring to  FIGS. 24 to 26 , the seat for the head in the holding member  2  can be designed in a different shape. Parts and portions that are identical to the previously described embodiment are marked with the same reference signs. For example, as depicted in  FIG. 24 , the holding member  2 ′ may have a seat  26 ′ that has a spherical segment shape  26   a ′ adjacent to the lower cylindrical portion which continues in a slightly conically widening portion  26   b ′ in a direction towards the second or distal end. As in the previous embodiment, the size of the seat portions is such that when the arms are compressed, the head experiences a force that presses it towards the abutment surface  14   a.    
         [0067]    In the embodiment shown in  FIG. 25 , the seat  26 ″ has a lower tapered portion  26   a ″ that tapers towards the first or proximal end and continues in a cylindrical portion  26   b ″ towards the second end. The inner diameter of the cylindrical portion is slightly larger than the largest diameter of the head  102 , so that when the arms are pressed together the tapered portion exerts a force onto the head that tries to move the head upward, however because of the abutment of the head against the abutment surface  14   a,  the head is firmly pressed against the abutment surface  14   a.    
         [0068]    In the embodiment shown in  FIG. 26 , the seat comprises a first tapered portion  26   a ″′ that is tapered towards the first end, followed by a cylindrical portion  26   b ″′which is followed by a second tapered portion  26   c ″′ that tapers towards the second end. The function is the same as for the other embodiments. 
         [0069]    In all embodiments the seat has such a size that when the arms are compressed, the head experiences a force that moves it upward in the direction of the abutment surface whereby it is pressed against the abutment surface. 
         [0070]    As a further example for a modification, the actuator can be realized by other means that allow to displace the displacement member to obtain the first and the second configuration. 
         [0071]    The actuator can also be omitted. For example, the displacement member may have an internal thread that cooperates with an external thread of the holding member and allows an advancement of the displacement member along the holding member. 
         [0072]    The abutment surface needs not to be circular or ring-shaped. Any abutment surface can be used that provides sufficient area to generate enough friction to eliminate the play between the engagement projection  13  and the engagement recess  104  of the head. 
         [0073]    The holding member can have more than two arms, for example three or more arms that can be pressed together in a resilient manner. 
         [0074]    Instead of the threaded connections shown in the embodiment, other detachable connections can be provided. 
         [0075]    For the bone anchor, any bone anchor can be used that has a shank and a head portion. It is not necessary that the bone anchor has to be screwed in. The instrument is also suitable for pushing a bone anchor into a hole. 
         [0076]    While the head of the bone anchor and the seat are shown to be designed for a polyaxial bone anchor, it is also possible that the head and the seat have flat side portions that render the bone anchor suitable for a monoplanar bone anchoring device that allows pivoting only in a single plane.