Abstract:
A method for assembling a bone anchoring device includes inserting a bone anchoring element in a first holder of a tool and inserting a receiving part in a second holder of the tool, actuating the tool from a first configuration towards a second configuration to insert a head of the bone anchoring element into the receiving part; continuing actuation of the tool towards the second configuration to move a locking ring and a receiving part body of the receiving part relative to each other until the locking ring assumes a second position with respect to the receiving part body in which the locking ring is latched to the receiving part body in a position where the locking ring compresses a portion of the receiving part body to compress the head, such that the head is held in and cannot be removed from the receiving part body and the bone anchoring element is held adjustably at a first angular position relative to the receiving part, and removing the attached bone anchoring element and receiving part from the tool. The tool is configured for the execution of the steps according to the method.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a continuation of U.S. patent application Ser. No. 13/101,997, filed May 5, 2011, which claims priority to and the benefit of U.S. Provisional Patent Application Ser. No. 61/416,658, filed Nov. 23, 2010, the contents of which are hereby incorporated by reference in their entirety, and claims priority from European Patent Application EP 10 192 278.9, filed Nov. 23, 2010, the contents of which are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     Field of the Invention 
     The invention relates to a method and a tool for assembling a bone anchoring device comprising a receiving part for receiving a rod, and for coupling the rod to a bone anchoring element and such a receiving part. The receiving part includes a receiving part body and a locking ring. The locking ring can assume a first position in which it is latched with respect to the receiving part body and in which a head of the bone anchoring element can be inserted, and a second position in which it is latched with respect to the receiving part body and in which the bone anchoring element is held in an adjustable angular position but is not fully locked. The bone anchoring device can be realized, for example, in the form of a polyaxial bone screw. The method of assembling includes the steps of providing the receiving part body and the locking ring in the first position, inserting the head and moving the locking ring to the second position. The tool is configured for the execution of the steps. 
     Description of Related Art 
     WO 2007/038350 A2 discloses an apparatus for connecting a bone anchor to a support rod, the apparatus including a connector body and a cap. The connector body has a socket for insertion, angulation and removal of a bone anchor. A sleeve is provided, which is configured to fit over the connector body in a temporary position, in which the sleeve permits insertion of the bone anchor, to move to a provisional locking position in which the sleeve permits angulation but prevents removal of the bone anchor, and to move to a locking position, in which the sleeve prevents both angulation and removal of the bone anchor. 
     SUMMARY 
     If a head of an anchoring element is freely pivotable with respect to the receiving part, alignment of the receiving part and insertion of a rod may be difficult in more complex clinical applications, for example, when multiple bone anchors are to be connected to the rod. 
     In some instances there is also a need to have a choice between different anchoring elements during surgery, to select the most appropriate anchoring elements for a specific clinical application. 
     It is the object of the invention to provide an improved method for assembling a bone anchoring device which can be easily carried out, and to provide an improved tool for assembly of such a bone anchoring device. 
     The receiving part according to an exemplary embodiment of the invention allows the insertion of the head of the bone anchoring element into the receiving part body when the locking ring is in a first position which is an insertion position. In this position, the locking ring is latched with respect to the receiving part body. Therefore, the locking ring will not be inadvertently moved to compress the head receiving portion of the receiving part body, in order to facilitate the insertion of the head. 
     In a second position, which is a pre-locking position, the locking ring is latched with respect to the receiving part body and the head receiving portion is compressed so that the bone anchoring element is held in an adjustable angular position but is not fully locked. This prevents inadvertent removal of the bone anchoring element and holds the receiving part body in an adjustable angular position with respect to the head of the bone anchoring element. Therefore, safe and convenient handling of the bone anchoring device during surgery can be assured. 
     In a third position, which is a locking position, the locking ring compresses the head receiving portion such that the bone anchoring element is fully locked and cannot pivot. 
     The receiving part body and the locking ring may be preassembled, and may be delivered after manufacture in a configuration in which the locking ring is latched in the first position to allow introduction of the head of the bone anchoring element. A suitable bone anchoring element, for example, a bone screw with a desired diameter and length, can be selected and inserted into the receiving part. Thereafter, the locking ring can be moved into the second position with respect to the receiving part, where the head is pre-locked. The latching of the locking ring in the receiving part body in the second position is audible, so that a person who assembles the bone anchoring device can be sure of correct assembly of the bone anchoring element in the receiving part. In the pre-locked condition, the screw element may only be pivotable with respect to the receiving part by applying an additional force, to overcome the frictional force of the clamping of the head. 
     With the bone anchoring device according to embodiments of the invention, a modular system can be provided, which allows for combinations of various anchoring elements with any suitable receiving part on demand, depending on the actual clinical requirements. This reduces the costs associated with polyaxial screws, reduces inventory, and gives the surgeon a wide choice of implants. 
     A method of assembling the bone anchoring device according to embodiments of the invention can be carried out by any specialist, for example, by a surgeon or any personnel assisting him or her before or during surgery. 
     A tool according to embodiments of the invention is easy to handle and provides for safer assembly of the bone anchoring device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of the invention will become apparent from the description of embodiments by means of the accompanying drawings. In the drawings: 
         FIG. 1  shows a perspective exploded view of a bone anchoring device according to an embodiment of the invention; 
         FIG. 2  shows a perspective view of the bone anchoring device of  FIG. 1  in an assembled state; 
         FIG. 3  shows an enlarged perspective view of a locking ring according to an embodiment of the invention; 
         FIG. 4  shows a cross-sectional view of the locking ring shown in  FIG. 3  along line A-A in  FIG. 3 ; 
         FIG. 5  shows a cross-sectional view of a bone anchoring device according to an embodiment of the invention in an assembled state, the section being taken perpendicular to a rod axis, where the locking ring is in a first position and is latched with respect to the receiving part body; 
         FIG. 6  shows a cross-sectional view of the bone anchoring device in an assembled state, the section being taken in a plane perpendicular to the rod axis, where the locking ring is in a second position and is latched with respect to the receiving part body; 
         FIG. 7  shows a cross-sectional view of the bone anchoring device in an assembled state, with a rod inserted and fixed, the section being taken in a plane perpendicular to the rod axis, and wherein the locking ring is in a third position; 
         FIG. 8  shows a cross-sectional view of a portion of the bone anchoring device in a first step of assembly, where a bone anchoring element is going to be inserted into the receiving part; 
         FIG. 9  shows a cross-sectional view of a portion of the bone anchoring device in a second step of assembly, where a head of the bone anchoring element has been introduced into the receiving part and is pre-locked; 
         FIG. 10  shows a cross-sectional view of a portion of the bone anchoring device where the head is locked; 
         FIG. 11  shows an enlarged cross-sectional view of a portion of the receiving part, where the locking ring is in the first position and is latched with respect to the receiving part body to allow for introduction of the head; 
         FIG. 12  shows an enlarged cross-sectional view of a portion of the bone anchoring device in a final locked state where additional clamping is effected by means of the locking ring; 
         FIG. 13  shows a perspective view of a tool for assembling a bone anchoring device according to a first embodiment of the invention; 
         FIG. 14  shows an enlarged perspective view of a portion of the tool according to the first embodiment, with a bone anchoring element inserted into a holder; 
         FIG. 15  shows a perspective view of a portion of the tool according to the first embodiment, with a receiving part to be inserted into a holder; 
         FIG. 16  shows a perspective view of a portion of the tool according to the first embodiment, with the receiving part introduced into the holder; 
         FIG. 17 a    shows a perspective view of the tool according to the first embodiment depicting a step of assembly, where the head of the bone anchoring element is going to be inserted into the receiving part; 
         FIG. 17 b    shows an enlarged view of a portion of  FIG. 17   a;    
         FIG. 18 a    shows a perspective view of the tool according to the first embodiment depicting another step of assembly, with the head of the bone anchoring element inserted into the receiving part; 
         FIG. 18 b    shows an enlarged portion of  FIG. 18   a;    
         FIG. 19  shows an enlarged perspective view of a portion of the tool according to the first embodiment with the bone anchoring device after insertion of the head and before entering the pre-locking position; 
         FIG. 20  shows a perspective view of the tool according to the first embodiment with the bone anchoring device in the pre-locking position; 
         FIG. 21  shows a perspective view of an enlarged portion of the tool according to the first embodiment, where the locking ring of the bone anchoring device has assumed the pre-locking position and clamps the head; 
         FIG. 22  shows a perspective view of a tool for assembling the bone anchoring device according to a second embodiment of the invention; 
         FIG. 23  shows a perspective view of a second holder according to the second embodiment; 
         FIG. 24  shows a perspective exploded view of the second holder according to the second embodiment; 
         FIG. 25 a    shows a cross-sectional view of the second holder in a first position according to the second embodiment; 
         FIG. 25 b    shows a cross-sectional view of the second holder in a second position according to the second embodiment; 
         FIG. 26 a    shows a perspective view of a portion of the tool in a first position of use according to the second embodiment; 
         FIG. 26 b    shows a perspective view of a portion of the tool in a second position of use according to the second embodiment; 
         FIG. 26 c    shows a perspective view of a portion of the tool in a third position of use according to the second embodiment; 
         FIG. 27  shows a cross-sectional view of a portion of the tool according to the second embodiment in the first position of use; 
         FIG. 28 a    shows a cross-sectional view of an enlarged portion of the tool in the first position of use according to the second embodiment; 
         FIG. 28 b    shows a cross-sectional view of an enlarged portion of the tool in the second position of use according to the second embodiment; 
         FIG. 28 c    shows a cross-sectional view of an enlarged portion of the tool in the third position of use according to the second embodiment; 
         FIG. 29  shows a perspective view of a tool according to a third embodiment of the invention, depicting a step of assembly where the head of the bone anchoring element is to be inserted into the receiving part; 
         FIG. 30 a    shows an insert for a first holder according to the third embodiment, in the form of a cylindrical section having several cylinder-shaped or U-shaped recesses; 
         FIG. 30 b    shows a first holder according to the third embodiment; 
         FIG. 31  shows the first holder and the insert in an assembled state according to the third embodiment; 
         FIG. 32  shows a cross-sectional view of the insert according to the third embodiment; and 
         FIG. 33  shows a cross-sectional view of a frame, the first holder, and the insert, the assembly holding a screw according to the third embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     As shown in  FIGS. 1 to 7 , a bone anchoring device, according to an embodiment of the invention includes a bone anchoring element  1  in the form of a bone screw having a threaded shaft  2  and a head  3  with a curved surface portion. In this embodiment the head  3  is spherical segment-shaped. The head  3  has a recess  4  for engagement with a tool. The bone anchoring device also includes a receiving part body  5  for receiving a rod  6  to connect it to the bone anchoring element  1 . Further, a fixation element  7  in the form of an inner screw is provided for fixing the rod  6  in the receiving part body  5 . The bone anchoring device also includes a locking ring  8  for locking the head  3  in the receiving part body  5 . 
     The receiving part body  5  includes a rod receiving portion  9 , which is substantially cylindrical and which has a first end  9   a  and a second end  9   b  opposite the first end  9   a . A coaxial first bore  10  is provided at the second end  9   b  as shown in  FIGS. 5 to 7 . The diameter of the first bore  10  is smaller than the diameter of the head  3  of the bone anchoring element  1 . The rod receiving portion  9  further has a coaxial second bore  11  extending from the first end  9   a  to a distance from the second end  9   b . The diameter of the second bore  11  is larger than that of the first bore  10 . A substantially U-shaped recess  12  extends from the first end  9   a  in the direction of the second end  9   b  in the rod receiving portion  9 , the diameter of the recess  12  being slightly larger than the diameter of the rod  6  in such a way that the rod  6  can be placed in the recess  12  and can be guided therein. By means of the recess  12 , two free legs  12   a ,  12   b  are formed on which an internal thread  13  is provided. The internal thread  13  can be a metric thread, a flat thread, a negative angle thread, a saw-tooth thread, or any other type of thread. Preferably, a thread such as a flat thread or negative angle thread is used, which prevents splaying of the legs  12   a ,  12   b  when the inner screw  7  is screwed-in. The depth of the recess  12  is such that the rod  6  and the inner screw  7  can be inserted between the legs  11   a  and  12   b.    
     As can be seen in  FIG. 1 , cut-outs  15  are provided in the rod receiving portion  9  on either end of the channel formed by the recess  12 . 
     On an outer surface of the rod receiving portion  9 , in the region of the legs  12   a ,  12   b , a groove  16  is provided, which extends in a circumferential direction and serves for engagement with a portion of the locking ring  8 . The groove  16  is asymmetric in such a way that it allows for disengagement between the locking ring  8  and the groove  16  when the locking ring  8  is shifted in one direction. The asymmetric shape of the groove  16  is realized by a downwardly inclined lower wall  16   a  and an upper wall  16   b  that is substantially perpendicular to an outer surface of the rod receiving portion  9 . 
     At the side of the second end  9   b  the receiving part body  5  has a head receiving portion  17  providing an accommodation space for the head  3  of the bone anchoring element  1 . A largest outer diameter of the head receiving portion  17  is smaller than a largest outer diameter of the rod receiving portion  9 . An internal hollow section  18  forms a seat for the head  3  of the bone anchoring element  1 , and is open via an opening  19  to a free end  17   b  of the head receiving portion  17 . The hollow section  18  corresponds in its shape to the shape of the head  3 . In the embodiment shown, the hollow section  18  is a spherical section to accommodate the spherical head  3 . Furthermore, the hollow section  18  is configured to encompass the head  3  of the bone anchoring element  1  from the side, covering a region including the largest diameter of the head  3 . 
     A plurality of slits  20  are provided in the head receiving portion  17 , which are open to the free end  17   b . The slits  20  make the head receiving portion  17  flexible so that it can be compressed to clamp and finally lock the head  3  in the hollow internal portion  18  by means of friction. The number and size of slits  20  is provided depending on the desired flexibility of the head receiving portion  17 . The flexibility of the head receiving portion  17  is such that the head  3  of the anchoring element  1  can be inserted by expanding the head receiving portion  17 , and the head  3  can be clamped by compressing the head receiving portion  17 . 
     The outer surface of the head receiving portion  17  has a first section  21 , with an outer diameter which increases towards free end  17   b , for example in an outwardly curved or conically widening manner. Adjacent to the first section  21 , there is a circumferential groove  22 , which is recessed with respect to the first section  21  and which serves for engagement with a corresponding portion of the locking ring  8 . The groove  22  is asymmetric to allow for disengagement between the locking ring  8  and the groove  22  when moving the locking ring  8  in one direction. The asymmetric shape of the groove  22  is realized by a lower downwardly inclined wall  22   a  and an upper wall  22   b  that is substantially perpendicular to an outer surface of the head receiving portion  17 . 
     Adjacent the groove  22  on a side opposite the first section  21 , there is a third portion  23  of the head receiving portion  17  with a substantially cylindrical outer surface. The third portion  23  is configured to cooperate with a portion of the locking ring  8  to enhance the clamping effect of the locking ring  8 . 
     The locking ring  8  will now be described with reference to  FIGS. 1 to 7 . The locking ring  8  is substantially cylindrical and has an upper end  8   a  and a lower end  8   b . In the mounted state the upper end  8   a  is oriented in the direction of the first end  9   a  of the rod receiving portion  9 , while the lower end  8   b  is oriented towards the free end  17   b  of the head receiving portion  17 . Near the lower end  8   b , a first portion  81  with an inner surface  81   a  is provided which cooperates with the first outer surface portion  21  of the head receiving portion  17  to compress the head receiving portion  17 . The outer surface of the first portion  81  may also be tapered to reduce an outer bottom diameter. The size of the first portion  81  is such that, for example, the tapered inner surface  81   a  can engage the outer surface portion  21  of the head receiving portion  17  to exert a compression force onto the head receiving portion  17 . The inner surface  81   a  of the first portion  81  of the locking ring  8  can also be curved with a curvature directed towards a center of the locking ring  8 . 
     At the lower end  8   b , the locking ring  8  includes an inwardly projecting edge  82 , the inner diameter of which is smaller than the inner diameter of the other portions of the locking ring  8 . The inwardly projecting edge  82  is configured to engage the groove  22  of the head receiving portion  17 . 
     The locking ring  8  further has a third portion  83  with upwardly extending wall portions  83   a , which are separated from each other by slits  84 . The upwardly extending wall portions  83   a  are arranged at an outer circumference of an inner circumferential shoulder  85  of the locking ring  8 , and render the third portion  83  of the locking ring  8  flexible. The number and size of the slits  84  and the thickness of the wall portions  83   a  are configured such that a desired flexibility is realized. At the free ends of the wall portions  83   a  are engagement sections  83   b  which are shaped so as to engage the groove  16  provided on the outer surface of the rod receiving portion  9 . The inner diameter of the third portion  83  of the locking ring  8  is only slightly larger than the outer diameter of the rod receiving portion  9 , as can be seen in  FIG. 5 . 
     The locking ring  8  is sized in such a way with respect to the head receiving portion  17 , that the head receiving portion  17  can expand within the locking ring  8  to allow the introduction of the head  3  when the locking ring  8  is in the first position, as shown in  FIG. 5 . 
     Two projections  86 , which are located diametrically opposite to each other, are formed in the third portion  83  of the locking ring  8 . The projections  86  have a height where they extend into the cut-outs  15  and project above the bottom of the substantially U-shaped recess  12  when the locking ring  8  is in a position in which the head  3  is not yet locked, as shown in  FIGS. 5 and 6 . A free end surface  86   a  of the projections  86  can be curved, particularly inwardly curved, with a curvature corresponding to an outer surface of the rod  6 . The locking ring  8  is arranged in such a manner around the head receiving portion  17  of the receiving part body  5 , that the projections  86  are located at the positions of (e.g., are aligned with) the recess  12 . Here, the projections  86  prevent the locking ring  8  from rotating when the rod  6  is not inserted. 
     The flexibility of the head receiving portion  17  and the size of the head receiving portion  17  at the open end  17   b  allows for mounting of the locking ring  8  by assembling the locking ring  8  from the free end  17   b  onto the head receiving portion  17 . Since the outer diameter of the head receiving portion  17  is smaller than that of the rod receiving portion  9 , the locking ring  8  may only project minimally beyond the rod receiving portion  9  in a radial direction. 
     The inner screw  7  has a thread corresponding to the internal thread  13  provided on the legs  12   a ,  12   b . If a thread, which prevents the legs  12   a ,  12   b  from splaying is used, a single fixation element such as the inner screw  7  is sufficient. This reduces the size of the bone anchoring device in a radial direction. Other fixation elements such as, for example, an outer nut are also possible. 
     The receiving part body  5 , the locking ring  8 , the inner screw  7  and the bone anchoring element  1  are made of bio-compatible materials, for example, titanium or stainless steel or a bio-compatible alloy such as nitinol or a bio-compatible plastic material, such as polyether ether ketone (PEEK). The parts can be made of the same or of different materials. 
     The function of the locking ring  8  is now explained with referenced  FIGS. 5 to 12 . As shown in  FIG. 5 , the locking ring  8  is in a first position, which is an insertion position and where the locking ring  8  is latched with respect to the receiving part body  5 . In the first position, the inwardly projecting edge  82  of the locking ring  8  engages groove  22  on the outer surface of the head receiving portion  17 . As can be seen in the figures, the inner diameter of the inwardly projecting edge  82  is larger than the outer diameter of the head receiving portion  17  at the position of the groove  22 , so as to allow for expansion of the head receiving portion  17  when the head  3  is introduced. In the first position, the locking ring  8  is additionally held by a clamping force between the rod receiving portion  9  of the receiving part body  5  and the flexible wall portions  83   a  of the locking ring  8 , which are slightly bent outwards, as can be seen in particular in  FIGS. 5, 8, and 11 . 
     When the locking ring  8  is in the first position, the head receiving portion  17  is not compressed. In this position, the introduction of the screw head  3  is possible as can be seen in  FIG. 8 . In the first position, the locking ring  8  is prevented from moving upwards towards the first end  9   a  of the rod receiving portion  9 , since the shoulder  85  of the locking ring  8  abuts against the second end  9   b  of the rod receiving portion  9   b , while the inwardly projecting edge  82  of the locking ring  8  abuts against the upper wall  22   b  of groove  22 . As shown in particular in  FIG. 8 , the abutment of the locking ring  8  against the second end  9   b  and against the upper wall of groove  22  holds the locking ring  8  in place against upward movement. The inclined lower wall  22   a  of the groove  22  prevents inadvertent downward movement of the locking ring  8  but allows downward movement of the locking ring  8 , upon exertion of an additional force. Since portions of the inner diameter of the locking ring  8  are larger than corresponding portions of the outer diameter of the head receiving portion  17  in a non-compressed state in the first position, an expansion of the head receiving portion  17  into a space between the locking ring  8  and the head receiving portion  17  is possible. In addition, in the first position, the head  3  can freely pivot. 
       FIGS. 6 and 9  illustrate the bone anchoring device in a second position in which the locking ring  8  is latched with respect to the receiving part body  5  in a pre-locking position. In the second position, the locking ring  8  has been shifted from the first position towards the free end  17   b  of the head receiving portion  17  until the engagement portions  83   b  of the flexible wall portions  83   a  resiliently snap into the groove  16  provided on the rod receiving portion  9 . Once in the second position, the free upper edge of the engagement portions  83   b  will abut against the upper wall  16   b  of the groove  16 , as shown in  FIGS. 6 and 9 , thereby preventing upward movement of the locking ring  8  out of the pre-locking position. On the other hand, the inclined lower wall surface  16   a  of the groove  16  prevents inadvertent downward movement of the locking ring  8  towards the free end  17   b , but allows for downward movement upon exertion of an additional force. 
     In the second position, as can be seen in particular in  FIGS. 6 and 9 , the inner inclined surface  81   a  of the locking ring  8  presses against the first outer surface portion  21  of the head receiving portion  17 , so as to compress the head receiving portion  17  to clamp the head  3  within the hollow internal portion  18  without fully locking the head  3 . In addition, the inwardly projecting edge  82  of the locking ring  8  presses against the third portion  23  of the head receiving portion  17 , resulting in an additional clamping force. Therefore, clamping of the head  3  can be effected not only from above and/or the sides of the head  3 , but also from a region around the lower portion of head  3 . Pre-locking means that under conditions arising during surgery, the angular position of the bone anchoring element  1  with respect to the receiving part body  5  is maintained, and can be loosened only by exerting an additional force onto the receiving part body  5  and/or the bone anchoring element  1  of the bone anchoring device. In the pre-locked position, the bone anchoring element  1  cannot be removed from the receiving part  5 . Hence, accidental or inadvertent removal of the head  3  is not possible. However, angulation of the bone anchoring device to be adjusted to a desired angle is still possible, for example, by manual adjustment. 
     A third position, which is the locking position, is shown in  FIGS. 7, 10, and 12 . The third position is defined as a position in which the screw head  3  is finally locked within the head receiving portion  17 . The inner surface  81   a  of the locking ring  8  engages the outer surface of the first portion  21  of the head receiving portion  17  in such a way that the head  3  is locked by compression of the head receiving portion  17 . In addition, the inwardly projecting edge  82  of the locking ring  8  further compresses the head receiving portion  17  at the third portion  23 , thereby enhancing the locking force. 
     The dimensions of the receiving part body  5  and the locking ring  8  are configured such that desired clamping forces can be achieved in the second position and in the third position, respectively. 
     The third position can be reached by shifting the locking ring  8  relative to the receiving part body  5  such that the engagement portions  83   b  and the inwardly projecting ring  82  slide along the lower inclined wall portions  16   a  and  22   a  of the grooves  16 ,  22 , respectively. 
     The bone anchoring device is preassembled as follows. First, the locking ring  8  is mounted onto the receiving part body  5  from the free end  17   b . This can be done, for example, by the manufacturer. Preferably, the locking ring  8  is in the first position, where it is latched by engagement or alignment of the inwardly projecting edge  82  with the groove  22 . 
     Thereafter, the head  3  of the anchoring element  1  can be introduced from the free end  17   b  into the hollow internal portion  18  of the head receiving portion  17 . Thereafter, the locking ring  8  is moved downwards relative to the receiving part body  5 , so that the inwardly projecting ring  82  slides out of the groove  22  and the engagement portions  83   b  of the flexible wall portions  83   a  snap into groove  16 , to reach the second position, in which the head  3  is pre-locked by frictional clamping. 
     A tool for assembling the bone anchoring device and its operation according to a first embodiment will now be described with reference to  FIGS. 13 to 21 . The tool  100  includes a frame  101  with a first holder  102  for the bone anchoring element  1  and a second holder  103  for the receiving part (including, for example, the receiving part body  5  and the locking ring  8 ). The holders  102 ,  103  are oriented such that a longitudinal axis of the bone anchoring element  1  is horizontal or parallel with respect to a surface on which the tool is placed or positioned. The first holder  102  has a recess  102   a  for the shank  2  of the bone anchoring element  1 , which serves for holding and guiding the shank  2 . The diameter of the recess  102   a  is smaller than the diameter of the head  3  in the area of a free end of the first holder  102  facing the second holder  103 . Therefore, a free end surface  102   b  of the first holder  102  serves as an abutment for the head  3  of the bone anchoring element  1 . The first holder  102  is supported on the frame  101 . 
     The second holder  103  for the receiving part is also supported on the frame  101 . It has a substantially circular recess  103   a  for accommodating a portion of the receiving part. The orientation of the second holder  103  with respect to the first holder  102  is such that a central axis of the receiving part is configured to be positioned coaxial with the axis of the bone anchoring element  1  when the receiving part and the bone anchoring element  1  are both inserted into their respective holders  102 ,  103 . The circular recess  103   a  can be adjusted to have two different depths. This may be realized by an insert  104  which is inserted in a corresponding slot provided in the second holder  103  and which can be shifted in a direction transverse to the direction of the central axis of the recess  103   a , to limit the depth of recess  103   a . The insert  104  has one circular recess  142 . In position  1 , as shown in  FIGS. 13 to 18 , the insert limits the depth of the recess  103   a  to a first depth  141 , and thus provides an abutment for the first end  9   a  of the receiving part body  5 . In position  2 , which is shown in  FIGS. 19 to 21 , the insert  104  is shifted such that its recess  142  forms or defines the bottom of the recess  103   a  of the second holder  103 , the depth of which is greater than the depth  141  of the recess  103   a  when the insert  104  is in the first position. As such, the recess  142  effectively increases the length of the recess  103   a . The receiving part can therefore be inserted deeper into the recess  103   a  until an outer surface  103   b  of the second holder  103  forms an abutment for the locking ring  8 , as shown in  FIGS. 19 to 21 . The shape of the recess  103   a  need not be circular, but can be otherwise shaped, and in particular, it can be adapted to correspond to the contour of the receiving part. 
     The first holder  102  for the bone anchoring element  1  is movable relative to the second holder  103  for the receiving part body  5  in an axial direction. The first holder  102  can be actuated via a lever  105  and a handle  106 . It is to be understood that the lever  105  is only an example, and that movement of the first holder  102  for the bone anchoring element  1  can be effected in many other ways, for example, by means of a toothed rack. 
     The dimensions of the tool  100  are configured such that, by movement of the first holder  102  for the bone anchoring element  1  with respect to the second holder  103  in which a receiving part is inserted, a sufficient force can be exerted to introduce the head  3  of the bone anchoring element  1  into the head receiving portion  17 , when the recess  103   a  is set to have the first depth  141  and the locking ring  8  is in the first position. It is further configured such that a sufficient force can be exerted onto the locking ring  8  when first holder  102  is moved again relative to the second holder  103  to move the locking ring  8  out of the first position into the second position, when the recess  103   a  is set to have the second depth  142 . 
     The operation of the tool according to the first embodiment is shown in  FIGS. 17 to 21 . As shown in  FIGS. 17 a  and 17 b    first, the bone anchoring element  1  is inserted into the first holder  102  and the receiving part body  5 , with mounted locking ring  8  in the first position, is mounted in the recess  103   a  of the second holder  103  when the recess  103   a  is set to have the first depth  141 . 
     In a next step, as shown in  FIGS. 18 a  and 18 b   , the handle  106  is actuated to actuate the lever  105  so that screw head  3  is pushed into the hollow internal portion  18  of the head receiving portion  17 . The bottom of the recess  103   a  of the second holder  103  serves as an abutment for the receiving part, so that the head receiving portion  17  can expand to allow the introduction of the head  3 . The handle  106  is actuated until the head  3  of the bone anchoring element  1  is latched or inserted in the hollow internal portion  18 . The latching may produce an audible sound. 
     Thereafter, as shown in  FIGS. 19 to 21 , the first holder  102  for the bone anchoring element  1  is shifted backward and the insert  104  is moved to the second position, in which the circular recess  142  forms or defines the bottom of recess  103   a  to provide and use outer surface  103   b  of holder  103  as an abutment for the locking ring  8 . 
     As shown in  FIGS. 20 and 21 , the handle  106  is then actuated to push the first holder  102  towards the second holder  103 . By means of this, the head  3  with the receiving part body  5  is pushed farther into the bottom of the recess  103   a , which has a depth where the free front surface  103   b  of the second holder  103  presses against the engagement portions  83   b  of the flexible wall portions  83   a  of the locking ring  8 , thereby moving the locking ring  8  out of the first position into the second position, where the engagement portions  83   b  are latched in the groove  16  on the rod receiving portion  9 . When the engagement portions  83   b  snap into the groove  16 , the latching of the locking ring  8  with respect to the receiving part body  5  is audible, which indicates that the correct pre-locking position is reached. 
     Thereafter, the first holder  102  is moved backward and the bone anchoring device is removed. 
     A tool for assembling the bone anchoring device and its operation according to a second embodiment will now be described with reference to  FIGS. 22 to 28   c . As can be seen from  FIG. 22 , the tool  200  includes a frame  201  with a first holder  202  for the bone anchoring element  1  and a second holder  203  for the receiving part (again including, for example, the receiving part body  5  and the locking ring  8 ). The tool  200  according to the second embodiment only differs from the tool  100  according to the first embodiment in that the second holder  203  is different from the second holder  103  according to the first embodiment. The holders  202 ,  203  may be oriented such that a longitudinal axis of the bone anchoring element  1  is horizontal or parallel with respect to a surface on which the tool is placed or positioned. The holder  202  has a recess  202   a  for the shank  2  of the bone anchoring element  1 , which serves for holding and guiding the shank  2 . The diameter of the recess  202   a  is smaller than the diameter of the head  3  in the area of a free end of the first holder  202  facing the second holder  203 . Therefore, a free end surface  202   b  of the first holder  202  serves as an abutment for the head  3  of the bone anchoring element  1 . The first holder  202  is supported on the frame  201 . 
     The second holder  203  for the receiving part is also supported on the frame  201 . The orientation of the second holder  203  with respect to the first holder  202  is such that a central axis of the receiving part is configured to be positioned coaxial with the axis of the bone anchoring element  1  when the receiving part and the bone anchoring element  1  are both inserted into their respective holders  202 ,  203 . 
     The first holder  202  for the bone anchoring element  1  is movable relative to the second holder  203  for the receiving part body  5  in an axial direction. The first holder  202  can be actuated via a lever  205  and a handle  206 . It is to be understood that the lever  205  is only an example, and that movement of the first holder  202  for the bone anchoring element  1  can be effected in many other ways, for example, by means of a toothed rack. 
     As can be seen from  FIG. 23  the second holder  203  includes a main body  210 , a first end  211 , a second end  212 , a bore  220 , and a substantially circular recess  203   a  for accommodating a portion of the receiving part. As can be seen from  FIG. 24  the second holder  203  further includes a thread  213 , a sleeve  214 , a spring  215 , a plate  216  and a screw  217 . As can be seen from  FIGS. 25 a  and 25 b   , the cup-shaped sleeve  214  is closed on a side extending towards the first end  211  and is configured to slide within the bore  220  of the main body  210  since its diameter is slightly smaller than the inner diameter of at least a portion of the bore  220 . The spring  215  is provided within the sleeve  214  and is supported by the plate  216  and the inner screw  217  which is screwed into the thread  213  of the main body  210 . Other designs for supporting the spring  215  are possible, such as for example, a base which is press-fitted into the second holder  203 . The spring  215  is shown as a helical spring  215  in the second embodiment. However other spring elements are possible, such as leaf springs, disk springs, elastomer cushions, etc. 
     The circular recess  203   a  can have varying depths. This is realized by the movable sleeve  214  which provides an abutment for the receiving part. In  FIG. 25 a    a first abutment position and in  FIG. 25 b    a second abutment position are shown, where a space  218  is shown in  FIG. 25 b    when the spring  215  is compressed. The sleeve  214  provides an abutment for the first end  9   a  (see  FIG. 1 ) of the receiving part body  5 . In the first abutment position, the closed side of the sleeve  214  forms the bottom of the recess  203   a  of the second holder  203 , the depth of which is less than a depth of the recess  203   a  in the second abutment position that is achieved when a greater force is applied by the first end  9   a  of the receiving part onto the sleeve  214  and the spring  215 , respectively. The force is exerted by the handle  206  and the lever  205  via the first holder  202 , the shaft  2  and the head  3  to the first end  9   a  of the receiving part and then to the sleeve  214 . The shape of the recess  203   a  need not be circular, but can be otherwise shaped, and in particular, it can be adapted to correspond to any possible contour of the receiving part. 
     The dimensions of the tool  200  and the spring force are configured such that, by means of moving the first holder  202  with the bone anchoring element  1  relative to the second holder  203  in which a receiving part is inserted, a first force can be exerted to introduce the head  3  of the bone anchoring element  1  into the head receiving portion  17 , when the recess  203   a  is set to have the first depth and the locking ring  8  is in the first position. It is further configured such that a second force can be exerted onto the locking ring  8  when the first holder  202  is moved closer relative to the second holder  203  to move the locking ring  8  out of the first position into the second position, where the recess  203   a  has the second depth. The tool  200  according to the second embodiment of the invention allows for mounting of various bone anchoring devices with, for example, receiving parts having different heights and/or various different locking ring positions relative to the receiving part. 
     The operation of the tool according to the second embodiment is shown in  FIGS. 26 a  to 28 c   . As shown in  FIGS. 26 a   ,  27  and  28   a  first, the bone anchoring element  1  is inserted into the first holder  202  and the receiving part body  5 , with mounted locking ring  8  in the first position, is mounted in the recess  203   a  of the second holder  203  when the recess is set to have the first depth. 
     In a next step, as shown in  FIGS. 26 b  and 28 b   , the handle  206  is actuated to actuate the lever  205  (see  FIG. 22 ) so that the screw head  3  is pushed into the hollow internal portion  18  (see  FIG. 5 ) of the head receiving portion  17 . The bottom of the recess  203   a  of the second holder  203 , i.e. the sleeve  214 , serves as an abutment for the receiving part, so that the head receiving portion  17  can expand to allow the introduction of the head  3 . The handle  206  is actuated until the head  3  of the bone anchoring element  1  is latched or inserted in the hollow internal portion  18 . The latching may produce an audible sound. The locking ring  8  may not yet contact a side wall of the first end  211 . The counterforce of the spring  215  is greater than the counterforce from insertion of the head  3  into the receiving part body  5 . Therefore, the spring  215  is not compressed during this step. 
     As shown in  FIGS. 26 c  and 28 c   , the handle  206  is then actuated further to push the first holder  202  closer to the second holder  203 . By means of this, the head  3  with the receiving part body  5  is pushed against the bottom of the recess  203   a , i.e. the sleeve  214 . The force which is needed for moving the locking ring  8  and the receiving part body  5  relative to each other is greater than the counterforce from the spring  215 . Therefore, the spring  215  is compressed. The recess  203   a  now has a depth where the side wall of the first end  211  of the second holder  203  presses against the engagement portions  83   b  of the flexible wall portions  83   a  of the locking ring  8  (see  FIG. 3 ), thereby moving the locking ring  8  out of the first position into the second position, where the engagement portions  83   b  are latched in the groove  16  on the rod receiving portion  9 . When the engagement portions  83   b  snap into the groove  16 , the latching of the locking ring  8  with respect to the receiving part body  5  is audible, which indicates that the correct pre-locking position is reached. 
     Thereafter, the first holder  202  is moved backward and the bone anchoring device is removed. One advantage of the second embodiment of the tool  200  as compared to the first embodiment of the tool  100  is that only a singe actuation of the handle  206  is needed, and no additional parts have to be moved during the mounting of the bone anchoring device. 
     By means of the tool  200  according to the second embodiment, assembly of the bone anchoring device can be easily completed in one step, including the introduction of the head and the mounting of the bone anchoring device into the pre-lock position. 
     A third embodiment of a first holder  302  for the bone anchoring element will now be described with reference to  FIGS. 29 to 33 . The tool  300  shown in  FIG. 29  can be the same or have similar features to the tools  100  or  200  described above, except for the first holder  302  which replaces the first holders in the previous embodiments. 
     The first holder  302 , shown in  FIGS. 30 b    and  31 , has a longitudinal U-shaped recess  302   a  which referring to  FIG. 30 b   , extends from one end towards another end in a longitudinal direction and from the side of the first holder  302  inwards. The diameter of the recess  302   a  is smaller than the diameter of a head  3  of a bone anchoring element  1  in the area of a free end of the first holder  302  facing a second holder  303 . Therefore, a free end surface  302   b  of the first holder  302  serves as an abutment for the head  3  of the bone anchoring element  1 . The first holder  302  is supported on the frame  301 . 
       FIG. 30 a    and  FIG. 32  show an insert  320  for the first holder in the form of a cylindrical section having a plurality of longitudinally extending recesses  322   a ,  322   b ,  322   c  that are arranged circumferentially around a central axis  323  of the insert  320 . The recesses are sized and/or adapted for the insertion of screws or other bone anchoring elements with different shanks. In the embodiment shown the insert  320  has a first cylinder-shaped or U-shaped recess  322   a  having a first radius, a second cylinder-shaped or U-shaped recess  322   b  having a second radius and a third cylinder-shaped or U-shaped recess  322   c  having a third radius, wherein the three radii may be different from each other. The three recesses  322   a ,  322   b ,  322   c  may extend the length of the insert  320 , as can be seen from  FIG. 30 a   . The insert  320  has a first end and a second end, wherein a first pin  321   a  is provided on the first end and a second pin  321   b  is provided on the second end. The pins  321   a ,  321   b  are in line with the central axis  323  of the insert  320 . 
     Referring to  FIG. 30 b   , the first holder  302  has a first end and a second end, where on the first end a first slot  311  is provided and near the second end a second slot  312  is provided which both extend from a side of the first holder  302  inwards. The second end of the first holder  302  is configured to be connected to a lever  305  and a handle  306  of the tool, as can be seen in  FIG. 29 . 
     The U-shaped recess  302   a  of the first holder  302  is provided for receiving the insert  320 . The radius of the circular portion of the U-shaped recess  302   a  may be substantially the same as the radius of the insert  320 . 
     As can be seen from  FIG. 31 , in a mounted state, the first pin  321   a  of the insert  320  fits into the first slot  311  of the first holder  302 , and the second pin  321   b  of the insert  320  fits into the second slot  312  of the first holder  302 . The insert  320  is supported and guided by the pins  321   a ,  321   b , and is also held by the U-shaped recess  302   a  of the first holder  302  as can be seen in  FIG. 33 . 
     The orientation of the first holder  302  with respect to a second holder  303  is such that a central axis of a receiving part  5  is configured to be positioned coaxial with an axis of the bone anchoring element  1  when the receiving part  5  is inserted into the second holder  303  and the bone anchoring element  1  is inserted into the first holder  302  in one of the recesses  322   a ,  322   b  or  322   c  of the insert  320 . 
     The three recesses  322   a ,  322   b  or  322   c  are adapted to receive different sized shanks of bone anchoring elements. With reference to  FIGS. 29, 31, and 33 , a particular recess  322   a ,  322   b ,  322   c  which is in use will face outwards towards the opening of the U-shaped recess  302   a . In this embodiment of the insert  320 , mounting of at least three different sized bone anchoring elements can be accommodated. For changing the recess  322   a ,  322   b ,  322   c  which is in use, the insert  320  can be rotated around its axis  323 . 
     In other embodiments, a cylinder or insert having more or less than three recesses can be provided. With the third embodiment of the holder, a user can combine screws or bone anchoring elements with different shanks to a receiving part. Hence, a modular system is provided that gives the user a wider selection of implant combinations depending on the actual clinical situation. 
     The bone anchoring device can be preassembled either by the manufacturer or in the course of preparation of surgery or at any other time. Advantageously, the surgeon can select prior to surgery the desired receiving parts and bone anchoring elements according to the specific requirements of the particular clinical application. The design of the bone anchoring device allows for selection of the appropriate bone anchoring elements in terms of diameter, length and other features of the anchoring section. Hence, a modular system can be provided, which includes receiving parts and various bone anchoring elements, which then can be individually chosen and adapted. 
     In use during surgery, the preassembled bone anchoring device including the receiving part body  5 , the bone anchoring element  1  and the locking ring  8  in the pre-locking position, is screwed into a bone. The recess  4  of the head  3  can be accessed with a screw tool through the first bore  10 . To correctly align the receiving part body  5  with respect to the rod  6 , to which it will be connected, an additional force can be exerted onto the receiving part, either manually or by application of an instrument. Once the correct position of the rod  6  with respect to other bone anchoring devices is also achieved, the inner screw  7  can be tightened for each bone anchoring device. Since the rod  6  abuts against the projections  86  of the locking ring  8 , the locking ring  8  is shifted downward into the third position, which is the locking position. When the locking ring  8  is moved towards the free end  17   b  of the head receiving portion  17 , it compresses the head receiving portion  17 , thereby locking a position of the head  3 . Final tightening of the inner screw  7  locks the rod  6  and the head  3  simultaneously. 
     In the pre-locking condition, the head  3  remains clamped when the inner screw  7  is loosened. This allows further adjustments with respect to positioning of the rod  6 . 
     Further modifications of the embodiments shown are possible. For example, the head of the bone anchoring element can have any other shape, for example, a cylindrical shape, whereby a monoaxial bone screw is provided, allowing rotation of the screw element with respect to the receiving part body around a single axis. The head can also be conically shaped or otherwise shaped, and the internal hollow section of the head receiving portion is adapted to correspond to this shape. In a further modification, the flexibility of the head receiving portion is based on properties of the material, for example, a plastic material, and the slits may be fully or partly omitted. 
     The projections of the locking ring which engage the rod can have another shape, for example, the surface of the free end can be flat or can be otherwise shaped. Alternatively, the projections can be omitted. 
     The head receiving portion can have an inclined open end, or can be otherwise asymmetric to allow for a greater angulation of the head in one direction. 
     The outer surface of the head receiving portion and the inner surface of the locking ring can have other shapes which allow for compression of the locking ring by means of an increasing force when the locking ring is shifted downwards relative to the receiving part body. 
     With respect to the tool, variations are also possible. For example, the tool can be configured such that the screw axis and the central axis of the receiving part extend perpendicular to the surface on which the tool is placed or positioned. The second holder for the receiving part body can be movable with respect to the first holder for the bone anchoring element. In addition, instead of a manual actuation of the tool, it may also be possible to actuate the tool by means of a mechanically or electronically operated device. 
     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.