# Patent

## Title
Bone anchoring device and tool cooperating with such a bone anchoring device

## Abstract

A bone anchoring device includes a receiving part including a rod receiving portion with a first end, a second end, and a U-shaped recess for receiving a rod, the recess defining two free legs, and a head receiving portion at the second end for introducing and clamping the head, and a locking ring configured to be arranged around the head receiving portion, the locking ring including an engagement structure for engagement with a tool, the engagement structure defining a width of the locking ring that is greater than a greatest width of the rod receiving portion, wherein the locking ring can assume a locking position exerting a greatest compressive force on the head receiving portion of the receiving part to lock an inserted head, and wherein the locking ring is movable out of the locking position towards the first end of the rod receiving portion.

## Background
CROSS-REFERENCE TO RELATED APPLICATION(S)
This application is a continuation of U.S. patent application Ser. No. 13/631,503, filed Sep. 28, 2012, which claims priority to and the benefit of U.S. Provisional Patent Application Ser. No. 61/541,511, filed Sep. 30, 2011, the contents of which are hereby incorporated by reference in their entirety. This application also claims priority to and the benefit of European Patent Application No. EP 11 183 616.9, filed Sep. 30, 2011, the contents of which are hereby incorporated by reference in their entirety.


BACKGROUND
Field of the Invention
The invention relates to a bone anchoring device including a bone anchoring element and a receiving part for coupling a rod to the bone anchoring element. The receiving part includes a rod receiving portion for receiving the rod and a head receiving portion that is flexible so as to allow introduction and clamping of a head of the bone anchoring element. The bone anchoring device further includes a locking ring arranged around the head receiving portion that can assume a locking position in which an inserted head is locked by compression of the head receiving portion of the receiving part. The locking ring has an outer surface with an engagement portion for engagement with a tool. The tool is configured to cooperate with the receiving part and the locking ring so as to allow releasing of the locking ring from a position in which the locking ring locks the head. The bone anchoring device can be realized, for example, in the form of a polyaxial bone screw.
Description of Related Art
US 2009/0149887 A1 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. Tools are provided for installing the connector body, the sleeve, a cap and the support rod.
SUMMARY
It is an object of the invention to provide a bone anchoring device that makes use of an outer locking ring for compressing a head receiving portion to lock a head of a bone anchoring element therein, that is improved with respect to its handling. Further, a tool can be provided that allows or facilitates such improved handling.
The bone anchoring device according to embodiments of the invention provide for safer loosening of a locking mechanism of the device. Also, the bone anchoring device according to embodiments of the invention enables a surgeon or other practioner to carry out revisions or further positioning or repositioning of the angular positioning of a receiving part with respect to the bone anchoring element. In one embodiment, a releasing of the locking mechanism may be possible with a rod still inserted.
The tool according to embodiments of the invention is configured to engage the receiving part and to be operated along a central axis of the receiving part. Therefore, it will not be necessary to have additional space for laterally applying the tool. Furthermore, the tool will not be jammed by, for example, forces acting only from one side.
Therefore, handling of the bone anchoring device can be simplified, because once a locking of the head is achieved, such locking can be released without applying larger forces that could result in damage to surrounding material, such as tissue, blood vessels, or nerves. Revisions or secondary adjustments of the rod and the receiving part can thus be performed in a more controlled manner.
With the bone anchoring device according to embodiments of the invention, a modular system can be provided that allows for combining of various anchoring elements with any suitable receiving part on demand, depending on the actual clinical requirements. This reduces the overall costs of using polyaxial screws, reduces inventory, and gives the surgeon a wider or more versatile choice of implants.



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 first embodiment of a bone anchoring device;
FIG. 2 shows a perspective view of the bone anchoring device of FIG. 1 in an assembled state;
FIG. 3 shows an enlarged side view of the bone anchoring device of FIGS. 1 and 2 in the assembled state;
FIG. 4 shows a cross-sectional view of the bone anchoring device shown in FIG. 3, the section being taken perpendicular to an axis of an inserted rod;
FIG. 5 shows a perspective view of a receiving part according to an embodiment of the invention;
FIG. 6 shows a cross-sectional view of the receiving part of FIG. 5, the section taken perpendicular to an axis of a channel for receiving a rod;
FIG. 7 shows a top view of the receiving part of FIG. 5;
FIG. 8 shows a perspective view of a locking ring of the bone anchoring device according to the first embodiment;
FIG. 9 shows a cross-sectional view of the locking ring of FIG. 8, the section taken perpendicular to an axis of a connected rod;
FIG. 10 shows a top view of the locking ring of FIG. 8;
FIG. 11 shows a perspective exploded view of an embodiment of a tool that cooperates with the bone anchoring device according to the first embodiment;
FIG. 12 shows a cross-sectional view of the tool of FIG. 11 in an enlarged view, the section taken through a central axis and perpendicular to a long side of a handle portion of the tool;
FIG. 13 shows a perspective view of a first step of mounting the tool of FIGS. 11 and 12 to the bone anchoring device of FIGS. 1 and 2;
FIG. 14 shows a cross-sectional view of a further step of mounting the tool of FIGS. 11 and 12 to the bone anchoring device according to the first embodiment;
FIG. 15 shows a perspective view of a further step of mounting the tool of FIGS. 11 and 12;
FIG. 16 shows an enlarged cross-sectional view of a portion of the tool attached to the bone anchoring device shown in FIG. 15;
FIG. 17 shows a perspective view of a further step of mounting the tool of FIGS. 11 and 12 to the bone anchoring device of FIGS. 1 and 2;
FIG. 18 shows an enlarged cross-sectional view of a portion of the tool mounted to the bone anchoring device as shown in FIG. 17;
FIG. 19 shows a further enlarged portion of FIG. 18;
FIG. 20 shows a perspective view of a first step of releasing a locking mechanism of the bone anchoring device of FIGS. 1 and 2 with the tool of FIGS. 11 and 12;
FIG. 21 shows an enlarged cross-sectional view of the tool and bone anchoring device shown in FIG. 20;
FIG. 22 shows an enlarged portion of FIG. 21;
FIG. 23 shows a perspective view of a step of removing the tool of FIGS. 11 and 12 from the bone anchoring device of FIGS. 1 and 2;
FIG. 24 shows an enlarged cross-sectional view of a portion of the tool and bone anchoring device shown in FIG. 23;
FIG. 25 shows a perspective view of a bone anchoring device according to a second embodiment in an assembled state;
FIG. 26 shows a side view of a locking ring of the bone anchoring device according to the second embodiment shown in FIG. 25;
FIG. 27 shows a top view of the locking ring of FIG. 26;
FIG. 28 shows a cross-sectional view of the locking ring of FIG. 26;
FIG. 29 shows a side view of the tool according to a second embodiment;
FIG. 30 shows a cross-sectional view of the tool of FIG. 29, the section taken along a central axis and perpendicular to a long side of a handle portion of the tool;
FIG. 31 shows a perspective view of a first step of mounting the tool to the bone anchoring device according to the second embodiment;
FIG. 32 shows a perspective view of a next step of mounting the tool to the bone anchoring device according to the second embodiment;
FIG. 33 shows a further step during the procedure of mounting the tool to the bone anchoring device according to the second embodiment;
FIG. 34 shows a perspective view of a next step of mounting according to the second embodiment, wherein ribs at the locking ring are engaged by the tool;
FIG. 35 shows a cross-sectional view of the bone anchoring device with the tool mounted thereon according to the second embodiment;
FIG. 36 shows an enlarged cross-sectional view of the device of FIG. 35 rotated by about 45°;
FIG. 37 shows an enlarged portion of FIG. 36; and
FIG. 38 shows a cross-sectional view of the bone anchoring device in a step of removing or detaching the tool according to the second embodiment.



DETAILED DESCRIPTION
As shown in FIGS. 1 and 2, a bone anchoring device according to an embodiment of the invention includes a bone anchoring element 1 in the form of, for example, a bone screw having a shank 2 with a threaded portion and a head 3 with a spherically-shaped outer surface portion. The head 3 has a recess 4 for engagement with a driver or tool. The bone anchoring device also includes a receiving part 5 for receiving a rod 6 to be connected to the bone anchoring element 1. Further, a fixation element 7 in the form of, for example, an inner screw may be provided for fixing the rod 6 in the receiving part 5. The bone anchoring device further includes a locking ring 8 for locking the head 3 in the receiving part 5.
Referring to FIGS. 1 to 7, the receiving part 5 includes a rod receiving portion 9 which may have a portion that is substantially cylindrical. The rod receiving portion 9 has a first end 9a and an opposite second end 9b, and a central axis C that passes through the first end 9a and the second end 9b. A coaxial first bore 10 is provided at the second end 9b as shown, for example, in FIGS. 6 and 7. A diameter of the first bore 10 is smaller than a diameter of the head 3 of the bone anchoring element 1. The rod receiving portion 9 further includes a coaxial second bore 11 extending from the first end 9a to a distance from the second end 9b. A diameter of the second bore 11 is larger than the diameter of the first bore 10. By the second bore 11, an abutment surface 11a is provided or formed inside the rod receiving portion 9 that may serve as an abutment for a tool, to be described in more detail below. A substantially U-shaped recess 12 extends from the first end 9a in the direction of the second end 9b in the rod receiving portion 9, wherein a width of the recess 12 is slightly larger than a 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 12a, 12b are formed, on which an internal thread 13 may be provided. The internal thread 13 can be, for example, a metric thread, a flat thread, a negative angle-thread, a saw-tooth thread, or any other thread form. Preferably, a thread form such as a flat thread or a negative angle thread that prevents or reduces splaying of the legs 12a, 12b when the inner screw 7 is screwed-in is used. A depth of the recess 12, which forms a channel for the rod 6, is such that the rod 6 and the inner screw 7 can be inserted between the legs 12a, 12b. Cut-outs 15 are provided in the rod receiving portion 9, extending from the second end 9b to the recess 12. The cut-outs 15 are provided on either end of the channel formed by the recess 12.
The inner screw 7 has a thread corresponding to the internal thread 13 provided on the legs 12a, 12b. If a thread form that prevents the legs from splaying is used, a single locking element such as the inner screw 7 may be sufficient.
On an outer surface of the rod receiving portion 9, in the region of the legs 12a, 12b, a groove 16 may be provided that extends in a circumferential direction and serves for engagement with a portion of the locking ring 8. The groove 16 may be asymmetric in such a way to allow for disengagement of the locking ring 8 and the groove 16 when the locking ring 8 is shifted downwards, away from the second end 9b. 
At the side of the second end 9b, the receiving part 5 further includes a head receiving portion 17 providing an accommodation space for the head 3 of the bone anchoring element 1. The head receiving portion 17 has a greatest outer diameter that is smaller than a greatest 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 17b of the head receiving portion 17. The internal hollow section 18 is adapted in its shape to the shape of the head 3. In the embodiment shown, section 18 is a spherical section to accommodate spherical head 3. Furthermore, the hollow section 18 is configured to encompass the head 3 of the bone anchoring element 1 from the side, to cover a region including a largest diameter of the head 3.
A plurality of slits 20 are provided in the head receiving portion 17 that are open to the free end 17b. The slits 20 render the head receiving portion 17 flexible so that the head receiving portion 17 can be compressed to clamp and finally lock an inserted head 3 in the hollow internal portion 18 by means of friction. A number and size of the 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 bone anchoring element 1 can be inserted by expanding the head receiving portion 17, and can be clamped by compressing the head receiving portion 17.
An outer surface of the head receiving portion 17 has a first section 21 with an outer diameter increasing towards the free end 17b, for example, in an outwardly curved or conically widening manner. Adjacent to the first section 21, there may be a circumferential groove 22 that is recessed with respect to the first section 21 and that serves for engagement with a portion of the locking ring 8. The groove 22 may be shaped so as to allow for disengagement of the locking ring 8 and the groove 22 when moving the locking ring 8 in a direction towards the free end 17b (or away from the first end 9a of the rod receiving portion). This is realized, for example, by having a lower wall of the groove 22 that is inclined towards the free end 17b. 
Adjacent the groove 22, there is a third portion 23 of the head receiving portion 17 with a substantially cylindrical outer surface. The third portion 23 may be configured to cooperate with a portion of the locking ring 8 to enhance a clamping effect of the locking ring 8.
The locking ring 8 will now be described in particular with reference to FIGS. 3, 4, and 8 to 10. The locking ring 8 may include a portion that is substantially cylindrical, and has an upper end 8a and a lower end 8b. In a mounted state, the upper end 8a is oriented in the direction of the first end 9a of the rod receiving portion 9 and the lower end 8b is oriented in the direction of the free end 17b of the head receiving portion 17. Approximately in a central region of the locking ring 8, at an inner wall, a first portion 81 is provided that cooperates with the first outer surface portion 21 of the head receiving portion 17 to compress the head receiving portion 17. The first portion 81 may be slightly tapered, may be straight, or may be curved with a curvature directed towards a center of the locking ring 8. Furthermore, at the lower end 8b, the locking ring has an inwardly projecting edge 82, an inner diameter of which is smaller than inner diameters of 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 and to finally engage the cylindrical portion 23 of the head receiving portion 17 when the bone anchoring device is in a locked state.
The locking ring 8 also has a third portion including upwardly extending wall portions 83a that are separated from each other by slits 84. The upwardly extending wall portions 83a are arranged at an outer circumference of an inner circumferential shoulder 85 of the locking ring 8, and render the third portion of the locking ring 8 flexible. A number and size of the slits 84 and a thickness of the wall portions 83a are configured such that a desired flexibility is obtained. At respective free ends of the wall portions 83a, the wall portions 83a are configured to engage the groove 16 provided on the outer surface of the rod receiving portion 9.
Two projections 86 that are located diametrically opposite to each other are also formed in the third portion of the locking ring 8. The projections 86 have such a height that they extend through the cut-outs 15 and project above a bottom of the substantially U-shaped recess 12 when the locking ring 8 is in a position where its shoulder 85 abuts the second end 9b of the rod receiving portion 9. A free end surface 86a of the projections 86 may be curved, for example, concave. The locking ring 8 is arranged around the head receiving portion 17 of the receiving part 5 such that the projections 86 are located at the positions of the recess 12. By means of this, the projections 86 prevent the locking ring 8 from rotating relative to the receiving part 5 when the rod 6 is not yet inserted.
The locking ring further includes on its outer surface portion adjacent the upwardly extending wall portions 83a an engagement portion 88 for engagement with a tool, described in more detail below. In a first embodiment, the engagement portion 88 is realized by an external thread. The thread extends in an axial direction of the locking ring 8 along a portion of the outer surface of the locking ring 8. A depth of the thread may be small, so that a violation of surrounding tissue or blood vessels can be avoided or minimized. Aligned with and just below the projections 86, the thread may be interrupted at two opposite thread-free sections 89. The thread may be, for example, flattened towards the center at the thread-free sections 89, respectively. The sections 89 may serve for applying a distraction or compression tool. The thread-free sections 89 can also be provided at other positions, or in some embodiments, more than two thread-free sections 89 can be provided.
The receiving part 5, the locking ring 8, the inner screw 7, and the bone anchoring element 1 may be made of bio-compatible materials, for example, of titanium or stainless steel, of a bio-compatible alloy such as Nitinol, or of a bio-compatible plastic material, such as PEEK (Polyetheretherketone). The parts can be made of the same or of different materials.
In an embodiment of the invention, the locking ring 8 can assume three main positions relative to the receiving part 5. In a first position (not shown), the inwardly projecting edge 82 of the locking ring 8 engages or is positioned adjacent to the groove 22 of the head receiving portion 17. In this position, the head 3 of the bone anchoring element 1 can be introduced into the internal hollow space 18 from the free end 17b of the head receiving portion 17. The locking ring 8 is prevented from moving further upwards towards the first end 9a of the rod receiving portion 9 because the shoulder 85 abuts against the second end 9b of the rod receiving portion 9.
In a second position, the locking ring 8 is shifted towards the free end 17b of the head receiving portion 17 until the flexible wall sections 83a snap with their free ends into the groove 16 of the rod receiving portion 9. In this position, the head 3 is not yet locked, but may be prevented from removal out of the internal hollow space 18. The head 3 may also be frictionally clamped in this state to such an extent that the bone anchoring element 1 is still movable relative to the receiving part 5 when a force, for example, a manually applied force, is applied to overcome the friction force. An example of the second position can be seen, for example, in FIGS. 21 and 22.
In a third position, the locking ring 8 is shifted further towards the free end 17b of the head receiving portion 17 (or away from the first end 9a of the rod receiving portion 9) such that the head 3 is finally locked. In this position, the head receiving portion 17 is compressed by the locking ring 8, so that the head 3 cannot move and is fixed at an angular position with respect to the receiving part 5. Between upper ends of the flexible wall sections 83a and an upper wall of the groove 16 of the rod receiving portion 9 is a gap 90, as shown, for example, in FIG. 4. In the locked condition of the head 3, the locking ring 8 cannot be loosened, or may be very difficult to loosen, under normal operating conditions.
A tool 50 according to a first embodiment will now be described with reference to FIGS. 11 and 12.
The tool 50 includes a tubular member 51 with a front end 52 and a rear end 53 opposite to the front end 52. Adjacent the front end 52, the tubular member 51 has at its inner wall an engagement portion 54 for engaging the engagement portion 88 of the locking ring 8. An inner diameter of the tubular member 51 at the front end 52 is therefore such that the tubular member 51 can be placed onto or around the receiving part 5 and can engage the locking ring 8. At a distance from the front end 52 a stop 55 is provided within the tubular member 51 that abuts against the upper edge 8a of the locking ring 8 when the engagement portion 54 engages the threaded portion 88, for example, as shown in FIG. 14. At its rear end 53, the tubular member 51 also has an internal threaded portion 56.
The tool 50 further includes a shaft 60 with a front end 61 and a rear end 62 that can be formed as a handle. The shaft 60 has an outer threaded portion 63 at a distance from the handle that cooperates with the internal threaded portion 56 of the tubular member 51. Adjacent the outer threaded surface portion 63 is a stop 64 that limits the insertion depth of the shaft 60 into the tubular member 51. The stop 64 can be, for example, a circumferential shoulder. The shaft 60 is insertable into the tubular member 51 and is movable therein. An insertion depth can be adjusted by the cooperation of the threads 56 and 63 of the tubular member 51 and the shaft 60, respectively. As shown in FIG. 12, when the shaft 60 is inserted and screwed into the tubular member 51 until the stop 64 abuts against the rear end 53, the front end 61 of the shaft 60 is at a distance from the front end 52 of the tubular member 51.
The shaft 60 has at its front end 61 a blind hole 65, in which a post 66 can be supported by a spring 67. An end surface 66a of the post 66 projects out of the front end 61 of the shaft (see, e.g., FIG. 16) and the post 66 is movable in the blind hole 65. The movement of the post 66 in the blind hole 65 may be limited in two directions by a stop. The stop may be formed by an axially elongated hole 68 that extends through the post 66 and through which a pin 69 is passed. The pin 69 extends through a transverse hole 70 provided in the shaft 60, as shown in particular in FIG. 11. By means of this, the post 66 is moveable between a first position in which the pin is at an end of the elongate hole 68 nearer to the spring 67, to a second position in which the pin 69 is at an end of the elongate hole 68 nearer to the front end 61 of the shaft 60. In the first position, the post 66 projects further outward from the front end 61 and may be biased by partial compression of the spring 67. In the second position, the post 66 is urged towards the end of the blind hole 65, thereby further compressing the spring 67. The spring 67 is shown as a helical spring, but can be any other spring or mechanism that accomplishes the same purpose. For example, the spring mechanism can also be realized by an elastomer cushion.
Operation of the tool 50 will now be described with reference to FIGS. 13 to 24. First, the tubular member 51 may be separated from the shaft 60. As shown in FIGS. 13 and 14, the tubular member 51 is placed over the receiving part 5 (of a locked bone anchoring device, for example) until the threaded portion 54 engages the threaded portion 88 of the locking ring 8.
In a next step, shown in FIGS. 15 and 16, the shaft 60 is inserted into the tubular member 51 until the front end 66a of the post 66 abuts against the abutment surface 11a in the receiving part 5 (see FIG. 16). In this position, the front end 61 of the shaft 60 is spaced apart from the first end 9a of the rod receiving portion 9 of the receiving part 5. The post 66 is in its first position, where the pin 69 is nearer to the spring 67. The shaft 60 is not yet fully introduced into the tubular member 51. As shown in FIGS. 17 and 18, the shaft 60 is thereafter further screwed into the tubular member 51 until the front end 61 of the shaft abuts against the first end 9a of the rod receiving portion 9 of the receiving part 5. Thereby, a counterforce acting onto the post 66 presses the post 66 into the blind hole 65 against the force of the spring 67. In this condition, the locking ring 8 is still in its third position, in which the head is locked. This can be seen more clearly in the enlarged representation according to FIG. 19 that shows the gap 90 between the upper end 8a of the locking ring 8 and the wall of the groove 16 facing the upper end 8a. 
For releasing the locking ring 8 from the locking position, the tubular member 51 can be gripped at a gripping portion 57 and held in this position while the shaft 60 is further screwed into the tubular member 51, as shown in FIG. 21. By means of this, the locking ring 8 is drawn upwards (i.e., towards first end 9a of rod receiving portion 9) until flexible wall section 83a abuts the upper wall of groove 16, as shown in detail in FIG. 22. Here, the locking ring 8 is in the second position, where the locking ring 8 does not lock the head 3.
A step of removing the tool is shown in FIGS. 23 and 24. The handle at end 62 can be turned in the other or opposite direction. The locking ring 8 abuts against an end of the groove 16, as shown in FIG. 22, and therefore, it can not be drawn further upwards (e.g., back towards the insertion position). Because the post 66 abuts against the abutment surface 11a of the receiving part, the screwing back of the shaft 60 does not result in simultaneous rotation of the receiving part 5 and the locking ring 8. Hence, the tool 50 can be removed easily. Then, the angular position between the bone anchoring element 1 and the receiving part 5 can be readjusted.
A second embodiment of the bone anchoring device is shown in FIGS. 25 to 28. The second embodiment differs from the bone anchoring device according to the first embodiment by the design of an engagement portion of the locking ring 8′. All other parts are identical or similar to the first embodiment and are marked with the same reference numerals. The description of the same or similar parts is not repeated.
The locking ring 8′ has an engagement portion for a tool that is in the form of diametrically opposite rib portions that extend circumferentially. In the embodiment shown, two rib portions 88a′ and 88b′ are arranged at an outer surface of the locking ring 8′ beneath the elastically deformable wall portions 83a. The rib portions 88a′, 88b′ extend over a segment of an outer circumference of the locking ring 8′ for approximately a quarter circle or less than a quarter circle. The rib portions 88a′, 88b′ may be arranged at an angle of approximately 45° with respect to the projections 86 that support the rod 6 around a circumference of the locking ring 8′.
Each rib portion 88a′, 88b′ includes two circumferentially extending ribs 881a′, 882a′ and 881b′, 882b′, respectively, that are spaced from each other in an axial direction of the locking ring 8′. The ribs 881a′, 882a′, 881b′, 882b′ may be slightly inclined towards the second end 8b of the locking ring 8′, as can be seen in particular in FIG. 28. The downward inclination of the ribs 881a′, 882a′, 881W, 882W provide an undercut for engagement with the tool described below that enhances the safety of the engagement with the tool. In the embodiment shown, two ribs in each rib portion 88a′, 88b′ are provided. It shall be noted, however, that one rib for each rib portion may be sufficient. Also, more than two ribs can be provided for each rib portion.
A tool for releasing the locking ring from a locked portion according to a second embodiment will be described, referring to FIGS. 29 and 30. The tool 50′ includes a tubular member 51′ with two substantially rectangular recesses 51a′, 51b′ at a front end 52′. Adjacent the front end 52′, engagement portions 54a′, 54b′ that correspond to and cooperate with the rib portions 88a′, 88b′ are located inside the tubular member 51′. The engagement portions 54a′, 54b′ are rib portions with an inclination or bias away from the front end 52′. At a distance from the front end 52′, a stop 55′ is provided. The substantially rectangular recesses 51a′, 51b′ have a size that may be approximately a size of rib-free portions of the locking ring in a circumferential direction, and have a depth in an axial direction that is such that when the tubular member 51′ is placed onto the receiving portion and engages the locking ring 8′, an inserted rod 6 can pass through the recesses 51a′ 51b′. 
The tubular member 51′ also has a rear end 53′ with an internally threaded portion 56′. At or in a vicinity of the rear end 53′, a grip portion 57′ is provided for facilitating gripping.
The tool 50′ further includes a shaft portion 60′ with a front end 61′ and a rear end 62′ that is provided with a handle. At a distance from the rear end 62′, an externally threaded portion 63′ is provided that cooperates with the internally threaded portion 56′ of the tubular member 51′, similarly as seen in the first embodiment. Furthermore, similar to the first embodiment, a stop in form of an annular shoulder 64′ may be provided at a distance from the rear end 62′. The shaft 60′ in this embodiment may not have a resiliently supported post as seen in the previous embodiment. A length of the shaft portion 60′ is such that when the shaft portion 60′ is inserted and screwed into the tubular member 51′ until the stop 64′ abuts against the rear end 53′ of the tubular member 51′, the recesses 51a′, 51b′ at the front end 52′ can cover or accommodate the receiving part 5 and the rod 6 inserted therein.
The tool 50′ according to the second embodiment can be applied, for example, when the rod 6 is inserted and the inner screw 7 is removed. As shown in FIG. 31, the tool 50′ is oriented such that the engagement portion in the form of the ribs 54a′, 54b′ is placed at or aligned with the rib-free portions of the locking ring 8′. The rod 6 may abut against one side of the recesses 51a′, 51b′. The tubular member 51′ is shifted downwards until the stop 55′ abuts against the upper edge 8a of the locking ring 8′. Thereafter, as shown in FIGS. 32 to 34, the tubular member 51′ is rotated so that the engagement portions 54a′, 54b′ inside the tubular member 51′ engage the rib portions 88a′, 88b′ on the outer surface of the locking ring 8′. Because the recesses 51a′, 51b′ are sufficiently large in a circumferential direction, the tubular member 51′ can be rotated such that the rod 6 may abut against the other side of the recesses 51a′, 51b′ after rotation, as shown in FIG. 34.
Thereafter, as shown in FIGS. 35 to 37, the shaft 60′ is screwed further into the tubular member 51′ until its front end 61′ abuts against the first end 9a of the rod receiving portion 9 of the receiving part 5. In a next step, as shown in FIG. 38, the shaft 60′ is further moved into the tubular member 51′ while the tubular member 51′ is held at the gripping portion 57′, for example, by a practitioner. By means of this, the locking ring 8′ is pressed upwards (i.e. towards the first end 9a of the rod receiving portion 9) and is released from the locking position.
For the removal of the tool 50′, the shaft 60′ is slightly screwed back, then the tubular member 51′ is rotated until the engagement portions 54a′, 54b′ of the tubular member no longer engage the engagement portions 88a′, 88b′ at the locking ring 8′. The tool 50′ can then be removed by pulling the tool 50′ upwards, away from the bone anchoring device.
It shall be noted that further modifications of the embodiments shown are also possible. For example, the tool according to the second embodiment may also include a post that is supported by a spring. Also, the tool according to the second embodiment may have a threaded engagement portion for engaging a threaded locking ring, similar to the first embodiment. The tool of the first embodiment may also be provided without a post.
For the engagement at the locking ring, other modifications are also conceivable. For example, it may be possible to have engagement portions with various other shapes.
The bone anchoring device according to other embodiments of the invention can be provided in a modified form. For example, the head of the bone anchoring element can have any other shape, such as, for example, a cylindrical shape, whereby a monoaxial bone screw is provided, allowing for rotation of a screw element with respect to the receiving part around a single axis. The head can also be conically shaped or otherwise shaped, with the internal hollow section of the head receiving portion adapted to the shape. In a further modification, flexibility of the head receiving portion may be based on or facilitated by properties of the material, for example, a plastic material may be used, and the slits may be fully or partly omitted.
The projections of the locking ring that engage the rod can also have various other shapes. The surface of the free end can be flat or can be otherwise shaped. The projections can be also omitted.
The head receiving portion can have an inclined open end or can be otherwise asymmetric to allow for a greater angulation of an inserted head in one direction.
The outer surface of the head receiving portion and the inner surface of the locking ring can also have other shapes that allow a compression of the locking ring by means of an increasing force when the locking ring is shifted downward.
The locking ring can also have various other designs. For example, the locking ring can be formed without the flexible wall sections.
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.

## Claims

1. A bone anchoring device comprising:
a receiving part for coupling a rod to a bone anchoring element, the receiving part comprising:
a rod receiving portion with a first end, a second end, and a U-shaped recess for receiving a rod, the recess extending from the first end in a direction of the second end and defining two free legs; and
a head receiving portion at the second end for accommodating a head of a bone anchoring element, the head receiving portion having a free end and being flexible for introducing and clamping the head; and

a locking ring configured to be arranged around the head receiving portion, wherein an engagement structure projects from an outer surface of the locking ring for engagement with a tool;
wherein when the locking ring is around the head receiving portion at a first position, an end of the locking ring abuts a wall of the receiving part to restrict movement of the locking ring towards the first end of the rod receiving portion, and the engagement structure projects farther radially outwardly than the wall extends relative to a central axis of the bone anchoring device;
wherein the locking ring can assume a locking position where the locking ring exerts a compressive force on the head receiving portion of the receiving part to lock the head of the bone anchoring element when the head is held in the head receiving portion; and
wherein the locking ring is movable out of the locking position towards the first end of the rod receiving portion.

2. The bone anchoring device of claim 1, wherein the engagement structure comprises a plurality of ribs extending in a circumferential direction of the locking ring.

3. The bone anchoring device of claim 2, wherein the ribs do not extend around an entire circumference of the locking ring.

4. The bone anchoring device of claim 1, wherein the engagement structure comprises at least two ribs spaced apart from each other in an axial direction of the locking ring.

5. The bone anchoring device of claim 1, wherein the engagement structure comprises a thread.

6. The bone anchoring device of claim 1, wherein the locking ring comprises two projections that are 180° offset from each other for engagement with a rod.

7. The bone anchoring device of claim 6, wherein the engagement structure is grouped into two groups to the left and right of the projections that are spaced apart in a circumferential direction of the locking ring.

8. The bone anchoring device of claim 7, wherein each of the groups is arranged closer to a corresponding one of the projections.

9. The bone anchoring device of claim 1, wherein the head receiving portion has an exterior surface with a tapered or curved portion and the locking ring has an interior surface with a tapered or curved portion which are configured to cooperate with one another, such that an inserted head is clamped when the locking ring is moved towards the free end.

10. The bone anchoring device of claim 1, further comprising a bone anchoring element having a shank for anchoring in a bone and a head.

11. The bone anchoring device of claim 1, wherein the engagement structure is spaced apart axially from the end of the locking ring.

12. The bone anchoring device of claim 1, wherein when the locking ring is around the head receiving portion, the engagement structure includes a surface that faces the free end of the head receiving portion in a direction parallel to a central axis of the bone anchoring device.

13. The bone anchoring device of claim 1, wherein the end of the locking ring comprises a flexible portion configured to flex radially outwardly.

14. A bone anchoring device comprising:
a receiving part for coupling a rod to a bone anchoring element, the receiving part comprising:
a rod receiving portion with a first end, a second end, and a U-shaped recess for receiving a rod, the recess extending from the first end in a direction of the second end and defining two free legs; and
a head receiving portion at the second end for accommodating a head of a bone anchoring element, the head receiving portion having a free end and being flexible for introducing and clamping the head; and

a locking ring configured to be arranged around the head receiving portion, wherein an engagement structure projects from an outer surface of the locking ring for engagement with a tool;
wherein when the locking ring is around the head receiving portion, two opposite portions of the locking ring extend above a bottom of the U-shaped recess towards the first end of the rod receiving portion to contact the rod, the engagement structure extends circumferentially around the locking ring by a distance that is less than a circumferential distance between the two opposite portions, and the locking ring can assume a locking position where the locking ring exerts a compressive force on the head receiving portion of the receiving part to lock the head of the bone anchoring element when the head is held in the head receiving portion; and
wherein the locking ring is movable out of the locking position towards the first end of the rod receiving portion.