# Patent

## Title
Receiving part for connecting a shank of a bone anchoring element to a rod and bone anchoring device with such a receiving part

## Abstract

A receiving part for connecting a shank of a bone anchoring element to a rod. The receiving part has a first end, a second end and a recess having a substantially U-shaped cross section. The recess extends from the first end in a direction of the second end forming two legs open at the first end. The recess defines a channel to receive the rod. The channel has a longitudinal axis. The legs have an inner thread to receive a securing element having an outer thread cooperating with the inner thread. The legs are provided with stiffening structures. In one embodiment, the stiffening structures are formed by the wall thickness of the outer edges of the legs being larger than the wall thickness of the legs at a location closest to the central axis of the receiving part.

## Background
CROSS-REFERENCE TO RELATED APPLICATION(S)
This application is a continuation of U.S. patent application Ser. No. 11/440,489, filed May 24, 2006, now U.S. Pat. No. 8,262,704, issued Sep. 11, 2012, which claims the benefit of U.S. Provisional Application No. 60/685,798 filed May 31, 2005 and claims the priority of European Patent Application No. 05 011 438.8, filed May 27, 2005, the entire contents of which are incorporated herein by reference.


BACKGROUND
The invention relates to a receiving part for connecting a shank of a bone anchoring element to a rod and to a bone anchoring device with such a receiving part to be used in spinal surgery or trauma surgery.
A known polyaxial bone screw 100 is shown in FIG. 11. It comprises a receiving part 101 for connecting a screw element 102 to a rod 103, the outer wall of the receiving part being substantially cylindrical. The receiving part 101 has a recess 104 with a substantially U-shaped cross-section forming two open legs 105, 106 defining a channel for inserting the rod 103. An inner thread 107 is provided at said legs for receiving a securing screw 108 to fix said rod in the channel. The inner thread 107 is a metric thread.
When screwing in the securing screw 108 as shown in FIG. 13, forces directed radially outwards which are depicted by arrows A act on said inner thread which cause said legs 105, 106 to splay. This may loosen the fixation of the rod.
Several approaches have been made so far to solve the problem of the splaying of the legs. One solution is to provide tube-shaped counter-holding tools. Another solution consists in providing an outer nut to be screwed onto an outer thread of said legs as, for example, disclosed in EP 0 614 649 A. Also, outer ring-shaped securing means are known. However, securing means such as outer nuts or rings enlarge the size of the bone screw and therefore limit the clinical applications.
A further approach consists in using a specific kind of thread shape for the inner thread 107 and the corresponding thread of the securing screw. It is known to use a saw tooth thread as disclosed in U.S. Pat. No. 5,005,562 or a reverse angle thread such as disclosed in U.S. Pat. No. 6,296,642 B1 or US 2002/0138076 A1 to eliminate the outwardly directed radial forces acting on said legs. A particularly advantageous thread shape is the flat thread as disclosed in US 2003/0100896 A1 which eliminates such radial forces and is easy to manufacture. The use of these specific thread shapes allows a use of the bone screw without a further outer securing device such as an outer nut or ring.
However, despite the above measures there is still a problem of a diagonal splaying of the legs at the time of final tightening of the securing screw. This splaying is caused mainly by the torque acting on said legs 105, 106 at the time of final tightening which causes a diagonal lateral deformation of the open ends of the legs in the longitudinal direction of the rod due to the friction between the thread flanks. The torque B acting on the legs is shown in FIG. 2. The deformation is shown in FIG. 14 which does not show the securing screw for the purpose of better illustration. The problem of the splaying of the legs is not restricted to polyaxial bone screws but also occurs in all so called top open bone anchoring devices including monoaxial screws which have two open legs and a slit in between for inserting the rod.
Providing flattened surfaces 109 at the sides of the receiving part which are oriented in a transverse direction of the channel, as shown in FIG. 12, does not solve the problem but contributes to minimize the size of the bone screw in a longitudinal direction of the rod.
SUMMARY
It is an object of the invention to provide a receiving part, in particular for a polyaxial bone anchoring device, and a bone anchoring device which is connected to a rod which is safe in use and at the same time has a compact design. Further it is an object to provide a tool for holding and/or guiding such a bone anchoring device.
The receiving part according to the invention has the advantage that it prevents or significantly reduces splaying of the legs without using any outer securing means. It is of a reduced size in a direction parallel to the longitudinal axis of said rod. Therefore it is more compact than the known screws. When designing it with the same size as a conventional screw in a direction parallel to the longitudinal axis of the rod, a rod with a larger diameter compared to that of the conventional screw can be used.
Further features and advantages of the invention will become apparent from the description of the embodiments using the drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a perspective view of a receiving part according to a first embodiment of the invention.
FIG. 2 shows a top view of the receiving part according to the first embodiment.
FIG. 3a shows a sectional view of the bone anchoring device with the receiving part of the first embodiment.
FIG. 3b shows a perspective view of the bone anchoring device with the receiving part of the first embodiment.
FIGS. 4a and 4b show schematically the principle of providing a stiffening structure according to the first embodiment of the receiving part.
FIG. 5a shows a schematic view of the cross section of the open legs of a receiving part according to a second embodiment of the invention.
FIG. 5b shows a perspective view of the receiving part according to the second embodiment.
FIG. 6a shows a schematic view of the cross section of the open legs of a receiving part according to a third embodiment of the invention.
FIG. 6b shows a perspective view of the receiving part according to the third embodiment.
FIG. 7 shows a side view of a counter holding tool.
FIG. 8 shows a perspective view of a holding portion of a counter holding tool according to the invention.
FIG. 9 shows a perspective view of a conventional trocar tool.
FIG. 10 shows a perspective view of a trocar tool according to the invention.
FIG. 11 shows a perspective view of a conventional polyaxial bone screw.
FIG. 12 shows a perspective view of a modified conventional polyaxial bone screw.
FIG. 13 shows a schematic sectional view illustrating a problem arising with the conventional receiving part.
FIG. 14 shows a schematic view illustrating a further problem arising with the conventional receiving part.



DETAILED DESCRIPTION
As shown in FIGS. 1 to 3b a receiving part 1 for connecting a shank 50 of a bone anchoring element 51 to a rod according to the invention comprises a first end 2 and a second end 3, a central axis M and a recess 4 having a substantially U-shaped cross-section. The recess 4 extends from the first end 2 towards the second end 3 and forms two legs 5, 6 open at the first end 2. The recess defines a channel having a longitudinal axis L for the insertion of a rod 7. The longitudinal axis L corresponds to the longitudinal axis of the rod when the rod is inserted. The width of the recess 4 is just as large that the rod can be placed in and fit into the bottom 8 of the recess while still being able to perform a sliding motion in the recess when it is not fixed. The legs 5, 6 project above the rod when the rod is inserted and have an inner thread 9 extending from a position adjacent to the first end 2 in a direction of the second end 3 over a predetermined length to allow a securing screw 10 to be screwed in to fix the rod. The inner thread 9 and the matching outer thread of the securing screw is preferably a flat thread, the upper and lower flanks of which each include an angle of 90 degrees with the screw axis.
At the outer wall surface of the legs 5, 6 two opposite flat surfaces 11, 12 are provided which extend parallel to the longitudinal axis L of the channel and parallel to the central axis M of the receiving part. The flat surfaces 11, 12 are parallel to each other and extend from a first position near the first end 2 to a second position towards the second end 3 which is located below the bottom 8 of the recess 4. Hence, the flat surfaces 11, 12 extend along the portions of said legs which enclose the rod from both sides. The receiving part 1 further comprises at its sides transverse to the longitudinal axis L of the recess two opposite flat surfaces 13, 14 extending from a distance from the first end 2 to a position below the bottom 8 of the recess. The flat surfaces 13, 14 are parallel to the central axis M and to each other. In this way, the flat surfaces 11, 12 and the flat surfaces 13, 14 include an angle of 90°, respectively. In the embodiment shown, the width W of the flat surfaces 11, 12, 13 and 14 in a direction perpendicular to the central axis M is the same so that an envelope E in a circumferential direction of the receiving part near the bottom 8 of the recess is substantially square shaped.
At the lateral edges of the legs 5, 6 portions 15, 16, 17, 18 with enhanced wall thickness compared to the wall thickness of the legs in the region of the flat surfaces 11, 12 are provided. These portions form stiffening structures preventing or significantly reducing a bending of the legs due to a diagonal splaying. The portions 15, 16, 17, 18 having the enhanced wall thickness have either a flattened outer surface as shown in FIGS. 1 and 2 or a rounded surface. They may extend along the whole length of the flat surfaces 11, 12, 13 or 14 or, as shown in FIGS. 1 and 2, go over in slanted portions 19 with diminishing wall thickness towards the first end 2.
FIGS. 3a and 3b show the receiving part according to the invention together with a bone screw 51 to form a polyaxial bone anchoring device. In this case the receiving part 1 further comprises, as shown in FIG. 3a, a coaxial bore 20 extending through the bottom 8 of the channel and tapering in a section 21 towards the second end 3 to provide an opening 22 in the second end 3 which has a diameter smaller than that of the bore 20. The diameter of said bore 20 is larger than the diameter of a head 52 of the screw element 51 to allow the shank 50 and the screw head 52 of the bone anchoring screw 51 to be guided through said bore. The diameter of the opening 22 is smaller than the diameter of the screw head 52 but larger than the diameter of the shank 50. Thus, the tapering section 21 and the screw head 52 form a ball and socket joint to flexibly connect the bone anchoring screw and the receiving part.
A pressure element 25 is provided which is slidable in the bore 20 and serves for exerting pressure on the head to fix the screw element in an angular position with respect to the receiving part when the pressure element is pressed against the head. The pressure element may be substantially disc shaped and may have a recess for encompassing the upper part of the screw head and a recess for accommodating the lower side of the rod 7. The pressure element may also have an opening for allowing a screwing-in tool to be guided through.
As can be seen from FIG. 3b the portion 15 of the receiving part and corresponding portions 16, 17, 18, which have the enhanced wall thickness, are located approximately at the height of the longitudinal axis of the rod when the rod is inserted into the receiving part and the flat surfaces 11, 12, 13, 14 extend over the area where the rod is enclosed by the legs 5, 6 from both sides.
In operation the screw element and the pressure element are inserted into the receiving part. Then the rod is placed in and fixed by means of the securing screw. In the embodiment shown the rod presses on the pressure element when the inner screw is tightened and the screw head and the rod are fixed simultaneously.
As shown in FIG. 2 by means of the arrows B the torque acting on the legs is more uniformly distributed so that a diagonal splaying of the legs can be almost fully prevented.
FIGS. 4a and 4b schematically show the principle of enhancing the stiffness of the legs 5, 6 according to the invention. In the conventional receiving part shown in FIG. 4a the legs have a cross-section in a direction transverse to the central axis M shaped like the shape of a section of a cylinder. Flattening the outer wall of the legs in a direction parallel to the longitudinal axis L of the recess 4 to generate the flat surfaces 11, 12 diminishes the wall thickness to a value d at a position closest to the central axis M. The thickness d is smaller than the wall thickness in the region of the lateral edges of the legs. Further flattening the surfaces perpendicular to the longitudinal axis L of the recess 4 to create the flat surfaces 13, 14 eliminates the sharp edges 30 and generates portions 15 with enhanced wall thickness D at the lateral edges of the legs. This provides a stiffening structure to prevent diagonal splaying of the legs. The following example illustrates the effect achieved with the receiving part according to the invention. By using a rod with a diameter drod of 5.5 mm and with an inner diameter of 7 mm of the inner threaded portion, the lateral width Wmax of the receiving part according to the invention in a direction transverse to the rod is reduced to 11.5 mm compared to 13 mm in the conventional receiving part. The geometrical moment of inertia which is a measure for the stiffness of the flanks, is enhanced to 380 mm4 for the receiving part according to the invention compared to 240 mm4 in the conventional case.
The reduced lateral width Wmax allows the construction of more anatomically shaped lower sides of the bone anchoring devices which contributes to reducing the load on and/or the contact with the facet joints.
On the contrary, if a receiving part according to the invention has the same lateral width Wmax as the conventional cylindrical receiving part, the receiving part according to the invention can have a larger recess 4 for the use of a rod with a larger diameter without decreased stiffness.
The receiving part 1 may be produced by providing a cylindrical part and producing the flat surfaces by milling.
The invention is not limited to the use of the receiving part in a bone anchoring device as shown in FIGS. 3a and 3b. Other polyaxial connections are possible, for example the head and the rod may be fixed separately. In this case separate rod fixation means and head fixation means are provided. Further, in the embodiment shown the screw head and the tapering section are of a spherical shape, but any other shape is possible as long as the head of the bone anchoring screw and the tapering section function as a ball and socket joint. For example, the tapering section can be conically shaped.
The invention is further not limited to polyaxial bone anchoring devices but is also applicable to monoaxial bone anchoring devices. In this case, the second end 3 of the receiving part is fixedly connected to a shank which is to be anchored in a bone.
The shank can be a threaded shank as shown in the Figures but may also be a pin or a hook or another device which is apt to be anchored in the bone.
The inner thread can have any shape, although a shape which reduces the forces directed radially outward is preferred.
A second embodiment of the receiving part is shown in FIGS. 5a and 5b. The second embodiment differs from the first embodiment shown in FIGS. 1 to 3b in that the surfaces 11′, 12′ corresponding to the flat surfaces 11, 12 are not flat but concavely shaped and that the portions 15′ having an enhanced wall thickness which are the outer edges of the concave surfaces 11′, 12′ extend along the whole length of legs up to the first end 2. Although stiffness is reduced compared to the first embodiment, the splaying of the legs is still considerably reduced with the receiving part of the second embodiment. The receiving part of the second embodiment has the advantage that the concave surfaces facilitate gripping of the receiving part by the surgeon.
A third embodiment of the receiving part is shown in FIGS. 6a and 6b. The third embodiment differs from the first embodiment shown in FIGS. 1 to 3b in that the flat surfaces 11, 12 are provided with longitudinal ribs 115 extending in a direction transverse to the longitudinal axis L of the channel and parallel to the central axis M of the receiving part over the whole surface of the flat surfaces. The longitudinal ribs facilitate gripping of the receiving part.
Other modifications are possible. The ribs do not need to extend over the whole outer surface of the legs. Further, also the second embodiment may have such longitudinal ribs on the concave surfaces 11′, 12′. It is even conceivable to provide the ribs at the outer surfaces of the conventional, convexly shaped outer surfaces of the known receiving parts.
In a further modification the stiffening structures are realized by adding to the basic material from which the receiving part is formed one or more additional materials to stiffen the basic material. This can be made, for example, by adding fibers, for example carbon fibers, or by adding glass beads.
FIG. 7 shows a conventional counter-holding tool for use with receiving parts of polyaxial screws. The counter-holding tool has a grip portion 30, a hollow shaft 32 which allows a screwing-in tool to be guided through and a holding portion 33 which has a circular shape for use with the conventional cylindrical receiving parts. The holding portion has opposite slits 34 to fit over the rod which allow counter-holding of the receiving part when the inner screw is screwed in.
According to the invention, the holding portion of the counter-holding tool is adapted to the shape of the receiving part. FIG. 8 shows the holding portion 40 for a counter-holding tool according to the invention. The holding portion 40 has a substantially square shaped contour with a hollow interior with an opening 43 which is connected to the hollow shaft of the counter-holding tool and two recesses 41 on opposite sides to fit onto the receiving part when the rod is inserted. The holding portion 40 further has two opposite parallel surfaces 42 which cooperate with the flat surfaces 11, 12 of the receiving part to provide a form-fit connection. The counter-holding tool is particularly suitable for percutaneous applications in minimally invasive surgery. By means of the form-fit connection between the holding portion and the receiving part it is possible to precisely align the receiving parts.
FIG. 9 shows a conventional tube part of a trocar for minimally invasive surgery which has a circular cross-section. FIG. 10 shows a tube part of a trocar according to the invention which has a shape adapted to the shape of the receiving part which is substantially square. With the tube part according to FIG. 10, due to the form-fit connection between the receiving part and the tube part, it is possible, for example, to precisely introduce and align the bone anchoring device by means of percutaneous introduction.

## Claims

1. A bone anchoring device comprising a bone anchoring element having a head and a shank, a rod, a receiving part for connecting the bone anchoring element to the rod, and a securing element, the receiving part comprising:
a first end, a second end, and a central axis extending between the first end and the second end,
a recess having a substantially U-shaped cross section extending transversely to the central axis through the entire receiving part, wherein the recess is open at the first end of the receiving part and extends from the first end in a direction of the second end, wherein the recess forms two legs that come together to form a bottom of the recess, and wherein the recess defines a channel to receive the rod, and
an outer surface that does not have a spherical profile extending continuously from the first end of the receiving part to the second end of the receiving part,
wherein the two legs are configured to receive the securing element to secure the rod, and wherein the securing element is rotatable to engage the two legs,
wherein the receiving part is polyaxially connectable to the bone anchoring element,
wherein for at least one vertical cross-sectional plane that includes the central axis and that extends through the legs, the resulting cross-section of the receiving part has an outer profile with a first section, wherein an entire axial length of the first section has a maximum horizontal width measured in a direction perpendicular to the central axis that is greater than every other horizontal width of the outer profile, and wherein at least part of the first section is above the bottom of the recess,
wherein the axial length of the first section is shorter than an axial distance between the first end of the receiving part and the first section, and
wherein the maximum horizontal width of the first section is greater than every other horizontal width of the entire portion of the receiving part between the first end of the receiving part and the first section.

2. The bone anchoring device of claim 1, wherein the two legs are provided with stiffening structures to prevent splaying caused by torque acting on the two legs, and wherein the stiffening structures are formed at an outer wall of the two legs.

3. The bone anchoring device of claim 2, wherein the stiffening structures comprise opposite flat surfaces, each of the surfaces formed in a respective outer wall of a leg of the two legs, the surfaces being parallel to a longitudinal axis of the channel.

4. The bone anchoring device of claim 3, wherein the flat surfaces extend from above a bottom of the recess to below the bottom of the recess.

5. The bone anchoring device of claim 4, wherein the flat surfaces extend substantially up to the first end.

6. The bone anchoring device of claim 3, wherein opposite flat surfaces are provided on sides of the receiving part perpendicular to the longitudinal axis of the channel.

7. The bone anchoring device of claim 1, wherein opposite flat surfaces are provided on sides of the receiving part perpendicular to a longitudinal axis of the channel.

8. The bone anchoring device of claim 1, wherein at least one envelope of the receiving part is substantially square shaped.

9. The bone anchoring device of claim 1, wherein the two legs are provided with stiffening structures to prevent splaying caused by torque acting on the two legs, and wherein the stiffening structures are rib-like sections of enhanced wall thickness.

10. The bone anchoring device of claim 9, wherein the rib-like sections extend substantially in a direction of the first end to the second end.

11. The bone anchoring device of claim 10, wherein the stiffening structures are provided at a location so as to enclose the rod from both sides.

12. The bone anchoring device of claim 9, wherein the stiffening structures are provided at a location so as to enclose the rod from both sides.

13. The bone anchoring device of claim 1, wherein lateral edges of the legs comprise a section of enhanced material thickness.

14. The bone anchoring device of claim 1, the receiving part further having a bore extending from the first end to the second end for passing through the shank of the bone anchoring element.

15. The bone anchoring device of claim 1, wherein the two legs are provided with stiffening structures to prevent splaying caused by torque acting on the two legs, and wherein the stiffening structures are formed by the wall thickness of outer edges of the legs being larger than a wall thickness of the legs at a location between the outer edges of the legs, the outer edges spaced from each other in a direction of a longitudinal axis of the channel.

16. The bone anchoring device of claim 1, wherein the two legs are provided with stiffening structures to prevent splaying caused by torque acting on the two legs, and wherein the stiffening structures comprise additional material to stiffen a base material of the legs.

17. The bone anchoring device of claim 16, wherein the additional material is located only in a portion of the legs.

18. The bone anchoring device of claim 1, wherein when at least a portion of the securing element is in the receiving part and an axial force is exerted on the securing element in a direction away from the second end of the receiving part, the securing element does not apply a force on the legs to urge the legs closer to the central axis.

19. A bone anchoring device comprising a bone anchoring element having a head and a shank, a rod, a receiving part for connecting the bone anchoring element to the rod, and a securing element, the receiving part comprising:
a first end, a second end, a central axis extending between the first end and the second end, and a recess having a substantially U-shaped cross section extending transversely to the central axis through the entire receiving part to define a channel for receiving the rod, wherein the recess is open at the first end of the receiving part and extends from the first end in a direction of the second end, and wherein the recess forms two legs that come together to form a bottom of the recess,
wherein the securing element is rotatable relative to the receiving part to engage respective engagement portions on the two legs for securing the rod and the head in the receiving part,
wherein the receiving part is polyaxially connectable to the bone anchoring element,
wherein along a first plane that is perpendicular to the central axis, that is spaced apart axially from the engagement portions, and that is positioned between the engagement portions and the bottom of the recess, each of the legs has a first thickness in a first radial direction from the central axis and a second thickness that is less than the first thickness in a second radial direction from the central axis, and
wherein along a second plane that is parallel to the first plane, that is spaced apart axially from the engagement portions, and that is positioned between the engagement portions and the first plane, respective thicknesses of each of the legs in every radial direction are less than the first thickness, while each of the legs maintains the second thickness in the second radial direction.

20. The bone anchoring device of claim 19, wherein the two legs are provided with stiffening structures to prevent splaying caused by torque acting on the two legs, and wherein the stiffening structures comprise opposite flat surfaces, each of the surfaces formed in a respective outer wall of a leg of the two legs, the surfaces being parallel to a longitudinal axis of the channel.

21. The bone anchoring device of claim 20, wherein the flat surfaces extend from above a bottom of the recess to below the bottom of the recess.

22. The bone anchoring device of claim 20, wherein opposite flat surfaces are provided on sides of the receiving part perpendicular to the longitudinal axis of the channel.

23. The bone anchoring device of claim 19, wherein opposite flat surfaces are provided on sides of the receiving part perpendicular to a longitudinal axis of the channel.

24. A bone anchoring kit, comprising a bone anchoring element having a head and a shank, a rod, a receiving part for connecting the bone anchoring element to the rod, a securing element, and a tool having a holding portion with a shape configured to receive the receiving part, the receiving part comprising:
a first end, a second end, and a central axis extending between the first end and the second end,
a recess having a substantially U-shaped cross section extending transversely to the central axis through the entire receiving part, wherein the recess is open at the first end of the receiving part and extends from the first end in a direction of the second end, wherein the recess forms two legs that come together to form a bottom of the recess, and wherein the recess defines a channel to receive the rod, and
an outer surface that does not have a spherical profile extending continuously from the first end of the receiving part to the second end of the receiving part,
wherein the two legs are configured to receive the securing element to secure the rod, and wherein the securing element is rotatable to engage the two legs,
wherein the receiving part is polyaxially connectable to the bone anchoring element,
wherein for at least one vertical cross-sectional plane that includes the central axis and that extends through the legs, the resulting cross-section of the receiving part has an outer profile with a first section, wherein an entire axial length of the first section has a maximum horizontal width measured in a direction perpendicular to the central axis that is greater than every other horizontal width of the outer profile, and wherein at least part of the first section is above the bottom of the recess, and
wherein the axial length of the first section is shorter than an axial distance between the first end of the receiving part and the first section, and wherein the maximum horizontal width of the first section is greater than every other horizontal width of the entire portion of the receiving part between the first end of the receiving part and the first section.

25. A bone anchoring device comprising a bone anchoring element having a head and a shank, a rod, a receiving part for connecting the bone anchoring element to the rod, and a securing element, the receiving part comprising:
a first end, a second end, and a central axis extending between the first end and the second end,
a recess having a substantially U-shaped cross section, wherein the recess is open at the first end of the receiving part and extends from the first end in a direction of the second end, wherein the recess forms two legs that come together to form a bottom of the recess, and wherein the recess defines a channel to receive the rod, and
wherein the two legs are configured to receive the securing element to secure the rod, and wherein the securing element is rotatable to engage the two legs,
wherein the receiving part is polyaxially connectable to the bone anchoring element, and
wherein for at least one vertical cross-sectional plane parallel to the central axis and that extends through the legs, the resulting cross-section of the receiving part has an outer profile with a portion that expands in width along a direction parallel to the central axis, with a rate of expansion that decreases and then increases as the cross-section extends from the second end of the receiving part towards the first end of the receiving part.

26. A bone anchoring device comprising a bone anchoring element having a head and a shank, a rod, a receiving part for connecting the bone anchoring element to the rod, and a securing element, the receiving part comprising:
a first end, a second end, and a central axis extending between the first end and the second end,
a recess having a substantially U-shaped cross section, wherein the recess is open at the first end of the receiving part and extends from the first end in a direction of the second end, wherein the recess forms two legs that come together to form a bottom of the recess, and wherein the recess defines a channel to receive the rod, the channel having a longitudinal axis, and
wherein the two legs are configured to receive the securing element to secure the rod and the securing element is rotatable to engage the two legs, and wherein when at least a portion of the securing element is in the receiving part and an axial force is exerted on the securing element in a direction away from the second end of the receiving part, the securing element does not apply a force on the legs to urge the legs closer to the central axis,
wherein the receiving part is polyaxially connectable to the bone anchoring element, and
wherein the receiving part has a horizontal first width that is a maximum outer width of the entire portion of the receiving part that is between the bottom of the recess and the second end, wherein for a vertical cross-sectional plane that includes the central axis and that is perpendicular to the longitudinal axis, a maximum outer width of the receiving part is less than the first width, while a maximum outer width of the receiving part in at least one other vertical cross-sectional plane that includes the central axis is greater than the first width.

27. A bone anchoring device comprising a bone anchoring element having a shank, a receiving part for connecting the shank of the bone anchoring element to a rod, and a securing element having an outer thread, the receiving part comprising:
a first end and a second end and a central axis extending from the first end in a direction of the second end,
a recess having a substantially U-shaped cross section, the recess extending from the first end in the direction of the second end and forming two legs open at the first end and coming together at a bottom of the recess, the recess defining a channel to receive the rod, the channel having a longitudinal axis, and
an outer surface that does not have a spherical profile extending continuously from the first end of the receiving part to the second end of the receiving part,
the two legs having an inner thread to receive the securing element, the outer thread configured to cooperate with the inner thread to secure the rod,
wherein the receiving part is polyaxially connectable to the bone anchoring element and the two legs are provided with stiffening structures to reduce splaying caused by a torque acting on the two legs at the time of final tightening of the rod by the securing element, such that along at least one vertical plane perpendicular to the longitudinal axis of the channel, a cross-section of the receiving part includes a first portion above the bottom of the recess and below the inner thread having a greatest outer width of the cross-section in a direction perpendicular to the central axis that is greater than respective outer widths of every portion of the cross-section below the bottom of the recess.

28. The bone anchoring device of claim 27, wherein the entire first portion having the greatest outer width is spaced apart axially from the inner thread.

29. The bone anchoring device of claim 27, wherein the width of the first portion is greater than respective outer widths of every portion of the cross-section above a bottom of the inner thread.

30. The bone anchoring device of claim 27, wherein the outer width of the cross-section of the receiving part becomes smaller from the first portion in a direction towards the first end of the receiving part.

31. A bone anchoring device comprising a bone anchoring element having a shank, a receiving part for connecting the shank of the bone anchoring element to a rod, and a securing element having an outer thread, the receiving part comprising:
a first end and a second end and a central axis extending from the first end in a direction of the second end,
a recess having a substantially U-shaped cross section, the recess extending from the first end in the direction of the second end and forming two legs open at the first end and coming together at a bottom of the recess, the recess defining a channel to receive the rod, the channel having a longitudinal axis, and
an outer surface that does not have a spherical profile extending continuously from the first end of the receiving part to the second end of the receiving part,
the two legs having an inner thread to receive the securing element, the outer thread configured to cooperate with the inner thread to secure the rod,
wherein the receiving part is polyaxially connectable to the bone anchoring element and the two legs are provided with stiffening structures to reduce splaying caused by a torque acting on the two legs at the time of final tightening of the rod by the securing element, such that along at least one vertical plane perpendicular to the longitudinal axis of the channel, a cross-section of the receiving part includes a portion above the bottom of the recess and below the inner thread where each of the legs has a greatest wall thickness in a direction perpendicular to the central axis that is greater than respective wall thicknesses of every portion of the cross-section below the bottom of the recess and of every portion of the cross-section above a bottom of the inner thread.

32. The bone anchoring device of claim 31, wherein the bone anchoring element further comprises a spherical head that is insertable into the receiving part from the second end of the receiving part.

33. The bone anchoring device of claim 32, wherein the head and the shank of the bone anchoring element are monolithic.

34. The bone anchoring device of claim 31, further comprising a pressure element that is movable axially in the receiving part when the bone anchoring element and the receiving part are connected to one another, wherein the pressure element is configured to exert pressure on the bone anchoring element to lock a position of the bone anchoring element relative to the receiving part.