# Patent

## Title
Polyaxial bone anchoring device

## Abstract

A polyaxial bone anchoring device includes a receiving part having a first end, an opposite second end, a channel at the first end, and an accommodation space at the second end for receiving a head of a bone anchoring element, and a pressure element configured to be positioned in the receiving part and to exert pressure on an inserted head, the pressure element including at least one flexible section. At least one of the receiving part or a free end of the flexible section of the pressure element has an inclined surface portion configured to engage the other one of the receiving part or the free end of the flexible section of the pressure element, such that a force directed towards the second end of the receiving part is exerted on the pressure element at the engagement to exert a pre-load on an inserted head of a bone anchoring element.

## Background
CROSS-REFERENCE TO RELATED APPLICATION(S)
The present application claims priority to and the benefits of U.S. Provisional Patent Application Ser. No. 61/673,110, filed Jul. 18, 2012 and U.S. Provisional Patent Application Ser. No. 61/789,431, filed Mar. 15, 2013, the contents of which are hereby incorporated by reference in their entirety, and claims priority from European Patent Application EP 12 176 959.0, filed Jul. 18, 2012, the contents of which are hereby incorporated by reference in their entirety.


BACKGROUND
1. Field
The invention relates to a polyaxial bone anchoring device that includes a bone anchoring element with a head and a shank, and a receiving part for coupling the bone anchoring element to a rod. In the receiving part, a pressure element is provided that comprises flexible sections each having an inclined surface portion that cooperates with a corresponding inclined surface portion provided at the receiving part in a configuration in which the flexible sections are flexed to hold the head in an angular position with low friction before the head is locked.
2. Description of Related Art
US 2010/0234902 A1 describes a receiving part for receiving a rod for coupling the rod to a bone anchoring element. The receiving part includes an accommodation space for accommodating a head of a bone anchoring element and a pressure element arranged at least partially in the accommodation space. In one embodiment, the pressure element has two upstanding resilient fingers and the receiving part comprises two pins to secure the pressure element in the receiving part. The pins form an abutment for the fingers of the pressure element when the pressure element is in a position beneath the pins and the resilient fingers assume their unflexed condition. In this configuration the pressure element exerts a pre-stress or pre-load onto the head that provides a friction fit of the head in the receiving part.
SUMMARY
It is the object of the invention to provide an improved polyaxial bone anchoring device with a friction fit of the head in the receiving part by exerting a pre-load onto the head with a pressure element.
The polyaxial bone anchoring device comprises a pressure element that is designed to achieve a friction fit of the head in the receiving part with a low frictional force. This improves the handling of the device during surgery because the receiving part can be pivoted relative to the bone anchoring element by applying a force, for example manually, to the receiving part to align it with the rod to be inserted.
Because the inclined surface portion of the pressure element and the corresponding inclined surface portion of the receiving part are in contact with each other when the pressure element moves downward, a chain of tolerances between the receiving part, the head and the pressure element can be balanced in such a way that a defined pre-load is exerted onto the head. Furthermore, the pre-load exerted via the pressure element onto the head is reproducible, i.e. does not depend on exact dimensions and exact relative positions of the pressure element and the receiving part.
The polyaxial bone anchoring device can be designed as a polyaxial bone anchoring device with an enlarged pivot angle of the anchoring element relative to the receiving part. In this case, the head of the bone anchoring element is seated in a sleeve-like insert piece that is rotatably and pivotably accommodated in the receiving part. With such a configuration, the chain of tolerances includes also the sleeve-like insert piece.
To achieve the friction fit of the head with a low frictional force, additional elements compared to known polyaxial bone anchoring devices are not necessary.



BRIEF DESCRIPTION OF DRAWINGS
Further features and advantages of the invention will become apparent from the description of embodiments using the accompanying drawings. In the drawings:
FIG. 1 shows a perspective exploded view of a polyaxial bone anchoring device with a spinal rod according to a first embodiment;
FIG. 2 shows a perspective view of the bone anchoring device of FIG. 1 in an assembled state;
FIG. 3 shows a perspective view from the bottom of the receiving part of the polyaxial bone anchoring device according to the first embodiment;
FIG. 4 shows a top view of the receiving part of FIG. 3;
FIG. 5 shows a cross-sectional view of the receiving part along line A-A in FIG. 4;
FIG. 6 shows an enlarged cross-sectional view of a portion of a sleeve-like insert piece of the polyaxial bone anchoring device according to the first embodiment;
FIG. 7 shows a perspective view from the top of a pressure element of the polyaxial bone anchoring device according to the first embodiment;
FIG. 8 shows a perspective view from the bottom of the pressure element of FIG. 7;
FIG. 9 shows a cross-sectional view of the pressure element, wherein the section has been taken in a plane containing the central axis and extending through the middle of resilient fingers of the pressure element in a first, unflexed configuration;
FIG. 10 shows a cross-sectional view of the pressure element in a second, flexed configuration;
FIG. 11a shows a cross-sectional view of the polyaxial bone anchoring device in an assembled state without inserted rod, the section taken along line A-A in FIG. 4 with respect to the receiving part;
FIG. 11b shows an enlarged cross-sectional view of a portion of FIG. 11a; 
FIG. 12 shows a perspective exploded view of the polyaxial bone anchoring device with a spinal rod according to a second embodiment;
FIG. 13 shows a top view of the receiving part according to the second embodiment;
FIG. 14 shows a cross-sectional view of the receiving part along line B-B in FIG. 13;
FIG. 15 shows a perspective view of the pressure element of the polyaxial bone anchoring device according to the second embodiment;
FIG. 16 shows a cross-sectional view of the pressure element in a first, unflexed configuration, wherein the section has been taken in a plane containing the central axis and extending through the middle of the resilient fingers of the pressure element;
FIG. 17 shows a cross-sectional view of the pressure element in a second, flexed configuration;
FIG. 18a shows a cross-sectional view of the polyaxial bone anchoring device according to the second embodiment without an inserted rod, wherein the section has been taken along line B-B in FIG. 13 with respect to the receiving part;
FIG. 18b shows an enlarged cross-sectional view of a portion of FIG. 18a; 
FIG. 19 shows a perspective exploded view of a polyaxial bone anchoring device with a spinal rod according to a third embodiment;
FIG. 20 shows a perspective view from the bottom of the receiving part of the polyaxial bone anchoring device according to the third embodiment;
FIG. 21 shows a cross-sectional view of the receiving part of FIG. 20, the cross section taken in a plane through the central axis and perpendicular to the rod axis;
FIG. 22 shows a perspective view of the pressure element of FIG. 19;
FIG. 23 shows another perspective view from the bottom of the pressure element of FIG. 19;
FIG. 24 shows a top view of the pressure element of FIG. 19;
FIG. 25 shows a cross-sectional view of the pressure element of FIGS. 19 and 22 along line C-C in FIG. 24 wherein the pressure element is in the first, unflexed configuration;
FIG. 26 shows a cross-sectional view of the pressure element along line C-C in FIG. 24, wherein the pressure element is in a second, flexed configuration;
FIGS. 27a to 27f show steps of assembling the polyaxial bone anchoring device according to the third embodiment;
FIG. 28a shows a cross-sectional view of the polyaxial bone anchoring device according to the third embodiment without an inserted rod, the section taken in a plane containing the central axis and extending through the middle of the resilient fingers of the pressure element;
FIG. 28b shows an enlarged cross-sectional view of a portion of FIG. 28a; 
FIG. 29a shows a cross-sectional view of a polyaxial bone anchoring device according to a fourth embodiment without an inserted rod, the section taken in a plane containing the central axis and extending through the middle of the resilient fingers of the pressure element; and
FIG. 29b shows an enlarged cross-sectional view of a portion of FIG. 29a. 



DETAILED DESCRIPTION
As shown in FIGS. 1 and 2, a polyaxial bone anchoring device according to a first embodiment includes a bone anchoring element 1 in the form of a bone screw having a shank 2 with a threaded portion and a head 3. The head 3 typically has a spherically-shaped outer surface portion and a recess 3a at its free end for engagement with a driver or another tool. The head 3 may be held in a receiving part 4 that couples the bone anchoring element 1 to a stabilization rod 100. In the receiving part 4, a sleeve-like insert piece 5 providing a seat for the head 3 and a pressure element 6 for exerting pressure onto the head 3 are arranged. Furthermore, a fixation element in the form of, for example, a fixation screw 7 is provided for securing and fixing the rod 100 in the receiving part 4.
Referring in particular to FIGS. 3 to 5, the receiving part 4 has a top end 4a and a bottom end 4b, a central axis C and a coaxial bore 8 extending from the top end 4a in the direction of the bottom end 4b. Adjacent to the top end 4a, a substantially U-shaped recess 9 is provided that forms a channel for receiving the rod 100. By means of the recess 9, two free legs are formed which are provided with an internal thread 10 for cooperating with the fixation screw 7.
The coaxial bore 8 opens into an accommodation space 11 provided in a lower part of the receiving part 4, e.g. nearer to the bottom end 4b. The accommodation space 11 has a lower opening 12 at the bottom end 4b of the receiving part 4. The accommodation space 11 further includes a seat portion 13 near the bottom end 4b of the receiving part in which the sleeve-like insert piece 5 may be seated. The seat portion has a spherical shape in order to provide a socket for a ball and socket joint that is formed by the sleeve-like insert piece 5 and the receiving part 4. It should be noted that the seat portion 13 can also be tapered, or can have various other shapes that can be used to realize a ball and socket joint. A smallest inner diameter of the lower opening 12 is smaller than an inner diameter of some of the other portions of the accommodation space 11. It shall also be noted that an inner diameter of the coaxial bore 8 can vary such that the coaxial bore 8 may have different portions with different diameters.
In order to allow the sleeve-like insert piece 5 to be introduced from the top end 4a, the receiving part 4 has in the inner wall of the coaxial bore 8 and the accommodation space 11 two recesses 14a, 14b. The recesses 14a, 14b are aligned with the U-shaped recess 9 and extend from a bottom of the U-shaped recess 9 into the accommodation space 11. The size of the recesses 14a, 14b is such that the sleeve-like insert piece 5 can be introduced from the top end 4a in a 90° tilted position, i.e. the width of the recesses 14a, 14b are greater than the height of the sleeve-like insert piece 5 in its axial direction. The recesses 14a, 14b extend into the accommodation space 11 to such an extent that tilting of the insert piece 5 into the seat 13 is possible.
As can be seen in particular in FIGS. 1 and 5, the receiving part 4 comprises a conically shaped cutout 15 that extends in a circumferential direction and is provided in both legs of the receiving part 4. The cutout 15 is located at an axial position with respect to the central axis C that is between the accommodation space 11 and the internal thread 10. The orientation of the conical cutout 15 is such that the cutout 15 widens in a direction towards the bottom end 4b, thereby providing on each leg a conically inclined surface portion 15a, 15b including an angle of inclination α with the central axis C (as can be seen, for example, in FIG. 11b). The angle of inclination α may be, for example, approximately 25° or at least less than 70°. A base surface 15c of the conical cutout may be oriented perpendicular to the central axis C. A maximum depth of the cutout 15 in a radial direction may be larger than the inner diameter of the bore 8. Between the portion with the internal thread 10 and the conical cutout 15 an undercut portion 16 may be provided with an internal diameter that is larger than the inner diameter of the upper edge of the conical cutout 15.
The sleeve-like insert piece 5 is shown in particular in FIGS. 1 and 6. The sleeve-insert piece 5 has an upper edge 5a and a lower edge 5b. Between the upper edge 5a and the lower edge 5b, the sleeve-like insert piece 5 may have a spherically-shaped outer surface portion 51. A largest outer diameter of the sleeve-like insert piece is greater than the smallest inner diameter of the lower opening 12 of the receiving part 4. Hence, the sleeve-like insert piece 5 cannot escape through the lower opening 12 when it is seated in the receiving part 4. The dimension or shape of the outer spherical surface portion 51 corresponds to that of the spherically-shaped seat portion 13 in the receiving part 4 in such a way that the sleeve-like insert piece 5 can pivot and rotate in the receiving part 4 when the insert piece 5 is seated in the seat portion 13. When the sleeve-like insert piece 5 rests in the seat portion 13 in a coaxial configuration such that its center axis is coaxial with the center axis C of the receiving part 4, the lower edge 5b projects out of the lower opening 12 as can be seen in FIG. 11a. When the sleeve-like insert piece 5 is pivoted or angled in the receiving part 4, at least a portion of the lower edge 5b still projects out of the lower opening 12.
The sleeve-insert piece 5 is hollow and has a central portion 52 that is spherically-shaped with a radius corresponding to a radius of the spherically-shaped outer surface portion of the head 3 of the bone anchoring element 1. A lower end of the central portion 52 forms a shoulder 53, an inner diameter of the shoulder 53 is smaller than a largest outer diameter of the spherical head 3, so that the head 3 can rotate and pivot in the central spherical portion 52 of the sleeve-like insert piece 5, similar to a ball and socket joint. Between the shoulder 53 and the lower edge 5b, a tapered portion 54 is provided that tapers outward to allow angulation of the bone anchoring element 1 until the shank 2 comes into contact with the lower edge 5b. Between the spherical central portion 52 and the upper edge 5a, a tapered portion 55 is provided which also tapers outward. An inner diameter of tapered portion 55 and of a transition between the tapered portion 55 and the spherical central portion 52 are greater than the largest outer diameter of the head 3, so that the head 3 can be inserted from the upper edge 5a. A height of the sleeve-like insert piece 5 in an axial direction is less than a height of the head 3 in an axial direction, such that when the head 3 is inserted into the sleeve-like insert piece 5, a portion of the spherical outer surface of the head 3 still projects from the upper edge 5a of the sleeve-like insert piece 5.
Referring to FIGS. 7 and 8, the pressure element 6 comprises a main body with a first cylindrical portion 61 with a first outer diameter and a second cylindrical portion 62 with a second outer diameter that is smaller than the first outer diameter. The first cylindrical portion 61 defines an upper surface 6a and the second cylindrical portion 62 defines a lower edge 6b. A spherical recess 63 is provided adjacent to the lower portion 6b. The recess 63 is sized such that it matches the shape of the spherical portion of the head 3. Furthermore, a coaxial through hole 64 extends from the upper surface 6a into the recess 63 to allow access with a tool to engage the recess 3a in the head 3 of the bone anchoring element 1. In the upper surface 6a, a shallow substantially cylinder segment-shaped recess 65 may be provided for guiding the rod.
The pressure element further comprises flexible sections in the form of two upstanding resilient fingers 66a, 66b extending from the upper surface 6a upwards. As shown in detail in FIG. 9, the outer wall of the resilient fingers 66a, 66b is adjacent to the top surface 6a and is substantially flush with the outer surface of the first cylindrical portion 61. The inner surface of the resilient fingers 66a, 66b is slightly concave in a portion adjacent to the upper surface 6a and substantially straight in a portion adjacent to the free end of the fingers. At their free ends, the fingers 66a, 66b each comprise an outwardly extending portion 67a, 67b, respectively, including a top surface 68a, 68b that extends substantially perpendicular to the central axis C and an outer conically inclined surface portion 69a, 69b wherein the cone widens towards the lower edge 6b. The angle of inclination of the inclined surface portion 69a, 69b with the central axis C is substantially 25°. It can be smaller than the angle of inclination α of the inclined surface portions 15a, 15b of the receiving part 4 or can be substantially the same. The outwardly extending portion 67a, 67b further comprises a downwardly facing surface 70a, 70b with an inclination opposite to the inclination of the inclined surface portion 69a, 69b that facilitates the insertion of the pressure element 6 into the receiving part 4. The dimensions of the pressure element 6 are such that the first outer diameter of the first cylindrical portion 61 is only slightly smaller than the inner diameter of the coaxial bore 8 so that the pressure element 6 can slide along the inner wall of the coaxial bore 8.
As can be seen in the cross-sectional view in FIG. 9, the thickness of the resilient fingers 66a, 66b in a radial direction is smallest at a position at substantially the middle between the top surface 68a, 68b of the fingers and the top surface 6a of the first cylindrical portion 61. The resilient fingers 66a, 66b are flexible towards the central axis C in such a manner that they can be slightly bent towards each other as shown in FIG. 10.
When the head 3 with the sleeve-like insert piece 5 is inserted into the receiving part 4 and seated in the seat portion 13 and the pressure element 6 is inserted, the height of the resilient fingers 66a, 66b is such that the inclined surface portion 69a, 69b engages an upper portion of the inclined surface portion 15a, 15b of the conical cutout 15, respectively, as shown in FIG. 11a. 
The bone anchoring device, as a whole or in parts, is made of a bio-compatible material, such as a bio-compatible metal, for example titanium or stainless steel, a bio-compatible alloy such as Nitinol, or of bio-compatible plastic materials, such as for example, polyetheretherketone (PEEK). The parts may be of the same material or of different materials.
The polyaxial bone anchoring device may be pre-assembled. In the following, the steps of assembling the anchoring device are described. First, the sleeve-like insert piece 5 may be introduced from the top end 4a into the receiving part 4 in such a manner that the sleeve-like insert piece 5 is tilted by 90 and then introduced into the receiving part 4 so that it extends into recesses 14a, 14b and thereafter is tilted to assume its position in the seat portion 13. Then the bone anchoring element 1 may be inserted so that the head 3 is seated in the spherical central portion 52 of the sleeve-like insert piece 5.
Thereafter, the pressure element 6 is inserted from the top end 4a with an orientation such that the resilient fingers 66a, 66b are arranged in the U-shaped recess 9. Then, the pressure element 6 is rotated so that its shallow recess 65 is aligned with the U-shaped recess 9 of the receiving part 4. The resilient fingers 66a, 66b are moved into the conical cutout 15. When the inclined portions 69a, 69b of the resilient fingers 66a, 66b engage the inclined surface portion 15a, 15b of the cutout 15, the fingers are slightly bent inward. Because in this configuration, the resilient fingers 66a, 66b tend to move apart from each other, a counterforce is generated when the inclined surface portion 69a, 69b contacts the inclined surface portion 15a, 15b of the cutout 15, respectively, as shown in FIG. 11a. By the elastic deformation of the resilient fingers 66a, 66b, a downward axial force F is generated that is transferred via the pressure element 6 onto the head 3 as depicted in FIGS. 11a and 11b. Hence, a pre-load is applied to the head 3 that holds the head 3 with a low friction force.
Because the surface area of the inclined surface portion 15a, 15b of the conical cutout 15 is greater than the surface area of the inclined surface portion 69a, 69b, respectively, the inclined surface portion 69a, 69b can move along the inclined surface portion 15a, 15b when the pressure element 6 is moved downward. By means of this, the dimensional tolerances between the head 3, the sleeve-like insert piece 5 and the pressure element 6 can be balanced by appropriate positioning of the pressure element 6. Therefore, a reproducible, low friction force acting onto the head 3 is generated.
The pressure element 6 may additionally be secured against escaping through the top end 4a. For example, the securing can be achieved by crimping (not shown). This may prevent the pressure element 6 from being pushed out through the top end 4a because the elastic deformation of the fingers 66a, 66b and the generated low friction force may not be sufficient to hold the pressure element 6 in the receiving part 4 in the pre-assembled state.
In use, the pre-assembled polyaxial bone anchoring device is inserted into a bone or a vertebra. Typically, at least two bone anchoring devices are used and connected with the rod 100. The receiving parts 4 are aligned by rotating and/or pivoting them with respect to the bone anchoring elements 1. The pre-load exerted onto the head 3 can be overcome by manually rotating and/or pivoting the receiving part 4. Because of the low friction between the head 3 and the pressure element 6, the anchoring device is convenient to handle.
Because the sleeve-like insert piece 5 is rotatable and pivotable within the receiving part 4, the polyaxial bone anchoring device has an enlarged range of angulation that can be achieved at any position within 360° of the receiving part 4 with respect to the bone anchoring element 1. When the receiving part 4 is pivoted with respect to the bone anchoring element 1 and the shank 2 of the bone anchoring element 1 comes into contact with the lower edge 5b of the sleeve-like insert piece 5, the sleeve-like insert piece 5 is also pivoted with the bone anchoring element 1. The shank 2 may push the insert piece 5 until the shank 2 abuts against the edge of the lower opening 12 of the receiving part 4. The maximum pivot angle that can be achieved depends on the dimensions of the sleeve-like insert piece 5, the receiving part 4 and the bone anchoring element 1, but is typically equal to or greater than 45° measured from a straight or zero angle position between the receiving part 4 and the bone anchoring element 1.
After alignment of the receiving parts 4, the rod 100 is inserted into the channel of each bone anchoring device and then the fixation screw 7 is inserted and tightened. Thereby, the pressure element 6 is moved downward slightly and presses the head 3 and the sleeve-like insert piece 5 into the seat portion 13. The inner screw 7 is inserted and tightened and the pressure element 6 and the rod 100 are locked.
A polyaxial bone anchoring device according to a second embodiment will be described with reference to FIGS. 12 to 18b. Parts and portions which are the same or similar to those of the first embodiment are designated with the same reference numerals, and the descriptions thereof are not repeated. The bone anchoring device of the second embodiment differs from the bone anchoring device of the first embodiment by the construction of the receiving part and the pressure element. All other parts may be identical or similar to those of the first embodiment.
As shown in FIGS. 12 to 14, the receiving part 4′ has an undercut portion 16′ but no conical cutout. At the lower end of the undercut portion 16′ two opposed transverse bores 20a, 20b extend fully through the wall of the receiving part 4′ and open into the undercut portion 16′. The bores 20a, 20b may have an enlarged section 21a, 21b with a greater diameter adjacent to the outside. Two pins 30a, 30b with a head portion 31a, 31b may be arranged in the bores 20a, 20b such that the head portions 31a, 31b rest in the enlarged portions 21a, 21b as depicted in FIG. 18a. When the pins 30a, 30b are inserted, they may form a part of the receiving part 4′. The free end of the pins 30a, 30b that extends into the inside of the receiving part 4′ has an inclined surface portion 32a, 32b, respectively. The inclined surface portions 32a, 32b may be flat, i.e. not conical when compared to the first embodiment. The arrangement of the pins 30a, 30b is such that the distance between the pins 30a, 30b along the inclined surface portions 32a, 32b increases towards the bottom end 4b when the pins 30a, 30b are inserted into the bores 20a, 20b. The angle of inclination is approximately 50° or less measured with respect to the central axis C. The pins 30a, 30b extend to such a length into the inside of the receiving part 4′ that the corresponding inclined surface portions 69a, 69b of the pressure element 6′ can press against the inclined surface portions 32a, 32b. 
The pressure element 6′ has resilient fingers 66a′, 66b′, the outer surfaces of which are not flush with the outer surface of the first cylindrical portion 61, but is offset from the outer surface of the first cylindrical portion 61. Hence, when the pressure element 6′ is inserted into the receiving part 4′, there is a space 17 between the inner wall of the coaxial bore 8 and the outside of the resilient fingers 66a′, 66b′. The inclined surface portions 69a′, 69b′ of the fingers are flat in this embodiment.
The resilient fingers 66a′, 66b′ are flexible towards the central axis C as shown in FIG. 17. When the pressure element 6′ is inserted into the receiving part 4′ that has the pins 30a, 30b mounted into the bores 20a, 20b, the inclined surface portions 69a′, 69b′ come into engagement with the inclined surface portions 32a, 32b of the pins 30a, 30b and the resilient fingers 66a′, 66b′ are slightly bent inward. In this configuration, the pressure element 6′ exerts a preload onto the head 3 that holds the head 3 in the sleeve-like insert piece 5 and in the seat portion 13 with a low friction force. When the pressure element 6′ is further pressed down onto the head 3 during tightening of the fixation screw 7, the inclined surface portions 69a′, 69b′ slide along the inclined surface portions 32a, 32b of the pins 30a, 30b, respectively. The outermost edge of the resilient fingers 66a′, 66b′ can extend into the space 17 when it is no longer in contact with the inclined surface portion 32a, 32b. 
A polyaxial bone anchoring device according to a third embodiment will be described with reference to FIGS. 19 to 28b. Parts and portions which are the same or similar to those of the first and second embodiment are designated with the same reference numerals and the descriptions thereof are not repeated. The bone anchoring device of the third embodiment differs from the bone anchoring device of the first and second embodiments by the design of the receiving part and the pressure element. All other parts may be identical or similar to those of the first and second embodiments.
As shown in FIGS. 19 to 21, the receiving part 4′′ comprises inclined surface portions 15a′′, 15b′′ at the inner side wall of the free legs formed by the U-shaped recess 9. The inclined surface portions 15a′′, 15b′′ are provided by a conical shape of the bore narrowing towards the top end 4a. Hence, the inclined surface portion 15a′′ and 15b′′ are arranged between the undercut 16 and the coaxial bore 8. They serve for cooperation with resilient fingers of the pressure element 6′′ as described below. Between the coaxial bore 8 and the accommodation space 11 there is a second coaxial cylindrical bore portion 8a with a larger diameter than the diameter of the coaxial bore 8. By means of this a stop 8b is provided that extends in a circumferential direction and serves as an abutment for a portion of the pressure element 6′′ to limit an upward movement of the pressure element 6′′. The stop 8b is formed monolithically with the receiving part 4′′. A separate crimping step to secure the pressure element 6′′ is therefore not needed.
Referring to FIGS. 22 to 26, the pressure element 6′′ comprises a main body with a cylindrical portion 62 adjacent to the lower edge 6b. Following the cylindrical portion 62, there is an upper portion 61 defining the upper surface 6a and comprising the cylinder segment-shaped recess 65 for guiding the rod 100. From the upper portion 61 two lateral projections 6c, 6d are provided that extend at 90° with respect to a longitudinal axis of the recess 65. From the lateral projections 6c, 6d two resilient fingers 66a′′, 66b′′ extend upwards from the upper surface 6a. A spherical recess 63 is provided adjacent to the lower edge 6b as in the first and second embodiments. Furthermore, a coaxial through-hole 64 extends from the upper surface 6a into recess 63 to allow the access with a tool to engage the head 3 of the bone anchoring element 1.
The resilient fingers 66a′′, 66b′′ have a decreasing thickness in a radial direction towards their free ends, respectively, to provide sufficient resiliency. They are arranged at approximately the centers of the lateral projection 6c, 6d so that a portion of the upper surface 6a surrounds the outside of the resilient fingers 66a′′, 66b′′. The upper free end 69a′′, 69b′′ of the resilient fingers 66a′′, 66b′′ is rounded in such a manner that an angle between an upper free end 66c of the resilient finger 66a′′ and the outer surface 66e of the resilient finger 66a′′ is approximately 90° and the edge 69a′′ between the upper free end 66c and the outer surface 66e is rounded. In the same way an angle between the upper free end 66d of the second resilient finger 66b′′ and the outer surface 66f of the second resilient finger 66b′′ is approximately 90° and the edge 69b′′ therebetween is rounded. As a result, the rounded edges 69a′′, 69b′′ act as inclined surface portions that cooperate with the inclined surface portions 15a′′ 15b′′ of the receiving part 4′′. The inclined surface portions of the rounded edges 69a′′, 69b′′ may be composed of infinitesimally small surface portions that have an inclination with respect to the outer surface 66e, 66f of the resilient fingers 66a′′, 66b′′. 
An outer diameter of the pressure element 6′′ in the region of the lateral projections 6c, 6d is greater than an inner diameter of the coaxial bore 8, but smaller than an inner diameter of the second cylindrical bore portion 8a so that the pressure element 6′′ fits into the receiving part 4′′ in such a manner that the lateral projections 6c, 6d can be housed in the second cylindrical bore portion 8a in the assembled state and the resilient fingers 66a′′, 66b′′ extend into the coaxial bore 8.
As shown in FIGS. 25 and 26, the resilient fingers 66a′′, 66b′′ are resilient in such a manner that they can be bent slightly inwards from a first configuration shown in FIG. 25 where the fingers are unflexed to a second configuration shown in FIG. 26 where the fingers are flexed inwards, when a radial force shown by the arrows acts on the fingers.
The steps of assembling the polyaxial bone anchoring device according to the third embodiment will be described referring to FIGS. 27a to 27f. In a first step shown in FIG. 27a the sleeve-like insert piece 5 is tilted by 90° so that its central axis 500 is perpendicular to the central axis C of the receiving part 4′′. In this orientation the insert piece 5 is inserted into the receiving part 4′′ from the top end 4a such that its central axis 500 is also perpendicular to the channel axis of the U-shaped recess 9. Because the sleeve-like insert piece 5 can be inserted into the recesses 14a, 14b of the receiving part 4′′ shown in FIGS. 19 and 21, it can be moved downward until it has reached to accommodation space 11. Then, as shown in FIG. 27b, the sleeve-like insert piece 5 is tilted so that its central axis 500 is coaxial with the central axis C of the receiving part 4′′ and the sleeve-like insert piece 5 is accommodated in the accommodation space 11.
Thereafter, as shown in FIG. 27c, the bone anchoring element 1 is introduced from the top end 4a into the receiving part 4′′ so that the threaded shank 2 extends through the lower opening of the receiving part 4′′ and finally the head 3 is received in the sleeve-like insert piece 5.
Then, the pressure element 6′′ is inserted into the receiving part from the top end 4a such that the resilient fingers 66a′′, 66b′′ extend upwards in the direction of the top end 4a and the lateral projections 6c, 6d extend into the direction of the channel axis of the U-shaped recess 9. Therefore, the pressure element 6′′ can be inserted with its lateral projection 6c, 6d engaging the recesses 14a, 14b of the receiving part 4′′ until the pressure element 6′′ reaches with its upper portion 61 the second coaxial bore portion 8a. As shown in FIG. 27e, the pressure element 6′′ is then rotated so that the lateral projection 6c, 6d are moved below the stops 8b and are accommodated in the second cylindrical bore portion 8a as depicted in FIG. 28a. Due to the stops 8b an upward movement of the pressure element 6′′ is limited when a portion of the upper surface 6a abuts the stop 8b in the receiving part 4′′.
In the assembled state as shown in FIGS. 27f, 28a and 28b, the resilient fingers 66a′′, 66b′′ extend through the coaxial bore 8 such that their upper rounded edges 69a′′, 69b′′ abut against the uppermost portion of the inclined surface portions 15a′′, 15b′′ of the receiving part 4′′, thereby being slightly flexed inwards.
In this configuration shown in detail in FIGS. 28a and 28b, the resilient fingers 66a′′, 66b′′ press onto the inclined surface portion 15a′′, 15b′′ with a force Fr shown by the arrow. They experience a counterforce from the inclined surface portions 15a′′, 15b′′ of the receiving part 4′′ and as a result an axial downward force Fd shown by the arrow acts onto the head 3 which clamps the head 3 by a low frictional force. Hence, a pre-load is applied to the head 3 that holds the head 3 in a certain angular position.
As in the previous embodiments, when the pressure element is moved downward, the rounded edges 69a′′, 69b′′ move along the inclined surface portions 15a′′, 15b′′. By means of this, the dimensional tolerances between the head 3, the sleeve-like insert piece 5 and the pressure element 6′′ can be balanced by appropriate positioning of the pressure element 6′′. Also in this embodiment, a reproducible low friction force acting onto the head 3 is generated. The rounded edges 69a′′, 69b′′ slide smoothly along the inclined surface portions 15a′′, 15b′′ of the receiving part 4′′ without being jammed.
A polyaxial bone anchoring device according to a fourth embodiment will be described with reference to FIGS. 29a and 29b. Parts and portions which are the same or similar to those of the first and second embodiment are designated with the same reference numerals and the descriptions thereof are not repeated. The bone anchoring device of the fourth embodiment comprises the pressure element 6 as in the first embodiment that comprises the outwardly inclined surfaces 69a, 69b at the upper end of the resilient fingers 66a, 66b. It shall be noted that the pressure element 6′ of the second embodiment can also be used in the fourth embodiment of the polyaxial bone anchoring device. The receiving part 4′′′ of the fourth embodiment differs from the receiving part of the first to third embodiments. All other parts may be identical or similar to those of the first to third embodiments.
The receiving part 4′′′ comprises a first undercut portion 16′′′ adjacent to the thread 10 and a second undercut portion 17 at a distance from the first undercut portion 16′′′ in the direction towards the bottom end 4b. Between the undercut portions there is a circumferentially extending projection 150a, 150b on each of the legs that projects towards the central axis C. The projection 150a, 150b has a substantially rectangular cross section so that a lower rectangular edge 151a, 151b is formed that may be rounded. An inner surface 152a, 152b of the projection extends substantially parallel to the central axis C. The size of the projections 150a, 150b is such that when the pressure element 6 is inserted and placed onto the head 3, the inclined surfaces 69a, 69b abut against the edges 151a, 151b and the resilient fingers 66a, 66b are slightly bent inwards. Through the resulting counterforce a downward axial force Fd is generated that acts onto the head 3 such that the head 3 is held by friction in a specific angular position.
Therefore, for all embodiments, through the interaction of an inclined surface or an edge provided on the pressure element or on the receiving part on the one side and an edge or another inclined surface at the corresponding position of the receiving part or the pressure element on the other side, a downward force is generated that holds the head by a low friction force. The inclined surface portions of either the pressure element or the receiving part have an upper end that is closer to the top end of the receiving part and a lower end that is closer to the bottom end of the receiving part when the pressure element is arranged in the receiving part and the inclination is such that the upper end is closer to the central axis C than the lower end
Modifications of the embodiments described may also be made. For example, more than two resilient fingers may be present. In the second embodiment, more than two pins may be present according to the number of resilient fingers. In a further modification, only one single flexible section may be provided on the pressure element. The lower portion of the pressure element may be designed differently. The sleeve-like insert piece may be omitted. In this case, the accommodation space of the receiving part may be smaller and the head may be seated directly in the seat portion of the receiving part. The receiving part may be constructed so as to allow the insertion of the bone anchoring element from the bottom end. Also, the pressure element may be constructed so as to allow the insertion into the receiving part from the bottom end. Furthermore, the inclined surface portions need not to be conically-shaped as in the first embodiment or flat as in the second embodiment. They may have any shape which allows to generate a downward force.
For the bone anchoring element, various different kinds of anchoring elements can be used and combined with the receiving part. These anchoring elements may be, for example, screws with different lengths, screws with different diameters, cannulated screws, screws with different thread forms, nails, hooks etc. For some anchoring elements, the head and the shaft may also be separate parts that are connectable to each other.
Other kinds of locking devices including outer nuts, outer caps, bayonet locking devices, or others are also possible. Also, a two part locking device may be used with an outer locking device that acts onto the pressure element only and an inner locking device that presses onto the rod. The inner surface portion of the pressure element may have any other shape that is suitable to exert pressure onto the head.
It shall also be noted that portions of the different described embodiments can also be combined with each other in various different combinations.
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 polyaxial bone anchoring device comprising:
a bone anchoring element comprising a head and a shank;
a receiving part for coupling a rod to the bone anchoring element, the receiving part having a first end, an opposite second end, a central axis extending through the first end and the second end, a channel at the first end for receiving the rod, and an accommodation space at the second end for receiving the head; and
a pressure element configured to be positioned in the receiving part and to exert pressure on the head, the pressure element comprising at least one flexible section having a free end;
wherein the receiving part or the free end of the flexible section of the pressure element has an inclined surface portion, such that when the pressure element is in the receiving part, the inclined surface portion widens away from the central axis as the inclined surface portion extends in a direction from the first end of the receiving part towards the second end of the receiving part; and
wherein when the head and the pressure element are in the receiving part, the receiving part and the free end of the flexible section of the pressure element are configured to engage one another to form a first engagement at the inclined surface portion, such that a force is exerted by the receiving part on the pressure element at the first engagement, the force being directed towards the second end of the receiving part for the pressure element to exert a pre-load on the head.

2. The polyaxial bone anchoring device of claim 1, wherein the at least one flexible section comprises at least one upstanding resilient finger.

3. The polyaxial bone anchoring device of claim 1, wherein the at least one flexible section is configured to flex towards the central axis of the receiving part when the pressure element is in the receiving part.

4. The polyaxial bone anchoring device of claim 1, wherein the inclined surface portion is at the free end of the flexible section of the pressure element.

5. The polyaxial bone anchoring device of claim 1, wherein the free end of the flexible section of the pressure element and a corresponding portion of the receiving part remain in engagement when the pressure element is moved along the central axis from a first position closer to the first end of the receiving part to a second position closer to the second end of the receiving part.

6. The polyaxial bone anchoring device of claim 1, wherein an angle of inclination of the inclined surface portion with respect to the central axis is such that by engagement of the free end of the flexible section of the pressure element and a corresponding portion of the receiving part, a force is generated onto the pressure element that is directed towards the second end of the receiving part.

7. The polyaxial bone anchoring device of claim 1, wherein an angle of inclination of the inclined surface portion is less than 70°.

8. The polyaxial bone anchoring device of claim 1, wherein the free end of the flexible section of the pressure element and the receiving part each has an inclined surface portion, and wherein an angle of inclination of the inclined surface portion of the pressure element is less than or equal to an angle of inclination of the inclined surface portion of the receiving part relative to the central axis.

9. The polyaxial bone anchoring device of claim 1, wherein two flexible sections are provided on the pressure element that are configured to enclose the rod therebetween.

10. The polyaxial bone anchoring device of claim 1, wherein the receiving part has an inner wall defining a coaxial bore for accommodating the pressure element, and wherein the inclined surface portion is formed on the inner wall and defines a conically-shaped recess in the inner wall.

11. The polyaxial bone anchoring device of claim 1, wherein the inclined surface portion is on the receiving part and is provided by pins extending through a wall of the receiving part.

12. The polyaxial bone anchoring device of claim 11, wherein a surface of the pins oriented towards the central axis is tapered.

13. The polyaxial bone anchoring device of claim 1, wherein the inclined surface portion is on the receiving part, and wherein the free end of the flexible section of the pressure element has a rounded edge configured to engage the inclined surface portion.

14. The polyaxial bone anchoring device of claim 1, wherein when the pressure element is in the receiving part, the pressure element is secured against escaping through the first end of the receiving part by a securing device.

15. The polyaxial bone anchoring device of claim 14, wherein the securing device comprises a step provided at an inner wall of the receiving part and being monolithic with the receiving part, the step extending in a circumferential direction forming a stop for the pressure element.

16. The polyaxial bone anchoring device of claim 1, further comprising a sleeve-like insert piece configured to be positioned in the accommodation space and to prevent the head of the bone anchoring element from escaping through an opening at the second end of the receiving part.

17. The polyaxial bone anchoring device of claim 1, further comprising a locking device configured to directly or indirectly exert pressure on the pressure element in the receiving part to lock the head relative to the receiving part.

18. The polyaxial bone anchoring device of claim 16, wherein the sleeve-like insert piece is configured to rotate and pivot in the accommodation space.

19. The polyaxial bone anchoring device of claim 1, wherein when the pressure element is in the receiving part, the flexible section is on a side of the pressure element closer to the first end of the receiving part.

20. The polyaxial bone anchoring device of claim 4, wherein when the inclined surface portion of the pressure element engages a corresponding portion of the receiving part, the flexible section of the pressure element is configured to be in a flexed configuration.

21. The polyaxial bone anchoring device of claim 7, wherein the angle of inclination of the inclined surface portion is approximately 25°.

22. A method for coupling a rod to a bone via a polyaxial bone anchoring device, the bone anchoring device comprising a bone anchoring element comprising a head and a shank, a receiving part for coupling a rod to the bone anchoring element, the receiving part having a first end, an opposite second end, a central axis extending through the first end and the second end, a channel at the first end for receiving a rod, and an accommodation space at the second end for receiving the head, a pressure element configured to be positioned in the receiving part and comprising at least one flexible section having a free end, wherein the receiving part or the free end of the flexible section of the pressure element has an inclined surface portion, such that when the pressure element is in the receiving part, the inclined surface portion widens away from the central axis as the inclined surface portion extends in a direction from the first end of the receiving part towards the second end of the receiving part, and a locking device, the method comprising:
inserting the bone anchoring element into a bone when the head and the pressure element are in the receiving part;
adjusting an angular position of the receiving part relative to the bone anchoring element, wherein the receiving part and the free end of the flexible section of the pressure element engage one another to form a first engagement at the inclined surface portion, such that a force is exerted by the receiving part on the pressure element at the first engagement, the force being directed towards the second end of the receiving part for the pressure element to exert a pre-load on the head to temporarily hold the angular position of the receiving part relative to the bone anchoring element;
inserting a rod into the channel of the receiving part; and
advancing the locking device in the receiving part to advance the rod and the pressure element away from the first end of the receiving part, such that the pressure element exerts pressure onto the head of the bone anchoring element, to lock the angular position of the bone anchoring element and a position of the rod relative to the receiving part.

23. The method of claim 22, further comprising:
inserting the head of the bone anchoring element into the receiving part; and
inserting the pressure element into the receiving part from the first end.

24. The method of claim 23, wherein inserting the pressure element into the receiving part from the first end comprises inserting the pressure element when the inclined surface portion and a corresponding engagement portion are rotationally offset from one another, and then rotating the pressure element to engage the inclined surface portion with the corresponding engagement portion.