Polyaxial bone anchoring device

1. A polyaxial bone anchoring device comprising:

an anchoring element having a shaft for anchoring in a bone, a head having a curved outer surface portion, and a longitudinal axis extending through the shaft and the head;

a receiving part having a top end and a bottom end, a channel for receiving a rod therein, a coaxial bore extending from the top end in a direction of the bottom end, and a seat portion for receiving the head at the bottom end; and

a pressure element configured to be arranged within the bore, the pressure element having a head contacting surface portion configured to contact at least a portion of the curved outer surface portion of the head having a greatest width or diameter in a direction perpendicular to the longitudinal axis of the anchoring element;

wherein when the head and the pressure element are in the receiving part, the head is pivotable with respect to the receiving part and can be locked at an angle by exerting a downward pressure via the pressure element onto the head to press the head against the seat portion of the receiving part;

wherein the greatest width or diameter of the head is greater than a greatest width or diameter of an inner space defined by the head contacting surface portion of the pressure element when the head contacting surface portion is unbiased, such that when the pressure element is in the receiving part and the head is fully inserted in the pressure element, the head biases the head contacting surface portion of the pressure element in a radially outward direction resulting in an interference fit wherein the head is not locked but is prevented from freely pivoting by friction; and

wherein the head and the pressure element are movable relative to each other to a plurality of different angular positions, and the interference fit results at each of the plurality of different angular positions before locking the head in the receiving part.

2. The polyaxial bone anchoring device of claim 1 , wherein the seat portion has a recess to accommodate the head contacting surface portion of the pressure element.

3. The polyaxial bone anchoring device of claim 1 , wherein the head contacting surface portion of the pressure element comprises at least two distinct portions spaced apart from one another for clamping the head from opposite sides.

4. The polyaxial bone anchoring device of claim 3 , wherein the distinct portions are formed as lugs or projections.

5. The polyaxial bone anchoring device of claim 1 , wherein the pressure element is substantially cylindrical and has an upper end, a lower end, and a recess at the lower end, wherein the head contacting surface portion is at the lower end.

6. The polyaxial bone anchoring device of claim 1 , wherein the receiving part is configured to hold the head at the seat portion and to prevent movement of the head towards the top end of the receiving part.

7. The polyaxial bone anchoring device of claim 1 , wherein the seat portion comprises an upper edge defining an opening having a greatest width or diameter that is less than the greatest width or diameter of the head, and wherein when the head is at the seat portion, the upper edge is positioned closer to the top end of the receiving part than the portion of the head having the greatest width or diameter to prevent movement of the head towards the top end.

8. The polyaxial bone anchoring device of claim 1 , wherein the pressure element comprises a spring element configured to snap into a recess at an inner wall of the receiving part, such that when the head and the pressure element are in the receiving part and the spring element is snapped into the recess, movement of the head towards the top end of the receiving part is prevented.

9. The polyaxial bone anchoring device of claim 1 , wherein the head contacting surface portion comprises a flap that is oversized compared to an inner wall of the receiving part and is configured to achieve an interference fit with the inner wall of the receiving part to prevent movement of the head towards the top end of the receiving part when the head and the pressure element are in the receiving part.

10. The polyaxial bone anchoring device of claim 1 , wherein the pressure element has a channel for receiving a rod.

11. The polyaxial bone anchoring device of claim 1 , wherein the outer surface portion of the head is spherical.

12. The polyaxial bone anchoring device of claim 1 , wherein the head contacting surface portion is spherical.

13. The polyaxial bone anchoring device of claim 1 , wherein the seat portion is spherical.

14. The polyaxial bone anchoring device of claim 1 , further comprising a fixation device configured to cooperate with the receiving part to lock the head in the seat portion and to fix a rod in the channel.

15. A method of coupling a rod to a bone via a polyaxial bone anchoring device, the bone anchoring device comprising an anchoring element having a shaft for anchoring in a bone, a head having a curved outer surface portion, and a longitudinal axis extending through the shaft and the head, a receiving part having a top end and a bottom end, a channel for receiving a rod, a coaxial bore extending from the top end in a direction of the bottom end, and a seat portion for receiving the head at the bottom end, a pressure element having a head contacting surface portion configured to contact at least a portion of the curved outer surface portion of the head having a greatest width or diameter in a direction perpendicular to the longitudinal axis of the anchoring element, wherein the greatest width or diameter of the head is greater than a greatest width or diameter of an inner space defined by the head contacting surface portion of the pressure element when the head contacting surface portion is unbiased, the method comprising:

inserting the bone anchoring device into a bone when the head is at the seat portion in the receiving part and is fully inserted in the pressure element, such that the head biases the head contacting surface portion of the pressure element in a radially outward direction resulting in an interference fit wherein the head is not locked but is prevented from freely pivoting by friction;

applying a force onto the receiving part or the anchoring element that is greater than a force of the friction to adjust an angular position between the anchoring element and the receiving part to one of a plurality of different angular positions, and the interference fit results at each of the different angular positions;

inserting a rod into the channel; and

advancing a fixation device into the channel to push the rod against the pressure element, the pressure element against the head, and the head against the seat portion of the receiving part to lock the angular position between the bone anchoring element and the receiving part.

16. The method of claim 15 , wherein prior to inserting the bone anchoring device into the bone, the method further comprises:

engaging the head and the pressure element such that the head contacting surface portion of the pressure element contacts the curved outer surface portion of the head; and

inserting the head and the pressure element into the receiving part such that the head is at the seat portion.

17. The method of claim 16 , wherein the head and the pressure element are engaged prior to inserting the head and the pressure element into the receiving part.

18. The method of claim 16 , wherein the head and the pressure element are engaged after inserting the head and the pressure element into the receiving part.

19. The method of claim 16 , wherein when the head and the pressure element are engaged and the head is at the seat portion, the head and the pressure element are prevented from moving towards the top end of the receiving part.