Bone anchoring device

1. A bone anchoring device comprising:

an anchoring element comprising a shaft for anchoring in the bone, and a head;

a receiving part for receiving the head and for receiving a rod to be connected to the anchoring element, the receiving part being formed in one piece and having a top end and a bottom end, a bore extending from the top end to the bottom end, an inner wall defined by the bore, a substantially U-shaped channel adjacent to the top end for receiving the rod, and a seat adjacent to the bottom end for accommodating the head, wherein the anchoring element is to be inserted from the top end;

a pressure element that is able to exert pressure onto the head,

wherein the anchoring element is pivotable with respect to the receiving part and can be fixed at an angle by exerting pressure via the pressure element onto the head; and

a spring element having a spring force;

wherein a bottom portion of the spring element facing the bottom end projects farther into the bore than a top portion of the spring element facing the top end;

wherein the spring element acts between the pressure element and the receiving part and allows the pressure element to be introduced from the top end but withstands against removal of the pressure element through the top end; and

wherein the spring element, the receiving part and the pressure element are separate parts.

2. The bone anchoring device of claim 1 , wherein the spring element is asymmetric in the direction of insertion.

3. The bone anchoring device of claim 1 , wherein the spring element is designed such that the pressure element can be moved into a first holding position and a second holding position,

wherein in the first holding position, the pressure element exerts a biasing force against the head sufficient to hold the anchoring element stationary with respect to the receiving part, yet still allow pivoting of the anchoring element when an external force is applied, and

wherein in the second holding position, the pressure element locks the anchoring element with respect to the receiving part.

4. The bone anchoring device of claim 3 , wherein the pressure element snaps into the first holding position against the spring force of the spring element.

5. The bone anchoring device of claim 1 , wherein the spring element is an elastic ring having an inner surface that is inserted in a groove of the pressure element and an outer surface that is inserted in a groove in the receiving part.

6. The bone anchoring element of claim 5 , wherein the elastic ring has one slit.

7. The bone anchoring bone anchoring device of claim 5 , wherein an inner diameter of the elastic ring is larger on a side facing the top end and smaller on a side facing the bottom end.

8. The bone anchoring device of claim 5 wherein at least a portion of the inner surface of the elastic ring is inclined between the top end and the bottom end.

9. The bone anchoring device of claim 5 , wherein the elastic ring has a circular shape.

10. The bone anchoring device of claim 5 , wherein the elastic ring has a wavy shape such as a star-like shape.

11. The bone anchoring device of claim 1 , wherein the spring element provides a stop surface to prevent removal of the pressure element through the top end.

12. The bone anchoring device of claim 1 , wherein the spring element has an inclined surface that allows the pressure element to gradually expand the spring element during insertion.

13. The bone anchoring device of claim 1 , wherein the pressure element is located in a preassembled condition at a position in which it exerts a slight biasing force against the head.

14. A modular system comprising the bone anchoring device according to claim 1 , wherein a plurality of different shanks and/or a plurality of different heads are provided.