Bone anchoring device

1. A bone anchoring device comprising:

an anchoring element comprising a shank to be anchored in a bone or a vertebra and a head, the shank having a conically shaped first end portion having a free end and a second end opposite the free end, the diameter of the conically shaped first end portion increasing in a direction from the second end to the first end portion, the head having an outwardly facing surface with a spherical segment-shaped portion and a hollow inner portion to receive the first end portion of the shank, wherein said shank and said head are separate parts;

a receiving portion comprising a first end, a second end opposite to the first end, a longitudinal axis passing through the two ends, a bore coaxial with the longitudinal axis, and a first region adjacent to the second end for receiving the spherical segment-shaped section of said head; and

a pressure element which contacts the outwardly facing surface of said head to exert pressure on said head and contacts the free end of the first end portion of the shank to lock said head in the receiving portion, the pressure element being of cylindrical construction and having a spherical recess on a side facing the head, which cooperates with the outwardly facing surface of the head when pressure is exerted;

wherein the first end portion of the shank is moveable from a first position wherein the first end portion of the shank is elastically clamped by the head in the hollow inner portion of the head and the head is moveable in the first region of the receiving portion to a second position wherein the first end portion of the shank is closer to the second end of the receiving portion and the shank is locked to the head and the head is locked in the receiving portion.

2. The bone anchoring device of claim 1 , wherein the hollow inner portion has a mating shape corresponding to the conically shaped end portion.

3. The bone anchoring device of claim 1 , wherein said conical shape is a truncated cone.

4. The bone anchoring device of claim 1 , wherein a cone angle of the conically shaped end portion is selected such that a self-locking occurs between the head and the end portion.

5. The bone anchoring device of claim 4 , wherein said angle is between about 2 and 10 degrees.

6. The bone anchoring device according to claim 1 , wherein the head comprises a spring-like edge facing towards the end portion of the shank.

7. The bone anchoring device of claim 1 , wherein the head comprises one or more slits which are directed substantially parallel to the rotational axis of the head and are open on a side facing the end portion of the shank.

8. The bone anchoring device of claim 1 , wherein the hollow inner portion of the head which receives the first end portion of the shank forms an abutment for the free end of the first end portion.

9. The bone anchoring device of claim 1 , wherein the hollow inner portion is shaped as a hollow truncated cone and extends from one end of the head to an opposite end.

10. The bone anchoring device of claim 1 , wherein said receiving portion has a U-shaped recess extending from the first end in the direction of the second end which forms a channel to receive a rod.

11. A bone anchoring assembly comprising:

a bone anchoring device comprising:

an anchoring element comprising a shank to be anchored in a bone or a vertebra and a head, the shank having a conically shaped first end portion having a free end and a second end opposite the free end, the diameter of the conically shaped first end portion increasing in a direction from the second end to the first end portion, the head having an outwardly facing surface with a spherical segment-shaped portion and a hollow inner portion to receive the first end portion of the shank, wherein said shank and said head are separate parts;

a receiving portion comprising a first end, a second end opposite to the first end, a longitudinal axis passing through the two ends, a bore coaxial with the longitudinal axis, and a first region adjacent to the second end for receiving the spherical segment-shaped section of said head; and

a pressure element which contacts the outwardly facing surface of said head to exert pressure on said head and contacts the free end of the first end portion of the shank to lock said head in the receiving portion, the pressure element being of cylindrical construction and having a spherical recess on a side facing the head, which cooperates with the outwardly facing surface of the head when pressure is exerted;

wherein the first end portion of the shank is moveable from a first position wherein the first end portion of the shank is elastically clamped by the head in the hollow inner portion of the head and the head is moveable in the first region of the receiving portion to a second position wherein the first end portion of the shank is closer to the second end of the receiving portion and the shank is locked to the head and the head is locked in the receiving portion;

a tool comprising:

a longitudinal member;

a first arm extending from the longitudinal member, the first arm having a fixed position on the longitudinal member; and

a second arm extending from the longitudinal member, the second arm being positionable relative to the first arm in a longitudinal direction of the longitudinal member.

12. The tool of claim 11 , wherein said second arm is slidingly mounted on said longitudinal member.

13. The tool of claim 11 , wherein said second arm or said first arm is fork-shaped.