Receiving part for receiving a rod for coupling the rod to a bone anchoring element and a bone anchoring device

A bone anchoring device comprising:

• a bone anchoring element having a shaft for anchoring in a bone and a head;
• and
• a receiving part for receiving a rod for coupling the rod to the bone anchoring element, the receiving part comprising:
• a receiving part body with a first end, a second end, and a central axis extending from the first end to the second end, a rod receiving portion at the first end and having a channel for receiving a rod, and a head receiving portion at the second end for accommodating the head, the head receiving portion having an open end for introducing the head;
• and
• a locking ring configured to be positioned around the head receiving portion,
• wherein the head receiving portion comprises a plurality of flexible wall portions, the flexible wall portions and the locking ring being configured to engage each other at circumferentially distinct pressure areas having positions corresponding to circumferentially separated projections on at least one of the flexible wall portions or the locking ring, and
• wherein in a plane perpendicular to the central axis and including the pressure areas, a surface of the at least one of the flexible wall portions or the locking ring on which the projections are located is substantially circular with a center of curvature located on the central axis, and wherein at least one of the projections is formed by a curved portion having a center of curvature spaced apart from the central axis.

A method of coupling a rod to a bone or vertebra via a bone anchoring device, the bone anchoring device comprising an anchoring element having a shaft for anchoring in a bone or vertebra and a head, and a receiving part comprising a receiving part body with a first end, a second end, and a central axis extending from the first end to the second end, a rod receiving portion at the first end and having a channel for receiving a rod, and a head receiving portion at the second end for accommodating the head, the head receiving portion having a plurality of flexible wall portions and an open end for introducing the head, and a locking ring configured to be positioned around the head receiving portion, the method comprising:

• inserting the bone anchoring device into a bone or vertebra;
• adjusting an angular position of the receiving part relative to the bone anchoring element;
• inserting a rod into the channel;
• and
• advancing a closure element into the channel to push the rod against the locking ring and to move the locking ring towards the second end of the receiving part body to a locking position to lock the angular position of the receiving part relative to the bone anchoring element, wherein in the locking position, the locking ring engages the head receiving portion at circumferentially distinct pressure areas having positions corresponding to circumferentially separated projections on at least one of the flexible wall portions or the locking ring, and wherein in a plane perpendicular to the central axis and including the pressure areas, a surface of the at least one of the flexible wall portions or the locking ring on which the projections are located is substantially circular with a center of curvature located on the central axis, and wherein at least one of the projections is formed by a curved portion having a center of curvature spaced apart from the central axis.