Coupling assembly and polyaxial bone anchoring device comprising the same
A coupling assembly for coupling a rod to a bone anchoring element is provided, wherein the coupling assembly includes a receiving part having a first end, a second end, a central axis extending through the first end and second end, an accommodation space for accommodating a head of an anchoring element, a bore extending from the accommodation space to the first end, and a recess for receiving a rod. The accommodation space has an opening at the second end sized to permit insertion of a head of the bone anchoring element. The coupling assembly further includes a retainer element configured to be positioned at least partially in the accommodation space and being radially expandable and/or compressible to allow retaining a head inserted through the opening. The retainer element is held in position adjacent the opening by an engagement structure provided at or in the accommodation space. The coupling assembly further includes a locking element configured to be arranged at least partially in the accommodation space. The locking element is movable from a first position, in which the retainer element is allowed to expand and release an inserted head, to a second position, in which radial expansion of the retainer element is hindered to prevent release of an inserted head. When the locking element is in the second position, a head of an anchoring element can be locked by exertion of a pressure force directly onto the head to press the head against the retainer element.
1. A coupling assembly for coupling a rod to a bone anchoring element, the coupling assembly comprising:
a receiving part having a first end, a second end below the first end, a central axis extending through the first end and the second end, an accommodation space for accommodating a head of the bone anchoring element, the accommodation space having an opening at the second end sized to permit insertion of the head into the receiving part, and a recess at the first end for receiving the rod;
a retainer element positionable at least partially in the accommodation space and radially expandable to facilitate insertion of the head; and
a locking element positionable at least partially in the accommodation space, the locking element having a lower end region with an inner surface that has a substantially constant inner width as the inner surface extends axially;
wherein when the retainer element and the locking element are in the receiving part, the lower end region of the locking element is directly engageable with the retainer element, and the locking element is movable to adjust the coupling assembly from a first configuration where the retainer element is expandable for inserting the head, to a second configuration where a vertical surface that extends substantially parallel to the central axis restricts radial expansion of the retainer element to prevent release of the inserted head.
2. The coupling assembly of claim 1 , wherein the retainer element defines an open or closed ring-shape.
3. The coupling assembly of claim 1 , wherein the retainer element includes one or more slits configured to allow the retainer element to expand.
4. The coupling assembly of claim 1 , wherein the retainer element defines a hollow spherical segment-shaped portion configured to receive the head.
5. The coupling assembly of claim 1 , wherein the inner surface of the locking element forms the vertical surface that restricts the radial expansion of the retainer element in the second configuration.
6. The coupling assembly of claim 1 , wherein the locking element is a pressure element configured to receive and transfer a pressure force from a fixation element and/or the rod received in the recess of the receiving part to lock the inserted head.
7. The coupling assembly of claim 6 , wherein the the locking element defines an inner hollow spherical segment-shaped recess configured to receive the inserted head.
8. The coupling assembly of claim 1 , wherein the locking element comprises a rod receiving portion configured to receive the rod.
9. The coupling assembly of claim 1 , wherein the locking element comprises a first surface configured to engage a second surface of the receiving part when the coupling assembly is in the second configuration to prevent adjustment of the coupling assembly back to the first configuration.
10. The coupling assembly of claim 1 , wherein the locking element is configured to directly engage the inserted head.
11. The coupling assembly of claim 1 , wherein an axial height of the locking element is the same in the first and second configurations.
12. The coupling assembly of claim 1 , wherein the coupling assembly can remain at the first configuration without any external forces acting on the coupling assembly.
13. The coupling assembly of claim 1 , wherein the lower end region of the locking element engages the retainer element to adjust the coupling assembly from the first configuration to the second configuration.
14. A polyaxial bone anchoring device comprising:
a bone anchoring element having a shank for anchoring to bone and a head; and
a coupling assembly for coupling a rod to a bone anchoring element, the coupling assembly comprising:
a receiving part having a first end, a second end below the first end, a central axis extending through the first end and the second end, an accommodation space for accommodating the head, the accommodation space having an opening at the second end sized to permit insertion of the head into the receiving part, and a recess at the first end for receiving the rod;
a retainer element positionable at least partially in the accommodation space and radially expandable to facilitate insertion of the head; and
a locking element positionable at least partially in the accommodation space;
wherein when the head, the retainer element, and the locking element are in the receiving part at a first configuration, an upper surface at an end of the locking element is configured to engage another surface of the coupling assembly to restrict further upward movement of the locking element relative to the receiving part, while the retainer element is expandable for inserting the head;
wherein the locking element is movable to adjust the coupling assembly from the first configuration to a second configuration where radial expansion of the retainer element is restricted to prevent release of the head, and wherein an axial height of the locking element is the same in the first and second configurations; and
wherein the head is configured to extend to an axial position that is closer to the first end of the receiving part than the retainer element and the upper surface of the locking element are to the first end of the receiving part.
15. The polyaxial bone anchoring device of claim 14 , wherein the locking element comprises a rod receiving portion formed on the upper surface to receive the rod.
16. The polyaxial bone anchoring device of claim 14 , wherein the locking element is configured to directly engage the head in the receiving part.
17. A polyaxial bone anchoring device comprising:
a bone anchoring element having a shank for anchoring to bone and a head; and
a coupling assembly for coupling a rod to a bone anchoring element, the coupling assembly comprising:
a receiving part having a first end, a second end, a central axis extending through the first end and the second end, an accommodation space for accommodating the head, the accommodation space having an opening at the second end sized to permit insertion of the head into the receiving part, and a recess at the first end for receiving the rod;
a retainer element positionable at least partially in the accommodation space and radially expandable to facilitate insertion of the head; and
a locking element positionable at least partially in the accommodation space, the locking element comprising a first region directly engageable with the retainer element and a second region directly engageable with the head;
wherein when the head, the retainer element, and the locking element are in the receiving part, the locking element is movable to adjust the coupling assembly from a first configuration where the retainer element is expandable for inserting the head, to a second configuration where the first region of the locking element surrounds at least part of the retainer element to directly block radial expansion of the retainer element and prevent release of the head.
18. The polyaxial bone anchoring device of claim 17 , wherein the retainer element comprises projections extending from an upper surface of the retainer element, and wherein the locking element is configured to surround the projections when the coupling assembly is at the second configuration.
19. The polyaxial bone anchoring device of claim 17 , wherein the locking element is monolithic.
20. The polyaxial bone anchoring device of claim 17 , wherein the locking element defines an inner hollow spherical segment-shaped recess configured to receive the head.