Coupling assembly and polyaxial bone anchoring device comprising the same

A polyaxial bone anchoring device comprising:

• a bone anchoring element having shank for anchoring to a bone and a head;
• 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 of the bone anchoring element, the accommodation space having an opening at the second end sized to permit the insertion of the head, a bore extending from the accommodation space to the first end, and a recess for receiving a rod;
• a retainer element configured to be positioned at least partially in the accommodation space and radially expandable and/or compressible to allow retaining the head;
• and
• a locking element configured to be arranged at least partially in the accommodation space, wherein the locking element is movable from a first position where the retainer element is configured to expand and release an inserted head to a second position where radial expansion of the retainer element is restricted to prevent release of an inserted head;
• wherein when the locking element is in the second position, the head can be locked relative to the receiving part by exertion of a pressure force directly onto the head by the locking element, and the head exerts a corresponding force against the retainer element;
• and
• wherein the retainer element is fixed axially relative to the receiving part in the first and second positions.

A method of coupling a rod to a bone via a polyaxial bone anchoring device, the polyaxial bone anchoring device comprising a bone anchoring element comprising a shank for anchoring to a bone and a head, a rod, 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 engagement structure and an opening at the second end sized to permit the insertion of the head, a bore extending from the accommodation space to the first end, and a recess for receiving the rod, a retainer element configured to be positioned at least partially in the accommodation space and radially expandable or compressible to allow retaining the head, a locking element configured to be arranged at least partially in the accommodation space, wherein the locking element is movable from a first position where the retainer element is configured to expand and release an inserted head to a second position where radial expansion of the retainer element is restricted to prevent release of an inserted head, the method comprising:

• inserting the head of the bone anchoring element into the accommodation space when the locking element is in the first position and the retainer element is held by the engagement structure in the receiving part in position relative to the receiving part adjacent the opening;
• and
• moving the locking element from the first position towards the second position, wherein when the locking element is in the second position and the head is in the accommodation space, at least one of the rod or the locking element contacts the head, and the locking element radially encompasses at least a portion of the retainer element.