A coupling assembly for coupling a rod to a bone anchoring element is provided, wherein the coupling assembly (4, 4') comprises a receiving part (5, 5') having a first end (5a), a second end (5b), a central axis (C) extending through the first end (5a) and second end (5b), an accommodation space (54) for accommodating a head (3, 3') of an anchoring element (1, 1'), the accommodation space having an opening (55) at the second end (5b) sized so as to permit the insertion of a head (3, 3') and a bore (51) extending from the accommodation space to the first end (5a), and a recess (52) for receiving a rod (100), and a retainer element (6) configured to be positioned at least partially in the accommodation space (54) and being radially expandable and/or compressible so as to allow retaining a head (3, 3') which is inserted through the opening (55). The retainer element (6) is held in position adjacent the opening (55) by an engagement structure (56) provided at or in the accommodation space (54). The coupling assembly further comprises a locking element (7, 7'), which is configured to be arranged at least partially in the accommodation space (54), the locking element (7, 7') being movable from a first position, in which the retainer element (6) is allowed to expand and release an inserted head (3, 3'), to a second position, in which radial expansion of the retainer element (6) is hindered so as to prevent release of an inserted head (3, 3'). When the locking element (7, 7') is in the second position, a head (3, 3') of an anchoring element (1, 1') can be locked by exertion of a pressure force directly onto the head (3, 3') such as to press the head against the retainer element (6).
