Anchor for attachment to a bony structure, comprising a hook (18), an anchor body (16) with a distal wall (17) facing the hook, and a fastening system for fastening a connection member to the anchor body (16). A slot (20) is defined between the hook (18) and the distal wall (17), for receiving a bony structure such as a vertebra portion. The anchor (10) further comprises a retention member (60) having a distal end (60A) with teeth (63), passing through the distal wall (17) and being movable relative to the anchor body (16), from a retracted position to a protracted position where the distal end (60A) protrudes from the distal wall (17), inside the slot (20). The proximal end (60B) of the retention member defines a seat (65) for the connection member.
