An extension device for a bone anchor is provided, where the bone anchor includes an anchoring section for anchoring to a bone and a receiving part connected to the anchoring section. The receiving part includes a central axis and a channel for receiving a rod, where sidewalls of the channel form two free legs. The extension device includes a first sleeve with a first sleeve axis that is coaxial with the central axis, where the first sleeve is configured to be detachably coupled to the receiving part. The extension device includes a second sleeve with a second sleeve axis, where the second sleeve is positioned within the first sleeve. The first sleeve is configured to be coupled to the receiving part to inhibit translational movement of the first sleeve relative to the receiving part along the central axis. The second sleeve is configured to be coupled to the receiving part to hinder or inhibit rotational movement of the second sleeve relative to the receiving part and the second sleeve is connected to the first sleeve through a coupling member that is configured to advance in the first sleeve together with the second sleeve in an axial direction.
