An extension device for a bone anchor is provided, wherein the bone anchor comprises an anchoring section (100) for anchoring in a bone and a receiving part (200, 200) connected to the anchoring section, the receiving part comprising a central axis (C) and a channel (204) for receiving a rod, wherein sidewalls of the channel form two free legs (205a, 205b).The extension device further comprises a first sleeve (1, 1, 1") with a first sleeve axis (c) that is coaxial with the central axis (C), wherein the first sleeve (1, 1, 1") is configured to be detachably coupled to the receiving part (200, 200), and a second sleeve (2, 2, 2", 2") with a second sleeve axis, wherein the second sleeve is positioned within the first sleeve (1, 1, 1").The first sleeve (1, 1, 1") is configured to contact to the receiving part (200, 200) and the second sleeve (2, 2, 2", 2") is configured to be coupled to the receiving part so as to hinder or inhibit a rotational movement of the second sleeve (2, 2, 2", 2") relative to the receiving part (200, 200).The second sleeve (2, 2, 2", 2") is connected to the first sleeve (1, 1) through a coupling member (3) that is configured to advance in the first sleeve (1, 1, 1") together with the second sleeve (2, 2, 2", 2") in an axial direction.The first sleeve (1, 1, 1") comprises a protrusion (120a, 120b) and/or a recess at an inner surface thereof that is configured to engage a complementary structure (206a, 206b) at an outer surface of the receiving element (200, 200) to provide a form-fit connection between the first sleeve (1, 1, 1") and the receiving part (200, 200) such as to inhibit a translational movement of the first sleeve relative to the receiving part along the central axis.
