A system of an extension device and a bone anchor, wherein the bone anchor comprises: an anchoring section for anchoring in a bone and a receiving part (200', 200") connected to the anchoring section, the receiving part comprising a central axis and a channel for receiving a rod, wherein sidewalls of the channel form two free legs (205a, 205b), and wherein the extension device comprises: a first sleeve (1', 1 ") with a first sleeve axis (c) that is coaxial with the central axis, wherein the first sleeve (1', 1") is configured to contact and be detachably coupled to the receiving part (200', 200"), and a second sleeve (2', 2", 2"') with a second sleeve axis, wherein the second sleeve is positioned within the first sleeve (1', 1 ") and 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"') relative to the receiving part (200', 200"), wherein the second sleeve (2', 2", 2"') is connected to the first sleeve (1, 1') through a coupling member that is configured to advance in the first sleeve (1', 1") together with the second sleeve (2', 2", 2"') in an axial direction; wherein the first sleeve (1', 1 ") comprises a circumferentially extending protrusion (120a, 120b) at an inner surface thereof that is configured to engage a complementary structure of a circumferentially extending groove (206a, 206b) at an outer surface of the receiving part (200', 200") such as to inhibit a translational movement of the first sleeve (1', 1") relative to the receiving part along the central axis, wherein the first sleeve (1', 1") further comprises at least one longitudinally extending rib (112a, 112b) or protrusion (129a, 129b) that engages a corresponding longitudinally extending groove (212a, 212b, 210a, 210b) formed at one of the legs of the receiving part (200', 200") so as to also inhibit a rotational movement of the receiving part relative to the first sleeve (1', 1") characterised in that the longitudinal grooves (212a, 212b, 210a, 210b) intersect the circumferentially extending grooves (206a, 206b) to provide a form-fit connection between the first sleeve (1', 1") and the receiving part (200', 200"). an anchoring section for anchoring in a bone and a receiving part (200', 200") connected to the anchoring section, the receiving part comprising a central axis and a channel for receiving a rod, wherein sidewalls of the channel form two free legs (205a, 205b), and a first sleeve (1', 1 ") with a first sleeve axis (c) that is coaxial with the central axis, wherein the first sleeve (1', 1") is configured to contact and be detachably coupled to the receiving part (200', 200"), and a second sleeve (2', 2", 2"') with a second sleeve axis, wherein the second sleeve is positioned within the first sleeve (1', 1 ") and 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"') relative to the receiving part (200', 200"), wherein the second sleeve (2', 2", 2"') is connected to the first sleeve (1, 1') through a coupling member that is configured to advance in the first sleeve (1', 1") together with the second sleeve (2', 2", 2"') in an axial direction;