An extension device for a bone anchor is provided, wherein the bone anchor comprises an anchoring section and a receiving part connected to the anchoring section, the receiving part having a central axis, the extension device comprising a first sleeve with a first sleeve axis that is coaxial to the central axis and being configured to be coupled to and decoupled from the receiving part a second sleeve with a second sleeve axis coaxial to the central axis, being positioned within the first sleeve and positionable relative to the first sleeve along the central axis, and being configured to be coupled to and decoupled from the receiving part a locking member that, in a first configuration, inhibits decoupling of the first sleeve or the second sleeve from the receiving part when the other one of the first sleeve or the second sleeve is coupled to the receiving part.
