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) 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 comprising a first sleeve (1) with a first sleeve axis (c) that is coaxial to the central axis (C), wherein the first sleeve (1) is configured to be coupled to the receiving part (200) and decoupled from the receiving part (200) a second sleeve (2) with a second sleeve axis (c) coaxial to the central axis (C), wherein the second sleeve (2) is positioned within the first sleeve (1) and positionable relative to the first sleeve along the central axis (C) and wherein the second sleeve (2) is configured to be coupled to the receiving part (200) and decoupled from the receiving part (200) a locking member (35) that in a first configuration inhibits decoupled of one of the first sleeve (1) or the second sleeve (2) from the receiving part (200) when the other one of the first sleeve (1) or the second sleeve (2) is still coupled to the receiving part.本案提供一種骨骼固定器之延伸裝置,其中該骨骼固定器包括:一固定段(100),供固定於一骨骼內,和一容納件(200),連接於該固定段,該容納件包括一中心軸線(C),和一槽溝(204),供容納一桿件,其中該槽溝之側壁形成二自由股部(205a,205b);該延伸裝置包括:一第一套筒(1),有一第一套筒軸線(c),與該中心軸線(C)共軸,其中該第一套筒(1)構成可聯結於該容納件(200),並可自該容納件(200)脫聯;一第二套筒(2),有一第二套筒軸線(c),與該中心軸線(C)共軸,其中該第二套筒(2)定位於該第一套筒(1)內,並可沿該中心軸線(C)相對於該第一套筒定位,且其中該第二套筒(2)構成可聯結於該容納件(200),並可自該容納件(200)脫聯;一鎖定構件(35),呈一第一組態時,抑制該第一套筒(1)或該第二套筒(2)之一,自該容納件(200)脫聯,而該第一套筒(1)或該第二套筒(2)之另一,仍然聯結於該容納件。1‧‧‧第一套筒2‧‧‧第二套筒3‧‧‧聯鎖軸襯15b‧‧‧第一橫向凹部16a‧‧‧第二橫向凹部18a,18b‧‧‧股部21‧‧‧第二套筒之第一部位22‧‧‧縱向開縫25a,25b‧‧‧股部31‧‧‧圓筒段35‧‧‧筒夾部37‧‧‧加厚環部112a,112b‧‧‧縱向翼肋
