A surgical assembly, comprising: - a bone anchoring screw having an anchoring part in the spine and a head in the form of a bush, and - an extension device intended to cap the bush of the bone anchoring screw to form a longitudinal extension thereof; - said extension device comprises a first tubular element, called inner tube, with an inner diameter greater than or equal to the inner diameter of the bush of the bone anchoring screw, and extending longitudinally by two diametrically opposite branches, said branches comprising a resiliency allowing them to be separated or brought closer to one another, and the free ends of the two branches form a clamp able to clamp the outer surface of the bush to be locked thereon; - the extension device comprises a second tubular element, called outer tube, mounted around the inner tube, and with an inner diameter adjusted to the outer diameter of said inner tube so as to be able to slide along said inner tube, between a so-called unlocked position and a so-called locked position of the bone anchoring screw, the outer tube extending longitudinally by two diametrically opposite branches and superimposed on the branches of the inner tube, the free ends of the two branches of the outer tube are facing the free ends of the two branches of the inner tube, characterized in that the free ends of the two branches of the outer tube each comprise retaining means able to cooperate, when the extension device caps the bush of the bone anchoring screw and when said outer tube is slid into its locked position, with complementary retaining means arranged on the bush to prevent said branches of the outer tube from moving away from one another, thus preventing the branches of the inner tube from moving away from one another so as to allow the maintenance of the locking of the extension device on the bush of the bone anchoring screw. - a bone anchoring screw having an anchoring part in the spine and a head in the form of a bush, and - an extension device intended to cap the bush of the bone anchoring screw to form a longitudinal extension thereof; - said extension device comprises a first tubular element, called inner tube, with an inner diameter greater than or equal to the inner diameter of the bush of the bone anchoring screw, and extending longitudinally by two diametrically opposite branches, said branches comprising a resiliency allowing them to be separated or brought closer to one another, and the free ends of the two branches form a clamp able to clamp the outer surface of the bush to be locked thereon; - the extension device comprises a second tubular element, called outer tube, mounted around the inner tube, and with an inner diameter adjusted to the outer diameter of said inner tube so as to be able to slide along said inner tube, between a so-called unlocked position and a so-called locked position of the bone anchoring screw, the outer tube extending longitudinally by two diametrically opposite branches and superimposed on the branches of the inner tube, the free ends of the two branches of the outer tube are facing the free ends of the two branches of the inner tube, characterized in that the free ends of the two branches of the outer tube each comprise retaining means able to cooperate, when the extension device caps the bush of the bone anchoring screw and when said outer tube is slid into its locked position, with complementary retaining means arranged on the bush to prevent said branches of the outer tube from moving away from one another, thus preventing the branches of the inner tube from moving away from one another so as to allow the maintenance of the locking of the extension device on the bush of the bone anchoring screw.