Endosseous screw assembly (100) for an internal fixation system (SC), comprising: a stem (1) extending longitudinally and having at least one threaded proximal portion (2) for allowing anchoring to a bone site of a patient; a connection sleeve (3) suitable for introduction inside a connection hole (42) of a fixation member (40); a ring nut (5) rotatably mounted on the connection sleeve (3), wherein the ring nut (5) is mounted on the connection sleeve (3) by means of a threaded connection and comprises a rim with teeth (51) designed to cooperate by means of contact with a step (41) formed on the fixation member (40), in the vicinity of the connection hole (42), so as to prevent rotation of said ring nut (5) with respect to the fixation member (40; 40') when the connection sleeve (3) is introduced into the connection hole (42) of the fixation member (40; 40'); said ring nut (5) further comprising a plurality of deformable interference means (52) which lie along a conical portion (31) of the connection sleeve (3); said conical portion (31) being designed to be seated inside the connection hole (42) when the connection sleeve (3) is introduced inside said connection hole (42) and being oriented so that relative sliding of the ring nut (5) and the connection sleeve (3) results in deformation of the deformable interference means (52).
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