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) and designed to be arranged making contact against the connection hole (42) of the fixation member (40)；where the ring nut (5) is mounted on the connection sleeve (3) by means of a threaded connection and comprises first engaging means (51) suitable for engagement with a first screwdriver (105) for screwing the ring nut (5) on said connection sleeve (3)；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), due to screwing of the ring nut (5) with respect to said connection sleeve (3), results in deformation of the deformable interference means (52) so as to obtain a contact force between the fixation member (40) and the ring nut (5) inside the connection hole (42) and lock the endosseous screw assembly (100) inside said connection hole (42) of the fixation member (40).
