An endosseous screw assembly for an internal fixation system, comprising: a stem extending longitudinally and having at least one threaded distal portion for allowing anchoring to a bone site of a patient; a connection sleeve suitable for introduction inside a connection hole of a fixation member; and a ring nut rotatably mounted on the connection sleeve. The ring nut may be threadably mounted on the connection sleeve and may comprise a rim with teeth designed to cooperate with a step formed on the fixation member, in the vicinity of the connection hole, so as to prevent rotation of the ring nut with respect to the fixation member when the connection sleeve is introduced into the connection hole of the fixation member. The ring nut may further comprise multiple deformable interference fit components which lie along a conical portion of the connection sleeve.