A screw anchor having a deformable central portion in which are formed
four main longitudinal slots defining in pairs on the central portion
respective deformable main longitudinal tongues; the screw anchor also
having a number of secondary longitudinal slots; and each secondary
longitudinal slot defines, on the central portion and with an adjacent
main longitudinal slot and/or an adjacent secondary longitudinal slot,
respective deformable secondary longitudinal anchoring tongues.