It incorporates, anchored with expansion studs or with stakes (2) a lower base (1) in the form of a centrally drilled spherical shell; a threaded rod (3), passing through this drilled hole, a foot (4), formed by a section of the shell (4.1) and which slides with respect to the base (1), followed by a short tubular section (4.2); an upper cap (5) with an axially drilled hole (5.1), a post (6) with an upper cover (7) and - two parallel rods (10) near to the edges of the mortise (6.1); a tensioning element (11).