The cage (1) has ribs (8) and a rotatable, elongate and cylindrical base body (4) including curved radial contact surfaces (9) at an outer circumference. The contact surfaces alternatively lie for rotatable mounting in corresponding channels (26) based on a spreading condition of the cage. The channels extend in a longitudinal direction of the cage, and are provided at inner sides of lower arms (18). A device for rotatably driving a spreading element (3) is provided in a rear part of the spreading element, where the rear part lies in a bridge-like connection (15) formed at ends of the arms.
