Expandable cage for intercorporal fusion of lumbar vertebrae - with two expendable arms running substantially parallel to each other, whose outsides are each constructed as supporting surfaces for adjacent vertebral bodies, - with a bridge-like connection present at one end of the arms, so that in the medial longitudinal section a substantially U-shaped profile of the main body of the cage is obtained, - with an expansion element, being located at the other, free end of the arms and being rotatably mounted between these, for pressing the arms apart and expanding the cage, and - with an opening provided in the bridge-like connection for connecting instruments to the cage, characterized in - that the expansion element comprises a rotatable elongated, in particular cylindrical, base body which has one or two rectangular plates which are integrally connected thereto and which have rounded edges running in the longitudinal direction of the cage and two opposing, radially aligned ribs running in the longitudinal direction of the cage, - that the rectangular plates engage with their narrow sides in corresponding grooves, which extend transversely in the insides of the arms, both in the unexpanded and expanded state of the cage, - that both the ribs and the base body each have curved, pair-wise opposing bearing surfaces along their respective outer circumference, which, alternatively, and depending on the expansion state of the cage, lie for rotatable support in corresponding channels which extend in the longitudinal direction of the cage and are present on the insides of the arms, and - that a device for rotatably driving the expansion element is present at least in the rear part of the expansion element located towards the bridge-like connection. - with two expendable arms running substantially parallel to each other, whose outsides are each constructed as supporting surfaces for adjacent vertebral bodies, - with a bridge-like connection present at one end of the arms, so that in the medial longitudinal section a substantially U-shaped profile of the main body of the cage is obtained, - with an expansion element, being located at the other, free end of the arms and being rotatably mounted between these, for pressing the arms apart and expanding the cage, and - with an opening provided in the bridge-like connection for connecting instruments to the cage, - that the expansion element comprises a rotatable elongated, in particular cylindrical, base body which has one or two rectangular plates which are integrally connected thereto and which have rounded edges running in the longitudinal direction of the cage and two opposing, radially aligned ribs running in the longitudinal direction of the cage, - that the rectangular plates engage with their narrow sides in corresponding grooves, which extend transversely in the insides of the arms, both in the unexpanded and expanded state of the cage, - that both the ribs and the base body each have curved, pair-wise opposing bearing surfaces along their respective outer circumference, which, alternatively, and depending on the expansion state of the cage, lie for rotatable support in corresponding channels which extend in the longitudinal direction of the cage and are present on the insides of the arms, and - that a device for rotatably driving the expansion element is present at least in the rear part of the expansion element located towards the bridge-like connection.