The cage (1) has sagittal walls (2, 3) interconnected with each other by anterior and posterior transverse walls (4, 5). The walls define, between them, a volume (7) opened downwards and upwards for receiving a bone graft. Rod shaped radio opaque markers (20, 23) are arranged in a material of the cage. The markers are in a same plane perpendicular to an antero-posterior direction, where the markers are distant from each other in a medio-lateral direction. The markers are formed at a level of lower and upper faces (8, 9) of the cage.
