An intervertebral cage includes a thin main wall having two return parts at opposite ends of the wall. The wall has an approximately constant thickness. The return parts are extended by facing end parts having a gap between them. The wall, the return parts and end parts delimit an inner cavity. The gap extends parallel to the wall and has a length between 50% and 100% of the cavity length. The gap length is preferably between 70% to 90% of the cavity length.