PCT No. PCT/FR97/01617 Sec. 371 Date Jul. 28, 1998 Sec. 102(e) Date Jul. 28, 1998 PCT Filed Sep. 12, 1997 PCT Pub. No. WO98/10722 PCT Pub. Date Mar. 19, 1998An expandable osteosynthesis implant has branches (5) each connected at one end to a seat (7) which is pierced by an orifice (8), suitable for being slid from a posterior direction between the facing faces of two consecutive vertebrae in order to hold them a given distance apart and restore stability of the spinal column. According to the invention, the branches (5) and the seat (7) define a hollow cage (1) which, in a "rest" position, has an outside general shape that is a cylinder of circular section, and a portion at least of the inside volume (9) of the cage (1) towards the distal ends of the branches (5) is in the form of a circular truncated cone whose large base is towards the seat (7), which implant has at least three branches (5) and, inside the inside volume (9) at least one spacer (2) suitable for passing through the orifice (8) and the large base of the truncated cone.