A spinal osteosynthesis device includes a plurality of implants, each having a yoke and an anchoring arrangement carried by the yoke for securing the implant to a selected vertebra. The device further has a connecting bar crossing each yoke; and a plurality of clamps tightening the bar to each yoke. Each clamp has a clamp body; and a pair of generally parallel-oriented claws spaced from one another by a slot and extending from the clamp body. The claws partially surround the respective yoke. The clamp body carries a tightening arrangement for rocking the clamp body about the connecting bar to tilt the claws toward the connecting bar so that the yoke is clamped thereagainst.