An implant device for performing posterior spinal arthrodesis at a facet joint, includes a flat element, of constant thickness, having, in order to receive a bone graft, a large central opening so as to form a substantially annular shape. The device is intended to be inserted into a cavity previously produced at the location of the facet joint that is to be immobilised. The element is D-shaped, the rounded side constituting the distal edge for insertion into the cavity, while the proximal opposing edge, which is straight, includes a device for securing to a holding and positioning tool. The opposing faces of the flat element have raised areas that include notches arranged perpendicular to the direction of insertion, each of the notches having a tooth profile with two sides.
