In one embodiment, the present invention includes a spinal osteosynthesis system comprising a connector, a connector member, a ring, and a bone anchor. The connector has an upper part disposed opposite of a lower part along a central axis. The upper part has a locking element and the lower part has a curved interior surface. The connector member can extend through the upper part along an opening axis transverse to the central axis. The ring may define a solid of revolution about the central axis. At least a portion of the ring may be receivable in the lower part of the connector. The bone anchor may have a head curved for polyaxial orientation with respect to the central axis. The locking element may be advanced towards the lower part to place the head of the bone anchor in contact with the connecting member, the ring, and the connector.