Abstract: The hook (32) comprises a body (33) and a laminar portion (37) which is suitably profiled and adapted to penetrate a foramen opening at the base of the occiput against which it bears. The body is arranged to receive an osteosynthesis rod or plate and has a plane of symmetry (P). The laminar portion (37) has a curvature adapted to the thickness of the occipito-cervical shell and is inclined relative to one side or the other of the plane of symmetry (P) so as to be adaptable to the anatomy of the edge of the occiput in the region of the foramen. This inclination may be for example substantially 45.degree.. This asymmetry of the laminar portion (37) relative to the body (33) of the hook (32) enables the surgeon to avoid having to previously remove a piece of the edge of the occiput of the patient and consequently damage the occiput.

Abstract: The instrumentation comprises two separate similar parts, namely a right part (1) and a left part (2), each formed by a cervical rod (3) having asperities and an elongate occipital plate (5) forming one piece with the rod and extending it toward the occiput (6) in the position of use, and adjustable means for anchoring the rod to the vertebrae and the plate to the occiput. Each part is so preangulated and shaped as to be adapted to the anatomy of the occipito-cervical connection. Each rod (3) has two rectilinear sections making therebetween an angle of between about 70.degree. and 90.degree., one of the sections being connected to the occipital plate (5), which is bent so as to correspond to the curvature of the occipital shell. The invention permits conveniently adapting the distance between the rods (3) to the anatomy of the patient and reduces the overall size of the instrumentation, which enables projections under the skin of the occiput to be considerably reduced.