A surgical instrument for fastening vertebrae via the posterior approach or via the posterolateral approach, the instrument comprising a tubular element formed of two legs separated by a recessed longitudinal zone, the legs being interconnected at their distal ends. Each of the legs having, in its inside surface, a guide channel of dovetail cross-section and an insert having two diametrically opposite longitudinal slideways of dovetail cross-section complementary to the cross-section of said guide channels. The instrument comprising the proximal end of each of the legs has anchor means suitable for coming to engage in complementary means by a transverse movement of the legs.
