The plate (1) has a generally triangular shape and includes two juxtaposed lower holes (7), intended to receive implantation screw (2) in the sacrum (100), and a first upper hole (8) for receiving an implantation screw (2) in the fifth lumbar vertebra (101). According to the invention, the plate (1) includes an extension (6) integral therewith, extending its upper side, i.e. protruding from the side opposite the two juxtaposed lower holes (7), the extension (6) having a second upper hole (15) formed in its entirety thereon, for receiving a second screw (2) intended to be implanted in the fifth lumbar vertebra (101), and having, at all points of its length, a width not greater than two times the diameter of the second upper hole (15); the direction in which the extension (6) protrudes from the plate (1) is such that the first upper hole (8) and the second upper hole (15) are placed on a first line (L1) substantially perpendicular to a second line (L2) passing through the two juxtaposed lower holes (7).