An osteosynthesis plate includes a plate body in which there is formed an anchor through-orifice having a first orifice region and the second orifice region which are secant and joined by an intermediate region, the first orifice region being of plain cylindrical overall form and coinciding with a first virtual cylinder, the second orifice region having a second upper segment of plain spherical or frustoconical overall shape centred on a second axis and a second lower segment of threaded cylindrical overall shape and coinciding with a second virtual cylinder intersecting the first virtual cylinder, a helical rib being formed in the second orifice region to define a screw thread having a screw thread external radius smaller than the major radius of the second upper segment, and the intermediate region being formed of two planar intermediate faces facing one another.