Polyaxial bone fixation assembly

On going from the bottom face (22) towards the top face (21) of the plate (2), the bore of the orifice (3) in the plate (2) comprises: an inner collar (9); a cylindrical portion (10); and the seat-forming female spherical surface (8); and on going between its male spherical surface (7) and its zone (11) connecting to the body (6) of the screw (4), the head (5) of the associated fastener screw (4) has a threaded portion (12) with a single-start thread, the thread being interrupted in at least two zones (13) in order to enable the inner collar (9) of the orifice (3) in the associated plate (2) to be machined while the screw (4) is being driven to turn inside said orifice (3) until it reaches a position of bearing contact between the male and female spherical surfaces (7, 8).