A receiver assembly includes a receiver with a central bore and a bottom opening, a channel for receiving a rod, and a discontinuous horizontal recess and one or more vertically-aligned side pockets formed into sidewall surfaces of the central bore. The assembly also includes a pressure insert with an upwardly-open rod-seating surface, opposite flanges projecting radially outward from an upper portion of an outer cylindrical surface, and one or more side protuberances projecting radially outward below the opposite flanges. The pressure insert is positionable into the central bore in a first position and then rotatable from the first position to a second position in which the side protuberances are interferingly forced into the side pocket recesses to inhibit further rotation of the pressure insert and the opposite flanges are rotated under downwardly-facing surfaces of the horizontal recess to inhibit upward movement of the pressure insert within the central bore.