A bone anchor assembly intended for securing an elongate rod to a bone of a patient, the bone anchor assembly comprising:
- a locking mechanism including a bottom surface and a helically wound first mating structure;
- a screw shank comprising a longitudinal axis, a capture portion at a proximal end having a partial spherical shape, an upper spherical surface at an upper end with an internal drive socket formed therein, an anchor portion at a distal end configured for fixation to the bone, a neck between the capture portion and the anchor portion defined by a circumferentially-extending surface of reduced radius, an annular planar top surface perpendicular to the longitudinal axis and entirely surrounding the internal drive socket, and a central opening extending from the internal drive socket an entire length of the anchor portion;
- a receiver comprising a base portion defining an internal cavity in communication with a bottom surface of the base portion through a bottom opening, and an upper portion defining a channel communicating with the internal cavity and configured to receive the elongate rod, the upper portion having a helically wound second mating structure formed therein configured to mate with the helically wound first mating structure of the locking mechanism, the bottom surface of the locking mechanism configured to apply a downwardly-directed force on the elongate rod when the elongate rod is received in the channel and the helically wound first mating structure of the locking mechanism mates with the helically wound second mating structure, the internal cavity having an inner support surface adjacent the bottom opening configured to prevent the screw shank from exiting the receiver through the bottom opening when the capture portion is positioned in the internal cavity of the receiver;
- and
- a separate element insertable within the receiver and configured to compressively engage with the capture portion of the screw shank so as to lock the bone anchor assembly with the locking mechanism in a locked configuration,
- wherein the annular planar top surface on the capture portion of the screw shank is configured to remain spaced apart from the separate element so as to not directly receive a downwardly-directed force from the locking mechanism to result in the locked configuration.