Pivotal bone anchor assembly with bottom-loaded shank, ring retainer and twist-in-place pressure insert

Abstract

A pivotal bone anchor assembly includes a receiver having a lower cavity with a circumferential groove adjacent a bottom opening, an upper first channel with a downward-facing surface formed therein below a closure mating structure, and a substantially cylindrical surface extending below the downward-facing surface with an alignment stop structure. The assembly also includes a shank having a capture portion configured for uploading into the cavity through the bottom opening, and an open ring retainer uploadable into the circumferential groove through the bottom opening after the capture portion so as to retain the capture portion within the cavity. The assembly further includes a pressure insert positionable into the first channel with a second channel of the pressure insert being transverse to the first channel of the receiver, after which the pressure insert is rotatable within the receiver until upwardly-facing surfaces on the insert rotate beneath the downward-facing surface to inhibit upward movement of the pressure insert, and a protruding rotational abutment structure on the insert engages the alignment stop structure to inhibit further rotation of the pressure insert when the second channel is aligned with the first channel.