A pivotal bone anchor assembly configured for deployment into forced internal interference engagement using a first tool and for being released therefrom using a second tool, prior to securing an elongate rod to a bone of a patient with a closure, the pivotal bone anchor assembly comprising:
- a bone anchor having a capture portion with a partially spherical outer surface and an integral anchor portion opposite the capture portion configured for fixation to the bone;
- the elongate rod having a circular cross-section defining a rod radius;
- and
- a receiver assembly in a pre-assembled configuration configured to receive the capture portion of the bone anchor, the receiver assembly in the pre-assembled configuration including:
- a receiver comprising a base defining an internal cavity centered about a vertical centerline axis and having a bottom opening in communication with a bottom surface of the receiver, and a pair of integral upright arms extending upwardly from the base to define a channel opening onto a front face and a back face of the receiver, the internal cavity extending upwardly through the channel to tops of the upright arms to define an axial bore, the channel having opposed U-shaped inner arm surfaces adjacent the front face and the back face on either side of the axial bore configured to receive the elongate rod, the axial bore including inwardly-facing interference wedging surfaces extending to the opposed U-shaped inner arm surfaces, the internal cavity configured to receive the capture portion of the bone anchor through the bottom opening and having a wider upper portion above a lower locking portion, the lower locking portion being defined by a narrowing restriction in a width of the internal cavity adjacent the bottom opening;
- a retainer having a concave interior surface operable to grip the partially spherical outer surface of the capture portion of the bone anchor, the retainer configured to be initially disposed and held within the wider upper portion of the internal cavity prior to the bone anchor being introduced into the receiver assembly, the retainer being further configured to separate to receive the capture portion therein as it is uploaded through the bottom opening, to then close around and secure the capture portion within the internal cavity while allowing pivotal motion of the bone anchor relative to the receiver, and to be non-pivoting within the internal cavity after securing the capture portion therein;
- and
- a pressure insert configured to be positioned within the axial bore above the retainer disposed within the wider upper portion of the internal cavity, the pressure insert having an upwardly-open rod seat with a curvate lower seating surface having a seat radius substantially equal to the rod radius and configured to downwardly receive the elongate rod, and a recess below the rod seat configured to receive an upper portion of the retainer prior to the capture portion of the bone anchor being uploaded into the receiver through the bottom opening,
- wherein after the capture portion of the bone anchor is uploaded into the receiver through the bottom opening and secured within the retainer, and prior to the installation of the elongate rod,
- the pressure insert with an empty rod seat is downwardly deployable with the first tool within the axial bore of the receiver until an outer surface of the pressure insert comes into a forced interference wedging contact with the interference wedging surfaces of the axial bore so as to inhibit the pressure insert from moving back up within the receiver, and
- the retainer together with the capture portion of the bone anchor secured therein are downwardly displaceable within the internal cavity along the vertical centerline axis so that the lower locking portion of the internal cavity impedes radial outward movement of the retainer, and
- wherein the empty rod seat and channel are configured to receive the elongate rod when the pressure insert, in the downwardly deployed position, is in the forced interference wedging contact with the interference wedging surfaces.