Pivotal bone anchor assembly with bottom loaded insert and pivotless retainer

A pivotal bone anchor assembly for securing an elongate rod to a bone, the pivotal bone anchor assembly comprising:

• a shank having an upper capture portion and an anchor portion extending distally from the upper capture portion for fixation to the bone;
• a receiver having a top, a bottom, and a longitudinal axis extending from the top to the bottom, a base defining a chamber communicating with the bottom of the receiver through a bottom opening, and a first pair of upstanding arms extending upwardly from the base to define a first channel opening onto the top of the receiver and configured to receive the elongate rod, the first channel communicating with the chamber, the chamber having an upper cylindrically shaped expansion portion with a first diameter and a lower cylindrically shaped locking portion with a second diameter smaller than the first diameter, the locking portion further defined in part by an upwardly-facing seating surface extending perpendicular to the longitudinal axis;
• an insert having a second pair of upstanding arms defining a second channel opening onto a top of the insert and configured to receive the elongate rod, the insert being uploaded into the receiver chamber through the bottom opening, the insert having an outer surface that is slidably engageable with a longitudinal alignment structure formed in the receiver to prevent rotation between the insert and the receiver and to maintain the second channel in alignment with the first channel;
• and
• an open retainer ring having a top surface, a bottom surface, an internal surface, and an external surface, and having a gap extending entirely through the ring from the top surface to the bottom surface and from the internal surface to the external surface, the retainer ring being uploaded into the receiver chamber through the bottom opening after the insert and positionable within the receiver chamber,
• wherein the shank upper capture portion is uploadable into the receiver chamber through the bottom opening after the insert and the retainer ring, in which the upward loading motion of the shank upper capture portion along the longitudinal axis urges the retainer ring into expansion within the upper cylindrically shaped expansion portion, after which the retainer ring resiliently returns to a non-expanded state within the lower cylindrically shaped locking portion so as to capture the shank upper capture portion within the receiver chamber, and
• wherein the insert is configured to engage the shank upper capture portion, with the retainer ring being seated against the upwardly-facing seating surface, so as to lock the shank with respect to the receiver.