Pivotal bone anchor assembly having a restrained pre-loaded shank head retainer

Abstract

A pivotal bone anchor assembly includes a receiver having a channel for receiving a rod and an internal chamber with a bottom opening, a lower region adjacent the bottom opening, and an upper region above the lower region. The assembly also includes a bone anchor with a capture portion and an anchor portion. The assembly further includes a resilient open retainer ring that is positioned and restrained in the upper region of the internal chamber prior uploading to the capture portion through the bottom opening, at which point the open retainer ring is configured to expand about and capture the capture portion of the bone anchor and then for downward displacement toward the lower region until an uppermost surface of the open retainer ring becomes engaged with a downwardly-facing surface located in the upper region to inhibit the open retainer ring from moving back up within the receiver.