1. A medical implant assembly for securing an elongate rod to a bone, the medical implant assembly comprising: a shank with a lower body portion for fixation to the bone and an integral upper portion having a top radiused surface above a hemisphere thereof and a capture structure thereon; a receiver having a base and a pair of upright arms extending upward from the base with opposed interior surfaces that define an open channel for receiving the elongate rod, the arm interior surfaces having a closure top mating feature formed thereon, the base defining a chamber in communication with the open channel and having an upper capture region, a lower locking region, and a lower opening in communication with a bottom surface of the receiver, the receiver including an interference engagement surface between the lower opening and the closure top mating feature; an insert positioned within the receiver prior to the shank and configured to engage the elongate rod when the elongate rod is received within the open channel, the insert being positioned in the receiver entirely above the receiver chamber upper capture region and initially being prevented from moving downwardly within the receiver by the interference engagement surface, the insert having a central opening: a retainer positioned in the receiver chamber prior to the shank, the retainer capturing the shank upper portion capture structure within the receiver chamber upper capture region when the shank is uploaded through the receiver lower opening, wherein after the shank upper portion is captured by the retainer, the insert is forced downwardly within the receiver by a tool into engagement with the interference engagement surface so as to prevent the insert from moving back up within the receiver, and wherein the shank has pivotal motion in only one plane with respect to the receiver prior to locking of the medical implant assembly.
3. The medical implant assembly of claim 1 , wherein the retainer is configured to widen within the receiver chamber upper capture region and then close back around the shank upper portion capture structure as the shank is uploaded through the receiver lower opening.