13. A medical implant assembly for securing an elongate rod to a bone, the medical implant assembly comprising: a shank with a lower body for fixation to a bone and an integral upper portion having an outer convex substantially spherical surface; a receiver having a base and a pair of upright arms extending upward from the base with inner surfaces defining an open channel for receiving the elongate rod, the arm inner surfaces having a closure top guide and advancement feature formed thereon, the base defining a chamber having an upper capture portion, a lower locking portion, and communicating with a receiver bottom surface through a lower opening, the open channel communicating with the chamber, the receiver having a central axis and a contact portion between the lower opening and the open channel and facing inwardly toward the central axis; a retainer positioned within the chamber before the shank is bottom loaded into the receiver through the lower opening, the retainer having a through-and-through gap extending from a top surface to a bottom surface of the retainer; and a lower pressure insert positioned within the receiver before the shank is bottom loaded into the receiver and engageable with the receiver inwardly-facing contact portion to prevent the insert from moving downwardly toward the chamber prior to the shank being capture by the retainer, wherein after the shank upper portion is bottom loaded into the receiver through the lower opening and captured by the retainer, the insert is forced downward toward the lower locking portion with a tool into an interference engagement with the receiver inwardly-facing contact portion that prevents the lower pressure insert from moving back up within the receiver, and wherein the wherein the insert and retainer cooperate with the receiver to limit pivotal motion of the shank with respect to the receiver to a single plane.
13. A medical implant assembly for securing an elongate rod to a bone, the medical implant assembly comprising: a shank with a lower body for fixation to a bone and an integral upper portion having an outer convex substantially spherical surface; a receiver having a base and a pair of upright arms extending upward from the base with inner surfaces defining an open channel for receiving the elongate rod, the arm inner surfaces having a closure top guide and advancement feature formed thereon, the base defining a chamber having an upper capture portion, a lower locking portion, and communicating with a receiver bottom surface through a lower opening, the open channel communicating with the chamber, the receiver having a central axis and a contact portion between the lower opening and the open channel and facing inwardly toward the central axis; a retainer positioned within the chamber before the shank is bottom loaded into the receiver through the lower opening, the retainer having a through-and-through gap extending from a top surface to a bottom surface of the retainer; and a lower pressure insert positioned within the receiver before the shank is bottom loaded into the receiver and engageable with the receiver inwardly-facing contact portion to prevent the insert from moving downwardly toward the chamber prior to the shank being capture by the retainer, wherein after the shank upper portion is bottom loaded into the receiver through the lower opening and captured by the retainer, the insert is forced downward toward the lower locking portion with a tool into an interference engagement with the receiver inwardly-facing contact portion that prevents the lower pressure insert from moving back up within the receiver, and wherein the wherein the insert and retainer cooperate with the receiver to limit pivotal motion of the shank with respect to the receiver to a single plane. 18. The medical implant assembly of claim 13 , wherein the shank is cannulated.