1. A pivotal bone anchor assembly for anchoring to patient bone and coupling with an elongate rod via a closure, the pivotal bone anchor assembly comprising: a shank having a longitudinal axis, a head with a partial spherical shape defining a hemisphere with an outer spherical surface extending above and below the hemisphere, and an anchor portion extending downwardly from the head for fixation to the patient bone; a receiver having a longitudinal axis, a base defining a bore centered around the longitudinal axis and in communication with a bottom surface of the receiver through a bottom opening, and a pair of upright arms extending upward from the base to define an open channel configured to receive the elongate rod, the open channel in communication with the bore, the bore having an upwardly-concave lower spherical seating surface proximate the bottom opening configured to receive the shank head outer spherical surface and provide for pivotal movement between the shank and the receiver; and an insert positionable within the receiver bore and having an upwardly-facing surface engageable with the elongate rod and a downwardly-facing surface engageable with the shank head above the hemisphere, wherein, prior to the engagement of the insert downwardly-facing surface with the shank head, the shank head outer spherical surface is frictionally engaged below and above the hemisphere, respectively, by the lower spherical seating surface of the receiver and by an inwardly-concave upper spherical seating surface located in the receiver bore below the insert, so as to provide a non-floppy pivotal frictional engagement between the shank and the receiver.
1. A pivotal bone anchor assembly for anchoring to patient bone and coupling with an elongate rod via a closure, the pivotal bone anchor assembly comprising: a shank having a longitudinal axis, a head with a partial spherical shape defining a hemisphere with an outer spherical surface extending above and below the hemisphere, and an anchor portion extending downwardly from the head for fixation to the patient bone; a receiver having a longitudinal axis, a base defining a bore centered around the longitudinal axis and in communication with a bottom surface of the receiver through a bottom opening, and a pair of upright arms extending upward from the base to define an open channel configured to receive the elongate rod, the open channel in communication with the bore, the bore having an upwardly-concave lower spherical seating surface proximate the bottom opening configured to receive the shank head outer spherical surface and provide for pivotal movement between the shank and the receiver; and an insert positionable within the receiver bore and having an upwardly-facing surface engageable with the elongate rod and a downwardly-facing surface engageable with the shank head above the hemisphere, wherein, prior to the engagement of the insert downwardly-facing surface with the shank head, the shank head outer spherical surface is frictionally engaged below and above the hemisphere, respectively, by the lower spherical seating surface of the receiver and by an inwardly-concave upper spherical seating surface located in the receiver bore below the insert, so as to provide a non-floppy pivotal frictional engagement between the shank and the receiver. 15. The pivotal bone anchor assembly of claim 1 , wherein the insert further comprises a cylindrically-shaped body with upstanding arms defining a second open channel for receiving the elongate rod.