Bone anchor having a snap-fit assembly

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

• a shank sub-assembly comprising;
• a shank having a shank distal end and a shank proximal end opposite the shank distal end, the proximal end of the shank comprising a spherically shaped shank head and a pair of opposed and spaced apart bores on the shank head;
• and
• a coupler comprising two symmetrical half ring collet members, each collet member comprising an outer surface with a circumferential locking groove and a rounded inner surface configured to surround the shank head, and a collet lock sleeve configured to press fit the two symmetrical half ring collet members around the shank head such that the coupler and shank head have a friction fit;
• a receiver sub-assembly having a receiver sub-assembly distal end and a receiver sub-assembly proximal end opposite the receiver sub-assembly distal end, the receiver sub-assembly comprising a receiver defining an opening that extends from the receiver sub-assembly proximal end to the receiver sub-assembly distal end and an internal snap-fit assembly comprising an insert, a retainer ring positioned within an expansion chamber of the receiver, and a positioner configured to hold the retainer ring in position,
• wherein the receiver sub-assembly is configured to couple to the shank sub-assembly via the snap-fit assembly in a bottom-loaded arrangement such that as the shank sub-assembly is at least partially received in the opening through the receiver sub-assembly distal end, the positioner is pushed up into a holding chamber and the retainer ring engages between the expansion chamber of the receiver and the circumferential locking groove on the outer surface of the collet.