A polyaxial bone screw assembly includes a threaded shank body having an
upper capture structure, a head and an open, compressible retainer
collar. The capture structure and retainer collar are both threaded for
rotatable attachment within a cavity of the head. The head has a U-shaped
cradle defining a channel for receiving a spinal fixation rod. The head
channel communicates with the cavity and further with a restrictive
opening that allows for loading the capture structure into the head but
prevents passage of the attached retainer collar out of the head. The
open retainer collar may be bottom- or top-loaded by compressing open
ends of the ring towards each other, the collar springing back into a
rounded shape after insertion in the head. The open collar has an
external substantially spherical surface that mates with an internal
surface of the head, providing a ball joint, enabling the head to be
disposed at an angle relative to the shank body. The threaded capture
structure includes a tool engagement formation and gripping surfaces for
non-slip engagement by a tool for driving the shank body into bone.