Method of assembling bone anchor assemblies with bi-spherical universal shank heads

A method of assembling a bone anchor assembly intended for securing a fixation rod to a bone of a patient, the method comprising:

• downloading a retaining structure into a central bore of a receiver,
• the receiver comprising a vertical centerline axis, an upper portion defining a channel configured to receive the fixation rod, and a base defining a lower portion of the central bore formed around the vertical centerline axis and communicating with a bottom surface of the receiver through a bottom opening, the central bore extending upwardly from the bottom opening through the channel to a top of the receiver and including a guide and advancement structure mateable with a closure proximate the top of the receiver and a spherical seating surface adjacent the bottom opening, and
• the retaining structure comprising a discontinuous outer spherical surface configured to slidably engage the seating surface of the receiver, a discontinuous inner spherical surface configured to frictionally engage an upper spherical surface of a shank head, and a plurality of slots extending upward from a discontinuous annular bottom surface to form a plurality of resiliently-expandable collet fingers,
• downloading a pressure insert into the central bore of the receiver, the insert comprising an upper surface for engaging the rod and a lower end configured to engage the retaining structure;
• and
• uploading the shank head of a bone anchor into the lower portion of the central bore of the receiver, the bone anchor comprising a longitudinal axis, the shank head at a proximal end, and an anchor portion opposite the shank head configured for fixation to the bone, the shank head including:
• an upper partial spherical portion comprising an annular planar top surface at an upper end of the shank head and the upper spherical surface having a first diameter extending downward from the planar top surface past a hemisphere plane to a circular inner edge of an upward-facing ledge;
• and
• a lower partial spherical portion comprising a lower spherical surface having second diameter greater than the first diameter extending downward from a circular outer edge of the upward-facing ledge toward a neck portion that connects the shank head to the anchor portion,
• wherein upon the uploading of the shank head of a bone anchor into the lower portion of the central bore,
• resiliently expanding the retaining structure to first receive and then to close around and capture the upper partial spherical portion of the shank head within the retaining structure;
• and
• lowering the retaining structure and enclosed shank head to the seating surface of the receiver to capture the shank head within the lower portion of the central bore of the receiver.