Uniplanar bone anchor assembly

A uniplanar bone anchor assembly for securing an elongate rod to a bone of a patient with a closure top, the uniplanar bone anchor assembly comprising:

• a shank having a shank head at a proximal end and a unitary anchor portion opposite the shank head configured for fixation to the bone, the shank head having a partial spherical shape with a flat planar outer surface portion on at least one side;
• a receiver having a longitudinal axis, a top portion with upwardly extending arms defining an open channel configured for receiving the elongate rod, and a base defining a cavity in communication with the open channel and having a distal opening in communication with a bottom surface of the base, the upwardly extending arms having opposed internal surfaces extending between a front side surface and a back side surface and including a mating structure formed therein and configured for mating with the closure top, the cavity including an inner groove extending at least partially circumferentially around the cavity;
• and
• a retainer having a central opening configured to receive the shank head, an outer surface sized and shaped for insertion into and non-pivotal engagement with the inner groove of the cavity of the receiver, a thickness between the outer surface and an inner surface, a vertical slot or gap extending through the thickness thereof configured to allow for compression of the retainer during installation of the retainer into the cavity, the vertical slot or gap separating two ends of the retainer adjacent the slot or gap into a non-overlapping arrangement, and at least one flat planar inner surface portion configured to contact the flat planar outer surface portion of the shank head so as to restrict a pivotal motion of the shank relative to the receiver to only one plane,
• wherein the inner groove and the retainer are configured together such that the retainer is positioned in the inner groove in a fixed predetermined rotational alignment orientation with respect to the longitudinal axis of the receiver.