A method of assembling a pivotal bone anchor assembly configured to secure an elongate rod to a bone of a patient via a closure, the method comprising:
- positioning a retaining structure into a base of a receiver having a central bore centered about a vertical centerline axis and a planar seating surface formed therein above a lower opening of the base, the retaining structure being operably slidably coupled with and supported by the planar seating surface and having a central opening, the receiver including a pair of upright arms extending upward from the base to define an open channel aligned along a transverse axis perpendicular to the vertical centerline axis and extending to front and back faces of the upright arms and configured to receive the elongate rod, and exterior surfaces comprising:
- opposite, at least partially cylindrical upper side surfaces of the upright arms extending downwardly from tops of the upright arms toward the base and defining a first width, as measured perpendicular to the transverse axis, from the upper side surface of one upright arm to the upper side surface of the other upright arm;
- opposite lower side surfaces of the base below the upper side surfaces of the upright arms, the lower side surfaces of the base defining a second width, as measured perpendicular to the transverse axis, that is greater than the first width;
- and
- opposite, at least partially radiused tool engaging surfaces formed in the upper side surfaces of the upright arms entirely above the base, the tool engaging surfaces being orthogonal to both the vertical centerline axis and the transverse axis;
- and
- positioning an at least partially spherical head portion of a shank into the central opening of the retaining structure, the shank comprising a longitudinal axis, a neck portion, and an anchor portion extending downward from the neck portion configured for fixation to the bone, the retaining structure extending above a hemisphere of the head portion when the longitudinal axis of the shank is aligned with the vertical centerline axis of the receiver and being configured to capture, at least partially surround, and pivotally retain and support the head portion of the shank in the retaining structure,
- wherein upon assembly of the retaining structure and the shank with the receiver, the retaining structure is slidably movable along the planar seating surface of the receiver, while maintaining the retaining structure perpendicular to the vertical centerline axis of the receiver, so as to provide for increased articulated movement between the shank and the receiver in at least one direction relative to the vertical centerline axis prior to locking the assembly with the elongate rod and the closure.