3. The improvement according to claim 1 wherein the first partial sphere has a concave region and the second partial sphere has a convex region that is operably received in the concave region of the first partial sphere.
1. In a polyaxial bone screw assembly for fixation to a bone, the assembly including a shank with a threaded body and an upwardly extending head portion, a retainer structure sized and shaped to matingly engage and pivotally rotate with the head portion, and a receiver pivotally engaging the head portion, the improvement comprising: a) a shank having a body for fixation to a bone, an upper head portion, a central cannulation bore extending the entire length of the body and upper head portion along a first axis the upper head portion defining a first partial sphere; b) a retainer structure, having a second central cannulation bore along a second axis and defining a second partial sphere, the first partial sphere mating in the receiver to with the second partial sphere so as to form a ball that polyaxially rotates within the receiver and wherein the second central bore aligns with the first central bore.