6. The assembly according to claim 5 wherein the first partial sphere has a concave region and the second partial sphere has a convex region that mates with the concave region.
5. 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 the head portion, a receiver pivotally engaging the head portion and having two spaced apart arms each being internally threaded and defining gaps therebetween, and an upper pressure insert receivable in the receiver and pivotally engaging the head portion, the assembly 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 the upper head portion defining a first partial sphere; b) a retainer structure, having a second partial sphere the first and second partial spheres mating n the receiver to form a ball like structure that polyaxially rotates in the receiver during position.