1. 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 join and polyaxially rotate with the shank, and a receiver that operably receives the head portion and retainer, the assembly further wherein: a) the head portion comprising a retainer engagement portion including a partially circumferential curvate groove with contoured upper and lower mating surfaces and a pair of spaced recesses; b) the retainer structure being sized and shaped to matingly engage the retainer engagement portion so as to form a substantially spherical ball-shaped pivoting surface with the head portion, the retainer structure comprising i) top and bottom mating surfaces complementary contoured for mating with the head portion upper and lower mating surfaces; and ii) a pair of spaced prongs sized and shaped to matingly engage the head portion recesses; c) the receiver defining an open channel and having a base with a lower seating surface partially defining a cavity, the seating surface being sized and shaped to engage at least a portion of upper head portion, the open channel communicating with the cavity, the cavity communicating with an exterior of the base through an opening sized and shaped to receive the shank upper head portion by uploading the upper head portion through the opening, where the upper head portion mates with the retainer within the cavity.
1. 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 join and polyaxially rotate with the shank, and a receiver that operably receives the head portion and retainer, the assembly further wherein: a) the head portion comprising a retainer engagement portion including a partially circumferential curvate groove with contoured upper and lower mating surfaces and a pair of spaced recesses; b) the retainer structure being sized and shaped to matingly engage the retainer engagement portion so as to form a substantially spherical ball-shaped pivoting surface with the head portion, the retainer structure comprising i) top and bottom mating surfaces complementary contoured for mating with the head portion upper and lower mating surfaces; and ii) a pair of spaced prongs sized and shaped to matingly engage the head portion recesses; c) the receiver defining an open channel and having a base with a lower seating surface partially defining a cavity, the seating surface being sized and shaped to engage at least a portion of upper head portion, the open channel communicating with the cavity, the cavity communicating with an exterior of the base through an opening sized and shaped to receive the shank upper head portion by uploading the upper head portion through the opening, where the upper head portion mates with the retainer within the cavity. 2. The assembly according to claim 1 including a pressure insert that pivotably engages the shank head portion comprising: i) a lower body with a pair of spaced opposed partially cylindrical surfaces and a pair of spaced vertical faces; and ii) a pair of opposed upwardly extending arm structures, each arm structure having a radially extending protrusion with a centrally located locking slot, the cavity of the receiver having an interior wall having a protrusion that mates with locking slot.