A bone fixation subassembly (75 - 675) configured to receive a fixation rod and a locking cap , the bone fixation subassembly comprising: an anchor seat (26 - 626) including an anchor seat body (38 - 638) extending along a central axis (A) and defining an upper end and a lower end , wherein the upper end comprises a pair of opposing fixation rod-receiving gaps (G) therebetween that are spaced along a longitudinal axis (L), and a bore (54 - 654) disposed between the rod-receiving gaps (G); a collet (28 - 628) including a collet body (45 - 645) configured to be disposed in the anchor seat (26 - 626), the collet body (45 - 645) defining an upper end (47 - 647) and an expandable lower end (49 - 649), the expandable lower end (49 - 649) comprising a plurality of fingers (69A, B - 669A, B) separated by slots ; and a bone anchor (30 - 630) that extends along an axis of rotation (B) and is configured to attach to the collet (28 - 628), wherein the bone anchor (30 - 630) defines a head (33 - 633) and the fingers (69A, B - 669A, B) of the expandable lower end (49 - 649) of the collet body (45 - 645) are sized to capture the head (33 - 633) of the bone anchor (30 - 630) therein; characterized in that the bone anchor (30 - 630) is configured to attach to the collet (28 - 628) such that the bone anchor (30 - 630) is permitted to rotate about the axis of rotation (B) relative to the collet (28 - 628) and the anchor seat (26 - 626), the bone anchor (30 - 630) is further permitted to pivot relative to the collet (28 - 628) in a desired plane, and the bone anchor (30 - 630) is prevented from pivoting in another plane that includes the central axis (A) and is angularly offset with respect to the desired plane. an anchor seat (26 - 626) including an anchor seat body (38 - 638) extending along a central axis (A) and defining an upper end and a lower end , wherein the upper end comprises a pair of opposing fixation rod-receiving gaps (G) therebetween that are spaced along a longitudinal axis (L), and a bore (54 - 654) disposed between the rod-receiving gaps (G); a collet (28 - 628) including a collet body (45 - 645) configured to be disposed in the anchor seat (26 - 626), the collet body (45 - 645) defining an upper end (47 - 647) and an expandable lower end (49 - 649), the expandable lower end (49 - 649) comprising a plurality of fingers (69A, B - 669A, B) separated by slots ; and a bone anchor (30 - 630) that extends along an axis of rotation (B) and is configured to attach to the collet (28 - 628), wherein the bone anchor (30 - 630) defines a head (33 - 633) and the fingers (69A, B - 669A, B) of the expandable lower end (49 - 649) of the collet body (45 - 645) are sized to capture the head (33 - 633) of the bone anchor (30 - 630) therein; characterized in that the bone anchor (30 - 630) is configured to attach to the collet (28 - 628) such that the bone anchor (30 - 630) is permitted to rotate about the axis of rotation (B) relative to the collet (28 - 628) and the anchor seat (26 - 626), the bone anchor (30 - 630) is further permitted to pivot relative to the collet (28 - 628) in a desired plane, and the bone anchor (30 - 630) is prevented from pivoting in another plane that includes the central axis (A) and is angularly offset with respect to the desired plane. <EOS>