A bone anchor assembly , comprising: an anchor seat for a bone fixation element , the anchor seat comprising: a pair of arms , that define an upper end of the anchor seat and that are spaced from one another along a first direction so as to define a rod receiving channel that extends between the arms along a second direction, perpendicular to the first direction; and a base that defines a lower end of the anchor seat and that is monolithic with the pair of arms such that the pair of arms extend from the base along a third direction, perpendicular to both the first and second directions, the anchor seat defining a bore that extends from the upper end and entirely through the lower end along a central axis (A) that extends in the third direction, the base including a pair of opposed guide walls spaced apart from one another along the first direction so as to define a lower opening between the pair of guide walls , and the base being configured to receive a head of a bone anchor therein such that the bone anchor extends through the lower opening , wherein the pair of guide walls are configured to directly engage opposing abutment surfaces of the bone anchor so as to permit the bone anchor to pivot along a select plane that extends in the second and third directions, and prevents the bone anchor from pivoting in any plane other than the select plane; and a collet including a collet body configured to be disposed in the anchor seat , the collet body defining an upper end and an expandable lower end , the expandable lower end configured to receive a head of the bone anchor , characterized in that the bone anchor assembly is configured such that inner guide surfaces of the pair of guide walls do not contact the collet when the bone anchor is received in the collet and the collet is seated in the anchor seat . a pair of arms , that define an upper end of the anchor seat and that are spaced from one another along a first direction so as to define a rod receiving channel that extends between the arms along a second direction, perpendicular to the first direction; and a base that defines a lower end of the anchor seat and that is monolithic with the pair of arms such that the pair of arms extend from the base along a third direction, perpendicular to both the first and second directions, the anchor seat defining a bore that extends from the upper end and entirely through the lower end along a central axis (A) that extends in the third direction, the base including a pair of opposed guide walls spaced apart from one another along the first direction so as to define a lower opening between the pair of guide walls , and the base being configured to receive a head of a bone anchor therein such that the bone anchor extends through the lower opening , wherein the pair of guide walls are configured to directly engage opposing abutment surfaces of the bone anchor so as to permit the bone anchor to pivot along a select plane that extends in the second and third directions, and prevents the bone anchor from pivoting in any plane other than the select plane; and a collet including a collet body configured to be disposed in the anchor seat , the collet body defining an upper end and an expandable lower end , the expandable lower end configured to receive a head of the bone anchor , characterized in that the bone anchor assembly is configured such that inner guide surfaces of the pair of guide walls do not contact the collet when the bone anchor is received in the collet and the collet is seated in the anchor seat . <EOS>