A bone anchor assembly, comprising: a bone anchor (104) having a proximal head and a distal bone engaging portion； and a receiving member (108) having a substantially U-shaped cross-section defined by two legs (124A, 124B), wherein a groove (130A, 130B) is formed on an exterior surface of the proximal end of each of the legs, the grooves each having a proximal surface (134) and a distal surface (132) opposite the proximal surface, the proximal surfaces each being oriented at angle (C) between approximately 5 degrees and approximately 30 degrees relative to an orthogonal line (142) that is oriented perpendicular to a longitudinal axis of the receiving member.
