A bone anchor assembly, comprising: a bone anchor (100) having a proximal head (104) and a coupling portion (106) to the distal bone； and a receiving member (108) having a proximal end (110) having a first hole (116), a distal end (112) opposite the proximal end and having a second hole (120), and a recess (114 ) to receive a spinal fixation element in communication with the first hole, the second hole being sized to receive the bone anchor head, the receiving member having a cross-section with a substantially U-shaped defined by two legs (124A, 124B) separated by the recess, each leg being free at the proximal end of the receiving member, in which a first groove (130A) is formed on an outer surface of the proximal end of a first (124A) of the legs and is formed a second groove (130B) on an outer surface of the proximal end of a second (124B) of the legs, wherein the first groove has a proximal surface (134) and a distal surface (132) opposite the proximal surface and the sec The groove has a proximal surface (134) and a distal surface (132) opposite the proximal surface, characterized in that: the proximal surface of the first groove and the distal surface of the first groove are generally oriented each with an angle greater than 0 degrees with respect to a line (142) that is oriented perpendicular to a longitudinal axis (118) of the receiving member, the proximal surface of the second groove and the distal surface of the second groove are generally oriented each with an angle greater than 0 degrees with with respect to a line (142) that is oriented perpendicular to the longitudinal axis of the receiving member, parts of the proximal surfaces of the first and second grooves that are furthest from the longitudinal axis are more distal than parts of the proximal surfaces found closer to the longitudinal axis； and parts of the distal surfaces of the first and second grooves that are furthest from the longitudinal axis are more distal than parts of the distal surfaces
