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-shape 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 there is formed a second groove (130B) on an outer surface of the proximal end of a second (124B) of the legs, the first groove having a proximal surface (134) and a distal surface (132) opposite the proximal surface, and having the second groove 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 proximal surface of the second groove are each oriented at an angle between approximately 5 degrees and approximately 30 degrees with respect to a line (142) that is oriented perpendicular to a longitudinal axis (118) of the receiving member, and the proximal surface of the first groove and the distal surface of the first groove, and the proximal surface of the second groove and the distal surface of the second groove are each inclined in the direction of the distal end of the receiving member, so that the surfaces descend in a direction radially inwards to a direction radially outward.Un conjunto para anclaje de huesos, que comprende: un anclaje de huesos (100) que tiene una cabeza proximal (104) y una porción (106) de acoplamiento al hueso distal； y un miembro (108) de recepción que tiene un extremo proximal (110) que tien
