A bone anchor assembly, comprising: a bone anchor having a proximal head and a distal bone engaging portion; and a receiving member having a proximal end having a first bore, a distal end opposite the proximal end and having a second bore, and a recess for receiving a spinal fixation element in communication with the first bore, the second bore being sized to receive the head of the bone anchor, the receiving member having a substantially U-shaped cross-section defined by two legs (124A, 124B) separated by the recess, each leg being free at the proximal end of the receiving member, wherein a first groove is formed on an exterior surface of the proximal end of a first one of the legs and a second groove is formed on an exterior surface of the proximal end of a second one of the legs, the first groove having a proximal surface and a distal surface opposite the proximal surface and the second groove having a proximal surface and a distal surface opposite the proximal surface, characterised 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 relative to a line that is oriented perpendicular to a longitudinal axis 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 angled in the direction of the distal end of the receiving member so that the surfaces decline in a radially inward to radially outward direction. a bone anchor having a proximal head and a distal bone engaging portion; and a receiving member having a proximal end having a first bore, a distal end opposite the proximal end and having a second bore, and a recess for receiving a spinal fixation element in communication with the first bore, the second bore being sized to receive the head of the bone anchor, the receiving member having a substantially U-shaped cross-section defined by two legs (124A, 124B) separated by the recess, each leg being free at the proximal end of the receiving member, wherein a first groove is formed on an exterior surface of the proximal end of a first one of the legs and a second groove is formed on an exterior surface of the proximal end of a second one of the legs, the first groove having a proximal surface and a distal surface opposite the proximal surface and the second groove having a proximal surface and a distal surface opposite the proximal surface, characterised 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 relative to a line that is oriented perpendicular to a longitudinal axis 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 angled in the direction of the distal end of the receiving member so that the surfaces decline in a radially inward to radially outward direction. 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 relative to a line that is oriented perpendicular to a longitudinal axis 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 angled in the direction of the distal end of the receiving member so that the surfaces decline in a radially inward to radially outward direction.