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, wherein the first groove has a proximal surface and a distal surface opposite the proximal surface and the second groove has a proximal surface and a distal surface opposite the proximal surface, characterised in that: the proximal surface of the first groove and the distal surface of the first groove are each generally oriented at an angle greater than 0 degrees relative to a line that is oriented perpendicular to a longitudinal axis of the receiving member, the proximal surface of the second groove and the distal surface of the second groove are each generally oriented at an angle greater than 0 degrees relative to a line that is oriented perpendicular to the longitudinal axis of the receiving member, parts of the proximal surfaces of the first and second grooves which are further from the longitudinal axis are more distal than parts of the proximal surfaces which are nearer the longitudinal axis, and parts of the distal surfaces of the first and second grooves which are further from the longitudinal axis are more distal than parts of the distal surfaces which are nearer the longitudinal axis. 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, wherein the first groove has a proximal surface and a distal surface opposite the proximal surface and the second groove has a proximal surface and a distal surface opposite the proximal surface, the proximal surface of the first groove and the distal surface of the first groove are each generally oriented at an angle greater than 0 degrees relative to a line that is oriented perpendicular to a longitudinal axis of the receiving member, the proximal surface of the second groove and the distal surface of the second groove are each generally oriented at an angle greater than 0 degrees relative to a line that is oriented perpendicular to the longitudinal axis of the receiving member, parts of the proximal surfaces of the first and second grooves which are further from the longitudinal axis are more distal than parts of the proximal surfaces which are nearer the longitudinal axis, and parts of the distal surfaces of the first and second grooves which are further from the longitudinal axis are more distal than parts of the distal surfaces which are nearer the longitudinal axis.