A bone anchor having a bone engaging portion adapted for anchoring to bone, and an elongate post portion extending from the bone engaging portion and arranged along a longitudinal axis. The elongate post portion includes a proximal end region connected to the bone engaging portion, a break-off region extending axially from the proximal end region, and a guide region extending axially from the break-off region and configured for sliding engagement with an implant, and with the break-off region including a pre-defined fracture initiator zone to facilitate initiation of a fracture therealong upon application of a sufficient torsional force onto the break-off region and removal of a length of the elongate post portion distal of the fracture.
