A bone anchor includes a shank for anchoring to a bone and a receiving part for coupling a rod to the shank. The receiving part includes a base and two legs each including a first portion extending away from the base, and a second portion extending from the first portion further away from the base, and an elongate extension member extending from the second portion still further away from the base. An internal engagement structure is provided on the first and second portions for engaging a fixation device. A breakaway portion facilitates breaking away of the second portion. The extension members are at least as long as the second portions. The second portions are stiffer than a region of the extension members adjacent the second portions.
