A system of a bone anchor and an elongate instrument is provided includinga bone anchor (1) comprising a shank (2) having a first end (3) and a second end (4) and a channel (7) extending through the shank (2) from the first end (3) to the second end (4), the channel (7) defining a channel axis (L)；wherein at or close to the second end (4) of the shank (2) an internal advancement structure (8) is provided at an inner wall of the channel (7)； andan elongate instrument (20, 30, 40, 50) that is configured to extend through the channel (7)；wherein the elongate instrument (20, 30, 40, 50) comprises a tip portion (23, 31, 42, 51) and an external advancement structure (25, 32, 43, 53) that cooperates with the internal advancement structure (8) of the shank (2) so as to permit the tip portion (23, 31, 42, 51) to be advanced in the channel (7) in the direction towards the second end (4) and to be moved back in the opposite direction towards the first end (3)；and wherein the elongate instrument with the tip portion (23, 31, 42, 51) is removable from the channel (7).
