A system includes a bone anchor including a shank having a first end, a second end, a channel extending through the shank with a channel axis, and an internal advancement structure having at least a portion being closer axially to the second end than to the first end, and an elongate instrument configured to extend through the channel, with a tip portion and an external advancement structure configured to cooperate with the internal advancement structure. Engagement between the advancement structures facilitates holding of the tip portion at a first axial position relative to the bone anchor and movement of the tip portion from the first axial position towards both the first and second ends of the shank. The tip portion and the external advancement structure of the elongate instrument are insertable into and removable from the channel via the first end of the shank.
