An adapter device for connecting an instrument to a bone anchor includes an elongate main portion having a longitudinal axis, a first end with a surface that is directly engageable with a head of the bone anchor, an opposite second end, and a channel that extends longitudinally through the main portion, and a holding member having an inner surface forming a seat for the head. The holding member is movable over a majority of the main portion, and is configured to be held at a position where a first portion of the seat is directed towards the second end of the main portion to hold the head in the seat against the first end of the main portion, while the holding member engages another part of the adapter device to keep the first portion of the seat spaced apart axially from the first end of the main portion.