An instrument assembly includes a downtube having a tubular body with a proximal end and a distal end. In one embodiment, the proximal end forms an opening for receiving a surgical tool, and the distal end forms at least two attachment members for attaching to a bone anchor. The tubular body forms a hollow passage extending from the proximal end to the distal end. The assembly includes a key for insertion into the proximal end of the downtube to attach the downtube to a bone anchor. The key includes an engagement end configured to engage the at least two attachment members when the key is inserted into the downtube. The key is rotatable within the tubular body between a clamping orientation and a releasing orientation.
