Re-treatment due to missed Mb2 is probably the most commonly referred procedure that I do. And almost every time, I leave the other canals alone. in this case, the referral did endo and placed a new crown, after which the patient had pain followed by a sinus tract. G.p cone tracing of the sinus tract led to the mesial root. In this case, the mb1 merged with the mb2. So, I re-treated the mb1 too. We managed to do this with the very conservative opening, thereby preserving the new crown.