Should dental work be avoided during the nine days or is it ok to do?
If the work being done does not involve any specific danger, i.e. general anesthetic etc, it is permitted. Even if it does involve an element of danger, it
is prefferable to postpone it until after Tisha B’av, however if it is needed it is permitted. By the way this idea also applies to the three weeks and not only to the nine days.
Emes L’Yackov ftnt. 515, Tzitz Eliezer 12-46- (10), Ashrei Hoish O:CH 3 69-5, Mbais Halevi 13- pg. 25, Nitei Gavriel (Bein Hamitzarim) 23-8, Yalkut Yosef ( KItzur Shulchan Aruch) Dinim Hashayachim Mrosh Chodesh Av -1. It is however brought in the name of R’ Moshe Feinstein zt”l that one does not have to be makpid on this.