Root apex resection is done when the conventional root canal treatment is not sufficient to save the infected tooth. A new (not older than 6 months) root canal filling is a precondition of the treatment. If the root canal filling is older it should be replaced.
We also do root apex resection if the root canal treatment is not perfect.
The main reasons are:
irregular anatomic conditions (tight, curved or branching root canal)
the X-ray shows infection on the pulp 6 months after the root canal treatment
The perfect function of the tooth could be achieved in 4-6 months with successful treatment.