Not familiar with this idea, I searched and found this
https://www.gearhartlaw.com/2016-9-8-a-way-to-defeat-a-troll...
IIUC, inter partes review is a relatively recent (9 y.o.?) patent law practice that allows 3rd parties to challenge patents directly to the patent office any time between patent award and expiry. I had thought there was only a short period this could be done after award of the patent.