By Gene Lee and Danielle Grant-Keane ​​ ( September 15, 2017, 4:52 PM EDT) -- Inter partes reviews came into effect in September 2012 as part of the enactment of the America Invents Act of 2011. In those five years, IPRs have had a radical effect on patent disputes in the United States. IPRs are being filed at a rate of well over 1,500 per year, making the Patent Trial and Appeal Board the most popular venue for litigating patent disputes. Notwithstanding their popularity, it is worthwhile to ask whether IPRs are achieving their intended policy goals and at what cost their popularity comes. Estoppel is one of the most important risks that most petitioners face when pursuing IPRs. Courts and the board have clarified the boundaries of estoppel, but questions remain. The importance of IPRs has caused the U.S. Supreme Court to take notice, with the court deciding one case and agreeing to hear two more in the past several years, including one case that questions the constitutionality of IPRs....