Estop Me Now: The Need for Extrinsic Evidence in Rebutting Prosecution History Estoppel via Tangential Amendments

In 2002, in Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., the U.S. Supreme Court significantly changed patent law when it rejected the Federal Circuit’s absolute bar approach to prosecution history estoppel. The decision expanded the doctrine of equivalents by creating three exceptions to prosecution history estoppel. Of the three exceptions, however, the Court provided de minimis guidance on the tangential amendment exception, leaving the Federal Circuit to sort out when a narrowing claim amendment is only tangentially related to a claimed equivalent. This ambiguity has resulted in pronounced inconsistency among Federal Circuit panels in deciphering the scope of this exception. Differing judicial approaches in interpreting gaps within the prosecution history record have further compounded this inconsistency. Moreover, the nuanced challenges that patents in complex and evolving fields pose has led to a more lenient application of the exception in these fields compared to patents in other domains. Consequently, the jurisprudence stands in a state of disarray, lacking the essential clarity needed for equitable implementation. This Note argues that to rectify this ambiguity and align with the Festo decision, patentees should be allowed to present extrinsic evidence. Relying solely on intrinsic evidence, which often offers limited and unreliable information, has overly emphasized the notice function of the prosecution record. The Warner-Jenkins presumptions, combined with extrinsic evidence to challenge estoppel through tangentiality, presents a balanced approach, promoting fairness and equity among patentees.