Patent Drafter Estoppel: Why Didn't Sage Products Create a New Foreseeability Limitation on the Application of the Doctrine of Equivalents?


This article reviews the 1997 Federal Circuit Case of Sage Products v. Devon and the case law that has followed it. There is some belief among patent practitioners that Sage Products created a new legal doctrine limiting the application of the doctrine of equivalents in patent infringement cases. The new doctrine, sometimes referred to as “patent drafter estoppel,” would bar the application of the doctrine of equivalents any time an accused equivalent structure should have been foreseen by a reasonable patentee. Federal Circuit case law since Sage Products has diverged into two lines of thought: one that supports the thinking that Sage Products did create a new limitation on the doctrine of equivalents, and one supporting the belief that Sage Products was an unremarkable decision merely applying existing limiting doctrines to the doctrine of equivalents. The distinction is not unimportant, since the doctrine of equivalents is used very frequently—in fact, almost universally—in patent infringement litigation. Thus, the existence of a new and poorly understood legal doctrine barring the application of the doctrine of equivalents in many cases would have an extraordinary impact on the practice of patent law. This article reviews the cases decided since Sage Products and determines that the Federal Circuit has not actually created any new foreseeability limitation on the doctrine of equivalents. Further, the article analyzes the purported new doctrine in light of United States patent law policy and determines that the patent system is better off without such a doctrine.


Intellectual Property Law

Date of this Version

February 2006