Patent Law Update

Profession:Kaye Scholer LLP

Supreme Court Holds That Narrowing Claim Amendment Made To Obtain a Patent-Whether To Avoid Prior Art or Not-Gives Rise To a "Flexible Bar" To Claiming Equivalents

In Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., No. 00-1543, 535 U.S.__ (May 28, 2002), the United States Supreme Court held that a patentee who narrows a claim element as a condition for obtaining a patent disavows claiming the broader subject matter of that claim element, whether the narrowing amendment was made to avoid prior art or to comply with some other patent law provision, such as 35 U.S.C. ß 112. The Supreme Court further held that making such an amendment does not necessarily serve as a complete bar to asserting the doctrine of equivalents.


Festo Corporation is the owner of two patents for an improved magnetic rodless cylinder, a piston-driven device that relies on magnets to move objects in a conveying system. Festo's patent applications were amended during prosecution. The claims of the first patent, U.S. Patent No. 4,354,125 ("the '125 patent"), were amended after the patent examiner rejected the claims under 35 U.S.C. ß 112 because the exact method of operation was unclear and some claims were drafted in an impermissible way. The claims of the second patent, U.S. Patent No. 3,779,401 ("the '401 patent"), were amended during a reexamination proceeding. In both cases, additional prior art references were made of record by the patentee.

After Festo began selling its rodless cylinder, respondent Shoketsu (also known as "SMC") entered the market with a device similar, but not identical, to the one disclosed by Festo's patents. Although SMC's device does not fall within the literal claims of either patent, Festo contended that SMC's device is so similar to Festo's claims that SMC infringes under the doctrine of equivalents. SMC asserted that Festo is estopped from relying on the doctrine of equivalents because, in SMC's view, the amendments to Festo's patents narrowed the earlier applications, thereby surrendering alternatives that are the very points of difference in the competing devices. SMC argued that, because Festo narrowed its claims in order to obtain the patents, Festo was estopped from saying that these differences are immaterial and that SMC's device is equivalent to its own.

Decision of the Courts Below

The district court held that Festo's amendments were not made to avoid prior art, and therefore the amendments were not the kind that gave rise to estoppel. The Federal Circuit Court of Appeals affirmed the lower court's decision.

The Supreme Court granted certiorari, vacated, and remanded the Federal Circuit's decision in light of the Supreme Court's intervening decision in Warner-Jenkinson Co. v. Hilton Davis Chemical Co., 520 U.S. 17 (1997), which held that prosecution history estoppel is a legal limitation on the doctrine of equivalents. The Court of Appeals ordered rehearing en banc.

The en banc court reversed, holding that prosecution history estoppel barred Festo from asserting that the accused device infringed its patents under the doctrine of equivalents. The court held that estoppel arises from any amendment that narrows a claim to comply with the Patent Act, not only from amendments made to avoid prior art. Further, the Federal Circuit held that, when estoppel applies, it stands as a "complete bar" to any claim of equivalence for the element that was amended.

The Supreme Court's Decision

Granting certiorari, the Supreme Court reversed and remanded the Federal Circuit's decision, and thereby rejected the holding that prosecution history estoppel concerning one or more claim elements effects a complete bar to the doctrine of equivalents with respect to those elements. The Court recognized uncertainty for competitors resulting from the doctrine of equivalents "as the price of ensuring the appropriate incentives for innovation."

The Supreme Court first considered the relation between two competing doctrines of U.S. patent law: prosecution history estoppel, which requires that the claims of a patent be...

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