Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co. — Flexible Bar Governs Prosecution History Estoppel

Background

Festo Corporation holds two patents on magnetically coupled rodless cylinders — industrial pneumatic devices used in manufacturing equipment ranging from sewing machines to theme park rides. During prosecution of both patents before the U.S. Patent and Trademark Office, Festo amended its claims to narrow them in response to the examiner’s rejections. Years later, Festo sued Shoketsu Kinzoku (SMC) for infringement, alleging that SMC’s products, while not literally falling within Festo’s narrowed claims, were nonetheless equivalent to what Festo had claimed.

The doctrine of equivalents allows a patentee to reach beyond the literal words of the claims to capture devices or processes that perform substantially the same function in substantially the same way to achieve substantially the same result. But the doctrine of prosecution history estoppel limits this reach: when a patentee narrows a claim during prosecution to secure allowance, the patentee may be estopped from later asserting that the surrendered territory is covered by the doctrine of equivalents.

The Federal Circuit, sitting en banc, adopted a strict “complete bar” rule: any narrowing amendment made for a reason related to patentability completely and irrebuttably bars the patentee from asserting equivalents for the amended element. The Supreme Court granted certiorari to evaluate this absolute rule.

The Court’s Holding

Justice Kennedy, writing for a unanimous Court, rejected the Federal Circuit’s complete bar but also rejected Festo’s position that estoppel should rarely apply. The Court instead adopted a middle path: a narrowing amendment for a patentability reason creates a rebuttable presumption that the patentee surrendered all equivalents within the territory between the original and amended claim scope.

Critically, the Court identified three circumstances in which the patentee can overcome the presumption and reclaim some equivalents: (1) the equivalent was unforeseeable at the time of the amendment, so the patentee could not reasonably have been expected to claim it; (2) the amendment bore only a tangential relation to the equivalent in question — it was made for a purpose unrelated to the element at issue; or (3) there is some other reason suggesting the patentee could not reasonably have been expected to describe the equivalent during prosecution.

The Court emphasized the importance of the public notice function of patents — competitors must be able to rely on the prosecution history to understand the scope of what is claimed — but also recognized that rigid rules can work injustice when inventors cannot anticipate every possible equivalent at the time they draft and amend claims.

Key Takeaways

• A narrowing claim amendment made for a patentability reason creates a presumption of surrender of all equivalents within the amended scope — but the presumption is rebuttable, not absolute.
• A patentee can overcome the presumption by showing the equivalent was unforeseeable, the amendment was tangential to the equivalent, or some other reasonable basis for not claiming the equivalent during prosecution.
• The Federal Circuit’s complete bar rule — under which any narrowing amendment categorically forfeits all equivalents — was rejected as too rigid.
• Patent prosecutors should document the specific, limited reasons for any claim amendments, as this record will determine the scope of estoppel and the available rebuttals.

Why It Matters

Prosecution history estoppel is one of the most practically significant doctrines in patent law — it determines how much room a patent gives its owner to capture infringers who design around the literal claim language. The Festo decision struck a careful balance between two competing values: the public’s right to rely on the prosecution record and know what territory is open, and the inventor’s right to protect the genuine scope of the invention against slightly-modified copiers.

For patent practitioners, Festo changed how claims are amended and prosecution is managed. Knowing that narrowing amendments create a presumption of surrender encourages drafters to provide explicit explanations whenever they amend claims — to preserve the ability to argue that a particular equivalent was unforeseeable or that the amendment was tangential. For litigants, it means that equivalents arguments after amendments must carefully engage with the prosecution record rather than simply asserting the substance of the invention was the same.