Festo (or more specifically, the U.S. Supreme Court case entitled Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., 535 U.S. 722 (2002) sets out the rule for when the patentee will be deemed to have surrendered equivalents as a result of earlier making a narrowing amendment. The rule is essentially as follows:
A court will first presume that any narrowing amendment offered during prosecution was occasioned by a "substantial reason related to patentability." The burden will be on the patent owner to rebut that presumption by showing that the amendment had another explanation sufficient to overcome prosecution history estoppel. Applicant may rebut this presumption with evidence showing that it was a "truly cosmetic" amendment, or a "clarifying amendment," or otherwise an amendment with no real impact on the scope of the claim.
But if the amendment was, in fact, related to patentability, the patentee then "bear[s] the burden of showing that the amendment [did] not surrender the particular equivalent in question." If the patentee cannot meet that burden, a court will presume that the patentee gave up, irretrievably, all of the subject matter excluded from the claim due to the amendment.
A court will first presume that any narrowing amendment offered during prosecution was occasioned by a "substantial reason related to patentability." The burden will be on the patent owner to rebut that presumption by showing that the amendment had another explanation sufficient to overcome prosecution history estoppel. Applicant may rebut this presumption with evidence showing that it was a "truly cosmetic" amendment, or a "clarifying amendment," or otherwise an amendment with no real impact on the scope of the claim.
But if the amendment was, in fact, related to patentability, the patentee then "bear[s] the burden of showing that the amendment [did] not surrender the particular equivalent in question." If the patentee cannot meet that burden, a court will presume that the patentee gave up, irretrievably, all of the subject matter excluded from the claim due to the amendment.
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