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What is Prosecution History Estoppel?

Prosecution history estoppel can arise when the patentee relinquishes subject matter during the prosecution of the patent—that is, during examination of a patent application at the U.S. Patent & Trademark Office (USPTO)—either by amendment or argument. A narrowing amendment made to satisfy any requirement for patentability (under 35 U.S.C. §§ 101-103, 112, 161, or 171) may give rise to an estoppel.  When it applies, prosecution history estoppel will prevent a patentee from relying on the Doctrine of Equivalents to establish infringement based on the particular equivalents surrendered. A given claim limitation might be given only its literal scope (as properly construed) but no more. This policy allows competitors to rely on prosecution history estoppel to ensure that their own devices/processes will not be found to infringe by equivalence

The Basic Rule

The Doctrine of Equivalents is an equitable doctrine created by courts long ago. It is premised on language’s inability to capture the essence of innovation, allowing some protection beyond the literal scope of a claim where the claimed invention and the accused product or process are equivalent.  Its purpose is to “temper unsparing logic” that “would place the inventor at the mercy of verbalism and would be subordinating substance to form” and it evolved in response to situations where accused infringers attempted to “practice a fraud on a patent” by introducing “minor variations to conceal and shelter the piracy.” However, there is a tension between such concerns and the need for patents to put the public on notice of what a patent does and does not cover.

Prosecution history estoppel is about interpreting patent claims by reference to the history that led to their grant. It limits the patentee’s ability to establish infringement by equivalence to help provide certainty to competitors about the enforceable scope of a U.S. patent. Accordingly, the Doctrine of Equivalents is not available where prosecution history shows the inventor was able to capture equivalent differences in words but chose narrower language. The patentee cannot later recapture what was previously surrendered under the guise of the Doctrine of Equivalents. Where the original patent application once embraced the purported equivalent but the patentee narrowed his claims to obtain the patent or to protect its validity, the patentee cannot assert that he or she lacked the words to describe the subject matter in question. 

Estoppel Can Arise Through Amendment or Argument

Prosecution history estoppel can arise two ways: (1) by making a narrowing amendment to the claim (amendment-based estoppel) or (2) by surrendering claim scope through argument to the patent examiner (argument-based estoppel). These two possibilities are discussed in turn.

How Amendment-Based Estoppel Can Arise

The first way that prosecution history estoppel can arise is by way of a narrowing amendment to a claim during prosecution. The question of what constitutes a narrowing amendment will depend on the context of a particular patent claim. But a classic example is adding words that narrow the limitations of a claim to avoid a prior art reference cited in a rejection in an office action. As another example, which is less intuitive, courts have said rewriting a dependent claim in independent form is an amendment adding a new claim limitation, which constitutes a narrowing amendment that may give rise to an estoppel. 

How Argument-Based Estoppel Can Arise

Argument-based estoppel arises when the prosecution history evinces a “clear and unmistakable” surrender of subject matter. Argument-based estoppel most often arises when the patentee tries to convince a patent examiner that the claims of an application recite something different from the prior art. For instance, a patentee may have argued that a claim term has a meaning that is narrow enough to avoid a disclosure or teaching in a cited prior art reference. In such a situation, the patentee will likely be estopped from later asserting that the Doctrine of Equivalents encompasses what was previously argued to be outside the scope of the claimed invention. Such arguments are treated like an acknowledgement that the patentee knew the meaning of the claim language and deliberately chose narrower language in order to obtain a patent.

Scope of the Estoppel

Though prosecution history estoppel can bar a patentee from challenging a wide range of alleged equivalents used by competitors, its reach requires examination of the particular subject matter surrendered. Even where prosecution history estoppel applies, the scope of the estoppel is not always absolute. These inquiries differ somewhat between amendment-based estoppel and argument-based estoppel.

Scope of Amendment-Based Estoppel

The scope of amendment-based estoppel depends on the claim language at issue and the reason for the amendment. For instance, where the reason for the amendment was not related to avoiding the prior art, it does not necessarily preclude infringement by equivalents of that element. But an amendment to clarify the recitation of the claimed invention to satisfy definiteness requirements for patentability would give rise to an estoppel, even though there was no prior art reference prompting the change. Also, cancellation of a claim can give rise to estoppel with regard to any the claim(s) that remain. However, the reason for an amendment is often not clear from the prosecution history.

When no explanation for an amendment is provided, there is a rebuttable presumption that the Doctrine of Equivalents is not available at all.  When the purpose underlying a narrowing amendment cannot be determined—and hence a rationale for limiting the estoppel to surrender of only particular equivalents—it is presumed that the patentee surrendered all subject matter between the broader and the narrower language.  But that presumption of the amendment-based estoppel can be overcome for a particular equivalent when (1) the equivalent in question was unforeseeable at the time of the application, (2) rationale underlying the amendment bears no more than a tangential relation to the equivalent in question, or (3) the patentee could not reasonably be expected to have described the insubstantial substitute in question. 

Scope of Argument-Based Estoppel

The scope of argument-based estoppel is based on the scope of particular arguments made during prosecution. Unlike amendment-based estoppel, there is no presumption-and-rebuttal approach. Rather, the question is how far argument-based estoppel applies in the first place, if at all. The question is whether there was a clear and unmistakable argument-based surrender of particular subject matter. Courts have said that simple arguments and explanations to the patent examiner do not surrender an entire field of equivalents. But any arguments can still surrender some equivalents, even if they do not surrender all possible equivalents. This is very context-dependent. The key to this inquiry is whether a competitor would reasonably believe that the patentee’s argument had surrendered the relevant subject matter.

However, courts have said that any clear assertions made during prosecution in support of patentability, whether or not those assertions were actually required to secure allowance of a claim, may still create an argument-based estoppel. Also, once an argument is made regarding a claim term that creates an estoppel, that estoppel will apply to that term in other claims in the patent too.

Prosecution Disclaimer

Prosecution disclaimer is a separate but related concept. Rather than relating to the availability of the Doctrine of Equivalents to establish infringement, prosecution disclaimer or disavowal can limit the proper construction of a claim’s literal scope. Thus, it may affect whether or not literal infringement is found—prior to any consideration of the Doctrine of Equivalents. For example, it can arise when a patentee makes arguments during prosecution about how the pending claims differ from the prior art. This is meant to prevent patentees from opportunistically taking different (and broader) positions in court when asserting infringement than when obtaining the patent in the first place. Statements made during the prosecution of family-related applications, even foreign counterparts, may sometimes result in prosecution disclaimer as well.

Questions about prosecution disclaimer pertain to proper claim construction. Claim terms are presumed to carry their full and customary meaning unless the patentee unequivocally imparted a novel meaning to those terms either in the patent application itself (called acting as her own lexicographer) or expressly relinquished claim scope during prosecution. Just as with argument-based estoppel, prosecution disclaimers must be “clear and unmistakable” in order to limit claim scope. An ambiguous statement during prosecution will not limit a claim term’s ordinary meaning.

An important difference between prosecution disclaimer and prosecution history estoppel is when they are taken into account. Prosecution disclaimer is a matter of claim construction. That is something courts do as a first step in patent infringement analyses, without reference to the specific product or process accused of infringement. In contrast, the Doctrine of Equivalents is bound up with the subsequent comparison of the accused product or process to the asserted patent claims as properly construed. Prosecution history estoppel is a question of whether a patent owner is permitted to invoke the Doctrine of Equivalents in particular ways in the second part of infringement analysis, after claim construction. But if literal infringement is found—despite any prosecution disclaimer—there is no need to reach the Doctrine of Equivalents.

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Austen Zuege is an attorney at law and registered U.S. patent attorney in Minneapolis whose practice encompasses patents, trademarks, copyrights, domain name cybersquatting, IP agreements and licensing, freedom-to-operate studies, client counseling, and IP litigation. If you have patent, trademark, or other IP issues, he can help.