The doctrine of prosecution history estoppel, formally known as “File Wrapper Estoppel”, holds that the inventor’s statements and amendments made during the prosecution of the patent application before the Patent Office can be used to narrow the scope of the claims of the patent.

Proponents of the doctrine argue that it prevents the inventor from speaking out of both sides of her mouth because she cannot argue during an infringement court action that which she expressly disavowed during prosecution of the patent application. Its adherents also point out that it helps determine the true intention of the inventor’s use of certain claim language. Its detractors, on the other hand, argue that the doctrine unnecessarily complicates and lengthens patent litigation, and pays too little attention to the patent itself at the expense of the “slag produced while crafting the patent.”

File wrapper estoppel is currently an artefact of US patent law, and has no application in Canada. Indeed, its potential intrusion into Canadian patent law was soundly rejected by the Supreme Court of Canada in the seminal decision Free World Trust. Justice Binnie had this to say:

In my view, those references to the inventor’s intention refer to an objective manifestation of that intent in the patent claims, as interpreted by the person skilled in the art, and do not contemplate extrinsic evidence such as statements or admissions made in the course of patent prosecution. To allow such extrinsic evidence for the purpose of defining the monopoly would undermine the public notice function of the claims, and increase uncertainty as well as fuelling the already overheated engines of patent litigation. The current emphasis on purposive construction, which keeps the focus on the language of the claims, seems also to be inconsistent with opening the Pandora’s box of file wrapper estoppel. If significant representations are made to the Patent Office touching the scope of the claims, the Patent Office should insist where necessary on an amendment to the claims to reflect the representation.

Settled law then, right? Not quite.

In Distrimedic inc. v. Dispill Inc., the Federal Court of Canada upheld the prohibition against resorting to file wrapper estoppel to construe (i.e. interpret) the claims of a Canadian patent. The Court does, however, draw “an inference” from an amendment made to the claims during prosecution. Although not determinative of the outcome of the case, this recourse to the prosecution history will raise eyebrows amongst many patent agents and cause them to reflect upon the wording they use to overcome claim rejections.

Dispill owns Canadian patent 2,207,045 (CA‘045) related to pill containers. Much of the case turned on the “positioning means” recited in some of the independent claims. Specifically, the Court needed to determine whether this positioning means was an essential element of the invention. This question was important because the Court found that Distrimedic’s competing product did not have a similar positioning means. Infringement can only occur if the competing product contains all of the essential elements of the claim in question. Therefore, if the Court found the positioning means to be essential, than Distrimedic’s product would not infringe the claims of CA‘045. If, however, the Court found that the positioning was a non-essential element, then perhaps Distrimedic’s product did infringe the claims.

In concluding that the positioning means was an essential element, the Court referred to a letter submitted by Dispill to the Patent Office during prosecution of CA’045. The letter was submitted in response to objections made by the patent Examiner. In this letter, Dispill amended the claims, and argued that the prior art reference cited by the Examiner did not teach the invention as defined by the new claims because it did not disclose the recited positioning means. Interestingly, Dispill described the positioning means in the letter as “the key feature of the present invention”.

Dispill argued that recourse to such extrinsic evidence as this response letter to construe the essentiality of a claim element was specifically prohibited by the Supreme Court. The Court agreed, but added:

I am not convinced that the letter […] falls squarely within the compass of that exclusion. While statements or admissions made in the course of patent prosecution shall not be used for the purpose of interpreting a claim, this is not what the Court is called upon to do in the case at bar. A change in the wording of a claim as a result of an objection from the Patent Office is an objective fact from which an inference may be drawn, and is not the same as representations made to the Patent Office. A purposive construction should obviously focus on the wording of a claim, obviously, but this is a far cry from saying that nothing else should be considered.

In other words, the Court is not relying upon the arguments or statements made by Dispill in the letter (i.e. its “representations”). Such a reliance would require applying the prohibited doctrine of file wrapper estoppel to interpret the claims. Instead, the Court relies upon the mere fact that a change occurred in the wording of the claims, and upon the fact that this change occurred in response to a rejection of the claims. A fine distinction indeed.

It should be noted that the Court’s findings on the doctrine were one factor among many that led it to conclude that Distrimedic did not infringe the claims of CA’045. In total, the Court’s discussion of file wrapper estoppel made up only five paragraphs of the 350 paragraph decision. Still, this decision of the Federal Court teaches us that the “slag” produced while negotiating with the Patent Office may be used to determine the scope of the claims, despite the express prohibition in Canada on applying the doctrine of file wrapper estoppel. While the Court did not specifically rely on the “statements or admissions made in the course of patent prosecution”, it did employ “extrinsic evidence (i.e. the letter) for the purpose of defining the monopoly” of the claims.

The wise inventor and patent agent will therefore think twice before commenting on the essentiality of elements of the claims during prosecution.