Unwrapping File Wrapper Estoppel: Some Effects of Bill C-86

Unwrapping File Wrapper Estoppel: Some Effects of Bill C-86

To the delight of patent litigators everywhere, the doctrine of file wrapper estoppel is about to be implemented in Canadian Patent Law. After more than 18 years since Justice Binnie rejected the doctrine in Canada in his ruling in Free World Trust v Electro Santé Inc., it will come into force via Bill C-86, likely in late 2019. While this move may help “fuel the overheated engines of patent litigation”, it is not likely to help many other members of the patent community.

File Wrapper Estoppel (or “Prosecution History Estoppel”) is a concept from American jurisprudence that was best described by the Supreme Court of the United States in Festo Corp.  It is an equitable power of the court designed to hold an inventor to the representations they made during the patent application process. It does this by allowing a patent-holder’s competitors to rely on the patent’s prosecution history to estop the patentee from recapturing subject matter surrendered by amendment as a condition of obtaining the patent.

The estoppel arises from any amendment that narrows a claim to comply with any patent legislation requirement, and when it applies it bars the doctrine of equivalence (the doctrine which deems any modification to an invention that performs an equivalent function to be infringement) for the element that was amended. When estoppel applies, it does not completely bar the patent-holder from claiming infringement of their amended claims, but only for the aspects of those claims that were amended.

There are many ways in which file wrapper estoppel can lead to increased litigation. Determining what the amended claim does or does not cover, through the lens of the patent prosecution dialogue, is where the largest amount of litigation arises.  There are also cases in which estoppel would normally arise, but doesn’t because the alleged infringing equivalent was unforeseeable at the time of the amendment, a fact which must be proven in court, through litigation.

It’s for the above reasons that Justice Binnie completely rejected the concept of file wrapper estoppel in Canada in his ruling in Free World Trust v Electro Santé Inc. He confined the scope of a patent to the “objective manifestation of that intent in the patent claims, as interpreted by the person skilled in the art”, rejecting “extrinsic evidence such as statements or admissions made in the course of patent prosecution”.  Justice Binnie reasoned that allowing file wrapper estoppel would weaken the public notice function of a patent’s claims, and that it would also introduce an uncertainty that could only be resolved with litigation.

While it is true that file wrapper estoppel has the ability to tie an inventor to their correspondence during patent prosecution, it is much simpler (as Justice Binnie stated) to just have the claims accurately reflect what was agreed upon in the negotiations instead of determining it through litigation. File wrapper estoppel diminishes the importance of the claims within the finalised and published patent in the determination of an invention’s limits, while subsequently elevating the importance of “rough drafts” of the patent and their surrounding correspondence. This has the very real effect of detracting from the primacy of the claims, and making the monopoly covered by the patent more uncertain.

The changes brought by Bill C-86 with the introduction of file wrapper estoppel will affect the patent prosecution process at every stage. Inventors will need to turn their minds to not just the prior art, but also to the prosecution histories of the prior art. Patent prosecutors will need to be much more exact and careful in their language during the prosecution process. Patent owners will have less certainty than before as to what their amended claims actually cover, and because of this, less certainty as to which acts are infringements of their patents. Potential patent infringers will now have a new tool of defence to any accusations of infringement. And, of course, the benefit of the newly-created doubt will go to the patent litigators, who will be needed to define the scope of the claims.


Written by Keenan Fast, IPilogue Editor and JD Candidate at Osgoode Hall Law School.