Abolade Owoeye is at Osgoode Hall and is taking the Patent Law Course.

Prosecution history estoppels also known as File-wrapper estoppel is a term used when an inventor has filed a patent application, but during prosecution of that patent application, the inventor amends the application in order to overcome a prior art. That inventor is subsequently estopped from claiming infringement of the claims that were amended at a later time based on this doctrine.

Different supreme courts of different countries either favor this doctrine or reject it. The United States Supreme Court blesses the use of doctrine of equivalence as enunciated in the case: Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., 535 U. S. 722 (2002).  In contrast, doctrine of equivalence is frowned upon by the Supreme Court of Canada. See, Free World Trust v. Électro Santé Inc., [2000] 2 S.C.R. 1024, 2000 SCC 66. These decisions raise the questions: should file-wrapper estoppel be employed by countries that don’t use it? Or is it against the fundamental principles of fairness and justice and does it aid towards the goal of patentability?

In Festo Corp. v. Shoketsu, the U.S. Supreme Court vacated the judgment of the Federal Circuit that created a bar to equivalent elements that had being narrowed by amendment during prosecution.  In so doing, the U.S. Supreme Court reasoned that a patentee’s decision to narrow a claim in order to comply with the Patent Act “assumes surrender of the territory between the original claim and the amended claim.” However, the Court still saw fit to place the onus on the patentee to show that the equivalents were not surrendered. Thus, it seems the U.S. Supreme Court is protective or gives deference to the patentee so that as long as the patentee can establish or submit evidence that “there was some reason why the patentee could not have recited the equivalent in the claim,” then the patentee will succeed in the infringement action.  This reasoning maybe beneficial to encourage inventors, that their patents will not be infringed merely by a conceivable equivalence. If that is the rationale, then it harmonizes with the goal of patents as expressed in the U.S. Constitution, which is to “promote the progress of science and useful arts.” 

However, Canada’s Supreme Court in Free World Trust v. Electro Sante Inc., emphatically rejected the doctrine of equivalence for previous claim negotiations with the patent office. Although the Court acknowledges that “it is obviously an important public policy to control the scope of ‘substantive infringement,’. . . [and that] a mere inconsequential variation in the device [may allow one] to appropriate the substance of the invention,” which the Court concluded will be “unfair to the public and unfair to competitors.” On the other hand, the Court did not want to “risk conferring on the patentee the benefit of inventions that he had not in fact made, but which could be deemed with hindsight to be ‘equivalent’ to what in fact was invented.”  The Court decided to restrict the patentee within the confines of their claims stating that the “Patent Act promotes adherence to the language of the claims” and “adherence to the language of the claims in turn promotes both fairness and predictability.” So, it seems the Court prefers to err by on the side of caution by not trying to construe whether the patentee conceived of an equivalent that is not stated in the claims or that has been dropped during prosecution.

It seems a more equitable balance can be achieved that is neither too lenient nor too restrictive. A mere substitution of equivalent elements may benefit an infringer, since the equivalent element is not stated in the body of claims of the patent. Also, a patentee should not be able to stretch their invention to cover every possible equivalent that they can now think of in hindsight. Nonetheless, a strict per se rule that disfavors any sort of equivalence while adhering to the strict letter of the law may not promote the spirit of the law.