The Doctrine of Equivalents and Prosecution History Estoppel play a vital role in concluding whether or not a case of Patent Infringement has been made out. Where there is an ambiguity as to which of the two shall be applied, the Prosecution history estoppel defense has supremacy over the latter since it is one of the factors that limit the application of the doctrine of equivalents.

Understanding Prosecution History Estoppel

Words and terms in English are logically fallible, which is why they can be used and molded in different ways to convey the same meaning. Consequently, inventors often play with words while drafting the claims of a Patent Application  sometimes to exaggerate the scope of their invention and sometimes to shrink into a tiny space sufficient to tick the checklist for a patentable subject matter. It not only happens while drafting claims initially but throughout Patent Prosecution.

Prosecution history estoppel equates that the claims in a patent application shall be interpreted and read in light of their rejections, cancellations, and amendments made during the process of patent prosecution when the proprietor makes representation to the examiner or when the inventor amends to narrow the claims to escape rejection. Adding a new technical feature to an independent claim

or swapping and substituting technical features from the specification and connecting them into an independent claim, or using a subordinate claim as a rather new and independent claim are a few of the most common modes of amending the structure of the claims, and therefore, resulting to the alternation of the scope of the independent claims and dependent claims, in summary. All these submissions made during the point of prosecution give rise to estoppels. Say, if the inventor surrenders certain aspects of his original patent application, then the same may not be subjected to protection by the same inventor in the same application by stating the doctrine of equivalents.

One important exception that applies is that where an inventor amends any claim of his application for a reason, which is not directly related to patentability, the claim will not be subjected to prosecution history estoppel. Therefore, overcoming the hurdles of this doctrine can be very challenging when the patent application is widely drafted. It is highly recommended to stay right on point and address the invention narrowly and specifically in the form of claims to enable easy interpretation.

The doctrine of prosecution history estoppel is popularly acclaimed in the realm of Patent Litigation since it poses a critical defense to an assertion of infringement. The doctrine usually comes to play concerning claim construction in addition to the determination as to the scope of infringement. Prosecution history estoppel is often applied in the following instances:

  • Where there is no literal infringement;
  • Where the patent owner asserts infringement by equivalents; and
  • Where the accused infringer affirmatively asserts the defense of prosecution history estoppel, preventing the patent holder from asserting the doctrine of equivalents based on that the relevant subject matter has been disclaimed during prosecution.

How is Prosecution History Estoppel Interrelated to the Doctrine of Equivalents?

The doctrine of equivalents is a mode of safeguarding patent owners from potential imitators making minute or insubstantial changes to an existent invention to claim Patent Rights over such same/similar subject matters. An element in a claim can be said to be an equivalent if the difference between the claimed element and the equivalent is insubstantial and interchangeable according to a person skilled in the art. It means that the difference is rather insubstantial since the elements perform the same function, in the same way, to obtain the same result as the prior claimed invention.

This doctrine is a result of judicial construction that recognizes the fallibility of dictionary words. The same was recognized in the case of Graver Tank & Manufacturing Co. v. Linde Air Products Co., where the court said that “courts have recognized that to permit imitation of a patented invention, which does not copy every little detail, would be to convert the protection of the patent grants into a hollow and a useless thing. Such a limitation would leave room for the unscrupulous copyist to make unimportant changes and substitutions in the patent, which, though adding nothing, would be enough to take the copied matter outside the claim, and hence, outside the reach of the law.”

How to Best Avoid or Walk Past Prosecution History?

As per the discussion above, patentees are advised to carefully maneuver the patent prosecution stage rather than making hasty choices to reserve patent rights in their favor. The following points of consideration may prove to be beneficial in the said regard:

  • Conduct extensive, thorough prior art search and limit the scope of your invention to claim what it is rather than overarching into a broad narration of an invention.
  • If, at all, the claim requires a suitable amendment by reason unrelated to the issue of its patentability, include a statement to such effect, adding a brief examination.
  • While amending to avoid being caught in the puddle of the prior art, re-draft carefully. Make amendments only to the extent necessary to avoid the prior art rather than introducing a narrowly assembled claim.
  • Since every argument and statement can be held up against the inventor, make to-the-point arguments rather than an extensive argument.
  • Before making amends in the patent application, try other routes like an examiner interview or appeals to argue and convince the examiner that there is no actual case of prima facie obviousness.


Concluding Remarks

The argument in support of prosecution history estoppel is often deployed against a patentee since it limits the scope of infringement deduced as per the application of the doctrine of equivalents used to recapture a subject matter, which might have been surrendered during prosecution from the literal scope of a patent claim. The ambit of prosecution history estoppel extends far enough to include even those representations that have been made to a foreign Patent Office, as observed in Caterpillar Tractor Co. v. Berco, S.p.A. The scope of limiting the doctrine of equivalents is also extended to prosecution in foreign borders. Therefore, the role of the two doctrines is often intertwined; however, the prosecution history estoppel always manages to have one foot beyond the latter.