top of page
  • Writer's pictureBrain Booster Articles


Author: Priyanka Gehlot, B.A.,LL.M.

The doctrine of equivalent

A patent breach is capable of being recognized in one of two ways. primary, if every ingredient and constraint of a patent claim, correctly construed, is established in the accused piece of equipment, the claim is supposed to interpret on the accused piece of equipment, & there is a factual violation. If there is no factual violation, the violation may still be established by the application of the “doctrine of equivalents.” Under this approach, which in its contemporary form is planned to avoid “fraud on a patent”, a procedure or creation may be detained to infringe if it performs-

  1. Substantially the same function.

  2. In substantially the same way.

  3. To give substantially the same result.

Legal Bars to the Application of the Doctrine of Equivalents

“Besides, legal equivalency requires the patent owner's assertion of infringement to fall outside various legal defences to the doctrine.[1] Legal bars define an area or products that lie outside the scope of the patent. They do not define what is within the scope of a patent; that is the function fulfilled by the test for equivalents. The legal bars and the test do not create one co-extensive line dividing what is and what is not equivalent. An element of an accused product that is not placed outside the range of equivalents by a legal bar might nonetheless fail to meet the test for equivalents.” On the other hand, an ingredient that meets the check for equivalent might be located outside the range of the doctrine of equivalents by a lawful bar. Therefore, in order to conclude if a picky part is equivalent to a claim limitation, the examination for equivalents must be useful, and the legal bars to equivalents must be examined. The Courts have renowned four legal bars:

  1. “prosecution history estoppels”

  2. “public dedication”

  3. “after- arising equivalents for means-plus-function claims” and

  4. “prior art patentability”.

In this paper, the researcher has explained only one limitation i.e PROSECUTION HISTORY ESTOPPEL.

Interpretation of Prosecution History Estoppel

The value of enlargement of a patentee's privileges enabled beneath the doctrine of equivalents is ambiguity. Innovators and competitors are left with no obvious suggestion of what is and are not covered by a patent. One useful basis of simplicity has been the prosecution history of the patent. Prosecution history estoppel dictates that the claims of a patent must be interpreted in the light of prosecution history, or the procedure during which the USPTO issued the patent based on a submitted application. More exclusively, the claims must be interpreted in the light of their rejections, cancellations and amendments. A patentee is estopped from asserting equivalents for a claim element when, for the reason of patentability, he has amended a claim in a way that surrenders the subject matter that he is now alleging is an equivalent.[2]

As an estoppel doctrine, prosecution history estoppel prevents a plaintiff from denying previous access when a new has relied upon it. In the framework of patent prosecution, the admission is the applicant's give up of claim scope to acquire a patent.[3] Prosecution history estoppel applies both to claim amendments to conquer rejections and to arguments submitted to obtain the patent.[4] therefore, the extent of the surrender, in general, must be resolute before the estoppel applies.

The Court in Warner-Jenkinson Company, Inc. v. Hilton Davis Chemical Co.,[5] illustrious that prosecution history estoppel mainly often arises when the applicant narrows a claim to overcome a rejection based on the prior art. It was held:

“Prosecution history estoppel continues to be available as a defence to infringement, but if the patent holder demonstrates that an amendment required during prosecution had a purpose unrelated to patentability, a court must consider that purpose to decide whether an estoppel is precluded.[6] The Court stated that the better rule would allow the patent holder to demonstrate that an amendment was made for a purpose unrelated to patentability.[7] If, however, the patent holder is unable to establish such a purpose, the court should presume that prosecution history estoppel applies”.

Prosecution history estoppel has expected a huge pact of interest from the Courts freshly. This slab prevents a patent owner from reclaiming, through the doctrine of equivalents, subject material that was agreed up for the period of prosecution of the patent. A claim alteration, claim termination, or dispute in support of patentability can form estoppel. A claim alteration creates an assumption that the altar was made for the reason of patentability. This assumption is refutable if the patent holder can demonstrate that the modification was made for reasons other than patentability.

Prosecution history estoppel is strongly linked to the notice function of the patent. “The arguments and amendments made during prosecution of the patent serve as sources of notice as to what the patent covers in addition to the notice provided by the issued patent and its claims. Prosecution history estoppel has been treated as a way to limit the range of equivalents, and not as a method to eliminate the doctrine of equivalents as a form of infringement.[8] In keeping with this tradition, prosecution history estoppel should remain subservient to the doctrine of equivalents.” This limitation has been altered by Festo Corp v Shoketsu Kinzoku Kogyo Kabushiki Co Ltd.[9]

Impact of Festo on Prosecution History Estoppel

In Festo, the Supreme Court of the United States, while continuing its support for the doctrine, unanimously agreed with the decision of Federal Circuit on the point “that prosecution history estoppel may apply any time an amendment is made to the application for reasons of patentability, not just when the amendment is made to avoid the prior art”.

On the other hand, the Court discarded an effort by the Federal Circuit to harshly limit the submission of the doctrine of equivalents in situations where prosecution history estoppel applies. It apprehended that opinion exists that prosecution history estoppel bars a verdict of likeness, but the patentee may deny that presumption. In sort to refute the supposition, a patentee has to demonstrate that at the instance of the alteration individual expert in the skill could not rationally be anticipated to have drafted a claim that would have exactly encompassed the suspected equivalent. The Court further held that not only is the burden on the patentee to show that an amendment was not made for patentability, but he or she must also show that the amendment does not surrender the particular equivalent in question.

The Supreme Court is renowned that usually a patentee's decision to narrow his claims through amendment should be presumed to be a general disclaimer of the territory between the original claim and the amended claim’. However, even if a patentee narrows a claim, he may rebut the presumption by showing that ‘at the time of the amendment one skilled in the art could not reasonably be expected to have drafted a claim that would have encompassed the alleged equivalent. Specifically, the Court states that the patentee can rebut the presumption that prosecution history estoppel bars a finding of equivalents if:

“The equivalent may have been unforeseeable at the time of the application; the rationale underlying the amendment may bear no more than a tangential relation to the equivalent in question, or there may be some other reason suggesting that the patentee could not reasonably be expected to have described the insubstantial substitute in question.”

In this regard, it is submitted that the complete bar would have had enormous consequences if it had been allowed to stand. Very few patents are issued in which the claims are not amended during prosecution.


“The doctrine of equivalence widens the scope of a patent and prevents any infringement made in the garb of an insubstantial alteration or substitution. The doctrine certainly strikes a balance between a fair scope for the patent and the notice the patent provides for the public, along with a balance between incentives to innovate and the costs of uncertainty.” The doctrine is further justified in that it is not absolute but well guided by the legal bars imposed on it, which see to it that “the doctrine does not take on a life of its own, unbounded by the patent claim’ providing for protection not within the scope of the patent, and thereby eliminating an important function of patents: clear public notice of patentee's scope of the exclusive right.”

[1] Festo Corp v Shoketsu Kinzoku Kogyo Kabushiki Co Ltd,535 US 722, the various legal limitations on the application of the doctrine of equivalents are to be determined by the court, either on a pretrial motion for partial summary judgment or on amotion for judgment as a matter of law at the close of the evidence and after the jury verdict.

[2] Miller Jennifer, Festo: Blessing to patent holders or the thorn in their sides? Duke Law & Technology Review, (2002) 17 para 11

[3] Festo Corp v Shoketsu Kinzoku Kogyo Kabushiki Co Ltd, 234 F 3d at 619-20.

[4] Townsend Eng'g Co v HiTec Co, 829 F 2d 1086, 1090 (Fed Cir 1987) (quoting Hughes Aircraft v United States, 717 F 2d 1351, 1362 (Fed Cir 1983)).

[5] 520 U.S. 17 (1997)

[6] ibid

[7] Hilton Davis Chemical Co v Warner-Jenkinson Co Inc, 520 US at 33. Patent claims serve both a definitional and a notice function, which is best served by placing the burden on the patent holder to establish the reason an amendment was made during patent prosecution

[8] Haynes Int'l Inc v Jessop Steel Co, 8 F 3d 1573, 1581 (Fed Cir 1993) (Newman J, concurring) (‘The broadening of prosecution history estoppel is a corollary to the narrowing of the doctrine of equivalents’).

[9] 535 US 722, 731-32 (2002).


bottom of page