Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., Ltd.
535 U.S. 722 (2002)
Stoll invented a kind of
piston and tried to get two patents on it. When the USPTO rejected the
original applications, Stoll went back and added a new limitation, and the
patents were approved.
The limitation that the
patent specified used a pair of sealing rings and that the outer shell must
be made out or magnetizing metal.
Stoll sold the patent to Festo
who began manufacturing the pistons. When SMC entered the market with a
very similar product, Festo sued for infringement.
SMC argued that their piston
used a different kind of sealing ring and was made out of non-magnetizing
metal, therefore it was outside of the scope of Stoll's patent.
Festo argued that the
differences were very minor, and so it was still infringing via the Doctrine
The Doctrine of
Equivalents says that a product or
process that does not literally infringe upon the express terms of a
patent claim may nonetheless be found to infringe if there is an equivalence between the elements of the accused product
or process and the claimed elements of the patented invention.
See Graver Tank v. Linde
Air Products Co. (339 U.S. 605
The Trial Court found for
Festo. SMC appealed.
The Appellate Court reversed.
The Appellate Court found
that Festo could not argue the Doctrine of Equivalents because of Prosecution History
Estoppel says that if you added a
new limitation to get your claim approved, then you can't go back later
and make an equivalents argument later.
The Court found that Prosecution
History Estoppel was a complete bar,
and there was no wiggle room for Festo.
The US Supreme Court vacated
The US Supreme Court found
that Prosecution History Estoppel
is not a complete bar, and that the amendments to the original patent
must be viewed in light of the reason for the change.
So, if the amendments were
made to the patent application just to clarify a translation, then there
would be no bar. On the other hand, if the amendments purposely limited
the claims in order to get around some prior art, then they would be a bar to asserting
For example, if you have a
chemical process that you original claim works up to a pH of 10, but
there is prior art at pH 10, so
you limit your claim to pH of 9 or lower, you can't then sue someone
for making a process at pH 10.
The Court found that you
have to look at the prosecution history (letters between the USPTO and
the inventor, the first office action, lab notes, etc), to determine why
someone amended their claim.
The Court found that there
is a presumption that an amendment that limits a claim will bar bringing
infringement claims via Prosecution History Estoppel. However, in the very narrow case that the
infringer is using some variation that was unforeseeable at the time the
original patent was filed, then the Doctrine of Equivalents might still apply.
"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 those cases the patentee can
overcome the presumption that prosecution history estoppel bars a finding of equivalence."
The Court remanded back to
the Trial Court to determine the exact reasons for why Stoll amended his
The Courts eventually
determined that Stoll amended his patents to get around the prior art, and that SMC's design was not unforeseeable.
Therefore Festo could not use the Doctrine of Equivalents because of Prosecution History
Is there really any difference
between what you originally put into the patent application and what you
add by amendment?
Using the above example, if
you knew about the prior art and
filed your original patent at pH 9, you could sue someone for making a process at pH 10.
Does that make sense?
According to my professor,
this case is the "Mani-Festo" for how to understand prosecution