Federal Circuit Further Defines Scope of Foreseeable Alternatives for Purposes of Prosecution History Estoppel

In Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., Ltd. 83 U.S.P.Q.2d 1385 (Fed. Cir. 2007) (Festo XIII), the Federal Circuit recently clarified some remaining issues surrounding prosecution history estoppel and the doctrine of equivalents in the most recent decision handed down in the on-going case of Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., Ltd.. In this decision, the thirteenth decision in the litigation, the Federal Circuit laid down rules for the “unforeseeability” exception to the bar raised by prosecution history estoppel.

In Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., 535 U.S. 722 (2001) (Festo VIII), the Supreme Court held that amendments to a claim for the purposes of patentability raised a complete bar to the application of the doctrine of equivalents. The Court laid down three exceptions to this rule: the alleged equivalent was unforeseeable at the time of the application for the patent, the rationale behind the amendment bore only a tangential relation to the alleged equivalent, or for some other reason suggesting that the patentee could not reasonably have been expected to have described the insubstantial substitute.

The origins of the suit stretch back to 1981, when Dr. Kurt Stoll filed an application for a magnetically coupled rodless cylinder, which can be used in automobiles. These cylinders include a piston forced through a cylinder and magnetically coupled to a driven member or driven assembly. The driven member can then be used to drive an automobile or other vehicle. When the piston moves through the cylinder, the magnetic force moves the driven member, and the driven member then drives the vehicle. The ‘125 patent was directed toward a “small gap” cylinder, which minimizes the distance between the piston and the driven member so as to increase the strength of the magnetic force.

After the examiner rejected the claims, Dr. Stoll responded by amending claim 1 to include “a cylindrical sleeve made of a non-magnetizable material”. The amendment also replaced the limitation of “sealing means” with “first sealing rings located axially outside said guide rings”. The purpose of the amendments was not explained in the response. After the amendment and a few other changes, the examiner allowed the application, which issued as U.S. Patent No. 4,354,125 (the ‘125 patent).

Festo sued SMC (Shokestu Kinzoku Kogyo Kabushiki Co.) for infringement of the ‘125 patent in August 1988. The accused device was also a magnetically coupled rodless cylinder; however, the device had two key differences. In the accused device, the sleeve is made of aluminum alloy, which is non-magnetizable material. In addition, the device contained two guide rings, but used only one sealing ring. Festo argued that the SMC device infringed under the doctrine of equivalents. SMC replied that the device did not infringe under the doctrine of equivalents, and that, in any event, prosecution history estoppel prevented the application of the doctrine of equivalents in this case.

Eventually, after nearly 15 years of litigation, the Supreme Court in Festo VIII remanded the case to determine whether any of the exceptions applied. Upon remand, the Federal Circuit, sitting en banc, held that whether a device met one of the named exceptions was a question of law for the judge, and that the only exception applicable to the facts of the case was the “unforeseeable” exception. Festo X, 344 F.3d 1359 (Fed. Cir. 2003).

After the remand from the Federal Circuit, the district court held a two-day bench trial. The district court then ruled that both of the differences in the SMC device were foreseeable. The aluminum alloy was foreseeable because containing magnetic leakage fields was not necessary to serve the purpose of the invention, and the sealing ring was foreseeable because, in light of Festo’s lack of evidence on the strength of magnets in 1981 (the time of filing) and 1988 (the time of the suit), the court was unwilling to credit Festo’s assertion that advances in magnetic technology had made the use of one sealing ring feasible. Festo appealed the decision to the Federal Circuit.


The Federal Circuit affirmed the district court, holding that the aluminum alloy was a foreseeable modification. The Federal Circuit did not reach the issue of whether the sealing ring was also a foreseeable modification. In reaching this conclusion, the Federal Circuit created a new rule to determine whether an alleged equivalent was foreseeable.

According to the Federal Circuit, an alleged equivalent is unforeseeable to one of ordinary skill in the art at the time of the amendment if “it is known in the field of the invention as reflected in the claim scope before amendment.” The rule applies even if “the suitability of the alternative for the particular purposes defined by the amended claim scope were unknown.” Festo had argued that the proper test was to determine whether the equivalent was foreseeable through application of the function/way/result test. In other words, the proper test, according to Festo, was whether the equivalent was foreseeable to a person of ordinary skill in the art to perform the same function as the claimed device in substantially the same way, to achieve substantially the same result.

As an initial matter, the Federal Circuit noted that Festo’s argument seemed to run counter to the purpose of the doctrine of equivalents. Under Festo’s proposed test, a device having novel elements (elements not known at the time of the invention as being substantially similar to elements in the claim) would be captured by the proposed test. However, the doctrine of equivalents is intended to protect the scope of the invention, not to expand the scope of the invention to cover a new invention.

In addition to pointing out the inconsistency of Festo’s proposed argument with the purposes of the doctrine of equivalents, the Federal Circuit went on to provide several additional reasons for rejecting Festo’s test. First, the function/way/result test is used to determine whether an element of an accused device has only insubstantial differences with an element of the claims, not whether prosecution history estoppel applies to a limiting element. Festo provided, according to the Federal Circuit, no reason to import the function/way/result test from the old context (determining equivalency) to the new context (determining foreseeability and the application of prosecution history estoppel.)

Second, Festo’s test would have the result of removing prosecution history estoppel as a serious restriction on the doctrine of equivalents. The Federal Circuit viewed Festo’s proposal as applying prosecution history estoppel only where an applicant was aware, or should have been aware, that the alleged equivalent would be an equivalent for the purposes of the invention. Under this scenario, prosecution history estoppel would be applied only rarely.

Third, Festo’s proposed test would lead to additional litigation and inconsistent positions taken by parties to a suit. In arguing for application of the doctrine of equivalents, the patentee would assert that the alleged equivalent performed the same function in substantially the same way, to achieve substantially the same result. However, in arguing against application of prosecution estoppel, the patentee would assert that no one skilled in the art would have foreseen the equivalent because the equivalent, at the time, did not perform the same function in substantially the same way, to achieve substantially the same result. The accused infringer would be placed in a similar position. The subtle difference between the two tests, the timing, might easily be lost in the rush of litigation.

Finally, the Federal Circuit faulted Festo’s timing. Whether an equivalent is foreseeable is determined as of before the narrowing amendment, not after the narrowing amendment, as argued by Festo. A foreseeable equivalent to the broad limitation does not become unforeseeable as a result of the narrowing amendment. Rather, it is the narrowing amendment that removes the foreseeable equivalent from the range of possible equivalents. An applicant who is aware of an equivalent under the broader claim, but chooses to amend the claim so as to eliminate the proposed equivalent from the scope of the claims, must have done so intentionally.


Having set forth the new test and turned aside Festo’s arguments, the Federal Circuit applied the test to the facts of the Festo case. In the Festo case, non-magnetizable sleeves were known in the art at the time Dr. Stoll amended the claims. In fact, two German patents, cited by Dr. Stoll in his response, disclosed a sleeve made of a non-magnetic material. In his amendment, Dr. Stoll argued that the amended claims clearly distinguished the invention of the ‘125 patent from the two German patents. Further, the ‘125 patent itself indicated that choosing a magnetic (magnetizable) sleeve over a non-magnetizable sleeve was merely “a matter of personal selection”. Since non-magnetizable sleeves were known in the art at the time of the amendment, the court held that the non-magnetizable sleeves were a foreseeable alternative to the magnetizable sleeves recited in the claim. Accordingly, prosecution history estoppel served to bar the use of the doctrine of equivalents to this element. Because this ground alone was enough to affirm the district court’s holding that the doctrine of equivalents did not apply, the Federal Circuit did not consider whether the one guide ring in SMC’s device was an equivalent to the two guide rings in the amended claim.


Judge Newman issued a forceful dissent. The most important distinction lay in whether the alleged equivalent had to be known as an equivalent at the time of the amendment, or whether the alleged equivalent had merely to exist. The majority opinion held that the alleged equivalent merely needed to exist, and to be known to exist, for the equivalent to be foreseeable. However, Judge Newman believed that the alleged equivalent must also have been known to be an equivalent. To do otherwise would be to apply hindsight reasoning. As Judge Newman pointed out, “hindsight is not foreseeability.”

Significance To Patent Owners

Festo XIII is yet another nail in the coffin for the doctrine of equivalents. Under the rule in this case, patent applicants will be placed in the difficult position of trying to determine whether a potential equivalent, which is not an equivalent at the time of making the amendment, may later become an equivalent due to advances in technology. The seemingly inconsistent result arrived at by the Federal Circuit may tempt lower courts to read the ruling narrowly. For example, Dr. Stoll gave no reason for the amendments at issue in Festo. Further, the specification itself indicated that non-magnetizable sleeves could be used. Given these facts, the Federal Circuit may have been less inclined to give Festo the benefit of the doubt. Avoiding the same fate may be as simple as carefully laying out the reasons for amendments to the claims, so as to limit the presumption of prosecution history estoppel. Any amendment should also be reviewed in light of the specification to see if any equivalents disclosed in the specification, even in passing, are being unnecessarily excluded from the narrower scope.


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