Recent decisions by the Federal Circuit Court of Appeals have breathed life into the doctrine of equivalents. This doctrine is judge-made law that allows patent owners to establish infringement when an accused product or method has skirted the literal wording of the patent claims but is only insubstantially different from the claimed invention.

The claims of a patent typically define the legal scope of the invention. The broader the claims, the more expansive the patent protection and the more difficult it is to get the claims allowed in the U.S. Patent and Trademark Office.

Due to the vagaries of language and invention and the unforeseeability of all variations on an inventive concept, any word or phrase in a patent claim offers aggressive competitors an opportunity to design around the words, while still benefiting from the inventive teachings. The doctrine of equivalents was developed to protect the invention when the claim language turns out to be too limiting.

The patent litigated in Deere & Co. v. Bush Hog, LLC (2012) was directed to a rotary cutter deck that houses one or more powered mower blades. The upper deck slopes downwardly “into engagement with, and being secured to” the lower deck. Rather than directly engaging the upper deck with the lower deck, the accused products included an intermediate structure connecting the upper deck to the lower deck.

The district court judge found no infringement because the upper deck of the accused product did not directly contact the lower deck. The Federal Circuit reversed, stating “into engagement with” did not require direct contact. Indirect contact could suffice.

More important, Chief Judge Rader took the opportunity to explain how a court should apply the doctrine of equivalents. He noted that the lower court judge believed that “into engagement with” was binary in nature, that is, the walls of the deck were either in contact or they were not. The district court feared that allowing a finding of infringement when there was no contact would “vitiate,” that is, give no effect to, that phrase of the patent claim.

Judge Rader cleared up a common misperception about what vitiation is and when it limits application of the doctrine of equivalents. To state the obvious, “in every case applying the doctrine of equivalents at least one claimed element is not literally present in the accused product.” 703 F. 3d at 1356. Thus, absence of an element is not sufficient to preclude the doctrine of equivalents, rather it is necessarily the case when doctrine of equivalents is applied.

Judge Rader declared that “vitiation” is not an exception to the doctrine of equivalents. It merely arises when no reasonable jury could find equivalence between the claim limitation and the corresponding element in the accused product.

Instead, the proper inquiry is “whether an asserted equivalent represents an ’insubstantial difference’ from the claimed element, or ‘whether the substitute element matches the function, way, and result of the claimed element.’” To the delight of patent owners, all district courts should now have a more open mind toward the application of the doctrine of equivalents.

The issue of vitiation came up again, in Brilliant Instruments, Inc. v. Guidetech, LLC (2013). Again, the Federal Circuit reversed a district court judge who had blocked a patentee from trying to prove infringement under the doctrine of equivalents. The case was returned to the district court to let a jury decide if infringement occurred on that basis.

The patent claim in Brilliant involved electronic circuitry that required a shunt and a capacitor that is “operatively disposed in parallel” to a “first current circuit.” The accused product also included a capacitor but it was located within the first circuit rather than parallel to it. For this reason, there was no literal infringement.

The patent owner submitted a technical expert’s report showing equivalents according to the function-way-result test. That is, the expert explained that the disposition of the shunt/capacitor in the accused device was equivalent to the claimed device,

“because it performs substantially the same function (allowing the shunt to control the path of current flow to or from the first current circuit) in substantially the same way (wherein an electrical path from the first current circuit can be traced to either the capacitor or the shunt) to achieve substantially the same result (providing an electrical relationship wherein, e.g., the shunt can direct current to flow from the first current circuit to the second current circuit or from the first current circuit to the capacitor).”

This analysis went unrebutted by the alleged infringer, so the Federal Circuit ordered that the patentee get its chance to convince a jury that its patent was infringed under the doctrine of equivalents. As in Bush Hog, the Court stressed that “vitiation”—raised defensively by the accused infringer–is not an exception to the doctrine of equivalents, but a legal determination that no reasonable jury could view the elements as equivalent.

Vitiation cannot be established simply by noting that an element is missing from the claimed structure, because, by definition, the doctrine of equivalents recognizes that an element is missing that must be supplied by an equivalent substitute. Eliminating the reflexive invocation of vitiation in response to a charge of infringement under the doctrine of equivalents lends further vitality to the enforcement strategies of patent owners.

For patent owners, these two Federal Circuit decisions are encouraging. The doctrine of equivalents, often criticized as unwieldy and thought to be of declining usefulness, has been rehabilitated and may prove once again to be an effective weapon against infringers.