Deere & Co. owns US patent No. 6,052,980 (the '980 patent), a patent concerning a dual wall deck for a rotary cutter. Bush Hog, LLC manufactures rotary cutters used to cut fields after harvest or clear weeds and brush along roadsides. The '980 patent claim involves the dual-wall deck with front and rear portions that come "into engagement with" the lower deck wall.

The US District Court for the Southern District of Iowa construed "into engagement with" to require direct contact and granted Bush Hog's motion for summary judgment of noninfringement because there was no direct contact. The district court also held that Deere could not assert infringement under the doctrine of equivalents because doing so would vitiate the "into engagement with" limitation. Because the "all elements rule" requires that each asserted patent claim limitation is found in the accused product or process, the doctrine of equivalents cannot apply if doing so vitiates a claim element. Deere appealed the district court's decision to the US Court of Appeals for the Federal Circuit.

In its December 4, 2012 opinion, the Federal Circuit affirmed some of the district court's claim construction rulings, but vacated the district court's construction of "into engagement with" and reversed the district court's grant of summary judgment of noninfringement.

On appeal, the Federal Circuit held that the district court's construction of "into engagement with" was incorrectly binary in nature, requiring that the upper deck wall either contacted the lower deck wall or did not. Instead, the Federal Circuit construed "into engagement with" to encompass indirect contact.

The court also vacated the district court's finding that Deere was barred from asserting infringement under the doctrine of equivalents. Based on its erroneous binary construction, the district court held that Deere could not assert infringement under the doctrine of equivalents because doing so would vitiate a claim element (that is, a claim that requires direct contact could not be satisfied by indirect contact).

In dicta, the Federal Circuit warned that the district court's construction did not preserve the proper scope of the doctrine of equivalents. Under the doctrine of equivalents, a claim element is by definition missing and must be satisfied by an equivalent element. Therefore, instead of assuming an element is either present or not, the Federal Circuit noted that courts should focus on the fundamental question of whether there is a genuine factual issue that the accused device, while literally omitting a claim element, nonetheless incorporates through an equivalent structure.

The vitiation test should therefore not serve as an exception to the doctrine of equivalents. Rather, the vitiation test serves as a legal determination that no reasonable jury could evidentially determine that two elements equivalent.